Investigation of Some Structural and Mechanical Properties of Ba_0.5Ca_xSr_0.5-xTiO₃ Ceramics

Lubna M. Sharaf El-Deen, Mohamed H. Badr, Abdel-Mageed H. Khafagy, Dalia U. Nassar

Physics Department, Faculty of Science, Minufiya University, Shibin El Kom, Egypt.

DOI: 10.4236/csta.2013.23018                

Abstract

Ba_0.5Ca_xSr_0.5-xTiO₃ (BCST) ceramics, where x = 0, 0.1, 0.2, 0.3 and 0.4, were prepared by the conventional solid state reaction technique. X-ray diffraction (XRD) analysis confirmed the formation of BST perovskite phase structure besides some calcium oxide peaks for samples with high Ca content, x. Scanning electron microscopy (SEM) results confirmed the XRD results, i.e., as x increased, the average grain size decreased. Energy dispersive X-ray (EDX) analysis verified the increase of the amount of Ca element with increasing of its content. Mechanical properties such as ultrasonic attenuation, longitudinal wave velocity, and longitudinal elastic modulus were studied by an ultrasonic pulse echo technique at 2 MHz frequency. Investigations of ceramic microstructures and mechanical properties showed their dependence on composition. Increasing of Ca content resulted in a decrease in bulk density and ultrasonic attenuation and an increase in porosity, velocity, and modulus. High temperature ultrasonic studies showed, in addition to Curie phase transition, three or more relaxation peaks and its origin was investigated.

Keywords

BST Ceramics; BCST Ceramics; Phase Transition; Ultrasonic Attenuation; SEM

1. Introduction

BaTiO₃ and SrTiO₃ are representatives for ABO₃-type perovskite materials. BaTiO₃ is usually a ferroelectric material with Curie temperature of 120˚C. SrTiO₃ is a paraelectric one with no ferroelectric phase transition [1]. Nevertheless, the combined production of BaTiO₃ and SrTiO₃ (i.e., Ba_1−xSr_xTiO₃) is a solid solution system between BaTiO₃ and SrTiO₃. Therefore, Ba_1−xSr_xTiO₃ (BST) has the simultaneous advantage of high dielectric constant of BaTiO₃ and the structural stability of SrTiO₃ [1,2]. These ferroelectric materials have attracted considerable attention owing to their unique properties such as chemical stability, high permittivity, high tunability, and low dielectric losses. Furthermore, BST has shown a great promise in applications, such as phase shifting elements in phased array antennas and as tuning elements in devices operating at microwave frequencies [1,3-9]. In view of their merits, the investigation on BST solid solution is therefore significantly important [1,2,9,10]. Furthermore, the Curie temperature of BST can be controlled by adjusting the Ba/Sr ratio and/or doping ions to substitute for A or B sites in the ABO₃ perovskite systems [11,12].

In BaTiO₃ materials, an elastic modulus anomaly and a mechanical loss peak are induced at three phase transitions: cubic-tetragonal (ferroelectric-paraelectric), tetragonal-orthorhombic, and orthorhombic-rhombohedra. Some losses due to relaxation processes have been observed in materials having coarse grains in ferroelectric phase [13]. These relaxations were ascribed to the interaction between domain walls and oxygen vacancies diffusion.

The Ba_0.5Sr_0.5TiO₃ (BST) material exhibits the transition from the ferroelectric state to the paraelectric state below room temperature [14]. In a recent work [15], we have investigated the impact of changing barium content on the mechanical properties (such as elastic modulus, attenuation, and velocity of ultrasonic waves) and Curie transition of Ba_xSr_1−xTiO₃ ceramics.  In this work, we aim to investigate thoroughly the effect of Ca-doping on the structural and mechanical properties of Ba_0.5Ca_xSr_0.5−xTiO₃ (BCST) ceramics with x = 0.0, 0.1, 0.2, 0.3 and 0.4. X-ray diffraction, SEM, EDX, and ultrasonic techniques (at a frequency of 2 MHz) were also used to characterize the structure and phase transitions of these ceramics.

2. Materials and Methods

Calcium-doped ceramics with the chemical formula Ba_0.5Ca_xSr_0.5−xTiO₃ (BCST), where x = 0.0, 0.1, 0.2, 0.3, and 0.4, were prepared by the conventional solid state reaction technique according to the following reaction:

For all prepared samples, the reagent grade chemicals of high purity (99.99%) BaCO₃, SrCO₃, CaO and TiO₂ powders were used as the raw materials and weighed according to the above indicated compositions.

