全文获取类型
收费全文 | 73521篇 |
免费 | 12853篇 |
国内免费 | 7310篇 |
专业分类
化学 | 50900篇 |
晶体学 | 657篇 |
力学 | 4757篇 |
综合类 | 311篇 |
数学 | 8211篇 |
物理学 | 28848篇 |
出版年
2024年 | 279篇 |
2023年 | 1601篇 |
2022年 | 2718篇 |
2021年 | 2792篇 |
2020年 | 3107篇 |
2019年 | 2908篇 |
2018年 | 2567篇 |
2017年 | 2339篇 |
2016年 | 3569篇 |
2015年 | 3473篇 |
2014年 | 4139篇 |
2013年 | 5305篇 |
2012年 | 6735篇 |
2011年 | 6729篇 |
2010年 | 4478篇 |
2009年 | 4259篇 |
2008年 | 4598篇 |
2007年 | 4107篇 |
2006年 | 3760篇 |
2005年 | 3132篇 |
2004年 | 2434篇 |
2003年 | 1921篇 |
2002年 | 1757篇 |
2001年 | 1472篇 |
2000年 | 1355篇 |
1999年 | 1541篇 |
1998年 | 1311篇 |
1997年 | 1294篇 |
1996年 | 1296篇 |
1995年 | 1059篇 |
1994年 | 928篇 |
1993年 | 766篇 |
1992年 | 676篇 |
1991年 | 584篇 |
1990年 | 507篇 |
1989年 | 386篇 |
1988年 | 341篇 |
1987年 | 278篇 |
1986年 | 249篇 |
1985年 | 205篇 |
1984年 | 147篇 |
1983年 | 102篇 |
1982年 | 81篇 |
1981年 | 68篇 |
1980年 | 65篇 |
1979年 | 33篇 |
1978年 | 30篇 |
1977年 | 40篇 |
1976年 | 30篇 |
1973年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Study on flame-retardant mechanism of polycarbonate containing sulfonate-silsesquioxane-fluoro retardants by TGA and FTIR 总被引:1,自引:0,他引:1
Shumei Liu Yongsheng Zhou Zhijie Jiang Xianbo Huang 《Polymer Degradation and Stability》2006,91(8):1808-1814
The flame retardancy of bisphenol A polycarbonate (PC) containing potassium diphenylsulfone sulfonate (KSS), poly(aminopropyl/phenylsilsesquioxane) (PAPSQ) and poly(vinylidenefluoride) (PVDF) was measured by limited oxygen index (LOI) and examined according to UL94. A high LOI and UL94 V-0 rating for 1.6 mm thickness samples were obtained by a combined use of equivalent KSS, PAPSQ and PVDF at 0.1-0.3 wt% loading, respectively. The improvement in flame retardancy of PC compositions arose from the synergistic interaction of three additives. Thermogravimetric analysis (TGA) indicated that the combination decreased the activation energy (E) of PC degradation and elevated the thermal degradation rate of PC to ensure the formation of an insulating carbon layer. FTIR analysis showed that the LOI char of PC containing the three additives took on a highly cross-linking aromatic ester and ether structure. 相似文献
992.
The solvent composition induced reentrant behavior of poly(N-isopropylacrylamide) (PNIPAM) chains grafted on a SiO2 surface in water-methanol mixtures was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) at 20 degrees C. The frequency and energy dissipation responses showed that the grafted PNIPAM chains sharply collapse when the methanol content (x(m)) reaches approximately 17 mol %. In the range 17-50 mol %, the grafted chains remain in a collapsed state. Further increase of the methanol content leads to an abrupt reswelling of the collapsed chains at x(m) > approximately 50 mol %. The sharp reentrant swelling-to-collapse-to-swelling transition was attributed to the water-methanol complexation instead of the preferential adsorption effect. Our results also suggest that the water-methanol complexation is not induced by hydrophobic interaction but by hydrogen bonding. 相似文献
993.
A capillary electrophoresis (CE) method was developed for the separation of heparin oligosaccharides compatible to study the interactions between the oligosaccharides and granulocyte-colony stimulating factor (G-CSF). Unfractionated heparin was eliminitively degraded to heparin oligosaccharides by an endolytic heparinase. The degraded smaller oligosaccharides (M(r) < 1000) were baseline-separated by CE under a 50 mM phosphate buffer (pH 9.0) in 10 min. Standard heparin disaccharides and larger oligosaccharides (1000 < M(r) < 8000) were all separated under optimized separation conditions. Compared with standard heparin disaccharides, smaller oligosaccharides contained one nonsulfated, two monosulfated, and two disulfated disaccharides, but trisulfated disaccharides were not found. The smaller oligosaccharides were also identified and molecular mass was deduced by electrospray ionization-mass spectrometry (ESI-MS). Furthermore, interactions between G-CSF and the oligosaccharides were studied by using capillary zone electrophoresis (CZE) under the above separation conditions. It was found that larger oligosaccharides could interact with G-CSF while smaller oligosaccharides were not observed to bind to G-CSF under the experimental conditions. In conclusion, the purified heparinase could selectively degrade heparin into oligosaccharides and the interaction between G-CSF and heparin was correlated with the chain length of heparin. 相似文献
994.
Growth of porous single-crystal Cr2O3 in a 3-D mesopore system 总被引:1,自引:0,他引:1
Jiao K Zhang B Yue B Ren Y Liu S Yan S Dickinson C Zhou W He H 《Chemical communications (Cambridge, England)》2005,(45):5618-5620
Single-crystal Cr2O3 with regular mesopores has been synthesized using mesoporous silica KIT-6 as a template and characterized by using XRD, HRTEM and nitrogen adsorption/desorption. 相似文献
995.
The conversion of phosphoenolpyruvate (PEP) to phosphonopyruvate (P-pyr) is catalyzed by PEP mutase via a dissociative mechanism. In this work, we investigate the uncatalyzed reaction using ab initio methods, density functional theory, and the semiempirical MNDO/d method. Comparisons of geometries and relative energies of stationary points (minima and transition states) with density functional results indicate that the semiempirical method is reasonably accurate. Solvent effects are examined using implicit solvent models, including the recently extended smooth conductor-like screening model. Due to the large negative charge carried by the system, solvation is found to drastically alter the location and energy of stationary points along the dissociative reaction pathways. The influence of substituting a nonbridging phosphoryl oxygen by sulfur (thio effects) was also investigated. Implications of these results for the enzymatic reaction are discussed. 相似文献
996.
997.
铋(Ⅲ)-溴邻苯三酚红配合物吸附波及其应用*刘家欣段友构黄诚朱苗力(吉首大学化学系湖南吉首416000)匡向阳(永州市水文站湖南永州425000)关键词铋溴邻苯三酚红配合物吸附波中图分类号O657.14近年来,在某些有机试剂存在下用极谱法测定铋已有一... 相似文献
998.
用硒与尿素化合,新生的由杂环有机硒所组成的Zs-93硒剂治疗移植性S180肉瘤,有显著的抑瘤作用(P<0.01),其作用表现出一定的剂量效应,以5μg/只小鼠为最适,抑瘤率达60.76%,治疗组小鼠的T细胞、SOD活性均有增强。 相似文献
999.
据不同地貌、地质条件在30个县市按剖面、同步、随机取土壤样品372份.测定硒等微量元素含量.作等级相关分析显示。土壤硒含量与消化系统癌呈负相关关系.作相对危险度、剂量-效应及多因素分析,认为此负相关关系并非偶然现象. 相似文献
1000.