Effect of substitution and processing on the morphology and dielectric properties of Pr2/3Cu3Ti4O12 (PCTO) ceramic

Abstract

Low‐cost, large dielectric constant, and resistivity materials have wide range of applications for radio frequency systems, flash memory devices to ferroelectric switches, and energy storage applications. The paper reports on the development and characterization of high‐resistivity Pr_2/3Cu₃Ti₄O₁₂ (PCTO) ceramics developed by doping with Ca²⁺ and Sr²⁺ ions under various processing conditions. In particular, synthesis and growth were performed at 1173 and 1273 K resulting in significant difference in both electrical and morphological properties. To alter the distortion and polarity of ceramics, low concentration (0.15 mole fraction) of Ca²⁺ and Sr²⁺ hetero‐valent ions with different size were substituted for Pr³⁺. The evolution and sizes of non‐spherical, non‐faceted grains altered to facet shapes due to changes in the anisotropy developed with increased copper concentration in the grains. The morphology analysis at these low processing temperatures revealed that grain growth occurs by engulfment of smaller grains by larger grains, necking process, and channel formation, with particles growing in nanometer size to greater than 10 µm and forming new layers on top of grains. The migration of copper‐rich phases above 1173 K from boundaries to grains causes faceted morphology in both pure and substituted PCTO. Finally, by substituting Ca²⁺ and Sr²⁺ into the ceramics, altering the processing temperature, resistivity, and dielectric constant significantly increased. The substituted ceramics showed a higher dielectric constant at applied frequencies of 100‒100 000 Hz and did not vary at bias voltages of 50‒1000 mV, indicating no breakdown of the ceramics.

BACKGROUND

Most industrial medium and high-voltage capacitors require dielectric materials with resistivities greater than 10⁷ Ω m to ensure low leakage current and avoid material breakdown. Over the past two decades, several research groups have developed and characterized variety of perovskites^1–6 using wet and semi-wet methods after Subramanian et al.⁷ first demonstrated that perovskites of the class ACu₃Ti₄O₁₂ (both composites and single crystals) have very large dielectric constants. Although perovskites were discovered almost seven decades ago,⁸ only recently researchers have been able to synthesize them with high dielectric constant and understand the effect of doping^9–13 on their electrical and optical properties. Gorzkowski et al.⁹ investigated barium strontium titanate for high energy density capacitors and observed significant differences in the energy density of the same stoichiometric compounds depending on whether they were glassy forms. In addition to experimental studies, different mechanisms have been proposed, including based on chemical redox, Maxwell–Wagner–Sillars processes,¹⁴ and the internal grain boundary barrier layer capacitance (IBLC) model of Sinclair et al.^15,16 Despite their large dielectric constant, low resistivity of this class of perovskites has been a hurdle for their widespread application in industrial devices and systems. Additionally, process variability and effect of processing conditions on these materials have resulted in different dielectric constant and resistivity values.

To achieve the high-resistivity doping and substitution^17,18 have also been investigated in CaCu₃Ti₄O₁₂ and Pr_2/3Cu₃Ti₄O₁₂ material systems. Most of these efforts were devoted to achieving resistivity in by CaCu₃Ti₄O₁₂. Rai et al.¹⁷ evaluated the effects of praseodymium substitution on electrical properties of CaCu₃Ti₄O₁₂ ceramics. In another important study, Sebald et al.¹⁸ investigated various other members of CaCu₃Ti₄O₁₂ class, including MCu₃Ti₄O₁₂, where M = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, or Tm and showed that the best choice for increasing resistivity was Pr_2/3Cu₃Ti₄O₁₂ (PCTO). Similar to CaCu₃Ti₄O₁₂ ceramic, they demonstrated that polarization at internal barriers, most likely grain boundaries, is the reason for the high dielectric constant. In their experiments they studied their materials at 1273 K for 3 h of sintering. However, we have shown¹⁹ that processing at 1173 K or higher temperatures results in significantly difference morphologies and electrical properties. To study these parameters and determine the effect of Ca²⁺ and Sr²⁺ ion-substituted PCTO, we processed at temperatures above and below 1173 K and examined the morphological and electrical properties of the resulting materials. This revealed significant difference in both morphology and electrical properties, which can be attributed to different growth mechanisms and doping.

