Piezoelectric ceramics efficiency

Recently, Petersson et al. [12] have developed a Lab-on-a-Chip device for particle separation based on this ultrasonic principle. In order to demonstrate the separation of particles with different physical properties in a continuous laminar flow, they have prepared a microfluidic device with a 750 xm wide and 250 p,m deep channel. As a test material, a mixture of milk and blood was used. The ultrasound half standing wave at 2 MHz was produced by a piezoelectric ceramic, which was glued directly on the bottom of the microfluidic device. Milk was used as an initial test medium (Fig. 9a and 9b) to confirm that the hpid particles within the milkfat were attracted toward the pressure anti-nodes of the half standing wave, which exist along the microfluidic channel wall. In addition, when they infused a mixture of milk and blood, they found efficient separation of blood cells from the mixture (Fig. 9c). This demonstration showed that a mixture of heterogeneous conponents can be effectively separated by utilizing ultrasonic standing waves, combined with the laminar flow in microfluidic channels. The separation efficiency of erythrocytes from the mixture was about 70%, while more than 80% of the milkfat lipid particles were... 

Compared with conventional heavily damped piezoelectric ceramic transducers, PVDF transmitters show lower efficiency however, the lower transmitting efficiency is fully adequate for narrow-band measurement systems such as time-delay spectrometry. Furthermore, the frequency response of the PVDF transmitters well exceeds the 40 MHz tested here and as such provide a very broadband alternative to traditional ceramic transmitters [19]. 

CH2—CI2—) —(—CF2— CFH—) (39). Ceramic crystals have a higher piezoelectric efficiency. Their high acoustic impedance compared to body tissues necessitates impedance matching layers between the piezoelectric and the tissue. These layers are similar in function to the antireflective coatings on a lens. Polymer piezoelectric materials possess a more favorable impedance relative to body tissues but have poorer performance characteristics. Newer transducer materials are piezoelectric composites containing ceramic crystals embedded in a polymer matrix (see Composite materials, polymer-MATRIX Piezoelectrics). 

These materials have shown piezoelectric responses after appropriate poling [18]. Their piezoelectric actuation properties are typically worse than ceramic piezoelectric crystals however, they have the advantages of being lightweight, flexible, easily formed, and not brittle. Additionally, while ceramics are limited to strains on the order of 0.1%, ferroelectric polymers are capable of strains of 10% [91] and very high electromechanical coupling efficiencies [93]. 

Piezoelectric transducers are the most common devices employed for the generation of ultrasound and utilise ceramics containing piezoelectric materials such as barium titanate or lead metaniobate. The piezoceramic element commonly used in ultrasonic cleaners and for probe systems is produced in the form of a disk with a central hole. Ceramic transducers are potentially brittle and so it is normal practice to clamp them between metal blocks. This serves both to protect the delicate crystalline material and to prevent it from overheating by acting as a heat sink. Usually two elements are combined so that their overall mechanical motion is additive (Figure 10.4). Piezoelectric transducers are better than 95% electrically efficient and can operate over the whole ultrasonic range. 

---