Directional Solidification Furnace Research Articles

Improving Heat Transfer Properties of DS furnace by the Geometrical Modifications for Enhancing the Multi Crystalline Silicon Ingot (mc-Si) Quality Using Transient Simulation

Heat transfer plays a main role on Directional Solidification (DS) process that determines the multi-crystalline silicon (mc-Si) ingot quality. The 2D axi-symmetric model based numerical simulations have been carried out to analyze the heat transfer properties of the DS furnace and to study the thermal effects on mc-Si ingot during the solidification process. For enhancing the heat transfer properties of the DS system and the quality of the mc-Si ingot, the conventional DS furnace was subjected to three types of modifications. The simulation has been made for conventional and modified furnaces and their results were compared and analyzed which confirms that the geometrical modification has influenced the thermal distribution, melt-crystal interface, growth rate, thermal stress and dislocation density of the mc-Si ingot. The following modifications have been taken in conventional DS furnace in sequence: a) For lowering the axial and radial temperature distributions, the bottom insulation was modified (DS-1). b) For melt-crystal interface optimization, the heat exchanger block is grooved in the center part (DS-2). c) Finally, the grooved surface has been subjected to Ar gas flow (DS-3). The evaluations are performed on conventional, DS-1, DS-2 and DS-3 furnaces and their results such as axial-radial temperature distributions, melt-crystal interface, growth rate and thermal stress and dislocation densities are compared and analyzed. The results revealed that the modified furnaces give lower thermal stress and dislocation densities than the conventional furnace. The lower thermal stress with lower dislocation density at the end of the solidification process can prevent the multiplication of dislocations during the cooling process that lead to a good quality mc-Si ingot.

Parameter study of traveling magnetic field for control of melt convection in directional solidification of crystalline silicon ingots

Melt convection plays a critical role in the quality of crystalline silicon ingots produced by directional solidification (DS), through influencing the solidification front shape and impurities distribution. The utilization of a traveling magnetic field (TMF) is a promising way to control melt convection pattern. Lorentz forces induced by a TMF will present various characteristics under different imposed electric current parameters, such as current amplitude, frequency and phase shift. Understanding the effects of current parameters on melt convection pattern is crucial for the selection of suitable parameters to achieve the enhancement of melt mixing and a preferable solidification front. Based on a coupled model of global heat transfer, this paper explores the effects of these current parameters on the melt convection pattern in a DS furnace of crystalline silicon ingots under the action of a downward TMF. The melt mixing effect and solidification front shape are further elucidated based on different melt convection patterns. The results indicate that the effects of current parameters on the melt mixing effect and solidification front shape is quite complicated, which is not monotonous. By precisely tailoring the combination of current parameters, a flat or slightly convex solidification front and decent melt mixing are simultaneously obtained. This study can facilitate the successful and optimum utilization of a TMF in the DS process of crystalline silicon ingots.

An efficient method to separate silicon from high-silicon aluminum alloy melts by electromagnetic directional solidification

Abstract In electromagnetic directional solidification, the silicon phase cannot always be completely separated, resulting in considerable waste of power and silicon. This study investigated the electromagnetic separation of silicon by using electromagnetic induction-heated directional solidification furnaces at varying frequencies. Two frequencies were applied to separate silicon from aluminum–silicon melts. Numerical simulation results indicated that a low frequency (3 kHz) could substantially enhance the separation of silicon from aluminum–silicon melts under an alternating electromagnetic field, which could increase the speed of the melts to 0.92 cm/s. Experimental results showed that separation efficiency could exceed 85% at a pulling rate of 10 μm/s when a furnace at a frequency of 3 kHz was used. This method can potentially meet the requirements of manufacturing low-cost solar cells for industrial use.

Microstructural evolution and mechanical properties of Sn-Bi-Cu ternary eutectic alloy produced by directional solidification

Sn-36Bi-22Cu (wt.%) ternary eutectic alloy was prepared using vacuum melting furnace and casting furnace. The samples were directionally solidified upwards solidification rate varying from 8.3 to 166 µm/s and at a constant temperature gradient (4.2 K/mm) in a Bridgman-type directional solidification furnace. The composition analysis of the phases and the intermetallics (Cu3Sn and Cu6Sn5) were determined from EDX and XRD analysis respectively. The variation of the lamellar spacing (Bi-rich phase) and the Cu3Sn phase spacing with the solidification rate were investigated. The dependence of microhardness, ultimate compressive strength and compressive yield strength on solidification rate were determined. The spacing and microhardness were measured from both longitudinal and transverse sections of the samples. The dependence of microhardness on the lamellar spacing and the Cu3Sn phase spacing were also determined. The relationships between phase spacings, solidification rate and mechanical properties were determined from linear regression analysis.

Numerical Modelling on Modified Directional Solidification Process of Multi-crystalline Silicon Growth for Photovoltaic Applications

A transient global model was used to investigate the effect of bottom grooved furnace upon the directional solidification (DS) process of multi-crystalline silicon (mc-Si). The numerical simulation assumed geometry is perfect 2D axis-symmetry. The temperature distribution, crystal-melt (c-m) interface, thermal stress and dislocation density have been simulated. The modified heat exchanger block system was used for controlling the temperature gradient at the bottom of the crucible. The obtained result shows convex shape of the c-m interface. The von Mises stress and dislocation density were reduced while using the bottom grooved furnace. This work was carried out in the different grooves of radius 30 and 60 mm of the heat exchanger block of the DS furnace.

Investigation of the thermoelectrical properties of the Sn91.2−x–Zn8.8–Agx alloys

Sn91.2−x–Zn8.8–Agx alloys (x = 0.15–10.0 wt%) were directionally solidified upwards at a constant G (4.16 K mm−1) and V (41.5 μm s−1) in a Bridgman-type directional solidification furnace. The electrical resistivity (ρ) measurements of the alloys depending on the temperature were performed using the standard four-point probe method, and the temperature coefficients of the resistivities (α) were calculated. Composition analyses of the alloys were carried out using energy-dispersive X-ray spectroscopy. The enthalpy (∆H) and the specific heat (∆Cp) values of the alloys were determined by differential scanning calorimetry analysis. The thermal conductivity (K) values were obtained from the Wiedemann–Franz equation. According to the experimental results, electrical resistivities increased up to 3.0 mass% Ag and decreased with further increase in Ag content. Enthalpy and specific heat values decreased with the increasing content of Ag. The results were compared with the previous works for Sn–Zn–Ag alloys.

Effect of cellular recrystallization on tensile properties of a nickel-based single crystal superalloy containing Re and Ru

AbstractA nickel-based single crystal superalloy containing Re and Ru was cast in a directional solidification furnace. The single crystal specimens after standard heat treatment were grit blasted with different pressures and then heat treated at 1100 °C for 4 h under vacuum condition. The evolution of recrystallized microstructure and its effect on the tensile properties at 850 and 980 °C were investigated. After heat treatment, the cellular microstructure was observed, and the thickness of the cellular recrystallization zone increases with the increase in grit blasting pressure. The appearance of the cellular structure undermines the tensile properties. Both the tensile strength and elongation decrease with increasing the thickness of the cellular structure. The recrystallized grain boundaries can act as the channels for the crack initiation and propagation during tensile test. The low bearing capacity of recrystallized layers and the local stress concentration resulting from the notch effect of cracking were the main reasons for the decrease of tensile properties.

Simulation of directional solidification furnace with bottom opening insulation to grow quality mc-Si ingot for PV applications

Abstract2D axi-symmetric transient simulation based on the thermo-plastic model was carried out for analyzing the thermal field of directional solidification (DS) furnaces during the solidification process. The simulation was carried out for two different furnaces: One conventional DS furnace and the other a modified DS furnace. In the modified DS furnace the bottom insulation was provided with a circular shape opening. The simulation was performed for conventional and modified furnaces which suggest that the lifting velocity of the side insulation in the conventional DS furnace significantly affects the solidification process of mc-Si ingot and the opening speed of the bottom insulation in the modified DS furnace has less influence on solidification and also gives optimal conditions to grow good quality mc-Si ingots. The results for conventional and modified DS grown mc-Si ingots were taken with two different insulation lifting profiles and their influence on thermally induced stress, dislocation density and growth rate were compared and analyzed.

