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   195861篇
  免费   5904篇
  国内免费   2882篇
化学   105840篇
晶体学   3326篇
力学   8654篇
综合类   112篇
数学   22405篇
物理学   64310篇
  2022年   1822篇
  2021年   2117篇
  2020年   2392篇
  2019年   2534篇
  2018年   2800篇
  2017年   2785篇
  2016年   4370篇
  2015年   3030篇
  2014年   4433篇
  2013年   9413篇
  2012年   8087篇
  2011年   9206篇
  2010年   6542篇
  2009年   6458篇
  2008年   8113篇
  2007年   7870篇
  2006年   7384篇
  2005年   6618篇
  2004年   5905篇
  2003年   5209篇
  2002年   5044篇
  2001年   5634篇
  2000年   4279篇
  1999年   3471篇
  1998年   2897篇
  1997年   2805篇
  1996年   2679篇
  1995年   2467篇
  1994年   2393篇
  1993年   2230篇
  1992年   2533篇
  1991年   2442篇
  1990年   2361篇
  1989年   2264篇
  1988年   2213篇
  1987年   2227篇
  1986年   2105篇
  1985年   2711篇
  1984年   2730篇
  1983年   2424篇
  1982年   2526篇
  1981年   2323篇
  1980年   2295篇
  1979年   2436篇
  1978年   2468篇
  1977年   2390篇
  1976年   2445篇
  1975年   2370篇
  1974年   2330篇
  1973年   2507篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
For the methylsilsesquioxane film whose optical birefringence is almost zero, it was recently reported that its vertical thermal expansion coefficient (CTE) was approximately one order of magnitude larger than the lateral CTE. Though the birefringence is not an absolute predictor of anisotropic behavior, the discrepancy in both the CTEs was so remarkable that it was essential to investigate whether the anisotropy was intrinsic property or not. If the effect of Poisson's ratio is considered in the calculation of the vertical CTE and when elastic modulus measured by surface acoustic wave spectroscopy is used in the assessment of the lateral CTE, both the CTEs are coincident with each other. Therefore, it can be concluded that the discrepancy in the CTEs can be attributed to a higher in‐plane polymer chain orientation but it can also arise from the misleadingly assumed modulus and Poisson's ratio. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3109–3120, 2006  相似文献   
993.
Laser Doppler vibrometer (LVD) has been the most favorite instrument for precision dynamics measurement due to its non-contact, high accuracy and high resolution. However, LDV can only give the dynamic data of a particular location on the entire feature. In order to get the whole field data, a laser beam-scanning mechanism has to be implemented. Currently, motor-driven scanning mirror is used to move the measurement probe from one point to another. The mechanical vibrations of the scanning mirror will reduce the measurement accuracy. This paper introduces a novel scanning LDV optical system embodied in an acousto-optic deflector scanning mechanism. It can improve the measurement accuracy since there is no mechanical motion involved. One main advantage of this system is that it generates a laser scanning beam in parallel that is different from the beam scanning in the conventional scanning laser Doppler vibrometer (SLDV). The new system has a board scanning range. The measurement target size ranges from few tens of millimeters down to 10 μm. We have demonstrated the capability of the novel system on scanning measurements of features as big as ultra-precision cutting tool to features as tiny as AFM cantilever. We believe that the novel SLDV will find profound potential applications in the precision engineering field.  相似文献   
994.
A new method for one-electron propagator calculations of molecular inization potentials is proposed, using a large matrix technique The results of some trial calculations on molecular nitrogen are given.  相似文献   
995.
This paper is based on the M. Sc. thesis written by the third author under the supervision of the first two authors. It was submitted to the University of Baghdad in 1986.  相似文献   
996.
This paper re-examines use of the linear programming (LP) formulation to solve the transportation problem (TP). The proposed method is a general-purpose algorithm which uses only one operation, the Gauss Jordan pivoting used in the simplex method. The final tableau can be used for post-optimality analysis of TP. This algorithm appears to be faster than simplex, more general than stepping-stone and simpler than both in solving general TP. A numerical example illustrates the methodology. It is assumed the reader is familiar with simplex terminology.  相似文献   
997.
The effect of a high pressure (up to 6 GPa) on the crystal and magnetic structure of the hexagonal manganite LuMnO3 is studied by neutron diffraction in the temperature range 10–295 K. It is found that, as the pressure increases, the ordered magnetic moment of Mn ions at T = 10 K decreases noticeably from 2.48 (0 GPa) to 1.98 μB (6 GPa). This decrease is due to an enhancement of the geometrical frustration effects on the triangular lattice. At the same time, the symmetry of the triangular antiferromagnetic state (the irreducible representation Γ2) remains unchanged. A correlation is revealed between the distortion parameter of the triangular lattice formed by Mn ions and the symmetry of the antiferromagnetic state of hexagonal manganites RMnO3. Based on this correlation, a generalized magnetic phase diagram of these compounds is constructed. The obtained phase diagram provides an explanation for the changes observed in the magnetic state of hexagonal manganites caused by high pressure and chemical substitution.  相似文献   
998.
Two matrix approximation problems are considered: approximation of a rectangular complex matrix by subunitary matrices with respect to unitarily invariant norms and a minimal rank approximation with respect to the spectral norm. A characterization of a subunitary approximant of a square matrix with respect to the Schatten norms, given by Maher, is extended to the case of rectangular matrices and arbitrary unitarily invariant norms. Iterative methods, based on the family of Gander methods and on Higham’s scaled method for polar decomposition of a matrix, are proposed for computing subunitary and minimal rank approximants. Properties of Gander methods are investigated in details. AMS subject classification (2000) 65F30, 15A18  相似文献   
999.
CrNx thin films have attracted much attention for semiconductor IC packaging molding dies and forming tools due to their excellent hardness, thermal stability and non-sticking properties (low surface free energy). However, few data has been published on the surface free energy (SFE) of CrNx films at temperatures in the range 20-170 °C. In this study CrNx thin films with CrN, Cr(N), Cr2N (and mixture of these phases) were prepared using closed field unbalanced magnetron sputtering at a wide range of Cr+2 emission intensity. The contact angles of water, di-iodomethane and ethylene glycol on the coated surfaces were measured at temperatures in the range 20-170 °C using a Dataphysics OCA-20 contact angle analyzer. The surface free energy of the CrNx films and their components (e.g., dispersion, polar) were calculated using the Owens-Wendt geometric mean approach. The influences of CrNx film surface roughness and microstructure on the surface free energy were investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. The experimental results showed that the lowest total SFE was obtained corresponding to CrN at temperature in 20 °C. This is lower than that of Cr(N), Cr2N (and mixture of these phases). The total SFE, dispersive SFE and polar SFE of CrNx films decreased with increasing surface temperature. The film roughness has an obvious effect on the SFE and there is tendency for the SFE to increase with increasing film surface roughness.  相似文献   
1000.
The performance of two external plastic scintillator detectors has been tested in an off-line mode using a source of106RuFe at UNISOR/NOF. The measurement of beta asymmetries at various temperatures has resulted in the reproduction of the previously-determined magnetic dipole moment for the 1+ ground state of106Rh and in addition the conclusion that the sign of this moment is positive.  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