Low-Temperature Sintering (Ba0.6Sr0.4)TiO3 Thick Film Prepared by Screen Printing

(Ba_0.6Sr_0.4) TiO₃ thick films doped with glass slurry were fabricated by the screen-printing technique. The dielectric properties and the sintering mechanism were investigated. The films can be sintered at 600°C. The dielectric constant is 88 and the dielectric loss is 0.002 with a tunability of 23.86% under 100 kV/cm. Higher dielectric constant and tunability were obtained in the samples sintered at higher temperatures. The highest tunability is 61.12% under 150 kV/cm in the sample sintered at 800°C. The low sintering temperature and dielectric loss of the glass-doped thick films make them potential candidates for LTCC and microwave tunable devices.     

Role of Liquid Phase in PTCR Characteristics of (Ba0.7Sr0.3)TiO3 Ceramics

The role of liquid phase in the enhancement of the PTCR (positive temperature coefficient of resistance) effect in (Ba_0.7Sr_0.3)TiO₃ (BST) with the addition of AST (4Al₂O₃· 9SiO₂· 3TiO₂) is investigated in this paper. The AST–BST samples were characterized with optical microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and impedance spectroscopy. Microscopic observations showed that slower cooling might facilitate the precipitation of the (Ba,Sr)TiO₃ phase from the liquid phase on matrix grains since the amount of liquid phase was reduced with a decreasing cooling rate. Impedance spectroscopy indicated that this variation accompanied the change in the intrinsic properties of grain boundaries, which could not be explained by well-known oxidation effects. With the aid of a brick-layer model and high-resolution transmission electron microscopy (HRTEM), it appeared that the change in electrical characteristics of grain boundaries with decreasing cooling rate originated from the precipitation of (Ba,Sr)TiO₃. Finally, the effect of precipitated (Ba,Sr)TiO₃ on the PTCR characteristics is discussed in terms of the acceptor-state density and the polarization state at grain boundaries.     

Constrained Film Sintering of Nanocrystalline TiO2

The sintering of thin, nanocrystalline TiO₂ films either 140 or 65 nm thick is characterized and compared with the sintering of bulk material. Grain size, pore size distribution, and density data are obtained. Observation of the microstructural evolution during sintering suggests that grain growth, as well as pore growth, at low density can be attributed to differential sintering. A continuum mechanical model for the intermediate stage is useful until the grain size becomes one-half the film thickness. From then on, the ratio of grain size to film thickness affects the degree by which both densification and grain growth decrease, as compared with the continuum computation results.     

Gas Pressure Sintering of Silicon Nitride Powder Coated with Al2O3 and TiO2

Gas pressure sintering kinetics of silicon nitride powder coated with 10 wt% (9:1) Al₂O₃ and TiO₂ have been studied at 1850°C with a pressure schedule of 0.3 MPa in the first stage and 1 MPa in the second stage. The rates have been analyzed with a liquid-phase sintering model. Diffusion-controlled intermediate-stage kinetics have been observed. The role of second-step pressurization with nitrogen and argon has been determined by monitoring the kinetics. Pressurization at an earlier stage (∼90% relative density) reduces the densification rate but produces a denser material at the final stage. Although final density is greater, a porous surface layer forms on samples sintered with argon pressurization at the second stage. No such porous layer is formed in the case of pressurization with nitrogen. The mechanism of the intermediate-stage kinetics has been discussed with respect to the nature of the product analyzed by XRD after sintering.     

