全文获取类型
收费全文 | 10417篇 |
免费 | 705篇 |
国内免费 | 394篇 |
专业分类
化学 | 3346篇 |
晶体学 | 37篇 |
力学 | 663篇 |
综合类 | 58篇 |
数学 | 5135篇 |
物理学 | 2277篇 |
出版年
2024年 | 16篇 |
2023年 | 156篇 |
2022年 | 207篇 |
2021年 | 311篇 |
2020年 | 389篇 |
2019年 | 352篇 |
2018年 | 353篇 |
2017年 | 396篇 |
2016年 | 340篇 |
2015年 | 311篇 |
2014年 | 557篇 |
2013年 | 1265篇 |
2012年 | 585篇 |
2011年 | 611篇 |
2010年 | 490篇 |
2009年 | 730篇 |
2008年 | 739篇 |
2007年 | 583篇 |
2006年 | 469篇 |
2005年 | 433篇 |
2004年 | 328篇 |
2003年 | 219篇 |
2002年 | 192篇 |
2001年 | 129篇 |
2000年 | 153篇 |
1999年 | 164篇 |
1998年 | 127篇 |
1997年 | 126篇 |
1996年 | 124篇 |
1995年 | 78篇 |
1994年 | 76篇 |
1993年 | 60篇 |
1992年 | 54篇 |
1991年 | 31篇 |
1990年 | 19篇 |
1989年 | 26篇 |
1988年 | 18篇 |
1987年 | 20篇 |
1986年 | 19篇 |
1985年 | 45篇 |
1984年 | 52篇 |
1983年 | 23篇 |
1982年 | 27篇 |
1981年 | 27篇 |
1980年 | 18篇 |
1979年 | 17篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 8篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
201.
Lei Wang Guang-ming Zhu Yue-zhong Meng Chun-mei Gao 《高分子科学》2007,(4):419-426
A novel sulfonated poly(arylene ether) containing triphenylmethane moieties was synthesized by the sulfonation of a designed parent polymer using chlorosulfonic acid as sulfonation agent. The sulfonation took place at the para position of the pendant phenyl rings because of the specially designed parent polymer. The position and degree of sulfonation were characterized by ^1H-NMR and elemental analysis. The sulfonated polymers are highly soluble in common organic solvents, such as dimethylsulfoxide, N,N'-dimethylacetamide, dimethylformamide, ethylene glycol monomethyl ether, and can be readily cast into tough and smooth films from solutions. The films showed good thermal and hydrolysis stabilities. Moreover, Fenton's reagent test revealed that the films exhibited superior stability to oxidation. The proton conductivities of the films were comparable with Nation 117 under same conditions. The membrane electrode assembly (MEA) prepared with the asmade film (706 EW, 100 μm dry thickness) shows better cell performance than Nation 115-MEA in the whole current density range. 相似文献
202.
203.
204.
205.
Kinsel GR Knochenmuss R Setz P Land CM Goh SK Archibong EF Hardesty JH Marynick DS 《Journal of mass spectrometry : JMS》2002,37(11):1131-1140
The photoionization of (pro)(n)DHB (pro = proline, DHB = 2,5-dihydroxybenzoic acid, n = 0, 1, 2 or 4) clusters was studied both experimentally and computationally. Experimentally the (pro)(n)DHB clusters are generated in the gas phase by laser desorption and supersonic jet entrainment. The photoionization thresholds are then determined by the mass-selective measurement of both one- and two-color photoionization efficiency curves. These experiments demonstrate that the ionization energies (IEs) of the (pro)(n)DHB clusters are substantially reduced in comparison with the IE of free DHB. Computational studies of the (pro)(n)DHB clusters provide insights into the mechanism of IE reduction. For the (pro)DHB system the IE reduction results from spin delocalization in the ion state of the cluster. In contrast, for the (pro)(2)DHB and (pro)(4)DHB clusters the IE reduction results from an inductive delocalization of electron density from pro to DHB in the ground state of the cluster. This latter effect, which is a result of the specific hydrogen-bonding interactions occurring in the mixed clusters, leads to IE reductions of >1 eV. Finally, determination of the energetics of the (pro)(2)DHB radical cation demonstrate that the DHB-to-proline proton transfer reaction is a barrierless, exoergic process in the ion state and that energetic demands for cluster dissociation to protonated (pro)(2) plus a deprotonated DHB radical are substantially lower than those for cluster dissociation to (pro)(2) plus DHB(+*). Cumulatively, these studies provide new energetic and mechanistic insights into both primary and secondary MALDI ionization processes. 相似文献
206.
