全文获取类型
收费全文 | 1082篇 |
免费 | 26篇 |
国内免费 | 121篇 |
专业分类
化学 | 1027篇 |
晶体学 | 16篇 |
力学 | 3篇 |
综合类 | 3篇 |
数学 | 1篇 |
物理学 | 179篇 |
出版年
2023年 | 9篇 |
2022年 | 11篇 |
2021年 | 10篇 |
2020年 | 14篇 |
2019年 | 17篇 |
2018年 | 14篇 |
2017年 | 16篇 |
2016年 | 22篇 |
2015年 | 16篇 |
2014年 | 25篇 |
2013年 | 63篇 |
2012年 | 67篇 |
2011年 | 69篇 |
2010年 | 30篇 |
2009年 | 81篇 |
2008年 | 62篇 |
2007年 | 78篇 |
2006年 | 80篇 |
2005年 | 64篇 |
2004年 | 72篇 |
2003年 | 48篇 |
2002年 | 32篇 |
2001年 | 30篇 |
2000年 | 31篇 |
1999年 | 42篇 |
1998年 | 40篇 |
1997年 | 27篇 |
1996年 | 24篇 |
1995年 | 23篇 |
1994年 | 24篇 |
1993年 | 16篇 |
1992年 | 16篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1980年 | 3篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有1229条查询结果,搜索用时 296 毫秒
11.
Zhong-Le Huang Wolfgang Bensch Hubert Ebert 《Journal of solid state chemistry》2004,177(10):3245-3253
The effect of substitution of the anion Te by Se in non-stoichiometric Cr5Te8 has been investigated with respect to its crystal structure, magnetic properties, and electronic structure. The compounds Cr(1+x)Q2 (Q=Te, Se; Te:Se=7:1; (1+x)=1.234(6), 1.264(6), 1.300(7)) were synthesized at elevated temperatures followed by quenching the samples to room temperature. The crystal structures have been refined with X-ray powder diffraction data with the Rietveld method in the trigonal space group with lattice parameters a=3.8651(1)-3.8831(1) Å and c=5.9917(2)-6.0528(2) Å. The structure is related to the NiAs structure with full and deficient metal layers stacking alternatively along the c-axis. The irreversibility in the field-cooled/zero-field-cooled magnetization suggests that the substitution effects of one Te by one Se is strong enough to cause cluster-glass behavior, from ferromagnetic Cr5Te8 to cluster-glass Cr(1+x)Q2. Non-saturation magnetizations at 5.5 T and the magnetic relaxation results further support the existence of cluster-glass behavior. Accompanying SPR-KKR (spin-polarized relativistic Korringa-Kohn-Rostoker) band structure calculations strongly support the observation that the Cr(1) sites are preferentially occupied by Cr atoms and predict that these compounds are metallic. Results for the spin-resolved DOS and magnetic moments on each crystallographic sites are presented. 相似文献
12.
In the present progress report literature data concerning homogeneous catalyst systems based on vanadium and chromium are reviewed and complemented by a combined study of the magnetic properties and the polymerization activity of such systems. The results show that vanadium forms active species as V (III ) and as V (II ), but chromium only as Cr (II ); models are suggested for the polymerization process. — A comparison with heterogeneous systems (Phillips catalyst) allows the establishment of some general principles concerning the activity of the catalyst and the properties of the polymers. 相似文献
13.
In order to explore the transfer mechanism of chromium(III) in mammals, a novel complex [Cr(ASA)(en)2]Cl· 2H2O, bis(ethylenediamine‐ κ 2 N,N′)(4‐aminosalicylic acid‐ κ 2 O,O′) chromium(III) monochloride dihydrate was synthesized (4‐aminosalicylic acid=H2ASA, ethylenediamine=en). The crystal structure belongs to orthorhombic system with the space group P212121 by means of X‐ray diffraction. The characteristic for transfer of Cr3+ from the compound to the low‐molecular‐mass chelator EDTA and the iron‐binding protein apoovotransferrin (apoOTf) was followed by UV‐visible (UV‐Vis) and fluorescence spectra in 0.01 mol·L?1 Hepes at pH 7.4. The second order rate constants were calculated. Those spectra in conjunction were used to obtain more accurate information about the interaction of chromium complex with apoOTf. The experimental results indicate that Cr3+ can be transferred from the complex to apoOTf with the retention of the 4‐aminosalicylic acid acting as a synergistic anion. 相似文献
14.
