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Monodispersed yttrium aluminum garnet ( YAG) powders have been synthesized via homogeneous precipitation method from the mixed solutions of yttrium nitrate, aluminum nitrate, and ammonium aluminum sulfate using urea as the precipitant. The molar ratio of aluminum nitrate to ammonium aluminum sulfate has a significant effect on morphology and particle size of the precursor powders. It was found that spherical precursor particles can be obtained when the ratio is 1:1. During the homogeneous precipitation process, aluminum ions precipitate first and form monodispered spherical powders, and then yttrium ions precipitate onto the surface of the existing spheres. Monodispersed spherical YAG particles with 500 nm in diameter were obtained by calcining the precursor powder at 1100°C for 5 h. The so-prepared monodispersed spherical Nd: YAG powders have good sinterability, and can be sintered into transparent ceramics by vacuum sintering at 1650°C for 3.5 h. [ABSTRACT FROM AUTHOR]

Highly transparent Lu2O3 ceramics were prepared by using co‐precipitation combined with vacuum sintering method with ZrO2 as sintering aid. The effect of ZrO2 on densification, transmittance, and microstructure evolution of the Lu2O3 ceramics was carefully studied. It was found that the addition of ZrO2 was very effective in improving densification of Lu2O3. The highest transmittance (at 600 nm) of the 3 at.% ZrO2 doped Lu2O3 ceramic (1.6‐mm thickness) sintered at 1800°C reached 80.1%. Microstructural inhomogeneity was found after vacuum sintering with larger grain sizes at the central. The microstructural inhomogeneity mainly occurred in the final stage of sintering. Doping ZrO2 mitigated microstructural inhomogeneity and decreased markedly the grain size of Lu2O3 ceramics. Raman measurements indicated that the disordered structure was formed due to oxygen vacancy, and the oxygen vacancy concentration at the central was higher than at the peripheral. The oxygen vacancy concentration gradient is the dominant factor governing nonuniform microstructure evolution during vacuum sintering. Furthermore, a uniform microstructure was obtained by the inhibition of oxygen vacancy formation by the Lu2O3/ZrO2 double powder bed. [ABSTRACT FROM AUTHOR]

Coprecipitation with rare‐earth nitrate, ammonium sulfate, and ammonium hydroxide produced hydroxide‐type amorphous precursors incorporating sulfate and carbonate anions, from which [(Gd1−xLax)0.99Tb0.01]2O2SO4 and [(Gd1−xLax)0.99Tb0.01]2O2S (x = 0, 0.15, 0.3, 0.5, 0.65, 0.8, and 1) were obtained as two series of nanophosphors by calcination at 950°C in air and hydrogen, respectively. The detailed characterization by X‐ray diffractometer, scanning electron microscopy/transmission electron microscopy, Brunauer–Emmett–Teller, and particle sizing confirmed that solid solutions were directly formed and that the products have small crystallite size, unimodal size distribution, and high specific surface area, revealing the advantages of the synthesis method. Photoluminescence study revealed that La3+ admixture may significantly improve the 545 nm main emission of Tb3+ for both the phosphor series. Furthermore, the 545 nm main emission of [(Gd1−xLax)0.99Tb0.01]2O2SO4 was identified to have an excellent thermal stability, which retained over 90% of its room‐temperature intensity at 150°C (no quenching for Gd2O2SO4:Tb3+). The two series of phosphors were comparatively studied for their excitation and luminescence performances, as a function of temperature and La3+ content, and the results were rationalized by considering bandgap, crystal structure, UV absorption, and the character of chemical bonds. [ABSTRACT FROM AUTHOR]

