Results 1 - 10 of 14
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[en] We electroplated Co–P films with nanocrystalline, amorphous and nanocrystalline/amorphous mixed structures and used them as under-bump metallization (UBM) joined with Sn–Ag–Cu lead-free solder. We systematically investigated the interfacial reaction between the Sn–Ag–Cu solder and the Co–P UBM and analyzed the growth mechanisms of the intermetallic compounds formed at the interfaces of the Sn–Ag–Cu/Co–P joints through multiple reflows. Among the three kinds of Co–P films, the film with a mixed structure shows the best diffusion-barrier properties and is a good candidate for the UBM joined with the Sn–Ag–Cu solder. For the nanocrystalline Co–P UBM, Co diffuses quickly toward the solder and Sn does not diffuse into the UBM, whereas for the amorphous Co–P film not only does Co diffuse into the solder, but also Sn diffuses into the Co–P film with a large diffusion rate. In addition, the first-principles calculation shows that the exchange coupling between Co(3d74s2) and Sn(5s25p2) electrons and between Sn(5s25p2) and P(3s23p3) electrons results in the formation of CoSn and SnP3, which originate from the diffusion and reaction of Co and Sn atoms, respectively; this is consistent with the experimental data of transmission electron microscopy characterization
[en] The hybrid manufacturing technologies have provided the possibility of producing complex parts with good surface quality and complex structure that are difficult to manufacture using directed energy deposition (DED) or milling process. In order to keep the samples warm, avoid cooling shrinkage deformation and improve the metallurgical effect, thermal milling (TM) was carried immediately after DED. A serious of tensile specimens of 316L stainless steel were fabricated by hybrid DED and TM manufacturing, and the tensile properties were tested. The analysis of the macroscopic fracture and microstructure was combined to understand the fracture mechanism. There were many differences between the brittle fracture and ductile fracture including the fracture surface morphology and microstructures. Then, the tensile properties and hardness at different heights were compared. Finally, the effects of milling temperature on tool wear and residual stress were studied.
[en] A granular magnetic material, Co-Fe-Hf-O, has been developed-using dc pulsed magnetron reactive sputtering. The deposition rate is as high as 1.3 nm/s. The electrical and magnetic properties of Co-Fe-Hf-O film can be tuned by changing O2 during deposition. A highly resistive, magnetically soft film has been achieved in a small range of the O2/(Ar+O2) gas flow ratio. The origin of the dependence of magnetic and electrical properties of this material is studied and explained by monitoring the evolution of the film microstructure, using x-ray diffraction and transmission electron microscopy
[en] The temperature dependence of the Raman spectra of Bi2Te3 and Bi0.5Sb1.5Te3 thermoelectric films was investigated. The temperature coefficients of the Eg(2) peak positions were determined as -0.0137 cm-1/ C and -0.0156 cm-1/ C, respectively. The thermal expansion of the crystal caused a linear shift of the Raman peak induced by the temperature change. Based on the linear relation, a reliable and noninvasive micro-Raman scattering method was shown to measure the thermal conductivity of the thermoelectric films. (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
[en] Integration of magnetic passive components into package has been attracting more interests recently, but efficient package-compatible magnetic materials are needed. We have developed a package-compatible granular material, CoFeHfO, on a printed circuit board by reactive sputtering and investigated the substrate dependence of its soft magnetic property. Atomic force microscopy and grazing incidence x-ray-scattering-diffraction spectra show that a rough substrate surface degrades the magnetic property of CoFeHfO thin films with almost the same crystal microstructure. With surface planarization by chemical-mechanical polishing, soft magnetic material CoFeHfO can be realized on the package substrate. This material is promising for future applications in package
[en] Highlights: • Adsorption mechanisms of SeO_2 on CaO surface under O_2 were firstly studied by DFT. • The adsorption energies, bond length and electron density maps were calculated. • The electronic structure changes upon adsorption were studied. - Abstract: Selenium is a hazardous element in coal. During coal combustion, most of the selenium will convert to SeO_2 in the flue gas. Ca-based adsorbents, especially CaO, have been considered as a potential sorbent to adsorb SeO_2 due to its low cost. In this paper, the adsorption mechanisms of single SeO_2 on CaO surface were investigated by density functional theory (DFT) calculation. Both the physisorption and chemisorption structures were determined. It has been identified that the adsorption of SeO_2 on CaO surface is primarily chemisorption, while physisorption takes effects at the initial stage of the process. Under O_2 atmosphere, selenate is hard to form. Most of the adsorption products are selenite. Additionally, the electron density maps were obtained to reveal the surface active sites. The partial density of states (PDOS) was calculated for analyzing the electronic structural change of SeO_2 and CaO surface during adsorption. The results provide fundamental information of the adsorption process, which could be meaningful for the development of new absorbents.
