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Activation of ferroelectric implant ceramics by corona discharge poling
Magnus Rotan, Mikalai Zhuk, Julia Glaum
PII:
S0955-2219(20)30528-8
DOI:
https://doi.org/10.1016/j.jeurceramsoc.2020.06.058
Reference:
JECS 13385
To appear in:
Journal of the European Ceramic Society
Received Date:
31 January 2020
Revised Date:
19 June 2020
Accepted Date:
21 June 2020
Please cite this article as: Rotan M, Zhuk M, Glaum J, Activation of ferroelectric implant
ceramics by corona discharge poling, Journal of the European Ceramic Society (2020),
doi: https://doi.org/10.1016/j.jeurceramsoc.2020.06.058
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© 2020 Published by Elsevier.
Activation of ferroelectric implant ceramics by corona discharge poling
Magnus Rotan, Mikalai Zhuk, Julia Glaum*
Department of Materials Science and Engineering, Norwegian University of Science and Technology,
Trondheim, Norway
Address: K2-102, Sem Sælands vei 12, 7034 Trondheim, Norway
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Phone: +47 73593983
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*corresponding author: Julia Glaum, e-mail address: julia.glaum@ntnu.no
Abstract
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Ferroelectric ceramics show great potential for medical implants to augment bone regeneration and
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improve implant fixation. To induce an overall piezoelectric response prior to implantation, a clean,
contact-less and non-line-of-sight electrical poling method is preferred to allow poling of arbitrarily
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structured scaffolds without the need for electrodes or insulating liquids. A feasibility study was
conducted on the use of corona discharge, which is a contact-less poling method operating in air, to
piezoelectrically activate (Ba,Ca)(Zr,Ti)O3 (BCZT), (K,Na)NbO3 (KNN) and commercial Pb(Zr,Ti)O3 (PZT)
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ceramics. While both PZT and BCZT are readily poled by the corona technique, KNN requires a high
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electric field and an extended poling period to activate its piezoelectric potential. Comparing the
poling characteristics of samples with and without electrodes revealed that the same degree of poling
can be achieved. These results highlight that corona discharge is a feasible method to pole
piezoelectric ceramics intended for biomedical applications.
Keywords: Piezoelectric ceramics, Biomedical application, Corona discharge poling
1.
Introduction
The unique property of ferroelectric materials – their ability to transform a mechanical deformation
into an electric signal and vice versa – is exploited in many applications, ranging from ultrasound
devices to buzzers in mobile phones [1]. As many types of cells are sensitive to electric stimulation,
increasing research efforts are put into the development of piezoelectric scaffolds and implants that
could supply such charges at a certain location in vivo without the need for an external power source.
The development of surface charges can either be achieved through ultrasound stimulation or simply
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through the mechanical load applied by the body itself, e.g. during walking. Such devices might
enhance the lifetime of implants by increasing the adhesion between natural tissue (e.g. neural or
bone cells) and artificial material, thus reducing the recovery time and health care cost following a
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surgical intervention [2,3].
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When considering new materials for in vivo applications, it is essential that they are non-toxic. In the
past decade, due to legislation and restriction on the use of hazardous elements, large effort has been
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put into finding acceptable replacements for the commercially used lead-based piezoelectric materials
[4-8]. Among these, BaTiO3- and (K,Na)NbO3-derivatives appear to be the most promising [2, 8-14].
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Apart from toxicity concerns, the materials must express a macroscopic piezoelectric response to be
able to generate a net surface charge during mechanical straining. After sintering, the ferroelectric
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domains of a polycrystalline ceramic are randomly oriented and need to be aligned by a sufficiently
high electric field for the material to exhibit a macroscopic piezoelectric coefficient. In industrial
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fabrication, this step is conducted in a heated oil bath, where the oil prevents dielectric breakdown of
the surrounding medium [1]. However, for biomedical applications, it is crucial to avoid any foreign
substances that are not biocompatible as this would jeopardize the livability of the implant and would
ultimately be toxic to the body.
