Synthesis、Characterization Of Rare-Earth Oxides And Ferrites And Kinetics Of Thermal Process Of Precursor

Rare earth oxides and ferrite are multifunctional materials which are widely used in many fields and are always the research hotspot. Nd2(C2O4)3·10H2O, Co1-xMgxFe2O4 precursor,1.1/2Pr2(C2O4)3-MnC2O4·5.3H2O, and Li0.5xMn0.4Ni0.6-xFe2+0.5xO4 precursor have been successfully synthesized via the low-heating temperature solid-state reaction in this paper. Nd2O3, Co1-xMgxFe204, Pr1.1MnO3.15, and Li0.05xMn0.4Ni0.6-xFe2+0.5xO4 are obtained by calcining precursor in air, respectively. The precursor and its calcined products are characterized by thermogravimetry and differential scanning calorimetry (TG/DSC), fourier transform infrared spectroscopy (FTIR),X-ray powder diffraction (XRD),scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM).The kinetics of the thermal process of precursor is studied using TG-DSC technique. Based on non isothermal iso-conversional method and iterative method, the values of the activation energies of thermal process of precursor are obtained. The most probable mechanism function of single step reaction for thermal process of precursor is determined using the linear equation method and the comparison method.The experimental results show that:(1) A high-crystallized hexagonal Nd2O3 is obtained by calcining Nd2(C2O4)3·10H2O in the air at 1223 K for 2h. The crystallite size of the particles is about 48 nm. Thermal process of precursor in air experienced three steps, which involved, at first, the dehydration of ten crystal water molecules from Nd2(C2O4)3·10H2O, then decomposition of Nd2(C2O4)3 into Nd2O2CO3, and at last, decomposition of Nd2O2CO3 into hexagonal Nd2O3. Based on KAS equation, the values of the activation energies associated with the thermal process of Nd2(C2O4)3·10H2O are determined. The average values of activation energy are 67.94±143.31,135.49±13.16, and 453.42±44.78 kJ mol-1 for the steps 1,2, and 3, respectively. The thermal process for step 1 could be multi-step reaction mechanisms. By contrast, the thermal process for steps 2 and 3 are simple reaction mechanisms. The most probable mechanism function integral forms of the thermal process for steps 2 and 3 are g(α)= [-ln(l-α)]2/3and g(a)=-ln(1-a), respectively. Both of rate-determining mechanisms for steps 2 and 3 are assumed random nucleation and its subsequent growth.(2) Co1-xMgxFe2O4 precursor is synthesized by solid-state reaction at low temperatures using MgSO4·7H2O, CoSO4·7H2O, FeSO4·7H2O, and Na2C2O4 as raw materials. A high-crystallized Co1-xMgxFe2O4 with cubic structure is obtained by calcining the precursor over 500℃ in air for 2 h. Magnetic characterization indicates that the specific saturation magnetization, remanence, and coercivity of Co1-xMgxFe2O4 depend on the composition and calcination temperature. The specific saturation magnetization, remanence, and coercivity of Co0.9Mg0.1Fe204 obtained at 600 °C were 78.67 emu/g, 34.4 emu/g, 1098.7 Oe, respectively. Co2+ and Mg2+ ions in Co0.9Mg0.1Fe2O4 have a synergistic effect in improving the specific saturation magnetization of Co0.9Mg0.1Fe2O4. The thermal process of CoC2O4 - 2FeC2O4·5.77H2O from ambient temperature to 850 °C experiences two steps, namely, the dehydration of 5.77 crystal waters, the reaction of CoC2O4 - 2FeC2O4 with 2O2 into CoFe2O4 and the six CO2 molecules. The average values of activation energy are 124.67± 10.41 kJ mol-1 and 180.20±14.34 kJ mol-1 kJ mol-1 for the steps 1 and 2, respectively. The thermal process for steps 1 and 2 are simple reaction mechanisms. The most probable mechanism function integral forms of the thermal process for steps 1 and 2 are g(α) = 1-(1-α)1/2. Rate-determining mechanisms for steps 1 and 2 are contracting cylinder (cylindrical symmetry).(3) Pr1.1Mn03.15 precursor 1.1/2Pr2(C2O4)3-MnC2O4·5.3H2O is synthesized by solid-state reaction at low temperatures using Pr(NO3)3·6H2O, MnSO4·H2O, and Na2C2O4 as raw materials. A high-crystallized Pr1.1MnO3.15 with an orthorhombic structure was obtained when the precursor was calcined over 1,000℃ in air for 2 h. Magnetic characterization indicates that orthorhombic Pr1.1MnC)3.15 behaves with weak magnetic properties at the room temperature. The thermal process of the precursor from ambient temperature to 1,050℃ in air experiences four steps: the dehydration of 5.3 crystal waters; the reaction of MnC2O4 with 0.75 O2 into 1/2 Mn2O3 and the 2 CO2 molecules; the reaction of 1.1/2 Pr2(C2O4)3 with 0.825 O2 into 1.1/2 Pr2O2CO3 and of 2.75 CO2 molecules; and the reaction of 1.1/2 Pr2O2CO3 with 1/2 Mn2O3 into Pr1.1MnO3.15 and 0.55 CO2.(4) A high-crystallized Li0.5xMn0.4Ni0.6-xFe2+0.5xO4 (0.0≤x≤0.3)with a cubic structure is obtained by calcining precursor oxalates over 600℃ in air. The specific saturation magnetization of Li0.5xMn0.4Ni0.6-xFe2+0.5xO4 depends on the composition and calcination temperature.Li+ ions in Li0.1Mn0.4Ni0.4Fe2.1O4 can improve the specific saturation magnetization of Li0.1Mn0.4Ni0.4Fe2.1O4. Li0.1Mn0.4Ni0.4Fe2.1O4 obtained at 600℃ has the highest specific saturation magnetization value,57.94 emu/g. However, Mn0.4Ni0.6Fe2O4 obtained at 600℃ has the highest coercivity value,130.32 Oe. Thermal process of Mn0.4Ni0.6Fe2O4 precursor Mn0.4Ni0.6C2O4-2FeC2O4·8.3H2O from ambient temperature to 700℃ experiences two steps, namely, the dehydration of 8.3 crystal waters, the reaction of Mn0.4Ni0.6C204-2FeC204 with 2 O2 into Mn0.4Ni0.6Fe2O4 and the six CO2 molecules. The average values of activation energy are 60.45±2.01 kJ mol-1 and 87.48±6.64 kJ mol-1 for the steps 1 and 2,respectively.The thermal process for steps 1 and 2 are simple reaction mechanisms. The most probable mechanism function integral forms of the thermal process for steps 1 and 2 are g(a)=1-(1-α)1/2 and g(a)=α1/2,respectively.