Half-thickness inversion layer high-frequency ultrasonic transducers using LiNbO3 single crystal.

Half-thickness inversion layer high-frequency ultrasonic transducers were fabricated using lithium niobate (LiNbO3) single crystal plate. The transducers developed for this study used a 36 degrees rotated Y-cut LiNbO3 thin plate with an active element thickness of 115 microm. The designed center frequency was in the range of 30 to 60 MHz. Half-thickness inversion layer was formed after the sample was annealed at a high temperature, and it is shown that the inversion layer thickness can be controlled by the temperature. Silver powder/epoxy composite and parylene were used as acoustic matching layers. A lossy silver epoxy was used as the backing material. Using an analytical method, the electrical impedance for different inversion layer ratios was determined. The measured resonant frequency was consistent with the modeled data. Even-order higher frequency broadband ultrasonic transducers with a center frequency at 60 MHz was obtained using half-thickness inversion layer of LiNbO3 single crystal.     

Broadband ultrasonic transducers using a LiNbO3 plate with a ferroelectric inversion layer

Some heat treatment of a lithium niobate (LiNbO/sub 3/) plate induces domain inversion, thereby yielding a ferroelectric inversion layer. In such a piezoelectric plate with an inversion layer, even-order thickness-extensional modes, as well as odd-order modes, can be excited piezoelectrically. Therefore, the ultrasonic transducer using such a piezoelectric plate is expected to operate over a wide frequency range. In this paper, it is shown that broadband ultrasonic transducers can be obtained at a certain thickness ratio of inversion layer to plate, and that the transducer characteristics differ depending on whether the inversion layer is on the front side or on the backside. The broadband characteristics are experimentally demonstrated by fabricating transducers with 9 MHz or 75 MHz center frequency using 36/spl deg/ rotated Y-cut LiNbO/sub 3/ plates with a ferroelectric inversion layer.     

High temperature stable GHz-range low-loss wide band transducersand filter using SiO2/LiNbO3, LiTaO3

GHz-range low-loss transducers and filters are required for communication systems, especially mobile telephone communication systems. Many types of low insertion-loss transducers and filters utilizing the high electromechanical coupling coefficient (K²) materials such as LiNbO₃ and LiTaO₃ have been developed. Unfortunately, these materials have large temperature coefficients of the frequency (TCF). In this paper, SAW substrates with high coupling coefficients and low propagation attenuations and small temperature coefficient of frequency in the GHz-range are theoretically and experimentally investigated. The experimental results show very low propagation loss of 0.02 dB/λ ₀ and larger K² than those of the substrates of LiNbO₃ and LiTaO₃ at the TCF of below -5 ppm/°C at 1~2 GHz-range. The low-loss filter results using internal reflection types of IDT show the insertion loss of about 2.9 dB at 1 GHz and 4.9 dB at 2 GHz under the TCF's of 0 and +20 ppm/°C. These materials are applicable for devices at GHz-range because SiO₂ thickness is very thin such as below 1 μm and the center frequency shift of the filter versus SiO₂ thickness is very small     

A single-element transducer with nonuniform thickness for high-frequency broadband applications

The design, fabrication, and evaluation of a high-frequency single-element transducer are described. The transducer has an annular geometry, with the thickness of the piezoelectric material increasing from the center to the outside. This single-element annular transducer (SEAT) can provide a broader frequency range than a conventional single-element transducer with a uniform thickness (single-element uniform transducer, or SEUT). We compared the characteristics of a SEAT and a SEUT. Both transducers used 36deg-rotated, Y-cut lithium niobate (LiNbO₃) material. The SEAT had a diameter of 6 mm and comprised 6 subelements of equal area (electrically connected by a single electrode on each side) whose thickness ranged from 60 mum (center) to 110 mum (outside), which resulted in the center frequency of the subelements varying from 59.8 MHz to 25 MHz. The overall center frequency was 42.4 MHz. The annular pattern was constructed using an ultrasonic sculpturing machine that reduced the root-mean-square value of the surface roughness to 454.47 nm. The bandwidth of the SEAT was 19% larger than that of the SEUT. However, compared with the SEUT, the 2-way insertion loss of the SEAT was increased by 3.1 dB. The acoustic beam pattern of the SEAT was also evaluated numerically by finite-element simulations and experimentally by an ultrasound beam analyzer. At the focus (10.5 mm from the transducer surface), the -6 dB beam width was 108 mum. There was reasonable agreement between the data from simulations and experiments. The SEAT can be used for imaging applications that require a wider transducer bandwidth, such as harmonic imaging, and can be manufactured using the same techniques used to produce transducers with multiple frequency bands.     

