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Essay on Ceramics
Essay # 1. Introduction to Ceramics:
Ceramics cannot be defined under metal, non-metal or organic polymer. Ceramics are non-metallic, inorganic, amorphous solid and are mostly metallic oxide. Atomic bonding in them from purely ionic to totally co-valent. These are formed under earth in pressure extensive heat. These ceramics are extremely brittle, having high thermal stability, high chemical stability i.e., corrosion resistance, having high hardness.
E.g., Silica (SiO2) & Al2O3
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(a) Traditional Ceramics:
Such as white ware, tiles, brick, Sewer pipe, Pottery and abrasive wheel.
(b) Industrial Ceramics:
It also called engineering, high-tech or fine ceramics such as turbine, automotive and aerospace components, e.g. Al2O3, Si2, Si3N4 etc.
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Essay # 2. Structure of Ceramics:
The structure of ceramics is among the most complex of all materials. It contains various atoms of different size. The bonding between these atoms is generally covalent or ionic. These bonds are much strengthen than metallic bonds. Ceramics are available as single crystal or in polycrystalline form. These have finer grain which leads to more strength and toughness.
Essay # 3. Mechanical Properties of Ceramics:
Ceramic materials are somewhat limited in applicability by their mechanical properties, which in many respects are inferior to those of metals.
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i. Brittle Fracture of Ceramics:
The brittle fracture process consists of the formation and propagation of cracks through the cross section of material in a direction perpendicular to the applied load.
Crack growth in crystalline ceramics may be either transgranular (i.e. through the grains) or intergranular (i.e., along grain boundaries); for transgranular fracture, cracks propagate along specific crystallographic (or cleavage) planes, planes of high atomic density. Griffith theory brittle fracture hold goods for ceramics.
ii. Hardness:
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One beneficial mechanical property of ceramics is their hardness, which is often utilized when an abrasive or grinding action is required; in fact, the hardest known materials are ceramics. Their hardness normally lies in between 500BHN to 2000BHN.
iii. Creep:
Often ceramic materials experience creep deformation as a result of exposure to stresses (usually compressive) at elevated temperatures.
In general, the time deformation creep behavior of ceramics is similar to that of metals; however, creep occurs at higher temperatures in ceramics.
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High-temperature compressive creep tests are conducted on ceramic materials to ascertain creep deformation as a function of temperature and stress level.
Sialon (Si3H4 & AI2O3) used for gas turbine at 1300°C due to this property.
Essay # 4. Fabrication and Processing of Ceramics:
Ceramic melt at high temperatures and they exhibit a brittle behavior under tension.
As a result, the conventional melting, casting and thermo-mechanical processing routes are not suitable to process the polycrystalline ceramics.
Inorganic glasses, though, make use of lower melting temperatures due to formation of eutectic.
Hence, most ceramic products are made from ceramic powders through powder processing starting with ceramic powders.
The powder processing of ceramics is very close to that of metals, powder metallurgy. However there is an important consideration in ceramic-forming that is more prominent than in metal forming: it is dimensional tolerance.
Post forming shrinkage is much higher in ceramics processing because of the large differential between the final density and the as-formed density.
Glasses, however, are produced by heating the raw materials to an elevated temperature above which melting occurs.
Most commercial glasses are of the silica-soda-lime variety, where silica is supplied in form of common quartz sand, soda (Na2O) in form of soda ash (Na2CO3) while the lime (CaO) is supplied in form of limestone (CaCO3).
Different forming are widely in practice to fabricate glass products. Thick glass objects such as plates and dishes are produced by pressing, while the blowing is used to produce objects like jars, bottles and light bulbs.
Drawing is used to form long objects like tubes, rods, fibers, whiskers etc. The pressing and blowing process is shown in the figure.
i. Pressing:
ii. Blowing:
SiO2– It is the most simple silicate material. It is a 3D network that is generated when each corner oxygen atom of tetrahedra is shared by adjacent tetrahedra. Thus the material is electrically neutral.
Ratio of Si to O atoms is 1:2. If this array is extended in all directions a crystalline structure is obtained.
There are three polymorphic forms of silica; “quartz, cristobalite and tridymite.”
These structures are open structures i.e. they are not closed packed together.