Furthermore, complex shapes necessary to meet the design specifications of implants introduce
challenges for poling by the conventional procedure where two oppositely charged electrodes are
needed in order to create an electric field. Exploring alternatives that yield sufficiently high
piezoelectric coefficients while operating in air, preferably without electrodes and being applicable
for non-flat surfaces are therefore of interest.
In this study, the potential of utilizing corona discharge to pole ferroelectric ceramics intended for
biomedical applications was investigated. Corona discharge poling (CDP) is a non-contact method that
allows the alignment of ferroelectric domains by application of a large electric field extending between
a sharp tip electrode and a grounded plate on which the piezoelectric sample is placed. In this process
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a secondary, effective field arises across the sample as ions are produced around the corona electrode
and led onto the sample surface. Two lead-free ferroelectric ceramic systems – (Ba,Ca)(Zr,Ti)O3 and
(K,Na)NbO3 - were compared to a commercial Pb(Zr,Ti)O3 (PZT) ceramic. Polarization (P), strain (S)
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and piezoelectric coefficient (d33) hysteresis loops were determined to distinguish the characteristics
of the three compositions and to establish a basis for comparison with samples poled by the corona
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technique. To investigate the influence of sample surface electrodes on the piezoelectric
and compared.
Experimental
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2.
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characteristics established during CDP, samples with and without deposited electrodes were poled
For the synthesis of Ba0.9Ca0.1Zr0.1Ti0.9O3 (BCZT), powders of BaCO3 (99.98%; Sigma Aldrich), CaCO3
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(99.0%; Sigma Aldrich), TiO2 (99.99%; Sigma Aldrich) and ZrO2 (99.978%, Alfa Aeasar) were dried and
mixed in stoichiometric amounts prior to milling for 24 hours in 96% ethanol using 5mm zirconia balls
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as grinding media. Powders were then dried, sieved (250 µm sized mesh) and further pressed into 25
mm diameter disks applying 40 MPa in a uniaxial press. Disks were calcined in a cylindrical zirconia
crucible with a half-closed lid placed in a chamber furnace at 1300 °C for 2 hours using 350 °C/h and
400 °C/h as heating and cooling rate, respectively, before crushing them and repeating the milling,
drying and sieving procedure. The final samples were achieved by pressing the calcined powder
uniaxially into 10 mm diameter disks at 100 MPa before sintering. Sintering was done in Pt crucibles
covered with an alumina lid leaving a 1-2 mm gap at 1400 °C for 6 hours partially covered in a bed of
sacrificial powder using the same heating and cooling rate as for the calcination.
Raw spray pyrolyzed K0.5Na0.5NbO3 (KNN) powder (CerPoTech, Norway) was milled for 24 hours in
100% ethanol before drying and calcination in an alumina crucible at 650 °C for 5 h using 180 °C/h and
600 °C/h as heating and cooling rate, respectively. Subsequently, the powders followed the same
preparation procedure as the one for BCZT. Pressed pellets were sintered completely covered in a bed
of sacrificial powder in alumina crucibles with an almost closed lid (1-2 mm gap). Sintering was
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executed at 1125 °C for 2 h using 300 °C/h and 600 °C/h as heating and cooling rate, respectively.
Sintered pellets of both BCZT and KNN were ground from both sides down to approx. 1 mm thickness
using P1200 grit SiC paper and cleaned in an ultrasonic bath using ethanol for 10 minutes before they
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were dried at 100 °C for 24 h.
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Commercially available PIC151 PZT (10 mm diameter, 1 mm thickness; PI Ceramics, Germany) were
received both with co-fired silver electrodes and without. Samples without electrodes were polished
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with P1200 grit SiC paper to achieve the same surface roughness as the other two compositions prior
to corona poling.
