全文获取类型
收费全文 | 2600篇 |
免费 | 56篇 |
国内免费 | 6篇 |
专业分类
化学 | 1617篇 |
晶体学 | 10篇 |
力学 | 44篇 |
数学 | 546篇 |
物理学 | 445篇 |
出版年
2020年 | 26篇 |
2019年 | 21篇 |
2016年 | 35篇 |
2015年 | 43篇 |
2014年 | 36篇 |
2013年 | 68篇 |
2012年 | 48篇 |
2011年 | 63篇 |
2010年 | 43篇 |
2009年 | 47篇 |
2008年 | 67篇 |
2007年 | 63篇 |
2006年 | 63篇 |
2005年 | 52篇 |
2004年 | 40篇 |
2003年 | 49篇 |
2002年 | 36篇 |
2001年 | 31篇 |
2000年 | 39篇 |
1999年 | 32篇 |
1998年 | 31篇 |
1997年 | 33篇 |
1996年 | 34篇 |
1995年 | 29篇 |
1994年 | 28篇 |
1993年 | 29篇 |
1992年 | 24篇 |
1991年 | 41篇 |
1990年 | 39篇 |
1989年 | 32篇 |
1988年 | 34篇 |
1987年 | 28篇 |
1986年 | 25篇 |
1985年 | 34篇 |
1984年 | 36篇 |
1983年 | 27篇 |
1982年 | 42篇 |
1981年 | 45篇 |
1980年 | 44篇 |
1979年 | 46篇 |
1978年 | 42篇 |
1977年 | 28篇 |
1976年 | 23篇 |
1975年 | 30篇 |
1973年 | 25篇 |
1970年 | 21篇 |
1933年 | 22篇 |
1929年 | 28篇 |
1928年 | 21篇 |
1927年 | 21篇 |
排序方式: 共有2662条查询结果,搜索用时 15 毫秒
41.
Determination of the Enantiomeric Excess of Chiral Ammonium long Using Liquid Membrane Electrodes A cell assembly with two membranes each containing one enantiomer of a chiral ionophore is described. It is suitable for the direct potentiometric determination of the enantiomeric excess of ions. The method is used to determine the enantiomeric excess of ephedronium ions in aqueous solutions. 相似文献
42.
Daniel Ammann Ren Bissig Marc Güggi Ern Pretsch Wilhelm Simon Irving J. Borowitz Louis Weiss 《Helvetica chimica acta》1975,58(6):1535-1548
The preparation of a series of non-cyclic, uncharged ligands able to selectively complex alkali and alkaline earth metal cations is described. These molecules are designed to be used as carriers for cations through membranes. Some of the compounds show high Ca2+ and Na+ selectivity, respectively, in liquid membrane electrodes. 相似文献
43.
Abstract In the presence of the photosensitizer riboflavin at high fluence rates a photoproduct, most probably H2 O2 , is formed which causes negative phototaxis in the colorless flagellate Polytomella magna . The aim of this study was to find out whether H2 O2 is produced in a type I or II reaction. As has been shown, 1 O2 quenchers either do not influence the photodynamic action of riboflavin (furfuryl ethanol, DPBF, l -histidine, crocetin) or show slight quenching effects only at very high concentrations ≧ 10−2 M (DABCO, DMF, imidazole). D2 O is toxic to P. magna even in 1:1 and 1:2 mixtures with H2 O. On the other hand, the quenching effect of 1,4-benzoquinone, highly indicative for the type I pathway, is more than two orders of magnitude stronger than the one of the above mentioned 1 O2 quenchers. The results suggest that H2 O2 is produced in a type I reaction. Superoxide does not seem to be involved since superoxide dismutase does not diminish the photodynamic effect of riboflavin. 相似文献
44.
Mass spectral fragmentation patterns of dimethylsilyl (DMS) ethers of primary unbranched, branched, and secondary unbranched aliphatic alcohols in the C5 to C10 range are compared with those of the corresponding trimethylsilyl (TMS) derivatives. Unlike their TMS analogues, DMS ethers of primary alcohols exhibit pronounced rupture of the C? C bond adjacent to the oxygen atom within the alkyl moiety (loss of an alkyl radical R) in marked preference to cleavage within the silyl substituent (loss of CH3). Within this class of compounds, complementary preparation of DMS derivatives can therefore be used to establish or to confirm the site, and thus the primary nature of the hydroxyl group, whereas preparation of TMS ethers may be of advantage in deducing molecular size. For the derivatives of secondary alcohols this diagnostically useful difference in fragmentation behaviour is not observed. 相似文献
45.
The non-linear autonomous of differential equations which plays an important role in chemical kinetics and other fields of physics (turbulence and plasma physics) is investigated using the Carleman linearization procedure. 相似文献
46.
Wilhelm Niethammer 《Numerische Mathematik》1980,34(3):271-283
Summary A nonlinear generalizationÊ
z
of Euler's series transformation is compared with the (linear) Euler-Knopp transformationE
z
and a twoparametric methodE
. It is shown how to applyE
orE
, to compute the valuef(zo) of a functionf from the power series at 0 iff is holomorphic in a half plane or in the cut plane. BothE
andE
, are superior toÊ
z
. A compact recursive algorithm is given for computingE
andE
,. 相似文献
47.
