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All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Manifestations of Dynamic Strain Aging in Soft-Oriented NiAl Single Crystals

The tensile and compressive properties of six NiAl-base single-crystal alloys have been investigated at temperatures between 77 and 1200 K. The normalized critical resolved shear stresses (CRSS/E) and work-hardening rates (Theta/E) for these alloys generally decreased with increasing temperature. However, anomalous peaks or plateaus for these properties were observed in conventional purity (CPNiAl), Si-doped (NiAl-Si), C-doped low Si (UF-NiAl1), and Mo-doped (NiAl-Mo) alloys at intermediate temperatures (600 to 1000 K). This anomalous behavior was not observed in high-purity, low interstitial material (HP-NiAl). Low or negative strain-rate sensitivities (SRS) also were observed in all six alloys in this intermediate temperature range. Coincident with the occurrence of negative strain-rate sensitivities was the observation of serrated stress-strain curves in the CPNiAl and NiAl-Si alloys. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main specie responsible for strain aging in NiAl is C but indicate that residual Si impurities can enhance the strain aging effects. The corresponding dislocation microstructures at low temperatures (300 to 600 K) were composed of well-defined cells. At intermediate temperatures (600 to 900 K), either poorly defined cells or coarse bands of localized slip, reminiscent of the vein structures observed in low-cycle fatigue specimens deformed in the DSA regime, were observed in conventional purity, Si-doped, and in Mo-doped alloys. In contrast, a well-defined cell structure persisted in the low interstitial, high-purity alloy. At elevated temperatures (greater than or equal to 1000 K), more uniformly distributed dislocations and sub-boundaries were observed in all alloys. These observations are consistent with the occurrence of DSA in NiAl single-crystal alloys at intermediate temperatures.

Weaver, M. L.↗
NASA NTRS1997

Solar Cells for Lunar Application

In this work a preliminary study of the vacuum evaporation of silicon extracted from the lunar regolith has been undertaken. An electron gun vacuum evaporation system has been adapted for this purpose. Following the calibration of the system using ultra high purity silicon deposited on Al coated glass substrates, thin films of lunar Si were evaporated on a variety of crystalline substrates as well as on glass and lightweight 1 mil (25 microns) Al foil. Extremely smooth and featureless films with essentially semiconducting properties were obtained. Optical absorption analysis sets the bandgap (about 1.1 eV) and the refractive index (n=3.5) of the deposited thin films close to that of crystalline silicon. Secondary ion mass spectroscopy and energy dispersive spectroscopy analysis indicated that these films are essentially comparable to high purity silicon and that the evaporation process resulted in a substantial reduction of impurity levels. All layers exhibited a p-type conductivity suggesting the presence of a p-type dopant in the fabricated layers. While the purity of the 'lunar waste material' is below that of the 'microelectronic-grade silicon', the vacuum evaporated material properties seems to be adequate for the fabrication of average performance Si-based devices such as thin film solar cells. Taking into account solar cell thickness requirements (greater than 10 microns) and the small quantities of lunar material available for this study, solar cell fabrication was not possible. However, the high quality of the optical and electronic properties of evaporated thin films was found to be similar to those obtained using ultra-high purity silicon suggest that thin film solar cell production on the lunar surface with in situ resource utilization may be a viable approach for electric power generation on the moon.

Freundlich, Alex↗
NASA NTRS1998

The Effect of Protein Impurities on Lysozyme Crystal Growth

While bulk crystallization from impure solutions is used industrially as a purification step for a wide variety of materials, it is a technique that has rarely been used for proteins. Proteins have a reputation for being difficult to crystallize and high purity of the initial crystallization solution is considered paramount for success in the crystallization. Although little is written on the purifying capability of protein crystallization or of the effect of impurities on the various aspects of the crystallization process, recent published reports show that crystallization shows promise and feasibility as a purification technique for proteins. In order to further examine the issue of purity in macromolecule crystallization this study investigates the effect of the protein impurities, avidin, ovalbumin and conalbumin, at concentrations up to 50%, on the solubility, crystal face growth rates and crystal purity, of the protein lysozyme. Solubility was measured in batch experiments while a computer controlled video microscope system was used to measure the f {101} and {101} lysozyme crystal face growth rates. While little effect was observed on solubility and high crystal purity was obtained (>99.99%), the effect of the impurities on the face growth rates varied from no effect to a significant face specific effect leading to growth cessation, a phenomenon that is frequently observed in protein crystal growth. The results shed interesting light on the effect of protein impurities on protein crystal growth and strengthen the feasibility of using crystallization as a unit operation for protein purification.

