Relationship between gas transport properties and fractional free volume determined from dielectric constant in polyimide films containing the hexafluoroisopropylidene group

The dielectric constant and gas transport properties (i.e., permeability, diffusivity, and solubility) in 2,2′‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)‐based polyimides were systematically investigated in terms of their polymer fractional free volumes (FFVs) at 30°C. The permeability and diffusion coefficients of the 6FDA‐based polyimide films to hydrogen, oxygen, nitrogen, methane, and carbon dioxide were correlated with their FFVs estimated using van Krevelen's group contribution method. There appeared, however, small linear correlation coefficients. Linear correlations were also observed between the gas transport properties and dielectric constant of these polyimides. This study described FFVas a function of the dielectric constant based on the Clausius‐Mossotti equation. It was found that the gas permeability and diffusion coefficients of these 6FDA‐based polyimide films increased as their dielectric constant‐based FFV increased. A better linear relationship was observed between the gas transport properties and the FFV determined from the polymer dielectric constant in comparison to that estimated using the group contribution method. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

On the Role of Water in Dielectric Heating with Radio Waves

Beside the classical mechanism of orientation polarization which is sufficient to explain dielectric heating of aqueous liquids, a number of additional aspects have to be considered when humid solids are heated by microwaves or radio waves. The large relative dielectric constant of water can lead to shielding and, therefore, attenuation of the electrical field in humid matrices. The shielding effect is influenced by the geometric arrangement of compartments with varying humidity. On the other hand, the large dielectric loss of water results in an effective heating. As shown by modeling of appropriate experiments, the dielectric loss factor is significantly changed for water which is in intimate contact with surfaces. The dielectric loss factor is much larger for water in the surface layers compared with the bulk phase. For some solids dielectric heating is based on a completely different mechanism which is also influenced by water. The dominant mechanism leading to dielectric heating of zeolites is the migration of cations within the zeolite framework. Water has a strong effect on the dielectric loss factor, most likely by hydration or modifying the electrostatic interactions of cations with the lattice.     

The removal of dichloromethane from atmospheric pressure air streams using plasma-assisted catalysis

Plasma-assisted catalysis was used for the destruction of 500 ppm of dichloromethane, CH₂Cl₂ (DCM), in gas streams of air using a non-thermal, atmospheric pressure plasma utilising a dielectric packed bed. The combination of plasma and catalyst gave improved destruction of DCM. Eight catalysts which including alumina, TiO₂ and various zeolites were investigated with the finding that alumina in a one-stage reactor configuration and TiO₂ and HZSM-5 in two-stage configurations gave the best DCM destructions. The sodium zeolites are capable of reducing by 50% the unwanted NO_x by-products, formed by plasma processing in air. The nature of the catalyst is important in terms of the destruction efficiency, end-product selectivity and NO_x reduction.     

分子筛催化γ-丁内酯气相胺化合成N-乙烯吡咯烷酮单体

研究了ZSM-5、NaY分子筛对γ-丁内酯与乙醇胺气相反应的催化性能,结果表明,ZSM-5与Y型分子筛的初始产物收率分别为29·8%和43·0%以上。反应产物收率随温度升高而增大,然后趋于平缓,当超过280℃以后呈降低趋势;选择性在较低温度时变化不明显,温度较高时明显降低,合适的反应温度为260~270℃。再生性实验表明,两种分子筛的失活都是由于水蒸气使其部分结构遭到破坏而引起的,是不可恢复的失活。     

Linear dielectric thermodynamics: A new universal law for optical,dielectric constants

The thermodynamics of linear dielectric are formally developed to explore the isothermal and adiabatic temperature-pressure dependence of dielectric constants. The refractive index of optical materials is widely measured in the literature: it is both temperature and pressure dependent. The argument to establish the dielectric constant's isentropic temperature dependence is a thermodynamic one and is thus applicable to all physical models that describe electron clouds and electronic resonances within materials. The isentropic slope of the displacement field vs the electric field at all temperatures is described by an adiabatic dielectric constant in an energy-per-unit mass system. This slope is shown through the electronic part of the entropy to be unstable at high temperatures due to the change in the curvature of the temperature dependence of the dielectric constant. The electronic entropy contribution for optical, thermo-electro materials has negative heat capacities which are unacceptable. The dielectric constant's temperature and pressure dependence is predicted to be only dependent on the specific volume so isentropes are always positive. A new universal form for the dielectric constant follows from this hypothesis: the dielectric constant is proportional to the square root of the specific volume for fully dense solids.     

