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.     

Effect of different contents of zirconia and yttrium oxide on microstructure and properties of high alumina ceramics

The effects of zirconia and yttrium oxide addition on microstructure, bulk density, microhardness, flexural strength, and wear resistance of high alumina ceramics (>97 wt% Al₂O₃, MSA ceramics) composed of MgO–SiO₂–Al₂O₃ system have been investigated. The results show that the addition of zirconia makes the mechanical properties and wear properties of ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂ (MSAZ ceramics) system have been greatly improved compared with MSA ceramics. In addition, the ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂–Y₂O₃ (MSAZY ceramics) system have better mechanical properties and wear properties than MSAZ ceramics. With the contents of zirconia and yttrium oxide increase, the bulk density, microhardness, and flexural strength of MSAZ and MSAZY ceramics increased at first and then decreased. However, the wear rate shows the opposite. When 0.4 wt% ZrO₂ and 0.6 wt% Y₂O₃ were added to the matrix, the wear rate of MSAZY ceramics reached a minimum of 0.042%, and the wear resistance was improved by about 73.8% compared with MSA ceramics with a wear rate of 0.16%. In addition, the optimum additions of zirconia and yttria are 0.4% and 0.6%, respectively.     

EXTRUSION OF REFRACTORY OXIDE INSULATORS FOR VACWM TUBES1

A method is described whereby nonplastic materials such as refractory oxides are given a temporary plasticity, sufficient for extrusion. The organic plastic gel used consists of flour or starch paste with ammonia as an electrolyte. A short detailed account is given of the manufacture of twin-hole insulators made of MgO 98% and talc 2% for radio 227 vacuum tubes, with supplemental experimental results on other materials. The refractory oxides Al₂O₃, BeO, ZrO₂, MgO, and ThO₂ are compared from the standpoint of their physical properties and their ability to act as insulators in contact with hot tungsten filaments. Examples are given of the use of such extruded oxide insulators for hot filament supports.     

The influence of highly refractory oxides on the thermo-electromotive force of tungsten,molybdenum and tantalum in vacuum at 1500°C

Conclusions Oxides of beryllium, aluminum, and magnesium practically do not affect the change in thermo-emf of tungsten. Zirconia interacts with tungsten at 1500°C, which leads to a change in its original emf.The thermo-emf of molybdenum does not alter during heating with magnesia, alumina, and zirconia. Beryllia slightly affects the change in thermo-emf of molybdenum.The magnitude and nature of the change in thermo-emf of tungsten and molybdenum during heating in MgO are identical. The readings from tungsten-molybdenum thermocouples are constant.The thermo-emf of tantalum is greatly changed compared with the original value during heating in powdered Al₂O₃, BeO, MgO, and ZrO₂, which hinders its use for thermocouples.     

Phase evolution mechanism of non‐oxide bonded Al–Al2O3–MgO–ZrO2 composites at 1873 K in flowing nitrogen

In flowing nitrogen, non‐oxides such as Al₄O₄C, Al₂OC, Zr₂Al₃C₄, and MgAlON bonded Al₂O₃‐based composites were successfully prepared by a gaseous phase mass transfer pathway using aluminum, zirconia, alumina, and magnesia as raw materials at 1873 K, after an Al–AlN core‐shell structure was formed at 853 K. Resin bonded Al–Al₂O₃–MgO–ZrO₂ composites after sintering were characterized and analyzed by X‐ray diffraction (XRD), scanning electron microscope (SEM) and, energy dispersive spectrometer (EDS), and the influence of the MgO content on the sintered composites was studied. The results show that after sintering, the phase composition of the Al–Al₂O₃–ZrO₂ composite is Al₂O₃, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄, while the phase composition of the Al–Al₂O₃–ZrO₂ composite with the addition of MgO 6 wt% and MgO 12 wt% is Al₂O₃, MgAlON, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄ as well as Al₂O₃, MgAlON, Al₂OC, and Zr₂Al₃C₄, respectively. The addition of MgO changed the phase composition and distribution for the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites after sintering. When the added MgO content is equal to or more than 12 wt%, the Al₄O₄C in the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites is unable to exist in a stable phase.     

