Started in 1985 Semimonthly
ISSN 1005-0302
CN 21-1315/TG
Impact factor:6.155

The journal has been awarded the excellent periodical in China, and its articles are covered by SCI, EI, CA, SA, JST, RJ, CSA, MA, EMA, AIA etc., PASCAL web. ISI web of Science,SCOPUS.

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      28 May 2009, Volume 25 Issue 03 Previous Issue    Next Issue
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    Reviews
    Challenges in Atomic-Scale Characterization of High-k Dielectrics and Metal Gate Electrodes for Advanced CMOS Gate Stacks
    Xinhua Zhu,Jian-min Zhu,Aidong Li,Zhiguo Liu,Naiben Ming
    J. Mater. Sci. Technol., 2009, 25 (03): 289-313. 
    Abstract   HTML   PDF (3923KB)

    The decreasing feature sizes in complementary metal-oxide semiconductor (CMOS) transistor technology will require the replacement of SiO2 with gate dielectrics that have a high dielectric constant (high-k) because as the SiO2 gate thickness is reduced below 1.4 nm, electron tunnelling effects and high leakage currents occur in SiO2, which present serious obstacles to future device reliability. In recent years significant progress has been made on the screening and selection of high-k gate dielectrics, understanding their physical properties, and their integration into CMOS technology. Now the family of hafnium oxide-based materials has emerged as the leading candidate for high-k gate dielectrics due to their excellent physical properties. It is also realized that the high-k oxides must be implemented in conjunction with metal gate electrodes to get sufficient potential for CMOS continue scaling. In the advanced nanoscale Si-based CMOS devices, the composition and thickness of interfacial layers in the gate stacks determine the critical performance of devices. To build up a full atomic-scale understanding of high-k gate stacks, including their ultimate electrical properties, a thorough atomic-scale physical analysis of these ultrathin gate stacks are highly required. High-resolution microscopic and spectroscopic methods are central in facilitating high-k gate dielectrics to be integrated in CMOS devices and to continue scaling. In this review, we summarize the strengths and capabilities of several high-resolution electron, ion, and photon-based techniques currently used to characterize the high-k gate dielectrics at atomic scale. Particularly, we review the enormous progresses on characterizing interface behavior and structural evolution in the high-k gate dielectrics by high-resolution transmission electron microscopy (HRTEM), and the related techniques based on scanning transmission electron microscopy (STEM), including high-angle annular dark-field (HAADF) imaging (also known as Z-contrast imaging), electron energy-loss spectroscopy (EELS), and energy dispersive X-ray spectroscopy (EDS). This review is organized into five sections. In the first section, we briefly introduce the working principles of each technique and outline their key features. And then we critically review the advances on microstructural characterization of high-k gate dielectrics at atomic scale by electron microscopy, citing some recent results reported on high

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    Research Articles
    Electrical Properties of Li-doped P-type ZnO Ceramics
    A.H. Salama,F.F. Hammad
    J. Mater. Sci. Technol., 2009, 25 (03): 314-318. 
    Abstract   HTML   PDF (2609KB)

    Li-doped p-type ZnO ceramics were prepared by conventional methods according to the chemical formula Zn1-x LixO2 where x=0.5, 1.0, 1.5 and 2.0 mole fraction, respectively. The crystal structures of the prepared samples were studied by X-ray diffraction analysis. The dielectric properties (including dielectric constant ε'
    and dielectric loss ε") and dc-electrical conductivity [σ(Ω-1·cm-1)] were investigated. The dielectric constant ε' was sharply decreased at the low frequency range and independent on frequency at high frequency range. Otherwise, the dielectric loss "00 varied with frequency and showed absorption peak located from 200 Hz to
    4 kHz and moved to higher frequency as the concentration of Li+ doped increased. It was found that dc-electrical conductivity log¾ varied from -9 to-5 and the energy gap width were calculated by using Arrhenius equation. The p-type conductivity of Li-doped ZnO may be attributed to the formation of a LiZn-Lii donor complex, which is limited by reducing the amount of Lii.

