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Article

Studies on δ-Bi2O3 Based Nanocrystalline Glass-Ceramics Stabilized at Room Temperature by Novel Methods

by
Paulina Kruk-Fura
* and
Jerzy E. Garbarczyk
*
Faculty of Physics, Warsaw University of Technology, 00-661 Warsaw, Poland
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2022, 12(15), 7885; https://doi.org/10.3390/app12157885
Submission received: 12 May 2022 / Revised: 5 July 2022 / Accepted: 3 August 2022 / Published: 5 August 2022
(This article belongs to the Section Materials Science and Engineering)
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Abstract

:
This study demonstrated for the first time that it is possible to prepare nanocrystalline δ-Bi2O3 that is stable at room temperature by twin-rollers and free cooling methods, using a ceramic crucible. The phase composition of prepared samples and upper limit of the thermal stability of nanograins confined in an amorphous matrix were determined by the X-ray diffraction (XRD) method. The average size of crystallites and the microstructure of studied samples was determined by SEM and XRD methods. The average grain size varied from 38 to 85 nm, depending on the preparation technique; however, it was also observed that agglomerations consisted of smaller crystallites ca. 10–30 nm. Using the EDX method, it was found that a crucial role in the preparation of nanocrystalline δ-Bi2O3 glass-ceramics was played by Si and Al impurities and their glass forming oxides from ceramic crucible. By impedance spectroscopy (IS), the temperature dependencies of electric conductivity (via oxygen ions) were studied and the activation energies of conductivity were determined.

1. Introduction

Bismuth (III) oxide is a well-known multiphase compound that exists in polycrystalline form in α (monoclinic); β (tetragonal); γ (cubic); and δ (cubic, fluorite type) structures. All these materials were investigated and described due to their temperature range of occurrence, as well as their physical properties. Moreover, Bi2O3 seems to exhibit different properties depending on the phase in which it occurs, which makes it an exceptionally interesting subject of research in the field of solid-state physics, as well as for potential applications [1,2,3,4,5]. Of particular interest is the δ-Bi2O3 phase, showing the highest value of conductivity at high temperatures (1 S/cm at 750 °C) of all known oxygen ion conductors. Unfortunately, such an excellent conductivity value is accompanied by a serious drawback, which is a narrow temperature stability range, from 735 °C to 825 °C [6]. This severely limits the applicability of the material in devices based on oxygen ion conductors, such as fuel cells, oxygen pumps, and gas sensors. For this reason, numerous approaches, such as obtaining a solid solution of δ-Bi2O3 in selected oxides [7,8,9], or the deposition of thin films of spoken compound on various substrates [10,11], have been made to extend the range of stable δ phase to lower temperatures.
Glass-ceramic composite materials consist of an amorphous phase (the so-called matrix) and one or more crystalline phases. By their structure, the composites and nanocomposites share a number of physical and chemical properties with both glass and classical polycrystalline ceramics, making it possible to obtain functional materials [12,13,14,15,16] with properties such as good strength/stability at high temperatures [15,16,17,18], high chemical resistance, or high ionic conductivity [19,20]. Such composites are generally produced in a two-step process. First, the glass is formed by rapid cooling from the liquid phase. Then, it is heated back to induce partial crystallization or nanocrystallization [15]. The properties of the glass-ceramic composites in this case can be controlled by controlling the crystallization conditions and the composition of the base amorphous material [12,17].
The presented idea is to fabricate a glass-ceramic nanocomposite with δ-Bi2O3 based grains confined in an amorphous matrix at room temperature. For this purpose, the technique of rapid cooling from the liquid phase-twin rollers [21] was used in a novel way to obtain partially crystallized composite materials in a one-step process (without the need for a glass annealing step [16]), in which their morphology was influenced by controlling the melt temperature and cooling rate. To test the effect of grain size on the stabilization of bismuth oxide in the form of the desired δ-like phase, control samples were prepared by the free cooling method, in which a polycrystalline material with large grains was expected due to a slow cooling rate. To investigate the potential effect of impurities on the stabilization of the δ-like phase, ceramic and platinum-iridium crucibles were used for the melting processes.

