Properties of ramming compounds based on electrofused and sintered zirconia

Conclusions Electrofused ZrO₂ is the best filler for a phosphate-bonded zirconia ramming compound. The bond should contain an addition of fine-ground monoclinic ZrO₂.The hot strength, chemical inertness, and thermal-shock resistance of such compounds are higher than for compounds based on sintered ZrO₂.Translated from Ogneupory, No. 7, pp. 48–53, July, 1977.     

Li2ZrO3 based Li-ion conductors doped with halide ions & sintered in oxygen-deficient atmosphere

All-solid-state batteries have recently attracted much attention for their high energy density and safety. Li₂ZrO₃-based Li-ion conductors with high electrochemical stability have potential applications for electrolytes in all-solid-state batteries. In this work, comparative investigations of Li₂ZrO₃ and halogen doped Li₂ZrO₃ ceramics were conducted by sintering at 700 °C in air or in oxygen-deficient atmosphere which was induced by a simple setup covering with corundum crucible. The analysis of phase composition reveals that the undoped Li₂ZrO₃ ceramic sintered in air contains pure monoclinic phase, while halogen-doped Li₂ZrO₃ sintered in air and all ceramics sintered in oxygen-deficient atmosphere are simultaneously composed of monoclinic and tetragonal phases. Li₂ZrO₃ ceramic with tetragonal phases has higher conductivity (0.28 mS cm⁻¹ for undoped Li₂ZrO₃) than the pure monoclinic Li₂ZrO₃ (0.07 mS cm⁻¹), and halogen doping can further enhance the conductivity of Li₂ZrO₃ ceramics higher than 0.5 mS cm⁻¹ at room temperature.     

Experimental investigation on the mechanical,structural and thermal properties of Cu–ZrO2 nanocomposites hybridized by graphene nanoplatelets

Hybrid Cu–ZrO₂/GNPs nanocomposites were successfully produced using powder metallurgy technique. The effect of GNPs mass fraction, 0, 0.5, 1 and 1.5%, on the mechanical and electrical properties of the produced hybrid nanocomposite was investigated while maintaining ZrO₂ mass fraction constant at 5%. High-energy ball milling was applied for mixing powders followed by compaction and sintering. The morphological analysis of the produced powder showed acceleration of Cu particles fracture during ball milling with the addition of GNPs up to 0.5% with noticeable reduction of agglomeration size. Moreover, the crystallite size of Cu–5%ZrO₂/0.5%GNPs hybrid nanocomposites revealed smaller crystallite size, 142 nm, compared to 300 nm for Cu–5%ZrO₂ nanocomposite. Additionally, the hybrid nanocomposite with 0.5% GNPs shows homogeneous distribution of both reinforcement phases in the sintered samples. The compressive strength increased with the GNPs content and reached 504.6 MPa at 0.5%, 31% higher than the Cu-5%ZO₂. The thermal conductivity had the maximum value at 0.5 wt%GNPs and reached 345 W/m k. The results provide efficient manufacturing process for high strength and good conductivity hybrid nanocomposites, which is applicable in many structural applications such as heat exchange purposes.     

Effect of stabilizing additions on the thermal shock resistance of zirconia products

Conclusions To obtain thermal-shock resistant products from zirconia the amount of monoclinic phase in the fired specimens prepared on the basis of granular bodies should be not less than 15%.It is found that with an increase in the content of CaO from 7.0 to 20 mole %, the thermal-shock resistance of the articles made from ZrO₂ is diminished. Introduction of up to 20% monoclinic ZrO₂ in the batch increases the thermal-shock resistance, but less so the higher the content of CaO in the stabilized part of the material. This produces additional stabilization of the zirconia as a result of the migration of the calcium oxide from the stabilized grains into the monoclinic ZrO₂. Additional stabilization of monoclinic zirconia is observed also during cyclic heating in the range 20–1600–20°C.Specimens of zirconia stabilized with CaO possess higher thermal-shock resistance than those made from ZrO₂ stabilized with MgO with the same contents of monoclinic phase.Translated from Ogneupory, No.1, pp.50–55, January, 1967.     

