铌酸钠钾基无铅压电陶瓷制备技术新进展

碱金属铌酸盐系无铅压电陶瓷以其优越的压电性能和较高居里温度倍受关注.结合目前有关KNbO3-NaNbO3(KNN)基无铅压电陶瓷的报道,综述了近年来铌酸钠钾基无铅压电陶瓷在粉体制备、陶瓷的成型、烧结以及晶粒取向等制备技术研究的新进展,并从不同方面展望了今后铌酸钠钾基无铅压电陶瓷性能研究及其制备技术上可能的进展.     

铌酸钾钠基无铅压电陶瓷的相变特性和掺杂改性研究进展

人类社会可持续发展的要求以及一些发达国家有关电子产品无铅化法规的全面实施,近年来无铅压电陶瓷的研究与开发受到极大关注.碱金属铌酸盐系压电陶瓷以其优越的压电性能和较高居里温度受到人们高度重视.结合近年有关KNbO3-NaNbO3(KNN)基无铅压电陶瓷的报道,着重从KNN基压电陶瓷的相变特性、掺杂改性和引入第三组元等几个方面,介绍了相关研究的现状,并对KNN基压电陶瓷的发展趋势进行了展望.     

Modelling and simulation of heavy gas dispersion on the basis of modifications in plume path theory

An analytical model for heavy gas dispersion based on the modifications in plume path theory has been developed. The model takes into account the variations in temperature, density, and specific heat during the movement of heavy gas plume.

The model has been tested for three hazardous gases — chlorine, natural gas and liquefied petroleum gas. The results have been compared with the recently generated experimental data as also with the outputs of other models. A good agreement is observed qualitatively as well as quantitatively.

A study has also been carried out to simulate the effect of the wind speed, density of the gas, and venting speed on dispersion. Based on the simulation study a set of empirical equations has been developed. The equations are validated by theoretical as well as experimental studies.     

Modelling and simulation of gyrotrons

Modelling and simulation of gyrotrons have two interconnected aspects, namely beam formation in the electron-optical system (EOS) and beam-field interaction in the resonant cavity. In this paper we address both problems and outline the physical models and numerical techniques implemented in our problem-oriented package of computer codes. In order to illustrate our approach we present some results of numerical experiments carried out at the FIR FU centre and directed towards analysis and optimization of the existing devices of the Gyrotron FU Series as well as applied to the development of simulation-based design (SBD) of a novel high harmonic gyrotron with the electron beam encircling the axis and a permanent magnet system.     

Phase-matching properties of KNbO3 in the mid-infrared

KNbO(3) has been found to be phase matchable for type 1 second-harmonic generation up to 2.4 mum at 22 degrees C. The improved Sellmeier equations that correctly reproduce the nonlinear experiments thus far reported in the literature and our new experimental results for harmonic generation of CO(2) laser harmonics between 3.5303 and 5.2955 mum are presented.     

Modelling and simulation of high-speed machining

A Lagrangian finite element model of orthogonal high-speed machining is developed. Continuous remeshing and adaptive meshing are the principal tools which we employ for sidestepping the difficulties associated with deformation-induced element distortion, and for resolving fine-scale features in the solution. The model accounts for dynamic effects, heat conduction, mesh-on-mesh contact with friction, and full thermo-mechanical coupling. In addition, a fracture model has been implemented which allows for arbitrary crack initiation and propagation in the regime of shear localized chips. The model correctly exhibits the observed transition from continuous to segmented chips with increasing tool speed.     

Preparation and properties of KNbO3 via the sol-gel method

Sol-gel processing of a solution of niobium and potassium ethoxides in methanol was used to prepare polycrystalline potassium niobate ceramic discs. Gel powders completely crystallized after only 2 h at 600 ° C. Crystallized powders were hot-pressed at pressures less than half of that required in conventional methods. Densities of up to 99% were achieved without encountering the common problem associated with the evaporation of the potassium oxide and subsequent formation of a stable second phase. The variation of dielectric constant with temperature was measured up to several degrees above the Curie temperature (ã420b ° C). Dielectric constants of dense samples were found to be more than one and one-half times greater than that of their single crystal counterparts.     

Pure SH-SAW propagation, transduction and measurements on KNbO3

Potassium niobate (KNbO3) supports the electromechanically active pure shear horizontal surface acoustic wave (SH-SAW) mode along Z-axis cylinder orientations, Euler angles (phi, 90 degrees, 0 degrees), in which two uncoupled wave solutions exist: a purely mechanical sagittal Rayleigh SAW and a piezoelectrically stiffened pure SH-SAW. Within this family of cuts, a maximum electromechanical coupling coefficient for the pure SH-SAW, K2 = 53%, is observed along (0 degrees, 90 degrees, 0 degrees). This pure SH-SAW orientation also has the maximum value of electromechanical coupling observed along rotated Y-cut X propagation directions, Euler angles (0 degrees, theta, 0 degrees). The use of the pure SH-SAW mode is attractive for liquid-sensing applications because the SH-SAW is modestly attenuated by the adjacent liquid, unlike the generalized SAW (GSAW), which has particle displacement normal to the surface. This work investigates propagation and excitation properties of the SH-SAW and the shear horizontal bulk acoustic wave (SH-BAW) on single crystal KNbO3, Euler angles (0 degrees, 90 degrees, 0 degrees). Interdigital transducer (IDT) arrays are analyzed using boundary element method (BEM) techniques, addressing IDT properties such as: power partitioning between the SH-SAW and SH-BAW, SH-BAW radiation as a function of wave vector direction and radiation angle, and overall IDT impedance. The percentage of SH-SAW power to total input power is above 98% for IDTs containing 1.5 to 5.5 wavelengths of active electrodes with surrounding metalized regions. For nonmetalized regions outside the IDT, the ratio drops to between 1 and 2%, showing the importance of an energy trapping structure for efficient SH-SAW excitation and propagation along this orientation. Simulated and experimental IDT admittance results are compared, verifying the validity of the analysis performed. The reported measurements on the frequency variation with temperature indicate that the orientation considered is temperature compensated at about 8 degrees C. The surface of the SH-SAW devices fabricated have been loaded with deionized water and showed additional 1.6 dB transmission loss with respect to the unloaded surface, verifying the suitability of the pure SH-SAW mode on KNbO3 for liquid sensor applications.     

