Viscous Flow as the Driving Force for the Densification of Low-Temperature Co-Fired Ceramics

Filled glass–ceramic composites, like low-temperature co-fired ceramics (LTCC), must densify at temperatures <900°C. The densification mechanism of LTCC is often described by liquid-phase sintering. The results of this paper clearly show that densification of ceramic-filled glass–composites with a glass content above 60 wt% can be attributed to viscous sintering, which is decisively controlled by the viscosity of the glass during the heat treatment. This is demonstrated by the experimental determination of the viscosity of a MgO–Al₂O₃–B₂O₃–SiO₂ glass dependent on temperature, by investigation of the wetting behavior of the glass on the ceramic filler mullite, and of the microstructural development. It was found that the glass does not wet the filler material in a temperature range up to 1000°C. Therefore, liquid-phase sintering can be excluded. Independent of any wetting effect and therefore in the absence of capillary forces, densification starts at a temperature of 750°C, which corresponds to a viscosity of 10^9.5 dPa·s. This densification can be attributed to viscous flow of the glass matrix composite.     

Densification of Alumina-Silicon Carbide Powder Composites: I, Effects of a Polymer Coating on Silicon Carbide Particles

One possible approach to improving the densification of powder composites containing a major crystalline phase which densifies (e.g., Al₂O₃) and a difficult-to-sinter phase (e.g., SiC) is to accommodate the matrix volume shrinkage with a "disappearing" polymer coating. A polymer coating prevents contact between the nonsinterable particles and the surrounding matrix. The coating can be burned off before sintering, allowing the matrix phase to "shrink-fit" around the nonsinterable particles during sintering. The effects of a polymer coating on the densification of a two-phase particle system were tested using SiC powder dispersed in an Al₂O₃ matrix. The composites processed with a polymer coating showed more densification during equivalent firing cycles than did those processed without a polymer coating. Densification during sintering was approximately proportional to the amount of polymer adsorbed on SiC, suggesting that the Al₂O₃ matrix did shrink-fit into the gaps between the SiC particles and the surrounding Al₂O₃ matrix. Differences in the pore-size distributions of polymercoated green compacts and uncoated compacts indicated a perturbation of the green microstructure by the gaps. The estimated average thickness of the gap is approximately 20 nm, ∼8% of the average radius of the SiC powder used in this study.     

Low-Temperature, Single Step, Reactive Sintering of Lead Magnesium Niobate Using Mg(OH)2-Coated Nb2O5 Powders

A low-temperature, single step, reactive sintering method for Pb(Mg_1/3Nb_2/3)O₃ (PMN) and PMN–PbTiO₃ (PMN–PT) processing was developed based on the coating of Mg(OH)₂ on Nb₂O₅. This method simplified the processing of PMN and PMN–PT to a single step of heat-treatment and decreased the sintering temperature to 1000°C. It was found that the pyrochlore phase formation reaction at 500°C reduced the particle size to 130 nm. The overlap of the pyrochlor-perovskite phase transformation between 700° and 900°C and the densification process between 800° and 1000°C improved the sintering process. These two factors were the major reasons of the low temperature sintering.     

Two-Step Sintering of Ceramics with Constant Grain-Size, II: BaTiO3 and Ni–Cu–Zn Ferrite

We investigated the preparation of bulk dense nanocrystalline BaTiO₃ and Ni–Cu–Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO₃ with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y₂O₃, the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO₃ ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.     

Mg–Cu–Zn Ferrites for Multilayer Inductors

Mg–Cu–Zn ferrites can be sintered at T ≤950°C to sufficient density and display adequate permeability profiles for application in multilayer ferrite inductors. The permeability and Curie temperature have to be optimized by proper selection of composition. Ferrites with <50 mol% Fe₂O₃ reveal enhanced densification behavior. Submicrometer powders prepared by fine milling show good sintering activity and density after firing at 900°C. Nano-size ferrite powders prepared by coprecipitation or flame synthesis lead to high density; maximum shrinkage already occurs at T <800°C. The use of Bi₂O₃ as a sintering additive further improves the densification, but also affects the microstructure and, hence, the permeability. A maximum permeability of μ_i=450–500 is obtained.     

Silver-Modified CrO2 of Core–Shell Nanoparticles and their Magnetic and Impedance Properties

A chemical CrO₃→CrO₂ transformation reaction occurs in the presence of Ag⁺ species forming CrO₂ core–shell magnetic nanoparticles. The process involves heating CrO₃ with AgNO₃ in an aqueous solution in ambient air at a low temperature of 330–350 K. Shell thickness (diamagnetic amorphous silver) is controlled to be ∼2 nm tunable to the magnetic and impedance properties. Improved coercivity H _c=75.7 mT and the Curie temperature T _C=415 K are found with saturation magnetization M _s = 80 Am²/kg after annealing the sample at 573 K in air for 2 h. On heating, the impedance (or electrical resistance) varies steadily through a maximum in the T _C point in a typical half-metallic ceramic behavior. In a dynamic response to frequency f , the T _C increases from 418 K at 1 kHz to as high a value as 505 K at 1 MHz. A nearly f independent and low value of dissipation factor φ as 0.05 is promising for low power loss high-frequency applications of such CrO₂ particles.     

Characterization of silica-coated silver nanoparticles prepared by a reverse micelle and hydrolysis–condensation process

Described herein is the synthesis of individually silica-coated silver nanoparticles using a reverse micelle method followed by hydrolysis and condensation of tetraethoxysilane (TEOS). The size of a silica-coated silver nanoparticle can be controlled by changing the reaction time and the concentration of TEOS. By maintaining the size of a silver nanoparticle as a core particle at around 7 nm, the size of a silica-coated silver nanoparticle increased from 13 to 28 nm as the reaction time increased from 1 to 9 h due to an increase in silica thickness. The size of silica-coated silver nanoparticles also increased from 15 to 22 nm as the TEOS concentration increased from 7.8 to 40 mM. The size of a silica-coated silver nanoparticle can be accurately predicted using the rate of the hydrolysis reaction for TEOS. Neither the dispersion nor the film of silica-coated silver nanoparticles exhibited any peak shifting during surface plasmon resonance (SPR) at around 410 nm, whereas, without silica coating, the SPR peak of Ag film shifted to 466 nm.     

Catalytic Effect of CaF2 Nanoparticles on Sintering Behavior of Kaolin-Based Materials

The present work represents preliminary results concerning the effect of nanometric CaF₂ particles on the sintering behavior of kaolin. The nanoparticles were synthesized using aqueous solutions of CaCl₂ dissolved with an organic dispersant and HF. The morphology and particle size of the obtained nanoparticles were square-like, having a mean length of 25 nm. Kaolin was mixed with different amounts of nanometric CaF₂ (from 0.625 to 5.0 wt%) and the mixtures were milled, pressed, and sintered. From sintering experiments, it was observed that additions of CaF₂ nanoparticles in kaolin clay could shift the phase transition temperatures to lower values. Specifically, the densification temperature could be reduced by almost 150°C when 2.5 wt% of CaF₂ was incorporated. Besides, adding 1.25% of nanometric CaF₂ can achieve that compressive strength augments from 125 to 185 MPa in kaolin samples sintered at 1200°C, meaning an improvement of 40% in the properties of the final material.     

Soft Ferrite Materials for Multilayer Inductors

Various families of soft ferrites (Ni–Cu–Zn; Mg–Cu–Zn; Co₂–Z; Mn–Zn) with a sintering temperature of T ≤950°C are used for multilayer inductors. It is shown for Ni–Cu–Zn and Mg–Cu–Zn ferrites that composition, powder particle size, and sintering additive concentration strongly affect the shrinkage, sintering behavior, microstructure, and magnetic properties. A maximum permeability of μ _i =700–900 is observed with a Bi₂O₃ addition of about 0.5 wt%. The maximum shrinkage can be shifted down to 700°C using nanosize powders. Z-type hexaferrites with a permeability of 10 are used for high-frequency applications up to 2 GHz.     

Synthesis and Color Evolution of Silica-Coated Hematite Nanoparticles

The aim of this communication is to investigate the color evolution of silica-coated hematite nanoparticles versus treating temperature. The silica-coated hematite nanoparticles were prepared by first suspending commercial hematite nanoparticles in an ammonia–water–2-propanol system, and then depositing amorphous nano-SiO₂ onto the surface of hematite nanoparticles using the Stöber method. The silica-coated nanoparticles were dripped onto biscuits and heat treated at various temperatures. The color changes of heat treated samples were determined by the CIE L ^* a ^* b ^* parameters, and the possible mechanism of color evolution was discussed.     

Densification of Nanocrystalline Yttria by Low Temperature Spark Plasma Sintering

We investigated the densification of undoped, nanocrystalline yttria (Y₂O₃) powder by spark plasma sintering (SPS) at sintering temperatures between 650°C and 1050°C at a heating rate of 10°C/min and an applied stress of 83 MPa. In spite of the low sinterability of the undoped Y₂O₃, a remarkable densification of the powder started at about 600°C, and a theoretical density of more than 97% was achieved at a sintering temperature of 850°C with a grain size of about 500 nm. The low temperature SPS is effective for fabricating dense Y₂O₃ polycrystals.     

Synthesis of Nanoparticulate Silica Composite Membranes by the Pressurized Sol–Gel Technique

A crack-free silica composite membrane has been synthesized from a nanoparticulate silica sol (particle diameter <10 nm) by a pressurized sol–gel coating technique developed in this study. The microporous silica layers with an estimated pore radius of 0.78 nm were deposited inside the pores (average pore size of 0.1 μm) of slip cast a-alumina support tubes. The microstructure of the coated layer was controlled by adjusting sol properties and pressurizing conditions. The room-temperature intrinsic permeability of N₂ through the silica membrane layer after heat treatment at 200°C is about 4.9 × 10⁻¹² mol·m/m²·s· Pa, and the mechanism of gas transport is Knudsen flow. The thermal stability of the silica composite membrane is excellent up to 500°C.     

Hydrothermal Synthesis of Ba-Hexaferrite Nanoparticles

Barium hexaferrite BaFe₁₂O₁₉ nanoparticles with a single-domain size were synthesized using a controlled hydrothermal process involving the LaMer–Dinger principle and the Ostwald ripening mechanism. Nanocrystalline particles of BaFe₁₂O₁₉ were obtained when the molar ratio of the precursor composition Ba(OH)₂·8H₂O/γ-Fe₂O₃ was 0.3 and the concentration of the suspension was about 1 wt%. The as-synthesized crystalline BaFe₁₂O₁₉ platelets approximately 50 nm in length and 5 nm in thickness exhibited a saturation magnetization of 40 Am²/kg.     

Densification and Grain Growth of Al2O3 Nanoceramics During Pressureless Sintering

α - Al₂O₃ nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al₂O₃ seed polyacrylamide gel method were used to sinter bulk Al₂O₃ nanoceramics. The relative density of the Al₂O₃ nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al₂O₃ nanopowders. The densification and fast grain growth of the Al₂O₃ nanoceramics occur in different temperature ranges. The Al₂O₃ nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering.     

Crack Growth and Damage in Constrained Sintering Films

The constrained sintering of films on substrates leads to a reduction in densification rate and may lead to processing flaws. This paper reports on a study of damage and cracking in sintering films, with particular emphasis on the growth of preexisting cracks. Experiments have been conducted with glass and polycrystalline Al₂O₃ films on various substrates. The effect of important variables (viz., film thickness, crack length, and friction with the substrate) on crack growth is reported. The experiments with glass films show that cracking occurs above a critical film thickness which is in good quantitative agreement with a recent analysis for this problem. In the case of Al₂O₃ films, we observe a diffuse damage zone ahead of cracks. Crack growth occurs by the coalescence of microcracks with each other and with the main crack. Some possible reasons for this difference between the glass and Al₂O₃ films are presented. As a model for diffuse damage, the stability of a sintering film under spatial variations in constitutive parameters is analyzed. It is shown that the film is unstable to small perturbations only in the early stages of densification, and that for viscous sintering the films are usually kinetically stable.     