Low-Temperature Sintering and Dielectric Properties of Ag(Nb,Ta)O3 Composite Ceramics

The sintering behavior and dielectric properties of perovskite Ag(Nb_{1− x} Ta _x )O₃ (0 < x < 1) solid solutions and two-phase composite assemblages were explored. A small amount of CuO (1 wt%) was used for liquid-phase sintering and led to high densification at temperatures <950°C. The temperature coefficient of capacitance, TCC, was adjusted by varying the Nb:Ta ratio within the solid-solution series and by creating composite microstructures. Two-phase assemblages consisting of Ag(Nb_3/4Ta_1/4)O₃ and Ag(Nb_1/4Ta_3/4)O₃ were synthesized to achieve a temperature-stable dielectric material for high-frequency applications. The composite dielectric with CuO addition had an average dielectric constant of 390 and a Q × f factor of 410 GHz at 2 GHz, with a stable TCC (0 to −180 ppm/°C) in the temperature range from −20° to +60°C. In addition, process compatibility with a silver conductor was confirmed by high-frequency ring-resonator measurements and microstructural characterization. The Ag(Nb_{1− x} Ta _x )O₃ solid solutions and composites are promising candidates as embedded capacitors for radio-frequency/microwave applications.     

Sintering of (Cu0.25Ni0.25Zn0.50)Fe2O4 Ferrite

A study has been conducted on the sintering of a ceramic ferrite having the composition (Cu_0.25Ni_0.25Zn_0.50)Fe₂O₄. The study analyzes the evolution of ferrite relative to density and microstructure with peak sintering temperature and dwell time at peak temperature. The densification and grain-growth rates are correlated with average grain size, relative density, and temperature. Corresponding rate-controlling diffusion mechanisms are proposed.     

Laser Sintering of Bismuth Germanate (Bi4Ge3O12) Ceramics

A solid-state route to synthesize bismuth germanate (Bi₄Ge₃O₁₂, or BGO) ceramics, and the optimum conditions for the laser sintering of these ceramics, were investigated. Special emphasis was placed on the influence of laser power and irradiation time on densification and microstructure evolution. The procedure that was used involved preheating the samples and a laser-irradiation program, which resulted in sintered, crack-free BGO ceramics with good pore shrinkage and a relative density of 95% (±3%).     

Low-Temperature Sintering, Densification, and Properties of Z-type Hexaferrite with Bi2O3 Additives

The effect of the addition of Bi₂O₃ on the densification, low-temperature sintering, and electromagnetic properties of Z-type planar hexaferrite was investigated. The results show that Bi₂O₃ additives can improve the densification and promote low-temperature sintering of Z-type hexaferrite prepared by a solid-state reaction method. The presence of Bi₂O₃ in the grain boundaries and the generation of Fe²⁺ degrade the initial permeability of the samples but make the quality factor and cut-off frequency increase. Various possible mechanisms involved in generating these effects were also discussed.     

Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)

The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al₂O₃and MgO powders with the addition of Na₃AlF₆up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al₂O₃can be produced from the mixture of 72 wt% (50 at.%) Al₂O₃+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na₃AlF₆fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al₂O₃can be produced from either the mixture of 90 wt% (78 at.%) Al₂O₃+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al₂O₃+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na₃AlF₆in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al₂O₃. Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.     

Faceting of High-Angle Grain Boundaries in Titanium-Excess BaTiO3

The grain boundaries in BaTiO₃ with excess Ti of 0.5, 0.3, and 0.1 at.% sintered at 1300° or 1250°C have been examined by scanning electron microscopy (SEM), electron backscattered diffraction pattern (EBSP), and transmission electron microscopy (TEM). In the 0.1% Ti-excess specimen, large grains growing abnormally form high-angle grain boundaries when they impinge on each other as verified by EBSP. A large fraction of these grain boundaries are faceted with hill-and-valley shapes. In the 0.5% Ti-excess specimen, large grains growing abnormally are elongated in the directions of their {111} double twins. These grains often form flat grain boundaries parallel to their {111} planes with the fine matrix grains, and the grain-boundary segments between the large impinging grains with high misorientation angles are often also parallel to the {111} planes of one of the grains. These grain boundaries are expected to be singular. Most of the grain boundaries between the randomly oriented fine-matrix grains in the 0.3 at.% Ti-excess specimen are also faceted with hill-and-valley shapes at finer scales when observed under TEM. The facet planes are parallel to {111}, {011}, and {012} planes of one of the grain pairs and are also expected to be singular. These high-angle grain boundaries lying on low index planes of one of the grain pairs are similar to those observed in other oxides and metals.     

