Klemens Kelm

Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells

We report a newly discovered dense microstructure of dendrite-like biocalcite that is formed by marine organisms. High spatial resolution electron backscatter diffraction (EBSD) was carried out under specific analytical conditions (15 and 10 kV) on the primary layer of the modern brachiopod Gryphus vitreus. The primary layer of modern brachiopods, previously termed nanocrystalline, is formed by an array of concave/convex calcite grains with interdigitated recesses and protrusions of abutting crystals without any cavities in or between the dendrites. The interface topology of this structure ranges from a few tens of nanometres to tens of micrometres, giving a nanoscale structure to the material fabric. The dendritic grains show a spread of crystallographic orientation of several degrees and can thus be referred to as mesocrystals. Individual dendritic mesocrystals reach sizes in one dimension larger than 20 μm. The preferred crystallographic orientation is similar in the primary and adjacent fibrous shell layers, even though these two layers show completely different crystal morphologies and grain boundary topologies. This observation indicates that two separate control mechanisms are active when the primary and the fibrous shell layers are formed. We propose a growth model for the interdigitated dendritic calcite grain structure based on a precursor of vesicles filled with amorphous calcium carbonate (ACC).

Hierarchical fibre composite structure and micromechanical properties of phosphatic and calcitic brachiopod shell biomaterials – an overview

Abstract Brachiopods are a phylum of shell-forming sessile marine invertebrates which have existed since the early Cambrian. Two very different biomaterial design strategies for their shells evolved early in their history. Both strategies use hybrid fibre composites, however, one is based on mineral fibres embedded in ~2 wt.% of organic biopolymer sheaths and the inorganic fibres are calcite single crystals. In the second strategy the fibres are biopolymers and are reinforced with Ca-phosphate nanoparticles to form a fibrous nanocomposite. Here the organic component (chitin) dominates. The Ca-phosphate nanoparticle-reinforcement strategy is not unlike that in vertebrate bone, however, the microscale structure is laminated with alternating laminae which have a different degree of mineralization. The calcitic shells feature an outer compact layer of calcite micro- and nanoparticles protecting the inner fibrous layer from the outside. Transmission electron microscopy of the fibrous layer reveals intercrystalline and intracrystalline biopolymers. The calcitic shell material is stiff with nano-indentation E-moduli of 63±8 GPa and relatively hard (Vickers microhardness up to 400 HV 0.0005/10 and nanohardness 4±0.5 GPa). Compared to inorganic calcite the microhardness is doubled and the nanohardness increases by 60%. We attribute this increased hardness to intracrystalline biopolymers. The nano-indentation E-moduli of the chitinophosphatic shells range from 3 to 55 GPa as a result of the varying degree of mineralization between their laminae, and similarly their nanohardness varies between 0.1 and 3 GPa. For brachiopods burrowing inside the sediment, the alternation of non-mineralized laminae with thin, more strongly mineralized laminae provides abrasion-resistance, hardness and longitudinal stiffness while it preserves the flexibility provided by the organic component for bending movements.

Amorphous calcium carbonate in the shell material of the brachiopod Megerlia truncata

Terebratulide brachiopod shells have a thin, hard outer nanocrystallinecalcite layer and a hybrid fibre composite inner layer with CaCO(3)fibres embedded in an organic matrix. Here we report our observation bytransmission electron microscopy (TEM) of a large compartment filledwith amorphous calcium carbonate (ACC) in the shell of the modernbrachiopod Megerlia truncata. The compartment has the typical shape andsize of fibrous calcite crystals composing the inorganic component ofthe hybrid fibre composite secondary shell layer. The ACC compartment isadjacent to an inclusion of foreign material that is entirelyincorporated into the shell. It has most probably been produced in thecourse of shell reparation. Under TEM imaging conditions the amorphouscarbonate crystallized in situ to vaterite and calcite. The distributionpattern of the organic component of the shell material is spatiallydifferentiated. While in the outer, nanocrystalline primary shell layerwe do not observe any organic material between the crystallites by TEM,the CaCO(3) fibres of the secondary layer are surrounded by an organicsheath. In the innermost segment of the secondary layer, in addition tothe organic sheaths, thick organic membranes are present. Thecompartment containing ACC is located between two, 1-2 mu m thick,organic membranes. Our observations indicate that brachiopod shellformation may occur via an ACC precursor that is produced in an initialStage prior to the crystallization of calcite.

Hierarchical structure of marine shell biomaterials: biomechanical functionalization of calcite by brachiopods

Abstract Biologic structural materials for skeletons or teeth show a hierarchical architecture, where organic macromolecules and mineral substance form a hybrid composite material with its components inter-weaved on many length scales. On the nanostructure level brachiopods form hybrid composite mesocrystals of calcite with occluded organic molecules. On the microstructure level several types of materials are produced, on which the electron back-scatter diffraction (EBSD) technique gives insight in texture and architecture. We describe the calcite single-crystal fiber composite architecture of the secondary layer involving organic matrix membranes, the competitive-growth texture of the columnar layer and the nano-structuring of the primary layer. In the overall skeleton the organic biopolymers provide flexibility and tensile strength while the mineral provides a high elastic modulus, compressive strength, hardness and resistance to abrasion. The hierarchical composite architecture, from the nanostructure to the macroscopic level provides fracture toughness. The morphogenesis of the biomaterial as a whole and of the mineral crystals is guided by the organic matrix and most probably involves amorphous calcium carbonate (ACC) precursors. In this paper we review the hierarchical architecture of rhynchonelliform brachiopod shells, which is very distinct from mollusk nacre.

