Recrystallization of fine-crystalline barium titanate in low-density water medium

https://doi.org/10.1016/j.supflu.2020.104771Get rights and content

Highlights

  • BaTiO3 recrystallization occurred in subcritical vapor or supercritical water.

  • Reversible interaction of BaTiO3 with H2O enhanced its solid-phase mobility.

  • BaTiO3 recrystallization changed its crystal shape and dispersion composition.

  • BaTiO3 recrystallization was accompanied with increase in tetragonality.

Abstract

Fine-crystalline barium titanate was synthesized from a mixture of barium hydroxide monohydrate and titanium dioxide in water vapor medium at 200 °C and 1.55 MPa and subsequently treated in subcritical vapor and supercritical water at 200−400 °C and 1.55-26.3 MPa. Recrystallization of synthesized powder led to an increase in average crystal size and to tetragonal distortion of BaTiO3 structure. Formation of highly tetragonal BaTiO3 was observed after the treatment in supercritical water at 400 °C, 26.3 MPa. Treatment of BaTiO3 crystals in water vapor and supercritical water made for elimination of structural defects and for changes in the morphology and dispersion composition of the powder. The results are of importance for effective production of barium titanate powder of different dispersion as raw material for ferroelectric ceramics.

Introduction

Nowadays metal-oxide-based materials count a great deal in different branches of industry such as production of energy storage and memory devices, coatings, catalysis, etc. [[1], [2], [3]]. Metal oxides applications are closely related to possibility of tailoring their structural and morphological properties. Methods of synthesizing metal oxides appear of interest if they allow control of phase content, particle shape and size. Some of the state-of-the-art approaches involve the use of water vapor below the critical point and/or supercritical water as a medium for treatment of oxide or organometal precursors [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. The process could be realized either in a flow or in a batch reactor and yields pure crystalline product of size ranges from nano to microns. Depending on the nature of reagents, the reaction could be routed by hydrothermal-like or sol-gel-like mechanisms [4]. Sub- and supercritical hydrothermal synthesis seems to be more attractive from the economic and ecological viewpoints: the process requires less expensive reagents and auxiliary compounds and could be characterized by higher atomic efficiency. In this case the reaction of starting salts or hydroxides might proceed by a combination of two mechanisms: dissolution-precipitation and in situ transformation [[14], [15], [16], [17], [18], [19], [20]]. The former involves nucleation of the product from a solution, while the latest implies formation of product layer at the surface of initial reagent particles. Which of two mechanisms prevails, depends on the state of reaction medium and on the properties of reagents such as phase modification, crystallinity, particle size, etc. Activation of the reagents occurs via hydration and hydroxylation followed by their dissolution or remaining of the compound in a solid state. In sub- and supercritical conditions water molecules and hydroxyl groups play one of the key roles in reagents’ transformation to the product [21,22]. As it was demonstrated for the synthesis of different complex oxides (BaTiO3, PbTiO3, Pb(Zr,Ti)O3, SrTiO3, Zn2SnO4, etc.), when dissolution-precipitation mechanism is realized in a liquid medium or high-density supercritical water, the reaction occurs between hydrated and hydroxylated species in a solution [21,23,24]. On the other hand, simple metal oxides structure formation is known to proceed owing to solid-phase mobility in precursors which appears at elevated temperature in the medium of subcritical vapor or low-density supercritical water (thermovaporous treatment, TVT) [[25], [26], [27]]. For instance, well-crystallized α-Al2O3 was obtained from boehmite AlOOH as a precursor [28]. Solid-phase mobility promoted by TVT of simple oxides mixture provides formation of complex oxide structures such as Y3Al5O12, MgAl2O4, LaAlO3, ZnAl2O4, BaFe12O19, LiNbO3, BaTiO3 [26,[29], [30], [31]]. TVT is accompanied by dissociative adsorption of water molecules on the surface of simple oxide particles and gradual saturation of their volume with hydroxyl groups [26,32]. Finally, the processes of hydration and dehydration of metal oxide matrix achieve quasi-equilibrium. Numerous acts of formation and breaking of new Me-O-Me bonds are accompanied by local rearrangements, which determine the solid-phase mobility and result in ordering of the crystal structure. Impact of water on the structural transformation of solid oxides results in significant decrease in the reaction temperature compared to the processes in anhydrous media. For example, α-Al2O3 is formed from transition modifications of aluminum oxides in air on heating above 1100 °C [[33], [34], [35]]. In TVT conditions, formation of fine-crystalline α-Al2O3 occurs at 400 °C [25,26]. Barium titanate could be obtained by TVT of barium and titanium oxides mixture at temperature as low as 110 °C [36] while in anhydrous medium heating up to 850−1400 °C is required for this reaction [37,38].

