Glass-ceramic Li-Al-Si-O composition and process for its production

In the process according to the invention, a gel is prepared by hydrolysis and polycondensation of precursor compounds which comprise at least silicon alcoholate and aluminum alcoholate and inorganic compounds of lithium and/or magnesium, the solvent is removed, and the gel obtained is subjected to a heat treatment resulting in dehydration and oxidation, then to compacting and ceramisization. The composition obtained is substantially in the form of solid ceramic solutions, such as .beta.-spodumene, cordierite and mullite.

The invention relates to glass-ceramic compositions based on silica, 
alumina and lithium oxide, it being possible for the last-mentioned to be 
replaced completely or in part by magnesium oxide. 
The compositions of this type are conventionally obtained by fusing oxides 
and/or carbonates. Some time ago, another route called sol-gel was 
discovered, which is based on a hydrolysis and polycondensation reaction 
in dissolved medium of precursor compounds, of which at least the aluminum 
and silicon compounds are alcoholates, with the formation of a sol, then 
of a gel, which is followed by a heat treatment at elevated temperature. 
This sol-gel route has several advantages compared with the conventional 
route: the product is purer, more homogeneous and has a more accurate 
composition due to the absence of evaporation loss. The expenditure of 
energy is reduced, since the temperature used is lower. The composition 
obtained, which, if necessary, is ground in order to eliminate 
agglomerates, yields a powder of large and specific surface area and very 
reactive, enabling dense pieces, i.e. of low or zero porosity, to be 
produced more easily. 
The object of the invention is to provide such a glass-ceramic composition 
obtained by the sol-gel route, which has a low thermal expansion 
coefficient and can therefore tolerate without harm large or rapid 
variations in temperature (thermal shocks) when it is in the form of a 
dense piece. 
Another object of the invention is to provide a composition of this type 
which is suitable in particular for forming the matrix of fiber-reinforced 
ceramic composite materials. 
The invention provides a glass-ceramic composition whose main components 
are SiO.sub.2, Al.sub.2 O.sub.3 and Li.sub.2 O and/or MgO and which is 
obtained by the sol-gel route from silicon alcoholate: and aluminum 
alcoholate and lithium compounds and/or magnesium compounds, which 
composition is substantially in the form of solid ceramic solutions, such 
as .beta.-spodumene, cordierite and mullite. 
It has been found that when the glass-ceramic composition is substantially 
in the form of such solid solutions, which themselves have a low expansion 
coefficient, it does not give rise to phase transitions due to an increase 
in temperature, except for the conversion of a glass phase which may still 
remain into a solid solution so that as a whole it has a low expansion 
coefficient. This low expansion coefficient and other properties of the 
composition also favor the formation of composite materials with fiber 
reinforcements, which have good thermomechanical characteristics. 
The basic formulae for .beta.-spodumene, cordierite and mullite are 
Al.sub.2 O.sub.3 .multidot.Li.sub.2 O.multidot.4SiO.sub.2, 
2MgO.multidot.2Al.sub.2 O.sub.3 .multidot.5SiO.sub.2 and 2SiO.sub.2 
.multidot.3Al.sub.2 O.sub.3, respectively. Since they are solid solutions, 
the composition of these phases can of course deviate from these formulae. 
Apart from the main components indicated above, the composition according 
to the invention can contain at least one component chosen from BaO, 
Nb.sub.2 O.sub.5 and B.sub.2 O.sub.3 , this list being non-limiting. The 
effect of such additives is especially that they modify certain properties 
of the composition, and/or the composite materials which it makes it 
possible to obtain, as a function of the intended application. 
The production of the structure according to the invention depends on 
several factors, one of which is of course the chemical formula of the 
composition which has to be compatible with the predominant presence and 
stability of at least one solid solution. One skilled in the art can 
easily determine by means of the phase diagram of the system in question 
whether an envisaged formula may be suitable in this respect. 
The inventors have found that this is the case in particular for a 
composition comprising ax-b mol of Li.sub.2 O, c mol of SiO.sub.2, b mol 
of BaO and a(l-x) mol of MgO per mole of Al.sub.2 O.sub.3, it being 
possible for a, b, c and x to vary from 0.4 to 1, 0 to 0.1, 3 to 8 and 0 
to 1, respectively. 
The structure according to the invention likewise implies a development 
process which avoids maintaining an amorphous glass phase, likewise avoids 
the appearance of damaging crystalline phases, such as cristobalite, and 
furthermore allows complete conversion of the eucryptite by a heat 
treatment. 
