Paul R. Chalker

Embedded fibre Bragg grating sensors in advanced composite materials

Abstract Fibre Bragg grating (FBG) sensors have been embedded in a number of advanced composite materials and fibre/metal laminates (FMLs). The post-fabrication FBG spectra were studied to examine the influence of manufacturing variables (such as composite stacking sequence and resin flow during processing) on the final profile of the spectrum and the functionality of the FBG sensor. Distortion and broadening of the width of the FBG spectra were observed in several of the specimens. As a result of a strong non-uniformity of the strain field caused by local asymmetric loading of the sensor, pronounced splitting of the spectra into multiple peaks was noted for FBG sensors embedded in angle-ply configurations. In contrast, the FBG spectra for the unidirectional specimens exhibited a single well-defined peak. Tensile tests carried out on these specimens showed excellent linearity within the test regime. However, for specimens exhibiting a multi-peak spectrum, it was observed that these specimens showed a tendency to produce strain anomalies during the loading event.

A review of the methods for the evaluation of coating-substrate adhesion

Abstract The adhesion between coating and substrate is often the predominant factor in determining the performance and reliability of coated engineering components. Methods which have been developed for evaluating coating-substrate adhesion often have a number of advantages and disadvantages depending on the specific coating-substrate system. The performances of a number of these techniques, namely the pressure-sensitive tape test, indentation testing, the scractch test, laser spallation, etc., are reviewed.

The identification and characterisation of mixed oxidation states at oxidised titanium surfaces by analysis of X-ray photoelectron spectra

The interaction of oxygen with titanium has been studied by X-ray photoelectron spectroscopy. Methods for analysing composite photoelectron spectra (curve fitting, deconvolution and spectral subtraction) are discussed and examined critically and the advantages of the ‘difference spectra’ approach highlighted. Evidence for variable oxidation states (suboxides) is obtained from an analysis of the Ti(2p) spectra observed during the formation of thin oxide films (ca. 10 A). Angular-dependent studies established that the lower oxidation states Ti2+ and Ti3+ were formed preferentially at the metal–oxide interface, whereas Ti4+ species were dominant at the oxide–gas interface. The distribution of the oxidation states within the thin oxide overlayer may be altered by thermally induced diffusion of Ti0 species from the metal substrate; the latter can occur at low temperature (ca. 200 K) for the thin oxide film regime investigated.

Novel Mononuclear Alkoxide Precursors for the MOCVD of ZrO2 and HfO2 Thin Films

Thin films of ZrO 2 and HfO 2 have been deposited by liquid injection metal-organic (MO)CVD using a range of new alkoxide precursors, [Zr(O t Bu) 2 (mmp) 2 ] (1), [Hf(O t Bu) 2 (mmp) 2 ] (2), [Zr(mmp) 4 ] (3), and [Hf(mmp) 4 ] (4): (mmp = 1-methoxy-2-methyl-2-propanolate, OCMe 2 CH 2 OMe). The complexes are mononuclear and volatile, and are significantly less reactive to air and moisture than existing Zr and Hf alkoxide precursors such as [Zr(O t Bu) 4 ] and [Hf(O t Bu) 4 ]. The ZrO 2 and HfO 2 films were grown over a wide range of substrate temperatures (350-650 °C), and analysis by laser Raman spectroscopy shows that the films were deposited in the thermodynamically stable α- or monoclinic phase.

Charge transport in heavily B‐doped polycrystalline diamond films

Temperature‐dependent conductivity and Hall measurements have been carried out on heavily in situ B‐doped polycrystalline diamond films in a temperature range from ∼100 to 750 K. The slope of the conductivity is clearly non‐Arrhenius leading to a pronounced tail at low temperatures. Carrier transport at low temperatures is dominated by variable range hopping. The activation energy decreases with increasing doping concentration and the most heavily doped diamond films show metallic behavior above room temperature. Hole carrier concentrations up to 1.8×1021 cm−3 were measured in agreement with secondary‐ion‐mass spectroscopy investigations.

