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Sintering of naturally derived hydroxyapatite using high frequency microwave processing
Journal of Alloys and Compounds 682 (2016) 107e114
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Sintering of naturally derived hydroxyapatite using high frequency microwave processing Mohamad Nageeb Hassan a, b, c, Morsi Mohamed Mahmoud d, e, *, Guido Link a, Ahmed Abd El-Fattah b, Sherif Kandil b a
Institute for Pulsed Power and Microwave Technology (IHM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University, El-Shatby 21526 Alexandria, Egypt c Department of Dental Biomaterials, Faculty of Dentistry, Alexandria University, El-Azarita 21526, Alexandria, Egypt d Institute for Applied Materials e Applied Materials Physics (IAM-AWP), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany e Department of Fabrication Technology, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research and Technological Application (SRTA), New Borg Al-Arab City 21934, Alexandria, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2016 Received in revised form 11 April 2016 Accepted 26 April 2016 Available online 29 April 2016
The sinterability of naturally derived hydroxyapatite (HA) materials using 30 GHz high frequency microwave (MW) was explored and compared to conventional sintering. The progress of the MW sintering process of die-pressed HA compacts was investigated via in-situ MW dilatometry measurements. The linear shrinkage, microstructure, porosity and microhardness of the MW sintered naturally derived HA samples were investigated and compared with the conventionally heat-treated samples. The results revealed that the application of high frequency MW allows production of a more porous HA compact with increased microhardness as compared to conventional sintering. In addition, the MW sintering process was successfully performed without the need of hybrid heating and in a relatively shorter time compared to conventional heating. The MW-induced porosity, achieved using millimeter wave MW sintering conditions, can be considered as another added value to the prepared naturally derived HA for drug delivery and bone grafting applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite High frequency microwaves Sintering MW dilatometry Porosity
1. Introduction Calcium phosphate based biomaterials have frequently been used as drug delivery vehicles [1] or bone conducting scaffolds due to their chemical similarity to the inorganic phase of bone [2,3]. Hydroxyapatite (HA) is a calcium phosphate based biomaterial that could be prepared from natural sources [4] or synthesized via several processes [5,6]. Sintering of HA materials increases their density, mechanical strength and hardness to bear loads when used in bone grafting applications [7,8]. Microwave (MW) sintering provides an advantage over the other conventional heating methods, through direct internal heat generation instead of external surface heating and subsequent
* Corresponding author. Institute for Applied Materials e Applied Materials Physics (IAM-AWP), Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. E-mail address: [email protected] (M.M. Mahmoud). http://dx.doi.org/10.1016/j.jallcom.2016.04.266 0925-8388/© 2016 Elsevier B.V. All rights reserved.
conductive heat transfer into the volume [9]. In addition, MW sintering of bioceramics showed high efficiency in saving time and energy, improving the microstructure and thus enhancing the mechanical strength of the ceramics [8,10]. However, since HA is a weak MW-absorber at low temperatures, most of the previous studies have fabricated HA through MW sintering using the domestic MW frequency (2.45 GHz) with the assistance of different types of susceptors (hybrid heating) [8,11e15]. The naturally derived HA material possess a unique thermal behavior [16]. Recent studies have explored different heattreatment conditions during sintering of naturally derived HA (derived from bovine bone) using conventional sintering [17] as well as 2.45 GHz MW sintering [18]. Compared to the widely used domestic MW processing, the use of higher MW frequencies such as 30 GHz with wavelengths in the mm-range could provide significantly more volumetric heating, due to more homogenous field distribution. For low loss materials, such as many glasses and ceramics, mm-waves also provide better
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MW absorption in the material than the domestic MW frequency [9,19]. This allows more efficient MW heating at room temperature in a way that susceptors are not needed. For HA used in drug delivery or in bone grafting applications, it is still crucial to maintain or induce porosity inside the produced samples [20,21]. Porosity was previously induced in MW-sintered HA through incorporating other chemicals (H2O2 [22e24], ammonium carbonate [25] and carbon powder [26]) within the treated HA. Other methods to create porosity inside the HA bodies were performed using the sponge replica method [20] or through constructing the HA porous body via 3D printing [27]. The aim of this study is to explore the sintering of this naturally derived HA using high frequency MW processing. The morphology and phase transitions of the prepared HA bodies were characterized. To the best of our knowledge, this is the first study reporting the effect of high frequency MW (30 GHz) on the sintering of naturally derived HA materials. 