Judge, Russell A.↗
NASA NTRS2001

Flightweight Electro-Magnet Systems

NASA has a need for lightweight high performance magnets to be used in propulsion systems involving plasmas. We report the design, construction, and testing of a six inch diameter by twelve inch long solenoid using high purity aluminum wire operating at a temperature of 77 Kelvin (K) for the current carrying element. High purity aluminum is the material of choice because of three properties that make it optimal for magnetic construction. At 77 K high purity aluminum has one of the lowest resistivities at 77 K of any metal (p = 0.254 muOMEGA-cm), thus reducing the power requirements for creating magnetic fields. Aluminum is a low-density (2.6989 g/cc) material and the end product magnet will be of low total mass compared to similar designs involving copper or other elements. The magneto-resistance of aluminum saturates at low magnetic fields and does not increase indefinitely as is the case in copper. The magnet consists of four layers of closely wound wire and is approximately 150 mm in diameter by 300 mm long. A cylinder made from G - 10 was machined with a spiral groove to hold the high purity Al wire and the wire wound on it. Following the winding, each layer was potted in STYCAST high thermal conductivity epoxy to provide insulation between the turns of the coil and mechanical strength. The magneto-resistance of the coil has been measured at the National High Magnetic Field Laboratory (NHMFL), Tallahassee, FL in externally applied fields to 10 tesla. Following these tests it was energized to the full 2 tesla field it can produce using the facilities of the NHMFL at the Los Alamos National Laboratory. The results of all of these tests will be presented.

Goodrich, Roy G.↗
NASA NTRS2014

Determining the Source of Water Vapor in a Cerium Oxide Electrochemical Oxygen Separator to Achieve Aviator Grade Oxygen

More than a metric ton of water is transported to the International Space Station (ISS) each year to provide breathing oxygen for the astronauts. Water is a safe and compact form of stored oxygen. The water is electrolyzed on ISS and ambient pressure oxygen is delivered to the cabin. A much smaller amount of oxygen is used each year in spacesuits to conduct Extra Vehicular Activities (EVAs). Space suits need high pressure (>1000 psia) high purity oxygen (must meet Aviator Breathing Oxygen "ABO" specifications, >99.5% O2). The water / water electrolysis system cannot directly provide high pressure, high purity oxygen, so oxygen for EVAs is transported to ISS in high pressure gas tanks. The tanks are relatively large and heavy, and the majority of the system launch weight is for the tanks and not the oxygen. Extracting high purity oxygen from cabin air and mechanically compressing the oxygen might enable on-board production of EVA grade oxygen using the existing water / water electrolysis system. This capability might also benefit human spaceflight missions, where oxygen for EVAs could be stored in the form of water, and converted into high pressure oxygen on-demand. Cerium oxide solid electrolyte-based ion transport membranes have been shown to separate oxygen from air, and a supported monolithic wafer form of the CeO2 electrolyte membrane has been shown to deliver oxygen at pressures greater than 300 psia. These supported monolithic wafers can withstand high pressure differentials even though the membrane is very thin, because the ion transport membrane is supported on both sides (Fig 1). The monolithic supported wafers have six distinct layers, each with matched coefficients of thermal expansion. The wafers are assembled into a cell stack which allows easy air flow across the wafers, uniform current distribution, and uniform current density (Fig 2). The oxygen separation is reported to be "infinitely selective" to oxygen [1] with reported purity of 99.99% [2]. Combined with a mechanical compressor, a Solid Electrolyte Oxygen Separator (SEOS) should be capable of producing ABO grade oxygen at pressures >2400 psia, on the space station. Feasibility tests using a SEOS integrated with a mechanical compressor identified an unexpected contaminant in the oxygen: water vapour was found in the oxygen product, sometimes at concentrations higher than 40 ppm (the ABO limit for water vapour is 7 ppm). If solid electrolyte membranes are really "infinitely selective" to oxygen as they are reported to be, where did the water come from? If water is getting into the oxygen, what other contaminants might get into the oxygen? Microscopic analyses of wafers, welds, and oxygen delivery tubes were performed in an attempt to find the source of the water vapour contamination. Hot and cold pressure decay tests were performed. Measurements of water vapour as a function of O2 delivery rate, O2 delivery pressure, and process air humidity levels were the most instructive in finding the source of water contamination (Fig 3). Water contamination was directly affected by oxygen delivery rate (doubling the oxygen production rate cut the water level in half). Water was affected by process air humidity levels and delivery pressure in a way that indicates the water was diffusing into the oxygen delivery system.

