Lr Lodewijk Wolff

The titanium-nitrogen system

Abstract The Titanium-Nitrogen phase diagram was investigated. Preliminary results indicate that the TiN composition range narrows suddenly from 1400°C downwards in the direction of nitrogen rich compositions. Above that temperature the homogeneity region extends from 28.5–29.0 at%N, at the Ti-rich border, to approximately 50 at%N at the N-rich border. Below 1400°C the Ti-rich border shifts to a value of 36.5 at%N (found at 1000°C). Simultaneously the N-rich limit of the α-Ti (N) phase shows a marked increase in nitrogen content between 1000°C and 1300°C. Outside this temperature traject the results are in good agreement with previously published values. The homogeneity region of the Ti 2 N-phase was found to be somewhat larger than the values found in literature: At 900°–1000°C the preliminary results indicate that the Ti 2 N-phase extends from 28.5 to 33.7 at%N. Work is in progress to establish the exact phase relations in the temperature range of 1000° up to 1300°C and the composition range of 20 to 35 at% Nitrogen.

Crystallography of aligned Fe-Al eutectoid

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Corrosion of technical ceramics by molten aluminium

Abstract Corrosion investigations with various types of nonoxide technical ceramics, eight silicon nitride materials (1 grade of HIPRBSN, 3 grades of HPSN and 4 grades of RBSN), and two types of silicon carbide (HIPSiC and SiSiC) were carried out in aluminium melts (pure aluminium and AlZnMgCu1·5). HIPRBSN and HPSN materials exhibit nearly no corrosion attack under the most severe conditions applied (1000°C for 750 h). RBSN materials with relatively high open porosity are attacked after relatively short times. HIPSiC is considerably more attacked than HIPRBSN and the HPSN materials; however, it is also fairly corrosion resistant. SiSiC reveals the poorest corrosion behaviour of the materials investigated. The results are explained in terms of thermodynamics, diffusion through the metal/ceramic interface and wetting effects.

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...

Reactions in the CaOCs2OAl2O3 system at 900°C

Abstract Reactions in the system CaOCs 2 OAl 2 O 3 were investigated. On the basis of the experimental results, a compatibility diagram for this system at 900°C is proposed. In this system a new compound was found and formulated as Cs 2 O.2CaO.4Al 2 O 3 . The X-ray diffraction pattern was measured and the cell parameters were derived. The phase relation results can be used to draw some conclusions about the caesium compatibility of Al 2 O 3 containing calcia. We found that all calcium aluminates react with Cs 2 O forming a stable compound. From these results we conclude that calcium will have a negative effect on the caesium resistance of Al 2 O 3 .

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.

Upgrading technical ceramics

Abstract Technical ceramics, in particular engineering ceramics, could benefit substantially from the elimination of surface defects. This will result in a marked enhancement of their strength and corrosion resistance. This effect can be achieved by coating these ceramics by chemical vapour deposition (CVD). By coating a ceramic material using essentially the same material applied by a suitable CVD process surface porosity and surface cracks can be virtually eliminated. In many of todays technical ceramics densification is achieved by liquid-phase sintering. The resulting grain boundary phases are, in many cases, an important factor in the corrosion damage of ceramic materials. CVD coating can effectively shield off those grain boundary phases from the corrosive medium. This markedly enhances the corrosion resistance. At the Centre for Technical Ceramics at Eindhoven University of Technology a research program was recently initiated in order to determine systematically the potential of this approach. In this presentation a literature survey is given of results obtained by other workers together with some preliminary results of our own. The latter concern the upgrading of reaction bonded silicon nitride and sintered AIN. A number of potential applications of this approach are indicated. The limitations of this upgrading of technical ceramics by CVD are also discussed.