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ISSN 1005-0302
CN 21-1315/TG
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      30 December 2013, Volume 29 Issue 12 Previous Issue    Next Issue
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    Achieving Large-area Bulk Ultrafine Grained Cu via Submerged Multiple-pass Friction Stir Processing
    P. Xue, B.L. Xiao, Z.Y. Ma
    J. Mater. Sci. Technol., 2013, 29 (12): 1111-1115.  DOI: 10.1016/j.jmst.2013.09.021
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    Large-area bulk ultrafine grained (UFG) pure Cu was successfully prepared by multiple-pass overlapping friction stir processing (FSP) under additional rapid cooling. Overlapping FSP did not exert a significant effect on the microstructure and mechanical properties of the FSP UFG Cu. Similar average grain size was achieved in the transitional zone (TZ) of the multiple-pass FSP sample compared to that in the nugget zone of the single-pass FSP sample, and the TZ exhibited a strong {111}〈112〉112 type A fiber shear texture. Very weak softening occurred in the TZ of the multiple-pass FSP UFG Cu, resulting in a relatively uniform hardness distribution throughout the whole processed zone. A high yield strength of ∼310 MPa and a uniform elongation of ∼13% were achieved in the bulk FSP UFG Cu. This study provides an effective strategy to prepare large-area bulk UFG materials.

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    Effect of Yttrium Addition on Microstructural Characteristics and Superplastic Behavior of Friction Stir Processed ZK60 Alloy
    Z.A. Luo, G.M. Xie, Z.Y. Ma, G.L. Wang, G.D. Wang
    J. Mater. Sci. Technol., 2013, 29 (12): 1116-1122.  DOI: 10.1016/j.jmst.2013.10.031
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    As-extruded ZK60 and ZK60-Y magnesium alloy plates were successfully processed via friction stir processing (FSP) at a tool rotation rate of 1600 r/min and a traverse speed of 200 mm/min. FSP resulted in the formation of equiaxed recrystallized microstructures with the average grain sizes of w8.5 and w4.7 mm in the ZK60 and ZK60-Y alloys, respectively. Moreover, FSP broke and dispersed the MgZn2 and W-phase (Mg3Zn3Y2) particles and dissolved MgZn2 phase in the FSP ZK60 alloy. With the addition of rare earth element yttrium (Y) into the ZK60 alloy, the ratio of the high angle grain boundaries (HAGBs) in the FSP alloys increased from 64% to 90%, and a certain amount of twins appeared in the FSP ZK60-Y alloy. The maximum elongation of 1200% and optimum strain rate of 3 ×10-3 s-1 achieved at 450 ℃ in the FSP ZK60-Y alloy were substantially higher than those of the FSP ZK60 alloy. This is attributed to the fine grains with high ratio of HAGBs and the distribution of a large number of dispersed second phase particles with high thermal stability in the FSP ZK60-Y alloy. Grain boundary sliding was identified as the primary deformation mechanism in the FSP ZK60 and ZK60-Y alloys from the superplastic data analyses and surficial morphology observations.

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    Plasticity Induced by Twin Lamellar Structure in Magnesium Alloy
    Xiyan Zhang, Chao Lou, Jian Tu, Qing Liu
    J. Mater. Sci. Technol., 2013, 29 (12): 1123-1128.  DOI: 10.1016/j.jmst.2013.10.017
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    Effect of {10–12} twins on the mechanical properties of magnesium alloy has received considerable research interest. A hot-rolled AZ31 Mg alloy sheet was subjected to dynamic plastic deformation with the aim of introducing {10–12} twin lamellar structure. It has been found that higher strength and better ductility are obtained when tensile loading is perpendicular to the c axis of twin region of the twin lamellar structured sample, indicating that the plasticity improvement caused by twins depends on the special strain path. The fracture morphology of the twin lamellar structured sample shows a dimple fracture mode under tensile loading perpendicular to the c axis, while the cleavage fracture with river pattern has been observed in other fractured samples. Above experimental results indicate that the interaction of dislocations and twin lamellae may play an important role in improving mechanical properties of Mg alloy.

