全文获取类型
收费全文 | 9887篇 |
免费 | 2578篇 |
国内免费 | 1011篇 |
专业分类
化学 | 7012篇 |
晶体学 | 190篇 |
力学 | 72篇 |
综合类 | 62篇 |
数学 | 43篇 |
物理学 | 6097篇 |
出版年
2024年 | 9篇 |
2023年 | 85篇 |
2022年 | 201篇 |
2021年 | 271篇 |
2020年 | 364篇 |
2019年 | 281篇 |
2018年 | 283篇 |
2017年 | 279篇 |
2016年 | 435篇 |
2015年 | 476篇 |
2014年 | 560篇 |
2013年 | 991篇 |
2012年 | 646篇 |
2011年 | 682篇 |
2010年 | 556篇 |
2009年 | 625篇 |
2008年 | 627篇 |
2007年 | 754篇 |
2006年 | 743篇 |
2005年 | 565篇 |
2004年 | 503篇 |
2003年 | 495篇 |
2002年 | 408篇 |
2001年 | 384篇 |
2000年 | 340篇 |
1999年 | 279篇 |
1998年 | 252篇 |
1997年 | 238篇 |
1996年 | 184篇 |
1995年 | 168篇 |
1994年 | 147篇 |
1993年 | 122篇 |
1992年 | 87篇 |
1991年 | 62篇 |
1990年 | 66篇 |
1989年 | 42篇 |
1988年 | 45篇 |
1987年 | 38篇 |
1986年 | 40篇 |
1985年 | 26篇 |
1984年 | 32篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 4篇 |
1978年 | 10篇 |
1975年 | 5篇 |
1973年 | 5篇 |
1970年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
991.
Historical textiles in the Topkap? Museum, which are called silk kaftans and brocades by art historians from the sixteenth and nineteenth were characterized by scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDX) and high-performance liquid chromatography with diode array detection for identification of metal threads and dyestuffs. In the most Ottoman textiles, metal threads, especially belonging to the sultans, were used as the gold gild. Although the chemical composition of the samples on the surface may easily be obtained by SEM–EDX, the thickness of the thin gold layer on metal threads cannot be obtained directly. Hence, the goal of this project is to describe whether metal threads are gilded or not and to measure coating thickness. A new method was developed for measuring the thickness of gold layer, and the modeling was reformed. The SEM–EDX results were interpreted in accordance with the theoretical models. The coating thickness of metal threads was subsequently measured in ancient textiles. The thickness, depth, and valuable metal composition in the threads of the artistic objects were obtained by this approach. Dye analysis was used to characterize the presence of indigotin, carminic acid, ellagic acid, and luteolin in the historical textiles. 相似文献
992.
993.
Ultrafine grain formation and coating mechanism arising from a blast coating process: A transmission electron microscopy analysis 下载免费PDF全文
Conor F. Dunne Kevin Roche Arne Janssen Xiangli Zhong M.G. Burke Barry Twomey Kenneth T. Stanton 《Surface and interface analysis : SIA》2017,49(12):1271-1278
This article examines the substrate/coating interface of a coating deposited onto mild steel and stainless steel substrates using an ambient temperature blast coating technique known as CoBlast. The process uses a coincident stream of an abrasive blast medium and coating medium particles to modify the substrate surface. The hypothesis for the high bond strength is that the abrasive medium roughens the surface while simultaneously disrupting the passivating oxide layer of the substrate, thereby exposing the reactive metal that then reacts with the coating medium. The aim of this study is to provide greater insight into the coating/substrate bonding mechanism by analysing the interface between a hydroxyapatite coating on both mild and stainless steel substrates. The coating adhesion was measured via a tensile test, and bond strengths of approximately 45 MPa were measured. The substrate/coating interface was examined using transmission electron microscopy and selected area diffraction. The analysis of the substrate/coating interface revealed the presence of ultrafine grains in both the coating and substrate at interface associated with deformation at the interface caused by particle impaction during deposition. The chemical reactivity resulting from the creation of these ultrafine grains is proposed to explain the high adhesive strength of CoBlast coatings. 相似文献
994.
Dynamic mechanical behaviour of natural rubber-silica composites was studied by a frequency sweep method at different temperatures (40 °C,70 °C and 100 °C) using a dynamic mechanical analyzer and a rotorless rheometer, RPA, in an attempt to establish a correlation between the two. The composites with silica content up to 40 phr were studied. It was found that the dependence of dynamic modulus on the frequency as obtained from both the instruments followed a similar trend. This suggests that the dynamic mechanical properties of rubber compounds can be determined even during curing. A correlation could be arrived at between the two sets of data, making it possible to predict one set knowing the other. The impact of silane coupling agent, bis (3-triethoxysilylpropyl tetrasulphide), TESPT, on viscoelasticity was also investigated. The mechanical properties were improved in the presence of TESPT. Additionally, an increase in thermal stability was also observed in the presence of TESPT. Scanning electron micrographs showed the better filler dispersion in the case of silane-coupled silica composites. 相似文献
995.
