全文获取类型
收费全文 | 1811篇 |
免费 | 99篇 |
国内免费 | 9篇 |
专业分类
化学 | 1351篇 |
晶体学 | 25篇 |
力学 | 60篇 |
数学 | 198篇 |
物理学 | 285篇 |
出版年
2024年 | 9篇 |
2023年 | 24篇 |
2022年 | 74篇 |
2021年 | 67篇 |
2020年 | 56篇 |
2019年 | 91篇 |
2018年 | 64篇 |
2017年 | 46篇 |
2016年 | 101篇 |
2015年 | 51篇 |
2014年 | 59篇 |
2013年 | 145篇 |
2012年 | 121篇 |
2011年 | 101篇 |
2010年 | 82篇 |
2009年 | 73篇 |
2008年 | 57篇 |
2007年 | 59篇 |
2006年 | 53篇 |
2005年 | 47篇 |
2004年 | 55篇 |
2003年 | 40篇 |
2002年 | 57篇 |
2001年 | 22篇 |
2000年 | 19篇 |
1999年 | 14篇 |
1998年 | 7篇 |
1997年 | 12篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 18篇 |
1992年 | 23篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 14篇 |
1987年 | 14篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 17篇 |
1983年 | 11篇 |
1982年 | 16篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1973年 | 8篇 |
排序方式: 共有1919条查询结果,搜索用时 15 毫秒
121.
Three natural basaltic samples were collected and used as efficient catalysts for the liquid‐phase synthesis of n‐butyl acetate. The samples were characterized by X‐ray fluorescence analysis (XRF), X‐ray diffraction (XRD), thermogravimetry (TG), differential thermal analysis (DTA), Fourier transform infrared (FT‐IR), scanning electron microscopy (SEM), and N2 sorption. The acidity of the samples was determined using isopropanol dehydration, and the strength of the acid sites was measured using chemisorption of basic probes. The catalytic activity of the samples towards the esterification of acetic acid with n‐butanol was extensively examined. The influence of different parameters, such as the reaction refluxing time, molar ratio, catalyst loading, reusability, and calcination temperature, on the esterification reaction was studied in detail. The results revealed that all samples had high catalytic activity with a selectivity of 100% to n‐butyl acetate formation. In addition, the sample obtained from the top hill of Volcano had the highest activity with a 80% yield of n‐butyl acetate. Moreover, the significant catalytic performances were well correlated with the acidity of the samples and to the reaction rate constants. 相似文献
122.
123.
Undoped and doped ZnO with 1 at.% (atomic percentage) chromium (Cr) was synthesized by RF reactive co-sputtering for oxygen gas sensing applications. The prepared films showed a highly c-oriented phase with a dominant (0 0 2) peak at a Bragg angle of around 34.2°. The operating temperature of the prepared ZnO sensor was around 350 °C and shifted to around 250 °C for the doped ZnO sensor which is lower than that of previously reported work. The sensitivity of the sensor toward oxygen gas was enhanced by doping ZnO with 1 at.% Cr. Good stability and repeatability of the sensor were demonstrated when tested under different concentration of oxygen atmosphere. 相似文献
124.
Zirconacycles 1, obtained from diethyl 1-alkynylphosphonates, insert either acyl chlorides or nitriles to provide, after acidic workup, (E)-3-oxo-1-alkenylphosphonates, 2, in isolated yields of 55-83%. Insertion produces only one regio- and stereoisomer. The reaction is quite general and proceeds well with both aliphatic and aromatic acyl chlorides. Acetonitrile and p-methoxybenzonitrile also inserted efficiently. Insertion of isobutyl chloroformate produced the vinylphosphonocarboxylate, 2c, the first representative of this class of compounds. 相似文献
125.
126.
127.
128.
The spectral distributions of the visible absorption and fluorescence emission under electron beam excitation of Eu3+-doped (Y2O3) and (YVO4) powders have been detected and analyzed. (Y2O3: Eu3+) has a cubicC crystal structure with a unit cell dimension a=10·61 Å. Its observed transitions from7
F
0 to many upper states have been recognized; the observed number of Stark components is in agreement with that based on theC
2 site symmetry of the Eu3+ ion in Y2O3. Eu3+-doped yttrium vanadate has a typical zircon tetragonal crystal structure with unit cell dimensions ofc=6·29 Å anda=7·11 Å. The observed transitions in (Eu3+: YVO4) have been identified and assigned in accordance with theD
2d site symmetry of the Eu3+ ion in this lattice.The authors would like to express their deep gratitude to Professor G. F. J.Garlick, University of Hull, England, for offering experimental facilities in his Physics Department. 相似文献
129.
Multiparticle production data on proton-nucleus collisions have been analyzed taking the number of ‘created’ charged particles
instead of the observed number of shower particles as the variable. The mean normalized multiplicity,R
A
, has been found to be independent of energy in the energy range (7–8000) GeV and its mass number dependence has been obtained.
The modified analysis introduces some more regularities in the experimental results onp-nucleus collisions like the invariance with respect to energy of the relationshipR
A
= α + βN
h
and the KNO-like scaling of the multiplicity distributions of the created charged particles. The functional form of the scaling
function has been calculated. 相似文献
130.
Fixation mechanism between zeolite and some radioactive elements 总被引:1,自引:0,他引:1
M. B. Hafez A. F. Nazmy F. Salem M. Eldesoki 《Journal of Radioanalytical and Nuclear Chemistry》1978,47(1-2):115-119
Fixation of some fission products and actinide elements from aqueous solutions on zeolite was investigated. Studies showed
that the interaction between the radionuclides and zeolite followed the exchange and the adsorption mechanisms. The relative
importance of each depends on the type, the nature, and the size of the radioelement. 相似文献