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Magnetic submicron mullite coatings with oriented SiC whiskers Zhaoxi Chen, James Townsend, Pavel Aprelev, Yu Gu, Ruslan Burtovyy, Igor Luzinov, Konstantin G Kornev, and Fei Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16572 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Applied Materials & Interfaces

Magnetic submicron mullite coatings with oriented SiC whiskers Zhaoxi Chen1, James Townsend1, Pavel Aprelev1, Yu Gu2, Ruslan Burtovyy1, Igor Luzinov1, Konstantin G Kornev1*† and Fei Peng1*‡

Keywords: alignment, superparamagnetic nanorods, mullite, silicon carbide, magnetic rotation, ceramic thin film

1

Department of Materials Science and Engineering, Clemson University Institute of Optoelectronic and Nanomaterials College of Materials Science and Engineering, Nanjing University of Science and Technology * Corresponding authors † Email: [email protected] ‡ Email: [email protected] 2

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Abstract Addressing the challenge of making ceramic thin films with the in-plane-oriented nanorods, we propose to decorate the nanorods with magnetic nanoparticles and orient them using the external magnetic field. As an illustration, the mullite thin films with embedded and oriented SiC nanorods were synthesized. The SiC nanorods were decorated with the Fe3O4 nanoparticles. A two-step processing route was developed when the nanorods are first oriented in a sacrificial polymer layer. Then, the polymer film with the aligned nanorods was removed by heat-treatment. At the second step, a sol-gel/dip-coating method was applied to produce the mullite composite film. The main challenge was to guarantee that all the nanorods which were initially randomly distributed in the polymer would have time to rotate toward the field direction before complete solidification of the sacrificial layer. Theoretical and experimental analyses of the orientational distribution of the nanorod axes were conducted to identify a relationship between the polymer viscosity, magnetic and processing parameters of the system. In contrast to the ferromagnetic nanorods, the rate of rotation of paramagnetic nanorods and their time of alignment are more sensitive to the magnetic field. This methodology allows manufacturing different ceramic films with aligned nanorods and making non-magnetic ceramic coating magnetic.


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1.

Introduction

Ceramic thin films enjoy broad applications and especially important in applications where the temperature is high and the environment is hazardous and chemically harsh. It is therefore attractive to enrich these films with additional functionalities by embedding some functional inclusions. Thin films with embedded nanoparticles, nanowhiskers, nanoplates and nanorods are on demand by many technologies including catalysis, microwave engineering, magnetic recording and sensing, and solar cell engineering

1-5

. The embedded anisotropic nano-

inclusions, nanorods in particular, brings to the composite the superior mechanical properties and exclusive novel functionalities such as magneto-optic anisotropy, microwave absorbency and controlled heat dissipation

6-14

. The most popular existing approaches to attain the nanorods

ordering in ceramic thin films are 1) the growth of nanorods in a porous template with ordered arrays of uniform nanochannels 8-9, 15, and 2) the deposition of a ceramic thin film on a substrate with a pre-grown forest of nanorods 16-22. In the first approach, the nanoporous films with oriented pore channels serve as the templates for nanorods grown by vapor or electrochemical deposition 8-9, 15. However, processing of the porous templates is often tedious and time consuming

18

. In the second approach, the

arrays of vertically aligned nanorods were grown on substrates by the template-based growth, hydrothermal, patterned electrodeposition or laser deposition methods

16-23

. The most important

limitation of these two technologies is their inability to provide large area films with a uniform orientation of nanorods: uniform deposition over an area exceeding several cm2 appears to be difficult 23. The strategy for the in-plane orientation of anisotropic nanoparticles in ceramic films has not yet been developed and thus remains the main challenge in thin film technology 5. The idea of using electric or magnetic fields to orient an assembly of the field responsive



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nanorods offers many exciting opportunities to fabricate large volumes of composite materials with a desirable anisotropy

5, 24-26

. The main effort in the last decade has been directed on the

development of different strategies to manufacture polymer films with aligned nanostructures 5-6, 14, 24-28

. Ceramic films with the in-plane ordering of nanorods have not been reported yet. In many cases, the fillers of interest in the form of nanorods are diamagnetic, for example,

the Si3N4, SiC, Al2O3 particles are quite popular in applications. Extremely strong magnetic fields have been used for many years to fabricate the bulk ceramic composites out of these materials

29-31

. This approach combines the ceramic processing methods with electrophoretic

deposition of colloidal suspensions and slip casting of fine ceramic powders in strong magnetic fields of greater than 10T

29-31

. It is therefore attractive to adapt this technology for film

fabrication. However, it is desirable to relax the requirements on the field strength. Making the nanorods superparamagnetic or ferromagnetic, one can significantly decrease the applied magnetic field 5, 14, 24-25. Recent studies demonstrated a robust and attractive methodology to make many desirable fillers magnetic by decorating ceramic particles with magnetic nanoparticles responding to sufficiently weak magnetic fields 14, 26, 32. We will follow this methodology. The main challenge in the nanorod ordering during the film fabrication is that the nanorods subject to the forced rotation may not keep in pace with the applied field because of the gradually increased viscous drag of evaporating or curing film. Thus, some nanorods from an initially randomly distributed assembly might not have time to make the full revolution towards the desired field direction prior to the film solidification

