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1.
Results of differential scanning calorimetry (DSC) at different heating rates on Se2Ge0.2Sb0.8 chalcogenide glass are reported and discussed. As the heating rate () changed, also the glass transition temperature (T
g) and onset temperature of crystallization (T
c) changed. As the value of the transition activation energyE
t changed, the crystallization fraction (), heat flow (q and the crystallization peak temperature (T
p) also changed. The value of the effective activation energy of crystallizationE
c
was calculated by means of six different methods. The Se2Ge0.2Sb0.8 chalcogenide glass has two crystallization mechanisms, a one-dimensional and an other surface crystallization growth. The average value ofE
t for Se2Ge0.2Sb0.8 is equal to 194.95±3.9 kJ·mol–1 and the average value ofE
c
is equal to 164±3.3 kJ·mol–1. 相似文献
2.
Niculae I. Ionescu Monica Caldararu Cristian Hornoiu Cornel Munteanu 《Reaction Kinetics and Catalysis Letters》2007,91(2):257-262
The overall activation energy for the ignition of thermokinetic oscillations observed (in a dynamic calorimeter) during the
heterogeneous catalytic oxidation of methanol on Pd/LiAl5O8 were obtained and discussed by a PdOx redox cycle. 相似文献
3.
Lisardo Núñez F. Fraga A. Castro L. Fraga 《Journal of Thermal Analysis and Calorimetry》1998,52(3):1013-1022
The influence of the resin/diamine ratio on the properties of the system diglycidyl ether of bisphenol A (BADGE n=0/m-xylylenediamine)
(m-XDA) was studied. Variation of this ratio resulted in significant effects on the cure kinetics and final dynamic mechanical
properties of the product material.
The study was made in terms of storage modulus (E′), vss modulus (E″) and molecular mass between cross-links (Mc) at different ratios. Two geometries (cylindrical and rectangular) were considered. The influence of temperature was studied
through the activation energy (Ea>), which depends on the epoxy/amine ratio and the geometry of the samples. Glass transition temperatures (Tg>) and glass transition temperatures for thermosets with null degree of conversion (Tgo>) were determined by DSC. Tg> decreases when amounts of curing agent greatly in excess of the stoichiometric composition were used.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
The results of determination of activation energies (EA) of polymeric cable insulations obtained by conventional methods (usually based on the evaluation of changes of mechanical
properties of insulations after their ageing in thermal chamber at different temperatures) have been compared with results
obtained by methods employing the differential scanning calorimetry (DSC). Three DSC methods have been tested: the method
according the ASTM E 698; measuring of DSC characteristics in the isothermal mode at several different temperatures; and the
method based on evaluation of DSC characteristics of insulations after their thermal ageing in thermal chamber. The last method
— which can be called as a modified conventional method, because instead of mechanical properties, the DSC characteristics
are determined — has been found as most acceptable and giving similar values of EA as the other conventional methods.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
5.
F. Fraga T. Salgado J. A. Rodríguez Añon L. Nuñez Regueira 《Journal of Thermal Analysis and Calorimetry》1994,41(6):1543-1550
Mechanical behaviour play an important role in the election of an epoxidic formulation of well determined properties as it has a marked influence on both structural and external factors. Temperature and time strongly act on polymers properties owing to their viscoelastic nature. Knowledge of the dynamic moduli and properties of polymeric materials is indispensable for the design of this materials. At the same time, the influence of the temperature on polymers behaviour may be studied once the activation energy is known. In this paper the different dynamic moduli and activation energy are measured using a Perkin Elmer DMA 7. The relationships between the dynamic mechanical properties and the molecular weight of the polymers make possible the calculation of the molecular weight. Results reasonably agree with literature values. 相似文献
6.
V. Plaček 《Journal of Thermal Analysis and Calorimetry》2005,80(2):525-528
Summary Simulation of thermal ageing is an important part of qualification of materials designed for the use in nuclear power plants (NPP). According to standards, the simulation of long-term service thermal ageing is performed isothermally at elevated temperature using Arrhenius methodology. The samples or equipment are aged in thermal chamber, to bring them to the same state as after long-term service time. To proceed a reliable simulation, the testing parameters should be taken very carefully and the accelerator factors should not be too high. The testing temperature and time and the activation energy are the most important parameters. Determination of these factors and the limitations of their use in practice are discussed. 相似文献
7.
