|NDT.net Dec 2004 Vol. 9 No.12|
Operation of Highly Focussed Immersion Ultrasonic Transducers at Elevated TemperaturesAlex Karpelson - Kinectrics
Mike Trelinski - Ontario Power Generation Nuclear
Corresponding Author Contact:
I. IntroductionOntario Power Generation, Inspection & Maintenance Services Division operates a number of inspection systems designed to inspect CANDU reactor pressure tubes. The inspection systems contain a variety of various sensors but the "heart" of the system is a cluster of ultrasonic probes. The probes are operated in a relatively hostile environment: high radiation, elevated temperature, and primary circuit coolant (heavy water) used as a couplant. A significant experience in operating ultrasonic immersion probes in such conditions has been accumulated over almost 20 years, ever since the inspection systems have been in operation.
Operating temperature is only one of the environmental parameters, which affect ultrasonic probe performance and longevity. This paper will deal with temperature-related effects only. Although radiation effects will not be discussed in this paper, temperature influence on previously irradiated probes has been investigated and will be presented in this paper.
Any ultrasonic (UT) piezoelectric transducer has a range of working temperatures, where its main parameters should not be affected beyond the traditionally accepted tolerances. Typically, this range is determined by the Curie temperature of piezoelement and/or the temperature, at which disbonding between piezoelement and damper or acoustic lens starts. However, the experience shows that some negative effects occur at temperatures significantly lower than expected.
Some effects on probe performance can be observed at the temperature range of 20ºC to 70ºC, which is generally considered as "safe" operating envelope. Some of the effects are permanent; some are reversible (recoverable). In specific circumstances such negative effects may reveal themselves during UT inspection of pressure tubes in CANDU nuclear reactors, which can lead to significant time loss. In order to understand the physical cause of the phenomenon and prevent it in the future, the investigation of temperature effects on the UT piezoelectric probes currently used for pressure tube inspection was performed.
Another interesting phenomenon is the effect of temperature rate change on the probe performance. It has been determined that even at a low temperature range, a rapid temperature variation may damage immersion type probe by opening water ingress paths. Catastrophic probe failures due to this phenomenon have been observed with rapid coolant temperature changes by approximately 40 - 45ºC.
It is also worth noting that various couplant temperature effects may be associated with the testing rig (head) geometry and not the probe itself. Changing couplant temperature affects sound velocity. This in turn may change the incident angles. For highly focused transducers with relatively narrow beams the resulting incident angle changes may lead to significant variations in the sensitivity and coverage particularly by transducers working in pitch-catch mode of operation.
Accordingly, various types of probes manufactured from different piezoelectric materials (Keramos K83 Lead-Methaniobate and 1-3 Piezo-Composite) with different diameters, focal lengths and center frequencies underwent the temperature test to demonstrate that they can withstand temperatures from 200C to 700C without changes or with measurable changes, so that proper correction could be applied. The results presented below in sections 3 and 4 were obtained for probes with center frequency 10MHz, focal length in water 33mm, and aperture diameter 9.5mm.
2. Temperature effects - Testing procedure30 different UT probes used for pressure tubes inspection were selected for testing. Half of them (15 probes) underwent earlier the radiation test: 200 hours in gamma radiation field of 106 R/hour. They were temperature tested in order to simulate real conditions in the reactor where radiation field and high temperature exist simultaneously.
Tests at different temperatures were absolutely identical. They included visual examination of all probes in order to detect any visible physical changes and measurement of main technical parameters: sensitivity, focal length, acoustic beam diameter, depth of field, peak frequency, center frequency, lower frequency, upper frequency, and bandwidth (all frequency parameters were measured at -6dB level). These parameters were measured in accordance with procedure described in  on the calibrated computer-based UT test scanning system including WINSPECT data acquisition software, SONIX STR-81G digitizer card and UTEX UT-340 pulser-receiver.
