Aidan Westwood

Influence of infiltration temperature on the microstructure and oxidation behavior of SiC–ZrC ceramic coating on C/C composites prepared by reactive melt infiltration

SiC–ZrC ceramic coating on C/C composites was prepared by reactive melt infiltration (RMI) using a powder mixture composed of Zr, Si and C as the infiltrator. The phase composition and microstructure of the ceramic coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The oxidation resistance of the as-prepared composites was tested at 1550 °C in static air. The results indicate that the infiltration temperature has remarkable effects on the phase composition and microstructure of the ceramic coating, as well as on the oxidation resistance of the composites. The SiC–ZrC coated C/C composites prepared at 2000 °C exhibit an excellent oxidation resistance. They gain weight about 5.9 wt% after oxidation at 1550 °C in static air for 5 h, whereas the SiC–ZrC coated C/C composites prepared at 1800 °C lose weight about 3.2 wt%. As a comparison, SiC coated C/C composites prepared at 2000 °C by RMI show an inferior oxidation resistance. After 5 h oxidation, SiC coated C/C composites are severely damaged and their weight loss reaches up to 44.3 wt%. The outstanding oxidation resistance of the SiC–ZrC coated C/C composites prepared at 2000 °C can be attributed to the rapid formation of a continuous glass-like layer composed of ZrO2, ZrSiO4 and SiO2, which covers the surface of the composites and retards the oxygen diffusion and the attack on the underlying C/C substrate. For SiC coated C/C composites, the large SiC particles formed on the surface of the composites are difficult to oxidize rapidly and so a continuous and dense SiO2 layer cannot be formed in time to significantly hinder fast oxygen diffusion leading to the consequent severe oxidation of the C/C substrate.

Universal synthesis method for mixed phase TiO2(B)/anatase TiO2 thin films on substrates via a modified low pressure chemical vapour deposition (LPCVD) route

A universal method for the synthesis of mixed phase TiO2 bronze (B)/anatase titania thin films by Low Pressure Chemical Vapour Deposition (LPCVD) onto any substrate is presented. General LPCVD conditions were titanium isopropoxide (TTIP) and N2 gas as the precursor and carrier gas respectively, 600 °C nominal reaction temperature, and 15 min reaction time; a range of different substrates were investigated including: a silicon wafer, fused quartz, highly ordered pyrolytic graphite (HOPG) and pressed graphite flake (grafoil). X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy were used to characterise the thin films which exhibited a columnar morphology together with smaller equi-axed particles. Pre-treatment of substrates by spraying with a Na-containing solution was found to encourage the crystallization of TiO2(B) during the LPCVD process. Increasing the concentration of Na in the pre-treatment process resulted in a higher proportion of TiO2(B) in the thin films up to an optimum condition of 0.75% w/v of Na. Na diffusion from the substrate surface into the adjacent TiO2 is the proposed mechanism for promoting TiO2(B) formation as opposed to the anatase phase with Density Functional Theory (DFT) modelling suggesting the presence of Na stabilises the TiO2(B) phase. Dye degradation tests indicate an increased photocatalytic activity for mixed phase anatase/TiO2(B) thin films.

Performance of graphite nanoplatelet/silicone composites as thermal interface adhesives

Graphite nanoplatelets (GNP)/silicone composites are potential thermal interface materials due to their high thermal conductivity and compliance. In this study, performance as thermal interface materials is studied by measuring thermal contact resistance. The effect of surface roughness, particle size of GNPs, wt% GNPs, temperature and applied pressure on the thermal contact resistance of the composite coatings was determined. The GNP/silicone coating performed much better on rough surfaces than on smooth surfaces. The composite coating consisting of large GNPs is more effective than small GNPs probably due to the two times higher thermal conductivity of the former. The thermal contact resistance of the GNP/silicone composite increased by ~3–10% with an increase of temperature but remained unaffected by an increase of pressure. The comparison of GNP/silicone composite coatings with GNP-based thermal pastes showed that the former perform much better in thick bond lines.

Investigating the distribution and bonding of light elements alloyed in carbonaceous materials using EELS in the TEM/STEM

Abstract Carbon-boron-nitrogen (CBN) alloys prepared by pyrolysis have been characterized using electron energy loss spectroscopy (EELS). Particular attention was paid to the homogeneity in the chemical composition over the nanometre length scale and to the extraction of local bonding information which are both of prime importance for properties such as strength, oxidation resistance as well as for subsequent materials processing. Further examples of the power of EELS analysis are provided by studies of elemental distributions in C fibres prepared from polymeric precursors, the investigation of the structure of vapour grown C fibres and the identification of interfacial reactions in C fibre/SiC composites.