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    Role of β Phase during Microarc Oxidation of Mg Alloy AZ91D and Corrosion Resistance of the Oxidation Coating
    Yanqiu Wang, Xiaojun Wang, Tao Zhang, Kun Wu, Fuhui Wang
    J. Mater. Sci. Technol., 2013, 29 (12): 1129-1133.  DOI: 10.1016/j.jmst.2013.10.014
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    Selective growth of oxidation coating was observed on Mg alloy AZ91D when this alloy was treated by microarc oxidation (MAO) technique, and then the role of intermetallic phase Mg17Al12 (β phase) during MAO was investigated. Corrosion resistance and anti-corrosion mechanism of the MAO coating were also studied. Optical microscopy and scanning electron microscopy (SEM) were used to characterize β phase and coating microstructure. Corrosion properties of the coated alloy were studied by potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. Results showed that sparking discharge preferentially occurred on α-Mg phase rather than on β phase at the early stage of MAO; however selective growth of the coating disappeared gradually with the increasing oxidation time and β phase would not further inhibit coating growth at the prolonged stage of MAO. Mg17Al12 phase ultimately was unable to destroy the integrity and continuity of MAO coating. The MAO coating could restrain charge transfer process and then greatly enhance corrosion resistance of AZ91D alloy. Sealing treatment of MAO coating by stearic acid could further improve the corrosion resistance of AZ91D alloy.

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    Influence of Substrate Temperature on Stress and Morphology Characteristics of Co Doped ZnO Films Prepared by Laser-Molecular Beam Epitaxy
    Yunyan Liu, Shanying Yang, Gongxiang Wei, Jiaoqing Pan, Yuzhen Yuan, Chuanfu Cheng
    J. Mater. Sci. Technol., 2013, 29 (12): 1134-1138.  DOI: 10.1016/j.jmst.2013.10.005
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    Zn1–xCoxO (x = 0.05) thin films are deposited on sapphire (0001) substrates by laser-molecular beam epitaxy technique at different substrate temperatures. The structural, stress and morphology evolution features are investigated by means of X-ray diffraction and atomic force microscopy. The surface parameters of roughness exponent α, root mean square (RMS) roughness w and autocorrelation length ξ are calculated and the surface parameters are preliminarily analyzed. The values of α vary from 0.7 to 0.9. The RMS roughness w is less than 2.2 nm, and it increases with increasing Ts from 300 to 400 °C, and then decreases when Ts is 500 °C. The autocorrelation length ξ decreases monotonously with the increase in Ts from 300 to 500 °C, which indicates that the increase in Ts restrains the spread of the surface fluctuations until Ts is higher than 400 °C.

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    Fabrication and Characterization of Undoped and Cobalt-doped ZnO Based UV Photodetector Prepared by RF-sputtering
    Husam S. Al-Salman, M.J. Abdullah
    J. Mater. Sci. Technol., 2013, 29 (12): 1139-1145.  DOI: 10.1016/j.jmst.2013.10.007
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    Undoped and 1 at.% Co-doped ZnO nanostructure based UV photodetectors were successfully fabricated by RF-magnetron sputtering technique with comb like Pt electrodes. Cobalt ions were successfully incorporated into the lattice of the ZnO nanostructure without changing its wurtzite structure. It was indicated that Co-doping can effectively adjust the luminescence properties of the ZnO nanostructure. The undoped and Co-doped ZnO photodetectors were observed to have photosensitivities of 1.44 × 104 % and 8.57 × 102 % and low dark currents of 9.74 × 10−8 A and 1.18 × 10−7 A, respectively.

