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Role of ion substitution and lattice water in the densification of cold-sintered hydroxyapatite
Scripta Materialia 177 (2020) 141–145
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Role of ion substitution and lattice water in the densification of cold-sintered hydroxyapatite Hui-Zhen Shen, Ning Guo, Liang Zhao, Ping Shen∗ Key Laboratory of Automobile Materials (Ministry of Education), School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 15 August 2019 Revised 10 October 2019 Accepted 14 October 2019 Available online 24 October 2019 Keywords: Hydroxyapatite Cold sintering Lattice water Densification
a b s t r a c t High-density hydroxyapatite (HA) compacts were achieved by cold sintering of co-precipitated powders at temperatures lower than 200 °C without addition of any solvent. Critical factors affecting the densification behavior including the source of powders, the type of solvent, applied pressure, sintering temperature and time were investigated. The presence of lattice water together with ion substitution in the co-precipitated HA powders has a significant effect on the densification. A novel viewpoint that is essentially different from classical dissolution−precipitation mechanism in cold sintering was proposed; i.e., the formation of hydrogen bonds between the HA particles plays a critical role in the densification. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
In the nature of water-bearing environment, large-scale geological movement allows dense mineral materials (such as limestone) to be obtained under mild conditions [1]. In contrast, artificial ceramics need the temperatures over 1500 °C for densification. Inspired by the phenomena of natural mineral deposition and biomineralization, Randall et al. developed a cold sintering process (CSP) [2−5], which can achieve high densification of selected materials by adding transient solvent to powders under low temperatures (< 300 °C) and high pressures (350−500 MPa). As demonstrated, materials with higher solubility in solvents are easier to densify in CSP, and a dissolution−precipitation mechanism was thus proposed and has so far been widely accepted [4,5]. In the CSP, solvent is a crucial factor that provides the necessary environment for the dissolution and transportation among particles. The applied pressure promotes the rearrangement of particles and increasing temperature leads to the volatilization of the solvent. As a consequence, the solute is oversaturated and precipitates, and then the particles are densified under high pressure. However, Hassan and Ryu [6] have recently reported the synthesis of high-density iodine-substituted hydroxyapatite without adding any solvent via CSP, which obviously does not conform to the classical dissolution−precipitation mechanism. Nevertheless, they did not pay attention to this important issue. As known, hydroxyapatite (HA, Ca10 (PO4 )6 (OH)2 ) is a widely used biomaterial. Its chemical composition and crystal structure
∗
Corresponding author. E-mail address: [email protected] (P. Shen).
https://doi.org/10.1016/j.scriptamat.2019.10.024 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
are similar to those of human bones and teeth. Also, the solubility of HA in water is only 0.4 ppm at room temperature. Traditional sintering of HA requires a temperature of over 1200 °C for densification. However, high temperature not only consumes time and energy, but also causes HA to lose hydroxyl groups, destroy its structural stability and reduce biological activity [7]. In addition, in the case of some important yet volatile components (such as Mg and Zn) being doped, high temperature sintering tends to lose these components and leads to the change in stoichiometry. Therefore, low-temperature sintering techniques are preferred and nowadays receive wide attention [7–9]. In this work, we investigated the feasibility to obtain high-density HA via CSP and further unraveled its densification mechanism. The HA powders used in this work were mainly synthesized from diammonium hydrogen phosphate (0.5 M) and calcium nitrate (0.3 M) solutions at room temperature using a co-precipitation method (hereafter named HA1). In addition, several commercial HA powders (HA2−HA4) were also employed for comparison. Table 1 lists all kinds of the HA powders used in this work and their characteristics. In the co-precipitation synthesis of HA1, the pH value was maintained at 11 by addition of an ammonium hydroxide solution. The diammonium hydrogen phosphate solution was slowly added to the calcium nitrate solution and the mixture was stirred at 500 r/min for more than 2 h. After maturation for 5 h, the precipitates were repeatedly washed until the pH in the solution was near neutral. Then the precipitates were dried in a blast drying chamber at 80 °C for 12 h. The morphology and sizes of the home-made HA1 powders were observed with a field emission electron microscope (FE-SEM, JSM-6700F, JEOL, Japan) and a
