Temperature-dependent Phase Change Research Articles

Temperature dependent effect of nano-scale phase change materials on fresh and hardened cement based mortar

One of the smart and innovative materials used to produce environmentally friendly energy-saving concrete is temperature dependent phase change (PCM) materials. In this study, the fresh and hardened concrete properties of the mortars including PCM in different ratios (0%, 2.5%, 5%) were investigated experimentally. Rheological properties such as slump-flow, specific viscosity, and yield stress tests were determined at different temperatures (from 20 to 50 °C). Time-dependent physical, mechanical, and thermal properties such as ultrasound pulse velocity (UPV), electrical resistivity, dynamic modulus of elasticity, compressive and flexural strength, thermogravimetric (TGA), and thermal conductivity tests were carried out. According to the experimental findings, the workability and hardened properties of mortars were decreased by using PCM. However, the rheological properties of fresh mortar change depending on temperature due to PCM absorbing the heat. Therefore, the fresh properties of mortar with PCM can be controlled by temperature. It was also found that the phase change of mortars with PCM can be obtained by measuring the electrical or UPV tests at different temperatures.

A Meshless Method of Radial Basis Function-Finite Difference Approach to 3-Dimensional Numerical Simulation on Selective Laser Melting Process

Selective laser melting (SLM) is a rapidly evolving technology that requires extensive knowledge and management for broader industrial adoption due to the complexity of phenomena involved. The selection of parameters and numerical analysis for the SLM process are both costly and time-consuming. In this paper, a three-dimensional radial basis function-finite difference (RBF-FD) meshless model is introduced to accurately and efficiently simulate the molten pool size and temperature distribution during the SLM process for austenitic stainless steel (AISI 316L). Two different volumetric moving heat source models were presented, namely the ray-tracing method heat source model and the double-ellipsoidal shape heat source model. The temperature-dependent material properties and phase change process were also considered, based on experiments and effective models. Results of the model for the molten pool size were validated with those of the literature. The proposed approach can be used to predict the effect of different laser power and scan speed on the molten pool size and temperature gradient along the depth direction. The result reveals that the depth of the molten pool is more sensitive to laser power than scan speed. Under the same scan speed, a 22% change in laser power (45 ± 10 W) affects the maximum temperature proportionally by about 9%. The developed algorithm is computationally efficient and suitable for industrial applications.

Spatial inhomogeneity and molecular aggregation behavior in aqueous binary liquid mixtures

The component molecules in aqueous mixtures tend to aggregate with increasing concentration, and it has been conclusively reported that their spatial distribution is no longer uniform. Here, we attempt to measure the spatial inhomogeneity of constituent molecules in the cosolvent-water mixture by calculating the h value wherein the time-varying molecular configurations are obtained from molecular dynamics (MD) simulations conducted at various concentrations and temperatures. The results reveal that the miscible methanol solutions have a small h value, indicating the homogeneous distribution of components, while the immiscible dichloromethane-water mixtures have a relatively large h value, representing the localization of components due to the formation of two separated liquid phases. In the butanol-water mixture, the h value decreases significantly with increasing temperature, which reflects a temperature-dependent phase change from two liquid phases to a single phase, resulting in uniform distribution of molecular components at high temperatures. Meanwhile, in the graph theoretical analysis of cosolvent aggregates and water H-bond network, two prominent types of aggregation behavior are shown in the investigated binary liquid system, i.e., self-associated or spatially extended aggregates. The growth of self-associated cosolvent aggregates facilitates the liquid-liquid phase separation, resulting in a heterogeneous distribution of components with high h values. The development of a spatially extended network enables fully mixing with water H-bond network, bringing about the homogeneous distribution of constituent molecules and thus resulting in low h values. The concept of spatial inhomogeneity and bifurcating hypothesis on aggregation behavior is anticipated to contribute significantly to understanding the underlying issues, such as miscibility and liquid-liquid phase separation in aqueous mixtures.

A Tuneable Pressure-Based Energy Harvester for Powering the Environmental Internet of Things.

