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ScienceDirect Materials Today: Proceedings 3 (2016) 2869–2876

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5th International Advances in Applied Physics and Materials Science Congress & Exhibition (APMAS2015)

Various Parameters Affecting the Synthesis of the Hydroxyapatite Powders by the Wet Chemical Precipitation Technique Azade Yeltena, *, Suat Yilmaza a

Istanbul University, Department of Metallurgical and Materials Engineering, Avcilar-Istanbul, 34320, Turkey

Abstract Hydroxyapatite (HA) is the most well-known mineral among the bioactive ceramic materials. HA crystals can be described as the calcium and phosphorus source that provides the sufficient density and strength of the bone as well as the bioactivity. Wet chemical precipitation technique has practical advantages such as simplicity of the experimental stages, low reaction temperatures, and the possibility of controlling the chemical composition and microstructure properties of the final product with harmless by-products. Therefore in this study, HA powders were synthesized by the wet chemical precipitation technique where calcium hydroxide (Ca(OH)2) was used as the calcium source precursor while orthophosphoric acid (H3PO4) as the phosphorus source precursor. There are several factors -such as the pH value of the reaction solution, dropping rate of the acid solution into the alkaline solution, temperature of the reaction solution, the stirring rate, etc.- that play role in the wet chemical precipitation process and some of these parameters were investigated in this work. Each of these factors individually may influence the crystal and/or amorphous phases that are obtained at the end of the process. Preparation and reaction of the precursor solutions, aging of the final solution and precipitation, filtrating and washing, drying and eventually heat treating the precipitation are the main steps that were carried out throughout the synthesis process. X-Ray Diffraction (XRD), Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) and Fourier Transform Infrared Spectroscopy (FTIR) characterization studies were applied in order to determine the chemical, microstructural and molecular bonding properties of the powders produced. © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee Members of 5th International Advances in Applied Physics and Materials Science Congress & Exhibition (APMAS2015).

* Corresponding author. Tel.: +90-212-473-70-70 (17650); fax: +90-212-473-71-80. E-mail address:[email protected] 2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee Members of 5th International Advances in Applied Physics and Materials Science Congress & Exhibition (APMAS2015).

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Keywords: Hydroxyapatite; Wet Chemical Precipitation Technique; pH; Temperature; Characterization.

1. Introduction Biomaterials can be classified into 4 main groups as bioceramics, metallic biomaterials, biocomposites and polymeric biomaterials. Bioceramics draw attention due to their high compatibility with the body hard tissues such as bone and teeth [1,2]. Hydroxyapatite (Ca10(PO4)6(OH)2, HA) is one of the most important member of bioceramics family and readily found in human and animal bones as a natural component. Therefore usage of HA in different forms such as powders, coatings, composites, etc. can be described as an ideal way to create a safe and effective osseointegration with body tissues. Also, stoichiometric HA (Ca/P molar ratio: 1,67) is the most stable calcium phosphate phase against the effects of the corrosive and variable pH of the body fluid [1-3]. There are several ways to produce HA such as precipitation, solid state reactions, sol-gel technology, hydrothermal reactions, combination of mechanochemical reactions and obtaining from natural sources, i.e. egg shells, bovine bones, etc [2, 4-12]. Wet chemical precipitation technique is an appropriate method for synthesizing HA in large amounts economically and practically at low working temperatures. Moreover by product of the precipitation reaction is not harmful and the final product has high purity. However there are several process parameters that may influence the final properties (crystallinity, particle size and morphology, stoichiometry, etc.) of the achieved powders. Since most of the process parameters play a role on varying the final properties of the HA powders, a well-built control and evaluation of the experimental parameters and their effects on the HA powders are essentially required [2, 5, 8, 13-15]. This study focuses on the influence of NH4OH addition, working temperature and acid addition rate on the chemical, microstructural and molecular bonding properties of the CaP powders produced by wet chemical precipitation technique. 2. Materials and Method HA powders were synthesized by following the wet chemical precipitation process and heat treatment steps [2, 5, 14, 16]. A detailed experimental chart for HA powder preparation is given Fig. 1. The starting materials were Ca(OH)2 (Calcium Hydroxide, Sigma-Aldrich, ACS reagent , ≥95%) as the calcium source and H3PO4 (Phosphoric Acid, Sigma-Aldrich, ACS reagent, ≥85% wt. in H2O) as the phosphorus source. The stages in this experimental chart can be respectively explained as mixing the acid and alkaline solutions at room temperature or 50-60 °C by adding the alkaline solution to the acid solution drop wise with a rapid or slow addition rate, addition of an appropriate amount of NH4OH solution (Ammonia Solution 25% for analysis, Merck) to keep the pH value near 9-10, aging the CaP suspension obtained after continuously stirring (and heating) the solution mixture, filtrating the CaP suspension by using a Buhner funnel and a vacuum pump and washing the CaP precipitate with distilled water for two times, drying the CaP filter cake, grinding the dried precipitate to obtain CaP powders and finally heat treating the CaP powders in alumina crucibles at 900 °C with a 10 °C/min heating rate in a high temperature laboratory furnace to improve the crystallinity of the HA powders. NH4OH addition is an optional stage and was applied to some of the samples. HA powders were synthesized according to the precipitation reaction between the Ca and P sources given below [2, 5, 7, 15, 16]: 10Ca(OH)2 + 6H3(PO4) → Ca10(PO4)6(OH)2 + 18H2O

