Структурный переход и температурные зависимости коэффициентов теплового расширения NaNO-=SUB=-3-=/SUB=-, внедренного в нанопористое стекло

The temperature evolution of the crystal structure of a nanocomposite material obtained by introducing sodium nitrate NaNO3 from a melt under pressure into a nanoporous alkali borosilicate glass with an average pore diameter of 7 nm has been studied by the method of diffraction of synchrotron radiation in a wide temperature range upon heating. Analysis of the experimental diffraction patterns revealed a significant decrease in the temperature of the structural (orientational) transition by more than 50 K (up to 496 K) compared to bulk sodium nitrate. From the temperature dependence of the intensity of the superstructure peak (113), the dependence of the critical exponent β (T) for this transition was obtained and a significant difference from the critical exponent for a bulk material was found in the temperature range from 455 K to the transition temperature. Analysis of the broadening of Bragg reflections made it possible to estimate the average size (~ 40 nm) of sodium nitrate nanoparticles into the pores. An increase in the linear coefficient of thermal expansion in the [001] direction was found in NaNO3 nanoparticles in comparison with bulk material at temperatures above 450 K.

Inertial torques and a symmetry breaking orientational transition in the sedimentation of slender fibres

Experimental measurements of the force and torque on freely settling fibres are compared with predictions of the slender-body theory of Khayat & Cox (J. Fluid Mech., vol. 209, 1989, pp. 435–462). Although the flow is viscous dominated at the scale of the fibre diameter, fluid inertia is important on the scale of the fibre length, leading to inertial torques which tend to rotate symmetric fibres toward horizontal orientations. Experimentally, the torque on symmetric fibres is inferred from the measured rate of rotation of the fibres using a quasi-steady torque balance. It is shown theoretically that fibres with an asymmetric radius or mass density distribution undergo a supercritical pitch-fork bifurcation from vertical to oblique settling with increasing Archimedes number, increasing Reynolds number or decreasing asymmetry. This transition is observed in experiments with asymmetric mass density and we find good agreement with the predicted symmetry breaking transition. In these experiments, the steady orientation of the oblique settling fibres provides a means to measure the inertial torque in the absence of transient effects since it is balanced by the known gravitational torque.