全文获取类型
收费全文 | 7915篇 |
免费 | 666篇 |
国内免费 | 17篇 |
专业分类
系统科学 | 1094篇 |
丛书文集 | 16篇 |
教育与普及 | 1篇 |
理论与方法论 | 301篇 |
现状及发展 | 2150篇 |
研究方法 | 72篇 |
综合类 | 4958篇 |
自然研究 | 6篇 |
出版年
2018年 | 739篇 |
2017年 | 752篇 |
2016年 | 447篇 |
2015年 | 38篇 |
2012年 | 298篇 |
2011年 | 1000篇 |
2010年 | 837篇 |
2009年 | 468篇 |
2008年 | 553篇 |
2007年 | 1017篇 |
2006年 | 57篇 |
2005年 | 114篇 |
2004年 | 209篇 |
2003年 | 276篇 |
2002年 | 147篇 |
2001年 | 61篇 |
2000年 | 67篇 |
1999年 | 58篇 |
1994年 | 272篇 |
1992年 | 41篇 |
1991年 | 26篇 |
1990年 | 37篇 |
1989年 | 36篇 |
1988年 | 35篇 |
1987年 | 35篇 |
1986年 | 29篇 |
1985年 | 46篇 |
1984年 | 31篇 |
1983年 | 18篇 |
1982年 | 21篇 |
1981年 | 20篇 |
1980年 | 28篇 |
1979年 | 62篇 |
1978年 | 23篇 |
1977年 | 39篇 |
1976年 | 29篇 |
1975年 | 29篇 |
1974年 | 42篇 |
1973年 | 44篇 |
1972年 | 43篇 |
1971年 | 42篇 |
1970年 | 44篇 |
1969年 | 42篇 |
1968年 | 44篇 |
1967年 | 48篇 |
1966年 | 32篇 |
1965年 | 23篇 |
1964年 | 18篇 |
1958年 | 16篇 |
1957年 | 17篇 |
排序方式: 共有8598条查询结果,搜索用时 31 毫秒
991.
Aquaporins are implicated in a wide variety of plant physiological processes, although the mechanisms involved in their regulation are not fully understood. To gain further insight into the regulatory factors involved in this process, we used a yeast two-hybrid system to screen for potential binding partners to the Arabidopsis tonoplast intrinsic protein (TIP) AtTIP1;1. This was the first protein identified to be associated with high water permeability in vacuolar membranes from Arabidopsis thaliana. Using ... 相似文献
992.
Two letters from William Burnside have recently been found in the Nachlass of Robert Fricke that contain instances of the Burnside Problem prior to its first publication. We present these letters
as a whole to the public for the first time. We draw a picture of these two mathematicians and describe their activities leading
to their correspondence. We thus gain an insight into their respective motivations, reactions, and attitudes, which may sharpen
the current understanding of professional and social interactions of the mathematical community at the turn of the twentieth
century.
Dedicated to Heiko Harborth on the occasion of his seventieth birthday. 相似文献
993.
水蓄冷空调系统蓄冷槽中斜温层厚度的数值计算 总被引:2,自引:0,他引:2
介绍了水蓄冷空调系统蓄冷槽中温度场与斜温层厚度的数值求解及影响斜温层厚度的因素。 相似文献
994.
Sergio Porté Agrin Moeini Irene Reche Naeem Shafqat Udo Oppermann Jaume Farrés Xavier Parés 《Cellular and molecular life sciences : CMLS》2011,68(6):1065-1077
Human ζ-crystallin is a Zn2+-lacking medium-chain dehydrogenase/reductase (MDR) included in the quinone oxidoreductase (QOR) family because of its activity
with quinones. In the present work a novel enzymatic activity was characterized: the double bond α,β-hydrogenation of medium-chain
2-alkenals and 3-alkenones. The enzyme is especially active with lipid peroxidation products such as 4-hydroxyhexenal, and
a role in their detoxification is discussed. This specificity is novel in the QOR family, and it is similar to that described
in the distantly related alkenal/one reductase family. Moreover, we report the X-ray structure of ζ-crystallin, which represents
the first structure solved for a tetrameric Zn2+-lacking MDR, and which allowed the identification of the active-site lining residues. Docking simulations suggest a role
for Tyr53 and Tyr59 in catalysis. The kinetics of Tyr53Phe and Tyr59Phe mutants support the implication of Tyr53 in binding/catalysis
of alkenal/one substrates, while Tyr59 is involved in the recognition of 4-OH-alkenals. 相似文献
995.
The effects of Cl-concentration,temperature,pH,flow velocity,soluble oxygen content of seawater and anodic current on the potential of high purity zinc and Zn-Al-Cd reference electrodes were investigated. The results show that the investigated metal materials are liable to establish stable potential and act as reference electrodes in seawater,diluted seawater and urban tap water. Cl-concentration,temperature,seawater flow velocity and anodic current have an obvious effect on zinc potentials. However,seawate... 相似文献
996.
To better understand variation of Holocene Chinese mandiblular morphology, a study was conducted on 23 metric traits of Neolithic (n=54), Bronze-Iron Ages (n=184) and modern (n=92) adult male mandibles from northern China. Results indicate that the linear characters of these Chinese mandibles evolved in the past 7000 years. From the Neolithic to Bronze-Iron Ages to present day, the overall size of mandibles decreased. The linear characters of the mandiblular features varied between different time periods. The decrease of thickness and height of the mandibular corpus primarily occurred during the Neolithic to Bronze-Iron Ages. The decrease in main size was during the Bronze-Iron Ages to present day. It is possible that mandibles became thinner before the overall size decreased. Comparisons also indicate that the bottom part of the face may have decreased more greatly in breadth than the upper portion. The decrease in mandible size may be associated with changes in climate and diet, and with changes in the craniums size. 相似文献
997.
用中腔胞壁比值法测定棉纤维成熟度系数有明确的标准规定,但这些规定有些令人费解.为此作者用折射率跟棉纤维折射率相近的厚壁玻璃管模拟棉纤维在放大镜和高倍显微镜下观察,证实了标准规定的合理和必要性.中腔胞壁比值的理论计算值与实验值准确地符合. 相似文献
998.
Heavy meromyosin subfragment-1 (HMM S-1) was prepared by papain digestion of arterial myosin or actomyosin and was purified by agarose-ATP affinity chromatography. Proteolysis of crude arterial myosin suspensions was preceded by solubilization. HMM-S-1 thus obtained consisted mainly of a 90,000 dalton polypeptide and fully retained the K+- and Ca2+-ATPase of the parent myosin. Its affinity to agarose-ATP was comparable to that of skeletal muscle HMM S-1. 相似文献
999.
It has been found that high-order deformation (e.g.β6) can have important effects on the structures of superheavy nuclei. In the present work, we investigate octupole deformation effects on superheavy nuclei with an improved potential-energy-surface (PES) calculation by including reflection-asymmetric deformations in a space of (β2 ,β3 ,β4 , β5). The calculations give various deformations including highly deformed (β2 ≈ 0.4) and superdeformed (β2 ≈ 0.7) shapes. The octupole-deformation degree of freedom mainly affects the fission barrier beyond the second minimum of PES. 相似文献
1000.
<正> In this paper,a two-scale finite element approach is proposed and analyzed for approximationsof Green's function in three-dimensions.This approach is based on a two-scale finite elementspace defined,respectively,on the whole domain with size H and on some subdomain containing singularpoints with size h (h H).It is shown that this two-scale discretization approach is very efficient.In particular,the two-scale discretization approach is applied to solve Poisson-Boltzmann equationssuccessfully. 相似文献