FIELD OF THE INVENTION
This invention relates to s process for producing a transparent
st ab i 1 i z eel amorphous c e ram i c gel .
BACKGROUND OF THE INVENTION
Z ire: on is ZrO exists in three monoclinic polymorphis. ro-ZrO transforms to t-phase at .1.170 C and
o
then to c-phase at 2370 C. The high temperature c:-phase, which
h a s p o t e n t i a 1 a p p I i c a t i o n s a r e o >: y g e n s e n s o r s, s o I i d f u e 1 c e 1 1 s
and even several devices is difficult to retain at room tempera-
ture because the transformation is reversible. Efforts have been
made to stabilise it by addition of a small amount of ligO, CaO,
Y 0 , Si N or AIN. The additive controls a refined micro-
structure at a nanometer scale. The common methods of preparation
include vapour phase reactions, co-precipitation process, hydro.....
thermaI proc ess , a 1 ko>; i de or ge 1 proc esses , mech an i c a 1 att ri t i on
a n d c: o m b u s t i o n m e t hi o d s.
None of these methods is successful in producing a single phase
c-ZrO compound. This high energy phase exists in extremely small
crystallites of 'size below 30nm in support of their high surface
energy. It requires a strict control of a homogeneous micro-
structure at this scale and that is not feasible so easily in
these methods which involve a heterogeneous reaction at a re la.....
tively high temperature to ensure a complete decomposition of the
precursor and to ensure a complete elimination of byproduct
impuri t i es.
OBJECTS OF THE INVENTION
An object of this invention is to propose a process for producing
a transparent stabilised amorphous ceramic gel.
Another object of this invention is to propose a process far
producing a transparent stabilised ceramic gel which uses commer-
cially available raw materials.
Still another object of this invention is to propose a proces for
producing a transparent stabilised ceramic gel which involves
f e wer st e p s t ha n t h e p roc e ss e s of th e k n own art.
Yet another object of this invention is to propose a process for
producing a transparent stabilised ceramic gel which is a highly
pure c—ZrG powder.
j...
Further objects and advantages of this invention will be more
app a i-en t f rom th e ensu i ng d esc r i p t i on .
BRIEF DESCRIPTION OF THE INVENTION
According to this invention is provided a process for the prepa-
ration of a transparent amorphous ceramic gel comprising the
steps of
providing an aqueous solution of a zirconium salt, adding thereto
an aqueous solution of an aluminium salt with stirring, followed
by addition of ammonia solution with stirring to obtain a gel
which is dried to obtain the amorphous ceramic gel.
In this invention, a transparent amorphous ceramic gel is deve-
4+- 3+
loped by hydrolysing Zr cations with 5-10 mol% Al additive in
an aqueous solution. Al additive supports gelation of hydro-
4 +
lyzed Zr in a polymeric network structure. Thermal decompo-
o
sit ion and annealing of gel at 35(3—85*3 C results in recrystal 1 i-•
3+
zation of a single phase finely divided loose nanopowder in Al
stabilised c-ZrO . It occurs via a refined microstructure in an
amorphous ceramic precursor of decomposed gel at as low tempera-
o
ture as 200 C by a self-controlled process. The obtained sample
o
is stable as such to a temperature as high as B50 C.
A transparent gel is formed by a simple reaction of three
re act ants (i) ZrOCl . 6H 0, A1C1 „6H 0, and ; c e s s w a t e r a t r o o m t e m p era t u r e . W h e n dried, i t
appears in a transparent solid metalloceramie gel within a __n\orvi;h
or so depending on its composition. This is possible due to the
AIO(OH) additive as will be apparent from the ensuing examples.
3 +
5-20 moIK additive of Al cations effectively supports gela-tion
in a stagle amorphous polymeric structure as per its >: —ray
diffraction,. Several batches of the reactions are carried out tay
varying the AIO(OH) content in the final gel between 5 and 20
mol'/i. All of them result in a transparent solid gel or- polymer.
o
The gelation reaction occurs the best 25 C, or a preferably lower
o
temperature in the range of 5-10 C, with an average concen-
tration of the precursor solution in the 0.2—0.5M range.
o
On heating at r^ 200 C in air, a controlled thermal decomposition
of metalloceramic gel occurs in a refined amorphous ceramic
powder of decomposed gel. Latter is dispersed and boiled in water
and then filtered in order to remove away residual byproduct
impurities, especially CI anions, if any (tested by adding an
AgNO solution. The recovered powder recrystal1ised promptly into
3+
a further refined powder of Al stabilised c-ZrO at temperature
O
as small as 350 C. It has an average size of crystallites as
small as 5-10nm and that does not grow above 20nm until the
o
temperature is below 850 C. . ...
