Papers by Keyword: Thermal Stability

Abstract: The copolyesters derived from dimethyl ester of terephthalic acid, ethylene glycol, and 4-hydroxybenzoic acid (HBA) have been synthesized via catalytically promoted polycondensation omitting the acetylation step. FTIR spectroscopy results have evidenced an insertion of HBA along a polymer backbone. Of note, thermal gravimetric analysis has shown that the HBA moieties substantially improved the thermal stability of polyesters. As found by differential scanning calorimetry and polarizing microscopy, the copolyesters are capable of forming an anisotropic phase in a temperature range of 150-170 °C. Additionally, the free surface energy of the samples was determined to evaluate the compatibility of thermotropic copolyesters with other high-molecular compounds.

Abstract: Thermal stability in a stockpile of reactive materials is analyzed in this article. The combustion process is modelled in a long cylindrical pipe that is assumed to lose heat to the surrounding environment by convection and radiation. The study of effects of different kinetic parameters embedded on the governing differential equation, makes it easier to investigate the complicated combustion process. The combustion process results with nonlinear molecular interactions and as a result it is not easy to solve the differential equation exactly, and therefore the numerical approach by using the Finite Difference Method (FDM) is applied. The numerical solutions are depicted graphically for each parameter’s effect on the temperature of the system. In general, the results indicate that kinetic parameters like the reaction rate promote the exothermic chemical reaction process by increasing the temperature profiles, whilst kinetic parameters such as the order of the reaction show the tendency to retard the combustion process by lowering the temperature of the system.

Abstract: In this study, polymer composites were synthesized from date seeds (DS) powder as natural filler and low-density polyethylene (LLDPE) as polymer matrix. This composite was exposed into chemical modification using different content of Stearic Acid (SA) (3, 6 and 9 wt.%). The composite was fabricated by using the process of extrusion and injection molding respectively. The prepared samples were examined using TGA, DSC, FTIR, and Gel Content test. It can be clearly observed that all the treated samples have presented three-steps of decomposition as shown in TGA curves. It is also observed that the Tm, Tc, and degree of crystallinity of the modified LLDPE/DS biocomposites increased as SA increase. Thr FTIR spectra have shown different type of stretching bands, the band at 3346 - 3347 cm^-1 appeared because of hydroxyle (OH) groups that is described as a hydrophilicity measure. Beside, there was a minor decrease on the peak between 3346 and 3347 cm^-1 of modified composite, whichwas attributed to the hemicellulose removal from the modified composite. The gel content of the treated mples increased due to the increase of crosslinking between DS and LLDPE in existence of SA.

Abstract: Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Abstract: The performance of graphene/polymer nanocomposites depends on many factors but the major factor is a nanoparticles dispersion and distribution into the host matrix. The present work investigates the effect of the dispersion of graphene oxide upon the structure-property relations in metallocene linear low density polyethylene (PE), homo polypropylene (PP), and blends thereof. These nanocomposites were prepared by solvent processing, where DMF and o-xylene were used as solvents for Graphene Oxide (GO) powder and the polymers respectively, before the two components were combined to form a well-mixed initial state. Characterization of the structure and crystallization of the nanocomposites was carried out by small- and wide-angle X-ray scattering and diffraction (SAXS and WAXD). The chemical structures were characterized by Fourier transform infrared spectroscopy (FTIR) and by Raman spectroscopy, and the latter used to calculate the ID/IG value for a pure GO samples. The thermal properties of the resulting nanocomposites were investigated by DSC and TGA in order to obtain Melting temperature ( ), crystallization temperature ( ) and degree of crystallinity ( ) as well as a range of degradation temperatures. The effect of GO on the mechanical properties was studied via the ultimate tensile strength and elastic modulus.