During the tests, probes were positioned in water tank in front of steel ball-reflector and worked in pulse-echo mode. Water tank was set on hot plate Barnstead-Thermolyne-Cimarec-3. The temperature was varied by heating the water on this hot plate and kept comparatively uniform by means of a magnetic stirrer. To measure the temperature, T-type thermocouple with instrument W Omega Omni-Cal-8A-110 was used. Besides it, thermometer Fisher Scientific 15-043b was put in water close to probe tested.
Measurements were performed at temperatures 22°C, 30°C, 40°C, 50°C, 60°C, and 70°C. All probes were kept in water tank for the whole day at one of these specified temperatures. The following procedure was used for testing at each specified temperature. In the morning after visual inspection to detect any visible physical changes on probe faceplates, the sensitivity, focal length, beam diameter, depth of field, peak frequency, center frequency, lower frequency, upper frequency, and bandwidth of all probes were measured at room temperature 22°C. Definitions of all these parameters, except sensitivity, are given in . The sensitivity is determined in pulse-echo mode as amplitude of the response reflected from steel ball-target 1.5mm diameter positioned at the transducer focal point. Then, after initial inspection, the hot plate was turn on; temperature grew up and after ~ 1 hour reached 300C. After that all probes one by one were put in a special holder positioned in water tank and their parameters were measured.
Frequency characteristics and sensitivity were measured at actual focal spot at a given temperature. At the end of day after finishing all the measurements, hot plate was turn off and temperature in water tank with all probes slowly decreased to 22°C. After that, probes were again visually inspected and their parameters were measured. The next day procedure was repeated at 40°C, the third day at 50°C, then at 60°C, and at last at 70°C.
Using similar procedure, in addition to these measurements, the capacitance of every probe was measured at different temperatures. These measurements were performed to check the described further assumption regarding variation of probe piezoelement permittivity with temperature. For this purpose SENCORE capacitor-inductor analyzer, model LC-53, was employed.
3. Results of probes parameters measurementThe obtained results showed that such probe parameters as beam diameter, depth of field, peak frequency, lower frequency, and upper frequency vary with temperature, however no clear trend was detected between these parameters and temperature. Beam diameter varies within range ± 0.1mm; depth of field changes by ± 4mm and peak, lower and upper frequencies vary within 3MHz range, when temperature changes from 22°C to 70°C. The higher temperature, the greater parameter variation is. However, all these changes in parameters are partially reversible, that is after cooling to 22°C after high temperature the probe parameter partly recovers, i.e. returns to value close to the initial one preserving some "residual" change.
The average results of focal length, center frequency, bandwidth, sensitivity and capacitance measurements for different probes and their average visible physical changes are summarized in Figures 1-5 and Table 1. The average values for each parameter were obtained across the tested probes from a given manufacture. Changes of all parameters are given in comparison with values at initial temperature 22°C. Non-available data for probe parameters mean that UT response was extremely small (about the noise level) and could not be properly measured, i.e. probe failed. Non-available indications for capacitance mean that probe capacitance values could not be measured for this probe. Abbreviations 1 stand for probes made from 1-3 Piezo-Composite, 2 and 3 stand for probes made from Keramos K83 Lead-Methaniobate but using different technological processes (manufacture procedures), NR stand for non-radiated probes, and R stand for radiation-tested probes.
Blisters, mentioned in Table 1, are the raised bubbles caused by local disbonding of the faceplate from the piezoelement. They are clearly seen in Fig. 6.
4. Data analysisThe obtained results for acoustic field (Fig. 1) show that focal length of "average" probe (nominal focal length equals 33mm) at 50°C increases by 1-3mm, but after cooling to room temperature 22°C it recovers. At lower temperatures 30°C and 40°C focal length changes are insignificant. At 60°C focal length increases by 3-5.5mm, but after cooling to room temperature it partially recovers to 2-3mm increase. After test at 60°C some of the irradiated probes failed, i.e. probe output signals became very small, about noise level. At 70°C focal length of the remaining probes increased by 5-7mm, but after cooling to 22°C it partially recovered to 3-5mm increase. The most probable cause of such variations is probe acoustic lens deformation at high temperatures, which leads to its focal length change. There is a small difference between irradiated and non-irradiated probes, which can be attributed to some lens distortion during irradiation.