6, 12, 33-36

. The condition for complete

alignment of ferromagnetic nanorods in solidifying polymer films was investigated employing the distribution function theory

28, 33, 36

. A phase diagram specifying complete and partial


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alignment of ferromagnetic nanorods was introduced; in particular, the necessary processing conditions such as the strength of magnetic field and the rate of viscosity increase were identified for the nanorods with the given physical properties 5. The phase diagram for paramagnetic nanorods have not been analyzed yet which makes the experimental analysis of the in-plane ordering of nanorods costly and tedious. To capitalize on our understanding of physics of nanorod ordering in polymeric films, we have chosen the sol-gel method as a versatile ceramic processing method enjoying a large library of material compositions, yet having many common features with the processing of polymeric films

37-40

. In this method, ceramic films are formed from the polymer-like solutions

41-42

. Post-

processing of polymeric films, however, represents a great challenge: one has to heat treat the material to the extent when the materials composition changes significantly from being a soft polymeric (gel) to becoming a rigid ceramic. This transformation of the matrix material affects the nanorod ordering; without a special care, the ordering attained in a polymer film gets completely destroyed after firing the film. The challenges in the fabrication of ceramic films include the preservation of nanorods alignment, control of film thickness, elimination of defects caused by the film dewetting and cracking. In this paper we develop a methodology for fabrication of ceramic composite films carefully addressing all these challenges. The SiC nanowhiskers decorated with the iron oxide superparamagnetic particles will be used as the fillers. The properties of these composite fillers have been analyzed in detail in our earlier publication14. These SiC whiskers demonstrate excellent optical, semiconducting, field emission, thermal and mechanical properties, hence are attractive for the high-power, highfrequency and high-temperature applications

43-44

. Mullite (3Al2O3·2SiO2) is chosen as the

matrix material for the film. Mullite demonstrates excellent strength, creep resistance, thermal



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and chemical stability at high temperatures 45-47. It has been widely used in ceramics engineering. Moreover, the mullite-SiC system has been established as a gold standard in ceramics owing to its superior thermo-mechanical properties with the damage self-healing characteristics 48-50. The reliable processing routes for synthesis of bulk mullite-SiC nanocomposites have been developed 48

. However, the mullite thin films with embedded SiC nanorods has not been produced yet. With

the developed methodology supported by the theory of magnetic alignment, these mullite-SiC films are expected to pave the way for new exciting applications.


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2.

Materials preparation and characterization The SiC nanorods were coated with Fe3O4 nanoparticles. The detailed experimental

procedure for the nanorod functionalization can be found in our recent publication14. A single SiC-Fe3O4 nanorod is shown in Figure 1. As has been shown in Ref.

14

, these nanorods are

superparamagnetic.

Figure 1. SEM image of a silicon carbide nanorod decorated with Fe3O4 magnetic nanoparticles

Schematic of the processing rout for fabrication of mullite films with embedded SiCFe3O4 nanorods is shown in Figure 2. The procedure assumes the two important steps: 1) formation of a sacrificial polymeric layer with the aligned nanorods and 2) synthesizing ceramic films covering the aligned nanorods after removal of the polymer matrix by firing.



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Figure 2. Schematic showing the experimental approach

Step 1. The obtained magnetic nanorods were dispersed in a methanol- polyethylene oxide (PEO, Mw = 1,000,000 g/mol, Sigma Aldrich, St. Louis, MO) solution at a polymer concentration of 2.1 wt. %. The obtained suspension was used to prepare a polymer thin film on silicon wafers (1 × 4 cm rectangles) using the dip-coating method. The wafers were dip coated with the nanorod dispersion at the withdrawing speed of 5.4 mm/sec. The dip coated wafer was immediately transferred and dried in a glass vial between the two parallel neodymium magnets (K&J Magnetics, Pipersville, PA). During solidification of the liquid film, the nanorods were oriented under the static magnetic field generated by these magnets. After drying, the obtained samples were heated to 750°C at 0.5°C/min in the air to remove the polymer films. Step 2. Before deposition of the ceramic precursor, the samples were cleaned by plasma (pdc-32g, Harric Plasma, NY, USA) for 5 minutes. Dip-coating the wafer with the solution containing 2.5 wt. % mullite yield and 1.25 wt. % PVP , one forms a precursor ceramic film deposited on top of the cleaned surface 39. The films with different thickness obtained at different withdrawal velocities from 0.17 to 5.4 mm/sec were examined. After deposition, the films were dried for 24 hours at room temperature before heat treatment. The films were heated in argon at 5 °C/min and calcined at 1000°C for 2 hours. Repetitive applications of these two processing 8 ACS Paragon Plus Environment

