Metal oxidation, directed

Lanxide A process for making composites of metals with oxides. A molten metal reacts with an adjacent oxidant and is progressively drawn through its own oxidation product so as to yield a ceramic/metal composite. Fibres or other reinforcing materials can be placed in the path of the oxidation reaction and so incorporated in the final product. The Lanxide Corporation was founded in 1983 in Newark, DE, to exploit this invention. In 1990 it formed a joint venture with Du Pont to make electronic components by this process. Variations are Dimox (directed metal oxidation), for making ceramic metal composites, and Primex (pressureless infiltration by metal), for making metal matrix composites. 

High- temperature superconduc- tors Transmission cables, transformers, current limiters, motors and generators Rolling, direct metal oxidation, tape casting Greater energy efficiency, higher critical current density... 

This chapter, then, deals primarily with the directed metal oxidation process, although selected examples of stability in metal matrix composites are also discussed briefly. The focus is, of course, on the applications of phase equilibria, and more generally, thermodynamic principles that are applicable to the formation of composites in the presence of molten metals. Because these general principles are the same regardless of whether the end product is an MMC or a CMC, little generality is lost by focusing the discussion on CMCs formed by directed metal oxidation. 

Because the details of processing in each class of CMCs (e.g., oxide, carbide, or nitride matrix) are slightly different, the appropriate thermochemical approach for each class may also be different. For example, in the formation of alumina matrix materials by directed metal oxidation, the alumina product grows from a molten aluminum alloy by reaction with an oxygen-containing gas phase. On the other hand, in the formation of platlet-reinforced zirconium carbide, the gas phase is not involved in the reaction at all, being inert to the reactants and products. Thus, a general approach to deal with the myriad of possible products formed by the... 

Before describing in detail these specific examples, the next section provides a brief overview of the directed metal oxidation process. The information provided will allow a better understanding of the phase diagram and thermochemical applications that follow. 

In this section the preparation of ceramic composites by the directed metal oxidation process is described. First, in Section II.A, the aluminum oxide system is used as an example to explain the nature of the process, and a further example, a ZrB2 reinforced ZrC composite, is discussed in Section II.B. 

Fig. 1. A schematic illustration of CMC growth to net-shape using a directed metal oxidation process where the preform is formed by cold-pressing.

Fig. 3. A schematic illustration showing the various steps employed to form a tubular component by the directed metal oxidation process. The preform is formed by slip casting.

Fig. 4. A schematic illustration of the processing used to produce zirconium diboride reinforced zirconium carbide materials by directed metal oxidation.

Fig. 5. The microstructure of zirconium diboride-reinforced zirconium carbide composites produced by directed metal oxidation, (a) A composite prepared with 22 vol % metal, (b) A composite produced with less than 2 vol % metal. 

Typically, the directed metal oxidation process involves the simultaneous reaction of molten metal, e.g., A1 with Oz, and infiltration of the reaction product and metal into a porous preform of the desired reinforcement. The directed metal oxidation process can also form composites in the absence of a reinforcement phase, termed matrix-only growth. Although the former process is more interesting commercially because of the ability to tailor the composite properties and because the product does not show significant preferred orientation, the latter case is simpler conceptually and theoretically. Thus, the thermodynamic discussion will begin with growth in the absence of reinforcements and then cover the additional complications that arise from their presence. 

To illustrate the thermodynamic complexities that arise because of the presence of a molten metal in the directed metal oxidation process, a detailed analysis is presented both with and without the Si metal present. The former analysis is applicable to more traditional ceramic processing such as sintering or hot-pressing of SiC/Si3N4 composites, whereas the latter is applicable to the directed metal oxidation process, or any other composite process where molten Si may be present. 

This analysis must be modified for the directed metal oxidation process or other processing where a molten Si phase is present. When this phase is included in the analysis, reactions (16) and (17) must be modified, becoming ... 

Fig. 13. Micrograph of a SiC reinforced Si, N4 matrix composite prepared by directed metal oxidation. From Johnson [52].

With the analysis embodied in Fig. 12, the formation of carbon fiber reinforced Si3N4 matrix materials by directed metal oxidation of molten silicon can also be described. To avoid fiber degradation during processing, the fiber should, ideally, be thermodynamically stable with respect to... 

Using the Zr-B-C phase diagrams, some important features about the process of forming the ZrB2/ZrC/Zr composites by directed metal oxidation are apparent. With a maximum interface temperature of 2300 to... 

The Hf-B-C system presents a situation that falls somewhat between the Ti and Zr systems [60]. Although the HfB phase is stable in the Hf-B binary system, it melts at 2100°C, below the melting point of the Hf parent metal (2227°C). During a directed metal oxidation reaction of molten Hf with B4C at just above the melting point of Hf, e.g., 2400°C, the Hf-B-C isothermal ternary cross section (Fig. 18) indicates that the molten metal is... 

A. S. Nagelberg, A. S. Fareed, and D. J. Landini, Production of ceramic matrix composites for elevated temperature applications using the DIMOX directed metal oxidation process. In Processing and Fabrication of Advanced Materials for High Temperature Applications (V. A. Ravi and T. S. Srivatsan, eds.), pp. 127-142. Metallurgical Society, Warrensdale, PA, 1992. 

W. B. Johnson, Reinforced Si3N4 matrix composites formed by the directed metal oxidation process. Ceram. Eng. Sci. Proc. 13(7-8), 573-580 (1992). 

Microbial activities that produce sulfides, organic, or inorganic acids causing direct metal oxidation are major driving forces in biocorrosion. Biochemical corrosion is enhanced by stagnant water, soil, and organic products. 

Microbial activities producing sulfides, organic, or inorganic acids causing direct metal oxidation... 

Directed metal oxidation molten aluminum 950X (1,742 = F) 100... 

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