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   379篇
  免费   20篇
化学   247篇
晶体学   3篇
力学   9篇
数学   87篇
物理学   53篇
  2023年   1篇
  2022年   9篇
  2021年   9篇
  2020年   12篇
  2019年   7篇
  2018年   13篇
  2017年   13篇
  2016年   22篇
  2015年   18篇
  2014年   16篇
  2013年   29篇
  2012年   21篇
  2011年   20篇
  2010年   10篇
  2009年   18篇
  2008年   33篇
  2007年   16篇
  2006年   19篇
  2005年   13篇
  2004年   14篇
  2003年   24篇
  2002年   14篇
  2001年   9篇
  2000年   4篇
  1999年   5篇
  1998年   1篇
  1997年   2篇
  1996年   4篇
  1995年   4篇
  1994年   1篇
  1993年   3篇
  1992年   2篇
  1991年   2篇
  1990年   2篇
  1989年   1篇
  1988年   1篇
  1981年   2篇
  1980年   1篇
  1978年   1篇
  1975年   3篇
排序方式: 共有399条查询结果,搜索用时 765 毫秒
251.
Through the use of cyclic beta-diketones as supporting ligands, the copper-catalyzed coupling of aryl iodides with aliphatic amines occurs at room temperature in as little as 1 h. These high reaction rates allow for the coupling of a wide range of aryl and heteroaryl iodides at room temperature. This method is highly tolerant of a number of reactive functional groups, including -Br and aromatic -NH2 as well as phenolic and aliphatic -OH. The high selectivity of the CuI-beta-diketone catalyst for aliphatic amines represents a useful complement to the palladium-based methods.  相似文献   
252.
253.
For a graph G, let ${\sigma_2(G)=\min\{d(u)+d(v):uv\notin E(G)\}}$ . We prove that every n-vertex graph G with σ 2(G)?≥ 4n/3 ? 1 contains each 2-regular n-vertex graph. This extends a theorem due to Aigner and Brandt and to Alon and Fisher.  相似文献   
254.
A proper vertex coloring of a graph is equitable if the sizes of color classes differ by at most one. The celebrated Hajnal-Szemerédi Theorem states: For every positive integer r, every graph with maximum degree at most r has an equitable coloring with r+1 colors. We show that this coloring can be obtained in O(rn 2) time, where n is the number of vertices.  相似文献   
255.
Products of the reaction of C(60) with H(2) gas have been monitored by high-resolution atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS), X-ray diffraction, and IR spectroscopy as a function of hydrogenation period. Samples were synthesized at 673 K and 120 bar hydrogen pressure for hydrogenation periods between 300 and 5000 min, resulting in the formation of hydrofullerene mixtures with hydrogen content ranging from 1.6 to 5.3 wt %. Highly reduced C(60)H(x) (x > 36-40) and products of their fragmentation were identified in these samples by APPI FT-ICR MS. A sharp change in structure was observed for samples with at least 5.0 wt % of hydrogen. Low-mass (300-500 Da) hydrogenation products not observed by prior field desorption (FD) FT-ICR MS were detected by APPI FT-ICR MS and their elemental compositions obtained for the first time. Synthetic and analytical fragmentation pathways are discussed.  相似文献   
256.
Ultra‐low fouling and functionalizable coatings represent emerging surface platforms for various analytical and biomedical applications such as those involving examination of cellular interactions in their native environments. Ultra‐low fouling surface platforms as advanced interfaces enabling modulation of behavior of living cells via tuning surface physicochemical properties are presented and studied. The state‐of‐art ultra‐low fouling surface‐grafted polymer brushes of zwitterionic poly(carboxybetaine acrylamide), nonionic poly(N‐(2‐hydroxypropyl)methacrylamide), and random copolymers of carboxybetaine methacrylamide (CBMAA) and HPMAA [p(CBMAA‐co‐HPMAA)] with tunable molar contents of CBMAA and HPMAA are employed. Using a model Huh7 cell line, a systematic study of surface wettability, swelling, and charge effects on the cell growth, shape, and cytoskeleton distribution is performed. This study reveals that ultra‐low fouling interfaces with a high content of zwitterionic moieties (>65 mol%) modulate cell behavior in a distinctly different way compared to coatings with a high content of nonionic HPMAA. These differences are attributed mostly to the surface hydration capabilities. The results demonstrate a high potential of carboxybetaine‐rich ultra‐low fouling surfaces with high hydration capabilities and minimum background signal interferences to create next‐generation bioresponsive interfaces for advanced studies of living objects.  相似文献   
257.
First time the method of plasma-enhanced chemical vapor deposition was used for preparation of As–Te chalcogenide films of different chemical and phase composition. The samples were synthesized via direct interaction of arsenic and tellurium vapors into low-temperature non-equilibrium RF (40 MHz) plasma discharge at reduced pressure (0.1 Torr). The plasma parameters such as temperature and concentration of electrons were measured by moving double probe diagnostic system. The dependence of solid phase radial distribution on plasma characteristics was established. Besides, the phase and structural evolution of As–Te films based on equilibrium coexistence of two phases (AsTe and As2Te3) and implemented by changing of the ratio of the initial substances in gas phase has been studied and discussed.  相似文献   
258.
259.
260.
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