2TiO3–BaTiO3 based thick-film capacitors for high-temperature applications

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Journal of the European Ceramic Society 34 (2014) 37–43

Bi1/2Na1/2TiO3–BaTiO3 based thick-film capacitors for high-temperature applications Jiadong Zang, Wook Jo ∗ , Haibo Zhang, Jürgen Rödel Institute of Materials Science, Technische Universität Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany Received 4 June 2013; received in revised form 10 July 2013; accepted 22 July 2013 Available online 12 August 2013

Abstract Thick films with compositions (1 − x)(0.94Bi1/2 Na1/2 TiO3 –0.06BaTiO3 )–x(K0.5 Na0.5 NbO3 ) (x = 0, 0.03, 0.09 and 0.18) have been prepared and their structural and electrical properties have been investigated. Dielectric properties show that these films are well suited for high-temperature applications due to their low variance in permittivity (<15%) over large temperature ranges. The thick film with x = 0.18 offers an operational temperature range from room temperature to 350 ◦ C. Films with x = 0.03 and 0.09 also possess a stabile permittivity up to 400 ◦ C. The improvement in the thermal stability of the permittivity is attributed to the addition of K0.5 Na0.5 NbO3 which leads to breaking of the long-range order in the materials. However, the polarizability of the materials was found to decrease possibly due to the clamping effect of the substrate. The characteristics of each film are discussed based on dielectric and electrical properties. © 2013 Elsevier Ltd. All rights reserved. Keywords: Ferroelectrics; High temperature dielectrics; Capacitor; Thick films

1. Introduction Ferroelectric ceramic capacitors are widely used as passive components for applications such as bypassing, coupling, decoupling, voltage smoothing and pulse discharge1,2 with the global market growing rapidly.3 In the meantime, cutting-edge technology requires electronic components to work at high temperatures ranging from 200 to 500 ◦ C in such industries as automotives, aerospace and oil exploration.4–6 However, most capacitors currently available are restricted to a maximum working temperature of ∼150 ◦ C,7 which constrains further development of high-temperature electronics. Furthermore, recent regulations on the use of hazardous elements in electronics have imposed new limitations on the development of electroceramics.8 Therefore, it is important to develop capacitor materials using non-toxic elements to show excellent performance at temperatures above 200 ◦ C. Several requirements need to be met by these materials. Firstly, the permittivity should be temperature-insensitive, ideally within the variation of 10–15%.7 Secondly, a high RC-constant is required to ensure

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Corresponding author. Tel.: +49 6151 16 6316; fax: +49 6151 16 6314. E-mail address: [email protected] (W. Jo).

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the performance of the capacitors and reduce the distortion of the signals passing through the circuits. Finally, it is important for the materials to have relatively high permittivity in order to allow for miniaturization of the devices. Several material systems have been developed for hightemperature applications. For BaTiO3 based materials, (Ba0.8 Bi0.2 )(Zn0.1 Ti0.9 )O3 and BiScO3 –BaTiO3 were reported as promising capacitor materials for temperatures ranging from 100 to 300 ◦ C.9,10 Similarly, Bi1/2 Na1/2 TiO3 (BNT) based materials were found to be excellent candidates.11 For example, Bi1/2 Na1/2 TiO3 –BaTiO3 (BNT-BT) systems modified with CaTiO3 were characterized to have an operational temperature range between −55 and 150 ◦ C.12 Recently, K0.5 Na0.5 NbO3 (KNN)-modified BNT-BT13 as well as KNN-modified Bi1/2 Na1/2 TiO3 –Bi1/2 K1/2 TiO3 14 systems were reported to have exceptionally large operational temperature range from room temperature to >350 ◦ C. The best properties have been achieved by BNT-BT modified with both KNN and CaZrO3 with an operational temperature range between −100 ◦ C and 500 ◦ C.15 The development of thick-film capacitors is important due to its wide use in the microelectronic industry.16 Since the capacitor materials are frequently used in the form of thick films or multilayers, it is worth investigating the behavior of

