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.     

Microwave Dielectric Properties of Low Firing Temperature Bi2W2O9 Ceramics

Phase stability, sinterability, and microwave dielectric properties of Bi₂W₂O₉ ceramics and their cofireability with Ag, Cu, and Au electrodes have been investigated. Single-phase Bi₂W₂O₉ powder was synthesized by solid-state reaction in air at 800°C for 3 days. X-ray powder diffraction data show Bi₂W₂O₉ to have an orthorhombic crystal structure described by the noncentrosymmetric space group Pna 2₁, with lattice parameters a =5.4401(8), b =5.4191(8), c =23.713(4) Å. Ceramics fired at temperatures up to 865°C remain single-phase but above this temperature ferroelectric Bi₂WO₆ appears as a secondary phase. The measured relative permittivity of Bi₂W₂O₉ ceramics increases continuously from 28.6 to 40.7 for compacts fired between 860° and 885°C. The bulk relative permittivity of Bi₂W₂O₉ corrected for porosity was calculated as 41.3. Bi₂W₂O₉ ceramics fired up to 875°C exhibit moderate quality factors, Q × f _r, ∼7500–7700 GHz and negative temperature coefficient of resonant frequency, ∼−54 to −63 ppm/°C. Chemical compatibility experiments show Bi₂W₂O₉ ceramics to react with both Ag and Cu electrodes, but to form good contacts with Au electrodes.     

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.     

Factors Affecting the Electrical Conductivity of Donor-Doped Bi4Ti3O12 Piezoelectric Ceramics

High-temperature piezoelectric ceramics based on W⁶⁺-doped Bi₄Ti₃O₁₂ (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800° and 1150°C in air. A rapid densification, >99% of the theoretical density (rho_th) at 900°C/2 h, took place in the chemically prepared W⁶⁺-doped Bi₄Ti₃O₁₂ ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of rho_th, at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W⁶⁺-doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO₂ produced a further decrease of the electrical conductivity.     

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.     

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.     

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Grain growth in a high-purity ZnO and for the same ZnO with Bi₂O₃ additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ— G ⁿ₀= K ₀ t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn²⁺ lattice diffusion mechanism. Additions of Bi₂O₃ to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi₂O₃ content. The preexponential term K ₀ was also independent of Bi₂O₃ content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi₂O₃ ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi₂O₃-rich liquid phase.     

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.     

Effects of B2O3 Addition on the Dielectric and Ferroelectric Properties of Ba0.7Sr0.3TiO3 Ceramics

The effects of B₂O₃ addition on the sintering behavior and the dielectric and ferroelectric properties of Ba_0.7Sr_0.3TiO₃ (BST) ceramics were investigated. The dielectric and ferroelectric properties of a BST sample with 0.5 wt% B₂O₃ sintered at <1150°C were as good as those of undoped BST sintered at 1350°C, and the dielectric loss was better. When >1.0 wt% B₂O₃ was added to BST, the overdoped B₂O₃ did not form a liquid phase or volatilize; it remained in the samples and formed a secondary phase that lowered the sintering behavior and the dielectric and ferroelectric properties of the BST.     

Elevated-Temperature Toughness and Hardness of a Hot-Pressed Al2O3—WC—Co Composite

The fracture toughness and hardness of an Al₂O₃80WC10Co composite were investigated in air at elevated temperatures. The primary phases in the composite were WC, α-Al₂O₃, and Co₃W₃C, but small amounts of Co and C (graphite) appeared at elevated temperatures, related to decomposition of the Co₃W₃C phase. The fracture toughness of the composite was constant with increasing temperature up to 330°C and then increased in the range 400° to 550°C. A transition of brittle to ductile behavior occurred at about 700°C. The enhancement of fracture toughness at elevated temperature is attributed to the decomposition of Co₃W₃C to Co and C, and enhanced crack deflection and bridging. Decreases in hardness with increasing temperature are attributed to the softening of WC matrix and decomposition of Co₃W₃C.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.     

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.     

Phase Equilibria in the Systems SrO-CuO and SrO-1/2Bi2O3

The phase equilibria of the systems SrO-CuO and SrO-1/2Bi₂O₃ were studied by X-ray diffraction analysis of quenched powder samples. The compounds SrCuO₂ and Sr₂CuO₃ melt incongruently at 1085° and 1225°C, respectively. The newly found compound Sr₆Bi₂O₉ decomposes at 965°C into SrO and Sr₃Bi₂O₆ melts incongruently into SrO and liquid at 1210°C. SrBi₂O₄ undergoes a phase transition at ∼825°C, and although both are nonstoichiometric, the low-temperature phase is slightly poorer in SrO with 33.5 mol% SrO than the high-temperature phase.     

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.     

Retrograde Densification in Bi2Sr2CaCu2O8 Superconductors

Bi₂Sr₂CaCu₂O₈ was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850° to 911°C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850° to 890°C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900° to 905°C does the material densify then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion, thus pushing the structure apart. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.     

Mechanical Activation-Assisted Synthesis of Pb(Fe2/3W1/3)O3

Perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) was prepared via a mechanical activation-assisted synthesis route from mixed oxides of PbO, Fe₂O₃, and WO₃. The mechanically activated oxide mixture, which exhibited a specific area of >10 m²/g, underwent phase conversion from nanocrystalline lead tungstate (PbWO₄) and pyrochlore (Pb₂FeWO_6.5) phases on sintering to yield perovskite PFW, although the formation of perovskite phase was not triggered by mechanical activation. When heated to 700°C, >98% perovskite phase was formed in the mechanically activated oxide mixture. The perovskite phase was sintered to a density of ∼99% of theoretical density at 870°C for 2 h. The sintered PFW exhibited a dielectric constant of 9800 at 10 kHz, which was ∼30% higher than that of the PFW derived from the oxide mixture that was not subjected to mechanical activation.     

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.     

Preparation and Characterization of Fine-Grained 3Y-TZP/BaFe12O19 In Situ Composites

Composites of BaFe₁₂O₁₉ particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO₃, and γ-Fe₂O₃ powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe₁₂O₁₉ particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe₁₂O₁₉ particles was ∼200-500 nm. Based on the grain size, most of the BaFe₁₂O₁₉ grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.     

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.