Zhen-Yong Man

A new type of thermoelectric material, EuZn2Sb2.

Polycrystalline EuZn(2)Sb(2) is prepared by direct reaction of the elements. Its composition, structure, magnetism, heat capacity, and thermoelectric properties have been investigated. EuZn(2)Sb(2) crystallizes in p3m space group with a=4.4932(7) A and c=7.6170(10) A. Antiferromagnetic ordering is detected at the Neel temperature of 13.06 K, and the saturation magnetization reaches 6.87mu(B)Eu at 2 K and 7 T. Eu ion has +2 valence. Its Hall effects are characterized by a high positive Hall coefficient of +0.226 cm(3)C, proper carrier concentration of 2.77x10(19)cm(3), and high carrier mobility of 257 cm(2)V s at 300 K. This compound shows high p-type Seebeck coefficient (+122 to +181 muVK), low lattice thermal conductivity (1.60-0.40 Wm K), and high electrical conductivity (1137-524 Scm). The obtained figure of merit and powder factor reach 0.92 and 20.72 muWcm K(2), respectively. The thermoelectric properties of EuZn(2)Sb(2) are encouraging.

Thermoelectric properties of YbxEu1−xCd2Sb2

The thermoelectric performance of EuCd(2)Sb(2) and YbCd(2)Sb(2) was improved by mixed cation occupation. The composition, structure, and thermoelectric properties of Yb(x)Eu(1-x)Cd(2)Sb(2) (x=0, 0.5, 0.75, and 1) have been investigated. Polycrystalline samples are prepared by direct reaction of the elements. Thermoelectric properties were investigated after densification of the materials by spark plasma sintering. Yb(x)Eu(1-x)Cd(2)Sb(2) crystallizes in the P3m1 space group. The lattice parameters increase with the europium content. These materials show low electrical resistivity, high Seebeck coefficient, and low thermal conductivity together with high carrier concentration and high carrier mobility. ZT values of 0.88 and 0.97 are obtained for Yb(0.5)Eu(0.5)Cd(2)Sb(2) and Yb(0.75)Eu(0.25)Cd(2)Sb(2) at 650 K, respectively.

Preparation, electronic structure and photoluminescence properties of RE (RE = Ce, Yb)-activated SrAlSi4N7 phosphors

The electronic structure of SrAlSi4N7 was calculated using the CASTEP code and SrAlSi4N7 is an intermediate band gap semiconductor with an indirect energy gap of 3.6 eV. Ce3+ and Yb2+-activated SrAlSi4N7 samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. SrAlSi4N7:Ce3+ shows a broad band emission in the wavelength range of 450–700 nm, originating from the 5d1–4f1 transition of Ce3+. The emission band of Ce3+ shifts to longer wavelength with an increase of Ce3+ concentration due to the increased Stokes shift and reabsorption by Ce3+. SrAlSi4N7:Yb2+ can be excited efficiently over a broad spectral range between 300 and 550 nm, and exhibits a single intense red emission at 600 nm with a full width at half maximum of 95 nm due to the electronic transitions from the excited state of 4f135d to the ground state 4f14 of Yb2+. The low energy of Yb2+ emission in SrAlSi4N7 can be attributed to the large nephelauxetic effect and crystal field splitting due to the coordination of Yb2+ by nitrogen. In addition, Sr1−2xCexLixAlSi4N7 shows higher thermal stability in air than that of Sr1−yYbyAlSi4N7 (0 ≤ x, y ≤ 0.1). A white-light LED can be generated by using single SrAlSi4N7:Ce3+ as the wavelength conversion phosphor combined with a blue LED chip (InGaN). The obtained LED exhibits a luminous efficiency of 74.3 lm W−1 at a corrected color temperature (CCT) up to 6350 K, and the color rendering index (CRI Ra) is around 78. These novel developed yellow-red phosphors have potential applications in spectral conversion materials for white-LEDs.

Preparation and Characterization of Non-Stoichiometric Yttrium Aluminum Garnet (YAG) with Antisite Defects as a Potential Scintillator

Non-stoichiometric YAG powder samples were prepared via a simple sol-gel combustion method by introducing antisite defects YAl,16a intentionally. The investigation of structure and luminescence properties confirmed that the antisite defects with different concentrations can be created through adjusting the composition in YAG at relatively low (1000°C) temperature. The X-ray excited luminescence and vacuum ultra-violet spectroscopies show that the luminescence intensity of non-stoichiometric YAG increases with increasing antisite defects concentration, which is much stronger than that of the stoichiometric counterpart under the same conditions. It indicates that non-stoichiometric YAG with antisite defects YAl,16a might be used as a potential candidate for scintillator.

Morphology controllable synthesis of yttrium oxide-based phosphors from yttrium citrate precursors

Abstract A novel yttrium citrate-templated conversion method for morphology controlled synthesis of Y2O3 microspheres, microflowers and microsheets was reported for the first time. The precursors with controllable morphologies were synthesized with a homogenous precipitation method in aqueous solution without any surfactant. Y2O3 samples with well-preserved morphological architectures were obtained by a subsequent thermal transformation strategy. The chemical formula of the precursor was identified and a two-stage growth mechanism was proposed. The effects of the aging time, reaction temperature, reactant concentration and molar ratio of yttrium nitrate to sodium citrate were discussed. The photoluminescence properties of the Y2O3:Eu3+ microspheres, microflowers and microsheets prepared were also studied.

Thermoelectric properties of Eu(Zn(1-x)Cd(x))2Sb2.

The thermoelectric performance of EuZn(2)Sb(2) and EuCd(2)Sb(2) was optimized by mixed occupation of the transition metal position. Samples in the solid solution Eu(Zn(1-x)Cd(x))(2)Sb(2) with the CaAl(2)Si(2)-type crystal structure (space group Pm1) were prepared from the elements for compositions with x = 0, 0.1, 0.3, 0.5 and 1. The thermoelectric properties were investigated after densification of the products by spark plasma sintering (SPS). The samples show low electrical resistivity, high thermopower and a low lattice thermoconductivity. The highest ZT value of 1.06 at 650 K is obtained for x = 0.1.