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Phase evolution of a novel silicon-alumina-fused mullite-containing Ti2O3 refractory at 1300 °C in N2
Abstract
A novel silicon-alumina-fused mullite-containing Ti2O3 composite refractory is prepared and sintered in the presence of solid carbon at 1300 °C in N2. The sintered samples exhibit a functional gradient characteristic. The phase evolution can be described as follows: Passive and active oxidation of Si to form SiO2 and SiO to reduce the partial pressure of oxygen. SiO(g) and Si react with N2 to form Si3N4 respectively. As the temperature increases and the partial pressure of oxygen decreases, Ti2O3 reacts with CO and N2 to form Ti(C,N)ss, which is accompanied by the release of O2. Si3N4 fixes the O2 and reacts to form Si2N2O, and Si2N2O reacts with Al2O3 to form O′-Sialon, thereby realizing the transformation from Si3N4 to Sialon. CO and residual carbon from the pyrolysis of phenolic resin react with SiO(s) and Si to form SiC. The dense layer formed by SiC and SiO2 blocks the diffusion of external gas to the central parts of the samples, there is still free Si which can continue to react and transform into a non-oxide reinforcing phase. In this paper, the reaction models are presented.
... As we all know that rutile TiO 2 is a very stable oxide chemi-cally, but when it is calcined with reducing agents such as C and Al at high temperatures, the presence of these reducing agents would reduce the partial pressure of oxygen in the reaction system, this urged the oxygen atoms to migrate outward continuously, and TiO 2 becomes unstable due to the breaking of Ti-O bond. Combined with the reports by Sun et al., 26 Zhu et al., 27 Wang and Zhou, 28 and Yue et al., 29,30 it can be inferred that the particle-like TiC 0.3 N 0.7 forms via the carbothermal reduction nitridation reactions (1) and (2), while the plate-like TiC 0.3 N 0.7 forms via the aluminothermic reduction nitration reaction (3). The Gibbs free energy of the reaction is an important means to judge the priority of different reactions. ...
The Al2O3–C refractories with low‐carbon content (w(C) ≤ 5%) are very crucial for the ultra–low‐carbon steelmaking. However, the properties of the Al2O3–C refractories like slag erosion resistance and thermal shock resistance deteriorated sharply if the content of graphite is reduced to below 5 wt%. To solve this problem, Al, Si, and TiO2 are used as additives to reconstruct a novel Al2O3–C refractory at elevated temperatures in nitrogen. The results indicate that Al4SiC4, (Al2OC)1–x(AlN)x and TiC0.3N0.7 phases were obtained in the matrix when the Al2O3–C refractories with Al/Si/TiO2 ratio of 9:2:2 were calcined above 1425°C. Benefiting from these new phases, the Al2O3–C refractory reconstructed at 1425°C exhibits excellent mechanical strength and thermal shock resistance; its cold modulus of rupture, cold crushing strength, and hot modulus of rupture are 20, 122, and 31 MPa, respectively, and its residual strength ratio after three times of air cooling at 1100°C reaches 75%. The thermodynamic calculations illustrated that the formation of flaky Al4SiC4 was the result of Si(l), Al(l), and C(s) through liquid–solid reaction. The novel Al2O3–C refractory prepared in this work has great application prospects in the field of ultra–low‐carbon steelmaking.
In order to obtain Al–Al 2 O 3 –ZrO 2 composite refractories with excellent oxidation resistance, erosion resistance and thermal shock stability, metallic aluminium and fused Al 2 O 3 –ZrO 2 were used as the raw materials, while phenolic resin was used as the binding agent. The microstructure was characterised by means of X-ray diffraction and scanning electron microscopy tests. Based on the reaction between zirconia and metallic aluminium under N 2 flowing at 1300 °C, the reaction mechanism of zirconia and metallic aluminium in-situ reaction at 1300 °C to generate (Al 2 OC) x (AlN) 1− x solid solution, zirconium nitride, and aluminium–zirconium alloy was revealed, and a model for the evolution of the phases in the aluminium-added Al–Al 2 O 3 –ZrO 2 specimen was constructed to realise the Al–Al 2 O 3 –ZrO 2 composite refractories phase controlling. Based on the good corrosion resistance of ZrO 2 to CaO and the excellent results of the in-situ reaction of metallic aluminium to generate a large number of non-oxide reinforcing phases capable of replacing elemental C, this work is of great significance for the study of refractory in erosion-heavy environments, such as calcium-treated steel.
This paper introduces a novel process to efficiently utilize medium-/low-grade or waste bauxite. The medium-/low-grade or waste bauxite is usually rich in iron oxide (total iron oxide content≥8%) or titanium dioxide (titanium dioxide content≥5%). In addition, both iron oxide and titanium dioxide have higher contents than typical bauxite (total iron oxide + titanium dioxide contents≥20%). It is difficult to obtain mullite with excellent properties using the process of beneficiation and sintering synthesis. Based on different reduction temperatures of K2O, Na2O, FeOx, SiO2 and TiO2, the reduction of K2O, Na2O and FeOx was set as the reduction end point. While SiO2 start to be reduced, TiO2 was unstable under high temperature and low oxygen partial pressure. The reason for the final formation of Ti2O3 in the TiO2→TiO2-x transformation process was analysed, and the morphology of the multiphase material obtained from the Ti2O3–3Al2O3·2SiO2 solid solution was characterized. The reaction model was established.
β-SiAlON/TiC0.3N0.7 composites containing 2.0-7.5 wt.% TiC0.3N0.7 are prepared by pressureless spark plasma sintering (SPS) with Al2O3-AlN-Y2O3 additives. The TiC0.3N0.7 phase particles are homogeneously dispersed in the β-SiAlON matrix. The reaction between the sample and the graphite mold can be avoided during the pressureless SPS. The β-SiAlON/TiC0.3N0.7 composite with 5 wt.% TiC0.3N0.7, fabricated at 1550°C, exhibits a high density of 3.28 g cm⁻³ (98% relative density), high Vickers hardness of 14.53 GPa, and high fracture toughness of 7.20 MPa m1/2. By further SPS under a pressure of 24 MPa at 1400 oC, these properties are enhanced to a higher density of 3.32 g cm⁻³ (full density), higher Vickers hardness of 15.87 GPa, and higher fracture toughness of 7.44 MPa m1/2. Obviously, the fracture toughness of the as-prepared β-SiAlON/TiC0.3N0.7 composites is apparently higher than those of the reported β-SiAlON-based ceramic composites prepared by pressurized SPS, which is mainly attributed to the highly dense microstructures of the interlocking lath β-SiAlON grains and the uniformly distributed TiC0.3N0.7 particles, and the disappearance of the oriented ununiform stress caused by uniaxial pressure in normal pressurized SPS. Accordingly, the pressureless SPS could be a potential technique for the fabrication of high-performance β-SiAlON matrix composites.
SiAlON/TiN composites with nominal composition of α:β = 25:75 were fabricated by microwave sintering. The effect of titanium nitride addition on the phases, microstructure, microwave absorption ability and mechanical properties (Vickers hardness and fracture toughness) of the SiAlON-based composites were studied. Finite Difference Time Domain (FDTD) software was used for the numerical simulation in order to assess the most suitable experimental setup. Sintering trials were performed in a single mode microwave furnace operating at 2.45 GHz and a power output of 660 W, for a reaction time of 30 min. SiC blocks were used as a susceptor to accelerate the microwave processing by hybrid heating, with reduced heat losses from the surface of the material of the α-β SiAlON/TiN composites. The optimum comprehensive mechanical properties, corresponding to a relative density of 96%, Vickers hardness of 12.98 ± 1.81 GPa and Vickers indentation fracture toughness of 5.52 ± 0.71 MPa.m 1/2 were obtained at 850 • C when the content of TiN was 5 wt.%.
There are limited studies on the tribological properties of SiAlON-TiN composites. Therefore, in the present study, tribology tests were conducted on a number of αⁱ/βⁱ-SiAlON-TiN composites with different αⁱ/βⁱ-SiAlON phase ratio and TiN content, fabricated with unique compositional design. The influence of αⁱ/βⁱ-SiAlON phase ratios, microstructure, mechanical properties and TiN content on friction and wear behavior was investigated and wear mechanisms were explained. Tribology tests were conducted on computer controlled tribometer under dry unlubricated ambient conditions with a linear reciprocating movement in a ball-on-disc sliding wear configuration and test parameters kept as constant. It was observed that TiN addition (17 wt.%) did not change the friction (CoF) of SiAlON and wear rate and wear volume were observed to increase. Wear mechanisms showed differences with αⁱ:βⁱ-SiAlON phase ratio. Fracture toughness had very pronounced effect on wear resistance.
