Boundary Conductivity Research Articles

Thermal boundary conductance between high thermal conductivity boron arsenide and silicon

Thermal boundary conductance (TBC) is important for heat dissipation in light-emitting diodes (LEDs). In this study, we predicted the TBC between the high thermal conductivity boron arsenide (BAs) and silicon (Si) by nonequilibrium molecular dynamics (MD) simulations. From the thermal conductivity accumulation function with respect to phonon frequency, the dominant phonon frequencies for heat conduction in BAs are extremely different from those in Si. However, our nonequilibrium MD simulations indicated that the TBC of the BAs/Si interface was still high compared to most other interfaces, even though there was a major frequency mismatch in the thermal conductivity accumulation function between BAs and Si. The primary reason for the high TBC is the overlap of phonon density of states between BAs and Si in the frequency range of 5–8 THz. The range of predicted TBC of the BAs/Si interface was between 200 and 300 MW/m2 K in the temperature range of 300–700 K, and the values of the TBC were not sensitive to the temperature. We also found that the TBCs in Si/BAs and Si/Ge interfaces were close to each other considering the simulation uncertainty. This work indicates BAs as an excellent material for heat dissipation across the interfaces.

Essential structural and experimental descriptors for bulk and grain boundary conductivities of Li solid electrolytes

ABSTRACT We present a computational approach for identifying the important descriptors of the ionic conductivities of lithium solid electrolytes. Our approach discriminates the factors of both bulk and grain boundary conductivities, which have been rarely reported. The effects of the interrelated structural (e.g. grain size, phase), material (e.g. Li ratio), chemical (e.g. electronegativity, polarizability) and experimental (e.g. sintering temperature, synthesis method) properties on the bulk and grain boundary conductivities are investigated via machine learning. The data are trained using the bulk and grain boundary conductivities of Li solid conductors at room temperature. The important descriptors are elucidated by their feature importance and predictive performances, as determined by a nonlinear XGBoost algorithm: (i) the experimental descriptors of sintering conditions are significant for both bulk and grain boundary, (ii) the material descriptors of Li site occupancy and Li ratio are the prior descriptors for bulk, (iii) the density and unit cell volume are the prior structural descriptors while the polarizability and electronegativity are the prior chemical descriptors for grain boundary, (iv) the grain size provides physical insights such as the thermodynamic condition and should be considered for determining grain boundary conductance in solid polycrystalline ionic conductors.

A case study of ceramic processing: Microstructural development and electrical properties of Ce0.8Gd0.2O1.9

The effects of powder characteristics and conformation methods on green microstructure have been investigated, and microstructural evolution was analyzed based on the green density and the different sintering techniques. Colloidal processing of nanopowders with narrow particle size distribution (15 ± 5 nm) can be more problematic than dry processing and can lead to more heterogeneous microstructures. Two-step sintering can be used to obtain dense nanostructured samples with more flexibility than microwave sintering and still at lower temperatures than conventional sintering. The sinterability of Ce0.8Gd0.2O1.9 is significantly improved by Zn addition, which can reduce sintering temperature to values as low as 800, 1150 and 1200 °C for two-step, microwave and conventional sintering, respectively, in order to achieve relative densities above 90%. Microscopic grain boundary conductivity is shown to be improved in nanostructured samples, despite the higher grain boundary density.

Interfacial Thermal Conductance across Room-Temperature-Bonded GaN/Diamond Interfaces for GaN-on-Diamond Devices.

The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications, such as wireless communication and radar systems. However, the performance and reliability of GaN-based high-electron-mobility transistors (HEMTs) are limited by the high channel temperature induced by Joule heating in the device channel. Integration of GaN with high thermal conductivity substrates can improve the heat extraction from GaN-based HEMTs and lower the operating temperature of the device. However, heterogeneous integration of GaN with diamond substrates presents technical challenges to maximize the heat dissipation potential brought by the ultrahigh thermal conductivity of diamond substrates. In this work, two modified room-temperature surface-activated bonding (SAB) techniques are used to bond GaN and single-crystal diamond. Time-domain thermoreflectance (TDTR) is used to measure the thermal properties from room temperature to 480 K. A relatively large thermal boundary conductance (TBC) of the GaN/diamond interfaces with a ∼4 nm interlayer (∼90 MW/(m2 K)) was observed and material characterization was performed to link the interfacial structure with the TBC. Device modeling shows that the measured TBC of the bonded GaN/diamond interfaces can enable high-power GaN devices by taking full advantage of the ultrahigh thermal conductivity of single-crystal diamond. For the modeled devices, the power density of GaN-on-diamond can reach values ∼2.5 times higher than that of GaN-on-SiC and ∼5.4 times higher than that of GaN-on-Si with a maximum device temperature of 250 °C. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling, especially for GaN-on-diamond devices.

