Prédiction et compréhension de la densification des poudres commerciales d'alumine et de fer grâce à une approche par réseau de neurones artificiels

EPFL, 2004
Thèse EPFL

Résumé

The influence of the properties of commercial powders on the densification during their packing, compaction and sintering process is still not understood in detail. With regard to the sintering process, neither the well-known sintering equation for the first sintering step nor the relation between the density and grain size at the final sintering step can describe the influence of powder characteristics on its densification behaviour. For improving the sintered density of a ceramic powder, it is known to be crucial to start with a highly dense and homogeneous green body. Therefore, the powder has to fulfil different requirements such as being agglomerate free, reasonably spherical and having a narrow size distribution (but not mono-dispersed). The aim of this work is to develop a better understanding of the relation between the powder properties and the densification behaviour during the packing, compaction and sintering process, of commercial, micron sized, metallic and ceramic powders. Another aim of this work is to evaluate if prediction of the packed, green and sintered densities based only on the known powder characteristics is possible via a neural network approach. The presented results show that a well learnt neural network is a useful tool for the prediction of green and sintered densities of granulated alumina powder produced either by milling (Bayer process) or by chemical processes. Moreover, the simulated influences of characteristics, on the green and sintered densifications, fit well literature models behaviours and intrinsic properties of such powders. Concerning the green densification, Bayer powders are denser for coarser particles and/or a broader size distribution. Relating to the chemically produced powders, those tendencies are inversed, due a stronger agglomeration with a broader size distribution and coarser particles. Regarding the sintered density, the neuronal approach highlights a better sinterability for finer powders. Limits of the artificial neural network tool are emphasized with its application to metallic powders: the learning stage seems to be primeval and simulated results are to be analysed and interpreted with care and inside the validity domain of each specific artificial neural network.

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https://infoscience.epfl.ch/record/33493?ln=fr

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EPFL, 2004
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/33493?ln=fr

À propos de ce résultat

Concepts associés (15)

Concepts associés

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Publications associées (4)

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Chargement

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

Chargement

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

Chargement

Nowadays thin films play an important role in everyday life and in industries. Thin film technology has been developed primarily for the growing demand for development of smaller and smaller devices that require advanced materials and new processing techniques suitable for micro and nano technology. One of the methods of producing thin films is by means of plasma technology. This is a technique by which plasma, also known as 'the fourth state of matter', is used to generate ions, electrons and radicals which deposit at a surface where they will form a thin film. The source of these depositing species could be either gaseous, liquid or solid in origin. Most of the time a plasma is generated by means of an electrical discharge. Plasma technologies are used for various applications. One of these is Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD systems have widespread applications in the electronic sector, because of their exibility in depositing different films such as silicon oxide thin films (SiOx). These films, currently deposited by organosilicon compounds, such as hexamethyldisiloxane (HMDSO), have dielectric and good barrier properties. The aim of the work at CRPP consists in optimizing PECVD processes for the deposition of these films on polymers. In this study, the species in gaseous phase and the powder produced in oxygen-diluted HMDSO plasmas were experimentally characterized in a radiofrequency (RF) capacitively-coupled reactor at 13.56 MHz. The gaseous phase of these plasmas and the particle formation were studied by in-situ infrared absorption spectroscopy and optical emission spectroscopy. The study of the powder formation is important because the particle deposition imposes an upper limit on the deposition rate. Particle formation was firstly studied as a function of HMDSO/O2 ratio. Analysis of the time evolution of the infrared absorption spectra gives the development of the size, the number density, the structure and the particle composition. At high oxygen dilution the formation of large particles is observed. The analysis of HMDSO/O2 plasma at low O2 content suggests that at the beginning of the process very small particles are formed by polymerization; the same particles do not grow much and keep sizes around 50 nm. The admixture of inert or non-reactant gases, such as Ar or N2, into the plasma promotes polymerization of the monomer; a high value of one of these inert or non-reactant gases leads to small powder particle size, since only polymerization takes place in this case. Particle size around 50 nm is also found in this case. At the same time we see a decrease of the carbon content and a weak reduction of the stoichiometry in SiOx indicated by the shift of the SiOSi asymmetric stretching vibration. This reduction also influences the deposited layers, less oxidized and, thus, quite far from a stoichiometric character. Hexamethyldisilazane (HMDSN) and tetramethylsilane (TMS) are other reagents that have been identified as alternatives to HMDSO. The in-situ infrared absorption spectra show that HMDSO/O2 and HMDSN/O2 plasmas are quite similar, but in the second case a lower SiOSi stretching peak intensity, due to NH and SiN peaks, is seen. TMS/O2 and HMDSN/O2 plasma spectra show a low SiOSi bending and stretching peak intensity, due also to the oxygen atom absence in their chemical structure. Two separate chapters are respectively devoted to the silicon nitride thin films (SiNx) and plasma crystal formation. SiNx layers are deposited by three different organosilicon compounds, such as HMDSN, TMS and bis(dimethylamino)dimethylsilane (BDMADMS). No powder formation is visualized in this contest. The ex-situ infrared absorption spectra show the difficulty to produce stable SiNx films; they present a low SiN peak intensity and are quite hygroscopic. Furthermore, in HMDSO/O2 plasmas at high oxygen dilution and high pressure, later in time into the discharge, accretion leads to larger and larger particles which can finish up in particles forming a plasma crystal. The study of RF harmonics shows the presence of instabilities, clear indications of the passage from small particulate to plasma crystal structure.

EPFL2011

Chargement

EPFL2011

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

Mehdi Mazaheri

EPFL2013

Preparation and properties of KNbO3-based piezoelectric ceramics

Evelyn Hollenstein

EPFL2007