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.     

Fabrication and mechanical behavior of ZrB2 strengthened SiCf/SiC composites prepared by hybrid processing route

A SiC fiber-reinforced composite containing a SiC-ZrB₂ mixed matrix (SiC_f/(SiC-ZrB₂)) with high density and enhanced mechanical properties was fabricated. ZrB₂ at 5 or 40?vol% was added to a (SiC + C) slurry to be infiltrated into the voids of 2D woven Tyranno?-SA grade-3 fabrics by electrophoretic deposition. Subsequent hot pressing at 1300?°C and 10?MPa for 1?h, followed by liquid silicon infiltration (LSI) at 1600?°C for 5?h in an Ar atmosphere resulted in the formation of the reaction-bonded SiC matrix, which revealed a composite density close to 97%. SiC_f/(SiC-ZrB₂) having open porosities of 0.2–0.6% showed peak strengths of 398 and 320?MPa for 5 and 40?vol% ZrB₂ addition, respectively. The large mismatch in the coefficient of thermal expansion and Young's modulus between the SiC and ZrB₂ phases was attributed to a reverse trend in the strength of composites. Brittle behavior of the composites in flexure can be explained by the strong bonding between the matrix and fibers formed by the reaction of interphase with molten Si during LSI. Strength retention after oxidation at 1000 and 1400?°C for 2?h was also compared in terms of ZrB₂ amount contained in the composites.     

Processing of ultrafine ZrO2 toughened WC composites

The interrelationships between the dispersion of the secondary ZrO₂ phase and the material properties of WC-based composites with up to 10 vol% of ZrO₂ are investigated. The homogeneity of the ultrafine WC–nanometric ZrO₂ powder mixtures was optimized by means of multidirectional milling and bead milling. In an alternative route, zirconium butoxide was used as a ZrO₂ source. The composites were fully densified by means of pulsed electric current sintering (PECS), also known as spark plasma sintering, within a few minutes at 1700 °C allowing to maintain an ultrafine grained microstructure combining a hardness of 2600 kg/mm² with an indentation toughness of 6 MPa m^1/2. The ZrO₂ content and Y₂O₃ stabilization were found to strongly influence the mechanical properties and especially strength of the WC–ZrO₂ composites.     

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.     

Studies on SnO2–ZrO2 solid solution

The solubilities of Sn in ZrO₂ and Zr in SnO₂ are investigated. The X-ray diffraction (XRD) studies show the solubility limit for Sn in zirconia to be 20 mol% whereas Zr in SnO₂ is around 25 mol%. All the compositions were prepared by the coprecipitation method. The average particle size for a typical composition was 25 nm as revealed by the transmission electron microscopy (TEM).     

CO oxidation on Pd/CeO2–ZrO2 catalysts

The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO₂–Pd on the kinetics of CO oxidation over Pd/CeO₂–ZrO₂. Under reducing-to-moderately oxidizing conditions, a zero-order O₂ pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO₂–ZrO₂ support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior.     

Synthesis and thermal expansion of ZrO2/ZrW2O8 composites

In this work, a ceramic composite of ZrW₂O₈ and ZrO₂ was synthesized, in order to investigate the possibility of compensating the positive thermal expansion of ZrO₂ with the negative thermal expansion (NTE) compound ZrW₂O₈, tailoring the thermal expansion of these composites. The NTE material was mixed with varying amounts of ZrO₂. The thermal expansion coefficients of this series of composites decrease with increasing amounts of ZrW₂O₈. Nevertheless, a negative deviation from the values expected by the rule of mixtures was found to be most pronounced in the middle of the compositional region.     

Phase diagram of the system: Al2O3–ZrO2

The system Al₂O₃–ZrO₂ was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO₂. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al₂O₃. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO₂ transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO₂, with 0.7±0.2% mass Al₂O₃ at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO₂ from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO₂ saturated solid solution.     

