Materials Science and Engineering C 43 (2014) 153–163

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Thermal and structural characterization of synthetic and natural nanocrystalline hydroxyapatite Ancuta M. Sofronia a, Radu Baies b, Elena M. Anghel a, Cornelia A. Marinescu a,⁎, Speranta Tanasescu a a b

Ilie Murgulescu Institute of Physical Chemistry of the Romanian Academy, 060021 Bucharest, Romania National Research Institute for Electrochemistry and Condensed Matter, 300224 Timisoara, Romania

a r t i c l e

i n f o

Article history: Received 18 December 2013 Received in revised form 15 May 2014 Accepted 3 July 2014 Available online 10 July 2014 Keywords: Hydroxyapatite Thermal analysis Fourier Transform Infrared spectroscopy Raman spectroscopy

a b s t r a c t The aim of this work was to study the thermal stability on heating and to obtain the processing parameters of synthetic and bone-derived hydroxyapatite over temperatures between room temperature and 1400 °C by thermal analysis (thermogravimetry (TG)/differential scanning calorimetry (DSC) and thermo-mechanical analysis—TMA). Structural and surface modifications related to samples origin and calcination temperature were investigated by Fourier transformed infrared (FTIR) and Raman spectroscopy, X-ray diffraction (XRD) and BET method. FTIR spectra indicated that the organic constituents and carbonate are no longer present in the natural sample calcined at 800 °C. Raman spectra highlighted the decomposition products of the hydroxyapatite. The calcination treatment modifies the processes kinetics of the synthetic samples, being able to isolate lattice water desorption processes of decarbonization and the dehydroxylation processes. Shrinkage of calcined synthetic sample increases by 10% compared to uncalcined synthetic powder. From the TMA correlated with TG analysis and heat capacity data it can be concluded that sintering temperature of the synthetic samples should be chosen in the temperature range of the onset of dehydroxylation and the temperature at which oxyapatite decomposition begins. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Hydroxyapatite (HAP), (Ca10(PO4)6(OH)2), is the main bone mineral component and is known as an implantable ceramic material for bone tissue and teeth reconstitution [1–6]. In addition to various synthetic chemical methods used to produce HAP, there are alternative ways to obtain HAP by its extraction from natural resources. The materials used as sources for the isolation of natural hydroxyapatite bone are varied: bovine [2,7], chicken [8], fish [9], pork [7,8], sheep bones [10], eggshells [11], coral [12] and seashells [13]. Natural HAP characteristics are different depending on the extraction applied method [4]. Various studies on HAP [5,7] indicate that there are considerable differences between synthetic hydroxyapatite and natural ones. Carbonate ions are a common impurity found in HAP and carbonated HAP appears to be an excellent material for bioresorbable bone substitutes [6]. High temperature processing of HAP-based materials is essential for biomedical applications and this implies the need for knowledge and understanding of thermal stability and transitions on heating. Thermal treatment of hydroxyapatite results in a series of physical and chemical processes that depend on the history and obtaining conditions of samples and significantly influence subsequent behavior of the material. ⁎ Corresponding author at: Ilie Murgulescu Institute of Physical Chemistry, Splaiul Independentei 202, 060021, Bucharest, Romania. Tel.: +40 213121147. E-mail address: alcorina@chimfiz.icf.ro (C.A. Marinescu).

http://dx.doi.org/10.1016/j.msec.2014.07.023 0928-4931/© 2014 Elsevier B.V. All rights reserved.

The HAP phase transformation on heating consists of following processes: dehydroxylation, which takes place in two stages, according to the following reactions Ca10 ðPO4 Þ6 ðOHÞ2 →Ca10 ðPO4 Þ6 ðOHÞ2−2x Ox □x þ xH2 O

ð1Þ

Ca10 ðPO4 Þ6 ðOHÞ2−2x Ox x →Ca10 ðPO4 Þ6 O þ ð1−xÞH2 O;

ð2Þ

where □ denotes hydrogen vacancies, Ca10(PO4)6(OH)2 − 2xOx x is oxyhydroxiapatite (OHA), Ca10(PO4)6O is oxyapatite (OA), and decomposition of OA (stable in a narrow temperature range around 800–1050 °C [5]) along with HAP in calcium phosphates, according to the following reactions [6]: Ca10 ðPO4 Þ6 O→2Ca3 ðPO4 Þ2 þ Ca4 ðPO4 Þ2 O

ð3Þ

Ca10 ðPO4 Þ6 ðOHÞ2 →2Ca3 ðPO4 Þ2 þ Ca4 ðPO4 Þ2 O þ H2 O

ð4Þ

where Ca3(PO4)2 is tricalcium phosphate (TCP) and Ca4(PO4)2O is tetracalcium phosphate (TTCP). Another possible decomposition reaction of OA [5] is: Ca10 ðPO4 Þ6 O→3Ca3 ðPO4 Þ2 þ CaO

