The invention is a process to manufacture a controlled-porosity ceramic material by synthesizing an aerogel produced by hypercritical drying of a high-porosity gel. Applications in the submarine acoustic field (active and passive sonars).

BACKGROUND OF THE INVENTION 
This invention is a method of producing a controlled-porosity piezoelectric 
ceramic whose properties offer advantages in submarine acoustics (active 
and passive sonars), medical applications (ultrasonic imagery) and signal 
processing (filters, delay lines, etc.). 
In general terms, when polarized by a voltage applied prior to their use, 
ferroelectric materials offer high piezoelectric charge coefficients 
resulting from the creation of an electrical charge when they are 
subjected to pressure. 
DESCRIPTION OF THE PRIOR ART 
These ferroelectric materials can be ceramics, such as barium titanate or 
lead zirconium-titanate (PZT) or polymer material such as poly vinylidene 
fluoride (PVDF). 
The various physical characteristics used to indicate the performance of 
these materials in the applications mentioned above are: 
the hydrostatic charge coefficient d.sub.h defined as the combination of 
coefficients d.sub.33 and d.sub.31 which represent the efficiency of the 
transverse and longitudinal modes respectively: d.sub.h =d.sub.33 
-2d.sub.31. 
the coupling coefficient K.sub.h which represents the efficiency with which 
the piezoelectric material converts an electrical signal to ultrasound and 
vice-versa: 
EQU k.sub.h =d.sub.h 2/.epsilon..S 
where 
.epsilon. is the dielectric constant 
S is the elastic compliance 
the tension coefficient g.sub.h (in the passive operating mode, i.e. 
detecting sound waves): 
EQU g.sub.h =dE/dP=d.sub.h /.epsilon. 
where E is the electrical field induced by pressure P. 
Ceramic materials which have low compliance because they are very rigid 
offer better coupling coefficients K.sub.h than polymers such as PVDF. 
Nonetheless, their very high dielectric constant (approx 1700 for PZT) and 
their high density makes it difficult to adapt their acoustic impedance to 
that of water and reduce the factor of merit g.sub.h. 
Work has therefore been done to improve the performance of ceramic 
materials, particularly to reduce the dielectric constant and increase the 
d.sub.h, k.sub.h and g.sub.h factors. Ceramics such as PZT are very dense 
materials which can be lightened by producing composite two-phase 
materials in which phase 1 is a ceramic and phase 2 a resin or air. There 
are various possible structures in which phases 1 and 2 are connected 
differently FIG. 1). If a phase 1 powder (non-interconnected grains) is 
dispersed in an elastic phase 2 material, interconnected in all three 
directions, the material is designated a 0-3 composite. 
The permittivity in this type of structure is reduced but to the detriment 
of factor K.sub.h, since compliance S is increased. 
A (1, 3) connectivity, obtained by placing PZT bars perpendicular to the 
electrodes, allows better transfer of stresses and polarization into the 
ceramic than can be obtained with (0, 3) connectivity. 
Complete connection to the ceramic improves performance even further. This 
can be achieved using the "lost wax" (investment casting) method and an 
extremely porous material: coral. 
With this process, the PZT material is interconnected in all three 
directions in space and the interstices are filled with resin. 
Approximately 50% of the volume is ceramic. The following table summarizes 
the performance offered by these different types of composite materials. 
__________________________________________________________________________ 
Pure 
(0,3)PZT chloroprene 
(1,3)PZT-epozy 
(3,3)coral 
PZT 
combination 
combination 
replica combination 
__________________________________________________________________________ 
.epsilon.33 
1700 
31 500 400 
d.sub.h (10.sup.-12 C/N) 
50 
34 20 100 
g.sub.h (10.sup.-3 V .multidot. m/N) 
3 
1080 40 250 
d.sub.n .multidot. g.sub.h (10.sup.-15 m/N) 
150 
36 800 25000 
__________________________________________________________________________ 
The factor of merit d.sub.h.g.sub.h is the predominant characteristic in 
selecting a piezoelectric material since it represents the overall 
transmission and reception performance (high transmission efficiency, high 
reception sensitivity). 
Despite the advantages of coral replica PZT, it is a fragile material 
difficult to use and whose porous structure is unchangeable; however, a 
model such as the Banno model [Jpn.J of Applied Phys. 24 (1985), suppl. 
24-2, 445-7] shows the effect of the pore shape on the various factors 
(d.sub.31, d.sub.33). FIGS. 2 to 5 show the variations in these factors 
depending on the porosity and pore shape. 
This model shows that it would be advantageous to produce highly porous 
ceramics with elongated pores. 
This invention is, therefore, a process to manufacture a piezoelectric 
ceramic whose porosity can be controlled and can be very high, unlike the 
process used to synthesize porous ceramics by burning binders while 
sintering the powder, which only allows porosities of roughly 15% to be 
obtained. The process complying with this invention includes the following 
steps: 
synthesis of a high-porosity gel in the presence of solvent and gel 
orientation agents 
synthesis of the aerogel by drying under conditions in which the solvent 
pressure and temperature are kept hypercritical to maintain the size and 
shape of pores obtained in the gel 
production of the ceramic by calcining the aerogel to form the required 
structure at high temperature. 
This invention is also a piezoelectronic ceramic manufactured using the 
process as per the invention. This porous ceramic is preferably barium 
titanate or lead zirconium titanate. The molecules used to orientate the 
gelling are preferably acetyl-acetone (acac) or acetic acid molecules. 
This invention will be better understood, and other advantages will become 
clear, upon reading the following description of the appended figures 
including:

DETAILED DESCRIPTION OF THE INVENTION 
It is known that the structure of gels produced from organic alcooxides is 
highly porous. However, these gels shrink considerably during drying at 
atmospheric pressure, considerably reducing the porosity. To avoid this 
shrinkage, this invention proposes to dry the gel under hypercritical 
temperature and pressure conditions where the liquid-vapour equilibrium no 
longer exists. The fluid to be eliminated passes, by adiabatic expansion, 
from the pores to the free space above the gel without changing phase and, 
therefore, without expanding, eliminating the stresses caused by pressures 
inside the gel. The porosity of the aerogels obtained can be up to 90%. By 
calcining this aerogel at a moderate temperature, a porous ceramic is 
obtained whose porosity can be controlled and can be up to 70%. The 
porosity is controlled during synthesis of the gel by including chelating 
molecules which block certain precursor functions and thus orientate the 
gel structure by preventing the formation of certain bonds. 
The synthesis of highly-porous PZT is described as an example: 
A PZT gel is synthesized by mixing lead, in the form of a carboxylate 
(acetate, 2-ethyl-hexanoate, etc.), titanium and zirconium in the form of 
alcooxides (iso or n.propoxide or butoxide) in an alcohol such as propanol 
1. After reaction in a reflux, the complex alcooxide is hydrolyzed with 
between approximately 1 and 10 moles of water per mole of lead by adding a 
water-propanol mixture. The gel obtained is placed in an autoclave with a 
pressure relief valve. The temperature is increased to more than 
263.5.degree. C. (the critical temperature of propanol) so that the 
pressure exceeds 51 atmospheres (the critical pressure); the volume is 
selected to satisfy this condition complying with the Van der Waals 
isothermal law. The pressure is then relieved in a larger volume but still 
remaining hypercritical and, on cooling, the 70% porous aerogel and the 
solvent are recovered separately. 
The aerogel is then calcined in an oven to crystallize the PZT phase 
without increasing its density, that is maintaining the same porosity, at 
a temperature between 300.degree. and 500.degree. C.