Properties of GaN Nanowires Grown by Molecular Beam Epitaxy

On Si(1 1 1) and Si(0 0 1), GaN nanowires (NWs) form in a self-induced way without the need for any external material. On sapphire, NW growth is induced by Ni collectors. Both types of NWs exhibit the wurtzite crystal structure and grow in the Ga-polar C-direction perpendicular to the substrate. The NW sidewalls are M-plane facets, although on the Ni-induced NWs also A-plane segments form, if the growth temperature is low. Both self-induced and collector-induced NWs are free of strain and epitaxially aligned to the substrate, but in particular the former show a significant spread in tilt and twist caused by a mostly amorphous interfacial layer of Si-N. The self-induced NWs are virtually free of extended defects, but the collector-induced NWs contain many stacking faults. The photoluminescence of the former is significantly brighter and sharper. The spectra of single, dispersed, self-induced NWs contain extremely sharp excitonic lines. Significant emission is caused by excitons bound to donors close to the surface whose binding energy is reduced compared to the bulk value. In comparison, both the microstructure and optical properties of the self-induced NWs are superior. The limited material quality of the collector-induced NWs can be explained by detrimental effects of the collector.

Some recent developments in the MOCVD and ALD of high-κ dielectric oxides

A range of high-permittivity (κ) dielectric oxides are currently being investigated as alternatives to SiO2 as the dielectric insulating layer in sub-0.1 μm CMOS technology. Metalorganic chemical vapour deposition (MOCVD) and atomic layer deposition (ALD) are promising techniques for the deposition of these high-κ dielectric oxides. Some recent developments in precursors for the MOCVD and ALD of ZrO2, HfO2, Zr- and Hf-silicate and the rare earth oxides M2O3 (M = Pr, La, Gd, Nd) are discussed.

Electrochemistry at boron doped diamond films grown on graphite substrates: redox-, adsorption and deposition processes

Highly boron-doped (atomic concentration ∼1021 cm−3) conducting diamond films were grown on graphite substrates by microwave assisted vapor deposition from a gaseous feed of hydrogen and methane and solid boron. These diamond films of ca. 5 to 10 μm thickness composed of crystals of up to 10 μm size were characterized by both surface analytical techniques such as Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), and electrochemical techniques. The reduction of Ru(NH3)63+, the oxidation of chlorpromazine, and the reduction of Pb2+ in aqueous media were studied in order to investigate the processes involving oxidation and reduction as well as adsorption and deposition. The one-electron oxidation of chlorpromazine in aqueous 0.1 M KCl and the one electron reduction of Ru(NH3)63+ in aqueous 0.1 M KCl gave well defined cyclic voltammetric responses at diamond electrodes with peak currents proportional to the square root of the scan rate consistent with diffusion control at a macroscopically uniformly active electrode. For the oxidation of chlorpromazine at concentrations less than 4 mM adsorption of the neutral chlorpromazine at the diamond electrode was additionally detected. At very low scan rates a transition to sigmoidally shaped responses occurred which may be attributed partly to the presence of areas of low or no conductivity on a microscopic level. The reduction of Pb2+ in aqueous 0.1 M HClO4 allowed the deposition of metallic lead on the diamond surface. Studies by ex-situ SEM and in-situ AFM show that Pb nuclei are formed and distributed inhomogeneously over the polycrystalline diamond film in the active surface areas, irrespective of grain boundaries. However, little or no Pb deposition occurred in some less active areas with dimensions of 2 to 100 μm. This effect may be attributed to regions of poor conductivity between the graphite substrate and the diamond film coupled to low lateral conductivity of the film itself. The size of the regions of lower activity decreases with higher applied over-potential. A characteristic delay in the re-dissolution process of the Pb deposit was observed and attributed to both poor adhesion and a resistive contact between deposit and electrode surface.

Photochemistry of refractive index structures in poly(methyl methacrylate) by femtosecond laser irradiation

Femtosecond, subablation threshold photomodification of poly(methyl methacrylate) (PMMA) at 387 nm is explored to enable fabrication of optical components. Volatile fragment analysis (thermal desorption gas chromatography-mass spectrometry) and molecular weight distribution monitoring (size exclusion chromatography) suggest photochemical modification, involving direct cleavage of the polymer backbone and propagation via chain unzipping under formation of monomers, similar to the pyrolytic degradation of PMMA. Waveguides were produced in undoped, clinical-grade PMMA, showing an increased refractive index in the laser focal region (Dnmax=4x10(-3)).

Transition from electron accumulation to depletion at InGaN surfaces

The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0⩽x⩽1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x=0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0⩽x⩽1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x=0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.