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Vol. 9. Issue 1.
Pages 76-88 (January - February 2020)
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Vol. 9. Issue 1.
Pages 76-88 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.032
Open Access
The enhancement of hydroxyapatite thermal stability by Al doping
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M.A. Goldberga,
Corresponding author
mgoldberg@imet.ac.ru

Corresponding author.
, P.V. Protsenkob, V.V. Smirnova, O.S. Antonovaa, S.V. Smirnova, A.A. Konovalova, K.G. Vorckacheva, E.A. Kudryavtsevc, S.M. Barinova, V.S. Komleva
a Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, 119334 Moscow, Russian Federation
b M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation
c National Research University “Belgorod State University”, 308015 Belgorod, Russian Federation
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Tables (4)
Table 1. Initial reagent ratio, powder composition and surface area.
Table 2. Phase composition.
Table 3. Lattice parameters in nanometers.
Table 4. Matched FTIR peaks presented at spectra (Fig.4) at different temperature.
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Abstract

Al-substituted hydroxyapatite (Al-HA) powders with apatite structure and particle size of 30–70 nm were obtained via precipitation method. The effect of Al content on specific surface area and morphology of powders was studied, and a formation of highly anisotropic phase due to Al doping was observed. The influence of heat treatment in 300–1400 °C range on the phase composition, lattice parameters, Fourier-transform infrared spectroscopy (FTIR) spectra and mass loss of powders was investigated. Introduction of Al in the hydroxyapatite (HA) lattice in the range 0.5–1.0 mol.% resulted in the improvement in thermal stability, which had not been reported previously. Incorporation of 5–10 mol.% of Al resulted in the formation of biphasic materials based on HA and whitlockite-like structure at 900 °C and apatite and α- tricalcium phosphate (α-TCP) phases at 1200–1400 °C. Introduction of 20 mol.% of Al resulted in the formation of isomorphic Al-substituted whitlockite phase Ca9Al(PO4)7. We have estimated the onset of Ca9Al(PO4)7 transformation into α-TCP phase at 1400 °C.

Keywords:
Al-substituted hydroxyapatite
Phase transformation
Thermal stability
Abbreviations:
HA
Al-HA
FTIR
α-TCP
β-TCP
XRD
AES-ICP
FEG SEM
TEM
ED
S
TG
Full Text
1Introduction

Hydroxyapatite (HA) is one of the most attractive biomaterials for bone reconstruction due to its good biological compatibility and osteoconductivity [1]. Introduction of different cations into HA structure can lead to beneficial effects on biomaterial properties such as the degree of structural order (i.e. crystallinity), morphology, mechanical properties, degradability, and dissolution rate under physiological conditions. Cation-substituted HA is known to provide many advantages for reconstructive surgery and bone tissue engineering applications [2]. Introduction of different cations into HA structure influences its behavior during heat treatment. Thermal stability of HA depends on several factors, including particle size, Ca/P ratio, and water vapor pressure. Available data on the HA thermal stability are controversial: according to Wang et al. decomposition of HA with the formation of β-tricalcium phosphate (β-TCP) begins at 1100 °C [3], while HA stability of HA up to 1250 °C was reported in [4]. HA rapidly lose water under vacuum starting from 850 °C [5] which was confirmed by the decrease of intensity of corresponding FTIR bands and detection of α-TCP peaks at 1000 °C by X-ray diffraction (XRD) [3].

Thermal stability of HA depends significantly upon the substitution of calcium by other cations and phosphate by halogens [6]. Specifically, mono- and divalent cations substitution was investigated in Refs. [7,8], due to their significant influence on the structure, properties and thermal behavior of HA and β-TCP ceramic materials. Incorporation of sodium did not deteriorate the HA phase stability up to 900 °C [9]. Increased thermal stability of the potassium substituted hydroxyapatites and biphasic mixtures of HA/β-TCP till 1300 °C without any phase decomposition was revealed [10]. Introduction of Mn in the HA structure resulted in a decrease of decomposition temperature: incorporation of 5 wt.% of Mn resulted in the onset of decomposition at 800 °C with complete transformation into α-TCP and Mn3O4 occurringat 1250 °C [11]. The decrease of thermal stability was also observed for Zn [12], Ni [13], and Cd [14] substituted HA. Influence of transition metal cations on thermal stability has been extensively studied for HA coatings on the Ti surface. The interaction between HA and Ti leads to a significant decrease of decomposition temperature, with the formation of the α-TCP at 900 °C [4].

The effect of trivalent metal ions has been studied in detail for Fe (III) substituted HA [15]. The formation of β-TCP at 800 °C was detected in all compounds with Fe concentration above 0.7 at.%. There are some data on the effect of Bi, La, Y and In substitution on thermal stability of HA and osteoblast response [16,17].

