We report 7SAs NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1-xPrxFe2As2. In both cases of x = 0.075, 0.15, a large increase of Vq upon cooling is consistent with the tetragonal-co...We report 7SAs NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1-xPrxFe2As2. In both cases of x = 0.075, 0.15, a large increase of Vq upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structure transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/75 T1 is enhanced and forms a peak at T ≈ 25 K, which may be caused by the magnetic ordering of the Pr3＋ moments or soin dynamics of mobile domain walls.展开更多
Lamellar crystal thickness lc of isotactic polybutene-1(it-PB1)have been investigated for crystal-lization in the melt over a wide range of crystallization temperature T from 40°C to 90°C by small angle X-ra...Lamellar crystal thickness lc of isotactic polybutene-1(it-PB1)have been investigated for crystal-lization in the melt over a wide range of crystallization temperature T from 40°C to 90°C by small angle X-ray scattering experiments and density measurements.The crystal thickness lc demonstrates two linear dependences on inverse supercooling and a transition from one dependence to the other has been observed around T=65°C.Each of the two dependences obeys the nucleation theory in the high and low supercooling ranges,respec-tively.Chain folding free energy q determined from the low supercooling range is larger than that determined from the high supercooling range.Possible mechanisms for the transition are discussed taking account of entropy of chain folding directions.展开更多
The morphology and lateral growth rate of isotactic polybutene-1(it-PB1)have been investigated for crystallization from the melt over a wide range of crystallization temperatures from 50 to 110℃.The morphology of it-...The morphology and lateral growth rate of isotactic polybutene-1(it-PB1)have been investigated for crystallization from the melt over a wide range of crystallization temperatures from 50 to 110℃.The morphology of it-PB1 crystals is a rounded shape at crystallization temperatures lower than 85℃,while lamellarsingle crystals possess faceted morphology at higher crystallization temperatures.The kinetic roughening transi-tion occurs around 85℃.The nucleation and growth mechanism for crystallization does not work below 85℃,since the growth face is rough.However,the growth rate shows the supercooling dependence derived from the nucleation and growth mechanism.The nucleation theory seems still to work even for rough surface growth.Possible mechanisms for the crystal growth of this polymer are discussed.展开更多
By taking tetragonal tungsten bronze(TTB)phase Nb_(18)W_(16)O_(93)as an example,an improved solid-state sintering method at lower temperature of 1000℃for 36 h was proposed via applying nanoscale raw materials.XRD,SEM...By taking tetragonal tungsten bronze(TTB)phase Nb_(18)W_(16)O_(93)as an example,an improved solid-state sintering method at lower temperature of 1000℃for 36 h was proposed via applying nanoscale raw materials.XRD,SEM and XPS confirm that the expected sample was produced.GITT results show that the lithium-ion diffusion coefficient of Nb_(18)W_(16)O_(93)(10−12 cm^(2)/s)is higher than that of the conventional titanium-based anode,ensuring a relatively superior electrochemical performance.The lithium-ion diffusion mechanism was thoroughly revealed by using density functional theory simulation.There are three diffusion paths in TTB phase,among which the interlayer diffusion with the smallest diffusion barrier(0.46 eV)has more advantages than other typical anodes(such as graphite,0.56 eV).The relatively smaller lithium-ion diffusion barrier makes TTB phase Nb_(18)W_(16)O_(93)become a potential highspecific-power anode material.展开更多
Zirconia-based bioceramics have been widely applied in the field of prosthodontics owing to its desir- able mechanical performance, biocompatibility and aesthetics. However, the low-temperature degradation （LTD） of ...Zirconia-based bioceramics have been widely applied in the field of prosthodontics owing to its desir- able mechanical performance, biocompatibility and aesthetics. However, the low-temperature degradation （LTD） of tetragonal zirconia （ZrOJ under intraoral condition can lead to the deterioration of mechanical properties of ZrO2 dental crowns, which contribute to many clinical failures in long-term observations. The long-term tetragonal phase stability and mechanical properties of yttria-stabilized tetragonal zir- conia polycrystal （Y-TZP） are influenced by grain size of ZrO2 crystals, distribution and properties of stabilizers, the humid environment, etc. However, it is still difficult to control the abovementioned factors at the same time. This review summarizes the major advances in researches dealing with LTD and clari- fies the obstacles to stabilization of the tetragonal ZrO2. Furthermore, the suggestions on improving the LTD resistance of tetragonal ZrO2 are proposed, which is the catalyst to promote the long-term stability of ZrO2-based all-ceramic crowns.展开更多