Graf, John↗
NASA NTRS1977

Performance and emissions of a catalytic reactor with propane, diesel, and Jet A fuels

Tests were made to determine the performance and emissions of a catalytic reactor operated with propane, No. 2 diesel, and Jet A fuels. A 12-cm diameter and 16-cm long catalytic reactor using a proprietary noble metal catalyst was operated at an inlet temperature of 800 K, a pressure of 300,000 Pa and reference velocities of 10 to 15 m/s. No significant differences between the performance of the three fuels were observed when 98.5 percent purity propane was used. The combustion efficiency for 99.8-percent purity propane tested later was significantly lower, however. The diesel fuel contained 135 ppm of bound nitrogen and consequently produced the highest NOx emissions of the three fuels. As much as 85 percent of the bound nitrogen was converted to NOx. Steady-state emissions goals based on half the most stringent proposed automotive standards were met when the reactor was operated at an adiabatic combustion temperature higher than 1350 K with all fuels except the 99.8-percent purity propane. With that fuel, a minimum temperature of 1480 K was required.

Anderson, D. N.↗
NASA NTRS1977

Orbiting molecular-beam laboratory

The composition of the atmosphere within the planned orbital envelope of the Space Shuttle and the velocity necessary to maintain a stable orbit within that envelope provide unique conditions for forming a high-purity, moderate energy beam (about 5 eV) of atomic oxygen. At 500 km, for example, atomic oxygen comprises approximately 90% of the atmosphere. Since the mean thermal speed of the ambient atomic oxygen is substantially less than the orbital speed, a high-purity beam can be generated by sweeping through the gas with a series of beam-forming truncated conical shells. Characteristics of the beam, including energy distribution, flux, and purity variation with orbital altitude and methods for lowering the mean energy, are presented. Gas-surface interaction experiments that have been proposed for this laboratory are also discussed.

Outlaw, R. A.↗
NASA NTRS1979

Development of an improved toughness hyperpure silica reflective heat shield

High purity three dimensionally woven silica-silica materials were evaluated for use as a tough reflective heat shield for planetary entry probes. A special weave design was selected to minimize light piping effects through the heat shield thickness. Various weave spacings were evaluated for densification efficiency with an 0.7 micron particle size high purity silica. Spectral hemispherical reflectance was measured from 0.2 to 2.5 microns at room temperature. Reflectance increases due to densification and purity of material were measured. Reflectance of 3D hyperpure silica was higher than 3D astroquartz silica for all wavelengths. Mechanical properties were measured in beam flexure and beam shear tests. Results indicated strengths lower than reported for slip cast fused silica. Low strengths were attributed to low densities achieved through vacuum impregnation.

Rusert, E. L.↗
NASA NTRS1981

Nitridation of silicon

Silicon powders with three levels of impurities, principally Fe, were sintered in He or H2. Non-densifying mechanisms of material transport were dominant in all cases. High purity Si showed coarsening in He while particle growth was suppressed in H2. Lower purity powder coarsened in both He and H2. The same three Si powders and Si /111/ single crystal wafers were nitrided in both N2 and N2/H2 atmospheres. Hydrogen increased the degree of nitridation of all three powders and the alpha/beta ratio of the lower purity powder. Some Si3N4 whiskers and open channels through the surface nitride layer were observed in the presence of Fe, correlating with the nitridation-enhancing effects of Fe. Thermodynamic calculations showed that when SiO2 is present on the Si, addition of H2 to the nitriding atmosphere decreases the amount of SiO2 and increases the partial pressure of Si-containing vapor species, that is, Si and SiO. Large amounts of NH3 and SiH4 were also predicted to form.

Shaw, N. J.↗
NASA NTRS1984

A fluidized-bed reactor for silane pyrolysis

The silane decomposition in a fluidized bed reactor was studied. The process feasibility and operating windows were determined. Long duration tests were conducted and silicon purity was demonstrated. A high purity linear was installed in the fluid bed reactor; the FBR product was melted and single crystallized. Product purity improvements are noted.

