|Title of Invention||
METHOD FOR THE PREPARATION OF AEROGELS
|Abstract||A PROCESS FOR THE PREPARATION OF AEROGELS INCLUDING : THE EXCHANGE OF THE LIQUID PHASE OF THE EQUAGEL WITH XENON ; THE EXTRACTION OF XENON AND THE POSSIBLE RECOVERY THEREOF.|
|Full Text||Method for the preparation of aerogels
The present invention is referred to a process for the
preparation of aerogels including an exchange phase of the
liquid present in the wet gel by xenon and a following
extraction of very xenon.
Aerogels are one of possible products of a sol-gel process.
Aerogels, up to now, have found application mainly in the
thermoacoustic insulation and in chemical catalysis, as
well as intermediate materials in production of glasses and
glass ceramics; a new application, currently under study,
is an insulation layer of very low dielectric constant in
the production of integrated circuits.
It is known that the sol-gel processes are chemical
processes according to which a material is produced from a
mixture of suitable precursors (called sol), such a
material being typically a simple or mixed oxide either as
a bulk or as a thin layer onto a carrier.
Sol-gel processes are the subject of important published
patent documents and are described, for example, in the
following patents: US-A-4.574.063, US-A-4.680.048, US-A-
4.810.674, US-A-4.961.767 and US-A-5.207.814.
Water, alcohols and water/alcohol mixtures are usually
employed as solvent/diluent for the starting solution, the
precursors may be soluble salts of metals and/or of
metalloids, for example nitrates, chlorides and acetates or
preferably they may be compounds of the general formula
M(OR)n where M is the metal or metalloid atom, O-R is an
alcoholic radical (typically from an alcohol containing
from one to four carbon atoms) and n is the valence of M.
Among the most frequently used precursors in sol-gel
processes are tetramethoxysilane (known as TMOS) with the
formula Si(OCH3)4 and tetraethoxysilane (known as TEOS)
with the formula Si(OCH2CH3)4.
The first phase of a sol-gel process is the precursor
hydrolysis from water that may be present as solvent or be
purposely added in the case of alcoholic solutions,
according to the reaction:
M(-0R)n+nH20- M(OH)n + n ROH (I)
This phase is generally helped by low pH values: typically
between 0 and 3, preferably between about 1 and 2.
The second phase in the sol-gel process is the condensation
of the M(OH)n species according to the following scheme:
M(OH)n + M(OH)n ? (OH)n-1 M-O-M(OH)n-1 + H2O (II)
This reaction, extended to all the M(OH)n species
originally present in solution, leads to an inorganic
oxidic polymer with an open structure inglobating within
its porosity all the solvent originally present or
generated during the hydrolysis. The inorganic, oxidic
polymer so produced is called gel.
To find practical application, the gel needs to be dried by
careful extraction of all liquid from its pores.
A possible method of drying a gel is by simple solvent
evaporation; the dry gel so produced is known as a.
"Xerogel". As it is known to the experts in the field, the
production of xerogels is extremely difficult because of
the strong capillary forces produced by the solvent on the
pore wall: during evaporation, that normally lead to the
destruction of the gel.
An alternative method for producing dry gels is the
supercritical (or hypercritical) extraction of the solvent.
Dry gels produced- by this technique are known as
During the hypercritical drying in suitable autoclaves the
liquid present in the gel is subjected to values of
temperature and pressure that exceeds the critical values
specific for that liquid. At that moment the whole liquid
volume passes from the liquid phase to that of
supercritical fluid and the related capillary forces inside
the pores decrease from the initial value to the reduced
value proper of the supercritical fluid. It is so avoided
the destructive phase of the; meniscus presence inside the
pores that is always produced by evaporations during the
preparation of xerogels. The technique of hypercritical
extraction of a liquid from a gel is described, for
US-A-4.432.956 e US-A-5.395.805. The main problem with this
technique is that alcohols, normally present in the gel
pores have critical pressure Pc typically above 60-70 bars
and critical temperatures Tc above 250oC. Such critical
values mandate the use of autoclaves of high resistance and
relatively high cost. Moreover, if the gel product is in
the form of film on support (for example in the case of a
dielectric insulating film on an integrated circuit), the
critical temperatures of alcohols and esters might be too
high and not compatible with the support or with other
materials present on it.
