K. Lu

Liquid−Solid Transition of Confined Water in Silica-Based Mesopores

Cooling and heating curves of water confined in partially filled Vycor porous glass were measured for both adsorption and desorption processes. One endothermic and two exothermic peaks were observed for almost all cases. The peak temperature and the enthalpy of the exothermic peak located below 232 K increased initially and then decreased with further increases in the filling factor. These abnormal changes were analyzed based on the liquid-solid transition of nanoconfined water using a core/shell model, and the initial adsorption process of water in this typical mesoporous material with disordered pores is discussed. In addition, an interesting observation is that different peak temperatures for the endothermic peak and an almost constant peak temperature for the exothermic peak were observed at the same filling factor obtained under different sample preparation conditions, that is, adsorption and desorption processes. To compare with the liquid-solid transition temperatures of confined water in fully filled silica-based mesopores of different pore radius, a parameter of the ratio of pore inner surface area to confined liquid volume is proposed in this paper. Referring to this parameter, the core part of confined water in silica-based nanopores has the same liquid-solid transition temperatures. This suggestion is valid for the freezing process of water confined in either fully filled ordered or fully or partially filled disordered pores. For the melting process, different linear changes of melting temperature with the ratio of pore inner surface area to liquid volume were observed for water in disordered and ordered pores.

Melting of superheated crystals initiated on vacancies

In a large variety of ideal crystals we found that when rapidly migrating atoms squash or annihilate a neighbouring vacancy and produce a disordered cluster, the heat of migration stored in the system exceeds the enthalpy increase required for the coordinating atoms of the vacancy to form a liquid phase, i.e. the liquid phase nucleates from vacancies. Furthermore, volumetric analysis supports this well. This vacancy-decomposition model provides quantitative information on the melting point, the latent heat and the volume change upon melting and hence clarifies the mechanism of melting.

Mechanical-milling-induced amorphization of Se: A crystallite destabilization model

Complete solid-state amorphization has been realized in elemental Se by means of mechanical milling of crystalline Se powder. Quantitative X-ray diffraction and thermal analyses were employed to characterize the amorphization process and indicated that the amorphization onset corresponds to a critical crystallite size and a drop in microstrain. During the major amorphization process, the remaining crystallite size remains unchanged with a constant lattice expansion. A new kinetics model of crystallite destabilization is proposed for the solid-state amorphization which satisfactorily explains the experimental observations.

Temperature effect of the local structure in liquid Sb studied with x-ray absorption spectroscopy

The temperature dependence of the local structure of liquid Sb has been studied by x-ray absorption spectroscopy. It is shown that about 10% of the atoms with coordination of 3 and weak Peierls distortion exist in liquid Sb just above its melting point. The Peierls distortion weakens gradually with increasing temperature and vanishes at about 750 degrees C. This structural variation in liquid Sb is different from the normal liquid-liquid phase transition. This work reveals the relationship between the variation in the local structure and the change in the physical properties, such as the electrical resisitvity of liquid Sb, with temperature. The complete agreement between the measured electrical resistivity values during heating and cooling processes suggests that the structural units with the features of a rhombohedron appear above the melting point of Sb during solidification.

Melting of metals: role of concentration and migration of vacancies at surfaces

The mechanism of melting is investigated by considering the role of surfaces with regard to the concentration and migration of vacancies. For many metals, it is found that while the vacancy concentration in the bulk is ∼0.37% at the equilibrium melting point (T 0), the vacancy concentration at the free surface is as high as 10%, i.e. similar to that in the bulk at the superheating limit. Melting is believed to be associated with a lattice instability induced at a critical vacancy concentration of ∼10%, both at the surface and within the crystal lattice. The abrupt increase in vacancy concentration from 0.37 to 10% on melting at T 0 can be explained as a result of melting of the surface. The surface pre-melting behaviour of metals is quantitatively interpreted by considering the vacancy migration there.

