Subliming consolidants

The vanadium alloy is purified and consolidated by one of two procedures, as shown in the flow diagram of the entire aluminothermic reduction process presented in Figure 1. In one procedure, the brittle alloy is cmshed and heated in a vacuum at 1790°C to sublime most of the aluminum, oxygen, and other impurities. The aluminum facilitates removal of the oxygen, which is the feature that makes this process superior to the calcium process. Further purification and consolidation of the metal is accomplished by electron-beam melting of pressed compacts of the vanadium sponge. 

Two approaches for the synthesis of nanostructured M50 type steel (composed of 4.0% Cr, 4.5% Mo, 1.0% V, 0.8% C and balance Fe) powders and their consolidation are reported in this chapter. One approach involved the sonochemical decomposition of organometallic precursors and the other involved the reduction of the metal halides with lithium triethyl borohydride followed by vaccum sublimation of the powders to remove lithium chloride. The as-synthesized powders are amorphous by X-ray diffraction (XRD) but the peaks corresponding to bcc a-Fe are observed in the compacts. The morphology and composition of the powders synthesized by both techniques, as well as the compacts, were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hardness, density, particle size and impurity contents were also determined for the compacts. In addition, pure nanosized iron particles obtained by the ultrasound decompositon of iron pentacarbonyl were consolidated and the properties of the latter were studied. 

The solid, subliming materials - camphene, tricyclene, menthol and cyclododecane - have been used as temporary, volatile consolidants and release agents (lagers lagers, 1999 Cleere, 2005). These have been chosen to be suitable because of their ease of use and resistance to polar solvents. Of these, cyclododecane is the preferred material because it is the most stable to degradation, has fewer impurities, and has fewer health and safety risks. It is important that the material applied contains as little as possible non-volatile material, which will remain after treatment as a contaminant. 

These materials are applied to fragile surfaces either in solution or molten. Films applied from different solvents tend to deposit as crystals so are less coherent and resistant to penetration by liquids, such as water. Films applied from molten materials are stronger, more coherent and solvent resistant. Mixtures of compounds may form strong films with smaller crystals, by analogy with similar findings from industrial practice (Araki and Halloran, 2004). The rate of sublimation of the consolidant depends on the vapour pressure of the material, the temperature, the rate of air movement and the depth of penetration into a porous surface. For the high-melting-point, low-volatility materials such as cyclododecane and menthol, it can take many weeks for the consolidant to leave. The more volatile materials will leave faster. 

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