Crucible Research Articles

Novel coatings for graphite materials in PV silicon applications: A study of the surface wettability and interface interactions

Coatings are one of the promising techniques that can be applied to modify the surface properties and tailor the surface interactions with the surrounding. The development of conformable coatings for graphite materials can expand their use in silicon PV applications with no compromise on the silicon quality. However, no studies are focused on the coatings for graphite materials in silicon crystallization applications. Here we introduce different coating techniques that promise minimal interactions with the graphite and suppress the liquid silicon penetration. Five coating methods were proposed in this study: (i) one-layer coating: a mixture of silicon nitride and colloidal silica was directly coated on graphite substrates, (ii-iv) two-layer coating: a porous coating layer of silicon nitride, silicon carbide, or a combination of both materials was deposited as a protective layer below the top layer of Si3N4 and colloidal silica, and (v) a dense SiC layer was deposited on the graphite by silicon infiltration method below the top layer. The coatings’ wettability and interactions with graphite were investigated via in-situ melting experiments. The two-layer coating approach revealed a considerable improvement in the non-wetting behavior, a decrease in the coating degradation rate, and a decrease in CO evolution during the isothermal holding. The best non-wetting conditions with minimal detrimental interaction between graphite and silicon oxides were achieved by applying a two-layer coating with a thickness of 400 ± 50 μm and an initial oxygen concentration of 8 wt.% in the top layer. These findings can be utilized for the application of reusable graphite crucibles in silicon crystallization processes.

Effect of Alloying on the Microstructure and Strength of the Ni Bronzes

In this study, Al was added to the Cu15Ni8Sn Ni-bronze alloy. The effects of the added amount of Al on the microstructure and strength of the alloy were investigated. As an approach, the Sn element has been replaced by Al. The element Al was added at 2, 4, 6, and 8 wt. %. Alloys were melted in a graphite crucible by induction melting and poured into a graphite mold. No heat treatment was applied to the cast alloys. The microstructure of the alloys was studied under a light optical microscope (LOM) and scanning electron microscope (SEM). Mechanical characterizations were made by hardness measurement and tensile tests. Metallographic studies showed that the microstructure was not changed with the amount of Al added. Chemical composition changes in the phases were reflected in the mechanical properties. It was determined that the composition with the highest hardness and strength among the alloys was the composition containing 15Ni, 4Sn, and 4Al wt.%. The alloy with the highest ductility and toughness is Cu15Ni8Al.

Effect of Ta and W Additions on Microstructure and Mechanical Properties of Tilt-Cast Ti-45Al-5Nb-2C Alloy

The effect of Ta and W additions on microstructure and mechanical properties of tilt-cast Ti-45Al-5Nb-2C (at.%) alloy was investigated. Three alloys with nominal composition Ti-45Al-5Nb-2C-2X (in at.%), where X is Ta or W, were prepared by vacuum induction melting in graphite crucibles followed by tilt casting into graphite moulds. The microstructure of the tilt-cast alloys consists of the α2(Ti3Al) + γ(TiAl) lamellar grains, single γ phase, (Ti,Nb,X)2AlC particles with a small amount of (Ti,Nb,X)C, and β/B2 phase identified only in W containing alloy. The EDS analysis shows that Ta segregates into the carbide particles and reduces dissolution of Nb in both (Ti,Nb,Ta)C and (Ti,Nb,Ta)2AlC phases. The alloying with W reduces Nb content in both carbide phases and leads to stabilisation of β/B2 phase in the lamellar α2 + γ regions. The alloying with Ta and W does not affect the volume fraction of the carbide particles but influences their size and morphology. While the alloying with Ta and W has no significant effect on Vickers hardness and the indentation elastic modulus of the studied alloys, the addition of Ta affects the nanohardness and elastic modulus of the (Ti,Nb,Ta)2AlC phase. The addition of W significantly increases the Vickers microhardness of the lamellar α2 + γ regions.

