全文获取类型
收费全文 | 35861篇 |
免费 | 290篇 |
国内免费 | 280篇 |
专业分类
化学 | 14176篇 |
晶体学 | 409篇 |
力学 | 1528篇 |
综合类 | 13篇 |
数学 | 10184篇 |
物理学 | 10121篇 |
出版年
2021年 | 144篇 |
2020年 | 121篇 |
2019年 | 126篇 |
2018年 | 1307篇 |
2017年 | 1603篇 |
2016年 | 959篇 |
2015年 | 746篇 |
2014年 | 684篇 |
2013年 | 1143篇 |
2012年 | 3432篇 |
2011年 | 2666篇 |
2010年 | 2023篇 |
2009年 | 1748篇 |
2008年 | 793篇 |
2007年 | 822篇 |
2006年 | 733篇 |
2005年 | 4543篇 |
2004年 | 4012篇 |
2003年 | 2408篇 |
2002年 | 596篇 |
2001年 | 395篇 |
2000年 | 195篇 |
1999年 | 273篇 |
1998年 | 208篇 |
1997年 | 152篇 |
1996年 | 139篇 |
1995年 | 126篇 |
1994年 | 130篇 |
1993年 | 109篇 |
1992年 | 255篇 |
1991年 | 225篇 |
1990年 | 223篇 |
1989年 | 198篇 |
1988年 | 172篇 |
1987年 | 146篇 |
1986年 | 96篇 |
1985年 | 127篇 |
1984年 | 121篇 |
1983年 | 113篇 |
1982年 | 110篇 |
1981年 | 97篇 |
1979年 | 119篇 |
1978年 | 109篇 |
1976年 | 168篇 |
1975年 | 115篇 |
1974年 | 116篇 |
1973年 | 108篇 |
1972年 | 91篇 |
1969年 | 91篇 |
1966年 | 99篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Syage JA 《Journal of the American Society for Mass Spectrometry》2004,15(11):1521-1533
In this paper we examine the mechanism of [M + H]+ (henceforth MH+) formation by direct photoionization. Based on comparisons of the relative abundance of M+ and MH+ ions for photoionization of a variety of compounds M as vapor in air versus in different solvents, we conclude that the mechanism is M + hnu --> M+ + e- followed by the reaction M+ + S --> MH+ + S(-H). The principal evidence for molecular radical ion formation M+ followed by hydrogen atom abstraction from protic solvent S are: (1) Nearly exclusive formation of M+ for headspace ionization of M in air, (2) significant relative abundance of MH+ in the presence of protic solvents (e.g., CH3OH, H2O, c-hexane), but not in aprotic solvents (e.g., CCl4-), (3) observation of induced equilibrium oscillations in the abundance of MH+ and M+, and (4) correlation of the ratio of MH+/M+ to reaction length in the photoionization source. Thermodynamic models are advanced that explain the qualitative dependence of the MH+/M+ equilibrium ratio on the properties of solvent S and analyte M. Though the hydrogen abstraction reaction is endothermic in most cases, it is shown that the equilibrium constant is still expected to be much greater than unity in most of the cases studied due to the very slow reverse reaction involving the very low abundant MH+ and S(-H) species. 相似文献
992.
Mojumdar S. C. Kozánková J. Chocholoušek J. Majling J. Nemecek V. 《Journal of Thermal Analysis and Calorimetry》2004,78(1):145-152
The conditions to fabricate the bulk porous specimens have been studied on account of sodium borosilicate (NBS) glasses. Glass
composition, heat treatment at phase separation and TiO2 addition have been considered in this study. Original glass samples of composition in mol%: sample A: 9.19 Na2O - 23.58 B2O3 - 67.23 SiO2, sample B: 9.29 Na2O - 3.17 TiO2 - 23.82 B2O3 - 63.72 SiO2 were prepared by melting reagent grade chemicals (Na2CO3, HBO3, SiO2 and AgNO3) in platinum crucibles at 1480°C for 1 h in air. The melts were poured onto stainless steel plates and were annealed at 500°C
for 0.5 h after cooling. Thus, obtained samples were phase separated at 700°C for 2, 15, 25 and 50 h to study their microstructure
by scanning electron microscope (SEM). Besides the direct study of the microstructure by SEM, information on glass structural
changes of samples are provided by measuring in situ changes by the optical transmittance thermal analysis. The isothermal
measurements were carried out at 700, 720 and 740°C. The temperature of phase separation, the leaching and nucleator addition
(TiO2), significantly influence the microstructure of the resulting leached product. TiO2 additive seems to suppress crystallization of cristobalite: especially at the extended above heat treatment phase separation
runs. The phase-separated domains of glasses containing above 80 moles of SiO2 are so small that it is very hard to observe them by SEM. The glass composition in our case was selected in a way to have
relatively large phase separated areas easily observed by SEM at magnification 20 000·. The influence of TiO2 is not too pronounced. It seems to suppress the cristobalite crystallization, especially of longer heating runs. The image
analysis of leached glasses shows the prevailing content of the skeletal phase in a comparison to pores. The TiO2 content diminishes the content of the skeletal phase.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
993.