Synthesis and characterization of a MgO-MgAl2O4-ZrO2 composite with a continuous network microstructure

Using analytically pure MgO, analytically pure Al₂O₃ and analytically pure ZrO₂ as raw materials, Mg_4.68Al_2.64Zr_1.68O₁₂ was prepared at 1993 K for 10 h, and then, a MgO-MgAl₂O₄-ZrO₂ composite with a continuous network was successfully obtained by controlling the cooling rate based on the in-situ decomposition reaction of Mg_4.68Al_2.64Zr_1.68O₁₂ at temperatures below 1887 K. The three phases of MgO, MgAl₂O₄ and ZrO₂ are highly dispersed in this continuous network microstructure, with ZrO₂ intertwined by MgO and MgAl₂O₄ and micropores with a size of less than 2 µm. Furthermore, the synthesis mechanism of Mg_4.68Al_2.64Zr_1.68O₁₂ is given as follows: first, MgAl₂O₄ is synthesized using the reaction: MgO(s)+Al₂O₃(s)=MgAl₂O₄(s) at temperatures below 1894 K; and then, Mg_4.68Al_2.64Zr_1.68O₁₂ is further prepared through MgO and ZrO₂ diffusion and dissolution into MgAl₂O₄ at temperatures above 1894 K, for example, at 1923 K or 1993 K in this work.     

DEVELOPMENT OF ZIRCONIA RESISTANT TO THERMAL SHOCK*

The effects on the crystal stabilization and other properties of pure zirconia were determined for additions of CaO, MgO, BeO, ThO₂, and CeO₂ in binary and ternary combinations. Complete stabilization was attained with the following compositions (in mol. %): 10 to 20 CaO, 14 MgO, 26 CeO₂, 10 CaO + 5 ThO₂, 10 CaO + 5 BeO, or 2 to 7 CaO + 4 to 6 to MgO. Cerium oxide produced partial stabilization, but BeO and ThO₂ used alone were ineffective at 1927°C. Some strong, dense bodies were found which had good resistance to thermal shock, namely, those containing (mol. %) 8 to 12 CaO, 6.7 CaO + 3.7 MgO, or 2.3 CaO + 6.4 MgO; all of these compositions still had a small amount of inversion present. This small inversion may be necessary for high resistance to thermal shock in ZrO₂ ware because (a) the over-all thermal expansion is then small and gradual and (b) with complete elimination of inversion, the expansion is large since the expansion curve is a straight line and the coefficient of expansion is rather high, 11 to 12 × 10^-6.     

High temperature wettability and corrosion of ZrO2, Al2O3, Al2O3-C,MgO and MgAlON ceramic substrates by an AZ91 magnesium alloy melt

Wettability and interfacial reactivity of various ceramics (ZrO₂, Al₂O₃, Al₂O₃-C, MgO, MgAlON) in contact with molten AZ91 were investigated in this paper. MgAlON and Al₂O₃-C are new materials to be used in magnesium melt filtration, a comparative study of its wettability and corrosion resistance is performed to evaluate their applicability. Modified sessile-drop tests were conducted, measuring contact angles between the ceramic substrates and molten AZ91 droplets at 680 °C; contact surfaces were analyzed by means of scanning electron microscopy. MgAlON showed the largest contact angles, indicating lowest wettability. Al₂O₃, Al₂O₃-C, and MgO proved non-wettable as well. Al₂O₃- and ZrO₂-containing substrates underwent noticeable interface reactions with molten AZ91.     

