Large thermoelastic effect in martensitic phase of ferroelastic alloys for high efficiency heat pumping

Solid state heat pumping using latent heat from first order ferroic phase transitions is a promising green alternative to traditional vapor compression technology. However, the intrinsic phase transition hysteresis poses a limitation on heat pumping energy efficiency. Here, we propose heat pumping using reversible heat from anhysteretic elastic deformation in martensitic phase of ferroelastic alloys. Conventionally, this thermoelastic effect (TeE) is considered too weak to be practical. But we find that in [100]-textured Ti₇₈Nb₂₂ martensitic polycrystals, the TeE can produce a large adiabatic temperature change (∆T_ad) of 4-5 K at 413-473 K due to macroscopic large linear thermal expansion (α_l = 10^-4/K). This large TeE not only far exceeds those of ordinary metals ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>a</mml:mi> <mml:mi>d</mml:mi></mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.2</mml:mn> <mml:mi>K</mml:mi></mml:math> ) but also brings a material-level energy efficiency that reaches about 90% of the Carnot theoretical limit. In other ferroelastic martensitic alloys with larger intrinsic α_l (up to 5.4 × 10^-4/K), the TeE is predicted to bring an even larger ∆T_ad (up to 22 K) while maintaining relatively high efficiency. Our findings offer a non-phase-transition-based way for high efficiency solid state heat pumping.