全文获取类型
收费全文 | 57184篇 |
免费 | 4526篇 |
国内免费 | 6028篇 |
出版年
2023年 | 938篇 |
2022年 | 1070篇 |
2021年 | 1447篇 |
2020年 | 1546篇 |
2019年 | 2261篇 |
2018年 | 2065篇 |
2017年 | 1734篇 |
2016年 | 1586篇 |
2015年 | 1634篇 |
2014年 | 2902篇 |
2013年 | 3683篇 |
2012年 | 2281篇 |
2011年 | 2761篇 |
2010年 | 2111篇 |
2009年 | 2686篇 |
2008年 | 2828篇 |
2007年 | 3000篇 |
2006年 | 2749篇 |
2005年 | 2258篇 |
2004年 | 1939篇 |
2003年 | 1918篇 |
2002年 | 1676篇 |
2001年 | 1452篇 |
2000年 | 1221篇 |
1999年 | 1034篇 |
1998年 | 961篇 |
1997年 | 882篇 |
1996年 | 866篇 |
1995年 | 890篇 |
1994年 | 874篇 |
1993年 | 762篇 |
1992年 | 747篇 |
1991年 | 745篇 |
1990年 | 553篇 |
1989年 | 556篇 |
1988年 | 505篇 |
1987年 | 497篇 |
1986年 | 449篇 |
1985年 | 725篇 |
1984年 | 967篇 |
1983年 | 782篇 |
1982年 | 839篇 |
1981年 | 661篇 |
1980年 | 669篇 |
1979年 | 600篇 |
1978年 | 482篇 |
1977年 | 439篇 |
1976年 | 393篇 |
1974年 | 272篇 |
1973年 | 272篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
A. V. Naumenko G. N. Kovalev V. Yu. Naumenko E. I. Zaraisky N. S. Snegireva 《Biophysics》2006,51(2):209-214
The interactions of model proteins with porous matrices in biosensors are considered. The viscoelastic properties of casein and albumin were assessed by a dynamic method of a piezoquartz resonator by applying thin layers of the studied solutions to a piezocrystal. The experimental data on the viscoelastic characteristics of protein solutions of various concentrations were compared with the characteristics of their tangential motion in the porous carriers of cellulose nitrate. It was demonstrated that the parameters of dynamic viscosity correlated with the motion time of the protein solutions in the porous polymeric carrier. 相似文献
62.
The synthesis and characterization of rare-earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) thiocyanate adducts with tripiperidinophosphine oxide (tpppO) with general formula (RE)(SCN)3(tpppO)3 are reported. Conductance measurements in acetonitrile indicate the non-electrolytic nature of the complexes. Infrared absorption spectra evidence that the SCN− ion coordinates through the nitrogen atom (isothiocyanate form) and that tpppO coordinates through the phosphoryl oxygen. X-ray powder patterns suggest the existence of three different crystal forms: (1) La; (2) an isomorphous series including Ce, Nd and Pr; and (3) another isomorphous series, including Sm, Gd, Eu, Ho, Er, Tb, Lu and Y. The visible spectra of the Nd adduct and the calculated parameters β = 0.98, b1/2 = 0.072 and δ = 1.06 indicate that the metal-ligand bonds are essentially electrostactic. The emission spectra of the Eu compound showed 5D0 → 7FJ bands (J = 0, 1, 2), suggesting a C3v symmetry for the coordination polyhedron. The lifetime of the 5D0 state is 1.28 ms. The emission spectra of the Tb complex presented 5D4 → 7FJ bands (J = 4, 5, 6) and the Dy complex showed the 4F9/2 → 6H13/2 band. The structure of the Pr complex showed that the coordination polyhedron is a trigonal antiprism, with the isothiocyanate anions in one base and three tpppO ligands in the other. Thermal analyses (TG-DTG) were carried out for the Ce, Nd and Gd adducts. Mass losses start between 250 and 334 °C. The final residues at 1300 °C are the corresponding phosphates. 相似文献
63.
Water scarcity is a widespread problem in many parts of the world. Most previous methods of water scarcity assessment only considered water quantity, and ignored water quality. In addition, the environmental flow requirement (EFR) was commonly not explicitly considered in the assessment. In this study, we developed an approach to assess water scarcity by considering both water quantity and quality, while at the same time explicitly considering EFR. We applied this quantity–quality-EFR (QQE) approach for the Huangqihai River Basin in Inner Mongolia, China. We found that to keep the river ecosystem health at a “good” level (i.e., suitable for swimming, fishing, and aquaculture), 26% of the total blue water resources should be allocated to meet the EFR. When such a “good” level is maintained, the quantity- and quality-based water scarcity indicators were 1.3 and 14.2, respectively; both were above the threshold of 1.0. The QQE water scarcity indicator thus can be expressed as 1.3(26%)|14.2, indicating that the basin was suffering from scarcity problems related to both water quantity and water quality for a given rate of EFR. The current water consumption has resulted in degradation of the basin's river ecosystems, and the EFR cannot be met in 3 months of a year. To reverse this situation, future policies should aim to reduce water use and pollution discharge, meet the EFR for maintaining healthy river ecosystems, and substantially improve pollution treatment. 相似文献
64.