Transparent (Y0.98-xTb0.02Eux)2O3 (x = 0-0.04) ceramics with color-tailorable emission have been successfully fabricated by vacuum sintering at the relatively low temperature of 1700°C for 4 h. These ceramics have the in-line transmittances of ~73%-76% at 613 nm, the wavelength of Eu3+ emission (the 5D07F2 transition). Thermodynamic calculation indicates that the Tb4+ ions in the starting oxide powder can essentially be reduced to Tb3+ under ~10-3 Pa (the pressure for vacuum sintering) when the temperature is above ~394°C. The photoluminescence excitation (PLE) spectra of the transparent (Y0.98-xTb0.02Eux)2O3 ceramics exhibit one spin-forbidden (high-spin, HS) band at ~323 nm and two spin-allowed (lowspin, LS) bands at ~303 and 281 nm. Improved emissions were observed for both Eu3+ and Tb3+ by varying the excitation wavelength from 270 to 323 nm, without notably changing the color coordinates of the whole emission. The transparent (Y0.98Tb0.02)2O3 ceramic exhibits the typical green emission of Tb3+ at 544 nm (the 5D4→7F5 transition). With increasing Eu3+ incorporation, the emission color of the (Y0.98-x Tb0.02Eux)2O3 ceramics can be precisely tailored from yellowish-green to reddish-orange via the effective energy transfer from Tb3+ to Eu3+ under the excitation with the peak wavelength of the HS band. At the maximum Eu3+ emission intensity (x = 0.02), the ceramic shows a high energy-transfer efficiency of ~85.3%. The fluorescence lifetimes of both the 544 nm Tb3+ and 613 nm Eu3+ emissions were found to decrease with increasing Eu3+ concentration. [ABSTRACT FROM AUTHOR]

Recently, study on Cr3+‐doped zinc gallate and zinc gallogermanate persistent phosphors has become a hot topic in persistent luminescence and bio imaging areas, because of their near infrared (NIR) emission and long afterglow. However, regulation of efficient traps and improvement of persistent luminescence through bottom‐up design are the key challenge. Here, we recommend a new paradigm of chemical unit co‐substitution with [Mg2+‐Ge4+] substituting for [Ga3+‐Ga3+] in ZnGa2O4, which contributes to the opposite charged and distorted octahedral defects of MgGa′ and GeGa· in pair around the CrN2 ions. The formed defect clusters of MgGa′‐CrN2‐GeGa·, which are closely related to the trap depth, can be accurately regulated through varying the doping content of Mg2+/Ge4+ in the resulting spinel solid solutions of ZnGa2−x(Mg/Ge)xO4:Cr3+ (x = 0–1.25). Moreover, the defect clusters cannot only store and recharge visible and UV radiations that contributes to the long lasting NIR persistent luminescence but also can enhance the NIR emission intensity at ~695 nm. The persistent luminescence induced by UV light excitation exhibits an improvement at a deeper trap depth, but it follows an opposite law through visible light excitation. The prepared nanoparticles have the advantages of intense NIR emission, long lasting afterglow, and excellent rechargeability for visible/UV radiations, so they are the potential nanoprobes for long‐term bio imaging in living animals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: Uniform spheres of (Yb0.98RE0.02)PO4 orthophosphate (RE=Ho, Er, and Tm, respectively) were synthesized via a homogeneous precipitation procedure mediated by SO42− anions. The as‐precipitated and 1100°C calcined products were determined via laser‐diffraction particle sizing to have the average diameters of ~2.04 ± 0.67 and 1.80 ± 0.93 μm, respectively. The upconversion luminescence of RE3+ under sensitization by the host Yb3+ was studied for the calcination products under 978 nm laser excitation, and it was found that the emissions are dominated by a red band at ~650 nm for Ho3+ (5F5 → 5I8 transition), similarly strong green (~515‐565 nm, 2H11/2/4S3/2 → 4I15/2 transition) and red (~640‐680 nm, 4F9/2 → 4I15/2 transition) bands for Er3+, and a near‐infrared band at ~800 nm for Tm3+ (3H4 → 3H6 transition). The number of laser photons needed to populate the emitting state was determined by varying the excitation power, and the possible photon reactions leading to the observed upconversion were discussed. [ABSTRACT FROM AUTHOR]

Abstract: The 3 new upconversion (UC) phosphors of La2O2SO4:RE/Yb (RE=Ho, Er, and Tm, respectively) were derived via facile dehydration of their layered hydroxide precursors that were hydrothermally synthesized at 100°C. Rietveld XRD refinement found contracting cell dimension with decreasing RE3+ size, confirming the direct crystallization of solid solution. The Er3+ and Ho3+ activators both exhibited simultaneous green and red (dominant) emissions under 978‐nm near‐infrared (NIR) laser excitation (NIR‐Vis UC). Particularly, Tm3+ produced a Gaussian‐shaped pure NIR emission band at ~812 nm via its 3H4 → 3H6 transition (NIR‐NIR UC), which is highly desired for NIR biological application. Analysis of the excitation‐power dependent UC properties manifested a 3‐photon mechanism for the 3 phosphors, and the possible photon reactions leading to UC were illustrated. [ABSTRACT FROM AUTHOR]

