Physics

Polysaccharide hydrogels are good candidates for skin scaffolds because of their inherent biocompatibility and water transport properties. In the current study, hydrogels were made from a mixture of four polysaccharides: xanthan gum, konjac gum, iota-carrageenan, and kappa-carrageenan. Gel formation, strength, and structure of these polysaccharides were studied using rheological and thermal techniques. All gel samples studied were strong gels at all times because of the gradual water loss. However, after 12 h of storage, elastic (G') and loss (G'') moduli of hydrogel mixture containing all the ingredients is of one to two orders of magnitude greater than that of mixtures not containing either xanthan gum or iota-carrageenan, which confirmed the varied levels of gel strength. This is mainly due to the rate of water loss in each of these mixtures, resulting in gels of varying structures and dynamic moduli over a period of time. Iota-carrageenan and xanthan gum differ in their effect on gel strength and stability in combination with konjac gum and kappacarrageenan.

Viscoelastic properties of j-carrageenan in saline solution at various concentrations and pH were investigated by dynamic rheological techniques, viscosity, elasticity measurements, and IR spectrometry. The viscosity and elasticity at low concentrations of j-carrageenan do not depend on pH, confirming that j-carrageenan is in a disordered conformation. At 0.7% j-carrageenan, the disordered confirmation transforms into an ordered helical confirmation with the possibility of weak-type gel formation. The transformation is also confirmed by dynamic measurements of loss and storage moduli. Furthermore, at this concentration, the viscosity and elasticity are highly dependent on pH. At higher concentrations of NaCl (0.5 M) at some pHs, we observed that storage moduli is greater than loss moduli for the entire frequency region. Hence, there is a possibility of structure transformation from weak-type gel to a somewhat intermediate gel. The lowest viscosity and elasticity were obtained at extreme pH, confirming that there are structural changes occurring at these pHs due to hydrolysis. This is confirmed by IR data.

Nanocomposites of polycarbonate (PC) containing low concentrations of pristine and COOH and OH functionalized single-walled carbon nanotubes (SWNTs, COOH-SWNTs and OH-SWNTs) were prepared by melt-mixing and analyzed using rheology and scanning electron microscopy (SEM). The steady state and linear viscoelastic behavior of each nanocomposite material is presented and compared to that of the neat PC. SEM analysis revealed that samples containing functionalized SWNTs were more dispersible than samples containing the pristine SWNTs.

Fast timing detectors are an essential element in the experimental setup for time-of-flight (ToF) mass measurements of unstable nuclei. We have upgraded the scintillator detectors used in experiments at the National Superconducting Cyclotron Laboratory (NSCL) by increasing the number of photomultiplier tubes that read out their light signals to four per detector, and characterized them in a test experiment with 48Ca beam at the NSCL. The new detectors achieved a time resolution ( σ ) of 7.5 ps. We systematically investigated different factors that affect their timing performance. In addition, we evaluated the ability of positioning the hitting points on the scintillator using the timing information and obtained a resolution ( σ ) below 1 mm for well-defined beam spots.

the limits of the nuclear landscape, nuclei may exhibit different ground state properties as the result of shell evolution. At the center of the N=20 island of inversion in Mg, a recently identified shape coexistent excited 0+ state was found with potentially large mixing with the ground state. Exploring the nature of these 0+ states, and the states that are built on top of them, is important for understanding shape coexistence and configuration mixing in this region. An experiment was performed at NSCL to study Mg via the decay of Na and Na (βn) which utilized a CeBrimplantation-decay detector along with ancillary detection arrays for energy and timing characterization. Preliminary results and a tentative level scheme produced from this work will be presented.

References: Generally, neutrons are more difficult to detect at higher energies. As neutrons approached 10MeV, interactions inside the simulation space decreased from 72.9% to 36%. This increase in energy also caused a 9% increase in forward scattered neutrons from 57% at 1 MeV to a maximum of 66% at 8 MeV. Gamma-rays are even less likely to interact than neutrons at equal energy. As gamma-rays approached 6 MeV, forward scattering increas -ed from 49% at 100 keV to a maximum of 57.6% at 5 MeV. Gammas interacting with the detector volume space in the simulation decreased from 49.9% at 100 keV to 12.9% at 6 MeV. In conclusion, a direct correlation between energy and forward scatter can be observed in higher statistic simulations. In order to understand the effectiveness of NEXT, a Geant4 based simulation software (NextSim) was implemented. NextSim tracks individual particles and stores pertinent information to the primary particle until it leaves or is absorbed by the material defined...

A new high precision time of flight neutron detector concept for beta-delayed neutron emission and direct reaction studies is proposed. The Neutron dEtector with Xn Tracking (NEXT) array aims to maintain high intrinsic neutron detection efficiency while reducing uncertainties in neutron energy measurements. A single NEXT module will be composed of thin segments of neutron-discriminating plastic scintillator, each optically separated, coupled to a position sensitive photo-detector. By incorporating a position sensitive photo-detector with a large optically separated scintillator, NEXT will achieve high precision determination of neutron time of arrival and interaction position within the active volume. A design study has been conducted based on simulations and experimental tests leading to construction of prototype units. First results from neutron measurements will be discussed.

The competitions between ferroelectric and rotational instabilities in rhombohedral PbZrxTi1−xO3 near x = 0.5 are investigated using first principles density functional supercell calculations. As expected, we find a strong ferroelectric instability. However, we also find a substantial R-point rotational instability, close to but not as deep as the ferroelectric one. This is similar to the situation in pure PbZrO3. These two instabilities are both strongly pressure dependent, but in opposite directions so that lattice compression of less than 1% is sufficient to change their ordering. Because of this, local stress fields due to B-site cation disorder may lead to coexistence of both types of instability are likely present in the alloy near the morphotropic phase boundary.

First principles calculations are used to investigate the band structure and the transport related properties of unfilled and filled 4d skutterudite antimonides. The calculations show that, while RhSb3 and p-type La(Rh,Ru)4Sb12 are unfavorable for thermoelectric application, n-type La(Rh,Ru)4Sb12 is very likely a high figure of merit thermoelectric material in the important temperature range 150-300 K.

We discuss the electronic structure of Na x CoO 2 from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5 þ x leading to a nearly full Co t 2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the e g bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from a g symmetry states and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh 2 O 4 . Like Na x CoO 2 it shows a large crystal field gap between narrow t 2g and e g manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.

We present a large-scale density functional theory (DFT) investigation of the ABO3 chemical space in the perovskite crystal structure, with the aim of identifying those that are relevant for forming piezoelectric materials. Screening criteria on the DFT results are used to select 49 compositions, which can be seen as the fundamental building blocks from which to create alloys with potentially good piezoelectric performance. This screening finds all the alloy end points used in three well-known high-performance piezoelectrics. The energy differences between different structural distortions, deformation, coupling between the displacement of the A and B sites, spontaneous polarization, Born effective charges, and stability is analyzed in each composition. We discuss the features that cause the high piezoelectric performance of the well-known piezoelectric lead zirconate titanate (PZT), and investigate to what extent these features occur in other compositions. We demonstrate how our results can be useful in the design of isovalent alloys with high piezoelectric performance.

We studied the dynamics of excitons excited in layered semiconductor PbI 2 nanoclusters (NCLs) using time-resolved photoluminescence (TRPL) spectroscopy. TRPL spectra reveal formation of self-trapped excitons (STEs). It was shown that the formation of the STEs for larger [more than the Bohr radius of exciton in bulk PbI 2 (R B ¼ 1.9 nm)] NCLs occurs in sub-nanosecond time scale, while in the case of small NCLs with sizes about R B , it takes place in nanosecond scale. The effective energy transfer from the small to the larger semiconductor NCLs, which arises from dipole-dipole intercluster interactions, takes place in sub-nanosecond scale. We demonstrate that the STEs are stable states and they define effective radioluminescence of the investigated Pb 1ÀX Cd X I 2 alloys. V

We investigate the electronic structure of over-coordinated defects in amorphous silicon via density-functional total-energy calculations, with the aim of understanding the relationship between topological and electronic properties on a microscopic scale. Maximally-localized Wannier functions are computed in order to characterize the bonding and the electronic properties of these defects. The five-fold coordination defects give rise to delocalized states extending over several nearest neighbors, and therefore to very polarizable bonds and anomalously high Born effective charges for the defective atoms.

We present a new code to evaluate thermoelectric and electronic transport properties of extended systems with a maximally-localized Wannier function basis set. The semiclassical Boltzmann transport equations for the homogeneous infinite system are solved in the constant relaxation-time approximation and band energies and band derivatives are obtained via Wannier interpolations. Thanks to the exponential localization of the Wannier functions obtained, very high accuracy in the Brillouin zone integrals can be achieved with very moderate computational costs. Moreover, the analytical expression for the band derivatives in the Wannier basis resolves any issues that may occur when evaluating derivatives near band crossings. The code is tested on binary and ternary skutterudites CoSb 3 and CoGe 3/2 S 3/2 .

We study in detail by means of ab-initio pseudopotential calculations the electronic structure of five-fold coordinated (T5) defects in a-Si and a-Si:H, also during their formation and their evolution upon hydrogenation. The atom-projected densities of states (DOS) and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of T5 defects in originating gap states through their nearest neighbors. The interaction with hydrogen can reduce the DOS in the gap annihilating T5 defects.