Temperature-induced Transformation Research Articles

Insights into the thermal responsiveness of W/O emulsions with nanoparticles and oleogels as building blocks

Understanding the thermal behavior of emulsions with complex structures was crucial for wider applications. However, the correlations between the structural units and emulsion properties at different temperatures are poorly understood. Herein, W/O emulsions stabilized by oleogels and nanoparticles were studied during thermal process, and dynamic interactions among the structural units occurred at micro-, meso-, and macroscales. Results revealed that the thermal behavior of W/O emulsions shifted from stable (25–50 °C) to sub-stable (50–60 °C), and unstable states (60–80 °C). Microstructural analysis confirmed that the particles desorbed from the interface and dissolved in the oil phase after heating, causing water droplet coalescence in the emulsions. This was attributed to the enhanced hydrophobic interactions between the particle dispersions and the oleogel, as the contact angle and interfacial tension were decreased during heating. At the mesoscale, the T2 values of oleogels and nanoparticle dispersion were increased more than three times from 25 °C to 80 °C, leading to the higher mobility of protons in emulsions. At the macroscale, the viscoelastic modulus of emulsions at higher temperature were higher, together with less energy dissipated by friction and increased nonlinear response. Furthermore, the temperature-induced transformation of emulsion stability can be reversibly controlled at least three times. Insights into the complex behavior of emulsions across different temperature stages can inspire the design of emulsions for specific applications.

Fabrication of Ln2Zr2O7 Fluorite and LnAlO3 Perovskite (Ln = La, Nd, Sm) Compounds to Catalyze the OCM Reaction: On the Temperature-Induced Phase Transformation and Oxygen Vacancy.

Through synthesizing Ln2Zr2O7 and LnAlO3 (Ln = La, Nd, Sm) catalysts, the origin of active sites for oxidative coupling of methane (OCM) on A2B2O7 fluorite and ABO3 perovskite compounds has been compared and elucidated. Ln2Zr2O7 catalysts show much better reaction performance than the respective LnAlO3 catalysts at low temperatures (500-600 °C), but the difference will be mitigated significantly above 600 °C. The reaction performance ranks in the order of La2Zr2O7 > Nd2Zr2O7 > Sm2Zr2O7 > LaAlO3 > NdAlO3 > SmAlO3. It is revealed that the unit cell free volume (Vf) plays an important role in affecting the catalytic activity, and the Ln2Zr2O7 catalysts with a disordered defect fluorite phase have inherent oxygen vacancies, which can directly activate gas-phase O2 molecules to generate OCM reactive O2- anions. However, the oxygen vacancies of LnAlO3 with a perovskite structure can only be generated by lattice distortion/transformation above 600 °C. Moreover, Ln2Zr2O7 fluorites have weaker B-O bonds than LnAlO3 perovskites, thus making it easier to generate surface vacancies as well as active O2- sites. The surface alkalinity is intimately relevant to the active oxygen species, which act together to decide the OCM performance on both types of catalysts. Indeed, this explains that LnAlO3 catalysts show much worse performance than Ln2Zr2O7 catalysts below 600 °C, which will be evidently improved at elevated temperatures due to phase transformation.

Development of a machine-learning interatomic potential for uranium under the moment tensor potential framework

Uranium is attracting growing interest across fundamental science and nuclear research, where atomistic simulations remain challenging. In this study, we developed a novel uranium interatomic potential based on a machine-learning approach embedded in the moment tensor potential package. An active learning framework was utilized in the potential training, using density-functional theory plus Hubbard U (DFT+U) modified data. The potential accurately reproduced the lattice parameters, cohesive energy, elastic, vibrational, and thermodynamic properties of α-U compared to DFT+U calculations and experimental results. The basic properties of other phases, including β, γ, HCP, and FCC phases, were also validated. In addition, molecular dynamics simulations were used to reproduce the temperature-induced allotropic transformation from α-U to γ-U, which agrees with experimental observations. Our potential provides a computationally efficient means to study the physical behavior of uranium with nearly DFT accuracy.

A Thermal, Mechanical, and Materials Framework for a Shape Memory Alloy Heat Engine for Thermal Management.

