全文获取类型
收费全文 | 912篇 |
免费 | 15篇 |
国内免费 | 4篇 |
专业分类
化学 | 528篇 |
晶体学 | 7篇 |
力学 | 15篇 |
数学 | 136篇 |
物理学 | 245篇 |
出版年
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 6篇 |
2017年 | 12篇 |
2016年 | 18篇 |
2015年 | 15篇 |
2014年 | 8篇 |
2013年 | 91篇 |
2012年 | 34篇 |
2011年 | 52篇 |
2010年 | 20篇 |
2009年 | 15篇 |
2008年 | 36篇 |
2007年 | 51篇 |
2006年 | 41篇 |
2005年 | 47篇 |
2004年 | 36篇 |
2003年 | 34篇 |
2002年 | 31篇 |
2001年 | 25篇 |
2000年 | 19篇 |
1999年 | 20篇 |
1998年 | 14篇 |
1997年 | 14篇 |
1996年 | 21篇 |
1995年 | 18篇 |
1994年 | 16篇 |
1993年 | 16篇 |
1992年 | 22篇 |
1991年 | 17篇 |
1990年 | 12篇 |
1989年 | 4篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1972年 | 6篇 |
1966年 | 3篇 |
1932年 | 3篇 |
1930年 | 6篇 |
1911年 | 3篇 |
排序方式: 共有931条查询结果,搜索用时 859 毫秒
31.
The purpose of the work was to characterize the thermomechanical behaviour of two well-known and widely-used thermoplastic polymers. Both dynamic and constant load measurements were made for high-density polyethylene (HDPE), low-density polyethylene (LDPE) and amorphous polystyrene (PS). The results of these measurements yield valuable information which can aid in both design applications and material processing.
Zusammenfassung Die Thermomechanische Analyse (TMA) wurde zur Untersuchung der thermoplastischen Polymere HDPE, LDPE und PS angewandt. Die Messungen an HDPE und LDPE wurden dynamisch im Temperaturbereich von –90C bis ca. +140C durchgeführt. PS wurde von –190C bis ca. + 130C statisch gemessen. Die Ergebnisse dieser Messungen zeigen deutlich die hervorragende Eignung der TMA bei der Bestimmung von Glasübergangs- und Schmelztemperaturen, der Charakterisierung der elastischen Eigenschaften sowie der Ausdehnungskoeffizienten. Zur Charakterisierung können weitere thermoanalytische Methoden (z. B. TG, DSC) herangezogen werden, wodurch zusÄtzliche Informationen erhalten und die TMA-Ergebnisse erhÄrtet werden können.相似文献
32.
Impact noise and the equal energy hypothesis 总被引:2,自引:0,他引:2
M Roberto R P Hamernik R J Salvi D Henderson R Milone 《The Journal of the Acoustical Society of America》1985,77(4):1514-1520
The equal energy hypothesis (EEH) was evaluated over a limited range of conditions by exposing four groups of chinchillas to impact noise (200-ms B duration) presented at a fixed rate of four impacts per second. The intensity of the impacts (107-125 dB peak SPL) and the duration (120-1.87 h) of the four exposure conditions were counterbalanced so that the four groups received the same total energy. The traumatic power of the exposures was assessed by measuring the threshold shift of the auditory evoked response and the amount of hair cell loss. Exposure between 107 and 119 dB were consistent with the EEH in that they produced roughly the same amount of permanent threshold shift (less than 20 dB) and hair cell loss (less than 20%). However, the 125-dB exposure produced substantially more threshold shift and hair cell loss than the three lower intensities. Thus, the EEH may be applicable only at lower impact intensities; above a "critical intensity" the amount of damage increases significantly. 相似文献
33.
34.
The silver(I) oxide mediated reactions of the gold(III) dichloride complex [{C6H3(CH2
uCl2] 2a with thiosalicylic or salicylic acid gives the respective complexes [{C6H3(CH2
)-2}] 3a (X=S) or 6b (X=O), containing chelating thiosalicylate or salicylate dianion ligands. X-ray studies show that for the thiosalicylate system, the thiosalicylate sulfur atom is trans to the N,N-dimethylamino group, whereas in the structure of the salicylate complex, it is the carboxylate group that is trans to NMe2. Both complexes show puckered metallacycles in the solid state. Electrospray mass spectrometry (ESMS) shows strong [M+H]+ and [2M+H]+ ions for both the gold-thiosalicylate and -salicylate complexes, and these ions possess a high stability towards cone voltage-induced fragmentation. ESMS was also used to identify a minor impurity, the bis(cyclo-aurated) cationic complex [A
Me2)-2-(OMe)-5}2]+ in the starting dihalide complex 2a and in the product 3a. This complex can be formed by reaction of Me4N[AuCl4] with 2 equivalents of the organomercury precursor [Hg{C6H3(CH2NMe2)-2-(OMe)-5}Cl]. The biological (antitumour, antimicrobial and antiviral) activities are also reported, and these reveal the complexes have moderate to high anti-tumour, antibacterial and antifungal activity. 相似文献
Full-size image
Full-size image
Full-size image
Full-size image
35.
