Interaction of calcium phosphates with calcium oxide or calcium hydroxide during the “soft” mechanochemical synthesis of hydroxyapatite

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Interaction of calcium phosphates with calcium oxide or calcium hydroxide during the “soft” mechanochemical synthesis of hydroxyapatite Marina V. Chaikinaa, Natalia V. Bulinaa,∗, Olga B. Vinokurovaa, Igor Yu. Prosanova, Dina V. Dudinab,a a b

Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze Street 18, Novosibirsk, 630128, Russia Lavrentyev Institute of Hydrodynamics SB RAS, Lavrentyev Avenue 15, Novosibirsk, 630090, Russia

ARTICLE INFO

ABSTRACT

Keywords: “Soft” mechanochemical synthesis Ball milling Hydroxyapatite Reaction kinetic Gibbs free energy

In the present work, the formation kinetics of hydroxyapatite Ca10(PO4)6(OH)2 during ball milling of mixtures of calcium phosphates with calcium oxide or calcium hydroxide was studied. The conversion degrees of reaction mixtures containing calcium dihydrogenphosphate/dicalcium phosphate/tricalcium phosphate/calcium pyrophosphate and calcium oxide/calcium hydroxide were determined for different milling times. The formation kinetic data were analyzed together with the Gibbs free energies of the corresponding reactions. It was found that the kinetic factors dominate at the initial formation stage of hydroxyapatite due to the acid-base interactions during the “soft” mechanochemical synthesis. A technologically attractive synthesis route of Ca10(PO4)6(OH)2 was suggested based on the “soft” mechanochemical synthesis via reactions between СаНРО4 and СаО or Са(ОН)2.

1. Introduction Hydroxyapatite (HA), Ca10(PO4)6(OH)2, an analogue of the mineral component of bone and dental tissues of humans and animals, is an important bioceramic material for medical applications [1-5]. HA can be synthesized by different methods, such as precipitation from a solution, sol-gel, hydrothermal method, biomimetic precipitation, and electrodeposition [4]. In the past few decades, the mechanochemical synthesis has been widely used for preparing HA and its derivatives containing different substitutions [6-15]. The perspectives of the mechanochemical synthesis of HA and HA-based composites have been discussed in detail in Refs. [9,10]. The mechanochemical processing has several advantages over traditional solvent-assisted methods. Indeed, in the mechanochemical processing, a solid-state reaction may be carried out in a solvent-free environment, which brings the yields of undesired side reactions down to a minimum. The target reactions can conveniently be carried out at ambient temperature and pressure. Furthermore, the use of the mechanochemical processing eliminates the need of reclaiming the solvent. In the mechanochemical synthesis, chemical reactions are initiated by the energy released during ball collisions and due to the action of friction forces. The released energy depends on the technical

∗

characteristics of the mill, namely, the kinetic energy of the balls (vial rotation speed). In this connection, the synthesis of HA occurring directly in the milling vials is possible only in ball mills working at a high rotation speed. The higher the rotation speed of the vial, the shorter the duration of the synthesis of HA. In a number of studies, the synthesis of HA is a lengthy procedure [6-11], which is a serious drawback from a technological perspective. As the synthesis completion depends on the input energy, treatment in low-energy mills has to last tens of hours. In Ref. [11], the mechanochemical synthesis of HA was carried out in a Frisch planetary ball mill using different rotation speeds (170, 270, and 370 rpm). The reaction mixture was composed of dicalcium phosphate CaHPO4 and calcium hydroxide Ca(OH)2; the milling time was 15 h. HA was synthesized at all rotation speeds used. However, the product obtained at 370 rpm was better crystallized than the products obtained at lower speeds. A bulk ceramic material produced from the powder synthesized at 370 rpm had a higher hardness and a higher relative density compared with materials obtained from HA synthesized at lower speeds. In our previous work, the synthesis was conducted in high-energy planetary ball mills of AGO-2 [16] and AGO-3 [17] types. In these mills, single-phase nanocrystalline HA can be obtained within tens of minutes under 1200 and 1800 rpm, respectively [13,18]. The mass of the HA product is 20 and 500 g, respectively. AGO-2 and AGO-3 mills can be

Corresponding author. E-mail address: [email protected] (N.V. Bulina).

https://doi.org/10.1016/j.ceramint.2019.05.239 Received 4 April 2019; Received in revised form 20 May 2019; Accepted 22 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Marina V. Chaikina, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.05.239

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used for synthesizing both stoichiometric and substituted hydroxyapatite. The synthesized materials were used to produce coatings by high-frequency magnetron sputtering [14] and micro-arc oxidation [15]. One of the important parameters of the synthesis is the chemical composition of the reaction mixture. The influence of the nature of the reactants on the thermodynamics and kinetics of the synthesis of HA was addressed in Refs. [6-12]. It was shown that HA can be mechanochemically synthesized from a mixture of Са2Р2О7 and СаСО3 in water or acetone [6]. Single-phase HA was obtained only when water was present in the system. When the synthesis was conducted in acetone, both the as-synthesized powder and the product obtained by annealing of the powder contained Ca3(PO4)2 as the second phase. The synthesis kinetics of HA in Retsch and Fritsch planetary ball mills was studied in Ref. [7]. In the experiments, different vial rotation speeds and vials differing in ellipticity were used. The influence of the vial lining and the material of the milling balls on the synthesis kinetics was studied. The kinetic constants of the interaction between СаНРО4⋅2Н2О and СаО during the dry mechanochemical synthesis of HA depended on the milling conditions and parameters of the milling devices. The effect of the vial rotation speed, the weight of the balls and their surface area on the kinetics of the mechanochemical synthesis was studied. The kinetic constant of the reaction and the inverse time of full consumption of CaO changed linearly with the square of the rotation frequency, the square of the vial ellipticity and with the product of the weight of the milling balls and their surface area. These observations correspond to theoretical models developed for mechanical activation [7]. It is suggested that, once these parameters are determined, it becomes possible to transfer the synthesis conditions from one mill to another and correctly compare experimental results obtained in ball mills of different design. In Ref. [8], a comparison between the dry and wet mechanochemical syntheses in a Retsch mill was made; different amounts of water were added to a mixture of СаНРО4⋅2Н2О and СаО. The dry processing was shown to have advantages of a faster synthesis and a lower contamination level while eliminating the need of filtering and drying the product. The “soft” mechanochemical processing is a promising synthesis method [19]. In this method, oxides and acid salts are used as reactants. The mechanochemical reaction between them is similar to acid-base interactions in liquid. The products of such reactions are the target compound and water. Unlike the synthesis from (NH4)2HPO4 as a reactant, in the processing described above, the synthesized product does not require washing [10-12]. The goal of this work was to study the “soft” mechanochemical synthesis of HA in reactions mixtures of calcium oxide or calcium hydroxide and calcium phosphates of different compositions and to determine, based on the thermodynamic calculations and kinetic data, the

