Recently, a scientific study led by Wu Junqiao of the American Chinese Science Prize found that vanadium dioxide material in the conductive, the heat generated is very small, much lower than the Weidmann – Franz law is expected to break the classic laws of physics. The scientific discovery, published in the January 27 issue of Science, will bring a wider range of applications, such as a thermoelectric system that converts waste heat from engines and equipment into electrical energy.
Weidman – Franz ‘s Law
Weidmann-Franz’s law is the law about the relationship between the material thermal conductivity λ and the conductivity σ. German physicist G. H. Wiederman and R. Franz is found by a large number of experimental facts, the thermal conductivity of many metals and the ratio of conductivity is a constant, this law is called Weidman-Franz law. 1891 H. A. Lorenz further found that the ratio λ / σT is a constant independent of the metal species, T is the absolute temperature, which is expressed by L, said Lorentz number.
Simply put, the law shows that the better the conductivity of the material, then the better the thermal conductivity.
However, vanadium dioxide material is an exception, it does not comply with this law.
Vanadium dioxide is a kind of metal oxide with phase transition property. The phase transition temperature is about 67 ℃, and it can change from the insulator to the metal when the phase transition temperature is reached. After the phase change, the infrared light can be generated by the transmission The reversible transformation.
At the United States Oak Ridge National Laboratory Nano-phase Materials Science Center, the University of Tennessee research assistant Alexander Teseloff and French scientists, with the theory of condensed physics successfully explained the phase behavior of vanadium dioxide. They found that the heterogeneous competition phenomenon of vanadium dioxide was caused by lattice symmetry, and that the vanadium dioxide lattice could be “folded” in different ways during cooling, so that the observed phenomenon was that vanadium dioxide was different Of the folded form.
Prof. Wu Junqiao, a researcher at the Department of Materials Science and Engineering at the University of California, Berkeley, and physicist at the Berkeley Laboratory in Materials Science, commented:
“This is a totally unexpected finding that the law of physics that is unbreakable for traditional conductors is disintegrated by this finding, which is of fundamental importance in understanding the basic electronic behavior of novel conductors.”
research process
Professor Wu and his research team and the United States Department of Energy Oak Ridge National Laboratory Duke University Associate Professor Olivier Delaire cooperation, launched a study of the characteristics of vanadium dioxide. Through simulation and X-ray scattering experiments, they combed the material lattice vibration (phonon) and electron motion, the impact rate for thermal conductivity.
The vanadium dioxide nanowires (VO2) synthesized by Berkeley researchers have singular conductive and thermal properties. In fake color scanning electron microscopy images, the thermal conductivity is calculated from the heat measurements conducted between the blue sensory panels that are blue-sensitive. The heat between the two plates is conducted by the VO2 nanowires.
For this phenomenon, Professor Wu explained:
“Electrons are in motion with each other, like a liquid, rather than a single particle in a common metal.For electrons, heat is a random motion. Ordinary metals can effectively conduct heat, because a single electron can In many different possibilities of micro-morphological jump between the relatively speaking, the electrons in the vanadium dioxide, consistent with each other, like the military band, for the heat conduction disadvantage.This is due to the electronic switch can change the form of less “”
In particular, by mixing vanadium dioxide with other substances, the amount of conductivity and heat transfer can be adjusted. When the researchers in the single crystal vanadium oxide samples doped with tungsten, vanadium dioxide into metal when the phase transition temperature becomes low. At the same time, the electrons in the metal phase become better thermal conductors. This allows the vanadium to undergo phase change from the insulator to the metal, the researchers can control the heat dissipation, and vice versa, the temperature can also be adjusted.
Related applications
The researchers said that this material can be used to eliminate or distribute the heat in the engine, or the development of glass coating to improve the utilization of building energy.
The research leader of the study, Berkeley laboratory researcher Yang Fan (transliteration) said:
“This material can be used to stabilize the temperature, by tuning the thermal conductivity, the material can be hot summer heat and efficient heat, because it has high thermal conductivity; but in the cold winter, you can prevent heat loss, because it is low When the low thermal conductivity.
Vanadium dioxide also has an advantage: in 30 degrees Celsius below is transparent, more than 60 degrees Celsius can absorb infrared.
Yang Fan said that before the commercialization of vanadium dioxide, there are some problems to be resolved, but this study highlights the material “strange electrical and thermal properties.” Although some of the other materials can also have similar characteristics, but they occur under the conditions of almost zero Baidu, it is difficult to carry out real business.
