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The contribution that a Physicist can make to Tribology will be greatly increased by effective and enlightened collaboration with the chemist, metallurgist and engineer. David Tabor (1913–2005)

The first meeting of the Institute of Physics Tribology Group was held at Cranfield, UK, in 1980 entitled 'Tribology: Contrasts in Frictional Processes'. The first chair of the Group was David Tabor who subsequently edited the proceedings of the 10th anniversary celebration of the formation of the Group which was held in Stratford-upon-Avon in 1991 and published by this journal as a special issue in 1992. This Special Cluster celebrates the continuation of this thriving Group and the work of some of our leading tribologists in the area who have contributed to the Group meetings in recent years. In particular, we are greatly pleased to have papers from young researchers in the field who will take this subject forward in different directions in the coming decades.

In 2000, due to popular demand, the Tribology Group was active in securing the reprint of Tabor's book The Hardness of Metals, originally published by Clarendon Press in 1951, and a very special memory for the Group was his presence at a one day event at Robinson College in Cambridge in July 2000 where, with characteristic humility and warmth, he asked the Tribology Group members present on the day to sign the blank page at the front of the book as his special memento of the event. He also gave a very illuminating account of his early research career. A tribute to David Tabor, who passed away in November 2005, was given in the opening address of the 25th anniversary meeting by Brian Briscoe, who described the importance of his seminal contributions to the subject which laid the foundation stones in our understanding of contact between solids, friction of metals and non-metals, frictional temperature rise, boundary lubrication and the science of adhesion and sliding wear.

Today, 25 years on, tribology, as Tabor anticipated above, is a very different subject, now with significant current interest in contact mechanics at the nano level and the interaction with corrosion and other chemical environments. The 25th anniversary meeting held in February 2006 included presentations in these areas, marking the milestones in the scientific developments in tribology during the past 25 years and was followed by a celebratory dinner in the headquarters of the Institute of Physics at Portland Place in London.

Thus, the papers in this Special Cluster of Journal of Physics D: Applied Physics celebrate the interdisciplinarity in the subject and some new horizons, authored by individuals who have contributed to IOP Tribology Group meetings in recent years. We are very grateful to the organizers of the 25th anniversary meeting, Mark Rainforth and Philip Shipway, and to the Institute of Physics for sponsorship of the reception on the day. We look forward to this subject moving in diverse directions in the coming years, and to the continuation of our lively and entertaining one day meeting series, where through 'human tribology', i.e. friction through debate and argument with some of our colleagues, tempered by the lubricating good humour of other colleagues, we progress this subject forward.

The different indentation fracture mechanisms of coated glass caused by a sharp tip (cube corner) at low loads (<4 mN) and a blunt tip (Berkovich) at high loads (up to 500 mN) have been analysed in this study. Existing indentation methods to estimate fracture toughness are unsuitable for very thin coatings (<500 nm). An alternative energy-based method can successfully be applied in the assessment of the ultra-small cracks produced in coated glass based on excursions in the load–displacement curve caused by indentation with a sharp tip. However, it was found that no excursions were observed associated with picture-frame cracks produced by a blunt tip in the same coatings; this is not unusual in other coated systems and makes the existing energy-based models invalid. Therefore, a new energy-based model is developed here to solve this problem and good results have been obtained. The existing models cannot be applied to the analysis of the indentation-induced interfacial failure in this study; therefore, new approaches have also been developed to assess the interfacial toughness and again reasonable results are obtained.

This study addresses the problem of the calculation of the elastic stress and displacement field within isotropic layered media in frictionless contact with rigid axisymmetric indenters. For a prescribed surface stress distribution, the integral transform approach is recalled using a matrix formulation which lends itself to generalizations to multilayered systems. It leads to an analytical solution for the Hankel transform of the elastic field which can readily be numerically inverted in the real space using available discrete Hankel transform algorithms. As an example, the shear stresses induced by the sphere indentation of a coated substrate are calculated as a function of the geometrical confinement of the contact and of the compressibility of the layer. The calculation was carried out using the surface pressure distribution provided by an exact solution to the coated contact problem. In addition, the elastic fields were also determined using an elliptic approximation of the contact pressure distribution. It is shown that the interface shear stress is strongly dependent on the details of the applied pressure profile close to the edge of the contact. In confined layers close to incompressibility, the elliptic approximation is found to result in a systematic overestimate of the interface shear stresses.

Diamond-like carbon (DLC) coatings are used in automotive engines for decreasing friction and increasing durability. There are many variants of DLC films which provide a wide range of mechanical, physical and tribological properties. The films can be extremely hard (>90 GPa), give low coefficients of friction against a number of counterfaces and exhibit low wear coefficients. The films are often considered to be chemically inert. The properties of DLC films depend to a large degree on the relative proportions of graphitically- (sp²) and diamond-like (sp³)-bonded carbon but the inclusion of elements such as hydrogen, nitrogen, silicon, tungsten, titanium, fluorine and sulphur can dramatically change their tribological response. Two different types of DLC, a WC/C amorphous hydrogenated DLC (WC/C a-C : H) coating and an amorphous hydrogenated DLC (a-C : H) have been investigated. The mechanical and tribological properties have been evaluated by nanoindentation, scratch and wear testing and friction testing in an instrumented cam–tappet testing rig. The deformation mechanisms and wear processes have been evaluated by scanning electron and atomic force microscopy. The results show that the harder a-C : H film was more wear resistant than the softer WC/C a-C : H film and performed better in the cam–tappet testing rig.

