AU613773B2 - Improvements in and relating to piezoelectric composites - Google Patents

Description

OPI DATE 12/12/89 AOJP DATE 25/01/90 APPLN. ID 36976 89 PCT NUMBER PCT/GB89/00561

PCT

INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 l te tion ub ipn Njer 7 WO 89/11738 H01L41/22 A uAl SHOL41/22 Al tetional blicon Dar: ovember 1989 (30.11.89) (21) International Application Number: PCT/GB89/00561 (81) Designated States: AT (European patent), AU, BE (European patent), CH (European patent), DE (European pa- (22) International Filing Date: 22 May 1989 (22.05.89) tent), FR (European patent), GB (European patent), IT (European patent), LU (European patent), NL (European patent), SE (European patent), US.

Priority data: 8812508.3 26 May 1988 (26.05.88) GB Published With international search report.

(71) Applicant (for all designated States except US): PLESSEY OVERSEAS LIMITED [GB/GB]; Vicarage Lane, IIford, Essex IG1 4AQ (GB).

(72) Inventors; and Inventors/Applicants (for US only) GOOSEY, Martin, Trevor [GB/GB]; The Cedars, 191 Weedon Road, Northampton NN5 5DA SETH, Philip, Neil, Andrew [GB/GB]; 28 The Holme, Great Broughton, Cleveland TS9 7IIF GARNER, Geoffrey, Michael [GB/GB]; Cheriton Way, Northamptonshire NNI 5SB (GB).

(74) Agent: ELLIOTT, Frank, Edward; The Plessey Company plc, Intellectual Property Department, Vicarage Lane, IIford, Essex IG1 4AQ (GB).

(54) Title: IMPROVEMENTS IN AND RELATING TO PIEZOELECTRIC COMPOSITES (57) Abstract The invention relates to piezoelectric composites and more particularly, but not exclusively, to a method of manufacturing a composite material in which a piezoelectric ceramic is dispersed as a powder within a polymer matrix. The method comprises forming a composite including a ceramic dispersed within a polymer matrix, partially curing the polymer matrix to a desired resistivity relative to the resistivity of the ceramic, applying a poling field to the composite to render the ceramic piezoelectric and increasing the resistivity of the polymer matrix by further curing. In one embodiment electrodes are bonded to the ceramic polymer composite by utilising the residual reactivity in the partially cured composite material to bond chemically the composite to the electrode material.

B

P/55324/P48 Title of the Invention IMPROVEMENTS IN AND RELATING TO PIEZOELECTRIC COMPOSITES S* Field of the Invention e The present invention relates to piezoelectric composites and more particularly, but not exclusively, to a method of manufacturing a composite material wherein a ceramic material is dispersed uniformly as a powder through a polymer matrix.

Background of the Invention It is known that piezoelectric ceramics are widely used in transducer applications ranging from simple piezoelectric buzzers to complex multilayer actuators. They are also extensively utilised in underwater transducer applications such as hydrophones despite a number of distinct disadvantages. Although they have large piezoelectric coefficients d in a first crystal axis direction (d 33 these materials often have poor hydrostatic piezoelectric coefficients dh because the coefficients in the second and third axial directions (d 31 are negative and Ro4 P/55324/P48 2 serve to cancel out the contribution made by the d 33 coefficient because dh d 33 2d 31 In some applications, their high permittivities are undesirable because this leads to a low voltage coefficient gh where g dh/a Their high densities also prevent good impedance matching to water (density Finally, being brittle ceramics, they require sintering or hot pressing and subsequent machining to achieve the required shapes and in many cases a more compliant material with greater shock resistance is preferred.

One possible way of obviating these undesirable o*I properties is to produce a composite material in which the ceramic powder is dispersed in a polymeric phase. One form of composite, a 0-3 composite, in which the ceramic particles are isolated from each other in all three dimensions, offers distinct advantages for many uses since the polymer phase lowers both the permittivity and the density, making the composites more suitable for underwater applications. The polymer phase also increases the compliance of the composite and offers the possibility of processing by more economical plastics processing technology such as compression, transfer and injection moulding. Using this approach the fabrication of complex shapes is much easier than by conventional ceramic processing and, in pa4VS N 4 -i P/55324/ 2 48 3 particular, Underwater simplified.

the manufacture of large area tiles for flank array application is considerably 94*t a.