Investigation of the electrical conductivity of sintered monoclinic zirconia (ZrO2)

High-density monoclinic ZrO₂ was manufactured through sintering at ~1200 °C by using nanosized powders. Then, the electrical conductivity was measured at a range of high temperatures (700–900 °C) by electrical impedance spectroscopy (EIS). For the as-sintered monoclinic ZrO₂, the measured electrical conductivity was 3.2×10⁻⁵ s/cm (for 80% TD) and 4.4×10⁻⁵ s/cm (for 89% TD) at 900 °C. After aging at 900 °C for 100 h, the electrical conductivity of the monoclinic ZrO₂ of 80%-TD decreased by more than 50%. However, after reheating at 1200 °C for 1 h, approximately 80% of the conductivity was recovered compared to the value of the as-sintered monoclinic ZrO₂. The pure monoclinic crystal structure was retained despite the aging and reheating treatment. Based on microstructural observations of the aged and reheated monoclinic ZrO₂, the changes in electrical conductivity after aging and reheating were explained by the formation and recovery of micro-cracks, respectively.     

Effect of Nano-ZrO2 on the Properties of Al-Al2O3 Nanocomposites Prepared by Mechanical Alloying

In the present work, mechanical alloying was used to prepare Al-20wt.% Al₂O₃ metal-matrix nanocomposites having up to 4wt.% ZrO₂ at the expense of Al₂O₃. The powders were milled for different time intervals. To characterize the powders after milling, x-ray diffraction and transmission electron microscopy were used to identify the phase composition, crystallite size and morphology. In order to study the sinterability, the milled powders were cold pressed and sintered in argon atmosphere at different firing temperatures up to 470 °C for 1 h. The relative density and apparent porosity of the sintered composites were determined according to Archimedes principle. Moreover, the microstructure was examined by a scanning electron microscope attached with an energy dispersive spectrometer (EDS). Microhardness and AC conductivity of sintered composites were also measured. The results pointed out that the increasing of milling time is responsible for uniform distribution of Al₂O₃-ZrO₂ particles in the Al matrix as well as remarkable increases in relative density, microhardness and AC conductivity of the sintered specimens. Also, the relative density was affected considerably by the increasing of sintering temperature. Moreover, increasing of ZrO₂ content led to a significant decrease in the crystal size of the milled powders and increase in the microhardness of the sintered compacts. No changes were observed on the conductivity after addition of ZrO₂.

     

Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO₂ and ZrO₂–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10⁻¹⁰ S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO₂/ZrO₂ boundaries.     

Preparation and properties of zirconium refractories stabilized by cerium dioxide

Conclusions We studied the reaction of zirconium dioxide with cerium dioxide in mixtures with CeO₂ contents of 6, 8, 10, 12, 15, 18, 20, 33, and 50%. The properties of the samples of these compositions were determined.On the addition of amounts of CeO₂ from 6–12% complete stabilization of ZrO₂ was not achieved by a single sintering at 1750°C. The samples with the composition 88% ZrO₂+12% CeO₂, sintered twice at 1750°C with an intermediate grinding, stabilized almost completely as a solid solution of tetragonal structure.The thermal stress resistance of dense, completely stabilized samples with CeO₂ contents of 15–18%, prepared from finely ground raw oxides, was 3–4 thermal cyclings. It improved when the CeO₂ content was decreased, or when more monoclinic ZrO₂ was added.It was found that the onset and the inversion temperature interval depend on the CeO₂ content, the granular composition of the original oxides, the temperature, and the gaseous sintering medium. We studied the properties of synthesized compositions and their dependence on the reducing or oxidizing conditions of sintering due to a change in the valency of cerium. In order to obtain zirconium-cerium refractories with definite properties it is necessary to have strict control of the gaseous medium during sintering.Translated from Ogneupory, No. 3, pp. 37–44, March, 1969.     

Conditions for Preparation of Oxide Components and Their Effect on Properties of Al2O3 – ZrO2 – SiC Composite

Results of a study of phase formation in the Al₂O₃ – ZrO₂ – SiC system sintered under vacuum and in a reducing medium at 1350 – 1550°C are reported. Conditions for preparation of Al₂O₃ and ZrO₂ powders from hydroxides are specified and the effect of specific surface, temperature, and holding time on the reaction between oxide and carbide components is considered. Results of microstructural, x-ray phase, thermogravimetric, and chemical analyses of precursor materials and end products are discussed. Optimum composition (20% SiC, 15% ZrO₂, 65% Al₂O₃ ) and dispersity of the mixture for obtaining a composite with a strength of about 200 MPa by a method other than high-temperature toughening are determined.     

