Controlled characterisation

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Bringing temperature and environmental control to ferro- and piezoelectric characterisation

Ferroelectric materials are defined as ‘crystalline materials that exhibit spontaneous electrical polarisations, switchable by an external electric field.’ These materials play a key role in modern society, with uses across electronic equipment, mobile phones, medical diagnostic instruments, cameras and military sensors, among others. Radiant Technologies was responsible for the creation of the first ferroelectric testing system in 1988, which has helped customers characterise these non-linear materials.

Piezoelectricity is the ability of a substance (particularly certain crystals) to generate an internal electrical field when subjected to mechanical stress. As a result, piezoelectricity is well suited to audio or medical applications, as the vibrations caused by placing the crystals under mechanical stress results in electrical energy that can be used in ultrasound scanning,  microphones and even record players. Radiant also specialises in the measurement of piezoelectric properties of non-linear materials. All of the company’s testers are equipped with high-impedance voltage input, to capture the output of the displacement sensor to measure piezoelectric properties. Radiant interfaces Linkam Scientific Instruments’ electrical stages with its Precision Multiferroic II Ferroelectric Test System, to bring temperature control to ferroelectric and piezoelectric characterisation.

“We began working together with Linkam when a mutual customer asked to interface the Linkam stage with our ferroelectric tester, to bring in temperature as a testing parameter,” says Michelle Bell at Radiant Technologies. “As we continued our work together, we paired our Precision Test Systems interface with the Linkam HFS600E-PB4 stage to test non-linear materials. As one of the biggest concerns in ferroelectric testing is electrical content from external factors, there are often difficulties when conducting temperature control as it can create additional electrical noise, leading to inaccuracies in results. This can be a particular concern for measuring thin films, when making contact with the sample itself. The probes that come with the stage make this process much simpler, and, as the stages have been configured to reduce electrical noise, both of these additional parasitic factors are removed, allowing users to bring temperature control into their testing without compromising results.”

The stage stabilises within 0.1°C of the assigned temperature but can change the temperature at a rate up to 150°C per minute, or 2.5°C per second. This high rate of change in combination with the internal electrical connections enables the instruments to measure the change in the electric charge following a temperature change of one degree – otherwise known as the pyroelectric coefficient.

Duncan Stacey at Linkam, comments: “The stage has been customised to further improve its functionality with the Radiant Precision Test systems. The sample pedestal inside the temperature stage chamber and the chamber enclosure are earth grounded, making the chamber an efficient Faraday cage that can shield the sample from ambient electrical noise. This, coupled with a passive ceramic insulator to hold the sample, provided by Radiant, results in quiet measurements of samples down to very small capacitance values.

The BNC connectors handle up to 300v, making it possible to execute all of the electrical measurements of thin ferroelectric and piezoelectric capacitors allowed by Radiant test instruments over the 600°C to -196°C temperature range. Bulk ceramic or single crystal capacitors may also be tested in this fixture up to 300 volts. This is enough voltage to generate full hysteresis loops on PMN-PT single crystal actuators and to test single layer or multilayer piezoelectric actuators over temperature.”

Increasing demand for piezoelectric testing

Piezoelectric actuators work by converting mechanical stress, such as a force or voltage, into a controlling motion. As these actuators are made up of simple designs, and use minimal movement, they are used in a variety of applications, such as medical, and consumer electronics.  These two applications in particular continue to grow and expand with technological advances, making piezoelectric actuators more in demand than ever.

As the option to bring temperature control into piezoelectric testing is becoming increasingly well known, Radiant has found that many users are opting to assess temperature against other measurements, such as hysteresis, leakage and breakdown voltage to get a more complete picture.

There is also an increase in research using piezoelectric nanoelectromechanical systems, with many users now focusing their efforts on the measurement of lead magnesium niobate-lead titanate (PMN-PT) single crystal actuators, to test single-layer and multi-layer piezoelectric actuators over temperature.

Real-world applications for ferroelectric testing

Ferroelectric materials have a range of applications that make them essential to researchers and consumers alike. For example, these materials are suitable for biological applications as ‘virtual electrodes’, as their spontaneous electric polarisation allows scientists to generate an electrical current under the right conditions, without an external electrical source.

Ferroelectric materials also have a place in infrared detection and thermal imaging, as small pyroelectric detector arrays form the basis of infrared burglar and intruder alarm products.

With both ferroelectric and piezoelectric materials serving a purpose across various technologies in modern life, research into them is sure to continue apace. Radiant and Linkam have developed an integrated measurement system that helps researchers to better understand how they behave, produce more efficient ferroelectric and piezoelectric materials and develop more applications for them in future.