As the internet of things expands to more remote locations, solutions are required for long-term remote powering of environmental sensing devices. In this publication, a device is presented which utilises the slow-moving diurnal temperature change present in many natural environments to produce electrical energy. This device utilises a novel actuator which harnesses temperature-dependent phase change to provide a variable force output, and this is combined with energy storage and release apparatus to convert the output force into electrical energy. Appropriate modelling is utilised to identify parameters for system tuning, and a final proof-of-concept solution is constructed and demonstrated to generate up to 10 mJ per 24 h period.

Tunable directional filter for mid-infrared optical transmission switching.

Controlling the spectral and angular response of infrared (IR) radiation is a challenging task of paramount importance to various emerging photonic applications. Here, we overcome these problems by proposing and analyzing a new design of a tunable narrowband directional optical transmission filter. The presented thermally controlled multilayer filter leverages the temperature dependent phase change properties of vanadium dioxide (VO2) to enable efficient and reversible fast optical switching by using a pump-probe laser excitation setup. More specifically, transmission is blocked for high intensity probe lasers due to the VO2 metallic properties induced at elevated temperatures while at low probe laser intensities high transmission through the filter occurs only for a narrowband IR range confined to near normal incident angles. The proposed multilayer composite dielectric filter is expected to have applications in optical communications, where it can act as dual functional infrared filter and optical switch.

Raman Spectromicroscopy: A Tool to “See” Subtle Aspects in Science, Technology, and Engineering

It will not be an exaggeration to say that there is a component of Raman spectroscopy in every scientist’s life, of course with different involvement levels. The “Raman-effect” based techniques have evolved over a period of time to cater to the needs of all researchers who work in a domain involving materials. Raman microscopy, one such technique, quantitatively displays dynamic variation in materials’ properties by amalgamating spectroscopic information collected by means of spatial, temporal and thermal imaging method. Spatial Raman imaging is one of the most widely used Raman microscopic tools, which enables one to image the Raman mode distribution over the sample and is of immense use, especially in biology and engineering. On the other hand, a Raman image evolution with time can be mapped to know how a Raman mode, and reasons therein, behaves with time. Similarly, a thermal Raman map gives information about the effect of temperature on the material to understand temperature dependent phase changes in a material. Here Raman spectromicroscopic techniques and their application in different aspects related to material science have been discussed to highlight how Raman microscopy can be used in areas where hesitancy is observed due to not much awareness about the potential of Raman-related spectromicroscopic techniques.

Thermo-hydraulic and entropy generation investigation of nano-encapsulated phase change material (NEPCM) slurry in hybrid wavy microchannel

PurposeThis study aims to capture the heat transfer and entropy generation characteristics of temperature-dependent nano-encapsulated phase change material (NEPCM) slurry in a hybrid wavy microchannel. In addition, the effect of substrate material combined with NEPCM slurry on conjugate heat transfer condition is captured for different microchannel heat sinks.Design/methodology/approachA novel “hybrid wavy microchannel” is proposed to enhance the overall heat transfer and reduce the pressure drop by combining wavy and raccoon geometry. NEPCM–water slurry is implied in the hybrid wavy, conventional wavy and raccoon microchannel. A user-defined function (UDF) is used to observe the effect of phase-change of paraffin material in thermophysical properties of NEPCM–water nanofluid. All three (hybrid, wavy, raccoon) microchannels are engraved on a rectangular substrate of 1.8 mm width (ωs) and 30 mm length (L), respectively. For hybrid, wavy and raccoon microchannel, waviness (γ) of 0.067 is selected for the investigation.FindingsThe result shows that NEPCM particle presence reduces the fluid domain temperature. The thermal performance of proposed Heat sink 2 is found better than the Heat sink 1. The effect of the geometrical modification, wall thermal conductivity, different volumetric concentrations of nanoparticles (ϕ ∼ 1 – 5%) and Reynolds number (Re ∼ 100 – 500) on thermodynamic irreversibility is also observed. Additionally, the effect of thermal and frictional entropy generation is reduced with a combination of NEPCM slurry and higher conductive material for all heat sinks.Practical implicationsA combination of NEPCM slurry with laminar flow microchannel cooling system emerged as a better alternative over other cooling techniques for higher power density devices such as microprocessors, electronic radar systems, aerospace applications, semiconductors, power electronics in modern electronic vehicles, high power lasers, etc.Originality/valueThe phase-change process of the NEPCM slurry is tracked under conjugate heat transfer in a hybrid wavy microchannel. Furthermore, the phase-change process of NEPCM slurry is captured with different heat sink materials (SS316, silicon and copper) under conjugate heat transfer situation for different heat sinks and concentrations (ϕ ∼ 1–5) of NEPCM.

