Initiation of smouldering combustion in biomass

Wildfires are naturally occurring phenomena that result in significant and catastrophic damage. Due to climate change, there has been a significant increase in the frequency, severity, and extent of wildfires. Therefore, there is a growing need to mitigate wildfire risk. In order to help mitigate the risk of wildfires, greater understanding is required. One particular gap in knowledge is the impact of smouldering combustion of potential fuel on wildfires. This thesis focuses on combustion of fuel beds in wildfires. Specifically, the thesis targets smouldering combustion. Smouldering combustion is a common type of combustion regime in wildfires and hazard reduction burning (a wildfire mitigation measure). Smouldering is a slow and low-temperature form of combustion, which shows no flame. Smouldering is a serious hazard because of its low ignition temperature, which makes it particularly relevant to fire initiation and spread. Smouldering plays a vital role in wildfires, as many forest biomass fuels such as grass, leaves and coarse woody debris are prone to smoulder. Most previous studies of smouldering combustion have only been carried out on polyurethane foam, due to its importance for residential fires. However, smouldering has been scarcely investigated from the point of view of wildfires. For example, smouldering combustion of forest fuel is scarcely studied. Hence, the project aims to develop a greater understanding of the initiation of smouldering combustion in biomass under different conditions with an emphasis on wildfire. Locating smouldering combustion in wildfires and hazard reduction burning is difficult and time-consuming, as there is no effective method to identify the initiation of smouldering combustion in biomass fuel beds. It is critical to know when and where smouldering combustion in a biomass fuel bed starts, as smouldering combustion could transition to flaming combustion under certain conditions. Radiation is one of the important heat transfer mechanisms in wildfires; however, there are few studies on smouldering combustion in biomass fuel beds started by external radiant heat flux. Although oxidiser flow rate and oxygen concentration have significant in influences on the propagation of smouldering front, their effects on the initiation of smouldering combustion in biomass fuels are not well understood. Hence, the effects of oxidiser flow rate and oxygen concentration on the initiation of smouldering combustion are investigated. Fuels in a forest are diverse, and it is essential to have a better understanding of what effects forest fuels have on smouldering combustion. Thus, the effects of plant species and plant parts on the initiation of smouldering in biomass fuel beds are also investigated. Within this framework, the work presented in this thesis can be split into two main topics: 1. Conditions required to initiate smouldering combustion in bio- mass fuel beds The required radiant heat flux and air flow rate for the initiation of smouldering and flaming combustion in a biomass fuel bed are investigated in an experimental testing rig. This investigation identifies and quantifies smouldering and flaming combustion in a biomass fuel bed based on the measurements of temperature, product gas concentration and mass change, and the required radiant heat flux and air flow rate for the initiation of smouldering and flaming combustion are determined. The effects of heating time and oxygen concentration on the initiation of radiation-aided and self-sustained smouldering combustion are investigated in the same testing rig. In this experimental study, the differences between radiation-aided and self-sustained smouldering combustion are characterised based on the measurements of temperature, product gas concentration and mass change, and the required heating time and oxygen concentration for radiation-aided and self-sustained smouldering combustion are determined. 2. Factors that influence smouldering combustion in biomass fuel beds The results from the first topic reveal that oxygen availability has significant effects on the initiation of smouldering combustion in a biomass fuel bed. The air permeability of a biomass fuel bed determines oxygen availability in that fuel bed. Hence, the air permeability of natural forest fuel beds is investigated in an air permeability testing rig. In this study, the air permeability of natural forest fuel beds is determined using experimental and theoretical methods. A comparison between the experimental and theoretical methods is made. The effects of Euca- lyptus species and plant parts on smouldering combustion are also investigated. In this study, the different plant parts from different Eucalyptus species are characterised based on the results of the thermogravimetric and ultimate analyses. The results of this study show that the differences among the different plant parts from different Eucalyptus can be characterised and quantified based on the results of the thermogravimetric and ultimate analyses. It is also found that Eucalyptus species and plant parts have significant effects on smouldering combustion. Although this thesis covers a series of experimental studies of the initiation of smouldering combustion in biomass fuel beds. There are still many important factors to be considered. For examples, the thesis focuses on small-scale laboratory experiments to better understand the fundamental studies of smouldering combustion of biomass. However, the real-world conditions could be much more complex. For example, forest fuel beds are composed with fuel particles with various sizes and shapes. These factors also have effects on smouldering combustion.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Mechanical Engineering, 201

