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1.
In this work, mass-transfer coefficients of a cross-corrugated plate have been determined by the limiting-current technique. A simple Microsoft Excel spreadsheet has been created, which allows students to correlate data. Students will be able to calculate the mass-transfer coefficient of different processes, for example, cross-flow membrane filtration processes or membrane reactors. The possibility of using the spreadsheet with different correlations and to discriminate between them is also validated by comparing the model results with published experimental data available in the recent literature. It is possible as well to study the influence of different turbulence promoters on the flow. 相似文献
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PO Bussiere S Therias JL Gardette M Murariu P Dubois M Baba 《Physical chemistry chemical physics : PCCP》2012,14(35):12301-12308
The crystallization of PLA-silane surface-treated ZnO nanocomposites was investigated by DSC and compared to that of neat PLA. Several modes of crystallization were considered: isothermal and non-isothermal cold crystallization and also isothermal and non-isothermal melt crystallization. The kinetics of cold crystallization were studied using different methods, namely the Avrami and Ozawa-Flynn-Wall models, to calculate activation energies and kinetic constants. In contrast to what is typically observed when the foreign particles are added in a polymer matrix, the silane surface-treated ZnO delayed the crystallization of PLA and made it more difficult to start. The nucleation activity of the ZnO nanoparticles, ?, was calculated and found to be greater than 1 (? = 1.7). This indicated that ZnO played an anti-nucleating role in the crystallization of PLA nanocomposites. This effect has been linked mainly to the interactions between the silane groups onto the surface of nanoparticles and PLA macromolecules. These interactions which reduce the mobility of polymer chains have been evidenced by rheological experiments. 相似文献
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低密度聚乙烯/乙丙烯三元共聚(LDPE/EPO)共混体系的结晶动力学 总被引:2,自引:0,他引:2
用DSC方法研究了LDPE/EPO共混体系的等温及非等温结晶动力学,对LDPE/EPO共混体系的等温结晶动力学研究表明,共混物是三维生长的异相成核,共混物在各个结晶温度下的结晶过程都是以方式K_g(Ⅱ)进行的.采用联系Avrami方程和Ozawa方程导出的新非等温结晶动力学方程,处理了LDPE/EPO共混体系,得到了非等温结晶过程的一些基本参数,新方程很好地描述了此共混体系的非等温结晶动力学过程. 相似文献
4.
Adam Al‐Mulla Johnson Mathew Robert Shanks 《Journal of Polymer Science.Polymer Physics》2007,45(11):1344-1353
Different crystallization kinetic models (Avrami and Tobin) have been applied to study the crystallization kinetics of virgin poly(butylene terephthalate) (PBT) and filled PBT systems under isothermal experimental conditions. The experimental data have been analyzed with a nonlinear, multivariable regression program. The kinetic parameters for the isothermal crystallization have been determined. The analysis results indicate that both models satisfactorily represent the isothermal crystallization kinetics. PBT crystallizes most slowly. The presence of nanoclays or nanofibers, added as fillers, enhances the crystallization rate of PBT composites. An analysis of the kinetic data with the Avrami and Tobin models has shown little change in the crystallization exponent compared with that of virgin PBT. The crystallization rate constant decreases with a rise in the temperature for the two models. This trend has been observed for similar polyester systems reported in the literature. The dispersion of the clay layers in the PBT nanocomposites has been characterized with wide‐angle X‐ray diffraction and transmission electron microscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1344–1353, 2007 相似文献
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V. A. Alvarez P. M. Stefani A. Vázquez 《Journal of Thermal Analysis and Calorimetry》2005,79(1):187-193
Non-isothermal crystallization of polyvinylalcohol-co-ethylene with different ethylene contents was studied. Several models were used to predict the crystallization behavior of these materials under non-isothermal conditions at a constant cooling rate. Kinetic parameters determined from isothermal date were employed. Experimental data were in accordance with model prediction at low cooling rate and relative degree of crystallization lower than 0.8, but it did not fit at high cooling rate. Kinetic parameters obtained by using a non-linear regression method, i.e, Kamals model and Dietzs modification, were able to describe better the non-isothermal crystallization behavior of the studied materials. The full model, that takes into account the induction and growth of the crystal during cooling under non-isothermal conditions was used to obtain a continuous cooling transformation diagrams for polyvinylalcohol -co-ethylene. Finally, non-isothermal models, coupled with the proposed expressions for induction time and kinetic constant, were used to represent the development of crystallinity during the processing of the polymer. 相似文献
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A general theoretical approach of the overall crystallization kinetics of thin polymer films is developed. This new model makes it possible to calculate the evolution of the transformed volume fraction anywhere in the film, whatever the cooling conditions are. In its isothermal limit this model is equivalent to previous approaches which have been well verified by a computer simulation. In conclusion, it is pointed out that both isothermal and anisothermal determinations of crystallization kinetic parameters are greatly dependent on the sample thickness. 相似文献
8.
