全文获取类型
收费全文 | 295篇 |
免费 | 10篇 |
国内免费 | 3篇 |
专业分类
化学 | 211篇 |
晶体学 | 5篇 |
力学 | 6篇 |
数学 | 15篇 |
物理学 | 71篇 |
出版年
2023年 | 3篇 |
2022年 | 2篇 |
2021年 | 2篇 |
2019年 | 3篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 10篇 |
2015年 | 6篇 |
2014年 | 12篇 |
2013年 | 13篇 |
2012年 | 17篇 |
2011年 | 19篇 |
2010年 | 15篇 |
2009年 | 16篇 |
2008年 | 16篇 |
2007年 | 13篇 |
2006年 | 13篇 |
2005年 | 12篇 |
2004年 | 10篇 |
2003年 | 10篇 |
2002年 | 14篇 |
2001年 | 10篇 |
2000年 | 7篇 |
1998年 | 3篇 |
1996年 | 2篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1967年 | 1篇 |
排序方式: 共有308条查询结果,搜索用时 15 毫秒
81.
Single crystals of the Ba-containing silicon clathrate superconductor Ba(7.76)Si(46) were prepared using a high-pressure and high-temperature condition (3 GPa, 1300 degrees C). It crystallized in the cubic space group Pm-3n with a = 10.3141(7) A and Z = 1. There are two crystallographically different types of Ba sites, at the centers of Si dodecahederal (Ba@Si(20)) and Si tetrakaidecahedral (Ba@Si(24)) cages. On evacuation at 527 degrees C, a part of Ba atoms were removed from the Ba@Si(20) sites. The superconducting transition temperature (T(c)) decreased from 9.0 to 6.0 K with the decrease of the Ba content from 7.76 to 6.63 Ba/Si(46). The Ba deficient sites and the deficiency were determined by the structural refinement in the single-crystal X-ray analyses. 相似文献
82.
Yagai S Nakajima T Kishikawa K Kohmoto S Karatsu T Kitamura A 《Journal of the American Chemical Society》2005,127(31):11134-11139
Hydrogen-bonded disk-shaped aggregates (rosettes) composed of azobenzene-appended melamine and barbiturate or cyanurate are investigated in view of their hierarchical organization and photoresponsive behavior by (1)H NMR and UV/vis spectroscopies, dynamic light scattering, and gelation behavior in aliphatic solvents and liquid crystalline behavior in bulk state. In the bulk state the rosette possessing a sterically bulky tridodecyloxyphenyl substituent in the barbiturate component stacks in an offset arrangement to form a rectangular columnar mesophase, whereas in aliphatic solvents it does not hierarchically organize into higher-order columnar aggregates. This drawback is improved by exchanging the barbiturate component into a more sterically nondemanding N-dodecylcyanurate component. The resulting new rosette stacks in a face-to-face arrangement to form a hexagonal columnar mesophase in the bulk state and hierarchically organizes into elongated fibrous aggregates in cyclohexane, which eventually leads to the formation of organogel. Dynamic light scattering and UV-vis experiments upon UV-irradiation of the columnar aggregates in cyclohexane revealed that the dissociation and the reformation of columnar aggregates can be controlled by the trans-cis isomerization of the azobenzene moiety. Molecular modeling indicates that the rosette possessing cis-azobenzene side chains loses its planarity. Using this photoinduced morphological change of the rosette, photoresponsive organogel is created by the use of a disk-shaped supramolecule the first time. 相似文献
83.
Morinao Fukuoka Yoshito Onoda Satoru Inoue Kenji Wada Akihiko Nukui Akio Makishima 《Journal of Sol-Gel Science and Technology》1993,1(1):47-56
Binary sols and gels of SiO2-Al2O3 were prepared using tetraethyl orthosilicate and each of four aluminum compounds; aluminum di (sec-butoxide) ethylacetoacetic ester chelate (AC), aluminum nitrate nonahydrate (AN), aluminum formoacetate (AF), and boehmite sol (BS) made from aluminum i-propoxide. The structure and the evolution of the Si-O-Al bonds in SiO2-Al2O3 sols and gels were investigated by 27Al nuclear magnetic resonance (NMR), Infrared absorption spectra, DTA, and X-ray diffraction. The formation of Si-O-Al bonds differs depending on the aluminum compounds used as raw materials. The ratio of Al(IV) to {Al(IV) + Al(VI)} is related to the microstructural homogeneity of the gels. When AC is used as a raw material, the Si-O-Al bonds are formed in the sol state and resultant gel shows good microstructural homogeneity. In case of AN, the Si-O-Al bonds are not formed either in the sol or the wet gel state. The bonds are formed by drying the gel before heat-treatment temperature reaches 300 400°C, resulting in good microstructural homogeneous gel. When AF is used, the Si-O-Al bonds are formed in the sol state but the ratio of Al(IV) to {Al(IV) + Al(VI)} is lower than when using AC. Microstructural homogeneity of the gel is ranked between AC or AN and BS. Using BS, the Si-O-Al bonds are not formed in the sol solution, and the change in the coordination number of the gel is similar to that of boehmite gel. The Microstructural homogeneity of the gel is the worst among the BS gels, which were prepared by using the four aluminum raw materials. 相似文献
84.