Study of Phase Changes in Lithium-Ion Battery Electrolytes via Spectroscopic Neutron Imaging

Fast charging is a key requirement for lithium-ion battery (LIBs) technology in a wide range of applications from portable devices to electric vehicles. However, fast charging impose high C-rates and temperature gradients to the system, which cause electrolyte degradation and polymerization, resulting in reduced performance, cycle life, and capacity [1,2]. Therefore, for a safe and efficient implementation of fast charging, it is critical to understand its effect on LIBs components, particularly in the electrolyte.There is a lack of non-invasive methods to elucidate changes in the electrolyte during LIBs operation, and it is commonly studied via post-mortem analysis or ex-situ degradation [3–5]. Neutron imaging (NI) is suitable for studying electrolyte distribution in LIBs, since hydrogen provides high contrast when interacting with the neutron beam, while casing materials like stainless steel or aluminum provide low contrast [6]. Furthermore, the neutron attenuation spectrum of organic molecules depend on the motion of an atom due to molecular vibrations or diffusion, making neutron spectroscopy a suitable tool to identify the chemical composition and aggregation state in batteries. Here, we introduce spectroscopic neutron imaging (SNI) as the new method to study these phenomena in a spatially resolved way.Imaging of electrolyte and solvent samples, performed at the V20 beamline of HZB in Berlin and the IMAT beamline of ISIS in UK (Figure 1-a), show that a liquid binary mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) exhibit similar attenuation spectra – though the different chemical composition and diffusivities result in small variations. On the other hand, solidified species (red region) present a noticeable contrast change due to the reduced molecular diffusion. At 17°C, the organic binary mixture, EC:DEC (1:1 volume ratio), exhibits liquid (EC and DEC) and solid (EC) phases. A similar behavior for solids is observed in the short wavelength (λ<3Ȧ) region of the normalized 1H cross-section spectra, while the curves in the diffusion region (λ>3Ȧ) are bounded to the mobility properties of each molecule (figure 1-b).Additionally, we will present measurements of different electrolytes and organic binary mixtures exposed to temperature-dependent phase changes, obtained via SNI at the ICON beam line of PSI. Setting a lower wavelength resolution requirement allows faster measurements, in order to capture the moment when the sample experiences a phase change. This novel method paves the way for in situ electrolyte behavior analysis, as it allows the detection of fine variations in the electrolyte linked to charge/discharge schemes that negatively affect LIBs performance.Understanding the electrolyte behavior will contribute to the improvement of battery materials to avoid issues in fast charging mechanisms.[1] Y. Liu, Y. Zhu, and Y. Cui, Nature Energy 4, (2019).[2] A. Tomaszewska et al., eTransportation 1, 100011 (2019).[3] G. Gachot et al., Journal of Power Sources 178, 409 (2008).[4] S. Nowak and M. Winter, Journal of The Electrochemical Society 162, 2500 (2015).[5] A. Friesen et al., Journal of Power Sources 334, 1 (2016).[6] K. V. Tran et al., Materials Today Advances 9, 100132 (2021).[7] M. S. Ding et al., Journal of The Electrochemical Society 148, A299 (2001). Figure 1

