The piezoelectric effect was discovered by brothers Pierre and Jacques Curie in 1880, and named after the Greek word piezein, that means to press or squeeze. They found that certain crystals were able to generate an electrical charge when mechanically loaded with tension or pressure. If exposed to an electric field, the same crystals would undergo a controlled deformation known as the inverse piezoelectric effect.
Compression and tension generate voltages of opposite polarity, so when compressed, the piezoelectric material decreases in volume and will have a voltage that has the same polarity as the material. When subjected to tension, the material will increase in volume and its voltage will be opposite to the polarity of the material.
Piezoelectric materials produce a voltage that is proportional to whatever mechanical pressure is applied. There are two types of material – crystals and ceramics. Most crystals occur naturally, while piezoelectric ceramics are manmade.
Piezoelectric systems manufactured today are primarily made from PZT, which is a mixture of lead zirconate and lead titanate crystals, or PZT and barium titanate. Making piezoelectric ceramics involves mixing the components in very specific proportions, then heating them to create a uniform powder. This powder is mixed with an organic binder before being formed into various structural elements.
Once the elements have been fired according to specified temperature and time requirements, they are shaped and trimmed, and electrodes are applied to surfaces where required. A variety of tools are used for this, but ID slicing saws are one of the most common and efficient solutions for machining piezoceramics.
Pushing ‘print’ on large-scale piezoelectric materials
Researchers have developed a revolutionary method to ‘print’ large-scale sheets of two dimensional piezoelectric material, opening new opportunities for piezo-sensors and energy harvesting. Importantly, the inexpensive process allows the integration of piezoelectric components directly onto silicon chips. Until now, no 2D piezoelectric material has been manufactured in large sheets, making it impossible to integrate into silicon chips or use in large-scale surface manufacturing.
This limitation meant that piezo accelerometer devices — such as vehicle air bag triggers or the devices that recognise orientation changes in mobile phones — have required separate, expensive components to be embedded onto silicon substrates, adding significant manufacturing costs. Now, FLEET researchers at RMIT University in Melbourne have demonstrated a method to produce large-scale 2D gallium phosphate sheets, allowing this material to be formed at large scales in low-cost, low-temperature manufacturing processes onto silicon substrates, or any other surface.
Gallium phosphate (GaPO4) is an important piezoelectric material commonly used in pressure sensors and microgram-scale mass measurement, particularly in high temperatures or other harsh environments. “As so often in science, this work builds on past successes,” lead researcher Professor Kourosh Kalantar-zadeh explains. “We adopted the liquid-metal material deposition technique we developed recently to create 2D films of GaPO4 through an easy, two-step process.”
The revolutionary new method allows easy, inexpensive growth of large-area (several centimetres), wide-bandgap, 2D GaPO4 nanosheets of unit cell thickness. It is the first demonstration of strong, out-of-plane piezoelectricity of the popular piezoelectric material.
THE TWO-STEP PROCESS
- Exfoliate self-limiting gallium oxide from the surface of liquid gallium made possible by the lack of affinity between oxide and the bulk of the liquid metal.
- ‘Print’ that film onto a substrate and transform it into 2D GaPO4 via exposure to phosphate vapour.
The new process is simple, scalable, low-temperature and cost effective, significantly expanding the range of materials available to industry at such scales and quality. The process is suitable for the synthesis of free standing GaPO4 nanosheets. The low temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future 2D piezoelectric materials. This simple, industry-compatible procedure to print large surface area 2D piezoelectric films onto any substrate offers tremendous opportunities for the development of piezo-sensors and energy harvesters.
Mechanical engineers develop process to 3-D print piezoelectric materials
Xiaoyu ‘Rayne’ Zheng, assistant professor of mechanical engineering in the College of Engineering, and a member of the Macromolecules Innovation Institute, and his team have developed methods to 3D print piezoelectric materials that can be custom-designed to convert movement, impact and stress from any directions to electrical energy.
