When Sustainability is Cool: Elastocaloric & Magnetocaloric Materials

Elastocaloric and magnetocaloric materials are promising candidates for environmentally friendly, solid-state cooling technologies. These materials exploit the principle of the caloric effect, which is the temperature change induced by an external field (mechanical stress or magnetic field) on the material. Both elastocaloric and magnetocaloric materials offer pathways toward energy-efficient refrigeration without the harmful refrigerants found in traditional vapor-compression systems.

The applications of elastocaloric and magnetocaloric shape memory alloys (SMAs) are expanding rapidly, especially within fields focused on sustainable and efficient cooling solutions. In 2024, the number of publications on elastocaloric and magnetocaloric materials and applications has increased tremendously. This highlights the potential of these materials for energy applications, particularly in refrigeration, heat pumping, air conditioning, and thermal energy harvesting systems.

Types of Caloric Effects

1. Elastocaloric Effect – Found in materials that undergo temperature changes when subjected to mechanical stress (e.g., stretching or compression). Shape memory alloys like nickel-titanium (NiTi) are common examples, with phase changes occurring under stress that result in heat being absorbed or released.
2. Magnetocaloric Effect – Occurs in materials that change temperature in response to a magnetic field. When exposed to a magnetic field, magnetic dipoles within the material align, releasing heat, while removing the field causes them to return to a random state, absorbing heat. Examples include gadolinium and certain rare-earth compounds.
3. Electrocaloric Effect – In this case, a material changes temperature when exposed to an electric field. This effect occurs in certain dielectric materials (often ferroelectric materials), which undergo a change in entropy when an electric field aligns the dipoles within the material. Examples include certain ceramic compounds like lead zirconate titanate (PZT) and polymers like polyvinylidene fluoride (PVDF).
4. Barocaloric Effect – These materials exhibit temperature changes when subjected to pressure changes. Similar to elastocaloric materials, barocaloric materials also undergo phase transitions, but the external stimulus is pressure rather than mechanical stress. Materials exhibiting this effect include some organic salts, plastic crystals, and certain polymers.
5. Thermocaloric Effect – This effect, often referred to as thermal expansion or contraction in materials, involves a direct response to temperature changes, affecting the material's volume or structure. Although it's not strictly caloric in nature, thermal responses are sometimes included for their relevance in managing heat.

Recent Advancements in Elastocaloric and Magnetocaloric Materials

Recent research has focused on overcoming the challenges associated with these materials to make them viable for practical applications.

1. Machine Learning Optimization: Machine learning models have been applied to predict and enhance the elastocaloric properties of NiMn-based alloys, facilitating the discovery of compositions with optimal temperature changes and entropy shifts. These predictive models help streamline the alloy development process, significantly reducing experimental demands[1]
2. Spark Plasma Sintering: In Ni-Co-Mn-Ti alloys, spark plasma sintering has been shown to enhance elastocaloric strength by improving grain refinement and mechanical durability. This method allows for the creation of alloys with high adiabatic temperature changes and robust mechanical properties, essential for practical applications in solid-state cooling[3].
3. Hierarchical Microstructures: Research on Ni-Mn-based Heusler alloys has revealed that controlling the material's microstructure, particularly through hierarchical martensite formation, can improve elastocaloric efficiency and cyclic stability. This hierarchical approach enables the material to maintain structural integrity under repeated stress cycles[4].
4. Multifunctional Thin Films: By integrating Ni-Mn-based Heusler alloy films into silicon microtechnology, researchers have enabled the use of elastocaloric materials in micro-scale devices. This development opens possibilities for efficient cooling in compact and portable electronic systems[5].
5. Combination of Positive and Negative Elastocaloric Effects: Combining materials with positive and negative elastocaloric effects has demonstrated potential for doubling cooling efficiency at room temperature. This approach is particularly effective in achieving higher temperature changes in a single device by using ferroelectric materials that operate under lower stress, improving overall system durability and efficiency [6].
6. Training of Alloys for Stability: In NiTiCuV alloys, mechanical and thermal training processes have been employed to enhance the cyclic stability and reduce hysteresis, thus increasing the homogeneity of the elastocaloric effect across repeated cycles [7].
7. Towards practical elastocaloric cooling" (2023): This article discusses the development of elastocaloric cooling as an environmentally friendly technology, focusing on material properties, manufacturing techniques, and actuators that drive elastocaloric materials. [8]
8. Continuous and efficient elastocaloric air cooling by coil-bending (2023): Researchers present a low-force, energy-efficient elastocaloric air cooling approach based on coil-bending of NiTi ribbons and wires, achieving continuous cold air output with a temperature drop of 5.2 K. [9]
9. Long-term stable compressive elastocaloric cooling system with latent heat recovery (2021): This study introduces a compressive elastocaloric cooling system that maintains long-term stability and incorporates latent heat recovery, enhancing the efficiency of elastocaloric cooling devices.[10]
10. Materials, physics and systems for multicaloric cooling (2022): The article reviews various caloric effects, including elastocaloric, barocaloric, and magnetocaloric, discussing the materials, physics, and systems involved in multicaloric cooling technologies. [11]
11. All-d-Metal Heusler Alloys: A class of shape memory alloys (SMAs) is composed entirely of d-block metals, specifically Ni(Co)-Mn-Ti alloys, which exhibit improved mechanical properties and significant magnetocaloric and elastocaloric effects. For instance, the directionally solidified Ni35.5Co14.5Mn35Ti15 alloy exhibits superior elastocaloric properties due to its unique microstructure and texture. Moreover, the combination of low critical stress, significant adiabatic temperature change, and moderate hysteresis makes it a promising candidate for solid-state cooling applications.[12]