Adsorption of Methane,Ethane, Ethylene,and Carbon Dioxide on X,Y, L,and M Zeolites Using a Gas Chromatography Pulse Technique

Abstract

Sorption of methane, ethane, ethylene and carbon dioxide in NaX, NaY, HY, CeNaY, KL, HKL, NaM, and HM zeolites at 303–473 K has been investigated using a gas chromatography pulse technique. The zeolites have been compared for the heat of sorption of the sorbates at near-zero sorbate loading and also for the specific retention volume (or thermodynamic sorption equilibrium constant) of ethane, ethylene, and carbon dioxide relative to that of methane.     

Structure and catalytic properties of thermally and hydrothermally treated zeolites: Acid strength distribution of REX and REY

Acidity distributions of REX and REY zeolites have been determined after pretreatment under various conditions. These acidities are correlated with the catalytic performance of the zeolites in gas oil cracking. It was found that hydrothermal treatment of the zeolites leads to a decreased number of strong acid sites, while the number of sites with intermediate and weak acid strength remain unchanged.The disappearance of the strong acid sites results in an increased cracking selectivity, especially for the REY type zeolites. This study shows that the selectivity in catalytic cracking for gasoline, gas and coke production is very much dependent on the total number of acid sites as also on the acidity distribution in the catalyst.     

Disproportionation of toluene over a metal-supported type L-zeolite catalyst

The disproportionation of toluene was studied using KHL zeolite, with and without metal supports. Incorporation of Ni and Pt to KHL increases significantly the surface acidity and catalytic activity. Their change in surface acidity due to metal-zeolite interactions predominantly influences disproportionation activity. The performance of metal-loaded zeolites is also better than that of KHL in terms of initial activity and stability. The effects of the metal, temperature and the carrier gas have been investigated. The bifunctional nature of metal zeolites as represented by the hydrogenation activity of the metallic component and disproportionation activity of acidic zeolites has been established.     

Dielectric properties and electromagnetic wave transmission performance of polycrystalline mullite fiberboard at 2.45 GHz

Refractory lining is an indispensable part of high temperature microwave heating equipment, and its wave transmission performance exerts an important impact on the mode and efficiency of microwave heating, while the complex dielectric constant (dielectric constant and dielectric loss) of the material is the decisive factor in determining the wave transmission performance of the material. In this work, we measured the complex dielectric constant of polycrystalline mullite fiber board (PMF) in the temperature range of 25–1000 °C, and the effect of temperature on dielectric constant and dielectric loss was analyzed; The wave-transmission properties of the material were calculated according to the theory of electromagnetic wave transmission line, and the effects of temperature, material thickness, polarization modes of electromagnetic wave and incident angle on the wave transmission performance were analyzed. The results reveal that the dielectric constant of PMF does not change much with the increase of temperature, which is about 1.6; The dielectric loss does not change much within 200 °C, but when the temperature is higher than 200 °C, the change presents approximately exponential increase with the rise of temperature. The wave transmission performance fluctuates with the increase of the thickness, and there are maximum value and minimum value, and the overall wave transmission performance decreases with the increase of the material thickness. In a transverse electric (TE) field, the overall wave transmission performance decreases with the increase of the incident angle, and better wave transmission performance can be obtained by priority selection of vertical incidence of electromagnetic wave. In a transverse magnetic (TM) field, with the increase in the incident angle, the wave transmission performance firstly climbs up then declines, and there is an optimal incident angle where total transmission can occur. Finally, this work selected the thickness corresponding to different temperature as the preferred thickness. This work is of important theoretical significance for understanding the mechanism of the dynamic change of the wave transmission performance of the thermal insulation materials in microwave heating, and provides important practical guidance for the design and optimization of microwave heating equipment.     

