全文获取类型
收费全文 | 7233篇 |
免费 | 814篇 |
国内免费 | 771篇 |
专业分类
化学 | 4643篇 |
晶体学 | 86篇 |
力学 | 902篇 |
综合类 | 71篇 |
数学 | 1123篇 |
物理学 | 1993篇 |
出版年
2024年 | 10篇 |
2023年 | 52篇 |
2022年 | 133篇 |
2021年 | 218篇 |
2020年 | 265篇 |
2019年 | 212篇 |
2018年 | 228篇 |
2017年 | 266篇 |
2016年 | 273篇 |
2015年 | 265篇 |
2014年 | 308篇 |
2013年 | 703篇 |
2012年 | 385篇 |
2011年 | 409篇 |
2010年 | 313篇 |
2009年 | 412篇 |
2008年 | 398篇 |
2007年 | 399篇 |
2006年 | 415篇 |
2005年 | 342篇 |
2004年 | 309篇 |
2003年 | 324篇 |
2002年 | 279篇 |
2001年 | 250篇 |
2000年 | 228篇 |
1999年 | 185篇 |
1998年 | 224篇 |
1997年 | 157篇 |
1996年 | 132篇 |
1995年 | 102篇 |
1994年 | 93篇 |
1993年 | 83篇 |
1992年 | 79篇 |
1991年 | 60篇 |
1990年 | 56篇 |
1989年 | 33篇 |
1988年 | 25篇 |
1987年 | 28篇 |
1986年 | 19篇 |
1985年 | 27篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 20篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1972年 | 8篇 |
排序方式: 共有8818条查询结果,搜索用时 312 毫秒
91.
92.
The freezing temperatures and densities (at 31°C) of solutions of octane, nonane, decane, 3,3-diethylpentane, and sodium oleate inN-methylacetamide (NMA) have been measured. The molality of the freezing solution was calculated from the density. The solubilities of octane, nonane, and decane inN-methylacetamide are also reported. Apparent molal volumes calculated from the densities are close to the values in the pure hydrocarbons and are not strong functions of the concentration. This indicates the absence of any unusual packing effect. The calculated free energies of transfer of the hydrocarbons from pure hydrocarbon to NMA solution are much less negative than the corresponding values for water, showing that the bulk solvophobic interaction inN-methylacetamide is smaller than in water. This is consistent with the freezing temperatures of sodium oleate which show that micelles do not form below 0.1 mole-kg–1. The osmotic coefficients of the hydrocarbons calculated from the freezing temperatures showed negative deviations from ideality that were larger for the hydrocarbons with the higher molecular weights. Two estimates of the pairwise solvophobic interaction inN-methylacetamide indicate that it is also smaller than in water. The solvophobic effect in this solvent does not include the large entropy and enthalpy effects found in aqueous solutions. 相似文献
93.
Tucker M. P. Farmer J. D. Keller F. A. Schell D. J. Nguyan Q. A. 《Applied biochemistry and biotechnology》1998,(1):25-35
Single-stage cocurrent dilute acid pretreatments were carried out on yellow poplar (Liriodendron tulipifera) sawdust using an as-installed and short residence time modified pilot-scale Sunds hydrolyzer and a 4-L bench-scale NREL
digester (steam explosion reactor). Pretreatment conditions for the Sunds hydrolyzer, installed in the NREL process development
unit (PDU), which operates at 1 t/d (bone-dry t) feed rate, spanned the temperature range of 160 – 210°C, 0.1 – 1.0% (w/w)
sulfuric acid, and 4-10-min residence times. The batch pretreatments of yellow poplar sawdust in the bench-scale digester
were carried out at 210 and 230°C, 0.26% (w/w) sulfuric acid, and 1-, 3-, and 4-min residence times. The dilute acid prehydrolysis
solubilized more than 90% of the hemicellulose, and increased the enzymatic digestibility of the cellulose that remained in
the solids. Compositional analysis of the pretreated solids and liquors and mass balance data show that the two pretreatment
devices had similar pretreatment performance. 相似文献
94.