Preparation and properties of C/SiC/ZrO2 porous composites by hot isostatic pressing the pyrolyzed preforms

Three phase mixture of C/SiC/ZrO₂ porous composites were prepared from commercially available phenolic resin, Si and ZrO₂ powders. In the first step, mixed powders were pyrolyzed at 850 °C in vacuum to obtain a carbonized microporous material and then hot isostatically pressed at 1200, 1300 and 1350 °C for 10 min in an argon pressure of 50 MPa to prepare C/SiC/ZrO₂ porous composites, in second step. The hot isostatic pressing led to the increase in density from 3.28 to 3.48 g/cm³ and reduction in porosity (from 32 to 20%) of the composites. X-ray diffraction analyses revealed the existence of β-SiC and carbon might be amorphous in the composites. According to the results of scanning electron microscopy, the crystal growth of β-SiC with facets was observed at 1350 °C. In addition, the energy dispersive spectroscopy showed that carbon/silicon atomic ratio was 1:1 in the crystals. X-ray photoelectron spectroscopy of the composites suggested that evolved gaseous molecules, due to the decomposition of phenolic resin, reacted with molecules containing Si to form β-SiC. The formation and growth of β-SiC in addition to the densification of matrix by hot isostatic pressing led to the increase in hardness (max.: 13.99 GPa) at higher temperatures.     

Structure characterization and mechanical properties of CeO2–ZrO2 solid solution system

The measured and calculated lattice parameters, microstructures, and mechanical properties (fracture toughness and microhardness) of CeO₂–ZrO₂ system ceramics are investigated, using CeO₂–ZrO₂ solid solution powder prepared by a microwave-induced combustion process. The CeO₂–ZrO₂ solid solution ceramics were sintered at 1500 °C for 6 h in air; the density of all specimens was greater than 94% of the theoretical density. For Ce_1−xZr_xO₂ (0.00  x  0.50), the measured lattice parameter is in accordance with that of Kim's doped CeO₂ model_. On the other hand, for x  0.50, the measured values fit Kim's doped ZrO₂ model. The fracture toughness and microhardness of CeO₂–ZrO₂ system ceramics with various compositions were investigated with Vickers indentation. The results showed that the crack mode of CeO₂–ZrO₂ solid solution was Palmqvist cracks under loads of 1 kg. Generally, the fracture toughness should increase with grain size at the submicron scale. However, larger grains may lead to spontaneous transformation, which should decrease the potential toughening at room temperature. This behavior was observed in the Ce_0.25Zr_0.75O₂ ceramic, which demonstrated a high fracture toughness that may be ascribed to two causes: (1) fine grain size and (2) transformation toughening.     

The SMSI between supported platinum and CeO2–ZrO2–La2O3 mixed oxides in oxidative atmosphere

CeO₂–ZrO₂–La₂O₃ (CZL) mixed oxides were prepared by citric acid sol–gel method. The as-received gel was calcined at 500, 700, 900 and 1050 °C to obtain the so-called C5, C7, C9 and CK, respectively. The C5, C7 and C9 powders were impregnated with H₂PtCl₆ and then calcined at 500 °C to prepare P5C5, P5C7 and P5C9, respectively. The impregnated CK powders were calcined at 500, 700 and 900 °C to prepare P5CK, P7CK and P9CK, respectively. The XRD and XPS analyses show that the surface distribution of Pt is evidently influenced by the structural and textural properties of the support. The CO adsorption followed by FTIR reveals that the dispersion and the chemisorption sites of Pt are reduced as the calcination temperature of CZL support increases. The chemisorption ability of the CK samples is even completely deactivated. The encapsulation mechanism, which has been applied to explain the so-called strong metal–support interaction (SMSI) after reductive treatment, is introduced here to demonstrate the abnormal observations though the samples were prepared in oxidative atmosphere. The HRTEM results also confirm this explanation. The effects of oxygen vacancies, the chemisorption sites on the Pt surface and Pt/Ce interfacial sites on the three-way catalytic activities are discussed.     

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.     

