Nonlinear Thermopower Behaviour of N-Type Carbon Nanofibres and Their Melt Mixed Polypropylene Composites

The temperature dependent electrical conductivity σ (T) and thermopower (Seebeck coefficient) S (T) from 303.15 K (30 °C) to 373.15 K (100 °C) of an as-received commercial n-type vapour grown carbon nanofibre (CNF) powder and its melt-mixed polypropylene (PP) composite with 5 wt.% of CNFs have been analysed. At 30 °C, the σ and S of the CNF powder are ~136 S m−1 and −5.1 μV K−1, respectively, whereas its PP/CNF composite showed lower conductivities and less negative S-values of ~15 S m−1 and −3.4 μV K−1, respectively. The σ (T) of both samples presents a dσ/dT < 0 character described by the 3D variable range hopping (VRH) model. In contrast, their S (T) shows a dS/dT > 0 character, also observed in some doped multiwall carbon nanotube (MWCNT) mats with nonlinear thermopower behaviour, and explained here from the contribution of impurities in the CNF structure such as oxygen and sulphur, which cause sharply varying and localized states at approximately 0.09 eV above their Fermi energy level (EF).

Electrode Nanomaterials and Electrolytes for Aluminium-ion Batteries

<p>Energy is one of the biggest challenges of the 21st century. Factors such as the decline in availability of non-renewable power sources, the alarming levels of atmospheric CO₂, and the steady increase of the worldwide demand for energy make the worldwide transition to a fully renewable source-based production an extremely urgent necessity. Because of the intermittent nature of most renewable energy sources, battery-based energy storage systems could be a useful tool for such transition. However, current battery technologies such as lithium-ion often lack the cost-effectiveness and safety requirements necessary for large-scale grid energy storage applications; it is therefore important to search for alternative technologies which are more suitable for this purpose. Aluminium-ion batteries have recently emerged as a very promising alternative to lithium-based systems, thanks to the low cost, non-flammability, and three-electron redox chemistry of aluminium. Al-ion batteries could, in principle, offer better cost-effectiveness, higer capacity and improved safety, which would lead to a substantial advance in energy storage technology. This PhD project deals with the investigation of novel electrode nanomaterials and electrolyte systems for Al-ion batteries. Particular emphasis is put on using the special properties of nanomaterials to improve the performance of batteries and on searching for low-cost compounds to be used as alternative electrolytes. Developing these areas will enhance the cost-effectiveness of the technology, and facilitate its commercial feasibility. Vanadium oxide nanofibres and carbon nanofibres were initially tested as cathode materials. The performance of such cathodes, however, did not meet expectations: V₂O₅ nanofibres showed poor reversibility, short cycling life, and underwhelming specific capacity, while carbon nanofibres displayed a mostly capacitive, adsorption-based energy storage mechanism, with no significant ion intercalation taking place. Nevertheless, the tests performed on the latter material led to the discovery of the phenomenon of solid-electrolyte interphase formation: this process was investigated in depth and found to be mainly caused by the presence of defects on the surface of the nanofibres, favouring the decomposition of the electrolyte into insoluble species during the charging phase. Two composite materials were then tested as cathode candidates: solvothermally-prepared core-sheath C/V₂O₅ nanofibres, and a layered nanostructured electrode. The former material showed an interesting behaviour as a battery cathode, as evidence for a multiple-ion intercalation mechanism was found; this phenomenon is however short-lived, as the cathode tends to disintegrate after the first few charge-discharge cycles. In a similar fashion, the fabrication methods used to create the layered electrode were shown to be unreliable: the poor adhesion of the active material to the underlying current collector resulted in highly unstable performance of the cathode, leading to the premature failure of the battery device. Within alternative electrolytes, mixtures of inorganic and non-ionic organic compounds were studied. Eutectic mixtures of aluminium trichloride with acetamide and other small amide analogues were found to achieve good performance as battery electrolytes. Reduction of viscosity was found to be the key to improve cycling performance: this was achieved either by dilution of the electrolytes with an appropriate solvent, or by using combinations of amides to weaken the inter-molecular interactions present in the melts.</p>

Electrode Nanomaterials and Electrolytes for Aluminium-ion Batteries

<p>Energy is one of the biggest challenges of the 21st century. Factors such as the decline in availability of non-renewable power sources, the alarming levels of atmospheric CO₂, and the steady increase of the worldwide demand for energy make the worldwide transition to a fully renewable source-based production an extremely urgent necessity. Because of the intermittent nature of most renewable energy sources, battery-based energy storage systems could be a useful tool for such transition. However, current battery technologies such as lithium-ion often lack the cost-effectiveness and safety requirements necessary for large-scale grid energy storage applications; it is therefore important to search for alternative technologies which are more suitable for this purpose. Aluminium-ion batteries have recently emerged as a very promising alternative to lithium-based systems, thanks to the low cost, non-flammability, and three-electron redox chemistry of aluminium. Al-ion batteries could, in principle, offer better cost-effectiveness, higer capacity and improved safety, which would lead to a substantial advance in energy storage technology. This PhD project deals with the investigation of novel electrode nanomaterials and electrolyte systems for Al-ion batteries. Particular emphasis is put on using the special properties of nanomaterials to improve the performance of batteries and on searching for low-cost compounds to be used as alternative electrolytes. Developing these areas will enhance the cost-effectiveness of the technology, and facilitate its commercial feasibility. Vanadium oxide nanofibres and carbon nanofibres were initially tested as cathode materials. The performance of such cathodes, however, did not meet expectations: V₂O₅ nanofibres showed poor reversibility, short cycling life, and underwhelming specific capacity, while carbon nanofibres displayed a mostly capacitive, adsorption-based energy storage mechanism, with no significant ion intercalation taking place. Nevertheless, the tests performed on the latter material led to the discovery of the phenomenon of solid-electrolyte interphase formation: this process was investigated in depth and found to be mainly caused by the presence of defects on the surface of the nanofibres, favouring the decomposition of the electrolyte into insoluble species during the charging phase. Two composite materials were then tested as cathode candidates: solvothermally-prepared core-sheath C/V₂O₅ nanofibres, and a layered nanostructured electrode. The former material showed an interesting behaviour as a battery cathode, as evidence for a multiple-ion intercalation mechanism was found; this phenomenon is however short-lived, as the cathode tends to disintegrate after the first few charge-discharge cycles. In a similar fashion, the fabrication methods used to create the layered electrode were shown to be unreliable: the poor adhesion of the active material to the underlying current collector resulted in highly unstable performance of the cathode, leading to the premature failure of the battery device. Within alternative electrolytes, mixtures of inorganic and non-ionic organic compounds were studied. Eutectic mixtures of aluminium trichloride with acetamide and other small amide analogues were found to achieve good performance as battery electrolytes. Reduction of viscosity was found to be the key to improve cycling performance: this was achieved either by dilution of the electrolytes with an appropriate solvent, or by using combinations of amides to weaken the inter-molecular interactions present in the melts.</p>