Tuning a binary ferromagnet into a multi-state synapse with spin-orbit torque induced plasticity

Inspired by ion-dominated synaptic plasticity in human brain, artificial synapses for neuromorphic computing adopt charge-related quantities as their weights. Despite the existing charge derived synaptic emulations, schemes of controlling electron spins in ferromagnetic devices have also attracted considerable interest due to their advantages of low energy consumption, unlimited endurance, and favorable CMOS compatibility. However, a generally applicable method of tuning a binary ferromagnet into a multi-state memory with pure spin-dominated synaptic plasticity in the absence of an external magnetic field is still missing. Here, we show how synaptic plasticity of a perpendicular ferromagnetic FM1 layer can be obtained when it is interlayer-exchange-coupled by another in-plane ferromagnetic FM2 layer, where a magnetic-field-free current-driven multi-state magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin-orbit torque. We use current pulses to set the perpendicular magnetization state which acts as the synapse weight, and demonstrate spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing-dependent plasticity. This functionality is made possible by the action of the in-plane interlayer exchange coupling field which leads to broadened, multi-state magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in-plane field component tuning the efficiency of the spin-orbit-torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside our Pt/FM1/Ta/FM2 structure with inherent in-plane coupling field is therefore tuned into a multi-state perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing.Comment: 37 pages with 11 figures, including 20 pages for manuscript and 17 pages for supplementary informatio

Tuning a binary ferromagnet into a multi-state synapse with spin-orbit-torque-induced plasticity

Ferromagnets with binary states are limited for applications as artificial synapses for neuromorphic computing. Here, it is shown how synaptic plasticity of a perpendicular ferromagnetic layer (FM1) can be obtained when it is interlayer exchange‐coupled by another in‐plane ferromagnetic layer (FM2), where a magnetic field‐free current‐driven multistate magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin–orbit torque. Current pulses are used to set the perpendicular magnetization state, which acts as the synapse weight, and spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing‐dependent plasticity are demonstrated. This functionality is made possible by the action of the in‐plane interlayer exchange coupling field which leads to broadened, multistate magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in‐plane field component tuning the efficiency of the spin–orbit torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside the Pt/FM1/Ta/FM2 structure with an inherent in‐plane coupling field is therefore tuned into a multistate perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing, demonstrating a significant pathway toward a combination of spintronics and synaptic electronics

Adjustable current-induced magnetization switching utilizing interlayer exchange coupling

Electrical current-induced deterministic magnetization switching in a magnetic multilayer structure without external magnetic field is realized by utilizing interlayer exchange coupling. Two ferromagnetic Co layers, with in-plane and out-of-plane anisotropy respectively, are separated by a spacer Ta layer, which plays a dual role of inducing antiferromagnetic interlayer coupling, and contributing to the current-induced effective magnetic field through the spin Hall effect. The current-induced magnetization switching behavior can be tuned by pre-magnetizing the in-plane Co layer. The antiferromagnetic exchange coupling field increases with decreasing thickness of the Ta layer, reaching 630 ±5 Oe for a Ta thickness of 1.5nm. The magnitude of the current-induced perpendicular effective magnetic field from spin-orbit torque is 9.2 Oe/(107Acm-2). The large spin Hall angle of Ta, opposite in sign to that of Pt, results in a low critical current density of 9×106A/cm2. This approach is promising for the electrical switching of magnetic memory elements without external magnetic field

Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors

A critical challenge for the integration of the optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. It is generally believed that a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the photoexcited carriers close to the metal-GaSe interface and the photocurrent active region is always close to the Schottky barrier with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happen, which was significantly enhanced up to 5,000 AW-1 for the bottom contacted device at bias voltage 8 V and wavelength of 410 nm. It is more than 1,700-fold improvement over the previously reported results. Besides the systematically experimental investigation of the dependence of the photoresponsivity on the spacing distance for both the bottom and top contacted MSM photodetectors, a theoretical model has also been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize shrinking the spacing distance and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of 2D semiconductor optoelectronics with high performances.Comment: 25 pages, 4 figure