4 a 4 a4 4. a 4 In a 0-3 composite, the dispersion of the ceramic powder matrix is such that the individual particles of ceramic are effectively isolated from each other in all three dimensions by a continuously connected polymer.

Because the ceramic particles are small compared with the thickness of typical composites, it is then difficult to achieve satisfactory poling of the composite at the fields typically used for poling a ceramic material per se. This is because the field is disproportionately applied across the polymer rather than the ceramic material as can be seen from the following equation which defines the field acting across an isolated ceramic particle in a low permittivity matrix: E, 3k 2

E

k, 3k 2 El, the proportion of the total field E o applied, appearing across the ceramic particle is determined by the permittivities (dielectric constants) of the ceramic k i and the polymer k 2 For typical values of 2000 for a 1

A

i I A4 4~k P/55324/P48 4 piezoelectric ceramic and 4.5 for a polymer such as an epoxide matrix, the field appearing across the ceramic could be less than 1% of the applied field Eg, and consequently very high fields would be required to achieve acceptable poled activity from the ceramic.

4 However, where the poling field can be applied to the composite for periods well in excess of the sample relaxation time, as in typical DC poling, the relationship between the electric fields appearing across the ceramic and polymer matrix can be defined by the Maxwell Wagner model as:- E/E2 P1/P2 4 0 where E, and E 2 are the fields across the ceramic and 4* polymer respectively and P 1 and P 2 are the resistivities of the ceramic and polymer. Consequently, if the resistivity of the ceramic and polymer matrix can be balanced it is possible to achieve reasonable poling of the ceramic. Most polymers have resistivities several orders of magnitude higher than the ceramic. The resistivity of a typical modified lead titanate ceramic at room temperature would be 1 x 1010 cm, whereas a suitable silicone or higher purity epoxide might be -1 x 1015-- cm.

I

I

11 1 1 1 t P/55324/P48 Various methods have been proposed to reduce the resistivity of the resin systems used, typical examples include loading the polymer with conducting or semiconducting materials such as carbon, germanium and silicon. Alternatively, icwer purity resins can be used closely to match the resistivity to the ceramic. In many oo transducer applications however, it is desirable to maintain the highest resistivity possible in the polymer matrix in order to minimise the dielectric loss of the composite and hence reduce the electrical noise generated. Also, the higher the resistivity of the polymer, the higher t)ie loading of the conductive phase of carbon, germanium or silicon that is required to achieve a resistivity balance.

At levels above 1.5 vol% loading, the dielectric loss increases rapidly, and the application of large fields becomes increasingly difficult.

Summary of the Invention According to the present invention, there is provided a method of manufacturing a piezoelectric composite, the method comprising forming a composite including a potentially piezoelectric ceramic material in particle form dispersed within a polymer matrix, partially curing the P/55324/P48 6 polymer matrix to a resistivity substantially the same as the resistivity of the ceramic, applying a poling field to the composite to render the ceramic piezoelectric and increasing the resistivity of the polymer matrix by further curing.

o S0- In one embodiment, electrodes are bonded to the ceramic polymer composite by utilising the residual reactivity in the partially cured composite material to bond chemically the composite to the electrode material.

In a preferred embodiment, the poling field is maintained during at least part of the further curing process.

Multilayer composite/electrode structures may be S. fabricated by applying the poling field to alternating layers of part-cured polymer/ceramic and electrode material, and then by applying the further curing process to provide the final composite multilayer structure.

The ceramic/polymer composite and the electrode material may have identical or similar part cured compositions. Alternatively, they may have dissimilar part cured compositions. For example, in such latter

II

i? I A I; i ;-lli-' P/55324/P48 7 embodiment, the ceramic/polymer composite may be an epoxide resin whilst the electrode material may be a rubber based polymer.

In yet a further embodiment, co-polymer or polymer blends are utilised for both the composition of the

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ceramic/polymer composite and the electrode polymer.

In a preferred embodiment, the polymer matrix of the composite exhibits a large resistivity change during curing, the resistivity of the polymer matrix being similar to that of the ceramic material in the partially cured state of the polymer but being significantly higher than that of the ceramic material after the full cure of the polymer.

