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排序方式: 共有433条查询结果,搜索用时 281 毫秒
1.
M. Georgelin A. Pocheau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):169-174
The polycrystalline perovskitelike manganese oxides La1-xAxMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation
state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates
for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie
temperature.
Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998 相似文献
2.
Chemical techniques were employed to synthesize CdS nanoparticles embedded in polymer (PEG 300) and sol-gel silica matrices. Systematic growth of particles (radius 3–9 nm) was obtained by adjusting post-deposition annealing temperature and time to examine the dependence of surface-state–related luminescence on particle size. Photoluminescence (PL) peak energy showed a linear dependence with a gentle slope in the weak confinement region and a steep slope in the strong confinement region, the divergence being observed near the excitonic Bohr radius for CdS. The empirical relation proposed for the weak confinement region could be used for estimating chemically prepared CdS nanoparticle size with a high degree of reliability from PL peak energy. 相似文献
3.
Summary The normal incidence reflectivity spectrum of excitons in GaAs/Ga1−x
Al
x
As multiple quantum wells is calculated within the local response approximation. It is shown that the reflectivity lineshape
strongly depends on the sample geometry. Using realistic parameters of a multiple quantum well structure, we obtain an excellent
fit of the experimental reflectivity curve, thus giving exciton energies, oscillator strengths and exciton broadening parameters.
Riassunto Lo spettro di riflettività ad incidenza normale degli eccitoni nei pozzi quantistici multipli di GaAs/Ga1−x Al x As è calcolato nell’ambito dell’approssimazione locale. Si mostra con la forma della linea di riflettività dipende in gran misura dalla forma geometrica del campione. Usando parametri realistici di una struttura a pozzi quantistici multipli, si ottiene un’ottima approssimazione della curva di riflettività sperimentale, che fornisce così energie eccitoniche, forze dell’oscillatore e parameri di ampliamento eccitonico.
Резюме В рамках приближения линейного отклика вычисляется спекрт отражательной способности экситонов при нормальном падении для множественных квантовых ям в GaAs/Ga1−x Al x As. Показывается, что форма линии отражательной способности сильно зависит от геометрии образца. Используя реалистические параметры структуры множественных квантовых ям, мы получаем хорошее соответствие с экспериментальной кривой для отражательной способности. Получаются энергии экситонов, силы осцилляторов и парам⪟тры экситонного уширения.相似文献
4.
金属蛋白研究中几个值得注意的动向 总被引:5,自引:0,他引:5
本文叙述了金属蛋白和金属酶研究中近年来几个值得注意的发展动向,即与金属离子相关的疾病(特别是神经退行性疾病)、金属离子在蛋白质的折叠、聚集和装配中的作用,金属伴侣分子、金属蛋白的设计和构建、金属蛋白与DNA相互作用。 相似文献
5.
A high-performance liquid chromatographic method with evaporative light-scattering detection (ELSD) has been developed for
analysis of spectinomycin and related impurities. Separation of spectinomycin from structurally related impurities was achieved
on a C18 column. The optimized mobile phase was 25 mmol L−1 ammonium acetate (pH 7.5)-methanol, 90:10 (v/v), at a flow rate of 0.6 mL min−1. The temperature of the drift tube of the ELSD was 95°C and the flow rate of carrier gas was 2.2 L min−1. The accuracy, specificity, precision, linearity, sensitivity, and robustness of the method were validated in accordance
with ICH guidelines. In addition to determination of spectinomycin and related impurities, the method is also ideal for determination
of the salts spectinomycin hydrochloride and spectinomycin sulfate. 相似文献
6.
近年来有不少报道从事于生理活性Drimane类倍半萜的合成,较多采用了分子间Diels-Alder反应来建立Drimane的骨架。本文首次采用内分子Diels-Alder反应作为关键反应合成了Drimane类化合物。同时还在引入了手性诱导基团后,首次完成了从无手性的原料化合物β-紫罗兰酮至光学活性(-)-drimenin1的不对称合成。 相似文献
7.
Heidar Raissi Effat Moshfeghi Abraham F. Jalbout Mohammad Saeid Hosseini Mustapha Fazli 《International journal of quantum chemistry》2007,107(9):1835-1845
The molecular structure and intramolecular hydrogen bond energy of 32 conformers of 4‐methylamino‐3‐penten‐2‐one were investigated at MP2 and B3LYP levels of theory using the standard 6–31G** basis set and AIM analyses. Furthermore, calculations for all the possible conformations of 4‐methylamino‐3‐penten‐2‐one in water solution were also carried out at B3LYP/6–31G** level of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the ketoamine conformers of this compound are more stable than the other conformers (i.e., enolimine and ketoimine). This stability is mainly due to the formation of a strong N? H···O intramolecular hydrogen bond, which is assisted by π‐electrons resonance. Hydrogen bond energies for all conformers of 4‐methylamino‐3‐penten‐2‐one were obtained from the related rotamers method. The nature of intramolecular hydrogen bond existing within 4‐methylamino‐3‐penten‐2‐one has been investigated by means of the Bader theory of atoms in molecules, which is based on topological properties of the electron density. The results of these calculations support the results which obtained by related rotamers method. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
8.
