Electric field-assisted solid-state reaction of BaCO3-TiO2 system

We report phase transition process during the solid-state reaction of BaCO₃-TiO₂ system under the assistance of electric field. Experiments were conducted at a constant heating rate with preset field strength and current limit. Solid-state reaction was completed upon reactive flash sintering taking place at ~1002℃ under 200 V/cm. Hexagonal BaTiO₃ phase, which rarely occurs at such temperatures, was obtained after flash sintering at a current density of 23.5 mA/mm². It is speculated that oxygen deficiency during flash sintering triggered cubic-to-hexagonal transition of BaTiO₃. Furthermore, X-ray diffraction results show that solid-state reaction takes place prior to flash sintering. Electric field could accelerate the reaction but did not alter the sequence of phase evolution.     

Reactive sintering of phosphorous coated BaTiO3

Doping BaTiO₃ with P⁵⁺ has been reported to decrease the sintering temperature of the ceramic, allowing homogeneous fine grained microstructures to be obtained. According to the present paper, the formation of a phosphorous-BaO rich phase covering the BaTiO₃ particles seems to be the origin of the improved porosity coalescence and removal observed at the earlier sintering stage of these materials. The formation and development of phosphorous-barium rich phases have been followed by means of DRX, and high temperature NMR. Phosphorous cations incorporated from phosphate ester form a surface layer covering the BaTiO₃ particles which reacts during heating to form Ba₂TiP₂O₉ and/or Ba₃(PO₄)₂. Formation of these phases seem to occur by solid-solid reaction involving phosphorous diffusion through the BaTiO₃ particles.     

Ultra‐Low Temperature Sintering and Dielectric Properties of SiO2‐Filled Glass Composites

Ultra low temperature co‐fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and SiO₂ filler was investigated with regard to the phase composition, the microstructure and the dielectric properties as functions of the filler content and sintering temperature. The softening temperature of 554°C and the crystallization temperature of around 650°C for the glass were confirmed by Differential Thermal Analysis result. The X‐ray diffraction results show that all SiO₂‐filled samples were made up of SiO₂, α‐Zn(BO₂)₂, Zn₃B₂O₆ phases. And there was no chemical reaction between SiO₂ and the glass during densification. And then the dielectric constant decreased with the increasing content of SiO₂. At the level of 15 wt% SiO₂ addition, the composites can be densified at a sintering temperature of 650°C for 30 min, and showed the optimal dielectric properties at 1 MHz with the dielectric constant of 6.1 and the dielectric loss of 1.3 × 10^?3, which demonstrates a good potential for use in LTCC technology.     

Effect of properties and crystallization behavior of BaO–ZnO–B2O3–SiO2 glass on the electrical conductivity of Cu paste

The extensive application of multilayer ceramic capacitors provides an attractive development for terminal electrode pastes. However, the growing requirement for advanced glass materials used in terminal electrode pastes is substantiated. Therefore, to advance the development of electrode pastes, better development and deeper exploration of glass powder are required. Here, a series of BaO–ZnO–B₂O₃–SiO₂ (BZBS) glasses were prepared by melt-quenching technique, which are used to investigate the effect of the introduction of BaO on structure and properties of the ZnO–B₂O₃–SiO₂ (ZBS) glass. With the introduction of BaO, the relative amount of [BO₄] was much less, which made the glass network structure loosen, decreased the glass transition temperature (T_g) and increased the coefficient of thermal expansion of the glass. The decreasing contact angle was observed on the surface of a barium titanate (BaTiO₃) substrate. When the BaO content was around 3–7 mol%, the stability of ZBS glass frit could be strengthened by inhibiting the precipitation of Zn₂SiO₄ crystal. In addition, to further characterize the effect of glass on terminal electrode paste, BZBS glass powder was adopted to prepare copper electrode paste, which was printed on the BaTiO₃ substrate and subsequently fired at 800°C for 10 min. With the related copper paste containing ZBS glass doped with 7 mol% BaO, the optimum value of acid resistance and sheet resistance (1.99 mΩ) were exhibited, at which the coated copper paste formed a dense conducting layer.     

