Lidong Chen

Preparation and upconversion luminescence of YAG(Y3Al5O12): Yb3+, Ho3+ nanocrystals

Cubic YAG: Yb3+, Ho3+ pure phase nanocrystals were synthesized by using coprecipition nitrate and ammonium hydrogen carbonate as raw materials. After calcining the precipitates at 800 °C, the resultant YAG: Yb3+, Ho3+ nanocrystals were nearly spheric and the particle size was about 40 nm. Intense upconversion spectra were observed on the powder compact pumped by a 980 nm continuous wave diode laser, and green emission centered at 549 nm, red emission centered at 667 nm, and NIR centered at 760 nm were all due to two photons process, which originated from 5S2 (5F4)→5I8, 5F5→5I8, and 5S2 (5F4)→5I7 transitions, respectively.

Effect of γ-Al2O3 Content on the Thermoelectric Performance of ZrNiSn/γ-Al2O3 Composites

ZrNiSn matrix composites containing -Al 2O3 nano particles with a relative density of 98% were prepared by spark plasma sintering technique. Thermal conductivity, Seebeck coefficient, electrical resistivity were measured. It was shown that the addition of small amount of -Al 2O3, which served as the additional phonon-scattering centers, led to a re lative great reduction in thermal conductivity without significantly affecting the power factor. The dimensionless figure of merit ZT of ZrNiSn half-Heusler thermoelectric material was improve d by introducing -Al 2O3 nano particles.

Preparation and Upconversion Luminescence of YAG: Er3+ Powders

Y2.85Er0.15Al5O3 was prepared by combustion process. Phase evolution of the synthesized powders was determined by XRD. Single-phase cubic YAG: Er3+ crystalline powders were obtained by calcining the combusted product at 1000°C and no intermidiate phase was observed. SEM morphology showed that the resultant YAG: Er3+ powders were porous. Upconversion emission spectra were measured on the powder compact pumped by a 980nm continuous wave diode laser. The relationship between pump power and luminescence intensity suggested that a two-photon process is active in this sample. The upconversion machanism of YAG: Er3+ system was also discussed.

Effect of annealing treatment on thermoelectric properties of n-type Bi-Te-Se sintered materials

In this study, n-type Bi-Te-Se crystals were prepared by the zone melting (ZM) method and bulk materials with high bending strength were fabricated by ZM followed by spark plasma sintering (SPS) technique. The annealing treatment was then carried out at 593 K with holding time of 36 hr for the sintered materials. Thermoelectric properties, including Seebeck coefficient (alpha), electrical conductivity (sigma) and thermal conductivity (kappa) were measured in the temperature range of 300-500 K. The results showed the maximum figure of merit (ZT = alpha2sigmaT/kappa) of 0.79 was obtained for the n-type Bi-Te-Se material through the annealing treatment, which was very close to that of the zone-melted crystal.

Infrared to Visible Upconversion Luminescence in Yb3+, Ho3+: Y2O3 Nanocrystalline Powders

Yb3+, Ho3+ co-doped Y2O3 nanocrystalline powders were synthesized via a reverse-strike co-precipitation method using nitrates and ammonia as raw materials. The as-prepared powders are cubic Y2O3 after fired at 7000C for 2 h. Field emission scanning electronic microscopy revealed that most of the particles are nearly spherical and the average diameter of the particles is in the range of 60-80 nm. Under the excitation with a 980 nm continuous wave diode laser, two main emission bands, i.e., green emission centered at 549 nm and red emission centered at 666 nm is observed, respectively. The green emission is attributed to the transition of (5S2, 5F4-5I8) levels of Ho3+; while for the red emission, the transition of 5F5-5I8 levels is suggested. In addition, near infrared emission centered at 755 nm is also detected, corresponding to the transition of (5S2, 5F4-5I7) levels. The dependence of the intensity of upconversion emissions upon the pump power of the diode laser suggests that two-photon absorption mechanism be involved in these processes.

Effect of C60 Particle Dispersion on Thermal Conductivity of CoSb3

CoSb3–C60 composites were prepared by solid state reaction and sintering process. The constituent phases of the samples were determined by XRD. SEM obse rvation showed that C 60 particles dispersed in CoSb 3 matrix uniformly. The thermal conductivity was reduced greatly by dispersing C60 particles as extended defects between 300K and 800K. Thermal conductivity decreased with increasing C 60 content. The effect of the particle distribution on thermal conductivity of skutterudites is discussed.

Thermoelectric Properties of n-type (Bi2Se3)x(Bi2Te3)1-x Crystals Prepared by Zone Melting

In the present study, n-type (Bi2Se3)x(Bi2Te3)1-x crystals with various chemical compositions were fabricated by the zone melting method. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ) and thermal conductivity (κ), were measured in the temperature range of 300-500 K. The influence of the variations of Bi2Te3 and Bi2Se3 content on thermoelectric properties was studied. The increase of Bi2Se3 content (x) caused an increase in carrier concentration and thus an increase of σ and a decrease of α. The maximum figure of merit (ZT = α2σT/κ) of 0.87 was obtained at about 325 K for the composition of 93%Bi2Te3-7%Bi2Se3 with doping TeI4.