Preparation Of High Performance Positive-Temperature-Coefficient Material And Its Thermal Control Characteristic

With the increasing miniaturization and lightweight,the heat dissipation,and the complexity of the tasks of spacecraft,there is a growing demand for a better performance of the active thermal control method based on electric heating.However,the traditional active thermal control method needs to cooperate with sensors and controllers by using the ordinary resistor as the heating element,which increase the complexity,unreliability and energy consumption of the thermal control system.It cannot be applied for micro-satellites that have strict requirements on volume and weight.The positive temperature coefficient（PTC）material is a kind of thermos-sensitive material whose resistance increases sharply with temperature.Based on the PTC effect,the PTC thermal control element can automatically adjust the heating power output according to the temperature of the controlled device and the change of external heat flow,so as to achieve adaptive temperature control.However,the Curie temperature of current PTC materials is mostly located in the high temperature zone（>60℃）,which cannot meet the requirements of adaptive temperature control of electrical devices in spacecraft.In addition,PTC materials still have the contradiction of simultaneously reducing the room-temperature（RT）resistivity and increasing the PTC effect.Therefore,there is a serious need for developing novel PTC materials with RT Curie point and high PTC effect.In this dissertation,perovskite-type Ba_1-xSr_xTi O₃（abbreviated as BST）is selected as the bulk-phase material to investigate on the adjustion of Curie temperature and resistivity,and reveals the doping mechanism of trace elements.The control methods of the low resistivity and high performance of polymer-based PTC materials is explored,and the synergistic mechanism of hybrid fillers is revealed.The intelligent thermal control element based on PTC material is developed,and the adaptive temperature control characteristic and high-precision thermal control are studied theoretically and experimentally.The contents of the thesis mainly include the following parts.1.Calculation on BST crystal structureDoping of trace elements is the key to prepare the BaTiO₃ with low RT resistivity and high PTC performance.Based on the first-principles method,the influence of doping elements（Sr,Ce,Nb,Mn,Y）on the crystal structure of BaTiO₃ was studied,and the feasibility of RT Curie point and low RT resistivity was revealed.The results show that the lattice constant of the BST material has a linear relationship with the Sr element content x,which conforms to Vegard’s law.By analysis of electronic structure and density of states,it is found that the hybridization between Ti 3d and O 2p orbitals of BST is stronger than the orbital hybridization between Ba-O and Sr-O,especially in the high energy range between the top of the valence band and the bottom of the conduction band（-5～0 e V and 1.5～5 e V）.The Ti-O orbital electron hybridization is the main reason for the spontaneous polarization and ferroelectricity of BST materials.However,the resistivity of BST ceramics are still high after doping with different contents of Sr,because of there has no obvious effect on the bandwidth.By calculating and analyzing the energy band structure of Ce/Nb-and Mn/Y-doped Ba_0.65Sr_0.35Ti O₃,it can be seen that due to the contribution of the transition metal element Nb3d and O 2p orbital electron hybridization,the Fermi level doped Ba_0.65Sr_0.35Ti O₃ moves upward to the bottom of conduction band,and part of the energy band passes through the Fermi level.Thus,the RT resistivity is greatly reduced.Nevertheless,the effect of Mn/Y doping on the band structure of Ba_0.65Sr_0.35Ti O₃ material is different.Due to the hybridization of the Mn 3d and O 2p orbital electrons,an impurity band is generated at the Fermi level,which causes the low RT resistivity.2.Preparation and performance-optimization of BaTiO₃-based PTC ceramicsBased on the results of theoretical calculations,Ba_1-xSr_xTi O₃（x=0.1,0.2,0.25,0.3,0.35）PTC ceramics were prepared by solid-phase synthesis.The RT resistivity and PTC effect are controlled by the double donor elements Ce/Nb.The effects of sintering temperature,sintering time and sintering atmosphere on the RT resistivity and PTC performance are investigated.XRD,TEM,EDS,mapping and other test methods are used to detect the crystal structure,surface morphology,element content and distribution.The PTC performance of the material is studied through the resistance-temperature performance test system.The results shows that the Curie temperature was moved to 22°C and 38°C by heavy-doping Sr.The RT resistivity and PTC effect are influenced by the sintering process.For the specimen sintered in a semi-reductive sintering atmosphere and at 1300℃for 3 h,the PTC strength reaches 10^2.1and the temperature coefficient reaches 2.6%/℃,but the RT resistivity（1.7?10⁵Ω·cm）was still high.After Mn/Y co-doping.When the T_C is lowered to 18℃,the doped BST material possesses the excellent PTC effect,including PTC intensity of 10^4.7 andа_PTCof 10.7%/℃.Meanwhile,the room temperature resistivity is relatively low.3.Preparation of polymer-based PTC materialsPTC materials are mainly divided into BaTiO₃-based ceramics and polymer-based PTC materials.PTC ceramics have the advantages of strong anti-aging and high reliability,while polymer-based PTC materials possess better sensitive PTC effect and mechanical flexibility.However,polymer-based PTC materials also need to manage the contradictory relationship of RT Curie point,high PTC effect and low RT resistivity to meet the thermal control requirements.In this paper,a series of polymer-based PTC materials with Curie temperature in the room temperature region were obtained by the preparation method of melt stirring and rapid cold pressing.The RT resistivity is lowered and the PTC effect is concurrently improved based on the synergistic effect of multi-walled carbon nanotubes（CNTs）and carbon black（CB）through hybrid fillers with different aspect ratios.SEM,TEM etc.are used to analyze the microstructure and filler distribution in the composite.The results show that the conductive particles CNTs and CB are mainly distributed in the paraffin phase,and CNTs play an important role in remote conduction between the conductive particles.The bubble-shape paraffin as the phase-transition is uniformly distributed in the EVA supporting framework.The temperature-resistance characteristic test shows that the sample 30EVA/70paraffin/5wt%CNTs/1wt%CB possesses the excellent PTC effect including the PTC intensity of up to10^5.2and theα_PTCof 58%/℃.Meanwhile,the RT resistivity is 1426?·cm,and Curie temperature is about 36.9℃.The Curie temperature of the PTC material is adjusted by the low melting point phase（n-hexadecane,n-eicosane）to 12°C and 23°C,respectively,while theа_PTC is as high as 117%/°C and the PTC intensity is up to 10^4.3.4.Adaptive thermal control performance of PTC materialsBased on PTC ceramics and polymer-based PTC materials,an adaptive temperature control experiment platform was built.The influence of ambient temperature,initial power,Curie temperature and resistance temperature coefficient on the effect of adaptive thermal control are investigated.Based on the experiment,a theoretical calculation model of the adaptive thermal control is established.The result shows that,compared with ordinary resistors,PTC materials have significant adaptive temperature control characteristics.With the resistance-temperature coefficient of the PTC material increasing,the equilibrium temperature of the controlled device is close to its Curie temperature,and shows the stronger the adaptive temperature control ability.The temperature control effect is more obvious in the fluctuation of the ambient temperature.Based on the PTC effect of the new polymer-based PTC materials,combined with PID（Proportional-Integral-Derivative）algorithm controller,the high-precision temperature control characteristics of PTC materials are investigated.The results show that the PTC element can obtain higher temperature control accuracy than ordinary resistors,and the highest temperature control accuracy can reach 0.086°C.