EXPERIMENTAL METHODS

Ceramic mixtures were synthesized using oxides and carbonates of the parent components and details are described in Refs.^12,20 Mixtures were homogenized by annealing grains without flux. The microstructural evolution from transient state to stable morphology was achieved. Following cooling and formation of stable ceramic, the electrical properties were measured, and results were compared for the pure and Ca²⁺ and Sr²⁺-substituted PCTO materials.

Synthesis of ceramic materials

Pure and substituted materials were prepared from Pr₂O₃, CaO, SrO, CuO, and TiO₂ as supplied by Aldrich Chemical Company. The purity of Pr₂O₃ was listed as 99.95%, CuO was listed for 99.9995%, and TiO₂ was listed for 99.9%. The particle sizes of these materials ranged from 20 to 50 nm. In a typical experiment, ceramic composites of approximately 10 g were synthesized. Stoichiometric mixture was thoroughly mixed followed by pressing uniaxially. Table 1 shows the compositions of materials used in this study. The synthesized powder was pressed into pellets before processing. The details of the pellets are described earlier.^10–13 The pressure applied for the preparation of the pellets ranged from 5000 to 7000 lb/inch².

TABLE 1 Weight (wt%) of the parent chemicals in the synthesis of substituted Pr_2/3Cu₃Ti₄O₁₂ (PCTO).

Parent chemicals used in the synthesis	Weight of parent components in Ca2+-substituted ceramic (wt%)	Weight of parent components in Sr2+-substituted ceramic (wt%)
Pr2O3	24.55	24.50
CuO	31.35	31.29
TiO2	42.00	42.20
CaCO3	2.10	0.00
SrO	0.00	2.01

Sintering and grain growth

The synthesized substituted samples were placed in a furnace maintained at 150°C for 50 h, followed by placing at 1173 K for the coarsening and grain growth. Since the goal was to study the effect of temperature on grain growth and performance, another set of samples was placed at 1273 K for coarsening and grain growth. These samples were also treated for a period of 96 h for grain growth.

Microstructural studies

Microstructural analysis was performed with a NANOSEM 450 scanning electron microscope. Operating voltage of 5‒10 kV was used for studying the microstructure of these materials. Energy dispersive X-ray spectroscopy (EDS) was used for the determination of the composition of grains and at the boundary (edges and spaces). The main focus was on the determination of concentration of copper since color of the samples treated at 1273 K was dark compared to the samples processed at 1173 K. Since copper oxide has lowest melting phase, variation in the concentration of the copper phase was found to be greater than that of the other oxides.

Dielectric and resistivity measurements

The dielectric constant and resistivity measurements were performed using parallel polished surfaces of pellets with electrode. The surfaces of the pellets were prepared by polishing with fine grit sandpapers, followed by cloth-based pads.²⁰ The surfaces were then cleaned using several solvents, including acetone. Silver paste was used as the electrode to determine the resistance and capacitance of the parallel polished pellets. A Hewlett Packard 4263A LCR meter was used to measure the capacitance, resistance, and loss tan δ. The dielectric constant was calculated from the measured capacitance based on the following equation: $$$\begin{equation*}C = \varepsilon {{\varepsilon }_0}\frac{A}{d}\end{equation*}$$$where ε is the dielectric constant of the material, ε₀ = 8.854 × 10⁻¹², C is the measured capacitance, A is the overlapping surface area of the pellet, and d is the thickness of the pellet. The dielectric capacitance of bulk samples was determined using applied bias voltages of 50‒1000 mV operating at frequencies ranging from 100 Hz to 100 000 kHz. Resistivity was inferred from the resistance measurements (a) using a parallel-resistance model of complex-impedance measurements and (b) measuring current from a DC voltage bias.

RESULTS AND DISCUSSION

Figure 1 shows as-synthesized Pr_2/3Cu₃Ti₄O₁₂. Samples processed at 1173 K (a) and 1273 K (b) reveal stark differences between the 1173 K processed metastable phases and the 1273 K processed stable phases. Compared to samples processed at 1173 K, the samples processed at 1273 K were very dark, as shown in Figure 1B.

