Low‐Frequency Phonons in High‐Pressure High‐Temperature C60 Polymers

Abstract

Inelastic neutron scattering investigations of the orthorhombic (O), tetragonal (T), and rhombohedral (R) pressure‐polymerized phases of C₆₀ as well as the pressure‐dimerized state give evidence for differences in the responses of low‐energy excitations between these phases. These differences are assigned to changes in low‐energy interball modes dynamics as a function of the structure of pressure‐polymerized phases. Far‐infrared transmittance measurements complete the information of the low‐frequency vibrational dynamics of these phases. The temperature dependence of these low‐frequency excitations reveals a strong anharmonicity and possible phase transitions.     

Determination of deuterium site occupancy in ZrCoD3 and its role in improved durability of Zr–Co–Ni deuterides against disproportionation

In the present study, we have for the first time reported the occupancy of deuterium in a new interstitial site of ZrCoD₃ which explain the hydrogen induced disproportionation behavior of ZrCo alloy. We have also reported the effect of Ni substitution on interstitial site occupancy of deuterium in ZrCo_1−xNi_xD₃, which in turn explains the improved durability of these Ni substituted deuterides against disproportionation. The crystal structure of the ZrCo_1−xNi_x (x = 0.0, 0.1, 0.2, 0.3) deuterides was investigated by X-ray powder diffraction and neutron diffraction methods. The XRD data reveals a single phase formation for all deuterides with varying Ni content (x). The neutron diffraction study shows that deuterium occupies a new site 8e in addition to 4c₂ and 8f₁. Additionally, the Zr–D distance in 8e site is shorter than that in ZrD₂. Therefore, increase in 8e site occupancy will in turn decreases the durability against disproportionation and vice-versa. Furthermore, the neutron diffraction reveals that occupancy of new 8e site decreases and its Zr–D distance increases with increase in Ni content, which explicate the higher durability against disproportionation for Ni rich compound.     

Development of paraffin and paraffin/bitumen composites with additions of B2O3 for thermal neutron shielding applications

In this work, paraffin and paraffin/bitumen composites with additions of boron oxide (B₂O₃) were prepared to evaluate the viscosity, flexural, and thermal neutron shielding properties for uses as thermal neutron shielding materials. The results showed that the addition of 3 wt% or 9 wt% bitumen to paraffin increased the overall flexural properties with the content of 9 wt% bitumen having the highest values. The improvement in flexural properties made the composites less brittle, stiffer, and longer-lasting. Furthermore, different contents of B₂O₃ (0, 7, 14, 21, 28, and 35 wt%) were added to paraffin and paraffin/bitumen composites to investigate the effects of the B₂O₃ contents. The results indicated that an increase in B₂O₃ contents improved the shielding properties but slightly reduced the flexural properties. Specifically for 5-mm paraffin and 5-mm paraffin/bitumen samples with 35 wt% of B₂O₃, both samples could reduce neutron flux by more than 70%. The overall results suggested that the paraffin and paraffin/bitumen composites with additions of B₂O₃ showed improved properties for utilization as effective thermal neutron shielding materials.     

Neutron flux characterization of the SM1 sub-critical multiplying complex of the Pavia University

SM1 is a thermal Sub-critical Multiplication complex located at the University of Pavia (Italy) and, since its installation in 1962, has been utilized mainly for radiochemistry research. This work focuses on the characterization, by means of the Monte Carlo code MCNP and direct measurements, of the neutron flux distribution inside the complex and on the calculation of the effective multiplicative coefficient (k_eff) in the current SM1 thermal configuration. For two specific irradiation channels, experimental measurements of the neutron fluxes were performed by foils activation technique and neutron spectrum de-convolution based on the SAND II code. Measurements have been compared with the simulation results showing a good agreement. Furthermore, a comparison between the preliminary results of the simulations of the SM1 plant in fast configuration, characterized by a solid lead diffuser, and the actual thermal configuration is also presented. The fast configuration of SM1, if implemented, will give the opportunity to carry out preliminary studies for the analysis of sub-critical fast-neutron installations and their applications.     

Structure and magnetic behavior of Zn doped NdMnO3 manganite: Neutron diffraction study

Effect of Zn doping on the structural and magnetic properties of NdMnO₃ has been investigated by neutron diffraction and dc magnetic susceptibility measurements. The partial replacement of Mn³⁺ by Zn²⁺ results in the decrease in T_N. In the temperature dependent magnetization measurements, a broad hump and a sharp peak has been observed around ~ 50 K and 10 K respectively for both the samples. Thermal hysteresis in magnetization between cooling and heating runs indicate first-order phase transition. Magnetization measurements on NdMn_0.95Zn_0.05O₃ sample clearly show that, 5% Zn-doping in NdMnO₃ results in the suppression in magnetism which is evident from the weakening of Nd–Mn interaction below T_N, and resulting in antiferromagnetic coupling of Mn³⁺ ions along x-axis with Mn³⁺ moments oriented parallel or antiparallel to the x-component of Nd³⁺ moments.     