Deterministic Magnetization Switching Using Lateral Spin–Orbit Torque

Current-induced magnetization switching by spin-orbit torque (SOT) holds considerable promise for next generation ultralow-power memory and logic applications. In most cases, generation of spin-orbit torques has relied on an external injection of out-of-plane spin currents into the magnetic layer, while an external magnetic field along the electric current direction is generally required for realizing deterministic switching by SOT. Here, we report deterministic current-induced SOT full magnetization switching by lateral spin-orbit torque in zero external magnetic field. The Pt/Co/Pt magnetic structure was locally annealed by a laser track along the in-plane current direction, resulting in a lateral Pt gradient within the ferromagnetic layer, as confirmed by microstructure and chemical composition analysis. In zero magnetic field, the direction of the deterministic current-induced magnetization switching depends on the location of the laser track, but shows no dependence on the net polarization of external out-of-plane spin currents. From the behavior under external magnetic fields, we identify two independent mechanisms giving rise to SOT, i.e. the lateral Pt-Co asymmetry as well as out-of-plane injected spin currents, where the polarization and the magnitude of the SOT in the former case depends on the relative location and the laser power of the annealing track. Our results demonstrate an efficient field-free deterministic full magnetization switching scheme, without requiring out-of-plane spin current injection or complex external stack structures.Comment: 39 Pages, 9 Figure

Toughness modification of SBS/CRMA on epoxy asphalt: curing behaviour and low-temperature cracking characteristic analysis

The limited utilisation of epoxy asphalt primarily stems from its inherent brittle cracking behaviour. This investigation aims to mitigate this issue by introducing styrene–butadiene-styrene/crumb rubber modified asphalt (SBS/CRMA). Furthermore, this research assesses the influence of SBS/CRMA on the curing behaviour of epoxy asphalt (EA) and the low-temperature fracture characteristics of epoxy asphalt concrete (EAC). The curing behaviour of EA was systematically examined using diverse analytical techniques, including attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR), rotational viscosity, and non-isothermal curing kinetics analysis. Notably, SBS/CRMA expedites the curing process of epoxy asphalt, possibly attributed to the amine constituents in the crumb rubber, which acts as catalyst. Subsequently, the study scrutinised the fracture characteristics of EAC through the application of the semicircle bending (SCB) test and acoustic emission (AE) methodology. The results substantiate that SBS/CRMEAC imparts a proficient toughening effect, underpinned by an augmented stress relaxation capacity due to the reinforcing influence of elastomers such as crumb rubber and SBS. The fracture process was delineated into three distinct stage and the AE signal in matrix epoxy asphalt (MEA) exhibited concentration during the macroscopic crack extension and final fracture stages. In stark contrast, SBS/CRMEAC exhibited a uniform distribution and showcased ductile fracture characteristics

Complementary Lateral‐Spin–Orbit Building Blocks for Programmable Logic and In‐Memory Computing

Current-driven switching of nonvolatile spintronic materials and devices based on spin-orbit torques offer fast data processing speed, low power consumption, and unlimited endurance for future information processing applications. Analogous to conventional CMOS technology, it is important to develop a pair of complementary spin-orbit devices with differentiated magnetization switching senses as elementary building blocks for realizing sophisticated logic functionalities. Various attempts using external magnetic field or complicated stack/circuit designs have been proposed, however, plainer and more feasible approaches are still strongly desired. Here we show that a pair of two locally laser annealed perpendicular Pt/Co/Pt devices with opposite laser track configurations and thereby inverse field-free lateral spin-orbit torques (LSOTs) induced switching senses can be adopted as such complementary spin-orbit building blocks. By electrically programming the initial magnetization states (spin down/up) of each sample, four Boolean logic gates of AND, OR, NAND and NOR, as well as a spin-orbit half adder containing an XOR gate, were obtained. Moreover, various initialization-free, working current intensity-programmable stateful logic operations, including material implication (IMP) gate, were also demonstrated by regarding the magnetization state as a logic input. Our complementary LSOT building blocks provide a potentially applicable way towards future efficient spin logics and in-memory computing architectures.