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At least 217 records · Page 12

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NASA NTRS2002

Thermoelectric properties of Nb3SbxTe7-x compounds

Niobium antimony telluride, Nb3Sbx,Te7-x, was synthesized and tested for thermoelectric properties in the Thermoelectrics group at the Jet Propulsion Laboratory. The forty atoms per unit cell of Nb3Sb2Te5 and its varied mixture of atoms yield acomplicated structure, suggesting that Nb3Sb2Te5 and related compounds may exhibit low thermal conductivity and hence a higher ZT value. Nb3SbxTe7-x, compounds were synthesized and subsequently analyzed for their Seebeck voltage, heat conduction, and electrical resistivity. Results indicate that Nb3Sb2Te5 is a heavily doped semiconductor whose thermoelectric properties are compromised by compensating n-type and p-type carriers. Attempts to dope in favor of either carrier by varying the Sb:Te ratio yielded samples containing secondary metallic phases that dominated the transport properties of the resulting compounds.

thermoelectric power generation segmented↗

NASA NTRS2002

Characterizing Thermal Properties of Melting Te Semiconductor: Thermal Diffusivity Measurements and Simulation

Tellurium is an element for many II-VI and I-III-VI(sub 2) compounds that are useful materials for fabricating many devices. In the melt growth techniques, the thermal properties of the molten phase are important parameter for controlling growth process to improve semiconducting crystal quality. In this study, thermal diffusivity of molten tellurium has been measured by a laser flash method in the temperature range from 500 C to 900 C. A pulsed laser with 1064 nm wavelength is focused on one side of the measured sample. The thermal diffusivity can be estimated from the temperature transient at the other side of the sample. A numerical simulation based on the thermal transport process has been also performed. By numerically fitting the experimental results, both the thermal conductivity and heat capacity can be derived. A relaxation phenomenon, which shows a slow drift of the measured thermal conductivity toward the equilibrium value after cooling of the sample, was observed for the first time. The error analysis and the comparison of the results to published data measured by other techniques will be discussed.

Zhu, Shen↗

NASA NTRS2002

Characterizing Thermal Properties of Melting Te Semiconductor: Thermal Diffusivity Measurements and Simulation

Tellurium is an element for many II-VI and I-III-VI(sub 2) compounds that are useful materials for fabricating many devises. In the melt growth techniques, the thermal properties of the molten phase are important parameter for controlling growth process to improve semiconducting crystal quality. In this study, thermal diffusivity of molten tellurium has been measured by a laser flash method in the temperature range from 500 C to 900 C. A pulsed laser with 1064 nm wavelength is focused on one side of the measured sample. The thermal diffusivity can be estimated from the temperature transient at the other side of the sample. A numerical simulation based on the thermal transport process has been also performed. By numerically fitting the experimental results, both the thermal conductivity and heat capacity can be derived. A relaxation phenomenon, which shows a slow drift of the measured thermal conductivity toward the equilibrium value after cooling of the sample, was observed for the first time. The error analysis and the comparison of the results to published data measured by other techniques will be discussed in the presentation.

Zhu, Shen↗

NASA NTRS2002

Wholly Aromatic Ether-imides. Potential Materials for n-Type Semiconductors

We report on the synthesis and characterization of a novel series low-molar-mass ether-imide rod-shaped model compounds. All ether-imides were obtained by terminating the appropriate rigid core dianhydride, i.e. pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and 3,3,4,4'-oxydiphthalic dianhydride (ODPA) with three flexible aryl-ether tails of different chain length. The mono-functional aryl-ether amines, i.e. 4-(3-phenoxy-phenoxy)-phenylamine (2) and 4-(3-phenoxy-3-phenoxy-phenoxy)-phenylamine (4), were synthesized using standard fluoro-displacement and Ullmann condensation techniques. The corresponding ether-imide model compounds were obtained in high yields using a one-step solution imidization procedure. Increasing the number of meta-substituted aryl-ether units reduces the melt transition temperatures and at the same time it increases the solubility of the model compounds. Most model compounds are crystalline solids and form isotropic melts upon heating. 2,7-Bis-(-4-phenoxy-phenyl)-benzo[Imn][3,8]phenanthroline1,3,6,8-tetraone (NDA-n0), however, displays a smectic A (SA) when cooled from the isotropic phase, followed by what appears to be either a highly ordered smectic phase or a, columnar phase. This is the first example, known to date, in which a mesophase is detected in a wholly aromatic ether-imide compound. For all compounds we present spectroscopic data and X-ray diffraction data. Cyclic voltammetry was used to determine the redox behavior and pertinent energy levels of the model compounds.

