Bridgman furnace

Figure 12. Axial segregation data for the growth of gallium-doped germanium in a vertical Bridgman furnace. The figure is taken from Wang (6).

Figure 34.25. Sketch of a Bridgman furnace which consists of a hot and a cold zone intenaipted by an adiabatic...

Aerogels thus allow a perfect control of the solidification process and an analysis of the intensity curves allows to extract further information, like the solid fraction or the eutectic fraction [31]. The photo of such a facility shown also in Figure 34.26, shows an additional feature easily build in close to the sample three pairs of Helmholtz coils are inserted, allowing to induce rotating magnetic fields and thus to stimulate fluid flow in the samples for mixing [32]. With conventional Bridgman furnaces this is only possible with... 

Bridgman furnace A furnace with at least three zones being at different temperamres. A hot zone and a cold zone are separated by an adiabatic zone. Directional solidification or crystal growth is achieved by either pulling the sample through the furnace from hot to cold or by moving the furnace... 

CeNi2Gc2 and CeCu2Si2 have also been crystallised from stoichiometric melts in BN crucibles in a Bridgman furnace slowly cooled at 8°h with a temperature gradient of 50°C maintained down the crucible (Kletowski 1983). 

Fig. 15.22 (a) Schematic illustration of the Bridgman furnace. T1 and T2 denote the temperatures at the crucible bottom and crucible wall, respectively, (b) Temperature distribution profile near the melt/solid interface. 

Related methods include the Bridgman and Stockbarger methods where a temperature gradient is maintained across a melt so that crystallization starts at the cooler end this can either be achieved using a furnace with a temperature gradient or by pulling the sample through a furnace. 

Analysis of Heat Transfer. In the vertical Bridgman-Stockbarger system shown in Figure la, the axial temperature gradient needed to induce solidification is created by separating hot and cold zones with a diabatic zone in which radial heat flow from the ampoule to the furnace is suppressed. Analyses of conductive heat transfer have focused on this geometry. 

A DTA apparatus that could be used up to a pressure of 4000 bar and temperatures to 500°C was described by Kuballa and Schneider (75). This apparatus, which is illustrated in Figure 6.22, contained a DTA cell that was constructed of stainless steel a maximum pressure of 4000 bar and a maximum temperature of 500CC were possible. The cell was enclosed by two Bridgman pistons which contained the thermocouples. Cell wells for the sample and reference materials were made from a 97% platinum-3% iridium alloy they were suspended on the sheathed thermocouple junctions. Upper and lower sides of the sample-holder area were thermally heated by Zr02 blocks. A furnace placed around the pressure vessel permitted linear heating rates from 0.2 to 10°C/min. 

Figure 5 Schematic diagram of a Bridgman two-zone furnace used for melt growths of single-crystal GaAs...

The growing of metal single crystals can be carried out via several methods. Tammann and Bridgman [12] have devised an apparatus for slow solidification of metallic melts. A tube filled with the melt is lowered slowly and at a uniform rate (e.g., by means of a clock mechanism) through a vertical, electrically heated tubular furnace. In order to force the crystallization process to occur at a fixed place and from only one crystallization center, the bottom of the tube is drawn out to a capillary point (Schubnikow Straumanis Cl2]. 

The Bridgman-Stockbarger technique is illustrated in Figure 29.8. The powdered charge is melted in a crucible, which is often a refractory metal Pt, Ir, or Mo. This technique necessitates that the melt does not significantly react with the crucible. The furnace is designed such that there is a sharp drop in temperature just below the bottom tip of the crucible in its initial position. 

An example of the crucible method is that due to Bridgman and Stockbarger (see Figure 7.20d). The melt is contained in a crucible with a conical bottom which is lowered slowly from a hot to a cold zone. The two sections of the furnace, which are kept at about 50 °C above and below the melting point of the substance, respectively, are isolated by a thermal shield. The bulk of the solidified material, apart from the tip of the cone where the nuclei gather, is a single crystal. This method has proved successful for many semiconductor materials and alkali halides. 

Figure 7.20. a) The Bridgman-Stockbarger technique , (b) the Verneuil boule furnace... 

Directional solidification of metals and alloys is often performed with Bridgman type furnaces shown schematically in Figure 34.23. Such a furnace consists of a hot and a cold zone separated by an adiabatic zone. The temperature profile in a sample has a typical S-shaped profile (see Figure 34.25, right). The sample, being in a cartridge, is withdrawn with a fixed speed fi om the hot into the cold zone. 

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