Thermionic effect

Jackson SE, Gunther D (2003) The nature and sources of laser induced isotopic fractionation in laser ablation-multicollector-inductively coupled plasma-mass spectrometry. J Anal At Spectrom 18 205-212 Jiang S-J, Houk RS, Stevens MA (1988) Alleviation of overlap interferences for determination of potassium isotope ratios by Inductively-Coupled Plasma Mass Spectrometry. Anal Chem 60 1217-1220 Lam JWH, Horlick G (1990) A comparison of argon and mixed gas plasmas for inductively coupled plasma-mass spectrometry. Spectrochim Acta Part B 45 1313-1325 Langmuir I, Kingdon KH(1925) Thermionic effects caused by vapours of alkali metals. Phil Trans R Soc A107 61-79... 

Thermionic effect The electrons within the electron fluid have a distribution of velocities very much like that of molecules in a gas. When a metal is heated sufficiently, a fraction of these electrons will acquire sufficient kinetic energy to escape the metal altogether some of the electrons are essentially boiled out of the metal. This thermionic effect, which was first observed by Thomas Edison, was utilized in vacuum tubes which served as the basis of electronics from its beginning around 1910 until semiconductors became dominant in the 1960 s. 

Langmuir I. and Kingdon K. H. 1925. Thermionic effects caused by vapours of alkali metals. PRoySocA 107, 61-79. 

I. Langmuir and K.H. Kingdon, Thermionic Effects Caused by Alkali Vapors in Vacuum Tubes , Science 57, 58 (1923). 

Figure 12.1 shows the electron beam generated on the cathode, typically a tungsten filament (W) or lanthanum hexaboride (LaBs), which is heated by an electric current. By thermionic effect, it creates an electron beam that is accelerated by an... 

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... 

It has been known for many years that strongly heating a metal wire in a vacuum causes emission of electrons from the metal surface. This effect is important for thermionic devices used to control or amplify electrical current, but this aspect of surface emission is not considered here. Rather, the discussion here focuses on the effect of heating a sample substance to a high temperature on a metal wire or ribbon. 

Thermionic Emission - Because of. the nonzero temperature of the cathode, free electrons are continuously bouncing inside. Some of these have sufficient energy to overcome the work function of the material and can be found in the vicinity of the surface. The cathode may be heated to increase this emission. Also to enhance this effect, cathodes are usually made of, or coated with, a low work-function material such as thorium. 

Shottky Emission - This is also a thermionic type of emission except that in this case, the applied electric field effectively decreases the work function of the material, and more electrons can then escape. 

The free-electron gas was first applied to a metal by A. Sommerfeld (1928) and this application is also known as the Sommerfeld model. Although the model does not give results that are in quantitative agreement with experiments, it does predict the qualitative behavior of the electronic contribution to the heat capacity, electrical and thermal conductivity, and thermionic emission. The reason for the success of this model is that the quantum effects due to the antisymmetric character of the electronic wave function are very large and dominate the effects of the Coulombic interactions. 

Nitenpyram and its metabolites. The metabolites of nitenpyram, CPMA and CPMF, are determined by HPLC under the same conditions as for the parent nitenpyram. The retention times of nitenpyram, CPMA, and CPMF are 9.2,7.9 and 5.3 min, respectively. However, these compounds are unstable and need to be derivatized to a more stable compound, CPF, prior to analysis. It is necessary to remove acetone from the extract before derivatization, because a by-product can be formed in the presence of acetone thus impacting the recovery of CPF. Nitenpyram is more effectively determined using HPLC, whereas CPF, as the analyte of nitenpyram and its metabolites, is more effective by gas chromatography/flame thermionic detection (GC/FTD). 

Covers charging due to relative motion or separation of two contacting phases in the absence of external electrostatic fields, radiation, or thermionic emission. The superimposition of such effects may serve to confuse the data in many cases, especially those involving natural phenomena. 

A mixed ion conductor, BaSnO, has also been tested as a contact layer on a Schottky sensor [90]. The BaSnOj/SiC sensor showed a response to oxygen and this was most pronounced at 400°C. The sensor was tested from 200°C to 700°C. Operated at 700°C, the sensor showed a negative resistance peak at a bias of 2V (Figure 2.8). This peak was accounted for by the tunneling or Esaki effect [91]. Up to an operation temperature of 400°C, thermionic emission was proposed to explain its behavior. At higher temperatures, a resistance connected in series with a Schottky diode can model the device [5, 73]. At temperatures of 500-600°C, the BaSn03 shows a mixed behavior of electronic and ion conduction, and the Nernst potential [92] can be added to the model. The complete proposed model is given in (2.9). 

The emission equation is valid for each of the patches on a nonuniform surface, but the total measured current from the various patches depends on the relative magnitudes of the collecting field and patch fields, as well as on the work function. Hence, the effect of some patches may be out of proportion to their area and the average work function of a polycrystalline surface measured thermionically may differ somewhat from the true average work function... 

A still more complete insight into the nature of adsorbed species can be obtained from experiments (I) on thermionic emission, ( ) with the field emission microscope, and (S) with the ion gauge. From some thermionic experiments, particularly with cesium adsorbed on tungsten, it is learned that (a) Cs can be adsorbed as positive ions as well as adatoms (b) as the concentration of adsorbed cesium increases, the ratio of adions to adatoms decreases (c) the forces produced by adions are long-range forces which have appreciable effects over distances of 10 to 20 atom diameters (d) adatoms and adions can migrate over the surface at much lower temperatures than those at which they evaporate from the surface. 

In the case of a metal-semiconductor junction the semiconductor surface is closely coupled to the metal. As a result electrons in the conduction band at the surface see a high density of empty metal states into which they can cross the interface isoenergetically. Similarly, valence band holes see a high density of filled electron states in the metal. As a result, electron transfer is usually treated as direct transfer in a thermionic process and surface states a re-rjs legated to a minor role in surface generation and recombination effects. -1—... 

The recombination current density, Jr, can be treated effectively as a Schottky barrier diode current density. Including both thermionic emission and diffusion charge transport mechanisms (13) Jr can be written as... 

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