全文获取类型
收费全文 | 15511篇 |
免费 | 433篇 |
国内免费 | 13篇 |
专业分类
化学 | 7647篇 |
晶体学 | 135篇 |
力学 | 336篇 |
综合类 | 1篇 |
数学 | 3558篇 |
物理学 | 4280篇 |
出版年
2021年 | 165篇 |
2020年 | 221篇 |
2019年 | 206篇 |
2018年 | 325篇 |
2017年 | 329篇 |
2016年 | 638篇 |
2015年 | 514篇 |
2014年 | 583篇 |
2013年 | 1184篇 |
2012年 | 779篇 |
2011年 | 942篇 |
2010年 | 581篇 |
2009年 | 528篇 |
2008年 | 520篇 |
2007年 | 449篇 |
2006年 | 308篇 |
2005年 | 204篇 |
2004年 | 252篇 |
2003年 | 249篇 |
2002年 | 264篇 |
2001年 | 221篇 |
2000年 | 225篇 |
1999年 | 207篇 |
1998年 | 192篇 |
1997年 | 189篇 |
1996年 | 173篇 |
1995年 | 162篇 |
1994年 | 159篇 |
1993年 | 149篇 |
1992年 | 161篇 |
1991年 | 136篇 |
1990年 | 147篇 |
1989年 | 134篇 |
1988年 | 141篇 |
1987年 | 131篇 |
1986年 | 127篇 |
1985年 | 179篇 |
1984年 | 185篇 |
1983年 | 141篇 |
1982年 | 159篇 |
1981年 | 158篇 |
1980年 | 158篇 |
1979年 | 164篇 |
1978年 | 160篇 |
1977年 | 159篇 |
1976年 | 160篇 |
1975年 | 139篇 |
1974年 | 156篇 |
1973年 | 144篇 |
1965年 | 126篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The reduction of chromium, nickel, and manganese oxides by hydrogen, CO, CH4, and model syngas (mixtures of CO + H2 or H2 + CO + CO2) and oxidation by water vapor has been studied from the thermodynamic and chemical equilibrium point of view. Attention was
concentrated not only on the convenient conditions for reduction of the relevant oxides to metals or lower oxides at temperatures
in the range 400–1000 K, but also on the possible formation of soot, carbides, and carbonates as precursors for the carbon
monoxide and carbon dioxide formation in the steam oxidation step. Reduction of very stable Cr2O3 to metallic Cr by hydrogen or CO at temperatures of 400–1000 K is thermodynamically excluded. Reduction of nickel oxide (NiO)
and manganese oxide (Mn3O4) by hydrogen or CO at such temperatures is feasible. The oxidation of MnO and Ni by steam and simultaneous production of
hydrogen at temperatures between 400 and 1000 K is a difficult step from the thermodynamics viewpoint. Assuming the Ni—NiO
system, the formation of nickel aluminum spinel could be used to increase the equilibrium hydrogen yield, thus, enabling the
hydrogen production via looping redox process. The equilibrium hydrogen yield under the conditions of steam oxidation of the Ni—NiO system is, however,
substantially lower than that for the Fe—Fe3O4 system. The system comprising nickel ferrite seems to be unsuitable for cyclic redox processes. Under strongly reducing conditions,
at high CO concentrations/partial pressures, formation of nickel carbide (Ni3C) is thermodynamically favored. Pressurized conditions during the reduction step with CO/CO2 containing gases enhance the formation of soot and carbon-containing compounds such as carbides and/or carbonates. 相似文献
992.
