Sm2Fe17 나노분말제조를 위한 새로운 환원-확산 공정에 관한 연구

Abstract

Sm2Fe17Nx 영구자석재료는 J. M. D. Coey에 의해 발견된 이래로 최근에는 우수한 내식성 및 내열성을 바탕으로 Nd2Fe14B를 대체할 수 있는 재료로 각광 받고 있다. 그러나 Sm2Fe17Nx는 고온 열처리 시 상분리가 일어나는 특성으로 인해 현재 본드자석의 형태로만 그 응용이 제한되고 있다. 이를 해결하기 위해 소결자석 제조연구가 펄스통전소결 및 HIP 등의 공정을 통해 수행되었으나 실양산화 및 복잡형상 응용의 어려움이라는 또 다른 한계점을 드러내었다. 이러한 복합적인 한계점을 극복하기 위한 새로운 접근 방법으로 Sm2Fe17Nx 나노분말을 이용하는 방안을 제시할 수 있다. 나노분말은 미세한 분말 크기에 따른 증가된 비표면적으로 인한 소결 구동력의 향상으로 저온영역에서의 소결공정을 기대할 수 있기 때문이다. 일반적으로 Sm2Fe17Nx 분말은 1100oC 이상의 고온영역에서 액상반응을 통해 Sm2Fe17 분말 제조 후 분쇄공정과 400-500oC에서 질화공정을 거쳐 제조되며, 분말의 크기는 최초 Sm2Fe17 분말의 크기에 의존하는 특성을 갖는다. 그러나 이러한 고온영역에서의 액상반응은 나노크기의 분말 제조에 적합하지 않아 분말의 입자 성장을 억제하기 위한 저온영역에서의 새로운 공정의 개발이 이루어져야 한다. 이에 본 연구에서는 타 공정에 비해 경제적이고 간단하며 단시간에 Sm2Fe17 분말을 제조할 수 있는 top-down 개념의 기존 환원-확산 공정을 바탕으로 Sm2Fe17 나노분말을 제조하는 새로운 환원-확산(modified reduction-diffusion) 공정 연구를 수행하였다. 새로운 환원-확산 공정은 Sm2O3, Fe2O3, Fe 나노분말과 고체환원제인 CaH2를 원료분말로 사용하여 나노크기의 다양한 조성의 혼합 분쇄분말을 제조하고 이를 저온영역에서 Sm2Fe17 나노분말을 제조하는 bottom-up 개념으로 설계되었다 . 최초 Sm2O3-Fe2O3-CaH2 혼합 분쇄분말을 사용한 새로운 환원-확산 공정에서 고체환원제로 사용되는 CaH2는 대기 중에서 취급 시 H2O와의 반응으로 인해 Ca(OH)2로의 상변화가 일어났으며, 이를 바탕으로 Ar의 보호분위기내에서 원료분말을 취급하고 공정 분위기 가스를 H2에서 Ar-5vol.%H2로 치환함으로써 수백 nm 크기의 결정립을 갖는 Sm2Fe17 화합물을 합성할 수 있었다. 그러나 공정 중 Fe2O3와 Sm2O3 및 CaO와의 반응에 의해 각각 SmFeO3 및 Ca2Fe2O5 상의 생성 반응이 Sm-Fe 화합물 생성 반응보다 우선적으로 일어남으로써 Sm2Fe17 화합물 생성 반응을 억제하는 것으로 확인되었다. Fe2O3의 반응에 의해 생성되는 SmFeO3와 Ca2Fe2O5의 복합산화물 생성 억제를 위해 Sm2O3-nano Fe-CaH2 혼합 분쇄분말을 이용하여 새로운 환원-확산 공정을 850-1100oC에서 진행한 결과, 혼합 분쇄분말 내 Fe 나노분말은 850oC 미만의 온도영역에서 입자간의 소결이 일어났으며, 기존 환원-확산 공정에서 보고된 바 없는 SmH2상이 850oC 부근에서 CaH2에 의해 환원된 Sm과 H2와의 반응에 의해 생성되었다. 이러한 SmH2는 공정온도가 950-1100oC로 증가함에 따라 Sm과 H2로 각각 분리가 일어났으며, 분리된 Sm 원자가 소결된 Fe 나노분말 응집체 표면으로 확산되면서 Sm2Fe17 화합물을 형성하였다. 이때, Sm2Fe17 화합물은 소결된 Fe 나노분말 응집체 표면으로부터 생성됨에 따라 Fe를 Sm2Fe17 화합물이 둘러싸고 있는 코어-쉘 구조의 분말형태를 나타내었다. SmH2의 분해특성을 고려하여 새로운 환원-확산 공정에 Ar-5vol.%H2의 가스분위기에서 약 1 Pa의 진공분위기로 치환하는 가스분위기를 적용한 결과, 800oC의 고상반응의 저온영역에서도 Sm2Fe17 화합물을 효과적으로 합성할 수 있었다. 또한 원료분말인 Sm2O3, Fe, CaH2가 균일하게 혼합될 수 있도록 미세구조를 제어한 수십 nm 크기의 Sm2O3-Fe-CaH2의 혼합 분쇄분말을 이용하여 새로운 환원-확산 공정을 600-800oC의 반응온도에 따라 진행한 결과, Th2Zn17형의 rhombohedral 구조를 가지며 500 nm 이하의 크기를 갖는 Sm2Fe17 나노분말을 700oC, 800oC 조건에서 안정적으로 제조할 수 있었다. 이렇게 제조된 Sm2Fe17 나노분말은 하이드라진을 이용하여 증류수 내 용존산소를 제거하는 개선된 수세공정을 통해 Sm2Fe17 나노분말 내 잔류산소를 2.53wt.%에서 0.85wt.%로 최소화할 수 있었다. 이때 분말의 자기적 특성은 보자력의 경우 1150 Oe, 포화자화의 경우 127 emu/g의 우수한 자기적 특성을 갖는 것으로 확인되었다. | Since J. M. D. Coey discovered the interstitial compound Sm2Fe17Nx ,it has attracted the attention of many researcher because as a permanent magnetic material, its theoretical magnetic properties are similar to those of Nd2Fe14B, and because it exhibits a high Curie temperature, strong uniaxial anisotropy, and good corrosion resistance. The usage of Sm2Fe17Nx, however, is limited to the zinc- or epoxy-resin-bonded magnet as it is decomposed into SmN and a-Fe at temperatures above 600oC. To overcome this problem, some previous researchers reported that sintered Sm2Fe17Nx magnets can be made through pulse-electric current sintering, HIP, etc., using Sm2Fe17Nx micron powder 3-20 mm in size. Their fabrication processes, however, are difficult to apply to net-shaped sintered magnets and are not suitable for mass production. To address this problem, the use of Sm2Fe17Nx nanopowders with excellent sinterability and magnetic property can be suggested as