Different
methods have been used by various workers to determine the activation energy
of thermal crystallization (Ec)
in chalcogenide glasses using non-isothermal DSC data. In the present work,
the crystallization kinetics of two important binary alloys Se80Te20
and Se80In20 is studied using
non-isothermal DSC data. DSC scans of these alloys have been taken at five
different heating rates. The values of activation energy of crystallization
(Ec) have been determined
by four different methods, i.e., Kissinger's method, Matusita-Sakka
method, Augis-Bennett's method and Ozawa's method, have
been used to calculate Ec.
The results obtained have been compared with each other to see the effect
of using different methods in the determination of Ec. 相似文献
9.
P. J. Sánchez-Soto M. Macías M. A. Avilés G. Colón J. A. Navío 《Journal of Sol-Gel Science and Technology》1994,2(1-3):353-357
Powder precursor gels with composition xZrO2·(100–x)SiO2, with selected values of x=8, 24, 43 and 75 mol%, were processed by sol-gel chemistry. Differential thermal analysis (DTA) was used to study crystallization in (cubic/tetragonal)-ZrO2 during the heating of the reactive amorphous precursors. Kinetic parameters such as activation energy, Avrami's exponent and frequency factor have been simultaneously calculated from the computed DTA data using a previously reported kinetic model. The crystallization temperature decreases relative to the increase in the amount of ZrO2, the value of the kinetic parameter of the crystallization being related to the value of x. 相似文献
10.
Syndiotactic E-1,2-poly(3-methyl-1,3-pentadiene) was synthesized with the catalyst system Fe(bipy)2Cl2-MAO. The thermal stability and kinetic parameters of degradation were determined by thermogravimetric analysis. The isothermal crystallization kinetics were described by means of the Avrami equation, which suggested a three-dimensional growth of crystalline units, developed by heterogeneous nucleation, followed by a secondary crystallization stage. Syndiotactic E-1,2-poly(3-methyl-1,3-pentadiene) isothermally crystallizes from the melt according to regime II of crystallization described by Lauritzen-Hoffman secondary nucleation theory. Non-isothermal crystallization kinetics were elaborated using different approaches. The equilibrium melting temperature was calculated. The kinetic and thermodynamic data were compared with those obtained from syndiotactic 1,2-poly(1,3-butadiene), which is the first example of 1,2 polydienes family. 相似文献
11.
