全文获取类型
收费全文 | 2520篇 |
免费 | 88篇 |
国内免费 | 33篇 |
专业分类
化学 | 1408篇 |
晶体学 | 25篇 |
力学 | 101篇 |
综合类 | 3篇 |
数学 | 542篇 |
物理学 | 562篇 |
出版年
2023年 | 19篇 |
2022年 | 22篇 |
2021年 | 30篇 |
2020年 | 42篇 |
2019年 | 41篇 |
2018年 | 55篇 |
2017年 | 56篇 |
2016年 | 86篇 |
2015年 | 66篇 |
2014年 | 92篇 |
2013年 | 177篇 |
2012年 | 143篇 |
2011年 | 153篇 |
2010年 | 113篇 |
2009年 | 89篇 |
2008年 | 136篇 |
2007年 | 113篇 |
2006年 | 120篇 |
2005年 | 82篇 |
2004年 | 86篇 |
2003年 | 69篇 |
2002年 | 62篇 |
2001年 | 40篇 |
2000年 | 50篇 |
1999年 | 33篇 |
1998年 | 36篇 |
1997年 | 26篇 |
1996年 | 38篇 |
1995年 | 35篇 |
1994年 | 33篇 |
1993年 | 18篇 |
1992年 | 16篇 |
1991年 | 24篇 |
1990年 | 15篇 |
1987年 | 12篇 |
1986年 | 15篇 |
1985年 | 29篇 |
1984年 | 20篇 |
1983年 | 13篇 |
1982年 | 20篇 |
1981年 | 22篇 |
1980年 | 19篇 |
1979年 | 14篇 |
1978年 | 23篇 |
1975年 | 17篇 |
1973年 | 12篇 |
1970年 | 11篇 |
1969年 | 14篇 |
1968年 | 13篇 |
1916年 | 10篇 |
排序方式: 共有2641条查询结果,搜索用时 135 毫秒
991.
HU Xiao-li LIU Sa LIU Shao-pu LIU Zhong-fang SONG Yan-qi . Key Laboratory on Luminescence Real-Time Analysis Ministry of Education School of Chemistry Chemical Engineering Southwest University Chongqing P. R. China . Chongqing Station National Water Quality Monitoring Net of City Water Supply Chongqing 《高等学校化学研究》2011,(2):208-211
In a pH=0.65―1.5 NaAc-HCl medium, methylene blue(MB) reacts with 12-tungstophosphoric acid (TPA) by virtue of electrostatic attraction and hydrophobic force to form a 3:2 ion-association complex. As a result, the intensities of resonance Rayleigh scattering(RRS), second-order scattering(SOS) and frequency doubling scatte- ring(FDS) are enhanced greatly. The maximum scattering wavelengths of RRS, SOS and FDS are located at 316, 647 and 311 nm. The increments of scattering intensity(△I) are directly proportio... 相似文献
992.
Eli Adjélé Wilson Saïd Ennahar Minjie Zhao Martine Bergaentzle Eric Marchioni Françoise Bindler 《Chromatographia》2011,73(1):137-142
Numerous isothiocyanates (ITCs) are poorly soluble in water which causes their precipitation in aqueous mobile phases used in reversed phase liquid chromatography (RP-LC), thus impacting the accuracy of the quantification. By comparing the amounts of ITCs injected and released from the column, losses could be estimated at 5–32% depending on polarities and concentrations. Results could be dramatically improved in terms of separation and quantification using RP-LC with a mercaptoethanol precolumn derivatization aimed at avoiding ITCs precipitation. The cancer chemoprotective allyl-ITC and sulforaphane were found in cabbage extracts at 1.2 and 2.7 μg g?1 fresh weight, respectively. 相似文献
993.
Meira M Quintella CM Tanajura Ados S da Silva HR Fernando JD da Costa Neto PR Pepe IM Santos MA Nascimento LL 《Talanta》2011,85(1):430-434
Oxidation stability is an important quality parameter for biodiesel. In general, the methods used to evaluate the oxidation stability of oils and biodiesels are time-consuming. This work reports the use of spectrofluorimetry, a fast analytical technique, associated with multivariate data analysis as a powerful analytical tool to prediction of the oxidation stability. The prediction of the oxidation stability showed a good agreement with the results obtained by the EN14112 reference method Rancimat. The models presented high correlation (0.99276 and 0.97951) between real and predicted values. The R2 values of 0.98557 and 0.95943 indicated the accuracy of the models to predict the oxidation stability of soy oil and soy biodiesel, respectively. The residual distribution does not follow a trend with respect to the predicted variables indicating the good quality of the fits. 相似文献
994.
Soares BM Alves OA Ferreira MV Amorim JC Sousa GR Silveira Lde B Prates RA Avila TV Baltazar Lde M de Souza Dda G Santos DA Modolo LV Cisalpino PS Pinotti M 《Photochemistry and photobiology》2011,87(2):357-364
Cryptococus gattii is an emergent primary human pathogen that causes meningismus, papilledema, high intracranial pressure and focal involvement of the central nervous system in immunocompetent hosts. Prolonged antifungal therapy is the conventional treatment, but it is highly toxic, selects for resistant strains, contributes to therapy failure and has a poor prognosis. Photodynamic inactivation (PDI) offers a promising possibility for the alternative treatment of cryptococcosis. The aim of this study was to test the effectiveness of toluidine blue O (TBO) and light-emitting diode (LED) against C. gattii strains with distinct susceptibility profile to antifungal drugs (amphotericin B: 0.015-1.0 μg mL(-1); itraconazole: 0.015-2 μg mL(-1); fluconazole: 4-64 μg mL(-1)). Using 25 μM (6.76 μg mL(-1)) TBO and LED energy density of 54 J cm(-2) these fungal isolates presented variable susceptibility to PDI. The production of reactive oxygen species (ROS)/peroxynitrite was determined, and the catalase and peroxidase activities were measured. After PDI, high amounts of ROS/peroxynitrite are produced and higher catalase and peroxidase activities could be correlated with a lower susceptibility of C. gattii isolates to PDI. These results indicate that PDI could be an alternative to C. gattii growth inhibition, even of isolates less susceptible to classical antifungal drugs, also pointing to mechanisms related to their variable susceptibility behavior. 相似文献
995.