Marita Wasner Wolfgang Pfleiderer Earl E. Henderson Robert J. Suhadolnik 《Helvetica chimica acta》1994,77(7):1757-1767
The antivirally active 3′-deoxyadenylyl-(2′–5′)-3′-deoxyadenylyl-(2′–5′)-3′-deoxyadenosine (cordycepin trimer core) was modified at the 2′- or 5′-terminus, by attachment of cholesterol via a carbonate bond (→ 15 ) or a succinate linker (→ 16 and 27 ) to improve cell permeability. The corresponding monomeric conjugates 4 , 7 , and 21 of cordycepin were prepared as model substances to study the applicability of the anticipated protecting groups – the monomethoxytrityl (MeOTr), the (tert-butyl)dimethylsilyl (tbds), and the β -eliminating 2-(4-nitrophenyl)ethyl (npe) and 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) groups – for the final deblocking steps without harming the ester bonds of the conjugate trimers. The syntheses were performed in solution using phosphoramidite chemistry. The fully protected trimer conjugates 13 , 14 , and 26 as well as all intermediates were characterized by elemental analyses, UV and 1H-NMR spectra. The deblocked conjugates 15 , 16 , and 27 were pure according to HPLC and showed the correct compositions by mass spectra. Comparative biological studies indicated that cordycepincholesterol conjugate trimers 16 and 27 were 333- and 1000-fold, respectively, more potent inhibitors of HIV-1-induced syncytia formation than cordycepin trimer core. 相似文献
36.
David Kessel Kathryn Woodburn† Barbara W. Henderson C. K. Chang 《Photochemistry and photobiology》1995,62(5):875-881
Abstract— Localization and photodynamic efficacy of a monocationic porphyrin (MCP) were assessed using murine leukemia cells in culture. This sensitizer localized at surface membrane loci and catalyzed selective photodamage to membrane structures. Although both cationic and hydrophobic, this porphyrin was not recognized by the multidrug transporter, which excludes many cationic agents from cells that express multidrug resistance. Photodynamic studies with the murine radiation-induced fibrosarcoma tumor model indicated moderate photosensitization of neoplastic lesions in vivo at 3 h, but not at 24 h after sensitizer administration. Pharmacokinetic studies indicate that plasma levels, not tissue levels were the major determinant of photodynamic therapy (PDT) response. Consistent with this observation, vascular damage and disturbances of tissue perfusion followed PDT. These effects were more pronounced in tumor-bearing skin than in normal skin. The therapeutic response to MCP appeared to be related mainly to secondary, probably vascular, effects. 相似文献
37.
PHOTOSENSITIZATION OF MURINE TUMOR, VASCULATURE and SKIN BY 5-AMINOLEVULINIC ACID-INDUCED PORPHYRIN 总被引:2,自引:0,他引:2
Barbara W. Henderson Lurink Vaughan David A. Bellnier Henricus van Leengoed Patricia G. Johnson Allan R. Oseroff 《Photochemistry and photobiology》1995,62(4):780-789
Abstract— The effects of topical and systemic administration of 5-aminolevulinic acid (ALA) were examined in several murine tumor systems with regard to porphyrin accumulation kinetics in tumor, skin and blood, vascular and tumor cell photosensitization and tumor response after light exposure. Marked, transient increases in porphyrin levels were observed in tumor and skin after systemic and topical ALA. Rapid, transient, dose-dependent porphyrin increases were also observed in blood; these were pronounced after systemic ALA injection and mild after topical application. They were highest within 1 h after ALA injection, thereafter declining rapidly. This matched the clearing kinetics of injected exogenous protoporphyrin IX (PpIX). Initially, vascular photosensitivity changed inversely to blood porphyrin levels, increasing gradually up to 5 h post-ALA, as porphyrin was clearing from the bloodstream. This pattern was again matched by injected, exogenous PpIX. After therapeutic tumor treatment vascular disruption of the tumor bed, while observed, was incomplete, especially at the tumor base. Minimal direct tumor cell kill was found at low photodynamic therapy (PDT) doses (250 mg/kg ALA, 135 J/cm2 light). Significant, but limited (<1 log) direct photodynamic tumor cell kill was obtained when the PDT dose was raised to 500 mg/kg systemic ALA, followed 3 h later by 270 J/cm2 , a dose that was however toxic to the animals. The further reduction of clonogenic tumor cells over 24 h following treatment was moderate and probably limited by the incomplete disruption of the vasculature. Tumor responses were highest when light treatment was carried out at the time of highest tumor porphyrin content rather than at the time of highest vascular photosensitivity. Tumor destruction did not reach the tumor base, regardless of treatment conditions. 相似文献
38.