Sintered glass-ceramics from Municipal Solid Waste-incinerator fly ashes—part I: the influence of the heating rate on the sinter-crystallisation

A glass composition, made by mixing 70% of MSW ashes and 30% of waste from feldspar production, was studied. The batch was melted at 1350 °C and the melt was quenched in water. The chemical stability of the glass was investigated by TCLP leaching test. DTA experiment, at 10 °C/min heating rate in the 20–1200 °C range, showed that the glass has a high crystallisation trend with a gehlenite (2CaO.Al₂O₃.SiO₂) formation at 935 °C. The percentage of crystal phase formed as a function of the heat treatment was measured by density variation and XRD. In order to obtain sintered glass ceramics, the frit was heat treated in Al₂O₃ moulds at 950, 1000 and 1050 °C by 2 and 20 °C/min heating rates. At high heating rate predominantly surface crystallisation occurred with fibre-like crystals growing perpendicularly from the surface; at low heating rate bulk crystallisation took place. At low rate, the sintering was inhibited by the crystallisation process. Non porous sintered samples were obtained by a 30 °C/min heating rate and a 40 min isothermal step at 1120 °C, near the liquidus temperature.     

Fabrication of piezoelectric particle-dispersed ceramic nanocomposite

The goal of this study is to fabricate perovskite type ferroelectric particles-dispersed ceramic nanocomposites though conventional hot-pressing or pulse electric current sintering (PECS). This type of nanocomposite is expected to show ferroelectricity or piezoelectricity with retaining mechanical properties. Magnesia (MgO) and barium titanate (BaTiO₃) were selected as a matrix and secondary phase dispersoid. From X-ray diffraction analysis, the BaTiO₃ was the phase compatible with the MgO matrix, and there were no reaction phases between the matrix and BaTiO₃. It was found that the BaTiO₃ enhanced the sinterability of the MgO ceramics. Relative density of pure MgO was lower than 80%, while dense MgO/10 vol% BaTiO₃ nanocomposites could be successfully prepared by sintering at 1200°C for 10 min through PECS method. Fine BaTiO₃ particles were homogeneously dispersed within the MgO matrix grain as well as at grain boundaries. Sintering behavior and microstructure development of the MgO/BaTiO₃ nanocomposites were discussed in terms of BaTiO₃ content and sintering temperatures.     

Fabrication and dielectric properties of barium titanate-based glass ceramics for tunable microwave LTCC application

Low-fired ferroelectric glass ceramics were fabricated from glass powders with a basic composition of 0.65BaTiO₃·0.27SiO₂·0.08Al₂O₃. The combined addition of SnO₂ (or ZrO₂) and SrCO₃ was conducted to modify the dielectric properties of the glass ceramics. The Sr-component could be incorporated preferentially in the perovskite structure after heating at 1000 °C. The bulk and thick film samples obtained by sintering glass powder with a starting composition of 0.65(Ba_0.7Sr_0.3)(Ti_0.85Sn_0.15)O₃·0.27SiO₂·0.08Al₂O₃ at 1000 °C for 24 h showed a broadened ɛ_r–T relation with T_c ≈ 10 °C and ɛ_r(max) ≈ 280 and microwave tunability of 32% at 3 GHz, respectively.     

Effect of step free energy on delayed abnormal grain growth in a liquid phase-sintered BaTiO3 model system

The changes in solid/liquid interface structure and grain growth behavior with oxygen partial pressure (PO₂) were systematically studied during liquid-phase sintering of 8TiO₂/2SiO₂-added BaTiO₃. As the PO₂ of the sintering atmosphere increased, the grain boundaries and solid/liquid interfaces showed increased faceting, indicating an increase in step free energy. This increase in PO₂ and step free energy caused a change in grain growth behavior as a function of sintering time. When samples were sintered in H₂, abnormal grain growth (AGG) occurred from the beginning, resulting in a coarse microstructure with a large average grain size. With increasing PO₂, the incubation time necessary for AGG also increased. Finally, for samples sintered in air, AGG did not occur even after 100 h. These changes in incubation time for abnormal grain growth demonstrate the effect of changing the step free energy on the microstructural development during liquid phase sintering of ceramic systems.     

