Structural, Magnetic, and MÃ¶ssbauer Investigation of Ordered Iron

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Structural, magnetic and Mössbauer investigation of ordered iron nitride with martensitic structure obtained from amorphous hematite synthesized via microwave route Petru Palade, Carmen Gabriela Plapcianu, Ionel Mercioniu, Cezar Catalin Comanescu, Gabriel Alexandru Schinteie, Aurel Leca, and Ruxandra Vidu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04574 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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Industrial & Engineering Chemistry Research

Structural, magnetic and Mössbauer investigation of ordered iron nitride with martensitic structure obtained from amorphous hematite synthesized via microwave route 1

1*

1

1

1

Petru Palade , Carmen Plapcianu , Ionel Mercioniu , Cezar Comanescu , Gabriel Schinteie , Aurel 1

Leca , Ruxandra Vidu 1

2

National Institute of Materials Physics, 405A Atomistilor Str., P O. Box MG-7, 077125, Magurele, Ilfov

Romania 2

*

University of California, 3123 Bainer Hall, Davis, CA 95616, USA

Corresponding Author: Dr. Carmen Plapcianu

National Institute of Materials Physics, 405A Atomistilor Str., P.O. Box MG-7, 077125 Magurele, Ilfov, Romania, Phone: +40 21 369 0170/146; Fax: +40 21 369 0177; E-mail: [email protected] Submitted to Industrial & Engineering Chemistry Research

1

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Abstract Amorphous hematite synthesized by a simple and fast microwave route was used to obtain Fe16N2 fine particles by reducing in 5% H2/Ar gas flow, followed by long time nitridation in ammonia gas flow at °

temperatures below 200 C. Depending on nitridation temperature, various amount of metallic iron was ’’

present along with α -Fe16N2 main phase. A small amount of iron oxide was observed by Mössbauer spectroscopy but it was undetected by X-ray diffraction due to its high degree of amorphization. °

Increased amounts of Fe3N and Fe4N phases were observed at a nitridation temperature above 150 C, which had detrimental effect on the magnetic properties. Structural information and phase composition were extracted from Rietveld refinement of the XRD data. Values of the magnetization at saturation °

’’

measured at 40 kOe and 25 C of 222 emu/g for α -Fe16N2 and 192 emu/g for metallic iron were obtained via magnetic measurements, Rietveld and Mössbauer analysis.

Keywords: microwave heating, iron nitride; martensite; ammonia; Mössbauer spectroscopy

1. Introduction The search for inexpensive permanent magnets to replace the expensive rare-earth elements became imperative in the last years due to increased price and limited resources of rare-earth elements. Fe16N2 is a promising material for permanent magnets that can be obtain by inexpensive methods. Microwave assisted technique represents a simple, mild, rapid and environmentally friendly way of processing various materials, which implies the use of microwave radiation for heating materials 1

2

containing electrical charges such as polar molecule in the solvent or charge ion in the solid state . Compared to the other heating techniques, the microwave assisted solution fabrication methods received an increased attention due to its rapid processing, high reaction rate, reduced reaction time and high yield 3

II

of the final product. Wang et al. reported the synthesis of high crystalline M Fe2O4 (M = Co, Mn, Ni) cubical spinel structure in a short time of just 10 min exposure of the precursors to microwave radiation. Microwave heating was also used for the synthesis of magnetite (Fe3O4) and hematite (α-Fe2O3), starting 4

from FeCl3, polyethylene glycol and N2H4.H2O as precursors . The microwave heating process of urea aqueous solution mixed with iron salt proved to be an environment friendly process and a high yield 5

synthesis method for the α-Fe2O3 nanoparticles . The Fe–N system shows a broad range of structural and magnetic properties depending on the ’’

nitrogen content. Among them, there is an ordered nitride compound in the iron-rich region, i.e. α -Fe16N2, 6

7

which crystallizes in the body centered tetragonal (bct) structure . Bhattacharyya has extensively 8

reviewed the iron nitrides system. Kim and Takahashi observed a giant magnetic moment of 3.0 µB per iron atom for Fe16N2 thin film that was deposited on glass substrate through a thermal evaporation synthetic process. Since then, the magnetic properties of Fe16N2 thin film have been investigated 9-13

extensively

. It has been found that the values of the average magnetic moment per iron atom of

2


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Fe16N2 are in a range of 2.4–3.4 µB, which makes a promising magnetic material for producing inexpensive rare-earth free magnets. The preparation of bulk materials containing high amounts of Fe16N2 is a difficult task. Only recently, 14

Kikkawa et al.

’’

synthesized high purity α -Fe16N2 powders using a low temperature nitridation process of

the commercially available γ-Fe2O3 of 30 nm in size. Up to date, the maximum value of the magnetization °

’’

15

at saturation was 226 emu/g at 25 C for fine particles of α -Fe16N2 . Various preparation methods such 16

17

as: coating with hydroxyapatite , preparation of nanocomposite fibers via magneto-electrospinning , synthesis of core-shell Fe16N2/Al2O3 by one-step radio-frequency thermal plasma process using iron 18

pentacarbonyl precursor , ball milling of iron powder together with ammonium nitrate followed by shock compaction approach

19

were investigated to produce Fe16N2 particles. Another technique based on

nitrogen ion-implantation was used by Jiang et al.

20

to obtain free-standing foils with energy product of 20

MGOe at ambient temperature. Densely consolidated samples were obtained by high-pressure sintering ’’

21

of α -Fe16N2 particles . ’’

Many theoretical works have been carried out to clarify the giant magnetic moments of α -Fe16N2. Most of the calculations provide an average magnetic moment in the range between 2.3–2.9 µB per Fe atom

22, 23

’’

for α -Fe16N2. The precursor used in the nitridation process is of maximum importance when ’’

high yield of α -Fe16N2 with good magnetic properties (coercivity and magnetization at saturation) is sought. Besides the size and shape of the particles, as resulted from Rietveld refinements

24

and SEM

images, composition and porosity are other factors which strongly influence the nitridation process. In the literature, various methods to obtain iron oxide precursors for the synthesis of α”-Fe16N2 phase 25

were reported. However, methods such as vapor growth of γ - Fe2O3 , complicated chemical synthesis using iron pentacarbonyl 200°C)

27

26

(toxic material), or long time preparation by hydrothermal technique (48 h at

are difficult. Instead, iron oxide precursors are synthesized in a very short time (few minutes) by

microwave method, the experimental set-up is inexpensive, it uses environmental friendly raw materials and allows the production of nanosize particles that are easy to reduce and to nitridate. ’’

The present work describes a microwave assisted process of preparing α -Fe16N2 nanoparticles starting from a mixture of iron nitrate and urea solutions. This approach was used to prove that the microwave assisted technique is a fast and simple processing method, allowing the synthesis of very ’’

small, amorphous, iron oxide particles, which are precursors for α -Fe16N2 with good magnetic properties. 2. Experimental Chemical reagents were provided by Alfa Aesar of analytical grade (99% purity) and were used without further purification. To obtain the iron oxide powder, a mixture of 50 ml of 0.26M Fe(NO3)39H2O and 0.4M of CO(NH2)2 was sonicated in an ultrasound bath for 5 minutes to help the urea dissolve fast. Double distilled water was used to solubilize both iron nitrate and urea.

3



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After complete dissolution, the solution was irradiated under 540W microwave for 5 min. Then, the precipitate was separated by centrifugation, washed with deionized distilled water, and dried in oven at 35°C for about 6h to obtain a solid yellow-brick powder sample. During this process, the chemical reaction that took place is as follows:

5 CO(NH2)2 + 2Fe(NO3)3

α-Fe2O3 + 5CO2 + 10 H2O + 8N2

(1)

The iron oxide obtained after the microwave treatment was further annealed in a tubular furnace °

at 420 C for 10 hours in a reducing atmosphere of 5%H2+95% Ar flow. ’’

The nitridation of the powder to synthesize the α -Fe16N2 type compounds was performed in °

°

ammonia flow at various temperatures in the range 130 C ÷160 C for 48 h. Regarding the possibility of shorten the reduction and nitridation processing time, it is quite difficult to obtain good magnetic properties of the final phase if the precursor processing time is reduced. For example, highly dispersive α”-Fe16N2 NP have been produced during 24 h nitridation time but the magnetization at saturation was only 173 emu/g at ambient temperature magnetization at saturation of 190 emu/g

18

16

while α”-Fe16N2 NP with

were obtained using a radio-frequency thermal plasma

process of the iron pentacarbonyl precursor and the nitridation time was 10 h. Starting from vapor grown γ 14

- Fe2O3 powder with crystallite size of 30 nm Kikkawa et al.

studied the influence of the nitridation time

and temperature on the magnetic properties of the resulted α”-Fe16N2 nanoparticles. They obtained the maximum value of the magnetization at saturation after nitridation time of 100 h while only three quarters of the maximum value was obtained for a 50 h nitridation time at the same temperature. It is worth mentioning that the nitridation process took place immediately after annealing in H2/Ar flow, without removing the sample from the furnace to prevent the oxidation of the metallic iron and to ’’

6

favor the synthesis of the α -Fe16N2. As reported in literature , at temperatures above 180°C, α”-Fe16N2 bulk phase starts to transform into Fe4N. For α”-Fe16N2 in nanosize form such as nanoparticles, the 26

temperature of this undesirable transformation is even lower . For safety, in order to avoid this °

transformation, the temperature before switching to ammonia flow must be below 100 C. Moreover, during changing the gas from H2/Ar to NH3, a small amount of oxygen can enter in the system. It is better to switch between gases at temperatures approaching ambient in order to diminish the danger of oxidation which increases with the temperature. After nitridation, the samples were cooled down to ambient temperature and purged with nitrogen gas before removing them from the furnace. The chart flow of the entire process is illustrated in Figure 1.

4


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Mixing and solubilization Fe(NO3)39H2O and urea CO(NH2)2 Microwave irradiation 540 W microwave for 5 min Drying 35 oC for 6 hours Annealing heating in 5%H2+95% Ar flow at 420 oC for 10 h Nitridation heating in amonia flow at 130-160 oC for 48 h Powder Fe16N2 Characterization XRD, SEM, Mössbauer Spectroscopy, SQUID ’’

Figure 1. Flowchart of the processes involved in the formation of α -Fe16N2 nanoparticles for permanent magnets.

The nanoparticles crystalline structure was investigated by X-ray diffraction using a Bruker D8 Advance diffractometer with Cu Kα radiation. The particle morphology in various stages of preparation has been investigated using a Lyra-Tescan SEM apparatus. Further on, the composition and the magnetic behavior of the iron nitride were analyzed by Mössbauer Spectroscopy using a Co

57

radioactive source

and an integrated system SEECO spectrometer operating under constant acceleration mode. The Mössbauer spectra measured at ambient temperature were decomposed in spectral components using

5



NORMOS program. Hysteresis loops at 25°C were acquired with SQUID magnetometer (Quantum Design) running under RSO (Reciprocate Sample Option) mode.

