Crucibles, heat losses

From Equation 1.1, it follows that the temperature difference between sample and reference is a measure of the difference in heat flow due to the presence of the sample in one of the crucibles, provided that the furnace and heat paths are truly symmetrical. Consequently, this differential heat flow is a measure of the properties of the sample, with all other influences (heat adsorption by the crucible, heat losses through convection, etc.) having been eliminated by use of the comparison with the reference. The AT signal requires calibration to provide a heat flow as a function of temperature, and this is usually carried out by use of standards that are usually pure metals with known enthalpies of melting and materials with known heat capacities (see Section 2.4 in Chapter 2). 

If tire crucible is assumed to be a cylinder, 1 m high by 1 m diameter, the total surface area for heat loss is 4 x 10" cm, and hence tire reactor loses 54 MJ during the reaction time. 

The size to use depends upon the susceptor being used, the temperature of operation desired and the heat losses within the system. For an Ir crucible, the 10 kc-20 KW generator works better than the 2 meg.- 75 KW generator. 

The next most importtmt parameters in Czochralski growth of crystals are the heat flow and heat losses in the system. Actually, aU of the parameters (with the possible exception of 2 and 9) are strongly ciffected by the heat flow within the crystal-pulling system. A tj pical heat-flow pattern in a Czochralski sjretem involves both the crucible and the melt. The pattern of heat-flow is important but we will not expemd upon this topic here. Let it suffice to point out that heat-flow is set up in the melt by the direction of rotation of the cr5rstal being pulled. It is also ctffected by the upper surface of the melt and how well it is thermally insulated from its surroundings. The circular heat flow pattern causes the surface to radiate heat. The crystal also absorbs heat and re-radiates it... 

The total heat requirement is thus around 599.98 kj, which is about 548.81 kj more than the heat available from the reaction. This calculation, however, does not take into account the inevitable heat losses due to the nonadiabatic conditions in the reactor. An estimate of these heat losses can be made by considering the industrial practice for aluminothermic chromium metal production. The charge is preheated to about 500 °C before loading into the aluminothermic crucible. This operation adds about 96.65 kj (i.e., 48.9 cal deg-1 475) of heat to the system. It, therefore, appears that around 41.84 kj (96.65 kj - 54.81 kj) of heat is lost due to radiation and convection for every mole of chromium sesquioxide reduced to the metal by the aluminothermic process. 

An upper limit of the heat losses through the reaction container wall, usually in the form of a cylindrical crucible with an increasing diameter from bottom to top, by assuming that the whole reaction mixture achieves the final reaction temperature immediately, and heat losses occur through the crucible refractory walls by conduction. The solution of Fourier s equation... 

Thermolysis on insulating wool. Kaowool or other refractory wools are valuable for reduced radiative heat losses from a hot crucible. However, their use causes some increase in the extent of pyrolysis of substrate vapor by the crucible assembly and this, in rare instances, may spoil a metal atom synthesis. Only one example is known at present. The reaction of palladium atoms with benzyl chloride gives very low yields of t73-benzylpalladium chloride when the palladium is evaporated from an alumina crucible insulated with Kaowool, but a 30-50% yield with an uninsulated crucible. It has been established that this is due to enhanced formation of product-destroying radicals on the hot Kaowool. 

Loss on Ignition Transfer about 1 g of sample, accurately weighed, and transfer it into a crucible, heat at 105° for 2 h, and then ignite in a muffle furnace at 450° 25° to constant weight. 

Loss on Drying (Volatile Matter) Transfer 1.5 to 2.5 g of colorant, accurately weighed, to a tared crucible. Heat in a vacuum oven at 135° for 12 to 15 h. Lower the pressure in the oven to -125 mm Hg, and continue heating for an additional 2 h. Cover the crucible, and allow to cool in a desiccator. Reweigh the crucible when cool. The loss of weight is defined as the volatile matter. 