The reports on the influence of Al on HA structure, phase composition, and thermal behavior are limited. Nie et al. synthesized aluminum substituted hydroxyapatite (Al-HA) powders by coprecipitation method and obtained nanoparticles without any impurity phases in as-synthesized materials [18]. Similarly, Wakamura et al. obtained pure Al-HA nanoparticles by coprecipitation and ion-exchange methods and reported that Al inhibits dental caries [19]. However, in these studies, thermal stability of substituted HA has not been investigated. The sol-gel synthesis of Al-HA followed by heat treatment at 900 °C was unsuccessful above 10 at.%: phase mixtures of AlPO4, CaP4O11, Al3(PO4)2(OH)3(H2O)5 and Ca2Al(PO4)2(OH) were obtained [20]. Kolekar et al. synthesized aluminum doped HA by solution combustion method with subsequent annealing at 950 °C: the powders were biphasic and contained HA and a small amount of Al2O3. Obtained samples did not have toxic effects on cell lines L929 [21].

The biocompatibility of aluminum contained HA is regarded a controversial question, which requires further investigation. The increased aluminum content in drinking water had been linked with the brain dementia including Alzheimer disease [22]. However, this assumption was criticized in later research [23], where no change in bulk aluminum content in the human brains of healthy people and Alzheimer disease patients was demonstrated and proven. In the same time, aluminum hydroxide and aluminum phosphate are considered as the important immune adjuvants approved by the FDA for use in humans due to its safety and efficacy [24–27]. Composite materials based on hydroxyapatite and Al2O3 containing clinoptilolite obtained by high-temperature sintering at 1300 °C demonstrated biocompatibility and absence of acute toxicity for SAOS-2 cells [28].

There are several potential applications of Al-HA outside the field of bioceramics. For example, Al-HA is reported to be an efficient adsorbent material for water purification [18]. The nanoscale powders of Al-HA are proposed as a drug career system [21]. Also, HA can serve as coating on the Al2O3[29,30] or Ti6Al14V substrates: its interaction with substrate materials could result in the Al-HA formation at high temperature [31]. Ceramics based on HA and Al2O3 possess enhanced mechanical properties [32]. Viswanath Ravishankar have demonstrated the formation of Ca aluminates in HA-Al2O3, but the heat treatment was limited by 1000–1200 °C [33].

This work is aimed at the investigation of the influence of Al content on the phase composition, morphology and thermal stability of powders obtained by precipitation method over the wide range of aluminum content up to 20 mol.% substitution.

2Experimental

Powder synthesis was carried out according to reaction 1 via precipitation method using reactants of analytical grade (LTD Labtech, Russia) and deionized water.

(10-x)Ca(NO3)2 + 6(NH4)2HPO4 + (2x/3)Al(NO3)3 + 8NH4OH→
(10-х)Al2x/3(PO4)6(OH)2 + 20NH4NO3 + 6H2O,
х = 0; 0.005; 0.01; 0.05; 0.1; 0.2

Calcium and aluminum nitrate salts were mixed with 250 ml deionized water to obtain the solutions with selected ratios (Table 1). The solutions of a diammonium hydrogen phosphate were prepared with 150 ml deionized water. The diammonium hydrogen phosphate solution were added by dropping to calcium and aluminum nitrate solution withthe mixing during 1 h at the room temperature. The pH of the reaction mixture was maintained at a level of 9.0–9.5 by adding aqueous ammonia. Powders were ripened at mother solution during 21 days at the temperature of 25 °C for full crystallization of precipitate [34]. The obtained powders were filtered, washed and dried at 60 °C for 24 h. The powders were heat-treated at 300 °C for 3 h in air to remove the ammonium nitrate, an impurity phase. Then the powders were heat-treated in the air for 3 h at T = 900 °C, 1200 °C, and 1400 °C. Further, the materials were ground in an agate mortar to a powder with the particle size under 100 μm.

Table 1.

Initial reagent ratio, powder composition and surface area.

Ca substitution by Al, mol.%  Stoichiometric formulae  m Ca(NO3)2*4H2O, g  m Al(NO3)3*9H2O, g  m (NH4)2HPO4, g  Al theoretical, mol.%  Al theoretical, wt.%  Al measured, wt.%  S, m2/g 
Ca10(PO4)6(OH)2  58.76  0.00  19.72  <10−4  91(3) 
0,5  Ca9,95Al0,033(PO4)6(OH)2  58.54  0.31  19.74  0.5  0.086  0.082 (0.003)  53(2) 
Ca9,9Al0,066(PO4)6(OH)2  58.31  0.62  19.77  1.0  0.173  0.17(0.02)  71(3) 
Ca9,5Al0,33(PO4)6(OH)2  56.46  3.14  19.95  0.876  0.84(0.03)  73(3) 
10  Ca9Al0,66(PO4)6(OH)2  54.11  6.35  20.18  10  1.772  1.79(0.03)  84(3) 
20  Ca8Al1,33(PO4)6(OH)2  49.23  13.01  20.65  20  3.629  3.62(0.04)  90(3) 

