An evaluation of the high temperature electron and photon emission behavior of several potential thermionic emitter materials

A material's effective work function is a critical parameter in the technology of thermionic energy conversion. A series of experiments was conducted to determine the effective work function of Mo-0.9Hf-0.06C, Mo-17W-3.6Re-0.9Hf-0.06C, Mo-3.8Re-0.9Hf-0.6C, and Mo-17W-0.9Hf-0.06C (all percentages given in weight percent) by the thermionic method. Temperature ramps were conducted in 100 K increments from 1600 to 2500 K. Additional experiments were conducted to determine the effective work function, as a function of temperature, of several refractory metal composites. The composite material's emission surfaces were mandrel side Chemically Vapor Deposited (CVD) tungsten, mandrel side CVD rhenium, freely CVD tungsten, and Vacuum Plasma Sprayed (VPS) tungsten. Each sample was tested by applying temperature ramps from 1573 to 1973 K in 50 K increments. Additionally, the effective work functions of the composite materials were measured at a temperature of 1950 K. Sample temperatures were determined by focusing a micro-optical disappearing filament pyrometer on a hohlraum (depth-to-diameter ration {dollar}ge{dollar}10) drilled radially into each sample. The total system pressure, in addition to the partial pressures of hydrogen, helium, water vapor, nitrogen/carbon monoxide, diatomic and monatomic oxygen, argon, and carbon dioxide, was monitored with a Residual Gas Analyzer (RGA) and recorded as a function of sample temperature and time. The effect of residual gas on the effective work function of each molybdenum alloy was established by introducing argon, carbon dioxide, helium, nitrogen, and oxygen through a precision leak valve into the Vacuum Emission Vehicle (VEV). An important material parameter required in radiant heat transfer calculations, the spectral normal emissivity, was determined as a function of surface texture and lay, temperature, time, residual gas pressure, and heat treatment. A detailed and comprehensive statistical and uncertainty analysis of the experimental apparatus and resulting effective work function and normal spectral emissivity data was presented. An attempt was made to correlate all data to conventional theory. Additionally, theoretical models relating the observed work function and emissivity data to the segregation, diffusion, and sublimation behaviors of the dilute solution constituents were presented where possible. Electron Probe Microanalysis (EPMA), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Optical Metallography (OM) were implemented to systematically and thoroughly characterize each of the alloys, both pre- and post test.