Barium titanate containing glass-ceramics - The effect of phase composition and microstructure on dielectric properties

Abstract

From a glass with the mol% composition 20.1 Na₂O-3 Al₂O₃-23.1 BaO-23 TiO₂-7.6 B₂O₃-17.4 SiO₂-5.8 Fe₂O₃ cubic barium titanate was crystallized. Scanning and transmission electron microscopy micrographs of the prepared glass-ceramics show fine crystals agglomerated to globular particles. With increasing crystallization time, their volume concentration increases and the particles form interconnected aggregates. From electrical impedance measurements of selected glass ceramics, the electrical conductivity and the dielectric permittivity of the prepared materials were determined for temperatures from −170 °C to 97 °C, and frequencies from 100 Hz to 100 kHz. The typical values of the resistance are in the order of 10⁸ Ω and the dielectric constants in the order of 40 at 100 kHz at room temperature. The two diffuse relaxor type phase transitions observed from −123 to −73 °C and from −33 to 0 °C are attributed to the transformation rhomboherdral - orthorhombic and orthorhombic - cubic BaTiO₃, respectively.

Introduction

Efficient energy storage is one of the most important problems nowadays and the synthesis of materials with controllable dielectric properties is a great challenge. During the past few decades, materials containing nano- and submicron-sized crystals from the perovskite type based on barium titanate (BaTiO₃) were intensely investigated [[1], [2], [3], [4], [5], [6]]. BaTiO₃ is known for its excellent dielectric properties [1,6] and when present as a single crystal, it may also exhibit particular optical properties [3]. It may occur in several allotropic modifications which in a different extent exhibit the above mentioned physical properties and depending on the specific application needed, different modifications are preferred. Thus, the respective materials can find applications as part of magnetic and resistive sensors [[7], [8], [9]], in electronics as an environmental friendly and cheap material for energy storage and as components of devices operating in the microwave frequency range [[10], [11], [12], [13], [14], [15]] and as components of solid oxide fuel cells operating at intermediate and higher temperatures [[16], [17], [18], [19], [20], [21]].

The two polymorphs of BaTiO₃ mostly preferred for practical applications are the tetragonal and cubic ones. Usually, the thermodynamically stable BaTiO₃ phase at room temperature is the tetragonal and, at temperatures higher than 120–130 °C, it transforms to the cubic modification [1,5,6]. The preparation procedure strongly affects the size of the BaTiO₃ crystals and the type of the formed polymorph and thus, determines the physical properties and possible applications [1,3,8]. Traditionally, barium titanate is prepared via solid-state syntheses at temperatures ≥1300 °C [2,22,23] which often leads to inhomogeneities in the composition and the structure. If high chemical homogeneity, a homogeneous particle distribution in the material volume and a controllable degree of crystallinity is required, other synthesis methods are advantageous. Among these methods is the controlled crystallization of glasses [8,[24], [25], [26], [27], [28], [29], [30]], which goes via a homogeneous state, the glass, as an intermediate. The proper selection of the glass composition as well of the subsequent thermal treatment regimes allow the preparation of barium titanate crystals with controllable size distribution and dielectric constants varying from moderate to very high values at room temperature. The value of the dielectric constant and hence also the potential application depends on the type of BaTiO₃ polymorph precipitated in glass. For a large number of applications, the paraelectric cubic phase is preferred. In order to stabilize the cubic barium titanate phase, to control the conduction mechanism and to decrease the ferroelectric-paraelectric transformation temperature, oxides of other elements, such as Fe₂O₃, Al₂O₃, K₂O, SrO or ZrO₂ are added to the raw materials [2,4,[9], [10], [11], [12],31]. Depending on the valency of the additives and the ionic radii, they may substitute either Ba²⁺ or Ti⁴⁺ which has a very different effect on the physical properties. For example, if present in the form of Fe²⁺, iron is likely to substitute Ba²⁺, however, as Fe³⁺, it will, most probably, be incorporated at a Ti⁴⁺ site. Depending on the type of substitution - donor or acceptor-on Ba²⁺ or Ti⁴⁺ sites, different temperature dependences of the dielectric properties and the conductivity will be obtained.

A special challenge is the coherent growth of BaTiO₃ and Fe₃O₄ because the obtained compound materials possess multiferroic properties [24] and find application in microelectronics (elaboration of magnetic memories and switches), and in sensor technology due to the possibility for miniaturization and in spintronics [24,31]. In the literature, reports on preparation and properties of nanocomposite multiferroic materials, such as combinations of BaTiO₃ and NiCuZn as well as of CoFe₂O₄ nanotubes incorporated into a BaTiO₃ matrix [31] are found. The synthesis of BaTiO₃ and thereon based materials containing 3d-transition elements from borate and borosilicate glasses [24,25,[27], [28], [29]] was reported as well. One glass system which enables the precipitation of barium titanate is Na₂O/Al₂O₃/BaO/TiO₂/B₂O₃/SiO₂/Fe₂O₃ which has previously been reported [8,[27], [28], [29]]. Here, due to the high concentration of alkaline-earth and transition metal oxides, the precipitation of barium titanate and iron containing barium titanate with controllable crystallite size is possible [29]. Based on former studies of the present group, the effect of glass composition on the phase composition and the microstructure was reported [8,[27], [28], [29]] especially of the ratio [Na₂O]/[Al₂O₃] [27]. Thus, the composition 20.1 Na₂O-3 Al₂O₃-23.1 BaO-23 TiO₂-7.6 B₂O₃-17.4 SiO₂-5.8 Fe₂O₃ was selected in which, depending on the applied time-temperature schedule, the crystallization of BaTiO₃ or a combination of BaTiO₃ and BaTi_1-xFe_xO_3-δ with controllable size and volume fraction was possible.

The present work reports on the synthesis of glasses with the composition 20.1 Na₂O-3 Al₂O₃-23.1 BaO-23 TiO₂-7.6 B₂O₃-17.4 SiO₂-5.8 Fe₂O₃ (in mol %) from which, after appropriate thermal treatment, barium titanate is crystallized. The phase composition, and microstructure are reported. The main part of the study is dedicated to the electrical properties of the glass-ceramics as a function of frequency and temperature and on possible phase transitions of barium titanate below room temperature.