Christopher David Cook

Graphene v2O5 nH2O xerogel composite cathodes for lithium ion batteries

A layer structured V2O5·nH2O xerogel was synthesized via a simple green hydrothermal technique by dissolving commercial V2O5 powder in de-ionized water and hydrogen peroxide. Graphene–V2O5·nH2O xerogel composites were then prepared by mixing and filtration of as-prepared V2O5·nH2O xerogel and graphene in the desired ratio. The method is a cost effective and energy saving way to prepare nanostructured composites. Structure and morphology were investigated by X-ray diffraction, thermogravimetric analysis, field emission scanning electron microscopy, and transmission electron microscopy. Heat treatment at different temperatures could yield V2O5·nH2O xerogels with different amounts of crystal water, and the presence of graphene in the composites enhanced the thermal stability of V2O5·nH2O, in which the phase transformation moved towards higher temperature compared with the sample without graphene. The pristine V2O5·nH2O xerogel consisted of thin layers of ribbons with widths around 100 nm. In the composites, the V2O5·nH2O ribbons were located on the surface of the graphene sheets. Increasing the graphene content in the composites resulted in better cycling stability when the composites were tested as cathodes in different voltage ranges for lithium ion batteries. The initial and the 50th discharge capacities of the composite cathode with 17.8% graphene are 299 and 174 mAh g−1, respectively, when cycled between 1.5 and 4.0 V. The capacities decreased to 227 and 156 mAh g−1, respectively, when cycled between 2.0 and 4.0 V. The initial and the 50th discharge capacities of the composite with 39.6% graphene are 212 and 190 mAh g−1 in the voltage range of 1.5–4.0 V, and the capacities are 143 and 163 mAh g−1 when cycled between 2.0 and 4.0 V, respectively. The outstanding electrochemical performance could be attributed to the graphene induced unique structure and morphology.

Reducing Wave-Based Teleoperator Reflections for Unknown Environments

Wave variables guarantee stability for delayed teleoperation. They also introduce reflections which can prove very disorientating for an operator. This paper presents a new method for reducing the wave reflections when operating in unknown environments. The new method does not only guarantee stability but also laboratory experiments show it also reduces wave reflections allowing for improved velocity tracking and force feedback in complex and unknown operating environments.

Validation of Resonant Frequency Model for Polypyrrole Trilayer Actuators

Conducting polymers are new materials that can be used as low-voltage actuators and active flexure joints, scaling down to the microscale; however, many devices based on these actuators are limited because they can only operate when submerged in an electrolyte. Conducting polymer trilayer actuators are laminated structures with a wide range of potential applications as they are capable of operating both in air and liquid environments, but their dynamic behavior is not yet fully understood. With the view to developing a comprehensive dynamic model of trilayer actuators, this paper presents the experimentally obtained frequency response of the actuator displacement, as measured using a laser displacement sensor. A model of the resonant frequency is also presented and then comprehensively validated using actuators of various geometry and loading. This model can be used to: 1) estimate properties of the trilayer actuator from experimental measurements; 2) quantify and optimize an actuators dynamic behavior as a function of geometry; and 3) facilitate the use of these actuators as a component in practical applications, such as force and motion control systems.

Inversion-Based Feedforward Control of Polypyrrole Trilayer Bender Actuators

Conducting polymer bending actuators show potential for unique manipulation devices, particularly at the microscale, given low actuation voltages, controllable manufacture, biocompatibility, and ability to operate in either air or liquid environments; however, the impracticalities of implementing feedback in these environments and at these scales can impede positional control of the actuator. This paper presents an application of inversion-based feedforward positional control to a trilayer bender actuator, which is shown to improve the performance without the use of feedback or adjustments to the chemistry of the device. The step and dynamic displacement responses have all been improved under the feedforward control system, while the response does not change significantly under large increases in external loads. This study contributes the first implementation of inversion-based feedforward control to the emerging area of conducting polymer actuators, paving the way toward their use in functional devices, particularly where the implementation of feedback is difficult.

Measurements of AC losses in HTSC wires exposed to an alternating field using calorimetric methods

Calorimetric methods for AC loss measurement for short superconducting wires have been investigated. The design, operation and results obtained from an experimental calorimeter are described. With these methods the total loss of a short superconducting sample exposed to a 50 Hz alternating field, both perpendicular and axial, have been measured with an accuracy of microwatts per centimeter. The sample is a 6 centimeter long (Bi,Pb)SrCaCuO-2223 silver-sheathed multifilamentary wire prepared by powder-in-tube techniques. The hysteresis part of the loss may be obtained by taking the eddy current component of the silver sheath from the total loss and by neglecting the coupling loss in the silver matrices. It is shown that the hysteresis losses are dominant in this frequency and its values correspond to the theoretical approximation.