To this end, the invention proposes a process for the production of a 
composition such as defined above, which comprises the following steps; 
a) preparation of a gel by hydrolysis and polycondensation of precursor 
metal compounds in solution in a solvent: 
b) removal of the solvent; 
c) grinding of the gel, if necessary; and 
d) dehydration and oxidation of the gel as well as compaction and 
ceramisization, by increasing the temperature. 
The precursors used are preferably silicon alcoholates and aluminum 
alcoholates, more particularly tetraethyl orthosilicate and aluminum 
sec-butoxide, and inorganic compounds of lithium and/or magnesium and, if 
the case arises, of barium and niobium, more particularly lithium nitrate 
and/or magnesium nitrate, and, if the case arises, barium nitrate and 
niobium chloride. 
Step a) is advantageously divided into two phases: 
a1) prehydrolysis of the silicon alcoholate by a portion of the total 
hydrolysis water; and 
a2) reaction of the product from phase a1) with the other precursors and 
the remainder of the hydrolysis water. 
Step d) can also be divided into two phases: 
d1) oxidation by gradual heating in air over a period of several hours up 
to a temperature of about 500.degree. C.; and 
d2) compaction and ceramisization treatment first at a temperature at the 
most slightly above the fusing temperature of the composition. 
The dehydration and oxidation phase yields, after grinding, a powder which 
has a large specific surface area, is particularly reactive and can easily 
and efficiently be compacted. 
This compaction can be achieved by subjecting the composition in the form 
of a powder to pressing during the ceramisization treatment. 
When the piece exhibits a glass phase after the compaction and 
ceramisization treatment, this phase can be converted into a solid ceramic 
solution by a second heat treatment at a temperature slightly below the 
fusing temperature of the composition. 
In order to obtain a fiber-reinforced composite material with refractory 
ceramic matrix, the fibers can be incorporated in the powder of the 
glass-ceramic composition before the compacting heat treatment. This 
procedure is applied in particular to fibers based on silicon carbide. 
When the silicon carbide fibers contain an appreciable amount of oxygen and 
carbon on the surface, the glass-ceramic composition preferably contains 
at least one component other than SiO.sub.2, Al.sub.2 O.sub.3 and Li.sub.2 
O, for example Nb.sub.2 O.sub.5, intended to limit adhesion of the 
composition to the fibers. 
Other characteristics and advantages of the invention will become evident 
from the detailed description given below of a few exemplary embodiments. 
First of all, the preparation method of the gel will be described. Only the 
list of precursor compounds and their quantities vary from one example to 
the next as a function of the formula of the composition to be prepared. 
These precursors are: 
tetraethyl orthosilicate Si(OC.sub.2 H.sub.5).sub.4 (TEOS) 
aluminum sec-butoxide Al(OCH(CH.sub.3)C.sub.2 H.sub.5).sub.3 (ASB) 
lithium nitrate LiNO.sub.3 and, if the case arises, 
magnesium nitrate Mg(NO.sub.3).sub.2.multidot. 6H.sub.2 O, containing 42.2% 
by weight of water 
barium nitrate Ba(NO.sub.3).sub.2 
niobium chloride NbCl.sub.5. 
The solvent chosen, which is the same for the precursors and the hydrolysis 
water, is isopropanol. 
Nitric acid is added in the form of a 70% by weight aqueous solution, which 
has the effect that precipitation of AlOOH in the course of hydrolysis is 
prevented and which favors the preparation of a microporous gel exhibiting 
high reactivity on sintering. 
Isopropyl alcohol is used in an amount of 10 mol per 1 mol of TEOS and the 
nitric acid in an amount of 0.2 mol per 1 mol of TEOS. 
The amount of prehydrolysis water, which includes that brought in by 
isopropanol and nitric acid, is 1 mol per mole of TEOS. The remaining 
amount of hydrolysis water, which includes that brought in by isopropanol 
and, if the case arises, by magnesium nitrate, is 3 mol per mole of ASB 
plus 4 mol per mole of TEOS. 
TEOS, half of the solvent, the prehydrolysis water and the nitric acid are 
introduced with stirring into a three-neck reactor equipped with a heating 
mantle, a stirrer and a distillation column. The mixture is refluxed 
(82.degree. C.) for 30 minutes. 
The purpose of this prehydrolysis phase is to avoid preferential hydrolysis 
of ASB with precipitation of AlOOH, which would take place if TEOS, ASB 
and water were to be brought together at the same time, due to the low 
reactivity of TEOS. In the course of this phase, TEOS is partially 
hydrolyzed according to the reaction: 
EQU Si(OR).sub.4 +H.sub.2 O.fwdarw.Si(OR).sub.3 OH+ROH 
and the product of partial hydrolysis will react with the other precursors 
in the following phase. 