Study of wavelength-shifting chemicals for use in large-scale water Cherenkov detectors

Cherenkov detectors employ various methods to maximize light collection at the photomultiplier tubes (PMTs). These generally involve the use of highly reflective materials lining the interior of the detector, reflective materials around the PMTs, or wavelength-shifting sheets around the PMTs. Recently, the use of water-soluble wavelength-shifters has been explored to increase the measurable light yield of Cherenkov radiation in water. These wave-shifting chemicals are capable of absorbing light in the ultraviolet and re-emitting the light in a range detectable by PMTs. Using a 250 L water Cherenkov detector, we have characterized the increase in light yield from three compounds in water: 4-Methylumbelliferone, Carbostyril-124, and Amino-G Salt. We report the gain in PMT response at a concentration of 1 ppm as 1.88±0.02 for 4-Methylumbelliferone, stable within 0.5% over 50 days, 1.37±0.03 for Carbostyril-124, and 1.20±0.02 for Amino-G Salt. The response of 4-Methylumbelliferone was modeled, resulting in a simulated gain within 9% of the experimental gain at 1 ppm concentration. Finally, we report an increase in neutron detection performance of a large-scale (3.5 kL) gadolinium-doped water Cherenkov detector at a 4-Methylumbelliferone concentration of 1 ppm.     

Modeling and characterization of grain structures and defects in solidification

The paper by Karma and Tourret (this volume) in this special issue focuses on multiscale modeling approaches ranging from atoms to microstructure. In the present one, the most recent and significant modeling contributions dealing with the scale of solidification from microstructure to grain structure are briefly reviewed. The paper also covers modeling of defect formation during the last stage of solidification, namely porosity and hot tearing. As will be shown, the field of solidification has taken advantage of several simulation and experimental tools which have become increasingly powerful and accessible over the past decade. The emphasis will be put on complex 2D and 3D models for which correlations with in situ observations using synchrotron radiation and/or combined orientation and metallography imaging have been made.     

Non-destructive investigation of aluminum alloy hemmed joints using neutron radiography and X-ray computed tomography

Methods for the non-destructive analysis of aluminum alloy hemmed joints for automotive applications were investigated to visualize the void size, volume fraction and shape within the adhesive fill. These defects can adversely affect the performance of hemmed joints by decreasing fatigue strength and corrosion resistance. Thermal neutron radiography and X-ray computed tomography were applied to visualize the voids. The void fraction within the adhesive fill was determined from the neutron images with the use of an image analysis thresholding method.     

Correlation between growth behavior,internal stress distribution and properties of ZnO:Zn crystals grown by carbon-assisted chemical vapor transport

Internal stress and stress-related defects are considered as the major obstacles that significantly hinder the growth of high-quality ZnO-based crystals. In this work, high-crystalline-quality ZnO:Zn bulk crystals were successfully grown by carbon-assisted chemical vapor transport (CVT). Internal stress in the crystal was directly measured by a neutron beam from a reactor, and stress distributions along the radial direction at different depths were obtained. The stress, temperature, and flow fields in the growth system were simulated by the finite element (FE) method, and the results agreed with the neutron stress analysis. The etch pit density (EPD), Hall properties, and optical transmittances of different crystal regions were studied in detail, and the distribution trend of the crystal properties was consistent with that of internal stress and stress-related defects in the crystal. It is found that the unique temperature filed in the growth system causes the crystal to bend to a slightly convex toward the growth direction and gives rise to a driving force for structural defect formation. The + c and –c faces of the crystal are subjected to tensile and compressive stress, respectively. The maximum stress values are about 280 MPa and -291 MPa near the central regions of ±c faces, while the crystal periphery is basically free of internal stress. The region near the center of +c face has an EPD of 7.5 × 10³ cm^-2 and a transmittance of 79.2% at 800 nm wavelength, while the corresponding carrier concentration and mobility are 2.27 × 10¹⁷ cm^?3 and 159 cm²/V·s, respectively. By comparison, the crystal periphery has an EPD of 10² cm^-2 with an 80.5% transmittance at 800 nm, while the carrier concentration and mobility are 1.85 × 10¹⁷ cm^?3 and 184 cm²/V·s, respectively.     

Defect control and ionic conductivity of oxynitride perovskite Sr0.83Li0.17Ta0.83O1.88N0.74

Perovskite lattice was tailored by introducing site vacancies and mixed anion composition, to produce Sr_0.83Li_0.17Ta_0.83O_1.88N_0.74 (Li02N). Further, Li02N was converted to a defect oxide Sr_0.83Li_0.17Ta_0.83O₃ (Li02O) by applying an optimized treatment: heating in air at 1173 K for 2 h. According to the neutron Rietveld refinement, Li02N and Li02O are tetragonal and orthorhombic, respectively, where the lattice volume of Li02O is significantly smaller than that of Li02N. The ionic conductivity (σ_ion) of Li02N and Li02O was evaluated by the ac impedance spectroscopy and the equivalent circuit analysis. Both Li02N (σ_ion = 10^?5.5 S/cm at 671 K) and Li02O (σ_ion = 10^?6.2 S/cm at 667 K) exhibited an Arrhenius behavior of ionic conductivity with activation energies of 0.87 eV and 0.75 eV, respectively. It is interpreted that the nitride component enhances the ionic conduction of Li02N, while the vacancy of the anion lattice makes an opposite effect.