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Article

Controlled-Atmosphere Sintering of KNbO3

by
Doan Thanh Trung
and
John G. Fisher
*
School of Materials Science and Engineering, Chonnam National University, Gwangju 61186, Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2020, 10(6), 2131; https://doi.org/10.3390/app10062131
Submission received: 17 February 2020 / Revised: 13 March 2020 / Accepted: 19 March 2020 / Published: 20 March 2020
(This article belongs to the Special Issue Sintering Phenomena and Microstructural Control)
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Abstract

:
The effect of sintering atmosphere (O2, air, N2, N2-5% H2, and H2) on the densification, grain growth, and structure of KNbO3 was studied. KNbO3 powder was prepared by solid state reaction, and samples were sintered at 1040 °C for 1–10 h. The sample microstructure was studied using Scanning Electron Microscopy (SEM). The sample structure was studied using X-Ray Diffraction (XRD). H2-sintered samples showed reduced density, whereas other sintering atmospheres did not affect density much. Samples sintered in N2-5% H2 showed abnormal grain growth, whereas sintering in other atmospheres caused stagnant (O2, air, N2) or pseudo-normal (H2) grain growth behavior. Samples sintered in reducing atmospheres showed decreased orthorhombic unit cell distortion. The grain growth behavior was explained by the mixed control theory. An increase in vacancy concentration caused by sintering in reducing atmospheres led to a decrease in the step free energy and the critical driving force for appreciable grain growth. This caused grain growth behavior to change from stagnant to abnormal and eventually pseudo-normal.

1. Introduction

KNbO3 is a ferroelectric perovskite material and is one of the end members of the KNbO3-NaNbO3 pseudo-binary system [1,2]. It has an orthorhombic Amm2 unit cell at room temperature, with phase transitions to a tetragonal P4mm and a cubic P m 3 ¯ m phase at ~225 and ~435 °C, respectively [3,4,5,6]. KNbO3 has many potential applications including electro-optic modulators and frequency convertors [7,8], photocatalysts [9,10], photovoltaic materials [10,11,12], phosphor hosts [13,14,15], and in biomedical applications [16,17,18,19]. KNbO3 has also been studied as a potential lead-free piezoelectric material for the replacement of Pb(Zr,Ti)O3 [12,20,21,22,23,24,25,26,27,28,29,30,31], although it has received less attention than its cousin (K0.5Na0.5)NbO3 due to its inferior piezoelectric properties [32]. KNbO3 shares the same challenges in processing (hygroscopic starting materials, difficulty in sintering to high density, the formation of water-soluble second phases, abnormal grain growth) as (K0.5Na0.5)NbO3 [21,25,26,30,33,34,35]. However, it has the advantages of a lower sintering temperature (1040 °C vs. 1100 °C) and less tendency towards abnormal grain growth. It also has unusually good shear mode piezoelectric properties (k15 = 0.55, d15 = 207 pC/N), making it a candidate for actuators and high-power applications [31]. An advantage of (K0.5Na0.5)NbO3 over Pb(Zr,Ti)O3 is its stability during sintering in reducing atmospheres [36,37,38]. This allows the possibility of co-firing with lower cost base-metal electrodes [39,40,41,42]. Sintering in an inert atmosphere also reduces the nonlinear behavior of (K0.5Na0.5)NbO3-based materials through the formation of oxygen vacancies that pin the domain walls [43]. It is known that the use of reducing sintering atmospheres can change the densification behavior, grain growth behavior, and also the structure of (K0.5Na0.5)NbO3 [44,45,46,47], but the behavior of KNbO3 during sintering in reducing atmospheres has not previously been studied. The effect of different reducing atmospheres on the densification, grain growth behavior, and structure of KNbO3 is examined in this work.

