Luminescent Er3+ doped transparent alumina ceramics

We report on successful preparation of Er³⁺ doped transparent alumina (0.1–0.17 at.%) exhibiting visible light photoluminescence using wet shaping method and hot isostatic pressing. The effects of dopant amount, type of doping powder and powder pre-treatment on final microstructure, real in-line transmittance and photoluminescence characteristics were studied.The real in-line transmittance ranged between 28 and 56%, depending on processing parameters. The transparency decreased with increased amount of dopant. The decrease is dependent on the type of doping powder and its pre-treatment.The photoluminescence spectra measured in both visible and NIR region showed typical emission bands due to the presence of Er³⁺ ions. The decay profiles of the ⁴S_3/2 → ⁴I_15/2 transition were fitted with a 2-exponential function, with faster component in the range of 360–700 ns and slower component around 1.6-2.4 μs. The intensity of emissions and lifetime of the ⁴S_3/2 level decrease significantly with increasing concentration of Er³⁺ ions.     

Luminescent Eu3+-doped transparent alumina ceramics with high hardness

The Eu³⁺-doped transparent aluminas were prepared by wet shaping technique followed by pressure-less sintering and hot isostatic pressing. The effect of dopant amount on microstructure, real in-line transmission (RIT), photoluminescence (PL) properties, hardness and fracture behaviour was studied. The RIT decreased with increasing amount of the dopant. The PL emission spectra of Al₂O₃:Eu³⁺ ceramics exhibited predominant red light emission with the highest intensity (under 394 nm excitation) for material containing 0.125 at.% of Eu³⁺ and colour coordinates (0.645, 0.355) comparable with commercial red phosphors. The doping resulted in hardness increase from 26.1 GPa for undoped alumina to 27.6 GPa for Eu-doped sample. The study of fracture surfaces showed predominantly intergranular crack propagation micro-mechanism.     

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.     

Luminescent Dy3+ and Dy3+/Cr3+ doped transparent Al2O3 ceramics: Microstructure and optical properties

Highly transparent Dy³⁺ and Dy³⁺/Cr³⁺ polycrystalline alumina ceramics were prepared with the real in-line transmittance (RIT) up to 55% (at λ = 632 nm), one of the highest values reported for luminescent rare-earth elements doped alumina. The RIT of doped alumina decreased more sharply with increasing mean grain size than predicted by the models for pure alumina. Co-doping with Dy³⁺ and Cr³⁺ resulted in a moderate increase of the RIT, which was independent on grain size. EDX analysis revealed that dysprosium segregated at grain boundaries and chromium was dispersed throughout the alumina matrix. A thermodynamic model for diffusion of multiple species in systems with multiple sorts of traps was proposed. The photoluminescence excitation and emission spectra of Dy³⁺-doped alumina showed peaks characteristic for Dy³⁺-doped materials. In co-doped Dy³⁺/Cr³⁺ alumina, Dy³⁺ emitted in the blue and yellow regions and Cr³⁺ in the red region, creating a suitable combination for obtaining white light.     

Effects of ZrO2 on the photoluminescence of 0.5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics

Zr-codoped 0.5 at% Eu: (Y_0.9La_0.1)₂O₃ ceramics sintered in H₂-reducing atomsphere, together with the ceramics with annealing treatment, were fabricated by solid-state reactions and the effects of Zr codoping on these materials’ photoluminescence examined. The obtained emission spectra showed that Zr codoping adjust the materials’ photoluminescence with UV excitation and a logical explanation was proposed. The results suggested that an Eu-doped, yttrium-lanthanum oxide transparent ceramic with Zr in low concentration appeared to have promising potential in modern lighting applications.     

Effect of Codoping on the R-Line Luminescence of Cr3+-Doped Alumina

The frequency of the R-line luminescence from Cr³⁺ ions in aluminum oxide is known to be sensitive to chromium concentration, stress, and temperature. In this work, the effect of simultaneously doping aluminum oxide with both Cr³⁺ and other ions (Fe, Ti, Ni, or Y) on the R-line frequency and intensity at room temperature is investigated. It is concluded that ions soluble in aluminum oxide cause systematic shifts in the luminescence frequency and intensity whereas those ions that are insoluble have no effect on the R-line luminescence. In addition, it is shown that contrary to previous investigations, the ratio of the N1 to R line intensities is not simply related to the chromium concentration and hence cannot be used as a measure of the chromium concentration.     

Optical and luminescent properties of quasi-stoichiometric YAG: Cr3+ ceramics

The impact of minor stoichiometric variations on the microstructure, optical characteristics, and luminescent properties of YAG:Cr ceramics, synthesized from chemically precipitated ceramic powders, was assessed for the first time. Transparent ceramics with over 70% transparency was produced with a nominal yttrium excess ranging from 0.47 to −1 mol.%. The phase composition, microstructure, and luminescent properties of quasi-stoichiometric YAG:Cr ceramics were examined, and the impact of stoichiometric deviations on the crystal lattice parameter and average grain size in ceramics was outlined. An examination of the optical characteristics of the ceramics revealed a specific absorption band in the case of yttrium excess. The effect of stoichiometry deviation on the luminescent properties of YAG:Cr ceramics was investigated. A change in stoichiometry from −1–0.47 mol.% excess yttrium resulted in a broadening of the luminescence R-line and a decrease in the lifetime of the excited state of Cr³⁺ from 1.91 to 1.81 ms.     

