arXiv subjects

Dorian A. H. Hanaor

Publications and source records attributed to Dorian A. H. Hanaor.

At least 19 recordsLinked to original sources

arXiv2024

Oxygen redox in battery cathodes: A brief overview

The participation of oxygen or other anionic species in redox activities in cathode materials for lithium and sodium-ion battery systems is known to play a role in governing the useful capacity of these batteries. Directly probing anionic redox mechanisms is not possible, rather the computational analysis by density functional theory poses the main approach towards gleaning insights into anionic redox activity and harnessing these effects to maximize capacity in future electrode materials. Here we showcase material systems exhibiting this mechanism of ion insertion and removal, and present the key computational considerations in studying anionic redox activities in battery materials. Aided by new computationally derived understandings of the role of anionic redox in emerging battery materials, increasingly greater levels of useable capacities can be extracted through informed materials design.

cond-mat.mtrl-sci↗

arXiv2023

Enhanced mechanical performance and bioactivity in strontium/copper co-substituted diopside scaffolds

Effective scaffolds for bone tissue-engineering are those that combine adequate mechanical and chemical performance with osseointegrative, angiogenetic and anti-bacterial modes of bioactivity. To address these requirements via a combined approach, we additively manufactured square strut scaffolds by robocasting precipitation-derived strontium/copper co-substituted diopside. Microstructure, mechanical performance, bioactivity, biocompatibility, and antibacterial activity were examined. The results show that the presence of strontium and copper in the diopside lattice reduces the grain size and increases the density of the ceramics. The compressive strength, hardness, and fracture toughness of the diopside showed improvement, attributed to a finer microstructure and improved sintering. Scaffolds had excellent compressive strength with a high porosity (68 to 72 %), which is attributed to the structure of the stacked square struts. All materials showed good in vitro bioactivity and favorable proliferation of osteogenic sarcoma cells, while strontium and copper co-doped materials exhibited the strongest anti-Escherichia coli activity. We show that across multiple indicators this system offers pathways towards high-performance bone substitutes.

cond-mat.mtrl-sci↗

arXiv2022

Charging/discharging mechanism in Mg3Bi2 anode for Mg-ion batteries; The role of the spin-orbit coupling

Using density functional calculations, we examine insertion/extraction of Mg ions in Mg3Bi2, an interesting Mg-ion battery anode. We found that a (1 1 0) facet is the most stable termination. Vacating a Mg2+ ion from the octahedral site is more favourable for both surface and bulk regions of the material. However, the diffusion barriers among the tetrahedral sites are around 3 times smaller than those among octahedral sites. Consequently, during the magnesiation/demagnesiation process, Mg ions first vacate the octahedral sites and then diffuse through the tetrahedral sites. The spin-orbit interaction lowers Mg's vacancy formation energy but has a minor effect on diffusion barriers.

cond-mat.mtrl-sci↗

arXiv2022

Contact behaviour of simulated rough spheres generated with spherical harmonics

Normal contact behaviour between non adhesive fractal rough particles is studied using a finite element method (FEM). A series of spherical grain surfaces with distinguished roughness features are generated by means of Spherical Harmonics. These surfaces are described by two roughness descriptors, namely, relative roughness (Rr) and fractal dimension (FD). The contact behaviour of rough spheres with a rigid flat surface is simulated using FEM to quantify the influences of surface structure and sphere morphology by focusing on contact stiffness and true contact area. The dependence of normal contact stiffness (k) on applied normal force (F) is found to follow a power law over four orders of magnitude, with both alpha and beta being highly correlated with Rr and FD. With increasing load, the power exponent converges to that of Hertzian contact, e.g., 1/3, independent of Rr. Regions of true contact evolve through the formation of new microcontacts and their progressive merging, meanwhile the area distributions of contact island induced by various forces tend to obey similar Weibull distributions due to fractal nature in their surfaces. Contacts with larger values of Rr are found to produce contact contours with higher fractal dimension as calculated by a 2D box-counting method. Our results suggest that the correlation between radial lengths in a quasi spherical particle should be considered in studying contact behaviour.

cond-mat.mtrl-sci↗

arXiv2022

A comparison of syntheses approaches towards functional polycrystalline silicate ceramics

