DEVELOPMENT OF COOLING SYSTEMS WITH ACTIVE ELASTOCALORIC REGENERATORS

The vapor compression cycle (VCC) has been developed and optimized over a century to provide cooling in residential and commercial buildings, and transport systems. However, its usage has resulted in unpredicted environmental damage such as depleting the ozone layer and promoting global warming when its refrigerant leaks into the atmosphere. Because of this, it is important to develop a superior technological alternative without the environmental costs. One way to tackle this problem is to develop heat pumping cycles using solid-state refrigerant since a solid is incapable of leaking into the atmosphere. However, a solid-refrigerant cannot flow to deliver cooling the same way a fluid-refrigerant does. This requires a system conceptual redesign, which started with near-room temperature cooling with magnetocaloric materials in 1976 and elastocaloric materials in 2012. In this work, four different system configurations were studied with the following objectives: 1) maximizing the system’s temperature lift and 2) measuring the cooling capacity as a function of the useful temperature lift of the system when operating as a water chiller. The first configuration was based on the thermowave heat recovery strategy, while the other three were based on a single stage, two-stage and reciprocating variants of the active regeneration cycle. From the studied configurations the thermowave-based cycle achieved a system’s temperature lift of 8 K, at large average strain of 4.5%. It produced a maximum useful temperature lift of 5 K and a maximum cooling capacity of 125 W. All active regeneration-based cycles achieved similar final results while the best results was a system’s temperature lift of 21.3 K at a low average strain of 3.5% and a maximum useful temperature lift of 6.5 K and a maximum cooling capacity between 16 W and 25 W. The advantage of the reciprocating system integration is that it can achieve these results at lower strain than the one- stage and two-stage configurations. This dissertation identified a fundamental limitation of the active regeneration cycles using single composition elastocaloric materials. It is due to the fact that the local strain is larger than the average strain where the temperature is lower, which limits the maximum applicable average strain to prevent premature failure. This directly affects both the temperature lift and cooling capacity of the system. Different alternatives to address this issue, as well as how to improve the overall thermal and structural performance of the system within the constraints of the materials commercially available are suggested

Two-membrane cavity optomechanics

We study the optomechanical behaviour of a driven Fabry-P\'erot cavity containing two vibrating dielectric membranes. We characterize the cavity-mode frequency shift as a function of the two-membrane positions, and report a $\sim 2.47$ gain in the optomechanical coupling strength of the membrane relative motion with respect to the single membrane case. This is achieved when the two membranes are properly positioned to form an inner cavity which is resonant with the driving field. We also show that this two-membrane system has the capability to tune the single-photon optomechanical coupling on demand, and represents a promising platform for implementing cavity optomechanics with distinct oscillators. Such a configuration has the potential to enable cavity optomechanics in the strong single-photon coupling regime, and to study synchronization in optically linked mechanical resonators

Membrane-Based Scanning Force Microscopy

We report the development of a scanning force microscope based on an ultrasensitive silicon nitride membrane optomechanical transducer. Our development is made possible by inverting the standard microscope geometry - in our instrument, the substrate is vibrating and the scanning tip is at rest. We present topography images of samples placed on the membrane surface. Our measurements demonstrate that the membrane retains an excellent force sensitivity when loaded with samples and in the presence of a scanning tip. We discuss the prospects and limitations of our instrument as a quantum-limited force sensor and imaging tool.</p

Friction Consolidation of Oxide Dispersion Strengthened Incoloy ® Alloy Ma956 Powder

INCOLOY® MA956 is a ferritic ODS alloy. It has very good oxidation resistance by virtue of its large chromium and aluminum concentrations and high mechanical strength and creep resistance at elevated temperatures thanks to oxide dispersion strengthening. The conventional processing route utilized to obtain this alloy involves two main multistep stages. The first (or front end) stage of the process consists of a dry, high-energy milling process which mixes very fine Y2O3 particles with elemental alloy powders by Mechanical Alloying (MA) in an attritor. The second (or back end) stage of the process consists of consolidating the mechanically alloyed powder by hot extrusion in vacuum-sealed cans at about 1000°C, or by degassing followed by hot isostatic pressing (HIP). The precipitation of a fine dispersion of yttrium-aluminum-rich oxides (Y-Al-O) during the consolidation is at the origin of the high temperature mechanical strength of this alloy. Three different thermodynamically stable oxides are known to exist for the binary Y2O3:Al2O3 system: Y4Al2O9, YAlO3 and Y3Al5O12. All three of them have been observed in this type of alloys when processed by the route described above. Their size ranges from just a few up to hundreds of nm. In this work, the applicability of Friction Consolidation to this ODS alloy was investigated in order to tackle the downsides of the conventional processing route (multisteps and extremely high raw material final cost). For this study, mechanically alloyed INCOLOY® MA956 powder was consolidated through Friction Consolidation under three different sets of processing conditions. As a result, three small compacts of low porosity have been achieved with a refined equiaxed ferritic grain structure smaller than 10 microns and the desired oxide dispersion. Two types of mixed Y-Al oxides were observed by different complementary techniques, Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD), YAlO3 and Y3Al5O12. Their size distribution was characterized using electron microscopy at different locations (for precipitates above 50 nm) and showed a larger average precipitate size for larger grain size. The total energy input during processing was correlated with the relative amount of each of the oxides in the disks (observed from XRD experiments): the higher the total processing energy input, the higher the relative proportion of Y3Al5O12 precipitates. The elemental composition of these precipitates was also probed individually by EDS showing an aluminum enrichment trend as precipitates grow in size. Overall, the Friction Consolidated material showed microstructural characteristics comparable to the ones observed in conventionally processed material, which makes it a very promising processing alternative

Overcoming fatigue through compression for advanced elastocaloric cooling

Elastocaloric materials exhibit extraordinary cooling potential, but the repetition of cyclic mechanical loadings during long-term operation of cooling systems requires the refrigerant material to have long fatigue life. This article reviews the fundamental cause of fatigue from aspects of initiation and propagation of fatigue cracks in shape-memory alloys (SMAs) that are used as elastocaloric materials, and highlights recent advances in using compression to overcome fatigue by curtailing the generation of surfaces associated with crack propagation. Compression is identified as a key means to extend fatigue lifetime in engineering design of elastocaloric cooling drive mechanisms. We summarize the state-of-the-art performance of different SMAs as elastocaloric materials and discuss the influence of low cyclic strains and high resistance to transformation. We present integration of compression-based material assemblies into a cooling system prototype and optimization of the system efficiency using work recovery and related measures.</p

Overcoming fatigue through compression for advanced elastocaloric cooling

Elastocaloric materials exhibit extraordinary cooling potential, but the repetition of cyclic mechanical loadings during long-term operation of cooling systems requires the refrigerant material to have long fatigue life. This article reviews the fundamental cause of fatigue from aspects of initiation and propagation of fatigue cracks in shape-memory alloys (SMAs) that are used as elastocaloric materials, and highlights recent advances in using compression to overcome fatigue by curtailing the generation of surfaces associated with crack propagation. Compression is identified as a key means to extend fatigue lifetime in engineering design of elastocaloric cooling drive mechanisms. We summarize the state-of-the-art performance of different SMAs as elastocaloric materials and discuss the influence of low cyclic strains and high resistance to transformation. We present integration of compression-based material assemblies into a cooling system prototype and optimization of the system efficiency using work recovery and related measures.</p