Transition Temperature Research Articles

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.

Effect of Tacticity on London Dispersive Surface Energy, Polar Free Energy and Lewis Acid-Base Surface Energies of Poly Methyl Methacrylate by Inverse Gas Chromatography

This research was devoted to study the effect of the tacticity on the surface physicochemical properties of PMMA. (1) Background: The determination of the surface free energy of polymers is generally carried out by inverse gas chromatography (IGC) at infinite dilution. The dispersive, polar and surface acid-base properties of PMMA at different tacticities were obtained via IGC technique with the help of the net retention time and volume of adsorbed. (2) Methods: The London dispersion equation was used to quantify the polar free energy of adsorption, while the London dispersive surface energy γsd(T) of PMMAs was determined using the thermal model. (3) Results: The results showed non-linear variations of γsd(T) of atactic, isotactic, and syndiotactic PMMAs with three maxima characterizing the three transition temperatures of PMMAs. The obtained values of the enthalpic and entropic Lewis’s acid-base parameters showed that the basicity of the atactic PMMA was about four times larger than its acidity. (4) Conclusions: A large difference in the behavior of the various PMMAs was proven in the different values of the polar acid and base surface energies of the three PMMAs with an important effect of the tacticity of PMMA on its acid-base surface energies.

Preparation and Performance of a Novel ZnO/TM/PET Composite Negative Ion Functional Fiber.

Using zinc oxide (ZnO), tourmaline (TM), and polyethylene terephthalate (PET) as main raw materials, a novel ZnO/TM/PET negative ion functional fiber was created. The rheological properties of a ZnO/TM/PET masterbatch were investigated; the morphology, XRD, and FT-IR of the fibers were observed; and the mechanical properties, thermal properties, and negative ion release properties of the new fiber were tested. The results showed that the average particle size of the ZnO/TM composite is nearly 365 nm, with an increase in negative ion emission efficiency by nearly 50% compared to the original TM. The apparent viscosity of fiber masterbatch decreases with the increase in the addition of the ZnO/TM composite, and the rheological properties of the PET fiber masterbatch are not significantly effected, still showing shear thinning characteristics when the amount of addition reaches 10%. The ZnO/TM composite disperses well in the interior and surface of the ZnO/TM/PET fiber matrix. The prepared ZnO/TM/PET fiber has excellent properties, such as fineness of 1.54 dtex, glass transition temperature of 122.4 °C, fracture strength of 3.31 cN/dtex, and negative ion release of 1640/cm3, which shows great industrialization potential.

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.

Colyophilized Sugar-Polymer Dispersions for Enhanced Processing and Storage Stability.

Sucrose and trehalose pharmaceutical excipients are employed to stabilize protein therapeutics in a dried state. The mechanism of therapeutic protein stabilization is dependent on the sugars being present in an amorphous solid-state. Colyophilization of sugars with high glass transition polymers, polyvinylpyrrolidone (PVP), and poly(vinylpyrrolidone vinyl acetate) (PVPVA), enhances amorphous sugar stability. This study investigates the stability of colyophilized sugar-polymer systems in the frozen solution state, dried state postlyophilization, and upon exposure to elevated humidity. Binary systems of sucrose or trehalose with PVP or PVPVA were lyophilized with sugar/polymer ratios ranging from 2:8 to 8:2. Frozen sugar-PVPVA solutions exhibited a higher glass transition temperature of the maximally freeze-concentrated amorphous phase (Tg') compared to sugar-PVP solutions, despite the glass transition temperature (Tg) of PVPVA being lower than PVP. Tg values of all colyophilized systems were in a similar temperature range irrespective of polymer type. Greater hydrogen bonding between sugars and PVP and the lower hygroscopicity of PVPVA influenced polymer antiplasticization effects and the plasticization effects of residual water. Plasticization due to water sorption was investigated in a dynamic vapor sorption humidity ramping experiment. Lyophilized sucrose systems exhibited increased amorphous stability compared to trehalose upon exposure to the humidity. Recrystallization of trehalose was observed and stabilized by polymer addition. Lower concentrations of PVP inhibited trehalose recrystallization compared to PVPVA. These stabilizing effects were attributed to the increased hydrogen bonding between trehalose and PVP compared to trehalose and PVPVA. Overall, the study demonstrated how differences in polymer hygroscopicity and hydrogen bonding with sugars influence the stability of colyophilized amorphous dispersions. These insights into excipient solid-state stability are relevant to the development of stabilized biopharmaceutical solid-state formulations.

Influence of Nickel on Microstructure and Mechanical Properties in Medium-Carbon Spring Steel.

The effects of adding nickel on the phase transition temperature, microstructure, and mechanical properties of medium-carbon spring steel have been investigated. The results show that adding nickel reduces the martensite start (Ms) temperature, improves hardenability, and refines the sub-microstructure of the martensite, thereby improving yield stress. The yield strength of martensitic steel increases by approximately 100 MPa due to a synergistic combination of grain refinement strengthening and dislocation strengthening, with an increase in the nickel content from 0 wt.% to 1 wt.%. The cryogenic impact toughness of martensitic steel also improved with a higher nickel content due to packet and block refinement and an increase in the proportion of high-angle grain boundaries (HAGBs).

Synthesis and Characterization of Graft Copolymers with Poly(ε-caprolactone) Side Chain Using Hydroxylated Poly(β-myrcene-co-α-methyl styrene).

