Phase Transitions and Dynamics Studies of Nanocomposite Materials

Dear Colleagues,

It is our honour to invite you to participate in a Special Issue of Crystals entitled "Phase Transitions and Dynamics Studies of Nanocomposite Materials". In the 21^st century, nanocomposites are a field of research that is constantly being developed and focuses on a promising group of materials with considerable possibilities. Nanocomposites have applications in optoelectronics, electronics, bioengineering, and electrical engineering. By combining their numerous properties, nanocomposites can be used to solve some of the important problems of recent years, such as reducing greenhouse gas emissions, developing renewable energy systems, or increasing the strength and biocompatibility of medical implants. Crystalline materials play an important role in nanocomposites. They are an important component of the system and usually show a change in physical properties. The nano size causes a change in behaviour under the influence of external factors. Therefore, it is important to study these properties in complex systems and under extreme conditions. It is also worth mentioning that nanocrystallites play a key role in electrical and thermal conductivity, as well as in the sintering process of ceramic materials, which directly affects the macroscopic properties of the materials. The aim of this Special Issue is therefore to gather experts from many fields to highlight the latest research trends, but above all to demonstrate the multidisciplinary nature of the subject and standardizing its legitimacy.

Dr. Szymon Starzonek
Prof. Dr. Aleš Iglič
Guest Editors

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Tittle 1: Nanoparticle-Imposed Giant Responses in Liquid Crystals

Authors: Arbresha Holbl, Kaushik Pal, Samo Kralj

Abstract: Liquid crystals (LCs) are extraordinary materials with unique coexistence of liquid character, orientational or translational order, softness, and optical anisotropy. In particular, their liquid character enables relatively easy preparation of stable mixtures consisting of a LC matrix and diverse nanoparticles (NPs). The resulting effective materials could exhibit desired combination of properties which are not displayed in isolated components. In our contribution we address mixtures of thermotropic nematic LCs and NPs exhibiting diverse geometries (particle-like, rod-like, platelets…). Bulk nematic LCs display in equilibrium long-range molecular orientational order which is established via discontinuous order-disorder phase transition on lovering temperature from the isotropic (i.e., ordinary liquid phase). We focus on mixtures in which the effective material could be driven towards a critical point, where systems in general exhibit anomalously strong responses to external stimuli. We determine different pathways to establish such conditions. We also suggest diverse applications in which proposed materials could be exploited.

Tittle 2: Order parameter-related changes of dielectric constant in nematic liquid crystals and its nanocolloids

Authors: S. J. Rzoska, A. Drozd-Rzoska, S. Starzonek

Abstract: For rod-like nematic liquid crystal (NLC) temperature scan of dielectric constant is directly coupled to the order parameter changes. Its component for samples oriented parallel and perpendicular to the long molecular axis and the measuring field is expected to cease changes as the function of temperature at a distance of several degrees below isotropic - nematic (I-N) phase transition temperature. This feature constitutes one of the basic tools for estimating the anisotropy of dielectric constant deltaEps=Eps(parallel) - Eps(perp). It is one of the most important material characterizations of NLC due to its direct-to-molecular properties of the rod-like molecule and essential significance for applications. For decades, the Haller relation is used as the basic tool for describing dielectric constant order parameter changes. [1-3] This report explicitly shows that the Haller relation can be considered only as a rough and effective tool in a limited range of temperatures. The report also indicates that the inherent feature of the fundamentally justified portrayal of dielectric constant components in the nematic phase is ‘bending’ both near and remote from I-N transition. The authors stress using the distortions-sensitive and derivative-based analysis to validate results. Finally, adding nanoparticles can act as a specific endogenic agent facilitating superior ‘parallel’ orientation of rhodium-like molecules. It leads to values of the parallel component of dielectric constant even 20 % larger than detected after the standard ‘exogenic’ arrangement by electric or strong magnetic fields. Such a result for NLC with a small admixture of NPs can be associated with reducing the influence of steric molecular hindrances and better effective orientation of rhodium-like molecules than using standard methods. The consequences of the results presented on the corresponding states scaling [4] are also discussed. I. Haller, Thermodynamic and static properties of liquid crystals, Prog. Solid State Chem. 10 (1975) 103-118. M. Geppi, A. Marini, C. A. Veracini, S. Urban, J. Czub, W.. Kuczyński, R Dabrowski, Orientational order of difluorinated liquid crystals: a comparative 13C-NMR, Optical, and dielectric study in nematic and smectic B phases, J. Phys. Chem. B 112 (2008) 9663-76. Z. Alipanah, M. S. Zakerhamidi, A. Ranjkesh, Temperature-dependent optical properties of some mixtures nematic liquid crystal, Sci Rep 12 (2022) 12676. M. Simões, D. S. Simeão, Corresponding states of order parameter in nematic liquid crystals, Phys. Rev. E 74 (2006) 051701.

Tittle 3: Giant, critical-like, premelting effect for the solid crystal - liquid crystal discontinuous transition

Authors: A. Drozd-Rzoska, S. J. Rzoska, S. Starzonek, J. Łoś1, J. Kalabiński

Abstract: Melting/freezing premelting transition between liquid and solid crystal remains a century-long cognitive challenge. It is a qualitative difference from continuous phase transitions, whose universalist description, leading to Critical Phenomena Physics, became one of the greatest successes of physics of the 20th century. In the last decade, the crucial experimental reference is the premelting effect appearing in the solid crystal phase of some systems on approaching the melting temperature. Unfortunately, the premelting effect is relatively small and appears only in the immediate vicinity of melting temperature Tm. Only recently, it has been shown that nitrobenzene can be significant and long-range but also have critical-like characteristics [J. Kalabiński, et al., Crystals 13 (2023) 247]. These results seem directly related to using dielectric methods for detecting the phenomenon. In this work, we show the experimental evidence of the giant premelting effect for nematic liquid crystal (NLC) -solid crystal phase transition and its nanocolloids with BaTiO3 nanoparticles (NPs), tested via temperature evolutions of the real part of dielectric permittivity in the static and low-frequency, hardly tested so far, domains. Test supplemented to the insight into dynamic properties related to the imaginary part of dielectric permittivity. Notable are critical-like changes of tested properties, with the singular (‘critical’) temperature almost coinciding with the melting temperature. The addition of nanoparticles amplifies premelting effects, leading to their appearance both on cooling and heating, i.e., regarding the freezing and melting temperatures. The report delivers explicit functional dependences of detected giant premelting effects, with links to properties of tested systems (NLC, NLC+NPs colloids) which can be the significant reference for ultimate modeling of melting/freezing discontinuous transitions, also indicating a link to Critical Phenomena Physics.