For determination of Al content in the precipitates, powders were dissolved in HCl–HNO3 mixture and analyzed by inductively coupled plasma atomic emission spectroscopy (HORIBA Jobin Yvon, ULTIMA 2) (ICP-AES). Field emission gun scanning electron microscopy (FEG SEM) assessed the particle morphology of synthesized powders (CrossBeam 1540 EsB, Carl Zeiss, gold sputtered specimens in scanning electron microscope mode, accelerating voltage of 7 kV, 5 mm WD, 30 μm aperture size) and by transmission electron microscopy TEM (JEOL JEM 2100, carbon sputtered specimens, accelerating voltage of 200 kV, 1000 mm of the diffraction length, 25.1). The parameters of lattice and phase composition were determined by the electron diffraction (ED) obtained by TEM. The specific surface area (S) of the as-synthesized powders was determined by low-temperature nitrogen adsorption measurements (BET, Micromeritics TriStar analyzer). The powder materials were characterized by the XRD method (Shimadzu XRD-6000, CuKα radiation, step = 0,02°) with identification of phase composition according to ICDD PDF database. The lattice parameters of HA, β – TCP, α – TCP and Ca9Al(PO4)7 (as a phase with the whitlockite-like structure similar to β – TCP) were determined by Rietveld refinement method using the GSAS/EXPGUI software package [35,36]. The Rietveld refinements were performed with fixed structural parameters of the P63/m structural model of partially substituted HA [37]. The model provides moderately acceptable fit quality for all datasets: reliability factors for the 1% of Al 900 °C sample are Rp = 13.3%, Rwp = 20.3%. However, to confirm the absence of systematic error which might be caused by imperfect fit of intensities, structureless (Le Bail) fit was also performed at one model composition, and the refined values of lattice parameters coincided within 1 e.s.d. Consequently, the lattice for all other compositions were evaluated by Rietveld method only as it provided superior computational stability.

Fourier-transform infrared spectroscopy (FTIR) analysis has been performed with a Nikolet Avatar FTIR spectrometer and spectra have been obtained in the range from 4000 to 400 cm−1 to evaluate the functional group of the specimens. Thermogravimetry (TG) data were obtained in a Netzsch STA 409 Luxx simultaneous thermal analysis system during continuous heating from 25 to 1400 °C under air flow, without holding at any fixed point temperatures. An empty crucible was used as a reference. The heating rate was 10 °C/min. The composition of the gases released during thermal decomposition was determined by mass spectrometry using a Netzsch QMS 403 C Aёolos quadrupole mass spectrometer with a capillary inlet system.

3Results3.1Powders chemical composition, morphology and surface area

Data on the Al content in the powders as determined by ICP-AES are presented in Table 1, the chemical composition of synthesized materials is close to the nominal one.

Formation of Al-HA powders structure was observed in pH 9–9.5 media formed by aqueous ammonia solution. In spite of Al amphoteric properties, we assumed the formation of Al(OH)3 in the aqueous ammonia solution media according to Ref. [38]. Al(OH)3 does demonstrate the acid behavior in the alkali conditions of NaOH (pH > 12), but does not dissolve in ammonia solution (pH 10–11) and could not form [Al(OH)4][39]. The formation of HA structure occurred during the ripening in the mother solution without [Al(OH)4]- anions and incorporation of [Al(OH)4]- in the HA structure in this conditions seems unlikely. In the same time, we did not observe [AlO4] bands in the FTIR spectra of as-synthesized powders and samples heat-treated at low temperature. As-synthesized Al(OH)3 had amorphous structure and preserved chemically active state. Its Al3+ cation participated in the formation of HA structure during the ripening and heat treatment.

According to FEG SEM and TEM data, as synthesized pure HA materials appeared as agglomerates of uniaxial and prismatic particles with average sizes of 20–30 nm (Fig. 1a). Materials with 0.5 mol.% of Al formed compact agglomerates consisted of spherical particles slightly larger than 30–50 nm with broad size distribution (Fig. 1b). As Al content increases up to 1 mol.%, the particle morphology evolved toward the formation of 70–100 nm length and ∼10 nm in diameter needle-like crystals and remained almost unchanged with the further increase of Al content (Fig. 1c, d). The TEM data confirmed FEG SEM observations and evolution from predominantly round-shaped and irregular-shaped to needle-like and prismatic particles was observed with increase of Al content (Fig. 2). The similar shape changes of as-synthesized by precipitation method HA particles with substitution up to 10 mol. % of Al has been described in Ref. [19].Thus, doping of HA by Al provoke the formation of highly anisotropic phase.

Fig. 1.

As synthesized precipitates before heat — treatment (FEG SEM), a — 0 mol.% of Al; b — 0.5 mol.% of Al; c — 1.0 mol.% of Al; d — 10 mol.% of Al. 2.

(0.62MB).
Fig. 2.

TEM investigations of powders before heat — treatment, a — 0 mol.% of Al; b — 0.5 mol.% of Al; c — 1.0 mol.% of Al; d — 10 mol.% of Al.

(0.85MB).

