全文获取类型
收费全文 | 1028篇 |
免费 | 34篇 |
专业分类
化学 | 558篇 |
晶体学 | 13篇 |
力学 | 26篇 |
数学 | 219篇 |
物理学 | 246篇 |
出版年
2023年 | 6篇 |
2022年 | 24篇 |
2021年 | 27篇 |
2020年 | 18篇 |
2019年 | 26篇 |
2018年 | 23篇 |
2017年 | 24篇 |
2016年 | 37篇 |
2015年 | 19篇 |
2014年 | 44篇 |
2013年 | 68篇 |
2012年 | 62篇 |
2011年 | 62篇 |
2010年 | 48篇 |
2009年 | 39篇 |
2008年 | 60篇 |
2007年 | 44篇 |
2006年 | 32篇 |
2005年 | 30篇 |
2004年 | 20篇 |
2003年 | 17篇 |
2002年 | 15篇 |
2001年 | 15篇 |
2000年 | 10篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1980年 | 11篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 9篇 |
1966年 | 8篇 |
1965年 | 6篇 |
1962年 | 5篇 |
1959年 | 5篇 |
1958年 | 9篇 |
1957年 | 8篇 |
1956年 | 8篇 |
1955年 | 10篇 |
1954年 | 10篇 |
1941年 | 6篇 |
1940年 | 5篇 |
1936年 | 10篇 |
1935年 | 6篇 |
排序方式: 共有1062条查询结果,搜索用时 15 毫秒
71.
P. Santhana Raman K.G.M. Nair R. Kesavamoorthy B.K. Panigrahi S. Dhara V. Ravichandran 《Applied Physics A: Materials Science & Processing》2007,87(4):709-713
Indium nanoclusters are synthesized in an amorphous silica matrix using an ion-implantation technique. Indium ions (In2+) with energy of 890 keV are implanted on silica to fluences in the range of 3×1016–3×1017 cm-2. The formation of indium nanoclusters is confirmed by optical absorption spectrometry and glancing incidence X-ray diffraction
studies. A low frequency Raman scattering technique is used to study the growth of embedded indium nanoclusters in the silica
matrix as a function of fluence and post-implantation annealing duration. Rutherford backscattering spectrometry studies show
the surface segregation of implanted indium. Photoluminescence studies indicate the formation of a small quantity of indium
oxide phase in the ion-implanted samples.
PACS 85.40.Ry; 78.67.Bf; 73.20.Mf; 82.75.Fq 相似文献
72.
A consistent LES/filtered-density function formulation for the simulation of turbulent flames with detailed chemistry 总被引:3,自引:0,他引:3
A hybrid large-Eddy simulation/filtered-density function (LES–FDF) methodology is formulated for simulating variable density turbulent reactive flows. An indirect feedback mechanism coupled with a consistency measure based on redundant density fields contained in the different solvers is used to construct a robust algorithm. Using this novel scheme, a partially premixed methane/air flame is simulated. To describe transport in composition space, a 16-species reduced chemistry mechanism is used along with the interaction-by-exchange with the mean (IEM) model. For the micro-mixing model, typically a constant ratio of scalar to mechanical time-scale is assumed. This parameter can have substantial variations and can strongly influence the combustion process. Here, a dynamic time-scale model is used to prescribe the mixing time-scale, which eliminates the time-scale ratio as a model constant. Two different flame configurations, namely, Sandia flames D and E are studied. Comparison of simulated radial profiles with experimental data show good agreement for both flames. The LES–FDF simulations accurately predict the increased extinction near the inlet and re-ignition further downstream. The conditional mean profiles show good agreement with experimental data for both flames. 相似文献
73.
Priyanka Trivedi Jonnala Kotesh Kumar Arvind Singh Negi Karuna Shanker 《Biomedical chromatography : BMC》2011,25(6):697-706
Extraction and pre‐concentration of a bioactive marker compound, phenyl‐1,3,5‐heptatriyne from Bidens pilosa, prior to HPLC has been demonstrated using both organic and ecofriendly solvents. Non‐ionic surfactants, viz. Triton X‐100, Triton X‐114 and Genapol X‐80, were used for extraction. No back‐extraction or liquid chromatographic steps were required to remove the target phytochemical from the surfactant‐rich extractant phase. The optimized cloud point extraction procedure has been shown to be a potentially useful methodology for the preconcentration of the target analyte, with a preconcentration factor of 4–99. Moreover, the method is simple, sensitive, rapid and consumes lesser solvent than traditional methods. An isocratic chromatographic separation and quantitation was accomplished on a C18 column with acetonitrile–acidified aqueous as mobile phase at a flow rate of 1.0 mL/min, UV detection at 254 nm and specificity with photo diode‐array detector (PDA) and MS. Under the optimum experimental conditions recovery was satisfactory (99.18–100.33%) without interference from the surfactant. The method seems to be reliable with intraday precision and interday precision below 2.0%. Good linearity was obtained in the working range from 7.5 to 30 µg/mL with correlation coefficient >0.99. The limits of detection and quantitation were 1.84 and 6.13 µg/mL, respectively. The method was validated following international guidelines and successfully applied for quantitative assays of cytotoxic compound phenyl‐1,3,5‐heptatriyne in Bidens pilosa. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
74.