2. Materials and methods 2.1. Preparation of naturally derived HA Samples were collected from the shaft of bovine femur (femur diaphysis) and cleaned to be used as a source for naturally derived hydroxyapatite powder (HA). Corresponding bones were cut into small pieces before immersion at room temperature in acetone for 24 h followed by sodium hypochlorite for 48 h to prepare pure inorganic naturally derived HA. The resulting pieces were dried at 80 C overnight. The dried samples were milled, sieved (particle size 250e500 mm) then calcined at 750 C for 4 h to further eliminate any organic remnants. The prepared HA powder was then pressed uniaxially in a 6.5 mm diameter die with around 10 mm in length mold using 20 kN force to produce the samples. 2.2. MW and conventional sintering The pressed HA samples were sintered in the 30 GHz MW processing gyrotron unit [9] at 900 C, 1100 C and 1300 C, respectively. Normally, samples were heated up to the peak temperatures with a constant heating rate of 25 C/min and followed by 5 min isothermal dwelling (holding time). In each group, different rates or holding times were explored as indicated. The HA samples processed in the gyrotron system were placed between alumina ceramic plates, covered by a thermal insulation made from mullite fiber boards to allow for controlled heating environment and to prevent surface heat loss. Temperature gradients on the sample surface were measured using S-type thermocouple. All samples were left for at least 2 h to cool down into room temperature. Control samples were sintered in a conventional resistance heated furnace (Nabertherm® e Germany) at 1100 and 1300 C with a constant heating rate of 10 C/min and 2 h holding time at peak temperature. All samples were left overnight to cool down to room temperature. 2.3. Weight loss and linear shrinkage measurements The weight loss after sintering was measured and compared to the original weight of the green bodies. A commercially available dilatometer system (type L75, Linseis Company e Germany) was adapted to the millimeter MW applicator so that information on linear expansion and shrinkage of specimens can be recorded insitu [28]. The actual sample surface temperature, linear dimensional changes and the used MW power were recorded simultaneously versus time by the use of closed-loop control and data
acquisition software. 2.4. Morphological analysis The morphology of all samples was examined using a scanning electron microscope (SEM) (XL 40, Philips e Netherlands). Sintered samples were embedded in polymethyl methacrylate - (Technovit® #4071) then cured, labeled, polished (TegraPol-31 e Struers Germany) at different grades and controlled with a stereo microscope before mounted on aluminum stubs and gold coated (Scan coat six, Edwards e UK). Samples were scanned at high voltage (20 kV) with a working distance of 20 mm to the backscatter detector. 2.5. Porosimetry Mercury intrusion porosimetry experiments (Porotech Pascal 140/440, Thermo Finnigan, Thermo Electron Cooperation e Germany) were performed with a maximum test pressure equal to 350 MPa at 25 C. The obtained data were analyzed for the total porosity (Vol.%) and the average pore size (nm) with the software (ThermoFisher scientific). 2.6. Crystal structure identification The sintered samples were ground for phase analysis via X-ray diffraction (XRD) with Cu-Ka radiation at a wavelength of 1.54056 Å (Seifert C 3000). Data (2q degree) were acquired in the range from 20 to 60 using a 2q step size of 0.02 . 2.7. Micro-indentation hardness testing The sintered samples were embedded in polymethyl methacrylate as mentioned earlier in Section 2.4. A microhardness tester (MHT-4, AP PAAR, Graz e Austria) was used to apply Vickers hardness test with a force (F) up to 900 N and 10 s indentation time. From the shape of the developed diamond indentation the hardness was estimated from the average length of the two diagonals (d) of each indentation, were hardness ¼ 1.854 F/d2. The mean value of 10 indentation points across the diameter of each sample was calculated in GPa. 3. Results and discussion Fig. 1 shows the XRD pattern and SEM micrograph of the naturally derived HA powder. It is clear that pure naturally derived HA particles were successfully prepared. The measured XRD spectra were compared to the HA reference data (PDF no. 74e0566) where both patterns matched closely and showed a good coincidence with no additional phases. The SEM micrograph showed that the prepared HA particles morphology was coarse with irregular shapes and their average particles sizes were in the range of 250e500 mm. The uniaxial pressed HA samples were sintered using both 30 GHz microwave processing as well as conventional furnace heating at different temperatures, 900 C, 1100 C and 1300 C, respectively. Fig. 2a shows the mean weight loss % of some sintered HA bodies at 1100 C and 1300 C using both MW (5 min) versus conventional (2 h) sintering. From this data, it was concluded that the weight loss was increased significantly from 1100 to 1300 C in both types of sintering. On the other hand, the MW sintered samples at both temperatures showed a slightly higher or comparable weight loss % values and were achieved in a relatively much shorter holding time (5 min) when compared to conventionally sintered samples (2 h). Fig. 2b shows the weight loss % of MW sintered HA
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Fig. 1. XRD pattern and SEM micrographs of the naturally derived HA powder.
Fig. 2. Mean weight loss % of the sintered HA bodies. (a) MW versus conventional sintered HA samples at 1100 C and 1300 C (b) MW sintered HA samples at 1100 C (25 C/min) at different holding times (1, 5, 10 and 25 min). (c) MW sintered HA samples at 1100 C (5 min) using different heating rates (10, 25 and 50 C/min).
