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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.

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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Resistance to Rolling in the Adhesive Contact of Two Elastic Spheres

For the stability of agglomerates of micron sized particles it is of considerable importance to study the effects of tangential forces on the contact of two particles. If the particles can slide or roll easily over each other, fractal structures of these agglomerates will not be stable. We use the description of contact forces by Johnson, Kendall and Roberts, along with arguments based on the atomic structure of the surfaces in contact, in order to calculate the resistance to rolling in such a contact. It is shown that the contact reacts elastically to torque forces up to a critical bending angle. Beyond that, irreversible rolling occurs. In the elastic regime, the moment opposing the attempt to roll is proportional to the bending angle and to the pull-off force P(sub c). Young's modulus of the involved materials has hardly any influence on the results. We show that agglomerates of sub-micron sized particles will in general be quite rigid and even long chains of particles cannot be bent easily. For very small particles, the contact will rather break than allow for rolling. We further discuss dynamic properties such as the possibility of vibrations in this degree of freedom and the typical amount of rolling during a collision of two particles.

Dominik, C.↗
NASA NTRS1995

Resistance to Rolling in the Adhesive Contact of Two Elastic Spheres

For the stability of agglomerates of micron sized particles it is of considerable importance to study the effects of tangential forces on the contact of two particles. If the particles can slide or roll easily over each other, fractal structures of these agglomerates will not be stable. We use the description of contact forces by Johnson, Kendall and Roberts, along with arguments based on the atomic structure of the surfaces in contact, in order to calculate the resistance to rolling in such a contact. It is shown that the contact reacts elastically to torque forces up to a critical bending angle. Beyond that, irreversible rolling occurs. In the elastic regime, the moment opposing the attempt to roll is proportional to the bending angle and to the pull-off force P(sub c). Young's modulus of the involved materials has hardly any influence on the results. We show that agglomerates of sub-micron sized particles will in general be quite rigid and even long chains of particles cannot be bent easily. For very small particles, the contact will rather break than allow for rolling. We further discuss dynamic properties such as the possibility of vibrations in this degree of freedom and the typical amount of rolling during a collision of two particles.

Dominik, C.↗
NASA NTRS2009

Expansion Compression Contacts for Thermoelectric Legs

In a proposed alternative to previous approaches to making hot-shoe contacts to the legs of thermoelectric devices, one relies on differential thermal expansion to increase contact pressures for the purpose of reducing the electrical resistances of contacts as temperatures increase. The proposed approach is particularly applicable to thermoelectric devices containing p-type (positive-charge-carrier) legs made of a Zintl compound (specifically, Yb14MnSb11) and n-type (negative charge-carrier) legs made of SiGe. This combination of thermoelectric materials has been selected for further development, primarily on the basis of projected thermoelectric performance. However, it is problematic to integrate, into a practical thermoelectric device, legs made of these materials along with a metal or semiconductor hot shoe that is required to be in thermal and electrical contact with the legs. This is partly because of the thermal-expansion mismatch of these materials: The coefficient of thermal expansion (CTE) of SiGe is 4.5 x 10(exp -6) C (exp -1), while the CTE of Yb14MnSb11 is 20 x 10(exp -6) C(exp -1). Simply joining a Yb14MnSb11 and a SiGe leg to a common hot shoe could be expected to result in significant thermal stresses in either or both legs during operation. Heretofore, such thermal stresses have been regarded as disadvantageous. In the proposed approach, stresses resulting from the CTE mismatch would be turned to advantage.

Sakamoto, Jeffrey↗
DOE OSTI2022

Investigation of Ga 2 O 3 as a new transparent conductive oxide for photovoltaics applications

This small innovation project intended to leverage recent activity and advances in ultrawide-bandgap oxide power electronics toward the investigation of Ga 2 O 3 as a transparent conductive oxide (TCO) for use in photovoltaics (PV). Ga 2 O 3 ’s theoretical advantage over incumbent TCOs is its very large bandgap of 4.8 eV, ensuring optical transparency of photons with λ ≥ 260 nm, effectively the full terrestrial solar spectrum. At the time of proposal writing, literature on Ga 2 O 3 as a TCO/optical material was relatively sparse — what little existed was focused on for UV sensors and/or emitters, with none related to PV. As such, this project was intended to help determine the potential of Ga 2 O 3 as a PV-oriented TCO by investigating its deposition using tools common to the PV and electronics industries — atomic layer deposition (ALD) and RF sputtering (RS), both of which are already used in PV manufacturing — and its resultant optical and electronic properties. Final project goals were to test its application to Si and III-V solar cells. Controllable film thickness and excellent uniformity was established for both methods deposition methods, with ALD providing higher precision for thinner films and RS more effective for growing thicker films. The as-deposited films were found to be amorphous in nature. Spectroscopic ellipsometry (SE) confirmed bandgaps of at least 4.8 eV and non-parasitic absorption across both AM0 and AM1.5G spectra; a refractive index approaching the expected value of 1.8 was also observed, with some degree of tunability based on process parameters. Using this initial optical data, a transfer matrix model, which interfaces with in-house EQE and LIV models, was developed to simulate Ga 2 O 3 optical effects on various solar cells, included potential in antireflection coatings (ARCs). Despite promising optical properties, the resultant resistivity / conductivity metrics did not meet expectations, regardless of deposition process. Although thick RS-deposited films, using both undoped and 1 at% Ge (n-type) doped sintered targets — demonstrated high net carrier concentrations (via C-V), and low specific resistance Ohmic contacts were demonstrated, transmission line measurements (TLM) showed very high resistivity. Further analysis indicated high vertical film conductivity, but very low lateral conductivity. Deeper characterization revealed a relatively high density of nano/microcrystalline inclusions that were ostensibly the source of the vertical conductivity, with the amorphous matrix serving as an effective insulator (likely due to high concentrations of electronic trap states). Despite extensive work to increase film polycrystallinity — demonstrated via high temperature annealing — sufficient lateral conductivity for TCO use was not achieved. The final phase of the project shifted focus toward investigation of ALD-deposited Ga 2 O 3 ’s potential as a passivant and/or passivating contact for both Si and III-V solar cells, with expectation of performance similar to Al 2 O 3 . However, initial rounds of testing using our baseline ALD process yielded minimal (but not quite zero) passivation of both GaInP and Si surfaces. Dielectric passivation is well-known to be highly process sensitive; given the unoptimized nature of the process used, this work was deemed inconclusive. The final outlook with respect to feasibility of Ga 2 O 3 as a PV-oriented TCO is, ultimately, still uncertain. The work performed in this project confirmed the optical properties and deposition methods, but the achieved electrical properties do not meet technological needs; literature reports in recent years are somewhat inconsistent and potentially untrustworthy, but generally appear to be in line with our results. At the very least it is clear that, should Ga 2 O 3 still be under consideration for PV TCO and/or selective contact applications, a significant amount of optimization and study is still needed.

14 SOLAR ENERGY↗
NASA NTRS1991

Semiconductor structural damage attendant to contact formation in III-V solar cells

In order to keep the resistive losses in solar cells to a minimum, it is often necessary for the ohmic contacts to be heat treated to lower the metal-semiconductor contact resistivity to acceptable values. Sintering of the contacts, however can result in extensive mechanical damage of the semiconductor surface under the metallization. An investigation of the detailed mechanisms involved in the process of contact formation during heat treatment may control the structural damage incurred by the semiconductor surface to acceptable levels, while achieving the desired values of contact resistivity for the ohmic contacts. The reaction kinetics of sintered gold contacts to InP were determined. It was found that the Au-InP interaction involves three consecutive stages marked by distinct color changes observed on the surface of the Au, and that each stage is governed by a different mechanism. A detailed description of these mechanisms and options to control them are presented.

Fatemi, Navid S.↗
NASA NTRS1989

Strong, Low-Resistance Bonds For AMTEC Electrodes

Heat of operating cell helps form contacts. Strong, low-electrical-resistance contacts between elements of current-collection grid in alkali-metal thermoelectric converter (AMTEC) cell obtained by simple in-place thermocompression bonding. Bonds formed between porous electrode of molybdenum film, nickel or cobalt strips overlying film, and molybdenum tie wires running at right angles to strips. Bonding method also works on films of tungsten or tungsten/platinum.

Williams, Roger M.↗
NASA NTRS1975

Boron Arsenide and Boron Phosphide for High Temperature and Luminescent Devices

The crystal growth of boron arsenide and boron phosphide in the form of bulk crystals and epitaxial layers on suitable substrates is discussed. The physical, chemical, and electrical properties of the crystals and epitaxial layers are examined. Bulk crystals of boron arsenide were prepared by the chemical transport technique, and their carrier concentration and Hall mobility were measured. The growth of boron arsenide crystals from high temperature solutions was attempted without success. Bulk crystals of boron phosphide were also prepared by chemical transport and solution growth techniques. Techniques required for the fabrication of boron phosphide devices such as junction shaping, diffusion, and contact formation were investigated. Alloying techniques were developed for the formation of low-resistance ohmic contacts to boron phosphide. Four types of boron phosphide devices were fabricated: (1) metal-insulator-boron phosphide structures, (2) Schottky barriers; (3) boron phosphide-silicon carbide heterojunctions; and (4) p-n homojunctions. Easily visible red electroluminescence was observed from both epitaxial and solution grown p-n junctions.

Chu, T. L.↗
NASA NTRS2006

Better Back Contacts for Solar Cells on Flexible Substrates

Improved low-resistance, semitransparent back contacts, and a method of fabricating them, have been developed for solar photovoltaic cells that are made from thin films of I-III-VI2 semiconductor materials on flexible, high-temperatureresistant polyimide substrates or superstrates. The innovative aspect of the present development lies in the extension, to polyimide substrates or superstrates, of a similar prior development of improved low-resistance, semitransparent back contacts for I-III-VI2 solar cells on glass substrates or superstrates. A cell incorporating this innovation can be used either as a stand-alone photovoltaic device or as part of a monolithic stack containing another photovoltaic device that utilizes light of longer wavelengths.

Woods, Lawrence M.↗
DOE OSTI2023

Ultrathin stable Ohmic contacts for high-temperature operation of β -Ga2O3 devices

Beta gallium oxide (β-Ga2O3) shows significant promise in high-temperature, high-power, and sensing electronics applications. However, long-term stable metallization layers for Ohmic contacts at high temperatures present unique thermodynamic challenges. The current most common Ohmic contact design based on 20 nm of Ti has been repeatedly demonstrated to fail at even moderately elevated temperatures (300–400 °C) due to a combination of nonstoichiometric Ti/Ga2O3 interfacial reactions and kinetically favored Ti diffusion processes. Here, we demonstrate stable Ohmic contacts for Ga2O3 devices operating up to 500–600 °C using ultrathin Ti layers with a self-limiting interfacial reaction. The ultrathin Ti layer in the 5 nm Ti/100 nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability. This novel contact design strategy results in an epitaxial conductive anatase titanium oxide interface layer that enables low-resistance Ohmic contacts that are stable both under long-term continuous operation (>500 h) at 600 °C in vacuum (≤10−4 Torr), as well as after repeated thermal cycling (15 times) between room temperature and 550 °C in flowing N2. This stable Ohmic contact design will accelerate the development of high-temperature devices by enabling research focus to shift toward rectifying interfaces and other interfacial layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2011

Thermally Conductive Tape Based on Carbon Nanotube Arrays

To increase contact conductance between two mating surfaces, a conductive tape has been developed by growing dense arrays of carbon nanotubes (CNTs, graphite layers folded into cylinders) on both sides of a thermally conductive metallic foil. When the two mating surfaces are brought into contact with the conductive tape in between, the CNT arrays will adhere to the mating surface. The van der Waals force between the contacting tubes and the mating surface provides adhesion between the two mating surfaces. Even though the thermal contact conductance of a single tube-to-tube contact is small, the tremendous amount of CNTs on the surface leads to a very large overall contact conductance. Interface contact thermal resistance rises from the microroughness and the macroscopic non-planar quality of mating surfaces. When two surfaces come into contact with each other, the actual contact area may be much less than the total area of the surfaces. The real area of contact depends on the load, the surface roughness, and the elastic and inelastic properties of the surface. This issue is even more important at cryogenic temperatures, where materials become hard and brittle and vacuum is used, which prevents any gas conduction through the interstitial region. A typical approach to increase thermal contact conductance is to use thermally conducting epoxies or greases, which are not always compatible with vacuum conditions. In addition, the thermal conductivities of these compounds are often relatively low. The CNTs used in this approach can be metallic or semiconducting, depending on the folding angle and diameter. The electrical resistivity of multiwalled carbon nanotubes (MWCNTs) has been reported. MWCNTs can pass a current density and remain stable at high temperatures in air. The thermal conductivity of a MWCNT at room temperature is measured to be approximately 3,000 W/m-K, which is much larger than that of diamond. At room temperature, the thermal conductance of a 0.3 sq cm array of CNTs was measured to be as high as 10 W/K. The high thermal conductivity and the nanoscale size make CNTs ideal as thermal interface materials. The CNT-based thermal tape can be used for the thermal management of microelectronic packages and electronic systems. It also can be integrated with current device technology and packaging. The material would allow for an efficient method to manage excess heat generation without requiring any additional power. Lastly, the CNT tape can be used to enhance thermal contact conductance across two mating surfaces on some NASA missions.

