Mengxian Zhang

In situ synthesis and formation mechanism of ZrC and ZrB2 by combustion synthesis from the Co-Zr-B4C system

Abstract ZrC-ZrB2-based composites were prepared by combustion synthesis (CS) reaction from 10 wt.% to 50 wt.% Co-Zr-B4C powder mixtures. With increasing Co contents, the particle sizes of near-spherical ZrC and platelet-like ZrB2 decreased from 1 μm to 0.5 μm and from 5 μm to 2 μm, respectively. In addition, the formation mechanism of ZrC and ZrB2 was explored by the phase transition and microstructure evolution on the combustion wave quenched sample in combination with differential scanning calorimeter analysis. The results showed that the production of ZrC was ascribed to the solid-solid reaction between Zr and C and the precipitation from the Co-Zr-B-C melt, while ZrB2 was prepared from the saturated liquid. The low B concentration in the Co-Zr-B-C liquid and high cooling rate during the CS process led to the presence of Co2B and ZrCo3B2 in the composites. The addition of Co in the Co-Zr-B4C system not only prevented ZrC and ZrB2 particulates from growing, but also promoted the occurrence of ZrC-ZrB2-forming reaction.

The effect of B4C particle size on the reaction process and product in the Cu–Zr–B4C system

Abstract ZrC and ZrB2 were prepared by self-propagating high-temperature synthesis in a Cu–Zr–B4C system. We explored the influence of B4C particle size on the reaction process, phase composition and microstructure of the products. The results show that the production of ZrC and ZrB2 is mainly controlled by the dissolution of B4C in Cu–Zr liquid. Increasing the size of B4C will not affect the reaction path, but will retard the formation of the Cu–Zr–B–C liquid. This leads to the incomplete conversion of ZrC and ZrB2 once the B4C exceeds 28 μm. In addition, ZrC and ZrB2 particle sizes greatly decrease from the micrometer dimension with a polyhedral morphology to the nanometer scale with a nearly spherical shape.

Characterization of Powders of Ceria-Zirconia Solid Solution Synthesized with Additives of Polymer-net Gel

Powders of ceria-zirconia solid solution were prepared by using different molar ratio of Ce(NO3)4·6H2O and Zr(NO3)4·5H2O as raw materials and appropriate additives of C3H5 NO, C7H10N2O2and(NH4)2S2O8as polymer-net gel. The influence of the polymer-net gel on the preparation process and the effect of Ce/Zr ratio on the properties of the powders of ceria-zirconia solid solution were investigated respectively. The results show that the additives of polymer are beneficial not only to the separation of the particulates of ceria-zirconia solid solution due to the formation of a three-dimensional network of the polymer- net gel, but also to the formation of nanoporous powders of ceria- zirconia solid solution due to the burn away of polymer during the subsequent calcinations. The powders of ceria- zirconia solid solution with particle size 10- 20 nm could be prepared when the mass ratio of C3H5 NO and C7H10N2O2 was 5: 1,the dissolvability of mixed solute was 0.04 mol/L and an appropriate heat treatment process was adopted. XRD analysis revealed that the ceria-zirconia solid solutions with molar ratio of Ce/Zr in a range of 3:7-5: 5 present tetragonal crystallographic structure, while those in a range of 6: 4-7: 3 present cubic structure respectively; their lattice constant decreased with the increasing Zr+content. N2 adsorption isotherm showed that the ceria- zirconia solid solution with Ce/Zr molar ratio of 6: 4 exhibited excellent features:specific surface area of 120 m2·g- 1; aperture of 8.12 nm; pore volume of 0.22 cm3/g. SEM images showed that the ceria-zirconia solid solution powders have an alveolate-like network structure.