B位空位补偿型钐掺杂PZT(54/46)陶瓷中的缺陷分析及其对压电性能的影响

-用固相反应法制备了B位空位补偿型钐掺杂非准同型相界组分PZT(54/46)陶瓷.通过正电子湮没寿命谱(PALS)和符合多普勒展宽能谱(CDBS)对陶瓷中的缺陷结构进行综合表征,结合常规表征手段如X射线衍射(XRD),电子扫描显微镜(SEM),介电、铁电和压电性能测量,研究缺陷对陶瓷压电性能的影响.XRD结果显示所有陶瓷均为纯钙钛矿相,掺杂诱导了菱方-四方(R-T)相变,准同型相界位于Sm掺杂量x=0.010.02.电学测量结果反映:介电、铁电和压电性能均先增强后减弱,MPB附近两个样品都有优异的介电和铁电性能,但其压电性能差别很大.x=0.01给出最优压电性能d_(33)=572 p C/N,较未掺杂样品增强了一倍.PALS结果表明掺杂使陶瓷中缺陷类型发生变化,x≤0.01,样品中同时含有A位空位与B位空位;x≥0.02,样品中以A位相关缺陷为主,B位空位浓度很低.CDBS结果进一步证实x=0.01和0.02中B位空位浓度分别是该体系中最高和最低的.由以上结果推断出:x=0.01获得的最优压电性能与其中较高浓度的B位空位有关,B位空位可稀释A位空位浓度,降低氧空位浓度,从而降低A位空位与氧空位形成缺陷偶极子的几率,促进畴壁运动,使压电性能增强.

掺杂B,Cr,Mo,Ti,W,Zr后金刚石中正电子湮灭寿命计算

金属基金刚石复合材料被广泛应用和研究,但金刚石表面预处理所导致的空位、掺杂等缺陷对金属基与金刚石界面性能有很大影响.尽管透射电子显微镜和能谱分析等技术已用于缺陷检测,但这些方法存在局限性.通过计算金刚石中正电子湮灭寿命,可以准确评估金刚石的界面缺陷.本文利用第一性原理计算方法,采用多种正电子湮灭算法和增强因子,计算了金刚石理想晶体、单空位、掺杂B,Cr,Mo,Ti,W和Zr后的正电子湮灭寿命.结果显示,在采用局域密度泛函时,结合Boronski&Nieminen算法以及随机相位近似限制作为湮灭增强因子时,计算得到的正电子湮灭寿命与文献的实验结果更吻合.同时,金刚石中B和Cr的掺杂使正电子湮灭寿命从由单空位119.87 ps增加为148.57 ps和156.82 ps.总体来说,金刚石中的空位和掺杂原子缺陷都会引起正电子湮灭寿命的变化.这些发现为理解和优化金刚石界面提供了有价值的理论依据.

脉冲电流作用下纯铁及RPV钢缺陷修复的正电子湮没研究

为探究电脉冲处理(Electropulsing Treatment,EPT)对金属材料缺陷的修复作用,设计了一套电脉冲处理设备和与其配套的处理方法,并对电子辐照样品与拉伸样品等含有缺陷的金属材料样品进行电脉冲处理。利用正电子谱学对原子尺度缺陷的十分灵敏的特点,对电脉冲处理后样品进行正电子寿命谱学等研究,获得纯铁及反应堆压力容器(Reactor Pressure Vessel,RPV)钢辐照或形变产生的缺陷及通过电脉冲处理后的部分“修复”情况。结果表明:缺陷“修复”的效果既与样品初始状态有关,又与电脉冲处理参数有关,电脉冲处理样品的正电子平均寿命和维氏硬度变化趋势一致。正电子湮没作为新的无损检测方法有望给出脉冲电流作用下材料损伤或缺陷“修复”的判据,可方便、快速及高灵敏探测实际工况部件的缺陷状态。

