Structural and electrical characteristic of crystalline barium titanate synthesized by low temperature aqueous method

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 171–177

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Structural and electrical characteristic of crystalline barium titanate synthesized by low temperature aqueous method Srimala Sreekantan a,∗ , Ahmad Fauzi Mohd Noor a , Zainal Arifin Ahmad a , Radzali Othman a , Anthony West b a

School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia (Engineering Campus), 14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia b Department of Engineering Materials, University of Sheffield, Sir Hadfield Building, Mappin Street, Sheffield S1 3JD, United Kingdom

a r t i c l e

i n f o

a b s t r a c t

Article history:

Barium titanate powder was processed at temperature 80 ◦ C by reacting TiO2 sol in aqueous

Received 2 March 2007

solutions that contained BaCl2 and NaOH at atmospheric pressure. Well-crystallized (cubic

Received in revised form

phase), spherical barium titanate powder with average size of 35 nm was formed by this

16 April 2007

method at 80 ◦ C. As for sintered barium titanate, observation via SEM showed bimodal dis-

Accepted 20 April 2007

tribution of grain size. The fine grains was in the range of 0.3–0.5 ␮m whereas the large ones approximately 1.5–2.0 ␮m. Furthermore, interesting features such as wedge shape lamellar domains, and {1 1 1} twins formation were observed in the sintered sample. Sintering

Keywords:

at 1300 ◦ C, led to the formation of a tetragonal structure BaTiO3 with a secondary phase

Barium titanate

of BaTi2 O5 and Ba6 Ti17 O40 . The ac response of sintered barium titanate indicated that the

Impedance spectroscopy

materials are electrically heterogeneous. Two regions are apparent in this material. The

Aqueous method

first element (R1 C1 ) and the second element (R2 C2 ) are roughly comparable volume fraction

Nanoparticle

because magnitudes of the capacitance value are similar but they differ in their resistances

Domain

by 1–2 orders of magnitude, depending on temperature. Activation energy for conductivity for R1 and R2 were 1.60 and 0.90 eV, respectively. C2 is a ferroelectric material but conclusive behaviour of C1 could not be determined. Therefore, these components are tentatively assigned to different composition regions: (R2 C2 ) is ferroelectric barium titanate whereas (R1 C1 ) is Ti-rich secondary phase. © 2007 Elsevier B.V. All rights reserved.

1.

Introduction

The excellent dielectric and ferroelectric properties of barium titanate make it attractive material in the field of electroceramic and microelectronics. Being a lead-free ferroelectric ceramic, barium titanate is an environmentally friendly material, thus making it a good candidate for various applications including capacitors, positive temperature coefficient resistors, high-density optical data storage, ultrasonic transducer,

∗

Corresponding author. E-mail address: [email protected] (S. Sreekantan). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.120

piezoelectric devices and semiconductors (Hennings, 1987). It is well known that the properties of barium titanate for electronic applications depend significantly on the microstructure of the sintered body. Sintered barium titanate having dense and fine grain microstructure shows better performance (Dawson et al., 1991). Therefore, nowadays enormous efforts have been devoted to develop a powder synthesis which produce well crystallized barium titanate particle with suitable particle size and morphology (Kleee and Brand, 1989).

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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 171–177

