全文获取类型
收费全文 | 9305篇 |
免费 | 1805篇 |
国内免费 | 602篇 |
专业分类
化学 | 2155篇 |
晶体学 | 82篇 |
力学 | 867篇 |
综合类 | 148篇 |
数学 | 4331篇 |
物理学 | 4129篇 |
出版年
2024年 | 9篇 |
2023年 | 89篇 |
2022年 | 145篇 |
2021年 | 273篇 |
2020年 | 285篇 |
2019年 | 324篇 |
2018年 | 309篇 |
2017年 | 389篇 |
2016年 | 426篇 |
2015年 | 346篇 |
2014年 | 489篇 |
2013年 | 849篇 |
2012年 | 568篇 |
2011年 | 602篇 |
2010年 | 531篇 |
2009年 | 578篇 |
2008年 | 644篇 |
2007年 | 561篇 |
2006年 | 499篇 |
2005年 | 456篇 |
2004年 | 374篇 |
2003年 | 393篇 |
2002年 | 364篇 |
2001年 | 298篇 |
2000年 | 260篇 |
1999年 | 249篇 |
1998年 | 181篇 |
1997年 | 201篇 |
1996年 | 156篇 |
1995年 | 130篇 |
1994年 | 99篇 |
1993年 | 72篇 |
1992年 | 67篇 |
1991年 | 58篇 |
1990年 | 51篇 |
1989年 | 45篇 |
1988年 | 34篇 |
1987年 | 35篇 |
1986年 | 32篇 |
1985年 | 49篇 |
1984年 | 31篇 |
1983年 | 18篇 |
1982年 | 27篇 |
1981年 | 25篇 |
1980年 | 9篇 |
1979年 | 19篇 |
1978年 | 14篇 |
1977年 | 14篇 |
1976年 | 11篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The Interval Correlation Optimised Shifting algorithm (icoshift) has recently been introduced for the alignment of nuclear magnetic resonance spectra. The method is based on an insertion/deletion model to shift intervals of spectra/chromatograms and relies on an efficient Fast Fourier Transform based computation core that allows the alignment of large data sets in a few seconds on a standard personal computer. The potential of this programme for the alignment of chromatographic data is outlined with focus on the model used for the correction function. The efficacy of the algorithm is demonstrated on a chromatographic data set with 45 chromatograms of 64,000 data points. Computation time is significantly reduced compared to the Correlation Optimised Warping (COW) algorithm, which is widely used for the alignment of chromatographic signals. Moreover, icoshift proved to perform better than COW in terms of quality of the alignment (viz. of simplicity and peak factor), but without the need for computationally expensive optimisations of the warping meta-parameters required by COW. Principal component analysis (PCA) is used to show how a significant reduction on data complexity was achieved, improving the ability to highlight chemical differences amongst the samples. 相似文献
992.
993.
994.
995.
2‐Acetylcyclopentanone (2‐ACP), which is a β‐dicarbonyl compound, undergoes keto–enol isomerization, and its enol tautomers are stabilized by a cyclic intramolecular hydrogen bond. 2‐ACP (keto form) has symmetric and asymmetric vibrational modes of the two carbonyl groups at 1748 and 1715 cm?1, respectively, which are well separated from the carbonyl modes of its enol tautomers in the FTIR spectrum. We have investigated 2‐ACP dissolved in carbon tetrachloride by 2D IR spectroscopy and IR pump–probe spectroscopy. Vibrational population transfer dynamics between the two carbonyl modes were observed by 2D IR spectroscopy. To extract the population exchange dynamics (i.e., the down‐ and uphill population transfer rate constants), we used the normalized volumes of the cross‐peaks with respect to the diagonal peaks at the same emission frequency and the survival and conditional probability functions. As expected, the downhill population transfer time constant (3.2 ps) was measured to be smaller than the uphill population transfer time constant (3.8 ps). In addition, the vibrational population relaxation dynamics of the two carbonyl modes were observed to be the same within the experimental error and were found to be much slower than vibrational population transfer between two carbonyl modes. 相似文献
996.
