Audio tapes, chromium

Properties. Pigments with a coercive field strength of 50-56 kA/m are used in video cassettes, high-bias audio cassettes (chromium dioxide operating point IEC II), and high-density floppy disks. Depending on the quality of the tape, the particle size varies between 0.2 and 0.4 pm (see Section 5.1.1, Table 36). 

The human ear normally can hear sounds up to about 15,000 cycles per second. To record such a high frequency without distortion requires extremely tiny magnetic particles. The magnetic material must be easy to align and should retain its pattern of orientation indefinitely. Better quality audio tapes use very fine grains of chromium dioxide instead of iron oxide. Inexpensive tapes are adequate for recording the spoken voice because its frequency range is much less than for music. 

High-temperature calcination of chromite ore and soda ash leads to oxidation of chromium to sodium chromate, which is leached out by water. After acidifying, sodium dichromate, the basic material for chromium chemicals, is formed. Chromium trioxide CrOj, chromic add, prepared from sodium dichromate, is used for preparing the baths for chromium plating. Hexavalent chromium chemicals are also still used to some extent in pigments, for corrosion protection and for timber treatment Chromic add is also used for video/audio tapes although iron-based tapes have taken over more than 90% of this market. 

The high hardness of chromium oxide resulting from its crystal structure is exploited in polishing agents for metals and in brake linings. Addition of a small amount of chromium oxide to magnetic materials of audio and video tapes imparts a self-cleaning effect to the sound heads. 

Chromium dioxide (CrOi) is at present the most important ferromagnetic ceramic. It is used as magnetic media in audio and video recording tapes. In this application, the Cr02 is not in the pure form but is usually doped to improve its properties. 

---