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   1531篇
  免费   28篇
  国内免费   2篇
化学   1194篇
晶体学   38篇
力学   13篇
数学   48篇
物理学   268篇
  2021年   12篇
  2020年   13篇
  2019年   15篇
  2018年   10篇
  2016年   14篇
  2015年   24篇
  2014年   25篇
  2013年   64篇
  2012年   65篇
  2011年   69篇
  2010年   55篇
  2009年   39篇
  2008年   93篇
  2007年   109篇
  2006年   103篇
  2005年   87篇
  2004年   66篇
  2003年   54篇
  2002年   68篇
  2001年   26篇
  2000年   24篇
  1999年   24篇
  1998年   17篇
  1997年   29篇
  1996年   14篇
  1995年   14篇
  1994年   13篇
  1993年   17篇
  1992年   11篇
  1991年   17篇
  1990年   12篇
  1989年   12篇
  1988年   13篇
  1987年   8篇
  1986年   12篇
  1985年   31篇
  1984年   25篇
  1983年   14篇
  1982年   16篇
  1981年   19篇
  1980年   23篇
  1979年   22篇
  1978年   17篇
  1977年   15篇
  1976年   19篇
  1975年   24篇
  1974年   11篇
  1973年   12篇
  1971年   10篇
  1968年   12篇
排序方式: 共有1561条查询结果,搜索用时 296 毫秒
1.
A detailed investigation of addition–fragmentation chain transfer (AFCT) in the free‐radical polymerization of methyl methacrylate (MMA) in the presence of methyl α‐(bromomethyl)acrylate (MBMA) was carried out to elucidate mechanistic details with efficient macromonomer synthesis as an underlying goal. Advanced modeling techniques were used in connection with the experimental work. Curve fitting of simulated and experimental molecular weight distributions with respect to the rate coefficient for addition of propagating radicals to MBMA (kadd) over 60–120 °C resulted in Eadd = 21.7 kJ mol?1 and Aadd = 2.18 × 106 M?1 s?1 and a very weak temperature dependence of the chain‐transfer constant (EaddEp). The rate coefficient for fragmentation of adduct radicals at 60 °C was estimated as kf ≈ 39 s?1 on the basis of experimental data of the MMA conversion and the concentration of 2‐carbomethoxy‐2‐propenyl end groups. The approach developed is generic and can be applied to any AFCT system in which copolymerization does not occur and in which the resulting unsaturated end groups do not undergo further reactions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2640–2650, 2004  相似文献   
2.
A novel focusing mirror based on the shifting of the beam phase according to the Gaussian beam optics enabled us to realize a focusing elements with different focal lengths in different directions and to convert the gyrotron output into a Gaussian-like beam. In this paper, we compare the quality of beams produced by the new system (which includes novel mirrors) and a conventional system consisting of a quasi-optical antenna, an ellipsoidal mirror and two parabolic cylinder mirrors.  相似文献   
3.
Sub-critical crack growth rates of soda-lime-silicate glass and less brittle glass with different fictive temperatures were compared using the DCDC method under both dry and humid atmospheres in order to investigate the origin of the unique mechanical features of the less brittle glass developed by Ito and his collaborators. In both dry and humid atmospheres, the crack velocity of the soda-lime-silicate glass was slower than that of the less brittle glass. For both glasses, the glass sample with higher fictive temperature showed a slower crack growth rate under both dry and humid atmospheres. These observations can be explained by the tendency for the plastic flow at the crack tip; the soda-lime-silicate glass is expected to show easier plastic flow under tension than the less brittle glass, and also the samples with higher fictive temperatures are expected to show easier plastic flow, leading to greater fracture toughness, KIC, and slower crack growth rate.  相似文献   
4.
The double-pass method is thought to obtain the point spread function (PSF) in human eyes based on two techniques: the symmetric double-pass method using the same pupil size and the asymmetric double-pass method using a different pupil size. The symmetric double-pass method provides autocorrelation of the retinal PSF and, thus, the modulation transfer function. The asymmetric double-pass method provides low-frequency partial-phase information and the partial phase-retrieval algorithm is applied to obtain the complete-phase information, and to estimate the PSF. The partial phase-retrieval algorithm is based on the iteration method proposed by Fineup and Kowalczyk and requires a lengthy computation. In this study, we propose a new high-speed phase-retrieval algorithm based on the property that the real and imaginary parts of optical transfer functions (OTFs) continuously change in value.  相似文献   
5.
This paper presents a method of measuring a light field of a light source with high directional resolution using a mirrored ball and a pinhole camera. The light field describes a spatial and directional distribution of radiances from the light source. The directional distribution is expanded by a reflection on the mirrored ball, and the radiances are measured by a charge-coupled device (CCD) camera with a pinhole lens. The light source is laterally moved by a robot arm to measure the directionally expanded light field, and each pixel on a CCD can obtain the radiances from the light source through the pinhole lens with high directional resolution. The light field is estimated from the pixel value and the position of each pixel using a ray tracing technique. The light field of a krypton lamp was experimentally measured by the proposed method, and the accuracy of the measurement was evaluated against the irradiances measured by a spectro-radiometer at sample points.  相似文献   
6.
7.
8.
A high magnetic field of 5 T was used to fabricate a magnetically aligned, optically anisotropic, liquid‐crystalline chitin/poly(acrylic acid) composite. The aligned mesophase was fixed by photoinitiated free‐radical polymerization. From an examination of polarized optical micrographs and an X‐ray diffraction study, a high degree of orientation of 0.70 was observed for the composite with a higher liquid‐crystalline chitin concentration (10.70 wt %); the orientation was reduced with a decreased chitin concentration at a given acrylic acid concentration. The X‐ray data for the developed composite showed a uniplanar orientation for the chitin crystallites, with its molecular long axes perpendicular to the direction of the magnetic field. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 711–714, 2003  相似文献   
9.
10.
Carbon based paramagnetic materials are frequently used for EPR oximetry, especiallyin vivo,but the EPR spectra of these materials often have more than one paramagnetic center and/or relatively low signal intensity. To determine whether the multi-components of carbon based materials could be separated and enriched in the active component, we used density gradient centrifugation to separate the materials into several fractions. We studied two types of coals, gloxy and Pocahontas, and found these materials to have large density distribution. The separated density fractions had very different EPR spectra and intensities. The active component from the coal material had a more homogeneous EPR signal and significantly increased EPR signal intensity, whereas for India ink, only slight changes were observed. This result can be very useful in the development of better probes for EPR oximetry.  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