全文获取类型
收费全文 | 69412篇 |
免费 | 12480篇 |
国内免费 | 9723篇 |
专业分类
化学 | 49685篇 |
晶体学 | 1051篇 |
力学 | 4483篇 |
综合类 | 675篇 |
数学 | 8274篇 |
物理学 | 27447篇 |
出版年
2024年 | 138篇 |
2023年 | 1285篇 |
2022年 | 1689篇 |
2021年 | 2297篇 |
2020年 | 2738篇 |
2019年 | 2698篇 |
2018年 | 2381篇 |
2017年 | 2346篇 |
2016年 | 3264篇 |
2015年 | 3407篇 |
2014年 | 4177篇 |
2013年 | 5271篇 |
2012年 | 6312篇 |
2011年 | 6627篇 |
2010年 | 4753篇 |
2009年 | 4758篇 |
2008年 | 5140篇 |
2007年 | 4431篇 |
2006年 | 4198篇 |
2005年 | 3325篇 |
2004年 | 2599篇 |
2003年 | 2149篇 |
2002年 | 2110篇 |
2001年 | 1714篇 |
2000年 | 1581篇 |
1999年 | 1386篇 |
1998年 | 1146篇 |
1997年 | 1062篇 |
1996年 | 1019篇 |
1995年 | 888篇 |
1994年 | 812篇 |
1993年 | 641篇 |
1992年 | 591篇 |
1991年 | 462篇 |
1990年 | 432篇 |
1989年 | 321篇 |
1988年 | 278篇 |
1987年 | 231篇 |
1986年 | 220篇 |
1985年 | 197篇 |
1984年 | 127篇 |
1983年 | 97篇 |
1982年 | 71篇 |
1981年 | 50篇 |
1980年 | 24篇 |
1979年 | 29篇 |
1978年 | 7篇 |
1977年 | 13篇 |
1957年 | 11篇 |
1925年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
离子液体萃取阿魏酸和咖啡酸的性能研究 总被引:4,自引:0,他引:4
以1-甲基-3-丁基咪唑六氟磷酸盐([C4mim][PF6])和1-甲基-3-己基咪唑六氟磷酸盐([C6mim][PF6])离子液体为萃取剂,采用紫外分光光度法研究了溶液pH值、温度、相比率及盐的种类和浓度对其萃取阿魏酸和咖啡酸效率的影响,考察了被萃取物的回收及离子液体的循环使用,探讨了两种离子液体替代传统有机溶剂阿魏酸和咖啡酸的可能性。实验结果表明:萃取温度和无机盐的种类及浓度对FA和CA萃取效率的影响较小;水相pH值对萃取效率有较大的影响,萃取FA适宜pH应小于3.67,萃取CA适宜pH应小于3.71;被萃取物浓度增大,萃取效率降低,而相体积比增大其萃取效率升高;两种离子液体对FA和CA的萃取效率[C4mim][PF6]大于[C6mim][PF6],同种离子液体对FA的萃取效率大于CA;在萃取相中的FA和CA可定量回收,且被萃取物中离子液体的残留[C6mim][PF6]小于[C4mim][PF6],离子液体可循环使用。 相似文献
993.
The goal of the present study is to test the hypothesis that LPA induces proliferation of astrocytes in hippocampus in vivo via phosphorylation of ERK 1/2. We first characterized the expression of GFAP, a special marker fiber protein of astrocytes,
in brain slices after direct injection of LPA into hippocampus by immunohistochemistry, and found that LPA induced a remarkable
proliferation of astrocytes. Then double-lablled immunofluorescence was used to detect GFAP and phosphorylation ERK 1/2 (p-ERK
1/2), LPA induced an immediate (10 min) and transient (<30 min) phosphorylation of ERK 1/2, and sequence sustained activation
of ERK 1/2 was observed, which last for at least 3 weeks after injection of LPA. Reactions are inhibited by U0126, a specific
pharmacological mitogen-activated protein kinase (MEK) inhibitor. Laser confocal scanning was used to study spatial relationship
of p-ERK and astrocytes. Amazingly, the early (<7 days) phosphorylation of ERK 1/2 is not expressed in astrocytes but in area
where neurons and/or in other cell type(s) occupied, expression of p-ERK 1/2 in astrocytes is not detected until 14 days after
LPA injection and lasts for at least 3 weeks. Taken together, these data suggest that LPA play an important role in proliferation
of astrocytes through phosphorylation of ERK 1/2 in hippocampus. It provides further proof for the functions of LPA in CNS
injury, and may contribute to clinical therapy for relative diseases. 相似文献
994.
Wei Ge ZHANG Ying Hua JIN Ping QI Kai BAO Jin Guang LIN Nai Li WANG Xin Shen YAO 《中国化学快报》2006,17(1):9-11
Erythromycin has been extensively used in the treatment of bacterial infections for over 50 years1. In addition to the antimicrobial effect, the quite unique antiinflammatory activity of the erythromycin derivatives has attracted much attention as new the… 相似文献
995.
