Qu Xuanhui

Review of metal matrix composites with high thermal conductivity for thermal management applications

Abstract Metal matrix composites with high thermal conductivity and tailorable coefficient of thermal expansion are found widespread applications in electronic package and thermal management. The latest advances in manufacturing process, thermal properties and brazing technology of SiC/metal, carbon/metal and diamond/metal composites were presented. Key factors controlling the thermo-physical properties were discussed in detail. The problems involved in the fabrication and the brazing of these composites were elucidated and the main focus was put on the discussion of the methods to overcome these difficulties. This review shows that the combination of pressureless infiltration and powder injection molding offers the benefits to produce near-net shape composites. Improving wettability and optimizing interfacial structure are prerequisites for successful fabrication and further enhancement of thermal properties. A new Ag-Cu-Sn brazing alloy with low melting point is found to be effective to braze Al-matrix composites.

Enhanced hydrogen storage performance for MgH2–NaAlH4 system—the effects of stoichiometry and Nb2O5 nanoparticles on cycling behaviour

Nowadays, the technological utilization of reactive hydride composites (RHC) as promising hydrogen storage materials is hampered by their reaction kinetics. In the present work, effects of reactant stoichiometry on ensuing hydrogen sorption properties and pathway of the MgH2–NaAlH4 (mole ratios 1:2, 1:1 and 2:1) system, both undoped and doped with Nb2O5 nanoparticles, were investigated. It was found that the as-prepared reactant stoichiometry of MgH2/NaAlH4 system had a profound impact on its dehydrogenation kinetics and reaction mechanism. Variable temperature dehydrogenation data revealed that undoped binary composites possessed enhanced hydrogen desorption properties compared to that of pristine NaAlH4 and MgH2. The use of Nb2O5 displayed superior catalytic effects in terms of enhancing dehydriding/rehydriding kinetics and reducing the dehydrogenation temperature of MgH2–NaAlH4 system. Isothermal volumetric measurements at 300 °C revealed that enhancements arising upon adding Nb2O5 were almost double that of undoped MgH2–NaAlH4 composites. The apparent activation energies for NaAlH4, Na3AlH6, MgH2, and NaH relevant decompositions in doped composite were found to be much lower than that for the undoped one. Moreover, Nb2O5 doping also markedly enhanced the reversible capacity of MgH2–NaAlH4 composites under moderate conditions, persisting well during three de/rehydrogenation cycles. XRD, XPS, and FESEM-EDS analyses demonstrated that reduction of Nb2O5 during first desorption was coupled to the migration of reduced niobium oxide species from the bulk to the surface of the material. It was suggested that these finely dispersed oxygen-deficient niobium species might contribute to kinetic improvement by serving as the active sites to facilitate hydrogen diffusion through the diffusion barriers both during dehydrogenation and rehydrogenation.

Effect of porosity on wear resistance of SiCp/Cu composites prepared by pressureless infiltration

The influence of porosity on the wear behavior of high volume fraction (61%) SiC(subscript p)/Cu composite produced by pressureless infiltration was studied using a sliding, reciprocating and vibrating (SRV) machine. SiC(subscript p)/Cu composites slid against hardened GCr15 bearing steel ball in the load range of 40-200 N. The results show that the wear rate increases with increasing porosity. The composite containing low porosity shows excellent wear resistance, which is attributed to the presence of mechanically mixed layer on the worn surface. In this case, the dominant wear mechanism is oxidative wear. Comparatively, the composite containing high porosity exhibits inferior wear resistance. Fracture and spalling of the particles are considered as the main causes of severe wear. Third body abrasion is the controlling wear mechanism. In addition, porosity has more important influence on wear rate at high load than at low load. This is associated with the fact that the fracture and spalling of particles is a process of crack initiation and propagation. At lower load, the pores beneath the worn surface can not propagate significantly, while the pores become unstable and easily propagate under high load, which results in a higher wear rate.

Effects of Ti and Fe Additives on Hydrogen Release from Lithium Alanate

Abstract LiAlH 4 doped with Ti and Fe additives have been studied by PCT experiment and X-ray diffraction. Doping with Ti and Fe significantly decreased the amount, the temperature and the rate of hydrogen release. Doping with Fe induced the largest decrease in the temperature; however, the amount of hydrogen release was significantly higher than that of the samples doped with Ti. Compared with the sample doped with 5 mol% Ti, the sample doped with 5 mol% Fe presented a lower dehydrogenation rate in the first step. The result obtained by XRD patterns for LiAlH 4 doped with 5mol% Fe and LiAlH 4 doped with 1, 3 and 5 mol% Ti heated at 250 °C showed that doping additives did not cause any structure change. No Ti and Fe or the secondary Ti and Fe-containing phases were found.