According to N2 absorption data, the pure as-synthesized HA had the specific area S (BET) of 91 m2/g. The introduction of Al resulted in a noticeable decrease of S down to 53 m2/g. This effect may be connected with the changes in particle size and morphology due to the increase of spherical particles diameter as detected by microscopy (see above). Additionally, the decrease of S can be connected with the change of agglomerates density. (Table 1).

Further increase of Al concentration has resulted in a gradual increase of S up to 90 m2/g for 20 mol.% of Al due to formation of needle-like particles with twice lower thickness compared to the diameter of spherical particles.

3.2Powder TEM ED data and XRD

According to TEM ED data, the as-synthesized powder of pure HA presented in Fig. 2a were formed by the apatite phase (PDF #9-432) with interplanar spacing (d value) of the most intense reflections of d1 = 3.48 Å (002); d2 = 3.13 Å (102); d3 = 2.78 Å (112). The introduction of 0.5 mol.% of Al resulted in formation of HA phase with slightly decreased values of d, with the corresponding ED reflections at d1 = 3.34 A (002) and d2 = 3.14 Å (102) and d3 = 2.77A(112). The increase of Al content up to 1 mol.% did not result in any distinguishable changes in the diffraction patterns compared to 0.5 mol.% (Fig. 2b, c). Similarly, the EED of materials with 10 mol. of Al % revealed the only phase of HA with d1 = 3.34 Å (002), d2 = 3.06 Å (102), d3 = 2.78 Å (112) (Fig. 2d). No additional lines were observed indicating the absence of impurity phases.

According to the XRD analysis, all powders preheated at 300 °C consisted of apatite-like phases with a low degree of crystallinity; the peak width increased with Al content growth, confirming the data of Ref. [40] (Fig. 3a). The only detected phase in all these samples was hydroxyapatite (PDF #9-432) consistent with the electron diffraction data of as-synthesized powders. The shape of triplet between 30,5° and 33,5° became smoother and transformed into the doublet for the sample containing 20 mol.% of Al.

Fig. 3.

XRD spectra of Al — HA powders: + — HA (PDF No: 9 -432), ■ — β-TCP (PDF No: 09–0169), ▼ — Ca9Al(PO4)7 (PDF No: 48-1192), ◊ — α-TCP (PDF No: 29-359); a — 300 °C; b — 900 °C; c — 1200 °C; d — 1400 °C.

(0.39MB).

After the heat treatment at 900 °C the degree of crystallinity of powders increased significantly (Fig. 3b). For Al content below 1 mol.% the only detected phase was HA (Table 2), with the position of main peaks shifted toward larger diffraction angles indicating the decrease of unit cell parameters. Formation of β-TCP (PDF # 09–0169) phase was detected starting from 5 mol.% of Al. We attribute the formation of β-TCP in the samples containing more than 1 mol.% of Al to the limited solubility of Al in HA lattice and starting of dehydration processes confirmed by FTIR and TG data. Further increase of Al content up to 20 mol.% resulted in the full transformation of HA to the whitlockite-like phase. In our previous work on the Al-substituted β-TCP, we had demonstrated the partial transformation of β-TCP into Ca9Al(PO4)7 (PDF # 48-1192) with Al doping and further co-existence of these two phases [41]. Here, XRD data do not allow to determine the phase composition unambiguously due to broad peaks and strong overlap - hence, we further assumed the existence of the only Al-contained whitlockite-like phase - β-TCP observed with HA at 5–10 mol. % of Al; however the presence of some Ca9Al(PO4)7 could not be excluded. At 20 mol.% of Al Ca9Al(PO4)7 was the only detected phase. Reflections corresponded to AlPO4 (PDF # 72-1161) and Al2O3 (PDF # 83-2081) were not detected in all compositions. Refined lattice parameters of β-TCP for materials with 5–10 mol.% of Al and of Ca9Al(PO4)7 at 20 mol.% of Al are summarized in Table 3.

Table 2.

Phase composition.

Ca substitution by Al, mol.%  300 °C  900 °C1200 °C1400 °C
  Ca10(PO4)6(OH)2wt. %  Ca10(PO4)6(OH)2wt. %  β- Ca3(PO4)2wt. % (Ca9Al(PO4)7aCa10(PO4)6(OH)2wt. %  α-Ca3(PO4)2 wt. %  β- Ca3(PO4)2wt. % (Ca9Al(PO4)7aCa10(PO4)6(OH)2wt. %  α-Ca3(PO4)2 wt. %  β- Ca3(PO4)2wt. % (Ca9Al(PO4)7a
100  100  90  10  72  28 
0.5  100  100  100  80  20 
1.0  100  100  100  88  12 
100  78  22  35  65  44  56 
10  100  63  37  13  87  94 
20  100  100  100  95 
a

β-Ca3(PO4)2 transformed into Ca9Al(PO4)7 when 20 mol.% of Al was introduced.

Table 3.

Lattice parameters in nanometers.

