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LatinAmerican Journal 01 Metallurgy and Materials, Vol. 8 Nos. 1 & 2 (1988) About Si0 2 Additions to NiZnCo Ferrites A. González Arias and T. Rodríguez Dpto, Física Aplicada, Fac. de Física, Universidad de La Habana. Se estudia la influencia de pequeñas cantidades de Si().¿en la permeabilidad y el factor de pérdidas de ferritas de NiZnCo, para dos como. posiciones básicas diferentes. En ambos casos. se obsaerva un cambio abrupto en la permeabilidad y en las pérdidas, en una región muy estrecha de alrededor de 0.025%en peso de Si().¿.Los resultados sugieren dos mecanismos diferentes para la influencia del Si().¿en las propiedades magnéticas: a) por disolución en la red espinela por debajo del 0.05%,y b) estorbando el crecimiento de los granos cuando se adiciona una mayor cantidad. About Si0 2 Additions to NiZnCo Ferrites The influence of small amounts of silica on the permeability and loss factor ofNiZnCo ferrites is studied, for two different basic cornposí- tions. An abrupt change in permeability and losses is observed. in both cases, in a very narrow region of about 0.025 wt % Si().¿.The results suggest two different mechanisms for the influence of Si().¿on the magnetic properties: a) by dissolution in the spinel lattice below 0.05 wt %, and b) by impeding grain growth when larger amounts are added. 1. INTRODUCTION Small amounts of silica, added to polycrystalline spinel or hexagonal ferrites, are known to affect noti- ceably one or another physical property. In the case of hard Ba and Sr M-type hexaferrites, commonly used as materials for permanent magnets, silica has been recom- mended as a sintering additive, able to improve the mate- rial properties through the promotion of shrinkage or by impeding grain growth, providing materials with larger volume magnetization or higher coercitivity. A more or less abrupt change in the smooth variation of the magne- tic and electric properties with added silica is observed below 1wt % Si0 2 , and this behavior may be correlated with the Si0 2 solubility-limit in the ferrite crystallattice. For Ba M-type hexaferrite, this limit is around 0.55 wt %, with a similar behavior for Sr-M hexaferrite [1]. In soft Mn-Zn spinel ferrites, used mostly as high- permeability cores for inductor and transformers in the kHz frequency range, much smaller amounts of SiO z are known to affect properties such as resistivity and per- meability (¡..t), Results obtained by Akashi [2]show that a maximum of resistivity and a mínimum value of the loss factor (tan o) are found when 0.02 to 0.04 wt % Si0 2 is added to a MnZn ferrite of composition Mllo. 66 Zllo. 21 F~.1104' Lescroel and Pierrot [3] obtained a quite un- usual behavior when analyzing the effect of Si0 2 on the permeability of a MnZn ferrite; this parameter reduces from 1800 to 500 at about 0.05% of Si0 2 increasing abruptly up to 2400 for 0.08%, and decresing again when more silica is added. Promoted grain growth and large grains were reported above 0.05 wt % Si0 2 , while below 0.05% the grain size was very smal!. It has been conside- red that, in MnZn ferrites, Si0 2 below 0.05% forms inclu- sions which impede grain growth and that, above 0.05%, it redissolves enhancing grain growth [4]. Silica has also been added to high frequency NiZnCo ferrites [5],but the literature on the subject is scant, and does not provide a clear understanding about the in- fluence of this additive, or about the mechanism invol- ved. In iron excess NiZnCo ferrites the C0 2 + ions, below the Curie temperature, become placed in preferential sites under the action of local forces, with their magnetic moments oriented along preferred directions, impeding the displacement of -stabilizing or "pinning" - the domain walls. The pinning ofthe walls causes a decrease in the permeability, and also in the magnetic losses [6,7]. In iron excess MnZn ferrites, the addition of trivalent or tetravalent ions such as Ti H , Sn H , ~+ ,Ge H or A1 3 +cause an increase in the Fe 2 + content [8]. Since a certain amount of Fe 2 + is also expected in iron excess NiZnCo ferrites sintered in air, the influence of Si0 2 in these ferri- tes could be related to one or several of the following mechanisms: i) reduction of eddy current losses through the increase in resistivity at the grain boundaries, ii) grain growth control, which is also known to have a strong influence in high frequency losses [6],iii) interac- tion with the Cc}+diffusion to stabilize domain walls, iv) changes in the FEf+ contento 2. EXPERIMENTAL To study the effect of Si0 2 in NiZnCo ferrites, 20 mm diameter toroidal samples were prepared from carbona- tes and oxides by conventional ceramíc methods, with mixing and millingtimes of 1and 2hours, respectively, in a vertical rotatory ball mil!. Calcination and sintering temperatures were set at 950 and 1200°C, and cornpac- tion pressure at 2.5 'I'on/crn". After sintering, cooling was carried out slowly with the furnace to room tempera-