Shape memory alloy (SMA) heat engines possess an inherent property of sensing a change in temperature, performing work, and rejecting heat through the shape memory effect resulting from a temperature-induced phase transformation. This work presents a framework for the design and implementation of an SMA-based Stirling heat engine for maximum torque or speed incorporating and combining mechanical, thermal, and material aspects. There is a growing need for such engines for reliable thermal management and energy recovery in both ground and space applications. Mechanical aspects were addressed from force balances in the SMA element and focused on the resulting stress distribution. Thermal aspects considered heat transfer between the SMA element and both the heat source and the heat sink. Materials aspects considered the chemical, elastic, and frictional contributions to the enthalpy of the transformation. The roles of nano- and microstructure through composition, precipitates, variant interfaces, training, cycling, texture, defects, nucleation sites (bulk vs. surface), and multi-step transformations (e.g., a trigonal R-phase transformation) in NiTi based-alloys are also emphasized. The aforementioned aspects were combined to present a figure of merit to aid in the design and implementation of a Nitinol Stirling heat engine operating to maximize torque or maximize speed.

An interatomic potential for ternary NiTiHf shape memory alloys based on modified embedded atom method

NiTi-based high temperature shape memory alloys (HTSMAs) can operate in high temperature applications, where binary NiTi cannot function. NiTiHf is among the most popular HTSMAs due to its lower preparation cost and good thermal stability. Various aspects of material properties of NiTiHf and the effect of different factors, such as composition, heat treatment and precipitates, on its behavior is yet to be understood. Molecular dynamics (MD) simulations can provide insights to acquire such an understanding. However, MD simulations of NiTiHf have not been possible due to the absence of a reliable interatomic potential, which is a necessary tool for such simulations. In this study, density functional theory (DFT) simulations were employed and a ternary Second Nearest Neighbor Modified Embedded Atom Method (2NN MEAM) potential was calibrated for NiTiHf. To perform this study, the related unary and binary potentials were calibrated by fitting the values of their reproduced physical properties to the DFT results. The final ternary MEAM potential was checked for reliability and transferability by performing MD simulations. The results showed that the developed potential can appropriately capture the temperature-induced and stress-induced martensitic phase transformations in NiTiHf.

Manganese, Fluorine, and Nitrogen Co-Doped Bronze Titanium Dioxide Nanotubes with Improved Lithium-Ion Storage Properties

Because of the unique crystal framework, bronze TiO2 (or TiO2(B)) is considered the prospective choice for high-performance lithium-ion battery anodes. Nevertheless, TiO2(B) requires efficient modification, e.g., suitable doping with other elements, to improve the electronic properties and enhance the stability upon insertion/extraction of guest ions. However, due to the metastability of TiO2(B), doping is challenging. Herein, for the first time, TiO2(B) co-doped with Mn, F, and N were synthesized through a successive method based on a hydrothermal technique. The prepared doped TiO2(B) consists of ultrathin nanotubes (outer diameter of 10 nm, wall thickness of 2–3 nm) and exhibits a highly porous structure (pore volume of up to 1 cm3 g−1) with a large specific surface area near 200 m2 g−1. The incorporation of Mn, F, and N into TiO2(B) expands its crystal lattice and modifies its electronic structure. The band gap of TiO2(B) narrows from 3.14 to 2.18 eV upon Mn- and N-doping and electronic conductivity improves more than 40 times. Doping with fluorine improves the thermal stability of TiO2(B) and prevents its temperature-induced transformation into anatase. It was found that the diffusivity of Li is about two times faster in doped TiO2(B). These properties make Mn, F, and N co-doped TiO2(B) nanotubes promising for application as high-performance anodes in advanced lithium-ion batteries. In particular, it possesses a good reversible capacity (231.5 mAh g−1 after 100 cycles at 70 mA g−1) and prominent rate capability (134 mAh g−1 at 1500 mA g−1) in the half-cell configuration. The (Mn, F, N)-doped TiO2(B) possesses a remarkable low-temperature Li storage performance, keeping 70% of capacity at −20 °C and demonstrating potentialities to be employed in full-cell configuration with LiMn2O4 cathode delivering a reversible capacity of 123 and 79 mAh g−1 at 35 and 1500 mA g−1, respectively, at a voltage of ~2.5 V. This research underlies that regulation of electronic and crystal structure is desired to uncover capabilities of nanoparticulate TiO2(B) for electrochemical energy storage and conversion.

