Synthesis and Sintering of ZnO Nanopowders

Nanopowders are continuously under investigation as they open new perspectives in numerous fields. There are two main challenges to stimulating their development: sufficient low-cost, high throughput synthesis methods which lead to a production with well-defined and reproducible properties; and for ceramics specifically, the conservation of the powders' nanostructure after sintering. In this context, this paper presents the synthesis of a pure nanosized powder of ZnO (dv(50)similar to 60 nm, easily redispersable) by using a continuous Segmented Flow Tubular Reactor (SFTR), which has previously shown its versatility and its robustness, ensuring a high powder quality and reproducibility over time. A higher scale of production can be achieved based on a "scale-out" concept by replicating the tubular reactors. The sinterability of ZnO nanopowders synthesized by the SFTR was studied, by natural sintering at 900 degrees C and 1100 degrees C, and Spark Plasma Sintering (SPS) at 900 degrees C. The performance of the synthesized nanopowder was compared to a commercial ZnO nanopowder of high quality. The samples obtained from the synthesized nanopowder could not be densified at low temperature by traditional sintering, whereas SPS led to a fully dense material after only 5 min at 900 degrees C, while also limiting the grain growth, thus leading to a nanostructured material.

Preparation and properties of KNbO3-based piezoelectric ceramics

Evelyn Hollenstein

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.

EPFL2007

Atomistic simulation in powder technology

Ulrich Aschauer

Despite many years of intensive research there still remain many unresolved questions in powder and ceramic technology. A majority of these issues are linked to interfacial phenomena of atomic scale origin, which makes their experimental investigation very difficult due to limitations in the spatial resolution of the available analysis techniques. Computer simulations at the atomic scale provide with the advent of more and more advanced methods and increasing computer power an ever more powerful tool for the investigation of these phenomena. The understanding of experimental systems gained at a fundamental theoretical level will help target key experiments in a knowledge based fashion for the rapid advance in top-performance materials development, saving time and resources with simulations showing the most promising routes and key parameters to be explored in experiments. The present thesis investigates several key steps in the production cycle of a ceramic material using atomic scale computer simulation techniques, leading to advances in the understanding of the underlying atomic scale phenomena in each case. Instead of treating a single material throughout the ceramic production process, a series of different systems of experimental interest are looked at in order to show the generic nature of the approach. Very often ceramic powders are produced by precipitation from solution, where many powders properties can be modified, amongst them the particle size, morphology and state of agglomeration, all of which have an important effect on the quality of the final ceramic. Growth can be modified by the reactive environment (temperature, pH) or extrinsic species such as ions or molecular additives. Growth modification of hematite (α-Fe2O3) by phosphonic acid molecules was simulated by energy minimization and experimentally validated, making the method useful as a predictive tool for other phosphonic acid molecules. Significant changes in morphology were predicted and experimentally observed, the main reason for preferential adsorption being the surface geometry and topology, major distortions of the molecule leading to unfavorably high adsorption energies on some faces. Since experiments showed calcite (CaCO3) particles grown in presence of polyaspartic acid (p-ASP) to have a higher specific surface area than with polyacrylic acid (PAA) despite the same functional groups on both additives, molecular dynamics simulations were used to investigate this difference. The presence of charged surface defects at steps was found to be the key requirement for adsorption, the molecules' approach being hindered by the highly coordinated water at the surface and the additive even desorbing without the attractive electrostatic force. PAA was found to form more complexes with counterions in solution and a more negative enthalpy of adsorption was found for p-ASP, both of which will lead to more marked growth suppression by binding of p-ASP to steps, which are expected to be the main growth sites. Further the adsorption conformation of p-ASP results in a better colloidal stabilization than PAA, preventing particle agglomeration. These three aspects can explain the higher specific surface area for powders precipitated in presence of p-ASP. These computational approaches can thus explain subtle differences between additives as well the same additive on different surfaces helping in the targeted use of additives and the design of new additives. Dopants may be added to ceramics during powder synthesis or before sintering either as sintering agents or to add specific properties to the final material. Often dopant ions are oversized compared those replaced in the crystal lattice leading to segregation towards defects where their incorporation is facilitated. Interface segregation was studied for industrially relevant dopant ions in alumina and zinc oxide using energy minimization and microstructural models to help compare with experimental findings. The results show a strong tendency for surface and a slightly lesser tendency for grain boundary segregation in all cases, however with important differences between different interfaces. As a key result one possible reason for the microstructural homogenization effect of magnesium was found to be its capacity to narrow the grain boundary energy distribution resulting in more equiaxed grains due to isotropic grain growth. The segregation of luminescent neodymium ions in YAG ceramics for laser applications was investigated and dopants were shown to accumulate at grain boundaries where their high concentration will make them inactive for lasing and result in light scattering due to slight local variations of the refractive index. Using microstructural models the proportion of inactive dopant ions was found to increase with decreasing grain size, making nanoceramic lasers less powerful. Optical models based on atomistic segregation results on the other hand show that light scattering is more severe for larger grains, nanoceramics thus being more transparent. There is therefore an optimal grain size where laser power and transparency will result in the best possible laser performance. Different microstructural models could be linked to the sintering procedure, predicting that the best laser performance should be obtained by slow conventional sintering rather than by novel rapid methods such as spark-plasma-sintering (SPS). These results are key in understanding the role of interfaces in this novel class of laser materials and will help improve their fabrication methods. Experiment and simulation of oxygen self-diffusion in alumina so far showed large discrepancies, the simulations predicting diffusive jump activation energies by a factor five smaller than experiments. In this thesis a novel simulation approach based on Metadynamics allowed to determine activation free energies of individual jumps, then using these in kinetic Monte Carlo simulations to predict the macroscopic diffusion coefficient. The key result is that the activation energy for the diffusion dominating jumps is of the order of the experimentally determined one, the so far reported simulated jump energies not forming a continuous diffusion network. The calculated diffusion coefficient correlates relatively well with experimental results. The developed approach is generic and can be applied to investigate a variety of diffusive phenomena in solids providing answers to many more of the remaining questions. While atomistic simulation approaches give very interesting results on local atomistic phenomena and allow understanding and elucidating which processes occur spontaneously in a system, for predictions on experimental length and time scales the use of larger scale models is immensely important to extrapolate atomistic results to these scales. The combination of techniques from the electronic structure through the atomistic and mesoscale to the continuum level in a multi-scale modeling approach seems very promising for material science applications, where the goal is to understand experimental observations and to target the next key experiment in a knowledge-based fashion.