A statistical design of experiment (DOE) was used to investigate the effects of medium frequency pulsed substrate bias voltage, pulse frequency and pulse width on 150 nm sputtered C coatings doped with Cr. Within the scope of this work, the DOE treatment of nanoindentation/nanoscratch, atomic force microscopy and x-ray photoelectron spectroscopy results were shown to be a successful method for investigating and potentially optimizing very thin sputter ion plated coatings. Bias voltage was shown to be the most significant of the three bias factors for mechanical properties. Bias frequency and pulse width showed effects that partly matched the voltage results and this was discussed with regard to an increase in peak voltage accompanying an increase in frequency and pulse width. For surface topography, the results were more complicated with the significance of each parameter varying according to the property measured. This work also demonstrated the complex inter-relationships that exist between the three bias parameters, meaning that any attempt to optimize the pulse bias condition for a given coating property would require a consideration of all three factors (within the available operating window of the pulse unit).

The majority of rolling element bearings in use today are lubricated by grease. Grease is a two-phase lubricant with complex rheological properties and poses severe challenges for the prediction for lubricating performance. Grease lubricated contacts are liable to starvation and as a result the film thickness is reduced, which can result in surface damage or premature bearing failure. It is important to know when starvation occurs and the effect of grease type, bearing design and operation on lubrication replenishment. The influence of bearing design and operation in controlling lubricant supply to the contact zone is examined in this paper. The aim is to develop a starvation parameter capable of predicting the operating limits for a particular bearing/grease system.

A number of bearing design parameters are examined in the paper; these include cage design, ball spin and bearing size. Ball spin and cage effects can be efficient mechanisms for maintaining the lubricant supply to the track. Increased bearing size, line contact geometries and high load result in reduced lubricant replenishment of the contact. Using this analysis it will be possible to establish operating limits for families of bearings.

NiTi shape memory alloys are very attractive for medical implants and devices (such as orthopaedic and orthodontic implants) and various actuators. However, wear is a major concern for such applications and a novel surface engineering process, ceramic conversion treatment, has recently been developed to address this problem. In this study, the tribological characteristics of ceramic conversion treated NiTi alloy have been systematically investigated under dry unidirectional wear, reciprocating-corrosion wear and fretting-corrosion wear condition. Based on the experimental results, the wear behaviour under different conditions is compared and wear mechanisms involved are discussed.

This paper, which forms part of a special issue of this journal marking a 25th year anniversary in tribology, aims to provide an appraisal of key issues in coating tribology over that period. Two main inter-related strands are emphasized. One is the continuing move down the length scale in terms of the fundamental understanding of tribological contacts. This has been particularly useful in aiding the development of new coatings by identifying their property requirements at different scale levels. A second strand relates to the ongoing imperative to be able to design and select coatings to meet practical friction and wear requirements. This selection problem requires a robust methodology, and one such is elaborated in the paper, which takes account of the requirements of different types of tribological contacts and uses a combination of theoretical and empirical information. Challenges still remain in this regard, and the paper seeks to provide a basis for further developments to improve coatings and to ensure their effective selection.

There has been much attention paid to the lubricant additives zinc dialkyldithiphosphate and molybdenum dialkyldithiocarbamate as the most commonly used antiwear/antioxidant and friction modifiers. The mechanism by which they function has been the subject of much research work. As a result of these efforts the tribofilms formed from the above additives are fully chemically characterized but understanding the physical properties and the dynamics of their formation, stability and removal is still not satisfactory and needs more research. This paper reviews the general characteristics of tribofilms formed from these additives in single component systems and also on their interactions as well as the current understanding of the dynamics of their formation. Experimental work is then presented alongside discussion of the literature to present a current status of understanding of the stability and the removal of tribofilms. The effect of temperature and additive interactions on the thickness of the steady state tribofilms, and consequently the effect on friction performance, is evaluated. The results of this study show that temperature and additive interactions play a significant role on the dynamic process of tribofilm formation as well as its chemical properties. This study also highlights the areas in which further development is needed to ensure progress in understanding of the tribofilm's formation and removal processes.

The objective of the work outlined in this paper was to increase the understanding of the wear mechanisms that occur within a soot contaminated contact zone, to help in future development of a predictive wear model to assist in the automotive engine valve train design process. The paper builds on previous work by the author, through testing of different lubricants and increased levels of soot contamination.

Wear testing has been carried out using specimens operating under realistic engine conditions, using a reciprocating test-rig specifically designed for this application, where a steel disc is held in a heated bath of oil and a steel ball is attached to a reciprocating arm (replicating a sliding elephant's foot valve train contact). Detailed analysis of the test specimens has been performed using scanning electron microscopy to identify wear features relating to the proposed wear mechanisms. Analysis of worn engine components from durability engine tests has also been carried out for a comparison between specimen tests and engine testing.

To assist the understanding of the wear test results obtained, the physical properties of contaminated lubricants were investigated, through viscosity, traction and friction measurements.

The results have revealed how varying lubrication conditions change the wear rate of engine components and determine the wear mechanism that dominates in specific situations. Testing has also shown the positive effects of advanced engine lubricants to reduce the amount of wear produced with soot present.

This paper reviews the available literature relating to the emerging research into the performance of coatings under combined wear and corrosion conditions. Understanding how coatings perform under these tribo-corrosion conditions is essential if the service life of equipment is to be predicted and to allow service life to be extended. Therefore, the tribo-corrosion performance of coatings deposited by a variety of techniques is discussed and the main mechanisms associated with their degradation under combined wear and corrosion highlighted. Coating composition, microstructure, defect level, adhesion, cohesion and substrate properties are seen as some of the critical elements in coating performance when subjected to tribo-corrosion contacts. The importance of post-coating deposition treatments such as laser resurfacing and sealing are also discussed. Interactions between wear and corrosion mechanisms are identified along with some models and mapping techniques that aim to inform coating selection and predict performance. Recent investigations into mono-layer as well as multilayered and functionally graded coatings are reviewed as candidates for wear–corrosion resistant surfaces. The review reveals the need for a more considered approach to tribo-corrosion testing and the way in which the results are analysed and presented. For example, the test conditions should be appropriate to the coating system under test; the level of in situ instrumentation deployed and the post-test analysis of in situ electrochemical data should be carefully selected as well as details given of the composition of any surface tribofilms formed and the identification of the degradation mechanisms.

