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The effect of rare-earth dopants on the texturing of alumina under high-strength magnetic field
Materials Chemistry and Physics 241 (2020) 122388
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The effect of rare-earth dopants on the texturing of alumina under high-strength magnetic field Carli Moorehead, Victoria Blair *, Nicholas Ku, Raymond Brennan Weapons and Materials Directorate, Army Research Laboratory, 6300 Rodman Rd, Aberdeen Proving Ground, MD, 21005, USA
H I G H L I G H T S
� Rare-earth dopants enhance the response of alumina to a magnetic field. � Dopant enhancement enables the use of weaker magnetic fields in ceramic processing. � Magnetically aligned bodies can be produced and sintered using colloidal casting. � Dopant type can affect the magnetic easy-axis of the alumina crystal. A R T I C L E I N F O
A B S T R A C T
Keywords: Al2O3 Magnetic alignment Microstructure texturing Colloidal casting ISOBAM
The use of magnetic fields in ceramic processing has been of interest recently for tailoring the microstructural grain alignment of ceramics in order to improve mechanical, optical, and other properties. However, a number of challenges exist, including the low magnetic response of most ceramic materials (e.g. alumina), and the lack of low viscosity bulk casting systems available for forming ceramics in a magnetic field. In this work, the low magnetic response of alumina was addressed by adding small amounts (~0.04 atom%) of various rare-earth dopants to phase-pure α-alumina to enhance the response of the bulk material, as supported by previous pre dictive modeling. After identifying the dopant with the highest magnetic response according to a novel, average facial angle alignment metric, a low-viscosity colloidal casting system, based on a single organic additive was selected to allow for the sintering of formed green bodies. This novel process was used to cast bulk ceramic parts under a magnetic field. The effect of magnetic alignment both in the green state and after sintering was inves tigated. It was found that both erbium and ytterbium improved the magnetic response of alumina, and that the alignment achieved in the green state was maintained through the sintering process, despite removal from the magnetic field.
1. Introduction High strength magnetic fields are increasingly being used to affect the microstructure of bulk ceramic materials [1–7] for applications that require improved mechanical properties [8] or polycrystalline trans parency [9]. The use of high strength magnetic fields leads to bulk ceramic parts with preferential alignment of the crystallographic planes along the magnetic field axes under which they were processed [1–7]. This phenomenon is possible since diamagnetic and paramagnetic ma terials, which cover the majority of bulk ceramics, still possess some, albeit low, degree of magnetic susceptibility [1,3–5,10]. Magnetic sus ceptibility is defined as the ratio of magnetization produced in a material in response to an applied magnetic field [11] and as such, is an indicator
of the magnetic field strength required to affect the crystallographic texture, as materials with lower susceptibilities require much stronger fields [12]. Many ceramics also exhibit magnetic susceptibility anisotropy, in which different susceptibilities are associated with different crystallo graphic directions [1–7]. Due to the magnetic susceptibility anisotropy, such crystals will align to the axis with the largest magnitude of mag netic susceptibility in the direction of the field. This provides a potential mechanism for preferred crystal orientation in non-ferromagnetic ma terials [1–7]. The response is governed by a set of primary equations derived from mechanics. The full derivation can be found in Ref. [13] where first principal mechanics were applied to hexagonal, non-ferromagnetic metal crystals in which the magnetic susceptibilities
* Corresponding author. E-mail address: [email protected] (V. Blair). https://doi.org/10.1016/j.matchemphys.2019.122388 Received 31 May 2018; Received in revised form 13 August 2019; Accepted 30 October 2019 Available online 4 November 2019 0254-0584/Published by Elsevier B.V.
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were equal in both the a- and b-axis directions, but not the c-axis di rection. However, this derivation is applicable to other ‘non-magnetic,’ hexagonal materials that also have equal a- and b-axis susceptibilities, and has been specifically applied to alumina previously (see Refs. [2,5,6, 14]). For alumina, the magnetic susceptibility difference (χ jj χ? , where || and ꓕ ? denote the susceptibilities parallel and perpendicular to the c-axis, respectively) is equal to 7.1 � 10 8 emu/mol, where both χ ? and χ jj are negative since alumina is diamagnetic [4]. While magnetic field processing can be applied to any ceramic pos sessing magnetic anisotropy, very high field strengths (10–12 T) neces sitating a superconducting magnet are often necessary [2,17]. Therefore, it is also of interest to determine if a ceramic material’s response to magnetic fields can be enhanced. Density functional theory (DFT) modeling has predicted that select dopants can both increase the magnetic susceptibility and change the crystallographic magnetic easy axis in alumina, and thus change which plane is aligned to the magnetic field direction [14,18]. Consequently, it is of interest to study the re sponses of various doped alumina materials cast under the same mag netic field. Furthermore, DFT predicts that the crystallographic phase of alumina can affect the magnetic easy axis [14,18]. For this reason, the contrast in response between a phase-pure α-alumina and a mixture of θand α-alumina, which is often used to produce nano-grained alumina parts, is of interest. Previous work has investigated the response of mixed θ-/α-phase alumina [14,16]. However, it was determined that the Lotgering factor (as defined in Refs. [8,14]) was an inaccurate repre sentation of the degree of alignment at lower values, as it relied on the ability to detect the (006) peak in order to distinguish alignment. Since the (006) peak is typically ~0.6% of the maximum peak height in a randomly aligned sample (powder diffraction file 00-010-0173, Syn thetic Corundum, space group R3 c), it is normally considered to be negligible at low alignment values. Therefore, regardless of any changes to primary peak intensities, the Lotgering factor indicated that no alignment had occurred. By replacing the Lotgering metric with a novel average facial angle, figure-of-merit (FOM) metric (as proposed by Asai et al., see Ref. [13]), which generates a percent alignment value ac counting for the deviation of all peaks from their normal heights, a more sensitive assessment was provided. For this reason, all of the previous results from the mixed-phase epoxy alignment study [14] were re-calculated using the average facial angle method, and presented here for comparison. In order to successfully implement a magnetic alignment technique, a low viscosity casting system is required that allows the grains to rotate in response to the magnetic field before hardening to lock them into place [13,15]. Previous success has been achieved using an epoxy-based system loaded with 20 wt% alumina powder [14]. However, phase-pure materials were not used [14,16] and the green parts were not sinterable due to their high organic content. This study examines the effect of rare-earth dopants on phase-pure α-alumina for chemically enhancing the magnetic response of alumina. In addition to undoped α-alumina samples, four rare-earth dopants (erbium, praseodymium, ytterbium, and gadolinium) were utilized to produce Er:Al2O3, Pr:Al2O3, Yb:Al2O3, and Gd:Al2O3 samples, respectively. The effect of dopant type was assessed by calculating a percentage of grains aligned from the X-ray diffraction (XRD) patterns according to the facial angle method proposed by Asai et al. After identifying Er as the dopant that most effectively enhanced the magnetic response, a novel colloidal casting system was successfully incorporated to cast, align, and sinter the powders to produce micro-textured, phasepure Er-doped α-Al2O3.
