Effect of the grain size on the formation of a nanophase structure and tribological properties of the friction surface of a ceramic made of partly stabilized zirconia

It is found that the wear of the ZrO₂ + 3 mol% Y₂O₃ ceramics with a grain size of 180 nm in the case of dry friction on steel is smaller by a factor of 20–60 than the wear of a ceramic with an average grain size of 700 nm. It is shown that this is due to the fact that a nanophase structure formed on the friction surface of the ceramic with a grain size of 180 nm consists of ferroelastic nanodomains of the nontransformable T′ phase.     

Electron-microscopy and electron-diffraction study of order-disorder phenomena in the intercalate C60(I2)2−x

Single crystals of iodine-intercalated C₆₀ have been examined by transmission electron microscopy. Tilting series of Electron Diffraction Patterns (EDP) confirm that it has a simple hexagonal structure with c/a1 in accordance with the results of X-ray diffraction, which allows higher precision c/a=1.0024. Cooling and heating experiments were performed in the vacuum of the microscope to investigate the order — disorder phenomena. High-Resolution Electron Microscopy (HREM) images and electron diffraction patterns reveal that the c/2-displacement along the c-direction plays an important role in the transition from the simple hexagonal structure into the fcc structure when iodine is lost from the specimen or vice versa. The orientation relationship between the intercalate and the C₆₀ structure is [01 10]_h[1 10]_fcc and (001)_h(110)_fcc. A model is proposed to interpret the features observed in EDP and HREM consistently.On leave from Department of Materials Science and Engineering, Zhejiang University, People's Republic of ChinaOn leave from Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, People's Republic of China     

An Electromagnetic Trinity from “Negative Permittivity” and “Negative Permeability”

An electromagnetic trinity comprising vacuum, anti–vacuum, and nihility is postulated — after making use of materials with negative permittivity and negative permeability — to illuminate the structure of electromagnetic theory, at least insofar as the relationship of phase velocity with Poynting vector is concerned.     

The lanthanideβ″ aluminas

The entire sodium ion content of sodium alumina (Na_1.67Mg_0.67Al_10.33O₁₇) can be replaced with a variety of lanthanide ions by simple diffusion reactions at moderate temperatures (500–700°C). Lanthanide alumina crystals are hard, clear, chemically stable, and have well-defined crystal structures. The fluorescence spectrum of Nd³⁺ in alumina is similar to that in YAG. The lifetime of the⁴ F _3/2 state of Nd³⁺ in completely-exchanged alumina (350s at 10²¹ Nd³⁺ cm^–3) is about 45% longer than in YAG (240s at 10²⁰Nd³⁺ cm^–3). The lanthanide aluminas may be of considerable interest as new phosphor and laser host materials.     

Recent67Zn-experiments

The 93.3keV transition in⁶⁷Zn is shown to offer extremely high resolution for the determination of small changes in -ray energy. The accuracies that have been achieved in high precision experiments with conventional and quantum beat methods are discussed. The⁶⁷Zn resonance is also highly favourable for studying lattice dynamical effects. This sensitivity has been used to investigate a combined effect in ZnO, the second-order Doppler effect of lattice dynamical origin and the temperature dependence of the isomer shift, caused by phonon-induced electron transfer from zinc to oxygen. The shift shows aT ⁴-dependence at low temperatures. High pressure experiments on Zn metal reveal an increase of the recoil-free fraction by a factor of 3 when the external pressure is increased to 58kbar, as well as large changes of the quadrupole interaction and of the center shift.This work has been funded by the German Federal Minister for Research and Technology (BMFT) under contract Nr. 03-KA1TUM-4 and by the Kernforschungszentrum Karlsruhe.     

Scaling-up the Production of Nanosized SiO2-particles in a Double Diffusion Flame Aerosol Reactor

Synthesis of nanophase silica (SiO₂) from hexamethyldisiloxane (HMDS) oxidation in a co-flow diffusion flame reactor at atmospheric pressure is investigated focusing on high production rates of powder. A new experimental set-up is introduced, including a diffusion burner which is operated with a ring-shape double diffusion flame. Significantly high HMDS concentrations are used resulting in SiO₂ production rates of up to 130g/h. Deposition of silica powder on the burner face is eliminated by the design of a special diffusion burner and higher collection rates are achieved using a baghouse filter. The specific surface area and the product powder composition are analyzed. Carbon black coated silica particles were produced at high production rates (130g/h) at low oxygen flow rates or using a mixture of nitrogen and oxygen as oxidant. The size of the product particles was controlled in the range of 15–170nm.     