The ether extract of the females of white-spotted longicorn beetle Anoplophora malasiaca showed activity as contact sex pheromone to males. The extract was fractionated, and a pheromonal activity was revealed only when three fractions; n-hexane, n-hexane/EtOAc 9:1, and EtOAc were blended. The relative structures of gomadalactone A, B, and C, three active components isolated from the EtOAc fraction, were determined by spectroscopic studies to be (1S∗,4R∗,5S∗)-5-hydroxy-4-[(E)-7-hydroxy-4-methylhept-3-enyl]-4,8-dimethyl-3-oxabicyclo[3.3.0]octan-7-en-2,6-dione, (1R∗,4R∗,5R∗)-5-hydroxy-4-[(E)-7-hydroxy-4-methylhept-3-enyl]-4,8-dimethyl-3-oxabicyclo[3.3.0]octan-7-en-2,6-dione, and (1S∗,4R∗,5S∗,8S∗)-5-hydroxy-4-[(E)-7-hydroxy-4-methylhept-3-enyl]-4,8-dimethyl-3-oxabicyclo[3.3.0]octan-2,6-dione, respectively. 相似文献
85.
Yuichi Hashimoto Toshiharu Ohta Koichi Shudo Toshihiko Okamoto 《Tetrahedron letters》1979,20(18):1611-1614
O-Acetates of 4-hydroxy and 4-hydroxy-7-methoxy-2H-1,4-benzoxazin-3-ones reacted with nucleophiles such as phenols, indoles and ethanethiol. The major reaction center is the positively-charged nitrogen atom. In the reaction with 2-methylindole, 6-position of the benzoxazinone ring is another major reaction center. 相似文献
86.
Recently, metal clusters supported on some solids which simultaneously exhibit many excellent properties of both metal clusters and supports have been intensively studled. Such surface organometallic chemistry, which concerns the synthesis, structure, reactivity and catalytic activity of surface-supported organometallfc compounds is a new 相似文献
87.
88.
Amane Mochizuki Michie Sakamoto Masahiro Yoshioka Takahiro Fukuoka Kazumasa Takeshi Mitsuru Ueda 《Journal of polymer science. Part A, Polymer chemistry》2000,38(2):329-336
A novel photosensitive polymer based on end‐capped polycarbodiimide (E‐PCD) and the photoamine generator N‐{[(4,5‐dimethoxy‐2‐nitrobenzyl)oxy]carbonyl}‐2,6‐dimethylpiperidine (1) was developed. The E‐PCD was prepared by polycondensation of tolylene‐2,4‐diisocyanate in the presence of 3‐methy‐1‐phenyl‐2‐phospholene‐1‐oxide as a catalyst. The reaction of carbodiimide and secondary amine was studied. The nucleophilic addition of amine to the carbodiimide moiety proceeded smoothly and provided a guanidine base that acted as a crosslinking agent. A lithographic evaluation of the system of E‐PCD and 1 revealed that the dissolution rate of the exposed area after the postexposure bake decreased drastically, although no obvious change of the dissolution rate in the unexposed area was observed. The E‐PCD containing 10 wt % 1 functioned as a negative working photosensitive polymer when it was exposed to 365‐nm UV light, postbaked at 120 °C for 5 min, and developed with toluene at 25 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 329–336, 2000 相似文献
89.
Keiji Itoh Masashi Sonobe Masaaki Sugiyama Kazuhiro Mori Toshiharu Fukunaga 《Journal of Non》2008,354(2-9):150-154
(Li2S)50(GeS2)50 glass was characterized by high-energy X-ray diffraction. The results were compared with previous neutron diffraction data. Moreover, Reverse Monte Carlo (RMC) modeling was performed using the X-ray and neutron data in order to obtain a three-dimensional structural model. The RMC model showed that the Ge–S framework structure was formed mainly by the connection of corner-sharing GeS4 tetrahedra. Furthermore, many lithium ions were coordinated by three non-bridging sulfurs and one bridging sulfur. 相似文献
90.