The synthesis of a novel class of functionalized benzophanes in which a (2′-hydroxy)ethoxy pendant arm is attached to the phenyl ring is reported. The reported approach, utilizes simple starting materials, and skillful organization of the synthetic steps allows for simultaneous transforms of the macrocyclic ring and the pendant arm. Binding studies of these systems with Pd2+ and Cd2+ cations is described. A chromatographic stationary phase containing the benzodiazacoronand moiety was also synthesized, and found to interact specifically with isomeric nitrobenzene derivatives. 相似文献
994.
Rafa? Bogel-?ukasik 《Monatshefte für Chemie / Chemical Monthly》2007,2(3):1137-1144
Secondary chiral alcohols are very attractive intermediates in organic synthesis of pharmaceutical and the fine-chemical industries.
The processes employing ionic liquids to obtain enantiomers of secondary alcohols have become sustainable. Furthermore, physico-chemical
properties of ionic liquids opened new possibilities to design “solvent free” processes for the resolution of racemic secondary
alcohols. This review is aimed to highlight some of the most important achievements in resolution of secondary alcohols. 相似文献
995.
Organic and inorganic anions are analyzed using capillary zone electrophoresis with indirect fluorescence detection. As electrolyte 2,5-dihydroxybenzoic acid is used. By adding Pb2+ to the electrolyte the migration time of SO4(2-) can be selectively influenced and the separation of Cl- and SO4(2-) considerably improved. The fluorescence intensity of 2,5-dihydroxybenzoic acid increases in the presence of Pb2+ up to a concentration of 6*10(-4) mol/L Pb2+. Limits of detection in the lower mumol/L range were obtained. 相似文献
996.
Arantzazu Zabala?Ruiz Dominik?Brühwiler Takayuki?Ban Gion?CalzaferriEmail author 《Monatshefte für Chemie / Chemical Monthly》2005,136(1):77-89
Summary. A convenient synthesis of zeolite L is presented. The size of the crystals can be tuned between 30 and 6000nm, spanning a volume range of seven orders of magnitude. The zeolite L crystals, which typically feature a cylindrical morphology, are synthesized with various aspect ratios ranging from elongated to disc-shaped. The importance of obtaining zeolite crystals with well-defined size and morphology is discussed in view of potential applications of zeolite L containing organic dye molecules as guests.This revised version was published online in February 2005. In the previous version the issue was not marked as a special issue, and the issue title and the editor was missing 相似文献
997.
Pinto FG Andrada D Magalhães CG Nunes BR de Amorim FR Franco MB Saint'pierre TD da Silva JB Curtius AJ 《Analytical and bioanalytical chemistry》2005,383(5):825-832
Different chemical modifiers for use with electrothermal atomic absorption spectrometry (ET AAS) were investigated in relation
to determining the selenium in human urine samples. The samples were diluted in a solution containing 1% v/v HNO3 and 0.02% m/v cetyltrimethylammonium chloride (CTAC). Studying the modifiers showed that the use of either Ru or Ir as the
permanent modifier gave low sensitivity to Se and the peak shape was very noisy, while Zr or Rh gave no peak at all. The same
occurred when Zr was used in solution. For mixtures of permanent modifiers, Ir plus Rh or Zr plus Rh gave very low sensitivity,
Zr plus Rh with co-injection of Ir in solution was also not efficient, Zr plus Rh in solution gave good sensitivity, but the
best results were obtained with a mixture of Zr and Rh as the permanent modifier and co-injection of Rh in solution. Using
this last modifier, the following dilutions with the HNO3 and CTAC were studied: 1:1, 1:2, 1:3 and 1:4. The best dilution was 1:1, which promoted good sensitivity and a more defined
peak shape and made it possible to correct for the background using a deuterium arc lamp. Under these conditions, a characteristic
mass of 26±0.2 pg was obtained for Se in aqueous solution. Six certified urine samples were analyzed using matrix matching
calibration and the measured concentrations were in agreement with the certified values, according to a t-test at the 95%
confidence level. Recovery tests were carried out and the recoveries were in the range 100–103%, with relative standard deviation
better than 9%. The limit of detection (LOD, 3 sd, n=10) was 3.0 μg L−1 in the sample. The treated graphite tube could be used for at least 600 atomization cycles without significant alteration
of the analytical signal. 相似文献
998.