Towards systematics of calcite biocrystals: insight from the inside

Abstract Biocrystals of calcite are frequent as they are employed by many phylae of organisms in shells, eggshells, teeth, spines or sensoric apparatus. The calcite phase in these materials occurs in a range of constitutions, from polycrystalline fabrics to “single-crystals”. We demonstrate systematics of calcite biocrystal architectures, from the hybrid composite mesocrystal fibres of brachiopod and mollusc shells, via the submillimeter-sized hybrid composite crystal aggregates formed by mesocrystal fibres with both morphological co-orientation and lattice co-oriäentation, to more complex purpose-oriented multiplex äcomposite crystals of echinoderm teeth, which feature a high degree of single-crystal-like 3D orientational correlation of microstructural elements of different morphology and composition. These systematics rely on observations by electron backscatter diffraction (EBSD) and TEM.

Mosaic structure in the spines of Holopneustes porossisimus

Abstract Sea urchin spines of Holopneustes porossisimus are porous single crystals, with the pores being filled with a material rich in carbon, silicon, fluorine and sodium. The magnesian calcite constituting the spine is highly strained. Even though the spines appear to be single crystalline on a macroscopic scale, the calcitic material exhibits an extended defect network. We find dislocations as well as rotational and other, not yet identified boundaries. We also observe within spine calcite a patterned distribution of sulphur. Both distributions, that of the defect network and that of sulphur resemble in their pattern to each other and have a similar mesh size of 50 nm. We conclude from these observations that they arise from the growth process of the spine and account for the mosaicity within the spine single crystals.

The infrastructure of brachiopod shells - A mechanically optimized material with hierarchical architecture

Brachiopod shells consist of low-magnesium calcite and belong to one of the most intriguing species for studies of marine paleoenvironments, variations in oceanographic conditions and ocean chemistry [6, 7, 11 - 13]. We have investigated the ultrastructure together with nano- and microhardness properties of modem brachiopod shells with transmission electron microscopy (TEM), scanning electron microscopy (SEM), nanoindentation and Vickers microhardness analyses. Brachiopod shells are structured into several layers, a thin, outer, hard, protective primary layer composed of randomly oriented nanocrystalline calcite, which is followed inward towards the soft tissue of the animal by a much softer shell segment (secondary layer) built of long calcite fibres, stacked parallely into blocks. The hardness distribution pattern within the shells is non-uniform and varies on scales as small as a few tens of microns, Our results show that the hardness of this biomaterial is controlled by two predominant features: (1.) The morphological orientation of the calcite fibres (not by the crystallographic orientation of the fibres), and (2.) the amount and distribution pattern of organic material between and within the calcite crystals.

The Symmetry of Ordered Cubic γ-Fe2O3 Investigated by TEM

Well-crystallized particles of cubic and tetragonal γ -Fe2O3 embedded in a Pd matrix were produced besides other oxides by internal oxidation of a Pd-Fe alloy in air. Particles of tetragonal γ -Fe2O3 consist of orientation domains with the c axes normal to each other. Particles of the ordered cubic γ -Fe2O3 appear single crystalline in bright field and in dark field images with reflections of the basic spinel structure. In dark field images enantiomorphous domains were observed using reflections of the ordered phase. From the analysis of electron diffraction patterns in the principal zone axes the description of ordered cubic γ -Fe2O3 in the enantiomorphous space groups P4132/P4332 follows without further presumptions. In the sequence from space group Fd3m of disordered cubic γ -Fe2O3 via P4132/P4332 of the ordered cubic phase to the pair P41212/P43212 of tetragonal γ -Fe2O3 a continuous group-subgroup relation can be derived. This relation shows that ordered cubic γ -Fe2O3 is an intermediate phase upon ordering of vacant octahedral sites towards tetragonal γ -Fe2O3

Combined HRTEM and EFTEM Study of Precipitates in Tungsten and Chromium-Containing TiB 2

The structure and chemical composition of two types of precipitates in the system TiB 2 -WB 2 -CrB 2 were studied by means of high-resolution TEM and energy filtering TEM. Type I particles (W 2 B 5 structure) are precipitated at the basal plane of the hexagonal matrix whereas type II precipitates are thin platelets lying parallel to the {1100} prism planes. Lattice imaging yields displacements of the metal positions with respect to the matrix. Information on the chemical composition at high lateral resolution is obtained from elemental maps of all chemical constituents using electron spectroscopic imaging (ESI). The type II precipitates show a decrease in the B and Ti concentration, whereas the tungsten concentration increases and the Cr is homogeneously distributed. The HRTEM results combined with the results of the elemental maps allow to develop a structural model based on the intergrowth of the β-WB structure in the TiB 2 -rich matrix. The two deficient boron layers in W 0.5 Ti 0.5 B with a spacing of 0.38 nm can be used to examine the resolution limit of ESI.

Creep strength of a binary Al62Ti38 alloy

Abstract Al-rich Ti – Al alloys, as compared to Ti-rich -TiAl-based alloys, offer an additional reduction in density of 20 %, better oxidation resistance and sufficient strength at high temperatures. High temperature creep of a binary Al62Ti28 alloy was studied in compression in the temperature range between 1 173 K and 1 323 K in air. It is shown that the alloy exhibits quite reasonable creep resistance at 1 173 K, especially in view of its low density of around 3.8 g cm– 3. Stress exponents calculated as the slope n = log (strain rate)/ log (stress) = 4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The values of the activation energies for creep for the as-cast and the annealed Al62Ti38 material coincides well with those found in the literature for interdiffusion of Al in -TiAl.