1,721,118 research outputs found
Enhancing compression-based elastocaloric cooling performance by structure design of refrigerant and architecture design of system
No full textSpace cooling consumes large amounts of electricity, and the market-dominant vapor-compression refrigeration relies on high global-warming-potential refrigerants. Elastocaloric cooling using shape memory alloys (SMA) is a promising alternative because of its high energy efficiency and greenhouse-gas-free properties. However, the limited temperature span (record of 31 K) and cooling power (record of 260 W) in existing elastocaloric devices restrict the commercialization of this technology. This thesis addresses the challenge by designing and fabricating tubular NiTi SMA with large specific heat transfer area (spiral cross-section of 7.8 cm2 g-1 and multi-cell cross-section of 12.5 cm2 g-1), and developing cascade-unit architectures to increase the system driving efficiency without exceeding the limit of Euler buckling under compression. Large specific heat transfer area of refrigerants enable fast heat exchange between NiTi and fluid, while a sufficient regenerative length facilitates a large temperature span along the fluid-flow direction. The effects of material austenite finish (Af) temperatures on system temperature span was investigated through constructing a multi-material layered elastocaloric heat pump. Through matching the working temperatures of NiTi units with their Af temperatures, a giant system temperature span of 75 K was achieved. The effects of operating frequency and heat exchange fluids on cooling density of NiTi was analyzed. Heat-exchange-enhanced NiTi and graphene nanofluid enable a large specific cooling power at high operating frequencies (12.3 W g-1 at 3.5 Hz operation), while the ‘SMA in series-fluid in parallel’ architecture ensures sufficient elastocaloric mass without increasing system fluid pressure. This device achieves a cooling power of 1284 W on the fluid side and endures over 10 million compressive phase-transition cycles. This thesis reveals the great importance of topology and material design of shape memory alloys and the importance of heat exchange enhanced nanofluids, paving the way for the large-scale commercialization of elastocaloric cooling.</p
Manipulating thermomechanical properties of NiTi shape memory alloy at microscale : fabrication and characterization
No full textNiTi shape memory alloy (SMA), as a typical functional material, has been widely used in many fields, such as biomedical devices, solid-state refrigeration and actuators. Recently, with the fast development of micro-electromechanical systems, the thermomechanical properties of the NiTi SMA at microscale have attracted wide attention but are very little explored so far. In this thesis, the microscale processing method was developed to manipulate the microstructure of NiTi for enhancing the thermomechanical performance, such as functional degradation, thermal expansion, and temperature dependent Young’s modulus. Firstly, the cuboidal micropillar with a dual-pillar method was exploited to measure the accurate stress-strain response of materials at microscale using the focused ion beam and nanoindentation. Then, a compression-based plastic deformation was conducted on cuboidal NiTi micropillars and it is found that, at the moderate plastic deformation with residual strain of 3.5%, the dense and saturated dislocation structures and residual martensite were created in the micropillars, so the functional fatigue resistance was significantly improved together with the reduced transformation stress and hysteresis loop area, which is attractive to elastocaloric refrigeration applications; at severe plastic deformation (residual strain of 50%), the polycrystalline NiTi was extremely grain-refined into 10-nm nanocrystals and the volume fractions of B2 and B19′ phases can be well-manipulated by deformation temperature, so that a temperature independent Young’s modulus and a giant coefficient of stress-induced thermal expansion were achieved in dual-phase and single-phase nanocrystalline NiTi micropillars, respectively. Such results provide a new insight via micro-fabrication to develop high performance materials. Keywords: NiTi shape memory alloy, microstructure manipulation, micropillar compression, functional fatigue, temperature dependent Young’s modulus, thermal expansion.</p
Compressive fatigue and elastocaloric cooling performance of NiTi tubes