alumina powders in an aqueous environment [14,19,20]. Salt solu tions of the appropriate starting materials were first prepared. The acidic solution (~pH ¼ 3) consisted of a 7.5 M aqueous solution of aluminum nitrate, 250 ppm of magnesium nitrate, and stoichiometric amounts of RE nitrates to obtain a composition of RE0.002Al1.998O3 (RE ¼ Er, Pr, Gd, Yb, or Al in the case of undoped powders). The basic solution consisted of an aqueous solution of 11 wt% ammonium bicarbonate and 3 wt% ammonium hydroxide. After the initial solutions were obtained, a third buffer solution of 2 wt% aqueous ammonium bicarbonate was prepared. The buffer pH was adjusted to ~7 using nitric acid. The prepared solutions were coneutralized in the buffer solution by adding each of them drop-wise while maintaining a pH of 7. A suspension of hydrated aluminum car bonate precipitate in buffer was obtained. Next, the solution was aged, stirring vigorously overnight. The suspension was filtered from the re action solution, and the resulting powder was washed twice with DI water and once with isopropyl alcohol before drying at 60 � C for approximately 96 h. The dry powder was then crushed and calcined for 30 min at 1350 � C for Er, Yb, Gd, and undoped powders, and 1400 � C for Pr-doped powders (using heating and cooling rates of 10 � C/min). Different temperatures were used to ensure that all powders converted to phase-pure α-alumina, since it was previously identified that the dopant type can affect the phase-transition temperature (see Ref. [20] for more information). Phases were confirmed with XRD, and all pow ders were found to be phase-pure α-Al2O3. The powders were re-ground in a glass mortar and pestle following calcination. 2.2. Epoxy sample preparation Two-part, room-temperature curing epoxy (EP1112NC Clear, Res inlab, Germantown, WI) was used to conduct initial magnetic alignment experiments and screen out dopants that did not lead to improvements [14]. Alumina-methanol slurries were prepared and ball milled with alumina milling media for 24 h to break up agglomerates in the calcined powders. After milling, the slurry was added to the resin component of the epoxy kit such that the final ratio of alumina to cured epoxy would yield 20 wt%. The mixture was combined using a wrist-action shaker, and the methanol evaporated off in a hot water bath (~60 � C). This resulted in an alumina powder suspended in resin, which was stored until ready to align. The alumina/resin suspension was added to the calculated amount of hardener immediately prior to magnetic align ment. The parts were mixed with a wooden stirring stick and separated into two lubricated one-inch cylindrical plastic potting cups, one aligned radially under a 1.8 T (T) magnetic field, and the other serving as the control (no field applied). The epoxies were allowed to cure for approximately 8 h until a soft cure had been achieved, then removed from the field. They were then allowed to cure completely overnight before demolding, cutting, and analyzing. Three samples of each con dition were prepared, cut, and analyzed (n ¼ 3 for both the control and the in-field conditions) unless otherwise noted. 2.3. Colloidal-casting sample preparation Commercial samples of an isobutylene and maleic anhydride 1:1 copolymer (trade name ISOBAM) were obtained from Kuraray (Kuraray America, Elastomer BU, Houston, TX). ISOBAM #104 with an average molecular weight of 55–65 kDa (kDa) was used. After initial experimentation, it was found that a slurry containing 70 wt% Al2O3, 0.3 powder wt% ISOBAM #104, 0.75 wt% DARVAN C–N (de-gassing agent; Vanderbilt Minerals, LLC. Norwalk, CT [21]), and the balance water resulted in good gelling properties (gelling time within 6 h; solids loading resulting in water-like viscosity, etc.). The slurry was produced by first preparing a pre-mixture in a glass beaker on a mag netic stirring plate. A stir bar was used to dissolve the ISOBAM in the appropriate amount of water before adding as much powder as possible without causing the slurry to seize. After mixing for approximately
2. Experimental procedure 2.1. Preparation of phase-pure α-alumina powders A co-precipitation method was used to synthesize amorphous, pre2
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20 min, the slurry was added to a Nalgene mill jar one-quarter full of 1 cm alumina milling media. The remainder of powder was gradually added, allowing a stable dispersion to form while preventing the slurry from seizing. Following the final addition, the jar was quickly sealed with parafilm™ and ball milled vigorously for 24 h to break up agglomerates. After milling, the slurry was cast and aligned using a 1.8 T magnetic field. Two, one-inch diameter potting cups were positioned for the alignment experiment, with one placed in the center of the static mag netic field, and another under standard lab conditions as a control. The molds were previously coated with a thin film of WD-40 (WD-40 Co., San Diego, CA) to prevent wall interaction stress that could lead to cracks in the green bodies as they gelled and dried. Next, the slurries were cast directly into the mold cups and allowed to set for ~4.5 h until a soft gel developed. For the slurry under magnetic field, this step was also designed to prevent misalignment once the field was removed. After initial gelling had been achieved, the casts were removed from the field and allowed to continue drying/gelling overnight before being demol ded and dried for another ~48 h at 60 � C.
2 a* ¼ pffiffi a 3
(2)
1 c
(3)
c* ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 dhkl ¼ � 2 2 h2 þ k2 þ hk a* þ l2 c*
(4)
where a and c are the unit cell parameters for the material. In the case of
α-alumina, a ¼ 4.758 Å and c ¼ 12.991 Å were used [18]. The (006) peak
was chosen as the reference peak (hereafter denoted the ‘alignment plane’) since the magnetic easy-axis of α-alumina is known to corre spond to the c-axis and the orientation of the samples and subsequent sectioning, would place the (006) plane parallel to the surface of the sample in a perfectly aligned specimen. By calculating the average interplanar angles (with respect to the (006) plane) for each XRD pattern, weighted on the basis of the indi vidual peak intensities (which varied with increasing alignment), an ‘average interplanar angle,’ henceforth referred to as the average facial angle (as proposed by Asai [13]), was determined for each specimen, as described in Equation (5) [13]. P ðIhkl Þ ðΦhkl Þi ΦF ¼ P i (5) ðIhkl Þi
2.4. Sintering of colloidally-cast samples An Er-doped, aligned green body cross-section was sintered and analyzed to determine the effects of densification on alignment pro duced during the green forming process. Sintering was carried out in an alumina boat according to the following temperature profile: heat at a rate of 5 � C/min to 1000 � C; hold for 60 min at 1000 � C before heating at 10 � C/min to 1550 � C; hold for 120 min at 1550 � C before cooling at 10 � C/min to room temperature. The sample was analyzed before and after sintering according to the XRD procedure described in section 2.5. Additionally, the polished cross-section was imaged under environ mental SEM.
where (Ihkl)i denoted the intensity of the ith peak. The (012), (104),
(110), (006), (113), (024), (116), (018), (214), (300), (1010), and (119) peaks (those visible in the experimental XRD pattern) were used in the calculation. The average angle changed with respect to the degree of alignment and could therefore be used to quantitatively track the amount of alignment achieved [13]. ΦF ¼ 0� occurs when there are only (00l) reflections present in the XRD pattern indicating a perfect align ment of the c-axis of all crystals along the applied magnetic field di rection. In contrast, ΦF ¼ 90� occurs when there are only (hk0) reflections in the XRD pattern indicating perfect alignment of the c-axis of all crystals perpendicular to the applied field direction. In both cases, preferential alignment occurred, but in fundamentally different align ment directions indicating a difference in the magnetic easy-axis of the crystals. Since the average facial angle, as defined by Asai [13], was denoted in units of degrees and varied between 0 and 90, it was difficult to delineate how much improvement was achieved by alignment over that of a randomly aligned sample. Decreased average facial angle with respect to the reference pattern, clearly indicates improved alignment, on average, but it is difficult to establish by how much without knowing the average facial angle for the reference pattern. Furthermore, a decrease in average facial angle by a given amount, does not necessarily correspond to an increase in alignment by the same amount since the reference average facial angle changes depending on the reference plane according to the inherent geometry of the crystal. Consequently, a figure-of-merit normalization was performed by adapting the Lotgering equation [8,14] to compare the observed average facial angle (ΦF ) with that of the average facial angle of a perfectly random specimen (ΦF r , as defined by the PDF card). This normalization results in a percent alignment value for each specimen and was calculated according to Equation (6):
