Is there any alternative that is environment friendly?

Piezoelectricity is a reversible property possessed by a selected group of materials that does not have a center of symmetry. When a dimensional change is imposed on the dielectric, polarization occurs and a voltage or field is created which is termed as direct effect. On the other hand, the application of an electric field to a dielectric may also result in dimensional a change which is termed as inverse effect. Dielectric materials that display this reversible behavior are piezoelectric.

This phenomenon was first discovered in 1880 by Pierre and Jacques Curie who demonstrated that when specially prepared crystals (such as quartz, topaz and Rochelle salt) were subjected to a mechanical stress they could measure a surface charge [1]. A year later, Gabriel Lippmann deduced from thermodynamics that they would also exhibit a strain in an applied electric field. The Curies later experimentally confirmed this effect and proved the linear and reversible nature of piezoelectricity.

One of the first applications of the piezoelectric effect was an ultrasonic submarine detector developed during the First World War. A mosaic of thin quartz crystals glued between two steel plates acted as a transducer that resonated at 50MHz. By submerging the device and applying a voltage they succeeded in emitting a high frequency 'chirp' underwater, which enabled them to measure the depth by timing the return echo. This was the basis for the invention of sonar and this development encouraged other applications using piezoelectric devices both resonating and non-resonating such as microphones, signal filters and ultrasonic transducers. However, many devices were not commercially viable due to the limited performance of the materials at the time.

The continued development of piezoelectric materials has led to a huge market of products ranging from those for everyday use to more specialized devices. In automotive industries they are found in air flow sensors, audible alarms, fuel atomizers, seat belt buzzers, knock sensors etc. Moreover, in computer industries they are found in the disc drives, inkjet printers etc. In medical sector they are used in foetal heart monitors, ultrasonic imaging, disposable patient monitors etc. They are also used in arm forces where they are found in hydrophones, guidance systems, sonar etc. Even they are substantially used in day to day appliances like-cigarette lighters, depth finders, fish finders, humidifiers, jewellery cleaners, musical instruments, speakers, telephones etc.

The most widely used piezoelectric ceramics is known by the acronym PZT which is actually a solid solutions between lead zirconate (PbZrO3) known as PZ and lead titanate (PbTiO3) known as PT. They are used extensively for their excellent piezoelectric properties (d33=374 pC/N, kp=0.67) at the morphotropic phase boundary (MPB) [2]. However, lead and its compounds are generally toxic. The important symptoms of lead poisoning are fatigue, aches in muscles and joints, abdominal discomfort, etc. Patients with poor dental hygiene may exhibit a blue line at the dental margin of the gums due to deposition of lead sulfide. Lead poisoning has long been considered as an environmental health hazard, for its adverse effects on intellectual and neurological development [3-5]. The main route of absorption in adults is the respiratory tract where 30–70% of inhaled lead (mostly the inorganic form like oxides and salts) goes into the circulatory system. For a reasonably well-controlled occupational exposure, blood lead value ranges between 1.45 and 2.4 molL-1 (30–50 µg 100 mL-1) with a provision that there is six monthly monitoring [6,7].

Due to the toxic effects of lead and its compounds, European Union (EU) is planning to restrict the use of hazardous substances such as lead as well as other heavy metals [8, 9]. However, there is no equivalent substitute for PZT; therefore, its use is still continued. This may be a temporary respite, but the legislation certainly impressed the researchers to develop alternative lead-free piezoelectric materials in order to replace lead-based materials [10, 11].

As a result, researchers are now looking into several classes of materials which are potentially attractive alternatives to PZT for applications like sensors, actuators etc. Some of them are: Bi-based piezoelectric ceramics such as perovskite-type (Bi0.5A0.5)TiO3 (A = Na, K) ceramics, M2NaNb5O15 (M = Sr, Ca, Ba) ceramics, BaTiO3 ceramics, Alkaline niobate-based perovskite-type ceramics [12]. Among those several candidates of lead-free piezoelectrics, alkaline niobate-based perovskite type ceramics are promising candidates for lead-free piezoelectric materials and most recent studies have concentrated on the development of KNN-based lead-free ceramics [13-15].

KNN has a lot of potential as a lead free piezoelectric material because similar to PZT it also exhibits MPB. However, there are some processing difficulties that prevent achievement of high piezoelectric properties. As a result, the piezoelectric properties of KNN are still not promising (d33=80 pC/N, kp=0.36).

Although in comparison with PZT, piezoelectric properties of KNN are not great. But, researchers are focusing on developing numerous alternative strategies to overcome the current processing difficulties associated with KNN for the greater sake of environment and mankind.

1.    Curie J, Curie P (1880) Bulletin de la Societe Mineralogique de France 3:90
2.    C. Barry. Carter, M. Grant Norton, “Ceramic materials, science and engineering”.
3.    Gordon JN, Taylor A, Bennette PN (2002) Br J Clin Pharmacol 53:451
4.    Barltrop D, Smith AM (1985) Postgrad Med J 51:770
5.    Rabinowitz MB, Wetherill GW, Kopple JD (1976) J Clin Invest 58:260
6.     Courtney D, Meekin SR (1985) Occup Med 35:128
7.    Control of Lead at Work Regulations (1998) London: The Stationery Office Ltd
8.    Ringgaard E, Wurlitzer T (2005) J Eur Ceram Soc 25:2701
9.    Takenaka T, Nagata H (2005) J Eur Ceram Soc 25:2693
10.    Yi L, Moon K, Wong CP (2005) Science 308:1419
11.    Shimamura K, Takeda H, Kohno T, Fakuda T (1996) J Crystal Growth 163:388
12.    Jungho Ryu, Jong-Jin Choi, Byung-Dong Hahn, Dong-Soo Park, Woon-Ha Yoon, and Kun-Young Kim,       “Sintering and Piezoelectric Properties of KNN Ceramics Doped with KZT”; IEEE transactions on ultrasonics,  ferroelectrics, and frequency control, vol. 54, no. 12, december 2007.
13.    S. Tashiro, H. Nagamatsu and K. Nagata, “Sinterability and Piezoelectric Properties of KNbO3 Ceramics after Substituting Pb and Na for K,” Japanese Journal of Applied Physics, Vol. 4, No. 11B, 2002, pp. 7113-7118. doi:10.1143/JJAP.41.7113
14.    E. Hollenstein, M. Davis, D. Damjanovic and N. Setter, “Piezoelectric Properties of Li- and Ta-Modified (Na0.5 K0.5) NbO3 Ceramics,” Applied Physics Letters, Vol. 87, No. 18, 2005, pp. 182905-182907. doi:10.1063/1.2123387
15.    M. Matsubara, T. Yamaguchi, K. Kikuta and S. Hirano, “Effect of Li Substitution on the Piezoelectric Properties of Potassium Sodium Niobate Ceramics,” Japanese Journal of Applied Physics, Vol. 44, No. 8, 2005, pp. 6136- 6142. doi:10.1143/JJAP.44.6136.

Adnan Mousharraf has won both University Merit Award and Dean’s list Scholarship from Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh.