Synthesis of BaTiO3-20wt%CoFe2O4 Nanocomposites viaSpark Plasma Sintering

Dipankar Ghosh,‡ Hyuksu Han,‡ Juan C. Nino,‡ Ghatu Subhash,§ and Jacob L. Jones‡,†

‡Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611

§Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611

Barium titanate-20wt% cobalt ferrite (BaTiO3-20wt%

CoFe2O4) nanocomposites were sintered from nanocrystalline

BaTiO3 and CoFe2O4 powders using spark plasma sintering

(SPS) and pressureless sintering (PS) techniques. Using SPS,dense polycrystalline composites were obtained at a sintering

temperature as low as 860°C and a time of 5 min whereas PS

required a higher sintering temperature (1150°C) and time(120 min) to obtain similarly dense composites. Microstruc-

tural analysis of the composites showed that both the tech-

niques retained nanocrystalline grain sizes after sintering. High

resolution X-ray diffraction measurements revealed that theBaTiO3-20wt%CoFe2O4 composites sintered by the SPS tech-

nique did not exhibit formation of any new phase(s) due to

reaction between the BaTiO3 and CoFe2O4 phases during sin-

tering. However, the PS technique resulted in the formation ofadditional phases (other than the BaTiO3 and CoFe2O4 phases)

in the composites. While the composites synthesized by SPS

were of superior phase-purity, evidence of Fe diffusion from thespinel to the perovskite phase was found from X-ray diffraction

and permittivity measurements.

I. Introduction

I N the class of ferroic materials, a particular interest hasgrown in the subset of multiferroics for multifunctional

devices, where a multiferroic material must possess at leasttwo ferroic orders such as ferroelectricity, ferromagnetism, orferroelasticity.1–6 The existence of such unique combinationof ferroic orders within one material allows a couplingbetween different physical fields, which can be utilized tomanipulate one ferroic order parameter by the other. How-ever, such coupling is known to be extremely weak in intrin-sic multiferroic compounds.2,3,5,6 Thus, an alternativecoupling mechanism has been explored using two-phase mul-tiferroic heterogeneous systems,1–6 where a strain coupling isengineered between the piezoelectric and the magnetostrictivephases. While each of the constituent phases exhibits onlyone ferroic order, coupling through the elastic interactionsbetween the phases can generate additional multiferroiceffects (e.g., magnetoelectricity). One promising architectureof the type is bulk multiferroic composites, which can beprocessed by mixing a piezoelectric phase and a magneto-strictive phase, and then co-sintering the phases.3,5,6 One sig-nificant challenge in such an approach is retaining phase

purity, for which it is necessary to limit the diffusion at thephase boundaries. Conventional sintering techniques such aspressureless sintering (PS) requires significantly high sinteringtemperature and long sintering time (on the order of hours)to obtain high density. This approach may result in signifi-cant diffusion between the piezoelectric and magnetostrictivephases, volatilization of species, and potentially may lead tothe formation of additional phase(s). Such processes can bedetrimental to the electrical properties of the composites aswell as the strain transfer across the piezoelectric-magneto-strictive phase boundaries.

Recently, spark plasma sintering (SPS) has emerged as apromising non-conventional sintering technique, where highamplitude of current (>1000 Amp) at a low voltage (5 V)level is used for powder particle sintering utilizing “Jouleheating” mechanism.7–11 The resistive heating mechanism inpresence of extremely high amplitude of direct current resultsin rapid temperature rise within the powder compact. UsingSPS, bonding of powder particles has been achieved at signif-icantly lower sintering temperatures than the conventionalsintering techniques.7–11 The low temperature sintering overa shorter period of time due to extremely high heating rate(e.g., >50–400°C/min) can effectively reduce the extent of dif-fusion at the phase boundaries and thereby enables the syn-thesis of higher purity materials. Most recently, Liu et al.12

has used SPS for synthesis of barium titanate–cobalt ferrite(BTO–CFO) microcrystalline composites. Here, the BTOconstitutes the piezoelectric phase whereas the CFO is themagnetostrictive phase. This composite is a promising multif-erroic system6,12,13 and has been explored in other architec-tures including core-shell structures, thin films andmultilayered architectures.6,14–20 Although the work by Liuet al.12 addresses the synthesis of microcrystalline BTO–CFOcomposites, questions remain as to the phase purity of suchcomposites and whether or not microcrystalline or nanocrys-talline composites can be produced in phase-pure form byconventional sintering.

