Numerical design of open-porous titanium scaffolds for Powder Bed Fusion using Laser Beam (PBF-LB).

The paper concerns the numerical design of novel three-dimensional titanium scaffolds with complex open-porous structures and desired mechanical properties for the Powder Bed Fusion using Laser Beam (PBF-LB). The 60 structures with a broad range of porosity (38-78%), strut diameters (0.70-1.15 mm), and coefficients of pore volume variation, CV(Vp), 0.35-5.35, were designed using the Laguerre-Voronoi tessellations (LVT). Their Young's moduli and Poisson's ratios were calculated using Finite Element Model (FEM) simulations. The experimental verification was performed on the representative designs additively manufactured (AM) from commercially pure titanium (CP Ti) which, after chemical polishing, were subjected to uniaxial compression tests. Scanning Electron Microscopy (SEM) observations and microtomography (µ-CT) confirmed the removal of the support structures and unmelted powder particles. PBF-LB structures after chemical polishing were in close agreement with the CAD models' dimensions having 4-12% more volume. The computational and experimental results show that elastic properties were predicted in very close agreement for the low CV(Vp), and with even 30-40% discrepancies for CV(Vp) higher than 4.0, mainly due to PBF-LB scaffold architecture drawbacks rather than CAD inaccuracy. Our research demonstrates the possibility of designing the open-porous scaffolds with pore volume diversity and tuning their elastic properties for biomedical applications.

Elastic Properties of Open Cell Metallic Foams-Modeling of Pore Size Variation Effect.

Elastic properties of open-cell metallic foams are investigated in correlation with relative density and pore size variation. A variety of foam architectures, with open porosity above 70% (relative density below 0.30) and various pore size distributions, were modeled using Laguerre-Voronoi tessellations (LVT). The coefficient of pore volume variation, CV(V), was introduced to quantify the uniformity of designed structures and ranged between 0.5 to 2.1. Elastic behavior of the modeled foams to uniaxial compression along three orthogonal directions was analyzed using the finite element (FE) method. It is shown that Young's modulus and Poisson's ratio of open-cell metals is not solely a function of relative density (porosity) but the pore size variation as well. For similar porosity (approx. 74-98%), Young's modulus and Poisson's ratio may be reduced by approx. 25-30% and 10-25%, respectively, when CV(V) increases from 0.5 to 2.1. Furthermore, the incorporation of a relationship between Young's modulus and the coefficient of pore volume variation to the Gibson-Ashby model is proposed.

Influence of Pore Size Variation on Thermal Conductivity of Open-Porous Foams.

This study addresses the influence of pore size variation on the effective thermal conductivity of open-cell foam structures. Numerical design procedure which renders it possible to control chosen structural parameters has been developed based on characterization of commercially available open-cell copper foams. Open-porous materials with various pore size distribution were numerically designed using the Laguerre-Voronoi Tessellations procedure. Heat transfer through an isolated structure was simulated with the finite element method. The results reveal that thermal conductivity is strongly related to porosity, which is in agreement with the literature. The influence of pore size distribution has also been observed and compared with analytical formulas proposed in the literature.

Toward a Comprehensive Understanding of Enhanced Photocatalytic Activity of the Bimetallic PdAu/TiO2 Catalyst for Selective Oxidation of Methanol to Methyl Formate.

Photocatalytic selective oxidation of alcohols over titania supported with bimetallic nanoparticles represents an energy efficient and sustainable route for the synthesis of esters. Specifically, the bimetallic PdAu/TiO2 system was found to be highly active and selective toward photocatalytic production of methyl formate (MF) from gas-phase methanol. In the current paper, we applied the electronic structure density functional theory method to understand the mechanistic aspects and corroborate our recent experimental measurements for the photocatalytic selective oxidation of methanol to MF over the PdAu/TiO2 catalyst. Our theoretical results revealed the preferential segregation of Pd atoms from initially mixed PdAu nanoclusters to the interface of PdAu/TiO2 and subsequent formation of a unique structure, resembling a core@shell architecture in close proximity to the interface. The analysis of the calculated band gap diagram provides an explanation of the superior electron-hole separation capability of PdAu nanoparticles deposited onto the anatase surface and hence the remarkably enhanced photocatalytic activity, in comparison to their monometallic counterparts. We demonstrated that facile dissociation of molecular oxygen at the triple-point boundary site gives rise to in situ oxidation of Pd. The in situ formed PdO/TiO2 is responsible for total oxidation of methanol to CO2 (no MF formation) in the gas phase. Our investigation provides theoretical guidance for designing highly selective and active bimetallic nanoparticles-TiO2 catalysts for the photocatalytic selective oxidation of methanol to MF.

Insight on the Interaction of Methanol-Selective Oxidation Intermediates with Au- or/and Pd-Containing Monometallic and Bimetallic Core@Shell Catalysts.

Using density functional theory (DFT), the interaction of crucial molecules involved in the selective partial oxidation of methanol to methyl formate (MF) with monometallic Au and Pd and bimetallic Au/Pd and Pd/Au core@shell catalysts is systematically investigated. The core@shell structures modeled in this study consist of Au(111) and Pd(111) cores covered by a monolayer of Pd and Au, respectively. Our results indicate that the adsorption strength of the molecules examined as a function of catalytic surface decreases in the order of Au/Pd(111) > Pd(111) > Au(111) > Pd/Au(111) and correlates well with the d-band center model. The preadsorption of oxygen is found to have a positive impact on the selective partial oxidation reaction because of the stabilization of CH3OH and HCHO on the catalyst surface and the simultaneous intensification of MF desorption. On the basis of a dynamical matrix approach combined with statistical thermodynamics, we propose a simple route for evaluating the Gibbs free energy of adsorption as a function of temperature. This method allows us to anticipate the relative temperature stability of molecules involved in the selective partial oxidation of methanol to MF in terms of catalytic surface.