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31.
James B. Gerken Claire Badger Christopher Bisbee Sasha Gardner Yan Qi Víctor Durà Vilà John D. Roberts 《Journal of Physical Organic Chemistry》2008,21(3):193-197
Conformational preferences of glutaric, 3‐hydroxyglutaric and 3‐methylglutaric acid, and their mono‐ and dianions have been investigated with the aid of NMR spectroscopy. In contrast to succinic acid, glutaric acid displays essentially statistical conformational equilibria in polar and non‐polar solutions of high and low hydrogen‐bonding ability with no clear evidence for intramolecular hydrogen‐bonding interactions. The acid ionization constant ratios, K 1/K2, in D2O and DMSO of glutaric, 3‐hydroxyglutaric, and 3‐methylglutaric acids also indicate that intramolecular interactions are much less important than, or indeed insignificant, for shorter‐chain acids. FTIR studies on 3‐methylglutaric acid indicate some preference for either association with solvent or dimerization, depending on the solvent, rather than intramolecular hydrogen bonding. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
32.
Superconducting YBa2Cu2.955Fe0.045O7 has been studied at temperatures between 300 and 1000 K. Spectra were recorded on a sample which was sealed as a pellet pressed with boron nitride. An atmosphere of oxygen was maintained in the sample region at all times. At temperatures up to 900 K, we observed the four iron sites A, (Cu(1) chain-site; square planar oxygen coordination with O(5) vacancy on thea-axis), B, (Cu(2) plane-site; square pyramidal oxygen coordination), C, (Cu(1) chain-site; square pyramidal oxygen coordination with O(5) site occupied) and D, (Cu(1) chain-site; quasi-tetrahedral oxygen coordination with O(4) vacancy along theb-axis). At temperatures up to 700 K, the relative area of the B and C sites remained nearly constant as the temperature was increased. However, the area of the A site decreased while, the D site area increased with increasing temperature. At all times, the total area of sites A and D remained constant, thereby indicating the possibility of oxygen atom hopping in thea-b plane. The quadrupole splitting of each site, except the B site, decreased linearly with increasing temperature.Research supported in part by the Old Dominion University Undergraduate Research Program. 相似文献
33.
A.B. Gardner S. Howard T.C. Waddington R.M. Richardson J. Tomkinson 《Chemical physics》1981,57(3):453-460
Incoherent quasi-elastic neutron scattering has been used to study the reorientational motions of the cyclopentadienyl rings in ferrocene, nickelocene and ruthenocene. The results for ferrocene show that the activation energy for ring rotation drops above the 164 K phase transition to 4.4 ± 0.5 kJ mol?1 (which is approximately half its low temperature value) but the rings still appear to jump between only five orientations on the observable time scale. At room temperature, the rings in nickelocene appear to behave the same as in ferrocene but in ruthenocene they reorientate much less frequently and resemble those in ferrocene below 164 K. 相似文献
34.
The short-term (up to 24 h) stability of estuarine water with high (22%?) and low (<21%?) salinity, stored unacidified in polythene containers prior to filtration, preservation and determination of dissolved trace metals, was assessed. For the-salinity sample a massive (140%) increase in copper concentration was observed, indicating desorption from the particulate phase; a similar, although less pronounced (11%) effect was found for the high-salinity sample, Evidence was also obtained for a significant loss (30%) of lead from a pre-filtered low-salinity sample, suggesting appreciable adsorption to the container walls. For cadmium and nickel, no substantial changes in concentration were observed. 相似文献
35.
Des-N-tetramethyltriostin A (1), a known DNA-intercalation agent, has been synthesized from tetra- and octapeptide intermediates that have Z-d-serine at the C-terminal position. The procedure thus allows the fragment coupling and cyclization reactions leading to the synthesis of the title compound to occur without racemization at the C-terminal amino acid. Esterification of Boc-Val-OH with the p-bromophenacyl ester of Z-d-serine provided didepsipeptide Z-d-Ser(Boc-Val)-OBpa (4). Stepwise addition of the requisite amino acids provided tetradepsipeptide Z-d-Ser[Boc-Ala-Cys(Acm)-Val]-OBpa (6). Fragment coupling of the respective C- and N-deprotected tetradepsipeptides 7 and 8, derived from 6, furnished linear octadepsipeptide 9, which upon cyclization and disulfide formation gave the bicyclic octadepsipeptide 11, a known synthetic precursor to 1. The degree of racemization incurred in the alanine and valine residues of selected depsipeptides was measured and the results compared with those obtained in previous studies. It was concluded that alanine, perhaps because of sequence effects, undergoes a degree of racemization (4–10%) during hydrolysis of tetradepsipeptide 6 and octadepsipeptide 9. 相似文献
36.
