全文获取类型
收费全文 | 1226篇 |
免费 | 118篇 |
国内免费 | 219篇 |
专业分类
化学 | 1255篇 |
晶体学 | 25篇 |
力学 | 3篇 |
综合类 | 9篇 |
数学 | 2篇 |
物理学 | 269篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 20篇 |
2021年 | 25篇 |
2020年 | 35篇 |
2019年 | 44篇 |
2018年 | 36篇 |
2017年 | 46篇 |
2016年 | 60篇 |
2015年 | 49篇 |
2014年 | 74篇 |
2013年 | 122篇 |
2012年 | 67篇 |
2011年 | 67篇 |
2010年 | 75篇 |
2009年 | 62篇 |
2008年 | 71篇 |
2007年 | 81篇 |
2006年 | 64篇 |
2005年 | 69篇 |
2004年 | 72篇 |
2003年 | 52篇 |
2002年 | 47篇 |
2001年 | 37篇 |
2000年 | 20篇 |
1999年 | 24篇 |
1998年 | 32篇 |
1997年 | 17篇 |
1996年 | 19篇 |
1995年 | 28篇 |
1994年 | 37篇 |
1993年 | 23篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1975年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有1563条查询结果,搜索用时 187 毫秒
101.
A one-pot synthesis of 3,5-disubstituted 1,2,4-oxadiazoles with two identical substituents directly from the reaction of nitriles and hydroxylamine hydrochloride in the presence of potassium fluoride as catalyst and solid support under solvent-free condition is described. Moreover, the formation of products has been discussed, and a plausible mechanism has been presented. Simplicity of the process, workup in aqueous media, and excellent yields are some advantages of this method. 相似文献
102.
A one-pot, fluoride-promoted Wittig reaction was developed. The reactions of ethyl α-bromoacetate with aliphatic, aromatic, and heteroaromatic aldehydes in the presence of tri-n-butylphosphine and tetrabutylammonium fluoride produced α,β-unsaturated esters in good to excellent yields and E-stereoselectivity. Under the same conditions, reactions of ethyl α-bromopropionate, α-bromo acetonitrile, and α-bromoacetophenone with aliphatic and aromatic aldehydes in the presence of tri-n-butylphosphine and tetrabutylammonium fluoride produced the expected α,β-unsaturated derivatives in good E-stereoselectivity. The protocol was extended to semistabilized ylides and applied to the synthesis of some combretastatin analogs. 相似文献
103.
A persistent perfluoroalkyl radical (PPFR), perfluoro‐3‐ethyl‐2,4‐dimethyl‐3‐pentyl, is shown to be a good source of •CF3 radicals and a useful radical capable of initiating the polymerization of vinylidene fluoride (VDF). NMR characterizations of the resulting PVDF homopolymers showed that polymerization of VDF was exclusively initiated by •CF3 radicals. The addition of •CF3 radical onto VDF was regioselective leading to CF3‐CH2‐CF2‐PVDF and the CF3 end‐group acted as an efficient label to assess the molecular weights by 19F NMR spectroscopy. Various [PPFR]0/[VDF]0 initial molar ratios lead to CF3–PVDF–CF3 of different molecular weights. When that ratio decreased, both the molecular weights and the thermostability of these PVDFs increased, showing less defects of chaining and higher crystallinity. 相似文献
104.
为了缓解AlGaN/GaNhighelectronmobilitytransistors(HEMT)器件n型GaN缓冲层高的泄漏电流,本文提出了具有氟离子注入新型Al0.25Ga0.75N/GaNHEMT器件新结构.首先分析得出n型GaN缓冲层没有受主型陷阱时,器件输出特性为欧姆特性,这样就从理论和仿真方面解释了文献生长GaN缓冲层掺杂Fe,Mg等离子的原因.利用器件输出特性分别分析了栅边缘有和没有低掺杂漏极时,氟离子分别注入源区、栅极区域和漏区的情况,得出当氟离子注入源区时,形成的受主型陷阱能有效俘获源极发射的电子而减小GaN缓冲层的泄漏电流,击穿电压达到262v通过减小GaN缓冲层体泄漏电流,提高器件击穿电压,设计具有一定输出功率新型A1GaN/GaNHEMT提供了科学依据. 相似文献
105.