Transparent alumina ceramics fabricated by 3D printing and vacuum sintering

Transparent alumina ceramics were fabricated using an extrusion-based 3D printer and post-processing steps including debinding, vacuum sintering, and polishing. Printable slurry recipes and 3D printing parameters were optimized to fabricate quality green bodies of varying shapes and sizes. Two-step vacuum sintering profiles were found to increase density while reducing grain size and thus improving the transparency of the sintered alumina ceramics over single-step sintering profiles. The 3D printed and two-step vacuum sintered alumina ceramics achieved greater than 99 % relative density and total transmittance values of about 70 % at 800 nm and above, which was comparable to that of conventional CIP processed alumina ceramics. This demonstrates the capability of 3D printing to compete with conventional transparent ceramic forming methods along with the additional benefit of freedom of design and production of complex shapes.     

Vacuum sintering of transparent Cr:Y2O3 ceramics

0–0.7 at% Cr:Y₂O₃ transparent ceramics were prepared by vacuum sintering. The optimum in-line transmittance in the visible and near infrared region is 78%, and the Vickers hardness of the sintered 0.1 at% Cr:Y₂O₃ is 10.1 GPa, respectively. The mechanism of Cr-doped and the optical properties has been discussed. The results indicated that the Cr:Y₂O₃ transparent ceramic is a promising laser material with enhanced mechanical property.     

Spark plasma sintering of Sm3+ doped Y2O3 transparent ceramics for visible light lasers

Transparent Sm:Y₂O₃ ceramics were fabricated by spark plasma sintering (SPS). The effects of LiF additive and sintering temperature on the microstructure and optical transmittance of the Sm:Y₂O₃ ceramics were investigated. The optimal content of LiF additive and sintering temperature was found to be 0.3 wt% and 1500 ℃. The transmittance of Sm:Y₂O₃ ceramics with a thickness of 1.7 mm reached 75.3% at 609 nm, which is about 94% of the theoretical value. The average grain size of the sample was about 50 µm.     

Near-infrared long-lasting phosphorescence in transparent glass ceramics embedding Cr3+-doped LiGa5O8 nanocrystals

Cr³⁺ doped transparent glass ceramics of SiO₂–Ga₂O₃–Li₂O were fabricated by melt-quenching and subsequent crystallization. X-ray diffraction and transmission electron microscopy analyses evidenced that cubic LiGa₅O₈ nanocrystals were homogeneously precipitated among the silicate glass matrix. The incorporation of Cr³⁺ ions into LiGa₅O₈ nanocrystals was evidenced by absorption, emission and time-resolved luminescence spectra. Impressively, the present Cr³⁺ doped glass ceramics were demonstrated to be a new near-infrared (∼720 nm) long-lasting bulk phosphor whose luminescence can last for more than 2 h after stoppage of UV (250–350 nm) irradiation. The occurring of Cr³⁺ long-lasting phosphorescence in the glass ceramics was confirmed to be mainly due to the precipitation of Cr³⁺:LiGa₅O₈ nanocrystals from glass matrix. The filling/releasing of electrons into/from the intrinsic traps of LiGa₅O₈ nanocrystals through the conduction band of host were proposed to be responsible for the realization of the long-lasting phosphorescence of the investigated Cr³⁺ doped glass ceramics.     

Scintillation and dosimeter properties of CaF2 transparent ceramics doped with Nd3+ produced by SPS

We have developed CaF₂:Nd transparent ceramics with varying doping concentrations of Nd by spark plasma sintering (SPS) and evaluated the optical, scintillation and dosimeter properties. The samples showed effective absorption peaks in the visible and near infrared regions due to the 4f-4f transitions of Nd³⁺. In scintillation properties, Nd³⁺ was also active and showed the 4f-4f radiative transitions of Nd³⁺ which appeared at 860 and 1064 nm under X-ray irradiation. In photoluminescence under 160 nm excitation, the samples showed emission peaks in the vacuum ultraviolet region due to the 5d-4f transitions of Nd³⁺. Furthermore, the samples showed thermally stimulated luminescence (TSL) exhibiting glow peaks at 100, 150 and 380 °C. Among the present samples the 5% Nd-doped sample showed the highest sensitivity which allowed to measure radiation dose from 0.1 to 1000 mGy with a good linear response.     

Plasma etching properties of various transparent ceramics

The etching properties of four types of transparent ceramics: sapphire (a single crystalline α-Al₂O₃), γ-AlON (γ-aluminum oxynitride), Mg-spinel (MgAl₂O₄), and Y₂O₃, as well as a polycrystalline opaque Al₂O₃ were examined using an inductively coupled plasma etcher under an incident plasma power of 2,000 W for up to 3 h. The transparent γ-AlON and opaque Al₂O₃ showed significant surface morphological changes, whereas sapphire, Mg-spinel, and Y₂O₃ revealed a relatively smooth surface upon etching. However, direct correlation between the surface morphological change and the degree of etching could not be drawn because sapphire showed a uniform surface etching despite its significant mass loss. Even though Y₂O₃ was found to be more plasma-resistant than Al₂O₃, overall, Mg-spinel was the most feasible transparent material for monitoring window application in a semiconductor processing chamber because of its minimal degree of erosion (≤0.4g/m²) and the transmittance change (≤2%) upon 3 h of fluorocarbon (CF₄) plasma etching.     

Simultaneous high transmittance and large tunability of up-conversion photoluminescence in Er3+ doped KNN-based ceramics

0.98(0.94K_0.51Na_0.5NbO₃-0.06SrZrO₃)-0.02Li_0.5La_0.5TiO₃+x mol Er³⁺ ceramics were prepared using a conventional solid-state reaction method. The transmittance of the sample x = 0.5% sintered at 1210 °C reaches 56% in at the wave length of 780 nm and 73% at the wave length of 2000 nm (the thickness of the sample is 0.3 mm). Under 980 nm excitation, two typical emission bands are obtained, which are the green emission band at 510 nm–580 nm and the red emission band at 645–695 nm. Under the irradiation of visible light, the up-conversion luminescence intensity of the sample is significantly reduced, showing a luminescence quenching characteristic. The PL quenching degree (ΔR) of up-conversion emission is up to 78.2% for the sample x = 0.5% sintered at 1210 °C. Besides, a behavior of photosensitive resistance is achieved at room temperature and 350 °C, which suggests that this system has a great application prospect in optical information storage.     

Crystal structure evolution and luminescence property of Ce3+-doped Y2O3-Al2O3-Sc2O3 ternary ceramics

Ce³⁺-doped Y₂O₃-Al₂O₃-Sc₂O₃ ternary ceramics with varying Sc₂O₃ were prepared via solid-state-reactive method. Their constitutions, crystal structures, microstructures and luminescence properties were thoroughly investigated by powder X-ray diffraction, ⁴⁵Sc and ²⁷Al solid-state nuclear magnetic resonance (NMR), scanning electron microscope (SEM) as well as photoluminescence (PL) characterizations. Single-phased YSAG transparent ceramics with garnet structure and homogeneous microstructures can be obtained. The NMR results suggest that partial [AlO₆] octahedrons are converted into [AlO₄] tetrahedrons with increased Sc³⁺. Partial Sc³⁺ occupies the remaining octahedral sites, the other Sc³⁺ occupies the dodecahedral sites. The luminescence spectra of the as-prepared ceramics show broadened emission bands with excessive Sc³⁺ incorporation. More significantly, the solubility of Ce³⁺ in garnet ceramics extensively increases due to the incorporation of Sc³⁺, leading to a considerable red-shift and broadening of emission spectra with little luminescence decline, indicating that the new ceramic phosphors are promising for white-light illumination with improved color rendering index.     

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.     

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.     

Effect of NaF doping on the transparency,microstructure and spectral properties of Yb3+: CaF2 transparent ceramics

Yb³⁺:CaF₂ transparent ceramics are promising laser gain media with outstanding performance. However, low transmittance in the visible range is the main challenge that restricts the application of Yb³⁺:CaF₂ ceramics in the laser system. In this paper, a new scheme to eliminate the residual pores in the Yb³⁺:CaF₂ transparent ceramics based on doping of NaF as a sintering aid is proposed. Microstructural characterization indicated that NaF could inhibit the grain growth and increase the transmittance in the visible range significantly. The corresponding transmittance was measured to be 85% at the wavelength of 400 nm. The spectra results showed that co-doped with Na⁺ ions could break the clusters of Yb³⁺ ions and modulate the spectroscopy properties of Yb³⁺: CaF₂ lattice efficiently. This paper proved that doping with NaF is an efficient strategy to improve the transmittance and fluorescence quantum efficiency of Yb³⁺:CaF₂ transparent ceramics.     

Trap-induced self-recoverable photochromism of rare-earth doped sodium niobate translucent ceramics

Most photochromic ceramics can back up post-irradiation to achieve color state transition. Unfortunately, color recovery usually requires exposure of material to external physical fields (such as light and heat), which severely limits the application as a convenient energy building material such as smart windows. Here, we report a kind of sodium niobate translucent ceramics whose photochromic effect can be completely self-recoverable without any external physical field stimulation. Based on the analysis of time-response transmittance, up-conversion emission spectra and thermoluminescence spectra, the behavior of carrier migration and trap capture/release in the process of self-recovery photochromic reaction has been proposed. It is believed that the emergence of intermediate trap level is an important factor to produce this spontaneous behavior. Moreover, by controlling the amount of rare earth doping and designing two types of materials with different carrier behaviors, Sm/Er-codoped NaNbO₃ ceramics are regarded as an appropriate candidate for tracing the time of light irradiation and intelligent regulation of smart building materials. This work can promote the fundamental understanding and practical applications of self-recoverable photochromic bulk materials.     

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⁺.