Preparation and characterization of corundum ceramics doped with Fe2O3 and TiO2 for high temperature thermal storage

High-temperature thermal storage materials have received urgent attention for efficient thermal transfer in solar thermal power generation. Corundum ceramics doped with Fe₂O₃ and TiO₂ were prepared via a pressureless sintering. A Fe₂O₃–TiO₂ system with different Fe₂O₃/TiO₂ ratios was applied to corundum ceramics. Phase composition, microstructural evolution, sintering properties, high temperature resistance and thermophysical properties were evaluated. The results indicated that Fe₂O₃ and TiO₂ rendered the grains highly active and enhanced the bonding between grains due to existing stably in the lattice of corundum. In addition, decrease in the Fe₂O₃/TiO₂ ratio led to a new phase of FeAlTiO₅, which refined the grains. These effects gave the samples good sintering properties and thermal shock resistance, but the thermal expansion coefficient mismatch between FeAlTiO₅ and corundum deteriorated the high-temperature (1300 °C) stability. Formula C1 (Fe₂O₃/TiO₂ ratio of 9:1) sintered at 1600 °C had the optimum comprehensive properties, possessing a bending strength loss rate of 1.54% after 30 cycles of thermal shock (1100 °C-room temperature, air cooling) and a constant strength retention rate of approximately 71.34% after 90 h high-temperature cycle. The corresponding thermal conductivity and specific heat capacity were 18.81 W/(m·K) and 1.02 J/(g·K) at 25 °C, which was suitable as a high-temperature thermal storage material.     

Effectively controlling the crystal growth of Cr2O3 using SiO2 as the second phase

The inevitable crystal growth of Cr₂O₃ during sintering causes the generation of cracks, which degrades the high-temperature properties. To solve this, SiO₂ is adopted as the second phase and the specimens are sintered at 1200-1500°C under buried carbon condition. The results show that the addition of approximate 20 wt% SiO₂ can effectively control the crystal growth of Cr₂O₃. The Cr₂O₃ particle size can keep uniform ranging from 4 μm to 12 μm even when the temperature increases to 1500°C. The sintering of Cr₂O₃ mainly follows the defect model, which depends on the reaction temperatures and atmospheres. This work should also contribute to the sintering of other oxide refractories with controlled crystal size for practical application.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.     

Microstructure and mechanical property of SiC whisker coating modified C/C composite/Ti2AlNb alloy dissimilar joints prepared by transient liquid phase diffusion bonding

SiC whisker coating was prepared on the surface of C/C composite successfully by CVD, and transient liquid phase (TLP) diffusion bonding was employed to realize the joining of SiC whisker coating modified C/C composite and Ti₂AlNb alloy using Ti–Ni–Nb foils as interlayer. The microstructure, shear strength and fracture behavior were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD) and universal testing machine. The results show that SiC has good compatibility with C/C composite, and gradient interface formed between SiC-modified C/C composite and Ti₂AlNb alloy. When the bonding experiment was carried out under bonding temperature of 1040 °C and holding time of 30min with 5 MPa pressure in vacuum, the joints formed well and no obvious defects can be observed. The typical microstructure of joints is C/C composite/SiC + TiC/Ti–Ni compounds + Ti–Ni–Nb solid solutions/residual Nb/diffusion reaction layer/Ti₂AlNb alloy. With the increasing of bonding temperature, the thickness of joining area increased due to sufficient element diffusion. However, when bonding temperature is elevated to 1060 °C, some defects such as cracks and slag inclusions exist in the interface layer between interlayer and Ti₂AlNb. The joints with maximum average shear strength of 32.06 MPa are bonded at 1040 °C for 30min. C, SiC and TiC can be found on the fracture surface of joints bonded at 1040 °C which indicated that fracture occurred at the interface layer adjacent SiC layer.     

