Heating-cooling Cycles Research Articles

Microstructural Investigation and Mechanical Properties of Resistance Spot Welding Joints of Mild Steel Sheets

Due to its high productivity, low cost, and increased work efficiency, resistance spot welding (RSW) can instantly join two or more steel sheets and it is widely used in the automotive industry and lately in the manufacturing of thin-walled cold-formed steel constructions. However, the RSW of zinc-coated mild steel sheets presents metallurgical challenges, especially when different thickness combinations are used. Therefore, in the present paper a resistance welding machine was used which uses direct current (MFDC) inverter technology combined with SMART AUTOSET technology to weld S250 GD and S350 GD galvanised mild steel sheets with different thicknesses. It is well known that, due to the elevated temperature that occurs during the welding process, followed by rapid cooling, defects such as cracking, porosity, lack of fusion, and an increased amount of brittle phases affect the welding quality. Therefore, the influence of the RSW process parameters established by an automatic sequence on the nugget geometry, microstructure, and mechanical properties was investigated. The phase transformations that took place during the heating-cooling cycle were analysed in detail through metallographic studies. The results showed that the microstructure of the weld nuggets was similar, characterised by columnar grains elongated in the direction of heat evacuation. Nevertheless, there were differences in terms of phase dispersion, defects and mechanical properties that have been linked to the RSW process parameters.

Exploring aluminum as a solid thermal storage medium for solar cooking application: An experimental investigation coupled with numerical modeling using OpenFOAM

The intermittent nature of solar energy presents a significant challenge to its reliability, particularly in applications that require a consistent energy supply, such as cooking. This issue is especially critical in emerging economies with abundant solar resources, where sustainable energy solutions are needed to reduce reliance on traditional fuels. To address this challenge, this study introduces a novel solar thermal storage (STS), utilizing a metal-based material to accumulate and retain heat for off-sunshine hour cooking. The research focuses on optimizing aluminium as the STS material, evaluating its temperature suitability, efficiency, and heat retention capabilities for household solar cooking applications. Numerical simulations using the OpenFOAM framework were conducted to analyze heat transfer within the cooker, determining the optimal size of the aluminium block based on existing literature and predefined parameters. Practical experiments, including solar-induced heating-cooling cycles and controlled cooking tests, were carried out to validate the findings. Experimental results demonstrate STS's ability to efficiently absorb and retain heat, reaching a maximum of 235 °C during a 5.5-h heating session. The water boiling experiment further confirmed STS's practical utility, effectively transferring stored heat to cooking tasks and sustaining temperatures up to 160 °C even after the test. Additionally, experiments with black lentils and chicken stew highlighted aluminium's suitability for practical cooking applications, showcasing its ability to sustain high temperatures and efficiently transfer stored heat despite longer cooking times. The study's novelty lies in integrating numerical modeling with experimental analysis to optimize STS systems, providing practical guidelines for efficient thermal storage in cooking applications. This research advances beyond previous efforts by providing a validated methodology for the design and optimization of thermal storage systems. It improves the reliability and adaptability of solar energy for cooking applications.

Preparation and thermal stability research of oxalic acid dihydrate-glutaric acid/PAMPS phase change gel for solar thermal energy utilization

Thermal energy storage technology based on phase change materials (PCMs) can address the temporal and spatial mismatches in solar thermal energy conversion, thereby enhancing solar energy utilization efficiency. However, the liquid flow and poor thermal reliability of PCMs limit their large-scale application in solar thermal systems. In this study, we employ a simple “one-pot method” to prepare a form-stable and thermally reliable oxalic acid dihydrate-glutaric acid/poly 2-Acrylamido-2-methyl-1-propanesulfonic acid (OAD-GA/PAMPS) phase change gel. The OAD-GA/PAMPS phase change gel melts at 64.5 °C with a phase change enthalpy of 193.6 kJ/kg, the tensile strength is 7.707 MPa, and the compressive strength is 16.940 MPa. By incorporating 5 wt% BN particles, the thermal conductivity of OAD-GA/PAMPS phase change gel reaches 0.63 W/(m·K). After 200 heating-cooling cycles, the phase change temperature of the OAD-GA/PAMPS phase change gel remains nearly unchanged, and the phase change enthalpy decreases by only 5.7 %. The photo-thermal conversion efficiency of the OAD-GA/PAMPS phase change gel is 82.7 %. The high thermal reliability and photo-thermal conversion efficiency of the OAD-GA/PAMPS phase change gel makes it suitable for solar thermal energy storage applications.

