Guiwu Liu

Nanowires as Building Blocks to Fabricate Flexible Thermoelectric Fabric: The Case of Copper Telluride Nanowires

A general approach to fabricate nanowires based inorganic/organic composite flexible thermoelectric fabric using a simple and efficacious five-step vacuum filtration process is proposed. As an excellent example, the performance of freestanding flexible thermoelectric thin film using copper telluride nanowires/polyvinylidene fluoride (Cu1.75Te NWs/PVDF = 2:1) as building block is demonstrated. By burying the Cu1.75Te NWs into the PVDF polymer agent, the flexible fabric exhibits room-temperature Seebeck coefficient and electric conductivity of 9.6 μV/K and 2490 S/cm, respectively, resulting in a power factor of 23 μW/(mK(2)) that is comparable to the bulk counterpart. Furthermore, this NW-based flexible fabric can endure hundreds of cycles of bending tests without significant performance degradation.

Morphology-Controllable Synthesis of Cobalt Telluride Branched Nanostructures on Carbon Fiber Paper as Electrocatalysts for Hydrogen Evolution Reaction

Cobalt telluride branched nanostructures on carbon fiber paper (CFP) with two different morphologies were synthesized via solution-based conversion reaction. Both the CoTe2 with nanodendrite and CoTe with nanosheet morphologies on the CoTe2 nanotube (CoTe2 NDs/CoTe2 NTs and CoTe NSs/CoTe2 NTs) supported by CFP exhibit high activities toward hydrogen evolution reaction (HER). Particularly, the CoTe NSs/CoTe2 NTs only require an overpotential of 230.0 mV to deliver the current density of 100 mA cm(-2) in acid solution. After cycling for 5000 cycles or 20 h continual electrolysis, only a small performance loss is observed.

Tuning the structures and electron transport properties of ultrathin Cu nanowires by size and bending stress using DFT and DFTB methods

The electron transport properties of ultrathin Cu nanowires (NWs) with diameters of 0.2–1.0 nm under different bending stresses are reported for potential future application in flexible displays and flexible solar cells. Density functional theory (DFT) and density-functional-based tight-binding (DFTB) approaches have been combined to systematically discover the ballistic transport and diffusive transport of ultrathin Cu NWs at the nanoscale. Our DFT calculations show that with an increase of bending stress (f), the structures of both nonhelical and helical wires become disordered, then exhibit a phase transition and eventually collapse. Therefore, the quantum conduction (G) values are reduced. In addition, as the size of the nanowires increases, the maximum bearable bending stress (fc) reduces. fc of a helical atomic strand is decided by its diameter, while fc of a nonhelical atomic strand is decided by the area of the cross section. Our DFTB calculations reveal that the intermediary atoms are the most important for forming the loop between two electrodes and implementing diffusion transport. Among the seven structures, 6-1b exhibits the best properties, after comprehensively considering the results of quantum transport, diffusive transport and collapse-resistance.

Preparation and protective surface coatings for low density C/C composite for harden insulation

Abstract To improve the anti-erosion and anti-oxidation performances is crucial for low density carbon/carbon (LD-C/C) composites to serve as a harden insulation material in high temperature equipment. Surface sealing treatment to the LD-C/C composite is an effective method. In the present study, first, diverse LD-C/C composite samples with different densities were fabricated, and then the effects of high temperature purification process on the density and thermal conductivity of the composites were briefly investigated. Furthermore, two protective coatings (graphite and SiC coatings) were respectively processed on the selected LD-C/C composite by painting graphite or silicon powder plus following heat-treatment, and the gas sealing and oxidation tests were performed on the coated samples. The experimental results indicated that the morphology (size and shape) of graphite powder particle played an important role in the microstructures of the coatings, resulting in varying gas sealing performance. The surface sealing process by using the silicon powder can also obtain a good gas sealing effect due to the effective infiltration of liquid silicon into the porous C/C composite and the formation of SiC, and thus the SiC coated sample presented the best oxidation resistance. However, there was no consistent relationship between the gas sealing performance and the oxidation resistance in the coated composite samples, which are closely related with the microstructural and physical characteristics of the coatings.

Microstructure and Strength of Alumina-Metal Joint Brazed by Activated Molybdenum–Manganese Method

High purity alumina/stainless steel joints were produced via activated molybdenummanganese (Mo-Mn) route using 72Ag-28Cu solder. Microstructures of the metallized ceramic and joint sections were observed by scanning electron microscopy. Joint strength was tested by shear-loading method. Some process factors were characterized and analyzed, which include temperature, holding time and heating and cooling rate in ceramic metallization process. The effects of Ni plating and succedent annealing were also investigated. Experimental results show that, migration of glassy phases is the main mechanism of the ceramic metallization. Glass migration direction is from metallizing layer to ceramic side. In the ranges of temperature and holding time of metallization, joint strength firstly increases and then falls with temperature raising and time extending. More fully sintered metallizing layer can be obtained while the temperature increases from 1200oC to 1500oC, and the time prolongs from 30min to 60min. Over-sintering of the metallizing layer will take place with metallizing temperature of 1600 oC and overlong holding time of 70min, which reduces the joint strength. The slower heating and cooling rate, and the annealing after Ni plating both help enhance the seal strength, due to relieving or eliminating interlayer residual thermal stress. However, too slow heating and cooling rate, such as 5 oC /min, is equivalent to overlong holding time and finally also decline the strength. A thin Ni coating helps solder wet metallizing surface, and stops solder erode metallizing layer.