Huixia Hu Yonggang Shangguan Min Zuo Qiang Zheng 《Journal of Polymer Science.Polymer Physics》2008,46(18):1923-1931
The liquid–liquid phase‐separation (LLPS) behavior of poly(n‐methyl methacrylimide)/poly(vinylidene fluoride) (PMMI/PVDF) blend was studied by using small‐angle laser light scattering (SALLS) and phase contrast microscopy (PCM). The cloud point (Tc) of PMMI/PVDF blend was obtained using SALLS at the heating rate of 1 °C min?1 and it was found that PMMI/PVDF exhibited a low critical solution temperature (LCST) behavior similar to that of PMMA/PVDF. Moreover, Tc of PMMI/PVDF is higher than its melting temperature (Tm) and a large temperature gap between Tc and Tm exists. At the early phase‐separation stage, the apparent diffusion coefficient (Dapp) and the product (2Mk) of the molecules mobility coefficient (M) and the energy gradient coefficient (k) arising from contributions of composition gradient to the energy for PMMI/PVDF (50/50 wt) blend were calculated on the basis of linearized Cahn‐Hilliard‐Cook theory. The kinetic results showed that LLPS of PMMI/PVDF blends followed the spinodal decomposition (SD) mechanism. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1923–1931, 2008 相似文献
106.
An efficient experimentally simple and inexpensive catalyst system for the selective amidation of aryl iodides using 15 mol% of CuI as catalyst, 15 mol% of L-proline as ligand and KF/Al2O3 as a base in toluene is described. 相似文献
107.
Temperature measurements of shock heated materials using multispectral pyrometry: Application to bismuth 总被引:1,自引:0,他引:1
A new device for temperature measurements of shock heated materials has been developed in order to refine their equations
of state. We present temperature measurements for bismuth samples in contact with a lithium fluoride window. The shock pressure
of these experiments ranges from 18 to 97 GPa.
Received 2 June 1997 / Accepted 26 January 1998 相似文献
108.
Fumiaki Ishii Toshio Ohga Akihiro Tsutsumi Hiroji Ohigashi 《Journal of Polymer Science.Polymer Physics》2002,40(10):1026-1037
The heterogeneous higher order structure and molecular motion in a single crystalline film of a vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer with 73 mol % VDF was investigated with the 1H–13C cross‐polarization/magic‐angle spinning NMR technique. A transient oscillation was observed in plots of the 13C peak intensity versus the contact time for the CH2, CHF, and CF2 groups. On the basis of the extended cross‐relaxation theory of spin diffusion, we determined that the oscillation behavior was caused by the TrFE‐rich segments in the chain and that the crystal consisted of VDF‐rich and TrFE‐rich domains. The former had TrFE‐rich segments in VDF and TrFE fractions of 0.24 and 0.27, respectively, and the latter had VDF‐rich segments in a VDF fraction of 0.49. The spin–lattice relaxation time T1ρH in the rotating frame for each group was minimal in the three temperature regions of β, αb, and αc (↑) on heating and in the two temperature regions of α1D and αc (↓) on cooling. The αc (↑) and αc (↓) processes depended on the first‐order ferroelectric phase‐transition regions on heating and cooling, respectively. The motional modes for the other processes were confirmed by the T1ρH minimum behavior of the VDF and TrFE groups in the TrFE‐rich domain and the VDF‐rich segments in the VDF‐rich domain. The β and αb processes were attributed to the flip–flop motion of the TrFE‐rich segments and the competitive motion of the TrFE‐ and VDF‐rich segments in the ferroelectric phase, respectively. The α1D process was due to the one‐dimensional diffusion motion of the conformational defects along the chain in the paraelectric phase, accompanied by the trans and gauche transformation of the VDF conformers of ttg+tg? and g+tg?tt. The effect of the competitive motion of the TrFE‐rich segment on the thermal stability of the VDF‐rich segment in the chain near the Curie temperature was examined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1026–1037, 2002 相似文献
109.
A new method to remove water adsorbed on flouride glass preforms is reported. This method, using NF3 plasma, can remove surface water on fluoride glasses 相似文献
110.
Poly(vinylidene fluoride)(PVDF)/Na+‐MMT composites have been successfully prepared utilizing sodium montmorillonite (Na+‐MMT) via N,N‐dimethylformamide (DMF) solution mixing. The dispersion of Na+‐MMT layers in composites were investigated by transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The effect of adding Na+‐MMT on crystallization behavior of PVDF was specifically studied. The β‐crystalline nucleation effect of Na+‐MMT was investigated and confirmed by differential scanning calorimetry (DSC), XRD, and Fourier transform infrared (FTIR) results. The interaction between PVDF and the surface of Na+‐MMT layers in DMF solution was confirmed by UV‐Vis absorbency. The effect of adding Na+‐MMT on rheological and electrical properties of PVDF/Na+‐MMT composites were also determined. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 903–911, 2009 相似文献