Study On Preparation And Performance Of Mannitol Phase Change Capsules

As a medium-temperature phase change energy storage material,sugar alcohol has various advantages including wide range of melting point,high energy density,and non-corrosiveness.As a non-toxic artificial sweetener and being widely used in industries such as pharmaceuticalindustry,sugar alcohol is also a material that isfriendly to human and theenvironment.However,duringthe phase change process,sugar alcohol is in direct contact with the oxygen in the environment,and the latent heat of the sugar alcohol has a tendency to decrease.Moreover,sugar alcohols also have problems such as easy sublimation at high temperatures,easy adhesion to instruments after phase change,and low thermal conductivity.Microencapsulation of phase change materials is an effective way to solve the above problems for sugar alcohols.At present,there are many researches on the microencapsulation of phase change materials,but there are few articles on the microencapsulation of sugar alcohols.There are mainlytwo shortage shortcomings in present research on mannitol microencapsulation.The microcapsules either failed after solid-liquid phase change cycles(no phase change),or required harshexperimental conditions for synthesis and the properties including the morphology werepoor.In view of the existing problems in the research on the synthesis and performance of mannitol capsules,in this study a method was proposed using silica to encapsulate mannitol to overcome the inherent defects of the phase change materials.Graphene oxide was usedfor dopingto improve the thermal properties and thermal stability of the capsules.Capsule suspensionswere prepared as atype of functional fluid,and the stability,and thermophysical properties were tested.The main work of the thesis is as follows:(1)A method forsynthesizing mannitol(DM)capsules DM@SiO₂(NEDM)for phase change energy storage was proposed,and the performance of the capsuleswas studied.The DM was encapsulated by the interfacial polymerization method and confirmed by Fourier infrared spectroscopy(FT-IR),field emission transmission electron microscope(TEM)and field emission scanning electron microscope(SEM).As shown by the results,DM was well encapsulated by the SiO₂shell,and there was no chemical interaction between the shell and DM.The size of the NEDM was uniformly distributed in the range of 100-200 nm.According to thedifferential scanning calorimeter(DSC)analysis,NEDM exhibited a melting point of 166.2°C,a latent heat of 220.3 k J/kg,and aencapsulation ratio of 76.5%.(2)On the basis of DM@SiO₂,SiO₂ was doped with graphene oxide to enhance thethermal stability and thermal conductivity of the capsules,and DM@SiO₂/GO(GONe DM)nanocapsules was prepared.The results show that the particle size of the GONe DM was uniformly distributed between 150 nm and 300 nm.The latent heat of melting was 216.7k J/kgand the latent heat of solidificationwas 174.4 k J/kg,respectively.Compared with the nanocapsules without graphene oxide,the addition of GO had little effect on the latent heat capacity of the nanocapsules.The encapsulation of the capsule was 75.2%.Compared with pure D-mannitol,the thermal conductivity of the GONe DM nanocapsules was increased to128.6%.(3)A two-step method was used to prepare suspensions of the capsules.The capsules were dispersed in heat-conducting oil to prepare suspensions with different mass concentrations.A stable upper layer of the capsule suspension was obtained after standingtime.The stability was characterized by a multiple light scattering instrument.The specific heat capacity,viscosity,thermal conductivity and other properties of the suspension were measured.The results show that the specific heat of the GONe DM suspension was as high as 221%of that of the base liquid.The thermal conductivity and viscosity of the GONe DM suspension were higher than those of the base fluid by up to 38%and 8%,respectively,which confirms the high efficiency of the GONe DM suspension as a heat transfer fluid.The GONe DM suspension with good thermophysical properties has potential applications in medium temperature energy storage and heat transfer.