Transition Temperature Research Articles

Experimental assessment of the heat shielding performance by the integration of phase change material and liquid cooling plate

This research focuses on innovative stealth equipment by integrating a liquid cooling plate and phase change material (PCM), and the temperature rise on the cold surface and PCM melting rate is reported to illustrate the PCM thermal behavior in lifting the heat shielding performance. Results show that the integrated PCM system is effective in delaying the temperature rise within a certain time. A slower temperature rise on the cold surface is found at the first 1200 s, and a significant acceleration in the temperature rise is given within 1200 s ∼ 2000 s as the heat flood breaks through the PCM layer. The PCM melting rate increases rapidly and then rises slowly, and the complete melting time is shortened as increasing heat power. The heat shielding performance is significantly reduced with increasing the liquid flow rate, and the influence of the coolant temperature on the temperature rise becomes more and more prominent as the completion of the PCM melting process. A slower temperature rise is observed at the lower PCM phase transition temperature, and a further increase in the PCM covering layer has little influence on the temperature rise at the initial stage.

First-principles thermodynamic investigation on the α phases in TiO and TiNb binary system.

O and Nb are two representative alloying elements of Ti to form high-temperature and corrosion resistance α Ti alloys. The investigation on the thermodynamic characteristics of α Ti-O and Ti-Nb has attracted much attention in recent years. However, in this regard, a satisfied experimental technique or modeling scheme is still yet to be developed due to the appearance of a variety of oxides in Ti-O and the mechanical instability present in Ti-Nb. Herein, we combined first-principles calculations with the cluster expansion method to investigate the ground-state characteristics for α Ti-O and α Ti-Nb systems. The atomic bonding interactions in these two systems were first revealed based on the calculated electronic structures. Afterward, the Debye-Grüneisen model and Monte Carlo simulations were employed together to investigate the thermodynamic properties of α phases in these two systems, and the effect of vibrational entropy on the order-disorder transition temperatures of the phases in α Ti-O system was first examined. A good agreement with experimentally reported phase boundaries is obtained in the Ti-Nb system by handling the mechanical instabilities introduced by the highly distorted structures. In addition, the cluster expansion coefficients for the Ti-O and Ti-Nb system offer a good starting point to investigate the phase equilibrium in Ti-Nb-O ternary alloy. We also believe the insights provided here would be helpful for those who would like to seek an efficient scheme they are confident with to investigate the phase thermodynamic properties of other hcp Ti-based alloys.

Photo-Healable Fabrics: Achieving Structural Control via Photochemical Solid-Liquid Transitions of Polystyrene/Azobenzene-Containing Polymer Blends.

While polymer fabrics are integral to a wide range of applications, their vulnerability to mechanical damage limits their sustainability and practicality. Addressing this challenge, our study introduces a versatile strategy to develop photohealable fabrics, utilizing a composite of polystyrene (PS) and an azobenzene-containing polymer (PAzo). This combination leverages the structural stability of PS to compensate for the mechanical weaknesses of PAzo, forming the fiber structures. Key to our approach is the reversible trans-cis photoisomerization of azobenzene groups within the PAzo under UV light exposure, enabling controlled morphological alterations in the PS/PAzo blend fibers. The transition of PAzo sections from a solid to a liquid state at a low glass transition temperature (Tg ∼ 13.7 °C) is followed by solidification under visible light, thus stabilizing the altered fiber structures. In this study, we explore various PS/PAzo blend ratios to optimize surface roughness and mechanical properties. Additionally, we demonstrate the capability of these fibers for photoinduced self-healing. When damaged fabrics are clamped and subjected to UV irradiation for 20 min and pressed for 24 h, the mobility of the cis-form PAzo sections facilitates healing while retaining the overall fabric structure. This innovative approach not only addresses the critical issue of durability in polymer fabrics but also offers a sustainable and practical solution, paving the way for its application in smart clothing and advanced fabric-based materials.