3. Results and discussion The amorphous powder obtained by microwave irradiation has been processed directly by annealing °

in H2/ Ar gas flow reducing atmosphere, for 10 hours at 420 C, followed by nitridation in ammonia gas °

°

flow in the temperature range 130 C ÷160 C. After purging with nitrogen the powder was extracted from the furnace and investigated to determine the phase composition, morphology, and magnetic properties.

3.1. XRD Characterization Figure 2 shows the XRD spectra of the material in each stage of the synthesis process.

* Fe16N2 + Fe α−Fe2O3

* +* Intensity (a.u.)

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*

*

+

*

* *+ (D)

+

+

+

(C)

(B)

(A) 20

30

40

50

60 0

70

80

90

2Θ( ) Figure 2. X-ray diffraction patterns of various samples: (A) - as prepared in microwave furnace, (B) - after o

0

heat treatment in air at 400 C, (C) – after annealing in 5% H2 + 95% Ar at 420 C and (D)- annealed and o

nitrided at 140 C for 48h.

The XRD spectrum presented in Figure 2A of the sample obtained after the microwave step is typical for °

an amorphous material. The amorphous powder heated in air for 4 h at 400 C undergoes a complete transformation to pure hematite (JCPDS file 71-5088) (Figure 2B). Pure and well crystallized bcc Fe °

(Figure 2C) was obtained by direct annealing for 10 h at 420 C in 5% H2/ 95% Ar (5.0 purity) gas flow of the sample obtained by microwave irradiation.

6


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(A)

(B)

(C)

(D)

0

Figure 3. Rietveld refinements of samples reduced at 420 C and nitrided under ammonia flow at 0 0 0 0 130 C (A), 140 C (B), 150 C (C), 160 C (D).

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’’

Ordered iron nitride powder of α -Fe16N2 (JCPDS file 78-1865) with martensite structure was °

obtained directly from amorphous hematite (Figure 2A) by heating for 10h at 420 C in 150 ml/min 5% H2 / °

95% Ar gas flow, followed by heating for 48 h at 130 C or higher temperatures in 50 ml/min NH3 (5.0 °

purity) gas flow. Therefore, the additional step of heating the amorphous hematite in air for 4 h at 400 C is ’’

not required for α -Fe16N2 synthesis.

Table 1. Calculated parameters for Rietveld refinements of X-Ray diffraction spectra for samples ° ° ° ° MW-130 C, MW-140 C, MW-150 C, MW-160 C and the fit reliability parameters (GOF is the goodness-of-fit test and Rwp and Rb are the weighted-profile and Bragg-intensity R-values, respectively)

Sample

MW-130 oC

Crystalline phases α’’-Fe16N2 Fe bcc

MW-140 oC

α’’-Fe16N2 Fe bcc α’’-Fe16N2

MW-150 oC

a=0.5717(2), c=0.6280(3)

size (nm)

48(5)

Relative

Fit

Amount

reliability

(%)

parameters

45(5)

GOF=1.033 Rwp=1.02%

a=0.2865(1) a=0.5715(2), c=0.6288(3) a=0.2866(2) a=0.5712(3), c=0.6286(4)

76(5)

55(5)

65(4)

75(5)

86(8)

25(2)

66(3)

83(5)

Rb=0.83% GOF=1.21 Rwp=1.19% Rb=0.95%

GOF=1.41

Fe bcc

a=0.2864(2)

91(8)

11(2)

Rwp=1.38%

Fe4N

a=0.3790

20

2(1)

Rb=1.10%

Fe3N

a=0.4717, c=0.4405

20(10)

4(2)

60(3)

64(5)

α’’-Fe16N2 MW-160 oC

Lattice constants (nm)

Crystallite

a=0.5716(2), c=0.6289(3)

Fe bcc

a=0.2866(2)

90(20)

12(2)

Fe4N

a=0.3789(4)

20(10)

6(2)

25(10)

18(4)

Fe3N

a=0.4717(9), c=0.4405(9) 8


GOF=1.065 Rwp=1.05% Rb=0.85%

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’’

A mixture of α -Fe16N2 + bcc Fe or even additional Fe3N (JCPDS file 83-0877) and Fe4N (JCPDS file 71-4452) were obtained, depending on the nitriding temperature. Figure 2D shows the XRD pattern of °

the sample that was annealed for 10 h at 420 C in 5% H2 / 95% Ar flow, cooled down to ambient °

temperature and then nitrided in ammonia gas flow at 140 C, without exposing the powder to air. The ’’

main phase obtained at this temperature was α -Fe16N2, but there is a detectable amount of bcc Fe. The ’’

0

0

two most intense peaks belonging to α -Fe16N2 are located at 2Θ = 42.698 (hkl = 202) and 2Θ = 44.778 ’’

(hkl = 220). The ratio of the intensities of (202) and (220) reflections for pure α -Fe16N2 phase should 0

approach 2:1. The main XRD peak of bcc Fe, positioned at 2Θ = 44.850 (hkl = 110), overlaps with the ’’

(220) reflection of α -Fe16N2 and causes a deviation from the theoretical intensity ratio of 2:1 between the ’’

’’

two peaks belonging to α -Fe16N2. As shown in Figure 2D, besides α -Fe16N2 and bcc Fe, XRD did not detect any other amorphous or crystalline phases. The nitridation process in ammonia flow of the annealed powder samples was studied at various temperatures to understand the formation of the Fe16N2 phase versus the heating temperature. Table 1 presents the parameters obtained from Rietveld refinements (MAUD °

°

°

24

program) of the XRD spectra for

°

the samples nitrided at 13 C, 140 C, 150 C and 160 C, respectively (Figure 3). Sample labeling °

°

°

°

corresponds to the nitridation temperature, i.e MW-130 C, MW-140 C, MW-150 C, MW-160 C. °

°

’’