Spatial distribution of ant workers and, notably their aggregation/segregation behaviour, is a key-element of the colony social
organization contributing to the efficiency of task performance and division of labour. In polymorphic species, specialized
worker castes notably differ in their intrinsic aggregation behaviour. In this context, knowing the preponderant role of minors
in brood care, we investigate how a stimulus such as brood can influence the spatial patterns of Pheidole pallidula worker castes. In a homogeneous area without brood, it was shown that minors display only a low level of aggregation while
majors form large clusters in the central area. Here we find out that these aggregation patterns of both minors and majors
can be deeply influenced by the presence of brood. For minors, it nucleates or enhances the formation of a large stable cluster.
Such high sensitivity of minors to brood stimuli fits well with their role as main brood tenders in the colony. For majors,
interattraction between individuals still remains the prevailing aggregation factor while brood strongly influences the localisation
of their cluster. We discuss how the balance between interattraction and sensitivity to environmental stimuli determines the
mobility of each worker castes and, consequently, the availability of minors and majors to participate in everyday colony
tasks. Moreover, we will evoke the functional value of majors’ cluster location close to the brood, namely with respect to
social regulation of the colony caste ratio.
Received 30 May 2005; revised 11 January 2006; accepted 13 January 2006. 相似文献
65.
Facultative joint colony founding by social insects (pleometrosis) provides an outstanding opportunity to analyze the costs
and benefits of sociality. Pleometrosis has been documented for a range of social insects, but most studies on the adaptive
benefits of this behavior are restricted to the Hymenoptera. In this study, we provide the first analysis of costs and benefits
associated with pleometrosis for Australian Dunatothrips, which form domiciles by glueing together phyllodes (leaves) of their Acacia host plant. In Dunatothrips aneurae, the distribution of foundress numbers per nest indicated that females formed associations non-randomly. Furthermore, average
group size was independent of both the number of foundresses on the host plant and the number of mature colonies, suggesting
that this behavior was not simply a response to limited availability of nesting sites. Although per capita reproduction declined
with increasing group size, we also identified two benefits of pleometrosis: (1) individual foundresses in groups had higher
survival than solitary foundresses during the brood development period, and (2) larger colony sizes resulting from pleometrosis
provided a benefit later in colony development, because a higher proportion of D. aneurae adults survived invasions by the kleptoparasite Xaniothrips mulga when colony size was larger. These results demonstrate that the reproductive costs of pleometrosis are at least partially
counterbalanced by survival benefits.
Received 4 April 2006; revised 9 September 2006; accepted 20 September 2006. 相似文献
66.
Susan Leekam 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2016,371(1686)
Early psychological theories of autism explained the clinical features of this condition in terms of perceptual and sensory processing impairments. The arrival of domain-specific social cognitive theories changed this focus, postulating a ‘primary’ and specific psychological impairment of social cognition. Across the years, evidence has been growing in support of social cognitive and social attention explanations in autism. However, there has also been evidence for general non-social cognitive impairments in representational understanding, attention allocation and sensory processing. Here, I review recent findings and consider the case for the specificity and primacy of the social cognitive impairment, proposing that we should focus more explicitly on clinically valid features for insights on the integration of ‘social’ and ‘non-social’ cognition. 相似文献
67.
A three-dimensional operational hydrodynamic model, developed at the Institute of Oceanography, University of Gdańsk was used
to forecast hydrological conditions in the Oder Estuary. The model was based on the coastal ocean circulation model known
as the Princeton Ocean Model (POM). Because of wind-driven water backup in the Oder mouth, a simplified operational model
of river discharge, based on water budget in a stream channel, was developed. Linking these two models into a single system
made it possible to forecast water levels, currents, water temperature, and salinity in the estuary. A good fit between the
observed and computed data allowed to consider the model as a reliable environmental tool. Obtaining a hydrological forecast
via a quick website access gives potential users an opportunity to predict the day-by-day course of processes that may affect
different areas of human life and activities, e.g., navigation, port operations, flood protection of coastal areas; the predictions
may also be used in studies of coastal processes in the estuary. 相似文献
68.
Gene flow, adaptive population divergence and comparative population structure across loci 总被引:3,自引:1,他引:2
Robert G. Latta 《The New phytologist》2004,161(1):51-58
69.
70.
Along-shore currents can propagate harmful algal blooms (HABs) over long distances in many coastal areas of the ocean. Harmful dinoflagellate blooms on the west coast of Iberia frequently occur when the Iberian poleward current (IPC) establishes on the continental slope. This has led to the suggestion that HABs could be transported northward by the IPC. To examine this possibility, the microplankton composition along the west coast of Iberia was studied in May 1993 coinciding with the presence of the IPC. The microplankton of the IPC was almost exclusively composed of small flagellates, with the notable absence of the harmful species usually associated with coastal waters. The primary influence of the IPC was to confine coastal microplankton populations to the shelf, where a downwelling convergence prevented their export from the coastal environment. Microplankton assemblages on the shelf revealed a north–south gradient related to different stages of succession. Earlier stages of succession in which diatoms were prominent were found on the northern shelf, whereas dinoflagellates were more abundant in the south. The toxic species Gymnodinium catenatum, which was only present in the southern shelf, did not show a northward transport associated with the IPC. It is suggested that the northward spreading of HABs along the west coast of Iberia must be related to the interaction between the IPC, which accumulates coastal populations on the shelf, and the latitudinal progress of microplankton succession that determines species composition. Thus, during the course of the season, HABs are likely to be observed in the south prior to their development in the north. 相似文献