Fig. 3. In-situ MW dilatometer measurements of some HA compacts: (a) Sintered samples at 1100 and 1300 C, (25 C/min) for 5 min (b) Sintered samples at 1100 C (25 C/min) at various holding time (5, 10, 25 min) compared with a sample heated at double higher heating rate (50 C/min).
bodies at 1100 C with a heating rate of 25 C/min but using different holding times (1, 5, 10 and 25 min) while Fig. 2c shows the weight loss % of MW sintered HA samples at 1100 C using 5 min
holding time but using different heating rates (10, 25 and 50 C/ min). It was concluded from these data that increasing either the holding time or the heating rate was found to significantly increase
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Fig. 4. Optical and SEM micrographs for the center of some HA samples sintered using: (a) Conventional heating at 1100 C (10 C/min) for 2 h (b) MW heating at 900 C (c) MW heating at 1100 C (d) MW heating at 1300 C using heating rate of (25 C/min) for 5 min. Note the presence of MW induced hotspots in the 1100 C and 1300 C treated samples (white arrows).
the weight loss of HA samples during the MW sintering process. Furthermore, the MW sintering process of some die-pressed HA compacts were investigated using in-situ MW dilatometry measurements as shown in Fig. 3. These MW in-situ dilatometer measurements of pressed HA compacts showed that by increasing the sintering temperature from 1100 to 1300 C, a significant increase in the total linear shrinkage was observed (Fig. 3a). This was in good agreements with the observations during the weight loss measurements. The amount of shrinkage also increased with the
increase of MW holding time [18] or sintering temperature [22,30]. However, the amount of total linear shrinkage in HA samples during the high frequency MW sintering process was significantly less than the previously reported values for the 2.45 GHz MW sintered HA samples [18,22,29,30]. On the other hand, increasing the MW holding time and increasing the heating rate from 25 to 50 C/min led to a significant reverse effect on the observed percent of shrinkage (Fig. 3b). This could be due to a thermal runaway effect and the generations of some hot spots inside the material and
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Fig. 5. Optical and SEM micrographs for the center of some MW sintered HA samples at 1100 C (25 C/min) at different holding times: (a) 5 min (b) 10 min (c) 25 min and (d) Compared to MW sintered sample using double heating rate (50 C/min) for 5 min. Note the presence of MW-induced hotspots (white arrows) where asterisks shows MW-induced macro pores formed due to HA partial thermal decomposition).
hence induction of some porosity which was also observed in SEM micrographs of these samples. Fig. 4 shows optical and SEM micrographs for the center of some sintered naturally derived HA samples using conventional heating at 1100 C using a heating rate of 10 C/min for 2 h (Fig. 4a) compared to some other MW sintered samples at different temperatures using a heating rate of 25 C/min for 5 min as 900 C (Fig. 4b), 1100 C (Fig. 4c) and 1300 C (Fig. 4d). The SEM micrograph of the conventionally sintered samples at 1100 revealed a
cracked structure with minimal particle consolidation (Fig. 4a) whereas similar cracked structure was also observed in the conventionally sintered sample at 1300 C. Furthermore, the MW sintered samples at 900 C showed comparable structural features with similar minimal particle consolidation (Fig. 4b). On the other hand, the MW sintered HA samples at 1100 and 1300 C showed slightly better coalescence of the particles (Fig. 4c and d) at shorter time in which some MW induced hotspots in these samples center were observed (white arrows) when compared to conventionally
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Fig. 6. Mercury intrusion porosimetry of HA samples sintered at 1100 C using: (a) Conventional heating (b) MW heating.
Fig. 7. XRD patterns of conventionally sintered and MW sintered HA samples compared to HA reference (PDF number 74e0566). Bold bullets mark the traces of b-TCP due to partial thermal decomposition of HA.
sintered samples. Fig. 5 shows optical and SEM micrographs for the center of some MW sintered HA samples at 1100 C using a heating rate of 25 /min with different holding times (5 min, 10, 25 min) and compared with a sintered sample at higher double heating rate (50 C/min for 5 min). From this data, it was concluded that increasing the MW holding time at the same peak temperature 1100 C (Fig. 5aec) or increasing the MW heating rate (Fig. 5d) would lead to better densification of the HA materials with the introduction of newly formed macro pores (closed porosity). The formation of these pores might be attributed to the MW thermal runway and the formation of some hot spots that could cause a rapid thermal decomposition of HA. This selective HA thermal decomposition, especially in the core (center) of the sample, is an indication that the MW thermal
runaway was due to a nonlinear increase of dielectric loss of HA that may have caused a local increase in temperature greater than 1450 C. On the other hand, the low heat conduction of HA materials may have promoted the pore formation through insufficient heat dissipation from the MW induced hot spots. In order to investigate the physical characteristics and nature of these formed macro pores under these specific conditions, mercury intrusion porosimetry was performed. Fig. 6 shows the mercury intrusion porosimetry of the conventionally sintered HA sample comapred to the MW sintered HA sample at 1100 C. When comparing the porosity of both sintered samples, it was clear that the conventionally sintered samples showed two types of pores: meso pores (pore radius around 700 nm) and macro pores (pore radius range 2000e4000 nm) as in Fig. 6a. This observation agreed
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Fig. 8. Mean hardness values of the sintered HA materials in (GPa) of the: (a) MW versus conventional sintered HA samples at 1100 C and 1300 C. (b) MW sintered HA samples at 1100 C (25 C/min) with different holding times (1, 5, 10 and 25 min) (c) MW sintered HA samples at 1100 C (5 min holding time) at three different heating rates (10, 25 and 50 C/ min).
