Phase diagram of the Al2O3–ZrO2–Nd2O3 system

The phase diagram of the Al₂O₃–ZrO₂–Nd₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1675 °C and it corresponds to the ternary eutectic Nd₂O₃·11Al₂O₃ + F-ZrO₂ + NdAlO₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–Nd₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system.     

Catalytic activity of PdO/ZrO2 catalyst for methane combustion

Catalytic activity of ZrO₂ supported PdO catalysts for methane combustion has been investigated in comparison with Al₂O₃ supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600–900°C) was suppressed by using ZrO₂ support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al₂O₃ catalyst was reduced at the temperature less than 100°C, whereas in PdO/ZrO₂ catalyst the consumption of hydrogen was also observed at 200–300°C. This result indicates that the stable PdO species were present in the PdO/ZrO₂ catalyst. In order to confirm the formation of the solid solution of PdO and ZrO₂, X-ray diffraction (XRD) analyses of the mixtures of ZrO₂ and PdO calcined at 700–900°C in air were carried out. The lattice volume of ZrO₂ in the mixture was larger than that of ZrO₂. Furthermore, the Pd thin film on ZrO₂ substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin film heated at 900°C in air. It was considered that the PdO on ZrO₂ support might be stabilized by the formation of the solid solution of PdO and ZrO₂.     

Mechanochemical synthesis and thermal evolution of La–ZrO2 cubic solid solutions

The formation of cubic solid solutions in the system La₂O₃–ZrO₂ by mechanochemical activation of a mixture of the oxides (molar ratio ZrO₂ 82%–La₂O₃ 18%) is studied. After 6 h of activation at room temperature, a poorly crystalline cubic solid solution is formed, with ultimate crystallite sizes in the nanometer range. The mixtures activated during 1–3 h form the solid solution on subsequent heating at 1000 °C, while the non-activated mixture does not react, even after thermal treatment at 1200 °C. The solid solution obtained at room temperature undergoes partial structural ordering at temperatures between 800 and 1000 °C. Long time heating at temperatures of 1000 °C and above results in the formation of La₂Zr₂O₇ and rejection of the excess ZrO₂. Mechanochemical activation offers interesting possibilities for the synthesis of these materials at temperatures lower than those used in conventional processing, and for the control of their physicochemical and microstructural properties.     

Sintering behavior of Al2TiO5 base ceramics and their thermal properties

Sintering behavior of Al₂TiO₅ without and with various additives and the thermal properties of the sintered material—thermal expansion and decomposition—were investigated. The precursors of Al₂TiO₅ powders were prepared by homogeneous precipitation and coprecipitation. Sintering of pure Al₂TiO₅ gave a fine grained-structure at 1300°C, but resulted in large-grained and cracked microstructures at 1400 and 1500°C. Addition of ZrO₂ or BaO gave fine-grained microstructures with a small increase in thermal expansion. Addition of ZrO₂, BaO or ZrSiO₄, especially ZrSiO₄, was effective in suppressing the thermal decomposition of Al₂TiO₅ at 1100°C. ©     

Grain growth inhibition and mechanical property enhancement by adding ZrO2 to Al2O3 matrix

The critical amount and critical size of ZrO₂ for effective dragging of grain boundary migration occurred at 8 vol% addition for Al₂O₃ ceramic composite, pressureless-sintered at 1600°C for 2h. The fracture toughness was increased from 4·1 to 5·4 MPa m^1/2, and the flexural strength from 290 to 410 MPa at optimal conditions. The enhancement of mechanical properties is attributed to the stress-induced phase transformation toughening when ZrO₂ particles were located intergranularly. Al₂O₃ grain growth is inhibited by ZrO₂ particles pinning at the grain boundaries.     

Internal friction behavior of an unidirectionally solidified Al2O3/YAG eutectic at high temperature

The internal friction behavior of a unidirectionally solidified Al₂O₃/YAG eutectic was examined between room temperature and 1400 °C. No internal friction was observed up to 1200 °C. Above 1200 °C, the internal friction drastically increased with increasing temperature and the number of torsional loading cycles. For the 1400 °C test, the internal friction gradually increased with the number of loading cycles and then saturated after 10³ cycles.     

