Search

This study focuses on the early stage of polymer-derived SiOC ceramic conversion. We demonstrate that the perceived SiOC phase separation is nonexistent. Instead, SiO2 and free carbon clusters form first and then carbothermal reduction sets in to induce SiOC formation. Such fundamental understanding is supported by both synchrotron X-ray diffraction study and reactive force field simulation. This work for the first time unifies the understanding of atomic evolution process of polysiloxane-based polymer to ceramic conversion. National Science Foundation; Air Force Office of Scientific Research; [CBET-2024546]; [FA9550-22-1-0081] Published version ACKNOWLEDGMENTS This work was supported by National Science Foundation under grant no. CBET-2024546 and Air Force Office of Scientific Research under grant no. FA9550-22-1-0081.

Melamine is a precursor and building block for graphitic carbon nitride (g-CN) materials, a group of layered materials showing great promise for catalytic applications. The synthetic pathway to g-CN includes a polycondensation reaction of melamine by evaporation of ammonia. Melamine molecules in the crystal organize into wave-like planes with an interlayer distance of 3.3 Å similar to that of g-CN. Here we present an extensive investigation of the experimental electron density of melamine obtained from modelling of synchrotron radiation X-ray single-crystal diffraction data measured at 25 K with special focus on the molecular geometry and intermolecular interactions. Both intra- and interlayer structures are dominated by hydrogen bonding and π-interactions. Theoretical gas-phase optimizations of the experimental molecular geometry show that bond lengths and angles for atoms in the same chemical environment (C−N bonds in the ring, amine groups) differ significantly more for the experimental geometry than for the gas-phase-optimized geometries, indicating that intermolecular interactions in the crystal affects the molecular geometry. In the experimental crystal geometry, one amine group has significantly more sp 3-like character than the others, hinting at a possible formation mechanism of g-CN. Topological analysis and energy frameworks show that the nitrogen atom in this amine group participates in weak intralayer hydrogen bonding. We hypothesize that melamine condenses to g-CN within the layers and that the unique amine group plays a key role in the condensation process.

The effect of a high-performance retarding additive in oil well cements was investigated under elevated temperature (165°C) and pressure (1000 psi) conditions via in situ synchrotron-based X-ray diffraction (XRD) and quasielastic neutron scattering (QENS) techniques. Under these temperature and pressure conditions, crystalline calcium silicate hydrates (C–S–H) are formed through the cement hydration process. From in situ XRD experiments, the retardation effect was observed by a change in the rate of the appearance of 11 Å tobermorites as well as a change in the rate of the α-C2SH generation and depletion. QENS analysis revealed that the retardation effect was related to the non-conversion of free water to chemical and constrained water components. A high presence of free water components was attributed to a decrease in 11 Å tobermorites along with slower consumption of the quartz and portlandite phases. Furthermore, QENS results infer that the water molecules experienced confinement in the restricted pore spaces. The retarder inhibited this initial water confinement by slowing the bulk diffusion of free water in the confined region.

The helix-sense reversal of poly(β-phenylpropyl l-aspartate) (3PLA) in the solid state was studied by synchrotron wide-angle X-ray diffraction and small-angle X-ray scattering. The direct determination of the characteristic helical pitch before and after the transition revealed that the transition takes place reversibly between the two α-helices having opposite screw-sense during the heating and cooling cycle. While the hexagonal packing remains unaltered, the helix-sense inversion causes discontinuous changes in the molecular arrangement and, by extension, the crystalline dimension. In this study, another transition was detected at a higher temperature from the left-handed α-helix to the π-helix, the molecular chirality being unaffected.

Advanced engineering materials 23(11), 2100130 (2021). doi:10.1002/adem.202100130, Nanoindentation of treated surfaces, thin films, and coatings is often used as a simple method to measure their hardness and stiffness. These quantities are technologically highly relevant and allow to qualitatively compare different material and surface treatments but fail to capture the entire extent of the highly complex mechanical interaction between indenter tip and the tested surface. Many studies have addressed this question by analytical or numerical modeling, but they must rely on verification by recalculating indentation curves or ex situ microscopy of surface deformation postexperiment. Herein, results from in situ measurements of the multiaxial stress distributions forming beneath an indenter tip while the tested sample is still under load are presented. A 9 ��m-thick TiN hard coating is tested in 1) as-deposited state and 2) shot-peened by Al2O3 particles, using two diamond wedges as indenter tips, with 60�� and 143�� opening angle, respectively. The results reveal a strong influence of the tip shape on the deformation behavior and the main stress component developing inside the sample while under load. In addition, a crack-closing effect can be attributed to the exponentially declining near-surface compressive residual stress gradient that is present in the shot-peened sample., Published by Deutsche Gesellschaft fu��r Materialkunde, Frankfurt, M.

