Study on electrical properties of strontium doped lanthanum manganate thin films

Rare earth perovskite lanthanum manganate (LaMnO_3) composite oxide has stable structure, stable physical and chemical properties, and negative temperature thermal effect. It is expected to be developed into thermistor thin film devices suitable for corrosive environment and high temperature heating. By comparing various preparation processes of lanthanum manganate based films, the composite Sol-gel method and screen printing method are selected for systematic study. The former is suitable for preparing curved and low-temperature sintered films. The latter is suitable for preparing flat film. The microstructure and electrical properties of strontium-doped lanthanum manganate films were studied by means of X-ray diffractometer, scanning electron microscope and DC four-probe tester. The whole process of slurry preparation, film formation and sintering was optimized. The deterioration of composite Sol-gel paste was studied, and the high temperature conduction mechanism of La_(1-x)Sr_xMnO_3 thin film prepared by screen printing was discussed. The main research contents and conclusions are as follows: 1. The effects of the composite Sol-gel process parameters on the microstructure and electrical properties of La_(0.8)Sr_(0.2)MnO_3 films were studied, the film forming mechanism was discussed, and the properties of the films prepared by acidic slurry and alkaline slurry were compared. The pure sol forms a film in the shape of an island, and then impregnated sol fills the inter-island channel to obtain a relatively continuous film. The powder added to the composite slurry can provide part of the particles for the nucleation of the gel components, which is conducive to reducing the number and size of the channel or crack. In the range of 0 ~ 70 wt%, with the increase of powder addition, the thickness of the films prepared by the two kinds of slurry increases, and the block resistance decreases. In the range of 5 ~ 15 impregnation times, with the increase of impregnation times, the block resistance decreases and the film thickness increases. The thickness of the film prepared by alkaline slurry increased rapidly, and the film thickness reached 100μm when impregnated 15 times. When adding the same amount of powder and impregnating times, the film prepared by alkaline slurry is thicker, the block resistance is smaller, and the surface cracks and channels are less. The sealing process can obviously improve the roughness of the surface. 2. La_(1-x)Sr_xMnO_3(x=0.2, 0.3, 0.4 and 0.5) films were prepared by composite Sol-gel method, and the effects of strontium content on the conductivity and crystal structure of the films were studied. With the increase of strontium content, the resistance of the film block decreases first and then increases, and reaches a minimum value when x=0.4, which indicates that the conductivity of the film can be adjusted by the amount of strontium content. The decrease of block resistance with the increase of strontium content can be attributed to the generation of Mn~(3+)-O~(2-)_Mn~(4+) small polaron caused by doping. When the amount of strontium content reaches a certain degree, the oxygen vacancy becomes the main way of charge compensation and the resistance value of the block becomes larger. Theoretical calculation shows that the stability of crystal structure increases with the increase of strontium content. From the XRD pattern, the rhomboid structure R(?) is obtained when x=0.2 ~ 0.4. 3c; At x=0.5, the lattice type changes to tetragonal I_(4/mcm). No other new phase was formed in the range of x=0.2 ~ 0.5, and the lattice constant decreased slightly and the cell volume shrank with the increase of strontium content. The Jahn-Teller distortion was obvious when the strontium content x=0.2 and 0.3. For La_(1-x)Sr_xMnO_3, the partial substitution of Sr~(2+)(0.144 nm) with larger ionic radius for La~(3+)(0.136 nm) with smaller ionic radius results in cell expansion. As charge compensation, some Mn~(3+)(0.0785 nm) with larger ionic radius will become smaller Mn~(4+)(0.067 nm) to shrink the cell. The effect of Sr~(2+) replacing La~(3+) is smaller than that of manganese ion change, and the result of the joint action of these two factors reduces the lattice distortion caused by Jahn-Teller effect and improves the lattice symmetry. A thin film element on the outer wall of the crucible was prepared by the optimized process. The resistance of the element in the range of room temperature to 800℃ showed obvious negative temperature thermal sensitive effect, which has the potential to be developed into a thermal sensitive device. 