By means of in situ small-angle x-ray diffraction experiments and semi-grand-canonical ensemble Monte Carlo simulations we demonstrate that sorption and condensation of a fluid confined within nanopores is capable of deforming the pore walls. At low pressures the pore is widened due to a repulsive interaction caused by collisions of the fluid molecules with the walls. At capillary condensation the pores contract abruptly on account of attractive fluid-wall interactions whereas for larger pressures they expand again. These features cannot solely be accounted for by effects related to pore-wall curvature but have to be attributed to fluid-wall dispersion forces instead.
The sorption of fluids in mesoporous silica is an important physical phenomenon with a wide range of applications. Traditionally, mesoporous materials have been considered as inert scaffolds for the sorption and condensation reaction of the fluid. Here we present in situ small angle X-ray diffraction experiments providing evidence for a sorption strain induced in the solid that manifests itself as a change in the lattice parameter of the ordered mesopore array as the pores gradually adsorb fluid material. The experimental data are analyzed by means of Monte Carlo simulations carried out in a grand canonical ensemble describing a fluid confined by deformable substrates. We show that-in agreement with experimental data-sorption of a nonpolar fluid causes the pores to expand initially, to shrink abruptly when capillary condensation sets in, and to expand again as more liquid-like fluid is adsorbed subsequently. We show that the pore pressure can be extracted from a thermodynamic analysis of sorption isotherms in the liquid-like regime and that this information can be used for an estimation of the Young's modulus of the porous silica material. In addition, our Monte Carlo simulations indicate that the phase behavior of confined fluids is considerably changed by the deformability of the confining solid. This is reflected by a change of the location of phase boundaries at sufficiently subcritical temperatures.
Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (⊥ b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.
Complementary experimental and theoretical results on the coalescence of nanoparticles demonstrate the importance of the crystallographic orientation on the coalescence process. In situ hot-stage transmission electron microscopy studies on self-supporting films consisting of indium tin oxide nanoparticles clearly show rotations of neighboring particles preceding their coalescence. Both rotation and coalescence are observed well below half the melting temperature. The coalescence of two adjacent nanoparticles is simulated by means of a combination of the kinetic Monte Carlo method for atomic diffusion with an integration of the equations of motion for the rigid body degrees of freedom of the two particles. This allows analyzing the reorientation of the two crystal lattices prior to the merging process. Thus, nanoparticle coalescence has theoretically as well as experimentally been shown to be a two-step process: first a reorientation of adjacent nanoparticles, and second their complete or incomplete coalescence depending on the matching of the crystallographic orientations.
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