The Preparation Technology, Structure And Performance Of La_1-xSr_xFeO₃ Films

It has been the target of researchers to explore the special physical and chemical properties of rare earth functional materials, and to exploit valuable devices based on them. La1-xSrxFeO3 films have negative temperature coefficient resistance and stable physical and chemical properties, which can be applied as thermistor element by adjusting the Sr doping content of La1-xSrxFeO3 films. The preparation technology, structure and performance of the functional La1-xSrxFeO3 films were summarized in this thesis, emphasizing on the current research progress, existing problems of negative temperature coefficient thermistor, electrothermal properties and microwave absorption. Currently, plenty of researches have been carried out on the preparation of La1-xSrxFeO3 films, but little researches were reported about the batch preparation technology and practical application of the La1-xSrxFeO3 films. This research aimed to develop the La1-xSrxFeO3 films into high accuracy temperature controlling component and electrothermal component with adjustable power density for adapting to different applications. Relevant theory and technical innovation were carried out. The preparation technology of the La1-xSrxFeO3 films with different Sr doping content were explored by optimizing the key process parameters of the screen printing method. The effect of the Sr doping on the structure, thermosensitive property and heating performance of the La1-xSrxFeO3 films was studied. To solve the problem of poor corrosion resistibility of the La1-xSrxFeO3 film, a layer of protective film was made to coat on the surface of the functional film as a protective layer to improve the corrosion and scour resistibility of the film. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to characterize the microstructure of the film. The grain growth mechanism of the La1-xSrxFeO3 film was discussed in depth which was heated at high temperature. The specific research results and conclusions are as follows:(1) The La1-xSrxFeO3 powders were prepared by a sol-gel method. During the synthesis process, metal ions and citric acid carboxylic formed a stable sol-gel network with single tooth coordination bond. The gel crystallized into lanthanum strontium ferrite phase when heated at the temperature above 600℃. As the temperature increased, the grains grew up significantly and the Fe-O bond had a tendency to be stronger. It was concluded that the La0.6Sr0.4FeO3 slurry of 60% terpilenol and 2～3% EC was suitable for batch printing process of the film through analyzing the influence of the key parameters of screen printing process on the microstructure and mechanical properties of the La1-xSrxFeO3 film.(2) The influence of Sr doping content on the structure of the La1-xSrxFeO3 films was studied. The phase transition from Pbnm to R3C was found when x= 0.3, which was caused by the lattice torsion of Fe-O octahedral due to the substitution of La3+ by Sr2+. La2SrFe2O7 precipitated from the La0.8Sr0.2FeO3 film after heated at 1300℃ for 16 h. The orientation relationship between La2SrFe2O7 and the parent phase (100)La0.8Sr0.2FeO3//(101)La2SrFe2O7 was observed. SrAl2O4 phase was generated in the La0.6Sr0.4FeO3 film after heated at 1400℃ for 16 h, which were resulted from the reaction between substrate Al2O3 and Lao.6Sro.4Fe03.(3) The grain growth kinetics of La0.6Sr0.4FeO3 films was studied which were heated at 1200℃ and 1300℃. The result revealed that the grain growth law was in accordance with the equation and grain growth index were 2.2 and 2.3, respectively. The grain growth activation energy was 454±10 kJ/mol from 1200℃ to 1300℃. When being heated to 1400℃, the grain growth deviated from the kinetics phenomenological theory, and the grains merged and grew up, which was in conformity with the "neck size control" mechanism. The grain growth mechanism is:the atoms of small grains go into large grains though grain boundaries; when there are micro-voids inside grain boundaries which alter the center of the curvature, the atoms can move from small grain boundaries to micro-voids, and then move from micro-voids to big grain boundaries. The driving force of grain growth is the decrease of the total interfacial free energy induced by the reduction of the total particle surface area. Grains grow by the step movement in in the crystal growth of La0.6Sr0.4FeO3.