全文获取类型
收费全文 | 3677篇 |
免费 | 200篇 |
国内免费 | 52篇 |
专业分类
化学 | 2612篇 |
晶体学 | 36篇 |
力学 | 141篇 |
综合类 | 2篇 |
数学 | 458篇 |
物理学 | 680篇 |
出版年
2024年 | 33篇 |
2023年 | 67篇 |
2022年 | 373篇 |
2021年 | 330篇 |
2020年 | 223篇 |
2019年 | 202篇 |
2018年 | 177篇 |
2017年 | 143篇 |
2016年 | 219篇 |
2015年 | 143篇 |
2014年 | 177篇 |
2013年 | 289篇 |
2012年 | 244篇 |
2011年 | 277篇 |
2010年 | 156篇 |
2009年 | 137篇 |
2008年 | 144篇 |
2007年 | 118篇 |
2006年 | 93篇 |
2005年 | 75篇 |
2004年 | 48篇 |
2003年 | 46篇 |
2002年 | 36篇 |
2001年 | 20篇 |
2000年 | 15篇 |
1999年 | 11篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 12篇 |
1995年 | 13篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1966年 | 2篇 |
排序方式: 共有3929条查询结果,搜索用时 31 毫秒
41.
The sorption behaviour of 2.5 × 10−5 M solution of Cd(II) on polyurethane foam (PUF) from iodide medium have been investigated. The conditions were optimized from aqueous solutions of different pH (1-10) and of acids of varied concentration (0.01-1.0 M). The maximum concentration of KI was found to be 0.24 M and equilibration time was established to be 20 min. The data successfully followed the Freundlich and Dubinin-Radushkevich (D-R) isotherms at low metal ion concentration while Langmuir isotherm followed at higher metal ion concentration. The Freundlich parameter 1/n = 0.66 ± 0.02 have been evaluated whereas D-R isotherm yields the sorption free energy E = 10.5 ± 0.1 kJ mol−1 indicating ion exchange type chemisorption. The monolayer coverage (XL) constant of Langmuir isotherm was found to be 23.7 ± 0.4 mg g−1. The numerical values of thermodynamics parameters enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) indicated the endothermic and spontaneous nature of sorption. The Scatchard plot analysis was tested to evaluate the binding sites of the PUF and stability constants of sorption were determined. On the basis of these parameters, the sorption mechanism was discussed. Among the foreign ions tested, Pb(II), Hg(II), cyanide and nitrite should be absent. The clean separation of Cd(II) from Zn(II) ions in the ratio 1:250, respectively, was achieved by column chromatography. 相似文献
42.
(+)-Xyloketal D was prepared in a one-pot multistep domino reaction by heating optically active 5-hydroxy-4-methyl-3-methylenepentan-2-one (R) in toluene with 2,4-dihydroxyacetophenone. The absolute configuration of the natural product was confirmed by preparation of the starting enone from a lactone of established absolute configuration. 相似文献
43.
The singlet oxygen (1Δg) photooxidations of angelic acid salt (1), tiglic acid salt (2), 2,3-dimethyl-2-butenoic acid salt (3), 3-ethoxycarbonyl-5,6-dihydro-2-methyl-4H-pyrane acid salt (4), cis-3-hexenoic acid salt (5), and trans-3-hexenoic acid salt (6) were conducted in deuterated water. The major and minor ene allylic hydroperoxide products were quantified and indicate that the allylic hydrogen geminal to the carboxylate group is preferentially abstracted in 1-4, whereas the allylic hydrogen α to the carboxylate is slightly favored for 5 and 6. We have attributed the observed regiochemistry in 1-4 to stabilizing hydrogen bonding interactions between the solvent and the perepoxide, which leads to the major ene product. 相似文献
44.
Nisar Ahmad M. Akram K.P. Gill S.P. Asdaq R.M. Akhtar M. Saleem M.A. Baig 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,41(3):159-163
A narrow bandwidth ring dye laser pumped by an argon ion laser has been used to investigate the hyperfine structure of the even and odd parity levels of tantalum by optogalvanic spectroscopy in the wavelength range 5640 to 6050 Å. Seventeen transitions have been observed and eight of these have not been reported in the literature so far. These transitions involve 27 levels with 15 odd and 12 even parity configurations. The magnetic dipole hyperfine interaction constants A and the electric quadrupole interaction constants B for these levels have been computed and compared with the data available in literature. The results for the levels at 34799.71 cm?1, 26960.46 cm?1 and 19657.78 cm?1 are reported for the first time. 相似文献
45.
The apparent association constant of hydrofluoric acid Q
HF
'
in 1m NaCl aqueous media has been determined potentiometrically at pressures up to 1 kbar and temperatures up to 250°C. A concentration cell employing hydrogen electrodes was used. The association of HF increases with increasing temperature but decreases with increasing pressure. 相似文献
46.