Brazing diamond/Cu composite to alumina using reactive Ag-Cu-Ti alloy

Abstract The novel properties of diamond/Cu composites such as low thermal expansion coefficient and high thermal conductivity have rendered the composites a valuable packaging material. The reactive brazing of diamond/Cu composites and alumina was performed using the 97%(72Ag-28Cu)-3%Ti alloy. The reactive brazing alloy displays good wettability with alumina and diamond film, and the equilibrium contact angle on both the substrates is found to be less than 5°. The influence of main bonding conditions such as peak heating temperature and holding time was investigated in detail. It is found that Ti element concentrates at the surface of diamond particle resulting in the formation of TiC compound. The morphology of TiC compound exhibits a close relationship with the shear strength of brazing joint. It is surmised that an optimal thickness of TiC layer on the diamond particle surface can ameliorate the shear strength of brazing joint. However, on the contrary, the particle-like shaped TiC compound or a thicker TiC compound layer can impair the shear strength. The maximum shear strength is found to be 117 MPa.

Micro powder injection molding—large scale production technology for micro-sized components

Micro powder injection molding (μPIM), a miniaturized variant of powder injection molding, has advantages of shape complexity, applicability to many materials and good mechanical properties. Co-injection molding has been realized between metals and ceramics on micro components, which become the first breakthrough within the PIM field. Combined with the prominent characteristics of high features/cost ratio, micro powder injection molding becomes a potential technique for large scale production of intricate and three-dimensional micro components or microstructured components in microsystems technology (MST) field.

The development of metal hydrides using as concentrating solar thermal storage materials

Metal hydrides high temperature thermal heat storage technique has great promising future prospects in solar power generation, industrial waste heat utilization and peak load regulating of power system. This article introduces basic principle of metal hydrides for thermal storage, and summarizes developments in advanced metal hydrides high-temperature thermal storage materials, numerical simulation and thermodynamic calculation in thermal storage systems, and metal hydrides thermal storage prototypes. Finally, the future metal hydrides high temperature thermal heat storage technique is been looked ahead.

Effect of Additives on the Reversibility of Lithium Alanate (LiAlH4)

Abstract The effect of various catalysts, such as Ti, Ni, Fe, Ce(SO 4 ) 2 and LaCl 3 , on the reversibility of LiAlH 4 was studied by Pressure-Content-Temperature (PCT) experiments. The result indicates that doping induces a marked decrease in the rate of hydrogen release. In addition, doping additives obviously decreases the initial temperature of LiAlH 4 of hydrogen release except for doping with LaCl 3 . However, at the same time, the amount of hydrogen release also decreases. In the study on the hydrogen absorption of LiAlH 4 doped with 1mol% Ni, 1mol% Ti, 1 mol% Ce(SO 4 ) 2 and 1mol%LaCl 3 at 180 °C under about 8 MPa, it has been found that the sample doped with 1 mol% Ni presents the largest hydrogen absorption amount with about 0.97wt%.

Thermodynamic consistent phase field model for sintering process with multiphase powders

Abstract A thermodynamic consistent phase field model is developed to describe the sintering process with multiphase powders. In this model, the interface region is assumed to be a mixture of different phases with the same chemical potential, but with different compositions. The interface diffusion and boundary diffusion are also considered in the model. As an example, the model is applied to the sintering process with Fe–Cu powders. The free energy of each phase is described by the well-developed thermodynamic models, together with the published optimized parameters. The microstructure and solute distribution during the sintering process can both be obtained quantitively.

Phase field simulation for non-isothermal solidification of multicomponent alloys coupled with thermodynamics database

Abstract In order to quantitively model the real solidification process of industrial multicomponent alloys, a non-isothermal phase field model was studied for multicomponent alloy fully coupled with thermodynamic and diffusion mobility database, which can accurately predict the phase equilibrium, solute diffusion coefficients, specific heat capacity and latent heat release in the whole system. The results show that these parameters are not constants and their values depend on local concentration and temperature. Quantitative simulation of solidification in multicomponent alloys is almost impossible without such parameters available. In this model, the interfacial region is assumed to be a mixture of solid and liquid with the same chemical potentials, but with different composition. The anti-trapping current is also considered in the model. And this model was successfully applied to industrial Al–Cu–Mg alloy for the free equiaxed dendrite solidification process.