全文获取类型
收费全文 | 454篇 |
免费 | 24篇 |
国内免费 | 3篇 |
专业分类
化学 | 428篇 |
晶体学 | 4篇 |
力学 | 1篇 |
数学 | 18篇 |
物理学 | 30篇 |
出版年
2022年 | 5篇 |
2021年 | 9篇 |
2020年 | 13篇 |
2019年 | 10篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 17篇 |
2015年 | 14篇 |
2014年 | 8篇 |
2013年 | 27篇 |
2012年 | 27篇 |
2011年 | 23篇 |
2010年 | 15篇 |
2009年 | 19篇 |
2008年 | 26篇 |
2007年 | 29篇 |
2006年 | 26篇 |
2005年 | 29篇 |
2004年 | 18篇 |
2003年 | 15篇 |
2002年 | 23篇 |
2001年 | 5篇 |
2000年 | 8篇 |
1999年 | 9篇 |
1998年 | 6篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 7篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1970年 | 2篇 |
1969年 | 1篇 |
1963年 | 2篇 |
排序方式: 共有481条查询结果,搜索用时 15 毫秒
1.
The hydrothermal syntheses of the alkali metal molybdenum bronzes from starting solids (HxMoO3) with structural affinities to the desired products were investigated. Single-phase potassium blue and red bronzes were prepared by the hydrothermal treatments at around 430 K, and characterized by powder X-ray diffraction, IR spectroscopy, and SEM. The formation processes of these two bronzes during the hydrothermal treatments were found to differ. The blue bronze was formed by a structure-inheriting solid-state route from HxMoO3 with x<0.3, whereas the red bronze was formed for x>0.3 through a solution dissolution/deposition route via the formation of MoO3+MoO2. 相似文献
2.
Yoshito Ikada Yasuko Nishizaki Hiroo Iwata Ichiro Sakurada 《Journal of polymer science. Part A, Polymer chemistry》1977,15(2):451-460
Poly(vinyl alcohol) (PVAI) was oxidized by ceric ion, Ce(IV), in aqueous HNO3 medium at different temperatures and found to be degraded as a result of selective cleavage of the 1,2-glycol unit existing in PVAl. The rate of oxidation increased with increasing temperature. The aldehyde groups formed at the ends of the degraded polymer upon oxidation were relatively stable at 0°C. With rise of temperature, the aldehyde groups reacted either with excess of Ce(IV) to carboxylic acids or with hydroxyl groups of PVAl molecules to give acetal linkage. When the acetalization predominated over the oxidation to carboxyl group, gelation of the reaction mixture was observed. Based on these results, a plausible mechanism of oxidation of PVAl with Ce(IV) and the subsequent reactions is discussed. 相似文献
3.
A simple synthesis of 3,3,3-trifluoropropenyl compounds by means of the TBAF-mediated Horner reaction is described. The reagent, 2,2,2-trifluoroethyldiphenylphosphine oxide, was readily prepared either by Arbuzov reaction of ethyl diphenylphosphinite with 2,2,2-trifluoroethyl iodide or by treating chlorodiphenylphosphine with trifluoroacetic acid and water. Treatment of the phosphine oxide with aromatic aldehydes in the presence of TBAF at room temperature afforded the corresponding 3,3,3-trifluoropropenyl compounds in good yields. The present method is very convenient for preparing 3,3,3-trifluoropropenyl compounds from aromatic aldehydes in terms of availability of the reagent, operational simplicity, and good yields of the products. 相似文献
4.
Bunzo Eda Machio Iwasaki 《Journal of polymer science. Part A, Polymer chemistry》1986,24(9):2119-2129
In the γ-irradiated aromatic epoxy resin, diglycidylether of bisphenol A cured with diaminodiphenylmethane, the cyclohexadienyl-type radical is produced as one of the main radical species. Although both the resin and the hardener contain benzene rings, it is concluded that the cyclohexadienyl-type radical is formed by selective addition of the OH hydrogen to the benzene ring on the resin side. The selectivity is accounted for in terms of electron capture by the benzene ring followed by proton transfer from the OH group which is located in front of the benzene ring. This reaction scheme is further supported by the effect of the addition of an electron scavenger on the radical yield as well as by the photobleaching behavior of the radicals involved. 相似文献
5.
[reaction: see text] A new catalytic cycle has been developed to effect all three subgroups of Cr-mediated couplings, i.e., (1) Ni/Cr-mediated alkenylation, alkynylation, and arylation, (2) Co/Cr-mediated 2-haloallylation, alkylation, and propargylation, and (3) Cr-mediated allylation. In the presence of chiral sulfonamide ligands, good asymmetric inductions can be achieved for some of the Ni/Cr-mediated alkenylation, Co/Cr-mediated 2-haloallylation and propargylation, and Cr-mediated allylation. 相似文献
6.
We have determined the crystal structure of the title compound, which has a triclinic cell with cell parameters of , , , α=76.617°, β=84.188°, γ=74.510° and space group . The crystal structure suggests the chemical formula CoMoO4·3/4H2O. The structure consists of MoO4 tetrahedra and CoO6 octahedra, confirming the earlier X-ray absorption near-edge spectroscopic (XANES) investigation on the hydrate. The comparison of the crystal structures of the hydrate and the α-,β-, and hp-phases shows that the hydrate exhibits metal cation coordinations similar to those of the β-phase, but had arrangements of CoO6 and MoOn polyhedra similar to those of the hp-phase. 相似文献
7.
The purpose of this study is to propose the application of a compression test to the determination of an optimal formulation for extrusion granulation. The electric current during extrusion was measured and the characteristics of the wet kneaded mass in the compression test were analyzed under various operating conditions, with different types of extruders and several formulations of kneaded mass. It was found that addition of a binder (HPC-L) to pharmaceutical powders lowered the load of a high-compressing type extruder, since the binder reduced the friction among the wet mass during extrusion. Also, the support stress was found to be proportional to the compression pressure without a binder, although an inflection point appeared on the support stress curve when a binder was present. This inflection point suggested large water retention of the wet kneaded mass, at which the medium of pressure was changed from a discontinuous solid powder to a continuous liquid, and large water retention contributed to the low friction of the wet mass. The friction of the wet kneaded mass and the aptitude of the formulation for extrusion were understood by using the compression test. The compression test is a very useful procedure at the first stage of a formulation study. 相似文献
8.
[reaction: see text] With use of 1 mol % of Cr catalyst 5, surprisingly efficient Cr-mediated couplings of aldehydes with various types of nucleophiles have been realized. The catalyst set of Cr catalyst 5 and Ni catalyst 4 is used for alkenylation, alkynylation, and arylation, whereas the catalyst set of Cr catalyst 5 and CoPc (cobalt phthalocyanine) is used for 2-haloallylation, alkylation, and propargylation. Only the Cr catalyst 5 is required for allylation. The reaction rates in DME and THF have been found significantly faster than that in MeCN. 相似文献
9.
[6n]Paracyclophenediynes 2a-d (n = 3-6) having [4.3.2]propellatriene units were prepared as precursors of the corresponding [6n]paracyclophynes. Laser irradiation of 2a-d produced the negative ions of [6n]paracyclophynes 1a-d (n = 3-6) by extrusion of the indan fragments, which were detected by time-of-flight mass spectrometry. 相似文献
10.
A study of structural modifications of MPB-07 was undertaken as part of a synthetic program aimed at discovering small molecules with CFTR activation potential. Solid-phase synthesis techniques were used to prepare derivatives of MPB-07 employing the Zincke reaction for the construction of aromatic, quaternary ammonium salts such as those found in 2 or 3. In this transformation, primary amines react with highly electrophilic N-2,4-dinitrophenylpyridinium (DNP) salt 4 to afford pyridinium salt 8 with release of 2,4-dinitroaniline 6. Thus, the reaction of 1-(2,4-dinitrophenyl)pyridinium salts with various polymer-bound amino ethers, followed by cleavage from the resin, delivers the desired salts in good yield and high purity. 相似文献