全文获取类型
收费全文 | 86191篇 |
免费 | 6499篇 |
国内免费 | 5170篇 |
专业分类
化学 | 26668篇 |
晶体学 | 1811篇 |
力学 | 3555篇 |
综合类 | 356篇 |
数学 | 21935篇 |
物理学 | 43535篇 |
出版年
2022年 | 182篇 |
2021年 | 328篇 |
2020年 | 652篇 |
2019年 | 952篇 |
2018年 | 926篇 |
2017年 | 636篇 |
2016年 | 487篇 |
2015年 | 420篇 |
2014年 | 1120篇 |
2013年 | 2264篇 |
2012年 | 1493篇 |
2011年 | 2043篇 |
2010年 | 2563篇 |
2009年 | 8001篇 |
2008年 | 8938篇 |
2007年 | 7006篇 |
2006年 | 6098篇 |
2005年 | 4236篇 |
2004年 | 4028篇 |
2003年 | 4167篇 |
2002年 | 4864篇 |
2001年 | 3799篇 |
2000年 | 3584篇 |
1999年 | 3405篇 |
1998年 | 2916篇 |
1997年 | 2075篇 |
1996年 | 1758篇 |
1995年 | 2326篇 |
1994年 | 2249篇 |
1993年 | 1667篇 |
1992年 | 1215篇 |
1991年 | 951篇 |
1990年 | 743篇 |
1989年 | 671篇 |
1988年 | 601篇 |
1987年 | 408篇 |
1986年 | 195篇 |
1985年 | 943篇 |
1984年 | 618篇 |
1983年 | 488篇 |
1982年 | 639篇 |
1981年 | 793篇 |
1980年 | 720篇 |
1979年 | 559篇 |
1978年 | 581篇 |
1977年 | 538篇 |
1976年 | 540篇 |
1975年 | 317篇 |
1974年 | 354篇 |
1973年 | 462篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
《Arabian Journal of Chemistry》2022,15(11):104208
Dendrobium nobile alkaloids (DNLA) and glycosides are the main active components extracted from Dendrobium nobile Lindl. (D. nobile) used for thousands of years in China. The pharmacological effects of the above chemical components are significantly different. D. nobile is mainly grown at an altitude ranging from 230 to 800 m in Chishui City, Northwest Guizhou Province. However, it is unclear whether the metabolite in D. nobile is influenced by the planting altitude. Hence, to reveal the different metabolite in D. nobile cultivated at the altitude of 336 m, 528 m, and 692 m, ultra-high performance liquid chromatography with Q/TOF-MS couple with multivariate analysis were developed. Using the orthogonal partial least squares-discriminant analysis, 19 different metabolites were discovered and then tentatively assigned their structures as alkaloids and glycosides by comparing mass spectrometry data with in-house database and literature. Moreover, the result of semiquantitative analysis showed the content of dendrobine that was belonged to alkaloids significantly increased at the altitude of 692 m, whereas the content of glycosides demonstrated an accumulation trend at the altitude of 528 m. The results could provide valuable information for the optimal clinical drug therapeutics and provide a reference for quality control. 相似文献
102.
103.
We investigate tail behavior of the supremum of a random walk in the case that Cramer's condition fails, namely, the intermediate case and the heavy-tailed ease. When the integrated distribution of the increment of the random walk belongs to the intersection of exponential distribution class and O-subexponential distribution class, under some other suitable conditions, we obtain some asymptotic estimates for the tail probability of the supremum and prove that the distribution of the supremum also belongs to the same distribution class. The obtained results generalize some corresponding results of N. Veraverbeke. Finally, these results are applied to renewal risk model, and asymptotic estimates for the ruin probability are presented. 相似文献
104.
《Journal of the Egyptian Mathematical Society》2014,22(3):517-523
Traveling wave solutions are obtained by using a relatively new technique which is called Tanh and extended Tanh method for Drinefel’d–Sokolov–Wilson Equations. Solution procedure and obtained results re-confirm the efficiency of the proposed scheme. 相似文献
105.
In this paper we present a new approach, based on the Nearest Interval Approximation Operator, for dealing with a multiobjective programming problem with fuzzy-valued objective functions. 相似文献
106.
