全文获取类型
收费全文 | 1273篇 |
免费 | 40篇 |
国内免费 | 91篇 |
专业分类
化学 | 691篇 |
力学 | 364篇 |
综合类 | 10篇 |
数学 | 54篇 |
物理学 | 285篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 19篇 |
2021年 | 21篇 |
2020年 | 28篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 27篇 |
2016年 | 29篇 |
2015年 | 38篇 |
2014年 | 23篇 |
2013年 | 63篇 |
2012年 | 99篇 |
2011年 | 79篇 |
2010年 | 58篇 |
2009年 | 76篇 |
2008年 | 55篇 |
2007年 | 72篇 |
2006年 | 87篇 |
2005年 | 76篇 |
2004年 | 78篇 |
2003年 | 77篇 |
2002年 | 40篇 |
2001年 | 43篇 |
2000年 | 31篇 |
1999年 | 29篇 |
1998年 | 23篇 |
1997年 | 27篇 |
1996年 | 20篇 |
1995年 | 28篇 |
1994年 | 20篇 |
1993年 | 19篇 |
1992年 | 10篇 |
1991年 | 10篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1979年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有1404条查询结果,搜索用时 15 毫秒
21.
Jian-bin Zhang De-zhu Ma Hua Zhong Xiao-lie Luo Department of Polymer Science Engineering University of Science Technology of China Hefei China 《高分子科学》2000,(6):569-572
The surface morphology of free-surface PCL ringed spherulites was investigated by using atomic forcemicroscopy. The spherulites were obtained by crystallization of PCL/PVC blends of different compositions. It was found thatthe ringed spherulite exhibited regularly fluctuating rings on its surface. Compared with the bright-dark ring pattern of thespherulite under a polarizing microscope, it was proved that the optical characteristics of the ringed spherulite underpolarizing microscope coincided with its surface characteristics. The bright rings in polarizing micrographs of the spherulite coincided with the convex rings on its surface, while the dark rings coincided with the concave rings. 相似文献
22.
Utilization of a spiropyran derivative in a polymeric film optode for selective fluorescent sensing of zinc ion 总被引:1,自引:0,他引:1
ZHANG Ying SHAO Na YANG Ronghua LI Ke’an LIU Feng CHAN Winghong & MO Tian . College of Chemistry Molecular Engineering Peking University Beijing China . Department of Chemistry Hong Kong Baptist University Hong Kong China 《中国科学B辑(英文版)》2006,49(3):246-255
Spiropyrans are the most studied families of func- tional materials due to their reversible structural con- version in response to external optical, chemical, and thermal stimulation[1]. Irradiation with ultraviolet light causes formation of an extended π-conjugation open form (merocyanine form) by heterolytic cleavage of the C (spiro)-O bond, which generates an intense ab- sorption in the visible region. Under the irradiating of visible light, the opened form will come back to the closed spi… 相似文献
23.
The core-shell structured grafted copolymer particles of polybutadiene grafted polymethyl methacrylate (PB-g-PMMA, MB) were prepared by emulsion polymerization. The MB particles were used to modify poly (vinyl chloride) (PVC) by melt blending. The mechanical properties of the PVC blends were investigated. The micro-morphology of the PVC blends was observed by scanning electron microscopy (SEM). The results indicated that the samples with the best impact strength could be obtained when the core-shell weight ratio of PB to PMMA is lower than 93:7, the mechanical properties correlated well with SEM morphologies, the addition of modifier with the ratio core to shell of 93:7 could reduce the domain size of the dispersed phase. Furthermore, the compatibility and properties of the blends were greatly enhanced and improved. The modifier particles could be well dispersed in the PVC matrix. 相似文献
24.
Samir Kamel 《先进技术聚合物》2004,15(10):612-616
Rice straw was employed for the preparation of lignocellulosic‐poly(vinyl chloride) (PVC) composites. The effect of pretreatment of rice straw, concentration of PVC, pressure as well as pressing temperature on the mechanical properties and water absorption was studied. Also, the effect of lignin as coupling agent on the mechanical properties and water absorption of composite was studied. Composites of rice straw comprising both PVC and a coupling agent offer superior properties compared to those made from only rice straw and PVC. The extent of improvement in the mechanical properties and dimensional stability of composites depended not only on the pretreatment of rice straw, concentration of PVC and lignin but also on pressure and pressing temperature. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
25.