Given that the strain transfer between the two phasesacross the phase boundary enables multiferroic behavior,synthesizing composites with high phase boundary area couldbe beneficial. This is because increase in the phase boundaryarea may facilitate the strain transfer and thus, the mechani-cal coupling between the piezoelectric and magnetostrictivephases. However, grain size reduction below 1 lm is knownto deteriorate the relative permittivity (er) of BTO ceram-ics.21,22 It is now commonly accepted that in the grain size of1–10 lm, er increases with decreasing grain size and reachesa maximum near 1 lm. However, in the submicron range(<1 lm) an opposite trend is observed where er decreaseswith decreasing grain size.21,22 Similarly, magnetic propertiesof CFO are also grain size dependent.23,24 Increasing grainsize of CFO has shown to increase the saturation magnetiza-tion and lower the coercive field.23 However, magnetostric-tive strain was observed to be higher for CFO ceramics withfine grain size and high density. To this end, fabrication of

S. Trolier-McKinstry—contributing editor

Manuscript No. 30476. Received October 11, 2011; approved March 21, 2012.This work was partially supported by the Florida Cluster for Advanced Smart Sen-

sor Technologies under “New Florida 2010” and partially by the U. S. Department ofArmy under contact No. W911NF-09-1-0435. Use of the Advanced Photon Source wassupported by the U. S. Department of Energy, Office of Science, Office of Basic EnergySciences, under Contract No. DE-AC02-06CH11357.

†Author to whom correspondence should be addressed. e-mail: jjones@mse.ufl.edu

1

J. Am. Ceram. Soc., 1–6 (2012)

DOI: 10.1111/j.1551-2916.2012.05221.x

© 2012 The American Ceramic Society

Journal

composites with grain sizes on the order of 1–100 nm is ofinterest, which is the focus of the present work. The presentstudy reveals that the shorter sintering time at lower sinteringtemperature in the SPS is highly advantageous for synthesisof nanocrystalline composites with superior phase-puritythan conventional sintering. It has also been shown thatwhile SPS was able to produce dense BTO–CFO compositesat a much lower temperature than conventional sintering,significant interphase diffusion occurred during processingand resulted in lowering of dielectric properties of the com-posite relative to pure BTO ceramics.

II. Experimental Procedure

(1) Raw MaterialsBoth single phase BTO ceramics and 80wt%BTO-20wt%CFO (BTO-20CFO) composites were sintered from commer-cially available nanocrystalline BTO (Alfa Aesar, Ward Hill,MA) and CFO (MTI Corporation, Richmond, CA) powdersusing both the PS and SPS techniques. To ensure the propermixing of both the BTO and CFO powders for synthesizingthe composites, they were first mixed in 80–20 wt% ratio ina mortar and pestle for 10 min. Then the mixture was balledmilled in ethanol for 24 h using 5 mm zirconia media. Theslurry was dried in an oven, again ground in a mortar andpestle and finally sieved through a 200 lm mesh. A limitedquantity of nanocrystalline BaTiO3-10wt%CoFe2O4 (BTO-10CFO) composites was produced by equivalent methodsand dielectric studies were conducted on both the BTO-10CFO and BTO-20CFO composites.

(2) Spark Plasma and Pressureless SinteringThe sintering conditions used in both the techniques are pro-vided in Table I. In the SPS technique, 5 g of powder wasplaced in a 20 mm graphite die and the sintering was con-ducted in an SPS system (Dr. Sinter 1020). During sintering,the sample was (i) first heated from room temperature to600°C at a rate of 200°C/min, (ii) then heated from 600°C tothe sintering temperature at a rate of 50°C/min, (iii) held atthe sintering temperature for 5 min, and (iv) finally cooled toroom temperature at a rate of 100°C/min. The maximumvoltage and current were 4 V and 800 Amp, respectively.Sintering was performed under an applied uniaxial pressureof 60 MPa and in a high vacuum (~6 Pa) atmosphere. In thePS technique, green pellets (diameter 10 mm and thickness2 mm) were made initially in a uniaxial press using a steeldie and then isostatically pressed at a pressure of 225 MPa.The green pellets were then (i) heated from room tempera-

ture to the sintering temperature at a rate of 3°C/min, (ii)held at the sintering temperature for 120 min, and (iii) finallycooled to room temperature at a rate of 3°C/min. In contrastto SPS, PS was performed in atmosphere air. BaTiO3 isknown to undergo partial reduction during sintering, espe-cially under non-oxidizing atmospheres (e.g., vacuum) and athigh temperatures.10,25 Also, due to use of the graphite die inthe SPS technique, diffusion of carbon is known to occur inthe sintered BTO disks, especially at the surfaces.21 There-fore, all the sintered samples were heat treated at 800°Cfor 6 h in a box furnace in an air atmosphere to remove thecarbon from the surfaces of the samples and re-oxidizethe BTO phase.

