全文获取类型
收费全文 | 1482篇 |
免费 | 158篇 |
国内免费 | 116篇 |
专业分类
化学 | 748篇 |
晶体学 | 10篇 |
力学 | 93篇 |
综合类 | 12篇 |
数学 | 409篇 |
物理学 | 484篇 |
出版年
2023年 | 9篇 |
2022年 | 15篇 |
2021年 | 22篇 |
2020年 | 22篇 |
2019年 | 171篇 |
2018年 | 48篇 |
2017年 | 61篇 |
2016年 | 38篇 |
2015年 | 37篇 |
2014年 | 43篇 |
2013年 | 59篇 |
2012年 | 40篇 |
2011年 | 76篇 |
2010年 | 53篇 |
2009年 | 82篇 |
2008年 | 73篇 |
2007年 | 82篇 |
2006年 | 64篇 |
2005年 | 58篇 |
2004年 | 45篇 |
2003年 | 64篇 |
2002年 | 85篇 |
2001年 | 30篇 |
2000年 | 25篇 |
1999年 | 28篇 |
1998年 | 25篇 |
1997年 | 27篇 |
1996年 | 18篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 18篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 13篇 |
1989年 | 16篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 18篇 |
1984年 | 18篇 |
1983年 | 9篇 |
1982年 | 20篇 |
1981年 | 20篇 |
1980年 | 12篇 |
1979年 | 36篇 |
1978年 | 14篇 |
1977年 | 22篇 |
1976年 | 13篇 |
1975年 | 15篇 |
1974年 | 16篇 |
1973年 | 19篇 |
排序方式: 共有1756条查询结果,搜索用时 78 毫秒
61.
62.
To investigate the pharmacokinetic mechanism of hepatobiliary excretion and brain distribution of caffeine, this study uses a method based on microdialysis technique and liquid chromatography that allows continuous and concurrent in vivo monitoring of extracellular caffeine in the blood, brain and bile of anesthetized rats following the administration of caffeine (3 or 10 mg/kg, i.v.) through the femoral vein. Dialysates of the blood, brain and bile were directly injected onto the liquid chromatographic system and no further clean-up procedures were required. The study design consisted of two groups of six rats in parallel: the rats of the control group received caffeine (3 or 10 mg/kg, i.v.) alone and those of the cyclosporine treated-group were injected cyclosporine (10 mg/kg, i.v.) 10 min prior to caffeine administration (3 or 10 mg/kg, i.v.). The decline of caffeine in the blood, brain striatum and bile suggested that caffeine had rapid exchange and equilibration between the peripheral compartment and the central nervous system. In addition, the results indicated that caffeine underwent hepatobiliary excretion and was distributed into brain. When cyclosporine was co-administered, the pharmacokinetic parameters were not significantly altered. The results of this study reveal that the pharmacokinetic mechanism of hepatobiliary excretion and brain distribution of caffeine might not relate to P-glycoprotein. 相似文献
63.
The Prigogine-Flory-Patterson theory of liquid mixtures has been qpplied to the H
m
E
and V
m
E
for binary mixtures of a bicyclic compound, benzene, cyclohexane and n-hexane with a cycloalkane, cyclohexene, a cycloalkadiene and benzene. Furthermore the Prigogine-Flory theory has been used to predict activity coefficients at infinite dilution from the experimentally determined H
m
E
at 25°C for the mixtures cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene and benzene with a bicyclic compound. The predictions are compared to experimental results. 相似文献
64.
磷酰氨基酸的酯交换反应研究 总被引:1,自引:0,他引:1
本文利用^3^1P NMR , 对比研究了磷酰化组氨酸与其它磷酰化氨基酸和一级醇的磷上酯交换的反应速度,实验表明磷酰化组氨酸的反应速度最快. 由此提出了以下机理:由于咪唑环的参与,磷可以形成六配位过渡态,从而加快了反应速度. 相似文献
65.
3-[1′(1′H)-Substituent-pyrazol-5′-yl]benzo[5,6]coumarins and 3-(1′,2′-oxazol-5′-yl)benzo[5,6]coumarin were prepared via condensation of 3-(2′-formyl-1′-chlorovinyl)benzo[5,6] coumarin with hydrazine derivatives or hydroxylamine.Reaction of 3-[1′(1′H)-pyrazol-5′-yl]benzo[5,6]coumarin with alkyl halides,olefinic compunds or acid chlorides are described. 相似文献
66.
The development of a stereospecific synthesis of a key intermediate in the synthesis of (±)-zoapatanol from the Weiland-Miescher ketone is described. 相似文献
67.
