Shao Hong Liu

Crystal Structure Stabilization of Gadolinium Aluminum Garnet (Gd3Al5O12) and Photoluminescence Properties

To suppress the thermal decomposition and to stabilize the crystal structure of Gd3Al5O12 (GdAG) garnet, doping GdAG with smaller Ln3+ (Ln=Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, respectively) to form (Gd,Ln)AG solid solutions was proposed in work. Carbonate precursors of (Gd,Ln)AG with an approximate composition of (NH4)x(Gd,Ln)3Al5(OH)y(CO3)z•nH2O were synthesized via coprecipitation from a mixed solution of ammonium aluminum sulfate and rare earth nitrate, using ammonium hydrogen carbonate as the precipitant. The precursors and the calcination derived oxides were characterized using FT-IR spectroscopy, DTA/TG, XRD, BET and FE-SEM. The results showed that smaller Ln3+ doping can indeed stabilize GdAG against its thermal decomposition to a mixture of GdAlO3 (GdAP) and Al2O3 phases at elevated temperatures and at the same time effectively lowers the temperature for garnet crystallization. The carbonate precursors are loosely agglomerated and the resultant (Gd,Ln)AG powders show good dispersion and a fairly uniform particle morphology. The (Gd,Ln)AG solid solutions exhibit decreasing lattice parameters along with decreasing radius of the dopant ions at the same dopant content of 50 at%. Photoluminescence properties of some of the garnet solid solutions are also studied. The materials developed herein may potentially be used for photoluminescent and scintillation applications.

Luminescence Behaviors of the New Upconversion Phosphors of Yb/Ho Co-Doped (Gd1-xLux)3Al5O12 (x=0.1-0.5) Garnet Solid Solutions

The metastable garnet lattice of Gd3Al5O12 (GdAG) was effectively stabilized via doping with significantly smaller Lu3+, and based on which (Gd,Lu)AG:Yb/Ho was developed in this work as a new type of upconversion phosphor. The phosphor particles calcined from the precursors synthesized via carbonate precipitation were observed to have good dispersion and fairly uniform morphologies. Optical spectroscopy found that the [(Gd1-xLux)0.948Yb0.05Ho0.002]3Al5O12 (x=0.1-0.5) garnet powders exhibit a green emission centered at ~543 nm (the 5F4,5S25I8 transition of Ho3+) and a red emission centered at ~668 nm (the 5F55I8 transition of Ho3+) under laser excitation at 978 nm. The upconversion emission intensity was found to decrease with increasing Lu3+ doping. Meanwhile, the dependence of up-conversion emission intensity on the pumping power was measured and the up-conversion mechanism was discussed in detail. The Yb/Ho codoped (Gd,Lu)AG garnet system developed herein may potentially be used as a new type of luminescent material.

Effect of SnO2 Particle Size on Properties of Ag-SnO2 Electrical Contact Materials Prepared by the Reductive Precipitation Method

Performances of Ag-SnO2 electrical contact materials can be strongly affected by the microstructure. In this work, Ag-SnO2 composite powders were synthesized by chemical reductive precipitation method. During the precipitation process, Ag particle was deposited onto the surface of SnO2 particle with the assistance of citric acid. The microstructure and properties were analyzed for the prepared Ag-SnO2 electrical contact materials. Our research reveals that the particle size of SnO2 has significant influence on the morphology of the Ag-SnO2 composite powders, and therefore on the microstructure and physical properties of the electrical contact materials. With the decrease of particle size of SnO2, hardness of the Ag-SnO2 electrical contact materials increases, while electrical conductivity decreases.

Luminescence Behavior of Tm3+ Activated GdAlO3 Phosphors Synthesized Using Solid-Reaction Method

Trivalent thulium ions (Tm3+) doped GdAlO3 (Gd1-xTmxAlO3) phosphors which show a blue luminescence of high color purity have been synthesized by using solid-state reaction method starting from nanosized powders. X-ray diffraction (XRD) measurements were used to analyze the phase transformations that take place during the preparation of the phosphors. The morphologies of the powders calcined at different temperatures were studied by using scanning electron microscopy (SEM). The luminescence properties of the compounds were investigated. Pure phase of orthorhombic type GdAlO3 (GAP) was yielded by calcining the phosphors at 1200°C for 8 h. The PL spectra showed representative Tm3+ emission. The strong band centered at ~488 nm and the weak one centered at 697 nm were attributed to the 1D2-3F4 and 1G4-3F4 transitions of Tm3+, respectively. The quenching concentration of Tm3+ was estimated to be ~0.75at.% (x=0.0075), for which can be ascribed to the exchange interactions. The decay curve was fitted to be a single exponent and the estimated fluorescent lifetime of the GdAlO3:Tm3+ phosphor was 1.73±0.08 ms.

Synthesis of Dispersed Y2O3 Nanopowder from Yttrium Stearate

Fine yttrium stearate powder was produced at a relatively low temperature using yttrium nitrate hexahydrate, ammonia and stearic acid as the raw materials. Dispersed Y2O3 nanopowder was synthesized by calcining the yttrium stearate. The formation mechanism of the precursor and the Y2O3 nanopowder was studied by means of XRD, TG-DTA, FT-IR, BET, FE-SEM and HR-TEM. Pure and dispersed Y2O3 nanopowder with an average particle size of 30 nm was produced by calcining the precursor at 600 °C. The particle size increases to about 60 nm with the increase of the calcination temperature to 1000 °C. In the preparation of Y2O3 from yttrium stearate, no water medium is involved, thus capillarity force and bridging of adjacent particles by hydrogen bonds can be avoided, resulting in good dispersion of the particles. The dispersed Y2O3 nanopowder prepared in this work has potential application in phosphors and transparent ceramic materials.

