| 1. |
- Wei, Kun, et al.
(författare)
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The Intergranular Microstructure of Magnesium Based Die-Cast AE Alloys
- 2009
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Ingår i: Magnesium Technology 2009. - : The Minerals, Metals & Materials Society (TMS). - 9780873397308 ; , s. 261-266
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Konferensbidrag (refereegranskat)abstract
- A systematic investigation on microstructures of magnesium based die-cast Mg-Al-RE alloys (AE alloys) has been performed by XRD, SEM and TEM. The alloys are with content of Al around 4wt% and various content of rare earth (RE). Samples of these alloys are in as-cast condition as well as in aged condition at 200°C and 250°C. The intergranular microstructure of the alloys has been studied in details. Three type binary Al-RE phases were found intergranularly. Al11RE3 is predominant intergranular phase in the as cast alloys. Al3RE particles and small amount of Al2RE phase were found in alloys with high RE content. Thermal stability of the Al-RE phases in AE alloys was suggested to decrease in sequence: Al2RE→ Al11RE3 →Al3RE. The Al/RE ratio of the die cast alloys determined their phase constitutions. Promising AE alloys for creep resistance is suggested to have an Al/RE ratio not higher than 1.8.
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| 2. |
- Wei, Liu-Ying, et al.
(författare)
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A TEM study of microstructural change in Mg alloys during creep
- 2007
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Ingår i: 8th Multinational Congress on Microscopy.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- M5-10 A TEM Study of Microstructural Change in Mg Alloys during Creep Wei L.-Y. Wei K. and Warren R. Division of Materials Science, School of Technology and Society Malmö University, 205 06 Malmö Sweden liu-ying.wei@ts.mah.se Due to their low densities and a good combination of castability and different properties, magnesium alloys have been attractive materials for automotive industry for weight saving. Die-cast AZ91 is one of the most widely used alloys. It exhibits excellent die castability, good strength properties and a fair ductility but its mechanical properties, especially the creep resistance, decrease on exposure at temperature above 120ºC [1]. Softening and coarsening of the interdendritic phase β-Al12Mg17 in AZ91 can readily occur at elevated temperatures due to its low melting temperature (Tm = 437°C) [2]. Rare earth in magnesium alloys usually brings about alloy strengthening through the formation of thermally stable intergranular phases and the complete suppression of the β-Al12Mg17 phase formation. In the present work tensile creep behavior of the die-cast magnesium alloys AZ91 and AE42 and their microstructural change during plastic deformation in creep have been studied. The objective of the work is to study the active creep mechanisms on the basis of the creep data in combination with the microstructural investigations. The SEM micrographs in Fig. 1 (a-b) illustrate the as-cast microstructures of the two die cast alloys showing a typical dendritic solidification structure with a fine-scale grain size. Segregation of Al adjacent to the dendrite boundaries can be seen clearly in AZ91D. Phases with a bright contrast and surrounded by high Al-segregated eutectic α-Mg are β-Al12Mg17 phase. Segregation of Al seems not obvious in AE42. The dominant interdendritic phase is Al11RE3. Mn-containing phases also formed during solidification in both AZ91 and AE42. The TEM micrographs in Fig. 2 (a-c) illustrate the structural changes during creep in AZ91 and AE42. Fig. 2 (a) shows the formation of denuded zones around the dendrite boundaries and the formation of discontinuous precipitates Al12Mg17. TEM/XEDS revealed that the Al content in the denuded zones decreased after creep test, from the original 5.5% to 2.5% (wt%). Fig. 2 (b-c) illustrates the formation of dislocation sources in the (0001) basal planes under a relatively high stress and the movement of dislocations from the sources towards the dendrite boundaries. The existence of the supersaturated Al-segregated zones adjacent to the dendrite boundaries is the main reason for the recovery of AZ91 at operating temperatures above 100ºC. Under loading plastic deformation occurred by the diffusion of Al atoms along the boundaries from compressive to tensile grain boundaries that caused the formation