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Preparation and properties of high-density Bi2O3 ceramics by cold sintering
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Preparation and properties of high-density Bi2O3 ceramics by cold sintering Jinjie Songa, Guisheng Zhua,∗, Huarui Xua,∗∗, Weining Fua, Jiwen Xua, Jian Zhanga, Shicheng Huangb, Aibing Yuc a Guangxi Key Laboratory of Information Materials, Engineering Research Center of Electronic Information Materials and Devices, Ministry of Education, Guilin University of Electronic Technology, Guilin, 541004, China b Guangxi Crystal Union Photoelectric Materials Co., Ltd., Liuzhou, 545036, China c ARC Hub for Computational Particle Technology, Monash University, Clayton, Victoria, 3800, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Cold sintering Bi2O3 ceramics High-density ceramics Dielectric properties
High-density Bi2O3 ceramics were prepared via a three-step cold sintering process (TS-CSP) at an ultra-low temperature. In the first step, the relative density of the Bi2O3 ceramic, subjected to 155 °C and 300 MPa for 25 min, is 96.23%, with the majority of the crystals inside the ceramic. The Bi2O3 ceramic was further densified with a corresponding increase in the relative density to 98.52%, after processing at 210 °C and 300 MPa for 25 min, in the second step. Finally, in the third stage of the processing, the temperature was raised from 210 °C to 270 °C and isothermally held for 25 min, which enabled the Bi2O3 ceramic grains to fully grow to yield a grain size of 4.02 μm, having a relative density of 99.13%. The densification mechanism of the TS-CSP Bi2O3 ceramics is via a dissolution-recrystallization-growth process. The relative permittivity, quality factor and grain size of the Bi2O3 ceramics are 33.44, 16,218 GHz and 4.02 μm, respectively. The materials and their preparation described herein provide a novel approach for the preparation of ultra-low temperature ceramics.
1. Introduction The cold sintering process (CSP) is a recent sintering method that can achieve high-density ceramics at processing temperatures < 300 °C [1–4]. CSP is principally used to sinter ceramics in the presence of a solution as a co-solvent. CSP was first used to prepare Li2MoO4 ceramics in 2014 [5], and formally named as “cold sintering” in 2016 [2]. CSP has attracted significant attention because of the demonstrated advantages related to the low cost of the process having a low energy consumption and the ease of working with metals, polymers and other materials. The CSP technology has been successfully applied across numerous materials including typical oxide ceramics [6], ceramic/ ceramic composites [7,8], microwave dielectric ceramics [2,3,5,9], ferroelectric and piezoelectric ceramics [1,10], solid electrolytes [11], thermoelectric materials [12], and ceramic/2D nanomaterial composites [13]. Bi2O3 ceramics having attractive dielectric properties are of significant importance in the field of wireless communication. However, the sintering temperature of Bi2O3 ceramics is relatively high. Zhou et al. [14] reported that Bi2O3 ceramics formed at 680 °C had a relative permittivity (εr) of ~32.8 and a quality factor (Qf) of ~17,900 GHz, and
∗
claimed a high relative density (ρ ~97%) that was achieved via conventional sintering (CS). Therefore, there is an urgent need to develop a method to reduce sintering temperature. Applying the CSP technology has hitherto not been used in the preparation of bismuth oxide ceramics, and hence the sintering results, sintering process and mechanism of cold sintering for the preparation of bismuth oxide ceramics still require elucidating. Solubility in the solution determines the suitability of a material for cold sintering. Bi2O3 is insoluble in water and alkali, but it is soluble in acetic acid, so this study chooses acetic acid solution as co-solvent to cold sinter Bi2O3 ceramic. In this study, high-density (ρ ~99.13%) Bi2O3 ceramics were sintered at 270 °C and 300 MPa via three-step CSP (TS-CSP). The densification process of the TS-CSP Bi2O3 ceramics was analyzed. The εr and Qf of the Bi2O3 ceramics are 33.44, 16,218 GHz, respectively, which are similar to the dielectric properties of Bi2O3 ceramics sintered at 680 °C by CS.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Zhu), [email protected] (H. Xu).
∗∗
https://doi.org/10.1016/j.ceramint.2020.02.177 Received 11 January 2020; Received in revised form 17 February 2020; Accepted 17 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Jinjie Song, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.177
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Fig. 1. (a) A schematic diagram of the cold sintering process (CSP) mold, (b) the three-step CSP (TS-CSP) sintering curve of the Bi2O3 ceramics.
2. Materials and methods
Table 2 Second group of experiments.
2.1. Experimental method The experimental set-up for the ceramic CSP required the use of a mold (not completely closed) and an outer heating jacket consisting of a mica band heater. The mold has the capability to exert uniaxial pressure and the jacketed band heater allows for the mold to be heated up to 450 °C. A straight thermocouple was positioned between the heater band and sample to measure the temperature of the ceramic slurry, as shown in Fig. 1(a). Additionally, the TS-CSP sintering temperature curve, at a heating rate of 6 °C/min, is shown in Fig. 1(b).
250
300
350
V (heating rate)/°C/min T (temperature)/°C t (holding time)/min
6 300 0
6 300 0
6 300 0
6 300 0
6 300 0
250
300
350
V/°C/min T/°C
6 230/235/ 240 /245 25
6 190/195/ 200 /205 25
6 160/165/ 170 /175 25
6 145/150/ 155 /160 25
6 142/147/ 152 /157 25
The crystalline structure was examined using x-ray diffraction (XRD, D8 Advance, Bruker Inc., Germany) with a CuKα source (λ = 0.15405 nm). The morphologies of the surface and cross-section of the ceramics were analyzed by field emission-scanning electron microscopy (FE-SEM, FEI Tecnai-450, USA). The density of the bulk samples was measured by determining the mass/dimension ratio as well as using the Archimedes’ method with water as the liquid medium. The microwave dielectric properties were tested using a Vector Network Analyzer (Agilent-N5230A). The mass loss of the ceramic materials was measured by thermogravimetric analysis (TGA, SDT Q600, TA, USA). Chemical element identification and valence state were analyzed by x-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250).
