Refractories
Refractories are materials that can withstand high temperatures without softening or deformation in shape. Refractories are mainly used for construction of lining in furnaces, kilns, converters, etc. The main function of a refractory is to withstand and maintain high temperatures and to resist the abrasive and corrosive action of molten metals, slags and gases.
Objective of a refractory
The main objective of a refractory is to resist heat losses and also to resist the abrasive and corrosive action of molten metals, slags and gases at higher temperatures, without softening or deformation in shape.
Uses of Refractories
1. Refractories are mostly used for the construction of the lining of the furnaces, tanks, converters, kilns, crucibles, ladles, etc.
2. They are employed for the manufacture of cement, glass, ceramics, paper, metals (both ferrous and non-ferrous ), etc.
Characteristics or requisites of a good refractory
(i) It should be infusible at the operating temperatures.
(ii) It nshould be chemically inert towards the corrosive gases, metallic slags and liquids.
(iii) It should resist the abrading action of flue gases, flames, etc.
(iv) It should not crack and suffer loss in size, at the operating temperatures.
(v) It should expand and contract uniformly, with temperature rise and fall respectively.
(vi) It should be able to withstand overlying load of structures, at operating temperatures.
(vii) It should have high refractoriness.
Classification of refractories
Refractories are classified in the following two ways
According to their chemical properties.
According to their refractoriness.
I. According to their chemical properties
According to the chemical properties, refractories are classified into three main types
Examples (i) Silicon (i) Magnesite (i) Graphite
(ii) Alumina (ii) Dolomite (ii) Carborundum
1. Acidic Refractories
Acidic refractories consists of acidic materials loke alumina(al203) and
Silica (SiO2). They are not attacked by acidic materials, but easily attacked by
basic materials.
Example:
Silica, alumina, fire clay refractories.
2. Basic Refractories
Basic Refractories consist of basic materials like Ca02, Mgo, etc. They are
not attacked by basic materials, but are easily attacked by acidic materials.
Example:
Magnesite, Dolamite refractories.
3. Neutral Refractories
Neutral refractories are made from weakly acidic and basic materials like Carbon, Chromite, Zirconia, etc. They are not attacked by both acidic and basic materials.
Example:
Graphite, Chromite, Zirconia, Carborundum refractories.
II. According to their refractoriness
According to the refractoriness, refractories are classified into four types.
Type of Refractories with PCE value
Example
1. Low heat duty refractories
2. Intermediate heat duty refractories
3. High heat duty refractories
4. Super heat duty refractories
Tuesday, November 17, 2009
Properties of refractories
Properties of refractories
1. Refractoriness
It is the ability of a material to withstand very high temperature without softening or deformation under particular service condition.
How to measure refractoriness
Since most of the refractories are mixtures of several metallic oxides, they do not have a sharp melting points. So the refractoriness of a refractory is generally measured as the softening temperature and is expressed interms of Pyrometric cone Equivalent (PCE).
Pyrometric cone Equivalent (PCE)
Pyrometric cone equivalent is the number which represents the softening temperature of a refractory specimen of standard dimension (38 mm and 19 mm triangular base) and composition.
Objectives of PCE Test
1. To determine the softening temperature of a test refractory material.
2. To classify the refractories.
3. To determine the purity the refractories .
4. To check whether the refractory can be used at the particular servicing temperature.
Measurement
Refractoriness is determined by comparing the softening temperature of a test cone with that of a series of segar cones. Segar cone (also called pyrometric cones) are pyramid shapes.
A test cone is prepared from a refractory, for which the softening temperature to be determined as same dimensions of segar cones. Then test cone is placed in an electric furnace along with segar cones. The furnace is heated at a standard rate of 10° C per minute during which softening of segar cone occur along with test cone. The temperature at which the apex of the cone touches the base is softening temperature.
1. Refractoriness
It is the ability of a material to withstand very high temperature without softening or deformation under particular service condition.
How to measure refractoriness
Since most of the refractories are mixtures of several metallic oxides, they do not have a sharp melting points. So the refractoriness of a refractory is generally measured as the softening temperature and is expressed interms of Pyrometric cone Equivalent (PCE).
Pyrometric cone Equivalent (PCE)
Pyrometric cone equivalent is the number which represents the softening temperature of a refractory specimen of standard dimension (38 mm and 19 mm triangular base) and composition.
Objectives of PCE Test
1. To determine the softening temperature of a test refractory material.
2. To classify the refractories.
3. To determine the purity the refractories .
4. To check whether the refractory can be used at the particular servicing temperature.
Measurement
Refractoriness is determined by comparing the softening temperature of a test cone with that of a series of segar cones. Segar cone (also called pyrometric cones) are pyramid shapes.
A test cone is prepared from a refractory, for which the softening temperature to be determined as same dimensions of segar cones. Then test cone is placed in an electric furnace along with segar cones. The furnace is heated at a standard rate of 10° C per minute during which softening of segar cone occur along with test cone. The temperature at which the apex of the cone touches the base is softening temperature.
characteristics of good refractory
A good refractory should have high refractoriness.
1. Refractoriness under load ( RUL) (or) strength
The temperature at which the refractory deforms by 10% is called refractoriness under load (RUL).
Refractories used in industries and in metallurgical operations, should bear varying loads. Hence refractories should have high mechanical strength under operating temperatures. Generally softening temperature decreases with increase of load. The load bearing capacity of a refractory can be measured by RUL test.
RUL Test
RUL test is conducted by applying a constant load of 3.5 or 1.75 kg/cm2 to the test refractory specimen of size base 5 cm2 and height 75 cm and heating in a furnace at a standard rate of 10°C per minute. A good refractory should have high RUL value.
2. Porosity
It is defined as the ratio of its pore volume to the bulk volume. Porosity is an important property of refractory bricks, because it affect many other characteristics like chemical stability, strength, abrasion- resistance and thermal conductivity.
Disadvantages of high porosity refractory
(i) It reduces the strength.
(ii) It reduces resistance to abrasion.
(iii) It reduces the resistance to corrosion.
Advantages of high porosity refractory
(i) Highly porous refractory possess lower thermal conductivity. This is due to presence of more air voids, which act as insulators and hence it be used for lining in furnaces, ovens, etc.
(ii) Highly porous refractory reduces thermal spalling.
A good refractory should have low porosity.
4. Thermal spalling
Thermal spelling is the property of breaking, cracking or peeling off a refractory material under high temperature.
Thermal spalling is mainly due to
(i) Rapid change in temperature
This causes uneven expansion and contraction within the mass of a refractory, and leads to development of uneven stresses and strains.
(ii) Slag penetration
This causes variation in the co-effcient of expansion and leads to spalling.
Thermal spalling can be decrased by
(i) Using high porosity , low co-effcient of expansion and good thermal conductivity refractory.
(ii) A voiding sudden temperature changes.
(iii) By modifying the furnace design.
A good refractory must show a very good resistance to thermal splling
5. Dimensional stability
It is the resistance of a refractory to any volume changes, when exposed to high temperature over a prolonged time.
These dimensional changes are of two types
(i) Reversible or (ii) Irreversible.
(i) Reversible dimensional changes
This may result due to the uniform expansion and contraction of a refractory material. So the dimensional changes of a good refractory must be reversible.
(ii) Irreversible dimensional changes
This may result either in the contraction or expansion of a refractory.
Example-1
Magnesite brick shrink in service
Magnesite is an amorphous material (specific gravity is 3.05). On heating it is gradually converted into more dense crystalline form of periclase (Sp. Gravity = 3.54)
Magnesite Periclase (Amorphous) (Crytalline)
Sp.gr = 3.05 sp.gr = 3.54
Example-2
Silica bricks expand in service.
Silica bricks expand on heating due to the transformation of one from to anther forms. This is accompanied by a considerable increase in volume. A good refractory should have high dimentional stability.
1. Refractoriness under load ( RUL) (or) strength
The temperature at which the refractory deforms by 10% is called refractoriness under load (RUL).
Refractories used in industries and in metallurgical operations, should bear varying loads. Hence refractories should have high mechanical strength under operating temperatures. Generally softening temperature decreases with increase of load. The load bearing capacity of a refractory can be measured by RUL test.
RUL Test
RUL test is conducted by applying a constant load of 3.5 or 1.75 kg/cm2 to the test refractory specimen of size base 5 cm2 and height 75 cm and heating in a furnace at a standard rate of 10°C per minute. A good refractory should have high RUL value.
2. Porosity
It is defined as the ratio of its pore volume to the bulk volume. Porosity is an important property of refractory bricks, because it affect many other characteristics like chemical stability, strength, abrasion- resistance and thermal conductivity.
Disadvantages of high porosity refractory
(i) It reduces the strength.
(ii) It reduces resistance to abrasion.
(iii) It reduces the resistance to corrosion.
Advantages of high porosity refractory
(i) Highly porous refractory possess lower thermal conductivity. This is due to presence of more air voids, which act as insulators and hence it be used for lining in furnaces, ovens, etc.
(ii) Highly porous refractory reduces thermal spalling.
A good refractory should have low porosity.
4. Thermal spalling
Thermal spelling is the property of breaking, cracking or peeling off a refractory material under high temperature.
Thermal spalling is mainly due to
(i) Rapid change in temperature
This causes uneven expansion and contraction within the mass of a refractory, and leads to development of uneven stresses and strains.
(ii) Slag penetration
This causes variation in the co-effcient of expansion and leads to spalling.
Thermal spalling can be decrased by
(i) Using high porosity , low co-effcient of expansion and good thermal conductivity refractory.
(ii) A voiding sudden temperature changes.
(iii) By modifying the furnace design.
A good refractory must show a very good resistance to thermal splling
5. Dimensional stability
It is the resistance of a refractory to any volume changes, when exposed to high temperature over a prolonged time.
These dimensional changes are of two types
(i) Reversible or (ii) Irreversible.
(i) Reversible dimensional changes
This may result due to the uniform expansion and contraction of a refractory material. So the dimensional changes of a good refractory must be reversible.
