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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 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effects of precipitates and its interface on thermal conductivity of Mg–12Gd alloy during aging treatment

Highlights: • Hardness and thermal conductivity of alloy can be improved by aging treatment. • The interface between precipitates and matrix transformed coherent into incoherent with increasing aging time. • Coherent interface would impede the improvement of thermal conductivity of alloy. - Abstract: How to simultaneously improve the thermal conductivity and mechanical properties of Mg alloys is an interesting and important issue topic. In the present study, the effect of aging treatment on the thermal conductivity and micro-hardness of Mg–12Gd alloy was investigated. Results indicated that the thermal conductivity and micro-hardness of the alloy could be significantly improved by aging treatment, and Mg–12Gd alloy exhibited a typical aging hardening response with the increasing aging time. At the early stage of aging (4 h), a large number of fine particles precipitated from α-Mg matrix, but thermal conductivity of alloy increased a little, which was attributed to the coherent interface hindering the increase of thermal conductivity. The thermal conductivity increased to the peak value of 75.7 W/(m·K) at the aging time of 300 h, which was about 2.17 times that of as-solutionized alloy. The huge increment in the thermal conductivity of as-aged Mg–12Gd alloy was related to the precipitation of particles and the incoherent interface between precipitates and α-Mg matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Investigation of the thermal conductivity of molten LiF-NaF-KF with experiments, theory, and equilibrium molecular dynamics

Molten salts are being proposed for numerous advanced energy applications, including advanced nuclear reactors, concentrating solar power plants, thermal energy storage, and fusion reactors. Accurate knowledge of the thermophysical properties of molten salts directly impact the performance of these energy systems and are essential for design and safety analyses. Thermal conductivity data for fluoride molten salts and mixtures are especially lacking. In this work, experimental measurements of thermal conductivity using the steady-state variable gap technique were performed on eutectic LiF-NaF-KF from 834 to 1195 K. The experiment accounts for radiative, convective, and conductive heat losses. In addition, theoretical and molecular dynamics models are used, from 750 K up to 1300 K, to estimate the thermal conductivity for comparison with the experimental results. The results of experiments show a weak negative deviation of thermal conductivity with temperature, unlike previous experimental results in the literature. The measured thermal conductivity magnitudes agree with the theoretical and molecular dynamics predictions, aside from the data above 1100 K, where heat losses and radiative errors are the most significant, having a 16% maximum deviation from theory. These experimental results provide new thermal conductivity data for the LiF-NaF-KF system and further validation of the predictive models. The theoretical model was used to map the composition and temperature dependent thermal conductivity of LiF-NaF-KF and the mapping’s deviation from a linear additivity estimation of thermal conductivity. Additionally, this mapping showed the highest deviations from linearity for KF-LiF rich mixtures and increasing deviation with temperature. Notably, the deviation from linearity near the LiF-NaF-KF eutectic composition was around 25%.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Thermal conductivity of ZnO thin film produced by reactive sputtering

ZnO thin films have been produced by reactive sputtering with different oxygen contents in the sputtering gas. As a result of transmission electronic microscopy observation, each film consist of two layers: an interfacial layer close to the substrate, with a thickness of about 200 nm, composed of very fine crystal grains and an upper layer above the interfacial layer, composed of column-shaped grains aligned along the out-plane direction. The grain diameter ranges from 35 to 100 nm depending on the oxygen partial pressure. The in-plane and out-plane thermal conductivity have been measured at room temperature. The out-plane thermal conductivity of the interfacial layer is 2.3 W m{sup -1} K{sup -1}, independent on the oxygen partial pressure. The out-plane thermal conductivity of the upper layer is 5.4, 7.1, and 4.0 W m{sup -1} K{sup -1}, and the in-plane thermal conductivity 4.86, 6.01, and 2.66 W m{sup -1} K{sup -1}, for the O{sub 2} 30%, 60%, and 90% ZnO film, respectively. Both out-plane and in-plane thermal conductivity decrease with the decrease of grain diameter. The thermal conductance of grain boundary has been estimated with the phonon diffusion mismatch model, and the intrinsic thermal conductivity within ZnO grains has been calculated with a cylinder-structured composite model. The result shows that the thermal conductivity of the ZnO thin films is dominated by the intrinsic thermal conductivity, which is a function of grain size.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Phonon thermal conductivity of monolayer MoS{sub 2}

