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1.
Jaedong Cho John Blackwell Sergei N. Chvalun Morton Litt Yuan Wang 《Journal of Polymer Science.Polymer Physics》2004,42(13):2576-2585
As‐cast films of poly(2,5‐benzimidazole) exhibit uniplanar orientation in which the planes of the aromatic rings lie parallel to the film surface. Upon doping with phosphoric acid, the original crystalline order is lost, but the doped film can be stretched to produce films with uniaxial orientation. After thermal annealing at 540 °C, nine Bragg reflections are resolved in the fiber diagram, and these are indexed by an orthorhombic unit cell with the dimensions a = 18.1 Å, b = 3.5 Å, and c = 11.4 Å, containing four monomer units of two chains. The absence of odd‐order 00l reflections points to a 21 chain conformation, which is probably planar so that the aromatic units can be stacked along the b axis. The water and phosphoric acid contents of the crystalline structure cannot be determined exactly because of the presence of extensive amorphous regions that probably have different solvation. The best agreement between the observed and calculated intensities is for an idealized structure containing two phosphoric acids and two water molecules per unit cell. However, the phosphoric acid is probably present mainly in the form of pyrophosphoric acid and its higher oligomers. In addition, the X‐ray data are consistent with a more disordered structure containing chains with random (up and down) polarity and a lack of c‐axis registry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2576–2585, 2004 相似文献
2.
Alexander G. Stepanov Sergei S. Arzumanov Horst Ernst Dieter Freude 《Chemical physics letters》2006,420(4-6):574-576
It has been demonstrated that 1H MAS NMR spectroscopy can be used as a tool for in situ monitoring the reaction kinetics of 13C-labeled carbon scrambling in alkane molecules adsorbed on zeolite catalysts at the reaction temperature of 540–570 K. The accuracy of the results and the time resolution are improved compared to 13C MAS NMR spectroscopy. 相似文献
3.
We address the problem of computing homotopic shortest paths in the presence of obstacles in the plane. Problems on homotopy of paths received attention very recently [Cabello et al., in: Proc. 18th Annu. ACM Sympos. Comput. Geom., 2002, pp. 160–169; Efrat et al., in: Proc. 10th Annu. European Sympos. Algorithms, 2002, pp. 411–423]. We present two output-sensitive algorithms, for simple paths and non-simple paths. The algorithm for simple paths improves the previous algorithm [Efrat et al., in: Proc. 10th Annu. European Sympos. Algorithms, 2002, pp. 411–423]. The algorithm for non-simple paths achieves O(log2n) time per output vertex which is an improvement by a factor of O(n/log2n) of the previous algorithm [Hershberger, Snoeyink, Comput. Geom. Theory Appl. 4 (1994) 63–98], where n is the number of obstacles. The running time has an overhead O(n2+) for any positive constant . In the case k<n2+, where k is the total size of the input and output, we improve the running to O((n+k+(nk)2/3)logO(1)n). 相似文献
5.
Vladimir V. Bazhanov Sergei L. Lukyanov Alexander B. Zamolodchikov 《Communications in Mathematical Physics》1997,190(2):247-278
This paper is a direct continuation of [1] where we began the study of the integrable structures in Conformal Field Theory.
We show here how to construct the operators ${\bf Q}_{\pm}(\lambda)$ which act in the highest weight Virasoro module and commute
for different values of the parameter λ. These operators appear to be the CFT analogs of the Q - matrix of Baxter [2], in particular they satisfy Baxter's famous T- Q equation. We also show that under natural assumptions about analytic properties of the operators as the functions of λ the Baxter's relation allows one to derive the nonlinear integral equations of Destri-de Vega (DDV)
[3] for the eigenvalues of the Q-operators. We then use the DDV equation to obtain the asymptotic expansions of the Q - operators at large λ; it is remarkable that unlike the expansions
of the T operators of [1], the asymptotic series for Q(λ) contains the “dual” nonlocal Integrals of Motion along with the local ones. We also discuss an intriguing relation between
the vacuum eigenvalues of the Q - operators and the stationary transport properties in the boundary sine-Gordon model. On this basis we propose a number
of new exact results about finite voltage charge transport through the point contact in the quantum Hall system.
Received: 2 December 1996 / Accepted: 11 March 1997 相似文献
6.
Xinyu Huang Roham Solasi Yue Zou Matthew Feshler Kenneth Reifsnider David Condit Sergei Burlatsky Thomas Madden 《Journal of Polymer Science.Polymer Physics》2006,44(16):2346-2357
The life of proton exchange membrane fuel cells (PEMFC) is currently limited by the mechanical endurance of polymer electrolyte membranes and membrane electrode assemblies (MEAs). In this paper, the authors report recent experimental and modeling work toward understanding the mechanisms of delayed mechanical failures of polymer electrolyte membranes and MEAs under relevant PEMFC operating conditions. Mechanical breach of membranes/MEAs in the form of pinholes and tears has been frequently observed after long‐term or accelerated testing of PEMFC cells/stacks. Catastrophic failure of cell/stack due to rapid gas crossover shortly follows the mechanical breach. Ex situ mechanical characterizations were performed on MEAs after being subjected to the accelerated chemical aging and relative humidity (RH) cycling tests. The results showed significant reduction of MEA ductility manifested as drastically reduced strain‐to‐failure of the chemically aged and RH‐cycled MEAs. Postmortem analysis revealed the formation and growth of mechanical defects such as cracks and crazing in the membranes and MEAs. A finite element model was used to estimate stress/strain states of an edge‐constrained MEA under rapid RH variations. Damage metrics for accelerated testing and life prediction of PEMFCs are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2346–2357, 2006 相似文献
7.
Alexei Yu. Lukin Sergei V. Shevyakov Alexei V. Laptev Olga I. Davydova Vitali I. Shvets Olga V. Demina Yury P. Strokach Valery A. Barachevsky Michail V. Alfimov Sergei P. Gromov Artem I. Vedernikov Johan Lugtenburg Andrey A. Khodonov 《Journal of inclusion phenomena and macrocyclic chemistry》2004,49(1-2):153-161
An effective synthetic approach to the preparation of a new crown-ether vinylogs involving the Horner–Emmons olefination of carbonyl precursors with the use of C2- and C5-phosphonates was proposed. The effects of the conjugation chain length and the nature of the terminal polar functions in the phosphonate reagent on the yield and process stereoselectivity were discussed. 相似文献
8.
Kudrik EV Theodoridis A van Eldik R Makarov SV 《Dalton transactions (Cambridge, England : 2003)》2005,(6):1117-1122
Catalytic oxidation of N,N'-dimethylthiourea and thiourea by dioxygen in water using a new cobalt(II) complex of octasulfophenyltetrapyrazinoporphyrazine was performed under mild conditions. The reaction is shown to include the formation of an intermediate anionic five-coordinate complex followed by an unusual two-electron oxidation to produce the corresponding urea and elemental sulfur (S8). Kinetic and thermodynamic parameters for the different reaction steps of the process were determined. Drastic differences in catalytic activity of cobalt and iron octasulfophenyltetrapyrazinoporphyrazines were observed. 相似文献
9.
A recently developed force-matching method for obtaining effective force fields for condensed matter systems from ab initio molecular dynamics (MD) simulations has been applied to fit a simple nonpolarizable two-site pairwise force field for liquid hydrogen fluoride. The ab initio MD in this case was a Car-Parrinello (CP) MD simulation of 64 HF molecules at nearly ambient conditions within the Becke-Lee-Yang-Parr approximation to the electronic density functional theory. The force-matching procedure included a fit of short-ranged nonbonded forces, bonded forces, and atomic partial charges. The performance of the force-match potential was examined for the gas-phase dimer and for the liquid phase at various temperatures. The model was able to reproduce correctly the bent structure and energetics of the gas-phase dimer, while the results for the structural properties, self-diffusion, vibrational spectra, density, and thermodynamic properties of liquid HF were compared to both experiment and the CP MD simulation. The force-matching model performs well in reproducing nearly all of the liquid properties as well as the aggregation behavior at different temperatures. The model is computationally cheap and compares favorably to many more computationally expensive potential energy functions for liquid HF. 相似文献
10.
Vladimir L. Kolesnichenko Victor B. Rybakov Leonid A. Aslanov Sergei V. Volkov 《Journal of Cluster Science》1997,8(1):27-45
Thioselenohalide complexes Mo2(μ-S2)2Cl6(SeCl2)2 (I), Mo2(μ-S2)2Br6(SeBr2)2 (II), and W2(μ-S2)2Br6(SeBr2)2 (III) were synthesized by the reactions of corresponding metal halides or carbonyls or molybdenum metal with excesses of S2 X 2+Se2 X 2 mixtures. The complex W2(μ-S2)2Cl6(SeCl2)2 (IV) was obtained by an exchange reaction between (III) and excess of Se2Cl2. Coordination of the neutral SeX 2 ligands to thiohalidesM 2(μ-S2)2 X 6 results in higher thermal stability, and suggests the possibility to synthesize SeX 2 complexes of the unstable parent tungsten thiohalides. An unusual oxidative addition reaction of (I) was detected: {fx27-1} Both (I) and (IV) were characterized by X-ray crystal structure analysis. They are isostructural and form discrete molecules. Bridging S 2 2? ligands are coordinated perpendicularly to the metal-metal bond;d(M?M)=2.8066 Å and 2.793 Å for I and IV, respectively. Nonequivalence of chlorine atoms which are bound to the metal atom, relate to nonequivalence of halogen atoms in the complexesM 2(μ?S2)2 X 8 2? . Chlorine atomstrans to SeCl2 ligands form short bonds with the metal; the corresponding35Cl NQR frequency is increased. The selenium dichloride ligand is ambidentate. The selenium atom binds as a donor to the metal and as an acceptor to two chlorine atoms which are also bound covalently to the same metal atom. 相似文献