Types and applications of ceramics
Ceramics offer a high temperature range. However, ceramics are not very strong. To compensate for their lack of strength ceramics are usually combined with some other material to form a ceramic composite.
1) Glasses and glass ceramics- The glasses are a familiar group of ceramics; containers, windows, lenses and fiberglass represent typical applications. The properties of standard vitrified products are insufficient for architectural applications and structural building components, insulation or other specialized applications. Yet there is an effective way to improve these properties without major alterations to the process itself - the introduction of a controlled crystallization process through a subsequent heat treatment, i.e. by forming a glass-ceramic.
Production of Glass-Ceramics
Glass-ceramic articles may be produced by three routes:
• The heat treatment of solid glass (the traditional route)
• The controlled cooling of a molten glass, known as the petrurgic method
• The sintering and crystallisation of glass powders.
In the latter case, the powders are densified at relatively low temperatures by exploiting a viscous flow sintering mechanism. After densification, the material is subjected to a crystallisation heat- treatment to obtain the required glass-ceramic microstructure. Alternatively, both densification and crystallisation may take place during a single sintering step. Along with the economic advantage of using relatively low processing temperatures, the powder technology route is suitable for the production of a range of advanced materials, including glass-ceramics with specified porosities and glass-ceramic matrix composites.
Using the petrurgic method, the slow cooling from the molten state causes nucleation and growth of certain crystalline phases. Therefore, the final microstructure, and hence the properties, depends mainly on the composition and the cooling rate.
Glass-Ceramics Based on Coal Ash
The very high iron oxide content of coal ash, table 1, indicates the potential for developing magnetic phases using appropriate processing - this was the aim of our work. We calcined the as- received ash at 800°C for two hours to remove any volatile Material compounds, including sulfur and carbon. The powder and petrurgic methods were explored, and gave us products with different phases and microstructures. For the sintering experiments, we mixed calcined ash powder with various amounts (10-50wt%) of borosilicate (Pyrex) glass. The powder mixtures were uniaxial cold pressed to a cylindrical shape and sintered in air at temperatures in the range of 1,000-1,500°C for periods of up to 15 hours. Using the petrurgic method, coal ash was mixed with sodalime glass powder. The mixture was melted at 1,500°C and cooled to room temperature at rates of between 1-10°C per minute.
Glass-Ceramic Composites
Work to date has largely concentrated on composites with a matrix of the slag-based Silceram glass-ceramic (a glass-ceramic for floor and wall tiles and wear components). We have investigated both particulate- (SiC and TiC) and fibre-reinforcement (SiC). Properties measured include the fundamental mechanical properties but also more complex properties such as thermal shock resistance and erosion resistance. As mentioned previously, the thermal shock resistance of glass-ceramics is superior to the parent glasses, and the shock resistance is further improved by particulate reinforcement. For example, monolithic Silceram has a thermal shock critical temperature of 180°C, whereas a 20wt%SiC composite has a value of 270°C. Erosion resistance may also be improved by particulate reinforcement, e.g., for TiC reinforced Silceram - the larger the reinforcement particle size and the greater the volume fraction, the lower the erosion rate. Results indicate a way for transforming vitrified silicate residues into useful products with broad application potential. The glass-ceramics obtained are candidate materials for applications in floors of industrial buildings and in construction, and for outside and inside facing walls. We are currently addressing issues associated with the effect of environmental influences on the chemical durability and toxic potential of the materials, which may be compromised by the presence of heavy metals incorporated in the glass or crystalline phases. Public acceptance of the use and exploitation of glass-ceramic-based materials in such applications will strongly depend on a satisfactory consideration of these issues.
2) Refractories -Refractories are materials needed for handling high temperature liquids, gases and solids, e.g., for industrial processing. Applications include solar furnaces, casting molds for molten materials, heat exchangers, and aerobraking heat shields. Industrial refractory needs can be satisfied by sintered calcia (CaO), silica (SiO2), magnesia (MgO), alumina (Al2O3) and titania (TiO2), with the desired porosity. Of course, these stable materials are commonly used on Earth for the same purposes, due to their great resistance to heat, oxidation (they are already fully oxidized), corrosion and abrasion. Minerals such as olivine [(MgFe)2SiO4] and anorthite (CaAl2Si2O8) are also useful for making refractory bricks and ceramics. Some refractories and their ceramics have low expansion due to heat and are attractive for space environments where a wide range of temperatures are experienced.
One particular application of refractories is in transportation for returning cargoes to low Earth orbit by aerobraking with the upper atmosphere, and perhaps slowing down some incoming asteroid payloads by aerobraking. Of course, this is the method used by spacecraft to return to Earth, including the reusable Space Shuttle. The Space Shuttle's tiles are made from silica (SiO2) (with a thin borosilicate coating to provide a smooth, aerodynamic surface for a smooth landing). Aerobraking tiles are produced from amorphous silica fibers which are pressed and sintered, with the resulting tile having as much as 93% porosity (i.e., very lightweight) and low thermal expansion, low thermal conductivity, and good thermal shock properties. This process can be readily performed in space when we can produce silica of the required purity. Cheaper materials besides silica fibers can be used. Silica fibers are used on the Space Shuttle in order to keep its weight down, thereby increasing cargo weight capacity. For resources already in space, we don't have this economic need. A number of other materials can be used for heat shields, e.g., alumina (Al2O3) or anorthite (CaAl2Si2O8).
3) Abrasives- Abrasive cements are used to wear, grind or cut away other material, which necessarily is softer. Therefor, the prime requisite for this group of materials is hardness or wear resistancr; in addition, a hig degree of toughness is essential to ensure that the abrasive particles do not easily fracture. Furthermore, high temperatures may be produced from abrasive frictional forces, so some refractoriness is also desirable. Diamonds, both natural and synthetic, are utilized as abrasives; however, they are relatively expensive. The more common ceramic abrasives include silicon carbide, tungsten carbide(WC), aluminium oxide and silica sand. Abrasives are used in several forms-bonded to grinding wheels, as coated abrasives and as loose grains. Coated abrasives are those in which an abrasive powder is coated on some type of paper or cloth material; sandpaper is probably the mostly familiar example. Wood, metals, ceramics and plastics are all frequently ground and polished using this form of abrasive. Grinding, lapping and polishing wheels often employ loose abrasive grains that are delivered in some type of oil or water based vehicle. Diamonds, corundum, silicon carbide and rouge are used in loose form over a variety of grain size ranges.
4) Cements: Several familiar ceramic materials are classified as inorganic cements:cements, plaster of paris, and lime, which as a group are produced in extremely large quantities. The characteristic feature of these materials is that when mixed with water, they form a paste that subsequently sets and hardens. This trait is especially useful in that solid and rigid structures having just about any shape may be expeditiously formed.