Air Driven Electrospinning of CNT Doped Conductive Polymer Fibers for Electronics

An electrostatic and air driven (EStAD) electrospinning device was used to achieve deposition of polymer fiber mats that carry electrical charge. The EStAD device does not require the polymer stream to contact a deposition electrode, thereby allowing enhanced control and processing versatility over production of conductive polymer materials. Direct current (DC) conductivity in the fiber mats was enabled through the use of a composite multi-walled carbon nanotube-polyethylene oxide (MWCNT-PEO) blend for electrospinning (ES). The electrospun fiber mats contained three different concentrations of MWCNTs. Conductivity and resistance were measured for each concentration as an electrospun fiber mat and compared to that of a drop-cast thin film. Results showed that at 7.51 wt% MWCNTs, conductivity in the electrospun fiber mats began to approach that of the drop-cast thin films at 1.76E-01 S/cm. At the lowest weight percent tested (3.37 wt%), conductivity was still measurable at approximately 8.48E-05 S/cm and was comparable to results reported previously using traditional ES methods.

Light Directed Electrophoretic Deposition for Additive Manufacturing: Spatially Localized Deposition Control with Photoconductive Counter Electrodes

Electrophoretic deposition (EPD) has traditionally been viewed as a thin film deposition technique for coating conductive surfaces. Recently, there have been reports of producing functional parts with EPD to near net shape, often containing gradients in material properties normal to the conductive deposition surface. By using reconfigurable electrode systems, a few researchers have gone beyond purely out-of-plane gradients and demonstrated gradients in material properties in the plane of the deposition electrode, a necessary condition for 3D additive manufacturing. In this work, we build upon a previously published technique called light directed electrophoretic deposition (LD-EPD) in which the deposition electrode is photoconductive and can be activated with light, leading to a patterned deposit. Here, we demonstrate that the LD-EPD technique can also lead to patterned deposits on any conductive surface by utilizing the photoconductive electrode as the counter electrode. This eliminates several issues with standard LD-EPD by allowing the potentially expensive photoconductive electrode to be reused, as well as mitigates post-processing material compatibility issues by allowing deposition on any conductive surface. We also detail the results of a finite element simulation of the deposition process in LD-EPD systems that captures key features seen experimentally in the final deposit.

Study of the Deposit Resistance during Electrophoretic Deposition

Deposition experiments in a Hull cell showed that high conductivity suspensions yield uniform deposits while low conductivity suspensions result in non-uniform deposits. This difference in deposition behavior is related to the resistance increase of the deposit during EPD. Impedance measurements during EPD showed that the ratio of the deposit resistance to the suspension resistance increases much more for high than for the low conductivity suspensions. They also showed that the total resistance of the EPD cell dropped almost to the suspension resistance after the electric field was turned off. This means that the deposit has no inherent resistance, but that its resistance during polarization is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. The change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.

Electrophoretic Deposition Mechanism of Mesoporous Silica Powder in Acetone

The preparation of a thick coating of mesoporous silica (MPS) on metal substrates by electrophoretic deposition (EPD) has been studied for application to an advanced desiccant cooling system. In this study, we investigated the EPD mechanism of MPS in acetone under a constant applied voltage for the fabrication of a thick MPS coating. We consider that the main contributors to the resistance between the deposition electrode and the counter electrode are the deposited MPS layer and the EPD bath. The current density was measured during EPD under a constant applied DC voltage. The current density decreased as the EPD progressed, and the resistance increased between the electrodes. The AC impedance between the deposition substrate and the counter electrode was measured in the range of 10 Hz-80 kHz. We observed a single semicircle in the impedance plots. We investigated the influence of the MPS powder concentration of the EPD bath and the amount of deposition layer on the total resistance. We found that the resistance caused by forming the MPS layer increased as EPD progressed whereas the resistance of EPD bath was almost constant through the EPD process. Moreover, we determined the EPD mechanism of MPS powder in acetone.

Textured α-Alumina through Electrophoretic Deposition and Templated Grain Growth

The development of texture was studied during electrophoretic deposition in alumina suspensions containing plate shaped alumina particles. The mechanism of platelet orientation during EPD was examined with respect to the influence of the electric field, gravity and hydrodynamic forces. This was realized by using two different deposition cells, with vertically or horizontally positioned deposition electrode. The texture of the green deposit was further enhanced during sintering by templated grain growth in which the platelet shaped alumina particles were growing at the expense of the fine grained matrix. The sharp ‘fiber texture’ obtained after templated grain growth during sintering of the deposit was characterized by means of x-ray diffraction and Electron Backscatter Diffraction (EBSD).

