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Bismuth Titanate Fabricated by Spray-on Deposition and Microwave Sintering For High-Temperature Ultrasonic Transducers Clifford T. Searfass, C. Pheil, K. Sinding, B. R. Tittmann, Life Fellow, IEEE, A. Baba, and D. K. Agrawal Abstract—Thick films of ferroelectric bismuth titanate (Bi4 Ti3 O12 ) have been fabricated by spray-on deposition in conjunction with microwave sintering for use as high-temperature ultrasonic transducers. The elastic modulus, density, permittivity, and conductivity of the films were characterized. Electro-mechanical properties of the films were estimated with a commercial d33 meter which gave 16 pC/N. This value is higher than typically reported for bulk bismuth titanate; however, these films withstand higher field strengths during poling which is correlated with higher d33 values. Films were capable of operating at 650 ◦ C for roughly 5 min before depoling and can operate at 600 ◦ C for at least 7 days. Index Terms—Spray-on deposition, ultrasonic transducers.

I. I NTRODUCTION

T

HE GOAL of this work was to fabricate ultrasonic transducers for high-temperature ultrasonic nondestructive evaluation (UNDE). The motivation springs from a growing interest for harsh environment UNDE from the aerospace and nuclear energy industries. The Energy Policy Act of 2005 included many new incentives for the construction of nextgeneration nuclear reactors and as many as 31 reactor license applications have been announced, which some speculate is the beginning of a “nuclear renaissance” [1]. The aerospace industry has pushed for sensors in health monitoring of turbine blades to reduce downtime from schedule-based maintenance. For instance, in 2003 the Air Force spent over 1 billion dollars a year on turbine maintenance [2]. Real-time monitoring of turbines would save millions of dollars annually and increase fleet availability. Using piezoelectric materials for high-temperature NDE is particularly advantageous in these fields because sensors can be made relatively lightweight and low profile. Here, we report progress on the use of bismuth titanate thick film transducers deposited with a spray-on technique for use as high-temperature ultrasonic transducers. Temperatures exceeding 150 ◦ C, the temperature commonly regarded as the

Manuscript received October 31, 2014; accepted November 13, 2015. Date of publication November 17, 2015; date of current version December 29, 2015. C. T. Searfass is with Structural Integrity Associates, State College, PA 16801 USA. C. Pheil is with Red Frog Events LLC, Chicago, IL 60654 USA. K. Sinding is with Pennsylvania State University, University Park, PA 16802 USA. B. R. Tittmann and D. K. Agrawal are with the Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802 USA (e-mail: [email protected]). A. Baba is with Hitachi, Ltd., Hitachi, Ibaraki 319-1221, Japan. Digital Object Identifier 10.1109/TUFFC.2015.2501241

operating limit of the commonly used lead zirconate–titanate (PZT)-based transducers, will be regarded as high temperature. The spray-on deposition technique was started by Barrow who used a modified sol–gel procedure [3], [4]. In that work, powders of PZT were dispersed into a sol–gel phase creating a slurry. Substrates were then dip-coated with the slurry creating 0–3 thick-film piezoelectric composite (the powder ideally having zero connectivity and the sol–gel phase being threedimensionally connected). This technique was later adapted by Kobayashi for the spray-on deposition of LiTaO3 /PZT and PZT/Al2 O3 composites [5]. With the spray-on deposition technique, transducers are sprayed directly onto a substrate using an air gun. This technique effectively eliminates the need to develop a suitable method for coupling the ultrasonic energy between the transducer and a structure requiring investigation at elevated temperatures. An additional advantage is that structures with complex surface geometries can have transducers attached without having to machine ceramics/crystals. This fabrication method has since been used to fabricate several singlephase and multiphase composites for use as high-temperature ultrasonic transducers, including single-phase bismuth titanate, as well as bismuth titanate-PZT, lithium niobate-PZT, lithium niobate–bismuth titanate-based composites, as well as several other materials [6]–[11]. Films fabricated utilizing this technique typically produce transducers having center frequencies within the 1–20-MHz region. Therefore, these films are of particular interest as a simple and cost-effective means of making transducers for applications such as thickness measurements, corrosion mapping, and phased array inspection, as these applications tend to utilize transducers operating within this frequency range. The spray-on deposition technique is capable of producing functional ultrasonic transducers. However, to date, little has been done to fully characterize materials fabricated using this technique. Quantification of parameters such as electromechanical strength has yet to be performed on several of the materials fabricated in this manner, and in some cases, X-ray diffraction of fabricated materials has not been reported. In such cases, either the X-ray diffraction spectrum was omitted without mention or the formation of phases was assumed during the firing process. In most cases, other material properties such as the elastic modulus and density have also not been reported. In the case of spray-on deposited films, their inherent porosity should lead to films whose properties differ from their denser bulk and thin film counterparts. Other interesting cases of multiphase materials have been reported where new intermediate phases