Corrosion resistance of AlN-based ceramics to molten uranium

A kind of AlN-based ceramics was prepared as a lining material of the high-density graphite crucible for high temperature melting uranium. During the corrosion experiment, the corrosion rate showed a downward trend with the increase of corrosion time. As the corrosion time increased from 2 h to 8 h, the corrosion rate showed a gradual downward trend from 0.25 mg cm−2 h−1 to 0.22 mg cm−2 h−1, and then decreased significantly to 0.07 mg cm−2 h−1 at 16 h. A dense layer consisting of UO2, Y2O3 and UC was formed between molten uranium and AlN-based ceramics, and the thickness increased with the extension of corrosion time, resulting in a significant decrease in corrosion rate.

Dehydroxylation processing and lasing properties of a Nd alumino-phosphate glass

In this work, we have studied the preparation and properties of an alumino-phosphate glass with composition 13Na2O-13K2O-16BaO-4Al2O3-54P2O5 (mol%). The first part of the work deals with the study of the processing conditions of the dehydroxylation of the phosphate glass, which was performed by remelting under N2 flow using graphite crucibles. Glass samples from 5 to 50 g and Nd2O3 doped were submitted to dehydroxylation and the influence of temperature, time, mass of glass and viscosity were correlated with the content of water in the glasses through the coefficient of absorption of OH ions. The network structure of the glasses was also determined by means of 31P and 27Al 1D/2D nuclear magnetic resonance and the local environment of Nd3+ ions was probed by electron paramagnetic resonance. The optimized conditions of processing were then used to obtain a dehydroxylated glass with a 2.5 wt% Nd2O3 whose spectroscopic and laser emission properties were studied. The spectroscopic properties of Nd3+ ions which include, Judd-Ofelt calculation, stimulated emission cross-section of the laser transition, lifetime, and quantum efficiency are presented. Site-selective laser spectroscopy and stimulated emission obtained under selective wavelength pumping along the 4I9/2→4F5/2 absorption band were performed to determine the distribution of crystal field in which the rare earth is located, together with its influence in the pump wavelength dependence of the spontaneous and laser emissions of Nd3+ in this glass matrix.

Synthesis of porous silicon, nickel and carbon layers by vapor phase dealloying

Porous thin films have various application fields, e.g., for energy conversion in fuel cells , energy storage in lithium ion batteries or supercapacitors as well for catalysis, filtration and sensing. We synthesized porous thin films by co-evaporating a low-vapor-pressure material (e.g., Si, Ni or C) together with zinc and depositing a compact layer of resulting composite. High-rate deposition process up to 100 nm/s was realized by electron beam physical vapor deposition (EB-PVD) of the materials from two graphite crucibles with a fast deflected electron beam in high vacuum. Immediately after deposition, the coated substrates were heated up in vacuum to a temperature above 500 °C and thereby zinc is removed selectively. Due to its higher vapor pressure against that of remaining component, zinc is expelled from the layer and vacancies are generated by so called vapor phase dealloying (VPD). We investigated the feasibility of VPD process for the elements silicon, nickel and carbon. The elemental composition and the morphology of the layers prior and after thermal annealing were analyzed by scanning electron microscopy, by energy-dispersive X-ray spectrometry and by X-ray diffraction. • High-rate deposition of compact multiphase layers by co-evaporation of Zn together with Si/Ni/C • Layer morphology can be adjusted by deposition parameters. • Subsequent thermal annealing in vacuum for expelling zinc and generating pores • Porous structure can be adjusted by layer morphology and annealing parameters.

Suppressing solvent compositional change during solution growth of SiC using SiC/C gradient crucible

To develop a long-term and stable growth technology for the solution growth of silicon carbide (SiC), the use of the newly fabricated SiC/C gradient crucible was studied. The fabrication of the SiC/C gradient structure was achieved by a two-step heat treatment, and a SiC/C structure with a high SiC areal ratio of more than 50% was obtained by Si infiltration into the porous graphite. Then, the top-seeded solution growth was performed using the fabricated SiC/C gradient crucible that revealed the suppression of the compositional change of the solution during the growth in comparison to the growth using a graphite crucible, which is the general method for the SiC solution growth. Furthermore, the SiC growth behaviors and crystal qualities were maintained when the SiC/C gradient crucible was used instead of the graphite crucible. The use of such a high-purity SiC/C gradient crucible was thus found to be effective in achieving crystal growth with less compositional change, which would enable long-term growth while maintaining a stable growth interface.

