Microwave good absorbers

The sintering of PEEK coatings can be achieved by a microwave sintering technique. It has been demonstrated that silicon carbide distributed in the polymer matrix is a good absorber for the microwave... 

Graphite has been used in military applications as radar absorbing material and in antielectromagnetic interference coatings for civil purposes since it is a good microwave radiation absorber. In this field, Fan et al. (2009) observed that microwave absorption maxima of milled flake graphite and carbon nanotubes was found in the 10-15 GHz frequency range. 

Phase-transfer catalytic (PTC) conditions, which are specific for anionic reactions (and anionic activation) are perfectly well tailored for microwave activation, because after ion exchange between a substrate and catalyst, the resulting nucleophilic ion pair is a highly polar species especially prone to interaction with microwaves [32]. Eventually, the mixture of neat reagents in an open vessel can lead to a reaction under microwave conditions provided that one of the reagents is liquid or a low melting solid that couples well with microwaves. On the other hand, even a small amount of a good microwave absorber (e.g,. H20, DMF for example, see Table 1.3) added to reaction mixtures that consist of substrates that do not absorb microwaves in the solid state can initiate an increase of reaction mixture temperature and then chemical reaction. 

First of all, it can be noticed the existence of temperature profile inversion with respeet to eonventional heating techniques. The air in proximity of the materials during the heat treatment, in fact, is not a good microwave absorber so that it ean be eonsidered that the atmosphere surroimding the material is essentially at low temperature. 

A new series of some spiro-1,4-dihydropyridines 2 has been synthesized by Hatamjafari [42] in good yields using a four-component, solvent-free process by the condensation of isatin, a primary amine, ethyl cyanoacetate and cyclohexanone absorbed on different solid supports under microwave irradiation applying a domestic oven. The report reveals that the application of montmorillonite KIO led to higher yields as compared to other solid supports (Scheme 2). 

Similarly, microwaves have been used for pyrolysis of coal, which is known to have very poor microwave absorption, by mixing it with inorganic oxides (very good microwave receptors) or with carbon. After the initial stages of pyrolysis the coal undergoes some graphitization, turning into carbon black that further absorbs microwaves [48, 49]. 

Scientific studies have found that the differences between microwave and conventional pyrolysis go beyond the obvious difference in the source of heat. Other differences arise from the very high rates of heat transfer from the microwave-absorbent to the waste, the amount heat received by the primary pyrolytic products once they leave the absorbent bed and the highly reducing environment. These three aspects have been shown to have an important effect in the final products since they modify the extent of secondary and tertiary reactions. Moreover, the scientific studies have shown that a nonthermal microwave effect in these processes is unlikely to exist. Tests have showed the potential of the microwave-induced pyrolysis process for the treatment of real plastic-containing wastes and it is believed that a commercial process could be developed, for example, to recover clean aluminium from plastic/aluminium laminates. Other materials, in particular tyres, coal and medical wastes are very good candidates to be treated/recycled using microwave pyrolysis and there have been a considerable number patents filed with this goal in mind. 

The polymer is used in microwave absorbers and as conductive coating with good abrasion resistance for electronic components. Polyaniline is not thermoplastic and cannot be melted or even softened by heat. It is still stable up to about 240° C and starts decomposing at higher temperatures. 

Finally, it must be emphasised that, as for hquid-phase extraction, it is possible to enhance thermal energy transfers, if desired, hy simply impregnating matrices that are not good microwave absorbers, or by dispersing within them, some strongly microwave-absorbing species. The selection of such a substance will be made according to the level of microwave absorption and heat dissipation exhibited by that substance. Furthermore, the added substance... 

The ionic conduction effect arises because ionic species in the presence of an electric field wiU migrate in one direction or the other. Energy is transferred firom the electric field, causing ionic interactions that speed up the heating of a solution. Ionic absorbers become stronger absorbers of microwave energy as they are heated since ionic conductance increases with temperature. Deionized water heats slowly, but if salt is added, it heats rapidly. Acids, of course, are good conductors and heat rapidly. 

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