Production and morphological and microstructural characterization of bulk composites or thick films for the study of multiphysics interactions

The surge of interest in multifunctional materials over the past 15 years has been driven by their fascinating physical properties and huge potential for technological applications such as sensors, microwave devices, energy harvesting, photovoltaic technologies, solid-state refrigeration, and data storage recording technologies. Among the others, magnetoelectric multiferroic composites are a special class of advanced solid-state compounds with coupled ferromagnetic and ferroelectric ferroic orders which allow to perform more than one task by combining electronic, magnetic and mechanical properties into a single device component. The production and characterization of lead zirconate titanate (PZT)- cobalt ferrite composites was the main topic of the thesis. During the PhD activity different ceramic processing and characterization technologies were studied and involved in order to optimize the produced materials as a function of the final microstructural and functional properties. The synthesis of cobalt ferrite (CF) and niobium-doped lead zirconate titanate (PZTN) powders by solid state reaction method and sol-gel technique, to control the particle size distributions and their microstructural and functional properties through calcination and milling treatments has been addressed first, followed by the mixing of the PZT and CF powders to produce particulate composites. The dispersion of PZT and CF in a liquid media, to produce layered composites by depositing the particles by electrophoretic deposition was an objective of the work as well. Key issues such as the lead loss during the sintering of PZTN-CF composites and the reaction between CF and titania have been addressed and have resulted in improvements in the sintering and characterization techniques leading to the production of fully dense PZTN-CF dual-particulate composites. In particular, the optimized sintering parameters have configured a new paradigm of ceramic sintering, which has been called quite-fast sintering, in respect to the traditional one, and the study of the PbO loss has led to propose an equation to calculate the PbO loss through XRD analysis. Further important achieved results were: the production of nanocobalt ferrite particles by multi-step milling, the correlation between the spin-canting angle with the microstrain and the average crystallite size of nanocobalt ferrite particles, the understanding of the CF growth mechanisms, the extension of the Globus model from small ferromagnetic grains “having no defect inside” to multiparallel-twinned overgrown ones, the understanding of heating rate effect on the interface nucleation onset of the anatase-to-rutile transformation and the anatase particle size, and the reaction products between CF and rutile at 1200 °C at the variation of CF/rutile ratio

Impact of residual stress on thermal damage accumulation, and Young's modulus of fiber-reinforced ultra-high temperature ceramics

Ultra-high temperature ceramic matrix composites (UHTCMCs) based on ZrB2-matrix reinforced with 45 vol% of unidirectional continuous carbon fibers are studied through the thermal mechanical hysteresis in order to investigate the thermal damage accumulation. The analysis carried out allowed to extrapolate the Young's modulus of the matrix from thermal expansion measures. It was found that the initial matrix Young's modulus of 195 GPa steadily decreases by thermal cycling the samples between RT and 1300 °C as a consequence of matrix cracking. On the other hand, the analysis suggested that carbon fibers keep their Young's modulus constant at 780 GPa. Finally, the residual stresses due to the different coefficient of thermal expansion between matrix and carbon fibers are discussed and let to justify the Young's modulus of 230 GPa, which cannot be explained with the so-called "rule of mixtures" generally valid and widely used in the composite science. Keywords: Boride, Ceramic matrix composite (CMC), Pitch-derived carbon fiber, Thermal expansion coefficient, Thermomechanical hysteresis loops, Linear elasticit

Thick composite magnetoelectric films by electrophoretic deposition

Electrophoretic deposition (EPD) from colloidal suspensions was utilized for the preparation of composite magneto-dielectric films on a conductive substrate. The present process is developed as a convenient forming process for the development of devices based on thick magneto-dielectric films [1]. The deposition parameters - using EPD - such as colloidal parameters, deposition voltage and deposition time and the post process parameters, such as drying velocity and sintering will be controlled. This work aims to control the fillers/matrix ratio during the deposition and obtain a good adhesion, compaction and functionality of the composite film after the heat treatment. Measurements results for the current transients during constant-voltage deposition and the correlated deposited mass are presented [2, 3].1] A.O. Karilainen, P.M.T. Ikonen, C.R. Simovski, S.A. Tretyakov, A.N. Lagarkov, S.A. Maklakov, K.N. Rozanov, and S.N. Starostenko, Experimental studies on antenna miniaturisation using magneto-dielectric and dielectric materials, IET Microw. Antennas Propag., vol. 5, no. 4, pp. 495–502, 2011. 2] C. Baldisserri, D. Gardini and C. Galassi, An analysis of current transients during electrophoretic deposition (EPD) from colloidal TiO2 suspensions, Journal of Colloid and Interface Science 347 (2010) 102–111 3] H. Farnoush, J.A. Mohandesi, D. H. Fatmehsari and F. Moztarzadeh, A kinetic study on the electrophoretic deposition of hydroxyapatite–titania nanocomposite based on a statistical approach, Ceramics International 38 (2012), 6753-676

