Microwave Syntheses in the Solid State

 © G. Whittaker, 1994 & 2007. This work, or extracts from this work may be reproduced only with the written permission of the author.

 In addition to the physical changes induced in ceramic materials, the microwave dielectric losses of many solid compounds may be used to provide sufficient heat to drive chemical changes. This is particularly true of a number of metal oxides. The high loss factor of copper oxide, for example, has been used alongside those of other materials to synthesise the high T_c superconductor YBa₂Cu₃O_x . A number of other materials also display excellent heating properties, for example boron and carbon, and whilst they may be used in conjunction with other lossy materials, their heating characteristics are good enough that they may be used as the only significant microwave absorber. In many cases, the microwave induced reactions require only a small percentage of the conventional syntheses. A summary of some of the published reactions is shown in Table 1

Reactants Products Conditions Ref. V2O5+ C VO2 5min / 500W 128 V2O5+ 2C V2O3 20min / 500W 128 U3O8+ C UO2 5 min / 500W 128 Fe3O4+SrCO3 +La2O3 LaSr2Fe3O8-x 36 min / 500W 129 Cr + B CrB 27 min (3 stages) 500W 130 Zr + 2B ZrB2 5 min / 500W 130 Cu + In + 2S CuInS2 1+3+3 min / 400W 131 Cu + In + 2Se CuInSe2 1+3+3 min / 400W 131

 Table 1 Inorganic solid state reactions under microwave irradiation

 The use of metals as microwave absorbent materials in solid state reactions is a special case with problems of its own. The improved syntheses of metal chalcogenides under microwave irradiation¹³² from my own work have been adapted for rapid syntheses of the semiconducting ternary sulphides CuInS₂ CuInSe₂ and CuInSSe.^{131, 133} The use of metals has also been recently reported in the synthesis of transition metal borides.

 Other work in this area includes the synthesis of submicron powders. Taking barium and titanium acetates in a 0.4M solution, microwave irradiation first gave a gel, and on further irradiation was pyrolysed via a dark brown phase to give submicron BaTiO₃ powder.¹³⁴ A more general alternative to this method uses a microwave plasma to produce powders from rapid indirect heating of aqueous solutions. A low pressure microwave induced plasma is fed with an aerosol formed from a suitable solution - usually contains the metal nitrate, although chlorides or even suspensions may be used.¹³⁵ Vaporisation of the solvent and subsequent chemical changes lead to the formation of nanometre-sized particles of ceramic oxides. By using mixed salts, for example, it is possible to form zirconia based ceramics such as (ZrO₂)_0.77(Y₂O₃)_0.03(Al₂O₃)_0.20.¹³⁶

 120. C.E. Holcombe & N.L. Dykes. J. Mat. Sci. Lett.9, 425 (1990).

 121. H.D. Kimrey, M.A. Janney & M.K. Ferber. Ceramic Technology Newsletter20, 3 (1988).

 122. S. Das & T.R. Curlee. Amer. Ceram. Soc. Bull.66, 1093 (1987).

 123. C. Shibata & H. Tamai. J. Microwave Power and Electromagnetic Energy25, 81 (1990).

 124. R.B. James, P.R. Bolton, R.A. Alvarez, W.H. Christie & R.E. Valiga.J. Appl. Phys.64, 3243 (1988).

 125. S. Komenarni & R. Roy. Materials Lett.4, 107 (1986).

 126. D. Palaith, R. Silberglitt, C.C.M. Wu, R. Kleiner & E.L. Libelo.MRS Symp. Proc.124, 255 (1988).

 127. H. Fukushima, T. Yamanaka & M. Matsui. MRS Symp. Proc.124, 267 (1988).

 128. D.R. Baghurst. Chemical Applications of Microwave Radiation Oxford, , (1993).

 129. D.R. Baghurst, A.M. Chippendale & D.M.P. Mingos. Nature332, 311 (1988).

 130. D.R. Baghurst & D.M.P. Mingos. Br. Ceram. Trans. J.91, 124-127 (1992).

 131. A.R. Barron & C.C. Landry. Science260, 1653 (1993).

 132. A.G. Whittaker & D.M.P. Mingos. J Chem Soc Dalton Trans 2751-2752 (1992).

 133. R. Kniep. Angew. Chem. Int. Ed. Engl.32, 1411-1412 (1993).

 134. W.F. Klandig & J.E. Horn. Ceramics International16, 99 (1990).

 135. D. Vollath & K.E. Sickafus. J. Mat. Sci.28, 5943-5948 (1993).