Xin-Xin Yang

Energy transfer mechanisms in Tb3+, Yb3+ codoped Y2O3 downconversion phosphor

Tb3+ and Yb3+ co-activated luminescent material that can cut one photon of around 483 nm into two NIR photons of around 1000 nm could be used as a downconversion luminescent convertor in front of crystalline silicon solar cell panels to reduce thermalization loss of the solar cell. The Tb3+ → Yb3+ energy transfer mechanisms in the UV–blue region in Y2O3 phosphor were studied by PL excitation spectra and time-resolved luminescence, from which the charge transfer mechanism and the cooperative transfer mechanism were identified. Tb3+ ions in the 4f75d1 state relax down to the 5D4 level and cooperatively transfer energy to two Yb3+ ions, which is followed by the emission of two photons (λ ~ 1000 nm). It was found in the (Y0.79Tb0.01Yb0.20)2O3 sample that 37% of the Tb3+ ions at the 5D4 level transfer energy to two neighbouring Yb3+ ions by the cooperative energy transfer mechanism Tb3+ (5D4) → 2Yb3+ (2F5/2). Unfortunately, the high Yb3+ concentration leads to severe concentration quenching that significantly reduces the external quantum efficiency. Moreover, the energy of the Tb3+ 4f75d1 state can also be lost non-radiatively or transferred to the Yb3+ 2F5/2 state via the charge transfer state Tb4+–Yb2+. In conclusion, RE3+ (RE = Ce, Pr, Tb) with strong absorption in the UV region is not an appropriate sensitizer of Tb3+ in Tb3+–Yb3+ codoped downconversion phosphor.

Synthesis and high thermoelectric efficiency of Zintl phase YbCd2-xZnxSb2

We synthesized a series of polycrystalline YbCd2−xZnxSb2 (x=0, 0.4, 0.8, 1, 1.2, 1.6, and 2) samples and measured their thermoelectric properties. Thermoelectric figure of merit ZT at 700K is higher than 1.0 for Cd-rich samples (x=0, 0.4, 0.8, and 1.0) and Zn substitute of Cd in YbCd2Sb2 can easily tune carrier concentration and reduce thermal conductivity. When x=0.4, sample exhibits the highest power factor (12–20μWcm−1K−2), the lowest lattice thermal conductivity (1.0Wm−1K−1 at 300K), highest ZT (1.2 at 700K), and best “self-compatibility.” The first principles calculations were performed to study the influences of bonding and electronic structures on physical properties.

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 and electronic structure of Zintl compound BaZn2Sb2

Polycrystalline sample of the title compound was prepared and its thermoelectric properties from 2to675K were investigated. This Zintl compound shows rather low thermal conductivity, 1.6Wm−1K−1, at room temperature. The value of its thermoelectric figure of merit ZT reaches 0.31 at 675K. Its electronic structure, calculated by ab initio methods, suggests that the electrical transport are mainly ascribe to [Zn2Sb2] framework for p-type BaZn2Sb2. The heat capacity curve at low temperature was fitted lineally to obtain Debye temperature (about 208K). It provides the authors with a host lattice for modification and optimization the thermoelectric properties through substitution and/or doping.

Zintl phase Yb1−xCaxCd2Sb2 with tunable thermoelectric properties induced by cation substitution

It has been shown previously that the thermoelectric properties of the Zintl phase compound YbCd2Sb2 can be finely tuned via Zn substitution at the Cd-site in the anionic (Cd2Sb2)2− framework. Here we report the results of the investigation of isoelectronic substitution of Yb by Ca. The p-type Yb1−xCaxCd2Sb2 (0.2≤x≤0.8) samples have been synthesized via a solid-state reaction followed by suitable cooling, annealing, grinding, and spark plasma sintering densification processes. In samples with x=0.2, 0.4, 0.5, 0.6, 0.8, the electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been carried out in the temperature range from 300 to 650 K. It is found that the Ca substitution effectively lowers the thermal conductivity for all samples at high temperature, while it significantly increases the Seebeck coefficient. As a result, the dimensionless figure of merit ZT of 0.96 has been attained at 650 K for samples with x=0.4, while the value is 0.78 for the unsubstituted YbCd2Sb2

