Insulation, comparison thermal conductivity

The dimensional stability of low density, water blown rigid PU foams for pour-in-place thermal insulation applications was improved by the use of a phthalic anhydride based polyester polyol containing a dispersed cell opening agent. The foam systems obtained allowed some of the carbon dioxide to be released through the cell windows immediately after filling of the cavity, and to be rapidly replaced by air. Studies were made of the flowability, density, open cell content, dimensional stability, mechanical properties, thermal conductivity and adhesion (particularly to flame treated PE) of these foams. These properties were examined in comparison with those of HCFC-141b blown foams. 21 refs. 

Introduction. It has been recognized that CFC-ll-blown rigid urethane foams are the insulation materials with the lowest thermal conductivities, in comparison with other insulation materials, such as water-blown rigid urethane foams, glass-fiber materials, or polystyrene foams. 

The absorption of impurity centres is observed in the transparency domains of semiconductors and insulators, which are limited by their intrinsic electronic and vibrational absorptions. Further, a brief account of the relevant physical processes and an overview of the intrinsic optical properties of these materials and of their dependence on temperature, pressure and magnetic field is given in this chapter. Some semiconductors have been or are now synthesized in quasi-monoisotopic (qmi) forms because of improvements in their physical properties like thermal conductivity. A comparison of their intrinsic optical properties with those of the crystals of natural isotopic composition is also given. The absorption related to free carriers, due mostly to doping is also discussed at the end of this chapter. A detailed account of the optical properties of semiconductors can be found in the books by Yu and Cardona [107] and by Balkanski and Wallis [4]. 

Thermal Conductivity. The results of the thermal conductivity, k, tests are given in Table VII for four different mixture ratios representing S/A ratios of 1.3 to 10. The data indicate that additional sulfur had little effect on the thermal conductivity, which averaged 11.7 X 10 4 cal-cm/ cm2-sec-°C (3.40 X Btu-in./ft2-hr-°F). A comparison with the value obtained for the A/C system of 15.77 cal-cm/cm2-sec-°C (4.57 Btu-in./ft2-hr-°F) would indicate that the thermal conductivity is about 25% less for S-A—S than for A/C. This is attributed to the higher air void contents in the former which add to the insulative characteristics of the material. 

Nanofillers have superb thermal and electrical properties. All nanotubes are expected to be very good thermal conductors along the tube axis, exhibiting a property known as ballistic conduction, but good insulators laterally to the tube axis. It has been reported that single-wall carbon nanotubes exhibit thermal conductivity (TC) values as high as 2000-6000 W mK [4] under ideal circumstances. The temperature stability of carbon nanotubes is estimated to be up to 2800 °C in a vacuum, and about 750 °C in air. By comparison, metals have TC values of several hundred W mK , and water and oil have TC values of only 0.6 W mK and 0.2 W mK respectively. Table 19.1 lists the thermal conductivities of various materials, including nanofillers (nanotubes), metals, and oils. 

The durian peel and coconut coir mixture was then used to develop low thermal conductivity particleboards. It was reported that the mixture ratio of durian peel and coconut coir was optimum at 10 90 by weight. In comparison with durian only particleboards and coconut only particleboards, the mixture particleboards showed better properties, except for the modulus of elasticity. The mixture particleboard is of lower thermal conductivity, which is suitable for ceiling and wall insulating materials. With more development, it will not be impossible to use this material for furniture applications. 

Organic aerogels (Part IV) are of special interest due to their thermal insulation properties. The organic solid backbone has a low solid thermal conductivity in comparison to inorganic aerogels [27]. Heat transfer within resorcinol-formaldehyde (RF) aerogels... 

Almost all steam and hot-process piping systems operate in the temperature range of 212 to 1000°F (100 to 538°C).The products most frequently used are calcium silicate, fiberglass, mineral wool, and expanded perhte. Choices are usually based on thermal conductivity and resistance to physical abuse where applicable. Hberglass products are the most thermally efficient at lower temperatures, with calcium sihcate being the best at higher temperatures. It is important to use the insulation mean temperature when comparing thermal conductivities in order to make the proper comparison. 

Microsphere Insulation. Good thermal-insulation performance has also been achieved by using packed, hollow glass spheres, typically of a size ranging from 15 to 150/xm in diameter and coated on the exterior with a film of low emittance material such as aluminum. These hollow spheres, which generally have a wall thickness of 0.5-2.0 m, substantially increase the conduction thermal resistance but markedly reduce the heat capacity and the mass relative to solid particles. Further, hollow microspheres offer a lightweight and low heat capacity alternative to MLI. A comparison of these two insulations is presented in Table 7.4. 

Semiconductors are low bandgap insulators. Low is defined qualitatively, so that an appreciable density of electrons can be thermally excited into the conduction band at temperatures that are technologically relevant. In silicon, a large gap semiconductor Eg = 1.12eV exp(—Eg/Agf) 1.6 x 10 at 300K), this density is very small at room temperature. Germanium (Eg = 0.67) and indium-antimonide (InSb, Eg = 0.16 eV exp(—Eg/ksT 2 x 10 at 300 K) are examples of lower gap semiconductors. For comparison, in diamond Eg = 5.5 eV. 

In the case of systems in which the filled valence band and the next unoccupied band are separated by a considerable energy gap the electric potential required for the transition and hence to produce conductivity is large. Such material behaves as insulators. The intermediate case where the energy gap exist but is relatively small in comparison with thermal energies correspond to... 

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