Fabrication and properties of transparent Tb2Ti2O7 magneto-optical ceramics

Transparent Tb₂Ti₂O₇ magneto-optical ceramics were fabricated from co-precipitated nano-powders by vacuum pre-sintering with hot isostatic pressing (HIP) post-treatment. The formation of pyrochlore phase, decomposition of the precursor, and the morphology of powders calcined at different temperatures were investigated. The in-line transmittance of Tb₂Ti₂O₇ ceramics, which were pre-sintered at 1350 ℃ for 2 h with HIP post-treatment at 1450 ℃ for 3 h and subsequently annealed at 800 ℃ for 20 h in NH₃ atmosphere, reaches 65.5 % at 1064 nm. The Verdet constant of Tb₂Ti₂O₇ ceramics is −229.0 ± 0.6 rad·T⁻¹ m⁻¹ at the wavelength of 633 nm, which is 71 % higher than that of the commercial Tb₃Ga₅O₁₂ crystals. Tb₂Ti₂O₇ magneto-optical ceramics show a promising application for Faraday rotators.     

Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Fabrication and characterizations of Ho3Sc2Al3O12 magneto-optical transparent ceramics

Currently, Faraday isolators and rotators for high-power lasers are popular research topics; therefore, finding magneto-optical materials with excellent Verdet constants and thermal properties is crucial. In this study, novel holmium scandium aluminum garnet (HoSAG) magneto-optical ceramics were fabricated using vacuum sintering. This study improved the traditional ball milling method and pre-synthesized powders with great sinterability after secondary ball milling and sieving. Different pre-synthesis temperatures were found to have significant effects on the particle size and micromorphology of the synthesized powders. HoSAG ceramics with 0.05 wt% MgO sintering aid and held at 1700°C for 30 h reached a transmittance of 76.7% at 1550 nm. Meanwhile, HoSAG transparent ceramics maintain higher transmittance in the infrared region (>1500 nm) than terbium gallium garnet (TGG) crystals, indicating better application prospects. The Verdet constants of HoSAG magneto-optical ceramics at 405, 532, 808, and 1064 nm were −421.5, −181.2, −65, and −32.3 rad T⁻¹ m⁻¹, respectively, which are slightly less than those of TGG magneto-optical ceramics. Moreover, the thermal conductivity of HoSAG ceramics at 300.00 K was 4.89 W m⁻¹ K⁻¹, which is comparable to that of the TGG ceramics.     

Preparation of (Tb1-xLux)2O3 transparent ceramics by solid solution for magneto-optical application

Novel (Tb_1-xLu_x)₂O₃ magneto-optical transparent ceramics were prepared through a solid-solution method via vacuum pre-sintering followed by hot isostatic pressing (HIP). The Lu-containing samples were solid-solution phases between Tb₂O₃ and Lu₂O₃, which effectively stabilized the phase transition of Tb₂O₃ during the sintering process. A typical component of (Tb_1-xLu_x)₂O₃ ceramic with x = 0.5 was selected to study the densification behavior and microstructure evolution in detail. The polished (Tb_0.5Lu_0.5)₂O₃ ceramics with a thickness of 3 mm vacuum pre-sintered at 1700 °C under 1.0 × 10^?3 Pa combined with HIP post-treatment at 1600 °C showed good magneto-optical property. The Verdet constant measured at 633 nm was -224.33 rad·T^?1 m^?1, 30% higher than that of Tb₃Al₅O₁₂ (TAG). However, the in-line transmittance was 73.6% at 633 nm, lower than that of TAG. The (Tb_1-xLu_x)₂O₃ ceramics will be a promising material for Faraday devices after further improving the optical quality.     

Fabrication and laser performances of Yb:Sc2O3 transparent ceramics from different combination of vacuum sintering and hot isostatic pressing conditions

In this paper we report on the preparation and laser performance of transparent 3at.% Yb:Sc₂O₃ ceramics by reactive sintering of commercially available powders under vacuum followed by hot isostatic pressing (HIP). Combinations of different vacuum sintering temperatures (1650 °C and 1750 °C) and different HIP treatments (1700 °C and 1800 °C at 200 MPa) were tested in order to understand how these steps influence the microstructure and thus the optical and lasing properties of the ceramic samples. All the samples showed a good optical quality. The microstructure analysis and the laser tests showed that the vacuum pre-sintering temperature is the key factor determining the quality of the samples and the laser performances. The best values of slope efficiency i.e. η_L = 50 % and output power i.e. P_out = 6.62 W were obtained for the sample pre-sintered under vacuum at 1650 °C and hot isostatically pressed at 1800 °C.     

