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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Contact in the Unitary Fermi Gas across the Superfluid Phase Transition

A quantity known as the contact is a fundamental thermodynamic property of quantum many-body systems with short-range interactions. Determination of the temperature dependence of the contact for the unitary Fermi gas of infinite scattering length has been a major challenge, with different calculations yielding qualitatively different results. Here we use finite-temperature auxiliary-field quantum Monte Carlo (AFMC) methods on the lattice within the canonical ensemble to calculate the temperature dependence of the contact for the homogeneous spin-balanced unitary Fermi gas. We extrapolate to the continuum limit for 40, 66, and 114 particles, eliminating systematic errors due to finite-range effects. We observe a dramatic decrease in the contact as the superfluid critical temperature is approached from below, followed by a gradual weak decrease as the temperature increases in the normal phase. Our theoretical results are in excellent agreement with the most recent precision ultracold atomic gas experiments. Here, we also present results for the energy as a function of temperature in the continuum limit.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

Significant reduction in errors associated with nonbonded contacts in protein crystal structures: automated all-atom refinement with PrimeX

All-atom models derived from moderate-resolution protein crystal structures contain a high frequency of close nonbonded contacts, independent of the major refinement program used for structure determination. All-atom refinement with PrimeX corrects many of these problematic interactions, producing models that are better suited for use in computational chemistry and related applications. All-atom models are essential for many applications in molecular modeling and computational chemistry. Nonbonded atomic contacts much closer than the sum of the van der Waals radii of the two atoms (clashes) are commonly observed in such models derived from protein crystal structures. A set of 94 recently deposited protein structures in the resolution range 1.5–2.8 Å were analyzed for clashes by the addition of all H atoms to the models followed by optimization and energy minimization of the positions of just these H atoms. The results were compared with the same set of structures after automated all-atom refinement with PrimeX and with nonbonded contacts in protein crystal structures at a resolution equal to or better than 0.9 Å. The additional PrimeX refinement produced structures with reasonable summary geometric statistics and similar R{sub free} values to the original structures. The frequency of clashes at less than 0.8 times the sum of van der Waals radii was reduced over fourfold compared with that found in the original structures, to a level approaching that found in the ultrahigh-resolution structures. Moreover, severe clashes at less than or equal to 0.7 times the sum of atomic radii were reduced 15-fold. All-atom refinement with PrimeX produced improved crystal structure models with respect to nonbonded contacts and yielded changes in structural details that dramatically impacted on the interpretation of some protein–ligand interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Non-Contact Printed Aluminum Metallization of Si Photovoltaic Devices

Alternative solution-based techniques such as aerosol jet printing offer the dual benefits of contactless pattern deposition and high material utilization. We have used aerosol jet printing to investigate non-contact printed Al metal ink as a replacement for screen printed Al back contacts on wafer Si solar cells. This particle-based ink can be prepared at high loadings of 60 weight % metal, which enables rapid deposition of 1 - 10 um thick lines. Al lines printed on Si wafers and heated between 550 and 800 degrees C form low resistance contacts suitable for current extraction. The effectiveness of these printed Al back contacts has further been demonstrated by incorporating them into a series of 21 cm 2 crystalline Si solar cells that produced a champion power conversion efficiencyof 13%.

14 SOLAR ENERGY↗
DOE OSTI2022

Local nm-Scale Imaging of Electrical Contact for Series Resistance Degradation of Silicon Solar Cells

We report on an electrical conduction mechanism for series resistance (Rs) degradation observed in a utility scale solar farm by nm-scale imaging of the local resistance at the Ag/Si interface of c-Si front metallization. Scanning spreading resistance microscopy imaging revealed that the number of point or small area electrical contacts decreased in a degraded cell compared to an unaffected cell, demonstrating the direct root cause of the Rs degradation. The degraded cell shows both a morphological and chemical difference in the screen-printed finger contact compared to the unaffected cell, which likely caused the degradation during the long-term field service. The reduction in electrical contact is likely caused by a structural change: The Ag particles in contact with the Si cell aggregate into bulk Ag, and a highly resistive ceramic oxide is formed in a "belt" shape at the Ag/Si interface. This resistive belt with a thickness of ~1 um blocks the current conduction from cell emitter to the Ag grid. Our results demonstrate an example of the multi-scale characterization approach for understanding degradation mechanisms in photovoltaics.