[en] The ceramic coatings containing Ca and P were prepared on AZ91D Mg alloy by plasma electrolytic oxidation technique in NaOH system and Na2SiO3 system, respectively. The phase composition, morphology and the element distribution of the coatings was studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The corrosion resistance of the coatings was examined by polarizing curve methods in a 0.9% NaCl solution. In NaOH system, there were a large number of micro-holes distributing evenly on the surface of the coating, and the coating was mainly composed of Mg, Al, P and Ca. In Na2SiO3 system, the micro-holes in the coatings were reduced greatly in number and the distribution of the micro-holes was uneven, and the coating was mainly composed of Mg, Al, Si, P and Ca. The ratio of Ca/P in the coating can be controlled by the adjustment of the technique parameters to a certain extent. The adjustment of the concentration of Ca2+ in the electrolyte was an effective method to change the ratio of Ca/P in the coating in both systems; the reaction time and the working voltage for the adjustment of the ratio of Ca/P in the coating was more suitable for the NaSi2O3 system than the NaOH system. The polarizing curve tests showed the coatings improved the corrosion resistance of the AZ91D Mg alloy in 0.9% NaCl solution by nearly two orders of magnitude.
[en] The mechanical properties of solder joints highly depend on the interfacial reaction between the solders and the metallization on substrates. In this work, we electroplated Co–P films with various compositions on the Cu pads of printed circuit boards and fabricated Sn-3.8 wt% Ag-0.7 wt% Cu/Co–P ball grid array (BGA) solder joints. The BGA solder joints were annealed at 150 °C for 100, 200, 500, and 1000 h and the shear strength of these joints was measured. When the P content of the Co–P metallization was increased from 2.3 to 18.8 at.%, the shear strength after 1000 h annealing initially rose to 107.9 MPa at a P content of 8.5 at.%, then decreased to 84.3 MPa at a P content of 12.5 at.%, and again increased to 96.0 MPa at a P content of 18.8 at.%. The enhancement of the shear strength of the joints with Co-8.5 at.% P, Co-12.5 at.% P, and Co-18.8 at.% P films was 109.5%, 63.7%, and 86.4% in comparison to the joints without Co–P metallization, respectively. The interfacial reaction between the Sn–Ag–Cu (SAC) solder and Co–P films during annealing and the fractured surfaces of the solder joints after the shear test were studied. For the joints with Co-8.5 at.% P and Co-18.8 at.% P films, a thick layer of CoSn3 was formed at the interfaces during annealing, which enhanced the shear strength. For the joints with Co-12.5 at.% P metallization, a thin layer of Co–Sn–P was formed at the interfaces and was peeled off layer by layer with prolongation of the annealing time. The spalled Co–Sn–P was mixed with the solder matrix, increasing the shear strength of the solder joints. The shear strength of the SAC/Co-12.5 at.% P joints was less than that of the joints with Co-8.5 at.% P and Co-18.8 at.% P films because no CoSn3 formed. Therefore, the composition of Co–P metallization played an important role in the interfacial reaction of the SAC/Co–P solder joints, which in turn affected the shear strength of the solder joints. Our experimental results show that the electroplated Co–P film is a promising candidate as the metallization for BGA solder joints.
[en] Highlights: • A double-layer adsorption mechanisms based on DFT analysis are figured out for the adsorption of SeO2 on CaO surface. • The experimental results of FE-SEM, XPS, ICP-AES and XRD match up with the DFT analysis, which proves the double-layer adsorption mechanisms. • A complete adsorption process is established according to a double-layer adsorption mechanisms. - Abstract: SeO2 adsorption mechanisms on CaO surface were firstly investigated by both density functional theory (DFT) calculations and adsorption experiments. Adsorption of multiple SeO2 on the CaO (001) surface was investigated using slab model. Based on the results of adsorption energy and surface property, a double-layer adsorption mechanisms were proposed. In experiments, the SeO2 adsorption products were prepared in a U-shaped quartz reactor at 200 °C. The surface morphology was investigated by field emission scanning electron microscopy (FE-SEM). The superficial and total SeO2 mass fractions were measured by X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES), respectively. The surface valence state and bulk structure are determined by XPS and X-Ray Diffraction (XRD). The experimental results are in good agreement with the DFT results. In conclusion, the fundamental SeO2 chemisorption mechanisms on CaO surface were suggested.
[en] We studied the thermal conduction of thermal interface materials (TIM) using silver nanoparticles (AgNP) and achieved ultra-low thermal resistance. The experimental data show that silver nanoparticles are very good candidates for TIM in power electronics applications in terms of the reduction in thermal resistance. The ultra-low thermal resistance of the AgNP-based TIM originates from the thinness, high thermal conductivity of silver and low temperature sintering properties of AgNP.