Thermal Conductivity of Ceramics and Other Insulating Materials

THERMAL CONDUCTIVITY OF CERAMICS AND OTHER INSULATING MATERIALS... 

Thermal Conductivity of Ceramics and Other Insulating Materials.12-225... 

Thermal Conductivity of Alloys as a Function of Temperature, 12-205 Thermal Conductivity of Ceramics and Other Insulating Materials,... 

One final note is appropriate for this section. Dne to the fact that many oxide ceramics are used as insulating materials, the term thermal resistivity is often used instead of thermal conductivity. As will be the case with electrical properties in Chapter 6, resistivity and conductivity are merely inverses of one another, and the appropriateness of one or the other is determined by the context in which it is used. Similarly, thermal conductance is often used to describe the thermal conductivity of materials with standard thicknesses (e.g., building materials). Thermal condnctance is the thermal conductivity divided by the thickness (C = k/L), and thermal resistance is the inverse of the prodnct of thermal conductance and area R = 1/C A). 

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials. 

Although most ceramics are thermal and electrical insulators, some, such as cubic boron nitride, are good conductors of heat, and others, such as rhenium oxide, conduct electricity as well as metals. Indium tin oxide is a transparent ceramic that conducts electricity and is used to make liquid crystal calculator displays. Some ceramics are semiconductors, with conductivities that become enhanced as the temperature increases. For example, silicon carbide, SiC, is used as a semiconductor material in high temperature applications. 

One of the main drawbaeks in the generation of thermal eonduetivity data on plasties remains the paucity of reference materials. Reference materials are important because they provide an important baseline for the calibration of instruments. Most thermal conductivity instruments require calibration to materials of known thermal conductivity. The National Institute of Standards (NIST) and other standards-setting institutions have characterized a number of metals, ceramics, glasses, and even insulation. These are not suitable as reference materials because they tend to possess a thermal conductivity that is higher or lower than those of plastics. For example, the NIST reference glass materials Pyrex 7740 and Pyroceram 9606 possess a thermal conductivity of 1 W/m-K which is five times greater than that of most plastics. The other reference materials are still more inapplicable. Many fluids, on the other hand, possess thermal conductivities similar to plastics. The transient line-source technique, however, is the only technique that can use such reference materials. 

Fillers may decrease thermal conductivity. The best insulation properties of composites are obtained with hollow spherical particles as a filler. Conversely, metal powders and other thermally conductive materials substantially increase the dissipation of thermal energy. Volume resistivity, static dissipation and other electrical properties can be influenced by the choice of filler. Conductive fillers in powder or fiber form, metal coated plastics and metal coated ceramics will increase the conductivity. Many fillers increase the electric resistivity. These are used in electric cable insulations. Ionic conductivity can be modified by silica fillers. 

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