Feasibility of in situ de‐agglomeration during powder consolidation

Abstract

Consolidation of nano-sized powders is a growing area in manufacturing of advanced materials, thanks to the reduced processing times, enhanced mechanical properties and high potential for the introduction of multi-functionality enabled by such reduced particle sizes. Nanopowders, however, are particularly prone to the agglomeration phenomena, and thus to the formation of hierarchical porous structures. The presence of pores differing up to several orders of magnitude in size leads to undesired differential shrinkage and localized grain growth. In order to avoid such issues, strategies for in situ de-agglomeration are proposed here. These optimization strategies are based on the development of an analytical model for shrinkage kinetics and mechanical properties of a hierarchical porous structure, containing both small-size intra-agglomerate pores and large-size inter-agglomerate ones. The modeling approach is an expansion of the continuum theory of sintering to the case of biporous materials presenting nonlinear viscous rheology, as expected for nano-sized crystalline powders. Considering the nonlinear viscous constitutive behavior of the solid phase also allows assessing the influence of the temperature on the microstructural evolution during processing, due to the dependence of the creep characteristic parameter, strain-rate sensitivity, on the thermal history. Material structure optimization strategies, aimed at de-agglomeration or at the design of tailored porous structures, become then possible and are here explored. K E Y W O R D Ssintering, agglomeration, nano-size, bi-modal porosity, modeling

Iron oxide-based nanostructured ceramics with tailored magnetic and mechanical properties: development of mechanically robust, bulk superparamagnetic materials

“…In any of such cases, SPS/CAPAD is characterized by high heating rates and overall greatly reduced processing times, enabling the development of novel materials with nanoscale grains, metastable phases, ne-tuned structures and constituents' combinations. [33][34][35][36][37][38][39][40][41][42][43] In the present work, SPS was applied to supercrystalline iron oxide/oleic acid nanocomposites, in order to obtain nanostructured iron oxide-based ceramics with ne-tuned mechanical and magnetic properties. The predominant iron oxide phase was magnetite (Fe 3 O 4 ).…”

Influence of Carbon Source on Microstructural and Mechanical Properties of High-Performance Reaction-Bonded Silicon Carbide

“…Figure 1 b CB presents a multi-modal distribution with the particle size on three gradients. The D 50 particle size of CB was 0.6 μm and D 10 was 200 nm, which may have caused the formation of hierarchical porous structures in the green body [ 22 ]. The graphite had a wide uni-modal distribution and D 50 was 1.2 μm, as shown in Figure 1 c. According to Figure 2 a,b, the SiC particles were mostly massive or flaky and a small amount of wrinkled impurities on the SiC-2 surface may have required pickling to remove.…”

“…The microspheres agglomerated to form larger spheres, which could explain the multi-modal structure in Figure 1 b. It could cause trouble when preparing slurries with better dispersivity and could create partial grain growth [ 22 ]. Figure 2 d reflects a layered structure of multi-layer graphite flakes.…”

Fabrication of Conductive and Gas-Sensing Microstructures Using Focused Deposition of Copper Nanoparticles Synthesized by Spark Discharge

“…Moreover, the resistivity of lines from spherical nanoparticles ~5 µΩ•cm (Table 3) is noticeably lower than the resistivity of lines from initial agglomerates 70-100 µΩ•cm (Figure 4), sintered at 550 • C. High residual porosity and high electrical resistivity of printed lines from initial agglomerates are associated with their high degree of agglomeration. It is known that agglomeration is the cause of the appearance of a bimodal porous structure with large inter-agglomerate and small intra-agglomerate pores [46], respectively. During sintering, small intra-agglomerate pores shrink faster than larger ones due to the higher local curvature.…”

“…During sintering, small intra-agglomerate pores shrink faster than larger ones due to the higher local curvature. Thus, large interagglomerate pores are more difficult to remove [46]. Consequently, agglomeration leads to residual voids and inhomogeneous microstructure during the sintering process.…”