全文获取类型
收费全文 | 58952篇 |
免费 | 6179篇 |
国内免费 | 11137篇 |
专业分类
化学 | 54686篇 |
晶体学 | 1941篇 |
力学 | 711篇 |
综合类 | 439篇 |
数学 | 3319篇 |
物理学 | 15172篇 |
出版年
2024年 | 182篇 |
2023年 | 1094篇 |
2022年 | 2192篇 |
2021年 | 2243篇 |
2020年 | 2432篇 |
2019年 | 2126篇 |
2018年 | 1773篇 |
2017年 | 2218篇 |
2016年 | 2507篇 |
2015年 | 2239篇 |
2014年 | 2858篇 |
2013年 | 5440篇 |
2012年 | 3952篇 |
2011年 | 4170篇 |
2010年 | 3524篇 |
2009年 | 4125篇 |
2008年 | 3856篇 |
2007年 | 3890篇 |
2006年 | 3608篇 |
2005年 | 3111篇 |
2004年 | 2894篇 |
2003年 | 2424篇 |
2002年 | 1992篇 |
2001年 | 1640篇 |
2000年 | 1493篇 |
1999年 | 1206篇 |
1998年 | 1009篇 |
1997年 | 858篇 |
1996年 | 749篇 |
1995年 | 753篇 |
1994年 | 665篇 |
1993年 | 529篇 |
1992年 | 501篇 |
1991年 | 382篇 |
1990年 | 264篇 |
1989年 | 224篇 |
1988年 | 191篇 |
1987年 | 127篇 |
1986年 | 98篇 |
1985年 | 114篇 |
1984年 | 90篇 |
1983年 | 39篇 |
1982年 | 65篇 |
1981年 | 91篇 |
1980年 | 64篇 |
1979年 | 68篇 |
1978年 | 44篇 |
1977年 | 50篇 |
1976年 | 30篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
XRD and XPS are used to study the dispersion state of CuO on ceria surface. The dispersion capacity values of CuO measured
by the two methods are consistent, which are of 1.20 mmol CuO/100 m2 CeO2. In addition, the results reveal that highly dispersed Cu2+ ions are formed at low CuO loadings and that increasing the CuO content to a value higher than its dispersion capacity produces
crystalline CuO after the surface vacant sites on CeO2 are filled. The atomic composition of the outermost layer of the CuO/CeO2 samples has been probed by using static secondary ion mass spectroscopy (SSIMS), and the ratim of Cu/Ce are found to be 0.93
and 0.46 for the 1.22 and 0.61 mmol CuO/CeO2 samples respectively. Temperature-programmed reduction (TPR) profile with two reduction peaks at 156 and 165°C suggests that
the reduction of highly dispersed Cu2+ ions consists of two steps and is easier than that of CuO crystallites, in which the TPR profile has only one reduction peak
at about 249°C. The above experimental results are in good agreement with the prediction of the incorporation model.
Project supported by the National Natural Science Foundation of China. 相似文献
982.
Wolfgang A. Herrmann Jo o D. G. Correia Georg R. J. Artus Richard W. Fischer Carlos C. Rom o 《Journal of organometallic chemistry》1996,520(1-2):139-142
Methyl(oxo)bis(η2-peroxo)rhenium(VII)1, the active species of the system CH3ReO3/H2O2 in the catalytic oxidation of different organic and organometallic compounds, is stabilized by a water molecule attached to the rhenium center. This water molecule can be removed and substituted by hexamethylphosphoramide (HMPA) to yield (hexamethylphosphoramide)methyl(oxo)bis(η2-peroxo rhenium(VII) (3). The synthesis, crystal structure (X-ray difraction study), and catalytic properties of which compound are reported. Crystal data are as follows: monoclinic, space group P21/n, A = 900.76(7) pm, B = 1229.80(11) pm, C = 1318.57(11) pm, β = 90.251(7)°, Rw = 0.034 for 1878 reflections. The catalytic properties of compound 3 in the oxidation of olefins with H2O2 are similar to those of 1. 相似文献
983.
Tetsuro Kato Ryosuke Nemoto Hisashi Mori Ryoetsu Abe Katsuo Unno Akio Goto Hideyuki Murota Masaoki Harada Motofumi Homma 《Applied biochemistry and biotechnology》1984,10(1-3):199-211
To achieve targeted distribution of anticancer drugs with sustained activity, ferromagnetic ethylcellulose microcapsules containing
an anticancer drug, mitomycin C (FM-MMC-mc), were prepared by a method based on phase separation principles. Two prototypes
of FM-MMC-mc were made: one with the drug as the core and zinc ferrite on its capsular surface (outer type); the other with
both the drug and zinc ferrite as the core (inner type). Both preparations provided a sustained-release property and a sensitive
response to conventional magnetic force, although certain differences in the release rate of drug, magnetic responsiveness,
and particle size were found between the two dosage forms. Animal studies showed that the magnetic microcapsules could be
magnetically controlled in the artery and urinary bladder. VX2 tumors in the rabbit hind limb and urinary bladder were successfully
treated with magnetic control of FM-MMC-mc. Pharmacokinetic study revealed that the targeting of the microcapsules markedly
enhanced the drug absorption into the surrounding tissues for a prolonged period of time. The results indicate the feasibility
and effectiveness of the magnetic microcapsules as a targeted drug delivery system. 相似文献
984.