Fabrication and characterizations of Eu2+-Dy3+ co-doped SrAl2O4 ceramics with persistent luminescence

(Sr_0.97Eu_0.01Dy_0.02)Al₂O₄ persistent luminescence (PersL) ceramics were fabricated by solid-state reactive sintering in vacuum combined with hot isostatic pressing (HIP) using H₃BO₃ as a sintering additive. The phase composition, microstructure, luminescence properties, trap state, and PersL performance of HIP post-treated (Sr_0.97Eu_0.01Dy_0.02)Al₂O₄ PersL ceramics were discussed. For the (Sr_0.97Eu_0.01Dy_0.02)Al₂O₄ PersL ceramics after HIP post-treatment, the initial luminescence intensity of the ceramics reached over 6400 mcd/m² with simulated daylight irradiation of 1000 lx for 5 min, and the persistent emission decay time > 17 h. This is much better than the SrAl₂O₄:Eu²⁺,Dy³⁺ PersL powders and the other luminescent ceramics. In addition, this method is a solid-state reactive sintering method for synthesizing ceramics, which has the advantages of low cost and simple operation, and is suitable for large-scale, high-volume industrial production.     

Fabrication and magneto-optical property of yttria stabilized Tb2O3 transparent ceramics

(Tb_0.5Y_0.5)₂O₃ transparent ceramics have been prepared by wet chemical co-precipitation route and flowing H₂ atmosphere sintering. The optical quality, microstructure and magneto-optical properties of the ceramics were investigated. No sintering aids or milling were adopted in the ceramic fabrication processing. The cold isostatic pressed green compact could be sintered to be transparent (Tb_0.5Y_0.5)₂O₃ ceramic at 1800 ℃ in flowing H₂ atmosphere. The mechanical polished ceramic showed the good transmittance from visible to near infrared wavelength, corresponding to a 71.9% transmittance at 1400 nm wavelength. The Verdet constant measured at 632.8 nm of the (Tb_0.5Y_0.5)₂O₃ transparent ceramics was -220.19 rad T⁻¹ m^-1, which was 1.64 times that of Tb₃Ga₅O₁₂ single crystal.     

Fabrication of highly transparent AlON ceramics by hot isostatic pressing post-treatment

Highly transparent aluminum oxynitride (AlON) ceramics were fabricated by pressureless sintering with hot isostatic pressing (HIP) post-treatment. The experimental results showed that the optical transparency of AlON ceramics was improved markedly over the visible and near-infrared range by HIP at 1825 °C for 3 h in 200 MPa argon gas, which derived from the elimination of residual pores in the prepared ceramics. For AlON ceramics pre-sintered at 1800 °C, the transmittances of the sample increased from 63.6% to 84.8% at 600 nm and from 75.4% to 86.1% at 2000 nm, respectively. The average grain size of the HIPed sample was about 47.9 μm.     

Preparation and properties of novel Tb3Sc2Al3O12 (TSAG) magneto-optical transparent ceramic

Novel Tb₃Sc₂Al₃O₁₂ (TSAG) magneto-optical transparent ceramic was fabricated by vacuum sintering. The phase composition, microstructure, optical quality, thermal properties, and magneto-optical properties of TSAG ceramic were measured. It is shown that the increase in holding time has an effect on the grain size of TSAG ceramic. It is noted that TSAG35 ceramic presents the highest transmittance, corresponding to 81.5 % at the wavelength of 1064 nm. The thermal properties of TSAG ceramic are close to or superior to that of the reported TSAG and TGG crystals. The Verdet constant of TSAG ceramic is comparable to that of reported TSAG crystal, and 1.2 times that of TGG crystal. The results indicate that the novel TSAG ceramic is comparable to TSAG crystal in terms of magneto-optical properties and superior to TGG crystal, making it a candidate material for magneto-optical materials to be used in high-power lasers.     