Fabrication of Yb:YAG Transparent Ceramics Using Stearate Melting Derived Ultrafine Powders

Yb rare earth doped YAG ultrafine particles were synthesized by the stearate melting method using yttrium stearate, ytterbium stearate and aluminum tristearate as starting materials. The phase formation of Yb:YAG, the properties and the sintering activity of the powders were investigated by means of XRD, SEM, dilatometry and vacuum sintering. The results show that pure Yb:YAG nanopowders can be obtained by calcining the co-melted precursor at a relatively low temperature of 800 °C for 4 h. The powders calcined at 1000°C have better sintering activity than the powders calcined at other temperatures. For the Yb:YAG powders doping with 0.5% TEOS, the compact can be sintered to 99.2% of the theoretical density at 1600 °C and 99.7% at 1700 °C. The transparent Yb:YAG ceramics obtained by vacuum sintering at 1700 °C for 5 h exhibit a pore-free and uniform microstructure.

Novel Direct Coagulation Casting of Alumina Suspensions Using Y3+-Ion Releasing Substances as Coagulant

Direct coagulation casting (DCC) is a relatively new ceramic near-net-shape forming process which can form homogeneous ceramic green body with complex shape and high density. Direct coagulation casting of aqueous alumina slurries by adding Y3+ ions, which have been frequently used as additive for the sintering of alumina ceramics, were studied. Two different kinds of Y3+-ion releasing substances, Y2O3 powder and Y(NO3)3, were used as coagulants and were introduced into stabilized alumina suspensions directly after dispersing. Compared with that of Y2O3 powder, both the coagulation time and the added amount required for a reasonable shaping were significantly reduced for using Y(NO3)3 as coagulant. The effects of the two coagulants on the properties of the consolidated green bodies and sintered ceramics were studied. The mechanisms of coagulation induced by the addition of Y3+-ions were discussed on the basis of interactions between Y3+ ions and the added polyelectrolyte dispersant.

Preparation and Thermoelectric Properties of Ca3Co4O9 Ceramics with Parallel Sheet Shaped Pores

Ca3Co4O9 powders were synthesized by a solid-state reaction method. Porous Ca3Co4O9 ceramics with parallel sheet shaped pores were prepared by a template sacrifice method using epispastic polystyrend (EPS) hollow spheres as the templates. During compaction of the green body, the EPS hollow spheres change into EPS discs due to the pressing force. After sintering, the pores in the Ca3Co4O9 ceramics are sheet shaped, well distributed and parallel to the pressing surface of compaction. The value of ZT merit of the porous Ca3Co4O9 sample obtained with 10 wt% EPS spheres is 0.0489. It was found that the ZT merit value can be improved by changing the density of sample to achieve a high ratio of electrical conductivity to thermal conductivity.

Structural Features and Color Tunable Photoluminescence of the Binary and Ternary Layered Rare-Earth Hydroxides of (Y,Ln)2(OH)5NO3·nH2O (Ln=Tb, Eu)

A series of layered rare-earth hydroxides (LRHs) of (Y,Ln)2(OH)5NO3•nH2O ( Ln=Tb,Eu), have been synthesized via a hydrothermal route. Crystal structures and optical properties of the materials have been investigated in detail by the combined techniques of XRD, FT-IR, FE-SEM, HR-TEM, and PLE/PL spectroscopies. It is shown that Tb3+ and Eu3+ are successfully incorporated into the Y-LRH host lattice to form solid solutions. Under UV excitation, the binary (Y0.97Tb0.03)- and (Y0.97Eu0.03)-LRHs exhibit their respective characteristic photoluminescence of the Eu3+ and Tb3+ activators. The ternary (Y0.965Eu0.005Tb0.03)-LRH simultaneously shows red and green emissions and both the intensity and emission color can be adjusted by changing the excitation wavelength. Compared with (Y0.995Eu0.005)-LRH, the ternary (Y0.965Eu0.005Tb0.03)-LRH exhibits appreciably enhanced 615nm red-emission, which indicates the existence of non- radiative energy transfer from Tb3+ to Eu3+. The ternary (Y0.965Eu0.005Tb0.03)-LRH also readily undergoes anion exchange with a series of inorganic and organic anions at room temperature.

Synthesis of Ultrafine Spherical Yttrium Aluminum Garnet Powders from Yttrium Nitrate and Aluminum Nitrate System

Ultrafine spherical yttrium aluminum garnet (YAG) powders have been synthesized via homogeneous precipitation method using urea as the precipitant. The precursor powders were calcinated at 1000°C or 1100°C for 4 hrs and then were studied by means of FE-SEM, XRD, FT-IR and TG-DTA. The result shows that the amount of ammonium sulfate has a significant effect on morphology and particle size of powders. Pure phase and spherical YAG particles with 350 nm in diameter can be obtained when the molar ratio of ammonium sulfate to aluminum nitrate is about 0.75 and the concentration of the metallic ions is 0.008M.