Thermal properties of ecological phosphate and silicate glasses

The present work deals with ecological phosphate and silicate glasses that belong to the oxide systems: Li₂O-MgO-P₂O₅, Li₂O-CaO-P₂O₅, Li₂O-MgO-P₂O₅-Fe₂O₃; Li₂O-CaO-P₂O₅-Fe₂O₃ and SiO₂-R₂O-R′O (R = Na, K; R′ = Mg, Ca), the last system contains certain amounts of ZrO₂, ZnO, TiO₂. These ecological glasses do not contain toxic substances as BaO, PbO, As₂O₃, As₂O₅, fluorine, CdS, CdSe and they have applications as regards the retention and counteracting action of the harmful compounds resulted from the nuclear plants. The replacement of MgO by CaO leads to an insignificant increasing of the thermal expansion index and a slight decreasing of the characteristic temperatures, except the softening point where the effect is opposite. Adding of iron oxide in the phosphate glass composition causes the increasing of characteristic temperatures and the decreasing of thermal expansion index, both in MgO and CaO-containing phosphate glasses. The ecological silicate glasses are used as opal glasses free of fluorine as well as for lead-free crystal glass (CFP) where BaO and PbO are replaced by non-toxic oxides as K₂O, MgO, ZrO₂, and TiO₂. The paper presents different glass compositions and the technological parameters to prepare the ecological glass samples. Both ecological phosphate and silicate glasses have been characterized as regards the characteristic temperatures (vitreous transition point, low and high annealing points, softening point) and the thermal expansion coefficient. This study presents the changes of the thermal parameters when CaO replaces MgO in phosphate glass samples and the role of iron oxide in the vitreous network. In the case of silicate glasses, the viscosity and wetting angle dependency of temperature are presented. The elemental analysis of the ecological glasses was made by XPS (X-ray photoelectron spectroscopy) which also put in evidence the iron species from the vitreous network.     

High temperature properties of B6O-materials

B₆O is a potential superhard material with a hardness of 45 GPa measured on single crystals. Recently it was found that different oxides can be utilized as an effective sintering additive which allows densification under low pressures. In this work the effect of addition of Y₂O₃/Al₂O₃ on high temperature properties was investigated using impulse excitation technique (IET), hardness measurements and dilatometric measurements. The IET technique reveals the softening of the residual B₂O₃ in the materials without additives at 450 °C; in the materials with Y₂O₃/Al₂O₃ the softening is observed at only about 800 °C. This data agrees with the values found for different borate glasses.The materials showed no pronounced reduction of hardness at these temperatures. This is additional evidence, supporting previous observations that the material consists of pure grain boundaries between B₆O grains. Hardness values (HV5) of up to 17 GPa at 1000 °C were observed.     

Composite Aluminum Oxide Ceramics with Fibrous Components

The reactions between oxides in the Al₂O₃ – ZrO₂ – MgO system in the production of ceramics using fibrous components are studied. It is established that under heat treatment of the ternary systems, the component stabilizing the tetragonal structure of zirconium dioxide reacts with the aluminum oxide matrix and forms spinel interlayers on the fiber – matrix interface. The use of highly disperse fibers as the initial component for producing ceramics and as a fibrous filler additive introduced into a gel-like matrix makes it possible to obtain composite ceramics of elevated strength.     

Effect of titanium dioxide on sintering and stabilization of ZrO2 in zirconium-alumina and spinel-zirconium mixtures

