Xun Cai

Shanghai Jiao Tong University, Shanghai, Shanghai Shi, China

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Publications (59)121.99 Total impact

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    ABSTRACT: Aluminum bipolar plates offer good mechanical performance and availability for mass production while allow up to 65% lighter than stainless steel. To improve the corrosion resistance and surface electrical conductivity of aluminum bipolar plates, several coatings, including TiN, CrN, C, C/TiN and C/CrN, are deposited on aluminum alloy 5052 (AA-5052) by close field unbalanced magnetron sputter ion plating. Scanning electron microscope (SEM) results show that the coatings containing carbon layer are denser than TiN and CrN. Although the potentiodynamic test results show improved corrosion resistance by all the coatings, the potentiostatic test results reveal different stability of these coatings in PEMFC environments. Comparing the SEM images of these coatings after potentiostatic test, C/CrN multilayer coating exhibits the best stability. C/CrN multilayer coated AA-5052 has the lowest metal ion concentration after potentiostatic test, being 11.12 ppm and 1.29 ppm in PEMFC cathodic and anodic environments, respectively. Furthermore, the interfacial contact resistance (ICR) of the bare AA-5052 is decreased from 61.58 m Omega-cm(2) to 4.08 m Omega-cm(2) by C/CrN multilayer coating at the compaction force of 150 N-cm(-2). Therefore, C/CrN multilayer coating is a good choice for surface modification of aluminum bipolar plate. Crown Copyright
    International Journal of Hydrogen Energy 05/2014; 39(16):8421–8430. DOI:10.1016/j.ijhydene.2014.03.136 · 2.93 Impact Factor
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    ABSTRACT: Corrosion resistance and electrical properties of carbon/chromium-titanium-nitride multilayer coatings on stainless steel, Journal of Power Sources (2013), doi: 10.1016/j.jpowsour.2013.10.103. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
    Journal of Power Sources 10/2013; 249. DOI:10.1016/j.jpowsour.2013.10.103 · 6.22 Impact Factor
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    ABSTRACT: Carbon films are deposited on 304 stainless steel (SS304) by close field unbalanced magnetron sputter ion plating using different substrate bias voltages and target currents to improve the corrosion resistance and electrical conductivity of bipolar plates made of SS304 in proton exchange membrane fuel cells (PEMFCs). The surface morphology, Raman scattering spectra, corrosion resistance, interfacial contact resistance (ICR), and contact angle with water of the carbon films are determined. A dense carbon film is produced on the SS304 by this technique and the corrosion resistance is improved significantly. The ICR value diminishes drastically and water contact angle increases after deposition. In addition, the passive current density in the simulated PEMFC environment decreases initially, increases as the substrate bias voltage is increased, and drops with decreasing target current. As the substrate bias is increased, the ICR between the carbon film and carbon paper exhibits an initial diminishing trend and then increases, but the effect of the target current on the ICR is not as substantial as that of the bias voltage.
    Thin Solid Films 03/2013; 531:320–327. DOI:10.1016/j.tsf.2013.01.036 · 2.13 Impact Factor
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    ABSTRACT: The two most important issues that plague wider use of stainless steel bipolar plates in polymer electrolyte membrane fuel cells (PEMFCs) are insufficient corrosion resistance and surface conductivity. In this study, C/CrN multilayer coatings are deposited on 316L stainless steel samples by close-field unbalanced magnetron sputtering ion plating. SEM shows that the C/CrN coatings are dense, continuous, and composing of carbon granules on the surface. Raman spectroscopy reveals an amorphous structure with a large sp2 constituent. The corrosion resistance and interfacial contact resistance (ICR) are investigated systematically. A superior ICR in the range of 2.6–2.9 mΩ-cm2 at a compaction force of 150 N/cm2 is achieved and it is even better than that of graphite. The deposited film possesses high chemical inertness thereby significantly enhancing the corrosion resistance of the coated SS316L. A thickness of 800 nm is sufficient to protect against corrosion. C/CrN multilayer coatings are beneficial in that it can lead to a faster PVD deposition process and lower material cost, while permitting a superior performance in terms of surface conductivity and corrosion resistance.
