Material Science: Chapter 5: Ceramics

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1. Introduction

  • Ceramics are compounds between metallic and nonmetallic elements for which the inter-atomic bonds are either ionic or predominantly ionic.
  • The term ceramics comes from the Greek word keramikos which means ‘burnt stuff’.
  • Characteristic properties of ceramics are, in fact, optimized through thermal treatments.
  • They exhibit physical properties those are different from that of metallic materials. Thus metallic materials, ceramics, and even polymers tend to complement each other in service

2. Characteristics of Ceramics

  • High temperature stability
  • High hardness
  • Brittleness
  • High mechanical strength
  • Low elongation under application of stress – low thermal and electrical conductivities

3. Classification of Ceramics

Ceramics greatly differ in their basic composition. The properties of ceramic materials also vary greatly due to differences in bonding, and thus found a wide range of engineering applications. Below is the classification of ceramics.

Classification type Examples
Based on their composition
  • Oxides
  • Carbides
  • Nitrides
  • Sulfides
  • Fluorides
Based on their specific applications
  • Glasses
  • Clay products
  • Refractories
  • Abrasives
  • Cements
  • Advanced ceramics for special applications
Traditional ceramics(made-up of clay, silica and feldspar) Bricks, tiles and porcelain articles
Engineering ceramics These consist of highly purified aluminium oxide (Al2O3), silicon carbide (SiC) and silicon nitride (Si3N4)

4. Crystal structure

  • Both crystalline and noncrystalline states are possible for ceramics.
  • The crystal structures of those materials for which the atomic bonding is predominantly ionic are determined by the charge magnitude and the radius of each kind of ion.

4.1 Silicates

Silicates are composed of primarily of silicon and oxygen. These two are most abundant elements in the earth’s crust.

  • Silicates structure is more conveniently represented by means of interconnecting SiO4-4 tetrahedra as shown below.

  • As in the figure, four oxygen atoms are situated at four corners of the tetrahedron with silicon at the center.
  • This is the basic unit of the silicates

Based on the sharing of corners of SiO4-4 tetrahedron with oxygen atom, below are different types of silicates

a) Silica

  • It is a three-dimensional network that is generated when every corner oxygen atom in each tetrahedron is shared by adjacent tetrahedra.
  • Chemically, the most simple silicate material is silicon dioxide, or silica (SiO2).
  • This material is electrically neutral and all atoms have stable electronic structures.
  • Under these circumstances the ratio of Si to O atoms is 1:2, as indicated by the chemical formula.

b) Silica Glasses

  • If above silica exists as a noncrystalline solid or glass, which is having a high degree of atomic randomness, such a material is called fused silica or silica glass.
  • It is a liquid in characteristic is liquid
  • such a material is called fused silica, or vitreous silica.

4.2 Carbon

Is carbon really a ceramic?

  • Carbon materials does not really fall within any one of the traditional metal, ceramic, polymer classification schemes.
  • However, we choose to discuss these materials in this chapter since graphite, one of the polymorphic forms[FAQ], is sometimes classified as a ceramic, and the crystal structure of diamond also a polymorph
  • Carbon exists in various polymorphic forms, as well as in the amorphous state.

Here brief introduction on diamond, graphite, the fullerenes, and carbon nanotubes are given below

a) Diamond

Diamond is a metastable[FAQ] carbon polymorph at normal conditions.

Structure:

  • Each carbon bonds to four other carbons, and these bonds are totally covalent.
  • This is appropriately called the diamond cubic crystal structure, which is also found for other elements like germanium, silicon, and gray tin those are in Group IVA of periodic table.

Properties:

  • Physical: Extremely hard (the hardest known material)
  • Electrical: Has a very low electrical conductivity
    • Due to its crystal structure and
    • The strong inter-atomic covalent bonds.
  • Thermal: Has a high thermal conductivity for a nonmetallic material,
  • Optical: Transparent in the visible and infrared regions of the electromagnetic spectrum

Applications:

  • Used as gemstones.
  • diamonds are utilized to grind or cut other softer materials.
  • Diamond films are having many uses like
    • Coating on the surfaces of drills, dies, bearings, knives etc. to increase surface hardness.
    • Surface of machine components such as gears, to optical recording heads and disks, and as substrates for semiconductor devices.