This study aims to shed light on processing pathways towards functional silicate ceramics, which show some promise in various emerging applications, including dielectrics and bioactive implant materials. Polycrystalline silicate ceramics of Neso, Soro and Inosilicate families were synthesised by three different techniques: (i) a co-precipitation method, (ii) a modified sol-gel method and (iii) standard solid-state reactions. Co-precipitated samples show increased sintering and densification behaviour compared to sol-gel and solid-state methods, with diametral shrinkage values during sintering of 28.8%, 13.3% and 25.0%, respectively. Well-controlled phase formation in these ceramics was most readily achieved through the steric entrapment of cations and shorter diffusion pathways afforded by the modified Pechini-type sol-gel method. Substituting Zn2+ for Mg2+ in enstatite samples was found to enhance the formation of orthoenstatite during cooling, which is otherwise very slow. We present guidelines for the design of synthesis methods that consider the requirements for different functional silicate ceramics in terms of phase formation and microstructure.

cond-mat.mtrl-sci↗

arXiv2022

High performing additively manufactured bone scaffolds based on copper substituted diopside

The inclusion of small amounts of copper is often reported to enhance the mechanical and biointegrative performance of bioceramics towards tissue engineering applications. In this work, 3D scaffolds were additively manufactured by robocasting of precipitation derived copper doped diopside. Compositions were chosen in which magnesium sites in diopside were substituted by copper up to 3 at.%. Microstructure, mechanical performance, bioactivity, biodegradability, drug release, biocompatibility, in vitro angiogenesis and antibacterial activity were studied. Results indicate that copper is incorporated in the diopside structure and improves materials fracture toughness. Scaffolds with more than 80% porosity exhibited compressive strengths exceeding that of cancellous bone. All compositions showed bioactivity and drug release functionalities. However, only samples with 0 to 1 at.% copper substitution showed favorable proliferation of osteogenic sarcoma cells, human umbilical vein endothelial cells and fibroblasts, while larger amounts of copper had cytotoxic behavior. In vitro angiogenesis was significantly enhanced by low levels of copper. Copper containing materials showed anti Escherichia coli activity, increasing with copper content. We show that across multiple indicators, copper substituted diopside of the composition CaMg0.99Cu0.01Si2O6, exhibits high performance as a synthetic bone substitute, comparing favorably with known bioceramics. These findings present a pathway for the enhancement of bioactivity and mechanical performance in printable bioceramics.

physics.med-ph↗

arXiv2022

Exceptionally high saturation magnetisation in Eu-doped magnetite stabilised by spin-orbit interaction

The significance of the spin-orbit interaction is very well known in compounds containing heavier elements such as the rare-earth Eu ion. Here, through density functional calculations, we investigated the effect of the spin-orbit interaction on the magnetic ground state of Eu doped magnetite ($\mathrm{Fe_3O_4:Eu_{Fe}}$). By examining all possible spin alignments between Eu and magnetite's Fe, we demonstrate that Eu, which is most stable when doped at the tetrahedral site, adapts a spin almost opposite the substituted Fe. Consequently, because of smaller spin cancellation between the cations on the tetrahedral site ($\mathrm{Fe_{Tet}}$ and $\mathrm{Eu_{Tet}}$) and the cations on the octahedral sites ($\mathrm{Fe_{Oct}}$), $\mathrm{Fe_3O_4:Eu_{Fe}}$ exhibits a maximum saturation magnetisation of 9.451 $μ_B/$f.u. which is significantly larger than that of undoped magnetite (calculated to be 3.929 $μ_B/$f.u.). We further show that this large magnetisation persists through additional electron doping. However, additional hole doping, which may unintentionally occur in Fe deficient magnetite, can reduce the magnetisation to values smaller than that of the undoped magnetite. The results presented here can aid in designing highly efficient magnetically recoverable catalysts for which both magnetite and rare earth dopants are common materials.

cond-mat.mtrl-sci↗

arXiv2021

Sand Supported Mixed-Phase TiO$_2$ Photocatalysts for Water Decontamination Applications