2.5. X-ray diffraction analysis of magnetic alignment XRD analysis of the cured epoxy pucks and green and sintered gelcast bodies was conducted using a previously established protocol [14]. First, the pucks were cross-sectioned perpendicular to the applied magnetic field direction and mounted for XRD analysis using Cu-Κα radiation at 30 kV, 15 mA (Rigaku MiniFlex II, Tokyo, Japan). XRD scans were performed over a 34–45� 2θ range at a step angle of 0.01� 2θ and a scan rate of 1� 2θ/min for all samples. Additionally, 20–80� 2θ scans at the same settings were performed on all the cured epoxy samples in order to investigate if the magnetic easy axis had shifted due to dopant type. The resulting scans were analyzed using Jade 8 software (MDI, Liv ermore, CA). For each scan, the background was automatically removed, the peaks were identified, and pattern fitting was conducted according to the peak height values. As described in the introduction, the percent alignment FOM was calculated based on the average facial angle (as proposed by Asai [13]) for each scan using the powder diffraction file #00-010-0173 (Synthetic Corundum, space group R3 c) and Equations (1)–(4). The interplanar angle, Φhkl , is the angle between two crystallo graphic planes and can be calculated by means of the following relation [22]: � � �� 1 2 2 ðΦhkl Þi ¼ cos 1 dhkl dh’k’l’ hh’ þ kk’ þ ðhk’ þ kh’ Þa* þ ll’c* (1) 2
% alignment ¼
where the subscript i indicates a specific angle between a particular plane (hkl), apparent in the XRD pattern of α-alumina, and a given reference plane (h’k’l’), and dhkl, a*, and c* are the spacings between successive planes and the reciprocal unit cell parameters, respectively, as defined for the hexagonal unit cell in Equations (2)–(4) below:
ΦF r ΦF x 100 ΦF r
(6)
where ΦF r denotes the average facial angle of the powder diffraction card with respect to the same reference plane. While this calculation results in a percentage alignment, it is recognized that this does not represent the percentage of planes fully aligned to a given direction, nor the percentage of rotation achieved for the majority of crystals in the 3
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Table 1 Average percentage alignment values with respect to the (006) plane � standard deviation for phase-pure α-alumina epoxy samples. The third column also lists the percent alignment values re-calculated using the average facial angle method from the previous mixed-phase alumina epoxy alignment samples for comparison. Dopant
Gd Yb Er Pr Undoped a
Phase-pure α-alumina Alignment (%)
Mixed-phase Alumina Alignment (%) [14]
0T 0.94 � 0.74 0.05 � 0.34 0.26 � 0.40 0.14 � 0.34 0.28 � 0.63
1.8 T 2.74 � 8.60 9.99 � 1.92 14.33 � 0.58a 1.13 � 2.80 0.65 � 0.21
1.8 T 2.27 � 1.14 11.55 � 0.88 16.15 � 5.05 1.61 � 0.37 1.97 � 0.91
Table 2 Magnetic easy-axes identified previously through DFT modeling [14] and the corresponding crystal planes with expected XRD peak enhancements relative to the baseline XRD patterns in aligned samples. Some directions did not corre spond to measurable XRD planes. Dopant
Predicted Magnetic Easy-axis
Gd Yb Er Pr Undoped
<0 <3 <1 <2 <1
0 1> 2 10> 2 0> 3 2> 2 0>
Corresponding Crystal Plane (006) (4130) (010) (146) (010)
Positive values indicate better average alignment to a given direction with respect to a randomly aligned sample, and negative values indicate preferential alignment in a different direction to that which was used as the reference direction. Two-tailed, two-sample t-tests were conducted to compare the alignment of the 1.8 T samples to the control samples. An alpha value of α ¼ 0.05 was used. Since it was of interest to determine if the magnetic easy-axis shifted with dopant type, the percent alignment was determined by changing the reference plane (h’, k’, and l’ indices from Eq. (1)) for each visible crystallographic plane in the XRD pattern of α-alumina, including the (012), (104), (110), (006), (113), (024), (116), (018), (214), (300), (1010), and (119) peaks. The percent alignment calculated with respect to each XRD peak (rather than just the (006) plane), was color-mapped into a three-dimensional schematic of the reduced alumina hexagonal unit cell using custom code written in MATLAB (2016b, Mathworks, Natick, MA) to show the degree of alignment corresponding to each plane in real space with color intensity contrast. This novel technique was developed as a visualization tool to confirm the axis of alignment. Since intermediate values of alignment were achieved, and additional planes exhibited a corresponding increase in alignment, it was necessary to confirm that the multiple peaks showing high alignment values were not an artifact produced by the assessment procedure.
Indicates n ¼ 2.
Fig. 1. Percentage alignment values for each dopant type. * indicates statisti cally significant difference from 0 T controls. # indicates statistically significant difference compared to the undoped 1.8 T controls.
3. Results and discussion 3.1. Degree of alignment for α-alumina samples vs. mixed-phase alumina samples Table 1 lists the average percent alignment values (with respect to the (006) plane) and standard deviations for each condition, while Fig. 1 compares the percent alignment across dopants. Only the Er, Yb, and Pr magnetically aligned samples had statistically significant percent alignment values compared to the undoped control samples, with Gd samples achieving similar alignment. As shown in Table 1 and Fig. 1, the Er-doped samples demonstrated the strongest response to the 1.8 T magnetic field, followed by Yb- and Gd-doped samples. These results were consistent with observations from the mixed-phase alumina epoxy samples (mixture of α- and θ-alumina) that were re-calculated using the average facial angle calculation in Table 1 [14]. However, it was diffi cult to draw strong conclusions from the mixed-phase samples due to the large standard deviations in alignment. In contrast, Pr exhibited a slightly negative alignment with respect to the random powder, which was also consistent with the mixed-phase samples at 1.8 T (Table 1) and 9 T ( 20.5%) [14], even after the percentage alignment value was re-calculated using the average facial alignment method. The negative alignment was likely due to a change in the alignment direction, as predicted by DFT modeling (see Section 3.2). This is reflected in Fig. 2, which shows representative XRD patterns for the Er and Pr epoxy samples, both aligned and unaligned. The Er sample shows an increase in the (006) peak with alignment, which is absent from the aligned Pr pattern. The (104), (110), and (113) peak heights also show changes in the aligned samples with respect to the control, which is captured by the average facial angle alignment metric as opposed to the Lotgering factor metric.
Fig. 2. Representative XRD patterns of the Pr and Er sampled, aligned and control. Curves were vertically translated for clarity.
part. Rather, this value represents the average texture of the specimen and the relative amount of texture improvement needed to result in perfect crystal alignment. As such, it is a useful figure-of-merit to quantitatively compare the alignment achieved across conditions, dop ants, and crystallographic directions. The percent alignment values for each condition were calculated for each sample and averaged across the sample repeats within each group. 4
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Fig. 3. Alumina crystal cell schematic showing the planes visible on the XRD pattern ((012), (104), (110), (006), (024), (116), (018), (214), (300), (1010), (119)) with associated percent alignment expressed as a color intensity and indicated by the color maps to the right of each drawing. The left column shows the controls while the right column shows the magnetically-aligned samples. All color scales are the same for all drawings. For reference, the (006) plane is outlined in black to show its proximity relative to the rest of the planes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
It is important to note that while it has been established that crys tallite radius affects the alignment time, with larger crystallites aligning faster (see Ref. [15]), the radius has been found to have a negligible impact when compared to other processing variables (e.g. viscosity,
magnetic field strength, and magnetic susceptibility anisotropy) when the electrical conductivity of the material is low, such as in alumina [15]. While the crystallite size distribution of each powder type was likely different (�~100–150 nm), and in theory could affect the 5
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Fig. 3. (continued).
Table 3 Average percent alignment values with respect to the (006) plane � standard deviation for phase-pure α-alumina gel casting samples. Dopant Er Undoped a
α-alumina casting Alignment (%)
α-alumina Epoxy Alignment (%)
0T
1.8 T
1.8 T a
0.55 � 0.22 0.78 � 0.32
21.32 � 5.57 1.13 � 0.82
a
16.15 � 5.05 1.97 � 0.91
Indicates n ¼ 2.
alignment since relatively long alignment times were used for these experiments (on the order of hours as opposed to seconds), the effects of these differences were negligible. Changes in alignment were attributed solely to the addition of the dopant. 3.2. Alignment of α-alumina vs. DFT predictions Table 2 lists the magnetic easy-axes for each of the dopants tested, as calculated by the DFT modeling described in Refs. [14,18]. The planes corresponding to the XRD peaks, which were expected to be enhanced (with respect to the baseline patterns) given the sectioning geometry used, were also identified for each of the dopants based on the DFT calculations (Table 2). However, since some of the predicted directions corresponded to planes that exhibited no distinguishable XRD peaks, it was not experimentally feasible to track alignment using the previously described XRD method. This was true for both the Yb and Pr dopants.
Fig. 4. Percent alignment values for each powder type. * indicates statistically significant difference from the 0 T controls. # indicates statistically significant difference compared to the undoped 1.8 T control.
