Nanocrystalline Barium Titanate Research Articles

Multiscale characterization of damage tolerance in barium titanate thin films

Barium titanate is a brittle, lead free ferroelectric and piezoelectric ceramic used in patterned and thin film forms in micro- and nano-scale electronic devices. Both during deposition and eventually during service, this material system develops stresses due to different loads acting on the system, which can lead to its failure due to cracking in the films and/or interface delamination. In situ microcantilever bending based fracture experiments and tensile tests based on shear lag tests in combination with digital image correlation were used to understand the cracking behavior of barium titanate films when deposited on flexible substrates. For the first time, the fracture behavior of these nanocrystalline barium titanate films has been quantified in terms of fracture toughness, fracture strength, and interface shear stresses for different film thicknesses. Critical defect size is estimated using the above information as a function of film thickness. It is found that damage tolerance in terms of fracture strength depends on film thickness. Furthermore, compared to a bulk single crystal, barium titanate fracture resistance of the nanocrystalline thin films is reduced. Both effects need to be considered in engineering design of reliable devices employing micro- and nano-scale barium titanate thin film structures.

Effect of heat treatment on internal stress in barium titanate nanocube assemblies and their dielectric property

Heat treatment is a key process to determine the dielectric properties of nanocrystalline barium titanate (BaTiO3) ceramics, which are a prospective candidate to realize further miniaturization of dielectric components in electric devices. Here, we use Raman spectroscopy and scanning microwave impedance microscopy to investigate the dependence of the internal stress and the relative permittivity of BaTiO3 nanocube three-dimensional assemblies on heat treatment temperature. We show that heat treatment in the temperature range from 700 °C to 1000 °C causes internal compressive stress in the nanocube assemblies without grain growth. The internal compressive stress is caused by formation of tight attachments between neighboring BaTiO3 nanocubes and of Ti-rich phases in the nanocube assemblies in the lower and higher temperature ranges, respectively. We also show that the relative permittivity of the nanocube assemblies at 3 GHz shows a positive correlation with the internal compressive stress. The result indicates that the internal compressive stress enhances the relative permittivity of BaTiO3 nanocubes.

Synthesis of nanocrystalline barium titanate: Effect of microwave power on phase evolution

Nanocrystalline BaTiO3 (BT) powder was synthesised using a polymer precursor route and the influence of conventional, microwave and combined hybrid heating methods on phase formation was investigated. A single-phase tetragonal BT (t-BT) nanocrystalline powder of about 20 nm primary particle size and decreased agglomeration were formed when high levels of microwave energy were used. This was accomplished at a lower processing condition of 700 °C for 30 min compared to conventional processing, which required 900 °C for 5 h, resulting in potential savings in time and energy. During the nano BT synthesis, the role of microwaves was determined by subjecting the samples to identical thermal histories, i.e. exactly the same time-temperature profiles, while using a range of different levels of microwave power. Significant reduction in the activation energy for the formation of the tetragonal phase was observed with increasing levels of microwave power and the results are explained in terms of a possible non-thermal mechanism. Furthermore, under otherwise identical thermodynamic conditions of temperature, time and (atmospheric) pressure, the co-occurrence of hexagonal crystal structure at < 200 W of additional microwave power along with formation of tetragonal crystal structure at ≥ 200 W and single phase tetragonal crystal structure at ∼1200 W was observed, demonstrating a new method of controlling the phase evolution during the synthesis of nanostructured barium titanate powder. The methodology could be applied to synthesise a variety of functional ceramic powders with tailored levels of crystallographic phases.

Following the Kinetics of Barium Titanate Nanocrystal Formation in Benzyl Alcohol Under Near-Ambient Conditions.

In complex chemical syntheses (e.g., coprecipitation reactions), nucleation, growth, and coarsening often occur concurrently, obscuring the individual processes. Improved knowledge of these processes will help to better understand and optimize the reaction protocol. Here, a form-free and model independent approach, based on a combination of time-resolved small/wide-angle X-ray scattering, is employed to elucidate the effect of reaction parameters (such as precursor concentration, reactant stoichiometry, and temperature) on the nucleation, crystallization, and growth phenomena during the formation of nanocrystalline barium titanate. The strength of this approach is that it relies solely on the total scattered intensity (i.e., scattering invariant) of the investigated system, and no prior knowledge is required. As such, it can be widely applied to other synthesis protocols and material's systems. Through the scattering invariant, it is found that the amorphous-to-crystalline transformation of barium titanate is predominantly determined by the total amount of water released from the gel-like barium hydroxide octahydrate precursor, and three rate-limiting regimes are established. As a result of this improved understanding of the effect of varying reaction conditions, elementary boundary conditions can be set up for a better control of the barium titanate nanocrystal synthesis.