2. Materials and Methods

The beginning of the process, regardless of the cooling technique used, was the same for each sample produced—a weighed amount of ceramic Bi2O3 powder (Sigma Aldrich, St. Louis, MO, USA, 99.96%) was placed in a ceramic or platinum-iridium (95% Pt, 5% Ir) crucible and inserted into a preheated chamber furnace where the batch remained under isothermal conditions in the atmospheric air for 15 min. Temperatures in a range from 900 to 1000 °C were used for melting to obtain the variety of samples. Then, in the case of slow cooling, the material was cooled freely with the furnace chamber to room temperature (25 °C). The average cooling rate was determined from the cooling time of the furnace chamber with the sample, and was approx. 10 °C/min. In the case of fast cooling using the twin rollers technique, after 15 min, the hot furnace was opened, and the molten batch poured onto stainless steel rollers—rotation speeds in the range of 400–800 RPM were used.
The final products were examined by XRD at room temperature to determine the presence of amorphous and crystalline phases and the crystallographic phase, together with the calculation of the lattice constant by the Rietveld technique using GSAS-II software [22]. The measurements were performed with the Malvern Panalytical Empyrean diffractometer, equipped with a copper X-ray tube with the wavelength λ = 1.54 Å on flat samples in Bragg-Brentano geometry. For preliminary characterization of the morphology of the samples, the average grain size was estimated using the Scherrer method. In order to investigate the thermal stability of the fabricated materials, XRD studies as a function of temperature were performed. The measurement was performed in the temperature range 25–775 °C with 25 °C increments and an angular range of 2θ 10–90°. In order to further describe the morphology of the samples, SEM micrographs were taken, which were used to determine the homogeneity of the material in a certain volume, as well as to determine the average grain size by using the opensource code for MATLAB software, created by A. Rabbani et al. [23]. An EDX microanalysis was used to estimate the amount of impurities that were present in the material due to the use of ceramic crucibles in the first step of the sample fabrication process.
In order to determine the conductivity and activation energy of the obtained materials, electrical measurements were performed using the impedance spectroscopy method. Due to the form of the samples produced (a sintered batch in the case of free cooling and thin, brittle ribbons in the case of twin rollers), the materials required several stages of preparation for measurements. For detailed information about this process, see Appendix A. IS measurements were performed on the sample pellets in the temperature range 25–500 °C.

3. Results

3.1. XRD Room Temperature Results

Figure 1 shows typical diffractograms for the two series, selected from the produced material. The samples were melted at (a) 900 °C, (b) 1000 °C, obtained by cooling the batch melted in the ceramic crucible with the twin rollers technique at (i) 1000 RPM, (ii) 800 RPM, (iii) by free cooling, and (iv) by cooling the batch melted in the PtIr crucible with the twin rollers technique (control sample). In both series presented, two components can be distinguished in the diffractograms, (i) and (ii)—the amorphous contribution to the scattering profile emerging from the background is visible at low angles and reflections coming from the crystalline phase in the material. In both series in diffractograms (iii) and (iv), mostly the crystalline phase is visible, whereby it should be noted that the latter diffraction pattern differs from the rest, which indicates that in these cases a different crystalline phase was obtained. The average crystallite size estimated by the Scherrer method (from four representative Bragg peaks) for the samples presented in Figure 1a is (i) 38 nm, (ii) 44 nm, (iii) 78 nm, (iv) 60 nm, while for those shown in Figure 1b—(i) 41 nm, (ii) 41 nm, (iii) 85 nm, (iv) 61 nm. In the presence of these results, taking into account the limited precision of the calculations in the Scherrer method, it cannot be stated unequivocally that a change in the melt temperature affected the obtained average crystallite size. However, it suggests that a significant decrease in the cooling rate allowed the preparation of crystalline materials with much larger crystallites for samples melted in the ceramic crucible, which is in agreement with theoretical predictions [24,25,26]. For the PtIr crucible melted samples, the average crystallite size is comparable to the freely cooled samples, despite the twin rollers technique, which, combined with the observed different crystalline phase, may suggest that there are differences in the chemical composition of the samples.
Figure 2 shows a representative phase matching performed by the Rietveld method for the sample melted at 900 °C, cooled by the twin rollers technique at 1000 RPM. On its basis, the presence of a crystalline δ-type phase (fluorite) in the material was identified (calculation based on ICDD no. 00-052-1007) and the lattice constant a = 5.537 Å was determined. The Rwp coefficient of the fit was 7.37%. (The Rwp stands for the weighted profile R-factor and is calculated from the square root of the quantity minimized, scaled by weighted intensities [27].) The analysis was performed for all the materials and its results are presented in Table 1 and Table 2. It should be noted that in the case of samples made by free cooling in the angular range 2θ 38–40° (Figure 1a,b Z1) and ca. 60° (Figure 1a,b Z2), there are visible reflections which cannot be attributed to the δ-type phase, but their intensity is too low to reliably identify the additional crystalline phase. Moreover, their intensity is lower for a higher melting temperature. A disparity in the ratio of the Bragg reflections intensities was also noted for these materials, which may indicate that, possibly due to cooling conditions, a preferred orientation of crystal growth appeared in the samples. This was taken into account when performing the Rietveld fits but is not discussed further in this paper.
It is remarkable that for materials obtained from PtIr crucible melts, despite using the same melting times and cooling rates as for ceramic crucible melts, a monoclinic phase (α) (calculation based on ICDD no. 00-041-1449) was identified (which is consistent with the current state-of-the-art of Bi2O3 phase stabilization in bulk materials [4]), instead of the preferred δ-type phase.
Based on the data presented, it can be concluded that the twin rollers technique was successfully used to produce glass-ceramic composites with nano-grains in a δ-type phase that was stable at room temperature. As expected, the free cooling method produced polycrystalline materials that were devoid of a significant glass matrix volume inherent in the other samples with larger grains; however, still not exceeding 100 nm. Interestingly, also in these samples, a fluorite-like crystalline phase that was stable at room temperature was found, but with visible impurities that are not observed in composite glass-ceramic materials. Obtaining polycrystalline materials in alpha phase from PtIr crucible melts that are cooled by the twin rollers technique may indicate firstly that impurities from the ceramic crucible are introduced into the samples, as the crucible type is only significant difference in that case, and ceramic crucibles are more susceptible to degradation factors present during this process than PtIr ones; secondly, that these impurities may be necessary to stabilize the fluorite-like phase in Bi2O3 based materials at room temperature.

