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991.
Abe K Abt I Ahn CJ Akagi T Allen NJ Ash WW Aston D Baird KG Baltay C Band HR Barakat MB Baranko G Bardon O Barklow T Bazarko AO Ben-David R Benvenuti AC Bienz T Bilei GM Bisello D Blaylock G Bogart JR Bolton T Bower GR Brau JE Breidenbach M Bugg WM Burke D Burnett TH Burrows PN Busza W Calcaterra A Caldwell DO Calloway D Camanzi B Carpinelli M Cassell R Castaldi R Castro A Cavalli-Sforza M Church E Cohn HO Coller JA Cook V Cotton R Cowan RF Coyne DG D'Oliveira A Damerell CJ Daoudi M 《Physical review letters》1995,75(20):3609-3613
992.
Electrochemically prepared films of conducting polymers of polypyrrole and polythiophene and their blends with polyamide have been investigated by X-ray photoelectron spectroscopy. In the N1s region of the spectra of films containing polypyrrole the peak corresponding to N(+) at 402.0 eV is separated from that of neutral N. The intensity of the N(+) peak can be correlated with the electrical conductivity of the films and the spectroscopically derived ratio of F/N(+) is close to 4 indicating that one BF(-)(4) dopant ion is incorporated for every oxidized nitrogen center. In the spectra of films of polythiophene and its blends peaks corresponding to S and S(+) can not be resolved but again the F/C ratio correlates with the electrical conductivity. 相似文献
993.
994.
1-Phenylthymine and the carbocyclic analog of thymidine were obtained in yields of 84-87% by cyclizing the appropriate 3-methoxy-2-methylacryloylureas in dilute sulfuric acid. High yields of 1-phenylthymine also resulted when the cyclization was carried out in trifluoroacetic acid, in acetic acid containing toluenesulfonic acid (TSA), or by fusion of the urea with a catalytic amount of TSA. In comparison, the typical aqueous-alkali catalyzed cyclizations gave lower yields of the two thymines, and cleavage of the acryloylureas was shown to occur. However, cyclization in concentrated aqueous ammonia produced high yields of both thymine derivatives. 相似文献
995.
Summary A computational method is described for mapping the volume within the DNA double helix accessible to the groove-binding antibiotic netropsin. Topological critical point analysis is used to locate maxima in electron density maps reconstructed from crystallographically determined atomic coordinates. The peaks obtained in this way are represented as ellipsoids with axes related to local curvature of the electron density function. Combining the ellipsoids produces a single electron density function which can be probed to estimate effective volumes of the interacting species. Close complementarity between host and ligand in this example shows the method to give a good representation of the electron density function at various resolutions. At the atomic level, the ellipsoid method gives results which are in close agreement with those from the conventional spherical van der Waals approach. 相似文献
996.
Stripping voltammetry with collection at a rotating ring-disk electrode having a platinum ring and a glassy-carbon disk was used for the determination of Hg(II) in 1.0M H(2)SO(4) medium in the range 0.10-4.00 ppM. Satisfactory results were obtained only after a thin film of Au (two monolayer equivalents) was electroplated on the disk electrode. The average relative deviation of the results for determination at the 0.10-ppM level was 7.5 %. The limit of detection for the technique is approximately 0.01 ppM. 相似文献
997.
Dipole interactions and electrical polarity in nanosystems: the Clausius-Mossotti and related models
Allen PB 《The Journal of chemical physics》2004,120(6):2951-2962
Point polarizable molecules at fixed spatial positions have solvable electrostatic properties in classical approximation, the most familiar being the Clausius-Mossotti (CM) formula. This paper generalizes the model and imagines various applications to nanosystems. The behavior is worked out for a sequence of octahedral fragments of simple cubic crystals, and the crossover to the bulk CM law is found. Some relations to fixed moment systems are discussed and exploited. The one-dimensional dipole stack is introduced as an important model system. The energy of interaction of parallel stacks is worked out, and clarifies the diverse behavior found in different crystal structures. It also suggests patterns of self-organization which polar molecules in solution might adopt. A sum rule on the stack interaction is found and tested. Stability of polarized states under thermal fluctuations is discussed, using the one-dimensional domain wall as an example. Possible structures for polar hard ellipsoids are considered. An idea is formulated for enhancing polarity of nanosystems by intentionally adding metallic coatings. 相似文献
998.
Norman S. Allen 《Polymer Degradation and Stability》1980,2(2):129-135
The effect of various primary and secondary anti-oxidants and light stabiliser systems on the photo-stabilising performance of a hindered piperidine compound bis[2,2,6,6-tetramethyl-4-piperidinyl]-sebacate, in polypropylene film has been examined. None of the anti-oxidants or light stabilisers gave an additive effect with the hindered piperidine compound. In many cases antagonistic effects were observed. The effects observed are discussed in relation to our current understanding of the mechanistic behaviour of these hindered piperidine systems. 相似文献
999.
Allen BD Benniston AC Harriman A Rostron SA Yu C 《Physical chemistry chemical physics : PCCP》2005,7(16):3035-3040
The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures. 相似文献
1000.
Norman S. Allen Alfonso Chirinis-Padron Terence J. Henman 《Polymer Degradation and Stability》1985,13(1):31-76
The incorporation of antioxidants and light stabilisers in polymers is important in order to preserve their long-term use in the environment. The mechanisms of photo-stabilisation are complex and with continued research grow even more in complexity. The processes involved are further complicated by the effects of processing which can in many cases dramatically influence the performance of the additives. This review presents an in-depth account of the complex mechanisms involved in both thermal and photochemical oxidation of polyolefins, with particular emphasis on polypropylene and the interactions that take place thermally. 相似文献