Ontwikkelingen in poeder- en keramische technologie

A review with 35 refs. is given of developments and trends in powder prepn. methods and shaping techniques. [on SciFinder (R)

Nieuwe inzichten in het sinterproces

A survey is given of developments in the last decennium, which have contributed to a better insight in the sintering process. The influence of sinter additives and the problems related to the sintering of inhomogeneous systems are also discussed

Electromagnetic properties of ferrites

Electromagnetic properties of ferrites are discussed, with special regard to the garnet and spinel structures. In the compound yttrium iron garnet, Y 3Fe50i2, small amounts of tetravalent dopants, like silicon, induce charge compensating Fe2+ ions. A discussion is given of the time-dependent magneto-crystalline anisotropy caused by the presence of the Fe2+ ions. At temperatures below about 100K, a number of physical properties of silicondoped iron garnets can be changed by irradiation with infrared light. Examples are given of photoinduced changes in magnetocrystalline anisotropy, magnetic susceptibility and coercive force. Further, we discuss related photoinduced changes in the optical absorption coefficient and in linear dichroism. The effects are divided into two different classes: I) photoinduced effects which depend on the direction of the magnetization with respect to the crystallographic axes and the polarization direction of the incident light, and II) photoinduced effects that occur regardless of the prevailing magnetization distribution. In case I the effects are due to an unequal distribution of Fe2+ ions over sites which have a different orientation of their local symmetry axis with respect to the magnetization direction. In case II the photoinduced effects are due to a redistribution of Fe2+ ions over sites at different distances from the electron donating centre, e.g. the Si4+ ion. Finally magneto-optic effects in ferrites are considered. After a discussion of the phenomenological theory, the Faraday effect in the microwave and infrared region and the Faraday and Kerr effects at optical frequencies are considered

Het gebruik van thermodynamische beschouwingen voor de beschrijving van keramische processen

A review and discussion with 6 refs. In many cases, the no. of expts. necessary for optimization of a process can be reduced by simple thermodn. predictions. Some examples are discussed for nonoxide ceramic materials

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