Processing and characterization of a Ni-Co ferrite for sensor applications

Processamento e caracterização de uma ferrita Ni-Co para aplicações em sensores

Abstract

Ni-Co ferrites are magnetostrictive ceramics that have potential application in magnetostrictive/ magnetoelastic sensors, as well as in magnetoelectric composites. Ni-rich Ni-Co spinel ferrite samples were processed by the ceramic method and bulk samples were sintered at 1350 ºC in the solid state and at 950 ºC with Bi₂O₃ liquid phase. The sintered samples were characterized by light microscopy, scanning electron microscopy, Raman spectroscopy, vibrating sample magnetometry and capacitance dilatometry. With Bi₂O₃ additions as small as 0.6 mol% it was possible to sinter the Ni-Co ferrite at 950 ºC, obtaining high-density samples; however, such liquid-phase sintered samples presented iron oxide particles. The studied samples presented magnetoelastic sensitivities very close to CoFe₂O₄, with significantly lower magnetic hysteresis. The results thus indicate that the magnetic properties of the samples analyzed are suitable for applications in magnetomechanical sensors.

Keywords:
ferrites; magnetoelastic sensors; magnetostriction; magnetic ceramics

Resumo

Ferritas Ni-Co são cerâmicas magnetostritivas que possuem potencial de aplicação em sensores magnetostrictivos/ magnetoelásticos, bem como em compósitos magnetoelétricos. Ferritas espinélio do tipo Ni-Co com alto teor de Ni foram processadas por meio do método cerâmico e amostras foram sinterizadas no estado sólido a 1350 ºC e via fase líquida a 950 ºC com adição de Bi₂O₃. As amostras sinterizadas foram caracterizadas por meio de microscopia óptica, microscopia eletrônica de varredura, espectroscopia Raman, magnetometria de amostra vibrante e dilatometria de capacitância. Utilizando-se pequenas adições de Bi₂O₃, na proporção de 0,6% molar, foi possível sinterizar a ferrita Ni-Co a 950 ºC, obtendo-se amostras de alta densidade relativa; no entanto, estas amostras sinterizadas via fase líquida apresentaram partículas de óxido de ferro na microestrutura. As amostras produzidas apresentaram sensibilidade magnetoelástica próxima à da esperada para CoFe₂O₄, com histerese magnética significativamente menor. Os resultados obtidos indicam que as propriedades magnéticas das amostras analisadas são adequadas para aplicações em sensores magnetomecânicos.

Palavras-chave:
ferritas; sensores magnetoelásticos; magnetostrição; cerâmicas magneticas

INTRODUCTION

The magnetostrictive properties of materials and their magnetoelastic effects have been subject of study for applications in several types of sensors, such as the ones for sensing force, magnetic field, viscosity, density, fluid flow rate, elastic modulus of materials, humidity, gas, chem-bio agents in liquids, and blood coagulation¹.

Amorphous ferromagnetic alloys and iron/rare-earth alloys are frequently used in magnetostrictive/ magnetoelastic sensors. Ferrites with cobalt in their composition have been pointed out as alternative magnetostrictive materials for this application because of their lower cost and better corrosion resistance compared to iron-based alloys. However, the low mechanical resistance of these ceramic materials may be a disadvantage in some cases. The use cermets' technology to increase the mechanical strength of cobalt ferrite has been proposed², making it suitable for use in torque transducers. Nickel-based ferrites are ferrimagnetic ceramic materials. Their electromagnetic and magnetostrictive properties are suitable for applications in high-frequency devices³, magnetic field sensors⁴and magnetoelectric transducers⁵. The substitution of Ni for Co, as in Ni_1-xCo_xFe₂O₄, is known to increase the saturation magnetization and magnetostriction magnitudes of these ferrites^{6 7}. The suitable magnetostrictive properties of these materials led Sedlar et al.⁴ to study the performance of Ni and Ni-Co ferrite in a magnetic field sensor and the best results were obtained for the Ni-Co ferrite. Ni-Co ferrite has also been pointed out as a material for use as anode material in lithium ion batteries⁸.