In this review, the potential of mode-locked lasers based on advanced quantum-dot (QD) active media to generate short optical pulses is analysed. A comprehensive review of experimental and theoretical work on related aspects is provided, including monolithic-cavity mode-locked QD lasers and external-cavity mode-locked QD lasers, as well as mode-locked solid-state and fibre lasers based on QD semiconductor saturable absorber mirrors. Performance comparisons are made for state-of-the-art experiments. Various methods for improving important characteristics of mode-locked pulses such as pulse duration, repetition rate, pulse power, and timing jitter through optimization of device design parameters or mode-locking methods are addressed. In addition, gain switching and self-pulsation of QD lasers are also briefly reviewed, concluding with the summary and prospects.

The martensitic transition, magnetocaloric effect (MCE) and shape memory effect (MSE) of ferromagnetic Heusler alloys Ni₅₀Mn_50−xSb_x (x = 12, 13 and 14) have been investigated. A large positive magnetic entropy change ΔS_M was observed in the vicinity of the martensitic transition. The maximum value of ΔS_M is 9.1 J kg⁻¹ K⁻¹ in Ni₅₀Mn₃₇Sb₁₃ at 287 K for a magnetic field change of 5 T. This change originates from the first-order transition from a low-temperature weak-magnetic martensitic phase to a high-temperature ferromagnetic parent phase. A magnetic-field-induced shape recovery strain of about 15 ppm at room temperature and at a relatively low magnetic field (1.2 T) was observed to accompany the reverse martensitic transformation. The large field-induced MCE and MSE in the NiMnSb system make it a promising material for room-temperature application.

Theoretical Overhauser-detected EPR spectra of ¹⁴N and ¹⁵N nitroxide systems in low magnetic field by field-cycled dynamic nuclear polarization (FC-DNP) were described by a combination of DNP theory and a model of FC-DNP. Spectra were simulated at magnetic fields between 0 and 8 mT. The simulations were able to predict both the EPR peak positions and their amplitudes, corresponding to those from FC-DNP experiments with ¹⁴N and ¹⁵N TEMPOL solutions. EPR irradiation was in the 45–133 MHz range while NMR signal detection occurred at a field of 59 mT. At this frequency range, four π transitions of a ¹⁴N system and three π transitions of a ¹⁵N system were observed. The simulation programmes were also used to predict the spectral amplitudes of the FC-DNP with EPR irradiation power in the 1–15 W range. Theoretical FC-DNP systems were in good agreement with experimental results; however, at low magnetic fields the inhomogeneity of our magnet system resulted in the EPR peaks being left-shifted and somewhat broader than those from the theoretical prediction.

Cobalt nanowire arrays were fabricated by ac (continuous and pulse) electrodeposition into anodic aluminium oxide templates. The effects of continuous electrodeposition waveform and frequency as well as the pulse feature on the structure and magnetic properties of the nanowire arrays were studied. For continuous electrodeposition into the low depth nanohole, the microstructures and magnetic properties of the Co nanowires are independent of the waveform and frequency due to relatively rapid electrodeposition growth. The hcp Co nanowires with high crystallinity were fabricated using the pulse technique. More than 50 ms off-time between the pulses induces the preferentially growth direction of Co nanowires (the c-axis parallel to the wire axis), thereby improving the magnetic properties. A coercive force of 2370 Oe was obtained for Co nanowires fabricated with a pulse of 100 ms off-time and 5 ms reductive/oxidative time.

We present experimental and theoretical investigations of the bias-dependent spectral shift of the photoresponse in InAs/In_xGa_1−xAs quantum-dots-in-a-well structures. Experimental results show that the wavelength response of the transition from the quantum dot ground state to quantum well states can be Stark-shifted by ∼15% by changing the applied bias between −1 V and +1 V. A theoretical model based on the 8-band k · p method fits our experimental data well using realistic dot parameters. We also demonstrate an increase in the operating wavelength and a reduced bias-dependent spectral shift for samples containing dots formed by depositing less InAs during growth.

The design and preparation of top-emitting organic light emitting devices with thick hole transport layers that effectively smooth the substrate were performed. Bilayer transfer with laser thermal patterning method was used to simplify the fabrication of hole transport layers with various thicknesses, with the aim of optimizing the cavity effect in the top-emitting devices. By carrying out optical simulations and experiments, the optimal thicknesses of the hole transport layers for our structure of blue, green and red devices were found to be 140 nm, 160 nm and 230 nm, respectively. We have briefly illustrated the underlying device physics of top-emission in terms of the resonance and microcavity effect. The resulting top-emitting devices with thick hole transport layers exhibit better colour purity than bottom-emitting devices and satisfactory luminous efficiency. The relatively high operating voltages can be further improved by the use of transport materials with enhanced charge mobility.

Infrared reflectography (IRR) is a non-destructive imaging technique based on the different optical behaviour of visible and near infrared (NIR) radiation through a thin pictorial layer. This effect is a consequence of both lower NIR absorption and reduced NIR scattering due to the particle size smaller than the wavelength. Here we present an interesting and, to our knowledge, new application of this technique to ancient multi-layered epigraphs, showing its useful application in archaeology. Both tests on laboratory samples and preliminary tests on the field show that it is possible to use IRR taken with a vidicon wide spectral range camera (let us call this NIR reflectography (NIRR) to underline the wide spectral range including almost all NIR, but excluding thermal infrared radiation), to read inscriptions 'deleted' by means of a whitewashing layer. The field tests carried out on a multi-layer inscription 'notice board' in Herculaneum have shown that NIRR can also be useful in integrating some gaps of the currently visible layer inscription and in reconstructing the layout of the inscription wiped off by mechanical abrasion. All these applications are of much interest since ancient Greek and Latin public documents were frequently written with black pigment on whitewashed boards and 'wiped off' when the inscription was no more required.

