The sol-gel synthesis of

barium strontium titanate ceramics

1University of Silesia, Department of Materials Science, ul. Śnieżna 2, 41-200 Sosnowiec, Poland

2University of Silesia, Institute of Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland

The sol-gel derived powders of the chemical composition (Ba0.6Sr0.4)TiO3 (BST) were used in the preparation of ceramic samples. Barium acetate, strontium acetate and tetra-butyl titanate were used as starting materials. The free sintering method was used for the final densification of ceramics. The ceramic samples were characterized in terms of their crystalline structure (X-ray diffraction), microstructure (scanning electron microscopy), chemical composition (energy dispersive spectroscopy), and dielectric properties.

Key words: (Ba0.6Sr0.4)TiO3; perovskite; ferroelectric ceramics; sol-gel method; X-ray studies; electric permittivity

1. Introduction

Among various ferroelectrics, oxides crystallizing in the perovskite structure are of particular importance [1, 2]. The perovskite structure is a relatively simple crystal structure that has the ABO₃ stoichiometry. Typically, the A-site cation is large (e.g., rare earth cation), and is coordinated by 12 anions in the lattice. The B-site ca-tion is typically smaller – frequently it is a transica-tion metal, being six-coordinated, forming BO₆ octahedral. It is usually through distortions of such octahedral that devia-tions from perfect cubic symmetry occur.

Perovskites are known to exhibit a lot of useful properties [3]. Thanks to their high dielectric coefficients over a wide temperature and frequency range, they are used as dielectrics in integrated or surface mounted device capacitors. The remarkable piezoe-lectric effect is applied in a variety of electromechanical sensors, actuators and trans-ducers [4]. Infrared sensors need a high pyroelectric coefficient which is available with __________

B.WODECKA-DUŚ et al. 792

this class of materials [5]. Tunable thermistor properties in semiconducting ferroelectrics are used in positive temperature coefficient resistors (PTCR) [6]. Significant non-linearities in mechanical behaviour, field tunable permittivity and refractive indices, and electrostrictive effects open up a broad field of various further applications. In addition, there is a growing interest in ferroelectric materials for memory applications as ferroelec-tric dynamic random access memories (FEDRAMs), where the direction of spontaneous polarization is used to store information digitally or as replacement for SiO₂/Si₃N₄ dielec-trics as the storage medium in conventional DRAMs [7, 8], etc.

ABO₃ oxides are very interesting because there are two cation sites which, upon substituting with lower valence cations, lead to a much wider range of possible oxygen ion conducting materials. A number of perovskite oxides are purely ionic conductors and, as such, have been used as solid electrolytes in devices such as solid oxide fuel cells (SOFCs). Of the perovskites investigated to date, lanthanum gallate (LaGaO₃ )-based materials have been found to be suitable for ionic applications [9]. Apart from oxide ion conduction, there are also materials of the perovskite structure conducting protons, such as barium cerate (BaCeO₃), barium zirconate (BaZrO₃), and related [10, 11]. It is conceivable that a perovskite-based proton conducting SOFC could be an attractive competitor to the leading oxide ion conducting systems [12].

Barium strontium titanate Ba_1–xSr_xTiO₃ (BST) which is known to adopt the ABO₃ -type structure, is a continuous solid solution of BaTiO₃ (BTO) and SrTiO₃ (STO) over the whole concentration range. The properties of Ba_1–xSr_xTiO₃ are known to de-pend dramatically on the composition x [13, 14]. Significant material modifications have to be introduced to transform pure components, such as BTO and STO, into for-mulae that have a suitable temperature coefficient of the electric permittivity. Partial substitution of either Ba ions or Ti ions in pure BaTiO₃ is often employed to modify the nature and temperature of the paraelectric-ferroelectric transition for a particular application. SrTiO₃ is usually added as a shifter in order to move the Curie point T_C to lower temperatures. It is well established that T_C of barium titanate decreases linearly with the amount of Sr2+ in place of Ba2+. For bulk Ba_1–x,Sr_xTiO₃ ceramics, the Curie point varies from 120 ºC to –240 ºC, whereas the relative electric permittivity (meas-ured at room temperature and electric field of frequency ν = 1kHz) decreases from ε' ~2000 to ε' ~300 for x from x = 0.0 to x = 1.0. As a result, the transition temperature, and hence the electrical and optical properties of Ba_1–xSr_xTiO₃, can be tailored over a broad range to meet the requirements of various electronic applications. The nonli-nearity of its dielectric properties with respect to applied DC voltage makes it attrac-tive for tunable microwave devices such as filters, varactors, delay lines, and phase shifters. The tunable ferroelectric devices offer the advantage of broad tuning range compared to ferrites, reduced resistive losses compared to p-n junction varactor dio-des, reciprocity, and fast switching times [15].

