Silicon alumina-based ceramics

Fig. 2.72 The influence of the temperature, T (diagrams a, c, e) at V h = 0.5 mm/min and of the speed V<-h, (diagrams b, d, f) at T = 1200 °C on the appearance of the load (P) versus deflection <5 a, b SN-1 c, d A-1 e, f Y-PSZ-3. SN-1 is a silicon nitride-based ceramic with additions of Y2O3 and AI2O3 A-1 is an alumina-based ceramic with an addition of MgO and Y-PSZ-3 is a Y2O3 stabilized zirconia based single crystal [18]. With kind permission of Elsevier...

The data base now contains 5497 test results on over 320 different batches of ceramic materials. Approximately 46% of these are on zirconia-based ceramics, 9% are on silicon carbides, 21% are on silicon nitrides, 6.7% are on whisker-reinforced silicon nitrides, 16.3% are on alumina-based ceramics (including whisker-reinforced aluminas and mullites), and 2% are on other ceramics. Table 1 gives a detailed breakdown, by material class, of the data stored in the system, A list of materials within a material class Is available on request. 

Now HlPing is used for a wide variety of ceramic (and metallic) components, such as alumina-based tool bits and the silicon nitride nozzles used in flue-gas desulfurization plants by the utility industry. The advantages of the HlPing process are becoming more important as interest in structural ceramics such as Si3N4 grows. 

From the point of view of the volume of production, polycrystalline alumina is the material most frequently used as ceramics for structural applications. However, in comparison with for example, silicon nitride, where the influence of various additives on microstructure and properties has been well characterized and understood, and despite several decades of lasting research effort, alumina remains a material with many unknown factors yet to be revealed. Alumina-based materials can be divided roughly into three groups ... 

Surface flaws are common in optical fiber because of the processing technique used for the fabrication of fused-silica fibers. They are also very common in other ceramic fibers such as alumina-based or silicon carbide-based fibers. Airborne particles as well as other elements tend to attach to the surface of the fiber during process or handling. 

Values for ceramics range from below 1 for glasses and most single crystals 1 to 3 for glass-ceramics, clay-based ceramics and MgO 2.5. to 4 for early engineering ceramics (alumina, boron carbide, silicon carbides, RBSN) 4 to 6 for hot-pressed silicon nitride and SRBSN and for transformation-toughened alumina... 

The traditional or conventional ceramics are generally in monolithic form. These include bricks, pottery, tiles and a variety of art objects. The advanced or high-performance monolithic ceramic materials represent a new and improved class of ceramic materials where, frequently, some sophisticated chemical processing route is used to obtain them. Generally, their characteristics are based on the high quality and purity of the raw materials used. Examples of these high-performance ceramics include oxides, nitrides, carbides of silicon, aluminium, titanium and zirconium, alumina, etc. 

Care has to be taken in selecting materials for the die and punches. Metals are of little use above 1000 °C because they become ductile, and the die bulges under pressure so that the compact can only be extracted by destroying the die. However, zinc sulphide (an infrared-transparent material) has been hot pressed at 700 °C in stainless steel moulds. Special alloys, mostly based on molybdenum, can be used up to 1000 °C at pressures of about 80 MPa (5 ton in-2). Alumina, silicon carbide and silicon nitride can be used up to about 1400 °C at similar pressures and are widely applied in the production of transparent electro-optical ceramics based on lead lanthanum zirconate as discussed in Section 8.2.1. 

The last quarter of the twentieth century saw tremendous advances in the processing of continuous, fine diameter ceramic fibers. Figure 6.4 provides a summary of some of the important synthetic ceramic fibers that are available commercially. We have included in Fig. 6.4 two elemental fibers, carbon and boron, while we have excluded the amorphous, silica-based glasses. Two main categories of synthetic ceramic fibers are oxide and nonoxides. A prime example of oxide fibers is alumina while that of nonoxide fibers is silicon carbide. An important subclass of oxide fibers are silica-based glass fibers and we devote a separate chapter to them because of their commercial importance (see chapter 7). There are also some borderline ceramic fibers such as the elemental boron and carbon fibers. Boron fiber is described in this chapter while carbon fiber is described separately, because of its commercial importance, in Chapter 8. 

In assembling hybrid microcircuits or multichip modules, ceramic interconnect substrates fabricated using thin-film or thick-film processes are attached to the inside base of a ceramic or metal package. Generally, film adhesives that have been cut to size are used to attach large substrates (greater than 1-inch square) while either paste or film adhesives may be used for smaller substrates. Substrates may be alumina, beryllia, aluminum nitride, or silicon. 

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