Method of making piezoelectric composites

A method of fabricating a piezoelectric composite comprising an array of piezoelectric elements and a reinforcement structure embedded in a material. The method includes the step of forming the reinforcement structure prior to assembly with the piezoelectric elements, the reinforcement structure defining an array of holes for receiving the piezoelectric elements.

The present invention relates to piezoelectric composite and more 
particularly to reinforced piezoelectric composites, and is a Continuation 
Application of U.S. patent application No. 804,565 (abandoned). 
Reinforced piezoelectric composites are known, the structure of which 
comprises an array of parallel, spaced rods of a piezoelectrc ceramic 
embedded in a matrix material, the piezoelectric rods being aligned so as 
to be all polarized in the same direction. Reinforcement members in the 
form of glass rods are also embedded in the matrix material, the glass 
rods being arranged in two mutually perpendicular arrays each of which 
intermeshes with and is perpendicular to the array of parallel 
piezoelectric ceramic rods. Such a structure is known as a three phase 
composite with a 3-1-1 connectivity by which is meant that the matrix 
material is a three dimensionally connected phase, the piezoelectric 
ceramic rods each represent a one dimensionally connected phase and the 
reinforced rods each represent a one dimensionally connected phase. 
It is known to assemble the above structure by first assembling the 
parallel array of ceramic rods and then assembling into position one by 
one the reinforcing glass rods. This is an extremely time consuming 
process as some of the glass rods may have diameters as low as 0.25 mm and 
there may be many hundreds of such rods that require assembly. 
One of the objects of the present invention is to reduce the fabrication 
time of a three phase composite. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a method of 
fabricating a piezoelectric composite comprising an array of piezoelectric 
elements and a reinforcement structure together embedded in a material, 
the method including the step of forming the reinforcement structure prior 
to assembly with the piezoelectric elements, the reinforcement structure 
defining an array of holes for receiving the piezoelectric elements. 
In one embodiment of the present invention the reinforcement structue has a 
mesh-lke configuration, and is preferably in the form of a woven mesh. A 
number of the mesh-like reinforcement structures are piles on top of one 
another to define layers of mesh having their holes in alignment to form a 
reinforcement cage-like structure defining an array of holes for receiving 
the piezoelectric elements. 
In another embodiment of the present invention each reinforcement structure 
is in the form of a sheet of material, with or without pre-ipregnation of 
a polymeric material, the sheet of material being punched to give a two 
dimensional array of holes for receiving the piezoelectric elements. In 
one form of the embodiment a number of sheets of the material are piles on 
top of one another to form a reinforcement cage-like structure defining 
the array of holes for receiving the piezoelectric elements. In another 
embodiment of the present invention the reinforcement structure is formed 
by piling together a plurality of sheets of reinforcement material, an 
array of holes being formed through the pile of sheets when assembled to 
define a reinforcement stucture having the array of holes for receiving 
the piezoelectric elements. 
In all of the above embodiments after the reiforcement structure or 
structures have been assembled, and the piezoelectric elements placed in 
the holes, the entire construction is cast in a suitable matrix material 
and is trimmed to shape. The final construction formed in a three phase 
composite having a 3-1-2 connectivity by which is meant that the matrix 
material is a three dimensionally connected phase, the piezoelectric 
elements each represent a one dimensionally connected phase and each 
reinforcement is a tow dimensionally connected phase. 
It is however within the scope of the invention to create a three phase 
composite having a 3-1-1 connectivity by using a one dimensionally 
connected reinforcement phase. This may be achieved for example by forming 
the reinforcement structure from a reinforcing member in the form of a 
strand of reinforcing fibrous material which is woven into a multi-layered 
cage-like structure prior to assembly with the piezoelectric elements. 
In a further embodiment of the present invention the fabrication process 
includes the step of first producing an array of parallel, aligned 
piezoelectric elements having spacings corresponding to the pitch between 
the holes in the reinforcement structures, the reinforcement structures 
being placed over the array of piezoeletric elements. 
An aspect of the present invention is the provision of a piezoelectric 
composite fabricated by any one of the methods according to the present 
invention described above. 
In one embodiment of the present invention there is provided a 
piezoelectric composite comprising an array of piezoelectric elements and 
a reinforcement structure embedded in a common material, at least some 
parts of the reinforcement structure extending in two directions 
substantially perpendicular to the direction of alignment of the 
piezoelectric elements to form a piezoelectric composite having a 3-1-2 
connectivity. 
In one preferred embodiment of the present invention each reinforcement 
structure defines an array of reinforcement elements which intermeshes 
with and is aligned substantailly perpendicular to the array of 
piezoelectric elements.