ð5Þ

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The study of decomposition mechanisms is thus of key importance to optimize the processing of HAP materials. Sintering is one of the most important manufacturing steps that a ceramic body is subject to. It determines, mostly, the final properties of the ceramic, its behavior in service. The achievement of the desired final properties is dependent of the processing parameters. Therefore data are needed at sintering temperatures. The aim of this work consists of thermal (TG-DSC, TMA), structural (X-ray diffraction, FTIR and Raman spectroscopy) and surface (BET, SEM, dynamic light scattering, DLS) characterization of the synthetic and natural HAP powders, emphasizing the influence of originating of samples and calcination temperature on the thermal stability of HAP samples. Thermal behavior of the HAP samples is discussed in terms of sample porosity and phase composition. 2. Materials and methods 2.1. Materials The investigated samples were synthetic, namely, commercial hydroxyapatite from Aldrich (HA) and calcined commercial hydroxyapatite at 900 °C, in air (HA900), and natural hydoxyapatite, extracted from bovine bone (HAN). 2.1.1. Bovine bone-derived hydroxyapatite A bovine tibia was obtained from a slaughterhouse and washed in water to remove visible tissues on the bone surface. A piece of cleaned bone of around 100 g was cut and ground to transform it into a fine powder (HANb). To remove the internal fats and proteins the bone powder was washed with deionized water until the pH of the washing water had the same value with the initial deionized water. This washed bone powder was treated with hydrogen peroxide 3% and then again rinsed with deionized water following the same previous protocol. The resulting powder was then dried in the oven under ambient conditions, at 100 °C, with 2 h holding time. Natural HAP was obtained after calcinations of the dried powder (HAN100) in an electric furnace under ambient conditions. Three calcination temperatures were tested, namely 700 °C, 800 °C and 900 °C, with one hour holding time [7]. The calcined powders of HAN100 were ground again and characterized by FTIR and TG-DSC techniques to find the optimal calcination temperature. All HAP powders were stored in air atmosphere without humidity/ carbon dioxide control. 2.2. X-ray diffraction Powder X-ray diffraction (XRD) patterns were recorded using a Panalytical XPERT-PRO diffractometer with Bragg–Brentano configuration, operating with a PIXcel detector using Cu Kα radiation. Data were acquired in the range 10–70° (2θ) using a step size of 0.0263° (2θ). The sample was rotated on a monocrystalline silicon support. XRD analysis was carried out to identify different phases present in the starting powders as well as the formation of any thermal decomposition product of HAP. To measure the instrumental line broadening a known silicon powder standard was used. The mean crystallite size (d) was calculated using Scherrer equation: Kλ  d¼ βsample −βstandard  cosθ

ð6Þ

where shape factor, K was taken 0.9, assuming that the crystallites were spherical particles, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM), and θ is the Bragg angle.

2.3. Thermal analysis For the thermal characterization a simultaneous high temperature thermogravimetry (TG) and differential scanning calorimetry (DSC) were employed. Thermal properties (temperature of transformations and mass change) of the HAP samples were measured by a TG-DSC Setaram Setsys Evolution 17 analyzer, in the temperature range from 40 to 1400 °C, with a scanning rate of 10 °C/min, in alumina crucibles, using Ar flow. Sample mass for simultaneous TG-DSC measurements was about 20 mg. The thermoanalyzer calibration was previously reported [14]. The error of TG measurement is ±0.154%. Heat capacities were determined in the temperature range of 400– 1450 °C, using a sample mass of about 100 mg, the same gas flow, Ar, at a rate of 16 ml/min, in Pt crucibles. The protocol of heat capacity running is similar to that used in paper [14]. To determine peak temperatures and heat capacities, instrument software (Calisto-AKTS) was used. Peak deconvolution was done using modified Gaussian function (CALISTO software). Linear expansion behavior of samples was measured from 40 to 1400 °C at a heating rate of 5 °C/min, in an Ar flow, using a thermomechanical analyzer (TMA) Setaram Setsys Evolution 17. Load is 0.049 N. By TMA technique the sample deformation under non-oscillating stress in function of a temperature program was measured. Powders were previously compacted at a pressure of 200 MPa in a 5 mm cylinder die. For each analyzed sample a blank curve obtained under the same conditions as those employed to test the samples was subtracted. 2.4. Surface and morphology characterization Specific surface area (S, m2 g−1) of HAP samples was measured by the Brunauer–Emmett–Teller (BET) nitrogen adsorption method in a Nova 2200e Quantachrome surface area analyzer. The samples were dried and degassed at 150 °C for 3 hours, and analyzed using a multipoint N2 adsorption/desorption method. The pore volume and average pore radius were calculated using Barrett–Joyner–Halanda (BJH) model. The primary particle size (DBET) was calculated by assuming the primary particles to be spherical [15]: DBET ¼