The electrodes, may, in one form, be screen printed onto the partially cured composite, the poling field being applied via the screen printed electrodes, and the electrode composite being fully cured with the poling field applied.

In one embodiment, the desired resistivities of the polymer matrix is achieved partly by loading the composite with a conductive filler and partly by exploiting the resistivity change which occurs on curing of the selected composite polymer mix.

4A u 4 P/55324/P48 8 Other aspects of the invention include the provision of a piezoelectric/polymer composite manufactured by embodiments of the invention described above; and of a transducer incorporating piezoelectric composites manufactured in accordance with the present invention. In one embodiment, the method of manufacturing the transducer includes bonding of the partially cured, poled composite to a metal substrate by utilising the residual reactivity of a partially cured resin component of the composite. In another embodiment, the partially cured composite is attached to a metal substrate during the final curing process whilst simultaneously applying the poling field.

CC

Detailed Description S" This invention is based on the fact that the resistivities of most polymers change between the cured and uncured states, and is particularly relevant when the changes are of several orders of magnitude. With the correct choice of polymer matrix, its resistivity in its uncured or partially cured state can be made to balance the cpramic materials resistivity. By partially curing the polymer to the required resistivity matching that of the ceramic material, poling can be achieved before final cure

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=2-9L i ",b

B

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P/55324/P48 9 0@S 0

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0 S. B 0 *0 S 0 of the resin is completed. In this way high purity, high resistivity polymers can be utilised in composite fabrication and these can provide both the resistivity balance required for poling to suitable activity and the high resistivities required to produce low loss composites providing low noise transducers.

The concept of utilising a partially cured polymer matrix offers the further advantage of allowing chemical bonding of electrode structures to the ceramic/polymer composite. In order to pole a typical composite structure, and also to allow the electrical connection required for any conceivable transducer application, electrodes have to be applied. Simple electroding can be achieved by painting on conductive silver electrodes such as silver DAG but for enhanced ruggedness, robust conductive polymeric electrodes are preferred. There is often difficulty in bonding these electrodes to the fully cured polymer ceramic composites resulting in poor interfacial integrity between the composite and the electrode. By utilising only a partial cure, the composite polymer matrix can have sufficient reactivity remaining to allow chemical bonding of the polymeric electrode system thereto. Various alternatives exist with this method, for example, the partially cured composite could have the polymeric electrode attached by I -J 1_ 1 P/55324/P48 10 further partial curing. The composite would then be poled and full cure completed after poling or even towards the end of poling, with the temperature being increased whilst the field was still applied. Alternatively poling could be achieved by the attachment of temporary electrodes, such as silver DAG, to the composite which would then be removed So .o prior to attachment of the robust electrodes and final cure.

Further possibilities exist for utilising the partial cure concept, since advanced device structures could be configured in such a way as to allow direct bonding of the transducers to their host vehicle, plane or vessel. An "example would be in the bonding of hydrophone tiles directly to submarine hulls in flank arrays. This method is particularly suitable for attaching larger area transducers to any metal surface such as aircraft fuselage or perhaps Sunderground piping systems. Poling during transducer attachment, by the use of suitable corona poling equipment would also be possible. A combined poling/curing apparatus is also feasible for certain applications.

As an example of a piezoelectric composite utilising this partial cure route, a ceramic material and a polymer could be formulated as follows.

iff 'I Brti P/55324/P48 11 For the piezoelectric ceramic material of the composite, a range of crystalline materials may be used. It is preferred to use ferroelectric crystals which are not piezoelectric when in the paraelectric phase. By analogy to ferromagnetic materials, the ferroelectric crystals can be considered as being composed of domains, application of a strong electric field serves to align the domains within the material whereafter strong piezoelectric effects are exhibited.

A preferred ferroelectric material of this type is a modified lead titanate.

*o 0 For the polymer, the preferred material is a thermosetting polymer system of which the curing can be arrested before completion but which is capable mechanical handling in its partly cured state. An epoxide resin system is preferred.

It is required, according to the invention, that the uncured or partially cured polymer system exhibits a resistivity of substantially the same order as that of the selected ceramic material but exhibits a resistivity, in the fully cured state, as high as possible and certainly several orders of magnitude higher than that of the selected ceramic P/55324/P48 -12 material.

