全文获取类型
收费全文 | 13679篇 |
免费 | 414篇 |
国内免费 | 492篇 |
专业分类
化学 | 6541篇 |
晶体学 | 101篇 |
力学 | 740篇 |
综合类 | 16篇 |
数学 | 4034篇 |
物理学 | 3153篇 |
出版年
2024年 | 125篇 |
2023年 | 723篇 |
2022年 | 482篇 |
2021年 | 465篇 |
2020年 | 1564篇 |
2019年 | 1178篇 |
2018年 | 1028篇 |
2017年 | 858篇 |
2016年 | 834篇 |
2015年 | 655篇 |
2014年 | 822篇 |
2013年 | 2945篇 |
2012年 | 647篇 |
2011年 | 177篇 |
2010年 | 131篇 |
2009年 | 171篇 |
2008年 | 182篇 |
2007年 | 213篇 |
2006年 | 158篇 |
2005年 | 168篇 |
2004年 | 174篇 |
2003年 | 120篇 |
2002年 | 118篇 |
2001年 | 98篇 |
2000年 | 81篇 |
1999年 | 57篇 |
1998年 | 70篇 |
1997年 | 55篇 |
1996年 | 50篇 |
1995年 | 50篇 |
1994年 | 26篇 |
1993年 | 19篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 6篇 |
1988年 | 10篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 12篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1974年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 528 毫秒
151.
Petrović A. F. Petrović D. M. Leovac V. M. Budimir M. 《Journal of Thermal Analysis and Calorimetry》1999,58(3):589-596
The paper describes the results of differential thermal analysis of the octahedral Fe(III) complexes of the general formula [Fe(HLn)2]Cl and Fe(HL3)L3, as well as of the corresponding ligands H2Ln (H2Ln — tridentate salicylaldehyde semi thiosemi- and S-methylisothiosemi-carabazones with n=1, 2 and 3 respectively). The decomposition of the complexes involving sulphur-containing ligands (H2L2 and H2L3) starts with sulphur elimination. In case of the complexes [Fe(HL2)]Cl and [Fe(HL3)]Cl sulphur evolves independently, whereas with Fe(HL3)L3 it is eliminated within the SCH3 group. In the former case, sulphur elimination takes place at the same temperature for both complexes. The change in the coordination mode, being a consequence of the replacement of O by S, has no essential effect on thermal stability of the coordination polyhedron. The complexes involving ONN coordination, realized with the H2L3 ligand, exhibit a comparatively highest thermal stability of the coordination polyhedron.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
152.
Fe3+/V5+/TiO2复合纳米微粒光催化性能的研究 总被引:29,自引:1,他引:29
采用溶胶凝胶法制备了Fe^3 /V^5 /TiO2复合纳米微粒作为光催化剂。光降解反应结果表明,其掺杂催化剂Fe^3 /V^5 /TiO2的光催化活性明显提高。光电化学研究显示,铁离子可以成为电荷陷阱,促进空穴的界面传递反应。适量钒离子掺杂使TiO2电极的光电流升高,导带中电子浓度的增大,加快了界面的电子传递反应。共掺杂催化剂中,Fe^3 、V^5 分别提供了空穴与电子的陷阱,同时加快了电子与空穴的界面传递反应,从 更有效地提高光催化活性。双组份共掺杂为提高TiO2光催化活性提供新的途径。 相似文献
153.
154.
Cellulose-based hydrogel materials were prepared and modified with tannic acid and l-methionine using ionic liquid as the solvent. The gels were prepared to develop a sustained release medium for selenourea (SeU). The drug delivery characteristics of selenourea-loaded cellulose (CSeU), selenourea-loaded tannic acid-modified cellulose (CTSeU), and selenourea-loaded L-methionine-modified cellulose (CMSeU) were investigated in aqueous media and simulated gastric fluid (SGF) media. This modified gel beads have been characterized using field emission scanning electron microscope, X-ray energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, thermogravimetry–differential thermal analysis and swelling properties and compared with those of the unmodified ones. We also investigated the inhibitory effects of SeU released from these gels on the activity of mushroom tyrosinase. Out of all the gel materials, CTSeU showed maximum SeU release both in water and SGF media. However, tyrosinase inhibitory action in PBS medium was comparable for all the three gel materials. 相似文献
155.
《Arabian Journal of Chemistry》2020,13(8):6568-6579
In this work, the sol–gel technique was used to prepare a new organic–inorganic hybrid from Epoxidized Natural Rubber (ENR-50) and Titanium dioxide (TiO2) by blending different content of titania precursors (10, 30, and 50 wt%) with an ENR-50 matrix. A wide range of analyses was conducted to understand the nature of this hybrid and also to evaluate its potential uses in applications required high refractive index such as micro optical and optoelectronic devices. Results indicated that the ring-opening reaction of epoxide groups in ENR-50 increased with the increase of titania content in the hybrid resulting a strong bonding between titania and ENR-50 through TiOC bond, which was observed in FTIR spectrum at 1027–1028 cm−1. It is also observed a slight decrease in the intensity of the amorphous peak along with a new crystalline peak appeared at 2θ = 23 and 27° due to the crystalline nature of titania. The hybrids showed three thermal degradation steps in the range of temperature 76 to 769 °C due to the existence of the Ti moieties with the mixture of polymer chains, which in turn shifted the Tg at 24.3, 26.9 and 28.1 °C for the hybrid at 10, 30, and 50 wt% TiO2 compared to the Tg of ENR-50 at −18.4 °C respectively. The morphology of the ENR-50 showed clear changes during of the synthesis of ENR-50/TiO2 hybrids, these changes were proven by SEM, TEM, and AFM analyses. Uv–Vis results showed that the higher wavelength peak at 293 nm has shifted to 296, 298 and 300 nm for the hybrid at 10, 30, and 50 wt% TiO2 respectively due to the strong interaction between titania precursors and ENR-50 matrix. Furthermore, the hybrids showed good optical transparency in the visible light range. 相似文献
156.
