全文获取类型
收费全文 | 8208篇 |
免费 | 432篇 |
国内免费 | 115篇 |
专业分类
化学 | 6146篇 |
晶体学 | 69篇 |
力学 | 438篇 |
综合类 | 2篇 |
数学 | 872篇 |
物理学 | 1228篇 |
出版年
2024年 | 10篇 |
2023年 | 59篇 |
2022年 | 151篇 |
2021年 | 393篇 |
2020年 | 398篇 |
2019年 | 402篇 |
2018年 | 408篇 |
2017年 | 387篇 |
2016年 | 541篇 |
2015年 | 383篇 |
2014年 | 503篇 |
2013年 | 927篇 |
2012年 | 704篇 |
2011年 | 636篇 |
2010年 | 411篇 |
2009年 | 393篇 |
2008年 | 386篇 |
2007年 | 350篇 |
2006年 | 247篇 |
2005年 | 209篇 |
2004年 | 139篇 |
2003年 | 139篇 |
2002年 | 117篇 |
2001年 | 38篇 |
2000年 | 18篇 |
1999年 | 31篇 |
1998年 | 27篇 |
1997年 | 25篇 |
1996年 | 33篇 |
1995年 | 19篇 |
1994年 | 19篇 |
1993年 | 21篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 17篇 |
1986年 | 8篇 |
1985年 | 21篇 |
1984年 | 14篇 |
1982年 | 23篇 |
1981年 | 11篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1974年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有8755条查询结果,搜索用时 234 毫秒
51.
Mohammad M. Mojtahedi Hassan Abbasi M. Saeed Abaee 《Journal of molecular catalysis. A, Chemical》2006,250(1-2):6-8
Various types of alcohols and phenols were rapidly protected by hexamethyldisilazane in good to excellent yields at room temperature in the presence of catalytic amount of magnesium bromide ethyl etherate under solvent-free conditions. Good to excellent chemoselectivity was demonstrated for competitive protection of primary hydroxyls in the presence of secondary and tertiary alcohols. Highly selective protection of phenols in the presence of aromatic amines was also demonstrated successfully. 相似文献
52.
A. Konjhodzic A. Adamczyk F. Vagizov Z. Hasan E E. Alp W. Sturhahn Jiyong Zhao J J. Carroll 《Hyperfine Interactions》2006,170(1-3):83-89
With the decrease in size of devices, rapid characterization of nano-devices is an inevitable necessity. It is shown that Mössbauer spectroscopy using synchrotron radiation from the advanced photon source provides such a tool of investigation. Results are presented and compared for conventional Mössbauer and Nuclear Forward Scattering for 151Eu-doped magnesium sulfide as an example, especially at low concentrations. 相似文献
53.
Sk. Asrof Ali Yunusa Umar B. F. Abu‐Sharkh Hasan A. Al‐Muallem 《Journal of polymer science. Part A, Polymer chemistry》2006,44(19):5480-5494
The cycloterpolymerizations of single‐, twin‐, and triple‐tailed hydrophobes with hydrophilic monomer N,N‐diallyl‐N‐carboethoxymethylammonium chloride and sulfur dioxide afforded a series of cationic polyelectrolytes (CPEs) in excellent yields. These CPEs, upon the acidic hydrolysis of the pendent ester groups, gave the corresponding pH‐responsive cationic acid salts, which, upon a treatment with sodium hydroxide, were converted to polybetaines (PBs), anionic polyelectrolytes (APEs), and PB/APE polymers containing various proportions of zwitterionic (PB) and anionic fractions (APE) in the polymer chain. At a shear rate of 0.36 s−1 at 30 °C, salt‐free water solutions of the CPEs (2 g/dL) containing 8, 4, and 2.67 mol % of the single‐, twin‐, and triple‐tailed hydrophobes (all having 8 mol % octyloxy tails) had apparent viscosity values of 70, 2800, and 396,000 cps, respectively. The PB/APE polymer with a ratio of 33:67 for the zwitterionic and anionic fractions in the polymer chain gave the highest viscosity value. The superior viscosity behavior of the polymers containing the triple‐tailed hydrophobe was attributed to the blocky nature of the comonomer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5480–5494, 2006 相似文献
54.
