Ceramic engineering | EPFL Graph Search

Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties. Ceramic materials may have a crystalline or partly crystalline structure, with long-range order on atomic scale. Glass ceramics may have an amorphous or glassy structure, with limited or short-range atomic order. They are either formed from a molten mass that solidifies on cooling, formed and matured by the action of heat, or chemically synthesized at low temperatures using, for example, hydrothermal or sol-gel synthesis. The special character of ceramic materials gives rise to many applications in materials engineering, electrical en

Titanium carbide-carbon porous nanocomposite materials for radioactive ion beam production

João Pedro Fernandes Pinto Ramos

The Isotope Separator OnLine (ISOL) technique is used at the ISOLDE - Isotope Separator OnLine DEvice facility at CERN, to produce radioactive ion beams for physics research. At CERN protons are accelerated to 1.4 GeV and made to collide with one of two targets located at ISOLDE facility. When the protons collide with the target material, nuclear reactions produce isotopes which are thermalized in the bulk of the target material grains. During irradiation the target is kept at high temperatures (up to 2300 °C) to promote diffusion and effusion of the produced isotopes into an ion source, to produce a radioactive ion beam. Ti-foils targets are currently used at ISOLDE to deliver beams of K, Ca and Sc, however they are operated at temperatures close to their melting point which brings target degradation, through sintering and/or melting which reduces the beam intensities over time. For the past 10 years, nanostructured target materials have been developed and have shown improved release rates of the produced isotopes, due to the short diffusion distances and high porosities. In here a new Ti-based refractory material is developed to replace the currently used Ti-foils. Since nanometric TiC can't be maintained at high temperatures (T>1200 °C) due to sintering, a processing route was developed to produce TiC-C nanocomposites where the carbon allotropes used were either graphite, carbon black or multi wall carbon nanotubes (MWCNT). The developed nanocomposites sinterability was tested up to 1800 °C and they were characterized according to dimensional changes, relative density, mass losses, surface area, TiC particle size and microstructure morphology. All carbon allotropes had a significant effect on the stabilization of the nanometric TiC where the best result was obtained for a 1:1 volume ratio of TiC:carbon black at 1800 °C where TiC crystallite sizes were of 76 nm (from 51 nm in green) and density of 55 %, followed by TiC:MWCNT with TiC of 138 nm (58 % dense). The processing introduced a ZrO2 contamination from the milling media, forming ZrC that solubilizes in the TiC phase, increasing its lattice parameter. TiC sintering kinetics were studied through the master sintering curve and the activation energy determined for sintering, 390 kJ/mol, were close to the ones obtained in the literature. Using the same method, the calculated activation energy for TiC-carbon black was 555 kJ/mol resulting from the carbon which reduces the TiC sintering, reducing its coordination number. The nanocomposites referred (and the TiC) were irradiated and studied in terms of isotope (Be, Na, Mg, K, Sc and Ca) release, where the nanocomposite with the highest isotope released fraction, TiC-carbon black was selected for the final target material. To produce a full target the processing was scaled up and a target prototype was build and tested at ISOLDE. Li, Na and K isotope intensities and release time-structure were measured from the target prototype, where in comparison with Ti-based materials, Na and Li intensities were higher, K were slightly lower and Ca were lower. The target presents an apparently longer release time structure when comparing with standard materials, as seen in other nanomaterial targets, which is likely related with effusion of the isotopes in the material porosity. Furthermore, contrarily to the Ti-foil targets, the obtained intensities were stable over the full operation time. At the end of this thesis suggestions for a future work which include a second iteration of the TiC-C nanocomposite (already developed) and further modeling.

EPFL2017

High volume fraction particulate and interpenetrating phase composites (IPC)

Ghodratollah Roudini

The role of reinforcing phase contiguity in high volume fraction particulate composites is investigated using model composites of pure aluminium reinforced with α-Al2O3 particles. To produce the composites, alumina particle (5µm) preforms were either loosely packed or, alternatively, packed and CIPed; these were then sintered, varying the sintering time and temperature, and infiltrated with pure liquid aluminium using gas pressure infiltration. The microstructure of the resulting composites was characterized using image analysis in terms of reinforcement phase contiguity (β), defined as the ratio of particle surface that is in contact with other particles to that in contact with the matrix. The influence of this parameter on several characteristics of the composites and the preforms they were made from was then studied, using theoretical models in the literature to isolate the influence of this parameter from that of other variables, such as the volume fraction ceramic. To this end, the thermal conductivity of the sintered preforms, basic composite mechanical properties (hardness, tensile strength, and damage evolution during straining) and the thermal expansion of the composites were characterized and compared with theory; furthermore, quenching and neutron diffraction were used to probe in more depth the thermal expansion behaviour of the composites. The results show that the thermal conductivity of the preform increases, as expected, upon increasing the relative density (volume fraction) and the contiguity of alumina particle after sintering. Comparison of data with theory shows that, while experimental values for the CIPed samples agree with the prediction by Argento and Bouvard for the thermal conductivity of powder preforms as a function of densification, neither this model, nor that of Montes et al. , fits all experimental values. Rather, when pooled the data show that the thermal conductivity of powder preforms, normalized for relative density using the differential effective medium model, obeys relatively well a simple linear relation between contiguity and conductivity. The hardness of the composite increases with increasing relative density and increasing contiguity. Here again, after normalization for the dependence of hardness on ceramic volume fraction using current theory, a nearly linear relation is found between hardness and contiguity. Tensile testing results show that the strength of the composite increases with increasing fraction ceramic and increasing contiguity while the elongation decreases, composites with an interconnected ceramic phase rapidly becoming quite brittle, with elongations falling below half a percent. No clear relationship is found between contiguity and Young's modulus after normalization for the volume fraction ceramic. Measurements of internal damage evolution show that damage increases very rapidly with deformation in the present interconnected composites, in agreement with their brittle character. The thermal expansion results show that the CTE of the present composites often varies with temperature at a rate higher than that predicted to result from physical parameter variations by existing theoretical models. With regard to their thermal expansion, the composites can be separated into three classes that are characterized by the level of contiguity of the low expansion phase: low contiguity (i.e. β ≤ 0.045), medium contiguity (i.e. 0.05 ≤ β < 0.074) and high contiguity (i.e. β ≤ 0.074 ). The CTE values during heating for the low contiguity composites lie between Schapery's upper bound and the rule of mixtures, starting upon heat-up from the Schapery upper bound at low temperature and transiting above roughly 150°C to the rule of mixture prediction; upon cool-down the same behaviour is observed in reverse. The CTEs of the composites with medium contiguity at low temperatures are somewhat below the Schapery upper bound, increasing somewhat faster than predicted for this bound with increasing temperature to reach Shapery's upper bound at around 200°C. For the composites with high contiguity, the CTEs lie between elastic and plastic bounds predicted using the self-consistent model, the latter model being obtained by assigning a zero shear stiffness to the matrix. Physical reasoning and comparison of data with theory indicate that these transitions in expansivity of the composites are mostly a result of matrix yield following fully elastic deformation past a certain thermal excursion, this transition being prevented by increased ceramic phase contiguity. This interpretation is confirmed by neutron diffraction results. Increased reinforcement contiguity in the composites also affects the amplitude of strain hysteresis during thermal cycling, the magnitude of hysteresis decreasing in nearly linear fashion with increasing contiguity, reaching zero near β = 15%. Samples that were quenched into liquid nitrogen after a relaxation heat treatment and reheated to room temperature yielded data globally similar to those of as-processed samples. Nitrogen-quenched samples with higher contiguity (β ≥ 0.05) exhibit furthermore signs of ratcheting upon cycling, which was not observed in the composites with low contiguity.

EPFL2008

Solid freeform fabrication of porous ceramic parts from ceramic powders and preceramic polymers

Thomas Alexandre

This dissertation addresses the development of a Solid Freeform Fabrication process, Selective Laser Pyrolysis, to fabricate three-dimensional porous ceramic preforms from ceramic powders and preceramic polymers. It contributes to a better understanding of the physical and chemical phenomena that intervene in this process. During this study, modeling tools were adapted and characterization techniques for porous parts were developed. The first part of this work describes the fabrication of porous preforms by using Selective Laser Pyrolysis. The choice of the fabrication process is discussed, as well as the choice of the materials used. The selected materials are characterized and methods to associate them are investigated. Finally, the microstructures of the resulting porous preforms are characterized. The second part of this study is devoted to the fundamental understanding of the various phenomena occurring during the fabrication process. These phenomena are characterized or modeled. The laser-matter interaction is investigated, and a thermal model adapted to the system is proposed. This model takes into account the laser-matter interaction, the non-linear heat diffusion process and the varying properties of the materials during the process. Results of simulations are compared with experimental data that empirically validate this model. Standard characterization techniques of mechanical properties are not adapted to porous preforms fabricated in this work. Therefore, an indentation technique is developed to evaluate the effect of process parameters on the mechanical behavior. Results of simulations with the thermal model and indentation tests are used to optimize the fabrication process.

EPFL2007