American Ceramic Society

Abstract Al 5 BO 9 is a promising thermal sealing material for hypersonic vehicles due to its low density, theoretically predicted low shear modulus, and low thermal conductivity. However, experimental investigations on the mechanical and thermal properties of bulk Al 5 BO 9 have not been carried out. Herein, we report the mechanical and thermal properties of bulk Al 5 BO 9 prepared by spark plasma sintering of solid‐state reaction synthesized Al 5 BO 9 powders. The bulk ( B ), shear ( G ), and Young's ( E ) moduli are 148 GPa, 85 GPa, and 214 GPa, respectively, which are close to the theoretical values. The Pugh's ratio G/B is 0.574, indicating its intrinsic damage tolerance, which is also revealed by Hertzian contact test. The Vickers hardness ( H v ) is 10.8 GPa, being lower than mullite. The flexural strength, compressive strength, and fracture toughness are, respectively, 277 ± 35 MPa, 814 ± 75 MPa, and 2.4 ± 0.3 MPa·m 1/2 , which are close to those of mullite. Al 5 BO 9 has anisotropic coefficient of thermal expansion (CTE) in three crystallographic directions, ie α a = (4.40 ± 0.21) × 10 −6 K −1 , α b = (7.11 ± 0.18) × 10 −6 K −1 , α c = (6.70 ± 0.29) × 10 −6 K −1 from Debye temperature to 1473 K, which are underpinned by its structural feature, ie lower α a is resulted from the edge‐shared AlO 6 octahedron chains along the [100] direction. The average CTE is (6.05 ± 0.06) × 10 −6 K −1 . The thermal conductivity declines with temperature as κ = 1336.39/T + 1.97, consisting with predicted trend from Slack's model. The low thermal conductivity and low density guarantee Al 5 BO 9 a promising candidate as ceramic wafer in the seal structure for hypersonic vehicles.

Vitrified inorganic wastes are often converted into glass–ceramics; although these products exhibit interesting properties, the processing is quite complex and expensive. This paper reports an alternative approach, based on substituting traditional feldspar fluxes in stoneware with a CaO–Al 2 O 3 –SiO 2 glass, from industrial plasma vitrification of municipal solid waste incinerator fly ashes. The new materials feature low sintering temperatures, <1000°C, associated to a substantial crystallization. For an optimized balance between crystallization and densification (by introducing soda‐lime glass as a sintering promoter), the products exhibit remarkable mechanical properties (e.g., bending strength exceeding 100 MPa), even superior to those of traditional porcelain stoneware.

Most different transparent grades of sintered and single crystalline spinel were investigated by Vickers and Knoop tests with loads of 1 kg (HK1, HV1) and of 10 kg (HV10). The hardness ranking of all samples was HK1<HV10<HV1. All measuring approaches revealed a negligible grain size effect from about 5 μm up to the single crystals. For finer microstructures, a more dominant improvement was observed to be limited to grain sizes <1 μm. Alumina‐rich sintered and single crystalline MgO· n Al 2 O 3 compositions with n ≥2 exhibited hardness values significantly below the common grain size–hardness relationship.

A novel process was demonstrated for the preparation of an epitaxial Bi0.5Na0.5TiO3 (BNT) single‐crystalline film on a SrTiO3 single crystal substrate by simply heating a composite consisting of a BNT powder layer on the substrate. The BNT powder layer was formed on the substrate by spin coating and the composite was heated at 1150°C for several hours in a lidded crucible containing the BNT powder to avoid evaporation of Na and Bi from the composite. This yielded an epitaxial BNT single‐crystalline film several micrometers thick. The film had a high degree of orientation with the rocking curve full width at half maximum of 0.064°. The mechanism of film formation was found to be solid‐state spreading of BNT powder particles on the substrate, and a dense film was obtained from the porous powder layer.

Nb‐doped Zr 2 [Al(Si)] 4 C 5 solid solutions were prepared by in situ hot pressing method and their high‐temperature oxidation behavior at 1000°–1300°C were investigated. The oxidation resistance of [Zr 0.9 (Nb 0.1 )] 2 [Al(Si)] 4 C 5 solid solutions is improved remarkably compared with Zr 2 [Al(Si)] 4 C 5 without dopant. The improved oxidation resistance is due to the formation of a more protective oxide scale consisting of (Zr,Nb)O 2 , Nb 2 Zr 6 O 17 , Al 2 O 3 , and mullite, in which (Zr,Nb)O 2 solid solution is more effective than ZrO 2 in depressing the inward diffusion of oxygen. In addition, the decrease of cracks in the oxide scale above 1100°C in Nb‐doped sample is also beneficial to the improved oxidation resistance.

The microwave dielectric properties and microstructures of (Mg1−xCox)1.8Ti1.1O4(x=0.03–1.00) solid solutions prepared by the conventional solid‐state route were investigated. Lattice parameters were also measured for the specimens with differentx. The forming of complete (Mg1−xCox)1.8Ti1.1O4solid solutions were confirmed by the X‐ray diffraction patterns analysis, the measured lattice parameters, and the cell volume, which linearly varied from 300.82 Å3forx=0.03–301.42 Å3forx=1.00. By increasingxfrom 0.00 to 0.05, theQ×fof the specimen can be tremendously boosted from 141 000 GHz to a maximum of 207 500 GHz. A fine combination of microwave dielectric properties (ɛr∼16.11,Q×f∼207 500 GHz at 10.72 GHz, τf∼−52.6 ppm/°C) were achieved for (Mg0.95Co0.05)1.8Ti1.1O4ceramics sintered at 1390°C for 4 h. Ilmenite‐structured (Mg0.95Co0.05)TiO3was detected as the second phase. The presence of this second phase did not cause any significant variation in the dielectric properties of the specimen. This may be because the second phase properties are also very similar to the main phase. Therefore, it is proposed as a very promising dielectric material for ultra‐high‐frequency applications.

Atmospheric cruise hypersonic vehicles are subject to high viscous heating over large surface areas. Acreage thermal protection systems (TPSs) must be stiff, strong, and light while withstanding large thermal gradients and protecting the cool interior of the vehicle. It is a challenge to design thermal protection to minimize the thermal stresses caused by thermal expansion mismatch. This paper uses a recent concept for low‐thermal‐expansion periodic lattices to propose a sandwich configuration for acreage TPSs. A key aspect of these concepts is that they can be attached to coll structures without inducing thermal stresses during heating. Sandwich TPSs are analyzed and optimized for minimum mass for required performance characteristics, and compared with an optimized baseline system. For performance requirements relevant to atmospheric hypersonic flight, the sandwich TPSs using low‐thermal‐expansion periodic lattices are superior to the baseline system for a large range of operating conditions.

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 β-tricalcium phosphate compositions. The process involved reaction with solutions of phosphoric acid (H3PO4) and 2-propanol, followed by heat treatment at high temperatures (up to 1300°C) in air. The crystalline composition of the BCP scaffolds could be controlled by varying the concentration of the H3PO4 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 >90% interconnected porosity. The structure consisted of continuous macroporous channels with smallest measured cross-sectional openings of 400 μm × 100 μm size. The BCP scaffolds prepared with 16 wt% H3PO4 solution had a measured compressive strength of 2.38 ± 0.24 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.