The Influences Of Doping Effect And Size Effect On The Phase Transitions Performance Of Sodium Molybdate (Na₂MoO₄)

Sodium molybdate (Na2MoO4) is a prospective candidate as a phase transition material for energy storage, since this material has relatively large enthalpy (ΔH= 52.6 kJ/mol) for its solid-solid phase transitions. Na2MoO4 exhibits three phase transitions among four solid phases: And it melts at 680℃. The a phase andβphase have a cubic cell and an orthorhombic cell, respectively. Theγphase also has an orthorhombic cell, which transforms at 638℃to the hexagonalδphase. In order to use Na2MoO4 flexibly, to modify its solid-solid transition temperatures is a practical way. This paper focuses on the influences of doping effect and size effect on the phase transition temperature and transition enthalpy of Na2MoO4. Experimental results also provided valuable information about how to keep relatively large phase transition enthalpy in the doping process. The main conclusions in this paper are summarized as follows: 1. The samples in two material systems Na2Mo1-xWxO4 and Na2Mo1-ySyO4 were prepared by using conventional solid reactions and characterized by XRD, EDX and DSC. The XRD and EDX data indicated that all the samples studied in both systems formed solid solutions. The DSC data indicated that in the system Na2Mo1-xWxO4, the solid-solid transition temperatures increased and in the system Na2Mo1-ySyO4, the solid-solid transition temperatures decreased. The total enthalpy of the solid-solid transitions in the system Na2Mo1-xWxO4, decreased much less than that in the system Na2Mo1-ySyO4.2. The variations of the phase transition temperature and transition enthalpy with the doping contents in the systems Na2Mo1-xWxO4 and Na2Mo1-ySyO4 were described and the mechanism of doping effect was discussed. In order to modify transition temperatures of Na2MoO4 and also to keep its relatively large total enthalpy, it is necessary to choose a doping material with large total enthalpy.3. Na2MoO4 nanoparticles were synthesized by sol-gel method and reverse microemulsion method. The powders were characterized in details by XRD, TEM, DSC and laser grain size analyzer, respectively. Influences of dispersant and ultrasonic treatment to the precursor solutions on the particles size were also investigated.4. The influences of size effect on the solid-solid transition temperatures and enthalpies are discussed. The results showed that smaller nanoparticles exhibit higher surface energy and less enthalpy of phase transition, resulting in the decreases of the phase transition temperature and melting point.