全文获取类型
收费全文 | 3561篇 |
免费 | 445篇 |
国内免费 | 276篇 |
专业分类
化学 | 395篇 |
晶体学 | 14篇 |
力学 | 908篇 |
综合类 | 25篇 |
数学 | 467篇 |
物理学 | 2473篇 |
出版年
2024年 | 8篇 |
2023年 | 17篇 |
2022年 | 65篇 |
2021年 | 62篇 |
2020年 | 66篇 |
2019年 | 56篇 |
2018年 | 72篇 |
2017年 | 120篇 |
2016年 | 112篇 |
2015年 | 81篇 |
2014年 | 202篇 |
2013年 | 256篇 |
2012年 | 173篇 |
2011年 | 240篇 |
2010年 | 176篇 |
2009年 | 217篇 |
2008年 | 210篇 |
2007年 | 212篇 |
2006年 | 230篇 |
2005年 | 181篇 |
2004年 | 195篇 |
2003年 | 171篇 |
2002年 | 131篇 |
2001年 | 149篇 |
2000年 | 141篇 |
1999年 | 115篇 |
1998年 | 95篇 |
1997年 | 73篇 |
1996年 | 73篇 |
1995年 | 68篇 |
1994年 | 44篇 |
1993年 | 46篇 |
1992年 | 46篇 |
1991年 | 44篇 |
1990年 | 22篇 |
1989年 | 28篇 |
1988年 | 16篇 |
1987年 | 18篇 |
1986年 | 11篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1977年 | 3篇 |
排序方式: 共有4282条查询结果,搜索用时 15 毫秒
931.
研究了Nd~(3+)和Ho~(3+)的内、外消旋-双(苯基亚砜)乙烷配合物的固体和溶液(非水溶剂)的电子光谱,分析了可见范围的电子光谱并计算了光谱参数(b~(1/2)、δ和Ω_1)和f-f跃迂强度,指出了“超灵敏”跃迁的振子强度与配合物的共价程度及配位数相关。 相似文献
932.
基于激光强度分布的激光雷达重叠因子计算及其敏感性分析 总被引:1,自引:0,他引:1
激光雷达系统是一种功能强大的遥感设备,具有较高的时空分辨率和精度,广泛应用于区域或全球尺度的大气探测。重叠因子是影响激光雷达系统探测近距离大气的一个重要因素,对其进行精确的计算有利于获得准确探测结果。基于激光强度分布的重叠因子计算方法具有简单实用的特点,可应用于任意指定激光强度分布和不同系统参数的激光雷达重叠因子求解。利用该算法对重叠因子进行了计算,并与实验法测量得到的重叠因子进行了对比分析。重点分析了重叠因子对各个参数的敏感性。分析结果为指导激光雷达的系统调试、优化配置和误差控制提供了一定的参考。 相似文献
933.
采用坩埚下降法生长了Tm3+掺杂浓度为0.45%,0.90%,1.63%与3.25%(摩尔分数,x)的LiLuF4单晶.测试了样品的电感耦合等离子体原子发射光谱(ICP-AES)、X射线衍射(XRD)谱、吸收光谱(1400-2000 nm),并且分析比较了808 nm半导体激光器(LD)激发下荧光光谱.结果表明:当Tm3+的浓度从0.45%变化到3.25%时,1800 nm处的荧光强度呈现了先增后减的趋势,当掺杂浓度约为0.90%时达到最大值,而位于1470 nm处的荧光强度则呈现了相反的趋势.Tm3+∶3F4能级的荧光衰减寿命随着掺杂浓度的增加不断减小.1800 nm处的这种荧光强度变化归结于Tm3+离子间的交叉驰豫效应(3H6,3H4→3F4,3F4)和自身的浓度猝灭效应.同时计算得到了浓度为0.90%的样品在1890 nm处的最大发射截面为0.392x 10-20 cm2.并且根据Judd-Ofelt理论所得寿命和测定的荧光寿命计算得到了3F→3H6的最大量子效率约为120%. 相似文献
934.
This study evaluates the feasibility of using high intensity focused ultrasound (HIFU) for the treatment of hydatid cysts of the liver. HIFU ablation was carried out in 62 patients with echinococcosis of the liver. The mean age of patients was 40.76 ± 14.84 (range: 17–72 years). The effectiveness of the treatment was monitored in real-time by changes in the gray-scale, and by morphological studies, computed tomography, magnetic resonance imaging, and ultrasound.Criteria for evaluating the effectiveness of treatment in real time were outlines. Cytomorphological picture of destructive changes of parasitic elements was presented as well. Loss of embryonic elements of the parasite was observed at the subcellular level after HIFU-ablation and underlines the effectiveness of HIFU. 相似文献
935.
The anti-plane fracture problem for a finite crack in a one-dimensional hexagonal quasicrystal strip is analyzed. By using Fourier transforms, the mixed boundary value problems are reduced to the dual integral equations. The solution of the dual integral equations is then expressed by the complete elliptic integrals of the first and the third kinds. The expressions for stress, strains, displacements and field intensity factors of the phonon and phason fields near the crack tip are obtained exactly. The path-independent integral derived by a conservation law equals the energy release rate, which can be used as a fracture criterion for a mode III fracture problem. 相似文献
936.
Viewing the effective k-space coverage of MR images: phantom experiments with fast Fourier transform
The purpose of this experimental study was to evaluate whether the effective k-space coverage of MR images can in principle be viewed after multidimensional Fourier transform back to k-space. 相似文献
937.
《International journal of quantum chemistry》2018,118(5)
We report a combined experimental and theoretical investigation on the Raman spectra of the polymorphs α, β, γ, and δ of 1,1,4,4‐tetraphenyl‐1,3‐butadiene (TPB), in the region of the intramolecular modes. The interpretation of the polarized spectra is supported by ab‐initio calculations for the isolated molecules and by lattice dynamics calculations for the crystals. The calculations reproduce the peculiar, and surprisingly large, differences among the spectra of the various polymorphs. The phenyl groups of 1,1,4,4‐tetraphenyl‐1,3‐butadiene may arrange themselves around the butadiene skeleton in 2 stable conformers, which have either inversion (Ci) or 2‐fold (C2) symmetry and therefore exhibit intramolecular vibrations with quite different Raman selection rules and spectra. The compound forms 4 crystalline polymorphs (α, β, γ, and δ) with different combinations of Ci and C2 conformers, and correspondingly different intramolecular spectra. The theoretical calculations provide a quantitative analysis of the various spectra. 相似文献
938.
939.
940.
The light intensity distribution (LID) reflected by a planar corner-cube retroreflector array with the size of centimeter and above (PCCRA) is calculated based on the Collins formula, coordinate transformation matrixes and vector algebra. The expression can be used for the PCCRAs with different parameters and different complex optical systems described by ABCD matrix. The expression indicates that the LID of a PCCRA is much different from a single corner-cube retroreflector (CCR) with equivalent aperture. Computer simulations are applied to explain the expression clearly. The results show that the LID of the PCCRA is more homogeneous than the single CCR with equivalent aperture under circumstances of both normal incident light and oblique incident light. In addition, the expression is proved by comparing the simulation result of the PCCRA only comprising a single CCR and the previous experimental result. 相似文献