全文获取类型
收费全文 | 1309篇 |
免费 | 37篇 |
国内免费 | 2篇 |
专业分类
化学 | 989篇 |
晶体学 | 9篇 |
力学 | 56篇 |
数学 | 87篇 |
物理学 | 207篇 |
出版年
2023年 | 11篇 |
2022年 | 12篇 |
2021年 | 17篇 |
2020年 | 28篇 |
2019年 | 36篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 43篇 |
2015年 | 44篇 |
2014年 | 38篇 |
2013年 | 87篇 |
2012年 | 74篇 |
2011年 | 99篇 |
2010年 | 59篇 |
2009年 | 43篇 |
2008年 | 74篇 |
2007年 | 87篇 |
2006年 | 62篇 |
2005年 | 66篇 |
2004年 | 56篇 |
2003年 | 41篇 |
2002年 | 39篇 |
2001年 | 17篇 |
2000年 | 11篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1986年 | 5篇 |
1985年 | 17篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1960年 | 5篇 |
1959年 | 6篇 |
1958年 | 5篇 |
1957年 | 5篇 |
1953年 | 4篇 |
排序方式: 共有1348条查询结果,搜索用时 15 毫秒
991.
992.
meso-Tetrakis(3,4,5-trimethoxyphenyl)cobalt(II) porphyrin [Co(TMPP)] (1) is synthesized by a new method. The X-ray structure of 1 grown in dichloromethane shows square-pyramidal coordination around the Co(II) ion, displaying a 1D polymeric network. When grown in chloroform, 1 displays an octahedral coordination around Co(II), resulting in a 2D coordination network. This solvent-dependent variation in ligation of the O-donor atom(s) of the 4-methoxy groups of the meso-phenyl rings in the axial position(s) of the central Co(II) is reversible, as shown by electronic spectroscopy. The magnetic and electron paramagnetic resonance (EPR) data of these two crystalline forms are dependent on the nature of the axial interaction. Increased axial coordination showed increased splitting between the e and a1 orbitals, resulting in further separation between 2A1 and 2E. The EPR data are consistent with this result. The difference in energy levels in these two forms is in agreement with the magnetic and spectroscopic data. 相似文献
993.
Dedicated to Professor John C. Sheehan on the occasion of his sixty-fifth birthday. The structures of cyclochlorotine and islanditoxin, cyclic peptides from Penicillium islandicum are reviewed. The peptide A isolated in these laboratories has been identified by gc/ms studies as cyclochlorotine. It is suggested that the structure of islanditoxin [cyclo-(L-dichloroprolyl-D-β-amino-β-phenylpropionyl-L-α-amino-n-butyryl-L-seryl-L-serine)] 1 should now be revised to that of cyclochlorotine, [cyclo-(L-dichloroprolyl-D-β-amino-n-butyryl-L-seryl-L-β-amino-β-phenyl-propionyl-L-serine)] 2 . 相似文献
994.
In this paper we examine computation of optimal control u* ofthe exact controllability problem (referred to as the constraintproblem) governed by the following type of linear parabolicdifferential equations: (y/t) + Ay = u in Q y = 0 on y(0) = y0 on where A is the second-order elliptic differential operator, is a bounded domain in k with smooth boundary , Q = (0, T)x , = (0, T) x and T > 0. This is achieved by approximatingu* through a sequence {un} of controls corresponding to unconstrainedproblems involving a penalty function arising from the controllabilityconstraint. 相似文献
995.
Sadana AK Liang F Brinson B Arepalli S Farhat S Hauge RH Smalley RE Billups WE 《The journal of physical chemistry. B》2005,109(10):4416-4418
Large fullerenes and carbon-coated metal nanoparticles that are formed during the synthesis of carbon nanotubes have been functionalized by the addition of alkyl radicals and isolated by extraction into chloroform. The soluble, functionalized fullerenes have been isolated from raw single-wall carbon nanotube (SWNT) material prepared by laser oven, direct current arc, and high-pressure carbon monoxide production methods. Analyses of the extracted large fullerenes were carried out by thermogravimetric analysis, UV-vis-near-IR, laser desorption ionization mass spectrometry, and high-resolution transmission electron microscopy. 相似文献
996.
Hyun Choi Young Eun Jeong Pullur Anil Kumar Kwan-Young Lee Heon Phil Ha 《Research on Chemical Intermediates》2018,44(6):3737-3751
Manganese-based catalysts have attracted much attention due to their excellent performance for NO reduction with NH3 (NH3-SCR) at low temperatures. In the current study, the novel metal Sb was modified into Mn/TiO2 and Fe–Mn/TiO2, and the NO x conversion was compared with those of Mn/TiO2 and Fe–Mn/TiO2 catalysts to investigate the effect of the Sb. The NO x reduction activities of the catalysts were evaluated in the temperature range of 100–250 °C at a space velocity of 60,000 h?1. The physicochemical properties of all the catalysts were characterized by Brunauer–Emmett–Teller surface area, temperature-programmed desorption of ammonia, temperature-programmed reduction, X-ray photoelectron spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy. Interestingly, the Sb-promoted Mn-based catalysts showed significantly higher NO x conversion than the other catalysts with or without 6 vol% of H2O. The high performance of the Sb-modified catalysts could be related to the increase of acid sites and redox properties. 相似文献
997.