We use nonequilibrium molecular dynamics modeling using Stillinger–Weber interatomic potential to investigate the thermal properties of monolayer molybdenum disulfide (MoS{sub 2}) nanoribbons. We study the impact of factors such as length, edge chirality, monovacancies, and uniaxial stretching on the thermal conductivity of MoS{sub 2} nanoribbons. Our results show that longer ribbons have a higher thermal conductivity, and the thermal conductivity of infinitely long zigzag and armchair MoS{sub 2} nanoribbons is, respectively, 54 W/mK and 33 W/mK. This is significantly lower than the thermal conductivity of some other graphene-like two-dimensional materials such as graphene and boron nitride. While the presence of molybdenum or sulfur vacancies reduces the thermal conductivity of ribbons, molybdenum vacancies have a more deteriorating effect on thermal conductivities. We also have studied the impact of uniaxial stretching on the thermal conductivity of MoS{sub 2} nanoribbons. The results show that in contrast to three dimensional materials, thermal conductivity of MoS{sub 2} is fairly insensitive to stretching. We have used the phonon dispersion curves and group velocities to investigate the mechanism of this unexpected behavior. Our results show that tensile strain does not alter the phonon dispersion curves and hence the thermal conductivity does not change.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Cosmological simulations of isotropic conduction in galaxy clusters

Simulations of galaxy clusters have a difficult time reproducing the radial gas-property gradients and red central galaxies observed to exist in the cores of galaxy clusters. Thermal conduction has been suggested as a mechanism that can help bring simulations of cluster cores into better alignment with observations by stabilizing the feedback processes that regulate gas cooling, but this idea has not yet been well tested with cosmological numerical simulations. Here we present cosmological simulations of 10 galaxy clusters performed with five different levels of isotropic Spitzer conduction, which alters both the cores and outskirts of clusters, though not dramatically. In the cores, conduction flattens central temperature gradients, making them nearly isothermal and slightly lowering the central density, but failing to prevent a cooling catastrophe there. Conduction has little effect on temperature gradients outside of cluster cores because outward conductive heat flow tends to inflate the outer parts of the intracluster medium (ICM), instead of raising its temperature. In general, conduction tends reduce temperature inhomogeneity in the ICM, but our simulations indicate that those homogenizing effects would be extremely difficult to observe in ∼5 keV clusters. Outside the virial radius, our conduction implementation lowers the gas densities and temperatures because it reduces the Mach numbers of accretion shocks. We conclude that, despite the numerous small ways in which conduction alters the structure of galaxy clusters, none of these effects are significant enough to make the efficiency of conduction easily measurable, unless its effects are more pronounced in clusters hotter than those we have simulated.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

A mini-review on proton conduction of BaZrO 3 -based perovskite electrolytes

Abstract Proton conducting ceramics show promise in fuel cells, electrolyzers, permeation membranes, sensor applications, and membrane reactors. Among several types of materials that exhibit proton conduction, perovskite oxides show high proton conductivity at intermediate temperatures, presenting potential benefits for long-term use and lower costs for energy applications. Doped barium zirconate, BaZrO 3 , is a material that has shown high proton conductivity with encouraging chemical stability. Therefore, it is considered a promising material especially for proton-conducting solid oxide electrochemical cells. Although the proton conduction of doped BaZrO 3 has been extensively characterized, the specific phenomena behind its proton conduction are not fully understood. Only recently have specialized techniques and computational tools begun to elucidate the phenomena that determine the conduction properties of the material. In this mini review, an evaluation of the factors affecting the proton conductivity of doped BaZrO 3 perovskites and the phenomena governing variations in proton concentration and mobility are presented. Special attention is given to proton interactions with dopants and their resulting effect on hydration and transport properties. Technical strategies are provided to give some guidance on the development of protonic ceramics in energy conversion applications.

