This article was downloaded by: [Akademia Pedagogiczna], [I. Jankowska-Sumara] On: 14 April 2015, At: 00:34 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gpht20 Composition-related structural, thermal and mechanical properties of Ba 1-x Sr x TiO 3 ceramics (0 x 0.4) W. Śmiga a, D. Sitko b, W. Piekarczyk c, I. Jankowska-Sumara b & M. Kalvane d a Institute of Technology, Pedagogical University, Kraków, Poland b Institute of Physics, Pedagogical University, Kraków, Poland Click for updates c Faculty of Materials Science and Ceramics, AGH-University of Science and Technology, Kraków, Poland d Institute of Solid State Physics, University of Latvia, Riga, Latvia Published online: 08 Apr 2015. To cite this article: W. Śmiga, D. Sitko, W. Piekarczyk, I. Jankowska-Sumara & M. Kalvane (2015): Composition-related structural, thermal and mechanical properties of Ba 1-x Sr x TiO 3 ceramics (0 x 0.4), Phase Transitions: A Multinational Journal, DOI: 10.1080/01411594.2015.1020310 To link to this article: http://dx.doi.org/10.1080/01411594.2015.1020310 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
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Phase Transitions, 2015 http://dx.doi.org/10.1080/01411594.2015.1020310 Composition-related structural, thermal and mechanical properties of Ba 1 x Sr x TiO 3 ceramics (0 x 0.4) W. Smiga a, D. Sitko b *, W. Piekarczyk c, I. Jankowska-Sumara b and M. Kalvane d a Institute of Technology, Pedagogical University, Krakow, Poland; b Institute of Physics, Pedagogical University, Krakow, Poland; c Faculty of Materials Science and Ceramics, AGH- University of Science and Technology, Krakow, Poland; d Institute of Solid State Physics, University of Latvia, Riga, Latvia (Received 17 December 2014; accepted 10 February 2015) The Ba 1 x Sr x TiO 3 (BST) ceramics were prepared by conventional ceramic method. The crystalline structure and morphology were studied by X-ray diffraction and scanning electron microscopy, respectively. Experimental results show that increase of sintering temperature leads to an uncontrolled precipitating of the phase with a lower content of Ti. Thedielectricconstantandspecificheatasafunction of composition and temperature were investigated. The increasing concentration of Sr ions leads to a shift of the Curie point below room temperature. To determine the elastic constants (the Young s modulus E, the shear modulus G and the Poisson s ratio v) ofbst,amethodofmeasurementof the longitudinal (n L ) and transverse (n T ) ultrasonic wave velocities for this type of material was developed. The structural, dielectric and mechanical properties of BST ceramics were discussed in terms of microstructure and chemical composition Keywords: ceramics; ferroelectricity; dielectric properties; elastic properties 1. Introduction Polycrystalline barium titanate BaTiO 3 (BT) for a long time has been widely used for highdielectric capacitors because of its very high dielectric constant at the Curie point (T c D 403 K).[1] Barium substitution by lead or strontium is used to raise or lower the Curie point in several applications. In order to reduce T c, strontium titanate SrTiO 3 has been associated with BaTiO 3 forming a well-known solid solution Ba 1 x Sr x TiO 3 (BST).[2] From practical point of view, barium strontium titanate ferroelectric solid solution is one of the attractive candidate materials for electronic applications such as capacitors with high dielectric constant, dynamic random access memories or tunable frequency devices. The dielectric properties of the BST compositions in the form of both ceramics [3-8] and single crystals [9] have been investigated since mid-1940s. However, much attention has been paid to the compositional effects on the Curie temperature and dielectric properties of BST ceramics.[10-12] Pure BaTiO 3 ceramic undergoes a para-ferro phase transition at 403 K from cubic (C) to tetragonal (T) phase, as a typical first-order phase transition of displacement type.[1] Other ferroelectric transitions around 278 K (or 273 K) from tetragonal (T) to orthorhombic (O) and the other, around 153 K (up to 173 K) between orthorhombic (O) and rhombohedral (R), also appear as first-order transitions. According to many publications,[11,13-15] it was indicated that optimal dielectric properties of BST occur near the composition x D 0.3, particularly that BST ceramics *Corresponding author. Email: dsitko@up.krakow.pl Ó 2015 Taylor & Francis