The conventional synthesis of barium titanate powder involves a high temperature (∼1200 ◦ C) calcination of solid mixtures of BaCO3 and TiO2 (Beauge et al., 1983). However, the conventional method suffers from contamination and grain growth problems during milling and calcinations process, respectively. Thus, there have been many investigations to overcome the limitation of the conventional method using wet chemical method, such as sol–gel method (Pfaff, 1992; Moreno et al., 1995; Kuo and Ling, 1994), Pechini processing (Pechini, 1966; Stockenhuber et al., 1993; Wada et al., 2003) using a citric or oxalate complex as the precursor, hydrothermal synthesis (Dutta and Gregg, 1992; Clark et al., 1999; Moon et al., 1999) and precipitation method (Flaschen, 1955; Kiss et al., 1966; Kumar, 1999). Among many advantages of such wet chemical synthesis are homogeneous mixing at all metal cations at a molecular level and facilitates crystallization with small grain-sized particles at relatively low temperature as compared to classic solid sate reaction. In the sol–gel process, barium titanate gels can be obtained by hydrolyzing the metal alkoxide. The most advantageous characteristics of this method are the high purity and the excellent control of the composition of the resulting powders. To crystallize barium titanate, the hydrolysis product normally will be calcined at temperatures above 500 ◦ C. The advantage of the Pechini process lies in the limitation of segregation of various metal ions, and is achieved by forming stable metal–chelate complexes with a stoichiometric Ba:Ti ratio of 1:1. The pyrolysis of the complexes often occurs at a temperature ranging from 500 to 1000 ◦ C, which results in aggregated nanosized barium titanate particles. In hydrothermal synthesis, processing temperature can be much lower, in the range of 100–250 ◦ C. The prepared barium titanate with narrow size distributions with the average particle size of larger than 100 nm can be obtained by hydrothermal method. As for the precipitation method, continuous large-scale production of small particle at low temperature can be obtained easily as compared to other synthesis routes. Based on the research done, it is apparent that the most of the investigations placed their aim mainly on the preparation of fine powders with high purity at low temperature. On the other hand, there have been few studies dealing with electrical characterization of sintered ceramics prepared by wet chemical methods. In the present study, the structural and electrical characteristic of barium titanate powder and the sintered body which was prepared by a simple chloride aqueous method at low temperatures 80 ◦ C at atmospheric pressure has been reported. The chloride aqueous method was established to produce barium titanate by reacting titania sol in alkaline aqueous solutions of BaCl2 and NaOH. As shown in this paper, the chloride aqueous method results in nanosized, crystalline particles with the formation of a metastable phase under certain condition.

2.

Experimental procedure

Titanium butoxide [Ti(OBu)4 ] (99% Fluka Chemical Co.) was used as a precursor and butanol [BuOH] (99.9% J.T. Baker)

as a solvent. Nitric acid (HNO3 ) (Merck) was used as the peptizing agent. All reagents and solvents were used in asreceived form, without any purification. TiO2 precipitation was obtained by adding 0.4 M of the titanium butoxide into distilled water. The mixture was stirred at a high speed (300 rpm) whilst the titanium butoxide was added dropwise. The amount of water was fixed at a [H2 O]/[Ti] molar ratio (r) of 110. The precipitates were washed with distilled water four times using a centrifuge. Subsequently, the peptization reaction was initiated by further addition of dilute HNO3 ([HNO3 ] = 0.25 M) in a reaction vessel and placed in a temperature control bath at 60 ◦ C. During peptization, the milky dispersion (titanium oxy-hydroxide) changed to a clear light-blue solution, which is indicative of the resuspension of the precipitates and the reduction of the particle size. Simultaneously, alkaline aqueous solutions that contained barium was prepared by initially boiling distilled water for at least 30 min to remove dissolved CO2 . The water was maintained at a temperature of 80 ◦ C under moderate stirring whilst appropriate amounts of BaCl2 ·2H2 O (Merck, 99%) and 1 M NaOH (Merck, 99.9%) were added. When the starting chemicals were dissolved, the solution was filtered using Buchner funnel. The filtered solution was kept in polyethylene bottle. After purging and backfilling with argon, polyethylene bottle was sealed and stored in an oven at a temperature of 80 ◦ C. Barium titanate was synthesized by adding the TiO2 sol as described previously, to solution that was kept in an oven. The initial ratio of barium in solution relative to titanium was 1. After mixing the TiO2 sol into the solution, the bottle was back-filled with argon, sealed and placed in an oven for reaction. The bottle was then removed from the oven and excess solution was decanted. The powder was washed with distilled water that was adjusted to pH 10 using NH4 OH to remove residual barium cations. Barium titanate powder was then placed in Petri dish and dried in an oven at a temperature of 80 ◦ C for 48 h. After drying, the powders was uniaxially pressed into pellet at about 100 MPa and sintered at 1300 ◦ C for 10 h. The microstructure was observed using JEOL JSM 6400 scanning electron microscope and transmission electron microscope (TEM). The details regarding the sample preparation for TEM have been reported elsewhere (Sreekantan et al., 2005). Phase identification was performed by high-resolution X-ray diffraction using a Stoe transmission powder diffractometer system (Stoe STADI P) operated at 40 kV and 40 mA. The electrical behaviour of the sintered pellets, which had been electroded with platinum paste and had platinum wire contacts attached prior to use, was measured using a Hewlett Packard 4192A impedance analyzer. The ability of impedance analyzer to characterize electrical microstructure and distinguish between the different regions (grain boundary, grain core, surface, electrode response, etc.) of the material (Irvine et al., 1990) is therefore explored to gather a better understanding of the electric properties of barium titanate ceramics. In this technique, ac impedance measurements were made over a frequency range of 5 Hz to13 MHz and the different regions of the material were characterized by a resistance and capacitance (Ri Ci ) which usually placed in parallel. In general, data obtained by impedance spectroscopy was analyzed in terms of four possible formalisms, the impedance (Z* ), the electric modulus (M* ), the admittance (Y* ) and permittivity ε* . These

