B2O3 katkısının doğrudan sinterlenen isotropik sert ferritlerin manyetik özellikleri üzerine etkisi

Abstract

Ferrit olarak bilinen ferrimanyetik oksitler, manyetik özelliklere sahip seramik malzemelere verilen bir isimdir. Genel olarak kimyasal bileşimi MeO. nFe2Oj olarak ifade edilen ferritler n=6 ve Me Mn, Sr, Pb2+ iyonlarını temsil ettiği zaman bu malzeme hegzagonal ferrit veya sert ferrit olarak tanımlanır. Sert ferritlerde eğer manyetik özellikler her doğrultuda aynı ise bunlar isotropik, belli bir doğrultuda geliştirilmişse anisotropik olarak isimlendirilir. Ferritlerde manyetik özellikler genellikle stokoimetrik oran olan n=6'dan biraz daha düşük oranlarda elde edilmektedir. Bu nedenle araştırmada BaO. 5.6Fe2Üj formülü esas alınmıştır. Yapılan bu çalışmada isotropik sert ferritlere B2O3 katkısının manyetik özelliklere etkisinin incelenmesi amaçlanmıştır. Bileşime BjCh katkısı molce 0, 0.01, 0.05, 0.1, 0.15 ve 0.2 olarak ilave edilmiştir. îkinci olarakta ferrit bileşiminin ağırlıkça yaklaşık %85'ini oluşturan hematitin de yerli kaynaktan kullanılması hedeflenmiştir. Sonuç olarak, isotropik sert ferritlere BjOj katkısının hem hematitin hegzagonal f err ite dönüşümünü kolaylaştırdığı, hem de klasik seramik yönteminin kademelerinden kalsinasyon aşamasının kaldırılmasına olanak tanıdığı saptanmıştır. 0.2 mol BjOj katkılı numunenin 1000°C'de x-ışını difraksiyon analizinde serbest hematitin tamamen yok olduğu bulunmuştur. Kalsinasyon aşamasından geçerek ve 1250°C'de sinterlenerek üretilen ferritlerde kalıcı manyetisasyon (Br) 0.2 Tesla (2125 Gauss), zorlayan kuvvet (Hc) 21*10^ Amper/m (2600 Oersted) manyetik enerji [(BH)ma£] 7200 Joule/m3 (0.9x10° Gauss-oersted) iken 12503C de doğrudan sinterlenen ferritlerde Br 0.2 Tesla (2000 Gauss), Hc 22*10* Amper/m (2800 Oersted), (BH)fflaî 7200 Joule/m3 (0.9>rlQ6 Gauss-Oersted) bulunmuş, 0.05 mol B2O3 katkısıyla 1250°C'de doğrudan sinterlenen ferritlerde ise Br 0.23 Tesla (2300 Gauss), Hc 19*104 Amper/m (2400 Oersted), (BH)aas 9600 Joule/m3 (1.2*10° Gauss-Oersted) olarak saptanmıştır.

