Magnetic polymer composites containing recycled neodymium-iron-boron (NdFeB) powder and natural rubber (NR) were prepared by the two-roll mill technique. Their mechanical and cure properties were studied as a function of NdFeB loading from 0-120 phr. With increasing magnetic loading, the cure time of the NdFeB-NR composites were exponentially decreased because of the reduction of the polymer chain crosslink. The tensile strength of the NR compound, related to the cure characteristics, was reduced by 40% by the addition of 10 phr NdFeB fillers because of the inhibition of the stress-induced crystallization. However, the variation in loading from 30-90 phr has modest effects on the tensile strength as well as elongation at break and the hardness. Furthermore, recycled NdFeB-NR composites had higher modulus and lower percentage of swelling in this magnetic loading regime. Simple tests confirmed the distribution of magnetic stray field around pieces of NdFeB-NR composites.

Cobalt ferrites of 0-45 wt% were incorporated in polypropylene by the extrusion technique. With increasing magnetic loading, the magnetizations were linearly increased according to the rule of mixture and the electrical permittivity was also proportional to the loading as described by Wagner's equation. The magnetic permeability may be increased but was reduced to minimal at high frequencies because of the substantial coercive field of cobalt ferrites after the extrusion. Without considerable adsorption of polymer matrix on magnetic fillers, the storage and loss modulus from 30 to 180 C of polypropylene were decreased by cobalt ferrites additions.

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Controlling magnetic properties of solgel synthesized cobalt ferrites (CoFe 2 O 4 ) have been studied by variations in amount of polyvinyl alcohol (PVA) and annealing temperature. PVA of 24, 32, 40 and 48 g in 10% w/v aqueous solutions was used in the reaction between 10.592 g of Co(NO 3 ) 2 ·6H 2 O and 29.407 g of Fe(NO 3 ) 3 ·9H 2 O. The decrease in PVA led to the reductions in coercive field and squareness. However, X-ray diffraction peaks indicated that iron oxide (¡-Fe 2 O 3 ) and iron oxide hydroxide [FeO(OH)] were also formed when the PVA was less than 48 g. The optimized amount for obtaining single phase CoFe 2 O 4 was therefore 48 g which was used for studies of heat treatment effect. Both coercive field and squareness were increased with the increase in annealing temperature from 280 to 680°C and then reduced with the further increasing temperature up to 900°C as a result of particles agglomeration.©2011 The Ceramic Society of Japan. All rights reserved.Key-words : Cobalt ferrites, Sol-gel method, Annealing, Coercive field, Squareness [Received February 22, 2011; Accepted April 8, 2011] Soft ferrites with the spinel structure are the important class of magnetic ceramics. 2+ and Fe 3+ was a major factor in regulating the reaction in both nitrate and chloride cases. Since magnetic properties and thus ultimate use of ferrites depend upon the conditions in the solgel preparation, we studied their dependence on the amount of PVA added in the solution as well as the annealing temperature.CoFe 2 O 4 powders were synthesized in polyvinyl alcohol (PVA) solution. The solution was prepared by slowly dissolving PVA powder in distilled water (10% w/v) under constant stirring. The PVA solution was kept at temperatures between 75 and 85°C until its viscosity was significantly increased. Then, Co(NO 3 ) 2 · 6H 2 O (10.592 g, 0.036 mol) and Fe(NO 3 ) 3 ·9H 2 O (29.407 g, 0.073 mol) were mixed with the PVA solution of varying volumes (240, 320, 400 and 480 cm 3 corresponding to PVA of 24, 32, 40 and 48 g). The reaction mixture was further stirred until the solution gelled and the result gel was dried. The obtained brown solid was annealed at 800°C for 4 h. The products were characterized by powder X-ray diffraction (XRD). A copper target was used as an X-ray source (K¡, wavelength = 1.54058 ¡) with 40 kV between the cathode and the copper target. The measurement was performed in the range of 2ª angles from 10 to 80 degrees with each rotating step 0.02 degree. To study effects of the amount of PVA on magnetic properties, samples were characterized by vibrating sample magnetometry (VSM) in an applied magnetic field up to 450 kA m ¹1

Cobalt ferrite (CoFe 2 O 4 ) powders synthesised by sol-gel techniques were incorporated into natural rubber (NR) and polyurethane (PU) by two-roll milling at different loadings from 0 to 45 phr. In either NR or PU composites, the magnetisations were proportional to the CoFe 2 O 4 loading, but the coercive field remained rather insensitive to the loading. The frequency response from 1 MHz to 1 GHz revealed that the real part of the magnetic permeability increased significantly only in the case of 45 phr CoFe 2 O 4 , while the imaginary part was modest in both NR and PU composites. In contrast, the electrical permittivity of CoFe 2 O 4 -PU was larger than that of CoFe 2 O 4 -NR composites, and both parts at 100 MHz had linear variations with the loading, in agreement with Wagner's equation.