No full textSuperelastic nanocrystalline NiTi tubes are promising candidates for eco-friendly elastocaloric cooling, but their cyclic stability suffers severely from functional degradation and the limited fatigue life of conventional coarse-grained NiTi remains a crucial bottleneck. First, the functional degradation and its effect on elastocaloric cooling performance were investigated. The results show that the functional degradation accompanies with progressive accumulation of residual strain and significant reduction in both hysteresis loop area (D) and forward transformation stress (σftr). Such functional degradation arises from phase transition-induced dislocations and dislocation-pinned residual martensite. The dislocations partition the original austenite grains into much smaller nanodomains, leading to the macroscopic residual strain and reduced D. The nanosized residual martensite can directly grow without overcoming martensite nucleation barrier and induce compressive residual stress in the austenite phase, contributing to the decrease in σftr . As a result of functional degradation, the material coefficient of performance was doubled for full phase transition and enhanced by 40% for partial phase transition compared with the first cycle, mainly due to the cyclically-decreased D. The study shows that the cyclic stability and elastocaloric cooling performance of NiTi can be improved via training at a suitable stress. Then, the ultrahigh fatigue life of nanocrystalline NiTi tubes was achieved via high frequency fatigue test after training, exceeding 120 million cycles under 800 MPa. The NiTi tubes demonstrate stable cyclic stress-strain responses and a stable adiabatic temperature drop of 6.6 °C in the lifespan. The material coefficient of performance increases from the initial 8.8 to 11.6 of the 108th compressive cycle. The high resistance to nucleation and growth of compression-parallel cracks results in the ultrahigh fatigue life of the tubes. The research shows the great potentials of nanocrystalline NiT tubes with both stable thermomechanical properties and long compressive fatigue life for reliable elastocaloric cooling. Keywords: Elastocaloric cooling; Cyclic response; NiTi; Shape memory alloy (SMA); Martensitic phase transformation; Ultrahigh fatigue life.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Controlling gradient microstructure and mechanical behavior by surface mechanical attrition treatment
No full textA surface nano-crystallization process, surface mechanical attrition treatment (SMAT), is used to tailer the microstructure of alloys and achieve ideal properties. To evaluate the effect of strengthening and fatigue resistance enhancement, two widely used alloy systems, NiTi shape memory alloy (SMA) and CoCrNi medium-entropy alloy are chosen to investigate. Many structures like medical stents made of superelastic NiTi SMA are subject to cyclic bending loads, where the material shows a limited fatigue life due to crack nucleation and growth in the surface layers under local tensile stress. In this thesis, the bending fatigue life of NiTi plates is enhanced by pre-strain warm surface mechanical attrition treatment (pw-SMAT) where the austenite phase is directly subject to severe plastic deformation and grain refinement without inducing phase transformation. Amorphous and grain size gradient (5-100 nm) microstructures as well as a maximum compressive residual stress of 1093 MPa are produced in the surface of the NiTi plates via the pw-SMAT. The compressive residual stress notably reduces the surface tensile stress from bending. The grain size gradient layers with improved hardness and reduced hysteresis have high fatigue crack nucleation resistance, whereas the middle large-grained layer has high fatigue crack growth resistance. The combined effects of the gradient nanostructure and the compressive residual stress substantially increase the bending fatigue life of the NiTi plates from an original 103 cycles to over 1.3x104 cycles. The results open up a new route to improve the bending fatigue life of NiTi plates by heterogenous nanostructures. SMAT was used to produce a grain-size gradient layer on the surface of spark-plasma-sintered medium-entropy alloy CoCrNi. In comparison with the as-sintered fine-grained CoCrNi, the SMAT alloy shows a 46% increase in yield strength and over three times higher strain hardening rate (dσ/dε). The ultimate tensile strength (σUTS) and the elongation (εf) reach 1420 MPa and 61.3%, respectively. The excellent strength-ductility synergy should be attributed to the Hall-Petch strengthening and the formation of high density of dislocations and nanotwins.</p
Surface polymorphlic crystalline layers in colloidal crystals
No full textMicrometer-sized colloidal particles have strong thermal motions and can form various phases, thus they can be used as a model of atoms. Their larger size and slow dynamics can be directly observed under optical microscopy. Therefore, colloidal model systems are ideal for investigating phase transitions, particularly on poorly understood kinetic processes. In this thesis, we first introduce the NIPA colloidal particles, which size and attraction can be finely tuned by temperature. It provided a platform to study surface physic at the single-particle level (Chapter 2). We observe the pre-solid-solid transition at the free surface in equilibrium and the growth front. In Chapter 3, we report the novel polymorphic crystalline layer on crystal surface, which is based on previous labmates Xipeng Wang’s simulations and Bo Li’s experiments about PMMA colloids. The surface of a thin-film 4△ NIPA crystal develops a layer of 4☐ lattice whose thickness increases in a power law as approaches the solid–solid transition point, analogous to premelting. Therefore, we name it as ”pre-solid-solid transition”. We show that surface crystals can appear during thermal equilibrium, melting, crystallization, and grain coarsening by forming a coherent interface. Additionally, we propose that a crystal surface can develop a crystalline layer and a liquid layer when the premelting and presolid- solid transition coexist. In Chapter 4, we focus on the pre-solid-solid transition during NIPA crystal growth. We find under slow cooling, the crystallization front of 4△ lattice develops a layer of 4☐ lattice whose thickness increases in a power law as the crystallization temperature decreases and can reach over 40 layers, which is much thicker than other surface-wetting layers in various non-equilibrium processes. In Chapter 5, we report the surprisingly large penetration depths of surface effects on crystals. Different parameters have different penetration depths, associated with the observed strong lattice dilation near the surface.</p
Enhancing elastocaloric cooling performance of NiTi-based shape memory alloys by thermomechanical processing
No full textThe elastocaloric cooling performance of NiTi-based shape memory alloys is enhanced by grain refinement (and partial amorphization), dislocation strengthening and alloying (combined with multiaxial forging and aging). Firstly, the effects of grain refinement and partial amorphization on temperature drop (ΔT), coefficient of performance (COP)and cyclic stability of NiTi are investigated. It is found that grain refinement and partial amorphization decrease ΔT by reducing the amount of B2 phase, increase COP by altering phase transition mode and enhance cyclic stability by grain boundary and amorphous phase strengthening. A good combination of large ΔT, enhanced cyclic stability and high COP is achieved in a 35 nm-grain-size NiTi. Secondly, to further enhance cyclic stability, nanocrystalline high-density-dislocation (HDD) NiTi is fabricated via cold rolling and annealing. The HDD NiTi exhibits large and stable elastocaloric effect with a ΔT of 16-18 ℃ and a residual strain of only 1.25% over 106 cycles under 1400 MPa. The nano-grains and the high-density dislocations suppress new dislocations and residual martensite, leading to the stable elastocaloric effect. Thirdly, to largely reduce transition stress, Cu and Co are added in NiTi to obtain quaternary NiTiCuCo. Precipitated NiTiCuCo is further successfully fabricated via multiaxial forging followed by aging. With a low transition stress of around 400 MPa at room temperature, the precipitated NiTiCuCo under the working stress of 550 MPa shows a large and stable ΔT of about 17 ℃, low residual strain of 0.19% and high COP of over 40 in 107 cyclic compressions. The excellent cyclic stability, on one hand, originates from the reduced sweep distance of the transformation interfaces due to introduction of dense precipitates, and on the other hand, from precipitation strengthening.</p
Mechanical properties of crystal-amorphous composites as a function of mean grain size and grain boundary thickness
No full textMechanical properties of polycrystals have been intensively studied as a function of mean grain diameter D, but rarely as a function of (D, l) where l is the mean grain-boundary thickness. Here we measure the mechanical properties in the (D, l) space by simulation for polycrystals with thick grain boundaries which are often called as crystalline-amorphous composites. The strength σy of a polycrystal increases with D at D ≳ 50 atoms (i.e., the famous Hall-Petch (HP) behavior) and decreases at D ≲ 50 (i.e., the inverse Hall-Petch (IHP) behavior). These behaviors generally hold for all kinds of polycrystals and have not been generalized to a function of other structural parameters before. Our simulations generalize the HP and IHP behaviors of σy(D) to σy(D, l). We demonstrate that increasing l and decreasing D have similar effects on reducing dislocation motions and promoting GB deformations. Consequently, the classical HP and IHP behaviors and our generalized HP-IHP behaviors share similar mechanisms and can be unified as σy (AGB∕Atot), where AGB∕Atot is the fraction of the amorphous region. In 2D, we only observe the IHP behavior because dislocations are rare in 2D. In 3D, we observed the HP and IHP behaviors in systems with various particle compositions and interactions. The maximum strength is robustly reached at (D, l) ≃ (50, 5) particles for single-component face-centered cubic solids and at (D, l) ≃ (50, 2) for bidispersed or body-centered cubic solids due to their different activation stresses for dislocation motions. The deformation mechanism changes from dislocation-dominated to grain-boundary-dominated as l increases. The results explain the recent alloy experiments and provide a route to exceed the maximum strength of polycrystals. Besides σy, the ductility and elastic moduli are similarly measured in the broad (D, l) space. The ductile and brittle regimes in the (D, l) space exhibit distinct fracture morphologies. Properly thickening grain boundaries can avoid the strength-ductility trade-off. These novel results in the (D, l) space can guide the fabrication of crystalline-amorphous composites with outstanding mechanical properties.</p
- …