Enhanced heat transfer in a PCM shell-and-tube thermal energy storage system

The dominant technology among latent heat thermal energy storage methods relies on solid–liquid phase change. Since the primary disadvantage of phase change materials is low thermal conductivity, heat transfer enhancement techniques are required for these types of systems. In this paper, we propose a new double-tube latent heat thermal energy storage units. The melting time and exergy efficiency are compared for eight different design solutions. 3D simulations of the phase change material melting process were performed using the enthalpy-porosity model, Boussinesq approximation, and select temperature-dependent phase change material properties. This study shows that the proposed latent heat thermal energy storage unit (M06) significantly reduces PCM melting time compared with vertical (76%), horizontal (66%), and helical-coiled (53%) systems. The helical-coiled unit with spiral fins (M05) has the highest exergy efficiency (0.77) at the end of melting time. The M05, M06 and M08 units have the highest exergy efficiency at times t = 1200 s and t = 3307 s, which is essential for climates with time constraints on latent heat thermal energy storage. • Proposition of a new helical-coiled tube enhanced with spiral fins and conical shell. • 60% PCM melting time reduction, compared with horizontal LHTES unit. • The helical-coiled unit with spiral fins has the highest exergy efficiency (0.77). • Analysis of the effect of fins and conical shell design on melting process.

Fire performance of masonry under various testing methods

Masonry, as a construction material, is known to perform well under elevated temperatures, which makes it an attractive choice for structural applications. This superior performance is a reflection of its inert thermal characteristics, good stability, and slow degradation of mechanical properties. Still, and similar to other construction materials, masonry undergoes a series of temperature-dependent physio-chemical and phase changes once exposed to high temperatures. Such changes are determined through temperature-dependent material models often obtained by means of physical tests on representative masonry specimens. A deep dive into the open literature shows that not only we lack standardized procedures for testing masonry under fire conditions, but existing researcher-derived methods vary significantly. As a result, available temperature-dependent material models also vary given their sensitivity to testing parameters (i.e., set-ups, heating history etc.). It is primarily due to the aforenoted observations that we continue to lack a holistic understanding of the fire behavior of masonry which also extends to limiting advancements in performance-based design of masonry structures. In order to bridge this knowledge gap, this paper reviews commonly adopted fire testing methods on masonry and the wide scatter of corresponding temperature-dependent material models to provide researchers and practitioners with much-needed knowledge that is currently missing in this domain. Findings from this review can then be used to develop modern and up-to-date temperature-dependent material models to facilitate the design of new masonry constructions or analysis of existing ones (including historical buildings).

Hydroxyethyl Cellulose As a Rheological Additive for Tuning the Extrusion Printability and Scaffold Properties.

Bioink, a key element of three-dimensional (3D) bioprinting, is frequently engineered to achieve improved printing performance. Viscoelasticity related to rheological properties is correlative of the printability of bioink for extrusion bioprinting, which affects the complexity of printing 3D structures. This article shows the use of hydroxyethyl cellulose (HEC) as a rheological additive for engineering bioink to improve the printability without reducing the biocompatibility. Different concentrations of HEC were added to four types of bioink, namely, reagent-crosslinked, temperature-dependent phase change, ultraviolet-polymerized, and composite hydrogel bioinks, to investigate the effect on the viscoelasticity properties, print fidelity, and other printed scaffold properties. The results indicate that HEC is able to increase the rheological properties by 100 times to stabilize complex structures and improve the printing fidelity to narrow the gap between the design value and theoretical value, even converting nonviscous ink into directly printable ink, as well as tune the swelling ratio for better molecular permeability. The degradation of bioink can also be tuned by the addition of HEC. Moreover, this bioink is biocompatible for cell lines and primary cells. HEC is expected to be widely used in 3D extrusion-based bioprinting.

Ab initio atomistic description of temperature-induced phase changes: The cases of zirconia and Ti-Y-co-doped zirconia

