Two-dimensional(2D)semiconductors possess strongly bound excitons,opening novel opportunities for engineering light-matter interaction at the nanoscale.However,their in-plane confinement leads to large non-radiative e...Two-dimensional(2D)semiconductors possess strongly bound excitons,opening novel opportunities for engineering light-matter interaction at the nanoscale.However,their in-plane confinement leads to large non-radiative exciton–exciton annihilation(EEA)processes,setting a fundamental limit for their photonic applications.In this work,we demonstrate suppression of EEA via enhancement of light-matter interaction in hybrid 2D semiconductor-dielectric nanophotonic platforms,by coupling excitons in WS2 monolayers with optical Mie resonances in dielectric nanoantennas.The hybrid system reaches an intermediate light-matter coupling regime,with photoluminescence enhancement factors up to 102.Probing the exciton ultrafast dynamics reveal suppressed EEA for coupled excitons,even under high exciton densities>10^(12)cm^(−2).We extract EEA coefficients in the order of 10^(−3),compared to 10^(−2)for uncoupled monolayers,as well as a Purcell factor of 4.5.Our results highlight engineering the photonic environment as a route to achieve higher quantum efficiencies,for low-power hybrid devices,and larger exciton densities,towards strongly correlated excitonic phases in 2D semiconductors.展开更多
The twist angle between two van der Waals coupled monolayers has emerged as a new and powerful degree of freedom for engineering physical properties of semiconductor homo-and hetero-bilayers.While the interlayer twist...The twist angle between two van der Waals coupled monolayers has emerged as a new and powerful degree of freedom for engineering physical properties of semiconductor homo-and hetero-bilayers.While the interlayer twist has shown prominent effect on electronic and optical properties of transition metal dichalcogenide(TMD)bilayers,it remains unclear how it could be used to manipulate the exciton dynamics,especially exciton-exciton annihilation(EEA)process which is the dominant energy loss channel in TMDs under moderate to high exciton density due to strong Coulomb interaction.Herein,we show that the twist angle in TMD bilayers can act as an effective knob to control the EEA process.Specifically,EEA rate constant increases from 1° twisted WSe_(2) bilayers(0.026 cm^(2)/s)by more than twice to 32° twisted bilayers(0.053 cm2/s)and then drops again in 60° twisted bilayers(0.019 cm^(2)/s).This twist-angle dependence can be attributed to the energy difference between indirect and direct excitons arising from the interlayer interaction.Our work opens up the possibility of artificially managing the exciton dynamics in TMD materials for optoelectronic applications via interlayer twist angle.展开更多
By means of an improved mapping method and a variable separation method, a series of variable separation solutions including solitary wave solutions, periodic wave solutions and rational function solutions) to the (...By means of an improved mapping method and a variable separation method, a series of variable separation solutions including solitary wave solutions, periodic wave solutions and rational function solutions) to the (2+1)-dimensional breaking soliton system is derived. Based on the derived solitary wave excitation, we obtain some special annihilation solitons and chaotic solitons in this short note.展开更多
The principles of positron annihilation and four positron experimental techniques are described. The application of positron annihilation technique in material science. atomic physics and other related fields are disc...The principles of positron annihilation and four positron experimental techniques are described. The application of positron annihilation technique in material science. atomic physics and other related fields are discussed.展开更多
Positron annihilation lifetime spectra measurements were performed on C_(60)-containing chitosan as a function of temperature ranging from 290 to 380K.Both the ortho-positronium annihilation lifetimeт_(3) and its int...Positron annihilation lifetime spectra measurements were performed on C_(60)-containing chitosan as a function of temperature ranging from 290 to 380K.Both the ortho-positronium annihilation lifetimeт_(3) and its intensity Is increase with increasing temperature,however,the values of I_(3) in C_(60)-containing chitosan are smaller than that in pure chitosan,and the slope ofт_(3) with temperature is also suppressed after C_(60) linked.The existing three models for positron annihilation in polymer have been compared to analyze the experiment result.It is realized that only the spur reaction model,combined with the free volume model,can explain it satisfactorily.The ortho-positronium lifetime and intensity were affected by the local physicochemical environment of material,i.e.,free-volume structure,electron scavenging and trapping sites,etc.The linked C_(60) that play a significant chemical inhibition and quenching role on positronium formation and annihilation isfirst observed.The microstructure change brought by C_(60) group has also been discussed.展开更多
