全文获取类型
收费全文 | 138969篇 |
免费 | 21952篇 |
国内免费 | 17274篇 |
专业分类
化学 | 96414篇 |
晶体学 | 1548篇 |
力学 | 9065篇 |
综合类 | 1102篇 |
数学 | 16229篇 |
物理学 | 53837篇 |
出版年
2024年 | 436篇 |
2023年 | 2554篇 |
2022年 | 4597篇 |
2021年 | 5009篇 |
2020年 | 5259篇 |
2019年 | 5044篇 |
2018年 | 4497篇 |
2017年 | 4281篇 |
2016年 | 6175篇 |
2015年 | 6365篇 |
2014年 | 7593篇 |
2013年 | 9848篇 |
2012年 | 11947篇 |
2011年 | 13178篇 |
2010年 | 8874篇 |
2009年 | 8753篇 |
2008年 | 9075篇 |
2007年 | 7936篇 |
2006年 | 7342篇 |
2005年 | 6383篇 |
2004年 | 5803篇 |
2003年 | 4405篇 |
2002年 | 4159篇 |
2001年 | 3306篇 |
2000年 | 2775篇 |
1999年 | 2764篇 |
1998年 | 2281篇 |
1997年 | 2146篇 |
1996年 | 2074篇 |
1995年 | 1764篇 |
1994年 | 1574篇 |
1993年 | 1612篇 |
1992年 | 1538篇 |
1991年 | 1193篇 |
1990年 | 1093篇 |
1989年 | 878篇 |
1988年 | 663篇 |
1987年 | 470篇 |
1986年 | 491篇 |
1985年 | 388篇 |
1984年 | 285篇 |
1983年 | 201篇 |
1982年 | 164篇 |
1981年 | 149篇 |
1980年 | 121篇 |
1979年 | 115篇 |
1978年 | 66篇 |
1977年 | 83篇 |
1976年 | 65篇 |
1973年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
A hybrid finite-element method, combining ideas from a modified method of characteristics and the streamline diffusion method, delivers accurate solutions to the advection–diffusion equation. An error analysis for the case of tensorial diffusion shows that the lowest-order version of the scheme, which allows one to use a symmetric linear solvers at each time step, possesses first-order accuracy in time and space. Numerical experiments demonstrate the scheme's ability to model advection-dominated transport of solute plumes without distorting sharp fronts. © 1995 John Wiley & Sons, Inc. 相似文献
112.
The growth mechanism of the peritectic η phase involving the peritectic reaction and peritectic transformation in Cu-70%Sn alloy was investigated under directional
solidification. The results show that a major growth mechanism in thickening of the peritectic η-layer is not the peritectic reaction but the peritectic transformation. The transformation temperature and isothermal time
play crucial roles in determining the volume fraction and the thickness of the peritectic η phase. With the increase of the temperature and isothermal time, the volume fraction of the peritectic η phase increases. The regressed data show that the relationship between the thickness of η phase (Δx) and the transformation temperature (T) meets the following equation In Δx=6.5−1673 1 / T. Additionally, there exists a relationship between the thickness of the η phase (Δx) and the isothermal time (t) at the 9 mm solidification distance below the peritectic reaction interface, Δx=0.72t
1/2, which is consistent with the theoretical model.
Supported by the National Science Foundation of China (Grant No. 50395102) 相似文献
113.
Characteristics and structures of weak efficient surfaces of production possibility sets 总被引:1,自引:0,他引:1
Quanling Wei Hong Yan Gang Hao 《Journal of Mathematical Analysis and Applications》2007,327(2):1055-1074
This paper studies the characteristics and structure of the weak surface of the production possibility set. We apply techniques and methods of transferring a polyhedral cone from its intersection form to its sum form, identify an intersection representation of the production possibility set. We give the structure theorem of weak surface of the production possibility set, which includes three complementary slackness conditions. We define the input weak efficiency and output weak efficiency for different DEA models according to the representation of the intersection form. It investigates the characteristics of the weak surfaces, and proves the structure theorems of input weak DEA efficiency and output weak DEA efficiency. The structure theorems establish weighted combination of inputs and outputs that are weak DEA efficient. Numerical examples are provided for illustration. 相似文献
114.
Guang-hua DONG~ Yan-pei LIU Department of Mathematics School of Science Beijing Jiaotong University Beijing China 《中国科学A辑(英文版)》2007,50(11):1563-1570
In this paper,we provide a new class of up-embeddable graphs,and obtain a tight lower bound on the maximum genus of a class of 2-connected pseudographs of diameter 2 and of a class of diameter 4 multi-graphs.This extends a result of Skoviera. 相似文献
115.
116.
In this work, the melting behaviors of nonisothermally and isothermally melt‐crystallized poly(L ‐lactic acid) (PLLA) from the melt were investigated with differential scanning calorimetry (DSC) and temperature‐modulated differential scanning calorimetry (TMDSC). The isothermal melt crystallizations of PLLA at a temperature in the range of 100–110 °C for 120 min or at 110 °C for a time in the range of 10–180 min appeared to exhibit double melting peaks in the DSC heating curves of 10 °C/min. TMDSC analysis revealed that the melting–recrystallization mechanism dominated the formation of the double melting peaks in PLLA samples following melt crystallizations at 110 °C for a shorter time (≤30 min) or at a lower temperature (100, 103, or 105 °C) for 120 min, whereas the double lamellar thickness model dominated the formation of the double melting peaks in those PLLA samples crystallized at a higher temperature (108 or 110 °C) for 120 min or at 110 °C for a longer time (≥45 min). © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 466–474, 2007 相似文献
117.
In the present paper, some types of general solutions of a first-order nonlinear ordinary differential equation with six degree are given and a new generalized algebra method is presented to find more exact solutions of nonlinear differential equations. As an application of the method and the solutions of this equation, we choose the (2 + 1) dimensional Boiti Leon Pempinelli equation to illustrate the validity and advantages of the method. As a consequence, more new types and general solutions are found which include rational solutions and irrational solutions and so on. The new method can also be applied to other nonlinear differential equations in mathematical physics. 相似文献
118.
Horng-Shyang Chen Shun-Lee Liu C. C. Yang Yean-Woei Kiang 《Optics Communications》2003,220(4-6):383-388
We report the reduced waveguiding efficiency for the signals around 1560 nm as the injection current of an GaAs/AlGaAs multiple quantum well laser diode (lasing wavelength at 840 nm) with a ridge-loading waveguide configuration increased. This reduction trend stopped when the injection current reached the threshold condition of the laser diode. The decreased waveguide transmission and the more expanded mode profile indicated the variation of the effective refractive index gradient in the lateral dimension with injection current. This variation was due to the refractive index decrease with increasing carrier density even below band gap. A slab waveguide model was used to simulate the lateral mode profile variation with injection current. The refractive index differences between the guiding layer and claddings in the slab waveguide model provided estimates of refractive index contrasts of the laser diode at a concerned wavelength under various injection conditions. 相似文献
119.
Yen-Shung TSAI 《中国科学A辑(英文版)》2007,50(7):1026-1034
By means of further investigation of solid codes,the problem“Is every fd-domain uni- formly dense”proposed by Yuqi Guo,C.M.Reis and G.Thierrin in 1988 is solved in this paper. 相似文献
120.