全文获取类型
收费全文 | 56559篇 |
免费 | 9665篇 |
国内免费 | 7756篇 |
专业分类
化学 | 40683篇 |
晶体学 | 879篇 |
力学 | 3227篇 |
综合类 | 549篇 |
数学 | 6776篇 |
物理学 | 21866篇 |
出版年
2024年 | 126篇 |
2023年 | 1109篇 |
2022年 | 1924篇 |
2021年 | 2165篇 |
2020年 | 2383篇 |
2019年 | 2259篇 |
2018年 | 1898篇 |
2017年 | 1981篇 |
2016年 | 2671篇 |
2015年 | 2775篇 |
2014年 | 3311篇 |
2013年 | 4226篇 |
2012年 | 5179篇 |
2011年 | 5239篇 |
2010年 | 3791篇 |
2009年 | 3606篇 |
2008年 | 3910篇 |
2007年 | 3443篇 |
2006年 | 3129篇 |
2005年 | 2675篇 |
2004年 | 2044篇 |
2003年 | 1608篇 |
2002年 | 1481篇 |
2001年 | 1220篇 |
2000年 | 1108篇 |
1999年 | 1194篇 |
1998年 | 946篇 |
1997年 | 868篇 |
1996年 | 844篇 |
1995年 | 779篇 |
1994年 | 658篇 |
1993年 | 571篇 |
1992年 | 469篇 |
1991年 | 426篇 |
1990年 | 363篇 |
1989年 | 272篇 |
1988年 | 215篇 |
1987年 | 194篇 |
1986年 | 185篇 |
1985年 | 167篇 |
1984年 | 101篇 |
1983年 | 107篇 |
1982年 | 64篇 |
1981年 | 44篇 |
1980年 | 35篇 |
1979年 | 19篇 |
1976年 | 24篇 |
1975年 | 22篇 |
1973年 | 18篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Liquid–liquid equilibria data of the quaternary system methyl isobutyl ketone (MIBK)–water–phenol–hydroquinone were measured at 25 °C under atmosphere pressure. The experimental data were correlated with the UNIQUAC and NRTL activity coefficient models on the base of the fixed binary interaction parameters that were obtained from two sub-ternary systems MIBK–water–phenol and MIBK–water–hydroquinone. The root mean square deviations (RMSD) show that the regressed results for the quaternary system were in good agreement with the experimental data for both UNIQUAC and NRTL models. The comparison between experimental and calculated distribution coefficient values of phenol and hydroquinone shows that a relative deviation of less than 5% is obtained. 相似文献
972.
采用XRD表征了水蒸气和磷联合改性的HZSM-5沸石分子筛的结构.通过NH3-TPD和N2吸附脱附研究了样品的酸性和比表面.采用正庚烷的裂化反应研究了样品的裂化活性,研究结果表明,磷改性样品与母体样品相比,经过水蒸气处理后显示出较高的酸量和正庚烷裂化活性.上述结果首次用模型簇和计算量子化学方法进行了解释.采用Gaussian94软件包和PM3半经验量子化学方法对模型簇进行了全优化和频率分析.计算结果显示磷改性后样品的脱铝补硅反应热大于母体样品脱铝补硅反应热,从而显示出磷对骨架的稳定化作用. 相似文献
973.
A second-order nonlinear optical polyurethane was formed with an X-type multi-dimensional charge-transfer (CT) chromophore (DNPIPDHPI) and 3,3′-dimethoxy-4,4′-diphenyl diisocyanate. Simultaneous poling and polymerization and in situ second harmonic generation (SHG) measurement technique was carried on to evaluate the thermal stability of the poling induced orientation. The SHG signal of the poled polymer film was not decay below 150 °C and remains 90% of relative d33 value at 200 °C, which is better than the results reported in literature. Comparison of thermal stability indicates that the X-type chromophores possess better property in controlling the decay of the SHG activity than classic chromophores. 相似文献
974.
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
975.
用ICP-AES法测定了东太平洋深海锰结核样品中二十多种主,次及微量元素,结果表明,表面瘤状和表面光滑两种类型锰结核的化学元素含量有较明显的区别,如表面光滑的锰结核TiO2含较高,而表面瘤状的锰结核Mn/Fe之值较高等。 相似文献
976.
Bo Jiang Lixing Yang Yong Zhou Guanglin Huang Libin Lin 《Radiation Physics and Chemistry》2002,63(3-6):469-473
The radiation-induced hybrid polymerization in the presence of the N-alkoxypyridinium salt having relatively stable nonnucleophilic anion (PF6−) has been investigated in the paper. Based on the analysis of experimental data and the GPC spectrum, the onium salts not only oxidize -alkoxyalkyl radicals, produced from IBVE in dichloromethane by irradiation, to the corresponding cations, but also give nonnucleophilic anions PF6− for the polymerization system. The experimental results clearly demonstrate that free radical and cationic polymerization mechanisms occurred simultaneously in IBVE/EMP+PF6−/CH2Cl2 systems on irradiation with γ-ray. 相似文献
977.
比较研究氰基钴胺素在光滑和粗糙化银电极上的电化学行为,发现在光滑银电极上氰基钴胺素的还原为一个二电子过程,而在粗糙化银电极上则为两个一电子过程,其原因是由于粗糙化银电极对氰根的强吸附作用。 相似文献
978.
Wei Lun-Cún Bao Shanglian Yang Qixiang Yang Xihong 《Journal of Radioanalytical and Nuclear Chemistry》1991,151(1):75-81
Combining the Charged Particle Activation Analysis (CPAA) and the Channeling Technique, the partial concentrations of the impurity carbon atoms in different crystal lattice locations of GaAs were claculated. The calculated results show that at lower total concentrations (0.3 ppm), carbon atoms occupy principally the octahedral and displaced octahedral interstitial positions, but at higher total concentrations (2 ppm), the substitutional carbon plays a principal role. 相似文献
979.
980.
The mercuration of a series of aryliminomethylferrocenes occurred predominantly in an ortho-position of the substituted ferrocenyl ring to yield 2-mercurated ferrocenylimines. The regiospecificity of this reaction suggests that the mercury is directed into the ortho-position by coordination of the mercury to imino nitrogen with subsequent electrophilic substitution. The chromatographic and spectral properties of the 2-mercurated products show the presence of an intramolecular N → Hg coordination via the five-membered ring in these molecules, which was further confirmed by the single-crystal structure analysis of 2-chloromercuro-1-[(4-methoxyphenylimino)methyl]ferrocene. 相似文献