The present paper is focused toward the preparation of the flexible and free-standing blend solid polymer electrolyte films based on PEO-PVP complexed with NaPF6 by the solution cast technique. The structural/morphological features of the synthesized polymer nanocomposite films have been investigated in detail using X-ray diffraction, Fourier transform infra-red spectroscopy, Field emission scanning electron microscope, and Atomic force microscopy techniques. The film PEO-PVP?+?NaPF6 (\( \ddot{\mathrm{O}}/{\mathrm{Na}}^{+}= \)8) exhibits highest ionic conductivity ~?5.92?×?10?6 S cm?1 at 40 °C and ~?2.46?×?10?4 S cm?1 at 100 °C. The temperature-dependent conductivity shows an Arrhenius type behavior and activation energy decreases with the addition of salt. The high temperature (100 °C) conductivity monitoring is done for the optimized PEO-PVP?+?NaPF6 (\( \ddot{\mathrm{O}}/{\mathrm{Na}}^{+}= \)8) highly conductive system and the conductivity is still maintained stable up to 160 h (approx. 7 days). The thermal transitions parameters were measured by the differential scanning calorimetry (DSC) measurements. The prepared polymer electrolyte film displays the smoother surface on addition of salt and a thermal stability up to 300 °C. The ion transference number (tion) for the highest conducting sample is found to be 0.997 and evidence that the present system is ion dominating with negligible electron contribution. Both linear sweep voltammetry and cyclic voltammetry supports the use of prepared polymer electrolyte with long-term cycle stability and thermal stability for the solid-state sodium ion batteries. Finally, a two peak percolation mechanism has been proposed on the basis of experimental findings. 相似文献
998.
Anil Kuwar Kundan Tayade Karunesh Keshav Suban K. Sahoo Mayank Narinder Singh 《Supramolecular chemistry》2018,30(1):52-60
The dinuclear copper complex receptor (R) was synthesized using the ligands 4-(di-ethyl-amino)-2-hydroxybenzaldehyde and benzo [d] thiazol-2-amine. The single crystal X-ray analysis of the homoleptic bis(μ-oxo)dicopper (II) complex indicated that the ligands are coordinated in an anti-configuration to each other and have adopted a distorted square-pyramidal environment around the Cu2+ ion. The X-ray solid state structural analysis and DFT reveals no direct Cu…..Cu linkage within the complex. The anion recognition ability of the R was studied by UV–vis and fluorescence spectroscopic in acetonitrile. The results indicated that the new complex R displays a sturdy sympathy and selectivity for hydroxide ion. 相似文献
999.
Ternary complexes of Co(II), Ni(II) and Cu(II) with pyridine (Py) or 2-methyl pyridine (2-MePy) (primary ligand) and some phenoxyacetic acid herbicides, such as phenoxyacetic acid (PAA), 2-chlorophenoxyacetic acid (2-CPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) (secondary ligands) have been studied in solution using a potentiometricpH-titration technique in order to test a complexation hypothesis of plant growth regulating activities of these herbicides. From stability constant data, the species distribution have been computed and it was found that the percentage of ternary complex (MAL) is about 60% of the totalM
2+ present at highpH indicating greater stability of these ternary complexes. TheMA
+ species (M
2+-Py andM
2+-2-MePy) have shown discriminating behaviour towards the next incoming secondary ligand containing O as donor atoms. Since the stabilities of biologically active (2-CPA, 2,4-D and2,4,5-T) and inactive (PAA) phenoxyacetic acid herbicides in binary (ML
2) and ternary (MAL) metal complexes were found approximately of the same magnitude, chelation may not thus be the probable mode of action of these plant growth regulators.
Ternäre Komplexe einiger Phenoxyessigsäure-Herbizide mit Pyridin und Metallionen von biologischer Bedeutung
Zusammenfassung Ternäre Komplexe von Co(II), Ni(II) und Cu(II) mit Pyridin (Py) oder 2-Methylpyridin (2-Me Py) als Primärliganden und einigen Herbiziden vom Phenoxyessigsäure-Typ [Phenoxyessigsäure (PAA), 2-Chlor-PAA (2-CPA), 2,4-Dichlor-PAA (2,4-D) und 2,4,5-Trichlor-PAA (2,4,5-T)] als Sekundärliganden wurden im Hinblick auf eine Komplexierungshypothese der pflanzenwuchssteuernden Wirkung dieser Herbizide mittels potentiometrischerpH-Titration untersucht. Aus den Stabilitätskonstanten wurden die Anteile der einzelnen Spezien in Lösung berechnet. Dabei wurden 60% an ternärem Komplex (MAL) — bezogen auf die totaleM 2–-Menge — bei hohenpH-Werten festgestellt und damit die größere Stabilität dieser Komplexe gezeigt. DieMA +-Spezies (M 2+-Py undM 2+-MePy) zeigte ein unterschiedliches Verhalten gegenüber dem nächsten chelierenden Liganden mit O als Donor. Da die Stabilitäten der biologisch aktiven (2-CPA, 2,4-D und2,4,5,-T) und inaktiven (PAA) Liganden in binären (ML 2) und ternären (MAL) Metallkomplexen in der gleichen Größenordnung liegen, scheint die Komplexierung bei der Regulierung des Pflanzenwuchses bei diesen Herbiziden kein entscheidender Faktor zu sein.相似文献
1000.
Homogeneous anisotropic cosmological models with variable gravitational and cosmological “Constants”
The Einstein field equations with perfect fluid source and variable andG for Bianchi-type universes are studied under the assumption of a power-law time variation of the expansion factor, achieved via a suitable power-law assumption for the Hubble parameter suggested by M. S. Berman. All the models have a power-law variation of pressure and density and are singular at the epocht=0. The variation ofG(t) as 1/t and (t) as 1/t
2 is consistent with these models. 相似文献