全文获取类型
收费全文 | 1004篇 |
免费 | 15篇 |
国内免费 | 5篇 |
专业分类
化学 | 722篇 |
晶体学 | 9篇 |
力学 | 29篇 |
数学 | 63篇 |
物理学 | 201篇 |
出版年
2023年 | 5篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 14篇 |
2015年 | 16篇 |
2014年 | 7篇 |
2013年 | 39篇 |
2012年 | 35篇 |
2011年 | 49篇 |
2010年 | 19篇 |
2009年 | 26篇 |
2008年 | 45篇 |
2007年 | 28篇 |
2006年 | 43篇 |
2005年 | 41篇 |
2004年 | 45篇 |
2003年 | 33篇 |
2002年 | 47篇 |
2001年 | 25篇 |
2000年 | 20篇 |
1999年 | 15篇 |
1998年 | 20篇 |
1997年 | 8篇 |
1996年 | 14篇 |
1995年 | 6篇 |
1994年 | 12篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 18篇 |
1990年 | 11篇 |
1989年 | 20篇 |
1988年 | 10篇 |
1987年 | 11篇 |
1986年 | 14篇 |
1985年 | 22篇 |
1984年 | 26篇 |
1983年 | 11篇 |
1982年 | 26篇 |
1981年 | 19篇 |
1980年 | 20篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 24篇 |
1976年 | 9篇 |
1975年 | 11篇 |
1974年 | 16篇 |
1973年 | 12篇 |
1972年 | 7篇 |
1971年 | 4篇 |
1967年 | 5篇 |
排序方式: 共有1024条查询结果,搜索用时 62 毫秒
41.
Novel thermally crosslinkable fluorine‐containing poly(arylene ether ketone)s comprised of 2,3,5, 6‐tetrafluoro‐1,4‐phenylene moiety were synthesized by the termination of polymer chain ends with propargyl ether groups in order to improve solvent resistance. Crosslinking reaction occurred over 250°C through the formation of both chromen ring and polyene structure. This structure change brought about not only the outstanding solvent resistance but also the increase in glass transition temperature (Tg). The cured films also exhibited excellent thermal stability, transparency and hydrophobicity derived from fluorine atoms. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
42.
Polymerization of allyl esters of unsaturated acids. V. Cyclopolymerization of methyl allyl fumarate
Kunio Urushido Akira Matsumoto Masayoshi Oiwa 《Journal of polymer science. Part A, Polymer chemistry》1980,18(6):1771-1785
The kinetics of radical polymerization of methyl allyl fumarate (MAF) is discussed in terms of cyclopolymerization and compared with the polymerization results of methyl allyl maleate (MAM) as a cis isomer. In the polymerization of MAF, the rate and degree of polymerization were quite enhanced compared with MAM, and gelation occurred at low conversion. The content of the unreacted allylic double bonds of the MAF polymer was quite large; whereas those of the unreacted fumaric double bonds and the cyclic structural units showed reverse tendencies. Only a slight presence of a five-membered ring was observed in the MAF polymer. The cyclization constants KA and KV, the ratios of the rate constants of the unimolecular cyclization reaction to those of the bimolecular propagation reaction of the uncyclized allylic and fumaric radicals, were estimated to be 2.73 and 1.48 mole/liter, respectively. These values suggest the great difference in the cyclopolymerization behavior between two isomeric monomers. These results are discussed in detail in connection with the high reactivity of the fumaric double bond compared to the maleic double bond. In addition, the formation mode and the sequence distribution of the structural units of the polymer produced are discussed on the basis of these analytical results. Thus, for the MAF polymer obtained in the bulk polymerization, about 60% of the cyclic structure can be formed via the intramolecular attack of the uncyclized fumaric radical on the allylic double bond, as opposed to the case of MAM via the predominant intramolecular attack (ca. 90%) of the uncyclized allylic radical on the maleic double bond; these results and the low probability for the succession of cyclic structures and the rather high probability of a vinyl-to-vinyl addition are presented. 相似文献
43.
Atsuyoshi Ohno Yuji Ishihara Satoshi Ushida Shinzaburo Oka 《Tetrahedron letters》1982,23(31):3185-3188
Benzaldehyde are reduced to benzyl alcohol by a model compound of NAD(P)H almost quantitatively. Reductions of some other aldehydes are also mentioned. 相似文献
44.
Takeo Iida Motoharu Nakanishi Kunio Got
《Journal of polymer science. Part A, Polymer chemistry》1974,12(4):737-749
Poly(vinyl chloride) (PVC) alone or mixed with 10 wt-% and 50 wt-% TiO2, SnO2, ZnO, and Al2O3 were pyrolyzed by using a pyrolysis gas chromatograph. Benzene, toluene, ethylbenzene, o-xylene, styrene, naphthalene, and various chlorobenzenes were identified. No hydrocarbons could be detected in pyrolysis products of any samples at 200°C. More aromatic hydrocarbons than aliphatic hydrocarbons are released from the PVC–TiO2 system and in preheated PVC. The contrary result is observed in the PVC–ZnO and PVC–SnO2 systems. Aromatics having methyl endgroups are easily released from the PVC–ZnO and PVC–SnO2 systems and at elevated pyrolysis temperature, because methylene groups are easily isolated along the chain by ZnO, SnO2 and the heating. The release of ethylbenzene o-xylene, and chlorobenzenes suggests a repeated dehydrochlorination and recombination of HCl and Cl2 to double bonds along the chain. Possible decomposition mechanisms of PVC are discussed. 相似文献
45.
Comparison of PAMAM-Au and PPI-Au nanocomposites and their catalytic activity for reduction of 4-nitrophenol 总被引:4,自引:0,他引:4
Dendrimer-Au nanocomposites are prepared in aqueous solutions using poly(amidoammine)dendrimers (PAMAM) (generation 2, 3, and 5) and poly(propyleneimine)dendrimers (PPI)(generation 2, 3, and 4) by wet chemical NaBH(4) method. The Au nanoparticles thus obtained are 2-4 nm in diameter for both dendrimers and no generation dependence on the particle size is observed, whereas the generations of the dendrimers are increased as stabilization of Au-nanoparticles is achieved with lower dendrimer concentrations. Studies of the reduction reaction of 4-nitrophenol using these nanocomposites show that the rate constants for the PAMAM dendrimers (generations 2 and 3) are higher than those for the PPI dendrimers (generations 2 and 3), while a distinct difference in the rate constants is not seen for the PAMAM dendrimer (generation 5) or the PPI dendrimer (generation 4). In addition, the rate constants for the reduction of 4-nitrophenol involving all the dendrimers decrease with increases in dendrimer concentrations. 相似文献
46.
The C-12˜C-17 segment
of (+)-aplasmomycin (
) was synthesized stereoselectively starting from (−)-malic acid based on the stereoselective ketone reduction. 相似文献
47.
Sumitra Tavornvipas Hidetoshi Arima Fumitoshi Hirayama Kaneto Uekama Toshihiro Ishiguro Masahide Oka Kenichi Hamayasu Hitoshi Hashimoto 《Journal of inclusion phenomena and macrocyclic chemistry》2002,44(1-4):391-394
Some physicochemical and biological properties of a new branched cyclodextrin, 6-O--(4-O--d-glucuronyl)-d-glucosyl--cyclodextrin GUG--CyD) were investigated. Further, theinteraction of GUG--CyD with several drugs was studied by the solubility and spectroscopic methods, and compared with those of parent -CyD and 6-O--maltosyl--CyD(2--CyD).The hemolytic activity of GUG--CyD on rabbit erythrocytes was lower than those of -CyD and 2--CyD. GUG--CyD and 2--CyD showed negligible cytotoxicity on Caco-2 cells up to at least 0.1 M. The inclusion ability of GUG--CyD to neutral and acidic drugs was comparable to or slightly smaller than those of -CyD and 2--CyD, probably because of a steric hindrance of the branched sugar. On the other hand, GUG--CyD showed greater affinity for the basic drugs, compared with -CyD and 2--CyD, owing to the electrostatic interaction of its carboxylate anion with positive charge of basic drugs. Thus GUG--CyD may be useful as a safe solubilizing agent particularly for basic drugs. 相似文献
48.
Frans C. Mijlhoff Gerrit H. Renes Kunio Kohata Kazuko Oyanagi Kozo Kuchitsu 《Journal of Molecular Structure》1977,39(2):241-252
The structure of 1,1-difluoroethylene was determined, from gas phase electron diffraction data obtained independently in Leiden and Tokyo and the rotational constants of F2CCH2, F2CCHD and F2CCD2 derived from the microwave study by Chauffoureaux. The two electron diffraction data agreed without significant discrepancy. From a joint least squares analysis of the diffraction and microwave data, the following rg bond distances and rz bond angles were derived: CC = 1.340 ± 0.006 Å, C-F = 1.315 ± 0.003 Å, C-H = 1.091 ± 0.010 Å, ∠C-C-F = 124.7 ± 0.3°, ∠C-C-H = 119.0 ± 0.4°, where the uncertainties represent estimated limits of error. 相似文献
49.
Kunio Fujiwara 《Foundations of Physics》1980,10(3-4):309-331
We investigate the novel problem of what happens in special relativity and in relativistic field theories whenthree-dimensional space is quantized. First we examine the equation for elastic waves on a linear chain, the simplest example of a quantized medium, and propose, on its analogy, a nonlinearp-k relationp=k(sinhkl)/kl for light and material waves. Here,kl is a new variable which represents the space-quantization effect on the plane wave of wave numberk=|k|. (Note thatkl=0 givesp=k.) This relation makes the light velocity in vacuum dependent onkl. We postulate, however, that the phase and group velocities of each individual light wave are still invariant, and try to generalize special relativity to the case ofkl 0. This can be simply done if the invariance ofkl is assumed. Our results suggest that localization might be a relative concept. One interesting consequence of our proposal is that relativistic field theories become automatically finite. This comes out without violating unitarity or causality. A precise measurement of velocities of high-energy photons or massive particles is desirable for checking our proposal. 相似文献
50.
Kunio Miki Yasushi Kai Noritake Yasuoka Nobutami Kasai 《Journal of organometallic chemistry》1979,165(1):79-91
The precise molecular structure of [PdCl(CH2SCH3)(PPh3)2] has been determined from three-dimensional X-ray diffraction data collected at ?160°C. The CH2Cl2 solvated crystal ([PdCl(CH2SCH3)(PPh3)2 · CH2Cl2]) belongs to the monoclinic system, space group P21/n, with four formula units in a cell of dimensions: a 14.973(3), b 15.333(3), c 17.377(3) Å and β 115.77(1)° at ?160°C. The structure was solved by the conventional heavy atom method and refined by the least-squares procedure to R = 0.035 for observed reflections. The geometry around the palladium atom is square-planar. The phosphorus atoms of the two triphenylphosphine ligands are mutually trans. The CH2SCH3 group is bonded to the palladium atom only through the PdC σ-bond and the sulfur atom is not bonded to the metal atom (PdC(1) 2.061(3), SC(1) 1.796(3), SC(2) 1.817(5), Pd?S 2.973(1) Å, PdC(1)S 100.64(14)° and C(1)SC(2) 101.28(18)°). The structure is in contrast to that of [PdCl(CH2SCH3)(PPh3)], in which both the carbon and sulfur atoms of the CH2SCH3 group are bonded to the palladium atom. 相似文献