Poleable Dielectric Elastomer Composites

As part of the recent efforts on renewable energies, energy harvesting by exploiting environmental motions and vibrations to generate electricity is being enthusiastically developed as a novel kind of green energy source. Piezoelectric materials are a logical choice for such applications, as they can readily generate electric charges when subjected to mechanical force without the need for additional equipment. While small deformations can be easily covered by traditional piezoelectrics, new materials have to be developed to access strains in excess of af few percent. This thesis lays out steps for the fabrication of a piezoelectric elastomer for use as active sensor and energy harvesting applications containing solely organic components. To achieve this a composite approach was chosen in which organic piezoelectric materials, so-called frozen-dipole electrets, were embedded as particulate fillers in a highly elastic polydimethyl siloxane (PDMS)matrix. Nanoparticles of a frozen-dipole electret material had to be prepared and characterized first, followed by their dispersion in the PDMS matrix, and finally the characterization of the composite elastomer. Poly-2-hydroxyethyl methacrylate (PHEMA) particles were produced byminiemulsion polymerization, and Disperse Red 1 (DR1) molecules were incorporated at the same time to turn the material into a frozen-dipole electret. The DR1 was introduced either as a blend or by copolymerization with amethacrylate-functionalized DR1 derivative. The study of PHEMA particles indicates that it would be very challenging to render them piezoelectric, due to their high conductivity, the environmental influence on their properties and unfavourable components introduced with the production method. In contrast, PMMA copolymer particles could easily be polarized. Polymethylmethacrylate (PMMA) copolymers with either methacrylate-functionalized DR1 or nitroaniline was used to produce nanoparticles via the nanoprecipitationmethod. Nanoparticles with up to 70 mol% DR1 could be obtained. These materials were chosen because of their known properties as electrets in order to serve as reference materials for the proof of concept of the organic piezoelectric elastomers. They were analysed by impedance spectroscopy and the thermally stimulated depolarization current technique to assess their relaxation and polarization behaviour. PMMA particles could be blended with PDMS without any additional additives to produce composite elastomers which can be turned into piezoelectric materials when poled in an electric field at high temperatures. Depending on the chain length of the PDMSmatrix, maximum strains of up to 600% could be reached. A custom built Berlincourt measuring setup was used to quantify the longitudinal piezoelectric coefficient d33 which was found to depend strongly on the force with which the composite is loaded. The transvere piezoelectric coefficient d31 was measured with a separate setup, based on a tensile testing machine. Piezoelectricity could be measured atmacroscopic strains as high as 200%. d31 initally decreases but stabilizes after a certain time. Most importantly, composites with PMMA-DR1 copolymer particles retain their piezoelectricity after exposure to elevated temperatures for several days.

