全文获取类型
收费全文 | 1002篇 |
免费 | 89篇 |
国内免费 | 75篇 |
专业分类
化学 | 501篇 |
晶体学 | 14篇 |
力学 | 57篇 |
综合类 | 9篇 |
数学 | 105篇 |
物理学 | 178篇 |
综合类 | 302篇 |
出版年
2024年 | 2篇 |
2023年 | 19篇 |
2022年 | 19篇 |
2021年 | 33篇 |
2020年 | 27篇 |
2019年 | 28篇 |
2018年 | 17篇 |
2017年 | 25篇 |
2016年 | 23篇 |
2015年 | 45篇 |
2014年 | 44篇 |
2013年 | 52篇 |
2012年 | 61篇 |
2011年 | 68篇 |
2010年 | 57篇 |
2009年 | 50篇 |
2008年 | 47篇 |
2007年 | 52篇 |
2006年 | 50篇 |
2005年 | 57篇 |
2004年 | 37篇 |
2003年 | 36篇 |
2002年 | 23篇 |
2001年 | 23篇 |
2000年 | 21篇 |
1999年 | 30篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1993年 | 15篇 |
1992年 | 26篇 |
1991年 | 9篇 |
1990年 | 14篇 |
1989年 | 5篇 |
1988年 | 1篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1976年 | 2篇 |
1974年 | 4篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1957年 | 2篇 |
1945年 | 1篇 |
排序方式: 共有1166条查询结果,搜索用时 15 毫秒
941.
Synthesis and application of water-soluble hyperbranched polyester modified by trimellitic anhydride
In this paper, we aimed to synthesize a water-soluble hyperbranched polyester with pentaerythritol and trimethylolpropane as branching centers and trimellitic anhydride as waterborne functional groups. The condition and principle of water-soluble hyperbranched resin synthesis were explored. The trimellitic anhydride was used to synthesize water-soluble hyperbranched resin at the most appropriate temperature of 160–170°C. The properties of the resin were characterized by FTIR, hydroxyl value, molecular weight, stability, viscosity, and then the relationship among them under different synthesis conditions was further studied. Results indicated that the water-soluble hyperbranched resin had no delamination for more than 6 months at a solid content higher than 60%. Moreover, the resin was mutual soluble with water at any proportion and kept clear and transparent at same time. After water-soluble hyperbranched resin mixing with amino resin in a ratio of 2.5:1, and curing at 130–135°C for 2 hours, the chemical and mechanical properties of the coating were further measured and characterized. 相似文献
942.
以天然胶乳为种子乳液 ,苯乙烯 (St)、甲基丙烯酸甲酯 (MMA)、丙烯腈 (AN)、丙烯酸异冰片酯 (IBA)等单体或它们的共混物为互穿单体 ,油酸为稳定剂 ,二乙烯苯为交联剂 ,叔丁基过氧化氢 /四乙烯五胺为氧化还原引发体系 ,采用种子乳液聚合法制备出乳胶粒外壳呈硬聚合物梯度分布的复合乳液 ,经絮凝干燥得固体聚合物。考察了互穿单体的种类和用量、交联剂用量及溶胀时间等对材料力学性能的影响。结果表明 ,溶胀时间对聚合物力学性能没有影响 ;交联剂对应硬单体的摩尔分数为 1 0 %时 ,聚合物力学性能最佳 ;硬单体配伍及其用量明显影响互穿聚合物的力学性能 ,其中IBA +St+AN为硬单体组分且质量分数为 2 0 %时 ,材料的力学性能较好 ,其拉伸强度8 7MPa ,撕裂强度 32 6kN/m ,伸长率 30 0 %~ 60 0 %。聚合物可热塑性加工。 相似文献
943.