The selected ceramic powder is roll milled with the polymer system to facilitate dispersion and this may (optionally) be assisted by the use of a chosen dispersion agent, such a soya bean lecithin. A suitable epoxide system would be epoxidised novolac cured by the tertiary amine accelerated reaction with a novolac resin. This type of system has the advantage that it could be easily achieved during the milling of the ceramic and polymeric components.

This system would also conveniently produce a B stage material which was solid at room temperature, thereby facilitating relatively easy processing. For example, if removed from the roll mill and formed into solid sheets, subsequent processing into suit.bility shaped tiles could be achieved at modest temperatures and pressures by, for example, compression moulding. The temperatures and times of this process would be determined in order to maintain or to adjust the resistivity balance between the part cured polymer and the ceramic material to the desired value.

Electrodes are then temporarily attached and poling performed. After electrode removal, permanent robust electrodes could then be attached to the partially reacted composite.

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a /y $7 P/55324/P48 13 A suitable electrode system would again be a B stage epoxide novolac (as used for the composite) but this time filled with a conductive material such as carbon, silver or copper. In addition, or alternatively, a metal mesh could be incorporated to facilitate ease of connection.

gee Alternatively, a conductive ink such as a copper or silver filled epoxide system could be screen printed on to 0. the partially cured composite.

In all of these examples the composite could be poled after the attachment of the final robust electrode system but prior to final curing. If the chosen resin system requires an elevated temperature cure this could be performed with the poling field still applied in order to prevent depoling although, for some ceramics, this would not be a problem.

If both the partially cured composite and the electrode polymer are identical or are chosen to be chemically compatible in terms of adhesive bonding, the two will react during the final cure, effectively eliminating the electrode/composite interface.

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Claims (13)

1. A method of manufacturing a piezoelectric composite, said composite comprising ceramic particles capable of exhibiting piezoelectricity, and a polymer, said particles having a first resistivity and said polymer h.ving a second resistivity several orders of magnitude higher than said first resistivity, said polymer resulting from a reaction mixture of polymer components, said mixture having a third resistivity at least as low as said first resistivity and rising to said second resistivity upon completion of the reaction, the method comprising the steps of dispersing the ceramic particles uniformly in said reaction mixture, applying a poling field across the reaction mixture containing the uniformly dispersed ceramic particles whilst said third resistivity is substantially the same as said first resistivity, said poling field serving to render said ceramic particles piezoelectric and to align domains thereof with said field, and thereafter allowing the reaction to proceed to completion whereby said polymer is fully cured and said piezoelectric composite is formed.

2. A method according to Claim 1 including the step i1 M^ C- I rP P/55324/P48 15 of applying electrodes to the composite before completion of said reaction, said electrodes being of a material such as to bond chemically with said composite due to said reaction.

3. A method according to Claim 2 wherein said electrodes are formed of said reaction mixture having a conductive filler selected from the group of conductive S0,: fillers including carbon, silver and copper. 00 Ct O

4. A method according to Claim 2 including the step of incorporating a metal mesh into said electrodes. i

5. A method according to Claim 1 including the step of applying temporary electrodes to said composite whereby to apply said poling field and removing said temporary electrodes and applying permanent electrodes to said S" composite before completion of said reaction.

6. A piezoelectric composite transducer comprising ceramic particles dispersed in a polymer natrix, the ceramic particles having been rendered piezoelectric by the application of a poling field to the composite at a stage of cure of the polymer during which the resistivity of the partially cured polymer was substantially the same as that of the ceramic particles. Rlj Pug I P/55324/P48 16

7. A piezoelectric transducer as claimed in claim 6 in the form of a 0-3 composite in which the ceramic particles are isolated from each other in all three dimensions by the continuously connected polymer.

8. A piezoelectric transducer as claimed in claim 6 or claim 7 having electrodes chemically bonded thereto by residual reactivity of the polymer during curing.