Zirconia powders doped with yttrium prepared by special liquid-phase precipitation method were sintered by spark plasma sintering （SPS） to obtain high performance samples. The microstructure, phase composition, and ...Zirconia powders doped with yttrium prepared by special liquid-phase precipitation method were sintered by spark plasma sintering （SPS） to obtain high performance samples. The microstructure, phase composition, and mechanical properties of the samples were studied. The results of X-ray diffraction （XRD）, Raman spectrum, and transmission electron microscope （TEM） show that the phase is tetragonal. The powders with large surface area and high sintering activity, due to small crystallite size, could be densified at 1100℃. The highest relative density of the sample obtained at 1300℃ is higher than 99% （the tetragonal phase is 6.08 g/cm^3）. The Hv and KIC are 13.76 GPa and 15.4 MPa.m^1/2, respectively.展开更多
Tailoring microstructure and microchemistry by altering elemental compositions and thermomechanical treatment parameters enables superior corrosion performance in zirconium alloys for nuclear applications.However,our ...Tailoring microstructure and microchemistry by altering elemental compositions and thermomechanical treatment parameters enables superior corrosion performance in zirconium alloys for nuclear applications.However,our understanding of the relationship between various defects and the corrosion process remains limited in the newly developed zirconium alloys.Here we report the oxide formation mechanism of a CZ1 zirconium alloy with corrosion resistance surpassing many other zirconium alloy systems,such as Zircaloy-4 and Zr-1Nb-1Sn alloys.Autoclave experiments of CZ1 alloy and Zr1Nb-1Sn model alloy were performed in 360°C water for up to 820 d.We quantitively determined oxide phases by transmission Kikuchi diffraction(TKD)and examined lateral cracks,nano-porosity,and second-phase particles in oxide scales by transmission electron microscopy(TEM).Compared to the Zr-1Nb-1Sn model alloy,CZ1 alloy with lower Nb and Sn concentrations has shown smaller and lower-density lateral cracks but slightly larger oxide grains,reducing the diffusion route for oxidating species.Using analytical scanning and transmission electron microscopy,we demonstrate that due to the lower content of Sn(∼0.9 wt.%),there is less tetragonal ZrO_(2) phase formed in the oxide,and the level of tetragonal to the monoclinic phase transition is reduced.Although the Nb content(0.1 wt.%–0.3 wt.%)is lower than the solid solution limit of Nb in Zr,by introducing minor elements such as Fe,Cr,and Cu,there are still a reasonable number of second-phase particles to relieve the high stress associated with the metal-to-oxide transformation.These mechanisms have substantially changed the density and distribution of lateral cracks in the oxide,thus reducing the corrosion rate of zirconium alloys.展开更多
The Y 2O 3-CeO 2-ZrO 2 powders were prepared using a co-precipitation process and the corresponding coatings were prepared by plasma spraying. The results show that an optimal composition exists in Y 2O 3-doped ...The Y 2O 3-CeO 2-ZrO 2 powders were prepared using a co-precipitation process and the corresponding coatings were prepared by plasma spraying. The results show that an optimal composition exists in Y 2O 3-doped CeO 2-ZrO 2, but not in CeO 2-doped Y 2O 3-ZrO 2. The powders mainly contain tetragonal phase and a trace amount of monoclinic phase. The homogeneity in composition, large agglomerate size, ideal particle size distribution and high flowability were obtained. The as-sprayed coatings are composed of non-transformable tetragonal phase, tz′structure, and resistant to transformation under thermal or mechanical stresses.展开更多