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    Fabrication and Characterization of FeNiCr Matrix-TiC Composite for Polishing CVD Diamond Film
    Yuan Zewei,ZhuJi Jin,Xingwei Ma,Boxian Dong
    J. Mater. Sci. Technol., 2009, 25 (03): 319-324. 
    Abstract   HTML   PDF (900KB)

    Dynamic friction polishing (DFP) is one of the most promising methods appropriate for polishing CVD diamond film with high efficiency and low cost. By this method CVD diamond film is polished through being simply pressed against a metal disc rotating at a high speed utilizing the thermochemical reaction occurring as a result of dynamic friction between them in the atmosphere. However, the relatively soft materials such as stainless steel, cast iron and nickel alloy widely used for polishing CVD diamond film are easy to wear and adhere to diamond film surface, which may further lead to low efficiency and poor polishing quality. In this paper, FeNiCr matrix-TiC composite used as grinding wheel for polishing CVD diamond film was obtained by combination of mechanical alloying (MA) and spark plasma sintering (SPS). The process of ball milling,
    composition, density, hardness, high-temperature oxidation resistance and wear resistance of the sintered piece were analyzed. The results show that TiC was introduced in MA-SPS process and had good combination with FeNiCr matrix and even distribution in the matrix. The density of composite can be improved by mechanical alloying. The FeNiCr matrix-TiC composite obtained at 1273 K was found to be superior to at 1173 K sintering in hardness, high-temperature oxidation resistance and wearability. These properties are more favorable than SUS304 for the preparation of high-performance grinding wheel for polishing CVD diamond film.

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    Effect of Elastic Modulus on Biomechanical Properties of Lumbar Interbody Fusion Cage
    Yue Zhu,Fusheng Li,Shujun Li,Yulin Hao,Rui Yang
    J. Mater. Sci. Technol., 2009, 25 (03): 325-328. 
    Abstract   HTML   PDF (379KB)

    This work focuses on the influence of elastic modulus on biomechanical properties of lumbar interbody fusion cages by selecting two titanium alloys with different elastic modulus. They were made by a new β type alloy with chemical composition of Ti-24Nb-4Zr-7.6Sn having low Young0s modulus ~50 GPa and by a conventional
    biomedical alloy Ti-6Al-4V having Young0s modulus »110 GPa. The results showed that the designed cages with low modulus (LMC) and high modulus (HMC) can keep identical compression load ~9.8 kN and endure fatigue cycles higher than 5×106 without functional or mechanical failure under 2.0 kN axial compression. The
    anti-subsidence ability of both group cages were examined by axial compression of thoracic spine specimens (T9~T10) dissected freshly from the calf with averaged age of 6 months. The results showed that the LMC has better anti-subsidence ability than the HMC (p<0.05). The above results suggest that the cage with low elastic modulus has great potential for clinical applications.

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    Effect of Reactivity of Quick Lime on the Properties of Hydrated Lime Sorbent for SO2 Removal
    H.G. Shin,H. Kim,Y.N. Kim,H.S. Lee
    J. Mater. Sci. Technol., 2009, 25 (03): 329-332. 
    Abstract   HTML   PDF (407KB)

    The hydration of quick lime and the sulfation of hydrated lime were carried out for verification of relationship between the reactivity of quick lime and the properties of hydrated lime as a sorbent. The effect of reactivity of quick lime was investigated with the change of calcination temperature and time. Results obtained showed that the temperature rise during the hydration of quick limes varied from 31 to 69°C with the variation of calcination temperature and time. The specific surface area and the sulfation ability of hydrated lime prepared by hydration of quick lime showed a proportional relationship with the reactivity of quick lime. The hydrated lime which was prepared by hydration of quick lime calcined at 1100°C had the highest reactivity and showed 41.53 m2/g of the speci¯c surface area, 0.16 cm3/g of the pore volume and 87% of the removal effciency for SO2 removal.

