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111.
112.
Carmen Branca 《Thermochimica Acta》2005,429(2):133-141
Thermogravimetric data on the devolatilization rate of beech wood are re-examined with the aim of incorporating the effects of high heating rates (up to 108 K min−1) in the global kinetics. The mechanism consisting of three independent parallel reactions, first-order in the amount of volatiles released from pseudo-components with chief contributions from hemicellulose, cellulose and lignin, is considered first. It is found that the set of activation energies estimated by Gronli et al. [M.G. Gronli, G. Varhegyi, C. Di Blasi, Ind. Eng. Chem. Res. 41 (2002) 4201-4208] (100, 236 and 46 kJ mol−1, respectively) for one slow heating rate results in very high deviations between predicted and measured rate curves. The agreement is significantly improved by a new set of data consisting of activation energies of 147, 193 and 181 kJ mol−1, respectively. In this case, the overlap is reduced between the reaction rates of the three pseudo-components whose chemical composition is also modified. In particular, instead of a slow decomposition rate over a broad range of temperatures, the activity of the third reaction is mainly explicated along the high-temperature (tail) region of the weight loss curves. The performances of more simplified mechanisms are also evaluated. One-step mechanisms, using literature values for the kinetic constants, produce large errors on either the conversion time (activation energy of 103 kJ mol−1) or the maximum devolatilization rate (activation energy of 149 kJ mol−1). On the other hand, these parameters are well predicted by two parallel reactions, with activation energies of 147 and 149 kJ mol−1. 相似文献
113.
D. Brault C. Vever-Bizet K. Kuzelova 《Journal of photochemistry and photobiology. B, Biology》1993,20(2-3):191-195
The interactions of dicarboxylic porphyrins with membrane systems are discussed with particular emphasis on the effect of the charge of the porphyrin and the nature of the side-chains. The incorporation of hematoporphyrin or related dicarboxylic porphyrins within small unilamellar vesicles as membrane models is favored by a decrease of the pH in the range of physiological pH values. This effect might play an important role in the retention of porphyrins by tumors, which are more acidic than normal tissues. Kinetics studies also show that the partition of the porphyrin between the lipidic bilayer and the aqueous phase is governed by its release rate rather than by its incorporation rate. 相似文献
114.
The stability of poly(N-methylaniline) (PNMA) as electrode material has been studied in aqueous solutions of sulfuric acid with the use of electrochemical
and in situ Raman spectroscopic techniques. It has been shown that the electrochemical decomposition of electrodeposited PNMA
films follows a first-order reaction kinetics. The decomposition rate constants vary between 1.2 × 10−5 and 2.0 × 10−3 s−1 for electrode potential varying between 0.2 and 1.0 V vs Ag/AgCl, respectively. In situ Raman spectroscopy has been applied
in obtaining kinetic data at selected electrode potentials, and good correlation of these data with the corresponding data
obtained by cyclic voltammetry has been found. As compared to polyaniline, the decomposition of PNMA proceeds at nearly the
same rate at electrode potentials not exceeding 0.5 V. The decomposition of PNMA proceeds faster within the potential limits
of 0.5 to 0.8 V and slower at electrode potentials exceeding 0.8 V as compared to polyaniline.
This article is dedicated to Professor Algirdas Vaškelis (Institute of Chemistry, Vilnius, Lithuania) on the occasion of his
70th birthday and in honour of his contributions to electrochemistry and physical chemistry. 相似文献
115.
《Journal of solid state chemistry》2004,177(10):3281-3289
A non-isothermal kinetic study of the oxidation of “carbon-modified MoO3” in the temperature range of 150-550°C by simultaneous TGA-DTA was investigated. During the oxidation process, two thermal events were detected, which are associated with the oxidation of carbon in MoOxCy and MoO2 to MoO3. The model-free and model-fitting kinetic approaches have been applied to TGA experimental data. The solid state-kinetics of the oxidation of MoOxCy to MoO3 is governed by F1 (unimolecular decay), which suggests that the reaction is of the first order with respect to oxygen concentration. The constant (Ea)α value (about 115±5 kJ/mol) for this first stage can be related to the nature of the reaction site in the MoO3 matrix. This indicates that oxidation occurs in well-defined lattice position sites (energetically equivalent). On the other hand, for the second stage of oxidation, MoO2 to MoO3, the isoconversional analysis shows a complex (Ea)α dependence on (α) and reveals a typical behavior for competitive reaction. A D2 (two-dimensional diffusion) mechanism with a variable activation energy value in the range 110-200 kJ/mol was obtained. This can be interpreted as an inter-layer oxygen diffusion in the solid bulk, which does not exclude other simultaneous mechanism reactions. 相似文献
116.
The kinetics of oxidation of allyl alcohol byN-bromosuccinimide (NBS) has been studied at 35 °C in aqueous medium. The reaction shows first order dependence on bothNBS and allyl alcohol. In fairly high acid concentration, there is no change in the rate of the reaction but at low acid concentration, the rate is considerably enhanced. There is no primary salt effect. At varying mercuric acetate concentrations, the rate constant remains the same. But in the absence of mercuric acetate, the rate is enhanced. The kinetic parameters,E
a,Arrhenius factorA, H, G and S have been calculated. A rate law in agreement with experimental results has been derived. A mechanism is proposed.