Conclusions The addition of titanium dioxide considerably reduces the sintering point of alumina and zirconia-alumina mixtures. Titanium dioxide does not have this effect on the sintering of the ternary equimolecular mixture ZrO₂MgO Al₂O₃. But the zero porosity was not even attained during firing up to 1700°. When the three-component mixture ZrO₂ + MgO + Al₂O₃ was sintered, spinel formed first, after which, at a higher temperature, there formed a solid solution ZrO₂ and MgO. In the presence of titanium dioxide some of the magnesium oxide apparently forms magnesium titanate with the TiO₂, and this impairs the stabilization of the zirconium dioxide, in view of which it is partially detected in monoclinic form in the fired mixtures. The addition of 2% TiO₂ reduces the temperature of polymorphous transitions of ZrO₂ by approximately 200°.Specimens of the composition 90% Al₂O₃ + 10% ZrO₂ and ZrO₂MgOAl₂O₃=111 show better spalling resistance than those made of alumina and those made of zirconium dioxide stabilized with magnesium or calcium oxide.Pure pre-synthesized spinel does not react with zirconium dioxide when fired up to 1600°, nor with titanium dioxide up to 1500°.The coefficient of thermal linear expansion of the equimolecular mixtures ZrO₂-MgO-Al₂O₃ and ZrO₂-CaO-Al₂O₃ is considerably lower than that of the corresponding mixtures without alumina.When the three component equimolecular mixtures ZrO₂-CaO-Al₂O₃ is sintered, calcium aluminate and the solid solution ZrO₂-CaO are formed.The two-component compositions Al₂O₃-ZrO₂ and three-component MgO-Al₂O₃-ZrO₂ have high refractoriness, satisfactory spalltng resistance, good stability-underload at high temperatures, and can be used as super duty refractories.     

Some principles of the recrystallization of oxides of the M2O3 composition

Conclusions In the series of oxides under discussion, A1₂O₃, Sc₂O₃, and Y₂O₃, the greatest tendency for recrystallization is found in Al₂O₃. Marked structural changes occur at 1400°C. The Y₂O₃ and Sc₂O₃ oxides are stable up to a temperature of 1600°C.The method of preparing the ceramic has a significant effect on the absolute recrystallization rates of the material. The structure of the sintered materials after a 20–50 h dwell at 1400–1800°C is stabilized while there is a monotonic increase in the size of the grains for hot-pressed specimens after heat treatment for 50 h.The addition of small quantities of MgO to A1₂O₃ and ThO₂ and ZrO₂ plus W to Y₂O₃ is very effective in preventing active recrystallization. A prolonged thermal dwell in vacuo leads in the case of an Al₂O₃-based ceramic to the intense redistribution of impurities and additives.Translated from Ogneupory, No. 6, pp. 46–51, June, 1980.     

Growth mechanism of AlN crystals via thermal nitridation of sintered Al2O3–ZrO2 plates

The crystal growth of AlN from aluminum oxides was studied using a thermal nitridation method. Four types of aluminum oxides, sintered Al₂O₃, ZrO₂-containing sintered Al₂O₃, and a- and c-plane sapphires, were used as a source material. As observed, millimeter-sized AlN crystal grains were successfully grown from the ZrO₂-containing sintered Al₂O₃ only at temperatures ranging from 2223 to 2323 K. The growth mechanism, including the role of ZrO₂ additive, was discussed from a thermodynamic viewpoint. The following growth model was proposed: predominant nitridation of ZrO₂ in Al₂O₃ suppresses Al₂O₃ nitridation, and the ZrO₂–Al₂O₃ liquid phase forms, which promotes the formation of Al₂O(g) and Al(g) from Al₂O₃. These Al-based gases react with CN(g) and/or N₂(g) to form AlN crystals on the Al₂O₃–ZrO₂ plate.     

Fabrication of highly dense Si3N4 via record low-content additive system for low-temperature pressureless sintering

Dense Si₃N₄ ceramics were fabricated by pressureless sintering at a low temperature of 1650°C with a short holding period of 1 h under a nitrogen atmosphere. The role of ternary oxide additives (Y₂O₃–MgO–Al₂O₃, Y₂O₃–MgO–SiO₂, Y₂O₃–MgO–ZrO₂) on the phase, microstructure, and mechanical properties of Si₃N₄ was examined. Only 5 wt.% of Y₂O₃–MgO–Al₂O₃ additive was sufficient to achieve >98% of theoretical density with remarkably high biaxial strength (∼1200 MPa) and prominent hardness (∼15.5 GPa). Among the three additives used, Y₂O₃–MgO–Al₂O₃ displayed the finest grain diameter (0.54 μm), whereas Y₂O₃–MgO–ZrO₂ produced the largest average grain diameter (∼0.95 μm); the influence was seen on their mechanical properties. The low additive content Si₃N₄ system is expected to have superior high-temperature properties compared to the system with high additive content. This study shows a cost-effective fabrication of highly dense Si₃N₄ with excellent mechanical properties.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Oxidative dimerisation of methane on supported palladium oxide catalysts

