首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   15341篇
  免费   1774篇
  国内免费   1573篇
化学   7234篇
晶体学   154篇
力学   467篇
综合类   113篇
数学   1122篇
物理学   3705篇
综合类   5893篇
  2024年   61篇
  2023年   228篇
  2022年   434篇
  2021年   463篇
  2020年   446篇
  2019年   502篇
  2018年   380篇
  2017年   380篇
  2016年   535篇
  2015年   650篇
  2014年   852篇
  2013年   1030篇
  2012年   1204篇
  2011年   1227篇
  2010年   1018篇
  2009年   1039篇
  2008年   1109篇
  2007年   1022篇
  2006年   859篇
  2005年   730篇
  2004年   590篇
  2003年   441篇
  2002年   423篇
  2001年   366篇
  2000年   290篇
  1999年   380篇
  1998年   304篇
  1997年   246篇
  1996年   245篇
  1995年   220篇
  1994年   191篇
  1993年   149篇
  1992年   147篇
  1991年   127篇
  1990年   102篇
  1989年   83篇
  1988年   60篇
  1987年   53篇
  1986年   39篇
  1985年   29篇
  1984年   9篇
  1983年   9篇
  1982年   3篇
  1981年   5篇
  1980年   2篇
  1979年   1篇
  1959年   1篇
  1957年   2篇
  1936年   2篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The chemical equilibrium and reaction kinetic behavior in the synthesis of polyoxymethylene dimethyl ethers(DMMn) were investigated over sulfated titania in order to reveal the decisive factor controlling the reaction. The results showed that the molar ratio of adjacent DMMn products in equilibrium solution had the same value, which depended absolutely on the reaction temperature. Meanwhile, the reactions had the same DMMn products distributions under varied reaction conditions. The equilibrium constants of the related step-wise reactions for DMMn formation were equal, which were calculated based on the bulk compositions of the reaction solution. And thus, the selectivity to DMMn was mainly controlled by the chemical equilibrium, i.e., thermodynamic control. In brief, the present results provide some guidance for future synthesis of DMMn.  相似文献   
992.
Two simple fluorescent anion receptors based on 1-phenyl-3-methylpyrozole-5-one-4-one phenylhydrazone (L1) and 1-phenyl-3-methylpyrozole-5-one-4-one p-nitrophenylhydrazone (L2) were designed, synthesized and characterized with 1H NMR, COSY spectrum, 13C NMR, ESI-mass and elemental analyse. Interestingly, two receptors with similar configuration exhibited different anion binding behaviors in DMSO solution. The results of Job plots and ESI-mass spectrum indicate that L1 bind anions such as F, AcO, H2PO4 to form 2:1 host-guest complexation, while L2 bind anions to form 1:1 host-guest complexation in the solution.  相似文献   
993.
In the present study, we have successfully set up a novel blue light source with the power density of 9 mW/cm2 and the wavelength of 435.8 nm and then the novel light source was used to investigate the effect of light radiation on curcumin-induced cell death. The cytotoxicity was investigated 24 h after the treatment of curcumin and blue light radiation together using MTT reduction assay. Nuclear chromatin was observed using a fluorescent microscopy with Hoechst33258 staining. The results showed blue light radiation could significantly enhance the cytotoxicity of curcumin on the MCF-7 cells and apoptosis induction. These findings demonstrated that blue light radiation could enhance curcumin-induced cell death of breast cancer cells, suggesting light radiation may be an efficient enhancer of curcumin in the management of breast cancer.  相似文献   
994.
A stable superhydrophobic polystyrene nanocomposite coating was fabricated by means of a very simple and easy method. The coating was characterized by scanning electron microscopy and X-ray photoelectron spectrum. The wettability of the products was also investigated. By adding the surface-modified SiO2 nanoparticles, the wettability of the coating changed to water-repellent superhydrophobic, not only for pure water, but also for a wide pH range of corrosive liquids. The influence of the drying temperature and SiO2 content on the wettability of the nanocomposite coating was also investigated. It was found that both factors had little or no significant effect on the wetting behavior of the coating surface.  相似文献   
995.

