全文获取类型
收费全文 | 597篇 |
免费 | 153篇 |
国内免费 | 193篇 |
专业分类
化学 | 493篇 |
晶体学 | 61篇 |
力学 | 35篇 |
综合类 | 3篇 |
数学 | 2篇 |
物理学 | 349篇 |
出版年
2023年 | 6篇 |
2022年 | 22篇 |
2021年 | 21篇 |
2020年 | 28篇 |
2019年 | 22篇 |
2018年 | 23篇 |
2017年 | 27篇 |
2016年 | 31篇 |
2015年 | 31篇 |
2014年 | 30篇 |
2013年 | 63篇 |
2012年 | 50篇 |
2011年 | 47篇 |
2010年 | 57篇 |
2009年 | 54篇 |
2008年 | 58篇 |
2007年 | 62篇 |
2006年 | 56篇 |
2005年 | 22篇 |
2004年 | 43篇 |
2003年 | 34篇 |
2002年 | 11篇 |
2001年 | 11篇 |
2000年 | 17篇 |
1999年 | 14篇 |
1998年 | 16篇 |
1997年 | 14篇 |
1996年 | 13篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1983年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有943条查询结果,搜索用时 328 毫秒
31.
以MoO_3为前驱物,CH_4/H_2为碳源,采用程序升温直接还原碳化法制备不同碳化终温(640、660、680、700和720℃)的碳化钼催化剂,通过XRD、N_2吸附-脱附、SEM、TEM、XPS和Raman表征研究碳化钼的物理性质和结构性质,并研究不同碳化终温碳化钼对喹啉加氢脱氮的催化性能。结果表明,不同碳化终温的碳化钼催化剂均为β-Mo_2C,碳化终温可显著改变碳化钼表面物种含量、平均孔径和介孔分布。碳化终温为680℃时,催化剂碳化程度较高,表面氧物种含量最低,表面C/Mo物质的量比最高,对应的催化活性也最佳,在340℃、4 MPa条件下,喹啉的转化率和脱氮率均高达99%以上,芳香族类化合物的选择性可达37.8%,显示出较低的芳环破坏性。表面组成尤其是表面氧对于β-Mo_2C上喹啉加氢脱氮反应途径的调控至关重要。 相似文献
32.
Growth of Single-Walled Carbon Nanotubes with Controlled Structure: Floating Carbide Solid Catalysts
Dr. Liu Qian Ying Xie Yue Yu Dr. Shanshan Wang Dr. Shuchen Zhang Prof. Jin Zhang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(27):10976-10979
Single-walled carbon nanotube (SWNT) horizontal arrays with specific chirality can be enriched using solid carbide catalysts on substrates. However, scale-up production by continuous loading of the solid catalysts onto the substrates is challenging. Described here is the preparation of a floating carbide solid catalyst (FSC) for the controlled growth of SWNTs. The FSC, titanium carbide (TiC) nanoparticle, was directly obtained in the carrier gas phase by decomposition and carbonization of the titanocene dichloride precursor at high temperature. By using the TiC nanoparticle FSC, both SWNT horizontal arrays and randomly distributed networks can be obtained. The chirality of the as-grown SWNTs were thermodynamically controlled to have fourfold symmetry. Further optimization of growth condition resulted in an abundance of (16,8) tubes with about a 74 % content. This FSC chemical vapor deposition (FSCCVD) method has potential for realizing mass growth of SWNTs with controlled structures. 相似文献
33.
Research on high-voltage 4H-SiC P-i-N diode with planar edge junction termination techniques 下载免费PDF全文
The planar edge termination techniques of junction termination extension (JTE) and offset field plates and field-limiting rings for the 4H-SiC P-i-N diode were investigated and optimized by using a two-dimensional device simulator ISE-TCAD10.0. By experimental verification, a good consistency between simulation and experiment can be observed. The results show that the reverse breakdown voltage for the 4H-SiC P-i-N diode with optimized JTE edge termination can accomplish near ideal breakdown voltage and much lower leakage current. The breakdown voltage can be near 1650 V, which achieves more than 90 percent of ideal parallel plane junction breakdown voltage and the leakage current density can be near 3 × 10-5 A/cm2. 相似文献
34.
