首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   653篇
  免费   100篇
  国内免费   142篇
化学   715篇
晶体学   26篇
力学   19篇
综合类   6篇
数学   2篇
物理学   127篇
  2024年   3篇
  2023年   20篇
  2022年   30篇
  2021年   40篇
  2020年   35篇
  2019年   41篇
  2018年   25篇
  2017年   35篇
  2016年   36篇
  2015年   27篇
  2014年   28篇
  2013年   46篇
  2012年   39篇
  2011年   27篇
  2010年   31篇
  2009年   38篇
  2008年   28篇
  2007年   53篇
  2006年   29篇
  2005年   38篇
  2004年   22篇
  2003年   29篇
  2002年   22篇
  2001年   23篇
  2000年   23篇
  1999年   16篇
  1998年   24篇
  1997年   18篇
  1996年   10篇
  1995年   8篇
  1994年   19篇
  1993年   7篇
  1992年   10篇
  1991年   3篇
  1990年   2篇
  1989年   5篇
  1986年   1篇
  1984年   2篇
  1982年   1篇
  1981年   1篇
排序方式: 共有895条查询结果,搜索用时 834 毫秒
881.
Two new bimetallic cyano-bridged complexes [Ce(DMSO)4(H2O)3Fe(CN)6]·H2O 1 and [La(DMSO)4(H2O)3Co(CN)6]·H2O 2 have been prepared by the ball milling reaction method and structurally characterized by X-ray single-crystal structure analyses. Crystallographic data for 1:C14H32CeFeN6O8S4, Mr = 736.67, monoclinic, space group P21/n, a = 14.952(1), b =13.7276(9), c = 15.392(1) (A), β = 108.288(1)°, V = 2999.6(4) (A)3, Z = 4, Dc= 1.631 g/cm3,μ =2.304 mm-1, F(000) = 1480, R = 0.0593 and wR = 0.1611; and those for 2: C14H32CoLaN6O8S4,Mr=738.54, monoclinic, space group P21/n, a = 14.945(3), b = 13.731(3), c = 15.300(3) (A), β=107.806(1)°, V= 2989.3(11) (A)3, Z = 4, Dc = 1.641 g/cm3,μ = 2.288 mm-1, F(000) = 1480, R =0.0383 and wR = 0.1132. In both complexes the lanthanide ion is eight-coordinated in a square antiprism arrangement, and the Fe(Ⅲ) or Co(Ⅲ) ion in a nearly regular octahedral environment.The [LnM(CN)6(DMSO)4(H2O)3]·H2O (Ln = Ce and M = Fe for 1; Ln = La and M = Co for 2)species are held together via hydrogen bonds by coordinated water molecules, lattice water molecules and nitrogen atoms of cyanide groups to form a three-dimensional framework.  相似文献   
882.
Traditional technologies applied for obtaining plant raw materials for cosmetic production are based primarily on high-level processing, which is reflected in the qualitative composition of the resulting materials. By using low-temperature drying, it is possible to retain in the raw materials a range of valuable ingredients. In this study, blue honeysuckle powder was used as an ingredient of cosmetic face masks. The stability of the masks was evaluated. Dynamic viscosity, yield point and texture analysis of the cosmetics was performed. The color of the emulsions and the level of skin hydration after face mask application was determined. Emulsions were found to be stable. A decrease in dynamic viscosity of the emulsions as a function of increasing concentrations of the additive and under the conditions of rising rotational speed were observed. Similarly, an increase in the concentration of blue honeysuckle in the emulsions resulted in a decrease in the value of the yield point. Based on the results, it can be stated that the addition of blue honeysuckle caused a decrease in hardness of the masks, while the opposite trend was observed for adhesive force. It was found that an increase in the concentration of blue honeysuckle gave a reddish-yellow color to the samples. Corneometric assessment confirmed proper skin hydration after the application of the emulsions.  相似文献   
883.