Ferrite is a general name given to a group of ceramic materials which show f errimagnetic properties. Ferrites can have a wide range of magnetic properties depending on their crystalline structure and chemical composition. The general chemical composition of ferrites can be expressed as Me0.nFe2Ö3' $hen n=l and Me represents the divalent ions such as Ni, Mn, Zn, they are referred as "Soft Ferrites". In this case the magnetic properties of the material can easily be changed with externally applied small magnetic fields. Although the magnetic properties are more inferior than the metallic materials, soft ferrites have been found to have a wide range of application in modern electronics, especially at high frequencies. In the general composition, when n=6 and Me represents divalent ions such as Ba, Sr, Pb permanent magnetic properties are developed. Such ferrites are named "Hexagonal" or "Hard Ferrites". These, too, have inferior magnetic properties compared to the metal ones. However due to their very low cost of production, hard ferrites have been found to have a wide application such as; magnetic clamps for domestic uses, in toy making, in small electrical motors, as loudspeaker magnets. According to the magnetic properties, hard ferrites are grouped into two categories termed as "isotropic" and "anisotropic" ferrites. In isotropic ferrites, the material shows equal magnetic properties in all directions. The permanent magnetization (Br) in these ferrites is in the order of 0.2 Tesla (2000 Gauss) and depending on the production method, the coercive force (He) ranges from 12.10r to 16.10* Ampere/m (1500-2000 Oersted), In anisotropic ferrites, the magnetic properties are developed in only one direction by employing special pressing techniques under high magnetic fields. Such ferrites have permanent magnetization in the range of 0.3 - 0.4 Tesla (3000-4000 Gauss) an coercivity varies between varies between 16sl0* to 24*10' Ampere/m (2000-3000 Oersted), The optimum magnetic properties in hard ferrites are usually obtained at Fe,0* contents slightly lower than the stoichiometric formulation (n<6). Further more, various oxide additions are employed in the formulation in order to enhance the desired magnetic properties. For example the addition of SİO2 and A^Oj to isotropic ferrites prevents the grain growth mechanism during the sintering and increases the coercive force. CaO is also employed as on enhancer of the permanent magnetization. It has also been reported that small additions of B2O3 developed beneficial effects in the magnetic properties. However, this particular study has been conducted in a very narrow range of a B^0% addition (0.005 - 0.035 mole). Hence the main objective of this study was the elucidate the effect of a B2O3 addition at a wider spectrum. The second objective of the study was to incorporate the Fe^O, content of the formulation with ingiginous material which accounts for - 85% by weight of the compound. For this purpose hematite produced by EVA, A.Ş. as a colour pigment for the dye industry was used in the study.. The chemical analysis of the hematite is given in Table 1. Table 1: Chemical analysis of EVA hematite (weight %) Isotropic hard ferrites are produced according to the conventional ceramic processing technique. In this method, the elemental oxides or carbonates, which are weighed according to the formulation and then ball milled in steel jars using steel balls and water as grinding media. After drying, the mixture is pressed into cakes which are then calcined between 800 - 1000 °C in order to form the ferrite phase. The calcined product is then crushed into small pieces (-1 mm) and milling operation is reapplied. After drying the mixture again, the powder is granulated using an appropriate binder. The granules are then pressed into the desired shape and then sintered. vi BaO. S.SFejOj formula was adopted as the basic formulation through the study. Pure grade barium carbonate (>99.5% purity, Merck, Germany) and hematite (EVA &.Ş.) were weighed according to the formulation. The raw materials were then ball milled in distilled water at 60 rpm for 8 hours, using a 3 1 capacity steel jar and steel cylindrical balls of 2 cm diameter; 3 cm length. The mixture was then dried at 110°C and kept in sealed jars as stock composition. Boric acid used as BjOj additive (hight purity, Etibank, Turkey), was weighed out in the quantities listed in Table 2, in order to achieve the following formula, BaO. 5. 6Fe20j+x(B20j), x=0, 0.01, 0.05, 0.1, 0.15 and 0.2 mole. Table 2: HjBOj used per 100 g. stock ferrite formulation. The weighed boric acid, 100 g stock ferrite powder and 1 g of PVA (used as a binder) were then mixed in 100 ml. of distilled water. The mixture was dried to 6-10% humidity and then granulated to -74um. From the granulated powders, cylindrical samples of 16 mm. diameter 8 mm. in thickness were pressed at 30 MPa. These samples were calcined at 750, 800, 850, 900 and 1000°C for 1 hour, using 200°C/h heating rate and self wooled in the furnace. The calcined samples were then tested by x-ray diffraction to determine the unreacted Fe20j phase. The amount of the unreacted Fe20j phases for the undopped and 0.2 mole B2O» doped samples which were calculated from the ratio of maximum intensity peaks of barium hexaferrite ( dQfln = 2.7 &fl) and a-Fe20j (dnfl|j = 2.782 AS ) are given in Table 3. Vll Table 3: Amount of unreacted a-Fe20j phase in calcined samples. As can be seen from table 3, the B2O3 addition clearly promotes the formation of the barium hexaferrite phase. Due to these encouraging results, it was decided to omit the calcination step from the manufacturing process. For this reason the research was focused on the effect of a B}0% addition on the magnetic properties of the direct-sintered samples. To study the direct sintering method, specimens in the above mentioned dimensions were pressed in a floating die system at a pressure of 100 MPa to a green density of 2,7«103 kg/m3. The sintering was carried out at 1100 - 1150 °C for 1-4 hours and at 1200, 1250 °C for 1 hour, in a SiC heated furnace in an atmospheric condition, employing a 200 °C/h heating rate. The bulk densities of the samples were determined from their mass and volume ratio. The microstructures were studied in a Jeol 840 scanning electron microscope and the magnetic properties were measured in permagraph equipment by Magnet-Physik (Köln - Germany).