3. Results and discussion Fig. 2(a) shows changes to the pressure profiles as a function of temperature during the ceramic preparation by CSP. An important sign of ceramic densification was the rapid reduction in pressure [15]. For example, when processing at 350 MPa and 300 MPa, the pressure rapidly reduced when subjected to a heating temperature of 152 °C and 155 °C, respectively. Similar observations were made when processing at 250 MPa, 200 MPa, and 150 MPa. Importantly, the temperature at which the densification process initiates (observed rapid drop in pressure) is determined to be pressure dependent. Reduced pressures indicate shrinkage of the bulk ceramic, which can be attributed to the growth of the grains by consumption of the surrounding powders when subjected to the necessary temperatures and pressures. Therefore, the temperature at which the rapid pressure drop is observed is considered to be the initial crystallization temperature of the Bi2O3 ceramics, which is similar to the results reported by Jiang et al. [15], were the initial crystallization temperature of a ZnO ceramic at 300 MPa was 150 °C, with an associated rapid pressure drop. Furthermore, from the data gathered herein, the ceramics are suggested to have different initial crystallization temperatures at different pressures. Fig. 2(b) further complements the authenticity of the results in Fig. 2(a) by using the
Table 1 First group of experiments. 200
200
2.3. Characterization
The starting Bi2O3 powder was obtained commercially from Alfa Aesar (Haverhill, MA) (99.9%). Typically, 7.0 g of Bi2O3 powder was placed in a mortar with an acetic acid solution (3 mol/L) added at 15 wt %. Thereafter, the mixture was uniformly ground with an agate mortar, and the slurry placed into the mold. Finally, the samples were prepared according to the following three groups of experimental conditions: in the first group, the mold was subjected to a uniaxial pressure of 150–350 MPa and heated up to 300 °C at a heating rate of 6 °C/min (the application of pressure for this group was one-time and noncontinuous), as shown in Table 1; in the second group, the mold was subjected to a uniaxial pressure of 350 MPa down to 150 MPa at 50 MPa intervals while being heated to 142–157 °C, 145–160 °C, 160–175 °C, 190–205 °C and 230–245 °C, respectively, all at a heating rate of 6 °C/min, where the pressure was isothermally held for 25 min, as shown in Table 2; in the third group, the mold was subjected to a uniaxial pressure of 300 MPa. Thereafter, the samples were sintered via the TS-CSP. This process first involves heating the mold to 155 °C at 300 MPa at a heating rate of 6 °C/min. The system was isothermally held as a function of time (0–45 min). Thereafter, the temperature of the mold was elevated further to 210 °C (300 MPa) and isothermally held for 25 min, before further increasing the temperature to 270 °C (300 MPa) and isothermally holding for 25 min to densify the product, as shown in Table 3. After allowing the samples to cool under ambient conditions the ceramic piece measured 10 mm in diameter and 5 mm in thickness.
150
150
t/min
2.2. Ceramic preparation
P(pressure)/MPa
P/MPa
2
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Table 3 Third group of experiments.
First stage
Second stage
Third stage
P /MPa
300
300
300
300
300
300
300
300
300
300
V/°C/min T/°C t/min V/°C/min T/°C t/min V/°C/min T/°C t/min
6 155 0 – -
6 155 10 – -
6 155 20 – -
6 155 25 6 180 -
6 155 25 6 190 -
6 155 25 6 200 -
6 155 25 6 210 25 6 240/270/300 25
6 155 25 6 270 -
6 155 35 – -
6 155 45 – -
Fig. 3(f). To further improve the crystallization process and compactness of the ceramics, there is a need to enhance the driving force of sintering. Fig. 3(b) shows the relationship between sintering temperature and density during the second stage of the process. Fig. 3(b) shows that when the sintering temperature of the second step was increased from 155 °C to 210 °C, the relative density value of the ceramic was also increased. However, when the sintering temperature of the second step exceeded 210 °C, the relative density value of the ceramic was not substantially changed. Fig. 3(g) shows an SEM micrograph of the ceramic prepared by CSP subjected to 300 MPa at 155 °C for 25 min, followed by 210 °C for 25 min. The grains of the ceramic prepared with the aforementioned parameters were well established with the grain boundaries being in close contact. Here, the relative density and the grain size of the ceramic approached 98.52% and 2.8 μm, respectively. Fig. 3(c) shows that when the sintering temperature of the third step was increased from 210 °C to 270 °C, the relative density increased, albeit to a small degree, from 98.52% to 99.13%. However, a significant change to the grain size was observed, increasing from 2.84 μm to 4.02 μm, as shown in Fig. 3(g–h). Fig. 3(h–i) shows the SEM micrographs at 210 °C and 270 °C for the third stage of the sintering process, respectively. Fig. 3(h–i) shows that the internal grains of the ceramic were fully grown, and obvious grain boundaries can be observed at both 270 °C and 300 °C. However, there was no significant difference in relative density, which demonstrates that at a CSP temperature of 270 °C, a high-dense ceramic was formed. Fig. 4 shows the XRD, TGA and XPS results of the ceramic sample prepared by the TS-CSP route subjected to the following conditions: 300 MPa at 155 °C for 25 min, followed by 210 °C for 25 min, and finally 270 °C for 25 min. Fig. 4(a) shows that the XRD pattern of the TS-CSP sample is in good agreement with the standard JCPDS data (PDF#76–1730), and that the sample can be attributed to pure Bi2O3 [16–18], which demonstrates that in the presence of 3 mol/L acetic acid the ceramics are phase pure with no impurities being identified. Fig. 4(b) shows the TGA profile of the sample from room temperature to