Yung-I Chen Bo-Lu LinYu-Chu Kuo Jen-Ching Huang Li-Chun Chang Yu-Ting Lin 《Applied Surface Science》2011,257(15):6741-6749
To prevent Co diffusion from cemented carbides at high temperatures, we fabricated TaNx coatings by reactive direct current (d.c.) magnetron sputtering onto 6 wt.% cobalt cemented carbide substrates, to form diffusion barrier layers. Varying the nitrogen flow ratio, N2/(Ar + N2), from 0.05 to 0.4 during the sputtering process had a significant effect on coating structure and content. Deposition rate reduced as the nitrogen flow ratio increased. The effects of nitrogen flow ratio on the crystalline characteristics of the TaNx coatings were examined by X-ray diffraction. The TaNx coatings annealing conditions were 500, 600, 700, and 800 °C for 4 h in air. We evaluated the performance of the diffusion barrier using both Auger electron spectroscopy depth-profiles and X-ray diffraction techniques. We also investigated oxidation resistance of the TaNx coatings annealed in air, and under a 50 ppm O2-N2 atmosphere, to evaluate the fabricated layers effectiveness as a protective coating for glass molding dies. 相似文献
35.
36.
The doped boron (B) atom in silicon carbide nanotube (SiCNT) can substitute carbon or silicon atom, forming two different structures. The transport properties of both B-doped SiCNT structures are investigated by the method combined non-equilibrium Green’s function with density functional theory (DFT). As the bias ranging from 0.8 to 1.0 V, the negative differential resistance (NDR) effect occurs, which is derived from the great difficulty for electrons tunneling from one electrode to another with the increasing of localization of molecular orbital. The high similar transport properties of both B-doped SiCNT indicate that boron is a suitable impurity for fabricating nano-scale SiCNT electronic devices. 相似文献
37.
In this paper, the viability of using a genetic algorithm to find band structure parameters for empirical pseudopotential method (EPM) calculations is demonstrated by applying a genetic algorithm to find the EPM parameters for 4H-SiC. The form of the pseudopotential for 4H-SiC and the 19 form factors found by the genetic algorithm to fit the band structure to experimentally measured indirect energy gap and direct optical gaps are given. In addition, the effective masses for the conduction band minimum are extracted from the calculated band structure. It is shown that the genetic algorithm provides an effective, automated way to find parameters that give reasonably good fits to both the band gaps and the effective masses simultaneously. 相似文献
38.
A. Bouifoulen M. EdelyA. Kassiba M. Makowska-JanusikA. Outzourhit J. SzadeA. Oueriagli 《Physica B: Condensed Matter》2011,406(23):4500-4504
Thin films based on silicon carbide and alumina were synthesized by means of rf-sputtering using a co-deposition process. Several nanostructures were created which consist of thin films (∼200 nm thick) with homogeneous distribution of SiC nanocrystals (∼5 nm mean diameter) in the host alumina matrices. Characterization methods including X-ray photoelectrons spectroscopy (XPS), UV-vis absorption and photoluminescence (PL) were used to identify the involved structures, compositions and optical features of these nanostructures. Thus, XPS investigations were relevant to point out the involved chemical bonding in the core SiC nanocrystals and in the host alumina environments. Additionally, mixed bonding such as Si-O-C was also shown and seems to correlate with the SiC-alumina interfaces. Optical properties of the nanostructures films such as UV-vis absorption and photoluminescence (PL) were measured in representative samples and compared to simulated PL responses obtained by a theoretical model. 相似文献
39.
刘磊 《中国光学与应用光学文摘》2011,(6)
为了满足高精度相机在外场环境下的检测要求,采用碳化硅光学材料制作反射镜,碳纤/环氧树脂基复合材料制作遮光筒,设计了一套重量轻、自身精度高、温度稳定性好的离轴平行光管。在二者线胀系数保持二倍关系的情况下,在一定温变范围内保持精度的稳定性。经检测,口径为400 mm,焦距为8 m的离轴平行光管的温变为(20±10)℃,系统波像差为1/5λ(P-V值,λ=632.8 nm)和1/27λ(RMS值),达到了设计要求,能够在外场环境下使用。 相似文献
40.
Zi‐Yu Li Zhen Yuan Dr. Yan‐Xia Zhao Prof. Dr. Sheng‐Gui He 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(14):4163-4169
Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas‐phase and condensed‐phase studies. Herein, methane activation by the diatomic cation MoC+ is presented. MoC+ ions have been prepared and mass‐selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time‐of‐flight mass spectrometer. Bare metal Mo+ and MoC2H2+ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C2H4 elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC+ with CH4. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the C?C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC+ plays a key role in methane activation because it becomes sp3 hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the C?H bond activation and C?C coupling involved in methane transformation over molybdenum carbide‐based catalysts. 相似文献