6 Sρ

ð7Þ

where ρ is the theoretical density of stoichiometric HAP (3.156 g/cm3) and S is the specific surface area (S). Size distribution and polydispersity index (PDI) of the powders were determined by dynamic lighting scattering (DLS, Malvern Nano ZS ZEN3600). PDI represents a dimensionless measure of the broadness of the size distribution calculated from the cumulants analysis (obtained by Zetasizer software). Surface characterization and size distribution of the sintered samples at 910 °C, 950 °C and 800 °C, 2 hours, in air, for HA, HA900 and respective HAN800 samples were also undertaken. Powder morphology was examined by scanning electron microscopy (SEM) using a high-resolution FEI Quanta 3D FEG (Dual Beam) microscope. 2.5. Spectroscopic analysis 2.5.1. FTIR spectroscopy Fourier Transform Infrared Analysis (FTIR) spectra were performed using the NICOLET IS10 equipment in the attenuated total reflection (ATR) mode, in the wavelength range of 525–4000 cm− 1, using 32 scans and a spectral resolution of 4 cm−1 to obtain information about the various chemical bonds. The peak locations and intensities were determined with the Omnic software (Nicolet Instrumentations Inc., Madison, WI, USA).

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Fig. 1. (a) FTIR spectra of HANb, HAN100 and HAN100 powder calcined at 700 °C, 800 °C and 900 °C. (b) DSC, TG and dTG curves of HAN100 powder.

2.5.2. Raman spectroscopy Unpolarized solid state Raman spectra were recorded by means of a LabRam HR spectrometer (Jobin–Yvon–Horiba) over 100–3600 cm−1 range. The 633 nm line of a Nd:YAG laser was used as exciting radiation through a 100× objective of an Olympus microscope in a backscattering geometry and at a confocal hole of 200 μm. The diameter of the laser spot on the sample surface amounted to about 2 μm providing a spectral resolution better than 2 cm−1. The resulting spectra were background corrected and curve fitted by Gaussian–Lorenzian profile using Igor software [16]. 3. Results 3.1. FTIR and TG-DSC analysis of the natural hydroxyapatite extraction process To analyze the changes that occur during the processing of bovine bone to extract natural HAP, FTIR analysis was done at various stages of the process on following powders: the initial bone powder (HANb), the washed and dried (100 °C) bone powder (HAN100) and bone powders calcined at different temperatures. In Fig. 1 the FTIR spectra of HANb, HAN100 and HAN100 powder calcined at 700 °C, 800 °C and 900 °C are presented. The HANb spectrum presents bands characteristic to both HAP (calcium phosphate groups) and amide groups of the proteins, namely 1560 and 1653 cm−1 [4,7] (Fig. 1). The bands specific to CO2− 3 group are present in HANb spectrum and belong to both A- and B-type CO2− (1540 cm−1 and 1418, 1456 cm−1 respectively) [6,17]. 3 The intensity of bands associated to amide groups (collagen) is much lower in the HAN100 spectrum than HANb. Although the bands

corresponding proteins groups disappear on calcination even at temperature of 700 °C, it can be seen that the bands corresponding carbonates are still present (Fig. 1). By calcination at 800 °C and 900 °C carbonate was removed completely (Fig. 1). Knowing the temperature range in which the thermal events occur and the transformations associated to them is important in establishing the calcination temperature of HANb powder for obtaining natural hydroxyapatite. According to the DSC-TG and dTG curves in Fig. 1 the mass loss of 32.62% of HAN100 sample takes place during the four stages: - 7.44% up to 217 °C due to loss of physisorbed water and to decomposition of a part of the organic matter - 17.56% in the temperature range 217–665 °C, due to liberation of chemically bonded water, decomposition of the organic matrix and carbonates - 6.52% for temperatures between 665–1121 °C, attributed both to decarbonization, process that can continue up to 900–1000 °C with different intensities depending on the source of the sample [5] and beginning of the dehydroxylation processes - 1.08% for dehydroxylation and decomposition in the 1121–1400 °C temperature range TG-DSC and FTIR data confirm the total removal of organic components and the carbonate by calcination at 800 °C, which leads to the conclusion that the optimal temperature for calcining the washed bone powder is 800 °C, while avoiding the HAP decomposition at higher temperatures than 800 °C. Therefore, the thermal analysis of natural HAP calcined at 800 °C (HAN800) is further discussed in the present paper.

Table 1 Phase composition and crystallite size calculated from X-ray measurements for thermal treated samples up to 1400 °C. Sample

Phase,%

Crystallite size, nm

HA HA900 HAN800 HA after DSC measurements

Ca5(PO4)3(OH) Ca5(PO4)3(OH) Ca5(PO4)3(OH) Ca4O(PO4)2, 88 Ca3(PO4)2, 12 Ca4O(PO4)2, 74 Ca5(PO4)3(OH), 26 α-Ca3(PO4)2, 51 Ca10(PO4)6O, 49

42 88 60 75 40 60 64 97 71

HA900 after DSC measurements HAN800 after DSC measurements Fig. 2. X-ray diffraction patterns of the HAP samples.