Tayyebeh Madrakian Akram Esmaeili Azizeh Abdolmaleki 《Journal of Analytical Chemistry》2004,59(1):28-32
A new method for the rapid and sensitive determination of trace quantities of thiocyanate based on its Landolt effect on the bromate-hydrochloric acid reaction was developed. The induction period of the reaction is proportional to the SCN– concentration. The decolorization of methyl orange by the reaction products was used to monitor the reaction spectrophotometrically at 525 nm. We were able to determine thiocyanate in the range 2 × 10–7–4 × 10–5 M by this method. The relative standard deviation for 10 determinations of 1.5 × 10–6 M thiocyanate ion is 0.19% and the detection limit of the method was 7.00 × 10–8 M. The method was applied to the determination of thiocyanate in human blood serum and of saliva samples with satisfactory results. 相似文献
47.
Muhammad Nawaz Tahir Lela Gorgishvili Jixue Li Tatiana Gorelik Ute Kolb Lutz Nasdala Wolfgang Tremel 《Solid State Sciences》2007,9(12):1105-1109
Crystalline ZrO2 nanoparticles were prepared from zirconium isopropoxide by slow hydrolysis and subsequent hydrothermal treatment of solutions containing various amounts of sodium hydroxide at 180 °C. Whereas moderately basic solutions lead to the formation of nanoparticles of monoclinic ZrO2 with plate-like morphology, and nanoparticles of the cubic ZrO2 high-temperature polymorph with diameters of approx. 5 nm were obtained from strongly basic solutions. The morphology, structure and properties of as-synthesized nanoparticles were characterized using HRTEM, XRD, Raman spectroscopy, UV–vis, PL spectroscopy and BET measurements. The formation of both, the monoclinic and the cubic polymorph, was confirmed by electron microscopy and Raman spectroscopy. The crystallinity and morphology of the resulting ZrO2 nanoparticles can be adjusted by the choice of the reaction conditions. The cubic ZrO2 nanoparticles have a high surface area (300 m2/g) and exhibit a strong photoluminescence in the UV region. 相似文献
48.
The sorptive potential of sunflower stem (180-300 μm) for Cr(III) ions has been investigated in detail. The maximum sorption (≥85%) of Cr(III) ions (70.2 μM) has been accomplished using 30 mg of high density sunflower stem in 10 min from 0.001 M nitric and 0.0001 M hydrochloric acid solutions. The accumulation of Cr(III) ions on the sorbent follows Dubinin-Radushkevich (D-R), Freundlich and Langmuir isotherms. The isotherm yields D-R saturation capacity Xm = 1.60 ± 0.23 mmol g−1, β = −0.00654 ± 0.00017 kJ2 mol−2, mean free energy E = 8.74 ± 0.12 kJ mol−1, Freundlich sorption capacity KF = 0.24 ± 0.11 mol g−1, 1/n = 0.90 ± 0.04 and of Langmuir constant KL = 6800 ± 600 dm3 mol−1 and Cm = 120 ± 18 μmol g−1. The variation of sorption with temperature (283-323 K) gives ΔH = −23.3 ± 0.8 kJ mol−1, ΔS = −64.0 ± 2.7 J mol−1 K−1 and ΔG298k = −4.04 ± 0.09 kJ mol−1. The negative enthalpy and free energy envisage exothermic and spontaneous nature of sorption, respectively. Bisulphate, Fe(III), molybdate, citrate, Fe(II), Y(III) suppress the sorption significantly. The selectivity studies indicate that Cr(III), Eu(III) and Tb(III) ions can be separated from Tc(VII) and I(I). Sunflower stem can be used for the preconcentration and removal of Cr(III) ions from aqueous medium. This cheaper and novel sorbent has potential applications in analytical and environmental chemistry, in water decontamination, industrial waste treatment and in pollution abatement. A possible mechanism of biosorption of Cr(III) ions onto the sunflower stem has been proposed. 相似文献
49.
Muhammad Ashram 《Journal of inclusion phenomena and macrocyclic chemistry》2007,59(3-4):315-321
Two novel chromogenic cone calix[4]dibenzothiacrown ethers 3 and 4 in which nitrophenylazo groups attached at the phenyl ring of dibenzothiacrown unit were described. The extraction properties
of 3 and 4 toward different transition metal ions have been studied using conductometric technique and found to exhibit Cu2+ and Hg2+ selectivity with very high stability constants range from log K
assoc
= 5.19 to log K
assoc
= 8.72. 相似文献
50.
ViqarUddin Ahmad MuhammadAthar Abbasi Muhammad Zubair Naheed Fatima Umar Farooq M.Iqbal Choudhary 《Helvetica chimica acta》2004,87(1):67-72
Phytochemical investigation of Symplocos racemosa resulted in the isolation of two new glycosides, symploracemoside ( 1 ) and symplomoside ( 2 ), which are structurally related to the reported benzoylsalireposide ( 3 ) and salireposide ( 4 ). The structure elucidation of the isolated compounds was based primarily on 1D‐ and 2D‐NMR analysis, including COSY, NOESY, HMQC, and HMBC correlations. These glycosides showed inhibitory activity against snake‐venom phosphodiesterase I. 相似文献