Preparation and properties of ceramics in the ZrO2-CeO2-CaO system

The effect of bivalent calcium cations introduced in the stage of homogenous precipitation on the phase and morphological compositions of a tetragonal zirconia powder and the properties of a polycrystalline material based on it is described. The experiment was conducted for a base system containing 12% CeO₂, (0.5–3%) CaO, and the remainder ZrO₂. The use of calcium oxide as a modifier in the ZrO₂–12% CeO₂ system changes the crystallization temperature of the amorphous hydroxide residue. In the presence of 0.8–1% calcium oxide the structure of the sintered ceramic material mostly contains fine round crystallites 0.3 µm in size. Simultaneously, the fracture toughness increases considerably (K _Ic=15 MPa · m^1/2), which seems to be caused by the changed mechanism of destabilization of tetragonal zirconia.Translated from Ogneupory, No. 11, pp. 14–17, November, 1995.     

Phase transformation kinetics of 3 mol% yttria partially stabilized zirconia (3Y-PSZ) nanopowders prepared by a non-isothermal process

A crystalline nanopowder of 3 mol% yttria-partially stabilized zirconia (3Y-PSZ) has been synthesized using ZrOCl₂ and Y(NO₃)₃ as raw materials throughout a co-precipitation process in an alcohol-water solution. The phase transformation kinetics of the 3Y-PSZ freeze dried precursor powders have been investigated by nonisothermal methods. Differential thermal and thermogravimetric analyses (DTA/TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been utilized to characterize the 3Y-PSZ nanocrystallites. When the 3Y-PSZ freeze dried powders are calcined in the range of 703-1073 K for 2 h, the crystal structure is composed of tetragonal and monoclinic ZrO₂. The BET specific surface area of the 3Y-PSZ freeze dried precursor powders calcined at 703 K for 2 h is 118.42 m²/g, which is equivalent to a crystallite size of 8.14 nm. The activation energy from tetragonal ZrO₂ converted to monoclinic ZrO₂ in the 3Y-PSZ freeze dried precursor powders was determined as 401.89 kJ/mol. The tetragonal (T) and monoclinic (M) ZrO₂ phases coexist with a spherical morphology, and based on TEM examination have a size distribution between 10 and 20 nm. When sintering green compacts of the 3Y-PSZ, a significant linear shrinkage of 8% is observed at about 1283 K. On sintering the densification cycle is complete at approximately 1623 K when a total shrinkage of 32% is observed and a final density above 99% of theoretical was achieved.     

Consolidation of undoped,monoclinic zirconia polycrystals by flash sintering

Consolidated, monoclinic ZrO₂ polycrystal was produced from undoped ZrO₂ powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO₂ was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal‐to‐monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO₂ body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO₂ without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain‐boundary second phase in the flash‐sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash‐sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that <001> directions of the monoclinic ZrO₂ grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash‐sintered specimen for 5 minutes. The alignment of ZrO₂ grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.     

Electrochemical characteristics and structural changes of molybdenum trioxide hydrates as cathode materials for lithium batteries

Several characteristics of MoO₃·2H₂O and MoO₃·H₂O, such as thermal behaviour and conductivity and the electrochemical behaviour and structural changes associated with discharge and charge have been investigated. The suitability of these substances as new cathode materials for non-aqueous lithium batteries has been assessed. MoO₃·H₂O, having only one coordinated water molecule, showed a discharge capacity of about 400 Ah kg^–1 of acid weight and a discharge potential around 2.5 V vs Li/Li⁺. This capacity was much higher than the 280 Ah kg^–1 of anhydrous MoO₃.MoO₃·H₂O showed good charge-discharge cyclic behaviour at a capacity below l e^–/Mo while keeping the original layered lattice on cycling. In addition, the crystal system of MoO₃·H₂O was found to be changed from a monoclinic system to orthorhombic with lattice parameters ofa=0.5285 nm,b=1.0824 nm,c=0.5237 nm on discharge to 0.5 e^–/Mo.This paper was originally presented at the Fall 1987 Meeting of the Electrochemical Society, Inc. held at Honolulu, Hawaii (Proceedings of the Symposium on Primary and Secondary Ambient Temperature Batteries, PV88-6, p. 484–493 (1987).     

