Electrical properties of niobium doped barium bismuth-titanate ceramics
Graphical abstract
Highlights
► Pure and doped BaBi4Ti4O15 were prepared via the solid-state reaction method. ► The grain size was suppressed in Nb-doped samples. ► The diffuseness of the dielectric peak increased with dopant concentration. ► Niobium affected on relaxor behavior of barium bismuth titanate ceramics. ► The conductivity change was noticed in doped samples.
Introduction
Ferroelectric materials with diffuse phase transition (DPT) characteristics and/or relaxor properties have been extensively studied in the last few decades mainly due to their very interesting and still not completely explained physical properties [1], [2], [3]. Relaxor materials are characterized by the diffuse phase transition (DPT) which has strong frequency dependence. Different models are proposed to explain the relaxor ferroelectric behavior and it is generally difficult to model and understand these dynamic relaxations in solid-state materials [4]. Lead zirconate titanate (PZT) based piezoelectric devices are widely employed in the range of 0–200 °C but these materials have a disadvantage due to toxicity of lead. Therefore, there is a growing need for new lead-free materials for various applications. Bi-based Aurivillius family of oxides as environmentally friendly materials is alternative to PZT [5], [6].
The crystal structure of BaBi4Ti4O15 is characterized by four perovskite-like unit cells sandwiched between (Bi2O2)2+ layers along the c-axis. It undergoes a ferroelectric to paraelectric phase transition at the Curie temperature (Tc = 418 °C) [7]. A high Curie temperature enables a wide applicability of these materials in electronic elements, such as transducers, piezoelectric and memory devices. The disadvantage of these layer-structured perovskite materials for high-temperature piezoelectric application is their relatively high conductivity. This conductivity is electronic p-type and, therefore, can be suppressed by donor doping [8]. Feature of the Aurivillius compounds resides in the compositional flexibility of the perovskite block which allows incorporating of various cations. It is thus possible to modify the ferroelectric properties according to the chemical composition [9]. The mostly investigated and typical representative of Aurivillius compounds is Bi4Ti3O12. There have been a lot of reports about Nb doped Bi4Ti3O12 ceramics [6], [10], [11], [12]. However, reports on Nb-doped BaBi4Ti4O15 ceramics are deficient [13]. Nb5+ (in which ionic radius is 0.64 Å) is exclusively incorporated at the Ti4+ (0.61 Å) site while it cannot substitute Ba2+ (1.35 Å) or Bi3+ (1.17 Å) due to its smaller ion radius.
In this work, the influence of niobium as a donor dopant on properties of barium bismuth titanate ceramics prepared by conventional solid-state reactions was studied. Structure formation and microstructure development were monitored and their influence on electrical properties of BBT ceramics modifications was investigated.
Section snippets
Experimental
The compositions of BaBi4Ti4–5/4xNbxO15 (x = 0.05, 0.15 and 0.30 denotes as BBNT5, BBNT15 and BBNT30) were prepared via the conventional solid-state reaction method. Stoichiometric amounts of high purity oxides: BaO, TiO2, Bi2O3 and Nb2O5 (Alfa Aesar, p.a. 99%) were homogenized in isopropanol medium for 24 h in a planetary ball mill (Fritsch Pulverisette 5). The obtained powder was calcined at 950 °C for 4 h and pressed into pellets (49 MPa) using a uniaxial press (8 mm in diameter and ∼3 mm thick).
Results and discussion
Fig. 1 shows the XRD patterns of BBT, BBNT5, BBNT15 and BBNT30 ceramics. No peaks corresponding to the reactant oxides or any other secondary phases were found in samples with various contents of the Nb5+. The X-ray diffraction pattern of all samples recorded using a conventional diffractometer, can be well fitted in a tetragonal structure (identified using JCPDS 35-0757) with non-polar space group I4/mmm. A number of authors [14], [15] have also suggested the existence of tetragonal crystal
Conclusions
Niobium doped BBT ceramics were fabricated by solid-state reactions. Nb5+ doping (up to 30 mol%) did not affect the crystal structure of BBT. According to SEM micrographs, the grain growth was suppressed in Nb-doped samples. There is a significant improvement in the density of the Nb-doped samples. The ferroelectric phase transition temperature, Tm, decreases with Nb contents increase, and the dielectric peak broadens (increase the parameter δ from 59.6 to 115.3) showing the increase of the
Acknowledgements
The authors gratefully acknowledge the Ministry of Education and Science Republic of Serbia for the financial support of this work (project III 45021), the Lithuanian Research Funding Institution – Faculty of Physics, Vilnius and COST MP 0904.
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