2 W. Smiga et al. exhibit a maximum dielectric constant of approximately 8000 at x D 0.3 at room temperature. The objective of this paper is, therefore, to investigate the effects of processing conditions during BST powder synthesis for microstructural, thermal, dielectric and mechanical properties of BST. Hence, we have prepared a set of polycrystalline BST ceramics in the composition range around x D 0.3, i.e., 0 x 0.4. 2. Experimental details Polycrystalline samples of Ba 1 x Sr x TiO 3 with x D 0, 0.1, 0.2, 0.3 and 0.4 were made by the conventional state reaction technology. The synthesis conditions were maintained to be the same for all samples in order to be able to compare their properties. The starting materials were TiO 2, SrCO 3 and BaCO 3 (Aldrich 99.9%). These were mixed in a ball mill with ethanol, then dried and calcined in alumina crucible. The synthesis temperature was 1573 K, for 2 hours. After that and before sintering the pellets were pressed at the pressure 100 MPa. During the time of sintering at the temperature of 1733 K (optionally 1773 K) the samples followed with simultaneous densification of the material. The X-ray diffraction (XRD) analysis was carried out on powders for phase identification and lattice parameter measurements. The measurements were performed using X Pert PRO diffractometer (PANalytical) using a goniometer filtered CuK-a radiation (λ D 0.154178 nm). Conditions for each measurement were as follows: angular range from 10 to 100 by 2 Q, work in steps of 0.01. To study the microstructure of the ceramic samples, a scanning electron microscope (SEM) Hitachi S-4700 coupled with an energy dispersive spectrometer (EDS) NORAN Vantage was used. An electron scanning microscope had also the ability to analyze the chemical composition of the sample surface with the help of EDS spectroscopy. The dielectric properties were measured using a HP 4363 LCR meter at an applied voltage of 1 V. Silver paint was used as an electrode of material. The measurement frequencies were varied from 100 Hz to 100 khz in the temperature range from 140 to 450 K. Thermal properties were studied using a Netzsch DSC F3 Maia calorimeter in the temperature range from 120 to 820 K under argon atmosphere at the flow rate 30 ml/min. The specimen consisted of a single piece of crystal of average mass 20 mg placed in an alumina crucible. The data were collected both on heating and cooling processes. The study of the mechanical properties was carried out in order to check how the material will work under load, because the load has an impact on the durability of a device. In technical applications, the elastic properties of the material are described by the following constants: Young s modulus E, shear modulus G, and Poisson s ratio n. Measurements of material constants were carried out using the ultrasound method on a prototype device PPO-1 produced by INCO-VERITAS. Two transducers with 1 and 2 MHz frequency were used to measure the velocity of longitudinal v L and transverse v T sound waves, respectively. Values of material constants were calculated knowing the velocity of the longitudinal and transverse ultrasonic wave propagation and the density of the samples using the formulas E D n2 Lrð1 nþð1 2nÞ ; 1 n G D n 2 T r; n D n2 L 2n2 T 2n 2 L ; 2n2 T (1)
Phase Transitions 3 where v L is the velocity of longitudinal wave, v T is the velocity of transverse wave, r is the density of specimen measured by the Archimedes method, E is the Young s modulus and G is the shear modulus and n is the Poisson ratio. 3. Results and discussion 3.1. Structure and composition The previous study showed that pure BT samples require the temperature of sintering as high as 1673 K.[13] It was also established that the addition of Sr C2 requires a higher sintering temperature. The SEM morphologies of BST samples sintered at two temperatures: 1733 and 1773 K are shown in Figure 1(a) and 1(b), respectively. Generally, the densities of ferroelectric ceramics increase with increasing sintering temperature; however, in case of BST, the increase of sintering temperature above 1773 K leads to an uncontrolled precipitating of the phase with a lower content of Ti which is illustrated in the inset of Figure 1(b). Moreover, elevated temperature of sintering also leads to an abnormal grain growth (so-called secondary grain growth). Figure 1. Elemental composition from EDS measurements for BST-0.4 sample sintered at 1733 K (a) and 1773 K (b) together with SEM microphotograph of the sample surface.