Influence of nitrogen ion implantation fluences on surface structure and tribological properties of SiC ceramics in water-lubrication

Nitrogen ions were implanted into SiC ceramics by using ion implantation technology (N⁺-SiC). The surface structure and chemical bonds of N⁺-SiC ceramics were determined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and their nanohardness was measured by nanoindenter. The friction and wear properties of the N⁺-SiC/SiC tribo-pairs were investigated and compared with those of SiC/SiC tribo-pairs in water using ball-on-disk tribo-meters. The wear tracks on the N⁺-SiC ceramics were observed by non-contact surface profilometer and scanning electron microscope (SEM) and their wear volumes were determined by non-contact surface profilometer. The results show that the N⁺-SiC ceramics were mainly composed of SiC and SiCN phase and SiN, CC, CN and CN bonds were formed in the implantation layer. The highest hardness of 22.3 GPa was obtained as the N⁺-SiC ceramics implanted at 50 keV and 1 × 10¹⁷ ions/cm². With an increase in nitrogen ion fluence, the running-in period of N⁺-SiC/SiC tribo-pairs decreased, and the mean stable friction coefficient decreased from 0.049 to 0.024. The N⁺-SiC ceramics implanted at 50 keV and 5 × 10¹⁷ ions/cm² exhibited the excellent tribological properties in water. In comparison of SiC/SiC ceramic tribo-pairs, the lower friction coefficient and lower wear rate for the N⁺-SiC/SiC tribo-pairs were acquired.     

Interface Microstructures of Si3N4/Fe and Fe-Cr Alloys Joints

Interfaces of solid state bonded Si₃N₄ceramics with Fe and Fe-(5, 10, 15 wt%)Cr alloy interlayers inArgon for 1 h at 1100°, 1200° and 1300°C have beencharacterized by electron probe microanalysis (EPMA) and transmissionelectron microscopy (TEM). Smooth interfaces with no evidence ofreactions products resulted when bonding at 1100°C. However, theinteraction between the ceramic and the metal increased at higherbonding temperatures and Cr-contents. In all samples Si and N fromthe ceramic dissolve and diffuse in the metal interlayer, whereas thesintering additives of the ceramic remain inactive.Low Cr-content (5%) interlayers resulted in the formationof an interfacial zone composed of two sublayer structures; adjacentto the ceramic was a thin one containing the sintering aids ofSi₃N₄ and fine precipitates of Fe₃Si and -Fe₄N in a bcc-Fe matrix. The second was thicker and includedfine Fe₃Si,-Fe₄N and-Fe₂N precipitates in abcc-Fe matrix. The bond region with high Cr-content interlayersincluded three sublayer structures. The first one next to the ceramicwas a bcc-Fe matrix containing sintering aids, fine dispersedFe₃Si and-Fe₄N, and CrN. The secondsublayer was similar but without any segregants from the ceramic. Thethird one, finally had a lamellae structure of Cr₂N/bcc-Fe and the Fe matrix contained also-Fe₄N.The interactionbetween the ceramic and the metal interlayer is believed to becontrolled by the solution rate of N in the alloy foils.     

The effects of slippage and diffraction in long wavelength operation of a free electron laser

The Free-Electron Laser user facility FELIX produces picosecond optical pulses in the wavelength range of 5–110m. The proposed installation of a new undulator with a larger magnetic period would allow extension towards considerably longer wavelengths. This would result in the production of extremely short, far-infrared pulses, with a duration of a single optical period or even less. In order to investigate the pulse propagation for free-electron lasers operating in the long wavelength limit, a three-dimensional simulation code was developed. Using the FELIX parameters, with the addition of a long-period undulator, the effects of slippage, diffraction losses, changes in the filling factor, as well as the effects of the optical cavity geometry were studied for wavelengths up to 300m, with electron pulses in the ps regime. It is shown that slippage effects are less restrictive for long wavelength operation than the increasing losses due to optical beam diffraction.     

