Granular Y2O3 ceramics obtained by sintering and high-frequency fusion

Conclusions Fused yttrium oxide and apparently other highly refractory oxides and compounds can be used to produce, by the direct high-frequency heating method, highly pure fused materials which together with sinered materials may find application in the production of granular structured refractories. An investigation of the sintering of granular ceramics from fused and sintered Y₂O₃ in relation to the grain size composition, temperature, and time of firing show that the sintering of granular bodies from Y₂O₃ generally obeys the law for the sintering of coarsely grained powders.We characterize the basic thermomechanical properties of granular structured ceramics from Y₂O₃. We detected no essential difference in the properties of ceramics based on fused and sintered Y₂O₃. The resulting refractories of granular structured Y₂O₃ have a sufficient density and strength, a high refractoriness under load of 2 kg/cm², low creep rates and volatilization rates, excellent electric insulating properties in vacuum and in air, and can be used as highly refractory construction materials.Translated from Ogneupory, No. 4, pp. 45–50, April, 1973.     

Phase relations and microstructure of molten specimens in the Al2O3-ZrO2—Mullite system

An installation for radiant heating and methods of x-ray phase analysis and microstructure investigation are used to study the phase relations and microstructure of molten cast specimens of the Al₂O₃ - ZrO₂ - mullite subsystem as a function of the melt's crystallization rate. The data obtained by thermal analysis are used to construct the liquidus and to refine the coordinates of invariant points in the given subsystem. The effect of Y₂O₃ and Na₂O on the phase composition and structure of the refractories and the optimal amounts of these admixtures for to manufacturing molten cast refractories under conditions of radiant heating are determined.Translated from Ogneupory, No. 6, pp. 19 – 22, June, 1994.     

Effects of different particle size of spinel and Y2O3 additive of the periclase-spinel refractory on the sintering densification and corrosion resistance to copper smelting slag

In order to analyze the sintering densification and copper smelting slag corrosion resistance of periclase-spinel refractories, the periclase-spinel refractories were prepared with fused magnesia, magnesia-rich spinel, industrial alumina, and yttrium oxide as the main raw materials. The different particle sizes of spinel in material and with or without Y₂O₃ additive were studied. The study demonstrated that: (1) The different particle sizes of spinel in periclase-spinel refractories can result in different effects. Adding particle spinel to the refractory can improve the strength and corrosion resistance of the periclase-spinel refractories. The addition of spinel and magnesia powders in the matrix resulted in cracks due to the great difference of coefficient of thermal expansion between magnesia and spinel. The reduction in bulk density and strength of the material decreased slag penetration resistance because of its poor sintering properties. While adding the alumina in the matrix can further fill the crack and prevent slag penetration by the volume expansion of in-situ reaction to form spinel. (2) The periclase-spinel refractories can be reacted with Cu slag to form a Mg₂FeO₄ insulating layer as the iron ion becomes oxidized. Adding Y₂O₃ in periclase-spinel refractories can result in grain boundary phase reconstruction, which can promote sintering densification, improve the slag physical infiltration resistance, and improve the chemical corrosion resistance of materials.     

Developments on the mechanical properties of MgO-MgAl2O4 composite refractories by ZrSiO4-3 mol% Y2O3 addition

Improvements and the effects of additions of ZrSiO₄-3 mol% Y₂O₃ into MgO-MgAl₂O₄ composite refractories on mechanical properties and thermal stress resistance parameters were investigated. Significant improvements were achieved on mechanical properties and R-R_st parameters up to ∼2 and ∼3-fold ratios. The major parameters improving mechanical properties and thermal behaviour of refractories were determined as follows: (i) the increase in resistance to crack initiation and propagation due to formation of Mg₂SiO₄ phase after decomposition of zircon; (ii) propagation of the microcracks formed in the structure for a short distance by interlinking each other; (iii) arresting or deviation of microcracks when reaching pores or ZrO₂ grains released after dissociation of zircon, located together with Y₂O₃ particles, and furthermore; (iv) co-presence of both intergranular and transgranular types of cracks, and with incorporation of zircon-Y₂O₃; (v) increase in density; and (vi) a significant reduction in MgO grain size.     

