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Scientists' breakthrough: How to achieve the wonderful phenomenon of negative thermal expansion in a single substance?
Negative thermal expansion (NTE) is an unusual physicochemical process whereby certain materials contract when heated, rather than expanding like most other materials. The most famous example of this phenomenon is water, which behaves between 0 and 3.98°C. The density of solid water (ice) is lower than that of liquid water at standard pressure, which is the reason for the NTE phenomenon of water, causing ice to float on the water surface instead of sinking.
Materials that can achieve negative thermal expansion may have broad potential in engineering, optoelectronics, electronics, and structural applications.
Research has shown that when some materials are used as thermal expansion compensating materials and mixed with materials that expand normally, composite materials with customized or near-zero thermal expansion can be produced.
Origin of negative thermal expansion
The origin of negative thermal expansion can be attributed to several physical processes, including lateral vibration modes, rigid unit modes, and phase changes. In 2011, researchers discovered that the NTE phenomenon arises from the existence of high-pressure, low-volume configurations that emerge through thermal fluctuations in a stable phase matrix. This allowed them to predict the huge positive thermal expansion (in molybdenum chloride) and infinite negative thermal expansion (in Fe3Pt) that occur in certain materials.
Negative thermal expansion in a closed system
Negative thermal expansion is usually observed in non-close-packed systems, such as ice, graphene, etc. But a recent paper shows that tightly packed lattices of a single component can also achieve NTE behavior. This paper proposes a potential sufficient condition for the NTE phenomenon to occur at fundamental distances, which requires taking into account the interactions between atoms.
This phenomenon is necessary and sufficient in one dimension, but only sufficient but not necessary in two and three dimensions.
Materials with negative thermal expansion
Perhaps the most studied negative thermal expansion material is niobium tungstate (ZrW2O8). The compound continues to shrink over a temperature range of 0.3 to 1050 K. Other materials that exhibit NTE properties include other members of the AM2O8 family as well as HfV2O7 and ZrV2O7, among others. The homogeneity of these materials makes them valuable in engineering applications because their NTE is consistent in three dimensions, facilitating their use as thermal expansion compensators.
Ice at low temperatures also exhibits the property of negative thermal expansion, which is quite useful in engineering.
Application prospects
Combining traditional positive thermal expansion materials with materials with abnormal negative thermal expansion properties will help to regulate the overall thermal expansion rate of the composite material, and even achieve a thermal expansion rate close to zero. This is particularly important in engineering, for example in precision instrumentation applications, when materials are required to maintain stable properties over a wide temperature range.
In daily life, the demand for zero thermal expansion materials is also very significant. For example, ceramic glass stoves have to withstand drastic temperature changes. Also, dental fillings that expand at a rate significantly different from that of the tooth can cause toothache, so using composite materials designed to expand in concert with tooth enamel can avoid this problem.
Faced with the miraculous phenomena of these materials, how will the scientific community further expand the boundaries of this research?
