Frontiers of Materials Science (v.3, #1)

Advances in self-assembled ultrathin polyoxomolybdates multilayers by Liang-bao Yang; Xiu-fang Wang; An-jian Xie; Gang Hu; Yu-hua Shen (1-8).
The research on the assembly and function of organized molecular films has gained more and more interest. Electrostatic interactions can be employed to assemble polyoxomolybdates in surface confined multilayers. Ultrathin multilayer films of polyoxomolybdates and organic molecules by the self-assembly method have been reviewed. At the same time, self-assemblies in aqueous solution are also reported, such as wheel-shaped clusters (Mo154), hollow spherical “blackberry”-like vesicles (Mo72Fe30) and Keggin structures. Polyoxomolybdate multilayers are promising candidates for diverse applications including electrocatalytic, photo- and electrochromic systems. The development in this particular field of materials science may be highlighted in the future.
Keywords: polyoxomolybdates; multilayer films; self-assembled

Vacancies as a constitutive element for the design of nanocluster-strengthened ferritic steels by M. K. Miller; C. L. Fu; M. Krcmar; D. T. Hoelzer; C. T. Liu (9-14).
The existence of nanoclusters that are thermodynamically stable at elevated temperatures is truly intriguing because of its scientific implications and potential applications. Highly stable nanoclusters have been observed by atom probe tomography in iron-based alloys at temperatures close to 1400°C (0.92T m) that appear to defy the stability constraints of artificially created nanostructured materials. The ∼4-nm-diameter Ti-, Y- and Oenrichednanoclusters are identified in the new form of a highly defective material state with vacancies as the critical alloying component and with (Ti + Y):O ratio different from the stable TiO2 and Y2Ti2O7 oxides. Vacancies play an indispensable role in enhancing the oxygen solubility and increasing the oxygen binding energy in the presence of Ti and Y, resulting in the stabilization of coherent nanoclusters. Atom probe tomography characterizations and theoretical predictions indicate that vacancies can be exploited for the first time as a nanoscale constituent to design materials with far superior high temperature properties.
Keywords: nanocluster; vacancy; ferritic alloy; nanoscale constituent; high temperature property

Fabrication of hydrophilic paclitaxel-loaded PLA-PEG-PLA microparticles via SEDS process by Ping Ouyang; Yun-qing Kang; Guang-fu Yin; Zhong-bing Huang; Ya-dong Yao; Xiao-ming Liao (15-24).
In this work, chemically bonded poly(D, L-lactide)-polyethylene glycol-poly(D, L-lactide) (PLA-PEG-PLA) triblock copolymers with various PEG contents and PLA homopolymer were synthesized via melt polymerization, and were confirmed by FTIR and 1H-NMR results. The molecular weight and polydispersity of the synthesized PLA and PLA-PEG-PLA copolymers were investigated by gel permeation chromatography. Hydrophilicity of the copolymers was identified by contact angle measurement. PLA-PEG-PLA and PLA microparticles loaded with and without PTX were then produced via solution enhanced dispersion by supercritical CO2 (SEDS) process. The effect of the PEG content on the particle size distribution, morphology, drug load, and encapsulation efficiency of the fabricated microparticles was also studied. Results indicate that PLA and PLA-PEG-PLA microparticles all exhibit sphere-like shape with smooth surface, when PEG content is relatively low. The produced microparticles have narrow particle size distributions and small particle sizes. The drug load and encapsulation efficiency of the produced microparticles decreases with higher PEG content in the copolymer matrix. Moreover, high hydrophilicity is found when PEG is chemically attached to originally hydrophobic PLA, providing the produced drug-loaded microparticles with high hydrophilicity, biocompatibility, and prolonged circulation time, which are considered of vital importance for vessel-circulating drug delivery system.
Keywords: supercritical CO2 (scCO2); SEDS; PLA-PEG-PLA; paclitaxel

A biocompatible diisocyanate, lysine ethyl ester diisocyanate, was prepared. Afterwards, biodegradable polyurethane (PU) was synthesized by the stepgrowth polymerization of this diisocyanate with hydroxyl terminated poly(ε-caprolactone) in the presence of 1,4-butanediol as a chain-extender. The resulting PU was characterized by GPC, IR and DSC measurements. Its mechanical strength was found to increase with increasing the hard segment content. The PU microfiber meshes with high porosity were obtained by solution electrospinning technique. Their degradation behavior in the PBS and enzymatic solution was also investigated.
Keywords: polyurethane; biodegradability; poly(ε-caprolactone); L-lysine ethyl ester diisocyanate; solution electrospinning

