Haiming Qin

Enhanced thermoelectric performance in p-type polycrystalline SnSe benefiting from texture modulation

Tin selenide (SnSe) compound has attracted much attention due to its unprecedented high ZT (∼2.6) in single crystals. The polycrystalline SnSe materials were then prepared to improve the mechanical performance for large-scaled application. However, the ZT values of 0.3–0.8 were much lower due to their poor electrical properties. In the present study, the zone melting method is employed to prepare the polycrystalline SnSe samples, which show highly textured structures and strong anisotropic thermoelectric performance. A maximum power factor (S2σ) of 9.8 μW cm−1 K−2 was obtained in the polycrystalline samples, which is comparable with that of SnSe single crystals, resulting in a peak ZT of 0.92 at 873 K. The zone-melted ingot was then pulverized into powders and the bulk material was prepared by the spark plasma sintering (SPS) technique. As a result, the ZT value was enhanced to be over 1.0, owing to the slight reduction of lattice thermal conductivity and maintenance of electrical performance. The present investigation indicates that the TE performance of the SnSe compound can be significantly improved by the texture modulation.

Ammonium sulfate regulation of morphology of Nd:Y2O3 precursor via urea precipitation method and its effect on the sintering properties of Nd:Y2O3 nanopowders

Ammonium sulfate has been widely used as a control agent in the preparation of yttrium-aluminium-garnet (YAG) transparent ceramics, however, research of its application in the preparation in transparent ceramic yttria has not been intensively studied. Neodymium–doped yttria (Nd:Y2O3) nanopowders with controlled morphology and size were synthesized via a urea precipitation method using ammonium sulfate as the additive. The effect of ammonium sulfate was intensively studied throughout the preparation process. Morphology of precursors was found to be evidently affected by the [(NH4)2SO4]/[Nd:Y2O3] ratio (measured by weight). Uniform spheres of Nd:Y2O3 precursor were obtained without the addition of ammonium sulfate. With increasing amounts of ammonium sulfate added, the scale of the Nd:Y2O3 precursors diminished which results in the aggregation of the Nd:Y2O3 precursor. Aggregates of coral like particles after precipitation and uniform well dispersed particles after calcinations were obtained as the dosage of ammonium sulfate reached 20 wt%. It was considered to be the optimum state for the preparation of highly sinterable Nd:Y2O3 nanopowders. Ammonium sulfate was proved to be a regulator that could mediate the nucleation and growth of the precursor as well as its decomposition behaviour. Results of this paper can contribute to the controllable synthesis of transparent ceramic yttria.

Enhanced thermoelectric figure of merit in p-type Bi0.48Sb1.52Te3 alloy with WSe2 addition

P-type Bi0.48Sb1.52Te3 alloys added with WSe2 were achieved via the zone melting method. The electrical performance was improved due to the increased carrier concentration, while the lattice thermal conductivity was simultaneously decreased, mainly due to the numerous types of nanoprecipitates produced, such as the second phase, antisite substitution and interfaces with different scales. As a result, a 23% improvement in the thermoelectric figure of merit zT was obtained with the addition of WSe2, making these composites more attractive for commercial applications.

Electrocatalytic oxidation of nucleobases by TiO2 nanobelts

Two kinds of TiO(2) nanobelts were prepared from commercial P-25 powders via an alkaline hydrothermal method with and without an acid etching process. The uncauterized nanobelts (TNs) exhibited a smooth surface, and mixed phases of anatase and TiO(2) (B), whereas the cauterized ones (CTNs) displayed a rough surface and a pure anatase structure. TNs and CTNs were then deposited onto a glassy carbon electrode (GCE) surface with a conductive adhesive (CA), and the resulting chemically modified electrodes exhibited electrocatalytic activities in the oxidation of nucleobases in a 0.1 M phosphate buffer solution (PBS) at pH 7.4. For guanine and adenine, well-defined oxidation peaks were observed in voltammetric measurements at about +0.62 and +0.89 V, respectively, at a potential sweep rate of 100 mV s(-1), whereas for cytosine, uracil and thymine, the voltammetric features were not obvious. The average surface coverages (Γ) of guanine and adenine on the CTNs/CA/GCE electrode were estimated to be 4.75 × 10(-10) and 7.44 × 10(-10) mol cm(-2), respectively, which were about twice those at the TNs/CA/GCE electrode. The enhanced activity of the CTN-based electrode towards purine nucleobase oxidation was ascribed to the large specific surface area and anatase structures with enhanced (001) facets of the CTN that facilitated adsorption of the analytes onto the electrode surface and charge transport through the electrode surface layer.

Effect of Yb3+ on the Crystal Structural Modification and Photoluminescence Properties of GGAG:Ce3+

Gadolinium gallium aluminum garnet (GGAG) is a very promising host for the highly efficient luminescence of Ce(3+) and shows potential in radiation detection applications. However, the thermodynamically metastable structure would be slanted against it from getting high transparency. To stabilize the crystal structure of GGAG, Yb(3+) ions were codoped at the Gd(3+) site. It is found that the decomposition of garnet was suppressed and the transparency of GGAG ceramic was evidently improved. Moreover, the photoluminescence of GGAG:Ce(3+),xYb(3+) with different Yb(3+) contents has been investigated. When the Ce(3+) ions were excited under 475 nm, a typical near-infrared region emission of Yb(3+) ions can be observed, where silicon solar cells have the strongest absorption. Basing on the lifetimes of Ce(3+) ions in the GGAG:Ce(3+),xYb(3+) sample, the transfer efficiency from Ce(3+) to Yb(3+) and the theoretical internal quantum efficiency can be calculated and reach up to 86% and 186%, respectively. This would make GGAG:Ce(3+),Yb(3+) a potential attractive downconversion candidate for improving the energy conversion efficiency of crystalline silicon (c-Si) solar cells.

