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41.
Neat poly(ethylene terephthalate) (PET) and PET/antimony doped tin oxide (ATO) nanocomposites were prepared by in situ polymerization. The study of the isothermal crystallization behaviors of neat PET and PET/ATO nanocomposites was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. For neat PET and PET/ATO nanocomposites, the Avrami exponent n both decreased with increasing crystallization temperature. In addition, for the same crystallization temperature, the value of n increased with increasing ATO content. These suggested that the crystallization types related to the values of n in the Avrami theory could not be suitable for the crystallization of PET and its nanocomposites. The change of the n values indicated that the addition of ATO resulted in the increase of the crystallizing growth points. That is a heterogeneous nucleating effect of ATO on crystallization of PET. In the DSC scan after isothermal crystallization process, multiple melting behavior was found. And the multiple endotherms could be attributed to melting of the recrystallized materials or the secondary lamellae produced during different crystallization processes. The equilibrium melting temperature of PET in the nanocomposites increased with increasing the ATO content. Surface free energy of PET chain folding for crystallization of PET/ATO nanocomposites was lower than that of neat PET, confirming the heterogeneous nucleation effect of ATO. 相似文献
42.
The crystallization properties of three regioregular poly(3‐alkyl thiophene)s (P3ATs) are studied: poly(3‐hexyl thiophene) (P3HT), poly(3‐octyl thiophene) (P3OT), and poly(3‐dodecyl thiophene) (P3DDT). The morphology of the isothermally crystallized samples is a whisker type. The values of the enthalpy of fusion of ideal crystals (ΔH), determined from the melting point depression in the polymer–diluent system, are 99, 73.6, and 52 J/g for P3HT, P3OT, and P3DDT, respectively. The values of the equilibrium melting point (T), determined from the Hoffman–Weeks extrapolation procedure, are 300, 230, and 180 °C for P3HT, P3OT, and P3DDT, respectively. From the linear extrapolation of the P3AT data, the T and ΔH values of unsubstituted polythiophene are predicted to be 400 °C and 139 J/g, respectively. The crystallization kinetics of these polymers are studied with differential scanning calorimetry, and the Avrami exponents vary between 0.6 and 1.4, indicating one‐dimensional heterogeneous nucleation with linear growth. As the P3AT whiskers are produced from the chain‐folding process, the Lauritzen–Hoffman growth rate theory is applied to analyze the temperature coefficient of the crystallization rate data. Graphical plots indicate a transition from regime I to regime II during isothermal crystallization for all the P3ATs studied. The fold surface energy and the work of chain folding calculated from the slopes of the graphical plots decrease with an increase in the number of carbon atoms of the side chain. The primary crystallization process of the side‐chain crystallization is very fast and is attributed to the zipping effect of the main‐chain crystals. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2073–2085, 2002 相似文献
43.
The crystallization kinetics and degree of crystallinity of polycaprolactone (PCL) were studied using the technique of differential scanning calorimetry (DSC). The crystallization exotherms measured using DSC were analyzed using a modified Avrami equation. The modification limited the analysis of the data to the primary crystallization process, where the Avrami equation is applicable. Both the degree of crystallinity and primary composite rate constant were found to decrease with increasing molecular weight. The observations have been explained in terms of the unified reptation‐nucleation theory. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
44.
N. T. Silva N. F. Nascimento L. S. Cividanes C. A. Bertran G. P. Thim 《Journal of Sol-Gel Science and Technology》2008,47(2):140-147
Diphasic cordierite gels were prepared from colloidal silica, aluminum and magnesium nitrates and citric acid. The mechanism
of xerogel decomposition was studied by infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA). The thermal
decomposition of the xerogel forms a solid mixture of MgO, Al2O3 and SiO2 at around 250 °C. Cordierite crystallization was studied by X-ray diffraction (XRD) and differential thermal analysis (DTA).
Xerogels were initially thermally treated, and this sample crystallized to μ-cordierite at 850 °C, at 900 °C α-cordierite
crystallizes and at 1150 °C α-cordierite is the major phase and μ-cordierite is totally consumed. The apparent activation
energy for cordierite crystallization process was determined based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, Ligero
methods and the Arrhenius law for dependence of activation energy with temperature. The apparent activation energy was (466.8 ± 34.3) kJ/mol,
the exponent of Avrami was (1.9 ± 0.2) and the frequency factor was (1.55 × 1020) s−1. The Avrami value indicates a nucleation controlled process, which can be a consequence of the high xerogel homogeneity,
a consequence of the early and simultaneous formation of the MgO, Al2O3 and SiO2 mixture. 相似文献
45.
