Composite thermal storage material fabrication by cold sintering process

Bilyaminu, Suleiman (2022). Composite thermal storage material fabrication by cold sintering process. University of Birmingham. Ph.D.

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Abstract

In the 21st century, energy is critical to the world overall development agenda. Its daily requirement is always increasing to satisfy the global population growth and the rapid change in the standard way of living. The world major source of energy is fossil fuel which is gradually depleting and is largely associated with numerous environmental consequences responsible for global temperature rise. To achieve sustainable economic development of our society, there is an urgent need for efficient energy utilization, development of reliable technologies and use of sustainable renewable and clean energy sources. The supply of electrical energy from renewable sources is intermittent and its integration into the existing supply network requires energy storage technology which is reliable and environmentally friendly. The present thermal energy storage technologies storing heat in either solid materials or phase change materials (PCM) cannot meet the conflicting requirements of high energy storage density, fast charging/discharging rate, and low temperature drop. Sintering method to fabricate composite PCMs is limited by high temperature requirement and mismatch in melting temperature limiting proper material sintering and integration. Thus, there is the need to develop a new innovative method for the fabrication of an efficient composite thermal energy storage materials with the desired mechanical, structural, and thermal properties.

In this research work, the fundamental mechanism of a highly innovative and novel cold sintering approach was proposed and studied for the fabrication of composite PCMs suitable for energy charging and discharging in a direct-contact heat transfer multi-layered thermal stores system. The study comprises of the fabrication of form stable composite Al\(_2\)O\(_3\)-NaCl storage materials at low temperature sintering ≤300 ℃ to overcome the inherent high temperature requirement. The limit and influence of the major cold sintering process (CSP) parameters were investigated. Also, the Microstructure, and mechanical properties of cold sintered porous alumina ceramics was studied as an alternative approach to develop a strong and porous structural scaffolding. The mechanism of the cold sintering process of Al\(_2\)O\(_3\)-NaCl composite is proposed from experimental understanding and theoretical defect chemistry analysis.

Results of this study showed that a very high dense Al\(_2\)O\(_3\)-NaCl composite thermal storage material with relative density of ⁓99% close to theoretical consistent with its microstructural observation. The composite mass ratio of 1:1 cold sintered with 30 wt% water had phase change enthalpy of 252 J/g consistent to the theoretical NaCl latent heat. The maximum limit of CSP parameters determined by single variable optimization to achieve desired composite density without failure during thermal cycle are the CSP temperature of 250℃, 400MPa pressure, 30 wt% water, and dwelling time of 90 minutes. The composite phase structure consists of Al\(_2\)O\(_3\) and NaCl species only and the peak intensity of all species reduced and disappeared in the case of alumina at high 2θ positions. The FTIR spectrum shows a gradual decrease in peak intensity from dry pressed to the mix and to the cold sintered pellet. The dry pressed and cold sintered composite dissolved when placed in water. The CSP-post annealed composite at 850℃ remain undissolved in water signifying the beginning of alumina sintering. It sintered more with itself with increase in annealing temperature, but NaCl volatilized leaving a complete sintered porous alumina structure. Several repeated SEM-EDS analyses of CSP-POST annealed samples at 850℃ shows presence of only Al, O and Na at many grain boundaries. NaCl completely volatilized at 1200℃ to form a highly porous alumina with 48% maximum porosity, good mechanical, and structural properties. At 48% maximum porosity, the porous alumina had mechanical strength of 17.63 MPa. Further increase in annealing temperature to 1300℃, 1400℃ and 1500℃, its excellent mechanical strength increases to 36.57, 111.47 and 137.21MPa with corresponding porosity of 41%, 29% and 28% respectively. Experimental evidence from phase structure, microstructure, EDS and FTIR analysis combined with literature evidence of alumina insolubility in molten salt provide guides to the proposed CSP mechanism from the perspectives of defect chemistry, a lattice phenomenon. Conclusively, at CSP stage, full composite densification was achieved, and species interaction created modifications which at elevated temperature of 850 ℃ begin to cause sintering of alumina at temperature much lower than its conventional sintering temperature. The NaCl complete volatilization at 1200℃ provides a new innovative way to fabricate a strong porous ceramic structure that could be used as matrix support for PCMs for medium to high temperature application. Meanwhile, the proposed mechanism from the perspective of defect chemistry in addition to plastic deformation and dissolution precipitation process revealed the beginning of CSP complex mechanism understanding. It requires further attention to fundamentally explore different chemistries behaviour under CSP conditions. Finally, fabrication of composite thermal energy storage materials is compatible to CSP but requires further investigation to fully encapsulate PCMs.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):