Effect of synthesis route on the microstructure of SiO 2 doped bismuth titanate ceramics

Synthesis and characterization of SiO2 doped bismuth titanate ceramics were investigated. Four investigated compositions belonging to the system Bi2O3-TiO2-SiO2 are located in the section Bi4Ti3O12-SiO2, near by it and in the binary Bi2O3-TiO2 system. Melt quenching was applied for synthesis of SiO2 doped bismuth titanate ceramics and for the sample 40Bi2O3·60TiO2. The binary sample 70Bi2O3·30TiO2 is prepared by gradient heating of bulk materials near the liquidus temperature (modified solid state reaction). The influence of the thermal treatment on the phase formation and microstructure was evaluated using XRD, EDS and SEM. The binary samples prepared by solid state reaction at low temperature (1000°C) possess poly-phased dense microstructure, while secondary crystallization combined with porosity formation is typical for the sample obtained at high temperature (1150°C). The ternary Bi2O3-TiO2-SiO2 samples, obtained by supercooling of the melts down to room temperature, are thermally treated at 700, 800°C. They consist of elongated crystals in amorphous matrix. The crystals have lower Bi2O3-content and higher TiO2-content than the nominal batch composition. The XRD data show that the main crystalline phases in the ceramics produced by melt quenching method and solid state reaction are β-Bi2O3, Bi4Ti3O12 and one unknown new phase. It is proved that the applied methods of synthesis are suitable for generation of different microstructures in the bulk SiO2 doped bismuth titanate ceramics, which is promising basis for modification of their electrical properties.


I. Introduction
The application of Aurivillius family of bismuthbased ferroelectric compounds with a layered structure [1] in capacitors, sensors, piezoelectric and electroop tic devices [2][3][4] is strongly influenced by the method of preparation.Doping with different cations is used to control the point defects.Various low-and high-temperature routes of synthesis are used: sol-gel method [5][6][7], hydrothermal crystallization [8], metal-organic decomposition [9] and others.Recently crystallization from melts and glasses is also applied [10][11][12][13][14].This method gives pos-sibility to control the particle size evolution during the transition from amorphous to crystalline state and to achieve suitable crystallographic orientation in the polycrystalline materials.
In our previous study, the phase formation in the system Bi 2 O 3 -TiO 2 -SiO 2 from fast quenched melts is investigated [15].It is established that the introduction of 20-40 mol% SiO 2 stimulates the partial amorphization of the samples.It is known, that the properties of some ferroelectric ceramics can be enhanced by texturing of their structure as in the case of partial grain orientation in polycrystalline Bi 4 Ti 3 O 12 ceramics obtained by sol-gel, magnetic alignment and other methods [16][17][18][19].All these facts motivate our further investigations.
The purpose of the present study is to elucidate the influenceoftheheattreatmentonthephaseformation and microstructure of SiO 2 doped bismuth titanate ceramics obtained by melting.

II. Experimental
The selection of compositions with high melting temperatures was made according to the data for the liquidus temperature in the system Bi 2 O 3 -TiO 2 [13,20,21] as well as to the glass formation in the system Bi 2 O 3 -SiO 2 [22,23].Four compositions (in mol%) were selected: one in the section Bi 4 Ti 3 O 12 -SiO 2  (24Bi 2 O 3 •36TiO 2 •40SiO 2 )  For all samples the phase formation was studied by X-raydiffractionanalysis(XRD-TURM62,Cu-Kα radiation) and energy dispersive spectroscopy (EDS -EDAX 9900).The microstructure was observed by scanning electron microscopy (SEM -525M, Philips).For the EDS analysis the samples were prepared by mechanical polishing and covered with thin carbon film,whiletheSEMobservationsweremadeonfresh fracturedsurfacesalsocoveredwiththincarbonfilm.

III. Results and discussion
The investigation started with a model binary composition 70Bi 2 O 3 •30TiO 2 obtained by solid state reaction with heating for 1 hour in the temperature range 1000-1150°С.TheSEManalysisillustratestheformationof different microstructures in the sample sintered at lowtemperature, 1000°С (Fig. 2a).Secondary crystallization combined with porosity formation is observed afterthehigh-temperature(1150°С)treatment(Fig.2b), where clearly shaped cubic-like crystals are formed.The sample prepared at low-temperature is poly-phased one and its XRD data contains the patterns of the phase β-Bi 2 O 3 andsomepatternsofothernon-identifiedphases(Fig.3a).Themaincrystallinephaseisβ-Bi 2 O 3 (Fig. 3b) for the sample treated at 1150°C.
The sample with composition corresponding to the phase Bi 4 Ti 3 O 12 (40Bi 2 O 3 •60TiO 2 ) was quenched from the melt at 1300°C.The XRD analysis shows mainly the formation of the phase Bi 4 Ti 3 O 12 without texturing effects (Fig. 4).The melt quenched sample with composition 24Bi 2 O 3 •36TiO 2 •40SiO 2 after subsequent thermal treatment at 700°C, consists of elongated crystals in amorphous matrix (Fig. 5).According to the EDS analysis, the crystals and matrix differ in composition: the crystals have lower Bi 2 O 3 -content and higher TiO 2 -content in comparison to the nominal composition (Figs.5a,b).For all detected EDS analysis the statistical uncertainty is less than 4-5 mol%.Full crystallization combined with predominating orientation of the crystals is achieved after batch heat treatment for 5 hours at 800°C (Fig. 5c).Before the thermal treatment according to the XRD results, the sample was amorphous (Fig. 6a), but after long heating, the crystallization has been achieved (Fig. 6b) and a new phase having XRD patterns close to those of the phase Bi 4 Ti 3 O 12 appears.The decreasing of the SiO 2 -content leads to more complex crystallization with participation of several phases including Bi 4 Ti 3 O 12 and some unknown phase (Fig. 7).

IV. Conclusions
The investigation shows for the first time that depending on the conditions of the melting and additional heat treatment of the supercooled samples, different poly phased ceramic materials with various microstructures could be obtained in the system Bi 2 O 3 -TiO 2 -SiO 2 .Formation of the phase Bi 4 Ti 3 O 12 , Bi 2 O 3 -polymorphs and one unknown phase was established.These results arepromisingbasisforcontrolandmodificationofthe electrical properties of the obtained bulk SiO 2 doped bismuth titanate ceramics.