全文获取类型
收费全文 | 3343篇 |
免费 | 173篇 |
国内免费 | 697篇 |
专业分类
化学 | 3862篇 |
晶体学 | 31篇 |
力学 | 13篇 |
综合类 | 27篇 |
数学 | 1篇 |
物理学 | 279篇 |
出版年
2024年 | 3篇 |
2023年 | 28篇 |
2022年 | 75篇 |
2021年 | 41篇 |
2020年 | 67篇 |
2019年 | 80篇 |
2018年 | 66篇 |
2017年 | 78篇 |
2016年 | 110篇 |
2015年 | 113篇 |
2014年 | 110篇 |
2013年 | 395篇 |
2012年 | 178篇 |
2011年 | 178篇 |
2010年 | 173篇 |
2009年 | 192篇 |
2008年 | 178篇 |
2007年 | 249篇 |
2006年 | 205篇 |
2005年 | 196篇 |
2004年 | 207篇 |
2003年 | 172篇 |
2002年 | 130篇 |
2001年 | 136篇 |
2000年 | 130篇 |
1999年 | 117篇 |
1998年 | 84篇 |
1997年 | 72篇 |
1996年 | 86篇 |
1995年 | 48篇 |
1994年 | 59篇 |
1993年 | 65篇 |
1992年 | 52篇 |
1991年 | 33篇 |
1990年 | 21篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 7篇 |
1986年 | 11篇 |
1985年 | 3篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有4213条查询结果,搜索用时 15 毫秒
991.
We report ignition delay time measurements for methyl propanoate (MP) and methyl acrylate (MA), carried out in a high-pressure shock tube. Experiments were performed behind reflected shock waves across a temperature range of 989-1 367 K, for fuel-air mixtures at equivalence ratios of ϕ = 0.5, 1.0, and 2.0, and nominal pressures of 1 and 4 MPa. Ignition delay times were found to decrease with increasing temperature, equivalence ratio, and pressure, and are well described with correlations involving Arrhenius temperature dependence and power-law dependence on equivalence ratio and pressure. Ignition delay times are compared with model predictions from literature kinetic models, with the models of Zhang et al (Energy & Fuels 2014; 28(11): 7194-7202) and Bennadji et al (International Journal of Chemical Kinetics 2011; 43(4): 204-218.) in good agreement with measured ignition delay times for MP and MA, respectively. Kinetic sensitivity analysis shows that the reactions most important for modeling ignition fall into two categories: initiation reactions (ie, decomposition and H-atom abstraction) and C0-C1 chemistry controlling the pool of small radicals. The unsaturated MA was observed to have lower reactivity than MP, due to its greater bond strengths for C─C and C─H bonds, resulting in slower rates for initiation reactions. 相似文献
992.
Xiao-Zhong Fu Feng-Jie Jiang Yu Ou Sheng Fu Yu-Feng Cha Shun Zhang Zong-Yuan Liu Wen Zhou Ai-Min Wanga Yong-Lin Wang 《中国化学快报》2014,25(1):115-118
A series of mono L-amino acid ester, mono non-steroid anti-inflammation drug(NSAID), carboxylic ester derivatives of acyclonucleoside phosphonates were prepared by using a ‘‘one pot synthesis' ' method and their in vitro anti-HBV activity were evaluated in HepG 2.2.15 cells. Compound 9a exhibited more potent anti-HBV activity and lower cytotoxicity than those of adefovir dipivoxil with IC50 and selective index(SI) values of 0.48 mmol/L and 763.72, respectively. 相似文献
993.
994.
Yuran Wang Sen Lian Xiangli Dong Caixia Wang Baohua Li Pingliang Li 《Biomedical chromatography : BMC》2019,33(2)
As one of the main fungicides for the apple leaf disease control, thiophanate‐methyl (TM) mainly exerts its fungicidal activity in the form of its metabolite carbendazim (MBC), whose dissipation kinetics is very distinct from that of its parent but has been paid little attention. The aim of this work was to investigate the dissipation kinetics of TM and its active metabolite MBC in apple leaves using a modified QuEChERS–UPLC–MS/MS method. The results showed that TM and MBC could be quickly extracted by this modified QuEChERS procedure with recoveries of 81.7–96.5%. The method linearity was in the range of 0.01–50.0 mg kg?1 with the quantification limit of 0.01 mg kg?1. Then this method was applied to the analysis of fungicide dissipation kinetics in apple leaves. The results showed that the dissipation kinetics of TM for the test in 3 months can be described by a first‐order kinetics model with a DT50 (dissipation half‐life) range of 5.23–6.03 days and the kinetics for MBC can be described by a first‐order absorption–dissipation model with the Tmax (time needed to reach peak concentration) range of 4.78–7.09 days. These models can scientifically describe the behavior of TM and MBC in apple leaves, which provides necessary data for scientific application. 相似文献
995.