Advanced engineering materials 2100151, 1-9 (2021). doi:10.1002/adem.202100151, Phase formation and microstructure selection during solidification in ��-TiAl alloysare highly relevant, both with respect to the microstructure and texture of castalloys and to directional solidification, which allows to obtain a unique combinationof properties. However, during cooling to room temperature, the solidifyingphases transform to low-temperature phases, which mask the priorsituation during solidification. A new inductive crucible-free zone melting furnaceis used in this work, which is specially designed for in situ investigations ofsolidification using synchrotron radiation. Herein, alloys with 45 and 48 at% Alare studied, and it can be shown that with varying withdrawal rate, a change from�� solidification to synchronous �� �� �� solidification occurs in Ti���48Al, as it ispredicted in the literature. Furthermore, it is observed that by increasing thewithdrawal rate, a preferred growth of the �� phase occurs in alloys with45 at% Al, which is interpreted as a transition to dendritic solidification. Theobservations are compared with a microstructure selection map calculatedaccording to the nucleation and constitutional undercooling (NCU) model proposedin the literature., Published by Deutsche Gesellschaft fu��r Materialkunde, Frankfurt, M.

Lithium (Li) metal serving as an anode has the potential to double or triple stored energies in rechargeable Li batteries. However, they typically have short cycling lifetimes due to parasitic reactions between the Li metal and electrolyte. It is critically required to develop early fault-detection methods for different failure mechanisms and quick lifetime-prediction methods to ensure rapid development. Prior efforts to determine the dominant failure mechanisms have typically required destructive cell disassembly. In this study, non-destructive diagnostic method based on rest voltages and coulombic efficiency are used to easily distinguish the different failure mechanisms-from loss of Li inventory, electrolyte depletion, and increased cell impedance-which are deeply understood and well validated by experiments and modeling. Using this new diagnostic method, the maximum lifetime of a Li metal cell can be quickly predicted from tests of corresponding anode-free cells, which is important for the screenings of electrolytes, anode stabilization, optimization of operating conditions, and rational battery design.

It has been generally accepted that any reaction between micropipes in silicon carbide (SiC) crystals requires a direct contact of the micropipes. We propose a new model of contact-free reactions that are realized through the emission and absorption of full-core dislocations by micropipes. This model can explain the correlated reduction in micropipe radii in the samples with low micropipe densities which has been observed in synchrotron radiation (SR) phase contrast images supported by computer simulations. We provide a theoretical description of a contact-free reaction between two parallel micropipes.

The molecular exchange of CH4 for CO2 in gas hydrates grown in confined nanospace has been evaluated for the first time using activated carbons as a host structure. The nano-confinement effects taking place inside the carbon cavities and the exceptional physicochemical properties of the carbon structure allows us to accelerate the formation and decomposition process of the gas hydrates from the conventional timescale of hours/days in artificial bulk systems to minutes in confined nanospace. The CH4/CO2 exchange process is fully reversible with high efficiency at practical temperature and pressure conditions. Furthermore, these activated carbons can be envisaged as promising materials for long-distance natural gas and CO2 transportation because of the combination of a high storage capacity, a high reversibility, and most important, with extremely fast kinetics for gas hydrate formation and release., F.R.R. and J.S.A. acknowledge the financial support from MINECO: Strategic Japanese-Spanish Cooperation Program (PLE2009-0052), Concert Project-NASEMS (PCIN-2013-057) and MAT2013-45008-p, and Generalitat Valenciana (PROMETEO/2009/002). F.R. and J.L.J. thanks financial support from MINECO through projects MAT2012-38567-C02-01, Consolider Ingenio 2010-Multicat CSD-2009-00050 and Sever Ochoa SEV-2012-0267, and Generalitat Valenciana (Prometeo). Authors thank synchrotron ALBA for beamtime availability (Project ID2015021151). E.V.R.F. thanks for his Ramon y Cajal grant (RyC-2012-11427).

Polycrystalline Sr2IrO4 is prepared by calcination of a stoichiometric mixture of SrCO3 and Ir in air followed by annealing (1.

Agglomerates of a few μm of the yellow phosphor La3BaSi5N9O2:Ce3+ are obtained by heating a mixture of LaF3, La(NH2)3, BaH2, Si(NH)2, and CeF3 in a radio frequency furnace under N2 (1600 °C, 10 h; cooling to 900 °C within 44 h).

A pressure-induced phase transition from β-BiB3O6 to a new polymorph, ζ-BiB3O6 between 11.5 and 18.5 GPa is identified by the combination of in situ Raman spectroscopy and synchrotron XRD.

The compounds Nb0.62Si0.38, Nb0.64Si0.30B0.06, Nb0.64Si0.24B0.12, Nb0.64Si0.18B0.18, and Nb0.64Si0.14B0.22 belonging to the solid solution based on Nb5Si3 in the Nb—Si—B system are characterized by synchrotron powder XRD and DFT calculations.

