Electron thermal emission

Application of an electric field between two metal electrodes causes a few ions and electrons to be desorbed and is surface or thermal emission (see Chapter 7 for more information on thermal ionization). Unless the electrodes are heated strongly, the number of electrons emitted is very small, but, even at normal temperatures, this emission does add to the small number of electrons caused by cosmic radiation and is continuous. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Fig. 10. DLTS spectrum for a Schottky-barrier diode on n-type ( 7 x 1015 P/cm3) silicon after hydrogenation (150°C, 50 min). The emission rate window e0 corresponds to delay times of 0.5 and 2.5 ms. Each peak is labeled with the measured activation energy for thermal emission of electrons (Johnson et al., 1987a).

Fig. 8-28. Cathodic polarization curves for several redox reactions of hydrated redox particles at an n-type semiconductor electrode of zinc oxide in aqueous solutions (1) = 1x10- MCe at pH 1.5 (2) = 1x10 M Ag(NH3) atpH12 (3) = 1x10- M Fe(CN)6 at pH 3.8 (4)= 1x10- M Mn04- at pH 4.5 IE = thermal emission of electrons as a function of the potential barrier E-Et, of the space charge layer. [From Memming, 1987.]...

An additional delay between excitation and emission is often introduced by metastable electron states known as electron traps, which are filled during excitation. Once an electron has become trapped in such a state it requires certain energy to release it but this can be provided thermally. Emission, which follows the emptying of such traps at a fixed temperature, is known as phosphorescence. 

A carrier thermally released from the trap into the transport band may be either retrapped by the same species of traps or a different one and, under the influence of an electric field, may contribute to an externally measurable current. It may either be swept out of the region being probed or recombined with a recombination center. Some of the electrons may even overcome the work function barrier and leave the solid. The traffic of these carriers from traps to the recombination centers or out of the material can be monitored at various stages, and thus, information on the thermal emission rates can be obtained indirectly. 

During the TSR process, the concentration of holes and electrons is determined by the balance between thermal emission and recapture by traps and capture by recombination centers, hi principle, integration of corresponding equations yields ric(t,T) and p t,T) for both isothermal current transients (ICTs) or during irreversible thermal scans. Obviously, the trapping parameters hsted together with the capture rates of carriers in recombination centers determine these concentrations. Measurement of the current density J = exp(/in c + yUpP) will provide trap-spectroscopic information. The experimental techniques employed in an attempt to perform trap level spectroscopy on this basis are known as Isothermal Current Transients (ICTs) [6], TSC [7]. 

Clyne, Thrush, and Wayne107 reexamined the chemiluminescence from the nitric oxide-ozone reaction and found its spectrum to be similar to that of the thermal emission of N02 at 1200°K. They concluded that the spectra represented transitions from similar low-lying vibration levels of the same excited electronic state of NOa to the ground state. By measuring the decay in chemiluminescence down a flow tube, they obtained the value of the rate constant between 216 and 322°K. The partial pressures of ozone and nitric oxide were 5 x 10-3 and 2 x 10 2 torr, respectively, in an argon carrier at a total pressure of 2 torr. In the presence of excess nitric oxide, they assumed the logarithmic disappearance of ozone proportional to [NO], so that... 

Several galaxy clusters show also an emission of extreme UV (Lieu et al. 1996, Durret et al. 2002) and soft X-ray (Bonamente et al. 2002, Kaastra et al. 2002) radiation in excess w.r.t. the thermal bremsstrahlung emission. This EUV emission excess may be consistent with both ICS of CMB photons off a non-thermal electron population (e.g., Lieu et al. 1999, Bowyer 2000) with Ee = 608.5 MeV (hv/keV)1/2 149 MeV for hv 60 eV, and with thermal emission from a warm gas at ksTe V 1 keV (Bonamente et al. 2002). In the case of Coma, the simple extrapolation of the ICS spectrum which fits the HXR excess down to energies 0.25 keV does not fit the EUV excess measured in Coma because it is too steep and yields a too high flux compared to the measured flux by the EUV satellite in the 0.065 — 0.245 keV band (Ensslin Biermann 1998). Thus, under the assumption that the HXR and the EUV emission of Coma is produced by ICS of CMB photons, the minimal requirement is that a break in the electron spectrum should be present in the range 0.3 — 2.8 GeV in order to avoid an excessive EUV contribution by the ICS emission and to be consistent with the radio halo spectrum. 

Nonlinear Thomson scattering can only be observed for the highest laser intensities, as shown in Fig. 11.7. As expected from the theory, when ao < 1, the nonlinear Thomson scattering vanishes and the collisional radiative processes from the thermal plasma prevail (Bremsstrahlung and radiative recombination). For ao > 1, these latter processes are still effective, as shown by the quadratic dependency on the electronic density of the plasma observed for 9 = 40°. However, this isotropic thermal emission remains less intense than the collimated nonlinear Thomson scattering emission and becomes detectable only at a large angle of observation (> 40°). 

Fig. 5. (a) Bulk electronic concentration at the metal—oxide interface and electron-hole concentration at the oxide—oxygen interface associated with equilibrium interfacial reactions, (b) Electronic energy-level diagram illustrating the dielectric (or semiconducting) nature of the oxide, with the possibility of electron transport (e.g. by tunneling or thermal emission) from the metal to fill O levels at the oxide—oxygen interface to create a potential difference, VM, across the oxide. 

In all cases of electron transport, whether it be hopping, thermal emission, or quantum tunneling, the effect of the electric field in the oxide film is extremely important. In fact, the electric field effect on ion motion is the primary reason the electronic species must be considered at all in most real metal oxidation reactions. This can be understood better when we discuss the coupled-currents approach [10,11] in Sect. 1.15. 

The same energy, then called work function, enters the formula for the thermal emission of metals such as happens in heated filaments (direct heated kathodes) of radio valves (thermionic emission). The thermal energy of the electrons is still very small but it has to be taken into account for this phenomenon, since only those rare electrons with a kinetic energy equal to V0 — Wi can escape. In Table 26 we have used the experimental values of this quantity to calculate Vo, which is very difficult to determine. 

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