White organic light emitting diodes (WOLEDs) with copper phthalocyanine (CuPc), 4,4',4''-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA), tungsten oxide (WO₃) and molybdenum oxide (MoOx) as buffer layers have been investigated. The MoOx based device shows superior performance with low driving voltage, high power efficiency and much longer lifetime than those with other buffer layers. For the Cell using MoOx as buffer layer and 4,7-diphenyl-1,10-phenanthroline (Bphen) as electron transporting layer (ETL), at the luminance of 1000 cd m⁻², the driving voltage is 4.9 V, which is 4.2 V, 2 V and 0.7 V lower than that of the devices using CuPc (Cell-CuPc), m-MTDATA (Cell-m-MTDATA) and WO₃ (Cell-WO₃) as buffer layers, respectively. Its power efficiency is 7.67 Lm W⁻¹, which is 2.37 times higher than that of Cell-CuPc and a little higher than that of Cell-m-MTDATA. The projected half-life under the initial luminance of 100 cd m⁻² is 55 260 h, which is more than 4.6 times longer than that of Cell-m-MTDATA and Cell-CuPc. The superior performance of Cell-MoOx is attributed to its high hole injection ability and the stable interface between MoOx and organic material. The work function of MoOx has been measured by the contact potential difference method. The J–V curves of 'hole-only' devices indicate that a small hole injection barrier between MoOx/N,'-bis(naphthalene-1-y1)-N, N'-bis(phenyl)-benzidine (NPB) leads to a strong hole injection, resulting in a low driving voltage and a high stability.

A method to measure built-in voltages on the material interfaces in capacitive MEMS-devices inside closed cavities is presented. The method is based on a vibrating capacitor (Kelvin-probe) principle and it can even be used to measure closed cavity samples. The suggested set-up is tested by measuring various capacitive accelerometers and the results are compared with those obtained from capacitance–voltage (C–V) measurements. The potential of the method for high-speed measurements is explored by demonstrating an accurate determination of built-in voltages by measuring only a few data points for a device due to a very highly linear response of the method.

As a result of capillary forces, animal hairs, carbon nanotubes or nanowires of a periodically or randomly distributed array often assemble into hierarchical structures. In this paper, the energy method is adopted to analyse the capillary adhesion of microsized hairs, which are modelled as clamped microcantilevers wetted by liquids. The critical conditions for capillary adhesion of two hairs, three hairs or two bundles of hairs are derived in terms of Young's contact angle, elastic modulus and geometric sizes of the beams. Then, the hierarchical capillary adhesion of hairs is addressed. It is found that for multiple hairs or microcantilevers, the system tends to take a hierarchical structure as a result of the minimization of the total potential energy of the system. The level number of structural hierarchy increases with the increase in the number of hairs if they are sufficiently long. Additionally, we performed experiments to verify our theoretical solutions for the adhesion of microbeams.

A two-dimensional silicon photonic crystal (PhC), which is designed to provide a modified dispersion for photon energies of less than half of the electronic band gap of silicon, and which has been fabricated by laser interference lithography (LIL), is studied by angular dependent infrared reflectivity measurements. The resonance features, which we observe in the polarized reflectivity spectra, and which arise from resonant coupling of the infrared radiation to the leaky modes, are used to derive the probed leaky modes' quality factor of the PhC fabricated by LIL. We also present photonic band structure calculations to reveal the bandgaps of the guided modes.

A large lateral photoeffect (LPE) has been observed in an oxidized film Co_x Mn_y O deposited on an n-type Si substrate by sputtering. Under the nonuniform illumination of a laser beam, the lateral photovoltage shows a high sensitivity to the spot position on the Co_x Mn_yO surface. The largest open-circuit position sensitivity is about 34.3 mV mm⁻¹. These phenomena were discussed in terms of the metal–semiconductor junction which exists between the oxidized film and the Si substrate. The large LPE is expected to make the oxidized film a new candidate for position-sensitive photodectors.

A method is introduced to optimize the acoustic band structures of two-dimensional phononic crystals (PCs). The acoustic band gap (ABG) between any two bands could be maximized by introducing an additional rod at a suitable position in a unit cell. The results also show that the symmetry reduction of the system is more efficient in creating the ABGs except for the first one. This additional rod method will be useful in designing the ABGs of PCs.

Temperature-dependent photoluminescence properties of undoped, N-doped and N-In codoped p-type ZnO thin films have been investigated in detail. The yielded temperature dependences of ultraviolet peak energy, width and intensity for several resolved emissions exhibit the different carrier recombination processes associated with doping mechanisms. We have revealed the acceptor binding energy of 113 meV, 140 meV and 112 meV and donor one of 56 meV, 82 meV and 112 meV for undoped, N-doped and N-In codoped ZnO, respectively, together with the broadening of the acceptor levels in N-doped and N-In codoped ZnO. We have also clarified the origin of the ZnO deep-level visible emission.

Yttrium-doped ZnO thin films were deposited on silica glass substrates by the sol–gel method. The structural, electrical and optical properties of yttrium-doped ZnO thin films were investigated systematically and in detail. All the thin films have a preferred (0 0 2) orientation. When compared with the electrical resistivity values of films without annealing treatment, the values of films annealed in the reducing atmosphere were decreased by about three orders of magnitude. The lowest electrical resistivity value was 6.75 × 10⁻³ Ω cm, which was obtained in the 0.5 at% yttrium-doped ZnO thin film annealed in nitrogen with 5% hydrogen at 500 °C. In room-temperature photoluminescence (PL) spectra, two PL emission peaks are found in the pure ZnO thin film; one is the near-band-edge (NBE) emission at 3.22 eV and the other is a green emission at about 2.38 eV. Nevertheless, the green emission is not found in the PL of the yttrium-doped ZnO thin films. The low-temperature PL spectrum of the undoped ZnO thin film at 83 K is split into well-resolved free and bound excition emission peaks in the ultraviolet region, but the NBE emission of the 5 at% yttrium-doped ZnO thin film at 83 K has only one broad emission peak.