composition of the reaction mixture allowing for a fast synthesis of single-phase non-agglomerated HA. 2. Materials and methods The mechanochemical synthesis was conducted in a planetary ball mill of AGO-2 type [16] with water-cooled vials 150 ml each at a rotation speed of 1200 rpm. The weight of the milling balls was 200 g. The weight ratio of the reaction mixture to the milling balls was 1:20. The duration of milling was 3, 5, 10 and 20 min. In order to avoid contamination of the product by the material of the balls and vials, the working zone of the vial was lined with the reaction mixture. The concentration of iron in the products of mechanochemical synthesis obtained after 30 min of milling was 0.03–0.05 wt%, as was determined by the atomic absorption method. The products of the mechanochemical synthesis were analyzed by the X-ray diffraction (XRD) and infrared (IR) spectroscopy. The XRD patterns were recorded using a D8 Advance powder diffractometer in Bragg-Brentano geometry with CuKα radiation equipped with a nickel Кβ-filter and ultrafast position-sensitive one-dimensional Lynx-Eye detector with a capture angle of 3о. The phase analysis of the compounds was carried out using ICDD PDF-4 database (2011). The concentrations of the phases in the products of mechanosynthesis were determined by the Rietveld method in Topas 4.2 software (Bruker, Germany). FTIR spectra were recorded using an Infralum-801 device. The specimens were prepared by a traditional KBr pellet method. Specific surface area (SSA) was measured by an automatic system ThermosorbTPD 1200 (Katakon, Russia) using nitrogen gas. 3. Results and discussion The “soft” mechanosynthesis of HA was investigated in reaction mixtures containing calcium phosphates and calcium oxide/calcium hydroxide (Table 1). The calculated Gibbs free energy of the reactions shows that all considered reactions are exothermic. Among reactions given in Table 1, the most energetically favorable is the reaction between Ca(H2PO4)2⋅H2O and CaO (reaction 1). The least favorable reactions are those involving tricalcium phosphates (reactions 7 and 8). According to the thermodynamic data, reactions involving calcium oxide are more energetically favorable than those involving calcium hydroxide. However, experiments show that reactions between calcium dihydrogenphosphate or dicalcium phosphate and Ca(OH)2 are faster than those between the above mentioned phosphates and CaO. This is explained by the kinetics of the “soft” mechanochemical synthesis, which is similar to acid-base interaction in liquid. Hydroxides interact more easily with acidic groups of salts than oxides. The SSA values show an advantage of reactions between phosphates and calcium

Table 1 Reactions of mechanosynthesis of HA from different calcium phosphates, Gibbs free energy and SSA of the HA product Reaction number

Mechanosynthesis reaction of HA

ΔrG (kJ/mol)

SSA (m2/g)

1 2 3 4 5 6 7 8 9 10

3(Ca(H2PO4)2⋅H2O) + 7CaO → Ca10(PO4)6(OH)2 + 8H2O 3(Ca(H2PO4)2⋅H2O) + 7Ca(OH)2 → Ca10(PO4)6(OH)2+15H2O 6(CaHPO4⋅2H2O) + 4CaO → Ca10(PO4)6(OH)2+14H2O 6(CaHPO4⋅2H2O) + 4Ca(OH)2 → Ca10(PO4)6(OH)2+18H2O 6CaHPO4 + 4CaO → Ca10(PO4)6(OH)2+2H2O 6CaHPO4 + 4Ca(OH)2 → Ca10(PO4)6(OH)2+6H2O 3Ca3(PO4)2 + CaO + H2O → Ca10(PO4)6(OH)2 3Ca3(PO4)2 + Ca(OH)2 → Ca10(PO4)6(OH)2 3Ca2P2O7 + 4CaO + H2O → Ca10(PO4)6(OH)2 3Ca2P2O7 + 4Ca(OH)2 → Ca10(PO4)6(OH)2 + 3H2O

-

29.9 101.0 46.2 57.8 44.1 56.0 <1 <1 ∼1 30.7

2

1164.59 765.96 690.28 462.60 670.06 442.38 166.17 109.23 631.24 403.57

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hydroxide (reactions 1-6, Table 1). Indeed, the SSA of HA formed by reactions of phosphates with calcium hydroxide is greater than that of HA formed by reactions of phosphates with calcium hydroxide. Water released in the reaction prevents the HA product from agglomeration and improves the process of interaction between the components of the

reaction mixture. The HA products formed by reactions 7-9 have a very low SSA; in these reactions, no water is released during the synthesis. This fact explains the differences in the thermodynamic and the kinetic data pertaining to the synthesis of HA from reaction mixtures of different compositions and will be discussed below.

