Relation of process parameters and green parts quality in the Layerwise Slurry Deposition Print process of SiSiC

Die Nachfrage nach großen, dichten keramischen Bauteilen mit komplexen Strukturen steigt. Solche Teile werden beispielsweise in der Halbleiterindustrie, aber auch in der Luft- und Raumfahrt gebraucht. Bisher gibt es kaum kommerzielle additive Fertigungsverfahren, die neben der Produktion von Bauteilen mit den benötigten Geometrien und Größen die Anforderungen an die Materialeigenschaften der Teile erfüllen können. Die Methode des schlickerbasierten Binder Jettings hat das Potential diese Lücke zu füllen. In dieser Dissertation wird der Einfluss der verwendeten Materialien im schlickerbasierten Binder Jetting, insbesondere dem schichtweisen Schlickerauftragsverfahren, engl.: layerwise slurry deposition print (LSD-print), auf die Grünbauteileigenschaften untersucht. Statt mit einem Pulver wie im klassischen Binder Jetting wird bei diesem Verfahren mit einem Schlicker gearbeitet, der in dünnen Schichten aufgebracht wird. In jeder Schicht wird der Querschnitt des zu druckenden Objekts durch Aufdrucken einer Tinte, die den Zusammenhalt der Partikel erzeugt, eingebracht. Die Verwendung eines Schlickers statt eines Pulvers ermöglicht eine hohe Packungsdichte und somit die Herstellung dichter keramischer Bauteile. Zum Schichtauftrag mittels Schlicker gibt es bereits einige Publikationen. Die Erkenntnisse daraus sind in dieser Arbeit zusammengefasst und hinsichtlich der verschiedenen Parameter übersichtlich dargestellt. Im Gegensatz dazu ist die Wechselwirkung zwischen Tinte und Pulverbett bisher kaum betrachtet worden. In dieser Dissertation wird, beispielhaft für andere Schlickeradditive, insbesondere der Einfluss des Dispergatorgehalts auf die Interaktion zwischen Pulverbett und Tinte experimentell anhand von Kontaktwinkelmessungen, Festigkeitstests sowie weiteren chemischen und physikalischen Analysen untersucht. Es zeigt sich, dass ein zu hoher Dispergatorgehalt dazu führt, dass die Auflösung der Druckprozesses und die Festigkeit der Teile gegenüber Teilen mit einem geringeren Dispergatorgehalt reduziert wird. Hier muss ein Kompromiss zwischen der Sedimentationsstabilität des Schlickers und den Eigenschaften der gedruckten Bauteile gefunden werden. In einem weiteren Teil werden aufbauend auf den Erkenntnissen zu möglichen Wechselwirkungen zwischen Schlickerkomponenten und Tinte die besonderen Anforderungen an die Tinte im schlickerbasierten Binder Jetting erarbeitet. In Erweiterung zu üblichen Testverfahren im pulverbasierten Binder Jetting ist unter anderem ein Kompatibilitätstests zwischen Tinte und Schlicker anhand von gegossenen Proben für Festigkeitstests aus einer definierten Mischung beider Materialien notwendig. Die teilweise aus der Literature bekannten Einflussfaktoren sind mittels eines Fischgrätendiagramms dargestellt und werden anhand von eigenen Experimenten genauer untersucht oder mittels Literaturrecherche kapitelweise erörtert. Die so erweiterte Wissensbasis ist Grundlage für die Industrialisierung des Prozesses. Der nächste Schritt vor dem industriellen Einsatz des LSD-print Verfahrens ist die Arbeit mit größeren (kommerziellen) Maschinen, um die Ergebnisse auf eine breitere statistische Basis zu stellen und die geforderten Bauteilgrößen fertigen zu können.Additive manufacturing (AM) can enable the production of complex-shaped components which, among other benefits, enables lightweight constructions for the aerospace and semiconductor industries. The production of thick-walled dense ceramic parts using these manufacturing methods is still a challenge and, especially for large-sized components with a reasonable surface finish, no suitable industrial solution is currently available. The layerwise slurry deposition print (LSD-print) process, which is a slurry-based binder jetting process (S-BJT) designed for ceramic part production, has the potential to produce such parts. In this approach, a part is built up by inscribing the cross-section of each part on a deposited slurry layer with ink applied by ink jet printing, whereby the ink drops bond the particles of a defined part to each other. To enable the production of thick-walled dense ceramic parts the green part properties play a major role. The packing density of the formed powder bed is one of the major influencing factors on the part quality and a literature review was done to summarize the influences on powder bed density when using a slurry or powder feedstock. The results indicate that the benefit of a high packing density of the slurry-based process is counteracted by a more time-consuming printing approach due to the necessary drying steps of the slurry layer and the redispersion process to remove the excess powder. Little research has been conducted on the interaction of the ink with the slurry. In this dissertation, the impact of the concentration of the dispersant on the reciprocal action between the ink and powder bed as an example of the influence of other slurry additives is investigated and the findings show that a large amount of dispersant can lead to a low resolution and even dissolution of the part during the removal of unbound particles by redispersion in water. As a result, a significant decrease in the biaxial strength of printed parts is observed. Based on these findings, tests were developed to prove the compatibility between the ink and slurry. For example, mixtures of slurry and ink were cast, and the biaxial strength of the green parts was tested by the ball on three ball method whereby the entire ink selection procedure for the S-BJT is elaborated. This dissertation summarizes the current state of knowledge concerning the LSD-print process and introduces novel aspects, especially concerning the interaction between the ink and the powder bed. The influencing parameters are presented in a Fishbone diagram and each branch is discussed in a chapter. The knowledge derived from this work is relevant for promoting the industrial application of the LSD-print process. In the next step (commercial) machines with a larger build volume will ensure the generation of a broader statistical basis for the results and enable the printing of large-sized, thick-walled dense ceramic parts with complex geometries as required in several industrial applications

