Determination of the representative volume-of-interest (REVOI) in ceramic replica foams

The investigations on ceramic foams with porous, heterogeneous microstructure are based on the standards and dimensions for dense materials due to the lack of alternatives. It is known that variations in pore size, shape and porosity have a significant effect on the physical properties, however, the considered sample volume is treated here as a secondary interest. To determine the representative volume-of-interest (REVOI), image data from micro tomography measurements of 30,45 and 60 ppi (pore per inch) Al2O3 foams were used. The REVOI is the minimum volume that comprehensively represents the microstructure and can thus be considered a unit volume. The Minkowski functionals allow an invariant microstructure description by single value using stereology approaches. Based on these values of defined volume sets, the REVOI was determined as a novelty in microstructure characterization. Additional analysis of the pore sizes in the 3D volume and determination of tortuosity and permeability at the previously defined volumes allowed validation of the REVOI

Open-Cellular Alumina Foams with Hierarchical Strut Porosity by Ice Templating: A Thickening Agent Study

Alumina replica foams were manufactured by the Schwartzwalder sponge replication technique and were provided with an additional strut porosity by a freeze-drying/ice-templating step prior to thermal processing. A variety of thickeners in combination with different alumina solid loads in the dispersion used for polyurethane foam template coating were studied. An additional strut porosity as generated by freeze-drying was found to be in the order of ~20%, and the spacings between the strut pores generated by ice-templating were in the range between 20 µm and 32 µm. In spite of the lamellar strut pore structure and a total porosity exceeding 90%, the compressive strength was found to be up to 1.3 MPa. Combining the replica process with freeze-drying proves to be a suitable method to enhance foams with respect to their surface area accessible for active coatings while preserving the advantageous flow properties of the cellular structure. A two-to-threefold object surface-to-object volume ratio of 55 to 77 mm−1 was achieved for samples with 30 vol% solid load compared to 26 mm−1 for non-freeze-dried samples. The freeze-drying technique allows the control of the proportion and properties of the introduced pores in an uncomplicated and predictable way by adjusting the process parameters. Nevertheless, the present article demonstrates that a suitable thickener in the dispersion used for the Schwartzwalder process is inevitable to obtain ceramic foams with sufficient mechanical strength due to the necessarily increased water content of the ceramic dispersion used for foam manufacturing

Gene expression profiling reveals consistent differences between clinical samples of human leukaemias and their model cell lines

Microarray gene expression profiles of fresh clinical samples of chronic myeloid leukaemia in chronic phase, acute promyelocytic leukaemia and acute monocytic leukaemia were compared with profiles from cell lines representing the corresponding types of leukaemia (K562, NB4, HL60). In a hierarchical clustering analysis, all clinical samples clustered separately from the cell lines, regardless of leukaemic subtype. Gene ontology analysis showed that cell lines chiefly overexpressed genes related to macromolecular metabolism, whereas in clinical samples genes related to the immune response were abundantly expressed. These findings must be taken into consideration when conclusions from cell line-based studies are extrapolated to patients

Towards advanced piezoelectric metamaterial design via combined topology and shape optimization

Metamaterials open up a spectrum of artificially engineered properties otherwise unreachable in conventional bulk materials. For electromechanical energy conversion systems, lightweight materials with high hydrostatic piezoelectric coupling coefficients and negative Poisson’s ratio can be obtained. Thus, in this contribution, we explore the possibilities of piezoelectric metamaterials design by employing structural optimization. More specifically, we apply a sequential framework of topology and shape optimization to design piezoelectric metamaterials with negative Poisson’s ratio for electromechanical energy conversion under uniform pressure. Topology optimization is employed to generate the initial layout, whereas shape optimization fine tunes the design and improves durability and manufacturability of the structures with the help of a curvature constraint. An embedding domain discretization (EDD) method with adaptive domain and shape refinement is utilized for an efficient and accurate computation of the state problem in the shape optimization stage. Multiple case studies are conducted to determine the importance of desired stiffness characteristics, symmetry conditions and objective formulations on the design of piezoelectric metamaterials. Results show that the obtained designs are highly dependent on the desired stiffness characteristics. Moreover, the addition of the EDD-based shape optimization step introduces significant changes to the designs, confirming the usability of the sequential framework

Stress and Deformation Behavior of 2D Composite Cellular Actuator Structures of Ceramic Building Blocks and Epoxy Resins

