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Zirconia ceramic is a significant structural material, but its use under some extreme circumstances is limited by its mechanical properties. In this work, SiC particles (SiCp) were added into alumina toughened zirconia ceramics to prepare ZrO2–Al2O3-SiCp ceramics with high performance by using oscillatory pressure sintering (OPS). Results showed that the best OPS temperature of 1600 °C was obtained, and the optimal SiCp particle size and content were 200 nm and 10 vol% respectively. Under these conditions, the specimen exhibited higher mechanical properties including Vickers hardness of 15.43 GPa, bending strength of 1162 MPa and fracture toughness of 6.36 MPa m1/2. Moreover, it was found that the atomic matching between ZrO2/SiCp, Al2O3/SiCp, and ZrO2/Al2O3 was much higher, showing the coherent interface relationship. Therefore, it was favorable for enhanced mechanical properties of as-prepared ZrO2–Al2O3-SiCp ceramics.

As main components of magnesia-based refractories, magnesia exhibits excellent properties such as high refractoriness and good basic slag corrosion resistance. However, magnesia produced from CaO containing cryptocrystalline magnesite has limited application owing to the low hydration resistance and poor thermal shock resistance (TSR). This work aimed to investigate the reinforcing effects of microscale monoclinic ZrO2 on free CaO containing magnesia for optimizing mechanical properties, TSR and hydration resistance. The results showed that adding ZrO2 could promote the removal of the open pores, strengthen the interface bonding between various grains and produce crack deflection, which improved flexural strength and fracture toughness. As a result, the TSR of the specimens was enhanced effectively due to increased strength and toughness and reduction in the thermal expansion coefficient. Besides, as the ZrO2 was introduced, hydration resistance of the specimens improved significantly, mainly attributing to the decrease in apparent porosity and elimination of the free CaO by forming CaZrO3 and cubic ZrO2 phases.

SiC porous ceramics (SPCs) are key functional medium materials, which are widely applied as porous burners. However, SPCs produced via the polyurethane sponge replica technique usually exhibit a randomly distributed cellular structure, leading to vulnerability and a low combustion efficiency for the porous burners. In this study, SPCs were structurally designed via novel 3D printed resin template technique; this endowed the SPCs with synergistically optimized strength and combustion characteristics. SPCs containing tetrakaidecahedron, octahedron and cubic cells were successfully prepared via SiC slurry coating and sintering in air. The cell parameters determined the strength and combustion properties of the SPCs. The SPCs containing cubic cells exhibited the largest strength and strain, because more struts were present parallel to the loading direction; this is more advantageous for stress bearing compared to those vertical to the loading direction. The tetrakaidecahedron cell facilitated the formation of a disturbance effect within the SPCs to a higher extent than the cubic and octahedron cells, thereby enhancing the heat exchange performance between the fluid and porous framework. Hence, the SPC with tetrakaidecahedron cells exhibited the best combustion properties, which resulted in the highest surface temperature and lowest pollution emission.

MgO-based refractories have been regarded as ideal linings for various furnaces owing to high refractoriness and excellent corrosion resistance to basic slag. However, thermal shock damage resulting from poor thermal shock resistance (TSR) of magnesia is the common failing. The present work aimed at improving TSR of magnesia aggregates by introducing microscale monoclinic ZrO2. The results showed that the ZrO2 added could increase cation vacancy concentration and inhibit abnormal growth of MgO grains, which therefore enhanced densification of the specimens. However, when the ZrO2 amount exceeded 15 wt%, densification decreased slightly due to agglomeration of ZrO2 and formation of more microcracks at the MgO grain boundaries. With increasing ZrO2 content, although the flexural strength degraded, the fracture toughness increased constantly because of the toughening effects of crack deflection and crack branching. Besides, although the addition of ZrO2 decreased the thermal conductivity, TSR of the specimens was significantly improved with increasing ZrO2 content due to the increase in toughness and decrease in strength, thermal expansion coefficient as well as Young's modulus. After thermal shock tests, 15 wt% ZrO2 containing specimen exhibited the highest TSR, whose residual strength ratio was about triple of that of the pure magnesia specimen. However, further increasing ZrO2 content to 20 wt% reduced the TSR attributing to the spalling of intergranular ZrO2 agglomerations.