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 171–177

173

are interrelated (Sinclair et al., 2000; Sinclair and West, 1989): M∗ = jwC0 Z∗ ε∗ = (M∗ ) Y ∗ = (Z∗ )

−1

−1

Y ∗ = jwC0 ε∗

(1) (2) (3) (4)

where ω is the angular frequency, 2f and C0 are the vacuum capacitance of the measuring cell and electrodes with an air gap in place of the sample. The temperature of measurement was varied from room temperature to 800 ◦ C in an interval of 50 ◦ C. All the data measured using ac impedance was corrected for sample geometry. The geometric factor (GF) of the sample was calculated according to Eq. (5): GF =

l A

(5)

where l is the thickness of the sample (cm) and A is the crosssectional area (cm2 ). This enables the instrument software to give the output in resistivity ( cm) rather than resistance value. The term capacitance (F) has been used sometimes although more rigorously this should be termed geometric capacitance (F cm−1 ).

3.

Results and discussions

Fig. 1(a) shows the XRD pattern of barium titanate synthesized at 80 ◦ C for 48 h. The pattern fit well with cubic phase barium titanate. The space group of the barium titanate particle is Pm3m with lattice constant of 0.4029 nm. Crystallite size was determined from X-ray line broadening with the help of a APDW software using Scherrer’s formula (t = 0.9/B cos ), where t is the diameter of the crystallite size,  the wavelength and B = (B2M − B2S ), BM and Bs are the full widths at halfmaximum of the sample and the standard. The standard used

Fig. 1 – XRD pattern of BT powder synthesized (a) at 80 ◦ C and (b) at 1300 ◦ C [() BT (cubic), (䊉) carbonate contamination, () BT (tetragonal), ( ) B6 T17 (monoclinic), ( ) BT2 (monoclinic)].

Fig. 2 – TEM micrographs of BT powder prepared at 80 ◦ C.

was quartz and it was chosen such that the peak of the sample and the standard have approximately similar 2 values (quartz gives a reflection which is near the (1 1 1) and (2 0 0) reflection of the barium titanate). Therefore, the (1 1 1) and (2 0 0) reflection of the observed X-ray data was chosen for calculating the crystallite size of the barium titanate. The crystallite size of the (1 1 1) and (2 0 0) plane was estimated to be 35 and 37 nm, respectively. There is no large difference between two crystallite size and therefore the crystal structure Pm3m is confirmed valid. However, small amount of carbonates contamination was observed in powder obtained at 80 ◦ C. The formation of barium carbonate is believed to be a result of the reaction between carbon dioxide that dissolved into the solutions from air and reacted with alkaline earth chloride during the process. XRD peaks that correspond to anatase or rutile titania were not detected. Sintering at 1300 ◦ C, leads to the formation of tetragonal barium titanate (Fig. 1(b)). There is a peak splitting of 0 0 2/2 0 0, 2 0 1/2 1 0, 1 1 2/2 1 1 which indicates that the structure transforms to a tetragonal phase (space group P4mm). The lattice parameter for barium titanate sintered at 1300 ◦ C is a = 0.3995 nm and c = 0.4030 nm. However, a small amount of monoclinic BaTi2 O5 with space group of C2/m and monoclinic Ba6 Ti17 O40 (space group A2/a) tends to appear as the secondary phase in the sintered sample. The BaTi2 O5 and Ba6 Ti17 O40 phase seems to originate from BaTiO3 with a Ba-deficient composition. The formation of the perovskite involves the reaction of the Ti with Ba ions left in solution and this reaction is mainly determined by the effective Ba concentration in the aqueous phase. When the reaction is incomplete, the powder obtained upon precipitation contained some residual Ti-rich amorphous phase and consequently Ba/Ti < 1. The excess of titanium leads to the formation of a second compound, BaTi2 O5 or Ba6 Ti17 O40 , depending on the temperature of thermal treatment. Fig. 2 shows the TEM bright-field image of the barium titanate powder prepared at 80 ◦ C. The particles were spherical with average particle size of 38 nm. The size obtained from TEM analysis is in a good agreement with the average crystallite size calculated using the Scherrer equation (35 nm). Fig. 3(a) shows SEM micrograph of as-sintered surfaces for barium titanate sample sintered at 1300 ◦ C for 10 h. As can be