Recent Advancements in Elastocaloric and Magnetocaloric Shape Memory Alloys (SMAs)

These advances have enhanced the potential for solid-state cooling applications. Here's an overview of the latest developments:

Elastocaloric Shape Memory Alloys:

Enhanced Elastocaloric Effect and Cyclic Stability:

Researchers have demonstrated a substantial elastocaloric effect with improved cyclic stability in a <001> textured polycrystalline (Ni??Mn??Ti??)??B? alloy. This material achieved an adiabatic temperature change (ΔT_ad) exceeding 15 K across a temperature range from 283 K to 373 K, with a peak ΔT_ad of ?27.2 K under a compressive stress of 412 MPa at 303 K. [13]

SMA Film-Based Elastocaloric Cooling Devices:

Shape memory alloy films have been explored for elastocaloric cooling due to their large surface-to-volume ratio, which facilitates efficient heat transfer. These films exhibit significant elastocaloric effects and enable solid-to-solid contact heat exchange, addressing limitations of bulk SMAs, such as low operational frequency. [14]

Development of TiZrNbAl Alloys:

A novel TiZrNbAl shape memory alloy has been developed, exhibiting excellent room-temperature superelasticity with a critical stress around 100 MPa and a small stress hysteresis of approximately 70 MPa. This alloy demonstrates a large elastocaloric effect over a broad temperature range, making it a promising candidate for elastocaloric refrigeration. [15]

Magnetocaloric Shape Memory Alloys:

Ni-Mn-In-Based Heusler Alloys:

Studies on Ni-Mn-In-based ferromagnetic shape memory alloys have shown that doping with elements like Cu and Fe can enhance magnetocaloric properties. These modifications improve the coupling between martensitic and magnetic transformations, leading to significant magnetocaloric effects suitable for solid-state refrigeration. [16]

Improved Magnetocaloric Reversibility:

Research on Ni?Mn?.?In?.? alloys with Pt substitution has demonstrated improved crystallographic compatibility between austenite and martensite phases. This enhancement results in reduced thermal hysteresis and reversible magnetocaloric effects, which are crucial for practical magnetic refrigeration applications. [17]

Additive Manufacturing of Magnetocaloric Materials:

Advancements in additive manufacturing techniques have enabled the production of Co??Ni??Ga?? ferromagnetic shape memory alloys with properties comparable to as-cast counterparts. This approach allows for precise control over material composition and microstructure, facilitating the development of magnetocaloric materials with tailored properties. [18]

Challenges of Magnetocaloric Materials:

The performance of magnetocaloric refrigeration devices depends not only on the magnitude of the MCE but also on factors like hysteresis, thermal conductivity, and mechanical stability. [19]

