Directional Solidification Furnace Research Articles

Dependence of microstructural, mechanical and electrical properties on growth rates in directional solidified Zn—Al—Bi eutectic alloy

Zn—5%Al—0.2%Bi (mass fraction) alloy was directionally solidified upward at a constant temperature gradient with a wide range of growth rates using a Bridgman-type directional solidification furnace. The eutectic spacings, microhardness, ultimate tensile strength and electrical resistivity for directionally solidified Zn—Al—Bi alloy were measured. Dependence of eutectic spacings, microhardness, ultimate tensile strength and electrical resistivity on growth rates was obtained by linear regression analysis. The results obtained in the present work for low growth rates (smaller than 450.0 μm/s) are in good agreement with experimental results obtained in previous work for directional solidified Zn—Al eutectic alloy with a similar growth rate but differs from the Jackson—Hunt eutectic theory and those obtained in previous experimental works at higher growth rates. The critical growth rate might be 450.0 μm/s for the Zn—Al—Bi eutectic alloy. From the plot of heat flow versus temperature, enthalpy of fusion, specific heat difference between liquid and solid phases and melting temperature for the Zn—Al—Bi alloy are found to be 112.55 J/g, 0.291 J/(g·K) and 660.20 K, respectively.

Effects of the location of a cast in the furnace on flatness of the solidification front in directional solidification

Many defects of single crystals are caused by the nonplanar solidification front. The transverse temperature gradient at melt–crystal interface results in nonplanar solidification fronts. The location of a cast in the directional solidification furnace affects heat dissipation and thus influences the transverse temperature gradient. This paper presents a criterion and a searching algorithm to find the optimal location of the cast for flattening the solidification front. A numerical simulation was employed for the verification of our method. Additionally, the effects of the size of the cooling device of the furnace on the optimal location, the transverse temperature gradient and the solidification time were discussed. The transverse temperature gradient is reduced about 50% without increasing much solidification time when setting the cast with a varying thickness mould at the optimal location. In addition, the optimal location is mainly influenced by the radius of the cooling ring.

Numerical modelling on stress and dislocation generation in multi-crystalline silicon during directional solidification for PV applications

Numerical modelling has emerged as a powerful tool for the development and optimization of directional solidification process for mass production of multicrystalline silicon. A transient global heat transfer model is performed to investigate the effect of bottom grooved furnace upon the directional solidification (DS) process of multi-crystalline silicon (mc-Si). The temperature distribution, von Mises stress, residual stress and dislocation density rate in multi-crystalline silicon ingots grown by modified directional solidification method have been investigated for five growth stages using finite volume method at the critical Prandtl number, Pr = 0.01. This paper discusses bottom groove furnace instead of seed crystal DS method. It achieves an advanced understanding of the thermal and mechanical behaviour in grown multi-crystalline ingot by bottom grooved directional solidification method. 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 mm, 60 mm and 90 mm of the heat exchanger block of the DS furnace. In this paper, the results are presented for 60 mm radius groove only because it has got better results compared to the other grooves. Also, the computational results of bottom grooved DS method show better performance compared the conventional DS method for stress and dislocation density in grown ingot.

Modeling of poly-crystalline silicon ingot crystallization during casting and theoretical suggestion for ingot quality improvement

Finite element (FE) and cellular automaton (CA) models are used in order to predict the microstructure of Poly-Crystalline Silicon (PC-Si) ingots during casting in a directional solidification furnace. During solidification, the 3D-FE model is used to simulate thermal field inside the furnace, while the nucleation and growth of PC-Si are simulated by the modified version of CA model. In this model both nucleation at silicon-crucible interface and nucleation of equiaxed grains on impurities are modeled. The origin of impurity is considered SiC particles when carbon segregates during the solidification and precipitates as SiC particles if carbon solubility limit is reached. The model is evaluated with experimental data for different parameters, such as: the temperature profile at the top and bottom of the PC-Si, grain size, and the height of solidified silicon during solidification. A one-dimensional form of grain growth is observed at the crucible-silicon interface, while a more two-dimensional form of growth is predicted at higher silicon height. The microstructure of PC-Si shows that the grain size increases as a function of PC-Si ingot height. The effects of the cooling rate and competitive growth on the final microstructure of PC-Si are also investigated. The results show that less nucleation sites are formed at the bottom of the crucible with a slower cooling rate, which leads to a larger grain size. Furthermore, the grains at the crucible side walls tend grow inward at a lower cooling rate, which can significantly change the microstructure of PC-Si. This study shows that the competitive growth phenomena plays an important role in the final microstructure of silicon ingots. A simulated model is proposed which shows that by changing the density and location of nucleation sites it is possible to achieve more effective control on final PC-Si micro structure. This simulation helps to explain the crystal behavior observed in ingots cast under different conditions.

