全文获取类型
收费全文 | 9424篇 |
免费 | 1276篇 |
国内免费 | 1323篇 |
专业分类
化学 | 5857篇 |
晶体学 | 88篇 |
力学 | 1353篇 |
综合类 | 63篇 |
数学 | 500篇 |
物理学 | 4162篇 |
出版年
2024年 | 23篇 |
2023年 | 116篇 |
2022年 | 256篇 |
2021年 | 278篇 |
2020年 | 384篇 |
2019年 | 322篇 |
2018年 | 298篇 |
2017年 | 382篇 |
2016年 | 397篇 |
2015年 | 384篇 |
2014年 | 487篇 |
2013年 | 780篇 |
2012年 | 603篇 |
2011年 | 666篇 |
2010年 | 506篇 |
2009年 | 575篇 |
2008年 | 532篇 |
2007年 | 590篇 |
2006年 | 494篇 |
2005年 | 509篇 |
2004年 | 439篇 |
2003年 | 374篇 |
2002年 | 327篇 |
2001年 | 322篇 |
2000年 | 256篇 |
1999年 | 233篇 |
1998年 | 206篇 |
1997年 | 172篇 |
1996年 | 151篇 |
1995年 | 152篇 |
1994年 | 125篇 |
1993年 | 92篇 |
1992年 | 78篇 |
1991年 | 72篇 |
1990年 | 65篇 |
1989年 | 41篇 |
1988年 | 46篇 |
1987年 | 33篇 |
1986年 | 30篇 |
1985年 | 32篇 |
1984年 | 31篇 |
1983年 | 12篇 |
1982年 | 25篇 |
1981年 | 16篇 |
1980年 | 17篇 |
1979年 | 20篇 |
1978年 | 10篇 |
1977年 | 22篇 |
1976年 | 14篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
73.
FTIR-ATR has been used for understanding the interaction between bacteria and surfaces in the adsorption progress. 相似文献
74.
采用分步浸渍法制备了系列Mg改性的Co/γ-Al_2O_3-TiO_2催化剂,通过X射线衍射(XRD)、紫外可见漫反射光谱(DR-UV-vis)、N_2吸附-脱附(BET)、X射线光电子能谱(XPS)和H_2程序升温还原(H_2-TPR)等技术对催化剂进行表征,并考察了其对丙烷燃烧的催化性能。结果表明,Co在原始γ-Al_2O_3-TiO_2载体和Mg改性MgO/γ-Al_2O_3-TiO_2载体上均以Co_3O_4的形式存在;Mg掺入后与Al_2O_3作用形成MgAl_2O_4尖晶石,改善了载体的织构性质,提升了Co_3O_4在催化剂载体表面的暴露数量和分散程度。此外,MgAl_2O_4与Co_3O_4相互作用提升了Co_3O_4颗粒表面Co~(3+)/Co~(2+)和O_(ads)/O_(latt)的比例,并削弱了Co-O键键能,从而提升了其对丙烷的催化燃烧活性。当Mg负载量为15%(质量分数)时,在Co/MgO(15%)/γ-Al_2O_3-TiO_2催化剂上进行丙烷燃烧,丙烷90%转化率的温度比无Mg掺杂的Co/γ-Al_2O_3-TiO_2催化剂的降低了45℃,并且连续反应40 h其活性保持稳定。 相似文献
75.
B. S. Narwade P. G. Gawali Rekha Pande G. M. Kalamse 《Journal of Chemical Sciences》2005,117(6):673-676
Dielectric constant (ε’) and dielectric loss (ε") of n-propyl alcohol (PA), ethylenediamine (EDA) and their binary mixtures,
for different mole fractions of ethylenediamine have been experimentally measured at 11.15 GHz microwave frequency. Values
of density (ρ), viscosity (η) and square refractive index (n
D
2
) of binary mixtures as well as those of pure liquids are reported. Excess square refractive index, viscosity and activation
energy of viscous flow have also been estimated. These parameters have been used to explain the formation of complexes in
the system. 相似文献
76.
Monodisperse poly(methyl methacrylate) (PMMA) particles containing various concentrations of stearyl methacrylate (SMA) were prepared, and a liquid crystal (LC) was swollen into the particles using a solute co-diffusion method (SCM). Phase separation behaviors between the polymer and LC were monitored by utilizing an optical and a polarized microscope (OM/POM). The monodisperse LC microcapsules were then applied to a polymer-dispersed liquid crystal (PDLC), and the electro-optical properties were investigated. As a result, the threshold and driving voltages were improved when the SMA content increased. The long alkyl chains of SMA in the capsules should exist at the interface of the LC and polymer resulting in an enhancement of phase separation between the polymer and LC, which largely influences the electro-optical properties of PDLC. 相似文献
77.
