全文获取类型
收费全文 | 66017篇 |
免费 | 6105篇 |
国内免费 | 22篇 |
专业分类
72144篇 |
出版年
2022年 | 364篇 |
2021年 | 701篇 |
2020年 | 614篇 |
2019年 | 664篇 |
2018年 | 956篇 |
2017年 | 858篇 |
2016年 | 1668篇 |
2015年 | 2787篇 |
2014年 | 2736篇 |
2013年 | 3748篇 |
2012年 | 4563篇 |
2011年 | 4110篇 |
2010年 | 2587篇 |
2009年 | 2124篇 |
2008年 | 3339篇 |
2007年 | 3221篇 |
2006年 | 3240篇 |
2005年 | 2921篇 |
2004年 | 3020篇 |
2003年 | 2821篇 |
2002年 | 2662篇 |
2001年 | 1725篇 |
2000年 | 1800篇 |
1999年 | 1363篇 |
1998年 | 731篇 |
1997年 | 652篇 |
1996年 | 641篇 |
1995年 | 641篇 |
1994年 | 546篇 |
1993年 | 576篇 |
1992年 | 1096篇 |
1991年 | 870篇 |
1990年 | 914篇 |
1989年 | 878篇 |
1988年 | 729篇 |
1987年 | 667篇 |
1986年 | 627篇 |
1985年 | 677篇 |
1984年 | 649篇 |
1983年 | 511篇 |
1982年 | 461篇 |
1981年 | 401篇 |
1980年 | 384篇 |
1979年 | 501篇 |
1978年 | 404篇 |
1977年 | 399篇 |
1976年 | 341篇 |
1975年 | 344篇 |
1974年 | 379篇 |
1973年 | 329篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Effect of Phosphate on the Corrosion of Carbon Steel and on the Composition of Corrosion Products in Two-Stage Continuous Cultures of Desulfovibrio desulfuricans 总被引:9,自引:4,他引:5 下载免费PDF全文
Paul J. Weimer Margaret J. Van Kavelaar Charles B. Michel Thomas K. Ng 《Applied microbiology》1988,54(2):386-396
A field isolate of Desulfovibrio desulfuricans was grown in defined medium in a two-stage continuous culture apparatus with different concentrations of phosphate in the feed medium. The first state (V1) was operated as a conventional chemostat (D = 0.045 h−1) that was limited in energy source (lactate) or phosphate. The second stage (V2) received effluent from V1 but no additional nutrients, and contained a healthy population of transiently starved or resting cells. An increase in the concentration of phosphate in the medium fed to V1 resulted in increased corrosion rates of carbon steel in both V1 and V2. Despite the more rapid corrosion observed in growing cultures relative to that in resting cultures, corrosion products that were isolated under strictly anaerobic conditions from the two culture modes had similar bulk compositions which varied with the phosphate content of the medium. Crystalline mackinawite (Fe9S8), vivianite [Fe3(PO4)2 · 8H2O], and goethite [FeO(OH)] were detected in amounts which varied with the culture conditions. Chemical analyses indicated that the S in the corrosion product was almost exclusively in the form of sulfides, while the P was present both as phosphate and as unidentified components, possibly reduced P species. Some differential localization of S and P was observed in intact corrosion products. Cells from lactate-limited, but not from phosphate-limited, cultures contained intracellular granules that were enriched in P and Fe. The results are discussed in terms of several proposed mechanisms of microbiologically influenced corrosion. 相似文献
92.
Hervé Seitz 《Current biology : CB》2010,20(3):R108-R110
93.
94.