Beam diameter (nominal ~ 0.8mm) insignificantly changes with temperature. Even at high temperatures 50°C-70°C it increases ~ 0.1mm, probably due to acoustic lens deformation. A small difference between irradiated and non-irradiated probes can be attributed to lens distortion during irradiation. Depth of field (nominal ~ 16mm) slightly increases with temperature. At 50°C-70°C the depth of field becomes ~ 18mm, probably due to acoustic lens deformation at high temperature. A small difference between irradiated and non-irradiated probes can be attributed to lens distortion during irradiation.
At low temperatures probe center frequency (Fig. 2) slightly varies with temperature. Even the largest change, about +0.5MHz (i.e. ~ 5% of initial nominal center frequency 10MHz), at 50°C is insignificant, and recoverable. These variations occur probably due to a change of adhesion between piezoelement and backing material used in these probes. Center frequency at 60°C varies by 1-2.5MHz, but after cooling to room temperature it partially recovers preserving 0.5-2MHz changes. After test at 60°C some of the irradiated probes failed. At 70°C center frequencies of the remaining probes varied by 1.5-3.5MHz, but after cooling to 22°C they partially recovered to 0.5-3MHz changes. The most probable cause of this significant variation of center frequency at higher temperatures is, at first, deterioration of adhesion, and then partial disbonding (particularly in transducers made from 1-3 Piezo-Composite) between probe backing material (damper) and piezoelement used in these probes. Such a delamination leads to a lesser damping of piezoelement. As a result, it oscillates longer and at its own higher resonant frequency. Fig. 7 represents responses in time domain for one of probes #1 at 22°C and 70°C.
It is clearly seen that pulse at 70°C is much longer and has higher frequency. These results and data related to center frequency (Fig. 2) and bandwidth changes (Fig. 3) confirm the assumption regarding disbonding between damper and piezoelement in probes #1 at high temperatures.
The behavior of other frequency characteristics (peak, lower and upper frequencies) is similar to center frequency variation. They increase with temperature and partially recover after cooling.
At low temperatures probe bandwidths (see Fig. 3) slightly vary with temperature. Even the largest change, ~ 15% at 50°C, is insignificant and recoverable. These variations occur probably due to a change of adhesion between piezoelement and backing material. Probe bandwidth at 60°C decreases by ~ 15-40%, but after cooling to room temperature 22°C it partially recovers to ~ 5-20% drop. After test at 60°C some of the irradiated probes failed. At 70°C bandwidths of the remaining probes decreased by ~ 25-60%, and partially recovered to ~ 20-40% drop after cooling to 22°C. The most probable cause of such a significant drop of bandwidth at higher temperatures is, at first, deterioration of adhesion, and then partial disbonding between probe backing material (damper) and piezoelement used in these probes. Such a delamination leads to a lesser damping of piezoelement. As a result, it oscillates longer at its higher resonant frequency and has narrower bandwidth.
As one can see from Fig.4, sensitivity drops 1-2dB at 30°C, but after cooling to 22°C it recovers. At 40°C sensitivity drops 2-3dB, but after cooling to 22°C it recovers. Sensitivity drops 3-9dB at 50°C, but after cooling to 22°C it partially recovers reaching 1-4dB drop. Sensitivity drops 5-13db at 60°C, but after cooling to 22°C it partially recovers to 2-7db drop. After test at 60°C some irradiated probes failed, i.e. their responses became so small that they could be hardly measured. At 70°C sensitivity drops 7-11dB, but after cooling to 22°C it partially recovers to 5-7dB drop. Since sensitivity drop at 30°C and 40°C is 100% recoverable, and at 50°C-70°C it is only partially recoverable, it was assumed that there are two separate mechanisms, which lead to sensitivity drop.
One of them is partial disbonding, which starts at higher temperature about 50°C between piezoelement and acoustic lens. And of course, this process is irreversible (non-recoverable). The higher temperature, the greater this disbonding is and the less probe response and sensitivity are, since acoustic pulses do not pass through such a delamination, which leads to blisters formation (see Fig. 6).
