Zr doped hydroxyapatite

Zr doped hydroxyapatite

Accepted Manuscript Influence of microwave processing and sintering temperature on the structure and properties of Sr/Zr doped hydroxyapatite Ravinde...

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Accepted Manuscript Influence of microwave processing and sintering temperature on the structure and properties of Sr/Zr doped hydroxyapatite

Ravinder Kumar Chadha, Anirudh P. Singh, Kanchan L. Singh, Chetan Sharma, Vandana Naithani PII:





MAC 21015

To appear in:

Materials Chemistry and Physics

Received Date:

25 July 2018

Accepted Date:

30 September 2018

Please cite this article as: Ravinder Kumar Chadha, Anirudh P. Singh, Kanchan L. Singh, Chetan Sharma, Vandana Naithani, Influence of microwave processing and sintering temperature on the structure and properties of Sr/Zr doped hydroxyapatite, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.09.086

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Influence of microwave processing and sintering temperature on the structure and properties of Sr/Zr doped hydroxyapatite Ravinder Kumar Chadhaa, Anirudh P. Singha*, Kanchan L. Singhb, Chetan Sharmaa, Vandana Naithania a b

I.K.G. Punjab Technical University, Jalandhar

D.A.V. Institute of Engineering and Technology, Jalandhar

*Corresponding Author Dr. Anirudh P. Singh E-mail: [email protected]


Abstract: Among the different cations that are substituted to modify the properties of calcium hydroxyapatite, Sr and Zr have gained interest because of their effect on bone formation and mechanical properties, respectively. In the present work, Sr and Zr substituted hydroxyapatite, Ca9.39Zr0.11Sr0.5(PO4)6(OH)2 are synthesized by conventional and microwave sintering at different temperatures. The effect of different sintering techniques and temperatures are investigated on the structural and morphological properties of as-synthesized samples. The X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) is used to study the variation in structural properties and scanning electron microscopy (SEM) is used to investigate the morphology with the variation in sintering conditions. The changes in lattice parameters clearly show that Sr and Zr are incorporated into hydroxyapatite lattice. Microwave sintered samples retain the higher amount of hydroxyapatite and experience higher density levels as compared to conventional sintering. As compared to conventional sintering, the grain size is more uniform and low percentage of secondary phase is observed in microwave sintered sample. Keywords: Calcium hydroxyapatite; Microwave sintering; Conventional sintering; X-ray diffraction


1. Introduction Increasing number of accidents, trauma, arthritis, and tumors have increased the interest of the scientific community to find new biomaterials [1-3]. Many research groups are working for the development and evaluation of new synthetic biomaterials for bone grafting. The main challenge in biomaterial engineering is the quest to find a material which has good biocompatibility, resorbability and mechanical strength [4-6]. Many materials are tried and tested, but they are found inappropriate regarding the above- mentioned properties. The most appropriate material to date is calcium hydroxyapatite (HA) [7-9]. HA is considered as a better option as compared to other calcium phosphates due to its biocompatibility and osteoconductive properties. It also exhibits excellent chemical and biological affinity with the bone tissue [10]. HA is basically calcium hydroxyapatite which makes most of the mineral part of bone and tooth enamel. But its stability in vivo and low mechanical strength as compared to bone has limited its use in loadbearing applications [11, 12]. To improve its mechanical properties, many approaches like the addition of dopants and different synthesizing routes have been investigated [13]. The best structural aspect of HA is its ability to accommodate a great variety of cations and anions while retaining its hexagonal structure. These substitutions can alter the thermal stability, surface reactivity and biological properties of HA [14-16]. Among the bivalent cations that can replace Ca in calcium hydroxyapatite, Sr (1.17 Å) has gained much interest due to its comparable size as that of Ca (1.06 Å) and its beneficial effect in the treatment of osteoporosis [17]. In vitro, Sr stimulates the bone formation and reduces the bone resorption. Therefore, the addition of Sr to HA increases the bioactivity of hydroxyapatite [18]. There is one another element Zr, which is incorporated in HA to increase the mechanical strength. This element is generally bio-inert and does not help in bonding with the natural bone,


