最新刊期
      卷 0 ， 期 0 ， 2020
      • Issue 10, (1994)
        摘要：Positron lifetime studies were performed for PTC material BaTiO3 doped with Nb5+ or Y3+.It was found that positron lifetime of BaTiO3 was sensitive to the changes of the valence state of impurities and the amount of impurities.The lifetime of positrons trapped in Ba2+ vacancy is estimated at 320ps.  
        关键词：Ceramics;Doping;Defects;Positron lifetime;PTC effect   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：The characteristics of positronium,a kind of exotic atoms,and the applicability of using it to probe the reaction mechanism and the active intermediates are briefly introduced.A synthesis reaction of catalyst Rh（I）from reducing Rh（III）by ethylene is chosen for testing this applicability.Based on the abnormal changes of positronium lifetime and intensity with the reaction time,the formation of intermediate Rh（II）and the reaction mechanism are discussed.  
        关键词：Positronium chemistry;Positron annihilation;Intermediates;Intermediate process   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：第五届全国正电子湮没谱学会议论文选编根据１９９０年召开的第四届全国正电子湮没谱学会议的决定，受中国核学会、中国物理学会、中国核物理学会的委托，由兰州大学材料科学系筹办的＂＂第五届全国正电子湮没谱学会议＂于１９９３年１０月１０日－１５日在甘肃省酒泉市举...  
          
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：Measurements of positron lifetime spectra have been performed for binary Ni3Al,Ni-22.33Al-1.37B,Ni-19.6Al-14.8Fe-0.17Zr and Ni-19.6Al-14.8Fe-0.26Hf alloys.Added to Ni3Al alloy,the B atoms have a strong tendency to segregate at grain boundaries and make the density of valence electrons in grain boundaies similar to that in bulk.Theτ2 values for both Ni-19.6Al-14.8Fe-0.17Zr and Ni-19.6Al-14.8Fe-0.26Hf alloys are smaller than that of binary Ni3Al,which indicates that the addition of metal elements such as Zr(or Had and Fe to Ni3Al alloy can increase the fraction of metallic bonds and density of free electrons participating in metallic bonds in grain boundaries and thus improve the structure of grainboundaries.  
        关键词：Grain boundary structure;Macroalloying;Microalloying   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：Positron lifetimes have been measured for 68wt% Cu-Zn brass containing 0.00.0.01, 0.03,0.05,0.07 and 0.10 wt% As respectively after dezincification in 1%, CuCl2aqueous solution.The vacancy mechanism of the brass dezincification and the inhibition of brass dezincification by arsenic have been discussed.  
          