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For micrograph imaging samples of all three compositions were polished down to 1 µm finish using
diamond spray followed by different etching procedures for each material. BCZT was chemically
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etched in a of solution deionized water and 37% hydrochloric acid (1:1 ratio) for 30 s and subsequently
thermally etched at 1300 °C for 5 min using 600 °C/h as heating and cooling rate. KNN was only
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thermally etched at 1070 °C for 7 min using 600 °C/h as heating and cooling rate. PZT was solely
chemically etched for 15 s in a solution of deionized water and hydrochloric acid (1:19 ratio) with 5
drops (≈ 0.25 ml) of hydrofluoric acid added.
The densities of BCZT and KNN were measured according to the Archimedes principle using
ISO5017:2013 [15].
Micrographs of the grain microstructure of the samples were obtained using a Zeiss Ultra 55 scanning
electron microscope (SEM) (Carl Zeiss AG, Germany).
Grain size measurements were performed following a linear intercept method using the Lince
software (Lince 2.4.2, Ceramics Group, TU Darmstadt, Germany). At least 100 intersections were
measured for the three compositions and expressed as mean value and standard deviation.
Gold electrodes were deposited on the BCZT and KNN samples on each side of the pellet. The
deposition was conducted in 0.1 bar Argon atmosphere for 1 min using Edwards S150B sputter coater
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(Edwards, UK).
An in-house corona discharge single tip electrode setup similar to the one described by Waller et al.
was utilized to pole the samples [16]. The point electrode was a cylindrical copper needle (28 mm in
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length and 5 mm in diameter) with a conical ending of 8 mm length and capped with a hemispherical
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tip with a radius of curvature of 150 µm. A schematic of the corona setup is given in Figure 1. An IKA
C-MAG HP4 hotplate (IKA®-Werke GmbH & Co. KG, Germany) was used to heat the samples and a K-
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type thermocouple (PeakTech Prüf- and Messtechnik, Germany) was used to verify the equilibrium
temperature of the sample surface prior to poling. The poling was conducted in a single step ramp-up
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of the DC voltage using less than 1 s to reach the setpoint for BCZT and PZT, whereas 3 s was used for
KNN. After poling, the specimens were removed from the hotplate and placed on a room temperature
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steel plate for quick dissipation of heat.
The piezoelectric coefficient d33 of the corona poled samples was measured by the direct method on
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a YE2730A d33 -meter (Sinocera Piezotronics Inc., China) applying 0.25 N at 100 Hz. Each sample was
measured on both sides and the average of the absolute value was noted. Samples poled without
electrodes were coated with gold electrodes prior to d33 -measurements.
The polarization and strain hysteresis loops were recorded at 1 Hz using a TF Analyzer 2000 (aixACCT,
Germany). The piezoelectric coefficient d33, determined by the converse effect, was measured using
the TF Analyzer’s small signal capacitance versus voltage (CV) function. The small signal frequency was
1000 Hz, and the vibration amplitude 3 V/mm for PZT and BCZT, 30 V/mm for KNN.
3.
Results
3.1 Microstructure
Scanning electron micrographs of PZT, BCZT and KNN are displayed in Figure 2 with their respective
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characteristics given in Table 1. For PZT, a dense and homogeneous microstructure can be observed.
The BCZT ceramic exhibits the largest grain size of all three compositions and its relative density
underlines the dense microstructure observed in the micrograph. Large grain size and high density are
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quite common for solid state synthesized BCZT [17, 18]. KNN reveals a much smaller average grain size
compared to the other two compositions, however, some abnormal grain growth is observed as well,
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which is commonly reported for this system [19, 20].
3.2 Ferroelectric hysteresis loops
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Polarization and strain as a function of electric field alongside their respective d33 loops are given in
Figure 3 for all compositions. Measurements were performed at selected temperatures below the
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individual Curie temperatures (Table 2). All compositions exhibit well-developed hysteresis loops
although with different appearances. Selected characteristics are extracted and plotted in Figure 4.