of the denuded grain boundaries and as a consequence the formation of the discontinuous precipitates Al12Mg17. Dislocation climb process also operated and tended to become a dominant process at stress above 60MPa.The low level of Al in AE42 suppresses the formation of Al supersaturated zones adjacent to the dendrite boundaries that, combining with the formation of the thermally stable interdendritic phase Al11RE3 (Tm~640ºC), provides AE42 a much better creep performance. Diffusion accompanying with the dislocation climb also occurred and became more dominant at a relatively high stress. Figure 1 SEM micrographs of the die cast alloys (a) AZ91D; (b) AE42. Fig. 2 TEM micrographs of the crept samples, (a-b) AZ91 after creep test at 100ºC/100MPa (n=3), (c) AE42 after creep at 175ºC/60MPa (n=2-7). References: 1. I.J. Polmer, 2nd Mg conference, DGM Informationsgesellschaft, Germany (1992), 201 2. M.S. Dargush, G.L. Dunlop, K. Pettersen, in: W. Higgins (Ed.), Trans. of 19th Intern. Die Casting Cong., North American Die Casting Association, Rosemont, IL, 1997, 131–137. Acknowledgements: Norsk Hydro ASA is gratefully acknowledged for providing the test bars
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| 3. |
- Wei, Liu-Ying, et al.
(författare)
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Creep behaviour and microstructure of magnesium die cast alloys AZ91 and AE42
- 2007
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Ingår i: Materials Science Forum. - : Trans Tech Publications. ; 546-549, s. 73-76
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Tidskriftsartikel (refereegranskat)abstract
- Tensile creep behaviour of the die-cast magnesium alloys AZ91 and AE42 has been studied at temperatures between 85oC and 200oC and at stresses in the range from 30 to 100 MPa. Microstructural investigations, mainly by TEM, have been performed on the selected crept samples to characterise the microstructural change during the plastic deformation, which reveals several phenomena related to the creep process including formation of dislocation sources, denuded zones around grain boundaries and microvoids, and changes in the nature of intermetallic phases. The active creep mechanisms have been discussed on the basis of the creep data in combination with the microstructural change during creep.
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| 4. |
- Adare, A., et al.
(författare)
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Gluon-Spin Contribution to the Proton Spin from the Double-Helicity Asymmetry in Inclusive pi(0) Production in Polarized p plus p Collisions at root s=200 GeV
- 2009
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Ingår i: Physical Review Letters. - 1079-7114. ; 103:1
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Tidskriftsartikel (refereegranskat)abstract
- The double helicity asymmetry in neutral pion production for p(T) = 1 to 12 GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, Delta G, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of mu 2 = 4 GeV2 a next to leading order QCD analysis gives Delta G([0.02,0.3]) = 0.2, with a constraint of -0.7 < Delta G([0.02,0.3]) < 0.5 at Delta chi(2) = 9 (similar to 3 sigma) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the Delta G constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.
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| 5. |
- Adare, A., et al.
(författare)
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Inclusive cross section and double helicity asymmetry for pi(0) production in p plus p collisions at root s=62.4 GeV
- 2009
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Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - 1550-2368. ; 79:1
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Tidskriftsartikel (refereegranskat)abstract
- The PHENIX experiment presents results from the RHIC 2006 run with polarized p + p collisions at root s = 62.4 GeV, for inclusive pi(0) production at midrapidity. Unpolarized cross section results are measured for transverse momenta p(T) = 0.5 to 7 GeV/c. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries A(LL) are presented for p(T) = 1 to 4 GeV/c and probe the higher range of Bjorken x of the gluon (x(g)) with better statistical precision than our previous measurements at root s = 200 GeV. These measurements are sensitive to the gluon polarization in the proton for 0.06 < x(g) < 0.4.
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| 6. |
- Adare, A., et al.
