全文获取类型
收费全文 | 2136篇 |
免费 | 77篇 |
国内免费 | 468篇 |
专业分类
化学 | 1512篇 |
晶体学 | 3篇 |
力学 | 149篇 |
综合类 | 3篇 |
数学 | 160篇 |
物理学 | 854篇 |
出版年
2021年 | 1篇 |
2016年 | 11篇 |
2015年 | 40篇 |
2013年 | 61篇 |
2012年 | 79篇 |
2011年 | 423篇 |
2010年 | 181篇 |
2009年 | 194篇 |
2008年 | 17篇 |
2007年 | 23篇 |
2006年 | 31篇 |
2005年 | 124篇 |
2004年 | 391篇 |
2003年 | 213篇 |
2002年 | 192篇 |
2001年 | 215篇 |
2000年 | 59篇 |
1999年 | 48篇 |
1998年 | 62篇 |
1997年 | 38篇 |
1996年 | 31篇 |
1995年 | 30篇 |
1994年 | 32篇 |
1993年 | 52篇 |
1992年 | 53篇 |
1991年 | 25篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1988年 | 3篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 2篇 |
1964年 | 1篇 |
1937年 | 2篇 |
排序方式: 共有2681条查询结果,搜索用时 15 毫秒
151.
Jiajian Gao Chunmiao Jia Jing Li Meiju Zhang Fangna Gu Guangwen Xu Ziyi Zhong Fabing Su 《天然气化学杂志》2013,(6):919-927
The correlation between phase structures and surface acidity of Al2O3 supports calcined at different temperatures and the catalytic performance of Ni/Al2O3 catalysts in the production of synthetic natural gas(SNG) via CO methanation was systematically investigated. A series of 10 wt% NiO/Al2O3 catalysts were prepared by the conventional impregnation method, and the phase structures and surface acidity of Al2O3 supports were adjusted by calcining the commercial γ-Al2O3 at different temperatures(600–1200 C). CO methanation reaction was carried out in the temperature range of 300–600 C at different weight hourly space velocities(WHSV = 30000 and 120000 mL·g-1h-1) and pressures(0.1 and 3.0 MPa). It was found that high calcination temperature not only led to the growth in Ni particle size, but also weakened the interaction between Ni nanoparticles and Al2O3 supports due to the rapid decrease of the specific surface area and acidity of Al2O3 supports. Interestingly, Ni catalysts supported on Al2O3 calcined at 1200 C(Ni/Al2O3-1200) exhibited the best catalytic activity for CO methanation under different reaction conditions. Lifetime reaction tests also indicated that Ni/Al2O3-1200 was the most active and stable catalyst compared with the other three catalysts, whose supports were calcined at lower temperatures(600, 800 and 1000 C). These findings would therefore be helpful to develop Ni/Al2O3 methanation catalyst for SNG production. 相似文献
152.
Tao Jia Weilong Zhou Fenghong Li Yajun Gao Lu Wang Jianxiong Han Jingying Zhang Yue Wang 《中国科学:化学(英文版)》2015,(2):323-330
Three alcohol/water-soluble porphyrins,Zn-TPy PMe I:zinc(II)meso-tetra(N-methyl-4-pyridyl)porphyrin tetra-iodide,ZnTPy PAd Br:zinc(II)meso-tetra[1-(1-adamantylmethyl ketone)-4-pyridyl]porphyrin tetra-bromide and Mn Cl-TPy PAd Br:manganese(III)meso-tetra[1-(1-adamantylmethyl ketone)-4-pyridyl]porphyrin tetra-bromide were employed as cathode interlayers to fabricate polymer solar cells(PSCs).The PC71BM([6,6]-phenyl C71 butyric acid methyl ester)and PCDTBT(poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)])-blend films were used as active layers in polymer solar cells(PSCs).The PSCs with alcohol/water-soluble porphyrins interlayer showed obviously higher power conversion efficiency(PCE)than those without interlayers.The highest PCE,6.86%,was achieved for the device with Mn ClTPy PAd Br as an interlayer.Ultraviolet photoemission spectroscopic(UPS),carrier mobility,atomic force microscopy(AFM)and contact angle( )characterizations demonstrated that the porphyrin molecules can result in the formation of interfacial dipole layer between active layer and cathode.The interfacial dipole layer can obviously improve the open-circuit voltage(Voc)and charge extraction,and sequentially lead to the increase of PCE. 相似文献
153.
