Determination of the center composition of gradient-activated lithium niobate crystals doped with magnesium and chromium

Investigations of the defect structure of a gradient-activated stoichiometric LiNbO₃:Mg²⁺, Cr³⁺ crystal, in which the chromium concentration has a constant value of 0.1 at. % along the crystal growth axis, and the magnesium concentration changes smoothly from 5.5 to 2.0 al. %. The threshold concentration of Mg²⁺ ions (2.35 at. %) in these crystals with a concentration width of change in the center composition (Cr optical center and its nearest neighbors) of 0.09 at. % Mg²⁺ ions is established..     

稀土掺杂钙钛矿型氟化物晶体生长及其光谱

用Bridgman-Stockbarger法长出了Eu²⁺:KMgF₃和LiBaF₃单晶。用区熔法长出Re:LiBaF₃(Re=Ce³⁺,Tb³⁺,Eu²⁺)晶体。X光衍射分析表明由相图得到的原料比BaF₂:LiF=40:60(mol%)时生长的是多晶,因为,最开始析出的是BaF₂。对掺杂晶体进行了光谱研究。氮分子激光器泵浦的Eu²⁺:KMgF₃荧光光谱显示了多中心结构。     

Spectral-luminescent properties of gradient-activated LiNbO<Subscript>3</Subscript> crystals with concentration profiles of Yb<Superscript>3+</Superscript> and Er<Superscript>3+</Superscript> ions

The luminescent properties of gradient-activated crystals of lithium niobate with concentration profiles of optical centers, ytterbium and erbium ions, are studied. It is shown that the spectral-luminescent properties of the gradient crystals are correlated with the concentration profiles of optical centers — donors Yb³⁺ and acceptors Er³⁺.     

An Optical Device for Determining the Local Characteristics of Crystals by Means of Counterpropagating Laser Beams

An optical device designed for nondestructive quality control and measurements of local characteristics in bulks of crystals and other optical elements with a spatial resolution of the order of 0.1 mm is described. The parameters determined experimentally are the concentration of impurity ions, the coefficient of scattering at an angle of 90°, and the light extinction coefficient within local regions. The operating principle of the device is based on the fluorescent method of counterpropagating laser beams. The distributions of characteristics along the lengths of LiYF₄: Er³⁺and LiYF₄: Nd³⁺crystals are studied.     

Measuring the lasing threshold of a passively Q-switched diode-pumped pulsed solid-state laser

A method for measuring the generation threshold of a passively Q-switched diode-pumped pulsed solid-state laser from the frequency-watt characteristic, which reflects the dependence of the laser-pulse repetition frequency on the absorbed pumping power, is proposed. The results of measuring the generation threshold of lasers based on Y₃Al₅O₁₂:Nd³⁺ and Ca₃Ga₂Ge₃O₁₂:Nd³⁺ crystals are presented.     

Growth of AlGaN/GaN heterostructures with a two-dimensional electron gas on AlN/Al2O3 substrates

The possibility of using AlN/Al₂O₃ substrates to grow AlGaN/GaN hetero-epitaxial structures with a two-dimensional electron gas is studied. A method of calibrating the temperature of the substrates by measuring the thermal radiation spectrum is proposed. Differences between AlN/Al₂O₃ substrates that lead to differences in the electrophysical parameters of the grown structures are determined. AlN/Al₂O₃ substrates were used to grow AlGaN/GaN samples with a two-dimensional electron gas mobility in excess of 1300 cm²/(V · s) at an electron concentration in the channel higher than 10¹³ cm^?2.     

Effect of the Concentration of Nd3+Ions in KGd(WO4)2: Nd3+Crystals on Laser Characteristics

Active KGd(WO₄)₂: Nd³⁺elements used in laser engineering have typical concentrations of Nd³⁺ions of 3 at. %. Our studies have shown that the optimal concentration is close to 4.5 at. %. This result is confirmed for active elements 3 mm in diameter and 50 mm long in two lasing modes at a wavelength of 1.067 m and in the stimulated Raman scattering (SRS) self-conversion mode at 1.538 m in a lamp-pumped laser.     

