NORMAS ISO

Barium ferrite has been prepared by a variety o f methods, some o f which are briefly described below.

BaFei20i9 fine particles have been prepared by a combustion method*^"*. A solution o f Ba(N03)2 and Fe(N0 3)2*9H2 0 in water was made to react with either oxaldihydrazide (ODH) or tetraformaltrisazine (TFTA) followed by warming to 300°C.

Wang and associates have produced barium ferrites by a hydrothermal method. This was done by reacting a-FeOOH or y-Fc203 with Ba(0H)2 or FeCb with BaCb in an alkaline solution (NaOH). This was kept at high temperature (240- 280 °C) and high pressure (425-950 psi) for several hours*^^'*^^.

Powders o f barium ferrite have been prepared by a self propagating chemical decomposition process by Mishra and co-workers*^**. This method consisted of mixing solutions o f barium and ferric nitrates and adding citric acid. The solution was stirred and was then neutralised with liquid ammonia. The solution was evaporated to dryness and calcined at 700 °C and 850 °C for 1 hour.

Other methods o f preparing barium ferrite include: sol-gel***, salt melt method*^^, flux method*^* co-precipitation*^^ and combustion*^**. Combustion has been effective in generating ultrafine powders.

Conventionally, ferrites have been prepared by the ceramic method. This involves the combination o f barium or strontium carbonate with iron oxide at high temperatures (1000-1200 ®C). This is followed by grinding and heating at 1000 °C or at 1200 o q15.131,132,133

However, the above methods o f preparation invariably produce materials with large particle sizes that are not suitable for synthesising perpendicular magnetic recording media^^^, and in some cases involve many steps. The ceramic method can lead to the formation o f large agglomerates with inhomogeneous com position^and there is also a considerable amount o f unreacted material remaining.

In this chapter the synthesis o f ferrites >vith potential for recording applications is investigated. The synthesis consists o f preparing M-type ferrites by SHS in large magnetic fields to investigate degree o f control over the magnetic properties without the need for doping.

2.2 Results.

2.3 Synthesis of barium ferrite; reaction of BaOz, Fe, and FeiOa in

pellet form.

Pellets o f the reaction green mixture were made by pressing pre-ground powders o f BaOa, Fe and Fe203 to 1 tonne o f pressure. The compacted discs o f starting materials aligned themselves with the magnetic field that they were placed into. (Figure 2.2). Reactions were studied in magnetic fields o f magnitude 0 T, 5 T, 10 T and 15 T.

Figure 2.2 Diagram of the pellet aligning itself with the magnetic field inside the quartz tube.

Discs o f starting materials were initiated by a hot wire and a propagation with an orange flame and a velocity of ca. 1 mms'^ was observed (See Figure 2.3). The wave propagated uniformly fi-om the point o f initiation. Reaction o f an initial 13 mm diameter disc (3 mm thickness) produced a 17 mm diameter disc (5 mm thickness).

The idealised reaction equation was

BaO] + 6Fe + 3Fe203 + 402 —^ BaFei20i9 (Eqn. 2.1)

However 10 % molar excess o f barium peroxide was used in the reaction. This reduces the amount o f Fe203 impurity in the final product. This is necessary to account for the higher volatility o f Ba0 2 (this mirrors standard practise in

conventional ceramic m ethods)’^. In actuality the following ratios were used 1:

5.46 : 2.73 o f BaOi : Fe : Fe2 0 3.

F i g u r e 2 . 3 P h o t o g r a p h s o f t h e r e a c t i o n o f B a O z , F e a n d F c z O s i n p e l l e t f o r m

i n z e r o f i e l d , a t i n i t i a t i o n ( l e f t ) a n d a t 1 . 8 s a f t e r i n i t i a t i o n ( r i g h t ) .

P h o t o g r a p h s a r e t y p i c a l f o r S H S r e a c t i o n s i n p e l l e t s .

After initial SHS all samples consisted o f two parts. One was partially reflective

and grey in colour and was termed “metallic” the other matt with a reddish look

was named “non-metallic”. The metallic part also appears to melt during the

reaction.

The metallic (shiny) and non-metallic (matt) components were separated and

analysed individually. Each sample (metallic and non-metallic) was further divided

into two equal portions. Half o f each was analysed as made and the other half was

sintered and then analysed.

It should be noted that both metallic and non-metallic components consisted o f

partially reacted material. Their composition is described in the next section.

Weighings o f the metallic and non-metallic parts revealed that the metallic part

consisted o f ca. 58% o f the total mass. Performing the reaction in the field did not change the percentage o f metallic part considerably.

2 . 3 . 1 X - r a y p o w d e r d i f f r a c t i o n

2 . 3 . 1 . 1 U n s i n t e r e d S a m p l e s .

The phases identified are detailed in the Table 2.1 below. These are the materials

observed before sintering. Sintering is usually needed in SHS to drive the reaction

to completion. T a b l e 2 . 1 P h a s e s i d e n t i f i e d i n t h e u n s i n t e r e d s a m p l e s o f b a r i u m f e r r i t e b y X - r a y d i f f r a c t i o n . S a m p l e P h a s e s I d e n t i f i e d O T M e t a l l i c B a F c2 0 4, a - F c2 0 3, F e , F c3 0 4, B a F e i 2 0 i 9 i O T N o n - m e t a l l i c j B a F c2 0 4, a - F c2 0 3, F e , F e O , B a F e , 2 0 , 9 : 5 T M e t a l l i c 1 B a F c 2 0 4 , a - F c 2 0 3 , F c 3 0 4 , B a F e , 2 0 i 9 5 T N o n - m e t a l l i c ! B a F c2 0 4, a - F c2 0 3, F e , F c3 0 4, B a F e , 2 0 i 9 ! l O T M e t a l l i c I 1 B a F c2 0 4, a - F c2 0 3, F e , F c3 0 4, B a F e i 2 0 i 9 1 l O T N o n - m e t a l l i c B a F c2 0 4, a - F c2 0 3, F e , F e O 1 1 S T M e t a l l i c B a F c2 0 4, a - F c2 0 3, F e , F e O , F c3 0 4, B a F e i 2 0 i 9 1 ! 1 S T N o n - m e t a l l i c B a F c2 0 4, a - F c 2 0 3, F e O , F e , B a F e , 2 0 i 9

From Table 2.1 it can be seen that before sintering barium ferrite is produced but

amongst a raft o f other phases. BaFc2 0 4 is produced in every case. This suggests

that BaFc2 0 4 is an intermediate that is formed during the SHS step. It was also

noticeable fi-om the X-ray dififaction patterns that magnetite (Fc3 0 4) was present

in all metallic samples and was also the most intense phase. Magnetite was only

present in one o f the non-metallic samples (5 Tesla). N o one phase was

predominant intensity wise in the non-metallic samples. It was also evident fi-om