50 research outputs found
High Thermal Conductivity in Wafer Scale Cubic Silicon Carbide Crystals
High thermal conductivity electronic materials are critical components for
high-performance electronic and photonic devices as either active functional
materials or thermal management materials. We report an isotropic high thermal
conductivity over 500 W m-1K-1 at room temperature in high-quality wafer-scale
cubic silicon carbide (3C-SiC) crystals, which is the second highest among
large crystals (only surpassed by diamond). Furthermore, the corresponding
3C-SiC thin films are found to have record-high in-plane and cross-plane
thermal conductivity, even higher than diamond thin films with equivalent
thicknesses. Our results resolve a long-lasting puzzle that the literature
values of thermal conductivity for 3C-SiC are perplexingly lower than the
structurally more complex 6H-SiC. Further analysis reveals that the observed
high thermal conductivity in this work arises from the high purity and high
crystal quality of 3C-SiC crystals which excludes the exceptionally strong
defect-phonon scatterings in 3C-SiC. Moreover, by integrating 3C-SiC with other
semiconductors by epitaxial growth, we show that the measured 3C-SiC-Si TBC is
among the highest for semiconductor interfaces. These findings not only provide
insights for fundamental phonon transport mechanisms, also suggest that 3C-SiC
may constitute an excellent wide-bandgap semiconductor for applications of
power electronics as either active components or substrates
Influence of gas environment and heating on atomic structures of platinum nanoparticle catalysts for proton-exchange membrane fuel cells
Atomic-scale relaxations of platinum nanoparticles (Pt NPs) for fuel-cell catalysts are evaluated by spherical-aberration corrected environmental transmission electron microscopy (ETEM) under reference high-vacuum and N2 atmospheres, and then under reactive H2, CO and O2 atmospheres, combined with ex situ durability test using an electrochemical half-cell. In high-vacuum, increasing roughness due to continuous relaxation of surface-adsorbed Pt atoms is quantified in real-space. Under H2 and N2 atmospheres at a critical partial pressure of 1 × 10-2 Pa the stability of the surface facets is for the first time found to be improved. The adsorption behaviour of CO molecules is investigated using experimentally measured Pt-Pt bond lengths on the topmost surface layer of Pt NPs. The deactivation of Pt NPs in the anode environment of a proton-exchange-membrane fuel-cell is demonstrated at the atomic-scale in the ETEM, and the transformation of NPs into disordered nanoclusters is systematically quantified using the partial size distribution of Pt atomic clusters under controlled heating experiments at 423, 573 and 723 K
Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations
In titanium alloys, the ω(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal ω transformation that can occur even above the ω transformation temperature. In body-centered cubic β-titanium alloyed with β-stabilizing elements, there are locally unstable regions having fewer β-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal ω transformation occurs even when the entire β region is stable on average so that athermal ω transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111]β longitudinal phonon, which cannot be suppressed by the stabilization of β phase on average. In the diffusionless isothermal and athermal ω transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111}β pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the ω-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent β/ω interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the β/ω interface energy is the rate-determining process in the diffusionless ω transformations.Tane M., Nishiyama H., Umeda A., et al. Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations. Physical Review Materials 3, 043604 (2019); https://doi.org/10.1103/PhysRevMaterials.3.043604
Impact of focused ion beam on structural and compositional analysis of interfaces fabricated by surface activated bonding
Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations
Characterization of Nanoscopic Cu/Diamond Interfaces Prepared by Surface-Activated Bonding: Implications for Thermal Management
Experimental inspection of a computationally-designed NiCrMnSi Heusler alloy with high Curie temperature
Nowadays advanced magnetic tunnel junction applications demand very high tunnel magnetoresistance at room temperature, thus it is quite important to explore high Curie temperature Tc half-metallic Heusler alloys. In this article rst-principles calculation unveiled that NiCrMnSi has Tc of 1200 K comparable to that of the traditional Co2MnSi Heusler alloys, even though it does not contain Co element. In addition, we examined whether NiCrMnSi Heulser phase lms can be obtained by a magnetron sputtering on MgO substrates. The results of the structural analysis and rst-principles calculations indicated that NiCrMnSi Heusler phase is metastable. A possible route to obtain metastable NiCrMnSi Heusler alloy is to utilize appropriate templates