Fe—Silicalite—2催化剂表面CO2加氢反应性能的研究 总被引:3,自引:0,他引:3
研究了Fe/Silicalite-2催化剂CO2加氢低碳烯烃反应性能,利用CO2-TPD,CO2/H2-TPSR和CO/H2-TPSR表征手段,考察了铁含量及MnO助剂对Fe/Silicalite-2催化剂CO2吸附脱附及加氢反应性能的影响,表明随铁含量增加可提高催化剂对CO2的吸附能力,有利于提高CO2加氢反应的转化率。 相似文献
157.
Gerald Giester 《Monatshefte für Chemie / Chemical Monthly》1992,123(11):957-963
Summary The crystal structure of the hydrothermally synthesized compound Fe(SeO2OH) (SeO4) · H2O was determined by single crystal diffraction methods:a=8.355(2) Å,b=8.696(2) Å,c=9.255(2) Å, =93.72(1)°,V=670.95 Å3;Z=4, space group P21/c,R=0.029,R
w=0.027 for 2430 independent reflections (sin /0.76 Å–1). Isolated FeO5(H2O)-octahedra share five corners with [SeO2OH] and [SeO4] groups to form sheets parallel to (100). These sheets are interconnected via hydrogen bonds only.
Die Kristallstruktur von Fe(SeO2OH)(SeO4)·H2O
Zusammenfassung Die Kristallstruktur der hydrothermal dargestellten Verbindung Fe(SeO2OH) (SeO4)·H2O wurde mittels Einkristallbeugungsmethoden bestimmt:a=8.355(2) Å,b=8.696(2) Å,c=9.255(2) Å, =93.72(1)°,V=670.95 Å3;Z=4, Raumgruppe P21/c,R=0.029,R w=0.027 für 2 430 unabhängige Reflexe (sin / 0.76 Å–1). Isolierte FeO5(H2O)-Oktaeder teilen fünf Ecken mit [SeO2OH]- und [SeO4]-Gruppen, wobei sie Schichten parallel (100) bilden. Diese Schichten sind nur über Wasserstoffbrücken miteinander verbunden.相似文献
158.
Bruce R. McGarvey 《Coordination chemistry reviews》1998,170(1):75-92
Using a newly proposed approach involving an internally consistent set of equations, the ligand field parameters Δ/ξ, V/ξ and k are obtained from literature values of the g matrix for strong field d5 systems of various conformations in which |Δ/ξ|≤10. Qualitative analysis of the observed results is done using the Angular Overlap Model, AOM. 相似文献
159.
《Talanta》2007,72(3):1157-1165
The chemical diversity of antioxidants in complex matrices such as plant extracts makes it difficult to separate and quantify antioxidants from these solutions. Therefore it is desirable to establish methods that can measure the total antioxidant capacity (TAC) levels directly from plant extracts. Iron(III)-based TAC assays, especially the most widely used FRAP (ferric-reducing antioxidant power), play an important role in this regard. However, many problems have been reported in the application of the FRAP assay, the most serious one being the incomplete oxidation of a number of antioxidants during the time protocol of the assay. Thus, six different ferric ion-based total antioxidant capacity (TAC) assays have been comparatively tested, modified, and improved so as to obtain more sensitive and precise results for complex mixtures, namely: 1,10-phenanthroline (o-phen) method (with incubation), batho-phenanthroline method (with incubation), original FRAP method, modified FRAP method (with incubation), original ferricyanide method, and modified ferricyanide method (with incubation). Two new assays in this regard (i.e., o-phen and batho-phen) have been established, and the existing assays (FRAP and ferricyanide) have been modified so as to let the oxidation reactions of antioxidants reach completion. The molar absorptivity for a variety of antioxidants was highest for modified FRAP, batho-phen, and original FRAP methods. The absorption maximum wavelength shifted batochromically to a higher extent for modified ferricyanide, FRAP, and batho-phen procedures, decreasing the possibility of interferences due to organics absorbing in the near-UV range of the visible spectrum where most antioxidant assays are performed. The linear concentration ranges were shown to be further extended and linear correlation coefficients improved with respect to the most widely used ferric-based assay, FRAP. Of the six assays tested and developed, only the modified ferricyanide procedure gave high intercept values and low addivitity of TAC values of constituents in complex mixtures, requiring further attention of method optimization. Thus, it was shown that the most widely used FRAP could be effectively modified, and o-phen, batho-phen, and ferricyanide methods constitute cheaper alternatives to FRAP under certain conditions, with partly improved molar absorptivity (and thus sensitivity) for antioxidants, lower intercept values (and higher precision), broader linear range (and higher flexibility), and better additivity of TAC values of antioxidant constituents in mixtures. 相似文献
160.
Yoshihiko Ito Yoshinori Inubushi Toru Sugaya Takeo Saegusa 《Journal of organometallic chemistry》1977,137(1):1-9
Reaction of carboxamides with Cu2O in the presence of t-butyl isocyanide gave new chelated copper(I) complexes, which probably are formed by the insertion of t-butyl isocyanide into the coppernitrogen bond of copper(I) amide isonitrile complexes, which were initially produced from the carboxamides and Cu2Ot-butyl isocyanide complex. The same chelated copper(I) complexes were prepared more readily by the reaction of the corresponding N-trimethylsilyl-carboxamides with Cu2Ot-butyl isocyanide complex. Reactions of the copper(I) complexes thus obtained with alkylating agents, such as alkyl halides, alkyl tosylates and triethyloxonium tetrafluoroborate, also were described. 相似文献