Effects of casting parameters and impurity concentrations on as-cast U–10Mo

This work investigates the effects of casting parameter changes on as-cast uranium-10 wt% molybdenum (U–10Mo) alloys on a 1–2 kg scale. A commercial tilt-pour vacuum induction melting system was utilized to study the effects of casting atmosphere, total heating time, hold time at maximum temperature, mold temperature, and crucible type on as-cast U–10Mo impurities and dendrite size. Fourteen plates of differing geometry were cast and analyzed for carbon, oxygen, hydrogen, and nitrogen impurities, as well as molybdenum content. Primary dendrite lengths (PDL) in the microstructure are quantified. A statistical evaluation was performed to identify key carbon and oxygen relationships with operating parameters, impurities, and PDL. Additional factors investigated include crucible material, time at maximum temperature, and total heating time. Zirconia crucibles reduced carbon content on average by 35 % over traditional graphite crucibles. There was statistically relevant associations between carbon and oxygen content, oxygen content and maximum temperature hold time, mold temperature and PDL, and between PDL, maximum temperature hold time, and mold temperature.

Alloying Elements Effects on Electrical Conductivity and Mechanical Properties of Newly Fabricated Al Based Alloys Produced by Conventional Casting Process.

The present investigation deals with a comprehensive study on the production of aluminum based alloys with the incorporation of different alloying elements and their effect on its electrical conductivity and mechanical properties. Casting of pure aluminum with different concentration and combinations of alloying additives such as cupper (Cu), magnesium (Mg) and silver (Ag) were carried out using a graphite crucible. The as-cast microstructure was modified by hot rolling followed by different heat-treated conditions viz., annealing, normalizing, quenching, and age hardening. The mechanical properties and electrical conductivity of the produced heat-treated alloys sheets under various processing conditions were carried out using tensile testing, hardness, and electrical resistivity measurements. It was found that by increasing the alloying elements content, yield strength results increased significantly by more than 250% and 500% for the as rolled and 8 h aged Al-Cu-Mg alloy, respectively. On the other hand, the electrical conductivity reduces slightly with −14.6% and −16.57% for the as rolled and 8 h aged of the same Al-Cu-Mg alloy, respectively.

Formation of Plasma Arc Anode Spot on the Rotating Graphite Crucible

Formation of Plasma Arc Anode Spot on the Rotating Graphite Crucible

Oxidation protective coating of Y2O3-Na2SixO2x+1 composite on graphite crucible for high temperature applications

Oxidation and corrosion protective coatings on graphite are essential for its high temperature applications. In the present work, oxidation protective coating on graphite was achieved with a combination of Y2O3 and SiO2 suspension with NaOH in methanol. Study showed that the optimized composition of the desired coating was Y2O3–15 wt% SiO2 (i.e., 2 g mixture of Y2O3 and SiO2 powder containing 0.3 g of SiO2 which is 15% of the total powder content if expressed in weight) with 5 M NaOH in 10 mL methanol. This protective coating on graphite was stable up to 1473 K for 30 min in Ar ambient before visible cracks appeared. The compositions of SiO2: NaOH were varied (5 mmol:5 M, 10 mmol:5 M and 5 mmol:10 M) to examine the roles of these two components in the overall adhesiveness of the coating. SiO2 and NaOH acted as inorganic binders and formed in-situ intermediate layers during sintering which held Y2O3 layer with the graphite surface. The coated graphite blocks and crucibles were characterized by X-ray diffractometer (XRD), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDS), differential thermal analysis and thermogravimetric analysis (DTA/TGA) and Fourier transform infrared spectroscopy (FTIR). The oxidation protective nature of the coating was tested by heating the coated crucible at 1273 K for 1-15 h in air atmosphere. The effective adhesion of oxidative coating on graphite was explained.