Preparation and Characterization of Nano-structured Ceramic Powders Synthesized by Emulsion Combustion Method

The emulsion combustion method (ECM), a novel powder production process, was originally developed to synthesize nano-structured metal-oxide powders. Metal ions in the aqueous droplets were rapidly oxidized by the combustion of the surrounding flammable liquid. The ECM achieved a small reaction field and a short reaction period to fabricate the submicron-sized hollow ceramic particles with extremely thin wall and chemically homogeneous ceramic powder. Alumina, zirconia, zirconia–ceria solid solutions and barium titanate were synthesized by the ECM process. Alumina and zirconia powders were characterized to be metastable in crystalline phase and hollow structure. The wall thickness of alumina was about 10nm. The zirconia–ceria powders were found to be single-phase solid solutions for a wide composition range. These powders were characterized as equiaxed-shape, submicron-sized chemically homogeneous materials. The powder formation mechanism was investigated through the synthesis of barium titanate powder with different metal sources.     

Excimer-laser-induced ablation of the high-T c superconductor Bi-Ca-Sr-Cu-O

The ablation of ceramic Bi-Ca-Sr-Cu-O by XeCl-excimer-laser projection has been investigated. In both air and vacuum, etching commences at about 2.4 J/cm² and then increases with fluence within the regime investigated ( <) 20 J/cm²). At 10 J/cm² the respective etch rates are around 1 m/pulse and 1.6 m/pulse.     

AC Poling study of lead zirconate titanate/vinylidene fluoride-trifluoroethylene composites

It is experimentally known that a polymer matrix phase in a composite of ferroelectric particles dispersed in a ferroelectric polymer can be polarized by using a few cycles of an ac field, without causing much disturbance to the state of polarization of the inclusion particles. This paper attempts to investigate this special poling process for a typical ferroelectric composite system of lead zirconate titanate (PZT) ceramic particles in a vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymer matrix, upon the application of 10 Hz ac fields of various amplitudes. Compared to a copolymer sample, the composite samples can be polarized at a lower field, and hence show a larger remanent polarization at the same poling field. Among the composites, the observed remanent polarization increases with increasing ceramic volume fraction. These experimental observations can be understood by a simple model, in which space charges are allowed to accumulate at the particle-matrix interfaces because of the electrical conductivity of the constituents. At a fast switching poling field of 10 Hz, the calculation shows that conductivity and charge accumulation effects in the composite are only minimal. Accordingly, although the PZT phase as well as the copolymer phase are both polarized under the ac field, the ceramic phase is only polarized to about 10% when the copolymer phase is almost fully polarized. Thus, one can still use an ac field to polarize only the copolymer phase of the composite without altering the polarization state of the ceramic phase significantly. PACS 77.22.Ej; 77.84.Lf     

电解质中添加氧化物颗粒对染料敏化太阳电池性能的增强

"在液态电解质中加入纳米TiO2、微米TiO2、Al2O3和ZrO2颗粒后,在一定的氧化物含量下,都不同程度地提高了DSC的光电特性,而以加入12.5%的纳米TiO2 (P25)得到最高的提高,达到5.5%,比没有加入氧化物颗粒的DSC效率提高达到51%以上．利用电化学阻抗谱和循环伏安法研究了在染料敏化太阳电池液态电解质中加入各种氧化物颗粒对电池电解质特性的影响,结果发现,加入氧化物颗粒后,由于界面效应,提供了I3-的快速扩散通道,从而I3-扩散系数得到较大提高,电解质/Pt电极间的电荷传输电阻下降,改善     

The SO(3,3) group as a common basis for Dirac's and Proca's equations

Dirac's and Proca's equations are unified in the sense that the algebras of Dirac -matrices and Duffin-Kemmer-matrices are shown to furnish two distinct matrix representations of the Lie algebra of the SO(3,3)-group. This fact is then interpreted as evidence for the classical picture of particles described by the above-mentioned equations to be a relativistic top. It is also argued that the shape of the top is rod-like.     