ZnO nanocrystals (NCs) embedded in a BaF₂ matrix have been prepared by a radio-frequency magnetron sputtering technique followed by a thermal annealing process from 400 to 1100 °C. X-ray diffraction results show that ZnO NCs with a hexagonal wurtzite structure are formed in a BaF₂ matrix. The average grain sizes are estimated ranging from 7.5 to 33 nm, corresponding to fabricated samples annealed at temperatures from 400 to 800 °C. Photoluminescence showed a very strong ultraviolet near-band-edge (NBE) emission located in the 364–383 nm spectral region. The emission intensity is enhanced and the linewidth is narrowed as the annealing temperature increases. The intensity ratio of ultraviolet NBE emission to deep-level visible is also increased with annealing temperature indicating that deep-level defects in ZnO are suppressed.

This paper is devoted to a plasma characterization with an experimental quantification of the energy transferred to the anode material in a transferred arc configuration. After spectroscopic measurements in an argon plasma medium, measurements in the anode material were performed using thermocouples. The flux applied by the plasma medium to the anode material was deduced from experimental measurements and the use of an iterative inverse method, the conjugate gradient method. The optimal temperature sensor positions were deduced from a previous theoretical study showing the suitability of the method for flux quantification using recommendations exposed in the paper. The measurements were made for various values of the current intensity around 100 A leading to fluxes close to 10⁷ W m⁻².

The species density distributions in a large post-discharge reactor placed downstream from a flowing microwave discharge in N₂–O₂ are calculated using a three-dimensional hydrodynamic model. The effects of surface losses of N(⁴S) and O(³P) atoms on the density distributions of different species in the reactor are investigated for three different wall materials: (i) Pyrex, (ii) aluminium and (iii) stainless steel. The effects produced by considering different surface loss probabilities corresponding to each one of these materials, as well as by assuming the production of NO from the wall, are evaluated and discussed. The simulation is conducted for the case of a 65 × 25 × 25 cm³ post-discharge reactor fed from a 2450 MHz discharge, at 2 Torr and 2 × 10³ sccm flow rate, in an N₂–xO₂ mixture composition, with x = 0.5–7%.

Streamer propagation in cyclohexane with various additives have been studied with fast impulses applied to a point-to-plane gap. The additives are perfluoro-n-hexane, N,N-dimethylaniline, 1-methylnaphthalene, di-n-propylether, 1,1-difluorocyclopentane and 1,4-benzoquinone, representing molecules with different electrochemical properties. We have used shadowgraphic imaging and a differential charge-measurement technique with a sensitivity of 0.1 pC to integrate currents induced to the point electrode during streamer propagation. The propagation is to various degrees facilitated by the additives. Perfluoro-n-hexane affects point cathode streamers, N,N-dimethylaniline and di-n-propylether affect point anode streamers, while 1-methylnaphthalene and 1,4-benzoquinone affect both point anode and point cathode streamers. 1,1-difluorocyclopentane is the only additive without a measurable effect in either point polarity. Largest effects are found for perfluoro-n-hexane, N,N-dimethylaniline and 1-methylnaphthalene, which are the only additives to produce a visible change in streamer appearances. In these cases, filaments become thinner and fewer, while propagating faster and sometimes further. The results follow expectations, considering point cathode streamers to be governed mainly by injection of thermal electrons from a gaseous phase, and point anode streamers to be governed mainly by more energetic electrons, extracted from the liquid at higher electric fields.

The need to improve plasma spraying processes has motivated the development of computational models capable of describing the arc dynamics inside plasma torches. Although progress has been made in the development of such models, the realistic simulation of the arc reattachment process, a central part of the arc dynamics inside plasma torches, is still an unsolved problem. This study presents a reattachment model capable of mimicking the physical reattachment process as part of a local thermodynamic equilibrium description of the plasma flow. The fluid and electromagnetic equations describing the plasma flow are solved in a fully-coupled approach by a variational multi-scale finite element method, which implicitly accounts for the multi-scale nature of the flow. The effectiveness of our modelling approach is demonstrated by simulations of a commercial plasma spraying torch operating with Ar–He under different operating conditions. The model is able to match the experimentally measured peak frequencies of the voltage signal, arc lengths and anode spot sizes, but produces voltage drops exceeding those measured. This finding, added to the apparent lack of a well-defined cold boundary layer all around the arc, points towards the importance of non-equilibrium effects inside the torch, especially in the anode attachment region.

The anode region of an argon high intensity arc with a superimposed cold flow has been investigated experimentally. Three-dimensional electron temperature and electron density distributions have been obtained with a laser Thomson scattering system, and the flow fields have been observed with a high speed Schlieren system. The transition from a steady mode to a takeover mode and finally to a restrike mode has been clearly identified. During the transition, the flow field becomes turbulent and the electron temperature and the electron density values in the major anode attachments increase significantly. Starting from a takeover mode, extended anode boundary layers have been found, which have high electron temperatures and low electron densities, indicating their non-equilibrium nature. In a restrike mode, the restrike behaviour has been found to be initiated by flow instabilities, which bring high electron temperature clouds into the extended anode boundary layer and encourage the electron overheating instabilities. Finally, together with the anode burn patterns, the experimental results are applied to plasma spray torches and a takeover mode is suggested as the favourable operation mode.

Thin films of CuInSe₂ have been fabricated by thermal annealing of evaporated elemental layers of Cu, In and Se onto Si (1 0 0) and Corning glass 7059 substrates at room temperature. Structural, optical and electrical properties of the layers were studied. X-ray diffraction revealed that the film was single phase with chalcopyrite structure and preferred orientation along the (1 1 2) plane. The temperature dependence of electrical conductivity exhibited two activation energies and the optical studies showed that the absorption coefficient of this film was above 3 × 10⁴ cm⁻¹ and the band gap was found to be 0.98 eV.