but it provides the required mechanical strength to HA [19]. Incorporation of the substituents Sr and Zr on the calcium site in HA requires removal of Ca from the site. In HA structure, there are ten total sites of Ca. When cations of different ionic radii replaces Ca at the site, there will be changes in the lattice parameters, dimensions, crystallinity and apatite solubility [20]. The increase in solubility makes HA more suitable in vivo. Even the substitution of these elements is supposed to increase the release of Ca from the lattice which also increases the cell response [21]. In spite of the presence of dopants, the processing conditions also affect the mechanical properties of the HA. To achieve the excellent mechanical strength, it is desirable to sinter the HA at high temperature. But at high-temperature HA decomposes into tri-calcium phosphate (TCP). The presence of TCP affects the mechanical and bioactive properties of HA [22]. In the past microwave processing has been reported to be a processing technique which reduces time and temperature of sintering [23]. Heating in MW ensures uniform heating with no thermal gradient. In MWS grain growth can be controlled easily which controls the mechanical strength [24]. Microwave sintered samples have uniform microstructure and higher density than the conventionally sintered products [25, 26]. There are many reports in the literature related to the doping of hydroxyapatite with different elements. But there are only a few reports which emphasize on the synthesis of doped hydroxyapatite by different routes and compare the physical properties. Considering all the above facts, in the present investigation, Sr and Zr doped HA, Ca9.39Zr0.11Sr0.5(PO4)6(OH)2 is synthesized through the conventional and microwave sintering, and the effect of dopant and synthesis conditions have been analyzed on the structural and morphological properties of HA.


2. Materials and Methods The precursor of composition Ca9.39Zr0.11Sr0.5(PO4)6(OH)2 (Sr-Zr HA) was prepared by mixed oxide method. The AR grade powders of CaCO3 (99.5%), P2O5 (99.99%), SrO (99.5%), ZrO2 (99%) were taken as the starting materials in the stoichiometric ratio in a polyethylene bottle. The mixture was ball milled for 6 h in acetone medium using zirconia balls of size 1.6-1.8 mm. After the milling, the acetone was removed by evaporation at room temperature. The precursor powder was characterized by differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis by using Netzsch DSC/TG system in the temperature range between 30-1000 °C. The precursor powder was calcined in a conventional electric furnace at 900 °C for 3 h. The completion of the reaction was ensured by reheating the powder at 900 °C for 30 min as no weight loss was found. The calcined powder was mixed with 2% PVA binder and consolidated into cylindrical pellets of diameter 13 mm at a pressure of 310 MPa. The green pellets five in number were sintered at 1100 °C and another batch of green pellets five in number were sintered at 1200°C for 3 h in a conventional electric furnace. The heating rate was 2ºC/minute. The green pellets five in number were sintered at 1100°C and another batch of green pellets five in number were sintered at 1200 °C for 30 minutes under microwave energy and the results were compared. The density of the sintered products was determined by the Archimedes principle. The calcined and sintered samples were characterized by X-ray diffraction to identify the phases formed. X-ray powder diffraction study was performed at room temperature using PANalytical X'Pert PRO system (Neitherlands) with Ni - filter. During the experiment, the step size was 0.0131 °/min. For more structural information, the prepared samples were also characterized by the Fourier Transform Infra-red spectroscopy (FT-IR) by using Perkin Elmer made FTIR. For the FTIR measurements, the samples were mixed with KBr and pelletized. The scanning electron


micrographs along with energy dispersive spectroscopy studies were recorded by Scanning electron microscope (JSM-6510LV, JEOL, Japan) to analyze the effect of heating on the grain development and size of the sample. The microhardness of the samples was determined by using Vicker hardness tester (HV-1000B, Russell Fraser Sales). The sintered samples were diamond polished to get an optical finish for the measurement. The hardness was measured by the ratio of applied load via a geometrically defined indenter to the contact area of the impression using the formulae: HV  1854.4 P / d 2 (GPa)

Where Hv is the Vicker hardness in GPa, P is the applied load in kg and d is the indentation diagonal length in mm [27] 3. Results and Discussion

