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991.
992.
Noise estimation is a challenging task in magnetic resonance imaging (MRI), with applications in quality assessment, filtering or diffusion tensor estimation. Main noise estimators based on the Rician model are revisited and classified in this article, and new useful methods are proposed. Additionally, all the surveyed estimators are extended to the noncentral chi model, which applies to multiple-coil MRI and some important parallel imaging algorithms for accelerated acquisitions. The proposed new noise estimation procedures, based on the distribution of local moments, show better performance in terms of smaller variance and unbiased estimation over a wide range of experiments, with the additional advantage of not needing to explicitly segment the background of the image. 相似文献
993.
The Eu2+and Dy3+ activated Sr3Al2O6 (S3A2O-ED) nanophosphors were synthesized by a new microwave method. The S3A2O-ED sample calcined in microwave oven at around 650 °C for 20 min possesses a cubic Sr3Al2O6 single phase. The sample showed small size (80–100 nm) and spherical shape. The excitation and emission spectra indicated that excitation broad band chiefly sited in visible range and the nanophosphors emitted strong light at 611 nm under around 473 nm excitation. Comparing with conventional method, the microwave synthesis of S3A2O-ED greatly decreased the calcining temperature and time. However, the brightness of S3A2O-ED nanophosphors was reduced. The change of luminescent intensity in S3A2O-ED nanophosphors could be attributed to the effect of surface energy. 相似文献
994.
995.
This Letter is concerned with the robust state estimation problem for uncertain time-delay Markovian jumping genetic regulatory networks (GRNs) with SUM logic, where the uncertainties enter into both the network parameters and the mode transition rate. The nonlinear functions describing the feedback regulation are assumed to satisfy the sector-like conditions. The main purpose of the problem addressed is to design a linear estimator to approximate the true concentrations of the mRNA and protein through available measurement outputs. By resorting to the Lyapunov functional method and some stochastic analysis tools, it is shown that if a set of linear matrix inequalities (LMIs) is feasible, the desired state estimator, that can ensure the estimation error dynamics to be globally robustly asymptotically stable in the mean square, exists. The obtained LMI conditions are dependent on both the lower and the upper bounds of the delays. An illustrative example is presented to demonstrate the feasibility of the proposed estimation schemes. 相似文献
996.
When the wavelength difference of two fiber lasers, sharing a common reflection mirror with the aid of an optical coupler, is tuned to fall into a certain range, the array lasers can be locked to generate coherent radiation with a wavelength somewhere between the two constituent field wavelengths. Based on the fact that the laser should oscillate at the wavelength with the lowest losses and the band widths of the reflection curves of the laser tuning elements are finite, the locking range of the laser array has been quantified. For the tuning element (i.e., fiber gratings) selected in this work, the calculations show that the locking range is mainly dependent on the coupling coefficients of the gratings which ever is smaller. 相似文献
997.
We demonstrate and explicate Bayesian methods for fitting the parameters that encode the impact of short-distance physics on observables in effective field theories (EFTs). We use Bayes’ theorem together with the principle of maximum entropy to account for the prior information that these parameters should be natural, i.e., O(1) in appropriate units. Marginalization can then be employed to integrate the resulting probability density function (pdf) over the EFT parameters that are not of specific interest in the fit. We also explore marginalization over the order of the EFT calculation, M, and over the variable, R, that encodes the inherent ambiguity in the notion that these parameters are O(1). This results in a very general formula for the pdf of the EFT parameters of interest given a data set, D. We use this formula and the simpler “augmented χ2” in a toy problem for which we generate pseudo-data. These Bayesian methods, when used in combination with the “naturalness prior”, facilitate reliable extractions of EFT parameters in cases where χ2 methods are ambiguous at best. We also examine the problem of extracting the nucleon mass in the chiral limit, M0, and the nucleon sigma term, from pseudo-data on the nucleon mass as a function of the pion mass. We find that Bayesian techniques can provide reliable information on M0, even if some of the data points used for the extraction lie outside the region of applicability of the EFT. 相似文献
998.
Krzysztof Bolejko 《General Relativity and Gravitation》2009,41(8):1737-1755
This paper presents the application of the Szekeres Swiss Cheese model to the analysis of observations of the cosmic microwave
background (CMB) radiation. The impact of inhomogeneous matter distribution on the CMB observations is in most cases studied
within the linear perturbations of the Friedmann model. However, since the density contrast and the Weyl curvature within
the cosmic structures are large, this issue is worth studying using another approach. The Szekeres model is an inhomogeneous,
non-symmetrical and exact solution of the Einstein equations. In this model, light propagation and matter evolution can be
exactly calculated, without such approximations as small amplitude of the density contrast. This allows to examine in more
realistic manner the contribution of the light propagation effect to the measured CMB temperature fluctuations. The results
of such analysis show that small-scale, non-linear inhomogeneities induce, via Rees-Sciama effect, temperature fluctuations
of amplitude 10−7–10−5 on angular scale ϑ < 0.24° (ℓ > 750). This is still much smaller than the measured temperature fluctuations on this angular scale. However, local and uncompensated
inhomogeneities can induce temperature fluctuations of amplitude as large as 10−3, and thus can be responsible the low multipoles anomalies observed in the angular CMB power spectrum. 相似文献
999.
Ido Adam 《General Relativity and Gravitation》2009,41(4):691-705
1000.
David K. Anthony 《Applied Acoustics》2009,70(9):1219-1225
For a single degree of freedom system, especially with non-light damping, the use of the real spectral part of either the displacement or velocity responses (or the transfer functions based on them) has the advantages of determining the natural frequency (fn) directly, independent of the response parameter, and providing an accurate measurement for damping (ζ). However, this method is sensitive to spectral phase errors due to temporal misalignment of the signal or due to net inter-channel delay differences caused by signal filtering. Two techniques are presented to correct for these errors; one based on the correct temporal alignment of the impulsive part, and the other on the infimum of the imaginary spectral part. These are first demonstrated using a numerical model, and are shown to facilitate the correct measurement of fn and bring ζ within the expected error limits due to quantisation. Secondly, they are applied to an experimental system and are seen to greatly improve the consistency between measurements using different methods. 相似文献