全文获取类型
收费全文 | 65080篇 |
免费 | 6080篇 |
国内免费 | 11188篇 |
专业分类
化学 | 49050篇 |
晶体学 | 1963篇 |
力学 | 781篇 |
综合类 | 447篇 |
数学 | 5343篇 |
物理学 | 14170篇 |
无线电 | 10594篇 |
出版年
2024年 | 221篇 |
2023年 | 1340篇 |
2022年 | 1931篇 |
2021年 | 2177篇 |
2020年 | 2209篇 |
2019年 | 2095篇 |
2018年 | 1697篇 |
2017年 | 2185篇 |
2016年 | 2274篇 |
2015年 | 2017篇 |
2014年 | 2702篇 |
2013年 | 5221篇 |
2012年 | 3874篇 |
2011年 | 4303篇 |
2010年 | 3635篇 |
2009年 | 4446篇 |
2008年 | 4238篇 |
2007年 | 4370篇 |
2006年 | 4099篇 |
2005年 | 3604篇 |
2004年 | 3446篇 |
2003年 | 2963篇 |
2002年 | 2466篇 |
2001年 | 2037篇 |
2000年 | 1841篇 |
1999年 | 1474篇 |
1998年 | 1252篇 |
1997年 | 1074篇 |
1996年 | 993篇 |
1995年 | 959篇 |
1994年 | 907篇 |
1993年 | 725篇 |
1992年 | 663篇 |
1991年 | 496篇 |
1990年 | 357篇 |
1989年 | 297篇 |
1988年 | 250篇 |
1987年 | 189篇 |
1986年 | 147篇 |
1985年 | 156篇 |
1984年 | 136篇 |
1983年 | 50篇 |
1982年 | 99篇 |
1981年 | 132篇 |
1980年 | 106篇 |
1979年 | 122篇 |
1978年 | 92篇 |
1977年 | 96篇 |
1976年 | 64篇 |
1973年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
In the present paper, we consider the problem of the optimal reconstruction of the solution of the wave equation from the approximate values of the Fourier coefficients of the function specifying the initial form of the string. For an operator defined on the weight space of vectors from l 2, we present the solution of the more general problem of reconstruction from the approximate values of the coordinates of these vectors. 相似文献
82.
This article studies some geometrical aspects of the semidefinite linear complementarity problem (SDLCP), which can be viewed as a generalization of the well-known linear complementarity problem (LCP). SDLCP is a special case of a complementarity problem over a closed convex cone, where the cone considered is the closed convex cone of positive semidefinite matrices. It arises naturally in the unified formulation of a pair of primal-dual semidefinite programming problems. In this article, we introduce the notion of complementary cones in the semidefinite setting using the faces of the cone of positive semidefinite matrices and show that unlike complementary cones induced by an LCP, semidefinite complementary cones need not be closed. However, under R0-property of the linear transformation, closedness of all the semidefinite complementary cones induced by L is ensured. We also introduce the notion of a principal subtransformation with respect to a face of the cone of positive semidefinite matrices and show that for a self-adjoint linear transformation, strict copositivity is equivalent to strict semimonotonicity of each principal subtransformation. Besides the above, various other solution properties of SDLCP will be interpreted and studied geometrically. 相似文献
83.
The 11 800-14 380 cm−1 frequency range has been scanned for rotationally resolved rovibronic transitions in the A2B2-X2A1 electronic band system of the symmetric (C2v) 16O14N16O and 18O14N18O isotopologues and in the corresponding electronic band system of the asymmetric (Cs) 18O14N16O isotopologue. The rotational analysis—reflecting minor differences in mass—in combination with symmetry induced spectral differences allows an identification of 68 16O14N16O vibronic levels, 26 18O14N18O vibronic levels and 51 18O14N16O vibronic levels. The bands are recorded using near infrared fluorescence spectroscopy and a piezo valve based pulsed molecular beam expansion of premixed 18O2 and 14N16O in Ar. The majority of the observed bands is rotationally assigned and can be identified as transitions starting from the vibrational ground state of one of the isotopologues. Numerous hot bands have also been identified. A comparison of the overall spectroscopic features of C2v vs. Cs symmetric species provides qualitative information on symmetry dependence of vibronic couplings. 相似文献
84.
Y. Fukuda T. Kobayashi H. Yoshida T. Sekizawa N. Sanada 《Applied Surface Science》2002,190(1-4):279-283
Adsorption and decomposition of triethylindium (TEI: (C2H5)3In) on a GaP(0 0 1)-(2×1) surface have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). It is found from the TPD result that ethyl radical and ethylene are evolved at about 300–400 and 450–550 K, respectively, as decomposition products of TEI on the surface. This result is quite different from that on the GaP(0 0 1)-(2×4) surface. The activation energy of desorption of ethyl radical is estimated to be about 93 kJ/mol. It is suggested that TEI is adsorbed molecularly on the surface at 100 K and that some of TEI molecules are dissociated into C2H5 to form P–C2H5 bonds at 300 K. The vibration modes related to ethyl group are decreased in intensity at about 300–400 and 450–550 K, which is consistent with the TPD result. The TEI molecules (including mono- and di-ethylindium) are not evolved from the surface. Based on the TPD and HREELS results, the decomposition mechanism of TEI on the GaP(0 0 1)-(2×1) surface is discussed and compared with that on the (2×4) surface. 相似文献
85.
掺杂Er^3+的TiO2的发射光谱研究 总被引:1,自引:1,他引:0
用无水乙醇、冰醋酸、钛酸丁酯凝胶法制备了掺杂Er^3+的TiO2粉末,测量了其在488m激发下的Stokes发射光谱和980nm激发下的上转换发光光谱。在可见光范围内,观察到了绿光和红光,绿光从500-570nm,对应Er^3+的^2H11/2,^4S3/2→^4I15/2,红光从650~690m,对应Er^3+的^4F9/2→^4I15/2的跃迁。由ln Ivis-In Iin曲线可知,绿光和红光均为双光子过程,光强正比于泵浦功率的二次方,即Iout ∝Iin。初步研究了此材料的上转换过程。 相似文献
86.
87.
We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1-Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg-Si interaction. 相似文献
88.
部分相干光束通过硬边光阑的推广光束传输M2因子 总被引:2,自引:2,他引:0
给出部分相干光通过硬边光阑后的强度二阶矩的计算公式,由此可得到部分相干光通过硬边光阑后的推广光束传输M^2因子。以部分相干高斯-谢尔模型光束为例,推导出相应的M^2因子,并作了数值计算和分析讨论。 相似文献
89.
90.
低温太阳热能与化学链燃烧相结合控制CO2分离动力系统 总被引:3,自引:1,他引:2
本文探索并提出控制CO2分离的低温太阳热能与清洁合成燃料甲醇-三氧化二铁化学链燃烧相结合的新颖能源动力系统。基于图象(?)分析方法,明确地指出甲醇化学链燃烧能量释放过程燃烧堋损失减小和低温太阳热能品位提升的机理。从能源有效利用和环境相容出发,研究和揭示化学链燃烧与太阳能有机整合共同减小CO2分离能耗的特性规律。相比不分离常规联合循环,新系统(?)效率提高约6.2个百分点;与分离CO2的联合循环相比,新系统媚效率提高约14.2个百分点。同时,低温太阳热能热转功效率可达到22.5%。 相似文献