Abstract: In this study, we propose a novel normally-off AlGaN/GaN HFET based on stack AlGaN barrier structure and p-type NiO gate. The residual thin AlGaN barrier (with low Al content) is adopted to alleviate mobility degradation. Besides, p-type conductive NiO formed by thermal oxidation at 500 °C was used as gate electrode, which contribute to the positive shift of threshold voltage. Combining NiO gate and thin barrier structure, normally-off device with a threshold voltage of +1.1 V is realized. Temperature dependent transfer characteristics show that the normally-off device presents good thermally stability within the temperature range from 25 to 150 °C.

Abstract: Dispersion strengthened Cu composites are studied over recent years to find an optimum processing route to obtain a high strength, thermal-stable copper alloy designed for modern applications in electrical engineering. The experimental Cu–4Al₂O₃–1MgO material was prepared by in situ thermo-chemical technique and mechanical milling followed by spark plasma sintering (SPS). The study analyses the influence of the Al₂O₃ and MgO secondary phases on strengthening the copper matrix. Microstructure of the composite was studied by X-ray diffraction analysis, scanning and transmission electron microscopy. The sintered microstructure shows a grain size distribution characterized by ultrafine grains/twins embedded inside the matrix of nanocrystalline grains. The microstructure is thermal stable up to 900 °C due to the dispersed alumina nano-particles that effectively strengthen crystallite/grain boundaries during the SPS process and annealing of the sintered compact at elevated temperatures. On the other hand, the coarsened MgO particles are responsible for ultrafine grains/twins formation. The obtained microstructure is important for practical utilization of the material because this structure is characterized by a good combination of strength and ductility.

Abstract: This work aims to reveal the effects of zeolite on properties of fly ash based geopolymer under high temperature at 300 °C, 600 °C and 900 °C. The specimens were prepared by alkali activation of fly ash, which was partially replaced by two different types of zeolite at 10%, 20% and 30% by weight. The specimens were analyzed for the maximum compressive strength, weight loss percentage, XRD and SEM. The results highlighted that the percentage of weight loss increased with the ratio of zeolite replacement. The compressive strength of geopolymer with synthetic zeolite and natural zeolite at 7, 28, 60 days were similar. The high-temperature exposure resulted in the reduction in compressive strength in all proportions. At the same temperature, compressive strength of all specimens were not significantly different.

Abstract: Boron nitride (BN) was modified by silane coupling agent (KH560) and used as heat conductive filler to prepare the modified BN (BN560)/epoxy composite. The effect of the BN560 filler content on the thermal conductivity and thermal stability of the epoxy composite was studied. The results show that BN560 can be uniformly dispersed in the epoxy matrix by an ultrasonic disperser. The BN560 added can effectively improve the thermal conductivity of the epoxy composite. With the increase of BN560 content to 20 wt.%, the thermal conductivity of the composite increases accordingly to 0.27 W/(m·K), 50% higher than that of pure epoxy, and a heat conductive network is formed. The BN560 added can improve the thermal stability of the composite. With increasing BN560 content, the thermal decomposition temperature and glass transition temperature of the composite increase. The composite with the BN560 content of 20 wt.% has the weight loss of 10 wt.% at 395.12 °C and the glass transition temperature of 144.59 °C.

Abstract: Platinum and iridium are known to belong to a family of partly miscible metals. On the other hand, these metals are high demanded as active components of the catalysts for a variety of industrially important processes. In the present work, a series of bimetallic Pt-Ir catalysts supported on alumina were prepared using a “single source precursor” concept. The ratio of metals in the alloy was varied from 1:3 to 3:1. The thermal stability and catalytic activity in CO oxidation of the samples were studied in a prompt thermal aging regime. Monometallic Pt/γ-Al₂O₃ and Ir/γ-Al₂O₃ samples were used as references. All the studied catalysts were characterized by UV-vis spectroscopy in initial state and after the aging treatment. It was found that the bimetallic nanoparticles being subjected to the high temperature aging at 600 and 800 °C undergo redistribution with further stabilization in a noticeably more active state. The observed increase in the catalytic activity is explained by an enrichment of the particles’ surface with platinum.