全文获取类型
收费全文 | 13411篇 |
免费 | 1332篇 |
国内免费 | 5篇 |
出版年
2023年 | 53篇 |
2022年 | 106篇 |
2021年 | 196篇 |
2020年 | 132篇 |
2019年 | 171篇 |
2018年 | 219篇 |
2017年 | 204篇 |
2016年 | 355篇 |
2015年 | 533篇 |
2014年 | 625篇 |
2013年 | 747篇 |
2012年 | 968篇 |
2011年 | 966篇 |
2010年 | 598篇 |
2009年 | 569篇 |
2008年 | 771篇 |
2007年 | 760篇 |
2006年 | 712篇 |
2005年 | 716篇 |
2004年 | 714篇 |
2003年 | 665篇 |
2002年 | 624篇 |
2001年 | 180篇 |
2000年 | 142篇 |
1999年 | 184篇 |
1998年 | 213篇 |
1997年 | 142篇 |
1996年 | 138篇 |
1995年 | 120篇 |
1994年 | 132篇 |
1993年 | 114篇 |
1992年 | 116篇 |
1991年 | 101篇 |
1990年 | 81篇 |
1989年 | 92篇 |
1988年 | 81篇 |
1987年 | 87篇 |
1986年 | 82篇 |
1985年 | 96篇 |
1984年 | 78篇 |
1983年 | 95篇 |
1982年 | 97篇 |
1981年 | 73篇 |
1980年 | 53篇 |
1979年 | 72篇 |
1978年 | 72篇 |
1977年 | 59篇 |
1976年 | 50篇 |
1975年 | 58篇 |
1969年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
1.
2.
F. H. Dawson 《Hydrobiologia》1981,77(1):71-76
The design and characteristics of inexpensive and simply constructed equal-energy response photosynthetic irradiance sensors is described for use particularly where several cells are required in comparative ecological studies either above or below water. The dimensions of the sensors can be changed proportionally to suit different applications or components. The response of the sensor to irradiance at varying angles corresponds very closely to that required by the cosine law. The sensor is comparatively insensitive to other environmental variables in field use and gave a stable output; the long term drift was proportional to electrical output but in continuous use, drift is regular and could reach -0.08 year-1 of the total. The spectral range and cosine response is discussed in comparison to other more expensive (x 5–10) commercially available, sensors and to local standards. 相似文献
3.
A. J. Edwards J. D. Hamilton W. D. Nichol G. W. Taylor A. M. Dawson 《BMJ (Clinical research ed.)》1970,1(5692):342-345
Seven patients with “coeliac axis compression syndrome” are reported. Five were treated surgically, but only two did well. A survey of 200 healthy adults showed epigastric bruits in 6·5%; only one of these had dyspepsia, though dyspepsia was present in 12·5% overall.Caution is urged in attributing a causal relationship between coeliac axis compression and pain and in proceeding to arteriography when compression is suspected on clinical grounds. 相似文献
4.
Pairs of radioimmunoassays, each of which include a two-dimensional matrix of standards, have been previously employed to resolve specificity problems in steroid immunoassay. In this study the bivariate radioimmunoassay principle has been applied to simultaneous measurement of plasma antidiuretic hormone, arginine vasopressin, and the synthetic antidiuretic agent 1-deamino-8-D-arginine vasopressin (desmopressin), by utilizing two arginine vasopressin antisera which show significantly different cross-reactivities with the synthetic analog. Data processing consists of mathematical representation of two curved dose-response surfaces followed by solution of this pair of nonlinear simultaneous equations for the unknown arginine vasopressin and desmopressin concentrations. Details of numerical procedures are given in the Appendix. The assay appears entirely adequate in terms of sensitivity, accuracy, and precision for measurement of these antidiuretic agents in clinical samples. No evidence of significant covariance in estimated concentrations could be detected but precision of estimation is (not unexpectedly) a function of the concentration of both agents. The plasma disappearance half-time of desmopressin (probably the second of a biphasic disappearance) was estimated as 37 min in one normal subject, which is in good agreement with a previously reported value of 30 min. 相似文献
5.
6.
Marianne E. Walsh Charles M. Collins Thomas F. Jenkins Alan D. Hewitt Jeff Stark Karen Myers 《Soil & Sediment Contamination》2003,12(5):631-645
Fort Greely, Alaska has an extensive complex of weapon training and testing areas located on lands withdrawn from the public domain under the Military Lands Withdrawal Act (PL106-65). The Army has pledged to implement a program to identify possible munitions contamination. Because of the large size (344,165,000 m2) of the high hazard impact areas, characterization of these constituents will be difficult. We used an authoritative sampling design to find locations most likely to contain explosives-residues on three impact areas. We focused our sampling on surface soils and collected multi-increment and discrete samples at locations of known firing events and from areas on the range that had craters, pieces of munitions, targets, or a designation as a firing point. In the two impact areas used primarily by the Army, RDX was the most frequently detected explosive. In the impact area that was also used by the Air Force, TNT was the most frequently detected explosive. Where detected, the explosives concentrations generally were low (<0.05 mg/kg) except in soils near low-order detonations, where the explosive-filler was in contact with the soil surface. These low-order detonations potentially can serve as localized sources for groundwater contamination if positioned in recharge areas. 相似文献
7.
8.
9.
10.
Birds use change in daylength during the year to time events during their annual cycles. Individual Eurasian siskins Carduelis spinus can breed and winter in widely separated areas in different years. Birds at different latitudes will experience different changes in photoperiod. So how does latitude affect photoperiodic control? Our aim in this study was to find whether Siskins caught from the wild in Britain and exposed to different photoperiodic regimes, typical of widely separated latitudes, would differ in the subsequent timing and duration of their moults and associated processes. Siskins were caught in late February and early March, and initially kept outside on natural photoperiods. From the spring equinox (21 March), they were divided into three groups kept under photoperiodic regimes that simulated latitudes 40°, 55° and 70°N respectively. All three groups showed highly significant subsequent changes in body mass, fat scores and cloacal protuberance size. Moult of the primary feathers started during June – August (mean 9 July), and lasted 61–99 days (mean 75 days). Birds that started to moult late in the season had shorter moult durations. All individuals showed lower mass and fat levels during moult than before or after moult. Crucially, there were no significant differences in the timing of these events between the three photoperiodic groups. Apparently these birds did not use prevailing absolute photoperiod or the prevailing rate of change in photoperiod to time moult‐related seasonal events, but used instead some other feature of the annual photoperiod cycle or some form of interval timer linked to photoperiod. 相似文献