A Hybrid Boundary Conduction Modulation for a Single-Phase H-bridge Inverter to Alleviate Zero-Crossing Distortion and Enable Reactive Power Capability

Boundary conduction mode (BCM) featuring zero-voltage switching has caught researchers’ eyes recently. In single-phase full-bridge inverter with one leg operating in high switching frequency and one leg in line frequency, it is easy to achieve high conversion efficiency for low-power applications. However, severe distortion will be introduced during zero-voltage crossing area due to too low switching frequency around this area, and reactive power generation is not allowed under this modulation scheme due to zero-voltage crossing issue. This article proposes a hybrid BCM strategy for a single-phase full-bridge inverter to both alleviate voltage zero-crossing distortion and enable reactive power generation by reshaping the triangular waveform of inductor current into quadrangle waveform through rearranging the driving signals during voltage zero-crossing area. This alleviates zero-crossing distortion by avoiding too low switching frequency and enables reactive power generation by employing the hybrid BCM around voltage zero-crossing area. High efficiency can be maintained by combining the proposed hybrid BCM employed only for a small portion of zero-crossing area and the conventional BCM for the rest. The principle of operation, theoretical analysis, and simulation results are presented in this article. A 300-W microinverter prototype was built to verify the feasibility and effectiveness of the proposed hybrid BCM scheme.

A single-stage soft-switching LED driver based on CLCL resonant converter and BCM circuit for lighting application

This paper proposes a single-stage LED driver based on the CLCL resonant circuit and boundary conduction mode (BCM) boost circuit. The input voltage of an LED driver is categorized into two capacitors, which will reduce the input peak voltage by making a single-stage LED driver to work under maximum input voltage modes. The proposed CLCL resonant circuit contains a soft-switching feature where the major and minor area works under zero voltage and zero current phases, respectively. Moreover, this approach has reduced very small turn-off loss, turn-off current of a switch, which in turn proves the system efficiency. In this paper, the proposed BCM circuit, as well as CLCL resonant converter circuit, has been explained with five operational phases, and experimentation and analysis were made for proposed and existing methodologies. For validation of the proposed system, a 100 W investigational prototype is used, and experimental results were analyzed with the theoretical result.

Quantification of grain boundary defect chemistry in a mixed proton‐electron conducting oxide composite

AbstractDual phase oxide membranes have shown promising hydrogen permeation fluxes in syngas applications due to their high mixed proton electron conduction (MPEC). However, the conductivity of grain boundaries can be many orders of magnitude lower than that of the bulk and so limits the total conductivity and hydrogen permeation. In this study, the three‐dimensional nanoscale oxygen and cation distributions around grain and phase boundaries in a BaCe0.8Y0.2O3‐δ‐Ce0.8Y0.2O2‐δ (BCY‐YDC) membrane were quantified by atom probe tomography (APT) and related to average grain boundary conductivity measured by electrochemical impedance spectroscopy (EIS). Segregation varied among the general high‐angle grain boundaries analyzed, but no trend from orientation analysis was determined. Correlative APT and electron energy loss spectroscopy (EELS) of one YDC grain boundary revealed composition and cerium valence information, respectively, allowing for the determination of vacancies at the grain boundary. While a specific MPEC membrane is characterized, the results are relevant to proton and electron conduction in a number of technologically important ceramics.

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho2Zr2O7) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho2Zr2O7 pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10−3 Scm−1) and grain boundary (2.50⨯10−3 Scm−1) conductivities of Ho2Zr2O7 prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.