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    Influence of Carrier Gas on Analysis of MgO Powders by Laser Ablation Inductively Coupled Plasma Mass Spectrometry
    J.S. Lee,Y.S. Lee,C.J. Park,H.S. Lee,D.S. Lee
    J. Mater. Sci. Technol., 2009, 25 (03): 333-335. 
    Abstract   HTML   PDF (380KB)

    Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been widely adopted for the direct multi-elemental analysis with high sensitivity. Especially analysis of fine ceramics by LA-ICP-MS without time-consuming sample decomposition process has been one of the most expecting application field. Small additive elements in MgO powders were analyzed by LA-ICP-MS. For precise and accurate analysis influence of carrier gas (Ar or He) was presented by the signal intensities of ICP-MS, relative standard deviation of signal intensities and ablated particle size distribution. Ablated particles were collected and analyzed by scanning electron microscopy (SEM) to investigate the particle size distribution, and the ablated sample surface was examined by camscope. In He gas atmosphere, the signal was more stable than in Ar gas atmosphere. The signal intensity was higher in Ar than in He. Examination of ablated particles and sample surface reveals that more particles were generated in Ar atmosphere and the distribution of particle size was larger.

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    Microstructure of Epitaxial La0.7Ca0.3MnO3 Thin Films Deposited by Direct Current Magnetron Sputtering on LaAlO3 Substrate
    Mingguang Wang,Hengqiang Ye
    J. Mater. Sci. Technol., 2009, 25 (03): 336-340. 
    Abstract   HTML   PDF (834KB)

    Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) have been used to study the microstructural properties of La0.7Ca0.3MnO3 films on (001) LaAlO3 substrates prepared by direct current magnetron sputtering technique. The as-grown thin films with different thickness are perfectly coherent with the substrates. The film suffers a tetragonal deformation in the area near the interface between the film and the substrate. With increasing thickness, the film is partially relaxed. It was found that La0.7Ca0.3MnO3 films consist of two types of oriented domains described as: (1) (110)f [001]f||(001)s[100]s and (1¹10)f [001]f||(001)s[100]s and (2) (110)f [001]f||(001)s[010]s and (1¹10)f [001]f//(001)s[010]s. Upon annealing, the film is relaxed by the formation of mis¯t dislocations. Other than mis¯t dislocations, two types of threading dislocations with Burgers vector of <100> and <110> were also identified.

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    Weibull Probability Model for Fracture Strength of Aluminium (1101)-Alumina Particle Reinforced Metal Matrix Composite
    A. Suresh Babu,V. Jayabalan
    J. Mater. Sci. Technol., 2009, 25 (03): 341-343. 
    Abstract   HTML   PDF (498KB)

    In recent times, conventional materials are replaced by metal matrix composites (MMCs) due to their high specific strength and modulus. Strength reliability, one of the key factors restricting wider use of composite materials in various applications, is commonly characterized by Weibull strength distribution function. In the present work, statistical analysis of the strength data of 15% volume alumina particle (mean size 15 μm) reinforced in aluminum alloy (1101 grade alloy) fabricated by stir casting method was carried out using Weibull probability model. Twelve tension tests were performed according to ASTM B577 standards and the test data, the corresponding Weibull distribution was obtained. Finally the reliability of the composite behavior in terms of its fracture strength was presented to ensure the reliability of composites for suitable applications. An important implication of the present study is that the Weibull distribution describes the experimentally measured strength data more appropriately.

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    Si-Al-Ir Oxidation Resistant Coating for Carbon/Carbon Composites by Slurry Dipping
    Min Huang,Kezhi Li,Hejun Li,Qianggang Fu,Yu Wang
    J. Mater. Sci. Technol., 2009, 25 (03): 344-346. 
    Abstract   HTML   PDF (335KB)

    A Si-Al-Ir oxidation resistant coating was prepared for SiC coated carbon/carbon composites by slurry dipping. The phase composition, microstructure and oxidation resistance of the as-prepared Si-Al-Ir coating were studied by XRD (X-ray diffraction), SEM (scanning electron microscopy), and isothermal oxidation test at 1773 K in
    air, respectively. The surface of the as-prepared Si-Al-Ir coating was dense and the thickness was approximately 100 μm. Its anti-oxidation property was superior to that of the inner SiC coating. The weight loss of SiC/Si-Al-Ir coated carbon/carbon composites was less than 5 wt. pct after oxidation at 1773 K in air for 79 h. The local oxidation defects in the coating may result in the failure of the SiC/Si-Al-Ir coating.