Graphite nanoplatelet/silicone composites for thermal interface applications

Thermally conducting and effectively electrically insulating nanocomposites for thermal interface applications were developed by dispersing graphite nanoplatelets (GNP) into a silicone matrix by dual asymmetric centrifuge mixing. Thermal conductivity, electrical conductivity, compression and hardness properties of the resulting composites were measured. The effects of GNP particle size and wt.% of GNP on the thermal conductivity and curing behaviour of the composites were also investigated. The results showed that the thermal conductivity of the GNP/silicone composites (having GNP with an average particle size of 15 µm), measured by the hot disk technique, reached 1.4 W/m.K at a loading of 20 wt.% (compared to 0.65 W/m.K for the, otherwise identical, system with an average particle size of 5 µm). The former represents a 7-fold increase compared to the thermal conductivity at 20°C of silicone alone. SEM analysis revealed that the composites consist of homogeneous randomly dispersed GNP in silicone at loading levels greater or equal to 15 wt.%, whereas at lower loadings a concentration gradient effect (due to settling) can be inferred. Differential scanning calorimeter (DSC) analysis showed that GNP addition increased the curing temperature of silicone from 92°C to 116°C, probably by hindering the free movement (mobility) of the silicone chains. Compression and Shore hardness testing results, perhaps unexpectedly, showed that the presence of the GNP did not increase the stiffness and compressive strength of the silicone. The GNP/silicone composites have thermal conductivities that are comparable to commercially available thermal interface materials but they have increased compliance, which is an advantage in gap-filling applications, whilst offering potential cost savings by using cheaper filler at lower loadings.

Structure of different grades of nuclear graphite

Owing to its low neutron absorption cross-section, large scattering cross section and thermal and chemical stability, graphite is a key component of operational nuclear reactors where it is used as a moderator, reflector and as major structural component for 90% of current UK nuclear plants. It is also of interest for use in developing the future high temperature gas-cooled reactors. The properties of the nuclear graphite are influenced by its structural characteristics, which change as a function of neutron irradiation, temperature and oxidation. The principal structural changes during neutron irradiation that affect the integrity and dimensions of nuclear graphite components, thereby affecting service lifetime, are that the a-axis contracts and the c-axis expands in the crystallites. Characterization of virgin graphite structure and of the damage evolution after irradiation of nuclear graphite has an important role to play in the understanding and development of materials used in current and future nuclear reactors, respectively.

The synthesis of single-walled carbon nanotubes over an Al2O3Fe2O3 binary aerogel catalyst

An Al2O3Fe2O3 binary aerogel was used as a catalyst for the synthesis of singlewalled carbon nanotubes. The carbon products were synthesized by catalytic decomposition of methane or a CH4/H2 mixture at 820-960°C for 30 min. The influence of preparation atmosphere on the growth of single-walled carbon nanotubes was investigated. The morphology and the structure of carbon products were investigated by TEM, HRTEM and Raman spectroscopy analyses. The carbon products prepared under a methane atmosphere are mainly amorphous. High-purity individual single walled carbon nanotubes (SWCNTs) and bundles were synthesized under the mixed atmosphere of CH4/H2. The results show that the atmospheres exhibit a close relationship to the yield and structure of carbon products formed.

Water assisted synthesis of clean single-walled carbon nanotubes over a Fe2O3/Al2O3 binary aerogel catalyst

Abstract A Fe 2 O 3 /Al 2 O 3 binary aerogel was used as a catalyst for the synthesis of single-walled carbon nanotubes (SWCNTs) by the catalytic decomposition of CH 4 -H 2 and a CH 4 -H 2 -water mixture at 900 °C for 30 min. The influence of water vapor on the growth of SWCNTs was investigated by characterizing the morphology and structure of the carbon products using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Results show that SWCNTs with a high purity were synthesized under the mixed atmosphere of CH 4 -H 2 -water when the amount of water vapor present was controlled by bubbling H 2 through water. The yield and microstructure of the as-formed carbon products can be correlated with the composition of the reaction mixture.

Nanostructural development of non-graphitising carbons probed using TEM/EELS: The importance of fullerenes?

This work presents a comprehensive study of a set of non-graphitising carbons as a function of heat treatment temperature (HTT) up to 3000 °C using a combination of density measurements, X-ray and electron diffraction, high resolution TEM and low loss and core loss EELS. Results are compared to those obtained from graphitising carbons. Of interest is the variation in plasmon energy with HTT and its relationship to density as well as the ratio of sp2to sp3 bonded carbon species derived from the C K-edge using a revised analytical procedure. Results are reviewed in light of the fairly recent suggestion that the intrinsic structure of non-graphitising carbons is based on the inclusion of fullerene-like units. We speculate as to whether this might have a broader significance for carbonaceous materials in general.

Nanostructure of a Glass-Like Carbon Characterized by X-Ray Diffraction and Electron Energy Loss Spectroscopy

In our present study, phenolic resins heated at different temperatures from 600 oC to 3000 oC were analysised in terms of phase structure and chemical structure using X-ray diffraction and high resolution transmission electron microscopy equipped with an electron energy loss spectrometer (EELS). It is shown that materials appear to be amorphous with many micro-pores surrounded by crystalline graphite layers; the formation of the pore is due to the gas evolution reaction. Using the intensities of peaks in EELS CK-ionization edge which arise from transition of an atomic 1s electron to the π* and σ* antibonding band-like states, the percentage of sp2-bonded carbon have been analyzed and the results reveal notable differences with heat treatment temperature.