Phase transition and high‐temperature properties of rare‐earth niobates (LnNbO4, where Ln = La, Dy and Y) were studied in situ at high temperatures using powder X‐ray diffraction and thermal analysis methods. These materials undergo a reversible, pure ferroelastic phase transition from a monoclinic (S.G. I2/a) phase at low temperatures to a tetragonal (S.G. I41/a) phase at high temperatures. While the size of the rare‐earth cation is identified as the key parameter, which determines the transition temperature in these materials, it is the niobium cation which defines the mechanism. Based on detailed crystallographic analysis, it was concluded that only distortion of the NbO4 tetrahedra is associated with the ferroelastic transition in the rare‐earth niobates, and no change in coordination of Nb5+ cation. The distorted NbO4 tetrahedron, it is proposed, is energetically more stable than a regular tetrahedron (in tetragonal symmetry) due to decrease in the average Nb–O bond distance. The distortion is affected by the movement of Nb5+ cation along the monoclinic b‐axis (tetragonal c‐axis before transition), and is in opposite directions in alternate layers parallel to the (010). The net effect on transition is a shear parallel to the monoclinic [100] and a contraction along the monoclinic b‐axis. In addition, anisotropic thermal expansion properties and specific heat capacity changes accompanying the transition in the studied rare‐earth niobate systems are also discussed.

The thermal expansion of a low symmetry crystal can be much more interesting than the lattice parameter expansion would suggest. Here, the complete thermal expansion tensors for monoclinic and tetragonal phases of ZrO 2 and HfO 2 have been measured in air, by high-resolution, high-temperature X-ray diffraction. These results reveal the highly anisotropic nature of thermal expansion in the monoclinic phase as well as a cooperative movement of ions and the existence of a zero thermal expansion plane.

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Cuttlefish bone is an inexpensive, readily available, morphologically complex natural material. It has an open structure, consisting of layers separated by pillar-like structures made of calcium carbonate. In this study natural bones from cuttlefish were successfully converted into porous biphasic calcium phosphate (BCP) scaffolds with a range of hydroxyapatite and b-tricalcium phosphate compositions. The process involved reaction with solutions of phosphoric acid (H 3 PO 4 ) and 2-propanol, followed by heat treatment at high temperatures (up to 13001C) in air. The crystalline composition of the BCP scaffolds could be controlled by varying the concentration of the H 3 PO 4 in solution, and the duration of reaction time at room temperature. The original microstructure of the cuttlefish bone was preserved in the BCP scaffolds which featured 490% interconnected porosity. The structure consisted of continuous macroporous channels with smallest measured cross-sectional openings of 400 lm  100 lm size. The BCP scaffolds prepared with 16 wt% H 3 PO 4 solution had a measured compressive strength of 2.3870.24 MPa, with a characteristic noncatastrophic failure behavior. The ability to tailor the composition of these BCP scaffolds allows development of implants with controlled biodegradation, while their superior mechanical and microstructural properties stand to benefit efficient osteointegration and osteoinduction.

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The rheological properties of commercially available polycarbosilane, SMP-10, were analyzed as a function of temperature, to guide development of thermal treatment processes for the improved yield and functionality of polymer ceramic precursors. The curing onset temperature for SMP-10 was determined to be as low as 100°C for a heating rate of 1°C/min enabling a heat treatment process at 90°C, where low molecular weight oligomers volatilize from the liquid precursor prior to curing. By driving off the low molecular weight oligomers before fabrication of a composite, the mass yield of SMP-10, from a room temperature liquid state was increased from 77% to 83%. The development of B-staging processes, or a semicure of SMP-10, were also demonstrated. B-staging processes were then applied to polymer infiltration and pyrolysis processing and compared with traditional wet layup CMC processing. It was determined that B-staging processes did not adversely affect ceramic matrix composite fabrication. B-staged processing methods were determined to result in less waste, allow ply-by-ply control of matrix compositions, and enable time independent processing when compared to traditional wet layup processing methods. K E Y W O R D Sceramic matrix composites, microstructure, pre-ceramic polymers, silicon carbide

The volatilization of polycarbosilanes is important to the processing and performance of polymer infiltration and pyrolysis‐based ceramic matrix composites. Low molecular weight (MW) polycarbosilane is often present in preceramic polymers and enhances viscosity for the purpose of composite infiltration. Due to the volatility of low MW chains, a model was developed to semi‐empirically determine the MW distribution and then predict the mass yield and evolution of the MW distribution as a function of temperature and time for StarPCS™ SMP‐10. The enthalpy of vaporization, the temperature dependence of the enthalpy of vaporization, the temperature dependence of the normal boiling point and a representation of the molecular weight distribution were fit using a series of thermogravimetric measurements, involving isothermal holds on a particular batch of SMP‐10. Once calibrated for SMP‐10 in this fashion, the molecular weight distribution of different batches of SMP‐10 could be fit using a thermogravimetric measurement involving a reduced temperature‐time series. The model was then predictive of mass loss over time for temperatures below the onset of curing (>90°C). Understanding this volatilization enables improved SiC yield, reduced processing time and minimizing void/bubble formation.

Preceramic polymers (PCPs) are a group of specialty macromolecules that serve as precursors for generating inorganics, including ceramic carbides, nitrides, and borides. PCPs represent interesting synthetic challenges for chemists due to the elements incorporated into their structure. This group of polymers is also of interest to engineers as PCPs enable the processing of polymer-derived ceramic products including high-performance ceramic fibers and composites. These finished ceramic materials are of growing significance for applications that experience extreme operating environments (e.g., aerospace propulsion and high-speed atmospheric flight). This Review provides an overview of advances in the synthesis and postpolymerization modification of macromolecules forming nonoxide ceramics. These PCPs include polycarbosilanes, polysilanes, polysilazanes, and precursors for ultrahigh-temperature ceramics. Following our review of PCP synthetic chemistry, we provide examples of the application and processing of these polymers, including their use in fiber spinning, composite fabrication, and additive manufacturing. The principal objective of this Review is to provide a resource that bridges the disciplines of synthetic chemistry and ceramic engineering while providing both insights and inspiration for future collaborative work that will ultimately drive the PCP field forward.