Ca substitution by Al. mol.%  900 °C1200 °C1400 °C
  Ca10(PO4)6(OH)2β- Ca3(PO4)2 (Ca9Al(PO4)7)aCa10(PO4)6(OH)2α-Ca3(PO4)2β- Ca3(PO4)2 (Ca9Al(PO4)7)aCa10(PO4)6(OH)2α-Ca3(PO4)2β- Ca3(PO4)2 (Ca9Al(PO4)7)a
  с 
0.9416(3)  0.6883(4)      0.9423(3)  0.6884(5)        1.0431(5)  3.7384(4)  0.9427(3)  0.6910(2)  1.5257(2)  2.7418(5)  1.2863(2)     
0.5  0.9410(2)  0.6874(3)      0.9421(4)  0.6885(2)            0.9414(2)  0.6902(1)  1.5219(4)  2.7281(2)  1.2886(4)     
1.0  0.9408(4)  0.6860(2)      0.9425(2)  0.6889(4)            0.9416(4)  0.6887(3)  1.5219(2)  2.7282(3)  1.2885(4)     
0.9417(6)  0.6872(2)  1.0392(4)  3.7360(2)  0.9427(3)  0.6894(5)  1.5235(4)  2.7364(6)  1.2859(2)      0.94133(5)  0.6916(4)  1.5231(3)  2.7379(1)  1.2856(2)     
10  0.9435(6)  0.6879(4)  1.0400(2)  3.7403(3)  0.9305(6)  0.6892(3)  1.5238(2)  2.7359(3)  1.2861(5)      0.9417(7)  0.6886(6)  1.5239(4)  2.7344(2)  1.2866(5)     
20      1.0339(3)  3.7340(6)            1.0364(4)  3.7432(2)      1.5220(4)  2.7280(4)  1.2886(3)  1.0372(2)  3.7388(3) 
a

β-Ca3(PO4)2 transformed into Ca9Al(PO4)7 when 20 mol.% of Al was introduced.

The heat treatment at 1200 °C resulted in a partial transformation of pure HA (0 mol.% of Al) into β-TCP (about 10 wt.%) due to dehydroxylation of HA structure (as discussed below in connection with FTIR observations) in agreement with literature data [42]. At the same time, the diffraction patterns of materials with 0.5 and 1.0 mol.% contained only reflections of HA without traces of β-TCP. Transformation of HA into the high-temperature modification – α-tricalcium phosphate (α-TCP) (PDF # 29-359) was observed for materials with 5 and 10 mol.% of Al (Fig. 3c). The amount of α-TCP in the resulting mixture was estimated as 65 wt.% and 87 wt.% for 5 and 10 mol.% of Al respectively. Formation of α-TCP for powder with 20 mol.% of Al was not observed, due to stabilization of Al-enriched Ca9Al(PO4)7 phase [43]. No reflections corresponding to any additional phases were detected.

Formation of α-TCP in all compositions including pure HA occurred at 1400 °C. Similar to the data on 1200 °C treatment described above, samples with 0.5 and 1.0 mol.% Al-HA demonstrated higher amount of HA and a lower content of α-TCP phase compared to pure HA (Fig. 3d). The increase of Al content above 5 mol.% resulted in a growth of α-TCP content, which reached 94 wt.% for the 10 mol.% of Al. Traces of α-TCP and whitlockite-like phase Ca9Al(PO4)7 were detected in a material with 20 mol.% of Al,indicating that Ca9Al(PO4)7 starts to transform into α-TCP between 1200 and 1400 °C. The temperature of this transformation was not reported in the literature up to date. No other crystalline phases including calcium aluminates or AlPO4 were detected in our samples.

Unit cell parameters were calculated for HA, β-TCP, Ca9Al(PO4)7 and α-TCP as a function of composition and annealing temperature presented in Table 3. Decrease of Al-HA cell parameters at 900 °C for concentration 0.5–1.0 mol.% of Al could be interpreted as a formation of solid solution due to introduction of Al3+ with lower ionic radii comparing to Ca2+ (0.051 and 0.104 nm respectively) into the HA lattice [44], but the observed magnitude of the effect is close to the detection limit of the XRD method (0.6860 compared to 0.6883 nm).

According to the density functional theory simulation of Wang [40] the most stable Al-HA structure of Ca8.5Al(PO4)6(OH)2 should demonstrate the decrease of parameter a in order of 0.001 nm and parameter с in order of 0.003 nm with respect to pure HA. We have obtained the same order of magnitude changes in lattice parameters refined from XRD data, justifying the formation of solid solution in the case of low Al concentration. We have attempted also to confirm the formation of the solid solution from the electron diffraction data of as-synthesized powders, but the resolution of the method was insufficient to reveal relatively small changes of cell parameters.