Fig. 1. XRD patterns of the products of mechanosynthesis via reactions (1-10) for different milling times: a – reaction 1, b – 2, c – 3, d – 4, e − 5, f – 6, g – 7, h – 8, i – 9, j – 10. 3

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Fig. 1. (continued)

The XRD analysis (Fig. 1 (a)) and FTIR spectroscopy (Fig. 2 (a)) show that, for reaction 1, during the first several minutes of mechanochemical processing, Ca(H2PO4)2⋅H2O loses lattice water and CaO transforms into Ca(OH)2. After 5 min of ball milling, the mixture contains 55 wt% of HA (Table 2). However, the peaks of the reactants disappear from XRD patterns only after 20 min of milling. As the milling continues, the lattice parameters of HA do not change indicating that the synthesis is complete. So, in order to synthesize HA in an AGO-2 mill, 20 min of ball milling is sufficient. Upon further milling, the energy delivered to the powder causes crystallization of the product, as indicated by an increase in the crystallite size of HA (Table 2). When CaO is replaced by Ca(OH)2 (reaction 2, Table 1), in the beginning of the mechanochemical processing, the formation of the HA structure is very fast: after 3 min of milling, the concentration of НА in the mixture equals to 77 wt% (Fig. 1(b), Table 2). This observation is not in agreement with the values of Gibbs free energy for these reactions (compare ΔrG for reactions 1 and 2), since kinetic factors dominate during the “soft” mechanochemical synthesis and determine the reaction advancement. Despite an earlier start of the HA formation, the synthesis by reaction 2 is completed only after 20 min, similar to the synthesis via reaction 1. The formation of HA during the first several

minutes of milling and a release of a significant amount of water leads to agglomeration and consolidation of the mixture. This results in a slower synthesis upon further milling. The crystallite size of HA obtained via reaction 2 is about half the size of the product synthesized by reaction 1 (Table 2). The lattice water contained in dicalcium phosphate hydrate (compare reaction pairs 3 and 5, 4 and 6) influences only the first stage of the formation of НА – the first 5 min of milling (Table 2). Despite the differences in the process kinetics at the initial stage of these reactions, the complete transformation of the reactants into HA required 20 min of milling (Fig. 1(с)-(f)). In reactions 7–10, for the synthesis of HA, CaO or Ca(OH)2 was used together with calcium salts that do not contain hydrogen cation – Са3(РО4)2 or Са2Р2О7. In reactions 7 and 9, 1 mol of water was introduced into the reaction mixtures, which, according to the FTIR spectroscopy, resulted in the formation of Ca(OH)2 (Fig. 2(b, c)). For these reactions, calcium hydroxide has not been detected by the XRD (Table 2) because of its amorphous state and a low concentration in the mixture. It was found that, although 1 mol of water was added, the synthesis was complete only after 30 min of milling. Interestingly, when Ca(OH)2 was substituted for CaO (reactions 8 and 10), the synthesis was complete after 20 min (Table 2). Consequently, 1 mol of water added to

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Fig. 2. FTIR spectra of the products of mechanosynthesis for different milling times: а – reaction 1, b –7, с – 9, d – 10

the 3(Ca3(PO4)2/Ca2P2O7)+CaO reaction mixture is not sufficient for accelerating the synthesis of the HA phase. Comparative analysis of the XRD patterns of the products of reactions 7-10 (Fig. 1(g-j)) shows that the synthesis of HA from Ca2P2O7 is more efficient than from Ca3(PO4)2. Furthermore, when the synthesis is conducted from a phosphate that does not contain lattice water, Ca (OH)2 is preferable as a source of calcium in comparison with CaO for accelerating the synthesis. Fig. 3 shows the conversion degrees of reaction mixtures corresponding to different milling times. It can be seen that the synthesis rates of HA are different for reaction mixtures of different compositions. The fastest synthesis is ensured by reactions 2 and 4. After 3 min of milling, the mixture contains more than 70 wt% of HA. Then the process slows down and, after 10 min of milling, the mixture contains 9798 wt% HA (Fig. 3(а-b)). However, full conversion of the reaction mixture into HA is achieved only after 20 min of milling. The synthesis slows down when СаНРО4 is used (Fig. 3 (c)) becoming even slower when the starting material is Са3(РО4)2 (Fig. 3(d)). As can be seen in Fig. 3, the mechanosynthesis of HA becomes slower as the number of water molecules and the number of hydroxyl groups of the phosphate reactant decrease (the conversion degree versus milling time profiles change their shape, the profile for Ca3(PO4)2 being close to linear, Fig. 3d). Although water promotes the formation of the HA structure at