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may be present and may possess new and unique material properties. The films fabricated in this work also took advantage of microwave sintering for the firing/sintering process. Microwave sintering differs greatly from conventional sintering in that the heating and solid-state reactions occur by way of different mechanisms. With conventional sintering, the heating process is a transfer of energy. With microwave sintering, heating is through the conversion of energy [12]. Microwave energy is converted into heat by friction from the motion of electric dipoles, electrons, and ions which try to stay in phase with the incident electromagnetic field. In a manner of speaking, the sample itself becomes the heat source. Depending on how well a material absorbs microwave radiation, extreme heating rates can be observed (> 400 ◦ C/min). Due to the dissimilarity of the physical nature between microwave sintering and conventional sintering, different properties are commonly observed between materials fabricated using microwave sintering compared to those sintered using conventional techniques at comparable times and temperatures [12]–[19]. The observed differences in material properties are sometimes referred to as the “microwave effect” [17]–[22]. These “microwave effects” have been attributed to several physical processes such as ponderomotive forces acting on ions at grain boundaries, which in turn encourage diffusion by the enhancement of surface diffusion from plasma formation in pores [19]. Others have speculated that anisothermal heating may be the cause for these observed differences in material properties [15], [16].

thick and six coatings produced a film with a thickness of approximately 120 µm (center frequency of ∼6 MHz). After sintering, electrodes were applied to the samples and then the samples were poled. For the electrodes, a gold layer of 150 nm was first sputtered onto the samples, followed by the application of an SPI conductive high-purity silver paint. If the silver was applied without the gold layer, conductivity of the sample became too high and poling became impossible. This is believed to occur because of silver ions infiltrating the film. Thus, the gold film acted as a conductive diffusion barrier [20]. All samples were poled at 70 kV/mm in an oil bath at 150 ◦ C. Higher field strengths could not be obtained without samples undergoing dielectric breakdown. Scanning electron microscope (SEM) images of fabricated films were taken using a Philips XL 30 Environmental SEM and a Nova NanoSEM 630 Field Emission Scanning Electron Microscope. The porosity in the films is clearly visible. Several images of typical microstructures are shown in Fig. 1(a)–(c). The higher magnification image in Fig. 1(c) shows that the average connected region of the microstructure is roughly 2 µm. There have been no cases of anisotropic growth platelets, which have been previously observed in bismuth titanate [21], [22]. Due to the porosity, a third inactive phase was technically introduced and the material is more accurately described as a {3(0–3)–0} composite via the convention outlined by Pilgrim (as opposed to simply being considered a 0–3 composite) [23]. X-ray diffraction of the samples using a Scintag powder X-ray diffractometer was performed and the results showed that phase pure formation of Bi4 Ti3 O12 was achieved. The measured spectra have been omitted for brevity

II. E XPERIMENTAL D ETAILS A. Film Fabrication: Procedure Outline

B. Film Fabrication: Microwave Sintering

To prepare the sol–gel solution, bismuth nitrate pentahydrate (Bi(NO3 )3 · 5H2 O) (Sigma-Aldrich, ≥ 98%) was dissolved in glacial acetic acid (Sigma-Aldrich, 99.99) at 80 ◦ C with a ratio of acid to bismuth nitrate pentahydrate being 2 mL:1 g. After the solution was allowed to cool to 30 ◦ C, titanium isopropoxide (Sigma-Aldrich, ≥ 99.99%) was then added, forming a clear and particle-free solution. Water at a temperature of 30 ◦ C was then added to the solution with a volumetric ratio of sol– gel phase to water being 2:1. Bismuth titanate powder was then added to the solution with a ratio of 2 g of powder for every 1.2 mL of solution. This solution-to-powder ratio creates a mixture having a viscosity suitable for spray-on deposition. Following the addition of powder, the slurry was sonocated with high intensity ultrasound to break up powder agglomerates and reduce particle size. It was found that using high-intensity ultrasound, when compared to ball milling, is better for mixing the powder and solution because it minimizes the amount of contaminants introduced. After being treated ultrasonically, the slurry was then sprayed onto 1-inch-thick, 1-inch-diameter stainless steel, 316 cylindrical substrates. After spraying, the samples were placed onto a hot plate at 400 ◦ C for 10 min to pyrolyze out water and additional organics introduced during the preparation of the solution. After 10 min, the samples were allowed to cool and the process was repeated until a desired thickness was achieved. Each coating was about 20 µm

