1. Beim feldaktivierten Sintern im Temperaturbereich von 500 bis 1000 °C fließen elektrische Ströme mit einer Dichte von 1 bis 3 A/mm². 2. Daraus folgt für die größten verwendeten Pulverteilchen mit einem Radius von 50 µm ein Strom je Teilchenkontakt von 10 bis 50 mA. 3. Die durch das Aufbringen des prozesstechnisch notwendigen Pressdruckes gebildeten relativen Kontaktradien (Kontaktradius geteilt durch Teilchenradius) haben eine Größe von 0,05 bis 0,3. 4. Die Einengung der Strompfade im Kontakt der Pulverteilchen erhöht, zusammen mit dem elektrischen Widerstand der Oxidschicht auf den Pulverteilchen, den elektrischen Widerstand des Pulverpresslings. 5. Der Stromfluss durch die Teilchenkontakte führt mit dem zusätzlichen elektrischen Widerstand dieser Teilchenkontakte zu einer lokalen Temperaturerhöhung (Übertemperatur) von 10-4 bis 1 Kelvin für Kupfer- respektive Stahlpulver. 6. Der zusätzliche elektrische Widerstand der Oxidschicht kann die Übertemperatur beim Kupferpulver auf bis zu 1 mK erhöhen. 7. Mit abnehmendem Teilchenradius sinkt die Übertemperatur quadratisch. 8. Das Wachstum der Teilchenkontakte im Verlauf der Verdichtung führt zu einer kontinuierlichen Verringerung der Übertemperatur. 9. Das Auftreten von schmelzflüssiger Phase, von Metalldampf oder von Plasma wird in den untersuchten metallischen Systemen ausgeschlossen. 10. Auch Elektromigration, Thermomigration oder andere Wirkungen des elektrischen Stromes spielen keine Rolle für die Verdichtung beim feldaktivierten Sintern. 11. Die Verwendung von gepulstem anstelle von kontinuierlichem Gleichstrom beeinflusst die Verdichtung der untersuchten Werkstoffe nicht. 12. Die Verdichtung vom Pulver zum kompakten Werkstoff findet für Pulverteilchen mit einem Radius größer als R = 10 µm über plastische Verformung durch verschiedene Formen des Kriechens statt. 13. Die Verformung ist im Anfangsstadium auf den Kontaktbereich begrenzt. 14. Bei Pulverteilen mit Teilchenradien unter R = 10 µm findet die Verdichtung zunächst als Folge von Leerstellenströmen in die Kontaktkorngrenze statt (Sintern). 15. Durch die schnelle Verdichtung bei niedriger homologer Temperatur werden Kornwachstum und Rekristallisation verringert.

Hypersonic aerospace vehicles are severely limited by the lack of adequate high temperature materials that can withstand the harsh hypersonic environment. Tantalum carbide (TaC), with a melting point of 3880°C, is an ultrahigh temperature ceramic (UHTC) with potential applications such as scramjet engines, leading edges, and zero erosion nozzles. However, consolidation of TaC to a dense structure and its low fracture toughness are major challenges that make it currently unviable for hypersonic applications. In this study, Graphene NanoPlatelets (GNP) reinforced TaC composites are synthesized by spark plasma sintering (SPS) at extreme conditions of 1850˚C and 80-100 MPa. The addition of GNP improves densification and enhances fracture toughness of TaC by up to ~100% through mechanisms such as GNP bending, sliding, pull-out, grain wrapping, crack bridging, and crack deflection. Also, TaC-GNP composites display improved oxidation behavior over TaC when exposed to a high temperature plasma flow exceeding 2500 ˚C.

