Glass-forming Liquid Crystals Research Articles

Modifying optical nonlinearities of ionic liquid crystal glass by adding gold and carbon nanoparticles

Future disruptive photonics technologies depend on the development of new types of nonlinear-optical glass composites containing nanoparticles. In this paper, we report the synthesis, structural characterization, spectral and nonlinear-optical properties of overlooked yet very promising glass nanocomposites made of glass forming ionic liquid crystals (cadmium caprylate) and two types of nanoparticles (gold and carbon). Nonlinear-optical properties of the prepared nanocomposites are studied by means of a standard Z-scan method and using visible (532 nm) nanosecond laser pulses. By exciting glass nanocomposites within their absorption band, an interplay between several nonlinear-optical mechanisms can be revealed. A simultaneous use of two types of nanoparticles (i.e. carbon and gold) enables a better control over effective optical nonlinearities of nanocomposites. More specifically, by changing an intensity of a laser beam, it is possible to vary the magnitude of the effective nonlinear refractive index and of the effective nonlinear absorption coefficient. In addition, even the sign of the effective nonlinear absorption coefficient can be changed. The proposed strategy to create novel nonlinear-optical materials using metal-alkanoates based glass-forming ionic liquid crystals and several types of nanoparticles will benefit rapidly growing nanophotonics and plasmonics technologies.

GlassNet: A multitask deep neural network for predicting many glass properties

A multitask deep neural network model was trained on more than 218k different glass compositions. This model, called GlassNet, can predict 85 different properties (such as optical, electrical, dielectric, mechanical, and thermal properties, as well as density, viscosity/relaxation, crystallization, surface tension, and liquidus temperature) of glasses and glass-forming liquids of different chemistries (such as oxides, chalcogenides, halides, and others). The model and the data used to train it are available in the GlassPy Python module as free and open source software for the community to use and build upon. As a proof of concept, GlassNet was used with the MYEGA viscosity equation to predict the temperature dependence of viscosity and outperformed another general purpose viscosity model available in the literature (ViscNet) on unseen data. An explainable AI algorithm (SHAP) was used to extract knowledge correlating the input (physicochemical information) and output (glass properties) of the model, providing valuable insights for glass manufacturing and design. It is hoped that GlassNet, with its free and open source nature, can be used to enable faster and better computer-aided design of new technological glasses.

Engineering the glass structure of a discotic liquid crystal by multiple kinetic arrests.

X-ray scattering has been used to characterize the columnar packing and the π stacking in a glass-forming discotic liquid crystal. In the equilibrium liquid state, the intensities of the scattering peaks for π stacking and columnar packing are proportional to each other, indicating concurrent development of the two orders. Upon cooling into the glassy state, the π-π distance shows a kinetic arrest with a change in the thermal expansion coefficient (TEC) from 321 to 109 ppm/K, while the intercolumnar spacing exhibits a constant TEC of 113 ppm/K. By changing the cooling rate, it is possible to prepare glasses with a wide range of columnar and π stacking orders, including zero order. For each glass, the columnar order and the π stacking order correspond to a much hotter liquid than its enthalpy and π-π distance, with the difference between the two internal (fictive) temperatures exceeding 100K. By comparison with the relaxation map obtained by dielectric spectroscopy, we find that the δ mode (disk tumbling within a column) controls the columnar order and the π stacking order trapped in the glass, while the α mode (disk spinning about its axis) controls the enthalpy and the π-π spacing. Our finding is relevant for controlling the different structural features of a molecular glass to optimize its properties.

Structural decomposition retarded crystal growth in the undercooled liquid of Zr70Al12.5Fe17.5 metallic glass-forming alloy

The recent attention on crystallization of undercooled glass-forming metallic liquids has been directing to the intriguing viscosity behaviors. The present study discloses that structural decomposition of the grown-out crystalline products plays a part in the crystal growth kinetics. In the supercooled liquid of Zr70Al12.5Fe17.5 metallic glass, the polymorphically crystallizing metastable τ2-Zr6Al2Fe phase decomposes into τ2 + τ2′ (space group:Amm2, a = 6.895 Å, b = 3.368 Å, c = 5.119 Å), and retards crystal growth. The structure decomposition also occurs in the solidified bulk alloys.

