Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform “volumetric” heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two‐dimensional (2‐D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.
The product ratios, ether: alcohol, for the solvolyses of the title compounds in ethanol-water solvent mixtures have been measured, and have been compared with ratios of the same products generated from the reactions of the corresponding diaryldiazomethanes in the same solvents. The ratios are similar although not always identical, and disparities are ascribed to solvation differences. The question of ' selectivities ' of intermediates towards ethanol and water is addressed, and it is concluded that product ratios can be used as selectivity measures only when a single intermediate yields stable products. The present results thus throw little light on the reality or otherwise of stability-selectivity relationships in the present series. Internal return is suggested as an important component in the unassisted solvolysis of the esters.THERE are two principal ways of providing information pertinent to the reactivity-selectivity principle (RSP) .2 One is to derive kinetic selectivity parameters for the reactions in question by comparing their rate responses to substituent or environmental change with rate or equilibrium behaviour of a model reaction. This approach is exemplified in the preceding paper. A second method is to generate an intermediate in an environment where two or more reagents can compete to form stable products. Analysis of the product mixture then affords a measure of selectivity of the intermediate.3 This method is also a powerful tool for the identification of intermediates: if two or more potential precursors of a reactive intermediate yield the same selectivities towards a given pair of reagents a common intermediate is strongly i m ~l i e d . ~In the preceding paper on the solvolysis of diarylmethyl P-nitrobenzoates in aqueous ethanol we were left with two alternatives as to mechanisms. One was that unassisted ionization (a k, process) is the rate-limiting step, in which case the RSP does not apply. The second alternative was that internal return of the tight ion pair occurred, so that further unassisted reaction of the ion pair becomes the rate-limiting step. If this is so, the apparent lack of dependence of selectivity upon reactivity can be rationalized in terms of competition between steps whose behaviour is in accordance with the RSP. The principal aim of the present work is to distinguish between the alternatives.
Rate constants for solvolysis of diarylmethyl p-nitrobenzoates in ethanol-water and trifluoroethanol-water mixtures have been measured. Hammett p values and Winstein-Grunwald m parameters have been evaluated. The former are insensitive to solvent composition and the latter to ring substiiuent. Mechanistic speculation is required before these results can be assessed as contributors to debate on the reactivity-selectivity principle, and it is concluded, on the basis of the trifluoroethanol-water probe, that solvent assistance is absent. 100 "C a AH: b AS: c 412 d 23.3 -7.4
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.