The sulfate-type layered hydroxides of (Gd1− xTb x)2( OH)5( SO4)0.5· nH2O ( x = 0.06-0.10) have been directly crystallized via hydrothermal reaction at 120°C and pH ~10.0, without the need of conventional anion exchange. The primary crystallites were found to be thin platelets having lateral sizes of ~450-1000 nm and thicknesses of ~65-85 nm. Combined analysis by DTA/ TG, FTIR, and XRD revealed that the layered compounds convert into cubic (Gd1− xTb x)2O3 solid-solutions through dehydration, dehydroxylation, and desulfuration up to ~1200°C and via a (Gd1− xTb x)2O2.5( SO4)0.5 intermediate phase. Optical spectroscopies found that the layered compounds and the derived oxides have their strongest 4 f8→4 f75 d1 transition excitations at ~248 and 305 nm and optimal Tb3+ contents of 9 at.% ( x = 0.09) and 8 at.%, respectively. Both the types of materials exhibit the strongest emission at ~544 nm (the 5D4→7F5 transition of Tb3+), but the oxide has an emission intensity ~3 times that of the hydroxide. Fluorescence lifetime of the 544 nm green emission was found to decrease from 2.45 to 2.15 ms along with increasing Tb3+ content for the oxide, and smaller values of ~1.55 ms were derived for the layered hydroxides. [ABSTRACT FROM AUTHOR]

Highly transparent (Y0.95− xGd xEu0.05)2O3 ( x = 0.15-0.55) ceramics have been fabricated by vacuum sintering at the relatively low temperature of 1700°C for 4 h with the in-line transmittances of 73.6%-79.5% at the Eu3+ emission wavelength of 613 nm (~91.9%-99.3% of the theoretical transmittance of Y1.34Gd0.6Eu0.06O3 single crystal), whereas the x = 0.65 ceramic undergoes a phase transformation at 1650°C and has a transparency of 53.4% at the lower sintering temperature of 1625°C. The effects of Gd3+ substitution for Y3+ on the particle characteristics, sintering kinetics, and optical performances of the materials were systematically studied. The results show that (1) calcining the layered rare-earth hydroxide precursors of the ternary Y-Gd-Eu system yielded rounded oxide particles with greatly reduced hard agglomeration and the particle/crystallite size slightly decreases along with increasing Gd3+ incorporation; (2) in the temperature range 1100°C-1480°C, the sintering kinetics of (Y0.95− xGd xEu0.05)2O3 is mainly controlled by grain-boundary diffusion with similar activation energies of ~230 kJ/mol; (3) Gd3+ addition promotes grain growth and densification in the temperature range 1100°C-1400°C; (4) the bandgap energies of the (Y0.95− xGd xEu0.05)2O3 ceramics generally decrease with increasing x; however, they are much lower than those of the oxide powders; (5) both the oxide powders and the transparent ceramics exhibit the typical red emission of Eu3+ at ~613 nm (the 5D0→7F2 transition) under charge transfer (CT) excitation. Gd3+ incorporation enhances the photoluminescence and shortens the fluorescence lifetime of Eu3+. [ABSTRACT FROM AUTHOR]

