Efficiency and purity control in the preparation of pure and/or aluminum-doped barium ferrites by hydrothermal methods using ferrous ions as reactants

Abstract

The synthesis of hexagonal barium ferrite (BaFe₁₂O₁₉) was studied under hydrothermal conditions by a method in which a significant amount of ferrous chloride was introduced alongside ferric chloride among the starting materials. Though all of the Fe²⁺ ions in the starting material were converted to Fe³⁺ ions in the final product, Fe²⁺ was confirmed to participate differently from the Fe³⁺ used in the conventional method in the mechanism of forming barium ferrite. Indeed the efficiency of the synthesis and the quality of the product and the lack of impurities such as Fe₂O₃ and BaFe₂O₄ were improved when Fe²⁺ was included. However, the amount of ferrous ions that could be included to obtain the desired product was limited with an optimum ratio of 2:8 for FeCl₂/FeCl₃ when only 2 h of reaction time were needed. It was also found that the role of trivalent Fe³⁺ could be successfully replaced by Al³⁺. Up to 50% of the iron could be replaced by Al³⁺ in the reactants to produce Al-doped products. It was also found that the ratio of Fe²⁺/M³⁺ could be increased in the presence of Al³⁺ to produce high quality barium ferrite.

Introduction

Hexagonal barium ferrite receives much attention in the field of recording material [1] due to its large magneto-crystalline anisotropy, relatively high saturation magnetization, non-toxicity, excellent chemical stability and good corrosion resistivity. The quality of the product is critical for successful technological applications. In particular it is necessary to provide ultrafine particle size, narrow size distribution and chemical homogeneity. The use of milling processes is not desirable as they cause broad particle size distribution and introduce impurities. Therefore it is particularly important to establish the conditions for the synthesis that will provide the particles with the best possible characteristics.

The mechanism [2] of formation of barium ferrite from barium oxide and ferric oxide was proposed to include the following three reactions:

BaO+Fe₂O₃=BaFe₂O₄

BaO+6Fe₂O₃=BaFe₁₂O₁₉

BaFe₂O₄+5Fe₂O₃=BaFe₁₂O₁₉

Attempts to synthesize barium hexaferrites by sol–gel [3] and co-precipitation [4], [5], [6] processes invariably lead to the partial formation of residual intermediates such as α-Fe₂O₃, γ-Fe₂O₃, BaCO₃ and BaFe₂O₄ and usually necessitate annealing at temperatures above 1000 °C for several hours which is undesirable as this induces agglomeration of the particles. The synthesis of nano-size barium ferrites at lower temperatures utilizing shorter times is therefore desirable to improve energy efficiency and cost-efficient maintenance of the equipment. The synthesis of barium ferrite by a hydrothermal reaction is preferable as it can be carried out at lower temperatures and the problem of aggregation is therefore reduced. An additional advantage of the hydrothermal method is that fewer unwanted intermediates and/or impurities are obtained. The impurities Fe₂O₃ and BaFe₂O₄ in the precursor materials were controlled in the condition optimizing process until they are not detected by XRD in this work.

Although barium hexaferrite has been hydrothermally synthesized using FeCl₃ as one of the starting materials, the syntheses were usually carried out using a relatively long time [7], [8], [9], [10], [11], [12], [13]. For example, successful hydrothermal syntheses have been carried out by Liu et al. [7] for 48 h at 230 °C who also noted that barium ferrite is the only XRD detectable phase at a reaction duration of 25 h. Li et al. [8] employed 10 h with temperatures ranging from 180 to 260 °C. Duong et al. [9] used 12 h at various temperatures between 150 and 200 °C. Yamauchi et al. [10] reduced the reaction time to within 1 h by using a microwave-induced hydrothermal method.

The efficiency of producing barium hexaferrite might be further increased if the mechanism of the reaction could be altered. Usually the starting material is Fe³⁺ and α-Fe₂O₃ is easier to form under the reported conditions. But it should be noted that BaFe₂O₄ is involved in the formation of barium hexaferrite as shown in Eq. (3). And Fe₃O₄ is a compound similar to BaFe₂O₄ in which Ba²⁺ is replaced by Fe²⁺. Furthermore, Fe₃O₄ possesses the inverse spinel-type structure, and also there are blocks of spinel structures in magneto-plumbite structure of barium ferrite. While α-Fe₂O₃ possesses a rhombohedral structure which is very different from the magneto-plumbite-type structure, the similarity in structures of Fe₃O₄ and barium ferrite would made the conversion of Fe₃O₄ to the product much easier than that of α-Fe₂O₃. Furthermore, α-Fe₂O₃ is more stable than Fe₃O₄ thermodynamically, especially at higher temperatures, so that α-Fe₂O₃ is easier to form and more difficult to convert to barium ferrite. If the reaction conditions were controlled such that Fe₃O₄ could be formed at some stage and the conversion of Fe₃O₄ to α-Fe₂O₃ is suppressed, then the efficiency of producing barium ferrite would be improved. Since Fe₃O₄ could be formed by the hydrothermal method with FeSO₄·7H₂O and FeCl₃ [14], introducing Fe²⁺ might improve the efficiency of preparing barium ferrite or induce the formation of γ-Fe₂O₃ the structure of which can be taken as the spinel structure oxide and it is less stable than α-Fe₂O₃. It can also be argued that lower temperature processes such as the hydrothermal method are much more likely to involve Fe₃O₄ than the higher temperature processes such as sol–gel and co-precipitation. Following this logic, the present work shows that the necessary temperature and time for synthesis can be reduced and the purity increased significantly by including FeCl₂ alongside with FeCl₃ among the reactants and that the presence of Fe²⁺ ions shows some unexpected benefits.

In spinel structure only 1/8 of the tetrahedral holes were occupied. In magneto-plumbite there are only 4f_IV and only 4f_VI holes in a unit cell and all of them are occupied by Fe³⁺. These two kinds of holes have opposite spins to Fe³⁺ in other holes. Only half of the 12k holes are occupied by Fe³⁺. The magnetic properties can be tailored if these Fe³⁺ are selectively substituted. Barium hexaferrites are often doped with metal ions to alter their magnetic properties as will be discussed later in this work. The substituted ions can weaken the super-exchange between the two sub-lattices in the ferrimagnetic barium ferrite structure that causes the moments to align in an anti-parallel fashion. Aluminum-substituted barium hexaferrites synthesized by hydrothermal precipitation and calcination have been reported by Mishra et al. [15]. However, samples calcined at 1200 °C by this method were found to contain significant amounts of α-Fe₂O₃ impurity. Aluminum-doped barium and strontium ferrites prepared via a sol–gel route using citric acid to complex the ions followed by an auto-combustion reaction have been reported by Shirtcliffe et al. [16]. In this present paper, we investigate the preparation of aluminum-doped barium ferrite BaFe_12−xAl_xO₁₉, x=0–6 by a pure hydrothermal reaction using ferrous and aluminum ions to replace all of the starting ferric ions together with barium chloride and sodium hydroxide, and establish the optimum conditions for the reaction.