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用浸渍法制备了一系列不同Mo含量的MoO3/Al2O3催化剂.测量了这些样品的正电子湮没寿命谱(PALS)与符合多普勒展宽(CDB)谱,以研究其孔洞结构以及Mo分散.正电子寿命测量结果表明,Al2O3载体中存在两种不同尺寸的孔洞.掺入MoO3之后,Mo原子主要进入Al2O3的大孔中,使孔洞体积减小.符合多普勒展宽谱结果表明,当MoO
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3/Al2O3催化剂')" href="#">MoO3/Al2O3催化剂
正电子湮没寿命谱
符合多普勒展宽
Mo 分散 相似文献
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钨合金中钾的掺杂会引入大量的缺陷,如尺寸几十纳米的钾泡、高密度的位错以及微米量级的晶粒带来的晶界等,这些缺陷的浓度和分布直接影响合金的服役性能.本文运用正电子湮没谱学方法研究钾掺杂钨合金中的缺陷信息,首先模拟计算了合金中各种缺陷的正电子湮没寿命,发现钾的嵌入对空位团、位错、晶界等缺陷的寿命影响很小;然后测量了不同钾含量掺杂钨合金样品的正电子湮没寿命谱,建立三态捕获模型,发现样品中有高的位错密度和低的空位团簇浓度,验证了钾对位错的钉扎作用,阐述了在钾泡形成初期是钾元素与空位团簇结合并逐渐长大的过程;最后使用慢正电子多普勒展宽谱技术表征了样品中缺陷随深度的均匀分布和大量存在,通过扩散长度的比较肯定了钾泡、晶界等缺陷的存在. 相似文献
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用浸渍法制备了一系列不同Mo含量的MoO3/Al2O3催化剂.测量了这些样品的正电子湮没寿命谱(PALS)与符合多普勒展宽(CDB)谱,以研究其孔洞结构以及Mo分散.正电子寿命测量结果表明,Al2O3载体中存在两种不同尺寸的孔洞.掺入MoO3之后,Mo原子主要进入Al2O3的大孔中,使孔洞体积减小.符合多普勒展宽谱结果表明,当MoO3的质量含量仅为3%时,多普勒展宽谱即发生了显著的改变.这表明Mo已分散至Al2O3的孔洞中,使得正电子测量所得到的电子动量分布发生改变.在MoO3含量达到18%之后,Al2O3中大孔的体积减小到尺寸与小孔相当,此后正电子寿命和多普勒展宽谱不再随MoO3含量变化,表明Mo分散逐渐达到饱和. 相似文献
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测量了不同C或B含量经不同烧结温度制备的Si基半导体、单晶Si、单晶SiO2、石墨和纯多晶B样品的正电子寿命谱和符合正电子湮没辐射Doppler展宽谱。结果表明,石墨的商谱谱峰最高,SiO2的谱峰次之,B的谱峰最低。随着B,C和O原子序数的增加,与正电子湮没的电子动量增加。含20%的C和含100ppm的B的样品的商谱的谱峰最高;含100ppm的B的样品的谱峰次之;含1ppm的B的样品的谱峰最低。随着烧结温度的升高,含100ppm的B的Si基半导体样品的商谱降低,正电子寿命增长,缺陷开空间和浓度升高。 相似文献
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用正电子湮没寿命测量研究了在3.2×1017cm-2和3.6×1016cm-2质子辐照硅单晶中的正电子捕获,观测到辐照诱导的捕获正电子的缺陷主要是双空位,辐照过程中高剂量样品的双空位基本上都捕获了氢,低剂量样品还有一小部分未捕获氢,在高剂量样品的两轮和低剂量样品的第一轮升温测量中都观测到双空位进一步捕获氢,含氢双空位的荷电态随温度升高在145K附近发生由负荷电向中性转变,它的负荷电态正电子寿命比中性态正电子寿命长,无论
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D. Herlach H. Stoll W. Trost H. Metz T. E. Jackman K. Maier H. E. Schaefer A. Seeger 《Applied Physics A: Materials Science & Processing》1977,12(1):59-67
By means of an integrated source-specimen technique the temperature dependence of positron lifetimes and annihilation lineshapes
has been measured, on the same specimens of gold and cadmium from 4.2K to the melting points, and also in electronirradiated
and quenched gold. The anomalous temperature dependence of positron annihilation at intermediate temperatures (200 to 350
K in Cd, 270 to 750 K in Au) discovered by Lichtenberger, Schulte, and MacKenzie is confirmed. The data are incompatible with
the idea that the intermediate temperature dependence is due to thermal expansion. They are well explained by an extension
of the trapping model which includes the formation of metastable self-trapped positrons. From lineshape measurements after
electron irradiation at 180 K and after quenching it is deduced that the trapping rate of positrons at vacancy-type defects
in Au is temperature independent below room temperature. 相似文献
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A. Jennane J. Bernardini P. Moser G. Moya 《Applied Physics A: Materials Science & Processing》1994,59(2):163-168
Samples of the near equiatomic NiSb compound were irradiated by 3 MeV electrons at 20 K or quenched from 1103 K and 1333 K and subsequently annealed isochronally. The behaviour of defects created by quench or irradiation were studied by the positron annihilation technique. Only one recovery stage was found around 425 K for quenched specimens, but two distinct stages (100 K and 425 K) were observed after irradiation. The 425 K stage is ascribed to the migration of Ni vacancies giving dislocation loops. The recombination of mobile interstitials with vacancies after irradiation is assumed to occur between 100 K and 250 K. Doppler broadening and lifetime variations of positrons as a function of the measuring temperature in these irradiation samples are discussed. 相似文献
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We have studied two-dimensional argon and nitrogen physisorbed on grafoil by conducting positron lifetime and Doppler broadening measurements as functions of adsorbate coverage and temperature. Positron lifetime and Doppler broadening parameters show turnarounds near one-half monolayer coverages at 77 K. The s parameter of the Doppler broadening spectra measured for one-half monolayer coverages of argon and nitrogen increases with temperature across the melting phase transition. We discuss the observed coverage and temperature effects following models based on Kosterlitz-Thouless-Nelson-Halperin-Young theory of two dimensional melting and positron localization in surface defects. 相似文献
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An Experimental Study of Mg Aggregation in AA5754 Alloys by Positron Annihilation Spectroscopy 下载免费PDF全文
Defects in an AA5754 (Al-3.0%Mg) alloy are investigated by coincidence Doppler broadening spectroscopy and positron lifetime spectroscopy. The results indicate enhancement of positron trapping by Mg atoms in this Al-Mg alloy after quenching treatment at 623K, which may be due to the formation of vacancy-Mg complexes or the aggregation of Mg near the vacancy sites. It is speculated that the aggregation of Mg atoms in the moderate temperature range is responsible for cracking in spot welding of AA5754 alloys. 相似文献