(3) Density Measurements, MicrostructuralCharacterization and Phase-PurityDensities of the sintered ceramics and composites were deter-mined from weight and dimensional measurements as well asusing the Archimedes technique. Using the theoreticaldensities of BTO (6.02 g/cm3) and CFO (5.29 g/cm3), theweight-average density of the BTO-20%CFO composite wascalculated to be 5.86 g/cm3. Scanning electron microscopy(SEM, JEOL 6335F FEG-SEM, Peabody, MA) was per-formed to reveal the particle size and morphology of the start-ing powders as well as the grain sizes of the sintered compacts.To check the phase purity of the sintered ceramics and com-posites, high-resolution X-ray diffraction (XRD) measure-ments in transmission geometry were performed using amonochromatic X-ray beam on beamline 11-BM at theAdvanced Photon Source at Argonne National Laboratory.All the measurements were performed in the powder form ofthe samples where the powders were loosely packed in 0.8 mmdiameter polyimide capillaries to minimize the absorption. Toimprove the powder crystallite averaging, the capillaries werespun at 60 Hz during data collection. XRD patterns for thepowders were measured using an X-ray wavelength of0.413262 A and for the sintered materials the X-ray wave-length used was 0.413395 A. The measurements were per-formed in the 2h range 3.0–30° using a 2h step size of 0.001°.

(4) Dielectric Property MeasurementsFor dielectric measurements, thin layers of gold (Au) werefirst deposited on the two parallel surfaces of the annealedBTO-10CFO and BTO-20CFO specimens using an Au sput-ter coater and then silver paste was applied on the Au coatedsurfaces, and finally dried in air for 5 min. Dielectric proper-ties (dielectric constant and loss) were measured from room

Table I. Sintering Conditions and Density Measurements of BaTiO3 Ceramics and BaTiO3-20CoFe2O4

Composites Synthesized by SPS and PS Techniques

Sintering method Material

Sintering temperature (°C),time (min) and pressure (MPa)

Density from

dimensions

Density from

Archimedes

g/cm3 % g/cm3 %

SPS BTO 800, 5, 60 3.9 65.1 5.19 86.2BTO 850, 5, 60 4.7 77.9 5.43 90.2BTO 900, 5, 60 5.7 94.5 5.67 94.2BTO-20CFO 860, 5, 60 5.7 96.4 5.62 95.9

PS BTO 1050, 120, 0 4.7 78.1 5.79 96.1BTO 1100, 120, 0 5.2 86.9 5.88 97.7BTO 1150, 120, 0 5.5 90.9 5.8 96.3BTO 1250, 120, 0 5.8 95.9 5.72 94.9BTO 1300, 120, 0 5.9 98.7 5.78 96BTO-20CFO 1100, 120, 0 5.2 88.7 5.41 92.4BTO-20CFO 1150, 120, 0 5.5 94.3 5.5 93.9BTO-20CFO 1250, 120, 0 5.3 90.9 5.33 90.9

BTO, barium titanate; CFO, cobalt ferrite; SPS, spark plasma sintering; PS, pressureless sintering.

2 Journal of the American Ceramic Society—Ghosh et al.

temperature to 175°C using conventional metal-insulator-metal, parallel plate capacitor techniques in an Agilent4284A LCR meter at the frequencies of 1, 10 and 100 kHz ina temperature chamber and controller from Delta Design,Inc., Poway, CA. In the present work, limited results ofdielectric measurements will be presented and more detailswill be published elsewhere.