Frequency response of polypyrrole trilayer actuator displacement

Conducting polymer trilayers are attractive for use in functional devices, given low actuation voltages, operation in air and potentially useful stresses and strains; however, their dynamic behavior must be understood from an engineering perspective before they can be effectively incorporated into a design. As a step towards the identification of the actuator dynamics, frequency response analysis has been performed to identify the magnitude and phase shift of displacement in response to a sinusoidal voltage input. The low damping of the trilayer operating in air and the use of a laser displacement sensor has allowed the frequency response to be continuously identified up to 100Hz, demonstrating a resonant peak at 80Hz for a 10mm long actuator. Two linear transfer function models have been fitted to the frequency response of the trilayer displacement (i) a 3rd order model to represent the dynamics below 20Hz and (ii) a higher complexity 6th order model to also include the resonant peak. In response to a random input signal, the 3rd order model coarsely follows the experimental identified displacement, while the 6th order model is able to fully simulate the real trilayer movement. Step responses have also been obtained for the 3rd and 6th order transfer functions, with both models capable of following the first 4 seconds of experimental displacement. The application of empirical transfer function models will facilitate accurate simulation and analysis of trilayer displacement, and will lead to the design of accurate positional control systems.

Towards the position control of conducting polymer trilayer bending actuators with integrated feedback sensor

Conducting polymers have a wide range of reversibly controllable properties, leading to a number of potentially useful devices for robotic applications, including actuators, sensors and batteries. Conducting polymers have the advantages of low weight, low cost, flexibility, small activation potentials (≪2V), biocompatibility and the ability to be manufactured using relatively straightforward techniques. Trilayer bending actuators which utilise the controllable change in volume of conducting polymers are potentially useful devices, but their speed and positioning ability must be improved. Significant research effort has been directed towards improving conducting polymer actuator performance through its chemistry, but the use of compensating control systems has had relatively little focus. This paper experimentally investigates three potential control systems for a trilayer bending actuator - feedforward gain control, feedforward inversion-based control and inversion-based PI control. It was found that the inversion-based PI control system provided the best performance, but the implementation utilised a large laser displacement sensor. To limit the requirement for such a large feedback sensor, a trilayer bending actuator with an integrated displacement sensor is proposed, exploiting the additional sensing capability of conducting polymers.

Critical current degradation caused by winding process of Bi-2223/Ag HTS wire in the form of a coil

High T/sub c/ superconducting (HTS) (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/ Cu/sub 3/O/sub 10+x/ Ag-clad wire has potential for practical applications in the form of a coil or a winding. This HTS wire has strong magnetic field-dependent and mechanical strain-dependent critical currents, consequently it has a severe problem of critical current degradation when it is used in the form of a coil. To design a winding with this conductor, the critical current degradation has to be identified with relation to the magnetic fields and the winding process. This is important to optimize an appropriate coil winding procedure using this HTS wire. A specially designed noninductive sample has been made with a (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/ Cu/sub 3/O/sub 10+x/ Ag-clad 27-filament wire, and the critical current degradation, which is caused by the magnetic field generated and the mechanical winding procedure used to form a coil, has been separated into two factors accordingly, and discussed in this paper.

Monitoring of wheel dressing operations for precision grinding

In grinding operation, on-line process monitoring of dressing will ensure the quality of the grinding wheel and will achieve reproducible surface finish. The results of a study conducted to evaluate the feasibility and effectiveness of Acoustic Emission (AE) sensors in online monitoring of dressing is reported. Two aspects of the process are addressed. The first is the detection of undesired wheel conditions, such as an out-of-round wheel and wheel contour errors. The other is improving the surface finish of the ground workpiece by monitoring the dressing parameters. Experimental results indicate that AE signal can be effectively used to identify the faults that might occur during dressing. This in turn will result in better surface finish in the machined workpiece.

Transport critical current of MgB2 wires: pulsed current of varying rate compared to direct current method

The measurement of transport critical current (Ic) for MgB2 wires and tapes has been investigated with two different techniques, the conventional four-probe arrangement with direct current (DC) power source, and a tailored triangle pulse at different rates of current change. The DC method has been widely used and practiced by various groups, but suffers from inevitable heating effects when high currents are used at low magnetic fields. The pulsed current method has no heating effects, but the critical current can depend on the rate of the current change (dI/dt) in the pulse. Our pulsed current measurements with varying dI/dt show that the same values of Ic are obtained as with the DC method, but without the artifacts of heating. Our method is particularly useful at low field regions which are often inaccessible by DC methods. We also performed a finite element method (FEM) analysis to obtain the time dependent heat distribution in MgB2 due to the electric potential produced at the current contacts to the superconducting sample and its gradient around the contacts. This gradient is defined as the current transfer length (CTL) of the samples and leads to Joule heating of the wire near the contacts. The FEM results provide further evidence of the limitation of the DC method in obtaining high transport critical current.