Rapid microwave synthesis and characterisation of group 13 carbides

Kennedy, Jennifer Louise (2015) Rapid microwave synthesis and characterisation of group 13 carbides. PhD thesis, University of Glasgow.

Abstract

This thesis describes the rapid microwave synthesis and subsequent structural characterisation of the group 13 carbides; specifically aluminium carbide, Al4C3 and boron carbide, typically B4C or B13C2. Due to practical considerations, which will be described in Chapters Three and Four, syntheses were conducted using elemental precursors.
Using a multi-mode microwave cavity (MMC) with an operating frequency of 2.45 GHz it was necessary to make use of a sealed and inert environment for the synthesis of oxide-free products. This was rationalised by the high stability of the oxides of aluminium (for example, Al2O3 ΔH= -1675.69 kJ mol-1, Al4C3 ΔH= -206.9 kJ mol-1). For boron carbide, this observation was explained by the tendency for the oxides of boron to volatilise (for example as B2O3). The use of sealed, evacuated tubes facilitated the synthesis of high-purity carbide products in 30 minute timescales. This represents the first report of successful Al4C3 synthesis in a microwave cavity. In addition, the synthesis of boron carbide was achieved in air in just 90 seconds using a 2.45 GHz single-mode microwave cavity (SMC) for the first time.
Following synthesis, products were characterised by powder X-ray diffraction (PXD), powder neutron diffraction (PND), Raman spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX). PXD was used for phase identification and preliminary structure refinement. Raman spectroscopy offered supporting information to PXD to confirm phase identity. SEM-EDX offered an insight into product morphology, for example, as a function of time, microwave power and cavity type, along with quantitative purity information. There was no PXD or EDX evidence for oxygen contamination across carbide samples. PND was used to probe defect structure and structural stability at elevated temperature. Microwave synthesised Al4C3 was structurally stable up to 1000 °C and boron carbide up to 400 °C (the maximum temperatures of the respective experiments).
Aluminium carbide is reported elsewhere to interact with water. Some initial experiments surrounding the nature of this interaction have been conducted. The preliminary results have indicated potential for intercalation of water into the aluminium carbide structure.
Obtaining a definitive structural model for boron carbide is complicated by the difficulty in distinguishing between boron and carbon experimentally. Boron and carbon are neighbours in the periodic table and near-isoelectronic. This complicates characterising the boron-carbon system by PXD, since PXD is mediated by electrons. There are also challenges posed by the system when using PND. The 10B isotope has an extremely high absorption cross section for neutrons; 3835.0(9) barn for 2200 m s-1 neutrons. In addition, the bound coherent neutron scattering length of 11B is very similar to that of carbon; C: 6.6460 fm and 11B: 6.65 fm. In this thesis, natural boron (abundance typically ~80% 11B and 20% 10B) was used to prepare boron carbide for PND experiments. By taking advantage of the contrast in scattering lengths of carbon and 10B, while obtaining high quality data by the presence of the non-absorbing isotope 11B, it was possible to derive a structural model for microwave-synthesised boron carbide.
It is expected that the synthetic methods developed in this thesis can be applied to more complex carbide systems and beyond. A particularly encouraging result from this work is the feasibility of synthesising high-purity, crystalline boron carbide in 90 second timescales in open air. Such a synthesis would be compatible with an SMC based continuous flow process which may offer a step reduction in energy usage compared to conventional batch processes.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Due to copyright restrictions the full text of this thesis cannot be made available online. Access to the printed version is available.
• Keywords: Microwave, carbide
• Subjects: Q Science > QD Chemistry; T Technology > TA Engineering (General). Civil engineering (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Supervisor's Name: Drysdale, Dr. Timothy D.
• Date of Award: 2015
• Depositing User: Miss Jennifer L Kennedy
• Unique ID: glathesis:2015-6504
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 06 Jul 2015 09:08
• Last Modified: 08 Jul 2015 09:15
• URI: https://theses.gla.ac.uk/id/eprint/6504