Power/Performance Bits: March 8

Non-toxic, printable piezoelectric; GaN MEMS resonator; verifying software models.


Non-toxic, printable piezoelectric
Researchers at RMIT University and University of New South Wales developed a flexible and printable piezoelectric material that could be used in self-powered electronics including wearables and implantables.

“Until now, the best performing nano-thin piezoelectrics have been based on lead, a toxic material that is not suitable for biomedical use,” said Dr Nasir Mahmood, a Vice-Chancellor’s Research Fellow at RMIT. “Our new material is based on non-toxic zinc oxide, which is also lightweight and compatible with silicon, making it easy to integrate into current electronics. It’s so efficient that all you need is a single 1.1 nanometer layer of our material to produce all the energy required for a fully self-powering nanodevice.”

The piezoelectric material, which the researchers say is 800% more efficient than other piezoelectrics based on non-toxic materials, is fabricated with a liquid metal printing approach. Zinc oxide is first heated until it becomes liquid. This liquid metal, once exposed to oxygen, forms a nano-thin layer on top. The metal is then rolled over a surface, to print off nano-thin sheets of the zinc oxide “skin.” The team said it can be used to rapidly produce large-scale sheets of the material and is compatible with any manufacturing process, including roll-to-roll processing.

The researchers are working on developing ultrasonic detectors for use in defense and infrastructure monitoring, as well as investigating the development of nanogenerators for harvesting mechanical energy. One such application could be devices that convert blood pressure into a power source for pacemakers.

GaN MEMS resonator
MEMS resonators can obtain the high temporal resolution and small phase noise needed to generate timing signals for 5G systems. However, silicon-based MEMS resonators suffer from poor stability at high temperatures. A researcher at the National Institute for Materials Science in Japan developed a MEMS resonator that stably operates even under high temperatures by using gallium nitride (GaN) to regulate the strain caused by heat.

A high-quality GaN epitaxial film was fabricated directly on a Si substrate using metal organic chemical vapor deposition (MOCVD) to fabricate the GaN resonator. Strain engineering was used to improve temporal performance, with strain created from the lattice mismatch and thermal mismatch between the GaN layer and its Si substrate.

Optimizing the temperature decrease method during MOCVD growth allowed the GaN to suffer no cracks and have crystalline quality comparable to the typical method of using a superlattice strain-removal layer between it and the Si.

The GaN-based MEMS resonator was able to operate stably at temperatures as high as 600K. Tests showed it had a high temporal resolution and good temporal stability with little frequency shift when the temperature was increased, thanks to the internal thermal strain compensating for the frequency shift and reducing the energy dissipation.

The device can be integrated with CMOS technology, and the researcher points to applications in 5G communication, IoT timing device, on-vehicle applications, and advanced driver assistance system.

Verifying software models
Researchers at Universitat Oberta de Catalunya are proposing a new method to reduce errors in software by ensuring the software models are correct throughout development.

Abstracted models that describe a software system can be more concise, easier to produce and understand than the source code itself, noted Robert Clarisó, a professor in UOC’s Faculty of Computer Science, Multimedia and Telecommunications. “The model type most frequently used is the UML (Unified Modelling Language) class diagram notation, which is used to describe the structure of a software system.” The aim is to make it easier to understand the system being developed and automate some repetitive elements.

However, there are some problems when verifying these software models. “We need to ensure the models are correct in order to minimize possible errors in the software that could occur as a result,” said Clarisó. Whenever the model is changed or modified, the whole system needs to be re-analyzed, leading to the verification typically only being conducted once a definitive model is in place.

The new technique for UML/Object Constraint Language (OCL) software models allows them to be verified during construction. “Our article outlines the application of incremental methods of verification, that is, we make it easier to verify a model any time changes are made,” said Clarisó.

In particular, the method focuses on incrementally verifying the internal consistency of UML class diagrams annotated with OCL constraints. It relies upon the use of certificates that illustrate the correct operation of the model. Rather than verifying the new model, a certificate from the original model could be adapted to the new one.

“When we modify a model, having a new certificate would remove the need for its verification,” said Clarisó. “It’s far less costly to adapt a certificate than it is to rerun the verification process.” The next challenge, the team said, is integrating these techniques into existing software modelling tools and environments.

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