Elsevier

Ceramics International

Volume 44, Issue 7, May 2018, Pages 7557-7568
Ceramics International

Eu3+ doped ferroelectric CaBi2Ta2O9 based glass-ceramic nanocomposites: Crystallization kinetics, optical and dielectric properties

https://doi.org/10.1016/j.ceramint.2018.01.164Get rights and content

Abstract

We report Eu3+ doped transparent glass-ceramics (GCs) containing bismuth layer-structured ferroelectric (BLSF) CaBi2Ta2O9 (CBT) as the major crystal phase. The CBT crystal phase was generated in a silica rich glass matrix of SiO2-K2O-CaO-Bi2O3-Ta2O5 glass system synthesized by melt quenching technique followed by controlled crystallization through ceramming heat-treatment. Non-isothermal DSC study was conducted to analyze crystallization kinetics of the glass in order to understand the crystallization mechanism. The optimum heat-treatment protocol for ceramization of precursor glass that has been determined through crystallization kinetics analysis was employed to fabricate transparent GCs containing CBT nanocrystals, which was otherwise difficult. Structural analysis of the GCs was carried out using XRD, TEM, FESEM and Raman spectroscopy and results confirmed the existence of CBT nanocrystals. The transmittance and optical band gap energies of the GCs were found to be less when compared to the precursor glass. The refractive indices of the GCs were increased monotonically with increase in heat-treatment time, signaling densification of samples upon heat-treatment. The dielectric constants (εr) of the GCs were progressively increased with increase in heat-treatment duration indicating evolution of ferroelectric CBT crystals phase upon heat-treatment.

Introduction

Mixed bismuth oxide layer compounds (known as Aurivillius compounds) were reported first by Bengt Aurivillius [1] in 1949, which was followed by investigation of ferroelectricity in these compounds by Smolensky et al. [2]. A member of the Aurivillius family, bismuth layered-structure ferroelectrics (BLSFs) has attracted great attention in recent years due to their unique properties, such as large dielectric breakdown strength (BDS), high Curie temperature (TC) and excellent fatigue free polarization [3], [4]. These properties make them a promising candidate for application in ultrasound transducers, high temperature piezoelectric devices, thin film capacitors, pyroelectric sensors, optical memories, displays and non-volatile ferroelectric random access memories (FE-RAM).

The BLSF crystals are represented by the general formula (Bi2O2)2+(Ax−1BxO3x+1)2−, where (Ax−1BxO3x+1)2- are the pseudo-perovskite unit cells interspaced between bismuth oxide ((Bi2O2)2+) layers along the c-axis. In the pseudo-perovskite block ‘A′ can be a monovalent (Na+, K+), bivalent (Ca2+, Ba2+, Sr2+, Pb2+) or trivalent cation (Bi3+), ‘B′ represents a tetravalent, pentavalent, or hexavalent ion (Ti4+, Nb5+, Ta5+, W6+, and V6+) and x can be any integer or ½ integer. The CaBi2Ta2O9 (CBT) crystal belongs to the BLSF family which consists of double TaO6 octahedral units within the (CaTa2O7)2− perovskite blocks stacked between the (Bi2O2)2+ layers [5]. It is a polar orthorhombic crystal phase with A21am space group [6]. The presence of small sized Ca2+ ion (ionic radius 1.00 Å) in the A-site of (ATa2O7)2− layers leads to a significant lattice mismatch between the CaO and TaO2 planes of the layers which caused structural distortion of crystals. Due to this structural distortion, the CBT crystals become non-centrosymmetric which contributes to larger spontaneous ferroelectric polarization [7]. CBT also exhibits higher Curie temperature (> 600 °C). Das et al. [8] synthesized CBT thin films and reported some of its ferroelectric and dielectric properties. A maximum polarization of 13.4 μC/cm2, remanent polarization of 3.44 μC/cm2, coercive field strength of 112 kV/cm, dielectric constant of 116 at 100 kHz and low dielectric loss of 0.01 were reported. Photoluminescence properties of rare earth doped CBT ferroelectrics synthesized through conventional solid state reactions technique have been reported in recent times. Ruirui et al. observed outstanding tunable emissions and generation of warm white color in Eu3+/Tb3+ co-doped CBT powders [9]. Strong red emission peaks at 622 nm for Pr3+ and 615 nm for Eu3+ were observed for CBT powders doped with 0.02 mol% of Pr3+ and 0.15 mol% of Eu3+ respectively [10]. They also reported significant enhancement of luminescence intensity of Eu3+ ion in CBT by co-doping with La3+ and the highest emission intensity was reported for europium ion concentration of 0.15 mol% [11]. Such rare earth doped non-centrosymmetric CBT crystals along with their inherent ferroelectric and non-linear optical (NLO) properties could be useful for designing of future generation opto-electronic devices [12].