Phononic crystals based on linbo/sub 3/realized using domain inversion by electron-beam irradiation

We report in this paper about the realization of domain inversion in z-cut lithium niobate by electron beam irradiation in order to perform phononic crystals. The fabrication of these phononic crystals on z-cut LiNbO₃, which is, in our case, a periodic repetition of voids and LiNbO₃, was achieved by domain inversion followed by wet etching, taking advantage of the large difference in etching rate between z+ and z- faces. A pertinent choice of irradiation conditions such as accelerating voltage, beam current, and charge density was determined and optimized. Two-dimensional structures at the micrometer scale were then realized on z-cut LiNbO₃. We demonstrate the achievement of hexagons with diameters between 2 mum and 18 mum, with a very important depth close to 30 mum, which depends on the etching time and the hole size. The obtained structures, which exhibit a filling fraction varying from 1% to 64%, were characterized before etching by polarizing microscope to visualize the inverted domains. After HF etching, scanning electron microscopy was used to observe the obtained phononic structures. Taking into account the obtained filling fraction values and the size of created hexagons, the frequency band gap of these structures is expected at a range of 200 to 350 MHz. As expected in this frequency range, we have proven experimentally the existence of the phononic band gap on z-cut LiNbO₃ by combination of a realized phononic crystal with a surface acoustic wave (SAW) device.     

Design, fabrication, and evaluation of high frequency, single-element transducers incorporating different materials

The performance of high frequency, single-element transducers depends greatly on the mechanical and electrical properties of the piezoelectric materials used. This study compares the design and performance of transducers incorporating different materials. The materials investigated include 1-3 lead zirconate titanate (PZT) fiber composite, lead titanate (PbTiO₃) ceramic, poly(vinylidene fluoride) (PVDF) film, and lithium niobate (LiNbO₃) single crystal. All transducers were constructed with a 3-mm aperture size and an f-number between 2 and 3. Backing and matching materials were selected based on design goals and fabrication limitations. A simplified coaxial cable tuning method was employed to match the transducer impedance to 50 Ω for the PZT fiber composite and PbTiO₃ ceramic transducers. Transducers were tested for two-way loss and -6 dB bandwidth using the pulse/echo response from a flat quartz target. Two-way loss varied from 21 to 46 dB, and bandwidths measured were in the range from 47 to 118%. In vitro ultrasonic backscatter microscope (UBM) images of an excised human eye were obtained for each device and used to compare imaging performance. Both press-focusing and application of a lens proved to be useful beam focusing methods for high frequency. Under equal gain schemes, the LiNbO ₃ and PbTiO₃ transducers provided better image contrast than the other materials     