It is difficult to interpret unambiguously the observed increase of lattice parameters at >5 mol. % Al content (Table 3). This effect could be attributed to the formation of amorphous Al-rich phase on the surface of the grains. Starting from 5 mol.% of Al β-TCP phase formed along with initial HA, with cell parameters independent on Al content. As the amount of Al increases up to 20 mol. %, the Al-enriched whitlockite-like phase Ca9Al(PO4)7 formed. This phase had significantly lower lattice parameters comparing to β-TCP. At 1200 °C formation of HA structure with lattice parameters close to theoretical ones (PDF #9-432) was observed. The difference between cell parameters of pure HA and Al-substituted HA was within the margin of error. Thus, XRD could not reveal definitely where Al was located — either in a solid solution based on HA structure or in a separate amorphous phase at the grain surfaces. After the heat treatment at 1400 °C, a slight decrease of cell parameters was observed similar to samples treated at 900 °C. Moreover, this effect was masked by partial dehydration of HA which was reported to influence on cell parameters [45].

Only in the case of β-TCP phase formation of Al-substituted crystal structure of 20 mol. % of Al samples was confirmed by the refined values of cell parameters, which were close to one reported for Ca9Al(PO4)7[46].

3.3FTIR investigations

The bands presented in the spectra of powders heated at different temperatures are listed in Table 4. The FTIR spectral bands of as-synthesized powders were wide due to amorphous state of powders and characterized by the presence of adsorbed H2O, OH, PO43−, NO3 and CO3 groups (Fig. 4a). The impurity anion bands of NO3 at 820 and 1380 cm−1 were attributed to residual nitrate groups resulting from nitrate synthesis precursors and traces of the impurity synthesis product phase NH4NO3[47]. The absorption bands at 1419 and 875 cm−1 could be linked with the presence of CO3- in the HA structure adsorbed from the air during the synthesis [48]. The small peak of the stretching vibration of OH at 3570 cm−1 and a slight librational band of OH at 635 cm−1 indicated the formation of the HA structure with low crystallinity and decreased in their intensity with the increase of Al amount. Adsorbed H2O appeared in the spectra by broadened regions at 2900–3400 cm−1 and 1780–1525 cm−1. The phosphate group was detected by significantly broadened peaks of stretching vibration modes ν1 at 962 cm−1 and ν3 region at 1170 cm−1, and 980 cm−1, by doublet at 562, and 602 cm−1 from the PO bending mode ν4, and by the slight triplet at 420–480 cm−1 of doubly generate bending mode ν2[49]. The absence of the bands at 840, 808, and 784 cm−1 which could be attributed to the stretching vibrations mode of AlO in AlO4 and 727, 690, and 646 cm−1 associated with those in AlO6 groups confirmed the introduction of Al as the cation substitute [50,51].

Table 4.

Matched FTIR peaks presented at spectra (Fig.4) at different temperature.

Peak assignment  Temperature/wavenumber
  60 °C  300 °C  900 °C  1200 °C  1400 °C 
Structural OH  3565, 631 cm−1  3565, 631 cm−1  3570, 631 cm−1  3569, 632 cm−1  3571, 631 cm−1 
H2O adsorbed  3510–2832, 1639 cm−1  3510–2832, 1639 cm−1  3510–2870, 1694–1561 cm−1     
CO3 group (ν31444–1416 cm−1  1444–1416 cm−1       
CO3 group  871 cm−1  871 cm−1       
NO3 group  1385, 820 cm−1  1385, 820 cm−1       
PO4 bend ν3  1089, 1044 cm−1  1089, 1044 cm−1  1092, 1044 cm−1  1083, 1024 cm−1  1170–981 cm−1 
PO4 stretch ν1  962 cm−1  962 cm−1  961 cm−1  962 cm−1  963 cm−1 
PO4 bend ν4  607, 568 cm−1  607, 568 cm−1  602, 572 cm−1  602, 573 cm−1  606, 572 cm−1 
PO4 bend ν2  468 cm−1  468 cm−1  467 cm−1  471 cm−1  486–461 cm−1 
P-O-P        828 cm−1  832 cm−1 
HPO42−      890  895   
Fig. 4.

FTIR spectra of Al - HA heated at different temperatures, a — 60 °C, b — 300 °C, с — 900 °C; d — 1200 °C; e — 1400 °C.

(0.66MB).

The spectra of powders heat-treated at 300 °C are similar to the spectra of as-synthesized ones, but with better resolution (Fig. 4b). Additionally, the intensity of NO3 bands at 820 and 1380 cm−1 decreased due to the removal of traces of NH4NO3 during the heat-treatment. The presence of librational and stretching weak bands of OH at 635 and 3572 cm−1 indicated the formation of the HA structure for all compositions including the 20 mol.% of Al.