Radoi A Compagnone D Batic M Klincar J Gorton L Palleschi G 《Analytical and bioanalytical chemistry》2007,387(3):1049-1058
Bulk screen-printed electrodes (bSPEs) modified with zirconium phosphate (ZrP) and Meldola blue (MB) and by electrochemical deposition of a Reineckate film
(bMBZrPRs-SPEs) have been constructed and used as NADH sensors. Cyclic voltammetric investigation of these bulk electrochemically
modified screen-printed electrodes revealed stable catalytic activity in oxidation of the reduced form of the coenzyme nicotinamide
adenine dinucleotide (NADH). Flow-injection analysis (FIA) coupled with amperometric detection confirmed the improved stability
of the bMBZrPRs-SPEs (10−4 mol L−1 NADH, %RSD = 4.2, n = 90, pH 7.0). Other conditions, for example applied working potential (+50 mV relative to Ag|AgCl), flow rate (0.30 mL min−1) and pH-dependence (range 4.0–10.0) were evaluated and optimized. A glycerol biosensor, prepared by immobilizing glycerol
dehydrogenase (GDH) on the working electrode area of a bMBZrPRs-SPE, was also assembled. The biosensor was most stable at pH 8.5 (%RSD = 5.6, n = 70, 0.25 mmol L−1 glycerol). The detection and quantification limits were 2.8 × 10−6 and 9.4 × 10−6 mol L−1, respectively, and the linear working range was between 1.0 × 10−5 and 1.0 × 10−4 mol L−1. To assess the effect of interferences, and recovery by the probe we analyzed samples taken during fermentation of chemically
defined grape juice medium and compared the results with those obtained by HPLC. 相似文献
993.
The geometries of N,N′-diphenylbenzene-1,4-diamine (DPPD), N-phenyl-N′-(1-phenylethyl)benzene-1,4-diamine (SPPD), N-(4-methylpentan-2-yl)-N′-phenylbenzene-1,4-diamine (6PPD), N-propan-2-yl-N′-phenylbenzene-1,4-diamine (IPPD), N-(2-methoxybenzyl)-N′-phenylbenzene-1,4-diamine (MBPPD), and N-phenyl-N′-(2-phenylpropan-2-yl)benzene-1,4-diamine (CPPD) as well as of their dehydrogenation products were optimized by the semiempirical
AM1 method. The results support the idea of stable NB=CX structures formation during the consecutive dehydrogenation of SPPD, 6PPD, IPPD, and MBPPD antioxidants. The biradicals formed
during the second step of dehydrogenation of substituted phenylenediamines might be important for their antioxidant effectiveness.
Dedicated to Professor Vladimír Kvasnička, DrSc., in honour of his 65th birthday 相似文献
994.
4-Amino-5-(4,6-diphenyl-2-pyrimidinyl)-3,4-dihydro-2H-1,2,4-trazole-3-thione is formed from the reaction of 4,6-diphenylpyrimidinecarboxylic
acid or its ethyl ester with thiocarbonyl hydrazide. Alkylation of the product leads to S-alkyl derivaties or 6-substituted
3-(4,6-diphenyl-2-pyriimidinyl)-7H-1,2,4-triazolo[3,4-b]-1,3,4-thiadiazine. Acetylation of 4-amino-5-(4,6-diphenyl-2-pyrimidinyl)-3,4-dihydro-2H-1,2,4-triazole-3-thione
gave under different conditions monoacetyl-, diacetyl, and triacetyl derivatives at the amino group and the N(2) atom, whereas benzoylation gave a benzoyl group at the amino group and 3-(4,6-diphenyl-2-pyrimidinyl)-6-phenyl-1,2,4-triazolo[3,4-b]-1,3,4-thiadiazole.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, 1088–1094, July, 2007. 相似文献
995.
A novel quartz device has been designed to trap arsine and selenium hydride and subsequently to volatilize the collected analyte
and atomize it for atomic-absorption spectrometric detection. The device is actually the multiple microflame quartz-tube atomizer
(multiatomizer) with inlet arm modified to serve as the trap and to accommodate the oxygen-delivery capillary used to combust
hydrogen during the trapping step. The effect of relevant experimental conditions (trap temperature during trapping and hydrogen
flow rate and trap temperature during volatilization) on collection and volatilization efficiency was investigated. Under
the optimum conditions collection and volatilization efficiency for arsenic and selenium were 50 and 70%, respectively.
