全文获取类型
收费全文 | 6307篇 |
免费 | 613篇 |
国内免费 | 462篇 |
专业分类
化学 | 1647篇 |
晶体学 | 56篇 |
力学 | 223篇 |
综合类 | 141篇 |
数学 | 2656篇 |
物理学 | 2659篇 |
出版年
2024年 | 16篇 |
2023年 | 56篇 |
2022年 | 134篇 |
2021年 | 118篇 |
2020年 | 127篇 |
2019年 | 154篇 |
2018年 | 162篇 |
2017年 | 187篇 |
2016年 | 207篇 |
2015年 | 169篇 |
2014年 | 363篇 |
2013年 | 497篇 |
2012年 | 319篇 |
2011年 | 374篇 |
2010年 | 315篇 |
2009年 | 392篇 |
2008年 | 432篇 |
2007年 | 434篇 |
2006年 | 347篇 |
2005年 | 308篇 |
2004年 | 283篇 |
2003年 | 294篇 |
2002年 | 231篇 |
2001年 | 177篇 |
2000年 | 192篇 |
1999年 | 170篇 |
1998年 | 138篇 |
1997年 | 144篇 |
1996年 | 138篇 |
1995年 | 90篇 |
1994年 | 58篇 |
1993年 | 54篇 |
1992年 | 45篇 |
1991年 | 32篇 |
1990年 | 34篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 22篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 23篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1973年 | 2篇 |
1969年 | 2篇 |
排序方式: 共有7382条查询结果,搜索用时 15 毫秒
161.
Comparison
of the fragility index of different eudragit polymers determined by activation
enthalpies
Gabriella Csóka A. Gelencsér Dorottya Kiss Eszter Pásztor I. Klebovich Romána Zelkó 《Journal of Thermal Analysis and Calorimetry》2007,87(2):469-473
The present study aimed to apply fragility
index (m) of polymers in the determination
of the optimal amount of plasticizer in polymer films. The fragility index
of different Eudragit polymers (RS, RL, EPO) was assessed by differential
scanning calorimerty (DSC), applying the Arrhenius connection (logq–1/T
g).
The fragility of Eudragit EPO films proved to be the highest, while in the
case of RS and RL, the increase of the alkyl-chain length caused the increase
of fragility. Studying the effect of plasticizer (triethyl citrate, TEC) on
the m value of Eudragit RL and RS films,
a near linear reduction of the fragility index could be observed between 5–30%
TEC concentration, but above 30%, this value leveled out to constant. 相似文献
162.
163.
A series of phenyl modified polydimethylsiloxane (PDMS) / polyhydrogenmethylsiloxane (PHMS) random copolymers containing both internal Si‐H and terminal SiH2 and T (MeSiO3/2) units was synthesized in one step through n‐BuLi‐catalyzed ring‐opening polymerization of cyclic comonomers and characterized by GPC, IR and 1H and 29Si NMR. Sequential microstructures of these copolymers were determined by 29Si‐NMR spectroscopy. Epoxy‐modified polysiloxanes were prepared and used as comparable standards for the assignment of the NMR spectra. A hydride‐transfer mechanism has been proposed to account for the formation of terminal Si‐H and T group. Detailed sequential analyses and chemical shifts of 29Si‐NMR for various siloxane units are reported for the first time. 相似文献
164.
A novel flow-injection system is proposed for the rapid measurement of the fish freshness indices K1 and K2: K1=[([HxR+[Hx])×100/([IMP]+[HxR]+[Hx])] and K2=[[Hx]×100/([HxR]+[Hx])], where [IMP], [HxR] and [Hx] are inosine-5′-monophosphate, inosine and hypoxanthine concentrations, respectively. For the estimation of index K1, 5′-nucleotidase immobilized reactor and nucleoside phosphorylase (NP)/xanthine oxidase (XO) coimmobilized reactor were incorporated in series in the flow-injection line made up by a 16-way switching valve with two sample loops. For the estimation of index K2, NP and XO immobilized reactors were also incorporated in the similar flow-line. Two sample portions passed through the flow-line with different residence times so that two peaks were obtained. The first and second peaks obtained in the K1-determining system corresponded to the total of HxR and Hx and the total of Hx, HxR and IMP, respectively. Similarly, the first and second peaks obtained in the K2-determining system corresponded to Hx and the total of Hx and HxR, respectively. Therefore, the indices K1 and K2 can be estimated by where i1 and i2 present the peak current of the first and second peaks, respectively, and f the ratio of the peak currents of the first and second peaks for a Hx standard solution. A sea bream was selected as a model fish and it was stored at 4 °C after death. A good correlation was found between the index K1 and the storage time over a period of 400 h, with a correlation coefficient of 0.992, but no correlation between the index K2 and the storage time. The measurements could be performed at a rate of 22 samples per hour with satisfactory precision (0.6–1.2% R.S.D.), without calibration for each species, accurate weighing of fish meat and any interferences in fish meat. 相似文献
165.
