全文获取类型
收费全文 | 93篇 |
免费 | 6篇 |
国内免费 | 129篇 |
专业分类
化学 | 193篇 |
晶体学 | 1篇 |
力学 | 1篇 |
综合类 | 1篇 |
数学 | 8篇 |
物理学 | 24篇 |
出版年
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2018年 | 8篇 |
2017年 | 1篇 |
2016年 | 2篇 |
2015年 | 2篇 |
2014年 | 10篇 |
2013年 | 7篇 |
2012年 | 5篇 |
2011年 | 6篇 |
2010年 | 2篇 |
2009年 | 3篇 |
2008年 | 1篇 |
2007年 | 6篇 |
2006年 | 3篇 |
2005年 | 3篇 |
2004年 | 4篇 |
2003年 | 6篇 |
2002年 | 8篇 |
2001年 | 20篇 |
2000年 | 5篇 |
1999年 | 9篇 |
1998年 | 2篇 |
1997年 | 21篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1978年 | 1篇 |
1975年 | 2篇 |
排序方式: 共有228条查询结果,搜索用时 15 毫秒
21.
Poly(ether ketone)s bearing pendent sulfonate groups (SPEK‐x/y/z) have been successfully synthesized via copolyacylation of a presulfonated monomer SBP and two isomeric AB‐type self‐condensable comonomers, that is, 4‐phenoxybenzoic acid (p‐POBA) and 3‐phenoxybenzoic acid (m‐POBA). Proton‐exchange membranes (PEMs) with precisely controlled ion‐exchange capacity (IEC) and high strength can be readily prepared from these ionomers. PEMs prepared from p‐POBA other than m‐POBA exhibit much higher dimensional stability and proton conductivity at elevated temperature above 60 °C, showing prominent isomeric (para vs. meta) effects of polymer structural units. Furthermore, properties of PEMs prepared from p‐POBA are optimized by tuning IEC. SPEK‐1.0/2.2/0 with an IEC of 1.84 mmol g?1 exhibits acceptable swelling, much higher proton conductivity, and lower methanol permeability than commercial Nafion 115, implying potential application in direct methanol fuel cells. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 200–207 相似文献
22.
Two naturally occurring rare 5-oxo β-triketones, AB5046A and AB5046B, were efficiently synthesized in five linear steps with overall yields of 36% and 28%, respectively. The synthetic strategy highlighted the readily preparation of the key intermediate, 3-ethoxy-5-hydroxycyclohex-2-enone, from commercially available phloroglucinol. 相似文献
23.
以NaBH4为硼源、氨基络合物Ni(NH3)6Cl2为氨源制备高储氢容量的氨硼烷(NH3BH3, Ammonia Borane, AB)及其放氢性能研究. 通过XRD, FTIR, 11B NMR, ICP等手段分析表征了所制备产物的组成和纯度, 在此基础上探究了原料比例、反应温度、时间和溶剂等因素对产物的影响. 同时, 对不同原料比制得氨硼烷的热解放氢性能进行了研究. 实验结果表明: 当物质的量NaBH4∶Ni(NH3)6Cl2=2∶1经过10 h的反应, 得到了纯度非常高的氨硼烷(纯度>99%)|以NaBH4∶Ni(NH3)6Cl2=3∶1得到的氨硼烷, 当以2 ℃/min进行升温时, 氢气释放主要集中在第一步, 并且没有硼烷和硼嗪等杂质气体的产生. 另外, 在产物中得到了金属Ni纳米颗粒, 经洗涤干燥后其粒径大小可控制在10 nm左右, 在催化氨硼烷等材料的水解放氢方面具有潜在的应用价值. 相似文献
24.
Shanhua Chen Shihu Han Yanhui Chen Dengpan Xue Xiaodong Zhu 《Composite Interfaces》2018,25(11):1019-1038
The Mg2B2O5 whiskers (Mg2B2O5w) were modified by boric acid ester (BE) coupling agent and used to prepare polycarbonate (PC) composites. Surface wettability test and evaluation of dispersibility of BE-modified Mg2B2O5w (BE-Mg2B2O5w) in n-heptane were carried out by water contact angle measurement and polarizing microscopy, respectively. The surface chemical characteristics of BE-Mg2B2O5w and estimation of the amount of tightly bonded organic modifier were examined by Fourier transform infrared spectrometry (FT-IR) and thermogravimetric analysis (TGA), respectively. The influence of BE-Mg2B2O5w on the mechanical property, morphology and thermostability of PC composites was investigated by using universal testing machine, scanning electron microscopy (SEM), dynamic mechanical analysis (DMA) and TGA, respectively. It was found that BE was successfully grafted to the surface of Mg2B2O5w, and the hydrophobic BE-Mg2B2O5w leads to high ratio of adhesion work to interfacial tension between PC and BE-Mg2B2O5w, and its better dispersibility in n-heptane. Furthermore, the incorporation of BE-Mg2B2O5w into PC matrix resulted in higher tensile properties of PC/BE-Mg2B2O5w than that of PC/Mg2B2O5w composites and lower maximum weight loss rate than that of PC. The storage modulus of the PC/Mg2B2O5w composites increased obviously as a function of whisker content, especially for the composites with BE-Mg2B2O5w, but the Tg value changed little. 相似文献
25.