Disposable PVDF ultrasonic transducers for nondestructive testingapplications

Disposable ultrasonic contact transducers have been constructed with inexpensive PVDF films for nondestructive testing (NDT) applications. This paper reports the temperature-dependent ultrasonic performance of commercial polyvinylidene fluoride (PVDF) films and PVDF sensors. PVDF film was evaluated for its material properties of interest for ultrasonic transducer performance including the relative dielectric constant ε_r, dielectric loss tangent tan δ_e , electromechanical coupling constant k_t, mechanical quality factor Q_m, and acoustic impedance Z. Disposable PVDF transducers were then coupled to both low and high acoustic impedance test panels and their ultrasonic insertion loss/sensitivity and frequency bandwidth measured as a function of temperature in both the pulse-echo and pitch-catch modes. The results yielded center frequencies in the 3.5-16 MHz range, with -6-dB fractional bandwidths for some of the transducers exceeding 100%. The temperature dependence of the transducers' performance is reported for the -40 to 80°C range and shows an approximate linear decrease in center frequency and increase in fractional bandwidth with increasing temperature. These measured results are compared with predictions from computer simulations based on Mason's model. These tests showed that the same disposable transducers can be used for both ultrasonic and acoustic emission sensing NDT applications. This paper summarizes the quantitative ultrasonic and vibration sensing performance for all of the disposable PVDF film transducers which were tested     

25 MHz ultrasonic transducers with lead- free piezoceramic, 1-3 PZT fiber-epoxy composite, and PVDF polymer active elements

This paper presents the fabrication and characterization of single-element ultrasonic transducers whose active elements are made of lead-free piezoceramic, 1-3 PZT/polymer composite and PVDF film. The lead free piezoelectric KNNLT- LS(K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.10S_0.06b)O₃ powders and ceramics were prepared under controlled humidity and oxygen flow rate during sintering. Due to its moderate longitudinal piezoelectric charge coefficient (175 pC/N) and k_t of 0.50, the KNN-LT-LS composition may be a good candidate for highfrequency transducer applications. PZT fibers with 25 μm diameter formed by the viscose suspension spinning process were incorporated into epoxy to fabricate 1-3 composites with the averaged k_t = 0.64 and d₃₃ = 400 pC/N. Using KNN-LS-LT ceramic, 1-3 PZT fiber composite, and PVDF film, 3 different unfocused single element transducers with center frequencies of 25 MHz were fabricated. The acoustic characterization of the transducers demonstrated that wideband and low insertion loss could be obtained employing KNN-LS-LT ceramic. The ?6 dB bandwidth and insertion loss were 70% and ?21 dB, respectively. In comparison, the insertion loss of the ceramic transducer was much smaller than those made with 1-3 composite and PVDF film. This was attributed to closer electrical impedance match to 50 Ω and higher thickness coupling coefficient of the ceramic transducer.     

Spectroscopic investigation of Nd ion in LiNbO3, MgO:LiNbO3 and LiTaO3 single crystals relevant for laser applications

The polarized absorption and luminescence properties of Nd³⁺ doped isostructural LiNbO₃, MgO:LiNbO₃ and LiTaO₃ nonlinear bulk single crystals are reported. Pump-probe experiments associated with the Judd-Ofelt approach are used to estimate two types of room temperature cross sections: polarized emission cross sections of the dominant ⁴F_3/2 → ⁴I_1//2 transition near 1085 and 1093 nm and polarized excited-state absorption cross sections in the same spectral domain and in the green spectral range corresponding to self frequency doubling. Self frequency-doubling results are also given in Nd:LiNbO₃ and Nd:MgO:LiNbO₃ versus sample temperature.     

RBS/channelling study of ion-implanted and proton-exchanged LiNbO3:Nd:MgO planar waveguides

We present a structural study of two Nd³⁺/MgO codoped LiNbO₃ waveguides fabricated by two different methods: ion-implantation and proton-exchange. Rutherford backscattering spectrometry in combination with channelling was used to study the influence of the waveguide fabrication method on the possible modification of the LiNbO₃ crystal structure within the waveguiding layer. The results show that the ion-implanted waveguide mainly maintains the same properties as the virgin crystal, apart from a deterioration of a thin surface layer, whilst the proton-exchanged waveguide exhibits a quite different behaviour, although its surface is less damaged.     

