Molar heat capacities of La_2Mo_2O_9 and La_(1.9)Sr_(0.1)Mo_2O_(9-δ)

The molar heat capacities of La2Mo209 and La1.9Sr0.1MO209-δ were obtained using the differential scanning calorimetry （DSC） technique in a temperature range from 298 to 1473 K. The DSC curve of La2Mo209 showed an endothermal peak around 834 K corresponding to a first-order monoclinic-cubic phase transition, and the enthalpy change accompanying this phase transition is 5.99 kJ/mol. No evident endothermal peak existed in the DSC curve of La1.9Sr0.1MO209-δ, but a broad thermal anomaly existed in its heat capacity curve at around 832 K. In addition, the heat capacity values of La2Mo209 and La1.9Sr0.1MO209-δ began to decrease at 1196 and 1330 K, respectively. The non-transitional heat capacity values of La2Mo209 and La1.9Sr0.1MO209-δ were formulated using multiple regression analysis in two temperature ranges.

Chemical synthesis and properties of La_(1.9)Ba_(0.1)Mo_(1.9)Mn_(0.1)O_9 as electrolyte for IT-SOFCs

The highly phase-pure electrolyte materials with compositionLa1.9Ba0.1MO1.9Mn0.1O9（LBMMO） was prepared by the sol-gel auto-combustion method for intermediate-temperature solid oxide fuel cells （IT-SOFCs）. The details ofgel＇s auto-combustion, phase evolution, sintering, thermal expansion and electrochemical performance of LBMMO were investigated by means of thermo-gravimetry （TG）, X-ray diffxaction （XRD）, scanning electron microscopy （SEM）, transmission electron spectroscopy （TEM）, thermal expansion curve （TEC） and complex impedance spectra. The results showed that the highly phase-pure electrolyte LBMMO could be obtained after calcining at 600 ℃. The sample sintered at 900 ℃ for 4 h in air exhibited a better sinterability, and the relative density of LBMMO was higher than 96%. The electrical conductivities of the sample were 6.7x 10-3 and 25.9× 10-3 S/cm at 700 and 800 ℃ in air, respectively. Results also showed that LBMMO had moderate thermal expansion （a=16.3×10-6 K-l, between room temperature and 800 ℃） and an electrical activation energy equal to 1.32 eV）.

Impedance Studies of La_2Mo_(2-x)Sn_xO(9-δ) Oxide Ion Conductors

A series of compounds La2Mo2-xSnxO9-6 （x=0-0.3） have been synthesized by solid-state reaction technique. Materials have been characterized by XRD, SEM, DSC and impedance study. In the temperature regime 520℃-590 ℃, the specimens with x〈0.05 have the conductivity higher than La2Mo2O9. Conductivity of Sn-doped compound decreases consistently with increasing Sn-doping, compared to the undoped compound both below and above phase transition, barring the specimens with x〈 0.05, where conductivity values remains almost same as that of undoped specimen in high temperature region. In the intermediate temperature regime （520℃-590℃）, the conductivity of doped compounds increases for x〈0.05 as compared to parent compound. Also, there is no indication of phase stabilization with Sn-doping in this compound even with the highest doping level, x=0.3. Electric modulus analysis suggests that thermally activated oxygen ion hopping mechanism is responsible for the conduction in Sn-doped compound.