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high-temperature dielectric materials in the form of thick films in comparison to bulk ceramics. For BNT-based materials, thick films realized by screen-printing techniques have been developed focusing on ferroelectric and piezoelectric properties, while their applications for high-temperature capacitors have not been studied so far.17–22 This study provides a systematic investigation of ceramic thick-film capacitors for high-temperature applications above 200 ◦ C. Thick films of compositions (1 − x)(0.94Bi1/2 Na1/2 TiO3 –0.06BaTiO3 )–x(K0.5 Na0.5 NbO3 ) with x = 0, 0.03, 0.09 and 0.18 are prepared for their structural and electrical properties. Temperature-dependent permittivity and dielectric loss demonstrate that these materials are suited for high-temperature applications. The differences in the dielectric properties between thick films and bulk ceramics as well as the influence of the addition of KNN are discussed. 2. Experimental Thick films and bulk ceramic samples of compositions (1 − x)(0.94Bi1/2 Na1/2 TiO3 –0.06BaTiO3 )–x(K0.5 Na0.5 NbO3 ) with x = 0, 0.03, 0.09 and 0.18 were prepared. The materials will be referred to as 0KNN, 3KNN, 9KNN and 18KNN in the following discussion. Ceramic powders were produced via a mixed oxide route using reagent grade oxides and carbonates (Alfa Aesar GmbH, Karlsruhe, Germany). Firstly, Bi2 O3 (purity 99.975%), Na2 CO3 (purity 99.5%), TiO2 (purity 99.9%), BaCO3 (purity 99.8%), K2 CO3 (purity 99.0%) and Nb2 O5 (purity 99.9%) were weighed according to the stoichiometric formula and ball-milled in a planetary ball mill (Fritsch Pulverisette 5, Idar-Oberstein, Germany) for 24 h at 250 rpm. The powder was calcined at 800 ◦ C for 3 h and ball-milled again with the same conditions as described above. For bulk ceramic samples, disk-shaped samples of 10 mm in diameter were compacted manually and then pressed isostatically at 300 MPa. The green bodies were sintered at 1150 ◦ C for 2 h. After grinding and polishing, silver electrodes were burnt in at 400 ◦ C for 2 h. Paste for thick film fabrication was prepared by mixing synthesized ceramic powders with an organic printing vehicle (ESL Electroscience, King of Prussia, USA). Thick films were screen-printed onto Al2 O3 substrates (CeramTec AG, Plochingen, Germany) coated with Pt as a bottom-electrode. The films were isostatically pressed at 300 MPa. The process of printing and pressing was repeated until the thickness reached about 40 ␮m. Then, the films were fired at 400 ◦ C for 30 min for binder burn-out and sintered at 1100 ◦ C for 1 h. Ag paste was screen-printed as a top-electrode. X-ray diffraction patterns were obtained with Cu K␣1 radiation (AXS D8, Bruker Corporation, Karlsruhe, Germany). The microstructure of the samples was examined with scanning electron microscopy (SEM XL 30FEG, Philips Corporation, Eindhoven, The Netherlands). Temperature and frequency dependent permittivity and dielectric loss were measured with an impedance analyzer (HP 4192A, Hewlett Packard Corporation, Palo Alto, USA). The data were collected at four frequencies (1 kHz, 10 kHz, 100 kHz and 1 MHz) for every 2 ◦ C from RT to 400 ◦ C with a heating rate of 2 ◦ C/min. For Vogel-Fulcher fitting, the same measurements were repeated for 15 different

Fig. 1. XRD patterns of (1 − x)(BNT-6BT)-xKNN (x = 0, 0.03, 0.09 and 0.18) thick films.