The piezoelectric materials come in only a few defined shapes and are made of brittle crystal and ceramic — the kind that require a clean room to manufacture. Zheng’s team has developed a technique to 3D print these materials so they are not restricted by shape or size. The material can also be activated — providing the next generation of intelligent infrastructures and smart materials for tactile sensing, impact and vibration monitoring, energy harvesting, and other applications.
Unleash the freedom to design piezoelectrics
Piezoelectric materials were originally discovered in the 19th century. Since then the advances in manufacturing technology has led to the requirement of clean-rooms and a complex procedure that produces films and blocks which are connected to electronics after machining. The expensive process and the inherent brittleness of the material, has limited the ability to maximize the material’s potential.
Zheng’s team developed a model that allows them to manipulate and design arbitrary piezoelectric constants, resulting in the material generating electric charge movement in response to incoming forces and vibrations from any direction, via a set of 3D printable topologies. Unlike conventional piezoelectrics where electric charge movements are prescribed by the intrinsic crystals, the new method allows users to prescribe and program voltage responses to be magnified, reversed or suppressed in any direction.
“We have developed a design method and printing platform to freely design the sensitivity and operational modes of piezoelectric materials,” Zheng said. “By programming the 3D active topology, you can achieve pretty much any combination of piezoelectric coefficients within a material, and use them as transducers and sensors that are not only flexible and strong, but also respond to pressure, vibrations and impacts via electric signals that tell the location, magnitude and direction of the impacts within any location of these materials.”
3D printing of piezoelectrics, sensors and transducers
A factor in current piezoelectric fabrication is the natural crystal used. At the atomic level, the orientation of atoms are fixed. Zheng’s team has produced a substitute that mimics the crystal but allows for the lattice orientation to be altered by design.
“We have synthesized a class of highly sensitive piezoelectric inks that can be sculpted into complex three-dimensional features with ultraviolet light. The inks contain highly concentrated piezoelectric nanocrystals bonded with UV-sensitive gels, which form a solution — a milky mixture like melted crystal — that we print with a high-resolution digital light 3D printer,” Zheng said.
The team demonstrated the 3D printed materials at a scale measuring fractions of the diameter of a human hair. “We can tailor the architecture to make them more flexible and use them, for instance, as energy harvesting devices, wrapping them around any arbitrary curvature,” Zheng said. “We can make them thick, and light, stiff or energy-absorbing.”
The material has sensitivities 5-fold higher than flexible piezoelectric polymers. The stiffness and shape of the material can be tuned and produced as a thin sheet resembling a strip of gauze, or as a stiff block. “We have a team making them into wearable devices, like rings, insoles, and fitting them into a boxing glove where we will be able to record impact forces and monitor the health of the user,” said Zheng.
“The ability to achieve the desired mechanical, electrical and thermal properties will significantly reduce the time and effort needed to develop practical materials,” said Shashank Priya, associate VP for research at Penn State and former professor of mechanical engineering at Virginia Tech.
The team has printed and demonstrated smart materials wrapped around curved surfaces, worn on hands and fingers to convert motion, and harvest the mechanical energy, but the applications go well beyond wearables and consumer electronics. Zheng sees the technology as a leap into robotics, energy harvesting, tactile sensing and intelligent infrastructure, where a structure is made entirely with piezoelectric material, sensing impacts, vibrations and motions, and allowing for those to be monitored and located. The team has printed a small smart bridge to demonstrate its applicability to sensing the locations of dropping impacts, as well as its magnitude, while robust enough to absorb the impact energy. The team also demonstrated their application of a smart transducer that converts underwater vibration signals to electric voltages.
“Traditionally, if you wanted to monitor the internal strength of a structure, you would need to have a lot of individual sensors placed all over the structure, each with a number of leads and connectors,” said Huachen Cui, a doctoral student with Zheng and first author of the Nature Materials paper. “Here, the structure itself is the sensor — it can monitor itself.”