Additionally, the DFT calculations did not predict alignment of the undoped alumina crystal with c-axis parallel to the field direction due to the non-magnetic perfect crystal obtained in DFT, which excluded the 6
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the (006) plane. This produced an inherent bias in the visualization technique used, although the marked improvement over the un-aligned sample indicates that this bias was likely negligible. In the samples that exhibited the lowest alignment values with respect to the (006) plane, either (a) a small amount of alignment was achieved with respect to every plane, as is the case in the Gd samples, or (b) alignment increased corresponding to a direction that was not the caxis direction, as in the case of Pr. When examining the alignment vi sualizations for Pr, alignment was observed to occur preferentially for directions, resulting in negative alignment values for the (006) and spatially-close planes. Furthermore, this alternative alignment axis did not correspond to the one predicted by the DFT calculations, again indicating that the model was accurate for predicting that some dopants could change the alignment direction of alumina crystals, but not to which axis the preferred alignment would shift. 3.3. Green body analysis Fig. 5. XRD pattern of the unaligned green body, aligned green body, and sintered aligned body. The (006) peak at approximately 41.5� 2θ increases with alignment. Curves were vertically translated for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 3 lists the calculated alignment values for each of the condi tions tested, and Fig. 4 compares the alignment of Er-doped alumina powders to undoped powders generated with 95% pure aluminum ni trate, both cast under a 1.8 T magnetic field. While the rare-earth doped samples showed a much higher degree of alignment, the undoped sample alignment was also statistically significant (α ¼ 0.05) with respect to the control. Furthermore, for both powders, it appeared that higher alignment was also achieved for the colloidally-cast samples than for the epoxy-cast samples, although this result is difficult to establish in the Er-doped powder due to the low number of samples. This apparent increase in alignment, compared to the epoxies, was attributed to better powder dispersion in the colloidally-cast slurry, as well as a lower vis cosity, since the colloidally-cast slurries (once milled) were much more fluid than the epoxy suspensions although further investigation would be necessary to confirm this trend. 3.4. Sintered body analysis Fig. 5 shows that the XRD pattern changed after sintering. In the green body state, the sample exhibited 21.3% alignment with respect to the (006) plane; after sintering, this unexpectedly increased to 32.4%. It had been hypothesized that the addition of thermal energy and the rearrangement of particles during sintering would lead to significantly lower alignment values with respect to the green part [15]. The findings do not support this hypothesis, demonstrating that alignment achieved during the green state could be maintained upon sintering. This was initially attributed to particle steric interactions impinging grain rota tion once water was removed from the green body and the part consolidated, despite the increase in thermal energy. However, the SEM cross-sectional image in Fig. 6 shows necking at the particle contact points with no further densification, indicating that the selected sin tering profile needs to be modified in order to densify the α-alumina nanopowders. Further optimization is required to determine whether or not complete densification will result in a reduction of the alignment achieved during colloidal green body processing.
Fig. 6. SEM image of polished cross-section of sintered, aligned, gel-cast part.
low concentration effects of point defects in experimental solids [14,18]. Due to inherent scaling limitations, the DFT calculations used to predict the axes of alignment and magnetocrystalline anisotropy energies were conducted for higher dopant concentrations (0.83 atom%). This likely gave rise to the deviations observed, though the overall prediction of dopant effects on alignment was realized. Fig. 3 shows the alignment visualizations in real space for each sample (based on an average of three samples), with the (006) plane outlined in black for reference. For the samples that showed higher alignment values, such as Er and Yb (Table 1), the planes corresponding to a high percent alignment were grouped in close proximity to the expected (006) alignment plane of undoped alumina. As the torque was applied, particles may have been prevented from moving into complete alignment for a number of reasons, including viscosity, steric in teractions between particles, agglomeration, mold-wall friction, etc. The increase in alignment for planes in close proximity to the expected alignment plane was believed to be indicative of crystals that began to rotate into alignment but were prevented from achieving complete alignment. However, it is important to note that due to the crystal structure of α-alumina, most of the visible peaks were located close to
4. Conclusions This work demonstrates that chemical augmentation of alumina by doping with rare-earth ions can significantly improve the magnetic response of alumina, enabling manipulation by lower strength magnetic fields. Er-doped alumina exhibited the highest response (~16% align ment with respect to the (006) plane), followed by Yb-doped, and Gddoped (despite the much higher total magnetic moment) [14,18]. Pr-doped alumina exhibited a reduction in alignment along the (006) direction. However, the alignment visualization results across the entire unit cell indicated that the negative alignment was indicative of align ment in an alternate direction (in contrast to negative alignment along 7
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References
<00l>). In general, the trends for the phase-pure α-alumina samples were consistent with those in the mixed-phase alumina samples analyzed previously [14], but further investigation with mixed-phase or phase-pure θ-alumina would be necessary to confirm this trend due to the large errors associated with the mixed-phase samples. While the DFT models were unable to predict the effects of specific dopants, they were able to predict that dopants, in general, could produce a change in the preferred alignment axis, as was observed for Pr. An assessment of additional dopants using DFT modeling and experimental evaluation would be necessary to study this effect further. After investigating the effect of different dopants on the magnetic properties of alumina using the epoxy system as a proof-of-concept, Erdoped alumina was successfully aligned using a colloidal casting system that allowed for sintering of the final part. It was found that the align ment achieved during the green forming process was maintained in the final sintered part, even without application of a magnetic field. While agglomerates were likely still present, despite considerable milling, this study demonstrated the efficacy of magnetic alignment as a processing method for fabricating bulk micro-textured parts without the need for an alternative processing methods such as templated grain growth [23]. Furthermore, the use of RE-dopants to enhance the magnetic response of alumina and allow weaker magnetic fields to effect alignment (acces sible through electromagnets rather than superconducting magnets), was also demonstrated. Further optimization is necessary to produce fully dense RE-doped parts so that the effect of alignment on mechanical and optical properties of alumina can be investigated.