an early stage, it hinders its crystallization upon the synthesis completion. As calculations of the Gibbs free energy showed, the most energetically favorable reaction is the one between calcium dihydrogen phosphate monohydrate and calcium oxide (reaction 1). However, the kinetic studies revealed that reactions 2 and 4 are faster (Fig. 3). Large quantities of lattice water released in the course of reactions 2 and 4 cause agglomeration and consolidation of HA accompanied by its severe sticking to the vial walls, which complicates the synthesis. Therefore, reaction 5 is a more attractive synthesis route: a non-agglomerated powder of nanocrystalline HA with a yield of 100% can be obtained after 20 min of ball milling. 4. Conclusions In this work, the thermodynamics of the formation of HA from and Са(Н2РО4)2⋅Н2О/СаНРО4⋅2Н2О/СаНРО4/Са3(РО4)2/Са2Р2О7 СаО/Са(ОН)2 as reactants and the kinetics of the mechanochemical synthesis in these mixtures were studied. Compositions of the reaction mixtures and conditions of the mechanochemical synthesis ensuring the formation of single-phase HA were determined. The most energetically favorable reaction is the reaction between Са(Н2РО4)2⋅Н2О and СаО. The fastest reactions are those in the Ca(H2PO4)2⋅H2O + Ca(OH)2 and

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Table 2 Results of the quantitative analysis of the products of mechanosynthesis occurring via reactions 1–10 and lattice parameters of the HA phase Duration of synthesis (min)

Concentration (wt.%) Ca(H2PO4)2xH2O

Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30 Reaction 3 5 10 20 30

CaHPO4x2H2O

Lattice parameters of the HA phase CaHPO4

Ca2P2O7

1: 3(Ca(H2PO4)2⋅H2O) + 7CaO → Ca10(PO4)6(OH)2 + 8H2O 58 – – 38 – 5 – – – – 2: 3(Ca(H2PO4)2⋅H2O) + 7Ca(OH)2 → Ca10(PO4)6(OH)2 + 15H2O – 9 12 – 7 5 – 2 – – – – – – – 3: 6(CaHPO4⋅2H2O) + 4CaO → Ca10(PO4)6(OH)2 + 14H2O 5 39 5 24 – 3 – – – – 4: 6(CaHPO4⋅2H2O) + 4Ca(OH)2 → Ca10(PO4)6(OH)2 + 18H2O 3 19 – 6 – 2 – – – – 5: 6CaHPO4 + 4CaO → Ca10(PO4)6(OH)2 + 2H2O 55 43 16 – – 6: 6CaHPO4 + 4Ca(OH)2 → Ca10(PO4)6(OH)2 + 6H2O 62 44 14 – – 7: 3Ca3(PO4)2 + CaO + H2O → Ca10(PO4)6(OH)2

10a: 3Ca2P2O7 + 4Ca(OH)2 → Ca10(PO4)6(OH)2 + 3H2O

Ca(OH)2

HA

a (Å)

с (Å)

Crystallite size (nm)

– – – – –

42 6 1 – –

– 55 94 100 100

– 9.425(3) 9.430(2) 9.430(1) 9.430(1)

– 6.893(3) 6.890(2) 6.888(1) 6.886(1)

– ∼20 20.6(4) 28.9(4) 32.7(4)

– – – – –

2 – – – –

77 89 98 100 100

9.422(3) 9.424(3) 9.435(4) 9.433(4) 9.434(2)

6.884(3) 6.885(3) 6.886(3) 6.886(3) 6.886(2)

