全文获取类型
收费全文 | 260篇 |
免费 | 11篇 |
专业分类
化学 | 217篇 |
晶体学 | 6篇 |
力学 | 3篇 |
数学 | 9篇 |
物理学 | 36篇 |
出版年
2023年 | 2篇 |
2022年 | 1篇 |
2021年 | 4篇 |
2020年 | 6篇 |
2019年 | 8篇 |
2018年 | 1篇 |
2017年 | 5篇 |
2016年 | 5篇 |
2015年 | 10篇 |
2014年 | 5篇 |
2013年 | 15篇 |
2012年 | 26篇 |
2011年 | 19篇 |
2010年 | 7篇 |
2009年 | 8篇 |
2008年 | 27篇 |
2007年 | 13篇 |
2006年 | 11篇 |
2005年 | 20篇 |
2004年 | 10篇 |
2003年 | 4篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 1篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1983年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1970年 | 2篇 |
1939年 | 1篇 |
排序方式: 共有271条查询结果,搜索用时 15 毫秒
91.
92.
Ito M Kosaka Y Kawabata Y Kato T 《Langmuir : the ACS journal of surfaces and colloids》2011,27(12):7400-7409
In a previous paper, we reported for the first time the lamellar-to-onion transition with increasing temperature at around 67 °C under a constant shear rate (0.3-10 s(-1)) in a nonionic surfactant C(16)E(7)/water system. In this study, the first temperature-shear rate diagram has been constructed in a wider range of shear rate (0.05-30 s(-1)) than in our previous study based on the temperature dependence of the shear stress at constant shear rate. The results suggest that the critical temperature above which the transition begins does not depend on the shear rate very much, although it takes a very shallow minimum. Then we have performed simultaneous measurements of small-angle X-ray scattering/shear stress (rheo-SAXS) with a stepwise increase in temperature of 0.1 K per 15 min at a constant shear rate of 3 s(-1) near the transition temperature. When the temperature exceeds 67 °C, just before the increase in the shear stress, the intensity of the Bragg peak for the velocity gradient direction (approximately proportional to the number of lamellae with their normal along this direction) is suddenly increased. As the temperature increases by 0.2 K, the shear stress begins to increase. At the same time, the peak intensity in the velocity gradient direction rapidly decreases and instead the intensity in the neutral direction increases. As the temperature increases further, the intensities in both the neutral and gradient directions decrease whereas the intensity in the flow direction increases, corresponding to the formation of onions. We have also performed rheo-SAXS experiments with a stepwise increase in shear rate at 72 °C. The sequence of the change in the intensity in each direction is almost the same in the temperature scan experiments at constant shear rate, suggesting that the transition mechanisms along these two paths are similar. The abrupt enhancement of the lamellar orientation with the layer normal along the velocity gradient direction just before the transition is the first finding and strongly supports the coherent buckling mechanism in the lamellar-to-onion transition proposed by Zilman and Granek (Zilman, A. G.; Granek, R. Eur. Phys. J. B 1999, 11, 593). 相似文献
93.
Harada J Ueki K Anada M Kawazoe Y Ogawa K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(50):14111-14119
Solid-state photochromism of benzopyrans and naphthopyrans (chromenes) was investigated in the temperature range between 300 and 80 K. Variable-temperature diffuse reflectance spectroscopy of microcrystalline powders showed that the extent of photocoloration was greatly enhanced at low temperatures. All the chromenes examined exhibited solid-state photochromism at low temperatures, even when they showed little or no photocoloration at room temperature. The solid-state photochromic properties of the chromenes were quite similar to those reported for analogous photochromic compounds of spiropyrans and spirooxazines, which indicates that these classes of compounds are generally photochromic even in the solid state. Photobleaching reactions of the colored merocyanine forms proceeded at low temperatures through the formation of a colorless intermediate, instead of directly resuming the original closed form. In addition to two stable planar merocyanine forms, which are usually observed in the photochromic reactions in solution, photoreactions at low temperatures allowed us to observe unstable colored species, which were tentatively assigned as nonplanar cisoid forms, and were stabilized in the solid state at low temperatures. 相似文献
94.