Low-carbon MgO–C refractories are facing great challenges with severe thermal shock and slag corrosion in service. Here, a new approach, based on the incorporation of nano-sized ZrO2–Al2O3 composite powder, is proposed to enhance the thermal shock resistance and slag resistance of such refractories in this work. The results showed that addition of ZrO2–Al2O3 composite powder was helpful for improving their comprehensive performances. Particularly, the thermal shock resistance of the specimen containing 0.5 wt% composite powder was enhanced significantly which was related to the transformation toughening of zirconia and in-situ formation of more spinel phases in the matrix; also, the slag resistance of the corresponding specimen was significantly improved, which was attributed to the optimization of pore structure and formation of much thicker MgO dense layer.

Fused silica (SiO2) ceramic crucibles with α-Si3N4 coating are commonly used for smelting photovoltaic silicon (Si). However, SiO2 ceramics will inevitably undergo crystallization and large volume change during the high-temperature service, which will lead to crucible cracking and deteriorate the quality and yield of the Si ingot. In this work, α-Si3N4/SiO2 ceramics are fabricated by introducing α-Si3N4 into SiO2 ceramics to inhibit crystallization. The results show that the introduction of α-Si3N4 can effectively inhibit crystallization of SiO2 ceramics at temperature higher than 1450 °C. Only 5 wt% cristobalite form in SiO2 ceramic with 20 wt% α-Si3N4 (heated at 1550 °C for 30min). The crystallization activation energy of SiO2 ceramic containing 20 wt% α-Si3N4 increases by 2.27 times to 931.2kJ/mol compared with that of pure SiO2 ceramic (409.6kJ/mol). The inhibition crystallization effect and increased activation energy derive from the in-situ formation of O–Si–N chemical bond and physical isolation of SiO2 particles by α-Si3N4 powders.

It is believed that the formation of hydration phase, MgO-SiO2-H2O (M-S-H), contributes to good workability and reliable comprehensive properties for magnesia based castables. In order to stimulate the formation of M-S-H in magnesia based castables and understand the minimum introduction of microslica amount, wet milling process was used to promote the dissolution of MgO and SiO2 in this work. The slurry containing different content of microsilica with wet milling technology and the castables with/without wet milling slurry were prepared. The effects of microsilica content on the formation of hydration phases were analyzed by XRD, FT-IR and TG/DSC and the properties of magnesia based castables were evaluated by explosion resistance, CMOR, HMOR and so on. The results showed that the formation of M-S-H was accelerated because of the dissolution of Mg2+ and HSiO3− in wet milling process. Higher amount of M-S-H led to a tight bonding in the early stage, and a denser structure after firing at high temperature due to the limited formation of brucite and in-situ formation of evenly distributed forsterite phase. In addition, much higher HMOR were obtained when less microsilica was added, attributing to the suppressed formation of low-melting-point liquid. Therefore, 2–3 wt% microsilica addition was recommended in this process.

In current work, effect of different carbon sources on densification and mechanical properties of WC ceramics was studied by using a new type of oscillatory pressure sintering technique. It was found that addition of 0.3 wt% carbon contributed not only to promoting densification of WC ceramics but also to inhibiting formation of W2C phase, thus considerably enhancing mechanical properties of such ceramics. The highest relative density of 99.76%, corresponding hardness of 26.31 GPa and fracture toughness of 6.85 MPa·m1/2 were obtained for WC ceramics with graphite oxide nanosheets as carbon source; also, crack deflection, crack bridging and pulling-out of graphite oxide nanosheets were main toughening mechanisms of such ceramics. It was suggested that WC ceramics with high performances can be prepared by combination of oscillatory pressure sintering and carbon addition.

