Preparation And Application In Gas Sensor Of LaF₃ Solid Electrolyte

The single crystalline and polycrystalline of LaF3 and LaF3 (mixed) are important solid electrolyte. The fluorine is most typical nonmetal element because of 4.0 electronegativity .The F-transport is very high speed under some conditions for small ion radius and one negative charge. The fluoride of alkali metals and alkaline earth metal all are ion crystal, ion conductivity of solid electrolyte next to liquid and has no electron transfer at high temperature. LaF3 solid electrolyte has been studied and found in field of application. LaF3solide electrolyte was used in ion electrode because of high sensitivity, simple operation, few trouble and continuous use .It is important use to measure ion concentration in solution .The material property will be changed when rare earth is added ,but it is also necessity to supervise its concentration. LaF3 solid electrolyte was used in component sensor of rare earth lanthanum of melting aluminium and melting iron, so that stable electromotive force is gained and activity of lanthanum can be calculated. LaF3 single crystalline and film also were used in gas sensor of fluorin , hydrogen ,oxygen and carbon monoxide at room temperature. LaF3 powder was prepared by deposition method sol-gel method and microwave heating method in the experiment. The purity and particle long were analyzed by XRD ,SEM and TEM. In the process of sol-gel method. some influence factors including concentration of La(CH3COOH)3 solution ,stirring intensity, pH value and dropping speed of NH4F to stability of LaF3 sol were studied. Under optimum condition of sol preparation, stable LaF3 sol was prepared. By seperating and microwave heating ,LaF3 superfine power below 300nm was obtained The superfine powder of La2(CO3)3 were synthesized by means of double pillar method using La2O3,HCl as raw material and LaF3 superfine powder were also synthesized by means of mixing solid La2(CO3)3 and NH4F under the microwave condition. The influence of different sattering agent to grain degree or pure degree was discussed. The crystal type and purity of LaF3 superfine powder was tested by XRD.The granularity and shape of LaF3 superfine powder was tested by SEM. LaF3 superfine powder of 50 nm was gotten in the direct solid synthesis. The ion conductivity were measured by means of impedance spectroscopy for different particle .It is result that LaF3 ion conductivity is increasing along with the particle is diminishing. The lanthanum fluoride single crystals[pure LaF3,mixed (Sm,Eu,Ga)F3 or CaF2] were prepared by means of temperature decrease at laboratory. Ionic conductivity of lanthanum fluoride single crystals made by oneself were measured by virtue of AC bridge in the range 20℃～100℃,it is same level of 10-6Scm-1.Impedance spectroscopy of lanthanum fluoride single crystals [ LaF3 mixed 1%(Sm,Eu,Ga)F3 2.05%CaF2, LaF3 mixed 0.2%EuF2 5%CaF2 ]also were measured in the range 100℃～400℃under the argon atmosphere,ionic conductivity of both single crystals is at the level of 10-4 Scm-1. The relationship between ionic conductivities of La1-XCaXF3-X singlecrystalline or polycrystalline and temperature ,gas concentration was studied. by the impedance spectroscopy technique in a range of temperature from 0～100 ℃in the SO2 atmosphere, the apparent conductive activation energy was obtained at same time. The results indicate that the ionic conductivity of the La1-XCaXF3-X in the SO2 atmosphere is higher than in the air .The conductivity is in the range of order 10-3～10-5Scm-1. The humidity sensors based on steam concentration cell were constituted by using small seramic tube of LaF3 doped with CaF2 as solid electrolyte , mixed H2C2O4-H2C2O4.2H2O or Na2SO4 -Na2SO4.10H2O as reference electrolyte, and pure water or KOH solution as working electrode . Sensitive experiment of two kind humidity sensor was investigated in the temperature ranges of 8～33℃and 6～24 ℃. The regular change was revealed between EMF and lg(PH2O(Ⅰ)/PH2O(Ⅱ))—EMF will become small along with temperature rise and EMF will become large along with work electrode PH20(Ⅰ) rise at constant temperature, there is a good liner relationship between EMF and lgPH2O(Ⅰ)/PH2O(Ⅱ) . A steam sensor composed of concentration cells has been developed using a small tube made from ZrO2 mixed with Y2O3, which was sintered to form solid electrolyte according to a certain regime. The reference electrode used was made from H2C2O4 mixedwith H2C2O4·2H2O or Na2SO4 mixed with Na2SO4·10H2O, of which either equilibrium steam pressure was given by a physic-chemical principle. It is revealed different partial pressures of steam can be obtained if adding some nonvolatile solutes into solvent. So, the other electrode for measurement was made from pure water or KOH solution of different concentrations, thus forming concentration cells for steam. Sensitivity experiments have been done separately for the cells as above in the ranges from 0 to 35℃and from 8 to 23℃. The results revealed that the EMF of cells, E, changes regularly with lg ( PH 2 O(Ι) / PH 2 O(Π)) where PH2O(Ⅰ) and PH2O(Ⅱ) are the partial pressure of steam at the electrode for measurement and at reference electrode, respectively, i.e., E decreases with increasing temperature and E increases with increasing PH 2 O(Ι) at constant temperature. A fit linear relationship is found between E and lg ( PH 2 O(Ι) / PH 2 O(Π)). The work cells of solid electrolyte LaF3 single crystalline (mixed) has been prepared with Pb-PbF2 reference electrode , Sn-SnF2 work electrode and Ag electrode wire by means of milling Pb,PbF2,Sn,SnF2 and pressing thin piece in ratio of 4:1 in a polymethyl methacrylate box of full Ar gas .this structure (+)Ag|Pb,PbF2|LaF3 single crystalline(mixed)|Sn,SnF2|Ag(-).The EMF of cell was measured in the temperature of 293.15K～353.15K .The relationships between SnF2 standard formation Gibbs free energy were calculated on the two kind of PbF2 standard formation Gibbs free energy datum as follow: f G m, SnF2( kJmol1)663.12145.42T? ?? =?+, 2( 1)675.81150.1;? f G mSnFkJ?mol? =?+T.Because the EMF measure errors are smaller than PbF2 Gibbs free energy error,sometime the difference of SnF2 standard formation Gibbs free energy in two kind of equations is very small each other , so as to get these results above are Correct. The sensors of Sn , SnF2 |LaF3 |Pt was prepared with Sn ,SnF2 reference electrode .LaF3 singlecrystalline piece and polycrystalline tube were used as solid electrolyte in the two kinds of sensors. sensitive experiments of the different concentration SO2 have been tested at the range of 303.15K～351.15K (used Ar as dilute gas ).The phenomenon was found in our experiments: the respond time of singlecrystalline sensor is shorter than polycrystalline sensor ,it is about the 5 min for singlecrystalline .The relationship between EMF and lgPSO2 or between EMF and T oftwo kinds of sensors were measured :the EMF of singlecrystalline sensor is very sensitive along with the change of temperature and concentration ;but the EMF of polycrystalline sensor is not sensitive at high temperature. The linear relationship is very good between EMF and lgPSO2 , it shows that Nernst equation is obeied under the condition of experiment . A gas sensor composed of Sn , SnF2 |LaF3(mixed) |Pt(Ni) (with Sn ,SnF2 reference electrode )has been developed using LaF3 singlecrystalline or polycrystalline . The relationship of two kinds of CO ,CO2 between the EMF and the concentration in the range of 5%～30% concentration ( Ar used as dilute gas ) ,the temperature in the range of 293.15K ～343.15K and responsetime which arrive at the cell equilibrium was respectively measured. Similar regularity was revealed that the EMF of cells to CO ,CO2 by the experimental results , changes regularly with the concentration or temperature of CO ,CO2 respectively, i.e., EMF increases with increasing the concentration under the condition of constant temperature; EMF increases with increasing temperature under the condition of constant concentration ;and EMF of singlecrystalline is higher than the polycrysyalline at same conditions. The responsetime when two kinds of gas arrived at the equilibrium is abount 2～5min.at constant temperature. LaF3solide electrolyte was used in ion electrode because of high sensitivity, simple operation, few trouble and continuous use .It is important use to measure ion concentration in solution .The material property will be changed when rare earth is added ,but it is also necessity to supervise its concentration. LaF3 solid electrolyte was used in component sensor of rare earth lanthanum of melting aluminium and melting iron, so that stable electromotive force is gained and activity of lanthanum can be calculated. LaF3 single crystalline and film also were used in gas sensor of fluorine ,hydrogen ,oxygen and carbon monoxide at room temperature.