Rapid consolidation of hydroxyapatite using intense millimeter-wave radiation

Materials Today: Proceedings xxx (xxxx) xxx

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Rapid consolidation of hydroxyapatite using intense millimeter-wave radiation S.V. Egorov, A.G. Eremeev, V.V. Kholoptsev, I.V. Plotnikov, K.I. Rybakov ⇑, A.A. Sorokin Institute of Applied Physics, Russian Academy of Sciences, 46 Ulyanov St., Nizhny Novgorod, 603950, Russia

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Article history: Received 5 November 2019 Received in revised form 26 November 2019 Accepted 11 December 2019 Available online xxxx Keywords: Microwave sintering Millimeter waves Hydroxyapatite Gyrotron Additive manufacturing

a b s t r a c t Hydroxyapatite powder compacts have been sintered to a relative density of about 95% using rapid 24 GHz microwave heating at rates 10–100 °C/min to 1300 °C with zero hold time. An optical system based on an infrared camera has been developed to measure shrinkage and temperature distributions over the surface of the samples. Ultra-rapid localized consolidation of hydroxyapatite powder, with application prospects in additive manufacturing, has been achieved using heating by a focused beam of 263 GHz millimeter-wave radiation. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the III All-Russian Conference (with International Participation) Hot Topics of Solid State Chemistry: From New Ideas to New Materials.

1. Introduction Ca10(PO4)6(OH)2, is the chemical formula for hydroxyapatite, is a candidate material for bioceramics fabrication due to its biocompatibility and good mechanical properties. The hydroxyapatite-based ceramics are used for bone replacement, dental implants and other biomedical applications [1–3]. Among many methods to produce hydroxyapatite ceramic, microwave sintering has been investigated over the recent decades in a number of studies (e.g., [4–9]). Microwave sintering is known to have a number of advantages, attributed to direct volumetric deposition of energy in the material undergoing sintering and to the electromagnetic field effects [10–13]. These include shorter processing times and reduced sintering temperature, which results in finer microstructure, improved mechanical and functional properties of the ceramics. These advantages in particular have been demonstrated in a number of studies of microwave sintering of hydroxyapatite [14–17]. The hydroxyapatite ceramic products for biomedical applications often require manufacturing in individual sizes and shapes, selection of additives composition, tailored grain size and porosity distributions. These requirements can be more readily met when using additive manufacturing methods [18–20] for the fabrication of the bioceramic parts. In application to the processing of ceram⇑ Corresponding author. E-mail address: [email protected] (K.I. Rybakov).

ics, one additive manufacturing approach can involve preparation of a green body according to a predefined shape model and its subsequent solidification in a high-temperature furnace. Within another approach the article is fabricated layer by layer, and the solidification of the deposited powder layers is accomplished using local heating by a concentrated energy flow – most commonly by a laser beam. Millimeter-wave radiation occupies an intermediate position in the electromagnetic spectrum between radio frequency and microwaves on one end and terahertz, infrared and optical ranges on the other. Like lower-frequency microwaves, millimeter waves can be used for volumetric heating of materials in multimode cavity applicators, providing better uniformity of the electromagnetic field distribution and more efficient coupling in low-absorbing materials. On the other hand, millimeter-wave radiation can be focused into beams with high radiation intensity and reasonable focal spot size (on the order of the wavelength) [12]. These properties make the processing using millimeter-wave radiation an attractive method for developing additive manufacturing applications. This paper reports the preliminary results of a study of millimeter-wave processing of hydroxyapatite. 2. Experimental The experimental samples were prepared from the hydroxyapatite powder GAP-85d (Polistom, Russia). The powder particle size

https://doi.org/10.1016/j.matpr.2019.12.081 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the III All-Russian Conference (with International Participation) Hot Topics of Solid State Chemistry: From New Ideas to New Materials.