Development of phyto-compound-based nanoformulation for skin regeneration and its in vitro evaluation

Skin regeneration is a process that regenerates damaged or aged skin cells, resulting in a healthier and younger appearance. The demand for nano dosage forms containing phyto-compounds in the cosmetics industry is increasing daily to control or prevent this process. Phyto-compounds with cell regenerative properties often show limited effectiveness in conventional use due to their low hydrophilicity and molecular weight, as well as their low absorption from the skin. Glycyrrhizic acid (GA), a phyto-compound obtained from licorice root, has many pharmacological properties, including skin regeneration properties. The aim of this study is to develop a highly stable GA-containing nanoemulsion (GA-NE) formulation for topical application, determine its anti-collagenase activity, and evaluate its safety and effectiveness by in vitro cell culture methods. In this context, GA-NEs were synthesized with sixteen production procedures using independent variables (oil, water percentage, surfactant, and homogenization time). The average droplet size, polydispersity index (PdI), and zeta potential (ZP) of the synthesized GA-NEs were determined by dynamic light scattering (DLS) analysis. The heating-cooling cycle (H-C) test result showed that the F4P3 formulation was the most stable formulation. Therefore, 90-day physicochemical stability tests were performed with the F4P3 formulation. In vitro release results revealed that GA was released from NEs at a rate of 93.15 ± 0.61 % within 24 h. An ex vivo permeation and penetration study was performed, and the permeation and penetration profile of F4P3 was determined. In vitro cell culture studies showed that the F4P3 formulation had higher cell viability and regeneration than GA in the HaCaT cell line. Anti-collagenase activity results also revealed a better activity of the F4P3 formulation compared to GA. Consequently, characterization, efficacy, and safety studies showed that the F4P3 formulation could be an innovative cell regeneration product candidate suitable for topical use with high stability.

Ultrasensitive and Adjustable Nanothermometers Based on Er3+-Sensitized Core@Shell Nanoparticles for Use in the First Biological Window.

In recent years, intensive research has focused on lanthanide-doped nanoparticles (NPs) used as noncontact temperature sensors, particularly in nanomedicine. These NPs must be capable of excitation and emission within biological windows, where biological materials usually show better transparency for radiation. In this article, we propose that NPs sensitized with Er3+ ions can be applied as temperature sensors in biological materials. We synthesized the NPs through a reaction in high-boiling solvents and confirmed their crystal structure and the formation of core@shell NPs by using X-ray diffraction, high-resolution transmission electron microscopy, and element distribution mapping within the NPs. NaErF4@NaYF4, NaYF4:12.5% Er3+, 2.5% Tm3+@NaYF4, NaYF4:7.5% Er3+@NaYF4, and NaYF4:12.5% Er3+, 2.5% Ho3+@NaYF4 exhibited intense upconversion (UC) emission under 1532 nm laser excitation detectable also in the whole human blood. We propose that this UC results from energy transfer between Er3+ ions and from Er3+ to Tm3+ or Ho3+ codopants. To determine the mechanism of UC, we measured the dependence of the emission band intensities on the laser power densities. Importantly, we also analyzed the temperature-dependent emission of the NPs within the 295-360 K range. Based on the collected emission spectra, we calculated the luminescence intensity ratios (LIRs) of the emission bands to assess their potential for optical temperature sensing. The temperature-sensing properties varied with the concentration of Er3+ ions and the presence of additional Tm3+ or Ho3+ codopants. Depending on the NP composition and the emission bands used for luminescence ratio calculations, the maximum relative temperature sensitivity ranged from 4.55%·K-1 to 1.12%·K-1, with temperature resolution between 0.05 and 2.53 K at room temperature. Finally, as proof of using NPs as temperature sensors in biomedicine, we successfully measured the temperature-dependent emission of NaYF4:7.5% Er3+@NaYF4 NPs dispersed in whole blood under 1532 nm excitation. We demonstrated that the ratio of Er3+ ion emission bands changes with temperature, indicating that these NPs have potential applications in temperature sensing within biological environments. We also confirmed the properties of NPs as temperature sensors by measuring the temperature reading uncertainty and the repeatability of the LIR readings during heating-cooling cycles, thereby confirming the excellent properties of the studied systems as temperature sensors.