Fantastic valence impacts of Pr on microstructures and Faraday magneto-optical effects of transparent (Ho,Pr)2O3 ceramics

Polycrystalline magneto-optical (Ho_1-xPr_x)₂O₃ (x = 0.05?0.2) ceramics were fabricated by vacuum sintering using layered rare-earth hydroxides as the precursors, among which the 5 at.% Pr doped specimen exhibits the highest in-line transmittance of ～76.1 % at 700 nm (～94.5 % of the theoretical value of defect-free Ho₂O₃ single crystal) and the largest Verdet constant of ?82 ± 6 rad?T^?1 m^?1 at 1064 nm (～2.3 times that of the commercial Tb₃Ga₅O₁₂ crystal and ～1.8 times that of the pure Ho₂O₃ ceramic). More Pr addition not only leads to a higher thermal decomposition temperature for the precursor but also a decreasing particle size for the oxide. A 5 at.% Pr dopant in Ho₂O₃ matrix generally exists in the oxidation state of +3, while an increasing Pr concentration up to 10 at.% induces coexisting valences of +3 and +4. The grain growth was suppressed by the present Pr⁴⁺ based on interstitial mechanism. The substitution of Pr³⁺ for Ho³⁺ is helpful for the rising Verdet constant of the binary ceramic, but Pr⁴⁺ has little positive contribution to it.     

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.     

Fabrication and optical characterizations of Yb,Er codoped CaF2 transparent ceramic

Nanoparticles of Yb, Er codoped calcium fluoride were obtained by a co-precipitation method. Scanning electron microscope (SEM) and X-ray powder diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grains size around 30–50 nm. Yb, Er:CaF₂ transparent ceramics were fabricated by hot pressing (HP) the nanoparticles at a temperature of 800 °C in a vacuum environment. For a 2 mm thickness ceramic sample, the transmittance at 1200 nm reached about 83%. Microstructures were characterized using SEM analysis, and the average grain size was about 700 nm. Grain boundaries of the ceramic sample were clean and no impurities were detected. The absorption, upconversion and infrared emission spectra of transparent ceramic sample under 978 nm excitation were measured and discussed.     

Fabrication and laser operation of Yb:Lu2O3 transparent ceramics from co-precipitated nano-powders

In this article, 5 at.% Yb:Lu₂O₃ transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) posttreatment using co-precipitated nano-powders. The influence of precipitant molar ratio, ammonium hydrogen carbonate, to metal ions (AHC/M³⁺, R value) on the properties of Yb:Lu₂O₃ precursors and calcined powders was investigated systematically. It was found that the powders with different R value calcined at 1100°C for 4 hours were pure cubic Lu₂O₃ but the morphologies of precursors and powders behaved differently. The opaque samples pre-sintered at 1500°C for 2 hours grew into transparent ceramics after HIP posttreatment at 1750°C for 1 hour. The final ceramic with R = 4.8 showed the best optical quality with the in-line transmittance of 79.7% at 1100 nm. The quasi-CW laser operation was performed at 1034 nm and 1080 nm with a maximum output power up to 8.15 W as well as a corresponding slope efficiency of 58.4%.     

Hot isostatic pressing of transparent Yb3+-doped Lu2O3 ceramics for laser applications

Yb³⁺-doped Lu₂O₃ nanoparticles produced by laser ablation were used to fabricate transparent ceramics by a combination of pressureless sintering in vacuum (PS) followed by a hot isostatic pressing (HIPing). The samples were subjected to various PS and HIPing conditions and the microstructure evolution and its correlation with the transmittance were investigated. Relative densities of over 97% were achieved after PS at the temperatures of 1250–1700 °C. Rapid grain growth occurred within PS and HIPing temperatures above 1500 °C leading to formation of intragranular porosity which is deleterious for optical quality. Higher transmittance (81.7% at λ = 1080 nm) and ultrafine microstructure with an average grain size of 0.35 μm were obtained by PS at 1250 °C followed by HIPing at 1400 °C for 5 h under 207 MPa. Output power of 2.02 W with a slope efficiency of 46.5% was obtained under a quasi-continuous wave end pumping at 929.4 nm.     

Influence of post-HIP temperature on microstructural and optical properties of pure MgAl2O4 spinel: From opaque to transparent ceramics

Transparent MgAl₂O₄ spinel ceramics were processed from sub-micrometric commercial powder by applying a two-step procedure: pressureless sintering under vacuum followed by hot isostatic pressing. To limit grain growth and to avoid secondary reactions or impurities, no additives or sintering aids were added to the powder. First, pressureless sintering at 1500 °C during 2 h under vacuum led to opaque samples due to a high level of porosity. To improve the optical quality of the MgAl₂O₄ ceramics and the in-line transmission in the visible range, a post-treatment by hot isostatic pressing was applied. Highly transparent ceramics were obtained after a post-treatment at 1800 °C for 10 h with an in-line transmission of 81% at 400 nm and 86% from 950 to 3000 nm for a thickness of 2 mm (98.8% of the theoretical transmission).     

