全文获取类型
收费全文 | 1345篇 |
免费 | 49篇 |
国内免费 | 8篇 |
专业分类
化学 | 1065篇 |
晶体学 | 25篇 |
力学 | 5篇 |
数学 | 94篇 |
物理学 | 145篇 |
无线电 | 68篇 |
出版年
2023年 | 10篇 |
2022年 | 15篇 |
2021年 | 24篇 |
2020年 | 25篇 |
2019年 | 23篇 |
2018年 | 12篇 |
2017年 | 12篇 |
2016年 | 38篇 |
2015年 | 30篇 |
2014年 | 37篇 |
2013年 | 83篇 |
2012年 | 84篇 |
2011年 | 123篇 |
2010年 | 69篇 |
2009年 | 51篇 |
2008年 | 70篇 |
2007年 | 79篇 |
2006年 | 79篇 |
2005年 | 85篇 |
2004年 | 81篇 |
2003年 | 70篇 |
2002年 | 56篇 |
2001年 | 35篇 |
2000年 | 11篇 |
1999年 | 10篇 |
1998年 | 14篇 |
1997年 | 7篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1993年 | 5篇 |
1992年 | 12篇 |
1991年 | 7篇 |
1989年 | 10篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 13篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1968年 | 3篇 |
1935年 | 4篇 |
排序方式: 共有1402条查询结果,搜索用时 881 毫秒
121.
122.
123.
124.
We describe an application of category theory to the theory of inverse semigroups: we prove the P-theorem for E-unitary inverse semigroups using groupoids of fractions of their associated division categories. 相似文献
125.
Helen L. Ngo 《Tetrahedron letters》2005,46(4):595-597
Polar bisphosphonic acid-derived Ru(BINAP)(DPEN)Cl2 precatalysts were synthesized and immobilized in room temperature ionic liquids (RTILs) for asymmetric hydrogenation of aromatic ketones with ee values of up to 98.7%. The performance of the Ru catalysts is highly dependent on the nature of imidazolium ILs. For the imidazolium ILs without acidic protons, both ILs and Ru catalysts were recycled by simple extraction and reused. Such a simple immobilization approach also prevented the leaching of Ru (and Ru catalysts) into the chiral secondary alcohol products, and should prove desirable for the production of pharmaceutical intermediates that are free from metal contaminants. 相似文献
126.
Xin-Gui Tang Jie Wang Xiao-Xing Wang Helen Lai-Wah Chan 《Solid State Communications》2004,130(6):373-377
Antiferroelectric PbZrO3 thin films were grown on Pt/Ti/SiO2/Si substrates with predominant (111) orientation using a sol-gel process. The Pt/PbZrO3/Pt film capacitor showed well-saturated hysteresis loops at an applied voltage of 5 V with remanent polarisation (Pr) and coercive electric field (Ec) values of 8.97 μC/cm2 and 162 kV/cm, respectively. The leakage current density of the highly (111)-oriented PbZrO3 film was less than 1.0×10−7 A/cm2 over electric field ranges from 0 to 105 kV/cm. The conduction current depended on the voltage polarity. The PbZrO3/Pt interface forms a Schottky barrier at electric fields from 20 to 160 kV/cm. The dielectric relaxation current behaviour of Pt/PbZrO3/Pt capacitor obeys the well-known Curie-Von Schweidler law at electric field of 20-80 kV/cm, the currents have contributions of both dielectric relaxation current and leakage current. 相似文献
127.
A radiotracer technique has been used to measure both mercury self-diffusion and surface concentration values in bulk and
liquid phase epitaxy, LPE, grown Hg1−xCdxTe. A high resolution sectioning technique has allowed profiling of thin epitaxial layers in submicron steps. Hg1-xCdxTe samples with composition values betweenx
Cd= 0.16 and 0.23 were isothermally annealed in carefully controlled and monitored diffusion con-ditions. Mercury reservoirs
containing Hg203 were used to provide vapour diffusion sources during closed tube isothermal anneals in the temperature range 300° C to 400°
C. Evidence has been found which may indicate the presence of two components in the radio-tracer profiles for both bulk and
epitaxially grown material. In some cases it was possible to estimate two diffusion coefficients,D
1andD
2, from the near surface and deeply penetrating components, respectively. Our results forD
1andD
2are compared with other work. For bulk material annealed at 400° C under a saturated mercury pressureD
1= 2.0 x 10-12cm2s-1 andD
2= 1.1 x 10-11cm2s−1. Diffusion coefficients at 310° C under saturated mercury pressure, have been measured in bulk and epitaxial material. Close
agreement was found between these results with an average value ofD
1= 1.4 x 10−13cm2s−1. We believe this to be the first time radiotracer results for epitaxial material have been presented. We have collated diffusion
data, as a function of reciprocal temperature, from several workers and suggest there is evidence for a change in the activation
energy for mercury diffusion around 350° C. This may be due to a change in the dominant diffusion mechanism. 相似文献
128.
Transnasal flexible laryngoscopy (TFL) is an examination of laryngeal anatomy and physiology using continuous light. TFL is being used increasingly by voice pathologists in treatment but with little scientific evidence to support it. The purpose of this study was to evaluate the effectiveness and efficiency of TFL as a therapeutic tool. The study used a prospective randomized controlled trial. Fifty dysphonic subjects were recruited and randomly assigned to either a traditional treatment group or a TFL-assisted treatment group. The effectiveness of voice therapy in both treatment groups was measured with a package of voice outcome measures. Subjects in both treatment groups demonstrated statistically significant improvements after voice therapy (perceptual auditory rating of voice quality measurement p < 0.01; instrumental electroglottographic measurement p < 0.01; patient questionnaire measurement p < 0.01). The time taken to complete treatment in both groups was recorded. The average (median) time taken to complete voice therapy in the TFL-assisted treatment group was 2 hours less than in the traditional treatment group (p < 0.01). Voice therapy with TFL as a therapy tool was effective and more efficient than traditional voice therapy. 相似文献
129.
Marc P. Maillard Maria-Carmen Recio-Iglesias Mahamane Saadou Helen Stoeckli-Evans Kurt Hostettmann 《Helvetica chimica acta》1991,74(4):791-799
As the root bark CH2Cl2 extract of Bauhinia rufescens showed antifungal activity in a bioassay with the plant pathogenic fungus Cladosporium cucumerinum, a phytochemical investigation was undertaken on material collected in Niger. Activity-guided fractionation of this extract, using different preparative chromatographic methods, allowed the isolation of the four new antifungal tetracyclic compounds 1 – 4 . Their structures were established by 1H- and 13C-NMR data, and single-crystal X-ray analyses were used to confirm the structures of 1 and 3 . The isolated compounds seem to be biogenetically related to a stilbene. 相似文献
130.
Dr. Concha Bosch‐Navarro Zachary P. L. Laker Helen R. Thomas Alexander J. Marsden Dr. Jeremy Sloan Dr. Neil R. Wilson Dr. Jonathan P. Rourke 《Angewandte Chemie (International ed. in English)》2015,54(33):9560-9563
Atomic‐resolution transmission electron microscopy was used to identify individual Au9 clusters on a sulfur‐functionalized graphene surface. The clusters were preformed in solution and covalently attached to the surface without any dispersion or aggregation. Comparison of the experimental images with simulations allowed the rotational motion, without lateral displacement, of individual clusters to be discerned, thereby demonstrating a robust covalent attachment of intact clusters to the graphene surface. 相似文献