The strain engineering in materials mechanics is known as the "touchstone" technique in the industry. One of its key methods is to grow another thin film material on a certain substrate through pulsed laser deposition technology, thereby achieving epitaxial strain caused by lattice structure mismatch.
In collaboration with various universities and research institutions both domestically and internationally such as the National University of Defense Technology, University of Cambridge, Beijing University of Science and Technology, University of Costa Rica, and University of Glasgow, inspired by the "touchstone" technique, they have successfully achieved strain-enhanced electric refrigeration effects in a ferroelectric precursor strontium titanate thin film for the first time. This has increased the efficiency of bulk strontium titanate material electric refrigeration cards by over 10 times, and near the Curie temperature (243 K), the refrigeration efficiency can even be increased by nearly a hundredfold.
This achievement was recently published in the journal "Nature Materials" in the form of a long article and a research brief, and was selected as the cover article of "Nature Materials" on May 4th Beijing time. The National University of Defense Technology is the first author unit, with Associate Professor Zhang Sen from the School of Science as the first author and corresponding author of the paper, and Professor Neil Mathur and Xavier Moya from the University of Cambridge, and Professor Gain Guzmán-Verri from the University of Costa Rica as co-corresponding authors.
Research findings selected as cover story for "Nature Materials". Image provided by interviewee.
Revolutionizing Refrigeration Technology Popular for Nearly a Century
Refrigeration technology stands as one of the most crucial inventions in the progression of human civilization, playing an indispensable role in modern production and daily life. For close to a century, vapor compression refrigeration technology has dominated the refrigeration market for appliances like air conditioners and refrigerators.
Hydrofluorocarbons serve as the core refrigerants in vapor compression refrigeration technology. However, their leakage during production, use, and disposal processes has led to irreversible damage to the ozone layer and greenhouse gas emissions. Issues such as low energy efficiency, low cooling power density, large size, and noise of vapor compression refrigeration systems, along with limitations in integration, have hindered their application in fields like electronics, healthcare, and new energy vehicles.
"The industry has been continuously developing emerging refrigeration technologies. Electrocaloric refrigeration, with its advantages in high efficiency, energy conservation, environmental friendliness, and rapid cooling, is considered one of the promising refrigeration solutions to replace traditional gas compression technologies," stated Zhang Sen. Challenges still remain in achieving greater electrocaloric effects and wider operating temperature ranges.
Zhang Sen (right) and team members discussing the principle of the electrocaloric effect. Image provided by interviewee.
What is the electrocaloric effect? It is an effect where the material undergoes an orderly or disorderly transformation of electric dipoles under the influence of an external electric field, leading to a change in thermodynamic entropy or temperature.
"In the microscopic world, electric dipoles are like a group of lively and cute children. During recess (no electric field), they move freely with low orderliness; during class (with an electric field), they sit attentively in their seats with high orderliness," explained Zhang Sen. He further illustrated that the level of order or disorder in this scenario represents entropy, which is related to heat or temperature. Therefore, changes in the electric field affecting the orderliness of microscopic dipoles will further result in changes in thermodynamic entropy or temperature, enabling heating or cooling effects.
Zhang Sen introduced that the search for new materials with higher electrocaloric effects or enhancing the electrocaloric performance of existing materials is a crucial step in developing novel devices based on electrocaloric refrigeration. Strontium titanate material itself is a material known as a "ferroelectric precursor" or "quantum paraelectric," exhibiting ferroelectric properties or electrocaloric cooling effects only near absolute zero (-273 degrees Celsius). How can strontium titanate exhibit a greater electrocaloric effect?
Significant enhancement in cooling efficiency with nanoscale thin films
The electrocaloric effect in natural materials is relatively small, and the optimal operating temperature is also low or narrow. The key to addressing these issues lies in continuing the search for or synthesizing new materials, as well as enhancing the electrocaloric effect or operating temperature in inorganic materials through classical methods such as doping or ion substitution. "However, this often leads to defects caused by doping, resulting in leakage current and even device breakdown, which is detrimental to industrial applications," noted Zhang Sen.
"Strontium titanate material has a good foundation. How to cultivate it well to produce new materials is a question the team has been pondering," Zhang Sen stated. The "alchemy" technique in the materials mechanics field has applications in the control of superconductivity, magnetism, and ferroelectric properties. Inspired by this, the team applied it to enhance the electrocaloric effect and achieved success.
The research, spanning over 7 years, involved over 30 revisions of the paper. The main challenges included high-quality thin film preparation, precise structural characterization, accurate testing of various electrical effects, and comprehensive and in-depth analysis of Landau theory. At each step, the research team meticulously verified and refined their work, striving for excellence.
Ultimately, they successfully grew high-quality strontium titanate thin films on dysprosium scandate single crystal substrates using pulsed laser deposition technology. This enhancement allowed the inherent electrocaloric effect of strontium titanate to increase by over 10 times in the temperature range of 172 K to 300 K, reaching even over a hundredfold enhancement near 243 K. This signifies that the electrocaloric cooling efficiency of bulk strontium titanate material has been increased by over 10 times, bringing it closer to practical applications.
"The strain enhancement approach we proposed is a new direction that can effectively avoid issues like leakage current and device breakdown mentioned earlier, but it also imposes higher demands on thin film growth processes," Zhang Sen explained. The proposed approach of strain-enhancing perovskite oxide thin films through epitaxy provides a new direction for expanding the research system of electrocaloric materials and offers important insights for future high-efficiency, energy-saving, environmentally friendly, and convenient new refrigeration technologies.
Schematic diagram of strain engineering design concept. Image provided by the interviewee.
During the submission process of the paper, all three peer reviewers gave very positive affirmations and support, considering the systematic characterization and realization of the electrocaloric effect in strontium titanate materials to be rare and remarkable. This research represents an innovative achievement and significant progress in the field of electrocalorics, serving as a model for enhancing or regulating electrocaloric effects through strain engineering to achieve efficient and energy-saving refrigeration applications.
Potential applications in fields such as infrared refrigeration.
How far away is the application of the research results from the team? Zhang Sen stated that the electrocaloric effect is already partially applied internationally, with a clear goal of developing a low-carbon, environmentally friendly new refrigeration technology. "Although our research significantly enhances the effect, the intrinsic effect of strontium titanate itself is too small, and the final enhanced electrocaloric effect is not very high, so there may still be a considerable distance from practical application."
Zhang Sen revealed that optimistically, preliminary applications in areas such as infrared refrigeration and on-chip in-situ thermal management may be possible in the next 5 to 10 years.
Zhang Sen is conducting atomic force microscopy structural characterization work. Interviewee provided images.
In 1987, Nobel laureate in Physics K. Alex Muller referred to strontium titanate as the "fruit fly of solid-state physics," as many crucial solid-state physics phenomena have been discovered in this material, some of which are still not fully understood.
"At present, the value of our research lies more in its inspirational significance for physics and materials science. Using strontium titanate as a model, our study physically demonstrates that epitaxial strain is an effective approach to enhancing the electrical properties of existing materials. This could inspire more related work in the future, promoting the development and progress of research on inorganic electrical properties," Zhang Sen stated.
This research was supported by the National Natural Science Foundation and the National Scholarship Fund.
Related paper information:
https://www.nature.com/articles/s41563-024-01831-1