Preparation and characterization of transparent magneto-optical Ho2O3 ceramics

Transparent magneto-optical Ho₂O₃ ceramics were successfully prepared with an in-line transmittance of ~73% at the wavelength of 1000 nm (~90% of the theoretical transmittance of Ho₂O₃ single crystal) and an average grain size of ~28 μm. The ceramics were fabricated using sulfate-exchanged nitrate-type layered rare-earth hydroxide as the precipitation precursor at a relatively low sintering temperature of 1700°C. The layered compound exhibited nanosheet morphology and fully collapsed into a round oxide powder with an average particle size of ~48 nm by pyrolysis. Calcination temperature for Ho₂O₃ powder significantly affected the optical quality of the sintered body and the optimum calcination temperature was found to be 1050°C. The transparent magneto-optical Ho₂O₃ ceramics displayed wavelength-dependent Verdet constants of −180, −46, and −20 rad/Tm at 632, 1064, and 1550 nm, respectively. Thus, the Ho₂O₃ ceramics show good potential for applications in high-power laser systems.     

Layered Y3Al5O12:Pr/Gd3(Ga,Al)5O12:Ce optical ceramics: Synthesis and photo-physical properties

We report a study of composite scintillating ceramics based on coupled layers of two different garnets, namely Ce-doped gadolinium gallium aluminium (GGAG:Ce) and Pr-doped yttrium aluminium (YAG:Pr), fabricated by hot isostatic pressing. Two samples were prepared, with different GGAG:Ce layer thickness, 120 µm and 690 µm respectively, but with a comparable overall thickness of 1.4 mm. The key finding is that the material architecture strongly determines the scintillation response. The radioluminescence is that expected from the irradiated material when a thick layer of ceramic is exposed to X-rays. Conversely, exposing a thin layer allows a non-null probability —about 0.3% for 120 µm of GGAG— of finding an X-ray photon in the underlying layer, and thus radioluminescence from both materials is recorded. We believe these results can extend the potential of layered optical ceramics for advanced devices, such as energy- and direction-sensitive X-ray detectors.     

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.     

Fabrication of Ti3SiC2 and Ti4SiC3 MAX phase ceramics through reduction of TiO2 with SiC

Ti₃SiC₂ and Ti₄SiC₃ MAX phase ceramics were fabricated through high-temperature vacuum reduction of TiO₂ using SiC as a reductant, followed by hot pressing of the products under 25 MPa of pressure at 1600 °C. It was found that both Ti₃SiC₂ and Ti₄SiC₃ may be obtained in good yields, depending on the annealing time during the reduction step. In addition to MAX phases, the products contained some amounts of TiC. The hot pressing step did not significantly affect the composition of the products, indicating good stability of Ti₃SiC₂ and Ti₄SiC₃ under these conditions. Analysis of the densification behavior of the samples revealed lower ductility in Ti₄SiC₃ compared to Ti₃SiC₂. The samples prepared herein exhibited the flexural strength, fracture toughness and microhardness typical of coarse-grained MAX-phase ceramics.     

Post-treatment of nanopowders-derived Nd:YAG transparent ceramics by hot isostatic pressing

Neodymium doped yttrium aluminum garnet (Nd:YAG) transparent ceramics were fabricated from Nd:YAG nanopowders synthesized via a reverse precipitation method by vacuum sintering and successive hot isostatic pressing (HIP) post-treatment. The powders obtained by calcining the precursor at 1100 °C for 4 h and then ball milling for 2 h with 0.5 wt% TEOS as sintering aid were used to fabricate Nd:YAG ceramics. The green bodies were vacuum sintered at 1500–1800 °C for 10 h, followed by the HIP at 1600 °C for 3 h in 200 MPa Ar atmosphere. Influence of the calcination temperature on the phase, morphology and particle size evolution of the nanopowders, as well as the optical transparency and microstructure of the obtained Nd:YAG ceramics before and after the HIP post-treatment was investigated in detail. It was found that for the post-treated 1800 °C-vacuum-sintered Nd:YAG ceramic sample, the in-line transmittance increased from 48.0% up to 81.2% at the lasing wavelength of 1064 nm.