Three-gamma annihilation of ortho-positronium in NiO/γ-Al2O3 catalysts detected by positron lifetime and coincidence Doppler broadening measurements

2012 ◽  
Vol 81 (7) ◽  
pp. 791-795 ◽  
Author(s):  
S.H. Huang ◽  
Z.Q. Chen ◽  
H.J. Zhang
Keyword(s):  
Positron Lifetime ◽  
Doppler Broadening ◽  
Coincidence Doppler Broadening

Ni-Substitution Induced Inversion in ZnFe2O4 Seen by Positron Annihilation

2012 ◽  
Vol 733 ◽  
pp. 219-223 ◽  
Author(s):  
P.M.G. Nambissan ◽  
O. Mondal ◽  
C. Chakrabarty ◽  
M. Pal
Keyword(s):  
Positron Annihilation ◽  
Spinel Structure ◽  
Positron Lifetime ◽  
Drastic Change ◽  
Doppler Broadening ◽  
Positron Trapping ◽  
Normal Spinel ◽  
Ni Substitution ◽  
Positron Lifetimes ◽  
Coincidence Doppler Broadening

We report in this work about the inversion of the spinel structure of ZnFe2O4induced by the substitution of Zn2+by Ni2+ions. Positron lifetimes were measured in Zn1-xNixFe2O4with different concentrations (x) of doped Ni2+ions and a drastic change across x = 0.4 – 0.6 was observed, which is attributed to this transformation. The interchange of positions of the cations on doping leaves a fraction of them unoccupied and these vacancies act as positron trapping centres. Since Ni2+is smaller in size than Zn2+, defects due to non-stoichiometry are less in NiFe2O4than in ZnFe2O4. The increase in positron lifetime implies the trapping of positrons being shifted from A- to B-sites and is an indication of the transformation from inverse to normal spinel configuration. Coincidence Doppler broadening measurements supported these findings.

Effect of Fe2O3 Dopant on Electronic Densities and Electrical Properties of ZnO-Based Varistors

2011 ◽  
Vol 415-417 ◽  
pp. 1042-1045
Author(s):  
Wen Deng ◽  
Zhen Quan Lei ◽  
Jiao Ling Zhao ◽  
Yan Qiong Lu ◽  
Ding Kang Xiong
Keyword(s):  
Electrical Properties ◽  
Positron Lifetime ◽  
Nonlinear Coefficient ◽  
Doppler Broadening ◽  
Donor Concentration ◽  
Current Voltage ◽  
Electronic Densities ◽  
Current Voltage Characteristics ◽  
Doppler Broadening Spectra ◽  
Coincidence Doppler Broadening

The effect of the Fe2O3dopant on the electronic densities and the electrical properties has been studied in a ZnO-Bi2O3-SnO2-Co2O3-MnO2-TiO2-Ni2O3-Fe2O3system by the measurements of positron lifetime spectra, coincidence Doppler broadening spectra and current-voltage characteristics. The results show that the 3d electron signal in the spectrum of the varistor increases with the Fe2O3content. The addition of small amount of Fe2O3into the ZnO-based varistor leads to an increase in the donor concentration in the bulk and the defects in the varistor. As the Fe2O3content increased, the threshold voltage (VT) and the nonlinear coefficient (α) of the varistor decreased monotonously.

Mn SUBSTITUTION EFFECTS AND ASSOCIATED DEFECTS IN ZnO NANOPARTICLES STUDIED BY POSITRON ANNIHILATION

NANO ◽  
2011 ◽  
Vol 06 (02) ◽  
pp. 173-183 ◽  
Author(s):  
B. ROY ◽  
B. KARMAKAR ◽  
P. M. G. NAMBISSAN ◽  
M. PAL
Keyword(s):  
Positron Lifetime ◽  
Substitution Effects ◽  
Doppler Broadening ◽  
X Ray Diffraction ◽  
X Ray ◽  
Octahedral Sites ◽  
Selected Area Electron Diffraction ◽  
Transmission Electron ◽  
New Phase ◽  
Coincidence Doppler Broadening