Direct Characterization of Grain-Boundary Electrical Activity in Doped (Ba0.6Sr0.4)TiO3 by Combined Imaging of Electron-Beam-Induced Current and Electron-Backscattered Diffraction

Simultaneous measurements of remote electron beam induced current (REBIC) and orientation imaging microscopy (OIM) in a scanning electron microscope (SEM) have been applied to a polycrystalline (Ba_0.6Sr_0.4)TiO₃ with a positive temperature coefficient of resistivity (PTCR) to elucidate a grain-boundary character dependence of the potential barrier formation. The absence of electrical activity in a coherent Σ3 twin boundary is clearly imaged. The resistivity of individual grain boundaries estimated from a resistive contrast image is interpreted in terms of geometrical coherency, which is defined by the degree of coincidence in the reciprocal lattice points.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Pulse Electric Current Sintering of Al2O3/3 vol% ZrO2 with Constrained Grains and High Strength

The pulse electric current sintering technique (PECS) was demonstrated to be effective in rapid densification of fine-grained Al₂O₃/3Y-ZrO₂ using available commercial powders. The composites attained full densification (>99% of TD) at 1450°C in less than 5 min. The composites sintered at a high heating rate had a fine microstructure. The incorporation of 3 vol% 3Y-ZrO₂ substantially increased the average fracture strength and the toughness of alumina to as high as 827 MPa and 6.1 MPa·m^1/2, respectively. A variation in the heating rate during the PECS process influenced grain size, microstructure, and strength, though there was little or no variation in the fracture toughness.     

The Structure and Dielectric Tunable Properties of Fine-Grained Ba0.6Sr0.4TiO3 Ceramics Prepared by Spark Plasma Sintering

Barium strontium titanate is a promising material for microwave-phased array applications. ^1,2 In this study, highly dense and fine-grained Ba_0.6Sr_0.4TiO₃ ceramics were prepared using the spark plasma sintering (SPS) technique. The structure and dielectric tunable properties of the samples were investigated. The "distorted nano-region" emerged in the interior of the grains of SPS samples, and resulted in the deterioration of the dielectric tunable properties of Ba _x Sr_{1− x} TiO₃. This phenomenon indirectly testified to the assumption of the "polar nano-region" mechanism. After the SPS samples were annealed, the "distorted nano-region" disappeared and better dielectric tunable properties were obtained. The dielectric constant was decreased to 1048, and the K value (Commutation Quality Factor) reached 7089.     

Microwave Dielectric Properties and Low-Temperature Sintering of Ba0.60Sr0.40TiO3 Ceramics

In the present work, the sintering behaviors and dielectric properties of Ba_0.60Sr_0.40TiO₃ (BST) ceramics with the addition of BaCu(B₂O₅) were investigated in detail. The results indicated that the addition reduced the sintering temperature of BST by about 500°C. It was suggested that a liquid phase BaCu(B₂O₅) assisted the densification of BST ceramics at lower temperatures. For a low-level BaCu(B₂O₅) addition (2.0 mol%), the BST sample sintered at 950°C for 5 h displayed good dielectric properties, with a moderate dielectric constant (ɛ=2553) and a low dielectric loss (tan δ=0.00305) at room temperature and at 10 kHz. The sample showed 45.9% tunability at 10 kHz under a dc electric field of 30 kV/cm. At the frequency of 0.984 GHz, BST-added 2.0 mol% BaCu(B₂O₅) possessed a dielectric constant of 2204 and a Q value of 146.7.     