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These results are consistent with previous work^2–4,17,18 suggesting that properties of perovskites and multinary oxides are very process dependent. It is well proven that the formation of boundaries, grain sizes, and shapes in materials are affected by the processing methods, temperature, flux used in system, and local compositional variation in the matrix. For this reason, attempts have been made^9–11 to achieve perovskites of high resistivity by doping or cation substitutions, which altered their properties. While interstitial substitution is difficult to perform since the perovskite have close packed structures, there is significant flexibility for substitution at A or B site of ABO₃ class of the perovskites employed in the present study. For these reasons,²¹ Ca²⁺ and Sr²⁺ were chosen to replace some of the Pr³⁺ in the PCTO ceramics. Generally, the resistivity of ceramics will be reduced due to the valence difference in either donor or acceptor substitution. However, ionic substitution changes the polarity, which results in dielectric and resistivity changes. Additionally, Ca²⁺ and Sr²⁺ substitutions were also made to understand the effect on morphology at 1173 K, where metastable materials have shown higher resistivity compared to materials processed at higher (e.g., 1173 K) temperatures. Studying these substitutions is a means of evaluating the difference in the size and valency have on dielectric constant and distortion in the resultant structure. The ionic sizes are extensively published in literature; however, Shannon RD²¹ has extensively revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. In a perovskite structure, the coordination number of Ca²⁺ and Sr²⁺ is typically 12 because the Ca or Sr cation occupies the “A” site in the perovskite structure, which is surrounded by 12 oxygen anions in a cuboctahedral arrangement. Similarly, the coordination number of a Pr³⁺ ion occupying the site “A” is 12 since each Pr³⁺ ion is surrounded by 12 oxygen anions. The ionic radius of Pr³⁺ in a perovskite structure is reported to be around 0.100 nm, with some sources listing it slightly smaller at approximately 0.099 nm. Similarly, the ionic radius of Ca²⁺ in a perovskite structure is reported to be around 0.100 nm, with some sources listing it slightly smaller at approximately 0.099 nm making it a potential substitute in a Pr-based perovskites without significantly disrupting the crystal structure. However, the ionic radius of Sr²⁺ in a perovskite structure is approximately 0.132 Å. Ca²⁺ and Sr²⁺ ions both have a lower valency than that of the Pr³⁺ cations they replace, and the solid compound get as electron acceptor. As a result, they create holes due to less contribution of electrons from the substituent ion they replace, or they generate oxygen vacancies, resulting in a greater chance of point defect creation and overall perovskite, which are slightly distorted with more defects. Finally, we evaluated if copper-rich phases, which change anisotropy and hence non-faceted to faceted morphology, behave in the same way as substituted materials.

Microstructural studies

Microstructural studies for pure and doped PCTO sintered at 1173 and 1273 K have shown significant difference in the internal boundary layers (IBLs), as observed by scanning electron microscopy (SEM). Figure 2 shows the microstructures of the PCTO ceramics and variations in the boundaries and shapes of the grains when sintered at 1173 K for 96 h. This image shows metastable grains with mostly non-faceted structures and several different shapes. However, in some regions, transitions from non-faceted to faceted appear. In addition, channel formation is also observed as is grain engulfing to form the larger non-faceted grains. Figure 3 shows the microstructure of PCTO ceramics processed at 1273 K for 96 h period. This image shows microstructures of ceramics in a relatively large area, with clearly developed faceted grains. In addition, some areas clearly revealed transition from non-faceted to faceted structures and engulfment of smaller grains into larger grains that were not complete.

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In the faceted area, channels and necks, which were very common in ceramics processed at 1173 K (Figure 2), were not observed. The observed no-nfaceted grains and transition of morphology is supported by EDS results discussed in later section. As temperature increased, copper-rich phases move from boundaries to grains, and change the characteristics of the grains, including surface anisotropy, similar to lead-doped CaCu₃Ti₄O₁₂ (CCTO) ceramic systems.^13,19 Figure 3 shows microstructure of PCTO observed for materials processed at 1273 K. The sizes of faceted grains shown in Figure 3 were in the range of 1‒2.5 µm. As shown in the shapes these grains, some faceted grains were very complex.