A bidentate ligand derived from cinnamaldehyde and acetylacetone and its copper(II) complex have been synthesized and characterized
by elemental analysis, UV-Vis, IR, ESR and magnetic susceptibility measurements. Magnetic susceptibility measurements, ESR
and electronic spectral data indicate the presence of six coordinated Cu(II) ion. The ligand and complex are tested for antibacterial
activity againstPseudomonas aeroginosa. They are found to show the antibacterial activity. 相似文献
75.
We consider the model of the one-dimensional cookie random walk when the initial cookie distribution is spatially uniform
and the number of cookies per site is finite. We give a criterion to decide whether the limiting speed of the walk is non-zero.
In particular, we show that a positive speed may be obtained for just three cookies per site. We also prove a result on the
continuity of the speed with respect to the initial cookie distribution.
相似文献
76.
A scheme to execute an n-bit Deutsch-Jozsa (DJ) algorithm using n qubits has been implemented for up to three qubits on an NMR quantum computer. For the one- and the two-bit Deutsch problem,
the qubits do not get entangled, and the NMR implementation is achieved without using spin-spin interactions. It is for the
three-bit case, that the manipulation of entangled states becomes essential. The interactions through scalar J-couplings in NMR spin systems have been exploited to implement entangling transformations required for the three bit DJ algorithm. 相似文献
77.
This paper provides an example of the application of Pressure Sensitive Paint (PSP) to complex internal suspersonic flows
and demonstrates the relationship between ejector wall pressure and aerodynamic performance. Details of such jet mixer-ejector
nozzles are relevant to jet noise reduction programs. Several ejector configurations with straight, convergent, and divergent
side walls were used in our experiments. The side-wall that was painted with PSP was also instrumented with an array of 156
pressure taps connected to Electronically Scanned Pressure (ESP) modules, enabling simultaneous measurement of “true” reference
pressures. The PSP results agreed very well with the “true” reference pressures and also provided a detailed map of the complicated
pressure patterns that could not be detected using the pressure taps. Finally, we also demonstrated the direct relationship
between ejector side-wall pressure distribution and ejector performance characteristics such as exit mean flow uniformity,
pumping, and thrust augmentation.
Received: 16 December 1997/Accepted: 21 August 1998 相似文献
78.
Knoevenagel condensate Schiff base ligands [L = 3‐cinnamalideneacetylacetone‐thiosemicarbazone (CAT)/3‐cinnama‐ lideneacetylacetoneethylthiosemicarbazone (CAET)/3‐cinnamalideneacetylacetonephenylthiosemicarbazone (CAPT)] and their copper/zinc complexes were synthesized. They were characterized by analytical and spectral techniques. From these data it was found that the ligands adopt square‐planar geometry on metalation with Cu2+ and Zn2+. To evaluate the antitumor and cytotoxic activity of the synthesized complexes in mice and human cancer cell lines, the antitumor activity of the complexes was evaluated against an Ehrlich ascites carcinoma (EAC) tumor model. The activity was assessed using survival time and short‐term in vitro cytotoxic activity. Oral administration of complexes (100 mg/kg) increased the survival time. The cytotoxic activity of complexes was evaluated using human breast cancer (MDA‐MB‐231), colon cancer (HCT‐116) and nonsmall lung cancer (NCI‐H‐23) cell lines. Both the complexes possessed significant antitumor and cytotoxic activity on EAC and human cancer cell lines. The in vitro antimicrobial screening effect of the investigated compounds was also tested against the various organisms by well diffusion method. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
79.
Yihao Tang Malik Hassanaly Venkat Raman Brandon A. Sforzo Jerry Seitzman 《Proceedings of the Combustion Institute》2021,38(2):2589-2596
Fast and reliable high altitude re-ignition is a critical requirement for the development of alternative jet fuels (AJFs). To achieve stable combustion, a spark kernel needs to transit in a partially or fully extinguished flow to develop a flame front. Understanding the relight characteristics of the AJFs is complicated by the chaoticity of the turbulent flow and variations in the spark properties. The focus of this study is the prediction of such characteristics by high-fidelity simulations, with a specific focus on fuel composition effect on the ignition process. For this purpose, a previously developed computational framework is applied, which utilizes high-fidelity LES simulations, a hybrid tabulation approach for modeling forced ignition and detailed quantification of uncertainty resulting from initial and boundary conditions to predict ignition probability. The method is applied to two alternative fuels (named C1 and C5) and Jet-A fuel (named A2) under gaseous conditions. Results show that the mixing of kernel and fuel–air mixture is not affected by the ignition process, but chemistry effects strongly dominate ignition probability. In particular, C1 exhibits much lower ignition probability than the other two fuels, especially at lean operating conditions. More importantly, this behavior is contradictory to ignition delay experiments which predict longer delay times for C5 compared to C1. Comparisons with experiments show that the comprehensive modeling approach captures the ignition trends. Analysis of kernel trajectories in composition space shows that the variations are caused by the relative effects of kernel mixing, response to strain, and ignition properties of the fuel. 相似文献
80.
An efficient [4+2] cyclization of N-methylanilines with maleimides to afford tetrahydroquinolines using N-hydroxyphthalimide as a metal-free and cheap organophotoredox catalyst is reported. The protocol involves C(sp3)H activation of N-methylanilines for the formation of α-amino radical without an oxidant at room temperature. The present method describes an easy preparation of tricyclic heterocycles in good to excellent yields under mild reaction conditions. 相似文献