Abstract
In this study, fully dense β-SiAlON / TiN composites were produced by Spark Plasma Sintering (SPS) method. Si3N4, Al2O3 , AlN and TiO2 powders were used as precursors. Starting powders were mixed with high energy ball milling and then were sintered by SPS method (at 1750 ̊C under pressure of 30 MPa for 12 min.). The milled powders had an average particle size of below ~ 155 nm. The XRD patterns of SPS-ed composites showed that the entire β-SiAlON phase constituent was in the form of Si4Al2O2N6 phase and cubic TiN phase can be formed by the phase transformation of TiO2 in relation with other precursors. FESEM micrographs confirmed that TiN particles were distributed homogeneously throughout β-SiAlON matrix. Mechanical properties evaluation revealed that by adding micro sized TiO2, optimal mechanical properties with a hardness ~ 14.6 GPa and a fracture toughness ~ 6.3 MPam1/2 were obtained. The improvement in the fracture toughness was attributed to the presence of the crack deflection as the dominant toughening mechanism in the SPS-ed β–SiAlON / TiN composites.
In modern steelmaking the duration of a working campaign for a blast furnace is related to the life of the crucible. Adding titanium oxide has been a frequent practice in the operation routines for modern blast furnaces, seeking the protection of the crucible walls, independently from its physical or chemical characteristics. These practices, as conventional operation of iron and steelmaking installations, present both advantages as well as undesirable consequences. The work proposes the incorporation of rutile (TiO 2) or illmenite (FeO·TiO 2) in the refractory matrix of the linings, as a practice that results in a protection of the crucible without altering, under any circumstance, the regular operation of the installation.
αâ²-Sialons are a relatively new class of ceramics that promise excellent high-temperature mechanical properties and thermal shock resistance. This report reviews the current status of research on αâ²-sialons, including phase equilibria, formation, sintering, and properties.
An effective approach for preparing electrically conductive SiAlON-TiN composites have been developed. Granules of a designed composition of alpha-beta SiAlON were obtained by spray drying and coated with varying amounts of TiO2 powder (0-10 vol%) homogenously by mechanical mixing. Fully dense composites were obtained by spark plasma sintering (SPS) under a pressure of 50 MPa at 1650 degrees C for 5min. According to the scanning electron microscope (SEM) analysis, unique microstructures containing continuously segregated in-situ formed TiN phase in 3D were achieved. Additionally, X-ray diffraction (XRD) studies revealed that all TiO2 was successfully converted to TiN. The resistivity of the alpha-beta SiAlON (1 x 10(11) Omega center dot m) was drastically reduced down to 2 x 10(-4) Omega center dot m at 5 vol % TiO2 addition.
Over recent years, owing to the need to increase productivity, it has been necessary among other things to increase the hot metal temperature in the blast furnace hearth. As a result, refractory lining wear becomes more intense. While publications present a host of comparative tests for hearth refractories and their performance, hardly any information is available in such publications regarding reaction mechanisms and further deterioration of these materials. Based upon comparative laboratory test results using two carbon materials of different concepts, the various reaction mechanisms and enhanced refractory lining deterioration for the blast furnace hearth have been identified. From an understanding of these wear mechanisms acting upon the hearth lining, it has been decided to introduce some practical operating measures, with a view to extending the blast furnace campaign.
The erosion of hearth refractory is a major limitation to the campaign life of a blast furnace. Titanium from titania addition
in the burden or tuyere injection can react with carbon and nitrogen in molten pig iron to form titanium carbonitride, giving
the so-called titanium-rich scaffold or buildup on the hearth surface, to protect the hearth from subsequent erosion. In the
current article, a mathematical model based on computational fluid dynamics is proposed to simulate the behavior of solid
particles in the liquid iron. The model considers the fluid/solid particle flow through a packed bed, conjugated heat transfer,
species transport, and thermodynamic of key chemical reactions. A region of high solid concentration is predicted at the hearth
bottom surface. Regions of solid formation and dissolution can be identified, which depend on the local temperature and chemical
equilibrium. The sensitivity to the key model parameters for the solid phase is analyzed. The model provides an insight into
the fundamental mechanism of solid particle formation, and it may form a basic model for subsequent development to study the
formation of titanium scaffold in the blast furnace hearth.