相似文献
996.
The perfect matching vector and forcing and the Kekulé-vector of cata-benzenoids are defined. Two theorems are given which
set the sufficient and necessary conditions for HKZ-vector (Harary et al. J Math Chem 6:295, 1991) and Kekulé-vector in cata-benzenoids.
Additional two theorems are obtained which give sharp bounds for the modules of HKZ- and Kekulé vectors.
Dedicated to Professor Tadeusz Marek Krygowski on the happy occasion of his 70th birthday. 相似文献
997.
Nebojša Banjac Gordana Ušćumlić Nataša Valentić Dušan Mijin 《Journal of solution chemistry》2007,36(7):869-878
Absorption spectra of eight 3-substituted-5,5-diphenylhydantoins have been recorded in fourteen solvents in the range 200–400 nm.
The effect of solvent dipolarity/polarizability and solvent/solute hydrogen bonding interactions are analyzed by means of
the linear solvation energy relationship (LSER) concept proposed by Kamlet and Taft. The lipophilic activity of the investigated
hydantoins was estimated by the calculation of log 10
P values with the Advanced Chemistry Development Software. The calculated values of log 10
P were correlated with the ratio of the contributions of specific solvent interactions, and, by employing the linear dependence
thus obtained, the pharmacological activity of the studied hydantoin derivatives is discussed. 相似文献
998.
The composting of organic solid waste, the mixture of fruit and vegetable leftovers enriched with night soil, was investigated
in a closed thermally insulated reactor. It was found that 80.9 % of the original substrate biodegraded after 14 days. A mathematical
model of the column reactor was proposed where the biodegradation rate of the organic solid waste was described using simple
n-thorder kinetics. A good prediction of process performance was obtained using the proposed kinetics and experimentally obtained
reaction heat.
Presented at the 33rd International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 22–26 May
2006. 相似文献
999.
Becerril R Gómez-Lus R Goñi P López P Nerín C 《Analytical and bioanalytical chemistry》2007,388(5-6):1003-1011
The aim of this work is the optimization and application of a group of analytical and microbiological techniques in the study
of the activity of essential oils (EOs) incorporated in a new antimicrobial packaging material and the research in depth of
the interaction between the microbial cells and the individual compounds present in the active material. For this purpose
the antimicrobial activity of the active packaging containing cinnamon or oregano was evaluated against E. coli and S. aureus. The vapour phase activity and the direct contact between the antimicrobial agents themselves, or once incorporated in the
packaging material, and the microbial cells have been studied. The direct contact was studied using a broth dilution method.
The vapour phase was evaluated by using a new method which involves the use of a filter disk containing the EOs. Furthermore,
the kill time assay was used to determine the exposure time for the maximum efficiency in packaging, and transmission electron
microscopy was used to investigate the antimicrobial activity and the possible mechanism of action against E. coli and S. aureus. Finally, the compounds absorbed by cells were identified. The results showed that the techniques used provide relevant information
about the antibacterial activity of cinnamon and oregano in direct contact as well as in the vapour phase. The antimicrobial
packaging showed a fast efficiency which supports its likely application as a food packaging material. Bacteria treated with
EOs exhibit a wide range of significant abnormalities; these include formation of blebs, coagulation of cytoplasmatic constituents,
collapse of the cell structure and lack of cytoplasmatic material. Some of these observations are correlated to the ability
of some of these substances to disrupt envelop structure, especially the inner membrane. After an extraction from dead cells,
cinnamaldehyde was detected by GC-MS in E. coli exposed to the active packaging containing cinnamon. 相似文献
1000.
Simple catalytic systems for the Suzuki—Miyaura reaction of aryl chlorides under microwave conditions were tested. Microwave
irradiation facilitated the reaction course. The catalyst, base, and solvent effect were studied and two reaction systems
offered reasonable to high yields within a short time. 相似文献