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During the transition from liquid to solid, the thermal conductivity coefficient λ of concrete decreases. Although λ of hardened concrete is well investigated, there is limited research on the transition from liquid to solid and how it depends on hydration. Currently, only simplified qualitative approaches exist for the liquid state and the transient process. An experimental method is not available. For this purpose, a test rig is designed to experimentally capture the evolution of λ for fine-grain concretes during transition. The performance of the test setup is evaluated on a characteristic high-performance concrete (HPC). The results are compared to theoretical predictions from the literature. The developed test rig is mapped in a digital twin to investigate extended boundary conditions, such as different heat sources and temperatures of the experimental setup. It allows the experiment to be repeated and optimized for different setups with little effort. The test principle is as follows: A liquid concrete sample is heated through a controlled external source, while the transient temperature distribution over the height is measured with a fiber optic sensor. The thermal conductivity is derived from the heat flux induced and the temperature distribution over an evaluation length. Experiments show that λ in the liquid state is approximately 1.4 times greater than in the solid state and exponentially decreases for the transient process. Numerical results on the digital twin indicate that the robustness of the results increases with the temperature of the heat source. Moreover, the derivation in λ turns out to be strongly dependent on the evaluation length. A length of three times the maximum grain diameter is recommended.

It is a critical issue to reduce the thermal conductivity and increase the thermal expansion coefficient of ceramic thermal barrier coating (TBC) materials in the course of their utilization. To synthesize samples with different composition and measure their thermal conductivity by the traditional experimental approaches is time-consuming and expensive. Most classic and empirical models work inefficiently and inaccurately when researchers attempt to predict the thermophysical properties of TBC materials. In this research project, we tentatively exploit a Genetic Algorithm-Support Vector Regression (GA-SVR) machine learning model to study the thermophysical properties, illustrated with the potential TBC materials ZrO2 doped DyTaO4, which has resulted in the lowest thermal conductivity in rare earth tantalates RETaO4 system. Meanwhile, we employ statistical parameters of correlation coefficient (R2) and mean square error (MSE) to evaluate the accuracy and reliability of the model. The results reveal that this model has brought about high correlation coefficients of thermal conductivity and thermal expansion coefficient (99.8% and 99.9%, respectively), while the MSE values are 0.00052 and 0.00019, respectively. The doping concentration of ZrO2 was optimized to reach as low as 0.085–0.095, so as to reduce their thermal conductivity further and increase their thermal expansion. This model provides an accurate and reliable option for researchers to design ceramic thermal barrier coating materials.

As global energy demand grows, more and more attention is being paid to efficient energy storage solutions, among which Phase Change Materials (PCMs) are particularly promising. They allow efficient thermal energy storage by using the latent heat that is absorbed or released during the transformation of the aggregate state. One of the most important properties of PCM for efficient heat transfer is the thermal conductivity. Unfortunately, it is not high for organic materials. This paper presents a study of RT58 PCM, presenting an experimental evaluation of the thermal conductivity of this material and the application of the material in a storage tank. The analytical evaluation focuses on the heat transfer under natural convection conditions, where the aim is to assess the influence of the thermal conductivity of the PCM on the amount of energy stored in the storage tank. In this case, the temperature of the heating surface, the magnitude of the thermal conductivity coefficient and the choice of the other PCM are considered. The experimental results allowed to refine the value of the thermal conductivity coefficient of the RT58 material used (0.188 W/(mK)) and the analytical calculations allowed to trend towards an increase of the heat exchange efficiency of the system with the PCM. Article in Lithuanian. RT58 fazinio virsmo medžiagos šilumos laidumo koeficiento tyrimas natūralios konvekcijos sąlygomis akumuliacinės talpos įkrovimo metu Santrauka Didėjant pasaulinei energijos paklausai, vis daugiau dėmesio skiriama efektyviems energijos kaupimo sprendimams, tarp kurių itin perspektyvios yra fazinio virsmo medžiagos (FVM). Jos leidžia efektyviai saugoti šiluminę energiją naudojant latentinę šilumą, kuri sugeriama arba išskiriama agregatinės būsenos virsmo metu. Viena iš svarbiausių FVM savybių efektyviems šilumos mainams yra šilumos laidumo koeficientas. Deja, organinių medžiagų atveju jis nėra aukštas. Šiame straipsnyje pateikiama RT58 FVM tyrimas, kuriame pristatomas eksperimentinis šios medžiagos šilumos laidumo įvertinimas ir medžiagos pritaikymas akumuliacinėje talpoje. Atliekant analitinį vertinimą nagrinėjami šilumos mainai natūralios konvekcijos sąlygomis, kai siekiama įvertinti FVM šilumos laidumo įtaką sukauptos energijos kiekiui akumuliacinėje talpoje. Šiuo atveju nagrinėjama šildomojo paviršiaus temperatūra, šilumos laidumo koeficiento dydis ir kitos FVM pasirinkimas. Eksperimentiniai rezultatai leido patikslinti naudojamos RT58 medžiagos šilumos laidumo koeficiento vertę (0,188 W/(mK), o analitiniai skaičiavimai – tendencijas didinti sistemos su FVM šilumos mainų efektyvumą. Reikšminiai žodžiai: fazinio virsmo medžiaga, natūrali konvekcija, šilumos atidavimas, šilumos laidumo koeficientas.

