全文获取类型
收费全文 | 2200篇 |
免费 | 105篇 |
国内免费 | 20篇 |
学科分类
数理化 | 2325篇 |
出版年
2024年 | 18篇 |
2023年 | 42篇 |
2022年 | 182篇 |
2021年 | 174篇 |
2020年 | 75篇 |
2019年 | 105篇 |
2018年 | 89篇 |
2017年 | 75篇 |
2016年 | 139篇 |
2015年 | 91篇 |
2014年 | 106篇 |
2013年 | 145篇 |
2012年 | 162篇 |
2011年 | 147篇 |
2010年 | 96篇 |
2009年 | 63篇 |
2008年 | 85篇 |
2007年 | 92篇 |
2006年 | 77篇 |
2005年 | 75篇 |
2004年 | 54篇 |
2003年 | 43篇 |
2002年 | 28篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 7篇 |
1998年 | 2篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 11篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1970年 | 3篇 |
排序方式: 共有2325条查询结果,搜索用时 15 毫秒
1.
2.
Preliminary results on the mechanical, optical and electrical properties of composite gel electrolytes (CGEs) with fumed silica
(SiO2) as a filler added to gel polymeric electrolyte (GPE) based on PMMA, LiCF3SO3 and PC are presented in this paper. Added fumed silica is seen to enhance the mechanical properties of the GPE without changing
the conductivity significantly. The high ionic conductivity (×10−3 S/cm), high transmission in the visible region and nominal variance of conductivity and viscosity over a wide temperature
window show that these CGEs are potential electrolytes for electrochromic windows (ECWs). 相似文献
3.
Md. Nur Alam Per B. Zetterlund Masayoshi Okubo 《Journal of polymer science. Part A, Polymer chemistry》2007,45(21):4995-5004
Bimolecular termination in nitroxide‐mediated radical polymerization in miniemulsion has been investigated through the heating of a polystyrene–2,2,6,6‐tetramethylpiperidinyl‐1‐oxy macroinitiator and its 4‐hydroxy‐2,2,6,6‐tetramethylpiperidinyl‐1‐oxy analogue in an aqueous toluene dispersion with sodium dodecyl benzenesulfonate as a surfactant at 125 °C. The level of bimolecular termination by combination, evaluated from the high‐molecular‐weight shoulder, was higher in miniemulsion than in solution and increased with decreasing particle size. Quantitative analysis revealed that these results cannot be rationalized solely by nitroxide partitioning to the aqueous phase. The results are explained by an interface effect, by which nitroxide is adsorbed or located at the aqueous–organic interface. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4995–5004, 2007 相似文献
4.
This paper is devoted to investigating the teleparallel versions of the Friedmann models as well as the Lewis–Papapetrou solution. We obtain the tetrad and the torsion fields for both spacetimes. It is shown that the axial-vector vanishes for the Friedmann models. We discuss the different possibilities for the axial-vector, depending on the arbitrary functions ω and ψ in the Lewis–Papapetrou metric. The vector related to spin has also been evaluated. 相似文献
5.
Bakhtar Ullah Yuli Zhou Jingwen Chen Zongbi Bao Yiwen Yang Qiwei Yang Qilong Ren Zhiguo Zhang 《Tetrahedron letters》2019,60(4):348-351
Thiourea itself has been introduced as a mild and efficient organocatalyst for the oxidative α -cyanation of N-aryltetrahydroisoquinolines (THIQs) with trimethylsilyl cyanide (TMSCN), giving the corresponding products in good to excellent yields. Experimental investigations demonstrated that thiourea acts as a radical initiator by abstracting hydroxyl radical (OH) from tert-butyl hydroperoxide (TBHP) directly instead of non-covalent hydrogen bondings (H-bondings) activation. The use of thiourea as a radical initiator offers a new avenue for innovative chemical transformations in organocatalyzed radical chemistry. 相似文献
6.
Himansu Sekhar Sahoo S. Mahalakshmi Md. Hedayetullah Mir R. Raghunathan 《Tetrahedron letters》2007,48(5):761-765
A remarkable upfield shift of the pyα protons of complexed 2,2′-bipyridine in [cis-Pd(bpy)(NO3)2] is observed which is considered to originate from the anisotropic influence of suitably positioned coordinated nitrate anions around the Pd(II) centre of the molecule. A typical complexation-induced downfield shift is observed for the NH2 protons in [cis-Pd(en)(NO3)2] where ‘en’ stands for ethylenediamine. 相似文献
7.
The new compound, Na[(Mo2
VO4)(pte)(OMe)(MeOH)2] (pte = anion of 6-acetonylisoxanthopterin), has been prepared using the redox non-innocent title ligand in MeOH–H2O, and characterized by elemental analysis and physico-chemical methods including e.s.m.s., i.r., u.v.–vis. and 1H-n.m.r. spectra. Rate constants data (1.4 × 10–3 s–1 at 300 K) for its oxygen atom transfer with dimethyl sulphoxide tally with those of synthetic analogue systems reported earlier by different authors. The negative activation entropy (–206.3 J mol–1 deg–1) is consistent with an associative mechanism for this reaction. 相似文献
8.
Norimah Yusof A.H. Ainul Hafiza Rozaini M. Zohdi Md Zuki A. Bakar 《Radiation Physics and Chemistry》2007,76(11-12):1767-1770
Radiation at 25 and 50 kGy showed no effect on the acidic pH of the local honey, Gelam, and its antimicrobial property against Staphylococcus aureus but significantly reduced the viscosity. Honey stored up to 2 years at room temperature retained all the properties studied. Radiation sterilized Gelam honey significantly stimulated the rate of burn wound healing in Sprague-Dawley rats as demonstrated by the increased rate of wound contraction and gross appearance. Gelam honey attenuates wound inflammation; and re-epithelialization was well advanced compared to the treatment using silver sulphadiazine (SSD) cream. To enhance further the use of honey in wound treatment and for easy handling, Gelam honey was incorporated into our hydrogel dressing formulation, which was then cross-linked and sterilized using electron beam at 25 kGy. Hydrogel with 6% of honey was selected based on the physical appearance. 相似文献
9.
Md. Sadequl Islam 《Tetrahedron》2007,63(5):1074-1079
(−)-Mellein, (+)-ramulosin, (−)-O-methylmellein, (−)-6-hydroxymellein, (−)-6-methoxymellein, and (+)-6-hydroxyramulosin were synthesized as optically active forms using one-pot esterification-Michael addition-aldol reaction of δ-hydroxy-α,β-unsaturated aldehyde and diketene as a key step. 相似文献
10.
Pradeep Mathur Bala Manimaran Md. Munkir Hossain Arnold L. Rheingold Louise M. Liable-Sands Glenn P.A. Yap 《Journal of organometallic chemistry》1997,540(1-2):165-168
The tris-methylene bridged compound (NO)4Fe2Se(μ-CH2)3 has been isolated. It has been characterised by IR and 1H, 13C, and 77Se NMR spectroscopy. Its structure has been determined by single-crystal X-ray diffraction methods. The structure consists of a heavy atom triangle consisting of one Se and two Fe atoms. The Fe-Fe and the two Fe-Se edges are bridged by methylene groups. 相似文献