全文获取类型
收费全文 | 14833篇 |
免费 | 1738篇 |
国内免费 | 664篇 |
专业分类
化学 | 4119篇 |
晶体学 | 30篇 |
力学 | 814篇 |
综合类 | 188篇 |
数学 | 1941篇 |
物理学 | 2450篇 |
无线电 | 7693篇 |
出版年
2024年 | 34篇 |
2023年 | 194篇 |
2022年 | 433篇 |
2021年 | 511篇 |
2020年 | 586篇 |
2019年 | 372篇 |
2018年 | 330篇 |
2017年 | 551篇 |
2016年 | 683篇 |
2015年 | 674篇 |
2014年 | 1106篇 |
2013年 | 894篇 |
2012年 | 1044篇 |
2011年 | 986篇 |
2010年 | 752篇 |
2009年 | 806篇 |
2008年 | 810篇 |
2007年 | 937篇 |
2006年 | 844篇 |
2005年 | 725篇 |
2004年 | 597篇 |
2003年 | 588篇 |
2002年 | 458篇 |
2001年 | 381篇 |
2000年 | 324篇 |
1999年 | 241篇 |
1998年 | 234篇 |
1997年 | 175篇 |
1996年 | 165篇 |
1995年 | 172篇 |
1994年 | 127篇 |
1993年 | 106篇 |
1992年 | 97篇 |
1991年 | 56篇 |
1990年 | 41篇 |
1989年 | 33篇 |
1988年 | 19篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 34篇 |
1984年 | 22篇 |
1983年 | 9篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 14篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1971年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and its pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that particle deposition can be realized under a lower gas pressure with a CB nozzle. 相似文献
82.
83.
Temperature effects on deposition rate of silicon nitride films were characterized by building a neural network prediction model. The silicon nitride films were deposited by using a plasma enhanced chemical vapor deposition system and process parameter effects were systematically characterized by 26−1 fractional factorial experiment. The process parameters involved include a radio frequency power, pressure, temperature, SiH4, N2, and NH3 flow rates. The prediction performance of generalized regression neural network was drastically improved by optimizing multi-valued training factors using a genetic algorithm. Several 3D plots were generated to investigate parameter effects at various temperatures. Predicted variations were experimentally validated. The temperature effect on the deposition rate was a complex function of parameters but N2 flow rate. Larger decreases in the deposition rate with the temperature were only noticed at lower SiH4 (or higher NH3) flow rates. Typical effects of SiH4 or NH3 flow rate were only observed at higher or lower temperatures. A comparison with the refractive index model facilitated a selective choice of either SiH4 or NH3 for process optimization. 相似文献
84.
Joshua Vaughan Joel Fortgang William Singhose Jeffrey Donnell Thomas Kurfess 《Mechatronics》2008,18(4):179-186
It is commonly accepted that hands-on experiences increase both learning and enjoyment during coursework. Mechatronics projects provide both interesting and relevant hands-on experiences for a wide range of topics including design processes, basic mechatronics concepts, technical communication, and working in a group environment. ME2110: Creative Decisions and Design at Georgia Tech integrates mechatronics and technical communication into a sophomore level mechanical design class. This paper describes the course in detail, highlighting the course goals and layout, tools provided to the students, industry involvement, and the main challenges of administering such a course. 相似文献
85.
86.
Using Lagrange's multiplier rule, we find upper and lower bounds of the energy of a bipartite graph G, in terms of the number of vertices, edges and the spectral moment of fourth order. Moreover, the upper bound is attained in a graph G if and only if G is the graph of a symmetric balanced incomplete block design (BIBD). Also, we determine the graphs for which the lower bound is sharp. 相似文献
87.
In this paper, three new direct Mutually Orthogonal Latin Squares (MOLS) constructions are presented for 7 MOLS(24), 7 MOLS(75) and 8 MOLS(36); then using recursive methods, several new constructions for 7 and 8 MOLS are obtained. These reduce the largest value for which 7 MOLS are unknown from 780 to 570, and the largest odd value for which 8 MOLS are unknown from 1935 to 419. © 2003 Wiley Periodicals, Inc. 相似文献
88.
David M. Evans 《组合设计杂志》2004,12(6):459-465
For all ‘reasonable’ finite t, k, and s, we construct a t‐(?0, k, 1) design and a group of automorphisms which is transitive on blocks and has s orbits on points. In particular, there is a 2‐(?0, 4, 1) design with a block‐transitive group of automorphisms having two point orbits. This answers a question of P. J. Cameron and C. E. Praeger. The construction is presented in a purely combinatorial way, but is a by‐product of a new way of looking at a model‐theoretic construction of E. Hrushovski. © 2004 Wiley Periodicals, Inc. 相似文献
89.
Marianne Gerny Burkhard Alpers 《International Journal of Computers for Mathematical Learning》2004,9(1):25-57
In this article we describe a mathematical microworld for investigating car motion on a racing course and its use with a group of grade 12 students. The microworld is concerned with the mathematical construction of courses and functions which describe car motion. It is implemented in the computer algebra system, Maple®, which provides the means to represent courses and functions symbolically and graphically. We describe the learning opportunities offered by the microworld in relation to the research literature on functions. Various facets and layers of the function concept are addressed in the microworld, and we suggest how work in the microworld might help in overcoming some well-known misconceptions.This revised version was published online in September 2005 with corrections to the Cover Date. 相似文献
90.