In comparison to ferroelectric ceramics, fabrication of ferroelectric glass-ceramics (FGCs) through melt-quenching technique offers several advantages like zero/low porosity, higher mechanical and dielectric breakdown strengths and the possibility of altering the properties by varying the volume fraction of the ferroelectric phase and size of the crystals dispersed in the glass matrix. Transparency in FGCs can be achieved through homogenous nucleation and controlling growth of crystallites in nanometer range that are too small for Rayleigh scattering. However, for application of transparent FGCs in integrated optical devices, the size of the crystallites must be large enough to generate a reasonably good ferroelectric response. A tradeoff between these two attributes remains a big challenge for the researchers [13]. Very recently we have reported the optical and dielectric properties of Eu3+ doped BaBi2Ta2O9 (BBTE) glass and glass-ceramics and observed favorable properties, like, a five-fold increase in photoluminescence intensity in the BBTE glass-ceramics and higher dielectric constant (εr) (> 100) along with low dielectric losses and dissipation factors [14]. To the best of our knowledge, there is no report available in the literature on rare earth doped transparent nanostructured glass-ceramics containing ferroelectric CBT (CaBi2Ta2O9) crystal phase, in spite of its favorable properties making it suitable for various advanced applications.

In the present work, we report for the first time synthesis of Eu3+ doped ferroelectric CaBi2Ta2O9 (CBTE) glass-ceramics using conventional melt quenching and ceramming technique. A comparative study of crystallization kinetics of the CBTE glass has been made through linear (Kissinger, Augis-Bennett, Ozawa, Matusita) and multivariate nonlinear (nth dimension Avrami-Erofeev) model-fitting approaches to ascertain the nature of nucleation and mechanism of crystal growth. A novel approach of isothermal prediction has been adopted through the nonlinear kinetics study in order to optimize the heat-treatment protocol for controlled crystallization. The theoretically optimized heat-treatment protocol was successfully exploited experimentally to synthesize transparent CBTE GCs by controlled crystallization. The microstructure, thermal, optical and dielectric properties of the CBTE GCs has been correlated to their processing conditions.

Section snippets

Experimental procedure

Precursor CBTE glass was prepared using a molar composition of 17.5K2O-51.3SiO2-10BaO-10Bi2O3-10Ta2O5 doped with 0.5 mol% of Eu2O3 and 0.7 mol% of CeO2. The glass was synthesized through melt quenching technique by preparing glass batch of 120 g and melting in a platinum crucible at 1475 °C for 2 h employing an electric furnace. High purity raw materials are used for synthesis of glass, such as silica, SiO2 (99.8%; Sipur A1 Bremtheler Quartzitwerk, Usingen, Germany), anhydrous potassium

Physical properties

The CBTE glass is a heavy metal oxide (HMO) glass containing 10 mol% of Bi2O3, prepared by melting at 1475 °C in a platinum crucible. When melted at above 1000 °C color of the glass changes to black from pale yellow [15]. The coloration was intensified and become black with the increase in melting temperature. This is due to the auto thermal reduction of Bi2O3 in air atmosphere, which could be contained by addition of some oxidizing agents like Sb2O3, As2O3 and CeO2. In the present glass

Conclusions

Eu3+ doped SiO2-K2O-CaO-Bi2O3-Ta2O5 (CBTE) glasses have been synthesized and reported for the first time. Detailed crystallization kinetics analysis of the glass was done and a homogenous nucleation with a diffusion controlled three dimensional crystal growth has been determined. The prediction of optimum crystallization temperature and time has been possible from the kinetics study, which was experimentally exploited to fabricate transparent CBTE glass-ceramics. The results of XRD, TEM, and

Acknowledgements

Anirban thanks UGC (212510213), India for his Junior Research Fellowship (JRF) grant. The authors are thankful to Dr. Ranjan Sen, Head, Glass Division and Dr. K. Muraleedharan, Director CSIR-CGCRI for their constant support and encouragement.

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