The new route for recycling fly ash was proposed to prepare SiC reticulated porous ceramics (SRPCs) with high strength and increased efficient filtration for molten metal filtration. The effects of fly ash on the rheological characteristics, microstructure evaluation and wetting behavior between SRPCs and molten metal were investigated. It was found that the fly ash was beneficial to thixotropic property of SiC slurry when its content was less than 30 wt%. Furthermore, fly ash in SRPCs was completely transformed into mullite with needle-shape at 1300 °C, forming a porous structure containing micro pores and windows. SRPCs containing 20 wt% fly ash exhibited a higher strength because of the improved rheological properties of SiC slurry and the optimized microstructure in skeleton. In addition, the added fly ash in SRPCs could increase the contact angle between skeleton substrate and molten metal via microporosization of skeleton, thus exhibiting the potential ability to improve the filtration efficiency.

The mechanical properties and oxidation resistance of the Al2O3-C refractories are of critical importance for iron and steel making processes. However, the evaporation of antioxidants related phases such as Al(g), Si(g), and SiO(g) would deteriorate these properties, especially during high-temperature treatment/application. Therefore, in the present work, a small amount of Ti3AlC2 compared with Al was introduced to overcome these problems. The phase compositions, microstructures, mechanical properties, and oxidation resistance of Ti3AlC2 containing refractories were investigated. The partial oxidation of Ti3AlC2 led to inherited lamellar structures such as Ti3Al1-xC2, TiC, and granular Al2TiO5 phases. The controlled oxidation of Ti3AlC2 and its volume expansion contributed to the compact-structure, thereby limiting the escape of Si and SiO vapors at high temperatures. Consequently, the mechanical properties and oxidation resistance of Ti3AlC2 containing Al2O3-C refractories treated at 1600 ℃ were improved.

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 (Al1-xCrx)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.

Al2O3-Cr2O3-CaO castables have been regarded as ideal linings for various furnaces owing to sufficient strength and excellent corrosion resistance. However, formation of toxic Cr(VI) is the common failing. The present work aimed at achieving in situ detoxification by introducing zeolite. The influence of zeolite on phase evolutions, mechanical properties and Cr(VI) formation were investigated. The results showed that leaching Cr(VI) at 110 °C was lowered by zeolite due to absorption, while chemical reduction of CaCrO4 and Ca4Al6CrO16 occurred at 500−1100 °C and 900−1300 °C, respectively. At 1300−1500 °C, zeolite promoted formation of (Al,Cr)2O3 solid solution (+3 state). Besides, after substituting partial cement with zeolite, CMOR of castables at 110 °C just decreased slightly due to Ca2(Al2SiO6)(OH)2 formation while at 1100−1500 °C, CMOR increased obviously because of enhanced densification. A comparable CMOR up to 68 MPa and a remarkable Cr(VI) detoxification rate of 96.3 % could be achieved for castables with zeolite.

Magnesia-carbon (MgO–C) refractory is considered as the most crucial lining material in steelmaking furnaces. To provide reference for producing high performance MgO–C refractory containing different carbon contents, the microstructure, mechanical strength, thermal shock resistance and thermal properties of this material with 1–18 wt% flake graphite were studied systematically in this work. The results showed that carbon content had small effect on microstructural evolution of MgO–C refractory after high temperature treatment, mainly MgAl2O4 particles, Al4C3 whiskers and plate-/whisker-shaped AlN formed in the matrix of all specimens. As carbon content added increased gradually, fracture strength of specimens exhibited a reduced tendency whereas the corresponding fracture displacements became higher and higher owing mainly to good sliding ability of flake graphite. Furthermore, the increase in carbon content reduced thermal expansion rate of specimens and correspondingly increased thermal conductivity, consequently contributing to enhanced resistance to thermal shock of MgO–C refractory.

Calcium aluminate cement-bonded Al2O3-SiC-C castables are widely used in practice due to sufficient initial strength and operability. However, a high amount of cement leads to deteriorated high-temperature properties due to the formation of low-melting-point phases. The present work aimed at optimizing the comprehensive properties of the ultra-low cement-bonded Al2O3-SiC-C castables with the introduction of high purity synthetic zeolite. The influences of additives on the properties of castables and the hydration behavior of cement were investigated. The results showed that zeolite had the following advantages: it promoted the hydration of cement through enhancing the physical absorption and potential chemical complexation of Ca2+ and eventually induced growth of the predominant plate-like hydrates (C2AH8) at 25 ℃. The formation of more C2AH8 was beneficial to improve the green strength even with less cement. The substitution of cement with 0.5 wt.% synthetic zeolite contributed to improved high-temperature properties by reducing CaO content.