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Fig. 1. Variations in the relative density of cold-sintered compacts with (a) powder type, (b) liquid type, (c) liquid content, (d) applied pressure, (e) sintering temperature and (f) heat-treatment temperature of the initial HA1 powders (The meanings of HA1−HA4 refer to Table 1). Table 1 Characteristics of hydroxyapatite powders. Type Particle size Manufacturer HA1 HA2 HA3 HA4
20 ± 6 nm 20−40 nm < 100 nm 30 μm
Purity
Prepared by co-precipitation method in this work − Shanghai McLean Biotechnology Co., Ltd. 97% Aladdin Reagent (Shanghai) Co., Ltd. 97% Shaanxi Kangyue Biotechnology Co., Ltd. 98%
transmission electron microscope (TEM, JEM-2100F, JEOL, Japan), showing round-rod shapes and an average size of 20 ± 6 nm. The phases in the precipitates were characterized by X-ray diffraction (XRD, D/max 2500 pc, Rigaku, Japan), revealing high purity of the product and weak crystallinity as compared with commercial powders (Fig. S1). The composition was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, Ulitima Expert LT, HORIBA, France), confirming a Ca/P ratio of 1.66−1.67. The thermal stability was studied by a thermogravimetric analyzer (TG/DTA, SDT Q600, TA, America), and the variation in the HA structure after heat treatment at different temperatures was investigated by Fourier transform infrared (FTIR, Nexus 670, Nicolet, America) spectroscopy in the range of 40 0−40 0 0 cm−1 with a resolution of 4 cm−1 . Fig. S2 shows the schematic of cold sintering device used in this work. The HA powders (weighing approx. 0.5 g) were placed in a mold with an inner diameter of 1/2 inch without addition of any binder or additive. The powders were then pressed under a uniaxial pressure and the mold was heated at 10 °C/min to a predetermined temperature, which was calibrated prior to the experiment. The holding time defined here refers to the duration after the temperature reached the target value. After cooling, the relative density of the cold-sintered compacts was calculated using a theoretical density of 3.156 g/cm3 [10] and an experimental value measured from mass and geometric sizes. The microstructures of the sintered samples were observed with FE-SEM.
Fig. 1 shows the effects of raw materials (HA1−HA4) and processing parameters on the relative density of the coldsintered compacts. Clearly, compared with commercial powders (HA2−HA4), only the HA1 powders can achieve high density (Fig. 1a) with a relative value of over 90%, which is generally consistent with the finding of Hassan and Ryu [6]. However, note that the type of solvents (deionized water, ammonium hydroxide (14 M) and acetic acid solution (1 M)) has an insignificant effect on the densification for HA1, HA2 and HA3 powders (Fig. 1b). In addition, the liquid content plays a mild role (Fig. 1c). Furthermore, the effect of holding time is also very weak. Noticeable densification can be achieved even after holding at 150 °C for 1 − 2 min under a uniaxial pressure of 500 MPa, but prolonging time does not yield a further significant increase (Fig. S3). In contrast, the pressure and temperature have a prominent effect. With an increase in the applied pressure, the relative density dramatically increases but later shows a plateau after the pressure reaches 350 MPa (Fig. 1d). The effect of temperature can be divided into three stages (Fig. 1e): (i) at 100 °C−175 °C, where the relative density increases with increasing temperature; (ii) at 175 °C−200 °C, where the relative density keeps almost constant; (iii) at 200 °C−250 °C, where the density slightly decreases with further increasing temperature. In addition, we found that heat treatment of the HA1 powders had a significant effect on the sintered density. If HA1 was heat-treated at 150 °C, the relative density still reached 90%. However, the density decreased with increasing heat-treatment temperature (Fig. 1f). When the heat-treatment temperature reached 450 °C−550 °C, the relative density stabilized at approx. 68%. The addition of 10 wt.% deionized water to the heat-treated powders only brought about a 2% − 5% promotion in the relative densities. This result clearly suggests that the densification ability of HA1 should be deteriorated by high-temperature heat treatment, and this change is irreversible; i.e., the densification ability cannot be fully restored by the later addition of water to the HA1 powders which were initially heat-treated at high temperatures. Fig. 2 shows the fracture
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Fig. 2. FE-SEM morphology of the fracture surfaces of the cold-sintered HA compacts using the HA1 powders heat-treated at different temperatures: (a) 80 °C−12 h; (b) 150 °C−1 h; (c) 250 °C−1 h; (d) 350 °C−1 h; (e) 450 °C−1 h and (f) 550 °C−1 h (the insets show the statistic diagrams of particle size distribution obtained by using Nano Measurer software).