(författare)
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Measurement of Bottom Versus Charm as a Function of Transverse Momentum with Electron-Hadron Correlations in p plus p Collisions at s=200 GeV
- 2009
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Ingår i: Physical Review Letters. - 1079-7114. ; 103:8
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Tidskriftsartikel (refereegranskat)abstract
- The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|< 0.35 in p+p collisions at s=200 GeV is measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range 2 < p(T)< 7 GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D -> e(+/-)K(-/+)X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4 GeV/c in p(T). A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is sigma(bb)=3.2(-1.1)(+1.2)(stat)(-1.3)(+1.4)(syst)mu b.
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| 7. |
- Adare, A., et al.
(författare)
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Onset of pi(0) Suppression Studied in Cu plus Cu Collisions at root s(NN)=22.4, 62.4, and 200 GeV
- 2008
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Ingår i: Physical Review Letters. - 1079-7114. ; 101:16
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Tidskriftsartikel (refereegranskat)abstract
- Neutral pion transverse momentum (p(T)) spectra at midrapidity (|y| less than or similar to 0.35) were measured in Cu + Cu collisions at root s(NN) = 22.4, 62.4, and 200 GeV. Relative to pi(0) yields in p + p collisions scaled by the number of inelastic nucleon-nucleon collisions (N-coll) the pi(0) yields for p(T) greater than or similar to 2 GeV/c in central Cu + Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu + Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.
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| 8. |
- Adare, A., et al.
(författare)
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Photon-hadron jet correlations in p plus p and Au plus Au collisions at s(NN)=200 GeV
- 2009
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Ingår i: Physical Review C (Nuclear Physics). - 0556-2813. ; 80:2
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Tidskriftsartikel (refereegranskat)abstract
- We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (gamma-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I-AA, in central Au+Au collisions, is 0.32 +/- 0.12(stat)+/- 0.09(syst) for hadrons of 3 < p(T)(h)< 5 in coincidence with photons of 5 < p(T)(gamma)< 15 GeV/c. The suppression is comparable to that observed for high-p(T) single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z(T)equivalent to p(T)(h)/p(T)(gamma), as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.
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| 9. |
- Adnan, Safdar, et al.
(författare)
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Microstructures of electron beam melted (EBM) biomaterial Ti-6Al-4V
- 2009
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Ingår i: Mechanics of Biological and Biomedical Materials;1132-Z09-06. - : The Materials Research Society (MRS).
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Konferensbidrag (refereegranskat)abstract
- Ti-6Al-4V alloy is an attractive biomaterial. The current work evaluates the microstructures of the solid and net-shape Ti-6Al-4V alloy produced by Electron Beam Melting (EBM) system using SEM/EDX and optical microscope. The microstructures are influenced by the cooling rate, processing parameters of the EBM system and re-heating of the existing layer during the melting of subsequent layers. Layer structure and columnar grains have been observed, with growing direction parallel to the built direction. The interior of these grains consists of alternating α / β phases. The β phase in the colonies resembles rod shape embedded in the α platelet. Along the grain boundaries more or less continuous α layers were observed. In comparison to solid samples uneven surfaces and pores were seen in the net shape structure. Microhardness evaluation of the EBM produced alloys was also carried out and compared with conventionally produced alloys.
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| 10. |
- Fu, Ying, et al.
(författare)
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Energy band structure and spectral gain characteristics of dilute-nitride zinc blende InGaNAs quantum wells embedded in GaAs and GaNAs barriers
- 2006
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 100:7
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Tidskriftsartikel (refereegranskat)abstract
- The spectral gain characteristics of dilute-nitride zinc blende InxGa1-xNyAs1-y quantum wells embedded in GaNy1As1-y1 barriers have been investigated experimentally and theoretically. Two samples, both with the gain peak at 1300 nm, were studied for comparison. One has a high nitrogen concentration in the quantum well with the surrounding barriers being pure GaAs. The other has a lower and uniform nitrogen concentration in the quantum well and the barriers (GaNAs barriers). Measurements show the redshift of the gain peak induced by the incorporation of nitrogen and difference in the spectral gain characteristics. The energy band structures and spectral gain characteristics are analyzed theoretically using the standard eight-band k center dot p theory. It is shown that the introduction of nitrogen atoms in the GaAs barriers reduces the barrier height for the central quantum well so that the energy sublevels in the conduction band becomes condensed. The condensation of the conduction-band energy sublevels reduces the peak gain and makes the gain spectrum narrower, in agreement with measurements.