Ca²⁺-substituted samples

The calcium-substituted ceramic with an original composition of Pr_0.57Ca_0.15Cu₃Ti₄O₁₂ had a very different morphology compared to the pure PCTO ceramic. After 96 h of processing at 1173 K, the ceramic showed microstructure is metastable. Figure 4 shows an SEM image of the microstructure of a typical calcium-substituted materials processed at 1173 K. The segregation of calcium-rich phases was observed in these samples as determined by EDS. As shown in Figure 4A, in several portions of the material, the calcium-rich phases were attached to non-faceted grains. Additionally, grain sizes were smaller compared to pure PCTO and less coarsened with large numbers of channels, necks, and ridges attached with other grains. This indicates that most of the grain growth is by merging of grains with necking and channel formations. Figure 4B shows the microstructure of a center part of the sample, which has lower calcium-rich phases. These metastable structures of the Ca²⁺-substituted ceramics processed at 1173 K are significantly different than the microstructures observed for the calcium-substituted materials processed at 1273 K (Figure 5).

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The morphology of the samples processed at 1273 K revealed a complete (Figure 5) transformation from a metastable to a faceted microstructure. Similar to that of pure PCTO ceramic, migration of copper-rich phases from grain boundaries to grains was observed. Although it was extremely difficult to focus only on boundaries or grains, EDS data for the ceramic processed at 1173 K indicated that the initial concentration of copper in boundaries ranged from 50.9 to 76.1 wt% with low concentrations of copper (2.5‒7.7 wt%) in the grains. However, the copper concentrations in the boundary area were in the range of 1.5‒2.2 wt% and 21.8‒22.7 wt% in the grains for the ceramic processed at 1273 K. Copper-rich concentrations in the semiconducting grains altered the anisotropy significantly¹⁹ in the ceramics at 1273 K, which was very similar to that of previous lead-doped CCTO grains,¹⁰ which also showed complete faceted microstructures. As shown in Figure 5, faceted structures were complex and showed signs of ridges on large crystals and grains that clearly grew in different sizes and shapes.

Sr²⁺-substituted samples

Figure 6 shows microstructures of the Pr_0.57Sr_0.15Cu₃Ti₄O₁₂ ceramics processed at 1173 K. The microstructures show that the Sr-rich phases did not completely dissolve even after 96 h of sintering at 1173 K. As shown in Figure 6A, the necks and channels connected to other grains and sizes were comparable to those observed in the Ca²⁺-substituted samples. However, as shown in Figure 6B, metastable grains diffused into larger grains and grew on the top of other grains, revealing different regions of some ceramic elongated grains with large aspect ratios in some area. Furthermore, in these Sr-substituted ceramics, channels and necks were more pronounced and their formation was observed throughout the samples. Additionally, Sr-rich phases segregated from the base material, and grains grew on the top of other grains in the form of layers.

