Insulation materials, thermal conduction

AH 103 EPS 303 Guidelines for Reporting Thermal Transmission Properties of Polystyrene Foam Insulating Materials, Thermal Conductance, and Transmittance of Built-Up Construction Systems. 

Thermal Conductivity (K Factor) Depending on the type of insulation, the thermal conductivity K factor) can vary with age, manufacturer, moisture content, and temperature. Typical published values are shown in Fig. 11-65. Mean temperature is equal to the arithmetic average of me temperatures on both sides of the insulating material. 

Naturally, not all of these properties are improved at the same time nor is there usually any requirement to do so. In fact, some of the properties are in conflict with one another, e.g., thermal insulation versus thermal conductivity. The objective is merely to create a material that has only the characteristics needed to perform the design task. 

Diamond is a material possessing unique properties. It has extraordinarily high atomic density, hardness, insulating ability, thermal conductivity, and chemical inertness (see Table 1). The history of its intensive electrophysical, physico-chemical, and optical studies covers many decades [1, 2], Its applications in materials science [3], microelectronics [4], and so on, are ever widening. 

The most important property for insulation is thermal conductivity. The following transport types participate in the transmission of heat heat conduction in PS, heat conduction in the filling gas (air), radiation heat transfer and heat convection by convection flows in the closed cells. The thermal conductivity of the air in the cells contributes the most to the total heat transport. The radiation fraction depends on the diameter of the cells formed. The thermal conductivity depends on the density of the foamed PS material. Thermal conductivity decreases with increasing bulk density, reaches a minimum and then rises again (Figure 9.15). The following processes are responsible for this characteristic. 

S4C Consider a. solid cylindrical rod whose side surface is maintained at a constant temperature while the end surfaces are perfectly insulated. The thermal conductivity of the rod material is constant and there is no heat generation. It is claimed that the temperature in the radial direction wiihin the rod will not vaty during steady heat conduction. Do you agree with this claim Why ... 

In the United States, the /f-values of insulation are expressed widiout any units, such as /f-19 and R- iO. These R-values are obtained by dividing the thickness of the material in feet by its thermal conductivity in the unit Btu/li ft F so that the/f-values actually have the unit h fi F/Btu. For example, the /f-value of 6-in-thick glass-fiber insulation whose thermal conductivity is 0.025 Btu/h ft F is (Fig. 7-37)... 

Electrically insulating and thermally conductive qualities are important in computer chips fabrication. One approach taken is based on boron nitride fillers which offers these two properties. There is also a need to develop materials which are thermally conductive but electrically insulating in high humidity conditions. Polyurethane composites filled with aluminum oxide or carbon fiber can be used for this application. Figure 19.15 shows the effect of the amount of filler on thermal... 

If a polymer is intended for use in applications requiring high thermal insulation, its thermal conductivity is very important. The low thermal conductivities of polymers, and of foams [12] made by expanding polymers and incorporating pockets of air or other gas molecules in the "cells" resulting from the expansion, when coupled with other desirable properties, have made polymers and polymeric foams the insulating materials of choice in many applications. 

Technically adequate thermal conductivity figures are in the region of 1 to 20 Wm" K and require the admixture of weighted quantities of fillers and additives. Filler systems can be ceramic materials (thermally conductive but electrically insulating) and metallic materials (thermally and electrically conductive) and diverse modified forms of carbon [4, 31]. The high proportion of fillers alters behavior radically compared to that of nonmodified thermoplastics see Fig. 2.10. 

Thermal Insulation. In addition to their low thermal conductivity, as discussed in the section above, siUca aerogels can be prepared to be highly transparent in the visible spectmm region. Thus, they are promising materials as superinsulating window-spacer. To take further advantage of its... 

Eig. 1. Thermal conductivity components vs density for a typical thermal insulation material at 300 K A, total conductivity B, air conduction C, radiation ... 

A low (<0.4 W / (m-K)) thermal conductivity polymer, fabricated iato alow density foam consisting of a multitude of tiny closed ceUs, provides good thermal performance. CeUular plastic thermal insulation can be used in the 4—350 K temperature range. CeUular plastic materials have been developed in... 

Thermal Conductivity and Aging. Thernial performance is governed by gas conduction and radiation (18—20). In most ceUular plastic insulations, radiation is reduced because normal densities of use ate 4-50 kg/m and the average cell size is <0.5 mm. For open-ceU and other materials containing air (at 24°C, 7 = 0.025 W/(m-K)) this results in total values of X at 0.029-0.0039 W/(m-K). 

The thermal conductivities of the most common insulation materials used in constmction are shown in Table 2. Values at different mean temperature are necessary for accurate design purposes at representative temperatures encountered during winter or summer. For example, under winter conditions with an outside temperature of -20 to -10°C, the mean temperature is 0—5°C. For summer, mean temperatures in excess of 40°C can be experienced. 

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. 

The most important properties of refractory fibers are thermal conductivity, resistance to thermal and physical degradation at high temperatures, tensile strength, and elastic modulus. Thermal conductivity is affected by the material s bulk density, its fiber diameter, the amount of unfiberized material in the product, and the mean temperature of the insulation. Products fabricated from fine fibers with few unfiberized additions have the lowest thermal conductivities at high temperatures. A plot of thermal conductivity versus mean temperature for three oxide fibers having equal bulk densities is shown in Figure 2. 

Polyurethane. Polyurethanes (pu) are predominantly thermosets. The preparation processes for polyurethane foams have several steps (see Urethane polymers) and many variations that lead to products of widely differing properties. Polyurethane foams can have quite low thermal conductivity values, among the lowest of all types of thermal insulation, and have replaced polystyrene and glass fiber as insulation in refrigeration. The sprayed-on foam can be appHed to walls, roofs, tanks, and pipes, and between walls or surfacing materials directly. The slabs can be used as insulation in the usual ways. 

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