全文获取类型
收费全文 | 269篇 |
免费 | 28篇 |
国内免费 | 34篇 |
专业分类
化学 | 185篇 |
晶体学 | 15篇 |
力学 | 9篇 |
综合类 | 11篇 |
数学 | 37篇 |
物理学 | 74篇 |
出版年
2023年 | 4篇 |
2022年 | 12篇 |
2021年 | 13篇 |
2020年 | 22篇 |
2019年 | 16篇 |
2018年 | 9篇 |
2017年 | 8篇 |
2016年 | 18篇 |
2015年 | 16篇 |
2014年 | 14篇 |
2013年 | 14篇 |
2012年 | 20篇 |
2011年 | 21篇 |
2010年 | 10篇 |
2009年 | 12篇 |
2008年 | 17篇 |
2007年 | 17篇 |
2006年 | 16篇 |
2005年 | 9篇 |
2004年 | 9篇 |
2003年 | 4篇 |
2002年 | 4篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1989年 | 3篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1983年 | 1篇 |
排序方式: 共有331条查询结果,搜索用时 890 毫秒
101.
Rapid determination of the volatile components in tobacco by ultrasound‐microwave synergistic extraction coupled to headspace solid‐phase microextraction with gas chromatography‐mass spectrometry
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Yanqin Yang Guohai Chu Guojun Zhou Jian Jiang Kailong Yuan Yuanjiang Pan Zhiyu Song Qian Xia Xinbo Lu Weiqiang Xiao 《Journal of separation science》2016,39(6):1173-1181
An ultrasound‐microwave synergistic extraction coupled to headspace solid‐phase microextraction was first employed to determine the volatile components in tobacco samples. The method combined the advantages of ultrasound, microwave, and headspace solid‐phase microextraction. The extraction, separation, and enrichment were performed in a single step, which could greatly simplify the operation and reduce the whole pretreatment time. In the developed method, several experimental parameters, such as fiber type, ultrasound power, and irradiation time, were optimized to improve sampling efficiency. Under the optimal conditions, there were 37, 36, 34, and 36 components identified in tobacco from Guizhou, Hunan, Yunnan, and Zimbabwe, respectively, including esters, heterocycles, alkanes, ketones, terpenoids, acids, phenols, and alcohols. The compound types were roughly the same while the contents were varied from different origins due to the disparity of their growing conditions, such as soil, water, and climate. In addition, the ultrasound‐microwave synergistic extraction coupled to headspace solid‐phase microextraction method was compared with the microwave‐assisted extraction coupled to headspace solid‐phase microextraction and headspace solid‐phase microextraction methods. More types of volatile components were obtained by using the ultrasound‐microwave synergistic extraction coupled to headspace solid‐phase microextraction method, moreover, the contents were high. The results indicated that the ultrasound‐microwave synergistic extraction coupled to headspace solid‐phase microextraction technique was a simple, time‐saving and highly efficient approach, which was especially suitable for analysis of the volatile components in tobacco. 相似文献
102.
Weiqiang Tan 《Journal of organometallic chemistry》2007,692(24):5395-5402
Phosphine-pyrazolyl based tripod ligands ROCH2C(CH2Pz)2(CH2PPh2) (R = H, Me, allyl; Pz = pyrazol-1-yl) were efficiently synthesized and characterized. Reactions of these ligands with [Ru(η6-p-cymene)Cl2]2 afforded complexes of the type [Ru(η6-p-cymene)Cl2](L) (6-8) in which the ligands exhibit κ1-P-coordination to the metal center. Complex [Ru(η6-p-cymene)Cl2{Ph2PCH2C(CH2OH)(CH2Pz)2}] (6) underwent chloride-dissociation in CH2Cl2/MeCN to give complex [RuCl(η6-p-cymene){κ2(P,N)-Ph2PCH2C(CH2OH)(CH2Pz)2}][Cl] (9). Complexes 6-9 demonstrated poor to moderate catalytic activity in the transfer hydrogenation of acetophenone. All these complexes were fully characterized by analytical and spectroscopic methods and their molecular structures were determined by X-ray crystallographic study. 相似文献
103.
Norcarbovir (1) and norabacavir (2), the desmethylene derivatives of anti-HIV agents carbovir and abacavir, were efficiently synthesized from a common intermediate . Their antitumor and antiviral activities were evaluated and the results indicate norabacavir showed comparable anti-HIV activity to that of abacavir. 相似文献
104.