174

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 171–177

Fig. 3 – (a) SEM micrographs of as-sintered surfaces for BT sintered at 1300 ◦ C for 10 h (b) SEM micrographs of polished and thermally etched (1 h at 1170 ◦ C) fracture surface for BT.

observed, there are bimodal distribution in the grain size, the fine ones is in the range of 0.3–0.5 ␮m where else the large ones approximately 1.5–2.0 ␮m. The same sample which was polished and thermally etched at 1170 ◦ C for 1 h shows the similar results; bimodal distribution (Fig. 3(b)). EDAX analysis shows the large grain is Ti-rich or Ba-rich but it is difficult to confirm which element due to the overlapping of Ba and Ti energy line. However based on XRD trace (Fig. 1) which shows

the presence of B6 T17 , implies that the grain growth is due to the Ti-rich phase. The sintered sample was also examined using TEM and the micrograph in Fig. 4 shows a bright-field image of barium titanate where wedge-shaped lamellar domains are clearly visible. The wedge-shaped domains are a common microstructural feature in most ferroelectric phase transition. It is well known that this type of domains appears when the 90◦ walls terminate within the grain (Forsbergh, 1949; Park and Chung, 1994) and the TEM image shown in Fig. 4 is in agreement with the reported observations. Its cause is not yet clear, although redistributing excess polarization charges had been proposed (Chou et al., 2000). A closer look at the other grains also reveals that the wedge-shaped domains appear to have stopped not only by grain boundaries but also by the other domains, as indicated by two different domains X and Y in Fig. 5(a) and {1 1 1} twin boundary as shown in Fig. 6(a). Impedance measurement of sample sintered at 1300 ◦ C for 10 h was done from room temperature to 800 ◦ C on the heating cycle. Fig. 7 shows the M and Z spectroscopic plots. The Z spectrum shows one peak but the M spectrum shows two peaks. The single Z peak corresponds to the lower frequency M peak. This element is the most resistive by far which dominates the overall sample resistivity at all temperatures measured. The associated resistivity for high frequency M peak is too small to be seen easily in the Z spectroscopic plot (Fig. 7). The values of R1 C1 (resistivity and capacitance of the low frequency element, respectively) and R2 C2 (resistivity and capacitance of the high frequency element, respectively) were obtained directly from the spectroscopic plot since at the peak  maximum Zmax = R1 /2. R2 was obtained from the maximum of the high frequency M peak using the relation 2fmax R2 C2 = 1.  C2 was obtained from the height of M peak at which Mmax =  1/2C2 and similarly C1 from the low frequency M peak. Values of R1 and R2 as a function of temperature are summarized in Arrhenius conductivity format in Fig. 8. Both resistivity, R1 and R2 show thermally activated process. Activation energy for conductivity for R1 and R2 were 1.60 and 0.90 eV, respectively. Plotting capacitance against temperature (Fig. 9) showed decrease in capacitance as temperature increases. Both capac-

Fig. 4 – TEM micrograph of the wedge-shaped lamellar domains in BT, (b) the SADP and the index showing the beam direction is [1 1 0].

175

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 171–177

Fig. 5 – (a) TEM micrograph of the wedge-shaped lamellar domains stopped by another domain, (b) the SADP and the index showing the beam direction is [1 1 1].

Fig. 6 – (a) TEM micrograph of the wedge-shaped lamellar domains stopped by twin boundary, (b) the SADP and the index showing the beam direction is [1 0 0].

1.5e10

-100000 611c.z

-2.0

-75000 -3.0

R2 , Ea = 0.90 eV

-50000

5.0e9

log σ /Scm -1

Mʺ

M''

Z” ohm cm

1.0e10

-4.0

-5.0 R1 , Ea = 1.60 eV

-25000 -6.0 Zʺ

0 100

101

102

103

104

105

106

107

0 108

Frequency (Hz) Fig. 7 – Spectroscopic plot for BT at 611 ◦ C.

-7.0 0.8

0.9

1.0

1.1

1.2

1.3

1000K/T

Fig. 8 – Arrhenius plot for resistances R1 and R2 .

1.4

176

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 171–177

1E-10

-500000

-400000

C2

6E-11 4E-11

Z” ohm cm

C/ Fcm-1

8E-11

C1

2E-11

-300000

-200000

0 400

500

600

700

800

T/ ºC

0V

-100000 15 V

Fig. 9 – Temperature dependence capacitance for C1 and C2 . 0

itances, C1 and C2 were similar within the range of 40–80 pF. This behaviour is seen in ferroelectric material where there is a rapid decrease in capacitance above Curie temperature (Tc ). However, conclusive ferroelectric behaviour could not be identified because data obtained from M spectroscopic plot were only attainable at high temperature (>400 ◦ C), which is well above Tc . Another method of getting more information regarding the capacitance value involves capacitance data, C versus log f. The capacitance data as a function of temperature based from this data show that C2 decreases above Tc , ∼126 ◦ C (Fig. 10a). Curie–Weiss behaviour of C2 , indicative of ferroelectric materials above Tc was confirmed by plotting reciprocal capacitance against temperature (Fig. 10b). A linear plot extrapolating to a Curie–Weiss temperature of ∼110 ◦ C was obtained for C2 . Unfortunately, the data for low frequency element C1 were not realistic and this is due to the dispersion (horizontal single plateau was not obtained). Voltage dependence work was also carried out in order to know whether the responses observed in the impedance plot (R1 and R2 ) belong to bulk or grain boundary. Impedance measurement was measured at 580 ◦ C, starting at 0, 5, 10 and 15 V. Biased measurements can cause local heating in the sample which strongly affects the impedance response; hence, the 3.5E-10

2.5E+10

3.0E-10

2.0E-10

(b)

1.5E+10

1/ ε

C (Fcm -1 )

(a)

1.5E-10

1.0E+10

1.0E-10 5.0E+09

5.0E-11 0.0E+00

0.0E+00 25

50

90

130 180 230 280 330

T ( oC) Fig. 10 – (a) Temperature dependence capacitance for C2 element (b) Curie–Weiss for C2 element for BT.

100000

200000

5V

300000

400000

500000

Z’ ohm cm Fig. 11 – Impedance plot shows the resistance is dependent of applied field.

sample was given 15 min cooling time between each measurement. Fig. 11 shows that the total resistivity of the component changes with dc bias. Since R1 is largely responsible for the total resistance of the sample, the changes with applied field is assumed to be associated with R1 rather than R2 . Normally, bulk response is independent of applied field, whereas the grain boundary exhibits decay due to the Schottky barrier mechanism at the grain boundary, whereby the barrier height (or resistance of the barrier) is decreased with increasing applied field Based on these results, it can be inferred that R1 response to some extent affected by the applied field and therefore it might be associated with grain boundary. At this stage there is no solid prove of R1 being a grain boundary for the capacitance value, 4 × 10−11 F cm−1 is virtually too small a value for a grain boundary. Based on the capacitance value we believe R1 might be the response of the Ti-rich secondary phase (Sreekantan et al., 2005) and it is voltage dependent.

4.

2.0E+10

2.5E-10

0

10 V

Conclusion

We have demonstrated the possibility of obtaining wellcrystallized, cubic and nano-sized barium titanate at 80 ◦ C using chloride aqueous method. Sintering at 1300 ◦ C, leads to tetragonal phase and crystallite growth. Evidence was obtained for the existence of the wedge shaped domains and it appeared to have stopped not only by grain boundaries but also by the other domains and {1 1 1} twin. The ac response of barium titanate sintered at 1300 ◦ C indicated the presence of two elements in this material. The capacitance values (C) for both elements were similar within the range of 40–80 pF, depending on the temperature. The element 2 (R2 C2 ) was confirmed to be a ferroelectric material but conclusive behaviour of element 1 (R1 C1 ) could not be determined. Furthermore, there is no direct information on the nature of the regions that are responsible for R1 and R2 . Therefore, tentatively assign

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 171–177

these components to different regions of the microstructure; (R2 C2 ) is ferroelectric barium titanate and (R1 C1 ) is probably Ti-rich secondary phase.

Acknowledgement The authors would like to thank Material Engineering, University of Sheffield, UK for their facilities in impedance related work for this study.

references

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