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：Positron lifetime spectra have been measured for monodirectional crystal,as-cast polycrystal and hot-rolled polycrystal of Ni-20Al-30Fe alloys.τ2 values for these samples are similar,indicating that lifetime of the positron annihilated in grain boundary is similar to that in a vacancy or a dislocation,and density of free electrons in grain boundaries is nearly equal to that in vacancies.It is concluded that the structure of grain boundaries in NiAl alloy can be improved by alloying with Fe.  
        关键词：Positron annihilation;Defects;Electron density   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：The recovery behavior of quenched-in or radiation-induced defects in the intermetallic compound NiAl has been studied by positron annihilation techniques.Two distinct recovery stages in NiAl samples quenched from temperature above 950℃ were found by Doppler broadening measurements;one at around 280℃and the other at around 500℃.The molecular dynamics was then further used to simulate mono-vacancies and divacancies in the NiAl of stoichiometric composition with"embedded atom method"（EAM） potentials. The vacancy ingration energies for three different mechanisms were calculated. The calculations indicated that the energetically favorable mechanism for vacancy migration was six-jump vacancy cycle.A comparison with the experiments showed that Al vacancies annealed out at the～280℃recovery stage while Ni vacancies at the～500℃ stage, and the vast majority of diffusion events in the ordered NiAl alloy were mainly attributed to the motion of vacancies.  
        关键词：NiAl;Yacancy;Recovery behavior;Migration mechanism   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：Variation of S-parameter with the strain [S（ε）curves] under the strain rate of 5×10 5/s in high purity iron was investigated by Doppler broadening measurements combined with slow tensile tests. In order to estimate dislocation density pd and vacancy concentration Cv,the deformed samples were annealed at o00K for 10h to eliminate the effect of vacancies induced by the strain and to retain the strain induced dislocations.A calculation based on the trapping model gave the dislocation density ρd on the order of 109-1011cm2 and the vacancy concentration Cv.on the order of 1017-1019cm3.both of which were increased with the strain.  
        关键词：High purity iron;Dislocation density;Vacancy concentration;S-parameter   
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        发布时间：2021-04-09
      • Wang Xiaogang
        Issue 10, (1994)
        摘要：The positron annihilation characteristics of vacancies in iron in different phases have been calculated by using the discrete variational method and embedded-cluster model based on the first-principle and two-component density-functional theory.The effect of lattice-expansion and lattice-relaxation on the positron annihilation characteristics is discussed.  
        关键词：Positron annihilation;Two-component density-functional theory;Iron;Vacancies   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：Hydrogen induced defects in annealed and plastically deformed pure aluminium have been studied mainly by the positron lifetime method and the transmission electron microscopy.It is revealed that the hydrogen charging can introduce blisters and dislocations in aluminium specimens.Inclusion particles in the specimens act as the nucleation sites of hydrogen induced defects. The effect of protons to shield defects from positron has been proved by the experimental results.  
        关键词：Hydrogen embrittlement;Aluminium;Positron annihilation;Electron microscopy   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：The positron lifetime spectra have been measured as a function of exposed air pressure for synthetic zeolite 13X and 3A,and as a function of calcination temperature for zeolite 10X.It is found that the adsorbed gases have a strong effect on positron annihilation.Moreover,the positron annihilation parameters varysignificantly with the amount of water existing in the zeolite cages. With the dehydration the free volume inside cages bacame larger, and when dehydrated to a certain degree,Bronsted acid, as the active center,begin to appear.  
        关键词：Positron annihilation;Zeolite;Spin exchange;Bronsted acid   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：ositron annihilation lifetime spectra have been measured for the samples of Na2O-B2O3 melt. B2O3,SiO2. V2O5 and Na2O-TiO2 melt during sharp cooling with a cooling rate>180K.s 1.The results show a quite well linear relation between optical basicity A and the first component τ1 of the lifetime spectra for non-transition metal oxides: A= 0.173 （1/ τ1） +0.060.With this relationship and τ1measured by PA the A values for ZnO.TiO2 and V2O5.are obtained to be 1.00. 0.63 and 0.75 respectively,approaching the literature values.  
          
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：The defect behavior of semi-insulating（SI） GaAs samples has been measured by the positron lifetime spectrometer at different isochronal annealing temperatures before and after neutron irradiation. It is found that vacancy-interstitial pairs and vacancy-antisite defects begin to recombine at 70℃,and vacancies and voids will disappear before 500℃, but new defects may reproduce after 740℃. The positron binding energy of shallow traps（VAs）is estimated to be 0.031-0.032eV.  
        关键词：GaAs;Positron annihilation   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：Positron lifetime spectra in samples of α-,β-,γ-and ε- phase Cu-Zn alloys before and after corrosion in 1% CuCl2 aqueous solution,have been measured and analysed.It is found that dezincification rate is increased with the rate of producing excess vacancies. The mechanism of volume diffusion of dezincification for Cu-Zn alloys,has been discussed.  
        关键词：Brass dezincification;Positron lifetime;Vacancy   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：The positron annihilation and perturbed angular correlation methods have been used to study the radiation damage and its thermal annealing behavior in the single crystal Si irradiated with fast neutrons of 1.45 ×1020/cm2 and 3.10×1017/cm2 and with 178W heavy ions of 5×1011/cm2,respectively.The measured positron lifetimes and quadrupole interaction frequencies show the existence of the monovacancysubstitutional oxygen complexes in the irradiated single crystal silicon.In silicon samples irradiated with 1.45×1020/cm2 neutron and heavy ions the combination of di-vacancies into quadri-vacancies has been observed by both methods.  
        关键词：Si single crystal;Lattice defects;Neutron and heavy ion irradiation;O-V complexes;Positron annihilation;Perturbed angular correlation   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：Positroniums can be formed and emitted to vacuum from an interaction of low energy positrons with the surface of metal or semiconductors. To reduce the large experimental error encountered in the peak method or the peak-valley method a direct measurement method of positronium fraction using the time-of-flight spectroscopy is proposed. The defects near Si（100）surface induced by Ar+ sputtering and their annealing behavior have been studied with this method.  
        关键词：Slow positron beam;Positronium fraction;Time-of-flight spectra;Ar+ sputtering   
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        发布时间：2021-04-09
      • Issue 10, (1994)
        摘要：The positron lifetime was measured as a function of doped F content for Bi2Sr2CaCu2Oy-xFx superconductor.Combined with the measurement of critical temperature Tc, the sites where F substituted for O and the change of electronic structures caused by local substitution of F in the samples were studied.  
        关键词：Positron lifetime;Bi-based superconductors   
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        发布时间：2021-04-09
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