Martin Schmitt

SAW-relevant material properties of langasite in the temperature range from 25 to 750 °C: New experimental results

The aim of this work is to determine the acoustical parameters of langasite up to the highest possible point of temperature. This paper presents measurements of transfer functions for delay lines on langasite at frequencies ranging from 150 MHz to 1 GHz, at temperatures from 25 to 750degC. Two cuts with Euler angles (0deg, 138.5deg, 26.6deg) and (0deg, 30.2deg, 26.6deg) have been studied. The devices were fabricated using langasite as substrate, with two different platinum (Pt) layer heights (45 nm and 75 nm), on a zirconium (Zr) adhesion layer (4 nm). A special signal processing algorithm utilizing cross-correlation was implemented in MATLAB and used for the analysis of measured data. The material parameters relevant for SAW devices, such as phase velocity, propagation loss, and electromechanical coupling coefficient, have been determined as a function of temperature.

SAW-properties of langasite at high temperatures: Measurement and analysis

The measurement range of temperature using SAW-devices is depending on the substrate material used in the measurements. This dependence is related to the temperature point, where the substrate loses its piezoelectric properties. This occurs by the SAW-materials such as quartz and lithium niobate at about 570°C or 350°C, respectively. The piezoelectric material langasite (La3Ga5SiO14) has a thermodynamically stable phase and do not lose its piezoelectric properties up to its melting point at about 1470 °C and can be used for SAW devices at high temperatures. This paper presents the measurements of acoustical parameters of langasite up to 750°C. Langasite is used as substrate in the measurements with Euler angles (0°, 138.5°, 26.6°) and two different platinum (Pt) layer heights (45 nm and 75 nm) top of a zirconium (Zr) adhesion layer (4 nm). The investigated acoustic properties of langasite are group and phase velocity, propagation loss, and electromechanical coupling coefficient as functions of temperature. The measured data is analyzed using a special signal processing algorithm to obtain these acoustic properties of the Langasite.

Multi Reflection of Lamb Wave Emission in an Acoustic Waveguide Sensor

Recently, an acoustic waveguide sensor based on multiple mode conversion of surface acoustic waves at the solid—liquid interfaces has been introduced for the concentration measurement of binary and ternary mixtures, liquid level sensing, investigation of spatial inhomogenities or bubble detection. In this contribution the sound wave propagation within this acoustic waveguide sensor is visualized by Schlieren imaging for continuous and burst operation the first time. In the acoustic waveguide the antisymmetrical zero order Lamb wave mode is excited by a single phase transducer of 1 MHz on thin glass plates of 1 mm thickness. By contact to the investigated liquid Lamb waves propagating on the first plate emit pressure waves into the adjacent liquid, which excites Lamb waves on the second plate, what again causes pressure waves traveling inside the liquid back to the first plate and so on. The Schlieren images prove this multi reflection within the acoustic waveguide, which confirms former considerations and calculations based on the receiver signal. With this knowledge the sensor concepts with the acoustic waveguide sensor can be interpreted in a better manner.

Propulsion of droplets on non-piezoelectric substrates via mode conversion of Lamb waves

The mode conversion of acoustic surface waves contacted by liquid droplets has been utilized for the propulsion of nanoliter droplets with respect to lab-on-a-chip applications. In this case, however, piezoelectric substrates equipped with interdigital transducers on the surface on which the droplets are moving have been used for the excitation of surface acoustic waves. Our approach is aiming at the acceleration of comparatively large droplets such as raindrops on non-piezoelectric substrates, e. g. glass plates. Following this approach, we have excited Lamb waves on glass plates by piezoelectric interdigital transducers attached at the rear side of the substrate, which are not in contact with the liquid. A propulsive action on water droplets of µl size has been observed, which could be improved by hydrophobic surface coatings. With continuous wave excitation at a frequency of 1 MHz and an amplitude of 200 Vpp an acceleration of about 1 m/s2 has been achieved so far with water droplets of 10 µl on a 1 mm thick glass plate. This concept allows numerous technical applications, including two-dimensional droplet relocation or “intelligent” droplet removal from glass panels in combination with droplet detection via a Lamb wave transmission line established by a second interdigital transducer.

Measurement of the emission of Lamb waves by a PVDF membrane hydrophone

The emission of sound waves resulting from mode conversion of Lamb waves propagating on thin glass plates submerged into water was investigated by a PVDF membrane hydrophone. The antisymmetrical zero order Lamb wave mode was excited by a piezoelectric single phase transducer operated at a frequency of 1 MHz. With the hydrophone the emission pressure distribution of the emitted sound waves in the liquid was recorded and the phase velocities of the Lamb waves were calculated from the direction of the emitted sound waves, i. e. the Lamb angles. Due to the dispersion of Lamb waves the velocity changes with the plate thickness. The measured phase velocities coincide well with the calculated values using the global matrix method for plate thicknesses between 0.5 and 4 mm.

Droplet propulsion on non-piezoelectric substrates induced by Lamb waves

Microliter droplet can be propelled on a non-piezoelectric substrate, on which antisymmetrical zero order Lamb waves with 1 MHz center frequency are propagating. Different droplet volume has different ratio of absorbed power, which is described by droplet propulsion distance/electrical input power. Moreover, with increasing droplet volume (10μl, 20μl, 50μl), the excitation frequency (1.03 MHz, 1.04 MHz, 1.06 MHz) must be slightly increased, for the optimization of the propulsion process.

Detection and removal of droplets on non-piezoelectric substrates via mode conversion of Lamb waves

A combination of detection and propulsion of droplets on glass substrates at macroscopic dimensions has been accomplished via mode conversion of 1 MHz Lamb waves excited and detected by piezoelectric interdigital transducers (IDT) attached to the substrates forming a delay line configuration. Experimental results with respect to water, oil and glycerin droplets with liquid volumes between 8 and 160 µl on hydrophobic and hydrophilic glass surfaces are reported. In order to switch from sensing to actuating the excitation voltage of the IDT has been increased from 20 Vpp to 300 Vpp and the operation mode has been changed from burst to continuous excitation. Applications of this approach are envisaged at mirrors, windows and other optical components whose function may be affected adversely by the presence of liquid droplets.

On-line surveillance of lubricants in bearings by means of surface acoustic waves

The acoustic wave propagation in a bearing filled with lubricants and driven by pulsed excitation of surface acoustic waves has been investigated with respect to the presence and the distribution of different lubricants. Experimental setups, which are based on the mode conversion between surface acoustic waves and compression waves at the interface between a solid substrate of the bearing and a lubricant are described. The results of preliminary measurements at linear friction bearings and rotation ball bearings are presented.

Characterization of langasite as a material for SAW based RFID and sensing systems at high temperatures

For langasite, a piezocrystal used as substrate material for SAW-based ID tags and sensors operating at high temperatures, the parameters of SAW propagation have been determined as a function of temperature. These parameters are the phase velocity, propagation loss, and electromechanical coupling coefficient. Two crystal cuts have been studied, with Euler angles (0°, 138.5°, 26.6°) and (0°, 30.2°, 26.6°). The SAW parameters were obtained from the measurements of transfer functions for delay lines on langasite chips at frequencies ranging from 150 MHz to 1 GHz and at temperatures from room temperature up to 750°C