David J. Cassidy

Crystallisation of zirconolite from an alkoxide precursor

Abstract Crystallisation of zirconolite (CaZrTi2O7) from a stoichiometric alkoxide precursor was studied by X-ray diffraction, differential thermal analysis and high resolution electron microscopy. The X-ray amorphous oxide formed by drying mixed ethanolic solutions crystallised to a disordered fluorite structure on heating to ~ 700 °C. Zirconolite with a pseudo-trigonal structure formed at ~ 900 °C, and at higher temperatures gradually transformed to fully ordered monoclinic zirconolite. Activation energies for the amorphous to fluorite and fluorite to pseudo-trigonal zirconolite transformations were obtained from the DTA measurements by the Ozawa method, but were not consistent with the rates of transformation observed by X-ray diffraction in isothermal measurements. The pseudo-trigonal zirconolite was found by high-resolution electron microscopy to be highly disordered monoclinic zirconolite.

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples.The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of γ-Al2O3 spinel and mullite, or simple γ-Al2O3 spinel, which subsequently transformed to mullite at 1260°C.Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of γ-Al2O3 spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.

Synthesis and characterisation of nanocomposite materials prepared by dispersion of functional TiO2 nanoparticles in PMMA matrix

Composite powders and thin films composed of poly(methyl methacrylate) (PMMA) and functionalised titania nanoparticles are successfully prepared by in situ bulk co-polymerisation using benzoyl peroxide (BPO) as the initiator. The functionalised titania nanoparticles are synthesised by an arrested hydrolysis of Ti(OiPr)4 with either undecylenic (UA) or undecenylphosphonic (UPA) acids used as the organic templates with the long hydrocarbon chains and functional (terminal double bond) groups. Surface-modified TiO2 nanoparticles could be easily dispersed in organic solvent due to the long hydrocarbon chain surrounding the titanium core, and engaged as a co-monomer in polymerisation with the MMA due to the presence of a terminal double bond. TEM and small angle X-ray scattering (SAXS) data presented support the homogeneous and consistent distribution of inorganic phase within the PMMA matrix, with the larger titania nanoparticles detected when the UPA was employed to modify a TiO2 nanoparticle. This is attributed to the UPA greater binding affinity towards the TiO2 surfaces and therefore particles aggregation to some extent.

High-temperature study of CaZrTi2O7

Abstract Previously reported anomalous thermal expansion effects in the 1200–1500°C range for hot-pressed CaZrTi 2 O 7 were shown to be due to irreversible bloating effects, from occluded gases. X-ray diffraction and differential thermal analysis of ordered CaZrTi 2 O 7 did not reveal evidence of a solid-state transformation at temperatures up to 1450°C.

Adhesion behaviour of organically-modified silicate coatings on stainless steel

The mechanical properties of organically modified silicate coatings on stainless steel substrates were investigated, using nano-indentation and simultaneous in situ microtensile testing/optical microscopy. The load-displacement response and fracture behaviour is examined to ascertain the effects of different organic groups on the film properties and adhesion characteristics. The relationship between the morphology and mechanical properties of the films is discussed, and it is demonstrated that the mechanical response of the coatings is significantly influenced by the nature of the organic group attached to the ormocer precursor.

Cracking and Decohesion of Sol-Gel Hybrid Coatings on Metallic Substrates

Thin film coatings based on organically modified silanes were synthesized using sol-gel technology. Various mixtures of tetraethoxysilane and glycidoxypropyltrimethoxysilane precursors were used to produce sol-gel coatings on as-received and thermally oxidised copper, aluminium and titanium substrates. The mechanical properties and adhesion behaviour of the coatings were assessed using nano-indentation and microtensile testing, respectively. The relationship between the film structure and its mechanical response is examined. It is shown that the mechanical properties (hardness and Youngs modulus) of the coatings are influenced dramatically by the organic substituent and the presence of an oxide layer thermally grown on the substrate material prior to deposition plays an important role on the film/substrate adhesion behaviour.

Low Temperature Bonding of Ceramics by Sol-Gel Processing

Sol-gel bonds were produced between smooth, clean silicon or polycrystalline alumina substrates by spin-coating solutions containing partially hydrolysed silicon alkoxides onto both substrates. The two coated substrates were assembled and the resulting sandwich was fired at temperatures ranging from 300 to 600°C. The influence of the sol-gel chemistry on the film microstructure and interfacial fracture energy was investigated using a wide range of techniques, including ellipsometry, FTIR, TG-DTA, rheology, TEM and micro-indentation. For silicon wafers, an optimum water-alkoxide molar ratio of 10 and hydrolysis water pH of 2 were found. Such conditions led to relatively dense films (>90%), resulting in bonds with significantly higher fracture energy (3.5 J/m2) than those obtained using classical water bonding (typically 1.5 J/m2). Aging of the coating solution was found to decrease the bond strength. Poly-crystalline alumina substrates were similarly bonded at 600°C; the optimised silica sol-gel chemistry yielded interfaces with fracture energy of 4 J/m2.

Evaluation of interfacial toughness and bond strength of sandwiched silicon structures

Abstract Results of a study to measure the interfacial strength and toughness in sandwiched silicon structures, using sol–gel processing as the bonding method, are examined. The interfacial bond strength was determined using a standard uniaxial tensile test, while a relative measure of interface toughness was ascertained using exploratory Vickers indentations. The specimens were positioned and aligned so that the indentations were made directly on the interface region, with the cracks emanating from one set of the impression diagonals at the free surface coinciding with the trace of the interface. The length of these radial cracks, having a penny-like configuration, required to cause debonding at the interface was measured in order to provide relative fracture toughness and fracture energy values. Indications of ‘local’ bond toughness were obtained by indenting at locations near the interface and following the path of the radial cracks. The applicability of the technique with reference to material interfaces is discussed.

Neutron irradiation effects in perovskite (CaTiO3)

Abstract Specimens of perovskite (CaTiO 3 ) irradiated with fast neutrons have been examined by X-ray diffraction. Expansion of the unit cell was anisotropic. The volume expansion increased with dose and approached a saturation value of 3.8%. X-ray diffraction lines showed both broadening and attenuation. The attenuation could be explained by postulating random atomic displacements, akin to thermal disorder, having an r.m.s. magnitude of ~ 0.02 nm. Recovery of the lattice parameters takes place in the temperature range 300–900°C, with a spectrum of activation energies. Effective activation energies of 2.52 ± 0.08 and 3.73 ± 0.13 eV were observed during the early and middle stages of recovery.

A comparative study of different IBA techniques for determining the thickness of thin SiO2 films

Abstract Thin nanometre films of SiO2 on Si have been measured using a variety of IBA techniques. High-energy non-Rutherford backscattering and NRA were found to be the most suitable methods for relative thickness determinations, giving precisions to ∼1 nm, comparable to that obtained using ellipsometry. The information obtained is used to determine the oxygen areal density and bulk density of sol–gel films.