Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze‐drying process. Freezing of a water‐based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze‐drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si3N4 had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si3N4 matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.

Previous experiments showed that γ‐Al2O3‐modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH− ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH− ions react with Al and release H2. Because of the limited H‐soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H+ species, H2 molecules accumulate and form small H2 gas bubbles at the Al:Al2O3 interface. When the reaction equilibrium pressure in H2 bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H2 bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as‐received Al powder could continuously react with water at temperatures above 40°C and under low vacuum, because the vacuum makes the critical gas pressure in H2 bubbles decrease as well.

A ZrB 2 -SiC composite was prepared from a mixture of zirconium, silicon, and B 4 C via reactive hot pressing. The three-point bending strength was 506 ؎ 43 MPa, and the fracture toughness was 4.0 MPa⅐m 1/2 . The microstructure of the composite was observed via scanning electron microscopy; the in-situ-formed ZrB 2 and SiC were found in agglomerates with a size that was in the particle-size ranges of the zirconium and silicon starting powders, respectively. A model of the microstructure formation mechanism of the composite was proposed, to explain the features of the phase distributions. It is considered that, in the reactive hot-pressing process, the B and C atoms in B 4 C will diffuse into the Zr and Si sites and form ZrB 2 and SiC in situ, respectively. Because the diffusion of Zr and Si atoms is slow, the microstructure (phase distributions) of the obtained composite shows the features of the zirconium and silicon starting powders.

The mechanical properties of Al 2 O 3 -based porous ceramics fabricated from pure Al 2 O 3 powder and the mixtures with Al(OH) 3 were investigated. The fracture strength of the porous Al 2 O 3 specimens sintered from the mixture was substantially higher than that of the pure Al 2 O 3 sintered specimens because of strong grain bonding that resulted from the fine Al 2 O 3 grains produced by the decomposition of Al(OH) 3 . However, the elastic modulus of the porous Al 2 O 3 specimens did not increase with the incorporation of Al(OH) 3 , so that the strain to failure of the porous Al 2 O 3 ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH) 3 particles. Fracture toughness also increased with the addition of Al(OH) 3 in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.

Unlike traditional batteries, small fuel cells have a high energy density and can work uninterruptedly, being better energy suppliers for portable devices. Such devices require an economically viable fuel. Recent findings showed that metal Al particle surfaces could be modified by fine ceramic oxide grains through a ceramic processing procedure, and the modified Al powder could continuously react with pure water and generate hydrogen under ambient conditions. The reaction of Al with water produces as much as 3.7-4.8 wt% hydrogen, and the reaction byproducts are chemically neutral. Metal Al is cheap and hydrogen generation from the reaction between surface-modified Al particles and water is a simple process. These features make this new process a cost-efficient way of generating hydrogen for small fuel cells in comparison with other portable hydrogengeneration materials and technologies. In this paper, the state of the art of portable hydrogen-generation materials is surveyed, and the ceramic oxide-modified Al-hydrogen technology and its potential are highlighted.