An internal view of a corner section 1 of a block of known reinforcement 
piezoelectric composite is shown in FIG. 1. The structure of the composite 
consists of an array of parallel, spaced rods 2 of a piezoelectric ceramic 
embedded in a matrix material 4, the piezoelectric rods 2 being aligned so 
as to be all polarized in the same direction. The matrix material 4 
consists of an epoxy resin and the reinforcement members are in the form 
of glass rods 6 arranged in two mutually perpendicular arrays each of 
which intermeshes with and is perpendicular to the array of parallel 
piezoelectric rods 2. 
The fabrication of the reinforced piezoelectric composite shown in FIG. 1 
involves first assembling into position the parallel array of spaced 
piezoelectric rods 2, next assembling each of the glass rods 6 into its 
respective position within the array of piezoelectric rods 2 and finally 
casting the assembled piezoelectric rod 2 and glass rods 6 in the matrix 
material 4. The fabrication can be extremely time consuming as it involves 
each of the glass rods being individually placed into its respective 
position. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
In order to reduce the fabrication time the reinforcement members and the 
piezoelectric rods 2 can be assembled together by a number of methods 
according to various embodiments of the present invention. One group of 
methods makes use of preassembled cages constructed of reinforcement 
members as will now be described with reference to FIGS. 2, 3 and 4. 
Referring to FIG. 2 a pre-woven glass fibre mesh, or other suitable 
mesh-like material, is cut into equal sized pieces to form reinforcement 
structures which are carefully piled on top of each other to form layers 
of mesh 8 having their holes in alignment. In this way a reinforcement 
cage is assembled having an array of parallel holes (10) for accepting the 
piezoelectric rods 2. 
Referring to FIG. 3 reinforcement structures of sheets of glass fibre mat 
14, or mat made from any other reinforcement fibres with or without 
preimpregnation of other polymeric materials such as polymeric resins, are 
punched to give a two dimensional array of holes. The sheets of mat 14 are 
cut into equally sized pieces and piled up to form the cage shown in FIG. 
3 for accepting the piezoelectric rods 2. 
During the method of cage assembly with reference to FIGS. 2 and 3 a metal 
or ceramic support peg 12 may be inserted in at least two corner holes to 
assist in securing the cage during assembly. 
A cage of glass thread, or other reinforcing fibrous material, can be woven 
using a support peg arrangement such as that shown in FIG. 4. The peg 
arrangement of FIG. 4 consists of two parallel rows and two parallel 
columns of pegs 20 there being five pegs 20 in each row and in each 
column. A thread or yarn 22 is wound around and between the two columns of 
pegs 20 to form a first layer 24 of thread or yarn. The thread or yarn 22 
is then wound around and between the pegs 20 defining the two rows so as 
to form a second layer 25 of thread or yarn. Thereafter the thread or yarn 
22 is again woven between the two columns of pegs to create a third layer 
and so on. In this way a reinforcement structure in the form of a woven 
cage is formed having an array of parallel holes 30 for accepting the 
piezoelectric rods. 
In another embodiment of cage construction layers of unpunched glass fibre, 
or other fibrous reinforcement material, mat preimpregnated with suitable 
matrix material are cut into equal sized pieces and piled on top of each 
other. The pile is trimmed to the required shape and a two dimensional 
array of parallel holes are drilled in the pile. In this way a 
reinforcement cage structure precoated in the matrix material is produced. 
When the reinforcement cages described above have been assembled, and the 
piezoelectric rods placed in the holes with all of the piezoelectric rods 
polarized in the same direction, the entire structure is cast in the 
matrix material and trimmed to shape. Good adhesion between the matrix 
material and the reinforcement members can be ensured by pretreating the 
reinforcement members with an adhesion promoter. One such adhesion product 
is Dow Corning's adhesion promotor Z6040. Other techniques for ensuring 
good adhesion include a surface etch of the reinforcement members, for 
example chromic acid etching of glass reinforcement rods, by precoating 
the glass reinforcement rods with a suitable secondary matrix material so 
as to form an intermediate layer situated between the reinforcement 
members and the main matrix material. 
Another method of fabricating a reinforced piezoelectric composite 
according to an embodiment of the present invention involves a comb type 
method of construction. The method of construction includes first 
producing a two dimensional array of parallel aligned piezoeledtric rods. 
This can be achieved by either placing pre-cut rods in a suitable array of 
holes in a metal base plate, having previously placed a soft material such 
as plasticine or wax in the holes to hold them firmly, or by forcing them 
into a bed of soft wax so that they stand up in a regular array or by saw 
cutting a two dimensional comb or array from a solid block of 
piezoelectric material. 
