Refractory metals requirements

Refractory metals require protection from oxidizing environment as they do not form protective oxide layers. Oxidation of Mo and W leads to a loss of material by the formation of volatile oxides above 600 °C, but without any significant impact on the mechanical properties. 

Elements in biomedical specimens which may be measured by FAAS are determined using the air-acetylene flame. The more refractory metals, requiring a higher temperature nitrous oxide-acetylene flame for... 

An important general trend is that interfaces with a higher reactivity require a lower temperature for formation of the sihcide [12]. For instance, refractory metals require temperatures in the range of600 °C to form disiUcides after reacting with Si, while quasi-noble metals react near 200 °C to form metal rich silicides. A summary of formation temperatures is recorded in Table 14.1. 

Some elements, such as the rare eartlrs and the refractory metals, have a high afflnity for oxygen, so vaporization of tlrese elements in a irormaT vacuum of about 10 " Pa, would lead to the formation of at least a surface layer of oxide on a deposited flhrr. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10 Pa. 

Ceramic boards are currently widely used in high-performance electronic modules as interconnection substrates. They are processed from conventional ceramic precursors and refractory metal precursors and are subsequently fired to the final shape. This is largely an art a much better fundamental understanding of the materials and chemical processes will be required if low-cost, high-yield production is to be realized (see Chapter 5). A good example of ceramic interconnection boards are the multilayer ceramic (MLC) stractures used in large IBM computers (Figure 4.11). These boards measure up to 100 cm in area and contain up to 33 layers. They can interconnect as many as 133 chips. Their fabrication involves hundreds of complex chemical processes that must be precisely controlled. 

The melting points of chromium (1857 °C) and of manganese (1244 °C) are considerably lower than that of alumina. The heat requirement in respect of the reactions leading to the formation of these metals was calculated by considering alumina melting as the objective. The objective changes when aluminothermy is applied to the production of the refractory metals niobium and tantalum. Niobium melts at 2468 °C and tantalum at 3020 °C. Thus, when these metals are the products, the heat requirements for reaching temperatures in excess of 2500 °C and 3050 °C have to be calculated. 

It should be noted that it is difficult to obtain models that can accurately predict thermal contact resistance and rapid solidification parameters, in addition to the difficulties in obtaining thermophysical properties of liquid metals/alloys, especially refractory metals/al-loys. These make the precise numerical modeling of flattening processes of molten metal droplets extremely difficult. Therefore, experimental studies are required. However, the scaling of the experimental results for millimeter-sized droplets to micrometer-sized droplets under rapid solidification conditions seems to be questionable if not impossible,13901 while experimental studies of micrometer-sized droplets under rapid solidification conditions are very difficult, and only inconclusive, sparse and scattered data are available. 

Electrical contact between the electrode and connecting wires can be made with solder if the electrode is a refractory metal, while lower-melting-point metals such as lead, and reactive metals such as magnesium, should be joined to a connection lead with commercially available conductive silver paint . Contact to ITO-coated electrodes will similarly require this conductive paint. 

Beryllia ceramic parts are frequendy used in electronic and microelectronic applications requiring thermal dissipation (see Ceramics as electrical materials). Beryllia substrates are commonly metallized using refractory metallizations such as molybdenum—manganese or using evaporated films of chromium, titanium, and nickel—chromium alloys. Semiconductor devices and integrated circuits (qv) can be bonded by such metallization for removal of heat. 

Refractory metals such as titanium and zirconium can be won from their oxides by reduction with metals which have oxides with a high heat of formation. Of these, only calcium (or calcium hydride) is capable of producing refractory metals in purities approaching those required for metallurgical uses. 

In our opinion non-stoichiometric metal halide compounds have to be expected for the electrodeposition of refractory and rare earth metals if the deposition is performed from halides as precursors. The electrodeposition of these metals requires... 

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