3.2. XRD Results as a Function of Temperature

Figure 3 shows representative diffractograms as a function of temperature for materials melted at 1000 °C in the ceramic crucible, produced by the twin rollers technique and cooled at 1000 RPM (Figure 3a—heating, Figure 3b—cooling). Diffractograms of other obtained samples, which can serve as comparisons, are included in Appendix B. Through this study, it was found that a δ-type phase stabilized at room temperature recrystallizes to a γ-like phase (identification based on ICDD no. 00-045-1344) at higher temperatures and, importantly, this is an irreversible process. The phase obtained after recrystallization remains stable when the samples are cooled down to room temperature. The initial temperatures of the recrystallization process are 575 °C for the sample cooled at 1000 RPM (Figure 3a), and 550 °C for the samples cooled at 800 RPM and the control sample (see Appendix B).

3.3. SEM with Microanalysis EDX Results

Figure 4 shows SEM micrographs taken to study the microstructure and morphology of fabricated glass-ceramic composite samples, melted at 1000 °C in ceramic crucibles, cooled at 1000 RPM (Figure 4a) and 800 RPM (Figure 4b), and polycrystalline material obtained from the batch melted in the PtIr crucible at 1000 °C, cooled at 1000 RPM (Figure 4c).
Materials melted in the ceramic crucible (Figure 4a,b) are characterized by a homogeneous grain distribution and a structure without visible cracks and pores. The grains are of nanometric size and do not have clearly defined boundaries, which probably confirms the significant contribution of the amorphous matrix in the structure of the glass-ceramic composite, revealed in the XRD results as amorphous contribution. It should be noted, however, that individual grains are often composed of several agglomerated much smaller crystallites. The morphology of the material obtained by cooling with twin rollers of the batch from the PtIr crucible (Figure 4c) differs significantly from the previously described samples. The grains are much larger, with sharply defined boundaries, which may indicate a negligible contribution of the amorphous part in the sample, a result that is consistent with the shape of the diffractograms for these materials. The structure of the sample is heterogeneous—it is possible to observe solid grains of micrometric size with nanometric partially separated precipitates on them, as well as areas where crystallites of less than 100 nm in size are agglomerated. Many cracks and pores are also visible.
To further analyze the grain size distribution and the occurrence frequency of crystallites of each size, a statistical analysis was performed on the SEM micrographs shown in Figure 4 using an algorithm in MATLAB software. The resulting histograms are presented in Figure 5.
It is very important to notice that the value obtained from XRD is the crystallite size, while the one obtained from the SEM is the grain size. Grain can consist of several crystallites. Nevertheless, based on this method, the determined average grain size for the samples from ceramic crucibles cooled by twin rollers at 1000 RPM and 800 RPM is 50 nm and 64 nm, respectively, which is in fairly good agreement with the values determined by the Scherrer equation. Histogram 5c differs greatly from those of Histograms 5a,b because it relates to the microstructure of the α-Bi2O3 sample obtained from the PtIr crucible. This difference clearly shows that only in the ceramic crucible is it possible to obtain δ-Bi2O3 by the twin rollers method. The large sample inhomogeneity described earlier, which may significantly affect the shape of the diffractograms, is most likely responsible for such a remarkable discrepancy.
