Refractory oxidation behavior

The macroscopic behavior of refractory oxides is controlled by both the bonding and crystal structure. In particular, the mechanical response and electrical behavior of materials are interpreted in terms of the symmetry of the constituent crystals using matrix or tensor algebra [27], Other characteristics such as melting temperature and... 

FI. Lavendel, R. A. Perkins, A. G. Elliot, J. Ong investigation of Modified Silicide Coatings for Refractory Metal Alloys with Improved Low pressure Oxidation Behavior AFML-TR-65-344.1965. 

C. E. Ramberg, P. Beatrice, K. Kurokawa, W.L. Worrell, High temperature oxidation behavior of stmctural silicides, in in C.L. Briant, J.J. Petrovic, B.P. Bewlay, A.K. Vasudevan, H.H. Lipsitt (Eds.), High Temperature Silicides and Refractory Alloys, vol. 332, Martials Research Society, Pittsburgh, 1994, pp. 243-254. 

Nichols and Howes have studied the behavior of refractory oxides and have found that certain oxides at specific temperatures emit vast amounts of light in a limited spectral range (they claim as much as 85,000 times the expected amount ) and that such radiation depends on previous heat treatment. 

Carniglia ( ), in a review of the mechanical properties of refractory oxides, has emphasized that small amounts of impurities can have pronounced effects on the material behavior when they segregate to grain boundaries and surfaces. The present experiments are likely to be affected by precisely this type of segregation. The impurity levels in all three of the materials are such that we cannot be assured that we are measuring the intrinsic behavior of alumina on any of them. However, it is possible that we may be approaching intrinsic behavior in the Linde alumina, which has a purity of about 99.99 %... 

S. C. Camiglia, Grain boundary and surface influence on mechanical behavior of refractory oxides Experimental and deductive evidence, in Materials Science Research, Vol. 3, W. W. Kriegel and H. Palmour, IB (Eds.), Plenum Press, New York, 1966, p. 425. 

Mitsui H, Habazaki H, Akiyama E, Kawashima A, Asami K, Hashimoto K, Mrowec S, High temperature sulfidation and oxidation behavior of sputter-deposited Al-refractory metal alloys , Mater Trans JIM (Japan), 1996 37 379-382. 

Uranium and thorium are actinide elements. Their chemical behavior is similar under most conditions. Both are refractory elements, both occur in nature in the +4 oxidation state, and their ionic radii are very similar (U+4 = 1.05 A, Th+4 = l.lOA). However, uranium can also exist in the +6 state as the uranyl ion (U02 2), which forms compounds that are soluble in water. Thus, under oxidizing conditions, uranium can be separated from thorium through the action of water. 

In contrast to refractory uranium and thorium, lead is a moderately volatile element. Uranium and thorium are lithophile, while lead can exhibit lithophile, siderophile, or chalcophile behavior. This means that in many cosmochemical situations, it is possible to strongly fractionate the daughter lead from parent uranium and thorium, a favorable situation for radiochronology. On the other hand, lead tends to be mobile at relatively low temperatures and can be either lost from a system or introduced at a later time. As already mentioned, uranium can also become mobile under oxidizing conditions. This means that the U-Th-Pb system is more susceptible to open-system behavior than several other commonly used dating techniques. However, as we discuss below, there are ways to recognize and account for the open-system behavior in many cases. 

Elements from selenium through the middle rare earths will be present in the mixed fission product population they exhibit a wide variety of volatilities (1). The elements Y, Zr, and Nb and the rare earth oxides are high boiling and condensable at low partial pressures, whereas the noble gases, and the alkali metals Mo, Tc, Pd, Ag, Cd, Sn, Sb, Te, Ru, and perhaps Rh, are very volatile in a relative sense Sr and Ba are predicted to be of refractory or intermediate behavior. 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

It appears, however, that support surfaces are not always as refractory to reduction as chemical intuition would dictate. An important new finding is that lanthanum oxide undergoes reduction, in the presence of a supported metal, to "LaO", and the properties of Pd/lanthana are similar in several respects to those of metal/titania (43,44). Even with alumina supports, surface reduction has been found in some instances (45-471. The reason for this anomalous behavior is not fully understood, although sulfur has been found capable of promoting the reduction (46). A recent report has described suppressed H2 chemisorption on Rh/zirconia (4 ) although this was not found in an earlier study of Ir/zirconia (2). One may suspect differences in surface reducibility between the supports used in the two cases. 

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