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.     

Sintering behavior of SiO2 aerogel composites reinforced by mullite fibers via in-situ rapid heating TEM observations

The direct in situ TEM imaging method is adopted to investigate the sintering behavior of SiO₂ aerogels during the rapid heating process. The structural evolution of SiO₂ aerogels and composites at different times during the heat treatment process are further investigated via SEM, FT-IR, BET and XRD. The results indicate that the shrinkage of the SiO₂ aerogels and composites primarily occurs during the initial stage of the heating process (within 20 min) with the shrinkage primarily linked to the fusion of small aerogel particles at high temperatures. The aerogel structure then stabilizes with no further shrinkage observed as the heating process continues. The heat treatment process only promotes the space rearrangement and fusion of small aerogel particles with no observed changes to the amorphous structure of the aerogels, and the small-sized particles fusion was the main causes for the structural evolution of SiO₂ aerogels and composites under rapid heating condition.     

Grain size effect and microstructure influence on the energy storage properties of fine‐grained BaTiO3‐based ceramics

The dependence of energy storage properties on grain size was investigated in BaTiO₃‐based ferroelectric ceramics. Modified BaTiO₃ ceramics with different grain size were fabricated by two‐step sintering method from BaTiO₃ powders doped with Al₂O₃ and SiO₂ by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al₂O₃‐SiO₂, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al₂O₃‐SiO₂‐ZnO‐doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm³. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM‐EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low‐cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.     

The influence of concentration on the formation of BaTiO3 by direct reaction of TiCl4 with Ba(OH)2 in aqueous solution

The formation of fine BaTiO₃ particles by reaction between liquid TiCl₄ and Ba(OH)₂ in aqueous solution at 85 °C and pH⩾13 has been studied for 0.062⩽[Ba²⁺]⩽0.51 mol l⁻¹. The concentration of Ba²⁺ ions has a strong influence on reaction kinetics, particle size and crystallite size. When [Ba²⁺]>≈0.12 mol l⁻¹, the precipitate consists of nanosized (≈30 nm) to submicron (100–300 nm) particles of crystalline BaTiO₃. At lower concentrations, the final product is a mixture of crystalline BaTiO₃ and a Ti-rich amorphous phase even for very long reaction times. A two-steps precipitation mechanism is proposed. Initially, a Ti-rich amorphous precipitate is rapidly produced. Reaction between the amorphous phase and the Ba²⁺ ions left in solution then leads to crystallisation of BaTiO₃. In addition to nucleation and growth of nanocrystals, the final size and morphology of BaTiO₃ particles obtained at low concentration can be determined by aggregation of nanocrystals and heterogeneous nucleation on existing crystal surfaces.     

Two-step sinter-crystallization of K2O–CaO–P2O5–SiO2 (45S5-K) bioactive glass

Bioactive glasses and glass-ceramics (GCs) effectively regenerate bone tissue, however most GCs show improved mechanical properties. In this work, we developed and tested a rarely studied bioactive glass composition (24.4K₂O-26.9CaO-46.1SiO₂-2.6P₂O₅ mol%, identified as 45S5-K) with different particle sizes and heating rates to obtain a sintered GC that combines good fracture strength, low elastic modulus, and bioactivity. We analyzed the influence of the sintering processing conditions in the elastic modulus, Vickers microhardness, density, and crystal phase formation in the GC. The best GC shows improved properties compared with its parent glass. This glass achieves a good densification degree with a two-step viscous flow sintering approach and the resulting GC shows as high bioactivity as that of the standard 45S5 Bioglass®. Furthermore, the GC elastic modulus (56 GPa) is relatively low, minimizing stress shielding. Therefore, we unveiled the glass sintering behavior with concurrent crystallization of this complex bioactive glass composition and developed a potential GC for bone regeneration.     