Efstathia G SakellariouAntonio Garrido Montalban Scott L BeallDavid Henderson Hubert G MeunierDavid Phillips Klaus SuhlingAnthony G.M Barrett Brian M Hoffman 《Tetrahedron》2003,59(46):9083-9090
Co-macrocyclizations of 2,3-dipropylmaleonitrile and 2,3-di-(4-(methoxycarbonyl)phenyl)maleonitrile, respectively, with N,N′-dibenzyl-N,N′-di-(11-tetrahydropyranyloxy-3,6,9-trioxo-undecyl))maleonitrile and N,N,N′,N′-tetramethylmaleonitrile were used to prepare derivatives of the 4,5-diamino-porphyrazine systems including the zinc(II) complexes. Subsequent oxidation of the macrocycles with potassium permanganate gave the corresponding seco-porphyrazines. These were shown to be efficient sensitizers for the production of singlet oxygen (ΦΔ=0.15-0.57) by the determination of their photophysical properties. 相似文献
39.
Lin P Clegg W Harrington RW Henderson RA 《Dalton transactions (Cambridge, England : 2003)》2005,(14):2388-2394
New Mn(II) complexes containing 5-(2-pyridyl)tetrazole, 5-(3-cyano-4-pyridyl)tetrazole or 5-(4-pyridyl)tetrazole ligands are described. The complexes are prepared by reaction of the corresponding cyanopyridines with sodium azide in the presence of Mn(II) salts. All the complexes have been characterized by X-ray crystallography, which reveals that 5-(pyridyl)tetrazole ligands can coordinate to Mn through either type of nitrogen atom in the tetrazole residue or via the pyridyl group. In the solid state, extended 2D and 3D structures are produced through networks of hydrogen bonding (involving water molecules and the tetrazolate residue). Acidification of the complexes produces the corresponding free 5-(pyridyl)-1H-tetrazole. 相似文献
40.
Marita Wasner Wolfgang Pfleiderer Robert J. Suhadolnik Susan E. Horvath Ning Kon Ming-Xu Guan Earl E. Henderson Robert J. Suhadolnik Earl E. Henderson Martin E. Adelson 《Helvetica chimica acta》1996,79(3):609-618
Monomeric 3′-deoxyadenosine (cordycepin) was modified at the 2′-O- ( 13–18 ) and 5′-O-position ( 25–29 ) by the vitamins E, D2, and A and by the two lipids 1,2-di-O-palmitoylglycerol and 1,2-di-O-hexadecylglycerol via succinate or carbonate linkages. The base-labile conjugates afforded protection groups like the 2-(4-nitro-phenyl)ethoxycarbonyl (npeoc) and monomethoxytrityl group (MeOTr) that are cleavable without harming the ester and carbonate bonds, respectively. Monomeric conjugates of cordycepin and vitamin E, vitamin D2, 1,2-di-O-palmitoylglycerol, and 1,2-di-O-hexadecylglycerol (see 13, 14, 17, 18, 25, 26, 28 , and 29 ) inhibited HIV-1-induced syncytia formation 1.7 to 6.2 fold compared to 1.5-fold for cordycepin (see Table); IC50 values for 25 and 28 were 257 and 267 m?M , respectively. In addition, the monomeric cordycepin-vitamin and -lipid conjugates inhibited HIV-1 RT activity 28–49% which compares with a 13% inhibition of HIV-1 RT observed for cordycepin. The minimal inhibition of HIV-1-induced syncytia formation and HIV-1 RT activity did not proceed by the activation of RNase L. The monomeric conjugates tested ( 13, 14 ) increased PKR expression. 相似文献