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Table 2 Specific surface area, porosity, average pore radius and particles dimension determined by BET technique for HAP samples. HAP Powders samples

Sintered samples

HA HA900 HAN800 HA at 910 °C HA900 at 950 °C HAN800 at 800 °C

DBET (primary particle size), nm

Specific surface area (BET), m2/g

Porosity at P/Po ~ 1, 10−2 cm3/g

Average pore radius, nm

99 200 138 247 453 322

19.2 9.5 13.7 7.7 4.2 5.9

4.3 for pores smaller than 173.4 nm 1.8 for pores smaller than 122.6 nm 13.7 for pores smaller than 105.5 nm 3.9 for pores smaller than 104.7 nm 1.5 for pores smaller than 177 nm 1.5 for pores smaller than 115.7 nm

4.5 3.7 19.9 10.1 6.9 5.2

3.2. Structure, morphology and specific surface area characterization XRD patterns of the HAP powders showed typical patterns for stoichiometric (Ca/P = 5:3) hexagonal HAP (Fig. 2). Small amount of CaCO3 (2%) was identified in the X-ray spectrum of HA900 sample. This is a consequence of yielding small quantity of CaO by calcination of the initial HA sample at 900 °C in air and poorly controlled storage of the HA samples in air atmosphere. CaO transforms into Ca(OH)2 and/or CaCO3 upon storage in air [6]. The X-ray diffraction analysis was performed on all synthetic and natural samples patterns, before and after thermal analysis (Table 1). BET surface area, total pore volume and average pore radius are presented in Table 2. N2 adsorption isotherms of HAP powders are shown in Fig. 3. This type of adsorption isotherms occurs on porous adsorbents with pores in the range of 1.5–100 nm. HA900 sample presents less adsorption than HA sample (inset of Fig. 3) in the whole range of pressure. The heat treatment at 900 °C of HA powder reduced by half the specific surface area from 19.2 to 9.5 m2/g. The behavior of HAN800 powder follows that of the HA sample at low P/Po values, except after the onset of a very sharp increase in adsorption due to capillary condensation at P/Po = 0.9. It can be observed that there are small adsorbate–absorbent interaction potentials (Fig. 3), and they are also associated with pores in a large

pore size distribution range with small micropores contributions and at the same time macropores domains (Fig. 3). Pore size distribution calculated from the desorption branch of HAP powders in Fig. 3 indicates that HAP powders have a wide pore size distribution which can be classified into three populations of sizes (trimodal). This finding is supported by SEM images of the powders (Fig. 4). Both synthetic HAP powders are inhomogeneous. Fig. 4 displays the SEM pictures and the corresponding DLS results of the samples. The DLS measurements showed a disparity in the particle size compared to the sizes obtained by BET method (due to the polydispersivity of the samples), but correlated well with the morphology observed in the SEM images (Fig. 4). HA particles are bidispersive (Fig. 4a) with size values between 59 and 531 nm (Fig. 4d). The smaller particles fill the pores between the larger particles. The calcination treatment narrowed the pore size distribution (b80 nm compared to 100 nm for HA sample) and enlarged the particle size distribution to the interval of 79–825 nm. The polydispersity index (PDI) values (around 0.3 for HAP powder samples) indicate that the particles formed as agglomerates [18] (Fig. 4). The round shape of the HA particles (Fig. 4a) is not destroyed by calcinations at 900 °C (Fig. 4b). Heat treatment of the HA samples leads to the particles coalescence initiation and subsequence the shrinkage of the pores. The mesopore contribution (average pore radius = 20 nm) to overall pore volume is higher for HAN800. The natural HAN800 sample

Fig. 3. (a) Adsorption–desorption isotherms and (b) pore size distribution for HAP powders.

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Fig. 4. SEM pictures (upper) and the corresponding DLS results (lower) of the samples (a, d) HA, (b, e) HA900, and (c, f) HAN800.

presents uniform pseudo-spherical shape particles (Fig. 4c), which are aggregated in large agglomerates with size values up to 955 nm. The tendency of HAN800 sample to form large agglomerates may be due to the small size of its primary particles. The mesoporosity in the HAN800 sample is formed due to a release of volatile material (hydrogen peroxide originated from HAP extraction stage from bone) and combustible burn-outs that are lost during calcination [19].

3.3. Spectroscopic characterization 3.3.1. FTIR analysis The IR spectra for both powder and sintered samples are illustrated in the Fig. 5 while their spectral deconvolution results and band assignments are listed in the Table 3 [20–25]. All spectra present the absorption bands corresponding to the specific groups of HAP [26]. FTIR

Fig. 5. FTIR spectra of HA, HA900 and HAN800 (a) powders and (b) sintered samples.

