Search Results (226)

In this work, we report a novel mechanochemical synthesis method for the synthesis of quinoxaline derivatives—a spiral gas–solid two-phase flow approach, which enables the efficient preparation of quinoxaline compounds. Compared to conventional synthetic methods, this approach eliminates the need for heating or solvents while significantly reducing reaction time. The structures of the synthesized compounds were characterized using nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV–Vis) absorption spectroscopy, powder X-ray diffraction (XRD), differential scanning calorimetry (DSC), and high-performance liquid chromatography (HPLC). Using the synthesis of 2,3-diphenylquinoxaline (1) as a model reaction, the synthetic process was investigated with UV–Vis spectroscopy. The results demonstrate that when the total feed amount was 2 g with a carrier gas pressure of 0.8 MPa, the reaction completed within 2 min, achieving a yield of 93%. Furthermore, kinetic analysis of the reaction mechanism was performed by monitoring the UV–Vis spectra of the products at different time intervals. The results indicate that the synthesis of 1 follows the A4 kinetic model, which describes a two-dimensional diffusion-controlled product growth process following decelerated nucleation. Full article

Concrete is a cornerstone material in the construction industry owing to its versatile performance; however, its inherent brittleness, low tensile strength, and poor permeability resistance limit its broader application. Graphene, with its exceptional thermal conductivity, stable lattice structure, and high specific surface area, presents a transformative solution to these challenges. Despite its promise, comprehensive studies on the multifunctional properties and underlying mechanisms of graphene-enhanced concrete remain scarce. In this study, we developed a novel concrete composite incorporating cement, coarse sand, crushed stone, water, and graphene, systematically investigating the effects of the graphene dosage and curing duration on its performance. Our results demonstrate that graphene incorporation markedly improves the material’s density, brittleness, thermal conductivity, and permeability resistance. Notably, a comprehensive analysis of scanning electron microscopy (SEM) images and thermogravimetric (TG) data demonstrates that graphene-modified concrete exhibits a denser microstructure and the enhanced formation of hydration products compared to conventional concrete. In addition, the graphene-reinforced concrete exhibited a 44% increase in compressive strength, a 0.7% enhancement in the photothermal absorption capacity, a 0.4% decrease in maximum heat release, a 0.8% increase in heat-storage capacity, and a 200% reduction in the maximum penetration depth. These findings underscore the significant potential of graphene-reinforced concrete for advanced construction applications, offering superior mechanical strength, thermal regulation, and durability. Full article

In this study, polymer matrix composites based on high-density polyethylene (HDPE) and recycled HDPE (HDPEr) were reinforced with zinc oxide nanoparticles (ZnO NPs). Four formulations (M1-M4) with HDPE/HDPEr/ZnO NP mass ratios of 50/50/0, 48/47/5, 45/45/10, and 43/42/15 were produced via melt injection molding. Disc-shaped samples (Ø30 ± 0.1 mm × 2 ± 0.1 mm) were evaluated in unaged and aged states (840 h at 100% humidity and 100 °C) using scanning electron microscopy, X-ray diffraction, ultraviolet–visible and Fourier-transform infrared spectroscopy, water absorption, thermal resistance, and mechanical and dielectric testing. Among all composites, M2 showed the best performance, with the highest aging resistance (estimated lifetime of 3891 h in humidity and 2361 h in heat). It also exhibited superior mechanical properties, with the highest indentation hardness, Vickers hardness, and elastic modulus before (0.042 GPa, 3.846 HV, and 0.732 GPa) and after aging under humidity (0.042 GPa, 3.932 HV, 0.706 GPa) and elevated temperature (0.085 GPa, 7.818 HV, 1.871 GPa). Although ZnO NPs slightly reduced electrical resistivity, M2 showed the most stable dielectric properties. In its unaged state, M2 had 22%, 30%, and 3% lower surface resistivity, volume resistivity, and dielectric strength, respectively, than M1 polymer. M2 was identified as the optimal formulation, combining mechanical strength, dielectric stability, and resistance to moisture and heat. Full article