Iya, S.↗
NASA NTRS1986

A metallurgical route to solar-grade silicon

The aim of the process is to produce silicon for crystallization into ingots that can be sliced to wafers for processing into photovoltaic cells. If the potential purity can be realized, the silicon will also be applicable for ribbon pulling techniques where the purification during crystallization is negligible. The process consists of several steps: selection and purification of raw materials, carbothermic reduction of silica, ladle treatment, casting, crushing, leaching, and melting. The leaching step is crucial for high purity, and the obtainable purity is determined by the solidification before leaching. The most difficult specifications to fulfill are the low contents of boron, phosphorus, and carbon. Boron and phosphorus can be excluded from the raw materials, but the carbothermic reduction will unavoidably saturate the silicon with carbon at high temperature. During cooling carbon will precipitate as silicon carbide crystals, which will be harmful in solar cells. The cost of this solar silicon will depend strongly on the scale of production. It is as yet premature to give exact figures, but with a scale of some thousand tons per year, the cost will only be a few times the cost of ordinary metallurgical silicon.

Schei, A.↗
NASA NTRS1996

Observations of Dynamic Strain Aging in Polycrystalline NiAl

Dynamic strain aging has been investigated at temperatures between 77 and 1100 K in eight polycrystalline NiAl alloys. The 0.2% offset yield stress and work hardening rates for these alloys generally decreased with increasing temperature. However, local plateaus or maxima were observed in conventional purity and carbon doped alloys at intermediate temperatures (600-900 K). This anomalous behavior was not observed in low interstitial high-purity, nitrogen doped, or in titanium doped materials. Low or negative strain rate sensitivities (SRS) were also observed in all eight alloys in this intermediate temperature range. Coincident with the occurrence of negative SRS was the occurrence of serrated flow in conventional purity alloys containing high concentrations of Si in addition to C. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main species causing strain aging in polycrystalline NiAl is C but indicate that residual Si impurities can enhance the strain aging effect.

Weaver, M. L.↗
NASA NTRS2004

Residual Gas Effects on Detached Solidification in Microgravity

Our long term goal has been to make detached solidification reproducible, which requires a full understanding of the mechanisms underlying it. Our Moving Meniscus Model of steady-state detachment predicts that it depends strongly on the surface tension of the melt and the advancing contact angle with the ampoule wall. Thus, the objective of the current project was to determine the influence of residual gases on the surface tension and contact angle of molten semiconductors on typical ampoule materials. Our focus was on the influence of oxygen on indium antimonide on clean silica ("quartz"). The research was performed by three chemical engineering graduate students, the third of whom will complete his research in the summer of 2005. Originally, we had planned to use a sealed silica cell containing a zirconia electrochemical element to control the oxygen partial pressure. However, zirconia requires an operating temperature above the 530 C melting point of InSb and is difficult to form a gas-tight seal with silica. Thus, we decided instead to flow an oxygen-containing gas through the cell. A special apparatus was designed, built and perfected. A piece of InSb was placed on a horizontal silica plate in a quartz cell. High purity argon, helium or hydrogen-containing gas is passed continuously through the cell while the oxygen concentration in the effluent gas is measured. The shape of the resulting drop was used to determine contact angle and surface tension of Ga-doped and high purity InSb. Oxygen appeared to decrease the contact angle, and definitely did not increase it. The following section gives the background for the research. Section 2 summarizes the results obtained on Ga-doped InSb with relatively high oxygen concentrations. Section 3 describes recent improvements made to the apparatus and methods of analysis. Section 4 gives recent results for high-purity InSb at low oxygen concentrations. Final results will be obtained only this summer (2005). Each section has its own references.

Wilcox, William R.↗
NASA NTRS2006

Chemical Characterization and Thermal Stressing Studies of Perfluorohexane Fluids for Space-Based Applications