Mo-doped Cr-Ti-Mo ternary o-MAX with ultra-low wear at elevated temperatures

In this work, the Cr-Ti-Mo ternary o-MAX ceramics based on the Cr₂TiAlC₂ phase are synthesized, and their tribological performance at elevated temperatures up to 800 ºC in the air is evaluated. With Si₃N₄ as a tribocouple, an unexpected ultra-low wear rate reaching 6.1 × 10^?8 mm³ N^?1 m^?1 is observed in Cr_1.9Ti_0.9Mo_0.2AlC₂ at 800 ºC, accompanied by a stable coefficient of friction (COF) around 0.33 in a wide temperature range. X-ray photoelectron spectroscopy (XPS) depth profiling confirms a tribofilm with gradient composition, which simultaneously offers fluid lubricating at elevated temperatures and self-healing after cooling. Particularly, confirmed by the theoretical simulations, the doping of Mo improves the interlayer binding as well as alters the oxidation behaviors of Cr₂TiAlC₂. With an optimal interlayer binding strength and oxidation rate, the Cr_1.9Ti_0.9Mo_0.2AlC₂ can generate a tribofilm possessing ideal composition, which simultaneously promotes lubrication and anti-wear performance at elevated temperatures.     

Synthesis and characterization of porous Y2SiO5 with low linear shrinkage,high porosity and high strength

The emerging porous Y₂SiO₅ ceramic is regarded as a promising candidate of thermal insulator owing to its very low thermal conductivity. However, recent works on porous Y₂SiO₅ are confronted with severe problems such as large linear shrinkage (18.51–20.8%), low porosity (47.74–62%) and low strength (24.45–16.51 MPa) at high sintering temperatures (1450–1500 °C). In this work, highly porous Y₂SiO₅ ceramic with low shrinkage and excellent high-temperature strength was fabricated by in-situ foam-gelcasting method at 1550 °C. The as-prepared sample has unique multiple pore structures, low linear shrinkages of 6.3–4.5%, controllable high porosities of 60.7–88.4%, high compressive strengths of 38.2–0.90 MPa, and low thermal conductivities of 0.126–0.513 W/(m K) (porosity: 87.1–60.2%). The effects of relative density on relative strength, as well as porosity on thermal conductivity were quantitatively discussed. The present results indicate that porous Y₂SiO₅ is the potential high-temperature thermal insulation material of light weight, low thermal conductivity, and high strength.     

A new ternary phase in the system CaO–Al2O3–Cr2O3: Crystal structure and thermal expansion of CaAl2Cr2O7

Polycrystalline material of a novel phase in the system CaO–Al₂O₃–Cr₂O₃ has been obtained by solid-state reactions. Chemical analysis indicated the composition CaAl₂Cr₂O₇. Single-crystal growth of the new compound using borax as a mineralizer was successful. Diffraction experiments at ambient conditions on a crystal with composition CaAl_2.13Cr_1.87O₇ yielded the following basic crystallographic data: space group P 3, a = 7.7690(5) Å, c = 7.6463(5) Å, V = 399.68(6) ų, Z = 3. Structure determination and subsequent least-squares refinements resulted in a residual of R(|F|) = 2.3% for 1440 independent observed reflections and 113 parameters. To the best of our knowledge, the structure of CaAl_2.13Cr_1.87O₇ or CaAl₂Cr₂O₇ represents a new structure type. It belongs to the group of double layer structures where individual double layers contain octahedrally and tetrahedrally coordinated cation positions. Linkage between neighboring sheet packages is provided by additional calcium cations. Furthermore, thermal expansion has been studied in the interval between 29 and 790°C using in situ high-temperature single-crystal diffraction. No indications for a structural phase transition were observed. From the evolution of the lattice parameters the thermal expansion tensor has been obtained. A pronounced anisotropy is evident. The response of structural building units to variable temperature has been discussed.     