G.R. Clark C.M. Cochrane W.R. Roper L.J. Wright 《Journal of organometallic chemistry》1980,199(2):C35-C38
The osmium carbyne complex, Os(CR)Cl(CO)(PPh3)2, (R p-tolyl) reacts with Group I halides to form the mixed dimetallocyclopropene species, ath>Os(Cul)(C ath>R)Cl(CO)(PPh3)2, ath>Os(AgCl)(C ath>R)Cl(CO)(PPh3)2, ath>Os(AuCl)(C ath>R)Cl(CO)(PPh3)2, and [ath>Os[Ag(OClO 3 )](C ath>R)Cl(CO)(MeCN)(PPh3)2] ClO4 X-ray crystal structure determination of ath>Os(AgCl)(C ath>R)Cl(CO)(PPh3)2 confirms the presence of a three-membered ring and the structure can be viewed as the “acetylene-like” interaction of an osmium—carbon triple bond with AgCl. In acid solution AgCl is precipitated and an alkylidene complex results from proton addition to the carbyne ligand. 相似文献
68.
Hesham Ibrahim Saleh 《中国化学会会志》2002,49(3):341-348
The reactions between strontium and iron nitrates have been studied in an open atmosphere system using three different molar ratios, 1:1 (I), 1:2 (II) and 2:1 (III) at different temperatures as pointed out from the DTA data. The reaction mechanism was discussed based on the chemical composition characterized by means of thermal analysis, X‐ray diffraction patterns, infrared spectra and magnetic susceptibility. It was found that the reaction products depend on both temperature of reaction and the ratio between reactants. The reaction products were found to be composed of a variety of iron compounds that possess different valences: SrFeO2.86, SrFeO2.97, SrFe2O4, SrFe12O19, Sr2Fe2O5 and Sr7Fe10O22 in addition to some accessory reaction products namely α‐Fe2O3 and FeO(OH). 相似文献
69.
The 57Fe Mössbauer effect in [Fe(pythiaz)2] (BF4)2 (I) and [Fe(pythiaz)2] (C&O4)2 (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔEQ(5T2) = 1.29 mm sec?1 and δ1S(5T2) = +0.93 mm sec?1 characteristic of a 5T2 ground state. At 236°K, a second doublet, typical for a 1A1 ground state appears. The transition 5T2 å 1A1 progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the 1A1 state, and ΔEQ(1A1) = 1.59 mm sec?1 and δ1S(1A1) = +0.26 mm sec?1. In an applied magnetic field, Vzz(1A1) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5T2 and f1A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the 5T2 and constant μeff for the 1A1 ground state. The resulting temperature dependence of f1A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions. 相似文献
70.
《Arabian Journal of Chemistry》2020,13(9):7115-7131
The impregnation of magnetite (Mt) nanoparticle (NPs) onto Musa acuminata peel (MApe), to form a novel magnetic combo (MApe-Mt) for the adsorption of anionic bromophenol blue (BPB) was studied. The SEM, EDX, BET, XRD, FTIR and TGA were used to characterize the adsorbents. The FTIR showed that the OH and Caticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/dbnd.gif" alt="double bond">O groups were the major sites for BPB uptake onto the adsorbent materials. The average Mt crystalline size on MApe-Mt was 21.13 nm. SEM analysis revealed that Mt NPs were agglomerated on the surface of the MApe biosorbent, with an average Mt diameter of 25.97 nm. After Mt impregnation, a decrease in BET surface area (14.89 to 3.80 m2/g) and an increase in pore diameter (2.25–3.11 nm), pore volume (0.0052–0.01418 cm3/g) and pH point of zero charge (6.4–7.2) was obtained. The presence of Pb(II) ions in solution significantly decreased the uptake of BPB onto both MApe (66.1–43.8%) and MApe-Mt (80.3–59.1%), compared to other competing ions (Zn(II), Cd(II), Ni(II)) in the solution. Isotherm modeling showed that the Freundlich model best fitted the adsorption data (R2 > 0.994 and SSE < 0.0013). In addition, maximum monolayer uptake was enhanced from 6.04 to 8.12 mg/g after Mt impregnation. Kinetics were well described by the pseudo-first order and liquid film diffusion models. Thermodynamics revealed a physical, endothermic adsorption of BPB onto the adsorbents, with ΔHo values of 15.87–16.49 kJ/mol, corroborated by high desorption (over 90%) of BPB from the loaded materials. The viability of the prepared adsorbents was also revealed in its reusability for BPB uptake. 相似文献