Thermionic Effect Research Articles

Surface-barrier analysis for niobium and tantalum and tantalum-on-niobium from periodic deviations in the thermionic Schottky effect

The Richardson emission constants and periodic deviations in the Schottky effect have been used to investigate the surface barrier for thermionic electron emission from polycrystalline tantalum and niobium and thin films of tantalum-on-niobium. The data have been fit to the Miller and Good theory and a Herring and Nichols complex reflection coefficient, μ, for the surface potential has been determined. The following conclusions can be drawn: The one-dimensional image nature of the barrier in the region of the barrier maximum has been confirmed. The tantalum and niobium experimental amplitudes increased faster than predicted by the theory. This would indicate: either a field-dependent error exists in the theoretical amplitude; or the surface reflection process should be described by an energy-dependent reflection coefficient. The average μ = 0.35 exp(0.63 i) for niobium and μ = 0.36 exp(0.75 i) for tantalum indicate the Sommerfield surface bandwidth model proposed to fit the periodic deviations from molybdenum and rhenium is inadequate for tantalum and niobium. The controlled deposition of tantalum-on-niobium lowered the work function of niobium, and the experimental results confirm work on the molybdenum-on-rhenium surface alloy system that : (a) Richardson emission constants are strongly dependent on the micropatch distribution of the emitter surface while (b) the periodic deviations parameters appear to be independent of it.

Response of the Flame lonization Detector to Carbon Disulphide

The response of the flame ionization detector to CS2 was examined under varying conditions of flow rates of fuel and carrier gases and compared with the responses for CH4, C2H6 and CCI4. The low value of the response for CS2, about 1% of that of CH4, is attributed to preoxidation of CS2 in the flame to substances not contributing to the ion forming process. Carbon disulphide was found to behave like organic compounds with respect to the hydrogen content of the flame, and a similar mechanism involving the formation of CH and its oxidation to CHO+ is proposed for the formation of ions from CS2. Under conditions of flow rates causing the burner jet to become near red hot a thermionic emission effect gives rise to positive response of the same order. The varying reports on the response of CS2 are discussed.

Surface-Barrier Analysis for Rhenium from Periodic Deviations in the Thermionic Schottky Effect

A precise experimental determination of the Herring-Nichols surface-reflection coefficient $\ensuremath{\mu}$ has been made from periodic deviations in the thermionic Schottky effect for rhenium. The value of $\ensuremath{\mu}=0.29\mathrm{exp}(i0.42)$ is in good agreement with a Sommerfeld box model in which the valence electrons of the atoms near the metallic surface are assumed to be free.

Electron Emission in Moderate Accelerating Fields

Investigations of Schottky emission from metals, including some recent work, are summarized and discussed. The following are some of the conclusions: The surface barrier outside a uniform metal surface can be represented accurately by a mirror-image form for distances greater than about 20 A from the surface. Patch effects can be correlated to some extent with known surface properties. The dependence of photoelectric emission on applied field is in agreement with the Fowler theory adjusted for a Schottky-type field-dependent threshold. Periodic deviations from the thermionic Schottky effect are in full agreement with the mirror-image model for the extra-surface barrier, and can be used in determining the barrier form in the immediate vicinity of the emitter surface.

The emission of electrons under the influence of chemical action Part V—The theory of the chemical electron emission and its application to certain reactions involving haloids

The chemical electron emission studied is that of the electrons emitted from the liquid alloy of sodium and potassium, K 2 Na, when it is acted an by different chemically active gases, such as the halogens, at very low pressures of the order of 10 -5 mm of Hg. The electron emission is looked upon as an immediate result of a bombardment, by free metallic electrons, of the unstable (excited) chemical bonds formed on the metal surface during collisions of the gas molecules with the metal. When an electron collides with an excited bond, the latter reverts spontaneously through an electronic rearrangement to the normal polar state and the available energy of the electronic rearrangement is simultaneously transferred to the electron which is thus enabled to escape from the metal. The fundamental physical problems underlying the observed phenomena, on this view, relate to (1) collisions of the second kind between free electrons and the electronically excited chemical bonds, and (2) the escape of electrons from the metal surface. Problem (2) is well known is connection with the thermionic and photo-electric effects. Problem (1) is treated by introducing, a priori , a mathematical expression, as general as possible, for the electron de-excitation, or excitation, function which represents the chance of a successful collision. An application of the theory to the experiments has enabled us to determine accurately the values of the constants included in the electron de-excitation, or excitation, function and also the value of an important constant—the total potential barrier of the alkali metal.

Experimental Atomic Physics

THIS introduction to atomic physics is based upon a course given at Princeton University, the purpose of the course having been to present the subject from a predominantly experimental point of view. Amongst the material included may be found radiation, atomicity of matter and electricity, thermionic and photoelectric effects, line spectra, atomic energy states, X-rays and radioactivity. Sufficient theoretical discussion is given to render the development systematic, but stress is laid on the experimental side and many experiments are described that are suitable for performance by students. Such experiments are by no means easy to arrange and this part of the book should make it of service to teachers who are planning a laboratory course on modern physics.

