Crucible Wall Research Articles

Solid phase formation during aluminium electrolysis

The paper illustrates the results of experimental study on the formation of the solid phase on the graphite crucible wall in the cryolite-alumina electrolyte depending on the temperature, rates of the liquid phase motion and heat flow in the laboratory electrolytic cell. The side ledge stability is shown to depend on the rate of the heat flow, which is caused by changes in the temperatures of the electrolyte, electrolytic cell walls and liquidus. When the temperature of the inside wall is higher than the electrolyte liquidus temperature the side ledge does not form, but in the case when it is lower than the liquidus temperature the side ledge formation proceeds until the temperatures equalize. As the electrolyte motion rate increases the thickness of the bottom ledge exceeds the thickness of the side ledge. The changes in the heat resistance of the electrolytic cell walls have a significant influence on the crust growth over the electrolyte and have less influence on the thickness of the bottom and side ledges.

Control of Melt Flow and Oxygen Distribution Using Traveling Magnetic Field during Directional Solidification of Silicon Ingots

Traveling magnetic field (TMF) is a potential method to control the melt flow and impurity transport during the directional solidification (DS) of silicon ingots. We numerically study the control mechanism of a downward and an upward TMF with different frequencies on the silicon melt flow and oxygen distribution in the DS process. Model experiment of generation and measurement of a TMF is first carried out to validate the magnetic field computing program. Then, the TMF frequency is varied to study its influence on the direction and magnitude of Lorentz force, pattern and intensity of silicon melt flow, and distribution of oxygen impurity. Results show that the downward TMF with large frequency can induce local melt flow above the melt-crystal interface and block the oxygen transport from the dominant flow to the interface. The upward TMF with small frequency and large amplitude can induce thoroughly upward melt flow along the crucible side wall and take the oxygen far away from the melt-crystal interface, which is better for reducing the oxygen content in the silicon melt and ingots.

Application of artificial neural network to optimize sensor positions for accurate monitoring: an example with thermocouples in a crystal growth furnace

We propose to utilize artificial neural network (ANN) to optimize positions of a limited number of sensors for accurate monitoring, and demonstrate its effectiveness by a case study of four thermocouples in a directional solidification furnace. Our concept consists of choosing the positions with ANN that has the lowest loss from a multiplicity of ANNs, which were trained by the simulated temperature distributions along the outer crucible wall. Interestingly, the top ten ranks of accurate predictions contain positions around the crucible’s bottom to suggest the importance of measuring temperatures carefully around high-temperature gradients that is the boundary between different materials.

Influence of submerged heating on vertical Bridgman crystal growth of silicon under travelling magnetic field

The effect of a submerged heater on the Bridgman crystal growth process under travelling magnetic field is studied. It is known that the magnetic field generates melt flow from crystal center to crucible wall which can suppress morphological instability development, but it also increases the curvature of the crystallization front. The advantage of the submerged heater method is the ability of better control over the temperature distribution in the vicinity of the solid-liquid interface, as well as the constancy of the growth zone geometry, in particular the height of the melt above the phase boundary, which helps reduce the axial heterogeneity of the resulting crystal. Within the steady state approach we perform numerical simulation of Bridgman growth process under simultaneous action of travelling magnetic field and a submerged heater. It is shown that the submerged heater can intensify the flow in the radial direction generated by travelling magnetic field near the growing crystal while the curvature of the crystallization interface remains almost unaffected. On the other hand, it is also possible to decrease significantly the flow close to the crystallization front.

The influence of Prandtl numbers of melts and crucible materials on the features of crystal growth by the Bridgman method

The processes of crystallization of silicon and heptadecan in flat-bottomed cylindrical crucibles in conjugate convective heat transfer regime were studied numerically by the finite element method. Crystallization of silicon was investigated in a graphite crucible. Crystallization of heptadecane was investigated in a Plexiglas crucible. The possibility of existence of two convective vortices over a solidification front during the crystallization of silicon and heptadecane has been discovered. Crystallization of heptadecane was studied at two rates of lowering the crucible into the cold zone. Lowering of the crucible was simulated by moving the breakpoint in the temperature distribution on the outer side of the crucible walls. The breakpoint was the boundary of transition from the wall area heated to the initial temperature to the area with a given temperature gradient. Adaptive grids on triangles tracking the position of the crystallization front at each time step were used. The software package of developed by the authors was used.

