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Nearly every material, whether it is solid, liquid, or gas, expands when its temperature goes up and contracts when its temperature goes down. This property, called thermal expansion, makes a hot air balloon float, and the phenomenon has been harnessed to create thermostats that automatically turn a home furnace on and off. Railroads, bridges, and buildings are designed with this property in mind, and they are given room to expand without buckling or breaking on a hot day.
Thermal expansion occurs because a material's atoms vibrate more as its temperature increases. The more its atoms vibrate, the more they push away from their neighboring atoms. As the space between the atoms increases, the density of the material decreases and its overall size increases.
There are a few exceptions, but by and large, materials conform strictly to this principle. There is, however, a class of metal alloys called Invars (think invariable), that stubbornly refuse to change in size and density over a large range of temperatures.
The new theory proposes that certain alloys, specifically those with a high degree of disorder in their atomic structure, do not change size when heated because of the presence of nanoscale defects called "point defects." These defects act as obstacles for the movement of atoms, preventing the alloy from expanding or contracting when heated.
Previous explanations for why certain alloys do not change size when heated focused on the presence of impurities or the arrangement of atoms in the alloy. This new theory shifts the focus to the role of point defects and their impact on the movement of atoms in the alloy.
Experiments have been conducted on various alloys, including iron-nickel and nickel-titanium, which have shown that the presence of point defects correlates with the alloys' ability to maintain their size when heated. Additionally, computer simulations have also supported the role of point defects in this phenomenon.
Understanding the role of point defects in certain alloys can have practical implications in industries such as aerospace and construction, where materials are subjected to extreme temperatures. This knowledge can help engineers select the appropriate alloys for specific applications to ensure stability and durability.
As with any scientific theory, there are limitations and areas that require further research. This theory is currently focused on certain alloys and does not account for other factors that may influence the size change of materials when heated, such as pressure or composition. Further studies and experiments are needed to fully understand the role of point defects in this phenomenon.