relationship between relative density and temperature. Fig. 2(b) shows that when applying a pressure of 350 MPa and 300 MPa, and increasing the temperature to 152 °C and 155 °C, respectively, relative density rapidly increased to a saturated value of 96.33% and 96.23%, respectively. Additionally, similar observations were observed for pressures of 250 MPa, 200 MPa and 150 MPa. Therefore, the data strongly suggest a correlation with pressure drop and relative density and a corresponding initial crystallization temperature at each pressure. Furthermore, the initial crystallization temperature, as a function pressure, decreases with increasing pressure. When the pressure exceeded 300 MPa, the increase of ceramic density and the decrease of initial crystallization temperature were observed not to be significantly affected, Fig. 2(a–b) Hence, 300 MPa and 155 °C were employed as the process conditions for the first stage of ceramic sintering to study the ceramic densification process. Fig. 3(a) shows the density trend for the samples at the first stage of processing (155 °C, 300 MPa) in the presence of 3 mol/L acetic acid sintered as a function of holding time. From Fig. 3(a), the relative density value of the ceramics increased from 85.51% to 96.23% when the holding time was increased from 0 min to 25 min. On further increasing the holding time beyond 25 min, the relative density did not change significantly. Fig. 3(d–f) shows SEM micrographs of the first stage of sintering, which systematically show the changes of the ceramic internal microstructure. Fig. 3(d) shows significant grain formation inside the ceramic, however, the crystallization was incomplete after being maintained at 155 °C for 5 min. A degree of the powder particles are not involved in the crystallization, and there are obvious atmospheric pores and voids. However, the grains were still not fully grown with evidence of obvious pores inside the ceramic. Fig. 3(e) shows a significant number of crystals and grain boundaries observed inside the ceramic with no obvious voids, however, small pores remain after holding at 155 °C for 15 min. The majority of the crystallization was completed after holding at 155 °C for 25 min, with no obvious sign of pores and a relatively good grain growth observed, as shown in
Fig. 2. (a) Pressure change of the ceramics formed by CSP as a function of temperature. The relationship between relative density and temperature for samples subjected to (b) 350 MPa, 300 MPa, 250 MPa, 200 MPa, and 150 MPa. 3
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Fig. 3. Density trend of (a) the pre-sintered samples at 155 °C and 300 MPa as a function of holding time, (b) the samples sintered during the second stage of sintering as a function of temperature, (c) the samples sintered during the third stage of sintering as a function of temperature. The corresponding scanning electron microscopy (SEM) images of the ceramic microstructures pre-sintered at 155 °C for (d) 5 min, (e) 15 min, and (f) 25 min. The corresponding SEM micrographs of the completely sintered ceramics at (g) 210 °C for 25 min, (h) 270 °C for 25 min, and (i) 300 °C for 25 min.
Fig. 4. (a) X-ray diffraction, (b) thermogravimetric analysis, (c–f) x-ray photoelectron spectroscopy spectra of (c) the overview spectrum, (d) C 1s, (e) Bi 4f, and (f) O 1s spectra of the TS-CPS sample. 4
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Fig. 5. Schematic illustration of the Bi2O3 ceramic microstructural evolution during the TS-CSP.
Fig. 3(a). At 300 MPa and 155 °C, the relative density value of the sample increased with increasing holding time. However, when the holding time was > 25 min, the relative density value of the ceramic did not substantially increase because within a holding time of 25 min there remained sufficient water inside the mold to ensure that the powder particles had sufficient time for crystal growth. Fig. 5(d–e) shows the second stage of the process, with the temperature increased from 155 °C to 210 °C, which further promoted the crystallization and grain growth of the ceramics. The void space was further reduced as the particles combined to form a clear grain-boundary. There are three mechanisms for grain growth: the first relates to the contact of the small grains with the large grains and epitaxial growth; the second mechanism derives from the Bi2O3 nuclei in the voids continuing to absorb Bi3+ from the solution with a concomitant subsequent growth; the third mechanism relates to the continuous growth of the nuclei on the surface of the particles by accepting ions from the solution. Fig. 5(e–f) shows the third stage of the process. When the temperature increases to 270 °C, the ceramics are subjected to higher driving forces, which increases the free energy at the grain boundaries. With further movement of the grain boundaries, the grains continue to absorb each other and grow resulting in the disappearance of the sub-grain boundaries. Based on the experimental observations herein, the first stage and second stage processes (0–155 °C) were mainly governed by dissolution-recrystallization processes, with the third stage (210–270 °C) following a grain growth process. In general, the TS-CSP densification mechanism of the high-density Bi2O3 ceramics followed a dissolution-recrystallization-growth process, which is similar to the liquid phase mass transfer process of conventional liquid phase sintering. During the third stage of the sintering process, the sintering temperature has a significant influence on the dielectric properties of the ceramics. Fig. 6(a) represents the εr and relative density of the Bi2O3 ceramics as a function of temperature. As the temperature is increased, εr improves and subsequently reaches an optimal value of 33.44 at 270 °C. The observed trend of relative density is consistent with that of εr. Thus, the change in εr is principally related to relative density. However, relative density is related to the porosity, which varies with temperature. As the temperature is increased, the defects and porosity in the ceramic crystal gradually decrease, and the density of the ceramic
