全文获取类型
收费全文 | 25750篇 |
免费 | 3946篇 |
国内免费 | 2385篇 |
专业分类
化学 | 17895篇 |
晶体学 | 309篇 |
力学 | 1530篇 |
综合类 | 109篇 |
数学 | 2914篇 |
物理学 | 9324篇 |
出版年
2024年 | 70篇 |
2023年 | 523篇 |
2022年 | 946篇 |
2021年 | 943篇 |
2020年 | 1006篇 |
2019年 | 991篇 |
2018年 | 827篇 |
2017年 | 727篇 |
2016年 | 1202篇 |
2015年 | 1150篇 |
2014年 | 1343篇 |
2013年 | 1789篇 |
2012年 | 2400篇 |
2011年 | 2406篇 |
2010年 | 1579篇 |
2009年 | 1500篇 |
2008年 | 1641篇 |
2007年 | 1504篇 |
2006年 | 1355篇 |
2005年 | 1151篇 |
2004年 | 841篇 |
2003年 | 643篇 |
2002年 | 607篇 |
2001年 | 444篇 |
2000年 | 434篇 |
1999年 | 513篇 |
1998年 | 427篇 |
1997年 | 437篇 |
1996年 | 451篇 |
1995年 | 358篇 |
1994年 | 306篇 |
1993年 | 235篇 |
1992年 | 234篇 |
1991年 | 198篇 |
1990年 | 169篇 |
1989年 | 143篇 |
1988年 | 97篇 |
1987年 | 109篇 |
1986年 | 87篇 |
1985年 | 82篇 |
1984年 | 47篇 |
1983年 | 43篇 |
1982年 | 35篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1976年 | 9篇 |
1975年 | 11篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Readily available Pd(II) chloride catalysts can catalyze selective and efficient oxidative coupling between N-aryl-2-aminopyridines and internal alkynes to yield N-(2-pyridyl)indoles. This process involves the ortho C-H activation of N-aryl-2-aminopyridines, and CuCl(2) was used as an oxidant. Compared to our previously reported Rh(III)-catalyzed synthesis of this class of product, this method is advantageous with a wider scope of alkynes and cost-effective Pd(II) catalysts. Molecular oxygen can be used as a terminal oxidant. 相似文献
992.
Titanate nanotubes were synthesized with hydrothermal reaction using TiO(2) and NaOH as the precursors and subsequent calcination at 400°C for 2h. The products were characterized with SEM and XRD. Adsorption and photocatalysis of methylene blue over titanate nanotubes and TiO(2) were investigated. The results indicated that titanate nanotubes exhibited a better photocatalytic degradation of methylene blue in a simultaneous adsorption and photodegradation system than that in equilibrium adsorption followed by a photodegradation system, whereas TiO(2) showed no significant differences in photocatalytic activity in the two systems. The methylene blue overall removal efficiency over TNTs in the first system even exceeded that over TiO(2). The different catalytic performances of titanate nanotubes in the two systems were tentatively attributed to different effects of adsorption of methylene blue, i.e., the promoting effect in the former and the inhibition effect in the latter. Decantation experiments showed that the titanate nanotube photocatalyst could be easily separated from the reaction medium by sedimentation. Thus titanate nanotubes with high sedimentation rates and concurrent adsorption represent a new catalyst system with a strong potential for commercial applications. 相似文献
993.
994.
合成了两种固态稀土丙氨酸配合物[Ho2(Ala)4(H2O)8]Cl6和[ErY(Ala)4(H2O)8](ClO4)6 (Ala为丙氨酸),用量热和热分析方法研究了这两种配合物的热力学性质.用全自动高精密绝热量热计测定了在78~377 K温区内的低温热容.对于[Ho2(Ala)4(H2O)8]Cl6,在214~255 K温区内发现一固-固相变,其相变温度为235.09 K.对于[ErY(Ala)4(H2O)8](ClO4)6,在99~121 K温区内也发现一固-固相变,其相变温度为115.78 K. [Ho2(Ala)4(H2O)8]Cl6固-固相变焓为3.02 kJ• mol-1,相变熵为12.83 J•K-1•mol-1; [ErY(Ala)4(H2O)8](ClO4)6 固-固相变焓为1.96 kJ•mol-1,相变熵为16.90 J•K-1•mol-1.同时,用TG技术在40~800 ℃温区研究了两配合物的热稳定性.由TG/DTG曲线分析可知, [Ho2(Ala)4(H2O)8]Cl6从80 ℃到479 ℃热分解分两步完成, [ErY(Ala)4(H2O)8](ClO4)6从120 ℃到430 ℃热分解分三步完成. 相似文献
995.
TiO2纳米膜表面结构形态特征 总被引:4,自引:2,他引:4
采用反胶束法制备TiO2纳米溶胶,用浸渍提拉法在不同的条件下制备了三种TiO2多孔纳米薄膜,并利用AFM、SEM、XRD等方法对膜表面结构物理化学特性进行表征.结果表明三种膜基本上由粒径约为59 nm的纳米粒子以不同的方式堆积而成,溶胶刚生成时浸提一次,干燥、焙烧得到膜上纳米粒子分布均匀,所生成的二次粒子粒径最小,二次粒子形成的二次表面粗糙度最小,浸提10次得到膜上纳米粒子间存在较丰富缝隙结构,二次粒子粒径及其形成的表面粗糙度较大,而溶胶制备好陈化6 h后浸提得到的膜上二次粒子粒径最大,表面粗糙度最高.由分形理论估算得到三种膜的分形维数分别是2.22、2.20和2.27. XRD测试表明,膜上TiO2为锐钛矿晶相.这些结果表明,采用不同制备步骤得到的膜,其表面结构形态存在较大的差异. 相似文献
996.