Digestion of solid samples with acid or solvent may be performed in small crucibles heated on a metal hot plate, or in an air bath (Fig. 11.38), or in the glass apparatus illustrated in Fig. 11.46. The last-named is heated over a micro burner, and then rotated so that supernatant liquid or solution may be poured off drop-wise without danger or loss. 

Here, we show only a bare outline of the individual components in the overall system. This SYSTEM is capable of operation in inert atmosphere or vacuum. A melt/seed contact monitor is provided as well as a CCTV camera for observing and controlling the crystal diameter as it grows. Note that both the crucible and crystal rotation can be controlled. In order to control the heat-convection patterns which normally appear in the melt, an external cryomagnet is supplied. Its magnetic field controls heat losses, plus it maintains a better control of the crystal growth. A slave micro-processor controls both crystal diameter and meniscus-contact of the growing crystal. 

For a tube 20 cm. high and 2.5 cm. in diameter, suitable quantities of reactants are 200 g. of CeCla, 103 g. of I3 (mole ratio IrCeCla = 0.625 1.0) and a 15% excess of verypureCa powder (particles 0.3-2 mm.). The reactants are mixed under anhydrous condition and placed in the crucible the iron cap is filled with CaO and screwed on. The tube is placed in a furnace heated to 650-750 °C. The reaction starts suddenly when the temperature inside the tube reaches 400 C. The yield of Ce metal is 93%. The reaction may be carried out on a larger scale, but due to smaller relative heat losses, only 0.5 mole of I3 per mole of CeCla and a 10% excess of Ca are needed. The use of sulfur or KCIO3 lowers the yield. The resultant Ce metal contains 1-5% Ca and 0.1-1% Mg. 

Medium frequency (250 Hz) furnaces have a higher power density (up to 1000 kW/tonne) than mains frequency (50 Hz) furnaces (300 kW/tonne). This allows the use of a smaller crucible (up to a factor of three smaller) which results in a smaller total heat loss. The thermal efficiency of medium frequency furnaces is 10 % higher than for the mains frequency types. Additionally, mains frequency units need to be operated with a molten heel of up to 2/3 of the crucible capacity to optimise specific energy consumption and also require specific starter-blocks for cold start-up. Medium frequency furnaces can readily be started with a cold charge and can be emptied at the end of each working shift or melting batch. 

Traditional Vanyukov furnace is designed to have a flat crucible bottom, while that of oxygen side blow furnace changes to a ladder-like model, which reduced the heat loss and is beneficial for steady production. 

When iodic acid is heated to I80°C, it loses water leaving I2OS. Weigh out accurately 0.1 g of your prepared iodic acid into a crucible. Heat gently on a small flame and then in an oven at 180 C to constant weight. Calculate fW>m the mass loss, the formula of the solid produced. 

Liner, pocket (e-beam evaporation) A crucible-Uke container that is sometimes used in the pocket of the e-beam evaporation hearth to lower the conductive heat loss from the melt and to allow easy removal of the charge from the hearth. 

Figure 11.3 A simplified schematic sketch of a typical effusion cell. Omitted are the top cap that protects the heater windings from the evaporant as well as improving the black body environment of the crucible, the support clamps and details of the power feedthroughs, etc. The thermocouple junction is pressed gently against the base of the crucible but the black body environment assures an accurate temperature measurement. Note that the heater coils are more closely spaced near the front of the crucible to compensate for greater heat loss in that portion of the cell.

To determine the exact Si02 content of the residue, moisten it with 1 mL water, add two or three drops of concentrated sulphuric acid and about 5 mL of the purest available hydrofluoric acid. (CARE ) Place the crucible in an air bath (Section 3.21) and evaporate the hydrofluoric acid in a fume cupboard (hood) with a small flame until the acid is completely expelled the liquid should not be boiled. (The crucible may also be directly heated with a small non-luminous flame.) Then increase the heat to volatilise the sulphuric acid, and finally heat with a Meker-type burner for 15 minutes. Allow to cool in a desiccator and weigh. Re-heat to constant weight. The loss in weight represents the weight of the silica (Note 2). 

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