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    An Investigation on Hall-Petch Relationship in Electrodeposited Nanocrystalline Cu-Ni-P Alloys
    Haiqing Sun,Yinong Shi
    J. Mater. Sci. Technol., 2009, 25 (03): 347-350. 
    Abstract   HTML   PDF (372KB)

    Nanocrystalline Cu-Ni-P alloys with average grain sizes of 7, 10 and 24 nm were synthesized by means of electrodeposition. The grain size dependences of tensile strength and hardness of the nanocrystalline Cu alloys were investigated. The breakdown of Hall-Petch relation was exhibited in both tensile strength and hardness.

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    Effect of Y Addition on Microstructure and Mechanical Properties of Friction Stir Welded ZK60 Alloy
    G.M. Xie,Z.Y. Ma,L. Geng
    J. Mater. Sci. Technol., 2009, 25 (03): 351-355. 
    Abstract   HTML   PDF (788KB)

    6 mm thick ZK60 and ZK60-Y alloy plates were successfully friction stir welded (FSW) at a tool rotation rate of 1200 r/min and a traverse speed of 100 mm/min. FSW resulted in the dissolution of MgZn2 particles in the ZK60 and the breakup and dispersion of W-phase (Mg3Zn3Y2) particles in the ZK60-Y alloy, thereby leading to a decrease in the hardness of the nugget zone (NZ) for the ZK60 alloy and an increase in the hardness of the NZ for the ZK60-Y alloy, respectively. While two FSW joints exhibited similar joint efficiency (87%{89% of ultimate tensile strengths of the parent materials), the yield strength of the FSW ZK60-Y joint was substantially higher than that of the FSW ZK60 joint. The fracture occurred in the NZ and the heat affected zone for the ZK60 and ZK60-Y joints, respectively, which were consistent with the lowest hardness distribution of the welded joints.

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    Microstructure and Mechanical Properties of Mg-3Al-1Zn-xRE Alloys
    Wei Qiu,Enhou Han,Lu Liu
    J. Mater. Sci. Technol., 2009, 25 (03): 356-360. 
    Abstract   HTML   PDF (1126KB)

    In this work, the influence of element RE on the microstructures and mechanical properties of the hot extuded Mg-3Al-1Zn-xRE alloys (with element RE content of 0.05, 0.1 and 0.2 wt pct) has been investigated and compared. It was found that RE can bring about precipitations phase that is identified as Al11RE3 by X-ray diffraction and transmission electron microscopy (TEM). The grain sizes would not be refined after adding RE element. Al11RE3 phase would increase strength and decrease the ductility. The addition of RE element affects dynamic recrystallized process and even reorientation of recrystallized grains. The results showed that the mechanical properties of AZ31+RE alloy are affected by combination of Al atoms, Mn atoms, Al11RE3 phase and grains orientation. It is important to consider the ratio of RE/Al when designing new Mg-Al-RE alloys.

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    Formation of Re-containing Carbides in a Second Generation Directionally Solidi¯ed Ni Base Superalloy
    Tan Zhao,Dong Wang,Jian Zhang,Guang Chen,Langhong Lou
    J. Mater. Sci. Technol., 2009, 25 (03): 361-364. 
    Abstract   HTML   PDF (907KB)

    The precipitates at grain boundary in a directionally solidified Ni base superalloy after heat treatment, aging at 975°C, and creep rupture test have been characterized. Besides the primary MC carbides and fine particles of  μ phase, the Re-containing M23C6 was observed. The precipitation kinetics revealed that the formation of M23C6 was associated with the dissolution of μ phase and MC carbides. TEM image shows that the continuous precipitation of M23C6 particles effectively hinders the dislocation  movement and strengthens the grain boundaries. The high strength of the alloy suggests that M23C6 carbides are beneficial to the properties although Re as an important matrix strengthening element was consumed.

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    Textural Evolution of AZ31B Magnesium Alloy Sheets Undergoing Repeated Unidirectional Bending at Room Temperature
    Guangsheng Huang,Wei Xu,Guangjie Huang,Hongcheng Li,Bo Song
    J. Mater. Sci. Technol., 2009, 25 (03): 365-369. 
    Abstract   HTML   PDF (902KB)

    Magnesium alloy sheets have poor plasticity and formability due to their strong (0002) basal textural component. In this paper, a new method, repeated unidirectional bending (RUB), was applied to improve the texture of AZ31B magnesium alloy sheets so as to enhance their stamping properties. The samples undergoing RUB were annealed at different temperatures. The mechanical properties, formability, textural components and microstructure of the samples before and after RUB were characterized and compared. It was found that the basal textural component was reduced dramatically by RUB and some other textural components appeared. The formability of the sheets was greatly enhanced after RUB, after which their percentage elongation and Erichsen value were considerably increased. The relationship between the improvement of the formability of the AZ31B magnesium alloy sheets and their textural evolution is discussed.

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    Amorphous Formation in an Undercooled Binary Ni-Si Alloy under Slow Cooling Rate
    Yiping Lu,Gencang Yang,Xiong Li,Yaohe Zhou
    J. Mater. Sci. Technol., 2009, 25 (03): 370-372. 
    Abstract   HTML   PDF (505KB)

    High undercooling up to 392 K was achieved in eutectic Ni70.2Si29.8 alloy melt by using glass fluxing combined with cyclic superheating. A small quantity of amorphous phase was obtained in bulk eutectic Ni70:2Si29:8 alloy when undercooling exceeds 240 K under slow cooling conditions (about 1 K/s). The amorphous phase was
    confirmed by high-resolution transmission electron microscopy and differential scanning calorimetry.

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    Hot Deformation Characteristics for a Nickel-base Superalloy GH742y
    Fuwei Kang. Jianfei Sun,Guoqing Zhang,Zhou Li
    J. Mater. Sci. Technol., 2009, 25 (03): 373-378. 
    Abstract   HTML   PDF (626KB)

    The hot deformation characteristics of as-cast nickel-base superalloy GH742y after hot isostatic pressing (HIP)(hereafter referred to as-cast alloy) have been investigated by hot compression tests in the temperature range of 1050 to 1140°C, strain rate range of 0.01 s-1 to 10 s-1 and strain range of 35% to 50% by means of Gleeble-3500 thermal mechanical simulator. The results show that the as-cast alloy exhibits the poor deformability,and shows wedge-shaped cracking beyond the strain of 35%. At strain rates less than 1.0 s-1, the stress-strain curves exhibit nearly steady-state behavior, while at strain rate of 10 s-1, a yield drop and serrated yielding occur. The activation energy values developed on the basis of the experimental data are divided into three domains. The first domain appears at lower strain rate (≤1.0 s-1) and lower  temperature (≤1080°C), with the lowest mean value of activation energy about 261.4 kJ/mol. The second domain appears at the same strain rate as the first domain, but higher temperature (>1080°C), with the intermediate mean value of activation energy about 328.8 kJ/mol. The third domain appears at higher strain rate (10 s-1) and temperature range of 1050 to 1140°C, with the largest mean value of activation energy about 605.05 kJ/mol. Three different constitutive equations are established in corresponding to domains. Microstructural observations in the third domain reveal non-uniform dynamic recrystallization (DRX) of homogeneous γ phase, which leads to the poor deformability and the highest Q value. In contrast, microstructures in the first domain show fully DRX of homogeneous γ phase, leading to the better deformability and the lowest Q value. It is noted that the grain size increases with the increment of strain rate or temperature. These results suggest that bulk metal working of this material may be carried out in the first domain where fully DRX of  γ homogeneous occurred.

