Hot Pressing of Infrared‐Transparent Y2O3–MgO Nanocomposites Using Sol–Gel Combustion Synthesized Powders

A sol–gel combustion method has been used to synthesize Y₂O₃–50 vol%MgO composite nanopowders. Solutions of the precursor nitrates were mixed with citric acid and ethylene glycol, heated from 200°C to a predetermined temperature gradually, giving nanocrystalline ceramic powders. The influence of the ratio of yttrium nitrate to the whole precursor mixture and the holding temperature on the properties of the composite nanopowder was investigated using a combination of thermal analysis, X‐ray diffraction, specific surface area analysis, and scanning electron microscopy techniques. When the ratio of yttrium nitrate to the whole precursor mixture reaches 22.5 mol%, the average particle size of synthesized composite nanopowder is 13 nm and the specific surface area is 45.9 m²/g. Then the synthesized Y₂O₃–MgO composite nanopowder was consolidated by the hot‐pressing technique at 1200°C with different dwell time. As a result, the nanocomposite ceramic prepared with a dwell time of 60 min got the highest transmittance of 75% at 5 μm wavelength. The cut‐off wavelength of Y₂O₃–MgO nanocomposite ceramic reaches 9.8 μm, which is superior to other mid‐IR transparent materials. In addition, the fabricated sample is more or less transparent in visible wavelengths and the transmittance at 0.8 μm is as high as 14.5%.     

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.     

Fabrication of Gd2O3‐MgO nanocomposite optical ceramics with varied crystallographic modifications of Gd2O3 constituent

The fabrication of Gd₂O₃‐MgO nanocomposite optical ceramics via hot‐pressing using sol‐gel derived cubic‐Gd₂O₃ and MgO nanopowders was investigated. The precursor powder calcined at 600°C had an average particle size of 12 nm. The effects of hot‐pressing temperature on constituent phases, microstructure, mid‐infrared transmittance, and microhardness were studied. The crystallographic modifications of Gd₂O₃ phase varied with the increase in sintering temperature from 1250 to 1350°C. The monoclinic‐Gd₂O₃ phase was retained for the composite sintered at 1350°C and the sample had an average grain size of 90 nm, excellent transmission (80.4%‐84.8%) over 3‐6 μm wavelength range, and enhanced hardness value of 14.1 GPa.     

High Reactive MgO‐Y2O3 Nanopowders via Microwave Combustion Method and Sintering Behavior

To decrease the sintering temperature of MgO‐Y₂O₃ composites to avoid undesired grain coarsening, high reactive MgO‐Y₂O₃ nanopowders were synthesized via microwave combustion method. The degree of combustion was enhanced effectively by adding an extra oxidant ammonium nitrate. The as‐synthesized MgO‐Y₂O₃ nanopowders, ~18 nm in size, showed high specific surface area of 64.55 m²/g and low agglomeration. Relative density of 98% was obtained when sintered at a low sintering temperature of 1350°C. The high reactivity can be attributed to the lower activation energy Q (131.13 kJ/mol), compared with samples without extra oxidant (192.97 kJ/mol).     

Preparation and study of the mechanical and optical properties of infrared transparent Y2O3–MgO composite ceramics

In this study, fine Y₂O₃–MgO composite nanopowders were synthesized via the sol–gel method. Dense Y₂O₃–MgO composite ceramics were fabricated by pre-sintering the green body in air at different temperatures for 1 h and then subjecting the sintered bodies to hot isostatic pressing at 1300°C for 1 h. The effects of pre-sintering temperature on the microstructural, mechanical, and optical properties of the resulting ceramics were studied. The average grain size of the ceramics was increased, whereas their hardness and fracture toughness were decreased with increasing pre-sintering temperature. A maximum fracture toughness of 1.42 MPa·m^1/2 and Vickers hardness of 10.4 GPa were obtained. The average flexural strength of the ceramics was 411 MPa at room temperature and reached 361 MPa at 600°C. A transmittance of 84% in the 3–5 µm region was obtained when the composite ceramics were sintered at 1400°C. Moreover, a transmittance of 76% in the 3–5 µm region was obtained at 500°C.     

Influence of synthesis conditions on the properties of Y2O3–MgO nanopowders and sintered nanocomposites

In this study, a citrate–nitrate combustion method was applied to synthesize composite Y₂O₃–MgO nanopowders. In order to optimize the synthesis condition to support sufficient combustion, the molar ratio of citric acid to nitrate (c/n molar ratio) used in the reaction mixtures was varied between 0.17 and 0.34. Nanopowders with an average particle size of 17 nm were achieved. The properties of these nanopowders indicated that the higher molar ratios decreased the unreacted organic components and increased the amount of carbide on the surface of the oxides, which helped to inhibit the formation of carbonate groups. The amount of carbonate groups was reduced with the increasing c/n molar ratio. Y₂O₃–MgO nanocomposites fabricated through hot-isostatic-pressing sintering showed a uniform distribution of Y₂O₃ and MgO grains, which had an average size of ∼180 nm. In addition, the absorption peaks at 1410 and 1511 cm⁻¹ disappeared until the c/n molar ratio reached 0.28. A high average infrared transmittance of 83% in the range of 4000–1667 cm⁻¹ (2.5–6 μm) was obtained in the nanocomposites.     

Enhanced near-infrared transmission of ZnO-doped Y2O3–MgO nanocomposites with reduced light scattering due to decreased refractive index difference

Y₂O₃–MgO nanocomposites have received attention owing to their high optical and mechanical properties. However, the inevitable light scattering stemming from the refractive index difference between the two phases limits their applications in the near-infrared region. In this study, the grain boundary light scattering was reduced by doping ZnO into MgO. Y₂O₃–Mg_1?xZn_xO nanocomposites were fabricated by hot-press sintered nanopowders synthesized via the sol-gel combustion process. The addition of ZnO reduced the sintering temperature by almost 300 °C and reduced the average grain size by more than 50 nm. Transmittance of 75%–85% was maintained for all samples in the wavelength range of 2–6 µm. The near-infrared cut-on wavelength shifted from 1222 to 843 nm when the ZnO concentration was up to 25 mol%. This work demonstrates the potential of ZnO-doped Y₂O₃–MgO nanocomposites as infrared transparent ceramics over a wide infrared transmission range.     

Synthesis of highly-infrared transparent Y2O3–MgO nanocomposites by colloidal technique and SPS

Infrared (IR) transparent Y₂O₃–MgO nanocomposites with a volume ratio of 50:50 were synthesized by combining colloidal and spark-plasma-sintering (SPS) techniques. In order to attain well-dispersed and homogeneous starting Y₂O₃–MgO nanopowder mixture, the effects of the pH value and the amount of polyetherimide (PEI) dispersant on the suspension stability were studied. Rheological measurement reveals that highly-dispersed and stable suspension was obtained at 7 wt% of PEI dispersant under pH = 10.6. The obtained nanopowders with particle size of 20–30 nm were densified using SPS at several sintering temperatures. The sintered composites show fine grains, narrow grain size distribution and uniform microstructure. The nanocomposite sintered at 1250 °C showed the maximum IR transmittance of 84% at a wavelength range of 2.5–6 μm. The Vickers hardness of the nanocomposite was about 11.9 ± 0.3 GPa, which is significantly higher than those of single phase MgO or Y₂O₃. Successful fabrication of the high-performance Y₂O₃–MgO nanocomposite indicates that i) the colloidal technique is an effect method to obtain highly dispersed and homogeneous nanopowders and ii) the SPS technique is a powerful tool to fabricate fine-grained dense transparent ceramics, which are suitable for fabricating IR transparent Y₂O₃–MgO composite ceramics from commercial starting powders.     

Effect of MgO and Y2O3 Doping on the Formation of Core–Shell Structure in BaTiO3 Ceramics

This study investigates the effects of doping BaTiO₃ with MgO and Y₂O₃ on the formation of core–shell structure. The MgO and Y₂O₃ enhanced the shrinkage upon sintering and inhibited the grain growth, respectively. However, increasing the amount of Y₂O₃ to 3.0 mol% suppressed the shrinkage upon sintering. The results of the diffusion experiment revealed that Y³⁺ was dissolved in the BaTiO₃ lattice to a depth of 5–10 nm inside the grains, whereas Mg²⁺ tended to remain close to the surfaces of the grains when sintered at 1150°C for 18 h, suggesting that Y³⁺ may have had a higher diffusion rate than Mg²⁺. The Mg²⁺ prevented the diffusion of Y³⁺ into the core during sintering. Therefore, Mg²⁺ plays an important role as a shell maker in the formation of the core–shell structure in the codoped system. The core–shell structure can be obtained in BaTiO₃ ceramics that are codoped with MgO and Y₂O₃ upon sintering at 1150°C for 3 h.     

Optimization of particle size,dispersity, and conductivity of 8 mol% Y2O3 doped tetragonal zirconia polycrystalline nanopowder prepared by modified sol-gel method via activated carbon absorption

8 mol% Y₂O₃ doped tetragonal zirconia polycrystalline (8Y-TZP) ceramic nanopowders were synthesized via a novel modified sol-gel method employing zirconium carbonate basic as zirconium resources. The activated carbon as a dispersant was added to the precursor solution during the formation of the sol. The phase behavior, thermal decomposition, microstructure morphology, and electrochemical performance of nanopowders with the addition of activated carbons were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), particles size distribution, and electrochemical impedance spectroscopy analysis (EIS). After adding the activated carbon, the average crystallite size of 8Y-TZP nanopowders decreased from about 53.16–33.51 nm when calcined at 900 ℃, and the 8Y-TZP nanopowders were produced loosely agglomerated. Meanwhile, compacts prepared by pressing the as-obtained 8Y-TZP nanopowders sintered to 98.8% relative density at 1600 ℃ and exhibited an average grain size of 0.89 µm, which brought a positive effect on ionic conductivity (0.079 S·cm^?1).     

Mid‐IR to Visible Photoluminescence,Dielectric, and Ferroelectric Properties of Er‐Doped KNLN Ceramics

Two mole percentage Er‐doped (K_0.5Na_0.5)_1 ? xLi_xNbO₃ ceramics have been prepared and their dielectric, ferroelectric, and photoluminescence (PL) properties have been investigated. Under an excitation of 980 nm, the ceramics exhibit intense up‐conversion luminescent emission at 548 nm (green), weak emission at 660 nm (red) as well as strong down‐conversion luminescent emission in near‐infrared (NIR) (1.40–1.65 μm) and mid‐infrared (2.60–2.85 μm) regions. Probably due to the induced structure distortion and reduced local symmetry, the PL intensities of the green, red as well as mid‐infrared emissions are enhanced by the doping of Li⁺. Our results show that the Li‐doping is effective in establishing a dynamic circulatory energy process to further enhance the PL intensity of the mid‐infrared emission at the expense of the NIR emission. At the optimum doping level of Li⁺ (~6 mol%), the full bandwidth at half maximum of the mid‐infrared emission reaches a very large value of ~250 nm. The ceramics also exhibit good ferroelectric properties, and thus they should have great potential for multifunctional optoelectronic applications.     

A Novel Top‐Seeded Infiltration Growth Technique for Fabricating Y–Ba–Cu–O Single‐Grain Superconductor Nano‐Composites

Top‐seed infiltration and growth technique (TSIG) is proposed to fabricate Y–Ba–Cu–O (YBCO) single‐grain superconductor nano‐composites, in which a solid source composition of nano‐Y₂O₃ + BaCuO₂ and a liquid source composition of Y₂O₃ + 10BaO + 16CuO are employed. As can be seen, this novel technique uses just one source of precursor powder of BaCuO₂, so it is more simplified and efficient. Microstructural observation indicates that fine Y₂BaCuO₅ (Y‐211) inclusions with a size from dozens of nanometers to about one hundred nanometers are successfully introduced in YBa₂Cu₃O_7?x (Y‐123) superconducting matrix, which can act as more effective pinning centers for improving the bulk performance. Superconducting property measurement shows that, a maximum trapped field of 0.36044 T is present at the center of the sample after magnetization by a permanent magnet (B = 0.5 T). These results prove that our proposed TSIG technique is a practical method for fabricating YBCO bulk superconductor nano‐composites with high performance.     

Er3+ ‐Doped Y2O3 Nanophosphors for Near‐Infrared Fluorescence Bioimaging Applications

Rare‐earth‐doped ceramic nanophosphor (RED‐CNP) materials are promising near‐infrared (NIR) fluorescence bioimaging (FBI) agents that can overcome problems of currently used organic dyes including photobleaching, phototoxicity, and light scattering. Here, we report a NIR–NIR bioimaging system by using NIR emission at 1550 nm under 980 nm excitation which can allow a deeper penetration depth into biological tissues than ultraviolet or visible light excitation. In this study, erbium‐doped yttrium oxide nanoparticles (Er³⁺:Y₂O₃) with an average particle size of 100 and 500 nm were synthesized by surfactant‐assisted homogeneous precipitation method. NIR emission properties of Er³⁺:Y₂O₃ were investigated under 980 nm excitation. The surface of Er³⁺:Y₂O₃ was electrostatically PEGylated using poly (ethylene glycol)‐b‐poly(acrylic acid) (PEG‐b‐PAAc) block copolymer to improve the chemical durability and dispersion stability of Er³⁺:Y₂O₃ under physiological conditions. In vitro cytotoxic effects of bare and PEG‐b‐PAAc‐modified Er³⁺:Y₂O₃ were investigated by incubation with mouse macrophage cells (J774). Microscopic and macroscopic FBI were demonstrated in vivo by injection of bare or PEG‐b‐PAAc‐modified Er³⁺:Y₂O₃ into C57BL/6 mice. The NIR fluorescence images showed that PEG‐b‐PAAc modification significantly reduced the agglomeration of Er³⁺:Y₂O₃ in mice and enhanced the distribution of Er³⁺:Y₂O₃.     

Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si₃N₄ (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu₂O₃–MgO–Y₂O₃ sintering additive. The addition of Eu₂O₃–MgO–Y₂O₃ was shown to promote full nitridation of the Si powder. The nitrided Si₃N₄ particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu₂O₃–MgO–Y₂O₃ were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu₂O₃ doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m^1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m^?1 K^?1, significantly higher than the counterpart without Eu₂O₃ doping, or with ZrO₂ doping by conventional methods.     

Effect of Li+ Ions Doping on Microstructure and Upconversion Emission of Y2Ti2O7:Er3+/Yb3+ Nanophosphors Synthesized Via a Sol–Gel Method

Er³⁺/Yb³⁺/Li⁺‐tridoped Y₂Ti₂O₇ nanophosphors were synthesized via a facile sol–gel process. The samples were characterized by the inductively coupled plasma atomic emission spectrometer (ICP‐AES), X‐ray diffraction (XRD), transmission electron microscopy (TEM), and infrared‐to‐visible upconversion (UC) luminescence spectra. XRD analysis showed that the crystallization temperature of pyrochore‐type Y₂Ti₂O₇ was reduced due to the flux effect of Li⁺ ions, whereas TEM measurements confirmed that the particles size of (Y_0.815Er_0.01Yb_0.075Li_0.10)₂Ti₂O₇ was about 30–40 nm when calcining at 800°C for 1.0 h. The calcining temperature and Li⁺ ion concentration dependence on UC luminescence spectra were investigated. It was found that, when incorporating 10.0 mol% Li⁺ ion, the UC red and green emission intensity was drastically increased by a factor of 18.6 and 8.3, respectively. The enhancement of UC emission may be mainly attributed to the modification of local symmetry around Er³⁺ ions by tridoping Li⁺ ions. And also, the pump power dependence of the emission intensity was investigated to understand the fundamental UC mechanism.     

Phase evolution mechanism of non‐oxide bonded Al–Al2O3–MgO–ZrO2 composites at 1873 K in flowing nitrogen

In flowing nitrogen, non‐oxides such as Al₄O₄C, Al₂OC, Zr₂Al₃C₄, and MgAlON bonded Al₂O₃‐based composites were successfully prepared by a gaseous phase mass transfer pathway using aluminum, zirconia, alumina, and magnesia as raw materials at 1873 K, after an Al–AlN core‐shell structure was formed at 853 K. Resin bonded Al–Al₂O₃–MgO–ZrO₂ composites after sintering were characterized and analyzed by X‐ray diffraction (XRD), scanning electron microscope (SEM) and, energy dispersive spectrometer (EDS), and the influence of the MgO content on the sintered composites was studied. The results show that after sintering, the phase composition of the Al–Al₂O₃–ZrO₂ composite is Al₂O₃, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄, while the phase composition of the Al–Al₂O₃–ZrO₂ composite with the addition of MgO 6 wt% and MgO 12 wt% is Al₂O₃, MgAlON, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄ as well as Al₂O₃, MgAlON, Al₂OC, and Zr₂Al₃C₄, respectively. The addition of MgO changed the phase composition and distribution for the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites after sintering. When the added MgO content is equal to or more than 12 wt%, the Al₄O₄C in the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites is unable to exist in a stable phase.     

Influence of Sintering Aid on the Translucency of Spark Plasma‐Sintered Silicon Nitride Ceramics

Single additives (Y₂O₃, MgO, and Al₂O₃) were used as sintering aid of Si₃N₄. The density, crystal phase, microstructure, transmittance, hardness, and fracture toughness of sintered Si₃N₄ were investigated. Highly densified sintered bodies were obtained in single Y₂O₃ and MgO systems, but not in single Al₂O₃ system. The XRD results indicated that sintered bodies were composed of β‐Si₃N₄. The SEM images showed that all the sintered bodies had a fine‐grained microstructure with an average diameter of 0.29–0.37 μm. The thickness of grain boundary was changed with additive content. The transmittance (T) and the wavelength (λ) followed the relationship of T∝ exp(?λ^?2.3) due to the light scattering. The transmittance was mainly influenced by the refractive index of additives and the thickness of grain boundary phase. The hardness and fracture toughness of sintered ceramics were 12.6–15.5 GPa and 6.2–7.2 MPam^1/2, respectively.     

Bottom‐up vs reactive sintering of Al2O3–YAG–YSZ composites via one or three‐phase nanoparticles (NPs). Bottom‐up processing wins this time

The bottom‐up approach describes the synthesis of bulk materials from the finest possible length scales to obtain the best global properties. This approach was adapted to the synthesis of multi‐phase ceramic composites produced from metal oxides produced by liquid‐feed flame spray pyrolysis (LF‐FSP). The effect of length scale of mixing was tested through two processing schemes, mixed single metal‐oxide nanopowders (NPs) and nanocomposite NPs having the desired composition within single particles. For the Al₂O₃–Y₂O₃–ZrO₂ ternary system, composites prepared from nanostructured nanoparticles sinter to finer grain sizes (<410 nm) at equivalent densities of 95%TD than those prepared from mixed nanoparticle processing. These contrast with our previous studies in this area where mixed NP processing gave the best or equivalent results. The nanocomposite NPs produced in this study exhibit novel nanostructures with three phases contained within single particles <26 nm average particle size (APS). This nanostructure may directly explain the enhanced sintering of the nanocomposite NPs and may provide an impetus for future synthesis of similarly structured NPs.     

High‐temperature stability of YSZ and MSZ ceramic materials in CaF2–MgF2–MgO molten salt system

The high‐temperature stability of YSZ and MSZ specimens was investigated in CaF₂–MgF₂–MgO molten salt at 1200°C. YSZ was mostly composed of m‐ZrO₂ and a small part of YF₃ in the early stages. The formation of YF₃ was attributed to the chemical reaction between Y₂O₃ and MgF₂, which can lead to the leaching of Y₂O₃ from YSZ. With an increase in exposure time, the degraded surface was coarser, and considerable amount of cracks, pores, and spallations were formed. Furthermore, no Y₂O₃ was found up to 120 μm of the YSZ bulk in the early stages. MSZ was composed of t‐ZrO₂ after 24 hours. However, the volume fraction of m‐ZrO₂ was 72% after 72 hours, and CaZrO₃ was formed by the chemical reaction between CaO and ZrO₂ after 168 hours. In addition, the volume fraction of m‐ZrO₂ was 60% in 2.5 wt% MgO and 49% in 10 wt% MgO. In 5 wt% MgO, CaZrO₃ was formed. We demonstrate that the high‐temperature stability of MSZ was better than that of YSZ, and that 10 wt% MgO was much more stable than the other concentrations of MgO.