X‐ray crystal structures and MMA polymerization of cadmium(II) complexes with bidentate pyrazole ligands: the formation of monomers or dimers as a function of a methyl substituent on the pyrazole and aniline rings 下载免费PDF全文
The reaction of CdBr2·4H2O with ancillary ligands, N,N‐bis(1H‐pyrazolyl‐1‐methyl)aniline ( L1 ), N,N‐bis(1H‐pyrazolyl‐1‐methyl)‐p‐methylaniline ( L2 ), N,N‐bis(1H‐pyrazolyl‐1‐methyl)‐3,5‐dimethylaniline ( L3 ), N,N‐bis(3,5‐dimethyl‐1H‐pyrazolyl‐1‐methyl)aniline ( L4 ) and N,N‐bis(1H‐pyrazolyl‐1‐methyl)‐2,6‐dimethylaniline ( L5 ) in ethanol yields novel Cd(II) bromide complexes, [L1CdBr2]2 , [L2CdBr2]2 , [L3CdBr2]2 , [L4CdBr2] and [L5CdBr2] . The X‐ray crystal structures of [L1CdBr2]2 , [L2CdBr2]2 and [L3CdBr2]2 reveal a bromo‐bridged dimeric species with crystallographic inversion symmetry. Conversely, [L4CdBr2] and [L5CdBr2] exist as monomeric complexes, presumably due to the steric hindrance between the methyl substituents of the two pyrazole groups in the ligand and cadmium centre for [L4CdBr2] , and crowding around the cadmium metal by methyl substituents on the aniline residue in the ligand for [L5CdBr2] . The geometry at each Cd(II) centre for [L1CdBr2]2 , [L2CdBr2]2 and [L3CdBr2]2 is best described as a distorted trigonal bipyramid. A distorted trigonal bipyramid is achieved in [L4CdBr2] by coordinative interaction of the nitrogen atom of the aniline unit and the cadmium atom with a σ plane of symmetry, based on the bond length of Cd―Naniline (2.759(7) Å). [L5CdBr2] exists with a distorted tetrahedral geometry involving non‐coordination of the nitrogen atom of aniline and the Cd centre, resulting in the formation of an eight‐membered chelate ring. The catalytic activity of monomeric, five‐coordinated [L4CdBr2] in the polymerization of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) at 60°C resulted in a higher molecular weight and a narrower polydispersity index (PDI) than those obtained with dimeric [LnCdBr2]2 (Ln = L1 , L2 , L3 ) or monomeric tetrahedral [L5CdBr2] . Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
996.
phosphonate‐based bimetallic metal‐ organic frameworks, namely STA‐12(M1, M2) (M1, M2 = Mn, Fe, Co), show photocatalytic activity for the degradation of Rhodamine B (RhB) and Methylene blue (MB) from aqueous solution under natural sunlight irradiation. The degradation of the dyes, appears to be faster with STA‐12(Fe, Mn) than other synthesized MOFs. Thus, photo‐Fenton oxidative discoloration of dyes has been studied by H2O2 catalyzed with the STA‐12(Fe, Mn). The process is first order with respect to dyes and the synergistic index in the STA‐12(Fe, Mn)/sunlight/H2O2system reached as high as 472%. Mineralization of dyes was discussed by spectroscopic and TOC measurement. Besides, the efficiency of STA‐12(Fe, Mn) used in photocatalytic process was attentively investigated through the characterization of reactive radicals, the stability and reusability of the photocatalyst, also the effect of operational parameters such as H2O2 dosage, solution pH and initial dye concentration. This work demonstrates the first example of facilitating photo‐Fenton‐like excitation of H2O2 via phosphonate based mixed metal organic frameworks as photocatalysts and explained a new opportunity for solar‐induced AOP environmental remediation and protection. 相似文献
997.
