全文获取类型
收费全文 | 442篇 |
免费 | 4篇 |
国内免费 | 1篇 |
专业分类
化学 | 274篇 |
晶体学 | 3篇 |
力学 | 5篇 |
数学 | 37篇 |
物理学 | 128篇 |
出版年
2019年 | 5篇 |
2015年 | 5篇 |
2014年 | 6篇 |
2013年 | 13篇 |
2012年 | 10篇 |
2011年 | 14篇 |
2010年 | 7篇 |
2008年 | 11篇 |
2007年 | 13篇 |
2006年 | 14篇 |
2005年 | 13篇 |
2004年 | 8篇 |
2003年 | 7篇 |
2002年 | 20篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 9篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1973年 | 7篇 |
1934年 | 3篇 |
1923年 | 4篇 |
1915年 | 3篇 |
1914年 | 4篇 |
1913年 | 3篇 |
1912年 | 3篇 |
1911年 | 10篇 |
1909年 | 8篇 |
1908年 | 4篇 |
1907年 | 6篇 |
1905年 | 4篇 |
1898年 | 3篇 |
排序方式: 共有447条查询结果,搜索用时 312 毫秒
21.
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
22.
Baxter PN 《The Journal of organic chemistry》2000,65(5):1257-1272
An efficient preparation of linear and curved bis- and branched tris-5-(2,2'-bipyridines) of nanoscopic dimensions possessing rigid conjugated bridges is presented. The synthesis, which avoids the need of protection/deprotection methodology, utilizes central bridge precursors which are outwardly di- and trifunctionalized with a 5-(2-chloropyridine) synthon via a chemoselective palladium-catalyzed Sonogashira or Negishi cross-coupling protocol to yield the bridged linear (5a-c, 5f,g) and curved (6, 7) bis- and branched (8) tris-5-(2-chloropyridines). Under more forcing conditions, the ethyne-bridged 5-(2-chloropyridines) undergo the Stille cross-coupling reacton with 2-trimethylstannylpyridines to afford the conjugatively bridged linear (1a,b, 1g-j) and curved (2a, b, 3a,b) bis- and branched, (4a,b) tris-5-(2,2'-bipyridines) in good overall yields. The phenyl- and biphenyl-bridged linear bis-5-(2, 2'-bipyridines) (1c-f) were best prepared from the bis-5-(2-bromopyridines) (5d,e) to ensure completion of the Stille cross-coupling reactions. The Stille cross-couplings showed a marked substituent effect in which the terminally phenylated bis- and tris-5-(2,2'-bipyridines) were formed in higher yields than the methyl-substituted analogues with the same bridge. The advantages of the methodology lie in its synthetic convenience and adaptibility for creating multitopic metal ion-binding scaffolds with a potentially very large variety of bridging units and substituents on the terminal pyridine rings. The bridged 5-(2-chloropyridines) may also serve as precursors for the fabrication of metal ion-coordinated conjugated polymers. 相似文献
23.
Inductively coupled plasma mass spectrometry (ICP-MS) was used in an isotope dilution mode to assay small-volume (0.25 ml) sediment pore waters for their uranium contents, using 236U as the spike. The only pretreatment required was a simple dilution by a factor of 20, which gave sufficient volume for three replicate analyses per sample. Rapid and accurate results were obtained for a variety of samples and standards, ranging in concentration from 0.05 to 10 ng U ml?1. A suite of 30 samples can be analysed in less than 6 h by this method. The relative standard deviation was better than 1.9%, with a detection limit, based on 3σ background, of 2 pg U ml?1 in solution (40 pg ml?1 in samples). Sea water is a difficult matrix for ICP-MS and thus the method is generally suitable for uranium determinations in many other sample solutions. 相似文献
24.
Ilia Rodushkin Emma Engström Douglas C. Baxter 《Analytical and bioanalytical chemistry》2013,405(9):2785-2797
This critical review focuses on inductively coupled plasma mass spectrometry (ICP-MS) based applications for isotope abundance ratio measurements in various clinical samples relevant to monitoring occupational or environmental exposure, human provenancing and reconstruction of migration pathways as well as metabolic research. It starts with a brief overview of recent advances in ICP-MS instrumentation, followed by selected examples that cover the fields of accurate analyte quantification using isotope dilution, tracer studies in nutrition and toxicology, and areas relying upon natural or man-made variations in isotope abundance ratios (Pb, Sr, actinides and stable heavy elements). Finally, some suggestions on future developments in the field are provided. 相似文献
25.
