Huiming Liu

Strong photoluminescence of nanostructured crystalline tungsten oxide thin films

Strong photoluminescence (PL) is observed in nanostructured crystalline tungsten oxide thin films that are prepared by thermal evaporation. Two kinds of films are investigated—one made of nanoparticles and another of nanowires. At room temperature, strong PL emissions at ultraviolet-visible and blue regions are found in both of the films. Compared with the complete absence of emission of bulk phase tungsten oxide powder under the same excitation conditions, our results clearly demonstrate the quantum-confinement-effect-induced photoluminescence in nanostructured tungsten oxides.Strong photoluminescence (PL) is observed in nanostructured crystalline tungsten oxide thin films that are prepared by thermal evaporation. Two kinds of films are investigated—one made of nanoparticles and another of nanowires. At room temperature, strong PL emissions at ultraviolet-visible and blue regions are found in both of the films. Compared with the complete absence of emission of bulk phase tungsten oxide powder under the same excitation conditions, our results clearly demonstrate the quantum-confinement-effect-induced photoluminescence in nanostructured tungsten oxides.

Advanced nanocrystalline Zr-based AB2 hydrogen storage electrode materials for NiMH EV batteries

The metallurgical microstructure, crystal-structure and electrochemical properties of Laves phase Zr-V-Mn-Ni system alloys (modified with Ti, Co, Sn, etc.) were investigated systematically in the present paper. Conventional polycrystalline Zr-based alloys, which consist of cubic C15 Laves phase, hexagonal C14 Laves phase and non-laves phase (such as Zr7Ni10, Zr9Ni11, Zr(NiMn)Sn-0.35), show the highest discharge capacity of 342 mAh g(-1) (at 60 mA g(-1) charge-discharge current), which decreases by 7.8% after 300 cycles. Amorphous phase alloys in melt-spun alloys exhibit poor electrochemical properties. Advanced nanocrystalline C15-Laves single-phase alloys were prepared by completely crystallizing the melt-spun amorphous Zr1-xTix[(NiVMnCo)(1-nu)Sn-nu](2+alpha) alloys. These alloys have a special microstructure composed of high-density interface phase and random-oriented grains varying from several nanometres to several dozens of nanometres. It was found that these materials had high discharge capacity (the maximum capacity is up to 379 mAh g(-1)) and long cycle life (the capacity only decreases 3% after 300 cycles). The maximum discharge capacities were found in the metallurgical microstructure and crystal-structure in Zr-based AB(2) alloys. The maximum discharge capacity increases in regular nanocrystalline/C15-Laves single-phase>polycrystalline/multi-phase (Laves and non-laves)>comorphous state/C15-Laves single-phase. It was shown that the complete crystallization method from amorphous solids is an effective way to greatly improve the electrochemical performance of Zr-based AB(2) hydrogen storage electrode materials, which is not only significant for academic research but also valuable for practical applications in the NiMH battery system for pure electric vehicles (PEV) and hybrid electric vehicles (HEV)

Interfacial properties of high-k dielectric CaZrOx films deposited by pulsed laser deposition

The interfacial properties of high-k dielectric CaZrOx thin films deposited by pulsed laser deposition in O-2 and N-2 ambient are investigated. The SiOx (x < 2) interfacial layer is observed for the films deposited at 300 degrees C in 20 Pa O-2. Rapid thermal annealing (RTA) of the films at 700 degrees C in N-2 for 10 s allows for oxidization of the interfacial layers into SiO2 and decomposition of the films into nano-ZrO2 crystals embedded in the matrix of amorphous CaO-rich zirconate. However, by the same RTA, the films deposited at 300 degrees C in 20 Pa N-2 remain amorphous with clean Si/CaZrOx interface and exhibit good electrical performances. (c) 2006 American Institute of Physics.

An apparatus for measurements of thermal conductivity and thermal expansion based on GM cryocooler

The thermophysical properties of matters are extremely important for engineering and materials science. This paper describes a multifunctional apparatus based on GM cryocooler for measurement of thermal conductivity and thermal expansion via steady-state longitudinal heat flow method and strain gauge technique respectively. The apparatus consists of a removable sample test rod on which bulk samples can easily be mounted and placed in the measurement device. Besides, the sample holder is easy to be replaced so that it suits various needs. All measurements are efficiently and accurately carried out at different temperatures by following a set of stability criteria the setup of the apparatus has been calibrated with sample stainless steel and copper, which gives an error within 6% around the published results in the literatures.

