全文获取类型
收费全文 | 609篇 |
免费 | 17篇 |
国内免费 | 5篇 |
专业分类
化学 | 361篇 |
晶体学 | 19篇 |
力学 | 35篇 |
数学 | 96篇 |
物理学 | 120篇 |
出版年
2024年 | 3篇 |
2023年 | 3篇 |
2022年 | 11篇 |
2021年 | 23篇 |
2020年 | 21篇 |
2019年 | 14篇 |
2018年 | 14篇 |
2017年 | 15篇 |
2016年 | 23篇 |
2015年 | 15篇 |
2014年 | 21篇 |
2013年 | 62篇 |
2012年 | 19篇 |
2011年 | 36篇 |
2010年 | 28篇 |
2009年 | 17篇 |
2008年 | 35篇 |
2007年 | 26篇 |
2006年 | 27篇 |
2005年 | 21篇 |
2004年 | 32篇 |
2003年 | 18篇 |
2002年 | 21篇 |
2001年 | 5篇 |
2000年 | 3篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1976年 | 7篇 |
1975年 | 6篇 |
1974年 | 2篇 |
1966年 | 2篇 |
1964年 | 2篇 |
排序方式: 共有631条查询结果,搜索用时 31 毫秒
601.
An indirect method for the determination of ca. 10?4 M amino acids is described which is based on the reaction of the latter with 2,4,6-trinitrobenzene-1-sulphonic acid at pH 11.7 and the measurement of the S2 emission in a hydrogen-nitrogen flame that is produced from the resulting compound after the addition of hydrochloric acid. The optimum conditions for the determination of amino acids were investigated. The amino acids examined were glycine, alanine, valine, methionine, serine, threonine and histidine. 相似文献
602.
Two oxide mixtures of clinker and its ferrite phase of compositions (66.5 wt.% CaO+24.5 wt.% SiO2+6.0 wt.% Al2O3+3.0 wt.% Fe2O3) and (60.4 wt.% CaO+15.4 wt.% Al2O3+24.2 wt.% Fe2O3) respectively were divided into protions and were mixed individually with 0.5, 1, 1.5 or 3 wt.% of LiF, MgF2, CaF2, CaCl2 or ZnO. Each portion of clinker was fired at 1450°C and each portion of ferrite was fired at 1350°C for 30 min. then quenched in air. Mössbauer effect and X-ray diffraction measurements were performed on each sample. The impurities doping produced small particle size. The LiF doping gave the smallest particle size and the highest blocking temperature. The ferrite with LiF exhibited two Fe3+ sites while the other used impurities gave one site only. The superparamagnetic relaxation appeared only in the spectra of ferrite with impurities, which means that the impurities in clinker have a tendency to combine with the calcium silicate phases not with C4AF. 相似文献
603.
604.
605.
606.
Edward J. Valente Jeffrey D. Zubkowski Ken S. Lee René A. J. Driessen Kees Goubitz Milco Numan Hendrik Schenk Henry J. Lee Donghoon Ko 《Journal of chemical crystallography》1997,27(4):219-222
The synthetic steroid 9α-fluoro-11β,17α,21-trihydroxy-1,4-pregnadiene-3,20-dione-21-acetate (9-fluoroprednisolone-21-acetate), formula C23H29O6F, is related to substances which are potent inducers of hepatic microsomal enzymes and anti-inflammatory agents. The structure was determined at 294 K from counter-collected data by direct methods. Crystals are tetragonal, space groupP41212,a, b=9.214(2),c=49.452(39) Å,V=4198(4), Å,D x =1.33μg/m3 forZ=8;R (onF) 0.063 for 915 independent intensities (I>2σ1). The structure shows H-bonding and packing of the mean molecular plane approximately perpendicular to the crystallographic 4-fold axis. 相似文献
607.