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    Effect of a high magnetic field on the shape of the γ′ precipitates in cast nickel-based superalloy K52
    X. Ren,G.Q. Chen,W.L. Zhou,C.W. Wu,C. Yuan,J.S. Zhang
    J. Mater. Sci. Technol., 2009, 25 (03): 379-382. 
    Abstract   HTML   PDF (416KB)

    The shape change of the  γ' precipitates of cast Ni-based superalloy K52 after aging treatment under a high magnetic field was investigated. The results show that duplex γ'  precipitates are present in the γ  matrix after aging treatment with or without the magnetic field. One is the coarse particles with average size of 500 nm; the other is fine spherical γ'  precipitates with average of 100 nm in diameter. The application of a 10T magnetic field only results in the shape of the coarse γ'  particles changing from spherical to cuboidal when the alloys subjected to the same heat treatments. This shape change was mainly discussed based on the strain energy increase caused by the difference in magnetostriction between the γ matrix and γ'  precipitates. The fine γ'  particles still keep spherical. Further TEM observations shows that a number of γ particles in nano-scale precipitate in the coarse γ'  particles in the specimens treated without the magnetic field. In addition, it was found that the magnetic field caused the decrease of the hardness in the alloy, and the hardness was associated with the field direction.

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    A Quantitative Analysis of Mn Segregation at Partitioned Ferrite/Austenite Interface in a Fe-C-Mn-Si Alloy
    H. Guo,S.W. Yang,C.J.Shang,X.M. Wang,X.L. He
    J. Mater. Sci. Technol., 2009, 25 (03): 383-388. 
    Abstract   HTML   PDF (394KB)

    Mn segregation at austenite/ferrite interface is studied in a Fe-C-Mn-Si alloy held at 656°C. Mn is partitioned during the growth of ferrite and as a result, a Mn pile-up exists in front of interface on the matrix side. An approach to evaluate Mn segregation quantitatively is developed by combining STEM raster window scanning and simulation of the interaction of the electron beam with the sample to subtract the contribution of Mn pile-up and obtain pure Mn segregation value. The evaluated maximum Mn interfacial segregation is in the order of a half-monolayer.

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    Investigation of Surface Damage in Forming of High Strength and Galvanized Steel Sheets
    Zhongqi Yu,Yingke Hou,Haomin Jiang,Xinping Chen,Weigang Zhang
    J. Mater. Sci. Technol., 2009, 25 (03): 389-394. 
    Abstract   HTML   PDF (711KB)

    Powdering/exfoliating of coatings and scratching are the main forms of surface damage in the forming of galvanized steels and high strength steels (HSS), which result in increased die maintenance cost and scrap rate. In this study, a special rectangular pan was developed to investigate the behavior and characteristics of surface damage in sheet metal forming (SMF) processes. U-channel forming tests were conducted to study the effect of tool hardness on surface damage in the forming of high strength steels and galvanized steels (hot-dip galvanized and galvannealed steels). Experimental results indicate that sheet deformation mode influences the severity of surface damage in SMF and surface damage occurs easily at the regions where sheet specimen deforms under the action of compressive stress. Die corner is the position where surface damage initiates. For HSS sheet, surface damage is of major interest due to high forming pressure. The HSS and hot-dip galvanized steels show improved ability of damage-resistance with increased hardness of the forming tool. However, for galvannealed steel it is not the forming tool with the highest hardness value that performs best.

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    Dynamical Solidification Behaviors and Metal Flow during Continuous Semisolid Extrusion Process of AZ31 Alloy
    Renguo Guan,Liqing Chen,Jiangping Li,Fuxing Wang
    J. Mater. Sci. Technol., 2009, 25 (03): 395-400. 
    Abstract   HTML   PDF (1717KB)

    In this paper, a novel near-net-shape forming process, continuous semisolid extrusion process (CSEP) of AZ31 alloy was proposed, and the dynamical solidification behaviors and metal flow during the process were firstly investigated. During casting AZ31 alloy by this process, non-uniform microstructure distributions and non-equilibrium solidification region near the roll surface were found in the roll-shoe gap. Microstructural evolution from dendrite to rosette and spherical grains was observed during the casting by CSEP. Casting temperature, roll-shoe gap width and cooling ability have great effect on casting process and metal flow, so these factors should be carefully controlled, a proper casting temperature of 710×750°C is suggested. The white ® phases were strongly stretched during the processing, and the remnant liquids are correspondingly distributes along the solid phase boundaries and also show stripped lines.