frequencies between 100 Hz and 1 MHz. Resistivity was determined at a dc voltage of 4 V from RT to 450 ◦ C using an electrometer (6517B, Keithley Instruments, Cleveland, USA). Polarization hysteresis loops under electric fields were measured by a ferro- and piezoelectric testing system (TF Analyser 2000, aixACCT Systems GmbH, Aachen, Germany). 3. Results and discussion Fig. 1 shows the XRD patterns of the thick films with different compositions. The XRD patterns confirm that the films have been successfully deposited as a single phase. All samples display a pseudo-cubic perovskite-type structure without an obvious non-cubic distortion, which is consistent with the XRD results reported for bulk materials of similar compositions.13,23 Fig. 2 displays the SEM images of the thick films. All samples show a homogeneous microstructure with similar average grain size. The average grain size is comparable to that of bulk ceramics with similar compositions.13–15 There is no clear tendency of change in microstructure with increasing amount of KNN. The grain size and porosity of the films are very similar for all samples. The porosity of the thick films is apparently higher than that of bulk ceramics, which is mainly due to constrained sintering. It is known that the presence of a rigid substrate hinders the radial shrinkage during sintering, leading to less dense films.24 The density is estimated to be within 80–85% from a stereological analysis on the SEM images. Similar microstructures with comparable grain size and porosity have been reported for thick films of other BNT-based materials.19,21 The same procedure applied to the bulk ceramics revealed the relative density of 97–98% and grain size of about 1 ␮m for all compositions. The temperature and frequency dependent permittivity and dielectric loss of the thick films are displayed in Fig. 3. It can be seen that the permittivity curve becomes flattened with increasing KNN content. In the 0KNN thick film (Fig. 3a), two anomalies at 150 ◦ C and 300 ◦ C can be observed with the first one showing typical relaxor behavior. The positions of both anomalies are consistent with those in bulk ceramics.11 Recently, it was suggested that these two anomalies in 0KNN be the consequence of polar nanoregions (PNRs) of two different crystallographic

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Fig. 2. SEM micrographs of (1 − x)(BNT-6BT)-xKNN thick films. (a) x = 0, (b) x = 0.03, (c) x = 0.09 and (d) x = 0.18.

symmetries, which can be referred to as low-temperature PNRs (LT-PNRs) and high-temperature PNRs (HT-PNRs).25,26 For both 0KNN bulk ceramics and thick films, the first anomaly at around 150 ◦ C is suggested to be due to thermal evolution of LT-PNRs while the second anomaly at around 300 ◦ C is given by both the phase transition between two PNRs and the thermal evolution of HT-PNRs. When the amount of KNN is increased, the thermal evolution of these two PNRs is significantly influenced. For 3KNN thick films (Fig. 3b), the position of the first anomaly is shifted slightly to the lower-temperature side, while the magnitude of permittivity decreases by around 30% compared with that of the 0KNN film. Given that the first anomaly is mainly

Fig. 3. Temperature and frequency dependent dielectric permittivity and dielectric loss of the (1 − x)(BNT-6BT)-xKNN thick films. (a) x = 0, (b) x = 0.03, (c) x = 0.09 and (d) x = 0.18.

contributed by LT-PNRs, the depression of the first anomaly indicates that the density of LT-PNRs decreases with the addition of KNN. The second anomaly of the 3KNN film also decreases in magnitude while its position remains almost unchanged. For thick films with 9KNN and 18KNN (Fig. 3c and d), while the fist anomaly shows no significant change, the second anomaly becomes less visible with increasing amount of KNN, which results in a smoothing of the permittivity curve at high temperatures. The fading out of the second anomaly for 9KNN and 18KNN films suggests that the addition of KNN eventually changes the volume ratio between LT-PNRs and HT-PNRs, as evidenced by structural analysis where the volume ratio of PNRs with different symmetries changes with KNN content.27,28 This implies that the added KNN favors a predominant existence of HT-PNRs at low temperatures, which weakens the contribution of the phase transition from LT-PNRs to HT-PNRs and the subsequent relaxation of transformed HT-PNRs at high temperatures. The permittivity and dielectric loss at 1 kHz of bulk ceramics (sintered from the same powder) and thick films are compared in Fig. 4. The positions of the first anomaly (the relaxor behavior) and the second anomaly (the dielectric maximum) are almost the same for bulk ceramics and thick films. This indicates that thick films with correct compositions have been fabricated. It can also be found that the thick films have a lower overall permittivity than bulk ceramics. As discussed earlier, the burning out of residual organics of the printing vehicle at high temperatures and the constrained sintering in radial direction24 inhibit the densification of the films, which results in lower permittivity values.29 On the other hand, the clamping effect of the substrate can be considered to play a role in reducing the polarizability of the materials in the direction of the electrical field.30 As well,

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Fig. 4. Comparison of temperature dependent permittivity and dielectric loss at 1 kHz between bulk ceramics and thick films of (1 − x)(BNT-6BT)-xKNN (x = 0, 0.03, 0.09 and 0.18).

