Natural structural materials are built at ambient temperature from a fairly limited selection of components. They usually comprise hard and soft phases arranged in complex hierarchical architectures, with characteristic dimensions spanning from the nanoscale to the macroscale. The resulting materials are lightweight and often display unique combinations of strength and toughness, but have proven difficult to mimic synthetically. Here, we review the common design motifs of a range of natural structural materials, and discuss the difficulties associated with the design and fabrication of synthetic structures that mimic the structural and mechanical characteristics of their natural counterparts.

Bioinspired structural materials

This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400042527

Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants

Composition of reactive powder concretes

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre-ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine-grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re-entry, and rocket propulsion.

Refractory Diborides of Zirconium and Hafnium

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.

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Cobalt particle size effects in the fischer- : Tropsch reaction studied with carbon nanofiber supported catalysts

THE major problem with the use of ceramics as structural materials is their brittleness. One way of overcoming this problem is to introduce weak interfaces which deflect a growing crack1. Polymer composites of this sort can be easily prepared by surrounding fibres with liquid plastic. To make similar structures with ceramic matrices and fibres is difficult and expensive, however, because traditional ceramic processing techniques of powder compaction and sintering prevent densification and cause cracking2–4. Here we describe a simple, inexpensive way of preparing a ceramic material that contains such weak interfaces. Silicon carbide powder is made into thin sheets which are coated with graphite to give weak interfaces and then pressed together and sintered without pressure. Relative to the monolithic material, the apparent fracture toughness for cracks propagating normal to the weak interfaces is increased more than fourfold, and the work required to break the samples increases by substantially more than a hundredfold. The technique should be readily applicable to other ceramics.

A simple way to make tough ceramics

Although the theoretical tensile strength of alumina is ∼46 GPa (ref 1), conventional powder processing rarely gives products better than 05 GPa because large defects, ∼50–100 µm in size, are invariably trapped during powder compaction2,3 Thin-film, fibrous, melt-processed and vapour-deposited ceramics can achieve higher strengths of several GPa (refs 4–6), but such processes are much more expensive than simple sintering of powder compacts Here we show how colloidal control of powders to remove agglomerates can be used to give high-strength sintered ceramics: for example, a commercial alumina powder can be improved from 037 to 104 GPa in bend strength

High-strength ceramics through colloidal control to remove defects

The physical and mechanical properties of YBa2Cu3O7−° superconductors are examined. These properties are related to powder preparation method, powder characteristics, sintering behaviour and sintered microstructure. The sintering atmosphere and sintering schedules affect the final microstructure very strongly and determine, in conjunction with starting powder characteristics, the sintered density. The mechanical properties such as Young's modulus, bend strength and critical stress intensity factor (fracture toughness) are measured and related to microstructure as determined by electron microscopy. Control of microstructure by careful powder selection and sintering schedule is seen as key to optimizing the physical and mechanical properties of the material. Finally attention is drawn to fabrication techniques and how these must be optimized in order to realize the mechanical properties which are necessary if these are to be useful as engineering materials. Comparisons between fabrication techniques show that uniaxial powder pressing suffers from limitations in terms of specimen complexity and densification whereas the favoured route, termed viscous processing, gives a more homogeneous microstructure, higher strength and allows near theoretical density to be achieved.

Physical and mechanical properties of YBa2Cu3O7-δ superconductors

It is demonstrated both theoretically and experimentally that fracture toughness does not directly influence the Weibull modulus of ceramic bending strength for materials that obey the Griffith criterion for crack propagation. Weibull modulus remains unchanged as toughness is increased. However, toughness variations with crack length do affect the Weibull modulus. Thus materials that display R-curve behavior or Dugdale character give an increased Weibull modulus and appear more reliable.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400096461

Influence of toughness on Weibull modulus of ceramic bending strength

Although the surface tension of liquids has been understood and measured since the time of Young1 and Laplace2, the surface energy of solids has eluded understanding and evaded measurement, despite its postulated importance to catalysis, crystal growth, colloidal behaviour, sintering and fracture. It is salutary to reflect that solid surface energies are even now more readily determined by theoretical argument than by experiment. Here we show that solid surface energies can be evaluated experimentally by measuring the elastic modulus of submicrometre powder assemblies, knowing the particle diameter, elastic modulus and volume fraction.

N. McN. Alford

Papers

A simple way to make tough ceramics

High-strength ceramics through colloidal control to remove defects

Physical and mechanical properties of YBa2Cu3O7-δ superconductors

Influence of toughness on Weibull modulus of ceramic bending strength

A new method for measuring the surface energy of solids