全文获取类型
收费全文 | 3853篇 |
免费 | 302篇 |
国内免费 | 669篇 |
专业分类
化学 | 4552篇 |
晶体学 | 28篇 |
力学 | 9篇 |
综合类 | 24篇 |
数学 | 1篇 |
物理学 | 210篇 |
出版年
2024年 | 10篇 |
2023年 | 29篇 |
2022年 | 72篇 |
2021年 | 113篇 |
2020年 | 192篇 |
2019年 | 146篇 |
2018年 | 114篇 |
2017年 | 137篇 |
2016年 | 142篇 |
2015年 | 165篇 |
2014年 | 207篇 |
2013年 | 366篇 |
2012年 | 254篇 |
2011年 | 207篇 |
2010年 | 187篇 |
2009年 | 217篇 |
2008年 | 215篇 |
2007年 | 197篇 |
2006年 | 196篇 |
2005年 | 179篇 |
2004年 | 190篇 |
2003年 | 185篇 |
2002年 | 117篇 |
2001年 | 92篇 |
2000年 | 90篇 |
1999年 | 89篇 |
1998年 | 93篇 |
1997年 | 57篇 |
1996年 | 82篇 |
1995年 | 64篇 |
1994年 | 62篇 |
1993年 | 73篇 |
1992年 | 55篇 |
1991年 | 39篇 |
1990年 | 30篇 |
1989年 | 26篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 10篇 |
1984年 | 16篇 |
1983年 | 7篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有4824条查询结果,搜索用时 31 毫秒
91.
Enthalpies of protonation in aqueous solution have been determined from calorimetric measurements between 0 and 100°C for methylpentylamine, butyldimethylamine, and quinuclidine; between 0 and 125°C for di-iso-propylamine; and between 0 and 75°C for 1-azacycloheptane. Values ofpKa at 25°C have been determined from pH measurements using a glass electrode for di-iso-propyl-, methylpentyl-, and butyldimethylamine. The calorimetric results are used together with values ofpKa at 25°C to derive expressions which allow the calculation of accuratepKa values over a large temperature range. The heat capacity and entropy changes for the protonation of aqueous alkylamines found in the present and in previous studies are discussed in terms of differences in hydrophobic solvation of the alkyl groups in the neutral amines and the ions. 相似文献
92.
A novel cross-linking process using two high molecular weight aromatic poly(thioether)s,which were synthesized by the reactions of 4,4′-thiobisbenzenethiol with 4,4′-difluorobenzophenone and 4,4′-difluorodiphenylsulfone,respectively, and commercially available lower molecular weight poly(p-phenylene sulfide)was investigated.These reactions were carried out in bulk by the addition of silver tetrafluroborate andα,α′-dibromo-p-xylene at 190℃over a period of 45 min. Furthermore,the same procedure could be modified to cross-link compression-molded films of these three polymers.The thermal and solubility behaviors of these polymers before and after cross-linking reactions,are presented. 相似文献
93.
Jiang Ke QIN Xian Zhang BU Yu Fang WANG Zhi Yun DU Xing Yuan WANG Zhi Shu HUANG Lin MA Lin Kun AN Lian Quan GU 《中国化学快报》2005,16(11):1451-1454
The reactions of tanshinone IIA with aromatic aldehydes have been investigated and several 2, T-(substituted arylmethylene) bis (1,6,6-trimethyl-6,7,8,9-tetrahydrophenanthro[1,2-b]- furan- 10, 11-dione) derivatives were obtained, 相似文献
94.
Application of matrix solid-phase dispersion in the analysis of priority polycyclic aromatic hydrocarbons in fish samples 总被引:1,自引:0,他引:1
The performance of matrix solid-phase dispersion (MSPD) for the extraction of polycyclic aromatic hydrocarbons (PAHs) in fish tissue is described. The suitability of different solid supports was tested as well as the influence on the extraction efficiency of the natural fat content in samples. Under optimal conditions 0.6-0.8 g of tissue sample, are dispersed with 2 g of octadecylsiloxane (C18) and 0.5 g of anhydrous sodium sulphate and transferred to the top of a polyethylene solid-phase extraction cartridge which already contains 2 g of florisil and 1 g of C18. Cartridges were eluted with acetonitrile. The analysis of the extracts was carried out by high-performance liquid chromatography (HPLC) coupled with fluorescence detection. The proposed method provides detection limits between 0.04 and 0.32 ng/g for the different considered PAHs, below the maximum levels established by the some regulatory bodies for the six PAHs after recent oil spill episodes and European Union regulations. Recoveries over 80% were obtained for all compounds. Accuracy validation was carried out using the US National Institute of Standards and Technology (NIST) SRM 2977 reference material. 相似文献
95.
Synthesis and binding studies of new bis-calix[4]arenes containing aromatic and heteroaromatic units
A series of new bis-calix[4]arenes containing different aromatic and heteroaromatic moieties have been synthesized. The complexing behavior of these bis-calix[4]arenes have been studied towards different metal ions and it has been found that these bis-calix[4]arenes bind silver ions selectively over other metal ions. The complexation has been studied by liquid-liquid extraction and by NMR and IR spectroscopy. 相似文献
96.
97.
98.
纳米多孔双金属氧化物在镍钛合金纤维上的原位生长及其对多环芳烃的选择性固相微萃取 总被引:1,自引:0,他引:1
采用阳极氧化法在镍钛合金(NiTi)纤维上原位生长了双金属氧化物纳米孔(NiTiONPs)涂层,通过扫描电镜(SEM)和能谱(EDS)考察了电解质组成和电压对形貌的影响。将NiTiONPs涂层的NiTi纤维与高效液相色谱-紫外检测器联用,研究了4种典型芳香分析物的萃取性能。结果表明,富含TiO2的NiTiONPs涂层对多环芳烃(PAHs)具有良好的萃取效率,尤其对苯并[a]芘的萃取选择性优于市售聚二甲基硅氧烷纤维和聚丙烯酸酯纤维。在优化条件下,PAHs的线性范围为0.05~200μg/L,相关系数均大于0.999,检出限为0.012~0.134μg/L。对单支纤维日内和日间分析的相对标准偏差(RSDs)分别为4.0%~5.5%和6.0%~6.8%,使用分批组装的5支纤维分析的RSDs为6.4%~7.6%。实际水样分析的加标回收率为84.5%~111.5%。所制备NiTi纤维至少可重复使用250次以上,重现性好。 相似文献
99.
100.