Effect of Alkyl Chain Length on the Phase Situation of Glass-Forming Liquid Crystals

The phase behaviour of the latest synthesised compound belonging to a family of (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[‘m’-(2,2,3,3,4,4,4-heptafluorobutoxy) ‘m’alkoxy]-benzoates (where ‘m’ means 3, 5 or 7 methylene groups) is described by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and Fourier-transform infrared absorption spectroscopy. It has been shown that as the length of the alkyl chain increases, a given liquid crystal possesses a greater number of mesophases and at a higher temperature it goes into the isotropic liquid phase. All examined compounds form a chiral smectic phase with antiferroelectric properties (SmCA* phase), in which the temperature range of occurrence increases with the length of the molecule. The number of methylene groups also affects the glass transition. The compound with the shortest alkyl chain (‘m’ = 3) is vitrified from the conformationally disordered crystal phase. For the compound with five -CH2- groups (‘m’ = 5), a glass transition from the monotropic high-order hexatic smectic SmXA* phase is observed. In the case of the liquid crystal with the longest carbon chain (‘m’ = 7), the vitrification from the less ordered SmCA* phase is visible. Differences in the crystallization kinetics, e.g., the nucleation-controlled mechanism for the compound with the shortest carbon chain vs. the complex phenomenon for its longer homologs, are discussed.

Shear-accelerated crystallization of glass-forming metallic liquids in high-pressure die casting

• Solidification behavior of Zr 55 Cu 30 Ni 5 Al 10 glass forming liquid under various flow rates was investigated. • Flow-induced crystallization was observed in glass forming liquid as injection rate exceeds 1.1 m/s. • The accelerated crystallization behavior is mainly ascribed to the abrupt perturbation at the end filling stage. • Flow related effect on glass forming ability must be considered in bulk metallic glasses produced by high pressure die casting. As one of the most important forming technologies for industrial bulk metallic glass (BMG) parts with complex shapes, high-pressure die casting (HPDC) can fill a die cavity with a glass-forming metallic liquid in milliseconds. However, to our knowledge, the correlation between flow and crystallization behavior in the HPDC process has never been established. In this study, we report on the solidification behavior of Zr 55 Cu 30 Ni 5 Al 10 glass forming liquid under various flow rates. Surprisingly, the resulting alloys display a decreasing content of amorphous phase with increase of flow rate, i.e. increase of cooling rate, suggesting that crystallization kinetics of glass-forming metallic liquids in the HPDC process is strongly dependent on the flow field. Analysis reveals that the accelerated crystallization behavior is mainly ascribed to the rapid increase in viscosity with a decreasing temperature as well as to the huge shear effect in the glass-forming liquid at the end stage of the filling process when the temperature is close to the glass-transition point; this results in a transition from diffusion- to advection-dominated transport. The current investigation suggests that flow-related crystallization must be considered to assess the intrinsic glass-forming ability of BMGs produced via HPDC. The obtained results will not only improve the understanding of crystallization dynamics but also promote the high-quality production and large-scale application of BMG parts.

Probing Optical Nonlinearities of Unconventional Glass Nanocomposites Made of Ionic Liquid Crystals and Bimetallic Nanoparticles.

In this paper, we report the synthesis and characterization of unconventional nanocomposites made of bimetallic nanoparticles dispersed in a liquid crystal glass. Core-shell bimetallic nanoparticles (Ag/Au or Au/Ag) and Ag-Au bimetallic nanoalloys are synthesized using cadmium alkanoate glass-forming liquid crystals as nanoreactors. Optical spectra of the produced glassy nanocomposites exhibit a distinctive absorption peak due to a surface plasmon resonance. In addition, these unusual materials demonstrate a strong nonlinear–optical response probed by means of the Z-scan technique. The use of near-infrared (1064 nm) and visible (532 nm) nanosecond laser pulses reveal a variety of nonlinear–optical mechanisms that depend on the composition of the studied nanocomposites. Our results indicate that metal alkanoate-based glass-forming ionic liquid crystals with embedded plasmonic nanoparticles are promising, yet they are overlooked photonic nanomaterials suitable for optical and nonlinear-optical applications.

