全文获取类型
收费全文 | 18641篇 |
免费 | 2469篇 |
国内免费 | 2252篇 |
专业分类
化学 | 5341篇 |
晶体学 | 226篇 |
力学 | 1061篇 |
综合类 | 216篇 |
数学 | 4677篇 |
物理学 | 11841篇 |
出版年
2024年 | 35篇 |
2023年 | 150篇 |
2022年 | 395篇 |
2021年 | 393篇 |
2020年 | 395篇 |
2019年 | 479篇 |
2018年 | 467篇 |
2017年 | 653篇 |
2016年 | 684篇 |
2015年 | 647篇 |
2014年 | 1081篇 |
2013年 | 1511篇 |
2012年 | 1095篇 |
2011年 | 1151篇 |
2010年 | 1008篇 |
2009年 | 1283篇 |
2008年 | 1398篇 |
2007年 | 1392篇 |
2006年 | 1235篇 |
2005年 | 1039篇 |
2004年 | 902篇 |
2003年 | 807篇 |
2002年 | 698篇 |
2001年 | 583篇 |
2000年 | 587篇 |
1999年 | 493篇 |
1998年 | 442篇 |
1997年 | 450篇 |
1996年 | 329篇 |
1995年 | 248篇 |
1994年 | 216篇 |
1993年 | 178篇 |
1992年 | 157篇 |
1991年 | 107篇 |
1990年 | 90篇 |
1989年 | 104篇 |
1988年 | 143篇 |
1987年 | 59篇 |
1986年 | 36篇 |
1985年 | 41篇 |
1984年 | 36篇 |
1983年 | 14篇 |
1982年 | 29篇 |
1981年 | 38篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 7篇 |
1977年 | 11篇 |
1974年 | 6篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Chu-Li Fu Xiang-Tuan Xiong Zhi Qian 《Journal of Mathematical Analysis and Applications》2007,331(1):472-480
In this paper a simple and convenient new regularization method for solving backward heat equation—Fourier regularization method is given. Meanwhile, some quite sharp error estimates between the approximate solution and exact solution are provided. A numerical example also shows that the method works effectively. 相似文献
52.
S. D. Gunapala S. V. Bandara J. K. Liu S. B. Rafol J. M. Mumolo C. A. Shott R. Jones J. Woolaway II J. M. Fastenau A. K. Liu M. Jhabvala K. K. Choi 《Infrared Physics & Technology》2003,44(5-6):411-425
A 9 μm cutoff 640 × 512 pixel hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated with excellent imagery. A noise equivalent differential temperature (NEDT) of 10.6 mK is expected at a 65 K operating temperature with f/2 optics at a 300 K background. This focal plane array has shown background limited performance at a 72 K operating temperature with the same optics and background conditions. In this paper, we discuss the development of this very sensitive long-wavelength infrared camera based on a GaAs/AlGaAs QWIP focal plane array and its performance in quantum efficiency, NEDT, uniformity, and operability. In the second section of this paper, we discuss the first demonstration of a monolithic spatially separated four-band 640 × 512 pixel QWIP focal plane array and its performance. The four spectral bands cover 4–5.5, 8.5–10, 10–12, and 13.5–15 μm spectral regions with 640 × 128 pixels in each band. In the last section, we discuss the array performance of a 640 × 512 pixel broad-band (10–16 μm full-width at half-maximum) QWIP focal plane. 相似文献
53.
By introducing an imaginary space transform curvature ρs, a complex space called Riemannian space is constructed, in which the light propagating in free space has the trajectory
of straight line while propagating. Moreover, this curvature couples with that of the wave front of the paraxial beam ρw, and therefore a complex curvature ρc is constructed, which can be employed to investigate the behavior of the light transmission and to generalize the ABCD law.
Project supported by the National Hi-Tech Inertial Confinement Fusion Committee, the Guangdong Natural Science Foundation
the Postdoctoral Foundation of Guangdong and National Postdoctoral Foundation of China. 相似文献
54.
