Radiation Properties of Materials

The authors of this book started working on chemical kinetics more than 10 years ago focusing on investigations of particular radiation - induced processes in solids and liquids. Condensed matter physics, however, treats point (radiation) defects as active particles whose individual characteristics define kinetics of possible processes and radiation properties of materials. A study of an ensemble of such particles (defects), especially if they are created in large concentrations under irradiation for a long time, has lead us to many-particle problems, common in statistical physics. However, the standard theory of diffusion-controlled reactions as developed by Smoluchowski... 

So fill, we have considered the nature of radiatiou, the radiation properties of materials, and the view factors, and we arc now in a position to consider the rate of heal transfer between surfaces by radiation. The analysis of radiation exchange between surfaces, in general, is complicated because of reflection a radiation beam leaving a surface may be reflected several times, with partial refleclion occurring at each surface, before it is completely absorbed. The analysis is simplified greatly when the surfaces involved can be approximated... 

Radiographic tests are made on pipeline welds, pressure vessels, nuclear fuel rods, and other critical materials and components that may contain three-dimensional voids, inclusions, gaps or cracks that are aligned so that the critical areas are parallel to the x-ray beam. Since penetrating radiation tests depend upon the absorption properties of materials on x-ray photons, the tests can reveal changes in thickness and density and the presence of inclusions in the material. 

There are numerous properties of materials which can be used as measures of composition, e.g. preferential adsorption of components (as in chromatography), absorption of electromagnetic waves (infra-red, ultra-violet, etc.), refractive index, pH, density, etc. In many cases, however, the property will not give a unique result if there are more than two components, e.g. there may be a number of different compositions of a particular ternary liquid mixture which will have the same refractive index or will exhibit the same infra-red radiation absorption characteristics. Other difficulties can make a particular physical property unsuitable as a measure of composition for a particular system, e.g. the dielectric constant cannot be used if water is present as the dielectric constant of water is very much greater than that of most other liquids. Instruments containing optical systems (e.g. refractometers) and/or electromechanical feedback systems (e.g. some infra-red analysers) can be sensitive to mechanical vibration. In cases where it is not practicable to measure composition directly, then indirect or inferential means of obtaining a measurement which itself is a function of composition may be employed (e.g. the use of boiling temperature in a distillation column as a measure of the liquid composition—see Section 7.3.1). 

In general it appears that the electrical properties of organic materials practically do not change, as long as the mechanical properties of materials withstand the action of radiation. 

From time to time we have mentioned that thermal conductivities of materials vary with temperature however, over a temperature range of 100 to 200°C the variation is not great (on the order of 5 to 10 percent) and we are justified in assuming constant values to simplify problem solutions. Convection and radiation boundary conditions are particularly notorious for their nonconstant behavior. Even worse is the fact that for many practical problems the basic uncertainty in our knowledge of convection heat-transfer coefficients may not be better than 20 percent. Uncertainties of surface-radiation properties of 10 percent are not unusual at all. For example, a highly polished aluminum plate, if allowed to oxidize heavily, will absorb as much as 300 percent more radiation than when it was polished. 

Refractive index — A fundamental physical property of materials through which light can travel. It is usually indicated by the symbol n, and it is defined as n = c/cQ, where c0 is the speed of light in vacuum and c corresponds to the speed at which the crests of electromagnetic radiation corresponding to a specific frequency propagate in a material [i,ii], A more rigorous definition for the refractive index of a dense and isotropic material composed of a unique kind of particles (atoms or... 

Knowledge of the optical properties of materials in relation to the solar spectrum is also important in measuring broadband solar radiation. For instance, a pyranome ter used to monitor total solar radiation for a renewable energy system has a spectral response (due to the special glass dome protecting the detector) that does not respond to the thermal infrared radiation of the sky beyond 3000 nm, as shown in Fig. 15. Flowever, there will be thermal infrared radiation exchanged between the radiometer and the sky dome, which will influence the measurement performance of the pyranometer.9... 

In this chapter we consider the thermodynamic properties of materials subjected to several external fields gravitational, centrifugal, surface, radiation, electric, and magnetic. The analysis gives rise to several new effects that are of intrinsic interest they also provide new insights on the systematics of thermodynamic analysis. The reader is urged to note these features in the subsequent discussion. 

The dielectric properties of materials are defined by two different parameters, namely the dielectric constant and the dielectric loss. The dielectric constant, e, describes the ability of a molecule to be polarized by the electric field. At low frequencies, e reaches a maximum as the maximum amount of energy can be stored in the material. The dielectric loss, s, measures the efficiency with which the energy of the electromagnetic radiation can be converted into heat. The dielectric loss goes through a maximum as the dielectric constant falls [16]. The dissipation factor (tan d) is the ratio of the dielectric loss of the sample, also called loss factor , to its dielectric constant tan 8 = e /s. 

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. 

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