(4) The glass phase reinforced film above the La1-xSrxFeO3 functional film surface was prepared, which enhanced the corrosion and scour resistibility of the functional films. The mass ratio of glass frit and the slurry composition were determined by comparative tests, which were:SiO2:Na2B4O7·10H2O:Al2O3:CaO= 1:1:0.26:0.08 and 70% terpineol+2% ethyl cellulose+28% reinforced material (functional phase:glass frit= 1:2.5) and 83% glycerin+17% reinforced material (La0.8Sr0.2FeO3:glass frit=1:6). The effect of sintering process on reinforced film morphology and phase composition was studied. The SiO2 powders were stacked in the reinforced film after sintered at 800℃ resulting in a discontinuous film. A small amount of B element of reinforced material went into Lao.6Sro.4Fe03 functional film to generate a small amount of LaBO3. After sintered at 900 ℃, a three-layer film structure was formed, which was composed of continuous reinforced film, La1-xSrxFeO3 film and alumina substrate. The LaBO3 content increased while the accumulation of SiO2 decreased significantly. After 1000℃ sintering, the film layer continuity of the reinforced film became better and the glassy luster was more apparent, but more glass phase permeated into the La0.8Sr0.2FeO3 film; After 1100 ℃ sintering, glass phase reacted with La0.8Sr0.2FeO3 violently, and no three-layer film structure existed. Four samples were sintered at 900℃ for 5,15,30,60 mins. It was showed that the obtained three-layer film structure of all the samples was similar. By multifactor optimization,900℃ heat treatment for 15-30 mins was ultimately determined as the sintering technology of the reinforced film. There were two kinds of influence of reinforced material on La1-xSrxFeO3 film:(1) The glass phase wetted La1-xSrxFeO3 and formed segmentation and hindering thin layers among the particles in La1-xSrxFeO3; (2) The glass phase, La1-xSrxFeO3 film and alumina substrate formed the three-layer film structure. The friction coefficient of the reinforced film was 0.22, which proved an excellent abrasion resistance; The resistance law of the exponential decaying with temperature in the range from 298 K to 500 K of the reinforced film was similar to the Lao.6Sro.4Fe03 film, and the thermal constant B value reduced from 3885 K to 2009 K, which can still meet the performance requirements for the preparation of thermistor.(5) The effect of Sr doping content on the absorbing performance of the La1-xSrxFeO3 film was investigated. In the wave band of 0.5～18 GHz, the value of ε" increased obviously when x= 0.3 and 0.5, and both the peaks were due to the dielectric relaxation caused by electric dipole polarization. The complex permittivity of La1-xSrxFeO3 was significantly higher than the complex permeability, which indicated that the dielectric loss was mainly responsible for the phenomenon. The reflectance peak width of La0.8Sr0.2FeO3 above 4 dB was 7.41 GHz～10.48 GHz, which presented a better absorbing performance.(6) The resistivity of La1-xSrxFeO3 film was significantly affected by the Sr doping content, which first decreased and then increased with the increase of Sr content, and a minimum value of 313.92 Ω·cm was obtained when x= 0.4. La1-xSrxFeO3 film showed obvious NTC thermal performance, and the value of thermal constant B of the film increased at first then decreased with the increase of the Sr content. The maximum B value of 3885 K appeared at x= 0.4. The films of x= 0.3,0.4 and 0.5 with high B values and low R values can be developed into NTC thermistor that can suppress surge current and the films of x=0.2 and 0.1 with low B values and high R values can widen the operating temperature range of heat sensitive components.(7) The conductive model of La1-xSrxFeO3 (x= 0.1～0.6) in the temperature range of 450～873 K was more in line with the non-adiabatic small polaron model. The carriers transited in the chain of Fe3+-O-Fe4+, forming small polaron hopping. With the increase of Sr content, the carrier concentration increased and activation energy decreased. When the doping amount of Sr was more than 0.3, the oxygen vacancies blocked part of small polarons of Fe3+-O-Fe4+, leading to the scattering of electrons and the increasing of activation energy. The conductive model of La1-xSrxFeO3 at low temperature was more accordance with VRH conductive mechanism. The transition mode of carriers transformed from neighbor transition into a different distance transition, and the existence of charge ordering also resulted in the VRH conduction mode.(8) The strip shaped La1-xSrxFeO3 film electric heating element used in accurate heating and temperature controlling in low temperature zone was prepared by the screen printing method. The technology parameters of electrothermal film were obtained which demonstrated different power density, through the research of the variation of the working temperature and power density of La1-xSrxFeO3 film along with the difference of doping content of Sr (x= 0.3,0.4,0.5) and load voltage. The La1-xSrxFeO3 film element was developed successfully which was used for voltage overload protection based on the data of overload.