Fig. 6 shows a generalized scheme of structural and chemical stages of zirconia
particles formation while preparing it from solutions.
Size of such primary particles is 20-30Å. These primary nano-particles of
zirconium hydroxopolimer complex are formed by spontaneous composition of
solution initial tetramers involving olation and oxolation processes. It is practically
impossible to extract these particles from the solution as they represent intermediate
cluster (supramolecular) heterostructures between true solutions and colloids. At this
level and with adequate delay the formed nanostructures can come to an equilibrium
state and to form defectless hydroxo polymer nanostructures amounting to 3 nm.
Formation patterns of such zirconium hydroxo polymer structures are discussed in
works [9-13].
A Generalized Scheme of Stabilized Zirconia Nanostructures Formation
While Preparing It From Hydroxides. If no preventive measures are taken up
aggregation of secondary and tertiary particles can proceed when ageing, keeping
hydroxide in mother solution before further usage, or when further thermal dehydration
of xerogels is occurring. In the long run this leads to formation of low-reactive
aggregated zirconia powders with low specific surface. That specific surface area of
the final powders (after being calcined at 750-800°C) amounts to 10-20 m
2
/g when
prepared by traditional coprecipitation method is one of the evidences of this effect. In
this case microparticles size is 35-60 nm according to X-ray data. Powders with such
properties rank among super disperse powders and this ensures decrease of ceramic
products sintering temperature up to 1450-1500°C while the ceramics density is 5.95-
6.01 g/cm
3
.
It is impossible to achieve such parameters of zirconia ceramics with
commercially and laboratory-prepared powders by coprecipitation method even if the
temperature increases to 1600°C.
This is accounted for by aggregated powders due to aggregate strength do not
break down when ceramics is mechanically formed and preserve or change their
structure only at high temperatures while being further sintered. On the basis of the
above study a fractographic scheme of transformations (Fig. 7) of solid phase thermal
decomposition of respective zirconium compounds was suggested. This is know-how
of technology and is not always covered by producers or researchers in scientific and
technical literature.
As shown in Fig. 7 in order to control the structure of zirconia powders it is
necessary to actively affect intermediary products structure with various physical and
chemical factors.
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