Crucible Lid Research Articles

Effects of particle size on the pyrolysis of spruce and poplar: Thermogravimetric analyses, DAEM modelling, validation, and prediction of secondary charring

Effects of particle size on the pyrolysis of spruce and poplar: Thermogravimetric analyses, DAEM modelling, validation, and prediction of secondary charring

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

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

Study on Evolution of Micropipes from Hexagonal Voids in 4H-SiC Crystals by Cathodoluminescence Imaging.

This paper presents an investigation on micropipe evolution from hexagonal voids in physical vapor transport-grown 4H-SiC single crystals using the cathodoluminescence (CL) imaging technique. Complementary techniques optical microscopy, scanning electron microscopy, and energy-dispersive spectroscopy (EDS) are also used to understand the formation mechanism of hexagonal voids along with the origin of pipes from these voids. The ability of CL to image variations along the depth of the sample provides new insights on how micropipes are attached to hexagonal voids that lie deep within the bulk single crystals. CL imaging confirms that multiple micropipes can originate from a single hexagonal void. EDS mapping shows that the inside of the micropipe walls exhibits higher levels of carbon. Investigation of the seed region by optical imaging shows that improper fixing of the seed to the crucible lid is the root cause for the formation of hexagonal voids that subsequently lead to micropipe formation.

Homogenization of Radial Temperature by a Tungsten Sink in Sublimation Growth of 45 mm AlN Single Crystal

To reduce the thermal stress during the sublimation growth of 45 mm AlN single crystal, a tungsten sink was put on the top of the crucible lid. Numerical experiments showed that the radial temperature gradient was reduced due to the homogenization effect on temperature as a result of the sink. Therefore, this simple tungsten sink method has the potential to grow large-size AlN ingots with fewer cracks. It also reveals that enhancing the heat exchange of the crucible lid is an effective way to improve the quality of crystal growth.

Modified Hot-Zone Design of Growth Cell for Reducing the Warpage of 6”-SiC Wafer

The modified hot-zone design, consisting of a new design and new materials for the backside of SiC seed holder was adopted for reducing stress in grown SiC crystal ingot and for reducing the warpage of 6-inch SiC wafer. Crucible lid on the backside of SiC seed holder was designed to be movable during the growth process. Based on the warp value and mapping measurement of FWHM (Full width at half maximum) value in X-ray rocking curve, the crystal quality of SiC crystals grown with new hot-zone design was observed to be better than conventional design.

A cooling fin to enhance the efficiency of crystal growth by physical vapor transport

Abstract In general for crystal growth, material should deposit on the seed crystal and not on any adjacent supporting structures. This efficiently uses the source material and avoids the possibility of spurious polycrystals encroaching on, and interfering with the single crystal growth. In this paper, a new crucible design with a cooling fin in contact with the seed was simulated and experimentally demonstrated on the physical vapor transport (PVT) crystal growth of scandium nitride (ScN). The heat transfer of the growth cavity for a conventional crucible and a modified crucible with the cooling fin were modeled theoretically via computational fluid dynamics (CFD) with FLUENT. The CFD results showed that the seed in the modified crucible was approximately 10 °C cooler than the crucible lid, while in the conventional crucible the temperature of the seed and lid were uniform. The experimental results showed that increasing the temperature gradient between the source and the seed by employing the cooling fin led to a dramatic increase in the growth rate of ScN on the seed and reduced growth on the lid. The relative growth rates were 80% and 20% on the seed and lid respectively, in the modified crucible, compared to 25% and 75% with the conventional crucible. Thus, the modified crucible improved the process by increasing the growth rate of single crystals grown by sublimation.

Investigation of the effect of Argon flow on the morphology of B4C nanoparticles synthesized by the VLS method

In this paper, new various morphologies of boron carbide were successfully synthesized using carbon black, activated carbon and boron oxide precursors as well as using cobalt nanoparticles as catalysts. Almost the whole morphology of synthesized boron carbide are consisted of smooth nanowires and nanobelts. With decreasing the carbon black particles size from 29 nm to 13 nm (29, 23, 17 and 13), the synthesis efficiency of nanowires and nanobelts are increased. With increasing the temperature from 1500 °C to 1700 °C, the amounts of nanowires and nanobelts are decreased and the amounts of nano particles and synthesizing the nanoworms are increased. By placing the catalyst nanoparticles on the crucible lid and being subjected to the argon gas flow, new morphologies of boron carbide are appeared (flower-shaped and needle-shaped are synthesized by using carbon black and activated carbon, respectively). The argon gas flow creates new branches on the main wires. The specimens were characterized by the X-ray diffraction and a scanning electron microscopy.

Thermal stress simulation of optimized SiC single crystal growth crucible structure

Abstract In order to reduce the dislocation density of SiC single crystal grown by PVT method, the thermal field was simulated based on finite element analysis. In this paper, VR-PVT SiC software was used to simulate the internal temperature gradient and thermal stress evolution caused by the different crucible structure. The change of crucible lid and sidewall structures has a better influence on radial temperature gradient and axial temperature gradient, respectively. Modified structure is a combination of these structures which improves both axial and radial temperature gradient. It reduces the maximum stress from 1.85 MPa to 1.58 MPa. The maximum shear stress and dislocation density distribution are shifted to the edge of the crystal. The optimization has only changed the external structure, so there is no conflict with the internal design.