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    βPreparation and Dielectric Properties of Nonstoichiometric β-SiC Powder by Combustion Synthesis
    Xiaolei Su,Wancheng Zhou,Zhimin Li,Fa Luo,Dongmei Zhu
    J. Mater. Sci. Technol., 2009, 25 (03): 401-404. 
    Abstract   HTML   PDF (309KB)

    The nonstoichiometric β-SiC powders were synthesized via combustion reaction of Si and C system in a 0.1 MPa nitrogen atmosphere, using Teflon as the chemical activator. The prepared powders were invistigated by XRD and Raman spectra. The results indicates that the cell parameters of all the prepared β-SiC powder
    are smaller than the standard value of β-SiC because of generation of CSi defects. The complex permittivity of prepared products was carried out in the frequency range of 8.2−12.4 GHz. It shows that the dielectric property of prepared β-SiC powder decrease with increasing PTFE content. The effect of CSi defects on dielectric property of -SiC powder has been discussed.

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    Doping Effects on Electronic Conductivity and Electrochemical Performance of LiFePO4
    Jiezi Hu,Jian Xie,Xinbing Zhao,Hongming Yu,Xin Zhou,Gaoshao Cao,Jiangping Tu
    J. Mater. Sci. Technol., 2009, 25 (03): 405-409. 
    Abstract   HTML   PDF (753KB)

    Olivine-structured pure LiFePO4 and doped Li(M, Fe)PO4 (M=La, Ce, Nd, Mn, Co, Ni) have been synthesized by a solvothermal method. X-ray diffraction and field emission scanning electron microscopy analyses indicate that the as-prepared LiFePO4 is well-crystallized nanopowders without any detectable impurity phases. The electronic conductivity of LiFePO4 is enhanced by around 1-3 orders by doping. It was found that doping alone is not sufficient for the high-rate performance of LiFePO4 and surface coating with such as carbon should be needed. The best dopant for LiFePO4 is Nd among those studied in the present work. Accordingly, doping with 1 mol fraction Nd leads to an increase in 70 mAh/g at 0.1 C for the hydrothermally synthesized sample and 50 mAh/g at 1.0 C after carbon-coating in comparison with the undoped samples.

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    Wetting Behavior and Interfacial Reactions in (Sn-9Zn)-2Cu/Ni Joints during Soldering and Isothermal Aging
    Ning Zhao,Haitao Ma,Haiping Xie,Lai Wang
    J. Mater. Sci. Technol., 2009, 25 (03): 410-414. 
    Abstract   HTML   PDF (541KB)

    The wetting property of (Sn-9Zn)-2Cu (wt pct) on Ni substrate and the evolution of interfacial microstructure in (Sn-9Zn)-2Cu/Ni joints during soldering as well as isothermal aging were studied. The wetting ability of eutectic Sn-9Zn solder on Ni substrate was markedly improved by adding 2 wt pct Cu into this solder alloy. Plate-like Cu5Zn8 intermetallic compounds (IMCs) were detected in (Sn-9Zn)-2Cu solder matrix. A continuous Ni5Zn21 IMC layer was formed at (Sn-9Zn)-2Cu/Ni interface after soldering. This IMC layer kept its type and integrality even after aging at 170°C for up to 1000 h. At the early aging stage (before 500 h), the IMC layer grew fast and its thickness followed a linear relationship with the square root of aging time. Thereafter, however, the thickness increased very slowly with longer aging time. When the joints were aged for 1000 h,
    a new IMC phase, (Cu,Ni)5Zn8, was found in the matrix near the interface. The formation of (Cu,Ni)5Zn8 phase can be attributed to the di®usion of Ni atoms into the solder matrix from the substrate.