Electrophoretically Deposited Porous Ceramics and their Characterisation by X-Ray Computer Tomography

Planar zirconia green bodies with unidirectionally aligned pore channels were prepared by an electrophoretic deposition method using an effect which is usually disadvantageous for the EPD from aqueous suspensions: the formation of gas bubbles by electrolysis. Aqueous ZrO2 suspensions containing acetic acid to enable a sufficient gas generation were used for the EPD experiments. The influence of selected parameters – electrolyte content, applied voltage, and kind of deposition electrode – on the pore structure has been investigated. The green bodies were sintered at 1450 °C in air. Optical microscopy and X-ray computer tomography (CT) were used for characterising the porous structures. CT-investigations have the advantage of a three-dimensional characterisation of the samples by a non-destructive method. The resulting pore structure depended to a high degree on the experimental conditions of the EPD. The kind of the deposition electrode – platinum foil or platinum gauze on foil – had a strong influence on the arrangement of the pores. The EPD on Pt-foil led to randomly arranged pores, whereas very regular pore structures were obtained by the use of Pt-gauze.

In-Situ Measurement of Actuation in Thin Films of Conducting Polymers

Conducting polymer materials can be developed as muscle-like actuators for applications in robotics, micro-electro mechanical systems, drug delivery systems etc. These materials are available in a large number of different varieties that can be synthesized and processed in different ways. However, their applications as actuators are limited due to the inability to create conducting polymer materials with robust mechanical properties. Currently most of the dynamic mechanical analysis technologies require the polymer created to be free standing and able to withstand large stresses. This severely limits the development of new materials with potential actuator applications. In this study, a technique to measure the actuation of polymers in the electrochemical deposition environment is described. This allows testing of an electrochemically grown conducting polymer sample on the surface of the deposition electrode itself. Thin polypyrrole films (2 to 20 microns thick) doped with tetraethylammonium hexaflourophosphate were grown on the surface of a glassy carbon electrode. These films were then tested on the surface of the glassy carbon using a custom built electrochemical dynamic mechanical analyzer. A square wave potential (+/- 0.8 V) is applied to the films that results in the actuation of the films. The films are able to generate a changing force of 3 mN of force against a 0.1 N sensor preloaded at 5 mN. The resulting magnitude of the measured force is a function of the film thickness while the change in force due to actuation is approximately constant.

Impregnation of 3D Woven Carbon Fibre Preforms by Electrophoretic Deposition of Single and Mix of Non Oxide Ceramic Nanoscale Powders, and Densification of the Composite Material

3D woven carbon fibre preforms (~ 2 mm thickness) were impregnated with single or mixture of non oxide ceramic nanoscale powders in ethanolic suspensions using electrophoretic deposition (EPD). The measurement of the ζ potential of the suspension compared with its behaviour in sedimentation permits to conclude about the optimal concentration of surfactant necessary to get the most stable suspension. The experimental results were in agreement with the theoretical ones obtained using DLVO (Derjaguin, Landau, Verwey and Overbeek) theory. EPDs were carried out by applying a constant voltage between the 3D carbon fibre preform serving as deposition electrode and counter electrodes in graphite. The effect of the powder concentration on the rate and the quality of impregnation was studied. A qualitative model based on the experimental results and literature was then proposed. Experiments were not only carried out on raw 3D fibrous preforms but also on preforms with interphases. SEM and optical micrographies of fractured and polished sections of the infiltrated fabrics revealed that a quite high degree of infiltration (rates of impregnation estimated between 50 and 70 %) was obtained. At last, the composite was densified using spark plasma sintering (SPS).

Electrophoretic Deposition as a Novel Near Net Shaping Technique for Functionally Graded Biomaterials

Due to their excellent properties of high strength, biocompatibility and stability in physiological environments, ceramics are investigated as bone substitute materials. In this way ceramic components have been used for total hip replacement components since the early 1970s. Alumina and zirconia monoliths are mainly used for these components. However, zirconia can undergo low temperature degradation in aqueous environment and alumina is brittle. To increase the strength, functionally graded Al2O3/ZrO2 ball-heads and acetabular cup inserts were made in this work by electrophoretic deposition (EPD). A composition gradient in alumina and zirconia was engineered to obtain a pure alumina surface region and a homogeneous alumina/zirconia core with intermediate continuously graded regions to generate thermal residual stresses at the surface after sintering. Experimental work revealed that a sequence of counter-electrodes was necessary to EPD complex shaped functionally graded material (FGM) components. To obtain deposits with a shape different from the deposition electrode, the deposit was grown up to the counter-electrode and the design of the counter-electrodes was supported by electrical field calculations to generate a constant electric field at the surface of the deposition electrode