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    Fabrication and Corrosion Resistance of SiC-coated Multi-walled Carbon Nanotubes
    Ning Song*, Hong Liu, Yongtao Yuan, Xing Li, Jingzhong Fang
    J. Mater. Sci. Technol., 2013, 29 (12): 1146-1150.  DOI: 10.1016/j.jmst.2013.10.006
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    Multi-walled carbon nanotubes (MWNTs) were coated with nanometer sized SiC layer by magnetron sputtering, and their corrosion resistances in oxygen and gaseous silicon were studied, respectively. X-ray diffraction (XRD) reveals that the as-coated MWNTs are comprised of the amorphous SiC and the phase could transfer from amorphous to polycrystalline SiC by annealing at 1360 °C. Scanning electron microscopy (SEM) observation indicates that SiC nanoparticles are uniformly coated on the MWNTs, forming a continuous SiC coating. Thermogravimetric analysis (TGA) confirms that the onset temperature of oxidation increases from 540 to 700 °C with the SiC coating. The morphologies of non-coated and SiC-coated MWNTs after corrosion by oxygen and gaseous silicon were also characterized by SEM. The results revealed that SiC coating could protect MWNTs from the corrosion with gaseous silicon effectively.

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    Morphological, Optical and DC Conduction Properties of a-GaSe Semiconductor Nanoparticle Thin Films
    Sushil Kumar, M.A. Majeed Khan
    J. Mater. Sci. Technol., 2013, 29 (12): 1151-1155.  DOI: 10.1016/j.jmst.2013.09.016
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    Amorphous gallium selenide (a-GaSe) semiconductor nanoparticle thin films were deposited onto well cleaned glass substrates by inert gas condensation (IGC) technique under a vacuum of 400 × 10−6 Pa (3 × 10−6 Torr). The films were characterized by different structural and optical techniques, including X-ray diffraction, field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), UV–visible absorption spectroscopy and IV measurements. The particle size and size distribution were determined by TEM images which show the presence of spherical particles in the range of 5–50 nm in size. SEM images indicate that the a-GaSe film grown on glass substrate is almost smooth and dense. The optical properties of a-GaSe nanoparticle thin films were determined by optical absorption spectra. The optical bandgap of the film was estimated to be 2.19 eV and the transitions are allowed direct type. The electrical conductivity of the deposited films has been studied as a function of temperature. In the higher temperature range the dominance of thermally activated band conduction was observed; whereas in the low temperature range the hopping conduction in the band tails of localized states was found to be dominated.

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    Large-scale Growth of Copper Oxide Nanowires on Various Copper Substrates
    Yumei Yue, Mingji Chen, Yang Ju, Shuai Wang
    J. Mater. Sci. Technol., 2013, 29 (12): 1156-1160.  DOI: 10.1016/j.jmst.2013.04.017
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    Large-scale growth of copper oxide nanowires was realized on surfaces of various copper-containing substrates, including copper grids, high-purity copper foils, and small copper blocks, by the stress-induced method. A relatively low heating temperature of 340 °C was demonstrated to give rise to dense nanowire growth with fine crystal structures and high aspect ratio of approximately 300. Gradual cooling process, which is positive for the growth of nanowires on multi-layer substrates, is shown to have no effect on the nanowire growth on other pure copper substrates. Diameter of as-obtained nanowires is mainly dependent on the heating temperature. Moreover, the nanowires growing on copper grids are much longer than those growing on two other substrates.