Measurements of Bleustein-Gulyaev waves in Li2B4O7 crystal

Properties of Bleustein-Gulyaev waves (BGW) in Li₂B₄O₇ crystal were determined from the measured parameters of a delay line with double electrode interdigital transducers (IDT). Since the frequency response at the third harmonic was in good agreement with the SAW equivalent circuit model, it was possible to determine the BGW parameters. It was found that the coupling coefficient of BGW is very sensitive to aluminum layer thickness variations. Good agreement was found between calculated and measured parameters of BGW from an extrapolation to the zero layer thickness     

Nano-sized domain inversion characteristics in LiNbO3 group single crystals using SNDM

This is, we believe, the first report on nano-sized domain inversion characteristics in congruent LiNbO₃ (CLN), MgO doped LiNbO₃ (MgO:LN) and stoichiometric LiNbO₃ (SLN) single crystals brought out using scanning dielectric nonlinear microscopy method. Thin crystal plates for forming nano-sized domain inversions were prepared by chemical mechanical polishing and an ion beam etching process. The small domain dot of 13 nm in radius was formed in CLN. However, the domain dots in CLN tended to disappear. The smallest domain dot of 6.1 nm in radius was obtained in MgO:LN. The domain dot array was also successfully formed at a density of 558 Gbit/in.². In case of higher application voltage, a unique result was obtained in MgO:LN showing that the sizes of domain inversions were almost constant against the voltage pulse width. It is not easy to form domain dot array in SLN since the sidewise growth speed in SLN is very fast.     

Equivalent networks for SAW interdigital transducers

An equivalent network approach is described for the analysis of surface-acoustic-wave interdigital transducers. Circuit parameters can he theoretically determined by applying the finite-element method to an infinite array. In this approach, all the effects of piezoelectric perturbation, mechanical perturbation, and energy storage are taken into account, To show the validity and usefulness of this approach, examples are computed for both single- and double-electrode interdigital transducers and one-port resonators. For single- and double-electrode interdigital transducers on 128°Y-X LiNbO₃, X-112°Y LiTaO₃,₄₅°X-Z Li₂B₄O₇, and ST-X quartz substrates, the dependence of excitation characteristics on electrode thickness and metallization ratio is investigated in detail. For a 128°Y-XLiNbO ₃ substrate, frequency responses of various one-port resonators are also investigated     

High-temperature and broadband immersion ultrasonic probes

Immersion ultrasonic probes for measurements and imaging at high temperature are presented. The probes consist of sol-gel-sprayed thick films as piezoelectric ultrasonic transducers (UTs) directly deposited onto steel buffer rods. They operate in pulse-echo mode at temperatures up to 500/spl deg/C. The operating ultrasonic frequency is between 5 MHz and 20 MHz, controlled by the film thickness. The ultrasonic thickness measurement of a steel plate with the probe fully immersed in molten zinc at 450/spl deg/C was demonstrated using ultrasonic plane waves. For imaging purposes, the probing end of the steel buffer rod was machined into a semispherical concave shape to form an ultrasonic lens and achieve high spatial resolution with focused ultrasound in liquids. Ultrasonic surface and subsurface imaging using a mechanical raster scan of the focused probe in silicone oil at 200/spl deg/C was also carried out. The importance of the signal-to-noise ratio (SNR) in the pulse-echo measurement is discussed.     

Liquid phase epitaxy of LiNbO3 thin films for integrated optics

LiNbO₃ thin films have been obtained by liquid phase epitaxy from a Li₂OV₂O₅ flux. Phase diagram, supersaturation domain, and growth conditions have been investigated. The propagation of light has been demonstrated in an Ag substituted LiNbO₃ film grown on a c plate LiNbO₃ substrate.     

Study of diamond-like carbon films on LiNbO3

Diamond-like carbon (DLC) films have been successfully deposited on Y-cut LiNbO₃ substrates using the plasma enhanced CVD technique. A thin interlayer of SiC between the DLC films and the LiNbO₃ is necessary to ensure a good adhesion of the DLC films to the LiNbO₃ substrate. The physical properties and structural network of the DLC films have been investigated in detail. It is observed that the film hardness is increased with increasing the film thickness, as is the adhesion of the DLC films to the LiNbO₃ substrates. The effect of accelerating surface acoustic wave by the DLC films has been confirmed.     

