全文获取类型
收费全文 | 22065篇 |
免费 | 2600篇 |
国内免费 | 2068篇 |
专业分类
化学 | 15874篇 |
晶体学 | 274篇 |
力学 | 1004篇 |
综合类 | 137篇 |
数学 | 2436篇 |
物理学 | 7008篇 |
出版年
2024年 | 66篇 |
2023年 | 356篇 |
2022年 | 680篇 |
2021年 | 653篇 |
2020年 | 792篇 |
2019年 | 817篇 |
2018年 | 611篇 |
2017年 | 646篇 |
2016年 | 875篇 |
2015年 | 933篇 |
2014年 | 1057篇 |
2013年 | 1482篇 |
2012年 | 1711篇 |
2011年 | 1899篇 |
2010年 | 1269篇 |
2009年 | 1228篇 |
2008年 | 1416篇 |
2007年 | 1244篇 |
2006年 | 1111篇 |
2005年 | 953篇 |
2004年 | 826篇 |
2003年 | 733篇 |
2002年 | 722篇 |
2001年 | 557篇 |
2000年 | 465篇 |
1999年 | 357篇 |
1998年 | 270篇 |
1997年 | 250篇 |
1996年 | 290篇 |
1995年 | 239篇 |
1994年 | 255篇 |
1993年 | 207篇 |
1992年 | 174篇 |
1991年 | 148篇 |
1990年 | 159篇 |
1989年 | 133篇 |
1988年 | 120篇 |
1987年 | 88篇 |
1986年 | 96篇 |
1985年 | 134篇 |
1984年 | 88篇 |
1983年 | 83篇 |
1982年 | 59篇 |
1981年 | 56篇 |
1980年 | 51篇 |
1979年 | 36篇 |
1977年 | 40篇 |
1976年 | 37篇 |
1974年 | 48篇 |
1973年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
971.
SPE HG-AAS method for the determination of inorganic arsenic in rice—results from method validation studies and a survey on rice products 总被引:1,自引:0,他引:1
Rie R. Rasmussen Yiting Qian Jens J. Sloth 《Analytical and bioanalytical chemistry》2013,405(24):7851-7857
The present paper describes the development, validation and application of a method for inorganic arsenic (iAs) determination in rice samples. The separation of iAs from organoarsenic compounds was done by off-line solid-phase extraction (SPE) followed by hydride generation atomic absorption spectrometry (HG-AAS) detection. This approach was earlier developed for seafood samples (Rasmussen et al., Anal Bioanal Chem 403:2825–2834, 2012) and has in the present work been tailored for rice products and further optimised for a higher sample throughput and a lower detection limit. Water bath heating (90 °C, 60 min) of samples with dilute HNO3 and H2O2 solubilised and oxidised all iAs to arsenate (AsV). Loading of buffered sample extracts (pH 6?±?1) followed by selective elution of arsenate from a strong anion exchange SPE cartridge enabled the selective iAs quantification by HG-AAS, measuring total arsenic (As) in the SPE eluate. The in-house validation gave mean recoveries of 101–106 % for spiked rice samples and in two reference samples. The limit of detection was 0.02 mg kg?1, and repeatability and intra-laboratory reproducibility were less than 6 and 9 %, respectively. The SPE HG-AAS method produced similar results compared to parallel high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (ICP-MS) analysis. The SPE separation step was tested collaboratively, where the laboratories (N?=?10) used either HG-AAS or ICP-MS for iAs determination in a wholemeal rice powder. The trial gave satisfactory results (HorRat value of 1.6) and did not reveal significant difference (t test, p?>?0.05) between HG-AAS and ICP-MS quantification. The iAs concentration in 36 rice samples purchased on the Danish retail market varied (0.03–0.60 mg kg?1), with the highest concentration found in a red rice sample. 相似文献
972.
Wanli Ma Kurunthachalam Kannan Qian Wu Erin M. Bell Charlotte M. Druschel Michele Caggana Kenneth M. Aldous 《Analytical and bioanalytical chemistry》2013,405(12):4127-4138
Dried blood spots (DBS), collected as part of the newborn screening program (NSP) in the USA, is a valuable resource for studies on environmental chemical exposures and associated health outcomes in newborns. Nevertheless, determination of concentrations of environmental chemicals in DBS requires assays with great sensitivity, as the typical volume of blood available on a DBS with 16-mm diameter disc is approximately 50 μL. In this study, we developed a liquid–liquid extraction and high-performance liquid chromatography/tandem mass spectrometry method for the detection of perfluorooctanesulfonate (PFOS), perfluorooctanoate (PFOA), and bisphenol A (BPA) in DBS. The method was validated for accuracy, precision, and sensitivity, by spiking of target chemicals at different levels on Whatman 903 filter cards, which is used in the collection of DBS by the NSP. Contamination arising from collection, storage, and handling of DBS is an important issue to be considered in the analysis of trace levels of environmental chemicals in DBS. For the evaluation of the magnitude of background contamination, field blanks were prepared from unspotted portions of DBS filter cards collected by the NSP. The method was applied for the measurement of PFOS, PFOA, and BPA in 192 DBS specimens provided by NSP of New York State. PFOS and PFOA were detected in 100 % of the specimens analyzed. The concentrations of PFOS and PFOA measured in DBS were similar to those reported earlier in the whole blood samples of newborns. BPA was also found in 86 % of the specimens at concentrations ranging from 0.2 to 36 ng/mL (excluding two outliers). Further studies are needed to evaluate the sources of BPA exposures and health outcomes in newborns. 相似文献
973.
