Preparation and characterizations of tetragonal barium titanate powders by hydrothermal method

Tetragonal barium titanate powder with an average particle size of 80 nm has been successfully prepared by a hydrothermal process at 240 °C for only 12 h. The influence of processing parameters on the phase and the particle size of hydrothermally derived BaTiO₃ powders was investigated. Increasing NaOH excess concentration and decreasing the initial titanium tetrachloride concentration promote to increase the particle size and are benefit for preparation of tetragonal BaTiO₃ powder, while increasing the reaction temperature and time also obtain same result. There is a relationship between the particle size and the phase of BaTiO₃ powder and a critical size of transformation of cubic-BaTiO₃ to tetragonal-BaTiO₃. In the present work the critical size was 80 nm and finer than other previous works.     

Identification of quality parameters for barium titanate powder

The authors describe the results of developing methods for determining some quality parameters of barium titanate powder, which is the main component in some ceramics that are used to produce capacitors. The degree of transformation of ZnO and BaTiO₃, the volume density of sintered powder, the degree of its dissolution in hydrochloric acid, and some diffraction characteristics of BaTiO₃ powders are determined according to the developed methods and can predict with a sufficient degree of reliability the quality of ceramic capacitors made from barium titanate powders.     

Effects of Initial Powder Size on the Densification of Barium Titanate Ceramics Prepared by Microwave‐Assisted Sintering

The effects of initial powder size on microwave‐assisted sintering (MWS) were investigated. BaTiO₃ powders with an average particle size of 50, 100, and 500 nm were prepared and sintered with MWS and conventional heating‐based sintering (CS). Samples of the 50 ‐ and 100‐nm‐sized BaTiO₃ powders were mechanically milled to study the effects of powder crystallinity on microwave absorption during the MWS process. The MWS of the 50‐nm‐sized BaTiO₃ powder resulted in a relative mass density of more than 90% when sintered at 1050°C, whereas the same density was achieved at 1200°C with CS. This difference between the optimal sintering temperatures, which is caused by the absorption of microwaves, was not observed when the 500‐nm‐sized BaTiO₃ powder was used. The sinterability of the BaTiO₃ ceramics prepared through the MWS of mechanically milled, 50‐nm‐sized powders decreased with increasing milling time. However, the sinterability was much higher than that of the BaTiO₃ ceramics prepared through the MWS of the 100‐ and 500‐nm‐sized unmilled powders. In conclusion, microwave absorption has significant effects on the sintering behavior of ~50‐nm‐sized powders, but is negligible for 500‐nm‐sized powders.     

Stability of Aqueous Barium Titanate Suspensions for MLCC Inkjet Printing

BaTiO₃‐based materials are currently used for the fabrication of multilayer ceramic capacitors (MLCC) because of their high dielectric properties. The inkjet printing (IJP) process can be used to fabricate MLCC of complex configurations by integrating internal electrodes and dielectric layers in a single step using a multi printing‐head system. Stabilized aqueous suspensions of BaTiO₃‐based powders are required to obtain dielectric inks adapted to IJP. This study investigates the influence of BaTiO₃ powder hydrolysis in water on the surface chemistry and stability in relationship with the milling step used to adjust the powder grain size to IJP. Optimum parameters for a good stability of BaTiO₃ suspensions are identified. The selected dispersant is a polyacrylate (PAA) for which the content is adjusted to minimize the sedimentation rates as required by IJP. Moreover, the addition of ethylene glycol is shown to be necessary to avoid the formation of a gel structure which could result from the interaction of borates ions leached from the surface of BaTiO₃ with the PAA dispersant. A mechanism of gel formation is proposed.     