Binary Nematic Liquid Crystals Mixture with Enhanced Electro-Optics Properties for Photonic Applications

Purpose: In this work, we mix two simple nematic liquid crystals (NLCs) and investigated the binaryNLCs mixtures of 7CB/PCH5 of different mixing ratios. Methodology: The pure liquid crystals 7CB and PCH5 and binary mixtures of them of high temperature stability were thermally analyzed by differential scanning calorimetry. The mixture 7CB/PCH5:30/70 wt% has the highest thermal stability with a nematic-isotropic (N-I) transition temperature at 50oC. The electrooptic properties of 7CB, PCH5, and the mixture 7CB/PCH5:30/70 wt% at room temperature were also investigated using an amplitude modulated electric signal (1 kHz - 100 Hz) by increasing diving peak voltage from 0 V to 10 V. The threshold volage is relatively reduced for the binary mixture in comparison to that value for PCH5. In comparison to the pure LCs, the mixture 7CB/PCH5:30/70 wt% has the fastest response times of values 2.36 ms total time response, 0.41 ms rise time, and 1.95 ms fall time. It has also the highest contrast ratio. Moreover, it has a maximum measured transmission that is higher than those for PCH5 and 7CB by about 17 % and 8%, respectively, at a field strength of 2V/mm. Findings: The obtained results indicate that the electrooptic properties of PCH5 was improved when mixed with a proper ratio of 7CB, of lower cost, more stablity , and higher potential for photonic applications. Unique Contriburibution to Theory, Practice and Policy: This expermental study shows that simply by mixing two relatively low cost NLCs materials, one of high thermal stability and low electro-optic properties with other one of low thermal stability and better electro-optic properties; this would improve the stability, response, and transmition of the binary mixture. If the a suitable driving method is applied, without doping with other orgnic or inorganic matrial.

A novel flame retardant epoxy thermoset based on renewable honokiol and furfuryl alcohol

In this study, honokiol epoxy resin was directly functionalized by bio-based material honokiol, which was combined with furfuryl alcohol-derived flame retardant by Diels-Alder reaction to prepare bio-based epoxy cross-linking system. After curing with 4,4′-diaminodiphenyl methane, HE1/FP0.5/DDM (121.37 °C) offered higher glass transition temperature than EP/DDM (91.75 °C). Meanwhile, the HE1/FP0.5/DDM exhibited excellent thermal stability and flame retardancy compared to EP/DDM. The char residue of HE1/FP0.5/DDM was obviously increased from 12.8% (EP/DDM) to 35.6% at 700 °C. Notably, the HE1/FP0.5/DDM achieved a limiting oxygen index value of 37.4% and UL-94 V-0 rating. Cone calorimeter demonstrated that the peak heat release rate, total heat release and fire growth rate of HE1/FP0.5/DDM were 400.29 kW/m2, 33.45 MJ/m2 and 4.40 kW/m2·s, reduced by 57.47%, 63.39% and 41.1% compared with EP/DDM, respectively. The satisfactory flame retardancy and smoke suppression were attributed to the flame-retardant function of FP in the condensed and gas phases. Thus, this work not only provided a simple but also eco-friendly approach to achieve flame retardant thermosets from honokiol to replace bisphenol A epoxy.

Pendant Group Functionalization of Cyclic Olefin for High Temperature and High-Density Energy Storage.

High-temperature flexible polymer dielectrics are critical for high-power-density energy storage and conversion under harsh operating conditions. These types of dielectrics will need to simultaneously possess a high bandgap, dielectric constant and glass transition temperature - a substantial challenge when designing novel dielectric polymers. In this work, by varying halogen substituents of an aromatic pendant hanging off a bicyclic mainchain polymer, a class of high-temperature olefins with adjustable thermal stability are obtained, all with uncompromised large bandgaps. Halogens substitution of the pendant groups at para or ortho position of polyoxanorborneneimides (PONB) imparts it with tunable high glass transition temperature from ∼220 to 245°C, while with also moderate dielectric constant of ∼ 2.8-3.0 and high breakdown strength of ∼625-800MV/m. A high energy density of 7.1 J/cc at 200°C is achieved with p-POClNB, representing the highest reported energy density among all-organic homo-polymer dielectrics. Molecular dynamic simulations and ultrafast infrared spectroscopy were used to probe the free volume element distribution and chain relaxations of the polymers to provide insights to the dielectric thermal properties. An increase in free volume element is observed with the change in the pendant group from fluorine to bromine at the para position; however, a decrease in free volume element is observed as we change the pendant group from fluorine to chlorine at the ortho position because of the steric hindrance. Overall, the dielectric constant and band gap remain stable while the glass transition temperature changes more obviously. Consequently, by proper designing the pendant groups, the thermal stability of PONB can be improved for harsh condition electrification. This article is protected by copyright. All rights reserved.