Baxter PN 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(22):5250-5264
The new twistophane macrocycles 2 and 3 have been synthesised; these compounds are composed of a cyclically conjugated dehydrobenzoannulene framework that incorporates 6,6'-connected-2,2'-bipyridine moieties for the purpose of coordinating metal ions. The cyclophanes were characterised by spectroscopic techniques, and shown by molecular mechanics calculations to be helically twisted and chiral molecules that may exist in several possible ground state conformations. UV/vis spectroscopic studies revealed that 2, 3 and precursor 9 bind with different selectivities to particular members of the following small group of metal analytes: CuII, AgI, HgII, Tl1 and PdII. Significantly, 2, 3 and 9 signal the presence of CuII ions through fluorescence emission quenching output responses. Furthermore, cyclophane 3 exhibited a particularly sensitive protontriggered chromogenic fluorescence response. With respect to their unique structural features, high analyte selectivity coupled with their enhanced and characteristic fluorescence emission responses, these molecules are among the first examples representing a new lead class of chemosensory materials. Compounds 2, 3 and 9 and derivatives thereof may, therefore, be expected to find many future applications in the detection of metal-based environmental pollutants, biologically important trace elements and monitoring proton fluxes. 相似文献
26.
Weissbuch I Baxter PN Kuzmenko I Cohen H Cohen S Kjaer K Howes PB Als-Nielsen J Lehn JM Leiserowitz L Lahav M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(4):725-734
Oriented crystalline monolayers, approximately 14 A thick, of a 2 x 2 Ag+ grid complex, self-assembled at the air-solution interface starting from an water-insoluble ligand 3,6-bis[2-(6-phenylpyridine)]pyridazine spread on silver-ion-containing solutions, were examined by grazing-incidence X-ray diffraction and specular X-ray reflectivity using synchrotron radiation. The monolayer structure was refined, including a determination of the positions of the counter-ions, with the SHELX-97 computer program. The monolayers were transferred from the interface onto various solid supports and visualized by scanning force microscopy, and characterized by X-ray photoelectron spectroscopy in terms of molecular structure. On surface compression, the initial self-assembled monolayer undergoes a transition to a crystalline bilayer in which the two layers, almost retaining the original arrangement, are in registry. Such a phase transition is of relevance to the understanding of crystal nucleation. 相似文献
27.
The surface shape of a bucket of sand rotating about its cylindrical axis is studied experimentally and theoretically. Focusing on fast time scales on which surface shape is determined by avalanches, we identify three regimes of behavior. At intermediate and high frequencies, the surface shape is always at its critical shape determined by the Coulomb yield condition. The low frequency behavior displays an unexpected subcritical region at the center of the bucket. To understand this central region, we adapt a continuum model of surface flow developed by Bouchaud et al. and Mehta et al. The model indicates that the subcritical region is due to a nonlinear instability mechanism. (c) 1999 American Institute of Physics. 相似文献
28.
Baxter RJ 《Physical review letters》2005,94(13):130602
We derive the order parameter of the chiral Potts model, using the method of Jimbo et al. The result agrees with previous conjectures. 相似文献
29.
R. J. Baxter 《Journal of statistical physics》2003,112(1-2):1-26
In a recent paper we derived the free energy or partition function of the N-state chiral Potts model by using the infinite lattice inversion relation method, together with a non-obvious extra symmetry. This gave us three recursion relations for the partition function per site T
pq
of the infinite lattice. Here we use these recursion relations to obtain the full Riemann surface of T
pq
. In terms of the t
p
,t
q
variables, it consists of an infinite number of Riemann sheets, each sheet corresponding to a point on a (2N–1)-dimensional lattice (for N>2). The function T
pq
is meromorphic on this surface: we obtain the orders of all the zeros and poles. For N odd, we show that these orders are determined by the usual inversion and rotation relations (without the extra symmetry), together with a simple linearity ansatz. For N even, this method does not give the orders uniquely, but leaves only [(N+4)/4] parameters to be determined. 相似文献
30.
We have conducted temperature and frequency dependent transport measurements in amorphous NbxSi1-x samples in the insulating regime. We find a temperature dependent dc conductivity consistent with variable range hopping in a Coulomb glass. The frequency dependent response in the millimeter-wave frequency range can be described by the expression sigma(omega) varies with (-iota omega)(alpha) with the exponent somewhat smaller than 1. Our ac results are not consistent with extant theories for the hopping transport. 相似文献