(ii) Irreversible dimensional changes
This may result either in the contraction or expansion of a refractory.
Example-1
Magnesite brick shrink in service
Magnesite is an amorphous material (specific gravity is 3.05). On heating it is gradually converted into more dense crystalline form of periclase (Sp. Gravity = 3.54)
Magnesite Periclase (Amorphous) (Crytalline)
Sp.gr = 3.05 sp.gr = 3.54
Example-2
Silica bricks expand in service.
Silica bricks expand on heating due to the transformation of one from to anther forms. This is accompanied by a considerable increase in volume. A good refractory should have high dimentional stability.
General methods of manufacture of refractories
General methods of manufacture of refractories
The manufacture of refractories involve the following steps.
1. Grinding
Raw materials crushed and ground to fine powder using crushers, pulveriser, ball mills.
2. Mixing
In order to alter the chemical properties of the refractories, two or more powdered raw materials are thoroughly mixed with a suitable binding material, which makes moulding easier.
3. Moulding
Moulding can be done either manually or mechanically by application of high pressure.
Hand-moulding produces refractories of low strength and low density. Mechanical-moulding produces refractories of high strength and high density
4. Drying
Drying is carried out slowly to the moisture from refractories.
5. Firing
It is done at a temperature as high as or higher than their use temperature . Firing is generally carried out in kilns.
The refractories are fired,
(i) to stabilize and strengthen their structures.
(ii) to remove water of hydration.
(iii) to facilitate development of stable mineral to form the finished products.
The manufacture of refractories involve the following steps.
1. Grinding
Raw materials crushed and ground to fine powder using crushers, pulveriser, ball mills.
2. Mixing
In order to alter the chemical properties of the refractories, two or more powdered raw materials are thoroughly mixed with a suitable binding material, which makes moulding easier.
3. Moulding
Moulding can be done either manually or mechanically by application of high pressure.
Hand-moulding produces refractories of low strength and low density. Mechanical-moulding produces refractories of high strength and high density
4. Drying
Drying is carried out slowly to the moisture from refractories.
5. Firing
It is done at a temperature as high as or higher than their use temperature . Firing is generally carried out in kilns.
The refractories are fired,
(i) to stabilize and strengthen their structures.
(ii) to remove water of hydration.
(iii) to facilitate development of stable mineral to form the finished products.
Refractory bricks
Some important refractory bricks
Alumina bricks (or) Fire clay bricks (Acidic refractories)
Alumina bricks contain 50% or more of Al2O3. They are generally manufactured by mixing calcined bauxite (Al2O3 ) with clay binder.
Manufacture
1. Grinding and mixing
The raw materials (calcined bauxite & SiO2 ) and grog (calcined fire clay ) are ground to fine poeder and mixed and mixed with required amount of water to convert it into pasty material.
2. Moulding
The pasty material is converted into bricks by the general moulding technique like machine pressing or slip casting.
3.Drying and Firing
The bricks after moulding is dried slowly to remove the moisture and then fired in continuous kiln or tunnel kiln to about 1200-1400 for 6-8 days.
Properties
(i) Alumina bricks are acidic refractories.
(ii) They posses very low coefficient of expansion.
(iii) They also posses high porosity, and high temperature load-bearing capacity.
(iv) They are also very stable to both in oxidizing and reducing conditions.
(v) Then posses better resistance to thermal to thermal spalling than silica bricks.
Uses
1. Medium-duty bricks (containing 50 to 60% Al2O3)
It is used in lining of cement rotary kilns soaking pita, reheating furnaces, hearts and walls, etc., which are subjected to high abrasion.
2. High-dury bricts (containing 75% Al2O3)
It is used in hottest zone of cement rotary kilns, lower parts of soaking pits, brass melting reverberatories, aluminium melting furnaces, etc.,
3. Fire clay refractories are larely using in steel industries
Magnesite bricks (Basic refractories )
Mangesite bricks contain mainly MgO. They are generally manufactured by mixing calcined magnesite with caustic magnesia or iron oxide as binding material.
Manufacture
1. Grinding and mixing
The raw materials (calcined magnesite) and binding materials (caustic magnesia) are ground to fine powder and mixed with water to a pasty material.
2. Moulding
Moulding is usually done by machine pressing to a required shape.
3. Drying and firing
Drying is carried out at ordinary temperature to remove the moisture. Firing is done in a kiln at 1500oC for 8 hours and then cooled slowly.
Properties
1. Magnesite bricks are basic refractories.
2. They can be used upto 2000oC without load and upto 1500oC Under a load of 3.5 kg/cm2.
3. They have good resistance to basic slags, but combine with H2O and CO2
4 . They possess good strength, little shrinkage and have lot of spalling.
5. They have poor resistance to abrasions.
Uses
1. They are used where high temperature is required to be maintained, together with great resistance to basic materials.
2. They are used in steel industry for the lining of basic converters and and open –hearth furnaces.
3. They are also used in hot mixer linings, copper converters and reverberatory furnaces.
Zirconia bricks ( Neutral refractories)
Manufacture
They are prepared by mixing zirconite mineral (ZrO2 ) with colloidal zirconia or alumina as binder and finally heated to 1700oC. Small amount of MgO or Cao is added as stabilizer because mineral zirconite undergoes volume changes on hearting and cooling.
Properties
1. Zirconia bricks are neutral refractories.
2. Though zirconia bricks are neutral, they are affected by acidic slags.
3. They can be used upto 2000oC and upto 1500oC under a load of 3.5 kg/cm2 .
4. They are also quie resistant to thermal shocks.
5. Their thermal expansion is low.
Uses
They are used only where very high temperature is maintained, e.g., high – frequency electric furnaces.
Alumina bricks (or) Fire clay bricks (Acidic refractories)
Alumina bricks contain 50% or more of Al2O3. They are generally manufactured by mixing calcined bauxite (Al2O3 ) with clay binder.
Manufacture
1. Grinding and mixing
The raw materials (calcined bauxite & SiO2 ) and grog (calcined fire clay ) are ground to fine poeder and mixed and mixed with required amount of water to convert it into pasty material.
2. Moulding
The pasty material is converted into bricks by the general moulding technique like machine pressing or slip casting.
3.Drying and Firing
The bricks after moulding is dried slowly to remove the moisture and then fired in continuous kiln or tunnel kiln to about 1200-1400 for 6-8 days.
Properties
(i) Alumina bricks are acidic refractories.
(ii) They posses very low coefficient of expansion.
(iii) They also posses high porosity, and high temperature load-bearing capacity.
(iv) They are also very stable to both in oxidizing and reducing conditions.
(v) Then posses better resistance to thermal to thermal spalling than silica bricks.
Uses
1. Medium-duty bricks (containing 50 to 60% Al2O3)
It is used in lining of cement rotary kilns soaking pita, reheating furnaces, hearts and walls, etc., which are subjected to high abrasion.
2. High-dury bricts (containing 75% Al2O3)
It is used in hottest zone of cement rotary kilns, lower parts of soaking pits, brass melting reverberatories, aluminium melting furnaces, etc.,
3. Fire clay refractories are larely using in steel industries
Magnesite bricks (Basic refractories )
Mangesite bricks contain mainly MgO. They are generally manufactured by mixing calcined magnesite with caustic magnesia or iron oxide as binding material.
Manufacture
1. Grinding and mixing
The raw materials (calcined magnesite) and binding materials (caustic magnesia) are ground to fine powder and mixed with water to a pasty material.
2. Moulding
Moulding is usually done by machine pressing to a required shape.
3. Drying and firing
Drying is carried out at ordinary temperature to remove the moisture. Firing is done in a kiln at 1500oC for 8 hours and then cooled slowly.
Properties
1. Magnesite bricks are basic refractories.
2. They can be used upto 2000oC without load and upto 1500oC Under a load of 3.5 kg/cm2.
3. They have good resistance to basic slags, but combine with H2O and CO2
4 . They possess good strength, little shrinkage and have lot of spalling.
5. They have poor resistance to abrasions.
Uses
1. They are used where high temperature is required to be maintained, together with great resistance to basic materials.
2. They are used in steel industry for the lining of basic converters and and open –hearth furnaces.
3. They are also used in hot mixer linings, copper converters and reverberatory furnaces.
Zirconia bricks ( Neutral refractories)
Manufacture
They are prepared by mixing zirconite mineral (ZrO2 ) with colloidal zirconia or alumina as binder and finally heated to 1700oC. Small amount of MgO or Cao is added as stabilizer because mineral zirconite undergoes volume changes on hearting and cooling.
Properties
1. Zirconia bricks are neutral refractories.
2. Though zirconia bricks are neutral, they are affected by acidic slags.
3. They can be used upto 2000oC and upto 1500oC under a load of 3.5 kg/cm2 .
4. They are also quie resistant to thermal shocks.
5. Their thermal expansion is low.
Uses
They are used only where very high temperature is maintained, e.g., high – frequency electric furnaces.
Abrasives
Abrasives
Definition
Abrasive are hard substances used for polishing shaping operations. They are characterized by high melting point high hardness chemically inactive.
Properties of abrasives
1. Hardness
It is the ability of an abrasive to grind or scratch away other materials. The harder the abrasive quicker will be its abrading action. Hardness of the abrasive is measured on Moh’s scale or Vicker’s scale.
Measurement of hardness using Moh’s scale.
Moh’s scale is a scale, in which common abrasives (natural or artificial) are arranged in the order of their increasing hardness.
Soft abrasives
Abrasives having their hardness 1-4 in Moh’s scale are know as abrasives.
2. Toughness
Abrasive are generally hard and brittle, which is otherwise known as toughness.
3. Abrasive power
It is the strength of an abrasive to grind away another materials. It depends on hardness, toughness and refractoriness.
Characteristics of abrasives
The following are the important characteristics of an abrasive.
(a) It should be very hard.
(b) It should resist the abrading action.
(c) It should be chemically inactive.
(d) It should possess high refractoriness.
(e) It should have high melting point.
(f) It should not be affected by frictional heat.