First attempts of firing/sintering of films using conventional tube furnaces resulted in poor film quality. Adhesion of the films to the substrate was generally very poor. It may be possible to improve film adhesion by exposing the substrate to various forms of chemical treatment; however, the use of microwave sintering proved to be a superior form of fabrication method and therefore the utilization of conventional sintering was abandoned. Samples were microwave sintered at 850 ◦ C for 10 min in a multimode, 2.45-GHz microwave chamber under atmospheric conditions. Samples were placed in a Fiberfrax insulation package to aid in maintaining a uniform temperature distribution throughout the film. Samples typically reached the soak temperature of 850 ◦ C in less than 10 min. Carbon powder was used as a susceptor to assist the microwave heating process. The films prepared in this fashion demonstrated excellent adhesion with approximately only 5% of samples delaminating after the sintering process. It is speculated that there are two reasons for enhanced film adhesion with microwave sintering. The first is that there is an anisothermal heat exchange between the film and the substrate. The metallic substrate, for the most part, reflects incoming microwave radiation, while the film itself heats up very rapidly. As it was shown by Peelamedu et al., anisothermal heating can result in unidirectional diffusion of species [15]. The case of Peelamedu dealt

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The second way this technique may promote adhesion is that due to lower processing times, and therefore less exposure to higher temperatures, the substrate may oxidize less during the sintering process.

III. C HARACTERIZATION P ROCEDURE A. Electrical, Dielectric, and Mechanical Properties Thickness measurements were made using an Olympus optical microscope which has a 100-nm defocusing sensitivity. Typical thicknesses for samples fabricated with six sprays were 120 µm, corresponding to a center frequency of approximately 6 MHz. The measured values were average values of the thickness, as surface roughness can be fairly large (±10 − 20 µm). Densities were measured by weighing the substrates on a scale with resolution greater than 1 µg before and postsintering following film deposition. The weight and the measured thickness were then used to calculate the density. With the density and thickness measured, it was then possible to calculate elastic modulus of the films via their ultrasonic response. Transducers were excited using a broadband voltage pulse (Panametrics 5052 PR) in pulse-echo mode and the center frequency of the loaded film was determined from the Fourier transform of the received echo which had reflected off of the bottom of the stainless steel substrate. With the center frequency of the loaded transducer known, and by knowing the material properties and dimensions of the substrate, the center frequency of the film if it were unloaded can be calculated from the following equation [24]:  fc

   Ms Ms + 1 = ft + fs Mt Mt

(1)

where Ms and Mt are the masses of the transducer and substrate, respectively, and ft and fs are the center frequencies of the substrate and the transducer if they are not loaded, respectively. Noting that the velocity v can be approximated by  v = λft ≈

c33 ρ

(2)

where c33 is the elastic modulus in the direction of poling and ρ is the density of the material. Noting that the thickness t is related to the fundamental resonant wavelength by λ = 2t, and by solving for t in (2) and substituting into (1), c33 can be solved for as

Fig. 1. (a)–(c) SEM images of the microstructure of a spray-deposited Bi4 Ti3 O12 film.

with an Fe3 O4 /Y2 O3 system; Fe3 O4 being a good microwave absorber and Y2 O3 being a poor absorber. In this case, it was shown that Fe species unidirectionally diffused from the hotter Fe3 O4 to the cooler Y2 O3 . It is suggested that a similar phenomenon may be occurring between the bismuth titanate film and stainless steel substrate resulting in better film adhesion.

c33 = 4ρt

2



 2 Ms Ms + 1 − fs . fc Mt Mt

(3)

