The Thermal Evolution of a Barium Ferrite Precursor Obtained by a New Chemical Coprecipitation Method

The evolution during thermal treatment of the precursor obtained by a new method to synthezise barium hexaferrite by chemical coprecipitation is described; the method involves the precipitation froma strongly alkaline ferrate(V1) solution containing barium chloride. Barium hexaferrite may be formed by heating of the original superparamagnetic precursor at temperatures as low as 700QC as shown by XRD, M6ssbauer spectra and DTA-TCTA behaviour. The precursor does not contain appreciable amounts of carbonate, thus favouring the formation of the hexaferrite atlowertemperatures. Scanning electron microscopy shows that the ferrite particles are less than 0.5 pm in diameter. The traditional synthetic procedure for barium hexafemte, Ba0.6 Fe20,, involves the solid state reaction of barium carbonate and hematite at 1200°C. The reaction proceeds through the formation of several intermediate phases, such as 2Ba0.Fe,03, BaO.Fq0, and Ba0.2Fe203 and Ba0.6FqQ [ 1-31, Other problems are the large crystal size and crystal strains, that lowers the coercitivity. Wet synthetic procedures usually involve the coprecipitation of fenihydrite, or amorphous femc hydrous oxide, and barium hydroxide, followed by calcination to relatively low temperatures. Under these conditions, BaFe,,O,, is formed at a relatively low temperatures and heating at 925'C leads to a highly coercive barium ferrite [4-51. In this paper we describe the characteristics of barium hexafemte synthesized by a procedure described by us [6], that involves coprecipitation of a precursor from a strongly alkaline solution in the presence of an oxidant (sodium hypochlorite) to generate ferrate(V1) species. 2.1 Synthesis of the precursor The procedure was described earlier [6]. Fe(N0,),.9H20 was added to an approximately 3 M solution of sodium hypochlorite ( also 3 M in NaC1) to yield a 0.55 M iron solution. BaCI,.Z%O was added to the filtered deep purple ferrate solution in adequate amounts to reach the desired Ba:Fe ratio. The mixture evolved rapidly into a sluny, with strong oxygen evolution. The slurry was left standing for 24 h, further txated at 80°C for one hour, and left in storage at room temperature. Aliquots were filtered through fritted glass, rinsed several times with r-l,O:C$I5OH (50:50) to remove residual chloride, and (C,H,),O to remove alcohol. The solid was left overnight in an oven at 1 10°C. 2.2 Thermal treatement and characterization of the solid. The evolution of the solid upon thermal treatment was followed by thermogravimetry, differential thermal analysis, X-Ray Diffraction (m), scanning electronmicroscopy (SEM),Fourier Transform Infrared Spectroscopy, and Mtissbauer spectroscopy. The instruments used for characterization were a Mettler TA 3000 Thermoanalyzer, a RIGAKU 0-20 X-Ray Powder Diffractometer (Cu,a =l .54A), Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19971126 JOURNAL DE PHYSIQUE IV a PHILIPS SEM model 5 15 with an EDS probe, a Nicolet FTIR apparatus, and a conventional "FeM~ssbauer transmission equipment (ELSCENT) ,using the constant aceleration method. Fe and Ba were determined by atomic absorption spectrometry. Ail reagents were analytical grade. The X-ray diffmtogram of the solid obtained after overnight dtylng at 1 10°C suggests the presence of crystalline Ba(OH),.Z&O. EDS shows that, witbin the sampling volume of the technique (l pm3), the composition is homogeneous and constant; washing removes all NaCl seen in the original samples. The mixture obtained is an excellent precursor for Ba0.6Fe20,. M e r six hours treatment at 7 10°C, the most intense dfiaction peaks are aIready due to the hexagonal ferrite, although unreacted barium oxide is still present. Figure 1 shows that heating at SOOT for six ilours produces highly clystalline pure barium hexafemte. The stoichiometric solid is best obtained when a slight barium excess is used in the original solution; the excess barium remains dissolved. BaO from unreacted Ba(OH),.2&0 is still iermted in the solid sintered at 710°C, L , . it dissapears upon further heating at higher tern) -.rltures. SEM examination of the precursor,and of the solid obtained by heating ..AOO°C for six hours show the evolution of the Ba(OH), platelets to barium hexafenite single crystal particles, the good cry~tallinit~ a d particle size range are adequate for good magnetic properties. Fig. 2 shows the hysteresiscurve. The Mossbauer spectra of samples calcined at diierent temperatures, in the range 1 10-800°C show the evolution from a superparamagnetic material to the typical hexafemte pattern of four sextuplets due to the (2a+4f,), 12k, 42 and 2b iron ions. The resolution of the peaks and the hfpameters demonstrate that aiready at 700°C, well crystallized barium hexafemte forms. FTIR spectra ofthe precursom and of a thermally treated sample demonstrates that carbonate contents is very low, as expected if due precautions are taken to protect the solutions from contact with air. Figure 1: XR diffractograrn of the synthesized hexafemte a J . ; . . ; . . ; . ; . ; . i , ; l l H (Testa) Fi y r e 2: Hysteresis eurve 4. DISCUSSION The amorphous iron oxide obtained in our conditions is fenihydrite, which is known to be structurally related to the hexagonal iron oxides svch as aFe,O,. The structural simiiarity with Ba-hexafenite, and the intimate mixture with barium hydroxide platelets, makes easy the evolution to the fenite. The use of a strongly oxidant alkaline solution, largely enhances iron solubility, and permits to precipitate simultaneously both ions. The hexafmitz is the only detected mixed oxide in the thermally treated samples, and the Mossbauer spectra demonstrate that 7OO0C suffices to produce it, well below the temperature of 925'C used by Ross [S]. The evolution from superparmagnetism to fenimagnetisn takes place at surprisingly low temperatures; a detailed study of the site mcupancies as a function of temperature is underway. DTA studies shows that the exothemic peak, characteristic of hexafemte crystallization is centered at 730"C, when the heating rate is 6 K mid (sample mass, ca. 50 mg). The absence of carbonate is probably responsible for the high reactivity of barium.