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

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

Temperature‐ and stress‐dependent electromechanical properties of phase‐boundary‐engineered KNN‐based piezoceramics

Abstract The influence of stress on the small‐signal dielectric permittivity and piezoelectric coefficient of polycrystalline lead‐free perovskite 0.92(Na1/2K1/2)NbO3–(0.08 − x)Bi1/2Li1/2TiO3–xBaZrO3 (x = 0, 0.02, 0.04, 0.06, and 0.07) was characterized under different constant uniaxial stress up to −200 MPa within a temperature range of −150 to 450°C, revealing stress‐induced suppression of the electromechanical response as well as shifts in the phase boundaries. For all compositions, the interferroelectric and ferroelectric–paraelectric phase transitions were shifted to higher temperatures under the uniaxial compressive stress. Interestingly, the sensitivity to the applied stress was found to increase with increasing BZ/BLT ratio in the system. The origin of a different extent of stress‐sensitivity with BZ/BLT ratio is suggested to be related to the change in the crystal structure. Additionally, at temperatures below −50°C, the relative permittivity showed a significant increase under applied compressive stress

Evaluating the Rate-determining Steps of Oxygen Permeation in Ba0.5La0.5FeO3−δ Perovskite

Perovskite oxides obtained from Ba1−xLaxFeO3−δ (BLF) are considered beneficial materials for electrodes of solid oxide fuel cells and oxygen permeation membranes because of their high oxygen permeability, which is a criterion of oxide ion (O2−)-electronic mixed conductivity. In this paper, the prime focus was to understand the oxygen permeation mechanism through surface exchange and bulk diffusion of the Ba0.5La0.5FeO3−δ (BLF55) sample. The permeated oxygen flux displayed higher than that of the typical mixed conductor La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), which was explored simultaneously with corresponding oxygen chemical potentials employing an especial experimental setup. This study found that the surface exchange reaction on the oxygen-lean side was the rate-determining step (RDS) of the oxygen permeation below 800 °C, resulting from lower hole concentration on the oxygen-lean side surface. Enhancing the charge transfer from the surface oxygen by increasing hole concentration is a prime important strategy to improve the surface exchange reaction