Tension-compression asymmetry of metastable austenitic stainless steel studied by in-situ high-energy X-ray diffraction

This work studies the tension-compression asymmetry (TCA) of metastable austenitic stainless steel (MASS) in uniaxial loading depending on temperature. In-situ high-energy X-ray diffraction was used to simultaneously probe phase fractions, transformation kinetics, crystallographic texture, lattice strains, strain and stress partitioning between austenite and martensites during quasi-static tensile and compressive deformation at 24, 60 and 100 °C. Complementary relaxed-constraint crystal plasticity simulations and calculations of the mechanical driving force related to the formation of α’ and ε martensites were performed. At 24 °C, martensitic transformations (MTs) prevail, while at 100 °C dislocation slip is the dominant deformation mechanism for both load senses. Macroscopic stress-strain response and transformation behaviour exhibit TCA, with compression promoting the conversion of ε into α’. Transformation kinetics were analyzed in relation to shear banding and the geometric alignment of ε lamellas depending on load sense and temperature. A strong TCA was found for crystallographic texture, bearing signatures of grain rotation due to plastic slip and of MT in case of austenite (γ). For both load senses, the relative strengths of austenite and martensite texture fibres were related to the driving force anisotropy for α’ formation calculated based on the phenomenological theory of martensite crystallography. Texture evolution of α’ is largely controlled by the MT itself, not by grain rotation. Analysis of differently orientated austenite grain families revealed a pronounced TCA of the lattice strains, linked to the γ → ε MT. This was found to be a direct consequence of driving force and volume change related to ε formation. Furthermore, stress is shared differently between austenite and martensites in tension vs. in compression. γ hardens more and hence carries a larger portion of the total stress in compression than in tension. The origin for this TCA could be found in the elasto-plastic accommodation of the volume change related to α’ formation. These findings can aid the development of new material laws for MASSs that are sensitive to load-sense and temperature for advanced forming simulations

Ordering of ionic liquids at a charged sapphire interface: Evolution with cationic chain length

HypothesisRoom Temperature Ionic Liquids (RTILs) bulk's molecular layering dominates their structure also at the RTIL/sapphire interface, increasing the layer spacing with the cationic alkyl chain length n. However, the negatively-charged sapphire surface compresses the layers, increases the layering range, and affects the intra-layer structure in yet unknown ways.ExperimentsX-ray reflectivity (XR) off the RTIL/sapphire interface, for a broad homologous RTIL series 1-alkyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide, hitherto unavailable for any RTIL.FindingsRTIL layers against the sapphire, exhibit two spacings:and . is n-varying, follows the behavior of the bulk spacing but exhibits a downshift, thus showing significant layer compression, and over twofold polar slab thinning. The latter suggests exclusion of anions from the interfacial region due to the negative sapphire charging by x-ray-released electrons. The layering range is larger than the bulk's. is short and near n-independent, suggesting polar moieties' layering, the coexistence mode of which with the -spaced layering is unclear. Comparing the present layering with the bulk's and the RTIL/air interface's provides insight into the Coulomb and dispersion interaction balance dominating the RTIL's structure and the impact thereon of the presence of a charged solid interface

Self‐Catalyzed Hydrogenated Carbon Nano‐Onions Facilitates Mild Synthesis of Transparent Nano‐Polycrystalline Diamond

Transparent nano-polycrystalline diamond (t-NPD) possesses superior mechanical properties compared to single and traditional polycrystalline diamonds. However, the harsh synthetic conditions significantly limit its synthesis and applications. In this study, a synthesis routine is presented for t-NPD under low pressure and low temperature conditions, 10 GPa, 1600 °C and 15 GPa, 1350 °C similar with the synthesis condition of organic precursor. Self-catalyzed hydrogenated carbon nano-onions (HCNOs) from the combustion of naphthalene enable synthesis under nearly industrial conditions, which are like organic precursor and much lower than that of graphite and other carbon allotropes. This is made possible thanks to the significant impact of hydrogen on the thermodynamics, as it chemically facilitates phase transition. Ubiquitous nanotwinned structures are observed throughout t-NPD due to the high concentration of puckered layers and stacking faults of HCNOs, which impart a Vickers hardness about 140 GPa. This high hardness and optical transparency can be attributed to the nanocrystalline grain size, thin intergranular films, absence of secondary phase and pore-free features. The facile and industrial-scale synthesis of the HCNOs precursor, and mild synthesis conditions make t-NPD suitable for a wide range of potential applications

Challenges and limitations of accelerated stress testing in GDE half-cell set-ups

