Fabrication and Luminescent Properties of Nd3+-Doped Lu2O3 Transparent Ceramics by Pressureless Sintering

The fabrication of transparent Nd³⁺ ion-doped Lu₂O₃ ceramics is investigated by pressureless sintering under a flowing H₂ atmosphere. The starting Nd-doped Lu₂O₃ nanocrystalline powder is synthesized by a modified coprecipitant processing using a NH₄OH+NH₄HCO₃ mixed solution as the precipitant. The thermal decomposition behavior of the precipitate precursor is studied by thermogravimetric analysis and differential thermal analysis. After calcination at 1000°C for 2 h, monodispersed Nd³⁺:Lu₂O₃ powder is obtained with a primary particle size of about 40 nm and a specific surface area of 13.7 m²/g. Green compacts, free of additives, are formed from the as-synthesized powder by dry pressing followed by cold isostatic pressing. Highly transparent Nd³⁺:Lu₂O₃ ceramics are obtained after being sintered under a dry H₂ atmosphere at 1880°C for 8 h. The linear optical transmittance of the polished transparent samples with a 1.4 mm thickness reaches 75.5% at the wavelength of 1080 nm. High-resolution transmission electron microscopy observations demonstrate a "clear" grain boundary between adjacent grains. The luminescent spectra showed that the absorption coefficient of the 3 at.% Nd-doped Lu₂O₃ ceramic at 807 nm reached 14 cm⁻¹, while the emission cross section at 1079 nm was 6.5 × 10⁻²⁰ cm².     

Rapid Rate Sintering of Nanocrystalline ZrO2−3 mol% Y2O3

Conventional ramp-and-hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO₂–3 mol% Y₂O₃ powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO₂–3 mol% Y₂O₃ produced mixed results. In the case of submicrometer ZrO₂–3 mol% Y₂O₃, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO₂–3 mol% Y₂O₃, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO₂–3 mol% Y₂O₃ specimens.     

Dense Amorphous Zirconia–Alumina by Low-Temperature Consolidation of Spray-Pyrolyzed Powders

Metastable amorphous ZrO₂-Al₂O₃ powders were hot-pressed at low temperatures (873 and 923 K), under moderately high pressures (500 and 750 MPa), and amorphous pellets with 1%-8% porosity were obtained. Crystallization of the amorphous pieces in the temperature range of 1173–1673 K produced a range of ultrafine microstructures, the finest of which had grains of tetragonal (ZrO₂-40-mol%-Al₂O₃) solid solution 6–8 nm in size that formed at 1173 K. Submicrometer grain sizes of the equilibrium monoclinic ZrO₂ and alpha-Al₂O₃ were stable against coarsening at 1673 K. The new technique was applied to produce a SiC-reinforced composite with an amorphous ZrO₂-80-mol%-Al₂O₃ matrix; the high matrix sinterability overcame the reinforcement constraint. The results suggest a possible solution to the difficulties in the bulk processing of amorphous, nanocrystalline, and other novel ceramics.     

Nonisothermal Synthesis of Yttria-Stabilized Zirconia Nanopowder through Oxalate Processing: I, Characteristics of Y-Zr Oxalate Synthesis and Its Decomposition

To obtain powder with a composition of 3 mol% Y₂O₃–97 mol% ZrO₂, a process of Y-Zr oxalate powder production has been optimized, to produce an oxalate with minimal particle size. The methodology of the nonisothermal decomposition of Y-Zr oxalate has been explained. Characteristics of the nonisothermal decomposition of different oxalates have been studied. Nanocrystalline Y₂O₃-stabilized ZrO₂ (YSZ) powder with a narrow size distribution of primary particles and aggregates was produced. The zirconia powder that was obtained from the smallest oxalate powder via nonisothermal decomposition had a particle size of 8–10 nm. The YSZ powder was weakly aggregated, with a narrow aggregate-size distribution of 70–90 nm.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

Effect of Powder Coating on Stabilizer Distribution in CeO2-Stabilized ZrO2 Ceramics

The phase and microstructure relationship of 12 mol% CeO₂-stabilized ZrO₂ ceramics prepared from coated powder was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersed X-ray spectroscopy (EDS). As compared with the sample prepared with co-precipitated method, which exhibited a similar grain size distribution, the EDS analysis revealed that the powder coating induced a wide distribution of CeO₂ solubility, which decreases monotonically with the increase of grain size. This variation of stabilizer content from grain to grain rendered many large grains in the monoclinic phase. Stronger cerium segregation to grain boundaries was observed between large grains, which often form thin amorphous films there. The inhomogeneous CeO₂ distribution keeps more tetragonal ZrO₂ grains close to the phase boundary to facilitate the transforming toughness. Addition of an Al₂O₃ precursor in coated powders effectively raises the overall CeO₂ stabilizer content in the grains and preserves more transformable tetragonal phase in the microstructure, which further enhanced the fracture toughness. The dependence of CeO₂ solubility on grain size may be explained in a simple coating-controlled diffusion and growth process that deserves further investigation.     

