全文获取类型
收费全文 | 46040篇 |
免费 | 3758篇 |
国内免费 | 5篇 |
专业分类
49803篇 |
出版年
2023年 | 226篇 |
2021年 | 379篇 |
2020年 | 315篇 |
2019年 | 316篇 |
2018年 | 890篇 |
2017年 | 923篇 |
2016年 | 997篇 |
2015年 | 924篇 |
2014年 | 1094篇 |
2013年 | 1935篇 |
2012年 | 3136篇 |
2011年 | 3483篇 |
2010年 | 1801篇 |
2009年 | 1241篇 |
2008年 | 2880篇 |
2007年 | 2941篇 |
2006年 | 2748篇 |
2005年 | 2495篇 |
2004年 | 2376篇 |
2003年 | 2227篇 |
2002年 | 2226篇 |
2001年 | 1516篇 |
2000年 | 1758篇 |
1999年 | 941篇 |
1998年 | 479篇 |
1997年 | 373篇 |
1996年 | 439篇 |
1995年 | 378篇 |
1994年 | 408篇 |
1993年 | 358篇 |
1992年 | 376篇 |
1991年 | 311篇 |
1990年 | 283篇 |
1989年 | 268篇 |
1988年 | 255篇 |
1987年 | 252篇 |
1986年 | 216篇 |
1985年 | 311篇 |
1984年 | 369篇 |
1983年 | 340篇 |
1982年 | 313篇 |
1981年 | 299篇 |
1980年 | 265篇 |
1979年 | 263篇 |
1978年 | 263篇 |
1977年 | 244篇 |
1976年 | 239篇 |
1975年 | 286篇 |
1974年 | 213篇 |
1973年 | 200篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
51.
52.
In soil micromorphology fissures are considered in vertical sections. To get information about the properties of the soil the joint distribution of spatial direction and width of these fissures is of interest. The fissures are mathematically generalized to flat bodies which are defined as stationary weighted surface processes with the weight “thickness”. In a typical point of the surface process suitable, joint parametric distributions of direction and thickness are assumed. The parameters have to be estimated from measurements on vertical sections which are taken from the soil. On these sections only a visible thickness and a visible angle can be observed. The joint distribution of these variables can be expressed by the joint distribution of spatial direction and thickness with the same parameters and in this indirect way the parameters can be estimated. The paper describes how to randomize the vertical section and how to measure the visible variables on the sections. The Chi-Square method is proposed for the parameter estimation. Further it is discussed how to derive good starting values for the numerical procedure. All this is demonstrated in a simulation study using the Bingham-Mardia distribution for the direction and the lognormal distribution for the thickness including a way to correlate the mean thickness and the direction. Finally an application in soil micromorphology is demonstrated for one soil horizon. 相似文献
53.
Johannes Schwerk Mario K?ster Hansj?rg Hauser Manfred Rohde Marcus Fulde Mathias W. Hornef Tobias May 《PloS one》2013,8(8)
Cell lines derived from the small intestine that reflect authentic properties of the originating intestinal epithelium are of high value for studies on mucosal immunology and host microbial homeostasis. A novel immortalization procedure was applied to generate continuously proliferating cell lines from murine E19 embryonic small intestinal tissue. The obtained cell lines form a tight and polarized epithelial cell layer, display characteristic tight junction, microvilli and surface protein expression and generate increasing transepithelial electrical resistance during in vitro culture. Significant up-regulation of Cxcl2 and Cxcl5 chemokine expression upon exposure to defined microbial innate immune stimuli and endogenous cytokines is observed. Cell lines were also generated from a transgenic interferon reporter (Mx2-Luciferase) mouse, allowing reporter technology-based quantification of the cellular response to type I and III interferon. Thus, the newly created cell lines mimic properties of the natural epithelium and can be used for diverse studies including testing of the absorption of drug candidates. The reproducibility of the method to create such cell lines from wild type and transgenic mice provides a new tool to study molecular and cellular processes of the epithelial barrier. 相似文献
54.
John A. Nyakatura Vivian R. Allen Jonas Laustr?er Amir Andikfar Marek Danczak Hans-Jürgen Ullrich Werner Hufenbach Thomas Martens Martin S. Fischer 《PloS one》2015,10(9)
Orobates pabsti, a basal diadectid from the lower Permian, is a key fossil for the understanding of early amniote evolution. Quantitative analysis of anatomical information suffers from fragmentation of fossil bones, plastic deformation due to diagenetic processes and fragile preservation within surrounding rock matrix, preventing further biomechanical investigation. Here we describe the steps taken to digitally reconstruct MNG 10181, the holotype specimen of Orobates pabsti, and subsequently use the digital reconstruction to assess body mass, position of the centre of mass in individual segments as well as the whole animal, and study joint mobility in the shoulder and hip joints. The shape of most fossil bone fragments could be recovered from micro-focus computed tomography scans. This also revealed structures that were hitherto hidden within the rock matrix. However, parts of the axial skeleton had to be modelled using relevant isolated bones from the same locality as templates. Based on the digital fossil, mass of MNG 10181 was estimated using a model of body shape that was varied within a plausible range to account for uncertainties of the dimension. In the mean estimate model the specimen had an estimated mass of circa 4 kg. Varying of the mass distribution amongst body segments further revealed that Orobates carried most of its weight on the hind limbs. Mostly unrestricted joint morphology further suggested that MNG 10181 was able to effectively generate propulsion with the pelvic limbs. The digital reconstruction is made available for future biomechanical studies. 相似文献
55.
Delia V?rg? Petru Lucian Cur?eu Lauren?iu Maricu?oiu Florin A. Sava Irina Macsinga Silvia M?gurean 《PloS one》2014,9(11)
This study seeks to explore whether neuroticism, agreeableness, and conscientiousness moderate the influence of relationship conflict experienced in groups on changes in group members'' evaluative cognitions related to teamwork quality (teamwork-related mental models). Data from 216 students, nested in 48 groups were analyzed using a multilevel modeling approach. Our results show that the experience of relationship conflict leads to a negative shift from the pre-task to the post-task teamwork-related mental models. Moreover, the results indicate that conscientiousness buffered the negative association between relationship conflict and the change in teamwork-related mental models. Our results did not support the hypothesized moderating effect of agreeableness and show that the detrimental effect of relationship conflict on the shift in teamwork-related mental models is accentuated for group members scoring low rather than high on neuroticism. These findings open new research venues for exploring the association between personality, coping styles and change in teamwork-related mental models. 相似文献
56.
57.
58.
Continuous coenzyme dependent stereoselective synthesis of sulcatol by alcohol dehydrogenase 总被引:2,自引:0,他引:2
Summary 6-methyl-5-hepten-2-one was reduced to sulcatol ((+)-6-methyl-5-hepten-2-ol) by using alcohol dehydrogenase fromThermoanaerobium brockii in a continuous process. The cofactor NADP(H) was retained by a charged UF-membrane and regenerated by oxidation of isopropanol to acetone. Use of native NADP in a charged UF-membrane reactor proved to be superior to use of PEG coupled NADP in a uncharged UF-membrane reactor. 相似文献
59.
60.