The raw materials were weighed and mixed in the appropriate ratios. Mixtures of required ceramics were first ground thoroughly by a ball milling for 4 h to insure homogeneity. Then, they were calcined at 1100˚C for 11 h in alumina crucibles opened to the air. The calcined compositions were again ground for 6 h. The produced fine calcinated powders were pressed into disc-shaped pellets with 10mm in diameter and 0.6 ~ 1.5 mm in thickness at an iso-static pressure of 5 tons with polyethylene glycol [(C₂H₄O)_n.H₂O] as an organic binder with 2.0% of the weight of the sample. The pelletized samples were finally sintered at 1250˚C for 6h.

The bulk density of ceramics was measured by the conventional Archimedean method. X-ray diffraction (XRD) patterns were recorded with Bruker AXS X-ray diffractometer (D8 Advance) using Cu Kα radiation. The two SEM and EDX measurements were made on JEOL scanning electron microscope (JXA-840A Electron Probe Microanalyzer, INCA x-sight, Oxford Instruments) for the elemental analysis and chemical characterization of the samples. Attenuation of ultrasonic waves (α), longitudinal velocity of wave propagation (V_L), and the longitudinal elastic modulus (L) of tested ceramics were determined by employing the conventional pulse-echo technique at room temperature and during heating as has been reported elsewhere [15].

3. Results

The Archimedean bulk density (ρ_exp) and the theoretical density (ρ_x) from the X-ray diffraction patterns of BCST prepared samples were used  to calculate the percentage porosity (Table 1) according to the equation % porosity = (ρ_x − ρ_exp)/ρ_x [16]. Figure 1 shows variations of both the bulk density and porosity of prepared BCST ceramics with the Ca content x where 0.0 ≤ x ≤ 0.4. This figure revealed a linear decease in the density from 4803 to 4339 kg/m³ as x increased from 0 to 0.4, in harmony with previously reported results [17-19]. Whereas, the percent porosity has increased linearly from 13.4% to 16.5%, for the same variation in Ca content.

Figure 2 shows the room temperature X-ray diffract-graphs for all tested BCST ceramics between 20˚ and 80˚. The intensity of peaks for different values of Ca content, x, were normalized and shifted for clarity pur-

Table 1. The variation of the density (bulk density, ρ_exp, and theoretical density, ρ_x) the percent porosity, the ultrasonic attenuation, longitudinal velocity, modulus, and lattice parameter of BCST ceramics with different values of Ca content, x.

Figure 1. Variation of the density and porosity with the Ca content of Ba_0.5Ca_xSr_0.5−xTiO₃ ceramics sintered at 1250˚C for 6 h.

Figure 2. XRD patterns of B_0.5Ca_xSr_0.5−xTiO₃ (BCST) ceramics with Ca content x = 0.0, 0.1, 0.2, 0.3 and 0.4.

poses. All peaks were indexed in the cubic structure due to their observed reflections of different polycrystalline orientations [20-22] as indicated by the (110) index; the major peak of highest intensity. Variation of the lattice parameter a (Å) with different Ca content was listed in Table 1.

The chemical compositions of tested BCST ceramics were determined from EDX spectra and listed in Table 2. The scanning electron microscopy (SEM) is a powerful experimental technique to determine the particle size, pore concentration, and inclusions in a material. Figures 3(a)-(d) show the scanning electron micrographs taken at room temperature of BCST ceramics with Ca content in the range 0.0 ≤ x ≤ 0.4. The SEM for x = 0.2 was not included in this figure for brevity reasons. These micrographs describe the surface property of samples, microstructure, size, and distribution of particles. Whereas, the average grain size dependence (as determined from XRD and SEM investigations) on the Ca content was illustrated in Figure 4.

Figure 5 shows the regression line which illustrates the variation of ultrasonic attenuation (α) of ultrasonic waves, measured at room temperature, with the Ca content of the prepared Ba_0.5CaSr_0.5-xTiO₃ ceramics sintered at 1250˚C for 6 h. Inspection of the figure reveals that α is dependent on the composition of the tested ceramic, i.e., it is exponentially decreased with increasing of Ca content over the above mentioned investigated range (see also Table 1).