This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of po...This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 ℃ annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600℃ annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.展开更多
We investigate the nonclassical properties of arbitrary number photon annihilation-then-creation operation (AC) and creation-then-annihilation operation (CA) to the thermal state (TS), whose normalization factor...We investigate the nonclassical properties of arbitrary number photon annihilation-then-creation operation (AC) and creation-then-annihilation operation (CA) to the thermal state (TS), whose normalization factors are related to the polylog- arithm function. Then we compare their quantum characters, such as photon number distribution, average photon number, Mandel Q-parameter, purity and the Wigner function. Because of the noncommutativity between the annihilation operator and the creation operator, the ACTS and the CATS have different nonclassical properties. It is found that nonclassical properties are exhibited more strongly after AC than after CA. In addition we also examine their non-Gaussianity. The result shows that the ACTS can present a slightly bigger non-Gaussianity than the CATS.展开更多
By incorporating the flavour-dependent quark--antiquark annihilation amplitude into the mass-squared matrix describing the mixing of the isoscalar states of a meson nonet,the new version of Schwinger's nonet mass ...By incorporating the flavour-dependent quark--antiquark annihilation amplitude into the mass-squared matrix describing the mixing of the isoscalar states of a meson nonet,the new version of Schwinger's nonet mass formula,which holds with a high accuracy for the 0^(-+),1^(--),2^(++),2^(-+)and 3^(--)nonets,is derived and the mixing angle of the isoscalar octet--singlet for these nonets is obtained.In particular,the mixing angle of the isoscalar octet-singlet for the pseudoscalar nonet is determined to take the value of-12.92°,which is in agreement with the value range from-13°to-17°deduced from a rather exhaustive and up-to-date analysis of data.It is also pointed out that the omission of the flavour-dependent qq annihilation effect might be a factor resulting in the invalidity of Schwinger’s original nonet mass formula for the pseudoscalar nonet.展开更多
The positron lifetime spectra and electrical conductivities have been measured for polyaniline as a function of protonation level ([H+] from 10-7-1000.8 mol/L). Weobserved that (1) the short lifetime τ1, which was re...The positron lifetime spectra and electrical conductivities have been measured for polyaniline as a function of protonation level ([H+] from 10-7-1000.8 mol/L). Weobserved that (1) the short lifetime τ1, which was related to electron density in bulk,decreased with the protonation level; (2) the intermediate lifetime τ2≈360ps, almost remaining constant, whereas its intensity I2 increased with increasing protonation level which was related to the conductivity of material. These results are discussed in terms of conducting island model.展开更多
Samples of the PM-355 polymeric solid state nuclear track detector were exposed to low gamma absorbed doses from 1 kGy (0.1 Mrad) up to 9 kGy (0.9 Mrad). Positron annihilation lifetime (PAL) in conjunction with transm...Samples of the PM-355 polymeric solid state nuclear track detector were exposed to low gamma absorbed doses from 1 kGy (0.1 Mrad) up to 9 kGy (0.9 Mrad). Positron annihilation lifetime (PAL) in conjunction with transmission electron microscopy (TEM) and Thermo-gravimetric analysis (TGA) were performed on irradiated and pristine samples at room temperature. The observed lifetime spectra were resolved into three components where the ortho-positronium (o-Ps) lifetime component was associated with the pick-off annihilation of positronium trapped by the free volume. PAL studies of irradiated PM-355 samples showed that ortho-positronium (o-Ps) lifetime increases with an increase in dose up to 4 kGy and decreases at higher doses. In contrast, the intensity of the o-Ps component, I3, decreases with the dose up to about 2 kGy, followed by a much smooth decrease up to 7 kGy, and then it levels off. TGA analysis indicated that the PM-355 detector decomposed in one main breakdown stage. These results are discussed on the basis of chemical and physical changes occurring at the microscopic level in the PM-355 due to irradiation. Crosslinking dominates for doses between 1 and 4 kGy, while the degradation mechanism (Chain scission) prevails for doses up to 9 kGy.展开更多