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    First-principles Study of Point Defects in Stoichiometric and Non-stoichiometric Y4Al2O9
    Z. Li, B. Liu, J.M. Wang, L.C. Sun, J.Y. Wang, Y.C. Zhou, Z.J. Hu
    J. Mater. Sci. Technol., 2013, 29 (12): 1161-1165.  DOI: 10.1016/j.jmst.2013.10.004
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    By using the first-principles calculation, we studied the mechanisms of point defects in Y4Al2O9 (YAM), a promising ternary oxide with excellent optical and thermal properties. It is found that the predominant native defect species is closely dependent on the chemical potentials of each constituent. In the case of O-rich condition, the oxygen interstitial has the very low defect formation energy, followed by the anti-site defects and Al vacancy; in the case of Al-rich condition, the oxygen vacancy yields the lowest defect formation energy, followed by the anti-site defects and Al interstitial. The present result shows that in all the possible chemical potential ranges, anti-site defects have relatively low defect formation energy and might exist in high concentration in YAM. Furthermore, AlY anti-site has relatively lower defect formation energy than the YAl anti-site throughout. The behaviors of defect complexes under non-stoichiometric condition, such as the Al2O3 or Y2O3 excess, are also investigated. The results provide helpful guide to optimize the experimental synthesizing of YAM.

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    Enhancement of Electric Conductivity in Transparent GlasseCeramic Nanocomposites of Bi2O3eBaTiO3 Glasses
    A.A. Bahgat, M.G. Moustafa, E.E. Shaisha
    J. Mater. Sci. Technol., 2013, 29 (12): 1166-1176.  DOI: 10.1016/j.jmst.2013.08.005
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    The frequency and temperature dependent electrical conductivity measurements for heat-treated binary glass system with composition of (100–x)Bi2O3xBaTiO3 (x = 20, 30, 40 and 50, in mol%) were carried out. The glass was prepared by melt quenching technique and their corresponding glass–ceramic nanocomposites were obtained by suitable heat treatment. Nanostructured behavior and electrical properties of these glasses and their corresponding glass–ceramic nanocomposites were studied. X-ray diffraction (XRD) and differential scanning calorimetry confirmed the amorphous nature of the glasses. Moreover, XRD patterns of the samples indicate nanocrystallites embedded in the glass matrix. The Fourier transform infrared spectroscopy (FT-IR) spectral analysis showed that the band positions of glass system are within the wave number range of BiO6, BiO3 and TiO6 structural units. It is observed that the electrical conductivity is enhanced by 102–103 times in the transparent glass–ceramic nanocomposite phase. With further heat treatment, the conductivity decreased considerably in the stage of glass–ceramic nanocomposite phase as compared with the glassy phase sample. Therefore, partially devitrified phase is more suitable as cathode material in secondary batteries compared to its vitreous or fully crystalline counterpart. The conduction mechanism was confirmed to obey the adiabatic small polaron hopping (SPH). AC conductivity measurements were performed as a function of temperature and frequency, showing a very slow increasing rate at low temperatures and then a fast rate at higher temperatures.

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    Microstructural Investigation of Alloy 617 Creep-ruptured at High Temperature in a Helium Environment
    Gyeong-Geun Lee*, Sujin Jung, Jae-Young Park, Woo-Gon Kim, Sung-Deok Hong, Yong-Wan Kim
    J. Mater. Sci. Technol., 2013, 29 (12): 1177-1183.  DOI: 10.1016/j.jmst.2013.09.024
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    A very high temperature reactor (VHTR) is one of the next-generation nuclear reactors chosen by the Generation IV International Forum. A Ni-base superalloy, Alloy 617, is considered as a primary candidate material for an intermediate heat exchanger (IHX) and hot gas duct (HGD) of the VHTR because of the superior creep resistance at a high temperature above 850 °C. In this study, the microstructures of the specimens creep-ruptured at high temperatures in a helium environment were investigated. The decrease in rupture time was more pronounced with increasing creep temperature. In the specimens crept at 950 °C, the external Cr-oxide layer of the specimens increased in thickness with increasing creep rupture time, and delaminated after a long-time creep. The high creep stress induced a deep penetration of carbide depletion along the grain boundaries at the early stage of the creep test. The creep temperature enhanced the growth rate of the decarburized zone depth clearly, but the temperature effect on the growth of the external oxide and internal oxide was not well understood as the surface reaction and creep stress affected the microstructures complexly.