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	K. Abe and S. Komatsu, “Ferroelectric Properties in Epitaxially Grown BaxSr1-xTiO3 Thin Films,” Journal of Applied Physics, Vol. 77, No. 12, 1995, pp. 6461-6465. doi:10.1063/1.359120
[2]	Z. L. Wang and Z. C. Kang, “Functional and Smart Materials—Structural Evolution and Structural Analysis,” Science Press, Beijing, 2002.
[3]	L. C. Sengupta and S. Sengupta, “Novel Ferroelectric Materials for Phased Array Antennas,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 44, No. 4, 1997, pp. 792-797. doi:10.1109/58.655193
[4]	S. S. Gevorgian and E. L. Kollberg, “Do We Really Need Ferroelectrics in Paraelectric Phase Only in Electrically Controlled Microwave Devices?” IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No. 11, 2001, pp. 2117-2124. doi:10.1109/22.963146
[5]	P. C. Joshi and M. W. Cole, “Mg-doped Ba0.6Sr0.4TiO3 Thin Films for Tunable Microwave Applications,” Applied Physics Letters, Vol. 77, No. 2, 2000, pp. 289-291. doi:10.1063/1.126953
[6]	W. Chang and L. Sengupta, “MgO-mixed Ba0.6Sr0.4TiO3 Bulk Ceramics and Thin Films for Tunable Microwave Applications,” Journal of Applied Physics, Vol. 92, No. 7, 2002, pp. 3941-3946. doi:10.1063/1.1505669
[7]	M. Kuwabara, H. Matsuda and Y. Ohba, “Varistor Characteristics in PTCR-Type (Ba,Sr)TiO3 Ceramics Prepared by Single-Step Firing in Air,” Journal of Materials Science, Vol. 34, No. 11, 1999, pp. 2635-2639. doi:10.1023/A:1004661018287
[8]	J. F. Scott, M. Azuma, E. Fujii, T. Otsuki, G. Kano, M. C. Scott, C. A. Paz de Araujo, L. D. McMillan and T. Roberts, “Microstructure-Induced Schottky Barrier Effects in Barium Strontium Titanate (BST) Thin Films for 16 and 64 Mbit (DRAM cells),” Proceedings of International Symposium on Integrated Ferroelectrics, New York, 1992, p. 356.
[9]	X. Weidong, L. Yanrong and Y. Chun, “First Principle Studies on Fine Structure for BaxSr1-xTiO3,” Chinese Journal of Chemical Physics, Vol. 18, No. 2, 2005, pp. 179-182.
[10]	T. Hu, H. Jantunen, A. Uusimaki and S. J. Leppavuori, “BST Powder with Sol-Gel Process in Tape Casting and Firing,” Journal of the European Ceramic Society, Vol. 24, No. 6, 2004, pp. 1111-1116. doi:10.1016/S0955-2219(03)00427-8
[11]	V. V. Lemanov, “Concentration Dependence of Phonon Mode Frequencies and the GrüNeisen Coefficients in BaxSr1-xTiO3 Solid Solutions,” Physics of the Solid State, Vol. 39, No. 2, 1997, pp. 318-322. doi:10.1134/1.1129842
[12]	B. Jaffe, W. R. Cook and H. Jaffe, “Piezoelectric Ceramics,” Academic Press, London, 1971.
[13]	B. L. Cheng, M. Gabbay, M. Maglione and G. Fantozzi, “Relaxation Motion and Possible Memory of Domain Structures in Barium Titanate Ceramics Studied by Mechanical and Dielectric Losses,” Journal of Electroceramics, Vol. 10, No. 1, 2003, pp. 5-18. doi:10.1023/A:1024007407033
[14]	A. Ioachim, M. I. Toacsan, M. G. Banciu, L. Nedelcu, C. Plapcianu, H. V. Alexandru, C. Berbecaru, D. Ghetu, G. Stoica and R. Ramer, “Frequency Agile BST Materials for Microwave Applications,” Journal of Optoelectronics and Advanced Materials, Vol. 5, No. 5, 2003, pp. 1389-1393.
[15]	M. H. Badr, L. M. Sharaf El-Deen, A. H. Khafagy and D. U. Nassar, “Structural and Mechanical Properties Characterization of Barium Strontium Titanate (BST) Ceramics,” Journal of Electroceramics, Vol. 27, No. 3-4, 2011, pp. 189-196. doi:10.1007/s10832-011-9664-5
