COERCIVITY ENHANCEMENT IN GAMMA-FE2O3 BY SURFACE-TREATMENT WITH PHOSPHATE IONS

Significant increases in coercivity were observed for stoichiometric g amma-Fe2O3 pigments that were treated with different phosphate ions. T he objectives of this investigation were to determine if the effect wa s specific to the phosphate anions as well as establishing the influen ce, if any, of induced compressive stress in coercivity enhancement. S toichiometric gamma-Fe2O3 pigments were treated with aqueous solutions containing the sodium salts of different anions and the changes in co ercivity measured by vibrating sample magnetometry. The surface treatm ents were done in an aqueous suspension, followed by water evaporation , and vacuum treatment at 150-160-degrees-C. Sodium phosphate, sodium pyrophosphate, sodium tripolyphosphate, sodium metaphosphate, poly(sod ium acrylate), and sodium p-toluenesulfonate were used and the weight ratio of salt to pigment was fixed at 1.5. Within the range of anions in this work, the coercivity enhancement was specific to the polyphosp hate anions. The coercivity for pigments treated with sodium p-toluene sulfonate (273 Oe) or poly(sodium acrylate) (280 Oe) was only slightly higher than 265 Oe for the as-received pigments. For the phosphate an ions, the coercivity enhancement increased in the order sodium phospha te (268 Oe), sodium pyrophosphate (287 Oe), sodium tripolyphosphate (3 16 Oe), sodium polyphosphate (336 Oe). To evaluate the effect of stres s on the coercivity, samples treated with sodium polyphosphate were gr ound to a powder and then pressed into pellets. The coercivity for the samples decreased linearly with increasing applied pressure during pe llet formation. Therefore applied stress does not account for the coer civity increase.