E. Fleury J. Dubois C. Léonard J. P. Joseleau H. Chanzy 《Cellulose (London, England)》1994,1(2):131-144
Solutions of cellulose diacetate (CDA) from two sources (cotton linters and wood pulp Floranier) were analysed in various solvents by size exclusion chromatography (SEC). Without special precautions, the SEC chromatograms presented three peaks — or prehumps — before the main polymer peak. The first prehump which could be eliminated by ultracentrifugation corresponded to microgels whose sugar composition was determined. These microgels were also investigated by electron microscopy, X-ray and electron diffraction analysis. They corresponded mainly to cellulose triacetate (CTA-II) in the case of CDA from cotton linters and a mixture of CTA-II and xylan diacetate (XDA) in the case of CDA from the wood pulp Floranier. The second and third prehumps could be attributed to ionic effects corresponding to the association of remaining sulfate groups on the CDA molecules with residual calcium. It was found that these ionic effects could be eliminated by the addition of LiBr or LiCl to the elution solvents. This led to chromatograms devoid of prehumps.Presented in part at the Cellulose '91 meeting in New Orleans. 相似文献
78.
K. V. Zaitsev S. S. Karlov M. V. Zabalov A. V. Churakov G. S. Zaitseva D. A. Lemenovskii 《Russian Chemical Bulletin》2005,54(12):2831-2840
Titanatrane CpTi(OCH(CH3)CH2)3N (3) was prepared by the reaction of CpTiCl3 with N(CH2CH(CH3)OH)3 in the presence of triethylamine. The reaction of CpTi(OMe)3 (8) with N(CH2CH2OH)2(CH2CHPhOH), erythro-N(CH2CH2OH)2(CHPhCHPhOH), and N(CH2CH2OH)2(CH2CPh2OH) gave cyclopentadienyltitanatranes 12–14. Compound 8 reacts with threo-N(CH2CH2OH)2(CHPhCHPhOH) to give a mixture of threo-CpTi(OCH2CH2)2(OCHPhCHPh)N and threo-MeOTi(OCH2CH2)2(OCHPhCHPh)N. Slow hydrolysis of the latter product gave threo-[Ti3O(OMe){(OCH2CH2)2− (OCHPhCHPh)N} 3]2, which was studied by X-ray diffraction analysis. The crystal structures of 3 and 12 were also determined by X-ray diffraction analysis. The titanium coordination polyhedron in these complexes is a distorted
trigonal bipyramid in which the equatorial positions are occupied by three oxygen atoms and the axial positions contain the
N atom and the Cp group. The titanium—nitrogen distances (2.313(2), 2.291(2) Å in two independent molecules of 3 and 2.271(2) Å in compound 12) confirm the presence of Ti←N transannular interaction.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2736–2744, December, 2005. 相似文献
79.
In order to study the influence of the side-chain orientation on the peptide backbone conformation we have synthesised the model dipeptides t-BuCO-l-Pro-(1S,2R)-c6Phe-NHMe and t-BuCO-l-Pro-(1R,2S)-c6Phe-NHMe, incorporating each enantiomer of the trans cyclohexane analogue of phenylalanine (trans-1-amino-2-phenylcyclohexanecarboxylic acid). The orientation of the aromatic side-chain determines the β-turn type accommodated by these peptides to the point that the (1S,2R)-c6Phe derivative retains the type I β-turn in the crystalline state, in contrast to the behaviour exhibited by the natural counterpart t-BuCO-l-Pro-l-Phe-NHMe. 相似文献
80.
Li-Ya Wang Xiao-Qiang Wang Kai Jiang Jiu-Li Chang Yu-Fang Wang 《Journal of Molecular Structure》2007,840(1-3):14-21
A new nitronyl nitroxide NIT2-bithph (1) and its manganese(II) compound [Mn(hfac)2(IMHBithph)]2·(NIT2-bithph)(C6H14) (2) (hfac = hexafluoroacetylacetonate; NIT2-bithph = 4,4,5,5-tetramethyl-2-(bithiophenal-2-yl)imidazoline-1-oxyl-3-oxide; IMHBithph = 1-hydroxy-2-bithiophenal-4,4,5,5-tetramethyl-4,5-dihydro- 1H-imidazole) have been synthesized and structurally characterized by X-ray diffraction methods. The units of compound 1 were connected as one-dimensional chain by the intermolecular hydrogen bonds which afford an intermolecular antiferromagnetic interaction between nitronyl nitroxide radicals within the chain (J = −1.89 cm−1). Compound 2 resulting from the reaction of Mn(hfac)2·2H2O and NIT2-bithph is dinuclear and includes the reduced amidino-oxide form of NIT2-bithph, it is made up of three parts: a [Mn(hfac)2(IMHBithph)]2 dimer unit, an uncoordinated NIT2-bithph radical and a noncoordinated solvent molecule of hexane, the molecule of radical is hydrogen bonded to its reduced form. Two reduced IMHBithph ligands bridge the two manganese(II) ions through their amidino-oxide oxygen atoms resulting in a small intramolecular antiferromagnetic interaction between the manganese ions (J = −1.55 cm−1). 相似文献