Topologically protected excitons in porphyrin thin films

The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyze a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogenous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.Comment: Submitted early March 2014 to a journal; currently in revisio

Fusion Zone Microstructure Evolution of Al-Alloyed TRIP Steel in Diode Laser Welding

TRansformation Induced Plasticity (TRIP) steels are promising materials to achieve a better combination of formability and strength than conventional steels due to their unique microstructural makeup. Though welding is a vital part of auto body manufacturing, the weldability of TRIP steels has some complex and poorly understood features, which has served to retard the growth of its applications in the automotive industry. In this study, autogeneous welds were carried out on Al-alloyed TRIP steel using a 4 kW diode laser. Both fusion zone solidification behavior and subsequent austenite transformation products were investigated with optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. In terms of solidification behavior, fusion zones solidified with high temperature -ferrite as the primary phase. Fusion zone microstructure at room temperature was composed of ferrite with a skeletal morphology characteristic of solidification, and austenite decomposition products almost all having a lath morphology. Skeletal ferrite covered about 30% fusion zone area. Upper bainite laths separated by retained austenite films comprised most of the transformed microstructure, about 65% of the fused area. Lower bainite with carbide particles dispersed in an aligned way, chunk shaped retained austenite, lath martensite and twinned martensite were also occasionally observed. The Al content was considered to be for a dominant influence on fusion zone microstructure evolution

Settleable engineered titanium dioxide nanomaterials for the removal of natural organic matter from drinking water

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.cej.2017.10.058 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Four linear engineered TiO2 nanomaterials (LENs) were synthesized and evaluated in terms of settlability and their ability to remove natural organic matter (NOM) from river water. The size, surface characteristics, and crystallinity of the LENs were manipulated by varying the temperatures used during the synthesis procedure. All four LENs settled out of purified water more effectively than standard Degussa Evonik Aeroxide P25 nanoparticles. The settling behaviour of the nanomaterials was impacted by surface charge effects and interactions with NOM and ionic species in the river water matrix. The reduction of two disinfection byproduct precursor surrogate parameters, DOC and UV254, by the LENs via adsorption (no irradiation) and photocatalytic degradation under UVA LED irradiation was compared to that by P25 nanoparticles. After 60 minutes of irradiation DOC removal by the LENs ranged from 20 to 50% and UV254 reduction ranged from 65 to 90%. Two of the most promising LENs were reused multiple times and although both materials experienced decreases in treatment efficacy over successive reuse cycles, the reusability of both LENs was equal to or superior to that of P25 nanoparticles. The electrical energy per order required to remove DOC and UV254 from the water ranged from 8 to 36 times higher than that required for UV/H2O2 treatment but comparable to results reported by other researchers using UV/TiO2 for NOM removal. A subset of the nanomaterials evaluated in this study may prove to be a viable alternative to standard TiO2 nanoparticles for the removal of DBP precursors from drinking water, but the characteristics of the water matrix have important effects on settling efficiency and will require site-specific evaluation.Natural Sciences and Engineering Research Council’s Strategic Project Grant program [STPGP 430654-12]Canada Graduate Scholarship program [Ontario

Dissimilar joining of carbon/carbon composites to Ti6Al4V using reactive resistance spot welding

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.jallcom.2018.09.018 © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/A 2D C/C composite with a high porosity (low strength) and a 3D C/C composite with a low porosity (high strength) were investigated for dissimilar joining to Ti6Al4V via reactive spot welding. It was determined that infiltration of melted metal into the composite and formation of a continuous thin TiC layer at the interface of the joints were the dominant joining mechanisms. The 2D C/C composite with a flat surface was successfully joined to Ti6Al4V due to the infiltration of the melted Ti6Al4V into its porous content. On the other hand, it was necessary to drill rectangular grooves onto the surface of the 3D C/C composite to facilitate the infiltration of the melted Ti into the composite, which produced high-strength joints. Surface patterning was determined to be necessary to join the components with mismatching coefficients of thermal expansion. The strength of the 2D C/C composite and Ti6Al4V joints was found to be 7 MPa, while the maximum strength of the groove-patterned 3D C/C composite and Ti6Al4V joints reached 46 MPa.Natural Sciences and Engineering Research Council of Canad

Effects of post-processing on the thermomechanical fatigue properties of laser modified NiTi

