Sintering of powders in the Al2O3-ZrO2-Y2O3 system obtained by the method of rapid solidification of the melt as a function of the initial heat treatment

The effect of heat treatment at 1340 — 1450°C on the sintering of ceramic powders of the A1₂O₃-ZrO₂-Y₂O₃ system obtained by the method of rapid solidification of the melt is considered. It is shown that in this temperature range powder particles undergo crystallization of the amorphous phase and an ε-A1₂O₃ → α-A1₂O₃ transformation with the formation of an intraparticle ultradisperse polycrystalline structure of an eutectic type. The system attains a thermodynamically equilibrium state. However, the sinterability of compacts from such powders increases substantially, which is associated with the elevated contribution of the intraparticle grain boundary diffusion into the diffusion mass transfer.     

Effect of a stabilizing Y2O3 additive on the strength properties of zirconia ceramics sintered from plasma chemical powders

The effect of the concentration of a stabilizing Y₂O₃ additive in zirconia ceramic sintered from ultradisperse plasma chemical powders on their properties is described. The optimum mole fraction (3%) of Y₂O₃ providing the best strength properties of the ceramic is established.     

Structure of glasses in the Na2O - Fe2O3 - B2O3 - SiO2 system

The structure of glasses of a local area of the Na₂O - Fe₂O₃ -B₂O₃ - SiO₂ system is studied. The sequence of formation of oxygen-containing tetrahedral groups in the presence of nonbridge oxygen ions is established. Formation of groups of a six-coordinated into cation facilitates crystallization of sodium-boron-silicate glass melts in cooling. Translated from Steklo i Keramika, No. 1, pp. 11–13, January, 2000.     

Improved thermal shock resistance of low-carbon Al2O3–C refractories fabricated with C/MgAl2O4 composite powders

The addition of C/MgAl₂O₄ composite powders can improve the thermal shock resistance of low-carbon Al₂O₃–C refractories attribute to the formation of microcracks in the agglomerated structure, thus consuming more thermal stress and strain energy. Moreover, C/MgAl₂O₄ composite powders additive promote the formation of short fibrous ceramic phases in the refractories, which suggest a bridging role in the interior of the refractories and increase its toughness. Furthermore, the C/MgAl₂O₄ composite powders also result in a remarkable enhancement of the slag corrosion resistance in the refractories.     

Co-precipitation synthesis of Y2W3O12 submicronic powder

Synthesis of negative and near-zero thermal expansion powders with low degree of agglomeration is one of the major challenges in their applications. In this research, Y₂W₃O₁₂ was synthesized via two soft-chemical approaches, co-precipitation by mixing reactants and modified reverse strike co-precipitation, to assess the effect of these methods on the agglomeration degree of powders. Chemical composition of amorphous precursors was estimated based on charge neutrality principle, carbon-hydrogen-nitrogen (CHN) and thermogravimetric analyses. It was demonstrated that both precursors are oxyhydroxynitrate compounds, determined as Y₂W₃O_y(OH)_z(NO₃)_x·nH₂O. Both co-precipitation methods presented similar particle sizes and agglomeration degrees. The applied calcination conditions caused the formation of necks and hard agglomerates and were critical factors for the final agglomerated state of the Y₂W₃O₁₂ powders. The formation of Y₂W₃O₁₂ submicronic powders in the range of 0.60–0.70 µm was verified by laser diffraction and field-emission scanning electron microscopy analyses. X-ray powder diffraction patterns, at room temperature, of hydrated Y₂W₃O₁₂ revealed predominantly the presence of monoclinic phase, while the monoclinic to orthorhombic phase transition was detected by dilatometry.     

Dielectric Properties of Glasses in the Systems Bi2O3-CdO-SiO2, Bi2O3-CdO-B2O3, and Bi2O3-CdO-GeO2 and Their Relation to the Structure of Glass

Areas of glass formation and regions of “neo-ceramic” glasses in the systems Biz03-CdO-Si02, Bi203-Cd0-B203, and Bi203-Cd0-GeO2 are demarcated. Properties of glasses in the neoceramic regions were determined before and after nucleation and heat treatment. There are no maxima or minima in the dielectric properties versus composition curves. The dielectric constants increase after nucleation and heat treatment. The dissipation factors show a remarkable change toward higher values. Transparent glasses with Bi₂O₃. CdO as the predominant constituents have unusual dielectric constants ranging from 30 to 42 and dissipation factors ranging from 3 to 50 × 10⁻⁴., Since these glasses contain only from 0.5 to 2.0 wt% SiO₂ or B₂O₃ or 5 wt% GeO₂, the conventional concept of glass structure composed of a random spacial network of SiO₄ tetrahedra, whose interstices are filled with network modifiers, cannot be sustained in this case. It is suggested that the dominant cations Bi^{3 +} and Cd²⁺ form the network and that the interstices are occupied by Si4 + cations. The function of the Si⁴⁺ ion apparently is to distort the highly polarizable Bi3 + ion enough to form a random network. It appears to be possible that Bi³⁺ and Cd²⁺ can form the network in sixfold coordination. The dielectric losses observed are explained in terms of the relative instability of the structure.     

One-step additive manufacturing and microstructure evolution of melt-grown Al2O3/GdAlO3/ZrO2 eutectic ceramics by laser directed energy deposition

Synchronized-powder-feeding-based laser directed energy deposition (LDED) has great application potential for the rapid fabrication of large-scale composite ceramics with complex shapes. In this study, near-full-density Al₂O₃/GdAlO₃/ZrO₂ ternary eutectic ceramics with different shapes and smooth surfaces were directly prepared by using an improved LDED device. Spherical ceramic powders with eutectic composition and good flowability were obtained by centrifugal spray drying. The microstructure characteristics and microstructure evolution of the rapidly solidified 3D-printed eutectic ceramic were systematically elucidated. In particular, the formation mechanism of the observed periodic banded structures was revealed through a unique laser partial remelting technique. The result indicated that the appearance of the banded structure is attributed to the drastic abnormal coarsening of the nanoscale microstructures adjacent to the molten pool. On the basis these results, a physical model was proposed to illustrate the microstructure evolution of the 3D-printed Al₂O₃/GdAlO₃/ZrO₂ eutectic ceramic.     

Comparison of dry sliding wear behavior of plasma sprayed FeCr slag coating with Cr2O3 and Al2O3-13TiO2 coatings

The microstructure and dry sliding wear performance of thermally sprayed FeCr slag coating were evaluated in comparison with those of commercially available Al₂O₃-13TiO₂ and Cr₂O₃ ceramic coating powders to assess the applicability of FeCr slag (FS) powder, fabricated from industrial waste, as a ceramic top-coating material against wear. Ceramic top coats and underlying NiCoCrAlY bond coats were deposited on AISI 316L samples via atmospheric plasma spraying (APS), and their tribological properties were assessed using a ball-on-disc test rig at room temperature. As a result, FS coating exhibited the lowest worn volume, although it has the lowest surface hardness. Tribolayer formation was observed on the surface of the samples which were subjected to dry sliding wear tests. Delamination type wear is the dominant wear mechanism for Cr₂O₃ and FS coatings, whereas local spallation areas arising from plastic deformation were observed on the surface of Al₂O₃-13TiO₂ coatings. The results suggested the applicability of FS powder as a candidate ceramic top coating material against wear.     

Synthesis and electrochemical properties of HT-LiCo0.8Ni0.2O2 prepared by molten salt synthesis method using 0.59LiNO3-0.41LiOH system

LiCo_0.8Ni_0.2O₂ powders were prepared by molten salt synthesis method using a 0.59LiNO₃-0.41LiOH melt. The physico-chemical properties of the HT-LiCo_0.8Ni_0.2O₂ powders were investigated by powder X-ray diffraction method, field emission scanning electron microscopy, cyclic voltammetry, and charge-discharge cycling. Uniform size nanocrystalline powder (~40 nm) could be obtained at temperature ranges of 280-480 °C. These nanocrystalline powders showed poor electrochemical properties because of presence of the low temperature phase. Pure single-phase HT-LiCo_0.8Ni_0.2O₂ powder could be obtained at 580 °C. Charge-discharge measurements indicated that this exhibited a good capacity and cyclability without further heat treatment at higher temperatures. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8-10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Reactive sintered highly transparent AlON ceramics with Y2O3-MgAl2O4-H3BO3 ternary additive

In this study, highly transparent aluminate oxynitride (AlON) ceramics were prepared via the reactive sintering of Al₂O₃ and AlN powders using a Y₂O₃-MgAl₂O₄-H₃BO₃ ternary sintering additive. The ternary doping process resulted in the efficient preparation of transparent AlON ceramics with small grains and high transmittance as compared to the binary doping (Y₂O₃-MgAl₂O₄) process. The addition of 0.1 wt.% Y₂O₃-0.4 wt.% MgAl₂O₄-0.12 wt.% H₃BO₃ resulted in the formation of a 4-mm-thick AlON ceramic with high transmittance (81% at 600 nm) and low haze (3.46%). This is the best performance in terms of the thickness and transmittance reported for AlON transparent ceramics prepared by the reactive sintering method.     

