全文获取类型
收费全文 | 10989篇 |
免费 | 2298篇 |
国内免费 | 1007篇 |
专业分类
化学 | 4215篇 |
晶体学 | 102篇 |
力学 | 1168篇 |
综合类 | 128篇 |
数学 | 687篇 |
物理学 | 7994篇 |
出版年
2024年 | 20篇 |
2023年 | 103篇 |
2022年 | 264篇 |
2021年 | 301篇 |
2020年 | 330篇 |
2019年 | 330篇 |
2018年 | 319篇 |
2017年 | 370篇 |
2016年 | 472篇 |
2015年 | 434篇 |
2014年 | 735篇 |
2013年 | 872篇 |
2012年 | 735篇 |
2011年 | 849篇 |
2010年 | 628篇 |
2009年 | 736篇 |
2008年 | 752篇 |
2007年 | 809篇 |
2006年 | 694篇 |
2005年 | 584篇 |
2004年 | 474篇 |
2003年 | 434篇 |
2002年 | 427篇 |
2001年 | 378篇 |
2000年 | 334篇 |
1999年 | 257篇 |
1998年 | 223篇 |
1997年 | 183篇 |
1996年 | 164篇 |
1995年 | 164篇 |
1994年 | 126篇 |
1993年 | 133篇 |
1992年 | 101篇 |
1991年 | 99篇 |
1990年 | 66篇 |
1989年 | 48篇 |
1988年 | 51篇 |
1987年 | 43篇 |
1986年 | 35篇 |
1985年 | 42篇 |
1984年 | 38篇 |
1983年 | 19篇 |
1982年 | 27篇 |
1981年 | 22篇 |
1980年 | 18篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
Qingfeng Zhang Shanjia Xu 《International Journal of Infrared and Millimeter Waves》2006,27(11):1539-1552
A novel uniplanar 2-D composite right/left-handed transmission line (CRLH-TL) structure is proposed based on a general transmission
line theory and the way to increase the relative operating bandwidth in the left-handed (LH) region and lower the LH operating
frequency is illustrated. In addition, a new method to extract parameters and calculate the Bloch impedance of the structure
is presented. Numerical results of the dispersion as well as the extracted parameters are given, which are calculated based
on full-wave simulation. The present 2-D uniplanar CRLH-TL structure is applicable to 2-D left-handed materials in lower and
wider LH frequency range.
Supported by the National Natural Science Foundation of China (No. 60371010) (No. 60471037) (No. 63531020) 相似文献
93.
采用一维粒子模拟(PIC)方法,研究了相对论效应对P偏振激光斜入射非均匀等离子体时产生的共振吸收的影响. 计算表明,弱相对论情况下,在临界面附近产生的电子等离子体波的相对论非线性效应占主要作用;随着入射光场的逐渐增大,吸收率逐渐降低. 当入射光强超过3.7×1017W/cm2时,由于超短激光脉冲本身在等离子体中产生相对论效应、等离子体波破裂效应,以及参量不稳定过程激发等,吸收系数随着激光强度又开始增加. 固定等离子体密度标长,取不同的激光入射角、电子初始温度,相对论效应对吸收系数的影响是一致的.
关键词:
激光等离子体
相对论效应
共振吸收
粒子模拟 相似文献
94.
核磁共振成像一维空间编码教学实验 总被引:2,自引:2,他引:0
利用梯度磁场实现检测信号的空间编码,是核磁共振成像(MRI)的关键技术.本文采用ccc系列样品将二维问题简化为一维,使用超小型教学用核磁共振成像仪进行了一维空间编码实验研究,并对实验过程及实验结果进行了计算模拟和分析. 相似文献
95.
