This paper provides a comprehensive account of the properties, development and extensive utilisation of Tibetan microcrystalline magnesite in industry. Tibetan microcrystalline magnesite has become a significant raw material for refractories, high-temperature insulating materials and magnesium chemical materials due to its high purity, low impurity content (mainly Si and Fe elements) and micrometre-sized crystallisation size (2~4 μm). The article presents a detailed analysis of the microstructure of Tibetan microcrystalline magnesite, its thermal decomposition behaviour and the key technologies employed in preparing high-purity magnesium oxide and sintered magnesia through light burning and electrofusion processes. Furthermore, this paper examines the potential applications of Tibetan microcrystalline magnesite in producing high-performance magnesium materials, including activated magnesium oxide, nano-magnesium oxide, and magnesium hydroxide, which are extensively utilized in environmental protection and high-temperature technology. It is demonstrated that the performance of Tibetan microcrystalline magnesite products can be markedly enhanced by optimising the process parameters and modification techniques, thereby further expanding their application prospects in industrial fields. This review offers a theoretical foundation and technical support for effectively utilising Tibetan microcrystalline magnesite, which possesses significant industrial application value and potential.
Considerable research has been done in the past on expensive, <50 nm particle size 3 mol% yttria-stabilized zirconia (3YSZ) using advanced sintering techniques. However, insights are still needed to reveal which factors among grain size and porosity, when both are changing simultaneously, more strongly control the hardness of conventionally sintered, relatively coarse, 250 nm 3YSZ powder, which can be used to make large industrial engineering ceramic parts at a lower cost. This investigation showed that elevating the sintering temperature from 1500 °C to 1650 °C increased the Rockwell hardness from 49.4 HRA to 86.0 HRA, which was concomitant with an increase in grain size and bulk density. A pseudo-inverse Hall-Petch relationship between hardness and grain size was observed given by H (in HRA) = 153.1 − 69.2/$$\small\sqrt{(\mathrm{grain}\,\mathrm{size})}$$ with a somewhat low R2 of 0.95, which was mainly due to the porosity being an additional important variable. Compared to grain size, the impact of open pore fraction (P) on hardness was stronger, inferred from a higher R2 of 0.99 while fitting the data into the well-known exponential decay equation, H = 92.9 exp(−11.1P). Finally, it was observed that the 3YSZ conventionally sintered at 1650 °C for 2 h had 0.8% open porosity, 6.08 g/cm3 bulk density, 960 nm grain size and consisted of only tetragonal ZrO2.
Fused zirconia-mullite (ZM) and zirconia-alumina (ZA) are expensive aggregates used in refractory formulations to enhance thermal shock tolerance and corrosion resistance, respectively. A cost-effective alternative approach was explored in this work to produce 37.4 wt% ZrO2 containing ZM utilizing conventional reaction sintering of siliceous clay, calcined alumina and monoclinic ZrO2. A series of chemical reactions ensued from 1200 °C, forming low quartz and cristobalite from the clay, in situ ZrSiO4, monoclinic ZrO2, α-Al2O3 and traces of leucite. 1600 °C was required to fully form mullite and monoclinic ZrO2 but it had 26.5% porosity even after firing at 1650 °C for 2 h. It consisted of small equiaxed primary mullite grains secondary mullite rods, and scattered and clustered, round ZrO2 grains. With 1.05% CaO addition, tetragonal ZrO2 formed, but 22.7% porosity remained despite the presence of 13.5% liquid phase having a low viscosity (0.6 Pa.s, from FactSage). With 2.11% CaO, porosity reduced to 10.7% but mullite partly dissolved, forming α-Al2O3 (ZMA aggregate). The added CaO mostly remained in the intergranular glassy phase rather than inside the ZrO2 grains but increased the thickness of the secondary mullite and the ZrO2 grains. Mullite was completely lost with 4.21% CaO doping but favorably formed cubic ZrO2 containing up to 0.26 at% Ca, interlinked α-Al2O3 rods and attained a low porosity of 0.2%. This ZA aggregate is limited to 1550 °C application temperature as excess liquid phase drained out beyond that. 7.37% CaO addition was detrimental as it formed an excessive anorthite-like liquid phase that percolated out at 1550 °C with 5.6% weight loss. Thus, in ZM-based calcium aluminate cement bonded refractory castables, the final CaO content should be restricted to below 2.1% to avoid partial dissolution of mullite.
Ablation resistance is a critical factor in evaluating the performance of BN-based ceramic composites under extreme service conditions. This study investigates the ablation behavior and underlying mechanisms of BN-MAS wave-transparent ceramic composites with varying magnesium aluminum silicate (MAS) content through oxyacetylene torch tests. The results reveal that increasing the MAS content reduces the mass ablation rate from 0.0298 g/s to 0.0176 g/s and the linear ablation rate from 0.149 mm/s to 0.112 mm/s. The incorporation of MAS into h-BN ceramics significantly lowers the surface ablation temperature, primarily due to the evaporation of B2O3 (g) and MAS ceramics. Cross-sectional analysis of the ablated composites indicates the presence of micro- and macro-spallation in the ablation center. The primary ablation products are magnesium-aluminum borosilicate glass and mullite. Key ablation mechanisms include the oxidation of h-BN under flame exposure, the erosion of viscous oxidation products, and the physical degradation of the matrix caused by the high-velocity gas flow.
Porous 430L stainless steel
components fabricated via tape casting underwent mechanical testing for
potential in-vehicle application as mechanical supports of solid oxide cells.
Tests included three-point bending up to 5% strain to assess flexural strength,
yield strength, Young’s modulus, indentation hardness, and microstructural
characterization. This study aimed to establish the relationship between pore
former size and volume fraction and the resulting yield strength. It also
compared sintered material without pore former, focusing on the influence of a
wide range of porosity of up to 46.5%. The materials exhibited an inverse
relationship for Young’s modulus, hardness and yield strength as a function of
porosity. The lowest flexural yield strength obtained was approximately 120 MPa
at the highest porosity of 46.5%, meeting the requirement of 59 MPa for the
bipolar plates of existing proton-exchange membrane fuel cells.