52 research outputs found
Hydrogen-assisted fracture of additively manufactured type 304L austenitic stainless steel
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FilamentâFree Bulk Resistive Memory Enables Deterministic Analogue Switching
Digital computing is nearing its physical limits as computing needs and energy consumption rapidly increase. Analogueâmemoryâbased neuromorphic computing can be orders of magnitude more energy efficient at dataâintensive tasks like deep neural networks, but has been limited by the inaccurate and unpredictable switching of analogue resistive memory. Filamentary resistive random access memory (RRAM) suffers from stochastic switching due to the random kinetic motion of discrete defects in the nanometerâsized filament. In this work, this stochasticity is overcome by incorporating a solid electrolyte interlayer, in this case, yttriaâstabilized zirconia (YSZ), toward eliminating filaments. Filamentâfree, bulkâRRAM cells instead store analogue states using the bulk point defect concentration, yielding predictable switching because the statistical ensemble behavior of oxygen vacancy defects is deterministic even when individual defects are stochastic. Both experiments and modeling show bulkâRRAM devices using TiO2âX switching layers and YSZ electrolytes yield deterministic and linear analogue switching for efficient inference and training. BulkâRRAM solves many outstanding issues with memristor unpredictability that have inhibited commercialization, and can, therefore, enable unprecedented new applications for energyâefficient neuromorphic computing. Beyond RRAM, this work shows how harnessing bulk point defects in ionic materials can be used to engineer deterministic nanoelectronic materials and devices.A resistive memory cell based on the electrochemical migration of oxygen vacancies for inâmemory neuromorphic computing is presented. By using the average statistical behavior of all oxygen vacancies to store analogue information states, this cell overcomes the stochastic and unpredictable switching plaguing filamentâforming memristors, and instead achieves linear, predictable, and deterministic switching.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163547/3/adma202003984_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163547/2/adma202003984-sup-0001-SuppMat.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163547/1/adma202003984.pd
Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes
Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode âlithium iron phosphate (âLiFePO4; âLFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 âLFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in âLFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes
Mapping genomic loci implicates genes and synaptic biology in schizophrenia
Schizophrenia has a heritability of 60-80%1, much of which is attributable to common risk alleles. Here, in a two-stage genome-wide association study of up to 76,755 individuals with schizophrenia and 243,649 control individuals, we report common variant associations at 287 distinct genomic loci. Associations were concentrated in genes that are expressed in excitatory and inhibitory neurons of the central nervous system, but not in other tissues or cell types. Using fine-mapping and functional genomic data, we identify 120 genes (106 protein-coding) that are likely to underpin associations at some of these loci, including 16 genes with credible causal non-synonymous or untranslated region variation. We also implicate fundamental processes related to neuronal function, including synaptic organization, differentiation and transmission. Fine-mapped candidates were enriched for genes associated with rare disruptive coding variants in people with schizophrenia, including the glutamate receptor subunit GRIN2A and transcription factor SP4, and were also enriched for genes implicated by such variants in neurodevelopmental disorders. We identify biological processes relevant to schizophrenia pathophysiology; show convergence of common and rare variant associations in schizophrenia and neurodevelopmental disorders; and provide a resource of prioritized genes and variants to advance mechanistic studies
Mapping genomic loci prioritises genes and implicates synaptic biology in schizophrenia
Schizophrenia has a heritability of 60â80%1, much of which is attributable to common risk alleles. Here, in a two-stage genome-wide association study of up to 76,755 individuals with schizophrenia and 243,649 control individuals, we report common variant associations at 287 distinct genomic loci. Associations were concentrated in genes that are expressed in excitatory and inhibitory neurons of the central nervous system, but not in other tissues or cell types. Using fine-mapping and functional genomic data, we identify 120 genes (106 protein-coding) that are likely to underpin associations at some of these loci, including 16 genes with credible causal non-synonymous or untranslated region variation. We also implicate fundamental processes related to neuronal function, including synaptic organization, differentiation and transmission. Fine-mapped candidates were enriched for genes associated with rare disruptive coding variants in people with schizophrenia, including the glutamate receptor subunit GRIN2A and transcription factor SP4, and were also enriched for genes implicated by such variants in neurodevelopmental disorders. We identify biological processes relevant to schizophrenia pathophysiology; show convergence of common and rare variant associations in schizophrenia and neurodevelopmental disorders; and provide a resource of prioritized genes and variants to advance mechanistic studies
Mechanisms of microstructure development at metallic-interlayer/ceramic interfaces during liquid-film-assisted bonding
Alumina has been bonded via copper/niobium/copper interlayers, and correlations have been made between various processing conditions (applied load, processing temperature, copper film thickness, surface roughness, etc.) and strength. Four-point bend strengths and micrographs of fracture surfaces have been used to determine the relationship between processing, microstructure, and properties. Transparent sapphire substrates bonded with copper/niobium/copper interlayers were used in model experiments to track the microstructural development of these ceramic/metalinterfaces and to identify the important mechanisms that contribute. High interfacial strengths were generally associated with small unbonded regions, extensive breakup of the copper film into isolated particles, ceramic pullout, and regions of niobium/alumina contact where the grain boundary grooves of the alumina are visible on both sides of the fracture surface. Experiments with sapphire substrates showed that asperities in the niobium and grain boundary grooves in the niobium play an important role in the initiation and growth of sapphire/niobium contact. The presence of a liquid film can enhance the kinetics of sapphire/niobium contact and growth by providing a low-temperature high-diffusivity path. The breakup of the copper film was described using two models that were in fairly close agreement. The breakup of the copper film depended on the asperity density in the niobium, niobium grain boundary density, liquid film redistribution, and the breakup of liquid patches via Rayleigh instabilities. The redistribution of the liquid was affected by defect geometry, local film thickness, and local interfacial crystallography. Thermal grooving effects of liquid copper on alumina and niobium were studied using conventional sessile drop experiments. The thermal grooving of one particular grain boundary in alumina when in contact with copper and niobium was studied using a fabricated bicrystal. Both diffusion mechanisms and the dissolution-precipitation reaction of alumina in niobium limited the kinetics of thermal grooving
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Processing of alumina-niobium interfaces via liquid-film-assisted joining
Alumina-niobium interfaces were fabricated at 1400 C via solid-state diffusion brazing of a 127-{micro}m-thick niobium foil between alumina blocks. Prior to brazing, some of the alumina mating surfaces, both polished and unpolished, were evaporation-coated with copper films {approx}1.4 {micro}m, {approx}3.0 {micro}m, and {approx}5.5 {micro}m thick to induce liquid-film-assisted joining at the brazing temperature. The effects of copper film thickness and surface roughness on fracture characteristics and ceramic-metal interfacial microstructure were investigated by room-temperature four-point bend tests, optical microscopy, profilometry, and atomic force microscopy. The average strength of bonds between niobium and polished alumina substrates increased with the introduction of copper film interlayers, and the scatter in strength tended to decrease, with an optimum combination of strength and Weibull modulus arising for a copper film thickness of 3.0 {micro}m. The strength characteristics of niobium bonded to unpolished alumina substrates were also improved by liquid-film-assisted joining, but were unaffected by the thickness of the copper interlayers
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