全文获取类型
收费全文 | 1640篇 |
免费 | 56篇 |
国内免费 | 4篇 |
专业分类
化学 | 1106篇 |
晶体学 | 20篇 |
力学 | 37篇 |
数学 | 236篇 |
物理学 | 301篇 |
出版年
2023年 | 8篇 |
2022年 | 23篇 |
2021年 | 59篇 |
2020年 | 49篇 |
2019年 | 44篇 |
2018年 | 42篇 |
2017年 | 34篇 |
2016年 | 73篇 |
2015年 | 72篇 |
2014年 | 67篇 |
2013年 | 118篇 |
2012年 | 112篇 |
2011年 | 129篇 |
2010年 | 79篇 |
2009年 | 75篇 |
2008年 | 111篇 |
2007年 | 85篇 |
2006年 | 80篇 |
2005年 | 55篇 |
2004年 | 58篇 |
2003年 | 27篇 |
2002年 | 43篇 |
2001年 | 31篇 |
2000年 | 24篇 |
1999年 | 20篇 |
1998年 | 19篇 |
1997年 | 15篇 |
1996年 | 15篇 |
1995年 | 13篇 |
1994年 | 16篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1974年 | 3篇 |
1966年 | 2篇 |
1890年 | 1篇 |
排序方式: 共有1700条查询结果,搜索用时 281 毫秒
951.
This work relates the development and validation of a simple reversed-phase high-performance liquid chromatographic (HPLC) method for the analysis of xanthone (XAN) and 3-methoxyxanthone (3-MeOXAN) in poly(D,L-lactide-co-glycolide) (PLGA) nanocapsule formulations. This method does not require any complex sample extraction procedure. Chromatographic separation is made with a reversed-phase C(18) column, using methanol-water (90:10, v/v) as a mobile phase at a flow rate of 1 mL/min. Identification is made by UV detection at 237 nm. The isocratic system operates at ambient temperature and requires 7 min of chromatographic time. The developed method is statistically validated according to United States Pharmacopoeia 25 and International Conference on Harmonization guidelines for its specificity, linearity, accuracy, and precision. The assay method proposed in this study is specific for XAN and 3-MeOXAN in the presence of nanocapsule excipients. Diode-array analyses confirm the homogeneity of XAN and 3-MeOXAN peaks in stressed conditions. Standard curves are linear (r > 0.999) over the concentration range of 0.4-2.5 and 1.0-5.8 micro g/mL for XAN and 3-MeOXAN, respectively. Recovery from nanocapsules ranges from 99.6% to 102.8% for XAN and 98.8% to 102.4% for 3-MeOXAN. Repeatability (intra-assay precision) is acceptable with relative standard deviation values of 1.2% for XAN and 0.3% for 3-MeOXAN. 相似文献
952.
Luiz Clio S. Rocha Mariana S. Rocha Paulo Rotella Junior Giancarlo Aquila Rogrio S. Peruchi Karel Janda Rmulo O. Azevêdo 《Entropy (Basel, Switzerland)》2021,23(2)
The high proportion of CO2/CH4 in low aggregated value natural gas compositions can be used strategically and intelligently to produce more hydrocarbons through oxidative methane coupling (OCM). The main goal of this study was to optimize direct low-value natural gas conversion via CO2-OCM on metal oxide catalysts using robust multi-objective optimization based on an entropic measure to choose the most preferred Pareto optimal point as the problem’s final solution. The responses of CH4 conversion, C2 selectivity, and C2 yield are modeled using the response surface methodology. In this methodology, decision variables, e.g., the CO2/CH4 ratio, reactor temperature, wt.% CaO and wt.% MnO in ceria catalyst, are all employed. The Pareto optimal solution was obtained via the following combination of process parameters: CO2/CH4 ratio = 2.50, reactor temperature = 1179.5 K, wt.% CaO in ceria catalyst = 17.2%, wt.% MnO in ceria catalyst = 6.0%. By using the optimal weighting strategy w1 = 0.2602, w2 = 0.3203, w3 = 0.4295, the simultaneous optimal values for the objective functions were: CH4 conversion = 8.806%, C2 selectivity = 51.468%, C2 yield = 3.275%. Finally, an entropic measure used as a decision-making criterion was found to be useful in mapping the regions of minimal variation among the Pareto optimal responses and the results obtained, and this demonstrates that the optimization weights exert influence on the forecast variation of the obtained response. 相似文献
953.