Kashani, Ali↗
DOE OSTI2022

Microdroplet Electrospray Localized Laser Printing and Sintering of Nanoparticles for Passivating, Carrier-Selective Contacts (Final Technical Report)

In this project, the team used a nanoparticle electrospray laser deposition (NELD) process developed at the University of Central Florida (UCF) to address current limitations of passivating, carrier-selective contact technologies for crystalline silicon (c-Si) photovoltaic (PV) cells. The NELD process was used to print low bulk line resistivity Ag contacts directly onto transparent conductive oxides (TCOs), like indium tin oxide (ITO), for hydrogenated amorphous silicon (a-Si:H) based c-Si heterojunction (SHJ) cells without any subsequent contact firing step needed.

14 SOLAR ENERGY↗
NASA NTRS1979

Intercell ohmic contacts for high efficiency multijunction solar converters

The monolithic multijunction converter is an attractive approach to achieving solar/electric conversion with greater than 30% efficiency. A major technical challenge in the development of such devices is the requirement for low resistance, optically transparent intercell contacts between adjacent junctions. These contacts should transmit, without significant loss, the spectral fraction of the incident sunlight which is not absorbed and converted in the overlying junction materials. Their contact resistances must be low enough to prevent significant I to the 2d power R loss at the designed current density levels. They should also exhibit adequate thermal conductivity to prevent device overheating when subjected to the designed illumination level. Recent encouraging results for the development of such contacts are presented.

Zehr, S. W.↗
DOE OSTI2024

Xylem-dwelling pathogen unaffected by local xylem vessel network properties in grapevines ( Vitis spp.)

Abstract Background and aims Xylella fastidiosa (Xf) is the xylem-dwelling bacterium associated with Pierce’s disease (PD), which causes mortality in agriculturally important species, such as grapevine (Vitis vinifera). The development of PD symptoms in grapevines depends on the ability of Xf to produce cell-wall-degrading enzymes to break up intervessel pit membranes and systematically spread through the xylem vessel network. Our objective here was to investigate whether PD resistance could be mechanistically linked to xylem vessel network local connectivity. Methods We used high-resolution X-ray micro-computed tomography (microCT) imaging to identify and describe the type, area and spatial distribution of intervessel connections for six different grapevine genotypes from three genetic backgrounds, with varying resistance to PD (four PD resistant and two PD susceptible). Key results Our results suggest that PD resistance is unlikely to derive from local xylem network connectivity. The intervessel pit area (Ai) varied from 0.07 ± 0.01 mm2 mm−3 in Lenoir to 0.17 ± 0.03 mm2 mm−3 in Blanc do Bois, both PD resistant. Intervessel contact fraction (Cp) was not statically significant, but the two PD-susceptible genotypes, Syrah (0.056 ± 0.015) and Chardonnay (0.041 ± 0.013), were among the most highly connected vessel networks. Neither Ai nor Cp explained differences in PD resistance among the six genotypes. Bayesian re-analysis of our data shows moderate evidence against the effects of the traits analysed: Ai (BF01 = 4.88), mean vessel density (4.86), relay diameter (4.30), relay density (3.31) and solitary vessel proportion (3.19). Conclusions Our results show that radial and tangential xylem network connectivity is highly conserved within the six different Vitis genotypes we sampled. The way that Xf traverses the vessel network may limit the importance of local network properties to its spread and may confer greater importance on host biochemical responses.

Plant Sciences↗
NASA NTRS1994

Making CoSi(2) Layers By Ion Implantation

Monolithic photovoltaic batteries containing vertical cells include buried CoSi(2) contact layers. Vertical-junction photovoltaic cells in series fabricated in monolithic structure. N- and p-doped silicon layers deposited epitaxially. The CoSi(2) layers, formed by ion implantation and annealing, serve as thin, low-resistance ohmic contacts between cells.

