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Ambitious electricity generation targets from renewable sources set by many governments have lead to the rapid growth of the wind energy industry over the last two decades. The need for larger wind turbine blades for increasing energy generation has considerably increased the demand and use of high performance composites in wind turbine applications, mainly in blades. A common type of failure in composite materials is delamination of adjacent layers, which can occur either due to manufacturing inconsistencies or due to in service loads. Understanding and characterising delamination is very important in order to implement damage tolerant design methodologies. The present research work focuses on the assessment of the delamination behaviour of composites for wind turbine applications. Several composite systems were tested and their fracture toughness and fatigue delamination propagation behaviour under mode I (peeling) loading conditions were evaluated. Quasi–static tests were performed and delamina...

The Microstructural Physics group at the Cavendish Laboratory is actively involved in a considerable number of research projects which cover a broad range of materials science. In this paper, we describe briefly several such projects, with particular emphasis given to the application of parallel-detection electron energy loss spectroscopy (PEELS) on a scanning transmission electron microscope (STEM) to the analysis of materials such as stainless steels, catalysts, and high temperature superconductors. In addition, we describe a number of related projects that are currently being carried out in the group, particularly those which utilise and develop novel STEM imaging and analytical techniques. © 1993 Wiley-Liss, Inc.

Upper Jurassic claystones of Kimmeridgian-Volgian(-Ryazanian) age are the main source rocks for oil in the Central Trough, North Sea. Drill cuttings and cores from the depth interval 2368–4548 m, the interval from early-mature to peak oil generation, have been investigated by high-resolution transmission electron microscopy (HRTEM) of ion-milled rock samples, and by HRTEM and powder X-ray diffraction (XRD) of dispersed illite-smectite (I-S) mixed-layers isolated from the µm fraction. Ordering parameters were determined by XRD profile simulation. TEM on shadowed particles, combined with XRD evidence, showed that dispersed I-S consists of a mixture of 10–70 Å thick particles, whereas HRTEM showed that intact bulk rock was dominated by 50–200 Å thick particles. Disruption, possibly along smectite planes, of the larger particles in the bulk rock formed the thin particles in dispersed I-S. HRTEM showed 20 and 30 Å periodic enhancement in the contrast of lattice fringes in bulk rock samples as well as in dispersed I-S crystals. These features were interpreted as ordered I-S crystals. Indications of disintegration of larger I-S particles during sample dispersion and of the presence of ordered I-S crystals in the intact bulk rock support the layer transformation mechanism for the conversion of smectite to illite.

We demonstrate an accurate method for measuring the lifetime of long-lived metastable magnetic states using a magneto-optical trap (MOT). Through optical pumping, the metastable $(3s3p)\text{ }{^{3}P}_{1}$ level is populated in a standard MOT. During the optical pumping process, a fraction of the population is captured in the magnetic quadrupole field of the MOT. When the metastable atoms decay to the $(3{s}^{2})\text{ }{^{1}S}_{0}$ ground state they are recaptured into the MOT. In this system no alternative cascading transition is possible. The lifetime of the metastable level is measured directly as an exponential load time of the MOT. We have experimentally tested our method by measuring the lifetime of the $(3s3p)\text{ }{^{3}P}_{1}$ of $^{24}\text{M}\text{g}$. This lifetime has been measured numerous times previously, but with quite different results. Using our method we find the $(3s3p)\text{ }{^{3}P}_{1}$ lifetime to be $(4.4\ifmmode\pm\else\textpm\fi{}0.2)\text{ }\text{ms}$. Theoretical values point toward a lower value for the lifetime.

Despite its importance, the understanding of ionic cloud distribution close to a charged macroion under physiological salt conditions has remained very limited especially for strongly coupled systems with, for instance, multivalent counterions. Here we present a formalism that predicts both counterion and coion distributions in the vicinity of a charged macroion for an arbitrary amount of added salt and in both limits of mean field and strong coupling. The distribution functions are calculated explicitly for ions next to an infinite planar charged wall. We present a schematic phase diagram identifying different physical regimes in terms of electrostatic coupling parameter and bulk salt concentration., 6 pages, 2 figures

A two-photon mechanism for cooling atoms below the Doppler temperature is analyzed. We consider the magnesium ladder system $(3{s}^{2})^{1}S_{0}\ensuremath{\rightarrow}(3s3p)^{1}P_{1}$ at $285.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ followed by the $(3s3p)^{1}P_{1}\ensuremath{\rightarrow}(3s3d)^{1}D_{2}$ transition at $880.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. For the ladder system quantum coherence effects may become important. Combined with the basic two-level Doppler cooling process this allows for reduction of the atomic sample temperature by more than a factor of 10 over a broad frequency range. First experimental evidence for the two-photon cooling process is presented and compared to model calculations. Agreement between theory and experiment is excellent. In addition, by properly choosing the Rabi frequencies of the two optical transitions a velocity independent atomic dark state is observed.

