Nucleation of a stable solid from melt in the presence of multiple metastable intermediate phases: Wetting, Ostwald step rule and vanishing polymorphs

In many systems, nucleation of a stable solid may occur in the presence of other (often more than one) metastable phases. These may be polymorphic solids or even liquid phases. In such cases, nucleation of the solid phase from the melt may be facilitated by the metastable phase because the latter can "wet" the interface between the parent and the daughter phases, even though there may be no signature of the existence of metastable phase in the thermodynamic properties of the parent liquid and the stable solid phase. Straightforward application of classical nucleation theory (CNT) is flawed here as it overestimates the nucleation barrier since surface tension is overestimated (by neglecting the metastable phases of intermediate order) while the thermodynamic free energy gap between daughter and parent phases remains unchanged. In this work we discuss a density functional theory (DFT) based statistical mechanical approach to explore and quantify such facilitation. We construct a simple order parameter dependent free energy surface that we then use in DFT to calculate (i) the order parameter profile, (ii) the overall nucleation free energy barrier and (iii) the surface tension between the parent liquid and the metastable solid and also parent liquid and stable solid phases. The theory indeed finds that the nucleation free energy barrier can decrease significantly in the presence of wetting. This approach can provide a microscopic explanation of Ostwald step rule and the well-known phenomenon of "disappearing polymorphs" that depends on temperature and other thermodynamic conditions. Theory reveals a diverse scenario for phase transformation kinetics some of which may be explored via modern nanoscopic synthetic methods

Solid-solid collapse transition in a two dimensional model molecular system

Solid-solid collapse transition in open framework structures is ubiquitous in nature. The real difficulty in understanding detailed microscopic aspects of such transitions in molecular systems arises from the interplay between different energy and length scales involved in molecular systems, often mediated through a solvent. In this work we employ Monte Carlo (MC) simulations to study the collapse transition in a model molecular system interacting via both isotropic as well as anisotropic interactions having different length and energy scales. The model we use is known as Mercedes-Benz (MB) which for a specific set of parameters sustains three solid phases: honeycomb, oblique and triangular. In order to study the temperature induced collapse transition, we start with a metastable honeycomb solid and induce transition by heating. High density oblique solid so formed has two characteristic length scales corresponding to isotropic and anisotropic parts of interaction potential. Contrary to the common believe and classical nucleation theory, interestingly, we find linear strip-like nucleating clusters having significantly different order and average coordination number than the bulk stable phase. In the early stage of growth, the cluster grows as linear strip followed by branched and ring-like strips. The geometry of growing cluster is a consequence of the delicate balance between two types of interactions which enables the dominance of stabilizing energy over the destabilizing surface energy. The nuclei of stable oblique phase are wetted by intermediate order particles which minimizes the surface free energy. We observe different pathways for pressure and temperature induced transitions

Design Fatigue Lives of Polypropylene Fibre Reinforced Polymer Concrete Composites

Flexural fatigue behavior of Poly-propylene fibre reinforced polymer concrete composites (PFRPCC) has been investigated at various stress levels and the statistical analysis of the data thus obtained has been carried out. Polymer Concrete Composite (PCC) samples without addition of any type of fibres were also tested for flexural fatigue.  Forty specimens of PCC and One hundred and Forty One specimens of PFRPCC containing 0.5%, 1.0% and 2.0% polypropylene fibres were tested in fatigue using a MTS servo controlled test system. Fatigue life distributions of PCC as well as PFRPCC are observed to approximately follow a two parameter Weibull distribution with correlation coefficient exceeding 0.9. The parameters of the Weibull distribution have been obtained by various methods. Failure probability, which is an important parameter in the fatigue design of materials, has been used to obtain the design fatigue lives for the material. Comparison of design fatigue life of PCC and PFRPCC has been carried out and it is observed that addition of fibres enhances the design fatigue life of PCC

Polymorph selection during crystallization of a model colloidal fluid with a free energy landscape containing a metastable solid

The free energy landscape responsible for crystallization can be complex even for relatively simple systems like hard sphere and charged stabilized colloids. In this work, using hard-core repulsive Yukawa model, which is known to show complex phase behavior consisting of fluid, FCC and BCC phases, we studied the interplay between the free energy landscape and polymorph selection during crystallization. When the stability of the BCC phase with respect to the fluid phase is gradually increased by changing the temperature and pressure at a fixed fluid-FCC stability, the final phase formed by crystallization is found to undergo a switch from the FCC to the BCC phase, even though FCC remains thermodynamically the most stable phase. We further show that the nature of local bond-orientational order parameter fluctuations in the metastable fluid phase as well as the composition of the critical cluster depend delicately on the free energy landscape, and play a decisive role in the polymorph selection during crystallization