Optimizing oxygen impurities using different heater design in the directional solidification of multi-crystalline silicon

The numerical simulations of heat and mass transport have been carried out. The heater design has been modified. The melt convection and oxygen impurities have been analysed. The simulation has been made for conventional and modified heater design using directional solidification (DS) furnace. The results were compared and the better heater design was used to control the oxygen impurities in the mc-Silicon ingot. The reduced melt convection is obtained in the silicon melt due to the modified heater design. The modified heater design reduces the oxygen impurities in the melt and crystal. The modified heater design improves the melt-crystal (m-c) interface. It gives less concave interface shape in the periphery region near the crucible wall. The m-c interface is mainly affected by flow pattern and melt convection. The numerical results are very useful for better understanding of thermal effect and melt convection in the directional solidification system for reduction of oxygen impurities in the mc-Silicon ingots.

Glass Formation in the PbO–GeO2 System

The glass-forming region in the PbO–GeO2 system is studied. To increase the glass-forming limit in this system, the tempering rate of the samples has to be increased using special methodological practices. The dependence of the temperature of the synthesis of lead germanate glass on the PbO content is obtained. It is shown that the corrosion of alumina crucibles proceeded during the synthesis of lead germanate glass. The dependence of the thickness of the corroded layer of the alumina crucible wall on the time of the synthesis of the glass of the 55PbO · 45GeO2 composition, mol % at 900°С is obtained. It is proved that the obtained glass is X-ray amorphous within the whole range of compositions in the PbO–GeO2 system.

Simulation to confirm the existence of distinct low-temperature regions in a Si melt using an insulating plate under the crucible bottom for the noncontact crucible method

The noncontact crucible (NOC) method has a large and deep low-temperature region in the upper central part of a Si melt to allow natural crystal growth inside it. Thanks to the low-temperature region, the NOC method can grow a uniform large Si single ingot without contact with the crucible wall. Using simulation of the temperature distribution in the Si melt, the establishment of the low-temperature region is clearly shown on basis of controlling the heat flow from the bottom heater into the Si melt using an insulating plate locally set under the crucible bottom. The heat from the bottom heater can be effectively weaken especially for the heat flow from the crucible center, resulting in the quite large low-temperature region in the melt. The effectiveness of such heat-flow control was proved by the growth of several Si single ingots using the NOC method. The effects of the heat power balance and the thickness of the insulating plate are also shown by the simulation. The temperature gradient in the NOC ingot is clearly smaller than that in an ingot grown by the Czochralski (CZ) growth mode.

Effect of Quartzite Heat Treatment on Induction Furnace Lining Failure Mechanism

The wear mechanism of a quartzite lining of induction crucible furnaces during iron and steel smelting is presented. It is established that the main cause of lining wear is a decrease in its strength due to transition of the tridymite joints into cristobalite, which is more readily washed off crucible walls by moving metal and slag. The second important factor of wear is the choice of quartzite raw material, its substance and grain size compositions, firing regime, and melting technology during furnace operation. Silicon reduction from silica of the carburizer lining and within the cast iron composition is a subsidiary factor of crucible lining wear.

Improving the compact crucible furnace performance by adding fins in heating chamber

A crucible furnace is developed continuously to obtain appropriate furnace in support casting practice at VHS. This research aimed at improving performance of compact crucible furnace by modification the heating chamber. Fins were placed at inner side of compact crucible furnace wall. The furnace performance determined by increasing of temperature rate to melt 3 kg of aluminium. Descriptive analysis is used to describe performance of modified compact crucible furnace. The fins addition on the inside wall improves the compact crucible furnace performance. The temperature rate is increase as the fins inclination angle increase. Fins with inclination angle of 800 could keep and maintain the temperature of heating chamber. Compact crucible furnace which added fin on the inside wall is able to melt 3 kg of aluminium in 35 minutes.

