Chris N. Christodoulou

Sm2Fe17-nitride-based permanent magnets produced by the hydrogenation-decomposition-desorption-recombination (HDDR) process

Abstract The hydrogenation-decomposition-desorption-recombination (HDDR) process in conjunction with nitrogenation has been employed to produce Sm2Fe17-nitride-based permanent magnets. The HDDR process has been studied in terms of the reaction characteristics, the phases present and the development of the magnetic properties at the different stages involved in the process. On heating under hydrogen gas, the Sm2Fe17 phase begins to absorb hydrogen at a temperature of about 185 °C and up to 250 °C, where the hydrogen concentration reaches a maximum value of about 2.4 H atoms per mole of Sm2Fe17. Subsequently, interstitial hydrogen desorption occurs gradually up to a temperature of 510 °C (Sm2Fe17H0.1), where a massive absorption of hydrogen takes place owing to the decomposition of the Sm2Fe17H0.1) into Sm hydride+α-Fe. Additional heat treatment of the “ Sm hydride + α- Fe ” mixture at temperatures of about 725 °C or above under vacuum or argon gas flow causes the simultaneous desorption of the Sm hydride (forming Sm + H2) and recombination of Sm + α-Fe to form microcrystalline grains of the Sm2Fe17 phase. Subsequent nitrogenation at about 450 °C produces Sm2Fe7-nitride which exhibits an intrinsic coercivity of 8.2 kOe, a Curie temperature of 470 °C and a maximum energy product of about 14 MG Oe.

Preparation, structural and magnetic properties and stability of interstitial Sm2Fe17-carbonitrohydrides

Abstract Sm 2 Fe 17 -carbides, -nitrides, -hydrides, -carbonitrides, -carbohydrides, -nitrohydrides and -carbonitrohydrides were synthesized by reacting Sm 2 Fe 17 powder with the appropriate gases (acetylene, nitrogen or hydrogen). Carbonation, nitrogenation and hydrogenation were performed at about 600 °C, 450 °C and 250 °C respectively. Low-carbon concentration carbides were prepared by conventional melting. The structural and magnetic properties were measured for each of the prepared compounds. Nitrogen and carbon can partially or fully occupy the 9(e) octahedral interstices. In addition to these, hydrogen can also partially occupy the 18(g) tetrahedral interstices. Interstitial carbon, nitrogen and hydrogen atoms cause an expansion of the lattice of the parent compound. Hydrogenation of the saturated Sm 2 Fe 17 -carbide or saturated Sm 2 Fe 17 -nitride causes preferential lattice expansion along the c -crystallographic direction. Hydrogen atoms cause the largest increase in saturation magnetization, followed by nitrogen and carbon atoms. The largest increase in the anisotropy field is caused by the nitrogen atoms followed by the carbon atoms. The hydrogen atoms cause a decrease in the anisotropy field. The nitrogen atoms cause the largest increase in the Curie temperature followed by the carbon and hydrogen atoms. Nitrogen atoms are very strongly bonded to the samarium atoms and weakly to the iron atoms, whereas carbon atoms are strongly bonded to both the samarium and iron atoms. This strong bonding makes the reversible removal of the nitrogen and carbon atoms impossible. However, hydrogen bonding is much weaker and therefore, hydrogen atoms can be absorbed and desorbed easily without any change in the Sm 2 Fe 17 structure. Nitrogen and carbon atoms can replace the hydrogen atoms in the 9(e) sites.

Oxidation of Fe‐R‐B powders during preparation of permanent magnets

Differential scanning calorimetry (DSC) has been employed to study the changes which occur in the powders used to prepare Fe‐R‐B permanent magnets upon heating. Three exothermic reactions are observed upon heating the powders. The correlation of DSC data with the magnetic properties of Fe‐R‐B powders shows that these reactions cause a significant deterioration of the hard magnetic properties of the resulting magnets. The most probable explanation of these exothermic reactions is the reaction between chemisorbed oxygen and rare‐earth metals to form bulk oxides. It appears that there is a critical oxygen concentration associated with a critical particle size which limits the development of hard magnetic properties.

Hydrogenation and nitrogenation of SmFe2

Abstract The characteristics of the reactions between SmFe 3 and H 2 or N 2 were studied by isochorothermal analysis, X-ray diffraction and thermomagnetic analysis. SmFe 3 absorbs about 3.1 atoms of hydrogen instantaneously at a temperature between 140 and 170 °C. Subsequent cooling to room temperature causes an increase in the hydrogen concentration to about 4 hydrogen atoms per mole of SmFe 3 . As the temperature is increased, the SmFe 3 hydride begins to desorb hydrogen and its composition becomes SmFe 3 H 0.2 at about 350 °C. The SmFe 3 hydride desorbs and absorbs hydrogen reversibly in the temperature range 25–400 °C, with a 16 °C temperature hysteresis occurring between 249 and 233 °C during heating and cooling. At about 425 °C, the hydride begins to decompose into Sm hydride and α-Fe. The decomposition reaction is accompanied by a massive hydrogen absorption. The SmFe 3 hydride has the rhombohedral crystal structure (same as the parent compound) with a 19% expanded lattice with parameters a = 5.378 A and c = 27.345 A . The stresses introduced by the huge lattice expansion cause the decrepitation of the SmFe 3 hydride samples. The Curie temperature and saturation magnetization are 90 °C and 70.8 e.m.u. g −1 for SmFe 3 H 4 compared with 390 °C and 80.6 e.m.u. g −1 for SmFe 3 . For both SmFe 3 and SmFe 3 hydride, the easy direction of magnetization is along the crystallographic c axis. SmFe 3 powder reacts with N 2 at about 250 °C forming SmN, α-Sm and α-Fe. The final products are SmN and α-Fe. No interstitial SmFe 3 nitride exists at any temperature.

