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It is confirmed that Fluorite-structured Entropy-Stabilized Oxides (F-ESO) can be obtained with multicomponent (5) equimolar systems based on cerium, zirconium, and other rare earth elements, selected according to the predictor already proposed by the authors. Indeed, in the present study, three different samples owning a standard deviation (SD in the following) of their cationic radii greater than the threshold value (i.e., SD > 0.095 with cationic radii measured in Å) needed to ensure the formation of the single-phase fluorite structure, were prepared via co-precipitation method. After a calcination step at 1500 °C for 1 h, the entropy-driven transition from multiple phases to single-phase fluorite-like structure has been actually confirmed. Thus, with the aim of defining the temperature at which such entropy-driven transition occurred, and identifying possible relation between such temperature and the actual value of SD, the phase evolution of all the prepared samples as a function of temperature (ranging from 800 °C to 1300 °C) was analyzed by in situ High Temperature X-ray Diffraction. An apparent inverse correlation between the standard deviation and the entropy-driven transition temperature has been identified, i.e., the higher the former, the lower the latter. These results, based on the conducted basic structural analysis, provide further support to the SD-based empirical predictor developed by the authors, suggesting that high values of SD could bring additional contribution to the overall entropy of the system, other than the configurational one. Thus, this SD-driven entropy contribution directly increases with the increasing of the standard deviation of the cationic radii of a given F-ESO. [ABSTRACT FROM AUTHOR]

In the present study, we discuss a possible extension of our previously proposed empirical predictor for the formation of fluorite-structured High-Entropy Oxides (HEOs) based on equimolar mixtures of Rare-Earth (RE) Oxides. Indeed, by using the so-called cluster-plus-glue atom model, a very recently proposed theoretical model to describe the crystal structure of inorganic compounds, we were able to confirm the results of the standard deviation predictor, i.e. the relation between the formed phase(s) and the standard deviation of cationic radii, justifying them based on the phases' thermodynamic stability and the valency of the involved cations. At the same time, we propose an explanation of the role of Cerium and Zirconium in the formation of fluorite-structured HEOs, designing and predicting the behavior of a completely novel non-equimolar system (based on ZrO 2 , La 2 O 3 Yb 2 O 3 , Sm 2 O 3 , Gd 2 O 3), stabilized in a bixbyite-like single phase. Based on the presented results, we believe that a link between the geometrical aspects of the involved cations and their chemical-thermodynamic properties exists, providing a new tool to predict the behavior of systems in the intricate and intriguing world of High Entropy Oxides. [ABSTRACT FROM AUTHOR]

High-Entropy Oxides (HEOs) are a totally new class of ceramic materials that have recently attracted many scientific attentions. However, the huge intrinsic complexity and the massive number of possible combinations characterizing such systems make it hard to predict a priori their properties and their crystal structures. Moreover, the idea of designing and engineering new materials by using entropy as a driving force is conceptually exciting and intellectually stimulating. Thus, we acknowledged that predicting and synthesizing unknown entropy-stabilized single-phases of a given formula in a given crystal structure could be of great interest to the HEOs research community and, through a systematic study of 18 samples of equimolar 5-component Rare Earths-based oxides, we were able to elaborate a simple and effective predictive model to design HEOs stabilized in a single-phase fluorite-like structure. The novelty of our model, other than its simplicity and immediacy, consists in pointing out that the "dispersion" of the cationic radii of the involved elements of a certain system (expressed in terms of their standard deviation) is crucial for stabilizing fluorite-structured HEOs. Definitely, for systems owning standard deviations of the involved elements cationic radii (coordination VIII) distribution higher than 0.095, single-phase fluorite-structured systems are formed; otherwise, for s < 0.095 firstly biphasic (fluorite and bixbyite) systems are formed and then single-phase bixbyite-structured systems are formed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this work, nanoparticles of cerium hydroxycarbonates were synthesized by a facile hydrothermal treatment at 120°C with ammonium carbonate as the precipitating/mineralizer agent in diluted solution. The as-formed amorphous coprecipitate undergoes several morphological and structural modifications as a function of the duration of the hydrothermal treatment, leading after 8 h to the formation of monosized nanoparticles of hexagonal CeCO3OH. A similar behavior has been found when neodymium-based precursors are used as well, whereas the same treatment produces very different results by using different lanthanides-based precursors in terms of formed phases and morphologies, thus leading to the formation of pure tengerite-type structure phases, biphasic systems (tengerite type and hexagonal), or even entirely amorphous systems. Furthermore, the hydrothermal transformation is influenced by the redox behavior of the rare-earth cation (i.e., cerium) too, eventually resulting in the formation of fluorite-like structures. Therefore, a specific pathway of Ce(III) precursor transformations during hydrothermal treatment is proposed in this paper. Definitely, our results show that ammonium carbonate can be used as the precipitating/mineralizer agent to obtain cerium, doped-cerium, and neodymium hydroxycarbonates, which show excellent morphologies (i.e., characterized by spherical, nanosized particles with monomodal size distribution). Therefore, they can be used as optimal precursors for oxide powders. Conversely, when tengerite-type carbonate precursors are formed, their morphology is characterized by large and acicular particles. [ABSTRACT FROM AUTHOR]