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Changes from non-faceted to faceted structures can be observed for the Sr²⁺-substituted ceramics processed at 1173 and 1273 K, respectively, which again can be explained based on differences in composition of the metastable and stable grains. Similar to that of Ca-substituted samples, copper concentration was measured by EDS for Sr²⁺-substituted materials, and a large difference was observed in the ceramics processed at 1173 and 1273 K. In the Sr-substituted samples sintered at 1173 K, the copper concentration in boundaries were determined to be 66.9‒69.1 wt% and 2.4‒6.8 wt%, respectively, in the grains. In contrast, in the samples processed at 1273 K, the trend was reversed, with concentrations of 0.7‒5.0 wt% in boundary area and 21.8‒24.2 wt% in the grains. However, smaller size grains compared to pure PCTO indicate that Ca and Sr substitution affected the migration of the copper-rich phases differently. The transition of non-faceted to faceted structures has been discussed in Ref.¹⁰ in details for the CCTO material system, in which lead oxide-doped CCTO ceramic showed changes in grain shape from non-faceted to faceted, indicating significant changes in interface energy. In the samples processed at 1173 K temperature, grain shapes were developing with the driving force for coarsening provided by the excess interfacial free energy and copper (from copper oxide) that migrated from boundaries to the grains. The local copper concentrations affects the anisotropy of grains significantly. While isotropic condition theories based on the classical theory of Lifshitz, Slyozov, and Wagner,^22,23 assume that the coarsening phase is infinitely dilute and the average chemical potential surrounding each crystal is identical. These are not the conditions for all these samples. As the grains grow, the characteristic critical radius of the particle r^* involves the surface energy per unit area γ given by the relationship r^* = 2γ/μ, where μ is the chemical potential. Grains larger than the critical radius, r^*, grow, and those smaller than r^* shrink and dissolve into larger grains. Shiflet et al.²⁴ discussed the kinetics of coarsening by the ledge mechanism proposing a very good approach for explaining the formation of faceted faces. Even more recently, various models were proposed by Voorhees and coworkers^25–27 to explain effects of growth parameters, as well as alloying and interface anisotropy parameters in various alloy systems and for variety of grain shapes. Pure Pr_2/3Cu₃Ti₄O₁₂ and Ca²⁺ and Sr²⁺-substituted ceramics processed at 1173 K showed that the shrinkage of smaller grains occurred by merging, elongation into other grains, channel formation, and other dislocation behavior near the junctions. During growth, changes in the properties, including the interface energy, may occur due to the variations in the local copper concentration, similar to that reported for lead doping,¹⁰ which changes the shape of grains. The addition of calcium and strontium ions created another complexity since rich phases of calcium and strontium did not completely dissolve in the temperature range of 1173 K. As the migration of copper took place from boundary area to grains, composition changed, and necks, channels, and distorted shapes developed. This clearly indicated that defects help in grain growth since transport is faster for these shapes compared to faceted shapes. In the copper-rich grains, anisotropy may have changed significantly like the case of PbO-doped CCTO ceramic,¹⁰ and microstructures completely altered faceted structures for the copper-rich grains.

Dielectric constant and resistivity

One of the goals of the substitution of Ca²⁺ and Sr²⁺ ion was to create distortion and hence polarity by valency difference in the PCTO perovskite and to determine its effect on the dielectric constant and resistivity. From the microstructural studies, it was clear that the compositional variation in materials processed at 1173 and 1273 K has tremendous effect on the morphology; hence, electrical properties and loss should be different since these properties depend on the material composition and the overall quality. Therefore, the dielectric constant and resistivity as a function of frequency were studied (Figures 7 and 8) to determine the effect of processing and substitution.

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Figures 9–13 show that there is significant difference between dielectric constant and resistivity of pure and substituted ceramic material processed at 1173 and 1273 K. Electrically insulating grain boundary (GB) and semiconducting grain interior areas are affected and evolution of the IBL capacitor structure occurs with increasing sintering temperature T_S (1173‒1373 K). Dielectric values indicate that the intrinsic bulk and GB permittivity changed significantly, and the resistivity decreased several orders of magnitude in the high-temperature processed material. These trends can be explained on the basis of increased Cu segregation in the boundary area or in the grains. The chemical changes due to possible Cu loss in ceramics with increasing temperature are small, and beyond the accurate quantitative detection limits of the copper-containing phase. However, in each case, trends show that at lower temperature (1173 K) processed samples, copper-rich phases are in the boundary area and in the samples processed at high temperatures (1273 K), higher copper concentrations are in grains. It was observed that copper-rich phases show liquidus phase above 1173 K. Although it was difficult to focus on individual boundaries and grains, EDS measurements showed migration of copper-rich phases from boundaries to grains. Along with transition to faceted structures, changes from metastable phases were observed for lower temperature materials processed at 1173 K. Also revealed, lower dielectric constant and higher resistivity compared to faceted materials processed at 1273 K. As shown in these figures, high dielectric constant and lower resistivity were always observed in the faceted ceramics processed at high-temperature copper-rich semiconducting grains.

