Preparation And Properties Of Multifunctional Composites Based On Paraffin@lead Tungstate Phase Change Microcapsules

With the rapid development of industrial society,the problems of energy shortages and environmental degradation have become increasingly prominent.Renewable energy,such as solar,tidal and geothermal energy,are considered desirable alternatives to fossil fuels.Nevertheless,renewable energy supply is discontinuous and unstable,which needs to be converted into other forms of energy for utilization.Phase change materials（PCMs）,as new energy storage materials,realize energy storage and utilization through latent heat absorbed or released during the phase change,solving the contradiction of mismatch between renewable energy in time and space.With strong latent heat storage capacity and high energy storage density,phase change materials show great potential in solar energy conversion systems,thermal management systems,and smart fabrics.Currently,organic solid-liquid PCMs（e.g.,paraffin）have gained widespread attention with high enthalpy,suitable phase change temperature,and good chemical stability.Nevertheless,the disadvantages of organic solid-liquid PCMs,such as easy leakage in the molten state and low thermal conductivity,greatly limit their application.As a fact,the microencapsulation of PCMs with shell materials to form phase change microcapsules increases heat-transfer area and prevents leakage of PCMs in the process of phase transition.Most of the studies on the synthesis of microcapsules only concern the monofunctional issue of thermal energy storage,which cannot satisfy the demands of more applications.With the widespread use of nuclear technology,the frequency of human exposure to ionizing radiation has increased.γ-rays,with their strong penetrating properties,can cause serious damage to humans and the environment.Therefore,it is essential to design and construct multifunctional phase change microcapsules with thermal energy storage andγ-radiation protection to meet the thermal management requirements of someγ-radiation shielding devices.In addition,based on application requirements,phase change microcapsules are blended with the polymer matrix to form flexible phase change composites.However,the obtained flexible composites suffer from poor thermal conductivity and photothermal conversion capacity,which results in poor photo response rate,and slow thermal conversion,transfer,and storage of composites.In this thesis,paraffin@lead tungstate（Pn@PWO）phase change microcapsules with thermal energy storage andγ-ray shielding functions were constructed by in-situ precipitation and self-assembly methods,investigating their growth and morphology regulation mechanism.Multi-shelled phase change microcapsules have been constructed based on Pn@PWO phase change microcapsules,and the impermeability,wettability,thermal cycling stability,and multifunctionality of multi-shelled phase change microcapsules have been systematically investigated.Flexible phase change composites based on Pn@PWO phase change microcapsules have been prepared and the interfacial interactions between the shell and the polymer matrix have been probed.The multifunctional flexible phase change composites with excellent comprehensive performance have been prepared by constructing an efficient thermal conductivity network and photothermal conversion system through filling carbon materials and structural design.The main results and conclusions are as follows:（1）The morphology-controllable Pn@PWO phase change microcapsules were successfully fabricated by in-situ precipitation and self-assembly methods.The morphology of microcapsules can be transformed from spindle to spherical by adjusting the structure-directing agent to control the growth of Pb WO₄ shell.The spindle microcapsules demonstrated a higher encapsulation ratio and lower supercooling degree than the spherical ones.The Pn@PWO microcapsules not only showed high latent heat-storage capacity over100 J·g^-1,but also exhibited high thermal conductivity over 0.596 W·m^-1·K^-1,good thermal storage ability,positiveγ-rays shielding performance.Meanwhile,the highest mass attenuation coefficients of Pn@PWO microcapsules at 86.5 ke V and 105.3 ke Vγ-rays energy reached 1.98 cm·g^-1 and 2.08 cm·g^-1 respectively.