全文获取类型
收费全文 | 2252篇 |
免费 | 78篇 |
国内免费 | 16篇 |
专业分类
化学 | 1579篇 |
晶体学 | 10篇 |
力学 | 57篇 |
数学 | 409篇 |
物理学 | 291篇 |
出版年
2023年 | 20篇 |
2022年 | 29篇 |
2021年 | 37篇 |
2020年 | 57篇 |
2019年 | 50篇 |
2018年 | 15篇 |
2017年 | 26篇 |
2016年 | 72篇 |
2015年 | 68篇 |
2014年 | 77篇 |
2013年 | 105篇 |
2012年 | 126篇 |
2011年 | 169篇 |
2010年 | 88篇 |
2009年 | 111篇 |
2008年 | 157篇 |
2007年 | 133篇 |
2006年 | 137篇 |
2005年 | 103篇 |
2004年 | 112篇 |
2003年 | 75篇 |
2002年 | 79篇 |
2001年 | 32篇 |
2000年 | 32篇 |
1999年 | 27篇 |
1998年 | 26篇 |
1997年 | 20篇 |
1996年 | 19篇 |
1995年 | 16篇 |
1994年 | 9篇 |
1993年 | 11篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 21篇 |
1985年 | 11篇 |
1984年 | 18篇 |
1983年 | 20篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1977年 | 10篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 8篇 |
1972年 | 9篇 |
1964年 | 7篇 |
排序方式: 共有2346条查询结果,搜索用时 31 毫秒
141.
142.
Certified reference materials (CRMs) are important quality assurance tools in the laboratory. It is, however, not certain they are always used properly. Several guidelines are available on the use of CRMs, but appear to be mostly unknown. The users thus get very little help on the way to proper use. Many of the problems with CRMs are well known, but that does not mean they have disappeared. Better use of CRMs will require the involvement of producers and analytical societies, e.g. the AOAC. Editors of international analytical journals could have a great influence on how CRM results and other QA data are treated in publications. 相似文献
143.
Primary- and secondary-alkyl aryl tellurides, prepared by arenetellurolate ring-opening of epoxides/ O-allylation, were found to undergo rapid (3-10 min) group-transfer cyclization to afford tetrahydrofuran derivatives in 60-74% yield when heated in a microwave cavity at 250 degrees C in ethylene glycol or at 180 degrees C in water. To go to completion, similar transformations had previously required extended photolysis in refluxing benzene containing a substantial amount of hexabutylditin. The only drawback of the microwave-assisted process was the loss in diastereoselectivity which is a consequence of the higher reaction temperature. Substitution in the Te-aryl moiety of the secondary-alkyl aryl tellurides (4-OMe, 4-H, 4-CF(3)) did not affect the outcome of the group-transfer reaction in ethylene glycol. However, at lower temperature, using water as a solvent, the CF(3) derivative failed to react. The microwave-assisted group-transfer cyclization was extended to benzylic but not to primary- and secondary-alkyl phenyl selenides. 相似文献
144.
L.?Landstr?mEmail author Zs.?Márton M.?Boman P.?Heszler 《Applied Physics A: Materials Science & Processing》2004,79(3):537-542
Excimer laser ablation of highly oriented pyrolytic graphite (HOPG) was performed at atmospheric pressure in an N2 and in an air ambient. During the ablation, nanoparticles condensed from the material ejecta, and their size distribution was monitored in the gas phase by a Differential Mobility Analyzer (DMA) in combination with a Condensation Particle Counter (CPC). Size distributions obtained at different laser repetition rates revealed that the interaction between subsequent laser pulses and formed particles became significant above 15 Hz. This interaction resulted in laser heating, leading to considerable evaporation and a decrease in the size of the particles. X-ray photoelectron spectroscopy revealed that approximately 8% nitrogen was incorporated into the CNx particles generated in the N2 ambient, and that the nitrogen was mostly bonded to sp3-hybridized carbon. Monodisperse particles were also deposited and were analyzed by means of Raman spectroscopy to monitor size-induced effects. PACS 81.07.-b; 61.46.+w; 79.70.+q 相似文献
145.
146.
Nielsen LP Stevenson CP Blackmond DG Jacobsen EN 《Journal of the American Chemical Society》2004,126(5):1360-1362
The mechanism of the hydrolytic kinetic resolution (HKR) of terminal epoxides was investigated by kinetic analysis using reaction calorimetry. The chiral (salen)Co-X complex (X = OAc, OTs, Cl) undergoes irreversible conversion to (salen)Co-OH during the course of the HKR and thus serves as both precatalyst and cocatalyst in a cooperative bimetallic catalytic mechanism. This insight led to the identification of more active catalysts for the HKR of synthetically useful terminal epoxides. 相似文献
147.
148.
Lars Andersson 《Annales Henri Poincare》2004,5(2):235-244
An energy estimate is proved for the Bel-Robinson energy along a constant
mean curvature foliation in a spatially compact vacuum spacetime, assuming an
bound on the second fundamental form, and a bound on a spacetime version
of Bel-Robinson energy.
Communicated by Sergiu KlainermanSubmitted 25/07/03, accepted 27/01/04 相似文献
149.
Aminoboranes have been shown to be highly efficient and mild iminium ion generators in the Mannich-type aminative coupling of aldehydes with silyl ketene acetals. By using aminoboranes bearing bulky amino groups, such as a diisopropylamino group, free secondary amines can be successfully used as the amino component in a three-component Mannich reaction with aldehydes and silyl ketene acetals. 相似文献
150.
Mala T Frich L Aurdal L Clausen OP Edwin B Soreide O Gladhaug I 《Magnetic resonance imaging》2003,21(7):733-740
This study evaluate intraoperative Magnetic Resonance Imaging (MRI) as predictor of tissue damage following cryoablation of porcine liver with and without concomitant hepatic vascular inflow occlusion.Inflow occlusion was used during freezing in 6 of 12 pigs included. The volumes of the procedural ice-balls were estimated from MR images. Immediately after thawing contrast (MnDPDP) enhanced MRI was performed to estimate the volume of the cryolesion. Four days after ablation MRI was repeated of the in-vivo and the ex-vivo liver. Photography was performed of the sliced liver specimens to estimate the volumes of the lesions. The intraoperative volume of the cryolesion as shown by contrast enhanced MRI corresponded well to the ice-ball volume for lesions made without vascular occlusion (difference 0.3 +/- 0.9 cm(3), p = 0.239). For lesions made during occlusion the volume of the intraoperative cryolesion was larger than the corresponding ice-ball (difference 7.5 +/- 3.3 cm(3), p = 0.003). The volume of the cryolesions as estimated from histopathology four days after freezing and contrast enhanced MRI immediately after freezing corresponded well for lesions made with (difference -2.6 +/- 4.5 cm(3), p = 0.110) and without vascular occlusion (difference -0.5 +/- 2.3 cm(3), p = 0.695).Intraoperative MnDPDP-enhanced MRI of the cryolesion is predictive of the tissue damage induced during cryoablation of porcine liver. The procedural ice-ball is not, if induced during inflow occlusion. 相似文献