Graft copolymers have unique application scenarios in the field of high-performance thermoplastic elastomers, resins and rubbers. β-myrcene (My) is a biomass monomer derived from renewable plant resources, and its homopolymer has a low glass transition temperature and high elasticity. In this work, a series of tapered copolymers P(My-co-AMS)k (k = 1, 2, 3) were first synthesized in cyclohexane by one-pot anionic polymerization of My and α-methyl styrene (AMS) using sec-BuLi as the initiator. PAMS chain would fracture when heated at high temperature and could endow the copolymer with thermal degradation property. The effect of the incorporation of AMS unit on the thermal stability and glass transition temperature of polymyrcene main chain was studied. Subsequently, the double bonds in the linear copolymers were partially epoxidized and hydroxylated into hydroxyl groups to obtain hydroxylated copolymer, which was finally used to initiate the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) to synthesize the graft copolymer with PCL as the side chain. All these copolymers before and after modifications were characterized by proton nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC), thermogravimetry analysis (TGA), and differential scanning calorimeter (DSC).

Exploring the nematic to twist-bend nematic phase transition: calorimetric and optical measurements

ABSTRACT We investigate the nematic (N) to twist-bend nematic (NTB) phase transition in binary mixtures of a symmetric liquid crystal dimer (CB7CB) and a calamitic bi-phenyl mesogen (7OCB). As the molar concentration of 7OCB increases, both the Iso–N and N–NTB transition temperatures decrease gradually, consequently, the width of the nematic phase above the NTB phase can suitably be varied from ~12 K to 46 K. Notably, as the nematic range expands, there is a remarkable reduction in transitional entropy, eventually approaching negligible or zero values. Additionally, the heat capacity peak and the phase shift disappear when transitional entropy reaches zero. This behaviour diverges from typical N–SmA and N–SmC phase transitions, where transitional entropy decreases near the tricritical point but the heat capacity peak persists. Our findings indicate a finite positive δ(∆n) across all concentrations at the N–NTB phase transition, suggesting a gradually weakening first-order nature. This contrasts with theoretical predictions of a second-order N–NTB phase transition, as our experimental data consistently supports a first-order nature. The self-consistent mean-field Landau Model successfully validates our experimental data, establishing the universal first-order nature of the N–NTB phase transition, with experimental entropy values in good agreement with theoretical prediction.

Free energy of a two-liquid system of charge carriers in strongly coupled electron and phonon fields and common nature of three phases in hole-doped cuprates

Hole-doped cuprates exhibit partially coexisting pseudogap (PG), charge ordering (CO) and superconductivity; we show that there exists a class of systems in which they have a single nature as it has recently been supposed. Since the charge-ordered phase exhibits large frozen deformation of the lattice, we develop a method for calculating the phase diagram of a system with strong long-range (Fröhlich) electron–phonon interaction. Using a variational approach, we calculate the free energy of a two-liquid system of carriers with cuprate-like dispersion comprising a liquid of autolocalized carriers (large polarons and bipolarons) and Fermi liquid of delocalized carriers. Comparing it with the free energy of pure Fermi liquid and calculating (with standard methods of Bose liquid theory) a temperature of the superfluid transition in the large-bipolaron liquid we identify regions in the phase diagram with the presence of PG (caused by the impact of the (bi)polarons potential on delocalized quasiparticles), CO and superconductivity. They are located in the same places in the diagram as in hole-doped cuprates, and, as in the latter, the shape of the calculated phase diagram is resistant to wide-range changes in the characteristics of the system. As in cuprates, the calculated temperature of the superconducting transition increases with the number of conducting planes in the unit cell, the superfluid density decreases with doping at overdoping, the bipolaron density (and bipolaronic plasmon energy) saturates at optimal doping. Thus, the similarity of the considered system with hole-doped cuprates is not limited to the phase diagram. The results obtained allow us to discuss ways of increasing the temperature of the superfluid transition in the large-bipolaron liquid and open up the possibility of studying the current-carrying state and properties of the bipolaron condensate.

The Influence of PVP Polymer Topology on the Liquid Crystalline Order of Itraconazole in Binary Systems.

This study presents a novel approach by utilizing poly(vinylpyrrolidone)s (PVPs) with various topologies as potential matrices for the liquid crystalline (LC) active pharmaceutical ingredient itraconazole (ITZ). We examined amorphous solid dispersions (ASDs) composed of ITZ and (i) self-synthesized linear PVP, (ii) self-synthesized star-shaped PVP, and (iii) commercial linear PVP K30. Differential scanning calorimetry, X-ray diffraction, and broad-band dielectric spectroscopy were employed to get a comprehensive insight into the thermal and structural properties, as well as global and local molecular dynamics of ITZ-PVP systems. The primary objective was to assess the influence of PVPs' topology and the composition of ASD on the LC ordering, changes in the temperature of transitions between mesophases, the rate of their restoration, and finally the solubility of ITZ in the prepared ASDs. Our research clearly showed that regardless of the PVP type, both LC transitions, from smectic (Sm) to nematic (N) and from N to isotropic (I) phases, are effectively suppressed. Moreover, a significant difference in the miscibility of different PVPs with the investigated API was found. This phenomenon also affected the solubility of API, which was the greatest, up to 100 μg/mL in the case of starPVP 85:15 w/w mixture in comparison to neat crystalline API (5 μg/mL). Obtained data emphasize the crucial role of the polymer's topology in designing new pharmaceutical formulations.