Study on the usage of a commercial software (Comsol-Multiphysics®) for dislocation multiplication model

Elaboration of silicon ingots for photovoltaic application in Directional Solidification furnace leads to formation of dislocations mainly due to thermoelastic stresses, which impact photovoltaic conversion rate. Several research teams have created numerical simulation models using home-made software in order to study dislocation multiplication and predict the dislocation density and residual stresses inside ingots after elaboration.In this study, the commercial software Comsol-Multiphysics® is used to calculate the evolution of dislocation density during the ingot solidification and cooling. Thermo-elastic stress, due to temperature field inside the ingot during elaboration, is linked to the evolution of the dislocation density by the Alexander and Haasen model (A&H model). The purpose of this study is to show relevance of commercial software to predict dislocation density in ingots.In a first approach, A&H physical model is introduced for a 2D axisymmetric geometry. After a short introduction, modification of Comsol® software is presented in order to include A&H equations. This numerical model calculates dislocation density and plastic stress continuously during ingot solidification and cooling. Results of this model are then compared to home-made simulation created by the teams at Kyushu university and NTNU. Results are also compared to characterization of a silicon ingot elaborated in a gradient freeze furnace. Both of these comparisons shows the relevance of using a commercial code, as Comsol®, to predict dislocations multiplication in a silicon ingot during elaboration.

Microstructure and room temperature mechanical properties of directionally solidified Mg–2.35Gd magnesium alloy

Directional solidification of Mg–2.35Gd (mass fraction, %) magnesium alloy was carried out to investigate the effects of the solidification parameters (growth rate v and temperature gradient G) on microstructure and room temperature mechanical properties under the controlled solidification conditions. The specimens were solidified under steady state conditions with different temperature gradients (G=20, 25 and 30 K/mm) in a wide range of growth rates (v=10–200 μm/s) by using a Bridgman-type directional solidification furnace with liquid metal cooling (LMC) technology. The cellular microstructures are observed. The cellular spacing λ decreases with increasing v for constant G or with increasing G for constant v. By using a linear regression analysis the relationships can be expressed as λ=136.216v−0.2440 (G=30 K/mm) and λ=626.5630G−0.5625 (v=10 μm/s), which are in a good agreement with Trivedi model. An improved tensile strength and a corresponding decreased elongation are achieved in the directionally solidified experimental alloy with increasing growth rate and tempertaure gradient. Furthermore, the directionally solidified experimental alloy exhibits higher room temperature tensile strength than the non-directionally solidified alloy.

Effect of Growth Rate on the Microstructure and Microhardness in a Directionally Solidified Al-Zn-Mg Alloy

The Al-5.5Zn-2.5Mg (wt pct) ternary alloy was prepared using a vacuum melting furnace and a casting furnace. Five samples were directionally solidified upwards at a constant temperature gradient (G = 5.5 K/mm) under different growth rates (V = 8.3–165 μm/s) in a Bridgman-type directional solidification furnace. The primary dendrite arm spacing, λ 1, secondary dendrite arm spacing, λ 2, and microhardness, HV, of the samples were measured. The effects of V on λ 1, λ 2 and HV properties of the Al-Zn-Mg alloy were studied by microstructure analysis and mechanical characterization. Microstructure characterization of the alloys was carried out using optical microscopy, scanning electron microscopy, wavelength-dispersive X-ray fluorescence spectrometry, and energy dispersive X-ray spectroscopy. From the experimental results, it is found that the λ 1, λ 2 values decrease, but HV values increase with the increase in V, and HV values decrease with the increase in λ 1 and λ 2. Dependencies of dendritic spacing and microhardness on the growth rate were determined using linear regression analysis. The growth rate, microstructure, and Hall–Petch-type relationships obtained in this work have been compared with the results of previous studies.