It is worth noting that BST bulk ceramics are mainly obtained by the conventional mixed oxide method (solid state reaction) [13, 14, 16], whereas the sol-gel method is rather rarely used for ceramics fabrication. Therefore, the goal of the present study

The sol-gel synthesis of barium strontium titanate ceramics 793

was to apply the sol-gel method for preparation of BST powders and utilize the sol-gel derived ceramic powders for sintering bulk ceramic materials with the Ba/Sr ratio of 60/40 (BST60/40). The obtained BST ceramics were characterized in terms of the crystalline structure, microstructure, chemical composition, and dielectric properties within the temperature region of the ferroelectric-paraelectric phase transition.

2. Experimental

Ceramic samples of the chemical composition BaO_0.6Sr_0.4TiO₃ were prepared by the free sintering method from the sol-gel derived powders. The sol-gel process used is shown schematically in Fig. 1.

Fig. 1. Flow diagram of the synthesis of BST60/40 powders by the sol-gel method

The starting materials were barium acetate (Ba(CH₃COO)₂, 99%), strontium ace-tate (Sr(CH₃COO)₂, 99%), and tetra-butyl titanate (Ti(OC₄H₉)₄, 97%). Glacial acetic acid (CH₃COOH, 99.9%) and butyl alcohol (C₄H₉OH, 99.9%) were used as solvents. The barium acetate and strontium acetate were dissolved in acetic acid and refluxed at 100 °C

B.WODECKA-DUŚ et al. 794

for 0.5 h. Tetra-butyl titanate was mixed in butyl alcohol. After cooling down to room temperature, the Ba–Sr solution was mixed with Ti solution with a magnetic blender for 0.5 h. Small amounts of acetyloacetone (CH₃COCH₂COCH₃) were added as a stabilizer, followed by hydrolysis. The sol was relatively stable and gelated in a few days. Dry gel was calcinated at 850 °C for 3 h.

The dry gel was analyzed by the thermogravimetric analysis (TGA) and by diffe-rential thermal analysis (DTA). Simultaneous measurements were performed in air with a derivatograph of Q-1500D type (Paulik–Paulik–Erdey system). The BST pow-der was milled and die-pressed into 2 mm thick disks of the diameter of 10 mm unpow-der 300 MPa. The samples were sintered at 1350 ºC and at 1450 ºC for 4 h. The heating rate K for each firing was 3 ºC/min which has been reported to be slow enough to pro-duce high density ceramics with small phase transition broadening [17].

The crystal structure was determined with an X-ray diffractometer (XRD, Philips PW 3710, CoK_α1α2 radiation). Microstructure was studied with a scanning electron microscope (SEM, Philips XL 30, ESEM/TMP, “Centaurus” BSE detector). The stoichiometry of BST ceramics was investigated using the chemical composition analysis system (EDS). All characterizations were carried out at room temperature.

For electrical measurements, the end surface electrodes were prepared using con-ducting silver paste. The dielectric properties were determined with the impedance analyzer HP41192A within the frequency range 100 Hz–1 MHz. Temperature depen-dence of the real (ε′) and imaginary (ε′′) part of electric permittivity was measured within the temperature range from –100 ºC to 100 ºC.

3. Results and discussion

The X-ray diffraction profiles of BST60/40 ceramic powders (Fig. 2) represent the influence of thermal treatment on 110 and 220 reflection of the perovskite cubic ele-mentary cell. The peaks corresponding to BST60/40 gel after calcination at 850 ºC (Fig. 2a, d) are broad and the application of Pearson VII function for the profile fitting can lead to the deconvolution of components. On the other hand, fitting profiles of the peaks corresponding to the finally sintered ceramics (Fig. 2c, f) revealed that a good crystalline ceramic was already obtained (Fig. 3).