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Table 3 Vibration frequencies in FTIR spectrum of HA, HA900 and HAN800 samples. Wave number, cm−1 HA 568, 601 630 877 962 1026, 1087 1417, 1456 3569

HA900 564, 599 630 876 962 1025, 1091 1456 3572

Assignment [22–25] HAN800 ν4 (O–P–O) OH– libration mode type B- v2 vibration mode; v2/HPO2− CO2− 3 4 962 ν1—P–O–P-stretching 1030, 1090 ν3 (P–O) vibration mode B-type CO2− 3 –v3 vibration mode 3572 OH− stretching mode 564, 599 630

spectrum of HAN800 powder presents sharp well defined peaks. The bands located at 1417 and 1456 cm− 1 observed in the IR spectra of HA and HA900 samples are attributed to the ν3 mode of the B-type CO23 − group (CO23 − groups that have replaced PO34 − groups). This indicates that the synthetic samples analyzed here are a type B sample, namely Ca10 − x(PO4)6 − x(CO3)x(OH)2 − x.

3.3.2. Raman analysis Given the fact that during high-temperature solid state processing of the HA materials non-uniform nanosized particles and extraneous compounds are formed [27], Raman investigations were conducted on HA, HA900 samples and HA heat treated sample at 1010 and 1400 °C. Raman spectra of these samples are illustrated in Fig. 6 while their results of deconvolution within 870–1180 cm−1 region and the band assignments are presented in Table 4. The very intense and narrow band at about 961 cm− 1 in these synthetic materials is assignable to the symmetric P–O stretch (ν1 PO34 −) in the stoichiometric HA with molar Ca/P ratio of 1.667 [28] and/or carbonate apatite (CAP) [29,30]. Symmetric bending, ν2, asymmetric stretching, ν3, and asymmetric bending, ν4, modes of the PO3– 4 groups are present in the 400–450, 1028–1076 and 579–610 cm −1 regions [28], respectively of the Fig. 6. The group of low intensity bands in the 50–350 cm−1 range derives from translations of the – 3– Ca2+, PO3– 4 , and OH ions and librations of the PO4 ions [31].

Table 4 Peak position/Full Width at Half Maximum (FWHM) and assignments of the Raman spectra for the synthetic samples. HA

HA900

– – 949/ 18 961/9 – – 1026/ 14 1038/ 13 1047/ 10 1053/ 10 1065/ 19 1073/ 12 1086/ 3 –

– – –

HA1010 947/8

HA1400 Assignments [22,23,27,28] 939/4 946/5 955/5

963/4 – – 1029/ 6 1042/ 9 1048/ 4 1054/ 12 –

960/7 966/6 1017/9 1028/15

961/6 968/7 1008/5 1026/ 14 –

1076/12 1075/6

–

1076/ 7 1087/ 3 – – –

–

–

α-TCP α-TCP Symmetric P–O stretching ν1

α-TCP Anti-symmetric P–O stretching triply degenerate, ν3

1048/18 1047/ 12 – –

– 1091/5 1121/14 1120/9 1164/32 1133/ 10 1164/32 –

ν1 CO3 P − O stretch (ν3) of the HPO2− 4 v3 PO4

If the full width at the half maximum (FWHM) of the stretching mode within 950–970 cm−1 region gives indication of the crystallinity of the hydroxyapatite materials, the hydroxyl band peaking up at 3573 cm− 1 (Fig. 6) is an indication of the dehydroxylation degree [32]. Upon heat treatment up to 900 °C, the FWHM of the 961 cm− 1 band of these synthetic samples was narrowed (Table 4) from 9 to 4 cm−1, pointing out that crystallinity degree increased in the HA900 sample. Conversely heating up to 1400 °C of the HA sample (HA1400) trigger formation of the new peaks around the ν1 mode of the PO3– 4 namely 939, 946, 955, and 968 cm−1 which might indicate the formation of the tricalcium phosphate α-TCP [29] with smaller Ca/P ratio

Fig. 6. Raman spectra of the synthetic samples: (a) HA, HA900 and (b) HA heat treated at 1010 and 1400 °C.

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Fig. 8. TMA curves for HA, HA900 and HAN800 powders. Fig. 7. (a) TG–DSC and (b) dTG curves for HA, HA900 and HAN800 powders.

