全文获取类型
收费全文 | 5095篇 |
免费 | 846篇 |
国内免费 | 591篇 |
专业分类
化学 | 3586篇 |
晶体学 | 59篇 |
力学 | 307篇 |
综合类 | 35篇 |
数学 | 587篇 |
物理学 | 1958篇 |
出版年
2024年 | 4篇 |
2023年 | 110篇 |
2022年 | 86篇 |
2021年 | 142篇 |
2020年 | 197篇 |
2019年 | 145篇 |
2018年 | 157篇 |
2017年 | 140篇 |
2016年 | 227篇 |
2015年 | 228篇 |
2014年 | 300篇 |
2013年 | 339篇 |
2012年 | 431篇 |
2011年 | 476篇 |
2010年 | 358篇 |
2009年 | 322篇 |
2008年 | 328篇 |
2007年 | 324篇 |
2006年 | 293篇 |
2005年 | 249篇 |
2004年 | 197篇 |
2003年 | 136篇 |
2002年 | 159篇 |
2001年 | 127篇 |
2000年 | 108篇 |
1999年 | 124篇 |
1998年 | 102篇 |
1997年 | 94篇 |
1996年 | 91篇 |
1995年 | 67篇 |
1994年 | 82篇 |
1993年 | 55篇 |
1992年 | 57篇 |
1991年 | 56篇 |
1990年 | 37篇 |
1989年 | 52篇 |
1988年 | 36篇 |
1987年 | 34篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1981年 | 8篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有6532条查询结果,搜索用时 15 毫秒
991.
阵列像素器件投影显示系统中自动对焦评价函数研究 总被引:13,自引:2,他引:11
数字投影显示已成为现代数字显示领域的主流,阵列像素器件投影成像的的精确会聚,直接决定了显示的效果。阵列像素器件的自动对焦,是实现生产自动化的关键技术。以CCD(电荷耦合器件)采集阵列像素器件的局部投影图像为基础,试验和分析了梯度算子、拉普拉斯算子、Sobel算子、Prewitt算子、方差函数和熵函数评价方法在阵列像素器件大屏幕投影显示系统中自动对焦的作用及性能。实验表明,方差算子具有良好的稳定性和对噪声的不敏感性,调节效果较好,可以作为这一特殊系统中的评价方法。 相似文献
992.
电磁力作用下发电机定子端部绕组的两自由度主共振 总被引:2,自引:1,他引:1
对压板松动时大型汽轮发电机定子端部单根绕组的两自由度主共振问题进行了研究.在给出定子端部绕组区域磁感应强度表达式、绕组所受电磁力以及与松动压板间摩擦力计算式的基础上,建立了研究绕组非线性电磁振动的力学分析模型.采用多尺度法对两自由度主、内共振问题进行求解,得到了稳态运动下的幅频响应方程和解的稳定性判定条件.通过算例,得到了反应系统跳跃现象和软硬特性的幅频响应曲线图,以及响应图、相图、Poincare映射图和频谱图,并阐述了系统可能存在的周期运动和锁模现象。 相似文献
993.
消化-石油醚提取氢化物-原子荧光光谱法测定食用油中砷 总被引:5,自引:0,他引:5
采用消化 -石油醚提取方法处理食用调味油样品 ,以氢化物 -原子荧光光谱法测定砷 ,研究了样品的处理方法、线性关系及检出限 ,建立了快速测定食用调味油中砷含量的原子荧光光谱法。相对标准偏差小于1.79% ,加标回收率 96 .7%— 10 3.8%。 相似文献
994.
995.
Porous stainless steel (PSS) supported TiO2 membrane was synthesized from colloidal TiO2 sol by the sol–gel technique. Morphology and phase structure of the obtained membranes were regulated through optimizing the synthesis parameters including organic binders, aging periods of the parent sol and concentrations of the casting solutions as well as the sintering temperatures. Polyvinyl alcohol (PVA) 1750 was found to be feasible to fabricate TiO2 membrane with relatively flat surface and homogeneous morphology without crack. The aging period of the parent sol, which was revealed to be very important to the morphology of the particles deposited in the membranes on PSS, was decided to be 24 h. The concentration, under which the membranes could avoid macro-pores and have a uniform thickness of approximately 8 μm, was regulated to 0.0036 mol/l. Besides, a homogeneous microstructure with grain sizes of 0.08–0.2 μm was obtained in the membrane with a pure rutile phase when sintered at 850 °C. The obtained PSS supported TiO2 membrane with homogeneous microstructure and rutile phase may be very promising for practical applications. 相似文献
996.