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| 11. |
- Gao, Xin-Fen, et al.
(författare)
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Three new species of Corydalis (Fumariaceae) from northwestern Sichuan China
- 2008
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Ingår i: Novon. - : Missouri Botanical Garden Press. - 1055-3177 .- 1945-6174. ; 18:3, s. 330-335
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Tidskriftsartikel (refereegranskat)abstract
- The jiuding Ridge (Sichuan, China) has been explored during the past several years by a team from chengdu Institute of Biology. Chinese Academy of Sciences, and three undescribed species of Corydalis DC. (Fumariaceae) have been revealed and are apparently endemic to this mountain. The new species were observed in forest (C. capitata X.F. Cao, Liden & Y.W. Wang), and alpine rocky limestone areas (C. schistostigma X.F. Cao, Liden, Y.W. Wang & Y.L. Peng). The two first species have their closest relatives in the Wolong-Balangshan District in the Wolong-Balangshan District (Wenchuan. Sichuan). Corydalis aeaeae differs from C. panada Liden & Y.W. Wang in its small size, few-flowered racemes, and broadly obtuse outer petals with low, short erests. Corydalis capitata differs from the C. flexuosa Franchet complex in the capitate racemes, small flowers with deeply serrate petals, and the peculiar, thin, strictly ecect lateral branches. corydalis schistostinma is unique in the C. curviflora Maximowicz ex Hemsley group, in the deeply cleft stigma and very forward-projecting crests to the inner petals, and possibly has iits affinities more to the north and northwest in the Hengduan Mountains.
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| 12. |
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| 13. |
- Liu, Haibin, et al.
(författare)
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Influence of substituents in the salicylaldehyde-derived Schiff bases on vanadium-catalyzed asymmetric oxidation of sulfides
- 2008
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Ingår i: Applied organometallic chemistry. - : Wiley. - 0268-2605 .- 1099-0739. ; 22:5, s. 253-257
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Tidskriftsartikel (refereegranskat)abstract
- A series of chiral Schiff bases (L-1-L-5) with different substituents in the salicylidenyl unit were prepared from condensation of 3-aryl-5-tert-butylsalicylaldehyde derivatives and optically active amino alcohols. Bromination of 3-phenyl-5-tert-butylsalicylaldehyde gave an unexpected product 3-(4-bromophenyl)-5-bromosalicylaldehyde, from which the corresponding Schiff base ligands L-6 and L-7, derived from (S)-valinol and (S)-tert-leucinol, respectively, were prepared. Ligands L-1-L-7 were applied to the vanadium-catalyzed asymmetric oxidation of aryl methyl sulfides. Under the optimal conditions, the oxidation of the thioanisole with H2O2 as oxidant in CH2Cl2 catalyzed by VO(acac)(2)-L-1-L-7 gives good yields (74-83%) with moderate enantioselectivity (58-77% ee). Ligand L-7, containing a 4-bromophenyl group on the 3-position and a Br atom on the 5-position of the salicylidenyl moiety, displays an 80-90% ee for vanadium-catalyzed oxidation of methyl 4-bromophenyl sulfide and methyl 2-naphthyl sulfide.
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| 14. |
- Liu, Jin-ming, et al.