Ferrite synthesis by means of chemical methods has made possible to obtain high-purity materials, with controlled particle size and morphology. There are many works about the synthesis of nanometric Ni-Co ferrites^{9 10}. These developments in the nanotechnology area are extremely important for these materials application advancement. On the other hand, the study of the traditional ceramic processing route is still important due to the suitability of large scale materials processing. Ferrites usually require sintering temperatures higher than 1200 ºC but the possibility of sintering in temperatures lower than 1000 ºC favors co-firing in electronic devices fabrication¹¹. Until now, numerous processing methods have been proposed to reduce the required sintering temperatures to obtain ferrites with highest mass density. The use of additives such as Bi₂O₃, V₂O₅and copper substitutions are procedures that result in ferrites with high mass density using low sintering temperatures^{12 13 14}. The presence of a liquid phase during sintering enhances diffusion and exerts capillary forces, favoring the densification at a lower sintering temperature compared to the temperature required for high densification without the liquid phase. The liquid-phase sintering of Ni-Co ferrites, using 5 wt.% Bi₂O₃ as the sintering additive, has been studied². The authors obtained samples with mass densities as high as 4.9 g/cm3 with sintering at 950 ºC for 2 h in air. However, there are very few works dealing with the effect of liquid-phase sintering on the magnetostriction of ferrites. This is important since the magnetostriction of ferrites is very sensitive to the microstructural changes that result from variations in both sintering and processing parameters^{14 15}. Also, it is known that secondary phases affect the magnetostrictive properties of ferrites^{16 17} and liquid-phase sintering generally introduces second phases in the microstructure of ceramics.

In this work, Ni-Co ferrite samples were liquid-phase sintered at low temperature and with small Bi₂O₃ additions (sintered at 950 ºC with 0.6 mol% Bi₂O₃). The microstructure and the magnetic properties of the samples were compared to those obtained from samples sintered in the solid state at 1350 ºC, which is within the usual temperature range for solid-state ferrite sintering.

EXPERIMENTAL PROCEDURE

Two batches of Ni-Co ferrite were fabricated by the ceramic method with 67.99% Fe₂O₃, 28.63% NiO and 3.38% Co₃O₄ oxides (wt.%) raw materials. The oxides were dry mixed in an agate mortar and calcined at 850 ºC for 4 h. The Ni:Co:Fe proportion of each batch of calcined powder was estimated from the atomic absorption spectrometry analysis results. After calcinations the powders were wet milled with ethyl alcohol in an eccentric mill for 1 h. For the fabrication of samples that were sintered with the liquid phase, 0.6 mol% Bi2O3 was added to the calcined powder before the milling step. Pellets with 8 mm diameter and thickness 2-3 mm were uniaxially pressed at 50 MPa and subsequently at 300 MPa in an isostatic press.

The pellets were sintered at 1350 ºC for 6 h (SSS) and the pellets with additives to obtain liquid-phase were sintered at 950 oC for 24 h (LPS). The densities of the sintered samples were evaluated by the Archimedes method. The crystal structures of the powders in calcined state and in a subsequently heat-treated one at 950 ºC/24 h were evaluated by X-ray diffraction (XRD).

The microstructures of the sintered samples were evaluated in a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer. The samples were prepared by a sequence of grinding and polishing steps, followed by thermal etching.

The chemical homogeneity of the microstructures of both SSS and LPS samples was also evaluated by micro-Raman spectroscopy with an argon laser (λ = 514.5 nm) of 8 mW.

Magnetostriction measurements were carried out in samples cut from the sintered pellets. The dimensions of the samples were 3.977 mm × 2.509 mm × 2.421 mm (SSS) and 3.083 mm × 2.364 mm × 2.090 mm (LPS). The measurement method was capacitance dilatometry, described in detail elsewhere¹⁸. Magnetization measurements were performed in the same cut samples by vibrating-sample magnetometry (VSM). Magnetostriction was measured in parallel and perpendicular directions to the applied field.

RESULTS AND DISCUSSION

The results of the AAS quantitative analysis of the calcined powders are shown on Table I. The batches presented similar Co content but the SSS batch presented a higher Fe:Ni ratio.

Table I/Tabela I
Results of AAS analysis (wt.%) of each element in the calcined powders and calculated Ni:Co:Fe atomic proportions. [Resultados da análise por espectrometria de absorção atômica (percentual em peso) para cada elemento previsto nos pós calcinados e proporções atômicas Ni:Co:Fe calculadas.]

Element	Fe	Ni	Co	Ni:Co:Fe
Batch for LPS	47.1 ± 0.8	21.4 ± 0.4	3.04 ± 0.20	0.90:0.13:1.97
Batch for SSS	48.0 ± 2.1	18.5 ± 1.0	3.32 ± 0.08	0.80:0.14:2.06

The X-ray diffraction patterns of the SSS batch after calcination and calcination followed by heat treatment are shown on Fig. 1. The peak expected for the (104) plane of hematite (α-Fe₂O₃ - rhombohedral) was detected in both powders, indicating that the reactions in solid state to form the spinel phase were not complete¹⁹. The calculated lattice parameters of the spinel phases were 8.343 Å for the calcined powder and 8.331 Å for the powder that was subsequently heat treated.

Figure 1/Figura 1
X-ray diffraction patterns of powders from the SSS batch. The red line indicates the peak that corresponds to the (104) plane of hematite (JCPDS file 89-0599) and the green lines indicate the typical peaks of the spinel phase. [Difratogramas de raios X dos pós calcinados utilizados nas amostras sinterizadas no estado sólido. A linha vermelha indica o pico correspondente ao plano (104) da hematita (JCPDS 89.0599) e as linhas verdes indicam os picos típicos da fase espinélio.]

The densities measured were 4.90 g/cm³ for the SSS sample and 5.02 g/cm³ for the LPS sample; LPS thus increased the density of the material, with a sintering temperature 400 ^oC lower.

Figs. 2 and 3 show SEM images of the microstructures of the SSS and LPS samples; the average grain sizes, estimated from these figures by a linear intercept method²⁰ were 3 µm for the LPS sample and 5 µm for the SSS sample. The LPS process resulted in a microstructure with pore-free grains, with porosity located at grain boundaries; the crystallized liquid phase appears as films and particles located at grain boundaries. The grain size of the SSS sample is fairly uniform and the intragranular porosity is very small: the porosity of the sample is predominantly intergranular.

Figure 2/Figura 2
SEM image of the SSS sample. [Imagem obtida em microscópio eletrônico de varredura da amostra sinterizada no estado sólido.]

Figure 3/Figura 3
SEM image of the LPS sample. [Imagem obtida em microscópio eletrônico de varredura da amostra sinterizada via fase líquida.]

The EDX spectrum shown in Fig. 4was taken from a white intergrain particle observed in the LPS sample in Fig. 3. This result suggests that this phase, rich in bismuth, has a high concentration of the Bi₂O₃ additive, as well as small amounts of impurities coming from the raw materials. However, due to the small thickness of these intergrain phases, much lower than 5 μm, which is the electron beam average size in SEM, there is the influence of the ferrite background on the measurement that do not allow to obtain a measurement only of the isolated intergrain phase.

Figure 4/Figura 4
Results of EDX analysis of one of the white particles observed in Fig. 3. [Resultados da análise por EDX de uma das partículas brancas observadas na Fig. 3.]

The EDX analysis in the centers of the ferrite grains of the LPS sample revealed two composition types: a major Fe-Co-Ni-O one and a cobalt-free one. Table II presents the typical composition of both phases. The (Ni+Fe):O proportion of the Co-free grains is near 2:3, as in Fe₂O₃, and Ni₂O₃. These results thus indicate that the Co-free grains are probably mainly constituted of Fe₂O₃.

Table II/Tabela II
Composition obtained from EDX analyses of the grains from the microstructure of the LPS sample (at.%). [Composição obtida por EDX (percentual atômico) de grãos da microestrutura da amostra sinterizada via fase líquida.]

Element	Ni-Co spinel grain	Co-free grain
Ni	11.91	3.76
Co	1.7	0.00
Fe	30.79	34.75
O	55.54	61.47
Bi	0.00	0.01
Ni:Co:Fe	0.80:0.12:2.08	0.29:0.00:2.71

Table III shows the results of the EDX analysis of the SSS sample, indicating the average and the standard deviation of the results obtained in the center of five grains. The results of the semi-quantitative analysis shown on Tabs. II and III indicated a Ni:Co:Fe approximate atomic proportion of 0.8:0.1:2.1 for the spinel Ni-Co phase in both SSS and LPS samples.