Effects of ZrO2/Al2O3 properties on the catalytic activity of Pd catalysts for methane combustion and CO oxidation

Zirconia supported on alumina was prepared and characterized by BET surface area, X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), temperature programmed desorption (TPD), and pulse reaction. 0.2% Pd/ZrO₂/Al₂O₃ catalyst were prepared by incipient wetness impregnation of supports with aqueous solution of Pd(NO₃)₂. The effects of support properties on catalytic activity for methane combustion and CO oxidation were investigated. The results show that ZrO₂ is highly dispersed on the surface of Al₂O₃ up to 10 wt.% ZrO₂, beyond this value tetragonal ZrO₂ is formed. The presence of a small amount of ZrO₂ can increase the surface area, pore volume and acidity of support. CO–TPD results show that the increase of CO adsorption capacity and the activation of CO bond after the presence of ZrO₂ lead to the increase of catalytic activity of Pd catalyst for CO oxidation. CO pulse reaction results indicate that the lattice oxygen of support can be activated at lower temperature following the presence of ZrO₂, but it does not accelerate the activity of 0.2% Pd/ZrO₂/Al₂O₃ for methane combustion. 0.2% Pd/ZrO₂/Al₂O₃ dried at 120 °C shows highest activity for CH₄ combustion, and the activity can be further enhanced following the repeat run. The increase of treatment temperature and pre-reduction can decrease the activity of catalyst for CH₄ combustion.     

Processing of hot pressed Al2O3–Cr2O3/Cr-carbide nanocomposite prepared by MOCVD in fluidized bed

Nanosized particles dispersed uniformly on Al₂O₃ particles were prepared from the decomposition of precursor Cr(CO)₆ by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr₂O₃, CrC_1−x, and C. A solid solution of Al₂O₃–Cr₂O₃ and an Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al₂O₃–Cr₂O₃/Cr-carbide (Cr₃C₂ and Cr₇C₃) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr₃C₂ and Al₂O₃ is non-coherent, while the interface between Cr₇C₃ and Al₂O₃ is semi-coherent.     

Study of n-hexane isomerization on mixed Al2O3–ZrO2/SO4 catalysts

Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr₂O were synthesized by means of sol–gel methods. The catalysts were sulfated with H₂SO₄ 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N₂-adsorption at 78 K, and SEM. The catalytic properties of the Al₂O₃–ZrO₂ series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al₂O₃–ZrO₂ solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane.     

Yb2O3 and Y2O3 co-doped zirconia ceramics

A suspension stabilizer-coating technique was employed to prepare x mol% Yb₂O₃ (x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y₂O₃ co-doped ZrO₂ powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb₂O₃ and Y₂O₃ co-doped zirconia ceramics. Fully dense ZrO₂ ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb₂O₃ content and the sintering parameters. Polycrystalline tetragonal ZrO₂ (TZP) and fully stabilised cubic ZrO₂ (FSZ) were achieved in the 1.0 mol% Y₂O₃ stabilised ceramic, co-doped with 1.0 mol% Yb₂O₃ and 4.0 mol% Yb₂O₃, respectively. The amount of stabilizer needed to form cubic ZrO₂ phase in the Yb₂O₃ and Y₂O₃ co-doped ZrO₂ ceramics was lower than that of single phase Y₂O₃-doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m^1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb₂O₃ and 1.0 mol% Y₂O₃ ceramic at 1450 °C for 1 h.     