08 HYDROGEN↗
DOE OSTI2011

Optical transparency and electrical conductivity of nonstoichiometric ultrathin In{sub x}O{sub y} films

The effect of thickness and composition on the electrical conductivity and optical transparency, mainly in the infrared, of ultrathin In{sub x}O{sub y} films was studied. In{sub x}O{sub y} films 35-470 A thick with oxygen atomic fractions of {approx}0.3 and {approx}0.5 were prepared via dc magnetron sputtering. All films were polycrystalline, consisting of only the cubic bixbiyte phase of In{sub 2}O{sub 3}. The average grain size of the films increased from 30 to 95 nm as the film thickness increased. The weak dependence of the electrical conductivity on the frequency and the low activation energies for conduction, a few hundredths of an eV, provided an indication that free band conduction was the primary electrical conduction mechanism in the case of all ultrathin In{sub x}O{sub y} films. It was found that introducing a high degree of nonstoichiometry in the form of oxygen deficiency did not help improve the electrical conductivity, since not all vacancies contributed two free electrons for conduction and due to impurity scattering. The optical nature of these films, studied mainly by ellipsometry, was found to be dependent on the film's composition and thickness. In the infrared, the dielectric function of all In{sub x}O{sub y} films was consistent with the Drude model, inferring that the transparency loss in this region was a result of free charge carriers. In the visible however, In{sub x}O{sub y} films under 170 A, which had an oxygen atomic fraction of {approx}0.5, were modeled by extending the Drude model to the shorter wavelengths. Films over 170 A, with the same composition, were modeled using the Cauchy dispersion model, meaning that no absorption was measured. These results indicate that, optically, under specific compositions, ultrathin In{sub x}O{sub y} films undergo a transition from metalliclike behavior to dielectric behavior with increasing film thickness. Using a figure of merit approach, it was determined that a nonstoichiometric 230 A thick In{sub x}O{sub y} film, with an oxygen atomic fraction of {approx}0.3, had the best combination of conductivity and transparency, namely, absorption of less than 20% in the infrared, about 10% in the visible, and electrical resistance of only 230 {Omega} at 20 kHz. Such a film may be classified as a highly conductive transparent oxide even in the infrared.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Conductivity degradation of polyvinylidene fluoride composite binder during cycling: Measurements and simulations for lithium-ion batteries

The polymer-composite binder used in lithium-ion battery electrodes must both hold the electrodes together and augment their electrical conductivity while subjected to mechanical stresses caused by active material volume changes due to lithiation and delithiation. We have discovered that cyclic mechanical stresses cause significant degradation in the binder electrical conductivity. After just 160 mechanical cycles, the conductivity of polyvinylidene fluoride (PVDF):carbon black binder dropped between 45–75%. This degradation in binder conductivity has been shown to be quite general, occurring over a range of carbon black concentrations, with and without absorbed electrolyte solvent and for different polymer manufacturers. Mechanical cycling of lithium cobalt oxide (LiCoO2) cathodes caused a similar degradation, reducing the effective electrical conductivity by 30–40%. Mesoscale simulations on a reconstructed experimental cathode geometry predicted the binder conductivity degradation will have a proportional impact on cathode electrical conductivity, in qualitative agreement with the experimental measurements. Lastly, ohmic resistance measurements were made on complete batteries. Direct comparisons between electrochemical cycling and mechanical cycling show consistent trends in the conductivity decline. This evidence supports a new mechanism for performance decline of rechargeable lithium-ion batteries during operation – electrochemically-induced mechanical stresses that degrade binder conductivity, increasing the internal resistance of the battery with cycling.

25 ENERGY STORAGE↗
DOE OSTI2011

Truncated multiGaussian fields and effective conductance of binary media.

Truncated Gaussian fields provide a flexible model for defining binary media with dispersed (as opposed to layered) inclusions. General properties of excursion sets on these truncated fields are coupled with a distance-based upscaling algorithm and approximations of point process theory to develop an estimation approach for effective conductivity in two-dimensions. Estimation of effective conductivity is derived directly from knowledge of the kernel size used to create the multiGaussian field, defined as the full-width at half maximum (FWHM), the truncation threshold and conductance values of the two modes. Therefore, instantiation of the multiGaussian field is not necessary for estimation of the effective conductance. The critical component of the effective medium approximation developed here is the mean distance between high conductivity inclusions. This mean distance is characterized as a function of the FWHM, the truncation threshold and the ratio of the two modal conductivities. Sensitivity of the resulting effective conductivity to this mean distance is examined for two levels of contrast in the two modal conductances and different FWHM sizes. Results demonstrate that the FWHM is a robust measure of mean travel distance in the background medium. The resulting effective conductivities are accurate when compared to numerical results and results obtained from effective media theory, distance-based upscaling and numerical simulation.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2017

