Thermal Transport and Transformation in Micro-Structured Materials
Environmentally sustainable forms of energy will be required to meet the aspirations of a growing world population. By 2050, an expected rise in global population from seven billion to ten billion and better living standards could lead to a two to threefold increase in energy consumption. No single technology will fulfill this demand. Therefore, the developments of sustainable power sources and energy-efficient technologies are of great importance to meet the challenges of world’s growing energy need. In this dissertation, thermal transport and transformation in micro-structured materials for applications in long-term source of power supply by nuclear fusion, as well as in energy-efficient thermal management for buildings are discussed.
Tungsten (W) has been chosen as one of the most promising plasma facing materials (PFMs) because of its high melting temperature, high thermal conductivity, high sputtering threshold energy, and low sputtering erosion yield. However, it is suffered from bombardment of neutrons, energetic ions, helium (He) plasma as well as hydrogen isotopes (H, D, T) plasmas. As a result, exposed surfaces exhibit nanostructured surface morphology change that in turn modify thermo-mechanical properties of materials and ultimately impact the performance of PFMs such as the surface temperature and erosion yield. A differential method, 3ω method, with improved measurement sensitivity was used to study the impact of heavy ions (as a surrogate of neutrons) and He plasma on the thermal conductivity of W. The results show a significant gradual reduction in thermal conductivity for W irradiated at room temperature, spanned from 10-3 to 0.6 dpa for Cu ions with increasing Cu ion damage level due to defects introduction, such as dislocation loops, self-interstitials (SIAs) and vacancies during irradiation, which scatters the electrons, the dominant heat carriers in W. When the Cu ion irradiation was performed at 1000 K, the reduced thermal conductivity for 0.2 dpa sample gets recovered to around 80% of the pristine value, attributing to the thermal annealing and annihilation of the irradiation induced defects, i.e. through vacancy/SIAs recombination. Meanwhile, at least ~80% reduction in thermal conductivity happens to W bulk and thin film irradiated by helium (He) plasma compared to that of undamaged W. This large reduction in thermal conductivity also comes from defects introduced during He-plasma irradiation. Those studies on thermal conductivity of PFMs under fusion relevant irradiation conditions need to be taken into consideration in the thermal design of future nuclear fusion reactors.
Residential and commercial buildings are one of the highest energy consumption sectors and over 40% of energy consumption and greenhouse gas emission are related to building temperature regulation. The second thrust of the dissertation is focused on the development and application of polymeric materials for thermal management in buildings to reduce the energy consumption. For the first time, the application of highly stretchable and tough double network hydrogels (DN-Gels) was proposed as durable and reusable ‘sweating skins’ for cooling buildings. These DN-Gels demonstrate outstanding cooling performance, reducing the top roof surface temperature of wooden house models by 25－30C for up to 7 hours after only a single water hydration charge. DN-Gels also exhibit extraordinary toughness and cyclability due to their interpenetrated ionically and covalently cross-linked networks, as demonstrated by constant cooling performance over more than 50 cycles. Our results suggest that bio-inspired sweat cooling, specifically using tough DN-Gel coatings, represents a promising energy-efficient technology for cooling buildings with reduction of ~290 kWh of annual electricity consumption for air conditioning and 160 kg of CO2 emission.
Besides, a thermo-responsive hydrogel composite (TRHC) desiccant is also synthesized by impregnating hygroscopic salt into porous thermo-responsive polymer matrix, for desiccant assisted air conditioning. Unlike traditional solid desiccant that has a tradeoff between its adsorption and desorption due to its fixed affinity to vapor, TRHC desiccant has drastically different affinities to water upon phase transition thanks to its thermo-responsive matrix. It achieves faster desorption at low-temperature (50C) as well as high adsorption capacity. With the attractive performance of TRHC desiccants, the COP of desiccant assisted air conditioning could be improved compared to that with silica gels, which consumes higher temperature/energy for reactivation under similar operating conditions.