Tetragonal zirconia polycrystalline (TZP) ceramics have been considered as the important structural materials in many key technology fields, but some of their mechanical properties do not meet the requirement under the extreme environmental conditions. In this work, SiC whiskers and Al2O3 particles were used as reinforcement additives to synergistically toughen TZP ceramics, and a newly developed oscillatory pressure sintering (OPS) technique was employed to fabricate the new zirconia-based ceramics. For comparison, the traditional hot pressing (HP) method was also used to prepare these composites. The results showed that with 15 vol% SiC whiskers addition, the toughening mechanisms such as whisker bridging, whisker pulling-out, crack deflection, and crack branching appeared apparently, thereby improving fracture toughness of such composites. In contrast to HP, OPS can eliminate defects, improve densification, obtain more uniform microstructure, and significantly enhance mechanical properties of the composites.

Tungsten carbide ceramics were fabricated by a novel oscillatory pressure sintering process and the influence of sintering temperature on the microstructure and mechanical properties of tungsten carbide ceramics was investigated. It was found that tungsten carbide would decarbonize to form W2C under high temperature environment. As the sintering temperature increased, the density and hardness of tungsten carbide ceramics increased apparently, and the growth of tungsten carbide grains became more obvious. The samples fabricated by oscillatory pressure sintering exhibited superior mechanical properties in comparison with some published data obtained by other pressure-assisted sintering methods. The optimum sintering temperature for tungsten carbide ceramics under oscillatory pressure was considered to be 1900 °C. Flexural strength, hardness and fracture toughness of the corresponding sample prepared at this temperature were found to be 1014 MPa, 26.98 GPa and 5.91 MPa·m1/2, respectively.

SiC reticulated porous ceramics (RPCs), as the key functional components, are widely applied in field of molten-metal filtration. However, the hollow struts with triangular tips caused by burnt out of polymer foam reduced the strength of SiC RPCs, which limited their engineering application. Aiming to reveal the effects of template strut morphology on the mechanical performance of SiC RPCs, templates with triangular, square and circular cross sections were designed by additive manufacturing. Subsequently, SiC RPCs with varied strut structures were prepared by slurry immersion, followed by sintering at elevated temperature. The blunt of template strut improved the mechanical property of SiC RPCs, samples with circular hollow struts showed the highest crushing strength and strain than that of triangular and square hollow struts. From the results of simulation calculation, triangular hollow struts led to the significant stress concentration in the connection of struts and their middle parts, while the smaller stress uniformly distributed in the whole SiC skeleton as hollow struts became circle, thereby the improved mechanical properties were obtained in SiC RPCs.

Magnesia-chrome bricks are commonly used as the lining of smelters in copper and lead industry because of their resistance against the thermal, mechanical and chemical loads in the pyrometallurgical production process. In fact, the pyrometallurgical production process of lead-copper matte is generally a reducing process with plenty of coke and iron fillings as reducing agent, which implies that the magnesia-chrome bricks service in stronger reducing atmosphere. In this work, the microstructure, mechanical properties and slag resistance of such materials were investigated in reducing atmosphere to simulate the influence of the atmosphere on the properties of magnesia-chrome refractories. The results show that obvious increase in porosity and decrease in strength occurred to the magnesia-chrome samples after treating in reducing atmosphere. This is related to the structural damage of samples, in which Fe3+/Fe2+ within spinel (Mg, Fe)(Cr, Al, Fe)2O4 were partly reduced to metallic Fe under reducing atmosphere. Consequently, the slag penetrated much easily into the matrix of the samples. Moreover, the slag corrosion also aggravated due to the aggressive FeO in slag and the formation of magnesiowustite in the penetration layer. Nevertheless, the high melt phases of spinel and forsterite were formed when the slag corrosion test was conducted in air atmosphere, thereby preventing further slag corrosion and penetration.