Crystallization and relaxation dynamics of glass-forming liquids at the Kauzmann temperature

If the entropy extrapolation of supercooled liquids (SCL) suggested by Kauzmann was correct, then they would have the same entropy as their stable crystalline phase at a certain low temperature, below the laboratory glass transition (Tg), known as the Kauzmann temperature (TK). Extrapolating even further, the liquid entropy would be null at a temperature above absolute zero, violating the Third Law of Thermodynamics and constituting a paradox. Several possibilities have been proposed over the past 70 years to solve this paradox with different degrees of success. Our objective here is to access liquid dynamics at deep supercoolings to test the so-called crystallization solution to the paradox. By comparing the relaxation and crystallization kinetics determined above Tg and extrapolated down to TK, a possible solution would be that the crystallization time is shorter than the relaxation time, which would mean that a SCL cannot reach the TK. In this case, the liquid stability limit or kinetic spinodal temperature (Tks) should be higher than TK. We tested two fragile glass-forming liquids (diopside and wollastonite) and two strong liquids (silica and germania). For the fragile substances, Tks ≫ TK, hence such a supercooled liquid cannot exist at TK, and the entropy crisis is averted. On the other hand, the results for the strong liquids were inconclusive. We hope the findings of this work encourage researchers to further investigate the liquid dynamics of different strong glass-forming systems at deep supercoolings.

On the relaxation dynamics of a double glass-forming antiferroelectric liquid crystal.

The relaxation dynamics in the thermodynamic states of the glass-forming antiferroelectric liquid crystal (S)-4'-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-benzoate (3F7HPhH7) was studied by broadband dielectric spectroscopy (BDS). Two glass transitions were found at Tg,1 = 259 K and Tg,2 = 239 K, which were associated with the freezing of anti-phase motions and reorientation around the long molecular axis in the antiferroelectric SmCA* phase, respectively. The low temperature α2-relaxation process shows a Vogel-Fulcher-Tammann (VFT)-type temperature dependence of its structural relaxation time τ(T). The two secondary β- and γ-relaxation modes ascribed to the intramolecular motions observed in the glassy state show Arrhenius behaviour of τ(T). Analysing the band shifts and the oscillator strengths of specific IR absorption bands and their temperature dependencies enables comparing them with the dielectrically determined relaxation dynamics. The kinetics of the isothermal cold crystallization in the temperature range between Tg,1 and Tg,2 was studied in detail using the Avrami and Avrami-Avramow models. This process depends primarily on the diffusion rate and the activation energy is equal to 132 kJ mol-1. The obtained values of the Avrami exponent nA suggest that the crystal growth is three-dimensional.

Vapor-Deposited Glass Structure Determined by Deposition Rate-Substrate Temperature Superposition Principle.

We show that deposition rate substantially affects the anisotropic structure of thin glassy films produced by physical vapor deposition. Itraconazole, a glass-forming liquid crystal, was deposited at rates spanning 3 orders of magnitude over a 25 K range of substrate temperatures, and structure was characterized by ellipsometry and X-ray scattering. Both the molecular orientation and the spacing of the smectic layers obey deposition rate-substrate temperature superposition, such that lowering the deposition rate is equivalent to raising the substrate temperature. We identify two different surface relaxations that are responsible for structural order in the vapor-deposited glasses and find that the process controlling molecular orientation is accelerated by more than 3 orders of magnitude at the surface relative to the bulk. The identification of distinct surface processes responsible for anisotropic structural features in vapor-deposited glasses will enable more precise control over the structure of glassy materials used in organic electronics.

Negative pressure effects on molecular dynamics and phase diagram of glass-forming nematic liquid crystal 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB) confined in nanopores

Here, we show the method to determine the pressure conditions for liquid crystal restricted in nanochannels. For this purpose, broadband dielectric spectroscopy studies of 4CFPB glass-forming nematic liquid crystal at various pressure conditions and confined in non-intersecting nanopores of mean diameters 6 nm and 8 nm were combined. The pressure dependence of 4CFPB clearing point occurred to be linear and enabled to find pressure conditions in restricted spatial geometry. The pressure evolution of glass transition temperature was non-linear and was described well using the Andersson-Andersson law. Based on the presented results, the temperature-pressure phase diagram was constructed.

Intensity dependent nonlinear absorption coefficients and nonlinear refractive indices of glass-forming ionic liquid crystals doped with gold and silver nanoparticles

In this paper, we report nonlinear-optical properties of the newly designed glassy nanocomposites made of mesomorphic cadmium octanoate (CdC8) doped with a small fraction of gold (Au) and silver (Ag) nanoparticles (NP). The nanosecond laser scanning measurements (Z-scan) were performed at the wavelength (532nm) within the absorption band of the studied materials. Both types of nanocomposites (CdC8|Au NP and CdC8|Ag NP) exhibit strongly pronounced dependence of the nonlinear absorption coefficient and nonlinear refractive index on the intensity of the laser beam. Physical mechanisms leading to this dependence are discussed.

Crystallization of glass-forming liquids: Thermodynamic driving force

The most general and accurate expressions for the thermodynamic driving force of critical cluster formation are derived employing both the classical and the generalized Gibbs approaches for the description of thermodynamic aspects of crystal nucleation and growth. The thermodynamic driving force of critical cluster formation in the form as applied commonly in classical nucleation theory can be obtained as a limiting case in both approaches introducing certain first-order approximations (truncated at first-order terms Taylor expansion with respect to pressure and temperature, incompressibility of the crystal phase). It is shown that in such approximation the thermodynamic driving force of crystallization has a maximum at the Kauzmann temperature. Improvements of the accuracy of the approximations by extending the Taylor expansion to second-order terms lead to negligible corrections for the thermodynamic driving force provided Gibbs classical treatment is employed. However, significant variations compared to the classical predictions are possible once the generalized Gibbs approach is utilized appropriately accounting for changes of the bulk state parameters of the critical clusters in dependence on the degree of undercooling. Supplemented by a generalization of the Stefan-Skapski-Turnbull relation, sketched briefly here and described in detail in an accompanying paper, the results give a new tool for the description of crystal nucleation and growth processes going considerably beyond the classical theory.

Crystallization of glass-forming liquids: Maxima of nucleation, growth, and overall crystallization rates

A set of equations for determining the temperatures and magnitudes of the maximum nucleation, growth, and overall crystallization rates of glass-forming liquids is derived and analyzed. The analysis is performed based on the classical theories of nucleation and growth, without introducing additional assumptions such as the Stokes–Einstein–Eyring (SEE) equation, models for specific kinetic mechanisms of aggregation, a specification of the type of temperature dependence of the diffusion coefficient, or specific models for the computation of the driving force of crystallization and the work of critical cluster formation. Such approximations are employed only for analytical estimates and to illustrate the general results. In particular, it is shown that the magnitude of the maximum of the steady-state nucleation rate Jmax decreases upon increasing the ratio Tmax(nucl)/Tm (Tmax(nucl): temperature of maximum of the steady-state nucleation rate, Tm: melting or liquidus temperature). Similarly, the maximum growth rate, umax, decreases with increasing values of the ratio Tmax(growth)/Tm (Tmax(growth): temperature of maximum of the growth rate). Several experimental results on the crystallization kinetics of glass-forming liquids are interpreted theoretically for the first time employing the concepts developed here.

Nanoscale size effects in crystallization of metallic glass nanorods.

Atomistic understanding of crystallization in solids is incomplete due to the lack of appropriate materials and direct experimental tools. Metallic glasses possess simple metallic bonds and slow crystallization kinetics, making them suitable to study crystallization. Here, we investigate crystallization of metallic glass-forming liquids by in-situ heating metallic glass nanorods inside a transmission electron microscope. We unveil that the crystallization kinetics is affected by the nanorod diameter. With decreasing diameters, crystallization temperature decreases initially, exhibiting a minimum at a certain diameter, and then rapidly increases below that. This unusual crystallization kinetics is a consequence of multiple competing factors: increase in apparent viscosity, reduced nucleation probability and enhanced heterogeneous nucleation. The first two are verified by slowed grain growth and scatter in crystallization temperature with decreasing diameters. Our findings provide insight into relevant length scales in crystallization of supercooled metallic glasses, thus offering accurate processing conditions for predictable metallic glass nanomolding.

Kinetics of Cold Crystallization of 4-Cyano-3-fluorophenyl 4-Butylbenzoate (4CFPB) Glass Forming Liquid Crystal. I. Nonisothermal Process As Studied by Microscopic, Calorimetric, and Dielectric Methods

For 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB), the process of the crystallization of the CrII phase was studied in microscopic (POM), calorimetric (DSC), and dielectric (BDS) nonisothermal experiments with various (0.5–50 K/min) heating of the metastable nematic phase obtained from its glass. Growth of areas of crystal CrII in the microscopic texture of nematic phase during heating allows estimation of degree of crystallinity D(T) vs temperature curves similar to these obtained basing on DSC heat flow curves and for slow heating with help of dielectric relaxation (BDS) method. Two types of CrII crystallization mechanisms seem to be identified: (1) strong ϕ dependence on temperature of full crystallization Tc(ϕ) and half time of crystallization t1/2(ϕ) on slow heating up to 5 K/min points to diffusion-controlled mechanism with the energy barrier 57 kJ/mol, and (2) small effect of faster heating on Tc(ϕ) and t1/2(ϕ) seems to illustrate thermodynamic mechanism with energy barrier 180 kJ/mol. The scenari...