    Journal of Power Sources 01/2013; 222:351–358. DOI:10.1016/j.jpowsour.2012.08.087 · 6.22 Impact Factor
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    ABSTRACT: The effects of dual Ti and C ion implantation on the corrosion resistance and surface conductivity of 316L stainless steel (SS) bipolar plates used in polymer electrolyte membrane fuel cells (PEMFC) are investigated. The ion-implanted SS bipolar plates are assembled into single cells to evaluate the performance and improvement. X-ray photoelectron spectroscopy (XPS) confirms the presence of the implanted layer and high-resolution transmission electron microscopy (HR-TEM) discloses an amorphous phase together with an underlying layer with fine grains on the surface of the Ti0.5C0.5 specimen due to ion collision. On the other hand, the surface microstructure of Ti0.5C2 consists of a precipitated nanophase layer, an amorphous underlying layer, and a zone with refined grains between the implanted surface and substrate. The interfacial contact resistance (ICR) of Ti0.5C0.5 is reduced by 6.5 times due to the Ti and C enriched surface and amorphous and fine grain underlayer. Electrochemical characterization and scanning electron microscopy (SEM) show reduced current density and improved corrosion resistance as a result of implantation of a proper ion fluence. According to the single cell evaluation, the peak power density of the ion-implanted bipolar plate increases from 566.5 mW cm− 2 to 709.8 mW cm− 2 and the power density at 0.6 V increases by about 50% compared to those measured from the single cell assembled with unimplanted SS bipolar plates.
    Surface and Coatings Technology 02/2012; 206(s 11–12):2914–2921. DOI:10.1016/j.surfcoat.2011.12.021 · 2.20 Impact Factor
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    ABSTRACT: The desirable properties of metallic bipolar plates in polymer electrolyte membrane fuel cells are good corrosion resistance and high electrical conductance. In this study, carbon-implanted SS316L stainless steel bipolar plates are evaluated by various ex situ and in situ methods. X-ray photoelectron spectroscopy and transmission electron microscopy reveal a carbon-enriched layer with a thickness of about 240nm thick. The structure depends on the ion implantation fluence. The interfacial contact resistance and electrochemical behavior are determined using ex situ techniques. The interfacial contact resistance decreases with increasing ion implantation fluence. The results obtained by potentiodynamic tests, potentiostatic tests, and inductively coupled plasma optical emission spectrometry measurements are consistent with each other confirming that the corrosion resistance is significantly improved after carbon ion implantation. The carbon-implanted stainless steel bipolar plates are assembled into single cells to undergo in situ evaluation. The peak power density of the carbon-implanted bipolar plate increases from 566.5mWcm−2 to 840.0mWcm−2 and the power density at 0.6V increases by a factor of two compared to those measured from a single cell made of unimplanted stainless steel bipolar plates.
    Journal of Power Sources 02/2012; 199. DOI:10.1016/j.jpowsour.2011.09.095 · 6.22 Impact Factor
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    ABSTRACT: Carbon film has been deposited on 304 stainless steel (SS304) using close field unbalanced magnetron sputter ion plating (CFUBMSIP) to improve the corrosion resistance and electrical conductivity of SS304 acting as bipolar plates for proton exchange membrane fuel cells (PEMFCs). The corrosion resistance, interfacial contact resistance (ICR), surface morphology and contact angle with water of the bare and carbon-coated SS304 are investigated. The carbon-coated SS304 shows good corrosion resistance in the simulated cathode and anode PEMFC environment. The ICR between the carbon-coated SS304 and the carbon paper is 8.28–2.59mΩcm2 under compaction forces between 75 and 360Ncm−2. The contact angle of the carbon-coated SS304 with water is 88.6°, which is beneficial to water management in the fuel cell stack. These results indicate that the carbon-coated SS304 exhibits high corrosion resistance, low ICR and hydrophobicity and is a promising candidate for bipolar plates.