Mechanism of ammonium sulfate regulation effect on microstructure of Y2O3 nanopowders via urea precipitation method

In the preparation of transparent ceramics and ultrafine oxide nanopowders, ammonium sulfate ((NH4)2SO4) has been widely used as the control agent. However, the mechanism of its regulation effect has not been well studied. In the present work, precursors of yttria (Y2O3) nanopowders with controlled morphology were synthesized via a urea precipitation method using ammonium sulfate as the additive. The effect of (NH4)2SO4 is discussed throughout the preparation process. Two opposite morphology transformation modes were noticed for the precursor samples made with and without (NH4)2SO4 during the calcination process. Flocculent precursors and flowers assembled by the plate-shaped particles were obtained in the systems without and with (NH4)2SO4. By calcination, the aggregated 200–400 nm particles with massive defects and 100 nm particles with homogeneous crystalline structure were obtained, respectively. The mechanism of (NH4)2SO4 as a regulator that could mediate the nucleation and growth of the precursors as well as its decomposition behavior is verified.

Microstructural characteristics of Nd:YAG powders leading to transparent ceramics

Abstract We reported on the successful synthesis of the Nd:YAG (Nd:Y3Al5O12) nano-powders by using urea as the precipitant with the microwave assisted homogeneous precipitation (MAHP) method. The different microstructural characteristics of the Nd:YAG nano-powders were affected by the concentrations of (Y3++Nd3+) and Al3+ ([Y3++Nd3+]=0.06 mol/L, [Al3+]=0.1 mol/L), aging time (6 d) and aging condition (in vessel). The optimum microstructural characteristics of the high quality Nd:YAG nano-powders leading to transparent Nd:YAG ceramics including the pure YAG phase, the smallest crystallite size, a uniform crystallite size distribution, less density defects, uniform micro-components and the proper molar ratio of (Y3++Nd3+) and Al3+ (0.6148) were discussed.

Effect of composition deviation on the microstructure and luminescence properties of Nd:YAG ceramics

Neodymium-doped yttrium aluminum garnet (Nd:YAG) ceramics with different Al2O3/Y2O3 deviation levels were prepared using a solid-state reaction-vacuum sintering method. Transparent ceramics were obtained with their composition in between stoichiometric Nd: YAG Y2.97Nd0.03Al5O12 and 2% Y2O3 excess Nd: YAG Y2.97Nd0.03Al5O12.2%Y2O3. The stoichiometric Nd: YAG ceramic obtains a homogeneous grain microstructure and clean grain boundaries. Non-stoichiometric compositions lead to ceramic microstructures different from that of the stoichiometric Nd: YAG. Nanograins of excess Al2O3 remain inside the Nd: YAG grains in Al2O3-excess ceramics, while Y-rich compounds recrystallize at the Nd: YAG grain boundaries in Y2O3-excess ceramics. Photoluminescence spectra of the sintered ceramics around 808 and 1064 nm were also studied. Peak positions of their luminescence spectra are little affected by composition deviation, while their intensity evidently declines due to the existence of a second phase arising from the composition deviation. The results of this work can contribute toward the controllable synthesis of highly transparent YAG ceramics and further exploration of the Nd ion spectra.

YAG phosphor with spatially separated luminescence centers

A micron size YAG:Ce/YAG:Cr core–shell structure was designed and accomplished via the urea homogeneous precipitation method using the YAG:Ce spherical core as the introduced second phase. A well dispersed gel like encapsulation structure can be achieved before the formation of YAG:Ce/YAG:Cr core–shell particles via a calcination process. As prepared YAG:Ce/YAG:Cr particles can emit a broad range of photons from 500 to 750 nm with excitation light of 433 nm. A schematic illustration showing the mechanism of excitation–emission of the core–shell particles is presented. The integral spectra are composed of three parts: emission photons of YAG:Cr, YAG:Ce, and emission light of YAG:Cr excited by the emission photons of the YAG:Ce core according to the proposed mechanism. The method accomplished in this work can significantly improve the exploration of full spectrum luminescent powder synthesis and spectra designation.

Fabrication of cerium-doped nonstoichiometric (Ce, Lu, Gd)3+δ(Ga, Al)5–δO12 transparent ceramics

Abstract Cerium-doped nonstoichiometric (Ce,Lu,Gd) 3+ δ (Ga,Al) 5– δ O 12 (LuGGAG) transparent garnet ceramic samples were fabricated via a solid state reaction method in this study. The ceramics were prepared via oxygen sintering followed by hot isostatic pressing (HIP). The phase and microstructure of the samples were analyzed by X-ray diffraction and scanning electron microscopy (SEM), respectively. The excitation, emission and transmission spectra were also measured. The total optical transmittance of the annealed LuGGAG ceramics with thickness of 3 mm reached 47% at the emission wavelength of 555 nm. The decay time was about 60 ns. Compact microstructure of polycrystalline grains with scale around 5 μm were gained according to scanning electron microscopy characterization. The successful preparation of the bulk ceramic material and implementation of the combined oxygen sintering-hot isostatic pressing treatment process provided an important method for the exploration of nonstoichiometric scintillator material.