Ante Graovac Ivan Gutman Oskar E. Polansky 《Monatshefte für Chemie / Chemical Monthly》1984,115(1):1-13
A novel general property of theS- andT-isomers (a concept which has been introduced and elaborated elsewhere1, 2) of alternant hydrocarbons is demonstrated, namely that due to the HMO total -electron energy theS-isomer should always be more stable than theT-isomer. Some other classes of conjugated isomers are also constructed, for which similar inequalities are derived.
Topologischer Effekt bei MO-Energien, 4. Mitt.: Die -Elektronen-Gesamtenergie vonS- undT-Isomeren
Zusammenfassung Es wird allgemein gezeigt, daß bei einemS-T-Isomerenpaar (beschrieben in1, 2) eines alternierenden Kohlenwasserstoffes infolge der HMO -Elektronen-Gesamtenergie dasS-Isomere stets stabiler sein sollte als dasT-Isomere. Weitere Klassen konjugierter Isomere werden konstruiert und ähnliche Ungleichungen angegeben.相似文献
9.
A structural study by means of X‐ray and electron diffraction of intermediate oxides between Eu2Mo3O12 and Eu2W3O12 shows the existence of a new Eu2(Mo1?xWx)3O12 solid solution with the scheelite structural type. The essential feature of the x = 1/3 and x = 2/3 compounds is the presence of isolated oxygen tetrahedra, in which, according to X‐ray and electron diffraction data, W and Mo are randomly distributed. Eu atoms occupy distorted square antiprisms sites. 相似文献
10.
Ivan Bernal D. C. Levendis Richard Fuchs G. M. Reisner Juanita M. Cassidy 《Structural chemistry》1997,8(4):275-285
The crystal structure of 4-cyclopropylacetanilide was investigated at room temperature (21C) and at –100C in order to determine the orientation of the phenyl ring with respect to the cyclopropane moiety and the effect of this substituent on the stereochemistry of the three-membered ring. The compound was chosen because it is one of the few species containing a simple phenyl ring as the sole cyclopropane ring substituent and whose crystals are suitable for X-ray diffraction at room temperature. The substance crystallizes in space groupP2l/c at either temperature (no phase transitions) with cell constants: (at 21C)a=9.725(2),b=10.934(3), andc=9.636(2) å,=106.13(1);V=984.21 å3 andd(calc;z=4)=1.182 g cm–3. The relevant parameters for the –100C structure area=9.557(4),b=10.980(2), andc=9.641(2) å,=106.34(3);V=970.76 å3 and d(calc;z=4)=1.199 g cm–3. Final values wereR(F)=0.042, Rw=0.035, using unit weights, and its nonhydrogen atoms were used to phase the low-temperature data, whose final discrepancy indices wereR(F)=0.051,R
w
=0.061. The phenyl substituent is almost exactly in the bisecting conformation with respect to the C-C-C angle at the point of attachment to cyclopropane and conjugative effects are clearly evident in the lengths of the cyclopropane ring [1.494(3), 1.498(3), and 1.474(4) å, the later being the distal bond]. If one omits the terminal methylene fragments at C10 and C11, the atoms comprising the acetanilide fragment and the substituted carbon of the cyclopropane ring lie in a nearly perfect plane. Molecular mechanics as well as semiempirical (AM1) calculations were carried out in order to determine the structure of the energy-minimized configurations in the two computational environments. The molecular conformations thus obtained are close to that experimentally observed from the X-ray diffraction experiment. In both theoretical models, the lowest energy conformation is that in which the plane of the phenyl ring bisects the cyclopropane C-C-C angle as was experimentally observed. Finally, the shape of the conformational barrier as a function of the orientation of the plane of the phenyl ring was computed, giving a maximum barrier to rotation of 2.2 kcal/mol. Similar calculations were carried out for two other aryl cyclopropanes, whose rings (naphthalene and anthracene) cannot adopt the bisecting position. Comparisons of experimental geometrical parameters as well as of the barriers to rotation are presented.on leave at the University of Houston, 1995–1996. 相似文献