全文获取类型
收费全文 | 726篇 |
免费 | 47篇 |
国内免费 | 9篇 |
专业分类
化学 | 562篇 |
晶体学 | 3篇 |
力学 | 15篇 |
数学 | 96篇 |
物理学 | 106篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 24篇 |
2020年 | 31篇 |
2019年 | 20篇 |
2018年 | 36篇 |
2017年 | 24篇 |
2016年 | 52篇 |
2015年 | 48篇 |
2014年 | 56篇 |
2013年 | 99篇 |
2012年 | 41篇 |
2011年 | 59篇 |
2010年 | 43篇 |
2009年 | 37篇 |
2008年 | 40篇 |
2007年 | 28篇 |
2006年 | 22篇 |
2005年 | 18篇 |
2004年 | 14篇 |
2003年 | 13篇 |
2002年 | 9篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1976年 | 2篇 |
排序方式: 共有782条查询结果,搜索用时 15 毫秒
131.
Ali Reza Fakhari Davood Nematollahi Somayeh Makarem Abdolali Alizadeh 《Tetrahedron》2007,63(18):3894-3898
Electrochemical oxidation of catechols (1a-c) has been studied in the presence of methyl acetoacetate (2a) and ethyl acetoacetate (2b) as nucleophiles in aqueous solution using cyclic voltammetry and controlled-potential coulometry. The results indicate that the quinones derived from catechols (1a-c) participate in Michael addition reactions with 2a and 2b to form the corresponding benzofuran derivatives (3a-f). The electrochemical synthesis of 3a-f has been successfully performed in an undivided cell in good yield and purity. The oxidation mechanism was deduced from voltammetric data and by coulometry at controlled potential. The products have been characterized after purification by IR, 1H NMR, 13C NMR, MS, and single crystal X-ray diffraction. 相似文献
132.
S. El‐Taher 《International journal of quantum chemistry》1999,71(3):273-283
The electrophilic additions of hydroperoxyl (HO ), alkylperoxyl (RO ), and halogenated alkylperoxyl radicals to ethylene were studied using the AM1 and PM3 semiempirical MO methods at the SCF/UHF level. Reactantlike transition states were predicted for the title additions. The AM1 activation enthalpies (ΔH ) were found to be increased in the order HO <CH3O <C2H5O <i‐C3H7O . The reactivity of an alkylperoxyl radical toward ethylene was found to be increased as the degree of halogen substitution on the alkyl group increased. A good correlation was established between ΔH and the Taft polar substituent constants, σ*. The Evans–Polanyi correlation between ΔH and ΔH was justified and the validity of the Hammond postulate was indicated. The calculated results were compared with the available experimental findings. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 273–283, 1999 相似文献
133.
134.
In this paper multivariate response surface methodology (RSM) has been used for the optimization of hydrodistillation-headspace solvent microextraction (HD-HSME) of thymol and carvacrol in Thymus transcaspicus. Quantitative determination of compounds of interest was performed simultaneously using gas chromatography coupled with flame ionization detector (GC-FID). Parameters affecting the extraction efficiency were assessed and the optimized values were 5 min, 2 μL and 3 min for the extraction time, micro-drop volume and cooling time after extraction, respectively. The amounts of analyte extracted increased with plant weight. The calibration curves were linear in the ranges of 6.25-81.25 and 1.25-87.50 mg L−1 for thymol and carvacrol, respectively. Limit of detection (LOD) for thymol and carvacrol was 1.87 and 0.23 mg L−1, respectively. Within-day and between-day precisions for both analytes were calculated in three different concentrations and recoveries obtained were in the range of 89-101% and 95-116% for thymol and carvacrol, respectively. 相似文献
135.
F. Hosseinzadeh Lotfi T. Allahviranloo M. Alimardani Jondabeh L. Alizadeh 《Applied Mathematical Modelling》2009
This paper discusses full fuzzy linear programming (FFLP) problems of which all parameters and variable are triangular fuzzy numbers. We use the concept of the symmetric triangular fuzzy number and introduce an approach to defuzzify a general fuzzy quantity. For such a problem, first, the fuzzy triangular number is approximated to its nearest symmetric triangular number, with the assumption that all decision variables are symmetric triangular. An optimal solution to the above-mentioned problem is a symmetric fuzzy solution. Every FLP models turned into two crisp complex linear problems; first a problem is designed in which the center objective value will be calculated and since the center of a fuzzy number is preferred to (its) margin. With a special ranking on fuzzy numbers, the FFLP transform to multi objective linear programming (MOLP) where all variables and parameters are crisp. 相似文献
136.
In the present contribution, the static strength of isostatic graphite using keyhole notch specimens under mixed mode loading is investigated. An experimental program was performed and in total, 18 new experimental data are provided. In addition, different loading mode ratios are considered by varying the inclination angle of the notch with respect to the direction of the applied load. The criterion based on the averaged value of the strain energy density over a control volume at the notch edge is applied to assess the static strength of specimens. A sound agreement is found between experimental data and the results obtained from strain energy density criterion. 相似文献
137.
Ladan Rashidi Ebrahim Vasheghani-Farahani Masoud Soleimani Amir Atashi Khosrow Rostami Fariba Gangi Masoud Fallahpour Mohammad Taher Tahouri 《Journal of nanoparticle research》2014,16(3):1-14
In this study, the effects of intracellular delivery of various concentrations of gallic acid (GA) as a semistable antioxidant, gallic acid-loaded mesoporous silica nanoparticles (MSNs-GA), and cellular uptake of nanoparticles into Caco-2 cells were investigated. MSNs were synthesized and loaded with GA, then characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, N2 adsorption isotherms, X-ray diffraction, and thermal gravimetric analysis. The cytotoxicity of MSNs and MSNs-GA at low and high concentrations were studied by means of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) test and flow cytometry. MSNs did not show significant toxicity in various concentrations (0–500 μg/ml) on Caco-2 cells. For MSNs-GA, cell viability was reduced as a function of incubation time and different concentrations of nanoparticles. The in vitro GA release from MSNs-GA exhibited the same antitumor properties as free GA on Caco-2 cells. Flow cytometry results confirmed those obtained using MTT assay. TEM and fluorescent microscopy confirmed the internalization of MSNs by Caco-2 cells through nonspecific cellular uptake. MSNs can easily internalize into Caco-2 cells without deleterious effects on cell viability. The cell viability of Caco-2 cells was affected during MSNs-GA uptake. MSNs could be designed as suitable nanocarriers for antioxidants delivery. 相似文献
138.
S. M. Hosseini R. Alizadeh E. Fatehifar A. Alizadehdakhel 《Heat and Mass Transfer》2014,50(10):1393-1403
This study is an attempt to investigate the chemical absorption of CO2 in aqueous monoethanolamine (MEA) solution in a wetted-wire column consisting of one wire. Computational fluid dynamics method along with volume of fluid model was employed for modeling of two-phase flow, mass transfer and chemical reaction inside the column. The modeling results were compared with available experimental data and very good agreement was achieved. The simulation results showed that the diameter and intervals of liquid beads increases by increasing the gas and liquid flow rates. The beads velocity increases by increasing the liquid flow rate and decreasing mass fraction of MEA in the liquid phase. Also, mass transfer resistance in the liquid phase reduces by formation of the beads. It was concluded that the developed model is capable to predict the effect of operating and physical parameters on the investigated chemical absorption process. 相似文献
139.
Seyed Sajad Sajadikhah Malek Taher Maghsoodlou Nourallah Hazeri Sayyed Mostafa Habibi-Khorassani Anthony C.Willis 《中国化学快报》2012,23(5):569-572
An efficient green protocol is described for the preparation of highly functionalized piperidines via a one-pot five-component reaction between aromatic aldehydes,anilines andβ-ketoesters in the presence of oxalic acid dihydrate as catalyst in ethanol at ambient temperature.The structure as well as the relative stereochemistry of these compounds was confirmed by single X-ray crystallographic analysis. 相似文献
140.
The present paper deals with the prediction of three-dimensional fluid flow and heat transfer in rib-roughened ducts of square
cross-section, which are either stationary, or rotate in orthogonal mode. The main objective is to assess how a recently developed
variant of a cubic non-linear k−ε model (proposed by Craft et al. Flow Turbul Combust 63:59–80, 1999) can predict three-dimensional flow and heat transfer characteristics through stationary and rotating ribbed ducts. The present
paper discusses turbulent air flow and heat transfer through two different configurations, namely: (I) a stationary square
duct with “in-line” normal and (II) a square duct with normal ribs in a “staggered” arrangement under stationary and rotating
conditions, with the axis of rotation normal to the flow direction and parallel to the ribs. In this paper the flow and thermal
predictions of the linear k−ε model (EVM) are also included, as a set of baseline predictions. The mean flow predictions show that both linear and non-linear
k−ε models can successfully reproduce most of the measured data for stream-wise and cross-stream velocity components. Moreover,
the non-linear model is able to produce better results for the turbulent stresses. The heat transfer predictions show that
both EVM and NLEVM2, the more recent variant of the non-linear k−ε, with the algebraic length-scale correction term, overestimate the measured Nusselt numbers for both geometries examined.
While the EVM with the differential length-scale correction term underestimates heat transfer levels, the Nusselt number predictions
with the NLEVM2 and the ‘NYP’ term are in close agreements with the measured data. Comparisons with our earlier work, Iacovides
and Raisee (Int J Heat Fluid Flow, 20:320–328, 1999), show that the NLEVM2 thermal predictions are of similar quality to those of a second-moment closure. 相似文献