The second mechanism leading to sensitivity drop is change of the permittivity (dielectric constant e ) of piezoelectric material with temperature. This process is the reversible (recoverable) one. Fig. 8 shows how dielectric constant changes with temperature for two most commonly used Lead-Zirconate-Titanate piezo-ceramics PZT -5A and PZT-5H and their modifications .
As one can see, dielectric constant varies substantially ( 50%) within temperature range from +20°C to +70°C. For any probe working in transmit-receive mode, the output voltage Uout in reception mode is approximately equal to 
where Y is the Young's modulus of the piezomaterial, M is the absorption coefficient of UT waves in all media, B is the coefficient depending on the multiple reflections within piezoelement, f is the UT linear frequency (piezoelement resonant frequency), c is the speed of ultrasound in piezomaterial, Zelgen-amp is the electric impedance of the generator/amplifier (typically this impedance is just a pure resistance, i.e. Zelgen-amp ~ Relgen-amp 50 W), S is the area of piezoelement, Uexc is the excitation voltage on probe in the transmission mode, d33 is the piezo-modulus of piezomaterial, T is the total transmission coefficient of the UT wave going through different media, e0=8.85·10-12F/m is the dielectric constant of vacuum, and is the dielectric constant of piezomaterial.
Thus, the greater , the lesser Vout and probe sensitivity are. Note that since Curie temperature (Tc) for these piezo-ceramics is 350°C, piezo-modulus d33 practically does not depend on temperature within range from +20°C to +70°C. Unfortunately, data analogous to ones presented in Fig. 8 are not available for piezoelectric materials Keramos K83 Lead-Methaniobate and 1-3 Piezo-Composite. However, there is no doubt, that similar trend exists in all piezoelectric ceramics (see e.g. [4-6]), since temperature increase makes easier molecules reorientation and, respectively, enhances polarization capacity of piezo-material. On the other hand, according to this explanation, such a process should be the reversible one, because when temperature decreases and piezo-material cools, the process of molecules reorientation becomes more difficult.
As a result of this physical reasoning, one can assert that only one quantity in equation (1) varies with temperature (within the range from 20°C to 70°C) and affects probe response and sensitivity. This quantity is the dielectric constant e of piezo-electric material. To check this assumption, very simple experiment was performed. Since, from electrical point of view, piezoelectric transducer is a pure capacitor, probe capacitance was measured at different temperatures. This capacitance C of the transducer depends on piezoelectric disk dimensions (radius R and thickness d) and dielectric constant of the piezomaterial:
Since thermal expansion coefficient for piezo-ceramics is about 10-6 mm/mm/°C, size variation of piezoelectric disk within temperature range from 20°C to 70°C can achieve maximum 1%. Such an increase in dimensions is negligible regarding its influence on piezoelement capacitance. Thus, the only factor, that may lead to probe capacitance change with temperature, is a permittivity. Fig. 5 shows that probe capacitance significantly varies with temperature: the higher temperature, the greater capacitance is. Moreover, this process is reversible. Obtained plots confirm the idea of permittivity variation with temperature. As one can see from Fig.5, capacitance increases ~ 50-100pF at 30°C, ~ 100-200pF at 40°C, ~ 200-500pF at 50°C, and ~ 400-800pF at 60°C, but after cooling to 22°C it recovers. Note that at 70°C we could not measure capacitance in some probes, while in others it increased ~ 500pF.
In order to check the assumption regarding cause of the reversible variations of essential probe parameters, particularly sensitivity, the correlation curves between these parameters and probe capacitance at various temperatures were generated. These correlations were obtained for reversible parts of parameters, since only these parts are related to piezoelement permittivity change with temperature. Disbondings at probe interfaces are responsible for the irreversible (irrecoverable) variations of probe parameters, however these disbondings do not affect probe capacitance. The results show that there is a weak correlation between capacitance and such functions as focal length; beam diameter; depth of field; bandwidth; center, peak, lower, and upper frequencies. However there is a strong 80-95% correlation between sensitivity and capacitance, as one can see in Fig. 9.