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

Strontium Bismuth Titanate is a very promising material for high temperature piezoelectric applications, its elevated ferroelectric phase transition (530°C), linear piezoelectric properties under low field and relatively low room temperature conductivity (compared to others Bismuth Titanates) make it very attractive for precision sensors. However, under severe conditions (low frequency, high field, high temperature or low oxygen partial pressure) some of those advantages disappear. Piezoelectric response is dominated by charge drift in general becomes unstable. Above all, at high temperature and low oxygen partial pressure, a large conductivity increase reduces the piezoelectric efficiency of the material, in this work, electrical conductivity, piezoelectric properties and dielectric permittivity of SrBIT ceramic have been investigated in conditions of high temperature, low oxygen partial pressure, low to moderate driving field and frequency. As a result of this research, a better understanding of SrBIT properties was achieved. Thanks to a careful study of SrBIT processing as a bulk ceramic, a reproducible route was established. Many basic mechanisms leading to both SrBIT crystallization and sintering have been identified. It was demonstrated that a detailed knowledge of the exact processing conditions is required in order to achieve high quality material. DC conductivity measurements were carried out as a function of temperature, oxygen partial pressure and dopant concentration. It was found that the apparent activation energy for conduction for undoped SrBIT was 1 eV between 140 and 220°C and 1.5 eV between 450 and 700°C. Decrease of the activation energy in the lower temperature range has been discussed considering grain boundary conductivity, as a transition from electronic to ionic conduction or as consequence of small polarons conduction. It was shown that lower activation energy resulted from Manganese doping (0.5 eV), this was interpreted as either growing influence of grain boundary conductivity as dopant concentration increases or as shallow hole trap formation. DC conductivity measurements and acceptor/donor doping experiments demonstrated p-type conductivity in the low temperature range (up to 220°C) as donor doping decreases conductivity, while oxygen partial pressure controlled measurements indicated n-type conductivity at higher temperature (above 700°C). An electrical impedance analysis was performed with several equivalent circuits. The aim of these models was to simulate the impedance of SrBIT. The best approximation was found with a distributed element of Havriliak-Negami type. However, as the physical justification for this circuit was not clear, the investigation of the grain, grain boundary and electrode impedances was performed with multiple discrete parallel RC elements. With temperature, grain size and oxygen activity variations, the identification of three separate contributions as grain, grain boundary and electrode was realized. The anisotropy of conductivity and permittivity was demonstrated with textured material and both DC and AC analysis. With the master curves built for the electrical modulus, it was found that the impedance probably related to Bismuth oxide layers produces an additional high frequency are. From this finding and by comparing characteristic relaxation frequencies of undoped, 2 mol.% Mn and 4 mol.% Nb SrBIT, it was determined that conductivity is higher in the ab plane direction than in the c direction within both perovskite units and Bismuth oxide layers. With conductivity measurements under controlled oxygen partial pressure, it was found that an acceptor-based (intrinsic or extrinsic) model could be used to describe the electrical conductivity under controlled oxygen partial pressure of both undoped and 2 mol.% Mn doped SrBIT. However, as neither a pO2-1/6 region (intrinsic oxygen vacancies compensated by electrons) for undoped SrBIT nor a pO2-1/4 region (oxygen vacancies compensated by singly ionized acceptors) for Mn doped SrBIT were seen, it was concluded that the acceptorcontrolled model is not sufficient for a complete description of SrBIT. For this reason and in order to include the low oxygen partial pressure behavior of undoped SrBIT, a donor-based (intrinsic) model was also considered. The source of intrinsic donors would be in that case Bi3+ cations sitting on Sr2+ sites in the perovskite sublattice. Considering Bismuth vacancies as the negative compensating species, both pO2-1/4 and pO2-independent regions could be predicted with the model. However, even if the donor-controlled model seems to better match conductivity measurements in the full PO2) range, rejecting the acceptor-based model would be an error. It is actually not demonstrated that in SrBIT the concentration of exchanged Bismuth cations is always (all temperature, pressure) larger than the natural acceptor impurity concentration. It is very likely that the cation exchange is dependent of the oxygen partial pressure. It is also not proved as suggested in the literature that direct compensation between exchanged Strontium and Bismuth exists, reducing the net donor-excess. With the acceptor-controlled model, the mass-action constants for reduction and for ionization of intrinsic carriers across the band gap were determined. The band gap of SrBIT was estimated to be 3.5 eV. The ionic conductivity of SrBIT was determined at high temperature with measurements under controlled oxygen partial pressure. It was found that the electrical conductivity of SrBIT is probably mixed (electronic and ionic) as the estimation of the transference number provided quite large values (t=0.8 at 800°C). From electronic and ionic conductivity data, mixed conduction can actually be predicted in a large temperature range (above room temperature). The piezoelectric measurements using direct effect demonstrated that it is actually possible to unlock piezoelectrically active ferroelectric domain walls and create non-linear piezoelectric properties in undoped SrBIT. This occurs above a threshold elastic field, which is thermally activated. With a piezoelectric composite, it was demonstrated that the electromechanical coupling between two different phases creates a piezoelectric relaxation. This one could be positive or negative depending on the respective properties of each composite's component. It was shown experimentally that a small temperature change is sufficient to transform a positive relaxation into a negative one. While these experiments did not provide a detailed microstructural explanation for the piezoelectric relaxation observed in 2 mol.% Mn doped SrBIT, they gave a first insight into an original phenomenological approach. Microstructure and piezoelectric properties were related with the calculation of a piezoelectric relaxation composite made of two textured samples. This demonstrated that a piezoelectric relaxation may occur just because of anisotropy. Microstructural reproduction of this coupling is actually an important feature of Aurivillius phases.

EPFL2000

On the relationship between the microstructure and the mechanical properties of an ODS ferritic/martensitic steel