Dingemans, Theo J.↗

NASA NTRS1994

Ampoule failure sensor time response testing: Experiment 1

The response time of an ampoule failure sensor exposed to a liquid or vapor gallium-arsenide (GaAs) is investigated. The experimental configuration represents the sample/ampoule cartridge assembly used in NASA's Crystal Growth Furnace (CGF). The sensor is a chemical fuse made from a metal with which the semiconductor material reacts more rapidly than it does with the containing cartridge. For the III-IV compound of GaAs, a platinum metal was chosen based on the reaction of platinum and arsenic at elevated temperatures which forms a low melting eutectic. Ampoule failure is indicated by a step change in resistance of the failure sensor on the order of megohms. The sensors will increase the safety of crystal growth experiments by providing an indication that an ampoule has failed. Experimental results indicate that the response times (after a known ampoule failure) for the 0.003 and 0.010 inch ampoule failure sensors are 2.4 and 3.6 minutes, respectively. This ampoule failure sensor will be utilized in the CGF during the second United States Microgravity Laboratory Mission (USML-2) and is the subject of a NASA patent application.

Johnson, M. L.↗

NASA NTRS1982

III-V semiconductor solid solution single crystal growth

The feasibility and desirability of space growth of bulk IR semiconductor crystals for use as substrates for epitaxial IR detector material were researched. A III-V ternary compound (GaInSb) and a II-VI binary compound were considered. Vapor epitaxy and quaternary epitaxy techniques were found to be sufficient to permit the use of ground based binary III-V crystals for all major device applications. Float zoning of CdTe was found to be a potentially successful approach to obtaining high quality substrate material, but further experiments were required.

Gertner, E. R.↗

NASA NTRS1984

Recent developments in semiconductor research

Recent findings drawn from various laboratories focused toward a new understanding of the nature of crystal growth and how impurities are incorporated or excluded from the lattice were considered. From these findings a heuristic approach is given for improving the uniformity and charge carrier concentration of thin semiconductor films. Such improvement is considered essential to achieve respectable yield on large scale integrated circuits. A tentative conclusion is drawn which indicates that if the host compound crystal reactants are provided to the growing crystal surface in a reasonably stoichiometric manner, it is virtually impossible to include impurities into a growing crystal surface held at a sufficiently low temperature; impurities are incorporated into the crystal by gettering action of crystal defects located one and two monlayers beneath the growing surface. Devices in 3-5 semiconductors based on heterojunctions and two dimensional electron gases are noted and reliability concerns are voiced. Alternatives and novel device structures such as truly single crystal high quality silicon on insulator and beta silicon carbide devices are presented.

Yoder, M. N.↗

NASA NTRS1995

Preparation of Thermoelectric Materials from Melts

The most common methods to prepare semiconductors are based on use of melts or vapor phase. A tendency to use thin film structures in modern semiconducting devices brought a variety of technologies based on vapor phase deposition. The preparation of chemical compounds and solid solutions from a melt begins with melting the elemental constituents together. Melt technology and techniques are discussed.

thermoelectric↗

NASA NTRS1984

High-temperature thermoelectric energy conversion. II - Materials survey

The current status of materials research for high-temperature thermoelectric energy conversion is reviewed. Two general classes of materials show promise for high temperature figure of merit (Z) values, viz, the rare-earth chalcogenides and the boron-rich borides. The electronic transport properties of the rare-earth chalcogenides are explicable on the basis of degenerate or partially degenerate n-type semiconductors. Boron and boron-rich borides exhibit p-type hopping conductivity, with detailed explanations proposed for the transport differing from compound to compound. Some discussion is presented on the reasons for the low thermal conductivities in these materials. Also, ZTs greater than one appear to have been realized at high temperature in many of these compounds.

