全文获取类型
收费全文 | 719篇 |
免费 | 19篇 |
国内免费 | 7篇 |
专业分类
化学 | 519篇 |
晶体学 | 8篇 |
力学 | 16篇 |
数学 | 26篇 |
物理学 | 176篇 |
出版年
2021年 | 7篇 |
2020年 | 14篇 |
2019年 | 17篇 |
2018年 | 9篇 |
2017年 | 7篇 |
2016年 | 12篇 |
2015年 | 9篇 |
2014年 | 11篇 |
2013年 | 30篇 |
2012年 | 37篇 |
2011年 | 42篇 |
2010年 | 17篇 |
2009年 | 19篇 |
2008年 | 42篇 |
2007年 | 37篇 |
2006年 | 39篇 |
2005年 | 30篇 |
2004年 | 26篇 |
2003年 | 20篇 |
2002年 | 26篇 |
2001年 | 9篇 |
2000年 | 16篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 11篇 |
1990年 | 16篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 17篇 |
1986年 | 25篇 |
1985年 | 15篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 11篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1974年 | 10篇 |
1971年 | 2篇 |
1969年 | 3篇 |
排序方式: 共有745条查询结果,搜索用时 250 毫秒
11.
Eietsu Hasegawa Shinya Takizawa Akira Yamaguchi Naoki Chiba Hiroshi Ikeda 《Tetrahedron》2006,62(27):6581-6588
Photoinduced electron-transfer reactions of several ketone substrates were studied to evaluate the utilities of 1,6-bis(dimethylamino)pyrene (BDMAP), 1,6-dimethoxypyrene (DMP), 9,10-bis(dimethylamino)anthracene (BDMAA), and 9,10-dimethoxyanthracene (DMA) as electron-donating sensitizers cooperating with 2-aryl-1,3-dimethylbenzimidazolines. BDMAP and DMP generally led higher conversion of ketones and better yield of reduction products compared to BDMAA and DMA. 相似文献
12.
In this letter, we report synthesis of branched polysaccharide 2 by glycosylation of glucal-type monomer 1 with two free hydroxy groups at position 3 and 4. Monomer 1 polymerized with N-halosuccinimide promoter in acetonitrile solvent at room temperature--50 degrees C. The product was isolated as a petroleum ether insoluble fraction. The structure was determined by 1H and 13C NMR spectra as well as elemental analysis to be a polysaccharide consisting of 2-halo-2-deoxy-alpha-D-mannoside units, indicating that the polymerization proceeded via stereoregular glycosylation manner. The molecular weights determined by GPC with DMF were 3,300-4,000. The degree of branching was estimated by the NMR data of the product from the reaction of 2 with 3,5-dinitrobenzoyl chloride. 相似文献
13.
Polysiloxane formation in dimethyldiethoxysilane (DMDES)-ethyl alcohol (EtOH(D))-oxalic acid (OA) (DMDES:EtOH(D):OA=1:2:0.5) and DMDES-dimethylsulfoxide (DMSO)-OA (DMDES:DMSO:OA=1:2:0.5) systems was investigated by gas chromatography-mass spectrometry and 29Si-nuclear magnetic resonance. While the DMDES-EtOH(D)-OA system was homogeneous, the DMDES-DMSO-OA system consisted of two immiscible phases. In both systems, ethoxy-terminated linear oligomers ((EtO)Me2SiO(Me2SiO)nSiMe2(OEt); n=0–4, Et = C2H5, Me = CH3) and cyclic tetramer ((Me2SiO)4) were identified. The reaction mechanism for polysiloxane formation is discussed. 相似文献
14.