Fiber–aerogel composites have gained significant attention as high-performance thermal insulation materials due to their unique microstructure, which suppresses conductive, convective, and radiative heat transfer. At room temperature, silica aerogels in particular exhibit ultralow thermal conductivity (<0.02 W/m·K), which is two to three times lower than that of still air (0.026 W/m·K). Their brittle skeleton and high infrared transparency, however, restrict how well they insulate, particularly at high temperatures (>300 °C). Incorporating microscale fibers into the aerogel matrix enhances mechanical strength and reduces radiative heat transfer by increasing scattering and absorption. For instance, it has been demonstrated that adding glass fibers reduces radiative heat transmission by around 40% because of increased infrared scattering. This review explores the fundamental mechanisms governing radiative heat transfer in fiber–aerogel composites, emphasizing absorption, scattering, and extinction coefficients. We discuss recent advancements in fiber-reinforced aerogels, focusing on material selection, structural modifications, and predictive heat transfer models. Recent studies indicate that incorporating fiber volume fractions as low as 10% can reduce the thermal conductivity of composites by up to 30%, without compromising their mechanical integrity. Key analytical and experimental methods for determining radiative properties, including Fourier transform infrared (FTIR) spectroscopy and numerical modeling approaches, are examined. The emissivity and transmittance of fiber–aerogel composites have been successfully measured using FTIR spectroscopy; tests show that fiber reinforcement at high temperatures reduces emissivity by about 15%. We conclude by outlining the present issues and potential avenues for future research to optimize fiber–aerogel composites for high-temperature applications, including energy-efficient buildings (where long-term thermal stability is necessary), electronics thermal management systems, and aerospace (where temperatures may surpass 1000 °C), with a focus on improving the materials’ affordability and scalability for industrial applications. Full article

Increasing the temperature of waste heat is crucial to enable its recovery. Vapor compression heat pumps and absorption heat transformers are the two heat upgrade technologies most commonly used for this purpose. Heat pumps have the advantage of entirely recovering the waste heat and the disadvantage of requiring electricity as input. Heat transformers need a negligible amount of electricity but reject at part of the waste heat input at low temperature. Due to these differences, the choice between the two options depends on the application. In this work, the environmental and economic performance of heat pumps and heat transformers are compared in some relevant applications. Indications about the most suitable technology are provided based on the availability of the waste heat, of the CO₂ content of the electricity and of the electricity–gas price ratio. Heat pumps perform better when the waste heat availability is limited compared to the upgraded heat requirements and has a better environmental profile when the electricity has low carbon content. Heat transformer results are often economically convenient, especially when the availability of waste heat is large. Full article

In this study, we produced gelatin from Skipjack tuna (Katsuwonus pelamis) bone. We used three heating levels to obtain the gelatin (80 °C, 90 °C, 100 °C), and its physicochemical and gel characteristics were thoroughly examined. The results revealed that the extraction temperature significantly affected the yield, gel strength, amino acid composition, antioxidant activity, and microstructure of the gelatin. Gelatin obtained at 80 °C yielded 6.53% with peak gel strength (59.72 ± 4.67 g), whereas the gelatin extracted at 100 °C had the highest yield (23.24%) but the lowest gel strength (13.71 ± 3.78 g). Fourier transform infrared (FTIR) scans of gelatin derived at different temperature levels showed subtle changes in the amide region, but all the samples presented the characteristic absorption peaks of gelatin. An amino acid analysis showed high glycine (18.51%) and proline (13.45%) contents in the gelatin. Antioxidant tests revealed that gelatin obtained at 80 °C displayed the greatest scavenging effects on DPPH, hydroxyl, and ABTS radicals. Scanning electron microscopy (SEM) revealed that the gelatin made with lower temperatures showed a smoother and tighter microstructure. Rheological analyses revealed that higher extraction temperatures resulted in lower gel temperatures and melting temperatures and weaker gel network stability. The results of this study suggest that lower extraction temperatures are more suitable for the preparation of high-quality skipjack tuna bone gelatin, which gives scientific support for its application in food and medicine. Full article

This study proposes a polygeneration system that integrates an absorption cooling system, a heat transformer, and an organic Rankine cycle for the simultaneous production of power, cooling, and freshwater—the former operating with a water/lithium bromide mixture while the organic Rankine cycle operates with R123. The system was simulated at temperatures between 100 °C and 150 °C and with heat supply capacities that a geothermal field in Nayarit, Mexico, could provide. A parametric analysis was conducted by developing a mathematical model using the Engineering Equation Solver software. The geothermal source temperatures, ambient temperatures, and mass ratios were varied to determine the power, cooling load, freshwater production, and energy utilization factor under different operating conditions. The results showed that the system could produce up to 13 kW of power, 100 kW of cooling, and 50 L per hour at a heat source temperature of 150 °C. The energy utilization factor varied from 0.32 to 0.43, which means that up to 43% of the energy supplied to the system could be utilized to produce cooling, freshwater, or power. Full article

This study introduces a novel in situ pulsed current-assisted resistance spot welding method, which differs fundamentally from conventional post-weld heat treatments and is designed to enhance the mechanical performance of 7075-T651 aluminum alloy joints. Immediately after welding, a short-duration pulsed current is applied while the weld remains in a high excess-vacancy state, effectively accelerating precipitation reactions within the weld region. Transmission electron microscopy (TEM) observations reveal that pulsed current treatment promotes the formation of band-like solute clusters, indicating a significant acceleration of the early-stage precipitation process. Interestingly, the formation of quasicrystalline phases—rare in Al-Zn-Mg-Cu alloy systems—is incidentally observed at grain boundaries, exhibiting characteristic fivefold symmetry. Selected area electron diffraction (SAED) patterns further show that these quasicrystals undergo partial dissolution under the influence of the pulsed current, transforming into short-range ordered cluster-like structures. Lap shear tests demonstrate that joints treated with pulsed current exhibit significantly higher peak load and energy absorption compared to untreated specimens. Statistical analysis of weld size confirms that both groups possess comparable weld diameters under identical welding currents, suggesting that the observed mechanical improvements are primarily attributed to microstructural evolution rather than geometric factors. Full article

4H-silicon carbide (4H-SiC) is a cornerstone for next-generation optoelectronic and power devices owing to its unparalleled thermal, electrical, and optical properties. However, its chemical inertness and low dopant diffusivity for most dopants have historically impeded effective doping. This study unveils a transformative laser-assisted boron doping technique for n-type 4H-SiC, employing a pulsed Nd:YAG laser (λ = 1064 nm) with a liquid-phase boron precursor. By leveraging a heat-transfer model to optimize laser process parameters, we achieved dopant incorporation while preserving the crystalline integrity of the substrate. A novel optical characterization framework was developed to probe laser-induced alterations in the optical constants—refraction index ($\mathrm{n}$) and attenuation index ($\mathrm{k}$)—across the MIDIR spectrum (λ = 3–5 µm). The optical properties pre- and post-laser doping were measured using Fourier-transform infrared spectrometry, and the corresponding complex refraction indices were extracted by solving a coupled system of nonlinear equations derived from single- and multi-layer absorption models. These models accounted for the angular dependence in the incident beam, enabling a more accurate determination of $\mathrm{n}$ and $\mathrm{k}$ values than conventional normal-incidence methods. Our findings indicate the formation of a boron-acceptor energy level at 0.29 eV above the 4H-SiC valence band, which corresponds to λ = 4.3 µm. This impurity level modulated the optical response of 4H-SiC, revealing a reduction in the refraction index from 2.857 (as-received) to 2.485 (doped) at λ = 4.3 µm. Structural characterization using Raman spectroscopy confirmed the retention of crystalline integrity post-doping, while secondary ion mass spectrometry exhibited a peak boron concentration of 1.29 × 10¹⁹ cm⁻³ and a junction depth of 450 nm. The laser-fabricated p–n junction diode demonstrated a reverse-breakdown voltage of 1668 V. These results validate the efficacy of laser doping in enabling MIDIR tunability through optical modulation and functional device fabrication in 4H-SiC. The absorption models and doping methodology together offer a comprehensive platform for paving the way for transformative advances in optoelectronics and infrared materials engineering. Full article

