Achieving fabrication of highly transparent Y2O3 ceramics via air pre-sintering by deionization treatment of suspension

An easy albeit quite effective deionization suspension treatment was adopted to alleviate the detrimental effects related to the hydrolysis of Y₂O₃ in an aqueous medium. Fabrication of highly transparent Y₂O₃ ceramics with a fine grain size via air pre-sintering and post–hot isostatic pressing (HIP) treatment without using any sintering additive was achieved using the treated suspensions. The hydrolysis issue of Y₂O₃ powder in an aqueous medium was effectively alleviated by using deionization treatment, and a well-dispersed suspension with a low concentration of dissolved Y³⁺ species was obtained. The dispersed suspensions were consolidated by the centrifugal casting method, and the green bodies derived from the suspension of 35.0 vol% solid loading showed an improved homogeneity with a relative density of 52.1%. Fully dense Y₂O₃ transparent ceramic with high transparency was obtained by pre-sintering consolidated green compacts at a low temperature of 1400°C for 16 h in air followed by a post-HIP treatment at 1550°C for 2 h under 200 MPa pressure. The sample had a fine average grain size of 690 nm. The in-line transmittance of the sample reached 83.3% and 81.8% at 1100 nm and 800 nm, respectively, very close to the theoretical values of Y₂O₃.     

Influences of Solid Loadings on the Microstructures and the Optical Properties of Yb:YAG Ceramics

Transparent Y_2.85Yb_0.15Al₅O₁₂ ceramics were prepared using an aqueous tape casting and vacuum sintering method. The rheological properties were measured by a rheometer. The results indicate high quality tapes, and ceramics can be obtained by increasing the solid loading of the corresponding slurries. The densities of the tapes increase from 2.42 to 2.69 g/cm³ by increasing the solid loadings from 35 to 50 vol%. The corresponding green body densities range from 52.7 to 57.1% of the theoretical. The solid loading suitable for fabricating transparent Yb:YAG ceramics should be higher than 45 vol%.     

ZrO2-doped Y2O3 transparent ceramics via slip casting and vacuum sintering

Commercial Y₂O₃ powder was used to fabricate highly transparent Y₂O₃ ceramics with the addition of ZrO₂ via slip casting and vacuum sintering. The effects of ZrO₂ addition on the transparency, grain size and lattice parameter of Y₂O₃ ceramics were studied. With addition of ZrO₂ the transparency of Y₂O₃ ceramics increased markedly and the grain size of Y₂O₃ ceramics decreased markedly by cation diffusivity mechanism and the lattice parameter of Y₂O₃ ceramics slightly decreased. The highest transmittance (at wavelength 1100 nm) of the 5.0 mol% ZrO₂–Y₂O₃ ceramic (1.0 mm thick) sintered at 1860 °C for 8 h reached 81.7%, very close to the theoretical value of Y₂O₃.     

Effect of solid loading on properties of Y2O3–Al2O3–Nd2O3 powder mixtures obtained by planetary ball milling and ceramics based on them

The effect of the solid loading (41–50 wt%) of the slurry on granulometric composition and physico-chemical characteristics of Y₂O₃–Al₂O₃–Nd₂O₃ powder mixtures obtained by planetary ball milling has been studied for the first time. It was shown that the particle size distribution of powder, its Zeta potential, and specific surface area depend on the solid loading of the milled slurry and, consequently, on the interparticle distance during milling. The interparticle distance decreases from 200 nm to 142 nm with an increase of solid loading in the range of 41–50 wt%. It was shown that for the solid loading of 47 wt%, the convergence of particles to a distance comparable to their median diameter promotes subsequent clustering of particles. This facilitates the sintering of highly-homogenous ceramics. It was found that solid loadings in the 46–50 wt% range is useful for obtaining high-quality Nd:YAG transparent ceramics. The lowest optical losses optical losses of 1 × 10^?3 cm^?1 and the highest in-line transmittance of 84.1%@1064 nm were obtained for 1 at.% Nd:YAG transparent ceramics (22 × 3 × 4 mm³) prepared from slurries with 47 wt% solid loading (taking all other ball milling parameters fixed). If the interparticle distance in the powder is higher (solid loading of 41 wt%) than the median particle diameter, the ceramics are characterized by significant residual porosity due to the survival of large particles (insufficient milling).     

Effects of starting powder on microstructure and thermal conductivity of pressureless-sintered fully ceramic microencapsulated fuels

The effects of the starting SiC powder (α or β) with the addition of 5.67 wt% AlN–Y₂O₃–CeO₂–MgO additives on the residual porosity and thermal conductivity of fully ceramic microencapsulated (FCM) fuels were investigated. FCM fuels containing ～41 vol% and ～37 vol% tristructural isotropic (TRISO) particles could be sintered at 1870 °C using α-SiC and β-SiC powders, respectively, via a pressureless sintering route. The residual porosities of the SiC matrices in the FCM fuels prepared using the α-SiC and β-SiC powders were 1.1% and 2.3%, respectively. The thermal conductivities of FCM pellets with ～41 vol% and ～37 vol% TRISO particles (prepared using the α-SiC and β-SiC powders, respectively) were 59 and 41 Wm^?1K^?1, respectively. The lower porosity and higher thermal conductivity of FCM fuels prepared using the α-SiC powder were attributed to the higher sinterability of the α-SiC powder than that of the β-SiC powder.     

Structural and Thermo‐Mechanical Properties of Nd: Y2O3 Transparent Ceramics

Various content of neodymia Nd: Y₂O₃ (Nd: 0.5–5.0 at.%) transparent ceramics were fabricated by vacuum sintering. The prepared Nd: Y₂O₃ ceramics exhibit high transmittance (~80%) at the wavelength of 1100 nm. It is found that the increase in Nd concentration enhances the grain size growth, while decreases the phonon energy, which is benefit for improving both the luminescence quantum and up‐conversion efficiency. The thermal conductivity and thermal expansion coefficient of the transparent 1.0 at.% Nd: Y₂O₃ ceramic is 5.51 W·(m·K)^?1 and 8.11 × 10^?6 K^?1, respectively. The hardness and the fracture toughness of the transparent ceramic is 9.18 GPa and 1.03 Mpa·m^1/2, respectively. The results indicate that the Nd: Y₂O₃ transparent ceramic is a potential candidate material for laser.     

Synthesis of Submicron Nanocrystalline Y<Subscript>2</Subscript>O<Subscript>3</Subscript>: Eu Particles via Solvothermal Approach Using Surface Modifiers

In this paper, solvothermal synthesis of submicron nanocrystalline Y₂O₃: Eu particles with and without surface modifier (β-alanine and Tween-80) is investigated. X-ray diffraction ananlysis confirms the one-step formation of Y₂(OH)₅NO₃H₂O phase during solvothermal process and its conversion to Y₂O₃: Eu after heat treatment at 600 °C. Fourier transformation infrared spectroscopy showed that C=C, C–C and C–H peaks are corresponded to the surface modifiers i.e. tween-80 and β-alanine. Scanning electron microscopy and transmission electron microscopy images also showed that the modifier results in the particle morphology improvement from sheet-like to submicron spherical particles. Photoluminescence experiments indicated that the emission intensity increases due to the morphology modification.     

Dispersion of nano-sized yttria powder using triammonium citrate dispersant for the fabrication of transparent ceramics

In the present work, transparent Y₂O₃ ceramics were prepared via colloidal processing method from nano-sized Y₂O₃ powders. The effects of triammonium citrate (TAC) on the colloid stability of aqueous suspensions of nano-sized Y₂O₃ powders were studied. The surface properties of yttria powders were notably affected by the addition of TAC dispersant. The adsorption of TAC on the particle surface shifts the IEP to lower pH values and increases the absolute zeta potential in alkaline region. Rheological characterization of the investigated system revealed an optimal dispersant concentration of 1 wt%, which correlated well with the saturation adsorption of TAC on Y₂O₃ powder surfaces. The suspensions with solid loadings up to 35 vol% were achieved with further addition of Tetramethylammonium hydroxide (TMAH) into the dispersing system. The consolidated green bodies were treated by cold isostatic pressing to further increase the green density. Transparent Y₂O₃ ceramics were prepared after vacuum-sintering at 1700 °C for 5 h. The transmittances of the sample were 74.5% at 800 nm and 79.8% at 2000 nm, respectively.     