    Journal of Power Sources 12/2011; 196(23):10032-10037. DOI:10.1016/j.jpowsour.2011.08.050 · 6.22 Impact Factor
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    ABSTRACT: The electrochemical behavior and change in the passive film formation of SS316L are investigated under polymer electrolyte membrane fuel cell (PEMFC) simulated (pH from 3 to 6 containing F−, SO42− and Cl− anions) and accelerated conditions (0.5 M and 1 M H2SO4 + 2 ppm HF). Potentiodynamic, potentiostatic, and EIS measurements are performed to investigate the electrochemical behavior of the SS316L specimens in both the anode and cathode PEMFC environments. The chemical composition of the passive film, surface topography of the specimens, and degree of metal ion release is characterized by XPS, SEM, and ICP-OES, respectively. The results reveal that the nature of the passive film depends on the pH value, external medium/environment, as well as applied potential during polarization. The corrosion behavior of SS316L is closely related to the chemical composition and structure of the passive film.
    International Journal of Hydrogen Energy 10/2011; 36(20):13032-13042. DOI:10.1016/j.ijhydene.2011.07.058 · 2.93 Impact Factor
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    ABSTRACT: The elastic properties of a thermally sprayed coating were determined by a three-point bending (3PB) test and nanoindentation, but different results were obtained by the two methods. The microstructure revealed by scanning electron microscopy shows the existence of pores which decrease the overall resistance of the coating to elastic deformation. Therefore, the overall mechanical properties determined by the 3PB test are influenced by pores. On the other hand, the mechanical properties of the materials determined by nanoindentation are rarely influenced by pores, as indentations are normally made far away from pores. As a result, the Young’s modulus determined by the 3PB test is lower than that determined by nanoindentation. Selection of the method to determine the Young’s modulus of the coating must be carried out with careful consideration. KeywordsThree-point bending–Nanoindentation–Young’s modulus–Thermally sprayed coating
    Journal of Coatings Technology and Research 05/2011; 8(3):355-361. DOI:10.1007/s11998-010-9313-y · 1.28 Impact Factor
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    ABSTRACT: The major concerns of bipolar plates in polymer electrolyte membrane fuel cells (PEMFCs) are their corrosion resistance and interfacial contact resistance (ICR). Silver is well known for its excellent conductivity and good corrosion resistance. In this study, the performance of austenitic stainless steel 316L (SS316L) implanted with Ag is evaluated in the simulated polymer electrolyte membrane fuel cell (PEMFC) environment. The potentiodynamic test reveals that Ag implant improves the corrosion resistance of SS316L. The corrosion potential of SS316L shifts towards the positive direction from −0.30 V vs SCE to −0.04 V vs SCE in the anode environment and the passivation current density at 0.6 V is reduced from 11.26 μA cm−2 to 8.25 μA cm−2 in the cathode environment. The potentiostatic tests reveal a significant decrease from 10 μA cm−2 to 0.7 μA cm−2 after Ag implantation. Furthermore, the chemical stability in the simulated cathode environment and conductivity are improved after Ag implantation. The beneficial effects can be attributed to the addition of silver to the surface and reduction in the passive layer thickness caused by the ion implantation.Research highlights▶ Ion implantation decreases the thickness of the passive layer and the silver-rich layer is about 45 nm thick. ▶ The corrosion resistance and polarization resistance are improved by the silver-rich surface layer which is chemically stable in the simulated PEMFC cathode environment. ▶ Silver ion implantation decreases the ICR of SS316L substantially due to reduction in the passive layer thickness and increased amounts of silver and nickel. ▶ Introduction of silver by ion implantation can improve the performance of stainless steel bipolar plates while needing a small dose of silver compared to other coating methods.