The effects of partial pressure of CO (PCO) on the product phases formed through titanium dioxide (TiO2) carbothermal reduction and nitridation were studied. Electrode graphite and anatase TiO2 were used as the raw materials, while the reaction was carried out under a flowing nitrogen gas atmosphere at 1,380 °C. The effects of PCO on the phase compositions, chemical compositions and microstructures of the nitridation products were investigated by adjusting the PCO in the system. The chemical mineral compositions, and microstructures of the products were characterised via scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction. The results demonstrate that the product phases are mainly titanium nitride (TiN)0.96 and residual graphite when the PCO is at 0.003 atm. As the value of PCO reached 0.08 atm in the furnace, a phase of Ti(O0.91, C0.53, N0.32) gradually began to form, while when the CO content in the atmosphere was over 0.12 atm, a phase of Ti(C0.2, N0.8) could be observed in the products. With the increase of PCO in the system, the evolution sequence of the reaction products was found to be TiO2→TiN→Ti(O, C, N) →Ti(C, N). By controlling the partial pressure, the synthesised temperature of titanium carbonitride (Ti[C, N]) can be significantly reduced, and the manipulation of the TiN and Ti(C, N) can be effectively realised, which could provide new ideas for experiments closely related to the partial pressure of gases.
Mullite-SiC-O′-SiAlON composites were prepared using bauxite-based homogenized mullite, silicon powder and phenolic resin as raw materials. The phase composition, microstructure and properties of samples with various Si addition calcined at 1500 °C under reducing atmosphere were investigated. The results indicated that the mullite-SiC-O′-SiAlON composites were obtained at 1500 °C in carbon embedded condition by firing Si and bauxite-based mullite mixtures. The whisker-like SiC and O′-SiAlON in the composites were formed because of the reactions between Si and C, CO or N2, and the growth of whiskers followed VS and VLS mechanisms. Both cold and high temperature strength, and thermal shock resistance of the composites increased with increase of silicon powder addition, and the optimized Si addition was 10 wt%. The improved properties at high temperature were due to the non-oxide whiskers filling in the skeleton of mullite and forming interlocking network structure as well as direct bonding between SiC whiskers and mullite, creating strengthening and toughening effects.
Sialon ceramics were prepared by the nitridation of Si, Al and Al2O3 powders with the transition metals (Fe, Co, Ni) catalysts. The phase composition, microstructure and mechanical properties of specimens were studied. Compared with the catalyst-free specimen, the addition of Fe, Co and Ni greatly promoted the nitridation process of Sialon ceramics and improved the weight gain rate and mechanical properties of specimens. With the increased content of catalysts, the apparent porosity of the products decreased first and then increased, contrary to flexural strength and compressive strength. The specimen with 2.5 wt% Fe had the highest flexural strength 145.8 MPa, and it exhibited the highest compressive strength of 594.6 MPa when Co content was 2.5 wt%, whereas they were 71.9 and 78.7 MPa, respectively, when without catalyst.
Brown fused alumina is a cost-effective alumina material, and the state of Ti in alumina has a great influence on its high-temperature performance. In this paper, the Ti-containing phases in brown fused alumina particles and Al-brown fused alumina refractory were successfully transformed into Ti(C,N) at 1973 K in flowing N2. The evolution of the Ti-containing phases in brown fused alumina under high temperature and nitrogen conditions was investigated by XRD, SEM and EDS. The results show that the Ti-containing phases in brown fused alumina include Ti2O3, Ti(C,N,O), TiFeSi2, Ca0·95Mg0·9Al10·1(Ti)O17 and a low-melting point Ca3Al2Si3(Mg,Ti)O12 phase. Under high temperature and nitrogen conditions, the TiO2[liquid], MgO[liquid] and SiO2[liquid] in the low-melting point phase are transformed into Ti(C,N), Mg(g) and SiO(g), while they are supplemented from Ti2O3, Ti(C,N,O) and Ca0·95Mg0·9Al10·1(Ti)O17. After heat treatment at 1973 K for 3 h, Ti2O3 and Ti(C,N,O) disappear, Ca0·95Mg0·9Al10·1(Ti)O17 is transformed into plate-like Ca0·55Al11O17.05, and Ti(C,N) is formed on the surface of the corundum particles. The formation of Ti(C,N) reduces the porosity of the brown fused alumina particles and increases their strength.
