Frontiers of Materials Science (v.3, #4)

New trends for solar cell development and recent progress of dye sensitized solar cells by Hong Lin; Wen-li Wang; Yi-zhu Liu; Xin Li; Jian-bao Li (345-352).
This article reviews the new concepts and new trends of solar cell development. To increase the photoelectric conversion efficiency, reduce the cost, and for application in a much broader field, thin film solar cell, flexible solar cell, and tandem solar cell have become important subjects to be studied. As the representative of the solar cells of the third generation, the progress and challenges of dye sensitized solar cell was also reviewed.
Keywords: solar cell; thin film solar cell; flexible solar cell; tandem solar cell; dye sensitized solar cell

Biomolecule sensitive hydrogel, as an important part of intelligent materials, has extensive applications in the biochemical and biomedical fields. In this paper, an antigen sensitive degradable hydrogel based on hyaluronic acid (HA) has been prepared. Antigen and antibody were covalently attached to the hydrogel simultaneous, while HA was cross-linked used adipic dihydrazide (ADH) as cross-linking agent primarily. We describe the synthesis, characteristics, and the antigen sensitive behavior of the hydrogel. The hydrogel can exhibit a reversible swelling behavior while the native antigen is present or not. This property of the antigen sensitive hydrogel may be applied as novel drug delivery system, which can release drug by the presence of native antigen.
Keywords: intelligent hydrogel; hyaluronic acid; antigen sensitive

A novel approach via combination of electrospinning and FDM for tri-leaflet heart valve scaffold fabrication by Rui Chen; Yosry Morsi; Shital Patel; Qin-fei Ke; Xiu-mei Mo (359-366).
In this paper, a novel combination method of electrospinning and rapid prototyping (RP) fused deposition modeling (FDM) is proposed for the fabrication of a tissue engineering heart valve (TEHV) scaffold. The scaffold preparation consisted of two steps: tri-leaflet scaffold fabrication and heart valve ring fabrication. With the purpose of mimicking the anisotropic mechanical properties of the natural heart valve leaflet, electrospun thermoplastic polyurethane (ES-TPU) was introduced as the tri-leaflet scaffold material. ES-TPU scaffolds can be fabricated to have a well-aligned fiber network, which is important for applications involving mechanically anisotropic soft tissues. We developed ES-TPU scaffolds as heart valve leaflet materials under variable speed conditions and measured fiber alignment by fast Fourier transform (FFT). By using FFT to assign relative alignment values to an electrospun matrix, it is possible to systematically evaluate how different processing variables affect the structure and material properties of a scaffold. TPU was suspended at certain concentrations and electrospun from 1,1,1,3,3,3-hexafluoro-2-propanol onto rotating mandrels (200–3000 rpm). The scaffold morphological property and mechanical anisotropic property are discussed in the paper as a function of fiber diameter and mandrel RPM. The induction of varying degrees of anisotropy imparted distinctive material properties to the electrospun scaffolds. A dynamic optimum design of the heart valve ring graft was constructed by FDM. Fabrication of a 3D heart valve ring was constructed using pro-engineer based on optimum hemodynamic analysis and was converted to an STL file format. The model was then created from PCL which was sewed and glued with electrospun nanofibrous leaflets. This proposed method was proven as a promising fabrication process in fabricating a specially designed graft with the correct physical and mechanical properties.
Keywords: tissue engineering heart valve; fused deposition modeling; electrospinning; thermoplastic polyurethane; nanofiber

Stabilization of horseradish peroxidase in silk materials by Shen-zhou Lu; Xiao-qin Wang; Neha Uppal; David L. Kaplan; Ming-zhong Li (367-373).
Horseradish peroxidase (HRP) is widely used as an indicator enzyme in enzyme immunoassays, enzyme electrodes, effluent treatment and synthetic organics. Previous studies showed that HRP was not stable in solution especially for low concentration solutions. It is important to prevent HRP from losing its activity. In the present study, we describe HRP stabilization in silk fibroin solution and in silk films. The results showed that HRP activity increased 30%–40% after being added into silk solution. The half-life time of HRP activity was 25 days at room temperature and 1 h at 60°C in silk solution while it was only 2.5 h at room temperature and 0.3 h at 60°C in PBS buffer. The HRP activity in silk film still remained at 24%, 22%, 17%, when compared with the original amount of activity, after being immobilized in silk films stored at 4°C, room temperature and 37°C for 5 months, respectively. Electrostatic interactions and hydrophobic interactions between silk and HRP might account for this improved stabilization. Silk fibroin can be used as an HRP protecting reagent in solution and in films.
Keywords: stabilization; horseradish peroxidase (HRP); enzyme; silk fibroin

