Electrical properties and structure of grain boundaries in n-conducting BaTiO3 ceramics

The electrical properties of positive temperature coefficient (PTC) ceramics are expected to strongly correlate with the potential barrier height at grain boundaries, which in turn may be influenced by the grain boundary structure and chemistry. In this study, n-conducting BaTiO₃ ceramics co-doped by La and Mn were prepared, and the electrical properties were determined by impedance spectroscopy and dc four-point van der Pauw measurements. Detailed analysis of the grain boundary structure was performed by electron microscopy techniques across different length scales. The study revealed that the randomly oriented polycrystalline microstructure was dominated by large angle grain boundaries, which in the present case were dry although a secondary crystalline and glass phase formed at triple junctions. The relationship between the observed grain boundary atomic structures and electrical properties is briefly discussed.     

Preparation of 1 1 1-textured BaTiO3 ceramics by templated grain growth method using novel template particles

Dense BaTiO₃ ceramics with 1 1 1-texture were prepared by the TGG process. Platelike BaTiO₃ particles with their 1 1 1 direction perpendicular to the plate face were prepared by the reaction of platelike Ba₆Ti₁₇O₄₀ particles with BaCO₃ particles in molten NaCl. A green compact was composed of the aligned, platelike BaTiO₃ template particles dispersed in the matrix of small, equiaxed BaTiO₃ particles. Sintering caused densification and also the growth of template particles at the expense of matrix particles, resulting in texture development. Densification prior to grain growth was found to be necessary to obtain highly textured ceramics, and the effect of pre-sintering conditions on texture development was examined.     

Reducing the solubility of Ag into BaTiO3 by alloying Ag with Pd

In the experiment reported in this study, a small amount of 70%Ag/30%Pd particles was mixed with BaTiO₃ powder and subsequently sintered at elevated temperature in nitrogen, air and oxygen. The solubility of Ag in the BaTiO₃ was determined using an electron probe microanalysis technique. The presence of Pd alloy in Ag can reduce its solubility and diffusion distance in BaTiO₃. Alloying with Ag significantly reduced the solubility of Pd in BaTiO₃.     

Synthesis and Characterization of Bowl-Like Single-Crystalline BaTiO3 Nanoparticles

Novel bowl-like single-crystalline BaTiO₃ nanoparticles were synthesized by a simple hydrothermal method using Ba(OH)₂·8H₂O and TiO₂ as precursors. The as-prepared products were characterized by XRD, Raman spectroscopy, SEM and TEM. The results show that the bowl-like BaTiO₃ nanoparticles are single-crystalline and have a size about 100–200 nm in diameter. Local piezoresponse force measurements indicate that the BaTiO₃ nanoparticles have switchable polarization at room temperature. The local effective piezoelectric coefficient d₃₃ ^* d_{33}^{ * } is approximately 28 pm/V.     

Phase structure and nano-domain in high performance of BaTiO3 piezoelectric ceramics

BaTiO₃ ceramics were prepared by conventional sintering technique with a special emphasis on the effects of sintering temperature (1100-1230 °C) on the crystalline structure and piezoelectric properties. XRD patterns indicated that the crystallographic structure changed from tetragonal phase to orthorhombic one with raising sintering temperature from 1160 °C to 1180 °C. Domains were shaped in a stripe and a herringbone in orthorhombic samples for BaTiO₃ ceramics. The domain width and domain density increased with raising sintering temperature. The BaTiO₃ ceramic sintered at 1190 °C showed the excellent electrical properties, d₃₃ = 355 pC/N, k_p = 40%, P_r = 10.2 μC/cm², respectively, which are originated to the contributions of both the crystallographic structure transition and nano-domain.     

Microstructure transition from normal to abnormal grains for BaTiO3

Quantitative characterization on the microstructure evolution during sintering of barium titanate has been conducted. The microstructure of the specimens sintered at 1320–1360 °C exhibits a typical mixture of normal and abnormal grains. The size of normal grains and abnormal grains differs by two orders of magnitude. No grain with an intermediate size is observed. A “pseudo-abnormal” region composing of many partially oriented normal grains is found instead. The formation of such pseudo-abnormal regions may help the transition from fine normal grains to coarse abnormal grains.     

