Heino Sieber

Biomorphic Cellular Ceramics

Biotemplating - a new concept for preparation of ceramic composite materials with biomorphic microstructures - will be described. Biological carbon preforms (C B -templates) were derived from different wood structures by high-temperature pyrolysis at temperatures between 800 and 1800°C and used as templates for infiltration with gaseous or liquid Si to form SiC- and SiSiC-ceramics. During high-temperature processing, the microstructural details of the bioorganic preforms were retained. Cellular ceramic composites with unidirectional porous morphologies and anisotrophic mechanical properties were obtained. The cellular composites show low density, but high specific strength and excellent high temperature stability.

Biomorphic SiC-ceramic prepared by Si-vapor phaseinfiltration of wood

Abstract The conversion of bioorganic structures like wood or ligninocellulosic fibers into porous microcellular SiC-ceramics is a novel manufacturing technology. Coniferous wood (pine) was transformed by high-temperature pyrolysis into carbon preforms and subsequently converted into biomorphic SiC-ceramics by Si-vapor infiltration under inert atmosphere. The morphology of the initial wood structure is retained. The infiltration of the Si-vapor phase into the C B -template and the kinetics of the SiC-formation were investigated. The amount of residual carbon was determined by means of mass balance calculation. The macroscopical properties of the final SiC-ceramics were characterized by porosity and mechanical property measurements.

Effect of microstructure on the fracture behavior of biomorphous silicon carbide ceramics

Abstract Highly porous cellular silicon carbide was prepared from native pine wood tissue by vapor infiltration of Si, SiO, and CH 3 SiCl 3 into the carbonized template. β-SiC at the biocarbon surface finally resulted in a complete conversion of the template into a cellular silicon carbide material. Due to the different reaction mechanisms, different strut microstructures were obtained. The strength of the biomorphous SiC was measured under biaxial tensile loading conditions perpendicular to the cell elongation (in-plane loading). A non-catastrophic stress-strain behavior was observed in the Si and CH 3 SiCl 3 derived materials which showed a high skeleton density of ⩾3 g/cm 3 . Extendend cell wall fracture (peeling) was observed in the Si derived material where the original intercellular lamella was retained in the ceramic material. FE calculations of the stress distribution in a representative structure model showed significantly lower levels of tensile stress in rectangular pore arrays (early wood tissue) compared to ellipsoidal pores (late wood tissue).

Glass formation versus nanocrystallization in AN Al92Sm8 alloy

Solid state glass formation is often viewed as a non-equilibrium process resulting from the destabilization of crystalline phases when the maximum metastable solubility is exceeded. In this work, glassy Al{sub 92}Sm{sub 8} alloys which have been obtained by both rapid quenching and deformation mixing techniques have been studied as a representative of a marginal glass forming material. The samples were examined to explore whether the nucleation of nanocrystals can be avoided by choosing an appropriate reaction pathway and whether the changes in the reaction pathway can influence the kinetic stability of the amorphous phase.

