全文获取类型
收费全文 | 925篇 |
免费 | 177篇 |
国内免费 | 124篇 |
专业分类
化学 | 410篇 |
晶体学 | 9篇 |
力学 | 12篇 |
综合类 | 6篇 |
数学 | 146篇 |
物理学 | 325篇 |
无线电 | 318篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 10篇 |
2021年 | 22篇 |
2020年 | 19篇 |
2019年 | 22篇 |
2018年 | 21篇 |
2017年 | 29篇 |
2016年 | 27篇 |
2015年 | 35篇 |
2014年 | 62篇 |
2013年 | 99篇 |
2012年 | 61篇 |
2011年 | 63篇 |
2010年 | 57篇 |
2009年 | 63篇 |
2008年 | 54篇 |
2007年 | 52篇 |
2006年 | 37篇 |
2005年 | 42篇 |
2004年 | 81篇 |
2003年 | 60篇 |
2002年 | 61篇 |
2001年 | 54篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 17篇 |
1997年 | 24篇 |
1996年 | 20篇 |
1995年 | 13篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1973年 | 3篇 |
排序方式: 共有1226条查询结果,搜索用时 0 毫秒
151.
基于集对分析的战场态势分析模型 总被引:4,自引:1,他引:3
在防空作战中,战场态势的分析非常重要,它是辅助决策的基础。针对战场态势的问题,以集对分析的同异反联系度和距离测度为基础,构造了战场态势的分析模型,进行了深入探讨。最后,通过实例计算表明了分析模型对于评估战场态势的可行性和有效性。 相似文献
152.
One-pot simultaneous debrominative decarboxylation and sulfamation of anti-2,3-dibromo-3-(4-chlorosulfonylphenyl)propa-noic acid in DMF (for alkyl amines) or DMF-pyridine (for aryl amines) using a diverse range of alkyl and aryl amines under microwave irradiation condition stereoselectively afforded (Z)-4-(2-bromovinyl)benzenesulfonamides in good yields. 相似文献
153.
This paper studies the number of limit cycles of some Z3-equivariant near-Hamiltonian systems of degrees 3 and 4,which are a perturbation of a cubic Hamiltonian system. By the Melnikov function method,we obtain 5 and 6 limit cycles respectively. 相似文献
154.
A semiring is an algebraic structure similar to a ring, but without the requirement that each element must have an additive inverse. A bounded semiring is a semiring equipped with a compatible bounded partial order. In this paper, properties of zero divisors and prime elements of a bounded semiring are studied. In particular, it is proved that under some mild assumption, the set Z(A) of nonzero zero divisors of A is A / {0, 1}, and each prime element of A is a maximal element. For a bounded semiring A with Z(A) = A / {0, 1}, it is proved that A has finitely many maximal elements if ACC holds either for elements of A or for principal annihilating ideals of A. As an application of prime elements, we show that the structure of a bounded semiring A is completely determined by the structure of integral bounded semirings if either |Z(A)| = 1 or |Z(A)| -- 2 and Z(A)2 ≠ 0. Applications to the ideal structure of commutative rings are also considered. In particular, when R has a finite number of ideals, it is shown that the chain complex of the poset I(R) is pure and shellable, where I(R) consists of all ideals of R. 相似文献
155.
156.
157.
158.
159.
为解决飞机技术出版物中公用信息的管理问题,提高信息的重用性和数据交换的便利性。通过对S1000D标准中公用信息库(CIR)概念的介绍,结合CIR在某大型客机维修类技术出版物编写中的实际应用情况,具体说明了CIR在公用数据管理、技术出版物编写、数据交换方面的优势,及其在提高技术出版物质量方面所起的重要作用。 相似文献
160.
Tairan Wang Ran Wei Yingting Feng Lijun Jin Yunpeng Jia Duxia Yang Zuonan Liang Mengge Han Xia Li Chenze Lu Xiangxian Ying 《Molecules (Basel, Switzerland)》2021,26(16)
The importance of yeast old yellow enzymes is increasingly recognized for direct asymmetric reduction of (E/Z)-citral to (R)-citronellal. As one of the most performing old yellow enzymes, the enzyme OYE3 from Saccharomyces cerevisiae S288C exhibited complementary enantioselectivity for the reduction of (E)-citral and (Z)-citral, resulting in lower e.e. value of (R)-citronellal in the reduction of (E/Z)-citral. To develop a novel approach for the direct synthesis of enantio-pure (R)-citronellal from the reduction of (E/Z)-citral, the enzyme OYE3 was firstly modified by semi-rational design to improve its (R)-enantioselectivity. The OYE3 variants W116A and S296F showed strict (R)-enantioselectivity in the reduction of (E)-citral, and significantly reversed the (S)-enantioselectivity in the reduction of (Z)-citral. Next, the double substitution of OYE3 led to the unique variant S296F/W116G, which exhibited strict (R)-enantioselectivity in the reduction of (E)-citral and (E/Z)-citral, but was not active on (Z)-citral. Relying on its capability discriminating (E)-citral and (Z)-citral, a new cascade reaction catalyzed by the OYE3 variant S296F/W116G and glucose dehydrogenase was developed, providing the enantio-pure (R)-citronellal and the retained (Z)-citral after complete reduction of (E)-citral. 相似文献