This result was expected, since it is based on a physical reasoning presented above. It confirms the principal correctness of the assumption that the recoverable variation of piezomaterial permittivity with temperature affects mainly probe capacitance and sensitivity. As it has already been mentioned, the irreversible part of probe sensitivity can be attributed to partial disbonding between piezoelement and acoustic lens.
Using data shown in Fig. 5 and assuming that, except permittivity e> , no other parameter affecting piezoelement capacitance C = e e0 p R2 / d, depends on temperature within range from +20°C to +70°C, one can obtain functions e (T) for piezoelectric materials Keramos K83 Lead-Methaniobate and 1-3 Piezo-Composite.
Fig. 10 shows that the absolute values and general trend confirm the assumption that permittivity of different piezo-materials significantly varies with temperature within range from +20°C to +70°C. Using the results presented in Figs. 4 and 10, the decrease of recoverable part of sensitivity vs. temperature was calculated and compared with experimental data (see Fig.11).
Fig. 11 demonstrates that sensitivity of various probes drops a few dB when temperature increases from +20°C to +70°C. Similar results are expected for transducers made from different piezoelectric materials including two most commonly used Lead-Zirconate-Titanate piezo-ceramics PZT -5A and PZT-5H and their modifications.
Close match between theoretical curves and experimental data in Fig. 11 for various probes (particularly for the non-radiated ones) proves correctness of the assumptions regarding drop in the recoverable part of sensitivity due to change of piezoelectric material permittivity with temperature. Moreover, this match corroborates rightness of calculations and accuracy of measurements.
The results presented in section 3 show that measured parameters of different probes have different variations with temperature and different degrees of recovering. The cause of this is differences in the materials and technological processes used for transducers manufacture. For instance, piezoelectric material "1-3 Piezo-Composite", is definitely more temperature sensitive than piezoelectric material "Keramos K83 Lead-Methaniobate".
After the detailed characterization of probes made from 1-3 Piezo-Composite and Keramos K83 Lead-Methaniobate was performed, it was decided to evaluate relation sensitivity vs. temperature for transducers made from most popular Lead-Zirconate-Titanate piezo-ceramics PZT-5A and PZT-5H. At first, using data presented in Fig. 8 and formula from , the curves sensitivity vs. temperature were calculated for these two materials (see solid lines in Fig. 12).
Then two probes (one made from PZT-5A and the other from PZT-5H, both were non-focused and had center frequency 2MHz and aperture diameter 16mm) were tested at different temperatures. Sensitivity in PE mode and capacitance of these transducers were measured according to procedure described in section 2. The obtained results were very similar to ones presented in Figs. 4 and 5. Using these data the recoverable parts of sensitivity vs. temperature were determined for both probes (see experimental points in Fig. 12).
In general, the results of sensitivity calculations and measurements for transducers made from piezo-ceramics PZT-5A and PZT-5H are very similar to data obtained for probes made from 1-3 Piezo-Composite and Keramos K83 Lead-Methaniobate. It confirms once again correctness of the assumptions regarding change in recoverable sensitivity due to change of piezomaterial permittivity with temperature, proves rightness of calculations and endorses accuracy of measurements.
5. Working temperature changes - operational experience
Operational experience and some limited qualitative testing indicates that immersion probes of the type currently used for CANDU reactor inspection may experience temperature change effects. Such effects are mostly associated with probe damage due to water ingress through water paths generated as a result of gaps between probe components. In one observed case a massive sensitivity loss (and other changes to probe parameters) was observed as a result of rapid water temperature variation from estimated 60ºC - 65ºC to ~ 22ºC. Due to differences between thermal expansion coefficients of probe case, acoustic lens and piezoelement, water ingress paths opened up, thus allowing the water to penetrate inside the transducer. It should be noted that the absolute temperature levels were not harmful to the probe and did not leave any significant lasting effects. The above phenomena were confirmed experimentally on similar probes from the same manufacturer.