Wood, C.↗

NASA NTRS1987

High temperature thermoelectric energy conversion

The theory and current status of materials research for high-temperature thermoelectric energy conversion are reviewed. Semiconductors are shown to be the preferred class of materials for this application. Optimization of the figure of merit of both broadband and narrow-band semiconductors is discussed as a function of temperature. Phonon scattering mechanisms are discussed, and basic material guidelines are given for reduction of thermal conductivity. Two general classes of materials show promise for high temperature figure of merit (Z) values, namely the rare earth chalcogenides and the boron-rich borides. The electronic transport properties of the rare earth chalcogenides are explicable on the basis of degenerate or partially degenerate n-type semiconductors. Boron and boron-rich borides exhibit p-type hopping conductivity, with detailed explanations proposed for the transport differing from compound to compound. Some discussion is presented on the reasons for the low thermal conductivities in these materials. Also, ZTs greater than one appear to have been realized at high temperature in many of these compounds.

Wood, Charles↗

NASA NTRS1978

The superconductivity of certain ternary molybdenum compounds

The objectives of this work were to measure the superconductivity and critical fields of (Cu1.5Mo4.5), (SmMo5S6), and (Pb0.9Mo5.1S6) in order to reproduce the published results, and by introduction of magnetic impurities into these semiconductors, observe the compensation effect.

Odermatt, R.↗

NASA NTRS2004

Thermophysical Properties of Selected II-VI Semiconducting Melts

Thermophysical properties are essential for the accurate predication of the crystal growth process by computational modeling. Currently, the temperature dependent thermophysical property data for the II-VI semiconductor melts are scarce. This paper reports the results of the temperature dependence of melt density, viscosity and electrical conductivity of selected II-VI compounds, including HgTe, HgCdTe and HgZnTe. The melt density was measured using a pycnometric method, and the viscosity and electrical conductivity were measured by a transient torque method. The results were compared with and showed good agreement with the existing data in the literature.

Li, C.↗

NASA NTRS2015

Body of Knowledge (BOK) for Copper Wire Bonds

Copper wire bonds have replaced gold wire bonds in the majority of commercial semiconductor devices for the latest technology nodes. Although economics has been the driving mechanism to lower semiconductor packaging costs for a savings of about 20% by replacing gold wire bonds with copper, copper also has materials property advantages over gold. When compared to gold, copper has approximately: 25% lower electrical resistivity, 30% higher thermal conductivity, 75% higher tensile strength and 45% higher modulus of elasticity. Copper wire bonds on aluminum bond pads are also more mechanically robust over time and elevated temperature due to the slower intermetallic formation rate - approximately 1/100th that of the gold to aluminum intermetallic formation rate. However, there are significant tradeoffs with copper wire bonding - copper has twice the hardness of gold which results in a narrower bonding manufacturing process window and requires that the semiconductor companies design more mechanically rigid bonding pads to prevent cratering to both the bond pad and underlying chip structure. Furthermore, copper is significantly more prone to corrosion issues. The semiconductor packaging industry has responded to this corrosion concern by creating a palladium coated copper bonding wire, which is more corrosion resistant than pure copper bonding wire. Also, the selection of the device molding compound is critical because use of environmentally friendly green compounds can result in internal CTE (Coefficient of Thermal Expansion) mismatches with the copper wire bonds that can eventually lead to device failures during thermal cycling. Despite the difficult problems associated with the changeover to copper bonding wire, there are billions of copper wire bonded devices delivered annually to customers. It is noteworthy that Texas Instruments announced in October of 2014 that they are shipping microcircuits containing copper wire bonds for safety critical automotive applications. An evaluation of copper wire bond technology for applicability to spaceflight hardware may be warranted along with concurrently compiling a comprehensive understanding of the failure mechanisms involved with copper wire bonded semiconductor devices.