∼10 10.8(5) 11.9(5) 14.9(4) 17.8(2)

1 1 – – –

11 9 – – –

44 61 97 100 100

9.436 9.436(2) 9.435(2) 9.433(1) 9.435(1)

6.892 6.892(2) 6.891(2) 6.886(1) 6.886(1)

∼13 15.1(4) 18.9(2) 26.5(2) 30.2(4)

4 – – – –

73 94 98 100 100

9.436(2) 9.437(3) 9.438(2) 9.436(2) 9.435(2)

6.891(2) 6.891(2) 6.891(2) 6.888(2) 6.885(1)

∼10 11.5(2) 15.5(2) 21.3(4) 27.9(4)

5 6 4 – –

20 35 78 100 100

9.441 9.441 9.441(2) 9.436(2) 9.433(1)

6.894 6.894 6.894(3) 6.891(2) 6.889(1)

∼13 13.7 (10) 21.8(4) 23.1(4) 27.6(4)

21 10 – – –

17 46 86 100 100

9.413 9.413(2) 9.422(2) 9.431(1) 9.431(1)

6.904 6.904(3) 6.902(2) 6.895(1) 6.890(1)

∼14 18.0(8) 19.0(4) 26.6(4) 31.1(4)

69 64 34 10 –

30 35 65 89 100

9.443 9.443 9.443(4) 9.440(3) 9.439(3)

6.893 6.893 6.893(4) 6.886(2) 6.884(2)

∼10 ∼10 13.9(6) 15.2(5) 15.6(5)

68 56 21 – –

32 44 79 100 100

9.433(5) 9.433(4) 9.433(4) 9.434(3) 9.440(2)

6.894(5) 6.894(4) 6.888(4) 6.883(2) 6.883(2)

13.9(8) 13.8(6) 16.8(6) 16.8(3) 17.6(2)

+– + + 98 100

not calculated 9.437 9.437(4) 9.433(3) 9.430(2)

not calculated 6.89 6.890(3) 6.885(2) 6.884(2)

not calculated ∼10 15.9(6) 16.6(4) 16.4(2)

+ + + 100 100

not calculated not calculated 9.421(2) 9.425(2) 9.426(2)

not calculated not calculated 6.902(2) 6.893(2) 6.888(1)

not calculated not calculated 21.1(4) 22.7(4) 24.6(4)

20 16 2 – –

1 1 1 1 –

8: 3Ca3(PO4)2 + Ca(OH)2 → Ca10(PO4)6(OH)2

9a: 3Ca2P2O7 + 4CaO + H2O → Ca10(PO4)6(OH)2

CaO

– – – – – + + +– – –

+ + + 2 –

+ + + – –

+ +– – – –

Ca3(PO4)2

a The quantitative analysis for reactions 9 and 10 (3-10 min) has not been carried out because of the absence of the crystallographic information for Ca2P2O7. For these reactions, information on the intensity of the main reflections of the components is provided: “+–“barely noticeable reflections, “+“high-intensity reflections

CaHPO4⋅2H2O + Ca(OH)2 reaction mixtures. However, a release of a significant amount of water leads to agglomeration and consolidation of the synthesized HA, which complicates the synthesis process. A technologically attractive route appears to be the synthesis via a reaction between СаНРО4 and СаО or Са(ОН)2. In those mixtures, the

mechanosynthesis is complete after 20 min of milling in an AGO-2 mill yielding a non-agglomerated powder of single-phase nanocrystalline HA. The synthesized HA has a specific surface of 40 – 50 m2/g, an average crystallite size of 25 nm and lattice parameters of а = 9.43 Å and с = 6.89 Å.

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Fig. 3. The conversion degree of the reaction mixtures versus milling time for the synthesis of HA from different reactants: а - Са(Н2РО4)2⋅Н2О; b - СаНРО4⋅2Н2О; c СаНРО4; d – Са3(РО4)2

Acknowledgment

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