全文获取类型
收费全文 | 19531篇 |
免费 | 3053篇 |
国内免费 | 3243篇 |
专业分类
化学 | 10961篇 |
晶体学 | 318篇 |
力学 | 638篇 |
综合类 | 247篇 |
数学 | 1418篇 |
物理学 | 4802篇 |
无线电 | 7443篇 |
出版年
2024年 | 87篇 |
2023年 | 379篇 |
2022年 | 636篇 |
2021年 | 709篇 |
2020年 | 716篇 |
2019年 | 716篇 |
2018年 | 653篇 |
2017年 | 725篇 |
2016年 | 798篇 |
2015年 | 1013篇 |
2014年 | 1162篇 |
2013年 | 1510篇 |
2012年 | 1675篇 |
2011年 | 1759篇 |
2010年 | 1557篇 |
2009年 | 1468篇 |
2008年 | 1554篇 |
2007年 | 1465篇 |
2006年 | 1241篇 |
2005年 | 1062篇 |
2004年 | 838篇 |
2003年 | 710篇 |
2002年 | 737篇 |
2001年 | 607篇 |
2000年 | 494篇 |
1999年 | 320篇 |
1998年 | 227篇 |
1997年 | 162篇 |
1996年 | 172篇 |
1995年 | 132篇 |
1994年 | 110篇 |
1993年 | 69篇 |
1992年 | 54篇 |
1991年 | 64篇 |
1990年 | 44篇 |
1989年 | 36篇 |
1988年 | 22篇 |
1987年 | 28篇 |
1986年 | 15篇 |
1985年 | 21篇 |
1984年 | 11篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1971年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
二氢吡唑是一类含有多种生物活性的五元氮杂环化合物,广泛存在于各种活性天然产物结构中。本文由4-氟苯乙酮和N-甲基哌嗪经取代反应后,再与2-噻吩甲醛发生羟醛缩合生成哌嗪取代噻吩查尔酮(2)。化合物2与水合肼环化得到中间体化合物3,最后经磺酰氯衍生化,合成得到8个未见报道的3-芳基-5-噻吩基二氢吡唑磺胺衍生物(4a~4h),其结构均经IR、~1H NMR和~(13)C NMR确证。采用小鼠巨噬细胞Raw264. 7模型初步测试了衍生物的抗炎活性,结果表明,化合物4a、4e和4h具有潜在的体外抗炎活性,特别是化合物4a抑制NO生成的IC_(50)值为12. 64μmol/L,与阳性对照药地塞米松活性相当。 相似文献
882.
The synthesis and crystal structure of γ‐aminobutyric acid naphthalene diimides derivative with the LaIII coordination complex, [La(L)(DMF)Cl]n, was reported, which is a twofold interpenetrating metal‐organic framework architecture. The coordination polymer was characterized by elemental analysis, infrared spectroscopy, thermal gravimetric analysis, and single‐crystal X‐ray diffraction. The optical properties of the crystallized complex were investigated both in solution and the solid state. 相似文献
883.
Wenpin Wang Qing Yao Jianhui Song Binbin Yao Hui Wang Zhibo Li 《Journal of Polymer Science.Polymer Physics》2019,57(11):662-669
The PeakForce Quantitative Nanomechanical Mapping based on atomic force microscope (AFM) is employed to first visualize and then quantify the elastic properties of a model nitrile rubber/poly(vinyl chloride) (NBR/PVC) blend at the nanoscale. This method allows us to consistently observe the changes in mechanical properties of each phase in polymer blends. Beyond measuring and discriminating elastic modulus and adhesion forces of each phase, we tune the AFM tips and the peak force parameters in order to reliably image samples. In view of viscoelastic difference in each phase, a three‐phase coexistence of an unmixed NBR phase, the mixed phase, and PVC microcrystallites is directly visualized in NBR/PVC blends. The nanomechanical investigation is also capable of recognizing the crosslinked rubber phase in cured rubber. The contribution of the mixed phase was quantified and it was found that the mechanical properties of blends are mainly determined by the homogeneity and stiffness of the mixed phase. This study furthers our understanding the structure–mechanical property relationship of thermoplastic elastomers, which is important for their potential design and applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 662–669 相似文献
884.