The red–ox treatments influence on the structure and properties of M2O3–CeO2–ZrO2 (M=Y, La) solid solutions

The introduction of trivalent cation — Y³⁺ or La³⁺ — into the lattice of CeO₂–ZrO₂ solid solutions allows to stabilise a cubic structure at low ceria content (30 mol%). The reducibility of the samples has been compared in the experiments of temperature-programmed reduction (TPR). The introduction of lanthanum cations decreases the amount of hydrogen consumed during TPR, while the introduction of yttrium ones increases this value. At the same time, the value of temperature of the maximum speed of reduction (T_max) is independent on the trivalent dopant. The reducibility of these solid solutions did not change during repeated red–ox treatments at temperature below 1220 K. It is connected with the high thermostability of all systems in this temperature interval. TPR up to 1470 K causes a significant shift of T_max value to higher temperature and a slight decrease of hydrogen consumption in two to three cycles. It is suggested that this alterations are connected with the sharp decrease of the specific surface area of all samples and partially phase decomposition of CeO₂–ZrO₂ and Y₂O₃–CeO₂–ZrO₂ solid solutions. Raman characterisation of the oxygen sublattice of the fresh samples and of the samples after TPR has been carried out.     

MoSi2–Al2O3 electroconductive ceramic composites

Al₂O₃–MoSi₂ composites with MoSi₂ volume fractions between 16 and 40% were fabricated from commercial ceramic Al₂O₃ and intermetallic MoSi₂ powders by granulation, cold isostatic pressing and vacuum-sintering. The addition of MoSi₂ had only a slight influence on the densification of the composites, with sintered densities of 98% for samples with 16 vol.% MoSi₂ and 94% for samples with 40 vol.% MoSi₂. Composites with MoSi₂ contents of 20 vol.% and higher were electroconductive due to the formation of a three-dimensional percolating network of the conductive MoSi₂ phase.     

Studies on structural properties, surface acidity and benzene isopropylation activity of sulphated ZrO2–TiO2 mixed oxide catalysts

A series of sulphated ZrO₂–TiO₂ mixed oxide with different nominal sulphate loadings in the range of 2–15 wt.% was prepared and characterized for their structural properties, surface acidity and benzene isopropylation activity. The catalyst with 10 wt.% nominal sulphate loading showed highest surface area and uniform pore size distributions. Surface acidity, measured by NH₃–TPD method, showed increase in acidity with sulphate loading and the 10 wt.% sulphate loaded catalyst showed highest acidity. The activities of these catalysts were tested for isopropylation of benzene to cumene using 2-propanol as the alkylating agent. The 10 wt.% sulphate-loaded catalyst also showed highest activity for this reaction with 97% cumene selectivity. The higher activity of this catalyst was attributed to its higher acidity.     

Thermal ageing of Pt on low-surface-area CeO2–ZrO2–La2O3 mixed oxides: Effect on the OSC performance

This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO₂–ZrO₂–La₂O₃ mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce³⁺ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce³⁺ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles.     

Autothermal reforming of methane over Ni catalysts supported over CaO–CeO2–ZrO2 solid solution

Ni catalysts supported on various solid solutions of ZrO₂ with alkaline earth oxide and/or rare earth oxide were synthesized. The catalytic activities were compared for partial oxidation of methane and autothermal reforming of methane. For partial oxidation of methane, the Ni catalyst supported on a CaO–ZrO₂ solid solution showed a high activity. Incorporation of CaO in the ZrO₂ matrix was effective for increasing the reduction rate of the NiO particles and for decreasing the coke formation. On the other hand, the Ni particles supported on the CaO–CeO₂–ZrO₂ solid solution had a strong interaction with the support, and the Ni particles showed high activity and stability for autothermal reforming of methane.     

Support effects in Pt/TiO2–ZrO2 catalysts for NO reduction with CH4

ZrO₂–TiO₂ mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH₄ were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773 K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f_7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773 K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O₂ co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH₄ reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface.     

Modification of the oxygen storage capacity of CeO2–ZrO2 mixed oxides after redox cycling aging

High surface area CeO₂–ZrO₂ mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H₂, TPO, magnetic susceptibility measurements to obtain the Ce³⁺ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides.

All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides.