Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Fabrication and characterizations of Cr3+-doped ZnGa2O4 transparent ceramics with persistent luminescence

0.5 at.% Cr:ZnGa₂O₄ precursor was synthesized by the co-precipitation method with nitrates as raw materials, using ammonium carbonate as the precipitant. Low-agglomerated Cr:ZnGa₂O₄ powders with an average particle size of 43 nm were obtained by calcining the precursor at 900℃ for 4 h. Using the powders as starting materials, 0.5 at.% Cr:ZnGa₂O₄ ceramics with an average grain size of about 515 nm were prepared by presintering at 1150℃ for 5 h in air and HIP post-treatment at 1100℃ for 3 h under 200 MPa Ar. The in-line transmittance of 0.5 at.% Cr:ZnGa₂O₄ ceramics with a thickness of 1.3 mm reaches 59.5% at the wavelength of 700 nm. The Cr:ZnGa₂O₄ ceramics can be effectively excited by visible light and produce persistent luminescence at 700 nm. For Cr:ZnGa₂O₄ transparent ceramics, the brightness of afterglow was larger than 0.32 mcd/m² after 30 min, which is far superior to that of Cr:ZnGa₂O₄ persistent luminescence powders.     

Photoluminescence and optical properties of Eu3+/Eu2+-doped transparent Al2O3 ceramics

Transparent Eu³⁺-doped (0.05–0.15 at. %) alumina ceramics with fine-grained microstructure were prepared and studied in terms of optical properties and photoluminescence (PL). The light transmission through ceramics up to dopant concentrations 0.125 at. % is dominated by birefringence scattering at grain boundaries. As confirmed by HRTEM/EDS element mapping, high photoluminescence intensity was achieved as the result of the dopant segregation at grain boundaries. The PL emission spectra of Al₂O₃:Eu³⁺ ceramics exhibited red light emissions with the highest intensity (394 nm excitation) for material containing 0.125 at. % of Eu³⁺. The luminescence decay was single-exponential with a lifetime ～1.5 ms. The post-sintering reduction of Eu³⁺→Eu²⁺ under an H₂ atmosphere (at 1300 °C) was difficult. Two simultaneously coexisting Eu²⁺ emitting PL centers were identified, one emitting blue light with average decay constant of 150 ns, and the other green light (more intense) with average decay constant of 1.3 μs.     

Preparation and optical properties of highly transparent MgAl1.9Ga0.1O4 ceramics via aqueous gel-casting method

The aqueous gel-casting technology has been widely used to prepare high-quality green body for various transparent ceramics with large dimension and complex shape. However, owing to the severe hydrolysis of MgAl₂O₄ powder, it is challenging to obtain thick aqueous slurry with high homogeneity and flowability. In this paper, the surface chemical state of MgAl₂O₄ powder was modified by introducing Ga³⁺, and stable MgAl_1.9Ga_0.1O₄ aqueous slurry with high solid-phase loading (52 vol. %) and low viscosity (136 mPa·s, at a shear rate of 50 s^-1) was successfully prepared. After pressureless presintering and hot isostatic pressing, the gel-cast sample exhibited much higher optical transmittance and more homogeneous microstructure than the dry-pressed sample, which is mainly derived from the improved homogeneity and densification of the green bodies and ceramics. The optical band gap, infrared cutoff wavelength, static refractive index and dispersion of both MgAl_1.9Ga_0.1O₄ and MgAl₂O₄ transparent ceramics were systematically compared. It is indicated that the transparent MgAl_1.9Ga_0.1O₄ ceramic has the increscent static refractive index of 1.695, the decrescent direct band gap energy of 6.15 eV and absorption coefficient of 0.49 cm^-1 at 5 µm, which could be ascribable to the fact that Ga³⁺ has different electronic structure, higher electronic polarizability and larger ionic radius in comparison with Al³⁺. This work provides a dependable solution for preparation of spinel oxide ceramics with superior optical properties and large dimension.     