偏压下弛豫型铁电陶瓷有效介电系数稳定方法的研究

本研究利用调整有效介电常数的互补公式     

Polyaniline/ethylene vinyl acetate composites as dielectric sensor

The compressive stress (pressure) sensitivity of dielectric properties has been studied on ethylene vinyl acetate (EVA)/polyaniline (Pani) composites prepared through In‐situ synthesis of polyaniline in the solution of insulating EVA matrix. It is observed that the dielectric constant and loss increase with the increase in applied pressure, that is some piezoelectric effect is observed for these composites. The dielectric properties are also found to increase with respect to time when subjected under constant pressure. It is seen that changes in dielectric constant and loss follow some exponential relationships with respect to applied pressure and time duration under constant stress, and the relaxation time for the composites can be calculated. The relaxation time decreases with the increase in concentration of Pani in a composite. However, a composite with lower Pani content exhibits relatively higher change in dielectric properties against applied pressure and time duration under compression compared to one with higher loading. Granular crew type morphology of Pani is observed through scanning electron microscopic (SEM) study. This study reveals that these EVA‐Pani composites can be used as dielectric sensor. POLYM. ENG. SCI., 54:1632–1639, 2014. © 2013 Society of Plastics Engineers     

Evolution of the intergranular phase during sintering of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics have been processed by solid state reaction and sintered at 1100 °C for different times. A clear increase of the dielectric constant of the material up to values of 6 × 10⁴ has been observed with the sintering time. This increase is accompanied by a limited grain growth and intergranular Cu-oxide phase thickness reduction. The disappearance of the Cu-oxide phase is caused by the incorporation of Cu cations into the grains, contributing to the increase of the dielectric constant. Raman spectroscopy shows the decrease of TiO₆ octahedral rotational modes with the sintering time due to the incorporation of Cu cations into the CCTO grains. XANES measurements show that the Cu main oxidation state is Cu²⁺ and does not change with the sintering time. The fitting of the experimental dielectric constant to the Internal Barrier Layer Capacitance (IBLC) model reveals the change of the intergranular phase dielectric constant, caused by a compositional change due to the incorporation of Cu into the CCTO grains.     

Changes in physical and chemical properties of shortenings used for commercial deep-fat frying

This study evaluated some of the changes that occur in shortenings used for commercial deep-fat frying in fast-service restaurants. Foods cooked in partially hydrogenated soybean oil were battered chicken parts and french fries. Sixty-five samples of fresh and used shortenings were collected from nine restaurants on three occasions over a three-month period. Frying periods varied from 0 to 300 hr, and most samples were taken just before the used fat was discarded. For fresh shortenings, percentages of polar materials, free fatty acids (FFA), materials not eluted by gas chromatography, and fatty acid profiles differed only slightly. For used samples, there were marked variations in these analyses and in increases of dielectric constant measurements. Frying times were highly correlated with increases in dielectric constant, polar materials and FFA. The greatest change in fatty acid profiles occurred intrans-C18 monoenes which decreased from over 40% to as low as 13%. Due to lipid exchange with chicken fat, both oleic and linoleic acids increased in the shortenings with hours of use, whereas stearic acid decreased. There were high correlations, among increases in dielectric constant, percentages of polar materials and FFA, demonstrating that each of these methods could predict degradation of the shortening. However, the increase in dielectric constant, as measured by a Foodoil Sensor (FOS), was the most convenient for quality control in restaurant situations. In most cases, used shortening was discarded before 100 hr of frying time; and only a few of these samples had FOS readings near 4.0, FFA over 1.00%, or percentages of polar materials over 27%. These values have been suggested as discard criteria. However, a number of samples used between 100 and 300 hr exceeded these limits. There is a need to specify suitable limits, related to quality and health factors, to determine at what point a cooking fat should be discarded.     

Dielectric properties of styrene — butadiene rubber/silica mixtures

Rice husk ash (RHA) obtained from rice mill and hydrated silica from RHA were used as a filler in vulcanized SBR 1502 and the dielectric properties were measured at a frequency of 1592 Hz at room temperature. The optimum hydrated silica content giving a good dielectric constant and conductivity was 125 parts/100 parts rubber and the dielectric loss was also high so that it could be a good insulater. There was no significant change in dielectric constant, dielectric loss and conductivity of SBR 1502 filled with RHA which could be used as high frequency dielectric due to low dielectric loss.     