A careful examination of the XRD reflection intensities further indicates that no preferred orientation could be found for any sample. The lattice parameters for BST60/40 were calculated for cubic phase using the Rietveld refinement [18], embed-ded into the computer programme PowderCell 2.4 [19]. A model structure used for diffraction pattern fitting exhibited the space group Pm3m (SG number: 221) (Fig. 3). Detailed information about the model structure used is given in Table 1, whereas the details of calculated X-ray spectrum are given in Table 2.

R-values of the Rietveld analysis obtained under the assumption of Pm3m space group (setting 1) are as follows: Rp = 13.72%, Rwp = 19.44%, Re = 3.67%. For fitting

The sol-gel synthesis of barium strontium titanate ceramics 795

the diffraction profile, the pseudoVoigt2 function was used and the following parame-ters were obtained: u = 0.1755, v = 0.0022, w = 0.0213. The calculated unit cell para-meter a for the supposed cubic symmetry space group Pm3m was determined to be 3.962(3) Ǻ.

Fig. 2. Results of the profile fitting for diffraction lines 110 (a, b, c) and 220 (d, e, f) for BST gel after calcination (a, d), powdered BST ceramics after sintering at 1350 ºC (b, e), and at 1450 ºC (c, f)

Although numerical criteria of appropriateness of the fit (i.e. R-values) are very important, it is necessary to point out that they do not fully reflect the quality of fit-ting. Graphical criteria such as plots of the calculated and observed intensities, as well as a plot of the difference between the calculated and observed intensities are also necessary. From the trace on the bottom of Fig. 3, one can see that there are no gross errors of fitting coming from bad scaling parameters or incorrect crystalline structure used for simulation or incorrect unit cell parameters.

B.WODECKA-DUŚ et al. 796

Fig. 3. Results of the X-ray pattern fitting for BST60/40 ceramics (circles – observed pattern, solid line – calculated pattern according to Pm3m space group) sintered at 1450 ºC. The trace on the

bottom is a plot of the difference between the calculated and observed intensities Table.1. Parameters of the model structure used for the XRD pattern fitting (Fig. 4)*

Space group number 221 Atoms in a unit cell ^{4.9, 5}

gen. positions

Space group P m4/ 32/m Volume of a cell 62.18Å3

Cell choice 1 Relative mass of a unit cell 211.64

Lattice parameter s 3.962(3) Å, 3.962(3) Å, 3.962(3) Å X-ray density 5.6519 g/cm3

Angles 90.00º, 90.00º, 90.00º Mass absorption coefficient 451.35 1/g·cm Atoms in an

asymmetric unit ³

Atom P. No. Ion Wyck. x y z SOF B

Ba1 56 Ba2+ 1b 0.5000 0.5000 0.5000 0.6000 0.0000

Sr1 38 Sr2+ 0.4000 0.0000

Ti1 22 Ti4+ 1a 0.0000 0.0000 0.0000 1.0000 0.0000

O1 8 O2– 3d 0.5000 0.0000 0.0000 0.9643 0.0000

*Abbreviations: P. No. – atomic number, Wyck. – Wyckoff position, SOF – site occupation factor, B – temperature factor,

x, y, z – relative atomic coordinates.

Table 2. Details of the calculated X-ray spectrum (Fig. 3) Source X-ray Co Kα1+2 1.789007 Å, 1.792892 Å (α2/α1 = 0.497) 2Θ 5.005º - 84.995º

Geometry Bragg–Brentano, fixed slit, anomalous dispersion

hkl 2Θ [deg] dhkl [Å] I [rel.] F(hkl) Mu FWHM 100 26.094 3.96231 13.73 17.27 6 0.1768 110 37.237 2.80178 100.00 48.34 12 0.2049 111 46.035 2.28764 25.89 38.17 8 0.2322 200 53.681 1.98116 37.89 63.50 6 0.2595 210 60.636 1.77200 6.93 15.58 24 0.2874 211 67.143 1.61761 38.42 41.00 24 0.3163 220 79.364 1.40089 22.23 51.64 12 0.3794

The sol-gel synthesis of barium strontium titanate ceramics 797

Point analysis of the chemical composition in the microarea for BST60/40 ceram-ics was studied by EDS. Stoichiometric ratios of the main metallic components of BST60/40 ceramics recalculated to simple oxides are as follows (in mass %): BaO – 43.125%, SrO – 19.430%, TiO₂ – 37.445%. Results of the measurement, given in Fig. 4, have shown the following chemical composition: BaO – 43.98%, SrO – 18.92%, TiO₂ – 37.10%. Thus, the conservation of the chemical composition of the BST60/40 ceramics was proved and the accuracy higher than ±3% was found.

