全文获取类型
收费全文 | 1501篇 |
免费 | 50篇 |
国内免费 | 16篇 |
专业分类
化学 | 1059篇 |
晶体学 | 11篇 |
力学 | 52篇 |
综合类 | 3篇 |
数学 | 105篇 |
物理学 | 337篇 |
出版年
2024年 | 11篇 |
2023年 | 13篇 |
2022年 | 61篇 |
2021年 | 67篇 |
2020年 | 41篇 |
2019年 | 47篇 |
2018年 | 41篇 |
2017年 | 28篇 |
2016年 | 69篇 |
2015年 | 52篇 |
2014年 | 58篇 |
2013年 | 126篇 |
2012年 | 106篇 |
2011年 | 119篇 |
2010年 | 59篇 |
2009年 | 65篇 |
2008年 | 63篇 |
2007年 | 53篇 |
2006年 | 61篇 |
2005年 | 51篇 |
2004年 | 44篇 |
2003年 | 39篇 |
2002年 | 32篇 |
2001年 | 17篇 |
2000年 | 16篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 11篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 9篇 |
1993年 | 12篇 |
1992年 | 9篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 12篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 10篇 |
1976年 | 6篇 |
1962年 | 3篇 |
排序方式: 共有1567条查询结果,搜索用时 0 毫秒
41.
M.Iqbal Choudhary SyedGhulam Musharraf Amtul Sami Atta‐ur‐Rahman 《Helvetica chimica acta》2004,87(10):2685-2694
The microbial transformation of (?)‐Ambrox® ( 1 ), a perfumery sesquiterpene, by a number of fungi, by means of standard two‐stage‐fermentation technique, afforded ambrox‐1α‐ol ( 2 ), ambrox‐1α,11α‐diol ( 3 ), ambrox‐1α,6α‐diol ( 4 ), ambrox‐1α,6α,11α‐triol ( 5 ), ambrox‐3‐one ( 6 ), ambrox‐3β‐ol ( 7 ), ambrox‐3β,6β‐diol ( 8 ), 13,14,15,16‐tetranorlabdane‐3,8,12‐triol ( 9 ), and sclareolide ( 10 ) (Schemes 1 and 2). Further incubation of compound 10 with Cunninghamella elegans afforded 3‐oxosclareolide ( 11 ), 3β‐hydroxysclareolide ( 12 ), 2α‐hydroxysclareolide ( 13 ), 2α,3β‐dihydroxysclareolide ( 14 ), 1α,3β‐dihydroxysclareolide ( 15 ), and 3β‐hydroxy‐8‐episclareolide ( 16 ) (Scheme 3). Metabolites 2 – 5, 12, 13 , and 16 were found to be new compounds. The major transformations include a reaction path involving hydroxylation, ether‐bond cleavage and inversion of configuration. Metabolites 11 – 16 of sclareolide showed significant phytotoxicity (Table 1). The structures of the metabolites were characterized on the basis of spectroscopic techniques. 相似文献
42.
Norimah Yusof Asnah Hassan M.N. Firdaus Abd Rahman Suzina A. Hamid 《Radiation Physics and Chemistry》2007,76(11-12):1756-1759
Most of the tissue banks in the Asia Pacific region have been using ionising radiation at 25 kGy to sterilise human tissues for save clinical usage. Under tissue banking quality system, any dose employed for sterilisation has to be validated and the validation exercise has to be a part of quality document. Tissue grafts, unlike medical items, are not produced in large number per each processing batch and tissues relatively have a different microbial population. A Code of Practice established by the International Atomic Energy Agency (IAEA) in 2004 offers several validation methods using smaller number of samples compared to ISO 11137 (1995), which is meant for medical products. The methods emphasise on bioburden determination, followed by sterility test on samples after they were exposed to verification dose for attaining of sterility assurance level (SAL) of 10−1. This paper describes our experience in using the IAEA Code of Practice in conducting the validation exercise for substantiating 25 kGy as sterilisation dose for both air-dried amnion and those preserved in 99% glycerol. 相似文献
43.