than HA. The calcium phosphate compounds have drawn attention as raw biomaterials for bone repairs [29]. 3.4. TG-DSC analysis The thermal behavior of HAP samples and mass loss associated to them are presented in Fig. 7 and Table 5. The mass loss for HA sample takes place slowly compared to HA900 that presents a decrease in mass that occurs in five well defined stages. The dTG curves of HAP samples are presented in Fig. 7. The theoretical mass loss for a complete HAP dehydroxylation is 1.793%. A mass loss bigger than theoretical value for the studied samples (3.840–1.793 = 2.047% for HA; 4.410– 1.793 = 2.617% for HA900; 3.500–1.793 = 1.707% for HAN800) is assigned to other processes than dehydroxylation (desorption of adsorbed water and carbonates). These processes overlap for HA and respective HAN800 samples in certain temperature ranges. 3.5. TMA analysis In order to investigate the linear shrinkage during sintering, thermomechanical studies were conducted and the shrinkage data are plotted in Fig. 8. The shrinkage rate in Fig. 9 is determined by differentiating the TMA curves and can be considered as the densification kinetics of HAP powders [6]. The onset temperatures of densification and the maximum shrinkage for HAP samples are presented in Table 6. For the HAN800 sample, which is stable in the temperature range of 200–700 °C the average CTE (coefficient of thermal expansion) value of 16.74 * 10−6 C−1 was obtained. 3.5.1. Apparent activation energy (apparent EA) of densification process Apparent EA was calculated based on thermomechanical results. For initial stage of sintering the apparent EA of densification can be calculated from linear shrinkage vs. temperature curve using the Arrhenius type equation: ½ðdL=L0 Þ=T  ¼ const:  expð−nEA =RT Þ

ð8Þ

where dL/L0 is the relative shrinkage at temperature T, EA is the activation energy, R is the universal gas constant and n is a constant describing the sintering mechanism (n = 1 for viscous diffusion, 0.5 for volume diffusion and 0.33 for grain-boundary diffusion) [33,34]. The plot of Ln[(dL/L0)/T] vs. 1000/T will be a straight line with slope equals to −nEA/R. Considering n = 0.33 [33] the values of apparent activation energy of sintering were obtained for all investigated samples and are presented in Table 6. 4. Discussion 4.1. Thermal behavior of the HAP samples Knowing the parameters of the thermal processes at heating has great importance in establishing the conditions for heat treatment, according to the desired characteristics and applications of the material. Thermal changes of the HAP samples and their corresponding weight loss on heating are the following: 1. HAP presents two types of water-adsorbed water and lattice water [5]. The first stage of mass loss in the ΔT1 temperature range for all samples is attributed to evolution of adsorbed water on the hydroxyapatite surface (physically bound water) with endotermic effect observed on heat flow curves (Fig. 7). In the ΔT2 temperature interval, the mass change represents the water loss within the network, lattice water (chemically bound water—water inside the pore or release of chemisorpted water), a phenomenon that occurs slower than desorption of physisorbed water (Fig. 7) [4,31]. The difference between mass loss attributed to lattice water desorption for the synthetic samples and HAN800 (Table 5) is given by the fact that the porosity and average pore radius of natural HAP are higher than those of synthetic samples (Table 2). 2. There are differences in thermal behavior of HA and HA900 samples and their processes in ΔT3 temperature range will be presented separately. For HA sample, in the temperature range of 606–1090 °C the mass loss is 2.30% and dTG curve shows an asymmetric peak. The deconvolution of this peak presented in Fig. 10a leads to three

Table 5 Mass loss of HAP samples obtained by TG analysis. Sample

ΔT1, °C

Δm1, %

ΔT2, °C

Δm2, %

ΔT3, °C

Δm3, %

ΔT4, °C

Δm4, %

ΔT5, °C

Δm5, %

Δm total, %

HA HA900 HAN800

40–245 40–220 40–253

−0.35 −0.20 −0.47

245–606 220–567 253–757

−0.25 −0.15 −0.63

606–1090 567−900 757−1400

−2.30 −2.32 −2.26

– 900–1125 −

– −0.89 –

1090−1350 1125−1350 –

−0.94 −0.85 –

−3.84 −4.41 −3.36

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Fig. 9. Heat capacity, derivative linear shrinkage curves and derivative thermogravimetric curves of HAP samples.

