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Tungsten (W) has one of the highest melting points (3410±20°C) among the elements in the periodic table and therefore has gained considerable attention for high temperature applications such as porous tungsten as a high current density cathode for flash lamps or electronic valves. Its brittle behaviour and high density have also been important factors in the processing methods employed. Through the years it has been processed mostly using PM techniques in order to obtain different forms. To obtain densification, it is necessary to apply high temperatures during sintering. This paper considers a novel approach to the pressing and sintering of tungsten powders and how their densification can be improved. One potential aid is to add very small amount of Al as a binder. The densification characteristics are presented using SEM analysis to develop the underlying mechanisms. [ABSTRACT FROM AUTHOR]

Porous tungsten as a high current density cathode is one of the important applications of the metal, which is mostly used in high temperature conditions due to its exceptional resistance to melting (Tm = 3410±20°C). Its porous form has been a crucial component of dispenser cathodes used in electronic valves and high power lamps. Porous tungsten skeleton forms the matrix, which is then impregnated with an electron emissive compound. Upon every emission from the surface, new material has to be fed into the surface pores via the open pore channels. Hence it may be proposed that a uniform porosity is needed for a better performance. However, a controlled porosity has not been achieved yet. Moreover, sintering of tungsten has always been difficult due to the extreme process conditions. A high sintering temperature (Ts≥2000°C) and a strong reductive atmosphere (hydrogen) have been the absolute necessity in making these parts. This study further explores an alternative sintering technique being developed. The idea is based on the reactive sintering concept. The energy output from the exothermic reactive system of tungsten oxide and aluminium has been the heat source for sintering porous tungsten. As a result, sintering temperature and time have been reduced considerably. Higher homogeneity, thus more uniform pore distribution, was observed. A better control of porosity related to the pressing and sintering conditions was achieved by the characterisation method previously developed. Microhardness has been a useful monitor of the scatter in porosity of the parts. Throughout the study, SEM was used to observe the porous structures and powder morphologies. DSC and XRD were useful to follow the microstructural evolution in the reactive system. [ABSTRACT FROM AUTHOR]

It has been shown recently that porous tungsten can be obtained at much lower temperatures T ≤ 1200°C than those in conventional techniques where T ≥ 2000°C. This has been achieved by employing the concept of reactive sintering. The method makes use of the addition of Al, a low melting point phase which decreases the process temperature while acting as a potential sintering aid. As a result, higher homogeneity and uniform porosity distribution have been obtained. In this study, attention has been paid to the swelling phenomenon observed in porous tungsten upon post-sintering heat treatment. It has already been shown that swelling occurred in sintering porous tungsten by this alternative technique, and upon further heat treatment swelling instead of shrinkage became even more pronounced. This article explores the microstructural evolution upon post-sintering heat treatment on these parts. The relation between swelling and porosity for different tungsten particle sizes has been demonstrated. An attempt has been made to explain the possible reasons behind the changes in the porous structure. SEM has been a useful tool for the purpose of this study. [ABSTRACT FROM AUTHOR]

One of the most important applications of tungsten is its use as a high current density cathode in the form of porous tungsten impregnated with electron emissive salts. Powder metallurgy is the usual processing route to make these parts since melting and casting of tungsten is very tedious due to its high melting point (3410±20°C). The overall porosity and porosity distribution are crucial parameters for the performance and lifetime of porous tungsten cathodes. Although there is an active debate over what constitutes the optimal porosity distribution, lack of homogeneity is observed in conventional parts. This may contribute to a shorter cathode and hence lamp lifetime. These parts are conventionally sintered at temperatures in excess of 200°C. Furthermore, due to the high temperatures involved, the conventional sintering is very costly and energy consuming. This paper briefly looks into an alternative sintering process being developed for porous tungsten technology and its outcomes in terms of porosity distribution across the parts. As a result, the sintering temperature is dropped down to 1150°C and more homogeneous porous structures have been obtained. A characterisation method previously developed by the authors based on microhardness measurements is proved to be a good measure of the homogeneity of porous tungsten parts. [ABSTRACT FROM AUTHOR]

An important application of tungsten is as a high current density cathode in porous form. It is used as a dispenser cathode after being impregnated with an electron emissive material. Therefore, porosity distribution in the part is a crucial parameter for its use since a uniform and controlled porosity will lead to a better performance.In this study, microhardness is used in order to determine the uniformity of the pore distribution and homogeneity of the structure. Effects of different pressing techniques on porosity of porous tungsten have been examined. Considerable attention has been given to die and cold isostatic pressed samples. A comparison has been made between the respective porous structures in view of the microhardness measurements. Optical microscopy and SEM are used to relate the results and porous tungsten structure for a better understanding of the applied method. [Copyright &y& Elsevier]