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Chapter 13:
Measurement of Material Properties
These include, as described in the taxonomy, emissivities, absorptivities, reflectivities, transmissivities, and refractive indices. They are discussed in turn. 13.1 Total Hemispherical Emissivity This measurement is usually made by invoking some form of energy conservation. The process is to suspend the unknown in a hohlraum (a complete enclosure that is in thermodynamic equilibrium) and generate heat electrically in the unknown. The temperature difference between the unknown and the walls of the hohlraum provide the emissivity information. The equation for total radiative transfer, the Stefan-Boltzmann equation, includes the constant, σ, the emissivity, and the temperature. Thus, proper measurement of the temperatures, the flux, and auxiliary parameters can give the emissivity. Drummeter and Goldstein have described their approach in the measurement of coatings for the Vanguard satellite. Their test body, a 10-cm diameter aluminum sphere with the unknown coating, was suspended in an evacuated, 40-cm, spherical, aluminum chamber that was kept at −75°C (198 K) with a dry-ice and alcohol bath. The arrangement is shown schematically in Figure 13-1. The inside of the chamber was coated with a Glyptal black, and electrical power was supplied to a cartridge heater in the sample. Both temperatures were measured with thermocouples. Both the power leads and the thermocouple leads to the unknown were maintained as thin as possible to minimize heat loss. The inside of the chamber was maintained at 10−5 torr. The power transfer is given by ΔΦ=EI=ε 1 ε 2 σA 1 A 2 (T 4 1 −T 4 2 ) ε 2 A 2 +ε 1 A 1 −ε 1 ε 2 A 1 +K(T 2 −T 1 )+CdT dt , where Φ = input power [W], E = heater voltage [V], I = heater current, ε1 = hemispheric surface emissivity, ε2 = emissivity of the walls, σ = Stefan-Boltzmann constant, A1 = sample surface area, A2 = chamber surface area, T1 = sample temperature [K], T2 = chamber temperature [K], K = conductive loss constant, and C = heat capacitance of the sphere.
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