All parameters on the right side of (3) are known, and therefore c33 can now be determined. Dielectric measurements of the capacitance and loss were performed using a Stanford Research Systems SR715 LCR meter. Measurements were made with a 1-V signal at 100 Hz, 120 Hz, 1 kHz, and 10 kHz. Measurements were performed at these frequencies to see if any dispersion of the capacitance and

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TABLE I M EASURED M ATERIAL P ROPERTIES OF B ISMUTH T ITANATE T HICK F ILMS

loss was present since the presence of dispersion would indicate a poor electrode condition. No dispersion was observed. For the particular case of bismuth titanate, conductivity is a topic of major importance. Bismuth titanate is somewhat notorious for having a high conductivity and this affects transducer performance in two ways [25], [26]. One such way is that the conductivity of a dielectric can dictate its breakdown strength, and therefore also dictates how much domain alignment can be achieved during the poling process. It is well known that the high coercive field of bismuth titanate compounded with the fact that bismuth titanate has high conductivity greatly affects the degree of domain alignment achievable during poling. Typically, field strengths of only 20 kV/mm can be achieved during poling without breakdown [25], [27]. This low field strength is typically capable of inducing a d33 of 3.5 pC/N or less, which is about 13% of the single crystal value for bismuth titanate. It is possible to reduce the conductivity of bismuth titanate by donor doping with Nb and thus obtain higher breakdown strengths, as well as employ the use of pulse poling to prevent breakdown [25]–[27]. Such methods have not been investigated here. The second effect the conductivity can have on a ferroelectric relates to a specific materials’ ability to operate at high temperatures. As the temperature of the sample increases, the conductivity also increases. If the conductivity is too high, the time constant of the material becomes too low and the length of time a charge can be maintained will be too small to allow for detection from electronic equipment [28].

B. Conductivity Conductivity measurements were taken using a charge– discharge method at field strengths where the material behaves ohmically, which was deduced to be at field strengths less than 10 kV/m. This matches numbers previously reported by Shulman [25]. The voltage source was a National Instruments PXI 5402 function generator, current was measured using a National Instruments PXI 4071 7.5 Digit Flex digital multimeter (pA resolution), and the current was measured using software developed in LabVIEW software package. Measurements were made at 200 ◦ C. At lower temperatures, it was not possible to obtain quality reliable values for the conductivity. Large fluctuations in the current were always present due to electrical noise and small fluctuations in the applied voltage. Spontaneous jumps in current may also be from random electronic and ionic conduction from thermal fluctuations within the material. At 200 ◦ C, the material conductivity was sufficiently high that small fluctuations were negligible and a quality measurement could be made. Unfortunately, 200 ◦ C was also the temperature limit for the experimental setup.

IV. R ESULTS AND D ISCUSSION A. Dielectric, Electrical, and Mechanical Properties Table I contains the measured values for electrical, dielectric, and mechanical properties of the films. Typical densities were between 2.9−3.2 g/cm3 . Typical values for the permittivity were found to be within the range of 90–95 and values for tan δ were typically 0.01. Both the permittivity and tan δ showed little or no dispersion indicating a high-quality electrode. Typical values for the calculated elastic modulus were 15 GPa. It was speculated a priori that the conductivity in the bismuth titanate films may differ from that of bulk bismuth titanate due to percolation of the flow of electrical charge due to the porosity. It was also speculated that differences in conductivity may arise from potential barriers which may form on the exposed surface areas within the pores [20], [29]. However, this was not the case and the conductivity of the films was typically 2 × 10−7 (Ω · m)−1 which very closely matches values previously reported for bulk bismuth titanate [25].

B. Piezoelectric Properties and Room Performance as an Ultrasonic Transducer

Temperature

Fig. 2(a) shows a waveform collected by a fabricated film operating in pulse-echo mode. The recorded waveforms are the echoes which have reflected off the backwall of the 1-inch-thick stainless steel 316 substrates. The waveform, which is shown, is representative of a typical transducer response for films being fabricated in the manner described in this work. Fig. 2(b) shows a close-up of the first backwall echo in the signal, Fig. 2(a). Fig. 2(c) shows the frequency response of the first backwall echo. The transducer was excited using an Ultran pulser– receiver on energy-level setting 3 and a gain of 40 dB. Fig. 2(c) illustrates the broadband nature of the pulse response of the transducers. The 3-dB Q is 1.3. This is a considerably low Qvalue and is presumed to be a result of the porosity of the films. The large signal strengths of the transducers led to speculation that the fabricated transducers have d33 values larger than the commonly reported value of ∼3.5 pC/N for bulk bismuth titanate. However, determining the electromechanical properties of a substrate-supported material is not a trivial task. Due to the mechanical boundary conditions imposed on the film by the substrate, i.e., the existence of a nonzero stress boundary condition on the interface between the film and substrate, the methods for determining electromechanical properties outlined in the IEEE standards are not applicable [30]. Another method is to determine the effective piezoelectric coefficients via laser interferometry [31]. However, due to the surface roughness of the films, this was also not an option. Another option for determining the electromechanical properties is to curve fit the