CeO_2 stabilized ZrO_2 ultra fine nanoparticles were successfully synthesized via a simple and effective sol-gel synthetic approach by using zirconylchloride octahydrate, cerium nitrate hexahydrate, and citric acid as...CeO_2 stabilized ZrO_2 ultra fine nanoparticles were successfully synthesized via a simple and effective sol-gel synthetic approach by using zirconylchloride octahydrate, cerium nitrate hexahydrate, and citric acid as starting materials. A series of techniques, including X-ray diffraction（XRD）, thermogravimetry（TG）, differential scanning calorimetry（DSC）, Fourier transform infrared spectroscopy（FTIR）, transmission electron microscopy（TEM）, and N_2-sorption analysis, were used to characterize the structure and morphology of the asprepared samples. XRD studies indicate that the as-synthesized sample is of well crystallized tetragonal phase of CeO_2 stabilized ZrO_2 with high purity. TEM images show that the as-synthesized sample is composed of a large number of fine dispersive nanoparticles with an average size about 10 nm. The as-synthesized tetragonal CeO_2 stabilized ZrO_2 sample was heated at different temperatures in order to evaluate its thermal stability. The exprimental results reveal that the as-synthesized tetragonal CeO_2 stabilized ZrO_2 sample exhibits excellent stability without the occurrence of phase transformation.展开更多
Strontium barium niobate/barium strontium titanate composite ceramics of xSrO·(1-x)BaO·0.5Nb2O5·0.5TiO2 (BSTN in short) with a range of Sr/Ba ratios were fabricated using a modified sol-gel method with ...Strontium barium niobate/barium strontium titanate composite ceramics of xSrO·(1-x)BaO·0.5Nb2O5·0.5TiO2 (BSTN in short) with a range of Sr/Ba ratios were fabricated using a modified sol-gel method with Nb2O5 fine powders suspended in the barium strontium titanate (BST in short) sol solution. Powders obtained from dried gels were calcined at 800 ℃ for 3 h. After preparing bulk ceramics from these powders by sintering at 1200 ℃ for 3 h, the tetragonal tungsten bronze(TTB) phase and perivoskite phase were co-present in compositions between 0.25≤x≤0.75, with the increasing of x value, the peaks of pervoskite phase shift to the high angle position slightly while no changes happened in the peak position of TTB phase. The peak intensity of both two phases were also changed.展开更多
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11074304 and 11222433)the National Basic Research Program of China (Grant Nos. 2010CB923004 and 2011CBA00112)supported by AFOSR-Multidisciplinary University, ResearchInitiative (Grant No. FA9550-09-1-0603)
文摘We report 7SAs NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1-xPrxFe2As2. In both cases of x = 0.075, 0.15, a large increase of Vq upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structure transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/75 T1 is enhanced and forms a peak at T ≈ 25 K, which may be caused by the magnetic ordering of the Pr3＋ moments or soin dynamics of mobile domain walls.
基金Professor Miyaji of Kyoto University for valuable advice and encouragement.
文摘Lamellar crystal thickness lc of isotactic polybutene-1(it-PB1)have been investigated for crystal-lization in the melt over a wide range of crystallization temperature T from 40°C to 90°C by small angle X-ray scattering experiments and density measurements.The crystal thickness lc demonstrates two linear dependences on inverse supercooling and a transition from one dependence to the other has been observed around T=65°C.Each of the two dependences obeys the nucleation theory in the high and low supercooling ranges,respec-tively.Chain folding free energy q determined from the low supercooling range is larger than that determined from the high supercooling range.Possible mechanisms for the transition are discussed taking account of entropy of chain folding directions.
文摘The morphology and lateral growth rate of isotactic polybutene-1(it-PB1)have been investigated for crystallization from the melt over a wide range of crystallization temperatures from 50 to 110℃.The morphology of it-PB1 crystals is a rounded shape at crystallization temperatures lower than 85℃,while lamellarsingle crystals possess faceted morphology at higher crystallization temperatures.The kinetic roughening transi-tion occurs around 85℃.The nucleation and growth mechanism for crystallization does not work below 85℃,since the growth face is rough.However,the growth rate shows the supercooling dependence derived from the nucleation and growth mechanism.The nucleation theory seems still to work even for rough surface growth.Possible mechanisms for the crystal growth of this polymer are discussed.