Supported palladium oxide catalysts are able to convert CH₄ to C₂H₆, CO, CO₂, H₂ and H₂O at temperatures 315 °C. Catalysts did not show any support effect when TiO₂, Al₂O₃, ZrO₂, La₂O₃ and MgO were used as supports. With sequential O₂ pulsing the catalyst showed long term activity when used at temperatures below 400 °C. Addition of Pt increased selectivity whereas with Ga it decreased. Results indicate participation of lattice O₂ from catalyst in the reaction pathway.     

The effect of natural dolomite admixtures on calcium zirconate–periclase materials microstructure evolution

The reaction sintering mechanism of dolomite–zirconia mixtures was investigated using fine grounded dolomite raw material and zirconium powder. The used dolomite raw materials differed by the content of impurities (SiO₂, Al₂O₃ and Fe₂O₃ oxides). The microstructure evolution of MgO–CaZrO₃ and CaZrO₃ sintered materials was presented as a temperature function. One- and two-step firing processes of calcium raw materials powder mixed with chemically pure zirconium oxide were applied. The kinetics of reaction of CaZrO₃ synthesis was estimated by determining the “free” calcium oxide by chemical and XRD analysis. The densification process was evaluated by firing shrinkage, apparent density, pore diameter and pore size distribution measurements. The microstructure of sintered materials was observed by SEM. It was observed that CaZrO₃ synthesis was definitely finished at temperature of 1500 °C in the both applied ways of the synthesis (one- or two-step process). The only phase present in the model material synthesized from chemically pure reagents (CaCO₃ and ZrO₂) after firing at temperature of 1500 °C was calcium zirconate.In the materials synthesized from natural dolomites and ZrO₂ two main phases were present—calcium zirconate and periclase. During firing of CaZrO₃–MgO materials at lower temperatures the presence of transient phases was detected (mainly ferrites and calcium aluminates, 4CaO·Al₂O₃·Fe₂O₃ or 2CaO·Fe₂O₃). These phases disappeared at higher temperatures. This is probably related to the dissolution of impurities in the main phases of CaZrO₃–MgO.The material obtained from the mixture of zirconium oxide and natural dolomite with the high impurities content has the highest densification level (～95% theoretical density of CaZrO₃–MgO) at 1500 and 1600 °C.     

Study of the phase composition and properties of periclase-zircon and spinel-zircon refractories

Conclusions Nd₂O₃ and MgO do not react at temperatures of up to 1600°C. ZrO₂, stabilized with 12% (mole) Nd₂O₃, forms with the MgO a cubic solid solution with limited solubility of periclase: the concentrations in this solid solution of magnesia at 1750° reach 12–13% (mole). It is confirmed that an addition of Nd₂O₃ increases the resistance to decomposition of cubic solid solutions ZrO₂-MgO.With an increase in temperature and in the presence of Nd₂O₃ there is rapid decomposition of MgAl₂O₄, as a result of which neodymium monoaluminate and periclase are formed. The neodymium oxide forming with the ZrO₂ a cubic solid solution does not react with the spinel. We note only a slight (1.5–2%) solubility of ZrO₂ in MgAl₂O₄.Studies were made of some properties of periclase-zircon and spinel-zircon refractories. The composition ZrO₂-MgAl₂O₄ is better in terms of thermal-shock resistance than products made from MgO and ZrO₂, stabilized with Nd₂O₃.Translated from Ogneupory, No. 4, pp. 48–52, April, 1972.