Novel processing route for the fabrication of bulk high-entropy metal diborides

A single high-entropy phase material with hexagonal structure is produced by a two-steps processing method. Elemental reactants are first remarkably converted by Self-propagating High-temperature Synthesis (SHS). The completion of the chemical transformation to the desired (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 phase and its concurrent consolidation up to 92.5% relative density is achieved by processing the SHS powders at 1950 °C via Spark Plasma Sintering. It is clearly evidenced that the use of the SHS technique is extremely beneficial to promote the formation of high-entropy ceramics, as compared to the time consuming ball milling treatment alternatively adopted

High-entropy transition metal diborides by reactive and non-reactive spark plasma sintering: A comparative investigation

The direct synthesis and consolidation by SPS (1950 °C, 20 min, 20 MPa) of high-entropy (Hf0.2Mo0.2Zr0.2Nb0.2Ti0.2)B2 from elemental powders resulted in a multiphase product. An increase of the heating rate determined a change of the mechanism governing the synthesis reaction from gradual solid-state diffusion to rapid combustion regime, while the final conversion degree was 67 wt.%. The sintered product displayed a non-uniform microstructure with the presence of 10–15 μm sized pores, due to volatilization phenomena occurring during the combustion synthesis reaction. In contrast, when the SPS process was preceded by powder synthesis via SHS, a homogeneous single-phase ceramic was obtained. Clear benefits are derived by the use of SHS, able to provide very shortly powders with elemental species very well intermixed, so that the obtainment of (Hf0.2Mo0.2Zr0.2Nb0.2Ti0.2)B2 during the subsequent SPS stage is strongly promoted. The resulting 92.5% dense product shows superior oxidation resistance with respect to individual borides prepared with the same method.ARCHIMEDES project sponsored by Regione Autonoma della Sardegna (Italy) - Fondo di Sviluppo e Coesione (FSC) 2014-2020 (Cod. RAS: RASSR88309, Cod. CUP: F76C18000980002). One of the authors (G.T.) performed her activity in the framework of the International PhD in Innovation Sciences and Technologies at the University of Cagliari, Italy. One of us (G.C.) acknowledges the results obtained in this manuscript as quite important for the “Ithermal” and “Generazione E” projects, sponsored by Sardegna Ricerche, Italy (Cod. CUP: F21I18000130006) and by the Italian Ministry of Education, University and Research, Italy (Cod. CUP: B96G18000560005), respectivel

SINTESI E SIMULTANEA DENSIFICAZIONE DI INTERMETALLICI MEDIANTE SINTERIZZAZIONE IN CORRENTE PULSATA

Sono state evidenziate alcune delle più recenti applicazioni in campo metallurgico della sinterizzazione attivata mediante corrente elettrica pulsata, SCP, meglio nota con l’acronimo inglese SPS (“Spark Plasma Sintering”). In particolare l’attenzione è stata focalizzata sull’impiego di tale tecnica per la sintesi e simultanea densificazione dei sistemi intermetallici NiTi e NbAl3 a partire da polveri dei metalli elementari. Nel caso della lega equiatomica NiTi, è stato dimostrato che, attraverso un processo ad un solo stadio, la tecnica SCP consente l’ottenimento di un prodotto finale già caratterizzato da un elevato grado di purezza (5.8-7.2 mol% di NiTi2) ed una elevata densità relativa (99%), più rapidamente (20 min) rispetto ai tempi richiesti dai processi convenzionali della metallurgia delle polveri. Sono state inoltre analizzate le potenzialità della combinazione di tecniche di attivazione meccanica con il processo SCP. Attraverso una opportuna analisi dell’effetto dei principali parametri operativi di entrambe le tecniche è stato possibile l’ottenimento di un prodotto NbAl3 puro e caratterizzato da densità molto elevate (97.5 ± 2.5% rispetto a quella teorica)

FIRB "SQUARE" project: nano-structured sensors for the detection of the polluting in engine exhaust gases and for indoor air quality monitoring

The present work is a final dissemination of activities carried out and main results obtained in the national founded project Firb "Square". The project is leaded by Centro Ricerche Fiat and it involves the most qualified national public Research Institutes and Universities active in the fields of nanomaterials synthesis, nanotechnology and gas sensors development

Influence of the heating rate on the in situ synthesis and consolidation of ZrB2 by reactive spark plasma sintering

The effect of the heating rate during reactive SPS to obtain dense ZrB2 from elemental reactants is investigated. When the temperature is increased at 500°C/min or faster, the synthesis reaction evolves under combustion regime. In contrast, if heating rates equal or lower than 200°C/min are considered, a gradual solid-state diffusion mechanism is established. In both cases, the synthesis reaction goes to completion during the non-isothermal heating step, although samples have to be processed for 20 min at 1850–2000°C and 50 MPa to reduce residual porosity. Despite of the combustion synthesis route allows to obtain pure dense products at milder temperature conditions, safety problems and product inhomogeneity make its exploitation not convenient. On the other hand, safety conditions are preserved during the gradual formation of ZrB2 by SPS. Correspondingly, 96% dense monolithic products can be obtained at temperature levels of about 2000°C within 20 min dwell time.Ltd. All rights reserved.Keywords: ZrB2; Reactive sintering; Spark Plasma Sintering; Solid-state diffusion; Combustion synthesis1. IntroductionThe peculiar combination of500◦C/min or faster, the synthesis reaction evolves under combustion regime. In contrast, if heating rates equal or lower than 200◦C/min areconsidered, a gradual solid-state diffusion mechanism is established. In both cases, the synthesis reaction goes to completion during the non-isothermal heating step, although samples have to be processed for 20 min at 1850–2000◦C and 50 MPa to reduce residual porosity. Despite of thecombustion synthesis route allows to obtain pure dense products at milder temperature conditions, safety problems and product inhomogeneity makeits exploitation not convenient. On the other hand, safety conditions are preserved during the gradual formation of ZrB2by SPS. Correspondingly,96% dense monolithic products can be obtained at temperature levels of about 2000◦C within 20 min dwell time

On the controversial formation of silver diboride: processing of Ag+2B powders by spark plasma sintering

Following recent theoretical studies based on electronic structure calculations conducted on Ag- and Au-borides which estimated their superconducting behaviour at elevated transition temperatures, the preparation of AgB(2) was attempted in this work by reactive Spark Plasma Sintering (SPS) from Ag and B elemental powders. It was found that, independently from the processing conditions adopted, the formation of a new phase, which corresponds to a small peak on the XRD pattern at 2 theta equal to about 28 degrees, cannot be attributed to AgB(2). In fact, it was shown that the new phase detected on sintered pellets is most likely boric acid which is formed only if the sample is left in contact with air. If SPSed samples were stored under inert (Ar) atmosphere, no additional phases other than unreacted elements were observed. (c) 2009 Elsevier B.V. All rights reserved

Effects of the Parent Alloy Microstructure on the Thermal Stability of Nanoporous Au

Nanoporous (NP) metals represent a unique class of materials with promising properties for a wide set of applications in advanced technology, from catalysis and sensing to lightweight structural materials. However, they typically suffer from low thermal stability, which results in a coarsening behavior not yet fully understood. In this work, we focused precisely on the coarsening process undergone by NP Au, starting from the analysis of data available in the literature and addressing specific issues with suitably designed experiments. We observe that annealing more easily induces densification in systems with short characteristic lengths. The NP Au structures obtained by dealloying of mechanically alloyed AuAg precursors exhibit lower thermal stability than several NP Au samples discussed in the literature. Similarly, NP Au samples prepared by annealing the precursor alloy before dealloying display enhanced resistance to coarsening. We suggest that the microstructure of the precursor alloy, and, in particular, the grain size of the metal phases, can significantly affect the thermal stability of the NP metal. Specifically, the smaller the grain size of the parent alloy, the lower the thermal stability