Spark Plasma Sintering of Alumina

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al₂O₃ powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al₂O₃ could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21–22 GPa and fracture toughness values of 3.5 ± 0.5 MPa·m^1/2 were found for the compacts containing submicrometer-sized Al₂O₃ grains.     

Novel Ferrimagnetic Material for Fabricating Multilayer Chip Inductors —Low-Temperature-Sintered Ba3Co2−xZnxFe24O41 Hexaferrites

The Zn-substituted planar hexaferrite Ba₃Co_{2− x} Zn _x Fe₂₄O₄₁ ( x =0–1.2) powders were prepared via a citrate precursor method. With an appropriate sintering aid, ceramics with high density could be obtained after sintering at a temperature of less than 900°C. These ceramics exhibit excellent high-frequency properties such as high initial permeability up to 10.0 with cut-off frequencies above 1.1 GHz. The results show that we have exploited novel promising ferrimagnetic materials well suited for multilayer chip inductors or multilayer chip beads applications in the high-frequency range.     

Dielectric Properties of Microstructure-Controlled Ba2Ti9O20 Resonators

Thermal schedules for sintering ZrO₂- and SnO₂-doped Ba₂Ti₉O₂₀ resonators to minimum porosity were developed using a shrinkage-rate-controlled dilatometer. Efficient sintering schedules were formulated that circumvented pore formation via grain-boundary/triple-point liquid-phase volatilization. A densification rate (0.5%/min) for the early stages of sintering, which minimized intragranular porosity, was chosen. For the later stages of sintering, a densification rate (0.01%/min) that minimized specimen slumping via the liquid phase permitted sintering to high density. These schedules were successfully upscaled to heat treatment in a conventional furnace. The dielectric constants, quality factors, and selected temperature coefficients of 0.82- and 1.64-mol%-SnO₂-, and 1.64-mol%-ZrO₂-doped monophase Ba₂Ti₉O₂₀ were reported.     

Spherical Rhabdophane Sols. II: Fiber Coating

A rhabdophane (LaPO₄·nH₂O) sol with fine spherical particles was used to coat Nextel™ 720 fiber tows continuously with monazite (LaPO₄). The coatings are compared with those made previously from rod-shaped particles. The coated fibers were heat-treated at 1000°–1300°C for 1, 10, and 100 h. The effect of heat treatment temperature and time on coating microstructure was characterized by scanning electron microscopy and transmission electron microscopy, and the strengths of the coated fibers were measured after coating and heat treatment. Grain shapes and grain growth rates were measured. Coating thickness uniformity was quantified by a fit to a truncated extreme-value distribution. Coating hermeticity was evaluated by analysis of grain growth rates. The spherical particles promote more rapid coating densification and local hermeticity, but introduce problems with sintering shrinkage cracking that are not present in coatings derived from rod-shaped particles.     

Enhancing Electrical Properties in NBT–KBT Lead-Free Piezoelectric Ceramics by Optimizing Sintering Temperature

Conventional sintering of (Na_{1− x} K _x )_0.5Bi_0.5TiO₃ (abbreviated as NKBT x , x =18–22 mol%) lead-free piezoelectric ceramics was investigated to clarify the optimal sintering temperature for densification and electrical properties. Both sintered density and electrical properties were sensitive to sintering temperature; particularly, the piezoelectric properties deteriorated when the ceramics were sintered above the optimum temperature. The NKBT20 and NKBT22 ceramics synthesized at 1110°–1170°C showed a phase transition from tetragonal to rhombohedral symmetry, which was similar to the morphotropic phase boundary (MPB). Because of such MPB-like behavior, the highest piezoelectric constant ( d ₃₃) of about 192 pC/N with a high electromechanical coupling factor ( k _p) of about 32% were obtained in the NKBT22 ceramics sintered at 1150°C.