全文获取类型
收费全文 | 79792篇 |
免费 | 14596篇 |
国内免费 | 4474篇 |
专业分类
化学 | 73312篇 |
晶体学 | 1579篇 |
力学 | 2415篇 |
综合类 | 228篇 |
数学 | 6196篇 |
物理学 | 15132篇 |
出版年
2024年 | 29篇 |
2023年 | 164篇 |
2022年 | 511篇 |
2021年 | 766篇 |
2020年 | 1185篇 |
2019年 | 2895篇 |
2018年 | 2754篇 |
2017年 | 3340篇 |
2016年 | 3787篇 |
2015年 | 5973篇 |
2014年 | 5806篇 |
2013年 | 8943篇 |
2012年 | 6586篇 |
2011年 | 6313篇 |
2010年 | 5229篇 |
2009年 | 5073篇 |
2008年 | 5441篇 |
2007年 | 4822篇 |
2006年 | 4549篇 |
2005年 | 4190篇 |
2004年 | 3682篇 |
2003年 | 3321篇 |
2002年 | 3873篇 |
2001年 | 2239篇 |
2000年 | 2107篇 |
1999年 | 1224篇 |
1998年 | 562篇 |
1997年 | 487篇 |
1996年 | 452篇 |
1995年 | 432篇 |
1994年 | 321篇 |
1993年 | 278篇 |
1992年 | 262篇 |
1991年 | 197篇 |
1990年 | 161篇 |
1989年 | 121篇 |
1988年 | 129篇 |
1987年 | 116篇 |
1986年 | 103篇 |
1985年 | 86篇 |
1984年 | 82篇 |
1983年 | 41篇 |
1982年 | 57篇 |
1981年 | 33篇 |
1980年 | 34篇 |
1979年 | 39篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
《Electroanalysis》2006,18(21):2115-2120
A new type of voltammetric sensor, Langmuir–Blodgett film of p‐tert‐butylthiacalix[4]arene modified glassy carbon electrode, was advanced and used for determining copper at trace levels by differential pulse stripping voltammetry. Calibration plot was found to be linear in the range of 2×10?8 M to 5×10?6 M; the detection limit was 2×10?9 M. Possible recognition mechanism was also discussed. From determination of Copper in real samples (river, lake and tap water) it can be concluded that the method is rapid, sensitive in determining of copper and can be used in the analysis of natural water samples. 相似文献
992.
采用低湿溶液缩聚的方法合成了对苯二甲酰氯,二甲基联苯胺和己二醇为单体的芳酯族液晶聚酯酰胺。用DSC,X光衍射分析和偏光显微镜等手段研究了该系列聚酯酰胺的热致液晶行为,确认了二甲基联苯胺单体用量在20%(mol)的情况下,所得聚酯酰胺仍为向列型液晶聚合物。由于聚酯酰胺分子间聚酰胺链段之间的氢键作用,随着二甲基联苯胺用量增加至60mol%时,所得的聚酯酰胺己无液晶转变温度,其液晶区间即从熔融温度直至分 相似文献
993.
With frontal analysis(FA),the dependence of adsorption isotherms of insulin on the composition of mobile phase in reversed phase liquid chromatography (RPLC) has been investigated,This is also a good example to employ the stoichiometric displacement theory (SDT) for ivestigating solute adsorption in physical chemistry.Six kinds of mobile phase in RPLC were employed to study the effects on the elution curves and adsorption isotherms of insulin.the key points of this paper are:(1) the stability of insulin due to delay time after preparing,the organic solvent concentration,the kind and the concentration of ion-pairing agent in mobile phase were found to affect both elution curve and adsorption isotherm very seriously.(2)To obtain a valid and comparable result,the composition of the mobile phase employed in FA must be as same as possible to that in usual RPLC of either analytical scale or preparative purpose.(3)Langmuir Equation and the SDT were employed to imitate these obtained adsorption isotherms.The expression for solute adsorption from solution of the SDT was found to have a better elucidation to the insulin adsorption from mobile phase in RPLC. 相似文献
994.
Chemical modification on the stereo‐regular poly(styrene‐co‐4‐methylstyrene) (sPS‐PMS) was attempted in this study. Metallocene copolymerization of styrene (St) and 4‐methylstyrene (MSt) was performed by using η5‐pentamethylcyclopentadienyl‐titanium(IV)tributoxide (Cp*Ti(OBu)3)/methylaluminoxane (MAO)/tri‐iso‐butylaluminum (TIBA) catalyst in the bulk state. Cobalt(II) catalyst was then applied to oxidize the benzylic methyl group on the MSt units of the resulting sPS‐PMS copolymer. Both aldehyde and carboxylic acid in the oxidized products were resolved by the FTIR and 1H NMR. The oxidized sPS‐PMSs exhibit a low and a high‐temperature Tg and Tm corresponding to the transitions in the amorphous and the crystalline regions. Hydrogen‐bond and polar interactions between the aldehyde and carboxylic acids tend to interrupt the regular chain packing of the oxidized sPS‐PMS, resulting in the lowering of Tm with oxidation level. The oxidized sPS‐PMS showed better adhesion to glass fiber than pure sPS‐PMS copolymer as evaluated from the respective SEM fractured micrographs. 相似文献
995.