Kinetik und Mechanismus der Oxidation von Allylalkohol mixN-Bromsuccinimid
Zusammenfassung Die Kinetik der Oxidation von Allylalkohol mitN-Bromsuccinimid (NBS) wurde bei 35 °C in wäßrigem Medium untersucht. Die Reaktion zeigt erste Ordnung gegenüberNBS und Allylalkohol. Bei relativ hoher Säurekonzentration zeigt sich keine Änderung der Reaktionsgeschwindigkeit, bei niedriger Säurekonzentration wird die Reaktionsgeschwindigkeit beträchtlich erhöht. Es wurde kein primärer Salzeffekt festgestellt. Bei varriierender Quecksilberacetatkonzentration bleibt die Reaktionsgeschwindigkeit gleich, bei Abwesenheit von Quecksilberacetat wird jedoch die Geschwindigkeitskonstante erhöht. Die kinetischen Parameter,E a, derArrheniusfaktorA, H , G und S wurden bestimmt. Ein Geschwindigkeitsgesetz in Übereinstimmung mit den experimentellen Befunden wurde abgeleitet und ein Mechanismus vorgeschlagen.相似文献
117.
118.
The effect of steric interactions on the regioselectivity of fluorination of para alkyl substituted phenols was investigated and the strong effect of size of the alkyl substituent, the structure of the F-N reagent and the solvent on the site of fluorination was established. The course of reaction obeyed a second order rate equation, while the fluorination process required higher ΔH≠ activation than oxidation or oxidative demethylation. Solvent polarity variations had a small effect on the rate of functionalisation, while an excellent Hammett correlation with ρ+=−2.3 was determined. 相似文献
119.
Anil K. Awasthi Santosh K. Upadhyay 《Monatshefte für Chemie / Chemical Monthly》1985,116(6-7):729-736
The kinetics of ruthenium(III) catalyzed oxidation of formaldehyde and acetaldehyde by alkaline hexacyanoferrate(III) has been studied spectrophotometrically. The rate of oxidation of formaldehyde is directly proportional to [Fe(CN)
3–
6
] while that of acetaldehyde is proportional tok[Fe(CN)
3–
6
]/{k +k[Fe(CN)
3–
6
]}, wherek, k andk are rate constants. The order of reaction in acetylaldehyde is unity while that in formaldehyde falls from 1 to 0. The rate of reaction is proportional to [Ru(III)]
T
in each case. A suitable mechanism is proposed and discussed.
Die Kinetik der Ru(III)-katalysierten Oxidation von Formaldehyd und Acetaldehyd mittels alkalischem Hexacyanoferrat(III)
Zusammenfassung Die Untersuchung der Kinetik erfolgte spektrophotometrisch. Die Geschwindigkeitskonstante der Oxidation von Formaldehyd ist direkt proportional zu [Fe(CN) 3– 6 ], währenddessen die entsprechende Konstante für Acetaldehyd proportional zuk[Fe(CN) 3– 6 ]/{k +k[Fe(CN) 3– 6 ]} ist, wobeik,k undk Geschwindigkeitskonstanten sind. Die Reaktionsordnung für Acetaldehyd ist eine erste, die für Formaldehyd fällt von erster bis zu nullter Ordnung. Die Geschwindigkeitskonstante ist in jedem Fall proportional zu [Ru(III)] T . Es wird ein passender Mechanismus vorgeschlagen.相似文献
120.
R. Baeza Luis M. Gugliotta A. M. R. Pilosof 《Colloids and surfaces. B, Biointerfaces》2003,31(1-4):81-93
The role of the non-gelling polysaccharide, propyleneglycol alginate (PGA), on the dynamics of gelation and gel properties of β-lactoglobulin (β-lg) under conditions where the protein alone does not gel (6%) was analyzed. To this end, the kinetics of gelation, aggregation and denaturation of β-lg in the mixed systems (pH 7) were studied at different temperatures (64–88 °C). The presence of PGA increased thermal stability of β-lg. The rate of β-lg denaturation was decreased and the onset and peak denaturation temperatures increased by 2.2–2.4 °C. PGA promoted the formation of larger aggregates that continued to grow in time. An average aggregate diameter of approximately 300 nm is reached at the gel point in the mixed β-lg+PGA systems, irrespective of the heating temperature. Comparing the activation energies for the aggregation (193 kJ/mol), denaturation (422 kJ/mol) and formation of the primary gel structure (1/tgel) (256 kJ/mol) processes in the mixed protein–polysaccharide system, it can be concluded that the rate determining step in the formation of the primary gel structure would be the aggregation of protein. Ea values for the processes after the gel point (solid phase gelation) suggest a diffusion limited process because of the high viscosity of the solid gelling matrix. The characteristics of the mixed β-lg+PGA gels in terms of rheological and textural parameters, water loss and microstructure were studied as a function of heating temperature and time. The extent of aggregation and the type of interactions involved, prior to denaturation seem to be very important in determining the gel structure and its properties. 相似文献