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Electrical conduction mechanisms in piezoelectric ceramics under harsh operating conditions
Sensors and Actuators A 167 (2011) 19–24
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Electrical conduction mechanisms in piezoelectric ceramics under harsh operating conditions Deyi Zheng a,∗ , Jonathan Swingler a , Paul M. Weaver b a b
School of Engineering Sciences, University of Southampton, University Road, Southampton SO17 1BJ, United Kingdom National Physical Laboratory, Hampton Road, Teddington Middle TW11 0LW, United Kingdom
a r t i c l e
i n f o
Article history: Received 3 July 2010 Received in revised form 6 October 2010 Accepted 31 October 2010 Available online 1 December 2010 Keywords: Piezoelectric ceramics Electrical conduction Mechanisms Harsh operating conditions Electrical breakdown
a b s t r a c t Piezoelectric ceramics are widely used in industry for sensor and actuator applications. Under an applied electric field over a long duration, the conductivity of the ceramic increases resulting in leakage currents and increased power consumption. This process is accelerated by harsh environments and operating conditions such as high temperature, high humidity and high electric field. In this study, a piezoelectric ceramic was stressed by exposure to high relative humidity, at an elevated temperature, with a continuously applied d.c. electric bias field. Periodically during the test, the bias field was removed and a low frequency a.c. electrical cycle applied, during which current and voltage were measured. Results show that the conductivity is both time and voltage dependent and indicative of a complex breakdown process in the ceramic. The breakdown field strength was observed to vary only slightly during the exposure to humidity and d.c. bias despite large increases in conductivity. The reestablishment of the conduction process on re-application of the d.c. bias field was also studied. Whilst these results are specific to the piezoelectric material system studied, the mechanisms and insights into the interaction of electrodes with a ceramic material under conditions of high humidity and high electric field are likely to have much wider relevance for electro-ceramics in general, including technologically important materials such as dielectrics for energy storage and electronics, electronic oxides, and ferroelectric memory devices. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Piezoelectric ceramics are used as electro-mechanical transducers in a wide range of applications both as actuators and as sensors [1]. Particularly for d.c. or low frequency applications, these materials are attractive for their low power consumption, making possible novel applications for remote or inaccessible locations, autonomous operation using energy harvesting [2], or long-lived miniaturised battery powered devices [3]. As these materials become more widely used, the demands on their performance increase, particularly with regard to lifetime and reliability in humid environments and at high electric field across a wide temperature range. Whilst there are applications in areas such as aerospace or power generation where temperatures of several hundred ◦ C are encountered, many commercially significant applications operate at temperatures in the range −40 ◦ C to +85 ◦ C. These lower temperature applications are likely to encounter conditions of prolonged exposure to high humidity, particularly for outdoor applications. This paper is therefore concerned with the effects of high humidity at temperatures close to ambient.
∗ Corresponding author. E-mail address: [email protected] (D. Zheng). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.10.022
The low power characteristic of a piezoelectric actuator depends on good insulating quality of the ceramic. Leakage currents can be a cause of significant power loss, and can ultimately lead to failure of the device. Ferroelectric materials are intrinsic wide band gap semi-conductors [4] resulting in thermally activated conductivity which is likely to be insignificant at room temperature [5]. PZT (Lead Zirconate Titanate) ceramics have been shown to exhibit p-type conductivity thought to be due to an excess of cation vacancies [6], but most PZT is doped with higher valence cations which reduces the conductivity to very low levels at temperatures of interest here. Conductivity also develops over a period of time at high temperature due to vacancy and defect migration [7]. If the d.c. electric field is removed, the piezoelectric materials can recover their dielectric strength as oxygen vacancies migrate from the cathode interface back to the bulk material [8]. However, these conduction mechanisms do not explain the changes in electrical resistance in humid atmospheres where rapid changes in conductivity occur at much more moderate temperatures (around room temperature) in a variety of materials including PZT monolithic [5,9] and multilayer [10,11] actuators, Barium Titanate capacitors [12] with Nickel [5,9] and Silver [10–12] based electrodes. These effects do not occur in the absence of humidity, so it is likely that mechanisms other than those discussed above are responsible.
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Fig. 1. Experimental arrangement.