Is artificial diamond preparation possible?

  • Techniques to produce synthetic diamonds have been developed, and today a large proportion of the industrial- quality materials are man-made, in addition to some of those of gem quality.
  • Over the last several years, diamond in the form of thin films has been produced. But none of the films yet produced has the long-range crystalline regularity of natural diamond.

b) Graphite

Graphite is also a polymorph of carbon.It has a crystal structure distinctly different from that of diamond and is also more stable than diamond at normal conditions.

Structure:

  • The graphite structure is composed of layers of hexagonally arranged carbon atoms
  • Within the layers, each carbon atom is bonded to three coplanar neighbor atoms by strong covalent bonds.
  • The fourth bonding electron participates in a weak van der Waals type of bond between the layers.

How Graphite became a lubricant?

  • Because of this weak vander waals bonds between the layers, it easy to achieve inter-planar cleavage.
  • This gives rise to the excellent lubricative properties of graphite.
  • Also, the electrical conductivity is relatively high in crystallographic directions parallel to the hexagonal sheets.

Properties:

  • Physical: High strength and good chemical stability at elevated temperatures
  • Thermal:
    • High thermal conductivity
    • Low coefficient of thermal expansion
    • High resistance to thermal shock
    • High adsorption of gases, and good machinability.

Applications:

  • Graphite is commonly used as heating elements for electric furnaces
  • As electrodes for arc welding, in metallurgical crucibles, in casting molds for metal alloys and ceramics.
  • For electrical contacts, brushes and resistors.
  • As electrodes in batteries.
  • In air purification devices.

c) Fullerenes

Another polymorphic form of carbon was discovered in 1985. It exists in discrete molecular form and consists of a hollow spherical cluster of sixty carbon atoms; a single molecule is denoted by C60.

Structure:

  • Each molecule is composed of groups of carbon atoms that are bonded to one another to form both
    • Hexagon (six-carbon atom) and
    • Pentagon (five-carbon atom) geometrical configurations.
  • These are arrayed in such a way that no two pentagons share a common side.
  • The molecular surface is like a soccer ball.

The material composed of C60 molecules is known as buckminsterfullerene, named in honor of R. Buckminster Fuller.

How this structure is different from that of diamond and graphite?

  • The carbon atoms in buckminsterfullerene bond together so as to form these spherical molecules.
  • But where as in diamond and graphite, all of the carbon atoms form primary bonds with adjacent atoms throughout the entirety of the solid.
  • In the solid state, the C60 units form a crystalline structure.

Properties:

  • As a pure crystalline solid, this material is electrically insulating.
  • But with proper impurity additions, it can be made highly conductive and semiconductive.

b) Carbon Nanotubes

Another molecular form of carbon has recently been discovered that has some unique and technologically promising properties.

Structure:

  • Its structure consists of a single sheet of graphite, rolled into a tube, both ends of which are capped with C60 fullerene hemispheres.
  • Each nanotube is a single molecule composed of millions of atoms
  • The length of this molecule is much greater (on the order of thousands of times greater) than its diameter.
  • Multiple-walled carbon nanotubes consisting of concentric cylinders have also been found to exist.

Properties:

  • These are extremely strong and stiff, and relatively ductile.
  • This is the strongest known material.

Applications:

  • The carbon nanotube has been termed the “ultimate fiber” and is extremely promising as a reinforcement in composite materials.
  • Nanotube displays in future would be cheaper and will have lower power requirements than CRT and liquid crystal displays.
  • Furthermore, it is anticipated that future electronic applications of carbon nanotubes will include diodes and transistors.

