Mechanical properties of biomaterials

Materials that are used for biomedical or clinical applications are known as biomaterials. The following article deals with fifth generation biomaterials that are used for bone structure replacement. For any material to be classified for biomedical applications, three requirements must be met. The first requirement is that the material must be biocompatible; it means that the organism should not treat it as a foreign object. Secondly, the material should be biodegradable (for in-graft only); the material should harmlessly degrade or dissolve in the body of the organism to allow it to resume natural functioning. Thirdly, the material should be mechanically sound; for the replacement of load-bearing structures, the material should possess equivalent or greater mechanical stability to ensure high reliability of the graft. Introduction The biomaterial term is used for materials that can be used in biomedical and clinical applications. They are bioactive and biocompatible in natu

Autogenous Shrinkage and Hydration Kinetics of SH-UHPFRC under Moderate to Low Temperature Curing Conditions

Mohammadhadi Kazemi Kamyab

Strain Hardening Ultra High Performance Fiber Reinforced Concrete (SH-UHPFRC) were used successfully over the last 8 years in numerous cast in place rehabilitation applications as long lasting waterproofing layers (20 to 30 mm thickness) and, combined with rebar, to increase the structural load bearing capacity of existing bridge decks or building slabs (50 to 70 mm thickness). In composite applications on reinforced concrete substrates, SH-UHPFRC provide a deformation capability (strain hardening) larger than their free shrinkage which makes localized macro cracking at service state very unlikely. On the other hand some specific conditions of microclimatic state of the substrate (moisture and thermal conditions) and climatological (temperature and humidity) of the site such as casting at low temperatures (winter conditions) might increase the autogenous shrinkage and thus eigenstresses of the freshly cast SH-UHPFRC and hinder the development of its tensile strength or/and deformability. Combination of these specific circumstances could compromise the SH-UHPFRC protective functions due to the occurrence of localized macro cracks, making the objective of using SH-UHPFRC as a onetime intervention strategy in rehabilitation application obsolete. Advancement in hydration at early age leads to formation of partially emptied capillary spaces contributing to self-desiccation, decrease of the relative humidity and increase of the capillary depression. Since water is under depression, the solid porous material is under compression forcing the material to shrink. From the aforementioned, it can be deduced that the autogenous shrinkage is a force driven phenomenon: a force is applied on the ageing viscoelastic porous skeleton and the apparent structural response of the bulk cementitious material is the autogenous shrinkage strain. Based on the aforementioned two levels of investigation were considered in this thesis: I. Material level: ageing viscoelastic porous skeleton of hydrating cementitious material II. Structural level: autogenous shrinkage development under the action of its driving forces The objective of this thesis was to study the development of the autogenous shrinkage and its relation with the development of microstructure and mechanical properties in SH-UHPFRC cured at moderate and low temperature conditions (1 to 20°C). Focus was put on: (1) kinetics and magnitude of hydration of silica fume and cement, (2) activation energy of the processes, (3) stiffness (E modulus and Poisson ratio) development, and (4) states of water and pore development. Finally a cross link analysis of information obtained from material and structural level was carried out to better highlight the mechanisms and driving forces of autogenous shrinkage. CM22-TKK, a SH-UHPFRC developed at MCS for rehabilitation applications, on the basis of CEMTECmultiscale® fibrous mixes, was used. Silica fume is one of the major constituents of CM22-TKK (26% by mass of cement). Extent of the silica fume pozzolanic reaction significantly affects the pore size distribution of the UHPFRC matrix which then influences the development of the relative humidity, self-desiccation and finally autogenous deformation. Detailed experimental investigation performed showed that only maximum 8% by mass of cement out of 26% silica fume used in the recipe reacts in moderate and low temperature curing conditions. Finally, a model of silica-fume degree of reaction as a function of curing temperature (5 to 250 °C) was proposed on the basis of a detailed literature survey of existing works with low W/B mixes. The activation energy of the thermo-mechanical properties was evaluated for thermo (cumulative heat of hydration) and mechanical properties development (such as dynamic Elastic modulus, dynamic Poisson’s ratio and compressive strength). A single activation energy Ea of 27.4 KJ/mol could successfully model the activation of all properties. Since maturity was not a proper reference to compare the autogenous deformations at various temperatures, a more fundamental reference such as degree of hydration was used. Experimental investigation was carried out using different techniques such as isothermal calorimetry and 29Si solid NMR to evaluate the development of the degree of hydration of cement and silica fume in UHPFRC at moderate and low temperature curing conditions. The ultimate degree of hydration of cement in CM22-TKK for a closed system was 0.34. There was a close correspondence between the experimental results and the predictions of the volumetric phase distribution models of Waller (0.28) and Jensen (0.32). Finally, the De Schutter et al. 1996 model was used to describe the mechanical properties (Emodulus and Poisson ratio) as function of degree of hydration. Nondestructive and non-invasive 1H-NMR was used to evaluate volumetric phase distribution development at 20°C, especially the classification of the water types (capillary, gel and C-S-H interlayer water) which brought fully new and valuable information about the hydration kinetics and pore size development. The results were also compared with Jensen volumetric phase distribution model to check the advantages and limitation of both approaches. Finally experimental investigation was carried out to follow the development of autogenous shrinkage and eigenstresses (in case of restrained shrinkage) of CM22-TKK cured in isotherm conditions (1 to 20°C). Eigenstresses were obtained and used to highlight the effect of the viscous response of the material at the different temperatures investigated. The previously mentioned results on the kinetics of the silica fume reaction, cement hydration and evolution of states of water and development of stiffness were then put into perspective with the trends observed on the autogenous shrinkage and eigenstresses development.

