TaC-Based Composites

The first carbonitride alloys based on Ti(C,N)—Ni—Mo were iatroduced ia 1970 foUowed by (Ti, Mo)(C,N)-based compositions having fine microstmctures that provided a balance of wear resistance and toughness (4). Continued research on the titanium carbonitride alloys, often called TiC—TiN cermets, ia the 1980s led to the developmeat of complex cermets having a variety of additives such as molybdeaum carbide(2 l) [12069-89-5] M02C, TaC, NbC, zirconium carbide [12020-14-3], ZrC, hafnium carbide [12069-85-1], HfC, WC, vanadium carbide [12070-10-9], VC, chromium carbide (3 2)... 

Hard materials on the basis of WC have usually a metallic binder phase of Co and/or Cr, Fe, Ni, and additions of other carbides (TiC, TaC, NbC), which determine the corrosion resistance. Accordingly, these composites are attacked by many acids and bases. The authors refer the reader to previous work [112,113] for more details. 

More recently, new quantitative structure-property relationships for Tg have been developed (1) they are based on the statistical analysis of experimental data for 320 linear (uncross-linked) polymers collected from many different sources, containing a vast variety of compositions and structural features. The Tg of the atactic form was used, whenever available, for polymers manifesting different tac-ticities. The Tg values of a subset of the polymers listed in this extensive tabulation are reproduced (with some minor revisions) in Table 1. (It is important to caution the reader here that these data were assembled from a wide variety of sources. Many different experimental techniques were used in obtaining these data.) The resulting relationship for Tg has the form of a weighted sum of structural terms mainly taking the effects of chain stiffness into account plus a term proportional to the solubility parameter S which takes the effects of cohesive interchain interactions in an explicit manner, as shown in equation (1) ... 

TaC shows a linear relationship between lattice parameter and composition, in contrast to the other carbides. A. L. Bowman (1961) fitted his data by a linear equation which has been used by a number of workers to convert from lattice parameter to composition. In view of the additional information, it is advisable to reexamine this property. Hence, the open points in Fig. 34, which are based on well-characterized material, were used to obtain the equation C/Ta( + 0.01) = -25.641 + 5.9757 by the method of least squares. This amounts to a minor change when compared to the equation given by A. L. Bowman (1961). As can be seen from the figure, the excellent agreement between numerous independent measurements gives overwhelming confirmation to this relationship. In fact, if a measurement is found to deviate from this line, the explanation is best sought in the experimental technique. For example, if a sample has... 

The base-case condition in Section 9.3 with no side-draw and no feed impurity is compared with the ones in this section. Figure 9.38 displays the column composition profiles for the base case in Section 9.3 and the optimum TAC case in this section with a side-draw flowrate of 50 kg/h. 

Presently, the majority of cermets are of titanium caibonitride base (with possible additions of M02C, WC, TaC, etc.) and Ni-Mo binder (with possible additions of Co, Al, etc.). The optimization of the hard phase and a careful control of the composition of the binder have given rise to significant improvements in the toughness and in the resistance to plastic deformation. The resultant microstractures are complex, with one or more hard phases with composite gradients (for example core of grain in titanium caibonitride and rim enriched with molybdenum carbide). 

Thus far, we have considered only the neat design. The excess reactant design may be preferable for systems with high TACs, especially for types I, II, and possibly IIIr. The motivation comes from the fact that some of the reactant concentrations are so low that a large reflux ratio and/or boilup ratio are required to achieved 95% conversion. We can refer to the composition profile of reactants A toward the column base and B toward the top in Figure 17.9 for type I. Other examples are reactant B in the reactive zone (Fig. 17.13) for type IIr and reactant A in the reactive zone (Fig. 17.17) for type III/. The excess reactant design is a simple means to achieve an improved reactant composition profile. 

---