Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Effect of Li2CO3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Mg2V2O7 Ceramics

Li₂CO₃ was added to Mg₂V₂O₇ ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li₂CO₃ changed the crystal structure of Mg₂V₂O₇ ceramics from triclinic to monoclinic. The 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ _r=8.2, Q × f =70 621 GHz, and τ_f=−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.     

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.     

Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics via a Soft Mechanochemical Route

Single-phase perovskite 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN–0.1 PT) from a stoichiometric mixture of starting materials was synthesized by applying a mechanochemical technique to the stage of a precursor. A stoichiometric mixture of PbO, TiO₂, Mg(OH)₂, and Nb₂O₅ was milled for 60 min and heated at temperatures as low as 850°C for 4 h to obtain a single phase. The maximum dielectric constant of the samples from the milled mixture increased as the sintering temperature increased, with the remarkable grain growth, and attained 24600 at 1200°C. In contrast, poor densification and coexistence of the pyrochlore phase were observed on the samples from the nonmilled mixture. Further observation suggested that the pyrochlore phase concentrated near the surface during sintering and then migrated into the PbZrO₃ packing powder, leading to a pyrochlore–free phase at 1250°C. The dielectric constant of the latter ceramics was explained by the series mixing rule for the dielectric constant of a diphasic solid.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Formation and Stability of Ferroelectric BaTi2O5

BaTi₂O₅ (BT₂) is thermodynamically stable over a very narrow temperature range between 1220° and 1230°C: a modification to the BaO–TiO₂ phase diagram is proposed. This thermodynamic stability was shown by constructing a time–temperature transformation diagram for the decomposition of BT₂. Once formed, BT₂ appears to be stable indefinitely at 1220°–1230°C; at higher temperatures, the decomposition rate increases with temperature; at lower temperatures, the decomposition rate increases with decreasing temperature and passes through a maximum at ∼1200°C; below ∼1150°C, BT₂ has long-lived kinetic stability. Kinetic considerations show a nucleation and growth mechanism for decomposition, with a nucleation induction period that is very temperature dependent. BT₂ can be prepared by various routes, including solid-state reaction of oxides below ∼1100°C; because it is metastable at all temperatures other than 1220°–1230°C, its formation is an example of Ostwald's rule of successive reactions. Discrepancies in the literature concerning the reported stability range of BT₂ can be explained by the complex dependence on temperature and time of both its formation and decomposition, for both of which, the nucleation stage is rate limiting.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

Sintering of Al2O3–TiC Composite in the Presence of Liquid Phase

The results obtained from the sintering of Al₂O₃–50TiC (in weight percent) composite in the temperature range from 1650° to 1800°C with addition of Y₂O₃ are presented. Densification is accelerated by the formation of liquid at temperatures above 1750°C, and 99% of theoretical density can be achieved by vacuum sintering at 1800°C for 15 min. The liquid presented at the sintering temperature is crystallized to YAG (Y₃Al₅O₁₂) during cooling.     

Effect of Excess PbO on the Sintering Characteristics and Dielectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3-Based Ceramics

Additions of excess PbO to the perovskite Pb[(Mg_1/3Nb_2/3)_0.92Ti_0.08]O₃ solid solution enhanced the formation of a liquid phase at 840°C, which served as a densification aid for the ceramics. The liquid phase allowed elimination of pores and promoted grain growth during sintering. With additions of 1 to 2 wt% excess PbO, densities in excess of 97% of theoretical were obtained at a sintering temperature of 950°C. The peak dielectric constants of the resulting ceramics were over 18 000 at 30°C and dissipation factors less than 1%. Additions of PbO in excess of 2 wt% resulted in inferior dielectric properties due mainly to the dilution of the ferroelectric phase.     

Retrograde Densification of Calcined and Glass Precursors of Approximate (Bi,Pb)2Sr2Ca2Cu3O10 Stoichiometry

Retrograde densification of pelletized calcines and glasses having an approximate (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ starting stoichiometry and sintered at ∼850°C can be described by first-order rate equations. Retrograde densification in the calcine precursors was largely due to the development of open pores, and was approximately proportional to the concentration of the (Bi,Pb)₂Sr₂CaCu₃O₁₀ phase. In the glasses, retrograde densification is mainly caused by porosity accompanying the growth of a needlelike Sr─Ca─Cu─O phase, together with (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ and (Bi,Pb)₂Sr₂CaCu₂O₈.     

Microstructure and Short-Term Oxidation of Hot-Pressed Si3N4/ZrO2(+Y2O3) Ceramics

Composite ceramic materials based on Si₃N₄ and ZrO₂ stabilized by 3 mol% Y₂O₃ have been formed using aluminum isopropoxide as a precursor for the Al₂O₃ sintering aid. Densification was carred out by hot-pressing at temperatures in the range 1650° to 1800°C, and the resulting micro-structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si₃N₄ matrix with consequent high room-temperature toughness and strength. Decomposition of the ZrO₂ grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO₂ incorporated some N during densification.     

Sintering of Si3N4 with the Addition of Rare-Earth Oxides

The effect of rare-earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N₂ at 1800° to 2000°C was studied. When a single-component oxide, such as CeO₂, Nd₂O₃, La₂O₃, Sm₂O₃, or Y₂O₃, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y₂O₃–Ln₂O₃ (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare-earth oxides is discussed in relation to the additive compositions.     

Hot Deformation of YBa2Cu3O7-δ and Composite YBa2Cu3O7-δ/Ag

The response of ceramic superconductors and ceramic composites to compressive stresses at high temperatures has been examined. Monolithic YBa₂Cu₃O_7-δ and composite YBa₂Cu₃O₇-δ₆/Ag were tested at constant true strain rates from 10^-6 to 10^-3 s^-1 at temperatures from 800° to 950°C. Fine-grained monolithic YBa₂Cu₃O_7-δ appears to have a regime of superplastic deformation between temperatures of 850° and 950°C at strain rates from 10^-6 to 10^-4 S^-1. The addition of 20 vol% Ag to a coarser-grained material enhances the ductility of the ceramic and lowers the flow stress by a factor of 3 to 10. However, there is no evidence of superplasticity in the composite material in the range of temperature and strain rate where it was tested.     

Activated Sintering of ThO2 and ThO2-Y2O3 with NiO

The effect of additives on the sintering of ThO₂ and ThO₂-Y₂O₃ compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high-density bodies at temperatures below 1500°C. The presence of Y₂O₃ as a separate phase increases the rate of sintering of ThO₂, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid-state mechanism may be responsible.     

Low-Temperature Sintering and Microwave Dielectric Properties of Li2MgSiO4 Ceramics

Development of a low-temperature sintered dielectric material derived from Li₂MgSiO₄ (LMS) for low-temperature cofired ceramic (LTCC) application is discussed in this paper. The LMS ceramics were prepared by the solid-state ceramic route. The calcination and sintering temperatures of LMS were optimized at 850°C/4 h and 1250°C/2 h, respectively, for the best density and dielectric properties. The crystal structure and microstructure of the ceramic were studied by the X-ray diffraction and scanning electron microscopic methods. The microwave dielectric properties of the ceramic were measured by the cavity perturbation method. The LMS sintered at 1250°C/2 h had ɛ_r=5.1 and tan δ=5.2 × 10⁻⁴ at 8 GHz. The sintering temperature of LMS is lowered from 1250°C/2 h to 850°C/2 h by the addition of both lithium borosilicate (LBS) and lithium magnesium zinc borosilicate (LMZBS) glasses. LMS mixed with 1 wt% LBS sintered at 925°C/2 h had ɛ_r=5.5 and tan δ=7 × 10⁻⁵ at 8 GHz. Two weight percent LMZBS mixed with LMS sintered at 875°C/2 h had ɛ_r=5.9 and tan δ=6.7 × 10⁻⁵ at 8 GHz.     

Improvement of Mechanical Properties and Corrosion Resistance of Porous β-SiAlON Ceramics by Low Y2O3 Additions

Addition of Y₂O₃ as a sintering additive to porous β-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0.5) ceramics has been investigated for improved mechanical properties. Porous SiAlON ceramics with 0.05–0.15 wt% (500–1500 wppm) Y₂O₃ were fabricated by pressureless sintering at temperatures of 1700°, 1800°, and 1850°C. The densification, microstructure, and mechanical properties were compared with those of Y₂O₃-free ceramics of the same chemical composition. Although this level of Y₂O₃ addition did not change the phase formation and grain size, the grain bonding appeared to be promoted, and the densification to be enhanced. There was a significant increase in the flexural strength of the SiAlON ceramics relative to the Y₂O₃-free counterpart. After exposure in 1 M hydrochloric acid solution at 70°C for 120 h, no remarkable weight loss and degradation of the mechanical properties (flexural and compression strength) was observed, which was attributed to the limited grain boundary phase, and with the minor Y₂O₃ addition the supposed formation of Y-α-SiAlON.     

Influence of Nd2O3 Doping on the Reaction Process and Sintering Behavior of BaCeO3 Ceramics

The influence of Nd₂O₃ doping on the reaction process and sintering behavior of BaCeO₃ is investigated. Formation of BaCeO₃ is initiated at 800°C and completed at 1000°C. When Nd₂O₃ is added to the starting materials, the formation of BaCe_1–xNd_xO_3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe_1-xNd_xO_3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd₂O₃ content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO₃ is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO₃ with Nd₂O₃ markedly enhances its sinterability. The enhancement of the sinterability of Nd₂O₃-doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain-boundary phase which inhibits sintering.     

Formation of a High-Temperature Liquid Phase During the Sintering of PbFe2/3W1/3O3

A high-temperature liquid phase (rather than a low-temperature liquid phase at 690°C as reported recently) has been demonstrated to form at 860°C on heating and to solidify at 840°C on cooling in PbFe_2/3O₃. This liquid phase not only promotes densification, but also induces the formation of rounded PbFe_2/3W_1/3O₃ grains during sintering at 870°C. Through slow cooling at a rate of 25°C/h after sintering, platelike grains, designated G phase, are found to form in a thin surface layer of specimens. This formation of platelike G phase is considered to be related to the solidification and recrystallization of the liquid phase exuded from the interior. The amount of the G phase on the surfaces decreases with the increase of cooling rates, indicating that fast cooling will lead the liquid phase to be solidified in the bulk of specimens. These results reveal that the microstructure of PbFe_2/3W_1/3O₃ is greatly affected by the high-temperature liquid phase; additionally, the slow cooling treatment seems to be a direct and effective method for removing the residual liquid phase from PbFe_2/3W_1/3O₃.