J Pak Mater Soc 2008; 2(2)

CHARACTERIZATION OF BARIUM TITANATE PREPARED VIA MIXED OXIDE SINTERING ROUTE Asad Jamal, Muhammad Naeem, Yaseen Iqbal, Department of Physics, University of Peshawar, N.W.F.P, Pakistan ABSTRACT The phase and microstructure of barium titanate (BaTiO3) prepared via mixed oxide sintering route was investigated. TG / DTA of the as mix-milled powder, revealed a weight loss of ~ 17.558% in the temperature range 770 to 890ºC and the crystallization exotherm occuring at ~905.04ºC. XRD of the sample calcined at 900ºC for 2 h revealed the presence of BaTiO3 (JCPDS Card#50626) as the major phase along with a couple of low intensity peaks due to Ba2TiO4 (JCPDS Card#350813) and BaCO3 (JCPDSCard#41-373). XRD of the sample sintered at 1250ºC for 2 h confirmed the presence of BaTiO3 as the major phase, however; a couple of low intensity peaks probably due to Ba2TiO4 were still present. Secondary electron scanning electron microscope images of thermally etched samples revealed the presence of irregular shaped grains varying in size from ~2 to 10µm. INTRODUCTION Barium titanate is a ferroelectric ceramic material with ABO3 type perovskite structure1. Being the simplest structure among this family of ferroelectric ceramics, it has been widely studied 2. It has four isomorphs, namely rhombohedral, orthorhombic, tetragonal and cubic. The tetragonal form of BaTiO3 is stable at room temperature whereas the cubic form is normally stable at high temperatures (>120°C). The cubic phase has been reported to exist even at room temperature but with decreased (~100 nm) crystal size. This phenomenon shows that the transformation of barium titanate is a function of temperature and crystal size1. The cubic form of BaTiO3 is non-polar. By decreasing temperature below 120oC (Curie temperature, Tc), spontaneous polarization occurs in BaTiO3 and the crystal goes through a phase transition to the ferroelectric state. In this process, the cubic structure transforms to tetragonal. A second phase transition in BaTiO3 occurs near 0oC (+5oC), below which the crystal remains ferroelectric having orthorhombic structure. A third phase transition occurs below -80oC resulting in the rhombohedral symmetry3. Below Tc (120°C) tetragonal, orthorhombic and rhombohedral phases are the three ferroelectric polymorphs of BaTiO3 having non-zero electric dipole moments. At temperatures above 1429°C, BaTiO3 transforms to the nonperovskite hexagonal phase 4. Barium titanate has relatively high dielectric constant ~12000 at Tc. Cubic polymorph of barium titanate has one dielectric constant denoted by ε, while its tetragonal polymorph has two dielectric constants denoted by εa and εc. At room

temperature, the numerical value of its spontaneous polarization is 26×10-2 C/cm2. The refractive index of its cubic phase is very large (n≈2.4). In cubic form, its refractive index increases with decreasing temperature because the electronic polarizibility increases at the Curie point (120°C). In the tetragonal form, the refractive index is practically independent of temperature. BaTiO3 has the ideal perovskite structure above the Curie point (120°C). The atomic coordinates of BaTiO3 are; 1 Ba at (0,0,0), 1 Ti at (½,½,½) and 3 Os at (½,½,0); (½,0,½); (0,½,½) The perovskite structure is visualized in a better way as a face-centered cubic closed packed arrangement of Ba2+ and O2- ions as shown in Figure 1. For orthorhombic phase, the lattice parameters at -10°C are a = 5.682Å, b=5.669Å and c=3.990Å. In the cubic unit cell of BaTiO3, there are fifteen degrees of freedom, of which nine are associated with vibrational motion, three with the translational motion and three with the tortional undergoes spontaneous motion. BaTiO3 polarization along the three directions perpendicular to one another. Various types of radiations affect the surface layer of BaTiO3. Long exposure to visible light affects the colour of crystallites within the surface layer. On the basis of crystallization, BaTiO3 is prepared in two forms namely; a) single crystal form and b) polycrystalline form. Some properties are identical in both the forms such as transition temperatures, specific heat, lattice constants

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Figure 1: Perovskite unit cell of BaTiO3.