Thermo‐mechanical properties of epoxy composites filled with inorganic particulate fillers for robust solder mask application

AbstractFiller components play a crucial role as thermo‐mechanical reinforcement to the epoxy‐based solder mask, ensuring effective insulation and passivation. This study investigates the impact of different inorganic filler types (talc, silica (SiO2), boron nitride (BN) and barium sulfate (BaSO4)) and loadings (5 and 15 wt%) on the tensile and thermal properties of the epoxy composites. The composites were blended, cast and thermally cured. Of these composites, only 15 wt% SiO2/epoxy composite exhibits notable increments in both tensile strength and modulus, surpassing unfilled epoxy by 11.65% and 31.9%, respectively. It also displays the highest elongation at break, supported by small particle size, high specific surface area and minimal void volume fraction. For thermal tests, the addition of all fillers increases the storage modulus in both glassy and rubbery regions, especially 15 wt% talc/epoxy composite with 30.09% increment at 25 °C. Additionally, all fillers raise the glass transition temperature (Tg), notably improving it by 9 °C in the 15 wt% BN/epoxy composite. Moreover, their inclusion generally enhances the onset decomposition temperature (Tonset) and reduces weight loss during decomposition. © 2024 Society of Chemical Industry.

Coevolution of superconductivity and Hall coefficient with anisotropic lattice shrinkage in compressed KCa2Fe4As4F2

Abstract It is well known that the stability of superconductivity in superconductors is widely recognized to be determined by various factors, including charge, spin, orbit, lattice, and other related degrees of freedom. Here, we report our findings on the pressure-induced coevolution of superconductivity and Hall coefficient in KCa2Fe4As4F2, an iron-based superconductor possessing a hybrid crystal structure combining KFe2As2 and CaFeAsF. Our investigation, involving high-pressure resistance, Hall effect and X-ray diffraction (XRD) measurements, allows us to observe the connection of the superconductivity and Hall coefficient with the anisotropic lattice shrinkage. We find that its ambient-pressure tetragonal (T) phase presents a collapse staring at around 18 GPa, where the sign of the Hall coefficient (R H ) changes from positive to negative. Upon further compression, both superconducting transition temperature (Tc) and R H exhibit a monotonous decrease. At around 41 GPa, the superconductivity is completely suppressed (T c =0), where the parameter a begins to decline again and the Hall coefficient remain nearly unchanged. Our experiment results clearly demonstrate that the pressure-induced anisotropic lattice collapse plays a curial role in tuning the interplay among multiple degrees of freedom in the superconducting system and, correspondingly, the stability of the superconductivity.

Exploring the thermal shielding performance by coupling phase change material to liquid cooling plate

To enhance the thermal shielding performance of high-temperature heat source target, an evolved cold shield system coupling phase change material (PCM) and liquid cooling plate with serpentine flow channel is developed. The thermal shielding effectiveness of the proposed system is illustrated by comparing the duration maintained at a lower temperature on the cold surface, and the temperature uniformity of the cold surface is analyzed to describe the role of PCM in the thermal shielding performance. The influences of the operating parameters of the liquid cooling plate and PCM physical parameters on the thermal shielding performance of the heat source targets are investigated. The results indicate that coupling PCM to a liquid cooling plate can effectively improve the thermal shielding performance. Increasing the heat power of the heat source target from 190 W to 231 W will lessen the advantage time from 3090 s to 1980 s for the coupled system. The advantage time is shortened by 1500 s as increasing the liquid volume flow rate from 0.26 L/min to 1.30 L/min. Lifting the starting temperature of the coolant from 15 °C to 25 °C prolongs the advantage time from 2130 s to 3090 s. The advantage time is improved by 660 s as increasing the PCM phase transition temperature from 26 °C to 30 °C, and increasing the PCM covering thickness from 8 mm to 10 mm extends the advantage time from 1830 s to 3090 s. Meanwhile, the temperature equalization of the cold surface for the coupled system is better than the single liquid cooling. These results indicate that the proposed coupled system has potential applications in improving thermal shielding performance as well as heat flow control, thermal insulation, and other areas of heat regulation.

