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Li, Z

Publications and source records attributed to Li, Z.

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

3D simulations of multipacting in the 56 MHz SRF cavity

The 56 MHz SRF Quarter-Wave Resonator (QWR) is designed for RHIC as a storage cavity to improve the collider performance. 2D multipacting simulation has been done for the cavity alone. Ripples were added to the outer body of the cavity for multipacting suppression based on the simulation findings. During operation, there will be four higher order mode (HOM) couplers. All of these components will be exposed to high RF fields. In this paper we compare 2D and 3D codes simulation results for multipacting in the cavity. We also report 3D simulation results for multipacting simulation at the couplers.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

VARIATIONS OF SOLAR ELECTRON AND PROTON FLUX IN MAGNETIC CLOUD BOUNDARY LAYERS AND COMPARISONS WITH THOSE ACROSS THE SHOCKS AND IN THE RECONNECTION EXHAUSTS

The magnetic cloud boundary layer (BL) is a dynamic region formed by the interaction of the magnetic cloud (MC) and the ambient solar wind. In the present study, we comparatively investigate the proton and electron mean flux variations in the BL, in the interplanetary reconnection exhaust (RE), and across the MC-driven shock by using the Wind data from 1995 to 2006. In general, the proton flux has higher increments at lower energy bands compared with the ambient solar wind. Inside the BL, the core electron flux increases quasi-isotropically and the increments decrease monotonously with energy from {approx}30% (at 18 eV) to {approx}10% (at 70 eV); the suprathermal electron flux usually increases in either parallel or antiparallel direction; the correlation coefficient of electron flux variations in parallel and antiparallel directions changes sharply from {approx}0.8 below 70 eV to {approx}0 above 70 eV. Similar results are also found for RE. However, different phenomena are found across the shock where the electron flux variations first increase and then decrease with a peak increment (>200%) near 100 eV. The correlation coefficient of electron flux variations in parallel and antiparallel directions is always around 0.8. The similar behavior of flux variations in BL and RE suggests that reconnection may commonly occur in BL. Our work also implies that the strong energy dependence and direction selectivity of electron flux variations, which were previously thought to have not enough relevance to magnetic reconnection, could be considered as an important signature of solar wind reconnection in the statistical point of view.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Chemical Convention in the Lunar Core from Melting Experiments on the Ironsulfur System

By reanalyzing Apollo lunar seismograms using array-processing methods, a recent study suggests that the Moon has a solid inner core and a fluid outer core, much like the Earth. The volume fraction of the lunar inner core is 38%, compared with 4% for the Earth. The pressure at the Moon's core-mantle boundary is 4.8 GPa, and that at the ICB is 5.2 GPa. The partially molten state of the lunar core provides constraints on the thermal and chemical states of the Moon: The temperature at the inner core boundary (ICB) corresponds to the liquidus of the outer core composition, and the mass fraction of the solid core allows us to infer the bulk composition of the core from an estimated thermal profile. Moreover, knowledge on the extent of core solidification can be used to evaluate the role of chemical convection in the origin of early lunar core dynamo. Sulfur is considered an antifreeze component in the lunar core. Here we investigate the melting behavior of the Fe-S system at the pressure conditions of the lunar core, using the multi-anvil apparatus and synchrotron and laboratory-based analytical methods. Our goal is to understand compositionally driven convection in the lunar core and assess its role in generating an internal magnetic field in the early history of the Moon.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A Low-Power, Radiation-Resistant, Silicon-Drift-Detector Array for Extraterrestrial Element Mapping

