Strontium Concentration Dependence of Selected ...

Integrated Ferroelectrics, 108: 77–88, 2009 Copyright © Taylor & Francis Group, LLC ISSN 1058-4587 print / 1607-8489 online DOI: 10.1080/10584580903324410

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Strontium Concentration Dependence of Selected Structural and Mechanical Properties of Polycrystalline Ba1−X SrX TiO3 1 ´ W. Smiga, B. Garbarz-Glos,1,∗ A. Kalvane,2 M. Antonova,2 W. Suchanicz,1 and A. Sternberg2 1

Institute of Physics, Pedagogical University, Podchora˙ zych 2, 30–084, Kraków, Poland 2 Institute of Solid State Physics, University of Latvia, Kengeraga 8, LV-1063, Riga, Latvia

ABSTRACT Microstructure and material constants such as Young modulus E, shear modulus G, Poisson ratioν of polycrystalline ferroelectric materials Ba1-x Srx TiO3 for 0≤x≤0.4 were investigated to determine their dependence on strontium concentration. The performed investigations showed that the material is chemically homogeneous. It was found that the strontium concentration had no significant influence on the grain size. The highest value of Young’s modulus is observed for Ba0.6 Sr0.4 TiO3 sample. The performed investigations of mechanical properties made it possible to evaluate the usability of this material for manufacturing transducers and sensors which could work under load. Keywords: barium-strontium titanate, structural properties, mechanical properties

INTRODUCTION Lead free barium-strontium titanate BaTiO3 -SrTiO3 solid solutions have been extensively measured and studied from both experimental and theoretical aspects [1–3]. The Curie temperature of the Ba1-x Srx TiO3 (BST) system decreases linearly with increasing the amount of Sr in a BaTiO3 lattice and since the solid solution can be formed over whole concentration range. BST is a promising candidate for wide range of applications as electromechanical sensors, transducers and actuators not to mention their superior piezoelectric coupling properties. Received March 31, 2009; in final form August 14, 2009. ∗ Corresponding author. E-mail: [email protected] 77


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BST has useful dielectric properties from low to microwave frequencies [4]. It makes this material ideal for various microwave devices such as microstrip line phase shifters, tuneable filters and high-Q resonators. Moreover the high dielectric permittivity value combined with low dissipation factors makes BST one of the promising candidates for applications in dynamic random access memories (DRAM). In this paper the results of microstructure, structure and mechanical properties investigations of Ba1-x Srx TiO3 for 0≤x≤0.4 polycrystalline samples have been presented and discussed.

EXPERIMENTAL METHODS 1. Preparation of Samples The Ba1-x Srx TiO3 ceramics used in this study were synthesized by a solid phase reaction from high purity grade oxide and carbonates: TiO2 , SrCO3 and BaCO3 . The mixture of starting raw materials was homogenized and ground in an agate ball mill in ethanol for 20 hours, dried and then calcined for 1–2 hours at the temperature from 1300 to 1350◦ C depending on composition. After the calcination the powder was homogenized and ground in ethanol, cold pressed (100MPa) and sintered using a conventional ceramic technology for 2 hours within the temperature range from 1400 to 1500◦ C. BaO constituent increased both calcination and sintering temperatures.

2. X-ray Measurements X-Ray powder diffraction measurements were performed by an X’Perrt PRO (PANalytical) diffractometer with CuKα radiation (λ = 0.154178 nm). The sample was placed in a nickel-plated copper sample holder of dimensions 18×9×0.2 mm3. Prior to the measurements the sample was annealed. Data were obtained using automatic slits and next recalculated to the fixed slits mode. All analysed patterns were made in the wide angle range of 2 angle from 10 to 100 degrees with step 0.01 deg. A profile-fitting program FULLPROF [5] based on the Rietveld method was used to analyse and fit the spectra.

3. Microstructure Investigations The investigations of microstructure of Ba1-x Srx TiO3 for 0≤x≤0.4 polycrystalline samples were performed on fractures and polished sections. They were carried out by means of SEM (Scanning Electron Microscope) with field

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emission Hitachi S4700 with microanalyses system Noran-Vantage located in Biology and Geology Science Scanning Laboratory of the Jagiellonian University. EDS (Energy-Dispersive X-Ray Spectroscopy) was applied to investigate the homogeneity of the composition and the EPMA (Electron Probe Microbeam Analysis) was applied to the analysis of the distribution of elements at the sample surface. Before the measurements the samples were sputtered with carbon.

4. Mechanical Properties of the Material Material constants: the Young s modulus E, the shear modulus G and the Poisson s ratio v were determined by ultrasonic method for Ba1-x Srx TiO3 for 0≤x≤0.4 polycrystalline samples. The measurements were carried out with the INCO -VERITAS Ultrasonic Measuring Set UZP-1. Two kinds of transducers were used. The 10 MHz frequency transducers connected to the sample by oil were used for longitudinal waves while the 2 MHz frequency transducers stuck on the sample with Canada balsam was applied for transverse waves. The investigations were performed on small samples of 10 mm height and 8 mm diameter. Since the wavelength of longitudinal ultrasonic waves was below 1mm, the tested samples could be treated as a three-dimensional medium [6, 7]. Material constants were calculated from the longitudinal and transverse ultrasonic wave propagation velocity and the apparent density of the samples, using the following formulas [7]: E = VL2 · ρ(1 + v)(1 − 2v)/(1 − v),

(1)

G = VT2 ρ, v = VL2 − 2VT2 / 2VL2 − 2VT2 ,

(2) (3)

where E is Young s modulus, G—shear modulus, ν — Poisson s ratio, ρ — density, VL velocity of the longitudinal wave, VT — velocity of the transverse wave.