As the key component for porous media burner, the sufficient strength of SiC reticulated porous ceramics (RPCs) is necessary due to the scour of hot flue gas. However, SiC RPCs prepared with template replica method often exhibited serious oxidation and low strength when the foam was burnt out. In the present work, a sol-coated Al powder with high hydration resistance was introduced into the silicon carbide slurry to promote mullite formation in SiC skeleton, meanwhile reduce the SiC oxidation ratio of sample during the heat treatment. Firstly, the silica-sol coated Al with uniform silica-sol was successfully prepared via dip coating to improve the ability of hydration resistance of Al powder. Furthermore, sol-coated Al with spherical morphology improved rheological properties of silicon carbide slurry, including viscosity and rheological behavior. In addition, the evolution mechanism of sol-coated Al in SiC RPCs was revealed. The AlO2(g) and AlO(g) was first formed in the system, thus significantly reducing the SiC oxidation ratio in SiC RPC. The partial pressure of AlO(g) increased with the sintering temperature, the needle-shaped mullite formed at 1300 ○C when it dissolved in the microsilica, while lumpy mullite formed as SiC was the silica source in SiC RPCs. The addition of sol-coated Al was beneficial to optimize the microstructure of SiC skeleton, resulting in higher compressive strength of SiC RPCs.

The insufficient strength and thermal shock resistance of mullite reticulated porous ceramics (RPC) limited their application in porous media combustion. To strengthen mullite RPC, the struts with three layers were prepared using the combined approaches of polymeric replica and vacuum infiltration. The effects of residual stress within the struts on thermal shock resistance of mullite RPC were investigated by finite element analysis (FEA). The results showed that the strut was composed of the coating of reaction-bonded mullite, middle layer of mullite skeleton and the triangular filling layer. The triangular voids and surface cracks were eliminated in the formed struts, resulting in a higher strength of mullite RPC than that of hollow strut. In addition, FEA results indicated that the residual compressive stress formed in the coating of strut after mullite RPC was sintered at 1450 °C and 1500 °C, while the residual tensile stress formed at 1550 °C. The larger residual compressive stress within the strut coating was beneficial to enhance the thermal shock resistance of mullite RPC.

Magnesia-chrome refractories have been essential materials for application as wear lining in copper production furnaces for many years. Formation of hexavalent chromium (Cr(VI)) has been a major concern despite its admirable corrosion resistance in copper slag. Magnesia-chrome refractory specimens (9, 12, 15 and 18 wt% Cr2O3) with addition of Al2O3 and TiO2 were fabricated and subsequent corrosion tests using copper slag was conducted in air. The corrosion mechanism between magnesia-chrome refractories fabricated and synthetic copper slag were investigated by means of X-ray diffraction (XRD) and scanning electron microscope (SEM), and concurrent formations of Cr(VI) were assessed effectively by leaching test. All the magnesia-chrome specimens prepared are composed of periclase and composite spinel Mg(Al,Cr,Ti)2O4 phases after firing at 1700 °C. Densification of the specimens decreases with the increase in Cr2O3 content due to the increased volume expansion, while TiO2 addition can improve their densification as mass transfer is enhanced during the sintering process. The corrosion resistance of the specimens decreases slightly with the Cr2O3 content due to the increased apparent porosity, which would result in penetration of more corrosive slag at the initial stage of the corrosion test while a newly formed (Mg,Cu)(Cr,Fe)2O4 spinel dense layer between the slag and penetration layer effectively hinders further penetration of slag and well protects the specimens. The Cr(VI) in the specimens prepared increases obviously with the Cr2O3 added content, while the TiO2 addition can suppress the formation of Cr(VI) effectively and the copper slag can cause the formed Cr(VI) to be reduced further during the corrosion process. For all the specimens, concentrations of Cr(VI) in the leachates is much lower than the US-EPA limit of 100 mg/kg but exceeds the European limit of 2 mg/kg after the corrosion test.

