全文获取类型
收费全文 | 308篇 |
免费 | 0篇 |
专业分类
化学 | 121篇 |
晶体学 | 1篇 |
力学 | 10篇 |
数学 | 38篇 |
物理学 | 138篇 |
出版年
2022年 | 3篇 |
2019年 | 2篇 |
2018年 | 3篇 |
2016年 | 2篇 |
2014年 | 2篇 |
2013年 | 17篇 |
2012年 | 7篇 |
2011年 | 10篇 |
2010年 | 6篇 |
2009年 | 7篇 |
2008年 | 14篇 |
2007年 | 14篇 |
2006年 | 17篇 |
2005年 | 11篇 |
2004年 | 18篇 |
2003年 | 20篇 |
2002年 | 6篇 |
2001年 | 11篇 |
2000年 | 11篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1994年 | 2篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 7篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 6篇 |
1970年 | 2篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1965年 | 2篇 |
1963年 | 1篇 |
1960年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有308条查询结果,搜索用时 328 毫秒
51.
Chemical investigations on the stem bark of Holarrhena antidysenterica resulted in the isolation of a new steroidal alkaloid designated as holadysenterine (1), together with three known steroidal alkaloids, conessine (2), isoconessimine (3) and kurchessine (4). Their structures were elucidated on the basis of 1D- and 2D-NMR techniques and high-resolution mass spectrometry. 相似文献
52.
Pamita Bhandari Neeraj Kumar Bikram Singh Ajai P. Gupta Vijay K. Kaul Paramvir S. Ahuja 《Chromatographia》2009,69(3-4):221-227
A liquid chromatography method was developed and validated for the determination of picroside-I and picroside-II in herbal preparations containing Picrorhiza kurroa as one of the ingradients. Resolution of picrosides was achieved on a reversed phase (C-18) endcapped bidentate column by using a mobile phase of acetonitrile: water (25:75 v/v). The detection of picrosides was carried out at 270 nm. The method was validated for precision, accuracy and robustness according to the International Conference on Harmonization (ICH) guidelines and is applicable for the quality control of preparations containing P. kurroa. Analysis of samples in forced degradation proved it to be applicable for stability evaluation. The linear regression analysis data showed good linear relationship (r 2 = 0.9999 ± 0.0010 for picroside-I and 0.9997 ± 0.0012 for picroside-II) in the concentration range of 0.4–4.0 μg. The limit of detection and quantification for picroside-I and picroside-II were recorded to be 28.1 and 73.1 ng and 85.2 and 221.5 ng, respectively. Satisfactory recovery results were observed from the herbal preparations (97.5–100.5%). Intra- and inter- day precision of the method was acceptable, with relative standard deviation (%RSD) values in the range of 0.04–1.16% and 0.03–0.27%, respectively. 相似文献
53.
54.
N. A. Babushkina A. N. Taldenkov S. V. Streltsov A. V. Kalinov T. G. Kuzmova A. A. Kamenev A. R. Kaul D. I. Khomskii K. I. Kugel 《Journal of Experimental and Theoretical Physics》2014,118(2):266-278
We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spin-state transitions in (Pr1 ? y Eu y )0.7Ca0.3CoO3 cobaltites. The Eu doping level y is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spin-state transitions (0.10 < y < 0.20). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope 18O, namely, with 90, 67, 43, 17, and 0% of 18O. Based on the measurements of the ac magnetic susceptibility χ(T) and electrical resistivity ρ(T), we analyze the evolution of the sample properties with a change of the Eu and 18O content. It is demonstrated that the effect of increasing the 18O content on the system is similar to that of increasing the Eu content. The band structure calculations of the energy gap between t 2g and e g bands including the renormalization of this gap due to the electron-phonon interaction reveals the physical mechanisms underlying this similarity. 相似文献
55.
Lakshmi Kaza Hany F. Sobhi Jeffrey A. Fruscella Chris Kaul Shravan Thakur Naullage I. Perera Kenneth Alexander Alan T. Riga 《Journal of Thermal Analysis and Calorimetry》2012,109(3):1365-1371
A standard protocol was developed to determine the water content by thermal analysis of milk of magnesia (MoM). Differential scanning calorimetry (DSC) and thermogravimetry (TG) were used in a novel manner for examining the physical characteristics of the commercial pharmaceutical suspensions. Moisture analyzer and oven-dry methods validate the proposed protocol. MoM consists primarily of water and magnesium hydroxide [Mg(OH)2]. Experimental design of the thermal analysis parameters were considered including sample size, flowing atmosphere, sample pan, and heating rate for both DSC and TG. The results established the optimum conditions for minimizing heat and mass transfer effect. Sample sizes used were: (5?C15?mg) for DSC and (30?C50?mg) for TG. DSC analysis used crimped crucibles with a pinhole, which allowed maximum resolution and gave well-defined mass (water) loss. TG analysis used a heating rate of 10?°C/min?1 in an atmosphere of nitrogen. The heat of crystallization, heat of fusion, and heat of vaporization of unbound water are 334, 334, and 2,257?Jg?1, respectively (Mitra et al. Proc NATAS Annu Conf Therm Anal Appl 30:203?C208, 2002). The DSC average water content of (MoM) was 80?wt% for name brand and 89.5?wt% for generic brand, based on the relative crystallization, melting and vaporization heats/Jg?1 of distilled water in the recently purchased (2011) MoM samples. The TG showed a two-step process, losing water at 80?C135?°C for unbound water and bound water (MgO·H2O) at 376?C404?°C, yielding a total average water loss of 91.9?% for name brand and 90.7?% for generic brand by mass. The difference between the high-temperature TG and the lower-temperature DSC can be attributed for the decomposition of magnesium hydroxide or MgO·H2O. Therefore in performing this new approach to water analysis by heating to a high temperature decomposed the magnesium hydroxide residue. It was determined that the TG method was the most accurate for determining bound and unbound water. 相似文献
56.
