首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   492701篇
  免费   35828篇
  国内免费   1123篇
  2021年   4214篇
  2019年   3462篇
  2018年   17655篇
  2017年   16398篇
  2016年   14846篇
  2015年   9986篇
  2014年   11341篇
  2013年   14511篇
  2012年   22117篇
  2011年   31912篇
  2010年   24840篇
  2009年   20267篇
  2008年   26381篇
  2007年   28639篇
  2006年   15040篇
  2005年   15302篇
  2004年   15056篇
  2003年   14735篇
  2002年   14036篇
  2001年   13979篇
  2000年   13552篇
  1999年   10838篇
  1998年   4706篇
  1997年   4149篇
  1996年   3941篇
  1995年   3649篇
  1994年   3504篇
  1993年   3511篇
  1992年   8376篇
  1991年   8018篇
  1990年   8041篇
  1989年   7806篇
  1988年   7262篇
  1987年   6844篇
  1986年   6141篇
  1985年   6580篇
  1984年   5528篇
  1983年   4838篇
  1982年   3752篇
  1981年   3455篇
  1980年   3296篇
  1979年   4767篇
  1978年   3852篇
  1977年   3506篇
  1976年   3455篇
  1975年   3754篇
  1974年   4048篇
  1973年   4044篇
  1972年   4071篇
  1971年   3786篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
With the use of anticoagulants to prevent recurrence of thrombosis it is now possible to open a major artery which has been occluded by a thrombus, remove the thickened lining with the clot and restore the continuity of the vessel. Free vein grafts have been used successfully to replace arterial segments in the extremities. Vein inlay grafts are preferred in dealing with aneurysms of the abdominal aorta. The aneurysmal wall can be used to support the venous segments. It has been possible to by-pass aneurysms of the abdominal aorta by anastomosing the splenic artery to the left iliac artery. The use of the left superficial femoral artery as a by-pass to supply blood to the right leg is described.  相似文献   
994.
995.
996.
As a result of selection of rats possessing a high threshold of neuro-muscular exitability in comparison with rats possessing a low threshold of neuro-muscular exitability, certain changes take place in the structures that are connected with the learning process: the visual cortex is wider, volumes of the pyramidal cells in the fields CA1 and CA3 of the hippocampus are increased, density of the glial elements in the fornix is essentially increased and that of neurons in the septal nucleus is decreased.  相似文献   
997.
Thermal resistance of Saccharomyces cerevisiae was found to be drastically dependent on the kinetics of heat perturbation. Yeasts were found to be more resistant to a plateau of 1 h at 50 degrees C after a slope of temperature increase (slow and linear temperature increments) than after a shock (sudden temperature change). Thermotolerance was mainly acquired between 40-50 degrees C during a heat slope, i.e., above the maximal temperature of growth. The death of the yeasts subjected to a heat shock might be related to the loss of membrane integrity: intracellular contents extrusion, i.e., membrane permeabilization, was found to precede cell death. However, the permeabilization did not precede cell death during a heat slope and, therefore, membrane permeabilization was a consequence rather than a cause of cell death. During a slow temperature increase, yeasts which remain viable may have time to adapt their plasma membrane and thus maintain membrane integrity.  相似文献   
998.
This article describes the calibration of a spectroscopic scanning instrument for the measurement of selected contaminants in a complex biological process stream. Its use is for the monitoring of a process in which contaminants are to be removed selectively by flocculation from yeast cell homogenate. The main contaminants are cell debris, protein, and RNA. A low-cost instrument has been developed for sensitivity in the region of the NIR spectrum (from 1900 to 2500 nm) where preliminary work found NIR signatures from cell debris, protein, and RNA. Calibration models have been derived using a multivariate method for concentrations of these contaminants, such as would be found after the flocculation process. Two strategies were compared for calibrating the NIR instrument. In one case, samples were prepared by adding materials representative of the contaminants to clarified yeast homogenate so the contaminant levels were well known but outside the range of interest. In the other case, where samples were like those from the process stream after flocculation and floc removal, there was uncertainty of analysis of contaminant level, but the calibration was in the range of interest. Calibration using process stream samples gave results close to those derived from traditional assays. When the calibration models were used to predict the contaminant concentrations in previously unseen samples, the correlation coefficients between measurements and predictions were above 90% in all cases but one. The prediction errors were similar to the errors in the traditional assays.  相似文献   
999.
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号