JOURNAL OF COLLOID AND INTERFACE SCIENCE 185, 402–410 (1997) ARTICLE NO. CS964525 Microstructural and Magnetic Studies on Hydrothermally Prepared Hematite KAMALA KANTA SAHU,* CHANDANA RATH,* NARESH CHANDRA MISHRA,* , ² SHASHI ANAND,* ,1 AND RADHANATH PRASAD DAS * * Regional Research Laboratory (Council of Scientific and Industrial Research), Bhubaneswar 751 013, India; and ² Department of Physics, Utkal University, Bhubaneswar 751 004, India Received April 18, 1996; accepted July 25, 1996 various processing techniques developed for the production Hematite particles of various shapes and sizes have been pre- of such particles are spray roasting (2, 3), gel–sol (4–11), pared hydrothermally at different pH values ranging from 3 to 10 and hydrothermal precipitation (12, 13). from ferric chloride solution at 180°C. Particle size decreases with Hydrothermal precipitation techniques have several ad- an increase in the pH of precipitation as observed from Transmis- vantages such as (i) effective control of size and shape of sion Electron Micrographs (TEM). A comparison of TEM and the particles, (ii) incorporation of fewer impurities in the mean crystallite diameter (MCD) from XRD data reveals that the hydrolyzed product, (iii) regeneration of the lixiviant, and particles prepared are polycrystalline in nature. The polycrys- ( iv ) relatively low reaction temperature. Optimization of tallinity decreases with an increase in the pH of precipitation, and processing parameters leading to hematite phase formation at pH 10 almost single crystalline nature of particles is obtained. Polycrystalline monodispersed pseudocubic particles obtained at hydrothermally from iron(III) nitrate and carboxylate solu- pH 3 exhibitvery high coercive force and remanent magnetization tion have been reported (12, 13). Preparation of pure hema- which decreases with an increase in pH, whereas Morin transition tite utilizing naturally abundant and waste materials like blue temperature, T m , shows an opposite trend. Magnetic a.c. suscepti- dust and iron scrap would favor a hydrochloric acid leaching bility increases with an increase in pH up to pH 7 and then de- route. Even in chloride media, direct precipitation of iron creases. A probable mechanism for the formation of polycrystalline oxide is difficult due to redissolution of precipitated iron particles has been suggested in orderto explain the above trends oxide. The present work reports a procedure for initial pre- in magnetic behavior.  1997 Academic Press cipitation of iron as hydroxide and its subsequent conversion Key Words: hematite; hydrothermal; subparticle; aggregation; to pure hematite through hydrothermal technique. coercivity;Morin transition temperature;a.c. magnetic suscepti- Further, it is also important to understand the magnetic bility. properties associated with the prepared iron oxides for realiz- ing a wide scope of applications in the field of science and technology. The magnetic features of iron oxides, both above INTRODUCTION and below Morin transition temperature ( T m ), are however influenced by the method of preparing the sample, which in The demand of raw materials for production of both mag- turn influences the size and shape of grains of the materials netic and data media components has increased dramatically (14). Since no systematic information is available on the because of enormous development in the electronic industry microstructural and magnetic properties of hydrothermally (1). Iron oxide is the major constituent of such materials, prepared hematite particles, the present work has been under- the quality of which controls the grain size, shape, hardness, taken to generate such information. permeability, magnetic losses, etc. of the ferrite products. High pure iron oxide is needed in order to produce high EXPERIMENTAL quality ferrites. Iron oxides also have other conventional applications as catalysts and pigments, and in cooling water Stock solution of ferric chloride was prepared by dissolv- treatment, besides being used extensively as anti-corrosive ing electrolytic iron powder ( 99.95% purity ) in concentrated material. Efforts are continuing to produce monodispersed HCl followed by oxidation of Fe(II) to Fe(III) with the iron oxide particles, uniform in shape, size, and composition addition of nitric acid. To ensure complete oxidation, the as ideal constituent for varieties of advanced materials. The contents were heated to boiling for about 2 h. The absence of Fe( II ) was confirmed volumetrically by dichromate method 1 To whom correspondence should be addressed. using BDAS as the indicator (15). Ammonia was added to 402 0021-9797/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.