全文获取类型
收费全文 | 4715篇 |
免费 | 521篇 |
国内免费 | 615篇 |
专业分类
化学 | 4210篇 |
晶体学 | 103篇 |
力学 | 16篇 |
综合类 | 32篇 |
数学 | 9篇 |
物理学 | 1481篇 |
出版年
2024年 | 8篇 |
2023年 | 34篇 |
2022年 | 119篇 |
2021年 | 109篇 |
2020年 | 182篇 |
2019年 | 139篇 |
2018年 | 99篇 |
2017年 | 136篇 |
2016年 | 172篇 |
2015年 | 166篇 |
2014年 | 198篇 |
2013年 | 449篇 |
2012年 | 269篇 |
2011年 | 253篇 |
2010年 | 201篇 |
2009年 | 256篇 |
2008年 | 284篇 |
2007年 | 318篇 |
2006年 | 294篇 |
2005年 | 242篇 |
2004年 | 206篇 |
2003年 | 223篇 |
2002年 | 163篇 |
2001年 | 192篇 |
2000年 | 128篇 |
1999年 | 153篇 |
1998年 | 125篇 |
1997年 | 94篇 |
1996年 | 97篇 |
1995年 | 84篇 |
1994年 | 58篇 |
1993年 | 57篇 |
1992年 | 56篇 |
1991年 | 39篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 27篇 |
1987年 | 23篇 |
1986年 | 14篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 15篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1976年 | 6篇 |
1974年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有5851条查询结果,搜索用时 437 毫秒
181.
Cory Momany Marvin L. Hackert Jyotsna Sharma Narinder S. Poonia 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(3):343-354
Sodium salicylate (NaSal where Sal=2-hydroxybenzoate), when mixed with dibenzo-24-crown-8 (DB24C8) yields a bimetallic complex [NaSal]2DB24C8 in most polar organic media, while potassium salicylate (KSal) under similar conditions shows a tendency to yield 11 or 21 complexes depending upon medium or synthesis. However, the presence of both NaSal and KSal together results in a unique mixed cation complex of composition NaKSal2DB24C8. This product melts sharply (190-92°C) without decomposition, displays IR spectral characteristics comparable to those of [Na(Sal)]2DB24C8, and is stable in aqueous media as shown by the detectable cation effect on the UV absorption bands of Sal and DB24C8. Single crystal X-ray analysis of NaK(Sal)2DB24C8 reveals that the system represents a co-crystallization complex of individual (KSal)2DB24C8 and (NaSal)2DB24C8 molecules. The crystals are monoclinic,P21/c,a=19.976(2) Å,b=9.031(1) Å,c=25.541(5) Å,=122.065(9)°, Å3,T=298 K,Z=2+2, CuK =1.5418 Å, and 2 (2.5°–100°). FinalR factor for the 3012 observed reflections (F>3) is 0.092. Both the Na2- and K2-molecules possess crystallographic centers of symmetry with one metal and its associated anion on each side of the crown ring. However, the conformations of the crowns are very different in the two molecules, with the K2-crown being nearly planar and the Na2-crown being quite puckered. Four oxygen atoms from the DB24C8 (KO, 2.680–2.908 Å) and three carboxyl oxygen atoms (KO, 2.472–2.708 Å) from separate salicylate ions coordinate with each potassium. Three oxygens from the crown (NaO, 2.536–2.65 Å) and three carboxyl oxygens (NaO, 2.31–2.563 Å) coordinate with each sodium. The salicylate ions lie on opposite sides and nearly perpendicular (77.2°, Na2-molecule; 82.7° K2-molecule) to each crown but coordinate to both of the metal ions within a molecule. The K+K+ and Na+Na+ distances in the respective molecules are 3.95 and 3.34 Å.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82044 (18 pages). 相似文献
182.
The present paper gives an account of different aspects of the tracer diffusion of Cs+ ions in alkali metal bromides. We have measured the diffusion coefficients, D, of cesium ions in 1% agar gel medium at 25 ∘C using a zone-diffusion technique over a concentration range of 5 × 10−5 to 0.1 mol,dm−3. The values of the diffusion coefficients were found to deviate from theory, which are explained on the basis of different
types of interactions occurring in the ion-gel-water system. The study is also focused on the effect of alkali metal bromides
on the obstruction effect and activation energy for the tracer-diffusion of cesium ions in agar gel medium. It is observed
that both parameters, extent of obstruction, ∝, and activation energy, E, decrease with increasing charge density of the cation of the supporting electrolyte. The influence of these trends is explained
on the basis of competitive hydration between the ions and agar molecules, and the relative distortion in the water structure
that is brought about by these different ions and agar molecules. 相似文献
183.
《Electroanalysis》2006,18(22):2243-2250
This work is focused on the voltammetric examination of the ion exchange properties of a smectite type clay, before and after its modification by the replacement of its native interlamellar cations (Na+, K+, Ca2+) by hexadecyltrimethylammonium cations (HDTMA+). The raw clay and its organically modified form were first characterized by X‐ray diffraction (XRD) and N2 adsorption–desorption isotherms (BET method) that confirmed the modification via an intercalation process. These materials were subsequently coated onto glassy carbon surfaces, and the resulting modified electrodes were evaluated for the uptake of [Ru(NH3)6]3+ and [Fe(CN)6]3? ions used as redox probes. Some experimental parameters affecting the incorporation of the probes within the film, including the ionic strength, the surfactant loading and the solution pH are thoroughly examined, in order to highlight the mechanism of the process. The possibility of using the surfactant‐intercalated clay modified electrode as an electrochemical sensor for [Fe(CN)6]3? is also evaluated. 相似文献
184.
