全文获取类型
收费全文 | 62162篇 |
免费 | 277篇 |
国内免费 | 549篇 |
专业分类
系统科学 | 1246篇 |
丛书文集 | 604篇 |
教育与普及 | 285篇 |
理论与方法论 | 538篇 |
现状及发展 | 33655篇 |
研究方法 | 1028篇 |
综合类 | 23441篇 |
自然研究 | 2191篇 |
出版年
2013年 | 806篇 |
2012年 | 658篇 |
2011年 | 2739篇 |
2009年 | 600篇 |
2008年 | 872篇 |
2007年 | 1013篇 |
2006年 | 1100篇 |
2005年 | 1251篇 |
2004年 | 2344篇 |
2003年 | 1972篇 |
2002年 | 1651篇 |
2001年 | 1625篇 |
2000年 | 1184篇 |
1999年 | 1072篇 |
1998年 | 650篇 |
1997年 | 774篇 |
1996年 | 535篇 |
1994年 | 686篇 |
1993年 | 695篇 |
1992年 | 1010篇 |
1991年 | 896篇 |
1990年 | 969篇 |
1989年 | 763篇 |
1988年 | 751篇 |
1987年 | 762篇 |
1986年 | 807篇 |
1985年 | 1037篇 |
1984年 | 860篇 |
1983年 | 741篇 |
1982年 | 824篇 |
1981年 | 864篇 |
1980年 | 980篇 |
1979年 | 1575篇 |
1978年 | 1411篇 |
1977年 | 1373篇 |
1976年 | 1227篇 |
1975年 | 1241篇 |
1974年 | 1200篇 |
1973年 | 1368篇 |
1972年 | 1437篇 |
1971年 | 1516篇 |
1970年 | 1801篇 |
1969年 | 1513篇 |
1968年 | 1407篇 |
1967年 | 1349篇 |
1966年 | 1152篇 |
1965年 | 900篇 |
1959年 | 551篇 |
1958年 | 733篇 |
1957年 | 618篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
221.
基于层面信息的路径优化 总被引:5,自引:0,他引:5
根据分层物体快速成型(LOM)中零件逐层累积的特点,从缩短加工时间、提高制件效率方面着手阐述了路径优化的必要性,针对LOM机的运动机理和软件数据结构,给出了扫描路径优化的原理,将图论中的便宜算法用于LOM机的扫描路径优化。 相似文献
222.
永磁同步直线电机弱磁调速时的设计 总被引:1,自引:0,他引:1
从凸极率角度出发,对内部磁极结构电机提出了新的设计方法,为使电机的特性具有可比性,假设电机均运行于相同的基值点,并采取相同的控制策略,着重了研究凸极率(ld/lq)对电机最大可能速度,逆变器功能(VA)和电流控制特性的影响,所得结论对电机及其驱动系统的选择和有积极的参考指导作用。 相似文献
223.
量子力学是当前物理学及其相关科学的重要科学分支,也是当前许多新科学技术中的重要应用的基础。本书包含了现代新科学技术工程师所需要的量子力学方面的新材料,如研究和了解原子核、原子、分子和固体材料,以及激光器和其他量子光学器件所需要的科学资料等。 相似文献
224.
R.E. Kastner 《Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics》2005,36(4):649-658
A modified version of Young's experiment by Shahriar Afshar demonstrates that, prior to what appears to be a “which-way” measurement, an interference pattern exists. Afshar has claimed that this result constitutes a violation of the Principle of Complementarity. This paper discusses the implications of this experiment and considers how Cramer's Transactional Interpretation easily accommodates the result. It is also shown that the Afshar experiment is analogous in key respects to a spin one-half particle prepared as “spin up along x”, subjected to a nondestructive confirmation of that preparation, and post-selected in a specific state of spin along z. The terminology “which-way” or “which-slit” is critiqued; it is argued that this usage by both Afshar and his critics is misleading and has contributed to confusion surrounding the interpretation of the experiment. Nevertheless, it is concluded that Bohr would have had no more problem accounting for the Afshar result than he would in accounting for the aforementioned pre- and post-selection spin experiment, in which the particle's preparation state is confirmed by a nondestructive measurement prior to post-selection. In addition, some new inferences about the interpretation of delayed choice experiments are drawn from the analysis. 相似文献
225.
226.
The fecundity of the blood-feeding insect,Rhodnius prolixus, was observed to increase in successive periods of egg production, each period being triggered by a single large blood meal. As previously published, the fecundity of mated animals was significantly higher than that of unmated animals for the first period of egg production. For a second period of egg production, fecundity increased significantly in both mated and unmated animals. By the fourth period, fecundity had returned to first-feed values for mated animals, but remained high for unmated animals, and the fecundity of mated and unmated animals was not significantly different. Thus, during successive periods of egg production, the processes which maintain fecundity of unmated animals below that of mated animals are overcome. 相似文献
227.
Induction of hemopoietic chimerism in the caprine fetus by intraperitoneal injection of fetal liver cells 总被引:1,自引:0,他引:1
R. D. Pearce D. Kiehm D. T. Armstrong P. B. Little J. W. Callahan L. R. Klunder J. T. R. Clarke 《Cellular and molecular life sciences : CMLS》1989,45(3):307-308
Summary Intraperitoneal injection of allogeneic liver cells from 43-day-old male fetuses into normal 60-day female goat fetuses resulted in persistent hemopoietic chimerism in surviving recipients without clinical evidence of graft-versus-host disease. Transplantation of normal fetal liver cells into preimmunocompetent goat fetuses affected with -D-mannosidosis may provide an alternative strategy for evaluating hemopoietic stem cell transplantation in the treatment of human lysosomal storage diseases. 相似文献
228.
P. Levy G. Cherqui A. Robert D. Wicek J. Picard 《Cellular and molecular life sciences : CMLS》1989,45(6):588-591
Summary During the spontaneous differentiation (day 5 to day 15 of the culture) of Caco-2 cells, the sulfation of cell layer glycosaminoglycans increased, whereas protein kinase C activity was concomitantly redistributed from the membrane to the cytosol. The protein kinase C activators, 4-phorbol 12-myristate, 13-acetate and 1,2-dioctanoyl-glycerol inhibited glycosaminoglycan sulfation. By contrast, 4-phorbol 12, 13 didecanoate was ineffective.These results suggest that membrane-bound PKC may exert a modulatory effect on glycosaminoglycan sulfation, and this effect is gradually attenuated as Caco-2 cell differentiation progresses. 相似文献
229.
S. Shiozawa K. Shiozawa S. Shimizu Y. Tanaka I. Morimoto Y. Kuroki R. Yoshihara T. Fujita 《Cellular and molecular life sciences : CMLS》1989,45(8):764-765
Summary A sensitive radioimmunoassay showed that circulating -interferon in the plasma of healthy individuals was low in children and reached the highest level in the young adult, then declined gradually with age. Circulating -interferon was 0.201±0.059 ng/ml in males (n=19) and 0.184±0.076 ng/ml in females (n=14) at ages 30–39 years old. It was noted that circulating -interferon was maintained up to a certain level even in elderly individuals. 相似文献
230.
Summary Calcium is released from the isolated heavy sarcoplasmic reticulum (SR) of frog skeletal muscle upon application of 0.1–1 mM diethylpyrocarbonate (DEP, an imidazolyl reagent). The Ca-ATPase activity of SR was suppressed by 20% in the presence of 1 mM DEP. More than 1 mM of free magnesium ion or 5 M ruthenium red eliminated the effect of DEP on calcium release but not on Ca-ATPase activity. A plausible site of DEP action is on the calcium channel. 相似文献