Entropy-stabilized oxides represent a novel family of advanced ceramic materials with attractive functional properties. In this work, entropy-stabilized oxides, in the system (Mg, Co, Ni, Cu, Zn)O, were produced by co-precipitation and hydrothermal synthesis. Although TG/DTA and XRD analyses of as-synthetized powders point out complex thermal evolution, in both cases the desired single-phase rock salt solid solution was obtained after a proper thermal treatment. The dilatometric analysis points out the excellent sinterability of the obtained powders, which were successfully consolidated for the first time reaching nearly full density (~ 97%) at relatively low temperature (1050 °C). [ABSTRACT FROM AUTHOR]

(Ba, Sr)-exchanged zeolite A with composition Ba0.74Sr0.22Na0.04Al2Si2O8 was prepared by cation exchange; a mild thermal treatment converts into an amorphous phase. Successive crystallization and sintering behavior was studied by XRD, DTA, and thermodilatometric analysis. The results point out the activation of viscous flow sintering mechanisms between 900°C and 1050°C. The densification process starts when the amorphous phase reaches its glass transition temperature (897°C) and finishes when the material crystallizes forming hexacelsian. The application of an external pressure in such temperature range allows to achieve an almost complete densification, the material transforming at 1300°C into dense monoclinic celsian much below the typical processing temperature. [ABSTRACT FROM AUTHOR]

Nanocrystalline 20 mol% samaria-doped ceria powders (Ce0.8Sm0.2O1.9) were synthesized by coprecipitation techniques using various precipitating agents in aqueous solution: ammonia, ammonium carbonate, tetramethylammonium hydroxide, and urea. The synthesized powders after calcination at 600°C possess a fluorite structure with nanometric size although they are characterized by a very different morphology and degree of agglomeration. Remarkable differences appear in the sintering behavior, especially because of the presence of hard agglomerates. The precipitating agent has therefore a crucial role in the coprecipitation process, which influences the morphology of the powders and in turn the sintering behavior. The obtained results clearly reveal that ammonium carbonate and urea are the best precipitating agents to obtain final dense products after sintering. [ABSTRACT FROM AUTHOR]

This work investigates the lithium doping effect on the structural and thermal behavior of (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2)O high entropy oxide (HEO) obtained by co-precipitation. The powders are characterized by differential thermal and thermogravimetric analyses (DTA-TG), mass spectroscopy, dilatometry, X-ray diffraction (XRD), magic angle spinning nuclear magnetic resonance (MAS NMR), Raman spectroscopy, X-ray photoelectron spectra (XPS) and electron spin resonance (ESR). The results point out that Co3+ ions (within a spinel phase) are reduced to Co2+ before the formation of the high entropy oxide in the absence of Li. Conversely, no reduction is observed in the case of Li-doping, thus indicating the presence of Co3+ within the high entropy rock salt lattice. At high temperature (>1050–1150 °C), the HEO phase loses oxygen changing the charge compensation mechanism for Li incorporation (mostly based on the presence of 3 + cations and oxygen vacancies at low and high temperatures, respectively). Moreover, it is found that lithium lies in two well-distinct chemical environments in HEO, which cannot be completely explained by assuming a random organization of the high entropy phase. This suggests the existence of some short-range order and possible M3+-Li+ pairs. • Li dissolves into the rock salt HEO phase in substitutional positions. • Single-phase HEO is formed without the Co3+ reduction in the case of Li-doping. • Li sits in two well-distinct chemical environments within the HEO structure. • The presence of some kind of short-range order within the Li-doped HEO is hypothesized. [ABSTRACT FROM AUTHOR]