Sintering of AlN Using CaO-Al2O3 as a Sintering Additive: Chemistry and Microstructural Development

The densification of aluminum nitride using Ca₁₂Al₁₄O₃₃ as a sintering aid has been studied with emphasis on the effect of using coarse or fine powder, the amount of sintering aid, the sintering temperature, and embedding. Both crystalline and amorphous grain boundary phases were observed. Significant weight losses were observed for coarse-grained samples, and if suitable embedding was not used. Porous and coarse-grained ceramics with high contiguity and minor amounts of secondary phases were obtained by enhanced evaporation while dense ceramics were obtained limiting the evaporation. High weight losses in the graphite environment resulted in formation of a dense AlN surface layer.     

Low-Temperature Hydrothermal Synthesis of Phase-Pure (Ba,Sr)TiO3 Perovskite using EDTA

A thermodynamic model was developed to describe the stability of (Ba,Sr)TiO₃ (BST) solid solutions in the Ba–Sr–Ti–K–(EDTA)–H₂O (EDTA = ethylenediaminetetraacetic acid) system. Phase diagrams were computed to identify the range of conditions suitable for making phase-pure BST. Hydrothermal experiments were performed to validate the thermodynamic model. The model was found to be more useful when an ideal solid solution was used to estimate the energetics for the BST phase instead of experimental thermodynamic data. In addition, EDTA was found to promote stable conditions for BST formation. When attempting to prepare Ba_0.50Sr_0.50TiO₃ without EDTA, BaTiO₃-rich and SrTiO₃-rich phases precipitated separately, at 70°–160°C. However, in the presence of EDTA, a phase-pure Ba _0.55Sr_0.45TiO₃ solid solution was obtained at 90°–120°C. EDTA is effective because it prevents phase heterogeneities from forming and equalizes the adsorption affinity of strontium and barium species.     

Sintering Behavior and Surface Microstructure of PbO-Rich PbNi1/3Nb2/3O3–PbTiO3–PbZrO3 Ceramics

The sintering behavior and surface microstructure of PbNi_1/3Nb_2/3O₃–PbTiO₃–PbZrO₃ (PNiNb-PT-PZ) ceramics were investigated. The PNiNb-PT-PZ ceramics with the stoichiometric composition and the addition of excess lead oxide (PbO-rich ceramics) were sintered by liquid-phase sintering in accordance with the solution-reprecipitation mechanism at temperatures below the melting point of PbO. The temperature at which the liquid phase forms fell to near the eutectic point of the PbO–Nb₂O₅ and the PbO–TiO₂ system (868°C) with the addition of 5 mol% PbO. As the calcination temperature influenced the sinterability of the stoichiometric PNiNb-PT-PZ ceramic, unreacted PbO was considered to be the source of the liquid phase in the sintering of the stoichiometric powder. The secondary phase was observed at the surface of PbO-rich ceramics and was suggested to be a liquid phase expelled from inside the ceramic. A sintering scheme of PNiNb-PT-PZ ceramics was proposed, and the high sinterability of PNiNb-PT-PZ ceramics was attributed to the low formation temperature of the liquid phase.     

Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder Compacts

The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa₂Cu₃O_{6+ x} and composite powder compacts (YBa₂Cu₃O_{6+ x} powder with 10 vol% dense inclusions of YBa₂Cu₃O_{6+ x} ) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured.     

Evolution of Microstructure and V-Shaped Positive Temperature Coefficient of Resistivity of (Pb0.6Sr0.4)TiO3 Materials

The effects of various liquid-phase sintering aids on (Pb_0.6SrO_0.4)TiO₃ ceramics have been investigated. The relationships between electrical properties and microstructures have been scrutinized. It has been found that, among the sintering aids studied, only SiO₂ exhibits a significant effect on the grain growth of (Pb_0.6SrO₄)TiO₃. The optimum firing profiles for sound microstructure and good electrical properties of (Pb_0.6SrO_0.4)TiO₃+ 5.0 mol% SiO₂ have been established. The V-shaped electrical behavior is prominent, and a PTCR jump of about 10^2.9 is observed. The formation of cation vacancies may increase the resistivity of the over-fired specimens. Various milling methods to pulverize the calcined powder and the optimum amount of packing protection powder during sintering are also discussed.