(författare)
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Mid-term effects of lung volume reduction surgery on pulmonary function in patients with chronic obstructive pulmonary disease
- 2007
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Ingår i: Chinese Medical Journal. - 0366-6999. ; 120:8, s. 658-662
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Tidskriftsartikel (refereegranskat)abstract
- Background Now lung volume reduction surgery (LVRS) has become one of the most effective methods for the management of some cases of severe chronic obstructive pulmonary disease (COPD). We evaluated the mid-term effects of LVRS on pulmonary function in patients with severe COPD. Methods Ten male patients with severe COPD aged 38-70 years underwent LVRS and their pulmonary function was assessed before, 3 months and 3 years after surgery. The spirometric and gas exchange parameters included residual volume, total lung capacity, inspiratory capacity, forced vital capacity, forced expiratory volume in one second, diffusion capacity for CO, and arterial blood gas. A 6-minute walk distance (6MWD) test was performed. Results As to preoperative assessment, most spirometric parameters and 6MWD were significantly improved after 3 months and slightly 3 years after LVRS. Gas exchange parameters were significantly improved 3 months after surgery, but returned to the preoperative levels after 3 years. Conclusions LVRS may significantly improve pulmonary function in patients with severe COPD indicating for LVRS. Mid-term pulmonary function 3 years after surgery can be decreased to the level at 3 months after surgery. Three years after LVRS, lung volume and pulmonary ventilation function can be significantly improved, but the improvement in gas exchange function was not significant.
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| 15. |
- Moore, Lynette M, et al.
(författare)
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IGFBP2 is a candidate biomarker for Ink4a-Arf status and a therapeutic target for high-grade gliomas
- 2009
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 106:39, s. 16675-16679
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Tidskriftsartikel (refereegranskat)abstract
- The levels of insulin-like growth factor-binding protein 2 (IGFBP2) are elevated during progression of many human cancers. By using a glial-specific transgenic mouse system (RCAS/Ntv-a), we reported previously that IGFBP2 is an oncogenic factor for glioma progression in combination with platelet-derived growth factor-beta (PDGFB). Because the INK4a-ARF locus is often deleted in high-grade gliomas (anaplastic oligodendroglioma and glioblastoma), we investigated the effect of the Ink4a-Arf-null background on IGFBP2-mediated progression of PDGFB-initiated oligodendroglioma. We demonstrate here that homozygous deletion of Ink4a-Arf bypasses the requirement of exogenously introduced IGFBP2 for glioma progression. Instead, absence of Ink4a-Arf resulted in elevated endogenous tumor cell IGFBP2. An inverse relationship between p16(INK4a) and IGFBP2 expression was also observed in human glioma tissue samples and in 90 different cancer cell lines by using Western blotting and reverse-phase protein lysate arrays. When endogenous IGFBP2 expression was attenuated by an RCAS vector expressing antisense IGFBP2 in our mouse model, a decreased incidence of anaplastic oligodendroglioma as well as prolonged survival was observed. Thus, p16(INK4a) is a negative regulator of the IGFBP2 oncogene. Loss of Ink4a-Arf results in increased IGFBP2, which contributes to glioma progression, thereby implicating IGFBP2 as a marker and potential therapeutic target for Ink4a-Arf-deleted gliomas.
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| 16. |
- Safdar, Adnan, et al.