4 W. Smiga et al. Figure 2. SEM microphotographs of the surface of ceramics with various x content. The images were taken at a magnification of 1000 : (a) x D 0, (b) x D 0.2, (c) x D 0.3, (d) x D 0.4. (GF indicates the fracture through the grain and IF the intergranular fracture.) In Figure 2, SEM images show that the surfaces of fracture run on both the grain and the inter-granular boundaries. It can be noticed that ceramics are well sintered with a crispy breakthrough. The problem of overgrown grains clearly concerns the samples with x D 0.3 and 0.4 which is illustrated in Figure 2(c) and 2(d). For most ferroelectric ceramics, there is a critical grain size below which their properties can be well developed and also a critical grain size beyond which most properties are saturated, i.e., the properties of ferroelectric materials do not always increase with grain size.[16] Both critical sizes depend on types of materials or compositions of given materials. In contrast to many other ferroelectrics, there is an anomalously high dielectric constant for BaTiO 3 ceramics of fine grains, which has not been fully understood till now. Several models, including the presence of internal stresses in fine-grained ceramics, which are due to the absence of 90 domain walls, increased domain wall contributions to the dielectric response in fine-grained ceramics, and shifts of the phase transition temperatures with grain size, have been suggested to explain this phenomenon.[17-19] The crystal structure of the obtained ceramics was examined using XRD analysis at room temperature and was assigned to be a tetragonal perovskite structure. The lattice constants a and c calculated from XRD patterns are presented in Figure 3. Analysis of plots leads to the conclusion that with the increase in the content of strontium from x D 0 to 0.3 the lattice constants a and c decrease in their values whereas for higher concentration of strontium (x D 0.4) an increase in the value of a (for this concentration, cubic phase is observed at 300 K) was observed. Thus, the
Phase Transitions 5 0,404 a,c (nm) c 0,402 0,400 a 0,398 0,396 composition x 0,0 0,1 0,2 0,3 0,4 Figure 3. Dependence of the lattice parameters a and c on concentration of strontium in the solid solution Ba 1 x Sr x TiO 3 for 0 x 0.4 (at T D 300 K). composition x D 0.3 in which structural anomaly was observed seems to play an important role for further characterization. 3.2. Dielectric properties The addition of Sr C2 in BT is known to increase the dielectric constant; however, a reduction of grain size should also play an important role in the improvement of dielectric constant.[20] Since grain boundary and porosity are the structural discontinuity, they are a natural barrier for electric dipoles to align in a parallel manner when an external electric field is applied. Figure 4 shows a dielectric constant of BST samples with different compositions. Dielectric peaks observed in the e 0 (T) graphs show typical ferroelectric behavior in all samples. From these peaks, the ferroelectric to paraelectric phase transition temperature (T emax ) and the other transitions can be determined (Table 1). Two (or three) peaks on e(t) dependence evidently correspond to the phase transformations that occur in BST, and the dielectric constant peaks are relatively sharp as would be expected for a normal ferroelectric material; however, for x > 0.2 the peaks become slightly diffuse. The highest dielectric constant was observed for pure BaTiO 3 ceramic, whereas for the BST samples the dielectric constant is considerably lower showing local minimum near composition x D 0.3. Unexpected low value of dielectric constant for BST 0.3 is most probably correlated to irregular grain size and existence of relatively large porosity in conjunction with the SEM observations presented in Figure 2. This experimental fact clearly indicates that a densely packed and low-void structure (like for pure BaTiO 3 )is essential for achieving good dielectric properties and thus shows how important the control of the sintering and process conditions is in achieving proper microstructure and in consequence the best dielectric properties. For BST compositions, the dielectric peak slightly broadens, and this broadening may be due to micro-compositional fluctuations resulting in different local transition temperatures which also results in decrease in dielectric maxima. The dielectric permittivity of the BST ceramics with x D 0.2 and 0.4 for a few frequencies is shown in Figure 5(a) and 5(b).