Effect of Y2O3 addition on tribological properties of plasma sprayed Al2O3-13% TiO2 coating

The effect of Y₂O₃ addition on structure, mechanical properties and tribological properties of Al₂O₃-13 wt% TiO₂ coating was investigated. The addition of 20 wt% Y₂O₃ resulted in better densification, stabilization of alpha (α) alumina phase and improvement in fracture toughness of Al₂O₃-13 wt% TiO₂ coating. Abrasive wear tests were performed over a range of loads and sliding speeds. The stabilization of α alumina phase further increased with an increase in severity of wear test conditions, as noted from X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) analysis of worn coatings. Al₂O₃-13 wt% TiO₂-20 wt% Y₂O₃ coating displayed lower friction coefficient and lower abrasive wear rate than Al₂O₃-13 wt% TiO₂ coating, which was due to synergistic effect of α alumina phase and formation of magneli phase oxide of titanium; Ti₂O₃. Friction energy map was used to rationalize observed wear rates, to identify different regimes of wear and degradation modes of coatings.     

Phase transformation of FeCr2O4 to (Fe,Cr)2O3 solid solution pigment powders: Effect of post-heating temperature

Iron chromite powders were synthesized via solution combustion route using iron(III) nitrate nonahydrate and chromium(III) nitrate nonahydrate as starting materials, as well as glycine–urea, glycine–citric acid, and glycine–ethylene glycol mixtures as fuels. The effect of postheating at different temperatures on the structure, molecular, microstructure, and chromatic properties of powders and tiles colored by in-glaze powders was studied. The X-ray diffraction patterns showed that as-synthesized powders were obtained in crystalline FeCr₂O₄ phases moreover, postheating of the powders led to d-space shift and oxidation and formation of (Fe,Cr)₂O₃ solid solution phase regardless of fuel type. Phase transformation of FeCr₂O₄ to (Fe,Cr)₂O₃ solid solution was observed at 500/750°C depending on the dominant phase of as-synthesized particles. Fourier transform infrared analysis illustrated that the band positions of octahedral M–O and tetrahedral M–O bonds were shifted due to Fe cations movement from their position and lattice shrinkage by increasing of post-heating temperature. Moreover, scanning electron micrographs showed that Fe_0.7Cr_1.3O₃ semispherical fine particles were formed from porous spongy FeCr₂O₄ particles due to oxidation and phase transformation during the postheating. Furthermore, chromatic properties of the samples were represented. The color properties of the pigments showed that the formation of brown pigments is provided with the phase transformation from FeCr₂O₄ to (Fe,Cr)₂O₃ at a temperature of up to 750°C. Moreover, increasing the color purity to this temperature is related to the removal of residual carbonaceous matters. The chromatic properties of the glazed tiles colored using the pigments showed that postheating between 250 and 500°C led to more brown appearance.     

CO2 laser post-treatment of sol-gel-derived Y3Al5O12:Ce phosphor ceramic powders

Cerium-doped yttrium aluminum garnet (Y₃Al₅O₁₂:Ce, YAG:Ce) was prepared using a sol-gel method and then fired for CO₂ laser post-treatments. Phase transformations and formation of impurities were not observed in YAG:Ce after CO₂ laser sintering. The shift of the diffraction peak and the appearance of another Raman peak indicate a more homogeneous distribution of Ce activators and enhanced crystallinity in laser-sintered YAG hosts. Larger spheres (100–200 μm) with tiny crystallites (<10 μm) were observed on the smoother surface in the laser-sintered YAG:Ce, unlike the irregular, porous, and layered powders in the sol-gel-derived YAG:Ce (1–100 μm). Photoluminescence (PL) measurements revealed an emission increase of 180% and a red shift of the emission peak for the laser-sintered YAG:Ce powders compared with the sol-gel-derived powders. Both have comparable thermal PL quenching behavior; however, the YAG:Ce powders with CO₂ laser treatment exhibited a PL efficiency improvement of approximately 4%.     

Thermal Reaction of Cristobalite in Nano‐SiO2/α‐Al2O3 Powder Systems for Mullite Synthesis

Nanoscaled cristobalite and α‐Al₂O₃ powders were used as the starting materials for synthesizing mullite by solid‐state reaction. The thermal reaction of the cristobalite with α‐Al₂O₃ during the thermal treatment was examined. Cristobalite powder with a D₅₀ value of 430 nm was adopted to mix with α‐Al₂O₃ powders with a D₅₀ values of 230, 310, and 400 nm in a stoichiometric composition of 3Al₂O₃?2SiO₂ (71.8 wt% α‐Al₂O₃ and 28.2 wt% SiO₂). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO₂/Al₂O₃ due to the different particle sizes of α‐Al₂O_3. Examinations were performed by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the α‐Al₂O₃ particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer‐sized α‐Al₂O₃ powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the α‐Al₂O₃ particle matrixes. The crystal orientation of the mullite was controlled by the α‐Al₂O₃ matrix, that is, [001] α‐Al₂O₃ → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO₂ with α‐Al₂O₃, initiating the nucleation of mullite. The α‐Al₂O₃ particles act as the hosts for mullite formation and determine the size of the mullite particles.     

