全文获取类型
收费全文 | 9023篇 |
免费 | 481篇 |
国内免费 | 37篇 |
专业分类
化学 | 6365篇 |
晶体学 | 36篇 |
力学 | 261篇 |
综合类 | 23篇 |
数学 | 1271篇 |
物理学 | 1585篇 |
出版年
2022年 | 68篇 |
2021年 | 98篇 |
2020年 | 108篇 |
2019年 | 134篇 |
2018年 | 120篇 |
2017年 | 114篇 |
2016年 | 252篇 |
2015年 | 239篇 |
2014年 | 271篇 |
2013年 | 436篇 |
2012年 | 465篇 |
2011年 | 541篇 |
2010年 | 366篇 |
2009年 | 326篇 |
2008年 | 456篇 |
2007年 | 436篇 |
2006年 | 438篇 |
2005年 | 393篇 |
2004年 | 344篇 |
2003年 | 353篇 |
2002年 | 445篇 |
2001年 | 227篇 |
2000年 | 263篇 |
1999年 | 187篇 |
1998年 | 148篇 |
1997年 | 116篇 |
1996年 | 137篇 |
1995年 | 115篇 |
1994年 | 101篇 |
1993年 | 76篇 |
1992年 | 96篇 |
1991年 | 89篇 |
1990年 | 82篇 |
1989年 | 87篇 |
1988年 | 71篇 |
1987年 | 52篇 |
1986年 | 48篇 |
1985年 | 95篇 |
1984年 | 97篇 |
1983年 | 61篇 |
1982年 | 100篇 |
1981年 | 71篇 |
1980年 | 86篇 |
1979年 | 64篇 |
1978年 | 95篇 |
1977年 | 65篇 |
1976年 | 73篇 |
1975年 | 52篇 |
1974年 | 56篇 |
1973年 | 38篇 |
排序方式: 共有9541条查询结果,搜索用时 15 毫秒
91.
Shibuya H Kitamura C Maehara S Nagahata M Winarno H Simanjuntak P Kim HS Wataya Y Ohashi K 《Chemical & pharmaceutical bulletin》2003,51(1):71-74
The microbial transformation of four Cinchona alkaloids (quinine, quinidine, cinchonidine, and cinchonine) by endophytic fungi isolated from Cinchona pubescens was investigated. The endophytic filamentous fungus Xylaria sp. was found to transform the Cinchona alkaloids into their 1-N-oxide derivatives. 相似文献
92.
Groothaert MH Smeets PJ Sels BF Jacobs PA Schoonheydt RA 《Journal of the American Chemical Society》2005,127(5):1394-1395
This work reports on the capability of the O2-activated Cu-ZSM-5 and Cu-MOR zeolites to selectively convert methane into methanol at a temperature of 398 K. A strong correlation between (i) the activity and (ii) the intensity of the 22 700 cm-1 UV-vis band, assigned to the bis(mu-oxo)dicopper core, is found (i) as a function of the reaction temperature, (ii) as a function of the Cu loading of the zeolite, and (iii) in comparison to other Cu materials. These three lines of evidence firmly support the key role of the bis(mu-oxo)dicopper core in this selective, low-temperature hydroxylation of methane. 相似文献
93.
Hydroboration of the syn, anti-[Fe(CO)3]2 double complex 24 of the readily available 5,6,7,8-tetramethylidene-2-bicyclo[2.2.2]octene ( 22 ) gave the corresponding doubly complexed 2-bicyclo[2.2.2]octanol 25. CrO3-oxidation furnished ketone 27 . The syn-Fe(CO)3-groups in 25 and 27 were oxidized selectively with trimethyl-amine oxide and yielded the corresponding anti-Fe(CO3)-monocomplexed tetraenes 26 and 28. The anti-Fe(CO)3-group in 28 could be removed, and 5,6,7,8-tetramethylidene-2-bicyclo[2.2.2]octanone ( 11 ) was obtained. NaBH4-reduction of 11 afforded tetraenol 10. TCE-cycloadditions to 10 and 11 (k1) were at least 10 times as fast as those (k2) to the corresponding monoadducts 35/36 and 34 , respectively. This Diels-Alder reactivity difference vanishes (k1 ≈ k2) with methyl propynoate. The latter dienophile added to the anti-Fe(CO)3-monocomplexed tetraenone 28 with ‘para’-regioselectivity. 相似文献
94.