ZrO2 Nanopowders Prepared by Low-Temperature Vapor-Phase Hydrolysis

ZrO₂ powder is prepared by low-temperature vapor-phase hydrolysis of ZrCl₄. TG-DTA, XRD, Raman, BET, and TEM methods are used to investigate the particle size, phase composition, and agglomeration before and after heat treatment. The results show that the as-prepared ZrO₂ powder is characterized by large surface area (150 m²/g), fine grain size (5.8 nm), and weak agglomeration. Additionally, the as-prepared ZrO₂ powder shows predominantly tetragonal phase attributed to a grain size effect. This route is free of powder drying and calcination processes that are essential for wet chemical preparation, contributing to less agglomeration.     

Low-Thermal-Expansion Polycrystalline Zirconyl Phosphate Ceramic: Solid-Solution and Microcracking-Related Properties

The effects of the addition of Ta205 or Nb205 on the sintering and refractory properties of 2ZrO₂·P₂O₅-based ceramics have been investigated. The solid solution with Nb₂O₅ has a lower average thermal expansion coefficient than the pure phase. The strength of the polycrystalline ceramic is reduced when the grain size exceeds the transition grain size for microcracking. The thermal expansion of the ceramic is abruptly lowered when the grain size exceeds a value which should be termed the "second transition grain size" for additional wider microcracking. SiO₂ was found most effective for inhibiting grain growth to prevent microcracking and for obtaining a strong ceramic.     

Densification and Phase Transformation During Pressureless Sintering of Nanocrystalline ZrO2–Y2O3–CuO Ternary System

ZrO₂–Y₂O₃–CuO nanocrystalline powders have been synthesized using a chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies show that the presence of CuO accelerates the densification process of ZrO₂(3Y). A fully dense (>96%) pellet of ZrO₂(3Y)/5 mol% CuO was obtained after sintering at 900°C, with a very small grain size of 44 nm calculated by X-ray line broadening.     

Low-Thermal-Expansion Polycrystalline Zirconyl Phosphate Ceramic

The synthesis of thermal-shock-resistant material from zirconyl phosphate, 2ZrO₂P₂O₅, was investigated. Modifications of the compound were examined; only the a phase appears to be thermodynamically stable. Sintering of a-phase powder to a dense ceramic was promoted by adding a metal oxide. Crystalline zirconyl phosphate has a low average thermal expansion coefficient. The relation of microcracking to grain size and strength was studied using specimens sintered under varied conditions. A strong low-thermal-expansion polycrystalline ceramic was obtained by controlling grain growth.     

Low-Temperature Processing and Mechanical Properties of Zirconia and Zirconia–Alumina Nanoceramics

The 1.5- to 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ (Y-TZP) and Al₂O₃/Y-TZP nanocomposite ceramics with 1 to 5 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the ceramic processing conditions, resulting density, microstructure, and the alumina content on the hardness and toughness were determined. The densification of the zirconia (Y-TZP) ceramic at low temperatures was possible only when a highly uniform packing of the nanoaggregates was achieved in the green compacts. The bulk nanostructured 3-mol%-yttria-stabilized zirconia ceramic with an average grain size of 112 nm was shown to reach a hardness of 12.2 GPa and a fracture toughness of 9.3 MPa·m^1/2. The addition of alumina allowed the sintering process to be intensified. A nanograined bulk alumina/zirconia composite ceramic with an average grain size of 94 nm was obtained, and the hardness increased to 16.2 GPa. Nanograined tetragonal zirconia ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughnesses between 12.6–14.8 MPa·m^1/2 (2Y-TZP) and 11.9–13.9 MPa·m^1/2 (1.5Y-TZP).     

Microstructure and Microwave Dielectric Properties of Ba(Zn1/3Ta2/3)O3 Ceramics with ZrO2 Addition

The effect of ZrO₂ on crystallographic order, microstructure, and microwave dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) ceramics was investigated. A small amount of ZrO₂ disturbed the 1:2 cation ordering. The average grain size of the BZT significantly increased with the addition of ZrO₂, which was attributed to liquid-phase formation. The relative density increased with the addition of a small amount of ZrO₂, but it decreased when the ZrO₂ content was increased. Variation of the dielectric constant with ZrO₂ addition ranged between 27 and 30, and the temperature coefficient of resonant frequency increased abruptly as the ZrO₂ amount exceeded 2.0 mol%. The Q value of the BZT significantly improved with the addition of ZrO₂, which could be explained by the increased relative density and grain size. The maximum Q × f value achieved in this investigation was ∼164 000 GHz for the BZT with 2.0 mol% ZrO₂ sintered at 1550°C for 10 h.     

Synthesis and Colloidal Processing of Zirconia Nanopowder

Nanosized tetragonal 3 mol% Y₂O₃-doped ZrO₂ powder was produced by hydrothermal precipitation from metal chlorides and urea sol followed by a washing–drying treatment and calcination. The effects on powder properties of powder washing by water and ethanol with subsequent centrifuging, with possible deagglomeration using microtip ultrasonication, were experimentally shown. Ultrasonic irradiation induced pressure waves, which generated cavities that could violently collapse, producing intense stress. This induced stress was effective in minimizing secondary particle size, deagglomerating the powder, redispersing the ZrO₂ after all the washing–centrifuging cycles, and minimizing mean aggregate size after final calcination. A uniformly aggregated tetragonal ZrO₂ nanopowder with a mean secondary particle size of ∼45 nm and without hard agglomerates was prepared. The properties of the nanopowders produced by colloidal processing and CIP were studied. Determination of the best suspension parameters allowed for low-temperature sinterability, which resulted in a nanograined ∼95 nm ceramic.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M _s for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO₂ content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M _s can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO₂ grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M _s temperature brought about by the increase in the ZrO₂ grain size, when T > M _s. In alumina containing 20 vol% ZrO₂ (12 mol% CeO₂), a toughness of ∼10 MPa. √m can be achieved for a ZrO₂ grain size of ∼2 μm ( M _s∼ 225 K). However, at a grain size of ∼2 μm, the alumina–40 vol% ZrO₂ (12 mol% CeO₂) has a toughness of only 8.5 MPa. √m ( M _s∼ 150 K) but reaches 12.3 MPa. ∼m ( M _s∼ 260 K) at a grain size of ∼3 μm. These findings show that composition (and matrix properties) play critical roles in determining the ZrO₂ grain size to optimize the transformation toughening in ZrO₂-toughened ceramics.     

Fabrication of Fine AlN Particles by Pulverizing with Very Small ZrO2 Beads

Nano-sized aluminum nitride (AlN) powders can enhance the sinterability of AlN ceramics. The present work examined the pulverization and dispersion of AlN powder using ZrO₂ beads of 0.1 mm in diameter as grinding medium in order to obtain nano-sized powder for low-temperature sintering. An attracting feature of the bead grinding is that the rotor creates centrifugal and shearing forces, which lead to efficient pulverization that exceeds the conventional grinding limit. The AlN powder with agglomerates and average particle size of 0.38 μm was broken up into more homogeneous particles with a narrow particle size distribution after 90 min of grinding. Qualitative examinations of the powder by transmission electron microscopy and BET indicated that the particle size was 50–100 nm and specific surface area was 70 m²/g.     

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.     

Preparation of Ultrafine Zirconia Particles

Ultrafine ZrO₂ particles have been prepared via a new sol-gel process. This process involves the addition of excess C₂H₄O into the aqueous ZrOCl₂ solution and reacting the mixture at room temperature; a glassy ZrO(OH)₂ gel is formed moments later. An ultrafine ZrO₂ powder is obtained after the gel is dried and calcined; the powder is monoclinic. The average particle size is ∼12 nm, and its specific surface area is 55.1 m²/g. In addition, partially stabilized ZrO₂ can be prepared in the same manner, yielding a good result.     

Grain-Size Distribution in Al2O3─ZrO2 Generated by High-Temperature Annealing

Grain-size distribution in various Al₂O₃─ZrO₂ (2.5 mol% Y₂O₃) ceramics during high-temperature annealing was examined. In alumina-rich alloys, the grain size of major and minor phases was very different, while grain size was almost uniform in zirconia-rich alloys. This difference in grainsize distribution was related to the difference in grain growth rate of the major phase and to the effectiveness of grain-boundary pinning by minor-phase grains.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).