Using an organometallic precursor, TiO$_2$ coatings were fabricated on surfaces of quartz, zircon and rutile sands. X ray Diffraction, X ray Fluorescence, UV Vis spectroscopy and surface area measurement were used to characterise support materials. The phase composition and morphology of the coatings were characterised by laser Raman spectroscopy and SEM respectively. A packed bed reactor was used to study the inactivation of Escherichia coli in recirculating water by the supported photocatalysts. It was found that the sand grains were well coated with a homogenous layer of TiO2 and coatings were well adhered, exhibiting a mixed anatase rutile composition after firing at 850C. Photocatalytic activity was highest in coatings applied to quartz sand, although sterilisation of the recirculating water was not achieved with any of the materials investigated. The advantages of quartz as a TiO$_2$ photocatalyst support material are likely a result of this materials higher purity and optical transmittance. Potential enhancement through Si doping cannot be ruled out.

cond-mat.mtrl-sci↗

arXiv2021

Improved oxidation resistance of high emissivity coatings on fibrous ceramic for reusable space system

To develop high emissivity coatings on fibrous ceramic substrates with improved thermal resistance for reusable space systems, WSi2-MoSi2-Si-SiB6-borosilicate glass coatings were prepared on fibrous ZrO2 by slurry dipping and subsequent high temperature rapid sintering. A coating with 50 wt% WSi2 and 50 wt% MoSi2 presents optimal thermal stability with only 10.06 mg/cm2 mass loss and 4.0 % emissivity decrease in the wavelength regime of 1.27 to 1.73 microns after 50 h oxidation at 1773 K. The advantages of double phase metal silicide coatings combining WSi2 and MoSi2 include improved thermal compatibility with the substrate and an enhanced glass mediated self healing ability.

physics.app-ph↗

arXiv2021

Scalable Surface Area Characterization by Electrokinetic Analysis of Complex Anion Adsorption

By means of the in situ electrokinetic assessment of aqueous particles in conjunction with the addition of anionic adsorbates, we develop and examine a new approach to the scalable characterization of the specific accessible surface area of particles in water. For alumina powders of differing morphology in mildly acidic aqueous suspensions, the effective surface charge was modified by carboxylate anion adsorption through the incremental addition of oxalic and citric acids. The observed zeta potential variation as a function of the proportional reagent additive was found to exhibit inverse hyperbolic sine-type behavior predicted to arise from monolayer adsorption following the Grahame-Langmuir model. Through parameter optimization by inverse problem solving, the zeta potential shift with relative adsorbate addition revealed a nearly linear correlation of a defined surface-area-dependent parameter with the conventionally measured surface area values of the powders, demonstrating that the proposed analytical framework is applicable for the in situ surface area characterization of aqueous particulate matter. The investigated methods have advantages over some conventional surface analysis techniques owing to their direct applicability in aqueous environments at ambient temperature and the ability to modify analysis scales by variation of the adsorption cross section.

cond-mat.soft↗

arXiv2021

Silicate dielectric ceramics for millimetre wave applications

Silicate ceramics are of considerable promise as high frequency dielectrics in emerging millimetre wave applications including high bandwidth wireless communication and sensing. In this review, we show how high quality factors and low, thermally stable permittivities arise in ordered silicate structures. On the basis of a large number of existing studies, the dielectric performance of silicate ceramics is comprehensively summarized and presented, showing how microstructure and SiO4 tetrahedral connectivity affect polarizability and dielectric losses. We critically examine the appropriateness of silicate materials in future applications as effective millimetre wave dielectrics with low losses and tuneable permittivities. The development of new soft chemistry based processing routes for silicate dielectric ceramics is identified as being instrumental towards the reduction of processing temperatures, thus enabling silicate ceramics to be co-fired in the production of components functioning in the mm wave regime.

cond-mat.mtrl-sci↗

arXiv2021

Cu Modified SrTiO3 Perovskites Toward Enhanced Water Gas Shift Catalysis: A Combined Experimental and Computational Study

