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X-ray diffraction and imaging techniques at the Advanced Photon Source (APS) have been used to characterize PbSe/MoSe 2 and PbSe/WSe 2 thin films and multilayers. Diffraction measurements with area detectors were made to demonstrate the nanoscale coherence size of the layers both in-plane and perpendicular to the surface. Higher-resolution diffraction measurements that exploit the tunability of synchrotron radiation for anomalous scattering contrast have been used to accurately determine layer spacings in these multilayers. We also discuss opportunities in other new and emerging x-ray techniques, such as time-resolved studies during growth or processing, and present x-ray imaging capabilities that can be used to investigate thermoelectric thin-film materials.

The synthesis of several new families of misfit-layered compounds is demonstrated. These compounds are crystalline along the c-axis and in the ab-plane, but show very short coherence lengths between ab-planes. This disorder leads to ultra-low and tunable thermal conductivity. Annealing isostructural samples under a chalcogen vapour until equilibrium is reached results in reproducible Seebeck and resistivity values. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The novel, partially filled compounds CexCo4Ge6Se6 were synthesized using two methods: low-temperature interdiffusion and nucleation of ultrathin elemental layers and a bulk, high-temperature technique. The compounds were characterized by differential scanning calorimetry, powder X-ray diffraction, and electron probe microanalysis. The lattice parameters for the samples synthesized using the high-temperature technique were calculated to be 8.30 A, while the samples synthesized using the elemental deposition technique showed lattice expansion proportional to the amount of incorporated rare earth element, with lattice parameters ranging from 8.30 A for x = 0 up to a lattice parameter of 8.34 A for x = 0.2. Rietveld analysis on X-ray diffraction data for both sets of samples showed incorporation of the rare earth element into the interstitial site, up to 20% occupancy.

Close attentions have been widely paid to the engineering textured and slip surfaces for improving bearing tribological performances. Comparison studies on the tribological characteristics of slip and textured surfaces are carried out in this work. The analysis results point out that the influences of surface texture and boundary slip on tribological performances of slider bearing are strongly similar. For the determinate surface textures, there is one and only value of slip velocity to make the tribological performances of textured and slip surfaces in agreement. The corresponding relation between the slip velocity and the texture structure parameters is also obtained, and the size of slip velocity is directly related to the texture geometry parameters including its position parameters. This study will help us to further understand the relationship between boundary slip and surface texture and also the slip phenomenon.

The targeted synthesis of multiple compounds with specific controlled nanostructures and identical composition is a grand challenge in materials chemistry. We report the synthesis of the new metastable compounds [(PbSe)1.00]m(MoSe2)n using precursors each designed to self-assemble into a specific compound. To form a compound with specific values for m and n, the number of atoms within each deposited elemental layer was carefully controlled to provide the correct absolute number of atoms to form complete layers of each component structural unit. On low-temperature annealing, these structures self-assemble with a specific crystallographic orientation between the component structural units with atomically abrupt interfaces. There is rotational disorder between the component structural units and between MoSe2 basal plane units within the MoSe2 layers themselves. The lead selenide constituent has a distorted rock salt structure exactly m bilayers thick leading to peaks in the off-axis diffraction pattern as a result of the finite size of and rotational disorder between the crystallites. The in-plane lattice parameters of the PbSe and MoSe2 components are independent of the value of m and n, suggesting little or no strain caused by the interface between them. These compounds are small band gap semiconductors with carrier properties dominated by defects and exhibit extremely low thermal conductivity as a result of the rotational disorder. The thermal conductivity can be tuned by varying the ratio of the number of ordered PbSe rock salt layers relative to the number of rotationally disordered MoSe2 layers. This approach, based on controlling the local composition of the precursor and low temperature to limit diffusion rates, provides a general route to the synthesis of new compounds containing alternating layers of constituents with designed nanoarchitecture.

The title compounds are prepared as thin films using the modulated elemental reactant method and their structures are characterized by synchrotron powder XRD and TEM.

The interaction between particles and a fluid is very important in many fields. The hydrodynamic characteristics of both spheroidal particles and non-spherical particles sediment in the fluid and their rebound dynamics were investigated by using the fictitious domain method-discrete element method (FDM-DEM) in this paper. The novelty lies in the combination of the Monte Carlo scheme with FDM to improve the accuracy of hydrodynamic force. A soft-sphere scheme of DEM is used to model the collision of particles. The hydrodynamic force on the particle is fully resolved by the FDM. The numerical results of spherical particle are verified by comparing the previous numerical and experimental results implemented by Cate. The results of non-spherical particles sediment show that the path instability occurs. Those results are in good agreement with the corresponding published data. The method presented in this paper can be used in practical applications.

We report a transformative, all inorganic synthesis method of preparing supported bimetallic Pd(3)Ag alloy nanoparticles. The method involves breaking down bulk Pd(3)Ag alloy into the nanoparticles in liquid lithium, converting metallic Li to LiOH, and transferring Pd(3)Ag nanoparticles/LiOH mixture onto non-water-soluble supports, followed by leaching off the LiOH with water under ambient conditions. The size of the resulting Pd(3)Ag nanoparticles was found narrowly distributed around 2.3 nm characterized by transmission electron microscope (TEM). In addition, studies by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy showed that the resulting Pd(3)Ag nanoparticles inherited similar atomic ratio and alloy structure as the starting material. The synthesized Pd(3)Ag nanoparticles exhibited excellent catalytic activity toward hydrogenation of acrolein to propanal.