Body of Knowledge (BOK)↗

NASA NTRS1981

Recent work in advanced hydrogen production concepts

The hydrogen photoelectrolytic conversion activity investigated the practicability of semiconductor electrolytic devises that use solar energy to decompose water into hydrogen and oxygen in an apparent single step process. The photocatalytic decomposition of inorganic hydrogen compounds; i.e., hydrobromic and hydriodic acids using rhodium organic bridge complexes were also studied. The feasibility of direct high temperature thermal decompositions of water with diffusion processes for separation of the equilibrium mixture of hydrogen and oxygen into usable energy sources was examined.

Lawson, D. D.↗

NASA NTRS2003

Improved Crystal Quality by Detached Solidification in Microgravity

The goals of our work on detached solidification have been to: Develop a complete understanding of all of the phenomena of detached solidification; Make it possible to achieve detached solidification reproducibly; Increase crystallographic perfection through detached solidification. Specifically, we aimed in this project to: Identify a system and develop methods that would allow viewing of the melt surface and convection in the melt during detached solidification in microgravity; Improve understanding of the origination and evolution of detachment through experiments and theoretical treatments; Achieve detachment on earth. The project resulted in 14 publications, 15 presentations, completion of 2 Ph.D. theses, and completion of 2 M.S. theses. Two additional papers are currently being reviewed for publication. Copies of most of the papers are attached as appendices. Among the accomplishments are: Achievement of detached solidification of InSb on earth and determination of the conditions favoring detachment on earth. Development of a new method for coating the interior of silica growth ampoules with transparent boron nitride, which yields the high contact angles for semiconductor melts that favor detachment. Development of a new coating for the interior of Pyrex ampoules yielding very high contact angles for water and molten organic compounds. Development of a material-balance model for steady detached solidification that provides greatly improved insight into the process, both in microgravity and on earth. Dimensionless parameters were found that clarify the role of operating conditions and physical properties. The reason for the occurrence of two steady states was clarified, along with their relative stability. Observation of periodic gas tubes during directional solidification of water and organic compounds. Gas bubbles did not propagate around the periphery to yield full detachment. Failure to obtain detachment is attributed to the ready plastic deformation of these materials, so that the frozen material continues to adhere to the ampoule wall during cooling.

Regel, Liya L.↗

NASA NTRS2004

Temperature Dependence of Density, Viscosity and Electrical Conductivity for Hg-Based II-VI Semiconductor Melts

The relaxation phenomenon of semiconductor melts, or the change of melt structure with time, impacts the crystal growth process and the eventual quality of the crystal. The thermophysical properties of the melt are good indicators of such changes in melt structure. Also, thermophysical properties are essential to the accurate predication of the crystal growth process by computational modeling. Currently, the temperature dependent thermophysical property data for the Hg-based II-VI semiconductor melts are scarce. This paper reports the results on the temperature dependence of melt density, viscosity and electrical conductivity of Hg-based II-VI compounds. The melt density was measured using a pycnometric method, and the viscosity and electrical conductivity were measured by a transient torque method. Results were compared with available published data and showed good agreement. The implication of the structural changes at different temperature ranges was also studied and discussed.

Li, C.↗

NASA NTRS1996

Solid Solutions Formation: Improving the Thermoelectric Properties of Skutterudites

Materials with skutterudite structure have been known for a long time. Some of them are semiconductors. A typical skutterudite is CoSb(sub 3) and its thermoelectric properties were partially studied in the 1960's. Recently, it has been discovered that many skutterudite compounds are thermoelectrics with promising future.

skutterudite skutterudite structure thermoelectric↗

NASA NTRS1990

Energetics of molecular-beam epitaxy models

The removal energies of constituent atoms from various unreconstructed semiconductor surfaces are calculated using a Green function method. An efficient difference-equation approach within the second-neighbor tight-binding model is employed. For a compound AB, binding energies for the A and B atoms on the (111), (-1 -1 -1), (100), and (110) surfaces are calculated. Analyses are made of the energy to remove an atom from the nearly full surface and from the nearly empty surface. Results are presented for Si, GaAs, CdTe, and HgTe; and the surface sublimation energies are found to depend on surface coverage and do not display a simple linear relationship to the number of bonds broken, as is often assumed in modeling growth by MBE.

Krishnamurthy, Srinivasan↗