全文获取类型
收费全文 | 37498篇 |
免费 | 6706篇 |
国内免费 | 6264篇 |
专业分类
化学 | 29586篇 |
晶体学 | 578篇 |
力学 | 1967篇 |
综合类 | 498篇 |
数学 | 3933篇 |
物理学 | 13906篇 |
出版年
2024年 | 143篇 |
2023年 | 794篇 |
2022年 | 1459篇 |
2021年 | 1488篇 |
2020年 | 1651篇 |
2019年 | 1790篇 |
2018年 | 1496篇 |
2017年 | 1411篇 |
2016年 | 2001篇 |
2015年 | 2029篇 |
2014年 | 2321篇 |
2013年 | 2977篇 |
2012年 | 3352篇 |
2011年 | 3355篇 |
2010年 | 2449篇 |
2009年 | 2353篇 |
2008年 | 2527篇 |
2007年 | 2162篇 |
2006年 | 2152篇 |
2005年 | 1929篇 |
2004年 | 1567篇 |
2003年 | 1316篇 |
2002年 | 1463篇 |
2001年 | 1197篇 |
2000年 | 945篇 |
1999年 | 751篇 |
1998年 | 538篇 |
1997年 | 392篇 |
1996年 | 424篇 |
1995年 | 338篇 |
1994年 | 308篇 |
1993年 | 274篇 |
1992年 | 239篇 |
1991年 | 189篇 |
1990年 | 152篇 |
1989年 | 115篇 |
1988年 | 78篇 |
1987年 | 73篇 |
1986年 | 73篇 |
1985年 | 59篇 |
1984年 | 24篇 |
1983年 | 35篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1971年 | 4篇 |
1959年 | 6篇 |
1957年 | 8篇 |
1936年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
801.
Xin Wang 《Journal of organometallic chemistry》2005,690(12):2934-2940
Preparation of two imidazolium salts, two monomeric nickel(II) and one cobalt(II) complexes bearing imidazolium ligands is described, The solid-state structures of these compounds have determined by single-crystal X-ray diffraction. After activation with methylaluminoxane (MAO) the nickel complexes show moderate catalytic activities of up to 6 × 105 g PE mol−1Ni h−1 for polymerization of ethylene. Catalytic activities, molecular weights have been investigated under the various reaction conditions. 相似文献
802.
新型稀土杂多蓝的合成及其抗艾滋病病毒(HIV-1)活性和毒性研究 总被引:14,自引:0,他引:14
合成Keggin结构钨硅、钨锗、钨磷、钨坤两电子及四电子稀土钐盐杂多蓝,铈量滴定及元素分析方法确定了化合物的还原电子数及化学组成,采用IR,UV-Vis,^183W NMR和ESR等对其结果进行了表征,在人T淋巴MT-4内,对合成的化合物进行了系统的抗艾滋病毒(HIV-1)活性及毒性测定,发现Keggin结构钨硅、钨锗四电子稀土钐盐杂多蓝具有较强的抗HIV-1活性,其中钨锗酸钐四电子杂多蓝(代号HPBR-2)具有较高的治疗指数。 相似文献
803.
Equilibrium gas-phase acidities of the six isomeric cyclohexanediols were measured in a Fourier transform ion cyclotron resonance mass spectrometer. Although all six cyclohexanediols have the same functional groups and similar structures, the acidities vary over 11 kcal/mol. This large difference is due mostly to the balance between hydrogen bonding and geometric strain. To understand the origins of the acidity differences in more detail, the conformations and energetics of the cyclohexanediols were studied using density functional theory, which gave good agreement with the experimental acidities. Finally, methanol-methoxide and methanol-methanol interactions were used as a model for the hydrogen bonding. 相似文献
804.
IntroductionThegenediagnosistechnology ,alsocallednucleicacidprobetechnology ,hasbecomeanattractivemethodinthefieldsofbiochemistryandclinicalmedicine .Itisprovedtobeconvenientandsafeusingnon radioactivereagentswithelectrochemicalandopticaldetector.Espe c… 相似文献
805.
Yu Ping ZHANG Wen Jun GONG Li Qun FAN Yi Jun ZHANG Kwang Pill LEE 《中国化学快报》2006,17(11):1485-1488
A porous monolithic sol-gel column with the solution of methacryloxypropyltrimethoxysilane in toluene with an acid catalyst was prepared in the presence and absence of sodium dodecyl sulfate. In situ polymerization was carried out byγ-ray irradiation within the capillary. Theγ-radiation-initiated synthesis could generate radicals directly on the monomer avoiding use of any initiator. The chromatographic behavior of the capillary monolithic columns were studied in the modes of CEC, p-CEC and low pressure-driven separation, all the tests exhibited reversed-phase character. It provided a viable alternative to either thermally initiated or photo polymerization method for the preparation of monolithic columns. 相似文献
806.