Ethylene dimerization over NiSO4 supported on Fe2O3-promoted ZrO2 catalyst

The NiSO₄ supported on Fe₂O₃-promoted ZrO₂ catalysts were prepared by the impregnation method. Fe₂O₃-promoted ZrO₂ was prepared by the coprecipitation method using a mixed aqueous solution of zirconium oxychloride and iron nitrate solution followed by adding an aqueous ammonia solution. No diffraction line of nickel sulfate was observed up to 20 wt.%, indicating good dispersion of nickel sulfate on the surface of Fe₂O₃–ZrO₂. The addition of nickel sulfate (or Fe₂O₃) to ZrO₂ shifted the phase transition of ZrO₂ (from amorphous to tetragonal) to higher temperatures because of the interaction between nickel sulfate (or Fe₂O₃) and ZrO₂. 15-NiSO₄/5-Fe₂O₃–ZrO₂ containing 15 wt.% NiSO₄ and 5 mol% Fe₂O₃, and calcined at 500 °C exhibited a maximum catalytic activity for ethylene dimerization. NiSO₄/Fe₂O₃–ZrO₂ catalysts was very effective for ethylene dimerization even at room temperature, but Fe₂O₃–ZrO₂ without NiSO₄ did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe₂O₃ up to 5 mol% enhanced the acidity, surface area, thermal property, and catalytic activities of catalysts gradually, due to the interaction between Fe₂O₃ and ZrO₂ and due to consequent formation of Fe–O–Zr bond.     

Effect of CeO2 on the sintering behaviour of zirconia–alumina composite

Precursor for a composite containing equimolar proportion of ZrO₂ and Al₂O₃ having higher reactivity has been synthesized by the wet interaction of inorganic salts in aqueous phase. Sintering of the compact body indicated that the densification rate increased from 1500 °C. The role of CeO₂ as a dopant was positive with respect to densification and retention of t-ZrO₂.     

ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming

Surface-phase ZrO₂ on SiO₂ (SZrOs) and surface-phase La₂O₃ on Al₂O₃ (SLaOs) were prepared with various loadings of ZrO₂ and La₂O₃, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH₄/CO₂ reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al₂O₃ and Pt/SiO₂ catalysts. CO₂ or CH₄ pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides.

The zirconia can be homogeneously dispersed on SiO₂ to form a stable surface-phase oxide. The lanthana cannot be spread well on Al₂O₃, but it forms a stable amorphous oxide with Al₂O₃. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO₂ and Pt/Al₂O₃ by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO₂ and Pt/Al₂O₃ catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO₂ adsorbed on the basic sites of SZrOs and SLaOs.     

NO reduction by CH4 in the presence of O2 over La2O3 supported on Al2O3

Dispersing La₂O₃ on δ- or γ-Al₂O₃ significantly enhances the rate of NO reduction by CH₄ in 1% O₂, compared to unsupported La₂O₃. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La₂O₃ precursor used, the pretreatment, and the La₂O₃ loading. The most active family of catalysts consisted of La₂O₃ on γ-Al₂O₃ prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m²) occurred between the addition of one and two theoretical monolayers of La₂O₃ on the γ-Al₂O₃ surface. The best catalyst, 40% La₂O₃/γ-Al₂O₃, had a turnover frequency at 700°C of 0.05 s⁻¹, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La₂O₃/Al₂O₃ catalysts exhibited stable activity under high conversion conditions as well as high CH₄ selectivity (CH₄ + NO vs. CH₄ + O₂). The addition of Sr to a 20% La₂O₃/γ-Al₂O₃ sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO⁼₄ to the latter Sr-promoted La₂O₃/Al₂O₃ catalyst decreased activity although sulfate increased the activity of Sr-promoted La₂O₃. Dispersing La₂O₃ on SiO₂ produced catalysts with extremely low specific activities, and rates were even lower than with pure La₂O₃. This is presumably due to water sensitivity and silicate formation. The La₂O₃/Al₂O₃ catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications.     

In-situ growth of needlelike LaAl11O18 for reinforcement of alumina composites

In situ growth of needlelike LaAl₁₁O₁₈ grains reinforcing Al₂O₃ composites can be fabricated by a coprecipitation method using La(NO₃)₃√6H₂O and Al(NO₃)₃√9H₂O as starting materials. The new two-step process involved firstly preparing needlelike LaAl₁₁O₁₈ grains distributed homogeneously in Al₂O₃ powder and then pressureless sintering the composite powders. The Al₂O₃/25 vol.%LaAl₁₁O₁₈ samples pressureless sintered at 1550°C for 4 h achieve relative density up to 96.5% and exhibit a bending strength of 420±30 MPa and a fracture toughness of 4.3±0.4 MPa m^1/2.     