The samples nitrided at 140 C and 150 C contain α -Fe16N2 (ICSD file No. 189827) and bcc Fe °

°

(ICSD file No. 76747). For the samples nitrided at 150 C and 160 C, besides Fe16N2 and bcc Fe phases, there are also present the Fe3N (ICSD file No. 24650) and the Fe4N (ICSD file No. No. 74747) phases. ’’

According to the Fe-N phase diagram, Fe4N will be formed by α -Fe16N2 decomposition into Fe and Fe4N at higher temperatures. However, Fe4N does not have any contribution in the right-side 0

’’

shoulder (at higher 2Θ) of the peak 2Θ = 42.698 (hkl = 202) corresponding to α -Fe16N2, but, exactly in this region, appears the main peak of Fe3N. The presence of Fe3N could be explained by long nitridation time (48 h) and small dimension of the particles. The amount of Fe3N and Fe4N additional phases is very °

°

low for MW-150 C, but becomes significant, especially concerning Fe3N, for sample MW-160 C. The lattice parameters, crystallite sizes and relative phase amount as obtained from Rietveld refinements with MAUD

24

software are given in Table 1. Looking at this table one can see that lattice

’’

parameters of α -Fe16N2 and bcc Fe phases do not essentially change from one sample to another. On the other hand, the relative amount of these two phases, changes significantly for different samples; a °

’’

maximum of bcc Fe phase was obtained for MW-130 C, while a maximum of α -Fe16N2 amount was found °

’’

for MW-150 C. The crystallite sizes of both α -Fe16N2 and bcc Fe increase with the nitridation ’’

temperature, but the crystallite size of bcc Fe is larger than that corresponding to α -Fe16N2. The Fe4N °

°

and Fe3N phases appear in samples nitrided at 150 C and 160 C, with a larger percent in sample °

°

’’

MW-160 C than MW-150 C, but their crystallite sizes are much smaller than that of α -Fe16N2 and bcc Fe.

9



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°

Based on these characterization results of powder crystallinity, we believe that nitriding at 130 C doesn’t allow phase transformation because the temperature is not high enough to get a significant amount of °

Fe16N2 phase. Unlike low temperature, nitriding at 150 C favors the formation of Fe4N and Fe3N phases.

3.2. Morphology Characterization by SEM Figure 4A and Figure 4B present the SEM images of the samples as obtained after irradiation in the microwave field. The morphology of the amorphous iron oxide powder consists in agglomerations of small grains with relatively narrow size distribution showing an average size of less than 50 nm.

Figure 4A. SEM image of as prepared sample.

After heat treatment in air (Figure 4C) the grain size increases to about 100 nm and their size °

distribution becomes broader. The sample reduced at 420 C in 5%H2/Ar gas mixture and afterwards °

nitrided at 140 C (Figure 4D) exhibits well separated grains with sizes in the range of 50÷100 nm or agglomerations of few grains linked together showing a macro-structure with empty spaces.

10


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Figure 4B. SEM image of as prepared sample.

Figure 4C. SEM image of as prepared sample heat °

treated in air at 400 C.

11



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°

Figure 4D. SEM image of sample reduced at 420 C and °

afterwards nitrided at 140 C. ’’

The SEM measurements are in the size range estimated for α -Fe16N2 and bcc Fe phases using Rietveld refinements (Table 1).

3.3. Mössbauer Spectroscopy 57

Fe Mössbauer spectroscopy is a useful technique suitable to study the electronic phenomena

and the magnetic behavior in connection to the electron configurations emerging from various compounds °

that contains iron. We analyzed the Mössbauer spectra recorded at 25 C and the results are presented in Figure 5. Table 2 presents the following experimental hyperfine parameters: isomer shift (IS), quadrupole splitting (QS), hyperfine magnetic field (H), which were obtained by fitting the Mössbauer spectra presented in Figure 5. In a complex Mössbauer pattern, a ferromagnetic phase such as the bcc Fe phase is assigned to a sextet (6-line pattern) while a paramagnetic phase is described by a doublet (2-line pattern). If the local structure generated by nearest neighbors of Fe contained in the paramagnetic phase has cubic symmetry the electric field gradient to the iron nucleus is zero and the doublet becomes singlet (single line). Each non-equivalent iron crystallographic position gives a spectral component (sextet, doublet or singlet) in the Mössbauer spectrum. Additionally, each crystalline or amorphous iron containing phase has various non-

12


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equivalent iron positions. A complex Mössbauer pattern can be decomposed in spectral components (sextets, doublets and singlets), each one of them corresponding to a non-equivalent Fe position,

9

0

0

Figure 5. Mössbauer spectra measured at 25 C for samples MW-140 C (A), MW-150 C (B), MW-160 0

C (C) nitrided at the corresponding temperatures together with the fitted curve and the decomposition in

spectral components.

13



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belonging to the same phase or to another one. A small amount of poorly crystallized or amorphous phase can be hardly observed in XRD pattern. On the contrary, an amount of only few % of such phase can be detected in Mössbauer spectrum. The Mössbauer spectrum depicted in Figure 5A – for sample °

’’

MW-140 C shows only the presence of α - Fe16N2, bcc Fe and superparamagnetic iron oxide. These °

°

phases appear in all analyzed samples, but MW-150 C and MW-160 C samples contain also Fe3N, Fe4N and superparamagnetic iron nitride.

Table 2. Hyperfine parameters: isomer shift (IS), quadrupole splitting (QS), hyperfine magnetic field (H), relative area (R.A.) for different crystallographic positions and phases corresponding to the spectral components of Mössbauer spectra and relative areas measured from XRD data refinements (taking into account that superparamagnetic (spm) iron oxide and nitride were not 0 0 0 evidenced from XRD due to line broadening) for samples MW-140 C, MW-150 C, MW-160 C

Sample

Phase

oxide Bcc Fe

0.16(2) 0.92(4) 0.00

Fe-N spm

0.38(3)

0.00

-

0.32 0.25 0.36 0.42

0.00 0.02 -0.03 -0.03

21.60 34.00 22.40 11.50

4e 8h 4d

oxide Bcc Fe ’’

oxide Bcc Fe Fe-N spm Fe4N Fe4N Fe3N Fe3N ’’

α -Fe16N2

Fe4N Fe4N Fe3N Fe3N

4e 8h 4d

1a 3c 6g(I) 6g(II) ) 4e 8h 4d

1a 3c 6g(I) 6g(II)

0.00 0.46(2) 0.21(2) 0.16(2) 0.95(3) 0.00 0.00 0.00 0.02 -0.03 -0.03 0.45(2) 0.22(2)

R.A. (%) (Mössbauer)

H (T) 29.59(3) 31.53(3) 40.16(3) 32.99(3) 29.67(3) 31.57(3) 40.28(3) 32.98(3) 21.60 34.00 22.40 11.50 29.66(3) 31.59(3) 40.32(3) 33.00(3)

α -Fe16N2

MWo 160 C

QS (mm/s) 0.44(2) 0.21(2) 0.17(2) 0.74(3)

α -Fe16N2

MWo 150 C

IS(mm/s) ref.Fe met 0.07(2) 0.19(2) 0.17(2) 0.39(2) 0.00 0.08(2) 0.20(2) 0.18(2) 0.39(2) 0.00 0.35(3) 0.32 0.25 0.36 0.42 0.08(2) 0.21(2) 0.18(2) 0.38(3) 0.00

’’

MWo 140 C

Fe pos.

17.1(3) 34.1(4) 17.1(3)

68.3(4)

R.A. norm spm oxide + spm nitride (XRD) 68(5)

10.0(2) 21.7(3) 17.8(3) 35.6(4) 17.8(3)

22(2)

71.2(4)

73(5)

9.6(2) 9.5(3)

9(1) 2.2(2)

1.3(2) 0.4(2) 4.4(3) 1.4(2) 13.9(3) 27.9(4) 13.9(3)

1.7(2)

2(1)

5.8(3)

4(2)

55.7(4)

54(5)

9.9(3) 13.1(3)

10(1) 4.9(3)

1.8(2) 0.6(2) 10.5(3) 3.5(2)

14


2.4(2)

6(2)

14.0(3)

15(4)

Page 15 of 22

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Most probably the bcc Fe belongs to larger grains, because the diffusivity of ammonia at these temperatures is low and insufficient to convert the entire Fe grain into Fe16N2. Both small superparamagnetic particles and external shell of Fe16N2 particles were easily oxidized during sample preparation and handling in air for XRD and Mössbauer measurements. The depth of surface oxide layer ’’

is only few nm for α -Fe16N2 nanoparticles and consequently it behaves as paramagnet at ambient ’’

temperature. The line-widths of α -Fe16N2 and metallic iron are sharp, indicating well-crystalized phases. ’’

The unit cell of α -Fe16N2 contains 3 non-equivalent iron positions: [4d], [8h] and [4e] with relative areas (given by the occupancy of Wyckoff positions) in the ratio 1:2:1, respectively. For these three nonequivalent Fe positions, the literature indicates hyperfine field values ranging from: 39.5 T for Fe (4d), 31.2 T for Fe (8h), 30.1 T for Fe (4e)

28

14

to 40.4 T for Fe (4d), 31.6 T for Fe (8h), 29.8 T for Fe (4e) . The ’’

hyperfine field values presented in Table 2 for the iron non-equivalent positions belonging to α -Fe16N2, about 40.25 T for Fe(4d), 31.55 T for Fe(8h) and 29.65 T for Fe(4e) fit well with literature data, which validates our fits. Moreover, the IS and QS values for Fe(4d), Fe(8h) and Fe(4e) are also given in Table 28

2, which are in good agreement with the same parameters from literature . Additionally, all Mössbauer ’’

patterns of α -Fe16N2 and bcc Fe phases (the sextet with hyperfine field of about 33 T) exhibit a broad doublet with isomer shift (IS) of about 0.38 mm/s, quadrupole splitting (QS) in the range 0.8÷0.9 mm/s and relative amount of about 10%. Low temperature measurements evidenced the transformation of this paramagnetic doublet from ambient temperature into magnetic sextet at liquid helium temperature. This behavior is typical for superparamagnetic fine particles below the blocking temperature. The hyperfine parameters indicate superparamagnetic magnetite. Even though its contribution in Mössbauer pattern is 10% the superparamagnetic magnetite has not been detected in any of the XRD patterns of the nitrided samples due to very broad line and small amount. Mössbauer pattern corresponding to Fe4N phase can be decomposed in two spectral components with hyperfine fields of 21.6 T and 34.0 T

28

and relative amount ratio 3:1, respectively, in

accordance to the occupation of the Wyckoff positions. Fe3N phase contains two non-equivalent iron sites, and their hyperfine fields correspond to 22.4 T and 11.5 T

28

with an occupation ratio of 3:1. The

hyperfine parameters of Fe4N and Fe3N were fixed during the fit due to the complexity of the Mössbauer °

°

pattern of MW-150 C and MW-160 C and only the relative amount was obtained by fitting. Besides these °

°

contributions, MW-150 C and MW-160 C contain also a small amount of superparamagnetic iron nitride (Fe-N spm in Table 2 - the singlet with IS of about 0.38 mm/s and QS=0 mm/s) with hyperfine parameters 29

close to superparamagnetic Fe3N nanoparticles . Comparing all samples from the same table it is obvious that the amount of Fe3N phase is higher than that of Fe4N phase. The content of both Fe3N and °

Fe4N as well as superparamagnetic iron nitride is maximum for MW-160 C sample, while the relative ’’

amount of α -Fe16N2 is the smallest for the same sample. This experimental evidence suggests that °

’’

increasing the temperature above 140 ÷ 150 C is detrimental for obtaining high amount of α -Fe16N2 phase.