A more plausible explanation of the conduction mechanism under d.c. bias at high humidity is that [5,9,11] the electrode material ionizes in the presence of water vapour to produce aqueous metallic ions at the anode. These ions migrate through pores and grain boundaries to the cathode where they deposit as metal. The metal filaments grow back towards the anode. When the metal filament has grown to within a critical distance of the anode, the electrical field reaches a level sufficient to cause breakdown, accompanied by an increase in the level of leakage current [9] and current transients [5]. However, no metal filaments have been observed. The reason for this can be understood by the following considerations. If it is assumed that the conduction of a piece of ceramic with electroded area A occurs through conductive elements with an effective total area AC embedded in an insulating material, then the resistance of the ceramic is given by R=
t V = AC i
where t is the thickness of the ceramic (assumed equal to the length of the conducting channels), is the resistivity of the conducting channel material, V is the voltage between the electrodes and i is the total current passing between the electrodes. Multiplying by the total electroded area A gives: V/t = AC /A i/A V/t is the electric field, E, and i/A is the current density, j, so AC j = A E AC /A is the fractional area of the conductive channel. After exposure to 90% RH at 55 ◦ C with an electric field of 1.6 kV mm−1 currents densities of order 1 A m−2 can be reached in timescales of order 10 h [9]. If the conductive channel was formed from Ni with a conductivity of 7 × 10−8 m, then the fractional area of the conductive channels would be 4 × 10−14 . For a ceramic sample of the area considered here (25 mm × 5 mm) this would correspond to a cylinder of diameter 2.6 nm. In fact this area would be divided over many conductive pathways distributed over the area of the ceramic, so each one would be much too small for easy observation and nearly impossible to locate. Dendrites have been observed on exposed surfaces in systems using silver based electrodes where quite high levels of electro-migration may occur [10] but physical evidence of the conducting pathways in the body of the material may be very difficult to obtain. Whilst we may not always be able to observe the conducting pathways directly, we can probe the electrical charac-
teristics of the conductive pathways, which may provide insight into their properties and characteristics. There is also evidence that the applied electric field has a significant effect on the leakage current, with much smaller conductivity at electric fields below approximately 0.5 kV mm−1 [9]. This paper therefore resents a study of the electrical characteristics of the conductive pathways in a piezoelectric ceramic obtained from measurement of the dependency of conductivity on applied electric field during exposure to humidity and d.c. bias field. 2. Experimental setup The ceramic used for these experiments was a commercial soft grade piezoelectric ceramic prepared in sheets 25 mm × 5 mm × 0.15 mm with Nickel electrodes (2 m–4 m thick) on the large faces. The coercive field for this material is approximately 500 V/mm and the dielectric constants ∈ T11 / ∈ 0 is 4400 and ∈ T33 / ∈ 0 is 5500. The experimental arrangement is as shown in Fig. 1. The sample was placed into a humidity chamber with a temperature of 55 ◦ C and a humidity of 90% relative humidity. A d.c. power supply under computer control was used to apply voltage to the ceramic sample. The polarity of the voltage could be changed by the relay indicated in Fig. 1. In this circuit, R1 and RV form a voltage divider with a ratio of 50:1 and RI is a current shunt. The voltage across the sample and the current through it were determined by measuring the voltages across RV and RI using an analogue-to-digital converter (ADC). The R1 , RV and RI are selected to produce a voltage within the operating range of the analogue-to-digital converter, which is ±5V. For most of the experiment a constant d.c. electric field (240 V, 1.6 kV mm−1 ) was applied between the electrodes, recreating the stress conditions used in previous work [5] (Stage 2 in Fig. 2). During this period the leakage current was measured every 10 s. Periodically, the d.c. field was interrupted and a single low frequency (0.04 Hz) cycle of alternating electric field (1.6 kV mm−1 ) was applied (Stage 1 in Fig. 2). The purpose of this was to measure the voltage dependent conductivity (I–V loop) of the sample at intervals during the test. The alternating cycle was preceded by a 30 s pulse of reverse bias field of 1.6 kV mm−1 (Stage 3 in Fig. 2). This was to ensure that at the start of the first half cycle of the voltage loop measurement (Stage 1) the direction of the applied electric field was reversed with respect to the direction of the ferroelectric polarisation, thus ensuring a similar polarisation state at the start of each of the two half cycles. During Stage 1 (Fig. 2) the voltage was changed in a series of 6 V steps. For a continuously varying voltage, the current would
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Fig. 4. Leakage current measurement results for 0 h to 12 h.
3. Results
Fig. 2. Illustration of the three steps of the voltage – leakage current characterisation experiment (a) three stages voltage waveform and (b) close up of Stage 1 triangular waveform.
include contributions from the charging of the sample, the changes in the ferroelectric polarization as well as the d.c. leakage current. To ensure that the charging of the sample and ferroelectric contribution were minimised, and therefore only the leakage current was measured, the voltage was applied as a series of 6 V steps up to a peak positive amplitude of 240 V, then a peak negative value of −240 V then back to zero. Measurement of the leakage current was made at a defined time after the step in voltage. To decide how long this time lag should be, the following experiment was conducted. Fig. 3 shows that when 240 V was applied suddenly across a new sample the charging process caused a current transient with a peak current of approximately 16 A. The current then decreased to approximately 2 A in about 300 ms. The leakage current levels of interest in this paper are in the region of 10–300 A. A 10 s time duration between the voltage step and the current measurement was therefore chosen to ensure that any currents due to switching transients were well below the leakage current levels of interest.
Fig. 3. Charging curve of PZT ceramic at 240 V d.c.
Results of the continuous current measurement during the d.c. stress phase of the experiment (Stage 2 in Fig. 2) from a 12 h experiment are shown in Fig. 4. Under the applied voltage of 240 V the leakage current increased over a period of several hours. These results are comparable to those previously reported [5,9] for similar conditions. The spikes in the current measurement identify the points where the loop measurement was taken. These were caused by the charging current transient which occurred when the d.c. bias was re-applied after the loop (the 0.44 h period during which the I–V loop was recorded is not shown in Fig. 4). Selected loop results taken over a period of 18 h are shown in Fig. 5. The amplitude of the current and the area of the I–V loop increases significantly with time. There is also a pronounced asymmetry between the positive and negative half cycles. 4. Discussion A typical I–V loop results measured after 18 h of d.c. bias is shown in Fig. 6. Results in Fig. 5 were all plotted on the same scale to illustrate the evolution over time of the I–V loop. When results were normalised to the peak current value, then all the loops showed a profile very similar to that shown in Fig. 6. The exceptions to this were the loops at 0 h and 4 h where the current level was so low that any structure was indiscernible from the background noise. These very low leakage current loops were therefore excluded from the following analysis. For the loop shown in Fig. 6, it is seen that gradually increasing the electric field from 0 V to approximately 170 V caused very little leakage current – comparable to that for a new sample. Above 170 V, the current rapidly increased with voltage, following an approximately linear trend. This is indicative of a breakdown occurring at a critical value of the applied voltage. The value of this breakdown voltage can be estimated by measuring the intercept with the V axis of a straight line fitted to the approximately linear portion of the data (marked as a in Fig. 6). As the voltage decreased from the maximum of 240 V, the current remained approximately constant. In some cases it even increased slightly (e.g. Fig. 5d) despite the decreasing voltage. There then followed another approximately linear portion of the curve, with the current decreasing to a low level at approximately 50 V. This is indicative of a minimum voltage required to maintain the conduction process (extinction voltage), and can be estimated from the intercept with the V axis of a straight line fitted to the data marked as region b in Fig. 6. A similar sequence is followed for the negative half cycle with values of the breakdown and extinction voltages similar to those of the positive half cycle. This is verified in Fig. 7 which com-
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Fig. 5. Voltage loop measurement results recorded at the start of the test (a) and then after 4 h (b), 6 h (c), 8 h (d), 10 h (e) and 18 h (f) of time under the applied d.c. field.
