Specific heat insulators

The cubic dependence of cph on temperature accounts for the small specific heat at low temperature of crystalline insulators. Specific heat of rare gas crystals is shown in Fig. 3.2. 

Heatshield thickness and weight requirements are determined using a thermal prediction model based on measured thermophysical properties. The models typically include transient heat conduction, surface ablation, and charring in a heatshield having multiple sublayers such as bond, insulation, and substmcture. These models can then be employed for any specific heating environment to determine material thickness requirements and to identify the lightest heatshield materials. 

Thermal Properties. Thermal properties include heat-deflection temperature (HDT), specific heat, continuous use temperature, thermal conductivity, coefficient of thermal expansion, and flammability ratings. Heat-deflection temperature is a measure of the minimum temperature that results in a specified deformation of a plastic beam under loads of 1.82 or 0.46 N/mm (264 or 67 psi, respectively). Eor an unreinforced plastic, this is typically ca 20°C below the glass-transition temperature, T, at which the molecular mobility is altered. Sometimes confused with HDT is the UL Thermal Index, which Underwriters Laboratories estabflshed as a safe continuous operation temperature for apparatus made of plastics (37). Typically, UL temperature indexes are significantly lower than HDTs. Specific heat and thermal conductivity relate to insulating properties. The coefficient of thermal expansion is an important component of mold shrinkage and must be considered when designing composite stmctures. 

The specific heats of polymers are large - typically 5 times more than those of metals when measured per kg. When measured per m, however, they are about the same because of the large differences in density. The coefficients of thermal expansion of polymers are enormous, 10 to 100 times larger than those of metals. This can lead to problems of thermal stress when polymers and metals are joined. And the thermal conductivities are small, 100 to 1000 times smaller than those of metals. This makes polymers attractive for thermal insulation, particularly when foamed. 

To maintain the target level for temperature, a specific amount of insulation may be needed, since too little insulation makes it impossible to keep the temperature levels. For each building it is necessary to make a detailed cost-benefit calculation of insulation and heating/cooling costs. The same discussion is applicable to temperature variation requirements, both for the rate of change and the period lengths (see Chapter 16). 

Equal masses of liquid A, initially at 100°C, and liquid B, initially at 50°C, are combined in an insulated container. The final temperature of the mixture is 80°C. All the heat flow occurs between the two liquids. The two liquids do not react with each other. Is the specific heat of liquid A larger than, equal to, or smaller than the specific heat of liquid B ... 

Though short fiber-reinforced mbber composites find application in hose, belt, tires, and automotives [57,98,133,164] recent attention has been focused on the suitability of such composites in high-performance applications. One of the most important recent applications of short fiber-mbber composite is as thermal insulators where the material will protect the metallic casing by undergoing a process called ablation, which is described in a broad sense as the sacrificial removal of material to protect stmcrnres subjected to high rates of heat transfer [190]. Fiber-reinforced polymer composites are potential ablative materials because of their high specific heat, low thermal conductivity, and ability of the fiber to retain the char formed during ablation [191-194]. 

Between 1 and 2 K, the 4He specific heat shows a steep dependence on temperature due to the so-called rotonic excitation. Below 0.6 K, 4He behaves like a Debye insulator with a specific heat proportional to T3. 

An electrical fault causes the heating of wire insulation producing fuel gases that mix instantly with air. The wire is in a narrow shaft in which air enters at a uniform velocity of 0.5 m/s at 25 °C (and density 1.18 kg/m3). The cross-sectional area of the shaft is 4 cm2. The gaseous fuel generated is at 300 °C and its heat of combustion is 25 kJ/g. Assume steady state conditions and constant specific heat at 1.0 J/g K. 

A 375 g plug of lead is heated and placed in an insulated container filled with 0.500 L of water. Prior to the immersion of the lead, the water is at 293 K. After a time, the lead and the water assume the same temperature, 297 K. The specific heat capacity of lead is 0.127 J/(g K), and the specific heat capacity of water is 4.18 J/(g K). How hot was the lead before it entered the water (Hint You ll need to use the density of water.)... 