3.1 Thermal Analysis The DSC/TGA curves of the precursor powder are shown in fig. 1. In TGA curve, the change in weight (%) with respect to temperature is given. The change in weight (%) can be divided into three main stages. In stage I, around 8% weight loss is observed between 30 and 150 °C. This weight loss can be due to the evaporation of water or moisture on the surface of the hydroxyapatite sample. In stage II, sample experience around 15% weight loss between 150 and 600 °C. This weight loss can be due to the mixed effect of evaporation of water from the closed pores and release of carbon dioxide from the sample. This loss of water in the second stage is related to the evaporation of water present within the network (chemically bound water—water inside the pore or release of chemisorbed water). This loss of water occurs slower than


desorption of physisorbed water [28]. This change can also be attributed to the formation of crystalline phases inside the sample. Finally, between 600-800 °C, a third stage abrupt weight loss of around 13 % is observed. This weight loss is attributed to the release of carbonates and the











. Even if the weight loss from the stoichiometric equation is calculated by considering the evaluation of carbon dioxide due to the conversion of carbonates from the precursors, the total weight loss comes out to be 28.51% which is comparable to the weight loss occurring in step II and step III. These steps involve more than one processes, so, this much difference occurs in theoretical and experimental values. The same changes with respect to temperature can also be observed in DSC curves. In DSC curve, mainly three peaks are found which are centered at 100, 500 and 660 °C. These three peaks correspond to three stages of weight loss. Out of these three peaks, peaks at 100 and 660 are quite sharp, but the peak centered at 500 °C is like a broad hump which indicates that many chemical processes are occurring in this temperature range as explained in the previous paragraph (TGA section).

3.2 X-ray Diffraction Fig. 2 shows the X-ray diffraction of the as-synthesized samples. The samples do not have single phase. Some percentage of calcium phosphate is present. All the present phases are tabulated in


table 1. The lattice parameters and crystallite size of the samples were calculated from the XRD data by using the formulae [30]: 1

 4 l2  2 2 2 d  ( h  hk  k )  Lattice parameter:  3a2 c 2  

Crystallite size:

(Scherrer’s formula)

Where D is crystallite size, k is shape factor (0.94), λ is the wavelength of the CuKα radiation, is the width of the peak at half maximum in radians and θ is peak position. The calculated parameters are summarized in table 2. The crystallinity (Xc) corresponds to the fraction of crystalline apatite phase in the studied volume of the powdered sample. The crystallinity is related to the full width at half maxima (βhkl) by the formula [14]:

Ka is set at a constant value of 0.24 and peak at (300) was used to calculate the FWHM. The calculated values are listed in table 2. With the increase in temperature, the crystallinity of the conventionally sintered samples increases which implies that with increasing temperature, the diffusion increases and results into the more crystalline phase [14]. Even the crystallinity of the microwave sintered samples are higher than the conventionally sintered samples at the comparative temperatures which is in accordance with the other XRD results and supports the fact that microwave sintering results in higher diffusion within small time as compared to conventional sintering.


As compared to undoped hydroxyapatite, the substitution of Sr2+ and Zr4+ has increased the size of the unit cell which is reflected with the increase in lattice parameters. This change can be attributed to the difference in ionic radii of Sr2+ and Ca2+ [31]. The ionic radius of Sr2+ (1.13 Å) is larger than the ionic radii of Ca2+ (0.99 Å). The larger ionic radii of Sr is responsible for the increase in unit cell parameters. Zr4+ is also substituted in the lattice, and it has smaller ionic radii (0.84 Å) as compared to Ca2+. But its concentration is very low as compared to Sr2+, so, Sr2+ effect dominates over the effect of Zr4+. Moreover, the change in lattice parameters of Sr-Zr HA demonstrates that the Sr2+ and Zr4+ ions are structurally incorporated in the lattice, and they do not just occupy the surface of the crystal. The substitution of Sr2+ and changes in the crystal structure can be explained more deeply after considering the substitution style of the dopant in place of Ca2+ in the lattice. The HA crystal structure has two non-equivalent Ca2+ sites (the site I and site II). Cations at the site I are coordinated to nine oxygen atoms of six PO43- forming the triangle and displaying columnar arrangement, whereas cations at site II are heptacoordinated by six oxygen atoms of five PO43anions and one OH- anion. The distance between Ca-Ca at the site I is found to be the smallest and the smallest distance between the cation and coordinated oxygen (anion) is at site II. Because of this geometry, the small ions are incorporated at the site I and bigger ions are incorporated at site II. That clearly indicates that the Zr4+ ion will be substituted at the site I and the Sr2+ ion at site II because of its larger ionic radii and increase the length and width of the crystal structure [32]. Apart from the effect of dopant, the effect of different sintering temperature and route can also be clearly seen on the unit cell volume and lattice parameters. In conventional sintering when the temperature is increased from 1100 °C to 1200 °C, the XRD peaks are shifting towards higher