The Future of Sustainable Refrigeration by Elastocaloric and Magnetocaloric Materials

Integrating theoretical modeling with machine learning is increasingly recognized as a transformative approach in the development of elastocaloric and magnetocaloric materials. Here’s a deep exploration of future research directions based on recent advances and insights drawn from the provided publications:

1. Machine Learning for Material Composition and Process Optimization

Machine learning (ML) provides a pathway to efficiently explore the vast parameter space in alloy composition and processing conditions for optimizing elastocaloric effects. For example, Yang et al. [1] employed ML to identify Ni50Mn33Ti17 as a high-potential composition, using the XGBoost Regressor model to predict an adiabatic temperature change of up to 10 K at room temperature. ML algorithms like Random Forest and Support Vector Regressor were also compared, demonstrating that ML models, especially XGBoost, can streamline the discovery process by accurately predicting performance metrics and identifying key alloying elements.

In the future, expanding databases of experimental and computational data will enable even more refined ML models that can predict not only elastocaloric properties but also material fatigue and stability under repeated cyclic loading. With higher computational power, real-time optimization of alloying compositions and manufacturing conditions could become feasible, making ML a cornerstone in the practical implementation of elastocaloric cooling technologies.

2. First-Principles Calculations to Refine Entropy Contributions

First-principles calculations based on density functional theory (DFT) have proven essential in understanding the entropy changes (e.g., vibrational and magnetic entropy) associated with the martensitic transformations in caloric materials. Huang et al. applied a first-principles approach to estimate the volume change ratio (ΔV/V0) in Ni-Mn-In-Ga alloys, which directly influences the entropy change and adiabatic temperature variation [20]. This model enabled the identification of Ga as an effective substitute for In, enhancing the volume change and, consequently, the elastocaloric effect.

Moving forward, hybrid models combining ML and first-principles calculations could allow researchers to dynamically adjust compositions based on real-time predictions of entropy changes and phase stability. This integration could make it possible to achieve precise control over the caloric effect by fine-tuning both alloy composition and microstructure.

3. Multicaloric Effects and Coupled Field Responses

The exploration of multicaloric effects—where multiple stimuli (e.g., stress and magnetic fields) are applied simultaneously—represents an exciting frontier in caloric materials. Ma?osa and Planes highlight the potential of magnetic and mechanical field combinations in shape memory alloys to induce high multicaloric responses, expanding the operational range of cooling systems [21]. This approach can optimize the cooling cycle efficiency by utilizing synergistic effects between elastocaloric and magnetocaloric responses.

Materials that exhibit both elastocaloric and magnetocaloric effects are of significant interest because they offer multiple avenues for solid-state cooling applications. These materials can respond to mechanical stress and magnetic fields, providing flexibility and efficiency in cooling technologies.

Future research may focus on developing theoretical models that can accurately predict the multicaloric responses based on coupled-field theories. Such models could be enhanced by ML to predict optimal stimulus combinations, leading to the development of adaptive cooling systems that can switch between different caloric effects depending on the operational requirements.

4. Crystallographic Texture and Microstructural Engineering

The texture and microstructure of elastocaloric materials significantly impact their cyclic stability and caloric performance. Huang et al. demonstrated the role of directional solidification in achieving strong crystallographic textures in Ni-Mn-In-Ga alloys, which enhance cyclic durability and reduce thermal hysteresis [20]. Such microstructural control can improve the energy efficiency of elastocaloric devices by minimizing energy losses due to material fatigue.

In future research, integrating ML algorithms with microstructure prediction models could allow for the rapid identification of texture and grain structure configurations that optimize the balance between thermal and mechanical properties. This approach could lead to the development of scalable fabrication techniques that produce microstructures with enhanced fatigue resistance and elastocaloric strength.

5. Expanding the Scope of Alloying Elements

The introduction of new alloying elements, guided by computational models and ML predictions, offers promising avenues for enhancing caloric effects. Substitutions such as Ga for In or Cu for Mn have shown to positively impact volume change ratios and reduce adverse magnetic entropy effects [1, 20]. Expanding the range of alloying elements could unlock higher-performing compositions that are stable across a broader temperature range, making them suitable for diverse applications, from room-temperature cooling to microelectronics.