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steps led to the multilayered structures. The direction of applied magnetic field was changed (e. g. by the 90°rotation) to alter orientations of nanorods within each layer. For fabrication of a free-standing film, the graphite substrate was used for the polymer film deposition following the same procedure. A ceramic precursor was directly deposited onto the polymer film that was dried for 24 hours at 25°C. The substrate was then removed by slowly heating (0.5°C/min) the coated substrate in the air at 750°C. The free-standing film was then heated in argon at 5 °C/min and calcined at 1000°C for 2 hours before the magnetic measurements. The Atomic Force Microscope (AFM) measurements were performed in the tapping mode on a Dimension 3100 (Veeco Instruments, Plainview, NY) microscope. Silicon probes with a spring constant of 50 N/m were used. Imaging was carried out at the 1 to 2 Hz scanning rates. The thickness was measured by the AFM scratch technique

51

. The polymer composites

were further investigated through imaging with a LEXT Optical Profiler (Olympus Scientific Solutions Americas Inc. Waltham, MA). An analysis of the nanorods orientation was carried out according to the previously reported protocol

33

. The microstructure was characterized using

scanning electron microscopy (SEM, Hitachi S4800, Hitachi, Ltd., Tokyo, Japan). The magnetic characterization was done by using the Alternating Gradient Magnetometer (AGM 2900, Princeton Measurements Inc., NJ, USA). Magnetic field strength was characterized using a magnetic probe sensor (DTM-133 Digital Teslameters, GMW associates, San Carlos, CA). For the magnets with the 8cm separation, the field varied from 0.0216 Tesla at the very center of the sample to 0.0320 Tesla at the ends of the wafer. The magnetic field at the center decreased as the distance between the magnets increased. The magnetic fields that were applied to study the nanorod alignment were



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0.0216, 0.0037 and 0.0006 Tesla corresponding to the magnets separation of 8, 14 and 28cm, respectively. Experimental histograms of the nanorods oriented in a particular direction were constructed by counting the number of nanorods present in the 10° sectors and normalizing it by the total number of nanorods in the field of view. To generate each diagram, the orientation angles of more than 500 nanorods were measured.


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3.

Theory of alignment of superparamagnetic nanorods in solidifying films

3.1

Model of the 2D rotation of a paramagnetic nanorod in Newtonian liquid

Alignment of magnetic nanorods in drying/curing liquid films is possible only when the nanorods rotate quickly enough so that the film is still mobile

12, 28, 33

. The distribution of

nanorods within the given angular sector has been recently studied for ferromagnetic nanorods in solidifying polymer films

12, 28, 33

. It remained unclear whether the theory would work for the

paramagnetic nanorods for which magnetic moment can be easily detached from the nanorod axis. Consider a single paramagnetic nanorod suspended in a liquid film; its motion is assumed confined within the plane of the liquid layer. The liquid is assumed Newtonian. Schematic of action of magnetic field and torques on a single rod is shown in Figure 3. We introduce the unit vectors ∥ and , oriented parallel and perpendicular to the nanorod long axis, respectively, with the positive ∥ pointing to the right. Magnetic field vector makes angle with vector ∥ . An essential difference between superparamagnetic and ferromagnetic nanorods is that when ) of superparamagnetic nanorod submitted to external magnetic field, the magnetic moment ( follows magnetic field: when the field vector deviates from the nanorod axis, the magnetic moment follows it leaving the nanorod axis as schematically illustrated in Figure 3. The magnetic moment of a paramagnetic nanorod is a function of the magnetic field B, angle , as well as the bulk susceptibility of the magnetic material. The detailed expressions for the can be found in Supplementary material, Eq. (S1). In contrast to magnetic moment ferromagnetic nanorods, for which magnetic moment does not depend on the magnitude of is proportional to the strength of the field applied field at weak fields, the magnitude of vector ( ∝ ). This approximation of a uniformly distributed magnetic material over the nanorod 11 ACS Paragon Plus Environment


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volume is expected to work well for the rods with thick magnetic shells; our nanorods belong to this case hence in our theory we will use this approximation.

Figure 3. As an illustration of the characteristic steps of the filed-induced alignment, the applied field is considered perpendicular to the nanorod axis at the first moment of time. The field direction does not change, but the nanorod tend to set their long axis parallel to the field.

An applied magnetic field exerts a torque on the nanorod. In contrast, for a ferromagnetic nanorod in a weak magnetic field when the magnetic moment is always aligned with the long axis of the particle (see Supplementary material, Eq. S3). Thus, the angular dependence of the

magnetic torque Γ sin2 for the superparamagnetic nanorods is different from that for the ferromagnetic ones. For a superparamagnetic nanorod the condition /2 is special: at this condition the corresponding magnetic torque is equal to zero. However, this equilibrium is unstable and a


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small perturbation of the angle leads to a non-zero torque; the nanorod rotates clockwise- or counterclockwise, depending on the sign of the perturbation as shown in Figure 3. The viscous drag from the polymer solution opposes the nanorod rotation. Balancing the magnetic torque by the viscous torque, Γη γφ!, where γ is the drag coefficient defined Supplementary material, one arrives at the following equation 52-54:

! −$%&' sin2

(1)

where the characteristic frequency $%&' is introduced as 53-54:

$%&'

()*+,⁄-/.01- 234

,

∙

(2)

where 6 and 7 are the diameter and length of the nanorod, respectively, 8 is the solution viscosity, 9: is the permeability of vacuum, and is the applied magnetic field. Dynamics of rotation of ferromagnetic nanorods is described by the following equation 5, 55

:

! −$%; sin

(3)

where the corresponding characteristic frequency $%; is given by

$%;

),2,?

∙

(4)



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The quadratic dependence of the characteristic frequency of rotation of a superparamagnetic rod on the magnetic field makes the superparamagnetic nanorods more sensitive to small changes in magnetic field as compared to the ferromagnetic rod. In contrast to the ferromagnetic nanorods which are always magnetized, the superparamagnetic nanorods are not magnetized in the absence of magnetic field. Upon application of an external magnetic field, the nanorods get magnetized with their magnetic moment aligned in either positive or negative direction along the long axis. This direction of magnetic moment depends on the initial orientation of the nanorod (Fig. 3). The magnitude of magnetic moment of a single nanorod is a linear function of B, which makes the nanorod very sensitive to the strength of applied field.

3.2.

Kinetics of alignment of an assembly of superparamagnetic nanorods in

Newtonian films. To study the kinetics of ordering of an assembly of non-interacting nanorods under an applied magnetic field, it is convenient to introduce the orientational distribution function F(φ,t) as dN (ϕ , t ) = N t F (ϕ , t ) d ϕ , where dN(φ) is the number of nanorods with the major axes oriented within the angle φ and φ+dφ, Nt is the total number of nanorods in the film and F(φ,t) is the distribution function. According to this definition, the distribution function describes the density of nanorods sitting within the angle φ and φ+dφ. If the nanorods are initially randomly distributed, the distribution function is constant, F(φ,0)=1/(2π). Detailed derivations of F can be found in Supplementary Material, Eq. (S4). This function depends only on parameter $%&' , time t and angle φ. The evolution of F(φ,t) as a function of dimensionless time 2$%&' @ is shown in Figure 4(a). The distribution function shows a non-monotonous behavior with time at angles φ=π/8 and 14 ACS Paragon Plus Environment

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φ=7π/8; a similar behavior was observed with the ferromagnetic nanorods

36

: the population of

nanorods at these angles first increases to reach a maximum, then decreases with time. At the other angles, F exhibits monotonous decrease with time.

Figure 4. (a) Dependence of F(π/8,t), F(π/4,t), F(π/2,t), F(3π/4,t) and F(7π/8,t) on dimensionless time $%IJ @. (b) The profile of F(φ,t) at different dimensionless time moments. (c) Dependence of K &' on viscosity under different magnetic field strength B, the drag coefficient was calculated with A=2.4.

The time at which F(φ,t) reaches its maximum is determined by setting dF(φ,t)/dt=0, which yields the following relation between dimensionless time $%&' @ and angle φ.

A

ACDEF

$%&' @ ln 0 A/CDEF

(5)

Since $%&' @ G 0, the maximum can be reached within (-π,-3π/4), (-π/4, π/4) and (3π/4, π). For ferromagnetic nanorods, the corresponding maximum in distribution function F is found within a semi-plane (-π/2, π/2). It can be seen from Eq. (S4) that F is an even π periodic function of φ. This makes the evolution of nanorods’ population at φ0, -φ0, π-φ0 and -π+φ0, identical, provided that the



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inequality 00, the constant _: has to be always greater than Δ. Figure 4c shows the dependence of K &' on viscosity 8 under different strengths of magnetic field B. The alignment was examined for a realistic set of parameters: the aspect ratio (l/d)=10, susceptibility =5.39, _: =0.45 and Δ=10º,

14

. This implies that 90% of the nanorods will be 17

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oriented parallel to the direction of the applied magnetic field within the angular sector Δ < 0 ± 10°. The time K &' is linearly proportional to 8, and it increases by two orders of magnitude with B increases by one. For ferromagnetic nanorods, the corresponding characteristic time evolves at a slower rate with the change in B, K2; ∝ 1/, see Supplementary material, Eq. (S11)5, 55

. Therefore, the dependence of time of alignment of superparamagnetic nanorods in Newtonian

film is stronger relative to that of ferromagnetic nanorods. The derived estimates for the time of alignment of superparamagnetic nanorods can be used to significantly shorten the process by increasing the field strength. For polymer films with viscosities of about ~10 times of water (~0.01 Pa s), the alignment of the paramagnetic nanorods is spontaneous (K &'

A

⋅ CDEwx⁄y YZ \/Ez+wx⁄

y4YZ \CDEF

4

.

(10)

For comparison, the equilibrium distribution function L, ∞ for ferromagnetic nanorods is given by 33:

L ; , ∞

A

>

A

⋅ CDEwxA⁄y { \/Ez+wxA⁄y { \CDEF 4

|

(11)



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The corresponding probability to find the SP nanorod in the 2] angular sector around is given as:

FF

_ &' , ], ∞ }F/F L′, ∞ ⋅ 6

A

>

arctan dtan ∙ exp

y4YZ

FF

h |F/F

(12)

For comparison, we show the corresponding probability for ferromagnetic nanorods 33:

A

_; , ], ∞ arctan dtan >

F

A

FF

∙ exp y { h |F/F |

(13)

In contrast to ferromagnetic nanorods for which the directions =0 and = are distinguishable implying different directions of the sample magnetization, the probability _, ], ∞ for superparamagnetic nanorods is − periodic. That is, if a superparamagnetic nanorod rotates over an angle , its new orientation will be identical to the previous one. Therefore to cover all orientations of nanorods we can multiply F or P by two with ranging from −/2 to /2. Figure 5 (b) shows the plot of the distribution function L &' , ∞ and L ; , ∞ with m:; m:&' m: 0.2 and m: 1 . For both types of nanorods, the peak value of the distribution function decreases as m: increases, which indicates that less nanorods are captured by the applied field. For example, at m: 0.2 , the peak value of L &' , ∞ increases by approximately three orders of magnitude compared to that of m: 1; the peak value of the peak value of L ; , ∞ increases by two orders of magnitude. The distribution function for superparamagnetic nanorods has a narrower spiky profile and a greater peak value as compared


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to that for the ferromagnetic nanorods, indicating that more superparamagnetic nanorods can be captured when all other parameters are the same. When m: goes to infinity, the peak values of both functions asymptotically approach 1/2.

3.5 Criterion of nanorod alignment in solidifying films Following Ref.

33

, we introduce a quantitative criterion specifying the condition of

nanorod alignment in solidifying films. Assigning a desirable probability level _: for the nanorod alignment and assuming that alignment is practically achieved when the inequality _0, ], ∞ G _: holds true, a critical value of m:&' _: m%&' is obtained by solving eq. (12). The expression for m%&' is given as: m%&'

(14)

Z4 / *+d h

The corresponding parameter for ferromagnetic nanorods was found in Ref. 33: m%;

A

(15)

Z4 / *+d h /

As an example, we take _: =0.9 and ] 10° as the alignment criteria. That is to say, 90% of the nanorods will be oriented parallel to the direction of the applied magnetic field within the angular sector < 0 ± 10°. We analyze the nanorods decorated with ferromagnetic and superparamagnetic nanoparticles of the same iron oxide. Saturation magnetization of the ferromagnetic iron oxide Fe3O4 (bulk) is 4.6×105 A m-1

59

. Susceptibility of superparamagnetic

SiC-Fe3O4 nanorods is of the order of ~5.39 (see Ref. 14 for the characterization details). Figure 6 shows the phase diagrams in terms of B vs. aspect ratio l/d , specifying the necessary conditions to achieve complete alignment of nanorods in thin films with different rheological and solidifying behaviors during solidification. The region of parameters above the line



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corresponding to the range of parameters where m: < m% (with the corresponding superscripts) holds will lead to the alignment of the nanorods before complete solidification. The requisite magnetic field for the defined complete alignment increases with the aspect ratio.

Figure 6. Phase diagrams specifying the range of parameters leading to the complete alignment of nanorods in solidifying films: (a) 8: /K: =0.02 Pa, (b) 8: /K: =2 Pa, (c) 8: /K: =200 Pa. For the small ratio 8: /K: = 0.02 Pa, the ferromagnetic SiC-Fe3O4 nanorods can be aligned more readily (B10-3T), provided that both types of nanorods have the same aspect ratio l/d (l/d 64: for the ferromagnetic nanorods, the required magnetic field is about two orders of magnitude greater than that for the superparamagnetic ones. This makes superparamagnetic large aspect ratio nanorods more attractive in achieving complete alignment in the highly viscous and fast curing thin films.

4. Experimental validation of the theory

In the theory, the nanorods are assumed non-interacting. This requires that the nanorods are well-separated in the drying solution. The measured average distance between nanorods at the higher concentration is 41 ± 20 µm, which is 4 times larger than the average length of the nanorod making the nanorod-to-nanorod interactions an unlikely source for disorientation. Moreover, because of the surface treatment of nanorods according to the protocol of Refs. 60, the probability of their aggregation was significantly reduced. The nanocomposite films with magnetically oriented SiC-Fe3O4 nanorods were obtained using two different PEO concentrations 0.01 wt. % and 0.038 wt. %. The concentration of SiCFe3O4 nanorods in the PEO dry matrix was estimated to be 0.5 wt. % (or ~0.16 vol. %) and 1.84 wt. % (or ~0.58 vol. %), respectively. Figure 7(a) is a stitched image of the composite film with 0.57 vol. % of SiC-Fe3O4 nanorods in the PEO film. The image is 2.4 mm long to show macro25 ACS Paragon Plus Environment


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scale dispersion and orientation. The center of the image is taken at the center of the applied magnetic field. The magnetic field gradient at the edge of the image is on the order of 2×10-4 Tesla/mm. The average nanorods orientation of the entire sample with respect to the direction of the magnetic field is 7.8°.

Figure 7. (a) a stitched image of the composite thin film with 0.57 vol% of SiC-Fe3O4 nanorods labeled as SiCW-MagNP; (b) the distribution of orientation from section I, II and III of the image. Figure 7(b) illustrates the distribution of nanorod orientations for the left, middle, and right portions. The Y-axis values (percent SiCW-MagNP) are the percentage of nanorods within 10° around the selected angles (X-axis values) normalized by the total number of nanorods. The middle of the Figure 7(a) has the highest degree of orientation, along with the lowest strength of applied magnetic field (0.0216 Tesla). The average deviation from the direction of magnetic field is 8.3°, 6.1°, and 8.8° for the left, middle and right portions of the stitched image.

Figure 8 shows the orientation of the SiC-Fe3O4 rods of two different concentrations. With a magnetic field strength of 0.0216 Tesla, the composite with 0.16vol. % of SiC-Fe3O4 rods has higher orientation with 83% of material oriented within 10o of the direction of the field. The 26 ACS Paragon Plus Environment

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composite with 0.58vol. % of SiC-Fe3O4 rods had 75% of the material oriented within 10o of the direction of the field. In general, the difference in the alignment for the two concentrations is quite small. The degree of orientation reduces as a weaker magnetic field (0.0037 Tesla) is applied. When the magnetic field is 0.0006 Tesla, the orientation of the nanorods is almost random. The fitting curves were generated from eq. (12) with adjustable parameter exp − The simulated values were exp −

y4YZ

y4YZ

.

=0.248 and exp − y YZ =0.307 for the low- and high4

concentration samples obtained at 0.0037 Tesla. The values obtained at 0.0216 Tesla were respectively 0.068 and 0.095 for the low- and high-concentration samples. A smaller value of exp −

y4YZ

corresponds to a stronger magnetic field, which is in good agreement with Eq. (S7).

Figure 8. The percentage of aligned of SiC-Fe3O4 nanorods labeled as SiC-MagNP (a) low concentration (0.16%vol) and (b) high concentration (0.58%vol) after removal of PEO.

5. Formation of ceramic thin films with nanorod assembly 5.1 Single and Multi-layered ceramic films



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The SiC-Fe3O4 nanorods remained separated during the polymer removal, the ceramic gel film deposition and the gel-to-ceramic film conversion. Figure 9 shows the orientational distribution of nanorods during each of those steps. Overall, the most significant alignment was achieved in the initial polymer films. The changes in the percentage (∆P) of nanorods within each 10° segment with respect to the initial distribution in polymer films are shown in figure 9 (c-d). For the majority of nanorods, ∆P is less than 5%. This deviation initially appeared after the polymer removal, which is possibly owing to the settlement of nanorods during the polymer decomposition. To better retain the nanorod orientation, the polymer removal is desired to be executed at a slow heating rate. The orientation remains fixed after the gel deposition and gel-to ceramic conversion. This indicates that the nanorods are getting less mobile after they get sintered to the substrate. In general, the differences in nanorod orientations between those processing stages are quite small.


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Figure 9. (a)-(b) The percentage of aligned nanorods in the films with different volume fraction of nanorods: (a) 0.16 vol. % and (b) 0.58 vol. %; (c) and (d) ∆_ – the change in the probability P(φ) of the corresponding samples compared to the distribution in the PEO film.

Figure 10 (a-b) shows the SEM images of a single-layered ceramic film with embedded magnetic SiC-Fe3O4 nanorods. The embedded SiC-Fe3O4 nanorods were well-separated and aligned. No significant segregation of nanorods was observed. The obtained mullite film with embedded magnetic nanorods is dense and crack-free, which is the desired goal for the hightemperature applications when the materials are exposed to the chemically aggressive environment

61

. The mullite matrix with very low oxygen diffusivity can effectively protect the

SiC-Fe3O4 rods from chemical attack such as oxidation 62.



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Figure 10. The SEM micrographs showing the top view of the thin film composite: a single layered film at low (a) and high (b) nanorod concentration; a triple layered film at low (c) and high (d) nanorod concentration; orthotropic layers with low (e) and high (f) nanorod concentration. (scale bar: 100µm)

The mullite matrix obtained from the sol-gel process is nanocrystalline. In our previous study, we demonstrated that the crystallite size of the film can be well controlled by annealing 30 ACS Paragon Plus Environment

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the material at 1000-1200ºC

39

. The hydrolysis and condensation reactions in the sol-gel

processing that has been carefully controlled at ambient temperature lead to the conversion of gel to phase-pure ceramic at relatively low temperatures

38

. Hence, the ceramic thin film matrix

crystallizes at temperature below 1000 °C, which gives rise to the formation of nanocrystalline thin film without further significant grain growth. Significant coarsening of the matrix material is not likely desired because nanorods might segregate at the grain boundaries which compromises the degree of orientations and the film mechanical properties 61. A higher temperature (>1400ºC) processing is not likely desired due to the phase instability of iron oxide nanoparticles in contact with the oxide matrix. Either aggregation to form large particles or undergoing chemical reaction with matrix to form Al3+ containing solid solution result in the changes of magnetic properties of the nanoparticles 63. Figure 10(c-f) shows the SEM images of multilayered mullite films with embedded nanorods. The nanorods remain well separated and aligned in the multilayered films. In Figure 10(c-d), the 3 layers (140nm×3) mullite-SiC-Fe3O4 thin films were obtained from the repetitive deposition. Figure 10(e-f) shows the SEM image of a bi-layer mullite-SiC-Fe3O4 film with embedded nanorods of orthotropic distribution. The distribution statistics of nanorods is shown in Figure 11. The major populations (> ~30%) of SiC-Fe3O4 nanorods are oriented at 0-10º and 80-90º to the direction of the magnetic field.