Ultrasonic studies of the binary mixtures of ethyl acetate and cresols—application of Kosower and Dimroth treatments

The ultrasonic velocity (ν) studies were carried out at a frequency of 2 MHz (transducer of x-cut quartz crystal) using ultrasonic pulse echo system (model UX4400-M) on cresols in ethyl acetate at constant temperature of 311 K. The values of internal pressure ( π_i) and molar free volume (V_f) were calculated from measured values of ultrasonic velocity (ν), viscosity (η) and density (ρ). An attempt is made to rationalize the ultrasonic velocity (ν), internal pressure ( π_i) and free volume (V_f) of binary mixtures using Kosower solvent parameter (Z), Dimroth solvent parameter (E_T) and Dielectric constant (). It is found that there is linear correlation between ultrasonic velocity and acidity constant pk⁻¹a, indicating the dependence of acidity. Correlation of Ksower and Dimroth parameters with ultrasonic velocity confirms that solvent polarity is an important factor in the variation of ultrasonic velocity in the present investigation.     

超薄单向纤维增强复合材料低频多模式超声兰姆波定征方法

提出低频多模式超声兰姆波定征方法来估计超薄层状单向纤维增强复合材料的密度、厚度、弹性常数等参数。这里"超薄"的概念是指材料厚度h远小于材料中的声波波长λ,导致在时域上材料前后界面的各次回波信号相互混迭。分析了沿平行和垂直于纤维两个方向上,在超薄层状单向纤维增强复合材料中传播的低频兰姆波色散特性,提出在最小二乘意义下以材料色散曲线为基础的反向算法对材料参数进行了估计。文中分析了影响估计准确性的各种因素,研究了该方法对材料参数的灵敏度及其在误差传递中的意义。结合超声漏兰姆波频域分析方法,并使用超声耦合剂耦合方式和一对中心频率为2MHz的宽带纵波换能器,实验结果证实,该方法能够准确估计h<0.05 λ的单向玻璃纤维增强双酚A型聚砜(PSF)复合材料的材料参数。      

Ultrasonic linear motor using surface acoustic waves

The first success in the operation of an ultrasonic linear motor at HF band driving frequency using the Rayleigh wave is described. The substrate material is a 127.8° Y-cut LiNbO₃ wafer whose diameter is three inches. Four interdigital transducers (IDT's) are arranged to excite x- and y-propagation waves in both directions. The dimensions of the IDT are 25 mm aperture size, 400 μm pitch, 100 μm strip width, and 10 pairs. The operation area is about 25 mm square, The driving frequency is about 9.6 MHz in the x direction and about 9.1 MHz in the y direction. The most important point of the success is the shape of the contact surface and slider materials. For the contact materials, small balls about 1 mm in diameter are introduced to obtain sufficient contact pressure around 100 MPa. The use of ruby balls, steel balls, and tungsten carbide balls is investigated. Each slider has three balls to enable stable contact at three points. The maximum transfer speed is about 20 cm/s. The transfer speed is controllable by changing the driving voltage     

Polyaniline Nanofiber Based Surface Acoustic Wave Gas Sensors—Effect of Nanofiber Diameter on H2 Response

A template-free rapidly mixed reaction was employed to synthesize polyaniline nanofibers using chemical oxidative polymerization of aniline. Hydrochloric acid (HCl) and camphor sulfonic acid (CSA) were used in the synthesis to obtain 30- and 50-nm average diameter polyaniline nanofibers. The nanofibers were deposited onto layered ZnO/64deg YX LiNbO₃ surface-acoustic-wave transducers. The sensors were tested toward hydrogen (H₂) gas while operating at room temperature. The dopant for the polyaniline nanofiber synthesis was found to have a significant effect on the device sensitivity. The sensor response was found to be larger for the 50-nm diameter CSA-doped nanofiber based sensors, while the response and recovery times were faster for the 30-nm diameter HCl-doped nanofibers