(författare)
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Effect of EBM processing parameters on surface roughness and microstructure of Ti-6Al-4V
- 2009
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Ingår i: Proceeding 22nd European Conference for Biomaterials. - : European Society for Biomaterials.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Titanium and its alloys especially Ti-6Al-4V is an attractive biomaterial due to their excellent biocompatibility. Electron Beam Melting (EBM) is one of the Solid Free Form Fabrication (SFFF) methods to build 3D solid and near-net shape objects for medical implants and aerospace industry. EBM system utilizes a high energy electron beam to selectively melt a powder layer according to CAD file in a vacuum chamber. EBM system can manufacture complex 3D geometries requiring no or very little machining before use. The EBM systems are energy and material efficient. The microstructures and surface properties of objects produced by EBM can be influenced by the setting of different processing parameters in the EBM system. In this study solid slabs of Ti-6Al-4V, approximately 5 x 5 cm with various thicknesses were produced with different sets of processing parameters such as beam current, offset focus, scan speed and scan direction. The effects of these parameters on surface roughness, surface morphology and microstructure of slabs have been evaluated by using confocal microscopy, SEM /EDX and optical microscopy. The samples for optical microscopy and SEM were prepared by using standard metallographic methods. Microstructures of Ti-6Al-4V alloy produced by EBM usually consist of columnar grains as shown in Figure 1. These grains always grow parallel to build direction. Layers of different contrasts were observed in the samples where the layer interface is perpendicular to the build direction as shown in figure 2. The growth of columnar grains and appearance of layers with different contrasts were observed irrespective of the parameter values. These two observed phenomena can be attributed to the partial reheating / re-melting of the solidified layer by the electron beam during the melting of subsequent layer. The diameter of individual grain and density of grains are not uniform and usually decreases with increase in build height. Upon cooling from the β-transus temperature, more or less continuous α-layers were found to form along the prior β grain boundaries. In the EBM produced Ti-6Al-4V alloy the β -phase was found to be in rod-like geometry, with a size of 0.05-0.1μm in diameter, imbedded in the α-plates. It was observed that the high value of off set focus can cause porosity in the sample. For example the resultant porosity could be up to 11%. On the other hand where the value of offset focus is relatively small no such phenomenon was observed. Reconstruction of 3D surface topography and roughness coefficient (Ra) were computed by using images taken from confocal microscope and novel computer program “COMSTAT’’ [A.Heydorn et al (2000)]. Figure 3 shows a 3D reconstructed surface of the EBM produced sample. The Ra is computed by using the equation below: Where Lfi is the thickness of ith point, Lf is the mean thickness and N is the number of measurements. It has been observed that the value of Ra is processing parameters dependent. A sample with bigger thickness or higher current values tends to have relatively higher values of Ra. The scan speed and scan direction can also influence the surface morphology and microstructures of the EBM produced alloys.
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| 17. |
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| 18. |
- Wei, Liu-Ying, et al.
(författare)
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Microstructural characterisation of several magnesium alloys in AM serie
- 2007
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Ingår i: Materials Science and Technology. - : Maney Publishing. - 0267-0836 .- 1743-2847. ; 23:6, s. 745-752
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Tidskriftsartikel (refereegranskat)abstract
- The as cast microstructures of several magnesium alloys in the AM series have been characterised, including the dendritic structure, Al segregation and the Mn containing phases. Two types of Mn containing particles were found and studied in the alloys mainly by using TEM. Type I is of equiaxed morphologies and a higher Mn/Al ratio in the composition. The flower shaped and dendritic type II phase has a lower Mn/Al ratio. The equiaxed type I could originate in the particle residues in the melts retained in the as cast alloy and the type II might form around a small type I particle. Electron diffraction has determined that both type I and type II have bcc crystal structure with lattice parameter a=0·91 nm. The higher Mn content in an alloy may lead to a higher amount of Mn containing particles.
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| 19. |
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| 20. |
- Xiao, Huan, et al.
