全文获取类型
收费全文 | 75134篇 |
免费 | 6006篇 |
国内免费 | 12918篇 |
专业分类
94058篇 |
出版年
2024年 | 378篇 |
2023年 | 1488篇 |
2022年 | 1848篇 |
2021年 | 2153篇 |
2020年 | 2328篇 |
2019年 | 3155篇 |
2018年 | 2886篇 |
2017年 | 2556篇 |
2016年 | 2412篇 |
2015年 | 2323篇 |
2014年 | 3639篇 |
2013年 | 4640篇 |
2012年 | 2992篇 |
2011年 | 3432篇 |
2010年 | 2709篇 |
2009年 | 3582篇 |
2008年 | 3537篇 |
2007年 | 3868篇 |
2006年 | 3512篇 |
2005年 | 3174篇 |
2004年 | 2926篇 |
2003年 | 2746篇 |
2002年 | 2402篇 |
2001年 | 2145篇 |
2000年 | 1792篇 |
1999年 | 1715篇 |
1998年 | 1495篇 |
1997年 | 1423篇 |
1996年 | 1453篇 |
1995年 | 1455篇 |
1994年 | 1406篇 |
1993年 | 1235篇 |
1992年 | 1208篇 |
1991年 | 1105篇 |
1990年 | 937篇 |
1989年 | 915篇 |
1988年 | 816篇 |
1987年 | 751篇 |
1986年 | 690篇 |
1985年 | 929篇 |
1984年 | 1141篇 |
1983年 | 840篇 |
1982年 | 1038篇 |
1981年 | 796篇 |
1980年 | 786篇 |
1979年 | 689篇 |
1978年 | 546篇 |
1977年 | 492篇 |
1976年 | 434篇 |
1973年 | 289篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
The arsenic ambient water quality criterion (AWQC) for protection of human health via ingestion of aquatic organisms is currently 0.14 μ g/L. This AWQC is derived using a bioconcentration factor (BCF) of 44, which is a consumption-weighted average based on two data points for oysters and fish that was proposed by the U.S. Environmental Protection Agency in 1980 for broad application to freshwater and marine environments. This BCF is based on the assumption that bioaccumulation is a simple linear function of the exposure concentration. In the nearly quarter of a century since this BCF was promulgated, there have been additions to the arsenic bioaccumulation database and a broader scientific understanding of bioaccumulation mechanisms and how they can be applied to estimating tissue concentrations in aquatic organisms. From this database, we identified 12 studies of arsenic bioaccumulation in freshwater fishes in order to explore differences in laboratory-generated BCFs and field-generated bioaccumulation factors (BAFs) and to assess their relationship to arsenic concentrations in water. Our analysis indicates that arsenic concentrations in tissue and arsenic BAFs may be power functions of arsenic concentration in water. A power function indicates that the highest BCF values may occur at low background levels and may decrease as environmental concentrations increase above the ambient range. 相似文献
92.
The variability of DNA content in turkey spermatozoa of four different lines and its correlation with body weight and sperm concentration were studied. In lines selected for lower body weight the DNA content was 2.034 and 2.036 pg per spermatozoon. In lines selected for higher body weight the DNA content was 2.267 and 2.370 pg per spermatozoon. Sperm concentration in 1 mm3 of semen, however, was higher in lines with a lower body weight (6.08–6.21 million) in comparison with lines selected for higher body weight (5.46–5.67 million). The correlations between the DNA content and sperm concentration were negative (r ranged from ?.457 to ?.860). 相似文献
93.
Abstract: Slices cut from five frozen human brains were dissected into 2-mm cubes and assayed for choline acetyltransferase (ChAT) activity and protein content. A pattern of enrichment of ChAT activity was found ventral to the anterior commissure; this finding is consistent with the location of the enzyme in the cells of the nucleus basalis of Meynert. The region beneath the anterior commissure was the only place a discrete enrichment of activity could be found, and the precise topography of the enrichment was somewhat variable from brain to brain. The results are discussed in the light of recent knowledge concerning the source of the cortical cholinergic innervation. 相似文献
94.
The estimation of soil moisture by using the backscattering coefficient of radar in a mountainous region is a challenging task due to the complex topography, which impacts the distribution of soil moisture and changes the backscattering coefficient. Complicated terrain can disturb empirical moisture estimation models, thereby, the resulting estimates of soil moisture are very unlikely reliable. This article proposed an innovative way of integration of the topographic wetness index (TWI) and the backscattering coefficient of soil obtained from the TerraSAR-X image, which improves the accuracy of measurement of the soil moisture. The standard estimation error and the coefficient of determination from the model were used to evaluate the performance of TWI. Our results show that the standard estimation error was decreased from: (1) 4.0% to 3.3% cm3 cm−3 at a depth of 5 cm and (2) 4.5% to 3.9% cm3 cm−3 at a depth of 10 cm. The most reliable estimation was observed at a depth of 5 cm, when it was compared with those of 0–5 cm, 10 cm and 15 cm. The TWI from the digital elevation model (DEM) is useful as a constraint condition for modeling work. This article concludes that the integration of the backscattering coefficient of soil with TWI can significantly reduce the uncertainty in the estimation of soil moisture in a mountainous region. 相似文献
95.
Abstract Both the intracellular and the extracellular concentration of cyclic AMP increases logarithmically in synchronously growing cultures of Escherichia coli . Thus, cyclic AMP by itself cannot regulate growth and division of the bacterium during the cell cycle. 相似文献
96.
97.
1. Time perception is seldom studied in invertebrates, with the limited experimental evidence being insufficient to provide a comprehensive pattern of the capacity of invertebrates to measure time and use it in decision‐making processes. 2. In this study, it was hypothesized that insect parasitoids have evolved the capacity to measure time precisely and to use it to optimize foraging decisions related to host exploitation. To examine time perception in females of the gregarious egg parasitoid Trichogramma euproctidis, the present study used their ability to adjust their investment (number of eggs laid) in a host to the initial transit duration (interval between the first contact with the host and the following contact with the substrate). Females utilize this method to assess host egg size, as a large egg necessarily requires more time to evaluate than a small host. In this study, the initial transit duration for a given sized egg was artificially extended by suspending it. 3. For similar sized hosts, female T. euproctidis significantly increased both oviposition duration and progeny allocation following a longer initial transit duration. 4. These results demonstrate the intrinsic capacity of this parasitoid to measure time and to adjust their progeny investment accordingly. This is believed to be one of the few demonstrations of a retrospective measure of time in an invertebrate. 相似文献
98.
99.
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
100.