Namavar, Fereydoon↗
NASA NTRS2004

LERCIP 2004 Summary

This summer I am continuing my project from the previous two summers. My work involves ohmic contacts to N-type silicon carbide (Sic) devices. My mentor, Dr. Robert Okojie, is developing the technology behind high performance sensors and actuators for harsh environments. Sic is useful because it is able to operate at temperatures up to 600 C and it is resistant to radiation damage. This allows sensors and electronics to be placed in new locations, such as inside a jet engine or in space application without using heavy shielding. Ultimately this results in more efficient, smarter engine technology, reduced launch weights for spacecraft, and high power and high temperature electronics. A fundamental part of Sic devices is the ohmic contact. The contact is the interface between the semiconductor (Sic) and external circuitry. The current flowing in and out the devices is through the contact. Ensuring that these contacts remain ohmic (linear I-V behavior) allows us to fabricate devices that do not waste power at the metallurgical junction. Another key part is maintaining a low contact resistance. It is desired to maintain minimum energy loss by avoiding a rectifying electrical characteristic. My project is to develop and implement a testing procedure for measuring the contact resistance while the device is operating at high temperature. It is important to measure the contacts while simulating the true operating environment as closely as possible. For this reason, measurements are taken while the device is heated at intervals up to 600 C in air. To test the long tern reliability of the devices, the high temperature measurements are repeated after heating the sample for long intervals in air. A new set of data is gathered after heating for a total of 100, 200 and then 400 hours. The current as a function of voltage and the contact resistance was measured using the four point probe technique. The four point probe method is chosen because it measures contact resistance while eliminating error due to wire resistance and calibration issues.

DeAngelis, David↗
DOE OSTI2021

Accumulation-Type Ohmic van der Waals Contacts to Nearly Intrinsic WSe 2 Nanosheet-Based Channels: Implications for Field-Effect Transistors

We report the fabrication of ohmic van der Waals (vdW) contacts to nearly intrinsic WSe 2 nanosheet-based channels in field-effect transistors (FETs) using degenerately p-doped MoS2 (p + -MoS2) as a contact metal. We demonstrate that accumulation-type ohmic contacts and the high device performance are achievable without electrostatically gating the drain/source contact regions despite the nearly intrinsic nature of WSe 2 . Back-gated WSe 2 FETs with p + -MoS 2 bottom contacts (which screen the back-gate electric field in the drain/source regions) exhibit linear output characteristics, a high on/off ratio of 10 8 , and a high two-terminal field-effect mobility up to ~200 cm2 V –1 s –1 at room temperature. Our theoretical modeling reveals that the p + -MoS 2 /WSe 2 vdW junction behaves like a metal/semiconductor ohmic contact signified by a vanishingly thin space-charge region of ~1 nm on the p + -MoS 2 side and a substantial accumulation layer of free holes on the WSe 2 side, which is further verified by additional temperature-dependent and dual-gated measurements of WSe 2 FETs. We attribute the formation of accumulation-type ohmic contacts free of a Schottky barrier to the near absence of Fermi-level pinning at the vdW interface and the work function of p + -MoS 2 being larger than the ionization energy of WSe 2 . This study represents an important step toward achieving low-resistance ohmic contacts to two-dimensional (2D) semiconductors by eliminating the Fermi-level pinning effects, which is expected to have significant implications for next-generation 2D semiconductor-based nanoelectronics.

36 MATERIALS SCIENCE↗
NASA NTRS1984

High temperature ceramic interface study

Monolithic SiC and Si3N4 are susceptible to contact stress damage at static and sliding interfaces. Transformation-toughened zirconia (TTZ) was evaluated under sliding contact conditions to determine if the higher material fracture toughness would reduce the susceptibility to contact stress damage. Contact stress tests were conducted on four commercially available TTZ materials at normal loads ranging from 0.455 to 22.7 kg (1 to 50 pounds) at temperatures ranging from room temperature to 1204C (2200 F). Static and dynamic friction were measured as a function of temperature. Flexural strength measurements after these tests determined that the contact stress exposure did not reduce the strength of TTZ at contact loads of 0.455, 4.55, and 11.3 kg (1, 10, and 25 pounds). Prior testing with the lower toughness SiC and Si3N4 materials resulted in a substantial strength reduction at loads of only 4.55 and 11.3 kg (10 and 25 pounds). An increase in material toughness appears to improve ceramic material resistance to contact stress damage. Baseline material flexure strength was established and the stress rupture capability of TTZ was evaluated. Stress rupture tests determined that TTZ materials are susceptible to deformation due to creep and that aging of TTZ materials at elevated temperatures results in a reduction of material strength.

Lindberg, L. J.↗