Screen printable
polymer piezoelectrics
Thomas Papakostas and
Neil White
Introduction
Polymer thick-film technology (Gilleo, 1995) is well known in the area of flexible circuit manufacturing. Thick-film pastes are printed on various substrates using screen masks, which contain the desired pattern. These pastes consist of an active phase like carbon, silver or copper particles dispersed in a polymer phase such as epoxies or phenolic resins. Solvents are also added to give the inks the correct
pseudoplastic property for screen-printing. Following the printing, the solvents are dried off and the pastes are cured at temperatures generally less than 2208C. The energy necessary for the cross-linking of the polymer chains is usually applied by convection heating in one to two hours or alternatively by infra-red radiation in less than ten minutes.
Over the last decade there has been a significant amount of research in the exploitation of polymer thick-films (PTF) as sensors and actuators. Carbon-based piezoresistive inks (Arshaket al., 1995), chemical membranes (Goldberget al., 1994) and heaters (Papakostas and White, 1999) have been developed and the range is still expanding. A novel polymer piezoelectric paste has been developed in the University of Southampton, opening up the way to an area of applications previously dominated by piezoceramics like lead zirconate titanate (PZT) or the polymer polyvinylidene fluoride (PVDF).
Piezoelectricity is the ability of some crystalline materials to develop an electric charge proportional to an applied mechanical stress (direct effect) or equivalently a strain proportional to an applied electric field (inverse effect). An essential requirement for the piezoelectric effect is the absence of a centre of symmetry in the crystal so that certain axes of the crystal possess polarity.
Both piezoceramics and PVDF are initially non-piezoelectric due to the random
orientation of their electric dipoles. A strong external electric field must be applied at high temperatures, a process called poling, to permanently align the dipoles to the direction of the electric field, thereby inducing a net polarisation to the material.
Need for a new sensor
Piezoceramics are generally very active materials but they are also brittle and possess The authors
Thomas Papakostasis Researcher in polymer thick-film sensors and hybrid microsystems andNeil Whiteis Senior Lecturer in the Department of Electronics and Computer Science, University of Southampton, Highfield, Southampton SO17 1BJ, UK.
Keywords
Polymer, Thick film, Piezoelectric, Sensors
Abstract
The use of polymer thick-film technology for the implementation of piezoelectric sensors opens up the way towards the low-cost production of piezoelectric sensing devices. A novel polymer thick-film piezoelectric paste is presented and compared with other piezoelectric materials. The main advantages of these films are the low processing temperature, their flexibility and the ease of creating patterns with feature sizes as small as 200m. Various applications are proposed demonstrating the potential of these screen printable polymer piezoelectrics.
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Research articles
135
Sensor Review
Volume 20 . Number 2 . 2000 . pp. 135±138
a high stiffness. Furthermore, the cost of tooling necessary for their shaping and mounting on substrates is high and therefore prohibitive for low-cost applications. PVDF, on the other hand, is flexible with good mechanical, chemical and piezoelectric properties but is usually available in thin sheets (Culshaw, 1996). Therefore,
complicated patterns with sizes in the micron scale are not easy to achieve. The low Curie point (1008C) and the high electric field necessary for their poling (80MV/m) are also important drawbacks.
The paste presented in this paper covers the need for low-cost piezoelectric applications by making the piezoelectric elements an integral part of thick-film technology. The latter may be used to implement both sensors and circuits on a wide range of substrates like Mylar polyester flexible substrates or PC boards that can withstand the curing temperatures.
Fabrication method
New pastes were formed by mixing PZT-5H grains (<6m size) with commercial PTF dielectric pastes in weight ratios ranging from 75 per cent to 95 per cent (Figure 1). The choice for the PZT as the filler material was based on its high charge piezoelectric constant d33(593pC/N). In order to improve the
homogeneity of the inks, these were further mixed in a triple-roll mill. The resulting pastes were polymer ferroelectric composites similar to those investigated by other
researchers (Dias and DasGupta, 1996). A capacitor test structure was printed on various substrates to investigate the properties of the pastes. The structure consisted of the piezoelectric film (25-90m thick)
sandwiched between two PTF conductive electrodes. These test devices were printed on alumina, Mylar and silicon substrates passivated with SiO2, in order to investigate compatibility issues.
Following the printing, the films printed on Mylar were cured at 1508C and the rest at 2008C. Initially, the cured films were not piezoelectric due to the random orientation of the electric dipoles in the paste. Therefore, it was necessary to polarise the films. The poling variables like applied electric field, poling time and poling temperature were varied in order to identify the optimum conditions.
