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Samples of flexible PU foam were prepared from a polyol (Elastoflex W 5516/115) and an isocyanate prepolymer (Iso 145/8), both commercial Elastogram products. For the thermooxidative stabilization, two phenolic compounds were used...
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Samples of flexible PU foam were prepared from a polyol (Elastoflex W 5516/115) and an isocyanate prepolymer (Iso 145/8), both commercial Elastogram products. For the thermooxidative stabilization, two phenolic compounds were used, separately or in mixture. These compounds were: 2,6-di-t-butyl-4-methyl-phenol (non-reactive) (AO-1), 3,5-di-t-butyl-4-hydroxy-benzyl alcohol (reactive, AO-2), used in total mass% of 0.3/1.5. The TG/DTG/DTA curves were drawn up in dynamic air, with a heating rate of 10 A degrees C min(-1), until 500 A degrees C. For the unstabilized sample a single thermodegradative TG step, with a maximum rate at 268-270 A degrees C was observed, whereas for the stabilized samples, supplementary steps at higher temperature were observed. The changes in the TG/DTG/DTA parameters are not in a single relationship with the mass% of the stabilizator, due to the following: AO-1 is easily migrated out from PV, especially, at higher concentrations. AO-2 had positive effect at all studied concentrations. But the most remarkable effect is the synergetic effect of a 1:1 mixture of AO-1 and AO-2.
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Synthetic and natural antioxidants are used in the biodiesel and food industries to increase product shelf lives. Rancimat, which is regulated by EN 14112, is the official analytical method used to determine the oxidation stabilit...
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Synthetic and natural antioxidants are used in the biodiesel and food industries to increase product shelf lives. Rancimat, which is regulated by EN 14112, is the official analytical method used to determine the oxidation stability of biodiesel, oils and fats. It involves subjecting samples to a temperature of 110 ℃ for prolonged periods. Some antioxidants decompose or volatilize below 110℃, leading to false results. A TG/DTA technique with both dynamic and isothermal (110℃) analysis methods was used to evaluate the thermal stability of commercial antioxidants. Synthetic antioxidants exhibited thermal resistances in the following order: PG > TBHQ> BHA > BHT. Initial thermal decomposition temperatures were lower than 110 ℃, and BHT, BHA and TBHQ volatilized during the first few hours of the analysis. Natural antioxidants are resistant to heat and displayed stabilities in the following order: α-tocopherol > caffeic acid > ferulic acid > gallic acid. Subjecting chelating agents to a temperature of 110 ℃ for prolonged periods resulted in the following order of thermal stabilities: ascorbic acid > citric acid > EDTA. The initial thermal decomposition temperatures for both gallic acid and EDTA were lower than 110 ℃.
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We consider a model of Dirac fermions coupled to flexural phonons to describe a graphene sheet fluctuating in dimension 2 + d. We derive the self-consistent screening equations for the quantum problem, exact in the limit of large ...
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We consider a model of Dirac fermions coupled to flexural phonons to describe a graphene sheet fluctuating in dimension 2 + d. We derive the self-consistent screening equations for the quantum problem, exact in the limit of large d. We first treat the membrane alone, and work out the quantum to classical and harmonic to anharmonic crossovers. For the coupled electron-membrane problem, we calculate the dressed two-particle propagators of the elastic and electron interactions and find that it exhibits a collective mode which becomes unstable at some wave vector q_c for large enough coupling g. The saddle-point analysis, exact at large d, indicates that this instability corresponds to spontaneous and simultaneous appearance of Gaussian curvature and electron puddles. The relevance to ripples in graphene is discussed.
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A novel thermal stabilizer for rigid poly (vinyl chloride) has been examined. Its high stabilizing efficiency is detected by the extent of discoloration and the mass loss rate of the degraded samples when compared with some of ref...
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A novel thermal stabilizer for rigid poly (vinyl chloride) has been examined. Its high stabilizing efficiency is detected by the extent of discoloration and the mass loss rate of the degraded samples when compared with some of reference stabilizers. A higher stabilizing efficiency is produced as the novel thermal stabilizer is mixed with different co-stabilizers. The study results for thermo oxidative stability of the degraded samples with novel thermal stabilizer have shown that the novel thermal stabilizer can prevent or delay the thermal oxidative degradation of PVC. A probable stabilization mechanism for the novel thermal stabilizer has been proposed.
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In this study, novel glasses based on SrO-La_2O_3-Al_2O_3-B_2O_3-SiO_2 system are investigated for solid oxide fuel and electrolyzer cells. The network structure evolution of the glasses with increasing B_2O_3:SiO_2 ratio was stud...
