SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
An application in which CCD optical imaging arrays with 250,000 elements are fabricated from the best commercially available float zone Si is described. Two image defects are described which appear to be related to the starting material. The striation effect appears to be caused by resistivity and recombination center variations. Up to 30 percent variations in resistivity was measured across a sample.
The surface control temperature required to properly pose the equations of state for the Bridgman-Stockbarger and float-zone techniques are investigated.
A finite-element method has been used to study thermocapillary convection in a model of relevance to the float-zone, crystal-growth process. The geometry consists of a pair of horizontal isothermal surfaces with a Boussinesq liquid suspended between them. Because of the variation of surface tension with temperature, the temperature gradient along the free surface drives thermocapillary convection. The free surface is allowed to deform and its location is calculated along with the velocity and temperature fields. Cases in which thermocapillarity dominates and others in which it interacts with an unstable axial buoyancy gradient are treated. In order to obtain information of possible interest to potential microgravity applications, the influence of the Grashof number is investigated.
The use of lithium-ion (Li(+)) drifting to study the properties of point defects in p-type Floating-Zone (FZ) silicon crystals is reported. The Li(+) drift technique is used to detect the presence of vacancy-related defects (D defects) in certain p-type FZ silicon crystals. SUPREM-IV modeling suggests that the silicon point defect diffusivities are considerably higher than those commonly accepted, but are in reasonable agreement with values recently proposed. These results demonstrate the utility of Li(+) drifting in the study of silicon point defect properties in p-type FZ crystals. Finally, a straightforward measurement of the Li(+) compensation depth is shown to yield estimates of the vacancy-related defect concentration in p-type FZ crystals.
The relationship between the flow and solute fields during steady mass transfer of a dilute component is analyzed for multi-cellular rotating flows in the floating zone process of semiconductor growth. When the recirculating flows are weak in relation to the rate of crystal growth, a closed-form solution clearly shows the link between the convection pattern in the melt and the solute distribution across the surface of the growing solid. In the limit of strong convection, finite element calculations demonstrate the tendency of the composition to become uniform over the majority of the melt. The solute segregation in the product crystal is greatest when the recirculating motion is comparable to the rate of crystal growth, and points to the danger in attempting to grow compositionally uniform materials from a nearly convectionless melt.
Computer simulation of steady-state axisymmetrical heat transfer and fluid flow was conducted to study thermocapillary flow and melt/solid interfaces in floating-zone crystal growth under microgravity. The effects of key variables on the extent of thermocapillary flow in the melt zone, the shapes of melt/solid interfaces and the length of the melt zone were discussed. These variables are: (1) the temperature coefficient of surface tension (or the Marangoni number), (2) the pulling speed (or the Peclet number), (3) the feed rod radius, (4) the ambient temperature distribution, (5) the heat transfer coefficient (or the Biot number), and (6) the thermal diffusivity of the material (or the Prandtl number).
This project was concerned with the determination of conditions of guaranteed stability and instability for thermocapillary convection in a model of the float-zone crystal-growth process. This model, referred to as the half-zone, was studied extensively, both experimentally and theoretically. Our own earlier research determined, using energy-stability theory, sufficient conditions for stability to axisymmetric disturbances. Nearly all results computed were for the case of a liquid with Prandtl Number Pr = 1. Attempts to compute cases for higher Prandtl numbers to allow comparison with the experimental results of other researchers were unsuccessful, but indicated that the condition guaranteeing stability against axisymmetric disturbances would be a value of the Marangoni number (Ma), significantly higher than that at which oscillatory convection was observed experimentally. Thus, additional results were needed to round out the stability picture for this model problem. The research performed under this grant consisted of the following: (1) computation of energy-stability limits for non-axisymmetric disturbances; (2) computation of linear-stability limits for axisymmetric and non-axisymmetric disturbances; (3) numerical simulation of the basic state for half- and full-zones with a deformable free surface; and (4) incorporation of radiation heat transfer into a model energy-stability problem. Each of these is summarized briefly below.
Gel-xSix (x ? 10 at.%) single crystals have been grown with the float-zone technique applied within a monoellipsoid mirror furnace. The feed crystal consisted of synthesized GeO.95siO.05 polycrystalline material. Two Boron-doped (much much more than 1-2 (raised dot) 1017 at/cm3) crystals were grown under identical conditions using <100> Ge seeds. One Ge:Ga-doped (much much more than 1018 at/cm3) reference crystal was also grown in the same furnace. Compositional profiles were measured with an energy dispersive analysis by X-ray (EDAX). The maximum silicon concentration attained was 10 at.%. Micrographs of the etched crystals show sharp changes in interface curvature at the crystal edges. These are considered to be caused by solutal Marangoni convection, whose strength is assumed to be a direct function of silicon concentration, Theoretical considerations show that the flow direction, velocity, and strength varies significantly from a solutal Marangoni convection regime directly in front of the solid-liquid interface to a thermal Marangoni convection regime within the bulk melt.