(författare)
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THE STUDY FOR FERROMAGNETIC PROPERTIES AND MICROSTRUCTURE OF Bi(0.9)Ho(0.1)FeO(3)
- 2008
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Ingår i: PROCEEDINGS OF THE 2008 SYMPOSIUM ON PIEZOELECTRICITY, ACOUSTIC WAVES AND DEVICE APPLICATIONS. - : IEEE. - 9781424428915 ; , s. 557-560
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Konferensbidrag (refereegranskat)abstract
- Bi(0.9)Ho(0.1)FeO(3) ceramic is prepared by sol-gel method. We investigated the effect of Ho element doping in the BiFeO(3). We studied the microstructure, morphology and ferromagnetism for Bi(0.9)Ho(0.1)FeO(3) ceramic by XRD, SEM and SQUID. It shows that Bi(0.9)Ho(0.1)FeO(3) ceramic exhibits single phase of perovskite structure. The c/a rate is elevated from 1 to 1.009 with respect to BiFeO(3). The grains of Bi(0.9)Ho(0.1)FeO(3) ceramic shrink sharply compared with the ones of BiFeO(3) ceramic, which is beneficial to the connection between grains and the density. Bi(0.9)Ho(0.1)FeO(3) ceramic exhibits obvious ferromagnetism while BiFeO(3) ceramic does not possess the macroscopic ferromagnetism at the RT. The magnetization sinks in the horizontal direction near the zero magnetic field. It indicates that Bi(0.9)Ho(0.1)FeO(3) ceramics do not only show obvious ferromagnetism, but also could exhibit lower magnetic hysteresis loss.
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| 21. |
- Zeng, Yong, et al.
(författare)
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Complete band gaps in three-dimensional quantum dot photonic crystals
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:11, s. 115325-
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Tidskriftsartikel (refereegranskat)abstract
- Nonlocal investigations have been performed about exciton-photon couplings in three-dimensional quantum-dot (QD) photonic crystals and a complete photonic band gap has been found in the band structure of a diamond lattice. The width of such a band gap can be broadened by increasing the filling ratio of the QDs (increasing the QD radius or/and decreasing the lattice constant of the photonic crystal). By decomposing the diamond lattice into two interlacing face-centered-cubic (fcc) sublattices, we have found that by significantly modifying the QD radius in one fcc sublattice (the diamond lattice therefore changed to the zinc blende lattice), the band structure of the zinc blende lattice is in principle the sum of two individual fcc sublattices. However, a huge exciton-photon coupling is observed near the band gaps of the two individual fcc sublattices when the radii of the QDs in the two fcc sublattices approach each other, resulting in the complete band gaps of the diamond structure.
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| 22. |
- Zeng, Yong, et al.
(författare)
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Dynamics of the damping oscillator formed by the collective generation of surface polaritons for extraordinary light transmission through subwavelength hole arrays in thin metal films
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:12
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Tidskriftsartikel (refereegranskat)abstract
- Light transmissions through a subwavelength hole array in a thin metal film are characterized by resonance peaks in the transmission spectrum. In this work we study the surface polaritons (SPs) in perforated thin metal films by analyzing the dynamic behavior of the Poynting vectors based on a full-vector finite-difference time-domain approach. It is shown that each resonance peak in the transmission spectrum is caused by a collective generation of SPs in the form of a dynamic damping oscillator which oscillates in space and time. The energy of the incident light is transported between the upper and the lower metal-air surfaces during the spatial oscillations largely through the air holes. This energy transport mechanism prevails even when the metal-film thickness becomes as thin as 100 nm, under which circumstance the wave functions of the SPs, localized separately on the two metal-air surfaces when the metal film is thick, begin to strongly overlap with each other. The lifetimes of the damping oscillators are different for different resonance peaks in the transmission spectrum and remain distinguishable by the order of their temporal development.
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| 23. |
- Zeng, Yong, et al.
(författare)
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Effective dielectric constant of two-dimensional photonic crystals in optical bands
- 2006
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Ingår i: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 138:4, s. 205-210
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Tidskriftsartikel (refereegranskat)abstract
- By using the Fourier expansion method, we have developed an approach to calculate the effective dielectric index of a two-dimensional photonic crystal. The approach is very general: it can take into account various Bravais lattice structure as well as arbitrary spatial variation of the dielectric index. It has been found that near a nondegenerate frequency w(n)(Gamma) at Gamma point, the transverse magnetic (TM) mode is ordinary, as it is independent of the propagation direction, whereas in general the transverse electric (TE) mode depends on the propagation direction, it is extraordinary. Therefore, a two-dimensional photonic crystal can always be described by an effective dielectric index for TM mode near the nondegenerate frequency w(n)(Gamma). However, the TE mode is much more complicated unless the lattice structure is highly symmetric. Moreover, a two-dimensional square photonic crystal has been identified as an effective birefringent crystal having two negative refractive indexes from the perspective of Snell's law.