The velocity autocorrelation function of a finite model system

We investigate in detail the dependence of the velocity autocorrelation function of a one-dimensional system of hard, point particles with a simple velocity distribution function (all particles have velocities ±c) on the size of the system. In the thermodynamic limit, when both the number of particlesN and the length of the boxL approach infinity andN/L , the velocity autocorrelation function(t) is given simply by c² exp(–2ct@#@). For a finite system, the function _N(t) is periodic with period 2L/c. We also show that for more general velocity distribution functions (particles can have velocities ±c_i,i = 1,...), _N(t) is an almost periodic function oft. These examples illustrate the role of the thermodynamic limit in nonequilibrium phenomena: We must keept fixed while letting the size of the system become infinite to obtain an auto-correlation function, such as(t), which decays for all times and can be integrated to obtain transport coefficients. For any finite system, our _N (t) will be very close to(t) as long ast is small compared to the effective size of the system, which is 2L/c for the first model.Supported in part by the AFOSR under Contract No. F44620-71-C-0013.     

Lyapunov Modes of Two-Dimensional Many-Body Systems; Soft Disks, Hard Disks, and Rotors

The dynamical instability of many-body systems can best be characterized through the local Lyapunov spectrum {}, its associated eigenvectors {}, and the time-averaged spectrum {}. Each local Lyapunov exponent describes the degree of instability associated with a well-defined direction—given by the associated unit vector —in the full many-body phase space. For a variety of hard-particle systems it is by now well-established that several of the vectors, all with relatively-small values of the time-averaged exponent , correspond to quite well-defined long-wavelength modes. We investigate soft particles from the same viewpoint here, and find no convincing evidence for corresponding modes. The situation is similar—no firm evidence for modes—in a simple two-dimensional lattice-rotor model. We believe that these differences are related to the form of the time-averaged Lyapunov spectrum near =0.     

Light Propagation through Weakly Absorbing Layers of Dielectric Cube-Shaped Particles

Light propagation through ensembles of dielectric particles with various packing density is studied by the method of physical modeling in the SHF range. Monolayers and multilayers of weakly absorbing cube-shaped particles with the refractive index n = 1.5 are investigated. Particle dimensions are comparable to the incident radiation wavelength. Dependences of the transmittance T and the scattering efficiency factor Q on the diffraction parameter are obtained for ensembles of particles with various overlap factors . For monolayers of particles, the effect of an enhanced transmittance with increasing packing density of particles is observed. Conditions are specified at which the monolayer transmittance is equal to zero. It is shown that the Mie formulas, derived for spherical particles, can be used to calculate the optical characteristics of an ensemble of particles with chaotic orientation of particle sides, if the diameter of an equivalent sphere is set equal to the cube edge length multiplied by a correction factor of 1.15. Transition to an ordered orientation of cube-shaped particle sides leads to increase in the scattering efficiency factor for > 2.     

Induced geometry and confinement

A classical, Poincaré invariant dynamical system is developed which contains, besides the natural metric _v , an induced metricg _v that is generated by a real scalar dynamical field. It is shown that scalar fields whose dynamics are governed by the induced metric can be consistently introduced. Also, point particles which follow timelike quasi-geodesic trajectories can be introduced. The reaction forces acting ong _v due to the presence of these fields and particles are computed. A discussion of causality and geometrical confinement is given.     

Exact solutions for stimulated emissions by external sources

The exact solutions for transition amplitudes are derived forstimulated emissions by external sources. More precisely, we obtain the exact expressions for transition amplitudes for the emission of an arbitrary number of particles by the sources when some particles are already present, in the process,prior to the switching on of the external sources. The solutions are given for an arbitrary number of particles with arbitrary configurations (of momenta, spin, etc.) and for particles of spin-0, spin-1/2, massive and massless (photons) spin-1 particles, and massless (gravitons) spin-2 particles. Applications are given as illustrations to the process Ø anything, and, in quantum electrodynamics, to the process e ⁺e^–+ any photons, in thepresence of external sources, where a (virtual) photon decays into the paire ⁺e^–.     

Mössbauer study of oxygen-related lattice instabilities in oriented 1-2-3 HTSC

⁵⁷Fe Mössbauer effect measurements as a function of temperature between 78 and 293 K in grain-aligned YBaCuO (T _c=89 K, T _c<2 K) with bothc andc orientations revealed a sequence of anomalies in the recoilless fraction, center shift and linewidth of all⁵⁷Fe quadrupole doublets in the host lattice. These anomalies are believed to arise due to antiferroelectric phase transitions driven by a nonlinear anharmonic vibrational motion of oxygen atoms in a breathing mode configuration. The results obtained are indicative of a possible coexistence of high-T _c superconductivity and antiferroelectricity in 1-2-3 ceramics.