张耀君  李聚源  张君涛  辛勤 《化学学报》2004,62(21):2205-2208
用微量热法技术测量CO的微分吸附热以探测碳负载的单金属Pt,Ru及双金属Pt-Ru催化剂的CO表面吸附位.结果表明,单金属Pt催化剂显示出最高的初始微分吸附热(qinitial=125 kJ·mol-1);单金属Ru催化剂具有最低的初始微分吸附热(qinitial=109 kJ·mol-1);三种双金属PtRu催化剂的初始微分吸附热(qinitial=124~112 kJ·mol-1)界于两种单金属之间.当双金属PtRu催化剂Pt:Ru原子比为1:2时,催化剂Pt原子表面上的强CO吸附位(> 112 kJ·mol-1)被Ru原子所覆盖而完全消失.  相似文献   
884.
D,L ‐3‐Methylglycolide (MG) was successfully polymerized with bimetallic (Al/Zn) μ‐oxo alkoxide as an initiator in toluene at 90 °C. The effect of the initiator concentration and monomer conversion on the molecular weight was studied. It is shown that the polymerization of MG follows a living process. A kinetic study indicated that the polymerization approximates the first order in the monomer, and no induction period was observed. 1H NMR spectroscopy showed that the ring‐opening polymerization proceeds through a coordination–insertion mechanism with selective cleavage of the acyl–oxygen bond of the monomer. On the basis of 1H NMR and 13C NMR analyses, the selective cleavage of the acyl–oxygen bond of the monomer mainly occurs at the least hindered carbonyl groups (P1 = 0.84, P2 = 0.16). Therefore, the main chain of poly(D,L ‐lactic acid‐co‐glycolic acid) (50/50 molar ratio) obtained from the homopolymerization of MG was primarily composed of alternating lactyl and glycolyl units. The diblock copolymers poly(ϵ‐caprolactone)‐b‐poly(D,L ‐lactic acid‐alt‐glycolic acid) and poly(L ‐lactide)‐b‐poly(D,L ‐lactic acid‐alt‐glycolic acid) were successfully synthesized by the sequential living polymerization of related lactones (ϵ‐caprolactone or L ‐lactide). 13C NMR spectra of diblock copolymers clearly show their pure diblock structures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 357–367, 2001  相似文献   
885.
Cadmium titanate, CdTiO3, was prepared by the sol–gel technique in bulk and in thin film form. The thermal evolution of the gels and the phase changes were studied by thermo- gravimetric analysis (TGA), X-ray diffractometry (XRD) and Raman and energy-dispersive (EDS) spectroscopies. The morphology of the samples was observed using scanning electron microscopy (SEM). Gels heated to 800 °C gave rise to powders with only the ilmenite-like phase. The orthorhombic perovskite phase is the only crystalline phase observed after a 4 h heat-treatment at 1100 °C. With respect to the conventional preparation method by solid-state reaction, by the sol–gel method it is possible to prepare the ilmenite phase at lower temperatures and the perovskite phase in a shorter time. Clear, homogeneous thin films were obtained by the dip-coating method. The refraction index and the thickness of the films were measured using ellipsometry. The humidity-sensitive electrical properties were measured for thin films deposited on alumina substrates with comb-type gold electrodes, heated to 200 °C and 450 °C. The films heated to 200 °C, which still contained organics, showed a variation of the resistance of six orders of magnitude in the relative humidity (RH) range tested (4–87% RH). The films heated to 450 °C, made of ilmenite-type CdTiO3, were nearly insensitive to RH. © 1997 by John Wiley & Sons, Ltd.  相似文献   
886.
887.
We report a bimetallic cooperatively catalyzed C−H activation strategy for the heteroarylation of easily accessed aryl fluorosulfates which were introduced to sulfur fluoride exchange (SuFEx) click chemistry by Sharpless. This novel catalytic system enables highly chemoselective synthesis of important heterobiaryl motifs through the coupling of aryl fluorosulfates in the absence of prefunclization and directing groups. Moreover, this strategy has good functional group tolerance and a wide range of substrates and could be successfully used for the synthesis of bioactive molecules tafamidis.  相似文献   
888.