Precut equal sized pieces of mesh or punched mat, of the same pitch as the 
rod spacing, can then be placed over the comb to build up the 
reinforcement cage, or an assembled cage, made as described above by 
drilling holes in a pile of unpunched layers of matrix material, can be 
placed over the rods. The assemlby is then cast in a suitable composite 
matrix material and trimmed to shape. 
The piezoelectic composites are used primarily as piezoelectic transducers 
and in some applications of piezoelectric transducers, such as 
hydrophones, entrapped bubbles are undesirable. To prevent them forming, 
it is advisable to ensure that the matrix material is vacuum out-gassed to 
remove dissolved air and water vapour, and that the matrix material is 
poured onto the assembled structure under vacuum to present the trapping 
of larger quantities of air. for example, oug-gassing of Eccogel (a 
Registered Trade Mark) 1365-80 epoxy resin (manufactured by Emerson and 
Cuming) was found to take approximately one hour at a vacuum of 740 mm Hg. 
This may vary, however, with the type, age and temperature of the epoxy 
resin used. 
When discussing the properties of piezoelectric materials, it is convenient 
to define an axial system to which they can be referred. For the purpose 
of this specification properties will be referred to a right-handed axial 
set in which the axis, and properties referred to them, are distinguished 
by the three subscripts `1`, `2` and `3`. The axis x.sub.3 is defined as 
being parallel to the polar direction in the material, (or the direction 
of poling in a piezoelectric ceramic). The x.sub.1 and x.sub.2 axes are 
defined as being mutually perpendicular to each other and to the x.sub.3 
axis. In the case of 3-1-1 piezoelectric composites containing glass rod 
reinforcement, the x.sub.1 and x.sub.2 axes are defined so that they are 
mutually perpendicular to each other and to the x.sub.3 axis and parallel 
to the directions in which the glass rods are laid. 
In a piezoelectric material, the charges generated across the poled faces 
are dependent upon the piezoelectric charge coefficients d.sub.ij ; here, 
use is made of the reduced subscript notation for the piezoelectric 
coefficients as defined by J. F. Nye in his publication "Physical 
Properties of Crystals", (Clarendon Press, Oxford) Chapter VII. In a 
hydrostatic system the stress, .sigma., in each of the three mutually 
perpendicular directions is the same and hence: 
EQU Q/A=.sigma.(d.sub.31 +d.sub.32 +d.sub.33)=.sigma.d.sub.H 
Where d.sub.H is they hydrostatic charge coefficient (d.sub.H =d.sub.31 
+d.sub.32 +d.sub.33), Q is the charge generated on the poled faces and A 
is the area of the poled faces. 
In a homogeneous cube of the piezoelectric material generically known as 
PZT (in the PZT family, the major constituent is a ceramic solid solution 
(PbZrO.sub.3).sub.x. (PbTiO.sub.3).sub.1-x where x 0.52)d.sub.31 
=d.sub.32 -d.sub.33 /2 and hence d.sub.H is close to zero, giving a 
small charge output when under hydrostatic stress. A two phase composite 
structure with 3-1 connectivity that consists of a two dimensional array 
of parallel aligned piezoelectric rods embedded in a continuous matrix, 
with no third phase reinforcement, has been shown to have an increased 
value of d.sub.H relative to the PZT materials, (see Table 1) below. This 
is due to the matrix introducing stress relief in the two directions 
mutually perpendicular to the rod direction, (the x.sub.1 and x.sub.2 
axes) resulting in smaller longitudinal stresses .sigma..sub.1 and 
.sigma..sub.2 being experienced by the piezoelectric rods. The hydrostatic 
charge coefficient of the composite as a whole is thus bigger than that of 
the piezoelectric material alone, hence giving an increased charge output 
for any particular pressure change. The lower volume fraction of 
piezoelectric material also produces a drop in the average dielectric 
constant .epsilon. of the composite which, with the increase in the 
d.sub.H gives an increase in the piezoelectric voltage coefficient g.sub.H 
(where g.sub.H =d.sub.H /.sub..epsilon.) relative to the piezoelectric 
material. This produces a greatly increased value for the piezoelectric 
figure of merit, d.sub.H.g.sub.H, which represents the energy output of 
the device per unit volume, per unit pressure change squared. Examples of 
suitable ceramic materials are PZT-5H and PZT-4. Examples of suitable 
polymeric materials are Eccogel 1365-80, Stycast 1264 (Trade Marks) (both 
epoxy resins manufactured by Emerson and Cuming), Araldite MY763 (a Trade 
Mark) (manufactured by Ciba-Giegy Ltd). 