2. Materials and Methods

KNbO3 powder was prepared via solid state reaction. K2CO3 (>99.5%) and Nb2O5 (>99.9%) (Daejung Chemicals) were dried (250 °C, 5 h) to eliminate adsorbed moisture. Stoichiometric amounts were weighed and ball milled for 24 h using high-purity ethanol (99.9%) and ZrO2 balls in a polypropylene jar. After milling, the slurry was heated on a hotplate/magnetic stirrer to evaporate the ethanol, followed by drying in an oven at 80 °C. After drying, the slurry was ground and passed through a 180 μm sieve to remove agglomerates, followed by calcination at 850 °C for 5 h in flowing O2 (flow rate 100 cc.min−1) in a covered alumina crucible. The calcined powder was examined by X-Ray Diffraction (XRD, X’Pert PRO, PANalytical, Almelo, the Netherlands) using CuKα radiation with 2θ = 20–90°, a scan speed of 3°.min−1, and a step size of 0.02°. The calcined powder was ball-milled again to reduce particle size.
Powder was hand pressed into pellets using a 6 mm die, then cold isostatically pressed at 147 MPa. Pressed pellets were buried in KNbO3 packing powder in a covered alumina crucible and sintered in atmospheres of O2, air, N2, N2-5 vol % H2, and H2 (flow rate 50 cc.min−1) at 1040 °C for 1–10 h. Heating and cooling rates were 5 °C.min−1. The sintered sample density was measured by the Archimedes method. Samples were vertically sectioned, polished with diamond suspension (1 μm finish), and thermally etched at 990 °C for 1 h in the same atmosphere in which they were sintered. The microstructure of Pt-coated samples was examined by Scanning Electron Microscopy (SEM, Hitachi S-4700 FE-SEM, Hitachi High-Tech, Tokyo, Japan) with an attached Energy Dispersive Spectrometer (EDS, EMAX Energy EX-200, Horiba, Kyoto, Japan) using standardless quantification. Mean matrix grain size and grain size distributions were analyzed using Image J v1.46 (National Institute of Mental Health, Bethesda, MD). Samples sintered at 1040 °C for 5 h were examined using XRD as before. Differential Thermal Analysis (DTA, DTG-60, Shimadzu, Kyoto, Japan) was carried out on samples sintered at 1040 °C for 3 h. Analysis was carried out in the temperature range 50–450 °C in N2 with heating and cooling rates of 10 °C.min−1.