Processing and mechanical properties of ZrO2–TiB2 composites

In this paper, a strategy is described to develop high toughness yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) composites reinforced with hard TiB₂ particles. The experimental results revealed that fully dense Y-TZP composites with 30 vol.% TiB₂ can be obtained with a moderate hardness of 13 GPa, a high strength up to 1280 MPa and an excellent indentation toughness up to 10 MPa m^1/2 by hot pressing in vacuum at 1450 °C. The toughness of the composites can be tailored between 4 and 10 MPa m^1/2 by varying the yttria stabiliser content of the ZrO₂ matrix between 3 and 2 mol%. An optimum composite toughness was achieved for a ZrO₂ matrix with an overall yttria content of 2.5 mol%, obtained by mixing pure monoclinic and 3 mol% Y₂O₃ co-precipitated ZrO₂ starting powders. An important observation is that the thermal residual tensile stress in the ZrO₂ matrix due to the TiB₂ addition, needs to be taken into account when optimising the transformability of the ZrO₂ matrix in order to develop high toughness Y-TZP composites.     

A new concept in high performance ceria–zirconia oxygen storage capacity material with Al2O3 as a diffusion barrier

CeO₂–ZrO₂ solid solution ((Ce,Zr)O₂) is an indispensable oxygen storage capacity (OSC) material for emission control in gasoline-fuelled automobiles. The high performance OSC material developed in this study is composed of Al₂O₃ as “a diffusion barrier” and (Ce,Zr)O₂ particles in intervening layers on a nanometer scale, and is abbreviated as “ACZ”. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of ACZ after durability testing in air at 1000 °C was 20 m²/g, which is higher than that of conventional CZ (2 m²/g) composed of (Ce,Zr)O₂ without Al₂O₃. After heat treatment at 1000 °C in air, the particle size of (Ce,Zr)O₂ in ACZ was about 10 nm and that without Al₂O₃ was one-half of the size in pure CZ. The OSC was roughly characterized by the total capacity (OSC-c1) and the oxygen release rate (OSC-r). In a fresh catalyst, ACZ and CZ had almost the same OSC-c1; however, the OSC-r of ACZ was twice as fast as CZ. After durability testing, the OSC-r of both ACZ and CZ were reduced significantly, but the OSC-r of ACZ was about five times as fast as CZ. While the OSC-c1 was hardly influenced by the (Ce,Zr)O₂ crystallite size and Pt particle size on the supports, the OSC-r was influenced by both of these parameters. The improvement of the OSC-r in the fresh catalyst and inhibition of the decrease in the OSC-r after durability testing were achieved by suppression of particle growth of (Ce,Zr)O₂ in ACZ by introducing Al₂O₃ as a diffusion barrier with resultant inhibition of sintering of Pt particles.     

Mechanical properties of Al2O3/ZrO2 composites

In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al₂O₃/(t-ZrO₂+m-ZrO₂) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO₂ inclusions are close to each other in the Al₂O₃ matrix, the yttrium ion originally in t-ZrO₂ particles can diffuse to nearby m-ZrO₂ particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al₂O₃/(t-ZrO₂+m-ZrO₂) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al₂O₃ alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al₂O₃ alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation.     

Formation of Y3Al5O12–Al2O3 eutectic microstructure with off-eutectic composition

Homogeneous-eutectic microstructure of Y₃Al₅O₁₂–Al₂O₃ system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y₂O₃ and Al₂O₃ having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y₃Al₅O₁₂ and Al₂O₃ phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al₂O₃), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed.     

Physicochemical surface and catalytic properties of CuO–ZnO/Al2O3 system

A series of CuO–ZnO/Al₂O₃ solids were prepared by wet impregnation using Al(OH)₃ solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O₂ at temperatures ranged between 125°C and 200°C.

The results showed that CuO interacts with Al₂O₃ to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl₂O₄ at 600°C while stimulates its production at 800°C. The treatment of CuO/Al₂O₃ solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol⁻¹ in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature.