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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Spall strength dependence on grain size and strain rate in tantalum

In this paper, we examine the effect of grain size on the dynamic failure tantalum during laser-shock compression and release and identify a significant effect of grain size on spall strength, which is opposite to the prediction of the Hall-Petch relationship because spall is primarily intergranular in both poly and nanocrystalline samples; thus, monocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultra-fine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra and intergranular in the poly and ultra-fine-grained crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. The length geometrically-necessary dislocations required to form a grain-boundary (intergranular) void is lower than that of grain-interior (intragranular) void with the same maximum diameter; thus, the energy required is lower. Consistent with prior literature and theory we also identify an increase with spall strength with strain rate from 6x10 6 to 5x10 7 s -1 . Molecular dynamics calculations agree with the experimental results and also predict grain-boundary separation in the spalling of polycrystals as well as an increase in spall strength with strain rate. Finally, an analytical model based on the kinetics of nucleation and growth of intra and intergranular voids and extending the Curran-Seaman-Shockey theory is applied which shows the competition between the two processes for polycrystals.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Simulation and analysis of the 3D double shock-induced recompression of spall using a hybrid PAGOSA with FLIP+MPM

In this work, an innovative hybrid algorithm PAGOSA with FLIP + MPM is first presented by coupling PAGOSA with the particle FLIP + MPM, and applied to systematically exploring the 3D recompression of spall induced by double-shock waves in ductile materials as the few existing studies are currently limited to the pure longitudinal recompression of spall, and ignore the 3D effects. This algorithm solves the difficulties the grid-based methods encounter when applied to capturing the fracture in material. We first validate the capabilities of PAGOSA with FLIP + MPM to predict different fracture/fragmentation cases by solving two benchmark problems and comparing with the analytical solutions or the experiment results. The convergences and the infinity-norm errors are also investigated. Subsequently, the 3D effects are demonstrated by simulating the spallation in material using PAGOSA with FLIP + MPM and comparing with those in the longitudinal cases. Additionally, some factors that affect the spallation with 3D effects are also discussed. Finally, the recompressions of spall in material with/without 3D effects are systematically simulated and analyzed. The recompression of spall driven by a high explosive is also simulated to show the ability of PAGOSA with FLIP + MPM to handle this kind of problem. The conditions that can lead to a recompression of spall are explored. Also, numerical results show that PAGOSA with FLIP + MPM can accurately predict different fracture cases, that the 3D effects play an important role in fracture/fragmentation, and the recompression of spall is very sensitive to the occurrence conditions, shows great differences when considering 3D effects in real applications. Moreover, the present hybrid method can be easily extended to other grid-based techniques employed for fracture in materials.

3D effects↗
DOE OSTI2018

III–V Solar Cells Grown on Unpolished and Reusable Spalled Ge Substrates

Substrate reuse offers a promising route toward enabling high-efficiency III-V solar cells to become cost-competitive for one-sun terrestrial applications. In this study, Ge films were spalled using an electroplated Ni stressor layer, and the fracture surface was characterized. Initial heteroepitaxial GaAs films grown on spalled Ge substrates by hydride vapor phase epitaxy were found to be single crystal with nearly equivalent growth rates between growth on epi-ready and unpolished spalled Ge substrates. We show the first demonstration of III-V solar cells (GaInAsP, E g ~ 1.7 eV) grown on unpolished spalled Ge substrates to assess the viability of substrate reuse. We demonstrate equivalent performance for a solar cell grown on spalled Ge and on a coloaded epi-ready Ge substrate, despite the residual roughness on the spalled Ge substrate and the lack of any substrate surface preparation. This initial demonstration of combining substrate reuse with the ability to grow III-V solar cells using a high growth rate and high throughput deposition process is a promising step toward low-cost III-V photovoltaics.

14 SOLAR ENERGY↗
DOE OSTI2022

(110)-Oriented GaAs Devices and Spalling as a Platform for Low-Cost III-V Photovoltaics

Here, we demonstrate the growth of GaAs solar cells by hydride vapor phase epitaxy (HVPE) on epi-ready and previously spalled (110) GaAs wafers as an advance towards a potentially low-cost (110)-based device platform. Controlled spalling offers a fracture-based path to substrate cost amortization, enabling device exfoliation and substrate reuse, but the faceted surface generated during the spalling of (100)-oriented GaAs presents hurdles to direct regrowth of subsequent devices. Spalling of (110) substrates instead eliminates faceting by aligning the substrate surface with the predominant crystal cleavage plane. III-V epitaxy of solar cells is significantly less developed on (110)-oriented substrates, however. Here, we develop (110)-based GaAs solar cells grown by HVPE on epi-ready substrates, demonstrating equal performance to devices grown on the more standard (100) orientation. We also characterize the surface of spalled, (110)-oriented substrates, revealing flat terraces separated by steps with sub-micron-scale height on average. Finally, we present an initial device grown on a previously spalled surface without additional surface re-preparation with nearly 16% efficiency under a simulated AM1.5G spectrum. Together, these results provide preliminary evidence of a potentially low-cost path to enable terrestrial III-V photovoltaics via the (110) substrate orientation.

14 SOLAR ENERGY↗
DOE OSTI2017

Low temperature spalling of silicon: A crack propagation study

Spalling is a promising kerfless method for cutting thin silicon wafers while doubling the yield of a silicon ingot. The main obstacle in this technology is the high total thickness variation of the spalled wafers, often as high as 100% of the wafer thickness. It has been suggested before that a strong correlation exists between low crack velocities and a smooth surface, but this correlation has never been shown during a spalling process in silicon. The reason lies in the challenge associated to measuring such velocities. In this contribution, we present a new approach to assess, in real time, the crack velocity as it propagates during a low temperature spalling process. Understanding the relationship between crack velocity and surface roughness during spalling can pave the way to attain full control on the surface quality of the spalled wafer.

14 SOLAR ENERGY↗
DOE OSTI2018

GaAs Solar Cells Grown on Unpolished, Spalled Ge Substrates: Preprint

Decreasing the cost of single-crystal substrates by wafer reuse techniques has long been sought for III-V solar cells. Controlled spalling of III-V devices is a possible pathway for epitaxial liftoff, which would help reduce costs, but chemo- mechanical polishing after liftoff tends to limit the potential cost savings. Growth on an unpolished spalled surface would be an additional step toward lower costs, but it is crucial to show high efficiency solar cell devices on these unprocessed substrates. In this study, we spalled 2-inch Ge wafers using a Ni stressor layer, and then grew GaAs solar cells by HVPE on the spalled Ge surface without any other surface treatment. We show a 12.8% efficient single-junction device, without anti-reflection coating, with quantum efficiency very close to identical devices grown by HVPE on non-spalled GaAs substrates. Demonstrating a high carrier collection on unpolished spalled wafers is a step toward reducing substrate-related liftoff and reuse costs.

14 SOLAR ENERGY↗
DOE OSTI2018

GaAs Solar Cells with High Carrier Collection Grown on Unpolished, Spalled Ge Substrates: Preprint

Decreasing the cost of single-crystal substrates has long been sought for III-V solar cells. Controlled spalling is a possible pathway for epitaxial liftoff, which would help reduce costs, but chemo-mechanical polishing after liftoff tends to limit the impact. Growth on an unpolished spalled surface would be an additional step toward lower costs, but it is crucial to show high efficiency solar cell devices on these unprocessed substrates. In this study, we spalled 2 inch Ge wafers using a Ni stressor layer, and GaAs solar cells are grown by HVPE on the spalled Ge surface without any other surface treatment. We show a 12.8% efficient device, without anti-reflection coating, with equivalent quantum efficiency as identical devices grown by HVPE on non-spalled GaAs substrates. Demonstrating a high carrier collection on unpolished spalled wafers is the first step toward reducing substrate-related liftoff and polishing costs.

14 SOLAR ENERGY↗
DOE OSTI2020

Spall and subsequent recompaction of copper under shock loading

Research was undertaken to study spall and subsequent recompaction of oxygen-free high-thermal-conductivity copper using a single-stage large-bore light gas gun capable of planar impacts. Gun experiments were conducted that produced an initial spall in the target with a subsequent recompaction of the spall damage/layer by the use of layered impactors. Symmetric spall experiments at similar conditions were also conducted as a control to the recompaction experiments. Photonic Doppler velocimetry was used to obtain the velocity history of the target’s back surface; these velocity histories were analyzed, and the results were compared to numerical simulations. After recovery of selected samples, the microstructure was analyzed using optical imaging microscopy and electron backscatter diffraction. Recompaction waves were clearly observed in the time-resolved data obtained at the back surface of the targets. Two different series of experiments were performed to understand the effect of damage morphology on the stress required for spall recompaction. In the initial series, the peak pressure of the first shock was systematically increased to alter the amount of damage in the material. While in the second series, the peak pressure of the second shock was increased, keeping the magnitude of the first shock relatively constant to understand the effect of peak stress on the recompaction process. Findings clearly show that recompaction results in a perturbed band of the microstructure at the location of the expected spall plane in all cases except where the peak stress of the second shock is lower than the critical value.

, recompaction↗
DOE OSTI2020

Characterization of Spall in Hard Rock from Observations and Simulations of the Source Physics Experiment Phase I

In this work, spall signals from the Source Physics Experiments are presented, analyzed, and modeled for insight to the explosion source. The observed signal is similar in nature to nearby historical nuclear explosions, and the surface force-time history or velocity can be interpreted with the same model. We use the models for peak spall velocity, spalled mass, and spall depth and radius derived from historical nuclear explosions to parameterize the physical force-time history model from Stump (1985) and show that this parameterized model can be used for spall prediction. The spall signal is also investigated with a numerical continuum model that incorporates gravity. Peak velocity and dwell time are well predicted, and the multiple slap-down phases are captured if one includes a weak near-surface layer similar to the geologic observation.

58 GEOSCIENCES↗
DOE OSTI2023

Mitigation of spall fracture by evolving porosity

Ductile materials subject to impact loading conditions can undergo spall fracture when an incoming compressive stress wave reflects off interfaces and free surfaces as a tensile stress wave. Experimental observations suggest that in ductile materials, spall fracture is driven by the evolution of porosity. However, the presence of initial porosity in ductile materials also introduces plastic compressibility, which can attenuate the incoming compressive stress wave and, as a result, reduce the amplitude of the reflected tensile stress wave. This, in turn, can mitigate or delay spall fracture. In this work, we report on finite deformation finite element calculations that analyze the response of porous ductile materials subjected to impact loading conditions. Two sets of calculations are carried out, in the first set the material contains initial porosity values ranging from 0% to 5% while in the second set the material also undergoes stress-controlled porosity nucleation. Both sets of calculations are carried out for a wide range of imposed impact velocities. Here, our results show that porosity in ductile materials can, under certain circumstances, mitigate spall fracture by attenuating stress waves. Results correlating the effects of impact velocity, initial porosity, and porosity nucleation on spall fracture are presented and the underlying mechanisms are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Increased fracture depth range in controlled spalling of (100)-oriented germanium via electroplating

Controlled spalling in (100)-oriented germanium using a nickel stressor layer shows promise for semiconductor device exfoliation and kerfless wafering. Demonstrated spall depths of 7-60 um using DC sputtering to deposit the stressor layer are appropriate for the latter application but spall depths < 5 um may be required to minimize waste for device applications. This work investigates the effect of tuning both electroplating current density and electrolyte chemistry on the residual stress in the nickel and on the achievable spall depth range for the Ni/Ge system as a lower-cost, higher-throughput alternative to sputtering. By tuning current density and electrolyte phosphorous concentration, it is shown that electroplating can successfully span the same range of spalled thicknesses as has previously been demonstrated by sputtering and can reach sufficiently high stresses to enter a regime of thickness (<7 um) appropriate to minimize substrate consumption for device applications.

14 SOLAR ENERGY↗
DOE OSTI2018

Effect of peak stress and tensile strain-rate on spall in tantalum

Materials subjected to dynamic environments experience a complex and wide range of stress, strain, and strain-rate conditions. To have confidence in material models, an accurate, predictive capability is required. In this study, we present a series of flyer-plate impact tests on well characterized, high purity tantalum. The shock-waves generated at impact release from the free-surfaces, reflect, and interact to produce incipient spall fracture. By varying the flyer-plate material and impact velocity, both the peak stress and the strain-rate in the samples were controlled independently. Velocimetry was used on the rear free-surface of the samples to measure the shock-response and the spall strength. While this measurement provided the same spall strength for all cases, at approximately 5.1 GPa, when the samples were sectioned during post-mortem, the quantity and distribution of internal damage was markedly different. For the high-strain rate cases, voids remained small and isolated, whereas in the lower strain-rate experiments, the spall damage was far more localized, with a well-defined continuous spall plane. With the use of hydrocode simulations, this was discovered to result from how the different release rates affect the interaction volume inside the sample. Here, these results highlight the importance of careful sample recovery, and the risks of relying solely on free-surface velocity measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2021

(110)-Oriented GaAs Devices and Spalling as a Platform for Low-Cost III-V Photovoltaics

We demonstrate the growth of (110) GaAs solar cells by hydride vapor phase epitaxy (HVPE) as an advance towards a (110)-oriented device platform with substrate reuse via spalling. Controlled spalling offers a fracture-based path to substrate amortization, allowing device removal and substrate reuse, but the faceted surface generated in spalling of (100)-GaAs presents hurdles to direct regrowth of subsequent devices. Spalling of (110)-oriented substrates takes advantage of the natural (110) cleavage plane in zinc-blende III-V materials, eliminating this faceting. The vast majority of III-V epitaxy development is on (100)-oriented substrates, but epitaxy is not limited to this orientation. Here, we demonstrate (110)-based GaAs solar cells grown by HVPE with equivalent performance to devices grown on the more standard (100) orientation. We also demonstrate spalling of (110) oriented substrates, revealing surfaces with flat terraces separated by steps. These results provide preliminary evidence of a potentially low-cost path for terrestrial III-V photovoltaics via the (110) substrate orientation.

(110) orientation↗
DOE OSTI2018

Combined curing as a novel approach to improve resistance of ultra-high performance concrete to explosive spalling under high temperature and its mechanical properties

Highlights: • The combined curing can significantly enhance the resistance of UHPC to explosive spalling under high temperature. • The combined curing can improve the mechanical properties of UHPC remarkably. • Both further hydration of residual cement and pozzolanic reaction are activated under a high temperature steam condition. - Abstract: The influence of a combined curing composed of precuring in hot water and heating in dry air on mechanical properties and explosive spalling of ultra-high performance concrete (UHPC) subjected to high temperature was investigated in this experiment. Meanwhile, the microstructure of UHPC was determined by means of X-ray diffraction (XRD) with Rietveld method, thermogravimetric analysis (TGA), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP). The results indicate that the combined curing improves the mechanical properties of UHPC remarkably. More importantly, it is quite efficient to enhance the resistance of UHPC to explosive spalling under high temperature, which should be a novel approach to prohibit spalling occurrence, different from adding polypropylene fiber. During dry air heating in the combined curing, a high temperature steam environment is established in the dense structure framework of UHPC formed in the preceding hot water curing. Both further hydration of residual cement and pozzolanic reaction of mineral admixtures can be activated during the dry air heating, leading to not only the formation of a large amount of dense hydrates (such as C-S-H gels, katoite, tobermorite and xonotlite, etc.), which accumulate in the aforementioned existing framework, but also the consumption of most of internal free water. Therefore, both mechanical properties of UHPC and its resistance to explosive spalling under high temperature can be enhanced significantly.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Controlled Spalling of 4H Silicon Carbide with Investigated Spin Coherence for Quantum Engineering Integration

