全文获取类型
收费全文 | 4890篇 |
免费 | 359篇 |
国内免费 | 245篇 |
专业分类
化学 | 1868篇 |
晶体学 | 89篇 |
力学 | 821篇 |
综合类 | 18篇 |
数学 | 675篇 |
物理学 | 2023篇 |
出版年
2024年 | 9篇 |
2023年 | 33篇 |
2022年 | 61篇 |
2021年 | 63篇 |
2020年 | 95篇 |
2019年 | 103篇 |
2018年 | 75篇 |
2017年 | 102篇 |
2016年 | 161篇 |
2015年 | 105篇 |
2014年 | 163篇 |
2013年 | 398篇 |
2012年 | 180篇 |
2011年 | 245篇 |
2010年 | 182篇 |
2009年 | 258篇 |
2008年 | 274篇 |
2007年 | 321篇 |
2006年 | 276篇 |
2005年 | 221篇 |
2004年 | 242篇 |
2003年 | 205篇 |
2002年 | 212篇 |
2001年 | 154篇 |
2000年 | 160篇 |
1999年 | 146篇 |
1998年 | 142篇 |
1997年 | 120篇 |
1996年 | 104篇 |
1995年 | 91篇 |
1994年 | 91篇 |
1993年 | 65篇 |
1992年 | 67篇 |
1991年 | 39篇 |
1990年 | 25篇 |
1989年 | 32篇 |
1988年 | 30篇 |
1987年 | 25篇 |
1986年 | 31篇 |
1985年 | 33篇 |
1984年 | 34篇 |
1983年 | 8篇 |
1982年 | 16篇 |
1981年 | 19篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 13篇 |
1977年 | 13篇 |
1976年 | 13篇 |
1973年 | 8篇 |
排序方式: 共有5494条查询结果,搜索用时 8 毫秒
61.
We consider the relaxation dynamics of two quantum levels coupled to a stochastic bath. We emphasize that even if the matrix elements of the fluctuating Hamiltonian are Gaussian, a second-order cumulant truncation is not exact. For various stochastic models, including the case of a spin-1/2 particle in a fluctuating magnetic field, we calculate 1/T
1, the population relaxation rate, and 1/T
2, the phase relaxation rate, up to fourth order in perturbation theory. We show that unlike the commonly accepted second-order result that 1/T
21/2T
1, when fourth-order terms are included, in some instances 1/T
2<1/2T
1. 相似文献
62.
Martin Macháček 《Journal of statistical physics》1987,47(5-6):949-952
We define mathematically a class of dynamical systems that exhibit relaxation corresponding to that observed in physical systems, and then show that this class is identical with the class ofK-mixing dynamical systems. 相似文献
63.
The dipole dynamics and α-relaxation behaviour of polyvinyl chloride PVC, poly(ethylene-co-vinyl-acetate) EVA70 and blend of them EVA70/PVC have been investigated by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). The differential scanning calorimetry (DSC) thermograms measured on samples show a single glass transition in the analysed temperature range. These three polymers are wholly amorphous and pure PVC and pure EVA70 are miscible in the ratio 1:1. The glass transition temperature Tg decreases significantly with presence of EVA70. Furthermore, the values of apparent activation energies for molecular motions at the α-relaxation and the values of fragility index have also been determined for each sample using Moynihan expression for DSC results and Vogel-Fulcher-Tammann-Hesse (VFTH) form for DRS results. It is shown a large dependence of all the values of these parameters with the content of EVA70. Comparing these three polymers, we found that the more fragile glass forming liquid is the PVC. Fragility decreases drastically with EVA70 content. EVA70 and EVA70/PVC blend exhibit practically the same behaviour. The effect of inter- and intra-molecular interactions on fragility is discussed to explain these variations. 相似文献
64.
Udo Kaatze 《Journal of solution chemistry》1997,26(11):1049-1112
A tutorial on dielectric (relaxation) spectrometry of liquids is given in this article. Some methods of measuring complex
(electric) permittivity spectra are briefly described. Results for water are presented and related to characteristic properties
of the liquid structure and to models of the molecular dynamics, particularly as resulting from computer simulation studies.
Dielectric spectra for aqueous solutions of low weight electrolytes, polyelectrolytes, small molecules, and polymers are discussed
to illustrate effects of kinetic depolarization, structure saturation, as well as positive, negative, and hydrophobic hydration.
Reference is also made to fluctuations in the hydrogen bond network of mixtures of water with liquids that are completely
miscible with this unique solvent. 相似文献
65.
A comparative quantum-chemical analysis of the electronic structures and spectroscopic parameters of the cycloalkanes C3H6, C4H8, C5H10, and C6H12 and their silicon analogs Si3H6, Si4H8, Si5H10 and Si6H12 was performed in the framework of the SCF MO LCAO method in the INDO approximation. Qualitative interpretation of “abnormal”
ionization potentials and energies of electronic absorption spectra of cyclopolysilanes has been given.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1105–1108, June, 1997. 相似文献
66.
