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Preparation and properties of pressureless-sintered dense calcite ceramics
Materials Chemistry and Physics 192 (2017) 304e310
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation and properties of pressureless-sintered dense calcite ceramics Jun Ito a, Yuta Matsushima a, Hidero Unuma a, *, Naohiro Horiuchi b, Kimihiro Yamashita b, Masahiko Tajika c a b c
Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, 992-8510, Japan Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo, 101-0062, Japan Shiraishi Central Laboratories Co., Ltd., 4-78 Motohama-cho, Amagasaki, 660-0085, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Calcite powder was sintered to 99% density in air. Sintering aid in the LiF - NaF - KF eutectic mixture was used. Mechanical, thermal, and electrical properties were evaluated.
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Article history: Received 19 December 2016 Received in revised form 17 January 2017 Accepted 22 January 2017 Available online 1 February 2017
This paper describes a method for pressureless sintering of calcite (CaCO3) powders and the physical properties of the obtained dense calcite ceramics. Eutectic mixture of the KF - NaF - LiF system was found to be a good sintering aid that promoted the densification and the grain growth of calcite. Calcite sample with high relative density of 99.0% was attained on sintering the calcite fine powder at temperatures as low as 723 K for 3 h in air, without calcite undergoing thermal decomposition. Particle size of the calcite powder was found to be an important factor in achieving densification. The thermal conductivity and dielectric constant were found to be close to those of calcite single crystals; however, Vickers hardness and Young’s modulus were lower than those of calcite single crystals. Thermally stimulated depolarization current of calcite ceramics was higher than that of calcite single crystals and hydroxyapatite ceramics. © 2017 Elsevier B.V. All rights reserved.
Keywords: Calcium carbonate Pressureless sintering Thermal properties Electrical properties Mechanical properties
1. Introduction
* Corresponding author. E-mail address: [email protected] (H. Unuma). http://dx.doi.org/10.1016/j.matchemphys.2017.01.062 0254-0584/© 2017 Elsevier B.V. All rights reserved.
Calcium carbonate, calcite, aragonite, and vaterite, is known to exhibit a high biocompatibility and a good bioresorbability [1e3]. The marine coral, consisting mainly of aragonite, and its phosphorylated derivatives, the framework of which closely resembles
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the human cancellous bones, have been clinically used as bone augmentation materials [4,5]. However, the synthetic calcium carbonate materials intended to be used for biomedical applications are either fine powders or microspheres because it is difficult to fabricate calcium carbonate monoliths in dense or porous forms. The main reason for the difficulty in consolidating calcium carbonate into bulk monoliths is that calcium carbonate decomposes into calcium oxide and carbon dioxide on heating above 900 K in ambient atmosphere [Eq. (1)], making it difficult to sinter calcium carbonate.
ethanol from an injection needle for 0.5 g powder mixture), and then uniaxially pressed into disc-shaped green compacts with a diameter of 15 mm under a pressure of 2.0 MPa. The role of ethanol was to reduce the friction among the CaCO3 particles for uniform compression. No other additives such as dispersants or binders were used. The green compacts were sintered in air at temperatures between 693 and 813 K for 3 h. A heating rate of 10 K/min was used. Fig. 1 shows the micrographs of both the powders obtained after mixing with the sintering aid.
CaCO3 %CaO þ CO2
2.2. Characterization
(1)
However, numerous potential advantages exhibited by the calcium carbonate monoliths including their biological compatibility have prompted many attempts to consolidate calcium carbonate into bulk monoliths using various methods. Ishikawa et al. [6e8] prepared a porous monolith of CaCO3 having a porosity of 40% by exposing Ca(OH)2 green bodies to an atmosphere of CO2 and H2O, with a strategy based on the setting phenomenon of lime plaster. Tempieri et al. [9] fabricated porous calcium carbonate monoliths having a microstructure from natural wood via a multistep process starting from pyrolysis of wood followed by the successive conversion of the resultant carbon to CaC2, CaO, and subsequently to CaCO3 without disturbing the microstructure. Attempts for preparing dense CaCO3 ceramics have been carried out employing an extremely high pressure of 1 GPa [10], pulsed direct current hotpressing [11], hydrothermal hot-pressing [12e14] or sintering of calcite under high-pressure CO2 atmosphere (0.2 MPa) [15,16], and the latter method resulted in the production of a material with a high relative density of 93.4% as compared to the theoretical density of calcite (2.711 g/cm3). Pressureless sintering of calcite in the presence of sintering aids has also been investigated. He et al. [17,18] sintered calcite at 923 K in air using phosphate glass as the sintering aid: however, no description on the sintered density was provided. Other sintering aids such as lithium fluoride [19,20], lithium carbonate [21], sodium carbonate [22,23], and lithium phosphate have been used [24]; however, the properties of the resultant ceramics have not been well described. In the present work, we prepared dense calcite ceramics by pressureless sintering in air. Eutectic mixture of the KF - NaF - LiF system was selected for the process for achieving the liquid phase formation at a temperature as low as possible. Monodisperse calcite fine powder was used for achieving a high sinterability. The physical, mechanical, thermal, and electrical properties of the resultant ceramics were evaluated. 2. Experimental 2.1. Sample synthesis Two kinds of calcite powders were used as the raw materials. Powder A, monodisperse calcite powder with an average diameter of approximately 150 nm and a chemical purity of 99.8%, was supplied from Shiraishi Kogyo Kaisha Ltd., Hyogo, Japan. Powder B, a calcite powder of commercial chemical reagent having an average diameter of 12e15 mm and a chemical purity of 99.5%, was obtained from Kanto Chemical Co., Inc., Tokyo, Japan. The sintering aid was prepared by mixing KF, NaF, and LiF in the molar ratio of 40: 11: 49, which corresponds to the eutectic composition of the ternary system with the eutectic temperature of 736 K [25]. The sintering aid was mixed with powder A or B in the ratios of 0.3, 0.6, 1.2, 2.5, and 5.0 mass%. The mixed powders were dry-milled overnight to ensure homogeneous mixing. The powder samples were then slightly wet with a small amount of ethanol (typically 3 drops of
In order to roughly estimate the liquid formation temperature, differential thermal analysis (DTA) measurements were carried out with the pristine sintering aid and 50: 50 mixture (mass ratio) of powder A and the sintering aid. The DTA measurements were conducted at a heating rate of 10 K/min with a thermal analyzer (Evo II, Rigaku, Tokyo, Japan). The sintered specimens were characterized using a scanning electron microscope (SEM, e-SEM, Shimadzu Rika Corp., Tokyo, Japan and JSM-6330F, JEOL, Ltd., Tokyo, Japan) for examining their microstructure, an X-ray diffractometer (XRD, Mini Flex, Rigaku, Tokyo, Japan) for determining the crystalline phases, and by the Archimedean method using ethanol as the substituting liquid for measuring the relative density. The density of calcite, 2711 kg/m3, was used as the theoretical density because of the following two reasons: the real density of the reaction products in the KF - NaF -
(a)
1µm (b)
10µm Fig. 1. SEM images of the CaCO3 powders mixed with the sintering aid; (a) powder A and (b) powder B.
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LiF - CaCO3 system was unknown and the optimum amount of the sintering aid was as low as 0.6 mass% as will be described later, resulting in a negligible perturbation of the theoretical density with the addition of the sintering aid. The most densely sintered specimen, powder A mixed with 0.6 mass% sintering aid and sintered at 783 K, was characterized for determining the following properties: dielectric constant and dielectric loss in the frequency range of 5 MHz - 1 GHz at room temperature measured with an impedance analyzer using a capacitance method (E4991B, Keysight Technologies, Tokyo, Japan), heat capacity and thermal diffusivity using the laser flash method (TC-9000, ULVAC Inc., Kanagawa, Japan and LFA 501, Kyoto Electronics Manufacturing Co., Ltd., Kyoto, Japan), Vickers hardness (HV) with a hardness tester under a load of 1.0 kg (HV-115, Mitutoyo Corp., Kanagawa, Japan), Young’s modulus (E) and Poisson ratio (n) using an ultrasonic pulse method (5900PR, GE sensing & Inspection Technologies, Tokyo, Japan), as well as the electrical conductivity and the electrical polarizability. Vicker’s hardness was measured 5 times and the thermal properties were measured 3 times. Elastic, dielectric and electric properties were measured once. The details of the electrical measurements are described elsewhere [26]. In brief, the AC impedance spectrometry measurements were carried out using an impedance analyzer (1260, Solarton Analytical, Leicester, UK) with an AC modulation at 1 Hz to 10 MHz in the temperature range of 300e630 K. The electrical conductivity was calculated using the relation: s ¼ d=ðARbulk Þ, where d, A, and Rbulk are the sample thickness, electrode area, and bulk resistance, respectively. The bulk resistance was obtained using the Nyquist plot method. Electric polarizability was evaluated by carrying out the thermally stimulated depolarization current (TSDC) measurements of the electrically polarized sample under constant heating up to 673 K at a heating rate of 5 K/min. The sample was electrically polarized under a DC voltage of 400 V (2.3 kV/cm) in air at 573 K for 30 min before being subjected to the TSDC measurements. 3. Results and discussion Fig. 2 shows the effect of sintering temperature on the relative density of the powders A and B. The powder A exhibited higher density than powder B. Regardless of the amount of the sintering aid added, the relative density of the compacts of powder A exceeded 98% after sintering above certain temperatures. The highest relative density of 99.0% was obtained for the powder A mixed with 0.6% of the sintering aid and sintered at 723 K. The highest relative density reported for the CaCO3 ceramics is 98.7% [24], for the sample sintered with 8% Li3PO4 as the sintering aid under CO2 atmosphere (4 kPa) at 973 K for 15 h. In the present study, a higher relative density was attained using a lesser amount of the sintering aid added at a lower sintering temperature and sintered for a shorter sintering time than the previous study. For powder A, the onset of the densification lowered with an increase in the amount of sintering aid used. In the case of the powders mixed with the sintering aid in 0.3, 0.6, and 1.2%, a sharp increase in the relative density was observed below 783, 723, and 693 K, respectively. Slight decreases in the relative density were observed with increasing sintering temperature; for example, the relative density of a sample with 1.2% sintering aid became lower above 750 K. Although we have not fully elucidated the reason, there are two possibilities for the slight decrease in the relative density; one is the evaporation of the sintering aid, and the other is the formation of CO2 through the reactions of alkali fluorides with calcium carbonate whose example is expressed in Eq. (2).
