Browsing by Author "Zhang, H."
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Item Elastic Wavefield Separation in Anisotropic Media Based on Eigenform Analysis and Its Application in Reverse-Time Migration(Oxford University Press, 2019-02-13) Yang, Jidong; Zhang, H.; Zhao, Y.; Zhu, Hejun; 0000-0002-7452-075X (Zhu, H); Yang, Jidong; Zhu, HejunSeparating compressional and shear wavefields is an important step in elastic reverse-time migration, which can remove wave-mode crosstalk artefacts and improve imaging quality. In vertical (VTI) and titled (TTI) transversely isotropic media, the state-of-the-art techniques for wavefield separation are based on either non-stationary filter or low-rank approximation. They both require intensive Fourier transforms for models with strong heterogeneity. Based on the eigenform analysis, we develop an efficient pseudo-Helmholtz decomposition method for the VTI and TTI media, which produces vector P and S wavefields with the same amplitudes, phases and physical units as the input elastic wavefields. Starting from the elastic VTI wave equations, we first derive the analytical eigenvalues and eigenvectors, then use the Taylor expansion to approximate the square-root term in the eigenvalues, and finally obtain a zero-order and a first-order pseudo-Helmholtz decomposition operator. Because the zero-order operator is the true solution for the case of ϵ = δ, it produces accurate wavefield separation results for elliptical anisotropic media. The first-order separation operator is more accurate for non-elliptical anisotropy. Since the proposed pseudo-Helmholtz decomposition requires solving an anisotropic Poisson's equation, we propose two fast numerical solvers. One is based on the sparse lower-upper (LU) factorization, which can be repeatedly applied to the input elastic wavefields once computing the lower and upper triangular matrices. The second solver assumes the model parameters are laterally homogeneous within a given migration aperture. This assumption allows us to efficiently solve the anisotropic Poisson's equation in the z k x domain, where k x and z denote the horizontal wavenumber and depth, respectively. Using the coordinate transform, we extend the pseudo-Helmholtz decomposition to the TTI media. The separated vector wavefields are used to produce PP and PS images by applying a dot-product imaging condition. Several numerical examples demonstrate the feasibility and applicability of the proposed methods. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.Item Enhanced Thermal Conductivity in Cu/Diamond Composites by Tailoring the Thickness of Interfacial TiC Layer(Elsevier Ltd) Wang, Luhua; Li, J.; Catalano, Massimo; Bai, G.; Li, N.; Dai, J.; Wang, X.; Zhang, H.; Wang, Jinguo; Kim, Moon J.; Wang, Luhua; Catalano, Massimo; Wang, Jinguo; Kim, Moon J.Diamond particles reinforced Cu matrix (Cu/diamond) composites were fabricated by gas pressure infiltration using Ti-coated diamond particles with Ti coating from 65 nm to 850 nm. The scanning transmission electron microscopy (STEM) characterizes that the Ti coating transforms from elemental Ti to TiC after infiltration, and the crystallographic orientation relationship between diamond and TiC is [1 1 0]_{diamond}//[1 1 0]_{TiC} and (1 1 1)_{diamond}//(1 1 1)_{TiC}. The thermal conductivity of the Cu/Ti-diamond composites firstly increases and then decreases with increasing Ti coating thickness, giving a maximal value of 811 W m⁻¹ K⁻¹ at 220 nm Ti-coating layer. The results clearly manifest the effect of interfacial layer thickness on the thermal conductivity of Cu/diamond composites.Item Power Splitting-Based SWIPT with Dual-Hop DF Relaying in the Presence of a Direct Link(Institute of Electrical and Electronics Engineers Inc.) Ye, Y.; Li, Y.; Zhou, F.; Al-Dhahir, Naofal; Zhang, H.; Al-Dhahir, NaofalThis letter investigates the outage performance of dual-hop decode and forward relay systems in the presence of a direct link between the source and the destination, where simultaneous wireless information and power transfer is exploited at the relay by using a static/dynamic power splitting (PS) scheme. An analytical expression for the outage probability is derived to determine the optimal static PS ratio. Moreover, we study the dynamic PS scheme and derive a closed-form expression for the optimal dynamic PS ratio to minimize the outage probability at a given target rate. Furthermore, an analytical expression for the outage probability with the optimal dynamic PS ratio is also presented. It is shown that a full diversity gain can be achieved in the investigated system with static/dynamic PS scheme, and that the dynamic PS scheme outperforms the static PS scheme. ©2018 IEEEItem Tailoring Interface Structure and Enhancing Thermal Conductivity of Cu/Diamond Composites by Alloying Boron to the Cu Matrix(Elsevier Inc.) Bai, G.; Wang, Luhua; Zhang, Y.; Wang, X.; Wang, Jinguo; Kim, Moon J.; Zhang, H.; Wang, Luhua; Wang, Jinguo; Kim, Moon J.Diamond particles reinforced Cu matrix (Cu/diamond) composites were prepared by alloying 0.1–1.0 wt% B to the Cu matrix in order to tailor the interface structure. The interface structure evolves from discrete triangular carbides into continuous jig-saw carbides depending on the availability of boron source in the Cu-B matrix. We report the highest thermal conductivity of 868 W/mK so far in boron-modified Cu/diamond composites, which originates from the discontinuous carbide interface in the Cu-B/diamond composites. The parallel connection of interfacial thermal resistances of the discontinuous carbide interface reduces the total interfacial thermal resistance and therefore promotes phonon transfer across the Cu/diamond interface. We clarify the formation mechanism of discontinuous carbide interface in the Cu-B/diamond composites and demonstrate the decisive role of discrete triangular carbides in enhancing thermal conductivity of Cu/diamond composites. The results help to establish the method of metal matrix alloying to prepare Cu/diamond composites with high thermal conductivity for thermal management applications. © 2019 Elsevier Inc.Item Tunable Coefficient of Thermal Expansion of Cu-B/Diamond Composites Prepared by Gas Pressure Infiltration(Elsevier Ltd) Bai, G.; Zhang, Y.; Dai, J.; Wang, L.; Wang, X.; Wang, Jinguo; Kim, Moon J.; Chen, X.; Zhang, H.; Wang, Jinguo; Kim, Moon J.Cu-B matrix composites reinforced with diamond particles (Cu-B/diamond) were prepared by gas pressure infiltration (GPI). The effect of boron addition in the range of 0–1.0 wt% on the thermal expansion behavior of the Cu-B/diamond composites was evaluated. The coefficient of thermal expansion (CTE) of the Cu-B/diamond composites initially decreases and then increases with increasing boron content. The minimum CTE value of 4.88 × 10 -6 /K is obtained at 0.5 wt% B addition, which satisfies the requirement of CTE of semiconductors (range of 4–8 × 10⁻⁶ /K) for electronic packaging applications. The variation of CTE of the Cu-B/diamond composites is attributed to the formation of interfacial carbides and their morphological evolution. The interface structure evolves from discrete triangular carbides into continuous carbide layer with increasing boron content. The increase in the quantity of discrete carbides enhances the interface, but the formation of continuous carbides impairs the interfacial bonding of the Cu-B/diamond composites. The results suggest that alloying B to Cu matrix is an effective route to tune the coefficient of thermal expansion of Cu/diamond composites. © 2019 Elsevier B.V.