Graphene oxide (GO) is an ideal adsorbent due to excellent physicochemical properties. Humic acid (HA) is ubiquitous in aquatic and soil environment, which can affect the migration of metal ions. In this study, we investigated the sorption mechanisms of U(VI) onto GO surfaces in the presence of HA. pH dependent and ionic strength independent sorption process were observed and the concentration of HA is positively proportional to U(VI) sorption capacities. Results also suggest that a pre-mixing HA + U(VI) gave better results than a pre-mixing of GO + HA, which can be explained by the size distribution of different GO systems.

  相似文献   
996.
选取自然资源环境、经济环境、基础设施环境、社会服务环境、人口劳动力环境和社会文化环境6个子系统的18个指标构建了城市投资环境指标体系,以陕西省下属的10个地级市为研究对象,采用因子分析的方法,对陕西省城市投资环境进行了实证分析。  相似文献   
997.
We studied the evolution of cooperation in the prisoner’s dilemma game on a square lattice where the size of the interaction neighborhood is considered. Firstly, the effects of noise and the cost-to-benefit ratio on the maintenance of cooperation were investigated. The results indicate that the cooperation frequency depends on the noise and cost-to-benefit ratio: cooperation reaches a climax as noise increases, but it monotonously decreases and even vanishes with the ratio increasing. Furthermore, we investigated how the size of the interaction neighborhood affects the emergence of cooperation in detail. Our study demonstrates that cooperation is remarkably enhanced by an increase in the size of the interaction neighborhood. However, cooperation died out when the size of the interaction neighborhood became too large since the system was similar to the mean-field system. On this basis, a cluster-forming mechanism acting among cooperators was also explored, and it showed that the moderate range of the neighborhood size is beneficial for forming larger cooperative clusters. Finally, large-scale Monte Carlo simulations were carried out to visualize and interpret these phenomena explicitly.  相似文献   
998.
根据效应分层原理,低阶效应比高阶效应更重要.在这种情况下,以别名集的结构为基础,提出了最小低阶效应混杂准则对2n-m设计进行排序.研究了新准则与其它最优性准则,即最大分辨度、最小低阶混杂、纯净效应和最大估计容量准则的联系.给出了具有16和32个水平组合的最小低阶效应混杂设计.  相似文献   
999.
当前的通信原理实验多为纸质版实验报告,需要学员将实验结果、信号波形进行徒手绘制,效率不高;且纸质实验报告还不便于档案的保存和管理。从提高实验课堂效率和档案记录的数字化存贮、管理角度出发,开发了通信原理实验系统。教学实践表明,该系统能满足实验报告计算机存贮、管理的功能,但有待进一步完善。  相似文献   
1000.
A novel azide-free asymmetric synthesis of oseltamivir phosphate 1 (Tamiflu®) starting from Roche’s epoxide is described. Roche epoxide 2 was converted into N-acetyl aminoalcohol 3 in 95% yield via a BF3·OEt2-catalyzed epoxide-opening with acetonitrile as a nucleophile. Compound 3 was then transformed into a methanesulfonate 4 in 98% yield. Compound 4 was converted into aziridine 5 in 91% yield. Aziridine 5 was subsequently converted into oseltamivir phosphate 1 via two paths (a and b). In the path a, compound 5 underwent aziridine-opening with diallylamine as a nucleophile to afford compound 7 in 93% yield; compound 7 could then be converted into oseltamivir phosphate 1 in 88% yield. In path b, compound 5 underwent aziridine-opening with isopropyl 2,2,2-trichloroacetimidate as a nucleophile to afford compound 8 in 94% yield, which was then converted into oseltamivir phosphate 1 in 82% yield.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号