Ceramics density

Fig. 3.5.5 Radially averaged density map of a uniform 75% Theoretically Dense (TD) Al203 ceramic cylinder, measured using NMR imaging of imbibed SF gas. The maximum and minimum ceramic densities were 3.10— 2.86 g cm-3.

This criterion, which is product rather than precursor-property driven, is critical to the design and synthesis of new precursors. The need for high ceramic yields arises because of the excessive volume changes associated with pyrolytic conversion to ceramic materials. Scheme 1 illustrates these changes for a SiC precursor with an 80% ceramic yield of phase pure SiC (3.2 gml-1). Most precursors densities are close to 1 gml-1, whereas most Si ceramic densities range from 2.5 to 3.5 gml-1. 

Colossal magnetoresistance studies of ceramics have demonstrated no significant influence of ceramic density on the magnetoresistance at H = 1.5T. However, ceramics obtained by the solid state synthesis method and containing inclusions of lanthanum-doped manganese oxide displayed substantially higher magnetoresistance values. This effect was later used for directed enhancement of low field... 

Zircon zor- kan, -kon [Gr, mod. of F jargon jargon, zircon, fr. I giargone] (1794) n. Zr02-Si02. Occurs as natural double oxide. It is produced by physical separation of beach sand (Florida, Australia). Used in vitreous enamels and ceramics. Density, 4.56 particle size, 1.0-2.5 pm refractive index, 2.0. Syn zirconium silicate. 

In previous papers (9,31-33) we have investigated the dielectric and electret properties of the composite pdy(methyl methacrylate) (PMMAVBaTiO, ceramic system. The 0-3 type composites used in the studies were prepared by polymerization of methyl methacrylate (MMA) in which fine particles of ferroelectric ceramics BaHO, were dispersed. The BaTiOt ceramics were ofarained by the known method of triple sintering of BaCO) and TK), in stoichioaietiic ratio. The final process was conducted at about 1620 K for 3 h. resulting in a ceramic density of 5.57 X 10 kg/m This was pulverized and sifted... 

Although this discussion has been in temis of molecules in solution, the same principles apply to other cases, such as precipitates in an alloy or composites of ceramic particles dispersed in a polymer. The density, p(r), is... 

SiHcon nitride (see Nitrides) is a key material for stmctural ceramic appHcations in environments of high mechanical and thermal stress such as in vehicular propulsion engines. Properties which make this material uniquely suitable are high mechanical strength at room and elevated temperatures, good oxidation and creep resistance at high temperatures, high thermal shock resistance, exceUent abrasion and corrosion resistance, low density, and, consequently, a low moment of inertia. Additionally, siHcon nitride is made from abundant raw materials. 

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

Heat treatment of related glasses melted under reducing conditions can yield a unique microfoamed material, or "gas-ceramic" (29). These materials consist of a matrix of BPO glass-ceramic filled with uniformly dispersed 1—10 p.m hydrogen-filled bubbles. The hydrogen evolves on ceranarning, most likely due to a redox reaction involving phosphite and hydroxyl ions. These materials can have densities as low as 0.5 g/cm and dielectric constants as low as 2. 

A more extensive comparison of many potential turbine blade materials is available (67). The refractory metals and a ceramic, sHicon nitride, provide a much higher value of 100 h stress—mpture life, normalised by density, than any of the cobalt- or nickel-base aHoys. Several intermetaHics and intermetaUic matrix composites, eg, aHoyed Nb Al and MoSi —SiC composites, also show very high creep resistance at 1100°C (68). Nevertheless, the superaHoys are expected to continue to dominate high temperature aHoy technology for some time. 

Inorganic membranes (29,36) are generaUy more stable than their polymeric counterparts. Mechanical property data have not been definitive for good comparisons. IndustriaUy, tube bundle and honeycomb constmctions predominate with surface areas 20 to 200 m. Cross-flow is generaUy the preferred mode of operation. Packing densities are greater than 1000 /m. Porous ceramics, sintered metal, and metal oxides on porous carbon support... 

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