EPFL2008

High Temperature Behavior of Nano-Structured Ceramic Composites Studied by Mechanical Spectroscopy

Mehdi Mazaheri

Silicon nitride based ceramics (SiAlONs); tetragonal polycrystalline zirconia (3Y‐TZP); alumina and their composites reinforced with different amount of multi‐ walled carbon‐nanotubes (CNTs) have been processed by Spark Plasma Sintering (SPS). High temperature mechanical spectroscopy measurements were performed in each material between room temperature and 1600 K. In each of these materials, anelastic and viscoplastic relaxation phenomena were investigated and responsible mechanisms were explained. In particular, grain boundary (GB) sliding, which is responsible for high temperature plasticity in fine grained ceramics, gives rise to a peak or an exponential increase in the high temperature mechanical loss. Peak and exponential background depend on two forces, which control the GB sliding: a friction force due to the GB viscosity and a restoring force due to the elasticity of the surrounding grains. In the present thesis, ceramics and composites have been processed, which allow one to study the role played by these forces on the thermo‐ mechanical behavior of these refractory materials. SiAlON ceramics were chosen for studying the viscous force due to the inter‐granular glassy phase. In zirconia and alumina, it is shown how the CNT reinforcements may improve the restoring force and consequently the creep resistance. Different grades of SiAlON ceramics, processed with different sintering aids (Ca2+, Y3+, Yb3+), with oxygen rich (CaO, Y2O3 and Yb2O3) and nitrogen rich (CaH2, YN and YbN) compounds, have been studied. Mechanical spectroscopy has been used to analyze the behavior of the residual glassy phase, present after sintering, either as grain‐boundary glassy (GB) films or glass pockets located in GB triple junctions. The mechanical loss spectra show a relaxation peak, which is due to a relaxation phenomenon (the so called “α‐relaxation”) associated with the glass transition in the amorphous phase. The peak position depends on the glass viscosity and the peak height is mainly affected by the glassy phase amount and the SiAlON bimodal microstructure, namely equiaxed versus elongated grains. Moreover the peak height depends on the restoring force provided by the neighboring grains, which limit the GB sliding process. As a matter of fact, a good correlation between the mechanical loss peak and plastic deformation in a compression test has been observed. It is concluded that the elongated grains in the bimodal microstructure provide a higher restoring force, which limits the GB sliding of smaller equiaxed grains. In the case of equiaxed fine grained oxide ceramics, such as zirconia and alumina, the restoring force has been increased by CNT additions, which can reinforce the GBs. 3Y‐TZPcomposites with a homogenous distributions of CNTs, ranging within 0.5 – 5 wt%, were processed by SPS. A significant improvement in room temperature fracture toughness and shear modulus as well as creep performance at high temperature were obtained, and interpreted as due to the GB reinforcing role of the CNTs. To support this interpretation, high‐resolution electron microscopy and Raman spectroscopy have been carried out. Moreover, a remarkable enhancement of the electrical conductivity up to ten orders of magnitude due to CNT additions has been obtained with respect to the pure ceramics. The isothermal spectrum of the 3Y‐TZP composites (measured at 1600 K) is composed of a mechanical loss peak at a frequency of about 0.1 Hz, which is superimposed on an exponential increase at lower frequency. This is interpreted as primarily due to GB sliding. The absence of a well‐marked peak in monolithic 3Y‐ TZP is justified by considering that the restoring force decreases at low frequencies or high temperatures. Therefore, GB sliding is no more restricted and the mechanical loss increases exponentially, which is correlated to macroscopic creep. With CNT additions the mechanical loss decreases and the relaxation peak is better resolved with respect to the background. This is interpreted by the pinning effect of CNTs on GBs, providing addition source of restoring force, which can hinder GB sliding at high temperature, resulting in a creep resistance improvement. Similarly to 3Y‐TZP based composites, alumina specimens (3 different types of alumina powders) reinforced with different amount of CNTs were sintered by SPS. It is shown how the initial particle size of the powders may affect the dispersion of CNTs. Measurement of different properties, such as hardness, fracture toughness and mechanical loss at high temperature, have evidenced the crucial role of the CNT dispersion in the obtained mechanical properties.

EPFL2013