pares the intercepts of the lines fitted to regions a, b, c, d of Fig. 6. Fig. 7 also shows that whilst there is a slight decrease in the critical voltages for breakdown they vary little over the whole test duration, at approximately 180 V (1.2 kV mm−1 ) for both positive and negative half cycles (the negative half cycle has a slightly lower breakdown voltage). This is strong evidence to support the model mechanism postulated in previous work [9] where metal filaments develop to within a critical distance from the anode where break-
Fig. 6. Current–voltage loop measurement result at t = 18 h (Fig. 5f). The arrows indicate the path through the full voltage cycle from 0 V increasing to peak positive voltage, decreasing back to 0 V, then to peak negative voltage before returning to 0 V. Red markers indicate the points used for straight line fits to the data in regions marked a, b, c, d. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
down occurs. In this model the gradual increase in the conductivity is caused mainly by an increasing number of conductive channels adding to the overall conductivity. The critical distance for breakdown and therefore the breakdown voltage would not be expected to vary significantly over the test duration. The slight decrease in breakdown voltage could be caused by reduction in the breakdown strength of the ceramic itself, possibly due to the effects of moisture on surfaces such as pore surfaces, cracks and grain boundaries. These results indicate that these effects are only minor as far as the breakdown process is concerned, although they are likely to play a significant role in the ionic migration and development of metallic filaments proposed in the model.
Fig. 7. Critical voltages for breakdown (a, c) and extinction (b, d) obtained from the intercept of least squares fit to the regions marked a, b, c, d in Fig. 6. The graph shows the amplitude of the voltage (the intercepts for the negative half cycles have negative values).
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Fig. 8. (a) Current (solid line) and voltage (dashed line) loop results (Stage 1 in Fig. 2) as functions of time; (b) conductance for the first half cycle (solid line) and for the period after re-application of the 240 V d.c. bias field (dashed line – Stage 2 in Fig. 2). Measurements were taken after 12 h exposure to d.c. bias.
As noted above, a critical voltage was also observed for extinguishing of the conduction process. With an average of 36 V (0.24 kV mm−1 ), this voltage was much smaller than the breakdown voltage. This could indicate a mechanism where the current helps maintain conduction even when the voltage is lower than that required to initiate conduction, or a time dependency in the breakdown voltage. It is also seen in Fig. 6 that when the applied voltage is past its peak value and is decreasing, the leakage current remains, for a time, approximately constant. This means that the conductance of the sample is increasing whilst the applied voltage is decreasing. In Fig. 6 the rate of decrease of voltage approximately balances the rate of increase of conductivity, for a time, during which the current stays approximately constant. This is probably fortuitous, and in some cases the current can increase despite the reducing electric field. This indicates a dependence of the conductance on time as well as voltage. To investigate the time dependency of the current in more detail, the current and voltage signals for a typical loop (Stage 1 in Fig. 2) are shown as functions of time in Fig. 8a. The conductance (the current divided by the voltage) of the first half cycle is plotted in Fig. 8b. The current measured on re-application of the d.c. bias field (Stage 2 in Fig. 2) is also plotted in Fig. 8b (the time axis origin was offset so that results could be compared directly with the Stage 1 loop results). Fig. 8b shows that the increase in conductivity after initiation of the conductive phase (a in Fig. 6) occurred at the same rate as the increase in conductivity after re-application of the 240 V d.c. bias field. The rates of rise of conductance stayed the same up to the point of maximum voltage in the a.c. cycle. As the a.c. cycle voltage decreased, the conductance remained constant or increased slightly. Where the d.c. bias was maintained, the conductance continued to increase at a gradually reducing rate. These results clearly demonstrate a dependency of the conductance on time for timescales over which the loops were recorded. The conductance continues to increase when the voltage is reversed resulting in the observed asymmetry in the loops. A similar time dependency of the conductance is also observed when the d.c. bias is re-applied. If there were no change in the conductivity then the current would return immediately to the level observed before the d.c. bias was removed. Fig. 8b shows that, in fact, the conductivity starts at a very low value – similar to that for a new sample, i.e., the conductivity of the electrical pathways has changed in the absence of the d.c. field. The conductivity recovers to the level observed before the break in the d.c. stress and continues to evolve thereafter as though the break had not occurred (Fig. 4). The timescale for re-establishment of the conductivity to the level before the break was just under 10 min in Fig. 8 (approximately 500 s). This is much shorter than