The oxide structures formed in sub- and supercritical water were revealed to contain residual hydroxyl groups trapped in oxygen vacancies as well as chemically or physically adsorbed at the surface of particles [22,26,[39], [40], [41]]. The residual structural hydroxyls were shown to favor recrystallization of oxides, for example, α-Al2O3 during TVT [42]. When discussing the recrystallization phenomenon, we imply the process of crystal growth in a single-phase powder which occurs owing to mass transfer from smaller to larger crystals and is accompanied by defect elimination and structure perfection. Practically, recrystallization is of high importance since it allows improvement of structure, morphology and functional properties of metal oxides [[43], [44], [45], [46]]. Current paper is devoted to study of recrystallization in fine-crystalline BaTiO3 in the medium of subcritical vapor and supercritical water.

Fine BaTiO3 powder is widely used as a raw material for capacitor ceramics, piezoceramic voltage transducers, PTC resistors, etc. [[47], [48], [49]]. Production lines require BaTiO3 powders of different fractional composition depending on functionality of final material. Additional processing of the synthesized BaTiO3 powder in the reaction medium of vapor or supercritical water studied in the present work would contribute to the improvement of its quality as a material for electronics industry.

Section snippets

Experimental

Standard commercially available reagents were utilized in the synthesis of BaTiO3: Ba(OH)2·H2O (>99 % purity) with particle size of less than 50 μm and TiO2 (>98 % purity) in anatase modification with average particle size of 0.122 ± 0.001 μm. TVT of the mixed reagents was carried out in laboratory autoclaves of 17 ml internal volume following the technique described elsewhere [31,50]. The mixture of starting oxides with the molar ratio BaO/TiO2 = 1.25 was prepared by their joint fivefold

Results and discussion

Fig. 1 shows SEM image, XRD pattern, and crystal size distribution for each of the synthesized BaTiO3 samples. The synthesis was carried out in the same conditions (the first stage of TVT). Conditions of the second stage of TVT differed among the samples. Single phase composition of barium titanate was revealed for all the samples s1-s4. The shape of crystals formed at 200 °C appeared nearly spherical (Fig. 1, sample 1). Prolonged treatment at 200 °C resulted in the increase of CSR size of BaTiO

Conclusion

Crystalline barium titanate synthesized from titanium dioxide and barium hydroxide monohydrate in a medium of water vapor (200 °C, 1.55 MPa) underwent recrystallization during subsequent prolonged treatment in vapor or supercritical water at 200−400 °C, 1.55–26.3 MPa. Formation of BaTiO3 powder proceeded in two stages: first, rapid formation of crystalline barium titanate, and second, slow mass redistribution between the crystals accompanied by ordering of their structure. While the average

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

The work was carried out with the financial support of The Ministry of Education and Science of the Russian Federation within the framework of the State Task “The Development of a Perspective Method for Obtaining of Ultra Dispersive Barium Titanate (BaTiO3) as a Main Component in the Production of Ceramic Capacitors” No. 0699-2017-0005 from May, 31 2017 and with the use of equipment of the Centre of Collective Usage “High Technology in Engineering” of Moscow Polytech (Number for publications:

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