Effects of thermomechanical treatments on Cu–Al–Mn Shape Memory Alloys

In this paper, the authors, building on the established state-of-the-art knowledge, explore the effects of thermomechanical treatments on the Shape Memory Effect (SME) and damping performance of the Cu–Al–Mn Shape Memory Alloys (SMA). Specifically, to explore the potential of application of this SMA in the form of sheets, for the purpose of designing lightweight structures with high damping performance. The casting and the thermomechanical treatment process were fully laid out in the paper. The characterization of the material in terms of chemical composition and microstructure was performed through optical microscopy and SEM coupled with EDX analysis. The temperature-induced phase transformation behavior of the alloy was explored using DSC. The damping performance was quantified through the loss factor which was measured at different levels of static tensile preload using a DMA machine. The alloy has shown significant workability which permits the fabrication of sheets. However, the material displays high sensitivity to oxidation due to a combined effect of impurities and the hot-rolling process necessary for obtaining thin SMA sheets. Nonetheless, the alloy shows competitive damping performance with respect to commercially available NiTi SMA.

Crossover from mechanical stabilization of martensite to spanning avalanche-type reverse transformation of deformed Cu-Al-Ni shape memory crystals

A new interpretation of factors controlling the kinetics of reverse transformation of martensites stabilized by prestrain is suggested. We investigate the effect of amount of prestrain in the β1´martensite and in the β-phase on temperature and kinetics of the reverse martensitic transformation in Cu-Al-Ni shape memory alloy single crystals. Calorimetry and video recording of the jumping samples, provoked by the burst strain recovery during temperature-induced reverse transformation, are used in experiments. For crystals deformed both in the β´1 martensite and in the β-phase, we report the existence of a sharp transition (crossover) from the mechanical stabilization of martensite to the burst reverse martensitic transformation which proceeds as a single avalanche. The crossover occurs for prestrain values close to the maximum transformation strain. The specific energy of the jump during burst strain recovery depends on the maximum stress applied during prestraining the sample. The crossover from a broad transformation range to an infinite or spanning avalanche is predicted by 3D random field Ising model for disordered systems undergoing the first order phase transition. Based on this model, we explain the crossover in deformed Cu-Al-Ni crystals by a rapid decrease of structural disorder towards the critical one at the final stage of the formation of detwinned γ´1martensite. A simple solution is obtained for the critical transformation rate which produces jumping of the sample during burst reverse transformation after prestrain.

Temperature-induced transformation between layered herringbone polymorphs in molecular bilayer organic semiconductors

Herein we investigated the temperature-induced transformation between distinct layered herringbone (LHB) polymorphs in model organic semiconductors (OSCs) of a series of $2\text{\ensuremath{-}}\mathrm{mono}\text{\ensuremath{-}}\mathrm{alkylated}\text{\ensuremath{-}}\mathrm{benzothieno}[3,2\text{\ensuremath{-}}b][1]\mathrm{benzothiophenes} (mono\text{\ensuremath{-}}{\mathrm{C}}_{n}\text{\ensuremath{-}}\mathrm{BTBTs})$. The component molecules are composed of a one-sided linkage between the BTBT core and alkyl chains of various lengths. We propose that the polymorphism originates from a unique feature of LHB structures in which two types of $T$-shaped contacts are possible for BTBT cores that exhibit relatively low symmetry. Based on polarized UV-visible absorption spectra and powder x-ray diffraction analyses, we show that four long-alkylated $mono\text{\ensuremath{-}}{\mathrm{C}}_{n}\text{\ensuremath{-}}\mathrm{BTBTs}$ ($n=8$, 9, 10, and 11) undergo mostly irreversible structural phase transformations into short-chain-type polymorphs at elevated temperatures of approximately $85\ensuremath{-}100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. These transformations allowed the investigation of the stability of each polymorph in terms of thermal analysis. Based on the chain-length dependent transition entropies, we demonstrate that the polymorphic transformations are triggered by the partial entropy gain of alkyl chains, followed by the total entropy gain at solid--liquid transitions observed at higher temperatures of approximately $110{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. We also analyzed the structure and intermolecular interactions of the four compounds. The results confirm that each molecular packing consists of a balance between the core--core and chain--chain interactions and also that the unique odd--even parity alkyl-chain-length effect of the transitions emerges as a result of the variation in interlayer stacking arrangements. We highlight the critical role of alkyl chains in terms of their large structural degrees of freedom in solution-processable and highly layered crystalline OSCs. These findings lead to a better understanding of molecular packing toward a more rational design of OSCs for efficient carrier transport.