Amuthan Ramar

In the quest of materials for the first wall of the future fusion reactor, it has been shown that oxide dispersion strengthened (ODS) ferritic / martensitic (F/M) steels appear to be promising candidates. The inherent good mechanical properties supported by a good thermal conductivity, swelling resistance and low radiation damage accumulation of the base material, such as EUROFER97, are further enhanced by the presence of a fine dispersion of nanometric oxide particles. They would allow in principle for a higher operating temperature of the fusion reactor, above 550°C, which improves its thermal efficiency. In effect, their strength remains higher than the base material at high temperatures. There are however limitations to their application in the reactor, which relate to the intimate link between the microstructure and the mechanical properties. These limitations, such as an increased ductile to brittle transition temperature above room temperature relative to the base material, come directly from the role played by the oxide particles but also from their indirect impact on the overall microstructure, stemming from the procedure of elaboration of these materials. In addition, there is a lack of knowledge on their response to irradiation, which, even though it appears to be promising with a reduced irradiation induced hardening, is not very well understood. In this work we have developed and evaluated ferritic/martensitic steel, EUROFER97, strengthened by a dispersion of yttria particles. (1) The powder metallurgical process used to obtain it was optimized, (2) the basic mechanisms of hardening were identified and (3) the impact of irradiation on its microstructure and mechanical properties were investigated. In order to understand the behaviour of this class of material other oxide dispersion strengthened (ODS) ferritic/martensitic steels based on EUROFER97 and model alloys were also investigated. For the elaboration of the ODS steel the powders of atomized EUROFER97 and yttria are ball milled in a planetary ball mill and the material is compacted by hot isostatic pressing (HIP). The mechanical properties are evaluated by uniaxial tensile test, Charpy test and micro-indentation while the microstructure is investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The focused ion beam (FIB) technique is also used. In the study of the ball milling it appears that the optimal time is 20 hours, as the X-ray peak broadening, crystallite size and hardness of the particles remain unchanged at longer times. Yttria nanoparticles clusters are no more detectable by X-ray after 2 hrs of milling, which is attributed here to their incorporation in the matrix, as opposed to dissolution in the matrix as is often suggested in the open literature. HR-TEM performed on the yttria particles before and after incorporation in the steel by ball milling shows that the yttria maintain its bcc equilibrium structure throughout the process. This could suggest that they are not dissolved during ball milling. We have clarified this point by a cross sectional examination of the EUROFER97 ball milled particles with 0.3wt% of yttria in an FIB. By X-ray EDS analysis it shows nanometric regions reach in yttrium and oxygen, which are the yttria particles embedded in the EUROFER97 particles, proving that they are not dissolved by ball milling. The mechanism at the origin of the alloy compaction during HIP was elaborated. The alloy compaction was done through HIPing at 1150°C for 2.5 hrs at a pressure of 190 MPa. This condition is shown to provide the highest bulk density, to a value close to 99.9%. In the optimization process, it is found that temperature plays a larger role than pressure for alloy compaction. The compaction appears to be mainly due to diffusion. Also, it appears that the ball milled particle size plays a major role in the alloy compaction. It is shown that smaller particles size induces better compaction, provided that the densification proceeds through diffusion flow. This further suggests to terminate the ball milling process by 20 hrs as the particles size reaches its minimum. The impact of the ball milling process on the mechanical properties was evaluated. EUROFER97 with and without ball milling, with and without oxide addition presents an average density around 99.8%. In the as received state, casted EUROFER97, atomized EUROFER97, ball milled EUROFER97 and ODS Yttria present microstructure morphology typical of tempered martensite, whereas ODS Ti presents a nano sized grain microstructure with an average grain size of 200 nm. Ball milling and HIPping does have a slight influence up to 400°C, due to the higher dislocation density induced by ball milling relative to casted EUROFER97. It becomes negligible above 400°C, since both casted and ball milled EUROFER97 present similar hardness and microstructure. In ODS Yttria, yttria presents an average particle size of 25 nm. In ODS Ti, Y-Ti-O presents an average particle size of 7 nm. In the following the identification of the hardening mechanisms in ODS F/M steels. A new model was developed to quantify the collective hardening due to different obstacles, assuming that they provide independent hardening mechanisms. The model helped identifying the dominating source of hardening in ODS F/M steels. The base of the model is the dispersion barrier strengthening model, which is given by Δσsparticle or defects = Mαμb(Nd)1/2 for the hardening due small defects or oxide particles and is given by Δσd-d = Mαμb(ρ)1/2 for the hardening due to dislocations. In this approximation, the total hardening is then given by Δσtotal = {(Δσd-d)2 + (Δσparticle or defects)2}1/2 . The model is used to explain the hardening due to oxides, dislocations and irradiation induced defects. It appears that dispersed oxides is dominating hardness up to 800°C, while following a heat treatment at 1000°C it is the dislocations that then dominate the hardness. Deformation mechanisms were investigated using differential strain rate experiments. The activation parameters, i.e. the activation volume and activation energy, were calculated. It appears that there is a decrease in the activation volume from room temperature to 400°C, which is less than 3%. At about 500°C there is a sudden drop in the activation volume that reaches a minimum at 600°C. With further increase in temperature, there is a significant raise in the activation volume. This abrupt decrease between 400°C and 600°C hints at a change in dislocation-rate controlling mechanism, marking two regimes, a low temperature one and a high temperature one. In situ TEM observations show that at low temperature the deformation is mainly due to the formation of the Orowan loops around the particle. At high temperature the activation energy is about 3.4 eV for ODS, whereas it is 1.3 eV for EUROFER97. The calculated activation energy corresponds well to the one of dislocation climb mechanism. This indicates that at high temperature the yttria particles could be overcome by climb, while at low temperature they are overcome by the Orowan mechanism. The peculiar response of the yield stress to irradiation for this class of material was rationalized using the total hardening model we developed. At a dose of 2 dpa, irradiation induced defects are higher in number density than the dispersed yttria particles, which although they are weaker than the yttria particles they can explain the significant hardening observed in the material. Conversely, the shallow impact of irradiation on ODS F/M steels at low doses, relative to base F/M steels, is explained by the fact that the amount of dispersoid is found to be higher than the density of irradiation induced defects.