The in-situ catalytic formation of nanocarbons and ceramic bonding phases in low–carbon magnesia–graphite refractories is one of the significant strategies for heightening their mechanical properties and thermal shock resistance. Here, effect of aluminum content and nickel–containing catalyst addition on microstructural evolution, mechanical and thermo-mechanical behavior of such refractories was explored. Under the function of the catalyst, addition of aluminum powders allowed the newly growth of plenty of nanocarbons (e.g., carbon nanotubes and carbon onions), and also accelerated the in–situ formation of more ceramic bonding phases (e.g., magnesia whiskers and spinel whiskers/particles, etc.) in samples. This occurrence optimized significantly the microstructure of samples, correspondingly giving rise to their superior mechanical properties and thermal shock resistance. This work might provide a path for exploring low–carbon magnesia–graphite refractories with high performances.

The ultra-low cement bonded Al2O3-SiC-C castables were prepared with the introduction of a novel Si2BC3N antioxidant. The microstructure evolution and the mechanical properties were evaluated in coke bed and air atmosphere. Besides, the thermal properties, including thermal shock, hot modulus of rupture, oxidation and refractories under load, were comparatively investigated. The results show that the Si2BC3N powder together with B4C and Si can satisfy the oxidation resistance requirements over a full temperature range. Si2BC3N has mainly two effects depending on the treating temperature: 1) it protects the carbon from oxidation and increases the structure integrity when the specimens are treated below 1100 °C; 2) it stimulates the growth of SiC whiskers under 1400 °C due to the enhanced reaction between SiO and CO. Consequently, the CMOR and CCS of the Si2BC3N containing specimens have been improved attributing to the structural integrity and more SiC whiskers formation, regardless of the treating atmosphere. Besides, the thermal properties such as the hot modulus of rupture, thermal shock and refractories under load are also optimized with Si2BC3N addition.

The phase composition and microstructural evolution of pitch-containing matrix sample with additive of silicon had been compared with pitch-free or resin-containing matrix sample to illustrate the strengthening effect of pitch. Two different pitch powders (CARBORES@P and High temperature pitch) were then incorporated into carbon blocks, respectively and the effect of pitch powder addition on microstructure and properties of carbon blocks fired at 1000 °C and 1400 °C in a coke bed was evaluated systematically. The results showed that compared with amorphous carbonized resin, carbonized pitch was a kind of highly graphitized carbon and could react with silicon and form SiC whiskers at 1400 °C. In carbon blocks, pitch powder accelerated the formation of AlN at 1000 °C and growth of β-SiC whiskers at 1400 °C, respectively, which enhanced the cold compressive strength, thermal conductivity and hot metal erosion resistance of carbon blocks. Moreover, carbon blocks containing CARBORES@P pitch with higher carbon yield exhibited better properties because of formation of more ceramic whiskers. The strengthening mechanism of pitch powder for carbon blocks was attributed to the pore-blocking effect of pitch carbonization and more in-situ formed whiskers derived from the reaction between carbonized pitch with silicon at 1400 °C.

Novel low carbon Al2O3-C refractories were prepared through adopting chemical vapour deposition (CVD) synthesized nano carbon decorated Al2O3 powder. The phase compositions, microstructures, mechanical properties and thermal shock resistance of Al2O3-C refractories were characterized and evaluated. The results show that the morphologies of nano carbon composites are mainly dominated by the concentration of catalyst. Specifically, the growth of MWCNTs is preferred with a Ni2+ concentration at 0.1 mol/L, while higher concentrations e.g. 0.3 mol/L would stimulate the formation of nano-onion like carbon. With the introduction of nano carbon decorated Al2O3 additives, the residual strength after thermal shock can reach 12.4 MPa, which is much higher than the 2 wt% nano carbon black containing specimens (6.4 MPa). The enhanced thermal shock resistance should be attributed to that the nano onion-like carbon reduces the cohesion between the matrix and the Al2O3 particles and decreases the thermal expansion coefficient.

