Gradient Furnace Research Articles

Advanced approaches to functional glasses

Two key technologies were reviewed to propose advanced approaches to functional glasses. The first is combinatorial glass research system. The combinatorial glass research system consists of an automatic batch preparation apparatus, two types of automatic apparatuses for melting of glass batches in crucibles and two types of apparatuses for evaluation of thermal stability, namely a DTA apparatus equipped with sample changer and a temperature gradient furnace. The combinatorial system was effective in searching for new glass forming systems and to speed up preparation of glass samples. The second technology is a new technique based on anodization of thin film aluminum. The anodic oxidation of sputtered aluminum film was applied to preparation of alumina nanostructures possessing nanopore arrays on glass surfaces. Oxides or metallic compounds could be introduced into nanopores using sol–gel technique or electro deposition technique. The introduction of TiO 2 by sol–gel process yielded high performance photo catalysts and the impregnation of Ni metal or Fe–Pt alloy into nanopores by electro deposition gave high density magnetic rod array of about 1 Tbit/in. 2. The new processes based on anodic oxidation of sputtered aluminum and titanium would be very effective to develop functional nanostructure layers on glass surfaces.

Effect of Travelling Magnetic Field on the Directional Solidification of Refined Al-3.5 wt%Ni Alloys

The specific objective of the paper is to investigate the effect of fluid flow on the development of a dendritic microstructure and more particularly on the columnar and equiaxed solidification regimes. A gradient furnace (BATMAF) with a possibility of applying a travelling magnetic field was designed for the solidification of aluminium-based alloys. The travelling magnetic field (TMF) drives a forced flow in the liquid above the solidifying interface. Some experiments are achieved in this furnace on an Al-Ni3.5wt% refined alloy. As a result, a significant change of the microstructure due to the transversal melt flow is observed. This work is performed in the framework of the CETSOL/MICAST Microgravity Application Projects of the European Space Agency (ESA).

Simulation of ESA's msl Furnace Inserts and Sample-Cartridge Assemblies Using the Thermal Modeling Tool Crysvun

has commissioned the Crystal Growth Laboratory in the framework of the Materials Science Laboratory (MSL) User Support Program to develop the MSL furnace inserts and samplecartridge assemblies Thermal Modeling Tool (TMT). The TMT assists the definition, preparation and analysis of the experiments onboard the International Space Station (ISS). The tool is based on the CrysVUn software, which was especially designed for global simulation of heat and mass transport processes during crystal growth and alloy solidification in high tem perature furnaces with complex (axi-symmetric) geometries. The main features of CrysVUn are briefly presented in this paper. The preliminary thermal model of the Low Gradient Furnace is illustrated and the model optimization strategy using genetic algorithms is briefly explained. Initial correlation results show that good agreement between simulated and measured axial temperature distributions is obtained. Direct modeling of the radiative heat transfer through the heater Multi-Layer Insulation (MLI) using view factors appears to be necessary for an accurate predict on of the resulting heater power. An example for the application of the TMT is presented.

Computational modeling of heat transport in a multi-zone high-pressure vertical electro-dynamic gradient CdZnTe furnace

The unsteady-state heat transport was computationally modeled in a multi-zone high-pressure vertical electro-dynamic gradient furnace using the Fluent software package. A 15,000-element, two-dimensional (2D) axisymmetric mesh and a 800,000-element 3D mesh were generated to model the solidification of CdZnTe during all phases of crystal growth. Temperature and flow fields were computed, and the effects of several materials properties and design parameters were quantified. The accuracy of the model was verified by experiment using both a graphite test fixture and a simulated load. Finally, the inclusion of a turbulent model to the simulation was shown to affect the gas phases, but not the molten CdZnTe.

Effect of forced melt flow on microstructure evolution in AlSi7Mg0.6 alloy during directional solidification

The AlSi7Mg0.6 alloy is commonly used in foundry industry to produce high stressed automotive components like motor frames and cylinder heads. The properties of the material correspond directly to their microstructure which is given by the parameters of the solidification process. The effect of forced melt flow on the microstructure evolution in AlSi7Mg0.6 alloy during directional solidification is investigated experimentally. For that purpose a Bridgman-type gradient furnace is equipped with a rotating magnetic field to induce flow in the melt. As a result the forced melt flow substantially changes the dendritic solidification microstructure. Increasing intensity of the melt flow leads to axial macrosegregation and enrichment of silicon in the centre part of a sample. Here, this yields to a transition to mixed columnar-equiaxed or even to equiaxed growth, whereas in the out-of-centre region columnar dendritic growth persists.

Directional solidification in a AgCuSn eutectic alloy

An experimental study of microstructures in a directionally solidified, near-eutectic Ag-Cu-Sn alloy has been completed. This material is an important candidate for use as a lead-free solder, and the studies show the origin and velocity dependence of some of the microstructures seen in solder joints. Quantitative stereology and microstructural observation were completed for directional solidification experiments where the sample was moved through a gradient furnace at velocities between 0.826 and 500 µm/s.

Effect of microstructure on mechanical properties of as-cast Mg-Al alloys

The mechanical properties of Mg-Al alloys are mainly determined by the microstructure, i.e., the amount and morphology of the phases, but also by the presence of defects arising from the melt handling and casting process. In order to obtain information about the isolated effect of the microstructure, it is, therefore, necessary to minimize the amounts of defects. In this study, this has been achieved by remelting and solidifying the alloys in a gradient furnace. The drawing rate was varied from 0.3 to 6 mm/s, which yielded a wide variety of microstructures. Three samples were produced for each parameter set, in order to have a statistical basis for the evaluation. The results showed that homogeneous and reproducible samples could be produced, and that the tensile properties obtained showed a very small scatter. The effects of microstructural parameters such as grain size, secondary dendrite arm spacing (SDAS), eutectic fraction, and eutectic morphology on the yield strength, ultimate tensile strength (UTS), fracture elongation, and hardness has been investigated.

Combinatorial fluorescence lifetime measuring system for developing Er-doped transparent glass ceramics

Fluorescence lifetime of Er 3+ was measured for F-doped tellurite glasses with parallel heat treatment under a temperature gradient atmosphere in order to find the annealing condition to make transparent glass ceramics in which Er 3+ ions are located in the precipitated crystals. The preparation and annealing of the samples were performed in a vertical temperature gradient furnace, where molten glass was sucked into a pre-heated Pyrex glass tube. The annealing temperature range is between 350 and 800 °C. After the annealing treatment, time-resolved fluorescence emission of Er 3+ (1.55 μm; excitation light source is 977 nm) were measured sequentially along the tube. The lifetime of the emission was about 2.6 ms for as-prepared glass. We have found that the lifetime increased to 5.2 ms when the glass was annealed at 470 °C for 5 min and 550 °C for 5 min successively, although its transparency was lost. This increase implies that the Er 3+ ions are embedded in fluorine-rich phase to bring about reduced non-radiative emission. We are now continuing to find the condition to get transparent glass ceramics.

Numerical modeling of crystal growth and solidification experiments carried out under microgravity conditions

AbstractIn this paper first examples for global thermal modeling of microgravity experiments in the field of crystal growth and alloy solidification are presented. The software program CrysVUn is used to simulate alloy solidification experiments carried out in TEXUS sounding rocket missions. Based on this software global thermal models are developed for the furnace inserts Low Gradient Furnace (LGF) and Solidification and Quenching Furnace (SQF) to be operated in the Materials Science Laboratory (MSL) onboard the International Space Station (ISS). These models will be used in order to optimize the process conditions and cartridge designs for the experiments to be carried onboard the ISS.

Fabrication of large-grain bulk superconductors in microgravity environment

The experiment to fabricate large-grain RE–Ba–Cu–O (RE: Rare-Earth element) bulk superconductors in microgravity environment will be performed in the Unmanned Space Experiment Recovery System project. It is difficult to grow high-quality large-grain superconductors on the ground due to the chemical contamination from the substrate and the segregation of liquid phase. Such problems can be overcome by growing large RE–Ba–Cu–O bulk in microgravity environment, in which the bulk can be supported by a seed crystal alone during the crystal growth. In this paper, we describe the experiment plan, the materials composition, design of a temperature gradient furnace and the conditions for the crystal growth.

Flux growth and charácterization of quasi cubic PZN-PT single crystals

Transparent single crystals of the perovskite Pb(Zn-1/3,Nb-2/3)O-3-PbTiO3 (PZN-PT) were grown from different lead fluxes in sealed platinum crucibles. A temperature gradient furnace was used (temperature gradient on crucible: 30-50 degreesC). Optimization of growth temperature range and cooling rates was carried out in fluxes of PbO, Pb3O4 with and without an addition of B2O3. The growth nature range was typically 1150 degreesC --> 930 degreesC (on crucible top) with cooling rates of 0.8 - 1 degreesC/h for the Pb3O4 flux. In these conditions, the best PZN-PT transparent single crystal was 4x4x2.5mm large and it showed a cubic habitus. The problem of crystal recovery from the residual flux Was solved by different methods based on liquid flux elimination at the end of the growth process.:The chemical composition and the homogeneity within the crystals were determined by electron probe micro-analysis and X-rays. Optical (polarized light microscope) and dielectric measurements were used for the characterization and determination of phase transitions, in single crystal with and without an electric field.

