Gianfranco Dell'Agli

Hydrothermal synthesis of ZrO2–Y2O3 solid solutions at low temperature

Abstract Ultrafine powders of ZrO2–Y2O3 solid solutions have been synthesized by hydrothermal treatment at 110°C. Zirconia gel, crystalline Y2O3 and various mineralizing solutions have been utilized as precursors for the hydrothermal synthesis. Yttria-stabilized zirconia (YSZ) with different Y2O3 content and characterized by different crystallite sizes have been produced by changing the hydrothermal treatment temperature, and the nature and concentration of the mineralizer solution. The role of mineralizer solutions on the crystallization-stabilization of zirconia gel at low temperature of hydrothermal treatment is discussed.

Agglomeration of 3 mol% Y–TZP powders sythesized by hydrothermal treatment

Mechanical mixtures of zirconia gel and crystalline Y2O3 (3 mol%) have been hydrothermally treated at 110C for 7 days in the presence of diAerent mineralizer solutions. Taking the K2CO3/KOH molar ratio constant and equal 3, the total concentration of the mineralizer solutions was changed between 0.25 and 3.0 M. Predominant tetragonal (T) ZrO2, characterized by diAerent crystal size and diAerent degree of agglomeration resulted by changing the concentration of mineralizer solution. The crystallization rate of YTPZ based powders, as well as the texture of the corresponding sintered products, are discussed in terms of concentration of the mineralizer solutions. # 2000 Elsevier Science Ltd. All rights reserved.

Low temperature hydrothermal synthesis of ZrO2-CaO solid solutions

Nanosized powders of ZrO2-CaO solid solutions have been synthesized by hydrothermal treatment at 110°C. Amorphous hydrous ZrO2, CaO freshly prepared by thermal decomposition of CaCO3 and MOH (M = Na, Li) mineralizer solutions were employed as precursors for the hydrothermal synthesis. Calcia-stabilized zirconia (CSZ) with different CaO content and characterized by different crystal sizes have been produced by changing the hydrothermal treatment temperature, the reaction time and the mixture composition of precursors. The combined effect of both MOH and CaO on the crystallization-stabilization of zirconia at low temperature of hydrothermal treatment is discussed.

Electrical and microstructural characterization of ceramic gadolinium-doped ceria electrolytes for ITSOFCs by sol-gel route

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.

Sinterability of 8Y–ZrO2 powders hydrothermally synthesized at low temperature

Abstract Nanometric yttria (8 mol%)-stabilized zirconia powders were hydrothermally synthesized at 110 °C for 7 days in the presence of dilute (0.20 M) or concentrated (2.0 M) solutions of (KOH+K2CO3) mineralizer. Zirconia xerogel, crystalline Y(OH)3, crystalline Y2O3 and a xerogel of coprecipitated (Y–Zr) hydroxide were used as starting materials. Setting the content of yttria constant and equal to 8 mol%, three types of mixtures were tested. Zirconia xerogel in mixture with crystalline Y2O3, zirconia xerogel in mixture with crystalline Y(OH)3 and, finally, a xerogel of coprecipitated (Y–Zr) hydroxide were hydrothermally treated. The different characteristics of the resulting powders are discussed in terms of both the mineralizer concentration and the type of Y-based precursor used in the hydrothermal treatments, respectively. Weakly agglomerated cubic ZrO2 powders with primary particles bigger in size and without any preliminary treatment show better performances when they are directly sintered at 1500 °C.

A study on the stability of superconducting YBa2Cu3O7−x phase

The thermal stability of the superconducting phase of nominal composition YBa2Cu3O7−x-sintered pellets has been studied with respect to different temperatures (ranging from 300 to 950° C), time (ranging from 1 to 72 h), oxygen partial pressure (from 4 Pa to 1 atm) and carbon dioxide partial pressure (from 10−4 Pa to 1 atm). Annealed samples were characterized by X-ray diffraction analysis, optical microscopy, and resistive measurements of the superconductive transition temperature. A stability field of the orthorhombic and tetragonal phases was obtained, showing a region of coexistance. The decomposition of the 1 2 3 phase is found to be strongly influenced by the presence of a small amount of CO2 (1 p.p.m.) in the sintering atmosphere. A sintering process is proposed to avoid the formation of by-products.

Thermal behaviour of (NH4)2V6O16 prepared by hydrothermal crystallization

Abstract Pure samples of (NH 4 ) 2 V 6 O 16 were prepared by hydrothermal crystallization of amorphous ammonium vanadate hydrate in the presence of LiOH solution. By changing the conditions of the hydrothermal treatment, crystallites differing by one order in size resulted. The ammonium hexavanadate thus prepared was thermally investigated by simultaneous TG and DTA, as a function of heating rate under a constant stream of different gases.

Electrical Behaviour and Microstructural Characterization of Magnesia Co-doped Scsz Nanopowders Synthesized by Urea Co-precipitation

Electrical Behaviour and Microstructural Characterization of Magnesia Co-doped ScSZ Nanopowders Synthesized by Urea Co-precipitation Grazia Accardo, Gianfranco Dell’Agli, Domenico Frattini, Luca Spiridigliozzi, Suk Woo Nam , Sung Pil Yoon a Fuel Cell Research Center, KIST-Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu 136-791, Seoul, South Korea b Department of Civil and Mechanical Engineering and INSTM Research Unit, University of Cassino and Southern Lazio, Via G. Di Biasio 43, 03043, Cassino (FR), Italy d16605@kist.re.kr

Proton Conductivity of Amorphous Hydrated Zirconia-Yttria Solid Solutions

Mechanical mixtures of zirconia xerogel with variable content of crystalline Y2O3 up to 25 mol%, were hydrothermally treated by microwave route at 110 °C for 2 hours in the presence of 0.2 M solution of (KOH+K2CO3) mineralizer. The resulting amorphous hydrated zirconia-yttria solid solutions with a maximum solubility of Y2O3 content between 20 ~ 25 mol%, showed a remarkable reduction of the surface area at the increasing Y2O3 content of the starting mixture. The as-synthesized products and the corresponding calcined powders at 400 °C were uniaxially pressed into pellets (10 x 7 x 2 ~ 4 mm, in width) at 150 MPa. Conductivities were measured at 25 °C by AC impedance method with a frequency range from 10 Hz to 1 MHz with the pellets equilibrated either under silica gel or under increasing relative humidity (RH) up to ~90 %. The effects of composition, surface area, calcination temperature and relative RH on the proton conductivity of the amorphous solid solutions are discussed.