Classical time-frequency analysis is based on the amplitude responses of bandpass filters, discarding phase information. Instantaneous frequency analysis, in contrast, is based on the derivatives of these phases. This method of frequency calculation is of interest for its high precision and for reasons of similarity to cochlear encoding of sound. This article describes a methodology for high resolution analysis of sparse sounds, based on instantaneous frequencies. In this method, a comparison between tonotopic and instantaneous frequency information is introduced to select filter positions that are well matched to the signal. Second, a cross-check that compares frequency estimates from neighboring channels is used to optimize filter bandwidth, and to signal the quality of the analysis. These cross-checks lead to an optimal time-frequency representation without requiring any prior information about the signal. When applied to a signal that is sufficiently sparse, the method decomposes the signal into separate time-frequency contours that are tracked with high precision. Alternatively, if the signal is spectrally too dense, neighboring channels generate inconsistent estimates-a feature that allows the method to assess its own validity in particular contexts. Similar optimization principles may be present in cochlear encoding. 相似文献
37.
Llobet A Christianson AD Bao W Gardner JS Swainson IP Lynn JW Mignot JM Prokes K Pagliuso PG Moreno NO Sarrao JL Thompson JD Lacerda AH 《Physical review letters》2005,95(21):217002
The heavy fermion system exhibits properties that range from an incommensurate antiferromagnet for small to an exotic superconductor on the Ir-rich end of the phase diagram. At intermediate where antiferromagnetism coexists with superconductivity, two types of magnetic order are observed: the incommensurate one of and a new, commensurate antiferromagnetism that orders separately. The coexistence of -electron superconductivity with two distinct -electron magnetic orders is unique among unconventional superconductors, adding a new variety to the usual coexistence found in magnetic superconductors. 相似文献
38.
Beauvilain TA Bolton JS Gardner BK 《The Journal of the Acoustical Society of America》2000,107(3):1189-1202
Theory related to global, free-field cancellation of a primary monopole field by the use of a displaced, secondary multipole was presented previously: a corresponding experimental investigation is presented here. The construction of multipole source components to octopole order is described, as are procedures for determining their source strengths. Dipoles, longitudinal quadrupoles, and longitudinal octopoles that conformed closely to their theoretical models were constructed using arrays of unbaffled loudspeakers. Two methods of calculating the multipole strengths required to cancel a primary monopole field were implemented in an open-loop manner: a "direct" approach based on a multipole expansion of the primary field, and a least-squares procedure based on fitting the secondary field to the primary field either along a circle enclosing a secondary source, or along a segment of that circle. Cancellation measurements were made on a 1-m-radius circle centered on the secondary source: the primary-to-secondary source separation was approximately 0.2 wavelengths. It was found both that a secondary multipole could provide far greater cancellation than a monopole placed at the same distance from the primary source and that the least-squares approach resulted in greater far-field cancellation than did the direct approach. 相似文献
39.
Gardner TN 《The Journal of the Acoustical Society of America》2000,107(1):163-176
The acoustic response of gassy seabed sediment is unique. It is a dispersive and extraordinarily attenuative natural material at frequencies which cause gas bubble resonance. It conceals the structure of the seabed from seismic profiling and it dampens acoustic signals that, for example, trigger acoustic mines. In the past, theoretical studies have formulated the probable cause of this response and crude experimental work has partially corroborated theory. This study measures compressional wave velocity and attenuation in a laboratory soil simulating natural gassy soil, and it investigates the structural properties that cause the unique acoustic response. It was confirmed that below the frequencies which cause resonance the soil behaves as a compressible material (containing gas), and above as a relatively incompressible material (containing no gas). Over the frequency range producing bubble resonance it is suggested that the soil should be modeled as a biphasic material of gas and a relatively incompressible saturated soil matrix (particles and fluid). Velocities for gassy soil were found to be as low as 220 m/s at frequencies below resonance and 1500 m/s above resonance; attenuations were found to be as high as 60 dB/cm for moderately gassy soil and as low as 1 dB/cm for soil with almost no gas. 相似文献
40.
Evenson William E. Lu Jun Winz Michele W. Gardner John A. Zacate Matthew O. Lee Teresa Mommer Niels 《Hyperfine Interactions》1999,120(1-8):427-431
In order to explain PAC data for tetragonal zirconia at temperatures between 900 and 1300oC, we have developed a four-state
stochastic model. The model simulates vacancies which trap and detrap at a PAC probe nucleus. While trapped, the vacancies
hop around the probe in equivalent sites. The four states in this Winkler–Gerdau stochastic theory are three trapped states
with equivalent electric field gradients (EFGs) of different orientations and a detrapped state with a weaker EFG whose axis
of symmetry is oriented along the diagonal between the three trapped EFGs. There are three hopping rates in this model: w,
the rate a trapped vacancy hops around the probe, wD, the detrapping rate, and wt, the trapping rate. We report results of calculations for values of these hopping rates implied by our tetragonal zirconia
data, and we report heuristic fitting functions which summarize the computer results and can be used to fit data efficiently
for a wide range of parameters.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献