Commercialisation of proton exchange membrane fuel cells (PEMFCs) depends on accurate and high throughput durability testing at the laboratory scale. With the rotating disk electrode method (RDE) unable to mimic the three-phase boundary scenario in the membrane electrode assembly (MEA), gas diffusion electrode half-cells were proposed for fundamental catalysis research. However, durability testing in such half-cell setups under realistic operational conditions has been limited, and in particular, not yet validated against RDE or FC data. In this paper, an attempt is made to fill part of this knowledge gap by performing accelerated stress tests in thin films, gas diffusion electrodes and membrane electrode assemblies. The results are compared for two selected catalysts with different Pt loading, expected to show broad variations in their degradation behaviour. Accelerated stress tests (ASTs) were performed with various start/stop potentials and load cycles, and the oxygen reduction reaction (ORR) performance studied before and after the AST protocols. The internal resistance of the half-cell was found to be influenced most significantly by gas coverage and temperature changes on the working electrode and must be compensated accordingly. The applied vertex potentials for ASTs after compensation have to be accurate in order to induce the intended degradation phenomena

Solving the Synthetic Riddle of Colloidal Two-Dimensional PbTe Nanoplatelets with Tunable Near-Infrared Emission

Near-infrared emitting colloidal two-dimensional (2D) PbX (X = S, Se) nanoplatelets (NPLs) have emerged as interesting materials with strong size quantization in the thickness dimension. They act as model systems for efficient charge carrier multiplication and hold potential as intriguing candidates for fiber-based photonic quantum applications. However, synthetic access to the third family member, 2D PbTe, remains elusive due to challenging precursor chemistry. Here, we report a direct synthesis for 2D PbTe NPLs with tunable photoluminescence [PL, 910–1460 nm (1.36–0.85 eV), PL quantum yields 1–15%], based on aminophosphine precursor chemistry. Ex situ transamination of tris(dimethylamino)phosphine telluride with octylamine is confirmed by 31P nuclear magnetic resonance and yields a reactive tellurium precursor for the formation of 2D PbTe NPLs at temperatures as low as 0 °C. The PL position of the PbTe NPLs is tunable by controlling the Pb/Te ratio in the reaction. Grazing-incidence wide-angle X-ray scattering confirms the 2D geometry of the NPLs and the formation of superlattices. The importance of a postsynthetic passivation of PbTe NPLs by PbI2 to ensure colloidal stability of the otherwise oxygen-sensitive samples is supported by X-ray photoelectron spectroscopy. Our results expand and complete the row of lead chalcogenide-based 2D NPLs, opening up new ways for further pushing the optical properties of 2D NPLs into the infrared and toward technologically relevant wavelengths

Small conformational changes in IgG1 detected as acidic charge variants by cation exchange chromatography

Fully characterizing the post-translational modifications present in charge variants of therapeutic monoclonal antibodies (mAbs), particularly acidic variants, is challenging and remains an open area of investigation. In this study, to test the possibility that chromatographically separated acidic fractions of therapeutic mAbs contain conformational variants, we undertook a mAb refolding approach using as a case study an IgG1 that contains many unidentified acidic peaks with few post-translational modifications, and examined whether different acidic peak fractions could be generated corresponding to these variants. The IgG1 drug substance was denatured by guanidine hydrochloride, without a reducing agent present, and gradually refolded by stepwise dialysis against arginine hydrochloride used as an aggregation suppressor. Each acidic chromatographic peak originally contained in the IgG1 drug substance was markedly increased by this stepwise refolding process, indicating that these acidic variants are conformational variants. However, no conformational changes were detected by small-angle X-ray scattering experiments for the whole IgG1, indicating that the conformational changes are minor. Chromatographic, thermal and fluorescence analyses suggested that the conformational changes are a localized denaturation effect centred around the aromatic amino acid regions. This study provides new insights into the characterization of acidic variants that are currently not fully understood

Hydrogen Evolution in Neutral Media by Differential Intermediate Binding at Charge‐Modulated Sites of a Bimetallic Alloy Electrocatalyst

The energy barrier to dissociate neutral water has been lowered by the differential intermediate binding on the charge-modulated metal centers of Co$_{85}$Mo$_{15}$ sheets supported on Ni-foam (NF), where the overpotential for hydrogen evolution reaction (HER) in 1 M phosphate buffer solution (PBS) is only 50±9 mV at −10 mA cm$^{−2}$. It has a turnover frequency (TOF) of 0.18 s$^{−1}$, mass activity of 13.2 A g$^{−1}$ at −200 mV vs. reversible hydrogen electrode (RHE), and produces 16 ml H$_2$ h$^{−1}$ at −300 mV vs. RHE, more than double that of 20 % Pt/C. The Mo$^{δ+}$ and Co$^{δ−}$ sites adsorb OH*, and H*, respectively, and the electron injection from Co to H−O−H cleaves the O−H bond to form the Mo−OH* intermediate. Operando spectral analyses indicate a weak H-bonded network for facilitating the H$_2$O*/OH* transport, and a potential-induced reversal of the charge density from Co to the more electronegative Mo, because of the electron withdrawing Co−H* and Mo−OH* species. Co85Mo15/NF can also drive the complete electrolysis of neutral water at only 1.73 V (10 mA cm$^{−2}$). In alkaline, and acidic media, it demonstrates a Pt-like HER activity, accomplishing −1000 mA cm$^{−2}$ at overpotentials of 161±7, and 175±22 mV, respectively