Power prediction for laser engineered net shaping of Al2O3 ceramic parts

Laser-aided additive manufacturing technique is a competitive method for direct fabrication of ceramic components. However, the optimal processing parameters are difficult to find because defects are easy to generate for ceramic parts. This paper proposes a mathematical model for predicting required laser power in direct fabrication of Al₂O₃ ceramic parts by laser engineered net shaping (LENS). The laser power model, which is derived based on energy balance of one deposition layer, reveals the relationship between laser power and other process conditions, such as powder flow rate, nozzle travel speed and physical properties of deposited material. The proposed model was then verified through a fabrication experiment of several single-bead wall Al₂O₃ ceramic parts with different laser power. Experimental results indicate that the laser power predicted by the model is accurate for different processing conditions. This model provides a new yet simple method for predicting required laser power accurately during LENS processes.     

Preparation of alumina/polystyrene core-shell composite powder via phase inversion process for indirect selective laser sintering applications

Indirect selective laser sintering (SLS) is one of the promising additive manufacturing (AM) methods that can process conventionally difficult or even impossible materials such as ceramics. In this work, an innovative phase inversion technique is used to fabricate spherical alumina particles coated with a thin layer of polystyrene (PS). Then, indirect SLS is used to fabricate green parts from the 6 wt% PS coated alumina particles via a Nd:YAG laser. The assessed SLS process parameters were the scan speed, laser power, scan spacing, pulse frequency, and pulse width. The characterization of the AL₂O₃/PS core-shell composite particles was described using techniques including SEM (for morphology), FT-IR (for chemical bonding at the interfaces), TGA (for mass loss), and DSC (for glass transition temperature, T_g). 3D green parts were then fabricated using proper process parameters as a proof of the feasibility of using SLS technique for AL₂O₃/PS core-shell composite powder. The results showed that using a Nd:YAG laser with less absorption by alumina and PS provides greater penetration through a powder bed. In addition, the possibility of sound connections among particles in every direction was observed due to the uniformity of the coating process in spite of a minimal amount of binder. In addition, green part density measurements show high values compared to previously reported results.     

3D printed ceramic phosphor and the photoluminescence property under blue laser excitation

An Al₂O₃/YAG: Ce³⁺ ceramic phosphor was fabricated for high-flux laser lighting using the digital lighting process (DLP)-based 3D printing method for the first time. The photocurable ceramic suspension for 3D printing was prepared by blending well-treated Al₂O₃/YAG: Ce³⁺ composite powders with photosensitive resin monomers and photo-initiators. The printing parameters, debinding and sintering processes were designed delicately to fabricated the dense sub-millimeter-sized cylinder ceramic with high dimensional accuracy. The ceramic showed excellent luminescence property under blue laser excitation with a threshold of 20.7 W/mm², higher than that prepared via dry-pressing followed by vacuum sintering. The luminescence properties and the microstructures of both ceramics were further comparatively investigated to find the possible interpretations for improvement of laser flux for the 3D-printed ceramic. The present work indicated that the new developed 3D printing method was promising for preparing luminescent ceramics for high-flux laser lighting in a rapid, effective, low-cost and precision-controlled manner.     

Direct selective laser sintering of hexagonal barium titanate ceramics

A direct selective laser sintering (SLS) process was combined with a laser preheating procedure to decrease the temperature gradient and thermal stress, which was demonstrated as a promising approach for additive manufacturing of BaTiO₃ ceramics. The phase compositions in BaTiO₃ ceramics fabricated by SLS were investigated by X-ray and neutron diffractions. The surface morphologies and cross-section microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A dense hexagonal h-BaTiO₃ layer was formed on the surface and extended to a depth of 500 μm, with a relative density higher than 97% and absence of pores or microcracks. SLS resulted in the formation of the high-temperature phase, h-BaTiO₃, which was retained at room temperature possibly due to the high cooling rate. The grain boundaries of SLSed h-BaTiO₃ ceramics consist of a Ti-rich secondary phase. Compared with that of the pressureless sintered t-BaTiO₃ ceramics, the Vickers hardness of SLSed h-BaTiO₃ is 70% higher.     