Rapid sintering method for highly conductive Li7La3Zr2O12 ceramic electrolyte

Solid electrolytes could address the increasingly urgent safety and energy density concerns of lithium-ion batteries. Among several kinds of solid electrolytes, Ta-doped Li7La3Zr2O12 (Ta-LLZO) became a research hotspot because of its high Li-ion conductivity and chemical stability against Li-metal and air. However, the preparation of high quality LLZO ceramic electrolyte via conventional air ambient sintering method is still a big challenge due to the serious “Li-loss” and abnormal grain growth phenomenon during the long-time high-temperature sintering process. Herein, a new rapid ultra-high-temperature air ambient sintering method without mother powder (MP) is put forward for the preparation of high quality Ta-LLZO ceramic electrolyte. The rapid sintering strategy can effectively restrain “Li-loss”. Furthermore, it is demonstrated that the none-mother-powder method is superior to traditional mother-powder method. Ta-LLZO ceramics sintered without MP via this rapid sintering method own small grain size, tight grain boundary, dense microstructure and high conductivity. Specifically, the Ta-LLZO ceramic sintered at 1360 °C for 10 min without MP exhibits high conductivity (8.5 × 10−4 S cm−1 at 25 °C) and high relative density (97%), which equate those made by hot pressing sintering method. Without MP, the sintering process can avoid a lot of material waste and simplify the operation process. Moreover, the rapid sintering process can sharply shorten sintering time and reduce energy consumption. Therefore, this low-cost high-efficient sintering strategy can be effectively used in the large-scale production of high-quality Ta-LLZO ceramic electrolyte.

Effect of interface density, quality and period on the lattice thermal conductivity of nanocomposite materials

We examine the effects of interface density, quality, and period size on the lattice thermal conductivity of nanocomposite materials within the framework of a recently developed extended modified effective medium approach. A density functional theory and Boltzmann equation based semi-ab initio approach is used to calculate the constituent thermal conductivities, and the effective thermal boundary conductance is computed by modeling interface roughness based on a realistic combination of acoustic mismatch and diffuse mismatch contributions, for systems with anisotropic (directionally dependent) and isotropic thermal conductivities. Results obtained for Si/Ge and MoS2/WS2 systems indicate that the effective cross-planar thermal conductivity of planar superlattice systems is closely related to the thermal boundary resistance of the system for small superlattice periods, whereas in nanodot superlattices, the effective thermal conductivity for small particles is primarily regulated through the effective scattering lengths used in the calculation of the insert and matrix conductivities.

Performance of GDC with alkali metal carbonates as sintering aids

Gd-doped ceria electrolytes (GDC, 10 at.% Gd) were prepared using a eutectic mixture of Li and Na carbonates as sintering aid (NLC - 0, 1, 3 and 5 mol%), using two processing routes (ceramic and chemical). Structural and microstructural characterization provided no evidence for significant solubility of alkali metal cations in GDC. Materials sintered at 1100 °C/4 h were similar in densification (>95%) and microstructure to those obtained from a commercial powder sintered at 1500 °C/4 h. Impedance spectroscopy in air (200–600 °C) and as a function of oxygen partial pressure (from 0.21 to about 10−25 atm, 600–750 °C) showed that the low temperature conductivity of materials prepared using chemical synthesis surpassed the conductivity of a commercial GDC due to higher grain boundary conductivity. The estimated n-type conductivity of materials with NLC was also slightly higher than for pure GDC. The role of sintering aid and processing routes is discussed taking into consideration decomposition and volatilization of alkali salts.

Investigation of Thermal Properties of β-Ga2O3 Nanomembranes on Diamond Heterostructure Using Raman Thermometry

The β-Ga2O3 nanomembrane (NM)/diamond heterostructure is one of the promising ultra-wide bandgap heterostructures that offers numerous complementary advantages from both materials. In this work, we have investigated the thermal properties of the β-Ga2O3 NM/diamond heterostructure with three different thicknesses of β-Ga2O3 nanomembranes (NMs), namely 100 nm, 1000 nm, and 4000 nm thick β-Ga2O3 NMs using Raman thermometry. The thermal property—temperature relationships of these β-Ga2O3 NM/diamond heterostructures, such as thermal conductivity and interfacial thermal boundary conductance were determined under different temperature conditions (from 100 K to 500 K with a 40 K interval). The result provides benchmark knowledge about the thermal conductivity of β-Ga2O3 NMs over a wide temperature range for the design of novel β-Ga2O3-based power electronics and optoelectronics.

