ORIGINAL ARTICLE
Sintering behavior of Ba/Sr celsian precursor obtained from
zeolite-A by ion-exchange method
Mattia Biesuz
1
| Luca Spiridigliozzi
2
| Antonello Marocco
2
|
Gianfranco Dell’Agli
2,3
| Vincenzo M. Sglavo
1,3
| Michele Pansini
2,3
1
Department of Industrial Engineering,
University of Trento, Trento, Italy
2
Department of Civil and Mechanical
Engineering, University of Cassino and
Southern Lazio, Cassino, FR, Italy
3
INSTM - National Interuniversity
Consortium of Materials Science and
Technology, Florence, Italy
Correspondence
Gianfranco Dell’Agli, Department of Civil
and Mechanical Engineering, University of
Cassino and Southern Lazio, Cassino, FR,
Italy.
Email: dellagli@unicas.it
Abstract
(Ba, Sr)-exchanged zeolite A with composition Ba
0.74
Sr
0.22
Na
0.04
Al
2
Si
2
O
8
was
prepared by cation exchange; a mild thermal treatment converts into an amor-
phous 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 densifica-
tion process starts when the amorphous phase reaches its glass transition tempera-
ture (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.
KEYWORDS
celsian ceramics, dilatation/dilatometry, sinter/sintering, spark plasma sintering, zeolites
1 | INTRODUCTION
The monoclinic polymorph of barium feldspar celsian,
BaAl
2
Si
2
O
8
, is a material of great technological interest
due to its peculiar thermal and electrical properties.
1,2
It
has found a variety of industrial applications because of its
unique physicochemical properties, such as high electrical
resistance, limited dielectric permittivity and loss, high
melting point, low thermal expansion coefficient, and does
not undergo to any phase transformation up to 1590°C.
1,2
Such properties have allowed the use of such material as
refractory, high temperature electrical insulator or substrate
for integrated circuits since a long time and their applica-
tion to the field of aeronautics and aerospace has also been
extensively studied.
1,3-5
Several attempts to obtain synthetic celsian have been
made in the past. These consisted in the electrofusion of
kaolinite and BaCO
3
6,7
or in the heat treatment at high
temperature for relatively long time of oxide mixtures,
8
of kaolinite clays and BaSO
4
,
9
of gel produced from
mixed alkoxides,
10
of mixtures of SrCO
3
, Al
2
O
3
, and
SiO
2
,
11,12
of Ba-exchanged geopolymer.
13,14
In other
works hydrothermal synthesis treatments were carried out
starting from gels
15
or synthetic cymrite (BaAl
2-
Si
2
O
8
H
2
O).
16
In most cases, very long processes, very
high temperature and the use of expensive raw materials
(like alkoxides) were required. In addition, one further
drawback of the reported synthesis procedures is that
they lead to the crystallization of the hexagonal celsian
polymorph (hexacelsian) which is stable above 1590°C.
17
Below such temperature, although the stable phase is
monoclinic celsian,
17
hexacelsian is the first polymorph
to nucleate, this effect being ascribed to the simplest
crystal structure which is associated to lower kinetic bar-
rier for nucleation.
18
The early crystallization of hexacelsian gives rise to two
different critical issues. At first, hexacelsian cannot be used
as a refractory material because it undergoes to reversible
transformation into orthorhombic structure at 300°C associ-
ated to detrimental (≥3%) volume change.
17
Then, the
transformation of hexacelsian into monoclinic celsian
occurs after prolonged heating treatments (more than
Received: 26 March 2017
|
Accepted: 28 July 2017
DOI: 10.1111/jace.15117
J Am Ceram Soc. 2017;1–11. wileyonlinelibrary.com/journal/jace © 2017 The American Ceramic Society
|
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