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Although substituted ceramics showed different values, this same trend was also observed, indicating that processing temperature can be used to achieve high dielectric constant or resistivity. Three models^14–17 that have been proposed to explain the large dielectric constant in perovskite materials are (a) redox model, which suggests that a small amount of Ti⁴⁺ ions occupy the Cu site when Cu²⁺ ions are partially reduced to Cu⁺ ions and other ions in some oxides at high temperatures,^28,29 (b) the Maxwell‒Wagner‒Sillars model,¹⁴ which suggests that insulating grain boundaries and high dielectric constant occur when charge carriers accumulate at the interface of two materials with varying conductivities, and (c) Sinclair et al.^15,16 model known as IBLC, which indicates that in the final processed perovskite the grains are semiconducting and grain boundaries are insulating. This IBLC model shows that in the final processed perovskites, the grains are semiconducting, the grain boundaries are insulating, and one can assume that resistors correspond to the bulk and GB resistivity, respectively, and the capacitance represents bulk and GB permittivity, respectively.

The loss tangent (tan δ) or dissipation factor (D_f) is the measure of the dielectric loss in material, and is defined by the following equation: $$$\begin{equation*}\tan \delta = \frac{{\varepsilon ^{\prime\prime}}}{{\varepsilon ^{\prime}}} + \frac{\sigma }{{2\pi f\varepsilon ^{\prime}}}\end{equation*}$$$where ε′ and ε″ are the real and imaginary part of the dielectric permittivity, respectively, and σ is the electrical conductivity of the materials, and f is the applied frequency. The magnitude of the dielectric constant and the loss in a dielectric material result from distortional, dipolar, interfacial, and conduction loss. Since the distortional loss is related with electronic and ionic polarization mechanisms, we expected substituted PCTO to show different values of the dielectric constant and resistivity. The data shown in Figures 9–13 showed that substitution changed these values significantly. The energy loss (W) refers to the energy dissipated in a dielectric material and it is proportional to the dielectric loss tangent and is defined as. $$$\begin{equation*}W \approx \pi \varepsilon ^{\prime}{{\xi }^2}f\tan \delta \end{equation*}$$$where ξ is the electric field strength and f is the frequency. Therefore, a low dielectric loss is preferred to reduce the energy dissipation and signal losses. The observed data for all systems indicated that as the frequency increased, dielectric constant and resistivity decreased significantly in these ceramics.

Figure 14 shows the loss tan δ observed for the Pr_0.50Ca_0.15Cu₃Ti₄O₁₂ ceramic. These values were determined for different frequencies in the range of 100‒100 000 Hz for the bias voltage of 250 and 500 mV, and the loss tan δ increased as function of the bias voltage and the frequency. The losses for this range were well below the loss tan δ of 1, indicating suitability of this material for applications in this frequency region.

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SUMMARY

The effect of processing temperature and the compositional substitution on the morphology and electrical properties of Pr_2/3Cu₃Ti₄O₁₂ were performed. From these studies, it was observed that migration of lowest melting copper phase in the pure and substituted Pr_2/3Cu₃Ti₄O₁₂ ceramic systems from the GB area to grains played significant role in the morphological transition from non-faceted to faceted structures. Furthermore, the migration of copper-rich phases in the grains created anisotropy in the interfacial energy, which played an important role in determining the shaped faces. EDS results indicated migration of copper-rich phases from boundaries to grains as the copper phase has the lowest melting temperature in these systems. The morphological observations showed that new layers grew by dislocation mechanism suggesting that nucleation-limited coarsening occurred by the development of a transient bimodal grain size consisting of grains with steps. In the metastable phases observed for lower temperature materials processed at 1173 K, a lower dielectric constant, and higher resistivity were observed when compared to well-developed faceted materials processed at 1273 K in the pure and substituted ceramics. However, for the Ca and Sr-substituted samples resistivities were higher than pure PCTO ceramics. The data for both samples processed at 1173 and 1273 K can be explained based on ILBC mechanism since copper composition changes showed different values of dielectric constant and resistivity for these samples.

ACKNOWLEDGMENTS

The authors would like to acknowledge the support from the Biological and Physical Science Division, Science Mission Directorate of NASA, Washington, DC, through NASA, Marshall Space Flight Center Huntsville, Alabama. The authors also thank Ms. Jane Henderson, Ms. Deb Waters and Ms. Michele Mullins for the administrative support during this study.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

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