全文获取类型
收费全文 | 29149篇 |
免费 | 2253篇 |
国内免费 | 7篇 |
出版年
2024年 | 23篇 |
2023年 | 205篇 |
2022年 | 331篇 |
2021年 | 713篇 |
2020年 | 463篇 |
2019年 | 587篇 |
2018年 | 837篇 |
2017年 | 740篇 |
2016年 | 1179篇 |
2015年 | 1771篇 |
2014年 | 1847篇 |
2013年 | 2288篇 |
2012年 | 2794篇 |
2011年 | 2541篇 |
2010年 | 1550篇 |
2009年 | 1263篇 |
2008年 | 1734篇 |
2007年 | 1675篇 |
2006年 | 1533篇 |
2005年 | 1369篇 |
2004年 | 1324篇 |
2003年 | 1118篇 |
2002年 | 1071篇 |
2001年 | 223篇 |
2000年 | 199篇 |
1999年 | 200篇 |
1998年 | 192篇 |
1997年 | 141篇 |
1996年 | 141篇 |
1995年 | 114篇 |
1994年 | 86篇 |
1993年 | 84篇 |
1992年 | 93篇 |
1991年 | 79篇 |
1990年 | 77篇 |
1989年 | 83篇 |
1988年 | 69篇 |
1987年 | 44篇 |
1986年 | 52篇 |
1985年 | 61篇 |
1984年 | 70篇 |
1983年 | 42篇 |
1982年 | 40篇 |
1981年 | 38篇 |
1980年 | 38篇 |
1979年 | 37篇 |
1978年 | 26篇 |
1977年 | 23篇 |
1976年 | 31篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
41.
42.
43.
44.
45.
In producing power, humans move the nutrients nitrogen (N) and phosphorus (P) from their long‐term geological and biological stocks and release or emit them in soil, water, and the atmosphere. In Finland, peat combustion is an important driver of N and P fluxes from the environment to human economy. The flows of N and P in the Finnish energy system were quantified with partial substance flow analysis, and the driving forces of emissions of nitrogen oxides (NOx) were analyzed using the ImPACT model. In the year 2000 in Finland, 140,000 tonnes of nitrogen entered the energy system, mainly in peat and hard coal. Combustion released an estimated 66,000 tonnes of N as nitrogen oxides (NOx) and nitrous oxides (N2O) and another 74,000 tonnes as elemental N2. Most of the emissions were borne in traffic. At the same time, 6,000 tonnes of P was estimated to enter the Finnish energy system, mostly in peat and wood. Ash was mainly used in earth construction and disposed in landfills; thus negligible levels of P were recycled back to nature. During the twentieth century, fuel‐borne input of N increased 20‐fold, and of P 8‐fold. In 1900–1950, the increasing use of hard coal slowly boosted N input, whereas wood fuels were the main carrier of P. Since 1970, the fluxes have been on the rise. NOx emissions leveled off in the 1980s, though, and then declined in conjunction with improvements in combustion technologies such as NOx removal (de‐NOx) technologies in energy production and catalytic converters in cars. 相似文献
46.
47.
J T Lecomte G N La Mar J D Smit K H Winterhalter K M Smith K C Langry H K Leung 《Journal of molecular biology》1987,197(1):101-110
Reconstitution of liver fluke (Dicrocoelium dendriticum) apo-hemoglobin with hemins selectively deuterated at specific positions has permitted the assignment of several heme resonances in the proton nuclear magnetic resonance spectrum of the Met-aquo and Met-cyano forms of the holoprotein. It was established that in the Met-aquo form the meso protons resonate at positions characteristic of a six-co-ordinated in-plane iron. From this, we deduced that the Met-aquo species retains a bound water molecule at pH values as low as 4.5. The orientation of the proximal histidine imidazole ring with respect to the heme group in the cavity was determined through the identification of the heme methyl signals and the analysis of the hyperfine shift pattern in the Met-cyano hemoglobin proton nuclear magnetic resonance spectrum. Compared to sperm whale myoglobin, the heme appears to be rotated by 180 degrees about the alpha, gamma meso-axis. Protein isomers with the heme group in a reversed orientation were not detected, even shortly after reconstitution. In the Met-cyano form, the resonances most affected by the Bohr transition were shown to arise from the heme propionates. 相似文献
48.
Gianfranco Canti Laura Ricci Ornella Marelli Paola Franco Angelo Nicolin 《Cancer immunology, immunotherapy : CII》1987,24(1):64-67
Summary New antigenic specificities, not detectable on parental cells, have been induced by many investigators in mouse lymphomas by treatment with the antitumor agent 5(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (DTIC). The antigens are transmissible, after withdrawal of the drug treatment, as an inheritable character. The mechanism of induction, the molecular nature, and the number of the new antigenic specificities have not been completely elucidated. Four clones from murine leukemia L1210 isolated and expanded in vitro were treated in vivo with DTIC and the new sublines were studied in detail. The four drug-treated sublines studied exhibited strong immunogenicity since they were rejected by syngeneic animals. Immunosuppressed animals challenged with 107 A/DTIC or P/DTIC cells were reciprocally protected by the adoptive transfer of spleen cells from donors that had rejected a lethal challenge of A/DTIC or P/DTIC clones. In a similar fashion, the adoptive transfer of spleen cells obtained from animals that had rejected the Q/DTIC or the R/DTIC clones protected immunosuppressed mice challenged with Q/DTIC or R/DTIC cells. No antitumor activity was observed in cross-protective schedules other than those indicated. It was been concluded that (a) the L1210 leukemia line does not have antigenic cells, (b) four DTIC-treated clone sublines were rejected by compatible hosts, and (c) two mutually exclusive sets of antigens were expressed in four antigenic clone sublines.Research supported in part by P.F.O. Contract Grant from C. N. R., Rome, Italy 相似文献
49.
50.
Purdy LM 《Bioethics》1989,3(1):18-34
The morality of surrogate mothering is analyzed from a "consequentialist" framework which attempts to separate those consequences that invariably accompany a given act from those that accompany it only in particular circumstances. Critics of surrogacy argue that it transfers the burden and risk of pregnancy onto another woman, separates sex and reproduction, and separates reproduction and childrearing; none of these acts is necessarily wrong, either morally or for women's or society's basic interests. While surrogate mothering can be rendered immoral if women are coerced into the practice or become victims of subordinating or penalizing contracts, it has the potential to empower women and increase their status in society by providing a job that is less risky and more enjoyable than other jobs women are forced to take and by achieving greater social recognition for reproductive labor. 相似文献