G. Mascolo

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.

The effect of mineralizers on the crystallization of zirconia gel under hydrothermal conditions

Abstract Solutions of different MOH mineralizers (M=Li + , Na + , K + and (CH 3 ) 4 N + ), with concentration level changing between 0.01 to 3.0 M, were utilized for the crystallization of zirconia gel under hydrothermal conditions at 110°C for 7 days. An increasing crystallization rate of zirconia resulted at increasing concentration of each basic mineralizer. The corresponding and progressive diminution in the crystallite size of products determined the formation of monoclinic, tetragonal and cubic sequence of zirconia. The crystallization rate of gel was found to depend also on the nature of mineralizer solution; higher cationic radius of MOH mineralizer, lower crystallization rate was observed. A nucleation and growth mechanism is proposed for the crystallization of zirconia gel during the hydrothermal treatment in the presence of different basic mineralizers.

Thermal shrinkage of various cation forms of zeolite A

Abstract The thermal shrinkage of Li, Na, K, Cs, ammonium, H, and Ca forms of zeolite A has been investigated by dilatometry up to 650°C. The shrinkages of Li, Na, K, Cs and Ca forms are attributed to the dehydration and partial structural collapse which predominates over the normal thermal expansion and cation migration from one site to another in the zeolitic structure. The ammonium form shows a noticeable shrinkage and a possible catalytic action of the acid sites produced in the zeolite A in the combustion of ammonia evolved upon heating. The H form undergoes collapse at temperatures higher than 350–400°C depending on the heating rate.

Synthesis of anionic clays by hydrothermal crystallization of amorphous precursors

Synthesis of anionic clays ([M2+1−xM3+x(OH)2]x+xnXn− · yH2O; M2+ = Mg, Zn, Ni, etc.; M3+ = Al, Cr, Fe, etc.; x = 0.20–0.50; X = anion; n = anionic charge) by crystallization of amorphous precursors is described. The method involves the crystallization of amorphous gels of M203 in presence of suitable crystallization agents, generally MO oxides. In C02 free conditions, anionic clays characterized by negatively charged interlayers containing OH− anions are produced. The synthesis of anionic clays is discussed in terms of the following parameters: nature and properties of both amorphous precursor and crystallization agent, pH of suspension, crystallization temperature, pressure and period of the hydrothermal treatment.

Crystallization of monoclinic zirconia from metastable phases

Abstract Zirconia gel has been hydrothermally treated at 110°C for 7 days in the presence of increasing concentrations of MOH basic mineralizers (M=Li, Na, K and (CH3)4N). The formation of crystalline products characterized by the presence of single or mixed phases of zirconia either as stable (monoclinic) or metastable forms (tetragonal or cubic) has been detected. At increasing temperatures of calcination, the structure of metastable cubic changes from cubic to tetragonal, then to stable monoclinic; while the structure of metastable tetragonal changes directly into monoclinic. The rate of crystallization and crystal growth of monoclinic zirconia, as well as the incorporation of M+ in the hydrothermally synthesized products are discussed in terms of nature and concentrations of mineralizers.

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.

Cement-lime mortars joining porous stones of masonries able to stop the capillary rise of water

The capillary rise of water in porous stones of masonries can be stopped by interleaving appropriate cement-lime mortars between the building stones. The type of cement in the mortar influences the time requested for the formation of an impermeable lime layer, responsible of the phenomena.

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.

Dilatometry of Na-, K-, Ca- and NH4-clinoptilolite

Abstract The dilatometric behaviour of Na-, K-, NH4- and Ca-clinoptilolite has been investigated. Na-, K- and NH4-clinoptilolite sensibly shrink (from 0.4 to 2.0%) up to about 350°C, while for higher temperatures only very small shrinkages (0.2%) have been detected. These dilatometric behaviours appear to be dominated by dehydration upon heating. Ca-clinoptilolite moderately shrinks up to 300°C (1.0%) and undergoes a more evident shrinkage (2.1%) at higher temperatures. The former shrinkage has been ascribed to dehydration upon heating, the latter higher one to structural damages. The different dilatometric behaviours have been interpreted in terms of the thermal properties of zeolites of the clinoptilolite type.