Preparation and properties of KNbO3-based piezoelectric ceramics

During the last several years, new lead-free piezoelectric materials have been developed to replace the lead-based materials, such as PZT. Presently, the family of lead-free ceramics showing the most promising piezoelectric properties is based on potassium sodium niobate (KNN). The KNN ceramics have been investigated since 1960s, but many problems arise, especially during the synthesis. Potassium and sodium based powders are moisture sensitive leading often to disintegration of samples after sintering and to moisture dependent properties. Another drawback is the poor densification during sintering. Elaborated procedures (hot pressing, special handling of powders) are needed to produce high quality KNN ceramics in a reproducible way, and their properties are inferior to those of PZT. Li, Ta and Sb modified KNN, however, were reported in 2004 to exhibit properties comparable to those of PZT. These modified compositions promise to be a new generation of environmentally safer piezoelectric materials. The goals of this thesis are to prepare selected compositions within this family and examine their properties relevant for applications in medical transducer. The emphasis of the work is on piezoelectric properties and their stability with respect to the temperature, humidity, and preparation conditions. The unmodified, and lithium (K, Na, Li)NbO3 and the lithium with tantalum modified KNN ceramics, (K, Na, Li)(Nb, Ta)O3 have been produced by the conventional solid state synthesis. The conventional processing steps have been adapted with a goal to obtain reproducible high quality samples without using complex techniques such as hot pressing or special powder handling. In particular, the particle grain size and particle size distribution have been controlled for all the steps; this control starts with the initial powders. To reduce the particle sizes the most efficient milling method has been found to be attrition milling. Another important point is the compositional homogeneity. To improve this homogeneity, a second calcination step has been added to the process. Finally, the sintering step is sensitive, the sintering temperature range in these compositions is as narrow as 5 °C and in some cases, the dwell time is reduced to minimum (several minutes) to avoid grain growth. The densities of the so-obtained ceramics are higher than 95%, but some compositional inhomogeneities have been observed in electron microscopy. The electromechanical properties at room temperature are promising for example d33 = 240 [pm/V], kt = 51%, kp = 45% with ε = 919 [-] and tanδ = 2.6% for ceramics modified with 7 at% lithium, and d33 = 310 [pm/V], kt = 46%, kp = 46% with ε = 829 [-] and tanδ = 2.4% for ceramics modified with 3 at% lithium and 20 at% tantalum. These properties are especially enhanced for compositions with the orthorhombic to tetragonal phase transition close to room temperature. Dielectric and piezoelectric (resonance, converse and direct) measurements as a function of temperature have demonstrated that the orthorhombic to tetragonal phase transition is strongly dependent on temperature unlike in PZT, where it is controlled primarily by the composition. The thermal behaviour of ceramics is thus influenced by the presence of the phase transition in the analysed temperature range. The thermal stability has been analysed in the perspective of medical transducers, which are subjected to sterilisation cycles up to 140 °C. A depolarisation after the first cycle has been observed for all the compositions, the smallest being for the lithium and tantalum modified KNN ceramics. After the second cycle, the properties stabilise, being the best for the compositions (K0.465Na0.465Li0.07)NbO3 and (K0.485Na0.485Li0.03)(Nb0.80Ta0.20)O3. As the enhanced piezoelectric properties of these materials are related to the presence of a phase transition, the determination of the contributions of the domain walls in either phase is then of practical importance. The intrinsic and extrinsic contributions of the piezoelectric response can be separated by analysing the piezoelectric response as a function of the applied stress (direct measurements). It has then been shown that the intrinsic contribution is always higher than the irreversible domain wall motion contribution. This latter contribution is nevertheless higher in the orthorhombic phase than in the tetragonal phase. Finally, as the potassium niobate based ceramics have shown in the past their moisture sensitivity, aging measurements have been done in different atmospheres. Surprisingly, the properties of the modified KNN ceramics studied are not found to be dependent on the moisture and only small aging has been noticed. After 100 days, the (K0.485Na0.485Li0.03)(Nb0.80Ta0.20)O3 ceramics show a decrease of the electromechanical properties below 10% and the coupling coefficient are almost stable with time. With these electromechanical properties and their time/thermal stabilities the modified KNN ceramics are promising substitutes to lead-based materials, in particular the lithium and tantalum modified ceramics.