At the end of the abovementioned 30 minutes, the temperature of the reactor 
is lowered by 10.degree. C. in order to interrupt the refluxing. The ASB 
fluidized by dilution in a small amount of isopropanol is then added in 
small quantities (four or five portions). Reflux is then continued for 
another 45 minutes. Heating is then discontinued, and lithium nitrate and, 
if the case arises, magnesium nitrate, dissolved in the remaining 
isopropanol, are introduced with continued stirring. 
When the mixture reaches ambient temperature, the hydrolysis water is 
added, in which, if the case arises, the barium nitrate is dissolved, and 
the mixture is stirred for another 5 to 10 minutes in order to obtain a 
homogeneous sol, which is introduced into sealed containers. A gel is 
formed within a few hours at 45.degree. C. or overnight at ambient 
temperature. After gelling, the containers are opened and kept in an oven 
at 90.degree. C. for 24 to 48 hours in order to evaporate the solvent. The 
gel is then, if necessary, ground in order to eliminate agglomerates and 
treated in a through-circulation oven for the oxidation step which leads 
to the glass-ceramic composition. 
The following heat cycle is suitable in particular for this step: heating 
at 3.degree. C./min from ambient temperature to 250.degree. C. and at 
10.degree. C./min from 250 to 500.degree. C. holding at temperature for 4 
hours every 50.degree. C. from 250 to 500.degree. C., then cooling at a 
rate of 20.degree. C./min down to ambient temperature. However, this cycle 
can be simplified by omitting certain temperature-holding stages. This 
gives a very reactive amorphous powder having a specific surface area of 
about 300 m.sup.2 /g. This powder is advantageously redispersed in a mill 
containing zirconium dioxide beads in order to break up the agglomerates 
of individual grains and screened so as to retain only the grains of 
diameter below 50 .mu.m.

EXAMPLE 1 
Composition Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 
The gel is prepared, then dehydrated and oxidized, and the resulting 
composition is ground and screened as just described, using amounts of 
precursors corresponding to the final formula 0.45 Li.sub.2 O-Al.sub.2 
O.sub.3-SiO.sub.2. In order to obtain a compact ceramic sintered piece, 
the powder obtained is subjected to a compacting and ceramisization heat 
treatment in the solid state, which is carried out under a vacuum for 
complete removal of water and in which the powder is pressed under a 
pressure of 35 MPa. The heat cycle used is as follows: heating at 
20.degree. C./min up to 500.degree. C., followed by holding at this 
temperature for 60 min; heating at 10.degree. C./min from 500 to 
1300.degree. C., interrupted by holding at 780.degree. C. for 20 min; 
cooling at 30.degree. C./min down to ambient temperature. 
The density of the product obtained is 2.5 with a closed porosity of 1% 
measured by immersion in water. The composition is made of 
.beta.-spodumene and mullite, and its average expansion coefficient 
between 20 and 1000.degree. C. is 2.0.multidot.10.sup.-7 /.degree. C. 
The breaking stress .sigma., the modulus of elasticity E and K.sub.1 C 
taken from the IS polynomial formulae, which were determined by the 
three-point bending test, are as follows: 
.sigma.=111 MPa 
E=80 GPa 
K.sub.1 C=1.45 MPa.multidot.m.sup.1/2 
For .sigma.and E, the force is applied perpendicularly to the direction of 
the pressing of the powder. The distance between supports is 16 mm and 
length/thickness ratio is equal to 15. For K.sub.1 C, the force is applied 
perpendicularly to the direction of pressing. The distance between 
supports is 15 mm, the notch depth 0.9 mm and its width 70 .mu.m. The 
height/notch depth ratio is 4, and the ratio of distance between supports 
to height is equal to 4.17. The rate of descent is 0.2 mm/min. These test 
conditions are likewise valid for the examples which follow, unless stated 
otherwise. 
EXAMPLE 2 
Composition Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 -Nb.sub.2 O.sub.5 
The amounts of reactants are the same as in Example 1, niobium chloride 
being added in such an amount that the final composition contains 3% by 
weight of Nb.sub.2 O.sub.5. The compacting and ceramisization heat 
treatment is carried out above the fusing temperature of the composition, 
the latter having the property of recrystallizing in a homogeneous manner 
in the form of solid solutions without addition of nucleating agents and 
after partial or complete fusing of the crystalline phases developed in 
the course of the first heat treatment. In order to avoid the formation of 
porosity as a result of the presence of combined residual water at the 
time of fusing, it is necessary to completely eliminate this water 
beforehand. 