It should also be noted that the expected value and, thus, the statistically most common grain size in the material for the samples from the ceramic crucibles cooled at 1000 RPM and 800 RPM is 23 nm (Figure 5a) and 34 nm (Figure 5b), respectively, which seems to confirm the assumption of the agglomeration of crystallites. Furthermore, it may indicate that the faster cooling resulted in smaller crystallites, an effect that was expected, but which could not be observed from the values determined by Scherrer’s equation.
For the samples melted in ceramic crucibles, an EDX microanalysis was performed for the presence of crucible-derived impurities. The obtained results indicate that the samples contain from 8.87 to 9.87 at.% Si and from 4.42 to 5.55 at.% Al. Considering a systematic error value of 0.14–0.38 at.%, it is fair to say that these values do not seem to depend significantly on the melting temperatures chosen in this study.

3.4. IS Results

Impedance spectroscopy measurements of the studied materials were performed in the temperature range up to 500 °C, taking into account the thermal stability of the studied samples, cf. Figure 3. Typical Arrhenius plots are presented in Figure 6, and corresponding values of activation energy are given in Table 3.
Values of sample resistance (and consequently conductivity) were determined by extrapolation of impedance diagrams to intersect with the Re(Z) axis at low frequencies (cf. inset in Figure 6). The total conductivity of the samples with a stabilized δ-type phase, regardless of the used quenching technique, was found to be about two orders of magnitude lower at 500 °C than that of the classical delta δ-Bi2O3 phase [6]. The determined activation energies are about 1 eV, which is quite a typical value for oxygen ion conductors. Moderate values of ionic conductivity, compared to polycrystalline δ-Bi2O3, are understandable, if we take into consideration that total conductivity was measured in this study. Apart from intra-grain δ resistivity, in the glass-ceramic samples we can distinguish the grain boundary resistivity and glassy matrix resistivity in series with the grain resistivity of the superionic δ phase. It should be underlined that because of the nanocrystalline nature of the studied samples, the effect of interfaces and glassy phase on the total resistivity/conductivity is high.

4. Discussion

New methods, which allow the stabilization of the δ-Bi2O3 type phase at room temperature, were described in this work. A common feature of both methods was the melting of α-Bi2O3 in a ceramic crucible. The first method was based on the fast quenching of the melt using the twin rollers technique, and the second one based on its free cooling.
Two hypotheses were proposed to explain why it was possible to stabilize by those methods a δ type phase, which is normally stable at 735 °C to 825 °C [6]. The first assumes that Si4+ and Al3+ ions play the role of stabilization dopants. The second hypothesis is that stabilization of δ type phase is possible due to the glassy matrix (initiated by aforementioned oxides) that is formed during synthesis. The confinement of nano grains of the δ type phase under mechanical stresses in the glassy matrix plays a crucial role in the stabilization process. This hypothesis is supported by the fact that SiO2 is an extremely effective glass former, and fast cooling is often not required in that case (free cooling may be sufficient).
A fact that does not fit to the first hypothesis is that the radii of Si4+ (0.4 Å) and Al3+ (0.54 Å) ions are too small compared to the ionic radius of Bi3+ and typical stabilizing dopants (about 1 Å). On the other hand, the samples prepared by the free cooling method, do not exhibit clear amorphous contribution to scattering profiles in the diffraction patterns, which is a new and surprising result. Further analysis related to the role and localization of Si and Al dopants in studied nanocomposites requires complementary experimental methods.