with the SEM micropraphs of this sample that showed minimum particles consolidation with larger cumulative pore volume. On the other hand, MW sintered samples showed only meso pores with peaks of pore radius around 200 and 700 nm (Fig. 6b). Thus, the MW sintered samples, even at a very short holding time of 5 min, showed a less overall total connected pore volume. This also matched with the slightly improved densification which was observed in case of MW sintering at much less time with a meso porous structure compared to conventional sintering. On the other hand, the MW-induced macro pores observed in the SEM micrographs for the samples with longer holding times or at higher heating rates (Fig. 5bed) were not detected by mercury intrusion porosimetry because it was a closed porosity. Since the stability of HA phase during sintering was studied in previous work [11], the XRD patterns of the current studied HA sintered samples using both types of heating were studied. Fig. 7 shows the XRD patterns of the conventionally sintered and the MW sintered HA samples compared to HA reference (PDF number 74e0566). Among all the sintered HA samples, traces of betatricalcium phosphate (b-TCP) were barely detected and only in the MW sintered HA samples at 1100 and 1300 C using a heating rate 25 C/min for shorter holding times (5 min). Furthermore, no other HA phase transformations were detected in the MW sintered samples at higher heating rates or at longer holding times. This could support the assumption of rapid thermal decomposition of HA during MW heating. Fig. 8 shows the mean microhardness values in GPa of the sintered HA materials as follows: (a) MW versus conventional sintered HA samples at 1100 C and 1300 C, (b) MW sintered HA samples at 1100 C (25 C/min) with different holding times (1, 5, 10 and 25 min) and (c) MW sintered HA samples at 1100 C (5 min holding time) and three different heating rates (10, 25 and 50 C/min). Due to the sample residual porosity, the standard deviation for the measured hardness values across the samples diagonal was rather large. Therefore, the following discussion provides a general trend of the mean hardness data under different process parameters. The MW sintered sample at 900 C showed the lowest hardness values in comparison to MW sintered samples at both 1100 and 1300 C. This could be attributed to the lack of sufficient sinterability at this temperature (Fig. 8a). This was due to the fact that HA sintering
could only be achieved at temperatures beyond 1000 C through conventional or MW heating [14,23]. Whereas, MW sintered samples at 1100 and 1300 C were found to have higher hardness at much shorter holing time (5 min) than its corresponding conventionally sintered samples at 2 h. Increasing the MW holding time from 1 to 10 min was found to increase the mean hardness values of the sintered samples. However, further extension of the holding period revealed a minor reduction in the mean hardness values (Fig. 8b). This could be attributed to the regional thermal decomposition of HA and the formation of more MW-induced pores as observed earlier via SEM and porosimetry data. Increasing the MW heating rate from 10 to 50 C/min had led to a linear increase in mean hardness values (Fig. 8c). Furthermore, the microhardness values obtained for all the high frequency MW sintered HA samples were higher than the previously reported values obtained by domestic 2.45 GHz MW sintering of naturally derived HA [30]. Generally, sintering of bioceramics is expected to enhance and assist bulk densification in order to achieve good mechanical properties. However, studies have indicated that porous biomaterials with interconnected porosity are highly desirable for bone tissue engineering applications [21,31]. At 2.45 GHz MW processing of HA, susceptors were needed to elevate the temperature of HA samples from room temperature to the critical temperature where HA materials start to absorb the MW energy more efficiently [13]. However, higher frequency (30 GHz) MW sintering of HA had more efficient MW power absorption and hence the sintering of HA was carried out without the need for susceptors. The MW thermal runaway and hot spots features that may arise in some low dielectric loss biomaterials, such as HA, could be useful to induce engineered controlled porosity that is needed for bone tissue engineering applications in which the thermal decomposition of HA under MW processing could be better achieved and controlled. 4. Conclusions Naturally derived compacts of HA were successfully sintered using 30 GHz high frequency MW heating without the assistance of susceptors and with relatively much shorter processing time when compared to conventional sintering. High frequency MW sintered
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samples revealed higher hardness, less linear shrinkage, and more porosity than the conventionally sintered HA samples. In addition, high frequency MW-induced macro porosity in HA samples were observed and could be influenced by changing the heating rate and/ or the holding time. These enhanced properties give the advantage to high frequency MW sintering of porous HA bodies to be optimized and used for bone grafting applications.
[13]
Author contributions
[14]
Mohamad N. Hassan, Morsi M. Mahmoud, Guido Link, Ahmed Abd El-Fattah and Sherif Kandil designed the experiment, interpreted and analyzed the experiment data. Mohamad N. Hassan prepared the samples and performed the experiments. Morsi M. Mahmoud and Mohamad N. Hassan wrote the paper.
[11]
[12]
[15]
[16]
[17]
Conflicts of interest The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Acknowledgements The authors would like to thank Ms. Simone Wadle for assisting in MW processing and Ms. Margarete Offermann for the porosimetry test. The authors would like also to deeply thank Dr. Alfons Weisenburger and Dr. Adrian Jianu for their kind assistance in SEM characterization. This work was funded through the GermanEgyptian Research Short-term Scholarships (GERSS) 2014 managed by theGerman Academic Exchange Service (DAAD).