On the Chemical Transport of Cr2O3 with CrI3/I2 – Experiments and Model-Calculations for Participation of CrOI2,g Gaseous chromium oxyiodides that were unknown up to now cause the migration of Cr2O3 in the temperature gradient 1 000°C→900°C when iodine (e. g. 0.1 mmol/ml) and CrI3 is added (eq. (1)). Transport agent for Cr2O3 is gaseous CrI4. With a smaller concentration of iodine (D(I2) ? 0.016 mmol/ml) and lower temperatures (e.g. T? = 850°C) the influence of H2O (from the wall of the silica ampoule) becomes more important. Under these conditions the transport of Cr2O3 is a result from the endothermic reactions (2), (3) and (4). H2,g has on the basis of the decomposition of HIg a positive difference of the solubility and H2,g should not to be considered as a transport agent. Because of the range of equilibrium-values the reaction 4 has to be taken into consideration. Estimated value of the enthalpie for CrOI2,g is fixed more precisely by thermodynamic model calculation to ΔfH°298(CrOI2,g) = ?51.4 kcal/mol. The estimated limit of error for the enthalpie of formation is smaller than ± 5 kcal/mol. Without an addition of CrI3 is in the system Cr2O3/I2 a migration of Cr2O3 not observable. 相似文献
15.
Krishna Gopal Dongol M. Cristina Melo e Silva Kouki Matsubara Taisuke Matsumoto Shuntaro Mataka Thies Thiemann 《无机化学与普通化学杂志》2003,629(6):945-947
The preparation of two η6‐estra‐1, 3, 5(10), 6‐tetraene tricarbonylchromium complexes 4 and 6 are described. In both cases only one stereoisomer can be isolated, in contrast to other estrane‐tricarbonylchromium complexes, where complexations are non‐stereoselective. X‐ray crystal structural analysis of 4 discloses that only the more sterically hindered β‐facial isomer is formed. It is assumed that the 6, 7‐olefinic moiety exerts a directive influence on the complexation. 相似文献
16.
Mateja
ikovec Mladen Franko Federico G. Cruz Sidney A. Katz 《Analytica chimica acta》1996,330(2-3):245-250
The applicability of thermal lens spectrometry (TLS) for quantification and routine determination of hexavalent chromium was investigated by using a collinear dual beam thermal lens spectrometer. In aqueous solutions the LOD of 0.1 μg l−1 was obtained for Cr(VI) by using 160 mW laser power. The performance of the technique was verified by the determination of hexavalent chromium in standard reference water samples (NIST SRM 1643a and NIST SRM 1643c) and comparing the results for Cr(VI) in CCA (Cr, Cu, As)-treated timber extracts to concentrations obtained by atomic absorption spectrometry (AAS). Good agreement between the TLS results and reported values for Cr(VI) in SRMs as well as AAS results for Cr(VI) in CCA-treated timbers confirmed that TLS is a reliable and accurate analytical technique applicable for the determination of Cr(VI) in aqueous solutions at concentration levels 0.5–100 μg l−1. 相似文献
17.
CO2氧化丙烷脱氢MoO3-V2O5/SiO2催化剂研究 总被引:4,自引:0,他引:4
采用表面改性和等体积浸渍法制备了MoO3-V2O5/SiO2复合氧化物,采用TPR、IR、TPD和微反技术表征了催化剂对CO2和C3H8的吸附性能和CO2部分氧化丙烷脱氢的反应性能.结果表明:随MoO3加入量的增加,V=O还原温度降低,晶格氧更易活化.CO2和丙烷的活化温度降低,丙烷转化率增高,丙烯选择性下降.V=O中晶格氧的活化是CO2氧化丙烷制丙烯反应的关键. 相似文献
18.