Ebrahim Sharifpour Ebrahim Alipanahpour Dil Arash Asfaram Mehrorang Ghaedi Alireza Goudarzi 《应用有机金属化学》2019,33(3)
In the present work, Mn‐doped CuO‐NPs‐AC was prepared by a simple method, characterized using various techniques such as FESEM, EDX, XRD, PSD, and pHpzc and finally used for the adsorption of malachite green (MG) and methyl orange (MO) in a number of single and binary solutions. A series of adsorption experiments were conducted to investigate and optimize the influence of various factors (such as different pH, concentration of MG and MO, adsorbent mass, and sonication time) on the simultaneous adsorption of MG and MO using response surface methodology. Under optimal conditions of pH 10, adsorbent dose of 0.02 g, MG concentration of 30 mg L?1, MO concentration of 30 mg L?1, and sonication time of 4.5 min at room temperature, the maximum predicted adsorption was observed to be 100.0%, for both MG and MO, showing that there is a favorable harmony between the experimental data and model predictions. The adsorption isotherm of MO and MG by Mn‐doped CuO‐NPs‐AC could be well clarified by the Langmuir model with maximum adsorption capacity of 320.69 mg g?1 and 290.11 mg g?1 in the single solution and 233.02 mg g?1 and 205.53 mg g?1 in the binary solution by 0.005 g of adsorbent mass for MG and MO, respectively. Kinetic studies also revealed that both MG and MO adsorption were better defined by the pseudo‐second order model for both solutions. In addition, the thermodynamic constant studies disclosed that the adsorption of MG and MO was likely to be influenced by a physisorption mechanism. Eventually, the reusability of the Mn‐doped CuO‐NPs‐AC after six times showed a reduction in the adsorption percentage of MG and MO. 相似文献
998.
Iodinated hydrocarbons are often used as precursors for hydrocarbon radicals in shock-tube experiments. The radicals are produced by C─I bond fission reaction, and their formation can be followed through time-resolved monitoring of the complementary I-atom concentrations, for example, by I-atom resonance absorption spectroscopy (I-ARAS). This very sensitive technique requires, however, an independent calibration. As a very clean source of I atoms, CH3I is particularly well suited as calibration system for I-ARAS presumed the yield of I atoms and the rate coefficient of I-atom formation from CH3I are known with sufficient accuracy. But if the formation of I atoms from CH3I by I-ARAS is to be characterized, an independent calibration system is required. In this study, we propose a cross-calibration approach for I-ARAS based on the simultaneous time-resolved monitoring of I and H atoms by ARAS in C2H5I pyrolysis experiments. For this reaction system, it can be shown that at sufficiently short reaction times very similar amounts of I and H atoms are formed (difference <1%). As calibration of H-ARAS, with mixtures of N2O and H2, is a well-established technique, we calibrated I-atom absorption–time profiles with respect to simultaneously recorded H-atom concentration–time profiles. Using this approach, we investigated the thermal decomposition of CH3I in the temperature range 950–2050 K behind reflected shock waves at two different nominal pressures (p ∼ 0.4 and 1.6 bar, bath gas: Ar). From the obtained absolute I-atom concentration–time profiles at temperatures T < 1250 K, we inferred a second-order rate coefficient k(T) = (1.7 ± 0.7) × 1015 exp(–20020 K/T) cm3 mol–1 s–1 for the reaction CH3I + Ar → CH3 + I + Ar. A small mechanism to describe the pyrolysis of CH3I under shock-tube conditions is presented and discussed. 相似文献
999.
The atom-efficient and green protocol for formation of pyrano[3,4-e][1,3]oxazines utilizing dimethyl carbonate under ultrasound irradiation in a presence of KF/basic alumina was reported. We provide a novel series of pyrano[3,4-e][1,3]oxazine derivatives interesting for biological screening tests. In general, it was found that ultrasound irradiations enable the reactions to occur which could not be carried out under silent conditions. These remarkable effects appeared in sonicated reactions can be reasonably interpreted in terms of acoustic cavitation phenomenon. Structures of the products were established on analytical and spectral data. This protocol offers several advantages attain many principles of green chemistry including, save energy, atom economy, clean reactions, inexpensive green reagent and use catalysts rather than stoichiometric reagents. 相似文献
1000.
Temperature effects on spreading of water nano‐droplet on poly(methyl methacrylate): A molecular dynamics simulation study 下载免费PDF全文
Masumeh Foroutan Hojat Zahedi Farshad Esmaeilian 《Journal of Polymer Science.Polymer Physics》2017,55(20):1532-1541
In this article, the effect of temperature on the spreading behavior of a water nano‐droplet on poly(methyl methacrylate) substrate is investigated. The contact angle analysis illustrates that the spreading process occurs in a stage‐like manner and the increase in temperature causes a regime change from partial to total wetting. The interaction energy distributions show that there exist sites on the surface which could trap water molecules and provide a better path for other molecules to overcome the asperities. Estimations of the coefficients of self‐diffusivity suggest that temperature has a major effect in the reorientation stage, which results in the formation of the interfacial layer. In the second stage of spreading, temperature affects the process by providing sufficient energy for water molecules to overcome the interactions with the substrate. Therefore, this stage is controlled by the movement of water molecules on the surface and is highly influenced by their interaction with the surface asperities, strong interaction sites, and the carbonyl groups. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1532–1541 相似文献