Fig. 3
Thermomagnetic Property and Phase Evolution of Nitrided Iron Powder S. Lan, Z. Feng, P. Moradifar, F. Ernst, D. H. Matthiesen, and M. A. Willard Materials Science and Engineering, Case Western Reserve University, Cleveland, OH Abstract
Introduction (continued) X-ray Diffraction
Iron nitrides are magnetic compounds with potential to replace rare earth permanent magnets in some applications. Our study examines the processing of iron nitrides to form the most desirable, yet metastable, magnetic iron nitride phase, α’’-Fe16N2. Nitridation has been performed on commercial Höganäs® AHC 100.29 iron powders by a custom nitridation furnace. X-ray diffraction and density measurement have been used to characterize the initial and nitrided materials. Processing window of nitridation for highly pure nitrogen austenite has been determined both by X-ray Diffractometer (XRD) and Vibrating Sample Magnetometry (VSM). Thermomagnetic property and temperature-dependent phase evolution of nitrided iron powders has been studied by high temperature VSM technique.
Thermomagnetic Measurement M(T)
Vol.% of NH3
I
IV
Fig. 5. Indexed X-ray diffraction patterns for iron powders after nitriding at 650 °C for 1 hour in NH3 / H2 gas with different volume ratios. Peaks for γ-(Fe,N) phase are present when vol.% of NH3 exceeds 5 vol.%. And peaks for γ’-Fe4N phase are present when vol.% of NH3 is higher than 12 vol.%. Fig. 3. Iron-nitrogen binary phase diagram with a schematic curve for Martensite starting temperature Ms. The yellow dashed line corresponds to nitrogen content in an iron nitride sample for thermomagnetic measurement.
Sample Synthesis
The sample nitrided at 650 °C for 1 hour in 11% NH3 / 89% H2 was used for further thermomagnetic measurement M(T). This sample is multiphase with 2% α, 96% γ and 2% γ’ in volume fraction. Based on lattice parameter of γ-(Fe,N) from XRD data, nitrogen content in γ-(Fe,N) is 9.8 at.%. CN(α) = 0 at.% CN(γ) = 9.8 at.% CN(γ’) = 20 at.%
2% α + 96% γ + 2% γ’ Fig. 4. A three-step procedure to synthesize bulk α’’-Fe16N2 phase.
Magnetization at Room Temperature
i. Sieving Höganäs® AHC iron powders to particle size less than 20 μm ii. Oxide removal in flowing H2 at 650 °C for 1 hour iii. Nitriding the reduced iron powder at 650 °C for 1 hour in flowing NH3 / H2 gas mixture with a specific volume ratio iv. Remove by chilled water
α
I Decomposition II Magnetic transition at 480°C III Hyper-eutectoid transformation IV Magnetic transition at 770 °C
γ-(Fe,N) → α + γ’ γ’(Ferro) → γ’(Para) α + γ’ → γ + γ’ α (Para) → α (Ferro)
CN ≈ 9.8 at.%
A processing window for highly pure γ-(Fe,N) was determined as 650 °C for 1 hour in 89% NH3 / 11% H2 using commercial iron powder. Near-zero magnetization at 1.5 T confirmed the purity of synthesized γ-(Fe,N) powders.
γ
Decomposition of metastable γ-(Fe,N) starts around 200 °C as determined by increase of magnetization during thermomagnetic measurement M(T). Magnetic transition of iron at 770 °C was only observed in M(T) cooling down curve of M(T).
Acknowledgement Dr. M. Daniil at CWRU for VSM measurement Project funded by ARPA-E
Experiments AccuPyc II 1340 Series Pycnometers
Selected References
Scintag X-1 advanced X-ray diffractometer Lake Shore 4710 Vibrating Sample Magnetometry
Results Density for as-received AHC 100.29 Iron Powder Measured Density (7.83 ± 0.05) g/cm3 Theoretical Density Fig. 2. Unit cell of α’’-Fe16N2
Possible reactions during M(T) measurement in Fig. 7
Conclusion
Fig. 1. Slater-Pauling curve with atomic Fe moment for α’’-Fe16N2
α’’-Fe16N2 is an ordered Martensite with tetragonal structure. Nitrogen atoms occupy octahedral sites preferentially in a specific way as shown by these blue octahedrons in an unit cell of α’’-Fe16N2 in Fig. 2.
Fig. 7. M(T) under applied field of 1 T up to 850 °C of AHC iron powder after nitriding at 650 °C for 1 hour in 11% NH3 / 89 %H2.
Average at.% N in Sample for M(T)
This study focuses on the first step to produce highly pure γ-(Fe,N)
4 𝐼 𝑚𝑚 𝑚
II
III
Introduction The figure of merit for permanent magnets is energy product (BH)max which could be improved by increasing saturation magnetization and coercivity. α’’-Fe16N2, which was reported with a giant magnetic moment 3.0 μB/Fe atom and magnetocrystalline anisotropy similar in magnitude (106 J/m3) to the hard magnet Fe14Nd2B, has potential to be designed as a good rare-earth-free permanent magnet.
Results (continued)
Results
(7.8766 ± 0.0003) g/cm3
Lower density could be caused by surface oxidation of iron particles
Fig. 6. Magnetization at 1.5 T plotted against volume fraction of NH3 in NH3 / H2 nitriding gas mixture for AHC iron powders after nitridation at 650 °C for 1hour. The observed decrease of magnetization is caused by formation of paramagnetic γ-(Fe,N) phase. Near-zero magnetization from 10 vol.% to 11.5 vol.% of NH3 indicates a highly pure γ-(Fe,N) phase. A magnetization increase after 12 vol.% of NH3 suggests a formation of a ferromagnetic phase, which is γ’-Fe4N according to XRD patterns in Fig 5.
[1] Coey, J. M. D. (2011). Magnetism and Magnetic Materials. Cambridge New York, Cambridge University Press. [2] Kim, T. K., and Takahashi, K. (1972). “New Magnetic Material Having Ultrahigh Magnetic Moment.” Appl. Phys. Lett., 20: 492-494. [3] Pearson, W. B., Villars, P., and Calvert, L. D. (1985). Pearson's Handbook of Crystallographic Data for Intermetallic Phases. Metals Park, Oh: American Society for Metals. [4] Wriedt, H. A., Gokcen, N. A., and Nafziger R. H. (1987). “The Fe-N (Iron-Nitrogen) System.” Bulletin of Alloy Phase Diagram 8: 355-377. [5] Bell, T. (1968). "Martensite Transformation Start Temperature in Iron-Nitrogen Alloys." J. Iron Steel Inst., 206: 1017-1021.
Introduction (continued) X-ray Diffraction
Iron nitrides are magnetic compounds with potential to replace rare earth permanent magnets in some applications. Our study examines the processing of iron nitrides to form the most desirable, yet metastable, magnetic iron nitride phase, α’’-Fe16N2. Nitridation has been performed on commercial Höganäs® AHC 100.29 iron powders by a custom nitridation furnace. X-ray diffraction and density measurement have been used to characterize the initial and nitrided materials. Processing window of nitridation for highly pure nitrogen austenite has been determined both by X-ray Diffractometer (XRD) and Vibrating Sample Magnetometry (VSM). Thermomagnetic property and temperature-dependent phase evolution of nitrided iron powders has been studied by high temperature VSM technique.
Thermomagnetic Measurement M(T)
Vol.% of NH3
I
IV
Fig. 5. Indexed X-ray diffraction patterns for iron powders after nitriding at 650 °C for 1 hour in NH3 / H2 gas with different volume ratios. Peaks for γ-(Fe,N) phase are present when vol.% of NH3 exceeds 5 vol.%. And peaks for γ’-Fe4N phase are present when vol.% of NH3 is higher than 12 vol.%. Fig. 3. Iron-nitrogen binary phase diagram with a schematic curve for Martensite starting temperature Ms. The yellow dashed line corresponds to nitrogen content in an iron nitride sample for thermomagnetic measurement.
Sample Synthesis
The sample nitrided at 650 °C for 1 hour in 11% NH3 / 89% H2 was used for further thermomagnetic measurement M(T). This sample is multiphase with 2% α, 96% γ and 2% γ’ in volume fraction. Based on lattice parameter of γ-(Fe,N) from XRD data, nitrogen content in γ-(Fe,N) is 9.8 at.%. CN(α) = 0 at.% CN(γ) = 9.8 at.% CN(γ’) = 20 at.%
2% α + 96% γ + 2% γ’ Fig. 4. A three-step procedure to synthesize bulk α’’-Fe16N2 phase.