600 °C. From the TGA curve, the total mass loss of the sample after TSCSP was 0.08%, which indicated that the Bi2O3 ceramic prepared by TS-CSP exhibited enhanced stability. As shown in Fig. 4(c), no additional peaks, corresponding to any binding energy elements, were observed, other than for C, Bi, and O, which also suggest the absence of any impurities remaining in the Bi2O3 ceramics sintered via TS-CSP. Fig. 4(d) shows the C 1s peak with a binding energy of 284.8 eV. The C 1s peak does not influence the interpretation of the present results, and hence was used for calibration of the binding energies for sample charging correction [19,20]. Fig. 4(e) represents the two Bi 4f peaks at binding energies of 159.3 eV and 164.5 eV. No shoulder peaks attributed to two, four and five valences were observed for the two main peaks of Bi 4f [17,21]. Hence, the spectrum indicated that the bismuth in Bi2O3, generated using ultra-low temperature sintering, existed as Bi3+. As observed in Fig. 4(f), the O 1s peaks at 530.0 eV and 531.0 eV are attributed to Bi–O− and O2− in Bi2O3, respectively [22,23], which indicated a phase pure Bi2O3, similar to the XRD findings. Fig. 5 shows the densification of the TS-CSP Bi2O3 ceramics. Cold sintering of the Bi2O3 ceramics is a nonequilibrium dynamic process similar to that of hydrothermal syntheses. The process can be divided into several main stages. Fig. 5(a–d) shows the first stage. The powder particles gradually approach each other, and in a liquid phase environment, were rearranged when subjected to a pressure of 300 MPa. Further increasing the temperature induced dissolution of the Bi2O3 powder surface resulting in Bi3+ ions transferring to the liquid solution, which thereafter combined with Ac− to form Bi(Ac)3, as shown in Fig. 5(a–b). When the liquid phase was evaporated at 155 °C, the concentration of the solution increased sufficiently to become a supersaturated solution. The supersaturated solution together with the applied pressure result in an environment that is similar to a hydrothermal reaction. The solute began as Bi(Ac)3 and finally formed Bi2O3 having small grains in this pseudo-hydrothermal environment. The small grains that were formed were uniformly deposited in the voids of the powder and on the particle surface, as shown in Fig. 5(b–c). When sintering at 300 MPa and 155 °C over a period of time, the pores are eventually filled with a large number of small particles, which continue to grow, thereby significantly reducing the porosity, as shown in Fig. 5(c–d). These observations are consistent with the description of
Fig. 6. (a) Relative permittivity and relative density, (b) quality factor and grain size of the TS-CSP Bi2O3 ceramics as a function of temperature. 5
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also increases, resulting in the dielectric loss of the ceramic being reduced [24–27], which increases the εr value. However, if the temperature exceeds a certain threshold, no change in the ceramic interior is observed, and hence relative density does not increase, thus, the εr value does not increase. As shown in Fig. 6(b), as the temperature was increased from 210 °C to 240 °C, and to 270 °C and 300 °C, the grain size of the Bi2O3 ceramic increased from 2.84 μm to 3.67 μm, 4.02 μm, and 4.05 μm, respectively, whereas the Qf of the Bi2O3 ceramics increased from 5631 GHz to 13,692 GHz, 16,218 GHz, and 16,316 GHz, respectively. The observed trend related to the grain size was consistent with that of Qf. Thus, changes to Qf were principally related to grain size. As the temperature is increased, the decreasing number of grain boundaries caused by the increase in grain size would also increase the Qf value of the ceramics [25,28]. However, if the processing temperature exceeded a threshold value of 270 °C, that resulted in there being no significant difference in grain size, the Qf value also failed to increase. As shown in Fig. 6(a–b), the third stage of sintering (270 °C, 300 MPa, 25 min) exhibited a relative density value of ~99.13%, a εr value of ~33.44 and a Qf value of ~16,218 GHz, which are comparable to a relative density of ~97%, a εr value of ~32.8 and a Qf value of ~17,900 GHz, derived from the CS samples sintered at 680 °C [14].
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4. Conclusions Bi2O3 ceramics having a relative density of 99.13%, εr of 33.44, and Qf of ~16,218 GHz were prepared via TS-CSP at 270 °C and 300 MPa. Pressure has a significant influence on the initial crystallization temperature. The initial crystallization temperature decreases as a function of increased pressure. The densification mechanism of the Bi2O3 ceramic material follows a dissolution-precipitation-growth process. Overall, the study provides a new approach for the ultra-low temperature sintering of ceramics. Declaration of competing interest The authors declare no known competing interests, either financial or personal relationships, that could have influenced the work reported in this study. Acknowledgments The work was financially supported by the Guangxi InnovationDriven Development Project (No. AA18118001) and Guangxi Key Laboratory of Information Materials Foundation (No. 191027-Z). We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. References [1] H. Guo, A. Baker, J. Guo, C.A. Randall, D. Johnson, Cold sintering process: a novel technique for low-temperature ceramic processing of ferroelectrics, J. Am. Ceram. Soc. 99 (11) (2016) 3489–3507. [2] J. Guo, H. Guo, A.L. Baker, M.T. Lanagan, E.R. Kupp, G.L. Messing, C.A. Randall, Cold sintering: a paradigm shift for processing and integration of ceramics, Angew. Chem. Int. Ed. 55 (38) (2016) 11457–11461. [3] J. Guo, A.L. Baker, H. Guo, M. Lanagan, C.A. Randall, Cold sintering process: a new era for ceramic packaging and microwave device development, J. Am. Ceram. Soc. 100 (2) (2017) 669–677. [4] J.P. Maria, X. Kang, R.D. Floyd, E.C. Dickey, H. Guo, J. Guo, A. Baker, S. Funihashi, C.A. Randall, Cold sintering: current status and prospects, J. Mater. Res. 32 (17)