Influences of Growth Velocity and Fe Content on Microstructure, Microhardness and Tensile Properties of Directionally Solidified Al-1.9Mn-xFe Ternary Alloys

In this study, influences of growth velocity and composition (Fe content) on the microstructure (rod spacing) and mechanical properties (microhardness, ultimate tensile strength and fracture surface) of Al-Mn-Fe ternary alloys have been investigated. Al-1.9 Mn-xFe (x=0.5, 1.5 and 5 wt. %) were prepared using metals of 99.99% high purity in the vacuum atmosphere. At a constant temperature gradient (6.7 K/mm), these alloys were directionally solidified upwards under various growth velocities (8.3-978 µm/s) using a Bridgman-type directional solidification furnace. The results show that two kinds of Al-rich α-Al phase and Fe-rich intermetallic (Al6FeMn) phase may be present in the final microstructures of the alloys when the Fe content increases from 0.5 wt.% to 5 wt.%. Al6FeMn intermetallic rod spacing, microhardness and ultimate tensile strength were measured and expressed as functions of growth velocity and Fe content by using a linear regression analysis method. According to experimental results, the microhardness and ultimate tensile strength of the solidified samples increase with increase in the growth velocity and Fe content and decrease in rod spacing. The elongations of the alloys decrease gradually with increasing growth velocity and Fe content.

Microstructure and mechanical properties of Ni3Al intermetallics prepared by directional solidification electromagnetic cold crucible technique

The present work focused on the Ni3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25Al alloy. Ni3Al intermetallics were prepared at different withdrawal rates by directional solidification (DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni3Al and NiAl phases. The primary dendritic spacing (λ) decreases with the increasing of withdrawal rate (V), and the volume fraction of NiAl phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.

Rotating bending fatigue property of the Ni3Al-based single crystal superalloy IC6SX at 900°C

The high cycle fatigue behavior of a Ni3Al base single crystal superalloy IC6SX has been investigated at 900°C in this work. The specimens used for the fatigue tests were prepared by screw selection crystal method in a directional solidification furnace. The rotating bending fatigue tests were carried out at 900°Cin air, the stress ratio of R(σmax/σmin) was -1, and the rotating speed of the fatigue tests was 6500r/min(108Hz). The stress-fatigue cycle life (S-Nf) curve was obtained based on the fatigue tests, and the fracture surfaces were examined using scanning electron microscopy (SEM). It has been found that the median fatigue strength is 457.5MPa and the safety fatigue strength is 413.93MPa. And the fatigue fracture was composed of three different characteristic regions.

Orientation Characteristics of Single Crystal Superalloys with Different Preparation Methods

Single crystal superalloys have been prepared separately by a grain selector and a seeding technique in a high temperature gradient directional solidification furnace. The orientation characteristics were measured by XRD. Results indicate that for the grain selector method, the crystal solidification begins at the cooling plate with random nucleation, and through mutual competitive growth at the starter block. The grains enter the spiral selector, and finally the single crystal superalloys which are close to <001> direction are obtained. By the seeding technique, by epitaxial growth of partially melted seed crystals, single crystal superalloys which have the same orientation as the seed crystal are obtained.

Feeding of liquid silicon for high performance multicrystalline silicon with increased ingot height and homogenized resistivity

Feeding of liquid silicon during the directional solidification process is a promising opportunity for cost reduction by increased throughput and improved material homogeneity due to constant resistivity over ingot height. In this work, a liquid feeding apparatus was developed for an industrial type directional solidification furnace. One n-type G2 sized High Performance multicrystalline ingot with liquid feeding of additional 14kg of undoped silicon feedstock was crystallized. The resistivity was kept within a range of ±0.1Ωcm of the target resistivity during the feeding sequence. A smaller mean grain area growth was observed during feeding, whereas the area fraction of recombination active dislocation structures was as low as in a reference ingot. Increased interstitial oxygen and substitutional carbon concentrations were measured for the ingot with liquid feeding. The measured mean bulk lifetime of 190µs for passivated wafers in the feeding sequence can probably be increased by further pre-melting crucible improvements. For this laboratory experiment, energy reductions of 2% per wafer and time savings of 16% per wafer were realized.

Global simulation of coupled oxygen and carbon transport in an industrial directional solidification furnace for crystalline silicon ingots: Effect of crucible cover coating

For accurate prediction of oxygen (O) and carbon (C) distributions in the crystalline silicon ingots grown by the industrial directional solidification (DS) furnace, we first performed transient global simulations of heat transfer based on a fully coupled calculation of the thermal and flow fields. The phase change problem was handled by an enthalpy formulation technique based on a fixed-grid methodology. The coupled C and O transport in the DS furnace was then carried out, taking into account five chemical reactions. Special attention was devoted to modeling the O and C impurity segregation during the entire solidification process. It was found that the developed model can successfully simulate the impurity segregation at the growth interface and the obtained boundary layer thickness is similar to that estimated by analytical calculation. The effect of the crucible cover coating on the coupled O and C transport was investigated. The numerical results show that the C concentration in the grown silicon ingot can be reduced by about 60% if the pure graphite cover was replaced by a graphite cover with an inert material coating on it. However, the cover coating did not significantly affect the O concentration in the grown ingot. The numerical predictions of the C concentration showed satisfactory agreement with the experimental measurements.

Hardness and electrical resistivity of Al–13 wt % Mg2Si pseudoeutectic alloy

In the present work, effect of growth rates on microhardness, electrical properties and microstructure for directionally solidified Al–13 wt % Mg2Si pseudoeutectic alloy at a constant temperature gradient were studied. Directional solidification process were carried out with five different growth rates (V = 8.33–175.0 μm/s) at a constant temperature gradient (G = 6.68 K/mm) by using a Bridgman type directional solidification furnace. Microstructure of directionally solidified Al–13 wt % Mg2Si pseudoeutectic alloy was observed as Mg2Si coral-like structure phase dispersed into primary α-Al phase matrix. The electrical resistivity for Al–13 wt % Mg2Si pseudoeutectic alloy, were measured by the d.c. four-point probe method. The dependency ofmicrohardness and electrical resistivity on growth rates were obtained as HV = 135.7 (V)0.09 and ρ = 17.30 × 10−8(V)0.08, respectively for Al–Mg2Si pseudoeutectic alloy. The results obtained in present work were compared with the previous similar experimental results.

Microstructures and Microsegregation of Directionally Solidified Mg-1.5Gd Magnesium Alloy with Different Growth Rates

Directional solidification of Mg-1.5Gd (wt%) magnesium alloy was carried out to investigate the effects of the growth rate on the microstructures under controlled solidification conditions. A Bridgman-type directional solidification furnace with a liquid metal cooling (LMC) technique was used to solidify the specimens, which could provide steady state conditions with a constant temperature gradient (40 K/mm) at a wide range of growth rate (10∼200 μm/s). Results show that the microstructures are cellular, and the relationship between cellular spacing (λ) and growth rate (V) is established in the form: λ = 130.2827V−0.2228 by a linear regression analysis, which is in good agreement with the calculated values by Trivedi model. The thermodynamics solidification path calculations by Scheil model and experimental observations confirm that the solidification microstructure in the alloy consists of primary α(Mg) phase and binary eutectic α(Mg)+Mg5Gd phase. Meanwhile, the microsegregation of the alloying element predicted by the Scheil model agrees reasonably with the electron probe microanalysis (EPMA) measurements.