The isothermal austenite-to-martensite transformation kinetics in a maragingsteel have been studied by time-dependent microbeam diffraction measure-ments with high-energy X-rays. The transformation kinetics are shown to beaccelerated significantly when a magnetic field of 8 T is applied. The averagephase behaviour, obtained from a Rietveld refinement of the powder-averageddiffraction data, demonstrates that the martensite formation does not lead to amacroscopic strain in the austenite and martensite phases. An analysis ofindividual austenite reflections in the microbeam diffraction patterns, however,indicates that within the transforming austenite grains a transformation straindevelops asaresult ofthe formed martensite.Thedevelopment ofelastic strainsduring the transformation isexplained bya partial strain confinement within theuntransformed part of the austenite grain. The strain relaxation to thesurrounding austenite grains is found to be dependent on the austenite volume.For a set of individual austenite grains the martensite nucleation is correlatedwith the initial austenite volume and the strain developed prior to thetransformation as a result of martensite formation in the neighbouring grains.1. IntroductionMaraging steels form a class of low-carbon high-alloyed steelsdeveloped in the 1960s for applications requiring ultra-highstrength combined with good fracture toughness and corro-sion resistance. Their remarkable properties are obtainedthrough aprocessof martensite formation followed byan age-hardening treatment to form fine precipitates in the marten-siticmatrix(Slunderetal.,1968).Forsomemaragingsteelsthemartensite can be formed by an isothermal phase transfor-mation below room temperature (Holmquist et al., 1995; SanMartin et al., 2010). The time-dependent formation ofmartensite at a constant temperature was first observed byKurdjumov & Maksimova (1948, 1950) in an Fe–Mn alloy andhassubsequentlybeenreportedforseveralothersystems,suchas Fe–Ni–Cr and Fe–Ni–Mn alloys (Kakeshita et al., 1993a;Borgenstam & Hillert, 1997). More recently, isothermal time-dependent martensite formation has also been observed inother metal alloys (Sordelet et al., 2007; Jeffries et al., 2009a,b;Kustov et al., 2010; Lee et al., 2011) and ceramics (Pee et al.,2006). In all these systems the martensite fraction depends onboth time and temperature. In contrast, for the more commonathermal martensitic transformations the martensite fractionis independent of time and only governed by the lowesttemperature reached (Porter et al., 2009).The isothermal transformation from the austenite phase( ), with a face-centred cubic structure, into the martensitephase (

Using synchrotron anomalous x-ray diffraction, the authors studied the structure of bimetallic Pt-Re catalysts with SiO[sub 2] as a support. The results reveal the presence of Re with a face-centered cubic structure in these catalysts. The authors propose that this form of Re is grown on Pt by an epitaxial effect.

The crystal structure of sodium pentamolybdyl tetradiphosphate Nax(MoO)5(P2O7)4 has been determined from synchrotron diffraction data collected at 293 K on two microcrystals. The compound crystallizes in a monoclinic space group I 1 1 2/a (no. 15, setting 11), with unit cell parameters a = 22.905(3), b = 23.069(2), c = 4.8537(2) A, γ = 90.641(9)° and a = 22.898(3), b = 23.056(2), c = 4.8551(2) A, γ = 90.82(1)°, for crystals I and II, respectively. The structure is pseudo-tetragonal, and the crystals are pseudo-merohedrally twinned by 90° rotation around the c-axis. The structure closely resembles the previously reported Li-deintercalated Mo1.3OP2O7 [V.V. Lisnyak, N.V. Stus, P. Popovich, D.A. Stratiychuk, Ya. Filinchuk, V.M. Davydov, J. Alloys Compd. 360 (2003) 81–84]. Comparison of the two structures led us to conclude that the Mo2 and Mo3 clusters were erroneously identified in Mo1.3OP2O7. A revised structure of Mo1.3OP2O7 contains a fully occupied oxygen site instead of the 16% occupied Mo(2) site, thus the revised formulae for the Li-deintercalated compound is (MoO)5(P2O7)4. In both structures, the (MoO)5(P2O7)4 framework strongly resembles the one in the earlier reported Ag(MoO)5(P2O7)4, while the location of Na and Ag atoms differ.

A procedure has been developed for following structural changes of pure synthetic calcium hydroxyapatite during its demineralization in a hydroxyethylcellulose solution. The procedure consists of demineralization and simultaneous powder diffraction patterns recorded with synchrotron X-ray radiation. Experimental details will be described. Es wird ein experimentelles Verfahren zur Diagnose struktureller Veranderungen von polykristallinem Calciumhydroxylapatit wahrend seiner Demineralisation in saurer Hydroxyathylzellulose-Losung beschrieben. Die Methode beinhaltet die Aufzeichnung von Pulverbeugungsdiagrammen wahrend der Demineralisation unter Anwendung von Synchrotron Rontgen-Strahlung. Experimentelle Details werden dargelegt.

The lattice structure of a pure and completely siliceous sample of zeolite ZSM-11 has been investigated by a combination of high-resolution solid-state {sup 29}Si MAS NMR and synchrotron-based X-ray powder diffraction techniques. The structure is temperature dependent in the range 263-363 K and a good fit to the diffraction data at 363 K is obtained with space group I{anti 4}m2 and a{sub 0} = 20.067 (1) and c{sub 0} = 13.411 (1). At room temperature the XRD data cannot be refined as the structure is intermediate and the lattice structure of the limiting low-temperature, low-symmetry form will only be deduced from data collected at 263 K.