New composites made of natural fiber polymers such as wasted date palm surface fiber (DPSF) and pineapple leaf fibers (PALFs) are developed in an attempt to lower the environmental impact worldwide and, at the same time, produce eco-friendly insulation materials. Composite samples of different compositions are obtained using wood adhesive as a binder. Seven samples are prepared: two for the loose natural polymers of PALF and DPSF, two for the composites bound by single materials of PALF and DPSF using wood adhesive as a binder, and three composites of both materials and the binder with different compositions. Sound absorption coefficients (SACs) are obtained for bound and hybrid composite samples for a wide range of frequencies. Flexural moment tests are determined for these composites. A thermogravimetric analysis test (TGA) and the moisture content are obtained for the natural polymers and composites. The results show that the average range of thermal conductivity coefficient is 0.042–0.06 W/(m K), 0.052–0.075 W/(m K), and 0.054–0.07 W/(m K) for the loose fiber polymers, bound composites, and hybrid composites, respectively. The bound composites of DPSF have a very good sound absorption coefficient (>0.5) for almost all frequencies greater than 300 Hz, followed by the hybrid composite ones for frequencies greater than 1000 Hz (SAC > 0.5). The loose fiber polymers of PALF are thermally stable up to 218 °C. Most bound and hybrid composites have a good flexure modulus (6.47–64.16 MPa) and flexure stress (0.43–1.67 Mpa). The loose fiber polymers and bound and hybrid composites have a low moisture content below 4%. These characteristics of the newly developed sustainable and biodegradable fiber polymers and their composites are considered promising thermal insulation and sound absorption materials in replacing synthetic and petrochemical insulation materials in buildings and other engineering applications.

The article discusses the impact of technological parameters on the degree of material widening in the hot rolling process. Physical simulation studies were conducted for S235JR steel and Grade 2 titanium. The result of the experiments was the determination of dependencies that allow the degree of material widening in the hot rolling process to be calculated. A heating furnace was used for the physical modelling studies to heat the samples in the temperature range of 750 to 1200°C, and a single-stand reversing duo/quarto rolling mill equipped with small-size sample feeding equipment (Fig. 1). The study results indicate considerable discrepancies in the calculations of expansion magnitude based on the widely applied formulas found in rolling literature. The computed values substantially deviate from those obtained through physical simulation.

We have proposed and developed a method for measuring the thermal conductivity of highly efficient thermal conductors. The measurement method was tested on pure metals with high thermal conductivity coefficients: aluminum (99.999 wt.% Al) and copper (99.990 wt.% Cu). It was demonstrated that their thermal conductivities at a temperature of T = 22 ± 1 °C were <λAl> = 243 ± 3 W/m·K and <λCu> = 405 ± 4 W/m·K, which was in good agreement with values reported in the literature. Artificial graphite (ρG1 = 1.8 g/cm3) and natural graphite (ρG2 = 1.7 g/cm3) were used as reference carbon materials; the measured thermal conductivities were <λG1> = 87 ± 1 W/m·K and <λG2> = 145 ± 3 W/m·K, respectively. It is well established that measuring the thermal conductivity coefficient of thin flexible graphite foils is a complex metrological task. We have proposed to manufacture a solid rectangular sample formed by alternating layers of thin graphite foils connected by layers of ultra-thin polyethylene films. Computer modelling showed that, for equal thermal conductivities of solid products made of compacted thermally exfoliated graphite and products made of a composite material consisting of 100 layers of thin graphite foil and 99 layers of polyethylene, the differences in temperature fields did not exceed 1%. The obtained result substantiates our proposed approach to measuring thermal conductivity of flexible graphite foil by creating a multi-layer composite material. The thermal conductivity coefficient of such a composite at room temperature was <λGF> = 184 ± 6 W/m·K, which aligns well with measurements by the laser flash method.

