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The mysterious world of negative thermal expansion: Why do some materials shrink when heated?
In daily life, we often observe that matter expands when heated. However, some materials exhibit the exact opposite behavior and actually shrink when they are heated. This phenomenon is called negative thermal expansion (NTE). This special physical change provides scientists and engineers with an exciting research topic and shows potential application prospects in many fields.
Negative thermal expansion is an unusual physicochemical process in which certain materials contract as they are heated, rather than the expanding behavior exhibited by most materials.
One of the most famous materials with negative thermal expansion is water, which exhibits this property in its behavior over the range of 0 to 3.98 degrees Celsius. Ice is less dense than liquid water, which causes ice cubes to float on water. This characteristic is not only of great significance in nature, but also provides many new ideas for engineering. Mixing negative thermal expansion materials and normal positive thermal expansion materials can be used as thermal expansion compensators to precisely adjust composite materials composed of different materials, or even achieve near-zero thermal expansion.
Origin of negative thermal expansion
The phenomenon of negative thermal expansion is caused by a variety of physical processes, including lateral vibration modes, rigid unit modes and phase changes. The study showed that negative thermal expansion arises from the high entropy presence of high-pressure, small-volume configurations that exist in a matrix of stable phases via thermal fluctuations. This process not only enables the prediction of huge positive thermal expansion of certain materials (such as barium) and zero and infinite negative thermal expansion (such as Fe3Pt), but also lays the foundation for the design of new materials.
Negative thermal expansion in close-packed systemsSome materials can exhibit large negative and positive thermal expansions due to the design of their internal microstructure.
Negative thermal expansion is usually observed in systems that are not tightly packed and have directional interactions, such as ice and graphene. However, the latest research indicates that negative thermal expansion can also be achieved in a single-component, close-packed crystal structure. This means that the behavior of NTE can also be observed in certain systems with specific interactions, thus expanding the horizons of scientific research.
Types of negative thermal expansion materials
In the research of negative thermal expansion materials, zirconate tantalate (ZrW2O8) is the one that attracts the most attention. It continues to shrink over the temperature range of 0.3 to 1050 K, even though the material would disintegrate at higher temperatures. In addition to zirconate tantalates, some other compounds, such as HfV2O7 and ZrV2O7, also exhibit NTE behavior, demonstrating their potential in engineering applications. Special ceramics, carbon fibers and certain mineral crystals (such as quartz) also exhibit negative thermal expansion within a certain temperature range.
Negative thermal expansion is observed in many materials. Some special materials, such as ALLVAR alloy 30, exhibit an instantaneous thermal expansion coefficient of -30 ppm/°C at 20°C, which makes it widely used in aerospace and optics. .
Application Prospects
Negative thermal expansion materials can form a composite with ordinary positive thermal expansion materials to adjust the overall thermal expansion coefficient of the material. This property is particularly important in engineering applications that require zero thermal expansion, such as precision instruments and thermally stable joints. For example, glass ceramic hobs are able to withstand the greatest temperature changes during cooking, using the expansion properties of the different phases to compensate and avoid the risk of cracking. Dental filling materials can also be precisely designed according to the thermal expansion characteristics of teeth in the form of composite materials to reduce patient discomfort.
With the advancement of science and technology, the research and application of negative thermal expansion has gradually received attention, providing new opportunities for future material design and engineering applications. How many mysterious scientific principles are there behind these materials full of potential that are still waiting for us to explore?
