Thermal expansion electrical insulators

Cases can be classified as either hermetic or nonhermetic, based on their permeabiUty to moisture. Ceramics and metals are usually used for hermetic cases, whereas plastic materials are used for nonhermetic appHcations. Cases should have good electrical insulation properties. The coefficient of thermal expansion of a particular case should closely match those of the substrate, die, and sealing materials to avoid excessive residual stresses and fatigue damage under thermal cycling loads. Moreover, since cases must provide a path for heat dissipation, high thermal conductivity is also desirable. 

Vitreous sihca has many exceptional properties. Most are the expected result of vitreous sihca being an extremely pure and strongly bonded glass. Inert to most common chemical agents, it has a high softening point, low thermal expansion, exceUent thermal shock resistance, and an exceUent optical transmission over a wide spectmm. Compared to other technical glasses, vitreous sihca is one of the best thermal and electrical insulators and has one of the lowest indexes of refraction. 

Uses. Hot-pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, especially in inert or reducing atmospheres, and for special materials such as IITV semiconductors (qv). Its low thermal expansion makes it resistant to thermal shock. The powder can be used as a mold release agent or as thermal insulation. Boron nitride is also available in fiber form (19). BN deposited pyrolyticaHy on refractory substrates at 1200—1800°C has a turbostratic stmcture and low porosity it has greater chemical resistance and is impervious to helium. 

Silicon dioxide (Si02), also known as silica, is a major industrial material with many applications particularly in the semiconductor industry in the form of coatings, which are produced mostly by CVD. It is an excellent electrical insulator with very low thermal expansion and good resistance to thermal shock. Its characteristics and properties are summarized in Table 11.4. 

Metals are characterized by their low coefficients of thermal expansion and their strong thermal and electric conductivities, whereas wood (except where there is excessive moisture), and neat polymers have high coefficients of thermal expansion and are electrical and thermal insulators. 

Aluminum nitride is one of the few materials that is both a good thermal conductor and a good electrical insulator. It is also a high-temperature ceramic, that has a low thermal expansion coefficient, and a low dielectric constant. It is also stable to molten metals such as aluminum, has good wear resistance, and good thermal shock resistance. 

Additions of BN powder to epoxies, urethanes, silicones, and other polymers are ideal for potting compounds. BN increases the thermal conductivity and reduces thermal expansion and makes the composites electrically insulating while not abrading delicate electronic parts and interconnections. BN additions reduce surface and dynamic friction of rubber parts. In epoxy resins, or generally resins, it is used to adjust the electrical conductivity, dielectric loss behavior, and thermal conductivity, to create ideal thermal and electrical behavior of the materials [146]. 

The problem of difference in thermal expansion coefficients between a silicon read-out substrate and an HgCdTe detector substrate is approached in US-A-4783594 by filling the space between the two substrates with a resilient electrically insulating polymeric material and thereafter separating the detector elements from each other by removing a layer of the HgCdTe detector substrate. 

Methods for Testing Shellac Used for Electrical Insulation Test Methods Relating to Moisture Absorption Test Methods Relating to Coefficient of Thermal Expansion Test Methods for Testing Rubber Cements... 

In comparison with metallic materials, the use of plastics is limited to relatively moderate temperatures and pressures (230°C is considered high for plastics). Plastics are also less resistant to mechanical abuse and have high expansion rates, low strengths (thermoplastics), and only fair resistance to solvents. However, they are lightweight, are good thermal and electrical insulators, are easy to fabricate and install, and have low friction factors. 

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