In this work, the enzymatic degradation of poly(butylene succinate-co-butylene terephthalate) (PBST) copolyesters was studied using the lipase from Pseudomonas (Lipase PS®). The biodegradation behavior was found to strongly depend on the overall impacts of several important factors as the BT comonomer structure and molar content, thermal characteristics, morphology, the enzyme-substrate, and so forth. Further, the biodegraded residual film samples were allowed to be analyzed by means of gel permeation chromatography (GPC), proton nuclear magnetic resonance (1H NMR), differential scanning calorimeter (DSC), small angle X-ray scattering (SAXS), and scanning electron microscope (SEM). On the experimental evidences, an exo-type mechanism of enzymatic chain hydrolysis preferentially occurring in the amorphous region was suggested for the PBST film samples. 相似文献
207.
Qing-de Chen Xing-hai Shen 《高分子科学》2005,(6):635-642
In the UV-Vis spectra of pure light-scattering systems, there is an exponential relationship between absorbance and wavelength (A = Kλ^-n). Here, the exponent n is named as flocculation-coagulation parameter. In the present paper, the effects of different additives on the stability of poly(N,N'-methylenebisacrylamide-co-4-vinylpyridine) (poly(Bis-co-4-VP)) microgel dispersion were studied in detail via this parameter. The results showed that the stability of the dispersion mainly comes from the ionization of pyridine groups, making the microgel positively charged on its surface. This was confirmed by the measurement of Zeta potential and the result of conductometric titration. The result of fluorescence analysis indicated that the hydrophobicity in the microgels is enhanced with the increase in total 4-VP unit content. 相似文献
208.
Fluorescence quantum yields and lifetimes of coumarin dyes are sharply reduced in polar solvents if amine substituent groups are free to rotate. The polar solvent effect is interpreted in terms of relaxation of excited dye from an initial planar conformation to a twisted zwitterionic state. 相似文献
209.
The possibility of simultaneous application of an electron capture (ECD) and a flame ionization detector (FID) connected to a glass capillary column for analyzing polynuclear aromatic hydrocarbons (PNA) has been investigated. The ECD/FID ratio is determined for 46 PNA compounds. The ratios vary from 0.02 to 117 with relative standard deviations better than 20 percent determined from 10 replicate analyses. The results suggest that the method may be used for obtaining additional evidence in identifying PNA in environmental samples. Impurities and transformation products in the standard were identified by computerized glass capillary gas chromatography/mass spectrometry. Quinones and diones are responsible for the high EC-response determined in some trace components in the standard. An application of the method is shown for PNA from particulates in urban atmospheres. 相似文献
210.
Fu-Sheng S 《Talanta》1983,30(6):446-448
The conditions for the reaction between phenylarsenazo (PAA) (2-[(2-arsonophenyl)azoj-7-(phenylazo)-1, 8-dihydroxynaphthalene-3,6-disulphonic acid) and chromium(III) have been studied. A blue 1:1 complex is formed at pH 2.2 on heating the reactants at 100 degrees for 15 min. It has maximum absorption at 635 nm and is stable for at least 24 hr. The molar absorptivity is 3.3 x 10(4) 1.mole(-1).cm(-1). Beer's law is obeyed in the chromium concentration range 0-1.4 mug ml . The reaction has been successfully applied to determination of chromium in alloy steel. 相似文献