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    Mechanism of Metal Transfer in DE-GMAW
    Kehai Li,Chuansong Wu
    J. Mater. Sci. Technol., 2009, 25 (03): 415-418. 
    Abstract   HTML   PDF (367KB)

    Modification of conventional gas metal arc welding (GMAW) process is of great potential to achieve high productivity with low cost and strong usability. Double-Electrode GMAW (DE-GMAW) is such a modified arc welding process which is formed by adding a bypass torch (gas tungsten arc welding torch) to a conventional GMAW system. The mechanism of metal transfer in DE-GMAW was proposed and verified in this paper. Experiments show that the critical current is decreased so that spray transfer can be obtained at a lower current level in DE-GMAW. Analysis of this significant change in metal transfer phenomena is conducted, and explanation is given out. It is found that the bypass arc in DE-GMAW lifts the anode point on the droplets such that the electromagnetic force becomes larger and squeezes the droplets so that spray transfer can take
    place under welding current lower than that in conventional GMAW.

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    First-Principle Calculations of Hardness and Melting Point of Mo2C
    X.R. Wang,M.F. Yan,H.T. Chen
    J. Mater. Sci. Technol., 2009, 25 (03): 419-422. 
    Abstract   HTML   PDF (497KB)

    This paper has constructed two kinds of atomic and electronic models for hexagonal β-Mo2C and orthorhombic α-Mo2C. The optimized lattice parameters, elastic constant matrixes and overlap population for Mo2C crystal cells have been obtained to realize the characterization of the hardness and melting point of the two structures by the first-principles plane wave pseudo potential method based on the density functional theory. The results reveal that the calculated lattice parameters of the Mo2C crystal cells agree with the experimental and other calculated data. The calculated melting point/hardness are 2715 K/11.38 GPa for β-Mo2C and 2699 K/10.57~12.67 GPa for α-Mo2C, respectively. The calculated results from the density of states (DOS) demonstrate that the hybridization effect between Mo-3d and C-2p states in α-Mo2C crystal cell is much
    stronger than that in β-Mo2C one.

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    Microstructure and Properties of M40 Carbon Fibre Reinforced Mg-Re-Zr Alloy Composites
    Gaohui Wu,Meihui Song,Ziyang Xiu,Ning Wang,Wenshu Yang
    J. Mater. Sci. Technol., 2009, 25 (03): 423-426. 
    Abstract   HTML   PDF (520KB)

    M40 carbon fibre reinforced rare earth magnesium alloy ZM6 composites with fibre volume fraction about 60% were fabricated by pressure infiltration method. The microstructure, interfacial morphology, and precipitation were studied by scanning electron microscopy, transmission electron microscopy, and energy dispersive X-
    ray spectrometer. It was shown that the interfaces between Mg alloy and fibres were well bonded and free from cracks. The Mg12Nd phase was preferentially precipitated at the fibre/matrix interfaces, leading to the segregation of Nd at the interfaces and the dramatic decrease of Mg12Nd precipitation in the matrix far from interfaces. Crystal defects such as high-density dislocations and twins were observed in the matrix near the fibre/matrix interface. A high bending strength (1393 MPa) and elastic modulus (190 GPa) were achieved in M40/ZM6 composite.

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    Growth and Photoluminescence Properties of Tetrapod-Shaped ZnO Microcrystals-Whiskers and Microrods
    Xiaojun Liang,Bingfa Liu,Nan Chen
    J. Mater. Sci. Technol., 2009, 25 (03): 427-432. 
    Abstract   HTML   PDF (782KB)

    Tetrapod-shaped ZnO microcrystals in forms of whiskers and microrods have been grown in the same crucible by thermal evaporation of Zn/C mixtures at a temperature of 930°C in air without using any catalyst. The tetrapod-shaped ZnO microrods were capped by hexagonal pyramids. It is for the first time observed that the tetrapod-shaped ZnO whiskers and microrods have quite different morphologies, and this is believed to be a result of different growth behaviors associated with these two forms of ZnO microcrystals. The octa-twin model has been used to discuss their growth behaviors. Photoluminescence properties of these two forms of tetrapod-shaped ZnO microcrystals have been investigated using different excitation wavelengths. Both of the two forms of ZnO microcrystals showed strong green emission and weak ultraviolet emission behaviors. The excitation spectrum of the tetrapod-shaped ZnO whiskers showed a strong excitation peak at 395 nm, which was not observed for the tetrapod-shaped ZnO microrods.

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ISSN: 1005-0302
CN: 21-1315/TG
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