Preparation and Property Study of Graphene Oxide Reinforced Epoxy Resin Insulation Nanocomposites with High Heat Conductivity

In this paper, graphene oxide reinforced epoxy resin nanocomposites were successfully prepared. Compared with unmodified epoxy resin, the heat conductivity of the graphene oxide reinforced epoxy resin nanocomposites had been improved while keeping the insulation performance. The tensile strength was investigated at both room temperature (300 K) and liquid nitrogen temperature (77 K). And the fracture surfaces were examined by scanning electron microscopy (SEM). Results showed that the materials had excellent mechanical properties, which could be advantages for the applications as insulating layer in low temperature superconducting magnets.

Giant isotropic magnetostriction in NaZn13-type LaFe13−xAlx compounds

The unusual low-temperature magnetostrictive property is of fundamental interest due to significant applications in rapidly developing cryogenic engineering. Here, we report a giant isotropic magnetostriction (λ = 1500 ppm, ω = 4500 ppm) over a wide temperature range (∼210 K), and the saturated volume magnetostriction can be up to 8400 ppm in cubic NaZn13-type LaFe13−xAlx compounds by optimizing the chemical composition. The large magnetostrictive effect often occurs in ferromagnetic materials. However, we discovered that the magnetic-field-induced volume expansion originates from the change of lattice parameters across the first-order transition from the low-volume antiferromagnetic ground state to the high-volume ferromagnetic state. Moreover, the magnetostrictive materials show an excellent zero thermal expansion (ZTE) property, which guarantees their reliability and stability operating at various temperatures. The present study suggests potential applications of La(Fe, Al)13-based compounds as ZTE and...

Dismountable sample holder apparatus for rapid thermal conductivity measurements based on cryocooler

A novel apparatus, based on cryocooler, for rapid thermal conductivity measurements between 8 K and 300 K is presented. It consists of a removable sample test bar on which bulk samples can easily be mounted and then placed in the described measurement device. This fast mounting measurement system uses a standard steady-state absolute thermal conductivity measurement and allows for excellent thermal stability and mechanical vibration isolation from the cryocooler. The distinction of this system is rapid mounting and measurement of thermal conductivity with high accuracy and precision in data acquisition. In addition, this system allows for versatility in its use, such as the specific heat and the linear thermal expansion measurement. The design of this apparatus, measurement specification, and thermal conductivity of standard materials measured in this system are presented.

Synthesis of atomic layers of hybridized h-BNC by depositing h-BN on graphene via ion beam sputtering

We report the deposition of hexagonal boron nitride (h-BN) on graphene by ion beam sputtering deposition. Both graphene domains and films synthesized by chemical vapor deposition were used as substrates. In the case of graphene domains, it was found that the h-BN domains were preferentially grown on the baked Cu surface instead of graphene due to the highly catalytic activity of Cu. On the other hand, the higher ejection energy of sputtered particles leads to the mixing of boron/nitrogen atoms and carbon atoms. Consequently, the h-BNC films consisting of the hybrid atomic layers of h-BN and graphene domains were formed when the graphene films were used as substrates. This work provides a promising and accessible route for the synthesis of hybridized h-BNC material.

Thermal Expansion and Magnetostriction Measurements at Cryogenic Temperature Using the Strain Gauge Method

Thermal expansion and magnetostriction, the strain responses of a material to temperature and a magnetic field, especially properties at low temperature, are extremely useful to study electronic and phononic properties, phase transitions, quantum criticality, and other interesting phenomena in cryogenic engineering and materials science. However, traditional dilatometers cannot provide magnetic field and ultra-low temperature (<77 K) environment easily. This paper describes the design and test results of thermal expansion and magnetostriction at cryogenic temperature using the strain gauge method based on a Physical Properties Measurements System (PPMS). The interfacing software and automation were developed using LabVIEW. The sample temperature range can be tuned continuously between 1.8 and 400 K. With this PPMS-aided measuring system, we can observe temperature and magnetic field dependence of the linear thermal expansion of different solid materials easily and accurately.

Design of High Voltage Electrical Breakdown Strength measuring system at 1.8K with a G-M cryocooler

Impregnating resins as electrical insulation materials for use in ITER magnets and feeder system are required to be radiation stable, good mechanical performance and high voltage electrical breakdown strength. In present ITER project, the breakdown strength need over 30 kV/mm, for future DEMO reactor, it will be greater than this value. In order to develop good property insulation materials to satisfy the requirements of future fusion reactor, high voltage breakdown strength measurement system at low temperature is necessary. In this paper, we will introduce our work on the design of this system. This measuring system has two parts: one is an electrical supply system which provides the high voltage from a high voltage power between two electrodes; the other is a cooling system which consists of a G-M cryocooler, a superfluid chamber and a heat switch. The two stage G-M cryocooler pre-cool down the system to 4K, the superfluid helium pot is used for a container to depress the helium to superfluid helium which cool down the sample to 1.8K and a mechanical heat switch connect or disconnect the cryocooler and the pot. In order to provide the sufficient time for the test, the cooling system is designed to keep the sample at 1.8K for 300 seconds.