Wear properties of two-phase Al2O3/ZrO2 (Y2O3) ceramics at temperatures from 296 to 1073 K

Reciprocating sliding friction experiments were conducted with various two-phase, directionally solidified Al₂O₃/ZrO₂ (Y₂O₃) pins sliding on B₄C flats in air at temperatures of 296, 873, and 1073 K under dry sliding conditions. Results indicate that all the Al₂O₃/ZrO₂ (Y₂O₃) ceramics, from highly Al₂O₃-rich to ZrO₂-rich, exceed the main wear criterion requirement of 10⁻⁶ mm³ N⁻¹ m⁻¹ or lower for effective wear-resistant applications. Particularly, the eutectics and Al₂O₃-rich ceramics showed superior wear properties. The composition and microstructure of Al₂O₃/ZrO₂ (Y₂O₃) ceramics played a dominant role in controlling the wear and friction properties. The controlling mechanism of the ceramic wear, friction, and hardness was an intrinsic effect involving the resistance to shear fracture of heterophase bonding and cohesive bonding and the interlocking microstructures at different scales in the ceramics.     

Studying tribological characteristics of alumina- and zirconia-based ceramics

Tribotests of ceramic specimens of various compositions (Al₂O₃-1% TiO₂, ZrO₂-5.3% Y₂O₃, Al₂O₃-15% (ZrO₂ + 5.3% Y₂O₃)) were carried out under dry friction conditions. It has been shown that all of the specimens have a high wear resistance, while the Al₂O₃-15% (ZrO₂ + 5.3% Y₂O₃) specimens have the lowest wear rate and the highest microhardness. This is due to the fine-grained structure of these specimens and their higher density compared to that of the other ceramic compositions.     

Fast-response thermistors made of synthetic single-crystal diamonds

Miniature thermistors are produced from boron-doped synthetic single-crystal diamonds grown under pressure using the thermal gradient method. It is shown that heavily doped diamonds with a boron concentration of 10¹⁹ cm^?3 or higher are most suitable for this purpose. In the temperature range of 300–700 K, coefficient β = ln(R ₁/R ₂)/(1/T ₁ ? 1/T ₂) is 2500 K. The characteristic response time of temperature-sensitive elements based on crystals with dimensions of 1 × 1 × 0.3 mm is ～100 µs; i.e., they can be used in monitoring systems with a response speed of up to 10 kHz.     

Internal and platinum initiated crystallization in Na2O.2 CaO. 3 SiO2 glasses: early stages of growth

Transmission electron microscopy at 100, 900 and 1000 kV was used to study the early stages of growth of internally nucleated crystals of the phase Na₂O.2 CaO. 3 SiO₂ (‘NC₂S₃’) in glasses close to the NC₂S₃ composition. Glass samples were partially crystallized by heat treatment in the range 833–873 K and thin foils for electron microscopy were prepared by ion beam thinning. The phase identified by electron diffraction from a number of small crystals (the low temperature form of NC₂S₃) was the same as that determined by X-ray diffraction of fully crystallized glass ceramics. Selected area diffraction and bright and dark field microscopy showed that in the early stages of development the crystallization centres were essentially single crystals but contained a significant concentration of defects. These crystals were nucleated, probably homogeneously, at temperatures higher than the well-established low to high reversible polymorphic transformation of the NC₂S₃ crystalline phase. In a glass of similar composition to which platinum was deliberately added there was clear evidence for heterogeneous nucleation of NC₂S₃ crystals on platinum particles distributed in the glass. Also the contact angle between NC₂S₃ crystals and the rounded platinum particles could be measured directly.     