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    Material Characterization of Ni Base Alloy for Very High Temperature Reactor
    Dong-Jin Kim, Gyeong-Geun Lee, Dae Jong Kim, Su Jin Jeong
    J. Mater. Sci. Technol., 2013, 29 (12): 1184-1190.  DOI: 10.1016/j.jmst.2013.09.022
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    The generation of highly efficient electricity and the production of massive hydrogen are possible using a very high temperature reactor (VHTR) among generation IV nuclear power plants. The structural material for an intermediate heat exchanger (IHX) among numerous components should be endurable at high temperature of up to 950 °C during long-term operation. Impurities inevitably introduced in helium as a coolant facilitate the material degradation by corrosion at high temperature. In the present work, the surface reactions available under controlled impure helium at 950 °C were investigated based on the thermodynamics and the corrosion tests were performed in a temperature range of 850–950 °C during 10–250 h for commercial Alloy 617 as a candidate material for an IHX. Moreover, the mechanical property and microstructure for nickel-based alloys fabricated in laboratory were evaluated as a function of the processing parameters such as hot rolling and heat treatment conditions. From the reaction rate constant obtained from an impure helium control system for a material evaluation, it was predicted that the outer oxide layer thickness, internal oxide depth, and carbide-depleted zone depth reach about 116, 600 and 1000 μm, respectively when Alloy 617 is exposed to an impure helium environment at 950 °C for 20 years. For Ni–Cr–Co–Mo alloy, subsequent annealing and a combination of cold working and subsequent annealing following solution annealing caused increases in the grain boundary carbide coverage and size. The angular distribution of the grain boundary as well as the carbide distribution was also changed leading to a consequent improvement of the mechanical property at 950 °C in air.

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    Microstructure Evolution of in situ (Ti3AlC D Ti5Si3)/Ti3Al Composite Sheet with a Novel Quasi-continuous Chain Reinforcement Distribution Architecture Prepared by Using Roll Bonding and Reaction Annealing
    J.C. Pang, G.H. Fan, X.P. Cui, A.B. Li, L. Geng, Z.Z. Zheng, Q.W. Wang
    J. Mater. Sci. Technol., 2013, 29 (12): 1191-1196.  DOI: 10.1016/j.jmst.2013.10.020
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    The Ti3Al matrix composite sheet with a novel quasi-continuous chain reinforcement distribution was prepared by using roll bonding and subsequent reaction annealing with pure Ti and SiCp/Al foils. The (Ti3AlC + Ti5Si3) reinforcements were produced by in situ reaction of the as-rolled Ti–(SiCp/Al) laminated composite sheet after a two-stage annealing treatment including first annealing at 660 °C and second annealing at 1250 °C. The microstructure evolution during the reaction annealing was investigated. In the first reaction annealing, TiAl3 was formed, and the SiC particles were pushed together due to the Kirkendall effect, and after the second reaction annealing, Ti3Al, Ti3AlC and Ti5Si3 were synthesized. In addition, the reinforcement distribution presents a quasi-continuous chain microstructure in Ti3Al matrix composite sheet.

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    Static Recrystallization and Precipitation Behavior of a Weathering Steel Microalloyed with Vanadium
    Hongyan Wu, Linxiu Du, Zhengrong Ai, Xianghua Liu
    J. Mater. Sci. Technol., 2013, 29 (12): 1197-1203.  DOI: 10.1016/j.jmst.2013.10.030
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    The static recrystallization (SRX) and precipitation behavior of a weathering steel microalloyed with vanadium were investigated through double-pass compression tests under controlled conditions using the MMS-300 thermal-mechanical simulator. The deformation temperatures ranged from 800 °C to 1000 °C, and the inter-pass time from 1 s to 500 s. The simulation results showed that SRX occurred after 5–10 s at the first compression deformation. The softening fraction of SRX was found to increase with increasing the deformation temperature and the pre-strain. However, the softening fraction scarcely changed during the process of strain-induced precipitation. In addition, the kinetics of SRX was described by the Avrami equation, and the Avrami exponent appeared to be closely associated with the deformation temperature. The microstructure evolution was investigated at the initiation and completion of recrystallization. The amount and distribution of the precipitates were analyzed. The relationship between the driving force of SRX and the pinning force of precipitation was discussed. Besides, the recrystallization inhibition was detected at the early stage of precipitation, and the pinning forces were found to be of a magnitude comparable to the driving force. Moreover, the pinning forces were found to increase with the degree of precipitation and reach a peak at the intermediate stage of precipitation, and finally reduce as the particles coarsened.