The behaviour of stationary, non-passive plumes can be simulated in a reasonably simple and accurate way by integral models.
One of the key requirements of these models, but also one of their less well-founded aspects, is the entrainment assumption,
which parameterizes turbulent mixing between the plume and the environment. The entrainment assumption developed by Schatzmann
and adjusted to a set of experimental results requires four constants and an ad hoc hypothesis to eliminate undesirable terms.
With this assumption, Schatzmann’s model exhibits numerical instability for certain cases of plumes with small velocity excesses,
due to very fast radius growth. The purpose of this paper is to present an alternative entrainment assumption based on a first-order
turbulence closure, which only requires two adjustable constants and seems to solve this problem. The asymptotic behaviour
of the new formulation is studied and compared to previous ones. The validation tests presented by Schatzmann are repeated
and it is found that the new formulation not only eliminates numerical instability but also predicts more plausible growth
rates for jets in co-flowing streams. 相似文献
954.
Mayara da Silva Araújo Túlio Rolim Barretto Juliana Costa Rolim Galvão César Ricardo Teixeira Tarley Luiz Henrrique Dall'Antônia Roberto de Matos Roberta Antigo Medeiros 《Electroanalysis》2021,33(3):663-671
A new and simple photoelectrochemical (PEC) sensor using a glassy carbon electrode (GCE) modified with bismuth vanadate (BiVO4) nanoparticles and dihexadecyl phosphate (DHP) film was useful for acetaminophen (AC) determination. In 0.2 mol L−1 phosphate buffer (pH=9), the GCE without modification exhibited the smaller photocurrent (0.86 μA) when compared with GCE modified with 1.0 mg mL−1 or 2.0 mg mL−1 BiVO4 nanoparticles suspension (5.9 and 34 μA, respectively). Based on the photocurrent signal generated through the interaction between GCE, BiVO4 and the energy of visible light a chronoamperometric method for AC determination was developed. The AC linear range concentration from 0.099 to 0.99 μmol L−1 and limits of detection and quantification of 0.027 and 0.091 μmol L−1, respectively, was obtained. The proposed method was applied to the AC determination in commercial drugs and tap water with satisfactory accuracy and precision. Moreover, the PEC construction was easy and had a short response time, which might confer higher sample throughput for the method. 相似文献
955.
Gabriel Lopes Marques Luana Rianne Rocha Maiyara Carolyne Prete Felipe Augusto Gorla Dayana Moscardi dos Santos Mariana Gava Segatelli César Ricardo Teixeira Tarley 《Electroanalysis》2021,33(3):568-578
This paper describes the development of a new electrochemical sensor for 17β-estradiol (E2) determination based on glassy carbon electrode (GCE) modified with molecularly imprinted polymer grafted onto iniferter-multiwall carbon nanotubes surface (MIP-MWCNT) and dihexadecyl-hydrogen-phosphate (DHP). The electrochemical method was based on closed-circuit preconcentration of E2 in 0.1 mol L−1 phosphate buffer (pH 7.0) during 500 s. Upon preconcentration, E2 was determined by differential pulse voltammetry (DPV) exhibiting a limit of detection of 0.01 μmol L−1. The sensor exhibited higher selectivity toward E2 and it was applied for E2 determination in natural water samples, with accuracy attested by HPLC-DAD. 相似文献
956.
José A. M. Nascimento Maysa L. Nascimento Carlos H. S. Mendes Severino Carlos B. Oliveira 《Electroanalysis》2021,33(1):216-225
The electrochemical oxidation of 3-nitro-tyrosine (3-NO2-Tyr) was studied in aqueous media at metallic electrodes (platinum and gold), using voltammetric techniques. The interaction between 3-NO2-Tyr and double helix DNA (dsDNA) in a physiological medium was also investigated. Electro-oxidation of 3-NO2-Tyr occurs in one single irreversible pH-dependent step with the transfer of one electron and one proton from the phenolic group to the formation of radicals, which preferably dimerize, fouling the electrode surfaces. The differential pulse voltammetry and gel electrophoresis results clearly demonstrated a strong interaction of 3-NO2-Tyr with the dsDNA for the formation of a stable 3-NO2-Tyr-dsDNA complex. 相似文献
957.