5. Conclusions

Two novel methods were proposed which allowed the stabilization of the δ-Bi2O3 type phase at room temperature (up to about 575 °C) in the form of nanocrystalline glass-ceramics. The first one is based on the fast quenching of melted α-Bi2O3 phase using the twin rollers technique. The second one is based on the free cooling of that melt. In both methods, ceramic crucibles were used, whose compositions contained glass forming SiO2 and Al2O3 oxides. Diffusion of these oxides into the melted sample was a key step during the synthesis of δ-Bi2O3 based glass-ceramics. Prepared nanocomposites exhibit fairly good conductivity of oxygen ions at 500 °C. Our studies on the preparation of δ-Bi2O3 based glass-ceramics in the PtIr crucible with controlled SiO2 and Al2O3 dopants are underway.

Author Contributions

Conceptualization, J.E.G.; methodology, P.K.-F.; validation, J.E.G. and P.K.-F.; formal analysis, P.K.-F.; investigation, P.K.-F.; resources, J.E.G. and P.K.-F.; writing—original draft preparation, P.K.-F. and J.E.G.; visualization, P.K.-F.; supervision, J.E.G.; project administration, P.K.-F.; funding acquisition, P.K.-F. and J.E.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Center, Poland, Grant Preludium-14 no. 2017/27/N/ST5/01943.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in Studies on δ-Bi2O3 based glass-ceramics stabilized by novel methods based on nanocrystallization of glasses (contained within the article).

Acknowledgments

The authors are grateful to Tomasz Pietrzak for his valuable ideas that assisted in the design of the present study, and to Michal Struzik for his technological guidance in preparing samples for the IS measurements and assistance with the SEM analysis and imaging. The authors would also like to extend their thanks to the Institute of High Pressure Physics of the Polish Academy of Sciences for the SEM micrographs and EDX analysis.

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, or in the decision to publish the results.

Appendix A

Preparation of the samples for IS measurements. The samples were initially crushed in a mortar and then ground in a planetary ball mill for 2 h at 240 RPM to homogenize the grains. In the next step, pellets were formed and compressed in a uniaxial press at 12 MPa for 2 min and 8 MPa for another 10 min, followed by sintering in a chamber furnace at 100–150 °C for 12 h to harden the sample. The lack of effect of the presented preparation on the crystalline phase of the samples was confirmed by an XRD study. In the last step, platinum electrodes were sputtered onto the pellets under high vacuum conditions. To ensure good surface contact with the electrodes, the samples were previously ground.

Appendix B

Thermal evolution of XRD patterns of the materials synthesized under different conditions.
Applsci 12 07885 g0a1 550
Figure A1. (a) Thermal evolution of XRD patterns of a sample prepared in ceramic crucible at 1000 °C and quenched by twin rollers method (800 RPM) during heating up to 775 °C. (b) Thermal evolution of the same sample during cooling down from 775 °C to room temperature. Blue and gray colors correspond to δ-like phase (fluorite structure); turquoise color indicates phase transition to γ-like phase; orange and maroon colors correspond to γ-like phase stability range.
Figure A1. (a) Thermal evolution of XRD patterns of a sample prepared in ceramic crucible at 1000 °C and quenched by twin rollers method (800 RPM) during heating up to 775 °C. (b) Thermal evolution of the same sample during cooling down from 775 °C to room temperature. Blue and gray colors correspond to δ-like phase (fluorite structure); turquoise color indicates phase transition to γ-like phase; orange and maroon colors correspond to γ-like phase stability range.
Applsci 12 07885 g0a1
Applsci 12 07885 g0a2 550
Figure A2. (a) Thermal evolution of XRD patterns of a sample prepared in ceramic crucible at 1000 °C and freely cooled during heating up to 800 °C. (b) Thermal evolution of the same sample during cooling down to room temperature. Blue and gray colors correspond to δ-like phase (fluorite structure); turquoise color indicates phase transition to γ-like phase; orange and maroon colors correspond to γ-like phase stability range.
Figure A2. (a) Thermal evolution of XRD patterns of a sample prepared in ceramic crucible at 1000 °C and freely cooled during heating up to 800 °C. (b) Thermal evolution of the same sample during cooling down to room temperature. Blue and gray colors correspond to δ-like phase (fluorite structure); turquoise color indicates phase transition to γ-like phase; orange and maroon colors correspond to γ-like phase stability range.
Applsci 12 07885 g0a2