9. A piezoelectric transducer as claimed in claim 8 wherein the electrodes comprise the same polymer as the composite but having a conductive filler. fees

10. A piezoelectric transducer as claimed in claim 8 or 9 wherein the electrodes incorporate a metal mesh.

11. A piezoelectric transducer as claimed in any of claims 6 to 10 comprising a plurality of layers of the composite stacked with interleaved electrodes. DATED this 21st Day of May 1991 PLESSEY OVERSEAS LIMITED Fellow Intine br Pct A Tcr s o po l .e 8 A pezolectic tansuceras caimd inclai 6 J Ii INTERNATIONAL SEARCH REPORT International Application No PCT/GB 89/00561 I. CLASSIFICATION OF SUBJECT MATTER (it several classfication symbols apply, indicate all) According to International Patent Classification (IPC) or to both National Classification and IPC IPC 4 H 01 L 41/22 II. FIELDS SEARCHED Minimum Documentation Searched 7 Classification System I Classification Symbols 4 IPC H 01 L Documeitation Searched other than Minimum Documentation to the Extent that auch Documents are Included In the Fields Searched III. DOCUMENTS CONSIDERED TO BE RELEVANT' Category Citation of Document, with indication, where aoprooriate, of the relevant passages 12 Relevant to Claim No. 13 A IEEE 1984 Ultrasonics Symposium, Proceedings, 1,12,17

14-16 November 1984, Dallas, Texas, vol. 1, IEEE, R.E. Newnham et al.: "Flexible composite piezoelectric sensors", pages 501-506 see page 502, column 1 A DE, A, 3138249 (SIEMENS AG) 2,3,13,14, 21 April 1983 17 see abstract; page 5, lines 5-11 A EP, A, 0061374 (COMMISSARIAT A L'ENERGIE 1,17 ATOMIQUE) 29 September 1982 see abstract A GB, A, 665452 (TECHNICAL ASSETS INC.) 1,17 23 January 1952 see page 2, line 126 page 3, line 88 SSpecial categories of cited documents: a later document put.lshed after the International filing date document defining the general state of the art which sa not or priority date and ot In conflict with the application but conidered to be of particular relevance cited to understand the principle or theory underlying the invention earlier document but published on or after the International X document of particular reevance the clamed i nvention filing date X" document if particular relevance: the claimed invention fg de cannot bi considered novel or cannot be considered to document which may throw doubts on priority claim(s) or involve an Inventive step which ia cited to establiah the publication data of another Y" documnt of pr la reI vanc te climed Inn citation or other aDIclal raaon (el lOlcified) document of particular relevance; the claimed Invention citation or other pecia reason (as specified)cannot be considered to Involve an Inventive sate when the document referring to an oral disclosure, use, exhibltlon or document is combined with one or more other such docu. other means ments, such combination being obvioua to a person skilled document published prior to the Internatlonal filing date but in the art. later than the priority date claimed document member of the same patent family IV. CERTIFICATION Date of the Actual Completion of the International Search Date of Mailing of this International Search Report July 1989 13. 09. 89 International Searthi:g Authority SIgnature Of Auctr EUROPEAN PATENT OFFICE -TK. WILLIS Form PCT/ISA/210 (second aheet) (January 195) f At il i.~j International Aoollcation No. PCT/GB 89 /00361.

111. DOCUMENTS CONSIDERED TO SE RELEVANT (CONTINUED FROM THE SECOND SHEET) Category Citation of Docurnieni, withl I cctiof, Wtmea CofPrlate, Of tne reievant passage% Relevant to Claim No CH, A, 278746 (TECHNICAL ASSETS) 16 February 1952 see page 2, lines 6-69; page 2, line 93 page 3, line 8; page 3, lines 51-69 I Form PCT ISA 210 (extra atteet) januery IM5) 3e. nots on accompanying sheest -w ANNEX TO THE INTERNATIONAL SEARCH REPORT ON INTERNATIONAL PATENT APPLICATION NO, GB 8900562 SA 28797 This annex lists the patcnt famiIN members relatino to the patent documents cited in the abo~e-mentioned international search report. The members are as contained in the European Patent Office 171)1 file on 07/09/89) The European Patent Office is in no Aia liable for these particulars -Ahich are merel% given for the purpose of information. Patent document Publication Patent famil Publication cited int search report date member(s) date DE-A- 3238249 21-04-83 None EP-A- 0061374 29-09-82 FR-A,B 2501916 17-09-82 JP-A- 57262377 06-10-82 GB-A- 665452 None CH-A- 278746 None LFor more details about this annex see Official Journal of tlte European Patent office, No. 12/82 0