Rapidly solidified Al92.3Fe4.3V0.7Si1.7Mm1.0 alloy has been studied by positron lifetime spectroscopy and the variations on the intedecial defects with the annealing temperature were revealed by an analysis of the lif...Rapidly solidified Al92.3Fe4.3V0.7Si1.7Mm1.0 alloy has been studied by positron lifetime spectroscopy and the variations on the intedecial defects with the annealing temperature were revealed by an analysis of the lifetime results. The intedece characteristics derived from the positron-lifetime results could be used to give a satisfactory interpretation of the dependence of mechanical properties on the annealing temperature展开更多
The interactions between solitoffs are extensively investigated. Besides the known solitoff fission and fusion interac- tions, two new types of solitoff interactions are discovered, named the solitoff reconnection and...The interactions between solitoffs are extensively investigated. Besides the known solitoff fission and fusion interac- tions, two new types of solitoff interactions are discovered, named the solitoff reconnection and the solitoff annihilation. Taking the asymmetric Nizhnik-Novikov Veselov equation as an illustrative system, five types of solitoff interactions are graphically revealed on the basis of the analytical solution obtained by the modified tanh function expansion method.展开更多
Measurements of the Doppler broadening S-lineshape parameter of positron annihilation and brittleness have been performed for two Co-based amorphous alloy prior to crystallization. It is shown that the brittleness is ...Measurements of the Doppler broadening S-lineshape parameter of positron annihilation and brittleness have been performed for two Co-based amorphous alloy prior to crystallization. It is shown that the brittleness is related to the S-parameter, that is, microdefects may be one of the important factors affecting the embrittlement of the Co-based amorphous alloys.展开更多
Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤2...Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤250, and then tend to constant levels when N≥250. The variations of the longest-lived component (I3,τ3) show that the size of free- volume holes has a minimum at N - 100 and the hole concentration decreases with N. These trends may be due to the variations in crystallinity, the chain ends, entanglements and the Van der Waals interaction between segments. The variations of the second component (I2,τ2) with N reflect the changes of defect properties in crystalline regions of PEG.展开更多
Thermal aging effects on surface of 2.5 Me V Fe ion-irradiated Fe–0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging ...Thermal aging effects on surface of 2.5 Me V Fe ion-irradiated Fe–0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging was performed at 573 K for 5, 50 and 100 h. From the results of Doppler broadening measurement, an obvious trough could be seen in near-surface region from the S parameters and inflection point form at S–W curves. This indicates changes in the annihilation mechanism of positrons in surface region after thermal aging. Coincident Doppler broadening indicates that the density of Cu precipitates in the thermal aged samples decreased, due to recovery of the vacancies.展开更多
The behuviour of B atoms added to mono-and poly-crystalline Ni_3AI alloys has been investi- gated hy positron annihilation technique(PA T).If the additive of B was less than 1.37 at-%, some of B atoms solid-solutioned...The behuviour of B atoms added to mono-and poly-crystalline Ni_3AI alloys has been investi- gated hy positron annihilation technique(PA T).If the additive of B was less than 1.37 at-%, some of B atoms solid-solutioned interstitially into matrix and distorted the lattice,thus the lifetime (τ) of positron (e^+) Or matrix dilated:the others of them segregated on vacancy-like defects and formed "filling effect",thus the mean lifetime (τ)and S parameters decreased.If B added was increased up to 2.22 at-%,more defects having larger free volume were induced by horide eutectic occurred along grain boundaries and grain interior,thus the τ in vacancy-like defects.τ and S parameters increased remarkably.展开更多