Synthesis and characterization of a pH/temperature responsive glycine-mediated hydrogel for drug release by Kui-lin Deng; Peng-fei Zhang; Xiao-bo Ren; Hai-bin Zhong; Yu-bo Gou; Li-rong Dong; Qian Li (374-379).
In this work, a pH/temperature responsive hydrogel (PMEA) from N-acryloylglycine methyl ester (NAGME), N-acryloylglycine ethyl ester (NAGEE), and acrylic acid (AAc) was synthesized by free radical polymerization. The swelling behaviors and drug release properties of hydrogels were systematically investigated at different temperature, pH, and AAc content. It was found that the hydrogel PMEA demonstrated pH and temperature responsive nature. The caffeine-release behaviors showed that only 49.1% caffeine was released from PMEA in pH 2.70 phosphate buffer solution (PBS) after 500 minutes, whereas more than 93.9% caffeine was gradually diffused into the medium in pH 7.49 PBS over the same time interval. In addition, the caffeine release was much higher at 37°C than that at 14°C in deionized water. As seen from the results, the PMEA seems to be a potential drug carrier with pH-temperature responsiveness.
Keywords: pH/temperature-responsive; N-acryloyl glycinate; drug release; polymer gels

A graft polymerization of glycidyl methacrylate (GMA) on the pretreated polyethylene (PE) sheet samples by oxygen capacitively coupled radio frequency (RF) plasma was carried out to improve the adhesive properties of PE. The PE samples were treated with a RF power of 200 W for a treatment time of 40 s and then exposed to an oxygen atmosphere for a saturation time of 10 min. The grafting of the plasma pretreated PE performed in an aqueous GMA solution with the monomer concentration from 20 vol.% to 100 vol.% at a temperature from 20°C to 90°C for a reaction period up to 50 h. The optimum wettability of the graft polymerized PE surface with the concentration of 40 vol.% at the temperature of 70°C and for the time of 24 h was obtained as the static contact angle decreased from 104.2° for the original PE to 67.6° for the graft polymerized. After the graft polymerization, a strong absorption peak of C-O bonding was shown at 1050 cm−1 in Fourier transform infrared spectrum, indicating an introduction of epoxy groups on the graft polymerized surface. Correspondingly, the surface roughness (Ra) increased from 0.137 μm for the original PE to 1.660 μm for the graft polymerized. The maximal lap adhesive strength of the graft polymerized PE samples lapped using a mixture of epoxy resin and curing agent was achieved to about 160 N·cm−2. The fractured surfaces by tearing of the PE sheet matrix were observed on the tensioned PE samples due to the higher adhesive strength than that of the PE matrix.
Keywords: polyethylene (PE); plasma-induced graft polymerization; adhesive properties; glycidyl methacrylate (GMA); capacitively coupled radio frequency plasma

The crystal morphology, impact strength and nonisothermal crystallization kinetics of poly(trimethylene terephthalate)/maleinized poly(ethylene-octene) (PTT/PEO-MA) copolymer blends were studied by using the polarized optical microscopy, impact tester and differential scanning calorimetry (DSC). Avrami theory modified by Jeziorny, Ozawa and Mo theories were used to study the non-isothermal crystallization kinetics of the blends, respectively. The results suggest that these methods are suitable for analyzing the crystallization kinetics of the PTT/PEO-MA blends. The PEO-MA component, serving as a nucleation agent in blends, can increase the start crystallization temperatures and accelerate the crystallization rate of the blends. The crystal dimensions are predominantly three-dimensional growths, judged from the Avrami exponent n and the Ozawa exponent m, but the spherulites in blends are much smaller than those in pure PTT. The crystallization active energy suggests that the PEO-MA component can make the PTT component easy to crystallize in blends. The blend has the highest Izod impact strength as PEO-MA content is 3 wt.%. Considering both the crystallization kinetic analyses results and the crystal morphology of the blends, the modified Avrami method is believed to be the most useful in reflecting the crystallization of the blends.
Keywords: poly(trimethylene terephthalate) (PTT); non-isothermal crystallization kinetics; maleinized poly (ethylene-octene) copolymer (PEO-MA); blend; DSC

