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151.
We study the dynamics of excitable integrate-and-fire neurons in a small-world network. At low densities p of directed random connections, a localized transient stimulus results either in self-sustained persistent activity or in a brief transient followed by failure. Averages over the quenched ensemble reveal that the probability of failure changes from 0 to 1 over a narrow range in p; this failure transition can be described analytically through an extension of an existing mean-field result. Exceedingly long transients emerge at higher densities p; their activity patterns are disordered, in contrast to the mostly periodic persistent patterns observed at low p. The times at which such patterns die out follow a stretched-exponential distribution, which depends sensitively on the propagation velocity of the excitation. 相似文献
152.
153.
Numerous products (such as paints, inks, foods, and beverages) and processes (such as coating, coagulation, sedimentation, lubrication, and paper-making) depend on the stability of the liquid films separating the dispersed phase from the continuous phase in the presence of nanoparticles or surfactant micelles. Nanoparticles exhibit a tendency to form ordered layers (i.e., stratification) and particle in-layer structures in these thin films at a sufficiently high concentration, resulting in a structural force that stabilizes the dispersion. The dispersion stability depends on the film size and the nanoparticle concentration. 相似文献
154.
Fan Y Mandelis A Spirou G Vitkin IA 《The Journal of the Acoustical Society of America》2004,116(6):3523-3533
In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging. 相似文献
155.
Optical monitoring relates the dynamic changes in measured light intensity to the extent of treatment-induced coagulation that occurs during laser interstitial thermal therapy. We utilized a two-region Monte Carlo simulation to elucidate the nature of the changes in interstitial radiance and fluence that result from the formation of a volume of thermal coagulation surrounding a cylindrical emitter. Using simulation results, we demonstrate that radiance sensors are more sensitive than traditional fluence sensors to coagulation-induced scattering changes. Radiance measurements take advantage of directional detection angles that are more receptive to the onset and passing of the coagulation boundary. We performed experiments with albumen phantoms to demonstrate the practicality of the radiance method for monitoring interstitial laser thermal therapy. 相似文献
156.
Yang VX Munce N Pekar J Gordon ML Lo S Marcon NE Wilson BC Vitkin IA 《Optics letters》2004,29(15):1754-1756
High-resolution optical coherence tomography demands a large detector bandwidth and a high numerical aperture for real-time imaging, which is difficult to achieve over a large imaging depth. To resolve these conflicting requirements we propose a novel multifocus fiber-based optical coherence tomography system with a micromachined array tip. We demonstrate the fabrication of a prototype four-channel tip that maintains a 9-14-microm spot diameter with more than 500 microm of imaging depth. Images of a resolution target and a human tooth were obtained with this tip by use of a four-channel cascaded Michelson fiber-optic interferometer, scanned simultaneously at 8 kHz with geometric power distribution across the four channels. 相似文献
157.
An analytical potential energy curve is developed from high quality ab initio calculations for the He+Li- interaction. The HeLi- electrostatic complex is found to have an Re of 18.5 bohrs and a De of 0.974 cm(-1). Numerical solution of the rovibrational Schr?dinger equation with this potential indicates two bound levels, (v,J)=(0,0) and (0,1), for all naturally occurring isotopologs (i.e., 4He7Li-, 4He6Li-, 3He7Li-, and 3He6Li-). For the common isotopolog, 4He7Li-, a D0 of 0.207 cm(-1) and an R0 of 26.5 bohrs is determined. 相似文献
158.
Auger rates are calculated for CdSe colloidal quantum dots using atomistic empirical pseudopotential wave functions. We predict the dependence of Auger electron cooling on size, on correlation effects (included via configuration interaction), and on the presence of a spectator exciton. Auger multiexciton recombination rates are predicted for biexcitons as well as for triexcitons. The results agree quantitatively with recent measurements and offer new predictions. 相似文献
159.
In this paper we study the L p ? L r boundedness of the extension operators associated with paraboloids in ${{\mathbb F}_{q}^{d}}In this paper we study the L
p
− L
r
boundedness of the extension operators associated with paraboloids in
\mathbb Fqd{{\mathbb F}_{q}^{d}} , where
\mathbbFq{\mathbb{F}_{q}} is a finite field of q elements. In even dimensions d ≥ 4, we estimate the number of additive quadruples in the subset E of the paraboloids, that is the number of quadruples (x,y,z,w) ? E4{(x,y,z,w) \in E^4} with x + y = z+w. As a result, in higher even dimensions, we obtain the sharp range of exponents p for which the extension operator is bounded, independently of q, from L
p
to L
4 in the case when −1 is a square number in
\mathbbFq{\mathbb{F}_{q}} . Using the sharp L
p
−L
4 result, we improve upon the range of exponents r, for which the L
2 − L
r
estimate holds, obtained by Mockenhaupt and Tao (Duke Math 121:35–74, 2004) in even dimensions d ≥ 4. In addition, assuming that −1 is not a square number in
\mathbbFq{\mathbb{F}_{q}}, we extend their work done in three dimension to specific odd dimensions d ≥ 7. The discrete Fourier analytic machinery and Gauss sum estimates make an important role in the proof. 相似文献
160.
Hetherington NB Kulak AN Sheard K Meldrum FC 《Langmuir : the ACS journal of surfaces and colloids》2006,22(5):1955-1958
Single crystals of calcite with regular patterned surfaces comprising close-packed arrays of hemispherical cavities or domes were produced by crystallization on colloidal monolayers or PDMS replicas of these monolayers, respectively. Perfect replication of the substrate topography was achieved for all colloidal particles, irrespective of their size and surface chemistry when the substrate geometry permitted unrestricted ion flow to the growing crystal. This work demonstrates that crystallization within a mould provides a very general route to producing single crystals with curved surfaces and unusual morphologies and that such patterning can be applied from the micro- to the nanoscale. 相似文献