Ball milling assisted preparation of nano La–Y/ZrO2 powder ternary oxide system: Influence of doping amounts

Zirconia powder with good dispersion, fine particle size, and stability is used as high-quality raw material in many fields, such as ceramic materials and refractories. In this paper, the influence of lanthanum oxide (La₂O₃) and yttrium oxide (Y₂O₃) co-doped zirconia (ZrO₂) on its phase transformation behavior, phase stability, and microstructure were investigated. The ball milling method is applied to fabricate different amounts of La₂O₃-doped yttrium oxide stabilized zirconia oxide. Then, the powder obtained from ball milling was roasted using the microwave sintering method. The samples were characterized using XRD, FT-IR, Raman, SEM and BET to determine the optimal conditions for La₂O₃–Y₂O₃ co-doped ZrO₂ powder. The results showed that replacing part of Y₂O₃ with La₂O₃ increases zirconia powder's tetragonal and cubic phase, enhancing the fracture strength of the subsequent synthesized materials. At the same time, the stability of zirconia stabilized with La₂O₃ doping is significantly improved compared to that of Y₂O₃ alone. According to all analysis methods, when the doping amount is 2.8Y0.2La, the powder's phase composition, stability, particle size distribution, and dispersion degree are the best compared with other doping amounts in our study. The obtained powder has a smaller specific surface area, a lower surface energy, a smaller porosity, and a higher density. The samples under this condition can be better used in subsequent materials. The enhancement of various properties of zirconia can significantly prolong the service life of materials in practical applications.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.     

MgO−Al2O3−Cr2O3 refractories of increased corrosion resistance for nonferrous metallurgical furnaces

The paper sums up a study that had as its objective to develop MgO−Al₂O₃−Cr₂O₃ refractories of increased corrosion resistance for harsh service conditions. The refractories thus developed, designated PShKhM-1 and PShKhM-2, have better high-temperature strength and abrasion resistance than PKhS refractories manufactured commercially. The experimental refractories will enhance the durability of furnace and converter linings, extend their campaigns, step up their productivity, and reduce consumption of refractories and repair costs. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 1, pp. 33–37, January, 1998.     

Arc melting plant for synthesizing and producing fusion-cast refractories

Data are given on arc melting plant for synthesizing and producing fusion-cast refractories. The technical characteristics of experimental plant type EDP-600 with moving electrodes are such as to allow one to synthesize fused materials on the basis of the oxides Al₂O₃, MgO, Cr₂O₃, and ZrO₂, and also allows one to produce refractory components for industrial glass-melting furnaces. The DSPM-1.5 arc furnace is recommended for the industrial production of fusion-cast baddeleyite-corundum, high-alumina, and chromium-bearing refractories.     

Formation and properties of solid solutions of zirconium dioxide with oxides of rare earth elements

Conclusions Interaction of zirconium dioxide with oxides of cerium, yttrium and lanthanum in solid phases occurs at 1400°C with the formation of solid solutions with the cubic structure.Sintering of the specimens may result at 1700–1750°C with a 3-h soak. At 1400°C and a 6-h soak the porosity of the specimens was 30–40%.Complete stabilization of the zirconia is attained by heating to 1700–1750°C with additions of 20 mol.% CeO₂, 15% Y₂O₃ or 25% La₂O₃. An addition of ceria and yttria displaces the effects of polymorphic inversion of the zirconia to the lower temperature region.New highly refractory materials may be obtained from solid solutions of ZrO₂-20% CeO₂, ZrO₂-80% CeO₂, ZrO₂-15% Y₂O₃, ZrO₂-80% Y₂O₃ and ZrO₂-25% La₂O₃ and firing to 1750°C. Some of them have a low coefficient of thermal expansion compared with ZrO₂, stabilized with calcium oxide and magnesium oxide, and apparently better thermal-shock resistance. The advantage in regard to resistance during prolonged heating at 1200°C is possessed by the solution ZrO₂-Y₂O₃. The region of the most effective use of goods made from solid solutions of ZrO₂ with CeO₂, Y₂O₃ and La₂O₃ as highly refractory materials should be determined by extra studies.The possibility of reducing CeO₂ (fusing temperature about 2700°C) to Ce₂O₃ (fusing temperature about 1700°C) limits the use of cerium-containing materials as refractories in chiefly oxidizing conditions.     