[1] Z.H.I. Sun, X. Guo, M. Guo, J. Vleugels, O. Van der Biest, B. Blanpain, Alignment of weakly magnetic metals during solidification in a strong magnetic field, J. Alloy. Comp. 551 (2013) 568–577. [2] T.S. Suzuki, Y. Sakka, K. Kitazawa, Orientation amplification of alumina by colloidal filtration in a strong magnetic field and sintering, Adv. Eng. Mater. 3 (7) (2001) 490–492. [3] N. Terada, H.S. Suzuki, T.S. Suzuki, H. Kitazawa, Y. Sakka, K. Kaneko, N. Metoki, In situ neutron diffraction study of aligning of crystal orientation in diamagnetic ceramics under magnetic fields, Appl. Phys. Lett. 92 (11) (2008) 3. [4] N. Terada, H.S. Suzuki, T.S. Suzuki, H. Kitazawa, Y. Sakka, K. Kaneko, N. Metoki, Neutron diffraction texture analysis for alpha-Al2O3 oriented by high magnetic field and sintering, J. Phys. D Appl. Phys. 42 (10) (2009) 5. [5] L. Zhang, J. Vleugels, O. Van der Biest, Slip casting of alumina suspensions in a strong magnetic field, J. Am. Ceram. Soc. 93 (10) (2010) 3148–3152. [6] A. Makiya, D. Shouji, S. Tanaka, N. Uchida, Ttp, Grain oriented microstructure made in high magnetic field, in: Euro Ceramics Vii, Pt 1-3, Trans Tech Publications Ltd, Zurich-Uetikon, 2002, pp. 445–448. [7] P. Gillon, Uses of intense d.c. magnetic fields in materials processing, Mater. Sci. Eng. A - Struct. Mater. Prop. Microstruct. Process. 287 (2) (2000) 146–152. [8] Z.G. Yang, J.B. Yu, K. Deng, L. Lan, H. Wang, Z.M. Ren, Q.L. Wang, Y.M. Dai, H. Wang, Fabrication of textured Si3N4 ceramics with beta-Si3N4 powders as raw material by gel-casting under strong magnetic field, Mater. Lett. 135 (2014) 218–221. [9] R. Apetz, M.P.B. van Bruggen, Transparent alumina: a light-scattering model, J. Am. Ceram. Soc. 86 (3) (2003) 480–486. [10] R. Libanori, F.B. Reusch, R.M. Erb, A.R. Studart, Ultrahigh magnetically responsive microplatelets with tunable fluorescence emission, Langmuir 29 (47) (2013) 14674–14680. [11] J. Baugh, D. Han, A. Kleinhammes, Y. Wu, Magnetic susceptibility and microstructure of hydrogenated amorphous silicon measured by nuclear magnetic resonance on a single thin film, Appl. Phys. Lett. 78 (4) (2001) 466–468. [12] J.F. Schenck, The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds, Med. Phys. 23 (6) (1996) 815–850. [13] T. Sugiyama, M. Tahashi, K. Sassa, S. Asai, The control of crystal orientation in non-magnetic metals by imposition of a high magnetic field (vol 43, pg 855, ISIJ Int. 43 (11) (2003) (2003) 1866-1866. [14] C.A. Moorehead, V.L. Blair, K.R. Limmer, R.E. Brennan, J.W. Adams, Preferred Orientation of Rare Earth (RE)-doped Alumina Crystallites by an Applied Magnetic Field, US Army Research Lab, Aberdeen Prooving Ground, MD, 2016, pp. 1–20. [15] C.A. Moorehead, M.M. Kornecki, V.L. Blair, R.E. Brennan, Sensitivity Analysis and Simulation of Theoretical Response of Ceramics to Strong Magnetic Fields, US Army Research Lab, Aberdeen Prooving Grounds, MD, 2016, pp. 1–18. [16] R. Brennan, C. Moorehead, V. Blair, K. Limmer, Energy Coupled to Matter for Magnetic Alignment of Rare Earth–Doped Alumina, Materials and Manufacturing Processes, 2016, pp. 1–6. [17] A. Szudarska, T. Mizerski, Y. Sakka, T.S. Suzuki, M. Szafran, Fabrication of textured alumina by magnetic alignment via gelcasting based on low-toxic system, J. Eur. Ceram. Soc. 34 (15) (2014) 3841–3848. [18] K.R. Limmer, M.R. Neupane, R.E. Brennan, T.L. Chantawansri, Rare-earth dopant effects on the structural, energetic, and magnetic properties of alumina from first principles, J. Am. Ceram. Soc. 99 (12) (2016) 4007–4012. [19] T. Sanamyan, R. Pavlacka, G. Gilde, M. Dubinskii, Spectroscopic properties of Er3 þ-doped α-Al2O3, Opt. Mater. 35 (5) (2013) 821–826. [20] K. Patel, V. Blair, J. Douglas, Q. Dai, Y. Liu, S. Ren, R. Brennan, Structural effects of lanthanide dopants on alumina, Sci. Rep. 7 (2017) 39946. [21] C.A. Moorehead, V.L. Blair, J.M. Sietins, Improved de-gassing and deagglomeration of isobam gel casting system for alumina using micro-computed tomography, in: Proceedings of the 41st International Conference on Advanced Ceramics and Composites, John Wiley & Sons, Inc., 2018, pp. 183–194. [22] A. Kelly, K.M. Knowles, Appendix 3: Interplanar Spacings and Interplanar Angles, Crystallography and Crystal Defects, John Wiley & Sons Ltd, 2012, pp. 469–472. [23] M.M. Seabaugh, I.H. Kerscht, G.L. Messing, Texture development by templated grain growth in liquid-phase-sintered α-alumina, J. Am. Ceram. Soc. 80 (5) (1997) 1181–1188.
Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of competing interest None. Many thanks to Dr. Krista Limmer for her work on the DFT modeling and discussion in the preparation of this paper. This research did not receive any specific grant from funding agencies in the public, com mercial, or not-for-profit sectors. Research by CM and NK was sponsored by the Army Research Laboratory and was accomplished under Coop erative Agreement Number W911NF-16-2-0008. The views and con clusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The effect of rare-earth dopants on the texturing of alumina under high-strength magnetic field Carli Moorehead, Victoria Blair *, Nicholas Ku, Raymond Brennan Weapons and Materials Directorate, Army Research Laboratory, 6300 Rodman Rd, Aberdeen Proving Ground, MD, 21005, USA
H I G H L I G H T S
� Rare-earth dopants enhance the response of alumina to a magnetic field. � Dopant enhancement enables the use of weaker magnetic fields in ceramic processing. � Magnetically aligned bodies can be produced and sintered using colloidal casting. � Dopant type can affect the magnetic easy-axis of the alumina crystal. A R T I C L E I N F O
A B S T R A C T
Keywords: Al2O3 Magnetic alignment Microstructure texturing Colloidal casting ISOBAM
The use of magnetic fields in ceramic processing has been of interest recently for tailoring the microstructural grain alignment of ceramics in order to improve mechanical, optical, and other properties. However, a number of challenges exist, including the low magnetic response of most ceramic materials (e.g. alumina), and the lack of low viscosity bulk casting systems available for forming ceramics in a magnetic field. In this work, the low magnetic response of alumina was addressed by adding small amounts (~0.04 atom%) of various rare-earth dopants to phase-pure α-alumina to enhance the response of the bulk material, as supported by previous pre dictive modeling. After identifying the dopant with the highest magnetic response according to a novel, average facial angle alignment metric, a low-viscosity colloidal casting system, based on a single organic additive was selected to allow for the sintering of formed green bodies. This novel process was used to cast bulk ceramic parts under a magnetic field. The effect of magnetic alignment both in the green state and after sintering was inves tigated. It was found that both erbium and ytterbium improved the magnetic response of alumina, and that the alignment achieved in the green state was maintained through the sintering process, despite removal from the magnetic field.
1. Introduction High strength magnetic fields are increasingly being used to affect the microstructure of bulk ceramic materials [1–7] for applications that require improved mechanical properties [8] or polycrystalline trans parency [9]. The use of high strength magnetic fields leads to bulk ceramic parts with preferential alignment of the crystallographic planes along the magnetic field axes under which they were processed [1–7]. This phenomenon is possible since diamagnetic and paramagnetic ma terials, which cover the majority of bulk ceramics, still possess some, albeit low, degree of magnetic susceptibility [1,3–5,10]. Magnetic sus ceptibility is defined as the ratio of magnetization produced in a material in response to an applied magnetic field [11] and as such, is an indicator
of the magnetic field strength required to affect the crystallographic texture, as materials with lower susceptibilities require much stronger fields [12]. Many ceramics also exhibit magnetic susceptibility anisotropy, in which different susceptibilities are associated with different crystallo graphic directions [1–7]. Due to the magnetic susceptibility anisotropy, such crystals will align to the axis with the largest magnitude of mag netic susceptibility in the direction of the field. This provides a potential mechanism for preferred crystal orientation in non-ferromagnetic ma terials [1–7]. The response is governed by a set of primary equations derived from mechanics. The full derivation can be found in Ref. [13] where first principal mechanics were applied to hexagonal, non-ferromagnetic metal crystals in which the magnetic susceptibilities
* Corresponding author. E-mail address: [email protected] (V. Blair). https://doi.org/10.1016/j.matchemphys.2019.122388 Received 31 May 2018; Received in revised form 13 August 2019; Accepted 30 October 2019 Available online 4 November 2019 0254-0584/Published by Elsevier B.V.