Chemical and microstructural characterisation of HfO2-Y2O3 ceramics with high amount of Y2O3 (33, 40 and 50 mol. %) manufactured using spark plasma sintering

Hafnia based ceramics are potential promising candidates to be used as thermal barrier coatings (TBC) for applications in the field of propulsion. In this study, Spark Plasma Sintering (SPS) of fully stabilised hafnia with yttrium oxide (yttria) was investigated to provide a better understanding of the effect of manufacturing parameters, on the crystallography, chemistry and microstructure of the material. Several hafnia powders, containing different amounts of yttria (33 mol. %, 40 mol. % or 50 mol. %), were sintered by SPS at different temperature levels ranging from 1600 °C to 1850 °C. On these materials, X-ray diffraction patterns associated with scanning electron micrographs have highlighted the influence of both the sintering temperature and the amount of yttria on the final composition, the lattice parameter and the microstructure of hafnia-based materials. In the end, it is established that, for all quantities of yttrium employed, the main phase is Y₂Hf₂O₇ with very high densification levels.     

Fabrication and characterization of highly transparent ZrO2-doped Tm2O3 ceramics

Highly transparent polycrystalline Tm₂O₃ ceramics were successfully fabricated by vacuum sintering at temperatures from 1650 to 1850 °C for 8 h using commercial Tm₂O₃ and ZrO₂ (1 at%) powders as starting materials. It is the first time that ZrO₂ was reported as a sintering additive to prepare Tm₂O₃ transparent ceramics. The effects of sintering temperature on the optical transmittance and microstructure of Tm₂O₃ transparent ceramics were studied. The desired Tm₂O₃ ceramics with relative density of 99.8% and an average grain size of approximately 9.7 μm were obtained at 1800 °C and the in-line transmittance reached 75% at 880 nm and fluctuated around 80% from 2100 to 2400 nm, respectively. This study demonstrated that Tm₂O₃ transparent ceramics with higher in-line transmittance and smaller grain size could be prepared by using ZrO₂ as sintering additive at a relatively lower vacuum sintering temperature compared to those already reported in open literatures.     

Co2+:MgAl2O4 saturable absorber transparent ceramics fabricated by high-pressure spark plasma sintering

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co²⁺:MgAl₂O₄ saturable absorbers for laser passive Q-switching. Densification of commercial MgAl₂O₄ powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co₉S₈ inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co²⁺:MgAl₂O₄. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co²⁺, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σ_gs) and excited (σ_es) cross-sections of 3.5×10^-19 and 0.8×10^-19 cm², respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co²⁺:MgAl₂O₄ ceramics.     

Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics

Ce³⁺-doped Gd₃(Al_1-xGa_x)₅O₁₂ (Ce:GAGG) transparent ceramics were successfully prepared via a solid state reaction/oxygen sintering method. The effects of Ga substitution on the structure and optical properties of the ceramics were investigated. The highest quantum yield and relatively high scintillation light yield were achieved in the Gd₃(Al_0.6Ga_0.4)₅O₁₂. The investigated processing technique demonstrated advantages such as increased flexibility and short processing time, thus being very cost effective. The investigated approach provides a much more economical alternative to the conventional melt growth processes used to fabricate single crystals.     

Mechanical and optical properties of MgAl2O4 ceramics and ballistic efficiency of spinel based armour

This study was devoted to the understanding of the influence of MgAl₂O₄ ceramic properties on their ballistic performances. By modifying the processing parameters, ceramics with different microstructures were obtained. Among them, a transparent MgAl₂O₄ spinel with an in-line transmission between 77% and 83% in the visible range, an average grain size of 8.6 μm and good mechanical properties (11.3 GPa in Knoop hardness and 2.5 MPa√m in fracture toughness) was produced. A thorough characterisation of the ceramics was accomplished in order to establish a link between microstructure, mechanical properties and ballistic protective performances against an armour piercing projectile of calibre 7.62x51 mm. The ballistic evaluation demonstrated the advantage of using a spinel layer as the strike face to stop a threat, while reducing drastically the thickness and the areal density of the transparent multilayer, compared to a simple glass armour. MgAl₂O₄ spinel with fine grains presented a better combination of mechanical properties compared to coarser microstructures, hence a better potential to damage a projectile at the impact.     