Kissinger method applied to the crystallization of glass-forming liquids: Regimes revealed by ultra-fast-heating calorimetry

Numerical simulation of DSC traces is used to study the validity and limitations of the Kissinger method for determining the temperature dependence of the crystal-growth rate on continuous heating of glasses from the glass transition to the melting temperature. A particular interest is to use the wide range of heating rates accessible with ultra-fast DSC to study systems such as the chalcogenide Ge2Sb2Te5 for which fast crystallization is of practical interest in phase-change memory. Kissinger plots are found to show three regimes: (i) at low heating rates the plot is straight, (ii) at medium heating rates the plot is curved as expected from the liquid fragility, and (iii) at the highest heating rates the crystallization rate is thermodynamically limited, and the plot has curvature of the opposite sign. The relative importance of these regimes is identified for different glass-forming systems, considered in terms of the liquid fragility and the reduced glass-transition temperature. The extraction of quantitative information on fundamental crystallization kinetics from Kissinger plots is discussed.

Dicholesteryl icosanedioate as a glass-forming cholesteric liquid crystal: properties, additive effects and application in color recording

A cholesteric ester dimer – dicholesteryl icosanedioate – was examined for color imaging. Cholesterol was introduced as one of the dopants which performed an important role reducing phase transition temperatures. The other dopants, 4,4′-dialkoxyazobenzene and 4,4′-dialkoxybiphenyl promoted formation of smectic domains in cholesteric phases which resulted in the expansion of the reflective band shift over the whole visible region. In particular, the photo-responsive dopant 4,4′-dialkoxyazobenzene allowed us to realize reversible RGB color tuning of the cholesteric film by both photon and thermal modes. The trans–cis photoisomerization led to a cholesteric pitch change which resulted in reflective color change, and recorded optical images that have been stable for more than 8 months in a glassy solid film state.

The Glassy Dynamics in Liquid Crystalline 4-n-Pentyl-4′-Cyanobiphenyl as Studied by Thermally Stimulated Currents

The slow molecular mobility in the model liquid crystal 4-n-pentyl-4′-cyanobiphenyl has been investigated by thermally stimulated depolarization currents and differential scanning calorimetry in the amorphous nematic solid state. The phase transition temperatures were determined, as well as the respective enthalpy changes. The glass transition relaxation was analyzed, and the fragility index of this glass-forming liquid crystal was determined.

Disentangling molecular motions involved in the glass transition of a twist-bend nematic liquid crystal through dielectric studies

Broadband dielectric spectroscopy spanning frequencies from 10(-2) to 1.9 × 10(9) Hz has been used to study the molecular orientational dynamics of the glass-forming liquid crystal 1",7"-bis (4-cyanobiphenyl-4'-yl)heptane (CB7CB) over a wide temperature range of the twist-bend nematic phase. In such a mesophase two different relaxation processes have been observed, as expected theoretically, to contribute to the imaginary part of the complex dielectric permittivity. For measurements on aligned samples, the processes contribute to the dielectric response to different extents depending on the orientation of the alignment axis (parallel or perpendicular) with respect to the probing electric field direction. The low-frequency relaxation mode (denoted by μ(1)) is attributed to a flip-flop motion of the dipolar groups parallel to the director. The high-frequency relaxation mode (denoted by μ(2)) is associated with precessional motions of the dipolar groups about the director. The μ(1)-and μ(2)-modes are predominant in the parallel and perpendicular alignments, respectively. Relaxation times for both modes in the different alignments have been obtained over a wide temperature range down to near the glass transition temperature. Different analytic functions used to characterize the temperature dependence of the relaxation times of the two modes are considered. Among them, the critical-like description via the dynamic scaling model seems to give not only quite good numerical fittings, but also provides a consistent physical picture of the orientational dynamics on approaching the glass transition.