15



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Page 16 of 22

For comparison, in Table 2 were gathered also the relative amount of various phases as resulted from Rietveld refinements of the XRD data. It’s worth mentioning that the superparamagnetic iron oxide and the superparamagnetic iron nitride were not observed in the XRD pattern due to cumulative factors such as small amount, small crystallite size (below 5 nm) and distorted structure while, in the Mössbauer spectra, they can be detected as doublet, respectively singlet. Also in Table 2, we normalized the amount of the relative phases obtained from Rietveld refinements considering also the presence of superparamagnetic iron oxide and superparamagnetic iron nitride, which are invisible in the XRD pattern. For example, concerning the values resulted from Rietveld °

’’

refinement in the case of MW-140 C sample, in Table 1 we’ve got an amount of 75% α -Fe16N2 and 25% °

bcc Fe. In Table 2, we can see that MW -140 C contains also 10% superparamagnetic iron oxide (magnetite) which can’t be detected by XRD measurements. That means the real composition of the ’’

sample should be 0.9 x 75% α -Fe16N2, 0.9 x 25% bcc Fe and 10% superparamagnetic iron oxide, and adding all three contributions we get 100%. By this procedure we performed a comparison that is required to confirm the relative phase amount obtained both from Mössbauer spectroscopy and Rietveld refinements of the XRD data. Consequently, the phase composition from Rietveld refinements of the XRD data is confirmed by Mössbauer spectroscopy, as can be seen in Table 2.

3.4 Magnetic Properties Hysteresis loops at ambient temperature obtained for the investigated samples are presented in Figure 6 and the corresponding parameters (magnetization at saturation measured at 40000 Oe, coercive field and remanence) are given in Table 3. The remanence of the samples does not change much with °

the nitridation temperature. The coercive field (Hc) is the lowest for MW-130 C, the sample which contains the maximum amount of bcc Fe with soft magnetic behavior. The magnetization at saturation (Ms) °

decreases significantly for MW-160 C, the sample which contains the maximum amount of Fe3N phase. o

Table 3. Magnetization at saturation, coercive field and remanence for samples MW-130 C, MWo

o

o

140 C, MW-150 C, MW-160 C Magnetization at saturation (Ms) (emu/g)

Coercive field (Hc) (Oe)

Remanence (%)

o

188.1

664

21

o

196.0

808

21

o

183.3

839

22

o

159.2

803

22.5

Sample

MW-130 C MW-140 C MW-150 C MW-160 C

16


Page 17 of 22

From a practical point of view we were interested to measure Ms at 40000 Oe, without extrapolating the magnetization curve at infinite field. Taking into consideration the relative amount of ’’

°

°

α -Fe16N2 and bcc Fe for the samples MW-130 C and MW-140 C from Table 1, the presence of about 10 % superparamagnetic iron oxide (the doublet from the Mössbauer spectra) and the values of Ms from ’’

Table 3 one can calculate Ms value for pure α -Fe16N2 and bcc Fe phases contained in the samples. The total Ms for each sample is the weighted sum of Ms for all pure phases considering their relative amount. The superparamagnetic iron oxide (most probable magnetite considering the values of the Mössbauer 30

°

spectral parameters) has Ms≈30 emu/g

’’

at 25 C and the relative amount of α -Fe16N2 and bcc Fe phases 0

°

can be extracted from Table 1 and Table 2 for MW-130 C and MW-140 C. Using this algorithm, we ’’

estimated the following values: Ms=222 emu/g for α -Fe16N2 and Ms=192 emu/g for bcc Fe contained in

0 -100 -200

50 25 0 -25 -50 -2000

-1000

0

1000

2000

Magnetic field (Oe)

200

MW-140 0C

100 0

Magnetization (emu/g)

100


MW - 130 0C

200



the nitrided samples.

-100 -200

50 25 0 -25 -50 -2000

-1000

0

1000

2000

Magnetic field (Oe)

-20000

0

20000

40000

-40000

Magnetic field (Oe)

MW-150 0C

100


0 -100 -200

50 25 0 -25 -50 -2000

-1000

0

0

1000

2000

200

-20000

0

40000

MW - 160 0C

100 0 -100 -200

20000

40000

-40000

Magnetic field (Oe)

50 25 0 -25 -50 -2000

-1000

0

1000

2000

Magnetic field (Oe)

Magnetic field (Oe)

-40000

20000

Magnetic field (Oe)


200

-20000


-40000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-20000

0

20000

40000

Magnetic field (Oe) o

o

o

o

Figure 6. Hysteresis loops measured at 25 C for samples MW-130 C, MW-140 C, MW-150 C, o

MW- 160 C nitrided at the corresponding temperatures. The inset presents the central part with higher magnification.