Classification of abrasives
Abrasive are classified into two types
1. Natural abrasive
(a) Nonsiliceous abrasive (d) Siliceous abrasives
2. Artificial or synthetic abrasive
Definition
Abrasive are hard substances used for polishing shaping operations. They are characterized by high melting point high hardness chemically inactive.
Properties of abrasives
1. Hardness
It is the ability of an abrasive to grind or scratch away other materials. The harder the abrasive quicker will be its abrading action. Hardness of the abrasive is measured on Moh’s scale or Vicker’s scale.
Measurement of hardness using Moh’s scale.
Moh’s scale is a scale, in which common abrasives (natural or artificial) are arranged in the order of their increasing hardness.
Soft abrasives
Abrasives having their hardness 1-4 in Moh’s scale are know as abrasives.
2. Toughness
Abrasive are generally hard and brittle, which is otherwise known as toughness.
3. Abrasive power
It is the strength of an abrasive to grind away another materials. It depends on hardness, toughness and refractoriness.
Characteristics of abrasives
The following are the important characteristics of an abrasive.
(a) It should be very hard.
(b) It should resist the abrading action.
(c) It should be chemically inactive.
(d) It should possess high refractoriness.
(e) It should have high melting point.
(f) It should not be affected by frictional heat.
Classification of abrasives
Abrasive are classified into two types
1. Natural abrasive
(a) Nonsiliceous abrasive (d) Siliceous abrasives
2. Artificial or synthetic abrasive
Natural abrasive
Natural abrasive
(a) Non- Siliceous Abrasives
It is a pure crystalline carbon. It is the hardest known substance. Its hardness is 10 on Moh’s scale. It is chemically inert and not affected by acids or alkalis. The off- colour diamond is called borts and black colour diamond is called carbonado.
Uses : It is used in drill points, cutting rocks, stones and grinding wheels.
2. Corundum
It is a pure crystalline alumina (Al2O3). Its hardness on Moh’s scale is 9.
3. Emery
It is a used for grinding glasses, gems, lenses, metals,etc.
(i) 55-75% crystalline alumina,
(ii) 20-40% magnesite,
(iii) 12% other minerals.
Uses : It is a used in the tip of cutting and drilling tools, and also it is used in making abrasive paper and cloth.
(b) Siliceous abrasive
1. Quartz
It is pure crystalline silica (SiO2). Its hardness is 7 on Moh’s scale.
Uses: It is used for grinding pigments in the paint industry and also as granules in grinding machines.
2. Garnet
It is a mixture of trisilicates of alumina magnesia and ferrous oxide. Its hardness ranges from 6-7.5 on Moh’s scale.
Uses : It is used in making abrasive paper and abrasive cloth and also in glass grinding and polishing metals.
(a) Non- Siliceous Abrasives
It is a pure crystalline carbon. It is the hardest known substance. Its hardness is 10 on Moh’s scale. It is chemically inert and not affected by acids or alkalis. The off- colour diamond is called borts and black colour diamond is called carbonado.
Uses : It is used in drill points, cutting rocks, stones and grinding wheels.
2. Corundum
It is a pure crystalline alumina (Al2O3). Its hardness on Moh’s scale is 9.
3. Emery
It is a used for grinding glasses, gems, lenses, metals,etc.
(i) 55-75% crystalline alumina,
(ii) 20-40% magnesite,
(iii) 12% other minerals.
Uses : It is a used in the tip of cutting and drilling tools, and also it is used in making abrasive paper and cloth.
(b) Siliceous abrasive
1. Quartz
It is pure crystalline silica (SiO2). Its hardness is 7 on Moh’s scale.
Uses: It is used for grinding pigments in the paint industry and also as granules in grinding machines.
2. Garnet
It is a mixture of trisilicates of alumina magnesia and ferrous oxide. Its hardness ranges from 6-7.5 on Moh’s scale.
Uses : It is used in making abrasive paper and abrasive cloth and also in glass grinding and polishing metals.
Synthetic abrasives
Synthetic abrasives
1. Silicon carbide (or) Carborundum (SiC)
Manufacture
Silicon carbide is manufactured by heating sand (60%) and coke (40%) with some saw-dust and a little salt in an electric furnace to about 1500 deg C.
Saw- dust evolves gases during burning which on circulation, increases the porosity of the charge. Salt reacts with iron and other similar impurities present in the raw materials, forming volatile chlorides. This also increases the porosity of the final product.
1500oC
SiO2 + 3C --------> SiC + 2CO
The silicon carbide, removed from the furnace, is then mixed with bonding agent (like clay, silicon nitride) and than shaped, dried and fired.
Properties
1. Silicon carbide possess a high thermal conductivity, low expansion and resistance to abrasion and spalling.
2. They are mechanically strong and with stand loads in furnaces upto 1650oC.
3. Heat conductivity of SiC is intermediate between metals and ceramic materials.
4. They are electrivity intermediate between conductors ans insulators.
5. The strength, density, abrasion-resistance, chemical resistance softening temperature of the various bonded refractories will be in the following order.
Self – bonded product > silicon nitride product > clay – bonbed product.
Uses
1. Silicon carbides are used as heating elements in furnace in the from of rods and bars.
2. They are also used for partition walls of chamber kilns, coke ovens muffle furnace and floors of heat-treatment funaces.
3. SiC bonded with tar are excellent for making high conductivity crucibles.
2. Norbide (or) Boron Carbide (B4C)
Manufacture
It is prepared by heating a mixture of boron oxide (B2O3) and coke (carbon ) in an electric furnace to about 2700oC.
2700oC
2 B2O3 + 7C ------> B4C + 6CO
Properties
1. Its hardness is 9 on moh’s scale.
2. It is light weigt and blackcoloured compound.
3. It is highly resistant to chemical attack and erosion.
4. It resists oxidation much better than diamond.
Uses
(i) It is used as hard materials for cutting and sharpening hard high- speed tools.
(ii) It is used to prepare scratch and wear resistant coatings.
Alundum (Al2O3)
Manufacture
It is prepared by heating a mixture of calcined bauxite, coke and iron in an electric furnace to about 4000oC. It is an artifical corundum. It is hard as carborundum but is less brittle tougher.
Uses
(i) It is used in grinding of hard steels and other materials of high tensile-strengths.
(ii) It is also used in the manufacture of abrasive wheels.
1. Silicon carbide (or) Carborundum (SiC)
Manufacture
Silicon carbide is manufactured by heating sand (60%) and coke (40%) with some saw-dust and a little salt in an electric furnace to about 1500 deg C.
Saw- dust evolves gases during burning which on circulation, increases the porosity of the charge. Salt reacts with iron and other similar impurities present in the raw materials, forming volatile chlorides. This also increases the porosity of the final product.
1500oC
SiO2 + 3C --------> SiC + 2CO
The silicon carbide, removed from the furnace, is then mixed with bonding agent (like clay, silicon nitride) and than shaped, dried and fired.
Properties
1. Silicon carbide possess a high thermal conductivity, low expansion and resistance to abrasion and spalling.
2. They are mechanically strong and with stand loads in furnaces upto 1650oC.
3. Heat conductivity of SiC is intermediate between metals and ceramic materials.
4. They are electrivity intermediate between conductors ans insulators.
5. The strength, density, abrasion-resistance, chemical resistance softening temperature of the various bonded refractories will be in the following order.
Self – bonded product > silicon nitride product > clay – bonbed product.
Uses
1. Silicon carbides are used as heating elements in furnace in the from of rods and bars.
2. They are also used for partition walls of chamber kilns, coke ovens muffle furnace and floors of heat-treatment funaces.
3. SiC bonded with tar are excellent for making high conductivity crucibles.
2. Norbide (or) Boron Carbide (B4C)
Manufacture
It is prepared by heating a mixture of boron oxide (B2O3) and coke (carbon ) in an electric furnace to about 2700oC.
2700oC
2 B2O3 + 7C ------> B4C + 6CO
Properties
1. Its hardness is 9 on moh’s scale.
2. It is light weigt and blackcoloured compound.
3. It is highly resistant to chemical attack and erosion.
4. It resists oxidation much better than diamond.
Uses
(i) It is used as hard materials for cutting and sharpening hard high- speed tools.
(ii) It is used to prepare scratch and wear resistant coatings.
Alundum (Al2O3)
Manufacture
It is prepared by heating a mixture of calcined bauxite, coke and iron in an electric furnace to about 4000oC. It is an artifical corundum. It is hard as carborundum but is less brittle tougher.
Uses
(i) It is used in grinding of hard steels and other materials of high tensile-strengths.
(ii) It is also used in the manufacture of abrasive wheels.
Applications of abrasives
Applications of abrasives
Abrasives are used in their forms
As loose powder
To clean the surface prior to coating, abrasive powders.
Example: Quartz and Garnet.
As abrasive paper (or) cloth
Manufacture of abrasive paper or cloth
The roll of paper or cloth is made to pass through a series of rollers, and a thin coating of glue is applied on its upper side. It is then passed under a hopper from which the grit abrasive is allowed to fall and spread evenly on the glued paper or cloth. Then it is dried in warm drying room.. Finally it is allowed to age few days, so that the glue sets firmly.
Uses: It is used to prepare smooth wood metal and plastic surfaces.
Example: Alumina and Silicon carbide.
As grinding wheels
Manufacture of grinding wheel
Grinding wheel is manufactured by mixing abrasive grains with binder.The mixture is moulded into desired shape and heated and cured.
Uses: It is used for the removal of rust from iron surfaces cutting tool sharpening.
Abrasives are used in their forms
As loose powder
To clean the surface prior to coating, abrasive powders.
Example: Quartz and Garnet.
As abrasive paper (or) cloth
Manufacture of abrasive paper or cloth
The roll of paper or cloth is made to pass through a series of rollers, and a thin coating of glue is applied on its upper side. It is then passed under a hopper from which the grit abrasive is allowed to fall and spread evenly on the glued paper or cloth. Then it is dried in warm drying room.. Finally it is allowed to age few days, so that the glue sets firmly.