Nilton Soares Camilo 《Tetrahedron letters》2004,45(13):2821-2823
The acid-catalyzed addition of 1,3-dicarbonyl compounds and activated olefins (silyl enol ethers and ethyl vinyl ether) to N-Boc-2-methoxypyrrolidine (1a) and N-Boc-2-methoxypiperidine (1b) in SDS/water medium is described. Good yields of the corresponding 2-substituted N-Boc pyrrolidines were generally observed from 1a while moderate yields prevailed from 1b. 相似文献
185.
2-{[1-(3,4-Dihydroxyphenyl)methylidene]amino}benzoic acid (DMABA) was loaded on Amberlite XAD-16 (AXAD-16) via azo linker and the resulting resin AXAD-16-DMABA explored for enrichment of Zn(II), Mn(II), Ni(II), Pb(II), Cd(II), Cu(II), Fe(III) and Co(II). The optimum pH values for extraction are 6.5-7.0, 5.0-6.0, 5.5-7.5, 5.0-6.5, 6.5-8.0, 5.5-7.0, 4.0-5.0 and 6.0-7.0, respectively. The sorption capacity was found between 97 and 515 μmol g−1 and the preconcentration factors from 100 to 450. Tolerance limits for foreign species are reported. The kinetics of sorption is fast as t1/2 is ≤5 min. The chelating resin can be reused for 50 cycles of sorption-desorption without any significant change (<1.5%) in the sorption capacity. The limit of detection values (blank +3 s) are 1.12, 1.38, 1.76, 0.67, 0.77, 2.52, 5.92 and 1.08 μg L−1 for Zn(II), Mn(II), Ni(II), Pb(II), Cd(II), Cu(II), Fe(III) and Co(II), respectively. The enrichment on AXAD-16-DMABA coupled with monitoring by flame atomic absorption spectrometry (FAAS) is used to determine all the metal ion ions in river and synthetic water samples, Co in vitamin tablets and Zn in milk samples. 相似文献
186.
Sorina M. Ulmeanu Henrik Jensen Zdenek Samec Graldine Bouchard Pierre-Alain Carrupt Hubert H. Girault 《Journal of Electroanalytical Chemistry》2002,530(1-2)
Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES). 相似文献
187.
金属离子对CdS量子点/聚酰胺-胺树形分子纳米复合材料光致发光性能的影响 总被引:1,自引:0,他引:1
以聚酰胺-胺树形分子为模板制备了分散好、尺寸均匀的CdS量子点,并用分光光度滴定法研究了Cd2+、Zn2+、Pb2+、Cu2+、Mn2+几种金属离子对其光致发光性能的影响。发现不同离子对CdS量子点的发光性能影响不同:Cd2+和Zn2+使量子点荧光增强,Pb2+、Cu2+和Mn2+使其荧光有不同程度淬灭。这归因于金属离子对CdS量子点表面的修饰作用。Cd2+能减少由S2-悬键构成的非辐射复合中心,增强树形分子对量子点表面缺陷的钝化作用,并能在量子点周围形成类肖特基能垒,从而显著增大CdS量子点的光致发光效率。由于ZnS与CdS的晶格参数非常接近,Zn2+能起到与Cd2+类似的作用,使CdS量子点的发光效率大大增强。Pb2+和Cu2+能取代Cd2+在CdS量子点表面生成窄带隙的壳层,对其发光有很强的淬灭作用。由于块体PbS的带隙比块体CuS窄,故Pb2+的淬灭能力强于Cu2+。Mn2+能破坏Cd2+与PAMAM树形分子的配位键,降低树形分子对CdS量子点表面缺陷的钝化作用,且其本身在量子点表面构成了新的荧光淬灭中心,但Mn2+也能形成较弱的类肖特基能垒,故对量子点的发光淬灭作用较弱。 相似文献
188.
碘酸钾存在下极谱催化波法测定陈皮甙 总被引:2,自引:0,他引:2
基于陈皮甙在KIO3存在下产生的极谱催化波,拟定了测定陈皮甙的新方法。在 0.55 mol/L HAc-0.01mol/L NaAc(pH 4.00±0.05)-1.0×10~(-3)mol/L KIO3缓冲溶液中,陈皮甙催化波的峰电位为-1.35 V(vs,SCE),它的一阶导数峰峰电流与陈皮甙浓度在2.6 × 10~(-7)~2.6 × 10~(-6)mol/L范围内呈线性关系(r=0.9970,n=6)。本方法比现有的电化学方法灵敏度高一个数量级。可用于直接测定中草药贯众水提液中陈皮甙含量。 相似文献
189.
《Chemphyschem》2003,4(12):1299-1307
Two classes of fullerene‐based donor–bridge–acceptor (D–B–A) systems containing donors of varying oxidation potentials have been synthesized. These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p‐anisidine/p‐toluidine). In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents. An increase in the rate constant and quantum yield of charge separation (kcs and Φcs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups. This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve. The long‐lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy. The small reorganization energy (λ) of C60 coupled with large negative free energy changes (‐ΔG°) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products. 相似文献
190.
Abdelfetah Mounir Mohamed El Guendouzi Abderrahim Dinane 《Journal of solution chemistry》2002,31(10):793-799
Ternary aqueous solutions of MgSO4 and K2SO4 have been studied by the hygrometric method at 25°C. The relative humidity of this system is measured at total molalities from 0.35 mol-kg–1 to about saturation for three ionic-strength fractions (y = 0.25, 0.50, and 0.80 of MgSO4. The data allow calculation of water activities and osmotic coefficients. From these measurements, the Pitzer ionic mixing parameters are determined and used to predict the solute activity coefficients in the mixture. The results are used to calculate the excess Gibbs energy at total molalities for ionic-strength fraction y. 相似文献