A thermal conductivity model for U-­Si compounds

U 3 Si 2 is a candidate for accident tolerant nuclear fuel being developed as an alternative to UO 2 in commercial light water reactors (LWRs). One of its main benefits compared to UO 2 is higher thermal conductivity that increases with temperature. This increase is contrary to UO 2 , for which the thermal conductivity decreases with temperature. The reason for the difference is the electronic origin of thermal conductivity in U 3 Si 2 , as compared to the phonon mechanism responsible for thermal transport in UO 2 . The phonon thermal conductivity in UO 2 is unusually low for a fluorite oxide due to the strong interaction with the spins in the paramagnetic phase. The thermal conductivity of U 3 Si 2 as well as other U-­Si compounds has been measured experimentally [1-­4]. However, for fuel performance simulations it is also critical to model the degradation of the thermal conductivity due to damage and microstructure evolution caused by the reactor environment (irradiation and high temperature). For UO 2 this reduction is substantial and it has been the topic of extensive NEAMS research resulting in several publications [5, 6]. There are no data or models for the evolution of the U 3 Si 2 thermal conductivity under irradiation. We know that the intrinsic thermal conductivities of UO 2 (semi-conductor) and U 3 Si 2 (metal) are very different, and we do not necessarily expect the dependence on damage to be the same either, which could present another advantage for the silicide fuel. In this report we summarize the first step in developing a model for the thermal conductivity of U-­Si compounds with the goal of capturing the effect of damage in U 3 Si 2 . Next year, we will focus on lattice damage. We will also attempt to assess the impact of fission gas bubbles.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Reconfigurable Electrical Networks within a Conductive Hydrogel Composite

Soft materials that exhibit compliance, programmability, and reconfigurability can have a transformative impact as electronic skin for applications in wearable electronics/soft robotics. There has been significant progress in soft conductive materials; however, achieving electrically controlled and reversible changes in conductivity and circuit connectivity remains challenging. To overcome this limitation, a soft material architecture with reconfigurable conductive networks of silver flakes embedded within a hydrogel matrix is presented. The conductive networks can be reversibly created/disconnected through various stimuli, including current, humidity, or temperature. Such stimuli affect electrical connectivity of the hydrogel by controlling its water content, which can be modulated by evaporation under ambient conditions (passive dehydration), evaporation through electrical Joule heating (active dehydration), or absorption of additional water (rehydration). The resulting change in electrical conductivity is reversible and repeatable, endowing the composite with on-demand reconfigurable conductivity. To highlight this material's unique properties, it is shown that conductive traces can be reconfigured after severe damage and revert to lower conductivity after rehydration. Additionally, a quadruped robot is demonstrated that can respond to stimuli by changing direction following exposure to excess water, thereby achieving reprogrammable locomotion behaviors.

36 MATERIALS SCIENCE↗
DOE OSTI2025

A simple centrifuge cell method for ex situ quantification of electrical conductivity of slurry electrode materials

We present the design, experimental procedure, and experimental evaluation of a system for fast, simple, and ex situ characterization of electrical conductivity of slurry electrode materials. The system uses a custom-designed electrochemical cell compatible with centrifugation in a swing-bucket centrifuge. The cell features cylindrical graphite electrodes that are partially sheathed so as to expose only 2 mm of the electrode surface to the bottom region of the packed particulate pellet. Also presented is a conduction model that provides a shape factor for estimating effective conductivity. We tested aqueous solutions of carbon black (CB), activated carbon (AC), and mixtures thereof. These particles were dispersed in 0.0 and 0.5 M NaCl solutions. Measurements show that the effective conductivity initially increases linearly with pellet mass and then saturates at higher masses. Notably, CB exhibited a fivefold increase in conductivity than AC at equal pellet masses. CB/AC mixtures at a fixed pellet mass were tested with CB mass fractions of 0 to 100%. Interestingly, the mixture conductivity was found to be a non-monotonic function of CB mass fraction, with a maximum conductivity at about 60 % CB mass fraction. At this maximum, the mixture conductivity is approximately 30 % higher than pure CB. NaCl concentration in the slurry solution had no effects on conductivity. These results highlight the interactions between slurry electrode composition and compaction, offering insights for optimizing slurry electrodes. Furthermore, the system and results may also be applicable to evaluation of particulate materials (including slurries) used for Li-ion batteries, capacitive deionization, fuel cells, and flow electrodes.

