全文获取类型
收费全文 | 4010篇 |
免费 | 303篇 |
国内免费 | 1334篇 |
专业分类
化学 | 4488篇 |
晶体学 | 108篇 |
力学 | 88篇 |
综合类 | 41篇 |
数学 | 244篇 |
物理学 | 678篇 |
出版年
2024年 | 3篇 |
2023年 | 34篇 |
2022年 | 89篇 |
2021年 | 95篇 |
2020年 | 97篇 |
2019年 | 111篇 |
2018年 | 89篇 |
2017年 | 122篇 |
2016年 | 133篇 |
2015年 | 134篇 |
2014年 | 181篇 |
2013年 | 330篇 |
2012年 | 280篇 |
2011年 | 246篇 |
2010年 | 208篇 |
2009年 | 227篇 |
2008年 | 273篇 |
2007年 | 334篇 |
2006年 | 300篇 |
2005年 | 251篇 |
2004年 | 267篇 |
2003年 | 194篇 |
2002年 | 188篇 |
2001年 | 138篇 |
2000年 | 152篇 |
1999年 | 149篇 |
1998年 | 106篇 |
1997年 | 117篇 |
1996年 | 107篇 |
1995年 | 97篇 |
1994年 | 91篇 |
1993年 | 63篇 |
1992年 | 85篇 |
1991年 | 55篇 |
1990年 | 52篇 |
1989年 | 64篇 |
1988年 | 38篇 |
1987年 | 24篇 |
1986年 | 21篇 |
1985年 | 30篇 |
1984年 | 15篇 |
1983年 | 23篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有5647条查询结果,搜索用时 46 毫秒
11.
YunXiaWANG LinJIA ZhenSHI 《中国化学快报》2003,14(6):561-564
The addition-hydrolysis reaction of benzimidazolium salt with some mono- and bifunctional amine nucleophiles is reported,and a novel method of biomimetic synthesis for formamides and heterocycle compounds is provided. 相似文献
12.
横向放大率法确定复合光学系统的基点 总被引:2,自引:0,他引:2
介绍了应用测量横向放大率确定两薄透镜组成的复合光学系统基点的方法。由于采用线阵光电耦合器件(CCD)测量物经光学系统成像的横向放大率,提高了测量精度。 相似文献
13.
14.
15.
We synthesized a new intercalation compound, 1,5-diaminonaphathalene(DAN)-saponite where intercalated DAN molecules were shown to have a formal charge of +0.67 The measurement of optical diffuse reflectance spectra revealed the formation of electronic bands with a gap of ca. 1 eV suggesting semiconducting behaviour of this system. From ESR measurements, the radical formation in DAN-saponite was confirmed and the spin concentration was determined to be 1 spin per 200 and 300 DAN-molecules at 290 and 7.9 K, respectively. This temperature dependence of the spin density also implies the semiconductive nature of DAN-saponite. 相似文献
16.
Standard ODE methods such as linear multistep methods encounter difficulties when applied to differential-algebraic equations (DAEs) of index greater than 1. In particular, previous results for index 2 DAEs have practically ruled out the use of all explicit methods and of implicit multistep methods other than backward difference formulas (BDFs) because of stability considerations. In this paper we embed known results for semi-explicit index 1 and 2 DAEs in a more comprehensive theory based on compound multistep and one-leg discretizations. This explains and characterizes the necessary requirements that a method must fulfill in order to be applicable to semi-explicit DAEs. Thus we conclude that the most useful discretizations are those that avoid discretization of the constraint. A freer use of e.g. explicit methods for the non-stiff differential part of the DAE is then possible.Dedicated to Germund Dahlquist on the occasion of his 70th birthdayThis author thanks the Centro de Estadística y Software Matemático de la Universidad Simón Bolivar (CESMa) for permitting her free use of its research facilities.Partial support by the Swedish Research Council for Engineering Sciences TFR under contract no. 222/91-405. 相似文献
17.
聚醚醚酮链结构与反应的模型化合物的量子化学研究 总被引:1,自引:1,他引:0
利用AM1方法对聚醚醚酮模型化合物全优化,结果为:芳环平均相互扭转角为33.0度,桥键角117.0-118.0度,其分子结构拓扑图形表明:所有苯环(核)为平面构型,但其内角扭曲;其氢原子对苯环构型无实质性贡献,在更长链的计算中,冻结苯核与氢原子也得出满意结果,根据Mulliken键序与电荷讨论了醚交换与磺化反应。 相似文献
18.
19.
Emrys A. Jones Nicholas P. Lockyer John C. Vickerman 《International journal of mass spectrometry》2007,260(2-3):146
Recent developments in desorption/ionisation mass spectrometry techniques have made their application to biological analysis a realistic and successful proposition. Developments in primary ion source technology, mainly through the advent of polyatomic ion beams, have meant that the technique of secondary ion mass spectrometry (SIMS) can now access the depths of information required to allow biological imaging to be a viable option.Here the role of the primary ion C60+ is assessed with regard to molecular imaging of lipids and pharmaceuticals within tissue sections. High secondary ion yields and low surface damage accumulation are demonstrated on both model and real biological samples, indicating the high secondary ion efficiency afforded to the analyst by this primary ion when compared to other cluster ion beams used in imaging. The newly developed 40 keV C60+ ion source allows the beam to be focused such that high resolution imaging is demonstrated on a tissue sample, and the greater yields allow the molecular signal from the drug raclopride to be imaged within tissue section following in vivo dosing.The localisation shown for this drug alludes to issues regarding the chemical environment affecting the ionisation probability of the molecule; the importance of this effect is demonstrated with model systems and the possibility of using laser post-ionisation as a method for reducing this consequence of bio-sample complexity is demonstrated and discussed. 相似文献
20.
蒋忠樟 《数学的实践与认识》2007,37(15):174-179
实对称正定矩阵的复合矩阵正定性的研究已有结论,但对于一般意义下的正定矩阵的复合矩阵是否仍然是正定的研究需要利用一般的正定矩阵的标准形的复合矩阵进行讨论,给出了一般公式及具体算法,为讨论其复合矩阵的正定性提供了基础条件. 相似文献