a breakthrough for the fabrication of a net-shaped bulk material that has near-full density at a low sintering temperature through a pressureless sintering process. In general, the Sm2Fe17Nx powders are produced through a two-step process consisting of the fabrication of Sm2Fe17 powder via the powder metallurgy method and the reduction-diffusion process at temperatures above 1100oC, and the subsequent nitrogenation process at 400-500oC, and the particle size of Sm2Fe17Nx depends on the particle size of Sm2Fe17 before nitrogenation. Most of the existing powder fabrication technologies, however, are thus far limited to the processing not of nano-sized powders but of micron-sized powders due to the high-temperature process of making Sm2Fe17 powder. The latter should certainly be performed through a low-temperature process to prevent the growth and agglomeration of Sm2Fe17 particles. In this study, to fabricate Sm2Fe17 nanopowder, the author considered a breakthrough technology, the so-called “modified reduction-diffusion (MRD) process” of the bottom-up concept, which is practicable through the use of a nanosized precursor and solid state reaction at a low temperature using Sm2O3, Fe2O3, and Fe nanopowder synthesized via ball-milling and hydrogen reduction, and CaH2 raw powder based on the conventional reduction-diffusion process of the top-down concept, which has economical advantages. The initial MRD process was performed to study the alloying behaviour of the Sm2Fe17 compound from the ball-milled powders of Sm2O3, Fe2O3, and a solid-reducing agent of CaH2 via SPEX mill. It was found that Sm2Fe17 can be produced by controlling the gas atmosphere in the process of powder preparation to a reduction-diffusion reaction. The powder handling of CaH2 in a protective atmosphere (Ar gas) is essential to prevent the formation of Ca(OH)2, which suppresses calcium formation. The switching-gas atmosphere of H2 to Ar-5vol.%H2 during the MRD process at 350oC resulted in the reduction of Fe2O3 and the alloying of Sm-Fe, consequently forming Sm2Fe17 several hundred nanometers in size. It was found, however, that the reduction of Sm2O3 and Fe2O3 is very difficult in the same MRD process because the SmFeO3 and Ca2Fe2O5 phases are formed earlier, through the reaction of Fe2O3, Sm2O3, and CaO, than the alloying of Sm-Fe. To prevent the formation of other compounds, such as SmFeO3 and Ca2Fe2O5, the MRD process was carried out to investigate the alloying process during the synthesis of Sm2Fe17 powder from ball-milled Sm2O3-CaH2 powder and Fe nanopowder (nano Fe). The Sm2O3-nano Fe-CaH2 mixed powders were subjected to heat treatment at 850-1100oC in Ar-5vol.%H2 for 5h. It was found that the Fe nanopowders in the mixed powders are sintered at temperatures below 850oC during the MRD process, and that SmH2 synthesized by reduced Sm combines with H2 at around 850oC. The results showed that SmH2 can separate Sm and H2 depending on the increase in the process temperature, and forms the Sm2Fe17 phase on the surface of the sintered Fe nanopowder agglomerates at temperatures of 950-1100oC. The formation of Sm2Fe17 layer is mainly due to the diffusion reaction of the Sm atoms into the sintered Fe nanopowder agglomerate at temperatures above 950oC. The Sm2Fe17 compound was effectively formed by the MRD process under the switching-gas atmosphere from Ar-5vol.