TiB2 is a promising material in several fields including impact resistant armor, seals, cutting tools, crucibles and wear resistant coatings given its physical, mechanical and chemical properties, in particular thanks to the combination of high hardness and exceptional wear resistance. It is however very difficult to sinter below 2000°C, also under mechanical pressure, and is limited by its low fracture toughness. By using sintering additives, it is possible to improve the sintering process and increase the mechanical properties since the additives react with oxidized layers to form secondary phases. In this study, we explored different preparation methods, various combinations of additives (B4C, Si3N4 and MoSi2), and sintering techniques (hot pressing and pressureless sintering). Thanks to the synergy between optimized process and tailored composition, an almost fully dense material was obtained at 1700°C with hardness of 24.4 ± 0.2 GPa and fracture toughness of 5.4 ± 0.2 MPa m0.5. However, the highest hardness value (30 ± 1 GPa) was obtained for samples sintered by pressureless sintering, featuring a core-shell grain structure.

TiB2 is a promising material for several fields including impact resistant armour, wear resistant coatings, cutting tools and crucibles given its physical, mechanical and chemical properties, in particular thanks to the combination of high hardness and exceptional wear resistance. It is however very difficult to sinter below 2000°C, also under mechanical pressure, and is limited by its low fracture toughness. By using sintering additives, it is possible to improve the sintering process and increase the mechanical properties since the additives react with oxidized layers to form secondary phases

A Spark Plasma Sintering (SPS) furnace was used to Flash-Sinter (FS) pure titanium diboride (TiB2) powder. A pre-sintering green body (Ø= 20 mm, relative density 60%) was used for the Flash-SPS using a dieless configuration with current passing entirely across the sample. The results show that the samples were densified in very short time (< 60 seconds) up to 90-95% of theoretical density. The rapid heating (?6000 °C/min ) prevented the complete evaporation of B2O3, leading to the formation of rarely seen segregation of boron at the grain boundaries. Compared to SPS or hot press, the rapid Flash-SPS processing promoted the formation boron rich grain boundaries during sintering, thus enhancing consolidation. TiB2 obtained by Flash-SPS was characterized using XRD and SEM analysis in order to quantify texturization induced by the FSPS hot forging effect. The Flash-SPS approach might be suitable to consolidate other refractory borides.

Abstract: Fluoride sintering additives are frequently utilized for the densification of various ceramics, but the current comprehension of the mechanism by which they affect densification is lacking behind empirical experience. A prominent example is LiF, which is commonly used in the preparation of transparent ceramics (MgAl2O4, Y2O3, YAG, MgO). It is generally accepted that LiF melt allows the rearrangement of particles, enhances densification and later in the process escapes from the system due to its high vapor pressure, so ideally no secondary phase is present in the final product. However, the second - and the most essential - step of enhanced densification is a source of scientific dispute. It is not clear if oxygen vacancies are responsible for this enhancement and if so, under what circumstances are they created. The present work tries to shed more light on fluoride additives and how they work throughout the whole process of preparation. The results suggest that the mutual dissolution of sintering additive and the base ceramic is the key aspect. Acknowledgements: This work was supported from the grant of Specific university research - grant No. A1_FCHT_2022_002

Alkali bonded ceramics are synthetic and amorphous alkali aluminosilicates, currently known as geopolymers. Since they can be regarded as the amorphous counterpart of zeolites, their application can be potentially extended in the chemical sector of molecular sieves. Geopolymers have a quite good CO2 adsorption capacity and selectivity up to 200 and 100 for CO2/N2 and CO2/CH4 separation, respectively, considerably higher than those of most of the adsorbent materials commonly accounted for such applications. The addition of zeolites or hydrotalcites as fillers can further improve the adsorption capacity for low and intermediate temperature CO2 capture applications. Na-based geopolymer-zeolite composites revealed a synergistic effect, as the CO2 capacity at low temperature was approximately 20% larger than the expected value. As well different types of hydrotalcites with different Mg/Al ratio can be used as fillers in geopolymer matrices for adsorption at intermediate temperature. Upon calcination, the structure of hydrotalcite changes, with loss of interlayered anions and water: the mixed oxide metaphase presents a large surface area and great affinity for CO2.

