Thermionic Emission Properties

The data regarding the thermoelectric properties will also have to be looked upon as tentative, since these properties depend materially on the method of measurement and also on the state and purity of the specimens. The thermionic emission properties are characterized by the more accurate data obtained in the USSR by investigators,principally of the school of Prof. B. M. Tsarev. 

BaBj Good thermionic emission properties Cathodes in electronics [25] 1951... 

The effect on the dissociation of hydrogen and on the emission of electrons was produced at extremely low pressures of oxygen, about lO 8 mm. of mercury, showing that the combination between the metal and the adsorbed layer of gas is extremely firm that the layer is monatomic was shown, as described previously, by the fact that the thermionic emission and hydrogen dissociation recommenced instantly, when the temperature was raised to a value at which the adsorbed film began to be removed this proved that the least removal of the screen of oxygen adsorbed left a clean surface, which could scarcely occur unless the layer were one atom thick if it were thicker, the removal of the screen and return of the properties of the clean metal surface would return in stages, not suddenly. 

The electrical properties and thermionic emission of molybdenite iia C been inx estipfatcd the mineral appears to exist in rtvo states between atmospherie teni]x rature and reel heat, and at temperatures above SIO C. a characteristic emission of positive molybdenum ions occurs. 

The optical properties of rare-earth doped yttrium oxide and other rare-earth oxides have been studied extensively for several years, since the oxides are excellent laser host materials. Laser action of EurYjOs has been observed at 0.6113/xm (Chang, 1963) Nd Y203 has also been studied as a laser crystal (Hoskins and Softer, 1964 Holloway et al., 1%6). Recently, a Nd tYjOs crystal was used as a room-temperature frequency converter laser output was observed at 1.07 /Lim and at 1.31 /i,m when the crystal was pumped by a Kr c.w. laser (Stone and Burrus, 1978). The possibility of an X-ray pumped laser using various rare-earth ions in Y2O3 has also been discussed (Ratinen, 1971). Other applications of Y2O3 such as thermionic emission (Kul varskaya, 1976) and electroluminescence (Tanaka et al., 1976) have also been described. 

The converter enclosure is hermetically sealed so that the atmosphere between the electrodes can be controlled. In the conventional thermionic converter, the interelectrode space is filled with cesium vapor from a liquid reservoir at a pressure of 1 torr. The cesium performs two functions. First, it adsorbs on the electrodes to provide the desired electron emission properties. Second, it provides positive ions to neutralize the electron space charge so that practical current densities can be obtained from the converter. 

These relations provide a smooth transition from the TFE regime to the field emission regime as the temperature is lowered, which hampers the thermionic emission. A unique property here is that the sum of the inverse of the forward and reverse ideality factors adds up to 1 ... 

While field ion microscopy has provided an effective means to visualize surface atoms and adsorbates, field emission is the preferred technique for measurement of the energetic properties of the surface. The effect of an applied field on the rate of electron emission was described by Fowler and Nordheim [65] and is shown schematically in Fig. Vlll 5. In the absence of a field, a barrier corresponding to the thermionic work function, prevents electrons from escaping from the Fermi level. An applied field, reduces this barrier to 4> - F, where the potential V decreases linearly with distance according to V = xF. Quantum-mechanical tunneling is now possible through this finite barrier, and the solufion for an electron in a finite potential box gives... 

Work Function (WF) plays a key role in the physics and chemistry of materials. Phenomena such as the semiconductor field effect, photo- and thermionic electron emission (Allen and Gobelli, 1962), catalysis (Vayenas et al 1996), and the like are dominated by the WF. This fundamental property of electronic materials is defined as the minimum work required to extract an electron from the Fermi level Ep of a conducting phase, through the surface and place it in vacuum just outside the reach of the electrostatic forces of that phase (Trasatti and Parsons, 1986). The reference level for this transfer is thus called the vacuum reference level. Because even a clean surface is a physical discontinuity, a surface dipole t] with its associated electric field always appears at the surface of the condensed phase. Thus, the work of extracting the electron can be conceptually divided between the work required to... 

Thermionic converters are high temperature devices which utilize electron emission and collection with two electrodes at different temperatures to convert heat into electric power directly with no moving parts. Most thermionic converters operate with a plasma of positive ions in the interelectrode space to neutralize space charge and permit electron current flow. Both the plasma characteristics and the surface properties of the electrodes are controlled by the use of cesium vapor in thermionic diodes. 

Mass spectrometry, which is the only technique that can be used to characterize met-cars and related metal-carbide clusters, implies that the detected clusters are ionized. This requirement opens a route to a variety of experimental procedures enabling insight to be gained into physical properties such as ionization energies, electron affinities, structure, and collective electronic properties such as thermionic electron emission and delayed atomic ion emission. 

Apart from transistors, several other solid state devices have been discussed [78], like junctions, photon and electron beam switches and various kinds of sensors. One property of diamond which has stimulated considerable interest in the recent years is the negative electron affinity (NEA) of suitably prepared surfaces [78,80]. The electron affinity, of a material is defined as the difference between the energy of a free electron in vacuum and the bottom of the conduction band Fyac - E. In Fig. 8 the electronic bands of p-doped clean and H-terminated (111) diamond surfaces near the surface are depicted, based on the results of UV-photoemission measurements. For the H-terminated surface, the electron affinity becomes negative once an electron is injected into the conduction band from a suitable contact or by UV excitation, it will easily leave the crystal and be emitted into vacuum. This effect, which is also observed on monohydride terminated (100) surfaces, is not unique to diamond but was also observed in a few other semieonductors with high band gaps [80]. Apart from a scientific interest, the NEA of diamond makes it an attractive eandidate for the replacement of thermionic emitters as electron beam sourees and as a miniature electron emitter for field emission displays. 

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