CaCO3 þ 2 NaF/ CaF2 þ Na2 O þ CO2
(2)
Fig. 2. The effect of the sintering temperature and the amount of the sintering aid on the relative density of compacts of (a) powder A and (b) powder B.
On the other hand, for powder B, a relative density higher than 92.0% could not be reached. The highest value obtained for the powder B was 91.7% when the powder was mixed with 1.2% sintering aid and sintered at 723 K. The addition of 2.5 and 5.0% sintering aid to both the powders caused swelling of the samples after the sintering under any condition (data not shown), which may be accounted for the reactions exemplified by Eq. (2). Therefore, the optimum amount of the sintering aid is smaller than 2%. The sintering behaviors of both the powders can be understood by observing the microstructural change of the sintered compacts. Fig. 3 shows the evolution of the microstructure of powder A containing 0.6% sintering aid. The sintered specimens were polished and thermally etched before the SEM observations. After sintering at 693 K, the particles coalesced to form grains of approximately 1 mm in average diameter [Fig. 3(a)]. This suggests that the liquid phase can be obtained below 693 K even though the eutectic temperature of the KF - NaF - LiF system is 736 K (discussed later). The grain size shown in Fig. 3 (a) is found to be quite homogeneous throughout the sintered specimen. With a further increase in the sintering temperature to 723 and 753 K, the sintering of the grains occurred while eliminating voids and without
J. Ito et al. / Materials Chemistry and Physics 192 (2017) 304e310
(a)
5µm (c)
5µm
(b)
5µm (d)
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temperature; however, numerous voids were still present in the specimen. After performing sintering at 783 K, an extensive grain growth occurred and a number of pores were observed within the grains. The sintering process aided by the KF - NaF - LiF eutectic mixture is discussed below. Fig. 5 shows the DTA curves of the pristine sintering aid and the sintering aid mixed with powder A. In the case of the pristine sintering aid, a large endothermic event initiated at around 731 K, in good accordance with the reported eutectic temperature, 736 K. When the sintering aid was mixed with CaCO3, the onset of the first endothermic event lowered to 684 K. This may account for the coalescence of the primary particles of powders A
5µm
Fig. 3. Evolution of the microstructure of powder A containing 0.6 mass% sintering aid with an increase in the sintering temperature. (a) 693, (b) 723, (c) 753, and (d) 783 K.
indicating a prominent grain growth; the average grain size obtained after sintering at 753 K was found to be approximately 2 mm. However, a sudden grain growth occurred to form grains larger than a few tens of micrometers at 783 K. The microstructure evolution for the powder B was found to be quite different from that for the powder A, as shown in Fig. 4. After being sintered at 693 K, grain growth in powder B proceeded in an inhomogeneous manner; some of the grains coalesced, thereby leaving large voids as shown in Fig. 4 (a). The size of the grains was not homogeneous in contrast to that obtained with the powder A [Fig. 3 (a)]. Sintering proceeded upon increasing the sintering
(a)
5µm
(c)
5µm
Fig. 5. DTA profiles of (a) the sintering aid and (b) sintering aid mixed with CaCO3 (mass ratio of 50: 50).
(b)
5µm
(d)
5µm
Fig. 4. Evolution of the microstructure of powder B containing 0.6 mass% sintering aid with an increase in the sintering temperature. (a) 693, (b) 723, (c) 753, and (d) 783 K.
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and B after being sintered at 693 K. The curve shown in Fig. 5 (b) also indicated second and third endothermic responses around 750 and 810 K, suggesting an increase in the amount of liquid in the powder compact. The extensive grain growth observed for the samples sintered at 783 K [Fig. 3 (d) and 4 (d)] could be attributed to the increase in the amount of the liquid and the solubility. Fig. 6 shows the XRD patterns of the powder A mixed with 0.6% sintering aid before and after sintering the sample at various temperatures. The peaks corresponding to the sintering aid could not be observed since its amount was too small. There was no indication of the decomposition of calcite to calcium oxide even after performing the sintering at 813 K. Table 1 summarizes the thermal and mechanical properties of 99.0% dense calcite ceramics. A thermal conductivity of 2.60 ± 0.003 W/(m$K) was obtained, in good accordance with the value of the calcite single crystal, 2.6 W/(m$K), as reported by Straube et al. [27]. The dielectric constant as a function of frequency is shown in Fig. 7, and the value obtained at 1 GHz, 8.56, is close to that of the calcite single crystal, 7.8, as reported by Veeraish et al. [28]. The difference in the values between the calcite ceramics and the single crystal might be attributed to the presence of the alkalirich grain boundary phase. Still, the addition of the sintering aid did not cause a heavily deteriorative effect on the dielectric properties. The hardness of the calcite single crystal indicates anisotropy and a dependence on the load [29]. Carter et al. reported the Knoop hardness (HK) of calcite single crystals measured under 10 g load to be in the range of 1.7e2.0 GPa depending on the cleavage plane while it decreased to 0.6e1.5 GPa under 100 g load [29]. Although HV and HK are different parameters and cannot be directly compared, these values have a similar order of magnitude, while HK is slightly lower than HV of the same material [30]. In the present work, HV was measured under a load of 1 kg in order to obtain sufficiently larger indentation marks than the grain sizes, resulting in lower HV values than those reported in the previous study [29]. The HV value could be affected by the presence of grain boundary phases, pores, and microcracks in the sintered ceramics. Considering these factors, an average HV value of 0.323 GPa is considered a rational value. The Young’s modulus, E and the Poisson ratio, n were measured to be 49 GPa and 0.25, respectively. According to Lin [31], the bulk (K) and shear (G) moduli of the calcite single crystal were 79.57 and 32.23 GPa, respectively, both with a very small temperature dependence, and a Poisson ratio was 0.322. E and n values calculated from the data by Lin using Eqs. (3) and (4) by assuming isotropic elastic response were 84 GPa and 0.322, respectively. Hence, the measured E and n were lower than the calculated values.
Fig. 6. XRD patterns of powder A mixed with 0.6% sintering aid (a) before and after sintering at (b) 693, (c) 753, and (d) 813 K.
Table 1 The physical properties of 99.0% dense calcite ceramics. Thermal conductivity
2.60 ± 0.003 W/(m K)
Dielectric constant (1 GHz) tan d (1 GHz) Vicker’s hardness (1 kg load) Young’s modulus Poisson ratio
8.56 0.0022 61.2 ± 3.4 Hv (0.323 GPa) 49 GPa 0.25
Fig. 7. Dielectric constant and tand of 99.0% dense calcite ceramics as a function of frequency.
E ¼ 9KG=ð3K þ GÞ
(3)
n ¼ ð3K 2GÞ=ð6K þ 2GÞ
(4)
The discrepancies of the measured and calculated E and n values should be attributed to the difference in the microstructures of the single crystal and sintered ceramics. The presence of pores and grain boundary phases might affect the elastic properties of the samples to a large extent. Fig. 8 shows the Arrhenius plot of electrical conductivity of calcite ceramics. The data corresponding to the calcite single crystal along (10.4) and (10.0) orientations obtained from Ref. [26] are also given in the figure. The activation energy for conduction of the calcite ceramics was 212.2 kJ/mol. The calcite ceramics exhibited
Fig. 8. Electrical conductivity of 99.0% dense calcite ceramics as a function of temperature. Data of the calcite single crystal along (10.4) and (10.0) orientations obtained from Ref. [26] are also shown.
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higher electrical conductivity and activation energy for conduction as compared with the single crystal calcite, 112e119 kJ/mol [26]. The higher conductivity suggests a higher concentration of electric carriers, possibly mobile ions and ionic vacancies in this case. The existence of a grain boundary might enhance the activation energy for conduction. However, since the nature of the grain boundary of the calcite ceramics is not well-understood, a speculative discussion on this cannot be provided. The TSDC spectrum of the calcite ceramics is shown in Fig. 9. The dissipation of electric current began on heating the specimen to approximately 580 K. The current density in the order of several hundred nA/cm2 (107 A/cm2) was obtained. Yamashita et al. measured the dissipated currents from calcite single crystal [26] and hydroxyapatite ceramics [32]. The dissipated current densities were typically in the order of several nA/cm2 for hydroxyapatite ceramics [32] and a few tens of nA/cm2 for calcite single crystal [26]. Therefore, our calcite ceramics dissipated much larger currents than the calcite single crystal or hydroxyapatite ceramics. This might imply that the calcite ceramics could be electrically polarized to larger extents than the single crystal or hydroxyapatite samples. The electrical polarization of hydroxyapatite enhanced the formation of bone-like apatite layers in simulated body fluid [32,33], osteoblast adhesion in vitro [34], and bone ingrowth into porous hydroxyapatite in vivo [35]. Hence, our calcite ceramics might exhibit enhanced biological properties on subjecting to electrical polarization treatment. The validation of the biological properties of the polarized calcite as well as the understanding the detailed mechanism of polarization should be carried out in the future.
4. Conclusions We demonstrated that calcite could be densely sintered up to a relative density of 99.0% pressureless sintering carried out in air. The eutectic mixture of the KF - NaF - LiF system was found to be a good sintering aid, which effectively promoted densification and grain growth when added at 0.3e1.2%. The highest relative density was attained by sintering the sample at 723 K for 3 h with 0.6% sintering aid. Some of the physical properties including the thermal conductivity, dielectric constant, Vickers hardness, and elastic properties were measured. Calcite ceramics could be electrically polarized, and the dissipated current density was higher than that of calcite single crystal and hydroxyapatite ceramics samples.
Fig. 9. TSDC spectrum of 99.0% dense calcite ceramics.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation and properties of pressureless-sintered dense calcite ceramics Jun Ito a, Yuta Matsushima a, Hidero Unuma a, *, Naohiro Horiuchi b, Kimihiro Yamashita b, Masahiko Tajika c a b c
Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, 992-8510, Japan Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo, 101-0062, Japan Shiraishi Central Laboratories Co., Ltd., 4-78 Motohama-cho, Amagasaki, 660-0085, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Calcite powder was sintered to 99% density in air. Sintering aid in the LiF - NaF - KF eutectic mixture was used. Mechanical, thermal, and electrical properties were evaluated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 December 2016 Received in revised form 17 January 2017 Accepted 22 January 2017 Available online 1 February 2017
This paper describes a method for pressureless sintering of calcite (CaCO3) powders and the physical properties of the obtained dense calcite ceramics. Eutectic mixture of the KF - NaF - LiF system was found to be a good sintering aid that promoted the densification and the grain growth of calcite. Calcite sample with high relative density of 99.0% was attained on sintering the calcite fine powder at temperatures as low as 723 K for 3 h in air, without calcite undergoing thermal decomposition. Particle size of the calcite powder was found to be an important factor in achieving densification. The thermal conductivity and dielectric constant were found to be close to those of calcite single crystals; however, Vickers hardness and Young’s modulus were lower than those of calcite single crystals. Thermally stimulated depolarization current of calcite ceramics was higher than that of calcite single crystals and hydroxyapatite ceramics. © 2017 Elsevier B.V. All rights reserved.
Keywords: Calcium carbonate Pressureless sintering Thermal properties Electrical properties Mechanical properties
1. Introduction
* Corresponding author. E-mail address: [email protected] (H. Unuma). http://dx.doi.org/10.1016/j.matchemphys.2017.01.062 0254-0584/© 2017 Elsevier B.V. All rights reserved.
Calcium carbonate, calcite, aragonite, and vaterite, is known to exhibit a high biocompatibility and a good bioresorbability [1e3]. The marine coral, consisting mainly of aragonite, and its phosphorylated derivatives, the framework of which closely resembles
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the human cancellous bones, have been clinically used as bone augmentation materials [4,5]. However, the synthetic calcium carbonate materials intended to be used for biomedical applications are either fine powders or microspheres because it is difficult to fabricate calcium carbonate monoliths in dense or porous forms. The main reason for the difficulty in consolidating calcium carbonate into bulk monoliths is that calcium carbonate decomposes into calcium oxide and carbon dioxide on heating above 900 K in ambient atmosphere [Eq. (1)], making it difficult to sinter calcium carbonate.
ethanol from an injection needle for 0.5 g powder mixture), and then uniaxially pressed into disc-shaped green compacts with a diameter of 15 mm under a pressure of 2.0 MPa. The role of ethanol was to reduce the friction among the CaCO3 particles for uniform compression. No other additives such as dispersants or binders were used. The green compacts were sintered in air at temperatures between 693 and 813 K for 3 h. A heating rate of 10 K/min was used. Fig. 1 shows the micrographs of both the powders obtained after mixing with the sintering aid.
CaCO3 %CaO þ CO2
2.2. Characterization
(1)
However, numerous potential advantages exhibited by the calcium carbonate monoliths including their biological compatibility have prompted many attempts to consolidate calcium carbonate into bulk monoliths using various methods. Ishikawa et al. [6e8] prepared a porous monolith of CaCO3 having a porosity of 40% by exposing Ca(OH)2 green bodies to an atmosphere of CO2 and H2O, with a strategy based on the setting phenomenon of lime plaster. Tempieri et al. [9] fabricated porous calcium carbonate monoliths having a microstructure from natural wood via a multistep process starting from pyrolysis of wood followed by the successive conversion of the resultant carbon to CaC2, CaO, and subsequently to CaCO3 without disturbing the microstructure. Attempts for preparing dense CaCO3 ceramics have been carried out employing an extremely high pressure of 1 GPa [10], pulsed direct current hotpressing [11], hydrothermal hot-pressing [12e14] or sintering of calcite under high-pressure CO2 atmosphere (0.2 MPa) [15,16], and the latter method resulted in the production of a material with a high relative density of 93.4% as compared to the theoretical density of calcite (2.711 g/cm3). Pressureless sintering of calcite in the presence of sintering aids has also been investigated. He et al. [17,18] sintered calcite at 923 K in air using phosphate glass as the sintering aid: however, no description on the sintered density was provided. Other sintering aids such as lithium fluoride [19,20], lithium carbonate [21], sodium carbonate [22,23], and lithium phosphate have been used [24]; however, the properties of the resultant ceramics have not been well described. In the present work, we prepared dense calcite ceramics by pressureless sintering in air. Eutectic mixture of the KF - NaF - LiF system was selected for the process for achieving the liquid phase formation at a temperature as low as possible. Monodisperse calcite fine powder was used for achieving a high sinterability. The physical, mechanical, thermal, and electrical properties of the resultant ceramics were evaluated. 