（2）Pn@PWO microcapsules show the drawbacks of low wettability and poor leakage-proof property and thermal reliability.Herein,the SiO₂ shell has been successfully constructed on the surface of Pn@PWO microcapsules by gel-sol method.And the Pn@PWO@SiO₂ microcapsules were applied in the waterborne polyurethane（WPU）to obtain WPU/（Pn@PWO@SiO₂）flexible phase change composites.The leakage rate of Pn@PWO@SiO₂ microcapsules decreased by at least 54.11%compared to Pn@PWO microcapsules.The SiO₂ layer with abundant polar groups ameliorated the wettability of microcapsules and the interfacial compatibility between microcapsules and the WPU matrix.The tensile strength of WPU/（Pn@PWO@SiO₂）composites reached 10.98 MPa,which was over 7 times greater than WPU/（Pn@PWO）composites.In addition,WPU/（Pn@PWO@SiO₂-2）composites with a latent heat capacity of over 41 J·g^-1 exhibited efficient phase change stability andγ-ray shielding properties.Also,the mass attenuation coefficients reached 1.38 cm²·g^-1 at 105.3 ke Vγ-rays energy and 1.12 cm²·g^-1 at 86.5 ke Vγ-rays energy,respectively.（3）Herein,Pn@PWO@SiO₂ microcapsules were modified with dopamine（DA）to constructparaffin@leadtungstate@silicondioxide@polydopamine（Pn@PWO@SiO₂@PDA）multi-shelled phase change microcapsules.Then,the tannic acid-modified graphene（TA-G）nanosheets and Pn@PWO@SiO₂@PDA microcapsules were filled with WPU to obtain WPU/（Pn@PWO@SiO₂@PDA）/TA-G phase change composites（WPG）.TA-G nanosheets have exhibited excellent hydrophilicity with a contact angle of58.8°and dispersed well in the WPU.The synergistic effect of SiO₂ and PDA layers has dramatically enhanced the leak-proof performance,thermal cycling stability as well as wettability of microcapsules.The multiple hydrogen bonding networks formed by the PDA and TA-G as well as WPU matrix have effectively enhanced the mechanical properties of composites.The tensile strength of WPG composites reached 7.51 MPa,which was over38.8%higher than WPU/（Pn@PWO@SiO₂）/TA-G composites.The thermal conductivity of the WPG composite was significantly improved by the addition of TA-G nanosheets,and the thermal conductivity of WPG composites reached 0.593 W·m^-1·K^-1.The synergistic effect of PDA and TA-G nanosheets is utilized to greatly capture visible light for conversion and to improve photothermal conversion efficiency.The WPG composites with latent heat capacity over 48 J·g^-1 have presented a superior photothermal conversion efficiency of 75.5%.In addition,at 105.3 ke V and 86.5 ke Vγ-rays energy,respectively,the mass attenuation coefficients of WPG composites reached 1.56 cm²·g^-1 and 1.28 cm²·g^-1,exhibiting efficientγ-rays shielding properties.（4）The paraffin@lead tungstate@silicon dioxide@carbon nanotubes（Pn@PWO@SiO₂@CNTs）phase change microcapsules were prepared by loading carbon nanotubes（CNTs）on the surface of Pn@PWO@SiO₂ microcapsules by electrostatic self-assembly.Subsequently,the Pn@PWO@SiO₂@CNTs microcapsules and TA-G nanosheets were incorporated into the aqueous polyurethane（WPU）to fabricate WPU/（Pn@PWO@SiO₂@CNTs）/TA-G phase change composites（WPCG）.The loading of CNTs significantly improved the thermal conductivity of Pn@PWO@SiO₂@CNTs microcapsules,reaching 1.221 W·m^-1·K^-1 at 2 wt%.The heterostructure design effectively addressed the issue of CNTs agglomeration and increased the filling volume of CNTs in the composites.Moreover,the staggered overlap between the microcapsules and TA-G nanosheets increased the contact area of the filler,promoting the formation of a 3D thermally conductive network.These improvements significantly enhanced the heat transfer of WPCG composites,with the highest thermal conductivity up to 0.766 W·m^-1·K^-1.Meanwhile,CNTs and TA-G nanosheets synergistically enhanced the absorption and conversion of solar light.The WPCG composites with latent heat capacity over 51 J·g^-1 achieved the highest photothermal conversion efficiency of 78.1%.Furthermore,the mass attenuation coefficients of the WPCG composites demonstrated efficientγ-rays shielding properties,reaching 1.59cm²·g^-1 and 1.31 cm²·g^-1 at 105.3 ke V and 86.5 ke Vγ-rays energy,respectively.This multifunctional phase change composite can efficiently absorb,convert and store solar light while shieldingγ-rays.It is expected to be applied toγ-rays shielding devices that require thermal regulation.