Tungsten(0) carbene complexes of the type (OC)5WC(NMeCH2CHCHCH2OH)R 2 (R=Me: 2a; R=Ph: 2b) were generated by aminolysis of (OC)5WC(OMe)R with cis-NHMeCH2CHCHCH2OH. Like their Cr-congeners 1, complexes 2 exist at room temperature as mixtures of Z- and E-isomers with regard to the C-N bond. The metallacyclic complexes (OC)4WC(η2-NMeCH2CHCHCH2OH)R (4) were obtained in good yields upon photo-decarbonylation of 2. Deprotonation/silylation of the complexes (OC)4MC(η2-NMeCH2CHCHCH2OH)Me (M=Cr: 3a; M=W: 4a) with one equivalent of nBuLi/Me3SiCl gave (OC)4MC(η2-NMeCH2CHCHCH2OSiMe3)CH3 (M=Cr: 5; M=W: 6), whereas with two equivalents of nBuLi/Me3SiCl complexes (OC)4MC(η2-NMeCH2CHCHCH2OSiMe3)CH2SiMe3 (M=Cr: 7; M=W: 8) were formed. Hydrolysis of the latter yielded selectively (OC)4MC(η2-NMeCH2CHCHCH2OH)CH2SiMe3 (M=Cr: 9; M=W: 10). The complexes 1-10 were analyzed in solution by one- and two-dimensional NMR spectroscopy (1H, 13C, 29Si, 1H/1H COSY, 1H/1H NOESY, 13C/1H HETCOR). 相似文献
19.
A superoxochromium(III) ion, CraqOO2+, acts as a catalyst for the co-oxidation of alcohols and nitrous acid with molecular oxygen according to the stoichiometry: CH3OH+HNO2+O2→CH2O + NO3− + H2O+H+. The kinetics are second order in [HNO2] and independent of the concentrations of the superoxochromium catalyst, substrate, and O2. The proposed mechanism features the disproportionation of HNO2 to NO and NO2, both of which react rapidly with CraqOO2+. The CraqOO2+/NO reaction generates another equivalent of NO2 and a mole of CraqO2+, the active oxidant. The two-electron oxidation of the alcohol by CraqO2+ produces Craq2+, which reacts rapidly with O2 to regenerate the catalyst, CraqOO2+. The NO2/CraqOO2+ reaction yields the peroxynitrato complex, CraqOONO22+, in a dead-end equilibrium process that has no effect on the catalytic reaction. The disproportionation of NO2 yields the final nitrogen-containing product, NO3−, and regenerates an equivalent of HNO2. Under a fixed set of conditions, the relative catalytic efficiency (CE) of CraqOO2+ decreases as its concentration increases owing to the competition between O2 and CraqOO2+ for the intermediate Craq2+. 相似文献
20.
The on-line incorporation of cloud point extraction (CPE) to flow injection analysis (FIA) is modified to extract and preconcentrate metal species rapidly, avoiding the formation of hydrophobic complexes, using a mixed micellar medium. Coupling the procedure with chemiluminescence (CL) detection based on the catalytic activity of metal species on the luminol-hydrogen peroxide reaction and enhancing the signal with the presence of a micellar carrier containing bromide ions produces a powerful tool for the preconcentration and determination of metal species at ng l−1 levels. As an analytical demonstration ultratrace concentrations of chromium were conveniently detected and quantified in samples with a complex matrix such as seawater and wastewater. The figures of merit for the determination of chromium were: 0.9-1.6% R.S.D. (n=5) with detection and quantification limits 0.5 and 2.0 ng l−1, respectively. The calibration graph was rectilinear from 2 to 200 ng l−1 (r=0.9996, n=6). Several other metal ions were determined in ideal situations, with analogous results. 相似文献