The Effect of the Crucible on the Temperature Distribution for the Growth of a Large Size AlN Single Crystal

The appropriate distribution of temperature in the growth system is critical for obtaining a large size high quality aluminum nitride (AlN) single crystal by the physical vapor transport (PVT) method. As the crystal size increases, the influence of the crucible on the temperature distribution inside the growth chamber becomes greater. In order to optimize the field of temperature and study the specific effects of various parts of the crucible on the large size AlN single crystal growth system, this study carried out a series of numerical simulations of the temperature field of two crucibles of different materials and put forward the concept of a composite crucible, which combines different materials in the crucible parts. Four composite crucible models were established with different proportions and positions of tantalum carbide (TaC) parts and graphite parts in the crucible. Calculations reveal that different parts of the crucible have different effects on the internal temperature distribution. The axial temperature gradient at the crystal was mainly governed by the crucible wall, whereas the temperature gradient was determined by the integrated effect of the crucible lid and the crucible wall in the radial direction. One type of composite crucible was chosen to minimize the thermal stress in grown AlN crystal, which is applicable to the growth of large sized AlN crystals in the future; it can also be used to grow AlN single crystals at present as well.

Control of Oxygen Impurities in a Continuous-Feeding Czochralski-Silicon Crystal Growth by the Double-Crucible Method

The continuous-feeding Czochralski method is a cost-effective method to grow single silicon crystals. An inner crucible is used to prevent the un-melted silicon feedstock from transferring to the melt-crystal interface in this method. A series of global simulations were carried out to investigate the impact of the inner crucible on the oxygen impurity distributions at the melt-crystal interface. The results indicate that, the inner crucible plays a more important role in affecting the O concentration at the melt-crystal interface than the outer crucible. It can prevent the oxygen impurities from being transported from the outer crucible wall effectively. Meanwhile, it also introduces as a new source of oxygen impurity in the melt, likely resulting in a high oxygen concentration zone under the melt-crystal interface. We proposed to enlarge the inner crucible diameter so that the oxygen concentration at the melt-crystal interface can be controlled at low levels.

WAYS TO IMPROVE INDUCTION CRUCIBLE FURNARES

Analysis of the main drawbacks caused by increased walls thickness of a lined crucible, presence of tubular copper single-layer inductor cooled from inside with standard water and absence or presence of core I-shaped magnetic circuits arranged around it forming a discrete ferromagnetic screen, was made for modern induction crucible furnaces. The first drawback is that a significant part of working electromagnetic flow Fwork is not used for effective heating, since it passes along the non-conductive lining of crucible, and not along the cage. Therefore, only 38.5 – 57.0 % of the flow Fwork is effectively used. The second drawback is increased cost and complexity of manufacturing of inductor coils from a special copper tube, which vibrate at twice the frequency, creating noise and weakening design of the furnace. Such inductors are characterized by reduced electrical efficiency and increased cost of preparation and cooling of conditioned water in systems that occupy an area several times greater than the area of furnace itself. The third drawback leads to the fact that a significant part of electromagnetic scattering flow of the Fconsupt does not participate in heating of charge and melt, but heats conductive elements of furnace, including surrounding magnetic inductor. Irrational use of total flow F, created by inductor, reduces its efficiency to almost 19 – 30 %, and the power factor cosφ to 0.03 – 0.10 and increases energy consumption. To reduce or eliminate disadvantages, three ways of improving these furnaces are proposed and justified: reducing thickness of crucible wall with its simultaneous hardening by installing a cylindrical shell between the crucible and the inductor, surrounding the inductor with an annular magnetic circuit and using a single or multiwire inductor instead of a tubular one. Combination of cylindrical shell, annular magnetic circuit, as well as the upper and lower plates of the furnace frame can form an annular closed cavity to accommodate wire inductor and circulating refrigerant, cooling the inductor and the magnetic circuit. As a result of the study, new design of induction crucible furnace with wire inductor and ring-type magnetic circuit developed at AltSTU is proposed, substantiated and patented. Based on experimental determination of effectiveness of the proposed structural elements, conclusion is made about the prospects for further research.

Effect of Stirring Speed and Flow Pattern on the Microstructure of a Rheocast Al-Mg Alloy

The effect of stirring speed and the flow pattern inside a processing crucible were examined in this study. This investigation was performed on the microstructure of mechanically stirred Al–8% Mg. Results reveal that by increasing the stirring speed, morphology of primary particles becomes finer and more spherical. By using baffles, the flow pattern inside the crucible changes. This change causes the melt to accelerate along the crucible wall and producing the finer structure, more spherical and nondendritic. the model, named as nucleation and separation from the wall, posses the ability to explain the microstructure evolution in semi solid fabricated slurry. It must be mentioned that the other model named “fragmentation-agglomeration’’ cannot explain the changes in microstructure by using baffles.

Thermal Profiles and Fraction Solid of Aluminium 7075 at Different Cooling Rate Conditions

In order to determine suitable processing conditions for semi-solid aluminium 7075 thermal analysis (TA) was performed in order to obtain the relationship between fraction solid and temperature. During experimental work, the alloy was heated to 750°C by induction furnace and solidified at various cooling rates. Cooling curves for the metal were recorded with two thermocouples, one at the centre of the melt volume and one beside the containing crucible wall. A specially designed chamber with kaowool blanket was used to achieve the slowest cooling rate. The faster cooling rate was achieved with the crucible in open atmosphere with a set air flow rate over the crucible. A Data Acquisition (DAQ) system controlled by LabVIEW software was used to record the temperature-time profiles. From these cooling curves, the phase change at any corresponding time and temperature was estimated. The temperature difference between centre and wall of crucible was used to determine dendritic coherency point (DCP). Results show that, the slowest cooling rate with the kaowool blanket was at 0.03°C/s. An intermediate cooling rate of 0.21°C/s was achieved by leaving the melt to cool without kaowool blanket or forced air flow, and the fastest cooling rate was 0.43°C/s. The change in cooling rate altered the temperatures at which phase changes occurred, including those for eutectic and solidus. It was found that for lower the cooling rates that the DCP occurred at lower temperatures. The DCP for the cooling rate of 0.03 °C/s was found to be 574°C (corresponding to 0.85 fraction solid) whereas the DCP for 0.43 °C/s was found to be 623°C (corresponding to 0.55 fraction solid).