全文获取类型
收费全文 | 103042篇 |
免费 | 19557篇 |
国内免费 | 29001篇 |
专业分类
化学 | 75096篇 |
晶体学 | 3314篇 |
力学 | 7524篇 |
综合类 | 3086篇 |
数学 | 15819篇 |
物理学 | 46761篇 |
出版年
2024年 | 295篇 |
2023年 | 1422篇 |
2022年 | 3646篇 |
2021年 | 3636篇 |
2020年 | 3798篇 |
2019年 | 3790篇 |
2018年 | 3311篇 |
2017年 | 4235篇 |
2016年 | 3979篇 |
2015年 | 5107篇 |
2014年 | 6241篇 |
2013年 | 7988篇 |
2012年 | 8599篇 |
2011年 | 9058篇 |
2010年 | 8125篇 |
2009年 | 8249篇 |
2008年 | 9140篇 |
2007年 | 8311篇 |
2006年 | 7978篇 |
2005年 | 6876篇 |
2004年 | 5364篇 |
2003年 | 4122篇 |
2002年 | 4241篇 |
2001年 | 4012篇 |
2000年 | 3910篇 |
1999年 | 2669篇 |
1998年 | 1618篇 |
1997年 | 1339篇 |
1996年 | 1289篇 |
1995年 | 1169篇 |
1994年 | 1107篇 |
1993年 | 1035篇 |
1992年 | 913篇 |
1991年 | 726篇 |
1990年 | 615篇 |
1989年 | 661篇 |
1988年 | 563篇 |
1987年 | 418篇 |
1986年 | 356篇 |
1985年 | 295篇 |
1984年 | 282篇 |
1983年 | 218篇 |
1982年 | 188篇 |
1981年 | 144篇 |
1980年 | 115篇 |
1979年 | 111篇 |
1978年 | 38篇 |
1977年 | 29篇 |
1965年 | 51篇 |
1959年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
51.
Journal of Thermal Analysis and Calorimetry - The fire behavior of one flame-retardant PVC cable used as building wire is investigated in this work. Bench scale tests were performed using a cone... 相似文献
52.
53.
54.
Pu Duan Bo Zhi Luke Coburn Christy L. Haynes Klaus Schmidt-Rohr 《Magnetic resonance in chemistry : MRC》2020,58(11):1130-1138
The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave-assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid-state nuclear magnetic resonance (NMR) experiments. 13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15N NMR resonances near 80 and 160 ppm proved the presence of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross-peaks in a 1H─13C HETCOR spectrum with brief 1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13C and 15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13C chemical-shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation. 相似文献
55.
静止轨道卫星差分吸收光谱仪采用摆扫成像方式对大气进行探测,针对其工作时CCD成像系统信噪比大于1 000、高速探测模式下探测周期小于10min、高分辨率模式下探测周期小于1h的要求,进行CCD成像系统设计.选取CCD47-20作为探测器,设计成像电路实现光谱图像信号的采集和上传.分析了帧叠加和像元合并对时间、空间分辨率的影响.结合帧转移CCD的特点设计了每个位置最后一帧读出时摆镜转动的成像方式,并合理设置了帧叠加数和像元合并数,达到优化成像周期的目的.1s曝光时间条件下,该CCD成像系统的高速、高分辨率模式探测周期分别为515s和3 315s,图像信噪比均大于1 000,污染物观测实验中未出现失帧或重复的现象.该CCD成像系统方案满足静止轨道星载差分吸收光谱仪的探测需求,为静止轨道环境监测仪器设计提供参考. 相似文献
57.
58.
59.
DFT mechanistic study of the H2‐assisted chain transfer copolymerization of propylene and p‐methylstyrene catalyzed by zirconocene complex 下载免费PDF全文
Cheng‐Gen Zhang Shu‐Yuan Yu Liaoyun Zhang Huayi Li Zhi‐Xiang Wang 《Journal of polymer science. Part A, Polymer chemistry》2015,53(4):576-585
DFT computations have been performed to investigate the mechanism of H2‐assisted chain transfer strategy to functionalize polypropylene via Zr‐catalyzed copolymerization of propylene and p‐methylstyrene (pMS). The study unveils the following: (i) propylene prefers 1,2‐insertion over 2,1‐insertion both kinetically and thermodynamically, explaining the observed 1,2‐insertion regioselectivity for propylene insertion. (ii) The 2,1‐inserion of pMS is kinetically less favorable but thermodynamically more favorable than 1,2‐insertion. The observation of 2,1‐insertion pMS at the end of polymer chain is due to thermodynamic control and that the barrier difference between the two insertion modes become smaller as the chain length becomes longer. (iii) The pMS insertion results in much higher barriers for subsequent either propylene or pMS insertion, which causes deactivation of the catalytic system. (iv) Small H2 can react with the deactivated [Zr]?pMS?PPn facilely, which displace functionalized pMS?PPn chain and regenerate [Zr]? H active catalyst to continue copolymerization. The effects of counterions are also discussed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 576–585 相似文献
60.
Dr. Feifan Wang Jie Tian Dr. Mengzhu Li Dr. Weizhen Li Dr. Lifang Chen Xiaozhi Liu Dr. Jian Li Aidaer Muhetaer Prof. Dr. Qi Li Prof. Dr. Yuan Wang Prof. Dr. Lin Gu Prof. Dr. Ding Ma Prof. Dr. Dongsheng Xu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(21):8280-8286
Fully utilizing solar energy for catalysis requires the integration of conversion mechanisms and therefore delicate design of catalyst structures and active species. Herein, a MOF crystal engineering method was developed to controllably synthesize a copper–ceria catalyst with well-dispersed photoactive Cu-[O]-Ce species. Using the preferential oxidation of CO as a model reaction, the catalyst showed remarkably efficient and stable photoactivated catalysis, which found practical application in feed gas treatment for fuel cell gas supply. The coexistence of photochemistry and thermochemistry effects contributes to the high efficiency. Our results demonstrate a catalyst design approach with atomic or molecular precision and a combinatorial photoactivation strategy for solar energy conversion. 相似文献