Cr3C2 assisted ultrafast high-temperature sintering of TiC

The strong covalent bonding of TiC renders its densification through conventional sintering difficult. Here, we propose a method involving liquid-phase-assisted ultrafast high-temperature sintering (UHS) for obtaining nearly full density of TiC ceramics by the addition of Cr₃C₂. The samples were heated at a rate of 600 °C/min to 2200 °C and held at this temperature for 1 min. The effects of sintering parameters and the Cr₃C₂ content on the relative density and microstructure of the sintered samples were investigated. The main causes of rapid densification were particle rearrangement associated with the Cr₃C₂ liquid phase, dissolution-reprecipitation associated with the solid solution, and the weak evaporation of Cr formed during UHS. In particular, the addition of Cr₃C₂ helped increase the hardness and elastic modulus of TiC significantly. This paper presents an effective and extensible method involving UHS for rapidly obtaining dense ceramics.     

High temperature strength and thermal stability for melt growth composite

Melt growth composites (MGCs) have a microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal oxide compounds (YAG (Y₃Al₅O₁₂), GAP (GdAlO₃)) interpenetrate without grain boundaries. Therefore, the MGCs have excellent high-temperature strength characteristics, creep resistance, superior oxidation resistance and thermal stability in the air atmosphere at very high temperature. To achieve ultra-high thermal efficiency and low NO_x emission for gas turbine systems, we produced turbine nozzle vanes that does not require cooling and heat shield panels for combustor liners. The thermal stability and mechanical properties of these parts have been studied. The high-temperature strength characteristics and the thermal stability of components were also no changes after heat treatment for 500 h at 1700 °C in an air atmosphere. The favorable properties of melt growth composite have been discussed for possible application in gas turbine system.     

Enhanced mechanical properties and thermal cycling stability of Al2O3-4J42 joints brazed using Ag–Cu–Ti/Cu/Ag–Cu composite filler

This study investigates the mechanical properties and microstructure evolution of Al₂O₃-4J42 joints brazed using Ag–Cu–Ti (ACT) and Ag–Cu–Ti/Cu/Ag–Cu (ACTCA) fillers during thermal cycling from 0 °C to 500 °C. The reaction products between Al₂O₃ and brazing filler of these two types of joints are mostly composed of Ti–O compounds, Ti₄Cu₂O and Al-based compounds. Brittle intermetallic compounds (IMCs) are observed in ACT joints, but not found in ACTCA joints. The reaction layer in ACT joints becomes thinner and discontinuous with thermal cycles, while that in the ACTCA joints hardly changes. Besides, the stress-induced cracks occur within the Al₂O₃ ceramic near the Al₂O₃/filler interface in the ACT joints, but no crack is found in the ACTCA joints. The mechanical tests show that the ACTCA joints maintain at least 217%, 154% and 144% higher shear strength than the ACT joints at 0, 10 and 20 thermal cycles, respectively. The Cu interlayer with low yield strength releases stress through plastic deformation, meanwhile acts as a barrier to prevent elements diffusion and the formation of the brittle IMCs, thus improving the mechanical properties and thermal cycling stability of the joints.     

High-temperature mechanical property and thermal shock resistance of Al2O3f/Al2O3 composites

Continuous alumina fiber–reinforced alumina matrix composites (Al₂O_3f/Al₂O₃ composites) were produced via sol–gel process, then the high-temperature mechanical property and thermal shock resistance of Al₂O_3f/Al₂O₃ composites were investigated. The results showed that the composites exhibited excellent high-temperature properties. The mechanical property of the composites was affected by heat treatment (prepared at 1100°C exhibited the most desirable mechanical property). The tensile strength of the composites abruptly decreased at higher temperatures. Although the mechanical property of the composites deteriorated after the thermal shock test was conducted at high temperatures, they exhibited excellent thermal shock resistance. After 50 thermal shock tests conducted at 1300 and 1500°C, the flexural strength of the composites was found to be 124.34 and 93.04 MPa, thus showing a decrease in strength with the increasing temperature.     