Geopolymers are recently been addressed as alternative, cost-effective, environmentally friendly adsorbents for the removal of pollutants from gaseous or liquid streams. Indeed, these materials have several properties which make them suitable as adsorbents. They are intrinsically mesoporous and their porosity can be further tailored, from the micro to the macro scale, employing several techniques. Different production processes allow geopolymers to be easily molded into different shapes as monoliths, granules or beads for easiness of operation and to facilitate the handling and storage. They have ionic exchange and electrostatic interaction properties deriving from their peculiar 3-dimensional structure, because of the presence of aluminium in tetrahedral coordination. To broaden the spectrum of applications, geopolymer matrices can be functionalized with fillers in order to create more performing composite materials. For all the mentioned reasons, geopolymer-composites, containing zeolites or hydrotalcites, were produced in the shape of monoliths or beads. The materials were then characterized as adsorbents for the removal of CO2 at different temperatures or for the treatment of wastewater.

Geopolymer-based beads were produced exploiting spherification processes in order to obtain millimeter-size porous beads useful for adsorption purposes. Adsorbents in form of beads offer some advantages as a good mobility, high packing density, ease of separation and reuse after regeneration. Furthermore, adsorption is considered one of the easiest and most effective techniques and alternative and low cost adsorbents, obtained by simple processes, are always researched. For these reasons, starting from different mixtures, obtained from metakaolin and potassium silicate solutions, geopolymer-based beads were formed through an injection-solidification method in different media: polyethylene glycol, liquid nitrogen and calcium chloride (using the ionotropic gelation of the alginate added to the mixture). The optimization of the processes allowed to obtain reproducible, round and mechanically resistant beads. The beads were characterized in term of morphology, macro- and microstructure, composition-stoichiometry, porosity distribution and specific surface area. Adsorption tests were carried out using dyes with different concentration and attempts to promote adsorption were made formulating composite beads containing hydrotalcite or TiO2 (exploiting its photodegradation effect).

Methane reforming is an important process for hydrogen production worldwide. The related CO2 footprint can be limited by ether using renewable methane, or recycling carbon dioxide from capture processes. Methane reforming in presence of a catalyst also acting as oxygen carrier was investigated, as an attractive process for production of hydrogen and carbon monoxide mixtures. The distinctive aspect of the research was the development of different granular materials based on CeO2 for utilization under fluidized bed conditions. Two granular materials were composed by CeO2 only, differing by the calcining temperature (900 and 1200°C), whereas a third one was a CeO2 and Al2O3 composite, thermally treated at 1200°C. Fluidization and comminution tests were performed at cold conditions and showed the best attrition resistance of materials calcined at the highest temperature. Conversely, thermogravimetric tests performed at 900 °C revealed the best performance of CeO2 granules with respect to the others in terms of oxygen supply capacity, achieving about 75% of the stoichiometric oxygen transfer capability of the material. Reforming tests in fluidized bed were performed at 900 °C with Ce-Al composite, giving rise to acceptable conversion of methane in syngas, achieving a value up to 80% in the initial time step of the reforming.

The paper will report on recent progresses on utilization geopolymer based active materials in i) catalytic cleaning/upgrading of syngas; ii) oxy-fuel chemical-looping combustion, and iii) fluidized bed behavior. Catalytic cleaning of syngas is relevant for removing undesired heavy hydrocarbons (tar) before its utilization as fuel or reactant [3]. Oxy-fuel combustion via chemical-looping is a very efficient and clean technology, allowing generation of flue gas with high CO2 concentration [4]. Fluidized bed technology is widely used for accomplishing processes requiring circulation of granular materials, e.g. catalyst, as well as high coefficients of heat and mass transfer. The results of materials characterization and experimental tests carried out at laboratory scale will be presented and discussed, also in comparison with standard catalysts of the process industry, in the perspective of a green approach.

The chemical homogeneity of single phase high-entropy AlB2-type Ti-Zr-Hf-TaTM diboride (TM = Cr, V, W, Mo), as well as Ti-Zr-Hf-Mo-W solid solutions was investigated using a new method based on the comparative examination of information provided by electron microscopy and structural parameters. The study of the densification behavior was accomplished, and strong correlations among densification rate-grain coarsening-long range chemical randomization were found. High-resolution synchrotron radiation X-ray diffraction supported by grain-scale chemical analyses by energy dispersive spectroscopy indicated that homogenization of the metals was incomplete, with direct impact on the refined lattice ?-strain. The chemical inhomogeneity was on the same length scale as the grain size, which makes it hardly detectable by typical chemical mapping using energy dispersive spectroscopy. Based on this analysis, the resulting ?-strain broadening is not an intrinsic property of the material, but strongly depends on its processing history.

