Powerful bonded nonconducting permanent magnet for downhole use

摘要

A transverse dipole NMR tool includes a non-conductive permanent magnet made of electrically conductive material such as SmCo. The concentration of SmCo is selected to be at a level low enough to prevent the formation of an electrically conductive path through the magnet. Disposed between the grains of conductive magnet material may be either a ferrite (to increase the remnant magnetization and coercive force) or an inert material. There is a tradeoff between increasing the grain size of the conductive material and the increased intragranular power dissipation. The volume fraction of the conductive material may be increased to a level somewhat higher than a theoretical percolation limit for packed materials; due to randomness in the grain distribution, as long as domains where the percolation limit is exceeded are separate, no conductive path through the magnet results.