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Ultra-high temperature ceramics (UHTCs) are generally referred to the carbides, nitrides, and borides of the transition metals, with the Group IVB compounds (Zr & Hf) and TaC as the main focus. The UHTCs are endowed with ultra-high melting points, excellent mechanical properties, and ablation resistance at elevated temperatures. These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles, particularly nozzles, leading edges, and engine components, etc. In addition to bulk UHTCs, UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics. Recently, highentropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials. This review presents the state of the art of processing approaches, microstructure design and properties of UHTCs from bulk materials to composites and coatings, as well as the future directions.

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Highly enhanced densification and grain growth of Ce 0.9 Gd 0.1 O 1.95-δ (CGO, gadolinium-doped ceria, with 10 mol% Gd) is achieved in low oxygen activity atmospheres. However, the material can suffer mechanical failures during cooling when the re-oxidation process is not controlled due to the large volume changes. In this work, the redox process of CGO is investigated using dilatometry, microscopy, electrochemical impedance spectroscopy and thermodynamic analysis. In addition, the conditions allowing controlled re-oxidation and cooling in order to preserve the mechanical integrity of the CGO component are defined: this can be achieved over a wide temperature range (800−1200 • C) by gradually increasing the oxygen content of the atmosphere. It is found that the electrical conductivity of the CGO, particularly at low temperature (<450 • C) is influenced by the sintering and controlled re-oxidation conditions. An increase in activation energy for conduction at low temperature is observed as the re-oxidation temperature decreases. Moreover it was observed that the ionic conductivity blocking effect, usually associated with grain boundary contributions, is not influenced by the grain size but rather by the chemical history of the material.Doped ceria (CeO 2 ) materials are being investigated for applications such as solid electrolytes for solid oxide fuel cells (SOFC), 1,2 catalytic substrates for chemical looping and hydrolysis at high temperature, 3,4 and oxygen transport membranes, 5,6 due to their good chemical stability, 1 catalytic properties in redox processes, 3,4 and high ionic conductivity at low temperatures (600-800 • C). 1 In ceria, doping in the range of 10-20 mol%, by trivalent lanthanides elements, results in compounds with high oxygen ion transport properties. 7 Ceria (both pure and doped) can be easily reduced under low oxygen partial pressure (P O 2 ) conditions by partial conversion of Ce 4+ ions to Ce 3+ to compensate oxygen vacancies produced when oxygen is lost from the lattice. The reduction process depends on both temperature and P O 2 , and introduces electronic/ionic conduction coupled with lattice expansion which both depend on the thermodynamic conditions. 1 Particularly, chemical expansion occurs as a net result of the larger radius of Ce 3+ ions (1.14 Å) compared to Ce 4+ ions (0.97 Å) and electrostatic interactions between defects (oxygen vacancies) and neighbouring atoms. 8,9 Such volume changes, corresponding to oxygen being incorporated and released from the lattice during redox cycles, are large and depending on the geometry induce large stresses which can easily result in mechanical failure of the ceramic. [10][11][12][13][14] The redox behavior of ceria has been studied in detail, 15-18 specifically with respect to the thermal and mechanical stability. 10,19 X-ray diffraction studies showed that ceria reduced in hydrogen at either 400 • C or 600 • C (level of reduction less than 60%) could be re-oxidized in air at room temperature. However, when ceria was reduced at higher temper...

Theoretical analyses of shrinkage and distortion kinetics during sintering of bilayered porous structures are carried out. The developed modeling framework is based on the continuum theory of sintering; it enables the direct assessment of the cofiring process outcomes and of the impact of process controlling parameters. The derived "master sintering curve"-type solutions are capable of describing and optimizing the generic sintering shrinkage and distortion kinetics for various material systems. The approach utilizes the material-specific parameters, which define the relative kinetics of layer shrinkages such as the relative intensity of sintering, and employs the conversion between real and specific times of sintering. A novel methodology is also developed for the determination of the ratio of the shear viscosities of the layer's fully dense materials. This new technique enables the determination of all input parameters necessary for modeling sintering of bilayers using experimental techniques similar to optical dilatometry applied to each individual layer and to a symmetric trilayered porous structure based on the two-layer materials utilized in the bilayered system. Examples of sintering different porous bilayered systems are presented to justify the capability of the model in predicting and optimizing sintering kinetics.R. Bordia-contributing editor Manuscript No. 32472.

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Phase pure hafnium diboride (HfB2) powder was synthesized by borothermal reduction of hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum. The synthesized HfB2 powder had an average particle size of 1.37 μm with an equiaxed shape, and a low oxygen content of 0.79 wt%. Using the as‐synthesized HfB2 powder and a commercial SiC, HfB2 monolithic, and HfB2–20 vol%SiC composite were hot pressed at 2000°C to relative densities of 95.7% and 99.2%, respectively. With the addition of SiC, the grain size decreased and the fracture behavior changed from intergranular to a mixed mode, which resulted in a high flexural strength of 993±90 MPa for the composite. Fracture toughness of the composite was 6.29±0.65 MPa m1/2, which was significantly higher than that of the HfB2 monolithic and the reported values in literature.

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