Sonochemical synthesis of monosized spherical BaTiO3 particles

Monosized spherical particles of BaTiO₃ have been successfully synthesized by a sonochemical method in a strong alkaline environment using BaCl₂·2H₂O as the barium source and TiCl₄ as the titanium source. The as-prepared BaTiO₃ powders were characterized by employing techniques including X-ray powder diffraction (XRD), scanning electron microscope (SEM), energy dispersive analysis of X-rays (EDAX) and laser particle size analyzer. The effects of reactant concentrations and Ba/Ti molar ratio on the precipitation of BaTiO₃ particles were briefly investigated. The particles have a monosized spherical morphology and the particle size ranges from submicron (600-800 nm) to nanometer (60-70 nm) by increasing the reactant concentration (from 0.072 mol/L to 0.72 mol/L). The studies indicated that increasing the Ba/Ti ratio can promote synthesis of BaTiO₃.     

Nano-sized Ag–BaTiO3 composite powders with various amount of Ag prepared by spray pyrolysis

The preparation of precursors of BaTiO₃ nanopowders with various amounts of Ag by spray pyrolysis is reported. The precursor powders obtained with hollow and thin-wall particles are composed of uniformly dispersed Ba, Ti, and Ag components. After post-treatment and a simple milling process, the precursor powders, irrespective of the amount of Ag, are transformed into Ag–BaTiO₃ composite nanoparticles. The mean particle size of the Ag (10 mol%)–BaTiO₃ powders is 142 nm. BaTiO₃ pellets containing Ag exhibit dense structures even at a low sintering temperature of 1000 °C. BaTiO₃ pellets with 10 mol% Ag show the highest dielectric constant of 2950, as opposed to the pure BaTiO₃ pellets (without Ag), whose dielectric constant is 1827.     

Synthesis and performance of tetragonal BaTiO3 fine powders via a facile tartaric acid assisted method

A facile tartaric acid assisted method using metatitanic acid, barium hydroxide and tartaric acid as starting materials is proposed to prepare tetragonal BaTiO₃ fine powders. Owing to the ultrafine character of the as-formed BaCO₃ with high chemical activity, and its arrangement coating the surface of intermediate TiO₂ nanoscale needles, pure cubic phase BaTiO₃ homogeneous powders with size of about 50 nm was obtained via calcining treatment at 650 ℃, which subsequently transformed as tetragonal BaTiO₃ uniform powders with size of about 240 nm and high tetragonality (c/a=1.0095) after calcining treatment at 1050 ℃. The BaTiO₃ ceramic prepared from the BaTiO₃ uniform powders displayed much improvement performances with high permittivity of about 5980 and low dielectric loss of about 0.014 at room temperature in comparison to the BaTiO₃ ceramic prepared respectively from the BaTiO₃ synthesized via a traditional solid-state method, and commercial BaTiO₃, suggesting it’s compatible in application of MLCCs with high performance.     

Embedded Capacitor Technology Using Aerosol Deposition

Embedded passives, which achieve miniaturization, cost reduction, and higher performance, are regarded as one of the most promising technologies for a future RF module substrate. Currently, a BaTiO₃/Polymer composite is being used for the embedded capacitors in printed wiring boards. One of the drawbacks of this composite is its relatively low dielectric constant, because the polymer component with a low dielectric constant suppresses the dielectric constant of the whole composite. We propose a resin build-up circuit board with passive functions embedded in a ceramic film without any polymer component for the next-generation low-cost RF modules. We have already manufactured a prototype board with ceramic capacitors embedded in an FR-4 substrate using a unique ceramic deposition technology: aerosol deposition (ASD) in which many kinds of ceramics can be deposited on a substrate at room temperature by making use of accelerated ceramic nanoparticle aerosol bombardment with a nozzle. In this study, first we examine the effects of the characteristics of raw ceramic powder on the crystal structure and the dielectric properties of ASD films. As a result, we confirmed that dense BaTiO₃ dielectric films can be deposited when raw powder without strain is used. From the resulting polarization versus electrical field (P–E curve), we confirmed that paraelectric was observed in the dense films, while the porous BaTiO₃ films deposited using milled powder exhibit a small hysteresis loop. We also clarified that dense BaTiO₃ dielectric films exhibit a nanostructure with a texture consisting of particles under 10 nm in diameter. We also examine the interfacial behavior between BaTiO₃ dielectric films and the Cu electrode, in order to investigate the deposition temperature and the reliability of a BaTiO₃ ASD film under high temperature (250°C), high humidity (100 Rh%), thermal cycle condition (−55°C to 150°C), and bias DC voltage (5 V). We clarified that the BaTiO₃ ASD film satisfies the criteria of reliability in the microelectronic packaging area.     