6 W. Smiga et al. ε' 7000 x=0 6000 x=0,2 5000 4000 3000 x=0,4 x=0,3 2000 1000 0 200 300 400 T (K) 500 Figure 4. Temperature dependences of the real part of e 0 (T) for different compositions of BST ceramics measured at the frequency 100 khz. For both samples, the permittivity presents a dispersion in the range of 100 Hz 100 khz. The composition x D 0.2 (Figure 5(a)) shows a permittivity vs. temperature dependence with a slightly diffuse character. The composition x D 0.4 has a more diffuse phase transition with a higher shift of the temperature corresponding to the permittivity maximum T m with increasing frequency. As expected for higher Sr content in BST, the relaxor character of phase transition is predominant. 3.3. Differential scanning calorimetry The measurements of specific heat in the range 120 820 K are presented in Figure 6.The anomalies observed as peaks on c p (T) dependencies correspond to the temperatures of phase transitions. Sharp and narrowly centered peaks are typical for the I-order phase transitions, however gradual decreasing of the transition energies DE together with increasing of Sr content suggest the transformation of the phase trasition character from sharp to diffuse. In the cubic phase above T c, the heat capacity reaches values close to 125 J/mol K. This value is nearly identical to a classical Dulong Petit limit c v c p D 3nK B N A D 124.7 J/mol K, where n is the number of atoms per unit cell, K B is the Boltzmann s constant Table 1. Temperatures of dielectric peaks corresponding to successive phase transitions in BST ceramics. T 1 T 2 T c Composition Heating Cooling Heating Cooling Heating Cooling x D 0 197 K 182 K 290 K 282 K 403 K 402 K x D 0.2 176 K 162 K 249 K 242 K 345 K 344 K x D 0.3 171 K 161 K 226 K 221 K 310 K 311 K x D 0.4 215 K 206 K 278 K 279 K
Phase Transitions 7 5000 ε' a) BST 0.2 5000 ε' b) BST 0.4 4000 4000 100 Hz 3000 1 khz 10 khz 100 khz 3000 100 Hz 1 khz 10 khz 100 khz 2000 270 275 280 285 Figure 5. T (K) 2000 335 340 345 350 T (K) Behavior of the dielectric constant for x D 0.2 and 0.4 with temperature and frequency. and N A is the Avogadro number. The small deviations from this value are most probably associated with a concentration of point defects and observed porosity. The calculated values of latent heat and entropies associated with the phase transitions in the function of composition are presented in Table 2. 3.4. Mechanical properties Mechanical properties as well as dielectric properties strongly depend on microstructure of the ceramic samples. Elastic deformation depends not only on the distance between atoms, type of bonds between the atoms, chemical composition but also on crystalline c p (J/mol*K) 150 100 50 0 150 100 50 150 0 100 50 0 150 100 50 BaTiO 3 BaTi 0,8 Sr 0,2 O 3 BaTi 0,7 Sr 0,3 O 3 BaTi 0,6 Sr 0,4 O 3 0 100 200 300 400 500 600 700 800 T (K) Figure 6. ceramics. Temperature dependences of the specific heat for different compositions of BST
8 W. Smiga et al. Table 2. The parameters of the phase transitions in BST ceramics taken from specific heat measurements. DSC T c (K) DE c (J/mol) T 2 (K) DE 2 (J/mol) T 1 (K) DE 1 (J/mol) BaTiO 3 396.30 345 287.30 156 205.50 103 BaTi 0.8 Sr 0.2 O 3 340.30 267 247.70 98 185.30 48 BaTi 0.7 Sr 0.3 O 3 309.00 244 224.00 92 x x BaTi 0.6 Sr 0.4 O 3 274.10 158 208.70 43 x x structure, structural anisotropy and on the material microstructure. Elastic properties of materials are usually characterized by Young s modulus E, shear modulus G, bulk modulus K and Poisson s ratio v. For isotropic materials, only two of these elastic constants are independent and other constants are calculated by using the relations given by the theory of elasticity. The best choice [21] for these two independent moduli is considered to be the shear modulus and the bulk modulus. The shear modulus, G, relates to strain response of a body to shear or torsional stress. It involves change of shape without change of volume. On the other hand, the bulk modulus, K, describes the strain response of a body to hydrostatic stress involving change in volume without change of shape. However, the best-known elastic constant is Young