A well known technique to overcome the problem is through
the liquid exchange in the wet gel before hypercritical
extraction, with a liquid of more favorable critical
constants, particularly of lower Tc. For example, it is
possible to use hydrocarbons, as pentane and hexane, that
have critical temperature in the range of 200°C. Even in
this case, however, the Tc value might be not compatible
with all the applications predictable with aerogels;
moreover the exchange of an hydroalcoholic liquid with an
hydrocarbon, because of the non mixibility of these liquids
would require an additional exchange with intermediate
liquids as, for example, acetone, with the consequent
expansion of the process time and of the cost of recycling
the organic solvents.
Still another possibility is to exchange the hydroalcoholic
liquid with liquid CO2, that has a very favorable Tc value
(about 35oC); also this liquid, however, is not miscible
with water and requires the use of an intermediate exchange
liquid. Acetone, in this case, is not suitable because, if
mixed with liquid CO2, prevents it from entering
hypercritical transition; it is possible to use, as an
intermediate exchange liquid, isoamylacetate; but, also in
this case, the double exchange (acetone first, amylacetate
after) requires process-time excessively long for an
industrial process and undesirable solvent-recycling costs.
The Applicant has now found that it is possible to prepare
the aerogels without no drawback among the ones belonging
to the known art and, according to a preferred embodiment,
through a hypercritical extraction step carried out at
moderate pressure and temperature values which,
furthermore, does not need long times in the preceding
liquid exchange in the wet gel.
As a matter of fact, an object of the present invention is
a method for the preparation of aerogels comprising the
exchange of the aquagel liquid phase with xenon, the
extraction of xenon and the possible recover thereof;
particular advantages are achievable by carrying out the
exchange with liquid xenon and the extraction thereof under,
The aquagel preparation can be made following one of the
preparative process reported in the state of the art; for
example, by hydrolysis of a suitable precursor. In this
case, the process will involve a preliminary step of
hydrolysis/condensation starting from the suitable
A peculiar embodiment of the present invention is the
preparation of the aerogels including:
a) hydrolysis/condensation starting from a precursor;
b) exchange of the liquid in the gel with xenon
c) supercritical extraction of xenon
d) possible xenon recovering.
The metallic precursor undergoing the hydrolysis reaction
may be whatever compound known in the art. Therefore, use
may be made of soluble salts such as, for instance,
nitrates, chlorides and acetate; moreover, use can be made,
according to the best carrying out, of alcoxydes or
alcoxyde mixtures having the general formula:
wherein Me is a metal of the 3rd, 4th and 5th Groups of the
Element Periodic System; n is Me valence; X is R or OR, R
being acid alkyl radical, linear or branched, having a
carbon atom number up to 10.
The hydrolysis is carried out in the presence of a
catalyst, preferably of the acid kind, and water may be the
solvent, or it may be added to the precursor alcoholic
solution; the relevant conditions and procedure are
reported in the known art such as, for instance, the one
corresponding to US patent no.5.207.814 according to which
the hydrolysis is carried out at room temperature and
preferred acid catalysts are chloride acid, nitric acid and
acetic acid. Metal oxides, mainly silicon oxide, can be
added to the prepared sol to modify the properties thereof,
according to, for instance, US patent no.5.207.814.
The liquid present in the wet gel is exchanged with xenon
having critical temperature, Tc = 16,6oC and critical
pressure, Pc = 58,4 bars, over very short times. Once the
exchange is completed, xenon is easily extracted without
any use of autoclave suitable for high temperature and
Xenon, as known, is a gas at atmospheric pressure and
temperature; it belong to the class of the so called rare
gases, and traditional is utilized in discharge lamps, in
solar lamps, in arch lamps for the production of U.V.
radiations, to excite laser cavities, for ionization
chambers, in the bubble chambers for the detection of
For the purposes of the current invention, the xenon is
maintained liquid with pressure above 58,4 bars and
temperature below 16,6°C, preferably below 10°C. To favor
interdiffusion processes in the liquid phase within the
pore structure, xenon temperature should be not as low,
usually not lower than 0°C.