The main goal of this thesis is to deposit metal particles on the surface of 2D nanoplatelets using a controlled cementation process. As a proof of concept, copper (Cu) and Graphene Nanoplatelets (GNP) were chosen as the representative metal and 2D nanoplatelets, respectively. Specific goals of this study include depositing nanometer scale Cu particles on the surface of GNP at a low concentration (approximately 5 vol.%) while maintaining clustering and impurities at a minimum. Parametric studies were done to attain these goals by investigating various metallic reducer types and morphologies, GNP surface activation process, acid volume % and copper (II) sulfate concentrations. Optimal conditions were obtained with Mg ribbon as a reducer, 3 minutes of activation, 1 vol.% of acetic acid and 0.01 M CuSO4. The GNP-Cu powder synthesized in this work is a precursor material to be consolidated via spark plasma sintering (SPS) to make a nacre-like, layered structure for future studies.

The aim of this thesis is preparation and characterization of bulk Mg–Ti based materials. In the first theoretical part properties of base materials and the complexity of preparation alloy from these metals is discussed. Second part is focused on powder metallurgy and its applicability on Mg–Ti system. In another part particle composites are described. In chapter current research another possible methods of alloy preparation from magnesium and titanium are mentioned. The experimental part of this thesis was the preparation of bulk Mg–Ti materials from metal powders. For sample preparation conventional methods of powder metallurgy and spark plasma sintering was employed. Furthermore a characterisation of these materials was done. Microstructure was observed. Present phases were found using X-ray diffraction analysis. Amounts of these phases were determined using a scanning electron microscope with energy–dispersive spectrometry and using X-ray fluorescence. Furthermore hardness was measured and bending test with evaluation was done. Significant difference in results of sample preparation using conventional methods of powder metallurgy and spark plasma sintering was observed.

Cold sintering process (CSP) je nová metoda pro slinování keramik a skel. Tato metoda vede ke snížení teploty (

Mechanical properties of aluminium alloys highly depend on their phase composition and microstructure. High strength can be achieved among others by introduction of a high volume fraction of fine, homogeneously distributed second phase particles and by a refinement of the grain size. Powder metallurgy allows to prepare fine grained materials with increased solid solubility which are favourable precursors for further precipitation strengthening. Gas atomization was used for the preparation of powders of the commercial Al7075 alloy and its modification containing 1 wt% Zr. A part of gas atomized powders was mechanically milled at different conditions. Mechanical milling reduced the grain size down to the nano-size range and the corresponding microhardness exceeded the value of 300 HV. Powders were consolidated by the spark plasma sintering method to nearly fully dense compacts. Due to a short time and relatively low temperature of sintering the favourable microstructure can be preserved in the bulk material. The grain size of compacts prepared from milled powder was retained in the submicrocrystalline range and the microhardness close to 200 HV exceeded that of the specially heat treated ingot metallurgical counterparts. The prepared compacts retained their fine grained structure and high...

This thesis is focused on carbide ceramics synthesis, more specifically on the synthesis of a Ti2AlC MAX Phase carbide using solution chemistry rather than powder metallurgy. Chloride and nitrate precursors have been used as a source of metals and citric acid as a source of carbon for carbothermal reduction and as a complexing agent. A new route of precursor synthesis has been developed based on peroxo-titanic acid, which helps retain aluminium. The syntheses were performed using the SPS facility to ensure high heating rates. Al4C3 and TiC carbides and their mixtures were prepared successfully. However, the Ti2AlC phase was not synthesized under used conditions. The phase composition, structure and grain sizes of the samples were investigated by powder X-ray diffraction, X-ray fluorescence and electron microscopy. The contents of carbon in the precursors were determined by thermogravimetric analysis.

No description available.