surface morphology of the cold-sintered compacts using the HA1 powders heat-treated at different temperatures. When the powders were initially treated at 80 °C or 150 °C, the compacts were dense and the grain boundaries were connected to each other. With increasing heat-treatment temperature, however, tiny pores were observed at the fracture surfaces. In addition, the particle sizes showed a moderate increase (Fig. 2). Fig. 3(a) presents the XRD patterns of the HA1 powders heattreated at different temperatures. When the heat-treatment temperature increased to 450 °C and 550 °C, some characteristic peaks became slightly sharper, suggesting better crystallization of the particles. The lattice parameter in the a-axis of the HA1 crystal decreased with increasing heat-treatment temperature (Fig. S4) until it reached the standard value (a = 9.432 A˚ in ICCD PDF #54–0022). This behavior is important, implying some structural changes in the HA crystal after heat treatment. Previously, LeGeros et al. [11] reported that the lattice parameter of a-axis increased due to the presence of lattice water in precipitated apatite and it decreased during continuous heating due to the loss of lattice water. They divided the TG curve of the precipitated apatite into two stages with a critical point at 200 °C, and attributed the mass reduction in the first stage to the loss of adsorbed water while that in the second stage to the loss of lattice water. Similar conclusion was also drawn by Reyes-Gasga et al. [12] in their TG analysis on the co-precipitated hydroxyapatite. Here, we examined the thermal stability of the HA1 and HA2 powders at a heating rate of 5 °C/min. As shown in Fig. 3(b), the mass reduction in HA1 was approx. 4.2% when temperature was not more than 200 °C, and nearly 2.3% at 200 °C−550 °C. We speculate that a certain amount of water was still adsorbed at the surface of the HA1 particles even though the powders were pre-treated at 80 °C for 12 h in a blast drying chamber. Therefore, the mass reduction in the first stage is due to the loss of adsorbed water. When the heat-treatment temperature was higher than 200 °C, the lattice water was gradually released. There-
fore, the mass decrease in the second stage is mainly attributed to the loss of lattice water. By contrast, HA2 has an almost negligible mass loss at temperatures above 200 °C. Clearly, the HA2 powders do not contain lattice water in their crystals, which is presumably related to the unsuccessful densification under CSP. In order to explore the mechanisms for the formation of lattice water in HA1 and its role in the densification in CSP, we performed FTIR analyses on the HA1 powders heat-treated at different temperatures as well as on HA2 for comparison. As shown in Fig. 3(c), HA1 treated at 80 °C and 150 °C displayed a distinct peak in the region of 850−880 cm−1 owing to the presence of HPO4 2− (P–O–H) group [11,13,14], suggesting the occurrence of partial replacement of PO4 3− (O–P–O group) by HPO4 2- when HA1 precipitated from aqueous solution. However, the intensity of the P–O– H group weakened while that of the O–P–O group strengthened with an increase in the heat-treatment temperature, implying the transformation from HPO4 2− to PO4 3− (or other ions belong to the O–P–O group) during the heat treatment. Compared with HA1, HA2 shows almost negligible peak intensity in this range (850−880 cm−1 ), similar to the case in the HA1 powders heat-treated at 450 °C or 550 °C. In addition, the broad valleys at 1620 cm−1 and 330 0−360 0 cm−1 (corresponding to H2 O) in HA1 gradually weakened with the increase in heat-treatment temperature. When the temperature reached 550 °C, the valley changed to a plateau. Most importantly, the intensity of the hydroxyl peaks at 631 cm−1 and 3650 cm−1 gradually increased with the increase in the heattreatment temperature, especially at 631 cm−1 . To illustrate the mechanism, Fig. 4(a) schematically shows the crystal structure of HA. Some calcium ions and portions of phosphate tetrahedra that lie beyond the boundaries (dashed lines) of the unit cell are included here to help define the channels occupied by hydroxyl ions [15]. Since HA1 is prepared in aqueous solution, it is reasonable that water molecules could partially replace hydroxyl groups [11], resulting in the formation of lattice
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Fig. 3. (a) XRD patterns for the HA1 powders heat-treated at different temperatures; (b) TG curves of HA1 and HA2 heated at 5 °C/min; (c) FTIR analysis on the HA1 powders heat-treated at different temperatures with that of the HA2 powders as a reference.