Generation and Evolution Mechanism of Cracks in HMX Single Crystals around Phase Transition Temperature

Generation and Evolution Mechanism of Cracks in HMX Single Crystals around Phase Transition Temperature

Curing characteristics and performance of a bio-based polyurethane engineered sealant with high bonding strength: Effect of R value and chain extender content

Polyurethane (PU) engineering sealants can be used to fill construction joints and pavement cracks with excellent practical results. However, traditional engineering sealants have the disadvantages of poor strength and raw materials that are not environmentally friendly. In this paper, a novel composition system of bio-based polyurethane engineering sealant was developed. The effects of the isocyanate/macroglycol molar ratio (nNCO/nOH, R value) for component A and the contents of amine chain extender (AME) on the curing characteristics and properties of the PU sealant were investigated using a series of experiments. Firstly, the curing process of the sealant was studied by Fourier transform infrared spectroscopy (FTIR) test, surface drying time test and gel time experiments. Secondly, the basic physical properties of the sealant were studied by tensile and hardness tests. And the water resistance of the sealant was judged. Then, the characteristics of molecular movement of polyurethane sealant were analyzed by dynamic mechanical analysis (DMA). The tensile cross-sections of the specimens were observed by scanning electron microscopy (SEM). Finally, the thermal stability of sealants was studied by thermogravimetric (TG) analysis. The results indicated that the material developed in this research can be used as a high-performance engineering sealant in actual projects. The increase in the R value and the decrease in AME content resulted in a longer curing process of the sealant. The rise of R value and AME content was able to raise the tensile strength, bonding strength and hardness of the sealant. The water resistance of the sealant was better. Also, the glass transition temperature (Tg) and storage modulus could also be increased and the thermal stability of the previous stage could be reduced by increasing the R value and AME content. Considering both the performance and construction requirements, the optimum R value and AME contents are recommended to be 2.1 and 1.4%, respectively.

Spontaneous time-reversal symmetry breaking by disorder in superconductors

A growing number of superconducting materials display evidence for spontaneous time-reversal symmetry breaking (TRSB) below their critical transition temperatures. Precisely what this implies for the nature of the superconducting ground state of such materials, however, is often not straightforward to infer. We review the experimental status and survey different theoretical mechanisms for the generation of TRSB in superconductors. In cases where a TRSB complex combination of two superconducting order parameter components is realized, defects, dislocations and sample edges may generate superflow patterns that can be picked up by magnetic probes. However, even single-component condensates that do not break time-reversal symmetry in their pure bulk phases can also support signatures of magnetism inside the superconducting state. This includes, for example, the generation of localized orbital current patterns or spin-polarization near atomic-scale impurities, twin boundaries and other defects. Signals of TRSB may also arise from a superconductivity-enhanced Ruderman-Kittel-Kasuya-Yosida exchange coupling between magnetic impurity moments present in the normal state. We discuss the relevance of these different mechanisms for TRSB in light of recent experiments on superconducting materials of current interest.

Impact of Sacrificial Hydrogen Bonds on the Structure and Properties of Rubber Materials: Insights from All-Atom Molecular Dynamics Simulations.

The inclusion of sacrificial hydrogen bonds is crucial for advancing high-performance rubber materials. However, the molecular mechanisms governing the impact of these bonds on material properties remain unclear, hindering progress in advanced rubber material research. This study employed all-atom molecular dynamics simulations to thoroughly investigate how hydrogen bonds affect the structure, dynamics, mechanics, and linear viscoelasticity of rubber materials. As the modified repeating unit ratio (β) increased, both interchain and intrachain hydrogen bond content rose, with interchain bonds playing a predominant role. This physical cross-linking network formed through interchain hydrogen bonds restricts molecular chain movement and relaxation and raises the glass transition temperature of rubber. Within a certain content of hydrogen bonds, the mechanical strength increases with increasing β. However, further increasing β leads to a subsequent decrease in the mechanical performance. Optimal mechanical properties were observed at β = 6%. On the other hand, a higher β value yields an elevated stress relaxation modulus and an extended stress relaxation plateau, signifying a more complex hydrogen-bond cross-linking network. Additionally, higher β increases the storage modulus, loss modulus, and complex viscosity while reducing the loss factor. In summary, this study successfully established the relationship between the structure and properties of natural rubber containing hydrogen bonds, providing a scientific foundation for the design of high-performance rubber materials.