Qiang Li Pingfan Wu Yongge Wei Hongyou Guo 《Journal of organometallic chemistry》2006,691(6):1223-1228
An organic-inorganic hybrid compound, (Bu4N)2[Mo6O18(NAr)] (Ar = p-BrC6H4) has been synthesized via the DCC dehydrating protocol of the reaction of [α-Mo8O26]4− with 4-bromoaniline hydrochloride in anhydrous acetonitrile, which has been characterized by UV-Vis spectra, 1H NMR and single crystal X-ray diffraction study. By comparing the UV-Vis spectra, which were used to monitor the reaction, the optimum preparative condition for this compound was also determined. This compound crystallizes in the monoclinic space group P21/n, which is featured in a terminal para-bromophenylimido group linked to an Mo atom of a hexamolybdate cluster by a Mo-N triply bond. In addition, there are π-π dimerization of the neighboring cluster anions though the parallel aromatic rings in their crystals. 相似文献
996.
新型含氟聚芳醚酮的合成与表征 总被引:10,自引:0,他引:10
聚芳醚酮具有很高的热稳定性和优良的电性能及机械性能 ,已经被广泛应用于宇航、电子及核能等高技术领域 [1] .氟元素的引入可以降低材料介电常数、折光指数和吸水率 ,提高热稳定性、溶解性和阻燃性 ,增加材料透明度 ,使这类聚合物在光电子、光学和微电子等应用领域的研究倍受关注 [2~ 4 ] .本文在合成含三氟甲基苯侧基的聚芳醚酮 [5] 的基础上 ,设计并合成了新型的含氟量更高的单体和聚合物 ,并对其性能进行了初步研究 .1 实验部分1 .1 试剂与仪器 [3,5 -二 (三氟甲基 ) ]苯代对苯醌 (自制 ) ;锌粉 ,A.R.级 ,天津化学试剂一厂产品 ;… 相似文献
997.
Xu-rong Xu Jun-ting Xu Lin-xian Feng Department of Polymer Science Engineering Zhejiang University Hangzhou China Department of Polymer Science Engineering College of Chemistry & Molecular Engineering Peking University Beijing China 《高分子科学》2002,(5)
The impact properties of two selected metallocene-catalyzed ethylene-butene copolymers and one conventionalcopolymer were evaluated using Izod impact test. It is found that the metallocene-catalyzed copolymer shows superior impactproperties. This result was explained on the basis of the more homogeneous inter-molecular composition distribution andnarrower molecular weight distribution, which leads to more homogeneous morphology with fewer defects. Stepwisecrystallization improves the impact properties, especially in the crack propagation process, to a large extent. This is due to thedecrease of entanglements by stepwise crystallization, which is advantageous for the chain slip and shear. The polymer withheterogeneous intra-molecular composition distribution exhibits a more evident improvement of impact properties understepwise crystallization. 相似文献
998.
999.
本文利用差示扫描量热仪(DSC)研究了聚醚醚酮(PEEK)和以PEEK为基体的碳纤维复合材料(APC-2)的热历史对它们熔融行为的影响。在200—315℃结晶并退火热处理后,试样的DSC曲线上出现了两个吸热峰。低温吸热峰(峰温T′m)较小,并与结晶和退火温度(Tc与在不同Tc下生成的不完整晶体的熔融和重结晶过程有关,而Tm的出现则与完整晶体的熔融过程有关。实验结果表明,T′m与Tc之间呈线性关系。按文献报道的方法,将T′m与Tc的直线外推至与T′m=Tc直线相交,其交点温度即为平衡熔点.结果表明,PEEK和APC-2试样的“平衡熔点”受不同热历史条件下生成晶体的完整程度的影响。当结晶并退火时间越长,求出的“平衡熔点”值越低,并与理论值接近。实验结果还表明,碳纤维表面具有促进PEEK树脂基体晶体完整化的作用,因而导致APC-2试样中PEEK树脂基体的熔点高于纯PEEK树脂。 相似文献
1000.
Zhou S Xu C Wang J Gao W Akhverdiyeva R Shah V Gross R 《Langmuir : the ACS journal of surfaces and colloids》2004,20(19):7926-7932
Acidic sophorolipid (SL) molecules derived from yeasts represent a novel type of asymmetrical bolaamphiphiles due to their unique structural features that include an asymmetrical polar head size (disaccharide vs COOH), a kinked hydrophobic core (cis-9-octadecenoic chain), and a non-amide polar-nonpolar linkage. Light microscopy, small- and wide-angle X-ray scattering, FT-IR spectroscopy, and dynamic laser light scattering were used to investigate the supramolecular structures of the self-assembled aggregates of SL molecules at different pH values. In acidic conditions (pH < 5.5), giant twisted and helical ribbons of 5-11 microm width and several hundreds of micrometers length were observed for the first time. Increase in solution pH values slowed ribbon formation, decreased ribbon yield, and increased the helicity and entanglements of the giant ribbons. An interdigitated lamellar packing model of acidic SL-COOH molecules with a long period of 2.78 nm, stabilized by both the strong hydrophobic association between the cis-9-octadecenoic chains and strong disaccharide-disaccharide hydrogen bonding, is proposed. The neutralization of SL-COOH in water to SL-COONa produced clear solutions with the formation of short-range ordered aggregates. At concentrations below 1.0 mg/mL, the size of self-assembled aggregates increased as the concentration increased. At concentrations above 1.0 mg/mL, narrowly distributed micellar aggregates with a constant hydrodynamic radius (R(h)) of about 100 nm are formed. The large micelles show strong angular dependence with the fast mode appearing at scattering angle theta >/= 60 degrees. 相似文献