Results are given for investigations into: (1) the physical limits which set the maximum practical diameters of Si crystals that can be processed by the float-zone method in a near weightless environment, and (2) the economic impact of large, space-produced Si crystals on the electronics industry. The stability of the melt is evaluated. Heat transfer and fluid flow within the melt as dependent on the crystal size and the degree and type of rotation imparted to the melt are studied. Methods of utilizing the weightless environment for the production of large, stress-free Si crystals of uniform composition are proposed. The economic effect of large size Si crystals, their potential applications, likely utilization and cost advantages in LSI, integrated circuits, and power devices are also evaluated. Foreseeable advantages of larger diameter wafers of good characteristics and the possibilities seen for greater perfection resulting from stress-free growth are discussed.
Single crystals of the metallic Ruddlesden-Popper trilayer nickelates R 4 Ni 3 O 10 ( R = La , Pr ) were successfully grown using an optical-image floating zone furnace under oxygen pressure ( p O 2 ) of 20 bar for La 4 Ni 3 O 10 and 140 bar for Pr 4 Ni 3 O 10 . Furthermore, a combination of synchrotron and laboratory x-ray single-crystal diffraction, high-resolution synchrotron x-ray powder diffraction and measurements of physical properties revealed that R 4 Ni 3 O 10 ( R = La , Pr ) crystallizes in the monoclinic P 2 1 / a ( Z = 2 ) space group at room temperature, and that a metastable orthorhombic phase ( Bmab ) can be trapped by postgrowth rapid cooling. Both La 4 Ni 3 O 10 and Pr 4 Ni 3 O 10 crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of Pr 4 Ni 3 O 10 , the MMT is found at 157.6 K. For La 4 Ni 3 O 10 , the MMT depends on the lattice symmetry: 147.5 K for Bmab vs 138.6 K for P 2 1 / a . Lattice anomalies were found at the MMT that, when considered together with the pronounced dependence of the transition temperature on subtle structural differences between Bmab and P 2 1 / a phases, demonstrate a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.
This research program focuses on analysis of the transport mechanisms in solidification processes, especially one of interest to the Microgravity Sciences and Applications Program of NASA. Research during the last year has focused on analysis of the dynamics of the floating zone process for growth of small-scale crystals, on studies of the effect of applied magnetic fields on convection and solute segregation in directional solidification, and on the dynamics of microscopic cell formation in two-dimensional solidification of binary alloys. Significant findings are given.
This award supports experimental materials physics research on a class of technologically relevant materials that cannot form under ambient conditions. With the complementary expertise at Louisiana State University (LSU) and infrastructure at Oak Ridge National Laboratory (ORNL), single crystals of these materials will be synthesized using the high-pressure floating-zone technique. Our ability to grow high-quality single crystals will facilitate advanced characterization of these materials and improve the scientific outcomes. In addition to in-house material characterization, the principle investigator will partner with scientists at ORNL and Brookhaven National Laboratory (BNL) to tackle materials problems using the state-of-the-art characterization techniques. This will allow us to explore the fundamental physics underlying the emergent phenomena only seen in materials prepared under extreme conditions. Our ultimate goal is to address one of the grand challenges of materials science: how do complex phenomena emerge from simple ingredients? The proposed project will also spearhead a unique opportunity to train “new breed” scientists, co-advised by experts at both LSU and U. S. Department of Energy National Laboratories. They will be essential for expanding materials physics research with ability to grow crystals, characterize their properties, and design new materials -- an imperative skill that is scarce in the U. S. scientific community.
In this research, the effect of carbon nanotube (CNT) addition on the structural and superconducting properties of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub y} + x wt% CNT materials with x = 0, 0.5, 1, and 3 prepared by solid state method, followed by directional grown using the laser floating zone (LFZ) technique has been investigated. XRD measurements showed that all samples are composed of almost single Bi-2212 phase. SEM images proved that Bi-2212 phase is the major one, with minor amounts of (Sr,Ca){sub 3}BiO{sub x}, and Bi-2201 secondary phases. According to the magnetic results, carbon nanotubes decrease T{sub c} values and significantly reduce the magnetic hysteresis loops area and the magnetic critical current density.
Uniaxial tungsten bronzes have received increasing interest because of their potential application as lead-free ferroelectrics as well as their flexibility-via chemical substitution. Here we demonstrate growth of single crystals of one such material, Ba 2 PrFeNb 4 O 15 , which possesses the tetragonal tungsten bronze structure, by the optical floating zone method. The challenge of this growth is to control the stability of the molten zone which allows the growth to persist for a long enough time to achieve large volume single crystals. Our work reveals that the use of a slight overpressure of air allows the molten zone to be maintained up to 7 h at a traveling speed of 4 mm/h. The obtained crystals were studied using high-resolution single crystal scattering, powder X-ray diffraction, and dielectric measurements. We find that Ba 2 PrFeNb 4 O 15 possesses a rather complex average structure with a monoclinic symmetry in a P 112/ m space group and unit cell parameters of a = 12.49188(8) Å, b = 12.49232(8) Å, c = 3.92863(1) Å, and γ = 90.056 °, with an additional incommensurate superstructural modulation superimposed. The temperature dependence of dielectric properties has been investigated by comparing as-grown, O 2 -annealed, and Ar-annealed samples. A broad relaxation dielectric peak is observed around 150 K for the O 2 -annealed sample, indicating a typical relaxor behavior below 150 K. A larger dielectric permittivity (ε' ≈ 1300) than those of corresponding ceramics (up to ε' ≈ 200) is found in the oxygen annealed crystals. In conclusion, we suggest that as-grown single crystals are oxygen deficient and that the oxygen vacancies can be eliminated by an oxygen annealing treatment.