A facile wet-chemical protocol for the synthesis of bimetallic CuPd alloy nanoparticles (NPs) anchored on mesoporous graphitic carbon nitride (m-gCN), serving as both stabilizer and support material, was presented herein. The presented protocol allowed to synthesize nearly monodisperse CuPd alloy NPs with an average particle size of 3.9 ± 0.9 nm without use of any additional surfactants and to prepare CuPd/m-gCN nanocatalysts with different Cu/Pd compositions (Cu25Pd75/m-gCN, Cu35Pd65/m-gCN, Cu16Pd74/m-gCN, Cu32Pd68/m-gCN, Cu10Pd90/m-gCN, and Cu50Pd50/m-gCN). After the detailed characterization of CuPd/m-gCN nanocatalysts, they were utilized as catalysts in the dehydrogenation of terpenes. Among all tested nanocatalysts, Cu50Pd50/m-gCN showed the highest activity in terms of the product yields within the same reaction time. Various parameters influencing the catalytic activity of Cu50Pd50/m-gCN were studied using himachalene as a model substrate and the optimum conditions were determined. Under the optimized reaction conditions, the catalytic application of Cu50Pd50/m-gCN nanocatalysts was extended to nine different terpenes and the corresponding products were obtained in high conversion yields (>90%) under mild conditions. A reusability test showed that Cu50Pd50/m-gCN nanocatalysts can be re-used up to four cycles without significant loss in their initial activity.  相似文献   
889.
Bimetallic palladium(II) complexes containing classical NHC donor ligands are becoming increasingly popular owing to their various catalytic applications. However, examples of the aforementioned complexes with mixed NHC/PPh3 ligands are still rare. Bimetallic palladium(II) complexes possessing these mixed ligands are described starting from a C2-symmetric bis-imidazolium salt containing 4,4′-substituted central biphenyl ring. All the palladium(II) complexes have been tested as precatalysts in α-arylation of oxindole and Suzuki–Miyaura coupling reactions. The complex composed of mixed NHC/PPh3 donor ligands shows superior catalytic activity compared with the corresponding PEPPSI type complexes when applied in α-arylation of oxindole. The dinuclear complexes display better activity compared with the mononuclear complexes. The preliminary electrochemical measurements show the facile oxidation of PdII in the presence of combined NHC/PPh3 ligands compared with a combination of NHC/Py ligands.  相似文献   
890.
The catalytic oxidation of benzyl alcohol (OBA) is one of the significant methods to produce benzaldehyde, an essential reagent in the chemical and pharmaceutical industries. However, developing an active and efficient catalyst for OBA is a tremendous challenge in commercialization. This research describes a simple, eco-friendly method for producing Fe, Pd, and Fe: Pd bimetallic nanoparticles fabricated by sol immobilization over graphene to conduct OBA. The resulting composite nano-alloys were then characterized using X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). The oxidation state and elemental composition of as-fabricated nanoparticles were analyzed using XPS. The point of zero charges (pHPZC) was analyzed and the PZC value indicated that the proposed adsorbent material tends to have a positive charge. The OBA reaction efficiency (87%) of bimetallic nanocatalysts stabilized in graphene support was increased through surface modification of the ratio of both metals. The experimental error was based on three parallel tests and the carbon balance (99.6%) was analyzed during the experiments A proposed reaction mechanism of OBA validated the β-hydride step's elimination by molecular oxygen converting the metal hydride into a water molecule, forming a peroxide intermediate to form water and oxygen molecules. The Fe on the nanocatalyst’s surface is preferentially responsible for the adsorption of the substrate molecule, resulting in the formation of metal-alkoxide. Since it lacks electrons, Fe is more likely to be oxidized, allowing it to perform better than monometallic catalysts in terms of catalytic activity. The present study has great potential to be applied on an industrial scale and studied for industrialists, researchers, and academicians.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号