To lower the carbon content but to exhibit a better thermal shock resistance with MgO-C refractories containing 14 wt% flaky graphite, a new approach, based on the addition of artificial graphite, is reported for enhancing the thermal shock resistance of such refractories with 10 wt% graphite in the present work. The addition of artificial graphite (not more than 2 wt%) has a slight influence on the flexural strength of the specimens, but apparently enhances their thermal shock resistance. In particular, the specimen containing 2 wt% artificial graphite has a higher flexural strength after thermal shocks and a relatively closer residual strength ratio as compared to the reference specimen with 14 wt% flaky graphite, as it is related with the formation of more AlN reinforced phases, decreased coefficient of thermal expansion as well as increased work of fracture.

Expanded graphite with nano SiC and amorphous SiCxOy coating was successfully prepared through pyrolysing silane coupling agent (SCA), where the grafting of SCA dominated the final products. The results show that mainly amorphous SiCxOy coating covers expanded graphite at 1000 °C, regardless of the SCA concentration. In comparison, nano SiC coating can be synthesized at 1200 °C depending on the good dispersion of SCA (with a SCA concentration of 50 vol%). The formed SiC coating contributes to much higher peak oxidation temperature (812.1 °C) than 678.0 °C of the pure expanded graphite. Meanwhile, the oxidation activation energies of expanded graphite are remarkably improved from 149.15 kJ/mol to 176.16 kJ/mol (based on Kissinger method), attributing to the derived nano SiC and SiCxOy coating.

Expanded graphite (EG) is a promising reinforcement for fabricating Al2O3-C refractories. However, the smooth surface of EG layers results in weak interfacial bonding strength between EG and matrix. Meanwhile, the EG is prone to react with antioxidants or reactive vapors (Al (g), Si (g) and SiO (g)) at high temperature. To overcome these problems, two kinds of silicon hybridized EG were introduced into Al2O3-C refractories. They are β-SiC whiskers hybridized EG (labeled as EG/SiC) and SiOx spheres hybridized EG (labeled as EG/SiOx), respectively. Afterward, the microstructure and mechanical properties of Al2O3-C refractories coked at the temperatures ranging from 800 °C to 1400 °C were investigated in this work. The results showed that the interfacial characterization of EG/SiC and EG/SiOx hybrids were remarkably improved compared with original EG. Moreover, the EG/SiC and EG/SiOx hybrid remained relative intact structure at high temperature. In addition, the mechanical properties such as cold modulus of rupture (CMOR), force and displacement of Al2O3-C specimens containing hybrids were improved in comparison with the specimen containing original EG. Furthermore, with the addition of EG/SiC or EG/SiOx hybrid, the thermal shock resistance of Al2O3-C refractories was considerably promoted. The above enhanced mechanical properties could attribute to the hybridized β-SiC whiskers and SiOx spheres on EG, which were pulled out to consume the fracture energy when the Al2O3-C specimens were bearing loading.

Boron and nitrogen doped expanded graphite (B-doped EG & N-doped EG) were synthesized for the first time by annealing the mixtures, obtained from the suspensions of expanded graphite (EG) with boric acid and urea, respectively. Afterwards, the doped EG were introduced into Al 2 O 3 -C refractories by partially replacing the graphite flake. Synthesized materials were characterized using SEM-EDS, XPS, XRD, TG-DSC and Raman Spectroscopy. Also, evaluation of various mechanical properties and oxidation resistance tests were conducted. Boron (B) and Nitrogen (N) were successfully introduced into the C skeleton via different C-B bonds (B4C, B-sub-C and BC2O) and C-N bonds (pyridine-N, amino-N and graphitic-N) respectively, maintaining the hexagonal graphitic lattice and lower reactivity. Compared to as-received EG, the B-doped EG and N-doped EG remained relatively intact structure in Al 2 O 3 -C refractories at high temperature due to their less defects and lower reactivity. Addition of B-doped EG and N-doped EG into Al 2 O 3 -C refractories showed improved mechanical properties such as modulus of rupture (CMOR), force, displacement and thermal shock resistance. Besides, the Al 2 O 3 -C specimens containing B-doped EG and N-doped EG exhibited significantly better oxidation resistance, which further promote the reinforcement effect of doped EG in Al 2 O 3 -C refractories during actual service condition.