Effect of Polypropylene Fibers on the Behavior of Dune Sand Concrete Subjected to Elevated Temperatures

In the field of civil engineering, concrete is one of the most used materials. Above a particular temperature, concrete can exhibit thermal instability during fire events. Improving the safety of buildings and structures is an important issue of this study. This paper investigates the effects of adding polypropylene fibers to dune sand concrete at high temperatures. Prismatic specimens (4 x 4 x 16 cm3) are produced, and stored until the concrete is hardened. The samples are then subjected to a heating-cooling cycle, starting from room temperature and increasing by 0.1 °C/min until reaching 600 °C. The length and the rate of polypropylene fibers are examined to determine their impact on compressive strength, porosity, and weight loss at temperatures of 80, 150, 300, 450, and 600 °C. Also, the partial replacement of dune sand (DS) with river sand (RS) is investigated. The results show that, for temperatures below 450 °C, the strengths of dune sand concretes varied from 29 to 39% compared to those obtained with concretes subjected to an ambient temperature (20 °C). However, it should be noted that residual compressive strength values decrease at most temperatures with the addition of polypropylene fibers. Furthermore, sand concrete with 50% DS and 50% RS shows an increase in compressive strength of 43.77% compared to that obtained by 100% DS concrete.

InP/ZnS Quantum Dots for High-Sensitivity Temperature Sensors.

In this work, we report the synthesis of high-quality, nontoxic InP/ZnS core-shell quantum dots (QDs). The temperature-dependent optical properties such as photoluminescence (PL), absorption, and PL decay with time have been investigated. Based on the temperature dependence of optical properties of QDs, three different configurations of temperature sensors using PMMA as the QD host material were fabricated. The temperature sensor configurations are planar thin-film (InP/ZnS QD)/PMMA deposited on a Si wafer (sensor 1), (InP/ZnS QD)/PMMA-filled borosilicate fibers (sensor 2), and InP/ZnS QD-doped electrospun PMMA nanofibers deposited on a Si wafer (sensor 3). After fabrication, the performance of the temperature sensors have been thoroughly investigated through photoluminescence reversibility tests in consecutive heating-cooling cycles, and the sensitivity of the sensors has been calculated. The three sensors have shown different critical reversible temperatures above which they go through irreversible structural damage and lose their reversibility. The highest critical reversible temperature of 95 °C and the highest sensitivity of 2.1% °C-1 have been achieved, which are comparable to Cd- and dye-based temperature sensors reported to date. The sensors were found to be highly stable, which demonstrated negligible degradation even after 30 days of exposure to ambient conditions. The InP/ZnS QD-based optical temperature sensors can be considered to be a potential replacement for toxic heavy-metal QD-based optical temperature sensors.

Preparation and thermal properties investigation of pentaerythritol based phase change microcapsules for low and medium temperature thermal energy storage

As one of classic organic compounds, pentaerythritol (PE) has been reported to have two phase-change processes with one solid-solid transition and other solid-liquid transition. To utilize these two phase-change enthalpies and avoid the liquid leakage over heat storage process, this work for the first time proposes to encapsulate PE with use of silicon dioxide (SiO2) as a shell substance through a so-called so-gel technology. The microstructure characterization and thermoproperties as well as the cycling performance of the PE- SiO2 composite are detailedly investigated. The results showed that PE could be successfully covered by SiO2 to form a spherical core-shell structure. Such an encapsulation has a mean size of 0.3 μm and an encapsulated rate of over 77.8 %. Due to the involvement of two phase-transition stages, the latent heat of the PE- SiO2 composite reached to 255 kJ/kg, which is higher than other organic microencapsulations. In addition, the thermal conductivity is largely enhanced by adjusting the ratio of SiO2. For a given SiO2 ratio of 77.8 %, the thermal conductivity of PE-SiO2 composite is 0.93 W/m·K, 36.8 % higher than that of pristine PE. Apart from these, the results also indicated that the composite achieves excellent thermal cycling performance. After one hundred repeated heating-cooling cycles, the latent heat of the PE- SiO2 composite decreases by less than 5 %, demonstrating that this composite material has a large potential for application in the field of thermal energy storage at low and medium temperatures.

Modeling of a Single-Band-Ratiometric Sensor Based on Lattice Positive Thermal Expansion in Eu3+-Activated Halide Perovskite Cs2NaEuCl6.

