Function Of Cathode Temperature Research Articles

Heavily phosphorus-doped nano-crystalline diamond electrode for thermionic emission application

Thermionic energy conversion is one of the promising technologies for transforming thermal energy into electrical energy. A stable cathode electrode with high conductivity and low work function is required to realize this application. It is already well known that hydrogen-terminated diamond exhibits negative electron affinity, which is a great advantage of an electron emission electrode. In this study, we focus on phosphorus doping, which gives n-type donors with 0.57eV in single crystal diamond to control the conductivity of diamond, in an attempt to develop a nano-crystalline diamond (NCD) electrode for thermionic emission by heavily phosphorus doping. Phosphorus concentration and structural characterization were performed by secondary ion mass spectroscopy and Raman spectroscopy, respectively. The thermionic emission properties were characterized in vacuum as a function of cathode temperature from 300 to 600°C. The work function of heavily phosphorus-doped NCD electrode was estimated based on the Richardson–Dushman equation, and the effect of heavily phosphorus doping on thermionic emission was briefly discussed.

Thermionic emission characterization of boron‐doped microcrystalline diamond films at elevated temperatures

AbstractA boron‐doped polycrystalline diamond film was deposited on a molybdenum substrate and resistively heated in a high vacuum environment. The thermally excited emission current as a function of cathode temperature was measured. This phenomenon, known as thermionic emission, is characterized by electron emission arising from a surface at elevated temperatures and is described by the Richardson equation. The observed thermionic emission current from the diamond sample followed the Richardson equation from which a work function of 4.43 eV and a Richardson constant of ∼60 A/cm2K2 were observed. This indicates boron‐doped polycrystalline diamond behaves as an interesting thermionic emitter for possible energy conversion and other electron emission applications.

Stabilization of electron emission from nanoneedles with two dimensional graphene sheet structure in a high residual pressure region

A stable field emission (FE) under a high residual pressure (10−5Torr) was obtained by the thermal field operation of a nanoneedle cathode with a two dimensional graphene sheet structure. A high brightness electron emission of the order of 1012Asr−1m−2, as well as stable emission, was achieved. The performance of the stabilized cathode was demonstrated by the construction of a compact FE scanning electron microscope (SEM) system, and clear FE-SEM images were obtained at a residual pressure above 10−5Torr. The emission current fluctuation as a function of cathode temperature was discussed based on the Poisson distribution.

Ion bombardment investigations of impregnated cathodes

Ion bombardment is one of the important factors limiting the performance of impregnated cathodes (=Ba dispenser cathodes) in high end television tubes or in colour monitor tubes. Hence, when designing a new gun with, e.g. higher electron beam current density, it is important also to model the influence of ion bombardment. Therefore, relations between basic parameters as a function of temperature need to be known quantitatively. In this paper, the emission slump of impregnated cathodes has been analyzed in a diode configuration in UHV with a differentially pumped Ar ion gun. The emission degeneration during and regeneration periods after ion bombardment have been investigated as function of cathode temperature, ion current and ion energy. One of the important results is, that the degeneration time coefficient is only weakly dependent on ion energy. The data matrix obtained can be used to improve the ion bombardment model applied in new electron gun design.

Observations of work function changes in field-emitter arrays

The current-voltage ( I–V) characteristics for arrays of microfabricated field emitter tips have been measured as a function of cathode temperature in a range of 30 to 300°C in an turbomolecular pumped vacuum system with a base pressure of <10 -10 Torr. The current at a fixed voltage was found to increase with increasing temperature. The change was reversible provided the temperature was held constant at its new value for about 40 min. This implies that the current increase with temperature is due to a change in work function rather than a change in the emitting area or tip shape. The I–V characteristic at each temperature followed the Fowler-Nordheim field emission equation over five orders of magnitude in current and indicated the occurence of work function changes between 0.5 and 2.0 eV. Such changes could be due to adsorption on the tip of electropositive atoms originating at the anode, which reached temperatures of approximately 670°C due to the electron bombardment at high anode voltages. Conversely, the lower work function could be due to desorption of electronegative atoms (such as oxygen) under the action of the heat and the increased H 2 and CO pressure associated with the relatively high anode temperature. Reversible desorption has been previously observed from the field emitting whiskers associated with high voltage vacuum breakdown between large area metal electrodes.

Theory of the retarding potential method of cathode temperature measurement

Retarding potential measurements can be used to determine the temperature of the cathode in a traveling wave tube. General expressions are derived for the current in a thermionic diode as a function of cathode temperature and applied voltage. An enumeration is made of the conditions under which such expressions can be reduced to a form that enables application of the retarding potential method of temperature measurement. An integral form is given for the factor needed to generalize the Richardson equation to more general geometry, and it is evaluated for a special case. Finally, there is a discussion of the case when, because of surface or other effects, the distribution function of emitted electrons is not given by the Fermi–Dirac distribution.

Cathode Activity Measurement: a Modification of the Dip Test

A method of cathode activity measurement which yields fundamental cathode parameters has been developed. Basically, it is a modified dip test and consists of plotting cathode current vs cathode temperature. A simple geometric analysis of the data plot yields data that can be related from one tube to another and to the cathode work function. Thus, experience gained by this simple and rapid technique may be translated from one tube type to another. This technique can be used to obtain the work function as a function of cathode temperature and a method is suggested for obtaining work function as a function of current density.

Chemical-thermionic auxiliary power systems

An analysis of the problems of matching the performance of a high-temperature plasma-thermionic static-energy conversion device to a heat source consisting of the combustion products of chemical fuels is presented. The thermodynamic relationships governing the efficiency and power densities of the thermionic converter as a function of cathode temperature and work function are established and the limitations imposed by positive ion space charge neutralization are pointed out. The temperature and heat flux limits set by the heat source are compared to the converter requirements and various matching problems are discussed.

Characteristics of the Glow to Arc Transition in Mercury Vapor

The probability of the transition from glow discharge to arc discharge at graphite electrodes has been investigated under carefully controlled cathode conditions. The transition probability was found to increase in an approximately linear fashion with an increase in cathode current density and glow voltage. The curve of transition probability as a function of cathode temperature was found to have a distinct maximum at a temperature of about 400° or 450°C. The probability of the transition occurring at temperatures 50° lower or higher was not more than about one-fourth the maximum. It was also found that the frequency with which the glow to arc transitions occurred depended greatly on the previous history of the discharge and the amount of high voltage positive ion bombardment experienced by the cathode under test. A possible explanation of these results has been based on the hypothesis that it is never possible to de-gas completely the cathode electrode even after prolonged operation in a high voltage discharge. It has been shown that the nonuniform emission of this gas from the cathode in the form of bursts from minute patches can lead to a greatly increased probability for transition from glow to arc.