Shape modeling is fundamental to the analysis of dynamic environment and motion around asteroid. Chang'E- 2 successfully made a flyby of Asteroid 4179 Toutatis and obtained plenty of high-resolution images during the mis- sion. In this paper, the modeling method and preliminary model of Asteroid Toutatis are discussed. First, the optical images obtained by Chang'E-2 are analyzed. Terrain and silhouette features in images are described. Then, the modeling method based on previous radar model and preliminary information from optical images is proposed. A preliminary polyhedron model of Asteroid Toutatis is established. Finally, the spherical harmonic coefficients of Asteroid Toutatis based on the polyhedron model are obtained. Some parameters of model are analyzed and compared. Although the model proposed in this paper is only a preliminary model, this work offers a valuable reference for future high-resolution models. 相似文献
107.
CUI Yuan-min FENG Jie-qing 《高校应用数学学报(英文版)》2014,(4):455-467
We implemented accurate FFD in terms of triangular Bezier surfaces as matrix multiplications in CUDA and rendered them via OpenGL. Experimental results show that the proposed algorithm is more efficient than the previous GPU acceleration algorithm and tessel- lation shader algorithms. 相似文献
108.
Jian Xu Qingsong Xu Lunzhao Yi Chi‐On Chan Daniel Kam‐Wah Mok 《Journal of Chemometrics》2016,30(1):37-45
High throughput data are frequently observed in contemporary chemical studies. Classification through spectral information is an important issue in chemometrics. Linear discriminant analysis (LDA) fails in the large‐p‐small‐n situation for two main reasons: (1) the sample covariance matrix is singular when p > n and (2) there is an accumulation of noise in the estimation of the class centroid in high dimensional feature space. The Independence Rule is a class of methods used to overcome these drawbacks by ignoring the correlation information between spectral variables. However, a strong correlation is an essential characteristic of spectral data. We proposed a new correlation‐assisted nearest shrunken centroid classifier (CA‐NSC) to incorporate correlation information into the classification. CA‐NSC combines two sources of information [class centroid (mean) and correlation structure (variance)] to generate the classification. We used two real data analyses and a simulation study to verify our CA‐NSC method. In addition to NSC, we also performed a comparison with the soft independent modeling of class analogy (SIMCA) approach, which uses only correlation structure information for classification. The results show that CA‐NSC consistently improves on NSC and SIMCA. The misclassification rate of CA‐NSC is reduced by almost half compared with NSC in one of the real data analyses. Generally, correlation among variables will worsen the performance of NSC, even though the discriminatory information contained in the class centroid remains unchanged. If only correlation structure information is used (as in the case of SIMCA), the result will be satisfactory only when the correlation structure alone can provide sufficient information for classification. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
109.
Cheng-Fang Wang Li Fan Mei Tian Shu-Shan Du Zhi-Wei Deng Jiang-Bin Feng 《Natural product research》2015,29(14):1380-1383
This work aimed to investigate benzophenanthridine from the roots of Zanthoxylum nitidum (Roxb.) DC. var. fastuosum How ex Huang for the first time. Thirteen benzophenanthridines were isolated, and our results of the cytotoxic activities indicated that compound 6 exhibited the best potency against A549, Hela, SMMC-7721 and EJ, with the IC50 values of 27.50, 37.50, 16.95 and 60.42 μM, respectively. Compounds 7 and 11 also showed strong cytotoxicity when tested against the four human cancer cell lines (A549, Hela, SMMC-7721 and EJ), while only compounds 12 and 13 displayed cytotoxicity in inhibiting BALL-1 proliferation among all the compounds. These results suggested that benzophenanthridines may become a valid alternative of potential basis for new anti-proliferative agents. 相似文献
110.
In this paper, for the first time, an inorganic–organic hybrid material based on ZnO nanoparticles was anchored to a composite made from polythiophene and hexagonally ordered silica (ZnO/PT/SBA-15) for use in solid-phase fibre microextraction (SPME) of medicinal plants. A homemade SPME apparatus was used for the extraction of volatile components of Ziziphora tenuior L. A simplex method was used for optimisation of five different parameters affecting the efficiency of the extraction. The main constituents extracted by ZnO/PT/SBA-15 and PDMS fibres and hydrodistillation (HD) methods, respectively, included pulegone (51.25%, 53.64% and 56.68%), limonene (6.73%, 6.58% and 8.3%), caryophyllene oxide (5.33%, 4.31% and 4.53%) and 1,8-cineole (4.21%, 3.31% and 3.18%). In comparison with the HD method, the proposed technique could equally monitor almost all the components of the sample, in an easier way, in a shorter time and requiring a much lower amount of the sample. 相似文献