Diagnostics of nanodisperse polycrystals based on the polarization bremsstrahlung of relativistic electrons

The spectra of polarization bremsstrahlung are measured in the backscattering geometry during the interaction of 7-MeV electrons with a polycrystalline Ni foil. Measurement is conducted under conditions such that the size of the region of coherent X-ray radiation scattering in a target is on the order of 10 nm. The obtained results make it possible to suggest that using polarization bremsstrahlung as a new method for the diagnostics of the atomic structures of nanodisperse polycrystals is effective.     

Bremsstrahlung,Cerenkovstrahlung und Übergangsstrahlung in einer dünnen Schicht

We present a formula for the coherent emission of Bremsstrahlung, Cerenkov radiation and transition radiation of a single electron traversing a thin foil at an oblique angle of incidence and changing its velocity (direction or amount) inside the foil. As an example we consider the bombardment of silicon foils with 50 keV electrons. The statistical scattering of the electrons inside the foil is taken into account by means of a measured scattering pattern.     

Radiation Emission of Fast Electrons in Collisions with “Ion‐Sphere” in Dense Plasmas

Radiative emission of fast electrons in collision with an “ion‐sphere” electron distribution in dense plasmas is under consideration. The electron structure of the ion sphere is calculated ab initio using self‐consistent solution of both bound and free electron distribution inside the sphere. Two radiation channels are included: emission of the colliding electron itself in static potential (conventional or static Bremsstrahlung) and the emission of “ion sphere” medium due to its polarization by the colliding electron (polarization Bremsstrahlung). The last one is calculated in the frame of local plasma density approximation. Interference between conventional and polarization Bremsstrahlung is taken into account. It is shown that spectral cross section of the process has characteristic features depending on plasma density and ionization stage of plasma ions. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectral structure of polarization radiation from relativistic electrons in aluminum

Collective effects in polarization bremsstrahlung generated by relativistic electrons in a polycrystalline aluminum foil are studied experimentally on the basis of the 2.4-MeV electron accelerator installed at the Institute of Nuclear Physics (Moscow State University, Moscow). A peak structure found in this polarization bremsstrahlung for the first time is in agreement with theoretical predictions. __________ Translated from Yadernaya Fizika, Vol. 63, No. 11, 2000, pp. 2097–2100. __________ Translated from Yadernaya Fizika, Vol. 63, No. 11, 2000, pp. 2097–2100.     

Photon polarization effects in inverse photoemission from Cu(001)

A study of the polarization of Bremsstrahlung emitted from a Cu(001) single crystal surface helps to identify surface states and allows to determine the parity of bulk final states involved in a particular radiative transition if the angle of electron incidence is varied in a mirror plane of the crystal. In case of overlapping peaks with different polarization, a polarization dependent light detection increases in addition the apparent experimental resolution considerably.     

Characterization Method of Polycrystalline Materials Using Conductive Atomic Force Microscopy

An apparatus for characterization of polycrystalline materials based on conductive atomic torce microscopy （cAFM） is developed and a quantitative measurement of electrical characteristics of individual grains in polycrystalline ZnO ceramic is demonstrated. Improvement of the experimental method is presented. Experimental results illuminate unambiguously the different electrical characteristics between individual grains, suggesting the suitability and maneuverability of this method in the study of local structure or properties and their relationship in polycrystalline materials such as semi-conducting ceramics.     

Potts-Ising model for simulation of polarization switching in polycrystalline ferroelectrics

This paper proposes a scheme based on the Potts and Ising models for simulating polarization switching of polycrystalline ferroelectrics using the Monte Carlo method. The polycrystalline texture with different average grain size is produced from the Potts model. Then Ising model is implemented in the polycrystalline texture to produce the domain pattern and hysteresis loop. The domain patterns and hysteresis loops have been obtained for polycrystalline texture with different average grain size. From the results of domain pattern evolution process under an applied electric field using this scheme, an extended domain, which covers more than one grain with polarization aligned roughly in the same direction, has been observed during the polarization reversal. This scheme can well reproduce the basic properties of polycrystalline ferroelectrics and is a valuable tool for exploring the physical properties of polycrystalline ferroelectrics.     

Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465?K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513?K. A change in the front propagation mechanism is proposed to explain the observed effects.     

Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition

The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH₄ flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.     

PT/PZT/PT铁电薄膜的铁电畴和畴壁

用sol-gel法制备出了具有良好铁电性、纯钙钛矿结构的PbTiO₃/Pb(Zr_0.3Ti_0.7)O₃/PbTiO₃(PT/PZT/PT)新型夹心结构铁电薄膜.用扫描力显微镜(SFM)的压电响应模式获得了薄膜铁电畴的垂直于膜平面方向(OPP)、膜平面内(IPP)及OPP的相位和幅度图像，结合理论分析指出薄膜的电畴主要由c畴和偏离垂直于膜平面方向上的c畴构成，薄膜取向的复杂性导致了复杂的畴结构.对于［111］取向的薄膜，当偏离垂直于膜平面方向上的c畴在垂直膜平面方向和面内方向都相反时构成180°的畴壁，在垂直膜平面方向上相同、面内方向相反或由垂直膜平面方向上相反、面内方向相同时构成90°畴壁. 关键词： 铁电薄膜 PT/PZT/PT 电畴和畴壁 扫描力显微镜(SFM)     

Role of polarization Bremsstrahlung in the formation of thick target Bremsstrahlung spectra in the energy range of 5–10 keV