Microscopy and microanalysis of crystalline glazes

Crystalline glazes on ceramic plates produced commercially in the U.K. and on ceramic pots produced commercially in Taiwan and Spain have been examined by X‐ray diffraction, conventional and polarized light microscopy, and scanning electron microscopy in order to identify the crystalline phases present in the glazes and to ascertain through X‐ray microanalysis the partitioning behaviour of the transition metal ions used to colour the glazes and the crystals within them. In each case examined, the macroscopic two‐dimensional spherulites within the glazes clearly seen by the naked eye were found to consist of large numbers of radially orientated acicular crystals each 5 µm or less in width embedded within the silica‐rich glaze. Energy dispersive X‐ray microanalysis and X‐ray diffraction of these crystals identified these crystals as willemite, α‐Zn₂SiO₄. The strong [001] texture of these crystals within the glaze evident from the X‐ray diffraction patterns was consistent with polarized light microscopy observations of the willemite crystals. In addition to willemite, small iron‐doped gahnite (ZnAl₂O₄) crystals were found in a honey‐coloured crystalline glaze and acicular rutile (TiO₂) crystals were found in the Portmeirion Pottery plates examined. Transition metal ions with a preference for tetrahedral coordination were observed to substitute for Zn²⁺ ions in willemite and to partition preferentially to the willemite crystals, whereas ions preferring octahedral coordination preferred to remain in the glaze.     

Energy dependence of the relative light output of YAlO<Subscript>3</Subscript>:Ce,Y<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Ce,and YPO<Subscript>4</Subscript>:Ce scintillators

The nonlinear dependence of the relative light output on the energy deposited in single-crystal scintillation materials YAlO₃:Ce (YAP:Ce), Y₂SiO₅:Ce (YSO:Ce), and YPO₄:Ce (YPO:Ce) has been studied. The investigations have been conducted under quasi-monochromatic X-ray excitation in the energy range of 9.5–100 keV. In addition to the standard technique for measuring the nonproportional scintillator response based on the dependence of the full-energy peak position on the energy of incident radiation, a method is proposed for measuring the light output by X-ray fluorescence peaks. Using this method for YAP:Ce, it is possible to investigate the nonlinear dependence of the light output on the photon energy in the energy range of 2–40 keV. Along with this method, the K-dip spectroscopy method has been proposed and tested by measuring the dependence of the relative light output on the electron energy in the range of 0.1–80.0 keV. The processes resulting in the loss of the scintillation material efficiency at a high ionization density are considered.     

Interactions of uranyl ions with lipid bilayer membranes

Uranyl ions (UO₂²⁺), once bound to the phosphate moieties of phospholipid head groups, stabilize bimolecular lipid membranes (BLMs) as well as decrease the nonactin-induced membrane conductance. UO₂²⁺ bind to a phosphatidyl choline-cholesterol (2:1, molar ratio) BLM surface with a dissociation constant of 2·3 μM and a maximum change in surface potential of 88 mV, which corresponds approximately to one uranyl ion per 31 nm² surface area. Furthermore, uranyl ions can penetrate the lipid bilayers as neutral complexes such as uranyl acetate.     

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti₂AlC grains in different materials as well as on large M₂₃C₆ precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the ¹³C isotope, and calculating the concentration of ¹²C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti₂AlC an additional difficulty arises from the overlap between C₂⁺, C₄²⁺ and Ti²⁺ at the mass-to-charge 24 Da.     

Evaluation of Y2O3 surface machinability using ultra-precision lapping process with IED