4.3 Other important Ceramic materials

Ceramic type Description
Glasses
  • They are non-crystalline silicates
  • When containing other oxides, usually CaO, Na2O, K2O and Al2O3 which influence the glass properties and its color.
  • Typical property of glasses that is important in engineering applications is its response to heating.
  • There is no definite temperature at which the liquid transforms to a solid as with crystalline materials. 
  • Eg: containers, windows, mirrors, lenses, etc.
Clay products
  • It is found in great abundance and popular because of ease with which products are made.
  • Clay products are mainly two kinds
    • Structural products (bricks, tiles, sewer pipes) and
    • White- wares (porcelain, chinaware, pottery, etc.)
Abrasive ceramics
  • The prime requisite for this group of materials is hardness or wear resistance in addition to high toughness.
  • They need to exhibit some refractoriness, as they may exposed to high temperature
  • Eg: Diamond, silicon carbide, tungsten carbide, silica sand, aluminium oxide etc.
Cements
  • When they are mixed with water, they form slurry which sets subsequently and hardens finally.
  • Thus it is possible to form virtually any shape.
  • They are also used as bonding phase, for example between construction bricks.
  • Eg: Cement, plaster of paris and lime.

5. Ceramics processing methods

  • The very specific character of ceramics – high temperature stability – makes conventional fabrication routes unsuitable for ceramic processing.
  • Inorganic glasses, though, make use of lower melting temperatures, most other ceramic products are manufactured through powder processing.

Ceramic powder processing consists of powder production by milling/grinding, followed by fabrication of green product, which is then consolidated to obtain the final product

  • Typical ceramic processing routes are
    • Powder synthesis
    • Green component
      • Casting
      • Extrusion
      • Compaction
    • Sintering / firing.

5.1 Processing Glasses

  • Most of them are silica-soda-lime variety.
  • Raw materials are heated to an elevated temperature, where melting occurs.

Glass melt is processed by different route to form different products:

Pressing:

  • To form shapes like plates and dishes

Blowing:

  • Used to produce objects like jars, bottles, light bulbs.

Drawing:

  • To form lengthier objects like tubes, rods, whiskers, etc

5.2 Ceramic Powder Processing

  • Ceramic powder processing route:
    • Synthesis of powder.
    • Followed by fabrication of green product.
    • Then consolidated to obtain the final product.

a) Synthesis of powder

  • It involves crushing, grinding, separating impurities, blending different powders.

b) Green component

  • It can be manufactured in different ways: tape casting, slip casting, extrusion, injection molding and cold-/hot- compaction.
  • Green component is then fired/sintered to get final product.

1. Ceramic powder processing using Casting

  • Slurry of ceramic powder is processed via casting routes – tape casting, and slip casting.

2. Ceramic powder processing using Extrusion & Injection molding

  • Extrusion – viscous mixture of ceramic particles, binder and other additives is fed through an extruder where continuous shape of green ceramic is produced. Then the product is dried and sintered.
  • Injection molding – Mixture of ceramic powder, plasticizer, thermoplastic polymer, and additives is injected into die with use of a extruder.
  • Then polymer is burnt off, before sintering rest of the ceramic shape.
    • It is suitable for producing complex shapes.

Extrusion and Injection molding are used to make ceramic tubes, bricks, and tiles.

3. Ceramic powder processing using Compaction

  • Ceramic powder is compacted to form green shapes of sufficient strength to handle and to machine those shapes.
  • Basis for compaction is application of external pressure from all directions.

c) Sintering

  • It is a process of subjecting the green ceramic to elevated temperatures with the purpose of gaining mechanical integrity.
  • Driving force for sintering is reduction in total surface area and thus energy.
  • Diffusion is responsible for growth of bonds at contact points of particles. This lead to coalescence of particles, and eventual mechanical integrity.

6. FAQ

Q: What is glass transition temperature?

Ans: Glass transition temperature or fictive temperature is defined based on viscosity above which material is named as super cooled liquid or liquid, and below it is termed as glass. There is no particular value for this but changes with composition.

 

References

  1. William D. Callister, Jr.David G. Rethwisch: Materials Science and Engineering: An Introduction, Wiley publication, 2014
  2. NPTEL material science material by Satish Vasu Kailas (IISc)

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