The azido sugar,GalNAz,was successfully used for imaging and perturbing protein glycosylation in triple-negative breast cancer cell line,MDA-MB-231.After the incorporation of GalNAz in the triple-negative breast cancer cell line,the tumorigenicity of these cells was decreased.Results from gene analysis and drug treatment suggest that the tumorigenicity decrease may be attributed to the reduction of cancer stem cell population.Possible mechanisms of GalNAz induced cancer stem cells(CSCs) proportion change are discussed. 相似文献
974.
Dan Tian SuiJun Liu DaShuai Zhang Ze Chang TongLiang Hu XianHe Bu 《中国科学:化学(英文版)》2013,56(12):1693-1700
Three new cobalt complexes, {[Co5(tci)2(bimb)3(µ3-O)2(H2O)2]·3DMF·4H2O} n (1), {[Co3(tci)2(bib)]·2DMF·2H2O} n (2) and {[Co(Htci)(bpea)0.5]·H2O} n (3) (H3tci = tris(2-carboxyethyl)isocyanurate, bimb = 4,4′-bis(imidazol-1-yl)biphenyl, bib = 1,4-bis(imidazol-1-yl)benzene, bpea = 1,2-bis(4-pyridyl)ethane, DMF = N,N′-dimethylformamide), have been successfully synthesized through the assembly of Co(II) ions, H3tci and different N-donor ligands, respectively. All complexes were structurally characterized by single crystal X-ray diffraction, elemental analyses, IR spectra, thermogravimetric (TG) analyses and X-ray powder diffraction (XRPD). Complex 1 exhibits a 3D three-fold parallel interpenetrated 3D → 3D structure with (65·8) CdSO4 topology. Complex 2 is built from [Co3(µ2-Ocarboxyl)2(CO2)4] clusters and linear bib ligands, displaying a two-fold parallel interpenetrated (3,8)-connected (43)2(46·618·84) topology, while complex 3 is a 3D pillar-layered structure involving an infinite -Co-(µ2-Ocarboxyl)(CO2)-Co-chain. The diverse structures of the three complexes indicate that the skeletons of different N-donor ligands play an important role in the assembly of such different frameworks. In addition, magnetic investigation indicates that besides spin-orbit coupling of Co(II) ions, there exist antiferromagnetic exchange interactions in Co5 and Co3 clusters of 1 and 2, respectively. 相似文献
975.
ZHU ZhenFeng ZHAO XinYuan QIN Wei CHEN GuangDi QIAN Jun XU ZhengPing 《中国科学:化学(英文版)》2013,56(9):1247-1252
2,3-Bis(4-(phenyl(4-(1,2,2-triphenylvinyl)phenyl)amino)phenyl) fumaronitrile (TPE-TPA-FN or TTF), which possesses aggregation-induced emission (AIE) characteristic, is doped in organically modified silica (ORMOSIL) nanoparticles. By increasing the weight ratio of TTF to the precursor of silica nanoparticles (the quantities of the precursors were kept the same), the fluorescence intensity of nanoparticles increased correspondingly, due to the formation of larger AIE dots in the cores of ORMOSIL nanoparticles. The fluorescent and biocompatible nanoprobes were then utilized for in vitro imaging of HeLa cells. Two-photon fluorescence microscopy clearly illustrated that the nanoparticles have the capacity of nucleus permeability, as well as cytoplasm staining towards tumor cells. Our experimental results may offer a promising method for fast and bright fluorescence imaging, as well as bio-molecule/drug delivery to cell nucleus. 相似文献
976.
