Hexagonal boron nitride nanotubes (BNNTs) have properties advantageous for the development of next generation materials and advanced composites that exhibit improved mechanical strength, high temperature resistance and thermal conductivity. The future exploitation of BNNTs depends on the development of economically accessible and safe production methods. These should also present a scope for BNNT structural tunability, and output a high purity of BNNTs versus other possible hexagonal boron nitride (h-BN) structures. In this thesis, a novel synthesis technique is validated that is primarily motivated by a demand to address these issues. Here, atmospheric pressure chemical vapour deposition (APCVD) is employed in conjunction with a safe and easy-to-handle compound, ammonia borane (H3N−BH3),to deposit h-BN coaxially onto the surface of multi-wall carbon nanotubes (MWCNTs). Subsequently, thermal oxidation is demonstrated to remove/sacrifice the MWCNTs, thereby leaving BNNTs as shells that exhibit diameter dependence on their MWCNT templates. By this method, BNNTs with walls 2-3 nm thick are synthesised, and nanotube outer diameters 16±2 nm and 40±16 nm are obtained using separate grades of MWCNT templates. The achieved BNNT diameter control exemplifies the versatility of MWCNT as a template material, which is justified by its structural compatibility to BNNTs, commercial scale availability and established morphological diversity. It is also argued here that use of H3N−BH3 precursor favours the van der Waals isolation of the deposited h-BN from the MWCNT template, which is advantageous for the h-BN purity of the BNNT shell. The work presented here also establishes that the proposed APCVD and sacrificial templating method, when implemented in combination with vacuum filtration, is a novel and adaptable means of fabricating macroscopic assemblies of BNNTs. This is achieved at the cm-scale for BNNTs randomly entangled within a low density, self-supporting and sheet-like architecture with thicknesses Throughout this work, newly synthesised nanomaterials (non-assembled and assembled) are characterised by multiple techniques including electron microscopy, spectroscopy and thermogravimetry, in order to determine their qualities such as dimensionality, crystallinity, chemical composition and thermal stability.