Development of Titanium based in-situ composites prepared by conventional powder metallurgy route and its future aspects

Harshpreet Singh^1,2, Muhammad Hayat², Ying Xu², Peng Cao²

¹NZ Product Accelerator, University of Auckland

²Chemical and Materials Engineering, University of Auckland

Titanium and its alloys are known to possess a good set of mechanical properties. High specific properties (e.g., strength/density, stiffness/density, modulus/density), and good creep and corrosion resistance of titanium make it an attractive candidate for many applications. By reinforcing high-temperature, lightweight materials such as titanium with ceramic particulates of high strength and high stiffness, a class of composite materials have been produced and commonly referred to as titanium-based metal matrix composites (Ti-MMCs). The effect of different reinforcements (borides, nitrides and carbides) have been studied in the development of Ti-MMCs by powder metallurgy route. We have used TiB2, Si3N4 and MWCNTs as our major reinforcements for the fabrication of titanium based MMCs. The effect of reinforcements on the microstructure evolution, interfacial bonding between reinforcement and matrix and mechanical properties have been studied in detail. A systematic study of the various physical and mechanical properties of the composites was also studied. Titanium based MMCs are going to change industrial fabrication for years to come. It’s being said that, researchers admit that they have only just begun to explore the possibilities of these new materials. With the dramatically growing market for additive manufacturing and consistent property performance of an increasing number of AM titanium components, the use of metal powders for the manufacture of structural components is expected to become more acceptable than before.

Altering the band bending at (01)- and (101)-terminated surfaces of β-Ga2O3 through chemical grafting methods

Liam Carroll¹, Rodrigo Martinez-Gazoni¹, Roger Reeves¹, Martin Allen², Alison Downard¹

¹School of Physical and Chemical Sciences, University of Canterbury

²Electrical and Computer Engineering, University of Canterbury

Monoclinic gallium oxide (β-Ga2O3) is an important transparent conducting oxide (TCO) with unusual electronic characteristics compared to other TCOs. In air, the β-Ga2O3 surface exhibits native upwards band bending and as such the surface is depleted of electrons, which is a barrier to some potential applications of the material. This work investigates chemical modification of two different surfaces of β-Ga2O3 via reaction with an aryldiazonium salt and a phosphonic acid as a route to controlling the surface band bending. We compare the surface layers formed by the electrochemical reduction of 4-nitrobenzenediazonium ions and the spontaneous grafting of octadecylphosphonic acid on (01)- and (101)-terminated β-Ga2O3 single crystals. Valence band investigations show that for all samples the NP layers increase the surface upward band bending with a shift of Δ(Vbb) = ~ + 0.5 eV observed on both surfaces compared to the as-received substrates. ODPA layers result in a decrease in upward band bending with a shift of Δ(Vbb) = ~ -0.3 eV for both surfaces, a large enough shift to observe negative band bending. The greatest band bending shift observed after X-ray radiation induced reduction of the NP-modified substrates with Δ(Vbb) = ~ + 0.7 eV compared to the as-received substrates. This phenomenon can be explained by the participation of electrons from the β-Ga2O3 surfaces and possibly hydrogen from subsurface donors in the reduction process. Our study shows that covalently bound organic modifiers can alter the fundamental nature of β-Ga2O3 surfaces and can controllably increase and decrease the electron depletion and band bending at these surfaces.

Hydrothermal design of rGO/TiO₂ nanocomposite for supercapacitors

Syeda Wishal Bokhari¹, Ahmad Hassan Siddique², Wei Gao¹

¹Chemical & Materials Engineering, University of Auckland

²Ningbo Institute of Materials Technology and Engineering, China

Supercapacitors (SCs) have gained tremendous interest due to their comparatively higher electrochemical performance than batteries, fuel cells, and electrocatalytic capacitors. Talking about the SC device components, electrodes make a major component as they are the sites for the chemical reactions to occur and charge storage during the electrochemical cycles. Henceforth, many materials have been so far explored as the active/support materials for SC electrodes.

There is an increased interest in designing hybrid electrode materials for all different types of energy storage and conversion devices as the hybrid materials offer improved electrochemical charge storage. One of the key parameters for hybrid materials is that they are designed in a way to incorporate advantageous traits of the parent materials. E.g. carbon-TMO hybrid composites (HCs) are said to demonstrate a high conductance and specific capacitance, long life cycle, and good capacitance retention which is far better than their parent materials.

Reduced graphene oxide (rGO) has unique properties that can revolutionize the performance of the functional devices. rGO hybrids can be designed with transition metal oxides for improved energy storage applications. Herein, a hybrid composite of conductive rGO with titanium dioxide, designed by a simple hydrothermal method, is reported to demonstrate a high double-layer capacitance in aqueous electrolyte systems. The mesoporous structure of the composite provides short ion diffusion pathways and the resultant capacitance of the material is 334 F/g with ~77% capacitance retention after 7000 charge-discharge cycles. The HC has also shown a low contact resistance (CR) of only Ω 3.8 and charge transfer resistance (CT) of Ω ~9.8 and capacitance retention of ~77% after 7,000 cycles, which demonstrate the potential of G-TiO₂ HC as SC electrode material.