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    Influence of Mg on Thermoplasticity of High-Temperature Stainless Bearing Steel Cr14Mo4
    Wei Gong, Cheng Wang, Haidong Wang, Zhouhua Jiang
    J. Mater. Sci. Technol., 2013, 29 (12): 1204-1208.  DOI: 10.1016/j.jmst.2013.10.029
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    Through the addition of appropriate amount of Mg (0.01–0.015 wt%) to the stainless bearing steel Cr14Mo4, the high-temperature thermoplasticity of steel was improved. The mechanism has been uncovered that the added Mg plays an important role in refining and uniforming the carbide precipitations in the steel. It has been found that the segregation of trace Mg is the key to improve the dispersed carbide. Moreover, considerable segregation of Mg in steel during annealing was evidenced by the theoretic analysis.

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    Determination of Vapor Pressure of Liquid Copper by Carrier Gas Method
    Y.J. Duan, B. Chen, Y.C. Ma, M. Gao, K. Liu
    J. Mater. Sci. Technol., 2013, 29 (12): 1209-1213.  DOI: 10.1016/j.jmst.2013.11.002
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    The relationship between the vapor pressure of liquid copper and the flow rate of carrier gas argon was discussed, when the carrier gas method was used to determine the vapor pressure of liquid copper at 1892 K. The proper argon flow rate range obtained was 150–500 mL/min and enough evidence was provided to verify the vapor pressure-flow rate of carrier gas relationship at the target temperature. Based on the proper flow rate range, the vapor pressure of liquid copper was measured at 1609–1892 K. The relationship of vapor pressure–temperature obtained by the method of regression analysis can be expressed as: ln(p/Pa) = (25.470 ± 0.903) − (39099.8 ± 1574.5)/T. Further, the thermodynamic properties including the heat of vaporization and the Gibbs free energy of the Cu (l) = Cu (g) reaction were calculated by the vapor pressure obtained over the temperature range covered.

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    Synthesis and Photovoltaic Properties of Poly(5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo-[c][1,2,5]-thiadiazole-9,9-dioctylfluorene)
    Liwei Wang, Feiyao Qing, Yeping Sun, Xiaoyu Li, Haiqiao Wang
    J. Mater. Sci. Technol., 2013, 29 (12): 1214-1218.  DOI: 10.1016/j.jmst.2013.10.015
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    A donor–acceptor type conjugated polymer poly(5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo-[c] [1,2,5]-thiadiazole-9,9-dioctylfluorene) (POTBTF) based on octyloxy-containing benzothiadiazole (OTBT) and octyl-containing fluorine (F) was synthesized by Suzuki coupling reaction. The polymer possesses a narrow bandgap and strong light harvesting ability with excellent thermal stability and reasonable solubility. POTBTF exhibited a broad absorption from 300 to 600 nm with an absorption peak at 543.8 nm. The power conversion efficiency of the polymer solar cell based on POTBTF/PC71BM (1:3, w/w) reached 1.77% with a short-circuit current density of 6.69 mA/cm2, an open-circuit voltage of 0.71 V, and a fill factor of 0.374 under AM 1.5 G irradiation (100 mW/cm2) without annealing or any additives. The results indicate that introducing long alkoxy side chains into benzothiadiazole would not only improve the solubility of the polymers but also maintain the planarity of molecule conformation, and therefore, enhance the performance of photovoltaic devices.

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