    Materials Chemistry and Physics 03/2011; 126(1-2):6-11. DOI:10.1016/j.matchemphys.2010.11.029 · 2.13 Impact Factor
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    ABSTRACT: The effects of titanium ion implantation on the surface chemical composition, roughness, and microstructure, as well as corrosion mechanism of 316L stainless steel in acid solution are investigated. The bare SS316L has a polycrystalline structure containing second-phase inclusions. These defects are prone to pitting corrosion and intergranular corrosion as observed by SEM. After Ti ion implantation, the outer surface is completely amorphized and homogenized and the region underneath is partly disordered. Consequently, localized corrosion is avoided and the sample undergoes general corrosion. With increasing ion implantation voltages, the surface roughness increases and the corrosion resistance decreases due to more extensive radiation damage as result of energetic ion bombardment.
    Materials Letters 01/2011; 68. DOI:10.1016/j.matlet.2011.11.014 · 2.27 Impact Factor
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    ABSTRACT: The desirable properties of the metallic bipolar plates in polymer electrolyte membrane fuel cells (PEMFC) are good corrosion resistance, high electrical conductance, hydrophobicity, and low cost. In this study, carbon films are deposited on stainless steel 316L (SS316L) samples by close field unbalanced magnetron sputtering. The AFM, SEM, and Raman results show that the carbon film is dense, continuous, and amorphous. The corrosion resistance, hydrophobicity, and interfacial contact resistance (ICR) of the carbon coated steel are investigated and compared to those of uncoated SS316L. The deposited carbon film has high chemical inertness thereby significantly enhancing the corrosion resistance of the coated SS316L. Furthermore, the carbon coated SS316L is more hydrophobic and the resulting ICR is elevated to that of graphite. Our results indicate that the properties of the carbon coated SS316L are better than those of conventional graphite bipolar plates.
    Diamond and Related Materials 11/2010; 19(11):1354-1361. DOI:10.1016/j.diamond.2010.07.003 · 1.57 Impact Factor
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    ABSTRACT: Nitrogen plasma immersion ion implantation (PIII), a non-line-of-sight surface treatment technique suitable for bipolar plates in polymer electrolyte membrane fuel cells, is conducted at low and high temperature to improve the corrosion resistance and conductivity of titanium sheets. X-ray photoelectron spectroscopy (XPS) shows that high-temperature (HT) nitrogen PIII produces a thick oxy-nitride layer on the titanium surface. This layer which provides good corrosion resistance and high electrical conductivity as verified by electrochemical tests, inductively coupled plasma optical emission spectroscopy, and interfacial contact resistance (ICR) measurements renders the materials suitable for polymer electrolyte membrane fuel cells. In comparison, the low-temperature (LT) PIII titanium sample exhibits poorer corrosion resistance and electrical conductivity than the untreated titanium control.
    Journal of Power Sources 10/2010; 195(19):6798-6804. DOI:10.1016/j.jpowsour.2010.04.053 · 6.22 Impact Factor
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    ABSTRACT: The corrosion behavior and interfacial contact resistance (ICR) of niobium implanted SS316L used as the bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC) are investigated. The ICR values of the bare and niobium implanted SS316L are measured to evaluate the electrical conductivity. The effects of ion implantation on the corrosion behavior are investigated by potentiodynamic and potentiostatic tests in the simulated PEMFC anode and cathode environments. The solutions after the potentiostatic test are analyzed by inductively-coupled plasma atomic emission spectrometry (ICP-AES). The surface topography of the samples before and after the potentiostatic test is monitored by SEM in order to investigate the mechanism and degree of corrosion. The XPS results indicate that the composition on the surface is altered by ion implantation. The electrochemical results reveal that the passivation current density of the Nb implanted SS316L decreases and has higher chemical stability in the simulated PEMFC environment. However, the ion implantation fluence affects the current density. The ICP results are in agreement with those of the electrochemical test disclosing that the bare SS316L has the highest dissolution rate in both the cathode and anode environments and niobium implantation reduces the dissolution rate significantly. SEM shows that the bare SS316L undergoes serious corrosion whereas after Nb ion implantation, corrosion is greatly retarded. The XPS depth profiles indicate that a passive film with a new composition consisting mainly of niobium oxide is formed after the potentiostatic test. Our results suggest that niobium implantation with proper ion fluences can significantly improve the corrosion resistance and the electric conductivity of SS316L in the simulated PEMFC environments.