Synthesis and characterization of nano scale YTiNbO6 – Energy material

Nano particles of YTiNbO6 are synthesized through a modified combustion technique. The compound have been studied for their properties and characterized structurally, optically and electrically. Phase purity, crystal structure and particle size of the nano powder are examined using X-Ray diffraction, FT Raman and transmission electron microscopic techniques. The X-Ray diffraction study reveals that the nanoparticles of YTiNbO6 ceramics crystallize in orthorhombic euxenite structure. The Structure is also confirmed by using FT Raman spectroscopic technique. The samples show a relatively narrow size and in nano size is observed in TEM which is good agreement with the result obtained from the XRD analysis. The absorption coefficient near the fundamental edge depends on the photon energy and it obeys the Tauc’s equation. The optical band gap is obtained as 2.43 eV. The sintering temperature of the nanocrystalline powder is found to be reduced by about 150 °C when compared to the other method of preparation. This is due to the high surface to volume ratio of the nano particles. It is also evident from the SEM that the sample has achieved high densification with little porosity. The impedance plot can be deconvolute into three semicircles indicating the contribution of grain and grain boundary conduction. The depressed semicircle suggests that the departure from the ideal Debye behavior. Highest ionic conductivity is observed for the nano scale YTiNbO6. These materials can be made useful in solid oxide fuel cells and in opto electronic devices.

Inherently Non-Pulsating Input Current DC-DC Converter for Battery Storage Systems

With an ever-growing number of batteries being integrated into the electric grid, bidirectional converters with non-pulsating dc input current are required to replace the existing bidirectional converters so as to extend the lifetime of such dc power suppliers. Bidirectional DC-DC converters are increasingly used in a variety of applications including uninterruptable power supplies, electric vehicles and renewable energy systems. In this study, we propose a new topology of bidirectional dc-dc converter with inherently non-pulsating input current (NPIC), which is intended to be used for battery energy storage systems. With simple modifications from the conventional converters, the proposed NPIC converter has no inherent pulsating input current in the step-down (buck) mode under both continuous and boundary conduction modes (CCM and BCM), whereas in the step-up (boost) mode, the proposed NPIC converter retains the desired voltage gain. Underpinning theories, operating principles and steady-state performances are analyzed and presented in detail, which are then corroborated by simulation and experimentation. The proposed converter topology, with simple design principle and ease for implementation, is likely to have wide-ranging applicability in interfacing electrochemically functioned dc sources to modern power systems.

Realising ZVS and ZCS in a continuous conduction boost converter with boundary mode control

Boost regulators working in the continuous conduction mode (CCM) are very popular in switch-mode power conversion. This configuration is widely adapted for power factor correction (PFC) in switch mode AC to DC converters. For lower power levels of up to 200 watts, boundary conduction mode (BCM) PFC is adapted, and for power requirements beyond about 200 W, CCM boost with average current control scheme is favoured. A novel technique is proposed in this paper to achieve complete soft-switching viz. Zero voltage switching and zero current switching for the main boost switch in CCM mode. The uniqueness of the scheme lies in not adding any auxiliary MOSFET switch and/or control algorithm. All the switching elements exhibit soft-switching, thus improving efficiency and reducing the EMI considerably. Most of the reported techniques for soft switching in CCM boost utilise additional active switches. A working model is fabricated to demonstrate and validate the scheme. The test results and the waveforms are presented and analysed. An 800 watts PFC converter exhibiting complete soft switching is built using the proposed technique and is evaluated for its performance.

Effects of MgO additions on the electrical conduction behavior of a CeO2-based electrolyte prepared by SPS process

Ce0.8Sm0.2O1.9 (SDC)-xMgO (x = 0, 5, 10, 15, and 20 mol%) composite powders were synthesized by a sol-gel method and then densified by spark plasma sintering (SPS) to investigate the effects of MgO additions on the electrical conduction behavior of CeO2-based ceramics. The microstructure and electrical performances of the SDC-MgO composite ceramics were characterized and measured by XRD, SEM, TEM, EDS and EIS. Results show that the second phase of MgO segregates at the grain boundaries of SDC when its content is higher than 5 mol% in the ceramics. The SPS process accelerates the formation of MgO lamellar grains with incomplete crystallization. As the MgO content rises from 0 to 20 mol%, the grain boundary and total electrical conductivities first increase and then decrease, and the SDC-5%MgO sample exhibits the highest total conductivity, i.e., 2.76 × 10−2 S/cm at 650 °C, which is 9.65% higher than that of pure SDC.