(1)

Chemical phase determination was realized by X-Ray Diffraction (XRD) analyses through monochromatic Cu-Kα radiation (λ= 0.154 nm) at Rigaku D/Max-2200/PC branded device. From the acquired X-Ray Diffraction data, the mean crystallite size (D) of the powders was calculated using the Scherrer Formula [17] given in Equation 2. In this formula, λ stands for the wavelength of CuKα, B for the full width at half maximum (FWHM) and θ for the diffraction angle.


D=

0.9λ B cosθ

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(2)

Fig. 1. Experimental steps respectively carried out in the wet chemical precipitation process to produce HA powders.

Microstructure investigation of the powder samples were performed with a Jeol branded JSM 5600 model Scanning Electron Microscope (SEM) and the elemental analyses were done with a XRF branded 550I model Energy Dispersive Spectroscopy (EDS) which is integrated to the SEM. The powder samples were coated with a thin layer of gold before being placed in the SEM. Molecular bonding properties of the powders were examined with the Fourier Transform Infrared Spectroscopy (FTIR) analyses using a Perkin Elmer Spectrum 100 branded device and the KBr method.


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Table 1. A summary of the process parameters considered for each sample. Sample Code

Acid Addition Rate

NH4OH Addition

Working Temperature (˚C)

Min. pH

900WCP1

10 ml/min (rapid)

with

room

9.92

900WCP2

1 ml/min (slow)

without

room

7.09

900WCP3

25 ml/min (rapid)

without

room

9.23

900WCP4

20 ml/min (rapid)

with

50-60

8.03

900WCP5

0.9 ml/min (slow)

without

50-60

4.91

900WCP6

20 ml/min (rapid)

without

50-60

6.27

Results of the characterization analyses were evaluated considering a comparison basis. Namely, the parameters for making a comparison between the samples are NH4OH addition for 900WCP1 and 900WCP3 at room temperature, acid addition rate for 900WCP2 and 900WCP3 at room temperature, NH4OH addition for 900WCP4 and 900WCP6 at 50-60 °C process temperature, acid addition rate for 900WCP5 and 900WCP6 at 50-60 °C process temperature. The process parameters considered for each sample were summarized in Table 1. “900” in the sample codes presents the heat treatment temperature. 3. Results and Discussion Firstly, it was found out that there is a distinct correlation between the temperature and the pH value. As the temperature increases, the pH value decreases. This situation can be realized also from the minimum pH values specified for each sample. pH variation was not significantly noticed when temperature effect was not active. Although the aim of NH4OH addition is to increase the amount of (OH-) ions in the solution, it is a difficult case to increase the pH of the system and keep it at 8-9 values even though NH4OH addition was applied for the samples WCP1 and WCP4. It is thought that higher temperatures may cause the evaporation of NH4OH. Moreover as the temperature goes up, ionization of water increases and more OH- and H+ ions will be released to the solution which leads to the decrease of the pH value. Temperature plays a role on the better solubility of the Ca source and rate of the precipitation reaction, i.e. reaction kinetics of HA formation, as well [16].