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The commercial PZT ceramics exhibit the highest remanent and maximum polarization, the highest
maximum and remanent strain as well as the highest piezoelectric response at room temperature
(Figure 3 a)-c)). The main characteristics of these hysteresis loops are stable up to at least 80 °C,
whereas at 200 °C a considerable reduction in the coercive field, maximum and remanent polarization
and negative strain is seen. In contrast to that, the remanent piezoelectric coefficient is persistent at
this temperature (Figure 3 c)).
Compared to PZT, BCZT exhibits a rather small coercive field that is stable in the measured
temperature range (Figure 3 d)-f)). The remanent polarization is about a third of that of PZT at room
temperature and continuously decreases with increasing temperature. The remanent piezoelectric
coefficient demonstrates an increase at 50 °C followed by a slight decrease at 70 °C. This observation
can be explained by the presence of a phase transition around 55 °C giving rise to more rotational
freedom of the electric dipoles [22-24].
KNN required the highest electric field to reach polarization saturation and in addition exhibits the
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lowest strain and the lowest piezoelectric coefficient of the three compositions (Figure 3 g)-i)). The
coercive field decreases only slightly within the measured temperature range, whereas the maximum
and remanent polarization increase.
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The characteristic shape of the polarization loop becomes more rounded as the temperature
increases, which can be rationalized by an increase in leakage current. These leakage currents might
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contribute to the development of the asymmetric appearance of the strain hysteresis loops with
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increasing temperature through charge accumulation. The remanent piezoelectric coefficient of KNN
increases slightly with temperature and reaches a maximum around 200 °C, which can be associated
3.3
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with the known phase transition from orthorhombic to tetragonal crystal structure [7].
Corona discharge poling
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The impact of corona discharge poling on the achieved piezoelectric coefficient depends on several
parameters: voltage, corona electrode distance, radius of the electrode tip, sample thickness,
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temperature and time under the corona [25, 26]. To evaluate the influence of the different variables
and to optimize the poling procedure for the individual material systems investigated, a wide
parameter space was investigated (Table 2). It is well known that poling is facilitated at higher
temperatures due to increased domain wall mobility. A set of temperatures in the range up to the
Curie temperature was therefore selected for the study [27-30].
For BCZT a poling temperature of 80 °C was deemed too close to TC as large scattering was observed
in the obtained d33 values and hence 70 °C was chosen as the highest temperature. KNN, having a high
coercive field and a relatively high TC, proved hard to pole leaving a limited parametric exploration
space. 330 °C was chosen as a limit for the highest temperature. Additionally, a high voltage in
combination with a short corona electrode distance, required the voltage to be ramped slower
compared to BCZT and PZT (in 3 s up to the maximum voltage as opposed to < 1 s for the other
compositions) to avoid instant sparking from the corona electrode.
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Corona discharge poling was performed on pristine PZT (co-fired silver electrodes), BCZT and KNN
(both with sputtered gold electrodes) and the measured d33 (determined by the direct method) as a
function of the applied voltage during poling is shown in Figure 5. The average value taken from three
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separate specimens is plotted with standard deviation.
Concerning PZT, d33 increases with increasing poling voltage for all temperatures (Fig. 5 a)), periods
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(Fig. 5 b)), and corona electrode distances (Fig. 5 c)) before leveling at a maximum of 610 pC/N. For
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samples poled at 200 °C (Fig. 5 a)), the maximum d33 is less than for those poled at 80 °C and 22 °C.
As expected, the piezoelectric coefficient increases with increasing time under the corona (Fig. 5 b))
and with decreasing distance between specimen and corona electrode (Fig. 5 c)). For the highest
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voltages, a high surface charge sometimes resulted in discharges in the form of sparks. However, the
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samples showed no physical sign of a dielectric breakdown post poling.