Perfluorohexane (PFH), C6F14, is a perfluorocarbon fluid. Several PFH fluids with different isomer concentrations were evaluated for use in an upcoming NASA space experiment. Samples tested included two commercially obtained high-purity n-perfluorohexane (n-PFH) fluids and a technical grade mixture of C6F14 branched and linear isomers (FC-72(TradeMark)). These fluids were evaluated for exact chemical composition, impurity purity and high temperature degradation behavior (pyrolysis). Our investigation involved simulated thermal stressing studies of PFH fluids under conditions likely to occur in the event of an atmospheric breach within the International Space Station (ISS) and subsequent exposure of the vapors to the high temperature and catalyst present in its Trace Contaminant Control Subsystem (TCCS). Exposure to temperatures in the temperature range of 200-450 C in an inert or oxidizing atmosphere, with and without the presence of catalyst was investigated. The most aggressive conditions studied were exposure of PFH vapors to 450 C in air and in the presence of TCCS (palladium) catalyst. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography (GC) analyses were conducted on the perfluorohexane samples before and after pyrolysis. The FC-72 and n-PFH samples showed no significant degradation following pyrolysis even under the most aggressive study conditions. Some trace level impurities associated with the PFH samples such as linear perfluorocarbon monohydrides or monoiodides were destroyed by pyrolysis at the upper limit. Other trace level impurities such as olefinic or cycloolefinic perfluorocarbons were converted into oxidation products by pyrolysis. The purity of PFH following pyrolysis actually increased slightly as a consequence since these trace contaminants were effectively scrubbed from the samples. However, since the initial concentrations of the thermally-impacted impurities were so low, the net effect was trivial. A potential byproduct of exposure of perfluorohexane fluids to high temperatures is the production of perfluoroisobutene (PFiB), which is extremely toxic. An ultra-high sensitivity PFiB-specific analysis based on GC-MS with negative ion chemical ionization (NICI) detection was used to evaluate the samples following thermal stressing. The perfluorohexanes examined here under conditions reflective of the ISS TCCS environment showed no signs of PFiB production with an analytical detection limit of 10 part per billion (ppb v/v).

Arnold, William A.↗
NASA NTRS2009

Distillation and Air Stripping Designs for the Lunar Surface

Air stripping and distillation are two different gravity-based methods, which may be applied to the purification of wastewater on the lunar base. These gravity-based solutions to water processing are robust physical separation techniques, which may be advantageous to many other techniques for their simplicity in design and operation. The two techniques can be used in conjunction with each other to obtain high purity water. The components and feed compositions for modeling waste water streams are presented in conjunction with the Aspen property system for traditional stage distillation models and air stripping models. While the individual components for each of the waste streams will vary naturally within certain bounds, an analog model for waste water processing is suggested based on typical concentration ranges for these components. Target purity levels for the for recycled water are determined for each individual component based on NASA s required maximum contaminant levels for potable water Distillation processes are modeled separately and in tandem with air stripping to demonstrate the potential effectiveness and utility of these methods in recycling wastewater on the Moon. Optimum parameters such as reflux ratio, feed stage location, and processing rates are determined with respect to the power consumption of the process. Multistage distillation is evaluated for components in wastewater to determine the minimum number of stages necessary for each of 65 components in humidity condensate and urine wastewater mixed streams. Components of the wastewater streams are ranked by Henry s Law Constant and the suitability of air stripping in the purification of wastewater in terms of component removal is evaluated. Scaling factors for distillation and air stripping columns are presented to account for the difference in the lunar gravitation environment. Commercially available distillation and air stripping units which are considered suitable for Exploration Life Support are presented. The advantages to the various designs are summarized with respect to water purity levels, power consumption, and processing rates.

Boul, Peter J.↗
NASA NTRS2011

A Cabin Air Separator for EVA Oxygen

Presently, the Extra-Vehicular Activities (EVAs) conducted from the Quest Joint Airlock on the International Space Station use high pressure, high purity oxygen that is delivered to the Space Station by the Space Shuttle. When the Space Shuttle retires, a new method of delivering high pressure, high purity oxygen to the High Pressure Gas Tanks (HPGTs) is needed. One method is to use a cabin air separator to sweep oxygen from the cabin air, generate a low pressure/high purity oxygen stream, and compress the oxygen with a multistage mechanical compressor. A main advantage to this type of system is that the existing low pressure oxygen supply infrastructure can be used as the source of cabin oxygen. ISS has two water electrolysis systems that deliver low pressure oxygen to the cabin, as well as chlorate candles and compressed gas tanks on cargo vehicles. Each of these systems can feed low pressure oxygen into the cabin, and any low pressure oxygen source can be used as an on-board source of oxygen. Three different oxygen separator systems were evaluated, and a two stage Pressure Swing Adsorption system was selected for reasons of technical maturity. Two different compressor designs were subjected to long term testing, and the compressor with better life performance and more favorable oxygen safety characteristics was selected. These technologies have been used as the basis of a design for a flight system located in Equipment Lock, and taken to Preliminary Design Review level of maturity. This paper describes the Cabin Air Separator for EVA Oxygen (CASEO) concept, describes the separator and compressor technology trades, highlights key technology risks, and describes the flight hardware concept as presented at Preliminary Design Review (PDR)

