全文获取类型
收费全文 | 88502篇 |
免费 | 2443篇 |
国内免费 | 1713篇 |
专业分类
化学 | 31524篇 |
晶体学 | 939篇 |
力学 | 7457篇 |
综合类 | 105篇 |
数学 | 33062篇 |
物理学 | 19571篇 |
出版年
2023年 | 268篇 |
2022年 | 361篇 |
2021年 | 473篇 |
2020年 | 488篇 |
2019年 | 495篇 |
2018年 | 10731篇 |
2017年 | 10547篇 |
2016年 | 6598篇 |
2015年 | 1386篇 |
2014年 | 953篇 |
2013年 | 1103篇 |
2012年 | 4782篇 |
2011年 | 11420篇 |
2010年 | 6370篇 |
2009年 | 6707篇 |
2008年 | 7249篇 |
2007年 | 9315篇 |
2006年 | 841篇 |
2005年 | 1840篇 |
2004年 | 1869篇 |
2003年 | 2256篇 |
2002年 | 1305篇 |
2001年 | 469篇 |
2000年 | 494篇 |
1999年 | 359篇 |
1998年 | 365篇 |
1997年 | 344篇 |
1996年 | 374篇 |
1995年 | 276篇 |
1994年 | 212篇 |
1993年 | 189篇 |
1992年 | 140篇 |
1991年 | 141篇 |
1990年 | 127篇 |
1989年 | 127篇 |
1988年 | 111篇 |
1987年 | 86篇 |
1986年 | 101篇 |
1985年 | 78篇 |
1984年 | 60篇 |
1983年 | 51篇 |
1982年 | 56篇 |
1981年 | 45篇 |
1980年 | 55篇 |
1979年 | 46篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
大庆渣油族组分非等温热反应的研究 总被引:2,自引:0,他引:2
采用动态热重分析法在升温速率为6.2K·min ̄(-1)及惰性气流量100cm ̄3·min ̄(-1)的条件下,对大庆渣油族组分的热转化行为进行了考察,获得了族组分的热转化速率峰值、速率峰值处的反应温度、转化率和剧裂分解温度区间等动力学基本数据以及各族组分的生焦性能。数据分析表明,族组分的热反应活性顺序为:饱和分>胶质>芳香分>沥青质。在大庆渣油族组分热转化的两个温区内,采用分段一级动力学模型较好地拟合了实验数据,确定了各族组分在两个温区内的热反应动力学参数。 相似文献
982.
Ramiro Pino Olivier Bokanowski Eduardo V. Ludeña Roberto López Boada 《Theoretical chemistry accounts》2007,118(3):557-561
Bearing in mind the insight into the Hohenberg–Kohn theorem for Coulomb systems provided recently by Kryachko (Int J Quantum
Chem 103:818, 2005), we present a re-statement of this theorem through an elaboration on Lieb’s proof as well as an extension
of this theorem to finite subspaces.
Contribution to the Serafin Fraga Memorial Issue. 相似文献
983.
Carolina A. Pinto P. M. Büchler J. Dweck 《Journal of Thermal Analysis and Calorimetry》2007,87(3):715-720
The catalyst
used in fluidized catalytic cracking (FCC) units of refineries after several
recovery cycles in regeneration units, reduces its activity and it is partially
substituted by new catalyst in the process. As it has a high silicon and aluminum
oxides content, the pozzolanic properties of a Brazilian FCC spent residual
catalyst, used in different substitution degrees to cement, were evaluated
by three thermal analysis techniques during the early stages of hydration
of a type II Portland cement. NCDTA curves show in real time that the residual
catalyst, accelerates the stages of cement hydration. TG and DSC curves of
respective pastes after 24 h of hydration evidence the pozzolanic activity
of the waste, respectively, by the lower water mass loss during the dehydroxylation
of the residual calcium hydroxide and by the lower dehydroxylation endothermal
effect. Within the analyzed period, the higher is the cement substitution
degree, the higher is the pozzolanic activity of the residual catalyst. 相似文献
984.
Changlun Chen Di Xu Xiaoli Tan Xiangke Wang 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(1):227-233
The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions.