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Figure 11. The percentage of aligned nanorods in the orthotropic samples

The layer-by-layer (LBL) assembly method offers the opportunity of creating 3D nanocomposites through sequential ordering of nanorods inside the films

64

. Lamination of

composites with a specific orientation of inclusions within each layer is desired for many applications

26

. For example, an orthotropic lattice structure is often desired for effective

mechanical reinforcement in all directions 26.

5.2.Morphology of the film surface

The magnified SEM images of the films with an embedded nanorod are shown in Figure 12. By varying the film thickness (t) and keeping the same nanorod diameter (d), different microstructures were observed. In general, mullite films are smooth, while the SiC nanorods have rough surfaces due to the presence of Fe3O4 NPs. When the film thickness is small as compared to the diameter of the nanorod, the coating does not mitigate the roughness of the SiC


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nanorods. The SiC-Fe3O4 nanorod was distinct from the surrounding film when the film thickness is small (60 nm) compared to the dimension of the rod, as shown in Figure 12 (a).

Figure 12. The SEM images showing the SiC-Fe3O4 nanorods embedded within mullite films. Images (a) and (b) of the embedded nanorods are taken from the top. The film thicknesses in (a) is 60nm and in (b) is 140 nm. (c) The protuberance observed on the film surface, the film thickness is ~500nm. With a slightly thicker ceramic film (140 nm), Figure 12 (b), the contour of the nanorod becomes less distinct from the film matrix. The grainy feature on the nanorod was gradually masked by the ceramic film on the top. The result indicates that nanorods could be embedded completely inside thicker films. The microstructure of thick films (~500 nm) with embedded nanorods is shown in Figure 12 (c). An interesting feature is the surface protuberance of the film induced by the rod. It is observed that surface protuberance can be gradually eliminated by depositing the films of greater thickness. The thickness of the sol-gel processed ceramic film is often restricted by the critical thickness, above which the materials failure occurs

39, 65

. The defect free films are typically less

than 1 µm thick, which is only slightly greater than the nanorod diameter

39, 66

. This restricts the

opportunity of building very thick films to reduce the protrusions. We observed that the composite films without cracks can be formed by controlling the layer thickness. Figure 13 shows the topographic profile of two types of thin films analyzed by 33 ACS Paragon Plus Environment


Page 34 of 55

AFM. The nanorods are placed parallel to the substrate. The zero level of the y-axis is chosen at the substrate surface. The center of the nanorod is positioned at x = 0. When the film thickness is greater than the diameter of the nanorods, a flattened surface with a small protuberance was observed as the film thickness increases. The protuberance becomes pronounced as the film thickness decreases. When the film thickness becomes comparable or smaller than the nanorod diameter, one observes a crack free film with the ceramic matrix conformally coating the nanorod with a very steep profile near the nanorod center.

Figure 13. The profiles of the film surfaces elevated above a 400 nm diameter nanorod. Comparing to the nanorod diameter, the roughness of the film itself is very low (evidenced by the AFM profile shown in Figure 13). Therefore the roughness of the film is determined by the relative height of the surface protuberance and its steepness. With the controlled alignment of nanorods, the protuberances are also aligned, making this material an


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attractive candidate for applications where one needs to control the film topography, for example, to enhance/ hinder wettability of the ceramic film by metals at high temperatures 67.

5.3 Magnetic properties

It is difficult to characterize the magnetic properties of thin films bonded to the silicon substrate, as the weight of the measured magnetic material is not well-defined. And the weight of the substrate is much greater than the film, making it difficult to detect the magnetic response from the film. In order to prove that the composite materials are highly magnetic, the freestanding magnetic film was obtained after removal of the graphite substrate by heating it in the air. The film can be attracted to and lifted by a permanent magnet. Figure 14 shows the magnetization-field (M-H) plot of the free standing mullite-SiC-Fe3O4 films. It shows that the ceramic thin film composites are superparamagnetic. The magnetization of the material is normalized by the total mass of the composite. For superparamagnetic materials, the magnetic moment m follows the Langevin dependence 68: : dcoth −

A

h

(25)

where B is the magnitude of the external magnetic field, : 9, and 9 ⁄ , 9 is the magnetic moment of a single magnetic domain, N the total number of domains in the composite, is the Boltzmann constant and T is the absolute temperature. The magnetic Fe3O4 nanoparticles in the film composite contribute to this superparamagnetism. From the fitting of experimentally measured magnetization curve, one finds that the composite film with an initial concentration of 0.58 vol.% SiC-Fe3O4 rods in PEO has saturation magnetization (Ms) of around 0.41 Am2/kg. The weight for the normalization equals to the weight of the measured freestanding film, which includes the mullite matrix, SiC nanorods and Fe3O4 nanoparticles. Since 35 ACS Paragon Plus Environment


Page 36 of 55

the Fe3O4 nanoparticles are the only component that is magnetic in the membrane, the small Ms is due to large weight percentage of the non-magnetic components. We assume the Ms of the superparamagnetic Fe3O4 nanoparticles being on the same order of magnitude of the reported Ms values, which are typically ~ 60 Am2/kg 69-70. As a rough estimate, the concentration of magnetic materials within the free-standing film is estimated as be on the order of 0.1-1 wt.%. This concentration can be further controlled by altering the ratio between the thicknesses of the polymer films and ceramic gel films. From the Langevin fitting, the magnetic moment of each domain is estimated to be on the order of 4.7×10-20 Am2.