M.Yu. Tretyakov V.V. Parshin M.A. Koshelev A.P. Shkaev A.F. Krupnov 《Journal of Molecular Spectroscopy》2006,238(1):91-97
Extension of the working frequency of modern resonator spectrometers into submillimeter wave range is described. Experimental record of atmospheric absorption spectrum covering 45-370 GHz range is demonstrated for the first time, and measured water vapor J′ka′, kc′-Jka, kc = 51,5-42,2 at 325 GHz line parameters are presented. For the first time pressure lineshift for the 325-GHz water vapor line is measured. Further extension of working range is discussed. New estimations of physical limits of time needed for measurements of absorption in the whole Backward Wave Oscillator (BWO) range are given for phase continuous synthesizer regime. Basic schemes of fast broadband continuous phase synthesized sources are discussed. Verification of the previous measurements of water vapor 31,3-22,0 at 183 GHz line parameters is presented. Comparisons with ringdown resonator spectrometers are given. 相似文献
96.
Motohiro Mizuno Yoji Aoki Darius Greenidge 《Journal of Physics and Chemistry of Solids》2006,67(4):705-709
The angular dependence of the 27Al NMR spectrum was measured for single crystals of smoky and colorless topaz, Al2SiO4(F,OH)2. Smoky topaz was obtained by irradiating high energy neutrons to colorless topaz. The quadrupole coupling constant e2Qq/h and the asymmetry parameter η were obtained from the analysis of the angular dependences of quadrupole splitting of the 27Al NMR spectrum. The local structures around the aluminum atoms in smoky and colorless topaz were discussed from the magnitude and the direction of the electric field gradient. The directions of principal axes of the EFG tensor of 27Al were close to the directions of Al-O and Al-F bonds. The difference in the bond lengths between Al(1)-F(1) and Al(1)-F(2) was found to affect the x and y components of the EFG tensor. 相似文献
97.
聚异丁烯高活性端基含量及相对分子质量测定方法的研究 总被引:2,自引:0,他引:2
综合使用核磁,VPO和近红外光谱分析技术对聚异丁烯活性端基含量以及相对分子质量的测定进行了详细研究,分别建立核测定聚异丁烯活性端基含量以及其他烯键含量和近红外光谱快速测量聚异聚丁烯活性端基含量。其他烯键含量和相对分子质量的分析方法。成对t检验结果表明,近红外光谱分析方法测定结果与核磁和VPO方法测定结果之间无显著性差异。 相似文献
98.
光谱色散匀滑技术与衍射光学器件联用性能的空间频谱分析 总被引:1,自引:0,他引:1
基于空间频谱分析方法,建立了光谱色散匀滑技术(SSD: Smoothing by Spectral Dispersion)与衍射光学器件(DOE: Diffractive Optical Element)联用性能的简化分析模型,为SSD与DOE联用时,SSD参量的优化提供了理论依据.数值模拟了SSD各参量,包括脉冲时间、调制频率、位相调制系数、光栅线色散系数等对束匀滑性能的影响.模拟结果表明,SSD参量经过优化选取,且波前畸变随时间快速变化时,能获得良好的束匀滑性能. 相似文献
99.
100.
Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr2O3 using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr2O3). A second phase, namely θ-(Al,Cr)2O3, was revealed by XRD in the sample containing 9.60 wt% Cr2O3. The structure of mullites was refined by the Rietveld method, location of Cr3+ was performed by the EPR spectroscopy. At low chromium doping level (Cr2O3 content less than ∼5 wt%) Cr3+ ions were substituted for Al3+ in the AlO6 octahedra of the mullite structure (M1 site). For higher doping level, Cr3+ ions were additionally substituted for Al3+ in the AlO6 octahedra of the second phase [θ-(Al,Cr)2O3 at 1400 °C, or α-(Al,Cr)2O3 at 1600 °C] which segregated in the system. Substitution of Cr3+ for Al3+ on M1 site in the mullite structure resulted in increase of average distances in (M1)O6 octahedron and decrease of average distances in T*O4 tetrahedron, while average distances in TO4 tetrahedron stayed almost constant. 相似文献