Gravity and temperature effects on particle distribution in Al–Si/SiC composites

During the EuroMIR’95 space flight four hypo-eutectic Al–Si alloy matrix composites with a SiC dispersion were made molten at different thermal gradients and solidified with a different cooling rate in the TITUS (Tubular furnace with Integrated Thermal analysis Under Space conditions) tube furnace. On comparison with the ground processed reference samples, it appears that the distribution of the particles in the molten zone is much more homogeneous in the zero gravity cast specimens. Also additional tests were performed in a high gradient furnace (ASPF — advanced solidification processing facility) and in a low gradient furnace to complement the results of the space processed samples. A higher temperature gradient, where the gradient is parallel to the direction of solidification and opposite to the gravity, gives less convection. A decrease in the cooling rate leads to larger dendrites and cells, and to a decrease of the homogeneity of the SiC particle distribution. The SiC particles settle in the terrestrial experiments, but the distribution differs as a function of the thermal profile. The SiC particles form a network in the liquid state but will, during resolidification, be pushed to the last solidified zone, between the α-Al dendrites, into the Al–Si eutectic zone.

New approach to the fabrication process of bulk high-Tcsuperconducting oxide materials with strong pinning of magneticflux

A new approach to the fabrication process of bulk HTSCmaterials is proposed, using the initial mixture of components (YBa2Cu3O7-x and non-superconducting inclusions) in the formof nanocrystalline powders. The preparation of ultra-fine powders is by meansof the pulse-plasma channel method, which allows metallic, oxide and alloymaterials etc (including HTSC) to be dispersed into spherical particles of20-200 nm size. Bulk specimens of YBa2Cu3O7-x(d~250 nm) + 2% ZrO2 (d~70 nm) were produced by pullingthe initial mixed powders through a gradient furnace. The results ofstructural, magnetic, and magneto-optical measurements are reported for thepowders and the bulk samples. The critical current density, jc, of the bulkcomposites range from 104 to 2×104 A cm-2 at 77 K inzero magnetic field. The magnetic measurements and magneto-opticalobservations reveal that jc is independent of direction in the compositeand corresponds to the transport current through the whole sample. The meandistance between pinning sites coincides with the mean grain size of YBa2Cu3O7-x, which confirms homogeneous distribution of theinclusion particles.

Vertical Bridgman growth of Al xGa 1− xSb single crystals

Al x Ga 1− x Sb crystals were grown by vertical Bridgman method with (1 1 1)B oriented GaSb seeds in a two-zone vertical gradient furnace. The growth was conducted using a two-step growth process, which consists of the growth under a constant temperature period and the growth in a vertical gradient freeze mode. During the period of the constant-temperature profile, the growth was realized by the transport of Al to the solid–liquid interface. Al x Ga 1− x Sb single crystals with a diameter of 14.5 mm and a length of more than 27 mm were successfully obtained in pyrolitic boron nitride (PBN) crucibles. The longitudinal cross sections were investigated by using a Nomarski differential interference contrast microscope (N-DICM) and it was found that strong striations were induced due to the convection on the ground. The AlSb distribution was measured by spatially resolved photoluminescence (SRPL) and it was found that AlSb composition reached 10.4%.

Growth and Properties of Transparent Pb(Zn1/3, Nb2/3)O3–PbTiO3 Single Crystals with a Cubic Habitus

Transparent single crystals of Pb(Zn1/3, Nb2/3)O3–PbTiO3 (PZN–PT) perovskite have been grown by the slow cooling method in a temperature gradient furnace. The optimization of growth techniques to crystallize PZN–PT with a cubic habitus is described in details. The growth temperature range and cooling rates was carried out in different lead fluxes. Crystal recovery from the residual flux was obtained by different techniques based on liquid flux elimination at the end of the growth process. The best transparent single crystals of PZN–PT, obtained by a cooling at 0.8°C/h from 1150°C to 930°C, had a cubic habitus and a size of approx.: 4×4×2.5 mm3. The chemical composition and the homogeneity of the crystals were tested by electron probe micro-analysis. Crystals characterization was performed using polarized light microscopy and dielectric and piezoelectric measurements. Phase transitions were determined by differential scanning calorimetry, optical and dielectric measurements.