The water gas shift reaction (WGS) is important and widely applied in the production of H2. Cu modified perovskites are promising catalysts for WGS reactions in hydrogen generation. However, the structure-dependent stability and reaction pathways of such materials remain unclear. Herein, we report catalytically active Cu modified SrTiO3 (nominally SrTi1-xCuxO3) prepared by a modified polymeric precursor method. Microstructural analysis revealed a partially segregated CuO phase in the as-prepared materials. Operando X-ray diffraction and absorption spectroscopy showed the reduction of CuO into a stable metallic phase under conditions of WGS reactions for all compositions. Among the characterized materials, the x = 0.20 composition showed the highest turnover frequency, lowest activation energy, and the highest WGS rate at 300C. According to density functional calculations, the formation of CuO is energetically less favorable compared with SrTiO3, explaining why the segregated CuO phase on the SrTiO3 surface is reduced to Cu during the catalytic reaction, while SrTiO3 remains. For x = 0.20, the size of the segregated CuO phase is optimum for facilitating the catalytic reaction. In contrast, a higher Cu content (x = 0.3) results in an aggregation of smaller CuO particles, resulting in fewer surface active sites and a net decrease in catalytic performance.

cond-mat.mtrl-sci↗

arXiv2021

Polymer-Derived SiOC Integrated with a Graphene Aerogel As a Highly Stable Li-Ion Battery Anode

Amorphous polymer derived silicon oxycarbide (SiOC) is an attractive candidate for Lithium ion battery anodes, as an alternative to graphite, which is limited to a theoretical capacity of 372 mAh/g. However, SiOC tends to exhibit poor transport properties and cycling performance as a result of sparsely distributed carbon clusters and inefficient active sites. To overcome these limitations, we designed and fabricated a layered graphene SiOC heterostructure by solvent assisted infiltration of a polymeric precursor into a modified three dimensional graphene aerogel skeleton. The use of a high melting point solvent facilitated the precursors freeze drying, which following pyrolysis yielded SiOC as a layer supported on the surface of nitrogen doped reduced graphene oxide aerogels. The fabrication method employed here modifies the composition and microstructure of the SiOC phase. Among the studied materials, the highest levels of performance were obtained for a sample of moderate SiOC content, in which the graphene network constituted 19.8 wt % of the system. In these materials, a stable reversible charge capacity of 751 mAh/g was achieved at low charge rates. At high charge rates of 1480 mA/g, the capacity retention was ca. 95% (352 mAh/g) after 1000 consecutive cycles. At all rates, Coulombic efficiencies greater than 99% were maintained following the first cycle. Performance across all indicators was majorly improved in the graphene aerogel SiOC nanocomposites, compared with unsupported SiOC. The performance was attributed to mechanisms across multiple length scales. The presence of oxygen rich SiO4-xCx tetrahedral units and a continuous free carbon network within the SiOC provides sites for reversible lithiation, while high ionic and electronic transport is provided by the layered graphene SiOC heterostructure.

cond-mat.mtrl-sci↗

arXiv2020

Enhancement of hydroxyapatite dissolution through structure modification by Krypton ion irradiation

Hydroxyapatite synthesized by a wet chemical route was subjected to heavy Krypton ion irradiation of 4MeV at various fluences. Glancing incidence Xray diffraction results confirmed the phase purity of irradiated HA with a moderate contraction in lattice parameters, and further indicated the irradiation induced structural disorder, evidenced by broadening of the diffraction peaks. High resolution transmission electron microscopy observations indicated that the applied Kr irradiation induced significant damage in the hydroxyapatite lattice. Specifically, cavities were observed with their diameter and density varying with the irradiation fluences, while a radiation induced crystalline to amorphous transition with increasing ion dose was identified. Raman and Xray photoelectron spectroscopy analysis further indicated the presence of irradiation induced defects. Ion release from pristine and irradiated materials following immersion in Trisbuffer showed that dissolution in vitro was enhanced by irradiation. We examined the effects of irradiation on the early stages of the mouse osteoblast like cells response. A cell counting kit 8 assay was carried out to investigate the cytotoxicity of samples, and viable cells can be observed on the irradiated materials.

physics.app-ph↗

arXiv2019

Two-dimensional modeling of the self-limiting oxidation in silicon and tungsten nanowires