Upgrade of the hot zone for large-size high-performance multi-crystalline silicon ingot casting

Casting larger silicon ingots by upgrading the hot zone is one of the main methods used to reduce the cost of multi-crystalline silicon wafers. In this paper, a new hot zone is designed and a transient global model is applied to investigate the effects of the new hot zone on the electricity consumption, the crystal growth rate, and the shape of the C–M interface during the solidification process. Based on the simulation results, a generation-five, directional-solidification furnace was upgraded and implemented in casting experiments. The experimental results show that the feedstock capacity increased by 77.8%, the crystal growth efficiency increased by 53.8%, and the average yield rate of the silicon ingots increased by 9%. The crystal–melt interface was flatter and the growth direction of the grains was almost straight upward.

Growth rate and composition of directionally solidified intermetallic TiAl–Nb alloys with different solidification conditions

Intermetallic Ti–xAl–8Nb (x = 41, 43, 45, 47, 49; at%) alloys were solidified unidirectionally upwards with a constant temperature gradient of G = 3.8 K·mm−1 at wide range of growth rates of v = 10–400 μm·s−1 using a Bridgman directional solidification (DS) furnace. Microstructural parameters including the primary dendrite arm spacing (λ 1), secondary dendrite arm spacing (λ 2), dendrite tip radius (R) and mushy zone depth (d) were measured statistically. The values of λ 1, λ 2, R and d decrease as the growth rate increases for a given composition (x). The values of λ 1, λ 2, R and v increase with the increase in x value, while the value of d firstly increases and then decreases with the increase in x value for a given v. The relationships between λ 1, λ 2 and R were analyzed by the linear regression. The average growth rate exponent of λ 1 is 0.29, which is in accordance with the previous experimental observations, and that of λ 2 is close to the previous experimental results, while those of R and d are lower than the results in other alloy systems. In addition, theoretical models for λ 1, λ 2 and R were compared with the experimental observations, and a comparison of the present experimental results with the theoretical models and previous experimental results was also made.

Crystal orientation control in TiAl–Nb alloys through a double directional solidification process

Intermetallic Ti–46Al–5Nb (at.%) alloys were directionally solidified by single directional solidification (SDS) process and double directional solidification (DDS) process using a Bridgman directional solidification furnace. Microstructure and crystal orientation neighboring the initial growth interface as well as the quenched mushy zone before starting directional solidification were investigated in the SDS and DDS processes, respectively. The results show that the dendrite morphology changes from equiaxial to columnar, but the directional dendrite segregation is almost keep the continuity across the initial growth interface after SDS and DDS processes, respectively. Crystal orientations in the regions neighboring the initial growth interface have preferential modifications along the growth direction. Compared with that in the first DS of DDS, the length of the initial mushy zone significantly decreases, but the β dendrite is well-orientated in the mushy zone in the second DS. Crystal orientation can be well-controlled in DDS by modifying the β growth morphology as well as the orientation in initial mushy zone of the two DS processes. The directional dendrite parallel to the growth direction and the preferred β orientation are considered as the main reasons to β seed crystallization. The process of the β seed crystallization is also discussed. Based on the β seeding, the DDS process can align the β dendrite orientation and promote the peritectic transformation undergoing completely, thus control the lamellar colony boundaries as well as the lamellar microstructures inside of the colonies.

Controlling solidification front shape and thermal stress in growing quasi-single-crystal silicon ingots: Process design for seeded directional solidification

To control the solidification front shape and thermal stress in the growing silicon ingot and reduce the small-grain region at its periphery, a moving insulation partition block was designed in an industrial seeded directional solidification furnace for quasi-single-crystal silicon ingots. We propose several moving process designs of the partition block. A transient global model of heat transfer in which all types of heat transfer are included was established to investigate the thermal field, solidification front evolution, and thermal stress in the growing ingot. Corresponding experiments were conducted to validate the simulation results through the relationship between solidification front shape and the small-grain region. It was found that the moving process can significantly influence the thermal field, solidification front shape, and thermal stress in the growing ingot during the solidification process. The moving partition block design is feasible to control the solidification front shape and reduce the small-grain region at the periphery of the solidified ingot. A favorable moving process design of the partition block was obtained that can simultaneously achieve a small small-grain region and low thermal stress in the solidified ingot.

Control of extended defects in cast and seed cast Si ingots for photovoltaic application

AbstractWe discuss the defect evolution in conventional cast and seed cast Si ingot growths for photovoltaic application. Three different cast Si ingots were grown in one directional solidification furnace. The two extremes are the seed cast ingots (mono‐Si), where growth starts from monocrystalline silicon seeds, and the multicrystalline silicon grown from small randomly oriented grains. The conventional multicrystalline (mc‐) cast Si ingots are grown without any seeds and have grain structures in between the two extremes. It was found that in mc‐Si the evolution of grain boundaries take place in several steps. On the other hand, the major defects in mono‐Si are dislocations and are generated by stress due to thermal gradient in the ingot. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Growth Rate and Composition on the Eutectic Spacing in the Directionally Solidified NiAl-Mo Eutectic Alloys

A series of stoichiometric NiAl alloys contain four different concentrations of Mo (7.8, 9, 13, 16 at.%) were directionally solidified in the Bridgman-type directional solidification furnace for four different growth rates (V = 1.5–6 μm/s) in a constant temperature gradient (G = 334 K/cm). the microstructures of the alloys produced with the different growth rates at a constant temperature gradient tended to form fully fibrous structures. The fiber spacings λ were measured from transverse section of the specimen. The variation of λ with respect to growth rate (V) and composition (C0) were determined by linear regression analysis. It has been found that λ decreases with the increasing values of V and C0. The results are compared with theoretical models and previous experimental published data.

Rotating bending fatigue behaviour of a Ni3Al-based single crystal alloy IC6SX at 760°C

The high cycle fatigue behaviour at 760°C of a Ni3Al-based single crystal alloy IC6SX has been studied in the present investigation. The specimens for the fatigue tests were prepared by screw selection crystal method in a directional solidification furnace. The rotating bending fatigue tests of the specimens was carried out at the temperature of 760°C in air, the stress ratio of R (σmax/σmin) was − 1, and the rotating speed of the fatigue tests was 6500 r min− 1 (108 Hz). The stress–fatigue life cycle (S − Nf) curve was obtained based on the fatigue tests, and the fracture surfaces were examined using scanning electron microscopy (SEM). The results showed that the fatigue cracks initiated from the surface defects of the specimens and the cracks propagated along the (111) crystal plane. The fatigue fracture was composed of three different characteristic regions corresponding to the three fatigue stages, including fatigue crack initiation stage, steady-state propagation stage and the fast propagation stage. The research results also showed that the rotating bending fatigue performance of alloy IC6SX is superior to that of the second-generation single crystal nickel-based superalloys containing 3 wt-% Re.

Elaboration of AlSi10Mg casting alloys using directional solidification processing

The effects of pulling velocity on the solidification behavior and microstructural parameters of AlSi10Mg alloys prepared in a Bridgman-type directional solidification furnace were investigated. The microstructure, particularly the secondary dendritic arm spacing (SDAS), and the Brinell hardness (BH) of the solidified AlSi10Mg alloys were characterized for samples with cylindrical shapes and different conicities (θ = 0°, 5°, and 10°). Microstructural studies revealed an increased density of α-Al phase dendrites and a decreased interdendritic distance with increasing pulling velocity. The dendrites were found to be preferentially oriented along the pulling direction for low pulling velocities. For larger pulling velocities, the dendrites grew first in the cooling direction but then broke as others nucleated and coarsened. The HB values of the solidified samples increased as the pulling velocity increased. In regard to sample conicity, smaller dendrites were observed for an apex angle of θ = 5°, resulting in the largest HB value. This result was interpreted in terms of the favorable orientation of the dendrite along the pulling direction.

슈퍼 듀플렉스 스테인리스강의 응고·냉각 시 상석출에 미치는 냉각속도의 영향

This work presents the effect of the cooling rate on the precipitation of super duplex stainless steel. Specimens of super duplex stainless steel with a specific composition were cooled at various cooling rates after being melted at <TEX>$1550^{\circ}C$</TEX> in a directional solidification furnace. Ferrite (<TEX>${\delta}$</TEX>), Austenite (<TEX>${\gamma}$</TEX>), Sigma (<TEX>${\sigma}$</TEX>), and Chi (<TEX>${\chi}$</TEX>) phases were precipitated when the cooling rate was lower than 0.22 K/s. When the cooling rate was 0.22 K/s or faster, <TEX>${\sigma}$</TEX> and <TEX>${\chi}$</TEX> phases were not precipitated.