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Figure 1. (a) XRD patterns of samples melted in ceramic crucible at 900 °C and prepared by: (i) twin roller quenching at 1000 RPM (red line); (ii) twin roller quenching at 800 RPM (dark grey line); (iii) free cooling (blue line). Pattern (iv) refers to sample melted in PtIr crucible at 900 °C (green line). (b) XRD patterns of samples melted in ceramic crucible at 1000 °C and prepared by: (i) twin roller quenching at 1000 RPM (red line); (ii) twin roller quenching at 800 RPM (dark grey line); (iii) free cooling (blue line). Pattern (iv) refers to sample melted in PtIr crucible at 1000 °C (green line).
Figure 1. (a) XRD patterns of samples melted in ceramic crucible at 900 °C and prepared by: (i) twin roller quenching at 1000 RPM (red line); (ii) twin roller quenching at 800 RPM (dark grey line); (iii) free cooling (blue line). Pattern (iv) refers to sample melted in PtIr crucible at 900 °C (green line). (b) XRD patterns of samples melted in ceramic crucible at 1000 °C and prepared by: (i) twin roller quenching at 1000 RPM (red line); (ii) twin roller quenching at 800 RPM (dark grey line); (iii) free cooling (blue line). Pattern (iv) refers to sample melted in PtIr crucible at 1000 °C (green line).
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Figure 2. Representative phase matching performed by the Rietveld method for the sample melted at 1000 °C, quenched by the twin rollers technique at 1000 RPM. Blue points (+) are the experimental data; red—the background function; green—the fitted fluorite phase model. Turquoise bottom line is a difference plot and black plot shows the weighted difference between the observed and calculated diffraction runs.
Figure 2. Representative phase matching performed by the Rietveld method for the sample melted at 1000 °C, quenched by the twin rollers technique at 1000 RPM. Blue points (+) are the experimental data; red—the background function; green—the fitted fluorite phase model. Turquoise bottom line is a difference plot and black plot shows the weighted difference between the observed and calculated diffraction runs.
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Figure 3. (a) Thermal evolution of XRD patterns of a sample prepared in ceramic crucible at 1000 °C and quenched by twin rollers method (1000 RPM) during heating up to 800 °C; (b) thermal evolution of the same sample during cooling down from 800 °C to room temperature. Blue and gray colors correspond to δ-Bi2O3 type phase (fluorite structure); turquoise color indicates phase transition to γ-like phase; orange and maroon colors correspond to γ-like phase stability range.
Figure 3. (a) Thermal evolution of XRD patterns of a sample prepared in ceramic crucible at 1000 °C and quenched by twin rollers method (1000 RPM) during heating up to 800 °C; (b) thermal evolution of the same sample during cooling down from 800 °C to room temperature. Blue and gray colors correspond to δ-Bi2O3 type phase (fluorite structure); turquoise color indicates phase transition to γ-like phase; orange and maroon colors correspond to γ-like phase stability range.
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Figure 4. SEM micrographs of the received samples: (a) melted in a ceramic crucible at 1000 °C, quenched by twin rollers method at 1000 RPM; (b) melted in ceramic crucible at 1000 °C, quenched by twin rollers method at 800 RPM; (c) melted in PtIr crucible at 1000 °C, quenched by twin rollers method at 1000 RPM.
Figure 4. SEM micrographs of the received samples: (a) melted in a ceramic crucible at 1000 °C, quenched by twin rollers method at 1000 RPM; (b) melted in ceramic crucible at 1000 °C, quenched by twin rollers method at 800 RPM; (c) melted in PtIr crucible at 1000 °C, quenched by twin rollers method at 1000 RPM.
