全文获取类型
收费全文 | 1362篇 |
免费 | 66篇 |
国内免费 | 7篇 |
专业分类
化学 | 865篇 |
晶体学 | 7篇 |
力学 | 34篇 |
数学 | 236篇 |
物理学 | 293篇 |
出版年
2023年 | 9篇 |
2022年 | 21篇 |
2021年 | 26篇 |
2020年 | 33篇 |
2019年 | 36篇 |
2018年 | 25篇 |
2017年 | 15篇 |
2016年 | 37篇 |
2015年 | 47篇 |
2014年 | 48篇 |
2013年 | 72篇 |
2012年 | 101篇 |
2011年 | 113篇 |
2010年 | 63篇 |
2009年 | 55篇 |
2008年 | 99篇 |
2007年 | 75篇 |
2006年 | 67篇 |
2005年 | 53篇 |
2004年 | 47篇 |
2003年 | 35篇 |
2002年 | 27篇 |
2001年 | 12篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 17篇 |
1997年 | 12篇 |
1995年 | 12篇 |
1994年 | 7篇 |
1993年 | 12篇 |
1992年 | 8篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 16篇 |
1983年 | 6篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1974年 | 9篇 |
1973年 | 6篇 |
1971年 | 6篇 |
排序方式: 共有1435条查询结果,搜索用时 15 毫秒
81.
82.
Mordecai Lewin 《Journal of Combinatorial Theory, Series A》1974,16(2):131-146
Subsets of a Cartesian product X × Y, where X and Y are arbitrary sets, are considered as a generalization of incidence matrices. Minimal cover, essential set etc. are introduced in a stronger sense and their properties discussed. The existence of a minimal cover for an arbitrary generalized incidence matrix is proved. As an application a previous result is extended. 相似文献
83.
Ohne Zusammenfassung 相似文献
84.
Ohne Zusammenfassung 相似文献
85.
[reaction: see text] Cyclic hydrazides function as asymmetric organocatalysts in aqueous Diels-Alder reactions. The hydrazide is employed as the catalytic machinery in a compact camphor-derived framework that imparts facial selectivity to the cycloadditions. Kinetic evidence suggests the reaction involves rapid iminium formation. 相似文献
86.
The magnetic and electrical properties of high-quality single crystals of A-site disordered (solid solution) Ln0.5Ba0.5MnO3 are investigated near the phase boundary between the spin-glass insulator and colossal-magnetoresistive ferromagnetic metal, locating near Ln=Sm. The temperature dependence of the ac susceptibility and the x-ray diffuse scattering of Eu0.5Ba0.5MnO3 are analyzed in detail. The uniformity of the random potential perturbation in Ln0.5Ba0.5MnO3 crystals with a small bandwidth yields, rather than the phase separation, an homogeneous short ranged charge or orbital order which gives rise to a nearly atomic spin-glass state. Remarkably, this microscopically disordered "charge-exchange-glass" state alone is able to bring forth the colossal magnetoresistance. 相似文献
87.
The multiconfiguration methods are the natural generalization of the well-known Hartree-Fock theory for atoms and molecules. By a variational method, we prove the existence of a minimum of the energy and of infinitely many solutions of the multiconfiguration equations, a finite number of them being interpreted as excited states of the molecule. Our results are valid when the total nuclear charge Z exceeds N–1 (N is the number of electrons) and cover most of the methods used by chemists. The saddle points are obtained with a min-max principle; we use a Palais-Smale condition with Morse-type information and a new and simple form of the Euler-Lagrange equations. 相似文献
88.
Knowing the low-frequency response of hydrophones, down to 100 kHz at least, is important for accurate biomedical ultrasound measurements. However, current international standards do not extend below 500 kHz. Furthermore, commercial hydrophone sources typically do not supply sensitivity data below 1-2 MHz. Therefore, to help identify and validate practical calibration methods below 2 MHz, the authors have extended their previous individual efforts in an interlaboratory evaluation of sensitivity calibration using the swept-frequency technique, time delay spectrometry (TDS). Calibrations were performed for needle and membrane PVDF hydrophones using each laboratory's TDS system. Each site employed the same purpose-built broadband source transducers, comprising both plano-concave and biconcave 1-3 piezocomposite elements 4 cm in diameter, with maximum and minimum thicknesses of approximately 1.5 and 0.1 cm. Agreement between laboratories was within the estimated measurement precision of +/-0.6 dB. The results demonstrated that a TDS system employing such transducers constitutes a viable method for hydrophone calibrations in this frequency range. 相似文献
89.
To facilitate the implementation and verification of the new ultrasound hydrophone calibration techniques described in the companion paper (somewhere in this issue) a nonlinear propagation model was developed. A brief outline of the theoretical considerations is presented and the model's advantages and disadvantages are discussed. The results of simulations yielding spatial and temporal acoustic pressure amplitude are also presented and compared with those obtained using KZK and Field II models. Excellent agreement between all models is evidenced. The applicability of the model in discrete wideband calibration of hydrophones is documented in the companion paper somewhere in this volume. 相似文献
90.
Optical sectioning in wide-field microscopy is achieved by illumination of the object with a continuously moving single-spatial-frequency pattern and detecting the image with a smart pixel detector array. This detector performs an on-chip electronic signal processing that extracts the optically sectioned image. The optically sectioned image is directly observed in real time without any additional postprocessing. 相似文献