the resulting stress can influence the permittivity by changing the mobility of PNRs or domain walls.31 The reduction in permittivity by 40–60% caused by porosity and clamping effect has also been reported for other lead containing32–34 and BNTbased thick films.17–19,21,22 Different from permittivity values, the dielectric loss of the thick films is only slightly affected at low temperatures. At higher temperatures, the dielectric loss increases rapidly due to the high conductivity of the films. The variance of permittivity at 1 kHz as a function of temperature is presented in Fig. 5. The permittivity value at 150 ◦ C was taken as the reference point, since it is at the midpoint of the desired operational temperature range and has also been used for other BNT-based high-temperature dielectrics.13,15 The 18KNN thick film has an operational temperature range from room temperature to around 350 ◦ C, which makes it most suitable for applications as high-temperature thick film capacitor. The 9KNN thick film is also suitable for applications ranging from 50 ◦ C up to 400 ◦ C. In Table 1, the high-temperature dielectric properties of 9KNN and 18KNN thick films are compared with those of bulk ceramics sintered from the same powder. The properties of the thick films are comparable to those of

Fig. 5. Variance of permittivity at 1 kHz of the (1 − x)(BNT-6BT)-xKNN (x = 0, 0.03, 0.09 and 0.18) thick films. The permittivity at 150 ◦ C was taken as the reference point.

Fig. 6. Temperature dependent dc resistivity of the (1 − x)(BNT-6BT)-xKNN (x = 0, 0.03, 0.09 and 0.18) thick films.

bulk ceramics with a slight improvement in the high-temperature range. Although the overall permittivity of thick films is no more than half of that of bulk ceramics, the absolute value is still high enough for applications. It is interesting to note that the RC constant of materials is improved for thick films. Fig. 6 presents the resistivity of the thick films as a function of temperature. No clear dependence of the resistivity on the composition is evident. The deviation in the absolute resistivity values from composition to composition appears to be related to the quality of films. A change in the conducting mechanism is identified from the appearance of a kink at around 300 ◦ C. Arrhenius-type analysis revealed that the activation energy at temperatures below 300 ◦ C is 0.8 ± 0.05 eV for all four compositions, which is comparable to the activation energy required for the short-range migration of oxygen vacancies in the BNTbased materials.35 At temperatures above 300 ◦ C, the activation energy is 1.5 ± 0.1 eV, which could be attributed to the diffusion of oxygen vacancies which enables long-range migration of oxygen vacancies.36 The change in the mechanism of dc conductivity into a thermally activated long-range nature explains the sharp increase of high dielectric loss observed in the thick films at temperatures above 300 ◦ C. The polarization of the thick films under electric field up to 10 kV/mm is shown in Fig. 7a. The figure indicates that the maximum polarization value at 10 kV/mm decreases with the increasing amount of KNN. This decrease in the absolute polarization value inducible at given electric field can be explained by the fact that the addition of KNN increases the threshold field required to induce a long-range order from the initial relaxor state, as revealed by PFM studies in the same materials.37 Therefore, the improvement in the thermal insensitivity of the permittivity can also be traced back to the role played by KNN in breaking the long-range order.38 The polarization of 0KNN has a well-shaped saturated hysteresis loop with a coercive field of 2.5 kV/mm, which is comparable to the value of bulk ceramics.11 The 3KNN thick film exhibits a slightly constricted P(E) loop with a coercive field of 1.6 kV/mm, which is close to that of bulk ceramics.37 Both 9KNN and 18KNN show no saturation in the P(E) loops. The remanent polarization values

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Table 1 Comparison of high-temperature dielectric properties between thick films and bulk ceramics sintered from same powders. Properties at 1 kHz

Variance < 10% Variance < 15% εr at 300 ◦ C tan δ at 300 ◦ C RC at 300 ◦ C

Thick films

Bulk ceramics

9KNN

18KNN

9KNN

18KNN

62–382 ◦ C

38–348 ◦ C

50–310 ◦ C

42–317 ◦ C 32–340 ◦ C 1916 0.01 1.11 s

49–387 ◦ C 1433 0.06 3.40 s

32–354 ◦ C 1266 0.06 1.68 s

39–343 ◦ C 2885 0.02 2.60 s

Fig. 7. (a) Electrical hysteresis loops of the (1 − x)(BNT-6BT)-xKNN thick films (x = 0, 0.03, 0.09 and 0.18). (b) Comparison of remanent polarization between bulk ceramics and thick films.

