Density Various ceramics

Table 4.3 Mean thermal expansion coefficients and theoretical densities of various ceramics...

With its low acoustic impedance, extreme bad width, high piezoelectric coefficient, and low density (only one-quarter the density of ceramic materials), PVDF is ideally suited as a transducer for hroad hand rmdenvater receivers in hghtweight hydrophones. The softness and flexibiHty of PVDF give it a comphance 30 times greater than ceramic. PVDF can thus he utilized in a hydrophone structure using various device configurations, such as compliant tubes, rolled cylinders, discs, and planar stacks of laminated material. 

Hot isostalic pressing (HIP) is a fabrication process that is used to reduce the porosity of metals and increase the density of various ceramic materials [70-74]. The HIP process subjects a component to both elevated temperature and isostatic gas pressure in a high-pressure containment vessel. The chamber is heated, causing the pressure inside the vessel to increase. Many systems use associated gas... 

The present chapter deals with experimental procedures and results achieved in the processing of various UHTCs. Because of strong covalent bonding and low self-diffusion, high temperatures and/or external pressure are required to density these ceramic materials. 

Table 2 shows the dielectric properties of a range of ceramic materials under various conditions and near the two frequencies for which Industrial equipment can be readily purchased. It is evident that the effective losses of various ceramics depend upon the material density and the temperature, frequency and field orientation, giving a range of effective loss factors from above 70 for silicon carbide to 3 x 10 for boron nitride. This latter cereimic can be considered as transparent to microwave energy and may be used as an insulating material in microwave Industrial equipment or as a microwave window in waveguides. In fact, any material with an effective loss factor... 

This chapter has described the various techniques of ceramic powder characterization. These characteristics include particle shape, surface area, pore size distribution, powder density and size distribution. Statistical methods to evaluate sampling and analysis error were presented as well as statistical methods to compare particle size distributions. Chemical analytical characterization although veiy important was not discussed. Surface chemical characterization is discussed separately in a later chapter. With these powder characterization techniques discussed, we can now move to methods of powder preparation, each of which 3uelds different powder characteristics. 

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