Please cite this article as: S. V. Egorov, A. G. Eremeev, V. V. Kholoptsev et al., Rapid consolidation of hydroxyapatite using intense millimeter-wave radiation, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.081

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S.V. Egorov et al. / Materials Today: Proceedings xxx (xxxx) xxx

was 0.7–2 mm, and the specific surface area was 3–10 m2/g. The samples were compacted by uniaxial pressing under a pressure of 200 MPa in the shape of disks, 8 mm in diameter and about 2.5 mm in thickness. The relative density of the obtained compacts was 48.0% of the theoretical value (3.16 g/cm3). The experiments on sintering under volumetric heating were carried out in the workchamber of a gyrotron system operating at a frequency of 24 GHz with a maximum power of 5 kW [21]. The compacted samples were sintered in air at normal atmospheric pressure. The samples were positioned in the middle of a channel with a diameter of 10 mm drilled through a block of low-absorption porous alumina (AL-30, ZIRCAR Ceramics, U.S.A.). The temperature of the samples was measured by two unshielded B-type thermocouples. One thermocouple head was inserted into a bore drilled in the center of the disk to half of its thickness, thereby measuring the maximum temperature of the sample during its volumetric heating. The head of the other thermocouple touched the disk surface at a distance of 0.6 mm from its edge. The constant rate of heating was sustained by automatically regulating the power input to the work chamber based on a feedback from the measured temperature signal. The preset heating rate was varied in the range 10–100 °C/min, and the maximum temperature of heating in the range 1000–1300 °C. There was no isothermal hold at the maximum temperature. For comparison between millimeter-wave and conventional sintering some samples were sintered in a resistive furnace (Termokeramika, Russia) with a heating rate of 2 °C/min and a 2 hr hold at the sintering temperature. The consolidation of hydroxyapatite powder layers under a focused millimeter-wave radiation beam was carried out in a gyrotron system operating at a frequency of 263 GHz [22]. The density of the sintered samples was determined by Archimedes weighing in distilled water. The microstructure was studied by scanning electron microscopy (JEOL JSM-6390 LV), and the phase composition was analyzed by X-ray difractometry (Rigaku Ultima IV). The microhardness and fracture toughness of the sintered samples were measured using a mechanical tester (Struers Duramin-5). Before measurements one side of the sintered samples was grinded off to a depth of about 100 mm and polished with diamond paste. The shrinkage of the samples during the sintering in the 24 GHz system and the temperature distribution on the surface of the samples were monitored using a purposely designed optical system. The samples were filmed through the channel in the thermoinsulating material by a digital infrared camera (3iCube IC4133IR) using a lens (VSZ-0745 with front converter lens VSZ-0.3X) and a water-filled window for protection against intense millimeterwave radiation. The automatic brightness control of the images

a

from the camera, the transfer and storage of the images to a computer, and the display on the monitor were carried out using the software Stream Pix 7 (NORPIX, Canada). The equivalent diameter of the sample was determined by approximating its visible shape with an ellipse and calculating the geometric average of its axes. To determine the temperature distribution over the surface of the sample, the brightness values of the pixels in the digital images were used. The spectral intensity of thermal radiation is linked to the temperature via the well-known Plank formula. The overall absolute brightness was determined as an integral of spectral intensity over the wavelength taking into account the spectral sensitivity of the camera and the transmission spectrum of the applied infrared filters. To restore the relationship between the brightness values and the temperature under the conditions of automatic exposure, a calibration procedure was applied for each image based on the brightness values of the areas adjacent to the thermocouple. 3. Results and discussion Under the described experimental conditions, millimeter-wave volumetric heating accompanied with heat removal from the surface of the samples gives rise to a significant temperature nonuniformity in the volume of the samples. The observation of the samples undergoing millimeter-wave sintering with the help of an infrared digital camera, along with the two-channel temperature measurements, has made it possible to analyze the dynamic temperature distribution during the process. An example of an infrared image from the camera, a brightness distribution along a vertical line and the obtained temperature distribution for a sample undergoing millimeter-wave heating at a rate of 10 °C/min are shown in Fig. 1. A drop in the brightness in the central area of the sample corresponds to the thermocouple that shades the thermal radiation from the sample. A spike of the brightness is due to a crack in the sample from which thermal radiation from the hotter interior of the sample reaches the camera. It can be seen that the temperature distribution over the sample surface is noticeably non-uniform, with a maximum temperature difference of about 150 °C. This reflects the competition between the volumetric heat deposition and the heat losses from surface of the sample. The degree of the temperature non-uniformity depends on the chosen heating rate and on the stage of the sintering process. The brighter areas change shape and migrate over the surface during the process, which is associated with the temperature dependence of the millimeter-wave absorption efficiency of the material [23]. Shown in Table 1 is a dependency of the relative density of hydroxyapatite samples on the sintering temperature. These

b

120

1140 1120

Temperature, °C

Brightness, a.u.

100

1100

80

1080

60

1060

40

1040 Brightness Temperature

20

1020

0

1000 -5

-4

-3

-2 -1 0 1 2 Vertical position, mm

3

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Fig. 1. (a) Example of an infrared image of the sample during millimeter-wave heating; (b) distribution of brightness and the calculated temperature along the vertical line shown in Fig. 1(a). The area shaded by the thermocouple has been excluded from the temperature distribution.

