全文获取类型
收费全文 | 584篇 |
免费 | 50篇 |
国内免费 | 8篇 |
专业分类
化学 | 535篇 |
晶体学 | 10篇 |
力学 | 12篇 |
数学 | 38篇 |
物理学 | 47篇 |
出版年
2023年 | 3篇 |
2022年 | 5篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 28篇 |
2018年 | 11篇 |
2017年 | 16篇 |
2016年 | 30篇 |
2015年 | 26篇 |
2014年 | 37篇 |
2013年 | 38篇 |
2012年 | 39篇 |
2011年 | 38篇 |
2010年 | 28篇 |
2009年 | 25篇 |
2008年 | 35篇 |
2007年 | 25篇 |
2006年 | 20篇 |
2005年 | 22篇 |
2004年 | 20篇 |
2003年 | 28篇 |
2002年 | 49篇 |
2001年 | 25篇 |
2000年 | 49篇 |
1999年 | 4篇 |
1998年 | 1篇 |
1996年 | 1篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1991年 | 1篇 |
1989年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1974年 | 1篇 |
1961年 | 2篇 |
排序方式: 共有642条查询结果,搜索用时 0 毫秒
581.
Construction of a cryogen‐free thermal desorption gas chromatographic system with off‐the‐shelf components for monitoring ambient volatile organic compounds 下载免费PDF全文
Chang‐Feng Ou‐Yang Wei‐Cheng Liao Pei‐Chieh Wang Gang‐Jei Fan Chien‐Cheng Hsiao Ming‐Tung Chuang Chih‐Chung Chang Neng‐Huei Lin Jia‐Lin Wang 《Journal of separation science》2016,39(8):1489-1499
An automated gas chromatographic system aimed at performing unattended measurements of ambient volatile organic compounds was configured and tested. By exploiting various off‐the‐shelf components, the thermal desorption unit was easily assembled and can be connected with any existing commercial gas chromatograph in the laboratory to minimize cost. The performance of the complete thermal desorption gas chromatographic system was assessed by analyzing a standard mixture containing 56 target nonmethane hydrocarbons from C2–C12 at sub‐ppb levels. Particular attention was given to the enrichment efficiency of the C2 compounds, such as ethane (b.p. = –88.6°C) and ethylene (b.p. = –104.2°C), due to their extremely high volatilities. Quality assurance was performed in terms of the linearity, precision and limits of detection of the target compounds. To further validate the system, field measurements of target compounds in ambient air were compared with those of a commercial total hydrocarbon analyzer and a carbon monoxide analyzer. Highly coherent results from the three instruments were observed during a two‐month period of synchronized measurements. Moreover, the phenomenon of opposite diurnal variations between the biogenic isoprene and anthropogenic species was exploited to help support the field applicability of the thermal desorption gas chromatographic method. 相似文献
582.
Sung‐Ho Jin Sung‐Hyun Kim Yeong‐Soon Gal 《Journal of polymer science. Part A, Polymer chemistry》2001,39(22):4025-4034
New crosslinkable polymers with a nonlinear optical (NLO) active chromophore as a pendant group were synthesized by condensation chain polymerization via palladium‐catalyzed carbon–carbon coupling reactions. The polymerization yields were almost quantitative between the diiodobenzene (DIB) and diethyldipropargyl malonate (DEDPM) or 4‐(dimethylamino)‐4′‐(6‐dipropargylacetoxypropylsulfonyl)stilbene (DASS‐6) monomers. To improve the molecular weight and mechanical properties of the NLO active polymer, we carried out the copolymerization with DIB and DASS‐6 with various feed ratios of DEDPM. The resulting polymers were soluble in organic solvents and spun‐cast onto indium tin oxide‐coated glass substrates to make thin films. The molecular structures of the resulting polymers were characterized with various instrumental methods to confirm the carbon–carbon coupling reactions between the DIB and diacetylene monomers. The absorption of the ultraviolet–visible spectrum of the resulting polymers was drastically reduced after thermal curing at 160 °C because of the crosslinking of the reactive acetylene group in the polymer backbone. The electrooptic coefficient (r33) measured at 1.3 μm ranged from 7 to 15 pm/V. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4025–4034, 2001 相似文献
583.