In this paper, we study an operator s which maps every n-by-n symmetric matrix A, to a matrix s(A_n) that minimizes || B_n-A_n || F over the set of all matrices B_n, that can be diagonalized by the sine transform. The matrix s(A_n), called the optimal sine transform preconditioner, is defined for any n-by-n symmetric matrices A_n. The cost of constructing s(A_n) is the same as that of optimal circulant preconditioner c(A_n) which is defined in [8], The s(A_n) has been proved in [6] to be a good preconditioner in solving symmetric Toeplitz systems with the preconditioned conjugate gradient (PCG) method. In this paper, we discuss the algebraic and geometric properties of the operator s, and compute its operator norms in Banach spaces of symmetric matrices. Some numerical tests and an application in image restoration are also given. 相似文献
944.
A method employing solid-phase extraction coupled with HPLC separation of thiol-monobromobimane (mBBr) derivatives was developed and optimized to quantify dissolved thiols at concentrations as low as 0.1 nM for glutathione (GSH) and gamma-glutamylcysteine (gammaEC) in natural waters. The reducing reagent, tri-n-butylphosphine (TBP), is needed for complete derivatization. At the optimal addition of TBP ([TBP]/[mBBr] = -0.4-1.6), no interference from copper was observed. The thiol fluorescence signal was totally suppressed if the mole ratio of TBP to mBBr was 2.6 or greater. Consistent recovery of thiols standards in a NaCl solution (0.5 M) was obtained using the Waters HLB reversed-phase resin, and blank levels of GSH and gammaEC were extremely low (less than 0.03 nM). The detection limits for GSH, gammaEC and phytochelatin-2 (PC-2) were 0.03, 0.03, and 0.06 nM, respectively. 相似文献
945.
946.
建立了离子色谱法测定休闲食品中柠檬酸和柠檬酸盐含量的方法。采用标准曲线法定量,柠檬酸浓度在1.0~20.0 μg/mL的范围内,线性方程为y=0.049 9x-0.017 4,线性相关系数为0.998 2,方法的检出限为0.015 μg/mL,加标回收率为96.9%~101.6%,方法具有简单、快速、重现性好等优点。 相似文献
947.
用毛细管色谱柱进行定量分析,必须测定正确的"定量校正值"。本文提出了用标准的二元混合物测定这种校正值的方法;并探讨了不同的操作条件对定量校正值的影响。 相似文献
948.
949.
应用光谱学方法以吖啶橙(AO)作探针研究了Ni(Ⅱ)-EDTA与DNA的作用机制.研究表明,Ni(Ⅱ)与EDTA形成配离子的摩尔结合比nNi(Ⅱ)∶nEDTA=1∶1,配合物Ni(Ⅱ)-EDTA与DNA的摩尔结合比为nNi(Ⅱ)-EDTA∶nDNA=2∶1,其结合常数K■B25℃=2.77×105 L.mol-1,K■B37℃=2.01×105 L.mol-1.该过程为熵和焓共同驱动,其ΔrS■m=171.39J.(mol.K)-1,ΔrH■m=-2.05×104 J.mol-1,25℃时ΔrG■m=-3.06×104 J.mol-1.Scatchard法和探针法等均表明Ni(Ⅱ)-EDTA与DNA之间的结合方式以沟区作用为主. 相似文献
950.
Xingming Zeng Yi Hu Rongbin Xie Sadaf Bashir Khan Shern-Long Lee 《Molecules (Basel, Switzerland)》2021,26(24)
In recent years, extending self-assembled structures from two-dimensions (2D) to three-dimensions (3D) has been a paradigm in surface supramolecular chemistry and contemporary nanotechnology. Using organic molecules of p-terphenyl-3,5,3′,5′-tetracarboxylic acid (TPTC), and scanning tunneling microscopy (STM), we present a simple route, that is the control of the solute solubility in a sample solution, to achieve the vertical growth of supramolecular self-assemblies, which would otherwise form monolayers at the organic solvent/graphite interface. Presumably, the bilayer formations were based on π-conjugated overlapped molecular dimers that worked as nuclei to induce the yielding of the second layer. We also tested other molecules, including trimesic acid (TMA) and 1,3,5-tris(4-carboxyphenyl)-benzene (BTB), as well as the further application of our methodology, demonstrating the facile preparation of layered assemblies. 相似文献