A recently discovered synthetic route to new kinetically stable [(MSe)y]m[TSe2]n layered intergrowths has been applied to prepare several different compositions (M = Pb or Sn, T = Ta, Nb, Mo, or W) with m = n = 1, in thin film form. Scanning transmission electron microscopy and synchrotron X-ray diffraction show the nanostructure of these materials is characterized by a combination of in-plane component crystallinity with misregistration and rotational mis-orientation between adjacent layers. Extremely low cross-plane thermal conductivity as low as 0.1 W m-1 K-1 are attributed to the turbostratic nanostructure. By appropriate choice of M and T, we demonstrate that a range of electrical transport properties are possible, from metallic to semiconducting. Annealing (PbSe)0.99WSe2 and (PbSe)1.00MoSe2 specimens in a controlled atmosphere of PbSe or WSe2 is observed to systematically influence carrier properties, and is interpreted in terms of reduction of the concentration of electrically active defects. Considering these observations and the large composition and structural space that can be explored in such [(MSe)y]m[TSe2]n intergrowths, these materials are of interest for further investigation as potential thermoelectric materials.

The novel, partially filled compounds CexCo4Ge6Se6 were synthesized using two methods: low-temperature interdiffusion and nucleation of ultrathin elemental layers and a bulk, high-temperature technique. The compounds were characterized by differential scanning calorimetry, powder X-ray diffraction, and electron probe microanalysis. The lattice parameters for the samples synthesized using the high-temperature technique were calculated to be 8.30 A, while the samples synthesized using the elemental deposition technique showed lattice expansion proportional to the amount of incorporated rare earth element, with lattice parameters ranging from 8.30 A for x = 0 up to a lattice parameter of 8.34 A for x = 0.2. Rietveld analysis on X-ray diffraction data for both sets of samples showed incorporation of the rare earth element into the interstitial site, up to 20% occupancy.

We report the room-temperature, cross-plane thermal conductivities, and longitudinal speeds of sound of multilayer films [(TiTe2)(3)(Bi2Te3)(x)(TiTe2)(3)(Sb2Te3)(y)](i) (x=1-5, y=1-5) and misfit-layer dichalcogenide films [(PbSe)(m)(TSe2)(n)](i) (T=W or Mo, m=1-5, and n=1-5) synthesized by the modulated elemental reactants method. The thermal conductivities of these nanoscale layered materials fall below the predicted minimum thermal conductivity of the component compounds: two times lower than the minimum thermal conductivity of Bi2Te3 for multilayer [(TiTe2)(3)(Bi2Te3)(x)(TiTe2)(3)(Sb2Te3)(y)](i) films and five to six times lower than the minimum thermal conductivity of PbSe for misfit-layer dichalcogenides [(PbSe)(m)(TSe2)(n)](i). We attribute the low thermal conductivities to the anisotropic bonding of the layered crystals and orientational disorder in the stacking of layered crystals along the direction perpendicular to the surface.

Numerical modeling of Hall effect data is used to demonstrate the existence of a conductive inversion layer at the surface of high-quality n-type single crystals of iron pyrite (cubic FeS2) grown by a flux technique. The presence of the inversion layer is corroborated by Hall measurements as a function of crystal thickness and photoemission spectroscopy. This hole-rich surface layer may explain both the low photovoltage of pyrite solar cells and the widely-observed high p-type conductivity of polycrystalline pyrite thin films that have together perplexed researchers for the past thirty years. We find that the thickness and conductivity of the inversion layer can be modified by mechanical and chemical treatments of the pyrite surface, suggesting that it may be possible to eliminate this hole-rich layer by passivating surface states and subsurface defects. Furthermore, modeling of the high-temperature electrical conductivity shows that the electronic band gap is 0.80 ± 0.05 eV at room temperature (compared to 0.94 eV according to optical transmission data), confirming that photovoltages of ∼500 mV should be attainable from pyrite under solar illumination.

The in-plane thermal conductivity is measured to be three times lower in misfit-layered [(PbSe)0.99]x(WSe2)x superlattice thin films than disordered-layered WSe2 because of interface scattering despite a higher cross-plane value in the former than the latter. While having little effect on the in-plane thermal conductivity, annealing the p-type [(PbSe)0.99]2(WSe2)2 films in Se increases the in-plane Seebeck coefficient and electrical conductivity because of decreased defect and hole concentrations. Increasing interface density of the annealed films by decreasing x from 4 to 2 has weak influence on the in-plane thermal conductivity but increases the Seebeck coefficient and decreases the room-temperature electrical conductivity.

For revealing the effects of slip on such characteristics as friction force and load-bearing capacity of sliding bearing, shear flows of a Newtonian fluid with a varying partial slip surface were computed using finite volume method. Calculation results showed that slip would decrease the friction force, which however was not affected by the location of slip region. The load-bearing capacity of sliding bearing closely depended on the location of slip region, especially the locations of the starting and end points of slip region. A well-designed partial slip surface can improve the load-bearing capacity, otherwise slip would cause lubrication failure.