Fabrication and optical properties of divalent Cu2+ ions incorporated Ce:YAG transparent ceramics for white LEDs

Transparent Ce:YAG ceramics via Cu²⁺ incorporating annealed at 1450?°C were successfully fabricated by the solid-state method to probe their potential applications in white light-emitting diodes (LEDs). The influence of Cu²⁺ concentration on the microstructure and optical properties of the Ce:YAG transparent ceramics were systematically investigated. The as-prepared ceramics possessed clean grain boundaries and homogeneous grain size distribution ranging from 3.7 to 6.5?µm. With the addition amount of Cu²⁺ increased, the red component of ceramics gradually increased and then decreased, it reached a maximum of 13.0% at 1.5?at% Cu²⁺ incorporation. By combining with commercially blue LED chips (465?nm) directly, the obtained optimal chromaticity coordinates (CIE) and correlated color temperature (CCT) of ceramics were (x?=?0.3335, y?=?0.3412) and 5450?K, respectively, while its color render index (CRI) was nearly 70 at the thickness of 1.0?mm. Therefore, this study provided an efficient approach to tailor the luminescence property of Ce:YAG ceramic for white LEDs.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Study of densification mechanisms during Spark Plasma Sintering of co-precipitated Ho:Lu2O3 nanopowders: Application to transparent ceramics for lasers

This work was focused on the determination of densification mechanisms during Spark Plasma Sintering (SPS) of Ho:Lu₂O₃ nanopowders. Strong variation of the stress exponent n was evidenced during the sintering process. At low relative density (i.e. ρ < 66 %), n = 3 and powder particles rearrangement and coalescence take place because of high value of effective stress and low size of primary nanoparticles. Then, for ρ between 66 % and 85 %, the stress exponent decreases to n = 2 then n = 1. Such values were related to Rachinger then Lifshitz sliding mechanisms, the last one was associated with an average activation energy of 565 kJ.mol⁻¹. At the final densification stage (ρ > 85 %), the stress exponent suddenly increases to 4 in accordance with a power-law creep. From these investigations, an optimized thermomechanical cycle was proposed to obtain highly transparent Ho:Lu₂O₃ ceramics suitable for laser applications.     

Effects of LiF on the microstructure and optical properties of hot-pressed MgAl2O4 ceramics

Transparent magnesium aluminate spinel (MgAl₂O₄) ceramics were fabricated by hot-pressing of the MgO and α-Al₂O₃ powder mixture using LiF as a sintering aid. Effects of the LiF additive on densification, microstructure and optical properties of MgAl₂O₄ ceramics were systematically investigated. It has been found that the addition of LiF can effectively remove the porosity and increase the optical transparency of MgAl₂O₄ ceramics. For the spinel ceramics HP-ed at 1550 °C for 3 h with 1 wt% LiF addition, the average grain size is about 36 µm and the in-line transmittance exceeds 60% at the wavelength of 800 nm.     

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Formation peculiarities and optical properties of highly-doped (Y0.86La0.09Yb0.05)2O3 transparent ceramics

Formation peculiarities of highly-doped (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been studied by X-ray diffraction and electron microscopy methods. The phase composition evolution of 1.81Y₂O₃∙0.18La₂O₃∙0.01Yb₂O₃ powder mixtures annealed at the temperatures of 1100, 1200, 1300, and 1400 °C has been studied by XRD. It has been shown that Yb₂O₃ phase dissolves in Y₂O₃ matrix in the calcination temperature range of 1300–1400 °C. Complete dissolution of La₂O₃ in Y₂O₃ matrix occurs at temperatures above 1400 °C. La³⁺ ions enter in Y₂O₃ and Yb₂O₃ crystal structures simultaneously in the 1200–1300 °C range, which leads to a remarkable increase in the volume of the corresponding crystal lattices. The possible reasons for suppressing the crystalline growth of Y₂O₃ and Yb₂O₃ cubic phases have been discussed. Finally, (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been obtained by solid-state vacuum sintering at 1650–1750 °C. Ceramics synthesized at a temperature of 1750 °C have been characterized by an in-line optical transmittance of 60% and a homogeneous distribution of constituent components within the volume and along the grain boundaries.     

The absorption and emission properties of highly transparent ZrO2-doped Yb3+: Y2O3 ceramics

Ultra-highly transparent ZrO₂-doped Yb³⁺: Y₂O₃ ceramics were prepared by slip casting and vacuum pressureless sintering and the transmittance reached the highest value of 80.9% for the sample doped with 8.0 at% Yb³⁺. There are three main absorption peaks at 905, 950, and 976 nm, corresponding to the transition from the lowest level of field splitting of ²F_7/2 crystal to every splitting energy levels of ²F_5/2 crystal field. We analyzed the absorption and emission spectra of transparent Yb³⁺: Y₂O₃ from the energy level structure of Yb³⁺, and the transmission, absorption, and emission spectra were systematically studied. There are three main absorption peaks at 905, 950, and 976 nm and four emission peaks at 1076, 1031, 1013, and 977 nm, respectively. The emission peaks at 977 and 1013 nm broaden and vanish for 8.0 and 10.0 at% Yb³⁺-doped Y₂O₃, which may be related to the change of Y₂O₃ crystal field caused by high concentration.     