Coupled Simulation of Current Flow and Residual Thermal Stress in ZnO Varistors

An electromechanically coupled modeling framework for the simulation of electric current flow in ZnO varistors is developed. The model is based on an equivalent circuit representation of the varistor microstructure with grain boundaries represented by nonlinear resistor branches of the circuit. This approach extends on previous circuit models by including the effect of mechanical stress on grain boundary conductivity. In this article, we consider, in particular, the effect of residual thermal stress in polycrystalline structures on electrical conductivity. The distribution of this stress throughout the material is computed by finite element method (FEM) simulations. Then, each grain boundary conductivity is determined by applying a self-consistent model for the trapped interface charge induced by piezoelectric polarization. Finally, the electric current flow patterns and the bulk conductivity of the material are computed using the nonlinear circuit model. The simulated I–V characteristics reveal a significant sensitivity of the electrical conductivity to internal thermal stress within the material. Furthermore, for realistic ZnO microstructures, the simulations demonstrate the effect of current concentration along thin conducting paths depending on the mechanical stress condition of the material.

Electrical Properties of Li-based NASICON Structured Ceramic Electrolytes Substituted With Chromium

Electrical properties of Li - ion conducting Li1+xCrxSn2-x(PO4)3 ceramic electrolytes with 0 < x < 1 were studied using electrical impedance spectroscopy in the frequency range of 1 Hz to 10 MHz at room temperature. Impedance analysis showed an increase in bulk and grain boundary conductivity with the increment of x up to x = 0.7. The highest bulk and grain boundary conductivity were 6.52 ×10-6 S cm-1 and 1.62 ×10-6 S cm-1 in the system of Li1.7Cr0.7Sn1.3(PO4)3 at room temperature. The charge carrier concentration,   mobile ion concentration, ionic hopping rate and ionic mobility were calculated by fitting the AC conductivity spectra. The ionic hopping rate and ionic mobility of the compound increased with the substitution of chromium due to the extra interstitial Li+ ions in the system. Additionally, the highest conducting sample with x = 0.7 had a negligible electronic conductivity based on transference number measurements. These results imply that the Li1+xCrxSn2-x(PO4)3 electrolytes obtained in this work can be considered as future candidates for solid state electrolytes. Â

Enhancement of ionic conductivity in Li0.5La0.5TiO3 with Ag nanoparticles

Li0.5La0.5TiO3 (LLTO)/nano-Ag composite electrolytes were synthesized by a sol–gel process. Through the addition of nano-Ag powder with a relatively low melting point, the lattice parameter, grain size and morphology of LLTO-based electrolytes were tailored. The results revealed that Ag was oxidized into Ag+ during high-temperature sintering in air, and partial Ag+ entered into LLTO lattice and improved the bulk ionic conductivity. Meanwhile, owing to the liquid-phase-assisted grain growth induced by Ag, the grain size of LLTO was increased and the grain boundary conductivity was substantially improved. As a consequence, the bulk and grain boundary conductivities of LLTO were simultaneously enhanced through a facile nano-Ag combination, thereby leading to the highest total conductivity of 4.2 × 10−5 S/cm in the sample with 5 wt% Ag, which is remarkably superior to that (2.8 × 10−5 S/cm) of pure LLTO.

Structural and electrical properties of BaZr0.7Ce0.2Y0.1O3−δ proton conducting ceramic fabricated by spark plasma sintering

Spark plasma sintering (SPS) is an attractive method to obtain fully densified polycrystalline ceramics at rapid sintering rates. Cerium- and yttrium-doped barium zirconate, BaZr0.7Ce0.2Y0.1O3−δ, was prepared by a SPS route, resulting in high values of density of above 98%. Investigations of microstructural and crystallographic properties with determination of grain sizes were performed by means of Scanning Electron Microscopy, X-ray Diffraction and Static Light Scattering and discussed in correlation to bulk and grain boundary conductivities obtained by AC impedance spectroscopy. Samples prepared by SPS were generally found to consist of small grains that exhibit high bulk conductivities, whereas their grain boundary conductivities were comparably small, possibly also dominated by the presence of minor phases.