Directed Assembly of BaFe12O19 Particles and the Formation of

ARTICLE pubs.acs.org/Langmuir

Directed Assembly of BaFe12O19 Particles and the Formation of Magnetically Oriented Films Darja Lisjak* and Simona Ovtar Department for Materials Synthesis, Jo
zef Stefan Institute, Ljubljana, Slovenia

bS Supporting Information ABSTRACT:

We have studied the preparation of oriented BaFe12O19 films produced using electrophoretic deposition (EPD). Highly anisotropic, platelike BaFe12O19 particles were synthesized under hydrothermal conditions, and from these particles, stable suspensions were prepared in 1-butanol by the addition of dodecylbenzene sulfonic acid as a surfactant. The interplay of the interaction forces between the suspended particles and the forces acting on the particles during the EPD directed the particles’ assembly in the plane of the substrate. The most significant effect on the orientation of the films was the diameter-to-thickness ratio of the particles, which was experimentally confirmed with X-ray analyses, electron microscopy, and magnetic measurements. The abnormal grain growth that accompanied the sintering at 1150 °C further improved the overall orientation of the films, which showed highly anisotropic magnetic behavior with a remanent-to-saturation magnetization ratio exceeding 0.8.

’ INTRODUCTION Barium ferrite (BaF) with the chemical formula BaFe12O19 is a well-known ferrimagnetic material. Its structure is of the magnetoplumbite type,1 where two structural blocks, R (BaFe6O112
) and S (Fe6O82+), are combined in an alternating fashion, RSR*S* (where * denotes a 180° rotation) in the direction of the hexagonal c axis, forming a highly anisotropic crystal lattice. BaF crystals preferentially grow in the ab plane with a high diameterto-thickness aspect ratio, forming thin hexagonal plates with their easy magnetic axis coinciding with the crystallographic c axis (i.e., perpendicular to the ab plane).1,2 Consequently, BaF also possesses a high magnetic anisotropy and a high ferromagnetic resonance and is therefore suitable for applications at millimeter wave frequencies in nonreciprocal devices or in electromagnetic absorbers.3
5 It was reported5 that the most suitable form of material for such millimeter wave frequency applications would be magnetically oriented (i.e., self-biased BaF films with a thicknesses of 10
100 μm). Such films could also be used in miniaturized electronic components where magnets are required. This is because BaF is also a known permanent-magnet material. Most of the widely used techniques for the preparation of films are suitable either for thin films only (i.e., pulsed-laser r 2011 American Chemical Society

deposition, sputtering)4,6,7 or for thick films with a limited orientation (i.e., sputtering, electron-beam evaporation).8,9 However, thick, oriented BaF films were previously prepared with liquid-phase epitaxy10 and with screen printing followed by annealing in a magnetic field.4,6,11 Another promising method for the preparation of thick films is electrophoretic deposition (EPD). EPD is a well-known method that is suitable for the preparation of films and coatings with various thicknesses and even bulk ceramics.12 Putting it simply, EPD can be regarded as an electrically driven assembly of charged particles. It is divided into two steps: (i) the flow of charged particles under an electric field toward an oppositely charged electrode (also called electrophoresis) and (ii) the deposition step, during which the charged particles deposit on the oppositely charged electrode substrate. It is important that the particles are dispersed in a solvent and not agglomerated. The attractive van der Waals forces, responsible for the agglomeration, can be overcome by electrostatic, steric, or Received: April 29, 2011 Revised: October 21, 2011 Published: October 24, 2011 14014

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Langmuir electrosteric repulsion with the selection of a suitable surfactant. However, magnetic particles are additionally attracted by the magnetic dipole
dipole forces that are stronger at larger separation distances than the van der Waals forces. Therefore, much stronger repulsive forces are required to stabilize the suspensions of magnetic particles than in the case of nonmagnetic particles of the same size. For this reason, stable magnetic suspensions, or ferrofluids, are most often prepared from superparamagnetic particles.13
15 Such particles show magnetic behavior only under an applied magnetic field. A much more challenging task is to prepare stable suspensions from hard magnetic particles that have a permanent magnetic moment. It was shown previously that stable suspensions of hard magnetic BaF particles can be prepared providing there is a strong electrosteric repulsion.16 The deposition step of the EPD involves electrochemical and agglomeration phenomena.12 The initial stage of the deposition from very dilute suspensions can be explained by the neutralization of the particles upon contact with the electrode.17 However, other mechanisms explain the formation of thick deposits: (i) the reduction of the repulsive forces between the particles due to the increased ionic strength next to the electrode,18 (ii) the double-layer distortion as a particle moves toward the electrode with a thinner double layer ahead and a wider double layer behind the particle,19 or (iii) the electroosmosis around the particles near the electrode.20 Provided there is an additional external force, an oriented deposition of the anisotropic particles can be achieved. For example, oriented Al2O3 and TiO2 films were obtained with EPD in a magnetic field (g10 T) strong enough to orient the diamagnetic particles with an anisotropic magnetic susceptibility.21,22 Oriented films of BaNd2Ti5O14 (BNT) were prepared with the EPD of highly anisotropic, elongated particles.23 However, previous attempts24 to prepare BaF films with EPD combined with an external magnetic field resulted in only a limited film orientation and thickness. The application of the magnetic field alone provided good orientation but poor film densities. Surprisingly, BaF films of comparable or even superior quality were obtained using EPD without any external magnetic field with an optimized size distribution of the original particles.25 The aim of this work was to study the orientation in BaF films prepared with classical EPD. In this study, we report on three mechanisms that have a crucial effect on the magnetic orientation of BaFe12O19 thick films prepared with EPD: (i) the interaction forces between the BaF nanoparticles in a polar solvent, (ii) the electrophoresis and the deposition of particles, and (iii) the anisotropic and abnormal growth of BaF grains during the sintering. The effect of those mechanisms on the orientation of the BaF film strongly depends on the particles’ shape anisotropy and the particle size distribution of the feedstock particles.

’ MATERIALS AND METHODS Powders. The BaFe12O19 (BaF) particles were synthesized hydrothermally as described in detail in ref 15. For the preparation of powder A, nitrates of Fe and Ba were dissolved in a 5:1 molar ratio in water. Then hydroxides were precipitated using excess Na hydroxide. This suspension of hydroxides was then decanted into an autoclave. The autoclave was heated to 160 °C with a heating rate of 3 °C/min and then cooled to room temperature. The resulting particles were washed with water to remove the Na ions and with nitric acid to dissolve the Ba carbonate. Powder B was prepared with a slightly modified procedure. The

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Table 1. Basic Data Used in the Calculationsa property

suspension A 2r = 11 ( 3

particles size (nm) Ms (particles) (emu/g)

suspension B 2r = 10
350

h=3

h = 5.6 ( 2.6

1015

35 (all particles) 6.4 (small fraction, 2r e 30 nm) 41.3 (large fraction)

γ (particles) (g/L)

7

7

|ζ| (mV)

86

120

σ (μS/cm) c (free DBSa) (mmol/L)

1 0.27

11 1.12

1/k (nm)

12.4

6.07

ϕi

0.092

0.166

γ denotes the concentration of particles in the suspension, σ denotes the conductivity, c denotes the molar concentration, ϕi denotes the volume fraction of adsorbed DBSa in an overlapping layer, and the other abbreviations are the same as used in the text. a

Figure 1. TEM images of the dried suspensions. Enlargements are shown on the right-hand side. synthesized yield was around 70% for both powders. Surfactant dodecylbenzene sulfonic acid (DBSa) was added to the suspension of hydroxides before the hydrothermal synthesis. The suspension was then decanted into the autoclave and heated to 240 °C. The resulting powder was washed with water. The adsorption of DBSa onto the particle surfaces occurred with the addition of nitric acid, which was otherwise also used to dissolve the Ba carbonate. The dried particles were deposited on a Cu grid and observed with a transmission electron microscope (TEM, Jeol 2100). The particles were platelets, and their equivalent diameter was determined from the surface with Gatan digital micrograph software. Although the statistics of the determined thicknesses were poorer than those of the diameters (because of the preferential alignment of the anisotropic particles in the plane), we managed to get reliable values by inspecting a sufficient number of TEM images. Powder A was homogeneous with the platelets having a diameter of 11 ( 3 nm and a thicknesses of around 3 nm (Table 1 and Figure 1). Powder B consisted of platelets having different sizes (Table 1, Figure 1, and Figure S1 in the Supporting Information). Almost half of the particles had a diameter of 20 nm; however, around 5% of the particles were smaller, with a diameter of 10 nm, and another 5% were very large particles, with diameters between 200 and 350 nm. The rest of the particles ranged between 30 and 190 nm in similar 14015

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Langmuir fractions. The thicknesses of all of the particles ranged between 3 and 10 nm (mean thickness = 5.6 ( 2.6 nm) regardless of the particle diameters, with the exception of the smallest particles (with a diameter of up to 20 nm) that were all around 3 nm thick. The magnetic properties of the powders were measured with a vibrating-sample magnetometer (VSM, Lake Shore 7312) to up to 10 kOe and with a Quantum Design SQUID magnetometer to up to 50 kOe. The high magnetic field of 50 kOe was necessary to saturate the samples fully, and the saturation magnetization (Ms) values are listed in Table 1. In our case, the estimated error in the magnetization values per mass was a maximum of 10% and the estimated error in the determination of coercivity (Hc) was (50 Oe. Powder B exhibited ferrimagnetic behavior, with an Ms of 35 emu/g, an Hc of 1620 Oe, which is typical for hard magnets, and a remanent magnetization (Mr) of 16 emu/g. Powder A showed inferior magnetic properties with respect to powder B, with an Ms of 10 emu/g, an Hc of 400 Oe, and an Mr of 2.5 emu/g.15 Such low magnetization values are typical for very small nanoparticles and are a consequence of the small particle size and the significant influence of the magnetically disordered particles’ surfaces.26 However, the Hc value was not as low as expected for such 10 nm BaF particles. A closer TEM inspection revealed that powder A contained a few larger particles with diameters of up to 100 nm (Figure 1), which was the reason for the relatively high Hc value. Suspensions. Powder A was dispersed in 0.032 M nitric acid, and 10% DBSa per powder mass was added. The adsorption of DBSa onto the particle surfaces was obtained when the suspension was held at 100 °C for 2.5 h. The powder was removed from the suspension by centrifugation and was then washed with water and acetone. To increase the adsorption of the surfactant onto the particles in powder B, its suspension in nitric acid was held at 100 °C for 2.5 h. After this, powders A and B were dispersed in 1-butanol under ultrasound. A pulsed ultrasound of 300 W (VCX500 Ultrasonic Processor, Sonics & Materials) was used for 5 min with a pulse of 2 s on and 1 s off. (See Figure S2 in the Supporting Information for photographs of both suspensions.) The zeta potentials of the saturated suspensions in 1-butanol were measured with single-point measurements (ZetaProbe Analyzer, North Attleboro and Zeta PALS Zeta Potential Analyzer, Brookhaven Instruments Corporation), taking into account the solvent’s dielectric constant (17.84), viscosity (2.99 mPa s), and refractive index (1.3993). The concentrations of the suspensions were analyzed after the preparation and after the centrifugation at 5000 rpm by weighing the suspension before and after a heat treatment at 460 °C. In this way, the total concentration of DBSa was also determined. The concentration of DBSa, dissolved in 1-butanol, was determined from a measurement of the conductivity (Conductometer Knick
Portamess, cell constant 0.475) using a standard addition method. To the BaF suspension in 1-butanol, a known amount of DBSa solution with a concentration of 10 mmol/L was added. During the addition of the DBSa solution, the conductivity of the suspension was measured. The concentration of free DBSa in the suspension (Table 1) was determined from a linear extrapolation of the conductivity versus the added DBSa concentration to 0 mS/cm. The volume fraction of adsorbed DBSa on the particle surfaces (Table 1) was calculated from the difference between the total and the free concentration of DBSa considering the volume of the double layer. Dried particles from the suspensions were observed with the TEM, as described above in the Powders section. In an additional experiment, the small particles from suspension B were magnetically separated from the larger particles with a Frantz Isodynamic Magnetic Separator (S. G. Frantz Co. Inc.). The applied voltage was 40 V, which gives a magnetic field of 3.7 kOe. Steel wool filler (6.0 g) was inserted into the separation column. A stable suspension (120 mL) with a concentration of 7 g/L was poured into the column when the magnetic field was applied. The smaller particles do not magnetically interact with the filler, so they flow out of the column. The

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Scheme 1. Presentation of the Interaction Energies between Two Approaching BaF Particles, Which Move/Rotate Freely Because of Brownian Motion (Large Separation Distance, l), Orient Because of Magnetic (Em) Attraction, and Agglomerate Because of Em and van der Waals (Evdw) Attractionsa

a

Possible stabilization mechanisms are electrostatic (ER) or steric repulsion (Es). Precise E and l values vary with the particle size. column was then washed with 1-butanol to remove all of the small particles. After this, the magnetic field was turned off and the larger particles were washed out of the column with the 1-butanol. The particle size distribution of both fractions is shown in Figure S3 in the Supporting Information. The small particles were no larger than 30 nm (Figure S4, Supporting Information). Suspension C was prepared from the small particles in a similar way to the other two suspensions. The particles were dispersed in water, and the pH was lowered to 1.5. The suspension was stirred for 2.5 h at 100 °C. Then the particles were separated from the suspension, washed, and dried. The dried particles were redispersed in 1-butanol. The magnetic properties of the two separated fractions were measured as described previously in the Powders section (Table 1 and Figure S5 in the Supporting Information). The small-particle fraction, with an average diameter of 10 ( 5 nm (used in suspension C), exhibited weak magnetic properties (Ms = 6.4 emu/g and Hc = 180 Oe) that were even poorer than those of powder A. As mentioned previously, the very low Ms values are a consequence of the small particle size and the significant influence of the magnetically disordered particle surfaces.26 The largeparticle fraction, however, exhibited typical ferrimagnetic behavior (Ms = 41.3 emu/g and Hc = 1500 Oe) similar to that of powder B. The latter shows a lower Ms because it is a mixture of the two separated fractions. Films. Stable suspensions A
C were used for the preparation of deposits A
C, respectively, using electrophoretic deposition (EPD). The electrophoretic cell was composed of the Al anode and the cathode substrate, which was coated with alumina with a 50-nm-thick Pt layer. The deposits were obtained at a constant voltage of 50 V and a separation distance between the electrodes of 7 mm during the deposition time of 15 min. The deposits were held at 460 °C for 2 h to remove the organic phase. After that, the deposits were sintered at 1150 °C for 5 h to obtain films. The grain sizes, the microstructures, and the thicknesses of the films were investigated with a scanning electron microscope (SEM, Jeol 7600F). The orientations of the grains in the films were calculated from the X-ray patterns obtained with an X’Pert PRO diffractometer (PANAnalytical) using Cu Kα1 radiation and the equation (P
P0)/(1
P0), where Por P0 = ∑I00L/∑ Ihkl is the peak area of the (00L) peaks and Ihkl is the area of all of the peaks. P0 corresponds to the randomly oriented BaHF powder, and P corresponds to the oriented film. The orientations of the grains in the films were also estimated from the magnetic measurements with the VSM, where the magnetic properties were measured out of plane (with the magnetic field applied 14016

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perpendicular to the film plane) and in plane (with the magnetic field applied parallel to the film plane).

eq 3b.34,35 ER ¼

’ THEORY AND CALCULATION

" εr ε0 r1 r2 1 þ e
kl 2ψ1 ψ2 ln ER ¼ 4ðr1 þ r2 Þ 1
e
kl  þ ðψ1 2 þ ψ2 2 Þ lnð1
e
2kl Þ

Interaction Forces between Particles in a Polar Solvent.

The interactions between the two approaching BaF particles are presented in Scheme 1. The studied BaF particles are so small that the Brownian motion prevents their sedimentation because of gravity (Table S1, Supporting Information), and at the same time they move and rotate randomly in the absence of any external or interparticle forces. In this case, an effective particle can be regarded as a sphere with a diameter of 2r. When two BaF particles in a solvent approach each other, they first become attracted by the magnetic dipole
dipole force. Single-domain BaF particles align with their large planes together and form columnar agglomerates. Namely, a single-domain magnetic particle is fully saturated, so it aligns in the direction of an applied magnetic field, which is in this case the magnetic easy axis of BaF that is perpendicular to the large plane of the particle. Singledomain BaF particles have sizes of between 10 and several hundred nanometers,27 like the sizes of the studied particles. Consequently, the BaF plates cannot be approximated with spheres anymore but rather with thin discs. At even smaller separation distances of up to a few nanometers, they are additionally attracted by the van der Waals force. The respective interaction energies, Em and Evdw, for the geometry shown in Scheme 1 can be calculated with eqs 1 and 2, respectively.28
31 Equation 2a is valid for two flat surfaces with the interacting area Sint, and eq 2b is valid for two spherical particles. Em ¼


μ0 πF2 Ms1 Ms2 V1 2 V2 2 4D3

Evdw ¼


Evdw

ASint 12πl2

ð1Þ

ð2aÞ


A y y þ 2 ¼
12 x2 þ xy þ x x þ xy þ x þ y  x2 þ xy þ x þ 2 ln 2 ð2bÞ x þ xy þ x þ y

where x = l/2r1 and y = r2/r1. Here we considered crude BaF particles with no surfactant layer having a thickness h and a radius r, where the suffix 1 stands for the larger of the two particles, 2 represents the smaller of the two particles, F = 5300 kg/m3 is the particle density, D is the separation distance between particle centers, and μ0 = 4π  10
7 J/A2 m is the permeability of a vacuum. The Hamaker constant (A) for the BaF/1-butanol system, which was estimated as in refs 32 and 33, was approximately A ≈ 5.3  10
20 J. Stable suspensions can be prepared when a large enough electrostatic and/or steric repulsion is provided between the particles by the application of surfactants or polymers.14,34
38 The Derjaguin
Landau
Verwey
Overbeek (DLVO) theory describes the electrostatic interaction between charged particles in polar solvents based on the formation of an electrical double layer (Scheme 1). The electrostatic interaction energy (ER) between two flat BaF particles can be calculated using eq 3a, and for two spherical particles it can be calculated using

εr ε0 ψ2 k 1
e
2kl Sint 2kl 2π e
e
2kl

ð3aÞ

ð3bÞ

where

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u ε ε kT u 0 r 1=k ¼ t 2 e0 ni Zi2

∑i

Here, ψ is the surface potential and can be approximated for organic media with the measured zeta potential (ζ),19,28 k is the reciprocal Debye
H€uckel parameter, ε0 = 8.854  10
12 As/Vm is the permittivity constant of a vacuum, εr is the relative permittivity of a solvent, ni is the number density of ion i in the medium, Zi is the charge of ion i, e0= 1.6022  10
19, As is the elementary electron charge, k = 1.38  10
23 J/K is the Boltzmann constant, and T is the absolute temperature. Macromolecules or polymers that are adsorbed or grafted onto the particle surfaces are responsible for the steric interaction. This steric interaction acts over only relatively short separations (i.e., up to double the thickness of the surfactant layer, 2t in Scheme 1) and consists of two contributions:36
38 (i) the mixing or osmotic interactions due to the mixing of adsorbed surfactant molecules in the overlapping layer (Scheme 1) and (ii) the volume restriction or elastic contribution due to the decrease in the configurational entropy when two surfactant layers approach each other. The mixing steric contribution is repulsive only when the surfactant is soluble in a solvent, meaning that the Flory
Huggins parameter is χ e 0.5.34 In contrast to this, the elastic steric contribution is always repulsive. An exact calculation of the steric energy is very difficult because of the complicated experimental determination of some of the parameters. Regardless of this problem, a strong steric repulsion occurs as soon as the separation distance between the two particles becomes smaller than double the thickness of the surfactant layer (l e 2t). Therefore, some assumptions that simplify the calculations can be applied without any significant influence on the final sign of the total interaction energy, repulsive or attractive. It was shown that in real systems the mixing mechanism alone based on osmotic repulsion can be applied as a good approximation to the estimation of steric repulsion energy.36,37 Using the hard-sphere approximation and considering a good solvent for the surfactant with steric repulsion (χ = 0), the steric repulsion energy (Es) can be calculated with eq 4.29,38 Equation 4a is valid for two discs that are attracted with their large planes (Scheme 1), and eq 4b is valid for two spheres. " ! # kT 1 Es ≈ ln ð4aÞ
ϕ Vint 12πa3 1
ϕ " ! #
   kT 1 πδ2 r1 þ r2 Es ≈ 3 ln 6 þ l
δ
ϕ a 1
ϕ 12 2 ð4bÞ Here, a is a monomer length (in this study, the length of the largest fragment, 0.24 nm, was considered), Vint is the interacting volume, 14017

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Scheme 2. Schematic Presentation of the Forces Acting on the BaF Particles during the EPDa

a

(a) Gravity, g; electrophoretic, El; electro-osmotic, Eo; and hydrodynamic, Hy. (b) In-plane view of an oriented deposit.

and ϕ is the volume fraction of the surfactant in the overlapping layer with a thickness t (2  2 nm in our study). The thickness of the overlapping layer is δ = 0 for l g 2t and δ = 2t
l for l < 2t. Obviously, the steric repulsion, Es > 0 J, occurs only at separation distances l < 2t. The total interaction energy was calculated to be ET = Evdw + Em + ER + Es. The Em increases significantly with the increasing particle volume and the consequent increase in their magnetic moment. In contrast, Evdw becomes significant with respect to Em but only for the smallest particles with small magnetic moments. At the same time, the surfaceto-volume ratio increases with the decreasing particle size and enables the adsorption of a larger fraction of macromolecules per particle volume. Therefore, it is to be expected that the stability of the suspensions will increase with a decreasing particle size. For the further orientation of the BaF particles during the EPD, it is crucial that the particles are dispersed in a solvent. Otherwise, agglomerates of randomly oriented particles would be deposited. Forces Acting on the Particles during the EPD. Three main forces act on the BaF particles during the EPD with the experimental setup used in this study (Scheme 2): gravity, electric, and hydrodynamic. Gravity, in general, orients the particles in parallel with the horizontally positioned electrode
substrate. However, its effect on the orientation of the studied particles is negligible because the rate of Brownian motion exceeds, by several orders of magnitude, the gravity-settling velocity (Table S1, Supporting Information). No significant effect on the particle orientation is expected for the electric (or electrophoretic) force, which is directly related to the electrophoretic mobility. Equation 512,39 shows that the electrophoretic mobility (μEP) is directly proportional to the zeta potential (ζ) and as such to the surface potential of the particles, whereas the permittivity and the viscosity (η) are properties of the solvent. X = 1 for particles with 2r , 1/k and X = 2/3 for particles with 2r > 1/k. μEP ¼ X

ε0 εr ζ η

ð5Þ

No correlation between the geometry of the particles and the electrophoretic mobility can be deduced from eq 5. Similar to this, no effect of the electric field on the orientation of BNT films consisting from anisotropic grains was observed.23 The theoretical and experimental study of Jimenez and Bellini41 on nonspherical (including disklike) colloid particles showed only a minor shape effect on the electrophoretic mobility for weakly charged particles. However, the shape effect increased with the particle charge and could not be neglected for highly charged particles. Mittal and Furst42 showed that a disklike particle can orient along an applied ac field’s direction or perpendicular to it, depending on the frequency. At low frequencies, the particles preferentially aligned perpendicular to the applied field by

connecting the large disk planes. In contrast to this, Oshima40 showed theoretically that cylindrical particles in an incompressible liquid possess a different electrophoretic mobility (up to 20%) in two directions, perpendicular and parallel to their long axis. Verde et al.45 showed that ZnO plates orient in plane with the substrate during the EPD. They explained such an orientation by the effect of hydrodynamic forces, which change their direction in the vicinity of a substrate/electrode, as is presented in Scheme 2a. In a regular flow, the hydrodynamic friction opposes such an orientation because the larger surface induces more friction than the side surfaces and favors the direction of the particle flow with their large plane perpendicular to the substrate.43 The hydrodynamic friction force (FF) depends on the orientation of arbitrarily shaped particles and is given by eq 6.44 FF ¼ fij vj , fij ¼

1 kTðln p þ Ct Þ 3 πηh

ð6Þ

Here, fij is the translational friction tensor or friction coefficient. The friction coefficient strongly depends on the shape of the particles, p = h/2r for disklike particles. Ct = 0.312 + 0.565/p + 0.100/p2 is a numerical constant and is valid for particles with p = 0.1
20. The hydrodynamic force changes its direction in the vicinity of the electrode, where particles tend to aggregate because of the convection caused by the electro-osmosis around the particles (Scheme 2a).20,45 Unfortunately, a high particle concentration diminishes or even prevents the ordering because the electroosmotic effect preferentially induces agglomeration. On the basis of the above, we can expect that the key to an oriented deposition is highly anisotropic and fully dispersed BaF particles in diluted suspensions. Anisotropic and Abnormal Grain Growth during Sintering. During normal grain growth, finer grains possess a much larger driving force for their growth than do large grains.46 From this point of view, one would prefer to use suspensions made from fine particles. However, fine particles with a lower shape anisotropy are expected to show a poorer in-plane orientation than are the larger particles with a larger shape anisotropy. (See the Forces Acting on the Particles during the EPD section.) Therefore, we expected that the optimum solution would be a combination of large particles, which are likely to orient preferentially in plane with the substrate, with fine particles that fill the empty space between the large particles (Scheme 2b). In this case, exaggerated or abnormal growth of the large particles at the expense of the small ones can be expected. Because the difference in the chemical potential across a curved grain boundary provides the driving force for the boundary to move to the center of the curvature, large grains with convex boundaries grow at the expense of small grains with concave boundaries.46 This abnormal grain growth can be additionally promoted by impurities, the presence of a liquid phase, and anisotropic grain growth as a result of the anisotropic crystal structure or the constrained sintering in films.2,23,47
50 Hexaferrites tend to grow anisotropically with extensive growth in the ab plane and limited growth in the c direction, thus forming hexagonal platelike grains.2 Assuming the preferential orientation of the as-deposited particles within the plane of the film, a higher degree of orientation can be expected for the larger grains than for the smaller ones. (See the Forces Acting on the Particles during the EPD section.) It has been shown that the orientation of grains tends to increase in the 14018

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Figure 2. Calculated interaction energy between BaF particles of different sizes with respect to the separation distance with an enlargement on the righthand side. (Legend) Two values show diameters of the interacting particles from suspension B.

direction of the preferred orientation of the large grains.47
49 This can be explained by the driving force for the growth of abnormal grains into normal, which is proportional to (1/rnormal)
(1/rabnormal).51 The abnormal grains can be several orders of magnitude larger than the matrix grains, and they will grow fast until all of the matrix grains are consumed. At this stage, the abnormal grains become normal. In contrast to this, the driving force for the growth of the grains with a homogeneous size is much smaller than for the former. In our study, the growth out of the plane of the film is additionally limited by the constraints of the substrate.23 On the basis of this, we would predict that any grain growth will increase the degree of orientation of the BaF film. Therefore, in an ideal case, the suspension should be composed of large and, at the same time, very thin plates that would assemble during the EPD in parallel with the substrate, together with very small particles that would fill the empty space between the large plates, as is schematically presented in Scheme 2b. However, this is possible only if the electrostatic attraction between the small particles and the electrode is stronger than the magnetic attraction between the deposited particles and the incoming small particles. In the opposite case, the small particles would deposit on top of the large particles, thus forming columns, as shown in Scheme 1. (See also the inset in Figure S2 in the Supporting Information.)

’ RESULTS AND DISCUSSION Interactions between the BaF Particles in a Polar Solvent. Figure 1 shows TEM images of the dried suspensions. Suspension A contains particles of a relatively homogeneous size: platelets with a diameter of 11 ( 3 nm. No agglomeration was observed between the small particles. The high-magnification image shows that the particles are not in contact with each other. This can be a consequence of a steric barrier, formed by the DBSa, adsorbed at the particle surfaces. A few particles with diameters of around 100 nm were also observed in suspension A, together with a limited agglomeration around them. This agglomeration is most likely to occur during drying because suspension A had a high stability with a high zeta potential of 86 mV (Table 1) and no sedimentation at all. (Figure S2, Supporting Information.) Suspension B (Figure 1) contains small particles as well as larger ones. (See also Figure S1 in the

Figure 3. SEM images of the surfaces of the deposits fired at 460 °C with encircled misoriented particles.

Supporting Information for the particle size distribution.) Most of the particles have diameters of around 20 nm, but a considerably smaller fraction of particles, with diameters larger than 200 nm, was observed. The measured thicknesses were in the range of 3
10 nm for all of the inspected particles. Some agglomeration around the large particles was observed, and it also affected the suspension’s stability. Suspension B remained stable for hours (Figure S2, Supporting Information), and the particles started to settle after 1 day. Consequently, the concentration of dispersed particles decreased from 7 g/L in fresh suspension B to 5.5 g/L in 1-week-old suspension B. Therefore, only fresh suspensions were used for the EPD. Figure 2 shows the calculated total interaction energy (ET = Evdw + Em + ER + Es from eqs 1
4) between two BaF particles of different sizes. The basic material parameters used in the calculations are listed in Table 1. As discussed in the previous section (see also Scheme 1), the BaF particles were considered to be spheres at large separations, where the magnetic attraction energy was negligible with respect to the thermal energy (|Em| < kT), whereas at smaller separation distances the BaF particles were considered to be thin discs. These calculations provide an estimation of the respective energies. (See details on the related assumptions and simplifications in the Theory and Calculation section.) Nevertheless, they explain the experimental results well and are therefore interpreted as such. In suspension A, the repulsion energy prevails over the attraction and the total interaction energy is repulsive (ET > 0 kT), regardless of the separation distance (l). Em in suspension A is negligible in comparison to Evdw. (See also Figure S6 and a related detailed explanation in the Supporting Information.) A high primary maximum (.5kT) confirms the experimentally 14019

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Figure 4. Magnetic hysteresis loops for the deposits fired at 460 °C.

observed high stability for suspension A. The stability of suspension A results mostly from the steric repulsion originating from a large DBSa molecule (benzene ring + C12 chain). Particles of various sizes constitute suspension B (Figures 1 and S1 in the Supporting Information). The stability of suspension B was estimated from the interaction energies between the particles of the main fractions (Figure 2; see also Figure S6 and the corresponding discussion in the Supporting Information). Apart from a primary maximum due to steric repulsion, a secondary minimum is also observed for the pair of particles, where at least one of them is larger than 30 nm (2r1 > 30 nm). The secondary minimum increases with the particle size because of the increasing Em. However, it is partially reduced by the electrostatic repulsion, which cannot be neglected for such large particles. The significant effect of the electrostatic repulsion was also observed experimentally because the rate of sedimentation increased with the increasing conductivity (i.e., with the increasing ionic strength). When the secondary minimum (absolute value) exceeds the thermal energy, the particles agglomerate before they approach close enough to be repelled by the steric barrier. Such an example is shown for a pair of particles with 2r1 = 80 nm and 2r2 = 20 nm (Figure S6, Supporting Information). Therefore, we can conclude that BaF particles that are larger than 50 nm cannot be dispersed in 1-butanol and will agglomerate. However, this agglomeration is not fast: suspension B remained stable for hours. This can be explained by the small concentration of the largest particles (Figures 1 and S1 together with the discussion related to Figure S6 in the Supporting Information). Assembly of the BaF Particles during the EPD. As discussed in the Theory and Calculation section, the hydrodynamic and electro-osmotic forces acting on anisotropic BaF particles in the vicinity of the electrode could induce a preferential orientation of the particles parallel to the substrate. Our results show that this was true and that the BaF particles indeed preferentially oriented in the plane of the substrate electrode. This is presented in Figure 3. We can see that most of the particles on the deposit surfaces are lying in the plane of the film. Some misoriented particles, highlighting the deposits’ surfaces, are also seen. Only a few particles with diameters of around 100 nm can be observed in the matrix of nanosized particles (hardly seen with the SEM because of their small sizes) in deposit A, as in dried suspension A (Figure 1). In contrast to this, a much larger fraction of particles

with diameters of 100
500 nm can be observed in deposit B than in dried suspension B (Figures 1 and 3), suggesting particle growth during the firing at 460 °C. The reason for the limited particle growth in deposit A when compared to that in deposit B is in the higher particle-size homogeneity of the former. The explanation is similar to that in the Anisotropic and Abnormal Grain Growth during Sintering section. The magnetic hysteresis loops (Figure 4) of deposit B measured in two directions clearly show anisotropic behavior, suggesting the preferential orientation of the deposited BaF particles. The larger Hc, Mr, and Mr/Ms ratio for the out-ofplane measurement when compared to those values measured in the plane (Table 2) further suggest that the BaF particles are preferentially oriented in the plane of the film. As mentioned before, the magnetic easy axis of BaF is perpendicular to the particle plane, and when it is parallel to the applied field (i.e., an out-of-plane measurement), larger Mr and Hc values are expected (see the Interactions between BaF Particles in a Polar Solvent in Theory and Calculation section for a more complete explanation) than when the applied field is perpendicular to the easy magnetic axis (i.e., an in-plane measurement). Some misoriented particles, which can be seen in Figure 3, contributed to the Mr and Hc values measured in the plane (Table 2). If we compare deposit A to deposit B, we can see that the former shows no distinct magnetic anisotropy and hence no significant particle orientation. As previously shown,45 the hydrodynamic force in the vicinity of the electrode orients anisotropic particles with their large dimensions in plane with the electrode. Solomentsev et al.20 showed that electro-osmosis favors agglomeration (i.e., the deposition of an incoming particle on an alreadydeposited particle rather than the deposition of a new incoming particle on a free space on the electrode). Because of the magnetic anisotropy of BaF platelets, columnar agglomerates are formed (Scheme 1). Therefore, once the first layer is preferentially deposited in a plane with a substrate the agglomeration should not affect the orientation significantly. The effect of the hydrodynamic forces during the EPD must have been much stronger on the larger particles with the higher shape anisotropy (i.e., a higher diameter-to-thickness aspect ratio) that were very rare in suspension A. The absence of any distinguishable magnetic orientation of deposit A suggests that the hydrodynamic forces had no significant effect on the small particles during the EPD because the shape anisotropy was too low. 14020

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Table 2. Magnetic Properties of the Deposits and Films Where OUT and IN Denote the Out-of-Plane and In-Plane Measurements, Respectivelya sample

a

deposit A

deposit B

film A

deposit C

film B

film C

MsOUT (emu/g)

3.8

11.5

no saturation

13.9

34

45

MrOUT (emu/g) Mr/MsOUT

1.6 0.40

4.6 0.40

2.3 0.30

11.7 0.84

28.5 0.83

10.1 0.45

HcOUT (Oe)

640

1185

1047

4688

3274

1396

MsIN (emu/g)

3.8

no saturation

no saturation

no saturation

no saturation

no saturation

MrIN (emu/g)

1.6

2.1

1.0

5.3

8.2

6.0

Mr/MmaxIN

0.40

0.17

0.11

0.43

0.29

0.15

HcIN (Oe)

640

294

173

3736

2544

1120

Mmax denotes the magnetization at 10 kOe when the magnetization did not saturate up to 10 kOe.

Figure 6. SEM images of the sintered film surfaces.

Figure 5. XRD patterns of sintered BaF films and randomly oriented BaF, where H denotes hematite, Pt denotes platinum, and * denotes corundum.

Anisotropic and Abnormal Grain Growth during Sintering. Figure 5 shows the XRD patterns of the sintered films. Both

patterns show the preferential orientation of the films in the plane because the relative intensities of the (00L) peaks are much higher than the others. For a comparison, the XRD of a randomly oriented BaF powder is also shown. Film B shows a higher degree of orientation, 78%, than film A, 46%. This is to be expected because deposit B showed a higher degree of in-plane orientation than deposit A (Assembly of the BaF Particles during the EPD section). The films also had different thicknesses, which could be the consequence of the different particle sizes. Film A, prepared from smaller particles, had a thickness of 6 μm and was thinner than film B with a thickness of 17 μm. The small thickness of film A is also the reason that the substrate peaks (Pt and * in Figure 5) can be observed together with the hexaferrite peaks in the corresponding XRD. At the same time, two hematite peaks are observed and are much more intense for film A than for film B. Hematite is antiferromagnetic and has no influence on the magnetic orientation of the films. We can see in Table 2 that all of the sintered films show a better orientation than the deposits as well as better magnetic properties in general. The latter can be explained by the improved crystallinity of the BaF grains and the higher density of the films with respect to the deposits, whereas the abnormal grain growth can explain the increased orientation after the

thermal treatment. Figure 6 shows the microstructures of the sintered films. It can be seen that the sintering was accompanied by grain growth (Figures 1 and 3). When compared to the prefired deposits at 460 °C (Figure 3), the grains grow from 10 to 100 nm to 0.8 ( 0.2 μm in film A and from 100 to 500 nm to 1.0 ( 0.3 μm in film B. Abnormal grain growth, which is typical for green compacts with an inhomogeneous grain size, can be observed in both films, although it was more significant in film B, which had a larger grain size distribution than did film A. This could be expected because the particle sizes were larger and less homogeneous in suspension B and in deposit B than in suspension A and deposit A. The diameter ratio between the abnormal and matrix grains is also larger in film B because of the larger particle size in the feedstock powder. The abnormal grain growth dominated in the film plane. It was shown previously that abnormal grain growth occurs in hexaferrites, regardless of the orientation of the large grains in the fine matrix.48 However, when some degree of preferential orientation was induced in a green compact, the orientation increased with the abnormal grain growth.2,47 Lee et al.49 showed experimentally that the large oriented grains formed from smaller ones, which were perfectly oriented in a green compact before sintering. Fu et al.23 used the elongated particles for the preparation of the oriented BNT films with the EPD and observed anisotropic grain growth during sintering that resulted in an increased orientation of the films. The increased orientation of the studied films during the thermal treatment can be explained by (i) the abnormal growth of the large grains that preferentially orient in the film plane during the EPD, as in refs 2 and 47, (ii) the sintering of the highly oriented agglomerates formed during the EPD as in ref 49, and (iii) the constrained grain growth perpendicular to the film plane due to the substrate. The last of these also causes constrained sintering and consequently residual porosity. The preferential orientation of BaF particles within the plane of the film was also confirmed by the magnetic measurements 14021

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Scheme 3. Representation of the Effect of the Alignment of a Single-Domain Particle on the Magnetic Hysteresisa

The arrow shows the direction of the applied magnetic field with respect to the particle.

a

Figure 7. Magnetic hysteresis loops of the sintered films.