Binderless tungsten carbide (BTC) ceramics are inherently difficult to process and very brittle. Most consolidation techniques for processing pure WC powder require long sintering times and intense energy consumption. High-T pressureless and pressure-assisted sintering processes often lead to low-quality and coarsened microstructures, thus limiting the use of WC ceramics to few niche applications. Field-assisted sintering techniques (FAST), like spark plasma sintering (SPS), significantly improve the densification of fine and ultrafine WC powders. However, SPS requires high current outputs and expensive apparatus. SPS ceramics still lack adequate toughness to extend the use of BTC components in heavy-duty applications requiring reliable load-bearing capability and/or resistance against rapid and unexpected impacts or temperature drops. This research work explored a new consolidation route capable of boosting the mass transport phenomena (accelerated sintering) and, simultaneously, introducing new microstructural features. The process called flash sintering (FS) offers great potential in accelerating diffusion phenomena and altering the crystallographic and/or the defect chemistry of the sintered ceramics. Many scientific studies reported structural alterations, enhanced plastic flow and material softening by introducing “out-of-equilibrium” characteristics. Currently, FS technology requires, for its activation, a negative dependence of the electrical resistivity with temperature (NTC) of the material to be sintered. This is a universal requirement for the flash event to occur thus theoretically inhibiting the flash sintering of conductive materials with a positive temperature coefficient for resistivity (PTC), like metals or WC. In the present work, we reported how during electrical resistance sintering (ERS) experiments conducted on pure WC nanopowders, a flash event was triggered during the first seconds of the process. This was demonstrated to occur thanks to the different evolution of the electrical properties of a granular compact with temperature. WC powders possess an initial NTC behaviour which can activate a transitory thermal runaway phenomenon which makes the activation of a flash event in these materials possible, intense enough to allow ultrafast densification in less than 10 s. This breakthrough allows to verify whether and how the flash event modifies the final sintered material. FS and SPS sintered ceramics were compared in their microstructural, physical and mechanical properties, thus pointing out how some peculiar modifications are exclusively present in the flash-sintered material. FS can stabilize the WC1-x metastable phase after cooling to room temperature, and this was demonstrated to alter the high-temperature deformation of WC micropillars during compression. In addition, FS BTC are inherently softer with respect to SPS ones, resulting in higher fracture toughness and slightly lower hardness. Even if not final, the results indicate how the flash sintering of WC can be explored further to process engineered BTC ceramics with an optimized hardness/toughness ratio and an enhanced deformability.

L'augmentation des températures de fonctionnement est une des évolutions actuelles de l'électronique de puissance. Ce fonctionnement entraine d’une part des changements de la structure des modules de puissance notamment des structures « 3D » pour assurer un refroidissement double face des composants de puissance, et d’autre part l’utilisation de matériaux qui permettent de réduire des contraintes thermomécaniques, liées à la différence de coefficient de dilatation des matériaux, lors d’une montée en température. Le travail réalisé au cours de cette thèse consiste à développer une nouvelle structure « 3D » basée sur une technique de contact par des micropoteaux en cuivre, élaborés par électrodéposition et ensuite assemblés à un substrat céramique métallisé (notamment, un DBC : Direct Bonding Copper). Pour réaliser ce contact, une technique de frittage par SPS (Spark Plasma Sintering) est utilisée. Nous étudions dans un premier temps le collage direct de cuivre sur des massifs, puis effectuons dans un deuxième temps le collage de cuivre entre les micropoteaux et le DBC. Cette technique SPS est aussi utilisée pour la réalisation d’un nouveau substrat céramique métallisé basé sur des matériaux avec des coefficients de dilatation thermique accordés, pour les applications à haute température. / The increase in operating temperature is one of the current trends in power electronics. This operation leads firstly to changes in the structure of power modules such as "3D" structures to provide a double-side cooling of power components, and secondly the use of materials that reduce thermomechanical stresses, related to the difference in coefficient of thermal expansion. The study realized during this thesis consisted in developing a new "3D" structure based on copper microposts prepared by electroplating, which are then assembled to a metallized ceramic substrate (eg, a DBC: Direct Bonding Copper). To realize this contact, a sintering machine (SPS: Spark Plasma Sintering) is used first to study the direct bonding of copper on solid, and second to perform the bonding between the copper microposts and the DBC. This technique is also used for the production of a new metallized ceramic substrate using materials with matching thermal expansion coefficients, for high temperature applications.