Development and application of a microelectrode based scanning voltammetric detector

A large part of the work presented in this thesis describes the development and use of a novel electrochemical detector designed to allow the electrochemical characterisation of compounds in flowing solution by means of cyclic voltammetry. The detector was microprocessor controlled, which provides digital generation of the potential waveform and collection of data for subsequent analysis. Microdisk working electrodes are employed to permit both thermodynamic and kinetically controlled processes to be studied under steady-state conditions in flowing solutions without the distortion or hysteresis normally encountered with larger sized electrodes. The effect of electrode size, potential scan rate, and solution flow rate are studied extensively with the oxidation of ferrocene used as an example of a thermodynamically controlled process and a series of catecholamines as examples of a kinetically controlled process. The performance of the detector was best demonstrated when used as a HPLC post-column detector. The 3-dimensional chromatovoltammograms obtained allow on-line characterisation of each fraction as it elutes from the column. The rest of the work presented in this thesis involves the study of the oxidative degradation pathway of dithranol. The oxidative pathway was shown to involve a complex free radical mechanism, dependent on the presence of both oxygen and, in particular light. The pathway is further complicated by the fact that dithranol may exist in either a keto or enol form, the enol being most susceptible to oxidation. A likely mechanism is proposed from studies performed with cyclic voltammetry and controlled potential electrolysis, then defined by subsequent kinetic studies.

具有微纳米结构超疏水表面润湿性的研究

In this paper, advance of wetting behavior research on the superhydrophobic surface with micro- and nano-structures is reviewed, including fabrication methods of superhydrophobic surfaces with micro- and nano-structures, influence of the micro- and nano-structures on wetting behavior, superhydrophobic criterions of surfaces with periodic structures, contact angle hysteresis and functionalization of superhydrophobic surfaces. Problems existed in this research field and possible prospects of superhydrophobic surface with micro- and nano-structures are discussed.

Anisotropic wetting characteristics on submicrometer-scale periodic grooved surface

Submicrometer-scale periodic structures consisting of parallel grooves were prepared on azobenzene-containing multiarm star polymer films by laser interference. The wetting characteristics on the patterned surfaces were studied by contact angle measurements. Macroscopic distortion of water drops was found on such small-scale surface structures, and the contact angles measured from the direction parallel to the grooves were larger than those measured from the perpendicular direction. A thermodynamic model was developed to calculate the change in the surface free energy as a function of the instantaneous contact angle when the three-phase contact line (TPCL) moves along the two orthogonal directions. It was found that the fluctuations, i.e., energy barriers, on the energy versus contact angle curves are crucial to the analysis of wetting anisotropy and contact angle hysteresis. The calculated advancing and receding contact angles from the energy versus contact angle curves were in good agreement with those measured experimentally. Furthermore, with the groove depth increasing, both the degree of wetting anisotropy and the contact angle hysteresis perpendicular to the grooves increased as a result of the increase in the energy barrier. The theoretical critical value of the groove depth, above which the anisotropic wetting appears, was determined to be 16 nm for the grooved surface with a wavelength of 396 nm. On the other hand, the effect of the groove wavelength on the contact angle hysteresis perpendicular to the grooves was also interpreted on the basis of the thermodynamic model. That is, with the wavelength decreasing, the contact angle hysteresis increased due to the increase in the number of energy barriers. These results may provide theoretical evidence for the design and application of anisotropic wetting surface.

Superhydrophobic properties of nonaligned boron nitride nanotube films

Superhydrophobicity is highly desirable for numerous applications. Here, we report that a semierect but nonaligned boron nitride nanotube (BNNT) film showed superhydrophobicity with contact angle above 170° and a small contact angle hysteresis. This superhydrophobicity was stable over a large range of drop sizes, and the measured critical transition pressure was about 10 kPa. However, the prostrate BNNT films only showed hydrophobicity. The drop retraction behavior during evaporation, the pressure effect on contact angle, the critical transition pressure, the drop impact behavior, and the self-cleaning efficiency between these two kinds of films were systematically investigated and compared.

Colloid probe AFM study of thermal collapse and protein interactions of poly(N-isopropylacrylamide) coatings

Graft coatings of poly(N-isopropylacrylamide) (pNIPAM) are of considerable interest for the reversible control of bio-interfacial interactions. In this study, graft coatings were prepared by free radical polymerisation from surface-bound polymerisable groups, on silicon wafers and quartz crystal microbalance (QCM) sensors. QCM with dissipation monitoring showed a gradual, extended phase change as the temperature increased. Colloid probe atomic force microscopy (CP-AFM) revealed a marked change in the compressibility of the coating below and above the lower critical solution temperature (LCST). Force curves showed an approximate 9-fold reduction in thickness between 24 °C and 38 °C, yet the collapsed coating at 38 °C still had a thickness significantly higher than the ellipsometrically determined dry thickness, indicating a residual extent of hydration above the LCST. At all temperatures, interaction force curves showed steric repulsion, though over different distances. There was little hysteresis between approach and retract force curves, which is evidence for almost instantaneous relaxation of the coating upon decompression. CP-AFM using a probe coated with bovine serum albumin (BSA) showed repulsive interactions with little approach/retraction hysteresis below the LCST, indicating lack of adhesion between the pNIPAM coating and the BSA-coated probe. In contrast, above the LCST the force curves on retraction were characteristic of adhesion, while the approach curves were still repulsive, and the adhesion increased in strength as the temperature was increased further beyond the LCST. Thus, QCM-D and CP-AFM data correlated well in showing a gradual nature of the phase transition and a concomitant gradual change in the interaction force with BSA.