Considering the high cost of xenon, the method of the
invention is preferably used with systems that provide the
recovery of xenon at the end of the extraction process.
A possible scheme of this type of system is indicated in
Fig.l, to which we here refer merely as an example
excluding any restrictive consideration.
System A of Fig.l is an example of process of
hypercritically drying an aquagel using exclusively xenon.
The system includes a reservoir of liquid xenon 1,
connected through a line 2 to at least one, but preferably
several moulds 3, (Fig.l shows a system of 3 moulds)
containing the original aquagels. The moulds are connected
with discharge lines 4, for the liquid exchanged in the
pores (water and/or alcohol) and gaseous xenon at the end
of the supercritical extraction process. All lines 4 are
converging into one suitable collector 5, with proper
temperature under which water and alcohol are solid while
xenon is in the liquid phase., for example at: a temperature
between about -30°C and -40°C. Finally collector 5 is
connected to a reservoir 6 for the recovered xenon, that in
certain models could be the same reservoir 1. The system is
supplied with open/closed valve, identified as V in Fig.l,
that provide selective insulation of any component of the
system. In addition, on the line 2 before each mould 3,
there can be connected flow-meters F, to regulate xenon
flux in each mould; to each mould is connected a pressure
gage P, to control pressure in each mould.
More in general, liquid xenon at temperatures between 0*C
and 16°C can be flown inside the aquagels to replace the
liquid originally contained so that a xenongel is obtained.
The xenongel is then subjected to hypercritical or
supercritical drying at a temperature above 16,6°C and
pressure above 58,4 bars.
1. A process for the preparation of aerogels including:
- the exchange of the liquid phase of the equagel with xenon;
- the extraction of xenon and the possible recovery thereof
wherein such an exchange is accomplished with liquid xenon
and the extraction thereof is accomplished under supercritical
2. The process for the preparation of aerogels as claimed in claim 1,
including a previous phase of hydrolysis/condensation of the suitable
3. The process for the preparation of aerogels as claimed in claim 2,
where the hydrolysis/condensation reaction is carried out starting
from an alkoxyde precursor of the formula:
In which Me is a metal belonging to the 3rd, 4th and 5th Groups of the
Element Periodic System; n is integer and represents the valence of
Me; X is either -OR or -R where -OR is an alkoxyde group and -R is
an organic radical linear or branched with a number of carbon atoms
up to 10.
4. The process for the preparation of aerogels as claimed in claim 3,
where the suitable precursor is preferably tetramethoxysilane,
5. The process for the preparation of aerogels as claimed in claim 3
where the hydrolysis reaction is accomplished in presence of an acid
selected among hydrochloric, nitric or acetic acid.
6. The process for the preparation of aerogels as claimed in claim 1,
where the exchange of the liquid in the aquagel is carried with
liquified xenon at temperature between 0 and 16.6 °C.
7. The process for the preparation of aerogels as claimed in claim 1
where the hypercritical extraction of xenon from the wet gel is carried
a temperature higher than 16.6 °C.
8. The process for the preparation of aerogels as claimed in claim 1
where the hypercritical extraction of xenon is carried at a pressure
higher than 58.4 bar.
9. The process for the preparation of aerogels including the exchange of
the aquagel liquid phase with xenon as claimed in claim 1 wherein it
comprises also a xenon recovering phase at the end of the extraction.
A process for the preparation of aerogels including: the exchange of the liquid
phase of the equagel with xenon; the extraction of xenon and the possible
|Indian Patent Application Number||177/KOLNP/2005|
|PG Journal Number||08/2008|
|Date of Filing||11-Feb-2005|
|Name of Patentee||NOVARA TECHNOLOGY S.R.L|
|Applicant Address||VIA FAUSER, 4, I 28100, NOVARA|
|PCT International Classification Number||C01B 33/158|
|PCT International Application Number||PCT/EP03/006606|
|PCT International Filing date||2003-06-24|