The addition of a third reinforcement phase to the composite matrix 
introduces reinforcement phase to the composite matrix introduces improved 
stress relief and hence increases d.sub.H, g.sub.H and d.sub.H g.sub.H 
further. Table 2(a) shows the characteristics of two examples of three 
phase composites made with 0.6 mm square PZT-5H ceramic rods embedded in 
Eccogel 1365-80 epoxy resin with 0.25 mm diameter borosilicate glass rod 
reinforcement and Table 2(b) shows the characteristics of two examples of 
three phase composites of similar construction to those in Table 2(a), 
except cast in a harder epoxy potting compound than Eccogel. 
Table 3 gives the values of d.sub.H, g.sub.H and d.sub.H gHfor a three 
phase composite made with 0.6 mm square PZT-5H ceramic rods and a ceramic 
volume fraction of 0.063. This sample had a matrix of Eccogel 1365-80 
epoxy resin and was reinforced with silica glass thread. The sample was 
approximately 24 mm square and 14 mm thick with a capacitance of 46 pF. 
The results show that all three samples were capable of high output when 
compared to an ordinary ceramic block (see Table 1), and are comparable in 
performance with the three phase composites of Table 2. 
Table 4 gives the values of d.sub.H, g.sub.H and d.sub.H g.sub.H for two 
three phase composites produced using a prewoven glass fibre mesh such as 
Automesh (Trade Mark) and Eccogel 1365-80 epoxy resin in accordance with 
one embodiment of our invention. Both samples had 1mm square PZT-5H 
ceramic rods with a ceramic volume fraction of approximately 0.08 and the 
results show that the construction process exhibits good reproducibility. 
Both samples were approximately 35 mm square and 15 mm thick with a 
capacitance of approximately 80 pF. 
TABLE 1 
______________________________________ 
Solid piezoelectric ceramic compared to a two 
phase piezoelectric composite. 
PZT-5H Two Phase Composite 
Solid (PZT-5H 24% by vol- 
Ceramic ume Araldite MY763 
Characteristic 
Block 76% by volume Units 
______________________________________ 
d.sub.H 
= 45 84.6 pCN.sup.-1 
g.sub.H 
= 1.48 19.0 10.sup.-3 VmN.sup.-1 
d.sub.H g.sub.H 
= 67 1610 10.sup.-15 Pa.sup.-1 
______________________________________ 
TABLE 2 
______________________________________ 
Characteristics of 3 phase composites 
(a) Eccogel 1365-80 epoxy resin matrix 
(b) Harder epoxy potting compound matrix 
than Eccogel 
______________________________________ 
(a) 
______________________________________ 
Ceramic Glass g.sub.H d.sub.H g.sub.H 
Volume Volume d.sub.H 10.sup.-3 
10.sup.-15 
Fraction Fraction pCN.sup.-1 
VmN.sup.-1 
Pa.sup.-1 
______________________________________ 
0.15 0.11 148 46.3 6850 
0.20 0.13 199 54.1 10760 
______________________________________ 
(b) 
______________________________________ 
Ceramic Glass g.sub.H d.sub.H g.sub.H 
Volume Volume d.sub.H 10.sup.-3 
10.sup.-15 
Fraction Fraction pCN.sup.-1 
VmN.sup.-1 
Pa.sup.-1 
______________________________________ 
0.25 0.15 122 22.3 2710 
0.30 0.16 125 16.1 2030 
______________________________________ 
TABLE 3 
______________________________________ 
Results obtained for a 3 phase composite made 
with glass thread reinforcement. 
______________________________________ 
d.sub.H 
= 50.5 pCN.sup.-1 
g.sub.H 
= 45.5 (.times. 10.sup.-3) Vmn.sup.-1 
d.sub.H g.sub.H 
= 2300 (.times. 10.sup.-15) Pa.sup.-1 
______________________________________ 
TABLE 4 
______________________________________ 
Results obtained for two three phase composites 
made with "Automesh" glass-fibre mesh reinforcement 
Characteristic 
Sample 1 Sample 2 Units 
______________________________________ 
d.sub.H = 71.8 71.2 pCN.sup.-1 
g.sub.H = 76.0 70.0 10.sup.-3 VmN.sup.-1 
d.sub.H g.sub.H 
= 5640 4990 10.sup.-15 Pa.sup.-1 
______________________________________ 
The reinforcement members in alternative embodiments include glass rods, 
glass fibres, glass threads, Kevlar (Trade Mark) fibres or other stiff 
fibrous material or combinations thereof, including for example chopped 
strand glass fibre mat. 
It will also be appreciated that the types of piezoelectric ceramic 
materials described above, are given by way of example only as it is 
possible to use other types of piezoelectric material for the purposes of 
the present invention, such as barium titanate ceramic and antimony 
sulphor iodide.