《Journal of computational chemistry》2017,38(32):2775-2783
A significant reduction in the computational effort for the evaluation of the electronic repulsion integrals (ERI) in ab initio quantum chemistry calculations is obtained by using Cholesky decomposition (CD), a numerical procedure that can remove the zero or small eigenvalues of the ERI positive (semi)definite matrix, while avoiding the calculation of the entire matrix. Conversely, due to its antisymmetric character, CD cannot be directly applied to the matrix representation of the spatial part of the two‐electron spin‐orbit coupling (2e‐SOC) integrals. Here, we present a computational strategy to achieve a Cholesky representation of the spatial part of the 2e‐SOC integrals, and propose a new efficient CD algorithm for both ERI and 2e‐SOC integrals. The proposed algorithm differs from previous CD implementations by the extensive use of a full‐pivoting design, which allows a univocal definition of the Cholesky basis, once the CD δ threshold is made explicit. We show that is the upper limit for the errors affecting the reconstructed 2e‐SOC integrals. The proposed strategy was implemented in the ab initio program Computational Emulator of Rare Earth Systems (CERES), and tested for computational performance on both the ERI and 2e‐SOC integrals evaluation. © 2017 Wiley Periodicals, Inc. 相似文献
996.
受电子束辐照的硬铝靶的喷射冲量耦合系数随能通量的变化规律 总被引:3,自引:0,他引:3
采用通光式传感法和直接测量特定时间间隔的探头原理,在闪光2号装置上进行了电子束辐照硬铝靶产生的喷射冲量耦合系数与能通量关系的研究。结果表明:在能通量为115~240J/cm2范围内,喷射冲量和亏损质量面密度都随能通量近似线性变化,而喷射冲量耦合系数随能通量增大而缓慢递增,并趋于一渐近值;硬铝产生喷射冲量的能通量阈值为40J/cm2左右;喷射冲量值为81~218Pas,亏损质量面密度为3.1 10-2~8.610-2g/cm2,喷射冲量耦合系数为0.70~0.98Pas/(J/cm 2)。 相似文献
997.
Yanli Zeng Xiaoyan Li Xueying Zhang Lingpeng Meng Prof. Dr. Shijun Zheng 《中国化学》2010,28(12):2345-2352
MP2/aug‐cc‐pVDZ calculations are carried out on the geometries, vibrational frequencies, interaction energies and topological properties for the π‐type halogen‐bonded complexes between propargyl radical and dihalogen molecules ClF, BrF and BrCl. There are two kinds of geometries: complex ( a ) involves the interaction between the X (X=Cl, Br) atom and the midpoint of C(1) –C(2) bond, complex ( b ) involves the interaction between the X atom and C(3) atom. The lengths of the halogen bond, the frequencies of the halogen bond, the elongation extent of the X–Y (XY=ClF, BrF, BrCl) bond, topological parameters at the BCPs of the halogen bond and X–Y bond are all well consistent with the interaction energies. The interaction of complex ( a ) is stronger than that of complex ( b ); the interaction of propargyl···BrF is stronger than that of propargyl···ClF and propargyl···BrCl. For the complexes ( a ) and ( b ), the charge transfer is observed from propargyl radical to XY, the atomic energy, the dipolar polarization, and the volume of the halogen atom X decrease upon complex formation. 相似文献
998.
999.
Heiko Rinderhagen Dr. Jochen Mattay Prof. Rafael Nussbaum Thomas Bally Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(24):7121-7124
In contrast to the structurally and configurationally stable alkyl‐ or aryl‐substituted cyclopropyl radical cations, cyclopropyl silyl ethers undergo spontaneous ring opening upon oxidation whereby the endocyclic C? C(O‐TMS) bond is cleaved with remarkable selectivity. DFT calculations on 1‐trimethylsilyloxybicyclo[4.1.0]heptane show that this selectivity arises from the topology of the potential surface of the corresponding radical cation which is initially generated in a very steep region of the potential surface from where the steepest descent leads to cleavage of the endocyclic rather than the lateral C? C(OTMS) bond. Cleavage of the lateral bond leads to interesting conformational changes which are explored in detail. 相似文献
1000.
Thomas Schwarze Wulfhard Mickler Dr. Carsten Dosche Dr. Roman Flehr Tillmann Klamroth Dr. Hans‐Gerd Löhmannsröben Prof. Dr. Peter Saalfrank Prof. Dr. Hans‐Jürgen Holdt Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(6):1819-1825
Fluoroionophores of fluorophore–spacer–receptor format were prepared for detection of PdCl2 by fluorescence enhancement. The fluorescent probes 1 – 13 consist of a fluorophore group, an alkyl spacer and a dithiomaleonitrile PdCl2 receptor. First, varying the length of the alkylene spacer (compounds 1 – 3 ) revealed a dominant through‐space pathway for oxidative photoinduced electron transfer (PET) in CH2‐bridged dithiomaleonitrile fluoroionophores. Second, fluorescent probes 4 – 9 containing two anthracene or pyrene fragments connected through CH2 bridges to the dithiomaleonitrile unit were synthesized. Modulation of the oxidation potential (EOx) through electron‐withdrawing or ‐donating groups on the anthracene moiety regulates the thermodynamic driving force for oxidative PET (ΔGPET) in bis(anthrylmethylthio)maleonitriles and therefore the fluorescence quantum yields (Φf), too. The new concept was confirmed and transferred to pyrenyl ligands, and fluorescence enhancements (FE) greater than 3.2 in the presence of PdCl2 were achieved by 7 and 8 (FE=5.4 and 5.2). Finally, for comparison, monofluorophore ligands 10 – 13 were synthesized. 相似文献