The invention will now be explained in greater detail with the
help of the following non-limiting examples.
EXAMPLE 1
The proposed hydrolysis reaction is carried out by adding NH Oh
4
(dissolved in water- in 0 „ 2M solution) to 0.5M ZrOCl .8H 0
2 2
o
solution in water at 25 C, or a preferably lower temperature in
o
the range of 5-10 C. NH OH is added in steps by stirring the
4
total solution with a magnetic stirrer. A white precipitate
occurs in a hydrolysed ZrO(OH) . >:H 0 product. It was washed in
4 2
o
distilled water, filtered, dried and then heated at 35*3 C to
decompose into a desired c--ZrO nanopowder. It is not very
2
c::<
stable. It converts to m-ZrO at 500 C.
2
EXAMPLE 2
The . above reaction is repeated by adding 0.5M A.1C1 . 6H 0 (in
1...' £..
o
water) to the (3.5M ZrOCl .SH 0 solution in water at 25 C. The
'"7" O
volumes in the two solutions were taken such that the mixture
contains 20 mo IV. of A1CI . 6H 0 and 80 mol*/ of ZrOCl . SH 0. A
3 2 2 2
white co.....precipitate occurs in a hydrolysed product in A10(0H)-
ZrO(OH) . >:H 0 on adding the NH OH. Stirring for 10-30 rains, helps
4 2 4
to get a clear transparent solution at pH 2,0. It dries into a
stable transparent gel at room temperature. On heating, it
3+ o
converts into an Al stabilised c-ZrO nanopowder at 350-850 C.
A pure single phase ceramic powder is obtained in this invention.
Furthermore, as no any other chemical additive has been used
during the reaction, it yields a highly pure powder in a single
single method if compared with sol gel or other conventional
methods in use to obtain similar ceramics. The final furnish c-
2 rO p roduc t i s ota t a i n ed i n a f e w s t ep s.
2 '-----—-----—__^^
WE CLAIM:
1. A process for the preparation of a Zirconia nanopowder comprising the steps of
providing a 5 to 20 mol % of an aqueous solution of a zirconium salt, adding
thereto a 5 to 20 mol % of an aqueous solution of an aluminium salt with stirring,
followed by addition of ammonia solution with stirring for upto 30 minutes to
obtain a gel which is dried to obtain an amorphous ceremic gel, the gel being
subjected to further heat treatment to obtain the zirconia nanopowder.
2. The process as claimed in claim 1 wherein said zirconium salt is zirconium
oxychloride and the aluminium salt is aluminium chloride.
3. The process as claimed in claim 1 wherein the average concentration of the
solution of the zirconium-aluminium salt is between 0.2and 0.5 M.
4. The process as claimed in claim 1 wherein a 0.2M ammonia solution is added.
5. The process as claimed in claim 1 wherein the zirconium and aluminium salt
solutions are mixed at a temperature in the range of 5-25°C, preferably 25°C.
6. The process as claimed in claim 1 wherein after addition of ammonia, the reaction
mixture is stirred for a period ranging from 10-30 mins at about pH 2.
7. The process as claimed in claim 1 wherein the ammonia solution is added
stepwise with stirring.
i
8. The process as claimed in claim 1 wherein the drying time of the gel is dried at
room temperature.
9. The process as claimed in claim 1 wherein the drying time of the gel is about 30
days.
10. The process as claimed in claim 1 wherein the thermal decomposition is carried
out at a temperature in the range of 350-850°C.

A process for the preparation of a Zirconia nanopowder comprising
the steps of providing a 5 to 20 mol % of an aqueous solution of a
zirconium salt, adding thereto a 5 to 20 mol % of an aqueous solution
of an aluminium salt with stirring, followed by addition of ammonia
solution with stirring for upto 30 minutes to obtain a gel which is
dried to obtain an amorphous ceremic gel, the gel being subjected to
further heat treatment to obtain the zirconia nanopowder.