To further this field, future research could employ generative adversarial networks (GANs) to propose entirely novel compositions, with ML models trained to predict their phase stability and caloric responses. This would allow researchers to explore previously inaccessible areas of the composition space, potentially identifying new classes of high-performance caloric materials.

Conclusion

Combining data-driven predictive methods with experimental microstructural optimization provides a powerful approach to designing materials exhibiting both elastocaloric and magnetocaloric effects. By integrating the insights from both studies:

• We can predict and optimize compositions that enhance both elastocaloric and magnetocaloric effects.
• We gain mechanistic understanding of how microstructure influences dual caloric effects.
• We can develop multifunctional materials for advanced solid-state cooling applications.

While SMAs and Heusler alloys are not known to exhibit a significant electrocaloric effect on their own, the exploration of multifunctional materials and composites continues. Researchers are investigating ways to integrate ferroelectric properties into these materials or to create composites that can harness multiple caloric effects simultaneously.

Several research efforts are required to overcome material and engineering challenges for elastocaloric and magnetocaloric technologies, including enhancing material properties and system integration.On the other hand, multidisciplinary collaboration and continued innovation are essential to realize the full potential of elastocaloric and magnetocaloric applications.

Last but not least, research efforts should aim to find cost-effective materials and manufacturing processes to make caloric devices economically competitive with traditional technologies.

Declaration of the use of artificial intelligence:

This article was developed with assistance from OpenAI’s language model, ChatGPT. The AI was utilized to synthesize recent research insights and present concepts related to elastocaloric and magnetocaloric materials in a clear, structured manner. Drawing on recent literature, AI generated concise summaries, organized thematic areas, and provided a draft for the LinkedIn article as part of the “#Materials Insights” newsletter. All content was then reviewed and customized to ensure accuracy and alignment with current research trends.