Instrumental neutron activation analysis (INAA) and atomic absorption spectrophotometry (AAS) were used to verify the protocols
for the quantification of zinc and magnesium in human whole blood. A study was conducted on group of volunteers consisting
of 131 patients having cardiovascular diseases (CVD) and 23 malignant hypertension (MH) patients along with 432 control subjects
with 218 male and 214 females. The elemental data of these elements has been exploited to establish the base line values in
control subjects. The possibility of any relationship between blood Mg and Zn levels with CVD and MH has also been investigated.
The mean blood Mg and Zn levels were found to be depleted in both CVD and MH patients as compared to normal subjects. In our
findings Mg and Zn were found to have an inverse relation with systolic and diastolic pressure. The reliability of the methods
was checked by the concurrent analysis of the IAEA reference material (RM) employing the optimized INAA and AAS protocols.
The determined values by both techniques were found to be in good agreement with the IAEA reference values. The elemental
data in whole blood samples of normal volunteers has also been compared with Mg and Zn contents reported by other countries. 相似文献
44.
M. Jafarzadeh I. A. Rahman C. S. Sipaut 《Journal of Sol-Gel Science and Technology》2009,50(3):328-336
A modified preparation of silica nanoparticles via sol–gel process was described. The ability to control the particle size
and distribution was found highly dependent on mixing modes of the reactants and drying techniques. The mixture of tetraethoxysilane
and ethanol followed by addition of water (Mode-A) produced monodispersed powder with an average particle size of 10.6 ± 1.40 nm
with a narrow size distribution. The freeze drying technique (FD) further improved the quality of powder. In addition, the
freeze dried samples have shown unique TGA decomposition steps which might be related to the well-defined structure of silica
nanoparticles as compared to the heat dried samples. DSC analysis showed that FD preserved the silica surface with low shrinkage
and generated remarkably well-order, narrow and bigger pore size and pore volume and also large endothermic enthalpies (ΔH
FD = −688 J g−1 vs. ΔH
HD = −617 J g−1) that lead to easy escape of physically adsorbed water from the pore at lower temperature. 相似文献
45.
A generalized technique for the encapsulation of nano‐sized NiO particles by styrene‐2‐hydroxyethyl methacrylate copolymer 下载免费PDF全文
M. Shamim Hossan M. Abdur Rahman Klaus Tauer Hideto Minami Hasan Ahmad 《先进技术聚合物》2015,26(9):1047-1052
Encapsulation of nickel oxide (NiO) particles is of great interest to the researchers as such modification produces remarkable improvement in properties and versatility in application potential. In this investigation, nanosized NiO particles were first prepared by calcination of nickel hydroxide precursor obtained using a simple liquid‐phase process. The produced NiO particles were stabilized with oleic acid and then treated with tetraethylorthosilicate to produce NiO/SiO2 composite seed particles. Finally tri‐layered inorganic/organic composite particles were prepared by seeded copolymerization of styrene and 2‐hydroxyethyl methacrylate (HEMA) in the presence of NiO/SiO2 composite seed particles. The produced composite particles named as NiO/SiO2/P(S‐HEMA) were colloidally stable, and the obtained particles were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy and thermogravimetric analyses. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
46.
Mohammad O. Tuhin Nazia Rahman M.E. Haque Ruhul A. Khan N.C. Dafader Rafiqul Islam Mohammad Nurnabi Wafa Tonny 《Radiation Physics and Chemistry》2012,81(10):1659-1668
Chitosan–starch blend films (thickness 0.2 mm) of different composition were prepared by casting and their mechanical properties were studied. To improve the properties of chitosan–starch films, glycerol and mustard oil of different composition were used. Chitosan–starch films, incorporated with glycerol and mustard oil, were further modified with monomer 2-hydroxyethyl methacrylate (HEMA) using gamma radiation. The modified films showed improvement in both tensile strength and elongation at break than the pure chitosan–starch films. Water uptake of the films reduced significantly than the pure chitosan–starch film. Thermo gravimetric analysis (TGA) and dynamic mechanical analysis (DMA) showed that the modified films experience less thermal degradation than the pure films. Scanning electron microscopy (SEM) and FTIR were used to investigate the morphology and molecular interaction of the blend film, respectively. 相似文献
47.