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

Contribution to the Production and Characterization of W-Y, W-Y2O3 and W-TiC Materials for Fusion Reactors

Lyubomira Veleva

This PhD Thesis work was aimed at investigating the potentiality of tungsten-base materials as structural materials for the future thermonuclear fusion reactors in attempting to develop reduced activation tungsten-base materials with high strength and sufficient ductility, especially in terms of fracture toughness and ductile-to-brittle transition temperature, on the basis of the following ideas: (1) nano-grained materials are expected to show an improved ductility with respect to normal grain-sized materials, and (2) nano-grained materials and materials reinforced with either oxide or carbide particles are expected to show improved strength and radiation resistance, as (i) the particles should act as obstacles for the propagation of mobile dislocations, (ii) the numerous grain boundaries and interfaces between the matrix and the particles should act as sinks for the irradiation-induced defects, and (iii) the particles should also help stabilizing the numerous grain boundaries upon thermal annealing and/or irradiation. A variety of W-base materials, namely W-(0.3-1.0-2.0)Y2O3, W-(0.3-1.0-2.0)Y and W-(0.3-0.9-1.7)TiC materials (in weight percent), have been successfully produced at the laboratory scale by powder metallurgy techniques including mechanical alloying followed by either cold pressing and sintering or hot isostatic pressing (HIPping). X-ray diffractometry and scanning and transmission electron microscopy observations combined with density and microhardness measurements as well as with tensile, Charpy impact, fracture toughness and non-standard three point bending tests allowed the identification of optimal chemical compositions and manufacturing conditions, among those investigated and on the basis of the experimental devices at disposal. Microscopic investigations showed that the materials produced at the laboratory scale are composed of small grains, with sizes between a few tens and a few hundreds nanometers, and contain a high density of small Y2O3 or TiC particles, with sizes between 1 and 50 nm. In the case of W-Y materials, all the Y transformed into Y2O3 during mechanical alloying, due to the high amount of O (around 1 wt.%) present in the milled powders, which is beneficial for reducing the excess O content in the materials. All materials also contain a residual porosity of a few percents, which is typical of materials compacted by HIPping. Macroscopic investigations showed that the HIPped materials exhibit high strength values and a promising resistance to irradiation but poor fracture properties up to a very high temperature of about 1100 °C, due mostly to air contamination of the mechanically alloyed powders and to residual porosity of the compacted materials. The materials produced entirely at the laboratory scale by mechanical alloying and HIPping are of better quality than a W-2Y material that was produced by mechanical alloying at the laboratory scale and compacted by sintering and forging at the Plansee company (Austria). Further studies should focus on reducing the O content in the mechanically alloyed powders and the residual porosity of the compacted materials. The grain size should be also optimized as well as the size distribution, number density and crystallographic features of the oxide and carbide particles.

