Enhanced Properties of Tin(IV) Oxide Based Materials by Field-Activated Sintering

The densification of SnO₂ (0.9 mol)–Sb₂O₃ (0.1 mol) solid solution without any additives was studied by conventional and field-activated sintering technique (FAST). FAST sintering achieved a relative density value of 92.4% at 1163 K for 10 min versus 61.3% in conventional sintering at 1273 K for 3 h. An abnormal reduction of the IR transmittance and a semiconductor defect structure with only one donor level in the SnO₂ energy gap were noticed in the FAST-sintered as compared with the conventionally sintered Sn_0.82Sb_0.18O₂ solid solution. A high charge carrier concentration (i.e., electronic conduction) was shown in the FAST-sintered sample by conductivity measurements and the negative values of the Seebeck coefficient.     

Effect of Manganese Oxide Additive on the Dielectric Properties of Mixed-Sintering Ceramics

The influence of internal electrode material on the sintering of mixed-sintering ceramics and the effect of MnO₂ additive on improving their dielectric properties were investigated for their application as multilayer ceramic capacitors. The resistivities of mixed-sintering ceramic pellets sintered with Ag-Pd electrode material decreased remarkably because of the expulsion of component elements during sintering. A small amount of MnO₂ additive improved the resistivities. Extended X-ray absorption fine-structure analysis showed that MnO₂ exists in the grain boundaries and compensates for the electrical valence of the expelled elements.     

Piezoelectric Properties of Pb(Mg1/3Nb2/3)O3 PbTiO3 -PbZrO3 Ceramics Modified with Certain Additives

Effects of additives on the piezoelectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃-PbZrO₃ ceramics in a perovskite-type structure are described. The tetragonality of Pb(Mg_1/3Nb_2/3)_0.375-Ti_0.375Zr_0.25O₃ ceramics increased with the addition of NiO, Cr₂O₃, or Fe₂O₃ but decreased with the addition of MnO₂ or CoO. The dielectric and piezoelectric properties of the base composition were improved markedly through selection of additives in proper amounts. Addition of NiO yielded a high dielectric constant and planar coupling coefficient for compositions at the morphotropic transition boundary. High mechanical Q -factors and low electrical dissipation factors were obtained by addition of MnO₂. Addition of both NiO and MnO₂ produced a mechanical Q -factor of 2051 and a planar coupling coefficient of 0.553. The resonant frequency of Pb(Mg_1/2Nb_2/3)_0.4375Ti_0.4375 zr_0.125O₃ containing MnO₂ had very low temperature and time dependence. The microstructure indicated that ceramics with a high mechanical Q -factor had a fine, uniform grain structure. Addition of Cr₂O₃ retarded grain growth and addition of MnO₂, NiO, CoO, or Fe₂O₃ promoted grain growth in the ternary system.     

Titania as a Sintering Additive in Indium Oxide Ceramics

The influence of TiO₂ additives on the sintering behavior of In₂O₃ ceramics has been investigated. TiO₂ increases the densification rate, decreases the grain growth during the intermediate stage of sintering, and hinders the pore/boundary breakaway that can affect the final stage of sintering. For a given grain size, TiO₂ shifts the grain size/density trajectory toward higher densities. TiO₂ mainly acts by a second-phase mechanism, but it also may decrease the decomposition rate of In₂O₃.     

Sintering of High-Purity Nickel Oxide, II

The effects of particle size, compacting pressure, additives, and mode of preparation on the sintering of nickel oxide are presented. A low calcining temperature results in a high bulk density of a sintered compact. No appreciable change in the rate of sintering is found above 1000 kg. per sq. cm., whereas the unheated density increases with increasing pressures. Additions of CaO promote sintering, additions of Na₂O, Fe₂O₃, and Al₂O₃ retard sintering, and CeO₂ and Ag₂O have no effect; B₂O₃ accelerates grain growth but retards densification. Nickel oxide from oxalate has a small rate of sintering as compared with nickel oxides from hydroxide, nitrate, and carbonate. The effects of the mode of preparation of nickel oxide are discussed in view of the activation energy of electronic conductivity and particle size.     

Grain Growth and Phase Transformation of Titanium Oxide During Calcination

Additions of NiO, CoO, MnO₂, Fe₂O₃, and CuO promote the anatase-rutile transformation and grain growth of TiO₂. Additions of Na₂O and WO₃ retard the transformation and have no effect on the grain growth. The addition of MoO₃ strongly promotes the grain growth but has only a slight effect on the phase transformation. Both the grain growth and transformation are promoted slightly by Cr₂O₃. The transformation is significantly affected by the method of preparing TiO₂. The grain size of TiO₂ heated in H₂ exceeds that of TiO₂ heated in O₂, in air, in argon, and in vacuum. The rate of transformation decreases with an increase in the partial pressure of oxygen of the atmosphere.     

Effect of Europium Concentration on Densification of Transparent Eu:Y2O3 Scintillator Ceramics Using Hot Pressing

Europium (Eu) was found to act as a solid-state sintering aid in Y₂O₃ optical ceramics by controlling ionic diffusivity, which in turn leads to enhanced optical transparency. Transparent ceramic samples of Eu-doped Y₂O₃, with no additional additives, were sintered by uniaxial vacuum hot pressing under 40 MPa and maximum temperature of 1580°C. Optical attenuation was found to decrease with increasing Eu concentrations between 0 and 5 at% for ceramics processed under the same sintering conditions. In order to study the effect of Eu concentration on ceramic densification, the strain rate and grain size during sintering at constant temperature and varied pressure were measured. A diffusional flow densification model was used to derive instantaneous effective diffusion constants for the densification process. Diffusion constants were found to increase with increasing Eu concentration according to a log–linear relationship. Eu²⁺ was detected in samples after hot pressing through fluorescence spectroscopy, and the extrinsic defect chemistry was found to be dominated by the reduced Eu in solid solution with Y₂O₃. A sintering model with diffusion rate limited by yttrium interstitial transport and controlled by the incorporation of Eu²⁺ onto the cation sublattice was found to be in good agreement with experimental diffusivity data.     

Phase Transitions and Electrical Properties of (Na1−xKx)(Nb1−ySby)O3 Lead-Free Piezoelectric Ceramics With a MnO2 Sintering Aid

Lead-free piezoelectric ceramics (Na_{1− x} K _x )(Nb_{1− y} Sb _y )O₃+ z mol% MnO₂ have been prepared by a conventional solid-state sintering technique. Our results reveal that Sb⁵⁺ diffuses into the K_0.5Na_0.5NbO₃ lattices to form a solid solution with a single-phase orthorhombic perovskite structure. The partial substitution of Sb⁵⁺ for B-site ion Nb⁵⁺ decreases the paraelectric cubic-ferroelectric tetragonal phase transition ( T _c) and the ferroelectric tetragonal-ferroelectric orthorhombic phase transition ( T _O–F), and retains strong ferroelectricity. A small amount of MnO₂ is enough to improve the densification of the ceramics. The co-effects of MnO₂ doping and Sb substitution lead to significant improvements in ferroelectric and piezoelectric properties. The ceramics with x =0.45–0.525, y =0.06–0.08, and z =0.5–1 exhibit excellent ferroelectric and piezoelectric properties: d ₃₃=163–204 pC/N, k _P=0.47–0.51, k _t=0.46–0.52, ɛ=640–1053, tan δ=1.3–3.0%, P _r=18.1–22.6 μC/cm², E _c=0.72–0.98 kV/mm, and T _C=269°–314°C.     

Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide

The phase change, densification, and microstructure development of ZnO doped with both Bi₂O₃ and Sb₂O₃ are studied to better understand the sintering behavior of ZnO varistors. The densification behavior is related to the formation of pyrochlore and liquid phases; the densification is retarded by the former and promoted by the latter. The pyrochlore phase, whose composition is Bi_3/2ZnSb_3/2O₇, appears below 700°C. The formation temperature of the liquid phase depends on the Sb/Bi ratio: about 750°C for Sb/Bi < 1 by the eutectic melting in the system ZnO—Bi₂O₃, and about 1000°C for Sb/Bi > 1 by the reaction of the pyrochlore phase with ZnO. Hence, the densification rate is determined virtually by the Sb/Bi ratio and not by the total amount of additives. The microstructure depends on the sintering temperature. Sintering at 1000°C forms intragrain pyrochlore particles in ZnO grains as well as intergranular layers, but the intragrain particles disappear at 1200°C by the increased amount of liquid phase, which enhances the mobility of the solid second phase.     

Effects of Additives on Microstructure and Properties of Alumina-Silicon Carbide Nanocomposites

Al₂O₃/5-vol%-SiC nanocomposites have been fabricated by using pressureless sintering with MgO and/or Y₂O₃ sintering aids and post-hot isostatic pressing (HIPing), which circumvents the limitations of hot pressing. Al₂O₃/SiC nanocomposites that have been doped with 0.1 wt% MgO and 0.1 wt% MgO + 0.1 wt% Y₂O₃ show an increased sintering density and a homogeneous microstructure, as well as a high fracture strength (1 GPa) after HIPing. In contrast, using Y₂O₃ as a dopant has a negative impact on the microstructure and the fracture strength. The results suggest that MgO, as a sintering additive, has a key role in improving the densification and controlling the microstructure of Al₂O₃/SiC nanocomposites.     

Electrochemical Chromium(III) Oxide Coatings on Non-oxide Ceramic Substrates

SiC and TiB₂ were electrochemically coated with Cr₂O₃ from a 0.1 M aqueous solution of chromium nitrate hydrate with ethanol additives. On both substrate materials poly-crystalline Cr₂O₃ was formed at current densities from 5 to 50 mA/cm² and deposition durations of 5 to 30 min. The coating weight increased with current density and with deposition time. The as-deposited coatings contained microcracks due to drying shrinkage. Microstructural observations indicate that sintering of the Cr₂O₃ coatings on TiB₂ at 1100°C for 1 n in a reducing atmosphere in a closed graphite crucible causes the densification of the coating via a liquid phase, which forms by oxidation of TiB₂. Under similar conditions, the Cr₂O₃ coatings on SiC may be sintered via an evaporation–condensation mechanism.     

Densification and Microstructural Development of the Reaction Sintering of Strontium Barium Niobate

SrNb₂O₆ and BaNb₂O₆ were selected as the precursor powders for the reaction sintering of strontium barium niobate of the composition Sr _x Ba_{1− x} Nb₂O₆ ( x = 0.5 and 0.6) (SBN). In comparing the densification behavior of the reaction sintering and the normal sintering of SBN, it was observed that the densification rate of the former was slower during reaction but became faster after reaction. It was also found that the chemical reaction occurred prior to densification, but a high-density and uniform microstructure could be obtained for the reaction-sintered sample. The densification rate of the reaction-sintered sample was pronouncedly enhanced when the reaction was almost completed. We also proposed the reason for the enhancement of the densification rate of the reaction-sintered sample when the reaction was almost completed. A high-density (>98% of the theoretical density), uniform, and fine-grained (3-4 µm) SBN microstructure could be achieved by using reaction sintering.     

Sintering Chromium Oxide with the Aid of TiO2

The influence of additives on the sintering behavior of Cr₂O₃ was studied. The oxygen pressure and temperature at which the sintering rate becomes significant are dramatically altered by adding ∼ 1 wt% TiO₂; the sintering temperature decreases, whereas the oxygen pressure for appreciable densification increases. At temperatures as low as 1280°C, densities of 92% of theoretical were obtained. Densification was examined as a function of temperature, oxygen pressure, and TiO₂ concentration. Several other transition metal oxides were tested for their ability to enhance sintering; none was successful.     

Properties of Modified Lead Zirconate Titanate Ceramics Prepared at Low Temperature (800°C) by Hot Isostatic Pressing

High-density lead zirconate titanate (PZT) ceramics were fabricated for the first time at a temperature as low as 800°C via the hot isostatic pressing (HIP) of a PZT powder with a modified composition of 0.92Pb(Zr_0.53Ti_0.47)O₃—0.05BiFeO₃—0.03Ba(Cu_0.5W_0.5)O₃ that contained 0.5 mass% MnO₂. The resultant PZT ceramics exhibited a microstructure that was denser and finer than that of PZT sintered at 935°C, which is the lowest temperature for the densification of the same composition via normal sintering. The relevant dielectric and piezoelectric properties of the HIPed PZT ceramics were as follows: coefficient of electromechanical coupling ( K ₃₁), 31.8%; mechanical quality factor ( Q _m), 1364; piezoelectric constant ( d ₃₁), −73.7 × 10⁻¹² C/N; relative dielectric constant (ɛ₃₃^T/ɛ₀), 633; dielectric loss factor (tan δ), 0.5%; Curie temperature ( T _c), 285°C; and density (ρ), 8.06 g/cm³. In addition to these reasonably good piezoelectric properties, the HIPed PZT exhibited better mechanical properties—particularly, higher fracture strength—than the normally sintered PZT.     

Effect of Sintering Atmosphere on Densification of MgO-Doped Al2O3

The densification behavior of Al₂O₃-MgO (0.1 wt%) has been studied in O₂ and N₂ atmospheres. Powder compacts have been sintered at 1600°C for 0.5 to 8 h. For some specimens the sintering atmosphere has been changed after 30 min of sintering. Irrespective of sintering atmosphere, sintered densities are approximately the same up to 99% relative density, implying that the capillary pressure effect dominates the atmosphere effect for most of the densification stage. During extended sintering treatment the density of specimens sintered in O₂ becomes higher than that in N₂. When the sintering atmosphere is changed from O₂ to N₂, enhanced densification results, and vice versa. Such an effect of sintering atmosphere is explained by the diffusiveness of gases entrapped in pores, as well as by oxygen potential differences inside and outside of the specimen. Differences in grain growth rate in various atmospheres are discussed on the basis of different densification rates.     

