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   15922篇
  免费   2951篇
  国内免费   2309篇
化学   12789篇
晶体学   189篇
力学   868篇
综合类   181篇
数学   1858篇
物理学   5297篇
  2024年   36篇
  2023年   342篇
  2022年   402篇
  2021年   543篇
  2020年   745篇
  2019年   830篇
  2018年   587篇
  2017年   484篇
  2016年   862篇
  2015年   884篇
  2014年   955篇
  2013年   1182篇
  2012年   1427篇
  2011年   1402篇
  2010年   1044篇
  2009年   963篇
  2008年   1094篇
  2007年   975篇
  2006年   829篇
  2005年   758篇
  2004年   652篇
  2003年   631篇
  2002年   729篇
  2001年   667篇
  2000年   432篇
  1999年   360篇
  1998年   216篇
  1997年   187篇
  1996年   153篇
  1995年   141篇
  1994年   91篇
  1993年   85篇
  1992年   85篇
  1991年   99篇
  1990年   86篇
  1989年   54篇
  1988年   40篇
  1987年   24篇
  1986年   31篇
  1985年   29篇
  1984年   19篇
  1983年   9篇
  1982年   3篇
  1981年   4篇
  1979年   3篇
  1977年   1篇
  1975年   1篇
  1971年   1篇
  1959年   1篇
  1936年   3篇
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
981.
Electrically conducting wires play a critical role in the advancement of modern electronics and in particular are an important key to the development of next‐generation wearable microelectronics. However, the thin conducting wires can easily break during use, and the whole device fails to function as a result. Herein, a new family of high‐performance conducting wires that can self‐heal after breaking has been developed by wrapping sheets of aligned carbon nanotubes around polymer fibers. The aligned carbon nanotubes offer an effective strategy for the self‐healing of the electric conductivity, whereas the polymer fiber recovers its mechanical strength. A self‐healable wire‐shaped supercapacitor fabricated from a wire electrode of this type maintained a high capacitance after breaking and self‐healing.  相似文献   
982.
Drug‐loaded nanoparticles (NPs) are of particular interest for efficient cancer therapy due to their improved drug delivery and therapeutic index in various types of cancer. However, the encapsulation of many chemotherapeutics into delivery NPs is often hampered by their unfavorable physicochemical properties. Here, we employed a drug reform strategy to construct a small library of SN‐38 (7‐ethyl‐10‐hydroxycamptothecin)‐derived prodrugs, in which the phenolate group was modified with a variety of hydrophobic moieties. This esterification fine‐tuned the polarity of the SN‐38 molecule and enhanced the lipophilicity of the formed prodrugs, thereby inducing their self‐assembly into biodegradable poly(ethylene glycol)‐block‐poly(d,l ‐lactic acid) (PEG‐PLA) nanoparticulate structures. Our strategy combining the rational engineering of prodrugs with the pre‐eminent features of conventionally used polymeric materials should open new avenues for designing more potent drug delivery systems as a therapeutic modality.  相似文献   
983.
To establish the structure–catalytic property relationships of heterogeneous catalysts, a detailed characterization of the three‐dimensional (3D) distribution of active sites on a single catalyst is essential. Single‐particle catalysis of a modular multilayer catalytic platform that consists of a solid silica core, a mesoporous silica shell, and uniformly distributed Pt nanoparticles sandwiched in between these layers is presented. The first 3D high‐resolution super‐localization imaging of single fluorescent molecules produced at active sites on the core‐shell model nanocatalysts is demonstrated. The 3D mapping is aided by the well‐defined geometry and a correlation study in scanning electron microscopy and total internal reflection fluorescence and scattering microscopy. This approach can be generalized to study other nano‐ and mesoscale structures.  相似文献   
984.
An efficient rhodium(III)‐catalyzed synthesis of 2H‐chromene from N‐phenoxyacetamides and cyclopropenes has been developed. The reaction represents the first example of using cyclopropenes as a three‐carbon unit in rhodium(III)‐catalyzed C(sp2)? H activations.  相似文献   
985.