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Preparation and properties of high-density Bi2O3 ceramics by cold sintering Jinjie Songa, Guisheng Zhua,∗, Huarui Xua,∗∗, Weining Fua, Jiwen Xua, Jian Zhanga, Shicheng Huangb, Aibing Yuc a Guangxi Key Laboratory of Information Materials, Engineering Research Center of Electronic Information Materials and Devices, Ministry of Education, Guilin University of Electronic Technology, Guilin, 541004, China b Guangxi Crystal Union Photoelectric Materials Co., Ltd., Liuzhou, 545036, China c ARC Hub for Computational Particle Technology, Monash University, Clayton, Victoria, 3800, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Cold sintering Bi2O3 ceramics High-density ceramics Dielectric properties
High-density Bi2O3 ceramics were prepared via a three-step cold sintering process (TS-CSP) at an ultra-low temperature. In the first step, the relative density of the Bi2O3 ceramic, subjected to 155 °C and 300 MPa for 25 min, is 96.23%, with the majority of the crystals inside the ceramic. The Bi2O3 ceramic was further densified with a corresponding increase in the relative density to 98.52%, after processing at 210 °C and 300 MPa for 25 min, in the second step. Finally, in the third stage of the processing, the temperature was raised from 210 °C to 270 °C and isothermally held for 25 min, which enabled the Bi2O3 ceramic grains to fully grow to yield a grain size of 4.02 μm, having a relative density of 99.13%. The densification mechanism of the TS-CSP Bi2O3 ceramics is via a dissolution-recrystallization-growth process. The relative permittivity, quality factor and grain size of the Bi2O3 ceramics are 33.44, 16,218 GHz and 4.02 μm, respectively. The materials and their preparation described herein provide a novel approach for the preparation of ultra-low temperature ceramics.
1. Introduction The cold sintering process (CSP) is a recent sintering method that can achieve high-density ceramics at processing temperatures < 300 °C [1–4]. CSP is principally used to sinter ceramics in the presence of a solution as a co-solvent. CSP was first used to prepare Li2MoO4 ceramics in 2014 [5], and formally named as “cold sintering” in 2016 [2]. CSP has attracted significant attention because of the demonstrated advantages related to the low cost of the process having a low energy consumption and the ease of working with metals, polymers and other materials. The CSP technology has been successfully applied across numerous materials including typical oxide ceramics [6], ceramic/ ceramic composites [7,8], microwave dielectric ceramics [2,3,5,9], ferroelectric and piezoelectric ceramics [1,10], solid electrolytes [11], thermoelectric materials [12], and ceramic/2D nanomaterial composites [13]. Bi2O3 ceramics having attractive dielectric properties are of significant importance in the field of wireless communication. However, the sintering temperature of Bi2O3 ceramics is relatively high. Zhou et al. [14] reported that Bi2O3 ceramics formed at 680 °C had a relative permittivity (εr) of ~32.8 and a quality factor (Qf) of ~17,900 GHz, and
∗
claimed a high relative density (ρ ~97%) that was achieved via conventional sintering (CS). Therefore, there is an urgent need to develop a method to reduce sintering temperature. Applying the CSP technology has hitherto not been used in the preparation of bismuth oxide ceramics, and hence the sintering results, sintering process and mechanism of cold sintering for the preparation of bismuth oxide ceramics still require elucidating. Solubility in the solution determines the suitability of a material for cold sintering. Bi2O3 is insoluble in water and alkali, but it is soluble in acetic acid, so this study chooses acetic acid solution as co-solvent to cold sinter Bi2O3 ceramic. In this study, high-density (ρ ~99.13%) Bi2O3 ceramics were sintered at 270 °C and 300 MPa via three-step CSP (TS-CSP). The densification process of the TS-CSP Bi2O3 ceramics was analyzed. The εr and Qf of the Bi2O3 ceramics are 33.44, 16,218 GHz, respectively, which are similar to the dielectric properties of Bi2O3 ceramics sintered at 680 °C by CS.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (G. Zhu), [email protected] (H. Xu).
∗∗
https://doi.org/10.1016/j.ceramint.2020.02.177 Received 11 January 2020; Received in revised form 17 February 2020; Accepted 17 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Jinjie Song, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.177
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Fig. 1. (a) A schematic diagram of the cold sintering process (CSP) mold, (b) the three-step CSP (TS-CSP) sintering curve of the Bi2O3 ceramics.
2. Materials and methods
Table 2 Second group of experiments.
2.1. Experimental method The experimental set-up for the ceramic CSP required the use of a mold (not completely closed) and an outer heating jacket consisting of a mica band heater. The mold has the capability to exert uniaxial pressure and the jacketed band heater allows for the mold to be heated up to 450 °C. A straight thermocouple was positioned between the heater band and sample to measure the temperature of the ceramic slurry, as shown in Fig. 1(a). Additionally, the TS-CSP sintering temperature curve, at a heating rate of 6 °C/min, is shown in Fig. 1(b).
250
300
350
V (heating rate)/°C/min T (temperature)/°C t (holding time)/min
6 300 0
6 300 0
6 300 0
6 300 0
6 300 0
250
300
350
V/°C/min T/°C
6 230/235/ 240 /245 25
6 190/195/ 200 /205 25
6 160/165/ 170 /175 25
6 145/150/ 155 /160 25
6 142/147/ 152 /157 25
The crystalline structure was examined using x-ray diffraction (XRD, D8 Advance, Bruker Inc., Germany) with a CuKα source (λ = 0.15405 nm). The morphologies of the surface and cross-section of the ceramics were analyzed by field emission-scanning electron microscopy (FE-SEM, FEI Tecnai-450, USA). The density of the bulk samples was measured by determining the mass/dimension ratio as well as using the Archimedes’ method with water as the liquid medium. The microwave dielectric properties were tested using a Vector Network Analyzer (Agilent-N5230A). The mass loss of the ceramic materials was measured by thermogravimetric analysis (TGA, SDT Q600, TA, USA). Chemical element identification and valence state were analyzed by x-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250).