Gas Phase Processing of Porous, Biomorphous TiC-Ceramics

Specimens of natural pine wood were converted into biomorphous TiC-ceramics by CVIR processing (chemical vapor infiltration – reaction). The wood samples were first pyrolysed in inert atmosphere at temperatures of 800°C to yield biocarbon-derived template structures (CBtemplates). Subsequently, the CB-templates were infiltrated with titanium-tetrachloride (TiCl4) in excess of hydrogen at temperatures above 1200°C by isothermal CVI processing. Elemental Ti was deposited on the surface of the CB-struts within the pores. During processing the carbon of the CBtemplate reacted with the deposited Ti to form TiC-ceramics. The infiltration of the Ti-species into the porous carbon template, the micro morphology and phase distribution of the TiC-ceramics were investigated by XRD, SEM/EDX-analysis as well as porosity measurements. The highly porous, biomorphous specimens was homogenous converted into TiC and exhibits of nano-crystalline TiCphase on the inner surface of the carbon struts. Residual carbon was found in the center of initial carbon struts, especially in late wood areas where the carbon strut thickness was more than 3 μm. Introduction In contrast to most conventionally produced foam structures, bioorganic cellular materials like wood are characterized by an unidirectional pore system used by the plant for water and nutrition transport. The bioorganic tissue can be converted into biocarbon preform or template structures (CBtemplates) by pyrolysis in an inert atmosphere. Subsequent transformation into porous carbide ceramics can be achieved using a variety of gas phase infiltration-reaction processes using carbide forming metals (e.g. Si, Ti). In the recent years various biotemplating processing technologies were developed for manufacturing of biomorphous SiC-based as well as oxidic ceramics [1-7]. Especially the manufacturing of porous SiC-ceramics by the conversion of bioorganic materials such as wood has recently become of particular interest. Ota et al. [2] e.g. investigated the infiltration of charcoal with TEOS (tetraethyl orthosilicate), which was then converted by high-temperature pyrolysis into a highly porous, biomorphous SiC-ceramic. Si-melt infiltration yields nearly dense SiSiC-ceramics [3-4]. Reactive gas phase infiltration of Si/SiO-vapor or MTS (methyltrichlorosilane) into carbonized wood yields highly porous and single-phase SiC-ceramic [5-7]. The inherent open porous structure of the biological derived materials is retained during the processing down to the sub micrometer level yielding highly porous ceramics with cell diameters from a few microns to several hundred microns. It results in an unique microcellular morphology that can not be produced by conventional ceramic processing technologies [1]. Biomorphous, SiC-based porous ceramics are interesting candidates for applications as high-temperature filter or catalyst support structures due to there high thermal conductivity, good oxidation and corrosion resistance as well as high strength at elevated temperatures. The materials properties of TiC-based ceramics are often inferior to those of SiC, however a higher hardness, improved corrosion resistance in phosphoric acid and, especially a high electrical conductivity favorably characterize them. However, for the processing of biomorphous TiO2/TiC-ceramics only few investigations are known. The infiltration of TTiP (titanium tetra-isopropoxide) into dried wood or charcoal and high-temperature treatment yields Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 2227-2230 doi:10.4028/www.scientific.net/KEM.264-268.2227

In Vitro Calcium Phosphate Formation on Cellulose – Based Materials

CaO-SiO2 sol-gel coatings were deposited on natural cellulose-based polymers with a 3D porous network structure. Changes in the sample surface after c o ting and subsequent soaking in simulated body fluid (SBF) were determined by SEM – EDX, FT-IR and X-ray diffraction analysis. Sample solution interactions were quantified on the basis of gravimetric and solution analysis. Within 3 days a homogeneous calcium phosphate layer was deposi ted on the sample surface. Furthermore, the porous structure of the samples was mainta ined. The Ca/P ratio in the deposited layer was 1.63, which is close to that in hydroxyl carbonated a patite (HCA). At the beginning of Ca-P layer precipitation calcium leaches out from the gel resulting in the formation of Si-OH groups in the surface. The Si-OH groups serve as favorable s ites for Ca and (PO4) 3absorption from SBF resulting in the formation of a Ca-P enriched la yer, which grows by consumption of calcium and phosphorus from the surrounding fluid. Sol-gel coatings of porous structures can be used to enhance osteointegration of porous bone replaceme nt materials and scaffolds for bone tissue engineering. Introduction Biomaterials are manufactured from all kinds of man-made materi als, including polymers, ceramics and metals as well as their composites. However, none of them can s erve a perfectly as the living tissue for a replacement. If used as bone repairing or replacing m aterial, metal implants will cause stress shielding. The low modulus of synthetic polymers limits thei r clinical application in bone reconstruction. For ceramic materials, the low fracture toughness is not favorable for bone-repairing material, thus the desire to search for new biomaterials is bene ficial. Natural cellulose based materials possess mechanical properties close to those of human bone [1]. The biocompatibility of cellulose and its derivatives is well established [2]. Neverthele ss, a complete mineralization could not be observed after implantation [3, 4]. The aim of the present work wa s to suggest a simple treatment that would induce calcium phosphate formation and mineralizat ion in the surface of biological cellulose-based structures via a biomimetic process. Materials and Methods Luffa aegyptiaca sponge was used as a source of cellulose. This natural material with a fibrous network, obtained from the matured dried fruit of Luffa aegyptiaca (syn. Luffa cylindrica), was reported to consist of cellulose, hemicellulose and small amount of ma nnan and galactan [5, 6, 7]. Pieces of Luffa approximately 15 mm in length were first extr acted in a 2:1 mixture of toluene and ethanol for 24 hours by Soxleth method and than dried for 24 hours at 110°C. The drie d samples were dip-coated with a sol containing 20 mol% CaO and 80 mol % SiO 2 [8]. The coated specimens were kept 3 days at room temperature, than 3 days at 70°C and subsequently 3 days at 120°C for hydrolysis, aging and drying of the coating, respectively. Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 1013-1016 doi:10.4028/www.scientific.net/KEM.254-256.1013