Investigation of the Some Physical Properties of the Directionally Solidified Al–Cu–Co Ternary Eutectic Alloy

The Al–23.9% Cu–1.2% Co (wt%) ternary eutectic alloy was prepared using a vacuum melting furnace and a casting furnace. The samples were directionally solidified upwards at a constant growth velocity (18.8 μm/s) under different temperature gradients (1.23–5.66 K/mm) and at a constant temperature gradient (5.66 K/mm) under different growth velocities (8.3–166 μm/s) in a Bridgman-type directional solidification furnace. The dependence of the eutectic spacing on the solidification parameters (temperature gradient and growth rate) and that of the microhardness and compressive strength on the eutectic spacing and solidification parameters were investigated. The electrical resistivity measurements of the studied alloy were performed and the temperature coefficient of the resistivity was calculated from the curve of the resulting ρ–Τ plot. The thermal conductivities of the sample grown at a constant temperature gradient and growth rate were obtained using the Wiedemann–Franz and Smith–Palmer equations.

Doping Optimization of Solar Grade (SOG) Silicon Ingots for Increasing Ingot Yield and Cell Efficiency

In the close future, use of SoG should become prominent 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). During this study, several kinds of silicon were compared with different rates of dopant content (mainly boron and phosphorus). Ingot yield and cell efficiency were optimized for each source of silicon at a production level (450 kg ingots) using boron or gallium doping. Starting from the resistivity specification given by the cell process, the doping level was 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 was checked: brick resistivity, lifetime of minority carriers and wafers were processed into solar cells. Optimizing of doping led to get comparable ingot yields and cell efficiencies using SoG and silicon purified by Siemens process or equivalent. The study was implemented at Kazakhstan Solar Silicon plant in Ust−Kamenogorsk using Kazakhstan SoG, SoG from a European manufacturer and polycrystalline Silicon purified by Siemens process. Directional solidification furnaces were manufactured by the French company ECM Technologies.

Control of the Gas Flow in an Industrial Directional Solidification Furnace for Production of High Purity Multicrystalline Silicon Ingots

A crucible cover was designed as gas guidance to control the gas flow in an industrial directional solidification furnace for producing high purity multicrystalline silicon. Three cover designs were compared to investigate their effect on impurity transport in the furnace and contamination of the silicon melt. Global simulations of coupled oxygen (O) and carbon (C) transport were carried out to predict the SiO and CO gases in the furnace as well as the O and C distributions in the silicon melt. Cases with and without chemical reaction on the cover surfaces were investigated. It was found that the cover design has little effect on the O concentration in the silicon melt; however, it significantly influences CO gas transport in the furnace chamber and C contamination in the melt. For covers made of metal or with a coating on their surfaces, an optimal cover design can produce a silicon melt free of C contamination. Even for a graphite cover without a coating, the carbon concentration in the silicon melt can be reduced by one order of magnitude. The simulation results demonstrate a method to control the contamination of C impurities in an industrial directional solidification furnace by crucible cover design.

Mechanical, electrical, and thermal properties of the directionally solidified Bi-Zn-Al ternary eutectic alloy

A Bi-2.0Zn-0.2Al (wt%) ternary eutectic alloy was prepared using a vacuum melting furnace and a casting furnace. The samples were directionally solidified upwards at a constant growth rate (V = 18.4 μm/s) under different temperature gradients (G = 1.15–3.44 K/mm) and at a constant temperature gradient (G = 2.66 K/mm) under different growth rates (V = 8.3–500 μm/s) in a Bridgman-type directional solidification furnace. The dependence of microstructure parameter (λ) on the solidification parameters (G and V) and that of the microhardness (Hv) on the microstructure and solidification parameters were investigated. The resistivity (ρ) measurements of the studied alloy were performed using the standard four-point-probe method, and the temperature coefficient of resistivity (α) was calculated from the ρ-T curve. The enthalpy (ΔH) and the specific heat (C p ) values were determined by differential scanning calorimetry analysis. In addition, the thermal conductivities of samples, obtained using the Wiedemann-Franz and Smith-Palmer equations, were compared with the experimental results. The results revealed that, the thermal conductivity values obtained using the Wiedemann-Franz and Smith-Palmer equations for the Bi-2.0Zn-0.2Al (wt%) alloy are in the range of 5.2–6.5 W/Km and 15.2–16.4 W/Km, respectively.