3. Results and discussion Fig. 2(a) shows changes to the pressure profiles as a function of temperature during the ceramic preparation by CSP. An important sign of ceramic densification was the rapid reduction in pressure [15]. For example, when processing at 350 MPa and 300 MPa, the pressure rapidly reduced when subjected to a heating temperature of 152 °C and 155 °C, respectively. Similar observations were made when processing at 250 MPa, 200 MPa, and 150 MPa. Importantly, the temperature at which the densification process initiates (observed rapid drop in pressure) is determined to be pressure dependent. Reduced pressures indicate shrinkage of the bulk ceramic, which can be attributed to the growth of the grains by consumption of the surrounding powders when subjected to the necessary temperatures and pressures. Therefore, the temperature at which the rapid pressure drop is observed is considered to be the initial crystallization temperature of the Bi2O3 ceramics, which is similar to the results reported by Jiang et al. [15], were the initial crystallization temperature of a ZnO ceramic at 300 MPa was 150 °C, with an associated rapid pressure drop. Furthermore, from the data gathered herein, the ceramics are suggested to have different initial crystallization temperatures at different pressures. Fig. 2(b) further complements the authenticity of the results in Fig. 2(a) by using the
Table 1 First group of experiments. 200
200
2.3. Characterization
The starting Bi2O3 powder was obtained commercially from Alfa Aesar (Haverhill, MA) (99.9%). Typically, 7.0 g of Bi2O3 powder was placed in a mortar with an acetic acid solution (3 mol/L) added at 15 wt %. Thereafter, the mixture was uniformly ground with an agate mortar, and the slurry placed into the mold. Finally, the samples were prepared according to the following three groups of experimental conditions: in the first group, the mold was subjected to a uniaxial pressure of 150–350 MPa and heated up to 300 °C at a heating rate of 6 °C/min (the application of pressure for this group was one-time and noncontinuous), as shown in Table 1; in the second group, the mold was subjected to a uniaxial pressure of 350 MPa down to 150 MPa at 50 MPa intervals while being heated to 142–157 °C, 145–160 °C, 160–175 °C, 190–205 °C and 230–245 °C, respectively, all at a heating rate of 6 °C/min, where the pressure was isothermally held for 25 min, as shown in Table 2; in the third group, the mold was subjected to a uniaxial pressure of 300 MPa. Thereafter, the samples were sintered via the TS-CSP. This process first involves heating the mold to 155 °C at 300 MPa at a heating rate of 6 °C/min. The system was isothermally held as a function of time (0–45 min). Thereafter, the temperature of the mold was elevated further to 210 °C (300 MPa) and isothermally held for 25 min, before further increasing the temperature to 270 °C (300 MPa) and isothermally holding for 25 min to densify the product, as shown in Table 3. After allowing the samples to cool under ambient conditions the ceramic piece measured 10 mm in diameter and 5 mm in thickness.
150
150
t/min
2.2. Ceramic preparation
P(pressure)/MPa
P/MPa
2
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Table 3 Third group of experiments.
First stage
Second stage
Third stage
P /MPa
300
300
300
300
300
300
300
300
300
300
V/°C/min T/°C t/min V/°C/min T/°C t/min V/°C/min T/°C t/min
6 155 0 – -
6 155 10 – -
6 155 20 – -
6 155 25 6 180 -
6 155 25 6 190 -
6 155 25 6 200 -
6 155 25 6 210 25 6 240/270/300 25
6 155 25 6 270 -
6 155 35 – -
6 155 45 – -
Fig. 3(f). To further improve the crystallization process and compactness of the ceramics, there is a need to enhance the driving force of sintering. Fig. 3(b) shows the relationship between sintering temperature and density during the second stage of the process. Fig. 3(b) shows that when the sintering temperature of the second step was increased from 155 °C to 210 °C, the relative density value of the ceramic was also increased. However, when the sintering temperature of the second step exceeded 210 °C, the relative density value of the ceramic was not substantially changed. Fig. 3(g) shows an SEM micrograph of the ceramic prepared by CSP subjected to 300 MPa at 155 °C for 25 min, followed by 210 °C for 25 min. The grains of the ceramic prepared with the aforementioned parameters were well established with the grain boundaries being in close contact. Here, the relative density and the grain size of the ceramic approached 98.52% and 2.8 μm, respectively. Fig. 3(c) shows that when the sintering temperature of the third step was increased from 210 °C to 270 °C, the relative density increased, albeit to a small degree, from 98.52% to 99.13%. However, a significant change to the grain size was observed, increasing from 2.84 μm to 4.02 μm, as shown in Fig. 3(g–h). Fig. 3(h–i) shows the SEM micrographs at 210 °C and 270 °C for the third stage of the sintering process, respectively. Fig. 3(h–i) shows that the internal grains of the ceramic were fully grown, and obvious grain boundaries can be observed at both 270 °C and 300 °C. However, there was no significant difference in relative density, which demonstrates that at a CSP temperature of 270 °C, a high-dense ceramic was formed. Fig. 4 shows the XRD, TGA and XPS results of the ceramic sample prepared by the TS-CSP route subjected to the following conditions: 300 MPa at 155 °C for 25 min, followed by 210 °C for 25 min, and finally 270 °C for 25 min. Fig. 4(a) shows that the XRD pattern of the TS-CSP sample is in good agreement with the standard JCPDS data (PDF#76–1730), and that the sample can be attributed to pure Bi2O3 [16–18], which demonstrates that in the presence of 3 mol/L acetic acid the ceramics are phase pure with no impurities being identified. Fig. 4(b) shows the TGA profile of the sample from room temperature to