Influence of a Magnetic Field on Structure Formation during the Crystallization and Physicomechanical Properties of Aluminum Alloys

The results of studying the structure and mechanical properties of A356.0 and A413.1 cast aluminum alloy subjected to a pulsed magnetic field of different strength during crystallization are presented. It is established during the experiments that the samples contain two phases each in their compositions which crystallize in definite temperature ranges and remain invariable even with the imposition of the magnetic field of the crystallizing melt. The temperature gradient between the crystallizer wall and outer crucible wall for both alloys, which varies from 14.3 to 16.0°C/mm, and the crystallization time of each phase are determined. The linear crystallization rate of both alloys is found using thermal approaches. It is shown that this rate decreases with a decrease in the temperature gradient, and the crystallization time of phases herewith increases. It is revealed that the magnetic field varies the distribution of dendrites in the bulk of A356.0 and A413.1 alloys, as well as their sizes and orientation in the metallographic specimen plane. A finer structure is formed in the alloy α phase with an increase in the magnetic-field induction amplitude. It homogeneously fills the metallographic specimen plane, which is reflected on the alloy mechanical properties. The hardness of alloys under study increases with an increase in the induction amplitude of the pulsed magnetic field for both alloys by 8–10% due to refining the dendritic structure and the more uniform distribution of dendrites of the α solid solution over the bulk of the crystallizing ingot. In addition, the magnetic field affects the ultimate tensile strength and almost does not vary the relative elongation during the uniaxial tension of the samples of A356.0 and A413.1 cast aluminum alloys.

Oxygen and Carbon Distribution in 80Kg Multicrystalline Silicon Ingot

Multicrystalline silicon (mc-Si) wafers produced by directional solidification still dominate the world market, due to the factor quality/price. The performance of solar cell depends directly to the quality of wafer and impurities distribution in mc-Si ingot. In our study we investigate the distribution of the interstitial oxygen (Oi) and substitutional carbon (Cs), from the bottom to top of the silicon ingot. During the solidification process the solid-liquid interface moves upward with an average growth velocity of 1.2 cm/h, with a slightly convex form. The determination of (Oi) and (Cs) concentrations were performed thanks to the Fourier Transform Infrared Spectrometry (FTIR) technique. The results show that oxygen concentration increases near the crucible wall to the maximum value of 6.3 × 1017 atoms/cm3, and the carbon concentration decrease from maximum value of 9.59 × 1017 atoms/cm3 in the top to the minimal value of 7.84 × 1017 atoms/cm3 in the bottom of ingot. The concentration of global carbon and oxygen in the centre and corner bricks was investigated using the Secondary Ion Mass Spectroscopy (SIMS) technique. The concentration of oxygen and carbon in the center bricks were 1.8 1018 and 2 1018 atoms/cm3, and in the corner bricks 4.6 × 1019 and 9 × 1019 atoms/cm3, respectively. These results provide quantitative information on the concentration of the light impurities in the as-grown mc-Si and allow an overview of their spatial distribution within the final ingot.

Air-isolated stir casting of homogeneous Al-SiC composite with no air entrapment and Al4C3

An air-isolated stir casting (AISC) technology was developed to solve the problems of air entrapment, inhomogeneous distribution of SiC particles and formation of weak compound Al4C3 in conventional stir casting of Al-SiC composite. To eliminate the air entrapment, a threaded cover screwed into the threaded crucible was used to evacuate the air in the crucible and a sealing plug on the cover was applied to isolate the Al-SiC slurry from the atmosphere. To realize the homogeneous distribution of SiC particles rapidly, a high-speed electromagnetic stirring was carried out to stir the Al-SiC slurry with the help of three uniform-distributed tilt blades on the crucible inner wall. Furthermore, the primary solid Al particles and the high viscosity of slurry were utilized to restrain the settling of SiC particles during casting. To avoid the formation of Al4C3 under short stirring time, the stirring temperature was dropped below 650 ℃. The theoretical analysis and the results of SEM, XRD, TEM and HRTEM show that the homogeneous Al-SiC composite with no air entrapment and Al4C3 can be processed by AISC technology. The mechanical and wear results and mechanism analysis show that the AISC Al-SiC composite possesses excellent performances.

