Heat Capacity Of Functional Oxide Materials

Heat capacity,one of the most important physical parameters of solid materials,is highly depedant on the features of electron,vacancy,magnetic interaction,lattice vibration,phonon and so on.Under reasonable approximation,the average energy of the studied material can be express as the function of heat capacity,which can reveal the contribution of elelctron,hole,phonon,etc to the energy and further explore their influence on the material performance.Although researchers have done extensive works in estiblishing the heat capacity measurement method and searching the fitting model,most of these works are carried out around calorimetry and the explanation of typical physical phenomena?such as superconductivity,polyiron phase transition,etc.?.If the advanced precise calorimetric methods and the reasonable physical models are extended to the studies of chemistry and materials,the relationship between structure,thermal stability,electrical transport and performance can be well understood,which will greatly promote the functionalization of new materials.In this dissertation,heat capacities was studied to correlate the preforamnce of metal oxide functional materials.Considering that the thermal stability is the primary feature of metal oxide functionalization,the standard thermodynamic functions of metal oxide materials in the temperature range of 0-300K were determined using calorimetric technology and physical analysis.Then,the delocalization of electrons in functional oxide materials was revealed by the measurement of electrical transport properties and the change of heat capacity.The results obtained in this dissertation will be helpful for promoting the application of functional materials.Four main achievements are summarize as following:1.Heat capacity and thermodynamic stability of TiO₂?B?:A mixture of TiO₂?B?nanowires and anatase TiO₂ was synthesized by hydrothermal method.The heat capacity of this mixture was measured using a Quantum Design Physical Property Measurement System?PPMS?over a temperature range from 1.9 to 302 K.After eliminating the contribution of anatase,the heat capacities for TiO₂?B?were fitted to a theoretical model in the low temperature range?T<9 K?,orthogonal polynomials in the middle temperature range?9<T<65 K?,and a combination of Debye and Einstein functions in the high temperature range?T>65 K?.The standard molar heat capacity,molar entropy,and molar enthalpy for TiO₂?B?at T=298.15 K were determined to be?58.18±0.81?J·K^-1·mol^-1,?52.31±0.69?J·K^-1·mol^-1,and?9.03±0.12?kJ·mol^-1,respectively,leading to a Gibbs free energy of–?6.56±0.09?kJ·mol^-1.These thermodynamic data show that TiO₂?B?is less stable than TiO₂ with rutile and anatase structure.2.Heat capacity and thermodynamic stability of TiO₂?H?:The pure phase TiO2?H?was successfully prepared by the combination of solid state reaction and aqua regia etching method.The heat capacity of TiO₂?H?was measured in the temperature range of 1.9 to 302 K.The experimental data was fitted with theoretical models,orthogonal polynomials and combination of Debye and Einstein function for the low,middle and high temperature range,respectively.Based on these fitted results,the standard molar heat capacity,entropy,enthalpy and Gibbs free energy at T=298.15 K are calculated as?55.51±0.56?J·K^-1·mol^-1,?50.55±0.51?J·K^-1·mol^-1,?8.65±0.09?kJ·mol^-1,and–?6.42±0.06?kJ·mol^-1,respectively.Comparing to other TiO₂ with different structures,the heat capacity of TiO₂?H?is similar to that of rutile and anatase,but significantly lower than those of brookite and TiO₂?B?at temperature above 100 K.The thermodynamic difference of these five TiO₂ was explained by the crystal structure and lattice vibration for the first time.3.Heat capacity and electrical transport properties of K_1.6Ti₈O₁₆:K_1.6Ti₈O₁₆ was synthesized by solid state reaction method.The experimental heat capacity of the this material was measured by PPMS.According to the contribution of electron,lattice vibration,Schottky anomaly and boson peak,the heat capacity in the low temperature range was fitted and the Debye temperature was calculated to be 583K.The conductivity mechanism of K_1.6Ti₈O₁₆ sample is determined to be variable range hopping?VRH?conduction and small polaron hopping?SPH?conduction in the temperature rang T<300K and T>300K,respectively.Based on the SPH mechanism,the relationship between the electrical transport properties and the temperature in K_1.6Ti₈O₁₆ is figured out,which lays a foundation for the functional application of this kind of material in a wider temperature range.4.Polaron delocalization in layered LiNi_0.4+xMn_0.4-xCo_0.2O₂:The layered LiNi_0.4+xMn_0.4-xCo_0.2O₂ samples were successfully prepared by sol-gel method.The heat capacity of these four samples were measured in the temperature range of195-300K,and the experimental heat capacity were fitted by Debye function.The Debye temperatures of the samples at X=0,0.1,0.2 and 0.3 were determined to be739,713,696 and 675K,respectively.The results of?_D/2 and the analysis of electronic conductivity show that the conduction mechanism in LiNi_0.4+xMn_0.4-xCo_0.2O₂ is adiabatic small polaron hopping conduction.The weakened electron–phonon interaction is the source of polaron delocalization in LiNi_0.4+xMn_0.4-xCo_0.2O₂,which became improved with increasing x due to the increased polaron sizes.These findings may provide more understanding for the design of layered oxide functional materials with high conductivity.