Estimation And Modulation On Optical And Thermal Property Of Carbon Nitride Photocatalyst And Molten Salts

Photocatalytic hydrogen production performance is mainly affected by the activity of photocatalysts.Recently,the design inspiration on optimizing photocatalytic performance by regulating the light absorption and heat transport properties inside the photocatalyst are attracting more and more attention.At present,optical characteristics analysis on photocatalysts is mainly based on geometric optics,which is not suitable for the cases when morphologies inside photocatalyst are in micro-nano scale.The optical constants of photocatalytic materials are also one of the most important factors that makes differences to the optical characteristics,estimating the optical constants of unknown photocatalytic materials and modulating optical properties of discovered photocatalytic materials are also necessary steps for photocatalyst optimization.The optical constants are mainly determined by the structure of the material.Therefore,it is also necessary to establish links between the crystal structure and the optical constants;the temperature is also an important factor affecting the photocatalytic activity,estimating and modulating the thermal conductivity of the photocatalyst is also an important aspect to improve the photocatalytic activity;Phase change is one of the main ways for thermal storage,as the working temperature of thermal storage medium is getting higher and higher,the radiative heat transfer is becoming much more important.On the other hand,the composition of some heat storage medium is changeable,and the high-temperature optical constants is difficult to measure,developing accurate prediction method for the high-temperature optical constants estimation is of great significance for the regulation on thermal storage performance.In this thesis,theoretically estimating and modulating optical and thermal properties of photocatalytic and heat storage materials are mainly focused.To estimate and modulate optical properties of carbon nitride photocatalysts,an analysis method for micro-nano scale optical absorption characteristics of catalysts is developed,and the pore size and diameter of photocatalysts are clarified.For the estimation and modulation on spectral radiation properties,first-principles calculations are introduce establishing the relationship between the crystal structure and radiation properties of photocatalytic materials,and this is helpful for the analysis of optical characteristics at the micro-nano scale;In this thesis,the thermal transport properties of carbon nitride materials is estimated and modulated;A theoretical calculation method for temperature-dependent optical properties of molten salt materials is developed,the main contents include the following aspects:For analyzing the optical characteristics of photocatalyst,the micro-nano-scale radiation transfer calculation was introduced into the analysis on energy absorption mechanism of photocatalysts.Taking carbon nitride photocatalytic materials as an example,the optical absorption characteristics for different morphologies are analyzed.The influence of morphologies on optical absorption characteristics is discussed,and it is found that when the pore size is less than 2μm,increasing the pore size will get the optical absorption performance of the photocatalyst improved;On the other hand,appropriate agglomeration of particles could enhance the photocatalytic activity,and this is of high importance for guiding the porous morphology,providing a new idea for the low-cost synthesis of photocatalysts.In order to modulate the optical constants of unknown photocatalytic materials and the optical absorption characteristics of known photocatalytic materials,the electronic structure of graphitic carbon nitride is modulated by alkali-halogen element co-doping,and the electronic structure,optical constants and catalytic activity of co-doped carbon nitride is estimated and calculated.It is found that the main role of halogen elements is to provide a new energy level in the band gap of the original graphitic carbon nitride,while alkali elements build an ionic channel between different C-N atomic layers to enhance the transport ability of charges.Then,base the micro-nano optical absorption analysis method that is combined with the morphology of the photocatalyst in this paper,the optical characteristics of photocatalyst are modulated at atomic-scale.Due to the structural variability of polymerized carbon nitride materials,this thesis estimates the thermal conductivity of polymerized carbon nitride to different degrees of polymerization,builds the relationship between the degree of polymerization and thermal conductivity of polymerized carbon nitride.On this basis,directional modulation on the thermal conductivity and elastic modulus of the polymerized carbon nitride is realized by strain engineering.It is found that when the volume of box is reduced to 1/2 of the original,the thermal conductivity can be improved up to be 155%.Since computation accuracy of classical molecular dynamics which is based on empirical potential field is the not high enough,and existing potential fields cannot cover all types of elements that may be considered,a deep potential model based on deep neural network for describing the graphitic carbon nitride system is constructed in this thesis,and the anisotropic thermal conductivity,mechanical properties of graphitic carbon nitride are modulated by adjusting the number of C-N layers.It is found that the thermal conductivity is 4.41 Wm^-1K^-1 and 4.12 Wm^-1K^-1 in x and y direction,respectively and the interlayer thermal conductivity is 1.40 Wm^-1K^-1 which is about 1/3 of that along the plane.The radiative heat transfer process of high-temperature molten salts has a great influence on thie general heat transfer process.Estimating the optical constants of high temperature molten salt at different temperatures is the premise of taking the radiation heat transfer process into consideration.Aiming at solving this problem that optical constants missing in the working state of high-temperature molten salt,this thesis develops a theoretical computation method for the temperature-dependent optical constants of high-temperature molten salt at different temperatures,with which,temperature-dependent optical constants of molten sloar salt,and chloride salts（Na Cl and KCl）are obtained.