全文获取类型
收费全文 | 88226篇 |
免费 | 407篇 |
国内免费 | 383篇 |
专业分类
化学 | 24679篇 |
晶体学 | 803篇 |
力学 | 6750篇 |
数学 | 32042篇 |
物理学 | 15186篇 |
无线电 | 9556篇 |
出版年
2021年 | 44篇 |
2020年 | 33篇 |
2019年 | 35篇 |
2018年 | 11835篇 |
2017年 | 11464篇 |
2016年 | 6978篇 |
2015年 | 933篇 |
2014年 | 387篇 |
2013年 | 415篇 |
2012年 | 4225篇 |
2011年 | 11722篇 |
2010年 | 6875篇 |
2009年 | 6817篇 |
2008年 | 7240篇 |
2007年 | 9661篇 |
2006年 | 280篇 |
2005年 | 1642篇 |
2004年 | 1809篇 |
2003年 | 2214篇 |
2002年 | 1106篇 |
2001年 | 271篇 |
2000年 | 316篇 |
1999年 | 169篇 |
1998年 | 211篇 |
1997年 | 148篇 |
1996年 | 200篇 |
1995年 | 119篇 |
1994年 | 77篇 |
1993年 | 96篇 |
1992年 | 61篇 |
1991年 | 66篇 |
1990年 | 54篇 |
1989年 | 61篇 |
1988年 | 61篇 |
1987年 | 59篇 |
1986年 | 57篇 |
1985年 | 49篇 |
1984年 | 43篇 |
1983年 | 36篇 |
1982年 | 41篇 |
1981年 | 41篇 |
1980年 | 46篇 |
1979年 | 44篇 |
1978年 | 34篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
1907年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
931.
932.
933.
O. Fatih Kilic M. Omer Sayin Ibrahim Delibalta Suleyman S. Kozat 《Signal, Image and Video Processing》2017,11(2):235-242
We introduce a new combination approach for the mixture of adaptive filters based on the set-membership filtering (SMF) framework. We perform SMF to combine the outputs of several parallel running adaptive algorithms and propose unconstrained, affinely constrained and convexly constrained combination weight configurations. Here, we achieve better trade-off in terms of the transient and steady-state convergence performance while providing significant computational reduction. Hence, through the introduced approaches, we can greatly enhance the convergence performance of the constituent filters with a slight increase in the computational load. In this sense, our approaches are suitable for big data applications where the data should be processed in streams with highly efficient algorithms. In the numerical examples, we demonstrate the superior performance of the proposed approaches over the state of the art using the well-known datasets in the machine learning literature. 相似文献
934.
Noemi Merayo Ana Balea Elena de la Fuente Ángeles Blanco Carlos Negro 《Cellulose (London, England)》2017,24(2):677-692
Although the positive effect that cellulose nanofibers (CNF) can have on paper strength is known, their effect on flocculation during papermaking is not well understood, and most relevant studies have been carried out in presence of only cationic starch. Flocculation is the key to ensuring retention of fibers, fines, and fillers, and furthermore floc properties have a great influence on paper quality. The aim of this research is to study the interactions between CNF and flocculants by assessing the effect of two types of CNF, from eucalyptus and corn, on the flocculation process induced by three different retention systems [a dual system, polyvinylamine (PVA), and cationic starch as reference]. The results showed that CNF interacted with the flocculants in different ways, affecting flocculation efficiency and floc properties. In general, addition of CNF increased floc stability and minimized overdosing effects. Moreover, presence of CNF increased floc size for given PVA dose; therefore, CNF addition could contribute to improve the wet end in the paper machine if combined with the optimal flocculant and dose. 相似文献
935.
Christelle Peillon Magali Estribeau Pierre Magnan Alice Pelamatti Olivier Saint-Pé Michel Breart de Boisanger 《Optical and Quantum Electronics》2017,49(11):378
The minority carrier continuity equation has been solved with the Green’s function approach in a N/P photodiode under the low-level injection assumption. The analytical solution obtained with this approach depends on the three spatial coordinates and on time. The diffusion and the collection of the excess minority carriers have been studied during the transitional period corresponding to very short integration times. The internal Quantum Efficiency and the diffusion Modulation Transfer Function have been calculated according to time. The simulations showed that they evolve with time until their steady-state values. When the integration time is very short, this evolution has to be taken into account for the estimation of the sensitivity of a photodiode and the contrast on an image of a sensor based on several adjacent N/P-type photodiodes. 相似文献
936.
937.