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Fig. 3. Measured (blue dots) and modeled (red line, with kt = 0.00001) impedance spectrum of bismuth titanate film.

Fig. 2. (a) Entire wave train; (b) close-up of first backwall; and (c) frequency spectrum of a fabricated transducer.

measured impedance spectrum to either the Mason equivalent circuit or to the exact solution that was derived by Lukacs [32], [33]. Unfortunately, this method also cannot be employed. Due to the relatively weak electromechanical coupling of bismuth titanate (when compared to PZT), the damping imposed on the film by the substrate, as well as because of the internal damping created by the film porosity, no resonance is observed in the impedance spectrum of the bismuth titanate films when investigated with conventional network analyzers. The effects of the substrate and porosity on the impedance spectrum of a bismuth titanate film can be illustrated using the Mason model. A computer code was written in the Mathematica software package to model the impedance spectrum of a bismuth titanate film using the experimentally determined material properties. Fig. 3 shows the measured impedance spectrum versus the modeled one (with kt arbitrarily set to 0.00001). The closeness of the model prediction to the measured data validates the model. To illustrate the adverse effects the substrate has on the ability to observe a resonance in the impedance spectrum, the model was used to predict what a film’s impedance would be when loaded and the hypothetical situation of the film being free standing. Fig. 4 shows a comparison of the modeled impedance spectrums of a loaded transducer and the hypothetical situation of a free-standing film. As it was expected, the resonant frequency is lowered and the magnitude of the resonance is decreased. This helps to illustrate to what degree the

Fig. 4. Comparison of modeled impedance spectrum for a substrate-loaded film (red) and a film without substrate loading (green). A value of kt = 0.05 was used.

magnitude of the effect the substrate has on detecting the resonance. Furthermore, due to film porosity, one would be inclined to expect significant mechanical losses, as is supported by how broadband the transducer responses are. This can be incorporated into the model by defining the elastic modulus as complex via the following convention [34]: D∗ cD∗ 33 = c33 (1 + i tan δm ) .

(4)

It was shown by Ritter that for fully dense PZT, tan δm can be as high as 0.1 [32]. The effect of mechanical losses on the magnitude of the resonance peak is shown in Fig. 5 where plots of the impedance spectrum for models having values for tan δm = 0 and tan δm = 0.1 are compared. As Fig. 5 shows, a value of tan δm = 0.1 almost completely eliminates the resonance peak. Since the thick films fabricated in this work are considerably more porous than the fully dense PZT film, the mechanical losses will be considerably higher than those for fully dense PZT (e.g., tan δm for these films will be much higher than 0.1). What these model comparisons demonstrate is that the loading on the film from the substrate, as well as large mechanical losses associated with the film porosity, is playing a significant role in not being able to detect a resonance peak in the films’ impedance spectrums. Therefore, the lack of a detectable resonance is not necessarily a reflection of weak electromechanical coupling.

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Fig. 5. Modeled impedance spectrum for a transducer with tan δm = 0 (green) and tan δm = 0.01 (red). A value of kt = 0.05 was used.