Recently, single-band ratiometric (SBR) thermometry has emerged as an innovative approach to traditional fluorescence thermometry, overcoming uncertainties associated with emission spectrum overlap or scattering while maintaining high spatial resolution and remote monitoring. This paper presents a novel Cs2NaEuCl6 perovskite prepared through a slow-cooling solution method. Additionally, it proposes a temperature sensor model that relies on the thermal quenching of charge-transfer state absorption. Mechanical studies highlight the role of lattice positive thermal expansion in affecting Eu3+ emission. Conversely, a significant emission enhancement is observed upon excitation corresponding to both the ground state and excited state absorption. The distinct luminescent behavior of this Eu3+-activated halide perovskite model makes it suitable for developing a highly sensitive SBR-type sensor with a relative sensitivity (Sr) exceeding 1.5% K-1 and temperature resolution (𝛿T) below 1 K at room temperature. Furthermore, it demonstrates the thermal stability during multiple heating-cooling cycles. Finally, the practical applicability of the proposed SBR model is demonstrated by employing a self-manufactured film sensor that enables precise real-time temperature detection for electronic components. The work is regarded as a significant stride toward the development of cutting-edge and exquisitely sensitive thermometers based on lanthanide-based halide double perovskites.

Ti3C2Tx MXene/Graphene hybrid frameworks for constructing thermally conductive phase change composites with solar-thermal conversion ability

Application of three-dimensional (3D) carbon framework in phase change material (PCM) has aroused a tremendous interest. However, implementing a high alignment carbon framework with low interfacial thermal resistance (ITR) remain challenging. Herein, we demonstrated a synergetic effect strategy by Ti3C2Tx MXene and graphene nanoplate (GNP) to achieve a 3D thermally conductive framework with high alignment skeleton and low filler-to-filler ITR. Typically, the 3D anisotropic carbon framework is fabricated by unidirectional ice template freezing cellulose nanofibers (CNF)/GNP solution with the assistance of MXene. The abundant functional groups of MXene and the similar layered structure promote the dispersion stability of GNP in CNF aqueous solution, resulting in the significant improvement in GNP alignment and the filler-to-filler ITR. Therefore, the final phase change composites obtained by impregnating polyethylene glycol (PEG) into the framework achieves a satisfactory thermal conductivity (TC) of up to 3.49 W/m K at only 6.25 vol% filler loading. Combining the solar-thermal conversion ability of graphene and MXene, the PCM reveals a rapid energy conversion, transfer and storage capability. More importantly, the phase change enthalpy of the PCM can be as high as 164.3 J/g, with little change even after 50 heating-cooling cycles, and the existence of carbon framework can effectively prevent the leakage phenomenon in solid-liquid conversion process, ensuring its shape stability.

Quality by Design Optimization of Telmisartan-loaded Self-nanoemulsifying Drug Delivery Systems

Aim: This study aimed to develop a QbD-based SNEDDS for Telmisartan to enhance its biopharmaceutical performance. Background: Quality by Design (QbD) was incorporated by selecting the Critical Process Pa-rameters (CPPs) as influential variables upon the desired responses. Using Design of Experiments (DoE) software, oil, and S/CoS were tested for pre-isotropic compatibility and formulation im-provement. A heating-cooling cycle and phase separation were used to determine the formula-tions' dispersibility, self-emulsifying time, mean globule size, and stability. Objective: Using Design of Experiments (DoE) software, oil (Capmul MCM C-10) and surfactant (Labrasol)/co-surfactant (Tween-20, and PEG-600) (S/CoS) ratios were optimized for pre-isotropic compatibility and formulation improvement. Methods: Aerosil-200, Sylysia-350, 550, and 730 were used as porous carriers, and Neusilin US2 as an adsorbing agent resulted in free-flowing granules of self-nano emulsifying drug delivery systems. The SNEDDS was further formulated into tablets using a direct compression technique with enhanced disintegration properties. Results: The prepared SNEDDS tablets exhibited a 71% increase in drug dissolution as compared to the pure drug and marketed formulation after the in vitro dissolution studies. Conclusion: This QbD approach offers a promising strategy for developing SNEDDS formula-tions for poorly soluble drugs like Telmisartan.

Thermal rock magnetic cycling (TRMC): a method to track thermal alteration details for palaeointensity interpretations

SUMMARY Accurate absolute palaeointensity is essential for understanding dynamo processes on the Earth and other planetary bodies. Although great efforts have been made to propose techniques to obtain magnetic field strength from rock samples, such as Thellier-series methods, the amount of high-fidelity palaeointensities remains limited. One primary reason for this is the thermal alteration of samples that pervasively occurred during palaeointensity experiments. In this study, we developed a comprehensive rock magnetic experiment, termed thermal rock magnetic cycling (TRMC), that can utilize measurements of critical rock magnetic properties at elevated temperatures during multiple heating-cooling cycles to track thermal changes in bulk samples and individual magnetic components with different Curie temperatures in samples for palaeointensity interpretations. We demonstrate this method on a Galapagos lava sample, GA 84.6. The results for this specimen revealed that GA 84.6v underwent thermophysical alteration throughout the TRMC experiment, resulting in changes in its remanence carrying capacity. These findings were then used to interpret the palaeointensity results of specimen GA 84.6c, which revealed that the two-slope Arai plot yielded two linear segments with distinct palaeointensity values that were both biased by thermophysical alteration. To further test the TRMC method, we selected another historical lava sample (HS 2) from Mt Lassen, detecting slight thermal-physical changes after heating the specimen HS 2–8C to a target temperature of 400 °C. We also isolated a stable magnetic component with a Curie temperature below 400 °C using the TRMC method, which may provide a more reliable palaeointensity estimate of 51 μT. By providing a method for tracking thermal alteration independent of palaeointensity experiments, the TRMC method can explore subtle, unrecognizable thermal alteration processes in less detailed palaeointensity measurements, which can help to assess the thermal stability of the measured samples and interpret the changes in the TRM unblocking spectrum and palaeointensity estimates, facilitating the acquisition of more reliable records for constrain the formation of the inner core and the evolution of Earth's magnetic field.

Post-fire structural material behaviour of high- and ultra-high-strength steels – Experimental study and aids for assessment for further use and potential reuse after fires

A thorough understanding and realistic modelling of the essential post-fire material behaviour is crucial for a safe assessment of the residual structural performance of steel components and the reliable further usage of steel structures after fires. This paper presents a comprehensive experimental study of the constitutive material behaviour after heating-cooling cycles of two quenched and tempered high-strength steels (S690QL, S960QL) as well as two thermomechanically rolled steels (S960M, S1100M). The results are compared to both the behaviour of grade S355J2+N mild steels and a comprehensive database of existing post-fire tests. Except for high- and ultra-high-strength steels, which are exposed to very high temperatures above the A1-phase-transformation-temperature during fires, the assumption of reversibility of the mechanical properties for structural designs of steel structures has been confirmed. At high and very high steel temperatures, the microstructure changes and the strength properties after fire are significantly lower than those of virgin high-strength steel specimens. The study thus provides the basis for reliable structural designs and future normative guidance of the post-fire structural steel designs.

Post-fire mechanical behavior of iron-based shape memory alloy used for structural damping

This study sheds light on the post-fire mechanical behavior of Fe-SMA by conducting a series of tests at material level, with a particular focus on the low-cycle fatigue (LCF) performance. Three temperatures (500 °C, 750 °C, 1000 °C) and three cooling methods (air cooling, water spray quenching, water quenching) are considered to simulate different fire-cooling scenarios. After the heating-cooling cycle, 10 monotonic tensile specimens and 20 LCF specimens are tested. Essential material parameters for Manson-Coffin model and combined hardening model are also calibrated based on the experimental data. The failure mechanism and possible factors affecting the post-fire LCF performance of Fe-SMA are further explored by scanning electron microscope (SEM), metallographic observation (MO), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) analysis. Results show that Fe-SMA could exhibit even better LCF resistance after undergoing the considered fire-cooling procedure, especially those heated up to 750 °C and 1000 °C. Even under rapid cooling, no signs of brittle fracture are observed, which is superior to traditional structural steels. Different heating temperatures and cooling methods affect the grain sizes and microstructure of Fe-SMA, leading to variations in its post-fire LCF behavior.