Fig. 4. Schematic illustrations of (a) HA structure and (b−c) two models of ion substitution and formation of hydrogen bond in HA1 during CSP (since Ca ions are not involved in ion substitution and formation of hydrogen bond, they are not shown in diagrams (b) and (c)).
water. As a consequence, the intensity of the hydroxyl peaks in the infrared spectra (631 and 3650 cm−1 ) decreased. Correspondingly, a hydrogen bond could form between the hydrogen atom in the lattice water and the oxygen atom in the nearby O–P–O group [16] (see Fig. 4(b)). Another possibility is that HPO4 2− partly replaced PO4 3− in HA [11,17]. Similarly, a hydrogen bond could develop between the oxygen atom in the hydroxyl group and the hydrogen atom in the HPO4 2− group [18,19], as illustrated in Fig. 4(c). Such ion substitutions led to an increase in the lattice param-
eter of a-axis and thus an unstable state of the HA structure. During CSP, the presence of a small amount of adsorbed water at the particle surface acted like a liquid film, reducing the friction between particles during compression [20]. The application of high pressure provided a driving force for the particle sliding and redistribution [6]. Moreover, the formation of hydrogen bonds greatly enhanced the attraction between the HA1 particles, shortened their distance, promoted their rearrangement, and thus led to the densification. However, when the HA1 powders were heat-
H.-Z. Shen, N. Guo and L. Zhao et al. / Scripta Materialia 177 (2020) 141–145
treated at temperatures over 200 °C, the lattice water was gradually discharged along the c-axis in a continuous manner by exchanging protons with the surrounding hydroxyl groups [21]; or alternatively, hydrogen phosphate decomposed to release lattice water in the form of 2HPO4 2− =P2 O7 4− +H2 O [11]. The discharge of the lattice water led to the strengthening of the hydroxyl and phosphate peaks and weakening of the hydrogen phosphate peak in the FTIR spectra as well as the recovery of the lattice parameter of HA1 to the standard value (Fig. S4). Simultaneously, the hydrogen bonds were weakened or even finally disappeared with the increase in the heat-treatment temperature, leading to significant deterioration in the densification of the samples (Fig. 1f). This effect was unrecoverable by the addition of deionized water to the heat-treated HA1 powders since the lattice water is essentially different from the physically absorbed water (Fig. 1f). On the other hand, the hydrogen bond did not exist in the HA2 powders because of the absence of ion substitution and lattice water. Therefore, the commercial HA2 powders failed to achieve high density under the CSP. In summary, high-density hydroxyapatite compacts were prepared by CSP using co-precipitated HA powders. The occurrence of ion substitution and presence of lattice water contribute to the formation of hydrogen bonds between the HA particles. The synergistic function of hydrogen bond and high pressure promotes the densification process. This finding broadens the materials selection for cold sintering and provides a potential roadmap for lowtemperature densification of the materials with negligible solubility. Declaration of Competing Interest We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the Changbai Mountain Scholars Program of Jilin Province (No. 2015011).
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Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.scriptamat.2019.10. 024. References [1] F. Bouville, A.R. Studart, Nat. Commun. 8 (2017) 14655. [2] J. Guo, H.Z. Guo, A.L. Baker, M.T. Lanagan, E.R. Kupp, G.L. Messing, C.A. Randall, Angew. Chem. Int. Ed. 55 (2016) 11457–11461. [3] J.P. Maria, X.Y. Kang, R.D. Floyd, E.C. Dickey, H.Z. Guo, J. Guo, A. Baker, S. Funihashi, C.A. Randall, J. Mater. Res. 32 (2017) 3205–3218. [4] H.Z. Guo, A. Baker, J. Guo, C.A. Randall, J. Am. Ceram. Soc. 99 (2016) 3489–3507. [5] J. Guo, R. Floyd, S. Lowum, J.P. Maria, T. Herisson de Beauvoir, J.H. Seo, C.A. Randall, Annu. Rev. Mater. Res. 49 (2019) 275–295. [6] M. Hassan, H.J. Ryu, J. Nucl. Mater. 514 (2019) 84–89. [7] E. Champion, Acta Biomater. 