Translucent yttrium oxide ceramics from low-density green bodies shaped by uniaxial pressing

Highly compact green bodies are usually required to fabricate transparent or translucent polycrystalline yttrium oxide. The present work reports on the fabrication of highly dense (ρ_rel >98%) translucent Y₂O₃ from low-density green bodies by exploiting a high sinterability of nano Y₂O₃ powder. A commercial nano Y₂O₃, with a crystallite size of ca. 30 nm, was shaped by uniaxial pressing and, afterwards, densified by pre-sintering at 1500 °C followed by hot isostatic pressing. The coarsening tendency of yttria nanoparticles causes the rearrangement of particles and results in the high sinterability of nano Y₂O₃ even at low green densities. This improves the packing density and enables sintering to high densities, eliminating pores, and, thus, obtaining translucent Y₂O₃.     

Preparation of silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction and liquid phase sintering

Dense silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction method and liquid phase sintering was reported. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) and ammonium carbonate ((NH₄)₂CO₃) were used as acid and base solutions to treat the silicon carbide powder, respectively. Influence of silicon carbide powder with chemical treatment on the preparation of silicon carbide suspension was studied. It was indicated that 50 vol% and 52 vol% silicon carbide suspensions with viscosities of 0.71 Pa s and 0.80 Pa s could be prepared using acid and base treated powders. Influence of silicon carbide powder with chemical treatment on the coagulation process and properties of green bodies and sintered ceramics were studied. It was indicated that silicon carbide green bodies with compressive strength of 1.13 MPa could be prepared using base treated powder. Dense silicon carbide ceramics with relative density above 99.3% and flexural strength of 697 ± 30 MPa had been prepared by DCC via dispersant reaction and liquid phase sintering using Al₂O₃ and Y₂O₃ as additives at 1950 °C for 2 h.     

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y₂O₃ addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 10¹ Ω·cm) that is larger by a factor of ~10² compared to that (1.3 × 10^?1 Ω·cm) of a baseline specimen sintered with Y₂O₃ only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10^?1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10^?2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.     

A novel approach of synthesizing nano Y2O3 powders for the fabrication of submicron IR transparent ceramics

A facile methodology of synthesizing highly reactive, round-edged, Sulfur–free nano Y₂O₃ powders to fabricate submicron IR transparent yttria ceramics having a unique combination of superior optical and mechanical properties are reported for the first time. Dispersion of yttrium hydroxide into aqueous sol and addition of seed particles produced near-spherical yttria powders having non – aggregated particles with narrow size distribution. The powder exhibited excellent sinterability reaching near-theoretical density at temperatures around 1400 °C in air. Effective inter-particle coordination and traces of Al additives assisted achieving superior densification. Sintered specimens showed average grain sizes closer to 700 nm. Post-sinter hot isostatic pressing eliminated the residual porosity from the sintered samples leading to exhibit IR transmissions up to 84% in the 2.0–9.0 μm regions, equivalent to single crystal Y₂O₃. Achieving densification through solid-state sintering and retaining the sintered grain sizes in the submicron regions significantly enhanced the mechanical properties. Sintered and HIPed Y₂O₃ specimens were further characterized for their thermal properties at temperature regions between ambient to 950 °C.     

Critical thickness of polymer-derived ceramic coatings with particulate fillers

In this study, we demonstrate a novel environmental barrier coating processed from polymer-derived ceramics (PDCs) with homogeneously distributed sub-micrometer Y₂O₃ as the filler. Under suitable conditions, dense and crack-free coatings can be achieved for all the designed compositions with the volumetric content of Y₂O₃ varied from 45 to 93 vol%. To process the PDC SiC–Y₂O₃ composite coatings, Y₂O₃ particles and SiC liquid precursor were uniformly dispersed in hexane and then dip-coated on SiC substrates. After cross-linking at 250°C and heat-treated at 900°C in argon, dense and crack-free PDC SiC–Y₂O₃ composite coatings were formed. The effect of coating thickness and heat-treatment temperature on the formation of cracks due to constrained pyrolysis was studied. The critical thickness for realizing crack-free coatings of three compositions (i.e., 93, 77, and 45 vol% Y₂O₃) was studied for heat treatment from 1000 to 1300°C using atomic force microscope and scanning electron microscopy. As heat-treatment temperature increases, the critical coating thickness decreases for the same coating compositions due to enhanced shrinkage at higher temperature. With higher Y₂O₃ content, the critical thickness of the coating increased. The inert Y₂O₃ particles reduce the amount of polymer leading to reduction in the overall constrained shrinkage of the coating during heat treatment.     