III. Results and Discussion

(1) Density and MicrostructureFigures 1(a) and (b) show the SEM micrographs of thenanocrystalline BTO and CFO powders, respectively, whichreveal that the particle sizes of both the powders were below100 nm. Table I shows the densities of the sintered ceramicsand composites. At lower temperatures, when significant levelof open porosity is present in sintered materials, a large vari-ation in the measured densities is observed between theArchimedes technique and the dimensional measurements.However, as the sintering temperature increases, both themeasurements yield comparable densities. The SPS techniqueproduced BTO ceramics with a relative density greater than94% at a sintering temperature as low as 900°C. The PStechnique, on the other hand, required sintering temperatureas high as 1250°C to achieve comparable densities of theBTO ceramics (i.e., 94%). Similarly, BTO-20CFO compositeswith relative densities greater than 94% could be achieved at860°C using the SPS technique whereas a sintering tempera-ture of 1150°C was required to achieve similar densities usingthe PS technique. Overall, the BTO-20CFO composites were

observed to achieve similar relative densities to the BTOceramics at lower sintering temperatures in both the SPS andPS techniques.

Figure 2 shows the SEM micrographs from the fracturedsurfaces of the BTO ceramics and BTO–CFO compositesrevealing the grain sizes and morphology. The average grainsizes of the BTO ceramics were measured from the SEMmicrographs of the fractured surfaces using the linear inter-cept technique as per ASTM E112 standard. The BTO cera-mic synthesized by the SPS technique at 900°C has anaverage grain size of 209 ± 11 nm [see Fig. 2(a)] where as theBTO ceramic synthesized at 1250°C by the PS technique hasan average grain size of 3.13 ± 1.1 lm. Thus, the PS tech-nique resulted in a significant grain growth of the BTO cera-mic relative to the SPS technique.

For the BTO-20CFO composites, both the SPS and PStechniques produced ultrafine-grained microstructures. Bycomparing the SEM micrographs of the composites inFigs. 2(b) and (d) with Fig. 2(a) for the BTO ceramic (sin-tered by the SPS), it is observed that the grain sizes in thecomposites are even finer than the BTO ceramic sintered bythe SPS technique. In SPS, this may be attributed to theslightly lower sintering temperature for the composite (860°C) than the BTO ceramic (900°C). In the PS technique, inaddition to the lower sintering temperature of the composite(1150°C) than the BTO ceramic (1250°C), it is possible thatgrain growth of the BTO phase in the compositewas restricted due to the presence of the CFO particulatephase, which is a commonly known effect in particulatecomposites.26

(2) Phase-PurityFigure 3 shows the XRD patterns of the starting BTO andCFO powders as well as of the sintered ceramics and com-posites. The XRD patterns of the BTO and CFO powderswere typical of tetragonal perovskite structure (JCPDS-05-0626) and cubic spinel structure (JCPDS-22-1086), respec-tively. However, small amount of barium carbonate (BaCO3,JCPDS-45-1471) was detected in the starting BTO powder,which is commonly used for synthesis of BTO powers. XRDpatterns of the BTO ceramics synthesized by the SPS (900°C)and PS (1250°C) techniques [Fig. 3(a)] demonstrate that onlythe BTO phase was present after sintering. Thus, the XRDanalysis confirmed that phase-pure BTO ceramics were suc-cessfully synthesized by both the sintering techniques. TheXRD patterns of the BTO-20CFO composites are shown inFig. 3(b). The XRD pattern of the BTO-20CFO compositesintered by SPS revealed presence of BTO and CFO phases.However, similar to the starting BTO powder, small amountof BaCO3 was also detected in the composite from the high-resolution XRD pattern. Interestingly, BaCO3 was notdetected in the BTO ceramic when sintered by SPS at 900°C.Therefore, it is possible that slightly lower temperature sin-tering of the BTO-20CFO composite (860°C) than the BTOceramic (900°C) in SPS resulted in incomplete decompositionof BaCO3 phase in the composite. However, the XRD pat-tern of the BTO-20CFO composite synthesized by SPS didnot show any phase as a result of the reaction between BTOand CFO phases during sintering.

In contrast, XRD patterns of the composite specimens sin-tered by the PS technique showed additional peaks otherthan the BTO and CFO phases; the additional peaks increasein intensity with increasing temperature (1100°C, 1150°C,and 1250°C). Note that BaCO3 phase was not detectedbecause of the higher sintering temperature. The additionalXRD peaks other than those corresponding to the BTO andCFO phases suggest a reaction between the BTO and CFOphases, generating additional phase(s) during sintering in thePS technique. The sintered density of the compositedecreased from 1150°C to 1250°C whereas the intensity ofthe additional phases increased with increasing temperature.

(a)

(b)

Fig. 1. SEM micrographs of nanocrystalline (a) BaTiO3 and (b)CoFe2O4 powders.