Dichang Zhang 《Journal of Thermal Analysis and Calorimetry》1995,45(1-2):141-150
A cracking catalyst designatedSRNY was manufactured from a commercialSRNY molecular sieve (M.S.). The support consisted of kaolin, clay and SiO2. The coking behaviour of theSRNY M.S., the support and the catalyst were examined with light diesel oil (LDO) as feedstock in a microreactor. The physico-chemical properties of both fresh and aged samples, subjected to or not subjected to the cracking reaction ofLDO, were sequentially characterized by means of pore structure determination and thermal analysis. The pore structure included the specific surface area and the pore volume or porosity. Thermal analysis methods used included TG and DSC. The results indicated that all coked samples exhibited obvious changes in surface pore structure and acidity in comparison with non-coked samples. Their specific surface area and acid amount decreased with increase in the coke content of the samples. The apparent activation energy data obtained from decoking samples in an air flow, using the temperature-programmed oxidation (TPO) method, showed that the kinetic parameters of theSRNY M.S. differed from those of theSRNY catalyst and its support. 相似文献
8.
乙二醇和丙三醇水溶液冻结特性的研究 总被引:16,自引:0,他引:16
利用差示扫描量热仪(Pyris-Diamond DSC),研究了乙二醇(甘醇)和丙三醇(甘油)水溶液的过冷行为、水合性质和它们的玻璃化转变温度及反玻璃化温度,分析了它们与分子中羟基个数的关系.进行了12组共24种不同浓度(质量分数)的溶液的差示扫描量热实验.过冷度的实验结果表明,在浓度相等的情况下,两种低温保护剂水溶液冻结的过冷度有相同的变化规律.水合实验的结果表明,浓度相同时,二者结合水的能力大体相当.玻璃化转变的实验表明,二者玻璃化转变温度与反玻璃化温度存在明显差异. 相似文献
9.
目前国内化学基础课教材关于碰撞理论中活化能的定义存在严重分歧,简单总结了目前中文教材中的3种定义。通过对碰撞理论形成过程中部分历史背景的梳理,总结出碰撞理论所具备的2个基本要素即气体分子运动论的应用以及碰撞活化假设,然后溯源3种不同定义来源的历史文献,从2个基本要素的角度分析3种定义。研究发现中文教材广泛采用的Lewis活化能解说不承认碰撞活化假设,采用了辐射活化假说,同时表述笼统不严谨,Tolman活化能解说是单纯的统计力学处理,没有应用气体分子运动论,不属于碰撞理论范畴。而简单碰撞理论提出的有效碰撞的最低能量作为活化能的定义更能承载碰撞理论的基本要素。 相似文献
10.
Giovanna Bruni C. Milanese G. Bellazzi V. Berbenni P. Cofrancesco A. Marini M. Villa 《Journal of Thermal Analysis and Calorimetry》2007,89(3):761-766
The processes of production of drugs and dosage forms in the solid state often cause unwanted transformation of portions of
the substances into amorphous state, with significant changes of properties such as stability and bio-availability. When this
amorphous fraction is of the order of a few percent, it usually goes unnoticed, but it should be accurately determined within
a quality control system. In this work, we consider a model drug, perphenazine, where partial amorphisation may be induced
by standard mechanical treatments.
We show that Differential Scanning Calorimetry (DSC) leads to consistent estimations of the amorphous fractions induced by
the treatment. Furthermore, DSC also yields the expected amounts of amorphous perphenazine when analysing known mixtures of
perfectly crystalline samples (untreated) and partially amorphous samples (treated). We show that even amorphous fractions
of the order of 1% are accurately estimated by our method. 相似文献
11.
Heating rate effect on the DSC kinetics of oil shales 总被引:1,自引:0,他引:1
M. V. Kök 《Journal of Thermal Analysis and Calorimetry》2007,90(3):817-821
This research was aimed to investigate the combustion and kinetics of oil shale samples (Mengen and Himmetoğlu) by differential
scanning calorimetry (DSC). Experiments were performed in air atmosphere up to 600°C at five different heating rates. The
DSC curves clearly demonstrate distinct reaction regions in the oil shale samples studied. Reaction intervals, peak and burn-out
temperatures of the oil shale samples are also determined. Arrhenius kinetic method was used to analyze the DSC data and it
was observed that the activation energies of the samples are varied in the range of 22.4–127.3 kJ mol−1 depending on the oil shale type and heating rate. 相似文献
12.
