全文获取类型
收费全文 | 27188篇 |
免费 | 3380篇 |
国内免费 | 3083篇 |
专业分类
化学 | 19129篇 |
晶体学 | 342篇 |
力学 | 1364篇 |
综合类 | 286篇 |
数学 | 3482篇 |
物理学 | 9048篇 |
出版年
2023年 | 308篇 |
2022年 | 546篇 |
2021年 | 658篇 |
2020年 | 736篇 |
2019年 | 824篇 |
2018年 | 700篇 |
2017年 | 719篇 |
2016年 | 1049篇 |
2015年 | 1105篇 |
2014年 | 1384篇 |
2013年 | 1838篇 |
2012年 | 2089篇 |
2011年 | 2386篇 |
2010年 | 1729篇 |
2009年 | 1787篇 |
2008年 | 1981篇 |
2007年 | 1841篇 |
2006年 | 1705篇 |
2005年 | 1491篇 |
2004年 | 1271篇 |
2003年 | 1055篇 |
2002年 | 1123篇 |
2001年 | 829篇 |
2000年 | 704篇 |
1999年 | 498篇 |
1998年 | 361篇 |
1997年 | 281篇 |
1996年 | 303篇 |
1995年 | 217篇 |
1994年 | 229篇 |
1993年 | 185篇 |
1992年 | 174篇 |
1991年 | 143篇 |
1990年 | 146篇 |
1989年 | 123篇 |
1988年 | 125篇 |
1987年 | 81篇 |
1986年 | 82篇 |
1985年 | 111篇 |
1984年 | 76篇 |
1983年 | 74篇 |
1982年 | 53篇 |
1981年 | 49篇 |
1980年 | 55篇 |
1979年 | 39篇 |
1978年 | 46篇 |
1977年 | 40篇 |
1976年 | 40篇 |
1974年 | 50篇 |
1973年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
101.
Yong LU Hong WANG Ye LIU Ming Yuan HE 《中国化学快报》2006,17(10):1397-1400
Recently, miniature H2 generator to power fuel cells for portable/micro electronic devices and passenger propulsion has been the focus of intense research activities1-3. One of the strategies is to find simple CO-free H2 production with novel microreactor… 相似文献
102.
103.
Longitudinal study has become one of the most commonly adopted designs in medical research. The generalized estimating equations (GEE) method and/or mixed effects models are employed very often in causal inferences. The related model diagnostic procedures are not yet fully formalized, and perhaps never will be. The potential causes of major problems are the high variety of the dependence within subjects and/or the number of repeated measurements. A single testing procedure, e.g., run test, is not possible to resolve all model diagnostics problems in longitudinal data analysis. Multiple quantitative indexes for model diagnostics are needed to take into account this variety. We propose eight testing procedures for randomness accompanied with some conventional and/or non-conventional plots to remedy model diagnostics in longitudinal data analysis. The proposed issue in this paper is well illustrated with four clinical studies in Taiwan. 相似文献
104.
105.
TieYong YangGuangjun 《分析论及其应用》2004,20(1):58-68
In this paper, we construct some continuous but non-differentiable functions defined by quinary decimal, that are Kiesswetter-like functions. We discuss their properties, then investigate the Hausdorff dimensions of graphs of these functions and give a detailed proof. 相似文献
106.
Whilst the space volume of muffler in noise control system is often constrained for maintenance in practical engineering work, the maximization on muffler’s performance becomes important and essential. In this paper, a novel approach genetic algorithms (GAs) based on the principles of natural biological evolution will be used to tackle this optimization of muffler design [M. Mitchell, An Introduction to Genetic Algorithms, The MIT Press, Cambridge, MA, 1996]. Here, the shape optimization of multi-segments muffler coupled with the GA searching technique is presented. The techniques of binary genetic algorithms (BGA) together with the commercial MATLAB package [G. Lindfield, J. Penny, Numerical Method Using Matlab, second ed., Prentice Hall, Englewood Cliffs, NJ, 2000] are applied in GA searching. In addition, a numerical case of pure tone elimination with 2-5 segments on muffler is introduced and fully discussed. To achieve the best optimization in GA, several GA parameters are on trial in various values. Results show that the GA operators, including crossover mutation and elitism, are essential in accuracy. Consequently, results verify that the optimal sound transmission loss at the designed frequency of 500 Hz is exactly maximized. The GA optimization on multi-segments muffler proposed in this study surely provides a quick and correct approach. 相似文献
107.
A novel measurement method of temperature based on the phenomena that the phase difference between principle polarization states in the optical retarder is function of temperature is described. The polarization state of optical beam is changed as it passes through the optical retarder, which depends on the temperature. The temperature of optical retarder is determined by comparison of the power difference between principal polarization states. We demonstrate successfully the temperature measurement by using a polarization maintaining fiber as the optical retarder. With a 100 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.236 rad/°C and the measurement error was ±0.038°C over the temperature range of −2.6 – +3.4°C. With a 11.5 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.021 rad/°C and the measurement error was ±0.79°C over the temperature range of −8.5 – +86.5°C. 相似文献
108.
Rong-Seng Chang Jin-Yi Sheu Ching-Huang Lin He-Chiang Liu 《Optics & Laser Technology》2003,35(1):43-47
A new approach based on the moiré theory and wavelet transform (WT) is proposed for measuring the micro-range distance between a charge-couple-device (CCD) camera and a two-dimensional reference grating. The micro-range distance is determined by measuring the pitch of the moiré pattern image, which is digitized by a CCD camera. A one-dimensional WT algorithm is applied to estimate the pitch of the moiré pattern. Experimental results prove that this technique is very efficient and highly accurate. The moiré range finder is an economic technique for measuring a micro-range distance. 相似文献
109.
研究了应用于锂二次电池正极的新型高能量密度存贮材料Li(AlxCo1-x)O2 (x=01—05)的磁性.发现Al3+的掺杂可导致Co3+中d电子自旋态发生变化,即有部分d电子进入高自旋态.伴随Co3+中电子状态的改变,材料结构演化也发生了相应变化,表现为c/a比增大明显减缓,较好地解释了材料结构对Vegard定律的正偏离.这对材料的微观结构与性能设计具有重要意义.
关键词:
锂电池材料
Li(AlxCo1-x)O2
磁性
自旋态
结构演化 相似文献
110.
A periodic one-dimensional four-state hopping model is proposed. In the model, the substeps betweenarbitrary adjacent states are unequal, and an explicit solution of the master equation is first obtained for the probabilitydistribution as a function of the time and position for any initial distribution with all the transients included. Next, thetransient behaviors in the initial period of time and the characteristic time to reach the steady state for the molecularmotor are discussed. Finally, we compare the steady state results to experiments and illustrate qualitatively the kineticbehaviors of a molecular motor under external load F. 相似文献