| Cobalt chromate (CoCr2O4) is one of spinel materials with wide application in various fields such as high performance pigment, catalysis and magnetic material, etc. In this paper, single-phase nanocrystalline CoCr2O4 has been synthesized by co-precipitation and low-temperature combustion synthesis(LCS) method using ammonium hydrogen carbonate(AHC) as precipitant and citric acid as reductant/fuel and metal nitrates as oxidants, respectively. TG-DTA, DSC, FTIR, XRD, Laser Particle Distribution Analyzer, SEM, (HR)TEM, Uv-V Spectrometer and Color-Eye 7000A Spectrophotometer were applied to characterize the thermo-performance and the composition of the precursors, particle size distribution, structure, morphology, micro-structure, Uv-V absorbance curve and the color parameters of the samples, respectively.In co-precipitation method, different precipitants, the manner of titration, water bath temperature, the concentration of AHC and calcination program were studied in terms of effect on the structural and component of the precursor, the crystallite formation temperature, the crystallite size, the lattice constant and color performance. The results show that, AHC is a good precipitant in this system with the concentration of 0.8mol/L at 75℃and using the reverse-strike method (i.e. adding salt solution to the precipitant solution). Crystalline with uniform morphology and about 35nm in size can be obtained by calcining the precursor at 800℃for 1h. Absorbtion bands at 575,615 and 670nm can be seen in the Uv-V curve due to the spin allowed transition 4Ar (F)â†'4T1 (P) of Co2+in tetrahedron and 4A2gâ†'4T2g of Cr3+in octahedron. As the increase of the calcination temperature, the pigment exhibit deeper green and blue hue and higher color saturation, indicating better color performance.In LCS, the impact of different reductant/oxidant ratio (CA/NO3'), calcination program, water bath temperature and different pH on the thermo performance of the precursor, lattice distortion ratio, gel formation time and the absorbtion curve were studied. The lattice distortion and crystalline size of the samples prepared by different calcination rates were estimated by Williamson-Hall method and Scherrer method. CoCr2O4 was also treated as a host to accept other metal ions, i.e. Zn2+, Ni2+and Al3+. The structure and Uv-V performance of the complex oxides were investigated.Results show that CA/NO3'=0.278 exhibits best thermo reaction performance, single phased and uniform cobalt chromate can be obtained by igniting the precursor at 400℃and then soaking at 800℃for 2h. Gel can be formed in the water bath at 90～95℃in 2.5h. Adjust the pH to 5 is suitable to gain single-phased samples with high crystallinity. Two absorbtion bands at 527 and 630cm-1 were witnessed in IR spectrum which were ascribed to the vibration of [CrO6] and [CoO4], respectively. Absorbtion bands at 578,618 and 670nm can be seen in the Uv-V curve due to the spin allowed transition 4A2(F)â†'4Ti(P) of Co2+ in tetrahedron and 4A2gâ†'4T2g of Cr3+ in octahedron. The W-H method was rekoned to be better to estimate the crystalline size obtained by different calcination rates and different soaking time and the lattice distortion is dependent on the calcination rate, which is evidenced by the HRTEM observation. Zn2+and Al3+ doped complex oxides were found to have better color performance without change in structure, which is meaningful both in economy and application. |
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