Bernhard R. Tittmann

High temperature ultrasonic transducer up to 1000 °C using lithium niobate single crystal

Structural health monitoring (SHM) techniques are needed to maintain the reliability of aging power plants. The high temperature transducers are necessary to realize SHM under the working condition of power plants. In this paper, a high temperature transducer was developed using lithium niobate (LiNbO3) single crystal, which is well known as a high Curie temperature piezoelectric material. The LiNbO3 was bonded onto a stainless steel substrate. The transducer was heated in an electric furnace while measuring the bottom echoes from the substrate. We confirmed that the high temperature transducer could work up to 1000 °C.

Cellulose microfibril orientation in onion (Allium cepa L.) epidermis studied by atomic force microscopy (AFM) and vibrational sum frequency generation (SFG) spectroscopy

Cellulose microfibril orientation in plant cell walls changes during cell expansion and development. The cellulose microfibril orientation in the abaxial epidermis of onion scales was studied by atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy. Onion epidermal cells in all scales are elongated along the onion bulb axis. AFM images showed that cellulose microfibrils exposed at the innermost surface of the abaxial epidermis are oriented perpendicular to the bulb axis in the outer scales and more dispersed in the inner scales of onion bulb. SFG analyses can determine the orientation of cellulose microfibrils averaged over the entire thickness of the cell wall. We found that the average orientation of cellulose microfibrils inside onion abaxial epidermal cell walls as revealed by SFG is similar to the orientation observed at the innermost cell wall surface by AFM. The capability to determine the average orientation of cellulose microfibrils in intact cell walls will be useful to study how cellulose microfibril orientation is related to biomechanical properties and the growth mechanism of plant cell walls.

Acoustic beamwidth compressor using gradient-index phononic crystals

We report a novel approach to effectively couple acoustic energy into a two-dimensional phononic-crystal waveguide by an acoustic beamwidth compressor using the concept of a gradient-index phononic crystal (GRIN PC). The GRIN PC-based beamwidth compressor is composed of a square array of solid scatterers embedded in epoxy. By gradually modulating the density and elastic modulus of the scatterers along the direction transverse to the phononic propagation, the beamwidth compressor can efficiently compress the wide acoustic beam to the scale of the phononic-crystal waveguide. This acoustic beamwidth compressor is investigated through a finite-difference time-domain method. A beam-size conversion ratio of 6.5?:?1 and a transmission efficiency of up to 90% is obtained over the working frequency range of the phononic-crystal waveguide. Potential applications for this device include acoustic biosensors and signal processors.

Radiation tolerance of piezoelectric bulk single-crystal aluminum nitride

For practical use in harsh radiation environments, we pose selection criteria for piezoelectric materials for non-destructive evaluation (NDE) and material characterization. Using these criteria, piezoelectric aluminum nitride is shown to be an excellent candidate. The results of tests on an aluminum-nitride- based transducer operating in a nuclear reactor are also presented. We demonstrate the tolerance of single-crystal piezoelectric aluminum nitride after fast and thermal neutron fluences of 1.85 x 1018 neutron/cm2 and 5.8 x 1018 neutron/cm2, respectively, and a gamma dose of 26.8 MGy. The radiation hardness of AlN is most evident from the unaltered piezoelectric coefficient d33, which measured 5.5 pC/N after a fast and thermal neutron exposure in a nuclear reactor core for over 120 MWh, in agreement with the published literature value. The results offer potential for improving reactor safety and furthering the understanding of radiation effects on materials by enabling structural health monitoring and NDE in spite of the high levels of radiation and high temperatures, which are known to destroy typical commercial ultrasonic transducers.

ALUMINUM NITRIDE AS A HIGH TEMPERATURE TRANSDUCER

The high temperature capabilities of bulk single crystal aluminum nitride are investigated experimentally. Temperatures in excess of 1100° Celsius are obtained and held for eight hours. Variation in the performance of single crystal samples is demonstrated.