Calculations on high-entropy diborides bring to speculate about the effects of lattice distortion on properties such as hardness, but none have provided any support for their assertions about strain within the unit cells. At the same time, studies on the homogeneity of distribution of metal atoms on the lattice sites remain sparse at best, with computational investigations suggesting segregation of some species to grain boundaries. We carried out an extensive and systematic study based on high-resolution synchrotron diffraction (HR-SD) and extended X-ray absorption fine structure analysis to test these points. Our analysis unequivocally shows that the picture of a random distribution of atoms with local strain on the d-metals site well describe the data. Additionally, a linear trend is observed between the average structure and the first neighbor distances, suggesting that any description of the properties of such materials should go beyond the simple dichotomy between long-range order and local structure. The long-range chemical homogeneity was also extensively investigated in several single-phase materials based on HR-SD and SEM-EDS: increasing lattice ?-strains resulted strongly correlated to an increasing lack of long-range chemical homogeneity. The thermal expansion study based on HR-SD analysis of a variety of high-entropy AlB2-type diboride solution solutions containing up 7 group IV-V-VI transition metals was finally done.

Cosa avranno mai in comune la riparazione ossea con l'energia solare o le esplorazioni spaziali? Sono tutti traguardi tecnologici che hanno preso ispirazione da fenomeni naturali: la biomimetica ci guida al progresso e, in tutto ciò, la ceramica, nelle sue infinite declinazioni, è il comune denominatore. Se parliamo di medicina rigenerativa, il componente ceramico principe è l'idrossiapatite. E' il basilare costituente delle nostre ossa che può essere utilizzato per creare dispositivi che stimolano le cellule alla ricostruzione dei tessuti o addirittura per realizzare "capsule intelligenti" che racchiudono farmaci da consegnare solo dove necessario, limitando gli effetti collaterali agli organi circostanti. Inoltre, i materiali ceramici si trovano in sempre più componenti che caratterizzano la nostra vita di tutti i giorni e, negli ultimi anni, hanno trovato ampio spazio in tecnologie e procedure innovative per ottenere energia elettrica da fonti rinnovabili come il sole. La fotosintesi clorofilliana è il più importante processo naturale che immagazzina energia solare e produce ossigeno facendo crescere le piante. Lo stesso meccanismo di trasformazione, semplice e sostenibile, può essere replicato artificialmente con materiali sintetici che sfruttano l'energia del sole per produrre energia alternativa rispetto ai combustibili fossili. Passando ad un settore più di nicchia, la ceramica è uno dei pochi materiali in grado di sopravvivere agli ambienti estremi tipici dell'aerospazio. In che modo i ceramici vadano ingegnerizzati ed assemblati insieme ad altri componenti ci viene insegnato dalla struttura di rettili, uccelli e insetti che, dopo processi evolutivi, hanno affinato sistemi organizzati in strutture gerarchiche che permettono loro di resistere a fortissimi impatti meccanici ed importanti sbalzi di temperatura.

Continuous carbon fibre ceramic matrix composites capable of tolerating multiple thermal-shock cycles and resisting ablation are needed for aerospace and hypersonic systems. Carbon fibre around 50 vol% and ultra-refractory matrices are fundamental parameters. The influence of rare earth (RE) oxides on the microstructure and mechanical properties of carbon fibre-ZrB/SiC composites was investigated. Materials were produced by slurry infiltration and hot pressing. The addition of YO, LaO and CeO led to the formation of lamellar boro-carbides that improved the densification, while ScO promoted the formation of (Zr,Sc)B solid solutions in the matrix. All these composites exhibited improved mechanical properties compared to a RE-free baseline, with room temperature strengths and toughness above 330 MPa and 9 MPa m, respectively, and strengths above 600 MPa at 1500 °C. The lamellar phase was identified as a fibre by-product with general formula REBC. Only CeO was detrimental on the long run due to its high reactivity with humidity which induced swelling and jeopardized the structural stability of the composite. This study revealed new fundamental insights into the microstructure evolution of carbon-fibre refractory composites and its impact on the mechanical properties, which will contribute to the development of new generation of reusable ceramic matrix composites for harsh environments.