Oxidation resistance,residual strength,and microstructural evolution in Al2O3-MgO–C refractory composites with YAG nanopowder

The decisive role of nanostructured yttrium aluminium garnet (YAG;Y₃Al₅O₁₂) powder addition on oxidation resistance, residual strength and microstructural evolution were studied in Al₂O₃-MgO–C refractory composites. Oxidation index and rate constant calculations indicated that the oxidation resistance was almost 70 % improved for the nano-YAG containing refractories oxidized in air at 1600 °C. Residual compressive strength (R_c) estimations showed that there was nearly 75 % strength retained in these oxidized refractories fortified with nano-YAG. Residual bending strength (R_b) estimations based on cyclic thermal shock, exhibited that there was almost 70 % thermal shock resistance enhancement in refractories reinforced with nano-YAG, showed a good agreement between R_b and R_c values. These beneficial properties were attributed to the formation of a well-sintered framework of YAG/Spinel bonding grains throughout the dense oxidized layer microstructure of these new class of refractories. The concept of interfacial toughening and implications of these results to practical applications are discussed.     

Study on the stability of different refractories during the melting process of a K4169 Ni-based superalloy

The composition of the refractory strongly affects the cleanliness of the alloy. K4169 Ni-based superalloys were melted in different types of refractories in this study. The cleanliness of the Ni-based superalloy and phase transformation of the refractory were observed by X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy energy dispersive spectroscopy (SEM‒EDS). The high-temperature stabilities of a Y₂O₃-based refractory, MgO-based refractory, and Al₂O₃-based refractory during melting with a Ni-based alloy were compared. The oxygen content was also lowest, and no Y₂O₃-containing inclusions were observed in the Ni-based alloy melted with the Y₂O₃-based refractory at 1823 K. Inclusions with 21%–29% MgO and a phase composed of Al, Mg and O with an area of approximately 1300 μm² were observed in the alloy. This indicates that the dissolution and erosion of the Y₂O₃-based refractory were weak, and obvious physical erosion and chemical dissolution of the MgO-based refractory occurred during the melting process of the Ni-based alloy. The width of the refractory phase adhered to the boundary of the Ni-based alloy increased in the order Y₂O₃-based refractory (15 μm- 23 μm)< Al₂O₃-based refractory (93 μm- 285 μm)< MgO-based refractory (3.5 mm–3.6 mm), indicating that the adhesive strength of the MgO-based refractory with the Ni-based alloy was strongest. The interaction between the refractory material, Ni-based alloy and inclusions was analyzed based on thermodynamic calculations by Factsage software. The effects of dissolution of the three refractory types on the formation and transformation of the new phases and inclusions were estimated. The thermodynamic results were in good agreement with the experimental results.     

Preliminary investigations of the production of MgAlON bonded refractories

The aim of the work was to find an appropriate composition for the formation of MgAlON bonding phase for Al₂O₃ and MgO based refractories. The first step was the preparation of pure MgAlON. AlN and Al₂O₃ were used as starting powders and either MgO or MgAl₂O₄ was added as a source of magnesium. The results verified the possibility to produce MgAlON under the prevailing conditions. Afterwards, MgAlON bonded alumina and magnesia refractories were investigated. The obtained results confirmed the possibility of the production of MgAlON bonded alumina refractories. However, in the case of magnesia-based samples MgAlON was not formed and instead stoichiometric spinel, AlN and alumina rich spinel were detected in the bonding phase. Additionally, the joining between the MgO grains and the matrix was poor with wide gaps on the interface.     

Change in the phase composition and properties of zirconia refractories under the prolonged effect of high temperatures

Conclusions A systematic study has been made of the behavior of ZrO₂ stabilized with various oxides under the prolonged action of temperatures up to 2300°C.The optimal concentrations of Y₂O₃, Nd₂O₃, CaO, and MgO in the zirconia refractories have been established; this ensures the successful service life of the articles under conditions of multiple prolonged action of high temperatures.The ultimately permissible temperature at which is it possible to operate the zirconia articles has been determined: with the stabilization of ZrO₂ by MgO it is 1900°C; with CaO, 2000°C; and with oxides of rare-earth elements, more than 2300°C.Translated from Ogneupory, No. 2, pp. 13–19, February, 1985.     

A novel approach to lightweight alumina-carbon refractories for flow control of molten steel

Functional refractory materials for flow control devices of molten steel in continuous casting used to be prepared from Al₂O₃–C refractories containing dense corundum aggregates. According to the traditional concept, the denser the refractories, the higher the strength of refractories. However, we prepared a new lightweight Al₂O₃–C refractory material using microporous corundum aggregates instead of dense corundum aggregates, which were reinforced by in situ formed SiC whiskers. A comparative analysis of microstructures and properties was carried out for conventional and lightweight Al₂O₃–C refractories with and without Si powder addition. We showed that microporous aggregates formed a better aggregate/matrix interface bonding and an improved distribution of SiC whiskers. The SiC whiskers formed not only in the matrix, but also inside of the microporous aggregates and at the aggregate/matrix interface by a vapor-solid reaction mechanism. Due to the formation of a microporous aggregate/matrix interface reinforced by SiC whiskers, the crack propagation along the aggregate/matrix interface was suppressed, whereas the percentage of cracks propagating within the aggregates was enhanced. Thus, the synergy between in situ formed SiC whiskers and microporous aggregates resulted in a significant higher strength of lightweight Al₂O₃–C refractories compared to conventional ones. The results therefore provide an original strategy to strengthen Al₂O₃–C refractories.     

Critical thickness of polymer-derived ceramic coatings with particulate fillers

In this study, we demonstrate a novel environmental barrier coating processed from polymer-derived ceramics (PDCs) with homogeneously distributed sub-micrometer Y₂O₃ as the filler. Under suitable conditions, dense and crack-free coatings can be achieved for all the designed compositions with the volumetric content of Y₂O₃ varied from 45 to 93 vol%. To process the PDC SiC–Y₂O₃ composite coatings, Y₂O₃ particles and SiC liquid precursor were uniformly dispersed in hexane and then dip-coated on SiC substrates. After cross-linking at 250°C and heat-treated at 900°C in argon, dense and crack-free PDC SiC–Y₂O₃ composite coatings were formed. The effect of coating thickness and heat-treatment temperature on the formation of cracks due to constrained pyrolysis was studied. The critical thickness for realizing crack-free coatings of three compositions (i.e., 93, 77, and 45 vol% Y₂O₃) was studied for heat treatment from 1000 to 1300°C using atomic force microscope and scanning electron microscopy. As heat-treatment temperature increases, the critical coating thickness decreases for the same coating compositions due to enhanced shrinkage at higher temperature. With higher Y₂O₃ content, the critical thickness of the coating increased. The inert Y₂O₃ particles reduce the amount of polymer leading to reduction in the overall constrained shrinkage of the coating during heat treatment.     

Mullite refractories from bauxites of the iksinskoe deposit

Bauxites with 76% Al₂O₃ and 2.65% Fe₂O₃ were used to produce mullite refractories and mixtures for steel-teeming ladles. Tests in service have shown that the strength of these refractories is no worse than or comparable with that of refractories based on commercial alumina and Chinese bauxites.Translated from Ogneupory, No. 2, pp. 28–31, February, 1995.     

Structural data from X-ray powder diffraction for new high-temperature phases (Y1–xLnx)2Si2O7 with Ln=Ce,Pr, Nd

Extending the investigation on possible mixed phases with composition (Y_1–xLn_x)₂Si₂O₇ formed at high temperature under oxidizing conditions in silicon nitride samples doped with Y₂O₃ and Ln₂O₃ as sintering aids, the compounds with Ln= Ce, Pr, Nd were successfully synthesized and structurally characterized by Rietveld whole powder pattern refinement. Some fluctuations in the x values were found as well, but all the compounds are monoclinic (space group P2₁/c), with lattice parameters close to those of the previously studied (Y_2/3La_1/3)₂Si₂O₇. Further efforts with Sm (and Dy) failed in producing the expected G-form phase, indicating that the family does not extend beyond Nd: in these cases Ln ions partially substitute Y in its already known α- and δ-disilicates. Under the adopted experimental conditions (1 atm, ambient air), an explanation of the impossibility to obtain the phase in question containing lanthanides with Z⩾62 was attempted on the basis of lattice strains introduced by foreign ions larger than Y.     