2. Experimental 2.1. Sample synthesis Two kinds of calcite powders were used as the raw materials. Powder A, monodisperse calcite powder with an average diameter of approximately 150 nm and a chemical purity of 99.8%, was supplied from Shiraishi Kogyo Kaisha Ltd., Hyogo, Japan. Powder B, a calcite powder of commercial chemical reagent having an average diameter of 12e15 mm and a chemical purity of 99.5%, was obtained from Kanto Chemical Co., Inc., Tokyo, Japan. The sintering aid was prepared by mixing KF, NaF, and LiF in the molar ratio of 40: 11: 49, which corresponds to the eutectic composition of the ternary system with the eutectic temperature of 736 K [25]. The sintering aid was mixed with powder A or B in the ratios of 0.3, 0.6, 1.2, 2.5, and 5.0 mass%. The mixed powders were dry-milled overnight to ensure homogeneous mixing. The powder samples were then slightly wet with a small amount of ethanol (typically 3 drops of
In order to roughly estimate the liquid formation temperature, differential thermal analysis (DTA) measurements were carried out with the pristine sintering aid and 50: 50 mixture (mass ratio) of powder A and the sintering aid. The DTA measurements were conducted at a heating rate of 10 K/min with a thermal analyzer (Evo II, Rigaku, Tokyo, Japan). The sintered specimens were characterized using a scanning electron microscope (SEM, e-SEM, Shimadzu Rika Corp., Tokyo, Japan and JSM-6330F, JEOL, Ltd., Tokyo, Japan) for examining their microstructure, an X-ray diffractometer (XRD, Mini Flex, Rigaku, Tokyo, Japan) for determining the crystalline phases, and by the Archimedean method using ethanol as the substituting liquid for measuring the relative density. The density of calcite, 2711 kg/m3, was used as the theoretical density because of the following two reasons: the real density of the reaction products in the KF - NaF -
(a)
1µm (b)
10µm Fig. 1. SEM images of the CaCO3 powders mixed with the sintering aid; (a) powder A and (b) powder B.
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LiF - CaCO3 system was unknown and the optimum amount of the sintering aid was as low as 0.6 mass% as will be described later, resulting in a negligible perturbation of the theoretical density with the addition of the sintering aid. The most densely sintered specimen, powder A mixed with 0.6 mass% sintering aid and sintered at 783 K, was characterized for determining the following properties: dielectric constant and dielectric loss in the frequency range of 5 MHz - 1 GHz at room temperature measured with an impedance analyzer using a capacitance method (E4991B, Keysight Technologies, Tokyo, Japan), heat capacity and thermal diffusivity using the laser flash method (TC-9000, ULVAC Inc., Kanagawa, Japan and LFA 501, Kyoto Electronics Manufacturing Co., Ltd., Kyoto, Japan), Vickers hardness (HV) with a hardness tester under a load of 1.0 kg (HV-115, Mitutoyo Corp., Kanagawa, Japan), Young’s modulus (E) and Poisson ratio (n) using an ultrasonic pulse method (5900PR, GE sensing & Inspection Technologies, Tokyo, Japan), as well as the electrical conductivity and the electrical polarizability. Vicker’s hardness was measured 5 times and the thermal properties were measured 3 times. Elastic, dielectric and electric properties were measured once. The details of the electrical measurements are described elsewhere [26]. In brief, the AC impedance spectrometry measurements were carried out using an impedance analyzer (1260, Solarton Analytical, Leicester, UK) with an AC modulation at 1 Hz to 10 MHz in the temperature range of 300e630 K. The electrical conductivity was calculated using the relation: s ¼ d=ðARbulk Þ, where d, A, and Rbulk are the sample thickness, electrode area, and bulk resistance, respectively. The bulk resistance was obtained using the Nyquist plot method. Electric polarizability was evaluated by carrying out the thermally stimulated depolarization current (TSDC) measurements of the electrically polarized sample under constant heating up to 673 K at a heating rate of 5 K/min. The sample was electrically polarized under a DC voltage of 400 V (2.3 kV/cm) in air at 573 K for 30 min before being subjected to the TSDC measurements. 3. Results and discussion Fig. 2 shows the effect of sintering temperature on the relative density of the powders A and B. The powder A exhibited higher density than powder B. Regardless of the amount of the sintering aid added, the relative density of the compacts of powder A exceeded 98% after sintering above certain temperatures. The highest relative density of 99.0% was obtained for the powder A mixed with 0.6% of the sintering aid and sintered at 723 K. The highest relative density reported for the CaCO3 ceramics is 98.7% [24], for the sample sintered with 8% Li3PO4 as the sintering aid under CO2 atmosphere (4 kPa) at 973 K for 15 h. In the present study, a higher relative density was attained using a lesser amount of the sintering aid added at a lower sintering temperature and sintered for a shorter sintering time than the previous study. For powder A, the onset of the densification lowered with an increase in the amount of sintering aid used. In the case of the powders mixed with the sintering aid in 0.3, 0.6, and 1.2%, a sharp increase in the relative density was observed below 783, 723, and 693 K, respectively. Slight decreases in the relative density were observed with increasing sintering temperature; for example, the relative density of a sample with 1.2% sintering aid became lower above 750 K. Although we have not fully elucidated the reason, there are two possibilities for the slight decrease in the relative density; one is the evaporation of the sintering aid, and the other is the formation of CO2 through the reactions of alkali fluorides with calcium carbonate whose example is expressed in Eq. (2).