Bernaus A Gaona X Ivask A Kahru A Valiente M 《Analytical and bioanalytical chemistry》2005,382(7):1541-1548
The present work studies the adsorption behaviour of mercury species on different soil components (montmorillonite, kaolinite and humic acid) spiked with CH3HgCl and CH3HgOH at different pH values, by using XAS techniques and bacterial mercury sensors in order to evaluate the availability of methyl mercury on soil components. The study details and discusses different aspects of the adsorption process, including sample preparation (with analysis of adsorbed methyl mercury by ICP-OES), the various adsorption conditions, and the characterization of spiked samples by XAS techniques performed at two synchrotron facilities (ESRF in Grenoble, France and HASYLAB in Hamburg, Germany), as well as bioavailability studies using mercury-specific sensor bacteria. Results show that XAS is a valuable qualitative technique that can be used to identify the bonding character of the Hg in mercury environment. The amount of methyl in mercury adsorbed to montmorillonite was pH-dependent while for all soil components studied, the bond character was not affected by pH. On the other hand, clays exhibited more ionic bonding character than humic acids did with methyl mercury. This interaction has a higher covalent character and so it is more stable for CH3HgOH than for CH3HgCl, due to the higher reactivity of the hydroxyl group arising from the possible formation of hydrogen bonds.The bioavailability of methyl mercury adsorbed to montmorillonite, kaolinite and humic acids was measured using recombinant luminescent sensor bacterium Escherichia coli MC1061 (pmerBRBSluc). In case of contact exposure (suspension assays), the results showed that the bioavailability was higher than it was for exposure to particle-free extracts prepared from these suspensions. The highest bioavailability of methyl mercury was found in suspensions of montmorillonite (about 50% of the total amount), while the bioavailabilities of kaolinite and humic acids were five times lower (about 10%). The behaviour of methyl mercury in the presence of montmorillonite could be explained by the more ionic bonding character of this system, in contrast to the more covalent bonding character observed for humic acids. Thus, XAS techniques seem to provide promising tools for investigating the mechanisms behind the observed bioavailabilities of metals in various environmental matrices, an important topic in environmental toxicology. 相似文献
999.
Zusammenfassung Es werden zwei neue Wege zur herstellung des 4,4-Äthinylendiphenols beschrieben. Der erste beruht auf der Entmethylierung des Bis-(4-methoxyphenyl)acetylens, der zweite auf der Oxydation des 4,4-Dihydroxybenzildihydrazons nach derCurtiusschen Methode.
Two new syntheses of 4.4-ethynylene diphenol
Two new syntheses of 4.4-ethynylene diphenol are described: demethylation of bis-(4-methoxyphenyl)-acetylene, and oxidation of 4.4-dihydroxybenzildihydrazone according toCurtius.相似文献
1000.
Summary The equilibrium mechanical behaviour of weak diepoxide-monoepoxide-diamine networks, prepared with an excess of diamine and measured in the rubbery state, was compared with theoretical predictions obtained by using the theory of branching processes. The experimental equilibrium moduli fit well the shape of theoretical curves over a broad range of crosslinking density regardless of whether the contribution by trapped entanglements is considered or not. The data fit equally well the theoretical dependence for the front factor A = 1 without entanglement contribution and forA = (f
e
- 2)/f
e
, (f
e
is the average effective functionality of a junction) with an entanglement contribution based on the contact probability between any two units within elastically active network chains (Langley).Dedicated to Prof. Dr.G. Rehage on the occasion of his 60th birthday.With 5 figures 相似文献