Y3+替代Bi3+对Bi2(Zn1/3Nb2/3)2O7陶瓷显微结构和介电性能的影响

本文研究了Y3 替代Bi3 对Bi2O3-ZnO-Nb2O5系介质材料结构和性能的影响,并借助X射线、扫描电镜、LCR4284测试仪对其相结构和介电性能进行分析。研究结果表明,随着Y3 替代量的增加,Bi2O3-ZnO-Nb2O5系介质材料的晶粒尺寸、介电常数、介电损耗都有所变化,当替代量x=0.2时,介电性最佳,介电常数为79.4094,介电损耗为0.002125。     

正构烷烃在介孔5A沸石中的吸附

分别选取具有不同碳原子数的正庚烷、正辛烷和正癸烷为吸附质,微孔及具有不同晶内介孔孔隙度的介孔5A沸石为吸附剂,通过重量吸附法测定了3种正构烷烃在293、308和318 K 3个温度下在微孔和介孔5A沸石上的吸附等温线,并从热力学的角度对其进行了分析。Langmuir、Langmuir-Freundlich和Toth 3种吸附模型对从实验获得的吸附数据的拟合结果表明,不同于烷烃分子在微孔5A沸石内的吸附,3种烷烃在介孔5A沸石上的吸附可用Langmuir-Freundlich和Toth两种吸附模型得到较好的拟合,在同一温度下,3种正构烷烃在介孔5A沸石内的最大吸附量均大于各自在微孔5A内的最大吸附量,但吸附质与吸附剂相互作用力随中孔的引入而减小,并由于微介孔吸附的共存,使得表面吸附多相性增加;另外,基于吸附数据计算获得的烷烃分子在沸石表面的吸附亨利常数和初始吸附热数据表明,沸石晶内介孔的存在,使其亨利常数和初始吸附热均减小;另外,由Clausius-Clapeyron方程计算得到的等量吸附热随表面吸附量变化的趋势表明,3种正构烷烃在介孔5A沸石内吸附热均随吸附量的增加而减小,其中以癸烷的变化最为明显。     

Characterization of small- and intermediate-pore zeolites by means of temperature-programmed desorption of amines

This paper presents results of temperature-programmed desorption (TDP) as a method for studying the acid and shape-selective properties of small- and intermediate-pore zeolites. A combination of an in situ infrared spectrometer and a downstream gas chromatograph for analyzing the desorption products is used to detect the desorption process. This is followed by a separate, newly installed microbalance facility. The studies shows that a part of the amines, esp. dimethylamine (DMA) and trimethylamine (TMA), adsorbed on the zeolites undergo decomposition, disproportionation and carbonization reaction during the TDP measurements at elevated temperatures. Comparison of the microbalance measurements and the gas chromatographic results give a good picture of the extent of these reaction.     

Adsorption of Methane,Ethane, Ethylene,and Carbon Dioxide on High Silica Pentasil Zeolites and Zeolite-like Materials Using Gas Chromatography Pulse Technique

Abstract

Adsorption of methane, ethane, ethylene, and carbon dioxide in H-ZSM-5, Na-ZSM-5, H-ZSM-8, Na-ZSM-8, Silicalite, and ALPO-5 at 303–473 K has been investigated using a gas chromatography pulse technique. The zeolites have been compared for the heat of adsorption of the adsorbates at near zero adsorbate loading and also for the specific retention volume (or thermodynamic adsorption equilibrium constant) of ethane, ethylene, and carbon dioxide relative to that of methane. Among the zeolites, ALPO-5 has a high potential for the separation of methane, ethane, ethylene, and carbon dioxide from their mixture.     

改性钛酸锶/PVDF复合材料介电性能研究

采用具有强络合能力的酒石酸在碱性溶液中对SrTiO3陶瓷颗粒进行表面改性,将改性后的SrTiO3颗粒与聚偏二氟乙烯(PVDF)经热压共混成型,制备出系列陶瓷/聚合物基复合材料,对改性SrTiO3/PVDF复合材料进行了介电性能分析。结果表明:添加了改性SrTiO3的复合材料比未改性SrTiO3复合材料的介电常数增加值达34%以上,同时,改性复合材料的介电损耗仍保持较低水平;随着改性SrTiO3在复合材料中含量的增加,介电常数也随之增加,介电损耗仍保持不变,改性后的陶瓷/聚合物复合材料表现出优异的综合介电性能。     

锑替代铋对铋锌铌陶瓷介电性能的影响

研究了Sb3＋替代Bi3＋对Bi2O3-ZnO-Nb2O5（BZN）系介质材料结构和性能的影响,并借助X射线、扫描电镜、LCR4284测试仪对其相结构和介电性能进行分析。研究结果表明,经Sb3＋替代的BZN陶瓷样品成瓷温度仍为1 000℃;随着Sb3＋替代量的增加,Bi2O3-ZnO-Nb2O5系介质材料的晶粒尺寸、介电常数、介电损耗都有所变化。当替代量x=0.4时,介电性能最佳,介电常数为184,介电损耗为0.001。