Study On Characteristics And Thermal Control Mechanism Of Flexible Polymer-based Positive Temperature Coefficiant Material At Room Temperature

With the rapid development of science and technology,thermal control has gradually become the main problem restricting the development of electronic devices and other fields.The material that can achieve adaptive thermal control without the help of the control circuit is widely used in the thermal control system of aerospace,electric vehicles,household appliances,etc.,where,Polymer-based PTC materials have attracted extensive attention because of their higher PTC strength and their application in the thermal control of electronic devices at room temperature.However,the research on the polymer-based PTC materials and their thermal control performance at room temperature is not enough.In this paper,the existing polymer-based flexible PTC materials lack of flexibility,film and other problems,studied the preparation method of polymer-based flexible PTC materials at room temperature,prepared a new flexible PTC thermal control material;By introducing the thermal physical parameters and internal heat transfer of PTC material,the theoretical model of self-adaptive thermal control of PTC material is further improved.Based on the high equivalent heat capacity of polymer PTC materials,a PTC thermal control scheme combining active and passive was proposed,and the thermal control characteristics of building envelope were studied.In this paper,Olefin Block Copolymer(OBC)is used as a matrix to prepare a flexible PTC composite at room temperature.It is shown that the flexibility of PTC is related to the composition of OBC and paraffin in the composite and increases with increasing temperature.The low temperature resistivity of PTC decreased with the increase of Graphite Powder(GP)content,while the strength of PTC increased with the decrease of Graphite content.Paraffin content has the opposite effect.When the mass fraction of GP is 40%and the ratio of OBC to paraffin is 30:70,the Curie temperature of flexible PTC is 22℃,young’s modulus is 26 MPa,PTC strength and low temperature resistivity are 2.9 and 1800Ω·cm,respectively.The PTC strength and low temperature resistivity are better than other polymer PTC materials in the same temperature region.By introducing the internal heat transfer and the influence of thermophysical parameters of PTC material,the existing adaptive thermal control theoretical model of PTC material was modified and improved,and a new adaptive thermal control theoretical model of PTC material was established,and the accuracy of the theoretical model was verified by experiments.On the basis of the model,the influence of PTC material thermal parameters on the adaptive thermal control is studied.The results show that the equilibrium temperature of the controlled devices decreases with the increase of the contact thermal resistance between the interfaces.The heat capacity of PTC material has no effect on the equilibrium temperature of the controlled devices,but the equilibrium time of the controlled devices is significantly prolonged when the heat capacity of PTC material is large.PTC materials with lower thermal conductivity have lower equilibrium temperature and longer equilibrium time for the controlled devices in adaptive thermal control.In addition,based on the new model,the influence of ambient temperature dynamic change on the adaptive thermal control performance of PTC materials was studied.The results show that PTC materials have good thermal control performance under different dynamic ambient temperature conditions,and the temperature fluctuation range of controlled devices is less than 1/10 of the ambient temperature fluctuation range.Finally,based on the theoretical analysis of PTC material adaptive thermal control,the dimensionless number representing the heating characteristics of PTC material was proposed,and the dimensionless criterion relation of equilibrium temperature of controlled devices was obtained through fitting,and the accuracy of the criterion relation was verified.Based on the adaptive thermal control characteristics of PTC materials and the characteristics of high equivalent heat capacity of polymer PTC materials,an active and passive integrated thermal control method of polymer PTC materials was proposed,and theoretical and experimental studies were carried out.Firstly,the passive thermal control performance of PTC material is verified by experiments.The results show that the temperature of the controlled devices with passive thermal control using PTC material is obviously lower than that without PTC material,which can maintain the operating temperature of the controlled devices.Secondly,the principle of active and passive thermal control of PTC material is revealed through theoretical analysis.The analysis shows that PTC material has adaptive active thermal control property due to the change of macroscopic structure,and passive thermal control ability due to the large amount of latent heat absorbed during phase transformation.Based on the above research on active and passive thermal control of PTC material,the building envelope structure based on PTC material is designed,and the PTC wall material is prepared.The thermal control performance of PTC wall material under different climate conditions is studied theoretically.The results show that PTC wall material can effectively reduce the interior surface temperature and the heat flow into the interior of the envelope structure,and the heat flow can be reduced by up to 28%in the whole month.Under winter conditions,PTC wall material is used to heat the building envelope to maintain the interior side surface temperature of the envelope higher than the indoor environment temperature,and the temperature fluctuation is reduced by more than 50%compared with the ordinary envelope.