For BCZT, a small reduction in d33 was observed in the first few seconds after inserting each sample
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into the d33- meter. It has been reported that the piezoelectric performance of BCZT is quite
susceptible to mechanical loading and already low forces can lead to some reduction of the
piezoelectric coefficient [33]. For consistency, readings were, therefore, taken 10 s after insertion. The
results are plotted in Figure 5 d) and e). BCZT reaches saturated d33 values already after 1 min at 20
kV both for low and high temperatures. An increase in standard deviation is observed at 15 and 20 kV
demonstrating little difference between samples poled at room temperature and at 70 °C. No
significant difference is seen for 1-, 3- and 5- minutes poling at 70°C and 15 kV or above (Fig. 5 e)).
4.
Discussion
4.1 Corona discharge poling vs. traditional poling
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As the switching of ferroelectric domains is a stochastic process with the probability for a distinct
switching event increasing with time and electric field applied [34-38], poling in general is expected to
be more efficient for longer poling times and higher fields applied. While the piezoelectric coefficient
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of all compositions follows these trends (Figure 5), there are distinct differences due to the variation
in the poling dynamics related to their inherent properties. BCZT, having the smallest coercive field of
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the three, requires the shortest time and the lowest field to reach polarization saturation. KNN on the
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other hand requires high field, short electrode distance and long time to pole due to its higher
resistance towards ferroelectric switching as expected by the high coercive field.
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While the general dependencies between the poling parameters and the developed piezoelectric
coefficients are similar for samples poled via corona discharge and via conventional poling,
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achievement of the saturated poling state happens on different time scales. For the two techniques,
a comparison of the time dependency of the polarization development is difficult because longer time
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is required to build up the effective electric field acting on the samples in the CDP method. For all the
samples investigated, the hysteresis loops obtained in the conventional way show that saturated
polarization was achieved during a single loop measurement, which means that ferroelectric switching
occurred within ¼ of a second at their respective maximum electric fields (Fig. 3). However, the
effective field acting on the samples during CDP is more difficult to predict. For PZT, fully polarized
samples were obtained after 60 s for sufficiently high voltages indicated by the plateau reached in the
measured d33 values (Fig. 5). An estimate of the electric field that is experienced by the samples and
its development over time during corona poling can be made by comparing the present data to those
of Zhukov et al. who investigated the development of polarization for PZT as a function of time and
applied field in a conventional electrode-wired configuration [34]. The d33 values of PZT poled by
corona discharge at different voltages, 80 °C and 5 cm distance (Fig. 5 b)) are normalized (d33/d33,max,
with d33,max being the highest values for each composition in Fig. 5 b)) and plotted together with the
normalized data of Zhukov (d33/d33-max) in Figure 6. It should be noted that from Figure 3, it was
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observed that the coercive field at 80 °C is essentially the same as at room temperature where the
data of Zhukov were recorded. Samples that were poled under the corona at 6 kV do not show any
piezoelectric response even after 5 min and it can be argued that the accumulated surface charge was
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not sufficient to produce a field in the range of the composition’s coercive field (0.93 kV/mm). Using
the same argument, at 10 kV, the charge accumulates to a surface potential creating a field between
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0.7 and 0.8 kV/mm in the time frame of 1-5 minutes demonstrating a rather slow build up at this
corona potential. After 3 min at 20 kV the effective field has clearly exceeded the coercive field and
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saturation in the polarization is reached. In this range, however, the exact surface potential is hard to
estimate as all potentials above the coercive potential lead to saturation of d33 after 1 minute.