A comprehensive first‐principles theoretical study of the electronic properties and half‐metallic nature of zigzag edge‐oxidized graphene quantum dots (GQDs) is carried out by using density functional theory (DFT) with the screened exchange hybrid functional of Heyd, Scuseria and Ernzerhof (HSE06). The oxidation schemes include ‐OH, ‐COOH and ‐COO groups. We identify oxidized GQDs whose opposite spins are localized at the two zigzag edges in an antiferromagnetic‐type configuration, showing a spin‐polarized ground state. Oxidized GQDs are more stable than the corresponding fully hydrogenated GQDs. The partially hydroxylated and carboxylated GQDs with the same size exhibit half‐metallic state under almost the same electric‐field intensity whereas fully oxidized GQDs behave as spin‐selective semiconductors. The electric‐field intensity inducing the half metal increases with the length of the partially oxidized GQDs, ranging from M=4 to 7.  相似文献   
986.
提出用带有非接触电导检测的微芯片毛细管电泳法快速测定片剂中盐酸二甲双胍的含量。取盐酸二甲双胍片20片,剥除糖衣后混匀研细,称取0.100 0g,用水超声溶解、过滤,滤液定容至100mL供毛细管电泳分析。十字通道芯片使用前按规定进行清洗。试验中采用含5%(体积分数)乙醇和0.1mmol·L-1十二烷基磺酸钠的2.0mmol·L-1柠檬酸缓冲溶液作为分离介质,进时间为10.0s,分离电压为1.3kV,可在1min内实现分离和测定。盐酸二甲双胍的质量浓度在10.0~110.0mg·L-1范围内与相应峰高呈线性关系,检出限(3S/N)为1.0mg·L-1。应用此方法分析了3个片剂样品,并用标准加入法做回收试验,测得回收率在94.5%~103%之间,测定值的相对标准偏差(n=6)在0.63%~1.1%之间。  相似文献   
987.
应用气相色谱-质谱法测定酒中23种邻苯二甲酸酯残留量。样品在沸水浴中加热除去乙醇后用正己烷提取。离心分离后上清液经DB-5MS石英毛细管色谱柱分离,在选择离子检测模式下进行质谱测定。23种邻苯二甲酸酯的质量浓度均在0.1~10mg·L-1范围内与其峰面积呈线性关系,方法的测定下限(10S/N)为0.5mg·kg-1(DINP及DIDP)和0.1mg·kg-1(其余21种邻苯二甲酸酯)。以空白样品为基体进行加标回收试验,所得回收率在84.1%~100%之间。  相似文献   
988.
以SiO2为载体采用分步浸渍法制备了Fe/Ir/SiO2催化剂,考察了助剂Fe对Ir/SiO2催化剂气相巴豆醛选择性加氢性能的影响.采用X射线粉末衍射(XRD)、CO化学吸附、H2-程序升温还原(H2-TPR)和拉曼光谱(Raman)等技术对催化剂进行了表征.结果表明,助剂Fe能有效提高Ir/SiO2催化剂的巴豆醛转化率和巴豆醇选择性.Fe含量为0.087%的0.087Fe/Ir/SiO2催化剂的反应性能最佳,反应进行9 h的巴豆醛转化率36.9%,巴豆醇选择性83%.随着催化剂CO吸附量的下降(Fe覆盖的增加),催化剂的转换频率(turn over frequency,TOF)明显增加,这表明Fe促进Ir/SiO2催化剂表面活性位的加氢活性.然而,Fe的掺杂使得Ir/SiO2催化剂存在明显的活性下降现象,归因于Fe导致Ir/SiO2催化剂表面积炭和CO中毒.  相似文献   
989.
水热合成了2个以D-樟脑酸(D-H2cam)与1,4-二(咪唑-1-基)丁烷(1,4-bimb)为配体的配位聚合物:[Co(D-cam)(1,4-bimb)]n·4nH2O(1)和[Ni(D-cam)(1,4-bimb)]n(2)。通过X-射线单晶衍射,元素分析,红外光谱,XRD分析,热重,和磁性分析对其结构进行了表征。测定了这两个聚合物的磁性。结构分析表明,1为P2空间群,其链状结构由氢键作用连接形成二维结构;2为Pna21空间群,呈四重穿插的dia网络结构。  相似文献   
990.
A series of novel phthalic diamide derivatives containing 1,2,3‐triazole moiety were synthesized using one‐pot click chemistry approach and characterized by 1H NMR and HRMS. The insecticidal activity against armyworm (Mythimna separata), Tetranychu scinnabarinus and cowpea aphid (Aphis craccivora) was evaluated. Compounds 4II‐a and 4II‐i showed 50% insecticidal activity against armyworm (Mythimna separata) at the concentration of 4 mg/L and one‐third of the compounds had moderate activity against Tetranychus cinnabarinus at 500 mg/L.  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