[16]	O. P. Thakur, C. Prakash and D. K. Agrawal, “Dielectric Behavior of Ba0.95Sr0.05TiO3 Ceramics Sintered by Microwave,” Materials Science and Engineering: B, Vol. 96, No. 3, 2002, pp. 221-225. doi:10.1016/S0921-5107(02)00159-9
[17]	J. F. Scott, “High-Dielectric Constant Thin Films for Dynamic Random Access Memories (DRAM),” Annual Review of Materials Research, Vol. 28, No. 1, 1998, pp. 79-100. doi:10.1146/annurev.matsci.28.1.79
[18]	C. Berbecaru, H. V. Alexandru, C. Porosnicu, A. Velea, , A. Ioachim, L.Nedelcu and M. Toacsan, “Ceramic Materials Ba(1-x)SrxTiO3 for Electronics—Synthesis and Characterization,” Thin Solid Films, Vol. 516, No. 22, 2008, pp. 8210-8214. doi:10.1016/j.tsf.2008.04.031
[19]	S. Yun, X. Wang, B. Li and D. Xu, “Dielectric Properties Ca-Substituted Barium Strontium Titanate Ferroelectric Ceramics,” Solid State Communications, Vol. 143, No. 10, 2007, pp. 461-465. doi:10.1016/j.ssc.2007.06.031
[20]	V. V. Lemanov, A. V. Sotnikov, E. P. Smirnova, P. P. Syrnikov and E. A. Tarakanov, “Phase Transitions and Glasslike Behavior in Sr(1-x)BaxTiO3,” Physical Review B, Vol. 54, No. 5, 1996, pp. 3151-3157. doi:10.1103/PhysRevB.54.3151
[21]	A. K. Singh, Subrat K. Barik, R. N. P. Choudhary and P. K. Mahapatra, “Ac Conductivity and Relaxation Mechanism in Ba0.9Sr0.1TiO3,” Journal of Alloys and Compounds, Vol. 479, No. 1-2, 2009, pp. 39-42. doi:10.1016/j.jallcom.2008.12.130
[22]	Y.-C. Liou and C.-T. Wu, “Synthesis and Diffused Phase Transition of Ba0.7Sr0.3TiO3 Ceramics by a Reaction-Sintering Process,” Ceramics International, Vol. 34, No. 3, 2008, pp. 517-522. doi:10.1016/j.ceramint.2006.11.005
[23]	C. Fu, C. Yang, H. Chen, W. Wang, and L. Hu, “Microstructure and Dielectric Properties of BaxSr1-xTiO3 Ceramics,” Materials Science and Engineering: B, Vol. 119, No. 2, 2005, pp. 185-188. doi:10.1016/j.mseb.2005.02.056
[24]	A. Ioachim, R. Ramer, M. I. Toacsan, M. G. Banciu, L. Nedelcu, C. A. Dutu, F. Vasiliu, H. V. Alexandru, C. Berbecaru, G. Stoica and P. Nita, “Effect of the Sintering Temperature on the Ba(Zn 1/3Ta 2/3)O3 Dielectric Properties,” Journal of the European Ceramic Society, Vol. 27, No. 2-3, 2007, pp. 1117-1122. doi:10.1016/j.jeurceramsoc.2006.05.023
[25]	A. Ioachim, H. V. Alexandru, C. Berbecaru, S. Antohe, F. Stanculescu, M. G. Banciu, M. I. Toacsan, L. Nedelcu, D. Ghetu, A. Dutu and G. Stoica, “Dopant Influence on BST Ferroelectric Solid Solutions Family,” Materials Science and Engineering: C, Vol. 26, No. 5-7, 2006, pp. 1156-1161. doi:10.1016/j.msec.2005.09.045
[26]	G. Cigna, “Dynamic Mechanical Properties, Structure, and Composition of Impact Polystyrene,” Journal of Applied Polymer Science, Vol. 14, No. 7, 1970, pp. 1781-1793. doi:10.1002/app.1970.070140712
[27]	H. Frayssignes, B. L. Cheng, G. Fantozzi and T. W. Button, “Phase Transformation in BST Ceramics Investigated by Internal Friction Measurements,” Journal of the European Ceramic Society, Vol. 25, No. 13, 2005, pp. 3203-3206. doi:10.1016/j.jeurceramsoc.2004.07.030
[28]	B. L. Cheng, B. Su, J. E. Holmes, T. W. Button, M. Gabbay and G. Fantozzi, “Dielectric and Mechanical Losses in (Ba,Sr)TiO3 Systems,” Journal of Electroceramics, Vol. 9, No. 1, 2002, pp. 17-23. doi:10.1023/A:1021633917071
[29]	A. S. Nowick and B. S. Berry, “Anelastic Relaxation in Crystalline Solids,” Academic Press, New York, 1972.