Yttria Ceramics Research Articles

Synthesis of yttria nanopowders for transparent yttria ceramics

Abstract This paper describes a precipitation process for synthesizing nanocrystalline yttria powder and sintering transparent yttria ceramics. Hydroxide precursor of Y 2 O 3 with an approximate composition of Y 2 (OH) 5 NO 3 · H 2 O was synthesized by using ammonia water as precipitant and yttrium nitrate as the starting salt. Employing appropriate striking method and optimum synthetic conditions, yttrium hydroxide with a card-house or spherical structure can be formed. It was found that normal striking derived powders have higher sinterability than reverse striking derived powders. The addition of a small amount of ammonia sulfate in the yttrium nitrate solution reduces the agglomeration and particle size of the produced yttria powders. Nanocrystalline yttria powders (60 nm in average size) was obtained by calcining the precursor at 1100 °C for 4 h. Transparent yttria ceramics were fabricated from the nano Y 2 O 3 powders by vacuum sintering at 1700 °C for 4 h. In the wavelength of 1000 nm, the in-line transmittance reaches 52%.

Isothermal Tetragonal-to-Monoclinic Phase Transformation in a Zirconia–Yttria System

The isothermal tetragonal-to-monoclinic phase transformation in zirconia-0.5–4 mol% yttria ceramics with and without alumina addition is summarized. This transformation proceeds sigmoidally with increasing isothermal aging time. C-shaped T-T-T curves are obtained, and their nose positions shift to shorter times and higher temperatures as the yttria content decreases. The apparent activation energy obtained from the TT-T curves is almost coincident with that for diffusion of oxygen ions in zirconia–yttria ceramics, irrespective of the environment in water, air or vacuum. The isothermally transformed laths advance from the grain boundaries to the grain interiors and then propagate adjacent grains with increasing aging time. Furthermore, this growth is controlled by the diffusion of oxygen ion vacancies in zirconia. The alumina addition acts to reduce the anisotropy of the transformed monoclinic phase.

Precipitation synthesis and sintering of yttria nanopowders

Yttria nanopowders were synthesized using a chemical precipitation method. pH value at the end of the precipitation process has a significant effect on the size and morphology of the precursor and the yttria powders. Under the same calcination condition, the yttria powders made from the precursor obtained at a pH of 8 are smaller in mean particle size and narrower in size distribution than those made from the precursor obtained at a pH of 10. It was found that the optimum calcination temperature is 1000 °C, and the yttria powder obtained is fine (30 nm) and well dispersed. Using the yttria powder, transparent yttria ceramics was produced by vacuum sintering at 1700 °C for 4 h without any additives.

ジルコニア‐イットリア系セラミックスのＴ‐Ｍ変態に対するアルミナ添加の影響

Effect of 5-30 mol% alumina addition on the tetragonal(T)-to-monoclinic(M) (Martensitic) phase transformation in zirconia-2 and 3 mol% yttria ceramics was investigated. Powders were compressed at 100MPa, and sintered at 1923 K for 54ks. The sintered specimens were aged at 573 K. Mean grain sizes were increased by alumina addition. The amount of expansion and contraction during heating and cooling accompanied with transformation were increased by the addition of alumina. The amount of transformation increased with aging time as a sigmoidal curve and the curve sifted to shorter aging time by the addition of alumina. In higher alumina, long time was required for saturation in transformation, although the transformation started occurred earlier. Aged 2Y specimen has a texture of (111)M, while in the alumina added specimen such tendency became to be weak. AFM observations show that, after aging in 3Y-5A specimen, the transformation accompanied by aging time developed in the various directions with many branch compared with those in 3Y.

Yttria ceramics: cutting tool application

Yttrium oxide ceramic cutting tool insert is developed. Yttrium oxide powders are pressed and sintered in the form of cutting tool inserts. Mechanical properties of the inserts like hardness and fracture toughness are evaluated. Machining studies are conducted on grey cast iron work piece to evaluate the performance of yttria ceramic cutting tool and it is compared with commercial zirconia-toughened alumina (ZTA) ceramic cutting tool.

Effects of the Sulfate Ion Exerted on Densification of Yttria on Precursor Synthetic

Ammonium hydroxide was used at 10°C as the precipitant to synthesize thin flakes of yttrium hydroxide, agglomerating in a manner resembling houses of card. Onemol% to 50mol% of ammonium sulfate was added to the yttrium hydroxide. Doping of sulfate ion resulted in yttria particles with round edges, in contrast to undoped yttria particles with sharp edges. The amount of doped sulfer was reduced to S/Y=10-3 by calcinations at 1100°C, and to trace by sintering at 1700°C. Doping of 10mol% of ammonium sulfate resulted in the maximum transparency in those of the yttria ceramics. Transparency of the yttria ceramics related to brittleness of agglomerate of the calcined powder which was evaluated by collapsing tendency of the agglomerates by ultrasonic dispersion.

Powder synthesis and sintering of high density thoria–yttria ceramics

ThO 2– n mol% Y 2O 3 ceramic powders with submicron average particle size were prepared by the citrate technique using thorium oxalate, yttrium nitrate, citric acid and ethylene glycol. For n=3, 6, 9 and 12, reactive thoria–yttria solid solution powders with surface areas of 17.3, 11.1, 9.4 and 8.7 m 2/g, respectively, were obtained. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were carried out for characterization of the powders. Pressing and sintering at 1550 °C/2 h produced pellets with densities higher than 90% of the theoretical density. The pellets were also characterized by XRD and SEM.

Fabrication of Transparent Yttria Ceramics by the Low‐Temperature Synthesis of Yttrium Hydroxide

Thin flakes of yttrium hydroxide agglomerated in a manner resembling houses of cards with aging at 10°C. The agglomerate then dissociated into fine yttria particles with calcination at >800°C. The particle size of the calcined powder increased appreciably as the calcination temperature increased. The shrinkage curve indicated similar densification behavior among undoped yttria powders calcined at 800°–1000°C, despite considerable particle growth as the calcination temperature increased. Increasing the calcination temperature to >1000°C shifted the shrinkage curve appreciably to the high‐temperature region. Sulfate‐ion‐doped yttria particles had round edges, irrespective of calcination temperature, in contrast to the sharp edges of the undoped yttria particles. A calcination temperature of <1000°C resulted in skeleton yttria particles, which exhibited poor sinterability. At a calcination temperature >1000°C, the skeleton particles dissociated into monodispersed particles that densified easily. When the calcination temperature was >1000°C and the average particle sizes were similar, the undoped and sulfate‐ion‐doped yttria showed similar densification rates. The transparency of the sintered yttria ceramics was dependent on both the calcination temperature and sulfate‐ion doping: that is, sulfate‐ion doping and calcining at 1100°C were both necessary conditions for the fabrication of a transparent body.

Silicon Nitride Joining Using Silica and Yttria Ceramic Interlayers

Dense hot‐pressed β‐Si3N4 blocks were joined using both SiO2 and SiO2‐Y2O3 powder slurries as bonding interlayers. The assembled specimens (Si3N4/interlayer/Si3N4) were heated in a flowing N2 atmosphere in the temperature range of 1500°–1650°C. The joining interlayer was clearly distinguished from the Si3N4 bulk. The microstructure and the reaction products found in the bonding interlayer were very different in both compositions. Reactions occurring between the Si3N4 and the ceramic joining compositions have been explained based on existing diagrams of the YN–Si3N4‐Y2O3‐SiO2 system.

Lattice parameters of thoria–yttria solid solutions

ThO 2: x mol% Y 2O 3 ( x=0, 3, 6, 9 and 12) ceramic powders have been prepared by the citrate technique. Dense specimens have been obtained after pressing and sintering at 1550°C/2 h. X-ray diffraction measurements were performed in the sintered pellets for structural phase analysis and lattice parameter determination. The experimental results suggest that an empirical equation, similar to the equation proposed for zirconia–yttria ceramics, is valid for thoria–yttria solid solutions.

Surface Relief Associated with Isothermal Martensite in Zirconia–3‐mol%‐Yttria Ceramics Observed by Atomic Force Microscopy

Surface relief associated with martensitic transformation—tetragonal‐to‐monoclinic phase transformation—that occurred during isothermal aging at temperatures of 400 and 473 K was observed via atomic force microscopy, using prepolished and thermally etched surfaces of zirconia–3 mol% yttria that was sintered at a temperature of 1923 K for 5 h. The isothermal martensitic transformation occurred first at grain boundaries and advanced into grain interiors as the aging time increased and then spread successively into the surrounding grains. The transformation area, which had an approximately round shape, increased as the aging time increased. The fine structure of the surface relief was composed of many tent shapes that were several nanometers high with an angle of ∼165°.

