Plasma sprayed tungsten as emitter material

The electron emission of etched and unetched plasma sprayed tungsten was measured in the unignited ion rich mode. Work functions were derived from the measured saturation current densities. In the ignited mode, I-V characteristics were measured. Using an unetched and anetched plasma sprayed tungsten emitter, barrier indexes of 2.35 and 2.05 eV respectively were found. At 1400°C, in the case of an unetched plasma sprayed emitter, a power density of 1.5 w/cm2 was found while for an etched plasma sprayed emitter a power density of 4.5 w/cm2 was measured

A thermionic energy converter with an electrolytically etched tungsten emitter

The bare work functions of etched and unetched plasma-sprayed tungsten were found to be 4.8 and 4.5 eV, respectively. The electron emission of plasma-sprayed tungsten, both etched and unetched, was measured over a wide range of temperatures in a cesium atmosphere. Work functions were derived from the saturation current densities. In the ignited mode, current-voltage (I-V) characteristics were measured. The influence of the emitter, collector, and cesium reservoir temperatures on the I-V characteristics was investigated. Barrier indexes of 2.06 and 2.30 eV were found for etched and unetched tungsten emitters, respectively. At an emitter temperature of 1400¿°C, in the case of an unetched tungsten emitter a power density of 1.5 W/cm2 was found, while for an etched tungsten emitter it was 4.5 W/cm2. This increased power density could be attributed to a lower collector work function. The lower cesiated collector work function resulted from the evaporation of oxygen, as WO3, from the etched tungsten emitter

On the effective bare work function of bcc thermionic electrode materials

An equation is derived for the effective work function of a polycrystalline metal with a fiber texture. This equation contains two parameters: the temperature and the maximal tilt angle, i.e. the maximal deviation from the fiber axis. A linear relationship is assumed between the work function of a uniform lattice plane and the angle of a low index plane with respect to the uniform lattice plane. The proportionality constant D in the [100] zone is evaluated from experimental data for tungsten: D=0.035 * (eV/degree). It is expected that D has the same value in other bcc metals. For a given maximal tilt angle, a higher temperature results in a higher effective work function. A reasonable agreement is found for the calculated effective work functions of tungsten with 110 fiber textures of various sharpness and the experimentally determined work functions from the literature. Furthermore, the effective work function of texture-free polycrystalline tungsten is calculated. The agreement with the experimentally determined value reported in the literature is excellent

WF6-CVD tungsten film as an emitter for a thermionic energy converter I. Production, texture and morphology of WF6-CVD tungsten films

The as-deposited WF6-CVD tungsten film consists of columnar grains about 3 µm in diameter and about 30 µm in length. The film has a 100 fiber texture with a clear tendency to the formation of a pseudo-mono-crystal. The overwhelming part of the grains have their 100 axes within 27° of the surface normal. The as-deposited film surface consists of {111} crystal planes. Two hours annealing at 2273 K resulted in grain growth leading to a grain diameter of more than 70 µm. Inspection of etch pits revealed that 62% of the surface area consists of grains which have their 100 axis within 16° of the surface normal

Electrochemical behavior of Ti/Al2O3 interfaces produced by diffusion bonding

In the field of biomedical applications a special interest exists regarding the study of the physicochemical and mechanical behaviour of materials, with special focus on the electrochemical degradation of metal/ceramic interfaces. In fact, etal/ceramic interfaces may be present in several biomedical devices, ranging from external or implantable sensors, to dental implants. Diffusion bonding represents an important technique since, in opposition to other production technologies, such as active metal brazing, avoid the possible liberation of certain chemical components harmful to health. The aim of this work is to study the electrochemical degradation of the interface formed between commercially pure Ti and Al2O3 produced by diffusion bonding, in contact with a physiological solution. The present approach included the evaluation of the contribution of individual and pairs of interfacial layers on the global degradation processes. For this propose d.c. electrochemical techniques were used to monitor the open-circuit potential, and to perform potentiodynamic polarization and galvanic corrosion evaluation. Also, electrochemical impedance spectroscopy was used as a complementary technique of the corrosion behaviour of the interface. Chemical composition and morphology of samples and corrosion products were evaluated by SEM and EDS analysis. According to experimental results, two principal reaction layers were formed in the interface: TiAl and Ti3Al. The TiAl layer appears to be the responsible for the strong increase in corrosion rate of the interface.Fundação para a Ciência e Tecnologia -POCTI/CTM/33384/2000; SFRH/BPD/ 5518/2001

Phase equilibria in the cadmium-gallium-mercury system

The critical temperature of a cadmium—gallium mixture is increased from 295 °C to 328 °C by the addition of mercury. This maximum critical temperature is attained at a mole fraction mercury XHg = 0.2 and a mole fraction gallium xGa = 0.5. Isothermal sections through the iternary phase diagram between 295 and 328 °C contain an isolated miscibility gap with two critical points. The ternary liquid is described by an Ising model. A hierarchy of three succes sively better mathematical approximations for the entropy function: point, pair and tetrahedron approximation, is briefly delineated. Equations are given to express the composition and the temperature of the ternary critical point in the three quasi-chemical interaction coefficients. Comparing the calculated miscibility gap with the experimentally determined one shows that the Ising model predicts too high iternary critical temperatures. Taking into account 3- and 4-particle interaction, the binary cadmium-gallium binodale is described in a better way. However, the calculated ternary maximal critical temperature is still too high

Phase equilibria in the gallium-mercury-indium system

The critical temperature of a gallium mercury mixture is increased by the addition of indium from 204°c to 211 °C. This maximal critical temperature is attained at the mole fraction indium x1, = 0.1. Six four-phase equilibria have been found. The ternary liquid alloy is described by an Ising model. A hierarchy of three successively better mathematical approximations forthe entropy function: point, pair and tetrahedron approximation, is briefly delineated, comparing the calculated conodes with the experimentally determined ones it is found that the agreement between the ternary miscibility gap calculated in the tetrahedron approximation and the experimentally determined miscibility gap is quantitative

Interfaces in composites of alumina in a molybdenum matrix

Composites of alumina in a molybdenum matrix are candidate electrode materials for thermionic energy converters. The composites are produced by sintering a mixture of alumina and molybdenum powders in vacuum. The polished composite surface was heated for 2000 h at 1400 °C in a caesium atmosphere (1 mbar). After heating, no alumina was present at the composite-atmosphere interface. No aluminium was detectable at the surface of the molybdenum matrix. Alumina particles were present in the bulk of the composite. Aluminium did not dissolve in the molybdenum matrix. Composites of Al2O3 and molybdenum were also sintered in dry hydrogen. In this case as well, no aluminium dissolved in the molybdenum matrix. The possibility of a chemical reaction of Al2O3 and molybdenum is thermodynamically investigated. An isothermal section of the phase diagram of the Mo---Al---O system is presented. The rate of the reaction of Al2O3 with molybdenum is determined by the oxygen permeability of the molybdenum matrix. The aluminium permeability of the molybdenum matrix is much higher. Because of the non-equilibrium conditions both in the testing procedure in the thermionic energy converter and during the sintering process in hydrogen or vacuum, the aluminium diffuses through the molybdenum matrix and evaporates at the composite surface. As a result no aluminium is detectable in the molybdenum matrix