Nanoporous hydroxyapatite (HA) ceramics were consolidated using conventional or free sintering, spark plasma sintering (SPS) and flash sintering (FS). Microstructures formed during electric field assisted sintering, which includes SPS and FS revealed that nanovoids were retained within HA grains during processing. After free sintering, however, no nanovoids were detected by transmission electron microscopy (TEM). The observed nanovoids were confirmed not induced by electron beam damaging, and showed either a round or faceted shape with diameters ranging between 5 and 10 nm. In-situ TEM heating experiments demonstrated that nanovoids remained stable up to 900°C but disappeared at a temperature as high as 1100°C.
LeGerosRZ, Ben-NissanB.Introduction to synthetic and biologic apatites. In: Ben-NissanB, editor. Advances in calcium phosphate biomaterials. Berlin, Heidelberg: Springer Berlin Heidelberg; 2014. p. 1–17. Ch. 1
2.
MoroniA, CajaV, EggerE, Histomorphometry of hydroxyapatite coated and uncoated porous titanium bone implants. Biomaterials. 1994;15:926–930. doi: 10.1016/0142-9612(94)90119-8
3.
VijayalakshmiU, RajeswariS.Preparation and characterization of microcrystalline hydroxyapatite using sol gel method. Trends Biomater Artif Organs. 2006;2006(2):57–62.
4.
MuralithranG, RameshS.The effects of sintering temperature on the properties of hydroxyapatite. Ceram Inter. 2000;26:221–230. doi: 10.1016/S0272-8842(99)00046-2
5.
MunierZA, QuachDV, OhyanagiM.Electric field and current effects on sintering. In: CastroR, van BenthemK, editors. Sintering, engineering materials. Springer Sci. & Business Media; 2012. p. 35.
6.
YuM, GrassoS, MckinnonR, Review of flash sintering: materials, mechanisms and modelling. Adv Appl Ceram. 2017;116(1):24–60. doi: 10.1080/17436753.2016.1251051
7.
DancerCEJ.Flash sintering of ceramic materials. Mater Res Express. 2016;3:102001. doi: 10.1088/2053-1591/3/10/102001
8.
YunJ, SonH, PrajatelistiaE, Characterization of transparent hydroxyapatite nanoceramics prepared by spark plasma sintering. Adv Appl Ceram. 2014;113(2):67–72. doi: 10.1179/1743676113Y.0000000079
9.
BajpaiI, HanY-H, YunJ, Preliminary investigation of hydroxyapatite microstructures prepared by flash sintering. Adv Appl Ceram. 2016;115(5):276–281. doi: 10.1080/17436753.2015.1136777
MeldrumA, WangLM, EwingRC.Electron-irradiation-induced phase segregation in crystalline and amorphous apatite: A TEM study. Amer Mineralogist. 1997;82:858–869. doi: 10.2138/am-1997-9-1003
13.
Reyes-GasgaJ, Garcia-GarciaR.Analysis of the electron-beam radiation damage of TEM samples in the acceleration energy range from 0.1 to 2 MeV using the standard theory for fast electrons. Radiat Phys Chem. 2002;64:359–367. doi: 10.1016/S0969-806X(01)00578-3
14.
Reyes-GasgaJ, Garcia-GarciaR, BresE.Electron beam interaction, damage and reconstruction of hydroxyapatite. Physica B. 2009;404:1867–1873. doi: 10.1016/j.physb.2009.03.008
15.
NicolopoulosS, Gonzalez-CalbetJM, AlonsoMP, Characterization by TEM of local crystalline changes during irradiation damage of hydroxyapatite compounds. J Sol State Chem. 1995;116:265–274. doi: 10.1006/jssc.1995.1212
16.
ChoiSH, ParkY, ChuS-H, Fabrication of advanced thermoelectric materials by hierarchical nanovoid generation. US Patent 2009/0185942.
17.
YaoY, YangY, ChenS.Size-dependent elasticity of nanoporous materials predicted by surface energy density-based theory. J Appl Mechanics. 2017;84:0610041–0610047. doi: 10.1115/1.4036345
18.
WilliamsDB, CarterCB.Transmission electron microscopy: A textbook for materials science. 2nd ed. Springer Sci.+Business Media; 2009. Ch.23, p. 397–404.
19.
HutchisonJL, BursillLA.Fresnel fringe contrast of faceted voids within gem-quality diamond. J Microscopy. 1983;131(1):63–66. doi: 10.1111/j.1365-2818.1983.tb04231.x
20.
KayMI, YoungRA.Crystal structure of hydroxyapatite. Nature. 1964;204:1050–1052. doi: 10.1038/2041050a0
21.
SengerB, BresEF, HutchisonJL, Ballistic damages induced by electrons in hydroxyapatite (OHAP). Phil Mag A. 1992;65(3):665–682. doi: 10.1080/01418619208201543
22.
BresEF, HutchisonJL, SengerB, HREM study of irradiation damage in human dental enamel crystals. Ultramicroscopy. 1991;35:305–322. doi: 10.1016/0304-3991(91)90083-I
23.
WangY, RenX, MaX, Alginate-Intervened hydrothermal synthesis of hydroxyapatite nanocrystals with nanopores. Cryst Growth Design. 2015;15:1949–1956. doi: 10.1021/acs.cgd.5b00113