and most of the properties in the cubic phase, while the remaining properties are different. Depending on the polarity of barium titanate, it has two forms, namely; a) polarized form and b) non-polarized form. The polar phase has tetragonal symmetry where as the non-polarized form is isotropic and non-piezoelectric. In contrast, its polarized form is an-isotropic and piezoelectric. The dielectric constant in the polarized form is 10 to 15% larger than that in the un-polarized form. Practical applications of barium titanate include electronic devices, such as multilayer ceramic capacitors (MLCCs), communication filters and piezoelectric sensors 5. It is also used in positive temperature coefficient resistors (PTCR) called thermistors, electroluminescent panels, pyroelectric elements, polymer matrix composites for embedded capacitance in printed circuit boards, heaters, TV degaussers, fuel evaporators, air conditioning equipment, transducers and optoelectronic elements. Its yearly production is approximately 11,000 tons. Only multilayer ceramic capacitors (MLCCs) is a multi-billion dollar industry in which more than 10 billion units are produced annually 6 - 9. BaTiO3 has been extensively investigated and several methods of preparation of high purity, homogeneous, reactive fine BaTiO3 powders at low temperatures have been reported 7, 10 - 11.

These methods include solid state sintering route, co-precipitation, sol-gel, hydrothermal and co-precipitation plus inverse micro-emulsion method. Impurities or additives doped into BaTiO3 even in small proportions have been reported to change its dielectric properties appreciably for potential applications. The addition of NaNbO3 to BaTiO3 inhibits grain growth and increases density and dielectric constant. Homogeneous distribution of both Na and Nb in the grain and grain boundaries take place at higher temperature or longer holding time, which is suited for raising the dielectric constant 12. In perovskite ferroelectrics, oxygen vacancies are the most mobile and abundant defects. BaTiO3 with oxygen vacancies is tetragonal but cause a little expansion in the volume of the unit cell. Oxygen vacancies play an important role in optical properties and polarization degradation phenomenon, for example fatigue and aging. An additional optical absorption peak is observed for BaTiO3 with oxygen vacancies which is absent in the case of defect-free BaTiO313. The best dispersing effects for barium titanate are with dispersants concentration of 0.3 wt%, ball milling time of 16 h and pH value of 9 under optimum conditions. In dispersing BaTiO3 in aqueous media (polyacrylic acid-co-maleic acid) is better than poly-DL-aspartic acid and polyacrylic acid, while ammonium citrate is the least effective14. The Curie temperature of

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J Pak Mater Soc 2008; 2(2) BaTiO3 depends on mechanical stress introduced by milling and barium to titanium ratio15.

media and 2-propanol as lubricant at Materials Research Laboratory (MRL), Department of Physics, University of Peshawar, Pakistan.

Dielectric permittivity of BaTiO3 depends on grain size. It increases with decrease in grain size, reaches a maximum at some critical size and then decreases with further decrease in grain size. Curie temperature is also influenced by grain size and boundary conditions such as formation of compressive and tensile stresses. BaTiO3 ceramic powders can be activated mechanically using high energy milling process to influence initial powder microstructure and sintering properties. Due to mechanical activation, structure disorder increases which results in changing the micro-structural evolution. The sintering process takes place in one step for non-activated samples while for activated samples it occurs in three steps. The grain morphology obtained for non-activated samples is characteristic for early stages (highly porous) of sintering and for activated samples the microstructure is less porous due to polyhedral grain shapes 16.

The resulting slurry was poured through a sieve into a glass beaker and dried at ~90ºC overnight. Thermal gravitmetry and differential thermal analysis (TG/DTA) was carried out to determine the temperatures at which significant phase transformation events occurred using a Perkin Elmer TG/DTA unit at 5ºC/min from 30 to 1200ºC at the Centralized Resource Laboratory (CRL), Department of Physics, University of Peshawar. The powder sample was then calcined at 900ºC for 2h at a cooling/heating rate of 10ºC/min. The calcined powders were pressed into 13 mm dia. pellets at 100MPa in a Carver Manual uniaxial Press. The pellets were finally sintered at 1250ºC for 2h at a cooling/heating rate of 5ºC/min. The phase analysis of the samples was carried out using a JEOL X-ray diffractometer (JDX-3532) with copper (Cu) kαradiationsλ=1.5418Å) operating at 40kV and 30mA with a step size of 0.03º/s from 10-70º after various heat treatments. The sintered sample was cut into two pieces by diamond fine cutting saw before fine polishing. The sample was thermally etched at 1100ºC for 30 min at 5ºC/min to resolve the grains. To provide a conducting medium and avoid charging in the SEM, the samples were mounted onto aluminum

EXPERIMENTAL WORK Carefully weighed 50g batch of GPR grade TiO2 and BaCO3 powder was prepared and mixmilled for 24h in a horizontal ball mill with ½″ diameter Yt-toughend ZrO2 balls as grinding

Figure 2: TG/DTA curve of the milled powders.