Modelling the dispersion of flashing jets using CFD

Risk assessments related to industrial environments where gas is kept in liquid form under high pressure rely on the results from predictive tools. Computational Fluid Dynamics (CFD) is one such predictive tool and it is currently used for a range of applications. One of the most challenging application areas is the simulation of multiphase flows resulting from a breach or leakage in a pressurised pipeline or a vessel containing liquefied gas. The present paper deals with the modelling of the post-flashing scenario of a jet emanating from a circular orifice. In addition to being based on the equations governing fluid flow, the models used are those related to turbulence, droplet transport, evaporation, break-up and coalescence. Some of these models are semi-empirical and based on the data from applications other than flashing. However, these are the only models that are currently available in commercial codes and that would be used by consulting engineers for the type of modelling discussed above, namely the dispersion of a flashing release. A method for calculating inlet boundary conditions after flashing is also presented and issues related to such calculations are discussed. The results from a number of CFD based studies are compared with available experimental results. The results show that whilst a number of features of the experimental results can be reproduced by the CFD model, there are also a number of important shortcomings. The shortcomings are highlighted and discussed. Finally, an optimum approach to modelling of this type is suggested and methods to overcome modelling difficulties are proposed.     

Modelling and simulation of just-in-time flexible systems

The increasing worldwide competition requires economical manufacture, high quality and short delivery time. The Just-In-time (JIT) philosophy of manufacturing is increasingly being considered by manufacturing organizations, as a response to the increased pressure to supply high quality products with short delivery times and at low cost. A very simple shop floor control system that was developed by Toyota in the 1970s specifically for their Just-In-Time assembly plant, has received considerable attention in the Western World, and is known as the ‘kanban’ system (literally translated as ‘card’ system). Japan’s success has prompted many scholars and practitioners, to turn their attention to Japanese management practices. This article is to address the modelling, simulation and implementation issues of Just-In-Time in flexible manufacturing environments. Priority nets are used for modelling and analysis of the kanban system. A large number of simulation runs are conducted/presented to probe the behaviour of the system, with respect to different parameter changes.     

Cooperative ordering of impurity dipoles in KNbO3 single crystals

The impurity atoms forming dipoles in the structure of KNbO₃ are arranged regularly in the structure. The locations of the dipole sites were observed by etching technique, when dilute HNO₃ was used as an etchant. The dipoles along with the domain structure with which they are associated are stable with respect to temperature, and retain the same sites at Curie temperature. Thus the regular cooperative ordering observed at room temperature also exists at the Curie transition. The importance of this ordering is discussed in terms of domain formation and the basic problem of ferroelectricity in crystals.     

Preparation and characterization of quenched KNbO3-Nb2O5 glass

KNbO₃-Nb₂O₅ glasses were prepared with molar ratio of K/(K + Nb) from 0.16 to 0.5 by using a twin-roller quenching apparatus. Most KNbO₃-Nb₂O₅ glasses crystallized to stable phases such as KNbO₃, K₂Nb₄O₁₁, K₃Nb₇O₁₉, KNb₃O₈ and K₂Nb₈O₂₁ via metastable phases after heat treatment and K₄Nb₆O₁₇ glass directly crystallized to K₄Nb₆O₁₇ crystal. KNbO₃ glass consisted of the corner-shared NbO₆ octahedra, while the amounts of the edge-shared NbO₆ octahedra in the KNbO₃-Nb₂O₅ glass structure increased with decreasing K⁺ ion content. The band gap energies and the ionic conductivity of the KNbO₃-Nb₂O₅ glasses increased with increasing K⁺ ion content, but the density and the activation energy of the ionic conduction decreased. The glasses possessed high dielectric constants appfoaching those of ferroelectric crystals at high temperature around the crystallization temperature. The dielectric constants of the glasses, however decreased greatly with decreasing temperature and with increasing frequency.     

Photorefractive response time of proton-implanted KNbO3:Rb crystal

Abstract

The photorefractive response time of Rb-doped KNbO₃ crystal has been measured after proton-implantation. The experimental results show that there is only one detectable photorefractive grating, with a response time of 434.41 s at erasure light intensity of 65.94μ W cm^?2 and 12.787s at 1 W cm^?2. This phenomenon has been analysed.