Possible pair-graphene structures govern the thermodynamic properties of arbitrarily stacked few-layer graphene

The thermodynamic properties of few-layer graphene arbitrarily stacked on LiNbO3 crystal were characterized by measuring the parameters of a surface acoustic wave as it passed through the graphene/LiNbO3 interface. The parameters considered included the propagation velocity, frequency, and attenuation. Mono-, bi-, tri-, tetra-, and penta-layer graphene samples were prepared by transferring individual graphene layers onto LiNbO3 crystal surfaces at room temperature. Intra-layer lattice deformation was observed in all five samples. Further inter-layer lattice deformation was confirmed in samples with odd numbers of layers. The inter-layer lattice deformation caused stick–slip friction at the graphene/LiNbO3 interface near the temperature at which the layers were stacked. The thermal expansion coefficient of the deformed few-layer graphene transitioned from positive to negative as the number of layers increased. To explain the experimental results, we proposed a few-layer graphene even–odd layer number stacking order effect. A stable pair-graphene structure formed preferentially in the few-layer graphene. In even-layer graphene, the pair-graphene structure formed directly on the LiNbO3 substrate. Contrasting phenomena were noted with odd-layer graphene. Single-layer graphene was bound to the substrate after the stable pair-graphene structure was formed. The pair-graphene structure affected the stacking order and inter-layer lattice deformation of few-layer graphene substantially.

Experimental Investigation of Lattice Deformation Behavior in S355 Steel Weldments Using Neutron Diffraction Technique

This study aims to investigate the influence of welding process on the elastic lattice deformation and its effects on fatigue and fracture behavior of S355 G10+M steel, which is widely used in fabrication of offshore wind turbine monopile structures. In situ neutron diffraction measurements were taken on cross-weld test samples at room temperature to monitor the evolution of intergranular strains under static and cyclic loading conditions. Both static and cyclic test results have shown that the {200} orientation exhibits the least load carrying capacity while {211} had the maximum stiffness. The hkl-specific response predicted using Reuss and Kröner model were found to agree well with experimental values obtained for the heat-affected zone for all the orientations; however, discrepancies between the experimental and model predictions have been observed for the base metal and weld metal. Moreover, the microstructural differences between the weld metal and heat-affected zone resulted in the maximum elastic–plastic strain mismatch at the interface of the two regions. The results from this experiment would be useful to understand the role of crystal-specific microstrains and lattice deformation on fatigue and fracture behavior of thick-walled monopile weldments.

Multiwavelength anomalous X-ray diffraction for combined imaging of atomic displacement and strain

The X-ray Bragg coherent diffractive imaging (CDI) technique assumes that the structure factor holds constant over the measured crystal. This approximation breaks down for materials exhibiting variations in the unit-cell configuration, such as piezo- and ferroelectrics. In that case, the strain field cannot be reliably determined from the reconstruction because the lattice deformation and the structure factor contribute concomitantly. Proposed here is a solution to this problem achieved by combining Bragg CDI and the multiwavelength anomalous diffraction approach that measures a Friedel pair of reflections at two different photon energies near an absorption edge. Comparing the obtained reconstructions with a parametric model that includes calculating the scattering amplitude as a function of wavelength and the unit-cell configuration, the contributions of the lattice deformation and the structure factor are separated. Simulations of the ferroelectric material BaTiO3 demonstrate the possibility of simultaneous probing of the strain and displacement of the Ti atoms. The proposed method opens up an opportunity to apply coherent X-ray diffraction for nanoscale-resolved 3D mapping of polarization domains in micro- and nanocrystals.