Thermionic electron emission

The saturation current observed in thermionic electron emission is related to temperature by the famous Richardson-Fowler law. 

Fi(i. 13. Distribution of thermionic electron emission of a tungsten-monocrystal wire in cesium vapor in the [Oil] zone at A 2,000°, B 900°, C 850°K. Cesium vapor pressure corresponds to room temperature [according to Johnson and Shockley (35)]. 

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... 

As is well known, the thermionic electron emission from the tungsten hot plate in the cesium gas has an excellent nature that the thermionic electron current from the tungsten hot plate becomes large at the lower hot plate temperature than 1000 K, although it depends on the hot plate temperature and cesium gas pressure. This is because the hot plate is covered by cesium thin layer, which reduces the work function of the hot plate to around 2.0 eV at low hot plate temperature. This nature seems to be very attractive from the view point of the development of thermionic energy converter because it enables to the operation of thermionic energy converter at the low emitter temperature. 

The thermionic electron emission current jT is described by the Dushman-Richardson relation [93,95] ... 

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. 

Solid state metal borides are characteristically extremely hard, involatUe, high melting and chemically inert materials which are industrially important with uses as refractory materials and in rocket cones and turbine blades, i.e. components that must withstand extreme stress, shock and high temperatures. The borides LaBg and CeB are excellent thermionic electron emission sources, and single crystals are used as cathode materials in electron microscopes (see Box 13.8). 

Furthermore, in 2013, two novel mechanisms for molecular oxygen activation hy MNPs have been ascertained. The first lies on an indirect photothermal pathway whereby induction of extreme heat on the gold nanoparticle surface through excitation of the plasmon resonance leads to the nanoparticle fragmentation and to an increased thermionic electron emission, responsible for generation. Alternatively, it has also been proved that molecular 0) gen adsorbed onto the metal nanoparticle surface can additionally be activated through electron transfer. 

The work functions of metal surfaces show a marked dependence on the crystal planes that constitute the surface. For polycrystal metal surfaces, the experimentally determined value of the work function is an average of the work functions of all the crystal planes that are present on the surface. It is made up of the contributions from the individual crystal planes. If /a, / and tp are defined as the mean effective work functions for thermionic electron emission, positive surface ionization, and negative surface ionization, respectively, it is generally evident that /+ > /, and / =... 

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... 

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

The beam of ionizing electrons is produced by thermionic emission from a resis-tively heated metal wire ox filament typically made of rhenium or tungsten. The filament reaches up to 2000 °C during operation. Some reduction of the working temperature without loss of electron emission (1-10 mA mm ) can be achieved by use of thoriated iridium or thoriated rhenium filaments. [22] There is a wide variety of filaments available from different manufactures working almost equally well, e.g., the filament can be a straight wire, a ribbon, or a small coil (Fig. 5.9). 

Modinos, A. (1984). Field, Thermionic, and Secondary Electron Emission Spectroscopy. Plenum, New York. 

Taylor and Langmuir (SO) followed the desorption of Cs ions and atoms from a W surface by a thermionic method. At low coverage the rate of desorption of positive ions was measured by the positive ion current. At higher coverages, the rate of desorption of atoms was determined by allowing a calculated fraction of the total number of atoms to impinge on an adjacent incandescent filament where they became ionized then the rate of evaporation of atoms from the original surface was calculated from the increase in the ion current. The electron emission was measured simultaneously so that the rate of evaporation of ions and atoms could be ex-... 

Bosworth and Rideal (67,68), too found with the contact-potential method an increase of the work function by 1.38 volts when nitrogen was adsorbed on tungsten at 90°K. Since the method described in section III,Id, was used, where a second tungsten filament is heated for the emission of thermionic electrons, it is to be expected that in this case, too, N atoms were formed, which reached the cold wire near the hot one and were adsorbed. 

Figure 5. Comparison of experimental and calculated electron emission rate constants for Cg and Cg. Experimental results are shown in solid circles connected by a dashed line. Rates are calculated using microcanonical statistical model for thermionic emission. Solid lines are results when ab initio vibrational frequencies are used. Dashed line (Cg only) is calculation in which the value of the lowest vibrational frequency in Cg was reduced by 30%.

Impurities in the interior of the filament can sometimes diffuse to the surface, and form there a surface film apparently just like those formed by combination with a gas. Thus thorium (i, p. 2280) diffuses to the surface between 2,000 and 2,500° K., forming a film which enormously increases the thermionic emission of electrons. This film grows till it is one thorium atom thick, and then ceases to grow. For further information as to the effect of layers of foreign atoms on electron emission, see Chap. VIII, 4. 

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. 

The activation energies for other electronic processes, such as photoelectric, thermionic and secondary electron emission, may also be used to deduce the relative positions of energy states. The relation between the activation energies for a donor defect as determined from the temperature dependence are given in Table 3. Here the symbols refer to the energies shown in Figure 31. 

This technique is used to study the surface or near-surface characteristics of specimens, and is one of the most versatile and widely used instruments in science. In this technique, a beam of electrons from a thermionic emission type tungsten filament is accelerated to 20-40 KeV, demagnified and reduced in diameter to 2-10nm on point of contact with a sample. The fine beam is scanned across the sample and a detector counts the number of low-energy secondary electrons or the radiation given off from each point on the surface (McHardy and Bimie, 1987 Newbury et al., 1987 Goodhew and Humphreys, 1988). Electron emission from a... 

Fullerenes and their derivatives not only represent the most massive and most complex single particles in interference experiments until recently, they also mark a qualitative step towards the mesoscopic world. In many aspects they resemble rather small solids than simple quantum systems they possess collective many-particle states like plasmons and excitons, and they exhibit thermionic electron, photon and particle emission [Mitzner 1995 Hansen 1998] - which may be regarded as microscopic analogs of glow emission, blackbody radiation and thermal evaporation. Fullerenes contain about two... 

Thermionic emission. The number of electrons which escape from the metal surface increases rapidly with temperature (thermionic emission). In general, the higher the temperature and the lower the work function, the higher is the electron emissivity. The current density can be calculated by the Richardson-Dushman equation (in the absence of an external electrical field), according to i — AT exp(—rp/kT), where A is the Richardson constant (A cm K ), T is the temperature (K), and

work function (eV). For pure tungsten A — 60.2 (A cm K ) [1.91]. The thermionic current (A cm ) can then be calculated as i — 60.2r exp(—52230/T) [1.37]. 

Electron Production Processes. The important electron production processes occur in the gas phase and, in the case of discharges with electrodes, at electrode surfaces. The major surface processes are (a) secondary electron emission on ion impact at the cathode, (b) field emission at sharp points on electrodes, and (c) thermionic emission in the case of arc-type discharges where electrodes become strongly heated. These are the sources of the primary electrons in d.c. and low frequency discharges. 

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