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were equal in both the a- and b-axis directions, but not the c-axis di rection. However, this derivation is applicable to other ‘non-magnetic,’ hexagonal materials that also have equal a- and b-axis susceptibilities, and has been specifically applied to alumina previously (see Refs. [2,5,6, 14]). For alumina, the magnetic susceptibility difference (χ jj χ? , where || and ꓕ ? denote the susceptibilities parallel and perpendicular to the c-axis, respectively) is equal to 7.1 � 10 8 emu/mol, where both χ ? and χ jj are negative since alumina is diamagnetic [4]. While magnetic field processing can be applied to any ceramic pos sessing magnetic anisotropy, very high field strengths (10–12 T) neces sitating a superconducting magnet are often necessary [2,17]. Therefore, it is also of interest to determine if a ceramic material’s response to magnetic fields can be enhanced. Density functional theory (DFT) modeling has predicted that select dopants can both increase the magnetic susceptibility and change the crystallographic magnetic easy axis in alumina, and thus change which plane is aligned to the magnetic field direction [14,18]. Consequently, it is of interest to study the re sponses of various doped alumina materials cast under the same mag netic field. Furthermore, DFT predicts that the crystallographic phase of alumina can affect the magnetic easy axis [14,18]. For this reason, the contrast in response between a phase-pure α-alumina and a mixture of θand α-alumina, which is often used to produce nano-grained alumina parts, is of interest. Previous work has investigated the response of mixed θ-/α-phase alumina [14,16]. However, it was determined that the Lotgering factor (as defined in Refs. [8,14]) was an inaccurate repre sentation of the degree of alignment at lower values, as it relied on the ability to detect the (006) peak in order to distinguish alignment. Since the (006) peak is typically ~0.6% of the maximum peak height in a randomly aligned sample (powder diffraction file 00-010-0173, Syn thetic Corundum, space group R3 c), it is normally considered to be negligible at low alignment values. Therefore, regardless of any changes to primary peak intensities, the Lotgering factor indicated that no alignment had occurred. By replacing the Lotgering metric with a novel average facial angle, figure-of-merit (FOM) metric (as proposed by Asai et al., see Ref. [13]), which generates a percent alignment value ac counting for the deviation of all peaks from their normal heights, a more sensitive assessment was provided. For this reason, all of the previous results from the mixed-phase epoxy alignment study [14] were re-calculated using the average facial angle method, and presented here for comparison. In order to successfully implement a magnetic alignment technique, a low viscosity casting system is required that allows the grains to rotate in response to the magnetic field before hardening to lock them into place [13,15]. Previous success has been achieved using an epoxy-based system loaded with 20 wt% alumina powder [14]. However, phase-pure materials were not used [14,16] and the green parts were not sinterable due to their high organic content. This study examines the effect of rare-earth dopants on phase-pure α-alumina for chemically enhancing the magnetic response of alumina. In addition to undoped α-alumina samples, four rare-earth dopants (erbium, praseodymium, ytterbium, and gadolinium) were utilized to produce Er:Al2O3, Pr:Al2O3, Yb:Al2O3, and Gd:Al2O3 samples, respectively. The effect of dopant type was assessed by calculating a percentage of grains aligned from the X-ray diffraction (XRD) patterns according to the facial angle method proposed by Asai et al. After identifying Er as the dopant that most effectively enhanced the magnetic response, a novel colloidal casting system was successfully incorporated to cast, align, and sinter the powders to produce micro-textured, phasepure Er-doped α-Al2O3.
alumina powders in an aqueous environment [14,19,20]. Salt solu tions of the appropriate starting materials were first prepared. The acidic solution (~pH ¼ 3) consisted of a 7.5 M aqueous solution of aluminum nitrate, 250 ppm of magnesium nitrate, and stoichiometric amounts of RE nitrates to obtain a composition of RE0.002Al1.998O3 (RE ¼ Er, Pr, Gd, Yb, or Al in the case of undoped powders). The basic solution consisted of an aqueous solution of 11 wt% ammonium bicarbonate and 3 wt% ammonium hydroxide. After the initial solutions were obtained, a third buffer solution of 2 wt% aqueous ammonium bicarbonate was prepared. The buffer pH was adjusted to ~7 using nitric acid. The prepared solutions were coneutralized in the buffer solution by adding each of them drop-wise while maintaining a pH of 7. A suspension of hydrated aluminum car bonate precipitate in buffer was obtained. Next, the solution was aged, stirring vigorously overnight. The suspension was filtered from the re action solution, and the resulting powder was washed twice with DI water and once with isopropyl alcohol before drying at 60 � C for approximately 96 h. The dry powder was then crushed and calcined for 30 min at 1350 � C for Er, Yb, Gd, and undoped powders, and 1400 � C for Pr-doped powders (using heating and cooling rates of 10 � C/min). Different temperatures were used to ensure that all powders converted to phase-pure α-alumina, since it was previously identified that the dopant type can affect the phase-transition temperature (see Ref. [20] for more information). Phases were confirmed with XRD, and all pow ders were found to be phase-pure α-Al2O3. The powders were re-ground in a glass mortar and pestle following calcination. 2.2. Epoxy sample preparation Two-part, room-temperature curing epoxy (EP1112NC Clear, Res inlab, Germantown, WI) was used to conduct initial magnetic alignment experiments and screen out dopants that did not lead to improvements [14]. Alumina-methanol slurries were prepared and ball milled with alumina milling media for 24 h to break up agglomerates in the calcined powders. After milling, the slurry was added to the resin component of the epoxy kit such that the final ratio of alumina to cured epoxy would yield 20 wt%. The mixture was combined using a wrist-action shaker, and the methanol evaporated off in a hot water bath (~60 � C). This resulted in an alumina powder suspended in resin, which was stored until ready to align. The alumina/resin suspension was added to the calculated amount of hardener immediately prior to magnetic align ment. The parts were mixed with a wooden stirring stick and separated into two lubricated one-inch cylindrical plastic potting cups, one aligned radially under a 1.8 T (T) magnetic field, and the other serving as the control (no field applied). The epoxies were allowed to cure for approximately 8 h until a soft cure had been achieved, then removed from the field. They were then allowed to cure completely overnight before demolding, cutting, and analyzing. Three samples of each con dition were prepared, cut, and analyzed (n ¼ 3 for both the control and the in-field conditions) unless otherwise noted. 2.3. Colloidal-casting sample preparation Commercial samples of an isobutylene and maleic anhydride 1:1 copolymer (trade name ISOBAM) were obtained from Kuraray (Kuraray America, Elastomer BU, Houston, TX). ISOBAM #104 with an average molecular weight of 55–65 kDa (kDa) was used. After initial experimentation, it was found that a slurry containing 70 wt% Al2O3, 0.3 powder wt% ISOBAM #104, 0.75 wt% DARVAN C–N (de-gassing agent; Vanderbilt Minerals, LLC. Norwalk, CT [21]), and the balance water resulted in good gelling properties (gelling time within 6 h; solids loading resulting in water-like viscosity, etc.). The slurry was produced by first preparing a pre-mixture in a glass beaker on a mag netic stirring plate. A stir bar was used to dissolve the ISOBAM in the appropriate amount of water before adding as much powder as possible without causing the slurry to seize. After mixing for approximately
2. Experimental procedure 2.1. Preparation of phase-pure α-alumina powders A co-precipitation method was used to synthesize amorphous, pre2
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20 min, the slurry was added to a Nalgene mill jar one-quarter full of 1 cm alumina milling media. The remainder of powder was gradually added, allowing a stable dispersion to form while preventing the slurry from seizing. Following the final addition, the jar was quickly sealed with parafilm™ and ball milled vigorously for 24 h to break up agglomerates. After milling, the slurry was cast and aligned using a 1.8 T magnetic field. Two, one-inch diameter potting cups were positioned for the alignment experiment, with one placed in the center of the static mag netic field, and another under standard lab conditions as a control. The molds were previously coated with a thin film of WD-40 (WD-40 Co., San Diego, CA) to prevent wall interaction stress that could lead to cracks in the green bodies as they gelled and dried. Next, the slurries were cast directly into the mold cups and allowed to set for ~4.5 h until a soft gel developed. For the slurry under magnetic field, this step was also designed to prevent misalignment once the field was removed. After initial gelling had been achieved, the casts were removed from the field and allowed to continue drying/gelling overnight before being demol ded and dried for another ~48 h at 60 � C.
2 a* ¼ pffiffi a 3
(2)
1 c
(3)
c* ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 dhkl ¼ � 2 2 h2 þ k2 þ hk a* þ l2 c*
(4)
where a and c are the unit cell parameters for the material. In the case of
α-alumina, a ¼ 4.758 Å and c ¼ 12.991 Å were used [18]. The (006) peak
was chosen as the reference peak (hereafter denoted the ‘alignment plane’) since the magnetic easy-axis of α-alumina is known to corre spond to the c-axis and the orientation of the samples and subsequent sectioning, would place the (006) plane parallel to the surface of the sample in a perfectly aligned specimen. By calculating the average interplanar angles (with respect to the (006) plane) for each XRD pattern, weighted on the basis of the indi vidual peak intensities (which varied with increasing alignment), an ‘average interplanar angle,’ henceforth referred to as the average facial angle (as proposed by Asai [13]), was determined for each specimen, as described in Equation (5) [13]. P ðIhkl Þ ðΦhkl Þi ΦF ¼ P i (5) ðIhkl Þi
2.4. Sintering of colloidally-cast samples An Er-doped, aligned green body cross-section was sintered and analyzed to determine the effects of densification on alignment pro duced during the green forming process. Sintering was carried out in an alumina boat according to the following temperature profile: heat at a rate of 5 � C/min to 1000 � C; hold for 60 min at 1000 � C before heating at 10 � C/min to 1550 � C; hold for 120 min at 1550 � C before cooling at 10 � C/min to room temperature. The sample was analyzed before and after sintering according to the XRD procedure described in section 2.5. Additionally, the polished cross-section was imaged under environ mental SEM.