ZBS glass synthesized containing both Eu3+-Eu2+ CsPbBr3 quantum dots with multi-color luminescence for white LEDs

In this article, zinc-borosilicate silicon（ZBS） glass containing europium (Eu) ions and CsPbBr3 quantum dots (QDs) (ZEQ) was synthesized via melting crystallization route. The composition and luminescent properties are investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and photoluminescence (PL) spectra. The XRD and XPS results imply that Eu 3+, Eu 2+ ions and CsPbBr3 QDs are coexisted in ZBS glass. The dual-mode emission from cyan to orange-red can be achieved by adjusting the melting temperature. Besides, the ZEQ has good water resistance, outstanding light irradiation and excellent heating-cooling cycle stability. Moreover, based on the ZEQ, the light-emitting diodes (LEDs) are designed to obtain the white light with the correlated color temperature (CCT) of 5255 K, the Coherent Infrared Energy (CIE) color coordinate of (0.337 0.313), the NSCT color gamut of 116 % and 87 % of Rec.2020. Thus, this article provides a novel material ZEQ, which is tunable luminescence and ultra-stable. The strategy provides the route that the luminescence of Eu ions and QDs can be observed at the same time, solving the difficulty of the rare earth ions entering into the QDs lattice. Moreover, the ZEQ is expected to be used as the light source for solid-state lighting and also provides valuable research results for future exploration of functional glass containing both rare-earth ions and QDs materials.

Investigating temperature-dependent gas permeation in PIM-1: Hysteresis, annealing effects, and thermal history impact

This study investigates the gas permeability, diffusion coefficients, and solubility coefficients of CO2 and N2 in PIM-1 over the temperature range of 35 °C–135 °C. During the first heating-cooling cycle, both gases' permeability exhibited hysteresis, which diminished during the second cycle. Thermal mechanical analysis confirmed volume relaxation occurring at temperatures above 95 °C, indicating physical aging. By calculating the relaxation time associated with volume relaxation, the influence of physical aging was minimized through measurements on samples annealed at 135 °C for 15 h. This approach enabled a successful evaluation of the temperature dependence of gas permeability under conditions where the impact of physical aging could be disregarded. The results revealed that the permeability of CO2 decreased, and that of N2 increased with increasing temperature. The diffusion coefficients showed positive temperature dependence, while the solubility coefficients decreased. Physical aging significantly affected the diffusion coefficients but had minimal impact on the solubility coefficients. Heat of sorption and activation energies for gas permeability and diffusion estimated, and it was found that the decrease in CO2 permeability with temperature was mainly attributed to the large heat of sorption for CO2.

Heating–cooling–heating cycles within ca. 70 Myr recorded in UHT granulites in the Khondalite Belt, North China Craton

Revealing the thermal evolution history of ultrahigh-temperature metamorphism (UHT) could help shed light on the genesis and evolution of the orogenic crust. However, it is generally difficult to constrain the duration of metamorphism, especially the heating stage due to the complex behavior of the datable accessory minerals (e.g., zircon and monazite). The Khondalite Belt of the North China Craton records Paleoproterozoic UHT metamorphic event which was previously constrained to be ca. 1920 Ma by using the weighted mean age of zircon U–Pb dating results, however, zircon could grow during both prograde and retrograde periods. Thus, the age of ca. 1920 Ma may be an oversimplified explanation and there could be a complex thermal evolution. In this study, combined with zircon U–Pb dating and Ti-in-zircon thermometry, the duration of the UHT metamorphism in the eastern Khondalite Belt was constrained to be 60–70 Myr with two short periods of decompression-heating (both lasting for ∼20 Myr) intervened by a period of cooling process (lasting for ∼30 Myr). This finding further expands our knowledge that there was a heating–cooling–heating cycle rather than a continuously prolonged cooling process in a long-lived UHT metamorphism. Our results show that the UHT metamorphism in the eastern Khondalite Belt requires two stages of lithosphere extension, which were possibly related to shallow slab breakoff and post-collisional lithospheric delamination, respectively. It further indicates that Paleoproterozoic orogenesis, although dominated by subduction of rheologically weak slab, is comparable to the formation of modern Himalaya orogens.

In-situ WAXS and SAXS microstructural investigation of Poly(vinylidene fluoride)- Fe3O4 coaxial-electrospun nanocomposites under thermal and mechanical loading: Nanoparticles size effects on fibers molecular orientation, mechanical properties and crystalline polymorphs

