19 research outputs found

    The dissociation of (a+c) misfit dislocations at the InGaN/GaN interface

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    (a+c) dislocations in hexagonal materials are typically observed to be dissociated into partial dislocations. Edge (a+c) dislocations are introduced into (0001) nitride semiconductor layers by the process of plastic relaxation. As there is an increasing interest in obtaining relaxed InGaN buffer layers for the deposition of high In content structures, the study of the dissociation mechanism of misfit (a+c) dislocations laying at the InGaN/GaN interface is then crucial for understanding their nucleation and glide mechanisms. In the case of the presented plastically relaxed InGaN layers deposited on GaN substrates we observe a trigonal network of (a+c) dislocations extending at the interface with a rotation of 3 degrees from directions. High resolution microscopy studies show that these dislocations are dissociated into two Frank-Shockley 1/6 partial dislocations with the I1 BSF spreading between them. Atomistic simulations of a dissociated edge (a+c) dislocation revealed a 3/5 atom ring structure for the cores of both partial dislocations. The observed separation between two partial dislocations must result from the climb of at least one of the dislocations during the dissociation process, possibly induced by the mismatch stress in the InGaN layer.Comment: This is a submitted version of the manuscript published in Journal of Microscop

    The effect of high-pressure plastic forming on the structure and strength of AA5083 and AA5754 alloys intended for fasteners

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    The presented results describe the effect of severe plastic deformation on the structure and mechanical properties of AA5083 and AA5754 alloys. Both materials were subjected to single hydrostatic extrusion (HE) and cumulative hydrostatic extrusion in the case of AA5083 and a combination of plastic deformation by equal-channel angular pressing (ECAP) with the next HE for AA5754. After the deformation, both alloys featured a homogeneous and finely divided microstructure with average grain size deq = 140 nm and 125 nm for AA5083 and AA5754, respectively. The selection of plastic forming parameters enabled a significant increase in the UTS tensile strength and YS yield stress in both alloys ÔÇô UTS = 510 MPa and YS = 500 MPa for alloy AA5083 after cumulative HE, and 450 MPa and 440 MPa for alloy AA5754 after the combination of ECAP and HE, respectively. It has been shown on the example of AA5083 alloy that after the deformation the threads of the fasteners made of this material are more accurate and workable at lower cutting speeds, which saves the cutting tools. The resultant properties of AA5083 and AA5754 alloys match the minimum requirements for the strongest Al-Zn alloys of the 7xxx series, which, however, due to the considerably lower corrosion resistance, can be replaced in many responsible structures by the AA5xxx series Al-Mg alloys presented in this paper

    Tarcie i zu┼╝ycie z nanocz─ůstkami WS2 w warunkach smarowania mieszanego i granicznego