We are developing a modular Silicon Drift Detector (SDD) X-Ray Spectrometer (XRS) for measuring the abundances of light surface elements (C to Fe) fluoresced by ambient radiation on remote airless bodies. The value of fluorescence spectrometry for surface element mapping is demonstrated by its inclusion on three recent lunar missions and by exciting new data that have recently been announced from the Messenger Mission to Mercury. The SDD-XRS instrument that we have been developing offers excellent energy resolution and an order of magnitude lower power requirement than conventional CCDs, making much higher sensitivities possible with modest spacecraft resources. In addition, it is significantly more radiation resistant than x-ray CCDs and therefore will not be subject to the degradation that befell recent lunar instruments. In fact, the intrinsic radiation resistance of the SDD makes it applicable even to the harsh environment of the Jovian system where it can be used to map the light surface elements of Europa. In this paper, we first discuss our element-mapping science-measurement goals. We then derive the necessary instrument requirements to meet these goals and discuss our current instrument development status with respect to these requirements.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

X-ray Diffraction of Cubic Gd2)3/Er under High Pressure

In this paper, we report the in situ high pressure X-ray diffraction studies on Er{sup 3+} doped Gd{sub 2}O{sub 3} in a diamond anvil cell up to 39.8 GPa at room temperature. Several phase transitions have been identified in our studies. The structural transformation from a starting cubic phase to a hexagonal phase occurred during the sample compression process, at 8.57 GPa. And the hexagonal phase was stable from 12.5 GPa up to the highest pressure in this study but was not quenchable and transformed to a monoclinic phase after pressure release. An anomalous high pressure behavior in the hexagonal type Gd{sub 2}O{sub 3} phase was observed, which might be caused by an electron transition influenced by Er{sup 3+} ions doping. By fitting the compression data to the Birch-Murnaghan equation of state, the bulk moduli of the cubic and two hexagonal (at p < 19.9 GPa and p > 27.0 GPa) Gd{sub 2}O{sub 3} phases were determined to be 164 {+-} 3, 185 {+-} 7, and 150 {+-} 10 GPa with B'{sub 0} = 4, respectively.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

The Structural Transition Behavior of CdSe/ZnS Core/shell Quantum Dots under High Pressure

The structural phase transition of CdSe/ZnS core/shell quantum dots (QDs) has been studied by in situ angle-dispersive X-ray diffraction under high pressure up to 53.6?GPa. The CdSe core transforms from wurtzite to rock-salt structure near 6.3?GPa and then to Cmcm or distorted Cmcm structure probably occurs at 45.1?GPa which has not been observed in CdSe nanomaterials before. The critical pressure from wurtzite to rock-salt and the bulk modulus of rock-salt phase are much higher than those for bulky and uncapped nanoparticle CdSe. The released sample can be kept in rock-salt phase for a certain time, verified by photoluminescence (PL) spectra, quite different from the reversible transition for pure CdSe. A reasonable interpretation of the experimental phenomena is given by comparing the bulk modulus of the core and shell and studying the stress sate of the core after decompression. Our study suggests that capping a hard shell is an effective approach to quench the high pressure phase of nanomaterial with a reversible phase transition.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Point Defects in CdZnTe Crystals Grown by Different Techniques

We studied, by current deep-level transient spectroscopy (I-DLTS), point defects in CdZnTe detectors grown by different techniques. We identified 12 different traps with energy levels from 7 meV to 1.1 eV. Although the levels of most of the identified defects were independent of the crystal growth techniques, nevertheless there were some associated differences in the traps energies and densities.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Rational Design Synthesis and Evaluation of First Generation Inhibitors of the Giardia Lamblia Fructose-1 6-biphosphate Aldolase

