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Home , Barium titanate

GRAIN SIZE EFFECT ON STRUCTURE AND PROPERTIES OF DOPED

B.D.Stojanovic, C.R. Foschini, M.A. Zaghete, Cilence M.(1); F.O.S.Viera, K.Peron J.A.Varela LIEC: Instituto de Química, UNESP, Araraquara, SP. Zip Code: 14.801-970 POBox: 355 Phone: +55 16 201-6712, Fax: +55 16 2227932 E-mail: [email protected]

Abstract It was found that the properties of BaTiO3 dependent of its grain size and that the coarse-grained of pure BT showed much lower dielectric constant at room temperature then fine-grained. In doped BT the effect is more complex. The doped barium titanate was prepared from powders obtained by of commercial barium titanate, and one or two steps procedure from organometallic complex using citrates as precursors. The grain size has strong influence on the microstructure and dielectric properties

Keywords: powder, Barium titanate, Doped, Grain size, Properties

1 INTRODUCTION

To date, barium titanate is the most extensively investigated ferroelectric material, because it is extremely interesting from the point of view of the practical applications [1, 2]. The BaTiO3 (BT) is chemically and mechanically very stable, and exhibits ferroelectric properties at and above room temperature. It can be easily prepared and used in form of ceramic polycrystalline samples. The study of size effect in barium titanate and other ferroelectric systems has lately very important because of their potential applications. It was found that the dielectric properties of BT strongly depend of its grain size. Coarse-grained ceramics ( 20-50 µm ) of pure BT showed lower dielectric constant at room temperature then fine-grained (0.8 to 1 µm). In the doped BT that effect is more complex, because it is necessary to consider also the influence of dopants [3]. Pure barium titanate is a highly insulating material. The semi conducting barium titanate can be produced by addition of dopants and by proper modification of the grains and grain boundary properties .The microstructure and grain boundary play a very important role in determining the electrical characteristics [4, 5]. The semi conducting barium titanate can be produced with replacing on Ba-ion sites by a trivalent ion (e.g. La3+,Sb3+, Y3+), or on the Ti-ion sites by a pentavalent ion (e.g. 5+ 5+ 5+ Nb , Sb , Ta ) on the Ti-ion sites. The powder preparation and conditions strongly affected the microstructure and properties obtained [6, 7]. In view of the above, our paper reports on work carried out on barium titanate doped with niobium, as donor dopant in the presence of small amount of manganese as an acceptor dopant. The influence of powders processing and concentration of dopants on the structure and dielectric properties were analysed.

2. EXPERIMENTAL PROCEDURE

The doped barium titanate (BT) were prepared from three procedures. In two cases BT powders were prepared from polymeric precursors method based on Pechini process [8]. As dopants were used niobium (0.2 up to 0.8 mol%) and manganese oxide (0.01 mol%). The ratio Ba/Ti was 1:1. As raw materials were used barium acetate, tetraisopropoxide, niobium oxide, manganese oxide, citric acid and ethylene glycol. In the first process the doped BT was carried out as a three-stage process using the citrate solutions of all components mixed together (PPM). In the second process a pure BT powder was obtained by PPM and the dopants were added after. This route was called two step procedure (TSP). In the third case BT was prepared starting from commercial BT powders (Aldrich, 99.9 % purity, grain size ~2 µm) mixed with dopants after (BCP). More details about procedures were reported previously [9,10,11]. The obtained powders were calcinated at 700° and 800°C for 2 hours After calcination and previously milling, the powders were pressed at 175 MPa into pellets of 10 x 2.5 mm2 using cold isostatic press. The sintering process was carried out at 1310°C and 1330oC for 2 hours with heating rate 5°C/min and 10°C/min during cooling in air atmosphere. The microstructure was investigated using a scanning electron microscopy (Topcon SM-300). The samples for microstructure were prepared by polishing and thermal etching during 20 minutes at 50 degrees lower than the sintering temperature. The dielectric properties were measured using a impedance analyzer (HP 4192A) coupled with a furnace.

3. RESULTS AND DISCUSSION

It is well known that precursor-prepared barium titanate powders can reach a high chemical homogeneity. The kinetic of phase formation (analyzed by XRD) and the carbonate formation (analyzed by infrared spectroscopy) were reported previously [10]. The tendency of CO2 to adsorb on surface is well established in literature data; however, during our previous experiments, the best conditions for successful elimination of carbonates before calcination process of powders were extrapolated. The carbonaceous compounds were not detected in the sintered bulks of doped barium titanate prepared by all used procedures The dopants have a profound effect on the densification and microstructure evolution of BaTiO3[12]. Niobium is a typical donor dopant and in lower concentration enhances grain growth, while close to or above the limit of dopant in barium titanate it inhibits grain growth [10]. Since the niobium ion has a different valence than that of the barium or titanium ion, substitution by niobium produce a charge imbalance the small amount of Mn could compensate the charge imbalance caused by adding of Nb in barium titanate The microstructure obtained in the PPM samples pointed that the relatively short time of sintering was not enough for significant grain growth and the differences in grain size for barium titanate doped with lower and higher concentrations of dopants were not so evident. The grain size was less than 500 nm for samples sintered at 1310°C (Fig.1a). The small amount of liquid phase, as a result of presence of secondary phase, did not generate dendrite morphology in the fine grains region. The microstructure of doped BT obtained by TSP showed that the grains were larger then in previous case ranging from 1 to 4 µm for the same sintering temperature (Fig.1b). The microstructure obtained in the BCP samples was not uniform and consisted from region with big grains with grain size more then 20 µm and with region with fine grained structure with grain size less then 1 µm (Fig.1c). This pronounced that changeover in the grain growth uniformity was caused by initial powder preparation procedure and that doped BT showed completely various microstructures. The variation of dielectric constant with temperature for samples with various compositions (Fig.2) showed that the niobium has a prominent influence on the dielectric properties. The samples doped with Nb and prepared by PPM showed a rather high dielectric constant from 4500 up to 10700 at the (Tc = 82°C for BT doped with 0.4 mol%, 77°C for BT doped with 0.6 mol% and 70°C for BT doped with 0.8 mol%) (Fig. 2a) and 3000, 6300 and 4500 at room temperature justifying the influence of niobium doping on barium titanate.

FIGURE 1: The micrographies of doped BT samples with 0.4 mol% Nb prepared from (a) PPM , (b) TSP and (c) BCP.

The samples doped with Nb prepared by TSP showed a rather low dielectric constant from 1400 up to 2600 at the Curie temperature (Tc = 98°C for BT doped with 0.2 mol%, 101°C for BT doped with 0.4 mol% , 100°C for BT doped with 0.6 mol% and 105°C doped with 0.8 mol%) (Fig. 3b). The addition of niobium leads to increase of dielectric constant; however the dielectric constant at room temperature was very low, from 400 up to 1000. The barium titanate prepared BCP and doped with 0.6 mol % Nb has been shown not rather well expressed Curie temperature of 119°C (Fig.2 with permissivity of around 1400. This confirms that the properties are strongly dependent on grain size.

12000 (a) 2500 (b) 10000 0.8% Nb 0.2% Nb 0.6% Nb 2000 0.4% Nb ) r

ε 8000 0.4% Nb 0.6% Nb 1500 0.8% Nb

6000

1000 4000

Permissivity ( 500 2000

0 0 0 50 100 150 200 250 300 60 80 100 120 140 160 180 200 Temperatura ( oC) o o Temperatura ( C) Temperature ( C)

1400

1200 ) r ε 1000

800 Commercial BT 100Hz

Permissivity ( 600 1kHz 10kHz 400 100kHz 1MHz 200

0 50 100 150 200 250 3 Temperature (°C)

FIGURE 2: Permissivity vs temperature for BT doped samples prepared from (a) PPM (b) TSP and (c) BCP.

4. SUMMARY

Nb doped barium titanate powder in the presence of small amount of manganese as acceptor dopant was synthesized by polymeric precursor method from citrate solutions by PPM and TSP. The microstructure development obtained by PPM was rather regular with a grain size less than 500 nm, while the grain size of samples prepared by TSP ranged from 1 to 4 µm. The microstructure of barium titanate prepared from commercial BT powders (BCP) showed nonuniform microstructure with region of grains with size more then 20 µm and with others less then 1 µm. The variation of dielectric constant with temperature confirmed that the niobium had a prominent influence on the dielectric properties. Barium titanate doped with Nb and prepared by PPM showed a rather high dielectric constant from 4500 up to 10700 at the Curie temperature, while the TSP route showed a rather low dielectric constant from 1400 up to 2600 at the Curie temperature. The dielectric constant of doped BT obtained from BCP showed the lowest permissivity regarding previous procedures Both the microstructure and dielectric properties depend strongly on powder processing and staring grain size, however the niobium concentration has more influence on the dielectric properties.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the Brazilian research funding institution CNPq (301007/99-3) and FAPESP (98/13678-3 and 00/01991-0) for the financial support for this work.

REFERENCES

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