The FTIR bands of powders after heat treatment at 900 °C are further improved in resolution reflecting the more crystallized structure of powders (Fig. 4c). The FTIR spectra of specimens with 0–1.0 mol.% of Al well corresponded to typical FTIR spectra of HA after heat treatment at 900 °C [52]. The intensity of NO3and CO32− bands decreased dramatically and almost disappeared for all samples. As the amount of Al increased, the librational and stretching band of OH at 635 and 3572 cm−1 became sharp, and gradually decreased in their intensity. For 10 mol% of Al the librational band noticeably diminished and for 20 mol.% of Al disappeared completely due to the formation of β-TCP. The bands related to the phosphate group of ν1- ν2 for materials 0–1.0 mol.% of Al were narrow and appeared at 570, 602, 1034 and 1086 cm−1 with broadening during further increase of Al content. For materials with 20 mol.% of Al a broad band of ν3 PO43-was observed in the region of 1200–950 cm−1 attributed to the structure of β-TCP [53]. The new band of at 890 cm−1 was assigned to HPO42−. The absence of ν2 CO32− mode with the same wavenumber (890 cm−1) is confirmed by the absence of absorption in the region 1545–1410 cm−1 where the much stronger ν3 bands would be seen [54]. Thus, heat treatment at 900 °C resulted in the formation of the HA structure of materials 0–1.0 mol.% of Al. Further increase of Al content stimulated formation of β-TCP due topartial dehydration of HA structure. This was confirmed by the appearance of the bands at 945 and 1025 cm−1 corresponding to oxyhydroxyapatite (OHA) functional group [55].

After the heat treatment at 1200 °C the OH bands observed at 3572 and 635 cm−1 were the most intense for the sample containing 0.5 mol.% of Al. The intensity of OH bands drops significantly over the concentration of 1.0 mol.% (Fig. 4d). Similar to the situation with the 900 °C heat-treated samples, this indicated the process of dehydroxylation in the HA structure with the formation of OHA. The detection of OHA by XRD is difficult due to overlapping of the characteristic peaks [6]. The transformation of HA into α-TCP is confirmed by the broadening of pure HA characteristic bands of PO43− group in the range 1175–950 cm−1, which extended up to 1200–950 cm−1. The band at 830 cm−1 observed at 5–20 mol.% of Al could be attributed to the formation of asymmetric stretching POP bond of P2O5[56] The band of HPO42− at 885 cm−1 demonstrated splitting analogous to the one described in Ref. [54].

At 1400 °C, the spectra of pure HA showed the disappearance of the librational band of OH at 635 cm–1 and a presence of a very slight stretching band at 3572 cm−1 (Fig. 4e). The introduction of Al in low concentration up to 1.0 mol.% resulted in the stabilization of HA structure as indicated by the increase of intensity for both librational and stretching OH- bands, in agreement with XRD data. The bands corresponding to PO43− groups demonstrated the broadening tendency with the increase of Al content. With the increase in the Al content, the absorption band ν2 PO4-group shifted toward higher wavenumbers from 461 to 486 cm−1. The broadband region with low intensity at 720–880 cm−1 at 10–20 mol.% of Al can be assigned to the asymmetric stretching POP [57].

3.4Thermogravimetry

According to the mass-spectroscopy data, the main emitted substance during the heating up to 1400 °C was H2O (18 m/z); additionally some N2O (44 m/z) was detected at temperature below 900 °C as a product of NH4NO3 traces decomposition [58]. TG curves of the powders in the temperature range of 400–1400 °C were presented in Fig. 5. The mass of samples have been normalized on the mass at 400 °C (after the loss of adsorbed water). HA and Al-HA have high adsorption capacity [59]. The addition of Al increases the adsorption capacity due to formation of Al−OH surface groups in addition to the surface POH as discussed by Wakamura et al. [19].

Fig. 5.

TGA curves of Al - HA heated from 400 to 1400 °C.

(0.14MB).

Pure HA did not show noticeable mass loss in the temperature range of 400–900 °C, crystallization of HA was the main process as confirmed by XRD. The HA phase started to lose the structure water after 900 °C and demonstrated increase of dehydroxylation rate with temperature growth, confirmed by the decrease of OH bands intensity in FTIR spectra (1.70% in the range of 900–1200 °C compared to 2.55% in the range of 1200–1400 °C). The first stage of mass loss of samples with 0.5–1.0% of Al was observed in the temperature range of 400–900 °C and can be linked with the removal of N2O and additional adsorbed water preserved by Al−OH surfaces (1.75% and 3.12%, respectively). The intensity of regions corresponding to adsorbed water (3505-2800 cm-1 and 1690-1535 cm−1) on FTIR spectra at 300 °C were remarkably higher compared to pure HA and had the highest intensity for the sample with 1.0 mol.% of Al. Samples with 0.5 and 1.0 mol.% of Al demonstrated TG patterns similar to the one for pure HA at the temperature above 900 °C. The mass loss of 1.38% and 1.13% respectively was linked with desorption of physically absorbed water. According to XRD and FTIR data on OH band at 3572 cm−1, these samples consisted of the apatite phase up to 1200 °C, and started to lose the structural water in the 1200–1400 °C temperature range. For 5 and 10 mol.% of Al the mass loss up to 900 °C was linked with the removal of traces of N2O, adsorbed water and structural OH. The samples with 5 and 10 mol.% of Al demonstrate considerable increase in mass loss at 820 °C which could be attributed to the onset of the thermal decomposition of amorphous HA with the formation of β-TCP. As the 5 mol.% of Al preserved the higher amount of Al-HA according to XRD at 900 °C, the additional effect of adsorbed water is higher compared to 10 mol.% of Al and the corresponding mass losses to 900 °C constituted 2.29% and 1.73%, respectively. The further gradual mass loss at higher temperatures was linked with transformation of Al-HA into TCP structures as confirmed by a decrease of the OH bands intensity in FTIR spectra compared to OH bands in the pure HA.