Nanocrystalline ZnO particles substituted with different concentrations (0–30%) of Mn were synthesized by using a modified ceramic route and characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis methods. Positron lifetime and coincidence Doppler broadening measurements were used as probes to identify the vacancy-type defects present in them and monitor the changes while doping. The predominant positron trapping center in the undoped ZnO is identified as the trivacancy-type cluster V Zn + O + Zn , which is negatively charged, and it transformed to the neutral divacancy V Zn + O on doping with Mn 2+ ions. The intensity of the defect-specific positron lifetime component got reduced initially indicating partial occupancy of the vacancies by the doped cations but then recovered on further doping due to the additional Zn vacancies created as a result of the increasing strain introduced by the Mn ions of larger radius. The creation of a new phase ZnMn 2 O 4 thereafter changed the course of variation of the annihilation parameters, as the positrons got increasingly trapped in the vacancies at the tetrahedral and octahedral sites of the spinel nanomanganite.

Positron Annihilation Studies on La1-xSrxMnO3 Manganites

2017 ◽  
Vol 373 ◽  
pp. 201-204
Author(s):  
Yu Yang Huang ◽  
Yan Meng ◽  
Wen Hua Huang ◽  
Shou Lei Xu ◽  
Ding Kang Xiong ◽  
...  
Keyword(s):  
Curie Temperature ◽  
Positron Annihilation ◽  
Electron Density ◽  
Exchange Coupling ◽  
Positron Lifetime ◽  
Double Exchange ◽  
Ferromagnetic Coupling ◽  
Doppler Broadening ◽  
Doppler Broadening Spectra ◽  
Coincidence Doppler Broadening

Positron lifetime and coincidence Doppler broadening spectra of La1-xSrxMnO3 and LaMnO3 manganites were measured. The effects of Sr contents on the microdefects of La1-xSrxMnO3 were investigated by means of positron lifetime results. The signals of 3d electrons of La1-xSrxMnO3 were extracted from the coincidence Doppler broadening spectra. The results show that the electron density of La1-xSrxMnO3 is larger than that of LaMnO3. The open volume of the defect in La1-xSrxMnO3 increases with the Sr contents. The signals of 3d electrons of ferromagnetic La1-xSrxMnO3 is larger than that of antiferromagnetic LaMnO3, and it reaches the maximum at x=0.33, that is, the signals of 3d electrons of the La0.67Sr0.33MnO3 is the highest in all testing samples, which corresponds to a lowest resistivity and a higher Curie temperature of the manganite. The influence of 3d electrons on the ferromagnetic double-exchange coupling and anti-ferromagnetic coupling of La1-xSrxMnO3 were discussed.

Positron lifetime and Doppler broadening study of defects in hydrogen charged Al–Mg alloys

2009 ◽  
Vol 24 (5) ◽  
pp. 1791-1794 ◽  
Author(s):  
Xiaowei Wang ◽  
Yurong Zhong ◽  
Baoyi Wang ◽  
Hongyan Zhang
Keyword(s):  
Grain Boundary ◽  
Positron Lifetime ◽  
Characteristic Change ◽  
Average Lifetime ◽  
Doppler Broadening ◽  
Before And After ◽  
Al Mg Alloys ◽  
Positron Lifetime Measurement ◽  
Momentum Region ◽  
Coincidence Doppler Broadening

The results of positron lifetime and Doppler broadening spectrum of defects in the hydrogen charged non-heat treatable 5xxx Al alloys are presented in this work. The yield stress of the sample was reduced for about 20 MPa after hydrogen was charged. A similar trend was observed in positron lifetime measurement, as the average lifetime τav descended remarkably to almost the level of Al matrix. The change in coincidence Doppler broadening (CDB) spectroscopy was also significant, exhibited by the characteristic change in CDB radio curves of a sample before and after hydrogen was charged. After hydrogen charging, there is an obvious enhancement in the high momentum region compensating dehancement in the low momentum region. This indicates the existence of hydrogen filling effect. The vacancies around the Mg atoms should be preferential filling sites for hydrogen because Mg has a strong affinity for hydrogen. The formation of an Mg–H bond parallel to a grain boundary is an important factor in weakening the grain boundary cohesion.