To this end, the product is treated, after dehydration and oxidation, with 
ammonia at 500.degree. C. (grinding and screening, if necessary, having 
been carried out beforehand). The powder is first maintained at 
320.degree. C. under nitrogen for 4 hours, in order to eliminate the water 
adsorbed on the surface. It is then flushed with ammonia, and the 
temperature is increased to 600.degree. C. at a rate between 1 and 
3.degree. C./min, in order to replace the remaining OH groups by 
nitrogen-containing groups. Heating under nitrogen is continued at a rate 
of 10.degree. C./min up to 1200.degree. C. The powder is then placed in a 
press and heated to 1000.degree. C. at a rate of 10.degree. C./min. At 
this temperature, a mechanical pressure between 16 and 35 MPa is applied 
while heating the powder at a rate of increase of 5.degree. C./min, during 
which the thickness of the product is monitored by means of a displacement 
transducer. 
When the temperature reaches 1330.degree. C. a decrease in thickness 
indicates the beginning of compaction. This temperature is kept constant 
until the thickness stabilizes, for example for 3 minutes. The composition 
then has a viscosity of about 1.5.multidot.10.sup.9 Pa.s. The pressing is 
interrupted and the product is cooled to ambient temperature. 
The product obtained has no detectable porosity. It is mainly made up of 
.beta.-spodumene and mullite with a small fraction of amorphous phase. 
Infrared spectography shows the absence of water, while the same method 
shows the presence of 224 ppm of water for the composition from Example 1. 
Nor was any presence of nitrogen detected. 
The product has the following properties: 
.alpha..sub.20.sup.1000 =2.41.multidot.10.sup.-6 /.degree. C. 
.sigma.=128 Mpa 
E=96 GPa .sigma.and E having been determned as in Example 1. 
Reheating at 1250.degree. C. for 5 minutes enables the amorphous phase to 
be made to disappear. The following properties are then obtained: 
.alpha..sub.20.sup.1000 =2.15.multidot.10-.sup.6 /.degree. C. 
.sigma.=137 MPa 
E=106 GPa. 
EXAMPLE 3 
Composition Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 +short fibers 
3 composite materials containing 5%, 10% and 20% by volume, respectively, 
of silicon carbide whiskers marketed by SUMITOMO are prepared. 
The dehydrated and oxidized glass-ceramic powder is prepared as in Example 
1, then dispersed under ultrasound in an isopropyl alcohol bath containing 
5% by volume of nitric acid for stabilizing the suspension. The latter is 
vigorously agitated, and the fibers likewise dispersed under ultrasound in 
isopropyl alcohol are added. The mixture obtained is agitated for another 
15 minutes under ultrasound, then filtered through a filter made of glass 
microfibres capable of retaining particles of a diameter of greater than 
0.5 .mu.m. 
The compacting and ceramisization treatment is carried out under the 
conditions below: increase in temperature by 20.degree. C./min up to 
450.degree. C. under vacuum, followed by holding this temperature for 45 
min; continuation of the heating at 10.degree. C./min up to 1000.degree. 
C. under nitrogen with pressing under 16 MPa and holding at 780.degree. C. 
for 20 min; continuation of the increase at 20.degree. C./min up to 
1300.degree. C. under nitrogen and pressing at 16 MPa. 
The table below gives the properties of the composite materials obtained as 
a function of the amount by volume of the fibers Vf. 
______________________________________ 
Vf Closed Open .sigma. 
E K.sub.1 C 
% porosity % porosity % 
MPa GPa MPa.m.sup.1/2 
______________________________________ 
5 9.8 4.6 99 70 1.55 
10 1.4 4.1 164 105 2.05 
20 1.9 1.23 278 135 2.94 
______________________________________ 
The expansion coefficient for a Vf of 20% is 2.84.multidot.10.sup.31 6 
/.degree. C. between 20.degree. and 1000.degree. C. 
EXAMPLE 4 
Composition Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 -BaO+short fibers 
The glass-ceramic powder is prepared as in Examples 1 and 3, but the amount 
of lithium nitrate reduced and barium nitrate is added such that the 
formula of the glass-ceramic composition is 0.05 BaO-0.4 Li.sub.2 
O-Al.sub.2 O.sub.3 -3 Sio.sub.2. The purpose of introducing BaO is to 
stabilize the composition beyond 1000.degree. C. by inhibiting the 
formation of cristobalite, which is a crystalline phase giving rise to a 
phase transition at about 300.degree. C., which could lead to cracking. 