Thermodynamic re-assessment of the Fe-Al-C system based on the Fe-rich experimental data by Lin Li; Shuigen Huang; Li Wang; Yan-lin He; Jef Vleugels; Omer van der Biest (33-37).
The influence of C and Al content on phase transformation temperatures, i.e., the A1 and A3 of Fe-rich alloys is investigated by dilatometric analysis. With the new set of experimental data, an updated thermodynamic description of the Fe-Al-C system is presented, by using the thermodynamic data of the Fe-C, Fe-Al and Al-C systems, as well as the parameters for the Fe-Al-C ternary system optimized in this study. The good compatibility of the thermodynamic parameters with experimental data is demonstrated by several calculated vertical sections. A well reproduced vertical section of the Fe-Mn-Si-Al-C system is also presented according to the thermodynamic description of the lower order systems.
Keywords: iron-aluminum-carbon; TRIP steel; phase diagram; thermal analysis

Simulation and analysis of the effective permittivity for two-phase composite medium by Jin-hui Peng; Jing-jing Yang; Ming Huang; Jun Sun; Zhong-yuan Wu (38-43).
Based on the effective medium theory, we calculated the complex effective permittivity of the twophase composite medium, consisting of circular inclusions embedded in a surrounding host. Results show that the maximum dielectric loss of the composite medium can be obtained by tuning the dielectric properties of the inclusion. This was confirmed by the results in literature. The local high temperature, as a consequence of the local electric field enhancement phenomenon, is the main reason that microwave heating can lead to a dramatic increase in the chemical reaction rate.
Keywords: composite medium; permittivity; microwave

Preparation of V2O5 thin film and its optical characteristics by Yu-quan Wang; Zheng-cao Li; Zheng-jun Zhang (44-47).
A two-step method was proposed to synthesize V2O5 films on planar substrates, i.e., depositing a V2O3 thin film at ∼160°C (by heating a pure sheet of vanadium in a rough vacuum) and then heat-treating it in air at ∼600°C. The films, made up of nano crystal particles, had good photoluminescence (PL) properties, and the peak was in the position of about 700 nm. Further, photosensitivity characteristics of these V2O5 thin films were investigated at room temperature. The results show that this film is sensitive to ultraviolet rays, which indicates good potential application for an ultraviolet photo detector (UVPD).
Keywords: vanadium oxides; thin films; photoluminescence; photosensitive

Oxidation phase growth diagram of vanadium oxides film fabricated by rapid thermal annealing by Tamura Kozo; Zheng-cao Li; Yu-quan Wang; Jie Ni; Yin Hu; Zheng-jun Zhang (48-52).
Thermal evaporation deposited vanadium oxide films were annealed in air by rapid thermal annealing (RTP). By adjusting the annealing temperature and time, a series of vanadium oxide films with various oxidation phases and surface morphologies were fabricated, and an oxidation phase growth diagram was established. It was observed that different oxidation phases appear at a limited and continuous annealing condition range, and the morphologic changes are related to the oxidation process.
Keywords: vanadium oxides (VO x ); rapid thermal annealing (RTA); phase growth diagram

The adherent strength of shearing is an important criterion in evaluating the quality of coating layers, and it is also the foundation to optimize manufacturing. The conventional way of adhesion strength test for thick layers is the pull-off method and lateral shearing method. The drawbacks of these two methods are discussed in this paper, and an evaluating method for coating adherence by beforehand embedded specimen is proposed. The shearing strength of samples is measured with two methods. The scatter band of the testing data is lower than that of the ASTM-C663-79 Standard.
Keywords: coating; shearing; adherent strength

Friction and wear behavior of carbon fiber reinforced brake materials by Du-qing Cheng; Xue-tao Wang; Jian Zhu; Dong-hua Qiu; Xiu-wei Cheng; Qing-feng Guan (56-60).
A new composite brake material was fabricated with metallic powders, barium sulphate and modified phenolic resin as the matrix and carbon fiber as the reinforced material. The friction, wear and fade characteristics of this composite were determined using a D-MS friction material testing machine. The surface structure of carbon fiber reinforced friction materials was analyzed by scanning electronic microscopy (SEM). Glass fiberreinforced and asbestos fiber-reinforced composites with the same matrix were also fabricated for comparison. The carbon fiber-reinforced friction materials (CFRFM) shows lower wear rate than those of glass fiber- and asbestos fiber-reinforced composites in the temperature range of 100°C-300°C. It is interesting that the frictional coefficient of the carbon fiber-reinforced friction materials increases as frictional temperature increases from 100°C to 300°C, while the frictional coefficients of the other two composites decrease during the increasing temperatures. Based on the SEM observation, the wear mechanism of CFRFM at low temperatures included fiber thinning and pull-out. At high temperature, the phenolic matrix was degraded and more pull-out enhanced fiber was demonstrated. The properties of carbon fiber may be the main reason that the CFRFM possess excellent tribological performances.
Keywords: carbon fiber; resins; wear; fibre/matrix bond; mechanical testing

