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   5724篇
  免费   314篇
  国内免费   25篇
化学   4550篇
晶体学   20篇
力学   114篇
数学   569篇
物理学   810篇
  2023年   61篇
  2022年   42篇
  2021年   80篇
  2020年   135篇
  2019年   155篇
  2018年   123篇
  2017年   105篇
  2016年   241篇
  2015年   185篇
  2014年   191篇
  2013年   297篇
  2012年   522篇
  2011年   593篇
  2010年   267篇
  2009年   202篇
  2008年   439篇
  2007年   436篇
  2006年   446篇
  2005年   407篇
  2004年   290篇
  2003年   248篇
  2002年   179篇
  2001年   46篇
  2000年   24篇
  1999年   19篇
  1998年   18篇
  1997年   31篇
  1996年   23篇
  1995年   21篇
  1994年   14篇
  1993年   22篇
  1992年   15篇
  1991年   15篇
  1990年   10篇
  1989年   8篇
  1988年   8篇
  1987年   9篇
  1986年   8篇
  1985年   18篇
  1984年   13篇
  1983年   6篇
  1982年   7篇
  1981年   10篇
  1980年   6篇
  1979年   11篇
  1977年   12篇
  1976年   7篇
  1973年   4篇
  1971年   4篇
  1967年   5篇
排序方式: 共有6063条查询结果,搜索用时 15 毫秒
1.

Production simulation from fractured shale reservoirs is often performed by simplifying the hydraulic fractures as rectangular planes with homogeneous aperture. This study investigates the effects of heterogeneous fracture aperture and proppant distribution in realistic, non-rectangular fractures on the multi-phase production from shales. The heterogeneous hydraulic fractures are generated with the GEOS multiphysics simulator under realistic 3D stress field. These fractures are embedded into the TOUGH+ multi-phase flow simulator for production simulation. The results emphasize the importance of flow barriers within the hydraulic fractures, due both to low-aperture regions caused by the stress-shadow effect and the settling of proppant. The production rate is particularly sensitive to aperture heterogeneity if the flow barriers are close to the wellbore such that a great portion of fracture volume is isolated from the well. A stage-to-stage comparison reveals that production from different stages could vary significantly because the local stress field leads to different fracture area and aperture. The use of proppant prevents fracture closure, but if the propped regions are far from the well, they do not enhance production because flow barriers between these regions and the well act as bottlenecks. The present study highlights the importance of incorporating aperture heterogeneity into production simulation, provides insights on the relationship between flow barriers, proppant concentration, and well production, and proposes a practical method to mitigate numerical difficulties when modeling heterogeneous fractures.

  相似文献   
2.
Gold(I) thiolate compounds (i.e. AuI-SR) are important precursors for the synthesis of atomically precise Aun(SR)m nanoclusters. However, the nature of the AuI-SR precursor remains elusive. Here, we report that the Au10(TBBT)10 complex is a universal precursor for the synthesis of Aun(TBBT)m nanoclusters (where TBBT=4-tertbutylbenzenethiol/thiolate). Interestingly, the Au10(TBBT)10 complex is also found to be re-generated through extended etching of the Aun(SR)m nanoclusters with excess of TBBT thiol and O2. The formation of well-defined Au10(TBBT)10 complex, instead of polymeric AuI-SR, is attributed to the bulkiness of the TBBT thiol. Through 1D and 2D NMR characterization, the structure of Au10(TBBT)10 is correlated with the previously reported X-ray structure, which contains two inter-penetrated Au5(TBBT)5 rings. The photophysical property of Au10(TBBT)10 complex is further probed by femtosecond transient absorption spectroscopy. The accessibility of the precise Au10(TBBT)10 precursor improves the efficiency of the synthesis of the Aun(TBBT)m nanoclusters and is expected to further facilitate excellent control and understanding of the reaction mechanisms of nanocluster synthesis.  相似文献   
3.
4.
The advancement of organic electronics has been continually pushed by the need for stable and high performance acceptor materials. By utilizing inexpensive and stable indigo dye as a starting material, Bay‐Annulated Indigo (BAI) provides a new motif for the development of semiconducting materials. Modular and straightforward synthesis makes BAI an outstanding platform for molecular design, while excellent stability, strong absorption, and high ambipolar mobility render BAI‐based materials excellent candidates for organic electronics. BAI‐based polymers and small molecules have taken advantage of these properties to show promising results in a variety of applications.  相似文献   
5.
Covalent organic frameworks (COFs) have garnered immense scientific interest among porous materials because of their structural tunability and diverse properties. However, the response of such materials toward laser‐induced nonlinear optical (NLO) applications is hardly understood and demands prompt attention. Three novel regioregular porphyrin (Por)‐based porous COFs—Por‐COF‐HH and its dual metalated congeners Por‐COF‐ZnCu and Por‐COF‐ZnNi—have been prepared and present excellent NLO properties. Notably, intensity‐dependent NLO switching behavior was observed for these Por‐COFs, which is highly desirable for optical switching and optical limiting devices. Moreover, the efficient π‐conjugation and charge‐transfer transition in ZnCu‐Por‐COF enabled a high nonlinear absorption coefficient (β=4470 cm/GW) and figure of merit (FOM=σ1o, 3565) value compared to other state‐of‐the‐art materials, including molecular porphyrins (β≈100–400 cm/GW), metal–organic frameworks (MOFs; β≈0.3–0.5 cm/GW), and graphene (β=900 cm/GW).  相似文献   
6.
The synthesis of two well‐defined rhodium(I) complexes of nitrous oxide (N2O) is reported. These normally elusive adducts are stable in the solid state and persist in solution at ambient temperature, enabling comprehensive structural interrogation by 15N NMR and IR spectroscopy, and single‐crystal X‐ray diffraction. These methods evidence coordination of N2O through the terminal nitrogen atom in a linear fashion and are supplemented by a computational energy decomposition analysis, which provides further insights into the nature of the Rh–N2O interaction.  相似文献   
7.
8.

The title metallacrown (MC) complexes LnIII[15-MCFeIIIN(shi)-5](OAc)2Cl(C5H5N)6 (Ln1), where OAc is acetate, shi3? is salicylhydroximate, and Ln?=?Gd and Dy, were synthesized via a self-assembly reaction in methanol and pyridine. Single crystals were grown using slow evaporation and characterized using X-ray diffraction. Seven-coordinate capped octahedron geometries were observed for the lanthanide ion in both complexes, which is uncommon for trivalent lanthanide species. The 15-MC-5 is a ruffled metallacrown archetype similar to previously reported mixed-valent manganese metallacrowns.

Graphic Abstract

The title metallacrown (MC) complexes LnIII[15-MCFeIIIN(shi)-5](OAc)2Cl(C5H5N)6 (Ln1), where OAc is acetate, shi3? is salicylhydroximate, and Ln?=?Gd and Dy, contains seven-coordinate capped octahedron geometries for the lanthanide ion in both complexes, which is uncommon for trivalent lanthanide species.

  相似文献   
9.
10.
A palladium-catalyzed enantioselective redox-relay Heck reaction of 2-indole triflates and disubstituted alkenes is reported. This process combines readily available indole triflates with a variety of alkenes to afford a range of indole derivatives bearing a stereocenter adjacent to C2. Enantioselectivity is achieved through use of a simple pyridine-oxazoline ligand. Tuning the electronics of the indole, through judicious choice of N-protecting group, is required to ensure selective β-hydride elimination away from the indole core. Utility of this method is highlighted in a modular formal synthesis of an S1P1 agonist precursor developed by Merck.  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