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[19]
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[21]
[22]
[23]
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Sintering of naturally derived hydroxyapatite using high frequency microwave processing Mohamad Nageeb Hassan a, b, c, Morsi Mohamed Mahmoud d, e, *, Guido Link a, Ahmed Abd El-Fattah b, Sherif Kandil b a
Institute for Pulsed Power and Microwave Technology (IHM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University, El-Shatby 21526 Alexandria, Egypt c Department of Dental Biomaterials, Faculty of Dentistry, Alexandria University, El-Azarita 21526, Alexandria, Egypt d Institute for Applied Materials e Applied Materials Physics (IAM-AWP), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany e Department of Fabrication Technology, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research and Technological Application (SRTA), New Borg Al-Arab City 21934, Alexandria, Egypt b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2016 Received in revised form 11 April 2016 Accepted 26 April 2016 Available online 29 April 2016
The sinterability of naturally derived hydroxyapatite (HA) materials using 30 GHz high frequency microwave (MW) was explored and compared to conventional sintering. The progress of the MW sintering process of die-pressed HA compacts was investigated via in-situ MW dilatometry measurements. The linear shrinkage, microstructure, porosity and microhardness of the MW sintered naturally derived HA samples were investigated and compared with the conventionally heat-treated samples. The results revealed that the application of high frequency MW allows production of a more porous HA compact with increased microhardness as compared to conventional sintering. In addition, the MW sintering process was successfully performed without the need of hybrid heating and in a relatively shorter time compared to conventional heating. The MW-induced porosity, achieved using millimeter wave MW sintering conditions, can be considered as another added value to the prepared naturally derived HA for drug delivery and bone grafting applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite High frequency microwaves Sintering MW dilatometry Porosity
1. Introduction Calcium phosphate based biomaterials have frequently been used as drug delivery vehicles [1] or bone conducting scaffolds due to their chemical similarity to the inorganic phase of bone [2,3]. Hydroxyapatite (HA) is a calcium phosphate based biomaterial that could be prepared from natural sources [4] or synthesized via several processes [5,6]. Sintering of HA materials increases their density, mechanical strength and hardness to bear loads when used in bone grafting applications [7,8]. Microwave (MW) sintering provides an advantage over the other conventional heating methods, through direct internal heat generation instead of external surface heating and subsequent
* Corresponding author. Institute for Applied Materials e Applied Materials Physics (IAM-AWP), Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. E-mail address: [email protected] (M.M. Mahmoud). http://dx.doi.org/10.1016/j.jallcom.2016.04.266 0925-8388/© 2016 Elsevier B.V. All rights reserved.
conductive heat transfer into the volume [9]. In addition, MW sintering of bioceramics showed high efficiency in saving time and energy, improving the microstructure and thus enhancing the mechanical strength of the ceramics [8,10]. However, since HA is a weak MW-absorber at low temperatures, most of the previous studies have fabricated HA through MW sintering using the domestic MW frequency (2.45 GHz) with the assistance of different types of susceptors (hybrid heating) [8,11e15]. The naturally derived HA material possess a unique thermal behavior [16]. Recent studies have explored different heattreatment conditions during sintering of naturally derived HA (derived from bovine bone) using conventional sintering [17] as well as 2.45 GHz MW sintering [18]. Compared to the widely used domestic MW processing, the use of higher MW frequencies such as 30 GHz with wavelengths in the mm-range could provide significantly more volumetric heating, due to more homogenous field distribution. For low loss materials, such as many glasses and ceramics, mm-waves also provide better
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MW absorption in the material than the domestic MW frequency [9,19]. This allows more efficient MW heating at room temperature in a way that susceptors are not needed. For HA used in drug delivery or in bone grafting applications, it is still crucial to maintain or induce porosity inside the produced samples [20,21]. Porosity was previously induced in MW-sintered HA through incorporating other chemicals (H2O2 [22e24], ammonium carbonate [25] and carbon powder [26]) within the treated HA. Other methods to create porosity inside the HA bodies were performed using the sponge replica method [20] or through constructing the HA porous body via 3D printing [27]. The aim of this study is to explore the sintering of this naturally derived HA using high frequency MW processing. The morphology and phase transitions of the prepared HA bodies were characterized. To the best of our knowledge, this is the first study reporting the effect of high frequency MW (30 GHz) on the sintering of naturally derived HA materials. 