Chemical precipitation at the freezing temperature of ~4°C has directly yielded layered rare-earth hydroxide [ LRH, Ln2( OH)5 NO3· nH2O, Ln = Y0.95Eu0.05] nanosheets (up to 7 nm thick) for the Y/Eu binary system, with the interlayer NO3− exchangeable with SO42−. Calcining the sulfate derivative at 1100°C for 4 h produces well-dispersed and readily sinterable Ln2O3 red phosphor powders (~14.8 m2/g) that can be densified into highly transparent ceramics via optimized vacuum sintering at the relatively low temperature of 1700°C for 4 h (average grain size ~14 μm; in-line transmittance ~80% at the 613 nm Eu3+ emission or ~99% of the theoretical transmittance of Y2O3 single crystal). Our systematic studies also found that (1) the extent of SO42− exchange and the interlayer distance of LRH are both affected by the SO42−/Ln3+ molar ratio ( R), and an almost complete exchange is achievable at R = 0.25 as expected from the chemical formula (one SO42− replaces two NO3− for charge balance). The optimal R value for sintering, however, was found to be 0.03; (2) The Ln3+ concentration for LRH synthesis substantially affects properties of the resultant oxides, and hard agglomeration has been significantly reduced at the optimized Ln3+ concentration of 0.05-0.075 mol/L; (3) Sulfate exchange significantly alters the thermal decomposition pathway of LRH, and was found essential to produce well-dispersed and highly sinterable oxide powders; (4) Both the oxide powders and transparent ceramics exhibit the typical red emission of Eu3+ at ~613 nm (the 5D0→7F2 transition) under charge-transfer ( CT) excitation. Red-shifted CT band center, stronger excitation/emission, and shorter fluorescence lifetime were, however, observed for the transparent ceramics. [ABSTRACT FROM AUTHOR]

This article presents a detailed study on the nanoscaled interface between microelongated gold particles ( GP) and biphase leucite/feldspar glass-ceramic matrix. The glass-ceramic composite with a nonuniform GP distribution was processed through hot-pressing under vacuum using a commercial dental ceramic furnace for glass-ceramic dental crown manufacturing. Heat treatments at 900°C, 1100°C, and 1300°C were conducted, and microstructural features along the interface were used to verify the chemical reactions between GP and glass-ceramic matrix. It was observed that the amorphous glass-ceramic matrix had nanoscaled biphase structures, and the distributed nanoscaled amorphous leucite phase was attracted to GP during hot-pressing, and was more reactive with GP than the feldspar phase. The thickness of the interfacial phase formed through chemical reactions between GP and glass-ceramic matrix is around 30 nm. The chemically bonded interface has contributed significantly toward the substantial improvements in both strength and toughness of the GP-reinforced glass-ceramic matrix composite. Characterization techniques, including X-ray diffraction and field-emission scanning electron Microscopy, incorporating X-ray microanalysis using energy dispersive spectrometry, have been employed in this study. [ABSTRACT FROM AUTHOR]

To suppress the thermal decomposition and stabilize the crystal structure of Gd3Al5O12 ( GdAG) garnet, GdAG was doped with significantly smaller Lu3+ to form (Gd,Lu)AG solid solutions. The precursors for ( Gd1− x Lu x)3 AG ( x = 0-0.5), with a general formula of ( NH4) x Ln3Al5(OH) y( CO3) z· n H2O ( Ln = Gd and Lu), were synthesized via carbonate co-precipitation from the mixed solutions of aluminum, lutetium, and gadolinium nitrates using ammonium hydrogen carbonate as the precipitant. The results showed that 10 at.% of Lu3+ ( x = 0.1) has been able to stabilize GdAG against its thermal decomposition to a mixture of GdAlO3 ( GdAP) and Al2O3 phases at temperatures above 1300°C and that Lu3+ doping effectively lowers the temperature for garnet crystallization. The precursors of x ≥ 0.3 convert to pure LnAG at a low temperature of ~1000°C via the intermediates of Ln4Al2O9 ( LnAM) and LnAP. The carbonate precursors are loosely agglomerated and the resultant LnAG powders show good dispersion and a fairly uniform particle morphology. The (Gd,Lu)AG solid solutions exhibit decreasing lattice parameters and increasing optical bandgaps along with more Lu3+ incorporation. [ABSTRACT FROM AUTHOR]

A novel Y3− x Si6 N11: x Ce3+ yellow phosphor was synthesized using the carbothermal reduction and nitridition method at 1550°C for 16 h in this letter. Photoluminescence spectra indicated that the phosphor showed broad excitation spectrum and had strong absorption in range of 350-450 nm. It also gave a broad emission band (Full width at half maximum = 153 nm) centered at 575 nm under 425-nm excitation. With increasing Ce3+ concentration, the strongest emission intensity was obtained at 5 mol% Ce3+ doping amount and a systematic redshift was observed as the Ce3+ concentration increased. The results indicate that this novel yellow phosphor is a promising candidate for using in blue-chip-excited white light-emitting diodes ( LEDs). [ABSTRACT FROM AUTHOR]