The powder/whisker mixture is prepared as in the previous example at an 
amount by volume of 20%. The compacting and ceramisization treatment 
comprises fusing of the glass-ceramic composition: heating by 10.degree. 
C./min under vacuum up to 980.degree. C., holding for 30 minutes at 
480.degree. C. and for 15 min at 780.degree. and 980.degree. C.; 
continuation of the heating at 10.degree. C./min under nitrogen with 
pressing at 16 MPa up to 1325.degree. C., holding this temperature for 5 
min; cooling at 30.degree. C./min under nitrogen down to ambient 
temperature without pressing. 
The composite material obtained has a relative density (apparent 
density/density of the completely dense compound) of 0.99, indicating 
excellent compaction. X-ray analysis shows the predominant presence of 
.beta.-spodumene and mullite and a small amount of amorphous phase. At 
this stage, the material has the following properties: 
.sigma.=317 MPa 
E=136 GPa 
K.sub.1 C=3.07 MPa.m.sup.1/2 
.alpha..sub.20.sup.1000 =4.61.multidot.10.sup.-6 /20 C. 
A subsequent heat treatment at 1280.degree. C. in air makes it possible to 
resorb the glass phase and leads to the appearance of a new phase 
identified as being barium aluminosilicate (defined in the ASTM standard 
12-926 under the name of hexacelsian). The composite material thus treated 
has an expansion coefficient between 20.degree. and 1000.degree. C. of 
3.2.multidot.10.sup.-6 /.degree. C., a density of 2.71 and remarkable 
mechanical properties: 
.sigma.=412 MPa 
E=130 GPa 
K.sub.1 C=2.98 MPa/m.sup.1/2. 
EXAMPLE 5 
Composition MgO-Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 +long fibers 
A glass-ceramic powder having the following formula: 
EQU 0.5 MgO - 0.5 Li.sub.2 O - Al.sub.2 O.sub.3 - 4 SiO.sub.2 
is prepared. 
The fibers used are long silicon carbide fibers produced by NIPPON CARBON 
under the reference NLM 202 and marketed by BROCHIER in the form of a 
bidirectional fabric under the reference E2140. The acrylic resin based 
coating of the fibres is removed by immersion of the fabric in a mixture 
of equal volumes of acetone and isopropyl alcohol, which is agitated every 
half hour for 15 minutes by means of an ultrasonic bath. The fabric is 
treated in this manner in two successive baths for two hours in each bath. 
70 g of the glass-ceramic powder dehydrated and oxidized at 500.degree. C. 
and deagglomerated through a screen of 50 .mu.m are mixed with a viscous 
solution of 5 g of polymethyl methacrylate in 100 cm.sup.3 of 
chlorobenzene. The suspension obtained is applied with a paintbrush in 
order to impregnate the fabric of fibers, after which the solvent is 
immediately evaporated, the polymer ensuring adhesion of the powder to the 
fibers. Several applications are carried out, followed by weighing, until 
an amount of powder between 80 and 90 mg per cm.sup.3 of fabric is 
obtained. Five 70.times.70 mm sheets are cut from the web of tissue thus 
impregnated and stacked in a graphite mould which has the same 
cross-section in order to carry out the compaction and ceramisization heat 
treatment: increase in temperature by 20.degree. C./min up to 
960.degree.-980.degree. C. under vacuum with a first 15-min holding of 
temperature at 350.degree. C. in order to remove the polymethyl 
methacrylate, followed by a second 45-min holding of temperature at 
450.degree. C. which favours the rearrangement, continuation of the 
heating at 20.degree. C./min under nitrogen up to 1310.degree. C. and a 
5-min holding of this temperature with pressing under 11 MPa; 
non-controlled cooling under nitrogen and without pressing. 
The composite material obtained has a fiber volume proportion of 36% and an 
apparent density of 2.49 (relative apparent density of 0.99). X-ray 
analysis shows the predominant presence of .beta.-spodumene and cordierite 
with a small amount of amorphous phase. 
The fracture resistance determined by the threepoint bending test with a 
specimen 10 mm wide and 2.38 mm thick at a distance between supports of 50 
mm is 290.+-.35 MPa. 
The above examples have no limiting character whatever. In particular, it 
is possible to combine differently, or to modify, the characteristics 
described, especially with respect to the formula of the glass-ceramic 
composition, the preparation method of the gel, the heat treatments of 
dehydration and oxidation as well as ceramisization, the presence or 
absence of fibers and their chemical nature and physical structure. More 
particularly, the use of silicon carbide fibers from other sources than 
those used in the examples, which have different surface properties, is 
taken into consideration.