2D actuator composite structures are fabricated of lead zirconate titanate (PZT) and Al2O3 building blocks with an epoxy resin matrix. This novel modular concept gives the possibility to create complex geometric structures where the structure itself tailors the physical properties. To improve the possibilities of excitation or loading and determine the influence of the geometric parameters as slenderness ratio t2 · g−1 and active area Aactive finite element (FE) simulations are used. The stress distribution σyy within the unit cell and the resulting strain amplification ay are tested with different mechanically coupled thermal excitation modes. A homogeneous excitation of the PZT building blocks and a maximum Aactive of 24% led to a maximum of strain amplification and a reduction of induced tensile stresses. In addition, zero deformation could be generated by modifying the structure design with a slenderness ratio of 0.5. Further geometric variations offer the potential to increase the strain amplification

Advanced Estimation of Compressive Strength and Fracture Behavior in Ceramic Honeycombs by Polarimetry Measurements of Similar Epoxy Resin Honeycombs

Finding a non-destructive characterization method for cellular ceramics’ compressive strength and fracture behavior has been a challenge for material scientists for years. However, for transparent materials, internal stresses can be determined by the non-destructive photoelastic measurements. We propose a novel approach to correlate the photoelastic stresses of polymer (epoxy resin) prototypes with the mechanical properties of ceramics (alumina). Regular and inverse epoxy honeycombs were 3D-printed via stereolithography with varying structure angles from −35° to 35°, with negative angles forming an auxetic and positive hexagonal lattice. Photoelastic measurements under mechanical loading revealed regions of excess stress, which directly corresponded to the initial fracture points of the ceramic honeycombs. These honeycombs were made by a combination of 3D printing and transfer molding from alumina. The photoelastic stress distribution was much more homogeneous for angles of a smaller magnitude, which led to highly increased compressive strengths of up to 446 ± 156 MPa at 0°. By adapting the geometric structural model from Gibson and Ashby, we showed that we could use a non-destructive technique to determine the compressive strength of alumina honeycombs from the median photoelastic stress measured on similar epoxy honeycomb structures

Porous Functional Graded Bioceramics with Integrated Interface Textures

Porous functional graded ceramics (porous FGCs) offer immense potential to overcome the low mechanical strengths of homogeneously porous bioceramics used as bone grafts. The tailored manipulation of the graded pore structure including the interfaces in these materials is of particular interest to locally control the microstructural and mechanical properties, as well as the biological response of the potential implant. In this work, porous FGCs with integrated interface textures were fabricated by a novel two-step transfer micro-molding technique using alumina and hydroxyapatite feedstocks with varied amounts of spherical pore formers (0–40 Vol%) to generate well-defined porosities. Defect-free interfaces could be realized for various porosity pairings, leading to porous FGCs with continuous and discontinuous transition of porosity. The microstructure of three different periodic interface patterns (planar, 2D-linear waves and 3D-Gaussian hills) was investigated by SEM and µCT and showed a shape accurate replication of the CAD-designed model in the ceramic sample. The Young’s modulus and flexural strength of bi-layered bending bars with 0 and 30 Vol% of pore formers were determined and compared to homogeneous porous alumina and hydroxyapaite containing 0–40 Vol% of pore formers. A significant reduction of the Young’s modulus was observed for the porous FGCs, attributed to damping effects at the interface. Flexural 4-point-testing revealed that the failure did not occur at the interface, but rather in the porous 30 Vol% layer, proving that the interface does not represent a source of weakness in the microstructure

Relation between Structure, Mechanical and Piezoelectric Properties in Cellular Ceramic Auxetic and Honeycomb Structures

Optimizing renewable energy harvesting is of major importance in the following decades. In order to increase performance and efficiency, an ideal balance of mechanical and piezoelectric properties must be targeted. For this purpose, the approach of ceramic auxetic and honeycomb structures made of (Ba,Ca)(Zr,Ti)O3 (BCZT) which is produced via injection molding is considered. The main design parameter is the structural angle θ which is varied between −35° and 35°. Its effect on compressive strength, Young's modulus, and Poisson's ratio are determined via uniaxial compression tests and digital image correlation (DIC). Maximum compressive strength of 95 MPa at 0° (porosity of 59%) is found, which is superior to conventional porous ceramics of the same porosity. The piezoelectric constants d33 (max. 296 pC N−1) and g33 (max. 0.068 Vm N−1) are measured via the Berlincourt method and also exceed expectations, regardless of the structure. The theoretical models of Gibson and Ashby (mechanical) and Okazaki (piezoelectrical), as well as finite element method simulations, strengthen and explain the experimental results