Table III/Tabela III
Results from the EDX analysis of the SSS sample (at.%). [Resultados (em percentual atômico) da análise de EDX da amostra sinterizada no estado sólido.]

Element	Ni	Co	Fe	O
Average	9.254	1.694	24.123	64.929
Std. deviation	0.251	0.108	0.369	0.494

Figs. 5 and 6 show light microscopy images of the sintered samples. It was observed that the LPS sample's microstructure was constituted of few and sparsely distributed white grains surrounded by a major phase, constituted of grey grains. The presence of such white phase was not observed in the SSS sample. The Raman spectra of the LPS sample were taken from three grains of each type.

Figure 5/Figura 5
Optical microscopy image of the LPS sample's microstructure. The arrows indicate the two types of grains from which the Raman spectra were taken: phase 1, grey grains; phase 2, white grains. [Imagem obtida por microscopia óptica da microestrutura da amostra sinterizada via fase líquida. As setas indicam os dois tipos de grãos nos quais foi realizada a análise por espectroscopia Raman: fase 1, grãos cinza; fase 2, grãos brancos.]

Figure 6/Figura 6
Optical microscopy image of the SSS sample's microstructure. [Imagem, obtida por microscopia óptica, da microestrutura da amostra sinterizada no estado sólido.]

Figs. 7 to 9 show Raman spectra of the LPS and SSS samples, taken from the centers of selected grains.

Figure 7/Figura 7
Raman spectra of the SSS sample. The sharper peaks are marked in red. [Espectros Raman obtidos na amostra sinterizada no estado sólido. Os picos mais agudos estão marcados em vermelho.]

Figure 8/Figura 8
Raman spectra of the grey grains observed in the LPS sample. The sharper peaks are marked in red. [Espectros Raman dos grãos cinza observados na amostra sinterizada via fase líquida. Os picos mais agudos estão numerados.]

Figure 9/Figura 9
Raman spectra of the white grains observed in the LPS sample. The sharper peaks are numbered. [Espectros Raman dos grãos brancos observados na amostra sinterizada via fase líquida. Os picos mais agudos estão numerados.]

The spectra of Figs. 7 and 8, for SSS and LPS samples, presented essentially the same pattern, which is typical of spinel Ni ferrites^{21 22}. In the case of the spectra of Fig. 9, taken from the white phases observed in optical microscope of the LPS sample (Fig. 5), the peaks numbered 1 to 5 match well with the phase α-Fe₂O₃ (hematite)^{23 24}. However, the peak number 6 has an average value of 655 cm^-1 of the Raman shift, which is close to the range of 663-706 cm^-1 range that is known to be the strongest mode of Fe₃O₄ (magnetite)²⁵. In addition, no spinel ferrite peak vestiges were observed in those white grains. These results thus indicate that the white grains are constituted of iron oxide, corroborating the previous chemical analyses, XRD and EDX experiments.

Figs. 10a-b and 11 show the results from the magnetization measurements of the sintered samples. The parameters obtained from such data are shown on Table IV. The saturation magnetization values of both samples were similar and the slightly higher coercivity of the LPS sample may be attributed to: the smaller grains (higher grain boundary area) of the sample, increasing the pinning effect on the magnetic domains' walls; the presence of non-magnetic second phase particles, such as diamagnetic α-Bi₂O₃ and antiferromagnetic α-Fe₂O₃. This factor may have also contributed to the slightly lower saturation magnetization of this sample.

Figure 10/Figura 10
(a) Hysteresis curve of the LPS sample; (b). Hysteresis curve of the SSS sample. [(a) Curva de histerese magnética da amostra sinterizada via fase líquida (a) e sinterizada no estado sólido (b).]

Figure 11/Figura 11
Initial magnetization curves. [Curvas de magnetização inicial.]

Table IV/Tabela IV
Results obtained from the magnetization measurements. [Resultados obtidos das medidas de magnetização.]

