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921.
A general method for the asymmetric synthesis of α-substituted β-amino secondary alcohols is described, which comprises the four-reaction sequence (1) the proline-mediated, asymmetric, three-component Mannich reaction of two different aldehydes, (2) nucleophilic carbon addition to aldehyde, (3) oxidation of the resulting alcohol to the corresponding ketone, and (4) diastereoselective reduction with LiAlH(O-t-Bu)3 or catecholborane. The former reductant afforded the 1,2-syn isomer, while the latter gave the 1,2-anti isomer stereoselectively. The present method was successfully applied to the efficient asymmetric synthesis of the N-terminal amino acid moiety of nikkomycin B and BX. 相似文献
922.
Platinum intercalated into a hydrotalcite-like solid, Mg0.74Al0.26(OH)2(NO3)0.26, was found to catalytically reduce interlayer nitrate (NO3-) to N2/N2O so as to give rise to a large surface area micro/mesoporous structure at lower temperature of ca. 300 degrees C, compared to 500 degrees C required for the decomposition of the pristine hydrotalcite phase. 相似文献
923.
W. Lyoo J. Yeum H. Ghim J. Park S. Lee J. Kim D. Shin J. Lee 《Colloid and polymer science》2003,281(5):416-422
To precisely identify the effect of the molecular weight of syndiotactic poly(vinyl alcohol) (s-PVA) on the water stability of a s-PVA/iodine complex film, we prepared four s-PVAs with similar syndiotactic diad (s-diad) contents of about 63% and with different number-average degrees of polymerization, Pn, of 900, 6,000, 10,000, and 17,000, respectively. The desorption behavior of iodine in the s-PVA/iodine complex film in water was investigated in relation to the solubility of s-PVA in water. The degree of solubility of a s-PVA film having different Pn in water at 80 °C was limited to about 0.3-10%, whereas the degrees of solubility of atactic PVA films with Pn of 6,000 and 10,000 were 100% at the same conditions. The degree of iodine desorption of the complex film decreased with increasing Pn of s-PVA. Especially, the degree of iodine desorption of a PVA drawn film having Pn of 17,000 was limited to 2%, regardless of soaking temperature from 40 to 80 °C. The desorption of iodine in water was strongly affected by the dissolution of PVA. In addition, the degree of iodine desorption of the drawn s-PVA/iodine film was larger than that of the undrawn one. 相似文献
924.
Fujio Mizukami Yoshimichi Kiyozumi Tsuneji Sano Shu-ich Niwa Makoto Toba Shigemitsu Shin 《Journal of Sol-Gel Science and Technology》1998,13(1-3):1027-1031
Amorphous alumina-silicas were prepared from a tetra-alkoxysilane and anhydrous aluminum trichloride or an aluminum alkoxide by a sol-gel process using 2-methyl-2,4-pentanediol, pinacol, 1,2-propanediol, 2,3-butanediol or ethylene glycol as the solvent or complexing agent, and the effect of diols and alkoxy groups on the physical and chemical properties of the alumina-silicas was examined. When the diol or the alkoxy group was bulky, the alumina-silicas had relatively larger micropores, a larger pore volume and higher surface areas. In the conversion of methanol catalyzed by the alumina-silicas, the bulkier diols and alkoxides gave catalysts that produced dimethyl ether in higher yield and hydrocarbons in lower yield. Thus, when ethylene glycol was used as the diol, the best catalyst for the production of hydrocarbons, especially the production of olefins such as ethylene, propylene and butene, was obtained. Furthermore, in comparison with alumina-silica prepared by a traditional kneading process, it was found that the sol-gel alumina-silica could efficiently convert methanol to dimethyl ether and hydrocarbons, but the material prepared by kneading had a very low conversion of methanol to other compounds. 相似文献
925.
Brill D Ahner W Baltes P Barth R Bormann C Cieslak M Debowski M Grosse E Henning W Koczon P Kohlmeyer B Miskowiec D Müntz C Oeschler H Pöppl H Pühlhofer F Sartorius S Schicker R Senger P Shin Y Speer J Stein J Stiebing K Stock R Ströbele H Völkel K Wagner A Walus W 《Physical review letters》1993,71(3):336-339
926.
Supramolecular chirality was optically induced in amorphous and achiral azobenzene polymer films by irradiation of a laser beam with elliptical polarization. The chirality resulted from helical orientation of azobenzene chromophores by a combined process of circular and linear polarization. The helix-handedness could be controlled by incident light-handedness. 相似文献
927.
