Material characteristics grain boundaries

The piopeities of a ceramic material that make it suitable for a given electronic appHcation are intimately related to such physical properties as crystal stmcture, crystallographic defects, grain boundaries, domain stmcture, microstmcture, and macrostmcture. The development of ceramics that possess desirable electronic properties requires an understanding of the relationship between material stmctural characteristics and electronic properties and how processing conditions maybe manipulated to control stmctural features. 

The performance characteristics of ceramic sensors are defined by one or more of the foUowing material properties bulk, grain boundary, interface, or surface. Sensor response arises from the nonelectrical input because the environmental variable effects charge generation and transport in the sensor material. 

The nature of the material to be studied, which means its degree of crystallinity and perfectness of crystal structure, may have a significant effect on the thermoanalytical behavior. In spite of identical chemical composition of a certain material the variations with respect to structure, imperfections, grain boundaries, etc. are almost infinite. Of course many of these will not show in normal thermogravimetric analysis, with very sensitive apparatus characteristically different TG curves18, 19 may be obtained however. As an example Fig. 26 shows the thermal decomposition of hydrozincite, Zn5(OH)6(003)2, whereby equal amounts of samples from natural origin and synthetic preparations are compared. 

These ordered array materials find interest not only in catalysis, but in several other applications, from optical materials, sensors, low-k materials, ionic conductors, photonic crystals, and bio-mimetic materials.Flowever, with respect to these applications, catalysis requires additional specific characteristics, such as the presence of a thermally stable nanostructure, the minimization of grain boundaries where side reactions may occur, and the presence of a porous structure which guarantees a high surface area coupled to low heat and mass transfer limitations. An ordered assembly of ID nanostructures for oxide materials could, in principle, meet these different requirements. 

Another distinctive characteristic of the PLZT materials is their fully dense, pore-free microstructure which is devoid of any second phases. This is reflected in measured bulk densities which routinely exceed 99.9% of theoretical density. The existence of pores or second phases in the volume of the grains or in the grain boundaries is undesirable since both act to increase light scattering and reduce optical transparency. 

It is practically impossible to prepare a stable HTSC material for which all the desired characteristics are present simultaneously. However, in specific applications, only a few of the above-mentioned characteristics are usually important. Moreover, advantage can be taken of certain effects which lead normally to unwanted characteristics of HTSC materials, for special purposes. For example, when the grain boundaries have microscopic areas with low conductivity, it is possible in certain cases to realize the Josephson and related effects. 

---