The Piezoelectric Effect and Long Range Order in Room Temperature Ionic Liquids

Michigan State University, East Lansing MI

Investigators

Abstract

Non-Technical Abstract Ionic liquids are a class of materials that have found wide use because of their unusual properties, ranging from the ability to sequester toxic gases to their use in certain energy storage devices, such as supercapacitors. Despite their wide use, ionic liquids exhibit a number of properties that are not well understood. It was discovered recently that ionic liquids exhibit the piezoelectric effect, which is the ability to produce electric charge when exposed to mechanical force. This effect was seen only in solids until this discovery, and the ability to use liquids to produce electric charge creates many opportunities for advances in technologies such as passive or wearable energy generation, and sensors. To realize such advances, it is essential to understand the fundamental physical and chemical basis for charge generation in ionic liquids. The goals of this project are to understand the size scale over which charge is generated in these materials, and how charge generation can be controlled and optimized based on the chemical structures and physical properties of ionic liquids. The work will be carried out by graduate students spanning the disciplinary areas of chemistry, physics and materials science. The training of scientists who can work at the interfaces between disciplinary areas is essential to the creation of a flexible workforce for science and technology that are in the National interest. Technical Abstract Room Temperature Ionic Liquids have found use in a variety of practical applications, but many fundamental issues remain to be resolved in understanding dynamics, structural order, and the response of these materials to external forces. RTILs exhibit the direct and converse piezoelectric effects. RTILs produce a potential upon the application of force and exhibit a free charge density gradient with a spatial extent of 50 micrometers when in contact with a charged surface. The existence of these effects in a fluid medium is without precedent and the reasons for these phenomena require a fundamental understanding. Experimental and theoretical investigations of RTILs have revealed organization of some type over a variety of length scales, but the “organization” is not well-defined. The existence of any type of organization over such distances is highly unusual for a liquid. Achieving a deeper understanding of this phenomenon and the recently-discovered direct piezoelectric effect in RTILs is essential to understanding how to utilize these materials. The goals of the proposed work are to establish a physical basis for the existence of the direct converse piezoelectric effects in RTILs, and to connect these effects to structural organization in RTILs. The primary goals of this project are to characterize the dependence and operative length scale of the piezoelectric effect on RTIL molecular structure and physical properties through a series of optical and electro-mechanical experiments, and to gain control over the magnitude of the piezoelectric effect in RTILs by deliberately inducing order through surface templating. This work is transformative because it will provide a new conceptual framework in which to understand ionic liquids and related materials, enabling technological advances in areas such as liquid phase, electronically tunable linear and nonlinear optical devices, and passive and wearable power generation. The larger purpose of this research project is to educate a diverse student cohort at the cutting edge of interdisciplinary science. It is imperative that the United States train students to be globally competitive for careers in science. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.