High pressure FAST of nanocrystalline barium titanate

This work studies the microstructural evolution of nanocrystalline (<1µm) barium titanate (BaTiO3), and presents high pressure in field-assisted sintering (FAST) as a robust methodology to obtain >100nm BaTiO3 compacts. Using FAST, two commercial ~50nm powders were consolidated into compacts of varying densities and grain sizes. Microstructural inhomogeneities were investigated for each case, and an interpretation is developed using a modified Monte Carlo Potts (MCP) simulation. Two recurrent microstructural inhomogeneities are highlighted, heterogeneous grain growth and low-density regions, both ubiqutously present in all samples to varying degrees. In the worst cases, HGG presents an area coverage of 52%. Because HGG is sporadic but homogenous throughout a sample, the catalyst (e.g., the local segregation of species) must be, correspondingly, distributed in a homogenous manner. MCP demonstrates that in such a case, a large distance between nucleating abnormal grains is required—otherwise abnormal grains prematurely impinge on each other, and their size is not distinguishable from that of normal grains. Compacts sintered with a pressure of 300MPa and temperatures of 900°C, were 99.5% dense and had a grain size of 90±24nm. These are unprecedented results for commercial BaTiO3 powders or any starting powder of 50nm particle size—other authors have used 16nm lab-produced powder to obtain similar results.

Formation of nanocrystalline barium titanate in benzyl alcohol at room temperature.

Nanocrystalline barium titanate (8-10 nm crystallite size) was prepared at temperatures of 23-78 °C through reaction of a modified titanium alkoxide precursor in benzyl alcohol with barium hydroxide octahydrate. The room temperature formation of a perovskite phase from solution is associated with the use of benzyl alcohol as solvent medium. The formation mechanism was elucidated by studying the stability and interaction of each precursor with the solvent and with each other using various experimental characterization techniques. Density functional theory (DFT) computational models which agreed well with our experimental data could explain the formation of the solid phase. The stability of the Ti precursor was enhanced by steric hindrance exerted by phenylmethoxy ligands that originated from the benzyl alcohol solvent. Electron microscopy and X-ray diffraction indicated that the crystallite sizes were independent of the reaction temperature. Crystal growth was inhibited by the stabilizing phenylmethoxy groups present on the surface of the crystallites.

A phase-field study on the hysteresis behaviors and domain patterns of nanocrystalline ferroelectric polycrystals

The overall hysteresis behavior of nanocrystalline ferroelectric polycrystals demonstrates unique characteristics against conventional ferroelectric ceramics. The existence of low-permittivity paraelectric grain boundary and its influence to the microstructure of grains can be a key factor leading to such characteristics, especially the grain size-dependent properties. A two dimensional (2D) polycrystalline phase-field model, which distinguishes the grain boundary from the ferroelectric grain, has been developed to investigate the microstructural evolution and hysteresis behavior of nanocrystalline barium titanate (BaTiO3) polycrystals. The results show apparent grain-size dependence on the hysteresis and noticeable vortex polarization structure that dominates the grains as the grain size reduces to tens of nanometers. By studying the hysteresis and domain patterns, it is observed that the grain size-dependent properties are significantly attributed to the grain boundary in two ways: the “dilution effect” due to its low permittivity and paraelectric property that are intensified with increased volume concentration, and the extrinsic effect due to the existence of depolarization field, leading to the superparaelectric domain structure. We conclude that this grain-size dependent microstructural mechanism can well explain various experimentally observed properties of nano-grained ferroelectric polycrystals.

Ultrafast Response Humidity Sensor Based on Electrospun Porous BaTiO&lt;sub&gt;3&lt;/sub&gt; Nanofibers

Nanocrystalline and porous barium titanate (BaTiO3) nanofibers with diameter 200-400 nm were synthesized via electrospinning and followed calcinations. The morphology and microstructure of the nanofibers were characterized using field emission scanning electron microscope, X-ray diffractometer and transmission electron microscope, respectively. And the electrical and humidity sensing properties of the nanofibers were also measured. The results reveal that the BaTiO3 nanofibers have a conductivity of about 0.3 S/cm, and show an ultrafast response time (~0.7 s) and a recovery time (~0.4 s) to humidity at room temperature. In addition, the sensing mechanism was also discussed briefly based on its nanocrystalline and porous microstructure of the electrospun material.

Spark plasma sintering of microwave processed nanocrystalline barium titanate and their characterisations

Nanocrystalline barium titanate (BT) was synthesized by the high-energy planetary mill assisted novel hybrid processing methodology. As-milled precursors were microwave calcined and subsequently sintered by spark plasma sintering (SPS). Powders were characterised by XRD, laser Raman spectroscopy, BET and TEM techniques. Microstructure, purity, thermal stability and dielectric characteristics of the sintered BT were analyzed using FE-SEM, EDX, TGA and LCZ meter. Nanocrystalline nature of BT was evident from the enhanced peak broadening noticed in the XRD pattern and from the TEM image. Raman spectra corroborated well with the XRD results. FE-SEM image revealed a fairly dense and uniform microstructure, which was attributed to the high reactivity and needle shaped morphology of the mechanically activated microwave calcined powders, which showed a surface area of about 20 m2 g−1. Microwave calcined SPS processed BT showed a thermal stability of 99.5% as evident from the thermo-gravimetric analysis. Superior dielectric response was shown by the hybrid processed BT which is highly suitable for capacitor applications.

Effect of Grain Size on Phase Transitions and Dielectric Properties of Nano-Crystalline Barium Titanate Ceramics

Barium titanate (BaTiO3) ceramics with grain size varied from 1000 to 8 nm were prepared by two step sintering method (TSS) and spark plasma sintering (SPS), respectively. Mixture structures of BaTiO3 ceramics were proved by in-situ temperature high resolution x-ray diffraction. Multiple ferroelectric domains present in nano-crystalline BaTiO3 ceramics were observed by transmission electron microscope. The evolution of phase transitions supported the existence of intrinsic mechanism. Dielectric loss of fine grain size BaTiO3 was higher than coarse grain size during Curie phase transition due to diffuse phase transition and grain boundary effects.

Influence of Ammonia on Properties of Nanocrystalline Barium Titanate Particles Prepared by a Hydrothermal Method

The influence of ammonia on BaTiO3 particles prepared via hydrothermal synthesis is discussed. Two kinds of Ti‐precursors, amorphous (with ammonia) and anatase (without ammonia), are synthesized by a hydrolysis reaction using Ti‐butoxide as the starting material. Amorphous Ti‐precursors are found to be excellent for this purpose and offer several advantages over anatase. Superior tetragonal properties of BaTiO3 powders with more uniform particle sizes can be achieved at 200°C after 24 h using amorphous Ti‐precursors. X‐ray photoelectron spectroscopy was carried out on both types of Ti‐precursor, and a decrease in H2O adsorption on the amorphous Ti‐precursors is confirmed. It is considered that the reduction in H2O adsorption on the Ti‐precursors results from the shielding effects of ammonia and is attributed to a decreased formation of intragranular pores in the BaTiO3 particles. These suggested mechanisms are clarified by the results of in situ transmission electron microscopy observations and density measurements of the hydrothermally synthesized BaTiO3.

Combined effects of milling and calcination methods on the characteristics of nanocrystalline barium titanate

Barium titanate (BT) nanoparticles were synthesized by high-energy planetary milling technique. Wet milling and dry milling effects were compared. The milled powders were calcined at 1000°C by microwave and conventional heating methodologies. The calcined powders were characterized by XRD, VP-SEM, EDX, BET, TGA and laser Raman spectroscopy techniques. Dry milled and microwave calcined BT showed the highest extent of tetragonality value of 1.021. VP-SEM micrograph confirmed the tetragonal morphology of microwave calcined BT(s) with an average grain size of 81nm. Raman Spectra corroborated well with the XRD results. Wet milled and microwave calcined powders showed relatively higher residual hydroxyl ion contamination than that of the dry milled powders. Dry milled and microwave calcined powders showed superior room temperature dielectric constant value with relatively lower dielectric loss tangent value which are very much essential for the fabrication of multi layered ceramic capacitors.