9 (2013) 5855–5875. [8] K.L. Montoya-Cisneros, J.C. Rendón-Angeles, Z. Matamoros-Veloza, A. Matamoros-Velozac, K. Yanagisawa, Ceram. Int. 43 (2017) 11907–11919. [9] Y.W. Gu, N.H. Loh, K.A. Khor, S.B. Tor, P. Cheang, Biomaterials 23 (2002) 37–43. [10] M. Eriksson, Y. Liu, J.F. Hu, L. Gao, M. Nygren, Z.J. Shen, J. Eur. Ceram. Soc. 31 (2011) 1533–1540. [11] R.Z. LeGeros, G. Bonel, R. Legros, Calcif. Tissue Res. 26 (1978) 111–118. [12] J. Reyes-Gasga, R. García-García, M.J. Arellano-Jiménez, E. Sanchez-Pastenes, G.E. Tiznado-Orozco, I.M. Gil-Chavarria, G. Gómez-Gasga, J. Phys. D: Appl. Phys. 41 (2008) 225407. [13] C. Rey, M. Shimizu, B. Collins, M.J. Glimcher, Calcif. Tissue Int. 46 (1990) 384–394. [14] J. Arends, C.L. Davidson, Calcif. Tissue Res. 18 (1975) 65–79. [15] J.D. Pasteris, C.H. Yoder, B. Wopenka, Am. Miner. 99 (2014) 16–27. [16] S.J. Joris, C.H. Amberg, J. Phys. Chem. 75 (1971) 3172–3178. [17] E.E. Berry, J. Inorg. Nucl. Chem. 29 (1967) 317–327. [18] E.E. Wilson, A. Awonusi, M.D. Morris, D.H. Kohn, M.M.J. Tecklenburg, L.W. Beck, Biophys. J. 90 (2006) 3722–3731. [19] S.J. Joris, C.H. Amberg, J. Phys. Chem. 75 (1971) 3167–3171. [20] X.Y. Kang, R. Floyd, S. Lowum, M. Cabral, E. Dickey, J.P. Maria, J. Am. Ceram. Soc. 102 (2019) 4459–4469. [21] C. Yoder, J. Pasteris, K. Worcester, D. Schermerhorn, M. Sternlieb, J. Goldenberg, Z. Wilt, Miner. 2 (2012) 100–117.
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Role of ion substitution and lattice water in the densification of cold-sintered hydroxyapatite Hui-Zhen Shen, Ning Guo, Liang Zhao, Ping Shen∗ Key Laboratory of Automobile Materials (Ministry of Education), School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 15 August 2019 Revised 10 October 2019 Accepted 14 October 2019 Available online 24 October 2019 Keywords: Hydroxyapatite Cold sintering Lattice water Densification
a b s t r a c t High-density hydroxyapatite (HA) compacts were achieved by cold sintering of co-precipitated powders at temperatures lower than 200 °C without addition of any solvent. Critical factors affecting the densification behavior including the source of powders, the type of solvent, applied pressure, sintering temperature and time were investigated. The presence of lattice water together with ion substitution in the co-precipitated HA powders has a significant effect on the densification. A novel viewpoint that is essentially different from classical dissolution−precipitation mechanism in cold sintering was proposed; i.e., the formation of hydrogen bonds between the HA particles plays a critical role in the densification. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
In the nature of water-bearing environment, large-scale geological movement allows dense mineral materials (such as limestone) to be obtained under mild conditions [1]. In contrast, artificial ceramics need the temperatures over 1500 °C for densification. Inspired by the phenomena of natural mineral deposition and biomineralization, Randall et al. developed a cold sintering process (CSP) [2−5], which can achieve high densification of selected materials by adding transient solvent to powders under low temperatures (< 300 °C) and high pressures (350−500 MPa). As demonstrated, materials with higher solubility in solvents are easier to densify in CSP, and a dissolution−precipitation mechanism was thus proposed and has so far been widely accepted [4,5]. In the CSP, solvent is a crucial factor that provides the necessary environment for the dissolution and transportation among particles. The applied pressure promotes the rearrangement of particles and increasing temperature leads to the volatilization of the solvent. As a consequence, the solute is oversaturated and precipitates, and then the particles are densified under high pressure. However, Hassan and Ryu [6] have recently reported the synthesis of high-density iodine-substituted hydroxyapatite without adding any solvent via CSP, which obviously does not conform to the classical dissolution−precipitation mechanism. Nevertheless, they did not pay attention to this important issue. As known, hydroxyapatite (HA, Ca10 (PO4 )6 (OH)2 ) is a widely used biomaterial. Its chemical composition and crystal structure
∗
Corresponding author. E-mail address: [email protected] (P. Shen).
