矩形辐射器压电多频超声换能器的研究

研究了一种具有多个共振频率的矩形辐射器夹心式超声换能器,换能器由圆柱形后盖板、压电陶瓷晶堆及矩形六面体辐射器前盖板组合而成。利用表观弹性法和一维近似理论给出了多频换能器横向及纵向理论共振频率方程。对一种特殊情况下的此类换能器进行了有限元及实验分析,给出了各自的频率输入导纳曲线。对理论和实验结果进行分析后表明,此类矩形辐射器夹心式超声换能器可以在不同的振动模态上工作,具有多个共振频率.     

Acousto-optic interaction in a non-homogeneous acoustic field excited by a wedge-shaped transducer

The article is devoted to theoretical analysis of light diffraction in a non-homogeneous acoustic field created by a wedge-shaped piezoelectric transducer. Relationships describing electrical, acoustic and acousto-optic properties of the acousto-optic cell are derived in the approximation of a small thickness of the piezoelectric plate. Principal characteristics of acousto-optic interaction are examined such as dependences of the light diffraction efficiency on the phase mismatch and the acoustic wave amplitude. It is shown that the acoustic field has a complicated amplitude-phase structure changing with the acoustic frequency. It is demonstrated that the diffraction efficiency in the Bragg regime can approach 100% in spite of a noticeable phase mismatch. The appropriate optimal values of ultrasound power and incidence angles of light are found.     

Fabrication and evaluation of fully-sampled, two-dimensional transducer array for "Sonic Window" imaging system

A low-cost, fully-sampled, 3600 element 2D transducer array operating at 5 MHz and designed for use in a hand-held ultrasound system is described here. Four array configurations are presented - (1) array with both matching and pedestal backing layers, (2) array with a matching layer but no backing pedestal, (3) array with a backing pedestal but no matching layer, and (4) array with neither matching layer nor backing pedestal. Each array was characterized in terms of impedance measurements, pulse-echo response, and experimental beamprofile. Comparative finite element analysis simulations are also presented. Average estimated active element yield for the four arrays was 94%. The array with pedestal layer proved the most promising, providing a 26% bandwidth and a 1.7 dB improvement in sensitivity with respect to the array with neither pedestal nor matching layer. Although this bandwidth is acceptable for our specific application (C-scan imaging), reverberations within the substrate material remain a potential challenge. We are currently working to fabricate a custom PCB material to address this concern, and may also consider using a pre-compensated transmit waveform or matched digital filter approach to further reduce the effects of such reverberations.     

A novel “sandwich” method for observation of the keyhole in deep penetration laser welding

A sandwich method was used to observe the keyhole in deep penetration laser welding, which provided an effective way to analyze both the Fresnel and inverse Bremsstrahlung absorption. In the transparent metal-analog system, different densities of metal vapor, ionized atoms, and free electrons in the keyhole can be simulated by changing the thickness of aluminum films. The research results show that inverse Bremsstrahlung absorption exerts a tremendous influence on the energy absorption of the laser beam for CO₂ laser welding. Low density of keyhole plasma benefits the incident laser energy coupling to the materials. However, excess density of keyhole plasma baffles the transmission of the incident laser beam to the interior material. By comparing inflow energy and outflow energy, there exits an energy balance on the keyhole wall by balancing the absorbed laser intensity and heat flux on the wall.     

铌酸钾钠基无铅压电陶瓷的X射线衍射与相变分析

分析了斜方相、四方相铌酸钾钠基无铅压电陶瓷材料的结构和X射线衍射图谱的特点. 对于铌酸钾钠基压电材料斜方相结构, 从构成晶胞的一个单斜原胞进行分析, 计算出X射线衍射谱上每个衍射角附近的衍射峰数目和相对强度. 提出了2θ在20°—60°范围内根据(1 0 2)衍射峰(52°附近)和(1 2 1)衍射峰(57°附近)劈裂的数目区分斜方和四方相的新方法. 对于多晶陶瓷粉末, 可以更简便的由22°(或45°)附近前后峰的相对高低来判断斜方、四方相. 关键词： 铌酸钾钠 无铅压电陶瓷 X射线衍射 相变     

Five port lattice model for simulation of piezoelectric structures possessing twin, compressional modes of vibration

A method is presented for predicting the performance of multilayered piezoelectric structures which demonstrate two, loosely coupled compressional modes of vibration. A linear systems, lattice approach is developed using a matrix formulation for implementation of the transducer transfer functions in the frequency domain. The technique is computationally efficient, providing good results over a key range of transducer dimensions and is especially useful for the practical evaluation of multilayered, linear or diced array structures.     

非晶态与晶态的压电热电铁电高聚物

本文综述了晶态与非晶态两类高聚物的压电性,热释电性和铁电性。这些现象近年来研究得愈来愈广泛。文中以聚氯乙烯作为非晶态高聚物的例子;以PVDF及其共聚物,还有尼龙11作为晶态高聚物的例子。按照偶极子模型,高聚物一般假定需具备四个条件,就可显示出较大的压电性,热释电性和铁电性。本文探讨了上述高聚物的结构与性能,这些对识别材料是很重要的。最后,列举了PVDF及其它材料的压电常数,热释电系数和介电常数,以供读者参考。     

A basic course on compiler principles

An attempt to devise a methodology of compiler design is described, and an outline is given for a possible course on this subject. The theoretical basis is formed by the concepts of phrase-structure language, finite-state- and stack-acceptor, and transducer. As their extension capable of processing context-dependent elements of languages, a so-called Table-Transducer is postulated, and it serves as the core-algorithm upon which compilers are based. The developed theory and method of compiler construction is applied to an example of a simple programming language.     

Nonlinear dielectric response of antiferroelectric liquid crystals in the smectic Cα * phase

Nonlinear dielectric response of antiferroelectric liquid crystals has been studied in the smectic C_α* (SmC_α*) phase. The linear dielectric spectrum shows a single relaxation of Debye type and its relaxation frequency is as high as one hundred kHz. The profile of the third-order nonlinear dielectric spectrum varies in complex fashion with temperature. In the vicinity of the SmA-SmC_α* phase transition, experimentally obtained nonlinear spectra are well described by those calculated with phenomenological theory of Landau type. The soft mode of the SmC_α* phase shows critical slowing-down near the transition temperature. In the lower-temperature region of the SmC_α* phase, the contribution from the soft mode of the SmC_α* phase reduces and the other relaxation mode with Debye-type spectrum appears at several hundred Hz in the nonlinear spectrum. The appearance of this low-frequency mode suggests that the cooperative fluctuation of directors over long range exists in the SmC_α* phase. Received 19 April 2002 and Received in final form 23 July 2002 RID="a" ID="a"e-mail: kimura@exp.t.u-tokyo.ac.jp     

Broad-band dielectric spectroscopy of Ba 2NaNb 5O 15 single crystal

Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain THz transmission spectroscopy, HF coaxial wave-guide technique and LF dielectric spectroscopy to cover the frequency range 10²-10¹⁴ Hz in a wide temperature interval. The dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric transition at T _c = 830 K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear on cooling in the ferroelectric phase. Below T _i the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling for the field direction has been explained by an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman active B₂-symmetry soft mode. Received 24 August 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: buixader@fzu.cz