While this analysis with the Mason model is insightful, the electromechanical properties of the film still remain elusive. To help shed light on this issue, the d33 of the film was approximated using a commercial d33 meter. However, such a measurement can only be assumed to be an approximation due to the mechanical boundary conditions, as well as from coupling in the direction of d31 due to the applied uniaxial stress. The d33 for substrate-supported films was derived by Lefki and is given as [35]   v 2d31 sE 13 + c (5) d33 = d33 − E sE 11 + s12 where d33 is the false measured value and d33 is the true d33 . sE ij is the elastic compliance of the film and ν and c are the substrate Poisson ratio and elastic modulus, respectively. Measured values of the d31 for bismuth titanate have previously been reported as 1.8 pC/N. Since this previously reported value is positive, (5) indicates that actual value for the d33 of the film would be slightly larger than what was measured using a d33 meter. The values obtained using the commercial d33 meter were typically 16 pC/N. This value is substantially larger than the commonly reported value of 3.5 pC/N for undoped bulk bismuth titanate when poled using conventional dc voltages [25], [26]. However, it was also noted that substantially larger field strengths could be achieved during the poling process than the more commonly reported maximum poling field strength of 20 kV/mm. Values as high as 70 kV/mm were achieved during poling without breakdown and thus the thick film transducers are capable of enduring field strengths 3.5 times greater than their bulk counterparts. It is also worth noting that Shulman reported d33 values similar to the values measured in this study for Nb-doped bismuth titanate [25]. In that study, values of the d33 were measured in the range of 18.7–20 pC/N for bismuth titanate films doped with Nb concentrations of 0.05–0.74 at. % and poled with the same field strengths that have been used in this work. This was primarily attributed to the fact that the addition of Nb substantially decreased the conductivity which resulted in raising the breakdown strength of the material. Comparing these results indicates how the breakdown strength can affect the poling process.

This was a very interesting observation and at first, a conclusion could not be drawn as to why higher field strengths could be achieved. In fact, due to their porosity, one may be inclined to assume that the films would only be able to withstand lower field strengths due to the air within the pores [36]. However, if one considers that the mechanism responsible for breakdown in bismuth titanate is thermal breakdown, an explanation presents itself. In dielectrics where common mechanism for dielectric breakdown is thermal breakdown, the breakdown strength of the material increases as thickness decreases [20]. This is because thermal breakdown can be avoided if localized heating in the material can be dissipated quickly enough. As a material’s thickness decreases, heat is also dissipated more rapidly, and thus, larger field strengths are achievable. For example, an exact equation for breakdown strength Eb for thermal breakdown as a function of thickness was derived by Smith and is described by [37]  1/3 3αcv Ro Δθ (6) Eb = t where α is the voltage ramp rate, cv is the material heat capacity, Ro is the material heat capacity, the product of Δθ are terms derived from the temperature dependence of the material resistivity, and t is the material thickness. From (6), it can be seen that, for materials which experience thermal breakdown, the breakdown strength increases inversely to the cube root of the material thickness. Considering this, it appears that the primary breakdown mechanism in bismuth titanate is thermal breakdown and, subsequently, by reducing the thickness of the material, a larger breakdown strength can be achieved. V. H IGH T EMPERATURE P ERFORMANCE OF THE F ILMS AS U LTRASONIC T RANSDUCERS To evaluate the potential for the fabricated transducers to perform at high temperature, samples were placed into a tube furnace and heated at 1 ◦ C/min. The low heating rate was used to ensure thermal equilibrium. A high-temperature thermocouple cable purchased from Omega was used as the electrical connection and was attached to the electrode of the transducer with SPI conductive silver paint. The stainless steel substrate was used as ground. Waveforms were recorded every 25 ◦ C and the pulser–receiver was only turned on when a measurement was taken to ensure that it would have as little of an effect as possible on the stability of the ferroelectric domain alignment during heating. Fig. 6(a) shows a plot of the measured signal amplitude normalized to the amplitude of the room temperature signal as a function of temperature. Fig. 6(b) shows a three-dimensional plot of the recorded waveforms. It is interesting to note that the signal strength is actually stronger than it is at room temperature throughout much of the experiment. Transducer strength remains at or above the room temperature strength until 650 ◦ C. The Curie point of bismuth titanate is accepted as being 675 ◦ C. Above 650 ◦ C, signal strength dies off very rapidly as temperature increases. Very low amplitude signals can be observed temporarily up to 685 ◦ C, but these signals disappear within minutes of reaching the temperature. Fig. 6(c)

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Fig. 7. Changes in the time (upper) and frequency domain (lower) for a transducer held at 500 ◦ C for 60 h.

and frequency domains of a signal received at 500 ◦ C recorded when the transducer first reached 500 ◦ C and 60 h later. This is a very interesting observation, and no explanation for this behavior has been determined. When the 7-day experiments ended, the transducer showed no sign of fatigue and temperature was brought back to room temperature.

