Thermal conductivity, low-density

He It has an excellent thermal conductivity, low density, inertness and it permits greater flow rates. It is highly expensive,... 

Carbon electrodes — Carbon is selected for many electrochemical applications because of its good electrical and thermal conductivity, low density, adequate corrosion resistance, low thermal expansion, low elasticity, and high purity In addition, carbon materials can be produced in a variety of structures, such as powders, fibers, large blocks, thin solid and porous sheets, nanotubes, fullerenes, graphite, and diamond. Furthermore, carbon materials are readily available and are generally low-cost materials. 

Foams, for insulation in buildings low thermal conductivity low density... 

The use of carbon materials in electrochemical systems started in 19 century, when carbon electrodes replaced copper ones in Volta batteries and Pt electrodes in Grove Cells. Nowadays, carbon materials are used in many electrochemical applications because of their high electrical and thermal conductivity, low density, high corrosion resistance, low elasticity adequate strength and high purity. In addition, carbon materials are available in a variety of physical structures (powders, fibres, cloths), have a low cost and can be fabricated into composite structures. 

Hence, the gel is able to re-expand as shown in Fig. 2.5. Rao et al. (2005) prepared two-step acid-base ambient pressure-dried silica aerogels successfully. He reported that aerogels obtained from APD have improved properties in terms of low thermal conductivity, low density, high porosity, and high optical transmittance than aerogels produced by HTSCD. 

Low thermal conductivity Low density Incombustible Good bonding strength... 

The primary attributes of a powder insulation include low thermal conductivity, low density, and particle size distribution to minimize shock and vibration effects. Powders can be utilized either evacuated or nonevacuated (gas filled), and one method of application is usually more suitable than the other for a given system. Generally, the small particle size of the material... 

The variation in total thermal conductivity with density has the same general nature for ah. cellular polymers (143,189). The increase in at low densities is owing to an increased radiant heat transfer the rise at high densities to an increasing contribution of k. ... 

Vermiculites are formed by the decomposition of mica. They contain layers of water and magnesium ions in place the potassium ions. When heated to 800°C-1100°C, vermiculite expands because of the conversion of the water to gas. The expanded vermiculite has a low thermal conductivity and density, and is used as a thermal and sound barrier as well as an aggregate in lightweight concrete. It is also used as a moisture-retaining soil conditioner in planting. 

For common thermally thick combustible materials (greater than a few millimeters) the time to ignition is proportional to the product k p c (where k is the thermal conductivity, p is the density, and c is the heat capacity), which represents the thermal inertia of the sample. Thermal inertia characterizes the rate of surface temperature rise of the material when exposed to heat. Low values of thermal inertia lead to a rapid temperature rise for a given applied heat flux and hence, to a rapid ignition.4 Polymeric foams have much lower thermal conductivity and density than the corresponding solid materials, thus the surface temperature of the first heats up more rapidly than that of the latter. Foam surface may reach the ignition temperature 10 times faster than the solid polymer.5... 

Transient conduction conditions occur in polymer processing. Appendix A derives Eq. (A.14) for one-dimensional transient heat flow, which contains the thermal diffusivity a. This is the combination k/pCp of the thermal conductivity k, density p and specific heat Cp. For most polymer melts a is approximately equal to O.lmm s" (Fig. 5.3). For the melting of low-density polyethylene in an extruder, typical conditions are a barrel temperature of To = 220 °C, an initial polymer temperature Tp = 20 °C, and a melting process complete at T = 120 °C. Consequently, using Eq. (C.19), after a contact time t, the melt front is at a distance from the barrel given by... 

Carbonaceous materials have found wide applications in electrochemical technologies. This fact, in part, may be attributed to their advantages like good thermal and electrical conductivities, low density, adequate corrosion resistance, low thermal... 

Silicon carbide has long been recognized as an ideal ceramic material for applications where high hardness and stiffness, mechanical strength at elevated temperatures, high thermal conductivity, low coefficient of thermal expansion, and resistance to wear and abrasion are of primary importance. Moreover, because of its low density it offers greater advantages compared to other ceramics. 

Reaction Bonded SIC (RBSC) is utilized in many applications for its high thermal conductivity, low coefficient of thermal expansion (CTE) and high specific stiffness. A large interest in this material for thermal management and precision applications is thus generated. The low density and high hardness of this material also leads to a large array of armor applications. 

Siace the pores ia an aerogel are comparable to, or smaller than, the mean free path of molecules at ambient conditions (about 70 nm), gaseous conduction of heat within them is iaefficient. Coupled with the fact that sohd conduction is suppressed due to the low density, a siUca aerogel has a typical thermal conductivity of 0.015 W/(m-K) without evacuation. This value is at least an order of magnitude lower than that of ordinary glass and considerably lower than that of CFC (chloro uorocarbon)-blown polyurethane foams (54). 

Euture appHcations may involve use of SiC as substrates for siHcon chips, making use of the high thermal conductivity of SiC and its close thermal expansion match to siHcon. The low density and high stiffness of siHcon carbides may also result in appHcations in space. One such appHcation is for space-based mirrors, making use of the high degree of surface poHsh possible on dense SiC. 

As a good first approximation (187), the heat conduction of low density foams through the soHd and gas phases can be expressed as the product of the thermal conductivity of each phase times its volume fraction. Most rigid polymers have thermal conductivities of 0.07-0.28 W/(m-K) and the corresponding conduction through the soHd phase of a 32 kg/m (2 lbs/fT) foam (3 vol %) ranges 0.003-0.009 W/(m-K). In most cellular polymers this value is deterrnined primarily by the density of the foam and the polymer-phase composition. Smaller variations can result from changes in cell stmcture. 

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