c-Si solar cell↗
DOE OSTI2013

Atomic layer etching removal of damaged layers in a contact hole for low sheet resistance

A damaged layer remains on silicon substrates after high-aspect-ratio contact (HARC) etching when using a fluorocarbon gas. Atomic layer etching (ALET) is a technique that can be applied to remove the damaged layer of silicon, removing about 1.36 Å per etch cycle. The characteristics of contact damage removal by ALET are investigated and compared with the conventional damage removal technique of low-power CF{sub 4} plasma etching. The low-power CF{sub 4} plasma etching technique not only has inadequate etch depth control, but also introduces secondary damage by implanting impurities about 25 Å into the contact bottom of the silicon surface. However, ALET allows contact damage to be removed effectively without introducing secondary damage to the substrate, and with precision etch depth control at the angstrom scale. When ALET is applied subsequent to low-power CF{sub 4} plasma etching, the fluorine- and carbon-damaged silicon is effectively removed in about 10 cycles. The sheet resistance of HARC etched silicon decreases from 142 to 137 Ω/□ after using low-power CF{sub 4} plasma etching, and subsequent ALET treatment further decreases the sheet resistance to 129 Ω/□, which is close to the reference value of 124 Ω/□.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Damaged silicon contact layer removal using atomic layer etching for deep-nanoscale semiconductor devices

Silicon atomic layer etching (ALET) using Cl{sub 2} is applied to remove the damaged layer on a 30 nm contact silicon surface formed by high-energy reactive ions during high aspect ratio contact etching, and its effects on the damage removal characteristics are investigated. Compared to a conventional damage removal method, such as the low-power CF{sub 4} plasma treatment technique, ALET produces less secondary damage to the substrate and gives exact etch depth control and extremely high etch selectivity to the contact SiO{sub 2} insulating pattern mold. When ALET is applied after a conventional damage removal technique, the sheet resistance of the damaged contact silicon surface is improved to a level close to that of a clean silicon surface, while exact atomic-scale depth control is maintained without changes in the pattern mold profile.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Improvement of Ohmic contacts on Ga 2 O 3 through use of ITO-interlayers

In this work, the use of ITO interlayers between Ga 2 O 3 and Ti/Au metallization is shown to produce Ohmic contacts after annealing in the range of 500–600 °C. Without the ITO, similar anneals do not lead to linear current–voltage characteristics. Transmission line measurements were used to extract the specific contact resistance of the Au/Ti/ITO/Ga 2 O 3 stacks as a function of annealing temperature. Sheet, specific contact, and transfer resistances all decreased sharply from as-deposited values with annealing. The minimum transfer resistance and specific contact resistance of 0.60 Ω mm and 6.3 × 10 -5 Ω cm 2 were achieved after 600 °C annealing, respectively. Lastly, the conduction band offset between ITO and Ga 2 O 3 is 0.32 eV and is consistent with the improved electron transport across the heterointerface.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Pulsed laser deposition nickel oxide on crystalline silicon as hole selective contacts

In this work, the authors report the use of pulsed laser deposition to fabricate nickel oxide layers on silicon and assess the applicability of this structure as a hole selective contact. Films were deposited at temperatures of 300, 500, 700, and 900 °C. Results from reflective high energy electron diffraction patterns indicate increasing crystallinity with temperature up to 500 °C. However, layers deposited at 900 °C are amorphous. Nevertheless, results from atomic force microscopy, contact resistance measurements, and photoluminescence lifetime imaging indicate that films deposited at 900 °C have the best film smoothness, lowest specific contact resistivity, and highest lifetimes, and therefore, they are best suited for hole selective contact applications. Kelvin probe force microscopy and x-ray photoelectron spectroscopy were also carried out to study the effects of deposition temperature.

42 ENGINEERING↗
DOE OSTI2012

Waterless Coupling of Ultrasound from Planar Contact Transducers to Curved and Irregular Surfaces during Non-destructive Ultrasonic Evaluations

The Applied Physics group at the Pacific The Applied Physics group at the Pacific Northwest National Laboratory (PNNL) in Richland, WA has evaluated a method for waterless/liquidless coupling of ultrasonic energy from planar ultrasonic contact transducers to irregular test surfaces for ultrasonic non-destructive evaluation applications. Dry couplant material placed between a planar transducer face and a curved or uneven steel or plastic surface allows for effective sound energy coupling and preserves the integrity of the planar transducer sound field by serving as an acoustic impedance matching layer, providing good surface area contact between geometrically dissimilar surfaces and conforming to rough and unsmooth surfaces. Sound fields radiating from planar ultrasonic contact transducers coupled to curved and uneven surfaces using the dry coupling method were scanned and mapped using a Pinducer receiver connected to a raster scanner. Transducer sound field coverage at several ultrasonic frequencies and several distances from the transducer contact locations were found to be in good agreement with theoretical beam divergence and sound field coverage predictions for planar transducers coupled to simple, planar surfaces. This method is valuable for applications that do not allow for the use of traditional liquid-based ultrasonic couplants due to the sensitivity of the test materials to liquids and for applications that might otherwise require curved transducers or custom coupling wedges. The selection of dry coupling material is reported along with the results of theoretical sound field predictions, the laboratory testing apparatus and the empirical sound field data.

Couplant↗
DOE OSTI2019

Features of the Temperature Dependence of the Specific Contact Resistance of Au–Ti–Pd–n{sup +}–n-Si Diffusion Silicon Structures

The temperature dependences of the specific contact resistance of silicon ρ{sub c} with a doping step are measured experimentally and described theoretically. The measurements are performed in the temperature range from 4.2 to 380 K. It is established that the contacts of the studied Au–Ti–Pd–n{sup +}–n-Si structures are ohmic. It is shown that minimal ρ{sub c} is implemented at T = 75 K. Its value rises both with a decrease in temperature (due to the freezing effect) and with an increase in temperature (due to the electron-enriched layer at the boundary with the bulk material). It is established that the bulk electron concentration strongly decreases in the near-contact region in a layer with a thickness on the order of one micron due to the compensation of silicon by deep acceptors appearing because of the formation of a rather high vacancy concentration during stress relaxation and the appearance of a high dislocation density, as well as due to their diffusion from the contact after heating to 450°C. The data on the occurrence of vacancy-type defects are confirmed by X-ray measurements. The dislocation density in the studied structures is also estimated from X-ray measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

The impact of the charge barrier height on Germanium (Ge) detectors with amorphous-Ge contacts for light dark matter searches

Germanium (Ge) detectors with ability of measuring a single electron–hole (e–h) pair are needed in searching for light dark matter (LDM) down to the MeV scale. We investigate the feasibility of Ge detectors with amorphous-Ge (a-Ge) contacts to achieve the sensitivity of measuring a single e-h pair, which requires extremely low leakage current. Three Ge detectors with a-Ge contacts are used to study the charge barrier height for blocking electrons and holes. Using the measured bulk leakage current and the Döhler–Brodsky model, we obtain the values for charge barrier height and the density of localized energy states near the Fermi energy level for the top and bottom contacts, respectively. We predict that the bulk leakage current is extremely small and can be neglected at helium temperature ( $$\sim $$ ~ 4 K). Thus, Ge detectors with a-Ge contacts possess the potential to measure a single e–h pair for detecting LDM particles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Novel Approach to Strengthening Ceramic Cathode Contact and Validation in a Generic Stack Test Fixture

The ceramic contact material at the cathode side has been identified as the weakest mechanical link in solid oxide fuel cells, due to poor sintering at low stack fabrication temperatures. In this work, a novel approach of mechanical interlocking with an engineered surface was proposed to strengthen LSM-type contacts. The engineered cathode surface was made by depositing large LSM20 granules onto a wet cathode print, followed by sintering. Granules of three sizes were tested (mesh #35, #60, and #100). Small coupons of anode-supported YSZ electrolyte with LSM cathode were joined at 850 and 950oC for 2h with LSM contact using either the engineered surface or plain surfaces. The results of contact strength measurements showed about 14 times increase with engineered surface compared to plain surfaces. Validation with a 2”x2” LSM-based cell in a generic stack fixture showed good thermal cycle stability with minimal change in ohmic impedance over ten cycles.

Chou, Yeong-Shyung↗
DOE OSTI2017

Experimental development of low-frequency shear modulus and attenuation measurements in mated rock fractures: Shear mechanics due to asperity contact area changes with normal stress

Reservoir core measurements can help guide seismic monitoring of fluid-induced pressure variations in tight fractured reservoirs including those targeted for supercritical CO 2 injection. We present the first seismic-frequency ‘room-dry’ measurements of fracture specific shear stiffness, using artificially fractured standard granite samples with different degrees of mating, a well-mated tensile fracture from a dolomite reservoir core, as well as simple roughened polymethyl methacrylate (PMMA) surfaces. We have adapted a low-frequency (0.01 to 100 Hz) shear modulus and attenuation apparatus to explore the seismic signature of fractures and understand the mechanics of asperity contacts under a range of normal stress conditions. Our instrument is unique in its ability to measure at low normal stresses (0.5 – 20 MPa), simulating 'open' fractures in shallow or high fluid pressure reservoirs. The accuracy of our instrument is demonstrated by calibration and comparison to ultrasonic measurements and low-frequency direct shear measurements of intact samples from the literature. Pressure sensitive film was used to measure real contact area of the fracture surfaces. The fractured shear modulus for the majority of the samples shows an exponential dependence on real contact area. A simple numerical model, with one bonded circular asperity, predicts this behavior and matches the data for the simple PMMA surfaces. The rock surfaces reach their intact moduli at lower contact area than the model predicts, likely due to more complex geometry. Lastly, we apply our results to a Linear-Slip Interface Model to estimate reflection coefficients and calculate shear wave time delays due to the lower wave velocities through the fractured zone. We find that cross-well surveys could detect even well-mated hard rock fractures assuming the availability of high repeatability acquisition systems.

58 GEOSCIENCES↗
DOE OSTI2021

Modeling Contact Angle vs. Temperature for the Quartz-Water-Decane System

Innovative approaches are needed to improve the efficiency of oil recovery technologies to meet the growing demands of fossil-fuel based energy consumption. Enhanced oil recovery (EOR) methods such as low-salinity waterflooding and chemically tuned waterflooding aim to optimize the reservoir’s wetting properties, detaching oil globules from rock surfaces and allowing easier oil flow through pore throats. This wetting behavior is commonly quantified by contact angle measurements of the rock-oil-brine interface, which have been thoroughly investigated and theorized for many systems at ambient temperatures and pressures. However, few studies exist for extending contact angle theories away from ambient conditions. In this paper, we model the contact angles of a quartz-water-decane system at elevated temperatures using the surface tension component (STC) approach. Temperature-dependent van der Waals [Lifshitz-van der Waals (LW)] interactions and hydrogen-bonding (acid-base) interactions were calculated and are incorporated into the model for the quartz-water-decane interface. Additionally, the Hough and White procedure was used to create temperature-dependent dielectric functions of quartz, water, and normal decane for calculations of Hamaker coefficients. Hamaker coefficients calculated this way are highly linear with temperature and agree well with Israelachvili’s approximation. The acid-base interactions likely contribute the most to system wettability changes. Resulting contact angles of the quartz-water-decane system shift from water-wet (16°) to slightly water-wet (57.4°) as temperature increases. The model was also successfully verified for the quartz-air-water system. Our results can be used in future studies to determine optimal injected water compositions for specific rock-oil-brine and other systems with consideration of reservoir temperature.

02 PETROLEUM↗
DOE OSTI2012

Ultracoatings: Enabling Energy and Power Solutions in High Contact Stress Environments through next-generation Nanocoatings Final Technical Report

A review of current commercially available, industrial-grade, low friction coatings will show that interfacial contact pressures nearing 1GPa (~150ksi) inherently limit surface engineering solutions like WC, TiN, TiAlN, and so forth. Extremely hard coatings, then, are often pursued as the principle path, although they too are not without significant limitations. A majority of these compounds are inherently brittle in nature or may not pair well with their mating substrate. In either case, their durability in high contact stress environments is compromised. In parallel to thin film coatings, many conventional surface treatments do not yield an interface hard enough to withstand extreme stresses under load. New research into advanced, nanocomposite materials like (Ti, Zr)B 2 shows great promise. Bulk compacts of this compound have demonstrated an order of magnitude better wear resistance than current offerings, notably materials like tungsten carbide. At a laboratory level, the (Ti,Zr)B 2 nanocomposite material exhibited abrasive and erosive wear resistance nearly ten times better than existing mixed-phase boride systems. In ASTM abrasion and erosion testing, these new compositions exhibit wear resistance superior to other known advanced materials such as RocTec 500 and 'Borazon' cubic boron nitride. Many significant challenges exist for mass production of (Ti, Zr)B 2 , one of which is the necessary processing technology that is capable of minimizing deleterious impurity phases. Secondly, this material's performance is derived from a synergistic effect of the two materials existing as a single phase structure. While the individual constituents of TiB 2 and ZrB 2 do yield improvements to wear resistance, their singular effects are not as significant. Lastly, deposition of this material on a commercial level requires thorough knowledge of nanocomposite boride solids; the benefits associated with these innovative new materials are just being realized. Advancing this technology, called Ultracoatings, through initial development, scale up, and commercialization to a variety of markets would represent a transformative leap to surface engineering. Several application spaces were considered for immediate implementation of the Ultracoatings technology, including, but not limited to, a drive shaft for an aerospace fuel pump, engine timing components, and dry solids pump hardware for an innovative coal gasifier. The primary focus of the program was to evaluate and screen the performance of the selected (Ti, Zr)B 2 Ultracoatings composition for future development. This process included synthesis of the material for physical vapor deposition, sputtering trials and coating characterization, friction and wear testing on sample coupons, and functional hardware testing. The main project deliverables used to gage the project's adherence to its original objective were: Development of a coating/substrate pairing that exhibits wear rate of 0.1 mg/hour or lower at a 1GPa contact pressure, while achieving a maximum coating cost of $0.10/cm 2 . Demonstrate the aforementioned wear rate in both lubricated and starved lubrication conditions. Although the (Ti, Zr) B 2 coating was not tailored for low friction performance, friction and wear evaluations of the material demonstrated a coefficient of sliding friction as low as 0.09. This suggests that varying the percentage of TiB 2 present in the composite could enhance the materials performance in water-based lubricants. In the aerospace drive shaft application, functional hardware coated with (Ti, Zr)B 2 survived a variety of abuse and long-range durability tests, with contact pressures exceeding 2 GPa. For engine timing components, further work is planned to evaluate the Ultracoatings technology in direct injection and diesel engine conditions. In the final identified application space the dry solids pump hardware, discussions continue on the application of the Ultracoatings technology for those specific components. Full implementation of the technology into the targeted markets equates to a U.S.-based energy savings potential of over 100 trillion BTU per year by 2030. This exceeds the original projection of 60 TBTU/year by 2030.

(Ti,Zr) B2↗
DOE OSTI2013

Inks for Ink Jet Printed Contacts for High Performance Silicon Solar Cells: Cooperative Research and Development Final Report, CRADA No. CRD-06-199

The work under the proposed CRADA will be a joint effort by BP Solar and NREL to develop new types of high performance inks for high quality contacts to silicon solar cells. NREL will develop inks that have electronic properties that will allow the formation of high quality ohmic contacts to n- and p-type crystalline silicon, and BP Solar will evaluate these contacts in test contact structures.

14 SOLAR ENERGY↗
DOE OSTI2017

Printed Nano Cu and NiSi Contacts and Metallization for Solar Cell Modules

There has long been a desire to replace the front-side silver contacts in silicon solar cells. There are two driving forces to do this. First, silver is an expensive precious metal. Secondly, the process to use silver requires that it be formulated into screen print pastes that need a lead-containing glass frit, and the use of lead is forbidden in many parts of the world. Because of the difficulty in replacing these pastes and the attendant processes, lead exemptions have granted to solar cells. Copper has been the replacement metal of choice because it is significantly cheaper than silver and is very close to silver in electrical conductivity. Using processes which do not use lead, obviates it as an environmental contaminant. However, copper cannot be in contact with the silicon of the cell since it migrates through the silicon and causes defects which severely damage the efficiency of the cell. Hence, a conductive barrier must be placed between the copper and silicon and nickel, and especially nickel silicide, have been shown to be materials of choice. However, nickel must be sputtered and annealed to create the nickel silicide barrier, and copper has either been sputtered or plated. All of these processes require expensive, specialized equipment and plating uses environmentally unfriendly chemicals. Therefore, Intrinsiq proposed using printed nano nickel silicide ink (which we had previously invented) and printed nano copper ink to create these electrodes and barriers. We found that nano copper ink could be readily printed and sintered under a reducing atmosphere to give highly conductive grids. We further showed that nano nickel silicide ink could be readily jetted into grids on top of the silicon cell. It could then be annealed to create a barrier. However, it was found that the combination of printed NiSi and printed Cu did not give contact resistivity good enough to produce efficient cells. Only plated copper on top of the printed NiSi gave useful contact resistivity, and that proved to five to ten times less conductive than the commercial silver grids. Even so, the NiSi layer was a very good barrier to copper migration, even under harsh environmental conditions. Additionally, both plated copper and printed copper could be soldered to. While it may be possible to produce an all printed copper/nickel silicide top electrode for silicon cells, it was not easily demonstrated within the time and monetary constraints of the present project. Additionally, potential customers have told us that having to laser ablate the anti-reflection coating of cells to create a connection for NiSi, and the addition of two printing and annealing (sintering for copper) steps, adds too much expense to compensate for any potential cost savings from using copper. The cost benefits of copper have been further eroded by the facts that over the lifetime of this project, the cost of silver electrodes decreased due to manufacturers finding ways to use less and less silver, and inventing pastes which use less costly silver materials to begin with. All of these factors were considered and led to the decision to stop the program before actual manufacturing scale was attempted.

14 SOLAR ENERGY↗
DOE OSTI2019

Tank Closure Cesium Removal Project CST Simulant Cesium Batch Contact Kinetics Test Results and Column Performance Predictions

To evaluate the cesium removal capabilities of the caustic-washed R9120-B version of crystalline silicotitanate (CST) ion exchange media from Savannah River Site (SRS) waste during Tank Closure Cesium Removal (TCCR) column processing, batch contact kinetics tests were performed at 23 °C in SRS Average simulant. Cesium loading kinetics data was also collected (in current and previous testing; King, et al., 2018a) using the same simulant for an older CST sample (IE- 911) which was caustic-washed by the vendor many years ago and was recently retrieved from drums stored at SRS. (Note that R9120-B and IE-911 are believed to be identical materials, after the R9120-B was caustic washed by SRNL, and differ only by particle size and manufacturing date, and thus can be considered different batches). Although cesium loading kinetics testing can be conducted more rigorously using dynamic column tests, the batch contact method requires less simulant and considerably less experimental effort. Average percent removal data for batch contact samples for the two CST batches are provided in Table ES-1. Faster cesium uptake was observed with IE-911, although the loading rates for the two media batches were similar. After 5.5 hours of CST contact, 81% cesium removal was observed with R9120-B versus 86% removal with IE-911. Although these differences are small, the trend is consistent across the timed results, with the larger bead material always loading slower. These seemingly small differences in percent removal are believed to have a large impact on the column performance. Cesium loading curves for the two CST batches are provided in Figure ES-1 below. Faster cesium loading with IE-911 CST is presumed to be associated with the smaller average particle diameter (408 μm mean volume diameter) versus R9120-B (572 μm diameter). Cesium loading kinetic data was needed to conduct column performance comparisons (i.e., model predictions) for the two CST batches.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