the timescale for the development of the conductivity in a new sample, which occurs over several hours (12 h in Fig. 4). This demonstrates that the conductive pathways, developed over several hours of exposure to high humidity and d.c. electric field, are already formed and persist when the bias conditions are removed. In terms of the metal filament breakdown model, the metal filaments are likely to remain in the absence of the d.c. field, and terminate close to the critical distance for breakdown from the anode. Most of the time taken to establish a high level of conductivity is associated with this metal filament growth phase. The critical distance appears unchanged during the test, but once breakdown has occurred the conductivity increases with time, reflecting time required to develop conductivity. Given the rapid timescale this is most likely due to moisture adsorption and surface diffusion of ionic charge carriers on pore surfaces, cracks and grain boundaries, rather than changes to the conducting pathways in the body of the ceramic. 5. Conclusions In this study the characteristics of the conducting pathways that are responsible for leakage currents in piezoelectric ceramics under conditions of high humidity and d.c. bias were investigated through analysis of the voltage dependency of the leakage current, examined periodically during exposure to the d.c. field and humidity. The resulting low frequency a.c. I–V loops reveal a complicated electrical characteristic and provide detailed information on the nature of the conduction mechanism. It was observed that the major contributions to the conduction process were only activated by electric fields above a certain critical electric field. The value of the critical electric field was found to vary little during exposure to the stress conditions. These observations support the proposition that conductive pathways evolve through the ceramic during exposure to the stress conditions but do not contribute significantly to the leakage current until they are within a critical distance of the anode whereupon breakdown occurs, establishing a conduction path. A critical field strength was also observed for extinguishing of the conduction process, again supporting the theory that internal breakdown processes play a significant role in the current leakage mechanism. It was observed that the conductivity was significantly dependent on time. The leakage current on re-application of the bias field after a gap was very small, but increased rapidly to its value before the interruption. Thereafter the leakage current continued to grow slowly over time as if the interruption had not occurred. It is thought that the conductive pathways created through the bulk of the ceramic remain in place when the bias field is removed (these have grown over a long period of time). The conductivity in the surface regions where breakdown occurs is much more time
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dependent with rapid diffusion of the conducting species when the current is removed, and rapid recreation of the conditions for conduction when the field is re-applied. Acknowledgements The authors would like to acknowledge the support of the Technology Strategy Board and the UK National Measurement Office. We would also like to acknowledge the support of the APAHOE project partners Servocell Ltd. and Diameter Ltd. References [1] W. Heywang, K. Lubitz, W. Wersing (Eds.), Piezoelectricity: Evolution and Future of a Technology, Springer Verlag, 2008. [2] S. Beeby, M. Tudor, N. White, Energy harvesting vibration sources for microsystems applications, Meas. Sci. Technol. 17 (2006) R175. [3] P.M. Weaver, Y. Zheng, Thermal response of large movement low power dc actuators for use in lock and valve mechanisms, in: Proc. Actuator, Bremen, Germany, 2004, p. 2004. [4] B. Nagaraj, et al., Leakage current mechanisms in lead-based thin-film ferroelectric capacitors, Phys. Rev. B 59 (24) (1999) 16022–16027. [5] D. Zheng, J. Swingler, P. Weaver, Current leakage and transients in ferroelectric ceramics under high humidity conditions, Sens. Actuators A: Phys. 158 (1) (2010) 106–111. [6] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971. [7] I. Stolichnov, et al., Dielectric breakdown in (Pb, La) (Zr, Ti)O3 ferroelectric thin films with Pt and oxide electrodes, J. Appl. Phys. 87 (4) (2000) 1925–1931. [8] E. Bouyssou, et al., Wafer level reliability and leakage current modeling of PZT capacitors, Mater. Sci. Eng. B 118 (1–3) (2005) 28–33. [9] I.P. Lipscomb, et al., The Effect of relative humidity temperature and electrical field on leakage currents in piezo-ceramic actuators under DC bias, Sens. Actuators A: Phys. 151 (2009) 179–186.
[10] Pertsch P., et al., Reliability of Piezoelectric Multilayer Actuators, 2006. [11] J. Thongrueng, T. Tsuchiya, K. Nagata, Lifetime and degradation mechanism of multilayer ceramic actuator, Jpn. J. Appl. Phys. 37 (Part 1 No. 9B) (1998) 5306–5310. [12] H.C. Ling, A.M. Jackson, Correlation of silver migration with temperaturehumidity-bias (THB) failures in multilayer ceramic capacitors, IEEE Trans. Components, Hybrids, Manuf. Technol. 12 (1) (1989) 130–137.