Graf, John C.↗
NASA NTRS2005

Atomic Clock Based on Opto-Electronic Oscillator

A proposed highly accurate clock or oscillator would be based on the concept of an opto-electronic oscillator (OEO) stabilized to an atomic transition. Opto-electronic oscillators, which have been described in a number of prior NASA Tech Briefs articles, generate signals at frequencies in the gigahertz range characterized by high spectral purity but not by longterm stability or accuracy. On the other hand, the signals generated by previously developed atomic clocks are characterized by long-term stability and accuracy but not by spectral purity. The proposed atomic clock would provide high spectral purity plus long-term stability and accuracy a combination of characteristics needed to realize advanced developments in communications and navigation. In addition, it should be possible to miniaturize the proposed atomic clock. When a laser beam is modulated by a microwave signal and applied to a photodetector, the electrical output of the photodetector includes a component at the microwave frequency. In atomic clocks of a type known as Raman clocks or coherent-population-trapping (CPT) clocks, microwave outputs are obtained from laser beams modulated, in each case, to create two sidebands that differ in frequency by the amount of a hyperfine transition in the ground state of atoms of an element in vapor form in a cell. The combination of these sidebands produces a transparency in the population of a higher electronic level that can be reached from either of the two ground-state hyperfine levels by absorption of a photon. The beam is transmitted through the vapor to a photodetector. The components of light scattered or transmitted by the atoms in the two hyperfine levels mix in the photodetector and thereby give rise to a signal at the hyperfine- transition frequency. The proposed atomic clock would include an OEO and a rubidium- or cesium- vapor cell operating in the CPT/Raman regime (see figure). In the OEO portion of this atomic clock, as in a typical prior OEO, a laser beam would pass through an electro-optical modulator, the modulated beam would be fed into a fiber-optic delay line, and the delayed beam would be fed to a photodetector. The electrical output of the photodetector would be detected, amplified, filtered, and fed back to the microwave input port of the modulator. The laser would be chosen to have the same wavelength as that of the pertinent ground-state/higher-state transition of the atoms in the vapor. The modulator/ filter combination would be designed to operate at the microwave frequency of the hyperfine transition. Part of the laser beam would be tapped from the fiberoptic loop of the OEO and introduced into the vapor cell. After passing through the cell, this portion of the beam would be detected differentially with a tapped portion of the fiber-optically-delayed beam. The electrical output of the photodetector would be amplified and filtered in a loop that would control a DC bias applied to the modulator. In this manner, the long-term stability and accuracy of the atomic transition would be transferred to the OEO.

Maleki, Lute↗
NASA NTRS2016

Design and Growth of Novel Compounds for Radiation Sensors: Multinary Chalcogenides

Increasing threats of radiological weapons have revitalized the researches for low cost large volume -ray and neutron ray sensors In the past few years we have designed and grown ternary and quaternary lead and thallium chalcogenides and lead selenoiodides for detectors to meet these challenges. These materials are congruent, can be tailored to enhance the parameters required for radiation sensors. In addition, this class of compounds can be grown by Bridgman method which promises for large volume productions. We have single crystals of several compounds from the melt including Tl3AsSe3, Tl3AsSe3-xSx, TlGaSe2, AgGaGe3Se8, AgxLi1-xAgGaGe3Se8 and PbTlI5-x Sex compounds. Experimental studies indicate that these have very low absorption coefficient, low defect density and can be fabricated in any shape and sizes. These crystals do not require post growth annealing and do not show any second phase precipitates when processed for electrode bonding and other fabrication steps. In this paper we report purification, growth and fabrication of large Tl3AsSe3 (TAS) crystals. We observed that TAS crystals grown by using further purification of as supplied high purity source materials followed by directionally solidified charge showed higher resistivity than previously reported values. TAS also showed constant value as the function of voltage. A low thermal gradient and high purity source material were used to reduce thermal stresses in large crystals. By improving the purification of the as supplied source materials very high quality thallium, selenium and arsenic was achieved for preparing stoichiometric Tl3AsSe3 compound. Low gradient (<20K/cm) and slow growth rate (1-2 cm/day) produced crystals with reduced stress. Crystals did not show any micro cracking during fabrication of crystals grown with high purity and at low thermal gradient. Since thallium is a major component and very sensitive to surface oxidation, removal of surface and bulk oxides is very important. Intentional increase in the growth rate from 1cm/day to higher speed (>5cm/day) showed very different morphologies on the surface of the crystals. Electrical resistivity was one order of magnitude higher than previously reported value and it was observed to be constant as the function of frequency.

Singh, N. B.↗