No abstract

The microwave synthesis of molybdenum carbide, Mo(2)C, from carbon and either molybdenum metal or the trioxide has been achieved on unprecedented timescales; Ex- and in-situ characterisation reveals key information as to how the reaction proceeds.

In the twenty-first century, it is clear that manufacturing processes will need to be energy efficient and sustainable in addition to being economically viable. Such requirements are particularly important for materials processing, as advances in functional and structural materials continue to be made. Rapid processing and energy efficiency are only two of many attractions that microwave (MW) methods offer to the materials chemist. [1][2][3][4][5] At least as attractive are the opportunities to access new (metastable) materials and to understand the interaction of electric (and magnetic) fields with solids. MW heating has emerged as an exciting development in organic and solution-based synthesis, [6,7] but constraints preventing major advances in solid-state synthesis and materials processing remain in place. Without a polar solvent one is reliant on the coupling of one or more dielectric or conducting solid components, often restricting the range of reactions one can perform, creating problems of variable heating rate and sample inhomogeneity, or demanding the use of a MW susceptor (which may need to be separated from the products if intimately mixed to give a high degree of homogeneity). Furthermore, although cheap and readily accessible, domestic microwave ovens (DMOs) with multimode cavities and crude halfrectified power supplies lead to unpredictable mode patterns and poor synthesis reproducibility. Many of the above issues can be addressed if the sample can be positioned in a position of known (preferably high) electric field intensity. High electric field strength single-mode cavities consist of a metallic enclosure in which a microwave signal of the correct electromagnetic field polarization will undergo multiple reflections. The superposition of the reflected and incident waves gives rise to a standing wave pattern that is very well defined in space. The precise knowledge of electromagnetic field configurations enables the dielectric material to be placed in the position of maximum electric field strength, allowing maximum heating rates to be achieved at all times. Microwave power density (P) is proportional to the electric field strength inside the material squared (Eq. 1), [8] therefore such cavities offer extremely rapid heating rates and the opportunity to heat materials that appear transparent to microwaves in ordinary multimode cavities. The use of singlemode cavities for materials synthesis therefore promises not only to overcome issues of reproducibility, but should also lead to vastly reduced reaction times. The power density can be expressed aswhere f is the frequency, e 0 is the permittivity of free space, e″ is the dielectric loss, and E i represents the electric field.We report here how a TE 10n single-mode cavity can be exploited as a general method for the production of refractory carbides, with the synthesis of tungsten carbide as an example. The technique is well-suited to carbide processing, with carbon acting as both starting material and microwave susceptor. WC, with a high melting po...