An online UV photolysis and UV/TiO2 photocatalysis reduction device (UV–UV/TiO2 PCRD) and an electrochemical vapor generation (ECVG) cell have been used for the first time as an interface between high-performance liquid chromatography (HPLC) and atomic fluorescence spectrometry (AFS) for selenium speciation. The newly designed ECVG cell of approximately 115 L dead volume consists of a carbon fiber cathode and a platinum loop anode; the atomic hydrogen generated on the cathode was used to reduce selenium to vapor species for AFS determination. The noise was greatly reduced compared with that obtained by use of the UV–UV/TiO2 PCRD–KBH4–acid interface. The detection limits obtained for seleno-DL-cystine (SeCys), selenite (SeIV), seleno-DL-methionine (SeMet), and selenate (SeVI) were 2.1, 2.9, 4.3, and 3.5 ng mL–1, respectively. The proposed method was successfully applied to the speciation of selenium in water-soluble extracts of garlic shoots cultured with different selenium species. The results obtained suggested that UV–UV/TiO2 PCRD–ECVG should be an effective interface between HPLC and AFS for the speciation of elements amenable to vapor generation, and is superior to methods involving KBH4. 相似文献
985.
Appu Aravind 《Tetrahedron letters》2005,46(17):2965-2968
A highly regioselective oxidative cleavage of 1,3:4,6-di-O-benzylidene-d-mannitol was carried out using NBS and the resultant product was readily converted to the C2-symmetric chiral ligand, (R,R)-3,4-dihydroxy-1,5-hexadiene. On the other hand, reductive cleavage of 1,3:4,6-di-O-benzylidene-d-mannitol was achieved in a highly regioselective manner using BF3·OEt2 and Et3SiH to give a highly functionalized benzyl ether, which was converted to a synthetically useful C2-symmetric bis-amino alcohol derivative. 相似文献
986.
Superoxide sensor based on cytochrome c immobilized on mixed-thiol SAM with a new calibration method
B GeF Lisdat 《Analytica chimica acta》2002,454(1):53-64
Cytochrome c was immobilized on a mixed-thiol (mercaptoundecanoic acid/mercaptoundecanol) modified gold electrode (MUA:MU/cyt c electrode). Characterization of the cyt c electrode showed a quasi-reversible, electrochemical redox behavior with a formal potential of −13±5 mV (versus Ag/AgCl) for the surface adsorbed protein and 3±5 mV for covalently immobilized cyt c. The heterogeneous electron transfer rate constants were determined to be about 70 and 40 s−1 for both states of the protein, respectively. They were found to be significantly higher than those of pure MUA-modified cyt c electrodes (MUA/cyt c electrodes). The interaction of superoxide radicals (O2−) with the (MUA:MU)/cyt c electrode was characterized and used for an amperometric O2− detection. The influence of H2O2 and uric acid on the sensor signal was investigated. The sensitivity of the (MUA:MU)/cyt c electrode to O2− was significantly improved compared with that of the MUA/cyt c electrode. Based on a kinetic model for the superoxide detection system, a new calibration method was established. This simple and fast method used the spontaneous dismutation of KO2 and was compared with the enzymatic superoxide generation system using xanthine oxidase. 相似文献
987.
T. I. Mukhanova L. M. Alekseeva A. S. Shashkov V. G. Granik 《Chemistry of Heterocyclic Compounds》2004,40(1):16-21
Derivatives of furo[2,3-f]quinoline were synthesized by the reaction of the enamines of acetylacetone and benzoylacetone with 2-methoxycarbonyl-4-oxo-5,8-quinolinequinone. A derivative of pyrrolo[2,3-h]quinoline was obtained from N-benzyl--aminocrotonic ester. 相似文献
988.
Acylation of 4-α-furyl-4-N-benzylaminobut-1-enes with maleic anhydride gave 4-oxo-3-aza-10-oxatricyclo[5.2.1.01,5]dec-8-ene-6-carboxylic acid via amide formation followed by intramolecular Diels-Alder reaction of furan (IMDAF). The cycloaddition proceeded under mild reaction conditions (25 °C) and provided only the exo-adduct in quantitative yield. Treatment of this compound with PPA gave isoindolo[2,1-b][2]benzazepine derivatives via ring opening, aromatization and intramolecular electrophilic alkylation. In order to extend the scope of the reaction sequence, 7-oxo-5,11b,12,13-tetrahydro-7H-isoindolo[2,1-b][2]benzazepine-8-carboxylic acids were further transformed into useful synthetic intermediates. 相似文献
989.
I. V. Ukrainets E. V. Mospanova L. V. Sidorenko 《Chemistry of Heterocyclic Compounds》2007,43(7):863-870
An improved method is reported for the synthesis of a series of 1-hydroxy-3-oxo-5,6-dihydro-3H-pyrrolo[3,2,1-ij]quinoline-2-carboxylic
acid hetarylamides. The antitubercular activity of all of the compounds prepared has been studied. The structure-biological
activity dependence revealed is discussed.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1023–1033, July, 2007. 相似文献
990.
Vera P. Shmachkova Nina S. Kotsarenko Vladislav V. Kanazhevskiy Galina N. Kryukova Dimitri I. Kochubey Jacques Vedrine 《Reaction Kinetics and Catalysis Letters》2007,91(1):177-185
Methods of the preparation of catalysts for alkane skeletal isomerization based on uniform nanoparticles of sulfated zirconia
anchored to different supports were investigated. These catalysts were characterized by using the ICP, HRTEM and BET techniques.
The activities of the catalysts in the reaction of n-butane isomerization were measured and compared with those of bulk catalysts. 相似文献