Please cite this article as: S. V. Egorov, A. G. Eremeev, V. V. Kholoptsev et al., Rapid consolidation of hydroxyapatite using intense millimeter-wave radiation, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.081

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S.V. Egorov et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 1 Relative density of hydroxyapatite (% th.d.) achieved under millimeter-wave and conventional heating. Temperature

1000 °C

1100 °C

1150 °C

1200 °C

1250 °C

Millimeter-wave sintering (50 °C/min, no isothermal hold) Conventional sintering (2 °C/min, hold for 2 h)

60.8 51.9

78.1 61.6

72.5

92.4 89.9

93.4

results demonstrate that the sintering is greatly enhanced in the case of millimeter-wave heating. It can be seen that although the duration of the millimeter-wave sintering process is much shorter, equal density values are obtained at temperatures that are 50– 100 °C smaller than in the case of conventional sintering. The temperature values listed in Table 1 for the millimeterwave sintering are those measured in the center of the samples. They are close to the maximum of the temperature distribution. However, the material was sintered to near-full density even in the peripheral regions of the samples, where the temperature was much lower, according to the temperature distribution measurements described above. It should be also noted that the time needed to achieve a relative density on the order of 95% in the described millimeter-wave sintering experiments was shorter by a factor of 102–103 compared to conventional sintering. These findings suggest that the mass transport in hydroxyapatite is enhanced during millimeter-wave sintering (see, e.g., [12,13] for a discussion of the possible mechanisms of such an enhancement). The Vickers microhardness Hl and fracture toughness R1C have been measured on the samples sintered under rapid millimeterwave and conventional heating. The measurements were carried out under a load of 0.3 kg. The mechanical properties were measured on five sintered samples, with five measurements accomplished on each of them. The values of the microhardness and fracture toughness of the millimeter-wave sintered samples virtually coincide with the respective properties of the samples obtained conventionally: Hl = 4.6 / 4.5 GPa, K1C = 0.7 / 0.6 MPa
m1/2 for millimeter-wave / conventional sintering, respectively. However, both parameters are by 30–60 % lower than the values from the literature [24], probably because of the properties of the powder used in this work. A separate series of experiments was designed to demonstrate feasibility of using millimeter-wave heating for localized ultrarapid consolidation of hydroxyapatite powder. For this purpose, electromagnetic radiation from a 263 GHz gyrotron was focused by a quasi-optical radiation transmission line [25] into a spot with a size of about 2.5 mm. At maximum gyrotron power, 1 kW, this would provide a radiation intensity of about 20 kW/cm2; however, for the experiments described here the power was reduced to 100 W and the intensity was therefore about 2 kW/cm2. The powder was poured loosely into a rectangular crucible made of porous alumina. The dimensions of the prepared powder layer were 57  12  2.5 mm. The crucible with the powder was positioned under the millimeter-wave radiation beam so that the focal point was close to the surface of the powder layer. The scanning of the wave beam over the powder surface was implemented by displacing the crucible at a velocity of about 0.5 mm/s, which was controlled to ensure uniform glowing of the heated powder. The displacement started after a 5 s hold needed for the initial heating of the powder. As a result of heating, localized consolidation has been achieved over the entire depth of the powder layer in the region that was exposed to the focused millimeter-wave radiation. The width of the consolidated region was 3–5 mm. These first results indicate that the use of focused millimeter-wave radiation beams has a potential of application in developing additive methods of controlled localized consolidation. The larger focal spot size compared to laser techniques is adequate for the fabrication of biocompatible ceramic products.