Zirconium dioxide, or zirconia, is a common and useful ceramic with a wide range of applications, from fuel cells to odontology. Its phase diagram is simple and well understood, having a structure which is monoclinic at temperatures up to 1500 K, tetragonal up to 2700 K and cubic up to 3000 K. Zirconia is rarely used in its pure form, being typically doped with ${\text{Y}}_{2}{\text{O}}_{3}$, MgO or ${\text{TiO}}_{2}$, and in this regime its phase diagram becomes much more complex. In this context, ab initio molecular dynamics (AIMD) can provide a detailed atomistic description of the phase diagram of this system, accurately describing its stable phases and transition regions. In this work, 3 mol-% ${\text{Y}}_{2}{\text{O}}_{3}$ (3YSZ) crystals doped with different Ti contents were studied at the density-functional level. For Ti contents varying from 0 to 30 at%, a global search algorithm was first used to explore the 0 K potential-energy surface and determine the most stable sites for the added Ti atoms. It was found that, at low Ti compositions ${X}_{\text{Ti}}$, small ${\text{TiO}}_{2}$ clusters form, followed by ${\text{TiO}}_{2}$ channels and infinite ${\text{TiO}}_{2}$ planes at larger ${X}_{\text{Ti}}$ values, and that the highest stability is achieved at 9% Ti. AIMD simulations within the isothermal-isobaric NPT ensemble were then performed to characterize the temperature-dependent phase changes as a function of the Ti content, where it was found that the Ti-doped structures presented considerably smaller volume changes near the phase-change critical temperatures. These findings suggest that YSZ materials doped with a small amount of Ti are both energetically and kinetically more stable than the undoped counterparts, in the ideal proportion of 3% ${\text{TiO}}_{2}$ for every 1% ${\text{Y}}_{2}{\text{O}}_{3}$ doping.

Controls on the present-day dynamic topography predicted from mantle flow models since 410 Ma

SUMMARYMantle convection induces dynamic topography, the lithosphere's surface deflections driven by the vertical stresses from sublithospheric mantle convection. Dynamic topography has important influences on a range of geophysical and geological observations. Here, we studied controls on the Earth's dynamic topography through 3-D spherical models of mantle convection, which use reconstructed past 410 Myr global plate motion history as time-dependent surface mechanical boundary condition. The numerical model assumes the extended-Boussinesq approximation and includes strongly depth- and temperature-dependent viscosity and phase changes in the mantle. Our results show that removing the chemical layer above the core–mantle boundary (CMB) and including depth-dependent thermal expansivity have both a limited influence on the predicted present-day dynamic topography. Considering phase transitions in our models increases the predicted amplitude of dynamic topography, which is mainly influenced by the 410 km exothermic phase transition. The predicted dynamic topography is very sensitive to shallow temperature-induced lateral viscosity variations (LVVs) and Rayleigh number. The preservation of LVVs significantly increases the negative dynamic topography at subduction zones. A decrease (or increase) of Rayleigh number increases (or decreases) the predicted present-day dynamic topography considerably. The dynamic topography predicted from the model considering LVVs and with a Rayleigh number of 6 × 108 is most compatible with residual topography models. This Rayleigh number is consistent with the convective vigor of the Earth as supported by generating more realistic lower mantle structure, slab sinking rate and surface and CMB heat fluxes. The evolution of the surface heat flux pattern is similar to the long-term eustatic sea level change. Before the formation of Pangea, large negative dynamic topography formed between the plate convergence region of Gondwana and Laurussia. The predicted dynamic topography similar to that of present-day has already emerged by about 262 Ma. Powers for degrees 1–3 dynamic topography at 337 and 104 Ma which correspond to times of higher plate velocities and higher surface heat fluxes are larger.

Temperature-dependent fluorescence emission of 4-cyano-4′-pentylbiphenyl and 4-cyano-4′-hexylbiphenyl liquid crystals and their bulk phase transitions

This study examined the fluorescence emission properties of bulk 4-Cyano-4′-pentylbiphenyl (5CB) and 4-Cyano-4′-hexylbiphenyl (6CB) and their variation with temperature. The temperature-dependent fluorescence emission spectra of these two thermotropic liquid crystals (LCs) were measured between 4 and 80 °C and the changes in fluorescence intensities and wavelengths were monitored. The fluorescence spectra of both liquid crystals are characterized by emissions of two structurally different excimers centred around 380–386 nm and 412 nm. The results show that the specific changes in intensity and wavelength differ between the two LCs and suggest presence of two different molecular conformers as the temperature changes. During nematic-isotropic phase transition, the fluorescence spectra show significant intensity increase, which can be exploited for sensitive analytical applications. The crystalline-nematic phase transition of 5CB was found to be 22 °C and the nematic-isotropic liquid transition occurred at 35.5 °C. On the other hand, the crystalline-nematic phase transition of 6CB occurred at 16 °C, while its nematic-isotropic phase transition was observed at 29 °C. The dramatic temperature-dependent phase changes observed in the bulk LCs were completely absent in a solution of 6CB. The observed temperature-dependent spectral evolutions are discussed in the context of possible excimer conformations that exist in different mesophases of the two liquid crystals as well as change in the angle between the two benzene rings, which can contribute to excimer behaviour during phase transitions.