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.

The Effect of Crucible Rotation and Crucible Size in Top‐Seeded Solution Growth of Single‐Crystal Silicon Carbide

AbstractThe top‐seeded solution growth method is a promising technique for growing high‐quality silicon carbide single crystal. Some inherent issues in this growth process, such as morphological instability, polycrystalline growth, and low growth rate, should be clarified. A high temperature difference between the seed and the crucible wall in this system is needed to enhance growth. However, such a high temperature gradient makes the radial growth rate profile non‐uniform due to the effect of Marangoni convection below the seed crystal, which leads to poor crystal quality. In the present work, the effects of crucible size and crucible rotation are numerically investigated to minimize the effect of Marangoni convection. The possibilities of the occurrence of growth‐rate non‐uniformity and undesired impurity incorporation are examined. A smaller crucible (in radius) leads to a more uniform growth rate profile. However, it gives rise to a higher possibility of impurity incorporation. It is also predicted that crucible rotation is ineffective in suppressing the Marangoni flow near the seed edge. This leads to a flow stagnation in the center of the melt, and consequently, it does not enhance the carbon transport below the seed. It also does not reduce the possibility of undesired impurity incorporation.

Synthesis of AlN nanopowder coated with a thin layer of C, using a thermal plasma reactor

High-purity nanocrystalline aluminum nitride powders were synthesized by using a 12 kW non-transferred arc plasma. The synthesis was conducted in a versatile, new designed, one-chamber thermal plasma reactor (TPR). The novel experimental assembly incorporated better working conditions like: high temperature gradient between the crucible and reactor's wall, and high super-saturation of the system by nitrogen and carbon. Thermodynamic modelling of the synthesis was conducted in order to achieve the best conditions for AlN formation. In this study, aluminum discs of Al 1100 were used as precursor material and pure nitrogen was the only gas used as reagent and plasmogenic gas. Nanopowders collected from reactor's wall were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-Ray diffraction (XRD). Synthesized h-AlN nano-powders were found to be free of oxides and aluminum metal. A thin carbon-layer around the particles was detected. TEM results indicated that the carbon-layer was around 5 and 10 nm. This outcome could make a significant difference with other synthesis reported in the literature since the occurrence of the carbon-layer, could delay AlN oxidation, prevent hydration, and could avoid the agglomeration of the particles.

The dependence of the crystallization front shape on the heat exchange regime in the Bridgman-Stockbarger method

The growth of a silicon ingot in the Bridgman-Stockbarger crystal-melt-crucible system, similar to that used in real technology is studied numerically in the axisymmetric formulation by the finite element method. The simulation was carried out taking into account the heat of phase transition under conditions of unsteady thermal conductivity and natural convection at the initial heating of the melt up to 40 K and two rates of crucible lowering. The temperature gradient along the lower part of the side wall of the crucible is linear and equal to 35 K/cm or 70 K/cm. A comparison between the crystallization processes in the unsteady heat conduction regime and the natural convection regime is carried out. The dependence of the shape of the crystallization front on the heat transfer modes is investigated. It has been shown that in all heat conduction modes being considered, the crystallization front has a convex shape during the entire crystallization process. Moreover, the shape of the crystallization front in the examined range of parameters weakly depends on the crucible lowering rate, but is drastically influenced by the temperature gradient on the crucible walls. Under conditions of hermogravitational convection the shape of the crystallization front strongly depends on the crucible lowering rate and temperature gradient on the side walls of the crucible. At some parameters, a secondary convective vortex can be generated over the crystal during the crystallization process. Depending on the combinations of temperature gradient on the side walls and the lowering rate of the crucible, the shape of the crystallization front can be either concave or convex.