The heat transfer mechanism of hollow glass microsphere/epoxy resin composites (HGM/ER) is intricate, and the formation of void structures during material preparation complicates the prediction of thermal conductivity. To investigate the microscopic heat transfer mechanisms of HGM/ER materials with void structures and analyze the impact of void variables on the overall thermal performance, this study addresses the issue of low packing density and poor uniformity in traditional cellular unit structures. An improved random sequential adsorption (RSA) algorithm is proposed, increasing the upper limit of particle fill rate by 25% relative to traditional RSA algorithms. The Benveniste equivalent microsphere thermal conductivity model is selected for thermal performance simulation, demonstrating its high correlation with the three-component model (air, glass, resin), with a maximum relative error of only 1.32%. A classification method for void types in HGM/ER materials is proposed, categorizing them into interfacial and free voids. The microscopic heat transfer mechanisms of HGM/ER materials are investigated under different voids levels and void types, and it was found that the effect of interfacial voids on thermal conductivity is 60% higher than that of free voids. Based on the measured voids of the material, this study provides a reference for the convenient prediction of thermal conductivity in practical engineering applications of HGM/ER composites.

Magnetic thin films and nanostructures present a unique challenge for a range of thermal measurements, with important consequences for both fundamental physics and material science and applications. This paper reviews the unique capabilities for measurement and control of these systems using thermal gradients applied using micro- and nanofabricated silicon-nitride membrane platforms. Supporting a thin film or nanostructure removes bulk heat sinks from the tiny structure, enabling otherwise challenging or impossible measurements including thermal conductivity, Seebeck coefficient, Peltier coefficient, magnon drag, both the anomalous and planar Nernst effect, specific heat, and novel manifestations of thermally assisted spin transport. After providing some historical context and motivation and overviewing the design and fabrication of silicon-nitride membrane thermal platforms, example data for each of the measurements above is reviewed, and the paper concludes with a consideration of the outlook for measurements enabled by these techniques.

This study describes the development and utilization of a novel setup for measuring the thermal conductivity of polyurethane composites with various nanoparticle contents. Measurements were conducted using both an experimental setup and a professional instrument, the TPS 2500 S, with results demonstrating high agreement with the precision of the measurements. The setup was further validated using a standard reference material with a thermal conductivity of 0.200 W/m/K. Additionally, the reliability of the setup was confirmed by its stability against ambient temperature variations between 20 and 30 degrees Celsius. This research presents a cost-effective method for measuring the thermal conductivity of polyurethane composites. Data processing involves noise reduction and smoothing techniques to ensure reliable results. The setup offers 5% accuracy and proves to be versatile for both research and educational applications.

Pineapple leaf fiber (PALF), striped sunflower seed fiber (SFSF), and watermelon seed (WMS) are considered natural waste polymer materials, which are biodegradable and sustainable. This study presents new novel thermal insulation and sound absorption materials using such waste as raw materials. PALF, SFSF, and WMS were used as loose, bound, and hybrid samples with different compositions to develop promising thermal insulation and sound-absorbing materials. Eleven sample boards were prepared: three were loose, three were bound, and five were hybrid between PALF with either SFSF or WMS. Wood adhesive was used as a binder for both the bound and hybrid sample boards. Laboratory scale sample boards of size 30 cm × 30 cm with variable thicknesses were prepared. The results show that the average thermal conductivity coefficient for the loose samples at the temperature range 20–80 °C is 0.04694 W/(m.K), 0.05611 W/(m.K), and 0.05976 W/m.K for PALF, SFSF, and WMS, respectively. Those for bound sample boards are 0.06344 W/(m.K), 0.07113 W/(m.K), and 0.08344 W/m.K for PALF, SFSF, and WMS, respectively. The hybrid ones between PALF and SFSF have 0.05921 W/m.K and 0.06845 W/(m.K) for two different compositions. The other hybrid between PALF and WMS has 0.06577 W/(m.K) and 0.07007 for two different compassions. The sound absorption coefficient for most of the bound and hybrid boards is above 0.5 and reaches higher values at some different frequencies. The thermogravimetric analysis for both SFSF and WMS shows that they are thermally stable up to 261 °C and 270 °C, respectively. The three-point bending moment test was also performed to test the mechanical properties of the bound and hybrid sample boards. It should be mentioned that using such waste materials as new sources of thermal insulation and sound absorption materials in buildings and other applications would lead the world to utilize the waste until zero agrowaste is reached, which will lower the environmental impact.