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    The mixture of Inorganic Fullerene-like (IF) WS2 and Graphene-like nanoparticles were tested under mixed and boundary lubrication in air. The mixture of nanoparticles was synthesized by chemical vapor condensation (CVC) by the pyrolysis of W(CO)6 carbonyls. The synthesized nanoparticles were not found to give an ultra-low friction coefficient compared to hexagonal 2H-WS2 nanoparticles made by special rolling-milling technique. However, under the conditions of our experiments using contact loads of 300-900N and sliding velocity 0.44m/sec it is found that all nanoparticles can dramatically improve the anti-wear properties of the base oil. Additionally, an unusual friction behavior was found for nanoparticle suspension applied in step-load tests. A sharp drop of friction coefficient (up to 0.015) was registered at the normal load increase while usually friction coefficient sharp increase (up to 0.025) occurs for oil lubrication case. The nanoparticles and the interface were investigated using High Resolution TEM, SEM, XRD and surface analyses (XPS) on the wear tracks. The WS2 graphene-like nanoparticles were found to present in amorphous-crystalline mixture. The WS2 films were generated at the interface. Shear-induced reorientation of the graphene-like nanoparticles at the increase of contact stress assured low friction and unusual friction behaviour as compared to those of oil lubrication.Badano mieszanin─Ö nieorganicznych cz─ůstek fullerenopodobnych (IF) WS2 i grafenopodobnych w warunkach smarowania mieszanego i granicznego w powietrzu. Mieszanin─Ö nanocz─ůstek syntetyzowano w drodze skraplania pary chemicznej (CVC) przez piroliz─Ö karbonyl├│w W(CO)6. Nie stwierdzono, by zsyntetyzowane nanocz─ůstki dawa┼éy bardzo niski wsp├│┼éczynnik tarcia w por├│wnaniu z sze┼Ťciok─ůtnymi nanocz─ůstkami 2H-WS2 sporz─ůdzonymi specjaln─ů technik─ů walcowania-mielenia. Jednak┼╝e, w warunkach naszych do┼Ťwiadcze┼ä z zastosowaniem obci─ů┼╝e┼ä stykowych 300-900 N i pr─Ödko┼Ťci po┼Ťlizgu 0,44 m/sek okazuje si─Ö, ┼╝e wszystkie nanocz─ůstki mog─ů znacznie poprawi─ç przeciwzu┼╝yciowe w┼éasno┼Ťci oleju bazowego. Ponadto, stwierdzono niezwyk┼ée zachowanie tarciowe dla zawiesiny nanocz─ůstek stosowanej w pr├│bach obci─ů┼╝ania krokowego. Du┼╝y spadek wsp├│┼éczynnika tarcia (do 0,015) zanotowano przy normalnym wzro┼Ťcie obci─ů┼╝enia, gdy tymczasem w przypadku smarowania olejem zwykle zdarza si─Ö du┼╝y wzrost wsp├│┼éczynnika tarcia (do 0,025). Nanocz─ůstki i powierzchni─Ö styku poddano badaniu za pomoc─ů TEM, SEM, XRD o wysokiej rozdzielczo┼Ťci i w drodze analizy powierzchniowej (XPS) na ┼Ťladach zu┼╝ycia. Stwierdzono obecno┼Ť─ç grafenopodobnych nanocz─ůstek WS2 w mieszaninie amorficzno-krystalicznej. Na powierzchni styku tworzy┼éy si─Ö filmy WS2. Spowodowana ┼Ťcinaniem reorientacja nanocz─ůstek grafenopodobnych przy wzro┼Ťcie napr─Ö┼╝enia stykowego zapewni┼éa niskie tarcie i niezwyk┼ée zachowanie tarciowe w por├│wnaniu ze smarowaniem olejowym

    Defects, strain relaxation, and compositional grading in high indium content InGaN epilayers grown by molecular beam epitaxy

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    We investigate the structural properties of a series of high alloy content InGaN epilayers grown by plasma-assisted molecular beam epitaxy, employing the deposition temperature as variable under invariant element fluxes. Using transmission electron microscopy methods, distinct strain relaxation modes were observed, depending on the indium content attained through temperature adjustment. At lower indium contents, strain relaxation by V-pit formation dominated, with concurrent formation of an indium-rich interfacial zone. With increasing indium content, this mechanism was gradually substituted by the introduction of a self-formed strained interfacial InGaN layer of lower indium content, as well as multiple intrinsic basal stacking faults and threading dislocations in the rest of the film. We show that this interfacial layer is not chemically abrupt and that major plastic strain relaxation through defect introduction commences upon reaching a critical indium concentration as a result of compositional pulling. Upon further increase of the indium content, this relaxation mode was again gradually succeeded by the increase in the density of misfit dislocations at the InGaN/GaN interface, leading eventually to the suppression of the strained InGaN layer and basal stacking faults

    InGaN laser diodes operating at 450-460 nm grown by rf-plasma MBE

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    This work demonstrates the first true blue laser diodes (LDs) grown by plasma assisted molecular beam epitaxy that operate at the region of 450-460 nm. The single quantum well LDs were grown on several types of c-plane bulk GaN substrates, with threading dislocation densities varying from 10 4 to 10 8cm -2. The key factors that allowed the authors to achieve lasing in true-blue wavelengths are improvements in the growth technology of the InGaN quantum wells attributed to the high nitrogen flux used and the design of the LD structure, which reduced the light losses in the cavity. The authors discuss the influence of the diodes' design on the parameters of LDs. \ua9 2012 American Vacuum Society.Peer reviewed: YesNRC publication: Ye