    Surface and Coatings Technology 09/2010; 205(1):85-91. DOI:10.1016/j.surfcoat.2010.06.009 · 2.20 Impact Factor
  • Xun Cai, Kai Feng
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    ABSTRACT: In the present work, austenitic stainless steel 316L (SS316L) samples were implanted with Ni and Ni-Cr. A nickel-rich layer about 100 nm in thickness and a Ni-Cr enriched layer about 60 nm thick are formed on the surface of SS316L. The effects of ion implantation on the corrosion performance of SS316L are investigated in a 0.5 M H2SO4 with 2 ppm HF solution at 80°C by open circuit potential (OCP), potentiodynamic and potentiostatic tests. The samples after the potentiostatic test are analyzed by XPS. The results indicate that the composition of the passive film change from a mixture of Fe oxides and Cr oxide to a Cr oxide dominated passive film after the potentiostatic test. The solutions after the potentiostatic test are analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The results reveal that Fe is selectively dissolved in all cases and a proper Ni and Ni-Cr implant fluence can greatly improve the corrosion resistance of SS316L in the simulated polymer electrolyte membrane fuel cells (PEMFCS) environment. They are in agreement with the electrochemical test results that the bare SS316L has the highest dissolution rate in both cathode and anode environments and the Ni and Ni-Cr implantation reduce markedly the dissolution rate. After the potentiostatic test the interfacial contact resistance (ICR) values are also measured. Ni and Ni-Cr are enriched in the passive film formed in the simulated PEMFC cathode environment after ion implantation thereby providing better conductivity than that formed in the anode one. A significant improvement of ICR is achieved for the SS316L implanted with Ni and Ni-Cr as compared to the bare SS316L, which is attributed to the reduction in passive layer thickness caused by Ni and Ni-Cr implantation. The ICR values for implanted specimens increase with increasing dose.
    Journal of Solid Mechanics and Materials Engineering 01/2010; 4(7):918-930. DOI:10.1299/jmmp.4.918
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    ABSTRACT: The interfacial adhesion measurement of a ceramic coating on a metal substrate is studied by three-point bending (3PB) technique. In the measurement, interfacial cracks are induced during the 3PB test, and the interfacial energy release rate is calculated from the released energy per unit crack surface area during crack extension under the fixed displacement conditions. A finite element analysis (FEA) model encompassing the plastic behavior of the metal substrate is developed to simulate the 3PB test and extract the energy data. The inputs to the FEA model include the crack length, the maximum and critical loads corresponding to crack initiation, and the mechanical properties of the coating and substrate. A MoB/CoCr ceramic coating/stainless steel substrate system is investigated by the technique for demonstrating the utility of the technique. KeywordsInterfacial adhesion-Coating-Three-point bending-Energy release rate-Finite element analysis
    Journal of Coatings Technology and Research 01/2010; 7(3):391-398. DOI:10.1007/s11998-009-9192-2 · 1.28 Impact Factor
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    ABSTRACT: A Ni–Cr enriched layer about 60 nm thick with improved conductivity is formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The electrochemistry results reveal that a proper Ni–Cr implant fluence can greatly improve the corrosion resistance of SS316L in the simulated PEMFC environment. The samples after the potentiostatic test are also analyzed by XPS and the ICR values are measured. The XPS results indicate that the composition of the passive film change from a mixture of Fe oxides and Cr oxide to a Cr oxide dominated passive film after the potentiostatic test. Hence, the ICR increases after polarization due to depletion of iron in the passive film. Nickel is enriched in the passive film formed in the simulated PEMFC cathode environment after ion implantation thereby providing better conductivity than that formed in the anode one.
