全文获取类型
收费全文 | 80799篇 |
免费 | 4729篇 |
国内免费 | 4338篇 |
专业分类
化学 | 7922篇 |
晶体学 | 233篇 |
力学 | 5426篇 |
综合类 | 647篇 |
数学 | 13421篇 |
物理学 | 8692篇 |
综合类 | 53525篇 |
出版年
2024年 | 184篇 |
2023年 | 553篇 |
2022年 | 973篇 |
2021年 | 1201篇 |
2020年 | 1348篇 |
2019年 | 1150篇 |
2018年 | 1265篇 |
2017年 | 1561篇 |
2016年 | 1751篇 |
2015年 | 2011篇 |
2014年 | 3725篇 |
2013年 | 4044篇 |
2012年 | 4500篇 |
2011年 | 4801篇 |
2010年 | 3930篇 |
2009年 | 4723篇 |
2008年 | 5056篇 |
2007年 | 5911篇 |
2006年 | 5693篇 |
2005年 | 4806篇 |
2004年 | 4278篇 |
2003年 | 3849篇 |
2002年 | 3444篇 |
2001年 | 2964篇 |
2000年 | 2488篇 |
1999年 | 2014篇 |
1998年 | 1551篇 |
1997年 | 1446篇 |
1996年 | 1262篇 |
1995年 | 1148篇 |
1994年 | 1001篇 |
1993年 | 882篇 |
1992年 | 765篇 |
1991年 | 718篇 |
1990年 | 626篇 |
1989年 | 533篇 |
1988年 | 401篇 |
1987年 | 313篇 |
1986年 | 176篇 |
1985年 | 144篇 |
1984年 | 102篇 |
1983年 | 59篇 |
1982年 | 90篇 |
1981年 | 64篇 |
1980年 | 62篇 |
1979年 | 83篇 |
1978年 | 49篇 |
1977年 | 49篇 |
1976年 | 37篇 |
1974年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
941.
用化学还原法制备了NiB、NiB/Al2O3和NiBSm/Al2O3三种催化剂,它们都呈非晶结构.用等离子耦合发射光谱仪(ICP)对催化剂组分进行了分析,采用差示扫描量热法(DSC)对三种催化剂进行了热稳定性分析.结果表明,载体的引入使非晶态NiB合金的体相组成中B的含量降低.将非晶态NiB合金负载到γ-Al2O3上,可以明显改善非晶态合金的热稳定性,少量Sm可以进一步提高晶化温度.晶化激活能数据亦表明了载体和Sm对非晶结构具有良好的稳定作用.载体和Sm可能通过不同的作用提高了非晶态合金的热稳定性. 相似文献
942.
Ryndina S. A. Kadushkin A. V. Solov"eva N. P. Granik V. G. 《Russian Chemical Bulletin》2002,51(5):854-859
Approaches to the synthesis of 3-aminothiophene-2,4-dicarboxylic acid derivatives and to their conversions into thieno[3,4-d]pyrimidines, thieno[3,4-d]-1,2,3-triazines, and thieno[3,2-d]pyrimidines are developed. 相似文献
943.
本文研究了氢硼酸根(B_(12)H_(12)~(2-)、B_(10)H_(10)~(1-)、B_(11)H_(14)~-)和π-芳烃-π-环戊二烯基铁阳离子的反应,合成了36个氢硼酸π-芳烃-π-环戊二烯基铁化合物。通过元素分析和红外光谱测定确定了化合物的组成。部份化合物还作了~1H NMR鉴定。对化合物的性质,包括它们的热行为,进行了较详细的研究。DTA仪测得的结果表明,(1)当阳离子相同时,化合物的热稳定性的顺序为:[ArFeC_5H_5]_2B_(12)H_(12)>[ArFeC_5H_5]_2B_(10)H_(10)>[ArFeC_5H_5]B_(11)H_(14);(2)当氢硼酸根相同时,其热稳定性受芳环上取代基的影响。 相似文献
944.
Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites 总被引:1,自引:0,他引:1
Walid H Awad Jeffrey W Gilman Marc Nyden Thomas E Sutto Paul C Trulove Douglas M Fox 《Thermochimica Acta》2004,409(1):3-11
Increasing the thermal stability of organically-modified layered silicates is one of the key points in the successful technical application of polymer-layered silicate nanocomposites on the industrial scale. To circumvent the detrimental effect of the lower thermal stability of alkyl ammonium-treated montmorillonite, a series of alkyl-imidazolium molten salts were prepared and characterized by elemental analysis, thermogravimetry (TGA) and thermal desorption mass spectroscopy (TDMS). The effect of counter ion, alkyl chain length and structural isomerism on the thermal stability of the imidazolium salts was investigated. Alkyl-imidazolium-treated montmorillonite clays were prepared by ion exchange of the imidazolium salts with Na-montmorillonite. These organically-modified clays were characterized by X-ray diffraction (XRD), TDMS and thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR), and compared to the conventional quaternary alkyl ammonium montmorillonite. Results indicate that the counter ion has an effect on the thermal stability of the imidazolium salts, and that imidazolium salts with PF6−, N(SO2CF3)2− and BF4− anions are thermally more stable than the halide salts. A relationship was observed between the chain length of the alkyl group and the thermo-oxidative stability; as the chain length increased from propyl, butyl, decyl, hexadecyl, octadecyl to eicosyl, the stability decreased. The results also show that the imidazolium-treated montmorillonite has greater thermal stability compared to the imidazolium halide. Analysis of the decomposition products by FTIR provides an insight about the decomposition products which are water, carbon dioxide and hydrocarbons. 相似文献
945.
946.
The thermal stability of representative hydrofluoropolyether (HFPE) and hydrofluoroether (HFE) compounds has been evaluated. The observed stability order appears to be correlated with the nature of the hydrogenated chain ends; in particular, molecules having fully hydrogenated chain ends (OCH3 and OC2H5) show a significantly lower stability compared with the OCF2H terminated compounds. The main degradation products suggest, however, that the same primary reaction is responsible for the decomposition of all the compounds examined; this reaction involves the fragmentation of the RfOCxHyFz bond with fluorine transfer between the two carbon atoms close to the oxygen, leading to the formation of a hydrofluorocarbon CxHyF(z+1) and an acyl fluoride or a ketone. 相似文献
947.
948.
The activation energy of the enzyme-catalyzed reaction for uric acid decreases markedly in the presence of o-phenanthroline, which activates the bioelectrochemicla activity of the polypyrrole uricase electrode. The response current of the enzyme electrodeis independent of the concentration of o-phenanthroline. Based on the experimental results, the mechamsm of the enzyme-catalyzed reaction for uric acid in the presence of o-phenanthroline is presented as follows: E+A→EA, EA+S EAS, EAS→EA+P, where E, A, S and P are the enzyme, activator, substrate and product, respectively. The effects of pH value, potential and the uric acid concentration on the response currents of the uricase electrode have been studied in the presence of o-phenanthroline. In the presence of o-phenanthroline, the response current of the enzyme electrode increase linearly with increasing concentration of uric acid in the region of 0.07 to 0.67 mmol·L~(-1), therefore the polypyrrole uricase electrode which has once lost its activity can be activated and used again to determine the substrate concentration. 相似文献
949.
950.
葡萄糖异构酶加速葡萄糖与果糖的平衡,平衡时果糖含量为42%左右,体系中加入一些硼酸盐,果糖的含量提高到80%左右,究其原因可能是由于葡萄糖和果糖与硼酸盐形成不同稳定性的配合物。虽然Roy,Conner,Daris,Belcher,Lorand等都已报导了这些配合物的稳定常数,概括结果分歧较大。Belcher曾首次报导了硼酸与D-果糖的配位数为 相似文献