The characterization of the barium titanate ceramic powders prepared by the Pechini type reaction route and mechanically assisted synthesis

BaTiO₃ ceramic powders were prepared by a complex method based on the Pechini type reaction route and mechanically assisted synthesis. In both ways BaTiO₃ ceramics were sintered after 120 min on 1300 °C without pre-calcination steps. The crystal structure was investigated by the XRD, IR and Raman spectroscopy. The particle size and morphology of BaTiO₃ were examined by XRD and SEM. The XRD results of powders indicate the formation of cubic phase of BaTiO₃. It can be observed that in the case of Pechini process BaTiO₃ powder is well crystallized but in the case of mechanochemistry process, significant amount of amorphous phase was detected. The sintered BaTiO₃ ceramic sample prepared by Pechini process, shows the formation of tetragonal phase. However, IR and Raman spectrum showed a mixture of cubic and tetragonal for BaTiO₃ obtained by Pechini process and tetragonal for BaTiO₃ obtained by mechanically assisted synthesis.     

High pressure FAST of nanocrystalline barium titanate

This work studies the microstructural evolution of nanocrystalline (<1 µm) barium titanate (BaTiO₃), and presents high pressure in field-assisted sintering (FAST) as a robust methodology to obtain >100 nm BaTiO₃ compacts. Using FAST, two commercial ~50 nm 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 300 MPa and temperatures of 900 °C, were 99.5% dense and had a grain size of 90±24 nm. These are unprecedented results for commercial BaTiO₃ powders or any starting powder of 50 nm particle size—other authors have used 16 nm lab-produced powder to obtain similar results.     

Shrinkage behavior and interfacial diffusion in Ni-based internal electrodes with BaTiO3 additive

The effects of particle size of starting materials and amount of a BaTiO₃ additive on the shrinkage behavior and elemental diffusion in Ni-based internal electrodes have been investigated in order to control the shrinkage of the internal electrode in multilayer ceramic capacitors (MLCCs). Two kinds of Ni and BaTiO₃ powders were used with different particle sizes. Volume shrinkage over the range of 700–1300 °C at 150 °C intervals and linear shrinkage during sintering were measured for starting materials and composites in a reducing atmosphere. The interfaces of Ni/BaTiO₃ composites with 90:10 and 70:30 volume ratios, respectively, were investigated using TEM. Composites with bimodal Ni powder show less shrinkage than those with monomodal Ni powder, showing less shrinkage in monolith Ni of bimodal particle size. The shrinkage behavior is changed during sintering with increasing amounts of BaTiO₃ additives in both Ni-based composites. The particle size of the BaTiO₃ additive affects the shrinkage behavior of composites, without the additional amount affecting the final shrinkage. A reaction layer of about 300 nm wide is observed at the interface between the Ni and BaTiO₃ powders in composites, in which elemental Ni diffuses into the BaTiO₃ without counterdiffusion.     

Synthesis and surface modification of submicron BaTiO3 powders via a facile surfactant-assisted method

Due to their unique properties, well-dispersed barium titanate (BaTiO₃) ultrafine powders can be used in wide-ranging fields. In the present work, by using barium hydroxide octahydrate (Ba(OH)₂·8H₂O) and α titanic acid (H₄TiO₄) as raw materials, uniform submicron BaTiO₃ powders with tetragonal structure and high degree of crystallinity were prepared via a solid-state reaction method at relatively low temperatures. Moreover, by simply using the stearic acid (St) as the modifier to modify the surface of the aggregated BaTiO₃ powders, well-dispersed BaTiO₃ particles could be obtained, which were then examined by complementary characterizations such as XRD, TEM, HRTEM, SEM, Raman, FT-IR, XPS and EDS. The results indicated that the tetragonal BaTiO₃ particles with submicron-size, good uniformity, and high crystallinity could be prepared at 800 °C for 1 h. Moreover, the addition of St for surface modification proved to be an effective way to avoid the agglomeration of the BaTiO₃ particles to get well-dispersed products, where 1 wt % of St was found to be the optimum concentration. The demonstrated surfactant-assisted surface modification method is expected to be applicable for other ultrafine powders to get well-dispersed particles.     

The isoelectric point of BaTiO3

The value of the isoelectric point (IEP) of BaTiO₃ is highly significant for the development of a fully rational basis for the successful processing of BaTiO₃ powder by aqueous routes. We report here experimental evidence that the true IEP value for BaTiO₃ is in the neutral pH range (6–7). However, Ba²⁺ ions are also selectively adsorbed at BaTiO₃ particle surfaces, raising the ζ-potential; IEP values greater than pH 7 can occur when the concentration of Ba²⁺ in the aqueous phase is high. Studies of the effects of Ba²⁺ and CO₃²⁻ additions on the ζ-potential of BaTiO₃ and on the rheology of aqueous suspensions, indicate that the reason for the wide range of ζ-potential and IEP values found for BaTiO₃ is in large measure the presence of impurity BaCO₃ in the BaTiO₃ powders used. Fully dehydrated BaTiO₃ powder has a low IEP value (∼4), which slowly rises on ageing in water. The IEP appears not to be influenced by Nb₂O₅ doping at the levels normally used.     

Exceptional reliability of MLCCs enabled by defect-engineered BaTiO3

It has generally been believed that the reliability of BaTiO₃-based multilayered ceramic capacitors (MLCCs) is mainly contributed by hydroxyl (OH⁻), and the contribution of CO₃²⁻ can be neglected. However, in this work, we demonstrated that the contributions of Ba/Ti ratio and CO₃²⁻ play important roles in the delivering high reliability for BaTiO₃-based MLCCs. The structure and performance of MLCC devices and ceramic chips based on BaTiO₃ powders prepared by different approaches were studied. It is found that the intracrystalline pores in ceramics or MLCCs are mainly derived from the decomposition of BaCO₃ during sintering, which has been demonstrated by ceramic derived from hydrothermal method powder and its modified powders. The point defects of Ba and Ti vacancies mainly originating from nonstoichiometric Ba/Ti rather than thermally stimulated have substantial influence on the migration of grain boundary that determines the grain size and whether the pores can be annihilated from the bulk material. Particularly, the Ti vacancies have a strong pinning effect and inhibit the migration of grain boundary effectively, due to their shorter migration distance comparing to Ba vacancies. Therefore, the synergetic effect of the second phase BaCO₃ and point defects leads to the differences in the structure and performance.     

Synthesis of barium titanate improved by modifications in the kinetics of the solid state reaction

In this work, the synthesis of BaTiO₃ powder from BaCO₃–TiO₂ reaction at high temperatures is studied. In order to improve the kinetics of this reaction, successive modifications on the powder processing have been implemented. The contribution of the mechanochemical activation to the BaTiO₃ formation is analysed. It was found that the use of a mechanochemical activation favours the decomposition of BaCO₃ at low temperatures and improves the barium ion diffusion through the BaTiO₃ layer. In consequence, a BaTiO₃ product free of secondary phases can be obtained at lower temperatures.