EPFL2013

Silicon nanowire AFM tips grown on released scanning probe cantilevers from stencil-deposited catalysts

Jürgen Brugger, Andreea Veronica Savu

We present a parallel, full wafer technique for deposition of catalyst on released scanning probe bodies for the growth of individual high aspect-ratio Si nanowire tips. 1-D probes are necessary for imaging high aspect-ratio nano-patterns, such as deep and narrow trench geometries, or nanometer lateral resolution in single cell probing [1]. Together with carbon nanotubes, Si nanowires are excellent candidates for providing such tips. The main challenge remains the efficient integration at full-wafer scale of these 1D nano-objects with the scanning probe bodies, while still maintaining their good mechanical properties as scanning tips [2]. We are integrating here NW growth with a shadow mask technique, a unique approach providing the capability of parallel nanopatterning on top of 3D substrates [3-4]. In this work stencil lithography is used to deposit nano-catalysts for Si nanowire growth at controlled positions on released cantilever bodies. We started by fabricating two wafers: the substrate with tip-less Si scanning probes connected only by a thin Si bridge to the wafer body, and the stencil, with 100 nm thin low-stress SiN membranes containing apertures, as shown in Fig. 1. A customized SUSS MA/BA6 stencil aligner was used to align the two and mechanically clamp them. In the aligned position, each aperture from one membrane corresponds to a position close to the tip of the cantilevers. The clamped set was introduced in an evaporator and 20 nm Au was deposited on the tip of the cantilevers through the 300 nm diameter circular stencil apertures, as illustrated in Fig. 2. Epitaxial Si nanowires were then grown from the Au catalysts by the vapor-liquid-solid method at 530 ºC with a hydrogen diluted SiH4 precursor with a partial pressure of 0.2 mbar for 5 minutes. Fig. 3 shows an example of Si nanowire grown at the tip of the cantilever. The grown nanowires were well suited for tapping mode atomic force microscopy measurements. We scanned Si nanotrenches fabricated by e-beam lithography, 100 nm wide, 275 nm apart, and 600 nm deep. The comparison between scans of this sample done with a super-sharp tip and with a Si nanowire tip is shown in Fig. 4. The high aspect-ratio of the nanowire tips provided a clear advantage for this geometry: the side walls of the trenches are much sharper when scanned with the nanowire tip, while a deconvolution technique has to be used if attempting to obtain the same information from the super-sharp tip image. We thus proved for the first time the use of stencil lithography for the deposition of catalysts at controlled positions on released micro-structures at full-wafer scale. Si nanowires were grown from the deposited catalysts on cantilever tips. These nanowires were used as scanning probes for 1:6 aspect-ratio Si nanotrenches and showed to perform much better than commercially-available super-sharp tips.

2010

Rapid processing of net-shape thermoplastic planar random composite preforms

Patrick Blanchard, Simon Jespersen, Jan-Anders Månson, Véronique Michaud, Daniel Schmäh

A novel thermoplastic composite preforming and moulding process is investigated to target cost issues in textile composite processing associated with trim waste, and the limited mechanical properties of current bulk flow- moulding composites. The thermoplastic programmable powdered preforming process (TP-P4) uses commingled glass and polypropylene yarns, which are cut to length before air assisted deposition onto a vacuum screen, enabling local preform areal weight tailoring. The as-placed fibres are heat- set for improved handling before an optional preconsolidation stage. The preforms are then preheated and press formed to obtain the final part. The process stages are examined to optimize part quality and throughput versus processing parameters. A viable processing route is proposed with typical cycle times below 40 s (for a plate 0.5 × 0.5 m2, weighing 2 kg), enabling high production capacity from one line. The mechanical performance is shown to surpass that of 40 wt.% GMT and has properties equivalent to those of 40 wt.% GMTex at both 20°C and 80°C.