Magnetization at Room Temperature
i. Sieving Höganäs® AHC iron powders to particle size less than 20 μm ii. Oxide removal in flowing H2 at 650 °C for 1 hour iii. Nitriding the reduced iron powder at 650 °C for 1 hour in flowing NH3 / H2 gas mixture with a specific volume ratio iv. Remove by chilled water
α
I Decomposition II Magnetic transition at 480°C III Hyper-eutectoid transformation IV Magnetic transition at 770 °C
γ-(Fe,N) → α + γ’ γ’(Ferro) → γ’(Para) α + γ’ → γ + γ’ α (Para) → α (Ferro)
CN ≈ 9.8 at.%
A processing window for highly pure γ-(Fe,N) was determined as 650 °C for 1 hour in 89% NH3 / 11% H2 using commercial iron powder. Near-zero magnetization at 1.5 T confirmed the purity of synthesized γ-(Fe,N) powders.
γ
Decomposition of metastable γ-(Fe,N) starts around 200 °C as determined by increase of magnetization during thermomagnetic measurement M(T). Magnetic transition of iron at 770 °C was only observed in M(T) cooling down curve of M(T).
Acknowledgement Dr. M. Daniil at CWRU for VSM measurement Project funded by ARPA-E
Experiments AccuPyc II 1340 Series Pycnometers
Selected References
Scintag X-1 advanced X-ray diffractometer Lake Shore 4710 Vibrating Sample Magnetometry
Results Density for as-received AHC 100.29 Iron Powder Measured Density (7.83 ± 0.05) g/cm3 Theoretical Density Fig. 2. Unit cell of α’’-Fe16N2
Possible reactions during M(T) measurement in Fig. 7
Conclusion
Fig. 1. Slater-Pauling curve with atomic Fe moment for α’’-Fe16N2
α’’-Fe16N2 is an ordered Martensite with tetragonal structure. Nitrogen atoms occupy octahedral sites preferentially in a specific way as shown by these blue octahedrons in an unit cell of α’’-Fe16N2 in Fig. 2.
Fig. 7. M(T) under applied field of 1 T up to 850 °C of AHC iron powder after nitriding at 650 °C for 1 hour in 11% NH3 / 89 %H2.
Average at.% N in Sample for M(T)
This study focuses on the first step to produce highly pure γ-(Fe,N)
4 𝐼 𝑚𝑚 𝑚
II
III
Introduction The figure of merit for permanent magnets is energy product (BH)max which could be improved by increasing saturation magnetization and coercivity. α’’-Fe16N2, which was reported with a giant magnetic moment 3.0 μB/Fe atom and magnetocrystalline anisotropy similar in magnitude (106 J/m3) to the hard magnet Fe14Nd2B, has potential to be designed as a good rare-earth-free permanent magnet.
Results (continued)
Results
(7.8766 ± 0.0003) g/cm3
Lower density could be caused by surface oxidation of iron particles
Fig. 6. Magnetization at 1.5 T plotted against volume fraction of NH3 in NH3 / H2 nitriding gas mixture for AHC iron powders after nitridation at 650 °C for 1hour. The observed decrease of magnetization is caused by formation of paramagnetic γ-(Fe,N) phase. Near-zero magnetization from 10 vol.% to 11.5 vol.% of NH3 indicates a highly pure γ-(Fe,N) phase. A magnetization increase after 12 vol.% of NH3 suggests a formation of a ferromagnetic phase, which is γ’-Fe4N according to XRD patterns in Fig 5.
[1] Coey, J. M. D. (2011). Magnetism and Magnetic Materials. Cambridge New York, Cambridge University Press. [2] Kim, T. K., and Takahashi, K. (1972). “New Magnetic Material Having Ultrahigh Magnetic Moment.” Appl. Phys. Lett., 20: 492-494. [3] Pearson, W. B., Villars, P., and Calvert, L. D. (1985). Pearson's Handbook of Crystallographic Data for Intermetallic Phases. Metals Park, Oh: American Society for Metals. [4] Wriedt, H. A., Gokcen, N. A., and Nafziger R. H. (1987). “The Fe-N (Iron-Nitrogen) System.” Bulletin of Alloy Phase Diagram 8: 355-377. [5] Bell, T. (1968). "Martensite Transformation Start Temperature in Iron-Nitrogen Alloys." J. Iron Steel Inst., 206: 1017-1021.