The extent of sorption of divalent metal cations is controlled by a number of factors including cosorbing or complexing. In
this work, the effects of pH, humic acid HA/Co(II) addition orders, ionic strength, concentration of HA, and foreign cations
on the Co(II) sorption on γ-Al2O3 in the presence of HA were investigated. The sorption isotherms of Co(II) on γ-Al2O3 in the absence and presence HA were also studied and described by using S-type sorption model. The experimental results showed
that the Co(II) sorption is strongly dependent on the pH values, concentration of HA, but independent of HA/Co(II) addition
orders, ionic strength, and foreign cations in the presence of HA under our experimental conditions. The results also indicated
that HA enhanced the Co(II) sorption at low pH, but reduced the Co(II) sorption at high pH. It was hypothesized that the significantly
positive influence of HA at low pH on the Co(II) sorption on γ-Al2O3 was attributed to strong surface binding of HA on γ-Al2O3 and subsequently the formation of ternary surface complexes such as ≡S-OOC-R-(COO−)
x
Co2−x
. Chemi-complexation may be the main mechanism of the Co(II) sorption on γ-Al2O3 in the presence of HA. 相似文献
985.
986.
通过对离子轰击下固体表面电离过程重新考虑认为,在固体表面覆氧或氧离子轰击下除表面原子的直接电离外,激发态双原子间电子交换和断键亦起重要作用。在此基础上修正了局部热力学平衡模型,得到了一个包含各类离子内配分函数、电离能、金属-氧原子键断键能以及表面金属原子与氧原子结合份数等参数决定的新电离几率分析表达式。应用该分析表达式解释了金属表面覆氧、氧离子轰击金属、化合物半导体表面二次离子发射中氧增强效应、充氧量对二次离子发射的影响及其基体效应等实验现象。并由此得到了元素相对灵敏度因子的分析表达式,对化合物半导体及一些陶瓷材料表面二次离子质谱分析中元素灵敏度因子随元素电离能变化曲线给予了相应的物理解释。 相似文献
987.
988.
In this study, a syringe was filled with silica gel loaded with 3-aminopropyltriethoxysilane, for the separation and preconcentration of copper, cadmium and chromium prior to their determination by graphite furnace atomic absorption spectrometry (GFAAS) in seawater. For this purpose, a syringe was filled with 0.5 g of modified silica gel and the sample solution was drawn into the syringe and ejected back again. The analyte elements were quantitatively retained at pH 5. Then, the elements sorbed by the silica gel were eluted with 2.0 M of HCl and determined by GFAAS. At optimum conditions, the recovery of Cu, Cd and Cr were 96-98%. Detection limits (3delta) were 6.6, 7.5 and 6.0 micro g L(-1) for Cu, Cd and Cr, respectively. The elements could be concentrated by drawing and discharging several portions of sample successively but eluting only once. Cu, Cd and Cr added to a seawater sample were quantitatively recovered (>95%) in the range of the 95% confidence level. The method proposed in this paper was compared with a column technique. Optimum experimental conditions, reproducibility, precision and recoveries of both techniques are the same, but the syringe technique is much faster, easier and more practical than the column technique. It is a portable system and allows one to make the sorption process in the source of sample. In addition, the risk of contamination is less than in the column technique. 相似文献
989.
The adsorption/desorption process of borate was studied at Pt(111) in acidic solution by cyclic voltammetry. A so-called butterfly
wave in the cyclic voltammogram of Pt(111) in HClO4 shifted to negative direction upon the addition of boric acid with the disappearance of its sharp spikes. The shift in potential
was found to be −57 mV with a tenfold increase of boric acid concentration. This illustrates that this anomalous wave is due
to borate adsorption/desorption by a one-electron transfer process. The borate adsorption/desorption wave was observed to
shift by −63 mV/pH. At pH>3, the anomalous wave splits forming two separate waves, depending on the pH and the scan rate.
The appearance of two waves is assigned to the change in the adsorption mode of borate or the participation of OH in the adsorption
process. 相似文献
990.
Vijayasarathi Nagarajan Barbara Müller Oksana Storcheva Klaus Köhler Andreas Pöppl 《Research on Chemical Intermediates》2007,33(8):705-724
Interactions and binding sites of the solvent molecules chloroform and ethanol to bis(acetylacetonate)oxovanadium(IV) (VO(acac)2) complexes in (frozen) solutions have been investigated by pulsed electron nuclear double resonance, sum peak electron spin
echo envelope modulation and hyperfine sublevel correlation spectroscopy. The experimental proton hyperfine coupling data
of coordinating solvent molecules have been interpreted using quantum chemical calculations (density functional theory). Experimental
and computed hyperfine couplings indicate that ethanol coordinates to vanadium in the equatorial plane of VO(acac)2 and chloroform interacts via hydrogen bonding to oxygens of the acac ligands. 相似文献