Chirlene Nascimento Botelho Daniele Pavão e Pavão Flávio Santos Damos Rita de Cássia Silva Luz 《Electroanalysis》2021,33(8):1936-1944
The present work describes the development of a photoelectrochemical sensor based on titanium dioxide, cadmium telluride quantum dots and the tris (2,2′-bipyridyl) ruthenium(II) chloride complex for detection of Isoniazid (INH). The Ru(bpy)32+/CdTe-QDs/TiO2/FTO photoelectrochemical platform was characterized by scanning electrochemical microscopy, electrochemical impedance spectroscopy and amperometry. The photoelectrochemical sensor presented two linear ranges for INH concentrations ranging from 0.5 to 150 μmol L−1 and 150 to 1270 μmol L−1, with a theoretical detection limit of 0.02 μmol L−1. The sensor was successfully applied for the determination of INH in drugs samples used in the treatment of tuberculosis. 相似文献
958.
do Nascimento Diego M. Nunes Yana L. de Almeida Jéssica S. Leitão Renato Carrhá Feitosa Judith P. A. Dufresne Alain Rosa Morsyleide de Freitas 《Cellulose (London, England)》2021,28(17):10891-10904
Cellulose - This work aimed to study the use of cellulose nanofibers (CNFs) from coir husk fibers to develop nanocomposite hydrogels. Cellulose and lignin were obtained from coir husk using the... 相似文献
959.
Fellype do Nascimento Antje Quade Mara Adriana Canesqui Konstantin Georgiev Kostov 《等离子体物理论文集》2023,63(1):e202200055
The employment of atmospheric pressure plasma jets (APPJs) in a large sort of applications is limited by the adversities related to the size of the treated area and the difficulty to reach the target. The use of devices that employ long tubes in their structure has contributed significantly to overcome these challenges. In this work, two different plasma systems employing the jet transfer technique are compared. The main difference between the two devices is how the long plastic tube was assembled. The first one uses a copper wire placed inside a long plastic tube. The other device has a metallic mesh installed in a concentric arrangement between two coaxial plastic tubes. As a result, the two APPJ systems exhibit different properties, with the wire assembly being more powerful, also presenting higher values for the electrical current and rotational temperature when compared to the mesh mounting. X-ray photoelectron spectroscopy (XPS) demonstrates that both configurations were capable of inserting O-containing functional groups on the polypropylene (PP) surface. However, the transferred plasma jet with wire assembly was able to add more functional groups on the PP surface. The results from XPS analysis were corroborated with water contact angle measurements (WCA), being that lower WCA values were obtained when the PP surface presented higher amounts of O-containing groups. Furthermore, the results suggest that the APPJ with wire configuration is more appropriate for material treatments, while the transferred jet with mesh arrangement tends to present lower electrical current values, being more suitable for biological applications. 相似文献
960.
Marcos Antonio Custdio Neto da Silva Jonas Henrique Costa Taícia Pacheco-Fill Ana Lúcia Tasca Gois Ruiz Flvia Castello Branco Vidal Ktia Regina Assuno Borges Sulayne Janaina Araújo Guimares Ana Paula Silva de Azevedo-Santos Kaio Eduardo Buglio Mary Ann Foglio Maria do Carmo Lacerda Barbosa Maria do Desterro Soares Brando Nascimento Joo Ernesto de Carvalho 《Molecules (Basel, Switzerland)》2021,26(12)
Euterpe oleracea Mart. (açai) is a native palm from the Amazon region. There are various chemical constituents of açai with bioactive properties. This study aimed to evaluate the chemical composition and cytotoxic effects of açai seed extract on breast cancer cell line (MCF-7). Global Natural Products Social Molecular Networking (GNPS) was applied to identify chemical compounds present in açai seed extract. LC-MS/MS and molecular networking were employed to detect the phenolic compounds of açai. The antioxidant activity of açai seed extract was measured by DPPH assay. MCF-7 breast cancer cell line viability was evaluated by MTT assay. Cell death was evaluated by flow cytometry and time-lapse microscopy. Autophagy was evaluated by orange acridin immunofluorescence assay. Reactive oxygen species (ROS) production was evaluated by DAF assay. From the molecular networking, fifteen compounds were identified, mainly phenolic compounds. The açai seed extract showed cytotoxic effects against MCF-7, induced morphologic changes in the cell line by autophagy and increased the ROS production pathway. The present study suggests that açai seed extract has a high cytotoxic capacity and may induce autophagy by increasing ROS production in breast cancer. Apart from its antioxidant activity, flavonoids with high radical scavenging activity present in açai also generated NO (nitric oxide), contributing to its cytotoxic effect and autophagy induction. 相似文献