Numerical Simulation and Experimental Study on Residual Stress in the Curved Surface Forming of 12CrNi2 Alloy Steel by Laser Melting Deposition.

The performance and service life of the nuclear emergency diesel engine shaft made of 12CrNi2 alloy steel is very important for the safety of nuclear power. Laser melting deposition (LMD) is a challenging camshaft-forming technology due to its high precision, rapid prototyping, and excellent parts performance. However, LMD is an unsteady process under the local action of laser, especially for curved surface forming, which is more likely to generate large residual stress on components, resulting in cracks and other defects. At present, the stress research on LMD curved surface forming is relatively insufficient. In the present paper, material parameter testing, high-temperature mechanical properties analysis, single-track sample preparation, and heat source checks are conducted. At the same time, the ABAQUS software and the DFLUX heat source subroutine are used to compile the curved double-ellipsoidal moving heat source, and the effects of the temperature-dependent thermophysical parameters and phase change latent heat on the temperature field are considered. A three-dimensional finite element model is established to analyze the thermal stress evolution and residual stress distribution of multi-track multi-layer on a curved surface by LMD, and the effect of the scanning method and interlayer cooling time on the residual stress of the formed components is studied. The results show that with the increase in temperature, the strength of the material reduces, and the fracture morphology of the material gradually transitions from ductile fracture to creep fracture. The material parameters provide a guarantee for the simulation, and the errors of the width and depth of the melt pool are 4% and 9.6%, respectively. The simulation and experiment fit well. After cooling, the maximum equivalent stress is 686 MPa, which appears at the junction of the substrate and the deposited layer. The larger residual stress is mainly concentrated in the lower part of the deposited layer, where the maximum circumferential stress and axial stress are the tensile stress. Compared with the axial parallel lap scanning method, the arc copying lap scanning method has a relatively smaller maximum thermal stress and residual stress after cooling. The residual stress in the deposited layer is increased to some extent with the increase in the interlayer cooling time.

Dynamic phase-change metafilm absorber for strong designer modulation of visible light

AbstractEffective dynamic modulation of visible light properties has been significantly desired for advanced imaging and sensing technologies. In particular, phase-change materials have attracted much attention as active material platforms owing to their broadband tunability of optical dielectric functions induced by the temperature-dependent phase-changes. However, their uses for visible light modulators are still limited to meet multi-objective high performance owing to the low material quality factor and active tunability in the visible regime. Here, a design strategy of phase-change metafilm absorber is demonstrated by making the use of the material drawbacks and extending design degree of freedom. By engineering tunability of effective anisotropic permittivity tensor of VO2-Ag metafilm around near-unity absorption conditions, strong dynamic modulation of reflection wave is achieved with near-unity modulation depth at desired wavelength regions without sacrificing bandwidth and efficiency. By leveraging effective medium theory of metamaterial and coupled mode theory, the intuitive design rules and theoretical backgrounds are suggested. It is also noteworthy that the dynamic optical applications of intensity modulation, coloring, and polarization rotation are enabled in a single device. By virtue of ultrathin flat configuration of a metafilm absorber, design extensibility of reflection spectrum is also verified. It is envisioned that our simple and powerful strategy would play a robust role in development of miniaturized light modulating pixels and a variety of photonic and optoelectronic applications.