In materials science, a detailed study of the microstructural characteristics, from the micro- down to the nano-scale, is fundamental for the identification of particular features, deriving from processing, which are responsible of specific thermo-mechanical behaviors. Only with an overall understanding of the microstructure evolution and behavior under extreme environment, corrective actions can be taken and materials performance ameliorated beyond current state of art. Extreme environment has a broad meaning, that might imply high-speed rate impact, corrosion, high temperature and ablation or a combination thereof. Here, the focus is on those structural ceramic materials that must withstand high thermo-mechanical loads at temperature above 1500°C. Thinking about the fields of machining and mining, WC-based ceramics play a major role, but are rarely used above 1000-1200°C due to the common addition of metallic phases. Moving then to the aero-space and hypersonic fields, the family of materials known as ultra-high temperature ceramics (UHTC) is the most suitable candidate. Each of the identified environment imposes a set of different structural requirements that are achievable only upon a careful tailoring of components, synthesis and processing. A series of ceramics and their unique microstructural features are presented and correlated to the observed specific properties.

A ZrB2-based ceramic, containing short Hi-Nicalon SiC fibers, was fabricated with a Mo-impermeable buffer layer sandwiched between bulk and the outermost oxidation resistant ZrB2-MoSi2 layer, in order to prevent inward Mo diffusion and associated fiber degradation reactions. This additional layer consisted of ZrB2 doped with either Si3N4 or with the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. Scanning electron microscopy imaging and elemental mapping via energy-dispersive X-ray spectroscopy showed that this tailored sample geometry provides an effective diffusion barrier to prevent the SiC fibers from deterioration due to reactions with Mo or Mo-compounds. In contrast, the structure of the SiC fibers in a reference sample without buffer layer is strongly degraded by MoSi2 diffusion into the fiber core. The comparison of the three buffer-layer systems showed a moderate alteration of the fiber structure in the case of Si3N4 addition, whereas in the PDC-doped samples hardly any structural change within the fibers was observed. A stepwise reaction mechanism is deduced, based on the continuous progression of a reaction zone that propagates toward the ZrB2-MoSi2 top layer. The progression of such a reaction zone as a consequence of the different eutectic melts forming in the different layers, that is, first in the SiC-fiber-containing bulk, then in the buffer layer itself, and finally in the top layer at high temperature, allows for an effective separation of the ZrB2-MoSi2 top layer from the SiC fibers. Subsequent oxidation at 1500 degrees C and 1650 degrees C for 15 min did not affect the efficiency of all three buffer layers, since no structural changes regarding buffer layer and fibers were observed, as compared to the non-oxidized samples.

A nominally pure and dense (TiCr)B ceramic was produced by spark plasma sintering of powders synthesized by boro/carbothermal reduction of oxides. The synthesized powders were a single phase and had an average particle of 0.4 ± 0.1 ?m and an oxygen content of 1.2 wt%. Average Vickers hardness values of the resulting ceramics increased from 25.9 ± 0.8 GPa at a load of 9.81 N, to 46.3 ± 0.8 GPa at a load of 0.49 N. Compared to the nominally pure TiB ceramic obtained under the same processing conditions, the (TiCr)B ceramic had higher values under the same load due to the finer average grain size (2.4 ± 1.0 ?m), higher relative density, and solid solution hardening. The results indicated that the Cr addition promoted densification, suppressed grain growth, and improved the hardness of TiB ceramics. This is the first report for dense and single-phase (Ti,Cr)B ceramics as superhard materials.

In this work, dye-sensitized solar cells containing seawater-based electrolytes were realized and investigated. The influence of the seawater composition on the electrochemical properties of the iodide/triiodide redox mediator was determined. High triiodide diffusion coefficient and ionic conductivity were assessed for seawater electrolytes through cyclic and linear voltammetry and impedance spectroscopy. Moreover, a notable influence of the seawater electrolyte on the charge transfer mechanism at the photoanode/dye/electrolyte interfaces was observed and deeply discussed. The ions, naturally present into seawater, reduce the charge recombination mechanism at the photoanode/electrolyte interface and promote a downward shift of the TiO conduction band, thus increasing the final DSSC efficiencies of 23% if compared with traditional devices containing water. The best seawater-based solar cell provides a photo-electrical conversion efficiency equal to 0.37% with 1.09 mA cm as short circuit current density. To the best of our knowledge, this is the first time that seawater is used as a key component for an energy production technology and the obtained results show the great potentiality of this green and recyclable element.

