Low temperature synthesis of SrxBa1−xTiO3 (x=0.0–0.2) powders

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Journal of Alloys and Compounds 335 (2002) 101–104

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Low temperature synthesis of Sr x Ba 12x TiO 3 (x50.0–0.2) powders Xiuling Jiao, Dairong Chen*, Yuting Zhao Department of Chemistry, Shandong University, Jinan 250100, PR China Received 9 May 2001; received in revised form 25 July 2001; accepted 25 July 2001

Abstract Srx Ba 12x TiO 3 (x50.0–0.2) powders have been hydrothermally synthesized from aqueous solution at temperatures as low as 808C using gel powders as precursors. Effects of the reaction conditions such as alkali concentration, temperature and strontium content of precursor on the crystallization of the oxide powders were studied in detail. The results indicated that the as-prepared powders had a small particle size of 20–30 nm, which was much smaller than those prepared from hydroxides.  2002 Elsevier Science B.V. All rights reserved. Keywords: Ferroelectrics; Nanostructures; Chemical synthesis; X-Ray diffraction; Scanning and transmission electron microscopy

1. Introduction

2. Experimental

Sr x Ba 12x TiO 3 powders have attracted great interest because of their extensive applications as multilayer capacitors, ferroelectric memories, surface acoustic wave devices, etc. [1]. The powders were conventionally synthesized via solid state reaction [2] and improved methods such as sol–gel [3,4] and co-precipitate techniques [5–7], which require a high temperature treatment to obtain crystalline oxide powders. The high temperature treatment usually leads to an irregular morphology and a high agglomeration of the resulting oxides, which is detrimental to the properties of the ceramics. As a novel route for preparing oxide powders, the hydrothermal method can be applied without a high temperature process, and the purity, morphology, shape and phase compositions of the resulting powders can be well-controlled [8,9]. Thus, this method is especially attractive for preparing both single- and multicomponent powders, and several oxides including Sr x Ba 12x TiO 3 powders have been successfully prepared by the hydrothermal method in the last decade [10–13]. However, a temperature higher than 1008C is always necessary for the hydrothermal synthesis of oxides. Here, a low temperature process, which is a combination of sol– gel and hydrothermal methods, is reported and crystalline Sr x Ba 12x TiO 3 particles with nanometer size were obtained at temperatures as low as 808C.

Before use as a reactant, Sr(NO 3 ) 2 ?nH 2 O was recrystallized from concentrated HNO 3 to obtain anhydrous Sr(NO 3 ) 2 . The general process applied to synthesize Srx Ba 12x TiO 3 powder is schematically shown as follows and all manipulations were conducted under N 2 atmosphere

*Corresponding author. Tel.: 186-531-856-4280; fax: 186-531-8568690. E-mail address: [email protected] (D. Chen).

Ti(OC 4 H 9 -n) 4 mixing and refluxing Sr(NO 3 ) 2 → mixed solution Ba(MOE) 2 / MOE adding 2,4-pentanedione

→

adding MOE – H 2 O mixture

→

precursor solution transformation

sol

→

gel

aging, drying

→

hydrothermal reaction

xerogel

→

Sr x Ba 12x TiO 3 powders (MOE: CH 3 OCH 2 CH 2 OH) XRD patterns of the prepared powders were recorded on a Rigaku D/ MAX III diffractometer with Cu Ka radiation ( l 50.15418 nm) and Ni filter. The percent crystallinity of the prepared powders was calculated according to the intensities of the 101 reflections of the XRD patterns (the area of the 101 reflection). The strongest intensity of this reflection was regarded as 100% crystallinity for convenient comparison. A JEM-100CXII transmission electron microscope was used to observe the powder particle size and morphology. Laser scattering and diffraction (LSD)

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X. Jiao et al. / Journal of Alloys and Compounds 335 (2002) 101 – 104

was applied to estimate the size distribution of the aggregates of the primary particles.

