Physical Properties of Insulators

This specification covers the composition and physical properties of insulating boards composed of wood-fiber insulation board laminated to rigid cellular polystyrene insulation boards, flat or tapered, used principally above structural roof decks for thermal insulation as well as for a base for roofing in building construction. 

Table 1. Thermal physical properties of insulation foams used in experiments.

Describe the physical properties of insulators, semiconductors, and conductors and show how some interchange can be made to occur. 

Mechanical Properties. The physical properties of a particular refractory product depend on its constituents and manner in which these were assembled. The physical properties may be varied to suit specific appHcations. For example, for thermal insulations highly porous products are employed, whereas dense products are used for slagging or abrasive conditions. 

Silicone foam thus formed has an open ceU stmcture and is a relatively poor insulating material. Cell size can be controlled by the selection of fillers, which serve as bubble nucleating sites. The addition of quartz as a filler gready improves the flame retardancy of the foam char yields of >65% can be achieved. Because of its excellent dammabiUty characteristics, siUcone foam is used in building and constmction fire-stop systems and as pipe insulation in power plants. Typical physical properties of siUcone foam are Hsted in Table 10. 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

Basic physical properties of sulfur, selenium, and tellurium are indicated in Table 1.3. Downward the sulfur sub-group, the metallic character increases from sulfur to polonium, so that whereas there exist various non-metallic allotropic states of elementary sulfur, only one allotropic form of selenium is (semi)metallic, and the (semi)metallic form of tellurium is the most common for this element. Polonium is a typical metal. Physically, this trend is reflected in the electrical properties of the elements oxygen and sulfur are insulators, selenium and tellurium behave as semiconductors, and polonium is a typical metallic conductor. The temperature coefficient of resistivity for S, Se, and Te is negative, which is usually considered... 

Reactions involving the creation, destruction, and elimination of defects can appear mysterious. In such cases it is useful to break the reaction down into hypothetical steps that can be represented by partial equations, rather akin to the half-reactions used to simplify redox reactions in chemistry. The complete defect formation equation is found by adding the partial equations together. The mles described above can be interpreted more flexibly in these partial equations but must be rigorously obeyed in the final equation. Finally, it is necessary to mention that a defect formation equation can often be written in terms of just structural (i.e., ionic) defects such as interstitials and vacancies or in terms of just electronic defects, electrons, and holes. Which of these alternatives is preferred will depend upon the physical properties of the solid. An insulator such as MgO is likely to utilize structural defects to compensate for the changes taking place, whereas a semiconducting transition-metal oxide with several easily accessible valence states is likely to prefer electronic compensation. 

Thermal conductivity is a physical property of the solid through which the heat is being transferred. It is a measure of the material s ability to conduct heat. Insulators have a low thermal conductivity and conductors have a high thermal conductivity. The rate of heat transfer has magnitude and direction. This is represented mathematically by the negative sign that appears in Fourier s law of heat conduction. 

To emphasize the importance of structural studies at LT or HP, let us take the example of TTF-TCNQ. This prototype charge density wave (CDW) system undergoes at ambient pressure a succession of three structural and electronic phase transitions, from a high-temperature metallic phase down to a low-temperature insulator phase. There has been a considerable debate about the mechanism of these transitions, and many distortional modes have been proposed to account for the physical properties of this material (e.g., rigid molecule displacement as translations [73,74] or librations [75,76] or even internal deformations of the molecules [77,78]. Indeed, an experi-... 

The purpose of this paper is the presentation of a brief overview of recent literature in which new models of electronic states in polymers and molecular solids have been proposed (, 2, 5-16). Since localized (e.g., molecular-ion) states seem prevalent in these materials, I indicate in Sec. II the physical phenomena which lead to localization. Sec. Ill is devoted to the description of a model which permits the quantitative analysis of the localized-extended character of electronic states and to the indication of the results of spectroscopic determinations of the parameters in this model for various classes of polymeric and molecular materials. I conclude with the mention in Sec. IV of an important practical application of these concepts and models The contact charge exchange properties of insulating polymers ( 7, 17, 18, 19). 

---