The palladium-catalyzed three-component coupling reaction (TCCR) of aryl isocyanides, allyl methyl carbonate, and trimethylsilyl azide was conducted in the presence of Pd(2)(dba)(3).CHCl(3) (2.5 mol %) and dppe (1,2-bis(diphenylphosphino)ethane) (10 mol %). Allyl aryl cyanamides with a wide variety of functional groups were obtained in excellent yields. This palladium-catalyzed TCCR was further utilized for the synthesis of N-cyanoindoles. The reaction of 2-alkynylisocyanobenzenes, allyl methyl carbonate, and trimethylsilyl azide in the presence of Pd(2)(dba)(3).CHCl(3) (2.5 mol %) and tri(2-furyl)phosphine (10 mol %) at higher temperatures afforded N-cyanoindoles in good to allowable yields. (eta(3)-Allyl)(eta(3)-cyanamido)palladium complex, an analogue of the bis-pi-allylpalladium complex, is a key intermediate in the TCCR, and a pi-allylpalladium mimic of the Curtius rearrangement is involved to generate the (eta(3)-allyl)(eta(3)-cyanamido)palladium intermediate. 相似文献
928.
Lee SH Kim HK Seo JM Kang HM Kim JH Son KH Lee H Kwon BM Shin J Seo Y 《The Journal of organic chemistry》2002,67(22):7670-7675
Arteminolides B-D (2-4), new farnesyl protein transferase inhibitors, were isolated together with a known arteminolide A (1) and new regioisomers (5-7) of the compounds from the aerial parts of Artemisia argyi. Structures of these compounds were elucidated by spectroscopic methods and chemical conversion. Arteminolides inhibited the farnesyl protein transferase with IC(50) values of 0.7-1 microM, while the regioisomers 5-7 were inactive. In addition, it was proved that the exocyclic double bond of sesquiterpene lactone did not affect the inhibitory activity of arteminolide. The effects of compound 2 on H-Ras processing and cellular growth in H-ras-transformed cells were also evaluated. 相似文献
929.
Yin D Horiuchi S Morita M Takahara A 《Langmuir : the ACS journal of surfaces and colloids》2005,21(20):9352-9358
The technique of patterning of surfaces with metal-rich structures on micro- or nanoscales was developed by assembling metal nanoparticles into a thin film of polymer in a controllable way. Palladium (Pd) nanoparticles were incorporated into a thin film of poly(methyl methacrylate) (PMMA) using palladium (II) bis(acetylacetonato), Pd(acac)(2), as a precursor vaporized in a nitrogen atmosphere. Depending upon its dose, the irradiation of a PMMA film by UV light or an electron beam (EB) enhances its reducing capability against Pd(acac)(2). This dependency on dose can be used to control the formation and assembly of Pd nanoparticles. Using this technique, binary patterns consisting of metal-rich and metal-poor regions in the polymer film can be created simply by irradiating the surface of the polymer through a binary photomask. Besides the creation of binary patterns, it is also possible to create grayscale patterns where the density of Pd nanoparticles can be tuned to provide shades of gray by the use of light with continuously modulated intensity. Because the electron beam also enhances the reducing power of PMMA against Pd(acac)(2), it is thus possible to obtain highly metallized films with nanoscale pattern features. The PMMA film can be selectively removed by oxygen plasma treatment or by pyrolysis. Thus, highly metallized surfaces with binary or grayscale patterns can be obtained by selective removal of the PMMA films. The metallized regions possess relatively high resistivity against CF(4) plasma compared to the bare silicon surface; therefore, the metallized surface patterns can be transferred onto the underlying silicon substrate by CF(4) plasma treatment. Because of the nanosize effect of metal nanoparticles, the thermal treatment at 900 degrees C, which is significantly lower than the melting temperature of the bulk Pd, yields continuous metallic features by binding the assembled nanoparticles. 相似文献
930.
Spectroscopic and chemical characterizations of molecular size fractionated humic acid 总被引:1,自引:0,他引:1
A sample of humic acid was divided by ultrafiltration into five fractions of different molecular size (F1; 300 000: F2; 100 000-300 000: F3; 50 000-100 000: F4; 10 000-50 000: F5; 1000-10 000 daltons). Characterization by IR, and CPMAS C-13 NMR spectroscopy indicated that the molecules of the fraction of >/=100 000 daltons were primarily aliphatic, while the smaller molecules of the >/=10 000 dalton fraction were predominantly aromatic. Titration (pH) data were consistent with an increase in the number of carboxylate groups per unit mass as molecular size became smaller. A comparative study with unpurified and purified (by treatment with ion exchange elution, acid precipitation and alkaline dissolution) humic acid samples showed chemical alteration with some loss of carboxyl groups in the humic acid. 相似文献