Comparison of field-cooling DC poling and AC poling for lead perovskite relaxor-PbTiO3 single crystals grown by a continuous feeding Bridgman process

We investigated the effectiveness of poling processes for Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals (SCs) produced using a continuous feeding Bridgman method, which is known to produce a high property uniformity. The four studied poling processes are: (I) standard direct current poling (STD-DCP); (II) low-voltage field-cooling DCP (LV-FCP); (III) high-voltage field-cooling DCP (HV-FCP); and (IV) mid-temperature alternating current poling (MT-ACP). The highest free dielectric constant (ε 33 T/ε 0) and piezoelectric constant (d 33) were obtained by MT-ACP (ε 33 T/ε 0 = 11 000, d 33 = 3000 pC/N), followed by LV-FCP (ε 33 T/ε 0 = 7500, d 33 = 2400 pC/N), HV-FCP (ε 33 T/ε 0 = 6250, d 33 = 1850 pC/N), and STD-DCP (ε 33 T/ε 0 = 6200, d 33 = 1800 pC/N). The LV-FCP SC showed a 21% and 33% increase in ε 33 T/ε 0 and d 33 compared to that of the STD-DCP SC; however, this was not as much as the 77% and 67% improvement of the MT-ACP SC. These results provide guidance for SC transducers.


Introduction
The piezoelectric effect discovered in 1880 by the Curie brothers refers to the electromechanical coupling that converts mechanical energy into electrical energy and vice versa.][6] Among these applications, ultrasound imaging probes have been taking a center position since early 2000. 7)These relaxer-based SCs have a high free dielectric constant (ε 33 T /ε 0 ) over 5000, a piezoelectric constant (d 33 ) over 1500 pC/N, and a bar-mode electromechanical coupling factor (k 33 ) over 90%, which are significantly higher than those of conventional Pb(Zr,Ti)O 3 (PZT) ceramics, and hence are very attractive for high-end ultrasonic transducer applications.These SCs have been mostly manufactured using the one-charge Bridgman (OC-BM) process since their initial development in the early 2000s.[10] In 2011, JFE Minerals Co. Chiba, Japan, succeeded in growing PMN-PT SCs by a continuous feeding Bridgman (CF-BM) method using ceramic raw material pellets feeding that solved the problems of OC-BM SCs. 11)A piezoelectric PMN-0.3PTSC boule with a size of 80 mm in diameter, 320 mm in length, and 13 kg in weight has been produced in the last decade.These CF-BM SCs show very uniform piezoelectric properties, within 10% within and between wafers in the ingot, as shown in Fig. 1. [11][12][13][14][15][16] Poling procedures for these SCs are essential to obtain high piezoelectric and dielectric properties such as d 33 , k 33 , and ε 33 T /ε 0 . Cnventional direct current poling (DCP) with high-temperature field cooling (FCP) has been used since the first discovery of these SCs by Kuwata et al. in 1982 and Shrout et al. in   1990.17,18) Lin et al. reported that outstanding d 33 (2200 pC/N) can be obtained by FCP from 260 °C to rRT at 6 kV cm −1 , which is superior to DCP at RT (d 33 = 1300 pC/N) for 0.1 mm thick PMN-PT SC. 19) Recently, alternating current poling (ACP), a readily controllable domain engineering method, has attracted many researchers' interest owing to enhanced d 33 and ε 33 T /ε 0 .However, there is no report on systematic comparisons regarding the effectiveness of FCP and ACP processes for PMN-PT CF-BM SCs with high composition and property uniformity.Therefore, the purpose of this study is to compare the electrical properties of DCP, FCP, and ACP conditions for PMN-0.30PTSCs grown by the CF-BM process.14,15) Our initial report only disclosed the temperature dependence of the dielectric constant of DCP SCs and did not mention the microstructure of the SCs, which greatly affects piezoelectric and dielectric properties.43) In this report, we have investigated in detail the electrical properties under four different poling conditions, and their relation to the microstructure is clarified.