Biographies Deyi Zheng received the B.Sc. degree in material sciences and technology from the Northeastern University, China, in 2003 and the M. Eng. degree in Materials with Business from the Queen Mary College, University of London, UK, in 2003. Since then, he has been studying as a PhD student in the School of Engineering Sciences, University of Southampton. Currently he is working as a research fellow in the School of Electronics and Computer Science, University of Southampton in the area of tactile devices for stroke rehabilitation. Research interests include micro-mechanism and application of function materials, tactile devices, 3D printing and energy harvesting. Paul Weaver is principal research scientist with the UK’s National Physical Laboratory’s multi-functional materials research group. Research interests include the development of novel electromechanical devices and the application of piezoelectrics and ferroelectrics for sensing and actuation. He is a visiting Reader at Southampton University and holds an MA in Natural Science from Cambridge University, and a PhD (Aeronautics) from Southampton University. He is a chartered engineer and member of the IET. Jonathan Swingler received the B.Sc. degree in physics and chemistry from the University of Keele, U.K., in 1990 and the PhD degree in the degradation of electrical contacts under low frequency fretting conditions from Loughborough University, Loughborough, U.K., in 1994. Since then, he has been pursuing research in the area of electrical conductance of materials at the University of Southampton, Southampton, U.K., and is currently an academic in the Electro-Mechanical Research Group. He lectures undergraduate Electrical Systems, Electromechanical Machine, and Automotive Electronics. He is a chartered scientist, chartered physicist, and a member of the Institute of Physics.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Electrical conduction mechanisms in piezoelectric ceramics under harsh operating conditions Deyi Zheng a,∗ , Jonathan Swingler a , Paul M. Weaver b a b
School of Engineering Sciences, University of Southampton, University Road, Southampton SO17 1BJ, United Kingdom National Physical Laboratory, Hampton Road, Teddington Middle TW11 0LW, United Kingdom
a r t i c l e
i n f o
Article history: Received 3 July 2010 Received in revised form 6 October 2010 Accepted 31 October 2010 Available online 1 December 2010 Keywords: Piezoelectric ceramics Electrical conduction Mechanisms Harsh operating conditions Electrical breakdown
a b s t r a c t Piezoelectric ceramics are widely used in industry for sensor and actuator applications. Under an applied electric field over a long duration, the conductivity of the ceramic increases resulting in leakage currents and increased power consumption. This process is accelerated by harsh environments and operating conditions such as high temperature, high humidity and high electric field. In this study, a piezoelectric ceramic was stressed by exposure to high relative humidity, at an elevated temperature, with a continuously applied d.c. electric bias field. Periodically during the test, the bias field was removed and a low frequency a.c. electrical cycle applied, during which current and voltage were measured. Results show that the conductivity is both time and voltage dependent and indicative of a complex breakdown process in the ceramic. The breakdown field strength was observed to vary only slightly during the exposure to humidity and d.c. bias despite large increases in conductivity. The reestablishment of the conduction process on re-application of the d.c. bias field was also studied. Whilst these results are specific to the piezoelectric material system studied, the mechanisms and insights into the interaction of electrodes with a ceramic material under conditions of high humidity and high electric field are likely to have much wider relevance for electro-ceramics in general, including technologically important materials such as dielectrics for energy storage and electronics, electronic oxides, and ferroelectric memory devices. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Piezoelectric ceramics are used as electro-mechanical transducers in a wide range of applications both as actuators and as sensors [1]. Particularly for d.c. or low frequency applications, these materials are attractive for their low power consumption, making possible novel applications for remote or inaccessible locations, autonomous operation using energy harvesting [2], or long-lived miniaturised battery powered devices [3]. As these materials become more widely used, the demands on their performance increase, particularly with regard to lifetime and reliability in humid environments and at high electric field across a wide temperature range. Whilst there are applications in areas such as aerospace or power generation where temperatures of several hundred ◦ C are encountered, many commercially significant applications operate at temperatures in the range −40 ◦ C to +85 ◦ C. These lower temperature applications are likely to encounter conditions of prolonged exposure to high humidity, particularly for outdoor applications. This paper is therefore concerned with the effects of high humidity at temperatures close to ambient.
∗ Corresponding author. E-mail address: [email protected] (D. Zheng). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.10.022
The low power characteristic of a piezoelectric actuator depends on good insulating quality of the ceramic. Leakage currents can be a cause of significant power loss, and can ultimately lead to failure of the device. Ferroelectric materials are intrinsic wide band gap semi-conductors [4] resulting in thermally activated conductivity which is likely to be insignificant at room temperature [5]. PZT (Lead Zirconate Titanate) ceramics have been shown to exhibit p-type conductivity thought to be due to an excess of cation vacancies [6], but most PZT is doped with higher valence cations which reduces the conductivity to very low levels at temperatures of interest here. Conductivity also develops over a period of time at high temperature due to vacancy and defect migration [7]. If the d.c. electric field is removed, the piezoelectric materials can recover their dielectric strength as oxygen vacancies migrate from the cathode interface back to the bulk material [8]. However, these conduction mechanisms do not explain the changes in electrical resistance in humid atmospheres where rapid changes in conductivity occur at much more moderate temperatures (around room temperature) in a variety of materials including PZT monolithic [5,9] and multilayer [10,11] actuators, Barium Titanate capacitors [12] with Nickel [5,9] and Silver [10–12] based electrodes. These effects do not occur in the absence of humidity, so it is likely that mechanisms other than those discussed above are responsible.
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Fig. 1. Experimental arrangement.