RESULTS 1. X-ray Measurements Basing on the obtained X-ray patterns a phase analysis has been performed. It confirms the mono-phase character of samples as to the chemical composition according to the assumed standards. All investigated samples have tetragonal symmetry at room temperature. Lattice parameters of Ba1-x Srx TiO3 unit cells


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Figure 1. The dependence of the lattice parameter a on Sr content in Ba1−x Srx TiO3 solid solution for 0 ≤ x ≤ 0.4 (at T = 300 K).

for 0 ≤ x ≤ 0.4 have been determined. The dependences of the lattice parameters a and c on Sr content are presented in Figs. 1, 2. An analysis of graphs leads to the conclusion that the values of lattice constants a and c decrease with the increase in Sr concentration from 0 to 30 mol-%. It could be explained with smaller ionic radius of Sr in comparison

Figure 2. The dependence of the lattice parameter c on Sr content in Ba1−x Srx TiO3 solid solution for 0 ≤ x ≤ 0.4 (at T = 300 K).


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with Ba (1.44Å and 1.61Å, respectively). However for higher Sr content the increase in the values of these parameters is observed.


2. Microstructure The investigations of the microstructure of BST ceramics were performed on fractures prepared in a way described above. The photographs taken at 1000x and 10000x magnification (Figs. 3–6) show that the surface of fracture runs both across grains and along intergranular boundaries. The ceramics are perfectly sintered and the fracture reveals a brittle nature. Pores have an irregular shape without curvings and narrowings. They also reveal no orientation related to the direction perpendicular to the direction of pressing. The BaTiO3 (BT) samples have the lowest porosity. The porosity and average size of grains slightly increase with the increase in Sr content. In the photographs taken at 1000x and 10000x magnification individual grains are clearly seen. The grain boundaries are also seen and the glassy phase is hard to find. In the pure BT 120◦ intergranular boundaries are visible. In the photographs growth terraces are seen. They indicate that the growth of grains occurred according to a layer mechanism, probably with a screw dislocation.

3. Material Constants The investigation of elastic properties was performed in order to determine the effect of the load of material on its properties, thus on the durability of the devices made of this material. Elastic properties characterize also directly interatomic bond force in the considered material. Figure 7 presents the dependence of the longitudinal and transverse ultrasonic wave velocities, measured along the diameter of a sample on strontium concentration. Figure 8 shows the dependence of the material constants (E, G, v) on strontium concentration in BST samples. The measurements of the longitudinal wave velocity in the formation direction of samples and in the perpendicular direction (along the diameter) indicate that a small anisotropy of velocity (up to 40% of Sr) occurred. Slightly higher values of velocity have been found for the samples measured along the diameter which can be explained by a weak texture of the elongated grains and pores with their longer dimension in the perpendicular direction to that of sample formation during a pressing process. Since the anisotropy is small, the samples are treated as three-dimensional media [7]. The ultrasonic wave velocity and the values of both Young s modulus E and the shear modulus G are the highest for Ba0.6 Sr0.4 TiO3 sample. It has been shown that the Young s modulus value decreases with the increase of strontium concentration in BT up to x = 0.2. However the further increase in strontium

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Figure 3. The photograph of microstructure of BaTiO3 sample surface, magn. 1000x (a) and 10000x (b).

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Figure 4. The photograph of microstructure of Ba0.8 Sr0.2 TiO3 sample surface, magn. 1000x (a) and 10000x (b).

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Figure 7. The influence of strontium concentration in Ba1-x Srx TiO3 samples (x = 0.0–0.4) on: a) longitudinal ultrasonic wave velocity VL , b) transverse ultrasonic wave velocity VT .

Figure 8. The influence of strontium concentration in Ba1-x Srx TiO3 samples (x = 0.0–0.4) on: a)Young s modulus E, b) shear modulus G, c) Poisson s ratio v.


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concentration up to x = 0.4 considerably increases this modulus. The dependence of shear modulus G on sample composition is similar to the respective dependences of Young’s modulus E whereas the Poisson s ratio ν decreases with the increase in strontium concentration. The lowest value has been found for Ba0.6 Sr0.4 TiO3 sample while the highest one for the pure BaTiO3 .

CONCLUSIONS The barium strontium titanate samples Ba1-x Srx TiO3 (BST) were prepared by a solid phase reaction. The performed investigations show that the material is chemically homogeneous. The examined samples are good quality, the grains are well shaped and there is a very small amount of a glassy phase. It is found that strontium concentration has no significant influence on the size of grains or pores. Young’s modulus E, shear modulus G and Poisson’s ratio ν were determined using an ultrasonic method. It has been shown that Young’s modulus E decreases with the increase in strontium concentration in BaTiO3 up to x = 0.2. However the further increase of strontium concentration up to x = 0.4 considerably increases this modulus. The highest value of Young’s modulus E is observed for Ba0.6 Sr0.4 TiO3 sample. The dependence of shear modulus G on sample composition is similar to the respective dependence of Young’s modulus E, whereas the Poisson’s ratio ν decreases in all interval of strontium concentration. The performed investigations of mechanical properties make it possible to evaluate the usability of this material for manufacturing transducers and sensors.

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5. J. Rodriguez-Carvajal, Abstract of the Satelite Meeting on Powder Diffraction, Congr. Int. Union of Crystallography. Toulouse. France (1990). 6. J. Piekarczyk, Optymalne wymiary próbek do badań ultrad´zwiekowych. In˙z. Mater. 13, 72–73 (1992). 7. J. Piekarczyk, H. W. Hennicke, and R. Pampuch, On determining the elastic constants of porous zinc ferrite materials. cfi/Ber.D.K.G. 59, 227–232 (1982).