Semisolid phospholipid preparations have been well known for several years and are still investigated as drug carrier systems,
e.g. for potential cancer therapy. They may be applied parenterally as semisolid vesicular phospholipid gels suitable as implants
for sustained drug release or as liposomal preparations after redisperging the stable storage form. Due to enhanced stability,
mixtures of hydrated phospholipids and cholesterol are more suitable than natural unsaturated phospholipids. In order to describe
characteristics of vesicular phospholipid gels, only a few techniques may be useful. Especially the structure of the semisolid
preparation is not yet completely understood. We tried to get some more information about these systems by using a combination
of freeze-fracture electron microscopy, differential scanning calorimetry and rheometry to elucidate, on the one hand, the
inner structure or homogeneity and, on the other, the thermotropic phase transition of the three-dimensional lipid network
and the temperature dependency of the fluidity/viscosity of the samples.
Using freeze-fracture electron microscopy we found coexisting phospholipid domains of lamellar sheets and vesicular structures.
With the help of differential scanning calorimetry the reasons for the different phase behaviour were elucidated. Rheometric
measurements show increased intermediate viscosity at the thermotropic phase transition of the lipid bilayers, possibly induced
by interacting membrane defects.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
13.
Karger-Kocsis J. Shang P. P. Moskala E. J. 《Journal of Thermal Analysis and Calorimetry》1999,55(1):21-28
The tensile loading-induced necking in notched specimens of an amorphous copolyester (aCOP) was studied by modulated differential
scanning calorimetry (MDSC). It was shown that necking occurred by cold drawing since the enthalpy of cold crystallization
and that of the subsequent melting agreed fairly with each other. Increasing deformation in the necking zone and increasing
deformation rate of the specimens shifted the onset of cold crystallization toward lower temperatures and yielded a slightly
higher glass transition temperature (Tg). This was attributed to the molecular orientation caused by mechanical loading. The finding that the melting contained a
non-reversing part was considered as appearance of possible microcrystallinity. The Tg range was strongly influenced by the deformation rate and reflects the thermomechanical history of the samples accordingly.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
Characterization of water in polysaccharide hydrogels by DSC 总被引:1,自引:0,他引:1
H. Yoshida T. Hatakeyama H. Hatakeyama 《Journal of Thermal Analysis and Calorimetry》1993,40(2):483-489
Water molecules in hydrogels were classified into three categories according to phase transition behavior; non-freezing, freezing
bound and free water. Melting, crystallization, and glass transition of water in hydrogels reflected the state of the water
interacting with polysaccharides. Freezing bound water formed metastable ice by slow cooling and formed amorphous ice by quenching.
From the isothermal crystallization measurement, nucleation rate and crystal growth rate were obtained. The crystal growth
rate of freezing bound water was about ten times slower than that of free water. The DSC characterization of water in hydrogels
was summarized.
Zusammenfassung In übereinstimmung mit dem Phasenum wandlungsverhalten werden Wassermoleküle in Hydrogelen in drei Kategorien eingestuft; nicht gefrierendes, gefrierendes gebundenes und freies Wasser. Das Schmelzen, die Kristallisation und die Glasumwandlung von Wasser in Hydrogen wiederspiegeln den Zustand von Wasser, welches in Wechselwirkung mit Polysacchariden tritt. Gefrierendes gebundenes Wasser bildet metastabiles Eis durch langsames Abkühlen und amorphes Eis durch Abschrecken. Anhand isothermer Kristallisationsmessungen wurden Keimbildungsgeschwindigkeit und Kristallwachstumsgeschwindigkeit erhalten. Die Kristallwachstumsgeschwindigkeit von gefrierendem gebundenem Wasser liegt zehnmal niedriger als die von freiem Wasser.相似文献
15.
在介绍阿仑尼乌斯速率方程基础上,从碰撞理论、过渡态理论和托尔曼统计等视角诠释活化能概念,认为仅以活化能大小作依据判断反应速率大小不严密,并从教学层面和试题角度进行了反思。 相似文献
16.
