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.

/pdf/cobalt-particle-size-effects-in-the-fischer-tropsch-reaction-nvtmg7gy4b.pdf

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

It has been observed that "the most important parameter of a superconductor... is the critical current density Jc" (ref. 1). In thin films or single crystals of YBa2Cu3Ox, Jc is high at ∼106A cm−2 but in bulk materials Jc is much lower, typically 102–103 A cm−2 (ref. 4). The sources of variation in Jc are important but vary. Chemical contamination such as SiO2 impurities is known to reduce Jc5 but even in the absence of such contaminants Jc varies widely. Some groups have experienced difficulty in maint-aining high (1,000 A cm-2 in bulk ceramic) current densities and sometimes even zero resistance at 77 K at high densities of the ceramic (>6.2 g cm−3). Jc is known to be anisotropic being greater in the a–b planes and least in the c-axis direction. In thin films a thirty-fold difference in Jc was reported demonstrating the anisotropy6. In bulk materials Jc has been increased to 7.5 x 103 A cm−2 by melt textured growth of the ceramic in order to induce preferred orientation7. Here we report the factors controlling Jc in sintered bulk ceramics that display no preferred orientation. We examine the relation between sintered density, oxygen content and Jc and show how Jc is strongly dependent on both the volume fraction of solid and on the oxygen stoichiometry, both being strongly influenced by the ceramic density. The problem we address is how to optimize the solid connectivity and simultaneously maintain a high oxygen content thereby maximizing Jc in anisotropic bulk ceramics.

The effect of density on critical current and oxygen stoichiometry of YBa2Cu3Ox superconductors

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.

A LOWsurface resistance,Rs, is the key to successful development of radio-frequency and microwave applications of high-temperature superconductors. Here we report the Rs of YBa2Cu3Ox thick films on yttria-stabilized zirconia substrates at frequencies up to 50 GHz. Films processed below the peritectic temperature are fine grained, have Rs similar to bulk YBa2Cu3Or, generally have low critical current density (Jc) and exhibit little preferred orientation of crystallographic axes. Films processed above the peritectic temperature exhibit preferred orientation in large spherulitic grains, have higher Jc and far lower Rs. For these films the crossover frequency at which Rs equals that of copper is 50 GHz, a factor of two higher than the best bulk material or thick film yet reported and only a factor of ∼4 lower than high-quality thin films. At the frequencies used for mobile communications (900 MHz and 1.8 GHz), the superconductor losses would be two orders of magnitude lower than those of normal metals. The particular advantages of the thick-film route are the speed and low cost of the process, and the ability to apply the films on curved surfaces and on large areas, the largest so far being >200 cm2.

Low surface resistance in YBa2Cu3Ox melt-processed thick films

Several techniques for measuring the porosity and pore-size distribution in hardened cement paste (HCP) have given conflicting results. This is partly because these methods require drying or pre-treatment of specimens. Small-angle neutron scattering (SANS) allows investigation of the fine-pore porosity without any such pre-treatment. Measurements using the Pluto SANS spectrometer indicate a bimodal distribution of near-spherical pores with diameters of 5 and 12 nm. The former have a well peaked size distribution and account for 1-1.5% porosity. The pores are relatively unaffected by the water-to-cement ratios investigated although drying has the effect of collapsing the structure.

https://iopscience.iop.org/article/10.1088/0022-3727/15/9/027/pdf

A small-angle neutron scattering study of cement porosities

IT is well-known that the mechanical properties of ceramics are sensitive to flaws which cause weakness1,2. The most damaging flaws are large gas bubbles or particulates which contaminate the ceramic moulding during the compaction process, and which are not healed by sintering3–5. But even when great care is taken to remove such extraneous flaws, for example by filtering or by fabricating under clean room conditions, the strength of the ceramic does not rise to the level expected from the fine structure of the material (M. Real, personal communication), suggesting that some more fundamental intrinsic problem limits the strength. Here we describe a new phenomenon, flocculation clustering, which can account for the low strengths observed. An equation is derived to predict the size of the clusters and is verified by experiments on flocculation of titania sols. We find that flocculation clustering can also be observed in other colloidal systems, such as silica and polystyrene dispersions.

N. McN. Alford

Papers

The effect of density on critical current and oxygen stoichiometry of YBa2Cu3Ox superconductors

A Simple Way to Make Tough Ceramics.

Low surface resistance in YBa2Cu3Ox melt-processed thick films

A small-angle neutron scattering study of cement porosities

Flocculation clustering and weakness of ceramics