全文获取类型
收费全文 | 1065篇 |
免费 | 48篇 |
国内免费 | 197篇 |
专业分类
化学 | 1152篇 |
晶体学 | 29篇 |
力学 | 6篇 |
综合类 | 9篇 |
数学 | 3篇 |
物理学 | 111篇 |
出版年
2023年 | 9篇 |
2022年 | 21篇 |
2021年 | 21篇 |
2020年 | 34篇 |
2019年 | 23篇 |
2018年 | 21篇 |
2017年 | 36篇 |
2016年 | 25篇 |
2015年 | 17篇 |
2014年 | 44篇 |
2013年 | 104篇 |
2012年 | 70篇 |
2011年 | 44篇 |
2010年 | 35篇 |
2009年 | 50篇 |
2008年 | 70篇 |
2007年 | 60篇 |
2006年 | 66篇 |
2005年 | 56篇 |
2004年 | 53篇 |
2003年 | 66篇 |
2002年 | 34篇 |
2001年 | 39篇 |
2000年 | 30篇 |
1999年 | 38篇 |
1998年 | 19篇 |
1997年 | 25篇 |
1996年 | 28篇 |
1995年 | 23篇 |
1994年 | 19篇 |
1993年 | 13篇 |
1992年 | 16篇 |
1991年 | 16篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 4篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1972年 | 3篇 |
排序方式: 共有1310条查询结果,搜索用时 15 毫秒
101.
When poly(ethylene terephthalate) was quenched from above Tg and then heated, it exhibited a step increase in thickness in the glass transition region at every rate tested. When the polymer was cooled more slowly than it was heated, a higher Tg and a slightly larger step increase in thickness were observed as the cooling rate was reduced. These experimental results appear to be adequately interpreted on the basis of the normal structural changes that occur in a glass as its thermal history is varied. Two observations, however, were not easily included in this view. First, the polymer, on cooling from above Tg, exhibited an abnormally high expansion coefficient over much of the range of temperature in which it exists as a fluid. Second, the polymer exhibited a step increase in thickness when it was heated at the same rate at which it had previously been cooled. 相似文献
102.
保存、分析方法等因素对土壤中硝态氮测定的影响 总被引:6,自引:0,他引:6
为探明土壤中硝态氮测定的影响因素,采用黑土、潮土、红壤为材料,研究样品的保存方法、浸提剂种类、分析方法、实验用水及试剂纯度对土壤中硝态氮测定的影响。结果表明,测定土壤硝态氮时,土样在冷冻条件下保存优于冷藏条件。冷冻条件下保存可在45 d内完成测试,而冷藏保存则需在7 d内完成测试。采用0.01 mol/L CaCl2和2 mol/L KCl为浸提剂时,对土壤中硝态氮的测定结果无显著差异。紫外比色法的测定结果与林业标准法(LY/T1230-1999)相比无显著差异,但显著高于流动注射法,且紫外比色法与流动注射法之间有显著相关性。流动注射法的测定结果与林业标准法相比也无显著差异。实验用水的杂质可使土壤中硝态氮的测定结果显著偏高,对其进行加热煮沸、蒸馏及无氨化蒸馏后可显著降低水中杂质的含量。 相似文献
103.
《Green Chemistry Letters and Reviews》2013,6(1):55-59
Abstract Ceric ammonium nitrate in polyethylene glycol has been used as a sustainable, non-volatile, and ecofriendly catalytic medium for the green synthesis of 2,5-disubstituted 1,3,4-oxadiazoles. This protocol is effective toward various substrates having different functionalities. The easy recyclability of the reaction medium makes the reaction economically and potentially viable for commercial applications. 相似文献
104.
A simple and convenient method for the aromatization of Hantzsch 1,4-dihydropyridines to the corresponding pyridines is achieved via combination of aluminum or ferric nitrates and silica sulfuric acid as environmentally friendly and novel oxidizing media. This oxidation was carried out in dichloromethane under heterogeneous conditions with good to excellent yields. 相似文献
105.
This paper describes the oxidation of benzylic and allylic alcohols under non-aqueous and aprotic conditions or microwave conditions using benzyltriphenylphosphonium chlorochromate (1) PhCH2PPh3CrO3.HCl which has been prepared by mixing an aqueous solution of benzyltriphenylphosphonium chloride with CrO3 in 6 N HCl at room temperature. This reagent is stable orange powder which may be stored for months without loss of its activity. This compound is readily soluble in organic solvents such as acetonitrile, chloroform and dichloromethane and slightly soluble in carbon tetrachloride, ether and hexane. the oxidation under microwave conditions is much more faster. 相似文献
106.
Katell Fiselier Florian Grundböck Karsten Schön Oliver Kappenstein Karla Pfaff Christoph Hutzler Andreas Luch Koni Grob 《Journal of chromatography. A》2013,1271(1):192-200
So far the majority of the measurements of mineral oil saturated hydrocarbons (MOSH) and mineral oil aromatic hydrocarbons (MOAH) were obtained from on-line high performance liquid chromatography–gas chromatography–flame ionization detection (on-line HPLC–GC–FID). Since this technique is not available in many laboratories, an alternative method with more easily available tools has been developed. Preseparation on a small conventional liquid chromatographic column was optimized to achieve robust separation between the MOSH and the MOAH, but also to keep out the wax esters from the MOAH fraction. This was achieved by mixing a small portion of silica gel with silver nitrate into highly activated silica gel and by adding toluene into the eluent for the MOAH. Toluene was also added to the MOSH fraction to facilitate reconcentration and to serve as a keeper preventing loss of volatiles during solvent evaporation. A 50 μl volume was injected on-column into GC–FID to achieve a detection limit for MOSH and MOAH below 1 mg/kg in most foods. 相似文献
107.
A selective and effective oxidation of alcohols into the corresponding aldehydes and ketones, respectively, with a new reagent, ferric(III) nitrate supported on kieselguhr, under heterogeneous conditions is reported. 相似文献
108.
Mekhman S. Yusubov Elena N. Tveryakova Elena A. Krasnokutskaya Irina A. Perederyna Viktor V. Zhdankin 《合成通讯》2013,43(8):1259-1265
A simple and environmentally safe general method of iodination of aromatic substrates under sovent‐free conditions using the I2/AgNO3 combination in a solid state is reported. Both activated and deactivated aromatic compounds afford the respective aryl iodides in generally high yields (80–90%). 相似文献
109.
110.
Ying Liu Ying‐Hui Yu Yi‐Fu Liu Guang‐Feng Hou Xiao‐Dan Wang Bo Wen Jin‐Sheng Gao 《无机化学与普通化学杂志》2013,639(1):193-196
The cage‐like complex, Ag4L4(NO3)4 ( 1 ) [L = 1, 4‐bis(pyridine‐2‐ylmethoxy)benzene] was synthesized by the reaction of the flexible bidentate ligand and silver nitrate. It was characterized by elemental analysis, IR spectroscopy, TG, and single‐crystal X‐ray analysis. Complex 1 is reported as the first cage‐like cluster constructed by four nitrate anions bridging two [2+2] macrocycles. A blue luminescent emission and luminescent enhancement effect are observed in complex 1 . 相似文献