Si-based phase change materials in thermal energy storage systems
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The main aim of this dissertation is to develop new phase change materials (PCMs) for high temperature energy storage in thermal energy storage (TES) devices, with special focus on Si-based PCMs due to silicon’s the high latent heat of fusion, high thermal conductivity, moderate melting point, abundance on earth, and low cost. The questions are, however, what kind of Si-based alloy can be used as PCM and what kind of refractory material can be used as PCM container at high temperatures. In order to comply with these objectives, experimental studies were carried out to investigate the thermodynamic properties of the selected PCMs and the interaction between PCMs and potential refractory materials. Ultimately, the results were used to decide the most suitable PCM and the most compatible refractory material. First, Si-B alloys were selected as the potential PCMs, due to the high latent heat of fusion of B. In addition, Si has a high volume expansion going from liquid to solid, and the volume change is expected to decrease with the addition of B. The challenge is to find the optimal B content for Si-B alloy as the PCM. For this purpose, the effect of B content on the phase formation in Si-B alloys were investigated, in which alloys with 2-11 mass % B were heated to 1450 oC, 1550 oC, 1650 oC, and 1750 oC, respectively. Moreover, the effect of B on volume change and heat of fusion in Si-B alloys were calculated theoretically. Microstructural analyses showed that phase formations were changed with the increase of B content in Si-B alloys, and, it was proved that volume change was minimally decreased with the increase of the B content, and around 25 mass % B must be added to obtain no volume change during solidification. Moreover, the sum of heat of fusion is increased with the increase of B content, while Si-3.25B eutectic alloy possessed the highest heat of fusion per unit volume. In order to find the most compatible refractory material for Si-B alloys, the interaction between graphite, Si3N4, and hexagonal BN (h-BN) and Si-B alloys were investigated. Using graphite showed that carbides were introduced to Si-B alloys. Carbides layers were formed at the interface between the Si-B alloys and the graphite, in which a single SiC layer was transformed to SiC and B4C layers with the increase of B content. The C solubilities were measured in Si-B alloys, in which alloys with 2, 3.25, and 5 mass % B were melted in the graphite and SiC crucibles in the temperature range 1450-1750 oC, and it was found C solubility is increased with the increase of the B content. When Si3N4 was used as the container for Si-B alloys with the B addition of 2-11 mass % at 1750 oC, the cross-sectional images and micro-analyses showed that BN precipitates were formed in Si-B alloys and a BN layer was formed at the interface. However, the amount of BN precipitate was negligible. The wetting behavior was investigated in Si-3.25B/Si3N4 system under 10-4 atm, and a contact angle of 134o showed non-wetting behavior between the alloy and the crucible material. When using a h-BN container, the wetting tests of Si-3.25B alloy were performed in a sessile drop furnace at temperatures below 1450 oC under 10-4 atm, and non-wetting behavior was observed. Microstructural analyses showed that no new phases are formed in the Si-3.25B alloy, neither at the interface. However, the relationship between partial equilibrium pressure of N2 and BN decomposition recommended that it is better to use BN in high N2 partial pressure. The study of Si-B alloys and the interaction between Si-B alloys and graphite, Si3N4, and h-BN definitively answers the question regarding the optimal B content in Si-B alloys and the most compatible refractory material. It is suggested that Si-3.25B alloy is used as the PCM and Si3N4 refractory material is used as the container. The advantage of the eutectic Si-3.25B alloy is that it will have a high enthalpy change over a small temperature area. If Si3N4 is used as a crucible, no carbide phases will be formed in the alloy. Si3N4 is also a resistant material towards the alloy, and relatively cheap compared h-BN. Secondly, Fe-26Si-9B alloy was selected as the potential PCM. The third element of Fe was added to Si-B based alloy aiming to decrease the volume change of Si-B alloys upon solidification. The main goal was to determine the chemical stability of the formed phase during thermal cycles, liquidus temperature, volume change, and value of the latent heat of fusion. In this regard, Fe-26Si-9B alloy was subjected to 1-12 temperature cycles between 1100 oC and 1300 oC in Si3N4 crucible under Ar. Microstructural analyses showed that FeSi, FeB, FeSiB3, and SiB6 were formed in the alloy, in which FeSi + FeSiB3 constituted the eutectic structure, and TEM analyses confirmed that FeSiB3 was a new ternary phase in Fe-Si-B system. Moreover, the formed phases were not changed with the increase of thermal cycles. Differential scanning calorimetry (DSC) tests of the Fe-26Si-9B alloys were performed in the temperature range from 25 °C to 1450 °C. DSC curves showed that solidus temperature was ~ 1223 oC and liquidus temperature was ~ 1254 oC in the heating step. Hence, Fe-26Si-9B alloy was believed not to be a eutectic alloy, while the eutectic composition was calculated to be 61 mass % Fe, 29 mass % Si, and 10 mass % B based on the formed morphology in the Fe-26Si-9B alloy. It was also found that the Fe-26Si-9B alloy slightly shrunk during cooling. The interaction between Fe-26Si-9B alloy and graphite, Al2O3, Si3N4, and h-BN were also investigated, aiming to find the most compatible refractory material. Using graphite as the substrate under different atmospheres (Ar, 10-4 atm, 10-5 atm, and 10-9 atm), it was observed that carbides were formed at the top of the Fe-26Si-9B alloy and carbide layers were formed at the surface, especially at high pressures. Interestingly, the penetration of liquid alloy into graphite was insignificant. Moreover, the alloy wets graphite with contact angle of 31o-48o under four different atmospheres. For Al2O3 testing, the DSC and wetting tests were performed. Microstructural analyses showed that no Al-based phases were formed in Fe-26Si-9B alloy and no new layer was formed at the interface. However, Al might be dissolved into the other phases in the Fe-26Si-9B alloy. The formed oxides at the metal surface and the oscillation phenomenon also proved that oxygen was dissolved into Fe-26Si-9B alloy, due to little Al2O3 dissolution. Si3N4 and h-BN were also tested by sessile drop approach under 10-4 atm. Microscopic analyses showed that no nitride phases was formed in Fe-26Si-9B alloy and no new layers were formed at the interface between Fe-26Si-9B alloy and Si3N4 and h-BN. High N2 partial pressure was recommended in the use of h-BN in the case of its decomposition. In conclusion, Fe-26Si-9B alloy is a desirable PCM, and graphite and Si3N4 were the most compatible refractory materials to be used as container for Fe-26Si-9B alloy. Finally, Cr-43Si-5B alloy was selected as a potential PCM. Thermal cycle experiments were performed in a resistance furnace, in which Cr-43Si-5B alloy was charged to graphite crucible and subjected to 4 thermal cycles in temperature range 1314-1514 oC. Micro-analyses showed that CrSi2, CrB2, Cr3B4, and Si were formed in Cr-43Si-5B alloy. However, the eutectic structure was not found in the solidified alloy. Moreover, substantial pores existed in Cr-43Si-5B alloy after experiments, which would affect its thermal conductivity. Therefore, Cr-43Si-5B alloy was not recommended to be the PCM in the TES systems.