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Example research essay topic: Raw Material High Temperature - 1,217 words

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Introduction First we will start with the definition of refractories and ceramics. Refractories and ceramics are non-metallic materials capable of maintaining physical and chemical stability at high temperatures. Refractories in modern practice are usually ceramic in nature, and are used in a wide variety of primary, secondary and tertiary industries. Wherever an industrial process involves heat in excess of 700 to 800 degrees Fahrenheit (roughly), one will find refractory material in place, either as a lining or forming the process vessel itself. Some common process vessels using refractories are; boiler combustion chambers, furnaces like the one in the foundry, incinerators, many emission control scrubbers, rotary kilns and so on. The list is by know means exhaustive.

For example, Launch Pads 39 A and 39 B at the Kennedy Space Center are refractory lined. The shuttles themselves are lined with ceramic tiles to protect them from the heat of re-entry into earth's atmosphere, these tiles are! unique to the shuttle, but are non-metallic and heat resistant. The Making of Refractory Materials Chart # 1 The first step in processing ceramics is crushing of the raw materials.

Crushing is usually done in a ball mill, either wet or dry. Wet crushing is more effective because it keeps the particles together and prevents the suspension of fine particles in air. The ground particles are then mixed with additives, the functions of which are one or more of the following: 1. Binder for the ceramic particles 2.

Lubrication for mold release and to reduce internal friction between particles 3. Wetting agent to improve mixing 4. Plasticizer to make the mix more plastic and formal. 5. Various agents to control foaming and sintering. 6. De-flocculent to make ceramic-water suspension. De-flocculent ion changes the electrical charges on the clay particles so that they repel instead of attract each other.

Next, it's time to begin the casting process. The shaping process for refractories are casting plastic forming and pressing. The most common casting process is slip casting. The slip is poured into a porous mold made usually of plaster of paris.

Then inverted and the remaining suspension is poured out for making hollow object much like slush casting. The part is then trimmed the mold opened and the part removed. The second process of shaping ceramics is plastic forming. We have various methods of plastic forming such as extrusion, injection molding and jiggering. Plastic forming tends to orient the layered structure of clays along the direction of material flow. This leads to anisotropic behavior of the material, both in subsequent processing and in the final properties of the ceramic product.

In extrusion, the clay mixture is forced through a die opening. The cross section of the extruded product is constant, and there are limitations to wall thickness for hollow extrusions. Finally the third process in shaping ceramics is pressing. Dry pressing is used for relatively simple shapes.

this process has high production rates and close control of tolerances. Dies are usually made of carbides or hardened steel. However the dies can be quite expensive as the must have a high wear resistance from the abrasive ceramic tiles. Wet pressing is used to make very complex shapes. Production rates are high but part size is limited, dimensional control is difficult because shrinking during drying, and tooling can be expensive. The third type of pressing is isostatic pressing mainly used to produce spark plug insulators, silicon nitride vanes for high temperature and so on.

Isostatic pressing allows one to obtain uniform density distribution throughout a part. Finally we have hot pressing which combines pressure and temperature. The die life is short as a result of the temperature, and usually protective atmosphere's are used along with graphite materials used in! the punch and die materials. One example of a hot pressing part is the vane for a gas turbine engine in a jet airplane. Finally, after the part has been cast in anyone of our methods above we begin drying and firing the part to give it strength.

Drying is very critical as the part may want to warp and crack from variations in moisture and thickness within the part and the complexity of the shape. Control of atmospheric humidity and temperature is very important to avoid warping and cracking. Next, the part must be fired, this is where the part gains it's strength and hardness. The improvement in the properties result from (a) development of a strong bond between the complex oxide particles in the ceramic and (b) reduced porosity. After firing, additional operations may be performed to give the part it's final shape, remove surface flaws, and improve the surface of the finish and tolerances.

Processes used include; grinding, lapping, and ultrasonic, chemical and electrical-discharge machining. The finer the finish the higher the parts strength will be. Most product are finally given a glossy coating with a glaze material the improve appearance, strength once again and to make them impermeable. Structure of Refractories Materials A refractory is a type of ceramic so I'll refer to ceramics once again. the structure of a ceramic crystal is among the most complex of all materials, containing various elements of different sizes.

The bonding between these atoms is generally covalent (electron sharing, hence strong bonds). and ionic (primary bonding between opposite charged ions, thus strong bonds). These bonds are much stronger than metallic bonds. Consequently, the properties of ceramics are significantly higher than those for metals, particularly their hardness and thermal and electrical resistance. Ceramics are available as a single crystal or in a polycrystalline form, consisting of many grains. Grain size plays a major role in strength and the properties of the part.

The finer the grain size the higher the strength and toughness- hence the term fine ceramics. Among the oldest raw material for ceramics is clay, a fine-grained sheet like structure, the most common being kaolinite. White clay, consists of silicate of aluminum and altering weakly bonded layers of silicon and aluminum ions. When added to kaolinite water attaches itself to the layers, makes them slippery, and gives wet clay it's well known softness and plastic properties that make it formal. Another major raw material for ceramics is flint and feldspar. In their general state these materials usually contain impurities and these impurities must be removed prior to further processing of materials into useful products with reliable performance.

Next well move on to some types of refractory materials and refractory metals most commonly used in industry. The types of ceramics I'll be talking about are carbides. Typical examples of carbides are tungsten and titanium. Tungsten carbide consists of tungsten-carbide particles with cobalt as a binder. The amount of the binder has a major influence on the material's properties. Toughness increases with cobalt content, whereas hardness, strength, and wear resistance decrease.

Titanium carbide has nickel and molybdenum as the binder and is not as tough as tungsten carbide. These metals are typically used as cutting tools and die materials, also abrasives on cutting wheels. Silicon carbide has good wear, thermal shock, and corrosion resistance. It has a low coefficient of friction and retains strength at elevated temperatures. It is suitable for high-temperature components in heat engines and is also used as an abrasive in grinding wheels.

Next, we have Nitrides; Cubic boron nitride, Titanium nitride and Silicon...


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