In this paper, martensitic transformation induced plasticity (TRIP) of 9Cr1Mo steel is analyzed by using a Gleeble 3500 test simulator. The values of transformation-induced plasticity under different applied loads are obtained by measuring the axial deformations of the specimen in a free dilatometric test and transformation plasticity test. The experimental results show that there is a good linear relationship between the final TRIP strain and the applied stress. On the basis of experimental results, some models describing TRIP due to theGreenwood-Johnson mechanism are considered. The TRIP coefficient, the evolution of TRIP strain duringmartensitic transformation as well as the dependence of TRIP strain on applied stress are all studied. The values of TRIP coefficient under different applied stresses are almost equal. As for the evolution of TRIP strain with the martensitic volume fraction, the description based on Leblond model is in good agreement with our experimental results.
Keywords: transformation induced plasticity; martensitic transformation; 9Cr1Mo steel

XRF inductive bead fusion and PLC based control system by Jin-hong Zhu; Ying-jie Wang; Hong-xin Shi; Qing-ling Chen; Yu-xi Chen (67-70).
In order to ensure high-quality X-ray fluorescence spectrometry (XRF) analysis, an inductive bead fusion machine was developed. The prototype consists of super-audio IGBT induction heating power supply, rotation and swing mechanisms, and programmable logic controller (PLC). The system can realize sequence control, mechanical movement control, output current and temperature control. Experimental results show that the power supply can operate at an ideal quasi-resonant state, in which the expected power output and the required temperature can be achieved for rapid heating and the uniform formation of glass beads respectively.
Keywords: XRF analysis; bead fusion; induction heating; PLC

Welding distortion control of automobile engine stator by finite element method by De-jun Yan; Xue-song Liu; Huan-yu Xu; Jian-guo Yang; Hong-yuan Fang; Jing-yang Lu (71-74).
In this paper, a thermal elastic-plastic finite element method (FEM) is used to simulate the plasma welding process in order to predict the welding distortion of automobile stator iron core. Six spatial symmetrical welding torches are adopted in the welding process so as to make the iron core rings welded firmly and the distortion symmetric. The effect of rigid clamp on actual welding process is replaced by the contact function between rigid body and deformation element in the MSC software, MARC. The welding process restrained by clamp and deformation analysis after the removal of clamp was successfully simulated. The predictions show good agreement with the test results when the rigid clamps are taken into account in the welding simulation, which satisfies the design requirement for manufacture.
Keywords: SPCC automobile engine stator; rigid clamp; numerical simulation; welding distortion

Discussion and calculation on welding residual longitudinal stress and plastic strain by finite element method by Hong-yuan Fang; Xue-qiu Zhang; Jian-guo Yang; Xue-song Liu; Shen Qu (75-77).
In recent years, some researchers have put forward the new viewpoint that the weld is merely formed during the cooling process, not concerned with the heating process. According to this view, it can be concluded that it is not the compressive but the tensile plastic strain that may remain in the weld. To analyze the formation mechanism of the longitudinal residual stress and plastic strain, finite element method (FEM) is employed in this paper to model the welding longitudinal residual stress and plastic strain. The calculation results show that both the residual compressive plastic strain and the tensile stress in the longitudinal direction can be found in the weld.
Keywords: residual stress; plastic strain; finite element method (FEM); temperature field

Dissimilar metals TIG welding-brazing of aluminum alloy to galvanized steel by San-bao Lin; Jian-ling Song; Guang-chao Ma; Chun-li Yang (78-83).
Dissimilar metals TIG welding-brazing of aluminum alloy to galvanized steel was investigated, and the wettability and spreadability of aluminum filler metal on the steel surface were analyzed. The resultant joint was characterized in order to determine the brittle intermetallic compound (IMC) in the interfacial layer, and the mechanical property of the joint was tested. The results show that the zinc coated layer can improve the wettability and spreadability of liquid aluminum filler metal on the surface of the steel, and the wetting angle can reach less than 20°. The lap joint has a dual characteristic and can be divided into a welding part on the aluminum side and a brazing part on the steel side. The interfacial IMC layer in the steel side is about 9.0 μm in thickness, which transfers from (α-Al + FeAl3) in the welded seam side to (Fe2Al5+ FeAl2) and (FeAl2+ FeAl) in the steel side. The crystal grain of the welded seam is obviously larger in size in the aluminum side. The local incomplete brazing is found at the root of the lap joint, which weakens the property of the joint. The fracture of the joint occurs at the root and the average tensile strength reaches 90 MPa.
Keywords: aluminum alloy/galvanized steel; TIG welding-brazing; intermetallic compound; microstructure; mechanical property

Numerical simulation on bucking distortion of aluminum alloy thin-plate weldment by Jun Li; Jian-guo Yang; Hai-long Li; De-jun Yan; Hong-yuan Fang (84-88).
In this paper, the welding residual distortion of aluminum alloy thin plates is predicted using the elasticity-plasticity finite element method (FEM). The factors contributing to the welding buckling distortion of thin plates are studied by investigating the formation and evolution process of welding stresses. Results of experiments and numerical simulations show that the buckling appearance of thin-plate aluminum alloy weldments is asymmetrical in the welding length direction, and the maximum longitudinal deflection appears at the position a certain distance from the middle point of the side edge towards the arc-starting end. The angular deformation direction of thin-plate weldments is not fixed, and such case as the angular deformation value of the arc-starting end being higher than that of the arc-blowout end exists.
Keywords: aluminum alloy; thin plate; welding; buckling distortion; numerical simulation

Magnetism-current inverse image reconstruction of the nugget in aluminum alloy spot welding based on ART algorithm by Zhi-qing Xue; Zhen Luo; Ping Shan; Xin-xin Tang; An Dong; San-san Ao; Xiao-yi Li (89-92).
In this work, the computer tomography (CT) theory and its reconstruction algorithm were used to deal with the magnetism-current inverse problem in the resistance spot welding (RSW). At first, the magnetic fields around the nugget were detected. Then, the current distribution of the nugget section was calculated by reconstruction algorithm. At last, we changed the current distribution data into a graph using Matlab. The inversed graph of the nugget-section current distribution in the inverted RSW can be achieved, and by this graph the details of the nugget can be observed directly, which can help evaluate the joint quality.
Keywords: resistance spot welding (RSW); reconstruct algorithm; inversed imagery; nugget; magnetic signals

Chaotic oscillator detection system about weak signals in spot welding by Kai-lei Song; Zhen Luo; Feng Ye; Xin-xin Tang; Shu-xian Yuan (93-97).
Spot welding is an efficient and shortcut processing method used in plate, and its quality detection is very important. However, there are many factors affecting the spot welding quality. Because of the low precision of traditional detection methods, spot welding has seldom been used in the aerospace industry which requires high welding quality. In this article, we give a new weak signal detection model based on chaotic oscillators. Using Melnikov methods and Lyapunov exponent, we can determine the critical values when the system enters in and out of chaos. Through lots of numerical simulations, it can be found that the lowest value of the weak sinusoidal signal the system can detect reach 10−11, and its signal-to-noise ratio (SNR) is −126 dB. Compared with other detection methods, chaos oscillator detection system not only has a lower threshold value, but also is easy to implement in practice. This model thus has good application prospects.
Keywords: spot welding; chaotic oscillators; Duffing equation; signal-to-noise ratio (SNR)

Study on quality of resistance spot welded aluminum alloys under various electrode pressures by San-san Ao; Zhen Luo; Xin-xin Tang; Lin-shu Zhou; Shu-xian Yuan; Rui Wang; Kai-lei Song; Xing-zheng Bu; Xiao-yi Li; Zhi-qing Xue (98-101).
The electrode force is one of the main parameters in resistance spot welding (RSW). It is very important to guarantee the quality of aluminum alloys and determine whether the electrode pressure is stable or adjustable in the welding process. With the drive set of a servo-motor, we conduct the RSW tests and tensile shear tests on the 5052 aluminum alloy sheets. Results of these tests show that all variable pressure curves are suitable for spot welding, and all have their own rules in affecting the tensile strength of the spot welded joints.
Keywords: electrode pressure; resistance spot welding (RSW); saddle-shaped curve; aluminum alloy