The polarization switching in methylammonium chlorobismuthate, (CH3NH3)5Bi2Cl11, single crystal was investigated by optical observations of the domain structure and switching current registration. It is shown that defects introduced during crystal growth influence the polarization switching kinetics. For the analysis of switching current data the statistical Avrami model of phase transformation kinetics was employed. Direct observation of the domain structure enabled us to verify the assumptions of the Avrami model in the case of non‐uniform (CH3NH3)5Bi2Cl11 crystals. It is found that application of this model to non‐uniform crystals can yield physically unclear values of some fitting parameters; e.g. the effective domain growth dimension is not constant with respect to the electric field. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
46.
Chain alignment has been induced in commercial atactic polymethyl methacrylate and atactic polystyrene by deformation just below the glass transition temperature. The resultant fiber-type X-ray diffraction patterns when azimuthally averaged are not distinguishable from the concentric diffuse halos typical of the “as-received” polymers. The conclusion is drawn that both the polymers examined contain domains within which neighboring chains are aligned parallel. The experiment also indicates that the extra structural information which can be extracted from the fiber-type diffraction pattern of the deformed polymers is also relevant to the unoriented state. 相似文献
47.
The isothermal crystallization behaviors in a newly developed CeGaCu bulk metallic glass have been investigated through the classic differential scanning calorimeter(DSC) method. It is found that the apparent activation energy(Ea) strongly depends on the fraction(x) of isothermal crystallization. Johnson-Mehl-Avrami(JMA) formula was used to analyze the mechanism of crystallization and the obtained Avrami exponent(n) was discovered to show an obvious correlation with the crystallization fraction x. With the help of the relation between Ea and n, the nucleation and growth activation energies, En and Eg, were estimated to be 214–304 kJ/mol and 91 kJ/mol, respectively. This result suggests that the main energy barrier against crystallization in the present glass should be the nucleation of nucleates, rather than the growth of crystals. Such a large En is also believed to be responsible for the good glass forming ability of the CeGaCu alloy. 相似文献
48.
The crystallisation behaviour of in situ polymerised cyclic butylene terephthalates (pCBT) and poly(butylene terephthalate)s
(PBT) were studied by differential scanning calorimetry (DSC) both under isothermal and non-isothermal conditions. The crystallisation
was analysed by adopting the Avrami, Ozawa and Kissinger methods for the isothermal and non-isothermal crystallisations, respectively.
An Avrami exponent n between 2 and 3 was found for the pCBTs whereas the exponent ranged between 3 and 4 for the PBTs. The Ozawa exponent m varied for all materials between 2 and 3. Differences in the crystallisation kinetics were also reflected in the related
activation energy data. 相似文献
49.
R. J. Williams 《Journal of Thermal Analysis and Calorimetry》1997,49(2):801-808
A cohort of human red cells of the same age persists in the circulation for about 110 days without access to repair but disappears over the next 10 days. Hyperosmotic stress accelerates the process exponentially. The kinetics are Avrami in all cases we have examined, withn=2. We have previously modelled this as a stress failure in a viscoelastic cytoskeleton, but because of the two dimensional long range order in the cytoskeleton, the data can also be interpreted as a state change in a crystalline material.We thank H. T. Meryman for his continuing encouragement and support. Naval Medical Research and Development Command Work Unit 1462. The opinions and assertions contained herein are the private ones of the writers and are not to be construed as official at large. 相似文献
50.
A kinetics study on PbGeO3 solid-solid phase transition was realised by means of differential scanning calorimetry and time-resolved X-ray powder diffraction. Isothermal and non-isothermal Johnson-Mehl-Avrami equations were applied to obtain both the activation energy Ea and the Avrami coefficient n. The latter parameter has been related to morphological evidences collected by scanning electron microscopy. The limits of the applied theory, i.e., the Arrhenian behaviour of the growth rate and a steady-state nucleation rate, are finally discussed in terms of recent theoretical developments. 相似文献