The effect of MgF2 as a sintering additive for the preparation of YAG ceramics via spark plasma sintering (SPS) is investigated with promising results, as nearly complete densification (0.58% porosity) is achieved at relatively low temperature and moderate pressure. Higher temperature and dwell time resulted in a translucent/trans-parent body. On the other side, significant grain growth was observed with MgF2 addition.

A new, innovative approach in the search for an effective and cheap carbon dioxide sorbent, in line with the circular economy and sustainable development principles, directs the attention of researchers to various types of waste ashes generated as a result of biomass combustion. In addition to the use of environmentally safe materials that have been experimentally identified, and that, in some way, have adjustable sorption capacity, it is also possible to rationally develop a widely applicable, simple, and inexpensive technology based on large amounts of this type of post-industrial waste, which is also an equally important issue for the natural environment (reducing the need for ash storage and accumulation). Even the lower sorption capacity can be successfully compensated for by their common availability and very low cost. Thus, the CO adsorption capability of the ashes from the combustion of straw biomass was experimentally investigated with the use of a high-pressure adsorption stand. The presented original technological concept has been positively verified on a laboratory scale, thus a functionalization-based approach to the combustion of substrate mixtures with nano-structural additives (raw, dried, calcined halloysite, kaolinite), introduced to improve the performance of straw biomass combustion and bottom ash formation in power boilers, clearly increased the CO adsorption capacity of the modified ashes. This allows for an advantageous synergy effect in the extra side-production of useful adsorbents in the closed-loop "cascade" scheme of the CE process. The addition of 4 wt.% kaolinite to straw biomass caused an over 2.5-fold increase in the CO adsorption capacity in relation to ash from the combustion of pure straw biomass (with a CO adsorption capacity of 0.132 mmol/g). In the case of addition of 4 wt.% nano-structured species to the straw combustion process, the best effects (ash adsorption capacity) were obtained in the following order: kaolinite (0.321 mmol/g), raw halloysite (0.310 mmol/g), calcined halloysite (0.298 mmol/g), and dried halloysite (0.288 mmol/g). Increasing the dose (in relation to all four tested substances) of the straw biomass additive from 2 to 4 wt.%, not only increase the adsorption capacity of the obtained ash, thus enriched with nano-structural additives, but also a showed a significant reduction in the differences between the maximum adsorption capacity of each ash is observed. The experimental results were analyzed using five models of adsorption isotherms: Freundlich, Langmuir, Jovanovi?, Temkin, and Hill. Moreover, selected samples of each ash were subjected to porosimetry tests and identification of the surface morphology (SEM). The obtained results can be used in the design of PSA processes or as permanent CO adsorbents, based on the environmentally beneficial option of using ashes from biomass combustion with appropriately selected additives.

Studio e analisi del processo più idoneo per la produzione di materiali di dimensioni maggiori in scala di laboratorio. In particolare è stata valutata la fattibilità di produrre un disco con diametro maggiore di 50 mm. La produzione del materiale di tali dimensioni è stata possibile grazie alla messa a punto del ciclo di sinterizzazione in pressione di gas (GPS). Per questa tecnica di sinterizzazione ISTEC possiede un forno prototipale con una camera utile di circa 7-8 litri in grado di sinterizzare materiali con dimensioni ad un livello tecnologico pari a TRL 4. Sono stati prodotti manufatti con forme squadrate o tonde di diverse dimensioni fino a raggiungere il diametro di 100 mm. Lo scale-up del processo di produzione e sinterizzazione di materiali a base di B4C-TiB2 ha permesso di produrre dei dimostratori tecnologici su cui effettuare una campagna di prove per i test balistici del progetto BALTIC. Questa attività è descritta all'interno del Rapporto Tecnico RT-2021/34, Protocollo ISTEC 980/2021 del 08/06/2021