ELECTROSTATIC FOCUSING AND IMPACT CONSOLIDATION OF AEROSOL PARTICLES

ELECTROSTATIC FOCUSING AND IMPACT CONSOLIDATION OF AEROSOL PARTICLE

Nanoenergetic Materials for MEMS: A Review

New energetic materials (EMs) are the key to great advances in microscale energy-demanding systems as actuation part, igniter, propulsion unit, and power. Nanoscale EMs (nEMs)particularly offer the promise of much higher energy densities, faster rate of energy release, greater stability, and more security sensitivity to unwanted initiation). nEMs could therefore give response to microenergetics challenges. This paper provides a comprehensive review of current research activities in nEMs for microenergetics application. While thermodynamic calculations of flame temperature and reaction enthalpies are tools to choose desirable EMs, they are not sufficient for the choice of good material for microscale application where thermal losses are very penalizing. A strategy to select nEM is therefore proposed based on an analysis of the material diffusivity and heat of reaction. Finally, after a description of the different nEMs synthesis approaches, some guidelines for future investigations are provided

Electro-spinning/netting: A strategy for the fabrication of three-dimensional polymer nano-fiber/nets.

Since 2006, a rapid development has been achieved in a subject area, so called electro-spinning/netting (ESN), which comprises the conventional electrospinning process and a unique electro-netting process. Electro-netting overcomes the bottleneck problem of electrospinning technique and provides a versatile method for generating spider-web-like nano-nets with ultrafine fiber diameter less than 20 nm. Nano-nets, supported by the conventional electrospun nanofibers in the nano-fiber/nets (NFN) membranes, exhibit numerious attractive characteristics such as extremely small diameter, high porosity, and Steiner tree network geometry, which make NFN membranes optimal candidates for many significant applications. The progress made during the last few years in the field of ESN is highlighted in this review, with particular emphasis on results obtained in the author's research units. After a brief description of the development of the electrospinning and ESN techniques, several fundamental properties of NFN nanomaterials are addressed. Subsequently, the used polymers and the state-of-the-art strategies for the controllable fabrication of NFN membranes are highlighted in terms of the ESN process. Additionally, we highlight some potential applications associated with the remarkable features of NFN nanostructure. Our discussion is concluded with some personal perspectives on the future development in which this wonderful technique could be pursued

Mechanical behaviour of additively manufactured lunar regolith simulant components

