全文获取类型
收费全文 | 1497篇 |
免费 | 63篇 |
国内免费 | 14篇 |
专业分类
化学 | 884篇 |
晶体学 | 10篇 |
力学 | 60篇 |
数学 | 265篇 |
物理学 | 355篇 |
出版年
2023年 | 10篇 |
2022年 | 12篇 |
2021年 | 22篇 |
2020年 | 24篇 |
2019年 | 28篇 |
2018年 | 19篇 |
2017年 | 20篇 |
2016年 | 32篇 |
2015年 | 29篇 |
2014年 | 36篇 |
2013年 | 59篇 |
2012年 | 57篇 |
2011年 | 76篇 |
2010年 | 39篇 |
2009年 | 32篇 |
2008年 | 52篇 |
2007年 | 65篇 |
2006年 | 68篇 |
2005年 | 88篇 |
2004年 | 67篇 |
2003年 | 42篇 |
2002年 | 37篇 |
2001年 | 22篇 |
2000年 | 29篇 |
1999年 | 21篇 |
1998年 | 25篇 |
1997年 | 17篇 |
1996年 | 26篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 27篇 |
1992年 | 20篇 |
1991年 | 17篇 |
1990年 | 25篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1985年 | 22篇 |
1984年 | 18篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 19篇 |
1980年 | 26篇 |
1979年 | 11篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 11篇 |
1975年 | 22篇 |
1974年 | 21篇 |
1973年 | 13篇 |
排序方式: 共有1574条查询结果,搜索用时 343 毫秒
21.
Yimin Jin J. Bonilla Ye-Gang Lin J. Morgan Linda McCracken J. Carnahan 《Journal of Thermal Analysis and Calorimetry》1996,46(3-4):1047-1059
Two poly(butylene terephthalate)/polycarbonate (PBT/PC) blends with different formulations were analyzed by modulated DSC (MDSC) and conventional DSC to determine differences in crystallization behavior. A significant difference (30°C in cold crystallization temperature) between the two samples was detectable by MDSC while no significant difference was seen by conventional DSC. That indicatesthe total heat flow from MDSC is not always equivalent to the heat flow from conventional DSC as we have assumed or seen before. The reason has not been fully understood, but may be related to unusual nucleation and crystallization induced by modulation. Alternative conventional DSC methods were developed and compared to the MDSC results.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayThe authors like to thank Drs. Bernhard Wunderlich and Robert Gallucci for helpful discussion, David Shaker and Mary Parsonage for some DSC experiments. Technical support from TA Instruments is also greatly appreciated. 相似文献
22.
Christopher J. Chandler Leslie W. Deady James A. Reiss Vasilios Tzimos 《Journal of heterocyclic chemistry》1982,19(5):1017-1019
The preparation of a series of macrocycles, formed by reaction of HO-(-CH2-CH2-O-)nH with 1,10-phenanthroline-2,9-dicarbonyl chloride (n = 2,3,4) and 1,8-naphthyridine-2,7-dicarbonyl chloride (n = 3,4), is described. An improved synthetic route to 2,7-dimethyl-1,8-naphthyridine 9 is also reported. 相似文献
23.
In this article we derive a segment-level formula for the matrix elements of the U(2n) generators in a basis symmetry adapted to the subgroup U(n) × U(2) (i.e., spin-orbit basis), for the representations appropriate to many-electron systems. This enables the direct evaluation of the matrix elements of spin-dependent Hamiltonians. 相似文献
24.
Mueller JA Cowell A Chandler BD Sigman MS 《Journal of the American Chemical Society》2005,127(42):14817-14824
A kinetic investigation into the origin of enantioselectivity for the Pd[(-)-sparteine]Cl(2)-catalyzed aerobic oxidative kinetic resolution (OKR) is reported. A mechanism to account for a newly discovered chloride dissociation from Pd[(-)-sparteine]Cl(2) prior to alcohol binding is proposed. The mechanism includes (1) chloride dissociation from Pd[(-)-sparteine]Cl(2) to form cationic Pd(-)-sparteine]Cl, (2) alcohol binding, (3) deprotonation of Pd-bound alcohol to form a Pd-alkoxide, and (4) beta-hydride elimination of Pd-alkoxide to form ketone product and a Pd-hydride. Utilizing the addition of (-)-sparteine HCl to control the [Cl(-)] and [H(+)] and the resulting derived rate law, the key microscopic kinetic and thermodynamic constants were extracted for each enantiomer of sec-phenethyl alcohol. These constants allow for the successful simulation of the oxidation rate in the presence of exogenous (-)-sparteine HCl. A rate law for oxidation of the racemic alcohol was derived that allows for the successful prediction of the experimentally measured k(rel) values when using the extracted constants. Besides a factor of 10 difference between the relative rates of beta-hydride elimination for the enantiomers, the main enhancement in enantiodetermination results from a concentration effect of (-)-sparteine HCl and the relative rates of reprotonation of the diastereomeric Pd-alkoxides. 相似文献
25.