Temperature- and pH-Responsive Schizophrenic Copolymer Brush Coatings with Enhanced Temperature Response in Pure Water

Novel brush coatings were fabricated with glass surface-grafted chains copolymerized using surface-initiated atom transfer radical polymerization (SI-ATRP) from 2-(2-methoxyethoxy)ethyl methacrylate (OEGMA188) and acrylamide (AAm), taken in different proportions. P(OEGMA188-co-AAm) brushes with AAm mole fraction >44% (determined with XPS and TOF-SIMS spectroscopy) and nearly constant with the depth copolymer composition (TOF-SIMS profiling) exhibit unusual temperature-induced transformations: The contact angle of water droplets on P(OEGMA188-co-AAm) coatings increases by ∼45° with temperature, compared to 17-18° for POEGMA188 and PAAm. The thickness of coatings immersed in water and the morphology of coatings imaged in air show a temperature response for POEGMA188 (using reflectance spectroscopy and AFM, respectively), but this response is weak for P(OEGMA188-co-AAm) and absent for PAAm. This suggests mechanisms more complex than a simple transition between hydrated loose coils and hydrophobic collapsed chains. For POEGMA188, the hydrogen bonds between the ether oxygens of poly(ethylene glycol) and water hydrogens are formed below the transition temperature Tc and disrupted above Tc when polymer-polymer interactions are favored. Different hydrogen bond structures of PAAm include free amide groups, cis-trans-multimers, and trans-multimers of amide groups. Here, hydrogen bonds between free amide groups and water dominate at T < Tc but structures favored at T > Tc, such as cis-trans-multimers and trans-multimers of amide groups, can still be hydrated. The enhanced temperature-dependent response of wettability for P(OEGMA188-co-AAm) with a high mole fraction of AAm suggests the formation at Tc of more hydrophobic structures, realized by hydrogen bonding between the ether oxygens of OEGMA188 and the amide fragments of AAm, where water molecules are caged. Furthermore, P(OEGMA188-co-AAm) coatings immersed in pH buffer solutions exhibit a 'schizophrenic' behavior in wettability, with transitions that mimic LCST and UCST for pH = 3, LCST for pH = 5 and 7, and any transition blocked for pH = 9.

Competition of interactions and a new high-temperature phase of selenourea.

The aggregation of molecules is usually associated with a specific type of interaction, which can be altered by thermodynamic conditions. Under normal conditions, the crystal structure of selenourea, SeC(NH2)2, phase α is trigonal, space group P31, Z = 27. Its large number of independent molecules (Zα' = 9) can be associated with the formation of an NH...N hydrogen bond substituting one of 36 independent NH...Se hydrogen bonds, which prevail among intermolecular interactions. Phase α approximates the trigonal structure with a threefold smaller unit cell (Z = 9), which in turn approximates another still threefold smaller unit cell (Z = 3). The temperature-induced transformations of selenourea have been characterized by calorimetry and by performing 21 single-crystal X-ray diffraction structural determinations as a function of temperature. At 381.0 K, phase α undergoes a first-order displacive transition to phase γ, with space group P3121 and Z reduced to 9, when the NH...N bond is broken and an NH...Se bond is formed in its place. Previously, an analogous competition was observed between NH...N and NH...O hydrogen bonds in high-pressure phase III of urea. The lattice vectors along the (001) plane in low- and high-temperature phases of selenourea are related by a similarity rule, while the lattice dimensions along direction c are not affected. This similarity rule also applies to the structures of phase γ and hypothetical phase δ (Z = 3). The thermally controlled transition between enantiomorphic phases of selenourea contrasts with its high-pressure transition at 0.21 GPa to a centrosymmetric phase β, where both the NH...Se and NH...N bonds are present. The compression and heating reduce the number of independent molecules from Z'= 9 in phase α, to Z' = 2 in phase β and to Z' = 1.5 in phase γ.

A multiphase phase-field study of three-dimensional martensitic twinned microstructures at large strains

The nanoscale multiphase phase-field model for stress and temperature-induced multivariant martensitic transformation under large strains developed by the authors in Basak and Levitas (J Mech Phys Solids 113:162–196, 2018) is revisited, the issues related to the gradient energy and coupled kinetic equations for the order parameters are resolved, and a thermodynamically consistent non-contradictory model for the same purpose is developed in this paper. The model considers \(N+1\) order parameters to describe austenite and N martensitic variants. One of the order parameters describes austenite\(\leftrightarrow \)martensite transformations, and the remaining N order parameters, whose summation is constrained to the unity, describe the transformations between the variants. A non-contradictory gradient energy is used within the free energy of the system to account for the energies of the interfaces. In addition, a kinetic relationship for the rate of the order parameters versus thermodynamic driving forces is suggested, which leads to a system of consistent coupled Ginzburg–Landau equations for the order parameters. An approximate general crystallographic solution for twins within twins is presented, and the explicit solution for the cubic to tetragonal transformations is derived. A large strain-based finite element method is developed for solving the coupled Ginzburg–Landau and elasticity equations, and it is used to simulate a 3D complex twins within twins microstructure. A comparative study between the crystallographic solution and the simulation results is presented.

