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排序方式: 共有142条查询结果,搜索用时 31 毫秒
21.
Peng-Peng Wang Sanboh Lee Julie P. Harmon 《Journal of Polymer Science.Polymer Physics》1994,32(7):1217-1227
The crack healing induced by ethanol in poly(methyl methacrylate) (PMMA) has been studied at temperatures of 40–60°C. Crack healing occurs because the effective glass transition temperature of PMMA is reduced to below the test temperature by ethanol plasticization. It is found that crack closure rate is constant at a given temperature. The fracture strength of healed PMMA is lower than that of the original samples. By comparing the fracture stress with the morphology of the crack edge on the PMMA surface, we found that a high degree of swelling is responsible for the incomplete recovery of mechanical strength. The fractography of the completely healed sample shows a very different fracture morphology from that of virgin PMMA. The transport of ethanol in PMMA also is studied. At lower temperatures, transport is described by ideal Case II behavior. As the temperature increases, the kinetics shift from ideal Case II to anomalous behavior. The first stage of crack healing is controlled by Case I transport. © 1994 John Wiley & Sons, Inc. 相似文献
22.
Mohomed K Abourahma H Zaworotko MJ Harmon JP 《Chemical communications (Cambridge, England)》2005,(26):3277-3279
The incorporation of self-assembled nanoparticles, a.k.a. hydroxylated nanoballs, into poly(2-hydroxyethyl methacrylate)(PHEMA) gives rise to a cross-linked network/hydrogel with enhanced interfacial interaction, whereas its inclusion in poly(methyl methacrylate)(PMMA) results in plasticization. 相似文献
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By identifying the key characteristic "structural scales" that dictate the resistance of a porous metallic glass against buckling and fracture, stochastic highly porous metallic-glass structures are designed capable of yielding plastically and inheriting the high plastic yield strength of the amorphous metal. The strengths attainable by the present foams appear to equal or exceed those by highly engineered metal foams such as Ti-6Al-4V or ferrous-metal foams at comparable levels of porosity, placing the present metallic-glass foams among the strongest foams known to date. 相似文献
25.
Hongmin Wang Shuting Fu Bo Shang Sungho Jeon Yiren Zhong Nia J. Harmon Chungseok Choi Eric A. Stach Hailiang Wang 《Angewandte Chemie (International ed. in English)》2023,62(30):e202305251
Photothermal CO2 reduction is one of the most promising routes to efficiently utilize solar energy for fuel production at high rates. However, this reaction is currently limited by underdeveloped catalysts with low photothermal conversion efficiency, insufficient exposure of active sites, low active material loading, and high material cost. Herein, we report a potassium-modified carbon-supported cobalt (K+−Co−C) catalyst mimicking the structure of a lotus pod that addresses these challenges. As a result of the designed lotus-pod structure which features an efficient photothermal C substrate with hierarchical pores, an intimate Co/C interface with covalent bonding, and exposed Co catalytic sites with optimized CO binding strength, the K+−Co−C catalyst shows a record-high photothermal CO2 hydrogenation rate of 758 mmol gcat−1 h−1 (2871 mmol gCo−1 h−1) with a 99.8 % selectivity for CO, three orders of magnitude higher than typical photochemical CO2 reduction reactions. We further demonstrate with this catalyst effective CO2 conversion under natural sunlight one hour before sunset during the winter season, putting forward an important step towards practical solar fuel production. 相似文献
26.
Shayla K. Emran George R. Newkome Claus D. Weis Julie P. Harmon 《Journal of Polymer Science.Polymer Physics》1999,37(16):2025-2038
This study utilized Matrix Assisted Laser Desorption/Ionization Time‐of‐Flight Mass Spectrometry, Thermogravimetric Analysis, Differential Scanning Calorimetry, X‐Ray Diffraction, and Dielectric Analysis to assess the viscoelastic and structural properties of three generations of tert‐butyl and methyl ester, amide‐based dendrimers. The effect of generation number and functionality on glass‐transition temperatures and corresponding apparent activation energies, obtained via adherence to WLF behavior, were determined. Both were found to increase with increasing generation number and bulkiness of terminal functionalities. WLF constants, C1 and C2, allowed the determination of free volume, and thermal expansion coefficients, respectively. Secondary transitions, conforming to Arrhenius behavior, were also characterized and increased in temperature with generation number. The apparent activation energy was greater when the matrix was crystalline. Dielectric relaxation responses were analyzed to yield dielectric strengths of the molecular relaxations which increased with generation number and were comparable for both tert‐butyl and methyl esters in the glass‐transition region. Electrical properties of the dendrimers were dominated by ionic conductivity in the high temperature region. In order to unmask the glass transition, the data were treated in terms of the electric modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2025–2038, 1999 相似文献
27.
Chang Liu Takeshi Kondo A.D. Palczewski G.D. Samolyuk Y. Lee M.E. Tillman Ni Ni E.D. Mun R. Gordon A.F. Santander-Syro S.L. Bud’ko J.L. McChesney E. Rotenberg A.V. Fedorov T. Valla O. Copie M.A. Tanatar C. Martin B.N. Harmon P.C. Canfield A. Kaminski 《Physica C: Superconductivity and its Applications》2009,469(9-12):491-497
We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFeAsO0.9F0.1 (R1111) and two layer Ba1?xKxFe2As2 (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dispersion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering significantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at Γ and electron pockets located in zone corners. The size and shape of the Fermi surface changes significantly with doping. Emergence of a coherent peak below the critical temperature Tc and diminished spectral weight at the chemical potential above Tc closely resembles the spectral characteristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of d-wave order parameter. Instead it points to s-wave or extended s-wave symmetry of the order parameter. 相似文献
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Pandey A Dhaka RS Lamsal J Lee Y Anand VK Kreyssig A Heitmann TW McQueeney RJ Goldman AI Harmon BN Kaminski A Johnston DC 《Physical review letters》2012,108(8):087005
The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature T(N)=625 K and a large ordered moment μ=3.9μ(B)/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K while retaining the same crystal and antiferromagnetic structures together with nearly the same high T(N) and large μ. Ba(1-x)K(x)Mn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local-moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba(1-x)K(x)Mn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high T(c) superconductivity. 相似文献
30.
N,N-Dimethyl-1-adamantamine-hydrogen fluoride (2MAAHF) does not dissociate in aqueous solution, while 2MAAHCl and 2MAAHBr readily ionize in H2O. Semi-empirical (AM1-SM2) calculations demonstrate that the enthalpic barrier to dissociation of 2MAAHF in aqueous solution is four times as high as that of 2MAAHCl and three times as high as that of 2MAAHBr. Ab initio calculations (3-21G(*) and 3-61G*) show that 2MAAHF is a tightly bound molecular complex between neutral HF and neutral amine, with a low dipole moment and little negative charge exposed on fluorine. 2MAAHCl and 2MAAHBr are ion-pairs with an electrostatic hydrogen bond between cation and anion, large dipole moments, and exposed halogen atoms with high electrostatic potential; these factors explain the ease of ionization of 2MAAHCl and 2MAAHBr. Parallel studies on other tertiary amines give similar results. 相似文献