Thermal emittance

This instrument [7] measures three types of ions in a sequential mode the positive and negative ions emitted from the surface of the ion emitter, and the neutral species volatilizing from the surface and ionized by electron impact (El). A commercially available quadrupole mass spectrometer equipped with an El source was modified to allot a specially designed thermal emitter to be just barely inserted into the ionization chamber. The chamber is much cooler than the emitter there-... 

The importance of the work function and temperature of the surface, the ionization potential for positive ion emission, and the electron affinity for negative ion emission are well established for conditions in which the S-L equations are valid. Experimentally, the IP and EA are also important for thermal emitters. For example, the alkali metals all have low IPs and are emitted in good yields from the zeolites impregnated with the corresponding alkali metal. The halide and perrhenate anions all have high EAs and are emitted in good yield from certain of the rare earth oxides. The temperature is also quite important, but possibly not for the same reasons as for the S-L conditions. Under S-L conditions a higher temperature is more likely to strip an electron or to add an electron to an atom. 

The brightness temperature correction for true LST is based on the radiative transfer equation [33]. The concept accounts for thermal emittance from an object in accordance with the blackbody theory, which states ... 

Tab. 10.6 Correlation of thermal emitter temperature with color (Boston Electronics Corporation www.boselec.com).

Roof type Initial solar reflectance Initial thermal emittance Rebate... 

Spectrally selective solar surfaces are commonly used for conversion of solar radiation into thermal energy. The optical properties of these surfaces are such that they possess high solar absorption (on the 400—2500 nm range) to maximize the fraction of solar energy transformed to heat, and low thermal emittance to limit infrared radiation losses. The energetic balance on a thermal solar absorber was detailed in fig 1. 

Other important derivative measurements are those of thermal Emittance, e and Solar Absorptance, Both can be derived directly from Reflectance measurements, and both have importance in radiative loss applications, e.g. in space. For instance, satellites and spacecraft ideally require high reflection of solar heat during sunfacing, implying a low a < 0.2, while at the same time having high radiative capability... 

Write out detailed definitions, with illustrations if appropriate, for the following terms Chronovoltabsorptometry oxidative and reductive Switching Times Electrochromic, Electrochemical and Charge Cyclabilities Charge Capacity Dynamic Range Open Circuit Memory Bandgap Solar Absorptance Thermal Emittance. 

Very large scale integration (VLSI) technology and electronic devices Carbides and diborides as field and thermal emitters, TiN as a diffusion barrier in metallization to Si semiconductors, resistive thermoconductive humidity sensors with TaN film, and Josephson tunnel junctions with NbN film. 

Thermal emittance from the surface of most solids approximates some fraction of the theoretical Planck blackbody distribution. The maximum of the distribution is temperature dependent according to Wien s law, shifting to shorter wavelengths and increasing in magnitude as the temperature increases. The blackbody emittance distributions for two temperatures are included in Fig. 1. For a temperature a little above the boiling point of water, such as 400 K, the maximum flux is at about 7 pm wavelength, with almost no flux below 3 pm but at 600 K the maximum flux is at about 5 pm with a small amount emitted below 2 pm. 

The spectral selectivity of a solar absorber can be judged by comparing its solar absorptance and thermal emittance with the ideal values. Because of the overlap of the solar and thermal spectra, as seen in Fig. 1, the solar absorptance of the ideal surface is limited to a maximum of about 0.99 and the total emittance (600 K) is limited to a minimum of about 0.01. 

A high stagnation temperature is desirable because it defines the upper limit at which heat may be extracted from a solar absorber. The stagnation temperature increases with increasing solar absorptance and decreasing thermal emittance therefore it is closely related to the ale ratio. 

The conversion efficiency is probably the best single indicator of the potential usefulness of a selective solar absorber. The conversion efficiency is dependent on both the solar absorptance and the thermal emittance. For the ideal surface mentioned before, operating at 600 K, CE = 0.98. Only a few absorbers have been developed that give a CE(600 K) greater than 0.3 without resorting to solar concentration. Many of the known selective surfaces are unable to give a CE(600 K) greater than 0. 

Several methods are used to determine solar absorptance and thermal emittance. Measurement of spectral reflectance is the easiest and most commonly used. This indirect method and three direct methods of measurement (1) are briefly discussed. 

Absorber Source Solar absorp. a(s) Thermal emitt. e(600 K) Ratio a(s)/ e(600 K) Stagnation Conversion efficiency, CE, at 600 K ... 

The thermal emittance (600 K) of the ZrNj film is a very low 0.039. The TiNj and ZrC cNy films show an emittance of about 0.05, which is similar to that of the best silicon film. The other Ti-and Zr-type films all have an emittance less than 0.1, except ZrCg, which has 0.123. It is believed that the emittance of the ZrC film could be reduced by deposition of a silver film between it and the stainless steel substrate, as used for the other Zr absorbers. The other types of absorbers shown in Table 1 have emittances of 0.09 or higher. 

Similarly, most IR detector applications depend on emission directly from the source of interest, so you may hear the term thermal emitter used interchangeably with IR source . Again, the two phrases are strongly related, but not synonymous. 

TABLE 1.1 Thermal Emitter Temperature and Color Correlation... 

DC or Pulsed thermal emitters (blackbody-like emitters) - electrically driven, in TO-8 packages, with internal reflector, and optional windows. 

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