Synthesis and Sintering of Cerium(II) Monosulfide

Cerium monosulfide (CeS) powder was synthesized by the reduction of Ce₂S₃ powder with metallic Ce, which was obtained from ceria (CeO₂) powder using carbon disulfide (CS₂) gas. To obtain the maximum amount of CeS from a mixture of Ce₂S₃ and Ce, an excess amount of metallic Ce, a stoichiometric composition, was necessary in the synthesis at 1273 K for 10.8 ks. The preliminary sintering experiments also were performed using a synthetic CeS powder containing a small amount of Ce, Ce₂O₂S, and β-Ce₂S₃ as impurities. It was found that the oxygen content in the sintered compact decreases gradually as the sintering temperature increases, because of the removal of the impurities due to the evaporation of the volatile CeO. Single-phase CeS was formed by sintering at 2173 K. To evaluate the activation energy for densification of single-phase CeS, a CeS powder was prepared by milling an initial sintered compact and was used as an ingredient for hot-press experiments. Densification data during hot-press sintering were analyzed using a kinetic equation, showing that boundary diffusion is a rate-limiting process. The results suggest that this boundary diffusion model can explain well the densification data, with an apparent activation energy of 479 kJ·mol^-1.     

Processing and Properties of TiB2 with MoSi2 Sinter-additive: A First Report

The densification of non-oxide ceramics like titanium boride (TiB₂) has always been a major challenge. The use of metallic binders to obtain a high density in liquid phase-sintered borides is investigated and reported. However, a non-metallic sintering additive needs to be used to obtain dense borides for high-temperature applications. This contribution, for the first time, reports the sintering, microstructure, and properties of TiB₂ materials densified using a MoSi₂ sinter-additive. The densification experiments were carried out using a hot-pressing and pressureless sintering route. The binderless densification of monolithic TiB₂ to 98% theoretical density with 2–5 μm grain size was achieved by hot pressing at 1800°C for 1 h in vacuum. The addition of 10–20 wt% MoSi₂ enables us to achieve 97%–99%ρ_th in the composites at 1700°C under similar hot-pressing conditions. The densification mechanism is dominated by liquid-phase sintering in the presence of TiSi₂. In the pressureless sintering route, a maximum of 90%ρ_th is achieved after sintering at 1900°C for 2 h in an (Ar+H₂) atmosphere. The hot-pressed TiB₂–10 wt% MoSi₂ composites exhibit high Vickers hardness (∼26–27 GPa) and modest indentation toughness (∼4–5 MPa·m^1/2).     

Sintering Behavior of In Situ Cobalt Oxide-Doped Cerium–Gadolinium Oxide Prepared by Flame Spray Pyrolysis

Ce_0.9Gd_0.1O_1.95 (CGO10) and in situ cobalt oxide-doped CGO10 were prepared by pilot-scale flame spray synthesis, yielding powders with an average particle size of 40 nm. Cobalt oxide was shown to be a very effective sintering aid for CGO10 and lowered the maximum sintering temperature from 1450° to about 1200°C. Sintering studies revealed that in situ cobalt oxide-doped CGO10 exhibited a temperature of maximum shrinkage rate of 880°C for a dopant concentration of 1 mol% CoO_{1− x} , whereas for conventionally cobalt oxide-doped CGO10, this temperature was 914°C. This decrease is believed to be a result of a more homogeneous dopant distribution of the in situ cobalt oxide-doped CGO nanopowders as compared with the powders in which the doping was introduced as nitrates.     

Microstructure and Current–Voltage Characteristics of Multicomponent Vanadium-Doped Zinc Oxide Varistors

The effects of the oxide additives MnO₂, Co₃O₄, and Sb₂O₃, commonly incorporated in commercial Bi₂O₃-doped ZnO varistors, on the current–voltage characteristics and microstructure of 0.25 mol% V₂O₅-doped ZnO varistors have been studied. MnO₂ is the most significant additive in terms of its effects on varistor performance. Varistor performance can also be improved by increasing the V₂O₅ content to 0.5 mol% in a ZnO ceramic containing 1 mol% MnO₂. Further increases in the V₂O₅ content of 1 mol% MnO₂-doped material cause a deterioration in varistor behavior. The microstructure of the samples consists mainly of ZnO grains with zinc vanadates as the minority secondary phases. Additional spinel phase is formed when Sb₂O₃ is incorporated.     

Mechanism of Alumina-Enhanced Sintering of Fine Zirconia Powder: Influence of Alumina Concentration on the Initial Stage Sintering

The isothermal shrinkage behavior of 2.9 mol% Y₂O₃-doped ZrO₂ powders with 0–1 mass% Al₂O₃ was investigated to clarify the effect of Al₂O₃ concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al₂O₃ addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β₀ ⁿ ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al₂O₃ addition and both nQ and β₀ ⁿ increased with increasing Al₂O₃ concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al₂O₃ addition largely depends on the increase of β₀ ⁿ , that is, the increases of n with GBD→VD change and β₀ with an increase in Al₂O₃ content, although the nQ also increases with Al₂O₃ addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al₂O₃ at ZrO₂ grain boundaries.