3. Results and discussion

3.1. Synthesis To investigate the effect of strontium content in the gel powders and reaction temperature on the crystallization of Sr x Ba 12x TiO 3 powders, precursors with different strontium contents were treated at different temperatures while the KOH concentration was held at 0.8 mol dm 23 . The results shown in Fig. 1 indicated that the required reaction temperature increased as the strontium content increased in the precursor. For example, well-crystallized Sr x Ba 12x TiO 3 powders could be obtained at 808C when the molar ratio of strontium to barium was 1:19, while 908C is necessary to prepare well-crystallized Sr x Ba 12x TiO 3 powders at a molar ratio of Sr / Ba51:4. The ionic radius of Sr 21 (0.127 nm) is much smaller than that of Ba 21 (0.143 nm) [14]. According to the formula ] t5(rA 1r O ) /Œ2(r B 1r O ), the tolerance factors t are, respectively 0.897 and 0.844 when the A atoms are Ba 21 and Sr 21 . With the replacement of Ba 21 by Sr 21 , the tolerance factor lay between 0.897 and 0.844, which showed that the perovskite structure could be kept in Ba 12x Sr x TiO 3 crystals. However, a distortion of the framework should take place and the tolerance factor exhibited a larger departure from 1 with the replacement of Ba 21 by

Fig. 1. Crystallinity of Sr x Ba 12x TiO 3 powders with different strontium content. KOH concentration: 0.8 mol dm 23 , x-values: (a) 0.05, (b) 0.10, (c) 0.15, (d) 0.20.

Sr 21 in the BaTiO 3 lattice due to their different ionic radius, which did not favor the formation of perovskite structure of Ba 12x Sr x TiO 3 . Thus, a higher reaction temperature was required with increasing strontium content in the gel powders. It is well known that alkali concentration greatly affects the crystallization of oxide powders during the hydrothermal process. In the present procedure, a suitable KOH concentration was necessary to obtain crystalline Sr x Ba 12x TiO 3 powders. Fig. 2 shows the XRD patterns of the prepared Sr x Ba 12x TiO 3 powders using different alkali concentrations at 808C while the molar ratio of Sr to Ba was held at 1:9. The crystallinity of the products gradually increased with the alkali concentrations increasing from 0.05 to 2.4 mol dm 23 , while it decreased when the alkali concentration was higher than 2.4 mol dm 23 . The optimal KOH concentration for preparing well-crystalline Sr 0.1 Ba 0.9 TiO 3 was 0.8–2.4 mol dm 23 at 808C from our experiments. In addition, the suitable alkali concentration changed as the strontium content in precursors changed. Table 1 lists the suitable alkali concentrations at different temperatures with different strontium contents. It revealed that the necessary alkali concentration increased as the molar ratio of Sr to Ba increased from 1:19 to 1:4, while higher temperatures were needed for low alkali concentrations. In terms of these results, a higher temperature or alkali concentration was needed to form Srx Ba 12x TiO 3 powder with increasing strontium content in the gel powder.

Fig. 2. XRD patterns of the Sr x Ba 12x TiO 3 powders with different KOH concentrations, x50.10, alkali concentration: (a) 0.05, (b) 0.8, (c) 1.6, (d) 2.4 mol dm 23 .

X. Jiao et al. / Journal of Alloys and Compounds 335 (2002) 101 – 104 Table 1 The optimal alkali concentration for preparing Sr x Ba 12x TiO 3 powders at different temperatures from precursors with different strontium content Sample

Sr 0.05 Ba 0.95 TiO 3 Sr 0.10 Ba 0.90 TiO 3 Sr 0.15 Ba 0.85 TiO 3 Sr 0.20 Ba 0.80 TiO 3

Alkali concentration (mol dm 23 ) A

B

C

0.2–1.6 0.8–2.4 0.8–2.6 –

0.1–1.0 0.2–1.3 0.4–1.2 0.8–2.0

0.1–0.8 0.2–1.0 0.3–1.0 0.4–1.4

The reaction temperature for A, B and C were, respectively 80, 90 and 1008C.