1300 °C 94.3

4. Conclusions Millimeter-wave sintering has been successfully used for rapid consolidation of hydroxyapatite. Using volumetric heating in a 24 GHz gyrotron system, a relative density on the order of 95% has been achieved with heating rates 50–100 °C/min, maximum temperature 1300 °C and zero hold time. Using a focused beam of 263 GHz radiation, rapid localized consolidation of hydroxyapatite powder has been demonstrated. CRediT authorship contribution statement S.V. Egorov: Investigation, Methodology. A.G. Eremeev: Methodology, Formal analysis. V.V. Kholoptsev: Investigation, Software. I.V. Plotnikov: Resources. K.I. Rybakov: Writing - original draft. A.A. Sorokin: Investigation. Acknowledgement This research was supported by Russian Science Foundation, grant # 17-19-01530 (development of an optical method to measure temperature distributions), and Russian Foundation for Basic Research, grant # 18-29-11045 (millimeter-wave volumetric and localized consolidation of hydroxyapatite). References [1] S. Samavedi, A.R. Whittington, A.S. Goldstein, Acta Biomater. 9 (2013) 8037– 8045. [2] M. Prakasam, J. Locs, K. Salma-Ancane, D. Loca, A. Largeteau, L. BerzinaCimdina, J. Funct. Biomater. 6 (2015) 1099–1140. [3] A. Haider, S. Haider, S.S. Han, I.-K. Kang, RSC Adv. 7 (2017) 7442–7458. [4] Y. Fang, D. Agrawal, D. Roy, R. Roy, J. Mater. Res. 9 (1994) 180–187. [5] S. Vijayan, H. Varma, Mater. Lett. 56 (2002) 827–831. [6] Y. Yang, J.L. Ong, J. Tian, J. Mater. Sci. Lett. 21 (2002) 67–69. [7] X. Wang, H. Fan, Y. Xiao, X. Zhang, Mater. Lett. 60 (2006) 455–458. [8] S. Bose, S. Dasgupta, S. Tarafder, A. Bandyopadhyay, Acta Biomater. 6 (2010) 3782–3790. [9] Q. Wu, X. Zhang, B. Wu, W. Huang, Ceram. Int. 39 (2013) 2389–2395. [10] J.D. Katz, Annu. Rev. Mater. Sci. 22 (1992) 153–170. [11] D.K. Agrawal, Curr. Opin. Solid State Mater. Sci. 3 (1998) 480–486. [12] Yu.V. Bykov, K.I. Rybakov, V.E. Semenov, J. Phys. D: Appl. Phys. 34 (2001) R55– R75. [13] K.I. Rybakov, E.A. Olevsky, E.V. Krikun, J. Am. Ceram. Soc. 96 (2013) 1003–1020. [14] S. Nath, B. Basu, A. Sinha, Trends Biomater, Artif. Organs 19 (2006) 93–98. [15] D. Veljovic´, E. Palcevskis, A. Dindune, S. Putic´, I. Balac´, R. Petrovic´, J. Mater. Sci. 45 (2010) 3175–3183. [16] S. Dasgupta, S. Tarafder, A. Bandyopadhyay, S. Bose, Mater. Sci. & Eng. C 33 (2013) 2846–2854. [17] A. Thuault, E. Savary, J.-C. Hornez, G. Moreau, M. Descamps, S. Marinel, J. Europ. Ceram. Soc. 34 (2014) 1865–1871. [18] W. Gao, Y. Zhang, D. Ramanujan, K. Ramani, Y. Chen, C.B. Williams, Comput. Aided Des. 69 (2015) 65–89. [19] H.A. Hegab, Manufacturing Rev. 3 (2016) 11–17. [20] T.D. Ngo, A. Kashani, G. Imbalzano, K.T.Q. Nguyen, D. Hui, Compos. B 143 (2018) 172–196. [21] Yu.V. Bykov, A.G. Eremeev, M.Yu. Glyavin, G.G. Denisov, G.I. Kalynova, E.A. Kopelovich, Radiophys. Quantum Electron. 61 (2019) 752–762. [22] M.Yu. Glyavin, M.V. Morozkin, A.I. Tsvetkov, L.V. Lubyako, G.Yu. Golubiatnikov, A.N. Kuftin, Radiophys. Quantum Electron. 58 (2016) 639–648. [23] A.G. Eremeev, S.V. Egorov, V.V. Kholoptsev, Proc., 17th, Int., Conf., Microwave and High Frequency Heating (AMPERE 2019), Editorial Universitat, Politècnica de València (2019) 310–317, https://doi.org/10.4995/Ampere2019.2019.9754. [24] R. Othman, B.D. Long, F.Y. Yeoh, J. Farah, Z. Azlila, A.F.M. Noor, ASEAN Eng. J. Part B 2 (2013) 12–34. [25] A.V. Vodopyanov, A.V. Samokhin, N.V. Alexeev, M.A. Sinayskiy, A.I. Tsvetkov, M.Yu. Glyavin, Vacuum 145 (2017) 340–346.

Please cite this article as: S. V. Egorov, A. G. Eremeev, V. V. Kholoptsev et al., Rapid consolidation of hydroxyapatite using intense millimeter-wave radiation, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.081