Enhanced fatigue property by suppressing surface cracking in a gradient nanostructured bearing steel

ABSTRACT A gradient nanostructured (GNS) surface layer was generated on an ultrahigh strength bearing steel (AISI 52100) by surface mechanical rolling treatment (SMRT). The initial martensite plates with cementite and retained austenite were transformed into nanograined martensite with a mean size of ∼24 nm at the topmost surface. Axial tension–compression fatigue tests showed that an enhanced fatigue property was achieved on the GNS sample due to the suppressed fatigue crack initiation at the surface. Effects from hardness, microstructure, and residual stress in the GNS surface layer were discussed in relation with the fatigue mechanism of the SMRT sample. GRAPHICAL ABSTRACT IMPACT STATEMENT A gradient nanostructured surface layer was generated on an ultrahigh strength bearing steel, and significantly improved the fatigue property of the steel by suppressing surface crack initiation at lower stresses.

The kinetic limit of superheating induced by semicoherent interfaces

The superheating behaviour of embedded particles induced by semicoherent interfaces has been observed in many circumstances. In this paper, a phenomeno‐ logical model for melt nucleation on misfit dislocations at a semicoherent interface is proposed. A kinetic limit for semicoherent-interface-induced superheating, which is in good agreement with the results of experiments and computer simulations, is derived from this model. Calculations and analyses based on the model reveal that melting prefers to initiate at the semicoherent interface and that superheating of embedded particles is possible for a melt nucleation contact angle less than 90°. Among the matrix-dependent parameters, the contact angle and the shear modulus of the matrix are found to be dominant in determining the superheating of embedded particles.

Influence of strain rate on the orientation dependence of microstructure in nickel single crystals

Abstract The deformation microstructures of nickel single crystals (99.945 wt.%) during dynamic plastic deformation and quasi-static compression to a true strain of 0.20 were comparatively investigated. The deformation microstructures are orientation dependent, forming cell structure, slip plane aligned or not slip plane aligned extended boundaries. It is found that the orientation spread decreases, remains unchanged and becomes enhanced when loading along 〈0 0 1〉, 〈0 1 1〉 and 〈1 1 1〉, respectively, as strain rate increases.

2d Granular Gas in Knudsen Regime and in Microgravity Excited by Vibration: Velocity and Position Distributions

Dynamics of quasi-2d dissipative granular gas is studied in microgravity condition (of the order of 10(-4) g) in the limit of Knudsen regime. The gas, made of 4 spheres, is confined in a square cell enforced to follow linear sinusoidal vibration in ten different vibration modes. The trajectory of one of the particles is tracked and reconstructed from the 2-hour video data. From statistical analysis, we find that (i) loss due to wall friction is small, (ii) trajectory looks ergodic in space, and (iii) distribution p(v) of speed follows an exponential distribution, i.e. p(v)approximate to exp(-v/v(xo,yo)), with v(xo,yo) being a characteristic velocity along a direction parallel (y) or perpendicular (x) to vibration direction. This law deviates strongly from the Boltzmann distribution of speed in molecular gas. Comparisons of this result with previous measurements in earth environment, and what was found in 3d cell [1] performed in 10(-2) g environment are given.

The global property of the dilute-to-dense transition of granular flows in a 2D channel

The dilute-to-dense transition of granular flow of particle size d 0 is studied experimentally in a two dimensional channel (width D) with confined exit (width d). It is found that there exists a maximum inflow rate Q c, above which the outflow changes from dilute to dense and the outflow rate Q(t) drops abruptly from Q c to a dense flow rate Q d. The re-scaled critical rate \( D/d_0 \) is found to be a function of a scaling variable λ only, i.e. q c ∼ F(λ), and \( \lambda \equiv \frac{d} {{d_0 }}\frac{d} {{D - d}} \) . This form of λ suggests that the dilute-to-dense transition is a global property of the flow; unlike the jamming transition, which depends only on \( \frac{d} {{d_0 }} \) . The transition is found to occur when the area fraction of particles near the exit reaches a critical value 0.65±0.03.