Experimental methods
Figure 2(a) shows the [011]-seeded and [001]-oriented CF-BM PMN-0.30PTSC of a large plate (L65 × W13 × T0.48 mm 3 , coercive field (E C ) = 2.2 kV cm −1 at RT), which was sputtered with NiCr/Au (30/200 nm) electrodes on its (001)-surfaces.The large plate SC was depoled at 250 °C for 30 min and was cut into 19 pieces of small plates for this experiment (L3 × W13 mm × T0.48 mm 3 ), as shown in Fig. 2(b), in order to minimize property variations.Figure 3 shows schematic images of the four poling types: (I) standard (STD)-DCP; (II) low-voltage (LV)-FCP; (III) high-voltage (HV)-FCP; and (IV) mid-temperature (MT)-ACP.In the high-temperature FCP process of (II) and (III), the SCs were poled with poling fields of 3 and 6 kV cm −1 at 200 °C for 5 min, and then slowly cooled down to RT under the same poling field at a temperaturecontrolled electric oven for 1000 min.For (IV), the MT-ACP process, a bipolar voltage of sine waves was generated by a multifunctional synthesizer (NF Electronic Instruments, 1930A, Yokohama, Japan) first, and then amplified by a high-voltage amplifier (Trek Inc., 609D-6, CA, USA).The MT-ACP processes were carried out using a bipolar sine wave of 3 kVrms/cm with a frequency of 0.1 Hz for 12 cycles at 60 °C, according to Wan and Sun et al.'s studies. 29,30)The (I) STD-DCP process was conducted at 5 kV cm −1 for 1 min according to the Institute of Electrical and Electronics Engineering standard for comparisons. 44)After aging for 48 h after each poling process, the piezoelectric and dielectric properties of the SC samples were evaluated with an impedance analyzer (Agilent Technology, 4194 A, CA, USA) and a piezo d 33 meter (ZJ-6B, Chinese Academy of Science, Beijing, China).
The temperature dependences of ε 33 T /ε 0 of the small plate SC samples were measured at 1 kHz using an LF impedance Analyzer (Yokogawa-Hewlett Packar.Ltd., 4192A, Tokyo, Japan).e /ε 0 is a free dielectric constant at 1 kHz and 33 S.
e /ε 0 is a clamped dielectric constant at twice the anti-resonance frequency (2f a ).The electromechanical coupling factor thickness mode (k t ) for the plate was calculated from a resonant frequency ( f r ) and anti-resonant frequency ( f a ) of the fundamental k t mode, as shown in Eq. ( 3): Frequency constants of the thickness mode (N t ) and length extensional mode N 31 (Hz•m) were calculated from f r and t or  l, where f r , t, and l are the resonance frequency, sample thickness, and length, respectively.The sound velocity (V ) and acoustic impedance (Z 33 ) were also calculated from the k t mode.
equations for material property calculations can be found in our previous report. 29)t least three sample plates were measured, and average values are used in this work.