The polarity factor in the kindling of electric impulse sparkover based upon lichtenberg figures

THE results of a photographic study of successive steps in the kindling process of impulse-flashover in unsymmetrical dielectric fields are presented in this paper. In order to keep the experimental conditions as simple as possible, the investigation was limited to the initial or kindling stage of impulse-sparkover and the energy content of the impressed potential impulse was kept very small to minimize, as much as possible, the thermionic effects.

Thermionic Phenomena Caused by Vapors of Rubidium and Potassium

Thermionic effects similar to those studied by Langmuir and Kingdon for caesium have been found for rubidium and potassium. Because of the difference in the electron affinity of tungsten (4.53 volts) and that of an atom of rubidium or potassium (4.16 and 4.32 volts respectively) positive ions are formed which at filament temperatures above 1000\ifmmode^\circ\else\textdegree\fi{} are drawn to the cylindrical collector from the coaxial filament. Below this temperature the ions are adsorbed on the surface of the filament and decrease its work function raising the electron emission. The degree of thermal ionization at various filament temperatures is found to be within the experimental error of that determined from Saha's equation. At low temperatures the filament is partially covered by adsorbed ions and the electron emission increases exponentially with the reciprocal of the temperature. A transition region is reached as the average life of an atom on the surface becomes shorter and then the emission decreases logarithmically with the reciprocal of the temperature. The curves and values are given. The positive ion emission increases logarithmically with the reciprocal of the temperature until a certain fraction of the atoms striking the filament is ionized. Further increase in temperature causes every atom to be ionized. The coincidence of experimentally determined points with the theoretical space charge curve shows each ion to have the mass of a single atom in the range of pressures studied.Vapor pressure of rubidium and potassium.---The vapor pressure of rubidium and potassium is calculated from the maximum ion current; for rubidium $logp=10.55\ensuremath{-}\frac{4132}{T}$ and for potassium $logp=11.83\ensuremath{-}\frac{4964}{T}$; here $p$ is in bars.

Thermionic Effects Caused by Alminium and Magnesium

Thermionic Effects Caused by Alminium and Magnesium

Thermionic effects caused by vapours of alkali metals

Early in 1923 it was shown that a tungsten filament heated to 1200° K or more in saturated cæsium vapour converts all cæsium atoms which strike it into cæsium ions. Thus when the filament is surrounded by a negatively charged cylinder a positive ion current flows from the filament, which is independent of the filament temperature (above 1200° K) and independent of the applied potential, if this is sufficient to overcome the space charge effect of the positive ions. At lower voltages the currents follow the 3/2 power law, and the currents are smaller than the corresponding electron currents obtainable from the same filament in the ratio of the square roots of the masses of the electrons and cæsium ions. The reason that the cæsium atoms lose their valence electrons so readily upon contact with the filament, is merely that the electron affinity of tungsten (Richardson work function) is 4·53 volts, while the electron affinity of a cæsium atom (ionising potential) is only 3·88 volts. Experiments showed in fact that if the work function for the filament is lowered to 2·69, by allowing a monatomic layer of thorium atoms to accumulate on the surface (by diffusion from the interior of a thoriated tungsten filament), the positive ion emission becomes negligible.

Societies and Academies

Societies and Academies

Thermionic Effects Caused by Alkali Vapors in Vacuum Tubes

Thermionic Effects Caused by Alkali Vapors in Vacuum Tubes

On the conductivity of salt vapours

It has been shown by Bettie, Garrett, and Garrett and Willows, that when certain salts are heated up to 300-400ºC. one can observe the discharge of positive and negative electricity. This phenomenon is observed more particularly in the case of the halogen salts of cadimum, zinc and ammonium. The researches of Schmidt and Sheard have shown that the discharge is due to two factors, (1) emission of the charges from the surface of the heated salt, and (2) to the conductivity of the salt vapours themselves. The thermionic effect of the above-mentioned salts was studied in detail by Prof. O. W. Richardson and his pupils, while some investigations of the electrical properties of the salt vapours were made by Schmidt in the papers referred to above. The object of the present research was to the current passing in the salt vapours. The Thermionic Effect of the Salt and the Conductivity of the Salt Vapours. Some experiments were carried out in order to test whether the conductivity of vapours is due to charges which have diffused to other parts of the apparatus from the surface of the heated salt. For this purpose the vapours were investigated after passing through a cylindrical condenser in which the electric force was strong enough to sweep all charged particles to the electrodes. The apparatus used consisted of a small bulb A (fig. 1), in which the cadmium iodide was placed. The vapour at first passed through the plug of glass wool B to stop the large ions which might be produced during the heating of salt, then entered the condenser K 1 , where the smaller ions could be removed by the electric field. The outer coating of the condenser K 1 was obtained by platinising the inner surface of gases tube with a "liquid platinum" (from Deutsche Gold und Silber Scheide Anstalt) and was connected to earth. As the inner coating of the condenser a glass rod covered with the same substance was used, and was connected by a a platinum wire the source of electromotive force.

LXXIV. The electron theory of thermoelectric and thermionic effects

(1912). LXXIV. The electron theory of thermoelectric and thermionic effects. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science: Vol. 24, No. 143, pp. 737-744.