基金the Key R&D Program of Shaanxi Province,China(No.2019ZDLGY04-05)the Natural Science Foundation of Shaanxi Province,China(No.2019JLZ-01)+1 种基金the Fundamental Research Funds for the Central Universities of China(Nos.19GH020302,3102019JC005,3102021ZD0401,3102021TS0406)the Science,Technology,and Innovation Commission of Shenzhen Municipality,China(No.JCYJ20180508151856806).
文摘By taking tetragonal tungsten bronze(TTB)phase Nb_(18)W_(16)O_(93)as an example,an improved solid-state sintering method at lower temperature of 1000℃for 36 h was proposed via applying nanoscale raw materials.XRD,SEM and XPS confirm that the expected sample was produced.GITT results show that the lithium-ion diffusion coefficient of Nb_(18)W_(16)O_(93)(10−12 cm^(2)/s)is higher than that of the conventional titanium-based anode,ensuring a relatively superior electrochemical performance.The lithium-ion diffusion mechanism was thoroughly revealed by using density functional theory simulation.There are three diffusion paths in TTB phase,among which the interlayer diffusion with the smallest diffusion barrier(0.46 eV)has more advantages than other typical anodes(such as graphite,0.56 eV).The relatively smaller lithium-ion diffusion barrier makes TTB phase Nb_(18)W_(16)O_(93)become a potential highspecific-power anode material.
基金?nancially supported by the National Natural Science Foundation of China(No.81500897)China Scholarship Council(No.201408210385)+1 种基金Foundation of the Education Department of Liaoning Province in China(No.L2013285)Science and Technology Planning Project of Shenyang City(No.F11-262-9-16)
文摘Zirconia-based bioceramics have been widely applied in the field of prosthodontics owing to its desir- able mechanical performance, biocompatibility and aesthetics. However, the low-temperature degradation （LTD） of tetragonal zirconia （ZrOJ under intraoral condition can lead to the deterioration of mechanical properties of ZrO2 dental crowns, which contribute to many clinical failures in long-term observations. The long-term tetragonal phase stability and mechanical properties of yttria-stabilized tetragonal zir- conia polycrystal （Y-TZP） are influenced by grain size of ZrO2 crystals, distribution and properties of stabilizers, the humid environment, etc. However, it is still difficult to control the abovementioned factors at the same time. This review summarizes the major advances in researches dealing with LTD and clari- fies the obstacles to stabilization of the tetragonal ZrO2. Furthermore, the suggestions on improving the LTD resistance of tetragonal ZrO2 are proposed, which is the catalyst to promote the long-term stability of ZrO2-based all-ceramic crowns.
文摘Zirconia powders doped with yttrium prepared by special liquid-phase precipitation method were sintered by spark plasma sintering （SPS） to obtain high performance samples. The microstructure, phase composition, and mechanical properties of the samples were studied. The results of X-ray diffraction （XRD）, Raman spectrum, and transmission electron microscope （TEM） show that the phase is tetragonal. The powders with large surface area and high sintering activity, due to small crystallite size, could be densified at 1100℃. The highest relative density of the sample obtained at 1300℃ is higher than 99% （the tetragonal phase is 6.08 g/cm^3）. The Hv and KIC are 13.76 GPa and 15.4 MPa.m^1/2, respectively.
文摘Tailoring microstructure and microchemistry by altering elemental compositions and thermomechanical treatment parameters enables superior corrosion performance in zirconium alloys for nuclear applications.However,our understanding of the relationship between various defects and the corrosion process remains limited in the newly developed zirconium alloys.Here we report the oxide formation mechanism of a CZ1 zirconium alloy with corrosion resistance surpassing many other zirconium alloy systems,such as Zircaloy-4 and Zr-1Nb-1Sn alloys.Autoclave experiments of CZ1 alloy and Zr1Nb-1Sn model alloy were performed in 360°C water for up to 820 d.We quantitively determined oxide phases by transmission Kikuchi diffraction(TKD)and examined lateral cracks,nano-porosity,and second-phase particles in oxide scales by transmission electron microscopy(TEM).Compared to the Zr-1Nb-1Sn model alloy,CZ1 alloy with lower Nb and Sn concentrations has shown smaller and lower-density lateral cracks but slightly larger oxide grains,reducing the diffusion route for oxidating species.Using analytical scanning and transmission electron microscopy,we demonstrate that due to the lower content of Sn(∼0.9 wt.%),there is less tetragonal ZrO_(2) phase formed in the oxide,and the level of tetragonal to the monoclinic phase transition is reduced.Although the Nb content(0.1 wt.%–0.3 wt.%)is lower than the solid solution limit of Nb in Zr,by introducing minor elements such as Fe,Cr,and Cu,there are still a reasonable number of second-phase particles to relieve the high stress associated with the metal-to-oxide transformation.These mechanisms have substantially changed the density and distribution of lateral cracks in the oxide,thus reducing the corrosion rate of zirconium alloys.
文摘The Y 2O 3-CeO 2-ZrO 2 powders were prepared using a co-precipitation process and the corresponding coatings were prepared by plasma spraying. The results show that an optimal composition exists in Y 2O 3-doped CeO 2-ZrO 2, but not in CeO 2-doped Y 2O 3-ZrO 2. The powders mainly contain tetragonal phase and a trace amount of monoclinic phase. The homogeneity in composition, large agglomerate size, ideal particle size distribution and high flowability were obtained. The as-sprayed coatings are composed of non-transformable tetragonal phase, tz′structure, and resistant to transformation under thermal or mechanical stresses.
基金Funded by the National Natural Science Foundation of China(Nos.U1304520 and U1404613)the State Key Lab of Materials Synthesis and Processing of Wuhan University of Technology for the fund support(2012-KF-5)+1 种基金the Education Department of Henan Province(2013GGJS-185)the program for New Century Excellent Talents in University(NECT-12-0655)
文摘CeO_2 stabilized ZrO_2 ultra fine nanoparticles were successfully synthesized via a simple and effective sol-gel synthetic approach by using zirconylchloride octahydrate, cerium nitrate hexahydrate, and citric acid as starting materials. A series of techniques, including X-ray diffraction（XRD）, thermogravimetry（TG）, differential scanning calorimetry（DSC）, Fourier transform infrared spectroscopy（FTIR）, transmission electron microscopy（TEM）, and N_2-sorption analysis, were used to characterize the structure and morphology of the asprepared samples. XRD studies indicate that the as-synthesized sample is of well crystallized tetragonal phase of CeO_2 stabilized ZrO_2 with high purity. TEM images show that the as-synthesized sample is composed of a large number of fine dispersive nanoparticles with an average size about 10 nm. The as-synthesized tetragonal CeO_2 stabilized ZrO_2 sample was heated at different temperatures in order to evaluate its thermal stability. The exprimental results reveal that the as-synthesized tetragonal CeO_2 stabilized ZrO_2 sample exhibits excellent stability without the occurrence of phase transformation.
文摘Strontium barium niobate/barium strontium titanate composite ceramics of xSrO·(1-x)BaO·0.5Nb2O5·0.5TiO2 (BSTN in short) with a range of Sr/Ba ratios were fabricated using a modified sol-gel method with Nb2O5 fine powders suspended in the barium strontium titanate (BST in short) sol solution. Powders obtained from dried gels were calcined at 800 ℃ for 3 h. After preparing bulk ceramics from these powders by sintering at 1200 ℃ for 3 h, the tetragonal tungsten bronze(TTB) phase and perivoskite phase were co-present in compositions between 0.25≤x≤0.75, with the increasing of x value, the peaks of pervoskite phase shift to the high angle position slightly while no changes happened in the peak position of TTB phase. The peak intensity of both two phases were also changed.
