全文获取类型
收费全文 | 2756篇 |
免费 | 813篇 |
国内免费 | 550篇 |
专业分类
化学 | 2552篇 |
晶体学 | 85篇 |
力学 | 18篇 |
综合类 | 24篇 |
数学 | 16篇 |
物理学 | 1424篇 |
出版年
2024年 | 8篇 |
2023年 | 22篇 |
2022年 | 56篇 |
2021年 | 89篇 |
2020年 | 88篇 |
2019年 | 77篇 |
2018年 | 72篇 |
2017年 | 130篇 |
2016年 | 149篇 |
2015年 | 114篇 |
2014年 | 200篇 |
2013年 | 534篇 |
2012年 | 255篇 |
2011年 | 247篇 |
2010年 | 195篇 |
2009年 | 150篇 |
2008年 | 176篇 |
2007年 | 200篇 |
2006年 | 204篇 |
2005年 | 203篇 |
2004年 | 179篇 |
2003年 | 159篇 |
2002年 | 173篇 |
2001年 | 72篇 |
2000年 | 71篇 |
1999年 | 57篇 |
1998年 | 34篇 |
1997年 | 31篇 |
1996年 | 25篇 |
1995年 | 29篇 |
1994年 | 22篇 |
1993年 | 18篇 |
1992年 | 16篇 |
1991年 | 8篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有4119条查询结果,搜索用时 62 毫秒
991.
提出一种时域微波层析成像新技术,分别通过非线性最优化方法、正则化方法、Cole-Cole建模方法,有效地克服了时域微波成像技术在筛查早期乳腺肿瘤出现的成像问题非线性、病态特性及乳腺组织色散特性等困难.在二维的数值算例中,考虑噪声影响,仿真结果初步证实:该技术检测小尺寸乳腺肿瘤是可行的;易于发现浅层肿瘤;且反演静态电导率的精度最高. 相似文献
992.
基于PIN限幅器的等效电路模型,构建了PIN限幅器HPM效应ADS等效电路仿真模型,利用HPM注入实验和等效电路仿真相结合的方法,研究了单个微波脉冲作用下PIN限幅器的响应规律,获取了HPM作用结束后限幅器限幅持续时间与注入脉冲功率、脉宽的对应关系,并对限幅器的限幅持续过程进行了分析。仿真与实验结果表明:PIN限幅器限幅持续时间随着微波脉冲功率和脉宽的增大而变大,实验和仿真结果趋势一致,该研究使用的ADS等效电路模型可以应用于PIN限幅器的高功率微波瞬态响应特性分析研究。 相似文献
993.
Significant suppression of residual nitrogen incorporation in diamond film with a novel susceptor geometry employed in MPCVD 下载免费PDF全文
Weikang Zhao 《中国物理 B》2022,31(11):118102-118102
This work proposed to change the structure of the sample susceptor of the microwave plasma chemical vapor deposition (MPCVD) reaction chamber, that is, to introduce a small hole in the center of the susceptor to study its suppression effect on the incorporation of residual nitrogen in the MPCVD diamond film. By using COMSOL multiphysics software simulation, the plasma characteristics and the concentration of chemical reactants in the cylindrical cavity of MPCVD system were studied, including electric field intensity, electron number density, electron temperature, the concentrations of atomic hydrogen, methyl, and nitrogenous substances, etc. After introducing a small hole in the center of the molybdenum support susceptor, we found that no significant changes were found in the center area of the plasma, but the electron state in the plasma changed greatly on the surface above the susceptor. The electron number density was reduced by about 40%, while the electron temperature was reduced by about 0.02 eV, and the concentration of atomic nitrogen was decreased by about an order of magnitude. Moreover, we found that if a specific lower microwave input power is used, and a susceptor structure without the small hole is introduced, the change results similar to those in the surface area of the susceptor will be obtained, but the spatial distribution of electromagnetic field and reactant concentration will be changed. 相似文献
994.
Behzad Zeynizadeh Samal Yeghaneh 《Phosphorus, sulfur, and silicon and the related elements》2013,188(2):362-368
Structurally different epoxides were efficiently converted to the corresponding thiiranes by Dowex-50WX8–supported thiourea under solvent-free conditions. The reactions were carried out either in an oil bath or under microwave irradiation to give the thiiranes in 75–98% yields within 30 sec–120 min. 相似文献
995.
Hossein Eshghi Zinat Gordi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(2):619-623
Abstract A rapid and efficient procedure is developed for a one-pot synthesis of nitriles by condensation of aldehydes with hydroxylamine hydrochloride in the presence of P2O5/SiO2 in solvent-free media under microwave irradiation. 相似文献
996.
Henry G. Heal Arthur J. Banister 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):95-100
Abstract The inadequacy of the present IUPAC Rules and the relative merits of the binary, coordination, thiazene and thiazyl types of nomenclature are discussed. Some recommendations are made for trivial and systematic names. A reformed systematic nomenclature should be of the additive type. 相似文献
997.
H. R. Prakash Naik H. S. Bhojya Naik T. R. Ravikumar Naik D. S. Lamani T. Aravinda 《Phosphorus, sulfur, and silicon and the related elements》2013,188(2):355-360
Quinolines/benzo[h]quinolines are associated with a broad spectrum of biological activities. In view of this, 3-(1,3-dioxolan-2-yl)quinoline/benzo[h]quinoline-2-thiol/selenols were prepared under microwave irradiation through one-pot reactions, and these quinolines/ benzo[h]quinolines were evaluated for potential antibacterial activity. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. 相似文献
998.
The millimeter wave spectrum of the Ar-CHF3 complex is measured. The microwave lines are split into two components due to tunneling between three equivalent minima. Aided by ab initio calculations, the bidimensional potential energy surface of the complex is investigated and the rotational dependence of the tunneling splitting is parameterized using an IAM-like approach. The analysis of the microwave data allows us to determine the parameters involved in this rotational dependence. The value obtained are consistent with a large amplitude motion involving a 2π/3 internal rotation of the CHF3 moiety about its threefold axis of symmetry. Information about the geometry of the intermediate configuration of this large amplitude motions is obtained. 相似文献
999.
Valentina Nascimento Melo de Oliveira Franciane Gonçalves dos Santos Vanessa Pinheiro Gonçalves Ferreira Héverton Mendes Araújo Cláudia do Ó Pessoa Roberto Nicolete 《合成通讯》2013,43(19):2522-2532
A facile synthesis of 3,5-disubstituted 1,2,4-oxadiazole derivatives under focused microwave irradiation (FMWI) is reported. Arylamidoximes 1a–i and dicyclohexylcarbodiimide (DCC) were carried out in DMF under FMWI to obtain 1,2,4-oxadiazoles 2a–i in 61–81% yields. All compounds exhibited antiproliferative activities in vitro against three human cancer cell lines HCT-116, PC-3, and SNB-19. 相似文献
1000.
A. Kh. Mamleev L. N. Gunderova R. V. Galeev A. A. Shapkin M. G. Faizullin A. P. Nikitina D. V. Shornikov E. A. Kantor 《Journal of Structural Chemistry》2007,48(6):1030-1035
The microwave spectra of five isotopomers with the 13C and 18O natural abundance isotopes of the 2-methyl-1,3-dioxane molecule (22–50 GHz) were studied. Rotational transitions of a and c types with 4 ≤ J ≤ 12 were identified. The rotational constants and the substitution r s and effective r o structural parameters of the molecule were determined. Ab initio calculations on 2-methyl-1,3-dioxane were performed with molecular structure optimization. The results of quantum-chemical calculations at different levels are compared with experimental data. 相似文献