Characterisation of piezoelectric materials
The reputation of NPL's expertise in functional materials has been built up through years of research and lies in the characterisation of materials that have some extra function apart from their obvious mechanical or chemical properties. Often it is a material that will couple one form of energy to another, for instance a piezoelectric will transform electrical energy into mechanical displacement in an ultrasonic cleaning bath. The group has built up an extensive suite of characterisation facilities for piezoelectric materials and the following lists some of these.
Polarisation Field (PE loop) analysis
The signature of a true ferroelectric material is the ability to switch electrically between two polarisation states, and this is characterised using the polarisation loop method. The functional materials group at NPL have over 20 years' experience in making these measurements and have used in house systems, commercial systems and also installed two in-situ synchrotron beamline systems at Diamond and ESRF. The current workhorse is a an Aixacct Piezoelectric Evaluation System (aixPES), which can test bulk samples at voltages up to 10kV, and at temperatures of up to 250C. This also has an integrated SIOS interferometer system for the simultaneous measurement of piezoelectric displacements. The software suite enables the automatic characterisation of the functional behaviour as a function of temperature, fatigue, and coupled with an impedance analyser (HP 4192A) the measurement of C-V behaviour.
Measurement of the Pyroelectric Effect
Measurement of the pyroelectric coefficient of materials can be measured via two methods. The standard method is Byer Roundy, where the temperature of the sample is cycled sinusoidally with an amplitude of a few degrees, whilst measuring the charge produced by the sample. This can be done above room temperature using our Aixacct system, but with addition of a Peltier Element the temperature of the sample can be cooled to below room temperature. A second method is to use the temperature dependence of PE loops, where the slope of a plot of the remnant polarisation against temperature also gives the pyroelectric coefficient.
Jamin Common Path Interferometer system
The displacements produced by piezoelectric materials under low field conditions can be smaller than is measureable using a standard Michelson Interferometer set-up such as the SIOS attached to the Aixacct system. For instance, piezoelectric quartz only displaces 2.3 pm with an applied voltage of 1V. To measure these sorts of displacements and smaller movements associated with thin film piezoelectrics, NPL has developed a Jamin Common path system. Using a common path set-up, coupled with lock-in techniques, it enables thermal and acoustic noise sources to be reduced enabling sub pm displacements to be measured.
In order to measure the piezoelectric coefficient of a thin film material on a substrate, it is necessary to be able to remove the bending contribution. Traditionally, this has been done using a beam path that interrogates the front and rear surface of a thin film sample. However, the extended beam path means the size of the interferometer is large and maintaining path stability over this size is more difficult. NPL has concentrated on design on the stability and registration of the sample holder such that a single sided interferometer can be used on a thin film sample, measuring the front and rear separately and subtracting the results.
Resonance analysis for low field piezoelectric characterisation
For virtually any kind of finite element modelling of active piezoelectric devices, the first stumbling block people have is where to get all the data to populate the model. For a model of a typical piezoelectric material with ∞mm symmetry, it requires six separate elastic constants, three piezoelectric coefficients and two dielectric constants. This is where resonance analysis characterisation comes in. By analysing the resonance modes of four simple shaped samples, these parameters can be derived. This method is defined in a European standard EN 50324-2 'Piezoelectric properties of ceramic materials and components Part 2: Methods of measurement Low power'. The Functional Materials Group at NPL were involved in defining this standard, and have written a best practice guide on the method. The method can also be extended to include losses where all the coefficients now have a real and imaginary part. These values are needed for some FEA codes that predict the loss.
Measurement of the piezoelectric coefficient using a Berlincourt tester
The measurement of the direct piezoelectric coefficient is a simple but effective test used in industry and academia to characterise the piezoelectric output of samples. Sometimes termed a 'Berlincourt tester', it applies an oscillating force and measures the resulting charge - the ratio is the piezoelectric coefficient. NPL has several Berlincourt type systems that can measure the d33, d31 and dh hydrostatic piezoelectric coefficient over a frequency range from 30-300Hz. They offer a rapid and cost-effective method of assessing the piezoelectric output of bulk materials in a variety of shapes and forms. NPL has written a best practice guide on the method, and are also developing a MEMS based tool that will perform a similar function on smaller samples.
LIMM measurement
In the most part ,characterisation of piezoelectric materials uses methods where sample sizes are of the order of millimetres or larger, and any quantitative results are an average value of these samples. It also assumes that the properties are homogeneous throughout the sample under test. Presently, the only technique where the spatial variation of the piezoelectric properties can be probed is piezoelectric mode atomic force microscopy, PFM, but here the technique is limited to use on very small sample sizes.
In the laser intensity modulation method, LIMM - an intensity modulated laser - is used as a periodic heat source which generates a pyroelectric current within the material that is detected using lock-in techniques. By changing the frequency of the intensity modulation, films can be probed to different depths. NPL has pioneered the use of this method to characterise piezoelectric materials, and has extended its use into scanning samples. The system has been used to characterise the polarisation variation of a number of different applications.
Dielectric breakdown testing
The dielectric breakdown strength of materials is important for many dielectric materials applications, but particularly for piezoelectric materials where high fields are needed to achieve the maximum output from a device. For breakdown tests on bulk materials to standards such as ASTM-149 or BS EN 60243-1, we have a 50kV Sefelec dielectric breakdown tester. We have also carried out less routine breakdown investigations on piezoelectric materials, for example withstand voltage over time whilst measuring leakage currents in high humidity.
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Publications
- Properties of morphotropic phase boundary Pb(Mg1/3Nb2/3)O3-PbTiO3 films with submicron range thickness on Si based substrates
M. Alguero, M. Stewart, M. G. Cain, P. Ramos, J. Ricote, M. L. Calzada
J. Phys. D.: App. Phys, 43 (20), 205401 (2010) - Spatial characterization of piezoelectric materials using scanning laser intensity modulation method (LIMM)
M. Stewart
J. Am. Ceram. Soc., 91 (7), pages 2176-2181 (2008) - Piezoelectric resonance
M. G. Cain, M. Stewart
Measurement Good Practice Guide No. 33 (2001) - Measuring piezoelectric d33 coefficients using the direct method
M. Stewart, W, Battrick, M. G. Cain
Measurement Good Practice Guide No. 44 (2001)