Better preparation process on formation of BaFe₁₂O₁₉ nanoparticles

Refs. 5 and 7 proposed a method for synthesizing ultrafine barium ferrite (BaFe₁₂O₁₉) by sol-gel technique that is excellent except that, in our opinion, they used ammonia as precipitating agent. We found experimentally that NaOH is better than ammonia except that a new problem is brought in, i.e., it brings in unwelcome sodium ions. We present a method that employs NaOH as precipitating agent without bringing in unwelcome sodium ions. BaFe₁₂O₁₉ nanoparticles were prepared for using sol-gel technique. Influence of preparation process on formation process of barium ferrite was studied. Experimental results show that, after the gel had been preheated at 500°C for 5 h and washed by water to eliminate sodium ions and then heat-treated at 900°C for 4 h, we could get high purity nanoparticles of BaFe₁₂O₁₉ with a coercive force of 29.95×10³ A/m, and a specific saturation magnetization M_s of 68.6 A·m²·kg^-1 with Fe/Ba=11.2. By selecting a different Fe/Ba and prolonging preheating and heat-treating time, we found it possible to make washing away sodium ions with water unnecessary. After the gel had been preheated at 500°C for 6 h and then heat-treated at 900°C for 6 h, we could get high purity nanoparticles of BaFe₁₂O₁₉ with a coercive force of 31.26×10³ A/m, and M_s=70.12 A·m²·kg^-1 with Fe/Ba=11.4. Besides, M_s=70.12 A·m²·kg^-1 is superior to M_s=62.7 A·m²·kg^-1 and M_s=65 A·m²· kg^-1 attained by products of Japanese firms Toshiba and Toda respectively.