Yttria Powders Research Articles

Mechanical alloying of alumina–yttria powder mixtures

Mechanical alloying has been used to prepare powder mixtures of alumina and yttria as a means to create composites with a dominant matrix phase together with small particles of a dispersed second phase. The yttria–alumina system, containing five possible phases, has the potential for creating eight combinations of matrix and dispersed phases. Here compositions designed to give YAlO 3 (YA) dispersed in Y 3Al 5O 12 (Y 3A 5 i.e. YAG) or Y 4Al 2O 9 (Y 2A) were studied. After milling with steel tools for times up to 8 h, the powders were subjected to thermal cycles up to 1500°C during which the phase evolution was monitored using X-ray diffractometry (including high-temperature XRD) and differential thermal analysis. During milling the original crystal structures were quickly broken down, in some cases partially replaced by an intermediate structure after milling. Upon subsequent heating the milled mixtures crystallized to give the expected phases, YA in Y 3A 5 and YA in Y 2A respectively, but the reaction route was seen to be different depending on the amount of amorphization of the yttria. Contamination by iron was seen to affect the phase distribution and the lattice parameters.

On droplet formation from capillary waves on a vibrating surface

The formation of sprays from a liquid film on a vibrating surface is used by ultrasonic atomizers for applications ranging from humidification to metal–powder manufacturing. The received opinion in the literature is that droplets are formed periodically from the apexes of an orderly pattern of standing capillary waves, with a wavelength that can be related to vibration frequency by stability analysis. It is described how this assumption may be incorrect in that, after droplet formation commences, the orderliness of the standing–wave pattern is lost due to one or more secondary instability phenomena. These phenomena, which lead to disorderliness, are investigated by using high–speed imaging techniques and a low–frequency vibrating film to model the high–frequency case, because of the difficulty of penetrating clouds of small droplets in the latter case. Different modes of droplet formation are identified and the flow patterns responsible for these modes are discussed. Physical mechanisms are proposed from which it is deduced that only a proportion of standing–wave crests can eject droplets, for a given wall–vibration period, and the identity of these ejecting waves should vary from period to period. The model thus developed demonstrates an apparently random ejection of droplets from one wave cell, even though the model itself is deterministic. The disorder of the capillary waves and the occurrence of several droplet–formation routes are sufficient to explain the range of droplet sizes that is produced by ultrasonic atomization.

Microstructural development during final-stage sintering of nanostructured zirconia based ceramics

The microstructural evolution of nanostructured zirconia based ceramics during pressureless final-stage sintering in vacuum was followed using X-ray diffraction, high resolution scanning electron microscopy and small-angle neutron scattering (SANS). The ultra-fine zirconia and yttria powders were synthesized by the inert gas condensation technique, the ultra-fine alumina by chemical vapor condensation. Powder compacts of ZrO 2 and dispersion mixed 5 mol% Y 2O 3 partially stabilized ZrO 2 and ZrO 2+14 wt.% Al 2O 3 were sintered at temperatures between 1000 and 1200°C for 2 h. Densification and concurrent grain growth as a function of temperature revealed a significant suppression of grain growth in the zirconia/alumina composite. Due to a homogeneous distribution of the alumina second phase in the zirconia matrix nearly full density and grain sizes <40 nm were achieved. Changes in the microstructure of monoclinic zirconia sintered at T>1050°C were detected by SANS and could be attributed to the detrimental effects of the monoclinic to tetragonal martensitic phase transformation. The log-normal pore size distributions were also evaluated for the ceramics.

Effect of Granule Properties on the Fracture Origin of Silicon Nitride Sintered Bodies.

The effect of granule properties on the microstructure of sintered silicon nitride bodies and their strength variation was examined. Commercially available silicon nitride, alumina and yttria powders were used as the raw materials. The granules were fabricated under three different spray drying conditions. Employed were several characterization techniques which are capable of directly observing the internal structures of granules, green bodies and of sintered bodies. The strength variation of the sintered bodies was correlated with the dimension of large pores detected by the direct observation of the internal structure of sintered bodies. These pores were originated from the inherent flaws in green compacts. These inherent flaws were introduced by non-uniform packing of powder particles via the incomplete collapse of dimples in granules and also from the cavities between granules. Furthermore, it was emphasized that these large pores in the sintered body was controlled by the granule size, dimple size, as well as the yield stress of granules.

Processing of β‐Silicon Nitride from Water‐Based alpha‐Silicon Nitride, Alumina, and Yttria Powder Suspensions

A water‐based route for processing ß‐Si3N4 from alpha‐Si3N4, Al2O3, and Y2O3 powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al2O3 and to pH 6 for Y2O3. Aluminum hydroxide (Al(OH)3) showed strong interaction with the Si3N4 powder, shifting the isoelectric point from pH 7 to pH 5.5. Low‐viscosity, high‐solids‐loading suspensions (60‐63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity &lt;1 Pa·s was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease‐catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green‐density distribution was homogeneous throughout all bodies, even for complex geometries.

A Simple Way to Synthesize Yttrium Aluminum Garnet by Dissolving Yttria Powder in Alumina Sol

A precursor for Y3Al5O12 was synthesized as a YAG sol by simply dissolving Y2O3 powder in an alumina sol. Phase‐pure Y3Al5O12 powder was obtained by precipitating the YAG sol with an aqueous dilute ammonia solution followed by calcination at 1100°C. TG/DTA analysis showed an exotherm at 938°C attributed to formation of YAG phase and weight loss of 44% at 1000°C. XRD and FT‐IR analysis showed that phase‐pure YAG can be formed through noncrystalline and metastable hexagonal YAlO3 without forming either yttrium or aluminum formate intermediate.

Nanostructured Yttria Powders Via Gel Combustion

Nanostructured yttria powders were prepared by a gel combustion technique. The technique involves exothermic decomposition of an aqueous citrate-nitrate gel. The decomposition is based on a thermally induced anionic redox reaction. A variety of yttria powders with different agglomerate structures can be made by altering the citrate-nitrate ratio γ. The gel with γ = 0.098 in situ yields nanostructured yttria powder at 258 °C that is porous and agglomerated with an average of 25 nm primary particles. Its specific surface area is 55 m2/g. The decomposition of the gels was investigated by simultaneous thermogravimetry analysis (TGA) and differential thermal analysis (DTA) experiments. The produced ashes and calcined powders are characterized by x-ray diffraction (XRD), ir spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer, Emmett, and Teller (BET) analysis.

Characteristics and sintering behavior of yttria powders synthesized by the combustion process

Characteristics and sintering behavior of yttria powders synthesized by the combustion process

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.

Effect of solvent, host precursor, dopant concentration and crystallite size on the fluorescence properties of Eu(III) doped yttria

A systematic spectroscopic study of Eu 3+ doped yttria powder phosphors are reported. Fluorescence study of this system has been investigated under the different solvent systems, e.g ethanol and water. In this study, the effect of the precursor of yttria, e.g. acetate, nitrate and chloride, on the fluorescence behaviour of Eu 3+ have been observed in particular. Attention was paid to their effect on the emission intensity of the 5 D 0→ 7 F 2 transition which is characteristic of the `red fluorescence' of the Eu 3+ ion. Results show that the CH3COO − ion has a tremendous effect on the enhancement of emission intensity, greater than NO 3 − and Cl − ions. The Fourier transformed infrared (FTIR) spectroscopy was used for chemical and structural analysis. It has also been stated that the concentration quenching of dopant was more pronounced in the powders derived from yttrium chloride than from yttria precursor. In the case of Y(CH 3COO) 3 as a precursor, the concentration quenching was totally absent and emission intensity increased with an increase in the concentration of dopant. This paper also includes some interesting results on the effect of modifiers, which are a mixture of Tween-80 with ε-caprolactum/ β-alanine/emulsogen-OG, on the 5 D 0→ 7 F 2 transition of Eu 3+ doped yttria in terms of crystallite size in brief.

Microstructural Characterization of Annealed Samples of Pure-Yttria Using Electron Microscopy

Abstract Yttria is an electroceramic material that has a negative temperature coefficient of resistivity (i.e. it is an NTC type semiconductor). This property, along with its high melting point and good stability, makes yttria a potential sensor material (thermistor) for measuring temperature differences in demanding applications. Recently there has been increasing interesting in studying this ceramic material. How-ever, there is very limited published information on the microstructural characteristics of pure yttria processed under various conditions. Furthermore, reported evidence on the effect of processing conditions on the electrical response of the material is still lacking. Studies within our Center have revealed a correlation between aging time and electrical behaviour in pure yttria samples. The present study examines the evolution of the microstructure of sintered yttria powder during different annealing treatments. Yttria pellets (6mm diameter, 2mm height) were cold pressed and then sintered at 1550°C for 24h in air. The sintered pellets were then annealed at 1100°C for different times. Specimens for SEM examination were prepared by grinding the surface of the pellets (using grit paper up to 400) and subsequently polishing them using l μm alumina powders.

EXAFS of Nanocrystalline Y2O3

A large volume fraction of atoms in nanocrystalline materials is located in surfaces and interfaces. Atoms facing the particle boundaries have a different local structure than atoms inside the particle. This heterogeneous disorder is investigated for nanocrystalline yttria powder by EXAFS spectroscopy.

Tensile properties of mechanically alloyed/milled Ods-Ni-Based alloys

Oxide dispersion strengthened (ODS) NiCr20 alloys were produced by ball milling blends of either prealloyed NiCr20 and yttria powders or elemental Ni and Cr powders and yttria powder. After milling, the powders were degassed in a vacuum furnace, sealed in steel cans, and consolidated by hot extrusion. The mechanical alloying process, which occurs during ball milling of elemental Ni and Cr powders, as well as the changes of microstructure, which occur during milling and extrusion, were characterized by X-ray diffraction (XRD) techniques. Measurements of microhardness, tensile strength, and elongation of extruded bars were done to gain information about the dispersoid partitioning and about the relation between milling parameters, microstructure, and tensile properties. These investigations showed that the attainable quality of ODS NiCr20 alloys is higher if they are produced by milling elemental Ni and Cr powders and yttria powders. Besides the dispersoid partitioning, the homogeneity of the mechanically alloyed powder strongly affects the quality. High-quality materials are only produced if the ball milling process yields a homogeneous dispersoid partitioning and a completely mechanically alloyed NiCr20 solid solution.