β-Sialon powder containing nanofibers was synthesized at 1300–1450 °C for 5 h in nitrogen by using transition metals (Fe, Co and Ni) as catalysts. Such synthesis conditions obviously reduced the nitriding temperature (~100 °C) of β-Sialon and increased the degree of nitridation. Observations from the weight gain rates of specimens relative to the content of catalysts suggest that the nitridation process was accelerated sharply via the addition of 0–2.5 wt% metallic catalysts. In addition, the amount of nanofibers of β-Sialon found in the specimens increased with the amount of transition metals, which was governed by the VLS formation mechanism with the help of transition metals. It could be concluded that transition metals not only greatly accelerated the nitridation of Si but also facilitated the formation of β-Sialon nanofibers in the specimens, as the formed metal-containing liquid greatly enhanced the amount of vapor generation.
Properties such as high hardness, low density, and high elastic modulus have made SiC ceramics proper choices for a variety of industrial applications. However, disadvantages such as low sinterability, and low fracture toughness have limited the fabrication of these ceramics. Past researches show that the use of Al 2 O 3 -Y 2 O 3 additives play an important role in improving the sinterability and the properties of the composites. The use of oxide, carbide, nitride and boride additives results in improved sinterability, physical and mechanical properties. The investigations show that the microstructure, porosities, amount of additives, reaction of additives with the matrix, grain size and, finally, the sintering temperature are the most important factors affecting the properties of SiC ceramics. In this paper, the effect of using various additives, the sintering temperature and the annealing heat treatment on sinterability, microstructure and properties of the SiC matrix composites fabricated by pressureless sintering method have been investigated.
The combined effect of carbon and Fe-Si alloys on Si3N4 was explored by heat treating Si3N4 materials at 1500 °C and 1600 °C in flowing nitrogen. The phase compositions and microstructures were characterized by XRD and SEM, respectively. The reaction degree was analysed based on the mass variation in the system. Combined with a thermodynamic assessment, the reaction mechanism was studied and proposed. The results show that the coexistence of Fe-Si alloys and carbon accelerates the phase transformation from Si3N4 to SiC and worsens the strength of Si3N4 materials. Fe-Si alloys accelerate the deposition of CO gas to free carbon and accelerate the decomposition of Si3N4 to Si. The in situ-formed Si can react with carbon, thus accelerating the thermodynamic and kinetic formation of SiC. Along with the growth of pores and the deterioration of the wettability of Fe-Si alloys during this process, the microstructure changes from a network constituted by Si3N4 columns/whiskers to porous SiC particles with weak linkages, which leads to the failure of Si3N4 materials. Therefore, the combined effect of Fe-Si alloys and carbon is harmful for Si3N4 materials at 1500–1600 °C.
Relationship between z value and intergranular phase (IGP) chemistry has not been completely developed to date for SiAlON based materials. In this study, the effect of z value of βı-SiAlON on crystallization and coalescence of IGP was investigated in αı/βı-SiAlON doped with Yb2O3-Sm2O3-CaO sintering additives and reinforced with TiN particles. αı/βı-SiAlON-TiN composites with compositions yielding different z values (0.3, 0.5, 0.7, 0.9 and 1.1) were prepared and sintered with gas pressure sintering route. Post sintering heat treatment was applied to sintered samples in order to enhance crystallization and coalescence of IGP. z value was proved an effective parameter on the intergranular phase chemistry and crystallization which are further influenced by post sintering heat treatment. No noticeable influence of heat treatment was observed on mechanical properties.