The mechanism of dimethyl oxalate hydrogenation to ethylene glycol over Cu/SiO2 catalyst was investigated by in situ Fourier transform infrared(FTIR)spectroscopy.It was found that dimethyl oxalate and methyl glycolate proceeded via dissociative adsorption on Cu/SiO2 catalyst,and four main intermediates,CH3OC(O)(O)C-M(1655 cm-1),M-C(O)(O)C-M(1618 cm-1),HOCH2(O)C-M(1682 cm-1)and CH3O-M(2924-2926 cm-1),were identified during the reaction.It was concluded that dimethyl oxalate hydrogenation to ethylene glycol mainly proceeded along the route:dimethyl oxalate →CH3OC(O)(O)C-M→ methyl glycolate →HOCH2(O)C-M→ ethylene glycol.Finally a schematic reaction network was proposed. 相似文献
154.
HAN Tao LU XiaoChun WANG Xue RAO YongNan ZOU DeCai YANG JianFei & WU YangYang National Time Service Center Xi'an China Key Laboratory of Precision Navigation Timing Technology Chinese Academy of Sciences Graduate University of the Chinese Academy of Sciences Beijing 《中国科学:物理学 力学 天文学(英文版)》2011,(6)
With the development of Global Navigation Satellite System (GNSS),the idea of GNSS interoperability is born and has become the focus of study in the field of satellite navigation.The popularity for GNSS to augment the interoperability with the existing ones necessitates the study of the assessment algorithm of this idea.In this paper,an assessment algorithm for interoperability comprehensive benefits based on the differential equation dynamical system is discussed.There are two important aspects in GNSS tha... 相似文献
155.
Recently,various efforts have been put forward on the development of technologies for the synthesis of methane from CO2 and H2,since it can offer a solution for renewable H2 storage and transportation.In parallel,this reaction is considered to be a critical step in reclaiming oxygen within a closed cycle.Over the years,extensive fundamental research works on CO2 methanation have been investigated and reported in the literatures.In this updated review,we present a comprehensive overview of recent publications during the last 3 years.Various aspects on this reaction system are described in detail,such as thermodynamic considerations,catalyst innovations,the influence of reaction conditions,overall catalytic performance,and reaction mechanism.Finally,the future development of CO2 methanation is discussed. 相似文献
156.
Wen Qiang Du Ze Ming Rong Yan Liang Yong Wang Xin Yi Lu Yi Fan Wang Lian Hai Lu 《中国化学快报》2012,23(7):773-776
Selective hydrogenation of α, β-unsaturated aldehydes with modified Pd/C catalyst was developed.The reduction of C=O bond could be efficiently inhibited by the addition of carbonates,and high selectivity to the corresponding saturated aldehydes was achieved under mild conditions.This protocol provides an alternative for efficient preparation of saturated aldehydes. 相似文献
157.
One new sesquiterpene lactone,1 α,8 α,9 α-trihydroxyeudesman-3(4),7(11)-dien-8 β,12-olide(1) and two new phenylpropanoidsubstituted catechin glycosides,glabraoside C(2) and glabraoside D(3) were isolated from the whole plant of Sarcandra glabra. Their structures were established by the analyses of spectral and chemical evidences. 相似文献
158.
Two new β-carboline-type alkaloids,dichotomine K(1) and dichotomine L(2),were isolated from the roots of Chinese medicinal plant Stellaria dichotoma L.var.lanceolata Bge.Structures of 1 and 2 were determined on the basis of chemical and spectroscopic means. 相似文献
159.
QI GuoPeng JIANG Feng SUN XueWen & ZHAO SuoQi School of Chemical Engineering Tianjin University Tianjin China State key Laboratory of Heavy Oil Processing Petroleum University of China Beijing China 《中国科学:化学》2010,(5)
The isotope exchange method was employed to investigate the catalytic mechanism of ionic liquid in alkylation of benzenes with olefins.It is proposed that alkylation was induced by the Lewis acid AlCl3 which attracted π electrons of 1-dodecene to shift toward 1-carbon,thus forming a carbonium ion.The carbonium ion further reacted with benzenes to form a complex.Due to unstabilit of the complex,a deuterated ring proton was transferred into an electronegative 1-carbon of the side chain to substitute for the A... 相似文献
160.
A novel supramolecular graft copolymer(SGP) composed of viologen-containing copolymer(P(DMA-co-diEV)) as the main chain and Np ended PNIPAM(Np-PN1PAm) as the grafts is prepared(DMA:N,N- dimethylacryamide,diEV:ethylviologen dimer,Np:naphthalene,PNIPAM:poly(N-isopropylacrylamide)). The grafting is based on the triple complexation among a host of cucurbit[8]uril(CB[8])and two guests of diEV and Np,which is characterized by UV-vis spectra and ITC.Temperature sensitive property of PNIPAm moiety allows SGP to self-assemble into non-covalently connected micelle(NCCM) at high temperature. The micelles are sensitive to reducing agents,for example Na2S2O3,which breaks the current inclusion complex pair and induces aggregation. 相似文献