Irradiation response of innovatively engineered metastable TRIP high entropy alloy

Properties and radiation responses of a metastable high entropy alloy (HEA) exhibiting the transformation induced plasticity (TRIP) effect were studied. Innovative engineering used to manufacture this HEA has shown superior mechanical and corrosion properties in 3.5% NaCl than most advanced stainless steels. The microstructural evolution and corresponding mechanical response after irradiation have been evaluated using detailed transmission electron microscopy, and nanoindentation. The study shows a change in metastability of the alloy with irradiation via a recovery mechanism, where the irradiation-induced transformation is reversed by the temperature-induced transformation, thereby introducing the concept of self-healing, made possible due to the TRIP behavior of HEA.

Nuclear Melt Glass from Experimental Field, Semipalatinsk Test Site

Investigation of shocked materials provides unique information about behavior of substances in extreme thermodynamic conditions. Near surface nuclear tests have induced multiple transformations of affected soils. Examination of nuclear glasses and relics of entrapped minerals provides a unique database on their behavior under an intense temperature flash. In this work, several types of nuclear fallout particles from historic tests at the Semipalatinsk test site are investigated using complementary analytical methods. Distribution of radionuclides in all types of samples is highly heterogeneous; domains with high content of radionuclides are often intermixed with non-active materials. There is no general correlation between chemical composition of the glassy matrix and content of radionuclides. In aerodynamic fallout, the main fraction of radionuclides is trapped in the outer glassy shell. Relics of quartz grains are always devoid of radionuclides, while glass regions of high activity have different composition. In contrast to underground tests, iron-rich minerals are not necessarily radioactive. In most cases, the glassy matrix in anhydrous and is strongly polymerized, and the Q3 silicate groups dominate. Temperature-induced transformations of entrapped minerals are discussed. Investigation of zircon grains shows absence of a direct correlation between degree of decomposition into constituting oxides, morphology of resulting baddeleyite, and maximum experienced temperature. For the first time, temperature history of a nuclear ground glass is estimated from Zr diffusion profiles from decomposing zircon grain.

A large compressive recoverable strain induced by heterogeneous microstructure in a Ni50.6Ti49.4 shape memory alloy via laser powder bed fusion and subsequent aging treatment

Laser induced highly metastable microstructure provides the possibility for the tunable precipitation behavior and mechanical/functional properties via direct aging treatment. In this paper, Ni-rich NiTi-based samples were fabricated by laser powder bed fusion (LPBF) and then were directly aged for different aging times that spanning three time-scales (10 min-100 h). The results showed that through the aging treatment, a heterogeneous microstructure consisting of Ti 4 Ni 2 O x nanoparticles, nanoscale Ni-rich precipitates, dislocation structures, and martensitic twin variants formed in the matrix. Due to the change in the elastic strain field, the short time aging treatment tended to induce (001) compound twins, while the long time aging assisted in the formation of< 011 > type II twins. The temperature-induced and deformation-induced phase transformation behavior were further studied. With the progress of precipitates, it was found that a rapid evolution from a single-stage phase transformation to a multi-stage one occurred. During the cyclic compression, the sample aged for 1 h showed the most excellent superelasticity with initial recoverable strain of 0.089 and steady recoverable strain of 0.087, as well as good cyclic stability. • Microstructure evolution of LPBF-ed Ni-rich NiTi SMA with the aging time was disclosed. • Effects of aged microstructure on transformation behavior and pseudoelasticity were elaborated. • A large compressive recoverable strain and its good cyclic stability were achieved via the proper aging treatment.

Peculiarities of phase diagram of Fe-Ni system at Ni concentrations 0 – 20 at.%.