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Figure 5. Histograms of distributions of grain sizes of the received samples (based on SEM micrographs presented in the Figure 4): (a) melted in a ceramic crucible at 1000 °C, quenched by twin rollers method at 1000 RPM; (b) melted in ceramic crucible at 1000 °C, quenched by twin rollers method at 800 RPM; (c) melted in PtIr crucible at 1000 °C, quenched by twin rollers method at 1000 RPM. Ordinate axis presents the relative frequency of occurrence of grains of a certain radius, and the equivalent grain radius value (nm) [23].
Figure 5. Histograms of distributions of grain sizes of the received samples (based on SEM micrographs presented in the Figure 4): (a) melted in a ceramic crucible at 1000 °C, quenched by twin rollers method at 1000 RPM; (b) melted in ceramic crucible at 1000 °C, quenched by twin rollers method at 800 RPM; (c) melted in PtIr crucible at 1000 °C, quenched by twin rollers method at 1000 RPM. Ordinate axis presents the relative frequency of occurrence of grains of a certain radius, and the equivalent grain radius value (nm) [23].
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Figure 6. Arrhenius plots of samples prepared in ceramic crucibles during heating or cooling (details are given in the legend on the plot). Samples were melted in ceramic crucibles. Inset presents a representative impedance diagram at 320 °C during heating for a sample prepared by free cooling method.
Figure 6. Arrhenius plots of samples prepared in ceramic crucibles during heating or cooling (details are given in the legend on the plot). Samples were melted in ceramic crucibles. Inset presents a representative impedance diagram at 320 °C during heating for a sample prepared by free cooling method.
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Table
Table 1. Results of calculations by the Rietveld method for samples melted at 900 °C.
Table 1. Results of calculations by the Rietveld method for samples melted at 900 °C.
SampleStructureSpace GroupLattice Constants [Å]Rwp Factor [%]
Twin rollers, 1000 RPM
Ceramic crucible		FluoriteFm3ma = 5.5377.37
Twin rollers, 800 RPM
Ceramic crucible		FluoriteFm3ma = 5.5367.56
Free cooling
Ceramic crucible		FluoriteFm3ma = 5.5366.56
Twin rollers, 1000 RPM
PtIr crucible		MonoclinicP21/ca = 5.8498.83
b = 8.166
c = 7.510
Table
Table 2. Results of calculations by the Rietveld method for samples melted at 1000 °C.
Table 2. Results of calculations by the Rietveld method for samples melted at 1000 °C.
SampleStructureSpace GroupLattice Constants [Å]Rwp Factor [%]
Twin rollers, 1000 RPM
Ceramic crucible		FluoriteFm3ma = 5.5377.91
Twin rollers, 800 RPM
Ceramic crucible		FluoriteFm3ma = 5.5416.50
Free cooling
Ceramic crucible		FluoriteFm3ma = 5.5376.78
Twin rollers, 1000 RPM
PtIr crucible		MonoclinicP21/ca = 5.8499.62
b = 8.167
c = 7.510
Table
Table 3. Activation energies for materials whose conductivity as a function of temperature is presented in Figure 6.
Table 3. Activation energies for materials whose conductivity as a function of temperature is presented in Figure 6.
SampleRampSymbolActivation Energy [eV]Total Conductivity Value (σ) at 500 °C [S/cm]
Twin rollers, 800 RPM
Ceramic crucible at 900 °C		HeatingRed triangle1.12 (0.01)3 × 10−2
CoolingBlue triangle1.12 (0.06)
Twin rollers, 800 RPM
Ceramic crucible at 1000 °C		HeatingGreen square0.89 (0.06)1 × 10−2
CoolingOrange square1.09 (0.01)
Free cooling
Ceramic crucible at 1000 °C		HeatingGreen square1.12 (0.02)3 × 10−2
CoolingOrange square1.06 (0.01)
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Kruk-Fura, P.; Garbarczyk, J.E. Studies on δ-Bi2O3 Based Nanocrystalline Glass-Ceramics Stabilized at Room Temperature by Novel Methods. Appl. Sci. 2022, 12, 7885. https://doi.org/10.3390/app12157885

AMA Style

Kruk-Fura P, Garbarczyk JE. Studies on δ-Bi2O3 Based Nanocrystalline Glass-Ceramics Stabilized at Room Temperature by Novel Methods. Applied Sciences. 2022; 12(15):7885. https://doi.org/10.3390/app12157885

Chicago/Turabian Style

Kruk-Fura, Paulina, and Jerzy E. Garbarczyk. 2022. "Studies on δ-Bi2O3 Based Nanocrystalline Glass-Ceramics Stabilized at Room Temperature by Novel Methods" Applied Sciences 12, no. 15: 7885. https://doi.org/10.3390/app12157885

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