Background: Gadolinium-doped ceria is an attractive electrolyte material for potential application in solid oxide fuel cells (SOFCs) operating at intermediate temperatures typically with 10%-20% substitution of Ce+4 by Gd+3. In particular, 10% gadolinium-doped ceria seems to have the highest values of conductivities among the other dopant compositions. Methods: Nanosized powders of gadolinium-doped ceria were prepared by hydrothermal treatment using coprecipitate as a precursor and in the presence of 3 different mineralizer solutions. The powders obtained were characterized by X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy and thermal analysis, while the electrical behavior of the corresponding pellets were ascertained by AC impedance spectroscopy. Results: Nanocrystalline gadolinium-doped ceria powders with fluorite cubic crystal structure were obtained by hydrothermal treatment. Independent of the mineralizer used, these powders were able to produce very dense ceramics, especially when selecting an optimized sintering cycle. In contrast, the electrical behavior of the samples was influenced by the mineralizer solution, and the samples synthesized in the neutral and alkaline solutions showed higher values of electrical conductivity, in the range of temperatures of interest. Conclusions: By the coprecipitation method, it has been possible to synthesize nanosized gadolinium-doped cerium oxide in a fluorite structure, stable in a wide range of temperatures. Hydrothermal treatment directly on the as-synthesized coprecipitates, without any drying step, had a very positive effect on the powders, which can be sintered with a high degree of densification, especially with an optimized sintering cycle. Furthermore, the electrical behavior of these samples was very interesting, especially for the samples synthesized using neutral mineralizer solution and basic mineralizer solution. [ABSTRACT FROM AUTHOR]

Single-phase high entropy fluorite (Ce 0.2 Zr 0.2 Y 0.2 Gd 0.2 La 0.2 O 2−δ) samples were synthesized by co-precipitation and consolidated by ultrafast high-temperature sintering (UHS) in less than 2 min. The chemical homogeneity of the sintered materials was confirmed by X-Ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDXS), high-resolution EDXS using Transmission Electron Microscopy (TEM), and Raman spectroscopy. Compared to conventional sintering, UHS of high entropy ceramics was a hundred times faster and it resulted in highly dense microstructures (relative density > 93%) with nanometric grains. Ce 0.2 Zr 0.2 Y 0.2 Gd 0.2 La 0.2 O 2−δ densified under an UHS current of 20–25 A. An optimized step-wise UHS schedule was proposed to obtain dense, single-phase pellets with a reduced amount of defects. The extreme heating rates were found beneficial to limit grain coarsening and to obtain a single phase. [Display omitted] [ABSTRACT FROM AUTHOR]

In the present work 10 mol% Samarium-doped Ceria nanopowders are densified by fast firing in 15–300 s, using a pre-heated furnace at temperature from 900 to 1100 °C. Outstanding sintering rates are achieved revealing that the densification mechanisms are well activated. The sintered pellets possess nanometric grain size, in almost all the cases below 100 nm. It is also shown that finer microstructures are obtained by increasing the firing temperature. The beneficial effect of an high heating rate process on densification and grain size is pointed out; allowing to define an optimum treating condition at 1100 °C using a soaking time of 30–60 s. [ABSTRACT FROM AUTHOR]

Ceramic electrolytes formed by Bi (4 mol%)-doped 8YSZ, i.e., Y2O3 (8 mol%)-doped ZrO2, were synthesized by a simple co-precipitation route, using ammonia solution as precipitating agent. The amorphous as-synthesized powders convert into zirconia-based single phase with fluorite structure through a mild calcination step at 500 °C. The calcined powders were sintered at very low temperatures (i.e., 900–1100 °C) achieving in both cases very high values of relative densities (i.e., >95%); the corresponding microstructures were highly homogeneous and characterized by micrometric grains or sub-micrometric grains for sintering at 1100 °C and 900 °C, respectively. Very interesting electrochemical properties were determined by Electrochemical Impedance Spectroscopy (EIS) in the best samples. In particular, their total ionic conductivity, recorded at 650 °C, are 6.06 × 10-2S/cm and 4.44 × 10-2S/cm for Bi (4 mol%)-doped 8YSZ sintered at 1100 °C and 900 °C, respectively. Therefore, Bi was proved to be an excellent sintering aid dopant for YSZ, highly improving its densification at lower temperatures while increasing its total ionic conductivity. [ABSTRACT FROM AUTHOR]