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J Pak Mater Soc 2008; 2(2) BT

BT-------BaTiO3 (JCPDS Card#50626) B2…….Ba2TiO4 (JCPDS Card#350813) B-------BaCO3 (JCPDS Card#5-378)

BT

T…….TiO2 (JCPDS Card#291360)

RELATIVE INTENSITY

4800

B2

BT

BT BT

B2

c) 2800

BT

b)

B2

BT BT

BT

BT

BT BT

BT

B B T B

B B

B

B

B

B B

B

a) 800 10

20

Figure 3:

30

2-THETA

40

50

60

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X-Ray diffractogram of a) as mix-milled, b) Calcined and c) sintered samples showing the presence of various phases after various heat treatments.

Buscaglia et al 5 reported a weight loss of 10% and a significant phase change at ~650ºC for the same composition. The d-spacings corresponding to XRD peaks from as-milled sample matched with the JCPDS Card#5-378 for BaCO3 and ICDD card#291360 for TiO2 (brookite) which confirmed the phases present in the initial ingredients. The major peaks labelled as “B” and “T”, correspond to BaCO3 and TiO2 (brookite) respectively (Figure 3a). The dspacings and relevant intensities corresponding

stubs with silver paint and then gold-coated for 120s. For micro-structural characterization a JEOL JSM-5910 SEM was used. RESULTS AND DISCUSSION A maximum of 17.55% weight loss was observed in the sample at 800-900ºC. Although, the DTA curve was not very clear, the calcination temperature (~905.04ºC) was selected on the basis of a rough approximation from the behaviour of the DTA curve [Figure 2].

o

o

Figure 4: SEI from BaTiO3 sample sintered at 1250 C and etched at 1100 C showing, a) general area and b) well-connected BaTiO3 grains.

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J Pak Mater Soc 2008; 2(2) to XRD peaks from the sample calcined at 900ºC matched with ICDD card # 50626 for BaTiO3. Some of the d-spacings corresponding to relatively week peaks from the same sample matched with ICDD card # 350813 for Ba2TiO4 and JCPDS card # 41-373 for BaCO3 (Figure 3b) which indicated incomplete reaction at the calcination temperature used in this study. The d-spacings and relevant intensities corresponding to the XRD peaks from the sample sintered at 1250ºC for 2h at 5º/min. matched with ICDD card # 50626 for BaTiO3. Again a small number of low intensity peaks matching ICDD card # 350813 for Ba2TiO4 were observed (Figure 3c) which were obviously showing the presence of Ba-rich regions in a small percentage indicating in-homogeneous crystallization5. Figure 4a shows a secondary electron SEM Image (SEI) of the sample sintered at ~1250ºC. It shows different grains and grain boundaries of BaTiO3. The average grain size is 5-10µm with polygonal shape. Figure 4b shows the grains and grain boundaries observed in the bulk of BaTiO3 sintered at 1250ºC. The average grain size in the bulk of the sample is ~5µm. CONCLUSION The wt% loss observed in the present study is higher than that reported previously. The XRD analysis of as mix-milled sample confirmed that the raw materials were almost pure. The phase formation after calcination at 900ºC for 2h was not complete as indicated by the presence of small peaks due to Ba2TiO4 and BaCO3, consistent with the previous study2. The variation in grain size may be due to bypassing some processing steps necessary for grain size homogenization and hence optimization of the density as the project time was limited. REFERENCES 1. Kim Y -II, Jung JK, Ryu KS. Structural study of nano BaTiO3 powder by Rietveld refinement. Mater Res Bull 2004; 39: 1045 1053. 2. Jona F, Shirine G. Ferroelectric Crystals. Pergamon GmbH volume 1, edition, 1962, pp. 108 -190. 3. He Y. Heat capacity, thermal conductivity, and thermal expansion of barium titanate based ceramics. Thermocimia Acta 2004; 419: 135 -141. 4. Licheri R, Fadda S, Orru R, Cao G, Buscaglia V. Self propagating high temperature synthesis of barium titanate and subsequent densification by spark plasma

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