N. N. Loshkareva Yu. P. Sukhorukov E. A. Gan’shina E. V. Mostovshchikova R. Yu. Kumaritova A. S. Moskvin Yu. D. Panov O. Yu. Gorbenko A. R. Kaul’ 《Journal of Experimental and Theoretical Physics》2001,92(3):462-473
A new approach to the study of phase separation in lanthanum manganites is proposed based on the combined investigation of their optical and magnetooptical characteristics providing information about the conducting and ferromagnetic regions, respectively. Effects of the 18O isotope substitution for 16O in the epitaxial films of (La0.5Pr0.5)0.7Ca0.3MnO3 (grown on SrTiO3 or LaAlO3 substrates) upon the IR absorption spectra and the equatorial Kerr effect measured in the 1.5–3.8 eV range were studied. A giant drop in the temperature of maximum resistance of the film grown on SrTiO3 and disappearance of the metal-insulator transition in the film on LaAlO3, observed upon the isotope exchange, are accompanied by a decrease in the contribution of free charge carriers to the absorption spectra, by the appearance of bands due to localized states, and by a decrease in magnitude of the equatorial Kerr effects. Measurements of the Kerr effect and the temperature variation of the optical transmission show evidence of the presence of ferromagnetic metal regions in the 18O-isotope-substituted (La0.5Pr0.5)0.7Ca0.3MnO3/LaAlO3 film at low temperatures, with a general semiconductor character of the resistivity behavior in the entire temperature range studied. Changes observed in the absorption spectra are explained based on a model of the pseudo-Jahn-Teller polar centers and phase separation. The optical and magnetooptical data show evidence of a percolation nature of the giant isotope effect in manganites. 相似文献
57.
R Kaul A R Angeles M J?ger E T Powers J W Kelly 《Journal of the American Chemical Society》2001,123(22):5206-5212
To probe the conformational requirements of loop 1 in the Pin1 WW domain, the residues at the i + 2 and i + 3 positions of a beta-turn within this loop were replaced by dPro-Gly and Asn-Gly, which are known to prefer the conformations required at the i + 1 and i + 2 positions of type II' and type I' beta-turns. Conformational specificity or lack thereof was further examined by incorporating into the i + 2 and i + 3 positions a non-alpha-amino acid-based beta-turn mimetic (4-(2'-aminoethyl)-6-dibenzofuran propionic acid residue, 1), which was designed to replace the i + 1 and i + 2 positions of beta-turns. All these Pin WW variants are monomeric and folded as discerned by analytical ultracentrifugation, NMR, and CD. They exhibit cooperative two-state transitions and display thermodynamic stability within 0.5 kcal/mol of the wild-type WW domain, demonstrating that the acquisition of native structure and stability does not require a specific sequence and, by extension, conformation within loop 1. However, it could be that these loop 1 mutations alter the kinetics of antiparallel beta-sheet folding, which will be addressed by subsequent kinetic studies. 相似文献
58.
Horst Schneider Gerd T. Puchta Franz A. R. Kaul Gabriele Raudaschl-Sieber Frdric Lefebvre Guillaume Saggio Dimitrios Mihalios Wolfgang A. Herrmann Jean Marie Basset 《Journal of molecular catalysis. A, Chemical》2001,170(1-2):127-141
The modification of a mesoporous silica surface with Si(Ind)(CH3)2Cl and the immobilization of CpZr(NMe2)3 on this surface was studied via IR-spectroscopy. To reduce side reactions, the indenyl-modified silica was reacted with hexamethyldisilazane (HMDS) under IR-control before the CpZr(NMe2)3-immobilization. The role of the hydroxyl group protection with HMDS is discussed. The surface modifications have been repeated via Schlenk technique at the same conditions and the surface modifications were studied with 13C CP MAS–NMR, 1H MAS–NMR, elemental-, SEM- and BET-analysis. The surface species of the resulting catalysts are discussed. The precatalysts have been treated with methylaluminoxane (MAO) (Al:Zr (mol:mol)=500:1) and the resulting Zr contents (leaching-effect) are discussed. All catalysts have been tested in ethylene and propylene polymerization. 相似文献
59.
A. I. Abramovich L. I. Koroleva A. V. Michurin O. Yu. Gorbenko A. R. Kaul’ 《Physics of the Solid State》2000,42(8):1494-1498
The magnetic, transport, and elastic properties of Sm0.55Sr0.45MnO3 have been established to be interrelated. At the Curie point, one observes a large volume compression ΔV/V≈0.1%, a sharp minimum in the temperature dependence of negative volume magnetostriction ω(T), and a maximum in the temperature dependence of the electrical resistivity. Giant negative volume magnetostriction ω=?5×10?4 has been found in a magnetic field H=0.9 T, which is accompanied by a colossal negative magnetoresistance of 44% in the same field. The results obtained are discussed in terms of a model of electronic phase separation. 相似文献
60.
The rates of gas‐phase thermal elimination reaction of esters of 2‐pyridine sulfonic acid and 8‐quinoline sulfonic acid have been measured over at least 50° for each compound. The relative rates of the first‐order unimolecular decomposition at 500 K for the primary:secondary esters suggest that C O bond breaking is kinetically more significant than C H bond breaking, leading to a transition state of carbocationic character. This is consistent with the electron‐donating effect provided by the methoxy substituent in 2‐p‐methoxyphenylethyl of 2‐pyridine and 8‐quinoline sulfonate esters stabilizing the carbocation center. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 771–775, 2000 相似文献