Prospects of Y₂O₃ have been more extended as a great promising and creditable material for optical, electronic and mechanical purposes. Y₂O₃ has been more observed as a fine ceramic which has great material properties: high light transparency, excellent thermal resistance and chemical inertness. But in terms of effective application of Y₂O₃, its hard and brittle nature needs to be overcome during the surface machining process. Therefore, the surface machining control of Y₂O₃ should be conducted carefully. The evaluation for stable and continuous machining should also be investigated in various industrial fields as there are only limited studies on the subject. The lapping process with in-process electrolytic dressing (IED) is widely used for surface machining of hard and brittle materials. In this study, Y₂O₃ surface machinability was evaluated by using the ultra-precision lapping process with IED method by changing three major variables: applied force, wheel speed and machining time. The most suitable value of Ra 92nm surface roughness was acquired with smooth surface quality from the following machining condition: 7kg of applied force, 60rpm of wheel speed and 30minutes of machining time. After the lapping process, the machining tendency and surface characteristics were analyzed with fracture toughness and Vickers hardness for the evaluation of Y₂O₃ surface machinability. This paper was recommended for publication in revised form by Associate Editor Dae-Eun Kim Eun-Sang Lee received B.S. and M.S. degrees in Mechanical Engineering from INHA University in 1985 and in 1987. After that time, he received a Ph.D. degree from Korea Advanced Institute of Science and Technology in 1998. Dr. Lee is currently a Professor at the School of Mechanical Engineering at INHA University in Incheon, Korea. His research fields are ultra-precision manufacturing, electro chemical micro machining and development of semiconductor wafer polishing system.     

可用于自倍频激光器的Cr3+:YAB和Cr3+:YGAB晶体光学特性的研究

Cr³⁺:YAB和Cr^3-:YGAB晶体具有较宽的荧光光谱,从红光到近红外光。Cr³⁺:YAB晶体在744nm到852.5nm之间被测到光学增益。最大单程增益为1.9,对应波长为820nm.Cr³⁺:YAB晶体的二次谐波产生(37nm)的转换效率是LBO晶体的1.5倍。     

Color cathodoluminescence of BGO crystals

The color cathodoluminescence (CL) mode in scanning electron microscopy (SEM) is promising in the study of a variety of defects in single crystals pulled from the melt by the Czochralski method. The regions of homogeneous and heterogeneous capture of impurities and of point and linear defects, which affect the luminescence in the undoped Bi₄Ge₃O₁₂ (BGO) and doped BGO:V, BGO:Yb, BGO:Cr³⁺, and BGO:Fe as grown single crystals, were observed. Depending on the doped impurities and dislocation configuration, a change was found in both the intensity and wavelength of CL.     

Upper-bound anisotropic yield locus calculations assuming 111-pencil glide

Deformation by 111-pencil glide has been analyzed by an upper-bound model which combines a least-shear analysis and Piehler's maximum virtual work analysis. The least-shear analysis gives exact solutions if three 111 slip systems are active, while the maximum work analysis provides solutions for the case of four active slip systems. Independent checks are used to determine which solution method is appropriate.Computer calculations using this model have been made to determine; (1) the orientation dependence of the Taylor factor for axisymmetric deformation; (2) the yield loci for textured materials having [100], [110] and [111] sheet metals and rotational symmetry; (3) the isotropic yield locus for randomly oriented materials; and (4) flow stresses along critical loading paths for various assumed textures with rotational symmetry. The latter calculations indicate that anisotropic yield loci of textured bcc metals with rotational symmetry are much better approximated by σ_x^a + σ_y^a + R¦σ_x − σ_y¦^a = (R + 1)Y^a where R is the strain ratio and Y is the tensile yield strength with an exponent a = 6 rather than with a = 2 as postulated by Hill. It is not known how well upper-bound calculations like these represent actual yielding behavior.     

Structure of superconducting thin films of YBa2Cu3O7−x grown on SrTiO3 and cubic zirconia

Thin films of the superconductive oxide YBa₂Cu₃O_7?x have been made by electron-beam coevaporation of the metals in an oxygen atmosphere onto single-crystal {001}-oriented SrTiO₃ and yttria-stabilized zirconia (YSZ) substrates. The oxide films were superconducting in the as-deposited state (T_c = 81–83K, J_c = 10⁶ A/cm² at 4.2K). Bright-field imaging, selectedarea diffraction (SAD), and high-resolution imaging in the transmission electron microscope were used to characterize the microstructure of these films. All of the films were polycrystalline. On SrTiO₃ the films were oriented, for the most part, with {110} parallel to the substrate surface. On YSZ, two microstructures were observed: one with smaller rectangular grains oriented with (100) or (010) parallel to the substrate surface and the other with (001) parallel to the surface (i.e., c-axis up).