A more plausible explanation of the conduction mechanism under d.c. bias at high humidity is that [5,9,11] the electrode material ionizes in the presence of water vapour to produce aqueous metallic ions at the anode. These ions migrate through pores and grain boundaries to the cathode where they deposit as metal. The metal filaments grow back towards the anode. When the metal filament has grown to within a critical distance of the anode, the electrical field reaches a level sufficient to cause breakdown, accompanied by an increase in the level of leakage current [9] and current transients [5]. However, no metal filaments have been observed. The reason for this can be understood by the following considerations. If it is assumed that the conduction of a piece of ceramic with electroded area A occurs through conductive elements with an effective total area AC embedded in an insulating material, then the resistance of the ceramic is given by R=
t V = AC i
where t is the thickness of the ceramic (assumed equal to the length of the conducting channels), is the resistivity of the conducting channel material, V is the voltage between the electrodes and i is the total current passing between the electrodes. Multiplying by the total electroded area A gives: V/t = AC /A i/A V/t is the electric field, E, and i/A is the current density, j, so AC j = A E AC /A is the fractional area of the conductive channel. After exposure to 90% RH at 55 ◦ C with an electric field of 1.6 kV mm−1 currents densities of order 1 A m−2 can be reached in timescales of order 10 h [9]. If the conductive channel was formed from Ni with a conductivity of 7 × 10−8 m, then the fractional area of the conductive channels would be 4 × 10−14 . For a ceramic sample of the area considered here (25 mm × 5 mm) this would correspond to a cylinder of diameter 2.6 nm. In fact this area would be divided over many conductive pathways distributed over the area of the ceramic, so each one would be much too small for easy observation and nearly impossible to locate. Dendrites have been observed on exposed surfaces in systems using silver based electrodes where quite high levels of electro-migration may occur [10] but physical evidence of the conducting pathways in the body of the material may be very difficult to obtain. Whilst we may not always be able to observe the conducting pathways directly, we can probe the electrical charac-
teristics of the conductive pathways, which may provide insight into their properties and characteristics. There is also evidence that the applied electric field has a significant effect on the leakage current, with much smaller conductivity at electric fields below approximately 0.5 kV mm−1 [9]. This paper therefore resents a study of the electrical characteristics of the conductive pathways in a piezoelectric ceramic obtained from measurement of the dependency of conductivity on applied electric field during exposure to humidity and d.c. bias field. 2. Experimental setup The ceramic used for these experiments was a commercial soft grade piezoelectric ceramic prepared in sheets 25 mm × 5 mm × 0.15 mm with Nickel electrodes (2 m–4 m thick) on the large faces. The coercive field for this material is approximately 500 V/mm and the dielectric constants ∈ T11 / ∈ 0 is 4400 and ∈ T33 / ∈ 0 is 5500. The experimental arrangement is as shown in Fig. 1. The sample was placed into a humidity chamber with a temperature of 55 ◦ C and a humidity of 90% relative humidity. A d.c. power supply under computer control was used to apply voltage to the ceramic sample. The polarity of the voltage could be changed by the relay indicated in Fig. 1. In this circuit, R1 and RV form a voltage divider with a ratio of 50:1 and RI is a current shunt. The voltage across the sample and the current through it were determined by measuring the voltages across RV and RI using an analogue-to-digital converter (ADC). The R1 , RV and RI are selected to produce a voltage within the operating range of the analogue-to-digital converter, which is ±5V. For most of the experiment a constant d.c. electric field (240 V, 1.6 kV mm−1 ) was applied between the electrodes, recreating the stress conditions used in previous work [5] (Stage 2 in Fig. 2). During this period the leakage current was measured every 10 s. Periodically, the d.c. field was interrupted and a single low frequency (0.04 Hz) cycle of alternating electric field (1.6 kV mm−1 ) was applied (Stage 1 in Fig. 2). The purpose of this was to measure the voltage dependent conductivity (I–V loop) of the sample at intervals during the test. The alternating cycle was preceded by a 30 s pulse of reverse bias field of 1.6 kV mm−1 (Stage 3 in Fig. 2). This was to ensure that at the start of the first half cycle of the voltage loop measurement (Stage 1) the direction of the applied electric field was reversed with respect to the direction of the ferroelectric polarisation, thus ensuring a similar polarisation state at the start of each of the two half cycles. During Stage 1 (Fig. 2) the voltage was changed in a series of 6 V steps. For a continuously varying voltage, the current would
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Fig. 4. Leakage current measurement results for 0 h to 12 h.
3. Results
Fig. 2. Illustration of the three steps of the voltage – leakage current characterisation experiment (a) three stages voltage waveform and (b) close up of Stage 1 triangular waveform.
include contributions from the charging of the sample, the changes in the ferroelectric polarization as well as the d.c. leakage current. To ensure that the charging of the sample and ferroelectric contribution were minimised, and therefore only the leakage current was measured, the voltage was applied as a series of 6 V steps up to a peak positive amplitude of 240 V, then a peak negative value of −240 V then back to zero. Measurement of the leakage current was made at a defined time after the step in voltage. To decide how long this time lag should be, the following experiment was conducted. Fig. 3 shows that when 240 V was applied suddenly across a new sample the charging process caused a current transient with a peak current of approximately 16 A. The current then decreased to approximately 2 A in about 300 ms. The leakage current levels of interest in this paper are in the region of 10–300 A. A 10 s time duration between the voltage step and the current measurement was therefore chosen to ensure that any currents due to switching transients were well below the leakage current levels of interest.
Fig. 3. Charging curve of PZT ceramic at 240 V d.c.
Results of the continuous current measurement during the d.c. stress phase of the experiment (Stage 2 in Fig. 2) from a 12 h experiment are shown in Fig. 4. Under the applied voltage of 240 V the leakage current increased over a period of several hours. These results are comparable to those previously reported [5,9] for similar conditions. The spikes in the current measurement identify the points where the loop measurement was taken. These were caused by the charging current transient which occurred when the d.c. bias was re-applied after the loop (the 0.44 h period during which the I–V loop was recorded is not shown in Fig. 4). Selected loop results taken over a period of 18 h are shown in Fig. 5. The amplitude of the current and the area of the I–V loop increases significantly with time. There is also a pronounced asymmetry between the positive and negative half cycles. 4. Discussion A typical I–V loop results measured after 18 h of d.c. bias is shown in Fig. 6. Results in Fig. 5 were all plotted on the same scale to illustrate the evolution over time of the I–V loop. When results were normalised to the peak current value, then all the loops showed a profile very similar to that shown in Fig. 6. The exceptions to this were the loops at 0 h and 4 h where the current level was so low that any structure was indiscernible from the background noise. These very low leakage current loops were therefore excluded from the following analysis. For the loop shown in Fig. 6, it is seen that gradually increasing the electric field from 0 V to approximately 170 V caused very little leakage current – comparable to that for a new sample. Above 170 V, the current rapidly increased with voltage, following an approximately linear trend. This is indicative of a breakdown occurring at a critical value of the applied voltage. The value of this breakdown voltage can be estimated by measuring the intercept with the V axis of a straight line fitted to the approximately linear portion of the data (marked as a in Fig. 6). As the voltage decreased from the maximum of 240 V, the current remained approximately constant. In some cases it even increased slightly (e.g. Fig. 5d) despite the decreasing voltage. There then followed another approximately linear portion of the curve, with the current decreasing to a low level at approximately 50 V. This is indicative of a minimum voltage required to maintain the conduction process (extinction voltage), and can be estimated from the intercept with the V axis of a straight line fitted to the data marked as region b in Fig. 6. A similar sequence is followed for the negative half cycle with values of the breakdown and extinction voltages similar to those of the positive half cycle. This is verified in Fig. 7 which com-
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Fig. 5. Voltage loop measurement results recorded at the start of the test (a) and then after 4 h (b), 6 h (c), 8 h (d), 10 h (e) and 18 h (f) of time under the applied d.c. field.