Magnetoelectric composite bilayer film by electrophoretic deposition

In the recent years the interest of the research community towards multiferroic composite materials was growing fast [1,2]. A number of papers relates to bulk materials while less attention is focused on films. Electrophoretic deposition (EPD) was applied to prepare magnetoelectric (ME) composite bilayer thick films based on perovskite phase and spinel cobalt ferrite as some of the best piezoelectric and magnetostrictive oxides belong these crystal groups. The co-deposition of titanium oxide (TO) and cobalt ferrite (CFO) nanoparticles and the deposition of niobium-doped lead titanate zirconate (PZTN) were made from colloidal suspensions in ethanol keeping constant voltage and recording the current. Good adhesion and compaction of the green film were achieved by optimization of deposition voltage and time while high density of the film and minimized interphase reactions occurred after sintering. The deposited volume, the mixing of dielectric and magnetic phases and the density and ordering of the films have been verified by electron scanning microscopy after heat treatment. No reactions between the different phases was found. The piezoelectric properties were measured on the sintered films. [1] N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. [2] R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21

Heterostructured ceramic materials based on PZTN-CFO compounds

Multiferroic composites are currently one of the hot research topics [1]. Particulate ceramic composites have the advantages of low cost, simple production technology, good magnetoelectric effect and easy control of electrical and magnetic properties if the ferroelectric phase (generally a perovskite) and the ferromagnetic one (a ferrite with spinel structure) are mixed in a favourable proportion under the percolation threshold of the ferromagnetic phase. A great research effort is in progress to improve the fabrication of PZT–CoFe2O4 (PZT–CF) composites due to the excellent piezoelectric properties showed by the PZT material class and the large magnetostrictive coefficient of the CF. Unfortunately unwanted reactions occur during densification of PZT–CF materials at 1100-1200 °C. They are promoted by initial PbO loss that is calculated through XRD analysis, considering the amount of ZrO2 and variation of perovskite's tetragonality. The resulting titania reacts with CF to form cobalt titanate [2]. The microstructure of the composites at 26-81 mol % CF content was thoroughly investigated; the CF grain size distribution can be mono- or bi-modal and overgrowth [3] occurs. By setting a quite-fast sintering full densification and prevention of unwanted reactions was achieved for the PZT:CF 74:26 composites. The high coercivity (789 Oe) shown by these composites is correlated to the good dispersion of euhedral CF grains (250 nm) in the PZT matrix that is affected as well by limited grain growth (240 nm). [1] G. Schileo, Recent developments in ceramic multiferroic composites based on core/shell and other heterostructures obtained by sol-gel routes. Prog. Solid State Ch. 41 (2013) 87–98. [2] P. Galizia, et al., PZT-cobalt ferrite particulate composites: Densification and lead loss controlled by quite-fast sintering. J. Eur. Ceram. Soc. (2016). doi:10.1016/j.jeurceramsoc.2016.08.025 [3] P. Galizia, C. Baldisserri, C. Capiani, C. Galassi Multiple parallel twinning overgrowth in nanostructured dense cobalt ferrite. Mater. Design 109 (2016) 19–26. doi:10.1016/j.matdes.2016.07.05

Structure analysis of cobalt ferrite/titania-silica composite

Magnetodielectric bulk composite of a magnetic phase in dielectric matrix have been studied. Silica coated titania powder was produced by elctrocoagulation and used as dielectric matrix; while the cobalt ferrite powder was selected as magnetic filler. This study aims at tailoring the magnetic and dielectric phases and their interfaces in order to obtain new electromagnetic properties. The microstructure of sintered titaniasilica/cobalt ferrite composites has been related to compositional modifications in terms of silica/titania weight ratio and titania-silica/cobalt ferrite volume ratio. The crystalline structure was studied by XRD analysis supported also by EDS investigationand image analysis performed on the electron micrographs. The image analysis supported the XRD and EDS interpretations, and contributed to evaluate the effective volume contentof the phases after heat treatment

Structure analysis of cobalt ferrite/titania composite

Magnetodielectric bulk composite of a magnetic phase and a dielectric one were tailored to tune the macroscopic properties of permittivity and permeability. The designing of heterostructure formed by magnetic and dielectric counterparts offers a versatile route for the production of isotropic composite material with unusual electromagnetic properties. This study investigates the microstructure of sintered titania (TO)/cobalt ferrite (CFO) composites related to compositional modifications. The crystalline structure was studied through the comparison of theXRD patterns withthe EDS analysis and the results of the image analysis done on the electron micrographs. The image analysis wasfundamental to verifythe XRD and EDS interpretations, and to measure the effective volume contentof the phasesafter heat treatment.Several TO/CFO ratios from 0.7 to 11.8were analyzed. The formation oftheternary compound (FCTO) was foundonly in the composite with the TO/CFO ratio equal or bigger than 3

Magnetoelectric composite bilayer films by electrophoretic deposition

In the recent years the interest of the research community towards multiferroic composite materials was growing fast [1,2]. A number of papers relates to bulk materials while less attention is focused on films. Electrophoretic deposition (EPD) was applied to prepare magnetoelectric (ME) composite bilayer thick films based on piezoelectric phases and cobalt ferrite. The deposition/co-depositionwere made from colloidal suspensions in ethanolkeeping constant voltage and recording the current. Good adhesion and compaction of the green film were achieved by optimization of deposition voltage and time while high density of the film and minimized interphase reactions occurred after sintering. The chemical activity between the two layers was controlled through the batches composition and it could lead to the synthesis of complex engineered structures.The deposited volume, the mixing of dielectric and magnetic phases and the density and ordering of the films have been verified by electron scanning microscopy after heat treatment. The piezoelectric properties were measured on the sintered films. [1] N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. [2] R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21