Inhibitors of the Giardia lamblia fructose 1,6-bisphosphate aldolase (GlFBPA), which transforms fructose 1,6-bisphosphate (FBP) to dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, were designed based on 3-hydroxy-2-pyridone and 1,2-dihydroxypyridine scaffolds that position two negatively charged tetrahedral groups for interaction with substrate phosphate binding residues, a hydrogen bond donor to the catalytic Asp83, and a Zn{sup 2+} binding group. The inhibition activities for the GlFBPA catalyzed reaction of FBP of the prepared alkyl phosphonate/phosphate substituted 3-hydroxy-2-pyridinones and a dihydroxypyridine were determined. The 3-hydroxy-2-pyridone inhibitor 8 was found to bind to GlFBPA with an affinity (K{sub i} = 14 {micro}M) that is comparable to that of FBP (K{sub m} = 2 {micro}M) or its inert analog TBP (K{sub i} = 1 {micro}M). The X-ray structure of the GlFBPA-inhibitor 8 complex (2.3 {angstrom}) shows that 8 binds to the active site in the manner predicted by in silico docking with the exception of coordination with Zn{sup 2+}. The observed distances and orientation of the pyridone ring O=C-C-OH relative to Zn{sup 2+} are not consistent with a strong interaction. To determine if Zn{sup 2+} coordination occurs in the GlFBPA-inhibitor 8 complex in solution, EXAFS spectra were measured. A four coordinate geometry comprised of the three enzyme histidine ligands and an oxygen atom from the pyridone ring O=C-C-OH was indicated. Analysis of the Zn{sup 2+} coordination geometries in recently reported structures of class II FBPAs suggests that strong Zn{sup 2+} coordination is reserved for the enediolate-like transition state, accounting for minimal contribution of Zn{sup 2+} coordination to binding of 8 to GlFBPA.

08 HYDROGEN↗
DOE OSTI2011

Decoy Strategies: The Structure of TL1A:DcR3 Complex

Decoy Receptor 3 (DcR3), a secreted member of the Tumor Necrosis Factor (TNF) receptor superfamily, neutralizes three different TNF ligands: FasL, LIGHT, and TL1A. Each of these ligands engages unique signaling receptors which direct distinct and critical immune responses. We report the crystal structures of the unliganded DcR3 ectodomain and its complex with TL1A, as well as complementary mutagenesis and biochemical studies. These analyses demonstrate that DcR3 interacts with invariant backbone and side-chain atoms in the membrane-proximal half of TL1A which supports recognition of its three distinct TNF ligands. Additional features serve as antideterminants that preclude interaction with other members of the TNF superfamily. This mode of interaction is unique among characterized TNF:TNFR family members and provides a mechanistic basis for the broadened specificity required to support the decoy function of DcR3, as well as for the rational manipulation of specificity and affinity of DcR3 and its ligands.

36 MATERIALS SCIENCE↗
DOE OSTI2011

The Study of Structural Transition of ZnS Nanorods under High Pressure

The study of pressure behavior for ZnS nanorods by X-ray diffraction measurements up to 37.2 GPa is carried out. ZnS nanorods transform from the initial wurtzite phase to rock salt phase at 19.6 GPa without undergoing the transition to zinc blende structure, and this is the first report about the direct phase transition from wurtzite to rock salt phase for ZnS. The longitudinal c-axis of ZnS nanorods exhibits more compressible behavior than that of radial a-axis direction, which is caused by the special rod morphology, and this induces the special direct phase transition to rock salt phase without zinc blende phase in the transition process. The results show the morphology of ZnS nanorods plays a crucial role in the special pressure behavior and also suggests we could explore the various pressure behaviors applying particular shaped nanomaterials under high pressure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

High-Pressure Studies on CeO2 Nano-Octahedrons with a (111)-Terminated Surface

In-situ high-pressure X-ray diffraction and Raman spectroscopies have been performed on well-shaped CeO{sub 2} nano-octahedrons enclosed by eight (111) planes. The CeO{sub 2} nano-octahedrons are shown to be more stable than their bulk counterparts and some other reported CeO{sub 2} nanocrystals of smaller size. The transition pressure from cubic to orthorhombic phase is approximately 10 GPa higher than that of 12 nm CeO{sub 2} nanocrystals even though they have similar volume expansion at ambient conditions. Additionally, the phase transition to {alpha}-PbCl2 phase is very sluggish and uncompleted even up to 55 GPa. The TEM image of the sample after decompression from 55 GPa clearly shows that the nano-octahedrons preserve the starting shape. Such distinct high-pressure behaviors in CeO{sub 2} nano-octahedrons have been discussed in terms of their special exposure surface. Further analysis shows that the lower compressibility of the exposed (111) planes in the nano-octahedrons is believed to be the major factor to the elevation of the phase transition pressure and the sluggishness of the transition.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Suppressing the Skin-Core Structure of Injection-Molded Isotactic Polypropylene via Combination of an in situ Microfibrillar Network and an Interfacial Compatibilizer