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.ijfatigue.2017.11.012 © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The multifunctional capabilities needed for advanced shape memory alloys (SMA) actuators has been shown to be achievable by locally tuning the properties through laser processing. Before the wide-spread use of these SMAs is realized, a detailed understanding on the long-term stability and functional life span of these material must be achieved. The current study systematically investigates the effects of thermomechanical treatment on laser modified NiTi wires, while comparing them to the original base material. Surface analysis was done using a scanning electron microscope (SEM), while microstructure analysis was performed using transmission electron microscopy (TEM). Mechanical properties were assessed using standard tensile tests and a custom built thermomechanical fatigue. Results showed that the coarse-grains, large inclusions and surface defects associated with as laser modified NiTi resulted in reduced mechanical performance. However, subsequent thermomechanical treatment restored the refined microstructure and mechanical performance similar to the base material while providing the added functionality, thus allowing for laser processed NiTi to be used for manufacturing multiple memory NiTi actuators.Natural Sciences and Engineering Research Council of CanadaCanada Research ChairsOntario Centres of Excellenc

Effect of coating weight on fiber laser welding of Galvanneal-coated 22MnB5 press hardening steel

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.surfcoat.2018.01.053 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Tensile strengths of up to 1.5 GPa and fracture elongation of 8% in press-hardening steel (PHS) opened up a great opportunity to improve the crash performance of vehicles. However, due to the increased susceptibility to welding defects and undesired transformation as a result of coating mixing into the weld pool, laser welding of PHS is still challenging. Hence, the present study focuses on the effect of Galvanneal (GA)-coating weight on the microstructure and mechanical performance of fiber laser welded PHS. It was observed that GA-coating weight considerably affects the laser welding process window as well as weld geometry. Weld penetration decreased and concavity increased with increasing coating weight, which is attributed to intensified Zn-plasma and laser interaction at higher coating weights. Moreover, a model has been developed to interpret the correlation between the GA-coating weight of the PHS and the penetration depth based on the heat input per unit thickness of the sheet. The size of fusion zone and heat affected zone decreased slightly with increasing coating weight due to lower energy absorption by the material. Furthermore, GA-coating weight did not affect the tensile strength of all welded joints as the failure happened in the base metal.National Science and Engineering Research Council (NSERC) of CanadaArcelorMittal Dofasco Inc. Hamilton, Canad

Deconvolution of overlapping peaks from differential scanning calorimetry analysis for multi-phase NiTi alloys

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.tca.2018.05.014 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/An adaptive function capable of fitting the curve of Differential Scanning Calorimetry (DSC) data for any NiTi phase transformation peak has been developed. A novel methodology was applied in conjunction with this new equation, allowing for the deconvolution of multiple overlapping NiTi phase transformation peaks. Characteristic transformation properties determine by this methodology closely matched those ascertainable by current analysis techniques. This novel analysis technique allows for better determination of characteristic properties of complex NiTi materials with overlapping phase transformations or multiple embedded memories.Natural Sciences and Engineering Research Council of CanadaCanadian Foundation for Innovatio

Dendritic coarsening model for rapid solidification of Ni-superalloy via electrospark deposition

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.jmatprotec.2018.03.023 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Control of splat thickness in an electrospark deposition (ESD) process can be used to improve the mechanical properties of deposited Inconel 718. The lower cooling rates of thicker deposition splats obtained through higher energy ESD parameters result in greater subgrain coarsening and lower microhardness. A subgrain growth model and Hall-Petch relationship are used to quantify the extent of subgrain coarsening and the influence of splat thickness on hardness, with a 4.5 times reduction in splat thickness achieving a 20% increase in microhardness.Natural Sciences and Engineering Research Council of Canada (NSERC)Canada Research Chairs (CRC) ProgramHuys Industrie

Resistive Switching Memory of TiO2 Nanowire Networks Grown on Ti Foil by a Single Hydrothermal Method

The resistive switching characteristics of TiO2 nanowire networks directly grown on Ti foil by a single-step hydrothermal technique are discussed in this paper. The Ti foil serves as the supply of Ti atoms for growth of the TiO2 nanowires, making the preparation straightforward. It also acts as a bottom electrode for the device. A top Al electrode was fabricated by e-beam evaporation process. The Al/TiO2 nanowire networks/Ti device fabricated in this way displayed a highly repeatable and electroforming-free bipolar resistive behavior with retention for more than 10(4) s and an OFF/ON ratio of approximately 70. The switching mechanism of this Al/TiO2 nanowire networks/Ti device is suggested to arise from the migration of oxygen vacancies under applied electric field. This provides a facile way to obtain metal oxide nanowire-based ReRAM device in the future.Natural Sciences and Engineering Research Council (NSERC) of CanadaState Scholarship Fund of China [201506160061