3. The acidic composite slurry prepared by the composite SOL-GEL method of strontium lanthanum manganate thin film has poor stability. After being placed for a period of time, particle dissolution, formation of white precipitate and Sol discolor appear, while the alkaline slurry only shows the deepening of the color of the basic sol components. Taking the stoichiometric formula La_(0.8)Sr_(0.2)MnO_3 as an example, the reasons for the deterioration of acidic slurry and the discoloration of alkaline sol were discussed. The results show that La_(0.8)Sr_(0.2)MnO_3 particles react with H~+, resulting in the formation of white precipitate [LaHCit·3H_2O]_2. MnO_3 Sr_ La_ (0.8) (0.2) and H ~ + of Mn ~ (3 +) and Mn ~ (4 +) ions in acid sol discoloration. The discoloration of alkaline sol is caused by complex complex complexes formed by Mn~(2+) ions and various ligands. In addition, white precipitate [LaHCit(NH_3)·6H_2O]_2 appears when pH=4 in the process of adjusting acidic sol to alkaline with ammonia. The white precipitate with higher alkalinity was completely dissolved because some carboxylic acid groups involved in the complexation were replaced by NH_3 with stronger complexation ability. The slurry made of alkaline sol with pH=9 has no particle dissolution and white precipitation after 6 months, which provides a technical way to prepare a stable slurry. 4. The effects of main parameters of screen printing on microstructure, crystal structure, electrical conductivity and adhesion of La_(1-x)Sr_xMnO_3(x=0.2 ~ 0.5) films were studied. The optimum process parameters were obtained as follows: the contents of ethyl cellulose and terpinol in the slurry were 1 ~ 2 wt% and 50 wt%, respectively; The leveling time after printing into wet film is 10 min. The sintering system of the film is first heated to 1000℃ at 10℃/min, then heated to 1200℃ at 4℃/min, then held for 2 h, and finally cooled with the furnace. In addition, the addition of low melting point frit powder can improve the bonding strength of the film, but significantly reduce the conductivity. The change of the resistance and crystal structure of the film blocks prepared by screen printing method is basically the same as that of the films prepared by composite Sol-gel method, and the conductivity of the films can be adjusted by the amount of strontium. The U-shaped La_(0.6)Sr_(0.4)MnO_3 thin film element was prepared by the optimized process. The resistance of La_(0.6) SR_ (0.4) MNO_3 thin film element showed obvious negative thermosensitive effect with the change of temperature in the range of room temperature to 800℃, which has the potential to be developed into a thermosensitive device. Different heating temperatures can be obtained by using the element as an electric thermal film at different loading voltages, and the power density can reach 20 W/cm~2 when the surface heating temperature is 800℃. 5. The high temperature conduction mechanism of La_(1-x)Sr_xMnO_3(x=0.2 ~ 0.5) film prepared by screen printing method in the range of room temperature to 800℃ was studied. The resistivity of the sample in this temperature range is consistent with the semiconductor conduction law. Four models of thermal activation, variable range transition, adiabatic small polaron transition and non-adiabatic small polaron transition are used to fit the resistivity data. The results show that the resistivity changes in the range of 135℃ ~ 600℃ conform to the adiabatic small polaron transition model. The activation energy values of the three samples with strontium content x=0.3 ~ 0.5 increased with the increase of strontium content. When x=0.2, the activation energy values were between the samples with x=0.4 and 0.5. The preexponential factor decreases as x increases. The conductive mechanism of La_(1-x)Ca_xCrO_3(x=0.01, 0.03 and 0.08) samples synthesized by solid phase method in the temperature range was compared. Unlike La_(1-x)Sr_xMnO_3, the conductive mechanism of La_(1-x)Ca_xCrO_3 samples was more consistent with the small nonadiabatic polaron In the transition model, the activation energy of conductance and the predigital factor decrease with the increase of calcium content. Although the two kinds of perovskite materials are conductive by small polaron transition at high temperature, their conductive mechanism is adiabatic or non-adiabatic, respectively, due to the difference in the configuration of ions and electrons that form small polarons.