Sachin Rawalekar Sreejith Kaniyankandy Sandeep Verma Dr. Hirendra N. Ghosh 《Chemphyschem》2011,12(9):1729-1735
Herein, we report the synthesis of aqueous CdTe/CdSe type‐II core–shell quantum dots (QDs) in which 3‐mercaptopropionic acid is used as the capping agent. The CdTe QDs and CdTe/CdSe core–shell QDs are characterized by X‐ray diffraction (XRD), high‐resolution transmission electron microscopy (HR‐TEM), steady‐state absorption, and emission spectroscopy. A red shift in the steady‐state absorption and emission bands is observed with increasing CdSe shell thickness over CdTe QDs. The XRD pattern indicates that the peaks are shifted to higher angles after growth of the CdSe shell on the CdTe QDs. HR‐TEM images of both CdTe and CdTe/CdSe QDs indicate that the particles are spherical, with a good shape homogeneity, and that the particle size increases by about 2 nm after shell formation. In the time‐resolved emission studies, we observe that the average emission lifetime (τav) increases to 23.5 ns for CdTe/CdSe (for the thickest shell) as compared to CdTe QDs (τav=12 ns). The twofold increment in the average emission lifetime indicates an efficient charge separation in type‐II CdTe/CdSe core–shell QDs. Transient absorption studies suggest that both the carrier cooling and the charge‐transfer dynamics are affected by the presence of traps in the CdTe QDs and CdTe/CdSe core–shell QDs. Carrier quenching experiments indicate that hole traps strongly affect the carrier cooling dynamics in CdTe/CdSe core–shell QDs. 相似文献
997.
Arnab Bagchi Yu‐Hsuan Huang Z. F. Xu P. Raghunath Yuan T. Lee Chi‐Kung Ni M. C. Lin Yuan‐Pern Lee 《化学:亚洲杂志》2011,6(11):2961-2976
The photodissociation of gaseous benzaldehyde (C6H5CHO) at 193, 248, and 266 nm using multimass ion imaging and step‐scan time‐resolved Fourier‐transform infrared emission techniques is investigated. We also characterize the potential energies with the CCSD(T)/6‐311+G(3df,2p) method and predict the branching ratios for various channels of dissociation. Upon photolysis at 248 and 266 nm, two major channels for formation of HCO and CO, with relative branching of 0.37:0.63 and 0.20:0.80, respectively, are observed. The C6H5+HCO channel has two components with large and small recoil velocities; the rapid component with average translational energy of approximately 25 kJ mol?1 dominates. The C6H6+CO channel has a similar distribution of translational energy for these two components. IR emission from internally excited C6H5CHO, ν3 (v=1) of HCO, and levels v≤2, J≤43 of CO are observed; the latter has an average rotational energy of approximately 13 kJ mol?1 and vibrational energy of approximately 6 kJ mol?1. Upon photolysis at 193 nm, similar distributions of energy are observed, except that the C6H5+HCO channel becomes the only major channel with a branching ratio of 0.82±0.10 and an increased proportion of the slow component; IR emission from levels ν1 (v=1) and ν3 (v=1 and 2) of HCO and v≤2, J≤43 of CO are observed; the latter has an average energy similar to that observed in photolysis at 248 nm. The observed product yields at different dissociation energies are compared to statistical‐theory predicted results based on the computed singlet and triplet potential‐energy surfaces. 相似文献
998.
The use of finite element and finite difference methods of spatial and temporal discretization for solving structural dynamics problems gives rise to purely numerical errors. Among the many numerical methods used to damp out the spurious oscillations occurring in the high frequency domain, it is proposed here to analyse and compare the well-known Bulk Viscosity method, which modifies the stresses calculations and a method recently presented by Tchamwa and Wielgosz, which is based on a modification of an explicit time integration algorithm. The efficiency of both methods is evaluated in a 2-D axisymmetric compressive test. 相似文献
999.
基于混合网格Navier-Stokes方程的并行隐式计算方法研究 总被引:2,自引:0,他引:2
针对结构网格很难处理复杂外形和非结构网格无法计算具有边界层的粘性流动的缺点,发展了基于混合网格格点的隐式算法,成功地解决了在工程应用中难于处理的复杂外形粘性流场计算和效率问题。同时针对大规模的工程问题,发展了基于MPI通信技术的染色分层通讯并行计算方法。其中空间离散采用基于Roe格式发展的三阶迎风HLLEW(Harten-Lax-Van Leer-Einfeldt-Wada)或AUSM格式,湍流模型采用k??两方程湍流模型,时间推进考虑到LU-SGS并行等效较困难则采用基于DP-LUR(Data-Parallel Lower-Upper Relaxation)格式的隐式算法,计算CFL数可取到105量级,从2个到128个CPU的并行加速效率都保持在90%以上,大大提高了计算效率。算例对标模M6机翼模型流场进行计算,验证了方法的可靠性;然后对标模DLR-F6翼身组合体进行混合网格粘性与无粘计算结果进行比较,进一步验证混合网格方法;最后计算了DLR-WBNP外挂发动机翼身组合体模型,准确模拟了外挂和超临界机翼的相互干扰流动问题,采用4 CPU 16 CORE到24 CPU 96 CORE,2000步计算时间都不超... 相似文献
1000.