Quality Improvement of 4’’ 4H-SiC Crystal by Using Modified Seed Adhesion Method

The method to attach seed to crucible lid as well as seed quality is very important for obtaining high quality crystals. Therefore, modified seeding method was developed for improving adhesive layer between seed and graphite crucible lid. SiC single crystal grown with modified seeding method definitely exhibited lower micropipe density (MPD) and lower full width at half maximum (FWHM) values comparing with values from conventional seeding method. Etch pit density of SiC crystal was successfully decreased with using the modified seeding method.

Separation of CsCl from a ternary CsCl–LiCl–KCl salt via a melt crystallization technique for pyroprocessing waste minimization

Abstract A parametric study has been conducted to identify the effects of several parameters on the separation of CsCl from molten LiCl–KCl salt via a melt crystallization process. A reverse vertical Bridgman technique was used to grow the salt ingots. The investigated parameters were: (1) the advancement rate, (2) the crucible lid configuration, (3) the amount of salt mixture, (4) the initial composition of CsCl, and (5) the temperature difference between the high and low furnace zones. From each grown salt ingot, samples were taken axially and analyzed using inductively coupled plasma mass spectrometry. Results show that CsCl concentrations at the top of the ingots were low and increased to a maximum at the bottom of the salt. Salt (LiCl–KCl) recycle percentages for the experiments ranged from 50% to 75% and the CsCl composition in the waste salt was low. To increase the recycle percentage and the concentration of CsCl in the waste form, multiple crystallization stages were explored showing that they were practical under the optimal experimental conditions at 5.0 mm/h rate with a lid configuration and temperature difference of 200 °C for a total of five crystallization stages. Up to 88% of the LiCl–KCl salt can be recycled under these proposed conditions.

Restraint of nucleation of SiC polycrystals surrounding the seed during SiC single crystal growth

The nucleation and growth of SiC polycrystals around seed crystals restrain the growth of SiC single crystals in the radial direction fabricated by physical vapor transport method in which the ordinary graphite is used as the crucible lid. Therefore, it is necessary to reduce the nucleation and growth of SiC polycrystals around the seed crystals. In order to effectively enlarge SiC single crystals, the authors propose the use of a graphite paper instead of graphite as the lid to restrain the nucleation of SiC polycrystals on the lid. The micrographs of SiC polycrystals on the graphite paper and graphite at the different growth stages show that the nucleation of SiC polycrystals on the graphite paper is more difficult than that on graphite. X-ray diffraction and scanning electron microscope investigations show that the graphite paper possesses high-macroscopic anisotropy, which induced that polycrystals can only grow on the surface of graphite paper and be easily removed.

Effect of the seed crystallographic orientation on AlN bulk crystal growth by PVT method

AbstractThe growth of AlN crystals by PVT method was investigated using TaC crucible in the temperature range of 2250‐2350 °C. AlN boules with 30 mm in diameter were successfully grown on the crucible lid by self‐seeded growth. The AlN boules consist of the spontaneously nucleated AlN single crystal grains with the {1010} natural crystalline face. The fast growth rate of more than 1 mm/h was achieved. AlN crystals grown on (11$ \bar 2 $0)‐, (10$ \bar 1 $0)‐, and (0001)‐face AlN seeds were investigated. Different experimental phenomena have been observed under particular condition. The crystal grown on (11$ \bar 2 $0)‐face seed has different natural crystalline face from the seed. For the crystal grown on (10$ \bar 1 $0) or (0001) seed, the crystal natural crystalline face is same as the crystallographic orientation of the seed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gas tight sintered material for high temperature sublimation setups

Abstract Fine powders of WC, TaC and TaC with W and WC additives were cold-isostatically pressed to ceramic discs and pressurelessly sintered at temperatures up to 2100 °C in a second step. Afterwards, the discs were tested as crucible lid under typical AlN growth conditions. The prepared discs should be gas tight and ensure a better alignment of the thermal expansion coefficients of TaC lid and AlN. Ceramic discs densified by this method reveal a relative density up to 97%. The TaC ceramic discs without additives show a microstructure with grain sizes in the range of 10–200 μm after sintering. The grain enlargement could be reduced by W and WC additives in the range of 1–5 wt.%. The results show that the AlN boules adhere only to WC lids and tungsten containing lids with W contents higher than or equal to 3 wt.%.

Numerical simulation of a new SiC growth system by the dual-directional sublimation method

A new SiC growth system using the dual-directional sublimation method was investigated in this study. Induction heating and thermal conditions were computed and analyzed by using a global simulation model, and then the values of growth rate and shear stress in a growing crystal were calculated and compared with those in a conventional system. The results showed that the growth rate of SiC single crystals can be increased by twofold by using the dual-directional sublimation method with little increase in electrical power consumption and that thermal stresses can be reduced due to no constraint of the crucible lid and low temperature gradient in crystals.

