The Research On Nano-ceramic Matrix Composites Infiltrated Phase Change Materials As Thermal Control Applications

Thermal protection systems (TPS) and heat insulation materials are required for a range of hypersonic vehicles ranging from ballistic reentry to hypersonic cruise vehicles, both within Erath's atmosphere and non-Earth atmospheres. Phase change materials (PCM) are one of the most preferrd methods to thermal control applications that can effctively delay or modify the temperature rise of the surface of the aircrafts subjected to high thermal flux. This work originately put PCMs to use in the internal themal insulated materilas of the hypersonic vehicles. Porous ceramic matrix serves as the supporting material, which provides structural strength and prevents the leakage of melted PCMs; and PCMs acts as thermal absorb material limiting the temperature abruptly rising of the aircrafts. This study demonstrates as below.Firstly, thermaldynamics, kinetics and statics equations of melted PCM infiltrated porous ceramic marix are derivated according to experimental model and CFD modeling, and then analyze the influence factors including infiltration temperature and infiltration time and so on. Results indicated the infiltraton temperature in theory is higher than the melt point of the PCM and lower than the sinter temperature of the porous cermic matrix, the calculated radius is less than 6.8×10^-5 mm. CFD modeling shows the pressure drop of the melted PCM in porous ceramics distinct and centerline velocity becomes lower along the directions. When infiltration time increases, infiltration depth will rise until it reaches equilibrium; and optimum infiltration depth needs to choose comprehesive infiltration conditions.Secondly, porous ceramics derived by sol-gel processing. Studies show the pore structure of porous ceramics was connected; its porosity was calculated as 90.4%, and surface area was measured as 527.8 m²/g and mean radius was measured as 30.3 nm. It indicates that high mesoporosity when the molar ratios of EtOH/TEOS is 2, 3 and 5. The pore size of porous ceramics becomes larger with the increase of EtOH/TEOS molar ratios; the average pore size of the specimen with E=10, 20 silica are 53.1nm and 56.0nm, respectively. Furthermore, PCM-porous ceramic composite were successfully prepared by melt infiltration pressurelessly; PCM and porous ceramics were chosen as thermal absorb material and supporting material, respectively. Compared with 3 types of pore structure porous silica using as supporting materials of the PCM, the mass percentage of the paraffin impregnated E=10, 20 silica matrices reached to 75% and over than those of the E=2 silica matrix, 68%. The E=10, 20 silica matrices are suitable to serve as supporting materials of the PCM for larger thermal absorption. For paraffin, the optimal infiltration time of preparing the composite was range from 180 to 210 seconds while the mass fraction of the paraffin reached to 75 per cent. For erythritol, the optimal infiltration time of preparing the composite was range from 250 to 350 seconds while the mass fraction of the PCM reached to 85 per cent. There was no reaction among the PCM-porous silica ceramic composite according by the structure analysis with SEM and the chemical analysis with FTIR. The measurement of latent heat, melting point of PCM-porous silica ceramic composite were 63²89.9 kJ/kg, 50¹20℃respectively. The flexural and compressive properties of 3 types of specimens were investigated by two techniques: 3-piont bending and uniaxial compression. Results indicated the bending strength of the erythritol/ceramic composite is 25.9±0.10MPa and paraffin -ceramic composite is 10.1±0.10MPa. The compressive strength of the erythritol-ceramic composite is 1.11 MPa corresponding 0.15% strain and paraffin-ceramic composite is 0.03 MPa corresponding 0.16% strain.Thirdly, experimental and numerical studies are proposed to predict and investigate the thermal absorb characteristics of porous silica infiltrated with phase change materials (PCM) for thermal protection applications. Several types of different solid-liquid phase change composites were introduced into a cylindrical enclosure while it experiences its heat from a heat source setting on the left of the enclosure. Studies show that the cold face temperature of the ceramic aerogel-composite is 184.2℃while the cold face temperatures of the paraffin-porous ceramic and erythritol-porous ceramic are 139.3℃, 86.9℃respectively. The numerical simulation of the porous ceramic composite indicates that transient heat transfer simulation shows good agreement with the experimental data. When heat tranfer time is 600,800,1000 seconds, the numerical caculated temperature is 115.0℃,161.4℃,201.2℃. Thus the simulation method can forecast the experimental conclusions and optimize the geometric structure of the heat insulation composites. The numerical simulation of the phase change compsoite was performed using the volume averaging technique and a finite volume technique was used to discretize the heat diffusion equation while the phase change process was modeled using the enthalpy porosity method. The results are portrayed in terms of temperature distribution and liquid fraction, and the numerical and experimental results showed good agreement. The results illustrated that the higher the latent heat storage capacity the more stability of the thermal performance of the phase change composite.Moreover, PCM-ceramic fiber composite were prepared by melt infiltration, particularly, to be embedded within the endotherms, ZrOCl₂·8H₂O or boric acid, to provide for nonreversible heat absorbing applications. Studies indicate endotherms to be in the fiber-PCM as crystal that dispersed in the PCM-ceramic fiber composite and has very large decomposition enthalpy. The thermal capacity of the PCM-ceramic composite embeded with ZrOCl₂·8H₂O is 536 kJ/kg happened at 56.3℃and 96¹58℃and the thermal capacity of the PCM-ceramic composite embeded with boric acid is 671kJ/kg happened at 51.9℃,151.2℃and 158.6℃. The thermal protection properties of the fiber-PCM composites were designed in the laboratory conditions via using a thermal measurement setup under a simulated thermal environment of the aircraft. The fiber-PCM composites produces here can be helpful for the thermal protection application and exhibited fairly efficient thermal regulation under very high temperatures and for long periods of time.