Processing and Characterization of Carbon Nanofibre Composites for Automotive Applications

Currently, numerous studies have shown that carbon nanofibres have mechanical properties that are replaced by other widely used fibres. The high tensile strength of the carbon fibres makes them ideal to use in polymer matrix composites. The high-strength fibres can be used in short form in a composite and mass-produced to meet the high demands of automotive applications. These composites are capable of addressing the strength requirement of nonstructural and structural components of the automotive industry. Due to these composite lightweight and high-strength weight ratios, the applications can be widely varying. The research for these materials is a never-ending process, as researchers and design engineers are yet to tap its full potential. This study fabricated phenolic resin with different wt% of carbon nanofibre (CNF). The percentage of the CNF as a filler material is varied from 1 to 4 wt%. Mechanical properties such as hardness, tensile strength, and XRD were investigated. Phenolic resin with 4 wt% of carbon nanofibre (CNF) exhibits maximum tensile strength and hardness of 43.8 MPa and 37.8 HV.

The Impact of Carbon Nanofibres on the Interfacial Properties of CFRPs Produced with Sized Carbon Fibres

In this work, different amounts of CNFs were added into a complex formulation to coat the CFs surfaces via sizing in order to enhance the bonding between the fibre and the resin in the CF-reinforced polymer composites. The sized CFs bundles were characterised by SEM and Raman. The nanomechanical properties of the composite materials produced were assessed by the nanoindentation test. The interfacial properties of the fibre and resin were evaluated by a push-out method developed on nanoindentation. The average interfacial shear strength of the fibre/matrix interface could be calculated by the critical load, sheet thickness and fibre diameter. The contact angle measurements and resin spreadability were performed prior to nanoindentation to investigate the wetting properties of the fibre. After the push-out tests, the characterisation via optical microscopy/SEM was carried out to ratify the results. It was found the CFs sizing with CNFs (1 to 10 wt%) could generally increase the interfacial shear strength but it was more cost-effective with a small amount of evenly distributed CNFs on CFs.

Occupational Exposure to Carbon Nanotubes and Carbon Nanofibres: More Than a Cobweb

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are erroneously considered as singular material entities. Instead, they should be regarded as a heterogeneous class of materials bearing different properties eliciting peculiar biological outcomes both in vitro and in vivo. Given the pace at which the industrial production of CNTs/CNFs is increasing, it is becoming of utmost importance to acquire comprehensive knowledge regarding their biological activity and their hazardous effects in humans. Animal studies carried out by inhalation showed that some CNTs/CNFs species can cause deleterious effects such as inflammation and lung tissue remodeling. Their physico-chemical properties, biological behavior and biopersistence make them similar to asbestos fibers. Human studies suggest some mild effects in workers handling CNT/CNF. However, owing to their cross-sectional design, researchers have been as yet unable to firmly demonstrate a causal relationship between such an exposure and the observed effects. Estimation of acceptable exposure levels should warrant a proper risk management. The aim of this review is to challenge the conception of CNTs/CNFs as a single, unified material entity and prompt the establishment of standardized hazard and exposure assessment methodologies able to properly feeding risk assessment and management frameworks.

Investigation of metallic nanoparticle distribution in PAN/magnetic nanocomposites fabricated with needleless electrospinning technique

Needleless electrospinning can be used to produce polyacrylonitrile nanofibres, for example, to which magnetic nanoparticles can additionally be added. Such composite nanofibres can then be stabilised and carbonised to produce carbon composite nanofibres. The magnetic nanoparticles have an influence not only on the structure but also on the mechanical and electrical properties of the finished carbon nanofibres, as does the heat treatment during stabilisation and incipient carbonisationThe present study reports on the fabrication, heat treatment and resulting properties of poly(acrylonitrile) (PAN)/magnetic nanofibre mats prepared by needleless electrospinning from polymer solutions. A variety of microscopic and thermal characterisation methods were used to investigate in detail the chemical and morphological transition during oxidative stabilisation (280 °C) and incipient carbonisation (500 °C). PAN and nanoparticles were analysed during all stages of heat treatment. Compared to pure PAN nanofibres, the PAN/ magnetic nanofibers showed larger fiber diameters and the presence of beads and agglomerations. In this study, magnetic nanofibers were investigated in more detail with the aim of detecting undesired agglomerations. Visual observation, for example with CLSM or SEM, does not provide conclusive evidence of agglomerations in the nanofibers. But based on the capabilities of SEM/EDS many different types of samples can be easily analysed where other analytical techniques simply cannot give the fast answer.