In-situ Observation of Martensite Decomposition in HAZ of Cr-Mo Steel Weldment

In-situ observation of martensite decomposition at Heat Affected Zone (HAZ) was investigated on a dissimilar joining between 2.25Cr-0.5Mo grade T22 as base material and ER90S-B9 as filler metal using GTAW process using LEEM at a synchrotron facility. A post weld heat treatment (PWHT) cycle was simulated on a welded specimen in high vacuum chamber by heating cartridge and electron bombardment. Both effects PWHT duration and weld areas were studied for comparisons. At the simulated PWHT between 690oC -700oC in CGHAZ, martensite started to decompose by the dissolution of carbide flakes. The prior-austenite grain boundaries were also shown during the process. The same phenomena were also observed in FGHAZ with different extent. In un-affected base material, ferrite and new pearlite grains presented and grew at the expense of old pearlite. Longer PWHT duration resulted in more ferrite formed in all weld areas. Raising PWHT temperature to 730oC could push the reaction above Eutectoid temperature as the new austenite formed at grain boundaries. The proposed mechanism for martensite decomposition would be in steps as dissolution of carbide followed by formation of ferrite and growth as PWHT proceeded

In-situ Observation of h-BN Formation on the Surface of Weld Dissimilar Joint Steels

In-situ observation of h-BN formation by surface precipitation on the surface of joined dissimilar steels is presented. Because the substrate consists of two different types of steels, different growth behaviors can be seen on different sides and also in the middle of the weld interface. This observation demonstrates that formations of 2D materials can occur on surfaces of steels under suitable conditions e.g. temperature, microstructures and concentrations of impurities. Characterizations by electron microscopy and synchrotron spectroscopy technics confirm that h-BN crystals that appear on the surface after annealing are of similar quality to those prepared by other methods such as chemical vapor deposition. Moreover, real-time observation during sample temperature swing above and below the phase transition temperature of Fe shows that h-BN islands reversibly form and dissociate on the surface. The results show that the formation of h-BN on steels is reversible and the analysis suggests that the process is likely affected by structural change of the steels near the phase transition temperature, which in-turn drives the diffusions of B and N atoms back and forth between surface and bulk

Planar Self-Assembly of Submicron and Nanoscale Wires and Grooves on III–V(110) Surfaces

Metallic Ga and In submicron and nanowires (NWs) tens of microns long naturally form via a self-propelled mechanism on the (110) surfaces of GaAs and InAs, respectively, during noncongruent sublimation in ultrahigh vacuum. Under stringent conditions, low-energy electron microscopy uncovers GaAs and InAs(110) surfaces on the brink of decomposition rapidly assemble planar wires of their cations in the ⟨1̅10⟩ direction. For InP(110), wire formation is unfavorable due to a smooth decomposing front but can be assisted by Au nanoparticles (NPs), which sacrifice themselves to form rough pits via solid–liquid–vapor etching. The resulting self-assembled and AuNP-assisted NWs grow crystallographically in a self-sustainable manner, unless they are obstructed and consumed by stationary microdroplets, leaving emptied grooves. The findings reveal a hitherto hidden natural process on the surfaces of binary crystals capable of producing elementary submicron and nanoscale wires without extrinsic materials, paving the way for the controlled fabrication of planar NWs, grooves, and NW/groove arrays with lengths approaching circuits or even chips scale and with potential applications in self-integrated circuits, plasmonics, and fluidics

Planar Self-Assembly of Submicron and Nanoscale Wires and Grooves on III–V(110) Surfaces

Metallic Ga and In submicron and nanowires (NWs) tens of microns long naturally form via a self-propelled mechanism on the (110) surfaces of GaAs and InAs, respectively, during noncongruent sublimation in ultrahigh vacuum. Under stringent conditions, low-energy electron microscopy uncovers GaAs and InAs(110) surfaces on the brink of decomposition rapidly assemble planar wires of their cations in the ⟨1̅10⟩ direction. For InP(110), wire formation is unfavorable due to a smooth decomposing front but can be assisted by Au nanoparticles (NPs), which sacrifice themselves to form rough pits via solid–liquid–vapor etching. The resulting self-assembled and AuNP-assisted NWs grow crystallographically in a self-sustainable manner, unless they are obstructed and consumed by stationary microdroplets, leaving emptied grooves. The findings reveal a hitherto hidden natural process on the surfaces of binary crystals capable of producing elementary submicron and nanoscale wires without extrinsic materials, paving the way for the controlled fabrication of planar NWs, grooves, and NW/groove arrays with lengths approaching circuits or even chips scale and with potential applications in self-integrated circuits, plasmonics, and fluidics

