The development of highly active nitrogendoped carbon-based transition metal(M-N-C)compounds for the oxygen reduction reaction(ORR)in proton exchange membrane fuel cells(PEMFCs)greatly helps reduce fuel cell cost,thus...The development of highly active nitrogendoped carbon-based transition metal(M-N-C)compounds for the oxygen reduction reaction(ORR)in proton exchange membrane fuel cells(PEMFCs)greatly helps reduce fuel cell cost,thus rapidly promoting their commercial applications.Among different M-N-C electrocatalysts,the series of Fe-N-C materials are highly favored because of their high ORR activity.However,there remains a debate on the effect of Fe,and rare investigations focus on the influence of Fe addition in the second heat treatment usually performed after acid leaching in the catalyst synthesis.It is thus very critical to explore the influences of Fe on the ORR electrocatalytic activity,which will,in turn,guide the design of Fe-N-C materials with enhanced performance.Herein,a series of Fe-N-C electrocatalysts are synthesize and the influence of Fe on the ORR activity are speculated both experimentally and theoretically.It is deduced that the active site lies in the structure of Fe-N4,accompanied with the addition of appropriate Fe,and the number of active sites increases without the occurrence of agglomeration particles.Moreover,it is speculated that Fe plays an important role in stabilizing N as well as constituting active sites in the second pyrolyzing process.展开更多
Al2O3 and Ti-6Al-4V alloy were brazed with Ag-Cu-Ti ＋B fillers in different brazing conditions. Effects of brazing temperature, holding time and additive Ti content on joints microstructure and shear strength were in...Al2O3 and Ti-6Al-4V alloy were brazed with Ag-Cu-Ti ＋B fillers in different brazing conditions. Effects of brazing temperature, holding time and additive Ti content on joints microstructure and shear strength were investigated by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, transmission electron microscopy and shear testing. Results indicate that TiCu and Ti（Cu,Al） decrease, but Ti2Cu and -Ti2（Cu,Al） increase in brazing seam with increasing brazing temperature, holding time and additive Ti content. Area consisting of Ti3（Cu,Al）30 and TiO near Al2O3 becomes gradually discontinuous from continuity when brazing temperature rises or holding time extends. As Ti additive content increases, TiO is absent near Al2O3, area consisting of only Ti3（Cu,Al）30 thickens. TiB whiskers are in situ synthesized by Ti and B atoms during brazing process. The brazing temperature, holding time and additive Ti content on joints microstructure influence the joints shear strength directly. The shear strength of joints, obtained at 850 ℃ holding for 10 min, reaches the maximum of 78 MPa. According to the experimental results, phase diagram and thermodynamics calculation, the interface evolution mechanism of the Al2O3/Ti-6Al-4V alloy joint was analyzed.展开更多
基金funded by the National Natural Science Foundation of China(Grant Nos.21533005 and 21802095)the National Key R&D Program of China(2016YFB0101201).
文摘The development of highly active nitrogendoped carbon-based transition metal(M-N-C)compounds for the oxygen reduction reaction(ORR)in proton exchange membrane fuel cells(PEMFCs)greatly helps reduce fuel cell cost,thus rapidly promoting their commercial applications.Among different M-N-C electrocatalysts,the series of Fe-N-C materials are highly favored because of their high ORR activity.However,there remains a debate on the effect of Fe,and rare investigations focus on the influence of Fe addition in the second heat treatment usually performed after acid leaching in the catalyst synthesis.It is thus very critical to explore the influences of Fe on the ORR electrocatalytic activity,which will,in turn,guide the design of Fe-N-C materials with enhanced performance.Herein,a series of Fe-N-C electrocatalysts are synthesize and the influence of Fe on the ORR activity are speculated both experimentally and theoretically.It is deduced that the active site lies in the structure of Fe-N4,accompanied with the addition of appropriate Fe,and the number of active sites increases without the occurrence of agglomeration particles.Moreover,it is speculated that Fe plays an important role in stabilizing N as well as constituting active sites in the second pyrolyzing process.
基金the National Natural Science Foundation of China (Grant Nos.51275135,51105107 and 51021002)the Natural Science Foundation of Heilongjiang Province,China (Grant No.QC2011C044)the Specialized Research Fund for the Doctoral Program of Higher Education,China (Grant No.20112302130005)
文摘Al2O3 and Ti-6Al-4V alloy were brazed with Ag-Cu-Ti ＋B fillers in different brazing conditions. Effects of brazing temperature, holding time and additive Ti content on joints microstructure and shear strength were investigated by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, transmission electron microscopy and shear testing. Results indicate that TiCu and Ti（Cu,Al） decrease, but Ti2Cu and -Ti2（Cu,Al） increase in brazing seam with increasing brazing temperature, holding time and additive Ti content. Area consisting of Ti3（Cu,Al）30 and TiO near Al2O3 becomes gradually discontinuous from continuity when brazing temperature rises or holding time extends. As Ti additive content increases, TiO is absent near Al2O3, area consisting of only Ti3（Cu,Al）30 thickens. TiB whiskers are in situ synthesized by Ti and B atoms during brazing process. The brazing temperature, holding time and additive Ti content on joints microstructure influence the joints shear strength directly. The shear strength of joints, obtained at 850 ℃ holding for 10 min, reaches the maximum of 78 MPa. According to the experimental results, phase diagram and thermodynamics calculation, the interface evolution mechanism of the Al2O3/Ti-6Al-4V alloy joint was analyzed.