在150℃下,仅以高锰酸钾溶液和无水乙醇为原料,通过水热反应合成前驱体γ-Mn OOH纳米棒.以γ-Mn OOH纳米棒为自牺牲模板,分别在350和600℃下煅烧90 min,制备出高纯度的β-Mn O2和α-Mn2O3纳米棒.采用X射线粉末衍射(XRD)、扫描电子显微镜(SEM)及热重分析(TGA)等对所制备的样品进行表征.结果表明,前驱物γ-Mn OOH为高纯度的纳米棒状晶体,直径约100~300 nm,长度可达数微米,且终产物β-Mn O2和α-Mn2O3均具有较高的纯度,也很好地保持了前驱物的纳米棒状结构.以二者为锰源,通过固相反应合成出尖晶石Li Mn2O4正极材料.当充放电倍率为0.5 C时,其首次放电比容量分别可达到120.4和123.9 m A·h/g,而且表现出良好的循环性能和倍率性能. 相似文献
105.
Efforts in realizing all-optical packet switching are overwhelming in the past decade. While optical packet switching remains an attractive switching paradigm in the long run, technical challenges prohibit it from becoming a practical solution for the ever-growing bandwidth hunger during the next few years. Finding a technically viable way to meet the increasing capacity requirement with good scalability and flexibility becomes a clear pursue for the community. Hybrid packet and circuit switching is considered to be one promising technique in realizing high performance switching at low cost and less energy consumption, by taking the advantage of both packet switching and circuit switching. In this paper, we review existing work in hybrid optical packet and circuit switching. We discuss the key technical challenges in realizing hybrid optical packet and circuit switching. We further introduce our ongoing efforts in building a seamlessly transformable packet/circuit-switching node with hybrid optical and electronic components. We show that in a hybrid node, the scheduling complexity with typical scheduling algorithms may be reduced to half of a node running in full packet switching mode. 相似文献
106.
We formulate and establish a super duality which connects parabolic categories O for the ortho-symplectic Lie superalgebras and classical Lie algebras of BCD types. This provides a complete and conceptual solution of the irreducible character problem for the ortho-symplectic Lie
superalgebras in a parabolic category O, which includes all finite dimensional irreducible modules, in terms of classical Kazhdan-Lusztig polynomials. 相似文献
107.
108.
A synthetic approach for synthesizing spherical aminopropyl-functionalized ethane-bridged periodic mesoporous organosilicas (APEPMOs) is reported. The mesoporous material was prepared by a one-step co-condensation of 1,2-bis(triethoxysilyl)ethane (BTSE) and 3-aminopropyltriethoxysilane (APTES) using cetyltrimethylammonium chlorine (C(18)TACl) as a template with the aid of a co-solvent (methanol) in basic medium. The APEPMOs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), powder X-ray diffraction (XRD), nitrogen sorption measurement, Fourier transform infrared spectroscopy (FT-IR) and elemental analysis. It was shown that this material exhibited spherical morphology, ordered cubic mesostructure and good mechanical strength. The APEPMOs were tested as a potential stationary phase for liquid chromatography (LC) because the column exhibited reduced back pressure. Moreover, they exhibited good chemical stability in basic mobile phase, which can be ascribed to the ethane groups in the mesoporous framework. 相似文献
109.
Lei Y Pang M Fan W Feng J Song S Dang S Zhang H 《Dalton transactions (Cambridge, England : 2003)》2011,40(1):142-145
Hydrophilic Ce, Tb doped BaYF(5) nanocrystals with uniform size were synthesized by a microwave-assisted route. The synthesized nanocrystals can be well dispersed in hydrophilic solutions (DMSO, DMF, EG, H(2)O). This synthesis procedure represents a less time consuming method, with high product yield and without using any assistant or/and template reagents, which may be expected to be a general method for rapid synthesis of other hydrophilic RE doped fluoride fluorescent nanocrystals. The Ce(3+), Tb(3+) codoped BaYF(5) nanocrystals show bright green fluorescence emission. The Ce(3+) acts as an effective energy transfer medium and the emission at the high (5)D(3) energy level of Tb is enhanced in this host material. 相似文献
110.