Material temperature, microwave heating

There is no temperature gradient in the materials during microwave heating, so the materials can be heated up more evenly due to volumetric heat absorption. 

Berteaud AJ, Badet JC. Fligh temperature microwave heating in refractory materials. Journal of Microwave Power Electromagnetic Energy 1976 11 315-20. 

Bertaud, A.J. and Badot, J.C., High temperature microwave heating in refractory materials. J Microwave Power, 11, [ ] 315-320, (1976). 

Fig. 2 shows the temperature as a function of irradiation time of Cu based material under microwave irradiation. CuO reached 792 K, whereas La2Cu04, CuTa20e and Cu-MOR gave only 325, 299 and 312 K, respectively. The performances of the perovskite type oxides were not very significant compared to the expectation from the paper reported by Will et al. [5]. This is probably because we used a single mode microwave oven whereas Will et al. employed multi-mode one. The multi-mode microwave oven is sometimes not very sensitive to sample s physical properties, such as electronic conductivity, crystal sizes. From the results by electric fixmace heating in Fig. 1, at least 400 K is necessary for NH3 removal. So, CuO was employed in the further experiments although other materials still reserve the possibility as active catalysts when we employ a multi-mode microwave oven. 

Several articles in the area of microwave-assisted parallel synthesis have described irradiation of 96-well filter-bottom polypropylene plates in conventional household microwave ovens for high-throughput synthesis. While some authors have not reported any difficulties in relation to the use of such equipment (see Scheme 4.24) [77], others have experienced problems in connection with the thermal instability of the polypropylene material itself [89], and with respect to the creation of temperature gradients between individual wells upon microwave heating [89, 90]. Figure 4.5 shows the temperature gradients after irradiation of a conventional 96-well plate for 1 min in a domestic microwave oven. For the particular chemistry involved (Scheme 7.45), the 20 °C difference between the inner and outer wells was, however, not critical. 

In conclusion, is it necessary to obtain a microwave athermal effect to justify microwave chemistry Obviously not - it is not necessary to present microwaves effects in a scientific disguise. There are many examples in which microwave heating results in specific time-temperature histories and gradients which cannot be achieved by other means especially for solid materials. Hence, rather than claiming nonther-mal effects it is better to claim a means or a tool which induces a specific thermal history. 

Chen et al. [70] suggested that temperature gradients may have been responsible for the more than 90 % selectivity of the formation of acetylene from methane in a microwave heated activated carbon bed. The authors believed that the highly nonisothermal nature of the packed bed might allow reaction intermediates formed on the surface to desorb into a relatively cool gas stream where they are transformed via a different reaction pathway than in a conventional isothermal reactor. The results indicated that temperature gradients were approximately 20 K. The nonisothermal nature of this packed bed resulted in an apparent rate enhancement and altered the activation energy and pre-exponential factor [94]. Formation of hot spots was modeled by calculation and, in the case of solid materials, studied by several authors [105-108],... 

Abstract The Zr (Ti)-Si-Al HDN Catalytic Materials have been synthesized rapidly with a new route heated by microwave. The synthesis conditions such as synthesis temperature, microwave oven pressure, pH value of synthesis solution and raw material were examined by experimentation. The thermostability, pore volume, surface area, surface Si/Al and hydrodenitrogen activity of the synthesis samples were also characteristiced. 

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