Won Ok Choi Mitsuo Sawamoto Toshinobu Higashimura 《Journal of polymer science. Part A, Polymer chemistry》1990,28(11):2923-2935
In the living cationic polymerization of isobutyl vinyl ether (IBVE) initiated by the hydrogen iodide/zinc halide (HI/ZnX2; X = I, Br, Cl) systems, the concentration ([P*]) of the living propagating species was determined by quenching with sodiomalonic ester ( 1 ). The quenching reaction was shown to be clean, instantaneous, and quantitative to give poly (IBVE) with a terminal malonate group from which [P*] was obtained by 1H-NMR spectroscopy. In the polymerizations in toluene below +25°C, [P*] was constant and equal to the initial concentration ([HI]0) of hydrogen iodide, independent of the type and concentrations of ZnX2 as well as monomer conversion. At 0 and +25°C, however, the living species started decaying immediately after the complete consumption of monomer. In contrast, such a decay process was absent at ?15°C even in the absence of monomer until about an hour (depending on the conditions) after the end of polymerization. The deactivation reaction was first order in [P*], and the lifetime (half-life) of the living species was longer at lower temperature and at lower ZnX2 concentration. On the basis of these [P*] and lifetime measurements, the HI/ZnX2 systems were also compared with the HI/I2 counterpart. 相似文献
15.
Hiroji Fukui Mitsuo Sawamoto Toshinobu Higashimura 《Journal of polymer science. Part A, Polymer chemistry》1994,32(14):2699-2708
Telechelic ( 8 ) and end-functionalized four-arm star polymers ( 9 ) were synthesized through the coupling reactions of end-functionalized living poly(isobutyl vinyl ether) ( 5; DP n ~ 10) with the bi-and tetrafunctional silyl enol ethers, H4-nC? [CH2OC6H4C(OSiMe3) = CH2]n ( 3: n = 2; 4: n = 4). The precursor polymers 5 were prepared by living cationic polymerization with functionalized initiators, CH3CH(Cl)OCH2CH2X(6), in conjunction with zinc chloride in methylene chloride at ?15°C. The initiators 6 were obtained by the addition of hydrogen chloride gas to vinyl ethers bearing pendant functional groups X , including acetoxy [? OC(O)CH3], styryl (? OCH2C6H4-p-CH = CH2), and methacryloyl [? OC(O)C(CH3) = CH2]. The coupling reactions with 3 and 4 in methylene chloride at ?15°C for 24 h afforded the end-functionalized multiarmed polymers ( 8 and 9 ) in high yield (>91%), where those with styryl or methacryloyl groups are new multifunctional macromonomers. © 1994 John Wiley & Sons, Inc. 相似文献
16.
K Ichimura H Yamanaka K Chiba T Shinozuka Y Shiki K Saito S Kusano S Ameniya K Oyama Y Nozaki 《Journal of chromatography. A》1986,374(1):5-16
Until now, there has been little work covering all of the main native adrenal-cortical steroids in blood. We therefore established a method for the simultaneous quantitative measurement of 14 native adrenal-cortical steroids, which involves capillary column gas chromatography-mass spectrometry (GC--MS). Serum steroids were purified from serum with the Extrelut mini-column and then converted into stable derivatives for GC-MS by a combination of boronic cyclization and trimethylsilyl and methyloxime derivatization. The sensitivities (with a signal-to-noise ratio greater than or equal to 7) of our GC-MS method ranged from 0.1 to 1.0 ng/ml of serum, and the coefficients of variation of intra- and inter-assays were less than 19% for each steroid. Our newly devised method involving a capillary column GC-MS system has been proven to be a simple and suitable method for a diagnosis requiring simultaneous detection of many native adrenal steroids in clinical practice. The analysis time is only 4 h. 相似文献
17.
Shin-Ichi Hirano Toshinobu Yogo Ko-Ichi Kikuta Wataru Sakamoto 《Journal of Sol-Gel Science and Technology》1994,2(1-3):329-334
The sol-gel processing is one of promising methods to fabricate well-derived integrated thin films at relatively low temperature. Ferroelectric niobates films do afford the possibilities for adding the values by the hybridization with semiconductor and/or electro-optic systems. The molecular level designing of the precursor solution was stressed as well as the control of key processing factors. The advantages of this sol-gel have been extended to prepare the ferroelectric Pb(Mg, Nb)O3-PbTiO3 solid solution films integrated on Si wafers. The crystallization of the films with desired crystal structure could be promoted by the controlled partial hydrolysis of designed alkoxide solution in the intermediate state and the pre-heated treatment of alkoxide-derived films in flow of water vapor and oxygen gas mixture. 相似文献
18.