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C. Lopes Gil A. P. De Lima N. Ayres De Campos P. Sperr G. Kögel W. Triftshäuser 《辐射效应与固体损伤》2013,168(4):111-118
Abstract Deuteron-irradiated and deformed stainless steel specimens were investigated by positron lifetime and Doppler broadening measurements. The evolution of the defect structures was studied as a function of the isochronal annealing temperature and for various degrees of deformation. A different behaviour was observed for deformed and irradiated stainless steel specimens. Evidence for vacancy clusters was found in the deuteron-irradiated steel. These clusters disappear after annealing around 900 K. 相似文献
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Abstract Positron lifetime and Doppler broadening measurements have been done to study the α-induced defects in stainless steel SS 302 and polycrystalline cobalt. For stainless steel the samples have been thin and the effect of the presence of helium on defect kinetics has been avoided by allowing the 30 MeV alpha particles to pass through the samples. The presence of impurity (carbon) atoms in steel is found to play an important role in the trapping and detrapping of vacancies in the temperature range 200°C to 450°C. Formation of vacancy-impurity complexes and their dissociation around 500°C have been observed. A steady decrease of the positron parameters has been seen beyond 700°C and they attain those of the reference sample around 1000°C. While annealing alpha irradiated cobalt we find migration of vacancies and rearrangement of dislocation loops below 650 K and then dissolution around 900 K. Helium-vacancy complexes form in the region 600 to 1000 K, leading to the growth of the He-bubble above 1000 K. The trapping model analysis shows strong interaction between He and vacancy clusters in the temperature region 600 to 1000 K. 相似文献
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H. C. Jamieson B. T. A. McKee A. T. Stewart 《Applied Physics A: Materials Science & Processing》1974,4(1):79-82
The precision of relative positron lifetime measurements is now sufficient to observe directly changes due to lattice thermal expansion. This information is necessary for the determination of accurate vacancy formation energies from positron annihilation data obtained over a higher temperature range. Measurements have been made of the change in positron lifetime in the metals Au, Ag, Cu, and Al over the temperature range 100–300 K where there is no contribution from positron trapping at vacancies. The results are compared with theoretical calculations which take account of positron annihilations with both valence and core electrons. 相似文献
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Positron lifetime and Doppler broadening of annihilation radiation have been measured in aluminium between 12 mK and 300 K. Positrons are not localized in aluminium in the whole temperature interval. The value of the positron lifetime is 167±4 ps. The data obtained for theS-parameter above 77 K is explained by simple thermal expansion. 相似文献
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W. Fuhs U. Holzhauer F. W. Richter 《Applied Physics A: Materials Science & Processing》1980,22(4):415-419
The positron lifetime in electron-irradiated undoped and doped silicon crystals is studied as a function of temperature between
90 and 300 K. We show that the temperature dependence of the two lifetime components does not arise from the escape, but from
the trapping rate at defects. The temperature dependences of the capture cross sections are deduced. It is concluded that
in undoped crystals the positrons interact with negatively charged and neutral defects, probably divacancies and vacancy-oxygen
complexes, respectively. In strongly P-doped crystals positron trapping occurs preferably in negatively charged centers. 相似文献
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Annealing Behaviour of Helium Bubbles in Titanium Films by Thermal Desorption Spectroscopy and Positron Beam Analysis 总被引:1,自引:0,他引:1 下载免费PDF全文
Helium-containing Ti Glms are prepared using magnetron sputtering in the helium-argon atmosphere. Isochronal annealing at different temperatures for an hour is employed to reveal the behaviour of helium bubble growth. Ion beam analysis is used to measure the retained helium content. Helium can release largely when annealing above 970K. A thermal helium desorption spectroscopy system is constructed for assessment of the evolution of helium bubbles in the annealed samples by linear heating (OAK/s) from room temperature to 1500K. Also, Doppler broadening measurements of positron annihilation radiation spectrum are performed by using changeable energy positron beam. Bubble coarsening evolves gradually below 680K, migration and coalescence of small bubbles dominates in the range of 68-970 K, and the Ostwald ripening mechanism enlarges the bubbles with a massive release above 970K. 相似文献