全文获取类型
收费全文 | 2023篇 |
免费 | 140篇 |
国内免费 | 136篇 |
专业分类
化学 | 293篇 |
晶体学 | 4篇 |
力学 | 189篇 |
综合类 | 15篇 |
数学 | 946篇 |
物理学 | 852篇 |
出版年
2024年 | 3篇 |
2023年 | 17篇 |
2022年 | 27篇 |
2021年 | 22篇 |
2020年 | 38篇 |
2019年 | 41篇 |
2018年 | 44篇 |
2017年 | 52篇 |
2016年 | 63篇 |
2015年 | 65篇 |
2014年 | 120篇 |
2013年 | 214篇 |
2012年 | 124篇 |
2011年 | 130篇 |
2010年 | 143篇 |
2009年 | 184篇 |
2008年 | 173篇 |
2007年 | 145篇 |
2006年 | 97篇 |
2005年 | 71篇 |
2004年 | 74篇 |
2003年 | 47篇 |
2002年 | 56篇 |
2001年 | 59篇 |
2000年 | 44篇 |
1999年 | 35篇 |
1998年 | 34篇 |
1997年 | 26篇 |
1996年 | 15篇 |
1995年 | 22篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1992年 | 19篇 |
1991年 | 13篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有2299条查询结果,搜索用时 15 毫秒
81.
A. A. Akhmetov S. S. Ivanov I. O. Shchegolev 《Physica C: Superconductivity and its Applications》1998,310(1-4):382-386
The matrix method was used to investigate the process of current decay in the samples of flat two-layer superconducting cables. The discrete spectrum of eigen-frequencies has been obtained. Each of these frequencies determines the rate of decay of the correspondent eigen-current. Despite of the increasing of the number of eigen-frequencies with the enlargement of the sample dimensions the spectrum remains finite, as the maximum and minimum frequencies tend to finite limits. An analysis made for the lowest eigen-frequencies showed the corresponding eigen-currents to be slowly decaying long current loops. Within the range of high frequencies the sinusoidal distribution of eigen-currents in the rows of the cable was observed. 相似文献
82.
Correlation integral as a tool for distinguishing between dynamics and statistics in time series data 总被引:2,自引:0,他引:2
We propose a new test for time series data for proper choice of processing technique: dynamical or statistical. It is based upon the normalized slope of the correlation integral (, m) = m−1d(ln C()) /d ln , where m is the embedding dimension. It is shown that when does not tend to 0 on the resolved range of scales as m grows, then there will be serious limitations for dynamical methods even if the data are dynamical by nature. In the latter case it means that the length of time series does not allow to resolve small scales, and on large scales the delay reconstruction for any m mixes true and false neighbours of points and therefore restricts the application of dynamical techniques, such as estimating Lyapunov exponents or predicting time series. 相似文献
83.
84.
The design and performance of an electron spin resonance spectrometer operating at 3 and 9 GHz microwave frequencies combined with a 9-T superconducting magnet are described. The probehead contains a compact two-loop, one gap resonator, and is inside the variable temperature insert of the magnet enabling measurements in the 0-9T magnetic field and 1.5-400 K temperature range. The spectrometer allows studies on systems where resonance occurs at fields far above the g approximately 2 paramagnetic condition such as in antiferromagnets. The low quality factor of the resonator allows time resolved experiments such as, e.g., longitudinally detected ESR. We demonstrate the performance of the spectrometer on the NaNiO2 antiferromagnet, the MgB2 superconductor, and the RbC60 conducting alkaline fulleride polymer. 相似文献
85.
Jeffrey J DaCunha Parmjeet K Singh 《Journal of Mathematical Analysis and Applications》2004,295(2):378-391
We prove the existence of a positive solution for the three point boundary value problem on time scale given by
86.
Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method 总被引:4,自引:0,他引:4
A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation. 相似文献
87.
Functional MRI using intravascular contrast agents: detrending of the relative cerebrovascular (rCBV) time course 总被引:3,自引:0,他引:3
In pharmacological fMRI experiments in animal models, blood pool contrast agents may be used to map cerebral blood volume change as a surrogate for neural activation. When the background signal drift due to contrast agent washout is non-negligible over the duration of the signal changes of interest, time-course detrending is essential for accurate interpretation of the experiment. Detrending approaches based on estimation of the background signal from a baseline period of the time course prior to pharmacological (or functional) challenge were evaluated with the aim of identifying a robust method of estimating the contrast agent washout contribution to the background signal drift. For fMRI studies in the rat, it was found that a constrained fit of a mono-exponential washout model was more accurate than a constant background approximation and unconstrained fits for experiments investigating the functional response to rapid pharmacological challenges such as cocaine and amphetamine. Moreover, the constrained fitting approach allows shorter baseline periods than unconstrained extrapolation, reducing the required duration of the experiment. 相似文献
88.
激光大气等离子体时间演化特性的光谱研究 总被引:8,自引:1,他引:7
采用延时光谱法和谱线演化法 ,对YAG脉冲激光器 1 0 6 μm光束击穿一个大气压的空气所产生的等离子体进行了时间分辨光谱研究。对激光大气等离子体连续光谱的短波带与长波带分别进行了时间分辨测量 ,结果表明两者的衰变速率均在等离子体激发约 0 5微秒以后明显变慢。分析认为这种衰变速率的变慢可能与空气中氧对自由电子的吸附与去吸附有关。对激光大气等离子体线状光谱所作的时间分辨测量则表明 ,大部分线状光谱的演化寿命大于其标称寿命 ,部分线状光谱还呈现“衰变—增涨—衰变”的复杂形式。分析认为线状光谱的这些演化特征可能与等离子体在衰变期间的各种复合过程和能量转移过程有关。 相似文献
89.
90.
We study the exact multiplicity and ordering properties of positive solutions of the p-Laplacian Dirichlet problem