Sm2Fe17-nitride-based permanent magnets produced by rapid solidification

Abstract The rapid solidification process in conjunction with nitrogenation has been employed to produce Sm 2 Fe 17 -nitride-based permanent magnets. In the as-quenched state, depending upon the alloy composition and process parameters, a mixture of the following phases was found: α-Fe, metastable SmFe 7 , Sm 2 Fe 17 , SmFe 3 , SmFe 2 , Sm and an amorphous phase. In general, Sm-rich alloys solidified at high quenching rates (55 m s −1 ) consist of a mixture of metastable SmFe 7 (TbCu 7 type) and an amorphous matrix. Subsequent heat treatment at 650 °C for 10 min forms Sm 2 Fe 17 (Th 2 Zn 17 type) in metastable equilibrium with SmFe 2 (MgCu 2 type). SmFe 3 (PuNi 3 type) also begins to precipitate and its amount increases for longer heat treatment times. The same mixture of phases (Sm 2 Fe 17 + SmFe 2 + SmFe 3 ) could also be produced directly in the as-quenched state at lower quenching rates (25 m s −1 ). Nitrogenation of the Sm 2 Fe 17 -based rapidly quenched alloys at about 450 °C forms Sm 2 Fe 17 nitride exhibiting coercivities as high as 17.7 kOe with a ( BH ) max of 12 MG Oe and a Curie temperature of 470 °C. The secondary phases present in the alloys (SmFe 2 + SmFe 3 ) decompose entirely into SmN + α-Fe.

Hydrogenation of Sm2Fe17

Abstract The characteristics of the reaction between Sm2Fe17 and H2 have been studied by means of isochorothermal analysis, X-ray diffraction and thermomagnetic analysis. Sm2Fe17 begins to absorb hydrogen at about 185 °C. At 250 °C the hydrogen content reaches the maximum value of 2.4 atoms of hydrogen per mole of Sm2Fe17. Subsequent cooling to room temperature causes an increase in the hydrogen concentration up to the value of 5 H atoms per mole of Sm2Fe17. Above 250 °C the Sm2Fe17 hydride begins to desorb hydrogen and its composition becomes Sm2Fe17H0.1 at about 500 °C. The Sm2Fe17 hydride desorbs and absorbs hydrogen reversibly in the temperature range between 25 and 500 °C. At about 510 °C the hydride begins to decompose into Sm hydride and α-Fe. The decomposition reaction is accompanied by massive hydrogen absorption. The Sm2Fe17 hydride has the rhombohedral crystal structure (the same as the parent compound) with a 3.4% expanded lattice with parameters α = 8.667 A and c=12.520 A . The stresses introduced by the lattice expansion cause crack formation along the Sm2Fe17 grain boundaries and the samples become friable. The Curie temperature and saturation magnetization are 280 °C and 159 e.m.u. g−1 for Sm2Fe17H5 as compared with 135 °C and 116 e.m.u. g−1 for Sm2Fe17. For both Sm2Fe17 and Sm2Fe17 hydride the easy direction of magnetization is perpendicular to the c direction in the rhombohedral structure.

High coercivity anisotropic Sm2Fe17N3 powders

Abstract Anisotropic Sm 2 Fe 17 N 3 powders can be prepared by introduction of nitrogen into Sm 2 Fe 17 single-grain powders produced by interstitial hydrogen absorption desorption. Nitrogenation can be performed with direct reaction with N 2 gas or alternatively by reaction with mixtures of N 2 H 2 or NH 3 and subsequent preferential dehydrogenation. The use of NH 3 was found to be particularly advantageous because particles can be completely nitrogenated at lower temperatures and in shorter times, in that way minimizing the decomposition of Sm 2 Fe 17 N 3 into undesirable SmN, FeN χ and α-Fe. The expected high nitrogen activity (from the dissociation of NH 3 ) on the surface of the particles together with the severe microstructural changes (cracks) are believed to be responsible for the observed high Sm 2 Fe 17 N 3 growth rate. With the use of NH 3 , high intrinsic coercivities (8 kOe or greater) can be achieved in a wide range of temperatures (375–500 °C).