The reduction of the sintering temperature of doped ceria ceramics remains an open challenge for their real exploitation as electrolytes for intermediate temperature solid oxide fuel cell (IT-SOFCs) at the industrial level. In this work, we have used Bi (0.5 and 2 mol %) as the sintering aid for Gd (20 mol %)-doped ceria. Nano-sized powders of Bi/Gd co-doped ceria were easily synthesized via a simple and cheap sol-gel combustion synthesis. The obtained powders showed high sinterability and very good electrochemical properties. More importantly, even after prolonged annealing at 700 °C, both of the powders and of the sintered pellets, no trace of structural modifications, phase instabilities, or Bi segregation appeared. Therefore, the use of a small amount of Bi can be taken into account for preparing ceria-based ceramic electrolytes at low sintering temperatures. [ABSTRACT FROM AUTHOR]

The rare-earth carbonates represent a class of materials with great research interest owing to their intrinsic properties and because they can be used as template materials for the formation of other rare earth phases, particularly of rare-earth oxides. However, most of the literature is focused on the synthesis and characterization of hydroxycarbonates. Conversely, in the present study we have synthesized both rare-earth carbonates—with the chemical formula RE2(CO3)3·2-3H2O, in which RE represents a generic rare-earth element, and a tengerite-type structure with a peculiar morphology—and rare-earth hydroxycarbonates with the chemical formula RECO3OH, by hydrothermal treatment at low temperature (120 °C), using metal nitrates and ammonium carbonates as raw materials, and without using any additive or template. We found that the nature of the rare-earth used plays a crucial role in relation to the formed phases, as predicted by the contraction law of lanthanides. In particular, the hydrothermal synthesis of rare-earth carbonates with a tengerite-type structure was obtained for the lanthanides from neodymium to erbium. A possible explanation of the different behaviors of lighter and heavier rare-earths is given. [ABSTRACT FROM AUTHOR]

In this work, ceria-based ceramics with the composition Gd0.14Pr0.06Ce0.8O2-δ and Sm0.14Pr0.06Ce0.8O2-δ, were synthesized by a simple co-precipitation process using either ammonium carbonate or ammonia solution as a precipitating agent. After the calcination, all of the produced samples were constituted by fluorite-structured ceria only, thus showing that both dopant and co-dopant cations were dissolved in the fluorite lattice. The ceria-based nanopowders were uniaxially compacted and consequently flash-sintered using different electrical cycles (including current-ramps). Different results were obtained as a function of both the adopted precipitating agent and the applied electrical cycle. In particular, highly densified products were obtained using current-ramps instead of "traditional" flash treatments (with the power source switching from voltage to current control at the flash event). Moreover, the powders that were synthesized using ammonia solution exhibited a low tendency to hotspot formation, whereas the materials obtained using carbonates as the precipitating agent were highly inhomogeneous. This points out for the first time the unexpected relevance of the precipitating agent (and of the powder shape/degree of agglomeration) for the flash sintering behavior. [ABSTRACT FROM AUTHOR]

Praseodymium-doped ceria electrolyte powders are synthesized by a co-precipitation method using ammonium carbonate in little excess to fabricate a stable electrolyte-supported solid oxide fuel cell able to operate in hydrogen conditions. Raman and X-ray Photoelectron Spectroscopy are employed for the electrolyte characterization to check the distribution of vacancies and the initial oxidation state of Pr that influences the transport mechanism under the real operation of SOFCs. The optimum Pr concentration in the electrolyte is found to be 6 mol% of Pr and 14 mol% of Gd (sample 6Pr). The electrolyte-supported cells fabricated with this composition are sintered at 1250 °C for 3 h and tested in different gas conditions and operating temperatures, showing a maximum power density of 305.31 mW·cm-2 at 530.36 mA·cm-2 (750 °C) in wet hydrogen conditions. Compared to standard cells fabricated with a gadolinium-doped ceria electrolyte sintered at 1500 °C, the 6Pr has long term stability performances with a power density loss of 17% after 100 h of operation. The results demonstrate the eligible use of this electrolyte under real operating environments. Image 1 • The vacancies distribution and the initial oxidation state of Pr were investigated. • The concentration of the O2- vacancies associated defect increase in 6Pr increases. • The Pr initial oxidation state influences the transport mechanism under SOFCs tests. • 305.31 mW·cm-2 power density at 750°C was obtained in SOFC operative conditions. [ABSTRACT FROM AUTHOR]