Role of polarization Bremsstrahlung in the formation of total Bremsstrahlung (BS) spectra in thick targets of Al, Ti, Sn and Pb, produced by complete absorption of ⁹⁰Sr beta particles having an energy range of 0–546 keV, are studied in the photon energy region of 5 to 10 keV. The theoretical BS spectral photon distributions, obtained from Elwert corrected (nonrelativistic) Bethe–Heitler theory, a modified Elwert factor (relativistic) Bethe–Heitler theory that describe ordinary Bremsstrahlung (OB) and a modified Elwert factor (relativistic) Bethe–Heitler theory for BS spectra that includes the polarization Bremsstrahlung (PB) into OB in stripped atom approximation, were compared with the experimentally measured BS spectral photon distributions. It has been observed that the experimental results are in agreement with the modified Elwert factor (relativistic) Bethe–Heitler theory at photon energy from 5 to 10 keV. It has been also observed that the contribution of PB into OB decreases with increase in end‐point energy of beta emitter and the energy of the emitted photon. Further, it has been found that the contribution of PB into OB increases with increase in atomic number of the target atom. This indicates the importance of PB in the formation of BS produced by continuous beta particle. Copyright © 2012 John Wiley & Sons, Ltd.     

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.     

金刚石薄膜在多晶铜和磷脱氧铜基片上的生长

分别采用99.99%的多晶铜片和99.95%的磷脱氧铜片作为沉积金刚石薄膜的基片，通过热丝化学汽相沉积法在两种基片上都获得了大面积、自支撑的多晶金刚石膜.使用高分辨率光学显微镜、扫描电子显微镜、Raman光谱和X射线衍射比较分析了两种铜基片上的金刚石膜.脱氧铜上的金刚石膜质量并不亚于多晶铜上的金刚石膜，而且它的成核密度、生长速率以及应力都高于多晶铜上金刚石膜的同类参数.特别采用了退火工艺和优化的生长条件来获得大面积的连续金刚石膜. 关键词：     

Information on neutral weak currents from circular polarization of bremsstrahlung

Bremsstrahlung of unpolarized electrons is analysed for information on neutral weak interactions. The coupling of axial lepton and polar hadron neutral currents should cause a circular polarization of about 10^?6 at electron and photon energies of a few hundred MeV and forward angles. The specific nuclear structure is of modest importance.     

Messung der zirkularen Polarisation der Inneren Bremsstrahlung von90Y

The circular polarization of the Internal Bremsstrahlung (IB) of⁹⁰Y was measured with a Compton polarimeter according to Beard and Rose. The results are compatible with the theoretical predictions of Ford and Martin [8], who include “detour transitions” in their calculations of the circular polarization. Measurements ofΒ-γ-correlations of the IB of⁹⁰Y are discussed, which suggest the consideration of detour transitions too.     

多晶材料晶粒生长粗化过程的相场方法模拟

基于采用晶体有序化程度参量ψ和晶体学取向θ来表示多晶粒结构的相场模型,利用自适应有限元方法模拟了多晶材料等温过程中的晶粒粗化现象.模拟结果显示,在曲率作用下,通过晶界迁移弯曲晶界逐渐平直化,小晶粒逐渐被大晶粒吞并,当晶界之间的取向差较小时,满足一定能量和几何条件的两晶粒在界面能作用下会发生转动,合并为单个晶粒.模拟结果与实验结果符合较好.因此,该相场模型可以很好地用来模拟固态相变中多晶材料的生长粗化等现象. 关键词： 相场 晶界迁移 晶粒转动 粗化     

Direct transitions in isochromat spectra from Ni(100)

Bremsstrahlung isochromat spectra at $\text{h}\text{?}\text{ω = 9.7 eV}$ were recorded from well characterized samples of polycrystalline nickel and Ni(100). Differences between spectra from the two samples are attributed to direct optical interband-transitions. Predictions from theoretical band structure calculations are in accord with observed features.     

Characterisation of a megavoltage linear accelerator Bremsstrahlung by means of Monte Carlo simulations

This work presents a method for megavoltage X‐ray Bremsstrahlung assessment by means of Monte Carlo (MC) (PENELOPE) simulations. The proposed method provides suitable spectrum characterisation by means of a few parameters, namely the acceleration potential and incident angle of electron beam on the anode. The obtained spectra have been used as incident beam for in‐depth dose distribution determination with the aim of performing further comparisons with experimental data. This comparison procedure has been employed for Bremsstrahlung assessment optimisation. Once the Linac Bremsstrahlung is well established, it may be used as incident beam for further dosimetric purposes, such as spatial dose distribution calculations. The whole process has been applied for the characterisation of the megavoltage Bremsstrahlung from a typical Linac, namely the 10 MV Varian Clinac 18. The obtained results for in‐depth dose distributions, when compared with experimental measurements, show an overall good agreement, which may suggest the feasibility of the method. Copyright © 2010 John Wiley & Sons, Ltd.     

Crystallographic analysis of lightly facetted cylindrical crystals

A method is described for determining the crystallographic orientation of cylindrical crystals from the geometry of surface facets. The method is applied to individual grains in polycrystalline wire samples having a bamboo structure, where classical methods of orientation determination, such as electron and X-ray diffraction, are inapplicable. In addition to providing information on the orientation of grains, the methods identifies the crystallographic orientation of the facets, which is of potential importance in studies of facet formation.