This study introduces an innovative analytical solution to the time-fractional Cattaneo heat conduction equation, which models photothermal transport in metallic thin films subjected to short laser pulse irradiation. The model integrates the Caputo fractional derivative of order 0 < p ≤ 1, addressing non-Fourier heat conduction characterized by finite wave speed and memory effects. The equation is nondimensionalized through suitable scaling, incorporating essential elements such as a newly specified laser absorption coefficient and uniform initial and boundary conditions. A hybrid approach utilizing the finite Fourier cosine transform (FFCT) in spatial dimensions and the Laplace transform in temporal dimensions produces a closed-form solution, which is analytically inverted using the two-parameter Mittag–Leffler function. This function inherently emerges from fractional-order systems and generalizes traditional exponential relaxation, providing enhanced understanding of anomalous thermal dynamics. The resultant temperature distribution reflects the spatiotemporal progression of heat from a spatially Gaussian and temporally pulsed laser source. Parametric research indicates that elevating the fractional order and relaxation time amplifies temporal damping and diminishes thermal wave velocity. Dynamic profiles demonstrate the responsiveness of heat transfer to thermal and optical variables. The innovation resides in the meticulous analytical formulation utilizing a realistic laser source, the clear significance of the absorption parameter that enhances the temperature amplitude, the incorporation of the Mittag–Leffler function, and a comprehensive investigation of fractional photothermal effects in metallic nano-systems. This method offers a comprehensive framework for examining intricate thermal dynamics that exceed experimental capabilities, pertinent to ultrafast laser processing and nanoscale heat transfer. Full article

Incorporating a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-based derivative (1,4-bis(diphenoxyphosphoryl)piperazine, DIDOPO) in combination with modified calcium sulfate whiskers (MCSWs) improved the flame retardancy, thermal stability, and rheological properties of a polyethylene oxide (PEO) composite. The synergistic flame-retardant effect of DIDOPO and MCSW on the PEO system was investigated. After introducing 5 wt.% MCSW and 10 wt.% DIDOPO into PEO, the UL-94 rating of the composite reached V-0, and the limiting oxygen index was increased to 26.5%. Additionally, the peak and average heat release rates and total heat release of the PEO/10% DIDOPO/5% MCSW composite decreased by 38.9%, 22%, and 20.07%, respectively. The results of a thermogravimetric analysis (TGA) revealed that PEO/10% DIDOPO/5% MCSW displayed an improved initial thermal stability and rate of char formation compared to those of the PEO matrix. The results of TGA/Fourier transform infrared analysis indicated that the composites exhibited phosphorus-containing groups during thermal degradation, based on the characteristic absorption peaks, and increased amounts of gas-phase volatiles. The morphologies and structures of the residues indicated that the PEO/10% DIDOPO/5% MCSW blend was less stable than PEO during combustion. The MCSW mixture formed a denser, more continuous carbon layer on the composite surface during combustion. The rheological behavior indicated that the high complex viscosity and moduli of PEO/10% DIDOPO/5% MCSW promoted the cross-linking network structure of the condensed phase during combustion. MCSW exhibited an excellent flame retardancy and improved thermal stability, which are potentially promising for use in fire safety applications. Full article

Glass-based microfluidic devices are essential for applications such as diagnostics and drug discovery, which utilize their optical clarity and chemical stability. This review systematically analyzes pulsed laser micromachining as a transformative technique for fabricating glass-based microfluidic devices, addressing the limitations of conventional methods. By examining three pulse regimes—long (≥nanosecond), short (picosecond), and ultrashort (femtosecond)—this study evaluates how laser parameters (fluence, scanning speed, pulse duration, repetition rate, wavelength) and glass properties influence ablation efficiency and quality. A higher fluence improves the material ablation efficiency across all the regimes but poses risks of thermal damage or plasma shielding in ultrashort pulses. Optimizing the scanning speed balances the depth and the surface quality, with slower speeds enhancing the channel depth but requiring heat accumulation mitigation. Shorter pulses (femtosecond regime) achieve greater precision (feature resolution) and minimal heat-affected zones through nonlinear absorption, while long pulses enable rapid deep-channel fabrication but with increased thermal stress. Elevating the repetition rate improves the material ablation rates but reduces the surface quality. The influence of wavelength on efficiency and quality varies across the three pulse regimes. Material selection is critical to outcomes and potential applications: fused silica demonstrates a superior surface quality due to low thermal expansion, while soda–lime glass provides cost-effective prototyping. The review emphasizes the advantages of laser micromachining and the benefits of a wide range of applications. Future directions should focus on optimizing the process parameters to improve the efficiency and quality of the produced devices at a lower cost to expand their uses in biomedical, environmental, and quantum applications. Full article