The formation mechanism and thermodynamics of Si3N4 in reaction-bonded Si3N4-SiC materials were analyzed. There are two kinds of Si3N4, fibroid α-Si3N4 and columnar β-Si3N4, which are formed by different processes in Si3N4-SiC materials. Silicon reacts with oxygen, forming gaseous SiO and reducing oxygen partial pressure. SiO(g) diffuses from central to peripheral sections of blocks and reacts with nitrogen, thus forming Si3N4, mainly in peripheral sections. The reaction between silicon and oxygen causes the consumption of oxygen and leads to low oxygen partial pressure in the sintering system, which allows silicon to react with nitrogen directly generatingSi3N4in situ. SiO(g) reacts with nitrogen forming Si3N4 at both central and peripheral sections of block. The non-uniform distribution of Si3N4 and uneven microstructure is caused by the generation process, indicating that it is unavoidable in Si3N4-SiC composites.
Coating of α-β SiAlON granules with TiCN powder led to a continuous 3D electrically conductive network formation. Here, the stability of TiCN grains following gas pressure sintering (GPS) and its effect on the electrical properties of an α-β SiAlON/TiCN composite were analytically investigated by using transmission electron microscopy (TEM) based techniques. In situ formation of nano-sized SiC grains adjacent to TiCN and α-β SiAlON grains were observed. These SiC grains may play a role on the high electrical conductivity of α-β SiAlON/TiCN composite. Ti:C:N ratios from TiCN grains along the network showed that TiCN grains did not preserve their initial composition after sintering. Finally, Ti diffusion from TiCN grains into α-β SiAlON and triple junction phases was observed. The incorporation of Ti into the SiAlON crystal lattices contributes to the high electrical conductivity of α-β SiAlON/TiCN composite.
Si3N4–TiN–SiC composites were synthesized from TiSi2 and SiC mixtures via the combustion reaction under high nitrogen pressure. The nitridation mechanism of TiSi2 was analyzed. The results show that the nitridation of TiSi2 produced TiN and Si firstly, and Si3N4 phase was formed by the further nitriding of Si. The molten eutectic phase and its agglomeration between Si and TiSi2 formed one core-shell structure and affected the nitridation process. Under higher nitrogen pressure, the nitridation reaction was complete and the relatively dense Si3N4–TiN–SiC composites obtained. TEM observation revealed inhomogeneous Si3N4 grain size, amorphous phase, cavities, microcracks and dislocations, and graphite from the nitridation of SiC in the microstructure.
A colloidal processing route has been developed for the pressureless sintering of dense SiC with a low content of sintering additives. In this route, a sol–gel solution precursor of the sintering additives is deposited onto the surface of the SiC particles, achieving a uniform distribution of the sintering additives in the green compact. This in turn promotes complete densification at short sintering times, which is not otherwise achievable when the batch is prepared by the standard method of mechanically mixing powders. It is also shown that the resulting ceramic has improved sliding-wear resistance compared to its counterpart prepared by the classical method, with essentially the same rate of mild and severe wear but a notably delayed transition from the mild to the severe wear regimes. This improvement is attributed to the reduction in the microstructural defect size achieved by the colloidal processing. Implications for the fabrication of low-cost SiC ceramics for wear-resistance applications are discussed.
A new method is described for the preparation of essentially monophase X-phase sialon, Si12Al18O39N8, by silicothermal reduction of mixtures of kaolinite, γ-Al2O3 and elemental Si in purified nitrogen at 1450–1500 °C. Powder X-ray diffraction and solid state 29Si and 27Al MAS-NMR of reaction mixtures heated to 1200–1570 °C suggest that the reaction proceeds in several parallel steps, including the nitridation of the elemental Si to Si3N4, thermal decomposition of the kaolinite to mullite and amorphous silica, reaction between the amorphous silica and γ-alumina, and reaction between silicon nitride and the aluminosilicate phase. Two forms of O′-sialon also appear as minor phases, possibly formed in the early stages by nitridation of the silica or aluminosilicate. The addition of up to 3 wt% of Y2O3 to the reaction mixture increases the rate of all the reactions occurring above 1100 °C without changing the reaction sequence, but the X-phase product formed in the presence of Y2O3 retains a high proportion of SiO2N2 groups, by comparison with X-phase formed without Y2O3, in which SiO3N groups predominate. XRD and MAS-NMR show the fully reacted silicothermal product to be comparable with reputable X-phase sialon prepared by a two-stage carbothermal method.