Uniform Coating of BaTiO3–Dy2O3–SiO2 Compound Nano Layer on Ni Particles for MLCC Electrode

Uniform coating of nanometer‐scale BaTiO₃–Dy₂O₃–SiO₂ layers on spherical Ni particles are achieved by controlled hydrolysis of tetrabutyl titanate (TBT), hydrothermal reaction with Ba(OH)₂, and co‐precipitation of tetraethylorthosilicate (TEOS) and Dy(NO₃)₃. The composition of the coating layer is similar to rare earth oxide‐silica–doped BaTiO₃, which is the main component of dielectric layer for base metal electrode (BME) multilayer ceramic capacitors (MLCCs). After coating, the shrinkage onset temperature of Ni particles is significantly increased. After sintered to pellets, the electrode has good electrical conductivity. This electrode material has good compatibility with rare earth oxide and silica‐doped BaTiO₃ dielectric materials, and could serve as promising candidate for application in the next generation BME‐MLCCs.     

Hydrothermal synthesis and formation mechanism of tetragonal barium titanate in a highly concentrated alkaline solution

Tetragonal cube-shaped barium titanate (BaTiO₃) was produced by the hydrothermal treatment of a peroxo-hydroxide precursor, a single-source amorphous barium titanate precursor, in a highly concentrated sodium hydroxide solution. Phase pure barium titanate with cube-shaped morphology and particle-sizes in the 0.2–0.5 µm range were formed at temperatures above 80 °C. Also, the cube-shaped morphology of the BaTiO₃ product was preceded by spherical- and plate-like morphologies with, respectively, a Ti-excess and Ba-excess. Coinciding with these morphological observations, changes in the reaction product were also observed. The formation of crystalline BaTiO₃ proceeded alongside secondary BaTi₂O₅ and Ba₂TiO₄ phases. These secondary phases disappeared as the reaction time was increased leaving only BaTiO₃ as the sole reaction product. Kinetic analysis of the formation of hydrothermal BaTiO₃ crystallization by the Johnson-Mehl-Avrami method showed that BaTiO₃ crystallization is a homogeneous dissolution-precipitation reaction. The mechanism is governed by nucleation and growth in the beginning of the reaction and dissolution-precipitation dominating throughout the hydrothermal reaction process.     

Liquid-phase sintering of ZrO2-based nanocrystalline glass–ceramics achieved by multielement co-doping

Liquid-phase sintering (LPS) is an effective pathway to assist the densification of ceramics. However, it has seldom been used to densify glass–ceramics. In the present study, a multielement co-doping strategy has been utilized to achieve LPS of a ZrO₂–SiO₂ nanocrystalline glass–ceramic. Compared with undoped samples densified by solid-state sintering, doping of equimolar Al, Y, and Ca promoted the densification of the glass–ceramic at lower temperatures with a faster densification rate. Ternary doping enhanced coarsening of ZrO₂ nanocrystallites during sintering and annealing. The distribution of dopants was carefully observed with X-ray energy-dispersive spectrometry technique in scanning electron transmission microscopy mode. Results showed that the three dopants showed different distribution behaviors. After sintering, Y dopants were predominately distributed in ZrO₂ nanocrystallites, whereas parts of Al and Ca dopants were distributed in ZrO₂ nanocrystallites and part of them co-segregated at the ZrO₂/SiO₂ heterointerfaces. Meanwhile, the segregation of Ca dopant at some intergranular films among ZrO₂ nanocrystallites was observed. Redistribution of dopants did not occur during annealing.     

Properties and rapid consolidation of nanostructured Al2O3-Al2SiO5 composites by high frequency induction heated sintering

The rapid sintering of nanostructured Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites was investigated by a high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites were produced with simultaneous application of a 80 MPa pressure and induced output current of a total power capacity (15 kW) within 3 min. The sintering behavior, grain size and mechanical properties of Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites were investigated.     

