Research On The Process Of Preparing La_1-xSr_xMnO₃ Films By Composite Sol-Gel Route

The perovskite material LaMO₃(M=Cr,Fe,Mn,et.al)which has obvious Negative Temperature Coefficient(NTC)characterization,could be used for the next generation material of the NTC thermally sensitive resistance.The resistance-temperature relationship could be controlled and the best performance could be achieved by such approach as doping.Due to the excellent high temperature performance and resistance-temperature characterization of LaMO₃ film,it is expected to be used as the next generation film material of thermo-sensitive,and exploit the direct contact type temperature probe that is resistant to acid,alkali and corrosive gas by being coated on the appropriate substrate.The purpose of this thesis is to prepare La_1-xSr_xMO₃(LSMO)film with dense structure and certain thickness on the Al₂O₃ substrate,and then study the electrical property of this kind of film.The structure,phase composition and surface and cross-section morphology of the films were analyzed by XRD,SEM testing methods et.al.The relationship between the processing parameters (including the concentration of the gel,the doped amount,the amount of the added powder,the plug densification,the dipping number,the presintering temperature,the sintering temperature and so on)and the structure of the film was studied;The electrical property of the La_1-xSr_xMO₃ film was researched to find out the key processing parameter that determines the electrical property;And the formation mechanism of the LaMO₃ film and the electrical mechanism of the doped peroveskite structure was also studied.Through systemic comparison,the composite sol-gel processing technology of preparing high quality La_1-xSr_xMO₃ film on the Al₂O₃ substrate was optimized, the key of this processing technology is as follows:first,prepare doped La_1-xSr_xMO₃ nano-powder and La_1-xSr_xMO₃ gel that is of the same component as the powder;then,mix the powder and the gel with certain proportion to make the composite slurry;and finally,coat the pretreated substrate with dipping sliding method and sinter to form the film.The main processing parameters that affect the quality of the film are the amount of the powders added,the dispersant,the number of the dipping,the plug processing,the heat treatment temperature and so on.It was shown by XRD and SEM that a single La_1-xSr_xMO₃ perovskite phase was obtained on the Al₂O₃ substrate with the composite sol-gel methods.With the increase of the added amount of the powder,the thickness of the film increases, but the adhesiveness becomes worse when too much powder was added;the dispersant accelerates the uniform disperse of the powder in the slurry,but its decomposition under high temperature is inductive to crack the film;the increase of the dipping time is helpful to improve the thickness of the film,but also prolongs the processing flow;the film is smoother after plugging,the cavity decreases obviously,and all the organic substance decomposes completely before 800℃.According to the study of the electrical property of different films,the amount of Sr dopped has a strong influence on the electrical property,within x=0.2,0.3,0.4,0.5,when x=0.4 the electrical property reaches its highest point. The increase of the added powder and the increase of the plug technology could increase the electrical property of the film.Based on the comparison of the influence of the added amount of the powder, the dispersant,the number of dipping,the plug technology and heat treat temperature on the film,the optimized processing parameter is obtained,that is, the amount of added powder 62.0%,polyethylene glycol as dispersant agent, added amount 0.2ml,the number of dipping 6,plug twice,pre-sintering at 600℃for 10 min,sintering at 800℃for 1 h in air.In addition,it is found that the stability of the slurry could be greatly improved by the adjustment of the pH value to 9.0.