of bulk ceramics and thick films are compared in Fig. 7b. Both curves reveal similar trends where the remanent polarization decreases with increasing amount of KNN. Remanent polarization is reduced from 0KNN to 3KNN for both bulk ceramics and thick films. However, it is apparent that remanent polarization of 0KNN is most affected by changing from bulk ceramics to thick film with the value decreasing by 70%, while other compositions undergo a smaller reduction (30–50%) in remanent polarization. The reduction in the polarization can be ascribed to the clamping effect caused by the in-plane tensile stress. This is further evidenced by the thickness dependence of polarization in other relaxor ferroelectric films39,40 and also by the depolarization induced by tensile stress in bulk ceramics where the non-180◦ domain switching is enhanced.41–43 The internal stress is mainly caused by the constrained sintering process24 and this in-plane tensile stress hinders the distortion of unit cell in the field direction and the formation of long-range order, as supported by the studies on other ferroelectric films.44–48 Fig. 8 compares the ferroelectric-to-relaxor transition temperature (TF-R ) of 0KNN bulk ceramics and thick film, which were poled at 6 kV/mm (bulk ceramics) and 10 kV/mm (thick film). This temperature can also be understood as the transition temperature where the material reverts back to the initial relaxor state.49 The TF-R of 0KNN bulk ceramics lies at around 82 ◦ C, while that of 0KNN thick films appears at around 70 ◦ C. The lower TF-R in thick films suggests that the field-induced ferroelectric phase is less stable in thick films. From the pattern of the transition process, it can also be concluded that the thick film was not fully poled even at 10 kV/mm. From our previous discussion, we can conclude that the internal stress induced during the constrained sintering process should be responsible for the destabilization of the field-induced long-range order. The other

compositions were not able to be poled since they were already in their ergodic state at room temperature.37 The transition temperature from ergodic relaxor phase to non-ergodic relaxor phase can be manifested by the freezing temperature TVF , which is determined from the Vogel-Fulcher law50   E , f = f0 exp − kB (Tm − TVF ) where f represents the measurment frequency, f0 a constant, E the activation energy, kB the Boltzmann constant, Tm the peak

Fig. 8. Ferroelectric-to-relaxor transition temperature (TF-R ) of (a) 0KNN bulk ceramics and (b) 0KNN thick film.

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References

Fig. 9. Freezing temperature (TVF ) from Vogel-Fulcher fitting of bulk ceramics and thick films.

position in permittivity and TVF the freezing temperature. The freezing temperature TVF of both bulk ceramics and thick films as a function of KNN-content is summarized in Fig. 9. It can be seen that with the addition of KNN, the freezing temperature decreases continuously due to the disruption of the long-range order and the reduction in the thermal stability of PNRs.37,38 The transition temperature of thick films is lower than that of bulk ceramics by 10–25 ◦ C. This is clearly visible in 0KNN, which is the only composition that can be poled to a stable ferroelectric state at room temperature. Here, the difference is 10 ◦ C, which is close to the difference in ferroelectric-to-relaxor transition temperature between bulk ceramics and thick film as shown in Fig. 8. This temperature difference again suggests that the internal stress in the thick films should contribute to destabilizing PNRs and reducing the polarizability of the materials in field direction. 4. Summary BNT-6BT-xKNN (x = 0, 0.03, 0.09 and 0.18) ceramics were successfully fabricated into thick films of good quality by screen printing technique, aiming at dielectric capacitive applications at temperatures above 200 ◦ C. Operational temperature range up to 400 ◦ C was achieved in both 3KNN and 9KNN, and from room temperature to 350 ◦ C in 18KNN. Fabrication of materials in the form of thick films was found to be effective in enhancing RC constant at the expense of absolute dielectric permittivity values and ferroelectric stability. Further investigations of electrical polarization and Vogel-Fulcher fitting suggest that addition of KNN contributes to destabilizing PNRs and thus gives rise to the improvement in the thermal stability of permittivity. The results also suggest that the internal stresses play a role in reducing the polarizability of the materials along the field direction. Acknowledgements The authors acknowledge the generous support by the Deutsche Forschungsgemeinschaft through the Leibniz program under RO 954/22. H.Z. also thanks the Alexander-vonHumboldt foundation for generous funding.

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