Advanced Materials Research Vols. 47-50

Abstract: Ultrafast optical pump-probe technique employed in a degenerate-four-wave-mixing configuration was used to study the light-induced optical property change during extremely short time period of femtoseconds (fs) to picoseconds (ps) for VO2 thin films. Dramatic change in optical properties serves as basis of light-induced functional materials in ultrafast applications. Upon light illumination the structural change was identified as a phase transition (PT) from insulator to metallic phases. After the light-induced PT a fast recovery process occurred, which was strongly dependent on optical pump energy. It could be governed by pure electronic relaxation excluding thermal contribution at sufficiently low excitation. The insulator and metallic phases are coexistent exhibiting fluctuations of dielectric constants. Different desorption activity was monitored for insulator and metallic VO2 thin solid films under femtosecond optical excitation.

Abstract: Low cost composite materials are widely used in civil and structural engineering applications. This project uses EPON to plasticize a commonly used resin, epoxy resin to lower the cost of the composite and to find out the mechanical and thermal properties of the plasticized epoxy resin to see if it is suitable for the said applications. Three point bending tests were carried out to evaluate the flexural properties of the plasticized resins. Differential scanning calorimetry and dynamic mechanical thermal analysis are used to evaluate the thermal properties of the plasticized epoxy resin. The study with epoxy and EPON showed that the mechanical properties of the epoxy composite were lowered but its ability to dissipate energy increased because of its improved thermal properties. As EPON is much cheaper that epoxy resin, the composite produced is therefore cheaper and provided the service requirements were not so demanding, it can be used in the said applications.

Abstract: CNx/TiNx composite films were prepared on high-speed steel (HSS) substrate by pulsed laser co-deposition process with the Ti/graphite combined targets and at a substrate temperature of 200 °C. The composition, morphology and microstructure of the films were characterized by energy dispersive X-ray spectrum (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). In the graphite/Ti range of 0.5–2.0 of the target, the composite films composed of TiNx, a-CNx and a small amount of metallic Ti. The TiNx disappeared in the films at a high graphite/Ti ratio of the target. The adhesion and tribological performance of the films were investigated using a conventional scratch tester and a ball-on-disk tribometer respectively. With increasing the graphite/Ti ratio of the target, the adhesion to substrate of the composite films deteriorated from 46 N to 26 N, and the friction coefficient drcreased from 0.23 to 0.17. The composite film deposited at the graphite/Ti ratio of 1.0 showed a low friction coefficient, good adhesion and wear rate of 3.2×10–7 mm3/Nm in humid air.

Abstract: The aim of this study was to put forward a new method to improve the ballistic impact performance of unidirectional plate (UD plate) by adding shell powder into matrix. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) spectra revealed that modified shell powder was dispersed uniformly in the matrix. The results of ballistic tests and temperature adaptation tests demonstrated that the bulletproof property of modified UD plate was improved about 20%. The heat-resistant temperature was increased approximately 5°C.

Abstract: a-CNx films were deposited onto silicon wafers at temperatures from RT up to 600 °C by using pulsed KrF excimer laser deposition. The composition, morphology and microstructure of the CNx films were characterized by X-ray photoelectron spectrum (XPS), scanning electron microscopy (SEM) and Raman spectrum. The tribological performance of the films was investigated using a ball-on-disk tribometer. With increasing the deposition temperature ranging from RT to 400 °C, the N content of films dropped from 36 at.% to 22 at.%, the ratio of N-sp3 C bonds, hardness and friction coefficient of the film decreased. Further increase of deposition temperature led to the lack of nitrogen and the increasing degree of order in ringed sp2 C=C bonds of the amorphous carbon film. The mechanical and tribological performances became worse. The film deposited at 300°C showed a low friction coefficient of 0.11 and a preferable wear resistance of 1.65×10–7 mm3 Nm–1 in humid air.

Abstract: Al2O3 ceramic coatings were directly prepared on 6063 aluminum alloy in borate electrolytes by micro-arc oxidation (MAO) technique. The phase composition, microstructure, elemental distribution, and micro-hardness of Al2O3 ceramic coatings were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and micro-hardness test. XRD results show that the coating consists of α-Al2O3 and γ-Al2O3. SEM results show that the pores with different sizes distributed all over the coating surface. Adhesion and tribological and wear tests were also performed. It was found that ceramic coatings with high hardness, excellent adhesion and wear resistance could be formed.

Abstract: We have developed a new method to fabricate poly(diphenylsilylenemethylene) (PDPhSM) matrix nanocomposite thin films containing copper nanoparticles produced by laser ablation in this paper. First of all, 1,1,3,3-tetra- phenyl-1,3-disilacyclobutane (TPDC) films were deposited on 4 cm2 silicon substrates cut from c-Si wafers by conventional vacuum evaporation under a pressure of 3.0×10-5 Torr; then copper nanoparticles were deposited onto the TPDC films by laser ablation; finally the TPDC films with copper nanoparticles were heated in an electric furnace in an air atmosphere at 553 K for 10 min to induce ring-opening polymerization of TPDC. The results indicated that it is possible to fabricate PDPhSM matrix nanocomposite thin films using copper nanoparticles produced by laser ablation. The morphology and size distribution of copper nanoparticles can be controlled by laser ablation conditions. Also, the polymerization efficiency depends on the size and chemical state of copper nanoparticles.

Abstract: Ba0.7Sr0.3TiO3 thin films with a thickness of 300~900 nm were prepared with sol-gel method directly on the copper foils. The films have been processed in almost inert atmosphere such that the oxidation of Cu foil was avoided while allowing the film to crystallize. The crystal structure, morphology and dielectric properties of the films were measured and analyzed. The effects of annealing temperatures and film thickness on the electrical properties were discussed.

Abstract: A carbon foam with high strength and high thermal conductivity was prepared through the incorporation of nano-titanium particle into mesophase pitch precursor. Results show that titanium act as catalysts to accelerate the graphitization of carbon, promote more perfect and larger crystallites and enhance the conductive and mechanical properties. Test results reveal that titanium doped carbon foam (TDCF) has excellent compressive strength and high thermal conductivity, with highest values reaching 29.6 MPa and 117.8 Wm-1 K-1 for a titanium concentration of 12 wt% in the precursor materials. More compact struts and cell walls stacked by more uniform were observed by scanning electron microscope in carbon foam. Correlation between the content of dopant and the properties and microstructure of TDCF was discussed.

Abstract: Ge1-xMnx (x=0.05, 0.07, 0.11, 0.15, 0.19, 0.23, 0.26, 0.29) thin films were prepared by magnetron sputtering. All the films had a Ge cubic structure, and no indication of a secondary phase was found in any sample using X-ray diffraction (XRD). The crystal lattice constant increases with the Mn concentration, in accordance with Vegard's law. No films show clear magnetic domain structure under a magnetic force microscope (MFM). Atom force microscope (AFM) measurements show that all films have a uniform particle size distribution, and a columnar growth pattern. X-ray photoelectron spectroscopy (XPS) measurements indicate that the Mn atoms are not singlely in the bivalent. Electrical transport properties show that the resistance of the films increases with increasing Mn concentration, suggesting that the Mn ions are in deep-level acceptor states, while resistance decreases with increasing temperature, which is a typical semiconductor property.