Binary transparent (Ho1-xDyx)2O3 ceramics: Compositional influences on particle properties,sintering kinetics and Faraday magneto-optical effects

Binary transparent magneto-optical (Ho_1-xDy_x)₂O₃ (x = 0.01–1) ceramics derived from layered rare-earth hydroxide (LRH) compounds were fabricated by vacuum sintering. They have in-line transmittances of ～67?77 % at the visible wavelength of 700 nm and ～77?84 % at the mid-infrared wavelength of 5 μm with similar maximal infrared cut-off at ～9.5 μm. The impacts of Dy³⁺ doping on particle properties, sintering kinetics and Faraday magneto-optical effects were systematically investigated. The results show that (1) The LRH precursors exhibit the nanosheet morphology with the thickness of ～6?10 nm. Dy³⁺ incorporation not only induces an expansion for the hydroxide host layer but also a contracted interlayer distance; (2) Upon calcination at 1100 °C, the LRH nanosheets collapse into sphere-like oxide particles. The addition of Dy³⁺ leads to increasing lattice constants and decreasing theoretical densities for the (Ho,Dy)₂O₃ solid solutions; (3) A smaller bandgap energy for Dy₂O₃ (～4.85 eV) was obtained relative to those of (Ho_0.9Dy_0.1)₂O₃ (～5.24 eV) and Ho₂O₃ (～5.31 eV); (4) Dy³⁺ dopant promotes grain growth and the pure Dy₂O₃ bulk has a rather smaller grain-boundary-diffusion controlled activation energy (～457 kJ/mol) than the (Ho_0.9Dy_0.1)₂O₃ counterpart (～626 kJ/mol); (5) The Verdet constants of magneto-optical (Ho_1-xDy_x)₂O₃ ceramics generally linearly increase with the rise of Dy³⁺ concentration.     

Microstructure and optical properties of Nd: YAG transparent ceramics with the addition of Lu3+ ions

The Nd:YAG transparent ceramics with addition of Lu³⁺ ions were fabricated by co-precipitation method and vacuum sintering. Pure YAG phases were obtained when Lu³⁺ ion content was kept under 4.5?at.-%. Lattice constant of polycrystalline ceramic with 0.8?at.-% Lu³⁺ calculated from XRD patterns was similar to that of YAG single crystal, and its fluorescent intensity arising from ⁴F3/2?→?⁴I9/2 transition of Nd³⁺ ions reached the maximum although the ceramic was opaque. The results indicated that Lu³⁺ ions under 1.5?at.-% relieved strains from lattice distortion and enhanced the fluorescent intensity.     

Near-infrared luminescent properties of Ln:LaGdZr2O7 (Ln = Nd,Yb) transparent ceramics for solid-state laser applications

Via a facile solid reactive method, transparent Ln_0.1La_0.9GdZr₂O₇ (Ln = Nd, Yb) ceramics were successfully fabricated for the first time. The highest in-line transmittances of Nd:LaGdZr₂O₇ and Yb:LaGdZr₂O₇ ceramics reached 68% and 69%, respectively, at 1100 nm. The microstructure and crystal structure of Ln_0.1La_0.9GdZr₂O₇ transparent ceramics were fully investigated, indicating that the solid reactive technique is a good method of industrially fabricating Ln_0.1La_0.9GdZr₂O₇ transparent ceramics. The PL spectra demonstrated that Ln_0.1La_0.9GdZr₂O₇ ceramics can effectively be excited at 808 nm and 976 nm, which correlates with the widely applied output wavelengths of AlGaAs and InGaAs laser diodes. The luminescence decay curves were also studied, showing that the average fluorescence lifetimes of Nd_0.1La_0.9GdZr₂O₇ and Yb_0.1La_0.9GdZr₂O₇ transparent ceramics was 355 μs and 663 μs, respectively. Combined with its high temperature resistance and good mechanical strength, Ln_0.1La_0.9GdZr₂O₇ (Ln = Nd, Yb) transparent ceramics can have potential applications in Nd/Yb solid-state laser construction.