诱导效应指数与含硫含氮的低碳直链有机化合物的标准汽化热 总被引:2,自引:0,他引:2
本文利用诱导效应指数建立了含硫含氮的低碳直链有机物的标准汽化热的简单估算方法,我们利用此法计算了一些含硫含氮的直链有机物的标准汽化热,41个可比较值的相对误差均在5%以内。 相似文献
166.
信息拓扑指数与烷烃分子热力学性质的关系 总被引:13,自引:0,他引:13
Two topological information indices were constructed based on Randic and Wiener indices, and the values of topological information indices for 85 alkanes were calculated. The thermodynamic properties such as the standard enthalpies of formation, the standard entropies and the standard free energies of formation for these alkanes were also correlated with these topological and information indices. It is found that the thermodynamic properties calculated for both gaseous and liquid states of the 85 alkanes are in excellent agreement with the experimental values through the regression analysis. 相似文献
167.
168.
《Arabian Journal of Chemistry》2020,13(10):7289-7301
Black pepper oils have been investigated frequently in the recent years. However, there is a significant variation in physicochemical properties and bioactivity of oils depended on extraction techniques. In this study, the systemic investigation of four various extraction methods was performed to evaluate the physicochemical characterizations, antioxidant and antibacterial activity. The investigation of 1H NMR, FTIR and UV–Vis spectra confirmed presence of non-volatile components in oils extracted through supercritical CO2 and hexane-soaking extractions which induced their typical thermal properties. The isothermal behaviour of extracted oils related to evaporation was within range of 3.2–7.3% (w/w) at 27 °C. The SEM images of the black pepper confirmed different operation manners of mechanism between extractions using the solvents and heating process. The lowest MIC for both essential oils from conventional hidrodistillation and microwave-assisted hidrodistillation against two bacteria including E. coli and B. subtilis were found to be 137 µg mL−1. The non-isothermal decomposition kinetics were investigated on the essential oil of microwave-assisted hydrodistillation extraction. The activation energies and pre-exponent factors of non-isothermal decomposition were found to be in range of 36.5–73.7 KJ mol−1 and 4.98 × 103–1.97 × 108 s−1, respectively, dependent on conversional fractions of the oil. The results revealed that chemical components, physicochemical properties and bioactivity of black pepper essential oils depended on the extraction techniques. 相似文献
169.
In this paper, excess thermodynamic functions have been computed from the measured values of density, viscosity, and refractive index at T = (298.15, 303.15, and 308.15) K, ultrasonic velocity at T = 298.15 K over the entire mixture composition range of (anisole with ethanol, propan-1-ol, propan-2-ol, butan-1-ol, pentan-1-ol, or 3-methyl butan-1-ol). Excess molar volume, VE has been calculated from densities, whereas deviations in viscosity, Δη, were computed from the measured viscosities. From ultrasonic velocities, isentropic compressibilities were calculated, from which deviations in isentropic compressibility, Δks have been computed. Lorenz-Lorentz mixture rule was used to compute molar refractivity, R from refractivity index data and from these data, deviations in molar refractivity, ΔR have been computed. Computed thermodynamic quantities have been fitted to Redlich and Kister polynomial equation to derive the coefficients and standard errors between experimental and predicted quantities. Intermolecular interactions between anisole and alkanols have been studied based on the computed excess thermodynamic quantities. 相似文献
170.
《Arabian Journal of Chemistry》2022,15(4):103712
Although recent decades have witnessed the synthesis of 1,3,4-thiadiazoles via phosphorus POCl3-promoted cyclization reaction, simultaneous access to 2-amino-1,3,4-thiadiazole and 2-amino-1,3,4-oxadiazole analogs remains unexpected and elusive. Herein, a detailed regiocontrolled synthesis of 2-amino-1,3,4-thiadiazoles in good to high yields with good regioselectivities from readily available thiosemicarbazides using POCl3 was disclosed. Meantime, to establish a comprehensive structure–activity relationship, 2-amino-1,3,4-oxadiazole derivatives as single regioisomers were prepared via EDCI·HCl-triggered cyclization of the thiosemicarbazide intermediates. The in vitro anti-influenza assays proved that the selected compounds with the pyrazine/pyridine ring exhibited certain inhibitory activities against influenza A virus strains A/HK/68 (H3N2) and A/PR/8/34 (H1N1) in MDCK cells. Among them, N-(adamantan-1-yl)-5-(5-(azepan-1-yl)pyrazin-2-yl)-1,3,4-thiadiazol-2-amine (4j) was the most active compound, and exhibited favorable activity with EC50 values of 3.5 μM and 7.5 μM, respectively. In addition, the molecular docking results explained the reason why compound 4j had dual inhibitory activity and revealed the reasonable binding mode of this compound with the M2-S31N and M2-WT ion channels. This compound had the potential to be further developed as an anti-influenza drug. 相似文献