Manzur J Mora H Vega A Spodine E Venegas-Yazigi D Garland MT El Fallah MS Escuer A 《Inorganic chemistry》2007,46(17):6924-6932
Two new ligands, 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol (HL) and 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methyl-6-(methylthio)phenol (HSL), were synthesized and were used to prepare the trinuclear copper(II) complex {[CuSL(Cl)]2Cu}(PF6)2.H2O (1) and the corresponding binuclear complexes [Cu2(SL)2](PF6)2 (2) and [Cu2L2](PF6)2 (3). The crystal structure of 1 shows two different coordination environments: two square base pyramidal centers (Cu1 and Cu1a, related by a C2 axes), acting as ligands of a distorted square planar copper center (Cu2) by means of the sulfur atom of the SCH3 substituent and the bridging phenoxo oxygen atom of the ligand (Cu2-S = 2.294 A). Compounds 2 and 3 show two equivalent distorted square base pyramidal copper(II) centers, bridged in an axial-equatorial fashion by two phenoxo groups, thus defining an asymmetric Cu2O2 core. A long copper-sulfur distance measured in 2 (2.9261(18) A) suggests a weak bonding interaction. This interaction induces a torsion angle between the methylthio group and the phenoxo plane resulting in a dihedral angle of 41.4(5) degrees. A still larger distortion is observed in 1 with a dihedral angle of 74.0(6) degrees. DFT calculations for 1 gave a ferromagnetic exchange between first neighbors interaction, the calculated J value for this interaction being +11.7 cm-1. In addition, an antiferromagnetic exchange for 1 was obtained for the second neighbor interaction with a J value of -0.05 cm-1. The Bleaney-Bowers equation was used to fit the experimental magnetic susceptibility data for 2 and 3; the best fit was obtained with J values of +3.4 and -16.7 cm-1, respectively. DFT calculations for 2 and 3 confirm the nature and the values of the J constants obtained by the fit of the experimental data. ESR and magnetic studies on the reported compounds show a weak exchange interaction between the copper(II) centers. The low values obtained for the coupling constants can be explained in terms of a poor overlap between the magnetic orbitals, due to the axial-equatorial phenoxo bridging mode observed in these complexes. 相似文献
608.
Moumita Biswas Georgina M. Rosair Guillaume Pilet M. Salah El Fallah Joan Ribas Samiran Mitra 《Polyhedron》2007
Five new compounds formulated as [NiII(dca)2(para-ABN)2(H2O)2] (1), [CuII(dca)2(para-ABN)2(H2O)2] (2), [CuII(dca)2(para-ABN)2]n, (3), [CuII(dca)2(ortho-ABN)2]n, (4) and [CdII(dca)2(meta-ABN)2]n (5), where dca = dicyanamide and ABN = aminobenzonitrile, have been synthesized and characterized by single crystal X-ray diffraction studies and low temperature (300–2 K) magnetic measurements. The structural analyses revealed that 1 and 2 are isomorphous where dca and para-ABN both act as monodentate ligands. 3 consists of infinite double stranded chains of Cu(II) ions connected through the para-ABN bridges whereas 4 and 5 consist of infinite double stranded chains of Cu(II) and Cd(II) respectively, connected through μ1,5-dca bridges. The compounds extend their geometries to three-dimensional for 1–3 and 5 and two-dimensional for 4 through hydrogen bonding interactions. All the metal ions Ni2+, Cu2+ and Cd2+ are located on inversion centres and have distorted octahedral coordination geometries. The variable temperature magnetic susceptibility measurements show that the global feature of the χMT versus T curves for 3 and 4 is characteristic of very weak antiferromagnetic interactions and between 300 and 2 K the best fit parameters were determined as J = −2.35 and −5.1 cm−1, respectively. 相似文献
609.
The optical properties of CdxZn1−xS thin films on glass substrate prepared by spray pyrolysis method
A series of CdxZn1−xS thin films have been deposited on glass substrates using spray pyrolysis technique. The crystallinity and microstructure of CdxZn1−xS thin films have been investigated by X-ray diffraction (XRD). Based on the results of Hall measurements, the films obtained were an n-type semiconductor. The X-ray data analysis of CdxZn1−xS thin films showed that the grain size of the CdxZn1−xS increased with increase in Cd composition. It is observed that the band gap increases as the Cd composition decreases. The results also showed a blue shift of absorption edge of optical transmission spectra is increases as Zn ratio increases. The effects of Cd composition on the structural and optical properties of CdxZn1−xS thin films were related to their grain size, stress and carrier concentration. 相似文献
610.
The hysteresis behaviors of the cylindrical Ising nanowire are investigated within the effective-field theory with correlations at temperatures below, around and above the critical temperature. The hysteresis curves are obtained for different reduced temperatures both ferromagnetic and antiferromagnetic interactions between the shell and the core. We find that the hysteresis loop areas decrease when the reduced temperatures increase, and the hysteresis loops disappear at certain reduced temperatures. Moreover, for the antiferromagnetic nanowire the hysteresis loop areas disappear earlier than the ferromagnetic case as the reduced temperature increases. An unusual form of triple hysteresis behaviors is observed for the antiferromagnetic nanowire system. The thermal behaviors of the coercivities and remanent magnetizations are also investigated. The results are in good agreement with some experimental and theoretical results. 相似文献