Melt containment

In the high pressure process, a resia melt containing a chemical blowiag ageat is iajected iato an expandable mold under high pressure. Foaming begias as the mold cavity expands. This process produces stmctural foam products with very smooth surfaces siace the skin is formed before expansion takes place. 

In the flux-growth method, crystals of the desired ceramic are precipitated from a melt containing the components of the product phase, often in addition to additives used to suppress the melting point of the flux. These additives remain in solution after crystal growth is complete. Crystals are precipitated onto seeds by slowly cooling the melt or the seed, or occasionally by evaporating volatile components of the melt such as alkaH haHdes, depressing the solubiHty of the product phase. 

Investigation of fluoride melts containing tantalum or niobium is a very difficult procedure that involves high temperatures, corrosion activity and the... 

Three conceptual steps can be discerned in the definition of the ionic structure of fluoride melts containing tantalum or niobium. Based on the very first thermodynamic calculations and melting diagram analysis, it was initially believed that the coordination numbers of tantalum and niobium, in a molten system containing alkali metal fluorides, increase up to 8. 

Analysis of the melting diagrams led to the conclusion that fluoride, fluoride-chloride and oxyfluoride-chloride melts containing niobium contain the complex ions NbF83 NbF7Cl3 Nb02F43 ... 

Physicochemical properties of fluoride melts containing niobium... 

The method based on a net melt-holder is used successfully for materials with relatively low melting points and is less promising for the investigation of fluoride melts containing tantalum and niobium. 

The formation of complexes in fluoride and oxyfluoride melts containing tantalum and niobium will be discussed later on in detail. 

Table 55 presents the results discussed above. Fluoride melts containing tantalum contain two types of complex ions, namely TaF6 and TaF72 . The equilibrium between the complexes depends on the concentration of fluoride ions in the system, but mostly upon the nature of the outer-sphere cations. The complex ionic structure of the melts can be adjusted by adding cations with a certain polarization potential. For instance, the presence of low polarization potential cations, such as cesium, leads primarily to the formation of TaF72 complexes, while the addition of cations with relatively high polarization potentials, such as lithium or sodium, shifts the equilibrium towards the formation of TaF6 ions. 

Additional confirmation of the above process can be found in the fact that some iodine separation is observed from melts containing more than 0.1 mol fraction KI [359, 360]. Chernov [361] noted a similar phenomenon of iodine separation when investigating the molten system K2ZrF6 - KI. 

Adding potassium hydroxide, KOH, to a melt containing KF and a 0.1 mol fraction of K2TaF7 leads to the appearance of an additional band at 900 cm 1, as shown in Fig. 79 [342]. This band corresponds to TaO bond vibrations in TaOF63 complex ions. Interpretation of IR spectra obtained from more concentrated melts is less clear (Fig. 80). The observed absorption in the range of 900-700 cm 1 indicates the formation of oxyfluoride polyanions with oxygen bridges. ..OTaO. The appearance of a fine band structure could be related to very low concentrations of some isolated components. These isolated conditions prevent resonance interaction between components and thus also prevent expansion of the bands by a mechanism of resonance [362]. 

Only two processes of tantalum metal production are of worldwide commercial significance. These are the electrolysis of fluoride-chloride melts containing potassium heptafluorotantalate, K TaF , and tantalum oxide, Ta20s, and the reduction with sodium of K-salt or K—salt that is dissolved in potassium fluoride-chloride melts. 

The majority of researchers, however, are inclined to believe that the tantalum reduction process takes place in a single step. Experimental results and discussions confirm that tantalum is reduced from fluoride, fluoride-chloride and oxyfluoride melts containing K2TaF7, via a single stage in which five electrons are transfered [546 - 548] ... 

Anode processes yield gaseous chlorine, fluorine, carbon chloride or fluoride. In the case of melts containing dissolved tantalum oxide, carbon oxides (mostly carbon dioxide) are formed on the graphite anode [28,37]. 

Density of melts containing niobium fluorides, 151 tantalum fluorides, 152, 165... 

Of course, measurements of amplitude dependence of the elastic modulus of a filled composition may, by itself, give an interesting material on thixotropic properties of such materials. Indeed, very strong effects are observed here. Thus, in a melt containing... 

In this method the melt contains boric oxide and the metal oxide in a suitable electrolyte, usually an alkali or alkaline-earth halide or fluoroborate. The cell is operated at 700-1000 C depending on electrolyte composition. To limit corrosion, the container serving as cathode is made of mild steel or of the metal whose boride is sought. The anode is graphite or Fe. Numerous borides are prepared in this way, e.g., alkaline-earth and rare-earth hexaborides and transition-metal borides, e.g, TiBj NijB, NiB and TaB... 

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