component peaks with temperature of 675 °C, 835 °C and 988 °C (maximum rate of mass loss). These peaks correspond to different processes that overlap. The difference in the TG behavior of the two synthetic samples is the temperature range and rate at which processes take place. For instance, a steepy mass loss in the temperature range of 567– 900 °C, representing about half of the total mass loss (2.32%) is observed for the HA900 sample (Fig. 7). In ΔT3 temperature range a large, sharp peak (Tpeak = 703 °C) on the dTG curve corresponds to removal of lattice water with a mass loss of 2.08% (the total amount of lattice water accounts for no more than about 2% of the total weight in apatite) [6] and a smaller peak (Tpeak = 835 °C) is attributed to decarbonatation. A loss of carbonate, an endothermic reaction, has a maximum rate at around 835 °C for both synthetic samples. In the Fig. 11 FTIR spectra of the synthetic samples heated up both to 835 °C (Fig. 11a and b), HA heated up to 1010 °C (Fig. 11c) and HA900 heated up to 900 °C (Fig. 11d), respectively, in an Ar flow, are presented. The presence of the carbonate bands in the FTIR spectra (Fig. 11) (a) and (b) and its absence in the (c) and (d) spectra, confirm the temperature interval (700–1010 °C for HA and 806–900 °C for HA900) of the decarbonatation process obtained from TG-DSC measurements. 3. Dehydroxylation and decomposition of the HAP samples. In the temperature range of 1090–1350 °C, the last stage of mass loss, 0.94% for HA is attributed to OHA already formed at lower temperatures and HAP dehydroxylation. Dehydroxylation process followed by the starting of the HAP decomposition to α-TCP, is observed in the Raman spectrum of HA sample heat treated at 1010 °C (Fig. 6) by the occurrence of a shoulder at 947 cm−1 corresponding to the α-TCP phosphate. Therefore, the three concurrent processes of HA sample that occur in the temperature range of 606–1090 °C are removal of lattice water (606–790 °C), decarbonization (700–1010 °C) and dehydroxylation (above 860 °C) (Fig. 10a). Dehydroxylation of HA900 is shifted to higher temperature (Tonset = 900 °C, Fig. 7), than that of the HA sample (Tonset = 860 °C, Fig. 10a). In the

Table 6 Onset temperatures of densification, maximum shrinkage and apparent activation energy of densification for HAP samples. Sample

Maximum shrinkage, %

Tonset, °C

EA, kJ/mol

HA HA900 HAN800

12.3 13.5 15

880 920 780

584 601 366

Fig. 10. Peak deconvolution of the dTG curves for (a) HA and (b) HAN800 samples.

temperature range of 900–1350 °C, mass loss is 1.74% for HA900 sample and coresponds to the theoretical one for a complete HAP dehydroxylation (1.793%). Above 1350 °C, the synthetic sample weight remains unchanged, i.e. dehydroxylation process ends to this temperature. As seen in Fig. 7, above 1100 °C, two successive endotherms are observed for the synthetic samples. According to the narrow OA stability range of 800–1050 °C [5], and the slope change at ~ 1050 °C on both DSC (Fig. 8) and Cp curves (Fig. 9), we tentatively assign the first endothermic peak to OA decomposition (Eq. (3)) formed during HAP dehydroxylation. The onset temperature of this peak is shifted to lower temperatures for HA sample consistent with earlier beginning of their dehydroxylation. The second endothermic less visible effect is assigned both to HAP direct decomposition into TTCP and β-TCP (Fig. 9) and to β-TCP → α-TCP phase transformation. A mass loss of 2.26% was encountered for the HAN800 sample within 757–1400 °C (Fig. 7). Moreover, the dTG deconvolution presented in Fig. 10b shows an overlapping of the dehydroxylation and decomposition reactions (Eqs. (1)–(4)) in the same temperature range. This finding indicates that HAN800 does not dehydrate completely before decomposition, and explains the superposition of several processes during heating [35]. On the corresponding Cp curve two peaks with similar shape as those of the synthetic samples but shifted to higher temperatures (1360 °C and 1415 °C, respectively) are originating from the same processes as for synthetic samples. 4.1.1. HAP reconstruction A very important property of the dehydroxylated HAP is that it could be rehydroxylated in a water vapor atmosphere as low as 400 °C [5] and is dependent on cooling rate. When the HA900 gradually cools from 1400 °C, a part (26%) of HAP is reconstructed, while in the case of HAN800, 51% of OA is reconstructed (Table 1). By calcination in air at 900 °C of the HA sample to HA900 a partial dehydroxylation took place and, at the same time, a rehydroxilation to HAP on cooling occured. This explains the higher mass loss of HA900 than HA during thermal analysis. Presence of a greater amount of OH groups (representing chemically bound hydroxyl groups) for the HA900 than the HA sample is confirmed by the more intense OH at 3570 cm−1 in the FTIR spectra (Fig. 5). Also, narrower and more intense Raman band, at 3572 cm−1 (Fig. 6a), proves a higher ordered lattice and a set position [32] for the hydroxyl ion in case of the HA900 sample. Since the FWHM of this band varies from 12.7 cm−1 in case of HA to 5.5 cm −1 for HA900, the area of this band is more informative regarding different hydroxylation degree of the two samples. Thus, a 1.63 times higher

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Fig. 11. FTIR spectra of the HA (a) and HA900 (b) samples heated up to 835 °C; HA heated up to 1010 °C (c); HA900 heated up to 900 °C (d).

