全文获取类型
收费全文 | 118089篇 |
免费 | 2356篇 |
国内免费 | 3963篇 |
专业分类
系统科学 | 4866篇 |
丛书文集 | 2583篇 |
教育与普及 | 2832篇 |
理论与方法论 | 871篇 |
现状及发展 | 29374篇 |
研究方法 | 208篇 |
综合类 | 81477篇 |
自然研究 | 2197篇 |
出版年
2023年 | 792篇 |
2022年 | 1212篇 |
2021年 | 1380篇 |
2020年 | 1057篇 |
2017年 | 994篇 |
2016年 | 1240篇 |
2015年 | 2067篇 |
2014年 | 2703篇 |
2013年 | 2951篇 |
2012年 | 2805篇 |
2011年 | 5699篇 |
2010年 | 3208篇 |
2009年 | 3962篇 |
2008年 | 4191篇 |
2007年 | 4155篇 |
2006年 | 3588篇 |
2005年 | 3790篇 |
2004年 | 4918篇 |
2003年 | 4118篇 |
2002年 | 4298篇 |
2001年 | 3733篇 |
2000年 | 2962篇 |
1999年 | 4309篇 |
1998年 | 3759篇 |
1997年 | 3808篇 |
1996年 | 3253篇 |
1995年 | 2854篇 |
1994年 | 2846篇 |
1993年 | 2879篇 |
1992年 | 2484篇 |
1991年 | 2283篇 |
1990年 | 2038篇 |
1989年 | 1729篇 |
1988年 | 1467篇 |
1987年 | 1102篇 |
1986年 | 897篇 |
1985年 | 769篇 |
1981年 | 756篇 |
1980年 | 818篇 |
1979年 | 1047篇 |
1978年 | 997篇 |
1977年 | 1006篇 |
1976年 | 909篇 |
1975年 | 871篇 |
1973年 | 939篇 |
1972年 | 1014篇 |
1971年 | 947篇 |
1970年 | 908篇 |
1969年 | 900篇 |
1968年 | 865篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
131.
莱钢特钢厂20CrMnTiH钢材成分波动较大,淬透性带HRC8~10,范围较大,制约了齿轮钢质量的进一步提高.通过研究,发现铸坯中心偏析尤其是碳偏析为主要问题.通过工艺优化采取各项措施,尤其增加电磁搅拌后,中心碳偏析得到明显改善;C控制在0.19%~0.20%的炉次达到了95.6%;淬透性带控制在HRC6内的达到了92.1%;在HRC4内的达到了86.2%;提高了产品质量,满足了用户需求. 相似文献
132.
H. v. Hayek 《Cellular and molecular life sciences : CMLS》1948,4(11):448-449
Ohne Zusammenfassung 相似文献
133.
Duan Jinqing Liu Hongbin Zhang Xinian Lu Hongmin Gou Ai Liu Zheng Wang Tao Ge Rili 《中国工程科学》2006,4(2):183-186
Study Objective: To compare and analyze the changes of the pulmonary-artery pressure of the migrants coming from different elevation in the hypoxic environment of 4636 - 4907 m extreme altitude. To explore the susceptibility to hypoxic pulmonary-artery hypertension (PH) in the subjects from different altitude and profession. Methods: By using Color Doppler Ultmsonography (CDU), measuring the pulmonary-artery pressure of 207 healthy men, who had continuously being lived and worked at the extreme altitude for more than six months, and then were divided into three groups according to their profession and the altitude of original living place. Results: There was a significant difference in the outcomes of pulmonary-artery pressure from the 3 groups. Conclusions: Altitude of original living place, labor intensity are some of factors that impact the pulmonary-artery pressure of the people who exposure to a hypoxic environment. The pulmonary- artery pressure of person without strenuously physical work experience was more sensitive to hypoxic surroundings than that of labor workers. It was not always the fact at an extreme altitude that the moderate altitude mountaineers were superior to other migrants from a lower altitude or plain. The higher PH was found in the groups of the moderate altitude mountaineers and labor workers. It is unlikely certain that one with PH would surfer from HAPE. 相似文献
134.
135.
Finding finer functions for partially characterized proteins by protein-protein interaction networks
LI YanHui GUO Zheng MA WenCai YANG Da WANG Dong ZHANG Min ZHU ding ZHONG GuoCai LI YongJin YAO Chen WANG Jing 《科学通报(英文版)》2007,52(24):3363-3370
Based on high-throughput data, numerous algorithms have been designed to find functions of novel proteins. However, the effectiveness of such algorithms is currently limited by some fundamental factors, including (1) the low a-priori probability of novel proteins participating in a detailed function; (2) the huge false data present in high-throughput datasets; (3) the incomplete data coverage of functional classes; (4) the abundant but heterogeneous negative samples for training the algorithms; and (5) the lack of detailed functional knowledge for training algorithms. Here, for partially characterized proteins, we suggest an approach to finding their finer functions based on protein interaction sub-networks or gene expression patterns, defined in function-specific subspaces. The proposed approach can lessen the above-mentioned problems by properly defining the prediction range and functionally filtering the noisy data, and thus can efficiently find proteins’ novel functions. For thousands of yeast and human proteins partially characterized, it is able to reliably find their finer functions (e.g., the translational functions) with more than 90% precision. The predicted finer functions are highly valuable both for guiding the follow-up wet-lab validation and for providing the necessary data for training algorithms to learn other proteins. 相似文献
136.
孟郊和贾岛是唐代两位著名的苦吟诗人,两人分别在五古和五律方面有着独特的建树,但贾岛的古体诗明显受到了孟郊诗歌的影响。具体来说,贾岛的古体诗只用五言的形式,呈现出奇而硬的风格,有着乐府化的倾向,这三方面都可以看出与孟郊诗歌的渊源关系。 相似文献
137.
揭示了城市技术创新系统和城乡统筹发展的内涵,并在技术创新、增长极和区域经济等理论框架下阐述了城市技术创新与城乡统筹发展的关系,进而提出城市技术创新促进城乡统筹的路径。 相似文献
138.
A. C. Muhr 《Cellular and molecular life sciences : CMLS》1948,4(9):355-355
Summary The fatty acids from dehydrated castor oil, mainly consisting of 9,11- and 9,12-linoleic acid, show in biological tests vitamin effects similar to those of the so-called essential fatty acids. 相似文献
139.
Summary Acetone and alcohol extracts of the posterior salivary glands ofOctopus vulgaris, Eledone moschata andEledone Aldrovandi contain a principle which strongly reduces the diuresis in hydrated rats.The antidiuretic principle is absent in all otherOctopus tissues as well as in the salivary glands ofOctopus macropus.It shows a high termostability and is strongly adsorbed by animal charcoal at any reaction.The salivary antidiuretic substance is not identical with the posterior pituitary principle nor with the other active substances found in the salivary glands of Octopoda.The injection of salivary extracts satisfactorily reduces polyuria and polydipsia in human diabetes insipidus.
Le presenti ricerche sono state in parte sovvenzionate da un contributo straordinario del Consiglio Nazionale delle Ricerche. 相似文献
Le presenti ricerche sono state in parte sovvenzionate da un contributo straordinario del Consiglio Nazionale delle Ricerche. 相似文献
140.
设 A是对称正定矩阵,λ_1是 A 的最大或最小特征值,χ_1是对应的特征向量.{zk}是用共轭斜量法求解方程组 Αχ=b时的近似解序列,ei=A~(_1)b-zi,本文给出了|x_1~Tei|较合理的上界估计式。从而为分析预处理共轭斜量法提供了进一步的理论基础。 相似文献