Kim do H Jang Y Park YD Cho K 《Langmuir : the ACS journal of surfaces and colloids》2005,21(8):3203-3206
With the aim of elucidating the surface-induced molecular ordering in regioregular poly(3-hexylthiophene) (P3HT) monolayer films, we have controlled the intermolecular interactions at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with two kinds of groups (-NH2 and -CH3). We have found that, depending on the surface properties of such modified insulator substrates, the P3HT chains in the monolayer films can adopt two different conformations (edge-on and face-on). This surprising variation in chain conformation arises because of the specific interactions of the P3HT chains with the modified insulator substrates, which can be explained in terms of the following factors: the unshared electron pairs of the SAM end groups (in the -NH2 system), the pi-H interactions between the thienyl backbone bearing pi systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs (in the -NH2 system). 相似文献
584.
Eun Yeong Nho Nargis Jamila Naeem Khan Yun Mi Dang Ga Hyun Lee Yu Min Park 《Analytical letters》2018,51(5):686-701
Recent research studies have confirmed that the dietary intake of vegetables and fruits has a positive and profound impact on human health, performance, and disease. This research was designed to determine the concentrations of essential trace elements; cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), selenium (Se), vanadium (V), and zinc (Zn), toxic metals; cadmium (Cd), mercury (Hg), and lead (Pb); and a metalloid; arsenic (As) in fruits of seven species of Cucurbitaceae family including gourd, zucchini, pumpkin, sweet pumpkin, melon, watermelon, and oriental melon commonly consumed in South Korea. The concentrations of As, Cd, Co, Cr, Cu, Pb, Se, and Zn were determined using inductively coupled plasma mass spectrometry, and Hg by a mercury analyzer. To analyze the samples after microwave digestion for the validation of analytical techniques, satisfactory values of all quality parameters of linearity, detection limits, precision, accuracy, and recovery experiments were obtained in all cases. On the whole, all analyzed elements were found in accordance to the published literature around the world and well within the critical recommended standards by CODEX and Ministry of Food and Drug Safety, Korea. The estimated daily intake values, and hazard quotients and indices of the analyzed samples were all below the World Health Organization, Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives, and European Food Safety Authority limits. 相似文献
585.
Mao‐Suan Huang Yuh‐Chyun Chiang Sheng‐Chieh Lin Hsin‐Chung Cheng Chiung‐Fang Huang Yung‐Kang Shen Yi Lin 《先进技术聚合物》2012,23(1):57-64
This study develops an improved method for generating aluminum mold inserts used in the replication of polymer‐based microfluidic chip. Since molding masters that are suitable for microfluidic chip replication must have features whose dimensions are of the order of tens to hundreds of microns, micro electrical discharge machining is employed herein to fabricate an aluminum mold insert of a microfluidic chip. The width and depth of the aluminum mold insert for the microfluidic chip are 61.50 and 49.61 µm, respectively. The surface roughness values of the microchannel and the sample reservoir in aluminum mold insert for the microfluidic chip are 53.9 and 34.3 nm, respectively. PMMA material is adopted as the molded microfluidic chip that is produced by micro‐hot embossing molding. The PMMA material can replicate the microchannel and sample reservoir very well when the aluminum mold insert is used in micro‐hot embossing molding. The results indicate that the most important parameter in the replication of molded microfluidic chip is the embossing pressure, which is also the most important parameter in determining the surface roughness of the molded microfluidic chip. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
586.
Seung Do Hong Man Yeong Ha S. Balachandar 《Journal of colloid and interface science》2009,339(1):187-195
The present study investigates the variation of static contact angle of a water droplet in equilibrium with a solid surface in the absence of a body force and the dynamic contact angles of water droplet moving on a solid surface for different characteristic energies using the molecular dynamics simulation. With increasing characteristic energy, the static contact angle in equilibrium with a solid surface in the absence of a body force decreases because the hydrophobic surface changes its characteristics to the hydrophilic surface. In order to consider the effect of moving water droplet on the dynamic contact angles, we apply the constant acceleration to an individual oxygen and hydrogen atom. In the presence of a body force, the water droplet changes its shape with larger advancing contact angle than the receding angle. The dynamic contact angles are compared with the static contact angle in order to see the effect of the presence of a body force. 相似文献
587.
Bernd E. Diebl Yang Li Mirza Cokoja Fritz E. Kühn Narayanan Radhakrishnan Stefan Zschoche Hartmut Komber Hui Yee Yeong Brigitte Voit Oskar Nuyken Phillip Hanefeld Hans‐Michael Walter 《Journal of polymer science. Part A, Polymer chemistry》2010,48(17):3775-3786
Acetonitrile ligated molybdenum (III) complexes of the structure [MoCl(NCCH3)5]2+ bearing different weakly coordinating anions [B(C6F5)4]? (WCA a), [B{C6H3(m‐CF3)2}4]? (WCA b) and [(C6F5)3B‐C3H3N2‐B(C6F5)3]? (WCA c) were applied as homogeneous catalysts of the polymerization of isobutylene. High monomer conversions were obtained in short reaction times (<30 min). The molecular weight of the resulting polyisobutylene is nearly independent of parameters such as temperature, solvent, monomer concentration, but is strongly influenced by the type of WCA and by chain transfer reactions which were observed in these systems. Highly reactive low molecular weight polyisobutylenes (Mn < 2000 g/mol) were obtained with a high content of exo double bond end groups as shown by 1H NMR analysis. Furthermore, experiments were performed to reduce the isomerization of these exo end groups into other internal double bonds by varying the polymerization parameters. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3775–3786, 2010 相似文献
588.
Ha-Jeong Kwon Ji-Seon Jeong Hee-Jung Sim Yong-Moon Lee Yeong Shik Kim Seon-Pyo Hong 《Journal of chromatography. A》2009,1216(20):4445-4450
We determined the minute amount of non-polar ginsenosides in red ginseng with a reversed-phase high-performance liquid chromatography-pulsed amperometric detection (RP-HPLC-PAD) method. Non-polar ginsenosides efficiently extracted by ethyl acetate were well separated in 40 min using a water–acetonitrile gradient eluent and detected by PAD under NaOH alkaline conditions. The ginsenoside detection limits (S/N = 3) were 0.03–0.10 ng. The coefficients of linear regression were 0.9972–0.9990. Intra- and inter-day precision (RSDs) was less than 8.34% and average recovery was 98.06–102.73%. The total amount of non-polar ginsenosides in hairy root of red ginseng was slightly higher than in the main root. 相似文献
589.
amil Ik Sema
ztürk Hoong‐Kun Fun Erbil Agar Selami amaz 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(1):95-96
The structure of the title compound, 4‐allyl‐2‐methoxy‐6‐[(4‐nitrophenyl)diazenyl]phenyl benzoate, C23H19N3O5, displays the characteristic features of azobenzene derivatives. The azobenzene moiety of the molecule has a trans configuration and in this moiety, average C—N and N=N bond lengths are 1.441 (3) and 1.241 (3) Å, respectively. 相似文献
590.
Shyamaprosad Goswami Reshmi Mukherjee Kumaresh Ghosh Ibrahim Abdul Razak S. Shanmuga Sundara Raj Hoong‐Kun Fun 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(6):716-717
In the title compound, N‐(6‐chloro‐4‐oxo‐3,4‐dihydropteridin‐2‐yl)‐2,2‐dimethylpropanamide, C11H12ClN5O2, the rings in the pterin moiety are planar. The amide carbonyl O atom is in syn‐periplanar conformation while the C—N—C—C propanamide linkage is antiperiplanar. The N—H?N and N—H?O intermolecular hydrogen bonds transform the molecules into infinite chains. 相似文献