We detail scientific and engineering advances which enable the controlled spalling and layer transfer of single crystal 4H silicon carbide (4H-SiC) from bulk substrates. 4HSiC’s properties, including high thermal conductivity and a wide bandgap, make it an ideal semiconductor for power electronics. Moreover, 4H-SiC is an excellent host of solid-state atomic defect qubits for quantum computing and quantum networking. Because 4H-SiC substrates are expensive (due to long growth times and limited yield), techniques for removal and transfer of bulk-quality films are desirable for substrate reuse and integration of the separated films. In this work, we utilize updated approaches for stressor layer thickness control and spalling crack initiation to demonstrate controlled spalling of 4H-SiC, the highest fracture toughness crystal spalled to date. We achieve coherent spin control of neutral divacancy (VV 0 ) qubit ensembles and measure a quasi-bulk spin T 2 of 79.7 μs in the spalled films.

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

Spall fracture in additive manufactured Ti-6Al-4V

Here, we present a study on the spall strength of additive manufactured (AM) Ti-6Al-4V. Samples were obtained from two pieces of selective laser melted (SLM, a powder bed fusion technique) Ti-6Al-4V such that the response to dynamic tensile loading could be investigated as a function of the orientation between the build layers and the loading direction. A sample of wrought bar-stock Ti-6Al-4V was also tested to act as a baseline representing the traditionally manufactured material response. A single-stage light gas-gun was used to launch a thin flyer plate into the samples, generating a region of intense tensile stress on a plane normal to the impact direction. The rear free surface velocity time history of each sample was recorded with laser-based velocimetry to allow the spall strength to be calculated. The samples were also soft recovered to enable post-mortem characterization of the spall damage evolution. Results showed that when the tensile load was applied normal to the interfaces between the build layers caused by the SLM fabrication process the spall strength was drastically reduced, dropping to 60% of that of the wrought material. However, when loaded parallel to the AM build layer interfaces the spall strength was found to remain at 95% of the wrought control, suggesting that when loading normal to the AM layer interfaces, void nucleation is facilitated more readily due to weaknesses along these boundaries. Quasi-static testing of the same sample orientations revealed a much lower degree of anisotropy, demonstrating the importance of rate-dependent studies for damage evolution in AM materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

GaAs Solar Cells Grown on Unpolished, Spalled Ge Substrates

We spalled 2-inch Ge wafers using a Ni stressor layer and grew GaAs solar cells by hydride vapor phase epitaxy on the spalled Ge surface without other surface treatments. The controlled spalling procedure leaves behind arrest lines, which are parallel surface striations, perpendicular to the crack front, caused by the crack propagation repeatedly halting and restarting during the fracture. We show that small arrest lines do not significantly affect device performance, but larger lines act as regions for carrier recombination. We demonstrate a 12.8% efficient single-junction device, without anti-reflection coating, grown on a spalled surface containing arrest lines. The quantum efficiency of this device is similar to devices grown on non-spalled GaAs and Ge substrates.

14 SOLAR ENERGY↗
DOE OSTI2019

Spall Strength of Shock-Heated Hafnium and the Equation of State of Its Polymorphic Modifications

We determined the spall strength of hafnium heated by a loading shock wave to thousands of degrees and subjected to transformations into denser polymorphic modifications. To achieve this goal, the following tasks were solved. The pressure profiles at the interface between the sample and soft buffer material in the spall experiments with flat, one-dimensional loading are measured. The equations of state of three polymorphic modifications of hafnium are constructed in the pressure range up to 100 GPa. The thermodynamic states of hafnium were calculated under the conditions of the performed experiments, which, together with the results of measurements of pressure profiles, made it possible to determine the spall strength of hafnium heated in a cycle of shock compression–unloading. The spall strength of hafnium was –4(1) GPa at a temperature of 1680(380) K. The spall experiments were mathematically simulated in the framework of a one-dimensional hydrocode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