Effects of dopants with various valences on densification behavior and phase composition of a ZrO2–SiO2 nanocrystalline glass-ceramic

Effects of dopants with different valences on the densification behavior and phase composition of a ZrO₂–SiO₂ nanocrystalline glass-ceramic (NCGC) during pressureless sintering were investigated in this study. The raw powder of Ca²⁺, La³⁺, Ce⁴⁺ and Ta⁵⁺ ions doped ZrO₂–SiO₂ (referred to as Ca-ZS, La-ZS, Ce-ZS, Ta-ZS, respectively) and pure ZrO₂–SiO₂ (PZS) sample were synthesized by sol-gel method, followed by pressureless sintering. Compared with the PZS sample, doping of Ca²⁺ and La³⁺ ions significantly promoted the densification of the NCGCs. The “densification promotion” effect was attributed to the formation of oxygen vacancies and the decrease of SiO₂ viscosity due to doping of aliovalent cations. The dopants with various valences showed significant effects on the phase compositions of the NCGCs during sintering. Doping of Ca²⁺ ion accelerated the reaction kinetics between ZrO₂ nanocrystallites and amorphous SiO₂ to yield ZrSiO₄. The La³⁺ ion acted as destabilizer of t-ZrO₂, which resulted in a rapid tetragonal (t) to monoclinic (m) ZrO₂ phase transformation during sintering, while in the Ta⁵⁺ and Ce⁴⁺ ions doped sample, the phase transformation occurred gradually. All the doping ions increased the lattice parameters and the volume of t-ZrO₂ unit cell, while the effects of the doping ions on the lattice parameters of m-ZrO₂ unit cell were more complex.     

AGGLOMERATION BEHAVIOR IN A BUBBLING FLUIDIZED BED AT HIGH TEMPERATURE

Minimum fluidization velocity and agglomeration behavior were investigated at high temperature in an 80?×?30?mm two-dimensional quartz fluidized bed and in an 82?mm i.d. circular fluidized bed. Bed materials tested were two sizes of glass beads as well as three sizes of fluidized bed combustor (FBC) ash. The minimum fluidization velocity decreased with increasing bed temperature, whereas the minimum sintering fluidization velocity increased with the bed temperature. The sintering of glass beads belongs to visco plastic sintering, the first type. FBC ash agglomerate has higher amounts of SiO₂, Al₂O₃, Na₂O, K₂O, and SiO₂ than in the original ash, indicating that low melting eutectics were formed and that the liquid phase in a silicate system was formed. The agglomeration of FBC ash belongs to the second type, an excessive quantity of liquid being formed by melting or chemical reaction.     

Far from equilibrium ultrafast high-temperature sintering of ZrO2–SiO2 nanocrystalline glass–ceramics

Ultrafast high-temperature sintering (UHS) is a novel sintering technique with ultrashort firing cycles (e.g., a few tens of seconds). The feasibility of UHS has been validated on several ceramics and metals; however, its potential in consolidating glass–ceramics has not yet been demonstrated. In this work, an optimized carbon-free UHS was utilized to prepare ZrO₂–SiO₂ nanocrystalline glass–ceramics (NCGCs). The phase composition, grain size, densification behavior, and microstructures of NCGCs prepared by UHS were investigated and compared with those of samples sintered by pressureless sintering. Results showed that NCGCs with a high relative density (～95%) can be obtained within ～50 s discharge time by UHS. The UHS processing not only hindered the formation of ZrSiO₄ and cristobalite but also enhanced the stabilization of t-ZrO₂. Meanwhile, owing to the ultrashort firing cycles, the UHS technology allowed the NCGCs to be consolidated in a far from equilibrium state. The NCGCs showed a microstructure of spherical monocrystalline ZrO₂ nanocrystallites embedded in an amorphous SiO₂ matrix.