Characterization of high-temperature PCMs for enhancing passive safety and heat removal capabilities in nuclear reactor systems

Suitable high temperature PCMs for thermal energy storage are characterized and proposed as a promising candidate for nuclear reactor safety applications in the containment structure in the event of LOCA accidents. PCM with phase transition temperature in the range 70 °C to 90 °C and latent heat of around 200 kJ/kg or higher can be utilized to reduce temperature and pressure in the reactor containment during accidents. Reviewing the available data for high-temperature PCMs indicated significant deficiencies in published data and further investigations revealed that a material misrepresentation has occurred in some cases. Octadecanoic acid was identified as the most promising PCM for this application and the temperature-dependent thermo-physical properties and phase change behavior of octadecanoic acid were studied in detail. The results closed the gaps in the existing experimental database, studied the feasibility of utilizing several PCM candidates, measured the currently unknown temperature-dependent properties for optimum PCM, measured the specific heat capacity in the overlap and phase transition regions, and provided a valuable database for future design concepts, experiments and predictive simulation and modelling models.

Development of Ionic Liquid-Water-Based Thermomorphic Solvent (TMS)-Systems for Biocatalytic Reactions.

The applicability of ionic liquid-water-based thermomorphic solvent (TMS)-systems with an upper critical solution temperature for homogeneous biocatalysis is investigated. Cholinium- and imidazolium-based ionic liquids are used to facilitate a temperature-dependent phase change, which can be easily fine-tuned by adding salts or polar organic solvents. Within the TMS-system, a high enzymatic activity and subsequent full conversion is achieved in the intermittent monophasic reaction system of the TMS-system. Therefore, the biocatalyst can be easily recycled after separating the phases at lower temperatures.

True Polymorphic Phase Transition or Dynamic Crystal Disorder? An Investigation into the Unusual Phase Behavior of Barbituric Acid Dihydrate

Crystalline barbituric acid dihydrate (BTADH) was previously thought to undergo a subtle temperature-dependent phase transition from a high-temperature orthorhombic Pnma space group to a nonmerohedrally twinned monoclinic P21/n phase below a transition temperature of ∼217 K. Questions remain on the true nature of the unusual transformation and the structure of the high-temperature crystal phase due to difficulties in resolving the Pnma structure from X-ray diffraction data and because of the subtlety of the structural transition. In this study, terahertz (THz) spectroscopy and solid-state density functional theory (DFT) were utilized to explore this suspected phase transition and uncover the principle physical mechanisms contributing to this anomalous transition. Our findings suggest that at temperatures above the previously reported phase transition temperature of 217 K, the crystal does not exist in the Pnma space group configuration. However, two equivalent favorable energetic states with P21/n symmetry related by the twinning plane lie on either side of the proposed Pnma structure. It is these same local potential energy minima that guide the crystal system to a nonmerohedrally twinned configuration identified in the low-temperature crystal structures. At temperatures above 217 K, the system possesses the thermal energy necessary to readily transition between these degenerate states separated by the low-lying Pnma structural barrier. The evidence acquired by THz spectroscopy and DFT indicate the absence of a true temperature-dependent phase change, and rather a system that exists in a tenuous thermally disordered state above the previously alleged transition temperature.

Exchange bias and the effect of phase competition in FePt3 single layer and bilayer films

FePt3(disordered)/FePt3(ordered) bilayers are ideal for studying exchange bias in FM/AFM heterostructures. The effects of phase competition between the two different antiferromagnetic phases in the bilayer system needs clarification to understand exchange bias in these bilayers. This is accomplished by preparing films with different crystallographic orientations. It is found that exchange bias fields in single layer films are dependent on their order parameters, whilst the exchange bias fields of bilayer films are determined by their film orientation. Bilayer films with a (001) orientation exhibit the largest exchange bias field at 20 K, whose value diminishes as the order parameter of the FePt3 layer decreases. This phenomenon can be explained by the formation of the Q2 phase in the ordered FePt3 layer at low temperatures. First principles calculations suggest that disorder defects in the interface and top FM layer play an important role in inducing a Q2 phase in the AFM layer. Therefore, bilayer films with a (001) orientation can undergo temperature dependent phase changes, showing that they are a good model system for studying exchange bias in FM/AFM heterostructures.