Coating Y2O3 nano-particles with ZrO2-additive via precipitation method for colloidal processing of highly transparent Y2O3 ceramics

In the present work, transparent Y₂O₃ ceramics were prepared via colloidal processing method using ZrO₂-coated nano-sized Y₂O₃ powders. The chemical precipitation method was adopted for the coating of Y₂O₃ raw powder. The evolution of the coated-ZrO₂ layer upon calcination was studied. The rheological behaviors of the slurries of Y₂O₃ powders coated with different content of ZrO₂-additive were investigated. The pH_IEP of ZrO₂-coated Y₂O₃ powders shows intermediate values between that of raw Y₂O₃ and ZrO₂ powders. As the ZrO₂-coating concentration increased from 0 to 5.0 at%, the magnitude of the negative zeta potential at pH > pH_IEP shows a general trend of increment, whereas it decreased at pH < pH_IEP. The viscosity decreases pronouncedly with the increase of ZrO₂ content from 0.5 at% to 3.0 at%. The suspensions with low viscosity and high stability was achieved for a solid loading of 35.0 vol% using Y₂O₃ powders coated with 5.0 at% ZrO₂. The dispersed suspensions were consolidated by centrifugal casting method and the green bodies shown improved homogeneity. Transparent Y₂O₃ ceramics were fabricated by vacuum sintering at 1800 ℃ for 5 h. Transmittance at wavelength 800 nm (1.0 mm thick) reached 80.8%, close to the theoretical value of Y₂O₃.     

Effect of La2O3 on the microstructure and optical performance of (Tb0.8Y0.2-xLax)2O3 ceramics

(Tb_0.8Y_0.2-xLa_x)₂O₃ transparent ceramics were prepared by using co-precipitation method combined with pressure-less sintering in flowing H₂ atmosphere. Microstructure, optical transmittance, elements composition, and Verdet constant of the (Tb_0.8Y_0.2-xLa_x)₂O₃ ceramics were studied. The amount of La₂O₃ is crucial for the formation of expected transparent (Tb_0.8Y_0.2)₂O₃. With increasing content of La₂O₃, the number of pores and the grain size of as-fabricated (Tb_0.8Y_0.2-xLa_x)₂O₃ ceramics both decrease. When 4 at.% La₂O₃ is doped, the (Tb_0.8Y_0.16)₂O₃ transparent ceramics shows the highest transmittance of 73.3% at 1400 nm wavelength. With holding time increasing from 8 h to 15 h, the average grain size of (Tb_0.8Y_0.16La_0.04)₂O₃ ceramics gradually increases from 5 μm to 13 μm. The Verdet constant measured at 633 nm is ?352 rad/T·m, which is 2.63 times higher than that of TGG. In addition, large-size ceramics with Φ 20 mm × 3 mm and Φ 30 mm × 3 mm were also successfully obtained.     

A Sucrose‐Mediated Sol–Gel Technique for the Synthesis of MgO–Y2O3 Nanocomposites

A sucrose‐mediated aqueous sol–gel procedure was developed to synthesize MgO–Y₂O₃ nanocomposite ceramics for potential optical applications. The synthesis involves the generation of a precursor foam containing Mg²⁺ and Y³⁺ cations via the chemical and thermal degradation of sucrose molecules in aqueous solution. Subsequent calcination and crushing of the foam gave MgO–Y₂O₃ nanocomposites in the form of thin mesoporous flake‐like powder particles with uniform composition and surface areas of 27–85 m² g ^{? 1}, depending on calcination conditions. The flakes exhibited a homogeneous microstructure comprising intimately mixed nanoscale grains of the cubic MgO and Y₂O₃ phases. This microstructure was resistant to grain coarsening with average grain sizes of less than 100 nm for calcination temperatures of up to 1200°C. The results indicate that the sucrose‐mediated sol–gel process is a simple effective method for making nanoscale mixed oxides.     

Highly transparent AlON ceramics sintered from powder synthesized by carbothermal reduction nitridation

Aluminum oxynitride (AlON) powders were synthesized by the carbothermal reduction and nitridation process using commercial γ-Al₂O₃ and carbon black powders as starting materials. And AlON transparent ceramics were fabricated by pressureless sintering under nitrogen atmosphere. The effects of ball milling time on morphology and particle size distribution of the AlON powders, as well as the microstructure and optical property of AlON transparent ceramics were investigated. It is found that single-phase AlON powder was obtained by calcining the γ-Al₂O₃/C mixture at 1550 °C for 1 h and a following heat treatment at 1750 °C for 2 h. The AlON powder ball milled for 24 h showed smaller particles and narrower particle size distribution compared with the 12 h one, which was benefit for the improvement of optical property of AlON transparent ceramics. With the sintering aids of 0.25 wt% MgO and 0.04 wt% Y₂O₃, highly transparent AlON ceramics with in-line transmittance above 80% from visible to infrared range were obtained through pressureless sintering at 1850 °C for 6 h.     

Rapidly fabricating Y2O3 transparent ceramics at low temperature by SPS with mesoporous powder

Fine-grained and dense highly transparent Y₂O₃ ceramics have been successfully prepared using high sintering activity mesoporous Y₂O₃ powders without any additive by spark plasma sintering (SPS). The influences of the sintering temperature on microstructure, density, optical, and mechanical properties of SPS-sintered Y₂O₃ ceramics were studied in detail. As results, the optimal Y₂O₃ ceramics with high relative density of 99.90% and fine average grain size of 140 nm were obtained at a low sintering temperature of 1140°C and a moderate load pressure of 60 MPa for 5 min. Meanwhile, the dense Y₂O₃ ceramics with 1 mm thickness after annealing show a high linear transmittance of 78% (close to 94% of the theoretical value) at 2.4–3 µm wavelength. In additions, the Vickers hardness and fracture toughness of samples can reach 8.48 GPa and 1.45 MPa m^1/2, respectively. This result proves that the high activity of mesoporous Y₂O₃ is considered to be an important means for preparing high-performance fine Y₂O₃ ceramics at low sintering temperature.     

The effects of microstructure on mechanical and electrical properties of W dispersed Al2O3 ceramics

This work aims to enhance the fracture toughness of brittle Al₂O₃ ceramics and apply insulated Al₂O₃ ceramics with electrical conductivity by dispersing second tungsten (W) metal particles. In order to investigate the effects of W dispersion on mechanical and electrical properties, Al₂O₃–W composites with various amounts of W (ranging from 5 vol% to 20 vol%) were fabricated by the hot-press sintering method at various sintering temperatures. Microstructure analysis revealed submicron Al₂O₃ matrix grains and W particles. The existence of three phases of Al₂O₃, W, and AlWO₄ was confirmed by X-ray diffraction patterns. All Al₂O₃–W composites showed higher fracture toughness than monolithic Al₂O₃. The toughening mechanism was attributed to crack deflection and crack bridging. Transgranular fracture was visible in all composites. Electrical resistivity dramatically lowered from 2.9 × 10¹² Ω cm of monolithic Al₂O₃ to 4.1 × 10² Ω cm of the composite with 20 vol% W addition. The percolation threshold is calculated as 18.5%. With the increase in sintering temperature, the amount of W particles was decreased and Al₂O₃ grains became large, leading to the reduced number of conductive pathways formed by the dispersed W particles. As a result, electrical conductivity was decreased.     

The influence of nano alumina additions on the coefficient of thermal expansion of a LZS glass–ceramic composition

In this work a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (mol%) glass–ceramic matrix was prepared and milled in order to determine its coefficient of thermal expansion (CTE) and to study how it is influenced by the addition of nanosized Al₂O₃ particles (1–5 vol%) and submicrometric Al₂O₃ particles (5 vol%). Comminution studies from the LZS parent glass frit showed that a powder with an adequate particle size (3.5 µm) is achieved after 120 min of dry milling followed by a second step of 60 h wet milling. The obtained LZS glass–ceramic samples (fired at 900 °C/30 min) showed an average relative density of ∼98% with zirconium silicate and lithium disilicate as main crystalline phases. Prepared composites with 1, 2.5 and 5 vol% of nanosized Al₂O₃ and 5 vol% submicrometric Al₂O₃ showed average relative densities varying from 97% to 94% as the alumina content increased. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 9.5 to 4.4×10⁻⁶ °C⁻¹. Composites with 5% nanosized alumina showed a CTE lower than that of the equivalent formulation with submicrometric alumina.