Application of Cr3C2/C composite powders synthesized via molten-salt method in low-carbon MgO–C refractories

High-performance and low-carbon MgO–C refractories are important refractories for smelting ultra-low carbon steel and clean steel. Based on this, Cr₃C₂/C composite powders were synthesized by the molten-salt method, and used as an additive to prepare low-carbon MgO–C refractories under nitrogen atmosphere. The phase, morphology and oxidation kinetics of Cr₃C₂/C composite powders were studied. In addition, the effect of Cr₃C₂/C composite powders on the morphology, mechanical properties, thermal shock resistance, and corrosion resistance of MgO–C refractories was investigated. The results indicated that the Cr₃C₂/C composite powders exhibited superior oxidation resistance than flake graphite. Moreover, the Cr₃C₂/C composite powders were introduced into the MgO–C refractories. Compared with the sample without Cr₃C₂/C composite powders, the introduction of 1 wt% Cr₃C₂/C composite powders significantly improved the thermomechanical properties and corrosion resistance of the material, its CMOR, CCS before and CCS after thermal shock were 9.06 MPa, 50.40 MPa and 32.60 MPa, respectively, and the corrosion index was significantly reduced from 44.6% to 26.5%.     

High-entropy La(Fe0.2Co0.2Ni0.2Cr0.2Mn0.2)O3 ceramic exhibiting high emissivity and low thermal conductivity

A novel, high-entropy, perovskite-structured, solid solution La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ ceramic was successfully synthesized via high-temperature solid-state reaction. The crystal structure, microstructure, infrared emissivity, and thermophysical properties were investigated. The experimental results indicated that La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ exhibited an infrared emissivity as high as .92 in the near-infrared region of .76–2.50 μm. The thermal conductivity was 1.38–1.72 W m⁻¹ K⁻¹ in the temperature range of 25–1200°C.     

Joining of SiC ceramics via a novel liquid preceramic polymer (V-PMS)

A novel liquid preceramic polymer (V-PMS) was synthsized by modifying polymethylsilane (PMS) with 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane ([CH₃(CH₂CH)SiO]₄, D4Vi), for joining SiC ceramics under ambient pressure. The obtained V-PMS with a viscosity of 125 Pas at room temperature exhibits excellent thermal properties and bonding strength. The ceramic yield of V-PMS treated at 1200 °C under Ar atmosphere is 84.5%, which is 38.3% higher than the original PMS. The shear strengths of the SiC joints joined by V-PMS at 800 °C, 1000 °C and 1200 °C under N₂ atmosphere are 11.9 MPa, 34.5 MPa and 29.9 MPa, respectively. The excellent performances make the obtained V-PMS promising candidates for joining SiC ceramics in high-temperature applications.     

Thermal emission properties of Al2O3/Er3Al5O12 eutectic ceramics

The mechanical properties and thermal stability of the Al₂O₃/Er₃Al₅O₁₂ (EAG) eutectic ceramics have been investigated at very high temperature. The emissive properties of this eutectic ceramics have also been measured and its possibilities of application to an emitter have been discussed. The present eutectic ceramic has excellent high-temperature strength characteristics, showing that tensile yielding stress is approximately 300 MPa at 1650 °C and superior thermal stability at 1700 °C in an air atmosphere. The present material shows strong selective emission bands at wavelength 1.5 μm due to Er³⁺ ion. The emission bands of this material are nearly coincident with the sensitive region of GaSb PV cell, therefore, the Al₂O₃/EAG eutectic ceramic can be regarded as one of the promising emitter materials in TPV systems.     

Preparation and mechanical properties of the 2.5D ASf/ZrO2 composites after high-temperature heat treatment

In the paper, the aluminosilicate fiber-reinforced zirconia (AS_f/ZrO₂) ceramic composites were successfully fabricated by polymer impregnation and pyrolysis (PIP) method. The microstructure and high-temperature mechanical properties of the original composites were well studied. The results revealed that the composites could maintain the stability of microstructure at 1000 °C. The flexural strength increased from 58.82 ± 2.83 MPa to 88.74 ± 6.20 MPa and the flexural modulus increased from 29.26 ± 4.67 GPa to 40.76 ± 8.76 GPa. The thermal exposure improved the interfacial bonding and made the load transfer more effective. After heat treatment from 1200 °C to 1400 °C, the flexural strength gradually declined due to the crystallization of the AS fibers and ZrO₂ matrix, while the flexural modulus increased in a completely different trend. After heat treatment at 1400 °C, the composites could maintain a flexural strength of 66.95 ± 4.24 MPa with a flexural modulus of 60.42 ± 7.25 GPa. But the fracture mode gradually evolved to brittleness.     