%H2 to a vacuum atmosphere, which led to the decomposition of SmH2 and the subsequent reaction of Sm and Fe at a solid state reaction temperature of 800oC. It was also found that Sm2Fe17 nanopowder was fabricated by the MRD process using Sm2O3-Fe-CaH2 powder with a particle size of several tens of nanometers, and that its mixing homogeneity is maintained at 700-800oC under a switched atmosphere. The Sm2Fe17 powder was identified as a nanoscale powder with an average size of below 500 nm and a rhombohedral Th2Zn17-type (2:17R) structure. It was determined that the residual oxygen contents of Sm2Fe17 nanopowder can be minimized from 2.53 to 0.85wt.% through the modified washing process, which involves removing the dissolved oxygen in DI water using hydrazine (N2H4). The magnetic-property result revealed that the coercivity and saturation magnetization of Sm2Fe17 nanopowder are 1152 Oe and 127 emu/g, respectively. It is concluded that the fabrication of Sm2Fe17 by the solid state MRD process using a nanoprecursor at a low temperature will pave the way for a breakthrough technology for the fabrication of Sm2Fe17Nx nanopowder.; Since J. M. D. Coey discovered the interstitial compound Sm2Fe17Nx ,it has attracted the attention of many researcher because as a permanent magnetic material, its theoretical magnetic properties are similar to those of Nd2Fe14B, and because it exhibits a high Curie temperature, strong uniaxial anisotropy, and good corrosion resistance. The usage of Sm2Fe17Nx, however, is limited to the zinc- or epoxy-resin-bonded magnet as it is decomposed into SmN and a-Fe at temperatures above 600oC. To overcome this problem, some previous researchers reported that sintered Sm2Fe17Nx magnets can be made through pulse-electric current sintering, HIP, etc., using Sm2Fe17Nx micron powder 3-20 mm in size. Their fabrication processes, however, are difficult to apply to net-shaped sintered magnets and are not suitable for mass production. To address this problem, the use of Sm2Fe17Nx nanopowders with excellent sinterability and magnetic property can be suggested as a breakthrough for the fabrication of a net-shaped bulk material that has near-full density at a low sintering temperature through a pressureless sintering process. In general, the Sm2Fe17Nx powders are produced through a two-step process consisting of the fabrication of Sm2Fe17 powder via the powder metallurgy method and the reduction-diffusion process at temperatures above 1100oC, and the subsequent nitrogenation process at 400-500oC, and the particle size of Sm2Fe17Nx depends on the particle size of Sm2Fe17 before nitrogenation. Most of the existing powder fabrication technologies, however, are thus far limited to the processing not of nano-sized powders but of micron-sized powders due to the high-temperature process of making Sm2Fe17 powder. The latter should certainly be performed through a low-temperature process to prevent the growth and agglomeration of Sm2Fe17 particles. In this study, to fabricate Sm2Fe17 nanopowder, the author considered a breakthrough technology, the so-called “modified reduction-diffusion (MRD) process” of the bottom-up concept, which is practicable through the use of a nanosized precursor and solid state reaction at a low temperature using