Relationship between thermal conductivity and structure of nacre from Haliotis fulgens
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Michel B. Johnson Institute for Research in Materials, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 Canada
Ulrike Werner-Zwanziger Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 Canada
Mary Anne Whitea) Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 Canada; and Institute for Research in Materials, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 Canada (Received 19 October 2010; accepted 1 March 2011)
The thermal conductivity of nacre from red abalone (Haliotis fulgens) has been determined as a function of temperature (2–300 K), direction, and treatment to partially demineralize or to remove a portion of the organic matrix. The room-temperature thermal conductivity and specific heat of nacre are ;1 W m1 K1 and 0.9 J K1 g1, respectively. The thermal conductivity of nacre is rather low and glass-like. It is not as anisotropic as one might expect on the basis of brick-and-mortar structure, in support of recent findings that the aragonite tablets are not monolithic. Partial removal of the mineral component reduces the thermal conductivity in both principal directions, whereas partial removal of the proteins (as observed by 13C NMR) only reduces the thermal conductivity across the aragonite layers.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.76
Consider the aragonite crystals. These can show, on average, long-range preferential orientation, with the c-axes parallel to each other and perpendicular to the sample shell, and the a- and b-axes statistically aligned, over areas as large as 25 mm 25 mm.2 However, close examination shows oriented domains of 3 to 10 aragonite tablets1 and differences in orientation at different growth stages5 and different growth rates.6 And the aragonite tablets are not monolithic. Each tablet is subdivided by organic matter that can be removed by heating to create pores.7 The intracrystalline organic matrix is in the form of an oriented, crystalline foam with very thin walls and closed cells, dividing each aragonite tablet into nanograins of approximate dimension 40 nm,2 and the nanograin structure likely plays a role in nacre’s mechanical properties.8 Furthermore, solid-state nuclear magnetic resonance (NMR spectroscopy) shows that the surface of the aragonite platelets is amorphous and there is no significant (chemical) interaction between this amorphous layer and the protein matrix that surrounds it.9 However, this amorphous layer surrounds only the plates and is not at the interfaces of the intraplate nanograins.9 Another subtle factor is that the crystal lattice of aragonite from biogenic sources, such as in nacre, has been found to be anisotropically distorted (up to 0.2% along the c-axis), a distortion that can be relieved by annealing.10
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Ó Materials Research Society 2011
Nacre from the interior of shells is a beautiful biomaterial, both at the macroscopic level, where it exhibits pearlescence colors because of interference effects,
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