2. Materials and methods 2.1. Preparation of naturally derived HA Samples were collected from the shaft of bovine femur (femur diaphysis) and cleaned to be used as a source for naturally derived hydroxyapatite powder (HA). Corresponding bones were cut into small pieces before immersion at room temperature in acetone for 24 h followed by sodium hypochlorite for 48 h to prepare pure inorganic naturally derived HA. The resulting pieces were dried at 80 C overnight. The dried samples were milled, sieved (particle size 250e500 mm) then calcined at 750 C for 4 h to further eliminate any organic remnants. The prepared HA powder was then pressed uniaxially in a 6.5 mm diameter die with around 10 mm in length mold using 20 kN force to produce the samples. 2.2. MW and conventional sintering The pressed HA samples were sintered in the 30 GHz MW processing gyrotron unit [9] at 900 C, 1100 C and 1300 C, respectively. Normally, samples were heated up to the peak temperatures with a constant heating rate of 25 C/min and followed by 5 min isothermal dwelling (holding time). In each group, different rates or holding times were explored as indicated. The HA samples processed in the gyrotron system were placed between alumina ceramic plates, covered by a thermal insulation made from mullite fiber boards to allow for controlled heating environment and to prevent surface heat loss. Temperature gradients on the sample surface were measured using S-type thermocouple. All samples were left for at least 2 h to cool down into room temperature. Control samples were sintered in a conventional resistance heated furnace (Nabertherm® e Germany) at 1100 and 1300 C with a constant heating rate of 10 C/min and 2 h holding time at peak temperature. All samples were left overnight to cool down to room temperature. 2.3. Weight loss and linear shrinkage measurements The weight loss after sintering was measured and compared to the original weight of the green bodies. A commercially available dilatometer system (type L75, Linseis Company e Germany) was adapted to the millimeter MW applicator so that information on linear expansion and shrinkage of specimens can be recorded insitu [28]. The actual sample surface temperature, linear dimensional changes and the used MW power were recorded simultaneously versus time by the use of closed-loop control and data
acquisition software. 2.4. Morphological analysis The morphology of all samples was examined using a scanning electron microscope (SEM) (XL 40, Philips e Netherlands). Sintered samples were embedded in polymethyl methacrylate - (Technovit® #4071) then cured, labeled, polished (TegraPol-31 e Struers Germany) at different grades and controlled with a stereo microscope before mounted on aluminum stubs and gold coated (Scan coat six, Edwards e UK). Samples were scanned at high voltage (20 kV) with a working distance of 20 mm to the backscatter detector. 2.5. Porosimetry Mercury intrusion porosimetry experiments (Porotech Pascal 140/440, Thermo Finnigan, Thermo Electron Cooperation e Germany) were performed with a maximum test pressure equal to 350 MPa at 25 C. The obtained data were analyzed for the total porosity (Vol.%) and the average pore size (nm) with the software (ThermoFisher scientific). 2.6. Crystal structure identification The sintered samples were ground for phase analysis via X-ray diffraction (XRD) with Cu-Ka radiation at a wavelength of 1.54056 Å (Seifert C 3000). Data (2q degree) were acquired in the range from 20 to 60 using a 2q step size of 0.02 . 2.7. Micro-indentation hardness testing The sintered samples were embedded in polymethyl methacrylate as mentioned earlier in Section 2.4. A microhardness tester (MHT-4, AP PAAR, Graz e Austria) was used to apply Vickers hardness test with a force (F) up to 900 N and 10 s indentation time. From the shape of the developed diamond indentation the hardness was estimated from the average length of the two diagonals (d) of each indentation, were hardness ¼ 1.854 F/d2. The mean value of 10 indentation points across the diameter of each sample was calculated in GPa. 3. Results and discussion Fig. 1 shows the XRD pattern and SEM micrograph of the naturally derived HA powder. It is clear that pure naturally derived HA particles were successfully prepared. The measured XRD spectra were compared to the HA reference data (PDF no. 74e0566) where both patterns matched closely and showed a good coincidence with no additional phases. The SEM micrograph showed that the prepared HA particles morphology was coarse with irregular shapes and their average particles sizes were in the range of 250e500 mm. The uniaxial pressed HA samples were sintered using both 30 GHz microwave processing as well as conventional furnace heating at different temperatures, 900 C, 1100 C and 1300 C, respectively. Fig. 2a shows the mean weight loss % of some sintered HA bodies at 1100 C and 1300 C using both MW (5 min) versus conventional (2 h) sintering. From this data, it was concluded that the weight loss was increased significantly from 1100 to 1300 C in both types of sintering. On the other hand, the MW sintered samples at both temperatures showed a slightly higher or comparable weight loss % values and were achieved in a relatively much shorter holding time (5 min) when compared to conventionally sintered samples (2 h). Fig. 2b shows the weight loss % of MW sintered HA
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Fig. 1. XRD pattern and SEM micrographs of the naturally derived HA powder.
Fig. 2. Mean weight loss % of the sintered HA bodies. (a) MW versus conventional sintered HA samples at 1100 C and 1300 C (b) MW sintered HA samples at 1100 C (25 C/min) at different holding times (1, 5, 10 and 25 min). (c) MW sintered HA samples at 1100 C (5 min) using different heating rates (10, 25 and 50 C/min).