Injection-molded semicrystalline polymer parts generally exhibited a so-called skin-core structure basically as a result of the large gradients of temperature, shear rate, stress, and pressure fields created by the boundary conditions of injection molding. Suppression of the skin-core structure is a long-term practical challenge. In the current work, the skin-core structure of the conventional injection-molded isotactic polypropylene (iPP) was largely relieved by the cooperative effects of an in situ microfibrillar network and interfacial compatibilizer. The in situ poly(ethylene terephthalate) microfibrils of 1-8 {micro}m in diameter and large aspect ratios of above 40 tended to entangle with each other to generate a microfibrillar network in the iPP melt. During injection molding, the iPP molecules experienced confined flow in the microchannels or pores formed by the microfibrillar network, which could redistribute and homogenize the flow field of polymer melt. Addition of the compatibilizer, glycidyl methacrylate-grafted iPP, restrained the molecular orientation but facilitated preservation of oriented molecules due to the chemical bonds at the interface between PET microfibrils and iPP. The cooperative effects of in situ microfibrillar network and interfacial compatibilizer led to almost the same molecular orientation across the whole thickness of the injection-molded parts. Additionally, the content of {beta} crystals in different layers of injection-molded iPP parts depended on the combined effects of the molecular orientation, the amount of oriented crystals, and the crystallization time between 105 and 140 C. The presence of the interfacial compatibilizer facilitated formation of the {beta} crystals because of preservation of the oriented molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Graphene Nanosheets and Shear Flow Induced Crystallization in Isotactic Polypropylene Nanocomposites

Combined effects of graphene nanosheets (GNSs) and shear flow on the crystallization behavior of isotactic polypropylene (iPP) were investigated by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. For crystallization under quiescent condition (at 145 C), the half-crystallization time (t{sub 1/2}) of nanocomposites containing 0.05 and 0.1 wt % GNSs was reduced to at least 50% compared to that of neat iPP, indicating the high nucleation ability of GNSs. The crystallization rate of iPP was directly proportional to the GNS content. Under a relatively weak shear flow (at a rate of 20 s{sup -1} for 5 s duration) and a low degree of supercooling, the neat iPP exhibited an isotropic structure due to the relaxation of row nuclei. However, visible antisotropic crystals appeared in sheared iPP/GNSs nanocomposites, indicating that GNSs induced a network structure hindering the mobility of iPP chains and allowing the survival of oriented row nuclei for a long period of time. The presence of GNSs clearly enhanced the effects of shear-induced nucleation as well as orientation of iPP crystals. Two kinds of nucleating origins coexisted in the sheared nanocomposite melt: heterogeneous nucleating sites initiated by GNSs and homogeneous nucleating sites (row nuclei) induced by shear. The difference of t{sub 1/2} of nanocomposites with and without shear was significantly larger than that of neat iPP. The presence of GNSs and shear flow exhibited a synergistic interaction on promoting crystallization kinetics of iPP, although the effect of GNS concentration was not apparent. From WAXD results of isothermal and nonisothermal crystallization of sheared iPP, it was found that the appearance of {beta}-crystals depended on the preservation of row nuclei, where the {alpha}-crystals were predominant in the iPP/GNSs nanocomposites, indicating that GNSs could directly induce {alpha}-crystals of iPP.

36 MATERIALS SCIENCE↗