Sintering of zirconia–yttria ceramics studied by impedance spectroscopy

Sintering of ZrO 2:Y 2O 3 ceramic compacts have been studied by impedance spectroscopy measurements at 400°C in the 5 Hz–13 MHz frequency range. Cold-pressed samples processed by a co-precipitation route were heat treated at 1350°C for different periods of time. The intragranular resistivity remains constant while the intergranular resistivity decreases for increasing sintering time. A correlation between sintering time and parameters related to pore density determined by impedance spectroscopy was found. The results show that details of the sintering process can be followed by impedance spectroscopy.

Fabrication of Transparent Yttria Ceramics through the Synthesis of Ytttrium Hydroxide at Low Temperature and Doping by Sulfate Ions.

Thin flakes of yttrium hydroxide coagulate in a manner similar to the houses of cards at an aging temperature lower than room temperature. Increasing the temperature results in an increase in the thickness of the flakes together with collapse of the structure of the house of cards. Relatively thick tablets of yttrium hydroxide appear at temperatures above approximately 50°C. Sulfate ions enhance the sinterability of yttria irrespective of the aging temperature of yttrium hydroxide, and have an influence on the transparency of yttria ceramics. Both the aging of yttrium hydroxide at temperatures lower than room temperature and the doping of sulfate ions are essential to the fabrication of transparent yttria ceramics using the present method.

Effects of Calcia and Zirconia on Densification and Grain Growth of Yttria Ceramics

Effects of Calcia and Zirconia on Densification and Grain Growth of Yttria Ceramics

ChemInform Abstract: Fabrication of Transparent Yttria Ceramics at Low Temperature Using Carbonate‐Derived Powder.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Fabrication of Transparent Yttria Ceramics at Low Temperature Using Carbonate‐Derived Powder

A new preparation method for a highly sinterable yttria powder has been developed, the resultant powder characterized, and its sinterability studied. As a precursor of the yttria powder, a fine and needle‐shaped yttrium carbonate was prepared by a precipitation and aging method. A fine and low‐agglomeration yttria powder was obtained by calcining the carbonate precursor at 1100°C. The primary crystallites measured ∼0.1 μm and weakly agglomerated to a size of ∼0.3 μm. The powder had a very high sinterability, so that the powder compact could be sintered to transparency by normal sintering under vacuum without additives at low temperatures, over 1600°C. The sintered transparent ceramics exhibited a homogeneous microstructure with no abnormal grain growth.

Densification of Yttria Ceramics

Densification of Yttria Ceramics

Effect of LiF Addition on the Preparation and Transparency of Vacuum Hot Pressed Y<SUB>2</SUB>O<SUB>3</SUB>

In the present work, the preparation of transparent yttria ceramics by vacuum hot pressing was investigated, using yttria powder whose mean diameter is 0.88 μm and whose purity is higher than 99.9%. Transparent yttria ceramics were fabricated by vacuum hot pressing with small additions of LiF (about 1 mass%). Hot pressing temperature and pressure were kept at 1573 K and 44 MPa, respectively. The processed samples were observed to have transparencies up to 78% at the wavelength of 644 nm and over 80% in the range of 860 nm-6 μm

真空ホットプレス法による透光性Y<SUB>2</SUB>O<SUB>3</SUB>の作製とそのスペクトル特性に及ぼすLiF添加の影響

In the present work, the preparation of transparent yttria ceramics by vacuum hot pressing was investigated, using yttria powder whose mean diameter is 0.88 μm and whose purity is higher than 99.9%.Transparent yttria ceramics were fabricated by vacuum hot pressing with small additions of LiF (∼1 mass%). Hot pressing temperature and pressure were kept at 1573 K and 44 MPa, respectively. The processed samples were observed to have transparencies up to 78% at 644 nm frequency and over 80% in the range of 860 nm-6 μm.

Superplastic forging procedures for manufacture of ceramic yttria stabilised tetragonal zirconia products

AbstractThe influence of initial grain size and density on the deformation behaviour of fine grained yttria stabilised tetragonal zirconia (YTZ) polycrystals has been studied. The presence of very fine (0·2–0·25 μm) grains at the start of straining, together with appreciable initial porosity, leads to an increase in the formability of these usually brittle ceramics. As a result, relatively high strains can be applied at sufficiently low temperatures and high strain rates, such that products of complex shape can be manufactured from YTZ compacts.