Fig. 3. In-situ MW dilatometer measurements of some HA compacts: (a) Sintered samples at 1100 and 1300 C, (25 C/min) for 5 min (b) Sintered samples at 1100 C (25 C/min) at various holding time (5, 10, 25 min) compared with a sample heated at double higher heating rate (50 C/min).
bodies at 1100 C with a heating rate of 25 C/min but using different holding times (1, 5, 10 and 25 min) while Fig. 2c shows the weight loss % of MW sintered HA samples at 1100 C using 5 min
holding time but using different heating rates (10, 25 and 50 C/ min). It was concluded from these data that increasing either the holding time or the heating rate was found to significantly increase
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Fig. 4. Optical and SEM micrographs for the center of some HA samples sintered using: (a) Conventional heating at 1100 C (10 C/min) for 2 h (b) MW heating at 900 C (c) MW heating at 1100 C (d) MW heating at 1300 C using heating rate of (25 C/min) for 5 min. Note the presence of MW induced hotspots in the 1100 C and 1300 C treated samples (white arrows).
the weight loss of HA samples during the MW sintering process. Furthermore, the MW sintering process of some die-pressed HA compacts were investigated using in-situ MW dilatometry measurements as shown in Fig. 3. These MW in-situ dilatometer measurements of pressed HA compacts showed that by increasing the sintering temperature from 1100 to 1300 C, a significant increase in the total linear shrinkage was observed (Fig. 3a). This was in good agreements with the observations during the weight loss measurements. The amount of shrinkage also increased with the
increase of MW holding time [18] or sintering temperature [22,30]. However, the amount of total linear shrinkage in HA samples during the high frequency MW sintering process was significantly less than the previously reported values for the 2.45 GHz MW sintered HA samples [18,22,29,30]. On the other hand, increasing the MW holding time and increasing the heating rate from 25 to 50 C/min led to a significant reverse effect on the observed percent of shrinkage (Fig. 3b). This could be due to a thermal runaway effect and the generations of some hot spots inside the material and
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Fig. 5. Optical and SEM micrographs for the center of some MW sintered HA samples at 1100 C (25 C/min) at different holding times: (a) 5 min (b) 10 min (c) 25 min and (d) Compared to MW sintered sample using double heating rate (50 C/min) for 5 min. Note the presence of MW-induced hotspots (white arrows) where asterisks shows MW-induced macro pores formed due to HA partial thermal decomposition).
hence induction of some porosity which was also observed in SEM micrographs of these samples. Fig. 4 shows optical and SEM micrographs for the center of some sintered naturally derived HA samples using conventional heating at 1100 C using a heating rate of 10 C/min for 2 h (Fig. 4a) compared to some other MW sintered samples at different temperatures using a heating rate of 25 C/min for 5 min as 900 C (Fig. 4b), 1100 C (Fig. 4c) and 1300 C (Fig. 4d). The SEM micrograph of the conventionally sintered samples at 1100 revealed a
cracked structure with minimal particle consolidation (Fig. 4a) whereas similar cracked structure was also observed in the conventionally sintered sample at 1300 C. Furthermore, the MW sintered samples at 900 C showed comparable structural features with similar minimal particle consolidation (Fig. 4b). On the other hand, the MW sintered HA samples at 1100 and 1300 C showed slightly better coalescence of the particles (Fig. 4c and d) at shorter time in which some MW induced hotspots in these samples center were observed (white arrows) when compared to conventionally
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Fig. 6. Mercury intrusion porosimetry of HA samples sintered at 1100 C using: (a) Conventional heating (b) MW heating.
Fig. 7. XRD patterns of conventionally sintered and MW sintered HA samples compared to HA reference (PDF number 74e0566). Bold bullets mark the traces of b-TCP due to partial thermal decomposition of HA.
sintered samples. Fig. 5 shows optical and SEM micrographs for the center of some MW sintered HA samples at 1100 C using a heating rate of 25 /min with different holding times (5 min, 10, 25 min) and compared with a sintered sample at higher double heating rate (50 C/min for 5 min). From this data, it was concluded that increasing the MW holding time at the same peak temperature 1100 C (Fig. 5aec) or increasing the MW heating rate (Fig. 5d) would lead to better densification of the HA materials with the introduction of newly formed macro pores (closed porosity). The formation of these pores might be attributed to the MW thermal runway and the formation of some hot spots that could cause a rapid thermal decomposition of HA. This selective HA thermal decomposition, especially in the core (center) of the sample, is an indication that the MW thermal
runaway was due to a nonlinear increase of dielectric loss of HA that may have caused a local increase in temperature greater than 1450 C. On the other hand, the low heat conduction of HA materials may have promoted the pore formation through insufficient heat dissipation from the MW induced hot spots. In order to investigate the physical characteristics and nature of these formed macro pores under these specific conditions, mercury intrusion porosimetry was performed. Fig. 6 shows the mercury intrusion porosimetry of the conventionally sintered HA sample comapred to the MW sintered HA sample at 1100 C. When comparing the porosity of both sintered samples, it was clear that the conventionally sintered samples showed two types of pores: meso pores (pore radius around 700 nm) and macro pores (pore radius range 2000e4000 nm) as in Fig. 6a. This observation agreed
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Fig. 8. Mean hardness values of the sintered HA materials in (GPa) of the: (a) MW versus conventional sintered HA samples at 1100 C and 1300 C. (b) MW sintered HA samples at 1100 C (25 C/min) with different holding times (1, 5, 10 and 25 min) (c) MW sintered HA samples at 1100 C (5 min holding time) at three different heating rates (10, 25 and 50 C/ min).
with the SEM micropraphs of this sample that showed minimum particles consolidation with larger cumulative pore volume. On the other hand, MW sintered samples showed only meso pores with peaks of pore radius around 200 and 700 nm (Fig. 6b). Thus, the MW sintered samples, even at a very short holding time of 5 min, showed a less overall total connected pore volume. This also matched with the slightly improved densification which was observed in case of MW sintering at much less time with a meso porous structure compared to conventional sintering. On the other hand, the MW-induced macro pores observed in the SEM micrographs for the samples with longer holding times or at higher heating rates (Fig. 5bed) were not detected by mercury intrusion porosimetry because it was a closed porosity. Since the stability of HA phase during sintering was studied in previous work [11], the XRD patterns of the current studied HA sintered samples using both types of heating were studied. Fig. 7 shows the XRD patterns of the conventionally sintered and the MW sintered HA samples compared to HA reference (PDF number 74e0566). Among all the sintered HA samples, traces of betatricalcium phosphate (b-TCP) were barely detected and only in the MW sintered HA samples at 1100 and 1300 C using a heating rate 25 C/min for shorter holding times (5 min). Furthermore, no other HA phase transformations were detected in the MW sintered samples at higher heating rates or at longer holding times. This could support the assumption of rapid thermal decomposition of HA during MW heating. Fig. 8 shows the mean microhardness values in GPa of the sintered HA materials as follows: (a) MW versus conventional sintered HA samples at 1100 C and 1300 C, (b) MW sintered HA samples at 1100 C (25 C/min) with different holding times (1, 5, 10 and 25 min) and (c) MW sintered HA samples at 1100 C (5 min holding time) and three different heating rates (10, 25 and 50 C/min). Due to the sample residual porosity, the standard deviation for the measured hardness values across the samples diagonal was rather large. Therefore, the following discussion provides a general trend of the mean hardness data under different process parameters. The MW sintered sample at 900 C showed the lowest hardness values in comparison to MW sintered samples at both 1100 and 1300 C. This could be attributed to the lack of sufficient sinterability at this temperature (Fig. 8a). This was due to the fact that HA sintering
could only be achieved at temperatures beyond 1000 C through conventional or MW heating [14,23]. Whereas, MW sintered samples at 1100 and 1300 C were found to have higher hardness at much shorter holing time (5 min) than its corresponding conventionally sintered samples at 2 h. Increasing the MW holding time from 1 to 10 min was found to increase the mean hardness values of the sintered samples. However, further extension of the holding period revealed a minor reduction in the mean hardness values (Fig. 8b). This could be attributed to the regional thermal decomposition of HA and the formation of more MW-induced pores as observed earlier via SEM and porosimetry data. Increasing the MW heating rate from 10 to 50 C/min had led to a linear increase in mean hardness values (Fig. 8c). Furthermore, the microhardness values obtained for all the high frequency MW sintered HA samples were higher than the previously reported values obtained by domestic 2.45 GHz MW sintering of naturally derived HA [30]. Generally, sintering of bioceramics is expected to enhance and assist bulk densification in order to achieve good mechanical properties. However, studies have indicated that porous biomaterials with interconnected porosity are highly desirable for bone tissue engineering applications [21,31]. At 2.45 GHz MW processing of HA, susceptors were needed to elevate the temperature of HA samples from room temperature to the critical temperature where HA materials start to absorb the MW energy more efficiently [13]. However, higher frequency (30 GHz) MW sintering of HA had more efficient MW power absorption and hence the sintering of HA was carried out without the need for susceptors. The MW thermal runaway and hot spots features that may arise in some low dielectric loss biomaterials, such as HA, could be useful to induce engineered controlled porosity that is needed for bone tissue engineering applications in which the thermal decomposition of HA under MW processing could be better achieved and controlled. 4. Conclusions Naturally derived compacts of HA were successfully sintered using 30 GHz high frequency MW heating without the assistance of susceptors and with relatively much shorter processing time when compared to conventional sintering. High frequency MW sintered
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samples revealed higher hardness, less linear shrinkage, and more porosity than the conventionally sintered HA samples. In addition, high frequency MW-induced macro porosity in HA samples were observed and could be influenced by changing the heating rate and/ or the holding time. These enhanced properties give the advantage to high frequency MW sintering of porous HA bodies to be optimized and used for bone grafting applications.
[13]
Author contributions
[14]
Mohamad N. Hassan, Morsi M. Mahmoud, Guido Link, Ahmed Abd El-Fattah and Sherif Kandil designed the experiment, interpreted and analyzed the experiment data. Mohamad N. Hassan prepared the samples and performed the experiments. Morsi M. Mahmoud and Mohamad N. Hassan wrote the paper.
[11]
[12]
[15]
[16]
[17]
Conflicts of interest The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Acknowledgements The authors would like to thank Ms. Simone Wadle for assisting in MW processing and Ms. Margarete Offermann for the porosimetry test. The authors would like also to deeply thank Dr. Alfons Weisenburger and Dr. Adrian Jianu for their kind assistance in SEM characterization. This work was funded through the GermanEgyptian Research Short-term Scholarships (GERSS) 2014 managed by theGerman Academic Exchange Service (DAAD).
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