Numerical simulation and experimental verification of electromagnetic field of continuous casting copper crucible

The electromagnetic field of continuous casting square copper crucible was simulated by numerical simulation using Ansoft Maxwell in order to clarify the influence of current frequency and intensity on the distribution of magnetic flux density in the crucible. The simulation results showed that as the current frequency increases or current intensity decreases, the distribution of the magnetic flux density in each vertical position of the crucible hardly changed, but the value of the magnetic flux density was gradually reduced, and the magnitude of the decrease was decremented from the maximum position of the magnetic flux density to the upper and lower ends. The maximum value of the magnetic flux density in different axial directions was proportional to the current intensity. In the radial direction, the magnetic flux density of the surface of the crucible was large, and gradually decreased toward the inner layer. The magnetic field at the slit was higher than the middle portion of the ridge due to the reinforcing effect at the slit. The small coil method was used to measure the actual electromagnetic field distribution in the crucible. It can be found that the magnetic flux density in the crucible decreased from the crucible wall to the crucible center, which is consistent with the results of the numerical simulation.

Upwards migration phenomenon on molten lunar regolith: New challenges and prospects for ISRU

As part of our research on the feasibility of producing commodities from lunar regolith by thermal-driven processes with minimal terrestrial precursors we need to characterize, reproduce, and understand thermophysical properties of the molten regolith still unforeseen under the lunar vacuum conditions at a scalable sample size. Two unanticipated phenomena, apparently caused by lunar melt’s surface tension under vacuum, have been revealed in our research work, vacuum void formation and upwards migration. In this paper we present our findings and thinkable explanation on the upwards migration phenomenon experimentally observed and consistently replicated as JSC-1A lunar regolith simulant melted at high vacuum. Upwards migration of molten lunar regolith will make future lunar ISRU’s melting processes both challenging as molten bulk material would migrate upwards along the container’s walls, and also promising on new opportunities for alternative ISRU’s sustainable processes as regolith’s upwards migration takes place in uniformed thin-film pattern. Among the potential ISRU’s processes that might use controlled thermal thin-film-based migration without the necessity of terrestrial precursors are production of feedstock for 3D printing, fractional separation of regolith’s component’s (O2, metals, and alloys) via pyrolysis, film coating, purification of valuables solid crystals including silicon, and fabrication of key elements for microfluidic, and MEMS devices. Thermal upwards migration phenomenon on JSC-1A’s melt is formulated and explained by the authors as due to thermal Marangoni effect (also known as thermo-capillarity) in which temperature gradients within the melt’s bulk and along the crucible’s wall yield the surface tension large enough to supersede the gravitational force and yield the experimentally observed upwards thin-film migration. As far as the authors know, upwards thermal migration of molten JSC-1A (or other lunar simulant regolith) under vacuum has not been reported in the literature. A thermal mathematical model accounting for thermal Marangoni effect on molten JSC-1A agrees with what experimentally was observed, the formation of the meniscus on the melt-wall surface interface along with an incipient upwards migration in thin-film pattern along the crucible wall that, according to the model, experiences large temperature gradient, an important factor to trigger the thermal Marangoni effect along with the fact that surface tension of the molten lunar regolith material is temperature dependent.

Petrological Study of Ferrous Burden-Crucible Interaction in Softening & Melting Experiments: Implications for the Relevance of Pressure Drop Measurements

Numerous tests are used worldwide to investigate the advanced high temperature reduction, softening, and melting (S&M) of blast furnace ferrous burden material. Commonly, the curve of pressure drop (dP) against temperature, measured over the experimental charge, is taken as directly indicative of the evolution of ferrous layer permeability. Previous authors have expressed concerns about the reproducibility and practical relevance of dP measurements due to narrow crucible diameter and wall effects. Petrological study of samples from interrupted Advanced Softening and Melting (ASAM) experiments, performed with ferrous burdens comprising a mixture of pellets (C/S 0.1–0.7) and highly basic sinter (C/S 2.4–3.5), sheds light on the nature and controlling mechanisms of systematic artefacts influencing the measurement of dP. The observations imply that the dP-T curves in ASAM, and likely other similar S&M, tests most immediately reflect the varying ease of gas flow in a sidewall bypass around the qualitatively impermeable ferrous burden layer, rather than through it. This is controlled by the formation of diverse oxide(-slag)-metal segregations at the exterior of the ferrous burden layer. The potential correlation of parameters derived from dP measurements with input burden characteristics is not in itself sufficient evidence that the measurements are indicative of the ferrous layer permeability.