# What are some materials that expand a lot proportionally to heat?

There's never any source on the internet that can simply give me a list of materials that have the kinds of properties I'm looking for. I'm wondering what materials, like for a solid block of something, in general expand either a lot or at least very quickly when exposed to heat. I also can't remember what property this is exactly, but I think it's the coefficient of thermal expansion.

Am I looking for something with a high coefficient or low coefficient to achieve this?

• Dry ice would vaporize and expand quickly and it's practical to use. Liquids such as hydrazine expand very quick but are dangerous to use. I'm not of the co efficient – Technetium Jun 24 '17 at 1:15
• Dry ice is an interesting idea, but I'm looking for something more stable like at room temperature. – RayOfHope Jun 24 '17 at 1:25
• Anything that is stable at room temperature most likely will not expand at a fast rate (violently) like what your describing. Hence the stability factor. Most materials will vaporize and expand at some rate when heat is applied tho. Some may require a lot of heat , some only a bit. – Technetium Jun 24 '17 at 1:53
• Right, but I'm telling you right now, I'm looking for "solid materials," something with which to construct something else. Last time I checked, you can't build a machine out of glycerin or ethanol. – RayOfHope Jun 24 '17 at 3:23
• Oh you want to build a machine! I thought you were reffering to solid phase (solid,liquid,gas) not specifically solid enough to build a machine. While there are plenty of solids that will expand quickly when heat is applied I'm not sure about a material solid enough to build a machine . There must be something that acts like that tho maybe a resin,plastic or fibreglass with explosive properties that could be molded – Technetium Jun 25 '17 at 4:35

You are looking for thermal expansion and its coefficient. Of course such tables, and diagrams exist

(source)

as it is of relevance in daily live. Thermometers based on liquid mercury, or alcohol, depend on this. Bridges take this into account in expansion joints, too:

(source)

Joining two materials of different expansion coefficient equally is of relevance for the design of bimetallic stripes, found not only in thermometers and mechanical clocks:

(source)

For solids you are looking for the linear thermal expansion coefficient $$α_L$$:

$$α_L = \frac{\mathrm{d}L}{L\cdot \mathrm{d}T}$$

where $$L$$ is length and can be uniform for isotropic materials (amorphous compounds, some polymers) and varied for anisotropic materials (crystals). For crystalline materials $$L$$ is often given for each axis $$a, b, c$$ of a crystal's unit cell resulting in direction-dependent thermal expansion. Obviously, the higher the $$α_L$$, the higher the value of expansion $$(\mathrm{d}L/L)$$ per temperature change $$\mathrm{d}T$$, so the expansion of the solid is directly proportional to $$α_L$$.

The Engineering ToolBox provides a decent list of thermal expansion coefficients for common solids at NTP. Looking for the materials with high thermal expansion coefficients, you probably have to look at polymers such as ethylene ethyl acrylate $$(α_L = \pu{205e-6 K-1})$$, ethylene vinyl acetate $$(α_L = \pu{180e-6 K-1})$$ or polyethylene $$(α_L = \pu{108e-6 K-1}~\text{to}~\pu{200e-6 K-1})$$.

Paraffin wax also has high $$α_L = \pu{106e-6 K-1}~\text{to}~\pu{480e-6 K-1}$$ (depends on exact composition) and expands drastically when reaches its melting point.

Among crystalline matter, the current record is held by silver(I) hexacyanocobaltate(III), $$\ce{Ag3[Co(CN)6]}$$ [1]. Due to mobile layer of silver atoms in between $$\ce{Co(CN)6}$$ sheets the crystal lattice can fold in one direction and stretch in another. Such high anisotropy results in $$α_a = \pu{130e-6 K-1}~\text{to}~\pu{150e-6 K-1}$$ and $$α_c = \pu{-130e-6 K-1}~\text{to}~\pu{-120e-6 K-1}$$. This is the record values for both negative and positive thermal expansion in crystals.

### References

1. Goodwin, A. L.; Calleja, M.; Conterio, M. J.; Dove, M. T.; Evans, J. S. O.; Keen, D. A.; Peters, L.; Tucker, M. G. Colossal Positive and Negative Thermal Expansion in the Framework Material $$\ce{Ag3[Co(CN)6]}$$. Science 2008, 319 (5864), 794–797. https://doi.org/10.1126/science.1151442.