Air-Coupled Ultrasonic Transducers
I. Air-coupled monolithic piezoelectric transducers.
Wide-band transducers
Narrow-band transducers
Water resistant transducers.
II. Air-coupled phased array piezoelectric transducers.
III. Air-coupled ferroelectret transducers.
Conventional transducers (350 kHz)
Miniaturized (9 mm diam) transducers (350 kHz)
IV. Mixed transducers: piezoelectric + ferroelectret
V. Spherically focused transducers.
VI. Embedded pair of wideband transducers and sample holder for RUS of plate/layered materials.
____________________________________________________________________
I. Air-coupled monolithic piezoelectric transducers.
We produce the wideband and air-coupled piezoelectric transducers that we requiere for our investigations in the fields of characterization of porous materials, NDT (CFRP, sandwich structures, etc.) and the study of vegetal tissues and food stuff, where the use of conventional ultrasonic coupling fluids is not permitted because the fluid can either contaminate or modify (or both) the properties of the material under investigation.
A main feature of our transducers is that they present a relatively large bandwidth (30-100%), while they also present an unrivalled sensitivity that ranges from -24 dB at 0.20 MHz to -40 dB at 2.00 MHz for wide badn transducers and as low as -19 dB for 250 kHz narrowband transducers.
This unique technology enable us to study wave dispersion in highly dispersive materials (foams, scaffolds, etc), to test highly attenuating materials (uncured CFRP, sandwich structures with foam or honeycomb cores) or high impedance materials (aluminium, steel, etc.) or biological samples were use of fluids is not possible: plant leaves, food stuff, etc.
We mainly produce plane monolithic transducers, but we have also manufactured spherically focused transducers and linear phased arrays.
Figure below shows the schemmatic design of the monolithic transducers for the frequency range 0.20-2.00 MHz
Figures below show the characterization of the transducers in through transmission operation mode.
WIDEBAND TRANSDUCERS
200 kHz tansducers
250 kHz transducers
400 kHz transducers
500 kHz transducers
650 kHz transducers
750 kHz transducers
NARROWBAND TRANSDUCERS
250 kHz transducers
WATER RESISTANT WIDEBAND AIR-COUPLED TRANSDUCERS
Unlike conventional air-coupled transducers, in this case, radiatting surface is fabricated of a completely hydrophobic material, so the transducer is water resistant: it is not affected by contact with water and does not absorb moisture.
500 kHz transducers
II. Air-coupled phased array piezoelectric transducers.
16 elements, 0.75 MHz array
III. Air-coupled ferroelectret transducers.
Conventional transducers (300 kHz)
These transducers are based on a ferroelectret film. Resonant frequency is 300 kHz. Compared with piezocomposite transducers they present a similar SNR though a more reduced sensitivity. An advantage is that smaller sizes can be achieved.
Miniaturized (9 mm diam) transducers (300 kHz)
Picture of the miniaturized 300 kHz ferroelectret transducers
Acoustic field of the miniaturized 300 kHz ferroelectret transducer. Receiver: flat transducer with a 0.7 mm pinhole aperture.
IV. Mixed transducers: piezoelectric + ferroelectret
V. Spherically focused transducers.
When high sensitivity is not enough and high spatial resolution is also needed (e.g. NDT applications) spherically focused transducers offer the possibility to get both.
We have developed a technology that permits to keep the sensitivity at figures similar to that shown before, and even improving them when operating in tone burst mode, while achieving a 6 dB focal spot under 5 mm.
Centre frequency Focal distance Focal spot (6 dB) Sensitivity Excitation
250 kHz 70 mm 5 mm -18 dB tone burst (15 cycles)
400 kHz 70 mm 4 mm -18 dB tone busrt (15 cycles)
Acoustic field of the 250 kHz focused transducer,
Transducer surface is at 22 mm at the left of the x-axis origin
VI. Embedded pair of wideband transducers and sample holder for RUS of plate/layered materials.
A pair of wideband tansducers (centre frequency of 0.25, 0.65 or 1.25 MHz, but a reduced size version) are mounted on a holder specially designed to perform through transmission measurements and for the characterization of solid plates by measuring the transmission coefficient at normal incidence. This holder allows to have the right transducers location, the right separation between them and also provides a slot for an easy location of the sample to be measured (see Sensors 2013, 13(5), 5996-6013; doi:10.3390/s130505996)
Left: Picture of the Tx-Rx pair (=.65 MHz centre freq) embedded in the sample holder, Right: Picture of the same device as used to monitor variation in the thickness resonances of Vitis vinifera leaves , see Ultrasoun in Med. & Biol. 2014. http://dx.doi.org/10.1016/j.ultrasmedbio.2014.04.004