This work studies the effects of the drain pn capacitance grading coefficient, duty cycle and supply modulation on the operation
of class E amplifier. The class E operation analysis with a combination of nonlinear and linear output capacitances is revisited
and duty cycle is added to the equations to allow further investigations for optimum class E circuit tuning. The presented
analysis method can be used for synthesis as well, where necessary load components are calculated for a transistor with a
specific grading coefficient both in the output capacitance and in the nonlinear feedback capacitance. Further, the grading
coefficient of the nonlinear output capacitance is varied in the paper while investigating the changes in normalized peak
voltages and harmonics. Drain efficiency, supply-to-amplitude modulation (V
dd
/AM) and supply-to-phase modulation (V
dd
/PM) are studied. It is shown that by varying the device duty cycle, one can scale the optimum load and the level of peak
voltages over the switching device. Also, the choice of grading coefficient affects the drain efficiency, V
dd
/AM and V
dd
/PM behaviour of the class E amplifier. Last, a case study about increasing the efficiency of a class E amplifier with hyperabrupt
output junction capacitance is shown. 相似文献
938.
The paper provides an analytical review of some fundamental problems of solid mechanics in high technologies. The review relies partly on the invited report presented by the authors at the 10th All-Russian Congress on Theoretical and Applied Mechanics held in Nizhny Novgorod in 2011. Attention is focused on the problems involved in technologies of materials with a bulk fine-grained (nano) structure and in technologies used to control mechanical properties of surfaces and interfaces. 相似文献
939.
Shinji Suzuki Furitsu Suzuki Yasumasa Kanie Koji Tsujitani Asako Hirai Hironori Kaji Fumitaka Horii 《Cellulose (London, England)》2012,19(3):713-727
The structure and crystallization of carefully isolated sub-elementary fibrils (SEFs) of bacterial cellulose have been investigated
using TEM, WAXD, and high-resolution solid-state 13C NMR. The addition of a suitable amount of fluorescent brightener (FB) to the incubation medium of Acetobacter xylinum effectively suppressed the aggregation of the SEFs into the microfibrils, as previously reported. However, this study confirmed
for the first time that serious structural change in the SEFs occurs during the removal of excess FB by washing with buffer
solutions having pH values higher than 6 or with the alkaline aqueous solution that was frequently used in previous studies.
In contrast, the isolation of unmodified SEFs was successfully performed by utilizing a washing protocol employing pH 7 citrate–phosphate
buffer solution containing 1% sodium dodecyl sulfate. High-resolution solid-state 13C NMR and WAXD measurements revealed that the SEFs thus isolated are in the noncrystalline state in which the pyranose rings
of the almost parallel cellulose chains appear to be stacked on each other. The respective CH2OH groups of the SEFs adopt the gt conformation instead of the tg conformation found in cellulose I
α
and I
β
crystals, and undergo significantly enhanced molecular motion in the absence of intermolecular hydrogen bonding associated
with these groups. The main chains are also subject to rapid motional fluctuations while maintaining the parallel orientation
of the respective chains, indicating that the SEFs have a liquid crystal-like structure with high molecular mobility. Moreover,
the SEFs crystallize into cellulose I
β
when the FB molecules that may adhere to the surface of the SEFs are removed by extraction with boiling 70 v/v% ethanol and
0.1N NaOH aqueous solution. On the basis of these results, the crystallization of the SEFs into the I
α
and I
β
forms is discussed, including the possible formation of the crystalline-noncrystalline periodic structure in native cellulose. 相似文献
940.
Kristin Voiges Roland Adden Marian Rinken Petra Mischnick 《Cellulose (London, England)》2012,19(3):993-1004
The alditol acetate method is a common procedure for sugar analysis, also applied to determine the substituent distribution
in monomer units of polysaccharide ethers like methyl cellulose by gas liquid chromatography. Consisting of several preparation
and work-up steps this procedure is both time consuming and prone to side reactions that promote discrimination of single
constituents, especially when no peralkylation step is performed prior to hydrolysis. As a consequence results scatter in
dependence on individual treatment and conditions. In the context of this work these critical points were overcome by strict
but simplified work-up procedures and using acid instead of alkaline catalyzed acetylation. Under the acidic conditions the
tedious removal of borate is no longer necessary and a reduced time requirement was achieved as well as good reproducibility.
Comparison with independent reference methods excluded a systematic error of the method and confirmed the results obtained.
Without peralkylation, i.e. in the presence of free hydroxyl groups, another fast modification of the method using DMSO as
solvent, no removal of borate, and 1-methylimidazole as catalyst for acetylation was found to produce a systematic error. 相似文献