Passive heating elements

The dihydropyrimidinethione 30 has been obtained by microwave-induced rearrangement of a dihydrothiazine, using silicon carbide as a passive heating element <06JOC4651>. 

Microwave-assisted reactions using nonpolar solvents such as toluene or hexane can still be heated to high temperatures using additives such as ionic liquids or passive heating elements. Ionic liquids heat extremely rapidly upon microwave irradiation. For example, heating 2 mL of pure hexane at 200 W in a monomode... 

The use of silicon carbide inserts has proved valuable in the uncatalyzed aza-Michael addition between piperazine and methyl acrylate. The reaction was low yielding using pure toluene as the solvent since the maximum temperature that could be attained was 170 °C. Adding the passive heating elements to the reaction mixture allowed for temperatures of 200 °C to be reached and, with this, a dramatic increase in the yield for the reaction was observed (Scheme 2.10). The product isolation procedure was trivial and consisted of physical removal of the heating element using a pair of tweezers followed by removal of the solvent. 

SiC heating elements, depending upon the grade, are routinely used at temperatures up to about 1500 °C in air, and even up to about 1650 °C for short periods. The unglazed variety rely on a thin native passivating silica film for their protection against oxidation. Service life therefore depends strongly on the atmosphere in which they operate, which affects the stability of the film, and on temperature which, of course, affects reaction rates. 

Many metals form conductive silicides, which, like SiC, are resistant to oxidation through the formation of stable passivating layers of silicates or silica on their surfaces at high temperatures. Molybdenum disilicide (MoSi2) has been developed as a heating element for use in air at temperatures above 1500 °C. Its resistivity behaves as is expected for a metal, increasing from about 2.5 x 10-7 fim at room temperature to about 4 x 10-6 Qm at 1800 °C. 

First shutdown system absorbing elements Second shutdown system boron injection Passive heat removal system Emergency injection system Containment system... 

Figure XIII-4 illustrates the basic concept of the present KAMADO design. The heat generated by a fuel rod is not removed directly with cooling water but via graphite blocks of a fuel element, i.e., cooling water is heated by high temperature graphite of the fuel element. In case of a loss of coolant or flow, such as a pipe break, turbine trip, etc., the decay heat is removed by passive heat transfer from surfaces of the fuel elements to the reactor water pool operating at atmospheric pressure and low temperature.

Strong oxidising acids, for example hot concentrated sulphuric acid and nitric acid, attack finely divided boron to give boric acid H3CO3. The metallic elements behave much as expected, the metal being oxidised whilst the acid is reduced. Bulk aluminium, however, is rendered passive by both dilute and concentrated nitric acid and no action occurs the passivity is due to the formation of an impervious oxide layer. Finely divided aluminium does dissolve slowly when heated in concentrated nitric acid. 

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