White luminescence of bismuth and samarium codoped Y2O3 phosphors

In this work Sm³⁺ (0–2.0 at%) and Bi³⁺ (0–2.0 at%) doped Y₂O₃ luminescent powders were prepared by a sol–gel method from yttrium acetylacetonate, samarium and bismuth nitrates as metal sources. The as prepared powders (chemical composition is close to stoichiometric Y₂O₃) present the cubic structure from 700 °C, and at 900 °C are characterized by the presence of rounded particles with heterogeneous size of 42.9 nm. Luminescent effect of ions of Sm³⁺ and Bi³⁺ into Y₂O₃ host as was studied on heat treated powders from 800 to 1100 °C. The combination of the red luminescence from the Sm³⁺ ions and the bluish from Bi³⁺, makes the synthesized phosphors candidates to be used in fabrication of phosphor-converted light-emitting diodes (LEDs).     

Dispersion efficiency of powders of the Al2O3-(ZrO2+Y2O3) system obtained by the plasma-chemical method

The method of electron microscopy is used to study the morphology and fraction composition of powders of the Al₂O₃-(ZrO₂+Y₂O₃) system subjected to mechanical, electric pulse, and explosion dispersion caused by the dosed addition of ammonium nitrate into the reaction zone during the oxide synthesis. The explosion method is given preference. The use of powders dispersed by the explosion method markedly improves the properties of sintered ceramics.Translated from Steklo i Keramika, No. 8, pp. 23–24, August, 1995.The authors are grateful to A. I. Slosman for cooperation in preparing this work.     

Tailoring the efficient laser-absorption-melting behavior of Bi2O3-B2O3-ZnO-Nd2O3 glass for the OLED encapsulation

The efficient 810 nm laser energy conversion of glass frit had been proven to be the key to the long-term hermetic encapsulation of Organic Light Emitting Display (OLED). A direct laser energy conversion laser-assisted Bi₂O₃-B₂O₃-ZnO-Nd₂O₃ sealing glass material without extra laser absorbent such as carbon black, was designed and systematically investigated. The addition of Nd₂O₃, as glass modifiers with higher cationic field strength, could be conducive to enhancing the polymerization of glass network structure, manifesting that the glass-transition temperature Tg, onset-crystallization temperature Tc and thermal stability ΔT (ΔT = Tc-Tg) increased, while thermal expansion coefficient CTE dropped to 9.72×10⁻⁶/°C and advantageously matched with the glass substrate (8±1×10⁻⁶/°C). More importantly, the absorption rate of BBZ-Nd glass was more than 50 % between 800∼810 nm owing to the 4f-4f electron transition of Nd³⁺ ions, and yet the reflectivity and transmittance of the wavelength at 800–810 nm were lower. As optimal compositions, the addition of 3.0 wt% Nd₂O₃ in Bi₂O₃-B₂O₃-ZnO-Nd₂O₃ glass frit with higher absorption coefficients (80 %) led to instantaneous bonding encapsulation between glass substrates without interfacial cracks or pores with the 808 nm wavelength of the laser at 20 W and 2.4 mm/s.     

Ultrasonic compacting of zirconium ceramics from ultradispersed powders

A method for compacting ultradisperse powders (UDP) of ZrO₂-3% Y₂O₃ by radially transmitted ultrasonic vibrations (USV) by imposing static pressing pressure is developed. The optimized USV regimes provide compacts from UDP with optimum properties.     

Enhancement of magnetic and dielectric properties of KNbO3–CoFe2O4 multiferroic composites via thermal treatment

In this work, multiferroic composites were produced from CoFe₂O₄ and KNbO₃ mixtures via control of the heat treatment temperature. For this, CoFe₂O₄ nanoparticles were produced by sol-gel method, while KNbO₃ was synthesized by microwave-assisted hydrothermal synthesis. The powders were homogenized and subjected to heat treatment at 300, 400 and 500 °C for 5 h. The structural, electrical and magnetic properties were characterized. The results of X-ray diffraction indicated that there was no formation of secondary phases with heat treatment. Raman vibrational modes confirmed the presence of KNbO₃ and CoFe₂O₄ in the prepared composites. SEM analysis showed that the composite microstructure consists of smaller ferrite particles arranged on the surface of largest cubic KNbO₃ particles. The improvement of coercivity (H_C = 382.1Oe) and dielectric constant (?’~7860) was observed for the composite thermally treated at 300 °C. The obtained results show the potential application of KN:CFO composites for multifunctional devices.