Hemmerlé J Roucoules V Fleith G Nardin M Ball V Lavalle P Marie P Voegel JC Schaaf P 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10328-10331
Mechanically responsive surfaces that allow to switch reversibly from a hydrophobic to a hydrophilic substrate are reported. The surfaces are constituted of polyelectrolyte multilayers deposited on modified charged silicone sheets. n bilayers of poly(allylamine)-Nafion (PAH-Naf) and m bilayers of poly(allylamine)-poly(acrylic acid) (PAH-PAA) composed the multilayers. A (PAH-Naf)(n) film possesses a water contact angle of around 105 degrees, whereas the contact angle of a (PAH-Naf)(4)-(PAH-PAA)(m) multilayer is around 50 degrees. When such a film with m < 5 and terminated by PAA is stretched out, its water contact angle increases up to around 100 degrees. Successive elongation/retraction cycles allow the water contact angle to alternate reversibly between 100 and 57 degrees indicating the reversible mechanical responsive nature of the film. 相似文献
95.
亚临床肝性脑病患者的血清锌,铜含量的测定及临床意义 总被引:1,自引:0,他引:1
检测了30例慢性活动性肝炎患者、30例肝硬化患者、30例亚临床肝性脑病患者血清中的铜、锌含量,并与正常对照组进行比较分析,探讨了微量元素铜、锌的血清含量改变及其影响因素,分析其临床意义。结果表明体内的铜锌含量与肝脏功能有密切的关系。在慢性活动性肝炎时,虽然肝脏功能有一定的损害,但由于肝脏的代偿能力强,铜,锌含量仍可保持在正常水平,而在肝硬化、亚临床肝性脑病时,铜锌明显降低,而铜锌含量的降低又导致机 相似文献
97.
Simultaneous separation of flavanone glycosides and polymethoxylated flavones in citrus juices using liquid chromatography 总被引:5,自引:0,他引:5
We present a simultaneous liquid chromatographic method for the separation of two flavonoid compound families, flavanone glycosides (FGs) and polymethoxylated flavones (PMFs), which are usually found in citrus fruit species and varieties. This technique permits the quantitation of six FGs (narirutin, naringin, hesperidin, neohesperidin, didymin, poncirin) and six PMFs (sinensetin, hexamethoxyflavone, nobiletin, scutellarein, heptamethoxyflavone and tangeretin). This technique, to be used to characterize a citrus juice by its polyphenolic profile, has been applied to the determination of flavonoid compounds in grapefruit- and orange juice. Differentiation of orange juice varieties and mixtures containing tangor juice using polyphenolic profiles and flavonoid content has been achieved. 相似文献
98.
The action of ultra violet rays of 260 nm on aqueous solutions of bases, nucleosides and nucleotides of DNA at 77°K is studied by electron spin resonance. It is shown that the free radicals observed are similar, with a few noteworthy exceptions to those induced by X-rays, under the same conditions of temperature, in the solid state. Contrary to what might be excepted on the basis of the results obtained by X-rays, the variation in the yield in paramagnetic centres in each sequence studied does not seem to be important. 相似文献
99.
测定了75例厌食小儿、101例呼吸道感染小儿和120例健康儿童发锌、铜,结果表明:厌食儿及易感儿平均发锌值分别为80.90×10-6和75.76×10-6,均极显著低于健康儿(P<0.01);发铜则无显著差异。发锌值在70×10-6以下者有60%以上身高、体重低于正常标准。说明微量元素锌与小儿食欲、免疫功能和生长发育有密切关系。 相似文献
100.
Sofiane Ben Hamouda Quang Trong Nguyen Pierre Schaetzel 《European Polymer Journal》2006,42(11):2994-3005
Ethylene/ethane sorption characteristics were determined for dry Pebax™ (poly(amide 12-block-tetramethylenoxide) copolymer)/AgBF4 membranes by using an electronic microbalance. The membranes containing 0.7 and 22 wt.% AgBF4 showed a dual-mode sorption isotherm. The ethane isotherms for all the membranes were of the Henry-type, which is the normal sorption for gases in rubbery polymers. The abnormal presence of Langmuir sorption sites only for ethylene in the rubbery copolymer, never reported sofar, is attributed to the silver-based specific complexation sites. The silver salt which dissolved in limited amounts in the rubbery copolymer had a much smaller Langmuir sorption capacity than the salt that crystallized in the copolymer. The sorption kinetics indicate that the crystallized salt did adsorb slowly ethylene according to a zeroth-order kinetics, but not ethane. The gas uptake kinetics resulting from a step of the pressure surrounding the copolymer exhibited one stage for ethane but two stages for ethylene. For the latter, there was first a fast Fickian sorption stage, then a drift of the zeroth-order sorption of ethylene on salt crystals, which contributes for a large part to the total uptake. The zeroth-order sorption suggests that the sorbed ethylene amount in the second-stage is independent of the crystal-surface coverage. The value of the Fickian diffusion coefficient calculated by fitting the kinetics with a solution of the second Fick’s law was 5 × 10−12 m2/s for both ethylene (the first stage) and ethane, and is typical for small organic compounds in a rubbery material. 相似文献