Fabrication of lightweight ZrO2 fiberboards using hollow ZrO2 fibers

ZrO₂ fiberboards with ultra-low densities (0.34–0.40 g/cm³) were fabricated using biomorphic ZrO₂ hollow fibers, which have a lower density and better thermal insulation than traditional ZrO₂ solid fibers. The effects of sol binder content, sintering temperature, and proportion of solid fibers on the density, microstructure, compressive strength, linear shrinkage, and thermal conductivity of lightweight ZrO₂ fiberboards were investigated. The results showed that the hollow features of biomorphic ZrO₂ fibers were successfully maintained after they were made into ZrO₂ fiberboards, which made them less dense and thermally conductive. The best conditions were found to be a sol binder content of 30 vol%, sintering temperature of 1400 °C, and 20 wt% sintered solid fibers to balance thermal insulation and compressive strength. The results show that the density and thermal conductivity of lightweight ZrO₂ fiberboard gives it obvious advantages as a heat-insulating ceramic. Specifically, when the sintering temperature was 1400 °C, the sample had an ultra-low density of 0.34–0.40 g/cm³, a thermal conductivity of 0.101–0.116 W/(m·K) (at 500 °C), a compressive strength of 0.05–0.24 MPa, and a linear shrinkage of 9.4–13%.     

Dense fused zirconia crucibles with high spalling resistance

Conclusions The use of wet-ground finely dispersed bodies, containing 50–70% fractions finer than 1, yields dense castings, and at moderate firing temperatures practically completely sintered crucibles based on fused stabilized ZrO₂ and combinations of it with monoclinic phase.The use as an additive of raw monoclinic ZrO₂ favorable affects the behavior of the crucibles in firing, only slightly increasing their hot shrinkage compared with that noted with the use of the same quantity of fused monoclinic dioxide.When we incorporate an addition of monoclinic zirconium dioxide into finely milled bodies, redistribution of the calcium oxide occurs during firing, and this intensely diffuses from the cubic phase into the monoclinic phase. In the same conditions the presence in the fired material of crystals of transition phase or monoclinic ZrO₂ depends mainly on the total content of CaO in the original body.A substantial increase in the spalling resistance of the crucibles is noted with a body composition guaranteeing the presence of a defect cubic phase and about 10% monoclinic phase in the fired material.Translated from Ogneupory, No. 1, pp. 50–54, January, 1969.     

Effect of preparation method on sinterability and properties of nanocrystalline MgAl2O4 and ZrO2-MgAl2O4 materials

Abstract

Nanocrystalline MgAl₂O₄ and ZrO₂-MgAl₂O₄ powders were synthesised by combustion and conventional solid state reaction routes. The synthesised powders were processed, dry pressed, and sintered for 3 h at temperatures ranging from 1550 to 1625°C. The sintered pellets were then characterised in terms of phase (XRD), microstructure (SEM), relative density, apparent porosity, water absorption, hardness, three point bend strength, and fracture toughness. The XRD studies revealed that ZrO₂ was present in tetragonal form in the case of combustion synthesised powders (CSP), whereas in powders obtained by solid state reaction (SSP) it was present in the monoclinic form. This study also revealed that the addition of ZrO₂ improved the mechanical properties of sintered MgAl₂O₄ samples: 20 wt-%ZrO₂-MgAl₂O₄ composites prepared from CSPs and conventional SSPs and sintered at 1625°C for 3 h had fracture toughness of 5·96 and 4·33 MPa m^1/2 and three point bend strength of 269 and 98 MPa respectively. Higher sintered density, the presence of tetragonal zirconia as a major phase, and the finer microstructure are probably responsible for the superior mechanical properties exhibited by sintered CSP materials as compared with the sintered SSPs.     

Mortars for zirconia refractories

Conclusions The composition was developed for a mortar based on zirconium dioxide and orthophosphoric acid intended for laying zirconia refractories. The filler should be stabilized ZrO₂ powder of a particle size below 0.5 mm with 10% monoclinic fine-grain ZrO₂ mixed with orthophosphoric acid added in the proportion of 2–4% in terms of P₂O₅.The mortar was tested with zirconia refractory linings and gave satisfactory results.Owing to its low water retention capacity and short setting time Ca₇Al₆ZrO₁₈ cement cannot be recommended for a zirconia base mortar.Translated from Ogneupory, No. 8, pp. 52–59, August, 1973.     

