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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

On Practical Aspects of Variational Consistency in Contact Dynamics

Usage of contact mechanics methodologies is a pervasive modeling requirement in dynamic simulations. While for some trivial problems, solutions taken from analytical geometry are available, use of a finite element framework is common to achieve formulation generality. This work explores two dynamic contact formulations: one based on the traditional node-to-segment (NTS) approach, and a variationally consistent segment-to-segment (STS) mortar formulation. The NTS formulation employed here enforces the constraints kinematically (i.e., the interpenetration is enforced to the solver tolerance), whereas the mortar approach uses Lagrange multipliers to enforce the contact constraints. Both approaches are implemented in the open-source finite element framework Multiphysics Object-Oriented Simulation Environment (MOOSE). The results highlight two relevant contact-interface-related dynamic phenomena in finite element simulations. First, stabilization of contact constraints is discussed, taking into account the evolution of the total energy in a benchmark problem. Second, the influence of finite element discretization on both of the aforementioned contact formulations is analyzed by exercising a large-deformation example with continuous relative sliding. Variationally consistent contact approaches such as the mortar formulation lead to improved energy preservation and avoid spurious excitation of the system's frequencies. This is especially relevant in settings where inertia and vibrations are of importance.

42 ENGINEERING↗
DOE OSTI2025

Methods to Observe Tribological Failures in Self-Mated Steel Contacts

Scuffing, a type of wear found in highly stressed or poorly lubricated contacts, is characterized by a rapid increase in friction and severe plastic deformation of the near-surface material. Scuffing has proven difficult to study because it initiates unpredictably, progresses rapidly, and typically develops within an inaccessible contact interface. Although there have been successful in-situ studies of scuffing in real-time, the transparent counter body needed for these studies changes the interactions between the surfaces and the lubricant, which affects the scuffing process in unknown ways. This paper describes the development of X-ray-compatible tribometry to study the scuffing of self-mated steels in-situ and in real-time. The method uses a crossed cylinders configuration with a thin (500 μm thick) stationary component and a small (≈200 μm) contact width to maximize X-ray interactions with atoms within the stress field generated by the contact. The resulting instrument and method are used to benchmark the scuffing response of self-mated 52,100 steel under tribologically challenging ‘oil-off’ lubrication conditions. The results demonstrate reliable scuffing in this configuration despite the relatively small contact areas and loads used. Following scuffing, gross plastic deformation was observed on both surfaces along with significant subsurface grain refinement and flow only on the stationary surface, which experienced constant contact. Interestingly, high friction initiated at specific locations of the migratory surface, which experienced intermittent contact, and then propagated across the track over time, suggesting that local conditions of the migratory surface dominated friction leading into the failure event.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Wear on Surface Crack Propagation in Rail–Wheel Wet Contact

The interaction between wear and rolling contact fatigue in wet contact was investigated by means of experiments on a two-disk test bench. Specimens of various railway wheel steels were coupled with specimens of the same rail steel, subjected to rolling and sliding wet contact with varying sliding/rolling ratio. Some tests with a dry rolling–sliding contact phase preceding wet contact were carried out as well (dry–wet tests). The pressurization of the fluid entrapped in the surface cracks was the cause of a rapid and severe damage in the dry–wet tests, due to the nucleation of surface cracks by ratcheting in the dry phase, which subsequently propagate in the wet phase. In the wet tests, the fluid pressurization effect was much mitigated due to the absence of initial surface cracks; the specimens subjected to higher sliding/rolling ratio showed the best performance against rolling contact fatigue, due to the effect of wear in reducing the length of surface cracks. A model for assessing the interaction between wear and rolling contact fatigue was proposed, based on the correction of the classical Paris law for crack propagation by taking into account the effect of wear on crack length reduction. The application of the model to the experimental tests allowed finding a correlation between the model predictions and the occurrence of fatigue failure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Wear Mechanisms in Contacts Involving Slippers in Axial Piston Pumps: A Multi-Technical Analysis

Axial piston pumps are widely used in actuation as power conversion systems, especially in aeronautics. Even though they combine compactness and efficiency, wear of their parts reduces their lifespan. Studies on these pumps often consist in analytic or numerical analyses of lubricated contact between their components. Piston slippers are in tribological contacts with swashplate, pistons and slipper retainer. This study aims to understand wear mechanisms in the contacts involving this central component. An experimental multi-technical analysis of helicopter pumps parts after several functioning times is presented. To determine wear mechanisms, worn surfaces are observed with SEM. 3D surface roughness measurements provide surface topography. Contact conditions and consequently wear severity and mechanisms differ from one contact to another. Detached coarse carbides from the swashplate surface act as an abrading third-body against slippers and swashplate. Debris generated in this contact is carried by the fluid to the other contacts. Although pistons are made of the same steel as swashplate, there is no carbide detachment and wear comes from debris polluting the fluid. Slipper retainer is almost not worn, and debris causes abrasive wear and craters generation in the slippers.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A multiscale model to understand the interface chemistry, contacts, and dynamics during lithium stripping

A reversible Li-metal electrode, paired with a solid electrolyte, is critical for attaining higher energy density and safer batteries beyond the current lithium-ion cells. A stable stripping process may be even harder to attain as the stripping process will remove Li-atoms from the surface, and naturally reduce surface contact area, if not self-corrected by other mechanisms, such as diffusion and plastic deformation under an applied external stack pressure. Here, we capture these mechanisms occurring at multiple length- and time- scales, i.e., interface interactions, vacancy hopping, and plastic deformation, by integrating density functional theory (DFT) simulations, kinetic Monte Carlo (KMC), and continuum finite element method (FEM). By assuming the self-affine nature of multiscale contacts, we predict the steady-state contact area as a function of stripping current density, interface wettability, and stack pressure. We further estimate the exponential increase of overpotential due to contact area loss to maintain the same stripping current density. We demonstrate that a lithiophilic interface requires less stack pressure to reach the same steady-state contact area fraction than a lithiophobic interface. A “tolerable steady-state” contact area loss for maintaining stable stripping is estimated at 20 %, corresponding to a 10 % increase in overpotential. To constrain contact loss within the tolerance, the required stack pressure is 0.1, 0.5, and 2 times the yield strength of lithium metal for three distinct interfaces, lithiophilic Li/lithium oxide(Li2O), Li/lithium lanthanum zirconium oxide(LLZO), and lithiophoblic Li/lithium fluoride(LiF), respectively. The modeling results agree with experiments on the impact of the stack pressure quantitatively, while the discrepancy in stripping rate sensitivity is attributed to the simplifying interface interaction in our simulations. Overall, this multiscale simulation framework demonstrates the importance of electrochemical-mechanical coupling in understanding the dynamics of the Li/SE interface during stripping.

Feng, Min↗
DOE OSTI2024

Thermodynamic driving forces in contact electrification between polymeric materials

Abstract Contact electrification, or contact charging, refers to the process of static charge accumulation after rubbing, or even simple touching, of two materials. Despite its relevance in static electricity, various natural phenomena, and numerous technologies, contact charging remains poorly understood. For insulating materials, even the species of charge carrier may be unknown, and the direction of charge-transfer lacks firm molecular-level explanation. Here, we use all-atom molecular dynamics simulations to investigate whether thermodynamics can explain contact charging between insulating polymers. Based on prior work suggesting that water-ions, such as hydronium and hydroxide ions, are potential charge carriers, we predict preferred directions of charge-transfer between polymer surfaces according to the free energy of water-ions within water droplets on such surfaces. Broad agreement between our predictions and experimental triboelectric series indicate that thermodynamically driven ion-transfer likely influences contact charging of polymers. Furthermore, simulation analyses reveal how specific interactions of water and water-ions proximate to the polymer-water interface explain observed trends. This study establishes relevance of thermodynamic driving forces in contact charging of insulators with new evidence informed by molecular-level interactions. These insights have direct implications for future mechanistic studies and applications of contact charging involving polymeric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Tunnel oxide passivating contact enabled by polysilicon on ultra-thin SiO 2 for advanced silicon radiation detectors

Conventional silicon junction detectors encounter significant carrier recombination within the heavily doped p⁺ and n⁺ layers, as well as beneath the metal contact regions, creating the so-called “dead layers”, especially on the detector side. In this study, we present the tunnel oxide passivating contact with doped polysilicon on oxide, which demonstrates exceptional surface passivation and carrier selectivity. The key innovation lies in an ultra-thin (~ 1.5 nm) interfacial oxide layer that facilitates efficient majority carrier transportation via tunneling while effectively block minority carriers. Remarkably low saturation current densities, ranging from 5 to 10 fA/cm² even with the metal contact, underscore the superiority of both n-type and p-type tunnel oxide passivating contacts. In contrast, conventional p–n junction or high-low junction exhibit saturation current densities ranging from 10 to 90 fA/cm² in the studied p⁺ and n⁺ layers with surface passivation schemes due to Auger recombination and surface recombination, and 1000–6000 fA/cm² with metal contacts due to intense metal-induced recombination at the interface. These findings indicate the potential and superiority of implementing n-type tunnel oxide passivating contact on the detector side and p-type contact on the back side for advanced silicon radiation detectors. This approach would enable thorough collection of generated charge carriers along the track of incident ionizing radiation particles, leading to improved energy resolution and reduced noise levels.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Method to improve the repeatability of dynamic contact resistance measurement test results for high-voltage circuit breakers

Dynamic contact resistance measurement (DRM) is an effective technique to evaluate the contact condition in substations without dismantling circuit breakers. However, performing DRM tests with unsuitable test parameters will cause low repeatability of the test results, and even lead to the misdiagnosis of contact condition. Performing the DRM tests at high DC injected current can improve the repeatability. This paper studies the mechanism of the DC injected current on the repeatability of the DRM test results, and it also promotes a new determination method of the DC injected current amplitude. The impact of the DC injected current on DRM test results is reviewed. The mechanism of the DC injected current on DRM test results is investigated by analysing the chemical composition of the contact surface, the force exerted on the contacts, and the temperatures of a-spots on the contacts' surface. Results indicate that not the Lorentz force, but the temperature of a-spots is the primary influencing factor. The amplitude of the DC injected current can be determined by calculating the temperatures of the a-spot on the contacts' surface. And this method can also be used for other SF6 circuit breakers.

Cheng, Tingting↗
DOE OSTI2014

Contact resistance improvement using interfacial silver nanoparticles in amorphous indium-zinc-oxide thin film transistors

We describe an approach to reduce the contact resistance at compositional conducting/semiconducting indium-zinc-oxide (IZO) homojunctions used for contacts in thin film transistors (TFTs). By introducing silver nanoparticles (Ag NPs) at the homojunction interface between the conducting IZO electrodes and the amorphous IZO channel, we reduce the specific contact resistance, obtained by transmission line model measurements, down to ∼10{sup −2 }Ω cm{sup 2}, ∼3 orders of magnitude lower than either NP-free homojunction contacts or solid Ag metal contacts. The resulting back-gated TFTs with Ag NP contacts exhibit good field effect mobility of ∼27 cm{sup 2}/V s and an on/off ratio >10{sup 7}. We attribute the improved contact resistance to electric field concentration by the Ag NPs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Novel vertical silicon photodiodes based on salicided polysilicon trenched contacts

The classical concept of silicon photodiodes comprises of a planar design characterized by heavily doped emitters. Such geometry has low collection efficiency of the photons absorbed close to the surface. An alternative, promising, approach is to use a vertical design. Nevertheless, realization of such design is technologically challenged, hence hardly explored. Herein, a novel type of silicon photodiodes, based on salicided polysilicon trenched contacts, is presented. These contacts can be prepared up to 10 μm in depth, without showing any leakage current associated with the increase in the contact area. Consequently, the trenched photodiodes revealed better performance than no-trench photodiodes. A simple two dimensional model was developed, allowing to estimate the conditions under which a vertical design has the potential to have better performance than that of a planar design. At large, the deeper the trench is, the better is the vertical design relative to the planar (up to 10 μm for silicon). The vertical design is more advantageous for materials characterized by short diffusion lengths of the carriers. Salicided polysilicon trenched contacts open new opportunities for the design of solar cells and image sensors. For example, these contacts may passivate high contact area buried contacts, by virtue of the conformity of polysilicon interlayer, thus lowering the via resistance induced recombination enhancement effect.

14 SOLAR ENERGY↗
DOE OSTI2016

Thermal contact resistance measurement of conduction cooled binary current lead joint block in cryocooler based self field I-V characterization facility

In the present study thermal resistance of conduction cooled current lead joint block employing two different interfacial material namely AlN sheet and Kapton Film have been studied in the temperature range 5K-35K. In each case, the performance of different interlayer materials e.g. Indium foil for moderately pressurized contacts (contact pressure <1 MPa), and Apiezon N Grease, GE varnish for low pressurized contact (contact pressure <1 MPa) is studied. The performances of AlN joint with Indium foil and with Apeizon N Grease are studied and it is observed that the contact resistance reduces more with indium foil as compared to greased contact. The contact resistance measurements of Kapton film with Apiezon N grease and with GE varnish were also carried out in the same temperature range. A comparative study of AlN joint with Indium foil and Kapton with GE varnish as filler material is carried out to demonstrate better candidate material among Kapton and AlN for a particular filler material in the same temperature range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Optimization of submicron Ni/Au/Ge contacts to an AlGaAs/GaAs two-dimensional electron gas

We report on fabrication and performance of sub-micrometer Ni/Au/Ge contacts to a two-dimensional electron gas in an AlGaAs/GaAs heterostructure. Utilizing scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and low temperature electrical measurements, we investigate the relationship between contact performance and the mechanical and chemical properties of the annealed metal stack. Contact geometry and crystallographic orientation significantly impact performance. Our results indicate that the spatial distribution of germanium in the annealed contact plays a central role in the creation of high transmission contacts. We characterize the transmission of our contacts at high magnetic fields in the quantum Hall regime. Our work establishes that contacts with an area of 0.5 μm2 and resistance less than 400 Ω can be fabricated with high yield.

Physics↗
DOE OSTI2015

Photometric investigation of the totally eclipsing contact binary V12 in the intermediate-age open cluster NGC 7789

NGC 7789 is an intermediate-age open cluster with an age similar to the mean age of contact binary stars. V12 is a bright W UMa-type binary star with an orbital period of 0.3917 days. The first complete light curves of V12 in the V, R, and I bands are presented and analyzed with the Wilson–Devinney (W-D) method. The results show that V12 is an intermediate-contact binary (f=43.0(±2.2)%) with a mass ratio of 3.848, and it is a W-type contact binary where the less massive component is slightly hotter than the more massive one. The asymmetry of the light curves is explained by the presence of a dark spot on the more massive component. The derived orbital inclination (i=83{sub .}{sup ∘}6) indicates that it is a totally eclipsing binary, which suggests that the determined parameters are reliable. The orbital period may show a long-term increase at a rate of P-dot =+2.48(±0.17)×10{sup −6} days yr{sup −1} that reveals a rapid mass transfer from the less massive component to the more massive one. However, more observations are needed to confirm this conclusion. The presence of an intermediate-contact binary in an intermediate-age open cluster may suggest that some contact binaries have a very short pre-contact timescale. The presence of a third body and/or stellar collision may help to shorten the pre-contact evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

A deep dilated convolutional residual network for predicting interchain contacts of protein homodimers