Properties of BaTiO3 synthesized from barium titanyl oxalate

In order to enhance the tetragonality of BaTiO₃ derived from barium titanyl oxalate (BTO), various treatments were carried out by considering the thermal decomposition mechanism of BTO in air. A multi-step heat treatment process and the addition of carbon black, as a particle growth inhibitor, were effective in increasing the tetragonality, whilst maintaining a particle size smaller than 200 nm. The synthesized BaTiO₃ powder with a mean particle size of 177 nm showed a tetragonality and K-factor of 1.0064 and approximately 3, respectively.     

Effect of TiO2 addition on grain shape and grain coarsening behavior in 95Na1/2Bi1/2TiO3-5BaTiO3

Grain coarsening behavior in the 95Na_1/2Bi_1/2TiO₃-5BaTiO₃ system has been studied as a function of the addition of TiO₂. As the amount of added TiO₂ was increased, the grain shape changed to a more faceted cube, indicating an increase in the step free energy of the facets, and hence a rise in the critical driving force for appreciable growth of grains. Grain coarsening behavior also changed from pseudo-normal to abnormal with an increasing TiO₂ concentration and thus increased faceting. The pseudo-normal behavior observed in the system without TiO₂ addition also changed to quite abnormal behavior during extended sintering. These observations support our theoretical prediction based on the coupling effects between the maximum driving force for growth and the critical driving force for appreciable growth.     

Spark plasma sintering of nanocrystalline BaTiO3-powders: Consolidation behavior and dielectric characteristics

BaTiO₃ nanopowders prepared by two different wet chemical routes, one based on microemulsion-mediated synthesis (M-BT) and the other one on the alkoxide-hydroxide method (A-BT) were consolidated by spark plasma sintering (SPS). The densification process, the linear shrinkage rates and the relative densities achieved were strongly dependant on the synthetic route. The results show that fully densified BaTiO₃ ceramics with a grain size of about 200 nm can be obtained in both cases by controlling the sintering temperature during the SPS process. The study of dielectric properties revealed that M-BT derived ceramics show higher permittivity values compared to those obtained for A-BT. The influence of the barium/titanium ratio on the sintering behavior and the dielectric properties is discussed.     

High piezoelectric properties of BaTiO3-xLiF ceramics sintered at low temperatures

BaTiO₃-xLiF ceramics were prepared by a conventional sintering method using BaTiO₃ powder about 100 nm in diameter. The effects of LiF content (x) and sintering temperature on density, crystalline structure and electrical properties were investigated. A phase transition from tetragonal to orthorhombic symmetry appeared as sintering temperatures were raised from 1100 °C to 1200 °C or as LiF was added from 0 mol% to 3 mol%. BaTiO₃-6 mol% LiF ceramic sintered at 1000 °C exhibited a high relative density of 95.5%, which was comparable to that for pure BaTiO₃ sintered at 1250 °C. BaTiO₃-4 mol% LiF ceramic sintered at 1100 °C exhibited excellent properties with a piezoelectric constant d₃₃ = 270 pC/N and a planar electromechanical coupling coefficient k_p = 45%, because it is close to the phase transition point in addition to high density.     

Structural investigation of mechanically activated nanocrystalline BaTiO3 powders

In this article, in order to obtain tetragonal nanocrystalline BaTiO₃, structural investigations of mechanically activated BaTiO₃ powder have been performed. A mercury porosimetry analysis and scanning electron microscopy method have been applied for determination of the specific pore volume, porosity and microstructure morphology of the samples. The lattice vibration spectra of nonactivated and activated powders, their phase composition, lattice microstrains and the mean size of coherently diffracting domains were examined by Raman spectroscopy and the X-ray powder diffraction method. The average crystal structure of obtained nanocrystalline powders, estimated from X-ray diffraction data, gave evidence of retained, but slightly sustained tetragonality of powders, even for particles as small as ∼30 nm. Raman spectroscopy also gave clear evidence for local tetragonal symmetries, in particular through the presence of a band at ∼307 cm⁻¹.     