The AB system is the basic integrable model to describe unstable baroclinic wave packets in geophysical fluids and the propagation of mesoscale gravity flows in nonlinear optics. On the basis of the spectral analysis of a Lax pair and the inverse scattering method, we establish the Riemann–Hilbert problem of the AB system. Then, the inverse problems are formulated and solved with the aid of the Riemann–Hilbert problem, from which the potentials can be reconstructed according to the asymptotic expansion of the sectional analytic function and the related symmetry relations. As an application, we obtain the multi-bright-dark soliton solutions to the AB system in the reflectionless case and discuss the dynamic behavior of elastic soliton collisions by choosing appropriate free parameters. 相似文献
26.
Using the Nikiforov–Uvarov (NU) method, the energy levels and the wave functions of an electron confined in a two-dimensional (2D) pseudoharmonic quantum dot are calculated under the influence of temperature and an external magnetic field inside dot and Aharonov–Bohm (AB) field inside a pseudodot. The exact solutions for energy eigenvalues and wave functions are computed as functions of the chemical potential parameters, applied magnetic field strength, AB flux field, magnetic quantum number and temperature. Analytical expression for the light interband absorption coefficient and absorption threshold frequency are found as functions of applied magnetic field and geometrical size of quantum pseudodot. The temperature dependence energy levels for GaAs semiconductor are also calculated. 相似文献
27.
Effects of external fields on a two-dimensional Klein-Gordon particle under pseudo-harmonic oscillator interaction 下载免费PDF全文
We study the effects of the perpendicular magnetic and Aharonov-Bohm(AB) flux fields on the energy levels of a two-dimensional(2D) Klein-Gordon(KG) particle subjected to an equal scalar and vector pseudo-harmonic oscillator(PHO).We calculate the exact energy eigenvalues and normalized wave functions in terms of chemical potential parameter,magnetic field strength,AB flux field,and magnetic quantum number by means of the Nikiforov-Uvarov(NU) method.The non-relativistic limit,PHO,and harmonic oscillator solutions in the existence and absence of external fields are also obtained. 相似文献
28.
Fang‐Iy Wu Ching‐Fong Shu 《Journal of polymer science. Part A, Polymer chemistry》2001,39(22):3851-3860
A new AB2 monomer was synthesized for use in the preparation of a hyperbranched poly(aryl ether oxadiazole) with terminal phenol functionality. The AB2 monomer contains two phenolic groups and a single aryl fluoride group that is activated toward nucleophilic displacement by the attached oxadiazole ring. The nucleophilic substitution of the fluoride with the phenolate groups led to the formation of an ether linkage. Subsequently, a hyperbranched poly(aryl ether oxadiazole) having approximately a 44% degree of branching, as determined by a combination of model compound studies and 1H NMR, was obtained. The terminal phenolic groups underwent facile functionalization, furnishing hyperbranched polymers with a variety of functional chain ends. The nature of the chain‐end groups had a significant influence on the physical properties of the polymers, such as the glass‐transition temperature and their solubility. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3851–3860, 2001 相似文献
29.
30.
Summary: Having developed a theoretical model of hyperbranched polymerization involving AB2 and B2 monomers we present preliminary results of a test of its validity for the system consisting of 4,4-bis(4-hydroxyphenyl)pentanoic acid (AB2 monomer) and bisphenol A (B2 monomer). The monomers reacted at room temperature to yield the title hyperbranched polyesters. Diisopropylcarbodiimide (condensation water remover) along with an effective catalyst were used for that purpose. The total of AB2 monomer was introduced at once to a reactor containing B2 core monomer or it was divided into 3 or 5 equal portions and each of them introduced after the monomers previously present have reacted to a predetermined conversion. GPC measurements of the resulting molecular mass distribution gave inconclusive results because of poor reproducibility of polymer isolation/purification procedure and/or insufficient accuracy of GPC technique. 相似文献