(Figure 7). A higher magnetic anisotropy (i.e., a larger difference between the measurements in the plane and out of the plane) was measured for film B than for film A. In a completely magnetically oriented material with an easy magnetization axis, the hysteresis loop is squarelike, with high Hc, Mr, and Mr/Ms values when measured out of plane (Scheme 3): the magnetization saturates (Ms) at a low applied field, and the remanent magnetization (Mr) approaches the Ms value. This means that the sample’s easy magnetic axis coincides with that of the applied field (i.e., it is perpendicular to the sample plane). Because the easy magnetic

axis of BaF is perpendicular to the particle plane, the preferential orientation of the particles in the film plane can be assumed from Figure 7. The Mr/Ms ratio is a measure of the magnetic orientation, which is 1 for a perfectly oriented sample. In contrast to this, the hysteresis loop measured in plane does not show magnetic saturation and has low Mr and Hc values. In this case, the magnetic easy axis is perpendicular to the applied field and a strong applied field is required to reverse the particles’ magnetic moments in the field direction (i.e., to saturate the sample). At the same time, the magnetic moments can flip easily within the plane, resulting in low Hc and Mr values. Therefore, the larger the difference between the magnetic properties measured in the two directions, the higher the magnetic anisotropy and the degree of orientation of the sample. The Mr and Hc values of the studied films measured in plane originate from the misaligned samples. Although Mr is a measure of the mass/volume fraction of misaligned particles, this is not so for Hc. The latter depends on the angle between the magnetic easy axis and the applied field (Scheme 3). Hc is largest when the magnetic easy axis is parallel to applied field (i.e., perpendicular to the particle plane), and it decreases with the increasing angle. Our films show moderate Hc values in plane, which are lower than those out of plane. This indicates that the misaligned particles are aligned at an angle smaller than 90°. Although the Mr/Ms ratio for the out-of-plane measurement (Table 2) was similar for films A and B (0.83 and 0.84), this value was lower for film B when measured in plane (0.29) than for film A (0.41), confirming the higher magnetic anisotropy of film B. The reason for the comparable orientation of the two films is that suspension A also contained some large particles with diameters of 100
300 nm (Figure 1). These large particles could orient in the plane of the film during the EPD. However, their fraction was too small for any measurable magnetic anisotropy in deposit A (Figure 4). Despite this, these large particles can induce abnormal grain growth during sintering (Figure 6). The high Mr/Ms ratio of film A indeed suggests that a large mass/volume fraction of the grains is oriented in the plane of the film. To verify the effect of large particles, suspension C was prepared from only the small-particle fraction obtained by the magnetic separation of suspension B (Suspension section in Materials and Methods). No particles larger than 30 nm in diameter were observed in suspension C (Figures S3 and S4 in the Supporting Information). Film C was prepared from suspension C in the same way as films A and B. The magnetic measurements (Figure 7, Table 2) showed a significantly lower 14022

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Langmuir magnetic anisotropy of film C when compared to those of the other two films: a smaller difference between the in-plane and out-of-plane measurements, a lower Mr/Ms ratio measured out of plane, and equal Hc values measured in both directions. At the same time, we can see that the overall magnetization is higher for film C than for films A and B, suggesting a higher density for this film. This can be a consequence of (i) the highest stability for suspension C, resulting in the highest green density of the deposit and/or (ii) the highest sintering rate for the finest particles. It was shown previously25 that the density of film B can be improved by increasing the suspension’s stability during the EPD that was performed at a lower applied voltage (i.e., at 30 instead of 50 V). In this case the Ms measured out of plane was as high as 55 emu/g and the Mr/Ms ratio was 0.87 whereas a 94% orientation was determined from the XRD analysis. We can conclude that dense BaF films with a high magnetic orientation can be obtained from stable suspensions with a limited fraction of large particles together with very small nanoparticles. Finally, we compare the mechanisms of the orientation of the most promising methods for the preparation of oriented BaF films: liquid-phase epitaxy (LPE), screen printing (SP), and EPD. The substrate with a structure similar to that of BaF induces the crystallization and growth of the BaF film from the melt in the preferred direction during the LPE.10 Prior to this, a BaF seed layer is often deposited on a substrate with pulsedlaser deposition in order to improve the orientation of the film. The orientation of the SP BaF films was not achieved during the SP process but during the subsequent low-temperature annealing under an applied magnetic field.11 Although in both the SP and the EPD presynthesized BaF particles are used as a feedstock, we showed in this study that magnetically oriented BaF films can be prepared without the applied magnetic field. Highly anisotropic BaF particles assemble during the EPD process in the plane of the substrate, and the net magnetic orientation can be further increased by the anisotropic grain growth and abnormal grain growth that accompany the sintering.

’ CONCLUSIONS The magnetic orientation of the Ba ferrite thick films prepared with EPD was studied. It was shown that these magnetically oriented films can be prepared without any external magnetic field. The basic condition for the high degree of orientation is a stable suspension of highly anisotropic particles. In this case, an individual particle orients in the plane with a substrate because of the hydrodynamic forces acting on a particle during the EPD in vicinity of the depositing electrode. The orientation of the deposits was further improved as a result of the abnormal grain growth and the anisotropic grain growth that accompany the sintering at 1150 °C. This was especially effective when a stable suspension of highly anisotropic platelike Ba ferrite particles with an inhomogeneous diameter was used for the EPD. The maximum orientation of the films determined with XRD was around 90%. The remanent magnetization nearly reached the saturation magnetization (i.e., it was >80%), suggesting that such films are suitable for applications in the remanent state (i.e., for self-biased applications) with no need for any external magnets. We propose that the same principle can be applied to the preparation of highly oriented films from thin plates in general.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Particle size distribution in powder B. Photographs of suspensions A and B. Characterization of the magnetically separated particles from suspension B. Effect of gravity versus Brownian motion on the particles. Detailed explanation of the interaction between the BaF particles in a polar solvent. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +386-1-4773-872. Fax: +386-12519-385.

’ ACKNOWLEDGMENT This work was financially supported by the Slovenian Research Agency. We acknowledge the CENN Nanocenter for the use of the TEM. We are grateful to Dr. Marko Jagodi
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