Synthesis and thermal oxidative degradation of a novel amorphous polyamide/nanoclay nanocomposite

A novel amorphous polyamide/montmorillonite nanocomposite based on poly(hexamethylene isophthalamide) was successfully prepared by melt intercalation. Wide angle X-ray diffraction and transmission electron microscopy showed that organoclay containing quaternary amine surfactant with phenyl groups was delaminated in the polymer matrix, resulting in well-dispersed morphologies even at high montmorillonite content. Thermal oxidation behavior of the polymer nanocomposites was studied by thermogravimetric analysis (TGA), and the chemical evolution in the solid residue was monitored by elemental analysis and Fourier transform infrared spectroscopy (FTIR). TGA results showed that the addition of well-dispersed organoclay resulted in a substantial increase (30 °C) in the onset degradation temperature of the nanocomposites as compared to the homopolymer. Elemental analysis on the solid residue indicated that the presence of nanoclay resulted in char formation with greater thermal stability. FTIR spectra showed that thermal degradation in air occurred via both oxidative and non-oxidative mechanisms simultaneously. In the homopolymer, the oxidative mechanism was more dominant. However, with the addition of well-dispersed organoclay, the non-oxidative pathway became more significant. Hence the presence of delaminated nanoclay layers could effectively retard thermo-oxidative degradation of the amorphous polymer by constraining the polymer chains and slowing down the rate of oxygen diffusion through the nanocomposites, but it was not as effective in hindering the non-oxidative degradation reaction pathway.     

Effect of heat-treatment and hydrostatic loading upon the poro-elastic properties of a mortar

This contribution aims at identifying experimentally the poro-elastic properties of a cement-based material under different levels of confining pressure, and after a heat-treatment up to 400 °C. The model material used is a normalized mortar, with a (W/C) ratio of 0.5. After a given heating/cooling cycle, drained bulk modulus K_b, solid matrix bulk modulus K_s and Biot's coefficient b are measured at different confining pressure levels (with a maximum of 25 MPa).Results show that under drained conditions, mortar stress-strain relationship evolves with increasing heat-treatment temperature from linear elastic with brittle failure (up to 105 °C heat treatment) to plastic and ductile (from 200 °C and above). Plastification testifies of material degradation under gradual confining pressure. At the microstructure scale, this is attributed to thermal damage after heat treatment above 105 °C, which consists mainly in various micro-cracking. This leads to easier failure of solid skeleton bridges (or trabecules), and to pore network collapse. Concomitantly to this, at given confining pressure P_c, secant drained bulk modulus K_b decreases monotonously, for heat-treatment temperatures above 105 °C. On the opposite, at given heat-treatment temperature above 105 °C, secant drained bulk modulus K_b increases when confining pressure is increased. This testifies of a solid matrix rigidification in the elastic domain, and it is attributed to increased skeleton compactness linked with pore network collapse. This is directly attributable to heat treatment followed by confinement.At given confining pressure P_c, matrix bulk modulus K_s and Biot's coefficient b increase with heat-treatment temperature above 105 °C. The increase in b means that mortar becomes less and less cohesive and more and more of a granular nature. Moreover, Biot's coefficient b and solid matrix bulk modulus K_s are independent of confining pressure P_c for intact mortar, whereas they decrease for heat-treated mortars when P_c increases. From literature analysis alone, it was quite unexpected that after heat treatment, K_s should vary under confinement. This is interpreted as the closure, under confining pressure, of micro-void connections and of micro-cracks created by heat-treatment. Therefore, increasing confinement induces more and more occluded pores in the solid matrix, whereby K_s diminishes.     

Analysis of pulsed microwave processing of polymer slabs supported with ceramic plates

A detailed theoretical analysis has been carried out to study the role of ceramic supports (alumina and SiC) and pulsed microwave heating of polymer (Natural Rubber, NR, and Nylon 66) slabs due to various distributions of microwave incidences. Ceramic plates are typical representations as they withstand high temperature without any deformation. It is found that ceramic plates influence the heating processes significantly and local hot spots within samples are governed by specific type of ceramic plates for various sample thicknesses and distributions of microwave incidence (one side or both sides). Optimized pulsing of microwave incidence has been employed to minimize the thermal runaway or hot spots in order to achieve uniform temperature distribution and pulsing is introduced based on two parameters: setpoint (ΔT_S) and on-off constraint (T^′). Detailed spatial distributions of power and temperature are illustrated for a few representative length scales to demonstrate the role of local maxima in power and temperature on heating rate as well as thermal runaway with or without pulsing. Pulsing ratio (PR) has been defined as PR=t_off/t_p, where t_off is power-off time and t_p is the total processing time such that smaller PR denotes large processing rates. It is found that one side incidence gives smaller values of PR for both the ceramic plates whereas SiC plate may be suitable for both sides incidence with large sample thicknesses of NR samples. It is also found that larger values of setpoints also minimize PR. The setpoints along with the on-off constraint play critical role to select the heating strategy as a function of ceramic plates and types of incidence. Pulsing may not be important for smaller thicknesses of Nylon samples and SiC or alumina plates may be recommended for processing larger thicknesses of Nylon samples in presence of pulsing. Current study recommends the efficient microwave heating methodologies for polymer processing attached with ceramic plates by means of optimized pulsing for the first time.     

Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes

Blends of a long-chain branched polypropylene (LCB-PP) and four linear polypropylenes (L-PP) having different molecular weights were prepared using a twin screw extruder. The linear viscoelastic properties suggested the immiscibility of the high molecular weight L-PP based blends, and the miscibility of the low molecular weight L-PP based blends. In addition, the Palierne emulsion model showed good predictions of the linear viscoelastic properties for both miscible and immiscible PP blends. However, as expected, the low-frequency results showed a clear effect of the interfacial tension on the elastic modulus of the blends for the high molecular weight L-PP based blends. A successful application of time-temperature superposition (TTS) was found for the blends and neat components. Uniaxial elongational properties were obtained using a SER unit mounted on an ARES rheometer. A significant strain hardening was observed for the neat LCB-PP as well as for all the blends. The influence of adding LCB-PP on the crystallinity, crystallization temperature, melting point, and rate of crystallization were studied using differential scanning calorimetry (DSC). It was found that the melting point and degree of crystallinity of the blends first increased by adding up to 20 wt% of the branched component but decreased by further addition. Adding a small amount of LCB-PP caused significant increase of the crystallization temperature while no dramatic changes were observed for blends containing 10 wt% LCB-PP and more. Furthermore, the crystalline morphology during and after crystallization of the various samples was monitored using polarized optical microscopy (POM). Compared to the neat linear polymers, finer and numerous spherulites were observed for the blends and LCB-PP. Dynamic mechanical (DMA) data of the blends and pure components were also analyzed and positive deviations from the Fox equation for the glass transition temperature, T_g, were observed for the blends.     

汽轮机转子热疲劳的计算方法

本文介绍了转子热疲劳计算方法,从理论上推导了计算额定弹性应力和应力集中系数的公式,阐明了利用通用周期应变特性和短时蠕变特性计算转子热疲劳寿命的步骤。该方法对苏制200、300和500MW机组的寿命计算和寿命管理提供了比较适当的公式。     

Effect of external bias voltage and coating thickness on the photocatalytic activity of thermal sprayed TiO2 coating

TiO₂ coatings of different thickness were prepared by a thermal spray process. It was found that the external bias applied to the as-sprayed TiO₂ coating could significantly improve its photocatalytic performance, which was characterized by decomposition of methylene blue (MB) solution. The decomposition efficiency increased with increasing external bias voltage. However, for voltage greater than 15 V, the decomposition efficiency remained constant. The TiO₂ coating with a thickness of 6 μm showed the best photocatalytic performance under an external bias voltage of 15 V.     

HEAT TRANSFER ANALYSIS FOR A VAPOR BUBBLE

In this work, we review the basic heat-transfer mechanisms of a vapor bubble immersed in an unbounded and subcooled liquid. The governing equations for the vapor and liquid phases, together with appropriate initial and boundary conditions, are presented in dimensionless form. In particular, if the characteristic inertial times are smaller than the characteristic process times, the evolution for the radius of the bubble depends basically on the Jakob number, Ja, and the vapor densities ratio, ρ. Recognizing that the Jakob number is very large compared with unity, the resulting governing equations are modeled by a thermal boundary layer, adjacent to the radius of the bubble. We show that higher order corrections due to curvature effects, associated with conductive and convective heat terms of the energy equation for the liquid, characterize the collapse of the bubble. For different values of Ja and ρ, the numerical results for the evolution of the nondimensional radius of the bubble,a, and for the corresponding nondimensional temperature profiles, θ, show that the ending collapse state has a singular behavior, which we have denoted as a “thermal runaway.”     

Effect of poly(ethylene glycol) additives on the photocatalytic activity of TiO2 films prepared by sol–gel processing and low temperature treatments

Photocatalytic activity of titania with poly(ethylene glycol) (PEG) in the sample films made under different operating conditions was investigated by kinetic analysis of photodegradation tests. The sample films, composed of PEG and nano-TiO₂ particles, were prepared by sol–gel processing and then treated thermally under an atmosphere of wet and dry air at different temperatures. After the thermal treatment, photocatalytic activities of the films were evaluated by a UV-exposure test. Results showed that the photoactivity was enhanced by processing in an atmosphere of wet air at 100 °C. Moreover, the presence of poly(ethylene glycol), and the change in surface morphology in the sample films were verified to be the most influential and significant factors to affect the photoactivitic activity.     

Comparative study of structural properties and NOx storage-reduction behavior of Pt/Ba/CeO2 and Pt/Ba/Al2O3

Differences in the NO_x storage-reduction (NSR) behavior of Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO₂ exhibits inferior NO_x storage and, particularly, reduction (regeneration) activity compared to the Al₂O₃ supported catalyst. The incomplete reduction of the stored NO_x-species in Pt/Ba/CeO₂ seems to be caused by a faster and more profound reoxidation of Pt particles during the lean period as evidenced by in situ X-ray absorption spectroscopy. Interestingly, the reduction activity could be significantly improved by a pre-reduction step at mild conditions. Exposure of the Pt/Ba/CeO₂ catalyst to reducing H₂ atmosphere in the temperature range 300–500 °C lead to a moderate increase of Pt particle size which beneficially influenced the regeneration activity. In contrast, pre-reduction at temperatures above 500 °C was unfavorable and resulted in a severe decrease of the regeneration activity, probably due to migration of the partially reduced CeO₂ onto the surface of Pt particles.     

桦甸页岩油热加工生产燃料油的研究

对桦甸页岩油重油馏分(＞360 ℃)进行热加工实验,研究了桦甸页岩油重馏分热加工反应的影响因素,并分析了裂解油的基本性质.结果表明:随着反应时间的延长、反应温度的提高,裂解油收率逐渐降低,焦炭收率和气体及损失率逐渐升高;当反应温度为395 ℃、反应时间为2 h时,裂解油的密度、凝点、闪点、运动粘度、残炭、灰分分别为0.875 g/mL、21.50 ℃、64 ℃、19.90 mm2/s、0.400、0.047,裂解油的质量已经达到了20#重柴油的使用要求.