High-performance multimode elastocaloric cooling system.

Developing zero-global warming potential refrigerants has emerged as one area that helps address global climate change concerns. Various high-efficiency caloric cooling techniques meet this goal, but scaling them up to technologically meaningful performance remains challenging. We have developed an elastocaloric cooling system with a maximum cooling power of 260 watts and a maximum temperature span of 22.5 kelvin. These values are among the highest reported for any caloric cooling system. Its key feature is the compression of fatigue-resistant elastocaloric nitinol (NiTi) tubes configured in a versatile multimode heat exchange architecture, which allows the harnessing of both high delivered cooling power and large temperature spans. Our system shows that elastocaloric cooling, which only emerged 8 years ago, is a promising direction for commercializing caloric cooling.

Combinatorial Exploration and Mapping of Phase Transformation in a Ni-Ti-Co Thin Film Library.

Combinatorial synthesis and high-throughput characterization of a Ni-Ti-Co thin film materials library are reported for exploration of reversible martensitic transformation. The library was prepared by magnetron co-sputtering, annealed in vacuum at 500 °C without atmospheric exposure, and evaluated for shape memory behavior as an indicator of transformation. Composition, structure, and transformation behavior of the 177 pads in the library were characterized using high-throughput wavelength dispersive spectroscopy (WDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and four-point probe temperature-dependent resistance (R(T)) measurements. A new, expanded composition space having phase transformation with low thermal hysteresis and Co > 10 at. % is found. Unsupervised machine learning methods of hierarchical clustering were employed to streamline data processing of the large XRD and XPS data sets. Through cluster analysis of XRD data, we identified and mapped the constituent structural phases. Composition-structure-property maps for the ternary system are made to correlate the functional properties to the local microstructure and composition of the Ni-Ti-Co thin film library.

Fatigue-resistant high-performance elastocaloric materials made by additive manufacturing.

Elastocaloric cooling, a solid-state cooling technology, exploits the latent heat released and absorbed by stress-induced phase transformations. Hysteresis associated with transformation, however, is detrimental to efficient energy conversion and functional durability. We have created thermodynamically efficient, low-hysteresis elastocaloric cooling materials by means of additive manufacturing of nickel-titanium. The use of a localized molten environment and near-eutectic mixing of elemental powders has led to the formation of nanocomposite microstructures composed of a nickel-rich intermetallic compound interspersed among a binary alloy matrix. The microstructure allowed extremely small hysteresis in quasi-linear stress-strain behaviors-enhancing the materials efficiency by a factor of four to seven-and repeatable elastocaloric performance over 1 million cycles. Implementing additive manufacturing to elastocaloric cooling materials enables distinct microstructure control of high-performance metallic refrigerants with long fatigue life.

Ultra-low-field magneto-elastocaloric cooling in a multiferroic composite device.

The advent of caloric materials for magnetocaloric, electrocaloric, and elastocaloric cooling is changing the landscape of solid state cooling technologies with potentials for high-efficiency and environmentally friendly residential and commercial cooling and heat-pumping applications. Given that caloric materials are ferroic materials that undergo first (or second) order phase transitions near room temperature, they open up intriguing possibilities for multiferroic devices with hitherto unexplored functionalities coupling their thermal properties with different fields (magnetic, electric, and stress) through composite configurations. Here we demonstrate a magneto-elastocaloric effect with ultra-low magnetic field (0.16 T) in a compact geometry to generate a cooling temperature change as large as 4 K using a magnetostriction/superelastic alloy composite. Such composite systems can be used to circumvent shortcomings of existing technologies such as the need for high-stress actuation mechanism for elastocaloric materials and the high magnetic field requirement of magnetocaloric materials, while enabling new applications such as compact remote cooling devices.

Conductive One- and Two-Dimensional Structures Fabricated Using Oxidation-Resistant Cu-Sn Particles.

Cu-Sn powders are promising alternatives to Ag and Au in applications including printed electronics because of their low cost and high oxidation resistance. Further development requires knowledge of the conductivity of their corresponding one- and two-dimensional structures. Herein, CuSny (y = atom ratio of Sn/Cu) wires and films were produced by direct printing. In situ measurements of structural resistivities with variation of the temperature from 2 to 400 K in oxygen-free conditions revealed that CuSn0.1 wires have resistivities comparable to those of Cu wires. Furthermore, CuSn0.1 films exhibited significantly lower resistivity increases after being heated at 573 K in ambient air, compared with Cu films.

Free-standing NiTi alloy nanowires fabricated by nanoskiving.

We report on free-standing NiTi alloy nanowires (120 nm × 75 nm) fabricated using a technique referred to as "nanoskiving", which complements conventional thin film sputter deposition with ultramicrotomy for thin sectioning. To date, the technique has been limited to pure metals without exploring metallic alloys. Leveraging the technique for the fabrication of shape memory alloy (SMA) nanostructures meets two critical requirements: compositional control (via film deposition) and controlled dimensions (via film deposition and programmable sectioning). Microstructure and composition analysis confirm continuity of the produced nanowires and Ni and Ti elemental uniformity. Free-standing NiTi nanowires are robust and remain intact throughout physical manipulation. The fabrication of NiTi alloy nanowires by nanoskiving will advance fundamental characterization of small scale SMA behavior.