A theoretical study is described on the effect of alkali attack on the sialon bond (Si6-zAlzOzN8-z) in silicon carbide refractories in the blast furnace environment. Phase diagram and thermochemical relationships indicate that raising the z-number of the sialon will confer increased resistance to alkali attack, due to the formation of higher melting point reaction products in the alkali condensation zone. However, at z-numbers greater than two there is also a greater possibility of deleterious α- and β-alumina formation in the lining. Theoretical and practical considerations indicate that the optimum bond composition to use in the lower stack is a sialon with a z-number of two.
Iron is shown to increase the tendency for Si3N4 to decompose by formation of stable Fe-Si alloys. The decomposition reaction is temperature- and pressure-sensitive, with increasing temperatures causing greater reaction. The liquid Fe-Si alloy produced at elevated temperatures solidifies on cooling and results in metallic inclusions that act as flaws and thus mechanical failure origins.
The kinetics of the reaction 35i(1)+2N2(g)=Si3N4(s) was studied by measuring the growth rate of Si3N4 crystals. Adding up to 1% Fe does not accelerate the reaction; 1% Ti and Hf enhance the reaction rate. The activation energies for the reaction for pure Si and for Si doped with 1% Ti and Hf are 460, 364, and 247 kJ/mol, respectively. It is suggested that solution of N2 in the molten Si, followed by its diffusion to the growing tip, is probably rate-controlling.
Recent developments in the understanding of the wear mechanisms of blast furnace refractories, and in laboratory testing methods, have been applied to the improvement of the resistance to alkali attack of silicon nitride-bonded silicon carbide refractories. Further work has led to the development of a SiC refractory bonded with a system consisting predominantly of a sialon phase, exhibiting enhanced physical properties and exceptional resistance to alkali attack and to the combined effects of oxidation and alkali aggression.
In this paper, a summary of the development of high-temperature silicon nitride (T>1200°C) is provided. The high-temperature capacity of various advanced commercial silicon nitrides and materials under development was analyzed in comparison with a silicon nitride without sintering additives produced by hot isostatic pressing. Based on this model Si3N4 composed of only crystalline Si3N4 grains and amorphous silica in the grain boundaries the influence of various sintering additive systems will be evaluated with focus on the high-temperature potential of the resulting materials. The specific design of the amorphous grain-boundary films is the key factor determining the properties at elevated temperatures. Advanced Si3N4 with Lu2O3 or Sc2O3 as sintering additive are characterized by a superior elevated temperature resistance caused by effective crystallization of the grain-boundary phase. Nearly clean amorphous films between the Si3N4 grains comparable to that of Si3N4 without sintering additives were found to be the reason of this behavior. Benefit in the long-term stability of Si3N4 at elevated temperatures was observed in composites with SiC and MoSi2 caused by a modified oxidation mechanism. The insufficient corrosion stability in hot gas environments at elevated temperatures was found to be the main problem of Si3N4 for application in advanced gas turbines. Progress has been achieved in the development of potential material systems for environmental barrier coatings (EBC) for Si3N4; however, the long-term stability of the whole system EBC-base Si3N4 has to be subject of comprehensive future studies. Besides the superior high-temperature properties, the whole application process from cost-effective industrial production, reliability and failure probability, industrial handling up to specific conditions during the application have to be focused in order to bring advanced Si3N4 currently available to industrial application.
Blast Furnace main trough is an industrial structure submitted to severe high temperature cyclic loading applied on its inner lining made of refractory concrete. The attempt to increase the lifetime of such a structure by numerical simulation requires a proper experimental characterisation of all materials involved and particularly of the refractory concrete. The present paper exposes an analysis of the conditions required for an experimental setup in accountancy with the material working conditions. Then, the development of a performing high temperature mechanical testing device aimed at characterising the castable behaviour in its service conditions is introduced. In particular an original extensometer allowing high temperature direct measurement of the specimen height variation has been developed. Lastly, results of an uniaxiale compression test carried out at intermediate temperature are presented and discussed.
Sialons and related nitrogen ceramics[J]
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Solid solubility of alumina in silicon nitride[J]
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