The thermal transformation of Ba-exchanged zeolite A into the monoclinic polymorph of Ba feldspar celsian (BaAl 2 Si 2 O 8 , a ceramic material of great technological interest) was found to be affected by a number of different parameter, such as the temperature and duration time of the thermal treatment, the forming pressure, the presence of the mineralizing cation Na + and Li + , and Sr 2+ doping. These facts were investigated in a long term study which is critically reviewed in this work, thus supplying the reader with an overall vision of this complex matter. Moreover, it is evidenced that the thermal transformation of Ba-exchanged zeolite A appears the most profitable and simple way to obtain the synthesis of the monoclinic polymorph of Ba feldspar celsian, which can be attained in few hours thermal treatments at 1000–1100 °C. The mechanisms on which these transformations are based and the sintering of monoclinic celsian powders obtained from Ba-exchanged zeolite A are discussed. [ABSTRACT FROM AUTHOR]

Starting from the knowledge of ancient materials, the present work aims at developing green eco-friendly restoration mortars. A multi-step approach is proposed for this purpose including a characterization of ancient mortars, followed by that of the raw materials, ending with an in-depth one of the new products. Therefore, new mortars have been produced following the ancient recipe used in the construction of the Aqua Traiana aqueduct (Rome, 2nd century CE). Minero-petrographic and mechanical features, along with the assessment of porosity, water absorption, hydraulic index, and the evaluation of phytotoxicity toward microalgae cells, have been considered to evaluate the new products. The results proved that the nature of the aggregate and of the binder influence the porosity and the formation of newly reaction phases. The growth of these minerals creates a porous network inside the binder paste which shows an efficient reactivity in the formation of C-S-H compounds. [ABSTRACT FROM AUTHOR]

Background: Gadolinium-doped ceria (GDC) is a promising alternative as a solid electrolyte for intermediate temperature solid oxide fuel cells (ITSOFCs) due to its low operating temperature and its high electrical conductivity. The traditional synthesis processes require extended time for powder preparation. Sol-gel methodology for electrolyte fabrication is more versatile and efficient. Methods: In this work, nanocrystalline ceria powders, with 10 and 20 mol% of gadolinium (Ce0.9Gd0.1O1.95 and Ce0.8Gd0.2O1.9) were synthesized by a modified sol-gel technique, featuring a nitrate-fuel exothermic reaction. GDC tablets were prepared from powders and sintered at 1500°C with a dwell time of 3 hours. The sintered pellets' microstructure (by SEM) and electrical conductivity (by EIS) were evaluated. The powder properties, such as crystalline structure (by XRD), thermal properties (TGA/DTA), particle size and morphology (TEM) and textural properties (BET method) were determined and, in addition, for the first time an accurate chemical structural evolution (FTIR) was studied. Results: Sintered GDC0.8 samples exhibited the maximum theoretical density of 97% and an average grain size of 700 nm. The electrical conductivity vs. temperature showed values ranging from 1.9⋅10-2 to 5.5⋅10-2 S·cm-1 at 600°C and 800°C for GDC with 20 mol% of gadolinium. Conclusions: The methodology investigated showed reduced reaction time, a better control of stoichiometry and low cost. Characterization results demonstrated that these materials can be applied in ITSOFCs due to high conductivity, even at 550°C-600°C. The increased conductivity is related to the improved mobility of gadolinium ions in a high-density structure, with nanometric grains. [ABSTRACT FROM AUTHOR]

To be suitable as electrolytes in intermediate temperature solid oxide fuel cell (IT-SOFC), ceramic precursors have to be characterized by high sintering aptitude for producing fully densified products which are needed for this kind of application. Therefore, synthesis processes able to prepare highly reactive powders with low costs are noteworthy to be highlighted. It has been shown that amorphous coprecipitates based on cerium doped (and codoped) hydrated hydroxycarbonates can lead to synthesized ceramics with such desired characteristics. These materials can be prepared by adopting a simple coprecipitation technique using ammonium carbonate as precipitating agent. As a function of both the molar ratio between carbonate anions and total metallic cations, and the adopted mixing speed, the coprecipitate can be either amorphous, owning a very good morphology, or crystalline, owning worse morphology, packing aptitude, and sinterability. The amorphous powders, upon a mild calcination step, gave rise to the formation of stable solid solutions of fluorite-structured ceria maintaining the same morphology of the starting powders. Such calcined powders are excellent precursors for sintering ceramic electrolytes at low temperatures and with very high electrical conductivity in the intermediate temperature range (i.e., 500–700 °C). Therefore, irrespective of the actual composition of ceria-based systems, by providing an accurate control of both chemical conditions and physical parameters, the coprecipitation in the presence of ammonium carbonate can be considered as one of the most promising synthesis route in terms of cost/effectiveness to prepare excellent ceramic precursors for the next generation of IT-SOFC solid electrolytes. [ABSTRACT FROM AUTHOR]