Empirical calibration method for the thermal simulation of Cu47Ti34Zr11Ni8 single tracks in laser powder bed fusion

AbstractBulk metallic glasses (BMGs) are materials that, due to their amorphous microstructure, offer a unique combination of high strength, hardness, and elasticity, making them attractive for various applications. Using laser powder bed fusion (PBF-LB/M) enables overcoming the current limitations of BMGs in size and shape imposed by traditional manufacturing methods such as casting. Despite its potential, challenges such as porosity, (nano-) crystallization, and impurities affect the mechanical performance of additively manufactured BMGs. This study focuses on the Cu–Ti-based alloy Vit101, known for its higher strength and improved cost-effectiveness compared to Zr-based BMGs. In-situ high-speed pyrometry and thermal simulations of single tracks are employed to enhance the understanding of processing and controlling the thermal cycling of Vit101. The proposed experimental calibration is performed through an off-axis integration of the pyrometer, allowing for in-situ temperature measurements. The acquired data show sufficient congruence with the simulated cooling profiles. Minimal cooling rates in the range of 104 K/s were measured and simulated above the glass transition temperature, indicating a large leeway for further development of glass-forming alloys. Scan track widths are evaluated for validation, resulting in minor deviations between 0.47% and 3.17%. However, challenges emerge at high scanning speeds, leading to higher deviations attributed to balling phenomena, which are not considered in the numerical model.

Synthesis of novel stimuli-responsive hydrogels based on polyurethane

This article describes the study of a thermo-responsive hybrid systems based on polyurethane (PU) modified with sensitive acrylamide derivates. The hybrid systems were synthesized using PU and two acrylamide derivatives (N-isopropylacrylamide (NIPA) and N-isopropylmethacrylamide (NIPMA)) in different proportions. The systems were characterized using infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic light scattering (DLS). Water-uptake capacity and sedimentation rate were also determined. The results showed that the hybrids with 30% acrylamide derivate (AD) showed a T50 of approximately 360 °C indicating that thermal degradation decreased with the addition of NIPA and NIPMA. Additionally, the incorporation of AD increases the glass transition temperature from −34 to −22 °C when 30% AD was used. The hybrids with 30% AD showed a variation in the diameters (above 60%) when the temperature was decreased from 50 to 22 °C. These changes were attributed to the hydrophilic → hydrophobic transition that occurs when measuring below and above the low critical solution temperature (LCST) of the polymer. Furthermore, the extra methyl group in the structure of NIPMA makes the collapse less pronounced than in NIPA, decreasing the relative diameter change by 10%. Sedimentation tests showed that the addition of the hybrid hydrogels in the sand increased the time of decantation by 60%. So, the combination of two thermo-responsive polymers to alter the hydrophilic/hydrophobic balance allows these polymers to modify their conformation at a specific temperature and could be potentially useful as self-suspending support agents or drug delivery systems.

Addition of calcium carbonate in the synthesis of flexible polyurethane foams

Within all their variability, foams constitute the largest segment in the polyurethane industry. Polyurethane is composed mainly of isocyanate (-N=C=O) and polyol (-OH) functional groups, presenting different characteristics by adding fillers. This polymer has a wide range of applications such as adhesives, coatings, varnishes, elastomers and foams. Aiming improvements in its thermal properties, calcium carbonate addition was investigated in proportions of 12.5%, 25% and 37.5% (w/w). This filler is widely used to change the properties of polymers, as it is easily found in nature and has a low cost. For the synthesis of polyurethane samples, isocyanate and polyol were reacted using "one shot" process. The obtained results demonstrated the change in the glass transition temperatures (Tg) of the material to higher values, which indicates a decrease in the mobility of the polymer chains. Also, an increase in decomposition temperature was observed via differential scanning calorimetry (DSC). With Fourier transform infrared spectroscopy (FTIR) analysis, it was possible to analyze that the material was successfully synthesized by the appearance of the characteristic bands of PU foams. Furthermore, with the addition of calcium carbonate, it was possible to identify the presence of two phases, which indicate that the material underwent changes in its crystallinity, identified through X-ray diffraction (XRD) analysis. Finally, it was possible to observe via optical microscopy (OM) analysis that the addition of filler significantly changed the morphology of the foams.

