Nacre (mother-of-pearl), made of inorganic and organic constituents (95 vol% aragonite calcium carbonate (CaCO3) platelets and 5 vol% elastic biopolymers), possesses a unique combination of remarkable strength and toughness, which is compatible for conventional high performance materials. The excellent mechanical properties are related to its hierarchical structure and precisely designed organic–inorganic interface. The rational design of aragonite platelet strength, aspect ratio of aragonite platelets, and interface strength ensures that the strength of nacre is maximized under platelet pull-out failure mode. At the same time, the synergy of strain hardening mechanisms acting over multiple scales results in platelets sliding on one another, and thus maximizes the energy dissipation of viscoplastic biopolymers. The excellent integrated mechanical properties with hierarchical structure have inspired chemists and materials scientists to develop biomimetic strategies for artificial nacre materials. This critical review presents a broad overview of the state-of-the-art work on the preparation of layered organic–inorganic nanocomposites inspired by nacre, in particular, the advantages and disadvantages of various biomimetic strategies. Discussion is focused on the effect of the layered structure, interface, and component loading on strength and toughness of nacre-mimic layered nanocomposites (148 references).

Layered nanocomposites inspired by the structure and mechanical properties of nacre

The thermal conductivity of porous alumina ceramics prepared using different types of starch (potato, wheat, corn, and rice starch) as pore-forming agents is investigated from room temperature up to 500 °C. The temperature dependence measured for alumina ceramics of different porosity (in the range 6–47%) is fitted with second-order polynomials and 1/ T -type relations, and compared to available literature data for dense alumina. It is found that the porosity dependence of the relative thermal conductivity k r  =  k / k 0 is well described by a modified exponential relation of the form k r  = exp(−1.5 ϕ /(1 −  ϕ )), where ϕ is the porosity. This finding is in agreement with other literature data and seems to indicate a common feature of all porous materials with microstructures resulting from fugitive pore-forming agents.

Thermal conductivity of porous alumina ceramics prepared using starch as a pore-forming agent

Porous ceramics of Y2O3-stabilized ZrO2 (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed.

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Synthesis of Highly Porous Yttria‐Stabilized Zirconia by Tape‐Casting Methods

Slurry rheology in wet ultrafine grinding of industrial minerals: a review

Porous silicon nitride is gaining interest for a number of applications including metal–ceramic thermal engineering components, biomaterials and catalyst supports. This paper describes the fabrication of porous silicon nitride ceramic materials using a fugitive additive, corn starch, which allows samples to be produced with different volume fractions of porosity from ∼0 to 0.25. The initial composition consisted of 92 wt.% Si 3 N 4 , 6 wt.% Y 2 O 3 and 2 wt.% Al 2 O 3 . Sintering was carried out at 1800 °C for 2 h under nitrogen. Relative density as a function of the fugitive additive content has been measured. Microstructural analysis reveals a dense matrix of elongated β-Si 3 N 4 grains surrounded by intergranular glass phase and containing large pores and cavities. Pore size, geometry and grain size have been measured for certain compositions. Young's modulus and modulus of rupture have been determined as a function of the volume fraction of porosity. The Young's modulus–porosity relationship has been compared with previous work in the literature and it was found that this dependency is close to that for a model for spherical pores in cubic stacking arrangement.

Characterisation of porous silicon nitride materials produced with starch

A simple and general method for making ceramic laminates with porous crack-deflecting interlayers is demonstrated. Both the strong laminae and the porous interlayers are made by tape casting suitable slurries using the same ceramic powders. Porosity is introduced into the interlayer by adding starch particles to the slurry. The effects of the starch on the burn-out and sintering behavior of the laminates has been fully described. The influence of resulting porosity on the ability of an interlayer to deflect a growing crack and to remain stable on prolonged heating has also been investigated. For the pore morphologies studied here, it is the volume fraction of pores that controls whether crack deflection takes place. Using existing mechanics solutions, a simple theoretical criterion for this volume fraction of porosity is given and is consistent with all the experimental observations.

Fabrication and Crack Deflection in Ceramic Laminates with Porous Interlayers

Aqueous tape casting of alumina with an acrylic latex binder has been studied in a continuous tape casting machine. The aim of the study was to evaluate this type of system in the different processing steps. Four slurry compositions were used in the experiments with varied binder level and solids content. The rheological properties of the slips were measured with a controlled stress rheometer and related to the casting behaviour. The critical cracking thickness, i.e. the thickness when spontaneous cracking during drying occurs, was also studied. Casting rates of 2 cm/s resulting in a total drying time of 3.1 min could be used for slips with a high solids content at a blade gap of 300 μm. All slurries gave good quality tapes with smooth surfaces. The tapes were possible to laminate at room temperature. A bending strength of 536 MPa and a Weibull modulus of 17 were reached for a material laminated from 200 μm green tapes.

Tape casting of alumina in water with an acrylic latex binder

Ceramic laminates have been made from alternating layers of silicon carbide and silicon carbide containing a fugitive polymer, which can be pyrolysed to produce porous interlayers. It is shown that such interlayers can be used to deflect cracks and that the volume fraction of porosity, due to the added polymer, that is required to cause crack deflection is approximately 0·4. A simple model has been developed which describes the fracture behaviour of porous solids and also predicts the volume fraction of porosity required to give crack deflection in the laminate and which is in good agreement with experiment.

Crack Deflection in Ceramic Laminates Using Porous Interlayers

The influence of particle concentration on the rheological properties of aqueous alumina suspensions is investigated under steady and oscillatory shear conditions. At solid volume fractions between 0·45 and 0·6, a high degree of particle stabilization is achieved when 0·2 wt% of polyacrylic acid is added as a dispersant. With increasing particle concentration the flow behavior changes from pseudoplastic to plastic. The apparent yield stress (from the generalized Casson model) is found to be a significant quantity for characterizing changes in structural conditions of the disperse phase with increasing solids loading even though more detailed information is provided by viscoelastic properties. The analysis of viscoelastic data based on the Friedrich–Braun fractional derivative model and the generalized Maxwell model showed that transitions of particle arrangements due to electrostatic repulsion take place at Φ=0·5 and 0·55. For these critical concentrations the influence of latex binder addition on the rheological properties is examined.

Influence of particle concentration on rheological properties of aqueous α-Al2O3 suspensions

Solid, homogenous or porous, shaped bodies are formed from different powders such as metals and ceramics by preparation of an aqueous slurry of the powder, which slurry comprises a protein substance which is soluble in water at ambient temperatures and capable of forming an irreversible gel on heating to a temperature not exceeding 90° C. The protein substance, for example albumin, is used in an amount sufficient for the formation of a gel. The slurry is formed into a body of the desired shape by methods such as molding, tapecasting or dropforming and heating to a temperature at least corresponding to the gelling temperature of the protein.

Annika Kristoffersson

Papers

Fabrication and Crack Deflection in Ceramic Laminates with Porous Interlayers

Tape casting of alumina in water with an acrylic latex binder

Crack Deflection in Ceramic Laminates Using Porous Interlayers

Influence of particle concentration on rheological properties of aqueous α-Al2O3 suspensions

Method for the production of solid shaped bodies