(a)

(c)

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(b)

(d)

Fig. 2. Comparison of the XRD results of the (a) 900WCP1 and 900WCP3, (b) 900WCP2 and 900WCP3, (c) 900WCP4 and 900WCP6, (d) 900WCP5 and 900WCP6 samples (PDF#09-432).

The precipitation reaction was clearly observed for the all samples especially for the ones which were prepared with NH4OH addition. The resultant precipitate was in a more aqueous state and needed a longer filtration time compared to the samples prepared with NH4OH addition. Acid addition rate influences the pH stability of the reaction solution. The final pH value recorded at the end of the synthesis process is definitely related to the acid addition rate [7,16]. It takes naturally a longer period to see the pH change and break the alkaline buffer when a slow acid-addition rate was preferred. It is easier to lower the pH of the system by applying a rapid acid addition rate, however it was observed that the pH of the solution still had a tendency to increase. It is determined from the XRD analyses (Fig. 2-a, b, c and d) that all samples showed the characteristic peaks of hexagonal synthetic Hydroxyapatite (HA, syn, (Ca5(PO4)3(OH)) phase and P63/m with PDF (Powder Diffraction Files) number 09-432 which was confirmed from several studies in literature [5, 6, 9, 10]. These results demonstrated that in all cases, HA phase could be obtained and the effect of process parameters i.e. acid addition rate, NH4OH addition and working temperature, on the produced phase was not evident. Therefore SEM-EDS and FTIR results become crucial in order to understand the influence of the process parameters on the properties of the HA powders. Crystallite size of each sample was calculated as 50.07 nm for 900WCP1, 58.61 nm for 900WCP2, 21.21 nm for 900WCP3, 71.90 nm for 900WCP4, 103.30 nm for 900WCP5 and 74.48 nm for 900WCP6. The crystallite sizes of the samples produced at room temperature are smaller than the samples obtained at 50-60 °C temperature. As the synthesis temperature increased, the peaks in XRD spectra become narrower and sharper. This situation explains the increase of the HA precipitate and therefore the crystallite size. Narrower and sharper peaks can be also interpreted as the sign of the increased size crystallinity [6].

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Fig. 3. SEM images of the (a) 900WCP1 (x1000), (b) 900WCP2 (x250), (c) 900WCP3 (x250), (d) 900WCP4 (x250), (e) 900WCP5 (x250) and (f) 900WCP6 (x250) samples.

HA powders have a high tendency to agglomerate and this case was confirmed by the SEM images (Fig. 3-a, b, c, d, e and f) from which it was also observed that HA phase has a flat agglomerated structure with different sizes [6]. Size of the HA agglomerates vary between 10-100 µm. It was thought that HA agglomerates were present in a platelike morphology due to the grinding effect. EDS analyses exhibited that Ca, P and O elements exist in the structure, as expected. (Ca/P) molar ratio was calculated close to 1,67 for most of the prepared samples.

Fig. 4. FTIR results of the WCP4, 900WCP4, WCP6 and 900WCP6 samples.


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FTIR analyses helped us to reveal the chemical bonding characteristics of the powder samples. FTIR spectra was also supported by the studies previously carried out [2, 5, 6, 8, 10, 13]. FTIR spectrum of WCP 4, 900WCP4, WCP6 and 900WCP6 samples (Fig. 4) showed the characteristic peaks of OH- (hydroxyl) group in the region of 3570 cm-1 and 620 cm-1. The band at ~1030-1040 cm-1 arise from the PO4-3 (phosphate) group and defined as the strongest peak of the HA phase. The peaks at ~560 cm-1, ~600 cm-1 and ~470 cm-1 were also assigned to the PO4-3 group. The weak absorption bands seen at 1420 cm-1 and 875-880 cm-1 indicates the presence of CO3-2 (carbonate) group. The reason for the formation of the CO3-2 ions can be considered as the absorption of CO2 from the atmosphere. Since the synthesis steps, i.e. mixing of the solutions, aging&precipitation, filtering&washing, were completed under air atmosphere, CO2 absorption and the reaction of it with the OH- ions in the solution is a naturally expected case [5]. The bands at ~1650 cm-1 and ~3400 cm-1 were attributed to H2O. 4. Conclusion Some important facts about this research can be summarized as:  HA powders were synthesized by following the wet chemical precipitation process. The influence of NH4OH addition, working temperature and acid addition rate on the chemical, microstructural and molecular bonding properties of the CaP powders was investigated.  The starting materials were Ca(OH)2 as the calcium source and H3PO4 as the phosphorus source.  It is determined from the XRD analyses that all samples showed the characteristic peaks of hexagonal synthetic Hydroxyapatite (HA, syn, (Ca5(PO4)3(OH)) phase with PDF number 09-432.  The crystallite sizes of the samples produced at room temperature are smaller than the samples obtained at 50-60 °C temperature.  HA powders have a high tendency to agglomerate and this case was confirmed by the SEM images.  Characteristic peaks of the OH-, PO4-3 and CO3-2 groups were detected from the FTIR spectrum. Acknowledgements The authors would like to thank the Research Fund (BAP) (Grant no: 37881) and Teaching Staff Training Program Office (ÖYP) of Istanbul University for providing financial support to this project. References [1] B.D. Ratner, A.S. Hoffman, F.J. Schoen, J.E. Lemons, in: B.D. Ratner, A.S. Hoffman, F.J. Schoen, J.E. Lemons (Eds.), Biomaterials Science An Introduction to Materials in Medicine, second ed., Elsevier Academic Press, New York-London, 2004, pp. 1-9. [2] K. Salma, L. Berzina-Cimdina, N. Borodajenko, Processing&Application of Ceramics. 4(1) (2010) 45-51. [3] A. Yelten, Properties and Characterization of Alumina-Bovine Hydroxyapatite (BHA) Composites Produced by Sol-Gel Method, M.Sc. Thesis, Istanbul University, 2010. [4] N. Monmaturapoj, J. Met. Mater. Miner. 18(1) (2008) 15-20. [5] A. Paz, D. Guadarrama, M. Lopez, J.E. Gonzales, N. Brizuela, J. Aragon, Quim. Nova. 35(9) (2012) 1724-1727. [6] S.S.A. Abidi, Q. Murtaza, J. Mater. Sci. Technol. 30(4) (2014) 307-310. [7] M.P. Ferraz, F.J. Monteiro, C.M. Manuel, J. Appl. Biomater. Biom. 2 (2004) 74-80. [8] C. Garcia, C. Paucar, J. Gaviria, A. Duran, Key Eng. Mater. 284-286 (2005) 47-50. [9] L.B. Kong, J. Ma, F. Boey, J. Mater. Sci. 37(6) (2002) 1131-1134. [10] Y. Liu, D. Hou, G. Wang, Mater. Chem. Phys. 86(1) (2004) 69-73. [11] C. Liu, Y. Huang, W. Shen, J. Cui, Biomaterials. 22(4) (2001) 301-306.

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[12] A. Yelten, S. Yilmaz, F.N. Oktar, Ceram. Int. 38(4) (2012) 2659-2665. [13] H. Eslami, M. Solati-Hashjin, M. Tahriri, Iranian Journal of Pharmaceutical Sciences. 4(2) (2008) 127-134. [14] M.H. Santos, M. de Oliveira, L.P. de Freitas Souza, H.S. Mansur, W L. Vasconcelos, Mat. Res. 7(4) (2004) 625-630. [15] W. Kim, F. Saito, Ultrason. Sonochem. 8(2) (2001) 85-88. [16] A.K. Nayak, Int. J. ChemTech. Res. 2(2) (2010) 903-907. [17] B.D. Cullity, Elements of X-Ray Diffraction, second ed., Addison-Wesley Publishing Company Inc., USA-Canada, 1978, pp. 102.