3. Results

The as-calcined KNbO3 powder’s XRD pattern is shown in Figure 1. The pattern was indexed with ICDD Card # 71-0946 for orthorhombic KNbO3 (space group Amm2). No secondary phase peaks were visible. The Archimedes density values of the KNbO3 samples sintered at 1040 °C for 1–10 h in different atmospheres are shown in Figure 2. All of the samples had densities between 95 and 98% theoretical density except for samples sintered in H2, which had densities between 89 and 92% (theoretical density of KNbO3 = 4.624 g.cm−3 [48]). Apart from H2, the sintering atmosphere did not have much effect on density. Sintering time also did not affect density.
SEM micrographs of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres are shown in Figure 3. The O2-sintered sample contained cube-shaped equiaxed grains 1–2 μm in diameter. The tiny particles on the surface of the grains are an artefact caused by the Pt coating. The air- and N2-sintered samples appeared similar, with slightly smaller grains. The N2-5%H2-sintered sample showed very different behavior. Many abnormal grains up to 30 μm in diameter were present. A few micron-sized grains were still visible in spaces between the abnormal grains. The abnormal grains contained many entrapped pores. In the H2-sintered sample, the grains were again micron-sized. The samples sintered for other sintering times showed similar behavior.
Figure 4 shows SEM micrographs taken from the edges of samples sintered at 1040 °C for 10 h. Abnormal grains were present at the edges of the O2-, air-, and N2-sintered samples. The degree of abnormal grain growth at the edges of the samples increased with sintering time. Samples sintered in N2-5%H2 showed the opposite behavior, with less abnormal grain growth taking place at the edges of the sample than in the bulk. The sample sintered in H2 contained large plate-like grains of a secondary phase at the edges. EDS analysis of these grains (mean and standard deviation of four point analyses) is shown in Table 1. The matrix grains appeared to be potassium-deficient. This may be due to potassium evaporation or to the difficulty in measuring alkali elements by EDS [49]. The K/Nb ratio of the secondary phase was much smaller than that of the matrix grains and was quite close to that of K4Nb6O17. Apart from this secondary phase, no abnormal grains were present at the edges of the sample sintered in H2. It is worth noting that the samples sintered in H2 were more fragile than the other samples.
The mean grain radius of KNbO3 samples sintered at 1040 °C for 1–10 h in different atmospheres is shown in Figure 5. Error bars represent the standard deviation. At least 200 grains were measured per sample sintered in O2, air, N2, and H2. For samples sintered in N2-5% H2, between 120 and 180 grains were measured per sample. Only grains from the bulk of the samples sintered in O2, air, and N2 were measured, i.e., abnormal grains found at the edges of these samples were not included. O2-, air-, and N2-sintered samples had similar values of mean grain radius, between 0.4 and 0.6 μm. The samples sintered in N2-5%H2 had a much larger mean grain radius between 6 and 8 μm, as well as wider error bars. The mean radius of H2-sintered samples was smaller than that of the other samples at ~0.35 μm. Sintering time did not have much effect on the mean grain radius of any of the samples.
Grain size distributions of KNbO3 samples sintered at 1040 °C for 1 h in different atmospheres are given in Figure 6. Dashed black lines indicate the radius value that was three times the mean radius. Grains larger than this value were classified as abnormal grains [50]. Samples sintered in O2, air and N2 showed narrow unimodal distributions. O2- and air-sintered samples did not have any abnormal grains. A small fraction of abnormal grains appeared in the sample sintered in N2. The sample sintered in N2-5% H2 showed a very broad size distribution with many abnormal grains. The grains were also much larger than in the previous samples. For the H2-sintered sample, the grain size distribution became narrow and unimodal again with no abnormal grains. The grain size distributions at increased sintering times did not change much. SEM micrographs of KNbO3 powder samples annealed for 1 h at 1040 °C in O2 and H2 are shown in Figure 7. The powder particles were of a cubic morphology with faceted faces and sharp edges and corners. The H2-annealed sample had rounder edges and corners than the O2-annealed sample.
Figure 8a shows the XRD patterns of KNbO3 samples sintered at 1040 °C for 5 h in different atmospheres. All patterns were indexed using ICDD Card # 71-0946 for orthorhombic KNbO3 (space group Amm2). No secondary phase peaks were visible in O2-, air-, N2-, and N2-5% H2-sintered samples. The H2-sintered sample contained Nb12O29, K4Nb6O17, and NbO2 secondary phases (Figure 8b). The (011)/(100) peak splitting at 22–23°, the (002)/(020)/(111) peak splitting at 31–32°, and the (022)/(200) peak splitting at 44–46° progressively diminished in reducing sintering atmospheres. Unit cell parameters were refined via the least-squares method (MDI Jade 6.5, Materials Data Inc., Livermore, CA). Results are shown in Figure 9. In progressively reducing sintering atmospheres, the a unit cell parameter tended to increase, whereas the b and c unit cell parameters decreased. The difference in value between the b and c unit cell parameters decreased noticeably for the H2-sintered sample.
Figure 10 shows DTA traces of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres. Endothermic peaks corresponding to transitions from orthorhombic to tetragonal phases and from tetragonal to cubic phases can be seen in the ascending traces (Figure 10a) [45,51]. The peak temperatures initially increased slightly as the sintering atmosphere changed from O2 to air and then progressively decreased in reducing sintering atmospheres. An orthorhombic-tetragonal phase transition peak could not be seen in the H2-sintered sample. Exothermic peaks corresponding to the transition from cubic to tetragonal phases could be seen in the descending traces (Figure 10b). Peak temperature decreased for the samples sintered in N2-5% H2 and H2 atmospheres. Peaks corresponding to the tetragonal-orthorhombic phase transition were not visible. Peak temperatures are given in Table 2. Cubic-tetragonal phase transition temperatures on cooling were lower than those of the tetragonal-cubic phase transitions on heating. This hysteresis indicated that the phase transitions were first order.

