Jean-Louis Desplat

Solar Thermionic Test in a Thermal Receiver

A single cell cylindrical inverted thermionic converter (CIC) was tested at the Solar Thermal Propulsion Test Facility of the NASA Marshall Space Flight Center (MSFC). The inverted design is well suited to heating via solar power. For testing the CIC was installed in a thermal receiver into which the concentrated solar flux was focused, achieving temperatures of ∼1700 K. A high temperature secondary concentrator was used at the entrance of the receiver to reduce re‐radiation losses and to help disperse the solar illumination within the receiver. The molybdenum secondary concentrator is a Winston cone design and reached operating temperatures approaching 1700 K. Ray tracing and thermal modeling of the receiver was performed to evaluate component operating temperatures and to develop a relation between input power and operating temperatures. Inefficiencies in the optical train coupled with a marginal solar irradiance peaking at 830 W/m2 resulted in lower than desired test temperatures. The maximum emitter t...

Cylindrical Inverted Multi‐Cell (CIM) Thermionic Converter for Solar Power and Propulsion Systems

Design and fabrication of a novel four cell cylindrically inverted multi‐cell (CIM) solar thermionic converter for space power applications is in progress. When heated externally, the converter (8 cm diameter and 35 cm long) is capable of producing up to 492 W of electric power. The emitters operate at 2000 K while the collectors operate at 1050 K. Key components include four polycrystalline rhenium (Re) emitters, a cesium (Cs) containment vessel, and a collector trilayer. The emitters and containment vessel are both made from Re tubing. Hot Isostatic press (HIP) processing is used to produce the collector trilayer which is made up of a niobium alloy (Nb‐1%Zr) base tube, plasma sprayed aluminum oxide insulation layer, and a niobium collector tube. After machining the Re emitters are brazed to the Nb collectors. The purpose of this paper is to describe the design and fabrication of the four cell CIM which will be tested at General Atomics (GA).

Tests of a conductively coupled multi-cell thermionic fuel element

Design and fabrication of a novel multi-cell thermionic fuel element (TFE) is in progress. The multi-cell approach helps minimize ohmic losses in the electrodes by reducing the electrode area of each cell. Series connected small cells result in higher output voltage and lower ohmic losses. A critical component is the intercell connector, which must accommodate any thermal expansion mismatch between the emitters and collectors and minimize electrical and thermal losses. Three designs for the intercell connector were evaluated. The multi-cell TFE incorporates one continuous fuel clad that is accessible with a single full-length TFE heater and is therefore testable with an electric heater. Furthermore, a multi-cell TFE system can be scaled to multi-megawatt power levels. With moderate design changes, the multi-cell concept can be incorporated into a solar thermal power system.

Evaluation of oxygen-dispensing collectors for thermionics

Collectors made by evaporating Mo onto Nb substrates in a low partial pressure of oxygen were studied in a demountable, fixed-spacing, planar thermionic converter featuring a common polycrystalline W emitter and an oxygen detector, which can be used to measure the effective oxygen pressure. Two collectors with oxygen contents of 2100 and 550 ppm were investigated. At spacings of 0.5 to 0.55 mm, their performances were similar with emitter bare work functions of 5.55 and 5.45 eV respectively. These values suggest that the effective oxygen pressure in the gap was around 4×10−5 Pa, while the oxygen detector indicated much lower values. Maximum electrode output power densities and calculated maximum electrode efficiencies at an emitter temperature of 1800 K were 9 W/cm2 and 17%.

Testing of a single cell cylindrical inverted converter

A Cylindrical Inverted Converter (CIC), made by Lutch, with the emitter on the outside was tested in a vacuum furnace supplying radiant heat to the emitter outer surface. The collector, coaxial with the emitter, has an integral heat pipe with sodium as the working fluid, which carries the heat dissipated in the collector to a radiating area with a coating of alumina and sub-stoichiometric TiO2. The CIC is a proof-of-principle device which will lead to the development of multi-cell inverted converter assemblies for space solar power systems. The thermionic performance at emitter temperatures of 1800 and 1900 K is presented.

Thermionic Performance of Rhenium Emitters

Polycrystalline and chemical‐vapor‐deposited (CVD) rhenium emitters were investigated in a demountable thermionic converter, against several collectors: polycrystalline Nb, polycrystalline W, polycrystalline Re and a single crystal Nb‐O alloy. For emitter performance comparison, the bare work function is the most important parameter: for polycrystalline Re the measured value was 5.20 eV, and for CVD Re it was 5.24 eV (fairly independent of the polycrystalline collector material). The bare work function of CVD Re increased to 5.50 eV, when tested opposite the oxygen‐dispensing Nb‐O alloy collector. However Re transport from the CVD Re to the oxygen‐dispensing collector occurred, which was unexpected. For the solar‐heated multi‐cell converter application, while CVD Re is the best emitter without oxygen, the difficulties involved in producing such surfaces inside a cylinder ruled against it in favor of polycrystalline Re. Similarly, consideration of ease of fabrication and of potential life‐limiting Re evapo...

Emitter tri-layer technology

The emitter tri-layer technology combines the best aspects of the single cell and multi-cell thermionic fuel element designs. The emitter tri-layer technology places strict requirements on materials especially the insulator. Emitter tri-layers consisting of scandium oxide and single crystal and polycrystalline tungsten were fabricated, machined into test samples, and tested. Scandium oxide has met the requirements of thermal stability and mechanical strength by surviving 20 thermal cycles between 300 and 1900 K (27 K/min heating and cooling rates). Electrical resistance lifetime tests currently under way at 1900 K and a 2.6 volt applied potential show that the electrical resistivity of the scandium oxide in the emitter trilayer, 1.03×102 ohms-cm, is acceptable for a conductively coupled multi-cell thermionic fuel element with a designed current density of 10 A/cm2.

Testing of a niobium-oxygen single crystal collector

A Nb-O single crystal alloy collector, L2, made by Lutch, with an oxygen content of 2.9 atom % was tested opposite a polycrystalline tungsten emitter. The emitter bare work function was raised to 5.40 eV, following oxygen evaporation from the collector and adsorption onto the emitter. This increase in the emitter bare work function was substantially higher that that obtained from another Lutch collector single crystal alloy, L1, which contained 1.2 atom %. The barrier index obtained with L2 was one of the lowest obtained with oxygen-dispensing collectors, 0.24 eV lower than with L1. At a spacing of 0.55 mm and an emitter temperature of 1800 K, the maximum electrode power density was 7.6 W/cm2 and the maximum electrode efficiency was 18%. After about 2000 hours of operation some tungsten condensation was detected on the collector.