pares the intercepts of the lines fitted to regions a, b, c, d of Fig. 6. Fig. 7 also shows that whilst there is a slight decrease in the critical voltages for breakdown they vary little over the whole test duration, at approximately 180 V (1.2 kV mm−1 ) for both positive and negative half cycles (the negative half cycle has a slightly lower breakdown voltage). This is strong evidence to support the model mechanism postulated in previous work [9] where metal filaments develop to within a critical distance from the anode where break-
Fig. 6. Current–voltage loop measurement result at t = 18 h (Fig. 5f). The arrows indicate the path through the full voltage cycle from 0 V increasing to peak positive voltage, decreasing back to 0 V, then to peak negative voltage before returning to 0 V. Red markers indicate the points used for straight line fits to the data in regions marked a, b, c, d. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
down occurs. In this model the gradual increase in the conductivity is caused mainly by an increasing number of conductive channels adding to the overall conductivity. The critical distance for breakdown and therefore the breakdown voltage would not be expected to vary significantly over the test duration. The slight decrease in breakdown voltage could be caused by reduction in the breakdown strength of the ceramic itself, possibly due to the effects of moisture on surfaces such as pore surfaces, cracks and grain boundaries. These results indicate that these effects are only minor as far as the breakdown process is concerned, although they are likely to play a significant role in the ionic migration and development of metallic filaments proposed in the model.
Fig. 7. Critical voltages for breakdown (a, c) and extinction (b, d) obtained from the intercept of least squares fit to the regions marked a, b, c, d in Fig. 6. The graph shows the amplitude of the voltage (the intercepts for the negative half cycles have negative values).
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Fig. 8. (a) Current (solid line) and voltage (dashed line) loop results (Stage 1 in Fig. 2) as functions of time; (b) conductance for the first half cycle (solid line) and for the period after re-application of the 240 V d.c. bias field (dashed line – Stage 2 in Fig. 2). Measurements were taken after 12 h exposure to d.c. bias.
As noted above, a critical voltage was also observed for extinguishing of the conduction process. With an average of 36 V (0.24 kV mm−1 ), this voltage was much smaller than the breakdown voltage. This could indicate a mechanism where the current helps maintain conduction even when the voltage is lower than that required to initiate conduction, or a time dependency in the breakdown voltage. It is also seen in Fig. 6 that when the applied voltage is past its peak value and is decreasing, the leakage current remains, for a time, approximately constant. This means that the conductance of the sample is increasing whilst the applied voltage is decreasing. In Fig. 6 the rate of decrease of voltage approximately balances the rate of increase of conductivity, for a time, during which the current stays approximately constant. This is probably fortuitous, and in some cases the current can increase despite the reducing electric field. This indicates a dependence of the conductance on time as well as voltage. To investigate the time dependency of the current in more detail, the current and voltage signals for a typical loop (Stage 1 in Fig. 2) are shown as functions of time in Fig. 8a. The conductance (the current divided by the voltage) of the first half cycle is plotted in Fig. 8b. The current measured on re-application of the d.c. bias field (Stage 2 in Fig. 2) is also plotted in Fig. 8b (the time axis origin was offset so that results could be compared directly with the Stage 1 loop results). Fig. 8b shows that the increase in conductivity after initiation of the conductive phase (a in Fig. 6) occurred at the same rate as the increase in conductivity after re-application of the 240 V d.c. bias field. The rates of rise of conductance stayed the same up to the point of maximum voltage in the a.c. cycle. As the a.c. cycle voltage decreased, the conductance remained constant or increased slightly. Where the d.c. bias was maintained, the conductance continued to increase at a gradually reducing rate. These results clearly demonstrate a dependency of the conductance on time for timescales over which the loops were recorded. The conductance continues to increase when the voltage is reversed resulting in the observed asymmetry in the loops. A similar time dependency of the conductance is also observed when the d.c. bias is re-applied. If there were no change in the conductivity then the current would return immediately to the level observed before the d.c. bias was removed. Fig. 8b shows that, in fact, the conductivity starts at a very low value – similar to that for a new sample, i.e., the conductivity of the electrical pathways has changed in the absence of the d.c. field. The conductivity recovers to the level observed before the break in the d.c. stress and continues to evolve thereafter as though the break had not occurred (Fig. 4). The timescale for re-establishment of the conductivity to the level before the break was just under 10 min in Fig. 8 (approximately 500 s). This is much shorter than