The propensity of (natural) polyols dispersed in heavy metal- and zinc-free PVC sheets to improve the initial colour, e.g. the colour during the first minutes of exposure to high processing temperatures (short-term stability), has been investigated. It is shown using W(Lab) values that the initial colour improves upon addition of polyols containing primary hydroxyl groups. The polyols act as HCl scavengers, presumably via an acid-catalysed SN2 substitution of the primary hydroxyl groups by chloride ions. In contrast, polyols with only secondary or tertiary hydroxyl groups accelerate the thermal degradation of PVC. Notwithstanding, the efficacy of the (natural) polyols containing primary hydroxyl groups will be reduced if the polyol is susceptible to competitive acid-catalysed intramolecular cyclodehydration reactions under the processing conditions. This is substantiated by a comparison of the behaviour of mannitol and 3,4-di-O-methyl-d-mannitol. The methylated derivative, which is less prone to undergo intramolecular cyclodehydration, improves the initial colour of heavy metal- and zinc-free PVC sheets more significantly than mannitol itself. 相似文献
26.
三苄基氯化锡最初用格氏法制得。Keiti等曾报道由锡和苄基氯直接制备,这时溶剂对反应的影响很大:用水作溶剂时得到三苄基锡;用甲苯作溶剂时得到二苄基二氯化锡;用正丁醇作溶剂时三苄基氯化锡的产率只有61%。在反应体系中加入KI、KBr或H_3PO_4等催化剂,三苄基氯化锡的产率也只有60%。本文用DMF作溶剂,AlCl_3-I_2作催化剂,并首次在反应体系中加入镁、锌、铝和钠等使副产物氯化亚锡还原成活性很高的金属锡,后者再与等基氯反应,可使三苄基氯化锡的产率提高到91.3%。文中还报导三种尚 相似文献
27.
A high-quality polyene can be obtained by exensive dehydrochlorination of poly(vinyl chloride) (PVC) in aliquid/solid two-phase system. The liquid phase is a tetrahydrofuran solution of PVC containing a small amount ofpoly(ethylene glycol) with molar mass of 400 g as a phase transfer catalys. The solid phase is potassium hydroxide particles.The structure of the polyene is polyacetylene-like and has a long conjugated C=C sequence and a narrow dispersity ofpolyene sequences according to its FT-infrared and Raman spectra. The environmental stability of the polyene was alsostudied by IR, Raman spectra and elemental analysis. Experimental results demonstrated that the polyene was susceptible toair and could be changed into a material containing high concentrations of hydroxyl and carbonyl groups. The polyenesequences were shortened and its dispersity became broader due to the effect of dioxygen. 相似文献
28.
《Electroanalysis》2005,17(11):1003-1007
A novel PVC membrane ion‐selective electrode based on tribenzyltin(IV) dithiocarbamate [Sn(IV)–TBDTB] as neutral carrier was developed for thiocyanate (SCN?) determination. The electrode exhibits a near‐Nernstian response for SCN? with a slope of 62.8±2.0 mV per decade over a wide concentration range 1.0×10?1–2.0×10?6 mol L?1 and a detection limit of 1.0×10?6 mol L?1 in MES–NaOH buffer, pH 6.0, at 25 °C. The electrode prepared with 1.5 wt.% Sn(IV)–TBDTB, 32.5 wt.% PVC and 66.0 wt.% 2‐nitrophenyloctyl ether (o‐NPOE) shows optimal response characteristics. Anti‐Hofmeister selectivity sequence for a series of anions shown by the electrode was as follows: SCN?>Sal?>I?>ClO >phCOO?>CH3COO?>Br?>Cl?>NO >NO >Citrate>SO42?. The useful pH range for the electrode was found to be 3–7 with a response time 30–40 s. The electrode has been used for direct determination of thiocyanate in wastewater with satisfactory results. 相似文献
29.
30.
Guo-xiang Xu Lu Qi Bi-tao Yu Lei Wen Department of Applied Chemistry College of Chemistry Molecular Engineering Peking University Beijing China Department of Inorganic Nonmetal Materials School of Materials Science Engineering University of Science Technology Beijing Beijing China 《高分子科学》2006,(3):307-313
PVC disulfide (2SPVC) was synthesized by solution crosslink and its molecular structure was confirmed by infrared spectrum. 2SPVC's specific area is 36.1 m2·g-1 tested by stand BET method, and granularity experiment gives out the particle size of d0.5= 11.3μm. With SEM (Scanning Electron Microscope) experiment the surface morphology and particle shape of 2SPVC were observed. Cyclic voltammetry (scan rate: 0.5 mV·s-1) shows that 2SPVC experience an obvious S-S redox reaction in charge-discharge process. When 2SPVC was used as cathode material for secondary lithium battery in a 1 mol·L-1 solution of lithium bis(trifluoromethylsulfonyl) imide (Li(CF3SO2)2N) in a 5:45:50 volume ratio mixture of o-xylene (oxy), diglyme (DG) and dimethoxymethane (DME) at 30℃, the first discharge capacity of 2SPVC is about 400.3 mAh·g-1 which is very close to its theoretical value (410.5 mAh·g-1) at a constant discharge current of 15 mA·g-1. It can retain at about 346.1 mAh·g-1 of discharge capacity after 30 charge-discharge cycles. So 2SPVC is a very promising cathode candidate for rechargeable lithium batteries. 相似文献