Uses: It is used to prepare smooth wood metal and plastic surfaces.
Example: Alumina and Silicon carbide.
As grinding wheels
Manufacture of grinding wheel
Grinding wheel is manufactured by mixing abrasive grains with binder.The mixture is moulded into desired shape and heated and cured.
Uses: It is used for the removal of rust from iron surfaces cutting tool sharpening.
Lubricants
Lubricants
Introduction
In all type of machines, the moving surfaces rub against each other. Due to this rubbing a resistance is offered to their movement. This resistance is known as Friction. This friction will cause a lot of wear and tear of surfaces of moving parts. Due to the friction large amount of energy is dissipated in the from of heat therby the efficiency of machine gets reduced.
Definitions
Lubricant
Lubricant is a substance used in between two moving surfaces to reduce the friction.
Lubrication
Lubrication is a process of reducing friction and wear between two moving surfaces by adding lubricant in between them.
Functions of a lubricant
1. It prevents the contact between the moving surfaces and reduces wear tear and surface deformation of the concerned parts.
2. It reduces the energy so that efficiency of the machine is enhanced.
3. It reduces the frictional heat and prevent the expansion of metals.
4. It acts as a coolant by removing the frictional heat generated due to the rubbing of surfaces.
5. At sometime it acts as a seal preventing the entry of dust and leakage of gases at high pressure.
6. It reduces the maintenance and running cost of the machine.
7. It minimizes corrosion.
Requirements (or) Characteristics of a lubricant
1. A good lubricant should not undergo any decomposition, oxidation, reduction at high temperature.
2. A good lubricant should have higher flash and fire point than the operating temperature.
3. A good lubricant should have high oiliness, viscosity index aniline point.
4. A good lubricant should not corrode machine parts.
Classification of lubricants
Lubricants are classified on the basis of their physical state as follows.
1. Liquid lubricants
(a) Vegetable oils – Palm oil, castor oil, etc.
(b) Animal oils - Whale oil, tallow oil, etc.
(c ) Mineral oils – Petroleum fractions.
(d) Synthetic lubricant – Silicones, polyglycol ethers, etc.
(e) Blended oils (or) Compounded – Mineral oils with various additives
2. Semi- solid lubricants
Greases, vaselines, etc.
3. Solid lubricants
Graphite, molybdenum-disulphide, etc.
4. Emulsions
(a) Oil in water type – Cutting emulsions.
(b) Water in oil type – Cooling liquids.
Introduction
In all type of machines, the moving surfaces rub against each other. Due to this rubbing a resistance is offered to their movement. This resistance is known as Friction. This friction will cause a lot of wear and tear of surfaces of moving parts. Due to the friction large amount of energy is dissipated in the from of heat therby the efficiency of machine gets reduced.
Definitions
Lubricant
Lubricant is a substance used in between two moving surfaces to reduce the friction.
Lubrication
Lubrication is a process of reducing friction and wear between two moving surfaces by adding lubricant in between them.
Functions of a lubricant
1. It prevents the contact between the moving surfaces and reduces wear tear and surface deformation of the concerned parts.
2. It reduces the energy so that efficiency of the machine is enhanced.
3. It reduces the frictional heat and prevent the expansion of metals.
4. It acts as a coolant by removing the frictional heat generated due to the rubbing of surfaces.
5. At sometime it acts as a seal preventing the entry of dust and leakage of gases at high pressure.
6. It reduces the maintenance and running cost of the machine.
7. It minimizes corrosion.
Requirements (or) Characteristics of a lubricant
1. A good lubricant should not undergo any decomposition, oxidation, reduction at high temperature.
2. A good lubricant should have higher flash and fire point than the operating temperature.
3. A good lubricant should have high oiliness, viscosity index aniline point.
4. A good lubricant should not corrode machine parts.
Classification of lubricants
Lubricants are classified on the basis of their physical state as follows.
1. Liquid lubricants
(a) Vegetable oils – Palm oil, castor oil, etc.
(b) Animal oils - Whale oil, tallow oil, etc.
(c ) Mineral oils – Petroleum fractions.
(d) Synthetic lubricant – Silicones, polyglycol ethers, etc.
(e) Blended oils (or) Compounded – Mineral oils with various additives
2. Semi- solid lubricants
Greases, vaselines, etc.
3. Solid lubricants
Graphite, molybdenum-disulphide, etc.
4. Emulsions
(a) Oil in water type – Cutting emulsions.
(b) Water in oil type – Cooling liquids.
Liquid lubricants
Liquid lubricants or lube oils
Vegetable and Animal oils
These are glycerides of higher fatty acids have very good oiliness. However, these oils cannot be used effectively, because
(i) They undergo oxidation at higher temperature and forms gummy and acidic products.
(ii) Also, they get hydrolysed easily under moist conditions. Actually, they are used as “blending agents” with other lubricating oils.
Mineral oils (or) Petroleum oils
It is obtained by fractional distillation of crude petroleum oil. The length of the hydrocarbon chain varies from C12 to C50. It is cheap and quite stable under normal operating condition. But it posses poor oilness. The oilness of which can be improved by mixing it with animal (or) vegetable oils.
The mineral oil obtained cannot be used as such because it contains a lot of impurities such as wax asphalt oxidisable impurities etc. These impurities have to remove from the mineral oil before using it as a lubricant. The impurities are generally removed by the following methods.
(a) Removal of waxes
Waxes get separated at lower temperature and interfere with lubricating properties.
The wax can be removed by dewaxing process in which the petroleum oil is mixed with a suitable solvent (propane trichloroethyl etc.,) and then cooled. The wax crystallises out and is removed by filtration.
(b) Removal of asphalt
Asphaltic and naphthenic materials tend to leave carbon deposits on the engine parts. These materials can be removed by acid refining process, in which the dewaxed oil is treated with con. H2SO4 and then agitated. Some of the unwanted impurities get dissolved in acid while other are converted into sludges. The sludges are removed by filtration. The filtrate is neutralized with calculated quantity of NaOH to neutralize the acid.
(c ) Removal of sulphur
Sulphur can be removed from the oil by desulphurization process in which the oil is treated with hydrogen in the presence of nickel as catalyst. During this process the unsaturated compounds are converted to satured compounds.
(d) Removel of coloured substance
The coloured and micro – crystalline waxes can be removed by filtration through Fuller’s earth.
Synthetic lubricants
Under severe operating condition ( -50 C to 250 C ) for example in air craft the lubricants are pumped at – 50 C but during take off and landing they get heated upto 120 – 150 C. Petroleum oils canot be effectively used because they tend to get oxidized at higher temperature while waxseparation will occur at lower temperatures. So synthetic lubricants have been developed range of ( -50 C to 250 C ) .
Example
Silicones, polyglycol, ethers, etc.
Blended (or) Compounded oils (or) Additives for lubricating oils
To improve the properties of the lubricating oils, certain substances called additives are added to the lubricating oils. The oil thus prepared are known as “ Blended oils (or) Compounded oils” .
Important additives and their functions
(i) Oiliness carriers - Fatty acids such as stearic acid palmatic acid, oleic acid. They increase the oilness adhering property of lubricants.
(ii) Extreme pressure additives - Organic chlorine compounds, organic sulphur compounds, organic phosphorous compounds. They react with metal surface film of lower shear strenge and high melting point.
(iii) Viscosity index improvers - n-hexanol, poly isobutlene, poly alkyl benzene. They prevent the oil from thinning higher temperatures and thickening at lower temperatures.
(iv) Pour-point depressants - Phenols, poly alkyl benzene. They prevent separation of wax from the lubricating oil.
(v) Thickeners - Polyesters, polystyrene. They increase the viscosity of the lubricants.
(vi) Anti oxidants - Aromatic amoino compounds, phenoilc, compounds. They retard the oxidation of the oil and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenols salts of carboxylic acids, sulphonates. They prevent foreign particles and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenls, salts of carboxylic acids, sulphonates. They prevent foreign particles and carbon deposits in engines which block the passage of oil.
(viii) Corrosion preventors or Corrosion inhibitors - Tricaresyl phosphates, organic compounds phosphorous antimony. They are adsorded on metal surfaces there by protecting the surface from attack by moisture.
Vegetable and Animal oils
These are glycerides of higher fatty acids have very good oiliness. However, these oils cannot be used effectively, because
(i) They undergo oxidation at higher temperature and forms gummy and acidic products.
(ii) Also, they get hydrolysed easily under moist conditions. Actually, they are used as “blending agents” with other lubricating oils.
Mineral oils (or) Petroleum oils
It is obtained by fractional distillation of crude petroleum oil. The length of the hydrocarbon chain varies from C12 to C50. It is cheap and quite stable under normal operating condition. But it posses poor oilness. The oilness of which can be improved by mixing it with animal (or) vegetable oils.
The mineral oil obtained cannot be used as such because it contains a lot of impurities such as wax asphalt oxidisable impurities etc. These impurities have to remove from the mineral oil before using it as a lubricant. The impurities are generally removed by the following methods.
(a) Removal of waxes
Waxes get separated at lower temperature and interfere with lubricating properties.
The wax can be removed by dewaxing process in which the petroleum oil is mixed with a suitable solvent (propane trichloroethyl etc.,) and then cooled. The wax crystallises out and is removed by filtration.
(b) Removal of asphalt
Asphaltic and naphthenic materials tend to leave carbon deposits on the engine parts. These materials can be removed by acid refining process, in which the dewaxed oil is treated with con. H2SO4 and then agitated. Some of the unwanted impurities get dissolved in acid while other are converted into sludges. The sludges are removed by filtration. The filtrate is neutralized with calculated quantity of NaOH to neutralize the acid.
(c ) Removal of sulphur
Sulphur can be removed from the oil by desulphurization process in which the oil is treated with hydrogen in the presence of nickel as catalyst. During this process the unsaturated compounds are converted to satured compounds.
(d) Removel of coloured substance
The coloured and micro – crystalline waxes can be removed by filtration through Fuller’s earth.
