High-energy ball milling was used to synthesize blue-green emitting Si nanocrystals from micron sized silicon particles.
We show that the surface of ice is scratch healing: micrometer-deep scratches in the ice surface spontaneously disappear by thermal relaxation on the time scale of roughly an hour. Following the dynamics and comparing it to different mass transfer mechanisms, we find that sublimation from and condensation onto the ice surface is the dominant scratch-healing mechanism. The scratch-healing kinetics shows a strong temperature dependence, following an Arrhenius behavior with an activation energy of Δ E = 58.6 ± 4.6 kJ/mol, agreeing with the proposed sublimation mechanism and at odds with surface diffusion or fluid flow or evaporation–condensation from a quasi-liquid layer.
Deliquescence is a first-order phase transition, happening when a salt absorbs water vapor. This has a major impact on the stability of crystalline powders that are important for example in pharmacology, food science and for our environment and climate. Here we show that during deliquescence, the abundant salt sodium sulfate decahydrate, mirabilite (Na2SO4·10H2O), behaves differently than anhydrous salts. Using various microscopy techniques combined with Raman spectroscopy, we show that mirabilite crystals not only lose their facets but also become soft and deformable. As a result, microcrystals of mirabilite simultaneously behave crystalline-like in the core bulk and liquid-like at the surface. Defects at the surface can heal at a speed much faster than the deliquescence rate by the mechanism of visco-capillary flow over the surface. While magnesium sulfate hexahydrate (MgSO4⋅6H2O) behaves similarly during deliquescence, a soft and deformable state is completely absent for the anhydrous salts sodium chloride (NaCl) and sodium sulfate thenardite (Na2SO4). The results highlight the effect of crystalline water, and its mobility in the crystalline structure on the observed softness during deliquescence. Controlled hydrated salts have potential applications such as thermal energy storage, where the key parameter is relative humidity rather than temperature.
The crystalline structure of minerals due to the highly ordered assembly of its constituent atoms, ions or molecules confers a considerable hardness and brittleness to the materials. As a result, they are generally subject to fracture. Here we report that microcrystals of natural inorganic salt hydrates such as sodium sulfate decahydrate (Na2SO4 • 10H2O) and magnesium sulfate hexahydrate (MgSO4 • 6H2O) can behave remarkably differently: instead of having a defined faceted geometrical shape and being hard or brittle, they lose their facets and become soft and deformable when in contact with their saturated salt solution at their deliquescence point. As a result, the hydrated crystals simultaneously behaves as crystalline and liquid-like. We show that the observed elasticity is a consequence of a trade-off between the excess capillary energy introduced by the deformation from the equilibrium shape and viscous flow over the surface of the microcrystals. This surprising, unusual mechanical properties reported here reveals the role of the water molecules present in the crystalline structure. Although many compounds can incorporate water molecules in their crystalline frameworks, the relationship between the different hydrates and anhydrate crystal forms are still poorly investigated. Our results on the floppy behaviour of such crystalline structure reveals some unexplored properties of water molecules entrapped in the crystalline structure and can open novel routes for their application in various fields such as pharmaceutical sciences , thermal energy storage and even the traceability of water on Mars
We show that the surface of ice is self-healing: micrometer deep scratches in the ice surface spontaneously disappear by relaxation on a time scale of roughly an hour.Following the dynamics and comparing it to different mass transfer mechanisms, we find that sublimation from and condensation onto the ice surface is the dominant selfhealing mechanism. The self-healing kinetics shows a strong temperature dependence, following an Arrhenius behavior with an activation energy of ∆E = 58.6 ± 4.6 kJ/mole, agreeing with the proposed sublimation mechanism, and at odds with surface diffusion or fluid flow or evaporation-condensation from a quasi-liquid layer.Ice is one of the most actively studied solids, 1-11 and many of its physical properties are still poorly understood. In particular, the structure and dynamics of the topmost layer of water molecules in ice has been subject of intense debate for more than 150 years. Already in 1860, Faraday observed that ice cubes sinter together, and he concluded that there is 1
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