Abstract Motivation Deep learning has revolutionized protein tertiary structure prediction recently. The cutting-edge deep learning methods such as AlphaFold can predict high-accuracy tertiary structures for most individual protein chains. However, the accuracy of predicting quaternary structures of protein complexes consisting of multiple chains is still relatively low due to lack of advanced deep learning methods in the field. Because interchain residue–residue contacts can be used as distance restraints to guide quaternary structure modeling, here we develop a deep dilated convolutional residual network method (DRCon) to predict interchain residue–residue contacts in homodimers from residue–residue co-evolutionary signals derived from multiple sequence alignments of monomers, intrachain residue–residue contacts of monomers extracted from true/predicted tertiary structures or predicted by deep learning, and other sequence and structural features. Results Tested on three homodimer test datasets (Homo_std dataset, DeepHomo dataset and CASP-CAPRI dataset), the precision of DRCon for top L/5 interchain contact predictions (L: length of monomer in a homodimer) is 43.46%, 47.10% and 33.50% respectively at 6 Å contact threshold, which is substantially better than DeepHomo and DNCON2_inter and similar to Glinter. Moreover, our experiments demonstrate that using predicted tertiary structure or intrachain contacts of monomers in the unbound state as input, DRCon still performs well, even though its accuracy is lower than using true tertiary structures in the bound state are used as input. Finally, our case study shows that good interchain contact predictions can be used to build high-accuracy quaternary structure models of homodimers. Availability and implementation The source code of DRCon is available at https://github.com/jianlin-cheng/DRCon. The datasets are available at https://zenodo.org/record/5998532#.YgF70vXMKsB. Supplementary information Supplementary data are available at Bioinformatics online.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Nickel Silicide Metallization for Passivated Tunneling Contacts for Silicon Solar Cells

Passivated tunneling contacts offer promise for applications in Interdigitated Back Passivated Contact (IBPC) high efficiency silicon solar cells. Metallization of these contacts remains a key research topic. This paper investigates NiSi/poly-Si/SiO2/c-Si passivated contacts using photoluminescence and contact resistivity measurements. An amorphous Si interlayer between the NiSi and poly-Si is observed to improve passivation, decreasing recombination. The overall recombination loss has a linear trend with the NiSi thickness. Implied Voc values close to 700 mV and contact resistivities below 10 mohm-cm2 have been achieved in NiSi/poly-Si:P/SiO2/c-Si contacts.

14 SOLAR ENERGY↗
DOE OSTI2017

Structure of the surface microrelief of a droplet evaporating from a rough substrate as a possible cause of contact angle hysteresis

Based on the refraction images of a droplet evaporating on a rough substrate, we simultaneously observed the dynamics of its surface microrelief, contact angle, and contact line deformations along the entire perimeter of the contact line. This has led us conclude that the microrelief structure is directly related to the phenomenon of contact angle hysteresis and the jump-like pattern of contact line deformation. We suggest a possible mechanism for the occurrence of contact angle hysteresis during droplet evaporation and derive the relations that specify the range of possible contact angles at known microrelief parameters.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Investigating the RTA Treatment of Ohmic Contacts to n-Layers of Heterobipolar Nanoheterostructures

The preparation of ohmic contacts to heterobipolar nanostructures has a number of characteristic features. In addition to the basic requirement of minimizing contact resistance, contacts to this type of structures have a transition layer whose depth of penetration must not exceed the emitter layer’s thickness, due to the possibility of short-circuiting the emitter base p–n junction. In this work, the effect the main technological parameters of rapid thermal annealing have on a contact’s characteristics are examined, and the process of obtaining a low-resistance ohmic contact to heterobipolar transistor layers is optimized. Ohmic contacts to the n-layers of heterobipolar nanoheterostructures based on gallium arsenide and produced via layer-by-layer electron-beam deposition of Ge/Au/Ni/Au are considered. The diffusion distribution profiles of doping with Ge impurities are calculated as a function of the time and temperature of rapid thermal annealing, and are examined via scanning electron microscopy. It is found that rapid thermal annealing for 60 s at a temperature of 398°C yields ohmic contacts with low resistance, smooth surface morphology, and the minimum size of the transition layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Structural Reliability of Cathode Contact Materials in Planar SOFCs

Despite significant developments in material and stack design technologies to enhance performance, the long-term mechanical reliability of solid oxide fuel cells remained an issue. The contact between electrodes and interconnects (especially the cathode side) was identified as one of the weakest links in present generation planar solid oxide fuel cells. Computational modeling was used to gain valuable insights on the cathode contact sintering mechanics and bond reliability. A continuum viscous sintering model was incorporated into a commercial finite element analysis code to simulate contact material densification and evaluate the bond and stack mechanical reliability due to developed stresses from the contact layer sintering. The parameters required for the cathode contact material constitutive model were extracted from experiments conducted for the contact material development at PNNL. The effects of contact layer thickness and sintering conditions like temperature and pressure on the stack stresses and reliability were investigated.

SOFC, FEA, Reliability↗