Homoepitaxial seeding and growth of bulk AlN by sublimation

AlN boules, 35 mm in diameter and up to 25-mm long, were grown on TaC crucible lid in an inductively heated reactor. The growth rates range between 100 and 300 μm/h. The boules grown on TaC show a columnar structure mostly composed of 〈0 0 0 1〉 grains. The largest grains (4–5 mm in diameter) were sliced and used for subsequent growth runs. Successful epitaxial seeding and growth on the starting AlN wafer was demonstrated and confirmed by electron back-scatter diffraction (EBSD) measurements. Crystals were grown on both Al and N surfaces of the seeds up to a maximum diameter of about 9 mm so far. Formation of oxy-nitride layers, very detrimental to the further AlN deposition, could be avoided when starting from pre-sintered source powder. Secondary ion mass spectroscopy (SIMS) measurements on axial cuts revealed a relatively low oxygen content, with variable distribution along the growth direction (290 ppm near seed, 100 ppm near external surface).

Sublimation growth of aluminum nitride crystals

The crystal growth of AlN unseeded on a tungsten crucible lid and seeded on a polycrystalline AlN wafer is compared. AlN crystals with a preferential (0 0 0 1) orientation were achieved in both methods, as demonstrated by electron backscattering diffraction. The AlN grain size increased with the thickness of the AlN crystals. Seeded growth produced larger grains than unseeded growth (average grain size of less than 500 μm compared to 2–3 mm). Photoluminescence confirmed the high quality of the resultant AlN crystals.

Thermoelastic stresses in SiC single crystals grown by the physical vapor transport method

A finite element-based thermoelastic anisotropic stress model for hexagonal silicon carbide polytype is developed for the calculation of thermal stresses in SiC crystals grown by the physical vapor transport method. The composite structure of the growing SiC crystal and graphite lid is considered in the model. The thermal expansion match between the crucible lid and SiC crystal is studied for the first time. The influence of thermal stress on the dislocation density and crystal quality is discussed.

Sublimation growth of AlN bulk crystals in Ta crucibles

Abstract AlN single crystals of 0.5 in diameter and up to 10–12 mm long have been grown by sublimation/recondensation in pre-treated tantalum crucibles. Growth of 45 mm diameter and 4 mm long polycrystalline AlN boules has also been demonstrated. After high-temperature pre-treatment in a carbon-containing atmosphere, the tantalum crucibles can be used for 300–400 h of AlN sublimation growth at 2200–2300 °C, without Ta impurities or additional C impurities in concentrations higher than 100 ppm appearing in the crystals. Both self-seeded growth of polycrystalline AlN on the crucible lid and seeded growth of single-crystal AlN on (0 0 0 1) SiC plates and AlN/SiC templates are demonstrated. X-ray diffractometry and topography of the grown crystals show a block structure with the characteristic block size >200 nm and the scatter of FWHMs of ω -scans in the range of 60–750 arcsec. Test slicing and polishing of the crystals and test MBE growth of AlGaN/AlN structures on the obtained AlN substrates have been successfully performed.

In situ fabrication of titanium carbide reinforced copper MMC

The in situ fabrication of titanium carbide reinforced copper and aluminium bronze (AB2) composites by carbothermal reduction of titanium in an induction furnace has been investigated. An inert atmosphere was maintained with carbon monoxide created as a byproduct from the heat of reaction between the induction field, graphite crucible and graphite lid. Titanium carbide particles of the order 1–3 μm were formed in aluminium bronze at approximately 1250°C and, in copper, particles of order 1–6 μm were produced at approximately 1330°C. Dispersion concentrations of titanium carbide of 20% and 6.5% were obtained for copper and aluminium bronze respectively. In addition, evidence is presented indicating that iron could be used as a dispersion medium for titanium carbide particulates in aluminium bronze alloys.

Virtual reactor as a new tool for modeling and optimization of SiC bulk crystal growth

We propose a new approach to optimization of SiC bulk crystal growth based on modeling. The idea is to employ a special software tool ‘‘virtual reactor’’ (VR) operated by the user of the code as an actual crystal growth system. The software tool includes the models necessary to simulate global heat transfer in the whole growth system and inside the crucible including radiative transport through the semi-transparent SiC crystal. It is known that accurate material properties are crucial for thermal modeling of SiC growth. A database with material properties of SiC crystal and powder, graphites and insulation is included into the VR-software. An advanced model of species transport during sublimation growth of SiC crystals is developed. The model includes convective and diffusive species transport, surface kinetics based on the Hertz–Knudsen equations and chemical models for all solid surfaces (SiC crystal, SiC source, graphite wall). A model to predict type of parasitic deposit and the corresponding deposition rate is combined with the mass transport model available in the VR-software. In this paper, we show the results of simulation of a large-size SiC bulk crystal growth using the VR-software tool with the focus on poly-SiC deposit formation on the graphite crucible lid around the crystal. # 2001 Elsevier Science B.V. All rights reserved.