Interfacial energy states of moisture-exposed cracks in mica

Results of crack growth observations on mica in water-containing environments are described. The study focuses on equilibrium crack states for reversed loading cycles, i.e., for initial propagation through virgin solid and subsequent retraction-repropagation through healed or misoriented-healed interfaces. Departures from these equilibrium states are manifest as steady-state forward or backward crack velocities at specific applied loads. The equilibria are thereby interpreted as quiescent, threshold configurations G = W_E, with G the Griffith mechanical-energy-release rate and W_E the Dupré work of adhesion, on crack velocity (v-G) diagrams. Generally, W_E is found to decrease with concentration of water, in accordance with a Gibbs formalism. Hysteresis is observed in the forward-backward-forward crack propagation cycle, signifying a reduction in the adhesion energy on exposure of the open interface to environmental species prior to healing. This hysteresis is especially marked for those interfaces that are misoriented before healing, indicating that the structure of the underlying solid substrate as well as of the intervening fluid is an important consideration in the interface energetics. The equilibrium states for different environments can be represented on a simple energy-level diagram, as differences between thermodynamic end-point states: initial, closed-interface states refer to crystallographic bonding configurations ahead of the crack-tip adhesion zone; final, open interface states refer to configurations behind the crack-tip zone. The significance of this diagram in relation to the fundamental atomic structure of interfaces in fracture and other adhesion geometries, including implications concerning kinetics, is discussed.

Space-for-time substitution to assess likely response of aquatic ecosystems to future climate-related change

Predicting and managing ecological response to a changing climate is often limited by an incomplete understanding of response thresholds and biogeographic differences. For example, step changes in rainfall and runoff, and threshold dynamics and hysteresis in ecological response make projection of future conditions difficult. To combat these constraints we propose that biophysical data across exiting climatic gradients can be used in a space-for-time substitution to predict climate-related ecological response elsewhere. This method builds on previous attempts at space-for-time substitution by using patterns in physical and physicochemical data to explain biological differences across the spatial gradient, then using those patterns to formulate hypotheses of temporal ecological response and finally testing those hypotheses on temporal data available in a second, similar region of interest. 

Combinational rate effects on the performance of nano-grained pseudoelastic Nitinols

Combinational loading-unloading rate effects were studied on the behavior of NiTi shape memory alloys (SMAs) under nanoindentation loads. While combinational loading rates showed negligible effects on the performance of NiTi SMAs, the combinational unloading rates did show significant effects on hysteresis energy. The heating-cooling phenomenon during the loading stage and the sole cooling during the unloading stage explain the effects. This study elucidates the nature of thermomechanical SMAs' behaviors during complex compressive loadings with the presence of solid-state phase transition.

Surface-scribed transparency-based microplates

Transparency sheets, which are normally associated with use on overhead projectors, offer lowered costs and high amenability for optical diagnostics in microplate instrumentation. An alternative microplate design in which circles are scribed on the surface of the transparency to create the boundaries to hold the drop in place is investigated here. The 3D profile of the scribed regions obtained optically showed strong likelihood of affecting three-phase contact line interactions. During dispensation, the contact angle (?95°) was larger than the drop advancing state (?80°) due to a period of nonadhesion, where the contact angle later reduced to the drop advancing state followed by increase in the liquid area coverage on the substrate. It was established that 50 ?L was needed to fill the well fully, and the maximum volume retainable before breaching was 190 ?L. While the tilt angle needed for displacement reduced significantly from 50 to 95 ?L, this was markedly better than nonscribed surfaces, where tilt angles always had to be kept to within 30°. It was found that there was greater ability to fill the well with smaller volumes with dispensation at the center. This was attributed to the growing contact line not meeting the scribed edge in parallel if liquid was dispensed closer to it, wherein pinning reduction in some directions permitted liquid travel along the scribed edge to undergo contact angle hysteresis. Fluorescence measurements conducted showed no performance compromise when using scribed transparency microplates over standard microplates.