Synthetic lubricants
Under severe operating condition ( -50 C to 250 C ) for example in air craft the lubricants are pumped at – 50 C but during take off and landing they get heated upto 120 – 150 C. Petroleum oils canot be effectively used because they tend to get oxidized at higher temperature while waxseparation will occur at lower temperatures. So synthetic lubricants have been developed range of ( -50 C to 250 C ) .
Example
Silicones, polyglycol, ethers, etc.
Blended (or) Compounded oils (or) Additives for lubricating oils
To improve the properties of the lubricating oils, certain substances called additives are added to the lubricating oils. The oil thus prepared are known as “ Blended oils (or) Compounded oils” .
Important additives and their functions
(i) Oiliness carriers - Fatty acids such as stearic acid palmatic acid, oleic acid. They increase the oilness adhering property of lubricants.
(ii) Extreme pressure additives - Organic chlorine compounds, organic sulphur compounds, organic phosphorous compounds. They react with metal surface film of lower shear strenge and high melting point.
(iii) Viscosity index improvers - n-hexanol, poly isobutlene, poly alkyl benzene. They prevent the oil from thinning higher temperatures and thickening at lower temperatures.
(iv) Pour-point depressants - Phenols, poly alkyl benzene. They prevent separation of wax from the lubricating oil.
(v) Thickeners - Polyesters, polystyrene. They increase the viscosity of the lubricants.
(vi) Anti oxidants - Aromatic amoino compounds, phenoilc, compounds. They retard the oxidation of the oil and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenols salts of carboxylic acids, sulphonates. They prevent foreign particles and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenls, salts of carboxylic acids, sulphonates. They prevent foreign particles and carbon deposits in engines which block the passage of oil.
(viii) Corrosion preventors or Corrosion inhibitors - Tricaresyl phosphates, organic compounds phosphorous antimony. They are adsorded on metal surfaces there by protecting the surface from attack by moisture.
Semi-solid lubricants
Semi-solid lubricants (Greases)
Preparation
Greases are semi – solid lubricants obtained by thickening of lubricating oil by the addition of metallic soaps. Soaps are prepared by saponification of vegetable oils (or) fats with alkali (like as thickner (or) gelling agents).
Example
Preparation of Lithum grease
1step
Lithium soap is prepared by the saponification of vegetable oils (or) fats with lithium hydroxide.
II Step
Lubricating oil thickened by adding lithium soap.
The nature of the soap determines,
(a) The temperature upto which the grease can be used .
(b) It acts as a thickener.
(c) It is water and oxidation resistant.
(d) It enables the grease to stick to the metal surface firmly.
Properties and Uses of different greases
1. Sodium – soap grease (soda – base grease)
(i) Slighty soluble in water.
(ii) Dropping point is very high. It can be upto 175 deg C in ball bearing.
2. Calcium – soap grease ( lime – bases grease, cup grease)
(i) It is water resistant.
(ii) Beyond 70o deg C the grease separates into soap and oil.
(iii) Lesser dropping point than soda – base grease. It is a general purpose grease, used for
lubricating water pumps, tractors, etc.
3. Barium – soap grease
(i) It is water – resistant.
(ii) Possesses good adhesiveness. Used for lubricating automotives.
4. Lithium – soap grease
(i) It is a resistant to water and heat.
(ii) Expensive and superior to all other types. It is used at lower temperature: used for
lubricating engines.
5. Aluminium – soap grease
(i) It possess very high adhesiveness.
(ii) Expensive and water resistance. Used for lubricating chains and oscillating surfaces.
6. Axle – grease (Resin grease)
It is cheap and water resistant. Used for less delicate equipments under high load and low speed.
Grease are used under the following situations
(i) Where oil is squeezed out due to heavier load or low speed.
(ii) Where the bearing and gears that work at high temperature.
(iii) Where the bearing need to be sealed against the entry of dirt and dust etc.
(iv) Where frequent application of lubricant is inconvenient, as in automobile wheel bearings.
Preparation
Greases are semi – solid lubricants obtained by thickening of lubricating oil by the addition of metallic soaps. Soaps are prepared by saponification of vegetable oils (or) fats with alkali (like as thickner (or) gelling agents).
Example
Preparation of Lithum grease
1step
Lithium soap is prepared by the saponification of vegetable oils (or) fats with lithium hydroxide.
II Step
Lubricating oil thickened by adding lithium soap.
The nature of the soap determines,
(a) The temperature upto which the grease can be used .
(b) It acts as a thickener.
(c) It is water and oxidation resistant.
(d) It enables the grease to stick to the metal surface firmly.
Properties and Uses of different greases
1. Sodium – soap grease (soda – base grease)
(i) Slighty soluble in water.
(ii) Dropping point is very high. It can be upto 175 deg C in ball bearing.
2. Calcium – soap grease ( lime – bases grease, cup grease)
(i) It is water resistant.
(ii) Beyond 70o deg C the grease separates into soap and oil.
(iii) Lesser dropping point than soda – base grease. It is a general purpose grease, used for
lubricating water pumps, tractors, etc.
3. Barium – soap grease
(i) It is water – resistant.
(ii) Possesses good adhesiveness. Used for lubricating automotives.
4. Lithium – soap grease
(i) It is a resistant to water and heat.
(ii) Expensive and superior to all other types. It is used at lower temperature: used for
lubricating engines.
5. Aluminium – soap grease
(i) It possess very high adhesiveness.
(ii) Expensive and water resistance. Used for lubricating chains and oscillating surfaces.
6. Axle – grease (Resin grease)
It is cheap and water resistant. Used for less delicate equipments under high load and low speed.
Grease are used under the following situations
(i) Where oil is squeezed out due to heavier load or low speed.
(ii) Where the bearing and gears that work at high temperature.
(iii) Where the bearing need to be sealed against the entry of dirt and dust etc.
(iv) Where frequent application of lubricant is inconvenient, as in automobile wheel bearings.
Solid lubricants
SOLID LUBRICANTS
Solid lubricants are used the following situations.
(i) where the operating temperature and load is too high.
(ii) where contamination of oils or greases by the entry of dust or grit particles are avoied.
(iii) where combustible lubricants must be avoided.
The most widely used solid lubricants are graphite and molybdenum disulphide
8.7.1. Graphite
Graphite consists of flat layers of hexagonal arrangement of carbon atoms.The carbon atmos in hexagone are bonded together by strong covalent bonds. The adjacents layers are held together by weak Vander Walls forces . Since the distance between the adjacent layers can slide easily one over the other co-efficent of friction .This property makes use of graphite as a lubricant.
Graphite is very soapy to touch and non – inflammable. It can be used upto 375oC above this temperature it gets oxidized. Graphite is used as a dry powder or as a colloidal dispersion . A dispersion of graphite in oil is called oil Aqua dag and a dispersion of graphite in oil is called oil dag.
Used
(i) Oil dag is used in internal combustion engines.
(ii) Aqua dag is used in air compressors and in food processing equipments.s
(iii) Graphite is as a lubricant generally used in lathes, machine shop work railway track joints, open gears chains.
8.7.2. Molybdeenum disulphide
Molydbenum disulphide has a sandwich like structure in which a layer of molybdenum atoms lies between two layers of sulphur atoms. The atoms in the layer are bonded together by strong covalent bonds but the layers are held together by weak Vander Waals forces of attraction
The molybdenum layers and sulphur layers silde over one another .As a result MoS2 possess very low co-efficient of friction. It is also used either as a dry powder or as a colloidal dispersion. It gets oxdised above 800oC. It is mainly in heavy machineries working under heavy load and high temperatures.
Uses
(i) Pure MoS2 is used in the vacuum of outer space.
(ii) It is used in heavy machinery working at higher temperatures.
Solid lubricants are used the following situations.
(i) where the operating temperature and load is too high.
(ii) where contamination of oils or greases by the entry of dust or grit particles are avoied.
(iii) where combustible lubricants must be avoided.
The most widely used solid lubricants are graphite and molybdenum disulphide
8.7.1. Graphite
Graphite consists of flat layers of hexagonal arrangement of carbon atoms.The carbon atmos in hexagone are bonded together by strong covalent bonds. The adjacents layers are held together by weak Vander Walls forces . Since the distance between the adjacent layers can slide easily one over the other co-efficent of friction .This property makes use of graphite as a lubricant.
Graphite is very soapy to touch and non – inflammable. It can be used upto 375oC above this temperature it gets oxidized. Graphite is used as a dry powder or as a colloidal dispersion . A dispersion of graphite in oil is called oil Aqua dag and a dispersion of graphite in oil is called oil dag.
Used
(i) Oil dag is used in internal combustion engines.
(ii) Aqua dag is used in air compressors and in food processing equipments.s
(iii) Graphite is as a lubricant generally used in lathes, machine shop work railway track joints, open gears chains.
8.7.2. Molybdeenum disulphide
Molydbenum disulphide has a sandwich like structure in which a layer of molybdenum atoms lies between two layers of sulphur atoms. The atoms in the layer are bonded together by strong covalent bonds but the layers are held together by weak Vander Waals forces of attraction
The molybdenum layers and sulphur layers silde over one another .As a result MoS2 possess very low co-efficient of friction. It is also used either as a dry powder or as a colloidal dispersion. It gets oxdised above 800oC. It is mainly in heavy machineries working under heavy load and high temperatures.
Uses
(i) Pure MoS2 is used in the vacuum of outer space.
(ii) It is used in heavy machinery working at higher temperatures.
Mechanism of lubrication (Types of lubrication)
Mechanism of lubrication(Types of lubrication)
Fluid film (or) Thick film (or) Hydrodynamic lubrication
Condition - Under low load and high speed
Under the condition of low load and high speed a thick fluid film of lubricant is maintained between the two solid surfaces. The thickness of fluid film is atleast 1000A. Since the thick fluid film separates the two solid surfaces there is no direct contact between solid surfaces this reduces wear and tear. The co-efficient of friction in such is as low as 0.001 to 0.03
Example
Consider the rotation of a shaft with respect to a stationary bearing.