hydroxylation degree, derived as aria ratio of the 3573 cm−1 band, was found for HA900 in comparison to HA. It is worth to notice that the calcination process enabled us to distinguish the following processes: the lattice water desorption (220–806 °C), the decarbonization (806–900 °C) and the dehydroxylation (900–1350 °C) (the kinetics of processes was changed). This emphasizes the importance of the heat treatment regime on the change in microstructure that can lead to differences in the resulting material properties. 4.2. Thermomechanical characterization of HAP samples Linear shrinkage measurement is used to simulate the process of sintering and to generate an optimized temperature profile. Comparing TMA curves corresponding to HA and HA900 (Fig. 8), it can be seen that densification begins for both samples around the onset temperature of dehydroxylation. Inflection point (around 1015 °C for HA, 1030 °C for HA900 and 1180 °C for HAN800 samples) on TMA curves corresponds to the slope change on Cp curves (OA decomposition temperature) and is characterized by a rapid increase in densification, as expected

as a consequence of the decomposition phenomena [36]. On the dTMA curves the splitting of peak for HA and HA900 samples with maximum temperatures corresponding to a variable rate of densification is observed in the temperature range of 980–1030 °C for HA and 990–1063 °C for HA900. The shape of the obtained TMA curves clearly indicates that there is a major solid phase sintering (broad peak in densification rate, Fig. 9) [37]. From the TMA correlated with TG analysis and heat capacity curves it can be concluded that sintering of the synthetic samples is appropriate to be carried out in the temperature range of the onset of dehydroxylation and the temperature at which oxyapatite decomposition begins (Fig. 9). This is explained by the fact that at the microscopic scale, solid-state diffusion in crystalline solids, which is responsible for sintering, requires the presence of point defects within the crystal structure [38]. Once HAP dehydroxylation began, OHA with hydrogen vacancies is formed according to the reaction (Eq. (1)), which facilitates the densification process. The creation or the presence of vacancies of the limiting species always activates the sintering [17]. This is in concordance with the earlier onset of sintering in the case of HA

Fig. 12. DSL results on HA (a), HA900 (b) and HAN800 (c) sintered samples.

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compared to the HA900 sample due to its lower onset temperature of dehydroxylation. For synthetic samples, the shrinkage of calcined samples increases by 10% compared to uncalcined synthetic powder. The HAN800 sample exhibits a different densification behavior due to its high porosity. Densification can be considered as a clearly twostage process. The first stage is characterized by an early onset at about 780 °C (the onset temperature of dehydroxylation) and a low densification rate. Since the HAN800 powder presents large agglomerates, it needs a smaller EA to initiate the densification process, the interparticle neck forming and growing, respectively. This stage occurs with a light densification, visible on the TMA curve. The second stage is characterized by a sharp increase in densification rate at a higher temperature with a maximum densification rate at 1180 °C. The densification does not end up to 1400 °C. For HAN800 sample, the shrinkage rate decreases compared to synthetic samples, which indicates that higher temperature is necessary to reach the final stage of sintering. The wider temperature range needed for shrinkage to proceed makes grain growth associated with sintering less possible [39]. These experimental data are of particular importance in controlling the sintering temperature program, aiming to avoid exceeding the threshold temperature where HAP decomposition begins. Accordingly, the following temperatures were selected for the sintering process: 910 °C for HA, 950 °C for HA900 and 800 °C for HAN800. FTIR spectra of the sintered samples (Fig. 5) present the absorption bands corresponding to the specific groups of HAP, confirming the fact that the samples do not decompose at the selected sintering temperatures. By sintering the increased average pore size can be observed as well as a small decreasing of the porosity due to the coalescence of the smaller particles for the synthetic samples. For the HAN800 sample with large agglomerates and mesopores (average pore size of 20 nm), a high decrease in both average pore size and porosity after sintering is recorded (Table 2). Instead, the particle size distribution of the HAN800 sample is narrowed and shifted to the greater values than 1 μm by sintering (Fig. 12).

5. Conclusions In this work, synthetic and natural HAP powders were subjected to a systematic study of thermal behavior of HAP as a function of history, particle size and porosity. FTIR spectra indicate that the organic constituents and carbonate are no longer present in the bone sample calcined at 800 °C, suggesting that this temperature is adequate to obtain both protein and carbonate free natural samples. Regarding porosity and morphology, it was clearly observed that the natural sample exhibits the highest porosity (average pore radius = 20 nm). Nevertheless, synthetic calcium phosphates do not show similar thermal and structural properties as in bone. Also, calcination temperature strongly influences thermal stability range of HAP and reactions kinetics, with consequences on the densification process. Correlating surface characterization (BET, SEM), DLS, FTIR, TMA, TG/DSC and heat capacities data the temperatures of powders sintering were obtained. In conclusion, the structural, surface and thermal properties exhibited by the HA900 sample indicate that it is appropriate for obtaining of HAP-based materials for bone tissue and teeth reconstitution.

Acknowledgements The financial support of the INFRANANOCHEM Nr. 19/01.03.2009 and National Program II—Partnership ctr. 72-184 grants are acknowledged. We thank Professor Oana Gingu from the University of Craiova for providing us the synthetic samples. We thank Florentina Maxim, PhD, from the Ilie Murgulescu Institute of Physical Chemistry, Bucharest for SEM imagines.

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