Bauer MD Sun Y Keough T Lacey MP 《Rapid communications in mass spectrometry : RCM》2000,14(10):924-929
We report the application of nanoelectrospray ionization tandem mass spectrometry (nES-MS/MS) and capillary LC/microelectrospray MS/MS (cLC/&mgr;ES-MS/MS) for sequencing sulfonic acid derivatized tryptic peptides. These derivatives were specifically prepared to facilitate low-energy charge-site-initiated fragmentation of C-terminal arginine-containing peptides, and to enhance the selective detection of a single series of y-type fragment ions. Both singly and doubly protonated peptides were analyzed by MS/MS and the results were compared with those from their derivatized counterparts. Model peptides and peptides from tryptic digests of gel-isolated proteins were analyzed. Derivatized singly protonated peptides fragment in the same way by nES-MS/MS as they do by post-source decay matrix-assisted laser desorption/ionization mass spectrometry (PSD-MALDI-MS). They produce fragment ion spectra dominated by y-ions, and the simplified spectra are readily interpreted de novo. Doubly protonated peptides fragment in much the same way as their non-derivatized doubly protonated counterparts. The fragmentation of doubly protonated derivatives is especially useful for sequencing peptides that possess a proline residue near the N-terminus of the molecule. The singly protonated forms of these proline-containing derivatives often show enhanced fragmentation on the N-terminal side of the proline and considerably reduced fragmentation on the C-terminal side. In addition, sulfonic acid derivatization increases the in-source fragmentation of arginine-containing peptides. This could be useful for sequence verification and sequence tagging for use in single stage mass spectrometry. Copyright 2000 John Wiley & Sons, Ltd. 相似文献
997.
Yanze Wu Huai Sun Liang Wu Joshua D. Deetz 《Journal of computational chemistry》2019,40(16):1586-1592
Determining reaction mechanisms and kinetic models, which can be used for chemical reaction engineering and design, from atomistic simulation is highly challenging. In this study, we develop a novel methodology to solve this problem. Our approach has three components: (1) a procedure for precisely identifying chemical species and elementary reactions and statistically calculating the reaction rate constants; (2) a reduction method to simplify the complex reaction network into a skeletal network which can be used directly for kinetic modeling; and (3) a deterministic method for validating the derived full and skeletal kinetic models. The methodology is demonstrated by analyzing simulation data of hydrogen combustion. The full reaction network comprises 69 species and 256 reactions, which is reduced into a skeletal network of 9 species and 30 reactions. The kinetic models of both the full and skeletal networks represent the simulation data well. In addition, the essential elementary reactions and their rate constants agree favorably with those obtained experimentally. © 2019 Wiley Periodicals, Inc. 相似文献
998.
Shuaifei Guo Cuihong Sun Lingpeng Meng Yanli Zeng 《Journal of computational chemistry》2019,40(32):2827-2833
The mechanism of ring-opening polymerization of L-lactide by iodine trichloride (ICl3) catalyst has been explored by using density functional theory (DFT) calculations and three catalytic pathways were proposed. The first and second pathways belong to the halogen bond catalysis, and the third pathway involves the ICl3 catalysts participating in reactions. When the carbonyl group was maintained involved in the reaction and activated catalytically by the halogen bond, there are two possible pathways. The first pathway involves only one transition state, and the second pathway requires two transition states. There is another pathway in which ICl3 directly participates in the reaction, it is named the third pathway. Two different transition states of the four-membered rings are generated successively, the transfer of I─O bonds determined the progress of the reaction. Theoretical calculations in this work provide the most basic understanding of ring-opening polymerization of L-lactide by ICl3 catalysts. © 2019 Wiley Periodicals, Inc. 相似文献
999.
Ensemble averaging measurements obscure the link between the electrochemical performance and the specific properties of an individual because of the interplay of inhomogeneity and heterogeneity. Nanocollision electrochemistry has attracted increasing interest because of its extremely high sensitivity, revealing the intrinsic properties of individual entities that are masked in the traditional ensemble measurements. In this perspective review, we summarized the recent developments in nanocollision-based single entity electrochemistry and photoelectrochemistry, the combined nanocollision electrochemistry with the other complementary techniques, as well as accurate data process. In closing, future challenges, opportunities, and destinations related to nanocollison electrochemistry were discussed. 相似文献
1000.
Photoionization and Dissociation Study of 2-Methyl-2-propen-1-ol: Experimental and Theoretical Insights 下载免费PDF全文
Ye-peng Yu Zhao-hui Li Xuan Lin Jun Chen Hang Zhang Yan-bo Li Huang-huang Wang Rui-rui Sun Qing-hui Meng Xiao-bin Shan Fu-yi Liu Liu-si Sheng 《化学物理学报(中文版)》2019,32(3):306-312
The photoionization and dissociation of 2-methyl-2-propen-1-ol (MPO) have been investigated by using molecular beam experimental apparatus with tunable vacuum ultravioletsynchrotron radiation in the photon energy region of 8.0-15.5 eV. The photoionization efficiency (PIE) curves for molecule ion and fragment ions: C4H8O+、C4H7O+、C3H5O+、C4H7+、C4H6+、C4H5+、C2H4O+、C2H3O+、C3H6+、C3H5+、C3H3+、CH3O+、CHO+ have been measured, and the ionization energy (IE) and the appearance energies (AEs) of the fragment ions have been obtained. The stable species and the first order saddle points have been calculated on the CCSD(T)/cc-pvTZ//B3LYP/6-31+G(d,p) level. With combination of theoretical and experimental results, the dissociative photoionization pathways of 2-methyl-2-propen-1-ol are proposed. Hydrogen migrations within the molecule are the dominant processes in most of the fragmentation pathways of MPO. 相似文献