Hole mobility in a copper-phthalocyanine (CuPc)-based top-contact transistor has been studied with various organic layer thicknesses. It is found that the transistor performance depends on the thickness of the CuPc layer, and the mobility increases with the increase in the CuPc layer and saturated at the thickness of 6 ML. The upper layers do not actively contribute to the carrier transport in the organic films. The morphology of the organic layer grown on the bare SiO₂/Si substrate is also presented. The analysis of spatial correlations shows that the CuPc films grow on the SiO₂ according to the mixed-layer mode.

In this paper, the fluorescence behaviour of nano colloids of ZnO has been studied as a function of the excitation wavelength. We have found that excitation at the tail of the absorption band gives rise to an emission that shifts with the change of the excitation wavelength. The excitation wavelength dependent shift of the fluorescence maximum is measured to be between 60 and 100 nm. This kind of excitation wavelength dependent fluorescence behaviour, which may appear to be in violation of Kasha's rule of excitation wavelength independence of the emission spectrum, has been observed for nano ZnO colloids prepared by two different chemical routes and different capping agents. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited state relaxation processes, namely, solvation and energy transfer, are responsible for the excitation wavelength dependent fluorescence behaviour of the systems.

The far field plasmonic behaviour of nanoporous gold films with void densities ranging from 60% to 90% has been investigated and modelled. These layers have good dc conductivity and quite different nanostructure to traditional porous layers in which the metal percolates. Our gold films with void density f above 70% have high thermal emittance for a conductor at their thicknesses and their flat spectral response at visible and near infrared wavelengths is not metal like. We derive effective optical constants which become plasmonic at wavelengths between 1.8 and 4 µm for f from 72 to 87%. This onset is much longer than that in bulk gold. For void densities below 70% the onset of plasmonic behaviour is much closer to the dense material. A simple test is implemented to test for surface plasmon polaritons (SPPs) under illumination. The more porous films show no evidence of SPP, while the less porous films display weak evidence. Thus by tailoring void content in these nanostructures we can tailor the onset of effective plasmonic response across a wide range from 0.8 to 4 µm and emittance from around 0.9 down to low values. An effective uniform metal response is thus found in the presence of surface nanostructure without the interface absorption found in dense gold layers with structured surfaces.

Weakly non-linear stability analysis of a thin liquid film falling down a heated plane with linear temperature variation has been investigated in a finite amplitude regime. Using the long wave expansion method, a non-linear evolution equation for the development of the free surface is derived. A normal mode approach and the method of multiple scales are used to obtain the linear and non-linear stability solution for the film flow. The study reveals that both supercritical stability and subcritical instability are possible for this type of thin film flow. The influence of thermocapillary force on the span of supercritical/subcritical regions are examined. It is noticed that the unconditional stable region vanishes after a cutoff Marangoni number, whereas other regions increase with the increase in Marangoni number for fixed values of other parameters. Finally, we also scrutinize the effect of Marangoni number on the amplitude and speed of waves. In the supercritical region amplitude and speed of the non-linear waves increase with the increase in Marangoni number, while in the subcritical region the threshold amplitude decreases with the increase in Marangoni number.

Photoinduced carrier injection through the aligning layer–liquid crystal (LC) interface or the electrode–LC interface has been suggested as the fundamental process involved in the surface mediated photorefractive effect in the LC cell. In this paper we construct LC cells with the gold film as one substrate as well as cells with the gold film covered with decanethiol and hexadecanethiol self-assembled monolayers (SAMs). We find that the current–voltage curves for the three kinds of cells are consistent with the equations describing carrier injection and bulk-ion-generation. The dark current and photocurrent are enhanced by the existence of alkanethiol SAM. The length of the alkane chain of alkanethiol SAM also affects the currents. The above experimental results indicate that the model which involves carrier injection from the gold electrode to the LC is more suitable and the role of the alkanethiol aligning layer is to enhance carrier injection.

Based on Landau–Ginzburg phenomenological theory, a first-order ferroelectric bilayer or superlattice with an antiferroelectric interface coupling has been studied by taking into account the spatial variation of polarization within each constituent film and the surface effect. The hysteresis loops are obtained and the size effect of a bilayer is discussed. The loop patterns vary between typically antiferroelectric and typically ferroelectric depending on the thickness ratio, the coupling constant, the thickness and the extrapolation length. The antiferroelectric coupling has a great effect on the phase transition of a bilayer. It is shown that the size-driven phase transition cannot be observed in a ferroelectric bilayer in the case of strong antiferroelectric coupling.

Transient photo-induced voltage characteristics have been studied in La_0.9Sr_0.1MnO₃ films on vicinal cut SrTiO₃ (0 0 1) substrates. The as-received thin films are epitaxially grown on the substrates and predominated by a single (1 0 1)-oriented growth. Under irradiation of a 308 nm laser pulse the voltage signal is pulsed and the response time agrees with the laser pulse width. The dependence of the voltage peak on the tilting angles from 0° to 45° is further measured. For the c-axis tilt angle, the largest signal does not appear at 45° and the optimum choice is ∼30°.

The preparation process of zinc aluminate (ZnAl₂ O₄) ceramic powder, as well as the sintering temperature have been consequently governed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques. A broad exothermic peak in the range 223–310 °C is observed due to the crystallization of ZnAl₂O₄ powder. Then the final resultant powder was irradiated with gamma rays at different doses from 30 to 150 kGy. The effect of gamma irradiation on the structure and the electrical behaviour of ZnAl₂O₄ ceramics has been obtained. The induced changes in the structure have been studied via SEM, XRD and FTIR spectrometers. The obtained results reveal no changes in the spinel phase of ZnAl₂O₄, while some displacements of the constituent individual atoms for the irradiated samples are observed. The I–V characteristic curves and the dielectric properties of the prepared ceramic powder have been measured for unirradiated and irradiated samples. These curves exhibit nonlinearity of this type of ceramics, where the dc current gradually increases with the increase in the dose. The irradiation of ZnAl₂O₄ with gamma radiation was found to increase the nonlinearity of the I–V curves. The dielectric constant and loss were found to decrease as the dose increases. Therefore, the irradiation of ZnAl₂O₄ with gamma rays can improve its utility as an electronic protector in electrical circuits against sudden overvoltage.

