Preparation Technology And Electrical Property Of Strontium Doped Lanthanum Manganate Films

The composite oxides lanthanum manganates(LaMnO₃),which have perovskite type structures,can show stable crystal structure,steady physicochemical properties,negative temperature thermal sensitive effect,and so on.They would be hopefully developed to thermal sensitive film elements used in the corrosive environment and heating film device for high temperature.By comparing with various preparation methods for LaMnO₃-based film,the composite Sol-gel method and screen-printing method were studied systematically in this paper.The composite Sol-gel method is suitable for preparation of the curved and low-temperature sintered films,and the screen-printing method is suitable for preparation of the flat-plate films.Microstructure and electrical conductivity of Sr-doped LaMnO₃ films have been investigated by using X-ray diffractometer(XRD),scanning electron microscope(SEM)and four-point probe analysis,and the film formation mechanism has been discussed.The two whole technical processes were optimized,including preparation of the slurries and films, and sintering schedule.Deterioration of the slurry for the composite Sol-gel method and high-temperature conductive mechanism of the La_1-xSr_xMnO₃ films have been further explored.The research contents and conclusions are follows:1.Effects of the technological parameters on the microstructure and electrical conductivity of La_0.8Sr(0.2)MnO₃ films prepared by the composite Sol-gel method were investigated,and the film formation mechanism was discussed.The properties of the films made from acidic slurries and alkaline slurries were compared.It is considered that,a kind of island film is formed with the pure sol as the slurry,and a continuous film is obtained while canals between the islands are gradually filled by subsequent sol.Furthermore,powder in the composite slurry can provide some sites for the nucleation of the gel.This would be beneficial to decrease number and dimension of the canals or cracks on the films.Thicknesses of the films made from acidic slurries and alkaline slurries increase with increasing amount of La_0.8Sr_0.2MnO₃ powder from 0 to 70wt%,while their sheet resistances decrease. Within 5 dippings to 15 dippings,sheet resistances of the films decrease with increasing number of dippings,while their thicknesses increase.Thicknesses of the films made from alkaline slurries increase faster than that made from acidic slurries. For instance,thickness of the film with 15 dippings can reach 100μm.With same amount of the powder and same number of dippings,the films made from alkaline slurries are thicker than that from acidic slurries,and they have lower sheet resistance,less cracks and canals on the surface.The sealing pore technique can improve surface roughness of the films obviously.2.A series of La_1-xSr_xMnO₃(x=0.2,0.3,0.4 and 0.5)films were prepared by the composite Sol-gel method,and effects of the content of strontium on electrical conductivity and crystal structure of the films were investigated.Sheet resistance of the films decreases first,and then increases with increasing the content of strontium. It reaches minimum as x=0.4.It is indicated that the electrical conductivity of La_1-xSr_xMnO₃ films can be regulated by dominating the content of strontium.The sheet resistance decreases first with increasing x,because dopant of strontium induces formation of the small polaron Mn³⁺-O²-Mn⁴⁺.While content of strontium reaches a certain extent,the sheet resistance increases on account of oxygen vacancy becoming main mode for the charge compensation.It is revealed from theoretical calculation that crystal structural stability increased with increasing content of strontium.Moreover,it is indicated from the XRD patterns of La_1-xSr_xMnO₃ films that,for x=0.2 to x=0.4 inclusive,they have orthorhombic structure,R(?)c,and for x=0.5 its structure changes to the tetragonal,I_4/mcm.For x=0.2 to x=0.5 inclusive,there is no other new phase created in La_1-xSr_xMnO₃ films. Their lattice parameters have a little decrease and unit cells shrinkage with increasing x.In addition,for x=0.2 and 0.3,effects of the Jahn-Teller distortion on their crystal structures are obvious.For the La_1-xSr_xMnO₃ compounds,the unit cells expand because of the substitution of Sr²⁺ions(0.144nm)for some small La³⁺ ions(0.136nm),on the other hand,the unit cells shrinkage owing to the Mn³⁺ions (0.0785nm)changing into small Mn⁴⁺ions(0.067nm).The former has less effect than the latter,so both aspects weaken the lattice distortion caused by the Jahn-Teller effect,and the lattice symmetry is improved.A kind of film on crucible ectotheca was prepared with the optimized parameters.Variation of its resistance with temperature