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   110145篇
  免费   12959篇
  国内免费   14607篇
化学   84931篇
晶体学   2471篇
力学   4255篇
综合类   589篇
数学   9718篇
物理学   35747篇
  2023年   1159篇
  2022年   1897篇
  2021年   3012篇
  2020年   4044篇
  2019年   3945篇
  2018年   2941篇
  2017年   3475篇
  2016年   4332篇
  2015年   4234篇
  2014年   5340篇
  2013年   8980篇
  2012年   7828篇
  2011年   7409篇
  2010年   5970篇
  2009年   7445篇
  2008年   7428篇
  2007年   7770篇
  2006年   6746篇
  2005年   5474篇
  2004年   5050篇
  2003年   4485篇
  2002年   3745篇
  2001年   2884篇
  2000年   2492篇
  1999年   2130篇
  1998年   1882篇
  1997年   1635篇
  1996年   1585篇
  1995年   1494篇
  1994年   1424篇
  1993年   1124篇
  1992年   1120篇
  1991年   697篇
  1990年   542篇
  1989年   418篇
  1988年   410篇
  1987年   371篇
  1986年   351篇
  1985年   472篇
  1984年   346篇
  1983年   196篇
  1982年   370篇
  1981年   535篇
  1980年   484篇
  1979年   510篇
  1978年   399篇
  1977年   298篇
  1976年   256篇
  1975年   93篇
  1973年   176篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
A branched [8]catenane from an efficient one-pot synthesis (72 % HPLC yield, 59 % isolated yield) featuring the simultaneous use of three kinds of templates and cucurbit[6]uril-mediated azide–alkyne cycloaddition (CBAAC) for ring-closing is reported. Design and assembly of the [8]catenane precursors are unexpectedly complex that can involve cooperating, competing and non-influencing interactions. Due to the branched structure, dynamics of the [8]catenane can be modulated in different extent by rigidifying/loosening the mechanical bonds at different regions by using solvent polarity, acid-base and metal ions as the stimuli. This work not only highlights the importance of understanding the delicate interplay of the weak and non-obvious supramolecular interactions in the synthesis of high-order [n]catenane, but also demonstrates a complex control of dynamics and flexibility for exploiting [n]catenanes applications.  相似文献   
2.
We report the synthesis and optoelectronic properties of TIPS-peri-pentacenopentacene ( TIPS-PPP ), a vertical extension of TIPS-pentacene ( TIPS-PEN ) and a low-band-gap material with remarkable stability. We found the synthetic conditions to avoid the competition between 1,2- and 1,4-addition of lithium acetylide on the large aromatic dione. The high stability of TIPS-PPP is due to the peri-fusion which increases the aromaticity by generating two localized aromatic sextets that are flanked with 2 diene fragments, similar to two fused-anthracenes. Like TIPS-PEN , TIPS-PPP shows the archetypal 2D brickwall motif in crystals with a larger transfer integral and smaller reorganization energy. The high mobility of up to 1 cm2 V?1 s?1 was obtained in an organic field-effect transistor fabricated by a wet process. Also, TIPS-PPP was used as a near-infrared (NIR) emitter for NIR organic-light-emitting-diode devices resulting in a high external quantum efficiency at 800 nm.  相似文献   
3.
Dr Martin Seah, NPL, was the initiator, founder, and first chairman of the Surface Analysis Working Group (SAWG) at the Consultative Committee for Amount of Substance, Metrology in Chemistry and Biology (CCQM) at the Bureau International des Poids et Mesures (BIPM), the international organization established by the Metre Convention. This tribute letter summarizes his achievements during his chairmanship and his long-running impact on the successful work of the group after his retirement.  相似文献   
4.
Quantification of the composition of binary mixtures in secondary ion mass spectrometry (SIMS) is required in the analyses of technological materials from organic electronics to drug delivery systems. In some instances, it is found that there is a linear dependence between the composition, expressed as a ratio of component volumes, and the secondary ion intensities, expressed as a ratio of intensities of ions from each component. However, this ideal relationship fails in the presence of matrix effects and linearity is observed only over small compositional ranges, particularly in the dilute limits. In this paper, we assess an empirical method, which introduces a power law dependence between the intensity ratio and the volume fraction ratio. A previously published physical model of the organic matrix effect is employed to test the limits of the method and a mixed system of 3,3′-bis(9-carbazolyl) biphenyl and tris(2-phenylpyridinato)iridium (III) is used to demonstrate the method. This paper introduces a two-point calibration, which determines both the exponent in the power law and the sensitivity factor for the conversion of ion intensity ratio into volume fraction ratio. We demonstrate that this provides significantly improved accuracy, compared with a one-point calibration, over a wide compositional range in SIMS quantification and with a weak dependence on matrix effects. Because the method enables the use of clearly identifiable secondary ions for quantitative purposes and mitigates commonly observed matrix effects in organic materials, the two-point calibration method could be of significant benefit to SIMS analysts.  相似文献   
5.
Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN-based materials as well-known. GaN-based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 × 108 cm−2 for Sample B with a lattice-matched In0.17Al0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4-nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high-electron mobility transistors (HEMTs).  相似文献   
6.
To systematically evaluate the quality of SiNx films in multi-stacked structures, we investigated the effects of post-deposition annealing (PDA) on the film properties of SiNx within the SiO2/SiNx/SiO2/Si stacked structure by performing X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), Fourier transform infrared (FT-IR) spectroscopy, and scanning transmission electron microscope–electron energy loss spectroscopy (STEM-EELS) analyses. The XPS results showed that PDA induces the oxidation of the SiNx layer. In particular, new finding is that Si-rich SiNx in the SiNx layer is preferentially oxidized by PDA even in multi-stacked structure. The XRR results showed that the SiNx layer becomes thinner, whereas the interface layer between the SiNx layer and Si becomes thicker. It is concluded by STEM-EELS and XPS that this interface layer is SiON layer. The density of N–H and Si–H bonding within the stacked structure strongly depends on the PDA temperature. Our study helps elucidate the properties of SiNx films in stacked structures from various perspectives.  相似文献   
7.
Obtaining superhydrophobic surfaces for their application in electronics and flexible wearable devices remains a significant challenge. Most previously reported methods for obtaining superhydrophobic surfaces involve complex and expensive preparation techniques and thus cannot be used for practical applications. Ion-beam irradiation is a simple and promising method for fabricating superhydrophobic nanostructures on large areas at a low cost. Ion-beam irradiation using argon and oxygen gases was used to prepare silica nanorod structures on glass substrates. This study is not just a modification of the surface of nanoparticles, but a change in nanoparticle shape. The nanorods were subsequently treated with perfluorooctyltriethoxysilane to obtain superhydrophobicity. The surface of the silica nanorods exhibited a static water contact angle of 153°, indicating superhydrophobicity. The combination of rough structures of silica nanorods and low surface energy resulted in superhydrophobicity. The surface properties were evaluated in detail using Fourier-transform infrared spectroscopy, field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy. The proposed method is facile, inexpensive, and can be used for the large-scale production of nanorod structures for potential industrial applications.  相似文献   
8.
The detection of cancer biomarkers is of great significance for the early screening of cancer. Detecting the content of sarcosine in blood or urine has been considered to provide a basis for the diagnosis of prostate cancer. However, it still lacks simple, high-precision and wide-ranging sarcosine detection methods. In this work, a Ti3C2TX/Pt–Pd nanocomposite with high stability and excellent electrochemical performance has been synthesized by a facile one-step alcohol reduction and then used on a glassy carbon electrode (GCE) with sarcosine oxidase (SOx) to form a sarcosine biosensor (GCE/Ti3C2TX/Pt–Pd/SOx). The prominent electrocatalytic activity and biocompatibility of Ti3C2TX/Pt–Pd enable the SOx to be highly active and sensitive to sarcosine. Under the optimized conditions, the prepared biosensor has a wide linear detection range to sarcosine from 1 to 1000 µM with a low limit of detection of 0.16 µM (S/N = 3) and a sensitivity of 84.1 µA/mM cm2. Besides, the reliable response in serum samples shows its potential in the early diagnosis of prostate cancer. More importantly, the successful construction and application of the amperometric biosensor based on Ti3C2TX/Pt–Pd will provide a meaningful reference for detecting other cancer biomarkers.  相似文献   
9.
Messenger RNA (mRNA)-based vaccines are advantageous because they can be relatively quicker and more cost efficient to manufacture compared to other traditional vaccine products. Lipid nanoparticles have three common purposes: delivery, self-adjuvanting properties, and mRNA protection. Faster vaccine development requires an efficient and fast assay to monitor mRNA purity and integrity. Microchip CE is known to be a robust technology that is capable of rapid separation. Here, we describe the development and optimization of a purity and integrity assay for mRNA-based vaccines encapsulated in lipid nanoparticles using commercial microchip-based separation. The analytical parameters of the optimized assay were assessed and the method is a stability indicating assay.  相似文献   
10.
A numerical model was developed and validated to investigate the fluid–structure interactions between fully developed pipe flow and core–shell-structured microcapsule in a microchannel. Different flow rates and microcapsule shell thicknesses were considered. A sixth-order rotational symmetric distribution of von Mises stress over the microcapsule shell can be observed on the microcapsule with a thinner shell configuration, especially at higher flow rate conditions. It is also observed that when being carried along in a fully developed pipe flow, the microcapsule with a thinner shell tends to accumulate stress at a higher rate compared to that with a thicker shell. In general, for the same microcapsule configuration, higher flow velocity would induce a higher stress level over the microcapsule shell. The deformation gradient was used to capture the microcapsule's deformation in the present study. The effect of Young's modulus on the microcapsule shell on the microcapsule deformation was investigated as well. Our findings will shed light on the understanding of the stability of core–shell-structured microcapsule when subjected to flow-induced shear stress in a microfluidic system, enabling a more exquisite control over the breakup dynamics of drug-loaded microcapsule for biomedical applications.  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