As a consequences these crystalline silicon have low densities.
Silica Glasses:
Silica can also be made to exist as non-crystalline solid or glass having high degrees of randomness. Such structure is called “fused Silica or vitreous Silica” Common glasses used for windows etc. are silica glasses.
Networking Modifiers:
Certain oxides such as Na2O and CaO are added to silica glass. These oxides do not form polyhedral networks rather their cations are incorporated within and modify the SiO44- network. For this reason, these oxide additives are called network modifiers.
Intermediate Oxides:
Such as TiO2 and Al2O3 which are not network modifiers, substitute for silicon and become part of and stabilize the network. The addition of these modifiers and intermediates lowers the melting point and viscosity of glass and makes it easier to form at lower temperature.
The Silicates (Simple Silicates):
Structurally simple and involves isolated tetra, hedra eg. (Mg2SiO4) for sterile where two Mg2+ ion associated with each tetrahedra such that Mg2+ ion has six oxygen nearest neighbours.
Layered Silicates:
These are two dimension sheet or layered structures and can be produced by sharing of three oxygen ions in each of the tetrahedra.
For these structures the repeating unit is (Si2O5)– the net negative charge is associated with unbounded oxygen atoms projecting out of the plane of the page. Electro neutrality is established by a second plane sheet containing excess of cations, which bond with excess unbounded oxygen atoms from the Si2O5 sheet.
Such materials are called the sheet or layered silicates, and their basic structure is characteristics of clays.
Silicon Carbide- Melting point 2400°C used in high temperature transistors. Its energy gap is large, about 3ev.
One of the most common clay minerals kaolinite, has a formula Al2 (Si2O5) (OH)4 in which the silica tetrahedral layer represented by (Si2O5)2- is made electrically neutral by an adjacent AI2(OH)42+ layer.
Diamond:
Here each carbon atoms bonds to four other carbon atoms by covalent bonds. Diamond is extremely hard and has a very low electrical conductivity, these characteristics are due to its crystal structure and strong interatomic covalent bonds.
It has high thermal conductivity and is optically transparent in visible and infrared regions and has high refractive index.
Industrially, diamonds are used to cut and grind soft materials. Diamond films are produced by vapour phase chemical reactions followed by film deposition.
These films do not have long range crystalline regularity of natural diamond.
Surface of drills, dies bearing. Knives and other tools are coated with diamond films to increase surface hardness. It is also applied on loudspeaker tweeter and high precision work.
Graphite:
It is more stable than diamond at room temperature and pressure. Its structure consists of layer of hexagonally arranged carbon atoms, within the layers, each carbon atom is bonded to their coplanar neighbour atoms by strong covalent bonds.
The further bonding electron participates in a weak vanderwaal type of bond between the layers. Thus center plane is fragile, which gives rise to excellent lubricative properties of graphite. Electrical conductivity is also high along hexagonal sheets.
Graphite has high strength and chemical stability at elevated temperature, high thermal conductivity, low coefficient of thermal expansion and high resistance to thermal shock and good machinability.
Graphite is commonly used as electrodes in welding, casting molds, refractories and insulations, rocket nozzles, chemical reactor vessels, electrical contacts, brushes and resistors, battery electrodes and in purification devices.
Essay # 5. Electrical Behaviour of Ceramics:
These are used for insulation purpose in electric power & transmission, electrical m/c etc. Mica, Poralian, Micanite, Glass bonded mica, glass tape etc. are example.
1. Piezoelectric Materials:
i. Quartz, SiO2 have piezoelectric coefficient.
ii. Ammonium dihydrogen phosphate (ADP) (NH4H2PO4)
iii. Lithium tantalite, LiTaO3
iv. Lithium niobate, LiNbO3
v. Potassium dihydrogen phosphate (KDP), KH2PO4
vi. Polyvinylidene fluoride (PVDF), (CH2-CF2)n
vii. Lead zirconate titanite (PZR), PbTi0.48Zr0.52O3
viii. Rochelle salt, NaKC4H4O6, 4H2O
2. Lead Zirconate Titanate (PZT): A Piezoelectric Ceramic:
It is a solid solution of lead zirconate (PbZrO3) and lead titanate (PbTiO3). Both can be mixed in different proportions to yield the properties of aspirated choices.
It has wide range of dielectric constants.
Have high Curie temperature that allows high temperature operations.
It has electromechanical coupling coefficients better than the BaTiO3.
They can be produced easily by sintering.
PbZrO3 is an antiferroelectric material. At room temperature, its structure is cubic which converts into orthorhombic above Curie temperature of 230°C. i.e.
PbTiO3 is a ferroelectric material. Below Curie temperature (Tc = 490°C), its structure is cubic but it converts to tetragonal structure above it. Thus;
PZT is prepared by mixing the zirconate and titanate in a ball mail and then calcimining above 1000°C.
Applications:
Mains applications of PZT in modern uses are the following:
a. Ultrasound
b. Sonar
c. Buzzers
d. Spark generators
e. Sensors
f. Transducers
g. Actuators
h. Filters
3. Lead Lanthanum Zirconate Titanate (PLZT):
It is a composition of lanthanum (La) with PZT. Lanthanum is added to modify the properties of PZT.
It results in:
i. Decreased Curie temperature
ii. Increased dielectric constant
iii. Increased squareness of hysteresis loop
iv. Higher electromechanical coupling coefficients
v. Decreased coercive field
vi. Better optical transparency
The proportion of La in PZT plays a vital role in deciding the property of PLZT.
It is added in different proportions. If the piezoelectric properties of PLZT is to be enhanced, La should be added less than 5%.
But for PLZT in electro-optical applications, a proportion of more than 5% is required.
PLZT is generally designated by La/PbZrO3/PbTiO3 combination e.g. 7/55/45, 2/65/35, 12/40/60 etc.
4. Barium Titanate: A Ferroelectric Ceramic:
Barium titanate (BaTiO3) is most important ceramic. It is a mixed oxide having perovskite structure as shown in figure. It is an artificial ferroelectric material. It’s εr > 2000. The curie temperature of BaTiO3 is 130°C.
Structure:
Its structure is ‘cubical’ above 130°C and ‘tetragonal’ below it. When BaTiO3 crystal is cooled below 130°C, the Ti4+ ion shifts on one side of the body centre and O2- ions in opposite side as shown in figure. Hence, the centres of positive and negative charges do not coincide, and local dipoles are created in entire crystal. Since the dipoles of neighbouring unit cells are all aligned, they induce a large polarization is solid.
Effect of Temperature on Structure of BaTiO3:
Dielectric Constant as a Function of Temperature:
The dielectric constant of BaTiO3 is affected considerably by temperature as shown in fig. It shows a rise with increasing temperature. There are abrupt and sharp rises at critical temperature viz. -90°C, 5°C and 130°C.
Modified Barium Titanate:
It is modified by adding Curie point shifter. Besides changing the Curie point to different operating temperature range, it also smoothens the profile of abrupt change in dielectric constant. The shifting in Curie point is accomplished by using some substitutions.
Main among them are given below:
a. BaTiO3 + PbTiO3:
This substitution results in formation of (Ba1-xPbx) TiO3 when 0<x<1. On mixing them, the Curie temperature rises at a rate of 3.7°C i.e. dTc/dx = 3.7°C. The value of Tc for BaTiO3 is 130°C and that for PbTiO3 is 490°C.
b. BaTiO3 + SrTiO3:
At room temperature, it is not a ferroelectric. Its structure is cubic perovskite. On mixing them, the Curie temperature lowers down by 3.7°C i.e. dTc/dx = -3.7°C.
c. BaTiO3 + CaTiO3:
This mixture has no effect on Curie temperature, but it lowers the tetragonal orthorhombic transition temperature.
Essay # 6. Recent Advances in Ceramics:
Chemically Bonded ceramics (CBC) are invented recently and have high performance, low cost. CBC materials are generally silicates, aluminates and phosphates.
Their low cost obtained from processing temperature for below the 1100°C needed for producing conventional ceramics.
CBC have 10 to 20 times the tensile strength of concrete about 32 times cheaper than Al. 20 times, steel and 4 to 6 times less expensive than plastics.
a. CBC armor brake lining.
b. CBC roofing, CBC flooring, CBC electrical fixture
c. CBC wall panel
Future Application:
Aircraft & rocket parts, cryogenic containers roadways etc..