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| 24. |
- Zeng, Yong, et al.
(författare)
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Extended plane-wave-based transfer-matrix approach to simulating dispersive photonic crystals
- 2006
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Ingår i: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 139:7, s. 328-333
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Tidskriftsartikel (refereegranskat)abstract
- It has been difficult to compute the band structures and transmission spectra for photonic crystals (PCs) with dispersive components included in the periodic units. Here we show that by using an extended plane-wave-based transfer-matrix method, we are able to formulate the problem for computing optical properties of dispersive PCs, including magnetic and left-handed PCs. This approach is very general, since it can treat PCs with arbitrary Bravais lattice composed of materials with arbitrary dielectric permittivities and magnetic permeabilities. Combined with the supercell method, this method can further simulate defective PCs such as PC-based waveguides and microcavities.
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| 25. |
- Zeng, Yong, et al.
(författare)
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Finite-difference time-domain simulations of exciton-polariton resonances in quantum-dot arrays
- 2008
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Ingår i: Optics Express. - 1094-4087. ; 16:7, s. 4507-4519
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Tidskriftsartikel (refereegranskat)abstract
- The optical properties of nanosize quantum-dot (QD) arrays are found to vary significantly around the exciton resonance frequency of the QDs. In order to simulate the interactions between electromagnetic waves and QD arrays, a general auxiliary-differential-equation, finite-difference time-domain approach is introduced and utilized in this article. Using this numerical method, the exciton-polariton resonances of single-layer and double-layer GaAs QD arrays are studied. The optical properties of a single-layer QD array are found to be characterized by the Mie resonance of its constituent QDs, while a double-layer QD array is characterized by the quasi-dipole formed by two QDs positioned in each of the two layers.
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| 26. |
- Zeng, Yong, et al.
(författare)
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Highly efficient generation of entangled photon pair by spontaneous parametric downconversion in defective photonic crystals
- 2007
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Ingår i: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 24:6, s. 1365-1368
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Tidskriftsartikel (refereegranskat)abstract
- We study the process of spontaneous parametric downconversion in a one-dimensional defective quadratic nonlinear photonic crystal. It is shown that the strong confinements of both the pump and signal waves around the defective layer result in a giant enhancement of the entangled photon-pair generation. An enhancement factor as high as 3.4 X 10(6) is obtained in our defective structure based on the dual-localized modes. Furthermore, the linewidth of the downconverted fields is only 0. 1 nm. Such a photonic crystal structure can be applied as a highly efficient source of entangled photon pairs for highly integrated all-optical circuits.
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| 27. |
- Zeng, Yong, et al.
(författare)
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Selective excitation of surface-polariton Bloch waves for efficient transmission of light through a subwavelength hole array in a thin metal film
- 2007
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 76:3, s. 035427-
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Tidskriftsartikel (refereegranskat)abstract
- Electromagnetic (EM) field was found to be able to transmit efficiently through a subwavelength hole array in a metal thin film at specific resonant frequencies. By analyzing the near-field distributions of EM fields in the array system, as well as the corresponding Fourier spectra, we show that the surface-polariton (SP) Bloch waves focus the energy of the incident plane-wave EM field to the vicinity of the hole at resonances (through SP scattering provided by the periodic hole). Furthermore, the wave vectors of the SP waves that contribute to the focusing effect are quantized as functions of the geometric shape of the holes in such a way that the focusing effect of the EM energy into the hole is maximal. The transmission efficiency and bandwidth at resonances are found to partially depend on the number of SP modes which contribute to the focusing effect.
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