BaTiO3-20wt%CoFe2O4 Nanocomposite by SPS 3

The decrease in the sintered density with increasing tempera-ture may be related to the formation of new phase(s) as wellas possible volatilization of species. The current resultsrevealed that while both the SPS and PS techniques retainthe grain sizes of the BTO-20CFO composites in the nano-meter range [see Figs. 2(b) and (d)], the SPS techniqueshowed the advantage of synthesizing phase-pure BTO-20CFO composites over the PS technique. This can be attrib-uted to the low temperature and rapid sintering in the SPStechnique than the PS technique, which needs higher temper-ature and longer sintering time to produce dense sinteredmaterials.

Figure 4 shows the {200}{200} reflections of the BTOceramic and the BTO-20CFO composite sintered by the SPS.Comparison of the XRD patterns of the BTO ceramic andBTO-20CFO composite (Fig. 4) synthesized by SPS suggeststhat the lattice aspect ratio of the BTO crystallites in theBTO-20CFO composite was smaller than the pure BTO cera-mic. From curve fitting, the lattice parameters c and a forthe BTO ceramic were calculated to be 4.029 and 3.9682 A,respectively, corresponding to a c/a ratio of 1.0153. On theother hand, for the BTO-20CFO composite, c and a valueswere calculated to be 4.0099 and 4.0076 A, respectively, andthe c/a ratio was 1.0005. The lower c/a value of the BTO

(a)

(c) (d)

(b)

Fig. 2. SEM micrographs revealing fractured surfaces of a (a) BTO ceramic and (b) BTO-20CFO composite processed by the SPS technique at900°C and 860°C, respectively; (c) BTO ceramic and (d) BTO-20CFO composite sintered by the PS technique at 1250°C and 1150°C,respectively.

Fig. 3. X-ray diffraction patterns from (a) BTO and CFO nanopowders, and sintered BTO ceramics, and (b) BTO-20CFO composites. Thepossible identified phases which might have been formed due to reactions between BTO and CFO phases are shown in (b).

4 Journal of the American Ceramic Society—Ghosh et al.

phase in the composite compared to the BTO ceramicsuggests the diffusion of Fe3+ or Co2+ into the BTO latticefrom the CFO phase. BaTiO3 is known to enable substitu-tion of different sizes of ions including Fe3+ and Co2+ andthe doping/substitution site depends on the size of the ions.27

The ionic radii of Fe3+ (0.64 A) and Co2+ (0.78 A) are com-parable to the ionic radius of Ti4+ (0.68 A) than that of Ba2+

(1.35 A) and therefore, Fe3+ and Co2+ substitution willoccur at the Ti-site.27 While substitution of Fe3+ can resultin a decrease of the BTO lattice parameters because of itssmaller radius than Ti4+,28 Co2+ substitution, on the otherhand, can increase the BTO lattice parameters due to its lar-ger radius than Ti4+. Therefore, the observed decrease in thelattice parameters of the BTO-20CFO composite in compari-son to the BTO ceramic (both synthesized by SPS) may beattributed to the diffusion of Fe3+ in to the BTO latticeduring sintering.

As discussed previously, the XRD patterns from the PSBTO-20CFO composite sintered at 1100°C showed severaladditional peaks, which increased in intensity with sinteringtemperature. Some of the possible phases that might have beenformed due to reaction between BTO and CFO phases duringPS have been indicated in Fig. 3. The possible phases thathave been identified here are BaTiFe2O6 (JCPDS-39-0810,tetragonal lattice), BaFe4Ti2O11 (JCPDS-26-1032, hexagonallattice), Ba2Co2Fe12O22 (JCPDS-19-0100, rhombohedrallattice) and Ba6Ti17O40�x (JCPDS-43-0559, monoclinic).

In the BTO-20CFO composites sintered at 1100°C and1150°C, the XRD peaks from the BTO phase were signifi-cantly stronger than the other phases (CFO and additionalphases). However, when the sintering temperature was raisedto 1250°C, intensity of the BTO phases was drasticallyreduced; see Fig. 3(b). Interestingly, as the intensity of theperovskite peaks (pure BTO phase) was reduced significantly,another set of perovskite peaks appeared in the BTO-20CFOcomposite sintered at 1250°C (not observed for at 1100°Cand 1150°C), which were much stronger in intensity than thepure BTO peaks. The new set of perovskite peaks appearedat higher d-spacings (lower 2h positions) suggesting anincrease in the lattice parameters than the pure BTO crystal-lites. The formation of the second perovskite phase at 1250°C could be due to the diffusion of large amount of Co fromthe CFO phase to the BTO lattice, which resulted in theincrease in the lattice parameters. Additionally, substitutionof Co2+ and Fe3+ at Ti4+ sites will cause in the formation

of oxygen vacancies to maintain the electroneutrality.27

Creation of oxygen vacancies can also contribute to theincrease of the lattice parameters.

