Chang-Peng Li

Low thermal conductivity and high thermoelectric figure of merit in n-type BaxYbyCo4Sb12 double-filled skutterudites

Filled skutterudites are one of the most promising thermoelectric materials for power generation applications. The choice and concentration of filler atoms are key aspects for achieving high thermoelectric figure of merit values. We report on the high temperature thermoelectric properties in the double-filled skutterudites BaxYbyCo4Sb12. The combination of Ba and Yb fillers inside the voids of the skutterudite structure provides a broad range of resonant phonon scattering and consequently a strong suppression in the lattice thermal conductivity is observed. A dimensionless thermoelectric figure of merit of 1.36 at 800K is achievable for n-type BaxYbyCo4Sb12.

Temperature induced single domain–vortex state transition in sub-100nm Fe nanodots

Magnetization reversal in nanomagnets via a vortex state, although often investigated at the remanent state, may not necessarily display a zero remanence or a highly pinched hysteresis loop. In contrast, the irreversible nucleation/annihilation events are clear indications of a vortex state. In this work, temperature induced single domain–vortex state transition has been investigated in 67nm Fe nanodots using a first-order reversal curve (FORC) technique. The two phase coexistence is manifested as different features in the FORC distribution. At lower temperatures, it becomes harder to nucleate and annihilate vortices and the amount of single domain dots increases.

Angular Dependence of Vortex Annihilation Fields in Asymmetric Co Dots

Shape asymmetries in nominally circular nanomagnets provide a potential means for vortex chirality control. However, in realistic arrays their effects are challenging to probe since asymmetric magnetization reversal processes are often averaged to include distributions over all angles. Here we investigate how shape asymmetry influences the vortex reversal in arrays of sub-micron edge-cut Co dots. We find that the vortices can be manipulated to annihilate at particular sites under different field orientations and cycle sequences. The vortex annihilation field and degree of chirality control depend sensitively on the angular position of the applied field relative to the flat edge of the dots. For small angles, the major loop annihilation field is significantly larger than that found from the half loop and the vortex chirality can be well controlled. At intermediate angles the chirality control is lost and an interesting crossover in the annihilation field is found: the half loop actually extrudes outside of the major loop, exhibiting a larger vortex annihilation field. At large angles the annihilation fields along major and half loops become degenerate.

Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area

Porous alumina masks are fabricated by anodization of aluminum films grown on both semiconducting and insulating substrates. For these self-assembled alumina masks, pore diameters and periodicities within the ranges of 10–130 and 20–200nm, respectively, can be controlled by varying anodization conditions. 20nm periodicities correspond to pore densities in excess of 1012 per square inch, close to the holy grail of media with 1Tbit∕in.2 density. With these alumina masks, ordered sub-100-nm planar ferromagnetic nanodot arrays covering over 1cm2 were fabricated by electron beam evaporation and subsequent mask lift-off. Moreover, exchange-biased bilayer nanodots were fabricated using argon-ion milling. The average dot diameter and periodicity are tuned between 25 and 130nm and between 45 and 200nm, respectively. Quantitative analyses of scanning electron microscopy (SEM) images of pore and dot arrays show a high degree of hexagonal ordering and narrow size distributions. The dot periodicity obtained from grazi...

Direct observation of cooperative effects in capillary condensation : The hysteretic origin

We tailor anodized alumina with pores with well-defined, simple geometries, in order to study the effects of constrictions in hysteretical capillary condensation, independently from other cooperative processes such as network effects. We confirm, by direct observation using optical interferometry, the occurrence of two cooperative phenomena: the classical pore blocking effect in nearly ideal “inkbottle” pores and the advanced adsorption in pores with a change in the cross section. They are relevant for the development of a theory of the poorly understood hysteresis in complex porous materials.

Effect of surface interactions on the hysteresis of capillary condensation in nanopores

Gas adsorption and liquid desorption of a number of organic vapors in anodized nanoporous alumina, with controlled geometry (cylindrical pore diameters from 10 to 60 nm), are studied using optical interferometry. The narrow-diameter distribution of disconnected pores allows checking the validity of the (long-predicted but not experimentally verified) Kelvin equation without any adjustable parameters, modeling or other assumptions. Evaporation occurs at liquid-vapor equilibrium according to this equation, whereas condensation occurs from metastable states of the vapor phase by nucleation, enhanced by surface defects inside the nanopores. This produces hysteresis, in qualitative agreement with theoretical models and simulations that use Van der Waals interactions between the fluid and the pore surface. The reproducibility of the hysteresis depends on the strength of these interactions, which play an important role in the dynamics of capillary condensation.

Figure of merit of quaternary (Sb0.75Bi0.25)2−xInxTe3 single crystals

Single crystals of a quaternary system based on (Sb0.75Bi0.25)2Te3 doped with In were prepared using the Bridgman technique. Samples with varying contents of In were characterized by the measurements of electrical conductivity σ⊥c, Hall coefficient RH(B∥c), Seebeck coefficient S(ΔT⊥c), and thermal conductivity κ(ΔT⊥c). The measurements indicate that by incorporating In in (Sb0.75Bi0.25)2Te3 one lowers the concentration of free holes. This effect is explained in terms of a point defect model in the crystal lattice. We also discuss the temperature dependence of the thermoelectric figure of merit Z=σS2∕κ of the samples. It is observed that low concentrations of In atoms in the (Sb0.75Bi0.25)2Te3 crystal lattice result in a substantial increase in the parameter Z in the temperature region 100–300K.

Deconvoluting reversal modes in exchange-biased nanodots

Ensemble-averaged exchange bias in arrays of Fe/FeF

Vortex lattice dynamics in Nb films with competing intrinsic random and artificial periodic pinning

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Thermal imaging of high power diode lasers subject to back-irradiance

nanodots has been deconvoluted into local, microscopic bias separately experienced by nanodots going through different reversal modes. The relative fraction of dots in each mode can be modified by exchange bias. Single-domain dots exhibit a simple loop shift, while vortex state dots have asymmetric shifts in the vortex nucleation and annihilation fields, manifesting local incomplete domain walls in these nanodots as magnetic vortices with tilted cores.