正已醇-邻、间、对二甲苯二元系固液相平衡 总被引:1,自引:0,他引:1
Melting temperatures have been measured and the solid-liquid phase diagrams constructed for 1-hexanol+o-xylene, 1-hexanol+m-xylene and 1-hexanol+p-xylene. They are simple eutectic systems. Excess mole Gibbs free energies were calculated at 298.15K, showing larger positive deviations from ideal-solution behavior. The largest values of GmE are 711、 650 and 800 J•mol-1 for {o-C6H4(CH3)2+C6H13OH}、 {m-C6H4(CH3)2 + C6H13OH} and {p-C6H4(CH3)2+C6H13OH} respectively. 相似文献
95.
96.
The synthesis of rod-coil diblock copolymers was achieved for the firsttime by TEMPO-mediated "living" free radical polymerization of styrene and 2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene(MPCS). The block architecture of the two diblockcopolymers thus prepared, MPCS-b-St(5400/2400) and MPCS-b-St(10800/8700), was con-firmed by GPC, DSC studies and the formation of multimolecular micelles. 相似文献
97.
The thinning of foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (average molecular weight 14,000 g/mol) is studied experimentally. The dependence of the surface forces on film thickness is obtained by the dynamic method of Scheludko and Exerowa.The total surface force measured in foam films (radius 60–70 m) from 10–5 M (0.014 wt%) polymer solution with 0.1 M NaCl is positive at thicknesses from about 800 down to 460 . The electrostatic repulsion is negligible while the contribution of van der Waals attraction is small (within 15%). Therefore a positive surface force component predominates. Most probably it arises from steric interactions between the hydrophilic polyethylene oxide tails of the polymer. The dynamic method appears to be a suitable technique for exploring the stabilization of foam films from ABA copolymers. 相似文献
98.
表面能与晶体生长/溶解动力学研究的新动向 总被引:3,自引:0,他引:3
界面现象使物质在结晶过程中出现了临界现象.但最近的研究指出在物质溶解过程中,在表面能量的控制下也存在着临界现象以及尺寸效应.实验发现,当晶体自身小到一 定的程度时(通常在纳米尺度上并和临界蚀坑的大小相近),在溶解过程中其速度会自发降 低,反应被抑制乃至停止.尽管在热力学上表面能的因素可以赋予小颗粒晶体较大的溶解度 ,但表面能却也能通过对临界条件的控制而使这些微粒在动力学上不被溶解.这个发现不仅 解决了纳米颗粒在水溶液中稳定性的问题,而且还从动力学的角度解释了生物矿物选择纳米 尺度作为其基本构成单元的原因.由于表面能和晶体生长/溶解的动力学有着密切的关系, 我们可以通过对表面能的调节来修改它们的动力学速度和晶体的形貌.反过来,也可以用动力学的方法来测定表面能及表面吸附/脱附常数等.相对于常规的界面研究手段,通过生 长和溶解动力学途径所得的数据有着很好的可靠性及重复性.我们认为,晶体生长和溶解的 动力学和表面能的研究相结合,不仅为界面研究提供了新的思路和方法,而且也会推动晶体生长和材料科学的发展. 相似文献
99.
In the absence of ligand, copper and amine, a recoverable nanoparticle palladium(0) catalyzed Sonogashira reaction of aryl iodides and bromides with terminal alkynes was developed. The protocol involved the use of an environmental-friendly reaction system with ethanol as the solvent, potassium carbonate as a base, and poly(vinylpyrrolidone) (PVP) supported nanosized palladium metal colloids as the catalyst. The palladium metal was recovered and recycled by a simple decantation of the reaction solution and used for eight consecutive trials without significant loss of its reactivity. 相似文献
100.
Interatomic distances in the transition state were estimated for the reactions of radical abstraction: H· + H2, H· + HCl, H· + CH4, N·H2 + NH3, HO· + H2O, HO2
· + HOOH, and C·H3 + SiH4. The calculation was performed by the quantum-chemical density functional method or coupled clusters method (QCH), as well as by the methods of intersecting parabolas (IPM) and Morse curves (IMM), using experimental data (activation energies and reaction enthalpies). The results of the latter two methods are close to the quantum-chemical calculation and differ only by the increment a: r(IPM or IMM) = a + r(QCH), where a = –4.5·10–12 m for IPM and a = +1.9·10–12 m for IMM. 相似文献