The phase boundaries of martensitic transformation in Fe-Ni alloys with the composition in the range of 0 - 20 Ni at.% have been determined by X-ray diffraction, differential scanning calorimetry, measurements of magnetic susceptibility and magnetization. It is shown that the alloys quenched from 1100 °C at room temperature are in a single-phase state of α2 martensite, except for Fe80Ni20, which additionally contains a small amount of γ phase. It is found that in the Fe97Ni3 alloy, the temperature-induced magnetic and structural transformations occur independently of each other. At a nickel concentration from 5 to 20%, the structural α → γ transformation upon heating is accompanied by the magnetic transition from the ferromagnetic to the paramagnetic phase, which is a first-order phase transition. When cooling the alloys with x ≤ 10, the paramagnet - ferromagnet transition starts several degrees ahead of the structural γ → α transformation. The lattice volume changes and thermal effects upon the martensitic transformation are determined. It is shown that the phase diagram including metastable martensitic transformation can be used together with the equilibrium phase diagram to determine the structural states of the Fe-Ni system.

High accuracy neural network interatomic potential for NiTi shape memory alloy

Nickel-titanium (NiTi) shape memory alloys (SMA) are widely used, however simulating the martensitic transformation of NiTi from first principles remains challenging. In this work, we developed a neural network interatomic potential (NNIP) for near-equiatomic Ni-Ti system through active-learning based acquisitions of density functional theory (DFT) training data, which achieves state-of-the-art accuracy. Phonon dispersion and potential-of-mean-force calculations of the temperature-dependent free energy have been carried out. This NNIP predicts temperature-induced, stress-induced, and defect-induced martensitic transformations from atomic simulations, in significant agreement with experiments. The NNIP can directly simulate the superelasticity of NiTi nanowires, providing a tool to guide their design.

Atomistic simulations of AuTi high-temperature shape memory alloys

Herein, the atomic-scale phase transformation and deformation processes of AuTi shape memory alloys for high-temperature applications were investigated using molecular dynamics simulations based on a newly developed interatomic potential for the Au-Ti binary system. The developed second nearest-neighbor modified embedded-atom method potential exhibited good accuracy and transferability to reproduce various physical properties of the target alloy system, particularly in properties related to the reversible phase transformation of AuTi high-temperature shape memory alloys. As a conceivable application of the interatomic potential, the temperature-induced phase transformation of AuTi single-crystal shape memory alloys and the effect of off-stoichiometry on the phase transformation behavior were examined. Molecular dynamics simulations revealed that the transformation temperature and thermal hysteresis decreased with increasing Ti content, consistent with the reported experimental trend, suggesting that the reduced volume difference between the austenite and martensite phases was closely related to the reduced thermal hysteresis. We further examined the mechanical response and resultant superelasticity of nanocrystalline AuTi alloys to demonstrate that the cyclic deformation of nanocrystalline alloys exhibited a transformation ratcheting behavior caused by the plastic deformation of amorphous-like grain boundary region.

A flexible and smart shape memory alloy composite sheet based on efficient and bidirectional thermal management

ABSTRACT Leveraging advances in shape memory alloys (SMAs) and flexible thermoelectric devices (f-TEDs), this paper presents a structural and functional integrity composite sheet to address the inefficient and bulky activation of 2-D SMAs. A series of experimental tests were performed to reveal the generation, change, and transfer mechanisms of different types of electrically-induced heat in the f-TED, as well as the temperature-induced SMA phase transformation behaviors. The results show that the composite sheet exhibited good bidirectional thermal management capacity. The austenite deformation can be completed within 10s at an operating current of 2 A. The cooling recovery, in particular, performs much better than in conventional modes (the temperature declines exponentially with time). Finally, through two functional prototypes, a light switch and a flexible gripper, the application potential of the proposed composite was further experimentally demonstrated.

Terahertz spectroscopy of temperature-induced transformation between glutamic acid, pyroglutamic acid and racemic pyroglutamic acid

Under heating conditions, L-Glutamic acid (L-Glu) can be dehydrated to form L-pyroglutamic acid (L-PGA), and L-PGA can racemize to form DL-PGA. Here, we characterized this transformation at different temperatures and times by terahertz time domain spectroscopy (THz-TDS). By Powder X-ray diffraction (PXRD), the validity of THz spectroscopy is verified. The results prove that the reaction rate of dehydration and racemization is significantly affected by temperature. The THz spectra divided the reactions into three stages. At 150–155 °C, the reaction changes drastically. Furthermore, we found that the absorption intensity at 0.97 and 1.55 THz has a good dependence on the reaction temperature and time, showing a non-linear relationship (R2 > 0.98). Our findings suggest that the chemical transformation and reaction rate can be sensitively probed by terahertz spectroscopy, which provides a potential method for the quantitative analysis of reaction products.