X-ray absorption study on nanostructured zirconia and yttria

Nanostructured zirconia and yttria powders with grain sizes between 5 and 30 nm have been studied by extended X-ray absorption fine structure (Y and Zr K-edges). Due to the high specific surface area of the powders (75 to 120 m2/g) the fraction of surface atoms is drastically increased. The effective coordination number of the zirconium and yttrium atoms is reduced (up to 40%) in the powders. The decrease of the coordination number is most obvious in the cation-cation-distances. Fully dense ceramic samples with a grain size of 80 nm have nearly the same coordination number compared to samples with conventional grain sizes of about 1 μm.

Synthesis of Yttria Powders by Electrospray Pyrolysis

Electrospray atomization of high‐concentration (∼400 g/L) chemical precursor solutions was applied to the synthesis of yttria powders. Conditions were found which led to high‐quality powders, composed of dense, spheroidal, submicrometer, and nanocrystalline oxide particles. The precursor solutions were hydrated yttrium nitrates dissolved in n‐propyl alcohol at concentrations ranging from 44.1 to 455 g/L. Electrospray atomization produced submicrometer precursor droplets which were dispersed in air and carried through an electric furnace for thermal decomposition at 500°C for several seconds residence time. X‐ray powder diffraction patterns indicated the expected cubic phase. Transmission electron micrographs showed that the particle structure varied with solution composition, ranging from hollow, inflated spheres for 6‐hydrated nitrates to dense spheroids for 5‐hydrated nitrates. The use of 6‐hydrated nitrates in the solutions appeared to form particle surfaces which were impermeable to alcohol vapor evolved during thermal decomposition, leading to hollow, inflated spheres.

Mechanical alloying process of the zirconia–8 mol % yttria ceramic powder

The phase transformation process of zirconia–8 mol % yttria powder mixtures during a high energy ball milling process has been studied by means of x-ray diffraction analysis. It has been found that the m-ZrO2 (monoclinic zirconia) transforms first to m-ZrO2 solid solution and then to t-ZrO2 (tetragonal zirconia) solid solution. Finally, a single cubic zirconia solid solution phase forms after prolonged milling. The structural transformation is discussed and explained in terms of the phase relations in zirconia-yttria ceramics and the nonequilibrium nature of the mechanical alloying.

Densification of Irregular Powders During Hot Isostatic Pressing

Abstract The densification of irregular powders during hot isostatic pressing was examined experimentally and compared to predicted densification rates using the HIP'ing model for monosized spheres proposed by Arzt, Ashby and Easterling (AAE model). Experimental densification rates for irregular powders of copper and yttria were found to be greater than those predicted by the AAE model. Empirical relationships were developed and incorporated into the AAE model to account for the shape of particles. These relationships incorporated the morphological characteristics into the description of effective pressure on the particle contacts. The modified HIP'ing model was then used to predict densification rates for irregular copper and yttria powders. The predicted rates are higher, thus reflecting the more rapid densification of the non-spherical-shaped particles.

Dispersion characteristics of ceramic powders in the application of cationic and anionic polyelectrolytes

Dispersion characteristics of silicon nitride, silicon carbide, alumina, aluminum nitride and yttria powders in aqueous solutions of anionic and cationic polyelectrolytes were studied by measuring particle size distribution using a gravity sedimentation technique. The influence of polyelectrolyte concentration and suspension pH on the dispersion was examined. All powders examined in this study could be well dispersed in the presence of either cationic or anionic polyelectrolyte by adjusting the dispersion pH to at least 2.5 pH units away from the effective pH iep (iep=isoelectric point) of the powder-polyelectrolyte system. The dispersion characteristics of these powders in the presence of polyelectrolytes are explained in terms of interfacial interactions in the powder-polyelectrolyte systems. These interactions between the powder and polyelectrolytes as a function of suspension pH, and possible mechanisms leading to the stability of particles in a dispersion, are discussed.

Effect of LiF Addition on the Preparation and Transparency of Vacuum Hot Pressed Y&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt;

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

Enhanced densification of yttria by addition of titanium oxide

Abstract The influence of several additives (ZrO 2 , CeO 2 , TiO 2 , SiO 2 ) on sintering of conventional yttria powders is investigated. Titanium oxide is the most efficient in promoting densification and in lowering sintering temperatures. Almost full densities can be obtained at 1400–1500°C if a small amount of dopant (0.2 wt% TiO 2 ) is dispersed homogeneously onto the surface of yttria during powder processing.