EPFL2011

High coupling materials for thin film bulk acoustic wave resonators

Janine Conde

Radio frequency (RF) filters based on bulk acoustic wave resonances in piezoelectric thin films have become indispensable components in mobile communications. The currently used material, AlN, exhibits many excellent properties for this purpose. However, its bandwidth is often a limiting factor. In addition, no tuning is possible with AlN. Ferroelectrics would offer both larger coupling to achieve larger bandwidths, and tunability. However, their acoustic properties are not well known, especially in the thin film case. The goal of this thesis is to investigate the potential and identify the limitations of ferroelectric thin films for thickness mode resonators in the 0.5 - 2 GHz range. The Pb(Zrx,Ti1-x)O3 (PZT) solid solution system is the main candidate, since it is known for its large piezoelectric constants and its growth is already well studied. As a main test vehicle, free standing thin film bulk acoustic resonator (TFBAR) structures with Pt/PZT/Pt/SiO2 membranes were successfully fabricated using silicon micro-machining techniques. The main drawback of ferroelectrics is the damping of acoustic waves by domain wall motion both in the RF electric field and in the pressure wave. For this reason films with varying orientations and compositions were investigated. From the device structures the electro-mechanical coupling constants kt2, the quality factors (Q-factors) and several materials parameters have been obtained. High coupling constants have been found for sol-gel Pb(Zr0.53,Ti0.47)O3 films with a {100} texture, kt2 is found to be 0.4 for a 1 µm thick film and 0.8 for a 3.8 µm thick film. However, the Q-factors of these films are low, 18 for the first film and 3 for the second film. The increase of kt2 and the decrease of the Q-factor with frequency indicates that the domains present in these films contribute to these characteristic parameters. It was generally observed that high coupling constant are associated to low Q-factors. This became evident when comparing films with 53/47 composition, where both tetragonal and rhombohedral phases are present, to tetragonal films as well as when comparing {100} textures with (111) textures. Both for the 53/47 composition and for the {100} texture, ferroelastic domain walls are thought to play a bigger role than for tetragonal compositions and (111) textures. The highest figure of merit (FOM) of about 15 was found when combining the composition leading to a high coupling constant (53/47) and the orientation leading to lower losses (111). However the losses even in this film are too high for RF-filter applications. On the other hand, films with low Q-factors but high coupling could prove very useful as transducers for ultrasonic imaging applications, where low Q-factors are desired. The stiffness coefficients of the studied PZT films were shown to be higher than expected from ceramics data. Most likely the stiffness of ceramics always contains domain contributions leading to softening. In contrast, in textured films the variety of domain orientation is very much reduced. In order to reduce losses due the presence of ferroelastic domains three different potential solutions were explored. The first idea was to manipulate the domain populations of the films deposited on silicon by using heat and vacuum treatments. Silicon substrates are known from previous works to be unfavourable for high c-domain fractions. It was discovered that an anneal in vacuum at 550 °C lead to a significant reduction of c-domains in tetragonal 30/70 PZT ({100}). On the contrary, if the sample was subjected to a compressive stress during cooling, the c-domain fraction could be increased. Analysis of the film stress versus temperature curves revealed a trend consistent with theoretical predictions, i.e. a phase boundary between the c/a/c/a and the a1/a2/a1/a2 domain patterns between room temperature and the Curie temperature θC. However, even though this method reveals interesting results, it can not be exploited as a method to achieve a sufficient c-domain population. The second idea explored was the implementation of a high thermal expansion material as a substrate. PZT films deposited on MgO are known to be compressive due to the difference in thermal expansion of the two materials. The compressive stress leads to highly c-axis oriented PZT films. Devices using MgO substrates were fabricated, however difficulties in the micro-machining of the MgO substrate inhibited a complete liberation of the membrane. Nevertheless, preliminary measurements indicate these devices could lead to both high coupling and high Q-factors, suggesting that further detailed study of this method is worthwhile. As a third method for avoiding ferroelastic domains, the uniaxial ferroelectric potassium lithium niobate (KLN) was explored. The unique ferroelectric axis in this material means that only 180° domain walls are present, which can theoretically be removed by poling. This material has been deposited in thin film form using pulsed laser deposition (PLD). KLN thin films with a {001} texture were deposited successfully on Pt/Si substrates. The films were piezoelectric with a d33,f value of around 10 pm/V and a dielectric constant of 250. This is the first time that piezoelectric properties were measured on KLN thin films. A columnar structure has been observed, however the small grain size and the rough surface currently make it difficult to apply this material to TFBAR's.

EPFL2009