In the past few years, a series of papers has examined the shock response of common engineering polymers as their microstructure changes. In this latest work, the behaviour of two polymers, polyoxymethylene and polyethylene, is investigated under conditions of shock loading in one-dimensional strain. The two are semi-crystalline thermoplastics differing only in the components of the chain. Equation of state data are investigated for the low pressure (u_p < 1 km s⁻¹) regime. Further, measurements of the lateral component of stress and knowledge of the impact conditions allow calculation of the shear strength. Variations both with impact stress amplitude and pulse duration are discussed in terms of the polymer chain structure.

In the framework of the maximum entropy production rate principle, a simple model for Teflon material ablation by arc radiation is introduced. The model predicts a vapour temperature T and an effective ablation enthalpy Δh_eff of about 3200 K and 11 MJ kg⁻¹, respectively. These values are constant for radiation fluxes Q_rad ≪ pΔh_eff/c (≈10¹⁰ W m⁻² at pressure p ≈ 10 bar and sound velocity c≈ 10³ m s⁻¹). For larger Q_rad the model is not self-consistent and must be generalized. In the range of self-consistency, the results are in accordance with observations reported in the relevant literature on nozzle ablation in gas circuit breakers or model tubes.

With the help of the entropy production rate, a Smoluchowski equation is derived for a model of particle diffusion in a nonuniform solid at local thermodynamic equilibrium. It turns out that the current density in the absence of an external potential is proportional to the gradient of the product temperature × density, which can be understood as a flow driven by the partial-pressure gradient of the particles. The result can also be obtained from nonequilibrium variational principles. As an example for a consequence of this type of thermo-diffusion, we discuss the occurrence of a current maximum at a finite temperature gradient in a solid polymer with a temperature dependent mobility.

Starting from elemental bismuth, tellurium and selenium powders, n-type Bi₂Te_2.85Se_0.15 solid solution with a fine microstructure was prepared by mechanical alloying and equal channel angular extrusion (ECAE) in the present work. The effect of extrusion temperature on the microstructure and thermoelectric properties of the as-ECAEed samples was investigated. A preferentially oriented microstructure with the basal planes (0 0 l) in the parallel direction to extrusion was formed, and the orientation factors F of the (0 0 l) planes of the 703 K and 753 K ECAEed Bi₂Te_2.85Se_0.15 alloys were 0.26 and 0.28, respectively. The electrical resistivity and the Seebeck coefficient decreased, and the thermal conductivity increased with increasing extrusion temperature. The electrical and thermal transmission performances were strongly affected by the preferentially oriented microstructure and the preferential orientation improved the thermoelectric properties of the ECAEed Bi₂Te_2.85Se_0.15 alloys in the parallel direction to extrusion. The maximum dimensionless figure of merit was obtained when extruded at 753 K at a testing temperature of 343 K, ZT = 0.66.

Optical tomography (OT) is a promising non-intrusive characterization technique of absorbing and scattering media that uses transmitted and/or reflected signals of samples irradiated with visible or near-infrared light. The quality of OT techniques is directly related to the accuracy of their forward models due to the use of inversion algorithms. In this paper, forward models for transient OT approaches are investigated. The system under study involves a one-dimensional absorbing and scattering medium illuminated by a short laser pulse; this problem is solved using a discrete ordinates–finite volume (DO–FV) method in both time and frequency domain. Previous works have shown that time-domain approaches coupled with first order spatial interpolation schemes cannot represent the physics of the problem adequately as transmitted fluxes emerge before the minimal physical time required to leave the medium. In this work, the Van Leer and Superbee flux limiters, combined with the second order Lax–Wendroff scheme, are used in an attempt to prevent this. Results show that despite significant improvement, flux limiters fail to completely eliminate the physically unrealistic behaviour. On the other hand, results for transmittance obtained from the frequency-based method are accurate, without physically unrealistic behaviours at early time periods. The frequency-dependent approach is however computationally expensive, since it requires approximately five times more computational time than its temporal counterpart when used as a forward model for transient OT. On the other hand, the great advantages of the frequency-based approach is that limited windows of temporal signals can be calculated efficiently (in transient OT), and it can also be used as a forward model for steady-state, frequency-based and transient OT techniques.

To apply mobile crystalline material 41 (MCM-41), which has a larger specific surface area than many other nanoporous silicas, for energy absorption, chlorotrimethylsilane treatment is performed to adjust the structure of inner surfaces of nanopores. As the treatment time is sufficiently long, the pore surface can be changed from hydrophilic to hydrophobic, leading to an energy absorption efficiency a few times higher than previously developed nanoporous-material-functionalized liquids. A strong effect of addition of electrolyte on infiltration pressure is observed.

Density functional theory calculations of the electronic and optical properties of the CaCO₃ calcite polymorph were performed within both the local density (LDA) and generalized gradient (GGA) approximations, respectively. The carriers effective masses are estimated, and the energy gap is shown to be indirect, with and (for comparison, the experimental value is 6.0 ± 0.35 eV). Two optical absorption regimes are predicted, and the dielectric function does not change considerably with the light polarization. The confinement features of excitons in Si@CaCO₃ and CaCO₃@SiO₂ spherical core-shell quantum dots are also presented.