The contribution of fractal nature to BaTiO3-ceramics microstructure analysis

The structure of BaTiO₃ based materials can be controlled by using different technological parameters and different additives. In this paper, microstructure and dielectrical properties of Er₂O₃ doped BaTiO₃-ceramics, sintered from 1320 °C to 1380 °C have been investigated. Microstructural investigations were carried out using scanning electron microscope equipped with energy dispersive spectrometer. Grain size distribution was determined by quantitative metallography method.The new correlation between microstructure and dielectric properties of doped BaTiO₃-ceramics, based on fractal geometry and contact surface probability, has been developed. Using the fractals and statistics of the grains contact surface, a reconstruction of microstructure configurations, as grains shapes or intergranular contacts, has been successfully done. Obtained results indicated that fractal analysis and statistics model for contact surfaces of different shapes were very important for the prognosis of BaTiO₃-ceramics microstructure and dielectric properties.The morphology of ceramics grains pointed out the validity of developing new structure analytical methods, based on different grains’ shape geometries. The grains contact model based on ellipsoidal geometry is presented as new modeling tool for structure research of BaTiO₃-ceramics materials. The directions of possible materials properties prognosis are determined according to the correlations synthesis-structure-property.The statistical approach to the investigation of BaTiO₃-ceramic grains concerning the relationship between the temperature and the area of the contact surface is also introduced.     

Gel casting of aqueous suspensions of BaTiO3 nanopowders

Commercial nano-BaTiO₃ powders have been formed into green bodies using colloidal forming routes. A study of the rheological behaviour of the suspensions as a function of dispersant concentration and homogenisation time was made in order to prepare stable concentrated suspensions of the nanopowders. Bulk components were then manufactured using aqueous slip and gel casting involving polysaccharides that gel on cooling, i.e. agar. The performance of theses consolidation techniques for obtaining dense green bodies from the BaTiO₃ nanopowders was studied. It was possible to prepare relatively big gel cast samples with a similar density and microstructure and in a shorter time compared to those obtained by slip casting.     

Solid state metathesis synthesis of BaTiO3, PbTiO3, K0.5Bi0.5TiO3 and Na0.5Bi0.5TiO3

A solid state metathesis approach has been applied to synthesize perovskite oxides such as BaTiO₃, PbTiO₃, K_0.5Bi_0.5TiO₃ and Na_0.5Bi_0.5TiO₃, these were characterized by powder XRD, IR and energy dispersive spectra (EDS). Potassium titanium oxalate and metal chlorides are used as the starting materials. X-ray analysis shows the formation of a single phase with tetragonal structure for BaTiO₃, PbTiO₃, K_0.5Bi_0.5TiO₃ and a monoclinic structure for Na_0.5Bi_0.5TiO₃. The Infrared spectra of these compounds show the characteristic band due to Ti–O octahedron for all the compounds. The EDS spectra show the relative ratio of the metal ions. The morphology of synthesized compounds was obtained from SEM measurements.     

Hydrothermal synthesis of BaTiO3 from different Ti-precursors and microstructural and electrical properties of sintered samples with submicrometric grain size

Barium titanate submicrometric particles were synthesized at 180 °C in a closed PTFE-lined stainless steel reactor with continuous stirring. Precursors used for titanium were Degussa P25 TiO₂ or titanium isopropoxide (TIP). Barium hydroxide (Ba(OH)₂) was added in a Ba/Ti = 1.1 molar ratio and KOH was used as mineralizer. The obtained powders were characterized by XRD and Raman spectroscopy. Powders were uniaxially pressed into discs and sintered at 1250 °C. The resulting microstructures were characterized by XRD, SEM, and Raman spectroscopy. Electrical measurements were carried out in order to characterize the ferroelectric behavior. The Ti precursor determined the sample density and grain size distribution and, consequently, the electrical response.     