Alfred Fischer Michael W. Morgan Colin Eaborn 《Journal of organometallic chemistry》1977,136(3):323-332
Rates of cleavage of some picoyl- and (quinolylmethyl)-trimethylsilanes (RSiMe3, where R = PyCH2 or QnCH2SiMe3) have been measured in “90%” aqueous methanolic sodium methoxide at 50°C. Relative reactivities are: 2-PyCH2, 1.0; 3-PyCH2, 0.030; 4-PyCH2, 8.9; 2-QnCH2, 41; 3-QnCH2, 0.161; 4-QnCH2, 37. The rates correlate well with those for base-catalysed hydrogen-exchange in the parent carbon acids RH. Approximate pKa's (based on the scale of ion-pair acidities in CsNHC6H11H2NC6H11, with pKa of 9-phenylfluorene = 18.6) for the carbon acids, RH, can be derived as follows: 2-PyCH3, 29.5; 3-PyCH3, 34; 4-PyCH3, 27; 2-QnCH3, 25; 3-QnCH3, 32; 4-QnCH3, 25.Rates of cleavage of pyridyl- and quinolyl-trimethylsilanes (PySiMe3 and QnSiMe3) by sodium hydroxide in 4 : 1 v/v Me2SO/H2O at 50°C have also been measured; and the relative reactivities are: 2-Py, 1.0; 3-Py, 2.9; 4-Py, 8.4; 2-Qn, 15.9; 3-Qn, 12.7; 4-Qn, 184. The sequence of reactivity differes from that for base-catalysed hydrogen-exchange at the relevant positions of pyridine and quinoline, indicating that the reactivities are not determined in both cases (if in either) solely by the stabilities of the corresponding carbanions. 相似文献
26.
Michael H. Gold Hidetoshi Kutsuki Meredith A. Morgan 《Photochemistry and photobiology》1983,38(6):647-651
Oxidation of specifically radiolabeled 14C-lignins by UV/H202. Fenton's reagent, photosesitizing riboflavin. UV- and γ-irradiation was examined. In the presence of UV/H2O2, a hydroxyl radical (·OH) generating system. 14C-methoxy. 2-[14C-sidechain] and 14C-ring labeled lignin were rapidly and extensively degraded as measured by gel filtration of the reaction products on Sephadex LH-20. This suggested that exposure to OH leads to rapid, nonspecific lignin degradation. Rapid degradation of 14C-methoxy, 2-[14C-sidechain] and 14C-ring labeled lignin also occurred in the presence of the ·OH generating system, Fenton's reagent, confirming the primary role of ·OH in these reactions. Photosensitizing riboflavin, also capable of effecting transformation of organic compounds via Type I hydrogen radical abstractions, caused extensive oxidative degradation of 14C-methoxy labeled lignin and significant degradation of 2-(14C-sidechain] and 14C-ring labeled lignin. In addition, UV- and γ-irradiation caused slower but extensive degradation of the polymers, probably via radical type mechanisms. All of these results indicate that ·OH as well as organic radical generating systems are effective agents for the purpose of degrading this heterogeneous, optically inactive and random biopolymer. 相似文献
27.
A coal tar pitch and a petroleum vacuum residue have been separated by TLC using pyridine, acetonitrile, toluene and pentane to develop the chromatograms. The bands of material detected were recovered in 1-methyl-2-pyrrolidinone (NMP) solvent and examined by size-exclusion chromatography (SEC) in NMP eluent. The relation between elution time in SEC and mobility on the TLC plate indicated that molecular size increased steadily with increasing immobility on the plate. This relation was reinforced by UV fluorescence spectroscopy in that the fluorescence moved to longer wavelengths with increasing immobility. The molecular size of the material excluded from the porosity of the SEC column remains undefined; some excluded material was found in all of the fractions from both samples. The valley of zero intensity separating the retained material from the excluded material may suggest a change of structure from near-planar in the retained region to three-dimensional in the excluded region. 相似文献
28.
A new technique is described for estimating the pure component spectra from a set of linearly independent spectra. The process is one of generalised spectral subtraction in which an iterative combination of multivariate linear least-squares analysis and matrix transformation is applied to the input data to give estimates of the number of independent components in the original mixed spectra. This technique is applicable to bipolar data (e.g. from e.p.r. spectra) as well as absorption spectra determined by any spectroscopic technique, provided that the spectra may be reasonably assumed to be an additive mixture of unknown pure components. Numerical model examples are given together with an experimental application to electron spin resonance. 相似文献
29.
Y. Li N. Obando F. Tschen R. J. Morgan 《Journal of Thermal Analysis and Calorimetry》2006,85(1):125-129
Thermal analysis of phenylethynyl end-capped imide oligomer AFR-PEPA-4
was performed to characterize cure reaction, thermal stabilities and semicrystalline
behavior of AFR-PEPA-4 oligomer and its cured polyimide. Cured AFR-PEPA-4
polyimide showed high T
gs
up to 418°C. Both AFR-PEPA-4 oligomer and polyimide exhibit excellent
thermal stabilities comparable to PETI-5 polyimides. AFR-PEPA-4 imide oligomer
has a T
m of 330°C
and exhibits spherulite crystalline morphology in the film. The crystallinity
in AFR-PEPA-4 films could not be regenerated under any annealing conditions
after the initial melt. 相似文献
30.
Joachim G. Liehr Richard M. Caprioli John H. Beynon Roger P. Morgan Wilhelm J. Richter 《Journal of mass spectrometry : JMS》1980,15(3):157-160
The oxygen rearrangement in molecular ions of 3-phenylpropionates has been investigated with the aid of mass analyzed ion kinetic energy spectra. Elimination of an allyl radical followed by expulsion of ketene from the molecular ion of allyl 3-phenylpropionate is shown to result in formation of protonated benzaldehyde. The oxygen rearrangement has been found to be inoperative in ionized methyl 3-methyl-3-phenylbutyrate. [M ? CH3 ? CH2CO]+ ions in the spectrum of the latter compound are formed by elimination of the 3-methyl substituent and subsequent methoxy migration. 相似文献