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81.
Quenched and annealed samples of linear low-density polyethylene (LLDPE) were γ irradiated in vacuo at 77 K; the kinetics of the alkyl free-radical decay reactions were studied at room temperature, and of the allyl free-radical reactions at 60, 70, and 80°C. The ESR signals saturate at a slightly higher microwave power in the LLDPE than in high-density polyethylene (HDPE), and the alkyl radicals start decaying at a lower temperature in the LLDPE than in the HDPE. As in the HDPE the decay of the alkyl free radicals at room temperature in the LLDPE follows the kinetic equation for two simultaneous first-order reactions with the fraction of the faster-decaying component being slightly greater in the quenched than in the annealed samples. In the case of the allyl free radicals the decay at 60°C follows the equation based on one fraction of the radicals decaying according to second-order kinetics in the presence of other nondecaying radicals. At higher temperatures the data are best understood in terms of a second-order rate equation with a continuously variable time-dependent rate constant as suggested by Hamill and Funabashi. 相似文献
82.
83.
Quenched and annealed samples of linear low-density polyethylene (LLDPE) were irradiated with 60Co γ rays in vacuo at room temperature. The data follow rather accurately Charlesby's equation s = k/r, where s is the soluble fraction, r the dose, and k a constant from which G(X), the G-value for crosslinks, was calculated. Crosslinking in the LLDPE is about twice as extensive at equal doses as in LHDPE. Production of vinylene unsaturation was approximately the same in the two types of polyethylene. 相似文献
84.
Marin-Montesinos I Brouwer DH Antonioli G Lai WC Brinkmann A Levitt MH 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,177(2):307-317
We examine the influence of continuous-wave heteronuclear decoupling on symmetry-based double-quantum homonuclear dipolar recoupling, using experimental measurements, numerical simulations, and average Hamiltonian theory. There are two distinct regimes in which the heteronuclear interference effects are minimized. The first regime utilizes a moderate homonuclear recoupling field and a strong heteronuclear decoupling field; the second regime utilizes a strong homonuclear recoupling field and a weak or absent heteronuclear decoupling field. The second regime is experimentally accessible at moderate or high magic-angle-spinning frequencies and is particularly relevant for many realistic applications of solid-state NMR recoupling experiments to organic or biological materials. 相似文献
85.
Raman scattering studies of the high-pressure stability of pentaerythritol tetranitrate, C(CH2ONO2)4
Lipinska-Kalita KE Pravica MG Nicol M 《The journal of physical chemistry. B》2005,109(41):19223-19227
High-pressure Raman scattering studies have been performed on a crystalline energetic material, pentaerythritol tetranitrate C(CH2ONO2)4 (PETN), an important secondary explosive. In situ, ambient-temperature investigations employed diamond anvil cell techniques and nitrogen as a quasi-hydrostatic-pressure-transmitting medium. The pressure-induced alterations in the profiles of the Raman lines, including positions, bandwidths, and intensities, were studied in a compression sequence up to about 31.3 GPa and in a subsequent decompression to ambient conditions. The observed changes of the Raman spectra implied that PETN gradually densified and compressed smoothly up to the highest investigated pressures. Compression below 12 GPa gradually shifted all Raman peaks to higher frequencies without significantly changing their relative intensities or bandwidths. At higher pressures, the peak intensities of the Raman spectra decreased considerably and the bands broadened significantly. The Raman spectrum of the material quenched from 31.3 GPa to ambient conditions indicated that no pressure-driven permanent reconstructive modification or decomposition of the PETN structure occurred. That is, the spectral changes were completely reversible upon compression and subsequent decompression to ambient conditions. 相似文献
86.
Pump-probe differencing technique for cavity-enhanced, noise-canceling saturation laser spectroscopy
We present an experimental technique that permits mechanical-noise-free, cavity-enhanced frequency measurements of an atomic transition and its hyperfine structure. We employ the 532-nm frequency-doubled output from a Nd:YAG laser and an iodine vapor cell. The cell is placed in a folded ring cavity (FRC) with counterpropagating pump and probe beams. The FRC is locked with the Pound-Drever-Hall technique. Mechanical noise is rejected by differencing the pump and probe signals. In addition, this differenced error signal provides a sensitive measure of differential nonlinearity within the FRC. 相似文献
87.
Bunch J Clench MR Richards DS 《Rapid communications in mass spectrometry : RCM》2004,18(24):3051-3060
Matrix-assisted laser desorption/ionisation (MALDI) quadrupole time-of-flight mass spectrometry (Q-TOFMS) has been used to detect and image the distribution of a xenobiotic substance in skin. Porcine epidermal tissue was treated with 'Nizoral', a medicated shampoo containing ketoconazole (+/-)-1-acetyl-4-[p-[[(2R,4S)-2-(2,4-dichlorophenyl)-2-(imidazol-1-ylmethyl)-1,3-dioxolan-4-yl]methoxy]phenyl]piperazine) as active ingredient. Following incubation for 1 h at 37 degrees C all excess formulation was washed from the surface. A cross-section of the drug-treated tissue was then blotted onto a cellulose membrane, precoated in matrix (alpha-cyano-4-hydroxycinnamic acid (CHCA)), by airspray deposition. In separate experiments the tissue surface was treated with Nizoral within a triangular former, and subsequently blotted onto a matrix-coated membrane. Sample membranes were then mounted into the recess of specialised MALDI targets with adhesive tape. All samples were analysed by MALDI-TOFMS using an Applied Biosystem 'Q-star Pulsar i' hybrid Q-TOF mass spectrometer fitted with an orthagonal MALDI ion source and imaging software. Detection of the protonated molecule was readily achievable by this technique. Treatment of the tissue within a template gave rise to images depicting the expected distribution of the drug, demonstrating that this technique is capable of producing spatially useful data. Ion images demonstrating the permeation of the applied compound into the skin were achieved by imaging a cross-sectional imprint of treated tissue. A calibration graph for the determination of ketoconazole was prepared using the sodium adduct of the matrix ion as an internal standard. This enabled construction of a quantitative profile of drug in skin. Conventional haematoxylin and eosin staining and microscopy methods were employed to obtain a histological image of the porcine epidermal tissue. Superimposing the mass spectrometric and histological images appeared to indicate drug permeation into the dermal tissue layer. 相似文献
88.
Nuclear spin order may be stored in a liquid for a much longer time than the longitudinal relaxation time T1, by using rf fields to isolate states of different symmetry. The method is demonstrated on a sample containing AX spin systems. 相似文献
89.
Hughes CE Carravetta M Levitt MH 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,167(2):306-265
Cogwheel phase cycles are often significantly shorter than traditional nested phase cycles. However, optimal solutions for cogwheel cycles are often difficult to find. This paper presents techniques and conjectures which the authors have found useful for generating cogwheel phase cycles either without the need for computer searches or with significantly smaller searches than would otherwise be necessary. The conjectures presently lack proofs but have been tested successfully for a large number of cases. 相似文献
90.
Chisholm MH Clark RJ Hadad CM Patmore NJ 《Chemical communications (Cambridge, England)》2004,(1):80-82
The preparation and characterisation of the tungsten-tungsten quadruply bonded, 3,6-dioxypyridazine bridged complex [((t)BuCO(2))(3)W(2)](2)([micro sign]-H(2)C(4)N(2)O(2)) and its single electron oxidised radical cation are reported and, when compared with related bridged dimolybdenum complexes, reveal a different mechanism of electronic coupling from that seen in related oxalate bridged systems. 相似文献