John H. Ingold

Thermionic Properties of Some Refractory Metal Carbides

The low‐field thermionic emission properties of vacuum hot‐pressed samples of MoC, Ta0.8Zr0.2C, TiC, HfC, TaC, and ZrC in polycrystalline bulk form have been measured over the temperature range 1300° to 1900°K. The samples were well characterized by metallographic examinations and x‐ray diffraction analyses. A correlation between microstructure and thermionic emission was observed. The MoC sample was found to be the most copious electron emitter, while the ZrC sample, which was carbon deficient, had the smallest electron emission over the temperature range investigated.

Calculation of the Maximum Efficiency of the Thermionic Converter

A theoretical analysis of the efficiency of a thermionic converter is made in terms of the following parameters: Va, the potential difference between the top of the potential barrier in the interelectrode space and the Fermi level of the anode; VL, the potential drop across a load impedance in series with the converter; and Vl, the potential drop in the necessary electrical connection to the cathode. The analysis is carried out by developing an expression for the efficiency of the converter and then maximizing this expression with respect to VL and Vl. This method yields optimum values of load impedance, cathode lead geometry, and cathode work function in terms of Va, cathode temperature, cathode emission constant (usually denoted by A), and effective emissivity of the cathode. A hypothetical example is worked out numerically and the results show that (1) a low value of Va is required for high efficiency, and (2) relatively low values of cathode work function are required for maximum efficiency at ordinar...

Diffusion Cooling of Electrons in Afterglow Plasmas

The results of mass‐identified measurements of the decay of positive atomic ion number densities in low‐pressure rare‐gas afterglow plasmas are reported. At high gas pressure, the loss of atomic ions is due to conversion into molecular ions and by ambipolar diffusion to the wall of the container. At low pressures, the loss of atomic ions is only by diffusion to the wall but at a slower rate than that of ambipolar diffusion; this slow loss rate is interpreted as a manifestation of electron diffusion cooling in heavy rare gases. A semi‐quantitative theoretical explanation of diffusion cooling is offered and shown to be in good agreement with the results of measurements of electron decay rates in Ne and Ar afterglows and of ion decay rates in Ne afterglows. Comparison with ion decay measurements in Ar, Kr, and Xe are also given.

Imprisonment of Resonance Radiation in a Planar Gas Discharge with Reflecting Walls

The imprisonment time of Doppler‐broadened resonance radiation in a planar gas discharge is calculated numerically and compared with the analytic approximation of Walsh. This comparison shows that Walshs results are higher than the numerical results by as much as 17% for no electronic collisional deexcitation, and by as much as 63% for copious electronic collisional deexcitation (at an optical thickness of 10). In addition, the effect of diffusely reflecting walls on the imprisonment time is calculated numerically and compared with the analytic approximation of Weinstein where possible. The numerical results are rigorously valid in the optically thick limit and approximately valid in the optically thin limit.

Electron Flow in Gas Diodes. I. Transition from Inertia‐Limited Flow to Mobility‐Limited Flow

The transition from inertia‐limited flow (vacuum) to mobility‐limited flow (high pressure) in gas‐filled diodes is studied theoretically by taking velocity moments of the Boltzmann equation for the electron‐velocity distribution function. It is shown that the momentum‐transfer equation can be integrated when νc(C), the frequency of elastic collisions between electrons and gas atoms, is independent of the electron speed c, and the hydrostatic‐pressure term is neglected. The resulting current‐voltage (J‐V) curve, which is valid for all gas pressures, reduces to the proper vacuum law (J ∝ V3/2) at extremely low gas pressure and to the proper high‐pressure law (J ∝ V2) at high gas pressure, while it is a mixture of the two laws for intermediate gas pressures. The importance of the ratio νc/νp, where νc is the average value of νc(C) and νp is the electron‐plasma frequency, is emphasized. It is shown that the current is inertia limited for νc/νp 1. It is shown further that m...

DECAY OF RESONANCE RADIATION IN A PLANAR AFTERGLOW WITH REFLECTING WALLS.

The decay time of Doppler‐broadened resonance radiation in a planar afterglow is calculated numerically and compared with the analytic approximation of Holstein. This comparison shows that Holsteins results are higher than the numerical results by as much as 18%, depending on the optical thickness. In addition, the effect of diffusely reflecting walls on the decay time is calculated numerically and compared with the analytic approximation of Weinstein where possible. The numerical results are rigorously valid in the optically thick limit and approximately valid in the optically thin limit.

Nonequilibrium Motion of Electrons and Ions Near Absorbing Boundaries

This paper deals with nonequilibrium transport of electrons and ions near absorbing boundaries where particle densities tend to decrease as the boundary is approached. This decrease affects the behavior of average velocity and average energy of the particles near the boundary. Consequently, transport and rate coefficients cannot be characterized by the local value of electric field. The purpose of this paper is to elucidate these nonequilibrium effects.

Electron Flow in Gas Diodes. II. Mobility‐Limited Flow for Collision Frequency Proportional to Electron Speed

The voltage (V) dependence of the mobility‐limited electron current density (J) in a gas‐filled diode is calculated for low pressure [(m/M)1/2νc/νp]<1 <νc/νp as well as for high pressure [1<(m/M)1/2νc/νp], where m/M is the ratio of electron mass to atom mass, νc is the average frequency of elastic collisions between electrons and gas atoms, and νp is the electron‐plasma frequency. It is assumed that νc(C) is proportional to the electron speed c, which corresponds to the case of an energy‐independent mean free path. It is shown that J should be proportional to V3/2p−1d−3 for low pressure, and to V3/2p−1/2d−6/2 for high pressure, where p is the gas pressure and d is the (planar) diode spacing. Corresponding expressions for cylindrical geometry are derived and compared with experiment.

Thermionic Properties of HfC

The thermionic emission properties of bulk HfC were measured at 1300 to 2100 deg K. The experimental apparatus, in addition to the external instrumentation, consisted of a plane diode with a copper collector and a hafnium carbide emitter. The saturation emission current was measured as a function of temperature with a constant voltage of 95 v applied to the collector, which was spaced about 1 mm from the emitter. An approximate least-squares fit to the experimental poiitts gives a work function of 4 v and a Richardson constant of 40 amp/cm2 deg2. A Richardson plot of the experimental data is given.