Qiu‐Yan Wang Xin Dong Juan Yang Xiao‐Ting Zhen Li‐Hong Ye Chu Chu Bin Wang Yu‐Han Hu Hui Zheng Jun Cao 《Journal of separation science》2019,42(23):3579-3588
The quantification of three alkaloids is important because quantitative study is a means of assessing the reliability of the experimental method, and three alkaloids of peimine, peiminine, and peimisine are main active ingredients in Chinese Pharmacopoeia 2015. An effective method based on the matrix solid‐phase dispersion microextraction was developed for the extraction of alkaloid compounds in Fritillariae Thunbergii Bulbus. Target analytes were analyzed by capillary electrophoresis coupled with quadrupole time‐of‐flight mass spectrometry. The optimized experimental condition was that 50 mg Fritillariae Thunbergii Bulbus was blended homogeneously with 10 mg citric acid for 5 min. Two hundred microliters of water acidized by 1 mol/L hydrochloric acid (pH = 4.5) was selected to elute tested alkaloids. The results demonstrated that the investigated method had low limits of detection (1.32–1.59 ng/mL), good recoveries (86.63–98.12%), and reproducibility (relative standard deviations of peak areas < 0.87%). The proposed matrix solid‐phase dispersion microextraction coupled with capillary electrophoresis combined with quadrupole time‐of‐flight mass spectrometry was successfully applied for the extraction of alkaloids in plants. 相似文献
885.
886.
Infrared Photodissociation Spectroscopic and Theoretical Study of the HC2nO+ (n=3-6) Cations 下载免费PDF全文
The carbon chain cations, HC2nO+ (n=3-6) are produced via a pulsed laser vaporization supersonic expansion ion source in the gas phase. Their infrared spectra are measured via mass-selected infrared photodissociation spectroscopy of the CO “tagged” [HC2nO·CO]+ cation complexes in 1600-3500 cm-1 frequency range. The geometric and electronic structures of the [HC2nO·CO]+ complexes and the core HC2nO+ (n=3-6) cations are determined with the aid of density functional theory calculations. These HC2nO+(n=3-6) ions are identified to be linear carbon chain derivatives terminally capped by hydrogen and oxygen. The triplet ground states are 10-15 kcal/mol lower in energy than the singlet states, indicating cumulene-like carbon chain structures. 相似文献
887.
We have implemented a kinetic Monte Carlo (KMC) simulation to study the effects of wafer miscut and wafer defects on the morphologies of Si (111) surfaces etched in NH4F. Although a conventional KMC simulation reproduced previously published results, it failed to produce the morphologies observed in our experiments. By introducing both dopant sites and lattice defect sites into the model, we are able to simulate samples having different dopant elements and densities as well as different defect concentrations. Using the modified KMC simulation, the simulated surface morphologies agree well with the morphologies observed in our experiments. The enhanced model also gives insights to the formation mechanism for multiple level stacking pits, a notable morphology on the etched surfaces of samples with very small miscut angles. 相似文献
888.
Epalrestat是一种治疗糖尿病晚期综合症的新药。它可以缓解糖尿病型视网膜病变,运动神经传导阻滞等症状。绕丹宁乙酸与α-甲基肉桂醛在乙酸中缩合得到了Epalrestat,并利用了高压液相色谱,X衍射,高分辨率核磁共振,对它的立体异构进行了研究,还研究了它的保存条件。发现Epalrestat的四个立体异构体中(Z,E)构型占96.5%。见光后易发生异构化,产物中(Z,Z)构型的含量可达到37.7%。而避光保存一年后(Z,E)仍能保持93.6%的含量。 相似文献
889.
890.
1,2环氧丙烷(下面简写成环氧丙烷)分子中含氧三员环具有很大的张力,且杂原子氧上含有未公享的电子对,分子中存在有很强的极性共价键,使其易于发生定向开环反应,常用于制造丙二醇、缩丙二醇、甘油等,是一种重要的有机合成化工原料和合成硝酸纤维脂等高分子材料良好的低沸点溶剂.近年来还发现,在含有的1,2环氧基团的天然产品中,这一结构单元对这些天然产品的生物活性起着十分重要的作用.环氧丙烷这种特殊结构,使其电离和解离过程与其它的杂环化合物(三员环以上)和一般的链状化合物有着明显的不同.这方面的研究工作仅有文献[1,2]报导,其方法是… 相似文献