全文获取类型
收费全文 | 1367篇 |
免费 | 82篇 |
国内免费 | 380篇 |
专业分类
化学 | 1698篇 |
晶体学 | 19篇 |
力学 | 6篇 |
综合类 | 8篇 |
数学 | 2篇 |
物理学 | 96篇 |
出版年
2024年 | 6篇 |
2023年 | 11篇 |
2022年 | 24篇 |
2021年 | 32篇 |
2020年 | 67篇 |
2019年 | 47篇 |
2018年 | 32篇 |
2017年 | 41篇 |
2016年 | 46篇 |
2015年 | 47篇 |
2014年 | 86篇 |
2013年 | 125篇 |
2012年 | 107篇 |
2011年 | 88篇 |
2010年 | 81篇 |
2009年 | 101篇 |
2008年 | 88篇 |
2007年 | 86篇 |
2006年 | 96篇 |
2005年 | 63篇 |
2004年 | 68篇 |
2003年 | 69篇 |
2002年 | 53篇 |
2001年 | 37篇 |
2000年 | 38篇 |
1999年 | 34篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 27篇 |
1995年 | 24篇 |
1994年 | 19篇 |
1993年 | 28篇 |
1992年 | 17篇 |
1991年 | 15篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1988年 | 18篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有1829条查询结果,搜索用时 15 毫秒
991.
A series of novel fluorinated polynaphthalimides (PNIs) (2a-g) were synthesized from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and trifluoromethyl (CF3)-substituted aromatic bis(ether amine)s (1a-g) by high-temperature solution polycondensation in m-cresol using isoquinoline as catalyst. Almost all the PNIs were readily soluble in polar solvents such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) and could be solution-cast to transparent and tough films with high tensile strengths. The PNIs exhibited high thermal stability, with glass-transition temperatures of 262-383 °C, 10% weight loss temperatures above 528 °C in nitrogen or air, and char yields at 800 °C in nitrogen higher than 50%. In comparison with analogous PNIs without the -CF3 substituents, these fluorinated PNIs revealed an enhanced solubility and better film-forming capability. 相似文献
992.
Anilkumar Raghavanpillai Stefan Reinartz Keith W. Hutchenson 《Journal of fluorine chemistry》2009,130(4):410-417
A series of fluorinated bis-urea and bis-amide derivatives were synthesized from fluorinated amines and explored as surface modifiers for nonwoven substrates. A majority of these derivatives showed excellent gelation properties both in organic solvents as well as in supercritical carbon dioxide (scCO2) at concentrations ranging from 0.3 to 3 wt%. Gelation in the presence of a nonwoven substrate led to a gel-impregnated surface, which upon drying produced a composite with porous microstructure morphology on the surface. The composites thus produced showed high water and hexadecane contact angles, indicative of excellent hydrophobic and lyophobic properties. The superior hydrophobic and oleophobic behaviors observed in these composites are attributed to a combination of increased surface roughness and the presence of fluoroalkyl functionalities in the gelator backbone. 相似文献
993.
Adsorption separation of carbon dioxide, methane and nitrogen on monoethanol amine modified B-zeolite 总被引:1,自引:0,他引:1
A new type of composite adsorbents was synthesized by incorporating monoethanol amine (MEA) into β-zeolite. The parent and MEA-functionalized β-zeolites were characterized by X-ray diffraction (XRD), N2 adsorption, and thermogravimetric analysis (TGA). The adsorption behavior of carbon dioxide (CO2), methane (CH4), and nitrogen (N2) on these adsorbents was investigated at 303 K. The results show that the structure of zeolite was well preserved after MEA modification. In comparison with CH4 and N2, CO2 was preferentially adsorbed on the adsorbents investigated. The introduction of MEA significantly improved the selectivity of both CO2/CH4 and CO2/N2, the optimal selectivity of CO2/CH4 can reach 7.70 on 40 wt% of MEA-functionalized β-zeolite (MEA(40)-β) at 1 atm. It is worth noticing that a very high selectivity of CO2/N2 of 25.67 was obtained on MEA(40)-β. Steric effect and chemical adsorbate-adsorbent interaction were responsible for such high adsorption selectivity of CO2. The present MEA-functionalized β-zeolite adsorbents may be a good candidate for applications in flue gas separation, as well as natural gas and landfill gas purifications. 相似文献
994.
J. Li Y. H. Xing M. F. Ge C. G. Wang Z. P. Li S. Y. Niu 《Journal of Structural Chemistry》2009,50(3):532-538
A new polyoxovanadium supramolecular complex, [(N(CH2CH2NH3)3)2(V6O18)]·3H2O (1), was synthesized as a crystalline product from aqueous solution at room temperature. From single crystal X-ray structural
analysis, the complex is monoclinic, space group P21/n, with a = 15.878(2) ?,b = 11.3309(15) ?, c = 19.469(3) ?, β = 102.244(2)°, V = 3423.0(8) ?3, Z = 4, final R = 0.0469. The crystal structure comprises 1D spiral vanadate chain as the oxide moiety, doubly protonated cations of tris(2-aminoethyl)amine
(tren) and three lattice molecules of water. There are numerous H bonds in the structure connecting the polyoxovanadium anion
moiety, protonated tren cation and lattice water molecules; the average O...O and N...O distances are 2.753 ? for O-H...O
and 2.879 ? for N-H...O, respectively.