Because of the complexity of several simultaneous physical processes, most heat transfer models of keyhole mode laser welding require some simplifications to make the calculations tractable. The simplifications often limit the applicability of each model to the specific materials systems for which the model is developed. In this work, a rigorous, yet computationally efficient, keyhole model is developed and tested on tantalum, Ti–6Al–4V, 304L stainless steel and vanadium. Unlike previous models, this one combines an existing model to calculate keyhole shape and size with numerical fluid flow and heat transfer calculations in the weld pool. The calculations of the keyhole profile involved a point-by-point heat balance at the keyhole walls considering multiple reflections of the laser beam in the vapour cavity. The equations of conservation of mass, momentum and energy are then solved in three dimensions assuming that the temperatures at the keyhole wall reach the boiling point of the different metals or alloys. A turbulence model based on Prandtl's mixing length hypothesis was used to estimate the effective viscosity and thermal conductivity in the liquid region. The calculated weld cross-sections agreed well with the experimental results for each metal and alloy system examined here. In each case, the weld pool geometry was affected by the thermal diffusivity, absorption coefficient, and the melting and boiling points, among the various physical properties of the alloy. The model was also used to better understand solidification phenomena and calculate the solidification parameters at the trailing edge of the weld pool. These calculations indicate that the solidification structure became less dendritic and coarser with decreasing weld velocities over the range of speeds investigated in this study. Overall, the keyhole weld model provides satisfactory simulations of the weld geometries and solidification sub-structures for diverse engineering metals and alloys.

Europium doped zinc-tellurite glasses modified with LiF, Na₂O + Li₂O and Na₂O + Li₂ O + Nb₂O₅ were prepared by the conventional melting procedure and their thermal, structural and optical properties were investigated. Differential thermal analysis curves in the temperature range 30–1200 °C at a heating rate of 10 °C min⁻¹ were used to determine the thermal properties such as glass transition, crystallization and melting temperature. FT-IR spectra were used to analyse the glass structure. The spectroscopic properties including absorption and emission spectra and fluorescence lifetime of Eu³⁺ ions were measured. A strong red fluorescence is observed from the ⁵ D₀ level of Eu³⁺ ions in these glasses. The relative luminescence intensity ratio (R) of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transitions has been evaluated to estimate the local site symmetry around the Eu³⁺ ions. The emission spectra of these glasses show a complete removal of degeneracy for the ⁵ D₀ → ⁷F₁ transition of Eu³⁺ ions. Based on the energy level data obtained from the absorption and emission measurements, free-ion energy level analysis has been carried out. Second rank crystal-field parameters have been calculated together with the crystal-field strength parameter by assuming the C_2v symmetry for the Eu³⁺ ions in these glasses. The crystal-field parameters are found to be higher in binary zinc-tellurite glasses. The trend of variation of crystal-field strength with glass composition is found to be more or less opposite to that of R in the present glasses. The effect of temperature on the luminescence from ⁵D₁ level is studied. The decay from the ⁵D₀ level is found to be exponential and the lifetime is shorter than those found in Eu³⁺-doped borate, phosphate and fluoride based glasses.

Oxygen ions are implanted into BiFeO₃ films on LaNiO₃/SrTiO₃(1 0 0) substrates. The evolution of films is characterized by x-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. The leakage current density of the implanted films is lowered by two orders of magnitude in comparison with unimplanted BFO films. The mechanisms for reduced leakage current in BiFeO₃ thin films are discussed.

The master Gd₅Si_1.8Ge_1.8Sn_0.4 MCE alloy was prepared by the arc-melt method. The alloy was re-melted and melt-spun by single-roller melt spinning at different copper wheel speeds. Temperature-dependent x-ray diffraction (XRD) and magnetic measurement confirm the first-order magnetic–crystallographic transition of as-cast Gd₅ Si_1.8Ge_1.8Sn_0.4. From powder XRD results, the melt-spun technique preserves the monoclinic phase of Gd₅Si_1.8Ge_1.8Sn_0.4 but broadens the corresponding Bragg reflection which implies the smaller crystalline size of the melt-spun ribbons. For the spun ribbons, the refrigerant capacity (RC) is comparable to that of the as-prepared Gd₅Si_1.8Ge_1.8Sn_0.4. The thermal (∼9.2 K) and magnetic hysteresis (∼62 J kg⁻¹ at 270 K) near T_C of the as-cast alloy have been significantly reduced (∼4 K and ∼5.7 J kg⁻¹ to ∼7.5 J kg⁻¹ for ribbons, respectively) with the help of the melt-spun technique. The low hysteresis and large RC values make the spun ribbons suitable for magnetic refrigeration application near room temperature.

This paper examines the reliability of commercial off the shelf (COTS) erasable programmable read only memory (EPROM) and electrically erasable programmable read only memory (EEPROM) components exposed to gamma radiation. Results obtained for 64 KB EPROM (NM27C512) and 16 KB EEPROM (M24128) components provide evidence that EPROMs have greater radiation hardness than EEPROMs. Moreover, the changes in EPROMs are reversible, and after erasure and reprogramming all EPROM components restore their functionality. On the other hand, changes in EEPROMs are irreversible. The obtained results are analysed and interpreted on the basis of gamma ray interaction with the oxide layer.

Embedding a thermoelectric generator (TEG) in a biological body is a promising way to supply electronic power in the long term for an implantable medical device (IMD). The unique merit of this method lies in its direct utilization of the temperature difference intrinsically existing throughout the whole biological body. However, little is known about the practicability of such a power generation strategy up to now. This paper attempts to evaluate the energy generation capacity of an implanted TEG subject to various physiological or environmental thermal conditions. Through theoretical analysis, it was found that the highest temperature gradient occurs near the skin surface of the human body, which suggested a candidate site for implanting and positioning the TEG. In addition, numerical simulations were performed on three-dimensional bioheat transfer problems in human bodies embedded with TEGs at different implantation depths and configurations. To further enhance energy generation of an implanted TEG, several external technical approaches by intentionally cooling or heating the skin surface were proposed and evaluated. Conceptual experiments either in vitro or in vivo were implemented to preliminarily test the theoretical predictions. Given the fact that an IMD generally require very little working energy, the TEG could serve well as a potential long-term energy supplier for such medical practices.