Luminescent metastable Y2WO6:Ln3+ (Ln = Eu, Er, Sm, and Dy) microspheres with controllable morphology via self-assembly

Three-dimensional (3D) Y2WO6 and Y2WO6: Ln3+ microspheres with novel controllable morphology have been obtained by the hierarchical self-assembly via a hydrothermal synthesis route with a subsequent heat treatment. The amount of citric acid and polyvinylpyrrolidone (PVP) play crucial roles in the controlling of the morphologies. Hollow, core/shell as well as waxberry-shape Y2WO6 microspheres can be prepared in different reaction systems under the same facile hydrothermal conditions. Metastable phases of the samples form when they are crystallized at low temperature, which transfer to the thermodynamically stable counterpart above 1100 °C. This phase transformation is irreversible. The mechanism of reaction and self-assembly evolution process are proposed. The as-prepared Y2WO6:Ln3+ (Ln = Eu, Er, Sm, and Dy) samples show strong multi-color visible light emission under ultraviolet-visible light excitation. Compared with the monoclinic one, this metastable Y2WO6: Eu3+ phase exhibits different luminescent properties. Due to multi-color luminescent properties, these Ln3+-doped (Ln = Eu, Er, Sm, and Dy) metastable Y2WO6 morophologies controllable microspherical samples may be promising for further fundamental research and find applications in color displays.

Luminescent properties of REBa3B9O18 (RE = Lu, Tb, Gd, Eu) under VUV excitation

The VUV excited luminescent properties of REBa3B9O18 (RE = Lu, Tb, Gd, Eu) were investigated. The bands near 160 nm in the VUV excitation spectra of those compounds are attributed to the host absorptions; at the same time, the energy of host absorption is shifted to a higher energy region with decreasing RE3+ radius. It is observed that there is energy transfer between the hosts and luminescent centres (e.g. Tb3+, Gd3+ and Eu3+). From the standpoints of colour purity and luminescent efficiency, the phosphor of TbBa3B9O18 is an attractive candidate for new green-emitting phosphors for plasma display panels and mercury-free fluorescent lamps.

VUV spectroscopic properties of rare-earth (RE3+ = Eu, Tb, Tm)-doped AZr2(PO4)3 (A+ = Li, Na, K) type phosphate

The excitation spectra of pure and rare-earth (RE3+ = Eu, Tb, Tm)-doped AZr2(PO4)3 (A+ = Li, Na, K) in the vacuum ultraviolet (VUV) regions are investigated. The results indicate that these samples show strong absorption in the VUV range. The band ranging from 130 to 160 nm is due to the absorption band of host lattice or PO4 groups; the band at 160–200 nm is related to the charge transfer (CT) transition of O–Zr. For Eu3+-doped samples, the weak band in the excitation spectra at about 208 nm is related to the CT band of O2−–Eu3+. As for Tb3+-doped samples, the strong band at 225 nm and the weak band at 261 nm in the excitation spectra are assumed to the spin-allowed and spin-forbidden f–d transitions (4f8→ 4f75 d1) of Tb3+. However, there is no obvious band of CT of O2−–Tm3+ in the Tm3+-doped samples.

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.

High quality thin film phosphors of Y2O3:Eu3+ deposited via chemical bath deposition

High transparency in visible region was required for red-light-emitting Y2O3:Eu3+ thin film phosphors. Such films were obtained via chemical bath deposition on bare SiO2 glass substrates through heterogeneous nucleation with further heat treatment. Thin amorphous yttrium basic carbonate films could be completely transformed to crystalline Y2O3 at 650 °C. X-ray diffraction and field-emission scanning electron microscopy were used to characterize these products. The deposition temperature and the post-annealing conditions were crucial factors for preparing high-quality thin films. The thin films prepared with optimized parameters emitted a bright red light around 612 nm. Meanwhile, these thin films exhibited optical transmittance of 90% to 95% in a wavelength range of 400 to 800 nm depending on the chemical deposition solution. These results clearly indicated that chemical solution deposition is a promising technique for large-scale production of thin film phosphors in application of display technology.