Results
Thed33coefficient of the films relates to the
charge generated for a given applied force. The subscript ``33'' refers to the fact that both the charge measured and the force applied are in the same axis, in this case along the thickness of the sample. A special testing rig was built in our laboratories for this purpose. Essentially the d33coefficient is measured by
applying a known force to the top electrode of the sample, and measuring the charge produced using a purpose-built charge amplifier. A schematic of the arrangement is shown in Figure 2. The values obtained for the various films were repeatable and varied between 5 and 20pC/N for the different pastes used. The piezoelectric activity of these films was found to depend on various processing variables like the PZT weight ratio, the poling conditions or the nature of polymer matrix used. According to theoretical models (Dias and DasGupta, 1996), the piezoelectric activity of these composites is improved by higher PZT loads, higher poling fields and
Figure 1Polymer piezoelectric fabrication method
Figure 2PTF piezoelectric capacitor test structure
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Sensor Review
temperatures, lower polymer resistivity and stiffness, higher polymer permittivity and larger PZT grain sizes. The effect of these parameters was confirmed in our
experiments.
In the case of the PTF composites, though, the need for a pseudoplastic paste viscosity restricts the choice of materials that can be used as well as the maximum PZT loading. A high PZT loading requires excessive thinning of the paste leading to a high short-circuit occurrence or low dielectric breakdown strength. Furthermore, the thickness of the printed films and the area of the screen mesh openings restrict the size of the PZT grains. A relatively big size will result in the blockage of the screen, poor resolution and high surface roughness. A rough film surface renders the quality of the top printed electrode poor.
So far, the best results in terms of rapid turnaround, quality of films and yield were obtained by 85 per cent wt PZT pastes with a d33constant of 15pC/N, which is a lot lower
than that of the bulk PZT. On the contrary, the voltage piezoelectric constantg33d33="
("is the permittivity of the paste) was around 30 10±3Vm/N and comparable to that of PZT. This is due to the very low permittivity of the composite compared to that of the bulk ceramic. Work is continuing to improve these values. The properties of PZT-5H, PVDF and piezoelectric PTF (P-PTF) are shown in Table I for comparison.
Furthermore, the pastes were compatible with all the substrates used, while test patterns with feature sizes of 200m were successfully printed on silicon and alumina. P-PTF printed on Mylar exhibited an outstanding flexibility as seen in Figure 3.
Applications
The potential of P-PTFs is great especially when they are part of a polymer thick-film circuit. In that way, low-cost piezoelectric smart sensors become feasible for applications such as force or pressure sensors. Simple force
sensors have been fabricated in our labs and proved to be very promising even for quasi-static measurements (Papakostas and White, 1999). Furthermore, flexible keyboard membranes (Hickset al., 1980) can be easily implemented on flexible substrates like Mylar as well as electronic keyboards with force sensing. Another application tested in our labs was an alarm sensor (Figure 4).
Mylar strips, with the structure shown in Figure 2 printed on, were fixed on one end forming a cantilever beam. The P-PTF picked up the vibrations induced to the beam creating a very low-cost motion sensor. Similarly, the same sensor printed on Mylar could be embedded in rigid structures for vibration monitoring as an alternative solution to the use of piezoelectric paints (Egusa and Iwasawa, 1998). Other applications like robotic fingertip sensors (Lorenzet al., 1990) or accelerometers (Oharaet al., 1993) could also benefit from PTF piezoelectrics. Generally, the flexibility of both P-PTF and substrate allow the conformation of the piezoelectric structure to uneven surfaces. In this case, the main advantage over PVDF is the ability to easily create complicated electrode-piezoelectric patterns like flexible interdigitated electrode planar capacitors or multilayer piezoelectrics (Figure 5).
Table IPiezoelectric material properties
Piezoelectric material
d33
(pC/N)
g33
(10±3mV/N)
PZT-5H 593 20
PVDF ±33 ±330
P-PTF 15 30
Figure 3P-PTF capacitors printed on Mylar
Figure 4Motion sensor
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Piezoelectric sensors do not need an external power source to operate and therefore they are ideal for low-power applications such as battery-powered devices. Furthermore, the combination of P-PTF with silicon micromachined structures is under investigation, since the low processing temperatures involved allow the inclusion of circuits on the wafer for the implementation of novel hybrid
microsystems.
References
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Figure 5(a) Planar or (b) vertical multilayer P-PTF sensors
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