Effects of Dispersive Agent and Alumina Micropowder on Workability of Al_2O_3-MgO Castables for Steel Ladles

This work aims at enhancing the workability of Al2 O3-Mg O castables. Al2 O3-Mg O castables were prepared using tabular alumina particles（ 5-3,3-1,≤1 mm）,tabular alumina fines（ ≤0. 088 mm）,fused magnesia（ ≤1 mm）,reactive alumina micropowder（ PFR40,PBR,and CL） and calcined alumina micropowder（ AC34 B5 and CT） as the main starting materials,calcium aluminate cement（ Secar71） as the binder,and polycarboxylic acid water reducer WS,FS,and AD as the dispersive agent,respectively. Effects of three dispersive agents addition（ 0. 4 mass% WS,0. 2 mass% FS and 1 mass% AD）,two kinds of reactive alumina micropowder addition（ PFR40 and PBR）,composite alumina micropowder addition（ PFR40-AC34 B5 and CL-CT,the mass ratio from 9/0 to 3/6）on the flow value,apparent porosity,bulk density,cold crushing strength and cold modulus of rupture of Al2 O3-MgO castables were investigated. The results show that：（ 1） with PBR or PFR40,the castables with0. 4 mass% WS own better performance than those with0. 2 mass% FS or 1 mass% AD;（ 2） compared with the castables with CL-CT composite powder, the castables with PFR40-AC34 B5 composite powder have slightly higher flow value; with the decreasing ratio of PFR40 and AC34 B5,or the decreasing ratio of CL andCT,the castables have declined flow value and the dried castable specimens have decreased density but the mildly changed strength; in consideration of the high price of reactive alumina,a proper compositing ratio shall be selected.

SiC WHISKER-TOUGHENED ALUMINIUM NITRIDE CERAMICS

Aluminium nitride ceramics containing 15 -30 w/o of SiC whiskers can be sintered to full density (by hot pressing at 1800℃)using 2-3 w/o of Y2O3 additions. The whiskers increase the toughness and strength of the composite, KIC increasing from 2. & to about 5.0 and flexural strength increasing by 30-50%. However, the whiskers must be well dispersed and if the dispersion is not satisfactory, toughness may increase but the strength decreases. The hot-pressing temperature can be reduced by up to 100℃ if Y(NO3)3 . 5H2O is used as the sintering additive instead of Y2O3, but some oxidation of the AIN occurs during heating. Isopropan-2-01 is a better dispersing agent than cyclohexane, but again some oxidation of the AIN does occur.The best sample prepared during this work contained 20 w/o of SiC whiskers and 2 w/o Y2O3 added as Y(NO3)3. 5H2O and mixed in isopropanol. This exhibited a mean strength of 453MPa (maximum 522MPa, measured by disc flexure) and a fracture toughness of5. 5MPam1/2.

Roles of polyacrylate dispersant in the synthesis of well-dispersed BaSO_4 nanoparticles by simple precipitation

Well-dispersed BaSO4 nanoparticles were synthesized in the presence of sodium polyacrylate （PAAS） by a simple precipitation method, with BaCl2 and （NH4）2SO4 as reactants. The different roles performed by PAAS in the synthesis of BaSO4 nanoparticles were investigated using X-ray diffractometry, Fourier transform infrared spectroscopy, and transmission electron microscopy. The results indicate that the assynthesized BaSO4 nanoparticles were spheres with an average diameter of 30 nm and that their surfaces were affected by the PAAS. Under a typical procedure employed, PAAS reacted with BaCl2 to yield an intermediate, serving as a control releasing agent and separating the nucleation and crystal growth processes of the BaSO4 nuclei. During formation of the BaSO4 nanospheres, the intermediate slowly dissolved and released barium and polyacrylate ions, inhibiting the growth and aggregation of newly formed BaSO4 seeds and resulting in particles of narrow diameter distribution and improved dispersibility. Moreover, these polyacrylate ions further modified the surfaces of the BaSO4 nanoparticles.