全文获取类型
收费全文 | 962篇 |
免费 | 44篇 |
专业分类
化学 | 759篇 |
晶体学 | 15篇 |
力学 | 15篇 |
数学 | 68篇 |
物理学 | 149篇 |
出版年
2024年 | 4篇 |
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 22篇 |
2020年 | 27篇 |
2019年 | 19篇 |
2018年 | 16篇 |
2017年 | 26篇 |
2016年 | 30篇 |
2015年 | 27篇 |
2014年 | 39篇 |
2013年 | 63篇 |
2012年 | 68篇 |
2011年 | 56篇 |
2010年 | 39篇 |
2009年 | 51篇 |
2008年 | 60篇 |
2007年 | 44篇 |
2006年 | 61篇 |
2005年 | 47篇 |
2004年 | 40篇 |
2003年 | 36篇 |
2002年 | 20篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1998年 | 3篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 8篇 |
1994年 | 10篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有1006条查询结果,搜索用时 171 毫秒
41.
42.
Anant Kant Shukla T.R. Ramamohan S. Srinivas 《Communications in Nonlinear Science & Numerical Simulation》2012,17(10):3776-3787
We demonstrate the efficiency of a modification of the normal homotopy analysis method (HAM) proposed by Liao [2] by including a non-homogeneous term in the auxiliary linear operator (this can be considered as a special case of “further generalization” of HAM given by Liao in [2]). We then apply the modified method to a few examples. It is observed that including a non-homogeneous term gives faster convergence in comparison to normal HAM. We also prove a convergence theorem, which shows that our technique yields the convergent solution. 相似文献
43.
We present measurements of Eulerian longitudinal velocity autocorrelations in homogeneous, isotropic, high-intensity (~9%) free-stream turbulence behind an active grid. Spatial correlations are measured using particle image velocimetry as well as with two-point hot-wire anemometry (HWA), while temporal correlations are measured using HWA. The temporal correlations are transformed into spatial correlations by using Taylor's ‘frozen’ hypothesis with both the mean as well as instantaneous velocities. A model relating Eulerian spatial and temporal autocorrelations is also used for this purpose. The differences from the measured spatial correlation resulting from the use of Taylor's hypothesis on the temporal correlation is quantified; even at this moderately high level of turbulent intensity, the result from the use of the instantaneous velocity as convection velocity is practically indistinguishable from that obtained using the mean velocity. Use of the model produces a good agreement between the estimates of the spatial correlation function. A relation between Eulerian spatial and temporal integral scales is also derived. 相似文献
44.
45.
A variety of aldehyde N,N-dimethylhydrazones are rapidly converted into the corresponding nitriles using oxone supported on wet Al2O3 under microwave irradiation in dry media. 相似文献
46.
An efficient and mild method for tetrahydropyranylation of alcohols and their detetrahydropranylation using NH4Cl is described. This protocol provides a useful alternative tetrahydropyranylation of alcohols and their deprotection at different pH. 相似文献
47.
Biswanath Das Yallamalla Srinivas Chittaluri Sudhakar Kongara Damodar Ravirala Narender 《合成通讯》2013,43(2):220-227
Bromination of alkenes and alkynes has efficiently been carried out at room temperature in short reaction times using KBr and diacetoxy iodobenzene in CH2Cl2-H2O (1:1) to prepare the corresponding trans-dibromo compounds in excellent yields. 相似文献
48.
Cellulose nanofibre aspect ratio controls the properties of sheets made from nanofibres and processing conditions, but aspect ratio is very difficult to measure. In this paper, aspect ratio was estimated from the gel point of a cellulose nanofibre suspension, the solids concentration at which the transition from a dilute to a semi-dilute suspension occurs. Four batches of cellulose nanofibres were tested. Two were produced from softwood fibres using ball milling. Commercially produced microfibrillated cellulose material was also used, both in as supplied form and after removal of the larger fibres by filtering. The average diameter measured from SEM images of fibres ranged from 33 to 73 nm. One sample was too heavily treated and an average dimension could not be measured. The gel-point was measured both from the height of a layer of cellulose nanofibres sedimented from a dilute suspension or from the lowest solids concentration at which a yield stress could be measured using a vane rheometer. The two methods were closely in agreement for all samples. Aspect ratio was then calculated using either the effective medium (EMT) or crowding number (CN) theories. Aspect ratio calculated with an assumed fibre density of 1,500 kg/m3, using the CN theory ranged from 155 to 60. Use of the EMT theory reduced the calculated aspect ratio by between 11 and 23 %. Reducing the assumed density in suspension from 1,500 to 1,166 kg/m3 reduced the calculated aspect ratio by 12–14 %. The heavily treated sample had by far the lowest aspect ratio. 相似文献
49.
A stereoselective total synthesis of penaresidin A has been accomplished involving Sharpless asymmetric epoxidation, regioselective ring-opening of epoxide, azetidine formation via SN2 reaction, Jung’s protocol, and Julia–Kocienski olefination. This approach has successfully demonstrated the synthetic utility of d-galactal in the construction of azetidine core of the natural product. 相似文献
50.
Quality by design: A systematic and rapid liquid chromatography and mass spectrometry method for eprosartan mesylate and its related impurities using a superficially porous particle column 下载免费PDF全文
Pradipbhai D. Kalariya Murali V. N. Kumar Talluri Vinay D. Gaitonde Prashant S. Devrukhakar Ragampeta Srinivas 《Journal of separation science》2014,37(16):2160-2171
The present work describes the systematic development of a robust, precise, and rapid reversed‐phase liquid chromatography method for the simultaneous determination of eprosartan mesylate and its six impurities using quality‐by‐design principles. The method was developed in two phases, screening and optimization. During the screening phase, the most suitable stationary phase, organic modifier, and pH were identified. The optimization was performed for secondary influential parameters—column temperature, gradient time, and flow rate using eight experiments—to examine multifactorial effects of parameters on the critical resolution and generated design space representing the robust region. A verification experiment was performed within the working design space and the model was found to be accurate. This study also describes other operating features of the column packed with superficially porous particles that allow very fast separations at pressures available in most liquid chromatography instruments. Successful chromatographic separation was achieved in less than 7 min using a fused‐core C18 (100 mm × 2.1 mm, 2.6 μm) column with linear gradient elution of 10 mM ammonium formate (pH 3.0) and acetonitrile as the mobile phase. The method was validated for specificity, linearity, accuracy, precision, and robustness in compliance with the International Conference on Harmonization Q2 (R1) guidelines. The impurities were identified by liquid chromatography with mass spectrometry. 相似文献