Figure 14. The M-H curve of a free-standing mullite-SiC-Fe3O4 film.


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6

Conclusions

A novel two step protocol for fabrication of ceramic films with aligned magnetic nanorods was developed. At the first step, the nanorods were aligned in a polymer sacrificial layer. Then the polymer was burned out and a ceramic precursor was deposited. A sol-gel processing was employed to synthesize mullite submicron films with embedded and aligned nanorods. We derived a quantitative criterion for nanorod alignment in solidifying liquid films. The alignment of superparamagnetic nanorods was theoretically studied to reveal the process features distinguishable from those for ferromagnetic nanorods. The characteristic time of rotation of superparamagnetic nanorods is more sensitive to the strength of applied magnetic field (quadratic dependence). We constructed a phase diagram identifying the process parameters when the nanorods can be fully aligned in solidifying polymeric films. This diagram was experimentally validated. The macroscopic alignment of superparamagnetic SiC-Fe3O4 nanorods was achieved in solidifying Polyethylene oxide films. The measured orientation distribution after polymer removal was compared with the theoretical predictions. Mullite thin film composite with aligned SiC-Fe3O4 nanorods was obtained through the sol-gel processing of a deposited ceramic precursor after removal of the polymer film. The alignment was retained through the process. This approach was further extended to develop multilayered structures through repetitive layer-by-layer deposition. The layer-by-layer formation of different ordered structures provides intriguing opportunities of making ceramic thin films and bulk composites for many advanced applications, such as controlled heat dissipation, polarization rotation and mechanical enhancement. The surface of ceramic films with the embedded nanorods demonstrates novel bumpy topography.



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This feature can be well controlled by varying the ratio between the film thickness and nanorod diameter. As refractory materials, these films offer new applications for control of the film wettability and fluid transport at high temperatures.


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Figure 1. SEM image of a silicon carbide nanorod decorated with Fe3O4 magnetic nanoparticles

Figure 2. Schematic showing the experimental approach


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Figure 3. As an illustration of the characteristic steps of the filed-induced alignment, the applied field is considered perpendicular to the nanorod axis at the first moment of time. The field direction does not change, but the nanorod tend to set their long axis parallel to the field.

Figure 4. (a) Dependence of F(π/8,t), F(π/4,t), F(π/2,t), F(3π/4,t) and F(7π/8,t) on dimensionless time $%IJ @. (b) The profile of F(φ,t) at different dimensionless time moments. (c) Dependence of K &' on viscosity under different magnetic field strength B, the drag coefficient was calculated with A=2.4.



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Figure 5. (a) Different dynamic regimes of nanorod rotation. The initial values at U =0 for different curves are 0 = 45°, 75°, 89.1° and 135°. The arrows indicate the directions of rotation of the FR and SP nanorods initially oriented at 89.1° and 135°. (b) distribution functions for ferromagnetic (FR) and superparamagnetic (SP) nanorods when m0 =mq&' mq; .

Figure 6. Phase diagrams specifying the range of parameters leading to the complete alignment of nanorods in solidifying films: (a) 8: /K: =0.02 Pa, (b) 8: /K: =2 Pa, (c) 8: /K: =200 Pa.


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Figure 7. (a) a stitched image of the composite thin film with 0.57 vol% of SiC-Fe3O4 nanorods labeled as SiCW-MagNP; (b) the distribution of orientation from section I, II and III of the image.

Figure 8. The percentage of aligned of SiC-Fe3O4 nanorods labeled as SiC-MagNP (a) low concentration (0.16%vol) and (b) high concentration (0.58%vol) after removal of PEO.



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Figure 9. (a)-(b) The percentage of aligned nanorods in the films with different volume fraction of nanorods: (a) 0.16 vol. % and (b) 0.58 vol. %; (c) and (d) ∆_ – the change in the probability P(φ) of the corresponding samples compared to the distribution in the PEO film.


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Figure 10. The SEM micrographs showing the top view of the thin film composite: a single layered film at low (a) and high (b) nanorod concentration; a triple layered film at low (c) and high (d) nanorod concentration; orthotropic layers with low (e) and high (f) nanorod concentration. (scale bar: 100µm)

Figure 11. The percentage of aligned nanorods in the orthotropic samples



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Figure 12. The SEM images showing the SiC-Fe3O4 nanorods embedded within mullite films. Images (a) and (b) of the embedded nanorods are taken from the top. The film thicknesses in (a) is 60nm and in (b) is 140 nm. (c) The protuberance observed on the film surface, the film thickness is ~500nm.

Figure 13. The profiles of the film surfaces elevated above a 400 nm diameter nanorod.

Figure 14. The M-H curve of a free-standing mullite-SiC-Fe3O4 film.


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Magnetic submicron mullite coatings with oriented SiC whiskers - ACS

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