Determination of Physical Property Gradients from Measured Surface Wave Dispersion

A technique for estimating the depth-dependent profiles of a physical property, such as case hardness, multilayers over a substrate, or surface corrosion, is presented. The technique uses surface wave pulses with sufficient bandwidth to allow the observation of the frequency spectrum over the range of frequencies required for the estimation procedure. Typically, this range must include those frequencies corresponding to wavelengths smaller and larger than the characteristic dimensions or depths of the physical property gradients in question. The technique uses pulse data acquisition at two locations of the receiving transducer and, through the use of one of the pulses as a reference, calculates the deconvolved Fourier transform of the other pulse and displays the dispersion diagram of the phase velocity. With the use of an inversion algorithm previously reported, the phase velocity variation with depth is obtained and is correlated to variations of the physical property gradient. The significance of this work is the ease and speed with which the data are obtained and processed to get the desired result. Another important consequence is that this technique provides a dense data set, thereby providing much greater precision than that obtainable by previously reported tone-burst measurements.

High-temperature (>500 /spl deg C) ultrasonic transducers: an experimental comparison among three candidate piezoelectric materials

High-temperature piezoelectric crystals, including YCa₄O(BO₃)₃, LiNbO₃, and AlN, have been studied for use in ultrasonic transducers under continuous operation for 55 h at 550°C. Additionally, thermal ratcheting tests were performed on the transducers by subjecting the crystals to heat treatments followed by ultrasonic performance testing at room temperature and 500_°C. The changes resulting from the heat treatments were less than the statistical spread obtained in repeated experiments and were thus considered negligible. Finally, in situ measurements of the pulse-echo response of YCa₄O(BO₃)₃ were performed at temperatures up to 950°C for the first time, showing stable characteristics up to these high temperatures.

Ultrasonic C-scan imaging for material characterization

Abstract Advances in the field have enabled ultrasonic C-scan imaging to go beyond its classical application of detecting large single flaws to meet the challenge of non-destructive evaluation for modern engineered structures and new materials. Increased frequency of operation and novel transducer designs have led to micrometre resolution limits for both surface and subsurface imaging. Improved image processing analysis methods allow more information to be extracted from the classical and contemporary application of the C-scan technique. Important results and developments are discussed with references made to industrial applications.

A Shear Wave Rheology Sensor

A rheology sensor for monitoring the dynamic viscosity of a thermally curing resin is described. The complex shear modulus of the resin at 1 MHz is derived from the measured reflection coefficient of shear wave pulses at the resin surface. A special transducer-buffer assembly has been developed that operates at high temperature (T = 145°C) and provides a reference calibration signal. With this assem- bly, absolute determinations are made throughout the cure cycle of the real and imaginary components of the shear modulus. From the latter high-frequency dynamic viscosity is calculated. The results are com- pared with data obtained at low shear rates with a 10 Hz torque vis- cometer.

Acoustic imaging of thick biological tissue

Up to now, biomedical imaging with ultrasound for observing a cellular tissue structure has been limited to very thinly sliced tissue at very high ultrasonic frequencies, i.e., 1 GHz. In this paper, we present the results of a systematic study to use a 150 to 200 MHz frequency range for thickly sliced biological tissue. A mechanical scanning reflection acoustic microscope (SAM) was used for obtaining horizontal cross-sectional images (C-scans) showing cellular structures. In the study, sectioned specimens of human breast cancer and tissues from the small intestine were prepared and examined. Some accessories for biomedical application were integrated into our SAM (Sonix HS-1000 and Olympus UH-3), which operated in pulse-wave and tone-burst wave modes, respectively. We found that the frequency 100 to 200 MHz provides optimal balance between resolution and penetration depth for examining the thickly sliced specimens. The images obtained with the lens focused at different depths revealed cellular structures whose morphology was very similar to that seen in the thinly sectioned specimens with optical and scanning acoustic microscopy. The SAM operation in the pulse-echo mode permits the imaging of tissue structure at the surface, and it also opens up the potential for attenuation imaging representing reflection from the substrate behind the thick specimen. We present such images of breast cancer proving the methods applicability to overall tumor detection. SAM with a high-frequency tone-burst ultrasonic wave reveals details of tissue structure, and both methods may serve as additional diagnostic tools in a hospital environment.