Sm2O3, Fe2O3, and Fe nanopowder synthesized via ball-milling and hydrogen reduction, and CaH2 raw powder based on the conventional reduction-diffusion process of the top-down concept, which has economical advantages. The initial MRD process was performed to study the alloying behaviour of the Sm2Fe17 compound from the ball-milled powders of Sm2O3, Fe2O3, and a solid-reducing agent of CaH2 via SPEX mill. It was found that Sm2Fe17 can be produced by controlling the gas atmosphere in the process of powder preparation to a reduction-diffusion reaction. The powder handling of CaH2 in a protective atmosphere (Ar gas) is essential to prevent the formation of Ca(OH)2, which suppresses calcium formation. The switching-gas atmosphere of H2 to Ar-5vol.%H2 during the MRD process at 350oC resulted in the reduction of Fe2O3 and the alloying of Sm-Fe, consequently forming Sm2Fe17 several hundred nanometers in size. It was found, however, that the reduction of Sm2O3 and Fe2O3 is very difficult in the same MRD process because the SmFeO3 and Ca2Fe2O5 phases are formed earlier, through the reaction of Fe2O3, Sm2O3, and CaO, than the alloying of Sm-Fe. To prevent the formation of other compounds, such as SmFeO3 and Ca2Fe2O5, the MRD process was carried out to investigate the alloying process during the synthesis of Sm2Fe17 powder from ball-milled Sm2O3-CaH2 powder and Fe nanopowder (nano Fe). The Sm2O3-nano Fe-CaH2 mixed powders were subjected to heat treatment at 850-1100oC in Ar-5vol.%H2 for 5h. It was found that the Fe nanopowders in the mixed powders are sintered at temperatures below 850oC during the MRD process, and that SmH2 synthesized by reduced Sm combines with H2 at around 850oC. The results showed that SmH2 can separate Sm and H2 depending on the increase in the process temperature, and forms the Sm2Fe17 phase on the surface of the sintered Fe nanopowder agglomerates at temperatures of 950-1100oC. The formation of Sm2Fe17 layer is mainly due to the diffusion reaction of the Sm atoms into the sintered Fe nanopowder agglomerate at temperatures above 950oC. The Sm2Fe17 compound was effectively formed by the MRD process under the switching-gas atmosphere from Ar-5vol.%H2 to a vacuum atmosphere, which led to the decomposition of SmH2 and the subsequent reaction of Sm and Fe at a solid state reaction temperature of 800oC. It was also found that Sm2Fe17 nanopowder was fabricated by the MRD process using Sm2O3-Fe-CaH2 powder with a particle size of several tens of nanometers, and that its mixing homogeneity is maintained at 700-800oC under a switched atmosphere. The Sm2Fe17 powder was identified as a nanoscale powder with an average size of below 500 nm and a rhombohedral Th2Zn17-type (2:17R) structure. It was determined that the residual oxygen contents of Sm2Fe17 nanopowder can be minimized from 2.53 to 0.85wt.% through the modified washing process, which involves removing the dissolved oxygen in DI water using hydrazine (N2H4). The magnetic-property result revealed that the coercivity and saturation magnetization of Sm2Fe17 nanopowder are 1152 Oe and 127 emu/g, respectively. It is concluded that the fabrication of Sm2Fe17 by the solid state MRD process using a nanoprecursor at a low temperature will pave the way for a breakthrough technology for the fabrication of Sm2Fe17Nx nanopowder.