Some properties of fine-grained mixtures of ZrSiO4-ZrO2

Conclusions The effect of additions of ZrO₂ in monoclinic and cubic modifications and also of Zr(OH)₄ on the swelling, thermal-shock resistance, and glass-resistance of high-density, zircon refractories has been studied.It is shown that the most effective addition for lowering the additional growth of the refractory almost fivefold was a mixture consisting of 9.5% monoclinic ZrO₂ and 0.5% Zr(OH)₄. This additive also increases significantly the thermal-shock resistance of the zircon refractories and the glass resistance as a result of the lower decomposition of ZrSiO₄, and the higher concentration of ZrO₂ in the contact zone. The density and electrical resistivity of the refractory is maintained at a high level.Translated from Ogneupory, No. 8, pp. 48–53, August, 1982.     

Enhanced phase stability in hydroxylapatite/zirconia composites with hot isostatic pressing

Hydroxylapatite (HA) composites with pure zirconia (ZrO₂), and 3 and 8% Y₂O₃ doped ZrO₂ were pressure-less sintered in air and hot isostatically pressed (under 120 MPa gas pressure) at 1100 °C for 2 h. The reactions and phase transformations were monitored by X-ray diffraction, thermal analysis, and Raman spectroscopy. HA/pure ZrO₂ composites were not thermally stable in air sintering; HA dissociated into α and β tricalcium phosphate while monoclinic ZrO₂ was transformed into tetragonal and cubic phases. No decomposition in HA or phase transformation in ZrO₂ were observed in hydroxylapatite/3% Y₂O₃ doped ZrO₂ or HA/8% Y₂O₃ doped ZrO₂ composites. On the other hand, HA and ZrO₂ phases in hot isostatically pressed composites remained stable. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ among the composites sintered in air. The densification of the composites decreased at lower sintering temperatures and higher ZrO₂ contents upon air-sintering. The HIPped composites were densified to about 99.5% of theoretical densities in all mixing ratios. The reactivity between ZrO₂ and HA was dependent on the amount of air in the sintering environment. Hot isostatic pressing with very limited retained air was proved to be a very convenient method to insure both phase stability and full densification during the production of hydroxylapatite zirconia composites.     

Thermal expansion and thermal conductivity of SmxZr1−xO2−x/2 (0.1 ≤ x ≤ 0.5) ceramics

A study was conducted of the effect of additions of samarium oxide on the thermal expansion and thermal conductivity of zirconium oxide for thermal barrier coatings. Sm_xZr_1?xO_2?x/2 (0.1 ≤ x ≤ 0.5) ceramic powders synthesized with a chemical-coprecipitation and calcination method were sintered at 1873 K for 15 h. Structures of the synthesized powders and sintered ceramics were identified by X-ray diffractometer. The morphologies of ceramic powders were observed by transmission electron microscope. The thermal expansion coefficients and thermal diffusion coefficients of Sm_xZr_1?xO_2?x/2 ceramics were studied with a high-temperature dilatometer and a laser flash diffusivity technique from room temperature to 1673 K. The thermal conductivity was calculated from thermal diffusivity, density and specific heat of bulk ceramics. Sm_0.1Zr_0.9O_1.95 ceramics consists of both monoclinic and tetragonal structures. However, Sm_0.2Zr_0.8O_1.9 and Sm_0.3Zr_0.7O_1.85 ceramics only exhibit a defect fluorite structure. Sm_0.4Zr_0.6O_1.8 and Sm_0.5Zr_0.5O_1.75 ceramics have a pyrochlore-type lattice. With the increase of Sm₂O₃ content, the linear thermal expansion of Sm_xZr_1?xO_2?x/2 ceramics increases except for Sm_0.1Zr_0.9O_1.95. The thermal conductivities of Sm_xZr_1?xO_2?x/2 ceramics ranged from 1.41 at 873 K to 1.86 W m^?1 K^?1 at room temperature in a test temperature range of room temperature to 1673 K, and the results can be explained by phonon scattering mechanism.