Ghm Gubbels

Toughening of calcium hydroxyapatite with silver particles

Calcium hydroxyapatite bioceramic was toughened by preparing composites with silver particles as reinforcements. The composites were fabricated from hydroxyapatite and silver oxide raw powders. The sintering behaviour was investigated using dilatometry. An optimized sintering programme was designed to promote densification and to suppress the decomposition of the hydroxyapatite matrix and the evaporation of silver. High density was achieved on both small cylinder samples and large block samples by pressureless sintering. The density of the composites is over 92.4% theoretical with silver inclusions up to 30 vol%. The strength of the composites is greater than 80 MPa as tested by four-point bending. Silver inclusions improve the toughness consistently, from 0.70 MPa m1/2 for the monolithic hydroxyapatite to 2.45 MPa m1/2 at 30 vol% silver. Studies on the toughening mechanism indicate that crack bridging and subsequent plastic work of silver are mostly responsible for the toughening, whereas crack deflection also makes some contribution.

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.

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

Abstract The as-deposited WF 6 -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.

A thermionic energy converter with polycrystalline molybdenum electrodes

A research diode with polycrystalline molybdenum electrodes is described. Voltage‐current characteristics are presented as a function of the cesium reservoir temperature. A power density of 4 W/cm2 is obtained at an emitter temperature of 1400 °C. The influence of the temperatures of the emitter, collector, and cesium reservoir and of the interelectrode distance is experimentally investigated. Physical explanations for the various effects are given. The work function of the polycrystalline molybdenum emitter in a cesium atmosphere is evaluated as 2.5–2.7 eV depending on the reduced emitter temperature (i.e., the emitter temperature divided by the cesium reservoir temperature). A barrier index Vb =2.0 eV and a cesium plasma drop Vd =0.4 eV are found.

Electron emission microscope measurements on cermet electrodes for thermionic converters

Abstract The electron emission is studied of a polycrystalline Mo-Al 2 O 3 cermet. Measurements with an electron emission microscope show that, both in vacuum or with Cs coverage, the Mo matrix contributes most to the emission. Work functions are measured at Cs effusion cell temperatures between 90°C and 130°C; the lowest work function measured is 1,2eV. The enhanced emission of the cermet as compared to pure Mo is attributed to Cs-O interaction at the Mo surface.

A thermionic energy converter with a molybdenum‐alumina cermet emitter

A study is made of the properties of cermets as electrode materials for thermionic energy converters. For thermodynamic reasons it is expected that all cermets composed of pure Mo and refractory oxides have the same bare work function. From data on the work function of Mo in an oxygen atmosphere this bare work function is estimated to be Φ=4.9 eV (at T=1400 °C). Experimentally, the bare work function of Al2O3‐Mo cermets was found to be Φ=4.5 eV, independent of the relative amounts of Al2O3 and Mo. The cesiated work function of the Al2O3‐Mo cermets was found to be 0.15 eV lower than the cesiated work function of pure Mo. The bare work function of Mo3Al was found to be Φ=4.0 eV. The cesiated work function of Mo3Al at collector temperature conditions was 0.3 eV lower than the cesiated work function of pure Mo. The electrical power density of a diode with an Al2O3‐Mo cermet emitter was 0.4 W/cm2 at 1300 °C. The barrier index at this temperature was 2.36 V. The high barrier index is attributed to a high plasma...

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 [lOO] zone is evaluated from experimental data for tungsten: D = 0.035 eV/degree. It is expected that D has the same value in other bee 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.

A WF6-CVD tungsten film as an emitter for a thermionic energy converter : II. Electron and Cs +-ion emission from WF6-CVD tungsten films

Abstract The workfunction of as-deposited WF6-CVD tungsten films in vacuum is found to be φe = 4.3 eV. This value corresponds with the workfunction of the {111} tungsten crystal faces. From the cesiated workfunction a bare workfunction of φ=e = 4.3 eV is deduced as well. The power density produced by a thermionic energy converter using a WF6-CVD emitter at a temperature of 1673 K is found to be 1.9 W/cm2. The barrier index at 1673 K is Vb = 2.06 V. For the plasma drop a value of Vd = 0.6 V is found.