Peter Van Bogaert Lieve Peremans Nadine Diltour Danny Van heusden Tinne Dilles Bart Van Rompaey Donna Sullivan Havens 《PloS one》2016,11(4)
The aim of the study reported in this article was to investigate staff nurses’ perceptions and experiences about structural empowerment and perceptions regarding the extent to which structural empowerment supports safe quality patient care. To address the complex needs of patients, staff nurse involvement in clinical and organizational decision-making processes within interdisciplinary care settings is crucial. A qualitative study was conducted using individual semi-structured interviews of 11 staff nurses assigned to medical or surgical units in a 600-bed university hospital in Belgium. During the study period, the hospital was going through an organizational transformation process to move from a classic hierarchical and departmental organizational structure to one that was flat and interdisciplinary. Staff nurses reported experiencing structural empowerment and they were willing to be involved in decision-making processes primarily about patient care within the context of their practice unit. However, participants were not always fully aware of the challenges and the effect of empowerment on their daily practice, the quality of care and patient safety. Ongoing hospital change initiatives supported staff nurses’ involvement in decision-making processes for certain matters but for some decisions, a classic hierarchical and departmental process still remained. Nurses perceived relatively high work demands and at times viewed empowerment as presenting additional. Staff nurses recognized the opportunities structural empowerment provided within their daily practice. Nurse managers and unit climate were seen as crucial for success while lack of time and perceived work demands were viewed as barriers to empowerment. 相似文献
95.
96.
Summary The influence of total nitrification to nitrate or partial nitrification to nitrite on the soil organic nitrogen status was examined. NH
4
+
–15N was added to the soil in the absence and the presence of NaClO3, respectively nitrapyrin. The first chemical inhibits only nitrate formation, the second inhibits total nitrification. The accumulation of nitrite nitrogen in the soil at levels up to 5 mg kg–1 increased the loss of nitrogen. Yet, it did not increase the binding of mineral nitrogen into soil organic matter, relative to the control soil. The data suggest that the biochemistry of the nitrite formation process, rather than the levels of nitrite ions formed, are of primary importance in the role of nitrification mediated nitrosation of soil organic matter. 相似文献
97.
Changes are reported of Crataegus-Betula dune woodlands between 1950–1980 from Meijendel, a calcareous coastal dune system near the city of The Hague, The Netherlands, which is used as a water catchment area. It concerns both woodlands in degradation stage and, more common, woodlands on the increase. Changes were recorded with help of successive vegetation maps, air photos and permanent plot observations. Woodland increase usually occurs on places with groundwater at or near the surface, either because of natural circumstances or as a result of artificial groundwater recharge through infiltration as part of the water catchment activities. The management of both types of woodland is discussed.Nomenclature follows Heukels-van der Meijden (1983), Flora van Nederland, 20th ed. Wolters-Noordhoff, Groningen. Nomenclature of mosses follows Margadant & During (1982), Beknopte Flora van Nederlandse Blad- en Levermossen, Thieme, Zutphen.The authors have pleasure in thanking their colleagues from the Dune Water Works who supplied data on hydrology. 相似文献
98.
Herman van der Putten Wim Quint Janny van Raaij Els Robanus Maandag Inder M. Verma Anton Berns 《Cell》1981,24(3):729-739
M-MuLV-specific DNA probes were used to establish the state of integration and amplification of recombinant proviral sequences in Moloney virus-induced tumors of Balb/Mo, Balb/c and 129 mice. The somatically acquired viral sequences contain both authentic M-MuLV genomes and recombinants of M-MuLV with endogenous viral sequences. All reintegrated genomes carry long terminal repeat (LTR) sequences at both termini of their genome. In the preleukemic stage a large population of cells exhibiting a random distribution of reintegrated M-MuLV genomes are seen, but during outgrowth of the tumor, selection of cells occurs leaving one or a few clonal descendants in the outgrown tumor. In this latter stage recombinant genomes can be detected. Although these recombinants constitute a heterogeneous group of proviruses, characteristic molecular markers are conserved among many individual proviral recombinants, lending credence to the notion that a certain recombinant structure is a prerequisite for the onset of neoplasia. The structure of these recombinants shows close structural similarities to the previously described mink cell focus-inducing (MCF)-type viruses. 相似文献
99.
In this paper, we present a model for the development of connections between muscle afferents and motoneurones in the human spinal cord. The model consists of a limb with six muscles, one motoneurone pool, one pooled (Ia-like) afferent for each muscle and a central programme generator. The weights of the connections between the afferents and the motoneurone pools are adapted during centrally induced movements of the limb. The connections between the afferents and the motoneurone pools adapt in a hebbian way, using only local information present at the synapses. This neural network is tested in two examples of a limb with two degrees of freedom and six muscles. Despite the simplifications, the model predicts the pattern of autogenic and heterogenic monosynaptic reflexes quite realistically. 相似文献