YAO Liang DENG Jing QU Bao-jun SHI Wen-fang 《高等学校化学研究》2006,22(1):118-122
IntroductionEpoxy resins are widely used in the fields of coa-tings,adhesives,insulating materials,etc..Diglycidylether of bisphenol A(DGEBA)is the most importantepoxy resin in industry because of its fluidity,physicaland mechanical properties after cure,… 相似文献
17.
L. Núñez-Regueira M. Villanueva I. Fraga-Rivas 《Journal of Thermal Analysis and Calorimetry》2006,83(3):727-733
The
study of the degradation of a polymer is important because it can determine
the upper temperature limit, the mechanism of a solid-state process, and the
life-time for this system. Since the behavior of thermosets is affected by
the selection of the curing cycle, it is important to investigate the changes
which take place during the thermal degradation of these materials when a
change on the sequence of time and temperature is introduced during the curing
reaction.
In
this work, the thermal degradation of two epoxy systems diglycidyl ether of
bisphenol A (BADGE n=0)/1, 2 diamine cyclohexane
(DCH) cured through different sequences of time and temperature was studied
by thermogravimetric analysis in order to determine the reaction mechanism
of the degradation processes, and also to check the influence of the curing
cycle on this mechanism. Values obtained using different kinetic methods were
compared to the value obtained by Kissinger’s method (differential method
which do not require a knowledge of the n-order
reaction mechanism), and to that obtained through Flynn–Wall–Ozawa
method in a previous work. 相似文献
18.
Ni原子活化氨分子理论研究 总被引:1,自引:0,他引:1
在UB3LYP/6-311++G(3df,3pd)水平下, 详细研究了Ni活化NH3分子的单重态和三重态势能面, 并用分子中的原子量子理论(Quantum Theory of Atom-in-Molecular, QTAIM)计算了势能面上所有驻点的性质. 计算结果表明, 单重态势能面有两条反应途径, 而三重态势能面仅有一条反应途径. 第一个N—H断开的活化能较低, 为99.96 kJ/mol, 活化自由能为100.86 kJ/mol,在常温下就可以进行; 第二个N—H键断裂所需能量高达200 kJ/mol, 不容易进行. 在合适温度下, Ni可以活化NH3得到三重态HNiNH2, 这表明Ni可以作为活化NH3分子的良好催化剂. 相似文献
19.
Ronny Pieters Hans E. Miltner Guy Van Assche Bruno Van Mele 《Macromolecular Symposia》2006,233(1):36-41
Temperature-induced phase separation of P(EO75-ran-PO25)/PES and reaction-induced phase separation of DGEBA/MDA modified with PVME are studied using MTDSC as an in-situ tool. Phase separation can be probed by the onset of an ‘excess’ contribution in the MTDSC heat capacity signal, in good correspondence with the cloud point temperature. This feature enables the complete construction of the state diagram of P(EO75-ran-PO25)/PES. The detection of phase separation-induced partial vitrification of the high-Tg phase (PES-rich phase) enables to sub-divide the LCST-type heterogeneous region in a zone 1 (no interference of partial vitrification) and a zone 2 (interference of partial vitrification of the PES-rich phase). This sub-division of the heterogeneous region has drastic implications on the remixing behavior of demixed blends. In DGEBA/MDA modified with PVME, reaction-induced phase separation accompanied by an increase in reaction rate, followed by a vitrification step of the epoxy-amine phase can be detected in-situ. In non-isothermal conditions, a diffusion-controlled reaction after vitrification and a final devitrification of the system is also observed. 相似文献
20.
Sergey Vyazovkin Nicolas Sbirrazzuoli Ion Dranca 《Macromolecular rapid communications》2004,25(19):1708-1713
Summary: An advanced isoconversional method has been applied to determine the effective activation energies (E) for the glass transition in polystyrene (PS), poly(ethylene terephthalate) (PET), and boron oxide (B2O3). The values of E decrease from 280 to 120 kJ · mol−1 in PS, from 1 270 to 550 kJ mol−1 in PET, and from 290 to 200 kJ mol−1 in B2O3. It is suggested that a significant variation in E should be observed for the fragile glasses that typically include polymers.