Additive manufacturing and its related techniques have frequently been put forward as a promising candidate for planetary in-situ manufacturing, from building life-sustaining habitats on the Moon to fabricating various replacements parts, aiming to support future extra-terrestrial human activity. This paper investigates the mechanical behaviour of lunar regolith simulant material components, which is a potential future space engineering material, manufactured by a laser-based powder bed fusion additive manufacturing system. The influence of laser energy input during processing was associated with the evolution of component porosity, measured via optical and scanning electron microscopy in combination with gas expansion pycnometry. The compressive strength performance and Vickers microhardness of the components were analysed and related back to the processing history and resultant microstructure of the lunar regolith simulant build material. Fabricated structures exhibited a relative porosity of 44 – 49% and densities ranging from 1.76 – 2.3 g cm-3, with a maximum compressive strength of 4.2 ± 0.1 MPa and elastic modulus of 287.3 ± 6.6 MPa, the former is comparable to a typical masonry clay brick (3.5 MPa). The 2AM parts also had an average hardness value of 657 ± 14 HV0.05/15, better than borosilicate glass (580 HV). This study has shed significant insight into realizing the potential of a laser-based powder bed fusion AM process to deliver functional engineering assets via in-situ and abundant material sources that can be potentially used for future engineering applications in aerospace and astronautics

Tailored Formulation of Capillary Suspensions as Precursor for Porous Sintered Materials

Makroporöse Sintermaterialien haben eine große technische Bedeutung als Werkstoffe für Filtrationsmembranen, Katalysatorträger, Implantate oder in Leichtbauanwendungen. Der Schwerpunkt dieser experimentellen Dissertation liegt auf der maßgeschneiderten Formulierung von Kapillarsuspensionen zur Herstellung von Produkten mit besonderen Materialeigenschaften. Kapillarsuspensionen wurden so unter anderem eingesetzt um hochporöse Glasfilter mit großer Permeabilität und 3D-gedruckte zelluläre Leichtbaustrukturen mit hoher spezifischer Festigkeit herzustellen

Continuous direct ink jet printing.

PhDThis thesis describes the preparation and continuous printing of zirconia ink under different conditions, as well as the development of silver inks, for the same purpose. The dispersion of sub-micrometer zirconia powder in industrial methylated spirit using other additives such as dispersant and binder was investigated with different mixing methods and at varying powder and binder contents. The use of high shear mixing by triple roll milling followed by ultrasonic disruption as well as adequate sedimentation and filtration produced a homogeneous and stable ink of 2.5 vol. % ZrO2. The ink could be printed directly and continuously on a commercial jet printer without interruption of any kind and the phenomena occurring during printing were investigated. The optimum modulation frequency for printing was determined with the generation of pear-shaped and symmetrical droplets. Printing was made on substrates of surface free energies lower and higher than the surface tension of the Zr02 ink. Powder migration was observed within a relic of the printed dot on the second type of substrate. Layers were also overprinted on the second type of substrate by varying the following: print resolution, printing interval, print area, drying conditions and ink powder loading. These series of prints were accompanied by the appearance of ridges, spattering and non-vertical walls and the effects were investigated. The wettability and shrinkage of droplets of the ceramic ink was also studied in-situ by monitoring the evolution of contact angle, width of ink-substrate interface and droplet height with a video camera. The shape of the droplet experienced different dynamics on different types of substrate. Lastly, the sedimentation behaviour of ethanol-based silver inks dispersed with different types of dispersant was investigated with respect to the sediment volume and half-value time. Deflocculated ink was obtained at a low dispersant level and powder loading

Nano-architectures with hierarchical porosity manufactured by colloidal techniques for application in ceramic semiconductor-based supercapacitors