New high temperature furnace for structure refinement by powder diffraction in controlled atmospheres using synchrotron radiation

A low thermal gradient furnace design is described which utilizes Debye–Scherrer geometry for performing high temperature x-ray powder diffraction with synchrotron radiation at medium and high energies (35–100 keV). The furnace has a maximum operating temperature of 1800 K with a variety of atmospheres including oxidizing, inert, and reducing. The capability for sample rotation, to ensure powder averaging, has been built into the design without compromising thermal stability or atmosphere control. The ability to perform high-resolution Rietveld refinement on data obtained at high temperatures has been demonstrated, and data collected on standard Al2O3 powder is presented. Time-resolved data on the orthorhombic to rhombohedral solid state phase transformation of SrCO3 is demonstrated using image plates. Rietveld refinable spectra, collected in as little as 8 s, opens the possibility of performing time-resolved structural refinements of phase transformations.

Morphology and etching studies on YBCO and CuO single crystals

Single crystals of YBCO (Y123) and CuO have been grown from a BaO–CuO (28:72) flux using a low axial gradient furnace (∼1°C/cm). Growth morphology of the grown crystals was studied using an optical microscope, scanning electron microscopy and atomic force microscopy. Cell parameter values and chemical composition of the grown crystals were determined from the X-ray diffraction data and inductively coupled plasma analysis, respectively. Etching studies were done for both the crystals using different etchants. Oxygen stoichiometry of the YBCO crystal was determined by iodometry titration analysis.

Materials with an Acoustic Impedance Gradient

This paper focusses on processing and characterization of aluminum- and copper-based materials which have a gradient in the acoustic impedance and can be used for ultrasonic diagnostics. In the copper-based material the amount of tungstencarbide is varied by solidification in a Bridgman-type gradient furnace vertically upwards and sedimentation of the tungstencarbide particles. The gradient in the aluminum-based material is produced by melt diffusion in a high temperature furnace by variation of the process time and succeeding quenching. To characterize the gradient materials the the acoustic impedance gradient is determined. The gradient couplers are tested in practice by using an ultrasonic set-up based on the pulse-echo method. It is found that the intensity of the detected echo can be increased up to a factor of two, i.e. about 12 dB. by using the gradient materials as acoustic couplers.

Planar to cellular transition in the system succinonitrile–acetone during directional solidification of a bulk sample

Directional solidification with a cellular solid–liquid interface morphology is characterized by two typical length scales – the size of the cells and the thickness of the solutal boundary layer ahead of the growing interface. The interaction of the solutal field with the morphology is investigated by using the model alloy succinonitrile–acetone which allows by its transparency the in situ observation of the interface structure. To avoid wall effects and decouple the solidification parameters temperature gradient and solidification velocity a rod-like bulk sample and a Bridgman Stockbarger-type gradient furnace are used. In the experiments the development of the interface morphology is affected by buoyancy driven melt convection which stabilizes significantly the planar morphology. At the planar to cellular transition quasi-hexagonal and elongated cells exist simultaneously, both at grain boundaries as well as inside the grains. With increasing solidification length a quasi-hexagonal pattern covers the interface which shows a random rearrangement of the cells. The size of the cells varies in a wide range and no wavelength selection is found.

Application of SiC‐Si functionally gradient material to thermoelectric energy conversion device

AbstractBecause of its high–temperature chemical stability, SiC ceramic is a promising material for high‐temperature device applications such as thermoelectric energy converters. However, the electrical conductivity of SiC ceramic is too low for it to be used as a thermoelectric energy converter at the cold junction. Therefore, we propose a SiC‐Si functionally gradient material (FGM) in order to improve the electrical conductivity of the SiC ceramic at the cold junction.An SiC rod was fired in a temperature gradient furnace. One end of the SiC rod was maintained at 2473 K and the other end was maintained at 1973 K for 30 min. After firing, the porous SiC edge fired at 1973 K was dipped into molten Si in order to infiltrate molten Si into the porous SiC.The microstructure of the FGM is classified into three regions: the SiC‐Si composite material; the porous SiC ceramic; and the densified SiC ceramic. The electrical conductivity, the Seebeck coefficient and the thermal conductivity for each region of SiC‐Si FGM was measured at 300 K; a figure of merit was calculated. The figure of merit of the SiC‐Si FGM at the cold junction, at room temperature, was 108 times higher than that of a nongradient SiC ceramic.