Progress in the study of giant autothermal effect materials in physics

Artificial refrigeration technology is one of the important symbols of modern industrial civilization. It has been widely used in daily life, industry, agriculture, commerce, medical care, aviation, military and other fields. Current gas compression refrigeration technologies consume a lot of energy and have a serious impact on the environment. The development of a new refrigeration technology that is environmentally friendly, energy-efficient and energy-efficient has become a problem that needs to be solved worldwide.

In the past 10 years, magnetic refrigeration technology based on the magnetocaloric effect has achieved considerable development. Several giant magneto-caloric effect materials have been discovered, including the Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter Physics. La(Fe,Si)13-based compounds, NiMnGa alloys, and the like found in the chambers. The common characteristic of these new materials is that they are intertwined with magnetic phase transitions and structural phase transitions. As a result, the giant magnetocaloric effect is often accompanied by the giant elastic thermal effect/macro-caloric effect caused by the structural phase transition (ie, the external field-driven Doka effect). . Classical thermodynamics states that any change in the degree of order in a substance (eg, magnetic order, lattice order, electrodeposition, etc.) is accompanied by a change in entropy and thus a thermal effect. In addition to the magnetocaloric effect caused by an external magnetic field, pressure can also cause autothermal effects. However, the autoclave effect of solid-state materials without structural phase transitions is usually relatively small, and rarely attracts attention.

Recently, Professor Hu Fengxia, a researcher at the Institute of Physics, Institute of Physics, Wang Jing, a researcher, and Wu Rongrong and Zhao Yingying, Ph.D. students, etc., based on the long-term research accumulation of giant magnetocaloric materials, and the National Institute of Standards (NIST) Professor Huang Qingzhen and Chao. Dong Xiaoli, Research Fellow of the National Key Laboratory, RUAN Changqing, Researcher of the Extreme Conditions Key Physics Laboratory, and Rao Guanghui, Professor of Guangxi Guilin University of Science and Technology, further studied the autoclave effect of magneto-structure coupling materials at room temperature and near room temperature. Research has made new progress. In the hexagonal Ni2In-type compounds, the giant autoclave effect due to the magnetic-structure coupled phase transition was first discovered.

It was found that the hydrostatic energy can drive the magneto-structural transition temperature Tmstr to a low temperature at a rate of 7.7 K/kbar (Fig. 1), resulting in a giant pressure thermal effect. At a pressure of 3kbar, the entropy change is as high as 52Jkg-1K-1 at 299K, and the maximum adiabatic temperature becomes 18.5K (Figure 2), far exceeding the performance of most of the entropy-varied materials under the same pressure or driven by a 5T magnetic field. 3). Further, high resolution neutron diffraction studies have found that the magnetic-structure coupled phase transition of the MnCoGe0.99In0.01 paramagnetic hexagonal phase to the ferromagnetic orthogonal phase is accompanied by a huge negative lattice expansion, with a unit cell volume change of 3.9% (Figure 4). , much higher than other giant magnetocaloric/thermostatic materials with lattice contribution reported. The crystal structure analysis of the neutron diffraction experiment shows that the Mn-Mn interlaminar spacing between MnCoMn and MnCoMnIn0.01 phase transitions under pressure is drastically increased (-2.9×10-2 Å/kbar) and Co-Ge remains unchanged. . The pressure-induced reduction of the Mn-Mn atomic spacing sharply increases the Mn-Mn covalent bond, stabilizes the hexagonal austenite phase and affects the effective 3d band width, and thus influences the magnetic exchange coupling energy before and after phase transformation. The lattice vibration energy has a giant pressure heating effect. Because the pressure of 3kbar is easily realized by using existing laboratory means, this result lays the foundation for the development of solid-state refrigeration technology based on the giant pressure-heat effect, and it is important to further explore the multi-field regulation of solid-state refrigerants and develop solid-state hybrid refrigeration technology. Inspiration. The results of this study are published in Scientific Reports, 5,18027 (2015); doi:10.1038/srep18027.

In relation to this, due to the urgent need of high precision optics and electronics industry for having a specific precise expansion coefficient or even zero-expansion composite material, materials with large negative thermal expansion have attracted people's extensive research interest. Hu Fengxia and Wang Jing have found that MnCoGe-based compounds have large lattice negative thermal expansion near the martensitic transformation in the study of the giant pressure heat effect. However, because the phase transition temperature window is not wide enough and the material is very fragile, this type of compound has not previously been considered as a negative thermal expansion material. They selected the MnCoGe-based series of samples with magnetic-structural phase transitions. Using stress intervention methods, a very small percentage of epoxy bonding was used to greatly improve the mechanical properties of the samples.

More importantly, it was found that the introduced residual stress can soften the acoustic phonons of the material and increase the instability of the crystal lattice, thereby greatly broadening the structural phase change window, and by keeping the preparation conditions as large as possible The crystal lattice is negatively thermal expansion, which in turn causes negative thermal expansion of the large lattice within a wide temperature range. The average coefficient of thermal expansion observed in the MnCo0.98Cr0.02Ge bond sample was as high as -51.5x10-6/K, and the operating temperature range was as wide as 210K. What is even more interesting is that the expansion coefficient (~-119 x 10-6/K) does not substantially change with temperature in the range of about 55K near room temperature (Fig. 5). This result greatly exceeds the vast majority of candidate negative thermal expansion materials that have been reported. Compared with the negatively-expanded material ZrW2O8 which has been widely used at present, the coefficient of linear expansion of the MnCoCrGe bonded sample is ten times its best value.

Even more than 50% of Bi0.95La0.05NiO3 material has been reported to have the largest negative expansion coefficient. The absolute value of the negative expansion coefficient of the MnCoGe-based bonding sample exceeds not only the relatively high aluminum alloy content in the metal (~23.8×10-6/K), but also the first time it reaches the organic glass (~130×10-6/K). The range of materials such as high molecular weight polymer plastics such as nylon (110-140×10-6/K) and engineering thermal plastics such as PTFE-polytetrafluoroethylene (140×10-6/K) can effectively compensate for such materials. With thermal expansion, it is possible to achieve precise control of the thermal expansion properties of organic materials and thus to prepare zero-expansion organic composites. This research has opened up new space for the study of negative thermal expansion materials, and the research results have excellent application prospects. The relevant results have been published in the recent Journal of the American Chemical Society (Journal of American Chemical Society 137, 1746−1749 (2015)), and follow-up studies are progressing steadily.

This series of work was supported by the Chinese Academy of Sciences, the Ministry of Science and Technology, and the National Natural Science Foundation of China.

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