A new route for the synthesis of calcium-deficient hydroxyapatites with low Ca/P ratio: Both spectroscopic and electric
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lier and I. Cachadin˜a Departamento de Fı´sica, Universidad de Extremadura, Avda. Elvas s/n. 06071-Badajoz, Spain
E. Matijevic´ Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5814 (Received 15 November 1999; accepted 17 August 2000)
A new route for obtaining calcium-deficient apatites with a Ca/P ratio lower than 1.5 is described, in order to study their proton conduction at temperatures lower than 400 °C. The process is based on the hydrolysis of a mixed solution of Ca(NO3)2 and NH4H2PO4 in the presence of hexamethylenetetramine at a pH of approximately 5 and temperatures of 85–90 °C. The resulting spherical particles of 14 m in average diameter were aggregates of smaller needles with approximate composition Ca8.5(HPO4)2(PO4)4OH ⭈ H2O. The effects of the reagent concentrations, pH, aging time, and temperature were studied, and the solids were characterized by x-ray diffraction, infrared absorption spectroscopy, and electron microscopy. The ionic conduction measured by alternating-current impedance spectroscopy yielded a value of 3 Sm−1 at 200 °C.
I. INTRODUCTION
Calcium phosphates encompass a large variety of structures and compositions, the variations of which have been compared with pure synthetic compounds used as standards. The most widely studied have been apatites, especially the stoichiometric calcium hydroxyapatite [HA, Ca10(PO4)6(OH)2], which can be taken as the structural prototype, with a large number of isostructural substitutions involving Ca2+, PO43−, and OH− ions. In addition to their significance in biology,1 owing to their unique surface structure and ionic substitutions, these compounds are useful as catalysts2,3 and ion exchangers.4,5 Recently, the electrical properties of calcium hydroxyapatites and related compounds have been investigated due to potential applications as gas sensors6 and proton conductors at high temperatures.7,8 The hydroxyl groups can be retained in the apatitic structure at temperatures as high as 800–900 °C and, therefore, a reasonably good proton conduction might be expected under such conditions. Indeed, the conductivity of approximately 10−4 Scm1 reported by Yamashita et al.7,8 for yttrium-doped hydroxyapatites over this range of temperature confirms the expectations, although this 2526
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J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 02 Apr 2015
value is clearly lower than that found in other ceramic electrolytes, such as doped zirconia systems (approximately 10−2 to 10−1 Scm−1). A review of possible proton conductors suitable for fuel cells indicated that good materials in the temperature range of 200–400 °C are still required.9 However, most proton conductors are unstable at greater than 150 °C and show low electrical conductivity. Calcium-deficient hydroxyapatites (CaDH) display structural characteristics, which might be suitable for adequate ionic conduction at temperatures less than 400 °C. Specifically, depending on the Ca/P ratio, according to Joris and Amberg,10,11 CaDH can be rep
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