Enhanced mechanical and dielectric behavior of BaTiO3/Cu composites

BaTiO₃/xCu composite ceramics with x = 0-30 wt.% were fabricated by the traditional mixing method in nitrogen gas. The mechanical properties and electric properties of the obtained composites were investigated as a function of the Cu mass fraction using a three bending test and impedance spectroscopy. The results indicated that the relative density of the sintered composites reached above 91%, the Cu-dispersed BaTiO₃ composites enhanced the mechanical properties, particularly the high fracture toughness (∼3.9 MPa m^1/2) and bending strength (∼134 MPa), compared to the monolithic BaTiO₃. Furthermore, the percolation threshold of BaTiO₃/Cu composites was x = 25 wt.%. The permittivity (?_r) markedly increased from ∼2000 for monolithic BaTiO₃ to ∼9000 with increasing Cu up to 30 wt.%. Additionally, the temperature coefficient of this system was less than 5% in the temperature range of 25-115.     

Microstructures and dielectric properties of spark plasma sintered Ba0.4Sr0.6TiO3/CaCu3Ti4O12 composite ceramics

(1 − x)Ba_0.4Sr_0.6TiO₃/xCaCu₃Ti₄O₁₂ composite ceramics were prepared by spark plasma sintering. Sintering behavior, microstructures and dielectric properties of the composite ceramics were investigated by XRD, SEM, EDS and dielectric spectrometer. Dense composite ceramics consisting of Ba_0.4Sr_0.6TiO₃ phase and CaCu₃Ti₄O₁₂ phase were prepared at 800 °C for 0 min. The dielectric loss of the composite ceramic decreased with increasing amount of Ba_0.4Sr_0.6TiO₃, and the high dielectric constant were retained. Moreover, the better temperature stability of dielectric constant was obtained. These improvements of dielectric characteristics have great scientific significance for potential application.     

Fabrication of BaTiO3-PTFE composite film for embedded capacitor employing aerosol deposition

The potential for using aerosol deposition (AD) as an alternative fabrication method to the conventional polymer composite process for embedded capacitors was examined. In order to achieve a high relative dielectric permittivity, BaTiO₃-polytetrafluoroethylene (PTFE) composite thick films were attempted by AD at room temperature. For the high dielectric constant, the BaTiO₃-PTFE composite films grown by AD should satisfied the following two critical conditions: a reduced decrement in ceramic particle size and a relieved distortion of the crystal structure. However, the relative permitivity of the composite films was too low compared with that of the BaTiO₃ films grown by AD. By predicting the dielectric constant in several composite models using the Hashin-Shtrikman bounds theory and 3-dimenstional (3-D) electrostatic simulation, we confirmed that the connectivity between ceramic particles is a highly critical factor for achieving a high dielectric constant in composite films.     

Inversion boundary induced grain growth in TiO2 or Sb2O3 doped ZnO-based varistor ceramics

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO₂/Bi₂O₃ ratio. The selection of a composition with an appropriate TiO₂/Bi₂O₃ ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi₂O₃ is important as the grain growth is slowed down by a larger amount of Bi₂O₃-rich liquid phase at the grain boundaries. Exaggerated grain growth in low-voltage varistor ceramics is related to the occurrence of the liquid phase and the presence of TiO₂ which triggers the formation of inversion boundaries (IBs) in only a limited number of grains, and as a result the final microstructure is coarse grained. The Zn₂TiO₄ spinel phase only affects grain growth in compositions with a TiO₂/Bi₂O₃ ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb₂O₃ trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb₂O₃/Bi₂O₃ ratio lower than 1, grain growth that is controlled entirely by an IBs-induced grain growth mechanism results in a fine-grained microstructure. The spinel phase interferes with the grain growth only at higher Sb₂O₃/Bi₂O₃ ratios.     

Quantitative analysis of oxidation–reduction behavior of Mn-doped BaTiO3

Effects of Mn addition on the oxidation–reduction reaction behavior of BaTiO₃ have been studied as a function of oxygen partial pressure and temperature. Oxygen vacancy concentration of Mn-doped BaTiO₃ significantly increased as the oxygen activity was lowered. The enthalpy of reduction reaction was estimated from the oxygen vacancy contents at various temperatures and increased with increasing Mn contents. The oxygen vacancy concentration of specimens co-doped with Mg and Mn at room temperature was much lower than that of the singly Mg-doped specimen. It was confirmed that the increased enthalpy of reduction reaction could suppress the degradation behavior and improve the reliability of dielectrics with Mn.