S Itagaki K Kamata K Yamaguchi N Mizuno 《Chemical communications (Cambridge, England)》2012,48(74):9269-9271
In the presence of Rh(2)(OAc)(4) (OAc = acetate) and TBA(2)WO(4) (TBA = tetra-n-butylammonium), the N-silylation of indole derivatives with hydrosilanes efficiently proceeded to give the corresponding N-silylated indoles in high yields. Pyrrole and carbazole were also N-silylated with the combined catalysts. 相似文献
95.
K Sugahara T Kimura K Kamata K Yamaguchi N Mizuno 《Chemical communications (Cambridge, England)》2012,48(67):8422-8424
A tetra-n-butylammonium (TBA) salt of a γ-Keggin -6-charged germanodecatungstate, [γ-H(2)GeW(10)O(36)](6-) (I), could act as an efficient homogeneous catalyst for Knoevenagel condensation of active methylene compounds with carbonyl compounds. 相似文献
96.
The potentiometric titration of a γ-Keggin tetra-protonated silicodecatungstate, [γ-SiW(10)O(34)(H(2)O)(2)](4-) (H(4)·I), with TBAOH (TBA = [(n-C(4)H(9))(4)N](+)) showed inflection points at 2 and 3 equiv of TBAOH. The (1)H, (29)Si, and (183)W NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [γ-SiW(10)O(34)(OH)(OH(2))](5-) (H(3)·I), [γ-SiW(10)O(34)(OH)(2)](6-) (H(2)·I), and [γ-SiW(10)O(35)(OH)](7-) (H·I), with C(1), C(2v), and C(2) symmetries, respectively. Single crystals of TBA(6)·H(2)·I suitable for the X-ray structure analysis were successfully obtained and the anion part was a monomeric γ-Keggin divacant silicodecatungstate with two protonated bridging oxygen atoms. Compounds H(3)·I, H(2)·I, and H·I were reversibly monoprotonated to form H(4)·I, H(3)·I, and H(2)·I, respectively. 相似文献
97.
Suzuki K Sugawa M Kikukawa Y Kamata K Yamaguchi K Mizuno N 《Inorganic chemistry》2012,51(12):6953-6961
Efficient polyoxometalate (POM)-based Lewis acid-base catalysts of the rare-earth-metal-containing POMs (TBA(6)RE-POM, RE = Y(3+), Nd(3+), Eu(3+), Gd(3+), Tb(3+), or Dy(3+)) were designed and synthesized by reactions of TBA(4)H(4)[γ-SiW(10)O(36)] (TBA = tetra-n-butylammonium) with RE(acac)(3) (acac = acetylacetonato). TBA(6)RE-POM consisted of two silicotungstate units pillared by two rare-earth-metal cations. Nucleophilic oxygen-enriched surfaces of negatively charged POMs and the incorporated rare-earth-metal cations could work as Lewis bases and Lewis acids, respectively. Consequently, cyanosilylation of carbonyl compounds with trimethylsilyl cyanide ((TMS)CN) was efficiently promoted in the presence of the rare-earth-metal-containing POMs via the simultaneous activation of coupling partners on the same POM molecules. POMs with larger metal cations showed higher catalytic activities for cyanosilylation because of the higher activation ability of C═O bonds (higher Lewis acidities) and sterically less hindered Lewis acid sites. Among the POM catalysts examined, the neodymium-containing POM showed remarkable catalytic performance for cyanosilylation of various kinds of structurally diverse ketones and aldehydes, giving the corresponding cyanohydrin trimethylsilyl ethers in high yields (13 substrates, 94-99%). In particular, the turnover frequency (714,000 h(-1)) and the turnover number (23,800) for the cyanosilylation of n-hexanal were of the highest level among those of previously reported catalysts. 相似文献
98.