996.
Martínez-Montero S Fernández S Sanghvi YS Gotor V Ferrero M 《Organic & biomolecular chemistry》2011,9(17):5960-5966
Preparation of abasic site precursors through a divergent chemoenzymatic synthesis has been accomplished. Several biocatalysts and acylating agents were studied furnishing a practical and scalable green method useful for industrial applications. Highly regioselective acylation and deacylation reactions with 1,2-dideoxy-D-ribose are described resulting in excellent yield. A fast, atom-efficient and convenient synthesis of 3-, and 5-O-DMTr-1,2-dideoxyribose 17 and 19 has been achieved. These compounds are useful precursors for the preparation of phosphoramidites required for the assembly of oligonucleotides containing the tetrahydrofuran abasic lesions. 相似文献
997.
Takahashi HT Novello CR Ueda-Nakamura T Filho BP Palazzo de Mello JC Nakamura CV 《Molecules (Basel, Switzerland)》2011,16(5):3469-3478
Porophyllum ruderale (Jacq.) Cass. is a plant native to Brazil and in the northwest region of the state of Paraná, Brazil, aerial parts of P. ruderale have been used popularly in the treatment of lesions caused by Leishmania sp.. In this study the antileishmanial and cytotoxic activities of the crude extract, fractions, and isolated compounds from aerial parts of P. ruderale was evaluated. The dichloromethane extract was submitted to chromatography to yield compounds active against Leishmania amazonensis. Their structures were established by comparison of their spectroscopic data with literature values. The activities of crude extract against promastigote and axenic amastigote forms of L. amazonensis (IC(50)) were 60.3 and 77.7 μg/mL, respectively. Its cytotoxic activity against macrophage cells (CC(50)) was 500 μg/mL. The thiophene derivatives isolated were: 5-methyl-2,2':5',2"-terthiophene (compound A) and 5'-methyl-[5-(4-acetoxy-1-butynyl)]-2,2'-bithiophene (compound B). The activity of compound A against promastigote and axenic amastigote forms were 7.7 and 19.0 μg/mL and of compound B were 21.3 and 28.7 μg/mL, respectively. The activity of the isolated compounds against promastigote and axenic amastigote forms was better than that of the crude extract and more selective against protozoa than for macrophage cells. 相似文献
998.
Francisco C. C. Arantes Antonio C. Moro Iolanda S. Klein Cristiana da Silva Adelino V. G. Netto Antonio E. Mauro Vânia M. Nogueira 《Journal of Thermal Analysis and Calorimetry》2011,106(2):379-383
The synthesis, spectroscopic characterization, and thermal analysis of the compounds [Pd(X)2(mtu)(PPh3)] (X = Cl− (1), SCN− (2); mtu = N-methylthiourea; PPh3 = triphenylphosphine) and [Pd(X)2(phtu)(PPh3)] (X = Cl− (3), SCN− (4); phtu = N-phenylthiourea) are described. The thermal decomposition of the compounds occurs in two, three, or four stages and the final
decomposition products were identified as Pd0 by X-ray powder diffraction. The thermal stability order of the complexes is 4 > 3>2 > 1. 相似文献
999.
In this work, we demonstrate an integrated sensor combining a grating-coupled plasmon resonance surface with a planar photodiode. Plasmon enhanced transmission is employed as a sensitive refractive index (RI) sensing mechanism. Enhanced transmission of light is monitored via the integrated photodiode by tuning the angle of incidence of a collimated beam near the sharp plasmon resonance condition. Slight changes of the effective refractive index (RI) shift the resonance angle, resulting in a change in the photocurrent. Owing to the planar sensing mechanism, the design permits a high areal density of sensing spots. In the design, absence of holes that facilitate resonant transmission of light, allows an easy-to-implement fabrication procedure and relative insensitivity to fabrication errors. Theoretical and experimental results agree well. An equivalent long-term RI noise of 6.3 × 10(-6) RIU/√Hz is obtained by using an 8 mW He-Ne laser, compared to a shot-noise limited theoretical sensitivity of 5.61 × 10(-9) RIU/√Hz. The device features full benefits of grating-coupled plasmon resonance, such as enhancement of sensitivity for non-zero azimuthal angle of incidence. Further sensitivity enhancement using balanced detection and optimal plasmon coupling conditions are discussed. 相似文献
1000.
Spectroscopic characterization of oxidized nanocellulose grafted with fluorescent amino acids 总被引:1,自引:0,他引:1
A hardwood bleached kraft pulp was oxidized by sodium hypochlorite using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)
and sodium bromide in water as catalysts to produce oxidized fibers. These oxidized fibers were mechanically disintegrated
into oxidized nano-elements which were separated from the coarse components by centrifugation to obtain oxidized nanocellulose
(ONC). The ONC was then coupled with fluorescent amino acids using a two step coupling procedure. First, the ONC was activated
by N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide hydrochloride, forming a stable active ester in the presence of N-hydroxysuccinimide.
Second, the active ester was reacted with the amino groups on the amino acids, forming an amide bond between the ONC and amino
acids. The products (ONC-amino acids) were characterized by transmission electron microscopy and by different spectroscopic
techniques such as absorption, emission, FTIR and XPS. 相似文献