By means of the positron annihilation technique (PAT), superfine TiC powders obtained by ball-milling process were characterized. The results showed that there was a clear correlation between the change in TiC powders...By means of the positron annihilation technique (PAT), superfine TiC powders obtained by ball-milling process were characterized. The results showed that there was a clear correlation between the change in TiC powders and that in the PAT parameters. In the high energy ball-milling process, TiC powders were severely deformed, which caused large increase in the amount of defects and severe distortion of lattice. On the other hand, the extreme refinement of grain size of powders resulted in the increase of grain boundary area and enhancement of system free energy. The spacing between deformed domains in 96 h milled samples is somewhat smaller than those in 48 h milled samples, indicating the size of Type II defects in 96 h milled samples is somewhat smaller than that in 48 h milled samples.展开更多
Positron annihilation lifetime (PAL) spectroscopy was applied to investigate the free volume properties of two types of simultaneous interpenetrating polymer networks (SINs), stoichiometric SINs and non--stoichiometri...Positron annihilation lifetime (PAL) spectroscopy was applied to investigate the free volume properties of two types of simultaneous interpenetrating polymer networks (SINs), stoichiometric SINs and non--stoichiometric SINs. PU/VER SINs are all composed of polyurethane(PU) and vinyl ester resin (VER)with different compositions. Mean free volume size in PU/VER deceased with the increase of VER weight percentage, and the relative intensity, relevant to free volume content, increased with VER contents. The effect of chemical bonds between PU networks and VER networks to free volume properties was observed. Results revealed that chemical bonds between the two networks led to molecular denser packing, free volume contraction and made stoichoimetric PU/VER SINs a miscible system.展开更多
The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseu- dopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atom...The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseu- dopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state. The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simu- lation, demonstrating the strong interplay between positron and electron. Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.展开更多
Positron annihilation lifetime spectroscopy (PALS) is a powerful technique for the study of free volume in polymers. The lifetime of ortho-positronium (o-Ps), a bound state of an electron and a positron, can be us...Positron annihilation lifetime spectroscopy (PALS) is a powerful technique for the study of free volume in polymers. The lifetime of ortho-positronium (o-Ps), a bound state of an electron and a positron, can be used to assess the pore size, while the intensity can be used to characterize the number of pores. Based on the values of the long-lived o-Ps components in the lifetime spectra, the radii and fractional free volumes of sulfonated poly (2,6-dimethyl-1,4- phenyleneoxide) (SPPO) membranes with added LiCl and SPPO-PES (Polyethersulfone) blend were compared. Free volume radii in both kinds of membranes are discussed.展开更多
基金S.A.M.acknowledges the Lee Lucas chair in physics and funding by the EPSRC(EP/WO1707511)the Australian Research Council(Centre of Excellence in Future Low-Energy Electronics Technologies-CE 170100039)+1 种基金L.S.further acknowledges funding support through a Humboldt Research Fellowship from the Alexander von Humboldt FoundationOur studies were partially supported by the Center for NanoScience(CeNS)-Faculty of Physics,Ludwig-Maximilians University Munich.
文摘Two-dimensional(2D)semiconductors possess strongly bound excitons,opening novel opportunities for engineering light-matter interaction at the nanoscale.However,their in-plane confinement leads to large non-radiative exciton–exciton annihilation(EEA)processes,setting a fundamental limit for their photonic applications.In this work,we demonstrate suppression of EEA via enhancement of light-matter interaction in hybrid 2D semiconductor-dielectric nanophotonic platforms,by coupling excitons in WS2 monolayers with optical Mie resonances in dielectric nanoantennas.The hybrid system reaches an intermediate light-matter coupling regime,with photoluminescence enhancement factors up to 102.Probing the exciton ultrafast dynamics reveal suppressed EEA for coupled excitons,even under high exciton densities>10^(12)cm^(−2).We extract EEA coefficients in the order of 10^(−3),compared to 10^(−2)for uncoupled monolayers,as well as a Purcell factor of 4.5.Our results highlight engineering the photonic environment as a route to achieve higher quantum efficiencies,for low-power hybrid devices,and larger exciton densities,towards strongly correlated excitonic phases in 2D semiconductors.
基金We thank the financial support from the National Natural Science Foundation of China(Nos.22022305,21773208,21922305,and 21873080)the Fundamental Research Funds for the Central Universities(No.2020XZZX002-06)National Key Research and Development Program of China(No.2017YFA0207700).