Dependence of microstructure, microhardness, tensile strength and electrical resistivity on growth rates for directionally solidified Zn-Al-Sb eutectic alloy

AbstractZn-5.3 wt.% Al-0.8 wt.% Sb alloy was directionally solidified upward at a constant temperature gradient (G = 2.39 K mm−1) with a wide range of growth rates (V) (9.70 – 2 013.40 μm s−1) by using a Bridgman type directional solidification furnace. The average values of eutectic spacing (λ), microhardness (HVT), ultimate tensile strength (σUTS) and electrical resistivity (ρ) were measured from transverse sections of the directionally solidified samples. The dependency of λ, HVT, σUTS and ρ on V were experimentally obtained by using linear regression analysis for low, high and all growth rates. The bulk growth rates were also determined by using the measured values of λ and V for low, high and all growth rates. The results obtained in the present work were compared with the Jackson–Hunt eutectic theory and similar experimental results in the literature. Also, the specific heat difference (ΔCP) and enthalpy of fusion (ΔH) for the Zn–Al–Sb alloy were determined by means of differential scanning calorimetry.

Effect of silicon content on microstructure, mechanical and electrical properties of the directionally solidified Al–based quaternary alloys

Effect of silicon content on the microstructure (lamellar and flake), mechanical (microhardness, ultimate tensile strength) and electrical resistivity properties of Al–Cu–Fe–Si quaternary alloys has been investigated. Al–26Cu–0.5Fe–xSi (x = 6.5, 8, 10, 12 and 14 wt %) were prepared using metals of 99.99% high purity in the vacuum atmosphere. These alloys were directionally solidified under constant temperature gradient (8.50 K/mm) and growth rate (8.25 μm/s) by using a Bridgman–type directional solidification furnace. Eutectic spacing, microhardness, ultimate tensile strength and electrical resistivity were expressed as functions of composition. The dependency of the eutectic spacing, microhardness, tensile strength and electrical resistivity on the composition (Si content) were determined. According to experimental results, the microhardness, ultimate tensile strength and electrical resistivity of the solidified samples increase with increasing the Si content, but decrease eutectic spacing. Variation of electrical resistivity with the temperature in the range of 300–650 K for studied alloys was also measured by using a standard d.c. four−point probe technique.

Numerical analysis of thermal stress and dislocation density distributions in large size multi-crystalline silicon ingots during the seeded growth process

In this study, a global transient numerical simulation of silicon growth from the beginning of the solidification process until the end of the cooling process is carried out modeling the growth of an 800kg ingot in an industrial seeded directional solidification furnace. The standard furnace is modified by the addition of insulating blocks in the hot zone. The simulation results show that there is a significant decrease in the thermal stress and dislocation density in the modified model as compared to the standard one (a maximal decrease of 23% and 75% along the center line of ingot for thermal stress and dislocation density, respectively). This modification reduces the heating power consumption for solidification of the silicon melt by about 17% and shortens the growth time by about 2.5h. Moreover, it is found that adjusting the operating conditions of modified model to obtain the lower growth rate during the early stages of the solidification process can lower dislocation density and total heater power.

Doping optimization of solar grade (SOG) silicon ingots for increasing ingot yield and cell efficiency

In the near future, SoG will become the principal material for photovoltaic ingot production as it requires much less energy for purification compared to silicon grades using gas transformation and purification (usually Siemens process or equivalent also used for electronic-grade preparation). In this study, several kinds of silicon have been compared with different dopant contents (mainly boron and phosphorus). Ingot yield and cell efficiency have been optimized for each source of silicon at a commercial level (450kg ingots) using boron or gallium doping. Starting from the resistivity specification given by the cell process, the doping level has been adjusted in order to maximize the ingot silicon yield (weight of silicon bricks used for wafer cutting/weight of silicon ingot). After doping adjustment, ingot quality has been checked, i.e. brick resistivity and lifetime of minority carriers, and wafers have been processed to solar cells. Doping optimization has led to comparable ingot yields and cell efficiencies using SoG and silicon purified by Siemens process or equivalent. The study has been implemented at the Kazakhstan Solar Silicon Plant in Ust-Kamenogorsk using Kazakhstan SoG, SoG has been received from a European manufacturer and polycrystalline silicon has been purified using the Siemens process. Directional solidification furnaces have been manufactured by ECM Technologies, France.