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In this study, novel glasses based on SrO-La_2O_3-Al_2O_3-B_2O_3-SiO_2 system are investigated for solid oxide fuel and electrolyzer cells. The network structure evolution of the glasses with increasing B_2O_3:SiO_2 ratio was studied using Raman spectroscopy. The thermal properties of the glasses, including glass transition temperature T_g and glass softening temperature T_d, were studied using dilatometry. The thermal stability of the glasses was investigated using X-ray diffraction. The study shows that as the B_2O_3iSiO_2 ratio increases, the SrO-La_2O_3-Al_2O_3-B_2O_3-SiO_2 glass micro-heterogeneity and the amount of non-bridging oxygen atoms increase. Correspondingly, the T_g of the SrO-La_2O_3-Al_2O_3-B_2O_3-SiO_2 glasses changes from 635 to 775℃, and the T_d changes from 670 to 815℃. Glass thermal stability decreases with B_2O_3:SiO_2 ratio increase. The glass without B_2O_3 is thermally stable after being kept at 850℃ for 200 hrs.
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Out of plane vibrations are suppressed in graphene layers placed on a substrate. These vibrations, in suspended samples, are relevant for the understanding of properties such as the electrical resistivity, the thermal expansion co...
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Out of plane vibrations are suppressed in graphene layers placed on a substrate. These vibrations, in suspended samples, are relevant for the understanding of properties such as the electrical resistivity, the thermal expansion coefficient, and others. We use a general framework to study the properties of the out of plane mode in graphene on different substrates, taking into account the dynamics of the substrate. We discuss broadening of this mode and how it hybridizes with the substrate Rayleigh mode, comparing our model with experimental observations. We use the model to estimate the substrate induced changes in the thermal expansion coefficient and in the temperature dependence of the electrical resistivity.
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Temperature fluctuations in the normal direction of planar crystals such as graphene are quite violent and may be expected to strongly influence their melting properties. In particular, they will modify the Lindemann melting crite...
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Temperature fluctuations in the normal direction of planar crystals such as graphene are quite violent and may be expected to strongly influence their melting properties. In particular, they will modify the Lindemann melting criterion. We calculate this modification in a self-consistent Born approximation. The result is applied to graphene and its wrapped version represented by single-walled carbon nanotubes. It is found that the out-of-plane fluctuations dominate over the in-plane fluctuations. This makes strong restrictions to possible Lindemann parameters. Astonishingly we find that these large out-of-plane fluctuations have only a small influence on the melting temperature.
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We use high pressure to reveal the dependence of interfacial heat transport on the stiffness of interfacial bonds. The combination of time-domain thermoreflectance and SiC anvil techniques is used to measure the pressure-dependent...
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We use high pressure to reveal the dependence of interfacial heat transport on the stiffness of interfacial bonds. The combination of time-domain thermoreflectance and SiC anvil techniques is used to measure the pressure-dependent thermal conductance G(P) of clean and modified Al/SiC interfaces at pressures as high as P = 12 GPa. We create low-stiffness, van der Waals-bonded interfaces by transferring a monolayer of graphene onto the SiC surface before depositing the Al film. For such weak interfaces, G(P) initially increases approximately linearly with P. At high pressures, P > 8 GPa, the thermal conductance of these weak interfaces approaches the high values characteristic of strongly bonded, clean interfaces.
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We report studies of thermal transport across the interface of a semiconductor heterostructure using x-ray diffraction to measure the time-dependent lattice expansion after ultrafast laser excitation. Femtosecond laser pulses are ...
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We report studies of thermal transport across the interface of a semiconductor heterostructure using x-ray diffraction to measure the time-dependent lattice expansion after ultrafast laser excitation. Femtosecond laser pulses are used to rapidly and locally heat the substrate at the buried interface of an Al_(0.3)Ga_(0.7)As/GaAs heterostructure grown by molecular-beam epitaxy. High-resolution time-resolved x-ray diffraction is used to study the heating and cooling of the film and substrate independently. The data are compared with a simple model for the thermal transport incorporated into dynamical diffraction calculations allowing us to extract the room-temperature cross-plane film thermal conductivity. The value is 40% lower than that extrapolated from prior results on liquid-phase epitaxy grown samples of varying concentrations.
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Combining the thermodynamics at the nanometer scale and the continuum mechanics, we established a universal and analytic thermodynamic model to elucidate the surface energy of nanocrystals. It was found that the surface energy dec...
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Combining the thermodynamics at the nanometer scale and the continuum mechanics, we established a universal and analytic thermodynamic model to elucidate the surface energy of nanocrystals. It was found that the surface energy decreases with decreasing the size of nanocrystals. The theoretical predictions were well consistent with the experimental data, implying that the thermodynamic model could be expected to be a general approach to understand surface energy in nanomaterials.
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