In order to predict the extraction ability of 12-crown-4 for different metallic ions, the complexes [M(12-crown-4)] and [M(H2O)4] (where M=Li+, Na+, K+, Be2+, Mg2+, Ca2+, Cu2+ and Zn2+) were investigated by the density functional theory without restrictions for their geometry. The metal binding capability
was evaluated using the binding energy, and the effect of nature of the metal on the binding properties was also studied.
The results of the calculations showed that the coordination ability of a donor molecule towards different metal ions increased
in proportion to their ionization potential. In addition, based on the extraction distribution coefficient, we found that
12-crown-4 can selectively extract Cu2+ and Be2+ ions from aqueous solutions of mixed cations. Obviously, the stability of complexes and the extraction power of extractants
depend greatly on the nature of the metal ions. Calculation results from our study could be used to predict the extraction
power of this crown ether and could play a guiding role in planning experiments. 相似文献
997.
Jian‐Ding Qiu San‐Hua Luo Jian‐Hua Huang Ru‐Ping Liang 《Journal of computational chemistry》2009,30(8):1344-1350
The prediction of secondary structure is a fundamental and important component in the analytical study of protein structure and functions. How to improve the predictive accuracy of protein structural classification by effectively incorporating the sequence‐order effects is an important and challenging problem. In this study, a new method, in which the support vector machine combines with discrete wavelet transform, is developed to predict the protein structural classes. Its performance is assessed by cross‐validation tests. The predicted results show that the proposed approach can remarkably improve the success rates, and might become a useful tool for predicting the other attributes of proteins as well. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009 相似文献
998.
Ru Qiao Dr. Xiao Li Zhang Dr. Ri Qiu Ju Chang Kim Prof. Young Soo Kang Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(8):1886-1892
Co(OH) 2 that blooms : Flowerlike hollow spherical architectures of cobalt hydroxide obtained through a facile solvothermal process are reported for the first time (see picture). Their growth process involves aggregation of cobalt hydroxide nanoparticles into solid spheres and a hollowing effect through subsequent dissolution‐diffusion‐redeposition of the smaller crystallites under the surface layer.
999.
Qing Sheng Zhao Xiao Jie Cheng Qiu Xia Ji Chuan Zhen Kang Xi Guang Chen 《Journal of Sol-Gel Science and Technology》2009,50(1):111-118
The influence of organic and inorganic acids on chitosan/glycerophosphate (CS/GP) hydrogel has been investigated by dissolving
chitosan in different acids. The results of gelation showed that all of the chitosan dissolved in monovalent acid solutions
(i.e., formic, acetic, propionic, butyric, isobutyric, lactic, nitric, hydrochloric, and chloroacetic acid), when neutralized
by GP solution, could transform into hydrogel after 2–5 min at 37 °C, while those dissolved in multivalent acids failed in
gel formation. The inner structures of CS/GP hydrogels prepared with monovalent acids depended on the ionic strength and chain
length of acids. Morphological examination by scanning electron microphotography demonstrated that large pores occurred during
the gel-forming process, and the aperture size was also related to different acids. The cytotoxicity studies indicated that
CS/GP systems prepared by dissolving chitosan in tested acids except chloroacetic acid were nontoxic to mouse embryonic fibroblasts
and Hela cells. 相似文献
1000.
Shuifa Qiu Yunyang Wei Prof. Dr. Guosheng Liu Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(12):2751-2754
Taking the air! A PdII‐catalyzed intramolecular hydroamination of allenes coupled to alcohol oxidation has been developed. This reaction is performed by using a nitrogen‐based ligand under aerobic conditions, under which the molecular oxygen is used as the terminal oxidant for the reoxidation of Pd0 species to complete the catalytic cycle.