The sample with 20 mol.% of Al lost the dominant amount of adsorbed water to 400 °C, which was confirmed by FTIR at 300 °C and mass-spectroscopy data. This sample demonstrated acceleration of mass loss at 780 °C attributed to transformation of amorphous HA into β-TCP and the loss of all water to 900 °C. According to XRD data and the absence of H2O and OH bands at FTIR spectra at 900 °C, this correlates with the complete transformation of amorphous Al-HA into β-TCP.

4Discussion

By means of the synthesis precipitation method, we obtained apatite phases at 300 °C and calcium phosphate Al-substituted phases for all Al contents (0–20 mol.%) at 900, 1200 and 1400 °C. According to FTIR data, there were no aluminates in obtained powders [60].

We investigated the kinetics of phase transformations and processes of dehydration of these Al-doped powders as a function of Al content and temperature. The influence of Al on the dehydration of HA observed in our experiments could be summarized in the schematic Fig. 6. We have plotted the amount of crystallized apatite phase – HA and OHA in the samples after heat treatment at 900, 1200 and 1400 °C. For each amount of Al, the predictive line of phase composition depending on temperature was drawn. It could be seen that a small amount of Al ( 0.5–1.0 mol.%) stabilizes apatite phase and the effect seems to be more pronounced in the case of lowest concentration – 0.5 mol.%. In the same time, addition of more than 5 mol. % of Al facilitate the formation of TCP phases. For 20 mol. % of Al complete transformation of HA into β-TCP was observed at a temperature less than 900 °C. This result is confirmed by FTIR spectra and TG curve. Such complex behavior was observed for the first time.

Fig. 6.

The schematic curves of the apatite phase amount (HA + OHA) in the mixture depending on the initial Al concentration.

(0.19MB).

Stabilization of the HA phase by a small amount of Al – 0.5 and 1.0 mol. % is the most interesting and unexpected result of our work. This result was demonstrated by both XRD data (Table 2) and marked differences in FTIR spectra for 1200 and 1400 °C. Previous studies observed the destabilization of HA due to the introduction of a high amount of Al at high temperature [20,33] and did not discuss the influence of a small amount of Al on the HA stability. The reason for this stabilization is the formation of Al solid solution based on HA or formation of an amorphous phase on the grain surface. The changes of lattice parameters and broadening of HA peaks at 0.5–1.0 mol.% of Al are indications of solid solution formation, but peaks broadening also could be interpreted as a decrease of crystalline size of HA. Formation of Al – containing amorphous phase could not be detected by XRD analysis, and consequently, this point needs more detailed investigations. FTIR data demonstrated that high amount of Al led to the dehydration of powders and in the same time the presence of low amount of Al resulted in a conservation of OH groups on the surface of Al-HA compared to pure HA.

It is logical to claim that the incorporation of Al could decreased the transformation rate at low concentration. This effect is connected with a noticeable decrease of specific surface area due to the introduction of a low amount of Al (from 91 to 53 m2/g): which would tend to slow down the dehydration of HA.

Thus, both thermodynamic and kinetic aspects are able to stabilize HA phase at high temperature in the presence of a low amount – 0.5–1.0 mol.% of Al. This result allows to improve the methodology of high-temperature synthesis of HA-based coatings for Al-containing subtracts (Ti6Al14V) due to an increase of HA thermal stability [31].

5Conclusions

Al-substituted HA powders with apatite structure and crystallite size of 30–70 nm were obtained by precipitation method. Introduction of Al in the HA lattice over the substitution range 0.5–1 mol.% resulted in the stabilization of the HA structure at high temperature. The increase of thermal stability due to the introduction of Al at low concentration was not reported previously. Introduction of 5–10 mol.% of Al resulted in the formation of biphasic materials based on HA and whitlockite-like structure at 900 °C and apatite and α-TCP phases at 1200–1400 °C. Introduction of 20 mol.% of Al resulted in the formation of isomorphic Al-substituted whitlockite phase Ca9Al(PO4)7. We estimated the onset of Ca9Al(PO4)7 transformation into α-TCP phase at 1400 °C. This result allows to increase the sintering temperature of Al-HA and produce high-temperature coatings with fixed phase composition.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors would like to thank the Russian Foundation for Basic Research for financial support under the grant No. 18-29-11053. The FEG SEM investigations were partly financially supported by State Assignment no 075-00746-19-00.

The authors thank A. A. Fomina from the Baikov Institute of Metallurgy and Materials Science for performing the ICP-AES investigations.

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