Preparation and characterization of spray-pyrolysed BaTiO 3 films

34 Materials Science and Engineering, B25 {, 1994) 34-38 Preparation and characterization of spray-pyrolysed BaTiO 3 films A. K. Tripathi, Vijayaraghavan Chariar, T. C. Goel and P. K. C. Pillai* Department of Physics, Indian Institute of Technology, New Delhi 110016 (India) (Received June 29, 1993; in revised form October 15, 1993) Abstract Thin films of BaTiO 3 were prepared by spray pyrolysis of BaTiO 3 sol. The structural and dielectric characteristics of these films are reported in this study. The results indicate that spray pyrolysis could be an useful technique to prepare homogeneous, capacitor-grade BaTiO 3 thin films. The pyroelectric characteristics of BaTiO3/PVDF composites prepared from spray-pyrolysed BaTiO 3 films are also reported. 1. Introduction The preparation of ultrafine homogeneous multipleoxide ceramics by hydrolysis of metal alkoxides is becoming an important technique these days [1-3]. The main advantage of this process is that, since the corresponding metal alkoxides are mixed in the liquid phase, homogeneous mixing at the molecular level is possible before subjecting the sol to hydrolysis and gelation. A number of ferroelectric ceramics, including BaTiOa [4, 5], PZT [6] and PLZT [7], have been prepared by the sol-gel method, both in the film and powder form. Thin films obtained by the sol-gel route have generally been prepared using the spin coating and dip coating techniques. One of the advantages of the spin and dip coating techniques is that optically transparent thin films can be obtained. However, the process is time consuming, because several coats are required to achieve the desired film thickness and the film has to be heat treated each time at 250-300 C, before a final sintering step at high temperature [8]. For device applications, such as thin film capacitors, surface acoustic wave (SAW) devices and IR detectors, where optically transparent films are not a rigid requirement, spray pyrolysis of sols has been found to be a useful preparation technique. This paper describes the preparation of BaTiO3 thin films by the spray pyrolysis technique. The dielectric, pyroelectric and structural characteristics of these films, as well as those of PVDF/BaTiO 3 composites *Author to whom correspondence should be addressed. prepared from the spray-pyrolysed BaTiO 3 films are reported. 2. Experimental details The BaTiO3 sol was prepared using acetic acid as a catalyst [9]. Propanol was added in barium acetate solution with an excess of acetic acid. An equimolecular propanolic solution of titanium isopropoxide was added slowly with constant stirring to give a clear sol of BaTiO 3. Deionized distilled water was added to adjust the viscosity of the sol. It was found that the sol prepared in propanol does not form a gel at room temperature (30 C), even when kept for one week. The sol was sprayed on substrates maintained at 450 C, using a spray gun and air compressor assembly. Care was taken to spray slowly and in a fine jet, by controlling the air pressure. The substrates used in the present experiment were optically flat glass slides, stainless steel and conducting glasses. After spraying, the substrates were cooled slowly to room temperature. The thickness of the films deposited in this way was 15-20/~m. Polymeric films 4/~m thick of PVDF were deposited onto a few BaTiO3 films by dipping them in a dilute solution of PVDF dissolved in DMF. This process, we find, is useful for filing the micropores observed in pure BaTiO a films. X-ray diffractograms of the BaTiO 3 films were recorded using CuKa radiation on a Rigaku diffractometer. The scan speed used was 10 deg min-i. The voltage and current ratings used were 40 kv and 30 ma respectively. 0921-5107/94/$7.00 1994 - Elsevier Sequoia. All rights reserved ~'~'/31 rio91 -~ 1117(Qg~O 109tq-7