In this study, 15 % w/w polyvinylidene fluoride (PVDF)–Fe3O4 nanoparticle (NP) nanocomposite scaffolds were prepared using coaxial electrospinning with a high-speed conductive rotating collector to obtain aligned fibers. The effects of varying the Fe3O4 NP size (6, 9, and 14 nm in diameter) on the fiber morphology, mechanical response, and piezoelectric phase content were investigated. In situ X-ray diffraction measurements during heating-cooling and tensile tests were carried out using synchrotron radiation to study the charge in the β-phase content and polymer chain orientation under different conditions. At zero strain, the presence of smaller NPs seems to impede the β-phase crystallization in PVDF. However, heating-cooling cycles revealed that the NPs acted as nucleation agents, promoting a higher β-phase content after cooling compared to pure PVDF. The mechanical response showed that nanocomposites with smaller NPs exhibited higher stiffness. While the initial degree of polymer chain orientation was higher in the nanocomposites, the presence of NPs hindered the α-to-β phase transformation and chain alignment under tensile deformation compared to pure PVDF. This study highlights the complex interplay among NP size, β-phase formation, mechanical properties, and chain orientation in PVDF-Fe3O4 nanocomposites, providing insights into tailoring their piezoelectric performance for various applications.

Variations in the Thermomechanical and Structural Properties during the Cooling of Shape-Memory R-PETG.

One of the useful features of 3D-printed specimens of recycled polyethylene terephthalate glycol (R-PETG) is the ability to repetitively develop free recovery as well as the work-generating, shape-memory effect. This behavior is enabled by the R-PETG's capacity to stiffen during cooling, thus allowing for a new temporary shape to be induced. Aiming to devise an explanation for the polymer's stiffening, in this study, the variation in some of the R-PETG's parameters during cooling are emphasized and discussed. The evolution of an R-PETG filament's shape was monitored during room-temperature-bending heating-cooling cycles. Straight-shape recovery and the complete loss of stiffness were observed at the start and the end of heating, respectively, followed by the forced straightening of the filament, performed by the operator, around 40 °C, during cooling. The tests performed by dynamic mechanical analysis disclosed the rise of the storage modulus (E') after 100 °C heating followed by either liquid-nitrogen- or air-cooling to room temperature, in such a way that E' was always larger after cooling than initially. Static tests emphasized a peculiar stress variation during a heating-cooling cycle applied in air, within the heating chamber of the tensile testing machine. Tensile-failure tests were performed at -10 °C at a rate of 100 mm/min, with specimens printed at various deposition directions between 10 and 40° to the transversal direction. The specimens printed at 40°, which had the largest ultimate strains, were broken with tensile rates between 100 and 500 mm/min. Deformation rate increase favored the shift from crazing to delamination failure modes. The correlation between the structural changes, the sharp E' increase on heating, and the stiffening induced by cooling represents a novel approach that enables the use of 3D-printed R-PETG for the fabrication of the active parts of low-priced lightweight resettable actuators.

Influence of Sputtering Power on the Properties of Magnetron Sputtered Tin Selenide Films.

The ecofriendly tin selenide (SnSe) is expected to find multiple applications in optoelectronic, photovoltaic, and thermoelectric systems. This work is focused on the thermoelectric properties of thin films. SnSe single crystals exhibit excellent thermoelectric properties, but it is not so in the case of polycrystalline bulk materials. The investigations were motivated by the fact that nanostructuring may lead to an improvement in thermoelectric efficiency, which is evaluated through a dimensionless figure of merit, ZT = S2 σ T/λ, where S is the Seebeck coefficient (V/K), σ is the electrical conductivity (S/m), λ is the thermal conductivity (W/mK), and T is the absolute temperature (K). The main objective of this work was to obtain SnSe films via magnetron sputtering of a single target. Instead of common radiofrequency (RF) magnetron sputtering with a high voltage alternating current (AC) power source, a modified direct current (DC) power supply was employed. This technique in the classical version is not suitable for sputtering targets with relatively low thermal and electrical conductivity, such as SnSe. The proposed solution enabled stable sputtering of this target without detrimental cracking and arcing and resulted in high-quality polycrystalline SnSe films with unprecedented high values of ZT equal to 0.5 at a relatively low temperature of 530 K. All parameters included in ZT were measured in one setup, i.e., Linseis Thin Film Analyzer (TFA). The SnSe films were deposited at sputtering powers of 120, 140, and 170 W. They had the same orthorhombic structure, as determined by X-ray diffraction (XRD), but the thickness and microstructure examined by scanning electron microscopy (SEM) were dependent on the sputtering power. It was demonstrated that thermoelectric efficiency improved with increasing sputtering power and stable values were attained after two heating-cooling cycles. This research additionally provides further insights into the DC sputtering process and opens up new possibilities for magnetron sputtering technology.