Large anomalous Hall effect in spin fluctuating devil’s staircase

Electrons in metals can show a giant anomalous Hall effect (AHE) when interacting with characteristic spin texture. The AHE has been discussed in terms of scalar-spin-chirality (SSC) in long-range-ordered noncollinear spin textures typified by Skyrmion. The SSC becomes effective even in the paramagnetic state with thermal fluctuations, but the resultant AHE has been limited to be very small. Here, we report the observation of large AHE caused by the spin fluctuation near the devil’s staircase transition in a collinear antiferromagnetic metal SrCo6O11. The AHE is prominent near and above the transition temperature at moderate magnetic fields, where the anomalous Hall angle becomes the highest level among known oxide collinear ferromagnets/antiferromagnets (>2%). Furthermore, the anomalous Hall conductivity is quadratically scaled to the conductivity. These results imply that the thermally induced solitonic spin defects inherent to the devil’s staircase transition promote SSC-induced skew scattering.

Finite dimension of the ion pathway networks in conducting glasses

In disordered materials, the ordinary understanding is that charge carriers tend to occupy energetically favorable sites known as ion-conducting channels. Many studies have revealed that the inherent fractal properties of such pathways lead to a sub-diffusive behavior. The linearity or branching of these pathways is crucial for determining how the charge carriers move. It can be thought that as the space dimensionality decreases, the average distance between the highest energy barriers along the conduction paths increases. In this study the finite dimension of those pathways is computed using an extended version of the classical Hausdorff dimension. Also, the Arrhenius behavior of the most mobile lithium ions is proved, confirming that such are responsible for conductivity behavior. The lithium ions mobility behavior in response to temperature changes and the finite dimension allowed to identify the ion diffusion regions fractal features. The reported results demonstrate that as the temperature increases the conducting channels become broadener, facilitating the transfer of electrical charge through the glassy matrix, below the transition temperature. The pathways behavior confirms the increase of the ionic conductivity when the temperature increases as it is experimentally observed.

Photorefractivity and photocurrent dynamics of triphenylamine-based polymer composites

The photorefractive properties of triphenylamine polymer-based composites with various composition ratios were investigated via optical diffraction, response time, asymmetric energy transfer, and transient photocurrent. The composite consisted of a photoconductive polymer of poly((4-diphenylamino)benzyl acrylate), a photoconductive plasticizer of (4-diphenylamino)phenyl)methanol, a sensitizer of [6,6]-phenyl-C61-butyric acid methyl ester, and a nonlinear optical dye of (4-(azepan-1-yl)-benzylidene)malononitrile. The photorefractive properties and related quantities were dependent on the composition, which was related to the glass transition temperature of the photorefractive polymers. The quantum efficiency (QE) of photocarrier generation was evaluated from the initial slope of the transient photocurrent. Transient photocurrents were measured and showed two unique peaks: one in the range of 10−4 to 10−3 s and the other in the range of 10−1 to 1 s. The transient photocurrents was well simulated (or reproduced) by the expanded two-trapping site model with two kinds of photocarrier generation and recombination processes and two different trapping sites. The obtained photorefractive quantity of trap density was significantly related to the photoconductive parameters of QE.

Gravitational Waves of Holographic QCD Phase Transition with Hyperscaling Violation

In this paper, we study the gravitational waves of holographic QCD phase transition with hyperscaling violation. We consider an Einstein–Maxwell Dilaton background and discuss the confinement–deconfinement phase transition between thermally charged AdS and AdS black holes. We find that hyperscaling violation reduces the phase transition temperature. In a further study, we discuss the effect of hyperscaling violation on the GW spectrum. We found that the hyperscaling violation exponent suppresses the peak frequency of the total GW spectrum. Moreover, the results of the GW spectrum may be detected by IPTA, SKA, BBO, and NANOGrav. We also find that the hyperscaling violation exponent suppresses the peak frequency of the bubble-collision spectrum h2Ωenv. Hyperscaling violation enhances the energy densities of the sound wave spectrum h2Ωsw and the MHD turbulence spectrum h2Ωturb. The total GW spectrum is dominated by the contribution of the bubble collision in runaway bubbles case.