https://doi.org/10.1016/j.scriptamat.2019.10.024 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
are similar to those of human bones and teeth. Also, the solubility of HA in water is only 0.4 ppm at room temperature. Traditional sintering of HA requires a temperature of over 1200 °C for densification. However, high temperature not only consumes time and energy, but also causes HA to lose hydroxyl groups, destroy its structural stability and reduce biological activity [7]. In addition, in the case of some important yet volatile components (such as Mg and Zn) being doped, high temperature sintering tends to lose these components and leads to the change in stoichiometry. Therefore, low-temperature sintering techniques are preferred and nowadays receive wide attention [7–9]. In this work, we investigated the feasibility to obtain high-density HA via CSP and further unraveled its densification mechanism. The HA powders used in this work were mainly synthesized from diammonium hydrogen phosphate (0.5 M) and calcium nitrate (0.3 M) solutions at room temperature using a co-precipitation method (hereafter named HA1). In addition, several commercial HA powders (HA2−HA4) were also employed for comparison. Table 1 lists all kinds of the HA powders used in this work and their characteristics. In the co-precipitation synthesis of HA1, the pH value was maintained at 11 by addition of an ammonium hydroxide solution. The diammonium hydrogen phosphate solution was slowly added to the calcium nitrate solution and the mixture was stirred at 500 r/min for more than 2 h. After maturation for 5 h, the precipitates were repeatedly washed until the pH in the solution was near neutral. Then the precipitates were dried in a blast drying chamber at 80 °C for 12 h. The morphology and sizes of the home-made HA1 powders were observed with a field emission electron microscope (FE-SEM, JSM-6700F, JEOL, Japan) and a
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Fig. 1. Variations in the relative density of cold-sintered compacts with (a) powder type, (b) liquid type, (c) liquid content, (d) applied pressure, (e) sintering temperature and (f) heat-treatment temperature of the initial HA1 powders (The meanings of HA1−HA4 refer to Table 1). Table 1 Characteristics of hydroxyapatite powders. Type Particle size Manufacturer HA1 HA2 HA3 HA4
20 ± 6 nm 20−40 nm < 100 nm 30 μm
Purity
Prepared by co-precipitation method in this work − Shanghai McLean Biotechnology Co., Ltd. 97% Aladdin Reagent (Shanghai) Co., Ltd. 97% Shaanxi Kangyue Biotechnology Co., Ltd. 98%
transmission electron microscope (TEM, JEM-2100F, JEOL, Japan), showing round-rod shapes and an average size of 20 ± 6 nm. The phases in the precipitates were characterized by X-ray diffraction (XRD, D/max 2500 pc, Rigaku, Japan), revealing high purity of the product and weak crystallinity as compared with commercial powders (Fig. S1). The composition was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, Ulitima Expert LT, HORIBA, France), confirming a Ca/P ratio of 1.66−1.67. The thermal stability was studied by a thermogravimetric analyzer (TG/DTA, SDT Q600, TA, America), and the variation in the HA structure after heat treatment at different temperatures was investigated by Fourier transform infrared (FTIR, Nexus 670, Nicolet, America) spectroscopy in the range of 40 0−40 0 0 cm−1 with a resolution of 4 cm−1 . Fig. S2 shows the schematic of cold sintering device used in this work. The HA powders (weighing approx. 0.5 g) were placed in a mold with an inner diameter of 1/2 inch without addition of any binder or additive. The powders were then pressed under a uniaxial pressure and the mold was heated at 10 °C/min to a predetermined temperature, which was calibrated prior to the experiment. The holding time defined here refers to the duration after the temperature reached the target value. After cooling, the relative density of the cold-sintered compacts was calculated using a theoretical density of 3.156 g/cm3 [10] and an experimental value measured from mass and geometric sizes. The microstructures of the sintered samples were observed with FE-SEM.
