G. Dell’Agli

Thermal transformation of Ba-exchanged A and X zeolites into monoclinic celsian

Abstract The thermal transformation of Ba- and (Ba+Li)-exchanged precursors of zeolites A and X into monoclinic celsian has been investigated by X-ray diffractometry, differential thermal analysis, and thermodilatometry. Upon the proper thermal treatment either Ba-exchanged zeolite A or X give rise to the following transformations: zeolite→amorphous phase→hexacelsian→monoclinic celsian. Zeolite A-based precursors are more reactive than zeolite X-based precursors allowing all the reported transformations to occur at far lower temperatures starting from Ba-exchanged zeolite A rather than from Ba-exchanged zeolite X. In particular, starting from zeolite X-based precursors, fully monoclinic celsian may be obtained after 24 h at 1550°C in the absence of Li and after 6 h at 1100°C in the presence of Li, whereas starting from zeolite A-based precursors it may be obtained after 6 h at 1100°C in the absence of Li and by slow heating (1°C/min) up to 900°C in the presence of Li.

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.

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.

Microstructural and electrical investigation of flash-sintered Gd/Sm-doped ceria

Ceria-based ceramics can be considered among the most promising solid electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFC). In the present work, variously doped nanocrystalline ceria powders were flash-sintered, and the role of doping (Gd and Sm, 5–20xa0mol%) and sintering aid (Li and Co) on the final microstructure and the electrical behavior was investigated. Gd- or Sm-doped nanocrystalline ceria powders were synthesized by co-precipitation method using ammonia solution as precipitating agent. The synthesized nanopowders were characterized by DSC-TG, XRD, and nitrogen physisorption analysis. The nanopowders were isostatically pressed and flash-sintered. The relative density was measured by hydrostatic balance, and the corresponding microstructure was observed by SEM. The electrical behavior was studied by EIS. Flash-sintered powder pellets showed different behaviors depending on the dopant and sintering aid. The electrical conductivity of the samples increased by increasing the relative density. Fully dense Gd-doped ceria samples, synthesized by co-precipitation, were obtained by flash sintering in very short times at 700xa0°C. The total conductivity was comparable to that measured on samples sintered with conventional route at much higher temperatures such as 1500xa0°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.

Effect of residual Na on the low temperature synthesis of monoclinic celsian from zeolite Ba-A

Publisher Summary This chapter presents the effect of residual Na on the low temperature synthesis of monoclinic celsian from zeolite Ba-A. Monoclinic celsian is used as a refractory, high temperature electrical insulator, or as substrate for integrated circuits on account of its good thermal and electrical properties, although its synthesis gives rise to considerable problems. A long term study on the thermal transformation of Ba-exchanged zeolites into monoclinic celsian was undertaken. This study started from the work of Subramanian who crystallized alkaline-earth and alkaline framework aluminosilicates by thermal treatment of cation exchanged zeolites at temperatures slightly higher than 1000 °C for some hours. This technique seemed to avoid the drawbacks arising in the traditional syntheses of monoclinic celsian. Interesting results were obtained in studying the thermal transformation of Ba-exchanged zeolites in monoclinic celsian. It was found that, upon thermal treatments at temperatures >1000 °C, zeolite Ba-A gave rise to the following transformations: zeolite → amorphous phase → hexacelsian → monoclinic celsian. It was found that thermal treatments at 1100 °C for 6 hours and 1300°C for 22 hours were sufficient to transform samples of zeolite Ba-A, containing 0.58 and 0.20 meq/g Na residual amount, respectively, into fully monoclinic celsian. These data reveal that zeolite Ba-A does appear a very reactive precursor as the full conversion into monoclinic celsian was obtained in far milder conditions than those reported in the literature.