Barrier index

Calculated Power and Efficiency. The simplified analytical models of thermionic characteristics have been used to project the converter efficiency and power density with the barrier index as a parameter. These projections are shown in Figures 8 and 9 as functions of the emitter temperature. The dashed lines in these two figures are for a constant current density of 10 A/cm. If the current density is adjusted to maximize the efficiency at each temperature, the calculated performance is represented by the solid lines. Typical present generation themionlc converters operate with Vg near 2.0. Ignited mode converters in laboratory experiments have demonstrated practical operation with 1.85 < Vg < 1.90. Other laboratory devices with auxiliary sources of ions and/or special electrode surfaces have achieved Vj < 1.5, but usually not under practical operating conditions. 

Parametric studies have shown that, in order to achieve high efficiencies and power densities at realistic spacings and emitter temperatures, it is necessary to have a high emitter bare work function 5>gQ at or above 5.2eV and a barrier index Vg at or below 2 eV. 

A thermionic converter with a plane parallel type of a polycrystalline W emitter and an AgO X collector, an interelectrode spacing at room temperature 0.1mm, was set up and the power generation experiments were conducted. The maximum power, 3.9W/cm, 0.6V, 6.5A/cm, was obtained under the unignited mode operation at T e =1583K. The barrier index was V b =1.5V at T e =1578K. Based on the experimental results, a new type of a FGM collector was proposed for a micro-gap thermionic converter. 

Barrier index Figure-of-merit parameter for characterizing the performance of a thermionic converter that is given by the sum of the arc drop and the collector work function, thus accounting for the plasma plus the collection losses. The lower this parameter, the higher is the converter performance. 

The barrier index, or Fb, is a eonvenieiit parameter for characterizing thermionic converter development, comparing experimental data, and evaluating converter concepts. It is an inverse figure of merit because the lower the Fb, the higher is the converter performance. The barrier index is defined as... 

The barrier index can be defined operationally. For any given emitter temperature and output current, it is possible to adjust cesium pressure, spacing, and collector temperature to maximize the power output. The spacing envelope of the optimized performance curves is shown in Fig. 6 for a converter with an emitter temperature of 1800 K. This envelope is shifted by a constant poterrtial difference from the Boltzmann line, which represents the ideal cmrent-voltage characteristic. This potential difference is defined as the barrier index. In Fig. 6, the barrier irrdex of 2.1 eV represents good performance for a thermiorric converter with bare metal electrodes, corresponding to Fd 0.5 eV and c — 1-6 eV. The equation for the Boltzmarm line is... 

FIGURE 6 Spacing envelope of the optimized perlbimance curves for a converter with a barrier index of 2.1 eV. Emitter, (110)W collector, niobium 7e = 1800 K Tq and Tr, optimum. 

Improvements in the barrier index can be translated into either higher efficiency at a given emitter temperature or the same efficiency at a lower temperature. 

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