When a lubricant is added to the system, It occupies the annular space between the shaft and the bearing and forms a hydrodynamic wedge so long as the shaft rotates, the hydrodynamic wedge will remain and prevent contact between the two solid surfaces. When the load becomes very high, the lubricant will be sequeezed out of the wedge and friction will occur.
Boundary lubrication (or) Thin film lubrication
Condition: under high load and slow speed
Under the conditions of high load and slow speed, a continuous fluid film cannot be maintained between the moving surfaces. Under such conditions, the thickness of the fluid film should be less than 1000A. Such a thin film, consists of 2 or 3 molecules thick. To form a thin film the lubricant has to be adsorbed on the metal surface by physical or chemical forces. In some cases, the lubricant will react chemically with the metal surface forming a thin film of metal soap, which will act as a lubricant. This thin film is known as boundary film. The co-efficient of friction in such cases is around 0.05 to 1.15.
The effectiveness of boundary lubrication depends on the oiliness of the lubricant. Oiliness is the ability of a lubricant to stick on to the surface. Vegetable oils and their fatty acids have more oiliness. (eg) Oleic acid (C17H33COOH), stearic acid (C17H35COOH) etc. The polar carbonyl group (-COOH ) of these oils reacts with the metal surface to form a continuous thin film of lubricant. Hydrocarbon chain of the fatty acid gets oriented outwards in a perpendicular direction.
Extreme pressure lubrication
Condition: Under high load and high pressure
Under the conditions of high load ( high pressure) and high speed, more heat is generated between the moving surfaces. As a result of this, the liquid lubricant fails to stick and undergoes decomposition or evaporation. Under these conditions, for effective lubrication, special additives known as extreme pressure additives are used along with the lubricants.
Import extreme pressure additives are organic compounds having active radicals or groups such as chlorine (e.g Sulphurized oils) etc. These compounds react with metallic surfaces to form metallic chlorides, sulphides etc. These metallic compounds possess high melting points and serve as good under extreme pressure conditions.
Fluid film (or) Thick film (or) Hydrodynamic lubrication
Condition - Under low load and high speed
Under the condition of low load and high speed a thick fluid film of lubricant is maintained between the two solid surfaces. The thickness of fluid film is atleast 1000A. Since the thick fluid film separates the two solid surfaces there is no direct contact between solid surfaces this reduces wear and tear. The co-efficient of friction in such is as low as 0.001 to 0.03
Example
Consider the rotation of a shaft with respect to a stationary bearing.
When a lubricant is added to the system, It occupies the annular space between the shaft and the bearing and forms a hydrodynamic wedge so long as the shaft rotates, the hydrodynamic wedge will remain and prevent contact between the two solid surfaces. When the load becomes very high, the lubricant will be sequeezed out of the wedge and friction will occur.
Boundary lubrication (or) Thin film lubrication
Condition: under high load and slow speed
Under the conditions of high load and slow speed, a continuous fluid film cannot be maintained between the moving surfaces. Under such conditions, the thickness of the fluid film should be less than 1000A. Such a thin film, consists of 2 or 3 molecules thick. To form a thin film the lubricant has to be adsorbed on the metal surface by physical or chemical forces. In some cases, the lubricant will react chemically with the metal surface forming a thin film of metal soap, which will act as a lubricant. This thin film is known as boundary film. The co-efficient of friction in such cases is around 0.05 to 1.15.
The effectiveness of boundary lubrication depends on the oiliness of the lubricant. Oiliness is the ability of a lubricant to stick on to the surface. Vegetable oils and their fatty acids have more oiliness. (eg) Oleic acid (C17H33COOH), stearic acid (C17H35COOH) etc. The polar carbonyl group (-COOH ) of these oils reacts with the metal surface to form a continuous thin film of lubricant. Hydrocarbon chain of the fatty acid gets oriented outwards in a perpendicular direction.
Extreme pressure lubrication
Condition: Under high load and high pressure
Under the conditions of high load ( high pressure) and high speed, more heat is generated between the moving surfaces. As a result of this, the liquid lubricant fails to stick and undergoes decomposition or evaporation. Under these conditions, for effective lubrication, special additives known as extreme pressure additives are used along with the lubricants.
Import extreme pressure additives are organic compounds having active radicals or groups such as chlorine (e.g Sulphurized oils) etc. These compounds react with metallic surfaces to form metallic chlorides, sulphides etc. These metallic compounds possess high melting points and serve as good under extreme pressure conditions.
Properties of lubricant
PROPERTIES OF LUBRICANTS
Viscosity
Viscosity is a measure of the internal resistance of a liquid during its flow. It is expressed in centipoise.The viscosity of an oil is the time in seconds for a given quantity of a oil to pass through a standard orifice under the specified conditions.
Determination
The viscosity of an oil is determined by
(i) Red wood viscometer.
(ii) Say bolt viscometer.
The time required for 50ml of the liquid to pass through the orifice of a red wood viscometer is called as Red wood seconds. The time required for 60 ml of the liquid to pass through the orifice of a say bolt viscometer is called Say bolt universal seconds.
Significance
(i) If the viscosity of the lubricating oil is too high, the movement of the machine is restricted due to excessive friction.
(ii) If the viscosity of the lubricating oil is too low, the liquid oil film can’t be maintained and excessive wear will take place.
A good lubricating oil must have moderate viscosity.
Viscosity index
The viscosity of an oil decreases with increase in temperature. The rate of change of viscosity with temperature is indicated by viscosity temperature curves or by a scale known as viscosity index(V.I). Viscosity index is defined as “ The average decreases in viscosity of an oil per degree rise in temperature between 100°F and 210°F”.
(i) If the viscosity of an oil decreases rapidly with the increase in temperature it has low V.I.
Example
Gulf coast oil (consists of naphthenic hydrocarbons) exhibits a larger change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as zer0.
(ii) If the viscosity of an oil is slightly affected with the increase in temperature, it has higher V.I.
Example
Pennsylvanian oil (consists of paraffinic hydrocarbons) exhibits a relatively a smaller change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as 100.
Determination
The V.I of a test oil is calculated by comparing with the above two standard oil. The test oil is compared at 38°C (100°F) with zero V.I oil (gulf coast ) and 100 V.I oil (pensylvanian oil ) both having the same viscosity as the test oil aty 99°C (210°F).
The V.I of the test oil is given by the following formula
V.I=L-U/L-H ×100
Where,
U is the viscosity of the test oil at 38°C.
L is the viscosity of the low V.I oil at 38°C
H is the viscosity of the high V.I oil at 38°C
The viscosity temperature curve is flatter for the oil of high V.I than the oil of low V.I
Significance
A good lubricant should have minimum change in viscosity for a wide range of temperature. A good lubricant should have a high V.I.
Improving viscosity index
V.I of a lubricant can be increased by the addition of linear polymers such as polyisobutylene, n-hexanol, etc.
Viscosity
Viscosity is a measure of the internal resistance of a liquid during its flow. It is expressed in centipoise.The viscosity of an oil is the time in seconds for a given quantity of a oil to pass through a standard orifice under the specified conditions.
Determination
The viscosity of an oil is determined by
(i) Red wood viscometer.
(ii) Say bolt viscometer.
The time required for 50ml of the liquid to pass through the orifice of a red wood viscometer is called as Red wood seconds. The time required for 60 ml of the liquid to pass through the orifice of a say bolt viscometer is called Say bolt universal seconds.
Significance
(i) If the viscosity of the lubricating oil is too high, the movement of the machine is restricted due to excessive friction.
(ii) If the viscosity of the lubricating oil is too low, the liquid oil film can’t be maintained and excessive wear will take place.
A good lubricating oil must have moderate viscosity.
Viscosity index
The viscosity of an oil decreases with increase in temperature. The rate of change of viscosity with temperature is indicated by viscosity temperature curves or by a scale known as viscosity index(V.I). Viscosity index is defined as “ The average decreases in viscosity of an oil per degree rise in temperature between 100°F and 210°F”.
(i) If the viscosity of an oil decreases rapidly with the increase in temperature it has low V.I.
Example
Gulf coast oil (consists of naphthenic hydrocarbons) exhibits a larger change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as zer0.
(ii) If the viscosity of an oil is slightly affected with the increase in temperature, it has higher V.I.
Example
Pennsylvanian oil (consists of paraffinic hydrocarbons) exhibits a relatively a smaller change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as 100.
Determination
The V.I of a test oil is calculated by comparing with the above two standard oil. The test oil is compared at 38°C (100°F) with zero V.I oil (gulf coast ) and 100 V.I oil (pensylvanian oil ) both having the same viscosity as the test oil aty 99°C (210°F).
The V.I of the test oil is given by the following formula
V.I=L-U/L-H ×100
Where,
U is the viscosity of the test oil at 38°C.
L is the viscosity of the low V.I oil at 38°C
H is the viscosity of the high V.I oil at 38°C
The viscosity temperature curve is flatter for the oil of high V.I than the oil of low V.I
Significance
A good lubricant should have minimum change in viscosity for a wide range of temperature. A good lubricant should have a high V.I.
Improving viscosity index
V.I of a lubricant can be increased by the addition of linear polymers such as polyisobutylene, n-hexanol, etc.
Flash point and Fire point
Flash point and Fire point
Flash point
It is lowest temperature at which the oil gives off enough vapour that ignite for a moment, when a small flame is brought near it.
Fire point
It is the lowest temperature at which the vapour of the oil burns continuously for at least 5 seconds, when a small flame is brought near it. Generally the fire point is 5-40°C higher than flash point.
Significance
A knowledge of flash and fire point is useful in providing protection against fire hazard during transport and storage. Lubricating oils of paraffinic base possess higher flash points than those of naphthenic base. Hence the determination of flash and fire points is helpful in identifying the type of lubricating oil.
A good lubricating oil should have flash and fire points higher than the operating temperature of the machine.
