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   2417篇
  免费   52篇
  国内免费   4篇
化学   1684篇
晶体学   41篇
力学   24篇
数学   77篇
物理学   647篇
  2021年   17篇
  2020年   21篇
  2019年   18篇
  2018年   21篇
  2017年   14篇
  2016年   25篇
  2015年   34篇
  2014年   33篇
  2013年   105篇
  2012年   99篇
  2011年   110篇
  2010年   76篇
  2009年   51篇
  2008年   122篇
  2007年   170篇
  2006年   142篇
  2005年   142篇
  2004年   125篇
  2003年   109篇
  2002年   116篇
  2001年   59篇
  2000年   42篇
  1999年   23篇
  1998年   23篇
  1997年   37篇
  1996年   32篇
  1995年   22篇
  1994年   30篇
  1993年   22篇
  1992年   23篇
  1991年   28篇
  1990年   21篇
  1989年   22篇
  1988年   24篇
  1986年   20篇
  1985年   50篇
  1984年   41篇
  1983年   29篇
  1982年   22篇
  1981年   35篇
  1980年   35篇
  1979年   42篇
  1978年   34篇
  1977年   36篇
  1976年   33篇
  1975年   24篇
  1974年   16篇
  1973年   16篇
  1968年   11篇
  1965年   9篇
排序方式: 共有2473条查询结果,搜索用时 15 毫秒
991.
We report new measurements of the decays B+-->rho+gamma, B0-->rho0gamma, and B0-->omegagamma using a data sample of 657x10(6) B meson pairs accumulated with the Belle detector at the KEKB e+e- collider. We measure branching fractions B(B+-->rho+gamma)=(8.7_-2.7-1.1;+2.9+0.9)x10(-7), B(B0-->rho0gamma)=(7.8_-1.6-1.0;+1.7+0.9)x10(-7), and B(B0-->omegagamma)=(4.0_-1.7;+1.9+/-1.3)x10(-7). We also report the isospin asymmetry Delta(rhogamma)=-0.48_-0.19-0.09;+0.21+0.08 and the first measurement of the direct CP-violating asymmetry ACP(B+-->rho+gamma)=-0.11+/-0.32+/-0.09, where the first and second errors are statistical and systematic, respectively.  相似文献   
992.
Using a 492 fb{-1} data sample collected near the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e{+}e{-} collider, we observe the decay B{0}-->ppK*0 with a branching fraction of (1.18{-0.25}{+0.29}(stat)+/-0.11(syst))x10{-6}. We study the decay dynamics of B{0}-->ppK*0 and compare with B{+}-->ppK*+. The K*0 meson is found to be almost 100% polarized (with a fraction of (101+/-13+/-3)% in the helicity zero state), while the K*+ meson has a (32+/-17+/-9)% fraction in the helicity zero state. The direct CP asymmetries for B{0}-->ppK*0 and B{+}-->ppK*+ are measured to be -0.08+/-0.20+/-0.02 and -0.01+/-0.19+/-0.02, respectively. In addition, we report improved measurements of the branching fractions B(B{+}-->ppK*+)=(3.38{-0.60}{+0.73}+/-0.39)x10{-6} and B(B{0}-->ppK{0})=(2.51{-0.29}{+0.35}+/-0.21)x10{-6}, which supersede our previous measurements.  相似文献   
993.
Poly (L ‐lactide) (PLLA)‐degrading enzyme was produced in a liquid culture of Amycolatopsis sp. (strain 41). In comparison with polyester substrates, silk powder from silkworm cocoons was the most effective in inducing enzyme production within 5 d. Application to DEAE and Superdex 75 columns resulted in a major protein with molecular weight estimated to be 42 kDa from size exclusion chromatography or 40 kDa from SDS‐PAGE analysis. Optimum pH and temperature are 6.0 and 37–45°C, respectively. Besides PLLA, the enzyme degrades casein, silk powder and Suc‐(Ala)3pNA at an even lower level than Proteinase‐K, but not Suc‐(Gly)3pNA, poly (ε‐caprolactone) and poly (β‐hydroxybutyrate).  相似文献   
994.
Combining Grüneisen's equation with our Young's modulus equation of glass, new formulae were semi-empirically derived for the calculation of bulk modulus, shear modulus and Poisson's ratio of glass. Considering packing density of atoms and bond energy in unit volume the elastic moduli of glass can be calculated. The agreements between calculated and observed values of the moduli of glass are satisfactory for more than 30 glasses.  相似文献   
995.
Iterative reconstruction of density pixel images from measured projections in computed tomography has attracted considerable attention. The ordered-subsets algorithm is an acceleration scheme that uses subsets of projections in a previously decided order. Several methods have been proposed to improve the convergence rate by permuting the order of the projections. However, they do not incorporate object information, such as shape, into the selection process. We propose a block-iterative reconstruction from sparse projection views with the dynamic selection of subsets based on an estimating function constructed by an extended power-divergence measure for decreasing the objective function as much as possible. We give a unified proposition for the inequality related to the difference between objective functions caused by one iteration as the theoretical basis of the proposed optimization strategy. Through the theory and numerical experiments, we show that nonuniform and sparse use of projection views leads to a reconstruction of higher-quality images and that an ordered subset is not the most effective for block-iterative reconstruction. The two-parameter class of extended power-divergence measures is the key to estimating an effective decrease in the objective function and plays a significant role in constructing a robust algorithm against noise.  相似文献   
996.
We are planning test experiments of fundamental symmetries based on the intrinsic properties of francium. It is expected that the laser cooling and trapping of francium will produce precision measurements. The pilot experiment using rubidium was performed with the goal of francium trapping. The ion beam generated with a francium ion source was investigated using a Wien filter. Each piece of equipment still must be studied in more detail, and the equipment should be upgraded in order to trap radioactive atoms.  相似文献   
997.
A simple pressure‐sensitive adhesion (PSA) system incorporating noncovalent interaction between thymine and adenine is presented. A copolymer having thymine moieties is combined with a low‐molecular‐weight bifunctional adenine cross‐linker. Molecular interactions caused by multiple hydrogen bonds between the thymine and adenine units are evaluated by FT‐IR spectral measurement. Mechanical properties of the PSA are examined by stress–strain curves and dynamic mechanical analysis. As the number of adenine cross‐linkers increases, Young's modulus increases from 0.24 to 3.0 MPa, and the glass transition temperature increases. Furthermore, it is found that the PSAs have adequate adhesive property from their shear strength test. Heat treatment at 80 °C is effective for reinforcement because of interchange of the hydrogen bonds. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1332‐1338  相似文献   
998.
999.
1000.
The variation of surface state of SUS-304L stainless steel with heat treatment in a vacuum of 10?9 Torr was observed by the ultra high vacuum low energy scanning electron microscope (UHVLESEM) by detecting the absorption electron current. The absorption electron current was very sensitive to the surface state and a large effect of the electron beam irradiation was observed. The variation of the absorption electron current with the surface segregation was measured within a microscopical dimension of 0.3 microm on a selected area of the scanning electron microscopic image. At the same time, the surface state was characterized by Auger electron spectroscopy and relations between surface states and the absorption electron current were discussed in relation to the secondary electron and backscattered electron yields.  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