Sol-Gel Processing of Transparent PLZT((Pb,La)(Zr,Ti)O3) Fibers 总被引:1,自引:0,他引:1
Kitaoka Kenji Takahara Koji Kozuka Hiromitsu Yoko Toshinobu 《Journal of Sol-Gel Science and Technology》1999,16(1-2):183-193
PLZT((Pb,La)(Zr,Ti)O3) ceramic fibers were prepared by the sol-gel method from a solution of lead acetate trihydrate, lanthanum isopropoxide, zirconium n-propoxide and titanium isopropoxide containing 2-methoxyethanol as the solvent. The as-drawn gel fibers were pretreated in an autoclave and preheated in flowing H2O vapor prior to the heat-treatment for crystallization. The effects of autoclaving and preheating in flowing H2O vapor on the structure and composition of the resultant fibers were investigated. Optically transparent single phase perovskite PLZT fibers of 5–15 m in thickness and 5 cm in length were successfully prepared when the autoclave-treatment and the preheating in flowing H2O vapor were conducted prior to crystallization. These treatments are thought to promote hydrolysis of the organic groups and polycondensation reaction in the gel fibers, leading to the successful formation of dense, crack-free transparent PLZT fibers. 相似文献
19.
Toshinobu Higashimura Osamu Kishiro 《Journal of polymer science. Part A, Polymer chemistry》1974,12(5):967-984
To clarify the nature of the propagating species in cationic polymerization of styrene catalyzed by acetyl perchlorate, the molecular weight distribution of the polymer was investigated under various conditions. The molecular weight distribution curve for the polymer obtained in methylene chloride at 0°C showed a double peak phenomenon. This suggests that two or more kinds of propagating species participate simultaneously in the propagation reaction. The weight fraction W(H) of the polymer corresponding to the higher molecular weight peak increased with increasing polarity of the solvent. W(H) decreased when the concentration of the ionic species was increased either by an increase of the catalyst concentration or by the addition of the common salt such as tetra-n-butylammonium perchlorate. On the other hand, the position of the peak in the molecular weight distribution curve was independent of polymerization conditions. It was concluded that the higher molecular weight part of the polymer was produced under conditions for conductive to dissociation of the propagating species and the less dissociated propagating species was responsible for the lower molecular weight part of the polymer. 相似文献
20.
Shokyoku Kanaoka Masahiko Minoda Mitsuo Sawamoto Toshinobu Higashimura 《Journal of polymer science. Part A, Polymer chemistry》1990,28(5):1127-1136
Amphiphilic block polymers of vinyl ethers (VEs). $\rlap{--} [{\rm CH}_{\rm 2} {\rm CH}\left( {{\rm OCH}_{\rm 2} {\rm CH}_{\rm 2} {\rm NH}_{\rm 2} } \right)\rlap{--} ]_m \rlap{--} [{\rm CH}_{\rm 2} {\rm CH}\left( {{\rm OR}} \right)\rlap{--} ]_n \left( {{\rm R: }n{\rm - C}_{{\rm 16}} {\rm H}_{{\rm 33}} ,{\rm }n{\rm - C}_{\rm 4} {\rm H}_{\rm 9} ;m \simeq 40,{\rm n} = 1 - 10} \right)$ were prepared, each of which consists of a hydrophilic segment with pendant primary amino groups and a hydrophobic poly(alkyl VE) segment. Their precursors were obtained by the HI/I2-initiated sequential living cationic polymerization of an alkyl VE and a VE with a phthalimide pendant (CH2 = CHOCH2CH2Im; Im; phthalimide group), where the segment molecular weights and compositions (m/n ratio) could be controlled by regulating the feed ratio of two monomers and the concentration of hydrogen iodide. Hydrazinolysis of the imide functions gave the target polymers which were readily soluble in water under neutral conditions at room temperature. These amphiphilic block polymers lowered the surface tension of their aqueous solutions (0.1 wt%, 25°C) to a minimum ? 30 dyn/cm when the hydrophobic pendant R was n-C4H9 (n = 4–9). The polymers with n-C4H9 pendants in the hydrophobic segment exhibited a higher surface activity than those with n-C16 H33 pendants. The surface activity of the polymers also depended on the pH of the polymer solutions; the surface activity increased in more basic solutions where the ionization of the amino group (? NH2)2? NH3⊕) is suppressed. 相似文献