Crucibles control

The true DSC uses a heating resistor placed under each crucible, controlling the crucibles temperature and maintaining them as equal [8]. The difference in heating power between these heating resistors directly delivers the thermal power of the sample. Thus, it is a method following the principles of ideal heat flux (see Section 4.2.3.2). 

The synthetic method used in preparing a particular boride phase depends primarily on its intended use. Whereas for basic research borides of high purity are desirable, for industrial applications, e.g., in coatings, tools and crucibles, as a refining agent in metallurgy or in control rods in nuclear energy plants, pure borides are unnecessary. 

The power source, relay, set-point controller and sensor need not be discussed here. However, the insulation, heating elements, and crucibles involve materials, and need to be examined in more detail. 

The above reaction shows that the oxychloride decomposes at the sublimation temperature into the volatile tetrachloride and the nonvolatile oxide. Reduction starts as soon as the chloride vapour contacts the molten magnesium, and this exothermic reaction raises the temperature of the reaction mixture. The temperature of the reduction crucible is maintained in the range of 800 to 875 °C. The process is carefully controlled by matching the sublimation rate of zirconium tetrachloride with the reduction rate. The conclusion of the reduction is indicated by a fall in temperature and pressure. 

Torsi et al. [395] have carried out a systematic investigation to establish the potential value of such an apparatus. The apparatus is basically an electrothermal device in which the furnace (or the rod) is replaced by a small crucible made of glassy carbon. Figure 5.10 provides an overall view of the apparatus. Figure 5.11 shows a block diagram of the electrolysis circuit the crucible (6) acts a cathode, while the anode is a platinum foil dipped into either the sample solution reservoir (1) or the washing solution reservoir (2). Pre-elecrolysis was performed at constant current with a 500 V dc variable power supply (5). Under these conditions, the cathode potential is not controlled, so that other metals can be codeposited with lead. 

The Fig. 13 g shows the sample holder with the two hot junctions of the DTA- and temperature control thermocouple. The two crucibles (sample and reference) must be placed in position so as to ensure satisfactory contact between hot junction and base of the crucible. After the experiment, any adhering cmcible should be loosened carefully by heating, and the holder freed of impurities by annealing at approx. 

Runs of other sizes may be made in the same apparatus. With half the quantity of sodium specified, temperature control demands much more attention. With larger quantities, the nickel crucible is dispensed with, and the carefully cleaned pot used. The checkers used 260-270 g. of sodium and averaged a yield of 94 per cent the reaction time was increased by only one-half hour. By arranging two series of apparatus in parallel, but connected to the same cylinder of ammonia, one operator can prepare twice as much amide in almost the same time. 

Figure 2.39. A sketch of a type of differential thermal analysis head is shown in (a). The two small crucibles (internal diameter 4 6 mm) contain the sample (S) and the reference (R, inert material). The junctions of two thermocouples (Ts, 7r) are inserted in the crucibles. They give the temperature of the sample and the AT between sample and reference. The DTA head is inserted inside a furnace (F), the heating and cooling regime of which may be conveniently controlled.

The turnaround time for a boule growth run is several days. It takes time to prepare the source, load the crucible, attach the seed, evacuate the system carefully, gradually heat up the crucible under controlled conditions, and finally grow the boule and then cool down. The turnaround time itself is not a problem, however, the cost of manufacturing a wafer needs to decrease so the price of the wafers can be reduced in order for SiC to gain acceptance in the market. 

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