Fig. 1 shows the effects of raw materials (HA1−HA4) and processing parameters on the relative density of the coldsintered compacts. Clearly, compared with commercial powders (HA2−HA4), only the HA1 powders can achieve high density (Fig. 1a) with a relative value of over 90%, which is generally consistent with the finding of Hassan and Ryu [6]. However, note that the type of solvents (deionized water, ammonium hydroxide (14 M) and acetic acid solution (1 M)) has an insignificant effect on the densification for HA1, HA2 and HA3 powders (Fig. 1b). In addition, the liquid content plays a mild role (Fig. 1c). Furthermore, the effect of holding time is also very weak. Noticeable densification can be achieved even after holding at 150 °C for 1 − 2 min under a uniaxial pressure of 500 MPa, but prolonging time does not yield a further significant increase (Fig. S3). In contrast, the pressure and temperature have a prominent effect. With an increase in the applied pressure, the relative density dramatically increases but later shows a plateau after the pressure reaches 350 MPa (Fig. 1d). The effect of temperature can be divided into three stages (Fig. 1e): (i) at 100 °C−175 °C, where the relative density increases with increasing temperature; (ii) at 175 °C−200 °C, where the relative density keeps almost constant; (iii) at 200 °C−250 °C, where the density slightly decreases with further increasing temperature. In addition, we found that heat treatment of the HA1 powders had a significant effect on the sintered density. If HA1 was heat-treated at 150 °C, the relative density still reached 90%. However, the density decreased with increasing heat-treatment temperature (Fig. 1f). When the heat-treatment temperature reached 450 °C−550 °C, the relative density stabilized at approx. 68%. The addition of 10 wt.% deionized water to the heat-treated powders only brought about a 2% − 5% promotion in the relative densities. This result clearly suggests that the densification ability of HA1 should be deteriorated by high-temperature heat treatment, and this change is irreversible; i.e., the densification ability cannot be fully restored by the later addition of water to the HA1 powders which were initially heat-treated at high temperatures. Fig. 2 shows the fracture
H.-Z. Shen, N. Guo and L. Zhao et al. / Scripta Materialia 177 (2020) 141–145
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Fig. 2. FE-SEM morphology of the fracture surfaces of the cold-sintered HA compacts using the HA1 powders heat-treated at different temperatures: (a) 80 °C−12 h; (b) 150 °C−1 h; (c) 250 °C−1 h; (d) 350 °C−1 h; (e) 450 °C−1 h and (f) 550 °C−1 h (the insets show the statistic diagrams of particle size distribution obtained by using Nano Measurer software).
surface morphology of the cold-sintered compacts using the HA1 powders heat-treated at different temperatures. When the powders were initially treated at 80 °C or 150 °C, the compacts were dense and the grain boundaries were connected to each other. With increasing heat-treatment temperature, however, tiny pores were observed at the fracture surfaces. In addition, the particle sizes showed a moderate increase (Fig. 2). Fig. 3(a) presents the XRD patterns of the HA1 powders heattreated at different temperatures. When the heat-treatment temperature increased to 450 °C and 550 °C, some characteristic peaks became slightly sharper, suggesting better crystallization of the particles. The lattice parameter in the a-axis of the HA1 crystal decreased with increasing heat-treatment temperature (Fig. S4) until it reached the standard value (a = 9.432 A˚ in ICCD PDF #54–0022). This behavior is important, implying some structural changes in the HA crystal after heat treatment. Previously, LeGeros et al. [11] reported that the lattice parameter of a-axis increased due to the presence of lattice water in precipitated apatite and it decreased during continuous heating due to the loss of lattice water. They divided the TG curve of the precipitated apatite into two stages with a critical point at 200 °C, and attributed the mass reduction in the first stage to the loss of adsorbed water while that in the second stage to the loss of lattice water. Similar conclusion was also drawn by Reyes-Gasga et al. [12] in their TG analysis on the co-precipitated hydroxyapatite. Here, we examined the thermal stability of the HA1 and HA2 powders at a heating rate of 5 °C/min. As shown in Fig. 3(b), the mass reduction in HA1 was approx. 4.2% when temperature was not more than 200 °C, and nearly 2.3% at 200 °C−550 °C. We speculate that a certain amount of water was still adsorbed at the surface of the HA1 particles even though the powders were pre-treated at 80 °C for 12 h in a blast drying chamber. Therefore, the mass reduction in the first stage is due to the loss of adsorbed water. When the heat-treatment temperature was higher than 200 °C, the lattice water was gradually released. There-