The present research deals with strength, porosity and hydric behavior of metakaolin cement admixtured with different types of water. The hydration of ordinary Portland cement in the presence of 0%, 10%, 20% and 30% metakaolin treated with distilled, ground and sea water with the water to cement ratio of 0.4 was studied. The experimental results on setting time, strength, porosity and hydric parameters are reported. The results show that, metakaolin percentage increases in strength with a decrease in porosity. The observed results are discussed with SEM micrographs. Further, sea water accelerates the cement hydration at the early stages but retards it in the latter stages of hydration.
Keywords: cement; metakaolin (MK); water; strength; porosity; hydric parameters

Nanostructured TiO2 photocatalyst and pump-probe spectroscopic study by Wen-can Zhou; Zheng-cao Li; Zheng-jun Zhang; Ken Onda; Sho Ogihara; Yoichi Okimoto; Shin-ya Koshihara (403-408).
Titanium dioxide (TiO2), with its large band gap, has attracted much attention due to its excellent photocatalytic activity. TiO2 ball-shaped nano-particles were deposited on silicon substrates by a thermal oxidation approach. With an increase in the annealing temperature the surface morphology and the structure of TiO2 remained stable, exhibiting good heat stability; meanwhile, the hydrogen production rate also increased. The femtosecond pump-probe spectroscopic study showed that the lifetime of carriers of the samples as- deposited and post-annealed at different temperatures were longer than 20 ps.
Keywords: TiO2 ; photocatalysis; pump-probe spectroscopy

Diamond-like carbon (DLC) films were successfully prepared on glass substrates and surfaces of selenium drums via radio frequency plasma enhanced chemical vapor deposition method. The microstructure, surface morphology, hardness, film adhesion, and tribological properties of the films were characterized and evaluated by X-ray photoelectron spectroscopy, atomic force microscopy, and micro-sclerometer and friction-wear spectrometer. The results showed that DLC films have smooth surfaces, homogeneous particle sizes, and excellent tribological properties, which can be used to improve the surface quality of the selenium drums and prolong their service life.
Keywords: diamond-like carbon (DLC) film; radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD); microstructure; tribological application

The Stillinger-Weber potential-based MD (Molecular dynamics) method is used to simulate the heating-up and axial tension of Si/Ge core-shell and superlattice nanowires; according to the simulative results, the differences in their thermal and mechanical properties are discussed. The results show the following: (1) The Si/Ge superlattice nanowire is more thermally stable than the core-shell one, and their melting points are 1160 and 1320 K, respectively. (2) The Si/Ge core-shell nanowire has higher elastic module than the super-lattice one. (3) Under tension, the super-lattice nanowire has better antideformation capability than the core-shell one but has comparative antiloading capability.
Keywords: core-shell; superlattice; Si/Ge nanowires; thermal stability; tensile properties

Application of contact element method in the numerical simulation of thermal stress by Hui Li; Jian-song Shi; Rong-rong Zong; Xiao-xia Wang (421-425).
The interaction relationship between casting and mold (core) decides the stress level of casting and mold (core). In this article, the stress field during the casting process of stress frame casting was simulated by contact element method provided of the professional casting simulation software, ProCAST, compared its results with the simulation results of sand mold with full rigidity. Meanwhile, the influence of shake-out temperature on residual stress was also in study. It showed that the stress result of full rigidity mechanical model is bigger than that of the contact element method. The casting residual stress first increases and then decreases along with the elevation of the shake-out temperature, and the residual stress reaches the maximum when the shake-out temperature is 600°C. The lower is the shake-out temperature, the smaller is the casting deformation.
Keywords: numerical simulation; casting/mold (core); contact element; residual stress; shake-out temperature