where (Ihkl)i denoted the intensity of the ith peak. The (012), (104),
(110), (006), (113), (024), (116), (018), (214), (300), (1010), and (119) peaks (those visible in the experimental XRD pattern) were used in the calculation. The average angle changed with respect to the degree of alignment and could therefore be used to quantitatively track the amount of alignment achieved [13]. ΦF ¼ 0� occurs when there are only (00l) reflections present in the XRD pattern indicating a perfect align ment of the c-axis of all crystals along the applied magnetic field di rection. In contrast, ΦF ¼ 90� occurs when there are only (hk0) reflections in the XRD pattern indicating perfect alignment of the c-axis of all crystals perpendicular to the applied field direction. In both cases, preferential alignment occurred, but in fundamentally different align ment directions indicating a difference in the magnetic easy-axis of the crystals. Since the average facial angle, as defined by Asai [13], was denoted in units of degrees and varied between 0 and 90, it was difficult to delineate how much improvement was achieved by alignment over that of a randomly aligned sample. Decreased average facial angle with respect to the reference pattern, clearly indicates improved alignment, on average, but it is difficult to establish by how much without knowing the average facial angle for the reference pattern. Furthermore, a decrease in average facial angle by a given amount, does not necessarily correspond to an increase in alignment by the same amount since the reference average facial angle changes depending on the reference plane according to the inherent geometry of the crystal. Consequently, a figure-of-merit normalization was performed by adapting the Lotgering equation [8,14] to compare the observed average facial angle (ΦF ) with that of the average facial angle of a perfectly random specimen (ΦF r , as defined by the PDF card). This normalization results in a percent alignment value for each specimen and was calculated according to Equation (6):
2.5. X-ray diffraction analysis of magnetic alignment XRD analysis of the cured epoxy pucks and green and sintered gelcast bodies was conducted using a previously established protocol [14]. First, the pucks were cross-sectioned perpendicular to the applied magnetic field direction and mounted for XRD analysis using Cu-Κα radiation at 30 kV, 15 mA (Rigaku MiniFlex II, Tokyo, Japan). XRD scans were performed over a 34–45� 2θ range at a step angle of 0.01� 2θ and a scan rate of 1� 2θ/min for all samples. Additionally, 20–80� 2θ scans at the same settings were performed on all the cured epoxy samples in order to investigate if the magnetic easy axis had shifted due to dopant type. The resulting scans were analyzed using Jade 8 software (MDI, Liv ermore, CA). For each scan, the background was automatically removed, the peaks were identified, and pattern fitting was conducted according to the peak height values. As described in the introduction, the percent alignment FOM was calculated based on the average facial angle (as proposed by Asai [13]) for each scan using the powder diffraction file #00-010-0173 (Synthetic Corundum, space group R3 c) and Equations (1)–(4). The interplanar angle, Φhkl , is the angle between two crystallo graphic planes and can be calculated by means of the following relation [22]: � � �� 1 2 2 ðΦhkl Þi ¼ cos 1 dhkl dh’k’l’ hh’ þ kk’ þ ðhk’ þ kh’ Þa* þ ll’c* (1) 2
% alignment ¼
where the subscript i indicates a specific angle between a particular plane (hkl), apparent in the XRD pattern of α-alumina, and a given reference plane (h’k’l’), and dhkl, a*, and c* are the spacings between successive planes and the reciprocal unit cell parameters, respectively, as defined for the hexagonal unit cell in Equations (2)–(4) below:
ΦF r ΦF x 100 ΦF r
(6)
where ΦF r denotes the average facial angle of the powder diffraction card with respect to the same reference plane. While this calculation results in a percentage alignment, it is recognized that this does not represent the percentage of planes fully aligned to a given direction, nor the percentage of rotation achieved for the majority of crystals in the 3
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Table 1 Average percentage alignment values with respect to the (006) plane � standard deviation for phase-pure α-alumina epoxy samples. The third column also lists the percent alignment values re-calculated using the average facial angle method from the previous mixed-phase alumina epoxy alignment samples for comparison. Dopant
Gd Yb Er Pr Undoped a
Phase-pure α-alumina Alignment (%)
Mixed-phase Alumina Alignment (%) [14]
0T 0.94 � 0.74 0.05 � 0.34 0.26 � 0.40 0.14 � 0.34 0.28 � 0.63
1.8 T 2.74 � 8.60 9.99 � 1.92 14.33 � 0.58a 1.13 � 2.80 0.65 � 0.21
1.8 T 2.27 � 1.14 11.55 � 0.88 16.15 � 5.05 1.61 � 0.37 1.97 � 0.91
Table 2 Magnetic easy-axes identified previously through DFT modeling [14] and the corresponding crystal planes with expected XRD peak enhancements relative to the baseline XRD patterns in aligned samples. Some directions did not corre spond to measurable XRD planes. Dopant
Predicted Magnetic Easy-axis
Gd Yb Er Pr Undoped
<0 <3 <1 <2 <1
0 1> 2 10> 2 0> 3 2> 2 0>
Corresponding Crystal Plane (006) (4130) (010) (146) (010)
Positive values indicate better average alignment to a given direction with respect to a randomly aligned sample, and negative values indicate preferential alignment in a different direction to that which was used as the reference direction. Two-tailed, two-sample t-tests were conducted to compare the alignment of the 1.8 T samples to the control samples. An alpha value of α ¼ 0.05 was used. Since it was of interest to determine if the magnetic easy-axis shifted with dopant type, the percent alignment was determined by changing the reference plane (h’, k’, and l’ indices from Eq. (1)) for each visible crystallographic plane in the XRD pattern of α-alumina, including the (012), (104), (110), (006), (113), (024), (116), (018), (214), (300), (1010), and (119) peaks. The percent alignment calculated with respect to each XRD peak (rather than just the (006) plane), was color-mapped into a three-dimensional schematic of the reduced alumina hexagonal unit cell using custom code written in MATLAB (2016b, Mathworks, Natick, MA) to show the degree of alignment corresponding to each plane in real space with color intensity contrast. This novel technique was developed as a visualization tool to confirm the axis of alignment. Since intermediate values of alignment were achieved, and additional planes exhibited a corresponding increase in alignment, it was necessary to confirm that the multiple peaks showing high alignment values were not an artifact produced by the assessment procedure.
Indicates n ¼ 2.
Fig. 1. Percentage alignment values for each dopant type. * indicates statisti cally significant difference from 0 T controls. # indicates statistically significant difference compared to the undoped 1.8 T controls.
3. Results and discussion 3.1. Degree of alignment for α-alumina samples vs. mixed-phase alumina samples Table 1 lists the average percent alignment values (with respect to the (006) plane) and standard deviations for each condition, while Fig. 1 compares the percent alignment across dopants. Only the Er, Yb, and Pr magnetically aligned samples had statistically significant percent alignment values compared to the undoped control samples, with Gd samples achieving similar alignment. As shown in Table 1 and Fig. 1, the Er-doped samples demonstrated the strongest response to the 1.8 T magnetic field, followed by Yb- and Gd-doped samples. These results were consistent with observations from the mixed-phase alumina epoxy samples (mixture of α- and θ-alumina) that were re-calculated using the average facial angle calculation in Table 1 [14]. However, it was diffi cult to draw strong conclusions from the mixed-phase samples due to the large standard deviations in alignment. In contrast, Pr exhibited a slightly negative alignment with respect to the random powder, which was also consistent with the mixed-phase samples at 1.8 T (Table 1) and 9 T ( 20.5%) [14], even after the percentage alignment value was re-calculated using the average facial alignment method. The negative alignment was likely due to a change in the alignment direction, as predicted by DFT modeling (see Section 3.2). This is reflected in Fig. 2, which shows representative XRD patterns for the Er and Pr epoxy samples, both aligned and unaligned. The Er sample shows an increase in the (006) peak with alignment, which is absent from the aligned Pr pattern. The (104), (110), and (113) peak heights also show changes in the aligned samples with respect to the control, which is captured by the average facial angle alignment metric as opposed to the Lotgering factor metric.