Ferromagnetic Ni_1-xFe_x nanofibers produced by electrospinning

Tehreema Nawaz¹, Grant Williams¹, Martyn Coles¹, Shen Chong²

¹School of Chemical and Physical Sciences , Victoria University of Wellington

²Robinson Research Institute, Victoria University of Wellington

Magnetic nanomaterials are well known for their intriguing properties including superparamagnetism, enhanced coercitivity, and exchange bias. These properties can be utilised in a variety of applications such as drug delivery, magnetic memory devices, spintronic devices, and in sensors.^{1, 2} Bimetallic Ni_1-xFe_x nanomaterials are particularly interesting because the bulk material has a very high magnetic permeability. They display magnetoresistance from the spin tunnelling effect between nanoparticles.³ Recently, one-dimensional nanofibers have gained popularity due to their uniaxial direction and narrow diameter that makes them favourable in nano-flux guiding and electromagnetic device systems.⁴

Herein, we report the fabrication of quasi-one-dimensional nanofibers with varying concentrations of x~0.1-0.2 using the electrospinning method. A clear difference in morphology and magnetic data was observed with a slight increase in concentration (from x~0.1 to x~0.2). Both concentrations have shown nucleation of a bimodal nanoparticle size distribution (see Fig. 1, top left inset for x~ 0.1). There are smaller nanoparticles embedded within the fiber and larger nanoparticles on the surface. They are structurally different since smaller nanoparticles were observed at the surface for x~0.2 (see Fig. 1, bottom right inset for x~ 0.2). The saturation moment per gram relative to the bulk of nanofibers (Fig. 1 hysteresis curve at 5 K for x~ 0.1) was high, which increased further in x~0.2 (Fig. 1 hysteresis curve at 5 K for x~ 0.2).

1. Williams, G. V. M.; Kennedy, J.; Murmu, P. P.; Rubanov, S. Applied Surface Science 2018, 449, 399-404.

2. Williams, G. V. M.; Kennedy, J.; Murmu, P. P.; Rubanov, S.; Chong, S. V. Journal of Magnetism and Magnetic Materials 2019, 473, 125-130.

3. Prakash, T.; Williams, G. V. M.; Kennedy, J.; Murmu, P. P.; Leveneur, J.; Chong, S. V.; Rubanov, S. Journal of Alloys and Compounds 2014, 608, 153-157.

4. Bayat, M.; Yang, H.; Ko, F. K.; Michelson, D.; Mei, A. Polymer 2014, 55, (3), 936-943.

Molecular design for surface immobilisation of spin switchable materials using Langmuir Blodgett technique

Sriram Sundaresan¹, Jonathan Kitchen², Sally Brooker¹

¹Department of Chemistry, University of Otago

²School of Natural and Computational Sciences, Massey University

Spin crossover (SCO) is a phenomenon exhibited by some octahedral d4-d7 transition metal complexes, whereby an external perturbation can switch the metal ion between the low-spin and high-spin states.¹ The transition is accompanied by changes in the chemical and physical properties of the material and so SCO-active compounds are of interest as sensors, displays, actuators and switches.² But in order to efficiently develop SCO complexes suitable for these applications it would be helpful to be able to (a) predictably tune the switching temperature (T_½)^{3, 4} and (b) immobilise them in some fashion so that they can be repeatedly addressed.⁵

The family of complexes, [Fe^II(L1^H-Me-OAlk)(NCBH₃)₂] (Figure, left), are SCO-active in the solid state:⁶ despite the alkyl tails no Langmuir work was reported. Our aim is to use the Langmuir-Blodgett technique⁵ to surface immobilise SCO-active complexes, so we have targeted adding a ‘tail’ off the literature HL2^H-OR ligand (Figure, middle, no Fe complexes),⁷ to form tailed complexes, [Fe^II(L2^H-OR)(NCE)₂] (Figure, right).⁸ This presentation will describe this study - from synthesis and characterisation, to predictable tuning of T_½ and successful surface immobilisation onto solid supports.

Towards efficient photon multiplication: harnessing luminescent perovskite nanocrystals

Matthew Brett¹, Mattie Timmer¹, Bridget Stocker¹, Nathaniel Davis¹

¹Chemical and Physical Sciences, Victoria University of Wellington

Solar cells suffer from an array of loss mechanisms, limiting their theoretical efficiency maximum to 34%.¹ A significant portion of this energy is lost via thermalisation. Singlet fission offers improvement to thermalisation, converting high energy photons into two lower energy excited states. The challenge limiting implementation of singlet fission into solar cells is efficiently accessing these excited states. These excited states can be transferred to emissive nanoparticles, forming the basis for singlet fission photon multiplication (Figure 1). Despite this, the efficiency is currently limited by the poorly emissive nanoparticles used.² This perovskite nanocrystals offer emission efficiencies approaching 100%,³ and if coupled with efficient singlet fission could offer photon multiplication close to 200% efficiency. This presentation will cover our work towards efficient photon multiplication, primarily discussing the challenges involved with ligation of singlet fission molecules to the surface of perovskite nanocrystals.