Xin Rong QIN* Quan LI Yu Yuan XIE State Key Laboratory of Drug Research Shanghai Institute of Materia Medica Shanghai Institutes for Biological Sciences Chinese Academy of Science Shanghai 《中国化学快报》2001,(9)
The proteinogenic (-amino acids constitute an important section of the "chiral pool", being inexpensive in the L-form (but available if necessary as the D enantiomers), structurally varied and chemically versatile1, they are useful starting materials for chiral reagents, auxiliaries and ligands2. Any high-yielding transformation of an (-amino acids which proceeds without racemization is thus of potential importance, especially if it generates another reactive center and may be applied to the s… 相似文献
807.
Runru Liu Dapeng Xu Shuang Li Zhe Lü Yanfeng Xue Deyong Wang Wenhui Su 《Frontiers of Chemistry in China》2006,1(4):398-401
In this paper, perovskite oxide SmCoO3 was prepared by the solid-state reaction method using Co2O3 and Sm2O3 as raw materials. The structure and properties of the samples were investigated by XRD, Raman spectral techniques, and DC
measurements and so on. The results of XRD and Raman spectra showed that the mixtures of Co2O3 and Sm2O3 can react to produce a single phase perovskite oxide SmCoO3 around 1353 K. The single-phase SmCoO3 changes from an insulator to a semi-conductor and transition occurs around 470 K. The thermal expansion coefficient (2.17
× 10−5 K−1) of the single-phase SmCoO3 is approximately equal to that of doped LaGaO3, but much bigger than that of SDC(Ce0.85Sm0.15O2) above 873 K. 相似文献
808.
Yong-Jiang Xu Weitao Pan Xin Chen Wai C. Wong Marc Labelle 《Tetrahedron letters》2005,46(44):7523-7526
Novel synthetic approaches toward 5,6-dihydro-pyrimido[4,5-b][1,4]oxazepines were reported that led to successful introduction of poorly reactive anilines and various 2-hydroxybenzaldehydes to this therapeutically relevant scaffold. More extensive SAR studies on this scaffold hence became possible. 相似文献
809.
含氟丙烯酸酯共聚乳液及其膜表面特性的研究 总被引:17,自引:0,他引:17
以十二烷基硫酸钠 (SDS)和OP 10混合乳化剂 ,制备了甲基丙烯酸全氟辛基乙酯 (FMA8) 甲基丙烯酸丁酯 (BMA) 甲基丙烯酸 (MAA)共聚乳液 .通过DSC、FT IR、1 H NMR对共聚物的结构、组成进行了表征研究 .采用JZHY 180界面张力仪研究了共聚乳液膜表面的性质 ,结果表明 ,随着共聚物中全氟单体含量的增加 ,共聚物膜的表面能显著降低 ,当全氟单体的含量达到 2 5wt %时 ,其表面能降低到 19 74mJ m2 .X ray光电子能谱(XPS)对共聚物表面原子组成的分析结果表明 ,共聚物表面氟的含量远高于其平均含量 ,证明了含氟基团的趋表现象 .经退火处理 ,共聚物膜表面的氟含量增加 ,表面自由能降低 相似文献
810.
In Situ IR Spectroscopic Studies on Molybdenum Nitride Catalysts: Active Sites and Surface Reactions
Recent IR spectroscopic studies on the surface properties of fresh Mo2N/-Al2O3 catalyst are presented in this paper. The surface sites of fresh Mo2N/-Al2O3, both Mo+ (0<<2) and N sites, are probed by CO adsorption. Two characteristic IR bands were observed at 2045 and 2200 cm-1, due to linearly adsorbed CO on Mo and N sites, respectively. The surface N sites are highly reactive and can react with adsorbed CO to form NCO species. Unlike adsorbed CO on reduced passivated one, the adsorbed CO on fresh Mo2N/-Al2O3 behaves similarly to that of group VIII metals, suggesting that fresh nitride resembles noble metals. It is found that the surface of Mo nitrides slowly transformed into sulfide under hydrotreating conditions, which could be the main reason for the activity drop of molybdenum nitride catalysts in the presence of sulfur-containing species. Some surface reactions, such as selective hydrogenation of 1,3-butadiene, isomerization of 1-butene, and hydrodesulfurization of thiophene, were studied on both fresh and reduced passivated Mo2N/-Al2O3 catalysts using IR spectroscopy. The mechanisms of these reactions are proposed. The adsorption and reaction behaviors of these molecules on fresh molybdenum nitride also resemble those on noble metals, manifesting the unique properties of fresh molybdenum nitride catalysts. Mo and N sites are found to play different roles in the adsorption and catalytic reactions on the fresh Mo2N/-Al2O3 catalyst. Generally, Mo sites are the main active sites for the adsorption and reactions of adsorbates; N sites are not directly involved in catalytic reactions but they modify the electronic properties of Mo sites. 相似文献