HDDR Process Research Articles

Hydrogen absorption and desorption study of Nd-TM-B system

Hydrogen absorption and desorption behaviour of Nd/sub 2.2/(Fe/sub 1-x-y/CoNi/sub y/)/sub 14/B/sub 1.05/ alloys (x=0.0/spl sim/1.0, y=0.0/spl sim/0.1) have been studied. The substituted alloys absorb less hydrogen possibly due to changes in cell dimensions and to an increase in alloy stability and the recombination reactions take place under hydrogen at lower temperature than non-substituted alloy. Substitution for Fe results in magnetically anisotropic features being retained in the HDDR process and processing temperatures are also influenced by substitution.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Origin of anisotropy in the HDDR process of Nd/sub 2/Fe/sub 14/B-based alloys

To clarify the origin of magnetic anisotropy induced in Nd/sub 2/Fe/sub 14/B-based magnet powders during hydrogenation, disproportionation, desorption, and recombination (HDDR) process, the microstructure of hydrogenated Nd/sub 12.5/Fe/sub 70-x/Co/sub 1/ Ga/sub x/B/sub 6/ powders (0<or=x<or=5) are investigated. As the hydrogenation temperature is increased over 1148 K, a small amount of Nd/sub 2/Fe/sub 14/B phase is observed to remain undecomposed in the Nd/sub 12.5/Fe/sub 69/Co/sub 11.5/Ga/sub 1/B/sub 6/ powder, even after the hydrogenation for 3.6 ks. This undecomposition becomes more evident when increasing the temperature up to 1198 K. The size of the undecomposed Nd/sub 2/Fe/sub 14/B particles in the powder hydrogenated at 1148 K is about 0.3 mu m. The powder with the undecomposed Nd/sub 2/Fe/sub 14/B exhibits magnetic anisotropy after the subsequent desorption process at 1123 K. The increase of the amount of Ga content also decelerates the decomposition. The origin of the anisotropy could therefore be the finely dispersed undecomposed Nd/sub 2/Fe/sub 14/B particles, which can be formed under the controlled hydrogenation condition and with the additives to decelerate the decomposition. Such dispersed particles should act as nuclei for a growth or recrystallization of the Nd/sub 2/Fe/sub 14/B phase during the desorption process, with the preferred orientation, i.e., the orientation of the original Nd/sub 2/Fe/sub 14/B phase.<<ETX>>

Development of HDDR Process and Anisotropic Nd-Fe-B Bonded Magnets

The intermetallic compound Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B undergoes four-step phase transformations in a hydrogen atmosphere, namely, hydrogenation, decomposition, desorption and recombination. This phenomenon was observed in a study of the sintering behavior of an Nd-Fe-B magnet in atmospheric hydrogen gas. The sinter produced consists of crystalline grains ranging from extremely large to very small, and are magnetically isotropic ( <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> approximately 7 kOe). The decomposition products first form fine crystalline grains and then grow into extremely large grains by an abnormal grain growth process when hydrogen is naturally desorbed. The forced low-temperature hydrogen desorption from the decomposition product creates powders with fine crystal grains (of order 0.3 μm), which have high coercivity (≫10 kOe). This process is called the HDDR process after the four steps involved. The addition of small amounts of elements such as Ga, Zr and Hf is effective for producing anisotropic magnet powders with good magnetic properties. It is now possible to prepare bonded magnets with (BH) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> =18 MGOe (140 kJ/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) from these anisotropic powders. This review describes the HDDR process and the uses of HDDR powders for anisotropic Nd-Fe-B bonded magnets and for full dense magnet production.

ＨＤＤＲ処理法とＮａ‐Ｆｅ‐Ｂ系異方性磁石粉末の開発

ＨＤＤＲ処理法とＮａ‐Ｆｅ‐Ｂ系異方性磁石粉末の開発

HDDR hot-pressed magnets: magnetic properties and microstructure

The recently developed HDDR process has now been extended to production of hot-pressed magnets. The samples prepared to date exhibit intrinsic coercivities of ≈1200 kA/m, remanence of around 690 mT and BH max values of ≈90 kJ/m 3. Careful studies of the microstructures reveal that the magnets consist of Nd 2Fe 14B grains predominantly in the range 0.1 μm⩽ x⩽1.0 μm with a very small number of grains in the range 10 μm ⩽ x⩽35 μm. These larger grains exhibit a very regular morphology and appear to be the result of a very rapid growth of some of the smaller sub-micron grains. The starting composition of the cast material Nd 16Fe 76B 8 ensures that the magnets have ≈12% Nd-rich intergranular material; this low melting point phase has been found to assist in the densification process. However, SEM studies have shown that the Nd-rich material is very unevenly distributed throughout the hot-pressed magnet and little evidence can be found for its presence between the grains of Nd 2Fe 14B.