The brown crystals of [NEt4]2[Se3Br8(Se2Br2)] ( 1 ) were obtained when selenium and bromine reacted in the solution of acetonitrile in the presence of tetraethylammonium bromide. The crystal structure of 1 has been determined by the X‐ray methods and refined to R = 0.0308 for 10433 reflections. The crystals are monoclinic, space group P21 with Z = 2 and a = 12.0393(3) Å, b = 11.8746(3) Å, c = 13.1946(3) Å, β = 96.561(1)° (123 K). In the solid state structure the anion of 1 is built up of Se3Br8 unit which consists of a triangular arrangement of three planar SeBr4 units sharing a common edge through two μ3‐bridging Br atoms, and one Se2Br2 molecule which is linked to one of μ3‐bridging Br atoms. The three SeII atoms form a triangle which is almost perpendicular to the planes given by three SeBr4 moieties. The contact between the μ3Br and the SeI atom of the Se2Br2 molecule is 3.1711(8) Å and can be interpreted as a bond of the donor‐acceptor type with the μ3Br as donor and the Se2Br2 molecule as acceptor. The terminal SeII‐Br and μ3Br‐SeII bond lengths are in the ranges 2.3537(7)–2.4737(7) Å and 2.7628(7)–3.1701(7) Å, respectively. The bond lengths in coordinated Se2Br2 molecule are: SeI‐SeI = 2.2636(9) Å, SeI‐Br = 2.3387(11) and 2.3936(8) Å. 相似文献
48.
A Lainez Emmerich Wilhelm G Roux-Desgranges J.-P.E Grolier 《The Journal of chemical thermodynamics》1985,17(12):1153-1161
Excess molar volumes VmE at 298.15 K were obtained, as a function of mole fraction x, for series I: {x1-C4H9Cl + (1 ? x)n-ClH2l + 2}, and II: {x1,4-C4H8Cl2 + (1 ? x)n-ClH2l + 2}, for l = 7, 10, and 14. 10, and 14. The instrument used was a vibrating-tube densimeter. For the same mixtures at the same temperature, a Picker flow calorimeter was used to measure excess molar heat capacities Cp, mE at constant pressure. VmE is positive for all mixtures in series I: at x = 0.5, VmE/(cm3 · mol?1) is 0.277 for l = 7, 0.388 for l = 10, and 0.411 for l = 14. For series II, VmE of {x1,4-C4H8Cl2 + (1 ? x)n-C7H16} is small and S-shaped, the maximum being situated at xmax = 0.178 with VmE(xmax)/(cm3 · mvl?1) = 0.095, and the minimum is at xmin = 0.772 with VmE(xmin)/(cm3 · mol?1) = ?0.087. The excess volumes of the other mixtures are all positive and fairly large: at x = 0.5, VmE/(cm3 · mol?1) is 0.458 for l = 10, and 0.771 for l = 14. The Cp, mEs of series I are all negative and |Cp, mE| increases with increasing l: at x = 0.5, Cp, mE/(J · K?1 · mol?1) is ?0.56 for l = 7, ?1.39 for l = 10, and ?3.12 for l = 14. Two minima are observed for Cp, mE of {x1,4-C4H8Cl2 + (1 ? x)n-C7H16}. The more prominent minimum is situated at x′min = 0.184 with Cp, mE(x′min)/(J · K?1 · mol?1) = ?0.62, and the less prominent at x″min = 0.703 with Cp, mE(x″min)/(J · K?1 · mol?1) = ?0.29. Each of the remaining two mixtures (l = 10 and 14) has a pronounced minimum at low mole fraction (xmin = 0.222 and 0.312, respectively) and a broad shoulder around x = 0.7. 相似文献
49.
Lipophilic trifluoroacetophenone derivatives incorporated in plasticized PVC membranes are able to selectively extract water and alcohols from the sample solution into the organic membrane phase, reversibly forming hydrates and hemi-acetals, respectively. Since this is accompanied by a change in the absorption spectrum of the acetophenone isologue, the chemical recognition process can directly be translated into an optical signal. With N-acetyl-N-dodecyl-4-trifluoroacetylaniline (ETH 6022) as the electrically neutral, lipophilized carrier ethanol can be determined from 0.5 to 35% (v/v) in aqueous solutions. The calibration curve for different ethanol-water mixtures shows a good correlation with the mathematically derived formalism and thus confirms the theoretically expected behavior. Besides high reproducibility of the optical signals, very short response times of less than 30 s were realized. The optode membrane presented exhibits a preference for ethanol compared to water by a factor greater than 11. The selectivities for several primary alcohols, such as methanol, ethanol, 1-propanol and 1-n-butanol, are comparable, but isopropanol and tert.-butanol are rejected by a factor of about 10. The alcohol concentration in different beverages was determined to evaluate the reliability of the system. The values obtained for wine, beer and different spirits show an excellent correlation with those obtained by a conventional approach involving distillation and density measurements. A residual standard deviation of ± 0.27% (v/v) over the 0.7–40% (v/v) range was found. 相似文献
50.
The behaviour under electron impact (70 eV) which includes some rearrangement processes of some tetraorganodiphosphanedisulfides R2P(S)-P(S)R2 (R ? CH3, C2H5, n-C3H7, n-C4H9, C3H5, C6H5) and CH3RP(S)–P(S)CH3R (R ? C2H5, n-C3H7, n-C4H9, C6H5, C6H5, C6H5,CH2) is reported and discussed. Fragmentation patterns which are consistent with direct analysis of daughter ions and defocusing metastable spectra are given. The atomic composition of many of the fragment ions was determined by precise mass measurements. In contrast to compounds R3P(S) loss of sulphur is not a common process here. The first step in the fragmentation of these compounds is cleavage of one P–C bond and loss of a substituent R?. The second step is elimination of RPS leading to [R2PS]+ from which the base peaks in nearly all the spectra arise. The phenyl substituted compounds give spectra with very abundant [(C6H5)3P]+. and [(C6H5)2CH3P]+. ions respectively, resulting from [M]+. by migration of C6H5. Rearrangement of [M]+. to a 4-membered P-S ring system prior to fragmentation is suggested. 相似文献