Results and discussion
Table I shows the poling conditions and the averaged material constants at 25.0 °C for the PMN-0.3PTSCs under the four different poling processes, (I) STD-DCP, (II) LV-FCP, (III) HV-FCP, and (IV) MT-ACP.
Figure 4 shows (a) ε 33 T /ε 0, (b) ε 33 S /ε 0 , and (c) calculated k 33 of the PMN-0.3PTSCs under the four different poling processes.The highest ε 33 T /ε 0 of 11 000 was obtained by the (IV) MT-ACP process, followed by 7500 with the (II) LV-FCP SC, 6250 with the (III) HV-FCP SC, and 6200 with the (I) STD-DCP SC.The ε 33 T /ε 0 and ε 33 S /ε 0 of the (II) LV-FCP SC showed 21% and −6% increases compared to those of the (I) STD-DCP SC, respectively; however, this was not as much as the 77% and 18% improvement by the (IV) MT-ACP process.
Figure 5 shows the temperature dependence of ε 33 T /ε 0 (a) from RT to 200 °C, (b) from 25 to 115 °C, and (c) these ε 33 T /ε 0 of 50 °C decreased from the phase change temperature (Tpc) for the PMN-0.3PTSCs for the different poling processes.Material constants for ordinary piezoelectric SCs are measured at RT, from about 20 to 25 °C.The material constants in Table I  T /ε 0 of 12 000 and d 33 of 4100 pc N −1 .However, the Tpc of this SC is  04SP37-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd 61 °C, which is problematic due to its low operating temperature range.For this reason, we proposed to compare the ε 33 T /ε 0 of piezoelectric materials with different Tpc at a constant temperature from the Tpc, in this case 50 °C lower, in order to make a fair comparison.By comparing these values, it is possible to objectively compare the intrinsic improvement of SCs poled by various poling methods.Figure 5(a) shows the dielectric loss peak temperature (T c ) at 143 °C and maximum ε 33 T /ε 0 temperature (T m ) at 150 °C, which are consistent with the JFE Mineral Co. report. 36)As shown in Fig. 5 Figure 6 shows the d 33 and g 33 of the PMN-0.3PTSCs for the different poling processes.The ACP SCs showed very   04SP37-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd high d 33 = 3000 pC/N, which is a 67% enhancement compared to those of the DCP SCs.However, the ACP SC showed g 33 = 31 × 10 -3 Vm/N, which is 4% lower than the DCP SC of 33 × 10 -3 Vm N −1 .However, the FCP SC showed g 33 = 36 × 10 -3 Vm N −1 , which was 10% larger than that of the DCP SC.Furthermore, over the poled conditions tested, under ACP conditions at RT, when the ACP SC was subjected to a voltage tenfold greater (35.4 kVrms cm −1 ) than that of the MT-ACP SC, there was a discernible 26% reduction in its performance (d 33 = 2285 pC/N, ε 33 T /ε 0 = 8390) due to an over-poled phenomenon, 41) but this is still higher than the (I) STD-DCP SC.These results suggest that the (II) LV-FCP SC is good for improving the receiving efficiency of transducers compared to ACP and STD-DCP SCs.
Figure 7 shows the (a) coupling factors k t and k 31 and (b) the frequency constants of the thickness mode N t and length mode N 31 of the PMN-0.3PTSCs for the four different poling processes.
The k t for the four poling processes were almost the same, approximately 60%, but the k 31 for FCP SCs were 53%-55%, clearly higher than those for DCP and ACP SCs, which were 45%-47%.The frequency constants N t for the four poling processes were almost equal at about 1900 Hz•m; however, It is also known that, for relaxor-based piezoelectric SCs in the rhombohedral phase, the maximum ε 33 T /ε 0 at the Tpc contributes most to the high ε 33 T /ε 0 and d 33 at RT.The maximum ε 33 T /ε 0 at a 50 °C decrease from the Tpc1 was 12500 for the (IV) MT-ACP SC, followed by the (II)    04SP37-5 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd LV-FCP SC, (7670), the (I) STD-DCP SC (6880), and the (III) HV-FCP SC (6470), respectively.It is important to note that the ε 33 T /ε 0 at a 50 °C decrease from the Tpc1 of the (I) STD-DCP SC (6880) is higher than the (III) HV-FCP SC (6470); even the RT value of the (I) STD-DCP SC (6200) was lower than the (III) HV-FCP SC (6250) due to a large shift of Tpc by the applied DC voltage.
Lin et al. reported that a 0.1 mm thick sample of PMN-PT had a ε 33 T /ε 0 of 3000 and d 33 = 1300 pC/N when FCP was performed at RT. 19) However, when this sample was subjected to FCP at 6 kV cm −1 , a higher ε 33 T /ε 0 of 5600 and d 33 of 2200 pC N −1 were obtained, representing an improvement of 87% and 69%, respectively.However, their experimental results offer limited information, and there are several aspects that warrant further investigation.
It should be noted that the PMN-PT SC samples used in Lin et al. were prepared by the OC-BM method, which has poor uniformity of composition and properties, and there is a large variation in properties among the samples.Furthermore, the exact amount of PT content is not given.
Secondly, the comparison of DCP and FCP was performed in Lin et al. by only using samples with a thickness of 0.1 mm.The piezoelectric and dielectric properties of SCs with a thickness of about 0.1 mm are strongly influenced processing damage to the surface layer.Recently, Takahashi and Namba reported that moderate heat treatment is essential to obtain good properties for PMM-PT SCs with a thickness of 0.1 mm or less because the ε 33 T /ε 0 is strongly affected by processing damage. 45,46)inally, the data of ε 33 T /ε 0 temperature dependence of DCP and FCP SCs samples were not disclosed in Lin et al.Therefore, it is not possible to accurately determine whether the large ε 33 T /ε 0 at RT is due to the decrease in Tpc.Wan et al. reported that the Trt decreased by 27 °C (from 80 °C to 53 °C) when the FCP was from 250 °C to RT by applying 3 kV cm −1 for 0.24PIN-0.46PMN-0.30PTSCs. 47)The result is similar to our PMN-0.3PTSC in these experimental results.
In our experiments, we used four different poling methods on 19 pieces of PMN-30PT SC plates prepared by the CF-BM method, which guarantees a high degree of uniformity in composition and properties.A thickness of about 0.5 mm was also used, which provides stable properties.Furthermore, the ε 33 T /ε 0 temperature dependences were precisely measured, and the ε 33 T /ε 0 was decreased by 50 °C from the Tpc, which eliminates the effect of ε 33 T /ε 0 improvement at RT, which was defined as ε 33 T /ε 0 at a 50 °C decrease from the Tpc, and this value is shown in Fig. 5(c).From this result, the ε 33 T /ε 0 improvement of (II) LV-FCP over (I) STD-DCP was as large as 11%, but was smaller than that of (IV) MT-ACP, which was 82%.The reliability of the effects of our four poling methods on piezoelectric and dielectric properties is considered to be extremely high.
Figure 8 shows scanning electron microscopy (SEM) microstructures of the (I) STD-DCP SC, (II) LV-FCP SC, (III) HV-FCP SC, and (IV) MT-ACP SC used in this experiment.These poled SC samples were broken manually and the fresh surface of the center portion was observed. 22)It is clear from this figure that (I) STD-DCP SCs have a low RT ε 33 T /ε 0 of 6000-7500 and Tpc ε 33 T /ε 0 of 12 000-17 000.The (II) LV-FCP SCs have a microstructure with a mixture of small and random, 79°and 109°domains similar to depolarized DEP SCs. 20,22)The (IV) MT-ACP SCs with high RT ε 33 T /ε 0 of 11 000 and Tpc ε 33 T /ε 0 of 34 000 showed distinct 109°domain layers with a width of several μm parallel to the electrode (perpendicular to the electric field). 20,22,23)][25]33) These fine domain layers parallel to the electrode in PMN-PT SCs are unique to the ACP process, and it is believed that the 109°domain layer of appropriate width contributes to the large  /ε 0 and d 33 of PMN-PT SCs.This fact has been reported by many researchers. 20,31,33)Furthermore, unlike all other SCs, the (III) HV-FCP SCs exhibit a complex and nonuniform domain structure that is nonparallel to the electrode.This is the inhomogeneous domain structure of PMN-PT SCs caused by over-poling, and the ε 33 T /ε 0 is almost the same as that of the STD-DCP SC. 35,41,48) Furthermore, as shown in Fig. 5(b), the STD-DCP SC was the only one that exhibited a rhombohedral to tetragonal phase transition point Trt of 88 °C.On the other hand, the MT-ACP SC showed complicated phase transition from a rhombohedral phase to a tetragonal phase via a monoclinic and/or orthorhombic phase at 87 °C and 101 °C. 49)This complicated phase transition 50) and the appearance of 109°l ayers domain morphology in the ACP process may contribute to the high dielectric and piezoelectric properties of PMN-PT SCs. 20,21,29,33,49)

