Microwave Processed NBT and SBT based Ferroelectric Composites for Multifunctional Device Applications

Abstract

Ferroelectric materials are widely employed in electronic devices such as sensors, actuators, and non-volatile random access memory (NVRAM), etc. due to their multifunctional properties. In these device applications, the ferroelectric materials are subjected to different loading cycles. This results in fatigue behaviour, which becomes a serious issue and restricts their commercial applications. This fatigue factor determines the stability and lifetime of ferroelectric devices. Among many ferroelectric materials, perovskite-based PbZr0.52Ti0.48O3 (PZT) and bismuth layered-based SrBi2Ta2O9 have been widely investigated for their use in non-volatile memory applications. Lead-based systems, despite their superior ferroelectric characteristics, are prohibited from being used in device applications owing to health and environmental concerns. Among various lead-free ferroelectrics, Na0.5Bi0.5TiO3 (NBT) system promises to be a potential replacement for lead-based perovskites. However, certain drawbacks of NBT system such as poor fatigue resistance and high coercive field need to be addressed. Though SrBi2Ta2O9 system has better fatigue resistance, the low switchable polarization (2Pr) and high processing temperature to synthesize this system make them incompatible to be used in device applications.
Synthesis of NBT and SrBi2Ta2O9 based materials by conventional solid-state reaction route necessitates high processing temperatures, which may result in the loss of volatile components due to their high volatilities, resulting in deterioration of material properties. Therefore, to lower the processing temperature and time, microwave processing technique is used. XRD study revealed that the optimal calcination and sintering temperatures for microwave synthesized NBT ceramics are 800 oC for 15 minutes and 1000 oC for 30 minutes. Whereas, the optimal calcination and sintering temperatures for conventionally synthesized NBT ceramics are 850 oC for 4 hours and 1150 oC for 4 hours, respectively. Similarly, for microwave synthesized Sr0.8Bi2.15Ta2O9 (SBT) ceramics, optimal calcination and sintering temperatures are 950 oC for 30 minutes and 1100 oC for 30 minutes, respectively, which is significantly lower than the same system synthesized by conventional processing routes. Because the processing temperature and time of microwave processing technique differ noticeably and lower than those of conventional synthesis processes, microwave processing technique offers a potential benefit in terms of time and energy savings. Dense microstructure with smaller and fine grains distribution was observed in microwave processed NBT and SBT based ceramics. Microwave processed NBT and SBT based ceramics showed lower loss and lower leakage current density that can be associated to their increased density.
There have been several attempts to enhance the characteristics of NBT system by different ionic substitution or doping at A or B-sites for wider applications. Donor doping has been shown to improve the ferroelectric characteristics of materials. To investigate the effect of doping, several dopants, based on their ionic radii and valency, to replace A and B-sites were chosen. Microwave-assisted solid-state reaction method was used to synthesize NBT doped systems with varying mol % of dopants, as given below,
1. (Na0.5Bi0.5)(1-x)LaxTi(1-x/4)O3 (where x=0.01,0.02,0.03)
2. (Na0.5Bi0.5)(1-x)SmxTi(1-x/4)O3 (where x=0.01,0.02,0.03)
3. (Na0.5Bi0.5)(1-x/2)Ti(1-x)NbxO3 (where x=0.005, 0.01, 0.015)
4. (Na0.5Bi0.5)(1-x)Ti(1-x)WxO3 (where x=0.005, 0.01, 0.015)
XRD analysis of A-site and B-site doped NBT systems indicated that the solubility limits of B-site dopants (around 1 mol %) were lower than those of A-site dopants (around 3 mol %). XRD patterns of both A-site and B-site doped NBT systems with single perovskite phase were matched with JCPDS file- 36-0340 of NBT having rhombohedral structure. Rietveld refinement indicated no structural change with doping, however, doping induced lattice deformation due to differences in dopant ionic radii and host atom radii. Substitution of La3+ and Sm3+ ions at A-sites tends to shrink the lattice due to their lower ionic radii than Na1+ and Bi3+ of host A-site ions. Whereas occupancy of Nb5+ and W6+ ions at B-site expands the lattice a little due to their comparable but slightly larger ionic radii than Ti4+ of host B-site ions. Density of all the doped systems was found to be lower than the parent system. Thus, higher sintering temperature is required to get densified modified NBT ceramics. Incorporation of donor dopants resulted in reduction in grain size as the cation vacancies accumulates at grain boundary, which inhibited grain growth, thus, all the dopants act as grain growth inhibitors in modified NBT systems. Minimal decrease in leakage current density was observed up to 2 mol% of A-site and 1 mol% of B-site doping and all the modified ceramics displayed Ohmic type conduction behaviour. Overall, 1mol% Sm-doped NBT (abbreviated as NBST1) showed the better ferroelectric characteristics among all the doped ceramics with 2Pr and Ec of ~20.65 μC/cm2and ~20.23 kV/cm, respectively compared to ~13.37 μC/cm2 and ~23.84 kV/cm for NBT. Hence NBST1 system was chosen as the matrix to synthesize composites with SBT system as filler.
Some researchers have put an effort to develop composites of efficient lead-free perovskite material with an appropriate bismuth-layered material to compensate for the required properties. In the present study, composites of perovskite NBST1 system and bismuth layered based SBT system have been prepared by microwave-assisted solid-state reaction route to improve their overall fatigue as well as ferroelectric properties. Two parent systems were prepared individually and were mixed by varying wt.% of each phase to produce the composite systems. The composites synthesized by microwave-assisted solid-state reaction route are;
(1-x) NBST1-xSBT (where x= 0, 4, 8, 12, 16wt.%)
X-ray diffraction patterns of (1-x)NBST1-xSBT composites revealed the coexistence of both perovskite and bismuth layered phases in all the composites. Change in peak intensity was thought to be induced by a change in composition, i.e., when SBT content increases, the peak intensity of the NBST1 perovskite phase decreased while the peak intensity of the bismuth layered SBT phase slowly increased. Density of all the composites was between that of matrix NBST1 (~5.28 g/cm3) system and filler SBT (~8.92 g/cm3) system. It was observed that well-developed grains of various sizes were densely distributed throughout the surface of ceramics. From the room temperature dielectric study, it was found that both dielectric constant and loss decreased with the increase of SBT content in composites. Increase in internal stress in composites owing to adjustment of two separate systems with considerably differing lattice parameters was evidenced by decrease in transition temperatures. Leakage current density and ferroelectric parameters like remnant polarization and coercive field decreased with the incorporation of SBT system in composites. Polarization fatigue study confirmed that fatigue endurance increased with the increase of SBT content. Among the studied (1-x)NBST1-xSBTcomposites, composite with x= 8 wt.% appeared to be the best in terms of dielectric, ferroelectric, and fatigue resistance for multifunctional device applications, especially for ferroelectric memory.