文摘The twist angle between two van der Waals coupled monolayers has emerged as a new and powerful degree of freedom for engineering physical properties of semiconductor homo-and hetero-bilayers.While the interlayer twist has shown prominent effect on electronic and optical properties of transition metal dichalcogenide(TMD)bilayers,it remains unclear how it could be used to manipulate the exciton dynamics,especially exciton-exciton annihilation(EEA)process which is the dominant energy loss channel in TMDs under moderate to high exciton density due to strong Coulomb interaction.Herein,we show that the twist angle in TMD bilayers can act as an effective knob to control the EEA process.Specifically,EEA rate constant increases from 1° twisted WSe_(2) bilayers(0.026 cm^(2)/s)by more than twice to 32° twisted bilayers(0.053 cm2/s)and then drops again in 60° twisted bilayers(0.019 cm^(2)/s).This twist-angle dependence can be attributed to the energy difference between indirect and direct excitons arising from the interlayer interaction.Our work opens up the possibility of artificially managing the exciton dynamics in TMD materials for optoelectronic applications via interlayer twist angle.
基金The project supported by the Natural Science Foundation of Zhejiang Province under Grant No. Y604106, the Foundation of New Century 151 Talent Engineering of Zhejiang Province, and the Natural Science Foundation of Zhejiang Lishui University under Grant No. KZ05010 Acknowledgments The authors would like to thank professor Chun-Long Zheng for his fruitful and helpful suggestions.
文摘By means of an improved mapping method and a variable separation method, a series of variable separation solutions including solitary wave solutions, periodic wave solutions and rational function solutions) to the (2+1)-dimensional breaking soliton system is derived. Based on the derived solitary wave excitation, we obtain some special annihilation solitons and chaotic solitons in this short note.
基金This work was supported by National Natural Science Foundation of China
文摘The principles of positron annihilation and four positron experimental techniques are described. The application of positron annihilation technique in material science. atomic physics and other related fields are discussed.
基金Supported by the National Natural Science Foundation of China under Grant No.29674020.
文摘Positron annihilation lifetime spectra measurements were performed on C_(60)-containing chitosan as a function of temperature ranging from 290 to 380K.Both the ortho-positronium annihilation lifetimeт_(3) and its intensity Is increase with increasing temperature,however,the values of I_(3) in C_(60)-containing chitosan are smaller than that in pure chitosan,and the slope ofт_(3) with temperature is also suppressed after C_(60) linked.The existing three models for positron annihilation in polymer have been compared to analyze the experiment result.It is realized that only the spur reaction model,combined with the free volume model,can explain it satisfactorily.The ortho-positronium lifetime and intensity were affected by the local physicochemical environment of material,i.e.,free-volume structure,electron scavenging and trapping sites,etc.The linked C_(60) that play a significant chemical inhibition and quenching role on positronium formation and annihilation isfirst observed.The microstructure change brought by C_(60) group has also been discussed.
文摘This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 ℃ annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600℃ annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.
基金supported by the National Natural Science Foundation of China(Grant Nos.11264018 and 11174114)the Research Foundation of the Education Department of Jiangxi Province of China(Grant Nos.GJJ12171 and GJJ12202)+2 种基金the Science Fund for Youth of Jiangxi Province,China(GrantNos.20132BAB212006 and 2011ZBAB215036)the Open Foundation of the Key Laboratory of Optoelectronic and Telecommunication of Jiangxi Province,China(Grant No.2013004)the National Science Foundation of Jiangsu Higher Education Institute of China(Grant No.12KJD140001)
文摘We investigate the nonclassical properties of arbitrary number photon annihilation-then-creation operation (AC) and creation-then-annihilation operation (CA) to the thermal state (TS), whose normalization factors are related to the polylog- arithm function. Then we compare their quantum characters, such as photon number distribution, average photon number, Mandel Q-parameter, purity and the Wigner function. Because of the noncommutativity between the annihilation operator and the creation operator, the ACTS and the CATS have different nonclassical properties. It is found that nonclassical properties are exhibited more strongly after AC than after CA. In addition we also examine their non-Gaussianity. The result shows that the ACTS can present a slightly bigger non-Gaussianity than the CATS.
基金Supported by the National Natural Science Foundation of China under Grant Nos.19991487,19677205 and 19835060the Foundation of the Chinese Academy of Sciences under Grant No.LVVTZ-1298.
