Research On The Basic Thermodynamic Parameters Of Rare Earth (La, Y) Silicate

Rare earth metal silicates are widely used in laser materials,high-energy fluorescent materials and anti-oxidation coatings because of their high melting point,low thermal expansion coefficient,low temperature oxygen permeability,chemical corrosion resistance and thermodynamic stability.Fundamental thermodynamics?specific heat,standard entropy,enthalpy of formation,Gibbs free energy?is the basic data for studying chemical reactions of materials,and plays an indispensable role in research,design and development in the fields of metallurgy,materials and chemicals..At present,although a large number of thermodynamic databases have been established,the fundamental thermodynamic data of some substances are still missing,such as rare earth silicates.Due to the lack of fundamental thermodynamic data,it is difficult to study the mechanism of such substances.In order to better study such substances,thesis studies the fundamental thermodynamic parameters of their silicates with the widely used of rare earth lanthanum?La?and yttrium?Y?.?1?The standard enthalpy of formation calculation model was established in this study.The established standard enthalpy of formation calculation model is based on the ion addition model and uses the standard enthalpy of cation,hydroxide ion and water molecules optimized in the Golam model,by collecting 95 groups of standard enthalpy of formation of silicates from the thermodynamic database.The parameter values in the estimation model are solved,and the linear relationship between the standard enthalpy of formation of silicate ions and the total number of siloxane ions is fitted.At the same time,the accuracy and applicability of the model were tested.The average relative error was 2.71%,indicating that the accuracy and exactness of the model is good.?2?This study established a standard entropy calculation model.The established standard entropy calculation model simplifies the calculation process by optimizing the Latimer model and the Mills model,and uses the cation standard entropy value optimized by the Mills model to optimize the standard entropy data of 95 groups of silicates in the thermodynamic database.The standard entropy of different silicate anions is obtained.At the same time,the accuracy and applicability of the model were tested.The average relative error was 6.00%,indicating that the accuracy and exactness of the model is good.?3?The specific heat capacity value of the rare earth silicate was measured by the sapphire three-stage method.Due to the complexity of the pure phase process of synthetic rare earth silicates,the pure phase of some rare earth silicates is even difficult to synthesize.Therefore,in this study,the specific heat capacity data of each pure phase was obtained indirectly by measuring the specific heat capacity of the mixture.According to the addition of specific heat capacity,as long as the mass ratio of each pure phase in the mixture is determined,the specific heat capacity value can be obtained.The mass ratio of each phase was obtained by Rietveld full-spectrum fitting analysis.From the measured specific heat capacity value of the mixture and the mass ratio of each phase,the simultaneous equations are used to obtain the specific heat capacity value of each temperature point.The specific heat capacity of rare earth metal silicates equation is obtained by data fitting method.The standard enthalpy of formation,standard entropy and Gibbs free energy equation of Y₂SiO₅,Y₁₀Si₆O₂₆,Y₂Si₂O₇,La₂SiO₅,La_9.33Si₆O₂₆ and La₂Si₂O₇ were calculated by the specific heat capacity equation and the standard entropy and standard calculated by the model.Fundamental thermodynamic data sheets for.Y₂SiO₅,Y₁₀Si₆O₂₆,Y₂Si₂O₇,La₂SiO₅,La_9.33Si₆O₂₆ and La₂Si₂O₇ were determined.