On the atomic diffusion mechanism and diffusivity of nitrogen atoms in Sm2Fe17

Abstract An atomic diffusion mechanism (voidal diffusion) of nitrogen atoms in Sm2Fe17 is proposed which clearly explains the low values of diffusivity observed. The nitrogen atoms are located inside 9(e) octahedra which share Sm corners but no faces. Migration of nitrogen atoms 9(e) sites cannot occur by direct jumping. The most probable way for nitrogen atoms to migrate is by jumping from a 9(e) site into a thermodynamically unstable tetrahedral 18(g) site and subsequently into a new 9(e) site. In such a migration path a nitrogen atom has to surmount an enormous energy barrier representing the energy needed to overcome the strong bonding with its nearest neighbours (Fe and especially Sm atoms) and more importantly the strain energy needed to break out through the octahedral face (Fe(f)-Sm(c)-Fe(h)) and in through the tetrahedral face (Fe(h)-Sm(c)-Fe(h)). Although the 18(g) sites cannot accommodate any nitrogen atoms under equilibrium conditions, their presence plays a key role in the diffusion of nitrogen atoms. The present atomic diffusion mechanism predicts that the anisotropic ratio of the planar (Dxx) to the axial (Dzz) diffusivity should be equal to about 0.3. It also predicts that atoms such as hydrogen, which can occupy the 18(g) tetrahedra under equilibrium conditions, can jump inside the “circular tunnel” formed by the adjacent 18(g) tetrahedra, resulting in an increase in the planar diffusivity Dxx, and consequently the anisotropic ratio D xx D zz can reach values as large as 1.07 (essentially isotropic behaviour).

Liquidus projection surface and isothermal section at 1000 °C of the Co-Pr-B (Co-rich) ternary phase diagram*

The liquidus projection surface and the isothermal section at 1000 °C of the Co-Pr-B (Co-rich) ternary phase diagram have been determined. The binary and ternary intermetallics (Pr2Co17, PrCo5, Pr5Co19, Pr2Co7, PrCo3, PrCo2, C03B, C02B, CoB, P2Co14B, PrCo4B, PrCo12B, Pr3Co11B4, and Pr2Co7B3) that were examined in the Co-rich portion of the Co-Pr-B ternary phase diagram were found to be true line compounds (no detectable solid solubility). The primary solidification phase field of the Pr2Co14B intermetallic compound shares boundaries with the primary solidification phase fields of αCo, Pr2Co17, PrCo5, P1-C04B, and PrCo12B6 intermetallics. There are eight reactions associated with the Pr2Co14B intermetallic compound: two ternary eutectic reactions (E1 = Liquid ↔ Pr2Co14B + PrCo12B6 + PrCo4B and E2 = Liquid ↔ Pr2Co14B + PrCo12B6 + αCo), two pseudobinary eutectic reactions (e3 = Liquid ↔ Pr2Co14B + PrCo4B and e4 = Liquid ↔ Pr2Co14B + PrCo12B6), three ternary quasi-peritectic reactions (P1 = Pr2Co17 + Liquid ↔ Pr2Co14B + αCo, P2 = Pr2Co17 + Liquid ↔ PrCo5 + Pr2Co14B, and P3 = Pr2Co14B + Liquid ↔ PrCo4B + PrCo5), and one pseudobinary peritectic reaction (p8 = Pr2Co17 + Liquid ↔ Pr2Co14B). The composition of the magnetically important Pr2Co14B intermetallic falls inside the primary solidification phase field of the Pr2Co17 intermetallic. The reaction through which the Pr2Co14B is produced is therefore the pseudobinary peritectic reaction Pr2Co17 + Liquid ↔ P2Co14B. The PrCo12B6 and PrCo4B compounds are found to form congruently from the melt At the temperature of 1000 °C and depending on the alloy composition, the P2Co14B intermetallic can be found in solid-state thermodynamic equilibrium with one or two of the following phases: αCo, Pr2Co17, PrCo5, PrCo4B, and PrCo12B6. The obtained information about the Co-Pr-B phase diagram can be used to explain correctly all the phases present in the P2Co14B-based permanent magnets. The present work also emphasizes the extreme importance and usefulness of thermomagnetic measurements as an aid in the determination of phase diagrams that involve ferromagnetic phases.

Reaction of samarium with hydrogen and nitrogen samarium oxides

Abstract The reaction characteristics between Sm metal and H2 or N2 gas have been studied by means of isochorothermal analysis and X-ray diffraction. Samarium was found to react with hydrogen forming SmH2+x (0⩽×⩽0.63). The reaction is very abrupt, exothermic and takes place at a temperature of between 300 and 525°C depending on the initial size, surface contamination and surface structural defects of the samarium metal. Sm hydrides are partially decomposed into b.c.c. Sm2O3 under ambient conditions. The reaction of Samarium with N2 initiates at about 600°C and proceeds very slowly forming SmN. SmN hydrolyzes in the presence of atmospheric moisture to form b.c.c. Sm2O3, hex-Sm(OH)3 and NH3. Sm oxide surface layers containing f.c.c. SmO, b.c.c. Sm2O3 and monoclinic Sm2O3 were found to form upon heat treatment of Sm metal under vacuum or argon gas flow.