Supriya Kasyap Ashmi T. Patel Arun Pratap 《Journal of Thermal Analysis and Calorimetry》2014,116(3):1325-1336
The study of crystallization kinetics of amorphous alloys has been a matter of great interest for material researchers for past few decades, since it provides information about the kinetic parameters i.e., activation energy of crystallization and the frequency factor. These kinetic parameters can be calculated by model-free isoconversional methods. Isoconversional methods allow calculating the activation energy as a function of degree of conversion, α. Hence, these methods provide accurate results for multistep processes like crystallization. Model-free methods are categorized as linear and non-linear isoconversional methods. Linear methods are further classified as linear differential and linear integral isoconversional methods. In present work, we have used these isoconversional methods to study the effect of non-linear heating rate, employed by modulated differential scanning calorimetry (MDSC), on the non-isothermal crystallization kinetics of Ti20Zr20Cu60 metallic glass. For Ti20Zr20Cu60, MDSC curves clearly indicate a two-step crystallization process. Both crystallization peaks were studied based on the modified expressions for isoconversional methods by non-linear heating rate. The term corresponding to non-linearity comes out to be (A T ω/2β)2. The effect of non-linear heating rate on measurement of kinetic parameters by isoconversional methods is studied. The activation energy of crystallization is calculated for Ti20Zr20Cu60 metallic glass for various degrees of conversion by linear integral isoconversional methods i.e., Ozawa–Flynn–Wall, Kissinger–Akahira–Sunose, and also with Friedman method which is a linear differential isoconversional method. 相似文献
12.
This review presents a number of mathematical expressions which the experimental scientist may find useful to fit to time-dependent luminescence intensity (and concentration) data for both decay and rise situations. Some of these expressions have been drawn from literature sources and are presented here in a unified form. In compiling these expressions, a number of gaps were found in the literature, and, in this review, new expressions have been derived to fill some of these gaps. The final section of the review summarizes both the existing and the new kinetic expressions and provides an explanation of how to use them. Some of the subtleties and pitfalls of using kinetic data fitting to determine reaction mechanisms are also discussed. 相似文献
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Silva D. V. Ribeiro C. A. Crespi M. S. 《Journal of Thermal Analysis and Calorimetry》2003,72(1):151-157
Fluoride glasses have been extensively studied due to their high transparency in the infrared wavelength. The crystallization
kinetics of these systems has been studied using DTA and DSC techniques. Most of the experimental data is frequently investigated
in terms of the Johnson-Mehl-Avrami (JMA) model in order to obtain kinetic parameters. In this work, DSC technique has been
used to study the crystallization of fluorozirconate glass under non-isothermal conditions. It was found that JMA model was
not fit to be applied directly to these systems, therefore, the method proposed by Málek has been applied and the Šesták-Berggren
(SB) model seems to be adequate to describe the crystallization process.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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Minqiao Ren Jianbin Song Chunlei Song Huiliang Zhang Xiaohong Sun Qingyong Chen Hongfang Zhang Zhishen Mo 《Journal of Polymer Science.Polymer Physics》2005,43(22):3231-3241
The crystallization kinetics of biodegradable poly(butylene succinate‐co‐adipate) (PBS/A) copolyester was investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The Avrami and Ozawa equations were used to analyze the isothermal and nonisothermal crystallization kinetics, respectively. By using wide‐angle X‐ray diffraction (WAXD), PBS/A was identified to have the same crystal structure with that of PBS. The spherulitic growth rates of PBS/A measured in isothermal conditions are very well comparable with those measured by nonisothermal procedures (cooling rates ranged from 0.5 to 15 °C/min). The kinetic data were examined with the Hoffman–Lauritzen nucleation theory. The observed spherulites of PBS/A with different shapes and textures strongly depend on the crystallization temperatures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3231–3241, 2005 相似文献
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采用动力学法研究了富硼浓缩盐卤稀释过程硼酸镁盐的结晶动力学,重点探讨了温度、稀释比和硼浓度对结晶过程的影响。利用单纯形优化法配合Runge-Kutta微分方程组数值解法对实验数据进行拟合,给出了结晶动力学方程和结晶速率。结果表明,低温、高硼浓度和中间稀释比有利于硼酸镁盐的结晶析出,最优条件下析硼率(以B_2O_3计)高达88%;结晶速率随硼浓度的增加和温度的降低快速增大;反应级数表明稀释结晶过程硼酸镁盐结晶主要受多核表面反应控制,同时提出了结晶相转化机理。 相似文献