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The polarization loops in Figure 3 a) and d) suggest that leakage currents during poling are low for
both PZT and BCZT. It is therefore reasonable to assume that charges are accumulated in a similar way
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and, hence, similar surface potentials are obtained during the corona charging of the two
compositions. Because the coercive field for BCZT is much lower than for PZT, the saturation
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polarization is reached in an even shorter time as seen in Figure 5 e). However, considering the low
coercive field, one would expect that the corona would produce saturated polarization and a leveling
in the d33 at even lower voltages than what is observed. For example, as in the argument above, 1
minute at 10 kV resulted in an electric field of at least 0.7 kV/mm over the sample. The coercive field
of BCZT at this temperature is approximately 0.13 kV/mm (Fig. 4 b)), meaning that the poling field is
more than five times the coercive field. This should be more than sufficient to pole the BCZT ceramics
to saturation. It has, however, been recognized that the poling mechanism in BCZT is somewhat
different from PZT and that the temporal development of domain realignment is of a slower character
[37, 38]. Higher fields are therefore necessary to achieve saturated polarization in short times for
BCZT.
In the case of KNN it was expected that poling would be more difficult compared to PZT and BCZT due
to its higher coercive field. In addition to that, the polarization loops in Figure 3 give evidence of a
significant leakage current during electric field application, indicating that the surface does not
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accumulate sufficient charge during corona poling in order to create a field strong enough to fully
polarize the samples within 30 minutes. This is supported by the observation that even at the highest
voltage applied, no dielectric breakdown of the air around the sample was observed. The dielectric
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strength of air is reported to be around 3 kV/mm in atmospheric conditions [35] and for PZT and BCZT,
prolonged corona charging resulted in timely sequenced sparks going between the sample surface
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and the ground plate. For poling of KNN this was not observed, neither at high temperatures nor for
preliminary low- and intermediate- temperature experiments. It should be noted that other factors
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such as air density, humidity and shape of electrode also influence the breakdown potential of air, but
these factors were consistent for all experiments conducted.
Electroded vs. non-electroded samples
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4.2
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For the purpose of biomedical implantation, implants should contain as few foreign substances as
possible to prevent unwanted side effects. Omitting the steps of electrode deposition from the
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manufacturing process would reduce this risk and also the cost of production. For these reasons, the
possibility to pole piezoelectric implants without the need for electrodes is of high interest. The
present results show that PZT, BCZT and KNN all display approximately the same piezoelectric
performance whether the samples were poled with or without electrodes using the corona discharge
method (Table 3).
While in the conventional procedure the electric charges are distributed through the highly conductive
electrodes giving rise to a homogeneous electric field throughout the sample disks, for non-electroded
and highly insulating ceramics, charges deposited on the surface have low mobility, which hinders the
formation of a uniform surface potential. The corona process must therefore make sure that charges
are deposited in a uniform manner to ensure proper poling of the whole specimen. Extensive research
has been carried out on the corona discharge method in order to describe the electric field
surrounding the corona tip, the charge transport, current density profiles, surface potentials and
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discharge currents for different setups [26, 40-43]. In the early work of Warburg, it is described that
the current density in both positive and negative point-to-plane coronas is distributed over the plane
according to what is now known as the Warburg distribution [40]. Figure 7 presents the Warburg
distribution as calculated for different applied voltages and 5 cm corona electrode distance. It is
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obvious that for using sample disks with a radius of 0.5 cm as used in the present study, a uniform
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distribution of charges across the surface is established, which is in line with our observation that
electroded and non-electroded samples exhibit the same piezoelectric performance after poling. For
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practical applications, however, where larger samples or non-flat surfaces need to be poled, care must
Conclusion
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5.
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be taken to assure a uniform surface potential.
Two piezoelectric material systems, BCZT and KNN, that have received much attention for their
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potential as active implant materials, were compared to commercial PZT. While both PZT and BCZT
are readily poled by the corona technique, KNN was found to be more reluctant and thus a high electric
field and an extended period of time had to be used to achieve a decent piezoelectric response. This
difference in poling behavior is attributed to the high coercive field as well as the significant
conductivity of the KNN samples studied, which hindered the proper development of an electric field
across the samples. Comparing the piezoelectric response between electroded and non-electroded
samples poled by the corona technique, led to the conclusion that under the experimental conditions
used, electrodes are not needed to achieve optimal piezoelectric performance. This work shows that
the corona technique can be successfully employed to pole lead-free piezoelectric ceramics without
the use of electrodes or insulating liquids, which makes it a very promising tool in the context of
biomedical applications.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
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that could have appeared to influence the work reported in this paper.
