Guanping Feng

Three operation modes of lateral-field-excited piezoelectric devices

Through theoretical calculations and experiments, three operation modes of lateral-field-excited piezoelectric bulk acoustic wave devices, which are referred to as the pure lateral field excitation (LFE), quasi-LFE, and pseudo-LFE modes, are presented. Several 4–6 MHz LFE devices with similar geometry using lithium niobate, lithium tantalate, and AT-cut quartz crystals are fabricated and tested in air and de-ionized water, respectively. The experimental results are in well agreement with the theoretical calculations.

Contributions of amplitude measurement in QCM sensors

The characteristics of the amplifier and the feedback loop of a quartz crystal series oscillating circuit are investigated. The fact that the change of the vibration amplitude of the quartz crystal is proportional to the change of its motional resistance is derived. The concept of a characteristic damping coefficient is introduced and the behavior of a quartz crystal vibrating in liquids is analyzed. The experiment shows that the effect of mass loading can be distinguished from that of the liquid damping of a quartz crystal microbalance (QCM) sensor in liquids by simultaneously measuring the amplitude and the frequency change.

Pseudo-LFE study in AT-cut quartz for sensing applications

Lateral field excitation (LFE) AT-cut quartz devices have recently been investigated for liquid sensing applications. However, previous studies concentrate on LFE device with Psi = plusmn90deg due to the maximum theoretical LFE coupling coefficient. Here Psi represents the angle between the lateral electric field and the crystallographic X-axes in the plate surface. In the present study, AT-cut LFE devices with different electric field directions were studied and a TSM (Thickness shear mode) was observed in the LFE device with Psi = 0deg, where the calculated LFE coupling coefficient is zero. This indicates that the observed TSM may not be excited by the lateral field but the thickness field with the liquid acting as an analog electrode, and the device is in fact a pseudo-LFE device. Moreover, experiments have also shown that LFE devices with different electric field directions are of similar sensitivities to liquid viscosity, relative permittivity and conductivity.

Energy-trapping mode in lateral-field-excited acoustic wave devices

We have analyzed the energy-trapping phenomenon of the thickness-shear vibration in lateral-field-excited (LFE) acoustic wave devices based on elastic wave propagation theory and the cutoff frequency. The standard LFE device does not support the energy-trapping effect, which is significantly different from thickness-field-excited devices. However, energy trapping in LFE devices can be achieved by placing an additional mass-loading layer in the gap region between the two working electrodes. A 5 MHz energy-trapping LFE device is fabricated and tested. The experimental results are in close agreement with the theoretical analysis.

Pseudo-LFE sensors with different electrode configurations on X-cut LiNbO 3

In the present study, several 4.5 MHz X-cut LiNbO3 LFE devices working on pseudo-LFE mode with different electrode configurations, including the dual-gap electrode, single-turn Archimedes spiral electrode, and two-turn symmetric spiral electrode, were fabricated and tested. Two common single-gap electrode LFE devices with different gap distance are also fabricated for comparison. All devices were tested in glycerol-water and isopropanol-water solutions and their resonant frequencies were recorded and analyzed. The result shows that the pseudo-LFE devices with the dual-gap electrode, single-turn Archimedes spiral electrode, and two-turn symmetric spiral electrode have a much higher sensitivity to the permittivity change of liquid than single-gap electrode LFE device. Theoretical analysis for these phenomena is given.

Broadband focusing ultrasonic transducers based on dimpled LiNbO 3 plate with inversion layer

A high-frequency broadband focusing transducer based on dimpled LiNbO3 inversion layer plate has been fabricated and characterized. A spherical surface with a curvature radius of 6 mm is formed on the half-thickness LiNbO3 inversion layer plate of Y36° cut orientation. The domain structure in the cross section is observed after a hydrofluoric acid etching process. For transducer fabrication, conductive epoxy is used as the backing material and polymer is deposited on the front face as the matching layer. The center frequency, bandwidth, and insertion loss of the focused transducer are measured to be 72 MHz, 136%, and -32 dB, respectively. The focused transducer has been successfully used for rabbit eyeball imaging and a better imaging capability compared with the planar transducer has been demonstrated. These promising results prove that the dimpled LiNbO3 inversion layer plate has great potential for fabrication of high-frequency broadband focusing ultrasonic transducers.

Investigation of pseudo-Lateral-Field-Excitation in (yxl)-16.5° LiTaO 3

In the present study, the LFE (lateral field excitation) coupling coefficient and phase velocity for (yxl)-16.5deg LiTaO3 were calculated as a function of the angle psi, which indicates the direction of the driving electric field with respect to the crystallographic x-axis of the piezoelectric plate. Several LFE devices of 5 MHz were designed and fabricated in two groups: psi = 0deg and psi = plusmn90deg. The result shows that for the LFE device of psi = 0deg operating in water, the thickness shear mode (TSM) could be excited both by LFE and TFE (thickness field excitation). For psi = plusmn90deg, the TSM is launched only by TFE and the device is in fact a pseudo-LFE device. Similar investigation has also been done to AT-cut quartz. The result suggests that the reported LFE AT-cut acoustic wave sensors may well be possible a pseudo-LFE device or a combination of TFE and LFE.

Optimal electrode shape and size of lateral-field-excited piezoelectric crystal resonators

We determine optimal electrode shape and size of lateral-field-excited (LFE) thickness-shear resonators. The determined electrodes are optimal in that they satisfy the criterion for Bechmanns number in every direction. Numerical and graphical results are provided for AT-cut quartz, (yxl)- 45°langasite, and (yxl)-16.5°LiTaO3 LFE resonators. The optimal electrodes of AT-cut quartz LFE resonators are also compared with those of AT-cut quartz thickness-field-excited (TFE) resonators.

Bandwidth improvement of LiNbO3 ultrasonic transducers by half-concaved inversion layer approach

A novel type of half-concaved LiNbO(3) plate with domain inversion layer has been proposed for fabricating high-frequency broadband ultrasonic transducers. Two opposite ferroelectric polarization layers with a curved boundary are presented after heat treatment in the half-concaved LiNbO(3) plate with a total thickness of 110 μm. Characterization of the transducers illustrates that, without a matching layer, the self-focusing transducer with the half-concaved LiNbO(3) plate has achieved 123% bandwidth at the center frequency of 60 MHz, which is a significant improvement over the planar inversion layer transducer.

Novel electrode configurations of lateral field excited acoustic wave devices on (yxl)-58° LiNbO 3

In the present study, Several 4.5 MHz LFE devices with different electrode configurations, including the dual-gap electrode, single-turn Archimedes spiral electrode, two-turn Archimedes spiral electrode and two-turn symmetric spiral electrode, were fabricated and tested. A common LFE device with the single-gap electrode is also fabricated for comparing their performances. A major resonance peak was observed in all devices in air. However, devices with the single-gap electrode and the two-turn symmetric spiral electrode offer higher Q value than other devices. The device with the dual-gap electrode shows the maximum frequency change compared to other devices when the sensing surface of these devices is exposed to water and 0.06 wt% NaCl water solution. Theoretical analysis for these phenomena is given.