Determination
Flash and fire points can be determined using the same apparatus. An oil is heated at a prescribed rate in an open cup (Cleve Land’s) apparatus or closed cup (pensky Martin’s) apparatus of standard dimensions. A small test flame is periodically applied over the surface of the oil. The temperature at which a distinct flash is seen is the flash point. The heating and periodical application of test flame is continued. The temperature at which the oil vapour catches fire and burns continuously for 5 seconds is noted as the fire point.
8.9.4 Cloud and pour point
Cloud point
When an oil is cooled slowly the temperature at which the oil becomes cloudy in temperature is called its cloud point.
Pour point
The temperature at which the oil ceases to flow or pour is called its pour point.
Significance
Most of the petroleum based lubricating oils contain dissolved paraffin wax and asphaltic impurities. When the oil is cooled these impurities undergoes solidification which cause jamming of the machine. So the cloud and pour points indicate the suitability of the lubricants in cold condition.
A good lubricant must have cloud point and pour point.
Determination
An oil is taken ina flat bottomed tube enclosed in an air jacket and it is cooled in a freezing mixture (ice + CaCl2 ). Thermometers are introduced into the oil and freezing mixture. As the cooling takes place via the air jacket temperature of the oil falls. The temperature at which the cloudiness appears is noted as the Cloud Point. The cooling is furthered continued. The temperature at which the oil does not flow in the test tube for 5 seconds on tilting it to the horizontal position is noted as the pour point.
How to improve cloud and pour point
Pour point of lubricating oil can be lowered by
(i) Dewaxing
(ii) Adding a pour point depressant.
Example
Poly alkyl benzene called “para flow’ is a commonly used pour point depressant.
8.9.5 Oiliness
Oiliness is the capacity of a lubricating oil to stick on to the surface of the machine parts under heavy load or pressure.
Significance
Lubricating which have oiliness stay in between the lubricated surfaces, when they are subjected to high load and pressure. But lubricants with poor oiliness will be squeezed out of the machine parts under this condition.
A good lubricant should have good oiliness.
How to improve oiliness
Mineral oils have poor oiliness where as vegetable and animal oils have high oiliness. Hence oiliness of mineral oil is improved by adding vegetable oils and higher fatty acids like oleic acid, stearic acid. etc.
Flash point
It is lowest temperature at which the oil gives off enough vapour that ignite for a moment, when a small flame is brought near it.
Fire point
It is the lowest temperature at which the vapour of the oil burns continuously for at least 5 seconds, when a small flame is brought near it. Generally the fire point is 5-40°C higher than flash point.
Significance
A knowledge of flash and fire point is useful in providing protection against fire hazard during transport and storage. Lubricating oils of paraffinic base possess higher flash points than those of naphthenic base. Hence the determination of flash and fire points is helpful in identifying the type of lubricating oil.
A good lubricating oil should have flash and fire points higher than the operating temperature of the machine.
Determination
Flash and fire points can be determined using the same apparatus. An oil is heated at a prescribed rate in an open cup (Cleve Land’s) apparatus or closed cup (pensky Martin’s) apparatus of standard dimensions. A small test flame is periodically applied over the surface of the oil. The temperature at which a distinct flash is seen is the flash point. The heating and periodical application of test flame is continued. The temperature at which the oil vapour catches fire and burns continuously for 5 seconds is noted as the fire point.
8.9.4 Cloud and pour point
Cloud point
When an oil is cooled slowly the temperature at which the oil becomes cloudy in temperature is called its cloud point.
Pour point
The temperature at which the oil ceases to flow or pour is called its pour point.
Significance
Most of the petroleum based lubricating oils contain dissolved paraffin wax and asphaltic impurities. When the oil is cooled these impurities undergoes solidification which cause jamming of the machine. So the cloud and pour points indicate the suitability of the lubricants in cold condition.
A good lubricant must have cloud point and pour point.
Determination
An oil is taken ina flat bottomed tube enclosed in an air jacket and it is cooled in a freezing mixture (ice + CaCl2 ). Thermometers are introduced into the oil and freezing mixture. As the cooling takes place via the air jacket temperature of the oil falls. The temperature at which the cloudiness appears is noted as the Cloud Point. The cooling is furthered continued. The temperature at which the oil does not flow in the test tube for 5 seconds on tilting it to the horizontal position is noted as the pour point.
How to improve cloud and pour point
Pour point of lubricating oil can be lowered by
(i) Dewaxing
(ii) Adding a pour point depressant.
Example
Poly alkyl benzene called “para flow’ is a commonly used pour point depressant.
8.9.5 Oiliness
Oiliness is the capacity of a lubricating oil to stick on to the surface of the machine parts under heavy load or pressure.
Significance
Lubricating which have oiliness stay in between the lubricated surfaces, when they are subjected to high load and pressure. But lubricants with poor oiliness will be squeezed out of the machine parts under this condition.
A good lubricant should have good oiliness.
How to improve oiliness
Mineral oils have poor oiliness where as vegetable and animal oils have high oiliness. Hence oiliness of mineral oil is improved by adding vegetable oils and higher fatty acids like oleic acid, stearic acid. etc.
Nanomaterials
NANOMATERIALS
The prefix (nano) in the word nanomaterials means a billionth (1×10-9 m).
Atoms are very small and the diameter of a single atom can vary from 0.1 to 0.5 nm. It deals with various structures of matter having dimensions of order of a billionth of meter.
1. Nanoparticles
Nanoparticles are the particles the size of which ranges from 1-50 nm. Generally they are obtained from colloids. The colloidal particles have nanocrystals. A large percentage of atoms in nanocrystals are present on the surface.
Nancocrystals possess electronic magnetic and optical properties. Since the nanoparticles exhibit an electronic behavior governed by the quantum physics they are also called as quantum dots.
2. Nanomaterials
Nanomaterials are the materials having components with size less than 100 nm.
• Nanomaterials in one dimension are layers such as a thin films or surface coatings.
• Nanomaterials in two dimension are tubes such as nanotubes and nanowires.
• Nanomaterials in three dimension are particles like precipitates, colloids andquantum dots.
3. Nanochemistry (or) Nanoscience
Nanoscience is defined as the study of phenomena and manipulation of materials at atomic molecular and macromolecular scales.
4. Nanotechnology
Nanotechnology is defined as the design characterization production and applications of structures systems and devices by controlling size and shape at 10-9 m scale or the single –atomic level.
The prefix (nano) in the word nanomaterials means a billionth (1×10-9 m).
Atoms are very small and the diameter of a single atom can vary from 0.1 to 0.5 nm. It deals with various structures of matter having dimensions of order of a billionth of meter.
1. Nanoparticles
Nanoparticles are the particles the size of which ranges from 1-50 nm. Generally they are obtained from colloids. The colloidal particles have nanocrystals. A large percentage of atoms in nanocrystals are present on the surface.
Nancocrystals possess electronic magnetic and optical properties. Since the nanoparticles exhibit an electronic behavior governed by the quantum physics they are also called as quantum dots.
2. Nanomaterials
Nanomaterials are the materials having components with size less than 100 nm.
• Nanomaterials in one dimension are layers such as a thin films or surface coatings.
• Nanomaterials in two dimension are tubes such as nanotubes and nanowires.
• Nanomaterials in three dimension are particles like precipitates, colloids andquantum dots.
3. Nanochemistry (or) Nanoscience
Nanoscience is defined as the study of phenomena and manipulation of materials at atomic molecular and macromolecular scales.
4. Nanotechnology
Nanotechnology is defined as the design characterization production and applications of structures systems and devices by controlling size and shape at 10-9 m scale or the single –atomic level.
Carbon nanotubes (CNT)
CARBON NANOTUBES (CNT)
Carbon nanotubes are allotropes of carbon with a nanostructure having a length-to-diameter ratio greater than 1,000,000. When graphite sheets rolled into a cylinder their edges joined and form carbon nanotubes. Carbon nanotubes are extended tubes of rolled graphite sheets.
Nanotubes naturally align into ropes and held together by Vander Waals forces. But each carbon atoms in the carbon nanotubes are linked by the covalent bond.
Structure (or) types of carbon nanotubes
Carbon nanotubes are lattice of carbon atoms, in which each carbon is bonded
to three other carbon atoms.
Depending upon the way in which graphite sheets are rolled two types of CNTs are formed.
1. Single - walled nanotubes (SWNTs)
2. Multi – walled nanotubes (MWNTs)
Single – walled nanotubes (SWNTs)
SWNTs consist of one layer of graphite. It is one-atom thick having a diameter of 2 nm and length of 100 um. SWNTs are very important because they exhibit important electrical properties. It is an excellent conductor.
Three kinds of nanotubes are resulted based on the orientation hexagon lattice.
(i) Armchair structures : The lines of hexagons are parallel to the axis of the nanotube.
(ii) Zig-zag structures: The lines of carbon bonds are down the centre.
(iii) Chiral nanotubes: It exhibits twist or spiral around the nanotubes.
It has been confirmed that armchair carbon nanotubes are metallic while zig- zag and chiral nanotubes are semiconducting.
2. Multi-walled nanotubes (MWNTs)
MWNTs (nested nanotubes) consist of multiple layers of graphite rolled on themselves to from a tube shape. It exhibits both metallic and semiconducting properties. It is used for storing fuels such as hydrogen and methane.
Synthesis of carbon nanotubes
Carbon nanotubes can be synthesized by any one of the following methods.
1. Pyrolysis of hydrocarbons.
2. Laser evaporation.
3. Carbon arc method.
4. Chemical vapour deposition
Pyrolysis
Carbon nanotubes are synthesized by the pyrolysis of hydrocarbons such as acetylene at about 700 deg C in the presence of Fe-silica or Fe-graphite under inert conditions.
Laser evaporation
It involves vapourization of graphite target containing small amount of cobalt and nickel by exposing it to an intense pulsed laser beam at higher temperature (1200 deg C) in a quarz tube reactor . An inert gas such as argon is simultaneously allowed to pass into the reactor to sweep the evaporated carbon atoms from the furnace to the colder copper collector nanotubes.
Carbon arc method
It is carried out by applying direct current (60-100 A and 20-25 V ) arc between graphite electrode of 10-20 um diameter.