A. K. Tripathi et al. / Spray-pyrolysed Ba TiO ~ films 35 A small amount of BaTiO3 powder was scratched from the spray-pyrolysed film and was used to record differential scanning calorimetry (DSC) thermograms. The DSC was performed on a Du Pont 2000 differential scanning calorimeter. Electrical and dielectric measurements were made on the films deposited on the conducting glass. Aluminium electrodes 1 cm in diameter were vacuum evaporated onto BaTiO 3 films and BaTiOa/PVDF composites. The dielectric measurements were carried out on a Hewlett packard LF impedance analyzer 4192A. Prior to making pyroelectric measurements, the films were corona poled at a corona voltage of 7 kv and corona current of 40/aA. The poling was carried out at 120 C for 5 min, and the samples were cooled (in the presence of the field) to room temperature in about 25 rain time. The pyroelectric current was measured on a Keithley Electrometer 610C. The heating rate for the pyrocurrent measurements was maintained at 3 C min -~, and the observations were recorded after the third heating cycle, which showed reproducible values. 3. Results and discussions An X-ray diffractogram of the spray-pyrolysed BaTiO3 film is shown in Fig. 1. It shows two prominent peaks at 20 = 31.2 and 38.1, with d values of 3.172 and 2.359 respectively. These peaks result from the (101) and (110) planes of BaTiO 3. The other peaks observed are identified in the diffractogram. Phule and Risbud [9] and Chaput and Boilet [19] have reported the trace presence of BaCO 3 when BaTiO 3 has been prepared using barium acetate and titanium isopropoxide as starting materials. Phule and Risbud [9] have also reported that, if sintering is performed above a temperature of 520 C, no traces of BaCO3 are observed. In our study, we have not observed any peaks corresponding to the (111) and (021) planes of BaCO 3. It seems that, in this study, as a result of the fine droplet size characteristic of the spray pyrolysis method, any BaCO3 that may have been formed decomposes at a lower temperature of around 450 C. A typical scanning electron micrograph of the BaTiO3 film ( 15/~m thick) is shown in Fig. 2. The particulate phase in the micrograph is the BaTiO 3 grains, and the voids seen in the micrograph result from the micropores in the film. The micrograph indicates that the average grain size of BaTiO 3 is 1-2/~m. Arlt et al. [11] have reported that BaTiO 3 exists predominantly in the tetragonal structure for grain sizes of 1.5/zm and above, and that, for an average grain size between 1.5 and 0.5 ~m, tetragonal and orthorhombic phases coexist. Therefore, it seems that BaTiO 3 films prepared in this study also exist as a mixed system of tetragonal and orthorhombic or pseudocubic structures. Our X-ray studies also support this conclusion, because the peaks corresponding to the (002) and (200) planes are not well resolved. The DSC thermogram in Fig. 3 clearly shows that the ferroelectric phase transition of spray-pyrolysed BaTiO3 films occurs at 149 C. The variations of the dielectric constant e and loss tangent tan 6 with temperature at various fixed frequencies are shown in Figs. 4 and 5 respectively. The dielectric constant shows a broad Curie peak at about 145 C, which is close to the phase transition temperature. The broad peak in the dielectric constant is indicative of stresses and strains in the sample [12]. For bulk BaTiO3, a transition temperature of 125 C has been reported. The shift in the Curie temperature to 145 C in our case, could be the result of the small size of BaTiO 3 grains in the film and the strain present in _ (101) )- I-- (110), (002)/(200) (112) 2O ~0 60 2 e ---P- Fig. 1. X-ray diffractogram of a spray-pyrolysed BaTiO 3 film. Fig. 2. Scanning electron micrograph of BaTiO 3 film 15/zm thick.

36 A. K. Tripathi et al. / Spray-pyrolysed Ba TiO films -0.2-0.t, 0.3 ------- (a) -0 6-0.8 ~-1.0 -I.~ -1,4 1,O'c /'-- 't ~ ~ " (b) 0.1 ~ (c) 0 I f I 30 50 70 90 110 130 150 170 TEHP ( C) Fig. 5. Variation of tan 6 with temperature for BaTiO 3 film: (a) 100 Hz; (b) 1 khz; (c) 10 khz. -1.6 I I I, I, I I, 60 so loo lzo 1~,o 18o TEHP ( C) 180 Fig. 3. DSC thermogram of BaTiO 3 film. (a) 16- i 15 (b) 2200 2000 1/,- 13- (c) 1800 1600 (a} 1~,00 1200 (b) I000 ~ ~(c) 800 I I I I I I L I 30 SO 70 90 110 130 1S0 170 190 TEMP ( C} Fig. 4. Variation of dielectric constant e' with temperature for BaTiO3 film: (a) 100 Hz; (b) 1 khz; (c) 10 khz. the sample. Several other studies have also reported a shift in the BaTiO 3 Curie temperature within wide limits, depending on the average grain size, inhomogeneities, internal stresses and sintering temperature [11, 13, 14]. The initial decrease in e' with temperature could be attributed to the fact that, in ceramics, the resultant dielectric constant is the sum total of e' along the a and c axes. The high value of flae dielectric constant of BaTiO 3 films obtained in this Way indicates that spray pyrolysis is a simple and powerful technique for making low voltage capacitor ceramics with high capacity-to-volume ratios. Such high values of the dielectric constant have also been observed by others for similar grain size [ 11 ]. The transition temperature observed in the dielectric studies was found to be slightly lower than that observed in the DSC. This shift in the peak could have resulted from the continuous heating of the sample in I 30 SO I I I I t I 70 90 110 130 150 170 TENP ( C) Fig. 6. Variation of dielectric constant e' and tan 6 with temperature for a PVDF/BaTiO 3 composite film: (a), (a') 100 Hz; (b), (b') 1 khz; (c) 10 ki-iz. the DSC, whereas the dielectric measurements were carried out at various fixed temperatures. The temperature dependence of tan 6 also shows a peak at the Curie temperature. The loss observed in these samples is slightly higher than that in bulk BaTiO 3 ceramic. This could be explained by the presence of micropores in the film. The presence of micropores hampers the process of corona poling of the film, leading to dielectric breakdown. Pyroelectric studies were made possible by coating the BaTiO3 films with a thin layer (4/~m thick) of PVDF. Dipping the BaTiO 3 films in PVDF facilitates poling by filling the micropores. An additional advantage of the polymer layer comes from the accepted fact that composite films have a lower dielectric constant than that of the parent ceramic film. A lower value of the dielectric constant is desirable for pyroelectric applications. The variations in the dielectric constant and loss tangent with temperature for the PVDF-coated BaTiO3 film are shown in Fig. 6. The maximum value of the dielectric constant for the composite film is found to be