Mohammad Mazloum Ardakani Pouran Pourhakak Masoud Salavati-Niasari 《Analytical sciences》2006,22(6):865-870
A coated-wire ion-selective electrode (CWISE), based on a Schiff base as a neutral carrier, was successfully developed for the detection of Pb(II) in aqueous solution. CWISE exhibited a linear response with a Nernstian slope of 29.4 +/- 0.5 mV/decade within the concentration range of 1.0 x 10(-5) - 1.0 x 10(-1) M lead ion. CWISE has shown detection limits of 5.0 x 10(-6) M. The electrode exhibited good selectivity over a number of alkali, alkaline earth, transition and heavy metal ions. This sensor yielded a steady potential within 10 to 20 s at a linear dynamic range. The electrode was suitable for use in aqueous solutions in a pH range of 2.0 to 5.0. Applications of this electrode for the determination of lead in real samples and as indicator electrode for potentiometric titration of Pb2+ ion using K2CrO4 are reported. 相似文献
55.
This paper proves that any set of n points in the plane contains two points such that any circle through those two points encloses at least
points of the set. The main ingredients used in the proof of this result are edge counting formulas for k-order Voronoi diagrams and a lower bound on the minimum number of semispaces of size at most k.Work on this paper by the first author has been supported by Amoco Fnd. Fac. Dev. Comput. Sci. 1-6-44862 and by the National Science Foundation under Grant CCR-8714565, by the second author has been partially supported by the Digital Equipment Corporation, by the fourth author has been partially supported by the Office of Naval Research under Grant N00014-86K-0416. 相似文献
56.
Trituration and capillary solid-state spot-tests for 25 anions with a number of organic and inorganic reagents have been examined. A comparative study of the corresponding reactions in solution has also been made. Incorporation of a glass-wool plug in the capillary improves the selectivity. Schemes are presented for possible use of the technique. 相似文献
57.
Titration calorimetry was used to construct the solid-liquid equilibrium line in ternary systems containing the solute and an aqueous mixed solvent by measuring the heat of dissolution of the solid solute during successive additions of the liquid solvent. The plot of cumulated heats versus the mole ratio, nsolvent/nsolute, yields two (almost) linear increases of different slopes. These two lines represent successively the enthalpy of dissolution then the enthalpy of dilution of the medium; their intersection gives the solubility and the enthalpy of dissolution of the solute. Phase diagrams have been established over the whole concentration range for o-anisaldehyde, 1,3,5-trimethoxybenzene and vanillin, in water + methanol, +ethanol, or +n-propanol at 303, 313 and 318 K. 相似文献
58.
59.
Mohammad H. Alizadeh Toktam Kermani Reza Tayebee 《Monatshefte für Chemie / Chemical Monthly》2007,138(2):165-170
Summary. Esterifications of acetic acid with some linear, secondary, tertiary, and benzylic alcohols mediated by catalytic amounts
of Keggin, Wells–Dawson, and Preyssler type heteropolyacids were carried out under reflux at mild reaction conditions with good to excellent yields. Among the examined
catalysts, H3PW12O40 and H14NaP5W30O110 revealed better results than other heteropolyacids. This work was performed with the aim of simplifying the esterification
process by omitting any solvents and mineral acid catalysts. Easy work-up, low cost, and acidic waste reduction, which are
all important features from the environmental and economical points of view, are distinct aspects of this protocol. Heteropolyacid
catalysts could be separated after a simple work-up and reused for several times. 相似文献
60.
Summary. A highly efficient, selective, fast, and cheap protocol is developed for oxidation of aromatic amines and alcohols utilizing
34% hydrogen peroxide in water catalyzed by some W- and Mo-based heteropolyoxometalates. Findings showed that dodecatungstophosphoric
acid, H3PW12O40, was the most efficient catalyst in the examined oxidation reactions. This methodology may prove to be a valuable alternative
for eco-friendly green oxidation. Inherent simplicity, easy work up, and using regenerable catalysts were other key aspects
of this oxidation protocol. 相似文献