CaCO3 þ 2 NaF/ CaF2 þ Na2 O þ CO2
(2)
Fig. 2. The effect of the sintering temperature and the amount of the sintering aid on the relative density of compacts of (a) powder A and (b) powder B.
On the other hand, for powder B, a relative density higher than 92.0% could not be reached. The highest value obtained for the powder B was 91.7% when the powder was mixed with 1.2% sintering aid and sintered at 723 K. The addition of 2.5 and 5.0% sintering aid to both the powders caused swelling of the samples after the sintering under any condition (data not shown), which may be accounted for the reactions exemplified by Eq. (2). Therefore, the optimum amount of the sintering aid is smaller than 2%. The sintering behaviors of both the powders can be understood by observing the microstructural change of the sintered compacts. Fig. 3 shows the evolution of the microstructure of powder A containing 0.6% sintering aid. The sintered specimens were polished and thermally etched before the SEM observations. After sintering at 693 K, the particles coalesced to form grains of approximately 1 mm in average diameter [Fig. 3(a)]. This suggests that the liquid phase can be obtained below 693 K even though the eutectic temperature of the KF - NaF - LiF system is 736 K (discussed later). The grain size shown in Fig. 3 (a) is found to be quite homogeneous throughout the sintered specimen. With a further increase in the sintering temperature to 723 and 753 K, the sintering of the grains occurred while eliminating voids and without
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(a)
5µm (c)
5µm
(b)
5µm (d)
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temperature; however, numerous voids were still present in the specimen. After performing sintering at 783 K, an extensive grain growth occurred and a number of pores were observed within the grains. The sintering process aided by the KF - NaF - LiF eutectic mixture is discussed below. Fig. 5 shows the DTA curves of the pristine sintering aid and the sintering aid mixed with powder A. In the case of the pristine sintering aid, a large endothermic event initiated at around 731 K, in good accordance with the reported eutectic temperature, 736 K. When the sintering aid was mixed with CaCO3, the onset of the first endothermic event lowered to 684 K. This may account for the coalescence of the primary particles of powders A
5µm
Fig. 3. Evolution of the microstructure of powder A containing 0.6 mass% sintering aid with an increase in the sintering temperature. (a) 693, (b) 723, (c) 753, and (d) 783 K.
indicating a prominent grain growth; the average grain size obtained after sintering at 753 K was found to be approximately 2 mm. However, a sudden grain growth occurred to form grains larger than a few tens of micrometers at 783 K. The microstructure evolution for the powder B was found to be quite different from that for the powder A, as shown in Fig. 4. After being sintered at 693 K, grain growth in powder B proceeded in an inhomogeneous manner; some of the grains coalesced, thereby leaving large voids as shown in Fig. 4 (a). The size of the grains was not homogeneous in contrast to that obtained with the powder A [Fig. 3 (a)]. Sintering proceeded upon increasing the sintering
(a)
5µm
(c)
5µm
Fig. 5. DTA profiles of (a) the sintering aid and (b) sintering aid mixed with CaCO3 (mass ratio of 50: 50).
(b)
5µm
(d)
5µm
Fig. 4. Evolution of the microstructure of powder B containing 0.6 mass% sintering aid with an increase in the sintering temperature. (a) 693, (b) 723, (c) 753, and (d) 783 K.
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and B after being sintered at 693 K. The curve shown in Fig. 5 (b) also indicated second and third endothermic responses around 750 and 810 K, suggesting an increase in the amount of liquid in the powder compact. The extensive grain growth observed for the samples sintered at 783 K [Fig. 3 (d) and 4 (d)] could be attributed to the increase in the amount of the liquid and the solubility. Fig. 6 shows the XRD patterns of the powder A mixed with 0.6% sintering aid before and after sintering the sample at various temperatures. The peaks corresponding to the sintering aid could not be observed since its amount was too small. There was no indication of the decomposition of calcite to calcium oxide even after performing the sintering at 813 K. Table 1 summarizes the thermal and mechanical properties of 99.0% dense calcite ceramics. A thermal conductivity of 2.60 ± 0.003 W/(m$K) was obtained, in good accordance with the value of the calcite single crystal, 2.6 W/(m$K), as reported by Straube et al. [27]. The dielectric constant as a function of frequency is shown in Fig. 7, and the value obtained at 1 GHz, 8.56, is close to that of the calcite single crystal, 7.8, as reported by Veeraish et al. [28]. The difference in the values between the calcite ceramics and the single crystal might be attributed to the presence of the alkalirich grain boundary phase. Still, the addition of the sintering aid did not cause a heavily deteriorative effect on the dielectric properties. The hardness of the calcite single crystal indicates anisotropy and a dependence on the load [29]. Carter et al. reported the Knoop hardness (HK) of calcite single crystals measured under 10 g load to be in the range of 1.7e2.0 GPa depending on the cleavage plane while it decreased to 0.6e1.5 GPa under 100 g load [29]. Although HV and HK are different parameters and cannot be directly compared, these values have a similar order of magnitude, while HK is slightly lower than HV of the same material [30]. In the present work, HV was measured under a load of 1 kg in order to obtain sufficiently larger indentation marks than the grain sizes, resulting in lower HV values than those reported in the previous study [29]. The HV value could be affected by the presence of grain boundary phases, pores, and microcracks in the sintered ceramics. Considering these factors, an average HV value of 0.323 GPa is considered a rational value. The Young’s modulus, E and the Poisson ratio, n were measured to be 49 GPa and 0.25, respectively. According to Lin [31], the bulk (K) and shear (G) moduli of the calcite single crystal were 79.57 and 32.23 GPa, respectively, both with a very small temperature dependence, and a Poisson ratio was 0.322. E and n values calculated from the data by Lin using Eqs. (3) and (4) by assuming isotropic elastic response were 84 GPa and 0.322, respectively. Hence, the measured E and n were lower than the calculated values.