对水热处理后得到的USY沸石作进一步的酸处理以及采用改进的氟硅酸盐溶液骨架富硅工艺,分别得到了经XPS剖面分析证实为铝分布均匀的超稳Y沸石HAY-Ⅰ和HAY-Ⅱ。IR分析表明,在酸处理过程中从USY沸石中去掉的那部分非骨架铝类与3690cm~(-1)处羟基有关,仍残留在HAY-Ⅰ沸石中的那部分非骨架铝类与3670cm~(-1)和3600cm~(-1)处羟基有关。XRD和化学分析表明HAY-Ⅰ沸石仍含有约50%的非骨架铝类,而HAY-Ⅱ沸石则基本上不含非骨架铝类,HAY-Ⅱ沸石还显示更高的结晶保留度。DTA分析表明,HAY-Ⅰ和HAY-Ⅱ两种沸石的热稳定性均高于USY沸石。 相似文献
55.
P. W. Hemker 《Advances in Computational Mathematics》1995,4(1):83-110
We introduce a multigrid algorithm for the solution of a second order elliptic equation in three dimensions. For the approximation of the solution we use a partially ordered hierarchy of finite-volume discretisations. We show that there is a relation with semicoarsening and approximation by more-dimensional Haar wavelets. By taking a proper subset of all possible meshes in the hierarchy, a sparse grid finite-volume discretisation can be constructed.The multigrid algorithm consists of a simple damped point-Jacobi relaxation as the smoothing procedure, while the coarse grid correction is made by interpolation from several coarser grid levels.The combination of sparse grids and multigrid with semi-coarsening leads to a relatively small number of degrees of freedom,N, to obtain an accurate approximation, together with anO(N) method for the solution. The algorithm is symmetric with respect to the three coordinate directions and it is fit for combination with adaptive techniques.To analyse the convergence of the multigrid algorithm we develop the necessary Fourier analysis tools. All techniques, designed for 3D-problems, can also be applied for the 2D case, and — for simplicity — we apply the tools to study the convergence behaviour for the anisotropic Poisson equation for this 2D case. 相似文献
56.
测量了Cr^4+,YAG、Cr^4+,Mg2SiO4晶体在室温和液氮温度下的荧光光谱,吸收光谱和激发态寿命,讨论了温度变化时,两种晶体中Cr^4+近红外辐射积分强度变化与激光发态寿命变化的关系,得出结论:在77K ̄300K范围内,Cr^4+的^3T2能级荧光辐射截面本身受温度影响不大,Cr^4+辐射荧光的变化,主要是由无辐射弛豫速率随温度变化而引起的。 相似文献
57.
A new method for rotation and brightness invariant pattern recognition was proposed by applying multiple circular harmonic expansions to the joint transform correlator. The amplitudes of the multiple orders of circular harmonic expansions made from a detecting image were synthetically modified to respond to the same auto-correlation peaks. These modified circular harmonic expansions were arranged in the input plane as reference patterns together with an arbitrary target pattern, and the correlation signals between them were calculated in the subtracted joint transform correlator. The fraction of the correlation-peak intensities between the target and the references were extracted as a new discrimination parameter. This new parameter performs pattern recognition under rotation and brightness invariance with good discriminability. Its high discriminability has been proved in computer simulations using the face image patterns of many individuals. 相似文献
58.
本文在相干理论的基础上,推导了光栅成像中的光强分布和物像位置关系,并给出一组具有不同参数的实验照片及一组借助计算机绘制的强度分布图。 相似文献
59.
讨论了横向位移双曝光散斑图维纳谱的信息分布,并由此导出杨氏纹图的一般表达式。讨论了条纹可见度与应变,面内转动,照明光束直径与条纹图图空间坐标的关系,指出了在条纹可见度影响下最大可见条纹数目。还讨论了散斑衬比与衍射晕的关系。 相似文献
60.
本文证明了d2k =δ2k =d2k ≥b2k,其中d2k ,δ2k , b2k分别表示A(BlMp)在lNq下的Kolmogorov,线性,Bernstein 2k-宽度,d2k 表示AT(BlN相似文献