Acknowledgements
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This work was funded by The Research Council of Norway under the FRINATEK project, with grant no.
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250098. Dr. James Roscow from the Department of Mechanical Engineering at University of Bath, UK
is kindly acknowledged for his input on the corona discharge setup. Birgitte Sofie Karlsen from Sintef
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Industry, Norway is acknowledged for chemical etching of PZT for micrograph imaging. Y.A. Genenko,
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Figure 1. Schematic diagram of the corona discharge poling setup. For the present study a negative
lP
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-p
ro
of
polarity was set to the tip electrode.
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Figure 2. Scanning Electron Micrographs of the polished and etched surfaces of PIC151 PZT,
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ur
Ba0.9Ca0.1Zr0.1Ti0.9O3 and Ka0.5Na0.5NbO3.
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of
-p
Figure 3. Polarization, strain and d33 hysteresis loops of PZT (a), (b) and (c), BCZT (d), (e) and (f) and
Jo
ur
na
lP
re
KNN (g), (h) and (i).
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of
-p
re
lP
Figure 4. Remanent polarization (a), coercive electric field (b), and small remanent signal d33 (c) as a
Jo
ur
na
function of temperature extracted from the hysteresis loops in Fig. 3.
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of
-p
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Figure 5. d33 of PZT (a), (b) and (c), BCZT (d) and (e) and KNN (f) after corona discharge poling at
Jo
ur
na
lP
different voltages, temperatures, times and corona electrode distances.
Figure 6. Normalized d33 values of corona discharge poled PZT samples compared to the data of Zhukov
et al. [34]. The measured data of Zhukov is reproduced with lines and labeled with arbitrarily set
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of
markers to guide the reader.
-p
Figure 7. Current density distribution according to the Warburg law calculated for 5 cm corona
Jo
ur
na
lP
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electrode distance [34, 36].
Table 1. Average grain size and density for PZT, BCZT and KNN ceramics. Densities are given in absolute
values as well as relative density calculated from extracted lattice parameters of their respective
powder X-ray diffraction patterns (not shown). Densities are given as an average of 5 samples.
Composition
Grain size (m)
Density/ Relative density (gcm-3 / %)
PZT
7±2
7.80 [21] / N.A.
BCZT
22 ± 6
5.59 ± 0.05 / 95.5
KNN
3.6 ± 0.6
4.15 ± 0.04 / 92.1
Temperature Voltage
(°C)
(kV)
6, 10, 15, 20,
22, 80, 200
25, 30
6, 10, 15, 20,
22, 70
25
1, 3, 5
BCZT
1, 3, 5
KNN
1, 3, 5, 10,
330
15, 30
30
4, 5, 7
5
250 [1]
90 [31, 32]
2.3
420 [7]
re
PZT
Corona electrode distance TC
(cm)
(°C)
-p
Sample Time
(min)
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of
Table 2. Parameter space investigated for PZT, BCZT and KNN on corona discharge
Sample Time Temperature
(min) (°C)
lP
Table 3. Piezoelectric coefficient d33 of PZT, BCZT and KNN poled by corona discharge. Stated values
PZT
3
70
20
5
595 ± 11
587 ± 3
BCZT
3
70
20
5
440 ± 22
421 ± 20
30
330
30
2.3
53 ± 17
43 ± 28
are an average of 15 samples except for PZT and KNN without electrodes, which are an average of 5.
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ur
Jo
KNN
Voltage Corona
(kV)
distance
(cm)
electrode d33
with d33
without
electrodes
electrodes
(pC/N)
(pC/N)