GAO Lei ZHANG Jing HE Chang SHEN SuLing ZHANG Yi LIU HongTao SUN QingJiang LI YongFang 《中国科学:化学(英文版)》2013,56(7):997-1003
A new star-shaped donor-acceptor molecule has been synthesized for application as the donor material in solution-processed bulk-heterojunction organic solar cells (OSCs). The molecule consists of a triphenylamine (TPA) unit as the core and a donor unit with three arms containing benzo[1,2,5]thiadiazole (BT) acceptor units and 5,5’’-dihexyl-2,2′:3′,2″-terthiophene (tTh) end groups. The molecule, denoted S(TPA-BT-tTh), exhibits a broad absorption band in the wavelength range 300-650 nm and high hole mobility of 1.1×10 -4 cm2 V -1 s 1 . An OSC device based on S(TPA-BT-tTh) as donor and [6,6]-phenyl C71 -butyric acid methyl ester (PC 70 BM) as the acceptor (1:3, w/w) exhibited a power conversion efficiency of 2.28% with a short circuit current density of 6.39 mA/cm2 under illumination of AM.1.5, 100 mW/cm2 . 相似文献
977.
From electronic excited state theory to the property predictions of organic optoelectronic materials
We introduce here a work package for a National Natural Science Foundation of China Major Project. We propose to develop computational methodology starting from the theory of electronic excitation processes to predicting the opto-electronic property for organic materials, in close collaborations with experiments. Through developing methods for the electron dynamics, considering superexchange electronic couplings, spin-orbit coupling elements between excited states, electron-phonon relaxation, intermolecular Coulomb and exchange terms we combine the statistical physics approaches including dynamic Monte Carlo, Boltzmann transport equation and Boltzmann statistics to predict the macroscopic properties of opto-electronic materials such as light-emitting efficiency, charge mobility, and exciton diffusion length. Experimental synthesis and characterization of D-A type ambipolar transport material as well as novel carbon based material will provide a test ground for the verification of theory. 相似文献
978.
979.
Ram B. Ambre Gao‐Fong Chang Manoj R. Zanwar Prof. Ching‐Fa Yao Prof. Eric Wei‐Guang Diau Dr. Chen‐Hsiung Hung 《化学:亚洲杂志》2013,8(9):2144-2153
A series of porphyrin sensitizers that featured two electron‐donating groups and dual anchoring groups that were connected through a porphine π‐bridging unit have been synthesized and successfully applied in dye‐sensitized solar cells (DSSCs). The presence of electron‐donating groups had a significant influence on their spectroscopic, electrochemical, and photovoltaic properties. Overall, the dual anchoring groups gave tunable electronic properties and stronger attachment to TiO2. These new dyes were readily synthesized in a minimum number of steps in gram‐scale quantities. Optical and electrochemical data confirmed the advantages of these dyes for use as sensitizers in DSSCs. Porphyrins with electron‐donating amino moieties provided improved charge separation and better charge‐injection efficiencies for the studied dual‐push–pull dyes. Attenuated total reflectance–Fourier‐transform infrared (ATR‐FTIR) and X‐ray photoelectron spectroscopy of the porphyrin dyes on TiO2 suggest that both p‐carboxyphenyl groups are attached onto TiO2, thereby resulting in strong attachment. Among these dyes, cis-Zn2BC2A , with two electron‐donating 3,6‐ditertbutyl‐phenyl‐carbazole groups and dual‐anchoring p‐carboxyphenyl groups, showed the highest efficiency of 4.07 %, with JSC=9.81 mA cm?2, VOC=0.63 V, and FF=66 %. Our results also indicated a better photostability of the studied dual‐anchored sensitizers compared to their mono‐anchored analogues under identical conditions. These results provide insight into the developments of a new generation of high‐efficiency and thermally stable porphyrin sensitizers. 相似文献
980.
Xiang Wang Dr. Qian Xu Dr. Fengtao Fan Dr. Xiuli Wang Dr. Mingrun Li Prof. Zhaochi Feng Prof. Can Li 《化学:亚洲杂志》2013,8(9):2189-2195
By taking advantage of UV‐Raman spectroscopy and high‐resolution TEM (HRTEM), combined with the focused ion beam (FIB) technique, the transformation from GaOOH into α‐Ga2O3 and then into β‐Ga2O3 was followed. We found that the stepwise transformations took place from the surface region before developing into the bulk of single particles without particle agglomeration and growth. During the transformation from GaOOH into α‐Ga2O3, the elimination of water vapor through the dehydroxylation of GaOOH resulted in the formation of micropores in the single particles, whilst maintaining their particle size. For the phase transformation from α‐Ga2O3 into β‐Ga2O3, the nucleation of β‐Ga2O3 was found to occur at the surface defects and this process could be retarded by occupying these defects with a small amount of La2O3. By finely controlling the process of the phase transformation, the β‐Ga2O3 domains gradually developed from the surface into the bulk of the single particles without particle agglomeration. Therefore, the surface structure of the α‐Ga2O3 single particles can be easily tuned and a particle with an α@β core–shell phase structure has been obtained. 相似文献