Synthesis of nanosized barium titanate/epoxy resin composites and measurement of microwave absorption

Barium titanate/epoxy resin composites have been synthesized and tested for microwave absorption/transmission. Nanocrystalline barium titanate (BaTiO3 or BT) was synthesized by the hydrothermal method and the composites of BT/epoxy resin were fabricated as thin solid slabs of four different weight ratios. BT was obtained in the cubic phase with an average particle size of 21 nm, deduced from the X-ray diffraction data. The reflection loss (RL) and transmission loss (TL) of the composite materials were measured by the reflection/transmission method using a vector network analyser R&S: ZVA40, in the frequency range 8·0–18·5 GHz (X and Ku-bands). The RL was found to be better than −10 dB over wide frequency bands. The higher RL for lower concentration of BT could be due to increase in impedance matching effects. Low TL values indicate that the absorption by BT is quite low. This could be due to formation of BT in the cubic paraelectric phase.

Synthesis and Characterization of Nano-Barium Titanate Prepared by Hydrothermal Process

Capacitors are the most convenient and economical devices for storage of electrical energy. Performance of a capacitor is primarily governed by solid-state characteristics of its dielectric material and the explicit value of the dielectric constant. Titanate ceramics such as Barium Titanate are popular capacitor dielectric materials because of their high dielectric constant and ferroelectric properties. In recent years, extensive research work has been done on Barium Titanate for multilayer ceramic capacitor application. Barium Titanate has been synthesized from high energy ball-milling process and sol-gel process or a thermo-chemical process. However, because of contamination from the grinding media, relatively wide particle size distribution and abnormal grain growth during the sintering, other novel synthesis processes have been developed. Nano-technology based hydrothermal synthesis is one such new process. In this paper, an effort has been made to synthesize nanocrystalline Barium Titanate through hydrothermal process. Characterization of the synthesized Barium Titanate is carried out by using X-ray diffraction for crystalline structure, scanning electron microscopy for microstructure and electron dispersive spectroscopy for chemical composition. Electric properties such as dielectric constant, dielectric dissipation factor and electrical resistivity have been measured. It has been observed that dielectric constant of about 4000 and dielectric dissipation factor of below 0.3 are obtained over a wide range of frequencies. The crystallite size of the optimized material is less than 50 nm.

A Phenomenological Model on Phase Transitions in Nanocrystalline Barium Titanate Ceramic

An innovative method is presented to estimate the size effect on the ferroelectric phase transitions in the barium titanate ceramic system. The method is based on the phenomenological thermodynamic theory with additional stress led by surface bond contraction. Two symmetry‐breaking factors are introduced to correct the stress. This method has enabled us to predict the changing trends in phase transitions and the critical grain size for the disappearance of ferroelectric phase, which is beyond the prediction of existing models.

Polymer/Ceramic Composite Hybrids Containing Multi-walled Carbon Nanotubes with High Dielectric Permittivity

Polymer-ceramic composites having polyvinylidene fluoride (PVDF) matrix filled with nanocrystalline barium titanate (BT) and multiwalled carbon nanotubes (MWNTs) were prepared by sonicated solution mixing method. The dielectric permittivity and electrical conductivity of such hybrids were measured over a wide range of frequencies. The results showed that the dielectric permittivity of PVDF/BT 50/50 composite at room temperature increases from ∼ 94 to ∼ 620 at 1 kHz by adding a small amount (2.5 vol. %) of nanotubes. Moreover, the dielectric permittivity of this hybrid composite tended to increase with increasing temperature and reached an apparent maximum value of 780 at 122 °C. The excellent dielectric performance of the composite was analyzed in terms of the percolation and dielectric relaxation concepts. Keywords: Dielectric permittivity, conductivity, nanocomposites, carbon nanotubes, barium titanate

