Method and device for producing a permanent magnet of a rare earth alloy powder

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PROBLEM TO BE SOLVED : To stabilize the amt. of hydrogen contained in the object to be treated and to uniformize its magnetic properties by measuring the thermal conductivity of atmospheric gas in a hydrogen discharging and recombination reaction stage and detecting the end point of the hydrogen discharging and recombination reaction in HDDR treatment on the basis of the thermal conductivity. SOLUTION : The smaller the density of gas is, the higher the thermal conductivity is made. Hence the increase and decrease of the partial pressure of hydrogen contained in atmospheric gas ceased can be detected by the change of its thermal conductivity. In particular, since, in the case the amt. of hydrogen in the atmospheric gas decreases in accordance with time in the last period of the hydrogen discharging and recombination reaction, the drop of the concn. of hydrogen can be detected as the drop of the thermal conductivity of the atmospheric gas, the time at which the drop of the thermal conductivity has approximately can be regarded as the end point of the reaction. Concretely, the time at which the temporal change of the thermal conductivity of the atmospheric gas measured by using a Pirani gauge has dropped to <=0.5% per min is judged as the end point of the hydrogen discharging and recombination reaction.