Developments in Ceramic Materials Research

Progress in Porous Piezoceramics 231
5. EVALUATION OF POROUS PIEZOCERAMIC HYDROPHONES
As seen in the previous sections, the 3-3 piezocomposites made up of porous
piezoceramics are useful materials for transducer applications, especially as hydrophones.
The lossy nature of the composite materials leads to wide-band response, which is an
important property of a hydrophone. Several commercially available hydrophones have flat
frequency response over a large band below the first resonance [41]. However, such a
hydrophone made up of dense piezoceramic material requires very careful selection of
dimensions of various components used in the device and meets conflicting design
challenges. Such a design gets greatly simplified if the piezoelectric component is wideband
in nature, like the piezocomposites. This section presents the construction and underwater
evaluation of hydrophones made up of 3-3 piezocomoposites and a comparison with
hydrophones made up of dense piezoceramic material of the same size [10].
5.1. Construction of Hydrophones
5.1.1. Sample Preparation
Samples required for constructing the hydrophones are prepared using Foam-replication
technique described in detail in Section 2. The foam replication method is well known for
making reticulated ceramics [43, 44, 45] and involves dipping a polyethylene foam (Foam
Engineers Ltd, UK, 10 ppi and 20 ppi) into a ceramic slurry. Since the hydrophones are for
passive mode only, the ceramic chosen for this study is PZT-5H (Morgan Electroceramics,
UK), a soft piezoelectric for sensor applications due to its high d33 piezoelectric coefficient.
The ceramic-coated foam is then heat treated to remove the polyethylene foam structure at
600 °C for 1 hour. The remaining ceramic skeleton is then sintered at 1200 °C for 2 hours.
Variation of the ceramic volume fraction, by changing the foam pore size or degree of
coating, can be used to tailor the piezoelectric charge coefficients (d33, d31), permittivity
( T
33
ε ) and acoustic impedance. The dH of the material reaches a maximum at approximately
50% ceramic volume fraction, while the gH is a maximum at approximately 10-20% ceramic
volume fraction [46]. Since gH is related to the electric field per unit hydrostatic stress,
hydrophone materials with high gH are produced for this work with 11% and 22% PZT
ceramic volume fractions by using 10 ppi and 20 ppi polyethylene foams, respectively.
After heat treating and sintering, the discs (11% PZT and 22% PZT) are ground by SiC
abrasive paper to dimensions of 40 mm diameter and 4±0.1 mm thickness. The 4 mm
thickness is larger than the pore size of the material and ensures sufficient ceramic material is
connected through the ceramic thickness. A dense PZT ceramic with same dimensions and
tolerance is also prepared from the same ceramic powder for direct comparison. Due to the
low strength of the 11% PZT disk, it is impregnated with epoxy (Specifix 40, Struers,
Germany) to improve its mechanical strength. It acts as a 3-3 piezocomposite.
Silver electrodes (~100 mm thick) are applied by brushing on silver conductive paint (RS
Products, UK, Product 186-3593) to the two faces. The discs are subsequently poled in the
thickness direction using a custom made corona poling using a 30 kV DC power supply
(Glassman Europe Ltd., Product FC30P4) at an electric field equivalent to 2 kVmm -1 .