A New Fossil Anolis Lizard in Hispaniolan Amber: Ecomorphology and Systematics

Pre-Pleistocene fossils of Anolis lizards are rare, although 20 Miocene fossils preserved in amber from the island of Hispaniola have been reported on previously. Using light-microscopy and computed-tomography imaging, we studied a new amber fossil Anolis lizard from Hispaniola. The fossil is likely a juvenile and preserves a largely intact left forelimb, including both scales and skeletal elements, as well as some additional skin, skin impressions, and fragmentary skeletal elements from other parts of the body. Using measurements and lamella counts from the forelimb of the fossil and other juvenile anoles, discriminant function analysis and three Euclidean-distance criteria derived from a principal components analysis consistently support classification of the fossil as a member of the trunk ecomorph category, and those results, in combination with two scale characters preserved in the fossil, suggest that it is a member of the Anolis distichus series within the Ctenonotus clade. These results represent only the third case of a well-supported assignment of an Anolis fossil to the trunk ecomorph category and the first to the A. distichus series. They also highlight that such assignments can sometimes be inferred from highly incomplete fossils

Nitride Synthesis under High-Pressure, High-Temperature Conditions: Unprecedented In Situ Insight into the Reaction

High-pressure, high-temperature (HP/HT) syntheses are essential for modern high-performance materials. Phosphorus nitride, nitridophosphate, and more generally nitride syntheses benefit greatly from HP/HT conditions. In this contribution, we present the first systematic in situ investigation of a nitridophosphate HP/HT synthesis using the reaction of zinc nitride Zn$_3$N$_2$ and phosphorus(V) nitride P$_3$N$_5$ to the nitride semiconductor Zn$_2$PN$_3$ as a case study. At a pressure of 8 GPa and temperatures up to 1300 °C, the reaction was monitored by energy-dispersive powder X-ray diffraction (ED-PXRD) in a large-volume press at beamline P61B at DESY. The experiments investigate the general behavior of the starting materials under extreme conditions and give insight into the reaction. During cold compression and subsequent heating, the starting materials remain crystalline above their ambient-pressure decomposition points, until a sufficient minimum temperature is reached and the reaction starts. The reaction proceeds via ion diffusion at grain boundaries with an exponential decay in the reaction rate. Raising the temperature above the minimum required value quickly completes the reaction and initiates single-crystal growth. After cooling and decompression, which did not influence the resulting product, the recovered sample was analyzed by energy-dispersive X-ray (EDX) spectroscopy

Thermal and combined high-temperature and high-pressure behavior of a natural intermediate scapolite

A natural intermediate member of the scapolite solid solution (Me$_{47}$; experimental chemical formula: (Na$_{1.86}$Ca$_{1.86}$K$_{0.23}$Fe$_0.01$)(Al$_{4.36}$Si$_{7.64}$)O$_{24}$[Cl$_{0.48}$(CO$_3$)$_{0.48}$(SO$_4$)$_{0.01}$]), with the unusual I4/m space group, has been studied at various temperatures and combined high-T and high-P by means of in situ single-crystal and powder X-ray diffraction, at both conventional and synchrotron sources. In addition, single-crystal neutron diffraction data were collected at ambient-T and 685 °C. A fit of the experimental V-T data with a thermal equation of state yielded a calculated thermal expansion coefficient at ambient conditions: $α_{V25°C}$ = 1/V$_{0·}$($∂$V/$∂$T)$_{P,25°C}$ = 1.74(3)·10$^{-5}$ K$^{-1}$. A comparative analysis of the elastic behavior of scapolite based on this study and other high-T XRD data reported in the literature suggests that a thorough re-investigation of the different members of the marialite-meionite solid solution is needed to fully understand the role of crystal chemistry on the thermal behavior of these complex non-binary solid solutions. The experimental data obtained within the full temperature range of analysis at ambient pressure confirm that the investigated sample always preserves the I4/m space group, and possible implications on the metastability of I4/m intermediate scapolite are discussed. Neutron diffraction data show that no significant Si and Al re-arrangement among the T sites occurs between 25 and 685 °C. The combined high-T and high-P data show that at 650 °C and between 10.30(5) and 10.71(5) GPa a phase transition towards a triclinic polymorph occurs, with a positive Clapeyron slope (i.e., dP/dT > 0). A comprehensive description of the atomic-scale structure deformation mechanisms induced by temperature and/or pressure, including those leading to structural instability, is provided based on single-crystal structure refinements