s modulus, E, which is most commonly used in engineering design. Thus, by using Young s modulus, which is the simplest elastic constant to measure experimentally, all other elastic constants can be evaluated easily. It was established that for polycrystalline metallic materials, a rough proportionality between G and E exists and is given by the relation [22,23], G E 3 8 : (2) The values of the Young s modulus (E) and shear modulus (G) calculated on the basis of Equations (1) in function of composition are presented in Figure 7(a) and the proportionality between E and G in Figure 7(b). G (GPa) 130 120 110 100 90 80 70 a) ν 0.30 0.27 0.24 0.0 0.1 0.2 0.3 0.4 x composition x 0.0 0.1 0.2 0.3 0.4 E (GPa) 50 45 40 35 30 25 E (GPa) 50 b) x=0,4 40 30 20 10 x=0,2 x=0,1 x=0 x=0,3 theory 0 0 20 40 60 80 100 120 G (GPa) Figure 7. The dependence of Young s modulus (E), shear modulus (G) and Poisson s ratio n (inset) on Sr content in the BST solid solution at room temperature (a). Variation of the shear modulus G with respect to Young s modulus E for different compositions (b).
Phase Transitions 9 The mechanical properties showed that the elastic modules are correlated simultaneously to the lattice constants and the microstructure of the material whereas Poisson s ratio seems to depend only on the microstructure. Poisson s ratio decreases markedly for the compositions with large porosity pointing to an increase in brittleness of the ceramic structure. The nature of variation of G with E as shown in Figure 7 seems to indicate that a similar relation like Equation (2) may also exist for oxide ceramics whether they are dense or porous. 4. Conclusions The effect of Sr ion at A-site in BaTiO 3 ferroelectric compound on its structural, mechanical and dielectric behavior has been studied. Sintering conditions and their influence on microstructure of the sample were also studied. For this reason, the polycrystalline single phase of Ba 1 x Sr x TiO 3 (x D 0, 0.1, 0.2, 0.3, 0.4) samples was successfully prepared by using solid-state reaction route. The XRD measurements confirmed the perovskite structure in the sintered samples. Specific heat measurements confirmed the correct sequence of phase transitions characteristic for this solid solution. Close relation between the microstructure and both dielectric and mechanical properties has been observed. The SEM photographs indicated that the microstructure changes dramatically with increase of Sr content and also with the temperature of synthesis, showing the abnormal grain growth with an inevitable increase in porosity of the sample. It is commonly known that the presence of exaggeratedly grown grains is harmful to the performance of most ferroelectric ceramics. As expected, large grains caused deterioration of both dielectric and mechanical properties. Despite significant porosity of BST ceramics the material constants E and G are roughly related by the relation E/G D 0.39.[23] This confirms that the samples are fully isotropic. Disclosure statement No potential conflict of interest was reported by the authors. Funding This research was partially financed by the European Regional Development Fund under the Infrastructure and Environment Programme [grant number UDA-POIS.13.01-023/09-00]. References [1] Jona F, Shirane G. Ferroelectric crystals. Pergamon: Oxford; 1962. [2] Jaffe B, Cook WR, Jaffe H. Ferroelectric ceramics. New York: Academic Press; 1971. [3] Jackson W, Reddish W. High permittivity crystalline aggregates. Nature. 1945;156:717 717. [4] Roberts S. Dielectric constants and polarizabilities of ions in simple crystals and barium titanate. Phys Rev. 1947;71:890 895. [5] Davis L Jr, Rubin LG. Some dielectric properties of barium-strontium titanate ceramics at 3000 megacycles. J Appl Phys. 1953;24:1194 1197. [6] Barb D, Barbulescu E, Barbulescu A. Diffuse phase transitions and ferroelectric-paraelectric diagram for the BaTiO 3 -SrTiO 3 system. Phys Stat Sol (a). 1978;74:79 83. [7] Liou JW, Chiou BS. Dielectric characteristics of doped Ba 1 x Sr x TiO 3 at the paraelectric state. Mater Chem Phys. 1997;51:59 63.
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