文摘By incorporating the flavour-dependent quark--antiquark annihilation amplitude into the mass-squared matrix describing the mixing of the isoscalar states of a meson nonet,the new version of Schwinger's nonet mass formula,which holds with a high accuracy for the 0^(-+),1^(--),2^(++),2^(-+)and 3^(--)nonets,is derived and the mixing angle of the isoscalar octet--singlet for these nonets is obtained.In particular,the mixing angle of the isoscalar octet-singlet for the pseudoscalar nonet is determined to take the value of-12.92°,which is in agreement with the value range from-13°to-17°deduced from a rather exhaustive and up-to-date analysis of data.It is also pointed out that the omission of the flavour-dependent qq annihilation effect might be a factor resulting in the invalidity of Schwinger’s original nonet mass formula for the pseudoscalar nonet.
文摘The positron lifetime spectra and electrical conductivities have been measured for polyaniline as a function of protonation level ([H+] from 10-7-1000.8 mol/L). Weobserved that (1) the short lifetime τ1, which was related to electron density in bulk,decreased with the protonation level; (2) the intermediate lifetime τ2≈360ps, almost remaining constant, whereas its intensity I2 increased with increasing protonation level which was related to the conductivity of material. These results are discussed in terms of conducting island model.
文摘Samples of the PM-355 polymeric solid state nuclear track detector were exposed to low gamma absorbed doses from 1 kGy (0.1 Mrad) up to 9 kGy (0.9 Mrad). Positron annihilation lifetime (PAL) in conjunction with transmission electron microscopy (TEM) and Thermo-gravimetric analysis (TGA) were performed on irradiated and pristine samples at room temperature. The observed lifetime spectra were resolved into three components where the ortho-positronium (o-Ps) lifetime component was associated with the pick-off annihilation of positronium trapped by the free volume. PAL studies of irradiated PM-355 samples showed that ortho-positronium (o-Ps) lifetime increases with an increase in dose up to 4 kGy and decreases at higher doses. In contrast, the intensity of the o-Ps component, I3, decreases with the dose up to about 2 kGy, followed by a much smooth decrease up to 7 kGy, and then it levels off. TGA analysis indicated that the PM-355 detector decomposed in one main breakdown stage. These results are discussed on the basis of chemical and physical changes occurring at the microscopic level in the PM-355 due to irradiation. Crosslinking dominates for doses between 1 and 4 kGy, while the degradation mechanism (Chain scission) prevails for doses up to 9 kGy.
文摘Rapidly solidified Al92.3Fe4.3V0.7Si1.7Mm1.0 alloy has been studied by positron lifetime spectroscopy and the variations on the intedecial defects with the annealing temperature were revealed by an analysis of the lifetime results. The intedece characteristics derived from the positron-lifetime results could be used to give a satisfactory interpretation of the dependence of mechanical properties on the annealing temperature
基金Project supported by the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20070248120)the Scientific Research Foundation for the Returned Overseas Chinese Scholars,State Education Ministry+1 种基金the National Natural Science Foundation of China (Grant No. 10905038)the Shanghai Rising-Star Programme, China (Grant No. 09QA1403300)
文摘The interactions between solitoffs are extensively investigated. Besides the known solitoff fission and fusion interac- tions, two new types of solitoff interactions are discovered, named the solitoff reconnection and the solitoff annihilation. Taking the asymmetric Nizhnik-Novikov Veselov equation as an illustrative system, five types of solitoff interactions are graphically revealed on the basis of the analytical solution obtained by the modified tanh function expansion method.
文摘Measurements of the Doppler broadening S-lineshape parameter of positron annihilation and brittleness have been performed for two Co-based amorphous alloy prior to crystallization. It is shown that the brittleness is related to the S-parameter, that is, microdefects may be one of the important factors affecting the embrittlement of the Co-based amorphous alloys.
基金The Project Supported by National Natural Science Foundation of China
文摘Positron lifetimes in polyethylene glycol (PEG) polymer have been measured as a function of polymerization degree N (N = 25 - 500). It is observed that all the positron parameters are strongly dependent on N when N≤250, and then tend to constant levels when N≥250. The variations of the longest-lived component (I3,τ3) show that the size of free- volume holes has a minimum at N - 100 and the hole concentration decreases with N. These trends may be due to the variations in crystallinity, the chain ends, entanglements and the Van der Waals interaction between segments. The variations of the second component (I2,τ2) with N reflect the changes of defect properties in crystalline regions of PEG.