    International Journal of Hydrogen Energy 01/2010; 35(2):690-700. DOI:10.1016/j.ijhydene.2009.10.106 · 2.93 Impact Factor
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    ABSTRACT: Amorphous carbon (a-C) film about 3 μm in thickness is coated on 316L stainless steel by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The AFM and Raman results reveal that the a-C coating is dense and compact with a small size of graphitic crystallite and large number of disordered band. Interfacial contact resistance (ICR) results show that the surface conductivity of the bare SS316L is significantly increased by the a-C coating, with values of 8.3–5.2 mΩ cm2 under 120–210 N/cm2. The corrosion potential (Ecorr) shifts from about −0.3 V vs SCE to about 0.2 V vs SCE in both the simulated anode and cathode environments. The passivation current density is reduced from 11.26 to 3.56 μA/cm2 with the aid of the a-C coating in the simulated cathode environment. The a-C coated SS316L is cathodically protected in the simulated anode environment thereby exhibiting a stable and lower current density compared to the uncoated one in the simulated anode environment as demonstrated by the potentiostatic results.
    International Journal of Hydrogen Energy 08/2009; 34(16):6771-6777. DOI:10.1016/j.ijhydene.2009.06.030 · 2.93 Impact Factor
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    ABSTRACT: Copper which has many favorable properties such as low cost, high thermal and electrical conductivity, as well as easy fabrication and joining is one of the main materials in lead frames, interconnects, and foils in flexible circuits. Furthermore, copper is one of the best antibacterial materials. However, unlike aluminum oxide or chromium oxide, the surface copper oxide layer does not render sufficient protection against oxidation. In this work, in order to improve the surface oxidation resistance of Cu, Al and N were introduced into copper by plasma immersion ion implantation (PIII) and beam-line ion implantation (BII). The implantation fluences of Al and N were 2×1017ionscm−2 and 5×1016ionscm−2, respectively. The implanted and untreated copper samples were oxidized in air at 260°C for 1h. The X-ray diffraction (XRD), scanning electron microscopy (SEM), as well as X-ray photoelectron spectroscopy (XPS) results indicate that both implantation methods can enhance the oxidation resistance of copper but to different extent. PIII is superior to BII in enhancing the oxidation resistance of copper. The effects and possible mechanisms are discussed.
    Materials Chemistry and Physics 08/2009; 116(2):519-522. DOI:10.1016/j.matchemphys.2009.04.023 · 2.13 Impact Factor
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    ABSTRACT: The in situ growth of ZnO nanosheets has been demonstrated by our research group and the excellent electrical contact between the ZnO nanosheets and brass substrate enables many potential applications in gas sensing and photocatalytic degradation. However, problems arising from corrosion, especially that arising from chloride ions, are inevitable in the field. In this work, the corrosion behavior of these ZnO nanosheets is investigated in chloride solutions. Our results show that in a NaCl solution, the chloride ions can react with ZnO to form ZnCl2. When the NaCl concentration is relatively low (1wt%), the structure exhibits a strong passivation behavior but a higher concentration of chloride ions can accelerate the transformation from ZnO to ZnCl2. The results also disclose that a high concentration of NaCl weakens the passivation performance and when the concentration reaches 3wt%, the passivation ability vanishes completely.
    Materials Chemistry and Physics 05/2009; 115(1):439-443. DOI:10.1016/j.matchemphys.2009.01.001 · 2.13 Impact Factor

Publication Stats

396 Citations
121.99 Total Impact Points

Institutions

  • 2000–2014
    • Shanghai Jiao Tong University
      • • Shanghai Key Laboratories of Laser Processing and Material Modification
      • • Department of Materials Science and Engineering (MSE)
      Shanghai, Shanghai Shi, China
  • 2002–2006
    • Shanghai University
      • School of Materials Science and Engineering
      Shanghai, Shanghai Shi, China