Carbon materials are, in general, very good absorbents of microwaves, i.e., they are easily heated by microwave radiation. This characteristic allows them to be transformed by microwave heating, giving rise to new carbons with tailored properties, to be used as microwave receptors, in order to heat other materials indirectly, or to act as a catalyst and microwave receptor in different heterogeneous reactions. In recent years, the number of processes that combine the use of carbons and microwave heating instead of other methods based on conventional heating has increased. In this paper some of the microwave-assisted processes in which carbon materials are produced, transformed or used in thermal treatments (generally, as microwave absorbers and catalysts) are reviewed and the main achievements of this technique are compared with those obtained by means of conventional (non microwave-assisted) methods in similar conditions.

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Resorcinol-Formaldehyde organic xerogels with an initial pH of 6.5 were synthesized in a microwave oven and then carbonised in a furnace. The influence of the carbonisation temperature, heating rate and the particle size of the organic xerogels upon the pore structure of the carbon xerogels was assessed. The textural properties of the organic and carbon xerogels were evaluated from N 2 adsorption-desorption isotherms at-196ºC and by measuring the true (helium) density. As the carbonisation temperature increased from 700 to 950ºC the S BET of the carbon xerogels decreased from 585 to 471 m 2 g-1 for the carbon xerogels derived from the organic xerogels crushed to a particle size > 212 µm. A decrease in the particle size to dp < 212 µm caused an increase in the BET specific surface area of the carbon xerogels of between 12 and 27% but also a decrease of the carbon xerogel yield of between 15 and 24%, depending on the carbonisation temperature. The pore structure of the carbon xerogels did not change significantly with the increase in the heating rate in the carbonisation process over the interval studied.

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Resorcinol−formaldehyde carbon xerogels were prepared by means of two different synthesis methods: conventional (C) and microwave heating (MW). The influence of the heating method and the pH of the precursor solution on the textural and chemical properties of the carbon xerogels was investigated. It was found that by modifying the initial pH, it is possible to control the porosity of carbon xerogels independent of the heating method used. The electrochemical performance of a selection of synthesized carbon xerogels as electrode materials in electric double-layer capacitors was studied by cyclic voltammetry and charge/discharge experiments in an acidic medium (1 M H2SO4). The electrochemical performance of the carbon xerogels was compared to that of an activated carbon commercialized for this application (Norit Super DLC-50), and it can be seen that the carbon xerogels display similar specific capacitances to those of the commercial carbon. Moreover, carbon xerogels have a good cycle durability after 18 000 galvanostatic cycles, with a drop in specific capacitance of around 10%. This excellent cycle durability, together with the attractive properties of carbon xerogels and the saving of time and energy achieved with microwave-assisted synthesis, would make resorcinol−formaldehyde carbon xerogels promising materials for applications of an electric double-layer capacitor (EDLC).

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The variety of methodologies used to determine the electrical conductivity of carbons makes it very difficult to compare samples and establish reference values. In this study, the electrical conductivity of a wide range of carbons was determined using two different methods: fourpoint probe and compression. Although the methodologies and the operating conditions are very different, linear correlations between the values measured by these two methods can be established for some of the materials studied. Only materials with a very high conductivity (graphite and carbon black) could not be correlated.