Capacitive deionization↗
DOE OSTI2024

Conducting polymer-based electrochemical sensors: Progress, challenges, and future perspectives

Conducting polymers are promising due to their unique properties, such as excellent electrical and optical properties, physical and chemical stability, high conductivity, and effective redox properties with high-temperature stability and biocompatibility. Due to these properties, conducting polymers are useful in diverse applications like sensors, batteries, oil industries, biosensors, biomedicines, catalysis, cancer treatment, etc. This review article aims to discuss the recent trends and analysis of conducting polymer-based electrochemical sensors in diverse areas with all required sensor characteristics, such as the derived limit of detection, utilized techniques for the sensing analysis and derived linear dynamic range with the stability of the sensors. Conducting polymers and their nanocomposites-based electrochemical sensors have demonstrated exceptional capabilities towards detecting various biomolecules, heavy metals, pesticides, and viruses like SARS-COV-2. Incorporation of redox mediators, use of conducting hydrogels, and molecular imprinting are promising strategies for better performance of the derived sensor. The article has demonstrated the existing challenges and limitations and provided solutions in the field. In the future, conducting polymers-based electrochemical sensors can be utilized in wearable sensors and integrated with IoT devices for better reach in real-time applications. They can also be made more accessible with precise control and data output by following specific methodologies. Utilizing green and sustainable conducting polymers can be crucial in advancing eco-friendly practices in the future. Conducting polymer-based electrochemical sensors has affectivity in neurochemical and pathogen sensing, which is essential for brain function and mental health.

42 ENGINEERING↗
DOE OSTI2019

Experimental and Theoretical Investigation of the Ion Conduction Mechanism of Tris(adiponitrile)perchloratosodium, a Self-Binding, Melt-Castable Crystalline Sodium Electrolyte

Sodium perchlorate (NaClO4) crystallizes with adiponitrile (ADN) as a 1:3 solvate to produce (ADN) 3 NaClO 4 , a solid electrolyte for sodium ion conduction. The solid possesses high thermal stability (up to 150 °C) and the ability to be melt-cast (T m = 81 °C). The pressed solid has a high ionic conductivity of 2.2 × 10 –4 S cm –1 at room temperature with a low activation barrier for ion conduction of 22 kJ mol –1 . The high conductivity is the result of low-affinity ion-conduction channels in the bulk based on the X-ray crystal structure, and by low grain-boundary resistance and possibly a grain-boundary percolating network due to a fluidlike nanoliquid layer between the grains, observable by scanning electron microscopy and differential scanning calorimetry. When the liquid nanolayer is rinsed away or removed by excessive drying, the bulk room temperature ionic conductivity is 4 × 10 –5 S cm –1 , activation energy for ionic conduction for an organic solid is 37 kJ mol –1 , and the sodium ion transference number is 0.71. Scanning electron microscopy and classical molecular dynamics simulations suggest that these cocrystals form a fluid layer of ADN at the surface, which facilitates the Na+ ion migration between the grains. Finally, density functional theory calculations are consistent with the possibility of ion conduction via a solvent–anion coordinated transition state through vacancy defects in the three symmetry-equivalent ion channels along separate directions, suggesting the possibility of ionic conductivity in three dimensions.

25 ENERGY STORAGE↗
DOE OSTI2016

Differentiation of surface and bulk conductivities in topological insulator via four-probe spectroscopy

Topological insulators, with characteristic topological surface states, have emerged as a new state of matter with rich potentials for both fundamental physics and device applications. However, the experimental detection of the surface transport has been hampered by the unavoidable extrinsic conductivity associated with the bulk crystals. Here we show that a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system can be used to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators. We derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allows quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi 2 Se 3 , Bi 2 Te 2 Se, and Sb-doped Bi 2 Se 3 with that of a pure 2D conductance of graphene on SiC substrate. We also report the 2D conductance enhancement due to the surface doping effect in topological insulators. This technique can be applied to reveal 2D to 3D crossover of conductance in other complex systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Anion chemical composition of poly(ethylene oxide)-based sulfonylimide and sulfonate lithium ionomers controls ion aggregation and conduction