17



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Page 18 of 22

These values are close to the values previously found in the literature, i.e. 226 emu/g for Fe16N2 nanoparticles

15

31

°

and about 200 emu/g for Fe nanoparticles . The Ms value at 25 C given in literature for

Fe3N particles with size of 16 nm is about 74 emu/g

32

33

and Ms=165 emu/g

°

at 25 C for Fe4N particles with

’’

size of about 30 nm. The Ms values for α -Fe16N2 and bcc Fe contained in investigated samples have °

°

been just detailed above and further used to calculate Ms for MW-150 C and MW- 160 C samples. In ’’

0

°

addition to α -Fe16N2, bcc Fe, Fe3N and Fe4N, the samples MW-150 C and MW-160 C contain also an 30

amount of 10% superparamagnetic iron oxide (magnetite) with Ms=30 emu/g 29

superparamagnetic iron nitride (with Ms≈3 emu/g

°

at 25 C and about 2÷5%

°

at 25 C). We follow the same procedure, extracting the

relative amount of all phases from Table 2 and using Ms values given in literature or derived for each °

individual phase one can obtain the total magnetization at saturation for samples nitrided at 150 C and °

°

160 C. Using this technique we can estimate Ms=186.3 emu/g for MW-150 C and Ms=166.3 emu/g for °

MW-160 C. These values are close to the experimental ones, given in Table 3.

Conclusions Very fine hematite amorphous nanoparticles obtained via microwave route starting from aqueous ’’

solutions of urea and iron nitrate were successfully used to prepare α -Fe16N2 nanoparticles. Depending °

’’

°

on nitriding temperature, at 130 C more bcc Fe phase is formed than α -Fe16N2, while above 150 C an increasing amount of Fe3N and Fe4N occurs by rising nitridation temperature, with detrimental effect on the magnetic properties. Besides crystalline phases, Mössbauer data indicates an amount of about 10% superparamagnetic iron oxide caused by sample handling in air, which is undetectable in XRD pattern due to high degree of amorphization. After nitridation, the grains remain well separated with sizes in the range of 50÷100 nm or small agglomerations of few grains linked together occur, showing a macro’’

structure with empty spaces. Rietveld refinements of XRD data show that crystallite sizes of α -Fe16N2 and bcc Fe increase with nitridation temperature, while the lattice parameters do not change significantly. The crystallite sizes of Fe4N and Fe3N are much smaller than those of Fe16N2 and bcc Fe. The amount of various phases obtained via Rietveld refinements are in good agreement with the Mössbauer spectroscopy results. Using Mössbauer and magnetic data we estimated that Fe16N2 prepared by this °

method has Ms=222 emu/g at 25 C and 40000 Oe, higher than that of metallic iron. Acknowledgements

This work was realized through the program Partnership in priority areas PN II

developed with the support of MEN - UEFISCDI, project PN-II-PT-PCCA-2013-4-0971 (project contract no. 275/2014) and Core Program 2016-2017 (PN16-480101). The technical support of Gheorghe Gheorghe is strongly acknowledged.

18


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References (1) Dallinger, D.; Kappe, C. O. Microwave-assisted synthesis in water as solvent. Chem. Rev. 2007, 107, 2563. (2) Plapcianu, C.; Agostino, A.; Badica, P.; Aldica, G. V.; Bonometti, E.; Ieluzzi, G.; Popa, S.; Truccato, M.; Cagliero, S.; Sakka, Y.; Vasylkiv, O.; Vidu, R. Microwave synthesis of fullerene-doped MgB2. Ind. Eng. Chem. Res. 2012, 51, 11005. II

(3) Wang, W. W. Microwave-induced polyol-process synthesis of M Fe2O4 (M= Mn, Co) nanoparticles and magnetic property. Mater. Chem. Phys. 2008, 108, 227. (4) Wang, W. W.; Zhu, Y. J.; Ruan, M. L. Microwave-assisted synthesis and magnetic property of magnetite and hematite nanoparticles. J. Nanopart. Res. 2007, 9, 419. (5) Chaudhari, N. K.; Chan Kim, H.; Son, D.; Yu, J. S. Easy synthesis and characterization of singlecrystalline hexagonal prism-shaped hematite α-Fe2O3 in aqueous media. CrystEngComm. 2009, 11, 2264. (6) Jack, K. H. The occurrence and the crystal structure of α”-iron nitride; a new type of interstitial alloy formed during the tempering of nitrogen-martensite. Proc. R. Soc. London Ser. A 1951, 208, 216. (7) Bhattacharyya, S. Iron nitride family at reduced dimensions: A review of their synthesis protocols and structural and magnetic properties. J. Phys. Chem. C 2015, 119, 1601. (8) Kim, T. K.; Takahashi, M. New magnetic material having ultrahigh magnetic moment. Appl. Phys. Lett. 1972, 20, 492. (9) Komuro, M.; Kozono, Y.; Hanazono, M.; Sugita, Y. Epitaxial growth and magnetic properties of Fe16N2 films with high saturation magnetic flux density. J. Appl. Phys. 1990, 67, 5126. (10) Takahashi, M.; Shoji, H.; Takahashi, H.; Wakiyama, T.; Kinoshita, M.; Ohta, W. Synthesis of Fe16N2 films by using reactive plasma. IEEE Trans. Mag. 1993, 29, 3040. (11) Nakajima, K.; Okamoto, S. Large magnetization induced in single crystalline iron films by high-dose nitrogen implantation. Appl. Phys. Lett. 1990, 56, 92. (12) Wang, H. Y.; Ma, Z. W.; Jiang, E. Y.; He, Y. J.; Huang, H. S. Effect of annealing on structure and magnetic properties of Fe-N and Fe-Ti-N thin films. Appl. Phys. A-Mater. 1999, 68, 559. (13) Atiq, S.; Ko, H. S.; Siddiqi, S. A.; Shin, S. C. Preparation and the influence of Co, Pt and Cr additions ’’

on the saturation magnetization of α -Fe16N2 thin films. J. Alloy Compd. 2009, 479, 755. (14) Kikkawa, S.; Yamada, A.; Masubuchi, Y.; Takeda, T. Fine Fe16N2 powder prepared by lowtemperature nitridation. Mater. Res. Bull. 2008, 43, 3352. (15) Ogawa, T.; Ogata, Y.; Gallage, R.; Kobayashi, N.; Hayashi, N.; Kusano, Y.; Yamamoto, S.; Kohara, ’’