Determining the Approximate Value of the Atomic Mass of Lead from Its Specific Heat Capacity. To determine the specific heat capacity of a metal, use a calorimeter and a device for heating the metal. A very simple calorimeter can be made from several beakers inserted one into another (Fig. 38). The inner beaker should have a volume of 100 ml, the middle one—300-400 ml, and the outer one—500 ml. Water is poured into the small beaker, while the others are needed to produce an air thermal-insulating layer. 

ASTM C351, 1999. Standard test method for the mean specific heat of thermal insulation. 

A piece of copper of mass 20.0 g at 100.0°C is placed in an insulated vessel of negligible heat capacity but containing 50.7 g of water at 22.0°C. Calculate the final temperature of the water. Assume that all the energy lost by the copper is gained by the water. The specific heat capacity of copper is 0.38 J-(°C) 1-g 1. 

ASTM C-351. Standard Test Method for Mean Specific Heat of Thermal Insulation. 

Two solutions, initially at 25.08°C, were mixed in an insulated bottle. One consisted of 400mL of a weak monoprotic acid solution of concentration 0.200 mL. The other consisted of lOOmL of a 0.800mol of NaOH per liter of solution. The temperature rose to 26.25°C. How much heat is evolved in the neutralization of one mole of the acid Assume that the densities of all solutions are 1.00 g/cm3 and that their specific heat capacities are all 4.2 J/g K. Actually, these assumptions are in error by several percent, but they nearly cancel each other. 

The specific heat of a good thermal insulator should be ... 

After finishing the measurement described in Example 5, the same iron resistor is placed in an insulated container with 75 g of an unknown substance. A current of 37 A through the resistor for 15 s raises the temperature of the system by 7.6°C. What is the specific heat of the unknown substance ... 

Systems may be in chemical or mechanical equilibrium, and they may also exhibit thermal equilibrium. If a hot object is placed in contact with a colder mass of the same material inside an insulated container, heat flows from the hot object into the colder object until the temperatures of the two are equal. Heat lost by the warm object is equal to the amount gained by the cold object. The amount of heat needed to raise the temperature of an object a certain amount is equal to the amount which that object would lose in cooling by the same amount. The amount of heat needed to warm or the amount lost when cooling equals the product of the specific heat (or heat capacity) of the substance, the mass, and the change in temperature. For example, if a 50-gram (1.8-ounce) piece of silver at 70°C (158°F) is placed in 50 grams (1.8 ounces) of water at 15°C (59°F), the principle of thermal equilibrium can be used to calculate the final temperature of the water and silver ... 

Low-molecular silicone elastomers are viscous liquids which do not contain solvents and solidify at room temperature. The specific properties of SKTN allow one to use them as insulation against heat, moisture and electricity in various miniature and large units of machines, mechanisms and devices, as well as for thermal, electrical and vibration sealing of various devices. The physiological inertness of elastomers accounts for their wide applications in medicine. 

The specific heat of a semiconductor has contributions from lattice vibrations, free carriers and point and extended defects. For good quality semi-insulating crystals only the lattice contribution is of major significance. Defect-free crystals of group III nitrides are difficult to obtain, and thus the specific heat measurements are affected by the contributions from the free carriers and the defects. While the specific heat of AIN is affected by the contribution of oxygen impurities, the data for GaN and InN are affected by free electrons, especially at very low temperatures. 

A transparent gas flows into and out of a black circular tube of length L and diameter D. The gas has a mean velocity um, specific heat at constant pressure cp and density p. The wall of the tube is thin, and the outer surface is insulated. The tube wall is heated electrically and a uniform input of heat is provided per unit area, per unit time. Determine the local wall temperature distribution along the tube length. Assume that the convective heat transfer coefficient h between the gas and the inside of the tube is constant. 

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