Opaque body

For a grey body, the emissivity and the absorptivity are, by definition, independent of temperature and hence equation 9.115 may be applied more generally showing that, where one radiation property (a, r or e) is specified for an opaque body, the other two may be obtained from equations 9.115 and 9.124. KirchofPs Law explains why a cavity with a small aperture approximates to a black body in that radiation entering is subjected to repeated internal absorption and reflection so that only a negligible amount of the incident radiation escapes through the aperture. In this way, a - e = 1 and, at T K, the emissive power of the aperture is aT4. 

In emission spectrometry, the sample is the infrared source. Materials emit infrared radiation by virtue of their temperature. KirchhofF s law states that the amounts of infrared radiation emitted and absorbed by a body in thermal equilibrium must be equal at each wavelength. A blackbody, which is a body having infinite absorptivity, must therefore produce a smooth emission spectrum that has the maximum possible emission intensity of any body at the same temperature. The emissivity, 8, of a sample is the ratio of its emission to that of a blackbody at the same temperature. Infrared-opaque bodies have the same emissivity at all wavelengths so they emit smooth, blackbody-like spectra. On the other hand, any sample dilute or thin enough for transmission spectrometry produces a structured emission spectrum that is analogous to its transmission spectrum because the emissivity is proportional to the absorptivity at each wavelength. The emissivity is calculated from the sample emission spectrum, E, by the relation... 

This igneous spirit, bom into bodies by the rays, is easily distinguished from them. The latter are communicated only as long as they find in their way no opaque bodies which arrest their course. The former penetrates even the most dense bodies, since we feel the heat on the side of a wall opposite to that on which the rays fall, although they have not been able to penetrate it. This heat exists even after the rays have disappeared with the luminous body. 

In the case of solid crystalline oxides, thermal conductivity decreases with increasing temperature but begins to rise above 1500— 1600 °C because transmission of heat by radiation (photons) begins to take a significant part besides the conduction of heat (phonon mechanism). In completely transparent materials (the coefficient of absorption a = O), no interaction with the radiation occurs in an opaque body (a = oo) the heat is transferred by conduction alone. With translucent materials, each element of the substance absorbs some of the incident radiation, and emits simultaneously,This internal radiation mechanism of heat transmission is characteristic for glasses. At high temperatures, a considerable proportion of heat is therefore transmitted by radiation the so-called apparent thermal conductivity is a sum of true conductivity with radiation conductivity ... 

The two material functions r x and a x of an opaque body are not independent of each other. The directional spectral reflectivity r x is determined by the directional spectral absorptivity a x. The similar relationship between the different absorptivities and reflectivities from Tables 5.1 and 5.2, respectively, mean that equations analogous to (5.41) are valid, with which the three other reflectivities can be found from the corresponding absorptivities. 

The directional spectral reflectivity r x of an opaque body can also be traced back to the directional spectral emissivity e x. According to (5.41) and (5.69), it holds that... 

This says that one single material function is sufficient for the description of the emission, absorption and reflective capabilities of an opaque body. Table 5.4 shows that it is possible to calculate the emissivities ex, s and from s x. Correspondingly, with known incident spectral intensity Kx of the incident radiation, this also holds for the calculation of ax, a and a from a x as well as of rx, r and r from r x, cf. Tables 5.1 and 5.2. So, only one single material function, e.g. e x = s x(X, f3,ip,T), is actually necessary to record all the radiation properties of a real body6. This is an example of how the laws of thermodynamics limit the number of possible material functions (equations of state) of a system. 

According to 5.3.2.1, the radiation properties of an opaque body are determined by its directional spectral emissivity e x = e x(A, f3,(p,T). In order to determine this material function experimentally numerous measurements are required, as the dependence on the wavelength, direction and temperature all have to be investigated. These extensive measurements have, so far, not been carried out for any substance. Measurements are frequently limited to the determination of the emissivity e x n normal to the surface (/ = 0), the emissivities for a few chosen wavelengths or only the hemispherical total emissivity e is measured. 

The spectral pure transmissivity decreases exponentially with the thickness of the body. An opaque body of thickness s has such a large spectral absorption coefficient that the product k(X)s reaches values over 7, then t 0 and a,[ ss 1. 

An opaque body, with the hemispherical total reflectivity r = 0.15, reflects diffusely. Determine the intensity LIef of the reflected radiation and the absorbed radiative power... 

The total radiation for a unit area of an opaque body of area emissivity Sj, and absolute temperature is... 

The white, opaque bodies fabricated in this manner have a density of 95-98% of theoretical and contain interconnected channels of uniformly sized pores of 50 nm average diameter, distributed throughout the dense amorphous silica matrix. These bodies are over twice as strong as optical grade fused sihca, as measured by transverse bend tests on specimens of equal size cut in the same way by diamond sawing. 

Fullman, R. L., Measurement of particle sizes in opaque bodies, Trans AIME, 197, 447-52, 1953. 

Absorptivity and black bodies. When thermal radiation (like light waves) falls upon a body, part is absorbed by the body in the form of heat, part is reflected back into space, and part may be actually transmitted through the body. Foremost cases in process engineering, bodies are opaque to transmission, so this will 6e neglected. Hence, for opaque bodies. 

From the Scope of Deformation Analysis of Opaque Bodies... 

That the contrast factor is the square of the difference of the scattering amplitudes is equivalent to Babinet s principle in optics, which states that the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape. For instance, a circular hole scatters like a circular disc of the same diameter. 

CAS 540-10-3 26636-40-8 (generic) 8001-78-3 123-944 Uses Lipid layer enhancer for emulsion-fomt and opaque body cleansing and hair care preparations... 

Uses Perfonnance wax dispersion for opaque body and hair cleansing preparations... 

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