For comparison, the metal hydroxide co-precipitation was used to prepare Sr x Ba 12x TiO 3 powders. After reacting at 1108C for 12 h with a KOH concentration of 1.0 mol dm 23 , crystalline Sr 0.1 Ba 0.9 TiO 3 formed, while 1308C for 8 h was necessary to prepare well-crystalline Sr 0.1 Ba 0.9 TiO 3 powders at the same KOH concentration. Further experiments (Fig. 3) showed that the necessary reaction time decreased from 11 to 6 h with the strontium content in the starting precipitate increasing from 0.05 to 0.20 as the reaction temperature and KOH concentration

103

were held at 1308C and 1.0 mol dm 23 . This is different from the preparative process using gel powders. The result revealed that the replacement of Ba 21 by Sr 21 in the BaTiO 3 lattice was thermodynamically unfavorable to the formation of the framework but it was favorable to the kinetic process using hydroxides as reagents. From the comparison of the preparative conditions using the above two precursors, it can be concluded that crystalline Sr x Ba 12x TiO 3 powders can be obtained at a lower temperature from the gel powder than from hydroxide co-precipitation. Thus, the gel powder had a higher reactivity compared with the hydroxide. The result can be explained as follows. Sr(II), Ba(II) and Ti(IV) atoms in the gel powder were homogeneous on the molecular scale through the reaction of Ti(OC 4 H 9 -n) 4 , Sr(NO 3 ) 2 , Ba(MOE) 2 and the hydrolysis and condensation in the mixed solvent, and some bonds similar to those in crystalline Sr x Ba 12x TiO 3 formed, which led to a smaller energy barrier to nucleation compared to the hydroxide co-precipitation [15,16]. In addition, the crystallization rate accelerated with increasing strontium content in hydroxides, which is different from the synthesis from gel powder. It indicated that increasing strontium content in hydroxides was kinetically favorable to the crystallization of the Sr x Ba 12x TiO 3 , while it was not for the gel powder.

3.2. Particulate property

Fig. 3. Crystallization time as a function of strontium content in hydroxide.

It is well known that the particle size and morphology, as well as its dispersibility greatly affect the electrical properties of ceramics. Table 2 lists the particulate properties of the powders from different starting materials. The particle size of powders from the gel was much smaller than that from hydroxide co-precipitation. The particles from gel powders had nanometer size with an irregular morphology, while that from hydroxides had a large particle size about 0.1–0.2 mm and cubic morphology. The formation of oxides by the hydrothermal method includes two processes — nucleation and grain growth, which compete with each other. The small size of the particles obtained from the gel powder may be due to rapid nucleation rather than grain growth, while the grain growth rate is relatively fast using hydroxide as reagent and large particles form. In addition, the LSD results (Fig. 4)

Table 2 Particulate properties of powders from gel powders and hydroxide Sample

From gel powders Particle size (nm)

Sr 0.05 Ba 0.95 TiO 3 Sr 0.10 Ba 0.90 TiO 3 Sr 0.15 Ba 0.85 TiO 3 Sr 0.20 Ba 0.80 TiO 3 a

2065 2065 30610 30610

Particle size taken from the TEM graphs.

a

From hydroxide BET surface area (m 2 g 21 )

Particle size a (nm)

BET surface area (m 2 g 21 )

38.2 37.6 24.3 24.7

100610 140620 140620 200630

9.2 6.4 6.6 4.2

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X. Jiao et al. / Journal of Alloys and Compounds 335 (2002) 101 – 104

tion at temperatures as low as 808C. The necessary reaction temperature and alkali concentration increased with increasing strontium content in the precursor and the borderline alkali concentration for preparing well-crystallized powders decreased with increasing temperature. Compared with those prepared via a conventional hydrothermal process from hydroxide, the powder particles had a much smaller size, an irregular morphology and displayed a higher agglomeration.

References

Fig. 4. Particle size distribution of Sr x Ba 12x TiO 3 powders by LSD measurement; x-values: (a) 0.05, (b) 0.10, (c) 0.15, (d) 0.20.

revealed that particles obtained from gel powders exhibited a larger particle size than that observed from the TEM graphs. TEM graphs revealed the real size of the particles, while the LSD method measured the size of the aggregates. The great difference of the size between the TEM graphs and the LSD measurements indicated that the Sr x Ba 12x TiO 3 nanoparticles had a high degree of agglomeration.

4. Conclusions Sr x Ba 12x TiO 3 (x50.0–0.2) powders with nanometer sizes have been successfully prepared from aqueous solu-

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