K.; Doi, M.; Takano, M.; Takahashi, M. Challenge to the synthesis of α -Fe16N2 compound nanoparticle with high saturation magnetization for rare earth free new permanent magnetic material. Appl. Phys. Express 2013, 6, 073007. ’’

(16) Nagai, D.; Kinemuchi, Y.; Suzuki, K.; Towata, A.; Yasuoka, M. Highly dispersive α -Fe16N2 particle synthesis using hydroxyapatite coating. J. Solid State Chem. 2015, 225, 455.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

(17) Kartikowati, C. W.; Suhendi, A.; Zulhijah, R.; Ogi, T.; Iwaki, T.; Okuyama, K. Preparation and ’’

evaluation of magnetic nanocomposite fibers containing α -Fe16N2 and α-Fe nanoparticles in polyvinylpyrrolidone via magneto-electrospinning. Nanotechnology 2015, 27, 025601. ’’

(18) Zulhijah, R.; Yoshimi, K.; Nandiyanto, A. B. D.; Ogi, T.; Iwaki, T.; Nakamura, K.; Okuyama, K. α Fe16N2 phase formation of plasma-synthesized core-shell type α-Fe nanoparticles under various conditions. Adv. Powder Technol. 2014, 25, 582. (19) Jiang, Y.; Liu, J.; Suri, P. K.; Kennedy, G.; Thadhani, N. N.; Flannigan, D. J.; Wang, J. P. Preparation ’’

of an α -Fe16N2 magnet via a ball milling and shock compaction approach. Adv. Eng. Mater. 2016, 18, 1009. ’’

(20) Jiang, Y.; Mehedi, M. A.; Fu, E.; Wang, Y.; Allard, L. F.; Wang, J. P. Synthesis of α -Fe16N2 compound free-standing foils with 20 MGOe magnetic energy product by nitrogen ion-implantation. Sci. Rep. 2016, 6, 25436. (21) Takagi, K.; Akada, M.; Ozaki, K.; Kobayashi, N.; Ogawa, T.; Ogata, Y.; Takahashi, M. High-pressure ’’

sintering behavior of α -Fe16N2 nanopowder. J. Appl. Phys. 2014, 115, 103905. (22) Tanaka, H.; Harima, H.; Yamamoto, T.; Katayama-Yoshida, H.; Nakata, Y.; Hirotsu, Y. Electronic band structure and magnetism of Fe16N2 calculated by the FLAPW method. Phys. Rev. B 2000, 62, 15042. (23) Sims, H.; Butler, W. H.; Richter, M.; Koepernik, K.; Aşloğlu, E.; Friedrich, C.; Blügel, S. Theoretical investigation into the possibility of very large moments in Fe16N2. Phys. Rev. B 2012, 86, 174422. (24) Lutterotti, L. Total pattern fitting for the combined size-strain-stress-texture determination in thin film diffraction. Nucl. Instrum. Meth. B 2010, 268, 334. (25) Yamanaka, K.; Onuma, Y.; Yamashita, S.; Masubuchi, Y.; Takeda, T.; Kikkawa, S. Humidity effects in Fe16N2 fine powder preparation by low-temperature nitridation. J. Solid State Chem. 2010, 183, 2236. (26) Kikkawa, S.; Kubota, K.; Takeda, T. Particle size dependence in low temperature nitridation reaction for Fe16N2. J. Alloy Compd. 2008, 449, 7. (27) Masubuchi, Y.; Yamashita S.; Motohashi, T.; Kikkawa, S.; Niederberger, M. Magnetite/maghemite mixture prepared in benzyl alcohol for the preparation of α”-Fe16N2 with α-Fe. J. Eur. Ceram. Soc. 2011, 31, 2417. (28) Qi, Q.; O’Donnell, K.; Touchais, E.; Coey, J. M. D. Mössbauer spectra and magnetic properties of iron nitrides. Hyperfine Interact. 1994, 94, 2067. (29) Ningthoujam, R. S.; Gajbhiye, N. S. Magnetic study of single domain ε-Fe3N nanoparticles synthesized by precursor technique. Mater. Res. Bull. 2008, 43, 1079. (30) Lee, Y.; Lee, J.; Bae, C. J.; Park, J. G.; Noh, H. J.; Park, J. H.; Hyeon, T. Erratum: Large-scale synthesis of uniform and crystalline magnetite nanoparticles using reverse micelles as nanoreactors under reflux conditions. Adv. Funct. Mater. 2005, 15, 2036.

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(31) Huang, M.; Qin, M.; Cao, Z.; Jia, B.; Chen, P.; Wu, H.; Wang, X.; Wan, Q.; Qu, X. Magnetic iron nanoparticles prepared by solution combustion synthesis and hydrogen reduction. Chem. Phys. Lett. 2016, 657, 33. (32) Huang, W.; Wu, J.; Guo, W.; Li, R.; Cui, L. Preparation and magnetic properties of nanoscale ε-Fe3N particles. J. Alloy Compd. 2007, 443, 48. (33) Grimes, C. A.; Qian, D.; Dickey, E. C.; Allen, J. L.; Eklund, P. C. Laser pyrolysis fabrication of ferromagnetic γ’-Fe4N and FeC nanoparticles. J. Appl. Phys. 2000, 87, 5642.

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Structural, Magnetic, and MÃ¶ssbauer Investigation of Ordered Iron

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