A. K. Tripathi et al. / Spray-pyrolysed Ba Ti03 films 37 16 at 100 Hz and 150 C. This value is considerably lower than that for the pure BaTiOa film. Similarly, the tan 6 values for the composite film have also reduced in comparison with that for pure BaTiO3. A broad transition peak at about 135 C is observed in the variation of the dielectric constant of the composite film with temperature at 10 khz. The variation of the pyroelectric current of a corona-charged BaTiO3/PVDF composite film with temperature is shown in Fig. 7. It was found that the 22 20 18 16 14. pyrocurrent increased gradually up to 90 C; however, thereafter, the increase was very rapid. The nature of the variation of the pyrocurrent is similar to that of bulk BaTiO3. The value of the pyrocoefficient is obtained using the relationship p=g/a (do/dt )-' where I is the pyroelectric current, A is the sample area and do/dt is the sample heating rate. A typical value of the pyroelectric coefficient is 9.55 nc cm -2 K -1 at50 C. FD and Fv, i.e. the pyroelectric figures of merit, are determined using the relationships [15] F o = p'/c'(e'tan 6) 1/2 Fv =p'/de' e where c' is the volume-specific heat and the other symbols have their usual meanings. Table 1 lists the values of F o and F v for composite films, assuming c to be 2.5 10-6 J m -3 K- 1. It can be seen from Table I that, the pyroelectric coefficient of spray-pyrolysed BaTiOa/PVDF composites are superior to those of pure PVDF films. In summary, our studies have demonstrated that spray pyrolysis of sols could be an excellent technique for preparing thin ceramic films. A high degree of compositional control could be obtained at lower processing temperatures without any additional sintering step. BaTiO3 films prepared by this method could have useful applications in thin film capacitors and pyroelectric devices. We are now continuing our efforts to control the microstructure of the film by optimizing the various parameters in the spray process. 6 m 4.- 2, I, L, I, I, r, I, 30 50 70 90!10 130 150 TEHP (oc) Fig. 7. Variation of pyroelectric current of the BaTiO3/PVDF composite film with temperature. References 1 C.J. Brinker, G. C. Frye, A. J. Hurd, in D. R. Uhlmann and D. R. Ulrich (eds.), Proc. 4th Int. Conf. on Ultrastructure Processing of Glasses, Ceramics and Composites, Wiley, New York, 1990. 2 S. P. Mukherjee, Sol-gel processing in glass science and technology, J. Non-Cryst. Solids, 42 (1980) 477. 3 H. Dislich, in L. C. Klen (ed.), Sol-gel technology for thin films, fibers, preforms, electronics and speciality shapes, Noyes, NewYork, 1990, p. 73. TABLE 1. Comparison of dielectric and pyroelectric properties of PVDF and BaTiO3/PVDF composite Material p' Freq) e tan 6 F v F D x 10-5 (/~C m -2 K- 1 ) (Hz) (m a C-1) (Pa- t/2) PVDF BaTiO3/PVDF 27 95.5 10 100 12 15.9 0.015 0.09 0.1 0.27 0.88 1.07 (50 C) 1000 14.4 0.06 0.30 1.38

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