Fig. 4. Results of EDS analysis of BST60/40 ceramics (left) and the SEM image of the surface taken at a magnification 3183× (right)

A typical SEM micrograph of as-sintered surface of BST60/40 ceramic sample sintered at 1350 ºC is shown in Fig. 4. A noticeable difference in grain size was ob-served. The smallest grain size is of about 1 μ whereas the largest grain size is about 10 μm. It is worth noting that the low heating rate used in our experiment (3 ºC/min) for sintering was favourable to the grain growth.

Fig. 5. Dependence of the real part ε' (a) and imaginary part ε”(b) of the electric permittivity on temperature for BST60/40 ceramics sintered at 1450 ºC

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Figure 5 shows the temperature dependences of electric permittivity (ε′) and the coef-ficient of dielectric losses (ε′′) for BST60/40 specimen sintered at 1450 ºC for 4 h for both cooling and heating processes. One can see from Fig. 5a that the real part of electric per-mittivity ε' shows some peculiarities. It reaches its maximum at temperature T1(ε_max′ ) = 10.2 ºC while cooling, whereas temperature of maximum ε′ during the heating cycle is shifted towards higher temperatures and amounts to T₂(ε_max′ ) = 11.8 ºC. It is worth noting that temperature of ε_max′ proves indirectly the molar ratio of Ba/Sr = 60/40 [13, 14].

Fig. 6. Temperature dependence of the reciprocal permittivity 1/ε' at 100 kHz of BST60/40 ceramics for the heating (a) and cooling (b) cycles. The solid and dashed lines represent fittings to the Curie–Weiss law

Fitting the temperature dependence of electric permittivity ε′ to the Curie–Weiss law made it possible to determine the Curie–Weiss temperature (θ) and the Curie –Weiss constant (C). It can be seen from Fig. 6 that for temperature T > T(ε_max′ ) two sets of parameters θ1, C1 and θ2, C2 were found for the heating (Fig. 6a) and cooling (Fig. 6b) cycles, respectively. The values of the parameters were as follows: θ1 = –1.95 ºC, C₁ = 1.09×105 K–1 and θ2 = 3.45 ºC, C₂ = 0.91×105 K–1 during the heating cycle, whereas for the cooling cycle: θ1 = 1.40 ºC, C₁ = 1.05×105 K–1 and θ2 = –21.84 ºC, C2 = 1.74×105 K^–1.

3. Conclusions

The sol-gel method was successfully employed for the synthesis of BST60/40 ce-ramic powder from barium acetate, strontium acetate and tetra-butyl titanate. Free sintering was used for the final sintering of ceramic samples. It was found that good crystalline ceramic samples exhibiting the regular symmetry of Pm3m space group were obtained after sintering at 1450 ºC for 4 hours. Analysis of the chemical compo-sition of the final ceramics have proved the conservation of stoichiometry with an accuracy better than ±3%. Temperature dependence of electric permittivity (ε’) was

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studied in the temperature range of ferroelectric–paraelectric phase transition. It was found that the temperature of electric permittivity maximum value T(ε_max′ ) fits well the Ba/Sr ion ratio. The Curie-Weiss temperature θ as well as the Curie-Weiss con-stant C were determined for BST60/40 ceramics from the dielectric measurements.

Acknowledgement

The authors wish to acknowledge the financial support of the Polish Ministry of Education and Sci-ence from the funds for sciSci-ence in 2006-2009, as research project N507 098 31/2319.

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Received 22 June 2006 Revised 12 January 2007

Materials Science-Poland, Vol. 25, No. 3, 2007

Phenomena responsible for energy dissipation