High-melting metals

Refractory Materials. Extremely high melting metals and those that are more resistant to deformation when hot are considered refractory... 

Detailed, critical surveys of the variants and complexities of crystal growth from the melt were published for low-melting metals by Goss (1963) and for high-melting metals (which present much greater difficulties) by Schadler (1963). 

Lloyd, E. D. in Plansee Proceedings 1958—High Melting Metals, Metallwork Plansee AG, Reutte Tyrol, 249-256 (1959)t... 

Lloyd, E.D., 1958 High Melting Metals , Plansee Proceedings, Metallwerk Plansee A.G., Reutte, Tyrol, 249-256 (1959)... 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

The ideal interface would be one which was smooth on an atomic scale. The only interfaces which can be produced which are as smooth as it is theoretically possible are interfaces between a single crystal of a high melting metal, e.g. platinum, and a deformable polymer. All interfaces between two solids will tend to be very rough on an atomic scale. The question therefore arises - How do real interfaces differ from idealised smooth interfaces" ... 

Ru (001) substrate in Fig. 2.11 [18J. Each trace corresponds to a different initial silver coverage. Systems of low melting metals such as Cu, Ag and Au, adsorbed on high melting metals such as Ru and W, are very suitable for fundamental desorption studies, because, firstly, readsorption on the sample surface does not occur and, secondly, the rate of desorption is measured without any systematic error due to background pressures, as would be the case with gases such as H2, CO and N2. Metallic overlayers have recently attracted attention as models for bimetallic surfaces in reactivity studies [19,20]. We use the data of Fig. 2.11 to show that TDS gives information on ... 

The silicides exhibit the lowest melting points and hardness values of the metallic hard materials (see Table 5.6-5). Their brittleness makes them unsuitable for utilization in hard metal alloys. Silicides have only been utilized industrially in metallurgical fields in which their scaling resistance and chemical resistance are important. They are also deposited using the CVD process e.g. as protective layers on high melting metallic surfaces. 

This anisotropy of the pzc tends to increase with the melting point of the metal. Therefore the influence of atomic steps and irregularities is less observable in electrochemical results obtained with low-melting metals than for results obtained with high-melting metals. 

The status of data at the beginning of 1984 is as follows. For the pzc, the situation is well advanced (Figs. 19 and 20) and the anisotropy of this parameter for high-melting metals is in agreement with the knowledge of the structure of the metal surface. 

For adsorption, the phenomenology is qualitatively what it is for polycrystalline electodes. Of course, the shift of the pzc with the CO (which increases with the melting point of the metal) entails differences which are not entirely reduced when the experimental results are analyzed as a function of charge density at the metal surface. Anyway, analysis of adsorption (especially for high-melting metals) is more valid for metal single-crystal faces than for polycrystals (which have a patchy, uncontrolled density-of-charge distribution at their surface). 

For a variety of applications, spherical particles are required. Many of these are associated with the field of powder metallurgy (Chapter 7). While it is relatively easy to produce spherical particles from low-melting materials by conventional techniques, in which melt droplets are produced and solidified in spray (prilling) towers [B.97], refractory solids in general and, specifically, high-melting metals can not be converted by this simple technique. However, if the solid is available in powder form (Section 13.3, refs. 82, 85, 89, 90, 94), various methods are available to produce spherical particles by agglomeration. 

Vapors of high melting metals and inorganic materials can be produced in situ by evaporation, either from resistively heated filaments of the material itself or else from bulk crucibles. These are all standard procedures 122). In a variant of this approach, permanent gases have been evolved by high temperature decomposition of a suitable solid, as for example H2 from ZrH2, 02 from CuO 44d), or from oxygen dissolved in silver 123), and CO from Mo(CO)a 124). In all these it is... 

The effective Debye temperature of the source matrix should be high so that the recoil-free fraction is substantial. High-melting metals and refractory materials such as oxides are the obvious choices. 

For films produced by evaporative techniques, it is approximately true that with metal films deposited on a substrate at room temperature and even higher, tensile stresses are predominant. The magnitude of the tensile stresses is between 108 and 10 dyn cm 2. Films of high-melting metals have the larger values, those of lower melting metals the smaller values in this range. Reactively sputter deposited dielectric films and metal films modified by chemical and/or physical gas incorporation frequently have compressive stresses. Compared with metal films, dielectric films often have smaller stress values. 

---