(3) Dielectric PropertiesThe BTO-10CFO was chosen for dielectric measurementsbecause BTO–CFO composites with higher CFO contentshowed significant dielectric loss. Figure 5 shows the relativepermittivity (er) and dielectric loss (tan d) of the annealedBTO ceramic, and BTO-10CFO and BTO-20CFO compositesynthesized by SPS. The top half of Fig. 5 shows the er andtan d of BTO ceramic whereas the er and tan d of BTO-10CFO and BTO-20CFO composites are shown in thebottom half of Fig. 5. In principle, the observed roomtemperature er of the BTO ceramic is consistent with that ofpolycrystalline BTO ceramics.21,22 However, a diffuse para-electric-ferroelectric phase transition is observed near Curietemperature (~125°C). Such a phenomenon has beenreported for fine-grain BTO ceramics (grain size below 1 lm)and therefore the diffuse phase transition observed is likelyto be associated with the submicron grain size of the SPSBTO ceramic.22 On the other hand, the BTO-10CFO com-posite showed significantly lower permittivity than the BTOceramic and the phase transition was highly diffuse with alarge reduction in Curie temperature. These characteristics(reduced permittivity, diffuse transition with suppression ofthe Curie temperature) resemble the permittivity response of

Fig. 4. The (111) reflection and splitting of {200} reflections ofBTO nanocrystalline powder, BTO ceramic sintered by the SPS andPS techniques, and BTO-20CFO composite sintered by the SPStechnique.

Fig. 5. Relative permittivity and dielectric loss measurements at 1,10 and 100 kHz of BTO ceramic, and BTO-10CFO and BTO-20CFO composites sintered by the SPS technique.

BaTiO3-20wt%CoFe2O4 Nanocomposite by SPS 5

Fe-modified BTO ceramics28 where a large drop in Curietemperature is observed. As such, the permittivity responseof the present samples, further supports the hypothesis thatFe diffusion occurs in the BTO–CFO composites during pro-cessing and significantly affects the properties of the compos-ites. The BTO-20CFO composite showed a significantlyhigher dielectric loss than both the BTO ceramic and BTO-10CFO composite. While the measured permittivity values ofBTO-10CFO are higher than the BTO-10CFO, the permittiv-ity values of the former at all the frequencies increased withincreasing temperature, suggesting a contribution of conduc-tivity to the measured permittivity of BTO-20CFO. Also, theparaelectric-ferroelectric phase transition is difficult to distin-guish in the composites due to the highly diffuse nature ofthe phase transition. This suggests a higher amount of Fediffusion into the BTO phase in the BTO-20CFO compositethan in the BTO-10CFO composite. This could result fromthe higher volume fraction of CFO in the former than thelatter. The current results suggest that composites synthesizedat these conditions may have been subjected to significantinterphase diffusion and therefore, appropriate processingcontrols are required to better control the properties of BTO–CFO composites.

IV. Conclusions

In summary, the current work successfully demonstrated syn-thesis of dense (95%) nanocrystalline BTO-20CFO compos-ites using SPS at a temperature as low as 860°C and a timeof 5 min. However, evidence of possible Fe diffusion in toBTO phase in the composite was obtained from the high-res-olution XRD measurements. When synthesized by PS, addi-tional phases were observed in the XRD patterns of theBTO-20CFO composites. Therefore, rapid sintering at lowertemperature in SPS was beneficial in limiting the reactivitybetween the BTO and CFO phases to prevent the formationof additional phases in the composites unlike conventionalPS. The dielectric response of the BTO-10CFO and BTO-20CFO composites revealed that limited interphase diffusionoccurs during synthesis and results in a substantial decreasein the dielectric permittivity.

Acknowledgments

The authors thank Mr. Akito Sakata and Mr. Ben Kowalski for their helpwith PS and Profs. Henry Sodano and Jennifer Andrew for comments andsuggestions on the research.

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