Fig. 6. XRD patterns of powder A mixed with 0.6% sintering aid (a) before and after sintering at (b) 693, (c) 753, and (d) 813 K.
Table 1 The physical properties of 99.0% dense calcite ceramics. Thermal conductivity
2.60 ± 0.003 W/(m K)
Dielectric constant (1 GHz) tan d (1 GHz) Vicker’s hardness (1 kg load) Young’s modulus Poisson ratio
8.56 0.0022 61.2 ± 3.4 Hv (0.323 GPa) 49 GPa 0.25
Fig. 7. Dielectric constant and tand of 99.0% dense calcite ceramics as a function of frequency.
E ¼ 9KG=ð3K þ GÞ
(3)
n ¼ ð3K 2GÞ=ð6K þ 2GÞ
(4)
The discrepancies of the measured and calculated E and n values should be attributed to the difference in the microstructures of the single crystal and sintered ceramics. The presence of pores and grain boundary phases might affect the elastic properties of the samples to a large extent. Fig. 8 shows the Arrhenius plot of electrical conductivity of calcite ceramics. The data corresponding to the calcite single crystal along (10.4) and (10.0) orientations obtained from Ref. [26] are also given in the figure. The activation energy for conduction of the calcite ceramics was 212.2 kJ/mol. The calcite ceramics exhibited
Fig. 8. Electrical conductivity of 99.0% dense calcite ceramics as a function of temperature. Data of the calcite single crystal along (10.4) and (10.0) orientations obtained from Ref. [26] are also shown.
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higher electrical conductivity and activation energy for conduction as compared with the single crystal calcite, 112e119 kJ/mol [26]. The higher conductivity suggests a higher concentration of electric carriers, possibly mobile ions and ionic vacancies in this case. The existence of a grain boundary might enhance the activation energy for conduction. However, since the nature of the grain boundary of the calcite ceramics is not well-understood, a speculative discussion on this cannot be provided. The TSDC spectrum of the calcite ceramics is shown in Fig. 9. The dissipation of electric current began on heating the specimen to approximately 580 K. The current density in the order of several hundred nA/cm2 (107 A/cm2) was obtained. Yamashita et al. measured the dissipated currents from calcite single crystal [26] and hydroxyapatite ceramics [32]. The dissipated current densities were typically in the order of several nA/cm2 for hydroxyapatite ceramics [32] and a few tens of nA/cm2 for calcite single crystal [26]. Therefore, our calcite ceramics dissipated much larger currents than the calcite single crystal or hydroxyapatite ceramics. This might imply that the calcite ceramics could be electrically polarized to larger extents than the single crystal or hydroxyapatite samples. The electrical polarization of hydroxyapatite enhanced the formation of bone-like apatite layers in simulated body fluid [32,33], osteoblast adhesion in vitro [34], and bone ingrowth into porous hydroxyapatite in vivo [35]. Hence, our calcite ceramics might exhibit enhanced biological properties on subjecting to electrical polarization treatment. The validation of the biological properties of the polarized calcite as well as the understanding the detailed mechanism of polarization should be carried out in the future.
4. Conclusions We demonstrated that calcite could be densely sintered up to a relative density of 99.0% pressureless sintering carried out in air. The eutectic mixture of the KF - NaF - LiF system was found to be a good sintering aid, which effectively promoted densification and grain growth when added at 0.3e1.2%. The highest relative density was attained by sintering the sample at 723 K for 3 h with 0.6% sintering aid. Some of the physical properties including the thermal conductivity, dielectric constant, Vickers hardness, and elastic properties were measured. Calcite ceramics could be electrically polarized, and the dissipated current density was higher than that of calcite single crystal and hydroxyapatite ceramics samples.
Fig. 9. TSDC spectrum of 99.0% dense calcite ceramics.
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