fore, the mass decrease in the second stage is mainly attributed to the loss of lattice water. By contrast, HA2 has an almost negligible mass loss at temperatures above 200 °C. Clearly, the HA2 powders do not contain lattice water in their crystals, which is presumably related to the unsuccessful densification under CSP. In order to explore the mechanisms for the formation of lattice water in HA1 and its role in the densification in CSP, we performed FTIR analyses on the HA1 powders heat-treated at different temperatures as well as on HA2 for comparison. As shown in Fig. 3(c), HA1 treated at 80 °C and 150 °C displayed a distinct peak in the region of 850−880 cm−1 owing to the presence of HPO4 2− (P–O–H) group [11,13,14], suggesting the occurrence of partial replacement of PO4 3− (O–P–O group) by HPO4 2- when HA1 precipitated from aqueous solution. However, the intensity of the P–O– H group weakened while that of the O–P–O group strengthened with an increase in the heat-treatment temperature, implying the transformation from HPO4 2− to PO4 3− (or other ions belong to the O–P–O group) during the heat treatment. Compared with HA1, HA2 shows almost negligible peak intensity in this range (850−880 cm−1 ), similar to the case in the HA1 powders heat-treated at 450 °C or 550 °C. In addition, the broad valleys at 1620 cm−1 and 330 0−360 0 cm−1 (corresponding to H2 O) in HA1 gradually weakened with the increase in heat-treatment temperature. When the temperature reached 550 °C, the valley changed to a plateau. Most importantly, the intensity of the hydroxyl peaks at 631 cm−1 and 3650 cm−1 gradually increased with the increase in the heattreatment temperature, especially at 631 cm−1 . To illustrate the mechanism, Fig. 4(a) schematically shows the crystal structure of HA. Some calcium ions and portions of phosphate tetrahedra that lie beyond the boundaries (dashed lines) of the unit cell are included here to help define the channels occupied by hydroxyl ions [15]. Since HA1 is prepared in aqueous solution, it is reasonable that water molecules could partially replace hydroxyl groups [11], resulting in the formation of lattice
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Fig. 3. (a) XRD patterns for the HA1 powders heat-treated at different temperatures; (b) TG curves of HA1 and HA2 heated at 5 °C/min; (c) FTIR analysis on the HA1 powders heat-treated at different temperatures with that of the HA2 powders as a reference.
Fig. 4. Schematic illustrations of (a) HA structure and (b−c) two models of ion substitution and formation of hydrogen bond in HA1 during CSP (since Ca ions are not involved in ion substitution and formation of hydrogen bond, they are not shown in diagrams (b) and (c)).
water. As a consequence, the intensity of the hydroxyl peaks in the infrared spectra (631 and 3650 cm−1 ) decreased. Correspondingly, a hydrogen bond could form between the hydrogen atom in the lattice water and the oxygen atom in the nearby O–P–O group [16] (see Fig. 4(b)). Another possibility is that HPO4 2− partly replaced PO4 3− in HA [11,17]. Similarly, a hydrogen bond could develop between the oxygen atom in the hydroxyl group and the hydrogen atom in the HPO4 2− group [18,19], as illustrated in Fig. 4(c). Such ion substitutions led to an increase in the lattice param-
eter of a-axis and thus an unstable state of the HA structure. During CSP, the presence of a small amount of adsorbed water at the particle surface acted like a liquid film, reducing the friction between particles during compression [20]. The application of high pressure provided a driving force for the particle sliding and redistribution [6]. Moreover, the formation of hydrogen bonds greatly enhanced the attraction between the HA1 particles, shortened their distance, promoted their rearrangement, and thus led to the densification. However, when the HA1 powders were heat-
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treated at temperatures over 200 °C, the lattice water was gradually discharged along the c-axis in a continuous manner by exchanging protons with the surrounding hydroxyl groups [21]; or alternatively, hydrogen phosphate decomposed to release lattice water in the form of 2HPO4 2− =P2 O7 4− +H2 O [11]. The discharge of the lattice water led to the strengthening of the hydroxyl and phosphate peaks and weakening of the hydrogen phosphate peak in the FTIR spectra as well as the recovery of the lattice parameter of HA1 to the standard value (Fig. S4). Simultaneously, the hydrogen bonds were weakened or even finally disappeared with the increase in the heat-treatment temperature, leading to significant deterioration in the densification of the samples (Fig. 1f). This effect was unrecoverable by the addition of deionized water to the heat-treated HA1 powders since the lattice water is essentially different from the physically absorbed water (Fig. 1f). On the other hand, the hydrogen bond did not exist in the HA2 powders because of the absence of ion substitution and lattice water. Therefore, the commercial HA2 powders failed to achieve high density under the CSP. In summary, high-density hydroxyapatite compacts were prepared by CSP using co-precipitated HA powders. The occurrence of ion substitution and presence of lattice water contribute to the formation of hydrogen bonds between the HA particles. The synergistic function of hydrogen bond and high pressure promotes the densification process. This finding broadens the materials selection for cold sintering and provides a potential roadmap for lowtemperature densification of the materials with negligible solubility. Declaration of Competing Interest We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the Changbai Mountain Scholars Program of Jilin Province (No. 2015011).
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