We investigate interactions between an elastic membrane and a substrate characterized by quenched positional disorder in the height function. We show that the positional disorder transforms the standard secondary DLVO minimum into two separate states: the hovering state characterized by a planar membrane at a finite separation from the interface and a pinned state where the membrane follows closely the asperities of the substrate and is as a consequence quite corrugated. The transition between the two states is continuous and depends on the parameters of the underlying DLVO potential as well as the parameters describing the quenched height-height correlation function of the substrate., 7 pages, incl. 1 figure

Accurate modelling of the transient behaviour of optical parametric oscillators (POPs) is of great importance for the understanding of many aspects of OPOs such as the build up of signal and idler fields from spontaneous parametric fluorescence, control and locking of the resonance mode, tuning and mode hopping and de-tuning due to heating effects. In this contribution we describe a numerical model, which takes into account all of the most important processes influencing the transient behaviour of an OPO. We then apply the model to two recent experimental observations: The growth of the signal and idler fields as an OPO is scanned through resonance by means of a piezoelectric mount for one of the resonators1 and the phenomenon of self-frequency locking.2,3 In both cases thermal effects due to even very small absorption losses in the nonlinear material modify the material properties of the OPO and lead to the observed complex behaviour. Comparison between the experimental data and the model data show that the model is able to account for all the observed features of the OPO output.

The Microstructural Physics group at the Cavendish Laboratory is actively involved in a considerable number of research projects which cover a broad range of materials science. In this paper, we describe briefly several such projects, with particular emphasis given to the application of parallel-detection electron energy loss spectroscopy (PEELS) on a scanning transmission electron microscope (STEM) to the analysis of materials such as stainless steels, catalysts, and high temperature superconductors. In addition, we describe a number of related projects that are currently being carried out in the group, particularly those which utilise and develop novel STEM imaging and analytical techniques.

The unique material properties of diamond1, including hardness, low coefficient of friction, high thermal conduc-tivity, electrical conductivity, and optical transmission, make diamond an attractive candidate for a wide range of industrial applications. As a consequence during the last decade, there have been extensive worldwide efforts toward the development of techniques for metastable synthesis. Diamond film synthesis has been demonstrated by using a wide range of low pressure chemical vapor deposition (CVD) techniques (see review by Spear2). Diamond synthesis by combustion of acetylene at atmospheric pressure was first reported by Hirose et al.3 and has been confirmed by others4,5,6,7,8. This technique represents the technological advantage of a low initial capital investment and shows that diamond can be grown at 1 atmosphere with the ability to coat large substrate areas. The drawbacks of this method are that the reactive species in the flame are non-uniformly distributed, and also that the combustion in the flame is responsible for a considerable heat flux to the substrate. Today, only a few applications of diamond films are on the market. For most potential applications, it is very important to synthesize a homogeneous morphology as well as to obtain good adhesion of the film to the substrate. To control the morphology of the film and the adhesion to the substrate, the nucleation and growth of diamond have to be controlled.

Nitriding of titanium alloys was performed in an r.f. plasma of nitrogen, hydrogen and argon at a substrate temperature of about 500 °C. The microstructure and the composition of the nitrided layers obtained were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). Three phases were identified: (a) a solid solution of nitrogen in titanium, α-(Ti, N), (b) tetragonal ϵ-Ti 2 N and (c) cubic δ-TiN (NaCl-type structure). Two distinct layers were formed on top of the plasma-nitrided Ti-6Al-4V alloy, followed by a solid solution of nitrogen in the titanium alloy. The outer layer was identified as δ-TiN while the inner layer was identified as a mixture of δ-TiN and ϵ-Ti 2 N. The ϵ-Ti 2 N was found to be highly oriented (002) while the δ-TiN was obtained as a randomly oriented polycrystalline layer. The outer δ-TiN was found to have a fine structure, while the inner δ-TiN plus ϵ-Ti 2 N comprised large and oriented grains. TEM studies showed that the crystallite size in the upper layer was tens to hundreds of angstroms, while in the inner layer it was one tenth to about half a micrometre. Energy-dispersive analysis performed with the TEM system and AES shows a variation in the bulk composition along the nitride layer. The microstructures and the relative phase content are presented and discussed in relation to the gas feed composition in the plasma.