4. Discussion

During solid state sintering of a polycrystalline ceramic, grain growth takes place due to grain boundary curvature-induced differences in pressure and atom chemical potential between grains of differing size [52,53,54]. Driving force ΔG for the growth of a grain is given by [54,55,56]:
Δ G = 2 γ V m ( 1 r * − 1 r )
where γ = grain boundary energy, Vm = molar volume, r* = critical grain radius (radius of a grain that does not grow or shrink, usually considered to be the mean grain radius), and r = radius of the grain of interest. For a faceted grain, r = perpendicular distance from the grain center to the surface and γ = specific surface energy [54]. The chemical potential (and hence, ΔG) of atoms in a grain with faceted surfaces also depends on grain size [57,58].
The grain growth rate also depends on the grain boundary structure, as well as the driving force. Grain boundaries have disordered (rough) or ordered (faceted) structures on an atomic scale [59,60]. For grains with disordered grain boundaries, there are many possible atom attachment sites on the grain surface. Grain growth is limited by atom diffusion across the grain boundary [54,56]. The grain growth rate shows a linear increase with ΔG. Grains with r > r* have positive values of ΔG and can grow (the black dotted line in Figure 11), whereas grains with r < r* have negative values of ΔG and will shrink [54]. Grain size distribution remains unimodal with sintering time, and abnormal grain growth will not occur [54].
If the grain boundaries are ordered, atoms attaching to the surface of a grain are unstable due to their many broken bonds. They will quickly detach from the grain unless they can migrate and attach to low-energy kink sites such as 2D nuclei, screw dislocations, steps, and re-entrant edges [54,60,61]. Grain growth is now limited by the rate of atom attachment at such kink sites (interface-reaction controlled growth). For a system in which grain growth is controlled by 2D nucleation and growth, the rate at which 2D nuclei form varies exponentially with ΔG [62,63]. Below a critical driving force ΔGc, the 2D nuclei form at a very slow rate, and atom attachment to the grain is difficult. At ΔG = ΔGc, the rate at which 2D nuclei form increases exponentially. For ΔG > ΔGc, kinetic roughening takes place [60,64,65]. Many 2D nuclei form, and atom attachment to the grain becomes easy. Grain growth becomes diffusion-limited, as in the case of a rough grain boundary. Hence, the grain growth rate is negligible at ΔG < ΔGc, increases exponentially at ΔG ≈ ΔGc (the dashed lines in Figure 11), and then becomes a linear function of ΔG (the solid lines in Figure 11) [66]. Grains with negative values of ΔG will shrink as a linear function of ΔG, as the energy requirement for atoms to detach from grain corners and edges is low [54,56,67]. If the grains contain screw dislocations, the grain growth rate increases parabolically with ΔG until ΔGc is reached, at which point grain growth becomes diffusion-limited again. According to the relative values of ΔGc and ΔGmax (where ΔGmax is the value of ΔG for the largest grain), different grain growth behaviors (pseudo-normal, abnormal, and stagnant) can be observed [54,56,67,68]. This grain growth theory, called the mixed control theory, was originally developed to describe liquid phase sintering, but similar behavior also takes place in single phase systems with faceted grain boundaries in which the formation and lateral spreading of steps governs grain boundary migration [54,66,67,69,70,71,72,73,74,75,76,77,78].
The critical driving force ΔGc is dependent on the step or edge free energy ε, which is the excess energy of the edge of the kink site [54,56,67,70,73]. The value of ε can vary depending on sintering atmosphere, sintering temperature, or dopant addition [59,60,79]. As ε decreases, ΔGc also decreases. Grain morphology is a qualitative indication of the value of ε [55,68,80]. The cubic morphology of the KNbO3 grains (Figure 3) with faceted faces, sharp edges, and corners (Figure 7) indicates a high value of ε. During sintering of KNbO3, evaporation of potassium can take place. However, the potassium vapor pressure over KNbO3 is much lower than that of lead oxide over PZT [81]. Oxygen vacancies will also form via the following defect reaction:
2 O O x → ↑ O 2 ( g ) + 2 V O • • + 4 e ′
The formation of potassium and oxygen vacancies increases the configurational entropy of the system, which in turn reduces step free energy ε and critical driving force ΔGc [82,83,84]. The decreased orthorhombic distortion in the unit cell of the H2-sintered sample (Figure 8 and Figure 9) and the decrease in temperature of the orthorhombic to tetragonal and tetragonal to cubic phase transitions in the samples sintered in N2-5%H2 and H2 (Figure 10) are caused by the increased concentration of oxygen vacancies [45,85,86,87,88,89,90]. The appearance of secondary phases in the H2-sintered samples also indicates increased evaporation of potassium (Figure 4 and Figure 8). By considering the rate of formation of oxygen vacancies in different sintering atmospheres, the grain growth behavior of KNbO3 can be explained. For O2- and air-sintered samples, the number of oxygen vacancies is relatively low. The values of ε and hence ΔGc are high. The value of ΔGc is higher than ΔGmax, the driving force of the largest grain (Figure 11). No grains in the samples had ΔG ≥ ΔGc, and so, no grains could grow appreciably. This caused stagnant grain growth behavior. For the N2-sintered sample, the lower P O 2 would push Equation (2) to the right and create more oxygen vacancies, increasing configurational entropy, lowering ε and ΔGc. A few grains now had ΔG ≈ ΔGc. These grains should be able to grow more rapidly than the surrounding matrix grains and become abnormal grains (Figure 6 and Figure 11). However, there was little size difference between the abnormal grains and matrix grains. For values of ΔG very close to ΔGc, the grain growth rate does not increase dramatically with small increases in ΔG (the region of the blue curve in Figure 11 which is beginning to curve upwards). With increasing sintering time, ΔG of the abnormal grains should slowly increase as they were slowly growing, while the matrix grains (which control the value of the critical radius r*) were barely growing [Equation (1)]. Once ΔG became large enough, the growth rate should then increase very rapidly (the steeply sloping region of the blue curve). However, this did not appear to happen as the grain size distributions of the N2-sintered samples did not change significantly as sintering time increased. A longer sintering time was possibly needed to allow the abnormal grains to grow large enough to reach the region of the growth rate vs. ΔG curve where the growth rate increases rapidly.