Original Russian Text Copyright ? 2009 by J. Li, Y. H. Xing, M. F. Ge, C. G. Wang, Z. P. Li, and S. Y. Niu
The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 50, No. 3, pp. 553–559, May–June, 2009. 相似文献
995.
Polyvinyl alcohol (PVOH) membrane, was modified both physically and chemically by incorporation of inorganic filler, sodium aluminosilicate and chemical crosslinking with maleic acid and glutaraldehyde. The change of morphology and crystallinity of PVOH by this physical and chemical modification was studied by FTIR, DSC, TGA, SEM and XRD. These membranes were evaluated in terms of its potential for dehydration of dioxane by preferential sorption and permeation using pervaporation (PV) technique. These membranes were cast in the laboratory by solution casting from the polymer and other additives. The performance of the unfilled (containing no filler) glutaraldehyde (GA) crosslinked PVOH-1 and maleic acid (MA) crosslinked PVOH-2 membranes were compared with filled (containing aluminosilicate filler) but GA crosslinked PVOH-3 and filled but MA crosslinked PVOH-4 membranes. The filled membranes were found to show higher flux and water selectivity. Among all the four used membranes, the MA crosslinked filled PVOH-4 membrane was found to show best results in terms of both water selectivity and flux. 相似文献
996.
A defect-free ultra thin PVAm/PVA blend facilitated transport membrane cast on a porous polysulfone (PSf) support was developed and evaluated in this study. The target membrane was prepared from commercial polyvinyl amine (PVAm) and polyvinyl alcohol (PVA). Effects of experimental conditions were investigated for a CO2–N2 mixed gas. A CO2/N2 separation factor of up to 174 and a CO2 permeance up to 0.58 m3(STP)/(m2 h bar) were documented. Experimental results suggest that CO2 is being transported according to the facilitated transport mechanism through this membrane. The fixed amino groups in the PVAm matrix function as CO2 carriers to facilitate the transport whereas the PVA adds mechanical strength to the blend by entanglement of the polymeric chains hence creating a supporting network. The good mechanical properties obtained from the blend of PVA with PVAm, enabled an ultra thin selective layer (down to 0.3 μm) to be formed on PSf support (with MWCO of 50,000), resulted in both high selectivity and permeance. The PVAm/PVA blend membrane also exhibited a good stability during a 400 h test. 相似文献
997.
998.
采用涂层法在玻璃基底上分别制备了纯聚氯乙烯(PVC)薄膜和添加水热法制备的钙钛矿型铌酸银(AgNbO3)光催化剂的复合薄膜(PVC-wAgNbO3, 其中w为AgNbO3的质量分数), 在500 W氙灯照射120 min条件下进行了薄膜的光催化降解实验. 利用X射线衍射仪(XRD)、 扫描电子显微镜(SEM)和傅里叶变换红外光谱仪(FTIR)等对光照前后薄膜的形貌及光催化降解过程进行了表征. 结果表明, 在光催化降解过程中纯PVC薄膜失重率为4.09%, 而PVC-3%AgNbO3, PVC-6%AgNbO3, PVC-9%AgNbO3和PVC-15%AgNbO3复合薄膜分别失重20.36%, 23.52%, 27.62%和33.83%. AgNbO3光催化剂加速了PVC薄膜的降解, 且随着AgNbO3光催化剂添加量的增加, PVC薄膜的光催化降解速率不断增大. 相似文献
999.
采用IR、XRD、SEM、EDS、DT-TG和滴定实验等技术手段研究主体三聚磷酸二氢铝(ATP)与客体甲胺、乙胺、正丙胺和正丁胺等有机胺的插层反应特性。 实验结果表明,ATP与甲胺、乙胺、正丙胺、正丁胺发生了化学反应,有机胺中的N与ATP层间-OH上的H形成配位键。 这些有机胺通过插层反应改变了ATP的酸性、层间距和热分解温度,但没有改变颗粒的层状形貌。 层间距从0.795 nm增大至1.71 nm,层间距d与有机胺的碳原子数Cn呈线性关系:d=0.229Cn+0.811,R2=0.9986。 有机胺分子链越长则越具有剥离倾向。 相似文献
1000.
聚氯乙烯/纳米水滑石复合材料的形态与力学性能 总被引:5,自引:0,他引:5
对由原位悬浮聚合制备的聚氯乙烯(PVC)纳米水滑石复合树脂加工得到的纳米复合材料的形态和力学性能进行了研究,并与直接熔融加工得到的PVC纳米水滑石复合材料进行比较.发现由前一方法得到的PVC纳米水滑石复合材料中纳米水滑石的分散性明显优于由后一方法得到的PVC纳米水滑石复合材料,水滑石以初级粒子形式存在,分散良好,无明显团聚体;与之对应,由前一方法得到的PVC纳米水滑石复合材料的力学性能也明显优于由后一方法得到的PVC纳米水滑石复合材料,当纳米水滑石含量小于5wt%时,复合材料的杨氏摸量、拉伸强度和缺口冲击强度均随水滑石含量增加而增大;纳米水滑石的引入可显著提高复合树脂的热稳定性;PVC纳米水滑石复合材料的储能和损耗模量略大于纯PVC材料,而损耗因子和玻璃化温度变化不大. 相似文献