the timescale for the development of the conductivity in a new sample, which occurs over several hours (12 h in Fig. 4). This demonstrates that the conductive pathways, developed over several hours of exposure to high humidity and d.c. electric field, are already formed and persist when the bias conditions are removed. In terms of the metal filament breakdown model, the metal filaments are likely to remain in the absence of the d.c. field, and terminate close to the critical distance for breakdown from the anode. Most of the time taken to establish a high level of conductivity is associated with this metal filament growth phase. The critical distance appears unchanged during the test, but once breakdown has occurred the conductivity increases with time, reflecting time required to develop conductivity. Given the rapid timescale this is most likely due to moisture adsorption and surface diffusion of ionic charge carriers on pore surfaces, cracks and grain boundaries, rather than changes to the conducting pathways in the body of the ceramic. 5. Conclusions In this study the characteristics of the conducting pathways that are responsible for leakage currents in piezoelectric ceramics under conditions of high humidity and d.c. bias were investigated through analysis of the voltage dependency of the leakage current, examined periodically during exposure to the d.c. field and humidity. The resulting low frequency a.c. I–V loops reveal a complicated electrical characteristic and provide detailed information on the nature of the conduction mechanism. It was observed that the major contributions to the conduction process were only activated by electric fields above a certain critical electric field. The value of the critical electric field was found to vary little during exposure to the stress conditions. These observations support the proposition that conductive pathways evolve through the ceramic during exposure to the stress conditions but do not contribute significantly to the leakage current until they are within a critical distance of the anode whereupon breakdown occurs, establishing a conduction path. A critical field strength was also observed for extinguishing of the conduction process, again supporting the theory that internal breakdown processes play a significant role in the current leakage mechanism. It was observed that the conductivity was significantly dependent on time. The leakage current on re-application of the bias field after a gap was very small, but increased rapidly to its value before the interruption. Thereafter the leakage current continued to grow slowly over time as if the interruption had not occurred. It is thought that the conductive pathways created through the bulk of the ceramic remain in place when the bias field is removed (these have grown over a long period of time). The conductivity in the surface regions where breakdown occurs is much more time
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dependent with rapid diffusion of the conducting species when the current is removed, and rapid recreation of the conditions for conduction when the field is re-applied. Acknowledgements The authors would like to acknowledge the support of the Technology Strategy Board and the UK National Measurement Office. We would also like to acknowledge the support of the APAHOE project partners Servocell Ltd. and Diameter Ltd. References [1] W. Heywang, K. Lubitz, W. Wersing (Eds.), Piezoelectricity: Evolution and Future of a Technology, Springer Verlag, 2008. [2] S. Beeby, M. Tudor, N. White, Energy harvesting vibration sources for microsystems applications, Meas. Sci. Technol. 17 (2006) R175. [3] P.M. Weaver, Y. Zheng, Thermal response of large movement low power dc actuators for use in lock and valve mechanisms, in: Proc. Actuator, Bremen, Germany, 2004, p. 2004. [4] B. Nagaraj, et al., Leakage current mechanisms in lead-based thin-film ferroelectric capacitors, Phys. Rev. B 59 (24) (1999) 16022–16027. [5] D. Zheng, J. Swingler, P. Weaver, Current leakage and transients in ferroelectric ceramics under high humidity conditions, Sens. Actuators A: Phys. 158 (1) (2010) 106–111. [6] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971. [7] I. Stolichnov, et al., Dielectric breakdown in (Pb, La) (Zr, Ti)O3 ferroelectric thin films with Pt and oxide electrodes, J. Appl. Phys. 87 (4) (2000) 1925–1931. [8] E. Bouyssou, et al., Wafer level reliability and leakage current modeling of PZT capacitors, Mater. Sci. Eng. B 118 (1–3) (2005) 28–33. [9] I.P. Lipscomb, et al., The Effect of relative humidity temperature and electrical field on leakage currents in piezo-ceramic actuators under DC bias, Sens. Actuators A: Phys. 151 (2009) 179–186.
[10] Pertsch P., et al., Reliability of Piezoelectric Multilayer Actuators, 2006. [11] J. Thongrueng, T. Tsuchiya, K. Nagata, Lifetime and degradation mechanism of multilayer ceramic actuator, Jpn. J. Appl. Phys. 37 (Part 1 No. 9B) (1998) 5306–5310. [12] H.C. Ling, A.M. Jackson, Correlation of silver migration with temperaturehumidity-bias (THB) failures in multilayer ceramic capacitors, IEEE Trans. Components, Hybrids, Manuf. Technol. 12 (1) (1989) 130–137.
Biographies Deyi Zheng received the B.Sc. degree in material sciences and technology from the Northeastern University, China, in 2003 and the M. Eng. degree in Materials with Business from the Queen Mary College, University of London, UK, in 2003. Since then, he has been studying as a PhD student in the School of Engineering Sciences, University of Southampton. Currently he is working as a research fellow in the School of Electronics and Computer Science, University of Southampton in the area of tactile devices for stroke rehabilitation. Research interests include micro-mechanism and application of function materials, tactile devices, 3D printing and energy harvesting. Paul Weaver is principal research scientist with the UK’s National Physical Laboratory’s multi-functional materials research group. Research interests include the development of novel electromechanical devices and the application of piezoelectrics and ferroelectrics for sensing and actuation. He is a visiting Reader at Southampton University and holds an MA in Natural Science from Cambridge University, and a PhD (Aeronautics) from Southampton University. He is a chartered engineer and member of the IET. Jonathan Swingler received the B.Sc. degree in physics and chemistry from the University of Keele, U.K., in 1990 and the PhD degree in the degradation of electrical contacts under low frequency fretting conditions from Loughborough University, Loughborough, U.K., in 1994. Since then, he has been pursuing research in the area of electrical conductance of materials at the University of Southampton, Southampton, U.K., and is currently an academic in the Electro-Mechanical Research Group. He lectures undergraduate Electrical Systems, Electromechanical Machine, and Automotive Electronics. He is a chartered scientist, chartered physicist, and a member of the Institute of Physics.