Fig. 1 .
Fig. 1.Schematic images of (a) OC-BM and (b) CF-BM processes, (c) SC ingot, and (d) piezoelectric constant d 33 distribution within an ingot.Note that the d 33 distribution of the CF-BM SCs is less than 10% within an ingot.

Fig. 2 .
Fig.2.CF-BM PMN-0.3PTSC samples; the dimension of (a) the initial plate was 65 × 13 × 0.48 mm, and (b) the test sample was 3 × 13 × 0.48 mm, respectively.Note that all evaluated small samples were obtained from one large plate to minimize property distributions between the samples.
(a), the rhombohedral to tetragonal phase transition (Trt) of the normal (I) STD-DCP SC was observed at Trt = 88 °C, its maximum ε 33 T /ε 0 was 17 000, and the RT ε 33 T /ε 0 was 6200.The (IV) MT-ACP SC shows two Tpcs, Tpc1 = 87 °C and Tpc2 = 101 °C, suggesting that a new phase (orthorhombic and/or monoclinic) was induced by the electric field.The maximum ε 33 T /ε 0 of Tpc1 was 34 000, about twice that of STD-DCP SC, and the RT ε 33 T /ε 0 was as large as 11 000.Figures 5(b) and 5(c) show the ε 33 T /ε 0 of 50 °C decreased from the Tpc for PMN-0.3PTSCs for the four poling processes.
the N 31 was 570 Hz•m for ACP SCs, 605 Hz•m, 630 Hz•m for LV-FCP and HV-FCP SCs, and 770 Hz•m for DCP SCs, showing a large difference.Since high k 33 and low k 31 are known to be effective in reducing unwanted vibrations (spurious mode vibration) in medical and underwater ultrasonic transducer applications, the ACP SC is considered to be the most useful.In general, the ε 33 T /ε 0 and coupling factor k 33 contribute to the piezoelectric d 33 of piezoelectric materials at RT, as shown in Eq. (7): where d 33 , k 33 , and S 33 E are the piezoelectric constants, electromechanical coupling factor bar mode, and elastic compliances, respectively.

Fig. 7 .
Fig. 7. (a) Coupling factors k t and k 31 and (b) frequency constants of the thickness mode N t and length mode N 31 of the PMN-0.3PTSCs for the different poling processes, (I)STD-DCP, (II) LV-FCP, (II) HV-FCP, and (IV) MT-ACP.

ε 33 T
in this paper were obtained from measurements at different RT, e.g., from 17 °C in winter to 28 °C in summer, all calculated at 25 °C using temperature 33 of the SC material are about 1%/ °C, which is considerably larger than the 0.3%-0.8%/°C of commercially available PZT ceramics.Therefore, high ε 33 T /ε 0 and d 33 can be easily obtained at RT by lowering the Tpc of piezoelectric SC below 65 °C.The PMN-30PT SC with Sm doping reported by Li et al. in 2019 showed an RT ε 33