全文获取类型
收费全文 | 5013篇 |
免费 | 926篇 |
国内免费 | 308篇 |
专业分类
化学 | 572篇 |
晶体学 | 15篇 |
力学 | 512篇 |
综合类 | 162篇 |
数学 | 592篇 |
物理学 | 4394篇 |
出版年
2024年 | 11篇 |
2023年 | 35篇 |
2022年 | 189篇 |
2021年 | 183篇 |
2020年 | 114篇 |
2019年 | 122篇 |
2018年 | 137篇 |
2017年 | 223篇 |
2016年 | 254篇 |
2015年 | 252篇 |
2014年 | 398篇 |
2013年 | 380篇 |
2012年 | 281篇 |
2011年 | 323篇 |
2010年 | 258篇 |
2009年 | 314篇 |
2008年 | 332篇 |
2007年 | 343篇 |
2006年 | 302篇 |
2005年 | 255篇 |
2004年 | 216篇 |
2003年 | 211篇 |
2002年 | 185篇 |
2001年 | 115篇 |
2000年 | 113篇 |
1999年 | 102篇 |
1998年 | 84篇 |
1997年 | 94篇 |
1996年 | 89篇 |
1995年 | 66篇 |
1994年 | 50篇 |
1993年 | 38篇 |
1992年 | 37篇 |
1991年 | 34篇 |
1990年 | 18篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1959年 | 5篇 |
排序方式: 共有6247条查询结果,搜索用时 15 毫秒
31.
全息散斑条纹的提取及图像处理 总被引:3,自引:1,他引:2
介绍了一种新的全息散斑条纹的提取方法.理论和实验表明:对全息散斑底片逐点再现时,两伴生亮斑条纹是全息散斑底片一级衍射光的杨氏干涉条纹,与中央的杨氏条纹是一致的,有较高的条纹对比度且没有背景光晕的影响.利用计算机数字图像处理技术对伴生散斑条纹进行处理,定量测试了受静载物体的面内位移. 相似文献
32.
33.
电子自旋共振准三维图象的重建方法 总被引:3,自引:0,他引:3
根据投影切片原理, 设计了一个简单而有效的ESR图象重建算法和程序, 可利用样品的一集ESR谱, 方便地绘制出样品在某截面的自旋密度立体轮廓图、等值图和散点密度分布图等, 为分析和研究自旋密度的分布及其性质, 提供了直观依据。作为应用, 文中给出了石英杜瓦受紫外光辐照后产生的局部晶格缺陷——F心的空间分布图, 图中清楚表明, F心的浓度与辐照方向和辐照强度的密切关系, 与试验结果完全相符。 相似文献
34.
Y. Sakaiguchi T. Shikata H. Urakami A. Tamura H. Hirata 《Colloid and polymer science》1987,265(8):750-753
Some cationic surfactants such as cetyltrimethylammonium bromide (CTAB) show a very spectacular viscoelasticity in solution, in even very diluted states, with some aromatic substances added, such as salicylic acid. Formerly, the authors established that the remarkable solution behaviour was the result of the entanglement of such enormously elongated giant micelles, based on recognition of the existence of large aggregates, through direct observation of them as substantial images under the electron microscope.In this report, we further confirm the relation between micelle length and solution viscoelasticity after arbitrarily obtaining different size aggregates by altering the media pH, or by admixing shorter chain length surfactants than CTAB, which alone did not induce viscoelasticity. 相似文献
35.
We derive exact properties of the inhomogeneous electron gas in the asymptotic classically forbidden region at a metal–vacuum interface within the framework of local effective potential energy theory. We derive a new expression for the asymptotic structure of the Kohn–Sham density functional theory (KS‐DFT) exchange‐correlation potential energy vxc(r) in terms of the irreducible electron self‐energy. We also derive the exact asymptotic structure of the orbitals, density, the Dirac density matrix, the kinetic energy density, and KS exchange energy density. We further obtain the exact expression for the Fermi hole and demonstrate its structure in this asymptotic limit. The exchange‐correlation potential energy is derived to be vxc(z → ∞) = ?αKS,xc/z, and its exchange and correlation components to be vx(z → ∞) = ?αKS,x/z and vc(z → ∞) = ?αKS,c/z, respectively. The analytical expressions for the coefficients αKS,xc and αKS,x show them to be dependent on the bulk‐metal Wigner–Seitz radius and the barrier height at the surface. The coefficient αKS,c = 1/4 is determined in the plasmon‐pole approximation and is independent of these metal parameters. Thus, the asymptotic structure of vxc(z) in the vacuum region is image‐potential‐like but not the commonly accepted one of ?1/4z. Furthermore, this structure depends on the properties of the metal. Additionally, an analysis of these results via quantal density functional theory (Q‐DFT) shows that both the Pauli Wx(z → ∞) and lowest‐order correlation‐kinetic W(z → ∞) components of the exchange potential energy vx(z → ∞), and the Coulomb Wc(z → ∞) and higher‐order correlation‐kinetic components of the correlation potential energy vc(z → ∞), all contribute terms of O(1/z) to the structure. Hence correlations attributable to the Pauli exclusion principle, Coulomb repulsion, and correlation‐kinetic effects all contribute to the asymptotic structure of the effective potential energy at a metal surface. The relevance of the results derived to the theory of image states and to KS‐DFT is also discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
36.
Paul Geladi 《Mikrochimica acta》1995,120(1-4):211-230
Local models are a very important concept for microscopic and macroscopic imaging. Different methods of sub-sampling a multivariate image are described both in general and for three examples. The need for sub-sampling and its influence on multivariate image analysis and visualization are studied. Examples from MRI (256 × 256), satellite imaging (7 × 512 × 512) and biofuel studies (6 × 512 × 512) are used to illustrate some of the principles involved. 相似文献
37.
38.
岩,心是油气储层的基本单元,其内部结构和渗流性质反映了油藏的基本特征.岩。心的精细描述是油藏描述、储层动态监测及设计强化采油方案的重要基础.本文利用核磁共振(NMR)自旋密度做成像,快速无损地观测到两类基本储油岩石——砂岩和灰岩的内部结构,提供了岩石渗透性、孔隙性及灰岩沉积环境等重要信息.1实验方法以自旋回波单片NMR成像脉冲序列为基础,设。P平面内样品的目施密度为P(。,v).演化期t。,在。方向梯度场G。作用下,各处横向磁化矢量获得的相移为式中,7为共振核(在成像中一般为质子)的旋磁比,g为坐标位置·… 相似文献
39.
40.
Anna B. DenisovaVyacheslav Ya. Sosnovskikh Wim Dehaen Suzanne ToppetLuc Van Meervelt Vasiliy A. Bakulev 《Journal of fluorine chemistry》2002,115(2):183-192
Reaction of 2-hydrazinothiazoles 1 with 1-thienyl- and 1-furyl-1,3-butanediones 2a,b in methanol in the presence of hydrochloric acid mainly leads to a mixture of pyrazoles 3 and pyrazolines 4 or pyrazoles 3 and 5 in strong acidic conditions. Isomeric hydrazones 6 and pyrazolines 4 were formed and isolated in these reactions in the absence of hydrochloric acid. It has been shown that the regioselectivity in the reaction of diketones 2 with hydrazine 1 is governed by both the concentration of acid and the nature of substituents in the 1,3-diketones 2. Cyclization of hydrazones 6 is shown to occur under milder conditions than dehydration for pyrazolines 4. The new heterocyclic compounds were prepared and fully characterized by NMR spectra and by X-ray analysis for 3c. 相似文献