Synthesis and ultra-high temperature ablation behavior of a ZrC/Cr2AlC composite

This work reports on the fabrication and high temperature ablation property of a new ZrC/Cr₂AlC composite. The ZrC/Cr₂AlC composite was obtained by hot pressing a mixture of 15 vol% ZrC and 85 vol% Cr₂AlC powders at 1300 °C with 20 MPa for 1 h in Ar atmosphere. The composite had a flexural strength of 622 MPa, higher than 400 MPa for Cr₂AlC. The high temperature ablation behavior of the composite was investigated using the oxyacetylene torch ablation test. During oxyacetylene torch testing, the composite underwent a series of thermal decomposition and oxidation. Microstructure and composition of the synthesized composite before and after the ablation test were characterized with scanning electron microscopy and X-ray diffractometry techniques.     

Sintering kinetics and properties of NiFe2O4-based ceramics inert anodes doped with TiN nanoparticles

Sintering kinetics of NiFe₂O₄-based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe₂O₄-based ceramics possessed high-temperature semiconductor essential characteristics. The electrical conductivity of NiFe₂O₄-based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe₂O₄-based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.     

Tailoring Magnetic Properties of MAX Phases,a Theoretical Investigation of (Cr2Ti)AlC2 and Cr2AlC

(Cr₂Ti)AlC₂ is a newly discovered MAX phase with ordered occupations of Ti and Cr atoms on M sites. The Cr‐containing MAX phase is expected showing magnetic property, which provides functional applications in spintronics and as self‐monitoring smart coating. The magnetic states of (Cr₂Ti)AlC₂ are predicted by GGA and GGA + U methods and compared to those of Cr₂AlC. The ground states are predicted as FM or AFM‐XX configurations depending on the calculation methods. Analysis of the electronic structure shows that the magnetic moments mainly originate from the net spins of Cr 3d valence electrons, whereas the contribution of other atoms is negligible. The calculated magnetic moments of Cr atoms in (Cr₂Ti)AlC₂ are higher than those in Cr₂AlC due to the larger distance between the out‐plane Cr atoms separated by the intercalated nonmagnetic Ti–C slab. This work provides an insight on tailoring magnetic properties of MAX phases by modifying the crystal structure.     

Microstructure and mechanical properties of ZrB2-SiC/Nb joints brazed with CoFeNiCrCuTix high-entropy alloy filler

ZrB₂-SiC ceramics and Nb alloy were brazed at 1160°C for 60 min with CoFeNiCrCuTi_x high-entropy alloy filler. The influence of Ti content on the interface structure and mechanical properties of ZrB₂-SiC/Nb joint was systematically studied. It is found that the rich-Ti Laves phase was formed due to the addition of large atomic size Ti fill into the filler alloy or brazing joint, and its content increases with Ti content. The joint brazed by high-entropy alloys filler without Ti can be divided into a tooth-shaped Cr₂B reaction layer and a central area composed of a eutectic mixed structure of FCC phase and rich-Nb lamellar Laves phase. Ti and Nb are mutual solid solution elements. The increase of Ti content in the joint makes the FCC phase and the rich-Nb lamellar Laves phase to transform into a big bulk Ti-rich Laves phase and the quadrilateral (Ti, Nb)B phase. The tooth-shaped Cr₂B was disappeared. The residual stress generated in the joint during the brazing process tends to cause defects such as holes and microcracks in the bulk Ti-rich brittle Laves phase. Therefore, with the addition of Ti, the normal temperature performance of the joint decreases from 216 MPa to 52 MPa. However, with the increase of Ti, the high-temperature mechanical properties of the joint first decrease, and then increase. It was mainly due to the formation of rich-Ti Laves phase and quadrilateral (Ti, Nb)B with excellent high-temperature mechanical properties. When brazing with CoFeNiCrCuTi_1.5 filler, the high-temperature performance of the joint reached 92% of its room temperature performance.