基金supported by the National Natural Science Foundation of China(Nos.91026006,91226103,11475193,11475197,11575205and 11505192)Beijing Natural Science Foundation(No.1164017)
文摘Thermal aging effects on surface of 2.5 Me V Fe ion-irradiated Fe–0.6%Cu alloy were investigated using positron annihilation techniques. The samples were irradiated at 573 K to a dose of 0.1 dpa. Their thermal aging was performed at 573 K for 5, 50 and 100 h. From the results of Doppler broadening measurement, an obvious trough could be seen in near-surface region from the S parameters and inflection point form at S–W curves. This indicates changes in the annihilation mechanism of positrons in surface region after thermal aging. Coincident Doppler broadening indicates that the density of Cu precipitates in the thermal aged samples decreased, due to recovery of the vacancies.
文摘The behuviour of B atoms added to mono-and poly-crystalline Ni_3AI alloys has been investi- gated hy positron annihilation technique(PA T).If the additive of B was less than 1.37 at-%, some of B atoms solid-solutioned interstitially into matrix and distorted the lattice,thus the lifetime (τ) of positron (e^+) Or matrix dilated:the others of them segregated on vacancy-like defects and formed "filling effect",thus the mean lifetime (τ)and S parameters decreased.If B added was increased up to 2.22 at-%,more defects having larger free volume were induced by horide eutectic occurred along grain boundaries and grain interior,thus the τ in vacancy-like defects.τ and S parameters increased remarkably.
基金This research is jointly financially supported by the National Natural Science Foundation of China, the Doctoral Edu-cation F
文摘By means of the positron annihilation technique (PAT), superfine TiC powders obtained by ball-milling process were characterized. The results showed that there was a clear correlation between the change in TiC powders and that in the PAT parameters. In the high energy ball-milling process, TiC powders were severely deformed, which caused large increase in the amount of defects and severe distortion of lattice. On the other hand, the extreme refinement of grain size of powders resulted in the increase of grain boundary area and enhancement of system free energy. The spacing between deformed domains in 96 h milled samples is somewhat smaller than those in 48 h milled samples, indicating the size of Type II defects in 96 h milled samples is somewhat smaller than that in 48 h milled samples.
文摘Positron annihilation lifetime (PAL) spectroscopy was applied to investigate the free volume properties of two types of simultaneous interpenetrating polymer networks (SINs), stoichiometric SINs and non--stoichiometric SINs. PU/VER SINs are all composed of polyurethane(PU) and vinyl ester resin (VER)with different compositions. Mean free volume size in PU/VER deceased with the increase of VER weight percentage, and the relative intensity, relevant to free volume content, increased with VER contents. The effect of chemical bonds between PU networks and VER networks to free volume properties was observed. Results revealed that chemical bonds between the two networks led to molecular denser packing, free volume contraction and made stoichoimetric PU/VER SINs a miscible system.
基金Project supported by the Fundamental Research Funds for the Central Universities and the National Natural Science Fundation of China(Grant Nos.11105139 and 11175171)
文摘The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseu- dopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state. The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simu- lation, demonstrating the strong interplay between positron and electron. Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.
基金supported by National Natural Science Foundation (No. 60602065)Innovation Funds of University of Science and Technology of China for Graduate Student (No.KD2006009)
文摘Positron annihilation lifetime spectroscopy (PALS) is a powerful technique for the study of free volume in polymers. The lifetime of ortho-positronium (o-Ps), a bound state of an electron and a positron, can be used to assess the pore size, while the intensity can be used to characterize the number of pores. Based on the values of the long-lived o-Ps components in the lifetime spectra, the radii and fractional free volumes of sulfonated poly (2,6-dimethyl-1,4- phenyleneoxide) (SPPO) membranes with added LiCl and SPPO-PES (Polyethersulfone) blend were compared. Free volume radii in both kinds of membranes are discussed.