Semiconducting ceramics produced using nanocrystalline barium titanate powder

Positive temperature coefficient of resistance ceramics of composition (Ba0.89Ca0.08Pb0.03)TiO3 + Y2O3 + MnO + SiO2 have been produced using barium titanate powder with an average crystallite size of 125 nm prepared by calcining barium titanyl oxalate at 900°C. The effect of firing temperature on their microstructure and electrical properties has been studied. The results demonstrate that the ceramics possess semiconducting properties starting at a firing temperature of 1205–1215°C. The room-temperature resistivity of the ceramics has a minimum at tfiring ≈ 1245–1250°C. The samples sintered at 1250–1260°C have the largest positive temperature coefficient of resistance. The highest electric strength (360 V/mm at ρ25°C = 290 Ω cm) is offered by the thermistor materials sintered at 1260°C, which is 60–70°C below the firing temperature of analogous ceramics produced by solid-state reaction.

Water-based sol–gel nanocrystalline barium titanate: controlling the crystal structure and phase transformation by Ba:Ti atomic ratio

Highly stable, water-based barium titanate (BaTiO3) sols were developed by a low cost and straightforward sol–gel process. Nanocrystalline barium titanate thin films and powders with various Ba:Ti atomic ratios were produced from the aqueous sols. The prepared sols had a narrow particle size distribution in the range 21–23 nm and they were stable over 5 months. X-ray diffraction pattern revealed that powders contained mixture of hexagonal- or perovskite-BaTiO3 as well as a trace of Ba2Ti13O22 and Ba4Ti2O27 phases, depending on annealing temperature and Ba:Ti atomic ratio. Highly pure barium titanate with cubic perovskite structure achieved with Ba:Ti = 50:50 atomic ratio at the high temperature of 800 °C, whereas pure barium titanate with hexagonal structure obtained for the same atomic ratio at the low temperature of 500 °C. Transmission electron microscope revealed that the crystallite size of both hexagonal- and perovskite-BaTiO3 phases reduced with increasing the Ba:Ti atomic ratio, being in the range 2–3 nm. Scanning electron microscope analysis revealed that the average grain size of barium titanate thin films decreased with an increase in the Ba:Ti atomic ratio, being in the range 28–35 nm. Moreover, based on atomic force microscope images, BaTiO3 thin films had a columnar-like morphology with high roughness. One of the highest specific surface area reported in the literature was obtained for annealed powders at 550 °C in the range 257–353 m2g−1.

Characterization, sintering and dielectric properties of nanocrystalline barium titanate synthesized through a modified combustion process

Nanocrystalline barium titanate has been synthesized through a modified combustion process in a single step for the first time. The as-prepared barium titanate powder is cubic perovskite with lattice constant a = 4.018 Å. The phase purity of the nanopowder was examined using thermo gravimetric analysis, differential thermal analysis and Fourier transform infrared spectroscopy. Transmission electron microscopic investigations have shown that the particle size of the as-prepared powder is in the range 20–40 nm. The agglomerate size distribution of the as-prepared powder was studied using atomic force microscopy. The nanoparticles of barium titanate were sintered to 97% of the theoretical density at a temperature of 1350 °C for 3 h. The microstructure of the sintered surface was examined using scanning electron microscopy. The dielectric constant and loss factor of the sintered pellets at 1 MHz measured at room temperature were 1223 and 3.5 × 10 − 3 respectively.

The hydroxyl concentration and the dielectric properties of barium titanate nano-powder synthesized by water-based ambient condition sol process

Nanocrystalline barium titanate, BaTiO3, powders have been successfully synthesized via water-based ambient condition sol (WACS) process, using barium hydroxide and titanium isopropoxide. The properties of the powder were investigated as a function of various processing parameters such as the concentration of Ba2+ ions ([Ba2+]), the concentration of base, and reaction time. The concentration of hydroxyl (OH−) groups in the BaTiO3 powder was greatly affected by changing the values of each processing parameter. The dielectric constant and tetragonality of the powders which contain lattice OH− more than 0.35 wt% were little affected with further increase in the lattice OH− concentration. The dielectric loss was highly varied with the concentration of the OH− groups, and it was increased with increasing OH− concentration. Calcination treatment significantly improved the dielectric properties of the powder. With higher calcination temperature, the dielectric constant increased and the dielectric loss decreased.