relationship between relative density and temperature. Fig. 2(b) shows that when applying a pressure of 350 MPa and 300 MPa, and increasing the temperature to 152 °C and 155 °C, respectively, relative density rapidly increased to a saturated value of 96.33% and 96.23%, respectively. Additionally, similar observations were observed for pressures of 250 MPa, 200 MPa and 150 MPa. Therefore, the data strongly suggest a correlation with pressure drop and relative density and a corresponding initial crystallization temperature at each pressure. Furthermore, the initial crystallization temperature, as a function pressure, decreases with increasing pressure. When the pressure exceeded 300 MPa, the increase of ceramic density and the decrease of initial crystallization temperature were observed not to be significantly affected, Fig. 2(a–b) Hence, 300 MPa and 155 °C were employed as the process conditions for the first stage of ceramic sintering to study the ceramic densification process. Fig. 3(a) shows the density trend for the samples at the first stage of processing (155 °C, 300 MPa) in the presence of 3 mol/L acetic acid sintered as a function of holding time. From Fig. 3(a), the relative density value of the ceramics increased from 85.51% to 96.23% when the holding time was increased from 0 min to 25 min. On further increasing the holding time beyond 25 min, the relative density did not change significantly. Fig. 3(d–f) shows SEM micrographs of the first stage of sintering, which systematically show the changes of the ceramic internal microstructure. Fig. 3(d) shows significant grain formation inside the ceramic, however, the crystallization was incomplete after being maintained at 155 °C for 5 min. A degree of the powder particles are not involved in the crystallization, and there are obvious atmospheric pores and voids. However, the grains were still not fully grown with evidence of obvious pores inside the ceramic. Fig. 3(e) shows a significant number of crystals and grain boundaries observed inside the ceramic with no obvious voids, however, small pores remain after holding at 155 °C for 15 min. The majority of the crystallization was completed after holding at 155 °C for 25 min, with no obvious sign of pores and a relatively good grain growth observed, as shown in
Fig. 2. (a) Pressure change of the ceramics formed by CSP as a function of temperature. The relationship between relative density and temperature for samples subjected to (b) 350 MPa, 300 MPa, 250 MPa, 200 MPa, and 150 MPa. 3
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Fig. 3. Density trend of (a) the pre-sintered samples at 155 °C and 300 MPa as a function of holding time, (b) the samples sintered during the second stage of sintering as a function of temperature, (c) the samples sintered during the third stage of sintering as a function of temperature. The corresponding scanning electron microscopy (SEM) images of the ceramic microstructures pre-sintered at 155 °C for (d) 5 min, (e) 15 min, and (f) 25 min. The corresponding SEM micrographs of the completely sintered ceramics at (g) 210 °C for 25 min, (h) 270 °C for 25 min, and (i) 300 °C for 25 min.
Fig. 4. (a) X-ray diffraction, (b) thermogravimetric analysis, (c–f) x-ray photoelectron spectroscopy spectra of (c) the overview spectrum, (d) C 1s, (e) Bi 4f, and (f) O 1s spectra of the TS-CPS sample. 4
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Fig. 5. Schematic illustration of the Bi2O3 ceramic microstructural evolution during the TS-CSP.
Fig. 3(a). At 300 MPa and 155 °C, the relative density value of the sample increased with increasing holding time. However, when the holding time was > 25 min, the relative density value of the ceramic did not substantially increase because within a holding time of 25 min there remained sufficient water inside the mold to ensure that the powder particles had sufficient time for crystal growth. Fig. 5(d–e) shows the second stage of the process, with the temperature increased from 155 °C to 210 °C, which further promoted the crystallization and grain growth of the ceramics. The void space was further reduced as the particles combined to form a clear grain-boundary. There are three mechanisms for grain growth: the first relates to the contact of the small grains with the large grains and epitaxial growth; the second mechanism derives from the Bi2O3 nuclei in the voids continuing to absorb Bi3+ from the solution with a concomitant subsequent growth; the third mechanism relates to the continuous growth of the nuclei on the surface of the particles by accepting ions from the solution. Fig. 5(e–f) shows the third stage of the process. When the temperature increases to 270 °C, the ceramics are subjected to higher driving forces, which increases the free energy at the grain boundaries. With further movement of the grain boundaries, the grains continue to absorb each other and grow resulting in the disappearance of the sub-grain boundaries. Based on the experimental observations herein, the first stage and second stage processes (0–155 °C) were mainly governed by dissolution-recrystallization processes, with the third stage (210–270 °C) following a grain growth process. In general, the TS-CSP densification mechanism of the high-density Bi2O3 ceramics followed a dissolution-recrystallization-growth process, which is similar to the liquid phase mass transfer process of conventional liquid phase sintering. During the third stage of the sintering process, the sintering temperature has a significant influence on the dielectric properties of the ceramics. Fig. 6(a) represents the εr and relative density of the Bi2O3 ceramics as a function of temperature. As the temperature is increased, εr improves and subsequently reaches an optimal value of 33.44 at 270 °C. The observed trend of relative density is consistent with that of εr. Thus, the change in εr is principally related to relative density. However, relative density is related to the porosity, which varies with temperature. As the temperature is increased, the defects and porosity in the ceramic crystal gradually decrease, and the density of the ceramic
600 °C. From the TGA curve, the total mass loss of the sample after TSCSP was 0.08%, which indicated that the Bi2O3 ceramic prepared by TS-CSP exhibited enhanced stability. As shown in Fig. 4(c), no additional peaks, corresponding to any binding energy elements, were observed, other than for C, Bi, and O, which also suggest the absence of any impurities remaining in the Bi2O3 ceramics sintered via TS-CSP. Fig. 4(d) shows the C 1s peak with a binding energy of 284.8 eV. The C 1s peak does not influence the interpretation of the present results, and hence was used for calibration of the binding energies for sample charging correction [19,20]. Fig. 4(e) represents the two Bi 4f peaks at binding energies of 159.3 eV and 164.5 eV. No shoulder peaks attributed to two, four and five valences were observed for the two main peaks of Bi 4f [17,21]. Hence, the spectrum indicated that the bismuth in Bi2O3, generated using ultra-low temperature sintering, existed as Bi3+. As observed in Fig. 4(f), the O 1s peaks at 530.0 eV and 531.0 eV are attributed to Bi–O− and O2− in Bi2O3, respectively [22,23], which indicated a phase pure Bi2O3, similar to the XRD findings. Fig. 5 shows the densification of the TS-CSP Bi2O3 ceramics. Cold sintering of the Bi2O3 ceramics is a nonequilibrium dynamic process similar to that of hydrothermal syntheses. The process can be divided into several main stages. Fig. 5(a–d) shows the first stage. The powder particles gradually approach each other, and in a liquid phase environment, were rearranged when subjected to a pressure of 300 MPa. Further increasing the temperature induced dissolution of the Bi2O3 powder surface resulting in Bi3+ ions transferring to the liquid solution, which thereafter combined with Ac− to form Bi(Ac)3, as shown in Fig. 5(a–b). When the liquid phase was evaporated at 155 °C, the concentration of the solution increased sufficiently to become a supersaturated solution. The supersaturated solution together with the applied pressure result in an environment that is similar to a hydrothermal reaction. The solute began as Bi(Ac)3 and finally formed Bi2O3 having small grains in this pseudo-hydrothermal environment. The small grains that were formed were uniformly deposited in the voids of the powder and on the particle surface, as shown in Fig. 5(b–c). When sintering at 300 MPa and 155 °C over a period of time, the pores are eventually filled with a large number of small particles, which continue to grow, thereby significantly reducing the porosity, as shown in Fig. 5(c–d). These observations are consistent with the description of
Fig. 6. (a) Relative permittivity and relative density, (b) quality factor and grain size of the TS-CSP Bi2O3 ceramics as a function of temperature. 5
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also increases, resulting in the dielectric loss of the ceramic being reduced [24–27], which increases the εr value. However, if the temperature exceeds a certain threshold, no change in the ceramic interior is observed, and hence relative density does not increase, thus, the εr value does not increase. As shown in Fig. 6(b), as the temperature was increased from 210 °C to 240 °C, and to 270 °C and 300 °C, the grain size of the Bi2O3 ceramic increased from 2.84 μm to 3.67 μm, 4.02 μm, and 4.05 μm, respectively, whereas the Qf of the Bi2O3 ceramics increased from 5631 GHz to 13,692 GHz, 16,218 GHz, and 16,316 GHz, respectively. The observed trend related to the grain size was consistent with that of Qf. Thus, changes to Qf were principally related to grain size. As the temperature is increased, the decreasing number of grain boundaries caused by the increase in grain size would also increase the Qf value of the ceramics [25,28]. However, if the processing temperature exceeded a threshold value of 270 °C, that resulted in there being no significant difference in grain size, the Qf value also failed to increase. As shown in Fig. 6(a–b), the third stage of sintering (270 °C, 300 MPa, 25 min) exhibited a relative density value of ~99.13%, a εr value of ~33.44 and a Qf value of ~16,218 GHz, which are comparable to a relative density of ~97%, a εr value of ~32.8 and a Qf value of ~17,900 GHz, derived from the CS samples sintered at 680 °C [14].
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4. Conclusions Bi2O3 ceramics having a relative density of 99.13%, εr of 33.44, and Qf of ~16,218 GHz were prepared via TS-CSP at 270 °C and 300 MPa. Pressure has a significant influence on the initial crystallization temperature. The initial crystallization temperature decreases as a function of increased pressure. The densification mechanism of the Bi2O3 ceramic material follows a dissolution-precipitation-growth process. Overall, the study provides a new approach for the ultra-low temperature sintering of ceramics. Declaration of competing interest The authors declare no known competing interests, either financial or personal relationships, that could have influenced the work reported in this study. Acknowledgments The work was financially supported by the Guangxi InnovationDriven Development Project (No. AA18118001) and Guangxi Key Laboratory of Information Materials Foundation (No. 191027-Z). We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. References [1] H. Guo, A. Baker, J. Guo, C.A. Randall, D. Johnson, Cold sintering process: a novel technique for low-temperature ceramic processing of ferroelectrics, J. Am. Ceram. Soc. 99 (11) (2016) 3489–3507. [2] J. Guo, H. Guo, A.L. Baker, M.T. Lanagan, E.R. Kupp, G.L. Messing, C.A. Randall, Cold sintering: a paradigm shift for processing and integration of ceramics, Angew. Chem. Int. Ed. 55 (38) (2016) 11457–11461. [3] J. Guo, A.L. Baker, H. Guo, M. Lanagan, C.A. Randall, Cold sintering process: a new era for ceramic packaging and microwave device development, J. Am. Ceram. Soc. 100 (2) (2017) 669–677. [4] J.P. Maria, X. Kang, R.D. Floyd, E.C. Dickey, H. Guo, J. Guo, A. Baker, S. Funihashi, C.A. Randall, Cold sintering: current status and prospects, J. Mater. Res. 32 (17)
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