between room temperature and 800℃shows a significant negative temperature effect,so the film has the potential to be thermal sensitive device.3.The acidic composite slurry for fabricating La_1-xSr_xMnO₃ films with the composite Sol-gel method has a poor stability,in which dissolvation of La_0.8Sr_0.2MnO₃ powder,appearance of white deposition and discoloration of sol, were observed after a resting time.However,the alkaline slurry only presents darkening of the alkline sol composition.Taking stoichiometry La_0.8Sr_0.2MnO₃ as an example,reasons for the deterioration of acidic slurry and discoloration of alkaline sol were discussed.It is indicated that,the reaction between La_0.8Sr_0.2MnO₃ powder and H⁺ ions brings on white deposition[LaHCit·3H₂O]₂ generated in acidic slurry.And Mn³⁺and Mn⁴⁺ions generated from the reaction lead to the discoloration of the acidic sol.The discoloration of alkaline sol is considered as a result of generation of the multicomponent complexes with Mn²⁺ions and various ligands.Moreover,in the process of the acidic sol being regulated to the alkaline sol with aqua ammonia,it was found that white deposition[LaHCit(NH₃)·6H₂O]₂ appeared in the sol as pH=4,then the deposition dissolved entirely as alkalinity of the sol increasing farther,because partial complexing citrate groups were replaced with NH₃,which had stronger complexing ability.Dissolvation of La_0.8Sr_0.2MnO₃ powder and appearance of white deposition were not observed in the alkaline slurry in the resting time of six months.This would provide a technique approach for preparing stable slurries.4.Effects of primary technological parameters of the screen-printing process on microstructure,crystal structure,conductivity and adhesion of the La_1-xSr_xMnO₃(x=0.2～0.5)films were investigated,and the process was optimized.The optimal details are as follows:contents of ethyl cellulose and terpilenol in the slurry are 1～2wt%and 50wt%respectively;wet film after being printed need 10 min for flow leveling;sintering schedule shows that the samples are heated to 1200℃with the rate of 10℃/min below 1000℃and with 4℃/min from 1000℃to 1200℃,then held for 2h at 1200℃.The final procedure is a furnance cooling.In addition,adhesions of the films can be obviously enhanced by adding frit powder as the low-melting phase,while their conductivities are reduced greatly.Variations of sheet resistance and crystal structure of the films prepared by the screen-printing method with different contents of strontium display a similar rule with the films by the composite Sol-gel method.Moreover,the electrical conductivity of these films can be regulated by dominating the content of strontium.A kind of U-type La_0.6Sr_0.4MnO₃ film was prepared with the optimized parameters.Variation of its resistance with temperature between room temperature and 800℃shows a significant negative temperature effect,so the film has the potential to be thermal sensitive device.As heating film,various heating temperatures can be obtained for the U-type film with different loading voltages.Its power density arrives at 20 W/cm² as the surface heating temperature displaying 800℃.5.It has been investigated that high-temperature conductive mechanism of the La_1-xSr_xMnO₃(x=0.2～0.5)films prepared by the screen-printing method from room temperature to 800℃.The resistivity-temperature relationships for these films are consistent with the conductive model of semiconductor.And these relationships are fitted with four models,thermally activated conductivity, variable-range hopping,adiabatic small polaron hopping,and non-adiabatic small polaron hopping.Fitting results indicate that,variation of the resistivity between 135℃and 600℃is consistent with the adiabatic small polaron hopping model, and conductivity activation energy values of the films for x=0.3 to x=0.5 inclusive increase with increasing x,while the activation energy value of La_0.8Sr_0.2MnO₃ film lies between the La_0.6Sr_0.4MnO₃ film and La_0.5Sr₍0.5_MnO₃ film.In addition, the pre-exponential factors decrease with increasing x.On the other hand,the high-temperature conductive mechanisms of La_1-xCa_xCrO₃(x=0.01,0.03 and 0.08) block samples prepared by the traditional solid state method in this temperature range are more suitable for the non-adiabatic small polaron hopping model,which are different with the La_1-xSr_xMnO₃ films.Their conductivity activation energies and pre-exponential factors decrease with increasing content of calcium for the La_1-xCa_xCrO₃ compound.Although the two kinds ofperovskite structure materials display the small polaron hopping model at high temperature,their true conductive mechanisms behave as the adiabatic state or the non-adiabatic state lying on the configurations of ions and electrons composing the small polaron.