Thermionic emission process

When solid particles are exposed to a high-temperature environment, typically for T > 1,000 K, charging by thermal electrification becomes important. The electrons inside the solid can acquire the energy from the high-temperature field and be freed by overcoming the energy barrier or the work function. By losing electrons in such a thermionic emission process, the particles are thermally electrified. 

Similar results have been obtained for the reduction of protons at n-GaAs electrodes. The thermionic emission process again limits the current (Uhlendorf et al. 

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

The boundary conditions are given by specifying the panicle currents at the boundaries. Holes can be injected into the polymer by thermionic emission and tunneling [32]. Holes in the polymer at the contact interface can also fall bach into the metal, a process usually called interlace recombination. Interface recombination is the time-reversed process of thermionic emission. At thermodynamic equilibrium the rates for these two time-reversed processes are the same by detailed balance. Thus, there are three current components to the hole current at a contact thermionic emission, a backflowing interface recombination current that is the time-reversed process of thermionic emission, and tunneling. Specifically, lake the contact at Jt=0 as the hole injecting contact and consider the hole current density at this contact. 

In an electromagnetic tube, electrons are produced by a hot filament. Electrons are emitted from the surface of the filament in a process known as thermionic emission. 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

In practice, unless

high temperatures. Thus, when the work function is increased by an adsorption process, the electronegative film may be partly evaporated before the requisite temperature for thermionic emission has been reached. An exceptional case is the adsorption of O2 on W. However, the thermionic method has proved very useful for stud5ring the electropositive films produced by alkali metals (39). Cs, for example, reduces the work function to such an extent that thermionic measurements may be made at temperatures as low as 150°. 

In the absence of an external field, electrons in the metal are confronted by a semi-infinite potential barrier (upper solid line in Fig. la), so that escape is possible only over the barrier. The process of thermionic emission consists of boiling electrons out of the Fermi sea with kinetic energy > x + M- The presence of a field F volts/cm. at and near the surface modifies the barrier as shown. It follows from elementary electrostatics that the potential V will not be noticed by electrons sufficiently far in the interior of the metal. However, electrons approaching the surface are now confronted by a finite potential barrier, so that tunneling can occur for sufficiently low and thin barriers. 

One possibility is to view the process as that of thermionic emission. Now thermionic emission is the oldest version of transition-state theory known to me. What one assumes is that the system is in thermal equilibrium and the rate is given by the rate at which thermal electrons will cross a (hypothetical) surface surrounding the cluster, where the barrier height is the work function. In other words, the theory takes the rate of crossing of the transition state to be rate determining. 

THERMIONIC CONVERSION. The process whereby electrons released by thermionic emission are collected and utilized as electric current. The simplest example of this is provided by a vacuum tube, in which the electrons released from a heated anode are collected at the cathode or plate. Used as a method of producing electrical power for spacecraft. 

In Chap. XX, Sec. 3, we spoke about the detachment of electrons from atoms, and in Sec. 4 of that chapter we took up the resulting chemical equilibrium, similar to chemical equilibrium in gases. But electrons can be detached not only from atoms but from matter in bulk, and particularly from metals. If the detachment is produced by heat, we have thermionic emission, a process very similar to the vaporization of a solid to form a gas. The equilibrium concerned is very similar to the equilibrium in problems of vapor pressure, and the equilibrium relations can be used, along with a direct calculation of the rate of condensation, to find the rate of thermionic emission. In connection with the equilibrium of a metal and its electron gas, we can find relations between the electrical potentials near two metals in an electron gas and derive information about the so-called Volta difference of potential, or contact potential difference, between the metals. We begin by a kinetic discussion of the collisions of electrons with metallic surfaces. 

It is evident from Eq. (94) that the maximum photovoltage depends critically on the exchange current Jo- In the case of pn-junctions, jo is determined by the injection and recombination (minority carrier device). Whereas in Schottky-type of cells jo can be derived from the thermionic emission model (majority carrier device). The analysis of solid state systems has shown that jo is always smaller for minority carrier devices [20,21]. Using semiconductor-liquid junctions, both types of cells can be realized. If in both processes, oxidation and reduction, minority carrier devices are involved, then jo is given by Eq. (37a), similarly as... 

Furthermore, in the time interval surrounding the rather complex nucleation from the gas-phase products to solid matter, processes such as thermionic emission contribute to the ion concentration in such a way that the recombination of chemi-ions is not noticeable in the gross positive charge structure. The mechanism discussed above is, of course, only speculative, and it is based on ion-molecule reactions suggested by others (1,12). A more detailed picture of the particular ions involved must await mass spectrometric analysis. 

Because the spacing between pores is always less than the width of the depletion layer and PS has a very high resistivity, Beale et al. proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. This intensification of field is attributed to the small radius of curvature at the pore tips. For lowly doped p-Si the charge transfer is by thermionic emission and the small radius of curvature reduces the height of the Schottky barrier and thus increases the current density at the pore tips. For heavily doped materials the current flow inside the semiconductor is by a tunneling process and depends on the width of the depletion layer. In this case the small radius of curvature results in a decrease of the width of the depletion layer and increases the current density at pore tips. The initiation was considered to be associated with the surface inhomogeneities, which provide the initial localized high current density at small surface depressions. 

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