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.
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.
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.
Superhard cubic boron nitride (cBN) cutting materials with different contents of cBN were investigated. The compositions of cBN-based materials included ceramic and metallic binders. The sintering of materials was performed by high-temperature hot pressing (HPHT) six-anvil apparatus at pressure 4.5 GPa and temperatures 1400–1450 °C. The process of compaction and processing of superhard cBN materials is followed by numerous chemical reactions. The chemical reactions are very important in compaction and sintering. The volume transformations during chemical reactions affect the shrinkage of the materials and may also impact the residual porosity of the finished products. The adhesion between the grains also depends on these chemical reactions. The research analyzed the volume transformations of various reactions during HPHT sintering of cBN materials, which may play a significant role in forming their structure and properties.
To meet the high-quality requirements for clean steel production and fully exploit the performance advantages of carbon-containing refractories, nanomaterial has been introduced into the matrix to develop advanced carbon-containing refractories. Nanomaterials, as critical additives, play a crucial role in developing novel refractories. The service performances of carbon-containing refractories are affected not only by their physical and chemical properties but also by their microstructure. This review provides a comprehensive overview of the latest research on oxide-carbon composite refractories containing nanomaterials, categorized by their composition: nanocarbons, nano oxides, and nano non-oxides. Incorporating nanomaterials can enhance the service performances of the refractories, optimizing phase composition and microstructure. Furthermore, future research directions in nanomaterial technology for carbon-containing refractories are discussed.
It is very important to clarify the mechanism of high-temperature superconductivity in strongly correlated electron systems. The mechanism of superconductivity in high temperature cuprate superconductors has been studied extensively since their discovery. We investigate the properties of correlated electron systems and mechanism of superconductivity by using the optimization quantum variational Monte Carlo method. The many-body wave function is constructed by multiplying by correlation operators of exponential type. We show that d-wave superconducting phase exists in the strongly correlated region where the on-site repulsive interaction is as large as the bandwidth or more than the bandwidth. The d-wave pairing correlation function is shown as a function of lattice sites, showing that the long-range order indeed exists.
In this paper, (100-m) BaZrO3-mY2O3 (m = 0, 20, 25, 33, 50, 100) crucibles were prepared, respectively. Then, the effect of crucible composition on the interaction between crucibles and highly active titanium alloys (Ti2Ni) was investigated. The degree of the erosion resistance of crucibles was compared before and after melting as well as the contaminated extent of the alloys. The results show that the two-phase crucibles consisting of BaZr1−xYxO3−δ and Y2O3(ZrO2), could be prepared after adding Y2O3 into the BaZrO3 crucible. As the amount of Y2O3 addition in the crucible was increased, the erosion resistance of the crucible to the alloy melt was gradually improved. The two-phase crucible with 50 wt.% Y2O3 addition exhibited the best erosion resistance with a 7 μm thick erosion layer, which was at the same level compared to the pure Y2O3 crucible (6.5 μm). However, the inclusion contaminants caused by this two-phase crucible were smaller than those of the pure Y2O3 crucible. This study provided a theoretical basis for further research on the preparation of highly stable crucibles for melting highly active titanium alloys.
A polycrystalline Cantor alloy, equimolar in Co, Cr, Fe, Mn and Ni, was cast. It was subjected to oxidation in a thermo-balance in a flow of synthetic dry air, at 1000, 1050, 1100 and 1150 °C. The mass gain was globally parabolic but rather irregular. The parabolic constants, ranging from 55 to 700 × 10−12·g2·cm−4·s−1, are much higher than for a chromia-forming alloy. They obey an Arrhenius law with an activation energy equal to 270 kJ/mol. The external oxide scales formed are composed of an outer part made of manganese oxide and an inner part made of (Cr, Mn) oxide containing a thin internal layer of chromia. The Mn and Cr-depleted depths and the Mn and Cr masses lost by the alloy increase with the oxidation temperature. Cr-rich acicular particles precipitated in subsurface at 1100 °C and internal oxidation along the grain boundaries are present in the whole thickness of the sample oxidized at 1150 °C. Oxide spallation occurred during the cooling, at temperatures in the 200–350 °C range, only for the alloys oxidized at 1050 and 1100 °C. Not too thick scale (1000 °C) or deep internal oxidation (1150 °C) may be favorable for scale adherence.