Chemical vapour deposition
It involves decomposition of vapour of hydrocarbons such as methane acetylene ethylene, etc. at high temperatures (100 deg C) in presence of metal nanoparticle catalysts like nickel cobalt iron supported on MgO or Al2O3.
Carbon nanotubes are allotropes of carbon with a nanostructure having a length-to-diameter ratio greater than 1,000,000. When graphite sheets rolled into a cylinder their edges joined and form carbon nanotubes. Carbon nanotubes are extended tubes of rolled graphite sheets.
Nanotubes naturally align into ropes and held together by Vander Waals forces. But each carbon atoms in the carbon nanotubes are linked by the covalent bond.
Structure (or) types of carbon nanotubes
Carbon nanotubes are lattice of carbon atoms, in which each carbon is bonded
to three other carbon atoms.
Depending upon the way in which graphite sheets are rolled two types of CNTs are formed.
1. Single - walled nanotubes (SWNTs)
2. Multi – walled nanotubes (MWNTs)
Single – walled nanotubes (SWNTs)
SWNTs consist of one layer of graphite. It is one-atom thick having a diameter of 2 nm and length of 100 um. SWNTs are very important because they exhibit important electrical properties. It is an excellent conductor.
Three kinds of nanotubes are resulted based on the orientation hexagon lattice.
(i) Armchair structures : The lines of hexagons are parallel to the axis of the nanotube.
(ii) Zig-zag structures: The lines of carbon bonds are down the centre.
(iii) Chiral nanotubes: It exhibits twist or spiral around the nanotubes.
It has been confirmed that armchair carbon nanotubes are metallic while zig- zag and chiral nanotubes are semiconducting.
2. Multi-walled nanotubes (MWNTs)
MWNTs (nested nanotubes) consist of multiple layers of graphite rolled on themselves to from a tube shape. It exhibits both metallic and semiconducting properties. It is used for storing fuels such as hydrogen and methane.
Synthesis of carbon nanotubes
Carbon nanotubes can be synthesized by any one of the following methods.
1. Pyrolysis of hydrocarbons.
2. Laser evaporation.
3. Carbon arc method.
4. Chemical vapour deposition
Pyrolysis
Carbon nanotubes are synthesized by the pyrolysis of hydrocarbons such as acetylene at about 700 deg C in the presence of Fe-silica or Fe-graphite under inert conditions.
Laser evaporation
It involves vapourization of graphite target containing small amount of cobalt and nickel by exposing it to an intense pulsed laser beam at higher temperature (1200 deg C) in a quarz tube reactor . An inert gas such as argon is simultaneously allowed to pass into the reactor to sweep the evaporated carbon atoms from the furnace to the colder copper collector nanotubes.
Carbon arc method
It is carried out by applying direct current (60-100 A and 20-25 V ) arc between graphite electrode of 10-20 um diameter.
Chemical vapour deposition
It involves decomposition of vapour of hydrocarbons such as methane acetylene ethylene, etc. at high temperatures (100 deg C) in presence of metal nanoparticle catalysts like nickel cobalt iron supported on MgO or Al2O3.
Properties of CNTs
PROPERTIES OF CNTs
Mechanical properties
(a) Carbon nanotubes are very strong and their elastic flexibility is indicated by young’s modulus. Young’s modulus of carbon nanotubes are 10 times than that of steel.
(b) CNTs withstand extreme strain and tension. Most of the materials fracture on bending because of the presence of more defects but CNTs possess only few defects in the structure.
Electrical properties
The electrical properties of CNTs vary between metallic to semiconducting materials. It depends on the diameter and chirality of the nanotubes. The very high electrical conductivity of CNTs is due to the minimum defects in the structure.
Thermal Conductivity
The thermal conductivity of CNTs is very high this is due to the vibration of covalent bonds. Its thermal conductivity is 10 time greater than the metal. The high thermal conductivity is due to minimum defects in the structure.
Vibrational properties
As in normal molecules the atoms in a CNT are continuously vibrating back and forth CNTs have two normal modes of vibration.
(i) A1g mode : It involves “ in and “out” oscillation.
(ii) E2g mode : It involves a squashing of the tube ( oscillation between sphere and an ellipse )
The frequencies of two modes of vibrations are Raman active.
Kinetic Properties
The inner tubes of MWNTs can slide without any friction within the outer nanotubes leading to an atomically perfect rotational bearing.
Mechanical properties
(a) Carbon nanotubes are very strong and their elastic flexibility is indicated by young’s modulus. Young’s modulus of carbon nanotubes are 10 times than that of steel.
(b) CNTs withstand extreme strain and tension. Most of the materials fracture on bending because of the presence of more defects but CNTs possess only few defects in the structure.
Electrical properties
The electrical properties of CNTs vary between metallic to semiconducting materials. It depends on the diameter and chirality of the nanotubes. The very high electrical conductivity of CNTs is due to the minimum defects in the structure.
Thermal Conductivity
The thermal conductivity of CNTs is very high this is due to the vibration of covalent bonds. Its thermal conductivity is 10 time greater than the metal. The high thermal conductivity is due to minimum defects in the structure.
Vibrational properties
As in normal molecules the atoms in a CNT are continuously vibrating back and forth CNTs have two normal modes of vibration.
(i) A1g mode : It involves “ in and “out” oscillation.
(ii) E2g mode : It involves a squashing of the tube ( oscillation between sphere and an ellipse )
The frequencies of two modes of vibrations are Raman active.
Kinetic Properties
The inner tubes of MWNTs can slide without any friction within the outer nanotubes leading to an atomically perfect rotational bearing.
Application of carbon nanotubes
APPLICATION OF CARBON NANOTUBES
Due to the unusual and unique properties of CNTs, they find potential applications in the following field.
1. CNTs in storage devices
Carbon nanotubes play an important role in the battery technology, because some charge carriers can be successfully store inside the nanotubes.
Examples
1. CNT in Fuel cells
Hydrogen can be stored in the CNT, which may be used for the development of fuel cells.
2. CNT in Lithium Battery
Lithium atoms act as a very good charge carrier can be stored inside the carbon nanotubes. It has been estimated that one lithium atom can be stored for every six carbon atoms of the CNT and hence can be used in lithium batteries.
2. CNT as protective shields
CNT s are poor transmitters of electromagnetic radiation and hence can be used as light weight shielding materials for protecting an electronic equipment againt electromagnetic radiation.
3. Sensors of gases
The gases like NO2 and NH3 can be detected on the basis of increase in electrical conducitivity of CNTs. When NO2 or NH3 is allowed to flow over CNT s, electrical conductivity is found to increase. This attributed to increase in whole concentration in CNTs due to charge transfer from CNTs to NO2 as the gas molecules bind to the CNTs.
4. Drug deliver vessels
CNTs canbe effectively used inside the body for drug delivery by placing the drugs within the tubes.
5. CNTs in microsocope
CNTs, attached to the tip of scanning probe microscope have been used to image biological and industrial specimens.
6. Reinforcing elements in composites
The composites of aluminium powder mixed with CNTs (5%) posseses a greater tensile strength compared to pure aluminium.
7. Components in integrated memory circuits
The integrated memory circuits, made of nanotube composites with conducting polymers have been found to be effective devices.
8. Quantum wires
The Quantum wires made of metallic carbon nanotubes are found to have electrical conductivity.
9. CNTs in catalysis
Carbon nanotubes serve as efficient catalysts for some chemical reaction.
Examples
(a) Hydrogenation reaction
Nested carbon nanotubes with ruthenium (Ru) metal bonded to the outside have been proved to have a strong catalytic effect in the hydrogenation reaction of cinnamaldehyde (C6H5CH = CHCHO) when compared with the effect of the same metal Ru attached to other carbon substrates.
(b) Other chemical reactions
Some chemical reactions are also been carried out inside the nanotubes.
Due to the unusual and unique properties of CNTs, they find potential applications in the following field.
1. CNTs in storage devices
Carbon nanotubes play an important role in the battery technology, because some charge carriers can be successfully store inside the nanotubes.
Examples
1. CNT in Fuel cells
Hydrogen can be stored in the CNT, which may be used for the development of fuel cells.
2. CNT in Lithium Battery
Lithium atoms act as a very good charge carrier can be stored inside the carbon nanotubes. It has been estimated that one lithium atom can be stored for every six carbon atoms of the CNT and hence can be used in lithium batteries.
2. CNT as protective shields
CNT s are poor transmitters of electromagnetic radiation and hence can be used as light weight shielding materials for protecting an electronic equipment againt electromagnetic radiation.
3. Sensors of gases
The gases like NO2 and NH3 can be detected on the basis of increase in electrical conducitivity of CNTs. When NO2 or NH3 is allowed to flow over CNT s, electrical conductivity is found to increase. This attributed to increase in whole concentration in CNTs due to charge transfer from CNTs to NO2 as the gas molecules bind to the CNTs.
4. Drug deliver vessels
CNTs canbe effectively used inside the body for drug delivery by placing the drugs within the tubes.
5. CNTs in microsocope
CNTs, attached to the tip of scanning probe microscope have been used to image biological and industrial specimens.
6. Reinforcing elements in composites
The composites of aluminium powder mixed with CNTs (5%) posseses a greater tensile strength compared to pure aluminium.
7. Components in integrated memory circuits
The integrated memory circuits, made of nanotube composites with conducting polymers have been found to be effective devices.
8. Quantum wires
The Quantum wires made of metallic carbon nanotubes are found to have electrical conductivity.
9. CNTs in catalysis
Carbon nanotubes serve as efficient catalysts for some chemical reaction.
Examples
(a) Hydrogenation reaction
Nested carbon nanotubes with ruthenium (Ru) metal bonded to the outside have been proved to have a strong catalytic effect in the hydrogenation reaction of cinnamaldehyde (C6H5CH = CHCHO) when compared with the effect of the same metal Ru attached to other carbon substrates.
(b) Other chemical reactions
Some chemical reactions are also been carried out inside the nanotubes.
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