全文获取类型
收费全文 | 44176篇 |
免费 | 4688篇 |
国内免费 | 13954篇 |
专业分类
62818篇 |
出版年
2024年 | 354篇 |
2023年 | 1079篇 |
2022年 | 1906篇 |
2021年 | 2469篇 |
2020年 | 1935篇 |
2019年 | 2240篇 |
2018年 | 1810篇 |
2017年 | 1513篇 |
2016年 | 1832篇 |
2015年 | 2589篇 |
2014年 | 3380篇 |
2013年 | 3302篇 |
2012年 | 4242篇 |
2011年 | 3903篇 |
2010年 | 2931篇 |
2009年 | 2722篇 |
2008年 | 3256篇 |
2007年 | 2946篇 |
2006年 | 2830篇 |
2005年 | 2310篇 |
2004年 | 1931篇 |
2003年 | 1698篇 |
2002年 | 1557篇 |
2001年 | 1406篇 |
2000年 | 1211篇 |
1999年 | 1025篇 |
1998年 | 545篇 |
1997年 | 399篇 |
1996年 | 405篇 |
1995年 | 298篇 |
1994年 | 347篇 |
1993年 | 232篇 |
1992年 | 292篇 |
1991年 | 276篇 |
1990年 | 241篇 |
1989年 | 176篇 |
1988年 | 150篇 |
1987年 | 114篇 |
1986年 | 91篇 |
1985年 | 112篇 |
1984年 | 94篇 |
1983年 | 64篇 |
1982年 | 94篇 |
1981年 | 49篇 |
1980年 | 30篇 |
1979年 | 32篇 |
1975年 | 24篇 |
1974年 | 23篇 |
1973年 | 24篇 |
1950年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
12.
Lexical gap in cQA search, resulted by the variability of languages, has been recognized as an important and widespread phenomenon. To address the problem, this paper presents a question reformulation scheme to enhance the question retrieval model by fully exploring the intelligence of paraphrase in phrase-level. It compensates for the existing paraphrasing research in a suitable granularity, which either falls into fine-grained lexical-level or coarse-grained sentence-level. Given a question in natural language, our scheme first detects the involved key-phrases by jointly integrating the corpus-dependent knowledge and question-aware cues. Next, it automatically extracts the paraphrases for each identified key-phrase utilizing multiple online translation engines, and then selects the most relevant reformulations from a large group of question rewrites, which is formed by full permutation and combination of the generated paraphrases. Extensive evaluations on a real world data set demonstrate that our model is able to characterize the complex questions and achieves promising performance as compared to the state-of-the-art methods. 相似文献
13.
14.
15.
16.
17.
我国广东路氏锥虫的研究Ⅰ.种的特性及其在实验寄主中的繁殖 总被引:1,自引:0,他引:1
本文对发现于广东的鼠锥虫进行了寄主特异性、形态学、实验寄主体内的感染过程、厦虫氨基酸组分等多方面的研究,并将其定为路氏锥虫在中国广东分布的一个不同地域株。本文研究了它在实验寄主体内繁殖消长的规律,整个发育过程的各期形态及成熟期的超微结构,并测出其16种氨基酸的存在和含量。 相似文献
18.
胆碱能神经对人餐后神经降压素释放的影响 总被引:2,自引:0,他引:2
本文比较了6名健康成人进食、餐前肌注阿托品以及单纯咀嚼食物后的血浆神经降压素样免疫活性物质(NTLI)水平的变化,以探讨胆碱能神经对神经降压素释放的影响。用放射免疫测定法分别测定NTLI和胰多肽(PP)的含量,以便同时比较两者释放的状态。6人的基础血浆NTLI和PP的水平平均分别为15.7±2.4和16.6±9 7pmol/L。进食后,血浆NTLI和PP水平均显著增高,并呈双相反应。第一个血浆NTLI高峰平均为60.7±13.2pmol/L,出现于餐后的20min。餐后90min,又出现另一个高峰,其平均水平为58.8±8.2pmol/L。在进食前肌注阿托品1mg,餐后的第一个血浆NTLI高峰消失,而第二个高峰仍存在。单纯咀嚼食物后,血浆PP水平明显增高,而对NTLI的释放无刺激作用。本文结果提示,餐后早期的神经降压素释放的调节是由非迷走胆碱能神经参与的,而后期的释放不受胆碱能神经的影响。 相似文献
19.
20.
P D Nguyen E A O'Rear A E Johnson R Lu B M Fung 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1989,192(3):261-269
The clot-lysing ability of streptokinase (SK) was examined using membrane-bound thrombi. Encapsulation of SK in large unilamellar phospholipid vesicles (liposomes) resulted in entrapping approximately 30% of its original activity. Measurements of streptokinase activity for liposomal-encapsulated streptokinase (LESK) indicated little loss of activity or leakage in Tris-buffered saline over a 24-hr period at temperatures of 4 and 23 degrees C. However, incubation of free SK and LESK in platelet-poor plasma (PPP) at 37 degrees C resulted in a decrease of SK activity. The retention of SK activity in LESK was considerably higher than that of unentrapped SK. Clot-dissolving time (CDT) was measured by monitoring the pressure drop during slow filtration in plasma through membrane-bound thrombi. The results indicated that both LESK and free SK were able to activate the fibrinolytic system. Without prior incubation in PPP at 37 degrees C, the CDT of a SK and PPP mixture (SK/PPP) was 10.7 +/- 1.9 min (n = 12), while that of a LESK and PPP mixture (LESK/PPP) was 12.4 +/- 1.7 min (n = 12). The CDT-detected clot-lysing abilities of both SK and LESK were diminished by incubation in PPP, but to different extents. After 15- and 30-min incubations, the CDT of SK/PPP increased significantly to 15.5 +/- 1.5 and 24.1 +/- 2.4 min (n = 5, P less than 0.05), respectively. In contrast, the CDT of LESK/PPP increased to 13.3 +/- 0.8 min (n = 5) after 15 min of incubation and to 16.0 +/- 1.1 min (n = 5, P less than 0.05) after a 30-min incubation. These results suggest that entrapment of SK in liposomes preserves the thrombolytic potential of the plasminogen activator by limiting its exposure to the components of the plasma. 相似文献