References

1. Yang, Y., Fu, H., Gao, W., Su, W., Sun, B., & Yi, X. (2024). Data-driven high elastocaloric NiMn-based shape memory alloy optimization with machine learning. Materials Letters, 371, 136948. https://doi.org/10.1016/j.matlet.2024.136948
2. Wu, Y., Guan, Z., Tang, H., Li, Z., & Lu, X. (2024). Excellent mechanical properties and giant room-temperature elastocaloric effect in spark plasma sintered Ni-Co-Mn-Ti shape memory alloy. Physica B, 696, 416611. https://doi.org/10.1016/j.physb.2024.416611
3. Schwabe, S., Niemann, R., Backen, A., Wolf, D., Damm, C., & F?hler, S. (2020). Building hierarchical martensite. Advanced Functional Materials, 31, 2005715. https://doi.org/10.1002/adfm.202005715
4. Fink, L., Kar, S., Lünser, K., Nielsch, K., Reith, H., & F?hler, S. (2023). Integration of multifunctional epitaxial magnetic shape memory films in silicon microtechnology. Advanced Functional Materials, 33, 2305273. https://doi.org/10.1002/adfm.202305273
5. Barman, A., Kar-Narayan, S., & Mukherjee, D. (2019). Caloric effects in perovskite oxides. Advanced Materials Interfaces, 6, 1900291. https://doi.org/10.1002/admi.201900291
6. Zhu, W., Shi, X., Gao, R., Wang, J., Zhang, G., & Huang, H. (2022). Enhancing the elastocaloric strength by combining positive and negative elastocaloric effects. Physica Status Solidi RRL, 16, 2200183. https://doi.org/10.1002/pssr.202200183
7. Schmidt, M., Ullrich, J., Wieczorek, A., Frenzel, J., Schütze, A., Eggeler, G., & Seelecke, S. (2015). Thermal stabilization of NiTiCuV shape memory alloys: Observations during elastocaloric training. Shape Memory and Superelasticity, 1, 132–141. https://doi.org/10.1007/s40830-015-0021-4
8. Wang, Y., Liu, Y., Xu, S., Zhou, G., Yu, J. and Qian, S., 2023. Towards practical elastocaloric cooling. Communications Engineering, 2(1), p.79. https://doi.org/10.1038/s44172-023-00129-5
9. Li, X., Hua, P. and Sun, Q., 2023. Continuous and efficient elastocaloric air cooling by coil-bending. Nature Communications, 14(1), p.7982. https://doi.org/10.1038/s41467-023-43611-6
10. Bachmann, N., Fitger, A., Maier, L.M., Mahlke, A., Sch?fer-Welsen, O., Koch, T. and Bartholomé, K., 2021. Long-term stable compressive elastocaloric cooling system with latent heat transfer. Communications Physics, 4(1), p.194. https://doi.org/10.1038/s42005-021-00697-y
11. Hou, H., Qian, S. and Takeuchi, I., 2022. Materials, physics and systems for multicaloric cooling. Nature Reviews Materials, 7(8), pp.633-652. https://doi.org/10.1038/s41578-022-00428-x
12. Zhang, Q., Qian, M. and Zhang, X., 2022. Magneto-Structural Transition and Refrigeration Property in All-D-Metal Heusler Alloys: A Critical Review. Journal of Solar Energy Research Updates, 9, pp.52-69. https://doi.org/10.31875/2410-2199.2022.09.6
13. Wang, H., Wang, Y., Zhang, G., Li, Z., Yang, J., Li, J., Yang, B., Yan, H. and Zuo, L., 2024. Giant Elastocaloric Effect and Improved Cyclic Stability in a Directionally Solidified (Ni50Mn31Ti19) 99B1 Alloy. Materials, 17(19), p.4756. https://doi.org/10.3390/ma17194756
14. Xu, J., Bruederlin, F., Bumke, L., Ossmer, H., Quandt, E., Miyazaki, S. and Kohl, M., 2024. SMA Film-Based Elastocaloric Cooling Devices. Shape Memory and Superelasticity, pp.1-15. https://doi.org/10.1007/s40830-024-00484-y
15. Lv, B.H., Xiang, H.Y., Gao, S., Guo, Y.X., Yang, J.H., Zou, N.F., Zhao, X., Li, Z., Yang, B., Jia, N. and Yan, H.L., 2024. Realization of Large Low-Stress Elastocaloric Effect in TiZrNbAl Alloy. Materials, 17(4), p.885. https://doi.org/10.3390/ma17040885
16. Ma, S., Zhang, X., Zheng, G., Qian, M. and Geng, L., 2024. Cu-and Fe-Doped Ni-Mn-Sn Shape Memory Alloys with Enhanced Mechanical and Magnetocaloric Properties. Materials, 17(13), p.3172. https://doi.org/10.3390/ma17133172
17. Dubey, K.K., Devi, P., Singh, A.K. and Singh, S., 2020. Improved crystallographic compatibility and magnetocaloric reversibility in Pt substituted Ni2Mn1. 4In0. 6 magnetic shape memory Heusler alloy. Journal of Magnetism and Magnetic Materials, 507, p.166818. https://doi.org/10.1016/j.jmmm.2020.166818
18. Law, J.Y., Moreno-Ramírez, L.M., Díaz-García, á. and Franco, V., 2023. Current perspective in magnetocaloric materials research. Journal of Applied Physics, 133(4). https://doi.org/10.1063/5.0130035
19. Smith, A., Bahl, C.R., Bj?rk, R., Engelbrecht, K., Nielsen, K.K. and Pryds, N., 2012. Materials challenges for high performance magnetocaloric refrigeration devices. Advanced Energy Materials, 2(11), pp.1288-1318. https://doi.org/10.1002/aenm.201200167
20. Huang, X.M., Zhao, Y., Yan, H.L., Tang, S., Yang, Y., Jia, N., Yang, B., Li, Z., Zhang, Y., Esling, C. and Zhao, X., 2023. A first-principle assisted framework for designing high elastocaloric Ni–Mn-based magnetic shape memory alloy. Journal of Materials Science & Technology, 134, pp.151-162. https://doi.org/10.1016/j.jmst.2022.06.041
21. Ma?osa, L. and Planes, A., 2024. Elastocaloric Effect in Shape-Memory Alloys. Shape Memory and Superelasticity, pp.1-10. https://doi.org/10.1007/s40830-024-00477-x