Inorganic/Inorganic Composites Through Emulsion Templating

Abstract

Inorganic/inorganic composites are found in multiple applications crucial for the energy transition, from nuclear reactors to energy storage devices. Their microstructures dictate their properties from mass transport to fracture resistance. Consequently, there has been a multitude of processes developed to control them, from powder mixing and the use of short or long fibers, to tape casting for laminates up to recent 3D printing. Here, emulsions and slip casting are combined into a simpler, broadly available, inexpensive processing platform that allows for in situ control of composite microstructure while also enabling complex 3D shaping. This study shows that slip casting of emulsions triggers a two-step solvent removal, opening the possibility for the conformal coating of pores. This process is showcased by producing strong and lightweight alumina scaffolds reinforced by a conformal zirconia coating. In addition, by manipulating magnetically responsive droplets, their distribution can be controlled, allowing for the formation of inorganic fibers inside an inorganic matrix in situ during slip casting. Using this approach, alumina has been reinforced with aligned metallic iron fibers to prepare composites with a work of fracture an order of magnitude higher than the pure ceramics.

Keywords: ceramic composites; emulsion; magnetic templating; porous materials.

Figure 1

Fabrication of inorganic/inorganic composites through emulsion templating. a) Schematic of the emulsion composition for slip casting. Picture: resulting emulsion after mixing. b) Representation of the slip casting process with and without the magnetic field. The presence of magnetic field induces droplet‐chain formation during the slip casting, leading to anisotropic fiber‐like microstructure.

Figure 2

Control of the stability and viscosity of inorganic/inorganic emulsions through rheology modifier. a) Storage G’ and loss G’’ moduli as a function of time for 20 vol.%. alumina slurry in water with 4 and 7 wt.% of PVA. SEM of the microstructure after drying and sintering of water‐alumina/decane‐zirconia emulsion with 4 b) and 7 wt.% c) of PVA. d) Viscosity as a function of shear rate of water‐alumina/decane‐zirconia for different amounts of decane‐zirconia phase ϕ. e) Storage G’ and loss G’’ moduli as a function of time for different amount of decane‐zirconia ϕ. Relative viscosity of emulsions as a function of volume fraction of decane‐zirconia ϕ with different fraction of zirconia φ _Z or iron in decane φ _F taken at a shear rate of f) 20 s⁻¹ and g) 100 s⁻¹. Microstructure of the water‐alumina/decane‐zirconia after slip casting and sintering for h) different amount of decane‐zirconia ϕ at constant zirconia fraction in decane φ _Z =  0.20 and i) different amount of zirconia in decane φ _Z at constant ratio of oil phase ϕ  =  0.50 and associate droplet size distribution after sintering.

Figure 3

In situ optical characterization of the solvent removal during slip casting of emulsion composites. Time series taken during the casting, a) first 160 min and c) from 160 min to 1360 min. Water contained rhodamine B as dye. Plot of the center slice of the image as function of time superimposed with fit of the position of the solvent front in b) first 80 min with water and d) from 160 to 1360 min with the oil‐based ferrofluid. e) Schematic representation of the two‐step slip casting mechanisms.

Figure 4

Porous and dense composites with controlled microstructure made by emulsion templating. a) Porosity as a function of the zirconia content at a constant initial volume fraction oil phase ϕ = 0.5. b) Stress‐strain curves in compression of the porous alumina zirconia composite from emulsion templating. c) Crushing strength versus porosity for porous emulsion‐based ceramic and our emulsion templated porous composites. Data obtained from references: BCS Emulsion Templating,^{[ 21 ]} Sacrificial templating,^{[ 45 ]} Direct foaming,^{[ 46} , ^{47 ]} Emulsion templating.^{[ 48} , ^{49 ]} The line denoted G‐A corresponds to the prediction from the Gibson‐Ashby model^{[ 38 ]} for porous alumina with fully closed‐cell λ  =  0 and fully open‐cell structure λ  =  1. d) Optical microscopy images of iron oxides and SPIONS containing oil droplet in water 7 wt.%. PVA solution with and without a static magnetic field applied. e) Microstructure of the alumina‐iron composite made from magnetically templated emulsion after sintering. f) Typical force–displacement curves obtained for the alumina iron templated composites tested in Single Edge Notch Bending. g) Optical images taken during the fracture testing showing the crack propagation. h) SEM of the cross‐section of the composites after testing.