全文获取类型
收费全文 | 1310篇 |
免费 | 264篇 |
国内免费 | 506篇 |
专业分类
化学 | 1210篇 |
晶体学 | 142篇 |
力学 | 117篇 |
综合类 | 13篇 |
数学 | 12篇 |
物理学 | 586篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 25篇 |
2021年 | 29篇 |
2020年 | 24篇 |
2019年 | 21篇 |
2018年 | 17篇 |
2017年 | 51篇 |
2016年 | 52篇 |
2015年 | 36篇 |
2014年 | 62篇 |
2013年 | 146篇 |
2012年 | 121篇 |
2011年 | 133篇 |
2010年 | 102篇 |
2009年 | 119篇 |
2008年 | 103篇 |
2007年 | 133篇 |
2006年 | 108篇 |
2005年 | 120篇 |
2004年 | 99篇 |
2003年 | 101篇 |
2002年 | 67篇 |
2001年 | 75篇 |
2000年 | 65篇 |
1999年 | 41篇 |
1998年 | 32篇 |
1997年 | 52篇 |
1996年 | 24篇 |
1995年 | 27篇 |
1994年 | 27篇 |
1993年 | 17篇 |
1992年 | 10篇 |
1991年 | 4篇 |
1990年 | 8篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1985年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
排序方式: 共有2080条查询结果,搜索用时 15 毫秒
91.
Arumugam Vasudevan Gayathri Devi Gurusamy Rajkumar Kathiresan Sakthipandi Venkatachlam Rajendran Nallaiyan Rajendran Mani Rajkumar 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):1434-1449
Abstract Phosphate-based bioactive glasses and their glass ceramics for 47P2O5– (30.5)CaO–(22.5 ? x)Na2O–xZrO2 for different ZrO2 contents (x = 0, 1.5, 3.0, 4.5, and 6.0 mol%) were prepared through melt quenching and controlled heat treatment procedures. The amorphous nature of glasses and the presence of crystalline phases in glass ceramics were studied by means of X-ray diffraction (XRD) studies. The density, molar volume, ultrasonic velocities, attenuation, elastic constants, and microhardness of glass and glass ceramics were used to study the structural changes. The formation of hydroxyapatite layer on the surface of glasses and glass ceramics after immersion in simulated body fluid (SBF) was explored through XRD, Fourier transform infrared (FTIR), and scanning electron microscopy (SEM) analyses. The results indicate that the added ZrO2 increases the crosslink density of glasses, resulting in network stability, and also induces the formation of an apatite layer on the surface of glasses. 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
92.
93.
Antonio Calvo-López Eva Arasa-Puig Mar Puyol Joan Manel Casalta Julián Alonso-Chamarro 《Analytica chimica acta》2013
The construction and evaluation of a Low Temperature Co-fired Ceramics (LTCC)-based continuous flow potentiometric microanalyzer prototype to simultaneously monitor the presence of two ions (potassium and nitrate) in samples from the water recycling process for future manned space missions is presented. The microsystem integrates microfluidics and the detection system in a single substrate and it is smaller than a credit card. The detection system is based on two ion-selective electrodes (ISEs), which are built using all-solid state nitrate and potassium polymeric membranes, and a screen-printed Ag/AgCl reference electrode. The obtained analytical features after the optimization of the microfluidic design and hydrodynamics are a linear range from 10 to 1000 mg L−1 and from 1.9 to 155 mg L−1 and a detection limit of 9.56 mg L−1 and 0.81 mg L−1 for nitrate and potassium ions respectively. 相似文献
94.
Torsten Kerber Rachel Nathaniel Kerber Xavier Rozanska Philippe Sautet Paul Fleurat‐Lessard 《Journal of computational chemistry》2013,34(13):1155-1163
We present a new software to easily perform QM:MM and QM:QM' calculations called QMX. It follows the subtraction scheme and it is implemented in the Atomic Simulation Environment (ASE). Special attention is paid to couple molecular calculations with periodic boundaries approaches. QMX inherits the flexibility and versatility of the ASE package: any combination of methods namely force field, semiempirical, first principle, and ab initio, can be used as hybrid potential energy surface (PES). Its ease of use is demonstrated by considering the adsorption of Al2Cl3Me3 on silica surface and by combining different levels of theory (from standard DFT to MP2 calculations) for the so‐called High Level cluster with standard PW91 density functional theory calculations for the Low Level environment. It is shown that the High Level cluster must contain the silanol group close to the aluminum atoms. The bridging adsorption is favored by 58 kJ mol?1 at the MP2:PW91 level with respect to the terminal position. Using large clusters at the MP2:PW91 level, it is shown that PW91 calculations are sufficient for structure optimization but that embedded methods are required for accurate energy profiles. © 2013 Wiley Periodicals, Inc. 相似文献
95.
采用盐酸回流法和氨水沉淀法合成了氧化铝载体,并通过络合真空浸渍法制备了不同来源氧化铝负载的Pt-Sn-K/γ-Al2O3催化剂。利用N2物理吸附、CO脉冲吸附、H2程序升温还原、NH3程序升温脱附、热重等手段对Pt-Sn-K/γ-Al2O3催化剂进行了表征,以异丁烷脱氢为探针反应研究了氧化铝载体对该催化剂脱氢性能的影响。结果表明,与由盐酸回流法合成的载体制备的催化剂相比,采用氨水沉淀法合成的氧化铝载体制备的Pt-Sn-K/γ-Al2O3催化剂表面具有较小的Pt颗粒和较弱的酸性分布,其脱氢活性及异丁烯选择性更优。14 d的连续运行数据显示,由氨水沉淀合成的载体制备的催化剂,其抗积炭能力更强,反应后催化剂的积炭石墨化程度更低,因而具有更好的稳定性;在该催化剂上,异丁烷初始转化率为56.67%,14 d后仍能达到34.71%,异丁烯初始选择性为80%,7 d后维持在94%左右。 相似文献
96.
97.
98.
The influence of principal parameters (reaction temperature, ratio of acetic acid and ammonia, composition of reactionary mixture and promotion of catalysts) on the selectivity and yield of the desired product was studied in the reaction of catalytic acetonitrile synthesis by ammonolysis of acetic acid. The processing of γ-Al2O3 by phosphoric acid increases amount of the centers, on which carries out reaction of acetamide dehydration. The kinetic model of a limiting stage of reaction – the acetamide dehydration to acetonitrile was suggested. In the process of ammonolysis of acetic acid it was demonstrated that the use of catalysts promoted by phosphoric acid and ratio NH3:CH3COOH=(3-4):1 at temperatures of a reactor 360-390°С leads to the increase of acetonitrile productivity to 0.7-0.8 g/cm3·h and allows to minimize formation of by-products. 相似文献
99.
100.
报道了基于半导体激光端面抽运Nd:YAG的4F3/2–4I13/2 跃迁的弱谱线多波长激光输出. 实验对比了透明陶瓷与单晶材料的激光输出特性, 表明透明陶瓷和单晶材料荧光谱强度的略微差异, 导致了多波长输出时相同两个波长之间的激光强度比在两种材料中的差异. 基于两种耦合输出镜片, 激光阈值都在2 W左右. 在13.5 W的抽运功率下, 基于Nd:YAG透明陶瓷获得了输出功率4.05 W、强度比1 :2的1338与1356 nm双波长激光和输出功率3.65 W、强度比13 : 1的1356与1414 nm 双波长激光, 斜率效率分别达33.9% 和31.9%. 相似文献