A diffusion absorption heat transformer is a completely thermally driven heat upgrading technology with significant application potential in low-grade thermal energy recovery. However, existing diffusion absorption heat transformers have problems such as complex circulation processes, limited solution flow rates, and insufficient stability due to their reliance on bubble pumps. A jet pump was proposed for application in a diffusion absorption heat transformer cycle to replace the bubble pumps in the original diffusion absorption heat transformer cycle. In the novel cycle, without electricity consumption, the diffusant gas was used as the primary flow of the jet pump to transport the solution, and the diffusion generation of the refrigerant was realized in the jet pump for more efficient and stable thermal energy upgrading. The performance of the novel cycle with H₂O/LiBr/C₅H₁₀ or H₂O/HCOOK/C₅H₁₀ as working fluids was analyzed based on a constructed theoretical model validated by numerical simulation. It was found that the performance of the jet pump was sensitive to the generator temperature and the pressure difference of the cycle. Increasing the temperature of the jet pump and reducing the temperature of the absorber were conducive to improving the COP. As a potential absorbent substitute for LiBr, HCOOK also led to slightly better performance in most cases. Full article

The crystal–chemical behavior of two layered titanosilicate minerals with porous crystal structures, kupletskite, K₂NaMn₇²⁺Ti₂(Si₄O₁₂)₂O₂(OH)₄F, and kupletskite-(Cs), Cs₂NaMn₇²⁺Ti₂(Si₄O₁₂)₂O₂(OH)₄F, was investigated under high-temperature conditions using single-crystal and powder X-ray diffraction; infrared and optical absorption spectroscopy and electron-microprobe analysis. Both minerals undergo topotactic transformation to dehydroxylated and oxidized high-temperature (HT) modifications at temperature above 500 °C while maintaining the basic bond topology of the astrophyllite structure-type. The high-temperature structures show contraction of the unit-cell parameters similar to that of Fe²⁺-dominant astrophyllite, indicating that Mn²⁺ oxidizes along with Fe²⁺ in M(2)–M(4) sites. The oxidation of Mn²⁺ is confirmed by the increase of the Mn³⁺-related absorption (in optical spectra) that is inversely correlated with the intensity of O–H bands in the infrared spectra. The Fe,Mn-oxidation is also evident by the contraction of the M(2), M(3), and M(4)O₆ octahedra. The M(1)–O bond length increases slightly, indicating a preference for mono- and divalent cations to occupy the M(1) site in the heated structure; this may be due to site-selective oxidation and/or migration of unoxidized cations (as previously shown for lobanovite) to this site. The role of extra framework A-site cations (K, Cs) in thermal expansion of these minerals is discussed. Full article

Based on a review of the existing literature on the use of diatomite and the functioning of phase-change heat accumulators, in this study, we conducted empirical research on the creation of a phase-change composite based on Carpathian diatomite. As part of our mineralogical research, we determined the phase composition of the Carpathian diatomites in this work. Their internal nanostructure was identified. Nanopores create regular systems that, depending on the variety of diatoms, may have sieve, tubular, or “honeycomb” shapes. Diatomites’ internal structure benefits the absorption capacity of phase-change materials (PCM). The obtained calorimetric thermograms of the organic phase-change material and the diatomite compound highlighted an extension of the temperature range in which phase transformation occurs from 4–5 °C (for pure PCM RT28HC) to 15–17 °C for the composites tested with weight proportions of 1:1 and 4:6. In the case of water-rich varieties, the presence of mixed-package minerals, i.e., montmorillonite, with its small size and specific 2:1 package structure, can hinder the penetration and accumulation of PCM. The ability to bind and accumulate heat will be influenced by the size of the diatomite particles or the relative size of the PCM and pores, i.e., structural and textural features. Full article