Microstructure and Properties of TiAl-Based Alloys Melted in Graphite Crucible

This paper presents the chemical and phase composition, microstructure, and selected properties both at room temperature and at the temperature corresponding to the expected operating conditions of three successive generations of TiAl-based alloys (Ti-47Al-2W-0.5Si, Ti-45Al-8Nb-0.5(B,C), and Ti-45Al-5Nb-2Cr-1Mo-0.5(B,C)-0.2Si) melted in a vacuum induction furnace with high-density isostatic pressed graphite crucibles. The obtained results of mechanical and physical properties of the produced alloys were compared to the properties of reference alloys with similar chemical composition and melted in a cold copper crucible furnace. The effect of increased carbon content in the produced alloys due to the degradation of the graphite crucible during melting is higher strength properties, lower plastic properties, higher coefficient of thermal expansion, and improved creep resistance. It was shown that the proposed technology could be successfully used in the production of different generation TiAl-based intermetallic alloys.

Cavitation-assisted decontamination of yttria from graphite of different densities

Yttria coated graphite crucibles are widely used to handle molten refractory and radioactive metals like uranium and plutonium. However, the coated layer suffers damages like cracking and peeling off owing to thermal cycles. As a result, removal of the yttria layer from the graphite surface is essential to ensure reuse of the crucible and minimization of radioactive waste. The present work investigates intensified dissolution of yttria from the coated graphite samples using ultrasound as a non-destructive decontamination technique to recycle the graphite substrate. The optimum conditions established for maximum dissolution were 8 M as acid strength, frequency of 30 kHz, temperature of 45 °C and power density of 8 W cm−2 that resulted in maximum dissolution of 52% in 30 min. Use of an oxidant H2O2 to the acid, did not yield any improvement in the dissolution kinetics, instead, increased oxidation of the graphite substrate was observed, leading to the anomalous weight gain of the graphite substrate despite surface erosion. Effect of ultrasound on the dissolution was pronounced, with almost a threefold increase compared to dissolution performed under silent conditions. Rates of dissolution of yttria from the substrate of different densities and pore size distribution were also studied. The dissolution was slowest from graphite of density 1.82 g cm−3 as the pore size distribution was conducive to accommodate the yttria particles. The dissolution in nitric acid followed ash layer diffusion controlled kinetics. The study has demonstrated the efficacy of application of ultrasound for accelerated decontamination of graphite substrates.

Preparation of high-purity TiSi2 and eutectic Si–Ti alloy by separation of Si–Ti alloy for clean utilization of Ti-bearing blast furnace slag

Titanium-bearing (Ti-bearing) blast furnace slag is not only industrial waste but also an important Ti resource. However, the cleaning of Ti-bearing blast furnace slag for utilization is a difficult problem. In this study, a new process is proposed to prepare Titanium disilicide (TiSi2) and eutectic Si–Ti alloy (Si/TiSi2), using Ti-bearing blast furnace slag and low-purity Si as raw materials. Firstly, Titanium oxide (TiO2) in the Ti-bearing blast furnace slag was reduced by Si and an Si–Ti alloy was obtained. The obtained Si–Ti alloy was then separated into TiSi2 and eutectic Si–Ti alloy by directional crystallization. In addition, MgO, Al2O3 and graphite crucibles were used and their influences on the results were investigated to obtain the optimal reaction container to prepare high-purity TiSi2 and eutectic Si–Ti alloy. The results show that the separation of Si–Ti alloy using an MgO crucible as a reaction container has little influence on the purity of the TiSi2 (>98%) and eutectic Si–Ti alloy (>97%) and that most of the impurities could be eliminated from the bulk TiSi2 crystal and eutectic Si–Ti alloy. The results of this study provide a theoretical and experimental basis for the simultaneous preparation of TiSi2 and eutectic Si–Ti alloy materials and proposes a new approach for the comprehensive utilization of Ti-bearing blast furnace slag.

Irradiation of thorium-bearing molten fluoride salt in graphite crucibles

Four fluoride fuel salt samples (78LiF-22ThF4) in graphite crucibles were irradiated in the HFR Petten for a duration of 508 Full Power Days under the name SALIENT-01 (SALt Irradiation ExperimeNT). Goal of the experiment was to gain experience with the design of liquid salt experiments and the handling of the salts before and after irradiation. Specific research goals for SALIENT-01 are (i) to confirm claims of good fission product retention in the salt, (ii) to obtain size distributions for noble metal particles using Transmission Electron Microscopy and (iii) to assess possible interactions between fuel salt and fine-grained nuclear graphite, as well as possible uptake of fission products by the graphite. Here the design and irradiation history of the experiment are discussed together with plans for post-irradiation examinations. Limitations in representativeness of this experiment and capsule irradiations in general are discussed as well as follow-up actions to improve the quality of future irradiations.