Research is aimed at the fundamental understanding of the interactions of heat and mass transport and fluid mechanics in crystal growth from the melt, especially in how it pertains to experiments supported by the Microgravity Science and Applications program. Emphasis has been on studies of the vertical Bridgman system, on small-scale floating zone systems proposed for space flight, and on quantitative prediction of nonlinear transitions in melt/solid interface morphologies leading to the formation of dendrites in directional solidification. The analysis of vertical Bridgman growth has lead to the complete numerical analysis of transport phenomena in directional solidification and to a detailed comparison of calculations to the GaGe growth experiments of Wang and Witt. Calculations show the central role of the ampoule material in determining the radial temperature gradients in the melt and thus, the intensity of convection. Results indicate that diffusion-controlled growth can not be achieved on Earth with conventional growth systems, but are feasible in microgravity
In this work, the effect of cesium substitution on the structural and superconducting properties of Bi{sub 2}Sr{sub 2}Ca{sub 1-x}Cs{sub x}Cu{sub 2}O{sub +y} (where x = 0.0, 0.025, 0.050, 0.075, 0.10, 0.125, and 0.15) samples prepared by the solid-state reaction route and by directional growth using the laser floating zone (LFZ) technique has been studied. X-ray diffraction has shown that both kinds of samples present the 2212 phase as the major one, with pseudo tetragonal structure, accompanied by minor amounts of (Sr,Ca)CuO{sub 2} and Bi{sub 2}(Sr,Ca){sub 4}O{sub 6 + d} secondary phases. SEM-EDX results indicate that Bi-2212 phase is the major one, with small amounts of secondary phases. Cs substitution diminishes the size and content of secondary phases. Magnetization measurements reflect different behavior for sintered and LFZ grown samples. Although the diamagnetic transition is very wide for the former samples, it is very sharp for the last ones. The critical onset temperatures, T{sub C}, are around 80 and 90 K, for sintered and LFZ grown samples, respectively. Moreover, M–H measurements indicate that LFZ grown samples exhibit larger areas than the sintered ones. The intragranular critical current, J{sub C}, calculated from the M–H loops and Bean’s formula, is maximum for x = 0.025Cs sintered and x = 0.05Cs LFZ grown samples. The maximum pinning forces, F{sub Pmax}, determined in these samples are higher than those obtained in undoped ones, reaching 9 × 10{sup 7} Oe*A/cm{sup 2} and 10 × 10{sup 8} Oe*A/cm{sup 2}, at 10 K, for sintered and LFZ grown samples, respectively.
The honeycomb TbInO 3 has attracted wide research attention due to its fascinating physical properties. However, TbInO 3 single crystal was difficult to grow owing to the high melting point and serious volatilization of indium during the crystal growth. In this study, the volatilization of the indium element was effectively suppressed by controlling the growth atmosphere and pressure. The excess ratio of indium oxide was determined, and pure hexagonal TbInO 3 crystal was obtained by the laser floating zone method. Systematic studies on the crystal structure and optical and ferroelectric properties were carried out. The structure distortion resulted in the improper geometric ferroelectric revealed by single crystal diffraction and Raman spectrum measurements. The topological vortex domains and P-E hysteresis loop demonstrated the presence of ferroelectricity. TbInO 3 crystal has great potential application in vortex memory.
Commercially available bulk silicon carbide (SiC) has a high number (>2000/sq cm) of screw dislocations (SD) that have been linked to degradation of high-field power device electrical performance properties. Researchers at the NASA Glenn Research Center have proposed a method to mass-produce significantly higher quality bulk SiC. In order for this bulk growth method to become reality, growth of long single crystal SiC fibers must first be achieved. Therefore, a new growth method, Solvent-Laser Heated Floating Zone (Solvent-LHFZ), has been implemented. While some of the initial Solvent-LHFZ results have recently been reported, this paper focuses on further characterization of grown crystals and their growth fronts. To this end, secondary ion mass spectroscopy (SIMS) depth profiles, cross section analysis by focused ion beam (FIB) milling and mechanical polishing, and orientation and structural characterization by x-ray transmission Laue diffraction patterns and x-ray topography were used. Results paint a picture of a chaotic growth front, with Fe incorporation dependant on C concentration.