Self-limiting oxidation of nanowires has been previously described as a reaction- or diffusion-controlled process. In this letter, the concept of finite reactive region is introduced into a diffusion-controlled model, based upon which a two-dimensional cylindrical kinetics model is developed for the oxidation of silicon nanowires and is extended for tungsten. In the model, diffusivity is affected by the expansive oxidation reaction induced stress. The dependency of the oxidation upon curvature and temperature is modeled. Good agreement between the model predictions and available experimental data is obtained. The developed model serves to quantify the oxidation in two-dimensional nanostructures and is expected to facilitate their fabrication via thermal oxidation techniques. https://doi.org/10.1016/j.taml.2016.08.002

cond-mat.mtrl-sci↗

arXiv2019

Theoretical insights into the hydrophobicity of low index CeO2 surfaces

The hydrophobicity of CeO2 surfaces is examined here. Since wettability measurements are extremely sensitive to experimental conditions, we propose a general approach to obtain contact angles between water and ceria surfaces of specified orientations based on density functional calculations. In particular, we analysed the low index surfaces of this oxide to establish their interactions with water. According to our calculations, the CeO2 (111) surface was the most hydrophobic with a contact angle of Θ = 112.53° followed by (100) with Θ = 93.91°. The CeO2 (110) surface was, on the other hand, mildly hydrophilic with Θ = 64.09°. By combining our calculations with an atomistic thermodynamic approach, we found that the O terminated (100) surface was unstable unless fully covered by molecularly adsorbed water. We also identified a strong attractive interaction between the hydrogen atoms in water molecules and surface oxygen, which gives rise to the hydrophilic behaviour of (110) surfaces. Interestingly, the adsorption of water molecules on the lower-energy (111) surface stabilises oxygen vacancies, which are expected to enhance the catalytic activity of this plane. The findings here shed light on the origin of the intrinsic wettability of rare earth oxides in general and CeO2 surfaces in particular and also explain why CeO2 (100) surface properties are so critically dependant on applied synthesis methods.

cond-mat.mtrl-sci↗

arXiv2018

The effects of copper doping on photocatalytic activity at (101) planes of anatase TiO2: A theoretical study

Copper dopants are varyingly reported to enhance photocatalytic activity at titanium dioxide surfaces through uncertain mechanisms. In order to interpret how copper doping might alter the performance of titanium dioxide photocatalysts in aqueous media we applied density functional theory methods to simulate surface units of doped anatase (101) planes. By including van der Waals interactions, we consider the energetics of adsorbed water at anatase surfaces in pristine and copper doped systems. Simulation results indicate that copper dopant at anatase (101) surfaces is most stable in a 2+ oxidation state and a disperse configuration, suggesting the formation of secondary CuO phases is energetically unfavourable. In agreement with previous reports, water at the studied surface is predicted to exhibit molecular adsorption with this tendency slightly enhanced by copper. Results imply that the enhancement of photoactivity at anatase surfaces through Cu doping is more likely to arise from electronic interactions mediated by charge transfer and inter-bandgap states increasing photoexcitation and extending surface-hole lifetimes rather than through the increased density of adsorbed hydroxyl groups.

cond-mat.mtrl-sci↗

arXiv2017

Dynamic contact angle hysteresis in liquid bridges

This work presents a combined experimental and theoretical study of dynamic contact angle hysteresis using liquid bridges under cyclic compression and stretching between two identical plates. Under various loading rates, contact angle hysteresis for three different liquids was measured by examination of advancing and receding angles in liquid bridges, and the capillary forces were recorded. It is found that, for a given liquid, the hysteretic phenomenon of the contact angle is more pronounced at higher loading rates. By unifying the behaviour of the three liquids, power-law correlations were proposed to describe the relationship between the dynamic contact angle and the capillary number for advancing and receding cases. It is found that the exponents of obtained power-law correlations differ from those derived through earlier methods (e.g., capillary rise), due to the different kinematics of the triple-line. The various hysteretic loops of capillary force in liquid bridges under varied cyclic loading rates were also observed, which can be captured quantitatively by the prediction of our developed model incorporating the dynamic contact angle hysteresis. These results illustrate the importance of varying triple-line geometries during dynamic wetting and dewetting processes, and warrant an improved modelling approach for higher level phenomena involving these processes, e.g., multiphase flow in porous media and liquid transfer between surfaces with moving contact lines.

cond-mat.soft↗