For samples sintered in N2-5% H2, the number of oxygen vacancies increased further. ε and ΔGc were further lowered (Figure 11). The number of grains with ΔG ≥ ΔGc increased, and their growth rate also increased compared to those in the N2-sintered samples. These grains underwent rapid growth and became abnormal grains (Figure 3 and Figure 6), causing a rapid increase in the mean grain size (Figure 5). Extensive abnormal grain growth took place by 1 h. After impingement of the abnormal grains, their value of ΔG decreased rapidly, and further grain growth was very limited. For the H2-sintered samples, ε and ΔGc were further lowered. The increased curvature at the corners and edges of the KNbO3 powder annealed in H2 is indicative of a reduced ε (Figure 7). The value of ΔGc became so low that many grains had ΔG ≥ ΔGc and could grow. Even grains with ΔG ≈ ΔGmax could not now grow rapidly to form abnormal grains as they had to compete for material with many other growing grains. A shift from abnormal to pseudo-normal grain growth took place, with the grain size distribution becoming narrow and unimodal [56,67,77,78].
For the O2-, air-, and N2-sintered samples, abnormal grain growth occurred at the edges of the samples. This was probably due to the increased evaporation of potassium at the sample edges, increasing the number of potassium vacancies. The concentration of oxygen vacancies at the sample edges was also likely to be larger than in the bulk. This caused ε and ΔGc to be lower at the sample edges than in the bulk. During prolonged sintering, some of the grains at the sample edges could grow sufficiently to have ΔG ≥ ΔGc and could then form abnormal grains. For the N2-5% H2-sintered samples, ε and ΔGc were also lower at the sample edges than in the bulk. The number of grains at the edges of the sample with ΔG ≥ ΔGc and therefore able to grow was larger than in the bulk, causing the grain growth behavior at the edges of the sample to transition towards pseudo-normal grain growth.
Comparing the SEM micrographs (Figure 3) with the mean grain size measurements (Figure 5), it can be seen that there are many large grains over 10 μm in radius in the samples sintered in N2-5% H2; however, the mean grain radius is only 6–8 μm. The measured mean grain radius will depend on the relative numbers of small matrix and large abnormal grains measured. Due to their large size, the abnormal grains were measured from micrographs taken at lower magnification, and their grain boundaries were not always clearly visible. It was easier to measure the size of the remaining small matrix grains, so the mean grain size and grain size distributions of the samples sintered in N2-5% H2 were likely skewed towards smaller values. Nonetheless, it is still clear that sintering in N2-5% H2 caused a large increase in mean grain size.
The appearance of secondary phases in the samples sintered in H2 indicates that KNbO3 is unstable when sintered in strongly reducing atmospheres (Figure 8b). Such instability was not noticed in (K0.5Na0.5)NbO3 samples sintered in H2 [45,46,47]. If one considers an Ellingham diagram, the Gibbs free energy vs. temperature line for the reaction 4K + O2 ⇆ 2K2O lies above the line for the reaction 4Na + O2 ⇆ 2Na2O, indicating that potassium oxide is more easily reduced than sodium oxide [91]. Shigemi and Wada calculated the formation energy of oxygen vacancies in strongly reducing atmospheres to be lower in KNbO3 (0.80 eV) than in NaNbO3 (0.84 eV) [92,93]. The vapor pressure of K over KNbO3 is also higher than that of Na over NaNbO3 [81,94]. Therefore, KNbO3 may be less stable than (K0.5Na0.5)NbO3 when sintered in reducing atmospheres. The appearance of the secondary phases may also explain the lower density of the H2-sintered samples. The density of K4Nb6O17 is 3.897 g.cm−3 [95] compared to 4.624 g.cm−3 for KNbO3 [48].
The present results show that KNbO3 samples sintered in N2-5% H2 underwent considerable abnormal grain growth. As abnormal grain growth is usually detrimental to the mechanical properties, KNbO3 may not be suitable for co-firing in reducing atmospheres with base metal electrodes. On the other hand, an increase in grain size is known to improve the piezoelectric properties of (K0.5Na0.5)NbO3-based ceramics due to reduced domain wall pinning (extrinsic contribution) [96,97,98]. Control of the orthorhombic-tetragonal phase transition temperature, which controls the relative amounts of orthorhombic and tetragonal phases, is also very important in improving the piezoelectric properties due to enhanced polarization extension and rotation (intrinsic contribution) [96,99,100,101], as well as enhanced extrinsic contributions [102]. By sintering KNbO3 in reducing atmospheres, it may be possible to control both grain size and orthorhombic-tetragonal phase transition temperature simultaneously, thereby enhancing the piezoelectric properties. The present study focused only on the effect of sintering atmosphere on the density, structure, and microstructure of KNbO3. The samples sintered in reducing atmospheres would have relatively high conductivity due to the free electrons formed by Equation (2). When sintering ferroelectric materials such as BaTiO3 or (K0.5Na0.5)NbO3 in reducing atmospheres, an acceptor dopant such as MnO2 or MnO is often added to trap these free electrons, enabling the materials to maintain high resistivity [103,104]. In the future, the effect of MnO2 doping on the grain growth behavior and electrical properties of KNbO3 sintered in reducing atmospheres needs to be studied.