SiSiC-Ceramic Composites from Biocarbon Powder

The reactive infiltration of carbonaceous materials derived from biological tissues with liquid Si (LSI-process) represents an interesting processing scheme for manufacturing of ceramic matrix composites (CMC). The LSI-processing of porous biocarbon preforms prepared from pressed powders results in nearly dense SiSiCor SiSiC/C-materials. The microstructure and material properties of the final CMC’s depend on the morphology of the biocarbon powders and preforms, e.g. density, specific surface area, pore size, wetting as well as reaction behaviour with the Si-melt. SiC-based ceramics were prepared by spontaneous Si-melt infiltration at 1600°C of different carbon powder preforms. As the biocarbon source, a waste carbon powder from the bio-fuel production was used. The phase distribution and interface morphology of the resulting biocarbon derived SiC-based ceramics were compared with SiSiC/Cmaterials prepared from conventionally available carbon powder materials (carbon black, graphite powder).

Structured SiSiC-zeolite ceramic composites for catalytic applications via a support self-transformation technique

The overall 3-step process for the manufacturing of SiSiC-zeolite structured cellular composites was studied. The process consists of a 2-step procedure for the ceramic monolith fabrication, followed by the functionalisation of the support surface with zeolite coating (3 rd step). SiSiC ceramic monoliths were prepared by reactive Liquid Silicon Infiltration (LSI) of carbon preforms from corrugated cardboard monoliths. The zeolite coating process consisted of a hydrothermal treatment of the SiSiC carrier in an alkaline solution containing the template and the Al-source, while the Si was provided from the ceramic substrate. MFI-type (ZSM-5) zeolite crystals were directly grown on the ceramic supports via a partial Si dissolution (from the SiSiC matrix) and zeolite crystallisation (support self-transformation). Cellular SiSiC-zeolite monoliths possess bimodal (micro-/macro-) porosity and high mechanical, chemical and thermal stabilities.

Modeling, Simulation, and Optimization of Microstructured Biomorphic Materials

We consider the modeling, simulation, and optimization of microstructured cellular biomorphic ceramics obtained by biotemplating. This is a process in biomimetics, a recently emerged discipline in materials science where engineers try to mimick or use biological materials for the design of innovative technological devices and systems.

Influência da temperatura de infiltração de alumínio gasoso em ligninocelulósicos nas propriedades de Al2O3 biomórfica

A conversao de materiais ligninocelulosicos em cerâmicas biomorficas tem despertado particular interesse, por tratar-se de uma nova classe de materiais cerâmicos de alto valor agregado, produzidos a partir de materias-primas de baixo custo e relevante papel economico. Cerâmicas biomorficas sao obtidas por meio do processo de biomodelagem, que consiste na reproducao ao nivel micrometrico da estrutura natural do material de partida. Neste trabalho, os materiais ligninocelulosicos utilizados como material de partida foram fibras de sisal, ratam e pinheiro. Estes foram convertidos por pirolise em pre-formas de carbono. Subsequentemente, essas pre-formas foram infiltradas com aluminio gasoso em temperaturas que variaram de 1400 oC a 1600 oC sob vacuo, obtendo-se assim carbeto de aluminio (Al4C3). Por ser um material fragil e de rapida decomposicao, O Al4C3 foi rapidamente oxidado a 1600 oC para sua conversao total em fibras de Al2O3 biomorfica. A microestrutura e morfologia das cerâmicas biomorficas obtidas foram caracterizadas por microscopia eletronica de varredura e por difracao de raios X. O estudo comparativo da infiltracao de aluminio gasoso foi realizado com base em resultados obtidos por analise termogravimetrica.