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Novel antiferroelectric glass-ceramics for energy storage applications

Final Report Summary - NAGCESA (Novel antiferroelectric glass-ceramics for energy storage applications)

NAGCESA, as the acronym for Novel Antiferroelectric Glass Ceramics for Energy Storage Application, was aimed at exploring novel antiferroelectric systems whose knowledge is still limited due to their recent discovery. NAGCESA was built based on the identification of a new class of materials, the so called “ABC compounds”, which have been predicted to be antiferroelectric only based on ab-initio simulations. Almost all of the ABC compounds have never been fabricated, so they represented an appealing family of new materials to study, in order to enlarge the current knowledge on antiferroelectric systems. The research activities were mainly concentrated on the MgSrSi compound, which is considered a prototype structure for other materials of similar type. Different techniques have been used in the attempts of synthetizing MgSrSi single phase. These included: i) RF sputtering using Mg, Sr and Si targets, ii) heat treatments in sealed fused silica tubes; iii) mechanical alloying of the stoichiometric mixture carried out in argon atmosphere to avoid oxidation reactions. The mechanical alloy process gave the most promising results and produced a nanostructured mixture with an overall amorphous phase. Subsequent heat treatments carried out on the high speed ball-milled mixture promoted crystallization and led to the formation of silicide phases. The combination of mechanical alloying and appropriate heat treatment is a feasible route that can be followed for the fabrication of other ABC compounds. This represents one of the major contributions of NAGCESA. In parallel to the experimental activity, the MgSrSi compound has also been investigated by ab-initio simulations, to study the stability of its phases and structures, the characteristics of the chemical bonds, the vibrational modes and the electronic properties, including electronic bands and density of electronic states. The simulations were performed using different functional such as LDA, PBE, PW91, HSE06 and B3LYP and indicated that the compound is thermodynamically stable in an orthorhombic structure (space group Pnma), which represents the ground state. The lattice parameters obtained from the optimization of the unit cell using the different functionals showed good agreement. The electronic behaviour predicted by PW91 suggested metallic characteristics, while a narrow band gap type semiconductor (0.98eV) was obtained using the B3LYP functional. The lattice dynamics studies identified 12 infrared and 18 Raman active vibrational modes, whose frequencies showed high consistency when simulated using B3LYP and PW91 functionals. The Born charges computed showed that Mg and Sr play as cations and the Si as anion suggesting an ionic-type bonding between the elements.
As alternatives to the ABC compounds, another group of perovskite oxides with possible antiferroelectric properties, as reported based on ab-initio calculations, has been identified in the literature. Four compositions among this group were chosen for the study, namely SrSnO3, SrZrO3, CaGeO3 and MgGeO3. The main contribution of this work consisted of establishing the optimized processing conditions and the understanding of their effects on phases, crystal structure, microstructure and dielectric properties for each material selected. Single phase SrSnO3 and SrZrO3 powders were obtained by ball milling and by a calcination treatment. The fabrication of high density SrSnO3 ceramics by conventional pressure-less sintering is difficult; only pellets of about 70% density were achieved. In order to improve density, Spark Plasma Sintering (SPS) was used. For the case of SrZrO3 high density pellets (about 97% of the theoretical density) with homogenous microstructure were prepared by conventional sintering. The preparation of CaGeO3 and MgGeO3 ceramics was carried out by solid state reaction method, while the melt-quenching route was used to obtain glass-ceramics. In the case of CaGeO3, the single phase powder can be reproducibly obtained between 1170°C and 1250°C temperature range. The main issues during conventional sintering are represented by the propensity to cracks formation and by the segregation of an additional phase identified as Ca5Ge3O11. The formation of this phase can be avoided with the addition of an excess of GeO2 which in turn, improved densification, but could not avoid cracking. In order to avoid the formation of secondary phases and cracking, spark plasma sintering was used, being a process that enables short sintering times which would prevent volatilization and excessive grain growth. It was demonstrated that single phase and crack-free ceramics can be obtained. The preparation of CaGeO3 glass-ceramics is challenging; X-ray diffraction carried out on melt-quenched systems under different conditions showed high crystallinity.
Guided by the MgO-GeO2 phase diagram, MgGeO3 ceramics were prepared with GeO2 in excess. The MgGeO3 phase mainly crystallizes with an orthorhombic symmetry (Pbca space group), with possible presence of monoclinic C2/c. The GeO2 in excess led to the formation of an amorphous phase. The study of the microstructure indicated that MgGeO3 grains present a whisker-like morphology, and they can be susceptible to produce grain-textured microstructure, by SPS hot forging.
The characterization of the dielectric properties provided the measurement of permittivity and dielectric loss as a function of frequency and of the current-polarization-electric field (I-P-E) hysteresis loops in all the compositions. All the materials prepared showed low permittivity (<30) and loss (in the order of 10^-3) and displayed linear, non-hysteretic P-E loops characterized by low polarization values and by the absence of the antiferroelectric-ferroelectric electric field-induced transformation. It was concluded that the electro-ceramics studied are not suitable for energy storage capacitors, but could have potential interest in microwave and high voltage applications.

NAGCESA has thriven a great hive of activities throughout the entire period of the Fellowship, which have led to the acquisition of additional skills in the Fellow’s background as a researcher, including ab-initio methodologies, crystal structure characterization and thermal analysis. Additionally, the Fellow has strengthened his knowledge on ceramics processing and has enlarged his experience being directly involved in the preparation of glass-ceramics and intermetallic systems. NAGCESA has resulted in the establishment of new research streamlines on electro-ceramics at the Department of Applied Science and Technology, the Politecnico di Torino (POLITO).
During the Fellowship period, the Fellow has consolidated the relationships with his previous collaborators through joined publications, reciprocal visits and meetings. He has developed a series of new collaborations, including a link with a group of expert metallurgists at University of Sassari, which have been involved in the mechanical alloy processing of MgSrSi, a group of theoretical chemists at the University of Torino where the Fellow has been trained on ab-initio calculations, a group in Belgium at the University of Leuven who shared their experience on the process of Mg-based intermetallic compounds. It is expected that these collaborations will lead to prolific and attractive research streamlines in the near future.
The research activities carried out in NAGCESA have contributed to the intense training of undergraduate students who have been involved in the experimental work, directly guided by the Fellow on the development of novel antiferroelectric oxides and in the processing of MgSrSi. This has led and will still lead to significant advancement in the Fellow’s supervision skills and will most likely lead to the continuation of the research activities started with NAGCESA. This is also guaranteed by the offer of a new research contract to the Fellow by the hosting group at POLITO. Additionally, the Fellow has been awarded of a KMM-VIN Fellowship based on the results obtained during NAGCESA.
An additional NAGCESA’s heritage is represented by the dissemination activities in which the Fellow has been involved. Thanks to NAGCESA’s funding the Fellow has managed to double the number of publications in his record that would have not happened without the support of NAGCESA funding. This represents a solid base to apply for future proposals. The Fellow is already looking for the best options among available fellowships and funding bodies that will guarantee further exploitation of the research activities started with NAGCESA.

Contact details of the Fellow: Dr Giuseppe Viola,