5. Conclusions

KNbO3 was sintered in different atmospheres and the effect on densification, grain growth, and structure studied. The sintering atmosphere did not have much effect on sample density except for H2, which caused a reduction in density. Oxygen-, air-, and N2-sintered samples showed bulk stagnant grain growth behavior (with abnormal grain growth at the sample edges); samples sintered in N2-5% H2 showed bulk abnormal grain growth behavior (with less abnormal grain growth at the sample edges), and samples sintered in H2 showed pseudo-normal grain growth behavior. Sintering in reducing atmospheres caused a reduction in the unit cell orthorhombic distortion and reduced orthorhombic-tetragonal and tetragonal-cubic phase transition temperatures. Samples sintered in H2 were unstable, with secondary phases and weak mechanical properties. Grain growth behavior in different sintering atmospheres was explained by the mixed control theory of grain growth. Sintering in reducing atmospheres caused increased formation of oxygen vacancies and increased configurational entropy, which caused a decrease in step free energy and in critical driving force for grain growth. This increased the number of appreciably growing grains, causing grain growth behavior to transition from stagnant to abnormal and eventually to pseudo-normal.

Author Contributions

Conceptualization, J.G.F.; methodology, J.G.F.; formal analysis, D.T.T.; investigation, D.T.T.; resources, J.G.F.; data curation, D.T.T. and J.G.F.; writing, original draft preparation, J.G.F.; writing, review and editing, J.G.F.; visualization, D.T.T. and J.G.F.; supervision, J.G.F.; project administration, J.G.F.; funding acquisition, J.G.F. All authors read and agreed to the published version of the manuscript.