Takehiko Yamato Carol Pérez-Casas Shofiur Rahman Zeng Xi Mark R. J. Elsegood Carl Redshaw 《Journal of inclusion phenomena and macrocyclic chemistry》2007,58(1-2):193-197
A novel receptor possessing two complexation sites and bearing 1,3-alternate conformation based on thiacalix[4]arene, confirmed by single crystal X-ray analysis, was prepared. The tetrathiacalix[4]arene
diamide shows strong intramolecular hydrogen bonding. The binding behavior towards K+ and halides has been examined by 1H NMR titration experiments. 相似文献
48.
The first stable crystalline geminal diol of an aldehyde lacking electron-withdrawing groups on the-carbon was synthesized fromd-sorbitol and characterized by1H and13C nuclear magnetic resonance (NMR) and single-crystal x-ray studies. Each hydroxyl is a donor for a single, unique intermolecular hydrogen bond. Only one hydroxyl acts as an acceptor. No intramolecular hydrogen bonding is observed in the crystal. 相似文献
49.
Md. Saifur Rahman Tamotsu Hashimoto Toshiyuki Kodaira 《Journal of polymer science. Part A, Polymer chemistry》2000,38(24):4362-4372
The cationic polymerization of 2‐[4‐(methoxycarbonyl)phenoxy] ethyl vinyl ether, a vinyl ether with a benzoate pendant, was carried out with an HCl/ZnCl2 initiating system in methylene chloride at −15 °C. The polymerization proceeded with living/long‐lived propagating species to produce polymers with controlled molecular weights and relatively narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight ≤ ∼1.4), despite the formation of a small amount of oligomeric products during the polymerization. The structural analysis showed that the lowest molecular weight oligomer had the structure CH3CH(OCH2CH2OC6H4COOCH3)OCH2CH2OC6H4COOCH3. The oligomer was formed by the reaction of the monomeric propagating species with the alcohol produced by the side reaction of the active species with water as an impurity during the early stage of polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4362–4372, 2000 相似文献
50.
Junes Ipaktschi Rahman Hosseinzadeh Peter Schlaf Thomas Eckert 《Helvetica chimica acta》2000,83(6):1224-1238
Self‐Assembly of Quinodimethanes through Covalent Bonds. Part III. Investigations on the Preparation of Nanostructures As part of our studies on the tetramerization of quinodimethane 1 to the macrocyclic compound 2 , the influence of substituents on this reaction was investigated. It was found that a large range of substituents such as 2‐phenylethyl, 2‐(4‐methoxyphenyl)ethyl, 2‐(4‐fluorophenyl)ethyl, 2‐[4‐(tert‐butyl)phenyl]ethyl, and 2‐[4‐(2‐phenylethyl)phenyl]ethyl, attached at positions 2 and 7 of 9H‐fluorene, do not prevent the tetramerization. The key step in the formation of the macrocylic compounds 13a – e is the debromination of 12a – e with mercury to the corresponding quinodimethanes which undergo a self‐assembly forming 13a – e in high yields. To study the conjugative influence of substituents on tetramerization, the effect of the hex‐1‐ynyl groups at positions 3 and 6 of the 9H‐fluorene rings was investigated. In this case, the corresponding macrocycle 17 was generated by the reaction of diol 16a with SnCl2. Although the expected tetramerization to 17 occurred, the yield was lower than in the case of 13a – e , due to the sensitivity of the product. 相似文献