Critical View on Buffer Layer Formation and Monolayer Graphene Properties in High-Temperature Sublimation

In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000 ∘C, and which is found to be independent of TAr. Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.

Phase Diagrams of Lead Difluoride Systems with Rare-Earth Fluorides

Phase equilibria in the PbF2–RF3 (R = La–Nd, Sm, Gd–Lu, Y, and Sc) systems were studied by X-ray powder diffraction in samples annealed under a fluorinating atmosphere and then quenched and by differential thermal analysis (DTA) under an inert atmosphere using unlidded thin-walled graphite crucibles. Stability areas were elucidated for Pb1 – xRxF2 + x fluorite solid solutions, for R1 – yPbyF3 – y solid solutions having the tysonite LaF3 structure, and for ordered phases of idealized composition Pb4R3F17 having trigonally distorted cubic unit cells. The topological evolution of the systems in response to the changing rare-earth ionic radius was considered.

Potential source of carbon contamination in U-Zr melt processed in Y2O3-plasma-spray-coated graphite

This study investigated carbon contamination in U-10wt.%Zr metallic fuel in the casting process based on the interaction between the U-Zr alloy melt and the Y2O3-plasma-spray-coated graphite. During the injection casting of U-10wt.%Zr, a ZrC reaction layer was found to form at the interface between the U-Zr melt and the Y2O3-plasma-coated graphite crucible. ZrC inclusions were also found in the matrix of the melt residue. This C contamination, evidenced by the formation of ZrC, is attributed to the migration of C through the Y2O3 coating from the graphite crucible. The results of TGA experiments using a Y2O3-plasma-coated graphite sample suggest that CO is produced by a reaction between Y2O3 and graphite at temperatures above 1773 K, which then migrates through defects in the coating to react with Zr in the U-Zr melt.

Enhancing High-Temperature Creep Resistance of <i>In Situ</i> TiAl-Based Matrix Composite by Low Volume Fraction of Ti<sub>2</sub>AlC Particles

Samples of TiAl-based matrix in-situ composite with the chemical composition Ti-46.4Al-5.1Nb-1C-0.2B (at.%) reinforced with a low volume fraction of primary Ti2AlC particles were prepared by vacuum induction melting in graphite crucibles and centrifugal casting into graphite moulds. The hot isostatic pressing (HIP) of the as-cast samples and subsequent heat treatments leads to the formation of equiaxed grains with fully lamellar α2(Ti3Al) + γ (TiAl) microstructure and uniformly distributed Ti2AlC and TiB particles. The minimum creep rates of the in-situ composite are significantly lower compared to those measured for the counterpart low carbon benchmark alloy with the chemical composition Ti-47Al-5.2Nb-0.2C-0.2B (at.%) at temperatures ranging from 800 to 900 °C and applied stress of 200 MPa. The studied in-situ composite shows also significantly improved creep resistance compared to that of some TiAl-based alloys with fully lamellar, convoluted and pseudo-duplex microstructures at a temperature of 800 °C and applied stress of 200 MPa.

Research on the Mechanism Model of Carbon Composite Material Damaged by Erosion in the Thermal Field of Single Crystal Silicon Furnace

Aiming at the problem of the short service life of the carbon/carbon pot wall caused by continuous silicification during the pulling process of single crystal silicon, the paper analyzes the failure mode of the carbon/carbon pot wall and analyzes the influence of the pot wall in the single crystal silicon furnace thermal field. The mechanism of erosion damage. It is concluded from the use situation that the carbon/carbon composite crucible can be used in the field of Czochralski single crystal silicon, and its performance is far better than that of the graphite crucible, and the destruction mechanism of the carbon/carbon composite crucible is analyzed, and solutions are proposed. Through the analysis of the entire industry, carbon/carbon composite crucibles will replace graphite crucibles and become the preferred material for the thermal field of Czochralski silicon single crystals in the future.