Planar Self-Assembly of Submicron and Nanoscale Wires and Grooves on III–V(110) Surfaces

Metallic Ga and In submicron and nanowires (NWs) tens of microns long naturally form via a self-propelled mechanism on the (110) surfaces of GaAs and InAs, respectively, during noncongruent sublimation in ultrahigh vacuum. Under stringent conditions, low-energy electron microscopy uncovers GaAs and InAs(110) surfaces on the brink of decomposition rapidly assemble planar wires of their cations in the ⟨1̅10⟩ direction. For InP(110), wire formation is unfavorable due to a smooth decomposing front but can be assisted by Au nanoparticles (NPs), which sacrifice themselves to form rough pits via solid–liquid–vapor etching. The resulting self-assembled and AuNP-assisted NWs grow crystallographically in a self-sustainable manner, unless they are obstructed and consumed by stationary microdroplets, leaving emptied grooves. The findings reveal a hitherto hidden natural process on the surfaces of binary crystals capable of producing elementary submicron and nanoscale wires without extrinsic materials, paving the way for the controlled fabrication of planar NWs, grooves, and NW/groove arrays with lengths approaching circuits or even chips scale and with potential applications in self-integrated circuits, plasmonics, and fluidics

Planar Self-Assembly of Submicron and Nanoscale Wires and Grooves on III–V(110) Surfaces

Metallic Ga and In submicron and nanowires (NWs) tens of microns long naturally form via a self-propelled mechanism on the (110) surfaces of GaAs and InAs, respectively, during noncongruent sublimation in ultrahigh vacuum. Under stringent conditions, low-energy electron microscopy uncovers GaAs and InAs(110) surfaces on the brink of decomposition rapidly assemble planar wires of their cations in the ⟨1̅10⟩ direction. For InP(110), wire formation is unfavorable due to a smooth decomposing front but can be assisted by Au nanoparticles (NPs), which sacrifice themselves to form rough pits via solid–liquid–vapor etching. The resulting self-assembled and AuNP-assisted NWs grow crystallographically in a self-sustainable manner, unless they are obstructed and consumed by stationary microdroplets, leaving emptied grooves. The findings reveal a hitherto hidden natural process on the surfaces of binary crystals capable of producing elementary submicron and nanoscale wires without extrinsic materials, paving the way for the controlled fabrication of planar NWs, grooves, and NW/groove arrays with lengths approaching circuits or even chips scale and with potential applications in self-integrated circuits, plasmonics, and fluidics

Diluted Magnetic Semiconductor Cobalt and Europium Implanted ZnO Thin Film

Diluted magnetic semiconductors (DMSs) have initiated enormous scientific interests because of their potential for multifunctional spintronics devices. ZnO based semiconductors have been identified to be the promising room temperature ferromagnetic materials with a wide band-gap. However, the intrinsic room temperature ferromagnetic spintronics materials are still far to be optimized. In this dissertation, the samples were prepared by using metal vacuum vapour arc (MEVVA) source ion implantation of cobalt and europium into ZnO/c-Al2O3 (0001) epitaxial thin films. The ion implantation is an effective technique for introducing dopants of heavy elements into thin film. The depth profile of as-prepared sample as well as dopants concentration was studied by ion beam analysis and transport of ions in matter (TRIM) calculation. It was found that the total magnet moment of Co doped ZnO was improved by additional Eu doping. The correlation between the properties of Zn1-xEuxO and Zn1-xCox-yEuyO system and local coordination chemical environment as well as the underlying mechanism was investigated in details.The superconducting quantum interface device (SQUID) magnetometer shows all as-prepared samples are ferromagnetic at room temperature. However, it is unclear whether such a phenomenon is an intrinsic property or caused by the Co metallic clusters. The X-ray magnetic circular dichroism (XMCD) shows that the strong spin polarization of localized Eu atoms observed near surface of the Zn1-xEuxO thin films. The XMCD results also suggest Eu implanting to ZnO:Co system has suppressed Co metallic clustering. X-ray absorption fine structure (XAFS) confirms that Eu3+ had substituted for Zn2+ and resided in tetrahedral geometry without changing the wurtzite structure of ZnO host lattice in Zn1-xEuxO; whereas substitutional Co2+ and Co metallic clusters are coexisting in Zn1-xCoxO. But the Co clustering fraction can be significantly decreased by adding Eu into Zn1-xCoxO through ion implantation. The experimental and theoretical studies suggest a short range interaction between the substitutional Eu3+ is anti-ferromagnetic in Zn1-xEuxO. The experimental results provide guidance to develop the new materials to enhance the intrinsic ferromagnetic properties of ZnO based DMSs via rare earth element implantation