angle side. A decrease in lattice parameters and increase in crystallite size is observed with the increase in the sintering temperature. This can be attributed to the increase in the percentage of the secondary phase Ca3 ( PO4 )2 along with the major phase of Ca5 ( PO4 )3 (OH). The increase in the percentage of secondary phase also reflects that increase in temperature enhances the growth of secondary phase. With the increase in temperature more diffusion occur which results in the increase of the crystallite size. In the microwave sintering (MWS), same phases got stabilized only in the 30 min, much less time as compared to 3 h conventional sintering (CS). When the temperature is increased from 1100 °C to 1200 °C in MWS, the XRD peaks are shifting towards lower angle side, and this is also reflected in the increase in lattice parameters. This indicates that with a rise in temperature the inter-particle interaction also increases and results in higher lattice parameters. As compared to conventional sintering, microwave sintered 1100 °C sample has shown a little increase in diffraction angle and a small decrease in unit cell parameters. It is quite possible that this microwave sintering temperature and time is not sufficient for the diffusion of the particles and hence results in this change. But as the temperature is increased up to 1200 °C, an increase in lattice parameters and crystallite size can be seen which demonstrates the proper inter-particle diffusion and results in the increase of secondary phase also. As compared to conventional sintering, in microwave sintered sample at 1100 °C, the percentage of secondary phase is quite low. So, it can be said that microwave sintering enhances the growth of Ca5 ( PO4 )3 (OH) and suppress the growth of secondary phase.


3.3 Fourier Transform Infrared Analysis Fig. 3 shows the FT-IR spectra of all the samples. The important bands observed in spectra are given in table 3. Small peaks around 2521 and 3429 cm-1 in the spectra correspond to the O-H vibrations of the hydroxyl group and adsorbed water. The peak around 3640 cm-1 corresponds to O-H stretching mode of surface P-OH groups. The peaks around 910, 1113 and 1805 cm-1 corresponds to asymmetric stretching of the PO4 group. The peaks around 862 and 1459 cm-1 are originated due to the O-H stretching of HPO42- group. The peak around 695 cm-1 mainly corresponds to the joint contribution of asymmetric stretching of P-O bond and vibrational mode of the hydroxyl group. The sharp bending mode doublet at 552 cm-1 indicates that samples are highly crystallized [33, 10]. In FT-IR spectra, when we move from calcined to sintered samples, no change in band position has been observed. But the bands become more diffused in sintered samples as compared to calcined sample. This change can be attributed to the increase in the percentage of secondary phase in the samples. No significant difference is observed in the FT-IR spectra of conventionally sintered and microwave sintered samples. This indicates that the sintering route affect the crystal structure, crystallite size but not the functional groups associated with the compound.

3.4 Density Measurement The measured value of the density of all the samples is given in table 4. It is apparent from the table that the density increases with the increase of sintering temperature during both the conventional sintering and microwave sintering. It is also apparent that the density of the microwave sintered samples are higher than the conventionally sintered samples at both temperatures 1100oC and 1200oC. This data is also well supported by scanning electron


micrographs (discussed in section 3.5). The difference in density of different samples can be due to the different surface exposure in MWS and conventionally sintered samples. Microwave sintering offers high diffusion which results in dense samples as compared to conventionally sintering [34].