Funding

This study was financially supported by Chonnam National University (Grant Number 2016-2518).

Acknowledgments

The authors would like to thank Kyeong-Kap Jeong (Centre for Research Facilities, Chonnam National University) for operating the XRD and Hey-Jeong Kim (Centre for Development of Fine Chemicals, Chonnam National University) for operating the SEM. The authors would like to thank Prof. Jong-Sook Lee and Thuy Linh Pham for experimental assistance.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; nor in the decision to publish the results.

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Figure 1. XRD pattern of calcined KNbO3 powder.
Figure 1. XRD pattern of calcined KNbO3 powder.
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Figure 2. Archimedes density of KNbO3 samples sintered at 1040 °C for 1–10 h in different atmospheres.
Figure 2. Archimedes density of KNbO3 samples sintered at 1040 °C for 1–10 h in different atmospheres.
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Figure 3. SEM micrographs of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres.
Figure 3. SEM micrographs of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres.
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Figure 4. SEM micrographs of the edges of KNbO3 samples sintered at 1040 °C for 10 h in different atmospheres.
Figure 4. SEM micrographs of the edges of KNbO3 samples sintered at 1040 °C for 10 h in different atmospheres.
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Figure 5. Mean grain radius of KNbO3 samples sintered at 1040 °C for 1–10 h in different atmospheres.
Figure 5. Mean grain radius of KNbO3 samples sintered at 1040 °C for 1–10 h in different atmospheres.
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Figure 6. Grain size distributions of KNbO3 samples sintered at 1040 °C for 1 h in different atmospheres.
Figure 6. Grain size distributions of KNbO3 samples sintered at 1040 °C for 1 h in different atmospheres.
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Figure 7. SEM micrographs of KNbO3 powder annealed at 1040 °C for 1 h in (a) O2 and (b) H2.
Figure 7. SEM micrographs of KNbO3 powder annealed at 1040 °C for 1 h in (a) O2 and (b) H2.
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Figure 8. (a) XRD patterns of KNbO3 samples sintered at 1040 °C for 5 h in different atmospheres; (b) magnified pattern of the H2-sintered sample.
Figure 8. (a) XRD patterns of KNbO3 samples sintered at 1040 °C for 5 h in different atmospheres; (b) magnified pattern of the H2-sintered sample.
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Figure 9. Unit cell parameters of KNbO3 samples sintered at 1040 °C for 5 h in different atmospheres.
Figure 9. Unit cell parameters of KNbO3 samples sintered at 1040 °C for 5 h in different atmospheres.
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Figure 10. (a) Ascending and (b) descending DTA traces of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres.
Figure 10. (a) Ascending and (b) descending DTA traces of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres.
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Figure 11. Schematic of growth rate vs. driving force for samples sintered in different atmospheres (2DNG = 2D nucleation-controlled growth).
Figure 11. Schematic of growth rate vs. driving force for samples sintered in different atmospheres (2DNG = 2D nucleation-controlled growth).
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Table
Table 1. EDS analysis of matrix grains and secondary phase in a KNbO3 sample sintered at 1040 °C for 10 h in H2.
Table 1. EDS analysis of matrix grains and secondary phase in a KNbO3 sample sintered at 1040 °C for 10 h in H2.
ElementMatrix GrainsSecondary Phase
K22.3 ± 5.118.8 ± 8.1
Nb24.1 ± 5.929.6 ± 12.3
O53.6 ± 10.551.6 ± 20.4
K/Nb ratio0.9 ± 0.10.6 ± 0.0
Table
Table 2. DTA phase transition temperatures of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres.
Table 2. DTA phase transition temperatures of KNbO3 samples sintered at 1040 °C for 3 h in different atmospheres.
Sintering AtmosphereOrthorhombic-Tetragonal Phase Transition Temperature (°C)Tetragonal-Cubic Phase Transition Temperature (°C)Cubic-Tetragonal Phase Transition Temperature (°C)
O2233407381
Air234408383
N2227406383
N2-5% H2159403376
H2-381358

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Trung, D.T.; Fisher, J.G. Controlled-Atmosphere Sintering of KNbO3. Appl. Sci. 2020, 10, 2131. https://doi.org/10.3390/app10062131

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