Mención Internacional en el título de doctorEsta tesis contiene artículos de investigación en anexoAll the scientific contributions presented in this thesis have a main goal: to achieve science and technology challenges in the manufacture of energy storage devices based on a ceramic semiconductor employing colloidal processing strategies. Needs of the renewable energy systems and portable devices have revolutionized research on energy storage systems. Development of more powerful devices able to store large amounts of energy is now the challenge. In this field, the research in supercapacitors (SC) based on ceramic semiconductors has been tackled as a relevant issue to improve the electrochemical response of the electrodes inducing better reactant–catalyst contacts through the design of complex structures with large surface-to-volume ratio. The structuring and electrochemical activity of the pseudocapacitors (PCs) electrodes can be controlled by tuning the physical properties of the electroactive material (particle size, crystalline phase, preferred orientation) but also, shaping and consolidating the ceramic microstructure (publication 1). As a promising electroactive material (and non-strategic raw material), the NiO presents an elevated theoretical capacitance, high thermal and chemical stability and easy availability (publication 1). For these reasons, in this research work, he ultrasound aided synthesis of Ni(OH)2 and NiO nanoplatelets was standardized, and the manufacture of tridimensional (3D) and bidimensional (2D) NiO-based electrodes for PCs was tackled throughout different colloidal processing strategies. The Electrophoretic Deposition (EPD) and inkjet printing (IJP) technologies have been employed as shaping techniques for the manufacturing of these electrodes. While EPD allows us to fully and homogeneously coating collectors with complex shapes, as 3D Ni foams, the IJP enables the miniaturization of 2D electrodes for their use in energy-storage microdevices (publication 5). Both techniques are based on the dispersion and stabilization of Ni(OH)2 and NiO nanoplatelets in colloidal water-based suspensions, which were optimized for each application, leading to PC electrodes with specific capacitances of 250 and 160 F/g with retentions of 71 and 100%, respectively, where the remarkable low charge transfer resistance (Rct = 0.23Ω) of NiO patterns (IJP) has to be highlighted. Moreover, the assembly of synthesized NiO nanoplatelets has been also addressed resulting in a cohort of semiconductor microarchitectures that improves the electrochemical response of PC electrodes shaped by EPD. EPD is a well-known colloidal technique with remarkable performance in the coating of electrodes with complex shapes. However, for PC electrodes shaping, the nature and shape of the substrate have to be considered since collector has a relevant influence on electron transfer phenomena as well as the ion diffusion at the electrode/electrolyte interface (publication 4 & 7). The deposition and consolidation of the electroactive nanoplatelets through a mild heat treatment (sintering) connects the particles by the formation of sintering necks reducing Rct of these 3D electrodes to 1.71Ω. (publication 2 & 7). To cover Ni-3D collectors improving electrochemical performance, one of the employed colloidal strategies in this thesis was the surface modification of NiO nanoplatelets by layer-by-layer deposition of polyelectrolytes, leading to the formation of organic/inorganic core-shell structures which strongly modified the NiO nanoplatelets assembly by EPD (publication 3). The resulting hierarchical nanostructure presents high Rct values (3.65 Ω), while the capacitive response (CPE-p = 0.90) step up due to the enhancement of electrolyte wetting and faster ion diffusion. This fully ceramic porous nanostructure shows a specific capacitance of 982 F/g, with 60% retention after 1000 cycles, and relaxation times in the rage of carbonaceous SC electrodes (τ0 = 18 s), demonstrating that the capacitance is strongly related to the surface exposure of the semiconductor to the electrolyte, in spite of a relative high Rct. This is because of the increase in resistance is compensated by a better capacitive behavior as well as a better rate capability (publication 7). Finally, we also demonstrate that Rct can be reduced by the inclusion of materials capable of sharing their free electrons within the semiconductor microstructure such as metallic Ni or reduced graphene Oxide (RGO) (publication 2, 6 & 7), without deteriorating the capacitive response. In this way, NiO/Ni electrodes with specific capacitances of 755 F/gr and a retention of 71% was shaped by EPD in Ni foams, leading to Rct values of 1.55Ω and relaxation times of 11 s; while RGO/NiO hybrid supercapacitors (HSC) were prepared also by EPD, exhibiting specific capacitances of 920 F/g at a current density of 2 A/g with a retention of 71% and a Rct as low as 1.13Ω and a relaxation time of 4 s.Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de MadridPresidente: María Eugenia Rabanal Jiménez.- Secretario: Jadra Mosa Ruiz.- Vocal: Sandra Cabañas Pol