Maximizing ion conduction in single-ion-conducting ionomers is essential for their application in energy-related technologies such as Li-ion batteries. Understanding the anion chemical composition impacts on ion conduction offers new perspectives to maximize ion transport, since the current approach of lowering T g has apparently reached a limit (lowest T g ~ 190 K, highest conductivity ~10 -5 –10 -4 S cm -1 ). Here, a series of random ionomers are synthesized by copolymerizing poly(ethylene glycol)methacrylate with either sulfonylimide lithium methacrylate (MTLi) or sulfonate lithium methacrylate (MSLi) using reversible addition–fragmentation chain transfer (RAFT) polymerization. Li-Ion conduction and self-diffusion coefficients (DLi+) of the ionomers are characterized with dielectric relaxation spectroscopy (DRS) and pulsed-field-gradient (PFG) NMR diffusometry, respectively. Increasing ion content decreases the Li-ion conductivity and D Li+ , as expected from the increased T g . Moreover, a considerably lower ionic conductivity and D Li+ are observed for MSLi compared to MTLi at constant ion content and T g /T. As revealed from X-ray scattering, strong ion aggregation in MSLi results in much lower conductivity and DLi+ compared with less aggregated MTLi based on the more delocalized sulfonylimide anion. These results emphasize the detrimental and molecularly specific role of ion aggregation in Li-ion conductivity, and highlight the necessity for minimizing ion aggregation via the rational choice of anion chemical composition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Ultrahigh thermal conductivity of isotopically enriched silicon

Most of the stable elements possess two and more stable isotopes. The physical properties of materials composed of such elements depend on the isotopic abundance to some extent. A remarkably robust isotope effect is observed in the phonon thermal conductivity, the principal mechanism of heat conduction in nonmetallic crystals. An isotopic disorder due to random distribution of the isotopes in the crystal lattice sites results in a rather strong phonon scattering and, consequently, in a reduction of thermal conductivity. In this paper, we present new results of accurate and precise measurements of thermal conductivity κ( T ) for silicon single crystals having three different isotopic compositions at temperatures T from 2.4 to 420 K. The highly enriched crystal containing 99.995% of 28 Si, which is one of the most perfect crystals ever synthesized, demonstrates a thermal conductivity of about 450 ± 10 W cm -1 K -1 at 24 K, the highest measured value among bulk dielectrics, which is ten times greater than the one for its counterpart nat Si with the natural isotopic constitution. For highly enriched crystal 28 Si and crystal nat Si, the measurements were performed for two orientations [001] and [011], a magnitude of the phonon focusing effect on thermal conductivity was determined accurately at low temperatures. The anisotropy of thermal conductivity disappears above 31 K. The influence of the boundary scattering on thermal conductivity persists sizable up to much higher temperatures (~80 K). The κ( T ) measured in this work gives the most accurate approximation of the intrinsic thermal conductivity of single crystal silicon which is determined solely by the anharmonic phonon processes and diffusive boundary scattering over a wide temperature range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Depth-dependent recovery of thermal conductivity after recrystallization of amorphous silicon

The depth-dependent recovery of silicon thermal conductivity was achieved after the recrystallization of silicon that had been partially amorphized due to ion implantation. Transmission electron microscopy revealed nanoscale amorphous pockets throughout a structurally distorted band of crystalline material. The minimum thermal conductivity of as-implanted composite material was 2.46 W m −1 K −1 and was found to be uniform through the partially amorphized region. X-ray diffraction measurements reveal 60% strain recovery of the crystalline regions after annealing at 450 °C for 30 min and almost full strain recovery and complete recrystallization after annealing at 700 °C for 30 min. In addition to strain recovery, the amorphous band thickness reduced from 240 to 180 nm after the 450 °C step with nanoscale recrystallization within the amorphous band. A novel depth-dependent thermal conductivity measurement technique correlated thermal conductivity with the structural changes, where, upon annealing, the low thermal conductivity region decreases with the distorted layer thickness reduction and the transformed material shows bulk-like thermal conductivity. Full recovery of bulk-like thermal conductivity in silicon was achieved after annealing at 700 °C for 30 min. After the 700 °C anneal, extended defects remain at the implant projected range, but not elsewhere in the layer. Previous results showed that high point-defect density led to reduced thermal conductivity, but here, we show that point defects can either reform into the lattice or evolve into extended defects, such as dislocation loops, and these very localized, low-density defects do not have a significant deleterious impact on thermal conductivity in silicon.

Huynh, Kenny↗