超快光纤激光器中可控脉冲产生与湮灭动力学

本文采用实时傅里叶变换光谱探测技术,研究了基于泵浦强度调制的超快光纤激光器中锁模脉冲产生与湮灭动力学过程.结果表明:当泵浦调制电压处于高电平时,激光器输出稳定的锁模脉冲.随着调制电压跳变至低电平,锁模脉冲的强度不断降低,而后经历一段衰减振荡阶段后发生湮灭,在~5μs后孤子从噪声中重建,这一过程伴随着调Q不稳定性的产生.在低电平阶段,激光腔内的湮灭过程连续发生,其周期为~55μs.通过改变调制泵浦的占空比,能够操控在低电平调制下孤子连续湮灭的次数.进一步,锁模与孤子湮灭的连续切换过程与泵浦调制频率有关,调制频率的升高能够有效缩短两种状态的持续时间从而减少孤子湮灭的次数.此外,通过减小低电平的值,能够降低激光腔内的增益,使得孤子连续湮灭的周期缩短.研究结果有利于深入了解孤子的形成与湮灭动力学,并为超快激光器各种运行机制的发展提供了新的视角.

锑化铟晶体空位缺陷的正电子湮灭研究

锑化铟晶体材料的电学性能是影响最终制备的红外探测器件性能的关键因素。材料内部的杂质以及点缺陷特别是空位缺陷会极大的影响材料的电学性能,有时甚至会导致材料反型。本文利用正电子湮灭谱对锑化铟晶体材料的空位缺陷类型进行了研究,同时还对不同晶体生长拉速、导电类型晶体材料的正电子湮灭寿命进行分析。结果表明其内部主要为V In型空位缺陷,且在一定拉速范围内,正电子湮灭寿命基本无变化,此外空位缺陷也不是导致N型锑化铟晶体材料导电类型反型的主要原因。

财政收支的货币效应:理论机制、经验证据及政策蕴含

财政收支的货币效应理论,在财政与金融之间搭建了一座理论桥梁,对新时期中国经济稳定与金融稳定具有重要的实践指导意义。采用部门资产负债表法,通过构建“多元双层”货币供给理论模型,分层揭示基础货币供给与存款货币供给的一般规律。采用联立方程模型、SVAR模型和梁氏因果检验法对财政收支的货币效应命题开展实证检验。结果表明,在国库单一账户制度下,财政收支与货币供给消长之间存在直接因果关系,即财政收支对货币供给具有“扰动效应”,财政收入引起基础货币与存款货币同时、等量湮灭,财政支出引起基础货币与存款货币同时、等量创造。相比于对存款货币的影响,财政收支对基础货币供给的扰动效应更剧烈。中国存款货币实际供给量已接近现存基础货币与法定存款准备金率所共同决定的货币上限,财政收入缴库引起基础货币湮灭可能会诱发银行间市场流动性紧张。基于此,应建立财政部门与央行部门的常态协调沟通机制,完善国库现金管理存款制度和国库现金余额目标管理制度,财政主动有为,更好发挥在稳金融、稳就业中的积极作用。

正电子湮没谱表征热处理工艺对聚碳酸酯力学性能的影响

考察了退火和淬火2种热处理方法所用的不同热处理温度和时间对聚碳酸酯力学性能的影响,并采用正电子湮没谱分析经过不同热处理工艺后聚碳酸酯内部自由体积的含量。结果表明,在玻璃化转变温度附近及以上进行一定时间的淬火可通过增大内部自由体积含量和尺寸来显著改善聚碳酸酯的韧性,降低材料的屈服强度和弯曲强度,提高缺口冲击强度和拉伸强度。在玻璃化转变温度以下,淬火不能改变材料内部的自由体积,不能提高材料的韧性。退火只能降低材料内部自由体积的含量和尺寸,但不受温度和时间的影响,且材料的拉伸强度、断裂伸长率和缺口冲击强度大幅降低。

