Text | China Science Daily reporter Wang Haohao correspondent Wang Jie
Cover of the current issue of Nature Materials. Photo courtesy of the interviewee
One of the important means of strain engineering in material mechanics is to epitaxial growth of another thin film material on a certain substrate through pulsed laser deposition technology, so as to realize the epitaxial strain caused by the mismatch of the unit cell structure.
The research team of the National University of Defense Technology, in cooperation with many universities and research institutions at home and abroad, was inspired by the technique of "turning stones into gold" to realize the strain-enhanced electric card effect in the aura ferroelectric strontium titanate oxide film for the first time, which increased the refrigeration efficiency of the bulk strontium titanate material electric card by more than 10 times, and even increased the refrigeration efficiency by nearly 100 times near the Curie temperature (243K). The results were recently published in Nature Materials in the form of a long article and a research brief, and were selected as the cover article.
The refrigeration technology that has been popular for nearly 100 years needs to be innovated
Refrigeration technology is one of the most important inventions in the process of human civilization and an indispensable technology in modern production and life. For nearly 100 years, vapor compression refrigeration technology has occupied the dominant position in the refrigeration market such as air conditioners and refrigerators.
Hydrofluorocarbons (HFCs) are the core refrigerants of vapor compression refrigeration technology, but their leakage during production, use, and disposal leads to irreversible ozone layer destruction and greenhouse gas emissions. The low energy efficiency and refrigeration power density, large volume and noise, and unfavorable integration of vapor compression refrigeration systems limit their application in the fields of electronics, medical treatment, and new energy vehicles.
"The industry is always developing new refrigeration technologies. Electric card refrigeration has many advantages such as high efficiency and energy saving, environmental friendliness, and rapid refrigeration, and is one of the refrigeration solutions that is expected to replace traditional gas compression technology. Zhang Sen, the first author and corresponding author of the paper and an associate professor at the National University of Defense Technology, said that it is still challenging to achieve a larger electric card effect and a wider operating temperature.
What is the card effect? Zhang Sen explained that it is an effect of the orderly and disordered transformation of the electric dipole of the material under the action of an external electric field, resulting in thermodynamic entropy or temperature changes.
"The electric dipole in the microcosm is like a group of lively and lovely children, moving freely when there is no electric field at the end of class, and the order is low; Sit in your seat when there is an electric field in 'class', and have a high degree of order. Zhang Sen said that the level of this order or the degree of chaos is entropy, which is associated with heat or temperature, so the change of the electric field brings about a change in the degree of order of the microscopic dipole, and further brings about a change in thermodynamic entropy or temperature, so as to achieve heating or cooling.
Zhang Sen introduced that finding new materials with higher power card effect or improving the performance of existing materials is the key link in the future research and development of new devices based on power card refrigeration.
Strontium titanate is a material known as a "premonitor" or "quantum paraelectric" that exhibits ferroelectric or electrothermal cooling effects only at close to absolute zero (-273°C). So, how to make strontium titanate exert a stronger electrical card effect?
Dramatically improve refrigeration efficiency
The electrical jam effect in natural materials is weak, and the optimal operating temperature is low or narrow. To solve these problems, on the one hand, it is necessary to continue to find or synthesize new materials, and on the other hand, it is necessary to increase the electric card effect or its operating temperature in inorganic materials through classical doping or ion substitution processes. "However, this often leads to leakage and even device breakdown damage due to defects caused by doping, which is not good for industrial applications." Zhang Sen said.
"The strontium titanate material has a 'good foundation', and how to cultivate it to produce new materials is a problem that the team has been thinking about." Zhang Sen said that the "turning stones into gold" technique in the field of material mechanics has applications in the regulation of superconductivity, magnetism and ferroelectric properties, and the team was inspired to use it in electrical card enhancement and achieved success.
According to reports, this research lasted for more than 7 years, and more than 30 drafts of the paper were revised alone, and the difficulty was mainly focused on the preparation of high-quality thin films, the fine characterization of structures, the accurate testing of various electrical effects, and the comprehensive and in-depth analysis of Landau theory.
The research team repeatedly verified and deliberated, and finally epitaxial growth of high-quality strontium titanate films on dysprosium scandate single crystal substrates by pulsed laser deposition technology, which increased the inherent electrical card effect of strontium titanate by more than 10 times in the temperature range of 172K to 300K, and reached a hundred-fold enhancement effect around 243K. This means that the refrigeration efficiency of the original bulk strontium titanate material electric card has been increased by more than 10 times, which is one step closer to practical application.
"The strain enhancement scheme we proposed is a new idea, which can better avoid problems such as leakage and device breakdown damage, but it also puts forward higher requirements for the thin film growth process." Zhang Sen said that the proposed scheme of enhancing the electrostatic effect of perovskite oxide thin films through epitaxial strain provides a new idea for expanding the research system of electrical card materials, and also provides an important reference for the new refrigeration technology with high efficiency, energy saving, environmental protection and convenience in the future.
According to reports, during the submission of the paper, the three reviewers all gave affirmation, believing that it is "rare" to systematically characterize and realize the electric card and its enhancement effect in the strontium titanate material, which represents an innovative achievement and important progress in the field of electric card.
Studying "Drosophila in Solid State Physics"
How far is the team's research from application? Zhang Sen said that at present, the electric card effect has been partially applied in the world, and the goal is also very clear, that is, to create a low-carbon, environmentally friendly new refrigeration technology. "Although the enhancement effect of our research results is strong, the effect of strontium titanate itself is too weak, and the final enhanced electric card effect is not very high, and there may be a long distance from practical application. It is optimistically estimated that in the next 5 to 10 years, it is expected to be initially applied in the fields of infrared refrigeration and chip in-situ thermal management. ”
Nobel Laureate in Physics K. Alex Muller calls strontium titanate "the fruit fly of solid state physics", and many important solid state physics phenomena have been discovered from this material, and even some of them are still not fully understood.
"At present, the value of this research is more in the sense of enlightenment for physics and materials science. Based on strontium titanate, our study physically proves that epitaxial strain is an effective solution to enhance the electrocardial effect of existing materials, which can inspire more related research work and promote the development and progress of the study of inorganic electromechanical tencatemia effect. Zhang Sen said.
Related Paper Information:
https://doi.org/10.1038/s41563-024-01831-1
https://doi.org/10.1038/s41563-024-01836-w