3.5 Morphology Study Scanning electron microscopy is used in the present study to analyze the effect of different sintering temperature and processing techniques. Conventional sintering process is a diffusion controlled process and diffusion occurs through various modes like volume diffusion, grain boundary diffusion and surface diffusion [12]. Whereas in microwave sintering process, highfrequency electromagnetic waves interact with the cations and ions of the iono-covalent solids and induce motion in them [35, 36]. This induced motion is resisted by the frictional, elastic and internal forces. Because of this, electric field associated with the microwave radiation is attenuated and cause volumetric heating of the material. As compared to CS, MWS requires less time due to the volumetric heating of the article. But within this short time, MWS results in proper grain growth and good densification of the material [37]. Scanning electron micrographs of CS and MWS samples are shown in fig. 4 along with their electron diffraction spectra (EDS). The presence of Ca, Sr, P, Zr and O elements in the EDS spectra clearly indicates that all the elements are present in the compound. Some of these elements were not reflected in the phases detected in XRD pattern because of their low concentration.


The SEM study reveals that MWS samples are denser as compared to CS samples. This difference can be due to the different surface exposure in both the routes. In conventional sintering, the sample is under normal temperature flow, and the exposed surface is the hottest part of the sample. At high temperature, the flow of ions increases which results in the closure of surface porosity but results in no remarkable change in density. Whereas microwave sintered samples are denser as compared to the conventional sintered samples. This increase in densification can be due to the increase in ion flow because microwave filed impart excess energy to the diffusion process [38, 39]. The grain size calculated for all the samples from the SEM micrographs are shown in fig. 5. For conventionally sintered samples at 1100 0C, the grain size lies in the range of 0.3-0.4 µm and increase to 0.5-0.6 µm at 1200 0C. This grain growth can be attributed merely to the increase in temperature. With the rise in temperature, more energy is supplied to the grain boundaries which results in higher ion diffusion and hence grain growth. For microwave sintered samples the same trend can be seen for samples sintered at 1100 0C and 1200 0C. In the present study, the same grain size with high densification is obtained with MWS sintering at 1100 0C and 1200 0C in only 30 min. Whereas, in conventional sintering, the grain size is achieved within a sintering time of 3h with low densification. Thus, it can be said that in CS samples, grain boundary diffusion does not reach a densification level corresponding to grain growth. The microwave sintering has a direct effect on the grain boundary which causes grain growth. For the grain growth, the leading energy is provided to the grain boundaries by the interaction between microwave field and sintered material. This energy makes the ions to flow across the grain boundaries and results in the ionic diffusion which is the crucial factor in grain growth. Due to these differences in CS and MWS samples, the difference in mechanical properties can also be


expected. The volume fraction, porosity and grain size equally determine the biaxial flexural strength. With increasing density and grain size, an increase in biaxial flexural strength can be expected in microwave sintered samples [40].

3.6 Microhardness The basic principle of Vicker hardness is to observe the material's ability to resist the permanent deformation. The hardness is determined from the load divided by the surface area of indentation. The calculated values of the hardness are given in table 3. The microwave sintered sample at 1200 0C shows the maximum values for hardness. With the increase in the sintering temperature for conventionally sintered samples, the hardness increases and same trend is observed for the microwave sintered samples. The microwave sintered samples has more hardness as compared to conventionally sintered samples. These results directly correlate with the density of the samples. With the increase in sintering temperature, the density of the samples also increases. The density of the microwave sintered samples is higher than the conventionally sintered samples and hence the hardness [41]. If the hardness values are correlated to the grain size distribution, then according to Hall Petch equation, the microhardness should decrease with the increase in the grain size distribution [42, 43]. But in the present studied samples opposite tend has been observed. With the increase in the grain size, hardness also increases. These contrary results can be due to the combined effect of secondary phase and density. Due to the presence of tricalcium in all the samples, the hardness increases. It is also reported in literature that the tricalcium phosphate exhibits maximum


hardness at 1200 0C [44]. Even in the present study the samples sintered at 1200 0C (both conventional and microwave sintered) exhibits high values of hardness.