The 3d Electrons and Microdefects in Binary Ti-Al Alloys Studied by Positron Annihilation

2011 ◽  
Vol 399-401 ◽  
pp. 99-102
Author(s):  
Wen Deng ◽  
Xiao Lei Guo ◽  
Bing Xie ◽  
Ding Kang Xiong ◽  
Yu Yang Huang
Keyword(s):  
Grain Boundary ◽  
Positron Annihilation ◽  
Free Electron ◽  
Positron Lifetime ◽  
Al Alloys ◽  
Doppler Broadening ◽  
Al Content ◽  
Open Volume ◽  
Ti Content ◽  
Coincidence Doppler Broadening

The behavior of 3d electrons and microdefects in binary Ti-Al alloys with Al contents from 47at.% to 53at.% have been studied by coincidence Doppler broadening and positron lifetime techniques. It has been found that the 3d electron signals in the spectra of binary Ti-Al alloys increase with Ti content. In Al-rich Ti-Al alloys, on increasing Al content, the open volume of defect on grain boundary increases while the density of free electron decreases. On the contrary, in Ti-rich Ti-Al alloys, on increasing Ti content, the open volume of defect on grain boundary decreases, and the electron density of the grain boundary increases.

Positron lifetime studies and coincidence Doppler broadening spectroscopy of Al–6Mg–xSc (x = 0 to 0.6 wt.%) alloy

2007 ◽  
Vol 42 (8) ◽  
pp. 2618-2629 ◽  
Author(s):  
M. S. Kaiser ◽  
P. M. G. Nambissan ◽  
M. K. Banerjee ◽  
A. Sachdeva ◽  
P. K. Pujari
Keyword(s):  
Positron Lifetime ◽  
Doppler Broadening ◽  
Lifetime Studies ◽  
Coincidence Doppler Broadening

Positron Annihilation in ZnO Based Varistor Doped with Semiconductor Additives

2015 ◽  
Vol 734 ◽  
pp. 834-840
Author(s):  
Tie Zhu Yang ◽  
Qi Tao Zhu ◽  
Juan Juan Wei ◽  
Zhi Qiang Zhang ◽  
Rui Rui Zhang ◽  
...  
Keyword(s):  
Positron Lifetime ◽  
Doppler Broadening ◽  
Electrical Characteristics ◽  
Maximum Probability ◽  
Pure Zinc ◽  
Ratio Curve ◽  
Different Characteristics ◽  
Doppler Broadening Spectra ◽  
Conventional Ceramic Processing ◽  
Coincidence Doppler Broadening

Coincidence Doppler broadening spectra measurements on ZnO-based varistor doped with semiconductor additives have been investigated, and the proposed samples exhibit different characteristics. The peak of the ratio curve of pure zinc is the highest, the Positron Lifetime in sample a03 doped with ZT is the longest. The maximum probability of the positron-3d electron occurs in sample doped with semi-ZnO (sintered at 1300 °C for 2h with conventional ceramic processing method). The electrical characteristics of the samples and the behavior of positrons in ZnO-based varistor have been discussed.

Influence of Sintering Temperature on Defects in Porous Silica Monoliths Studied by Positron Annihilation Techniques

2008 ◽  
Vol 607 ◽  
pp. 204-206 ◽  
Author(s):  
Yu Yang Huang ◽  
Chao Xiong ◽  
Yu Xin Wang ◽  
Yan Qiong Lu ◽  
Wen Deng
Keyword(s):  
Phase Transition ◽  
Organic Substance ◽  
Positron Lifetime ◽  
Sintering Temperature ◽  
Porous Silica ◽  
Doppler Broadening ◽  
Silica Monoliths ◽  
Ratio Curve ◽  
Doppler Broadening Spectra ◽  
Coincidence Doppler Broadening

Positron lifetime and coincidence Doppler broadening spectra for porous silica calcined at temperatures from 900 to 1500 oC have been measured. As the sintering temperatures increasing from 900 to 1000 oC, the height of the peaks of the ratio curves decrease, and the τ3 increases, which is due to the exclusion of the gas and the organic substance out of the cavities. As the sintering temperatures increasing from 1000 to 1250 oC, some of the defects were recovered, the grains of low quarts appearing and growing. This gives rise to the increase of the height of the peak of the ratio curve, and the decrease of lifetime (τ1,τ2 and τ3). As the sintering temperatures increasing from 1250 to 1500oC, the height of the peak of the ratio curve decreases, and the lifetime increases due to the phase transition of low quartz to cristobalite.

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