Effects of the U–O forming process on microstructure evolution of TA15 tubes

Abstract In view of the difficulty of manufacturing titanium alloy tubes, a U–O forming process was proposed to manufacture tubes with TA15 titanium alloy. The effect of the U–O forming process on the microstructure evolution of typical regions of TA15 tubes was investigated using electron backscattered diffraction technique. Results showed that the grain size of the U-shaped part was uniform when deformed at 750°С. Through microstructure evaluation of the O-shaped part, there were only slight changes concerning grain size compared with the initial plate because of the deformation sequence. However, in general, no significant changes concerning the grain size of most regions were observed when comparing the O-shaped tube with the initial plate, which indicated that the U–O forming process did not cause obvious changes in the initial microstructure in terms of grain size.

Preparation and Thermal-Physical Properties of Three Dimensional Bicontinuous SiC/Cu-Si Composite

A three dimensional (3D) SiC/Cu-Si composite with bicontinuous structure was fabricated by spontaneous infiltration method, using porous recrystallized SiC ceramic with porosity of 37% as 3D network reinforcement and Cu alloy (Cu-18Si) as matrix. The phase composition, microstructure, and thermo-physical properties of the as-prepared 3D-SiC/Cu-Si composite were investigated. The experimental results showed that the Cu-18Si alloy could fully penetrate into the porous SiC ceramic at 1600 °C for 2 h spontaneously. SiC and Cu15Si4 phases were identified in the as-prepared composites. The interfacial bond between SiC and Cu-Si alloy was tightly and no severe interfacial reaction was observed. The thermal conductivity and coefficient of thermal expansion of the as-prepared 3D-SiC/Cu-Si composite were changed from 89.8 to 55 W·m-1·K-1 and 7.512 to 9.64×10-6 °C-1 with the temperature increased from room temperature to 500 °C, respectively.

Molten Salt Synthesis of Mullite Whiskers from Silicon Carbide Precursor

Mullite whiskers were prepared from SiC powders in molten Al2(SO4)3-Na2SO4 mixture salts at different temperatures. The morphology and phase composition of resulting whiskers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) techniques. Mullite tiny fiber clusters with diameter about 50 nanometers and lengths of over several microns were obtained in 900°C mixture molten salts system. A new oxidation-dissolution mechanism was proposed for explanation mullite whiskers growth.

Microstructure Evolution during Combustion Synthesis of Ni/Ni-Aluminide Laminated Composites

Combustion reaction in laminated Ni and Al foils was ignited by plasma activated sintering (PAS) to synthesize metal-intermetallic laminated composites (MILCs). The microstructure evolution in the reaction and the post-heat treatment was investigated. The results showed that thermal explosion (TE) reaction were happened between Ni and Al foils at the melting point of Al. The reaction was incomplete due to the heat loss through the thick foils. The produced phases, Ni2Al3 and NiAl3, were converted to a compositionally gradient series of intermetallic phases in the nal microstructures of the intermetallic layers after the post heat treatment.

Low-Temperature and Rapid Fabrication of Bulk Nano-TiO2 Ceramic

A self-made sectional die made of high-performance graphite, SiC ceramic and Ni-based superalloy was firstly designed and developed. The TiO2 ceramic, with original TiO2 powders of average particle size ~25 nm and 80 wt.% anatase + 20 wt.% rutile, was fabricated by plasma activated sintering (PAS) at 500600 °C for 3 min under applied uniaxial pressure of 2001000 MPa using the sectional die. The influences of sintering temperature and applied pressure on the density, phase transformation and grain growth of the TiO2 ceramic were investigated. The results showed that the sintering temperature and applied pressure played key roles in determining the relative density, phase composition and grain size of the TiO2 ceramic. The relative density and grain size increased and the anatase phase transformed into the rutile phase slowly or quickly as the sintering temperature or the applied pressure increased. In particular, the increase of sintering temperature was very advantageous to the phase transformation, and the increase of applied pressure was quite effective to inhabit the grain growth. All the averaged grain sizes of TiO2 ceramics were less than 100 nm in the present experimental conditions. Moreover, the relative density of the sintered ceramic were over 95% when the optimized sintering parameters were 600 °C × 500MPa or 500 °C × 1000MPa. The TiO2 ceramics were composed of only the rutile phase when the applied pressure and the sintering temperature were not less than 300 MPa and 550 °C, respectively.