VI. C ONCLUSION Fig. 6. (a) Plot of measured amplitudes normalized to room temperature signal amplitude at 25 ◦ C intervals. (b) Plot of all waveforms measured at 25 ◦ C intervals. (c) Measured Q3 dB values at 25 ◦ C intervals. (d) Measured center frequency at 25 ◦ C intervals.

shows transducers measured Q3 dB as a function of temperature and Fig. 6(d) shows the measured center frequency as a function of temperature. It is interesting that the transducer center frequency increases with temperature. This is contrary to expectations, since thermal expansion, as well as softening of the elastic modulus as temperature increases, would lead one to believe the center frequency should decrease. However, the coupled-field nature of electromechanical systems is complex and an explanation for this has yet to be discovered. These results confirm that bismuth titanate thick films deposited with the spray-on deposition technique are capable of generating ultrasound at temperatures just below its Curie temperature, at least for short periods of time (typically observed to be 5 min or less). A limited number of experiments have been performed to see if long-term exposure to high temperatures is possible without signal loss. Samples have been held at both 500 ◦ C and 600 ◦ C for durations of 7 days. For these time periods, there was no loss in signal strength, and in fact, there was an increase in signal strength as samples were held at these temperatures. Again, the pulser–receiver was only turned on when a measurement was made (typically once every 12 h). Typically, the increase in signal amplitude occurred within the first 60 h at the soak temperature. The center frequency also increased in this time period. After roughly 60 h, no changes in the signal were observed. Fig. 7 shows a plot of the time

High-temperature ultrasonic transducers have been fabricated using a spray-on deposition technique with microwave sintering. Films were 120 µm thick having center frequencies of approximately 6 MHz. The films’ elastic modulus, density, permittivity, and conductivity were measured. The d33 constant was estimated to be 16 pC/N. The bismuth titanate films fabricated in this study were found to have withstood larger field strengths during poling, suggesting that the estimated d33 value was accurate. Transducers were capable of generating ultrasound at 650 ◦ C for short-time periods and showed that they were capable of operating at 600 ◦ C for 7 days and there is no evidence to suggest that they cannot operate at this temperature for longer time periods. Further experiments need to be performed to validate this. R EFERENCES [1] M. Holt, and C. E. Behrens. (Jan. 12, 2001). “IB88090: Nuclear energy policy. Congressional Research Service Issue Brief for Congress,” Washington, D.C., USA. [2] M. J. Schulz, M. J. Sundaresan, J. McMichael, D. Clayton, R. Sadler, and B. Nagel, “Piezoelectric materials at elevated temperatures,” J. Intell. Mater. Syst. Struct., vol. 14, pp. 693–705, 2003. [3] D. A. Barrow, T. E. Petroff, and M. Sayer, “Thick ceramic coatings using a sol gel phase based ceramic-ceramic 0–3 composite,” Surf. Coat. Technol., vol. 76–77, Part 1, pp. 113–118, 1995. [4] D. A. Barrow, T. E. Petroff, R. P. Tandon, and M. Sayer, “Characterization of thick lead zirconate titanate films fabricated using a new sol gel phase based process,” J. Appl. Phys., vol. 81, no. 2, pp. 876–881, 1997. [5] M. Kobayashi, T. R. Olding, L. Zou, M. Sayer, C.-K. Jen, and A. U. Rehmen, “Piezoelectric thick film ultrasonic transducers fabricated by a spray technique,” in Proc. IEEE Ultrason. Symp., 2000, pp. 985–989.

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Clifford T. Searfass received the B.S. degree (Hons.) in engineering science and mechanics, the M.S. degree in engineering science, and the Ph.D. degree in engineering science and mechanics (ESM), from Pennsylvania State University, State College, PA, USA, in 2004, 2007, and 2012, respectively. Currently, he works with the Division of Research, Development, and Integration, Structural Integrity Associates, Inc., State College, PA, USA. His research interests include solid state physics, with emphasis on piezoelectric materials and their applications in ultrasonic nondestructive evaluation (NDE).

C. Pheil, photograph and biography not available at the time of publication.

K. Sinding, photograph and biography not available at the time of publication.

B. R. Tittmann, (S’59–M’79-SM’84–F’90–LF’06) photograph and biography not available at the time of publication.

A. Baba, photograph and biography not available at the time of publication.

D. K. Agrawal, photograph and biography not available at the time of publication.