Phase transformation evolution in NiTi shape memory alloy under cyclic nanoindentation loadings at dissimilar rates

Hysteresis energy decreased significantly as nanocrystalline NiTi shape memory alloy was under triangular cyclic nanoindentation loadings at high rate. Jagged curves evidenced discrete stress relaxations. With a large recovery state of maximum deformation in each cycle, this behavior concluded in several nucleation sites of phase transformation in stressed bulk. Additionally, the higher initial propagation velocity of interface and thermal activation volume, and higher levels of phase transition stress in subsequent cycles explained the monotonic decreasing trend of dissipated energy. In contrast, the dissipated energy showed an opposite increasing trend during triangular cyclic loadings at a low rate and 60âsec holding time after each unloading stage. Due to the isothermal loading rate and the holding time, a major part of the released latent heat was transferred during the cyclic loading resulting in an unchanged phase transition stress. This fact with the reorientation phenomenon explained the monotonic increasing trend of hysteresis energy.

A high performance position sensorless surface permanent magnet synchronous motor drive based on flux angle

A position sensorless Surface Permanent Magnet Synchronous Motor (SPMSM) drive based on flux angle is presented in this paper. The motor equations are written in rotor fixed d-q reference frame. A PID controller is used to process the speed error to generate the reference torque current keeping the magnetizing current fixed. The estimated stator flux using Recurrent Neural Network (RNN) is used to find out the rotor position. The flux angle and the reference current phasor angle are used in vector rotator to generate the reference phase currents. Hysteresis current controller block controls the switching of the 3-phase inverter to apply voltage to the motor stator. Simulation studies on different operating conditions indicate the acceptability of the drive system. The drive system only requires a speed transducer and is free from position sensor requirement. The proposed control scheme is robust under load torque disturbances and motor parameter variations. It is also simple and low cost to implement in a practical environment.

Vector control of a position sensorless SPMSM drive with RNN based stator flux estimator

A position sensorless Surface Permanent Magnet Synchronous Motor (SPMSM) drive based on single layer Recurrent Neural Network (RNN) is presented in this paper. The motor equations are written in rotor fixed d-q reference frame. A PID controller is used to process the speed error to generate the reference torque current keeping the magnetizing current fixed. The RNN estimator is used to estimate flux components along the stator fixed stationary axes. The flux angle and the reference current phasor angle are used in vector rotator to generate the reference phase currents. Hysteresis current controller block controls the switching of the three phase inverter to apply voltage to the motor stator. Simulation studies on different operating conditions indicate the acceptability of the drive system. The proposed estimator can be used to accurately measure the motor fluxes and rotor angle over a wide speed range. The proposed control scheme is robust under load torque disturbances and motor parameter variations. It is also simple and low cost to implememnt in a practical environment

Carbon nanotube based elastomer composites-an approach towards multifunctional materials

The current study focuses on giving a basic understanding of tubular graphene sheets or carbon nanotubes (CNTs) and points towards their role in fabricating elastomer composites. Since the properties and the performance of CNT reinforced elastomer composites predominantly depend on the rate of dispersion of fillers in the matrix, the physical and chemical interaction of polymer chains with the nanotubes, crosslinking chemistry of rubbers and the orientation of the tubes within the matrix, here, a thorough study of these topics is carried out. For this, various techniques of composite manufacturing such as pulverization, heterocoagulation, freeze drying, etc. are discussed by emphasizing the dispersion and alignment of CNTs in elastomers. The importance of the functionalization technique as well as the confinement effect of nanotubes in elastomer media is derived. In a word, this article is aimed exclusively at addressing the prevailing problems related to the CNT dispersion in various rubber matrices, the solutions to produce advanced high-performance elastomeric composites and various fields of applications of such composites, especially electronics. Special attention has also been given to the non-linear viscoelasticity effects of elastomers such as the Payne effect, Mullin's effect and hysteresis in regulating the composite properties. Moreover, the current challenges and opportunities for efficiently translating the extraordinary electrical properties of CNTs to rubbery matrices are also dealt with.

Spin-crossover, mesomorphic and thermoelectrical properties of cobalt(II) complexes with alkylated N3-Schiff bases

Three new cobalt(ii) complexes, [Co(L12)2](BF4)2 (1), [Co(L14)2](BF4)2·H2O (2) and [Co(L16)2](BF4)2·H2O (3), where L12-16 are N3-Schiff bases appended with linear C12-16 carbon chains at the nitrogen atoms, were obtained in good yields by facile one-pot reactions. The single crystal X-ray structure of complex 1 shows a tetragonally compressed CoN6 coordination geometry. The melting temperatures of 1-3 were lower than 373 K, while their decomposition temperatures were above 473 K. All complexes have high-spin Co(ii) centres at 300 K and exhibit a columnar mesophase above 383 K. Complexes 1 and 3 showed normal thermal spin-crossover behaviour with weak hysteresis loops at about 320 K. Hence, these complexes showed uncoupled phase transitions (class iiia). The values for the Seebeck coefficient (Se) of the cobalt redox couples formed from 1 and 2 were 1.89 ± 0.02 mV K-1 and 1.92 ± 0.08 mV K-1, respectively, identifying them as potential thermoelectrochemical materials.