4. Conclusion The incorporation of Sr and Zr in place of Ca in hydroxyapatite allows the formation of calcium hydroxyapatite with larger unit cell parameters along with the secondary phase of tricalcium phosphate (TCP). In MWS samples, a decrease in secondary phase has been observed. As compared to CS samples at 1200 0C, microwave sintered samples shows an increase in lattice parameters due to proper inter-particle diffusion. FT-IR spectra do not show any significant change in both CS and MWS samples. Dense HA with a grain size in the range of 0.3-0.6 µm with conventional sintering and 0.4-0.8 µm with MWS has been obtained. Higher densification in MWS samples has been observed due to increase in ion diffusion. Higher grain size and less porosity also manifest that the MWS can have high flexural strength.

Acknowledgment: Authors are thankful to I.K.G. Punjab Technical University, Jalandhar Punjab, India, for providing facilities to carry out research work and Sophisticated Analytical Instruments Laboratories, Thapar Technology Campus, Thapar University, Patiala, India for carrying out different characterizations.


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ACCEPTED MANUSCRIPT Figure Captions Fig. 1: Differential scanning calorimetry and thermogravimetric analysis of precursor powder. Fig. 2: X-ray diffraction pattern of (a) Precursor powder (b) Calcined powder at 900 °C for 3 h (c) The product conventionally sintered at 1100 °C for 3 h (d) The product conventionally sintered at 1200 °C for 3 h (e) The product microwave sintered at 1100 °C for 30 min. (f) The product microwave sintered at 1200 °C for 30 min. Fig. 3: FTIR of (a) The powder calcined at 900 0C for 3 h (b) The product conventionally sintered at 1100 0C for 3h (c) The product conventionally sintered at 1200 0C for 3h (d) The product microwave sintered at 1100 0C for 30 min (e) The product microwave sintered at 1200 0C for 30 min Fig. 4: Scanning electron micrographs of (a) The product conventionally sintered at 1100 °C for 3h (b) The product conventionally sintered at 1200 °C for 3h (c) The product microwave sintered at 1100 °C for 30 min (d) The product microwave sintered at 1200 °C for 30 min (e) EDS image of the product conventionally sintered at 1100 °C for 3h (f) EDS image of the product microwave sintered at 1200 °C for 30 min. Fig. 5: Variation of grain size with standard deviation for (a) The product conventionally sintered at 1100 °C for 3h (b) The product conventionally sintered at 1200 °C for 3h (c) The product microwave sintered at 1100 °C for 30 min (d) The product microwave sintered at 1200 °C for 30 min.


Sr-Zr hydroxyapatite has been sintered by microwave and conventional methods.

The lattice parameters increase with temperature during conventional sintering.

Reverse trend for lattice parameters is observed during microwave sintering.

Microwave sintered products have higher density and hardness.

Microwave sintered product at 1100oC has a less percentage of secondary phase.

ACCEPTED MANUSCRIPT Table 1: Phases indexed in the XRD pattern.




ICDD card number 00-047-1743




Ca5 ( PO4 )3 ( OH )


Ca3 ( PO4 )2


Table 2: Lattice parameter, Crystallite Size, Crystallinity and Hardness

Sample Name

a (Å)

c (Å)



Percentage of Phases




size (nm)

Ca5(PO4)3( Ca3(PO4

y (Xc)




OH) Ca5 (PO4)3 (OH)


9.4180 6.8840 528.80

(ICDD card no.00-0090432) Calcined at 900°C









Conventionally Sintered

































at 1100°C Conventionally Sintered at 1200°C Microwave Sintered at 1100°C Microwave Sintered at 1200°C

ACCEPTED MANUSCRIPT Table 3: FT-IR assignments of functional group

Peak position (cm-1) 552 695 862 910 1113 1459 1805 2521 3429

Assignments O-P-O bending P-O asymmetric stretching, vibrational mode of hydroxyl group O-H stretching of HPO42P-O asymmetric stretching P-O asymmetric stretching HPO42- group vibrations P-O asymmetric stretching of PO4 O-H vibrations of adsorbed water O-H vibrations of adsorbed water


O-H stretching mode of surface P-OH groups

ACCEPTED MANUSCRIPT Table 4: Density of samples (gm/cm3)


Conventionally Sintered Sample

Microwave Sintered Sample