Sample:	SSS	LPS
Saturation magnetization -Ms(A.m2/kg)	9.56	53
Coercivity - μ0HC(mT)	5.4	8.6
Remanence - MR(A.m2/kg)	2.8	4.5
MR/Ms	0.050	0.162

The M_s values reported by other authors for Ni_xCo_1-xFe₂O₄vary within a wide range, depending on the fabrication route. For example, Ni_0.5Co_0.5Fe₂O₄has benn obtained by means of mechanical alloying and their samples presented a large variation of M_s, as a function of the annealing temperature after milling²⁶. Additionally, Table V shows M_s values of chemically synthesized Ni_xCo_1-xFe₂O₄ferrites from different sources^{6 7 10 27 28}; all these works show a tendency of Ms increasing values with the raise of cobalt content. In Fig. 12, a plot of Ms as a function of Ni content "x" in Ni_xCo_1-xFe₂O₄, elaborated with the data presented on Tab. V, shows clearly that the M_s values obtained in the present work (dashed line) approach the average of pure cobalt ferrite and are within the range reported for Ni_xCo_1-xFe₂O₄with x ≤ 0.5. The slight iron excess of the ferrite, as verified in the EDX analysis, probably contributed to this result.

Figure 12/Figura 12
Graphical representation of the M_s values reported in literature for Ni_xCo_1-xFe₂O₄. The dashed line indicates the M_s obtained in this work for the SSS sample. [Representação gráfica dos valores de M_s reportados na literatura para Ni_xCo_1-xFe₂O₄. A linha tracejada indica o valor de M_s da amostra sinterizada no estado sólido obtida neste trabalho.]

Table V/Tabela V
Saturation magnetization values [A.m²/kg] of chemically synthesized Ni_xCo_1-xFe₂O₄ reported by different authors. The data have been approximated to integers. [Valores de magnetização de saturação [A.m²/ kg] reportados por diferentes autores para amostras de Ni_xCo_1-xFe₂O₄ sintetizadas por meio químico. Os dados foram aproximados para números inteiros.]

x	Ref.27	Ref.7	Ref.28	Ref.10	Ref.6
0	66	37	70	56	-
0.1	65	-	69	-	-
0.2	58	37	65	50	-
0.3	-	-	63	-	-
0.4	-	45	60	47	-
0.5	35	-	57	-	54
0.6	29	34	55	42	47
0.7	-	-	52	-	42
0.8	15	27	51	36	40
0.9	-	-	47	-	25
1.0	14	13	45	30	12

According to Fig. 13, the trends reported in literature for the M_R/M_s ratio as a function of "x" in Ni_xCo_1-xFe₂O₄vary considerably. The M_R/M_s ratios of both LPS and SSS samples may be considered low when compared to the literature data. Also, in CoFe₂O₄ LPS samples previously studied¹⁴, the M_R/M_s ratios ranged from 0.197 to 0.420, which is larger than the values obtained for the Ni-Co ferrite samples. The ferrites produced in this work thus have low magnetic hysteresis and high saturation magnetization when compared to most Ni-Co ferrites presented in literature. These features make them interesting materials for sensors applications.

Figure 13/Figura
13: Values reported for M_R/M_s parameter of Ni_xCo_1-xFe₂O₄. [Valores do parâmetro M_R/M_s reportados para Ni_xCo_1-xFe₂O₄.]

Figs. 14-15 show the magnetostriction curves of the sintered samples and Table VI shows some parameters extracted from these curves.

Figure 14/Figura 14
Magnetostriction curves of the SSS sample: parallel (λ) and perpendicular (λ _┴) to the magnetic field. [Curvas de magnetostricção da amostra sinterizada no estado sólido: direção paralela (λ) e perpendicular (λ _┴) ao campo magnético.]

Figure 15/Figura 15
Magnetostriction curves of the LPS sample: parallel (λ) and perpendicular (λ _┴) to the magnetic field. [Curvas de magnetostricção da amostra sinterizada via fase líquida: direção paralela (λ) e perpendicular (λ _┴) ao campo magnético.]

Table VI/Tabela VI
Saturation magnetostrictions and saturation fields obtained from Figs. 14-15. [Valores de magnetostricção de saturação e campos magnéticos de saturação, obtidos das Figs. 14-15.]

Sample	(λ//)s	(λ┴)s	μ0H at saturation
SSS	-35 ppm	23 ppm	0.284 T
LPS	-40 ppm	24 ppm	0.353 T

Fig. 16 shows the magnetoelastic sensitivity (S) curves, calculated by differentiating the magnetostriction curves, and Tables VII and VIII show the moduli of the maximum magnetoelastic sensitivities observed. The LPS sample presented the lowest magnetoelastic sensitivity at low fields, but it is the most sensitive sample for μ₀H between 0.10-0.17T, approximately. It is important to mention that the derivative of both magnetostriction and magnetization curves are affected by the samples' geometry and dimensions, but the dimensions of both samples evaluated here are similar enough to allow a qualitative comparison between their magnetoelastic sensitivities.

Figure 16/Figura 16
Modulus of the magnetoelastic sensitivities of the sintered samples. [Módulo da sensibilidade magnetoelástica das amostras sinterizadas.]

Table VII/Tabela VII
Maximum magnetoelastic sensitivities (S_m)and corresponding μ₀H values. [Sensibilidades magnetoelásticas máximas (S_m)e valores de μ₀H correspondentes.]

Sample	(Sm)//[ppm/T]	μ0H [T]
LPS	341	0.096
SSS	463	0.068

Table VIII/Tabela VIII
Maximum magnetoelastic sensitivities (S_m)_┴ and corresponding μ₀H values. [Sensibilidades magnetoelásticas máximas (S_m)_┴ e valores de μ₀H correspondentes.]

Sample	(Sm)┴[ppm/T]	μ0H [T]
LPS	158	0.165
SSS	242	0.108

The excess iron detected in the EDX analysis may have different effects on the magnetic properties of the ferrite. For example, the saturation magnetization of the ferrite may be increased when the excess iron forms magnetite (Fe₃O₄), because this phase has a relatively high saturation magnetization (around 90 A.m²/kg²⁹. Moreover, Ni-Co ferrite and magnetite magnetostrictions have opposite signs in the direction parallel to the magnetic field, namely, Ni-Co ferrite retracts and magnetite expands. Therefore, the absolute value of the ferrite magnetostriction would be smaller due to the competition with the magnetite.

Comparing the results from this work with the ones obtained in CoFe₂O₄ tested under similar conditions ¹⁴, one can observe that the Ni-Co ferrite presented similar maximum magnetoelastic sensitivity values, but occurring at considerably lower fields. The (S_m) values of the CoFe₂O₄ samples in¹⁴ varied from 366 to 541 ppm/T and the minimum corresponding µ₀H found was 0.19 T. These results suggest that the Ni-rich Ni-Co ferrite here studied may be more sensitive than CoFe₂O₄ at low magnetic fields. When associated to a piezoelectric material, forming a magnetoelectric composite^{5 30}, the good electromagnetic³and magnetostrictive properties of Ni-Co ferrites make them candidates for applications in high-frequency tunable devices, for example.

CONCLUSIONS

The Ni-rich Ni-Co spinel ferrite samples presented magnetoelastic sensitivities very close to that of CoFe₂O₄, with significantly lower magnetic hysteresis than that ferrite. The M_R/M_s ratios were lower than the values reported in literature for Ni_xCo_1-xFe₂O₄ferrites. Liquid-phase sintering with Bi₂O₃ slightly increased the magnetic hysteresis and had little effect on the magnetostriction values; however, the magnetoelastic sensitivity of the liquid-phase sintered sample was lower than that obtained in the solid-state sintered sample. Liquid-phase sintering at 950 ^oC for 24 h resulted in chemically heterogeneous samples, with the presence of iron oxides. Because of its relatively high magnetoelastic sensitivity and low magnetic hysteresis, the ferrite is a candidate for application in magnetoelastic sensors and magnetoelectric composites.

ACKNOWLEDGMENTS

The authors acknowledge the support given by the following people/institutions: the Brazilian agencies FAPESP (Procs. 2009/09524-7, 2012/01448-2 and 2011/21258-0), CNPq (504282/2010-7) and CAPES for the financial support; students M. S. Hieda (Instituto Tecnológico de Aeronáutica, Brazil) and M. B. S. Dias (University of S. Paulo, Brazil) for obtaining the microstructure images; Materials Division of the Institute for Aeronautics and Space, Brazil, for the chemical and SEM analysis.

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Publication Dates

• Publication in this collection
  Jul-Sep 2015

History

• Received
  03 Dec 2014
• Accepted
  30 Apr 2015