EPFL2008

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

Since the first inductively coupled plasma (ICP) torch was developed in the 60's, RF thermal plasmas have found a variety of applications in material processing such as crystal growth, thermal spray coatings, spheroidization and vaporization of refractory materials. In recent decades, inductively coupled thermal plasmas have been used for the synthesis of high purity nanoparticles, thank to their remarkable advantages such as high energy density, variable operating pressures and low product contamination. The formation of nano-sized particles in thermal plasmas particularly using solid refractory precursor is a complex process. Hereby the injected precursor powders are heated by the high temperature of the thermal plasma. The heated powders are vaporized and decomposed into vapor species. During the following cooling, the vapor species are condensed to nano-sized particles. In this synthesis process, characterized by evaporation-condensation, the properties of the synthesized particles such as particle size, size distribution, phases and morphology are affected by various process parameters. Gas pressure, flow rates of the different torch gases, size of the precursor powders and powder loading in the plasma are considered as the influencing process parameters. In order to optimize and control the process for tailor-made nanoparticle synthesis, it is necessary to understand the effects of the process parameters on plasma properties and on the particle-plasma interactions resulting in final properties of the synthesized powders. In the present work, firstly, the enthalpy probe technique, a well-established plasma diagnostic tool for high pressure thermal plasma, has been applied to characterize the inductively coupled Ar-H2 thermal RF plasma used for alumina nanoparticle synthesis at different operating conditions. The plasma enthalpy has been determined, and the plasma temperature under a given gas pressure could subsequently be calculated assuming local thermodynamic equilibrium (LTE), once the gas composition has been determined by mass spectrometry. The gas velocity of the plasma jet could also be obtained by using the Bernoulli equation and stagnation pressure. Secondly, the influence of flow rates of central gas and precursor feed rates on the Al2O3 precursor evaporation has been investigated by process monitoring. Optical emission spectroscopy (OES) and laser light extinction (LE) measurements have been carried out to monitor in-situ the injection of the alumina precursors and to obtain information on the ongoing precursor evaporation. The emission line intensities of the aluminum vapor were used to determine the dependence of process parameters on precursor evaporation. Furthermore, the laser light extinction was used to calculate the number density of non plasma-treated powders, from which the number fraction of evaporated powders could be deduced. The size and morphology of the synthesized particles have been characterized ex-situ. Finally, the influence of quenching conditions on the condensation and formation of the nano-sized alumina particle has been studied by process analysis of the gas phase reactions of alumina. Optical emission spectroscopic was performed at different axial positions to monitor the gas phase reactions of the vaporized particles. Axial profiles of the emission line intensities of the main vapor species of alumina, Al(g) and AlO(g), were compared with the results of the enthalpy probe measurements and the thermal decomposition reactions of alumina found in literature. The results allow explanation of the alumina reactions in the thermal plasma as well as gives indications for the optimal axial position of the quenching gas injection. In addition to the determination of the optimal quenching position, the influence of the flow rates of quenching gas on the nanoparticle formation has been investigated. The synthesis of the submicron alumina using vapor-condensation principle predominantly leads to various thermodynamically metastable crystal structures called transition alumina. Therefore, the occurrence of different transition aluminas is discussed with regards to the flow rates of the quenching gas. The results presented in this thesis show that the synthesis process of the nanoparticle can be described on the basis of experimental results and basic information on the powder materials. The explanation of the influence of process parameters on powder evaporation and nanoparticle formation shows the usefulness of the in-situ plasma process monitoring, and the large potential for optimizing and controlling the nanopowder synthesis process in an inductively coupled RF thermal plasma.

EPFL2006