The effects of in situ formation of Y3Al5O12 on property improvement of magnesium aluminate spinel refractories

Magnesium aluminate spinel is widely used in cement rotary kilns, in the iron and steel industries, as well as in glass melting furnaces due to its excellent performance and chemical stability at both room temperature and elevated temperatures. In spite of these advantages, there are some practical problems during production of magnesium aluminate spinel refractories due to their poor sinterability: poor mechanical properties and poor creep resistance. These issues can cause problems during service. This study improved the sinterability of spinel refractories and in turn improved mechanical properties while decreasing the creep rate. This was done by forming a second low creep rate phase of yttrium aluminum garnet in the matrix structure. The addition of Y₂O₃ and reactive Al₂O₃ accelerated the densification process and increased the cold strength. There was a significant increase in the hot modulus of rupture due to the formation of YAG or the solid solution with spinel.     

Optimization of corrosion resistance and mechanical properties of Al2O3-Cr2O3 refractories for waste incinerator

The corrosion resistance and mechanical properties directly affects the operation and service life of Al₂O₃-Cr₂O₃ refractories used in waste incinerators. In this study, ZrO₂ particles were introduced via vacuum impregnation to adjust microstructure and properties of Al₂O₃-Cr₂O₃ refractories. The results showed that the impregnated ZrO₂ particles and increasing impregnation times resulted in the decreased median pore size and increased compactness, and mechanical strengths of refractories were elevated from the inhibited cracks propagation by ZrO₂ particles. The decreased amounts of large pores and increased amounts of small pores from the filled ZrO₂ particles inhibited penetration of low melting point phases, and the formed CaZrO₃ phase from the reactions between corrosion reagent and ZrO₂ particles increased the viscosity of penetrated corrosion reagent, resulting in the decreased penetration index from 8.57% to 2.58%. Meanwhile, the filled ZrO₂ particles around alumina particles prevented reactions between molten corrosion reagent and alumina, leading to the decreased corrosion index from 3.78% to .74%. The decreased pore size and formation of CaZrO₃ phase were primary factors that enhanced the penetration resistance. And formation of wrapped layers from ZrO₂ particles around alumina particles presented prominent effects on the strengthened corrosion resistance of Al₂O₃-Cr₂O₃ refractories.     

Coating Y2O3 nano-particles with ZrO2-additive via precipitation method for colloidal processing of highly transparent Y2O3 ceramics

In the present work, transparent Y₂O₃ ceramics were prepared via colloidal processing method using ZrO₂-coated nano-sized Y₂O₃ powders. The chemical precipitation method was adopted for the coating of Y₂O₃ raw powder. The evolution of the coated-ZrO₂ layer upon calcination was studied. The rheological behaviors of the slurries of Y₂O₃ powders coated with different content of ZrO₂-additive were investigated. The pH_IEP of ZrO₂-coated Y₂O₃ powders shows intermediate values between that of raw Y₂O₃ and ZrO₂ powders. As the ZrO₂-coating concentration increased from 0 to 5.0 at%, the magnitude of the negative zeta potential at pH > pH_IEP shows a general trend of increment, whereas it decreased at pH < pH_IEP. The viscosity decreases pronouncedly with the increase of ZrO₂ content from 0.5 at% to 3.0 at%. The suspensions with low viscosity and high stability was achieved for a solid loading of 35.0 vol% using Y₂O₃ powders coated with 5.0 at% ZrO₂. The dispersed suspensions were consolidated by centrifugal casting method and the green bodies shown improved homogeneity. Transparent Y₂O₃ ceramics were fabricated by vacuum sintering at 1800 ℃ for 5 h. Transmittance at wavelength 800 nm (1.0 mm thick) reached 80.8%, close to the theoretical value of Y₂O₃.