尼龙1010/尼龙6共聚物的表观相图研究 总被引:2,自引:0,他引:2
采用显微熔点法和DSC测定了尼龙1010/尼龙6共聚物的表观相图;分析了投料比与链节结构单元含量的关系。最低熔点时尼龙1010/尼龙6的理论重量投料比为60/40,摩尔比为33.3/66.7,实验结果与此相近。 相似文献
996.
997.
Single Crystal Investigations on Fluoroperovskites MPdF3 (M = Rb, K) and PdF2 Single crystal investigations on PdF2 (violet) confirm the tetragonale rutile structure [1, 2] with a = 495.44(3) pm, c = 338.48(3) pm, Z = 2, space group P42/mnm-D (No. 136) (xF = 0.3058(8)). RbPdF3 and KPdF3 have been obtained. RbPdF3 and KPdF3 both ruby-red single crystals crystallize cubic (perovskite-structure), space group Pm3m-O (No. 221) with a = 429.41(8) pm (RbPdF3) respectively a = 424.30(4) pm (KPdF3). 相似文献
998.
Summary The chromatographic behaviour of C6F5(CH2)3SiCl3, C6H5(CH2)3SiCl3, CF3(CF2)7CH2CH2SiCl3 and n-C10H21SiCl3 as well as the bonding of these ligands to silica is described. Phases with partially fluorinated ligands show entirely different chromatographic characteristics to analogous phases, where hydrogen replaces fluorine. The silica with the pentafluorophenyl-3-n-propyl-C6F5(CH2)3-ligand has higher k-values and a better selectivity for aromatic hydrocarbons than that with phenyl-3-n-propyl C6H5(CH2)3-groups, whereas the phase with the fluorinated carbon chain is less useful than the silica modified with n-decyltrichlorosilane n-C10H21SiCl3. The first effect can be explained by complex formation, and the second may be due to a different contact area between the ligand and the solute. The normal hydrocarbon chains hinder each other sterically and expose many adsorption sites, in contrast the perfluorinated chains are sterically fixed and are the first example of real brushes in HPLC. 相似文献
999.
Ohmiya H Tsuji T Yorimitsu H Oshima K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(22):5640-5648
Details of cobalt-catalyzed cross-coupling reactions of alkyl halides with allylic Grignard reagents are disclosed. A combination of cobalt(II) chloride and 1,2-bis(diphenylphosphino)ethane (DPPE) or 1,3-bis(diphenylphosphino)propane (DPPP) is suitable as a precatalyst and allows secondary and tertiary alkyl halides--as well as primary ones--to be employed as coupling partners for allyl Grignard reagents. The reaction offers a facile synthesis of quaternary carbon centers, which has practically never been possible with palladium, nickel, and copper catalysts. Benzyl, methallyl, and crotyl Grignard reagents can all couple with alkyl halides. The benzylation definitely requires DPPE or DPPP as a ligand. The reaction mechanism should include the generation of an alkyl radical from the parent alkyl halide. The mechanism can be interpreted in terms of a tandem radical cyclization/cross-coupling reaction. In addition, serendipitous tandem radical cyclization/cyclopropanation/carbonyl allylation of 5-alkoxy-6-halo-4-oxa-1-hexene derivatives is also described. The intermediacy of a carbon-centered radical results in the loss of the original stereochemistry of the parent alkyl halides, creating the potential for asymmetric cross-coupling of racemic alkyl halides. 相似文献
1000.
Imtiaz Ahmad M. Arsala Khan M. Shakirullah M. Ishaq Yousaf Iqbal Saeed ur Rehman 《中国化学会会志》2004,51(4):729-734
Influence of pretreatments, i.e., demineralization and pre‐soaking in some solvents on the yields of liquefied products obtained from hydro‐depolymerization of Pakistani coal in a pool of hydrogen has been discussed. Results of hydrogenation experiments demonstrated that demineralizing the coal sample prior to liquefaction caused the yields to decrease compared with the virgin coal samples. The contribution of mineral matter of coal has been evaluated by comparing the yields of liquid products of virgin and leached samples. Increase in the yields obtained from the hydrogenation experiments of swollen coal samples was not satisfactorily significant. 相似文献