Polycrystalline material insulator

Particles of ionic insulators suspended in solution can have charged surfaces because of an excess there of ions of one type as a result, e.g., of adsorption from the solution. The surface charge depends on the concentration in the solution of ions that can adsorb. The sol-gel technique is discussed in the next section as an example of a synthesis of nonmetallic inorganic polycrystalline materials that makes use of surface charge control of colloidal particles. 

Fig. 27.2 Young s modulus and linear thermal expansion coefficients (relative to 300 K) between 300 K open circles) and 1,000 K closed circles) of electrode metals, ionic conductors, and substrate/insulator materials for solid-state ionic micro gas sensors (polycrystalline materials). Young s modulus of NASICON and were not available (Reprinted with permission from Dubbe 2003, Copyright 2003 Elsevier)...

Single-crystal samples of conductors best other solid samples are suitable, including polycrystalline metals, polymeric materials, semiconductors, and insulators, ultrahigh vacuum compatible typically > 5 mm diameter, 1-3 mm thick... 

Grain boundaries have a significant effect upon the electrical properties of a polycrystalline solid, used to good effect in a number of devices, described below. In insulating materials, grain boundaries act so as to change the capacitance of the ceramic. This effect is often sensitive to water vapor or other gaseous components in the air because they can alter the capacitance when they are absorbed onto the ceramic. Measurement of the capacitance allows such materials to be used as a humidity or gas sensor. 

Compounds of the I—VII group in the periodic table are known to exhibit good ionic conductivity and have attracted much attention as possible candidates for solid electrolytes. A typical family of compounds is Lil, CuCl, CuBr, and Agl. Historically, polycrystalline solid electrolytes were noticed to show significantly higher ionic conductivity than bulk crystals, since a half century ago. Furthermore, a large increase in conductivity was reported for the system of the mixture of a solid electrolyte such as CuCl (1) and Agl (2) with submicrometer particles of several sorts of insulating materials. In this case, the size of the metal halide itself was on the order of a micrometer or larger. It was also reported that the enhanced conductivity was approximately proportional to the inverse of the size of the electrolyte substances (2). Hence it is natural to make an effort to obtain fine particles of metal halides in order to get better conductivity. 

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