基于Z变换和改进FRI的LFM信号参数估计方法研究

为了完成线性调频(linear frequency modulation,LFM)信号的稀疏采样,并利用稀疏数据对原始信号参数进行估计,本文提出了一种基于Z变换和改进有限新息率(finite rate of innovation,FRI)的LFM信号参数估计方法。以Z变换理论为基础,设计了一种数学模型,一旦信号能够表达成该数学模型的结构形式,就能通过Z变换和零化滤波器的方法估计信号参数。然后,利用了自相关延迟的FRI结构对LFM信号采样,该结构不仅完成了LFM信号的稀疏采样,而且稀疏采样结果能够与数学模型结构相符。在理论上通过数学论证的方式证明了所提方法能够用于获取LFM信号参数信息,并通过仿真和实测数据验证了所提方法的有效性,理论和实验结果表明该方法只需要4个采样点就能实现对LFM信号的参数估计,并且实验中的参数估计误差均在3%以内,极大的提高有限新息率采样的参数估计效率。

突发事故干扰背景下的铁路运输实时调度方法设计

通过对突发事故干扰背景下铁路运输实时调度问题构建数学模型,并引入烟花算法对调度方案进行寻优的方法,结果表明:该方法在30 min干扰时长下的目标函数值下降到3662;对影响11辆列车的干扰进行实时调度,只对其他2辆列车产生干扰,该方法不仅具有良好的运行性能,还能够有效进行实时调度,可应用于突发事故干扰背景下的铁路运输实时调度。

Annihilation Solitons and Chaotic Solitons for the （2＋1）-Dimensional Breaking Soliton System

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.

Positron Annihilation Mechanism Correlated with Microstructure of C60-Contained Chitosan

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.

APPLICATION OF THE POSITRON ANNIHILATION TECHNIQUE

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.

Effects of Flavour-Dependent qq Annihilation on the Mixing Angle of the Isoscalar Octet-Singlet and Schwinger’s Nonet Mass Formula

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.

Positron Annihilation Study on the Microstructural Stability in Rapidly Solidified Al_(92.3)Fe_(4.3)V_(0.7)Si_(1.7)Mm_(1.0) Alloy

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

POSITRON ANNIHILATION AND CONDUCTIVITY MEASUREMENTS ON POLYANILINE

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.

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

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.

Microstructural Investigation of PM-355 Nuclear Track Detector Subjected to Low-Dose Gamma Irradiation: A Positron Annihilation Lifetime Study

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.

Comparison between photon annihilation-then-creation and photon creation-then-annihilation thermal states:Non-classical and non-Gaussian properties

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.

Mapping of the lunar surface by average atomic number based on positron annihilation radiation from Chang'e-1

A map of the average atomic number of lunar rock and soil can be used to differentiate lithology and soil type on the lunar surface.This paper establishes a linear relationship between the average atomic number of lunar rock or soil and the flux of position annihilation radiation(0.512-Me V gamma-ray) from the lunar surface.The relationship is confirmed by Monte Carlo simulation with data from lunar rock or soil samples collected by Luna(Russia) and Apollo(USA) missions.A map of the average atomic number of the lunar rock and soil on the lunar surface has been derived from the Gamma-Ray Spectrometer data collected by Chang'e-1,an unmanned Chinese lunar-orbiting spacecraft.In the map,the higher average atomic numbers(ZA > 12.5),which are related to different types of basalt,are in the maria region;the highest ZA(13.2) readings are associated with Sinus Aestuum.The middle ZA(~12.1) regions,in the shape of irregular oval rings,are in West Oceanus Procellarum and Mare Frigoris,which seems to be consistent with the distribution of potassium,rare earth elements,and phosphorus as a unique feature on the lunar surface.The lower average atomic numbers(ZA < 11.5)are found to be correlated with the anorthosite on the far side of the Moon.

Effects of thermal aging on Fe ion-irradiated Fe–0.6%Cu alloy investigated by positron annihilation

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.