The vertical ionization potentials of bis(-allyl)nickel (see (1) in Fig. 1) are calculated by means of the Green's function approach within a semiempirical INDO extension to the first transition metal series. The computed ionization potentials are in good agreement with an experimentally deduced assignment. In contrast to earlier theoretical and experimental studies, the 7a
u
() level is predicted on top of the levels corresponding to the Ni 3d orbitals. Our approach leads to a complete assignment of the PE spectrum of (1) in the outer valence region. 相似文献
67.
Polystyrenes with different concentrations of side groups with cyano groups were prepared and complex dielectric constants were measured in the range of the glass transition temperature and the frequency range of 10–2–107 Hz.The GPC and DSC measurements showed that the molecular weight of these polystyrenes was about 10500 g/mole and the glass transition temperatures were 89.5°C for all samples.The dielectric relaxation spectra obtained for the side group polystyrene labels and also the chain-end polystyrene labels prepared before [9] were analyzed to find out the degree of coupling of the chain-end and side-group labels with the cooperative reorientation of the polymeric matrix. The analysis of the spectra was carried out using the analysis method developed by Mansour and Stoll [6].The results obtained showed that both end- and side-group labels are strongly coupled with the segmental reorientation and relax with relaxation times longer than that of the segments.The value of logf
m = (logf
m(label)) – logf
m(matrix)) was obtained from the recently designed comparison diagram suggested by Mansour and Stoll [6, 14]. The value of logf
m depends on the label length in the case of chain-end labels.It was surprising to find that the side groups relax slower than the segments by only 0.9 decades. These results obtained implied that the label relaxes through a multistep relaxation mechanism of the side and end groups and not through a diffusion mechanism of the whole chain. In addition, the effective lengths of the relaxing units were determined using the empirical equation obtained before in the case of rodlike molecules in polyisoprene [7]. 相似文献
68.
Tabushi Iwao Kuroda Yasuhisa Yamada Masahiko Sera Takashi 《Journal of inclusion phenomena and macrocyclic chemistry》1988,6(6):599-603
Deuterium relaxation times ofd- andl--d-tryptophan included in -cyclodextrin derivatives were directly measured by deuterium NMR spectroscopy. The results showed that the molecular motion of the tryptophan molecule was strongly restricted even in the cavity of unmodified -cyclodextrin and the additional recognition groupings — ammonium and carboxylate — on -cyclodextrin did not affect the molecular motion of tryptophan, though the association constants were significantly enhanced.Deceased on March 22, 1987. 相似文献
69.
Beate Hager Bettina Schwarzinger Heinz Falk 《Monatshefte für Chemie / Chemical Monthly》2006,137(2):163-168
Summary. Two model compounds for the green fluorescent protein chromophore were prepared. One of them incorporates the natural 4-hydroxybenzylidene
group of the natural tyrosin derived chromophore, the other one bears a methyl group instead of the hydroxy group. Whereas
the photochemically prepared (E)-diastereomer of the first compound very effectively reverted thermally (room temperature) to the thermodynamically stable
(Z)-diastereomer, the (E)-diastereomer of the second derivative proved to be stable even at elevated temperatures for more than a day. This finding
can be rationalized by constructing the appropriate resonance structures showing that only in the first case an effective
delocalization enables partial single bond character of the benzylidene double bond. From the standpoint of chemical etiology,
only Nature’s choice of the tyrosin derived chromophore of the green fluorescent protein provides an efficient radiationless
thermal relaxation channel for the unwanted photo-diastereomerization product formed after excitation besides the dominating
fluorescence channel of its chromophore. 相似文献
70.
A.R. Sousa 《Polymer Degradation and Stability》2006,91(7):1504-1512
This work aims to analyze the effects of photodegradation on the stress cracking resistance of polystyrene. Injection moulded samples were exposed to the ultraviolet light for various times in the laboratory prior to solvent contact. The bars were then stressed in a tensile testing machine under the presence of butanol. During this period the stress relaxation was monitored and the ultimate properties were evaluated after selected periods of stress cracking. Complementary tests were done by size exclusion chromatography and by scanning electron microscopy. The results indicated that butanol causes significant modification in polystyrene, with extensive surface crazing as well as reduction in mechanical properties. This is intensified under higher mechanical stress. The previous degraded samples showed a higher level of stress relaxation and a greater loss in tensile strength in comparison to the undegraded ones. The synergist action of photodegradation and stress cracking in polystyrene may be a consequence of the chemical changes caused by oxidation like the formation of polar chemical groups and the reduction in molecular weight. 相似文献