Shin-Ichi Shamoto Keigo Iizawa Yusuke Asano Kenji Ohoyama Tsuyoshi Kajitani 《Molecular Crystals and Liquid Crystals》2013,570(2):515-520
Abstract Superconducting critical temperature, Tc and the shielding volume fraction, SVF, of layered nitride superconductor NaxHfNCl have been studied as a function of x, i.e. Na concentration. Although Tc decreases gradually with increasing x from 20.0 K at x=0.11 to 16.5 K at x=0.85, SVF has a sharp peak around x=1/6, where strong coupling between local ordering of Na atoms and Fermi surface instability can be expected. Structural disorder in the samples above x=0.5, observed by powder neutron diffraction, does not affect the superconductivity appreciably. Electronic specific heat coefficient, γ, is estimated to be about 7.7 mJ/mol/K2 by its difference of magnetic susceptibility between HfNCl and Na0.5HfNCl. The γ value is relatively small compared with the high Tc value, revealing double honeycomb lattice system as new potential higher Tc superconductor series by intercalation. 相似文献
99.
Koji Kamagata Hiroyuki Tomiyama Yumiko Motoi Masayoshi Kano Osamu Abe Kenji Ito Keigo Shimoji Michimasa Suzuki Masaaki Hori Atsushi Nakanishi Ryohei Kuwatsuru Keisuke Sasai Shigeki Aoki Nobutaka Hattori 《Magnetic resonance imaging》2013
Objective
The pathological changes in Parkinson disease begin in the brainstem; reach the limbic system and ultimately spread to the cerebral cortex. In Parkinson disease (PD) patients, we evaluated the alteration of cingulate fibers, which comprise part of the limbic system, by using diffusional kurtosis imaging (DKI).Methods
Seventeen patients with PD and 15 age-matched healthy controls underwent DKI with a 3-T MR imager. Diffusion tensor tractography images of the anterior and posterior cingulum were generated. The mean kurtosis (MK) and conventional diffusion tensor parameters measured along the images in the anterior and posterior cingulum were compared between the groups. Receiver operating characteristic (ROC) analysis was also performed to compare the diagnostic abilities of the MK and conventional diffusion tensor parameters.Results
The MK and fractional anisotropy (FA) in the anterior cingulum were significantly lower in PD patients than in healthy controls. The area under the ROC curve was 0.912 for MK and 0.747 for FA in the anterior cingulum. MK in the anterior cingulum had the best diagnostic performance (mean cutoff, 0.967; sensitivity, 0.87; specificity, 0.94).Conclusions
DKI can detect alterations of the anterior cingulum in PD patients more sensitively than can conventional diffusion tensor imaging. Use of DKI can be expected to improve the ability to diagnose PD. 相似文献100.
This paper formulates a random-growth urban model with a notion of geographical fitness. Using techniques of complex-network theory, we study our system as a type of preferential-attachment model with fitness, and we analyze its macro behavior to clarify the properties of the city-size distributions it predicts. First, restricting the geographical fitness to take positive values and using a continuum approach, we show that the city-size distributions predicted by our model asymptotically approach Pareto distributions with coefficients greater than unity. Then, allowing the geographical fitness to take negative values, we perform local coefficient analysis to show that the predicted city-size distributions can deviate from Pareto distributions, as is often observed in actual city-size distributions. As a result, the model we propose can generate a generic class of city-size distributions, including but not limited to Pareto distributions. For applications to city-population projections, our simple model requires randomness only when new cities are created, not during their subsequent growth. This property leads to smooth trajectories of city population growth, in contrast to other models using Gibrat’s law. In addition, a discrete form of our dynamical equations can be used to estimate past city populations based on present-day data; this fact allows quantitative assessment of the performance of our model. Further study is needed to determine appropriate formulas for the geographical fitness. 相似文献