Metal transfer modes of twin-wire indirect arc welding by Shun-shan Zhang; Dong-ting Wu; Mei-qing Cao; Zeng-da Zou (426-433).
Metal transfer modes of twin-wire indirect arc welding are studied and the effects of welding parameters on transfer modes are investigated in this paper. The progress of transition is captured by a high speed camera system with a xenon lamp as light source in order to remove the intense arc light. An oscilloscope is used to record the values of the parameters. Results show that anode and cathode have different transfer modes under certain conditions because of different melting rates. All the transfer modes are classified as short-circuiting transfer and free-flight transfer. The latter could be subdivided into globular transfer, projected transfer, streaming transfer, etc. The metal transfer modes of TWIAWare influenced by the welding current, arc voltage and shielding gas. With an increase in welding current and arc voltage, the transition frequency increases and droplets become smaller. Images indicate that the metal transfer modes influence arc shape periodically because of the detaching force and retainable force changing periodically.
Keywords: indirect arc; twin-wire welding; metal transfer

Study on Laves phase in an advanced heat-resistant steel by Ping Hu; Wei Yan; Wei Sha; Wei Wang; Zhan-li Guo; Yi-yin Shan; Ke Yang (434-441).
The Laves phase is one of the most significant precipitates in ferritic/martensitic heat-resistant steels. Laves phase precipitates in the creep rupture specimens with different rupture life were studied on a 10 wt.% Cr heat-resistant steel. JMatPro thermodynamic and kinetic calculations were carried out to simulate and predict the precipitation behavior of the Laves phase in the steel at the equilibrium state. The morphologies of the Laves phase developed with creep time were characterized under both scanning electron microscope (SEM) and transmission electron microscope (TEM). Effects of Co on the growth behavior of Laves phase and the corresponding fracture mode were analyzed. It was found that the Laves phase in the steel grew to 200 nm in size after only 1598 h at 600°C, indicating that the addition of Co in the steel could accelerate the growth of Laves phase, and the coalescence of large Laves phase would lead to the brittle intergranular fracture.
Keywords: heat-resistant steel; Laves phase; precipitate; microstructure; creep

Development in high-grade dual phase steels with low C and Si design by Guo-hui Zhu; Xue-hui Zhang; Wei-min Mao (442-446).
Cold rolled dual phase steels with low C and Si addition were investigated in terms of combination of composition and processing in order to improve mechanical properties and workability including welding and galvanizing. Mo and Cr could be used as alloying elements to partially replace C and Si to assure enough hardening ability of the steels and also give solute-hardening. Mo addition is more effective than Cr addition in terms of obtaining the required volume fraction of martensite and mechanical strength. The ferrite grain was effectively refined by addition of Nb microalloying, which gives optimized mechanical properties. The experimental results show that it is possible to obtain the required mechanical properties of high grade 800 MPa dual phase steel, i.e., tensile strength > 780 MPa, elongation > 15%, and yield/tensile strength ratio < 0.6 in the condition of low carbon (C < 0.11 wt.%) and low silicon design (Si < 0.05 wt.%) through adequate combination of composition and processing.
Keywords: dual phase steels; composition design; processing; mechanical properties

Comparative research on high-velocity compaction and conventional rigid die compaction by Ming-jun Yi; Hai-qing Yin; Jian-zhong Wang; Xian-jie Yuan; Xuan-hui Qu (447-451).
Experiments were carried out with different apparatus to compact electrolytic copper powders at distinct loading speeds. It appears that green densities of compacts prepared by HVC out-number that by conventional compaction by one percent. Compacts by quasi-static compaction are almost as dense as those by HVC under comparable peak pressure. The relationship between green compact density and peak pressure accords with Hang Pei-yun formula well. Spring-backs of HVC compacts are far smaller than those of conventional compaction and quasi-static compaction. HVC compacts are harder than compacts by conventional compaction and quasi-static compaction when they have the same density.
Keywords: powder metallurgy; high-velocity compaction; convention die compaction; impact velocity