Fig. 2. Representative XRD patterns of the Pr and Er sampled, aligned and control. Curves were vertically translated for clarity.
part. Rather, this value represents the average texture of the specimen and the relative amount of texture improvement needed to result in perfect crystal alignment. As such, it is a useful figure-of-merit to quantitatively compare the alignment achieved across conditions, dop ants, and crystallographic directions. The percent alignment values for each condition were calculated for each sample and averaged across the sample repeats within each group. 4
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Fig. 3. Alumina crystal cell schematic showing the planes visible on the XRD pattern ((012), (104), (110), (006), (024), (116), (018), (214), (300), (1010), (119)) with associated percent alignment expressed as a color intensity and indicated by the color maps to the right of each drawing. The left column shows the controls while the right column shows the magnetically-aligned samples. All color scales are the same for all drawings. For reference, the (006) plane is outlined in black to show its proximity relative to the rest of the planes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
It is important to note that while it has been established that crys tallite radius affects the alignment time, with larger crystallites aligning faster (see Ref. [15]), the radius has been found to have a negligible impact when compared to other processing variables (e.g. viscosity,
magnetic field strength, and magnetic susceptibility anisotropy) when the electrical conductivity of the material is low, such as in alumina [15]. While the crystallite size distribution of each powder type was likely different (�~100–150 nm), and in theory could affect the 5
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Fig. 3. (continued).
Table 3 Average percent alignment values with respect to the (006) plane � standard deviation for phase-pure α-alumina gel casting samples. Dopant Er Undoped a
α-alumina casting Alignment (%)
α-alumina Epoxy Alignment (%)
0T
1.8 T
1.8 T a
0.55 � 0.22 0.78 � 0.32
21.32 � 5.57 1.13 � 0.82
a
16.15 � 5.05 1.97 � 0.91
Indicates n ¼ 2.
alignment since relatively long alignment times were used for these experiments (on the order of hours as opposed to seconds), the effects of these differences were negligible. Changes in alignment were attributed solely to the addition of the dopant. 3.2. Alignment of α-alumina vs. DFT predictions Table 2 lists the magnetic easy-axes for each of the dopants tested, as calculated by the DFT modeling described in Refs. [14,18]. The planes corresponding to the XRD peaks, which were expected to be enhanced (with respect to the baseline patterns) given the sectioning geometry used, were also identified for each of the dopants based on the DFT calculations (Table 2). However, since some of the predicted directions corresponded to planes that exhibited no distinguishable XRD peaks, it was not experimentally feasible to track alignment using the previously described XRD method. This was true for both the Yb and Pr dopants.
Fig. 4. Percent alignment values for each powder type. * indicates statistically significant difference from the 0 T controls. # indicates statistically significant difference compared to the undoped 1.8 T control.
Additionally, the DFT calculations did not predict alignment of the undoped alumina crystal with c-axis parallel to the field direction due to the non-magnetic perfect crystal obtained in DFT, which excluded the 6
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the (006) plane. This produced an inherent bias in the visualization technique used, although the marked improvement over the un-aligned sample indicates that this bias was likely negligible. In the samples that exhibited the lowest alignment values with respect to the (006) plane, either (a) a small amount of alignment was achieved with respect to every plane, as is the case in the Gd samples, or (b) alignment increased corresponding to a direction that was not the caxis direction, as in the case of Pr. When examining the alignment vi sualizations for Pr, alignment was observed to occur preferentially for
Table 3 lists the calculated alignment values for each of the condi tions tested, and Fig. 4 compares the alignment of Er-doped alumina powders to undoped powders generated with 95% pure aluminum ni trate, both cast under a 1.8 T magnetic field. While the rare-earth doped samples showed a much higher degree of alignment, the undoped sample alignment was also statistically significant (α ¼ 0.05) with respect to the control. Furthermore, for both powders, it appeared that higher alignment was also achieved for the colloidally-cast samples than for the epoxy-cast samples, although this result is difficult to establish in the Er-doped powder due to the low number of samples. This apparent increase in alignment, compared to the epoxies, was attributed to better powder dispersion in the colloidally-cast slurry, as well as a lower vis cosity, since the colloidally-cast slurries (once milled) were much more fluid than the epoxy suspensions although further investigation would be necessary to confirm this trend. 3.4. Sintered body analysis Fig. 5 shows that the XRD pattern changed after sintering. In the green body state, the sample exhibited 21.3% alignment with respect to the (006) plane; after sintering, this unexpectedly increased to 32.4%. It had been hypothesized that the addition of thermal energy and the rearrangement of particles during sintering would lead to significantly lower alignment values with respect to the green part [15]. The findings do not support this hypothesis, demonstrating that alignment achieved during the green state could be maintained upon sintering. This was initially attributed to particle steric interactions impinging grain rota tion once water was removed from the green body and the part consolidated, despite the increase in thermal energy. However, the SEM cross-sectional image in Fig. 6 shows necking at the particle contact points with no further densification, indicating that the selected sin tering profile needs to be modified in order to densify the α-alumina nanopowders. Further optimization is required to determine whether or not complete densification will result in a reduction of the alignment achieved during colloidal green body processing.
Fig. 6. SEM image of polished cross-section of sintered, aligned, gel-cast part.
low concentration effects of point defects in experimental solids [14,18]. Due to inherent scaling limitations, the DFT calculations used to predict the axes of alignment and magnetocrystalline anisotropy energies were conducted for higher dopant concentrations (0.83 atom%). This likely gave rise to the deviations observed, though the overall prediction of dopant effects on alignment was realized. Fig. 3 shows the alignment visualizations in real space for each sample (based on an average of three samples), with the (006) plane outlined in black for reference. For the samples that showed higher alignment values, such as Er and Yb (Table 1), the planes corresponding to a high percent alignment were grouped in close proximity to the expected (006) alignment plane of undoped alumina. As the torque was applied, particles may have been prevented from moving into complete alignment for a number of reasons, including viscosity, steric in teractions between particles, agglomeration, mold-wall friction, etc. The increase in alignment for planes in close proximity to the expected alignment plane was believed to be indicative of crystals that began to rotate into alignment but were prevented from achieving complete alignment. However, it is important to note that due to the crystal structure of α-alumina, most of the visible peaks were located close to
4. Conclusions This work demonstrates that chemical augmentation of alumina by doping with rare-earth ions can significantly improve the magnetic response of alumina, enabling manipulation by lower strength magnetic fields. Er-doped alumina exhibited the highest response (~16% align ment with respect to the (006) plane), followed by Yb-doped, and Gddoped (despite the much higher total magnetic moment) [14,18]. Pr-doped alumina exhibited a reduction in alignment along the (006) direction. However, the alignment visualization results across the entire unit cell indicated that the negative alignment was indicative of align ment in an alternate direction (in contrast to negative alignment along 7
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References
<00l>). In general, the trends for the phase-pure α-alumina samples were consistent with those in the mixed-phase alumina samples analyzed previously [14], but further investigation with mixed-phase or phase-pure θ-alumina would be necessary to confirm this trend due to the large errors associated with the mixed-phase samples. While the DFT models were unable to predict the effects of specific dopants, they were able to predict that dopants, in general, could produce a change in the preferred alignment axis, as was observed for Pr. An assessment of additional dopants using DFT modeling and experimental evaluation would be necessary to study this effect further. After investigating the effect of different dopants on the magnetic properties of alumina using the epoxy system as a proof-of-concept, Erdoped alumina was successfully aligned using a colloidal casting system that allowed for sintering of the final part. It was found that the align ment achieved during the green forming process was maintained in the final sintered part, even without application of a magnetic field. While agglomerates were likely still present, despite considerable milling, this study demonstrated the efficacy of magnetic alignment as a processing method for fabricating bulk micro-textured parts without the need for an alternative processing methods such as templated grain growth [23]. Furthermore, the use of RE-dopants to enhance the magnetic response of alumina and allow weaker magnetic fields to effect alignment (acces sible through electromagnets rather than superconducting magnets), was also demonstrated. Further optimization is necessary to produce fully dense RE-doped parts so that the effect of alignment on mechanical and optical properties of alumina can be investigated.
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Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of competing interest None. Many thanks to Dr. Krista Limmer for her work on the DFT modeling and discussion in the preparation of this paper. This research did not receive any specific grant from funding agencies in the public, com mercial, or not-for-profit sectors. Research by CM and NK was sponsored by the Army Research Laboratory and was accomplished under Coop erative Agreement Number W911NF-16-2-0008. The views and con clusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.
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