Effect of La2O3 promoter on NiO/Al2O3 catalyst in CO methanation

A series of NiO/Al2O3 catalysts promoted by different La₂O₃ contents were prepared by impregnation method. The physicochemical properties of NiO-La₂O₃/Al₂O₃ were characterized by N₂ adsorption-desorption, X-ray diffraction (XRD), H₂ temperature programmed reduction (H₂-TPR) and H₂ chemisorption. The effect of La₂O₃ on the activity of NiO/Al₂O₃ for CO methanation was investigated in a fixed bed reactor. A lifetime test, as well as thermogravimetric (TG) analysis, was performed to investigate the stability performance and anti-carbon deposition of catalysts. The results showed that the addition of La₂O₃ can restrain the growth of NiO particles, increase the H₂ uptake and Ni dispersion, and therefore enhance the activity of catalysts. When the La₂O₃ content was 3 wt%, a CO conversion of 98% and a selectivity to CH₄ of 96% were obtained at 400 °C. Furthermore, the catalyst NiO-La₂O₃/Al₂O₃ with 3 wt% La₂O₃ content displayed highly stable performance in long-term tests, especially exhibiting good anti-carbon deposition property.     

Special features of the formation of an ultradisperse state in ceramic powders of the ZrO2 - Y2O3 - Al2O3 system

The influence of heat and laser treatment during the formation of the ultradisperse structure of composite powders of the ZrO₂ - Y₂O₃ - Al₂O₃ system is investigated. The parameters of short-range order of amorphous powders in the process of heat and laser treatment are determined. The effect of surface modification of ultradisperse powders on the structure formation and phase composition of the ceramics is established.Translated from Ogneupory, No. 11, pp. 12 – 14, November, 1994.     

Influences of oxide chemical modified on microstructure and electrical properties of PbTiO3-Bi(Ni1/2Ti1/2)O3

The piezoelectric ceramics 0.45PbTiO₃-0.55Bi(Ni_1/2Ti_1/2)O₃ doped with PbO, Bi₂O₃, NiO and TiO₂ were synthesized by conventional solid state reaction method. Thereafter, the structural, ferroelectric and piezoelectric properties of modified ceramics were investigated systematically. By adding Bi₂O₃, NiO and TiO₂, the leakage current of ceramics was found to diminish pronouncedly. Moreover, the coercive field was observed to decrease by modification with TiO₂ in these ceramics. It was also proven that NiO can only be added in the B-site in PT-55BNT from the results of X-ray photoelectron spectroscopy analysis. Therefore, the modification with oxides can inhibit the volatilization of Pb during the sintering process that eventually improves the performance of materials.     

Phases in La2O3 and NiO doped (Zr,Sn)TiO4 microwave dielectric ceramics

Dielectric ceramics with composition in the ZrO₂–SnO₂–TiO₂ system containing La₂O₃ and NiO as sintering aids were prepared and investigated by XRD, XPS, SEM, EDS, and microwave dielectric measurements. Ceramics prepared with defloculed slurry using a Dyno-Mill^®, and sintered at 1370°C exhibit very good microwave dielectric characteristics: ε=37 and QF up to 62 000 at 4 GHz. We have observed a matrix phase with at least two or three different secondary phases in function of the employed grinding media (balls of zircon or magnesium stabilised zirconia). So, we have synthesised and characterised these phases.     

Doping behaviors of NiO and Nb2O5 in BaTiO3 and dielectric properties of BaTiO3-based X7R ceramics

Doping behaviors of NiO and Nb₂O₅ in BaTiO₃ in two doping ways and dielectric properties of BaTiO₃-based X7R ceramics were investigated. When doped in composite form, the additions rendered higher solubility than that doped separately due to the identical valence between the complex (Ni_1/3²⁺Nb_2/3⁵⁺)⁴⁺ and Ti⁴⁺. NiO–Nb₂O₅ composite oxide was more effective in broadening dielectric constant peaks which was responsible for the temperature-stability of BaTiO₃ ceramics. A reduction in grain size was observed in the specimens with 0.5–0.8 mol% NiO–Nb₂O₅ composite oxide, whereas the abnormal growth of individual grains took place in the 1.0 mol% NiO–Nb₂O₅ composite oxide-doped specimen. When the specimen of BaTiO₃ doped with 0.8 mol% NiO–Nb₂O₅ composite oxide was sintered at 1300 °C for 1.5 h in air, good dielectric properties were obtained and the requirement of (EIA) X7R specification with a dielectric constant of 4706 and dielectric loss lower than 1.5% were satisfied.     

ZrO2-doped Y2O3 transparent ceramics via slip casting and vacuum sintering

Commercial Y₂O₃ powder was used to fabricate highly transparent Y₂O₃ ceramics with the addition of ZrO₂ via slip casting and vacuum sintering. The effects of ZrO₂ addition on the transparency, grain size and lattice parameter of Y₂O₃ ceramics were studied. With addition of ZrO₂ the transparency of Y₂O₃ ceramics increased markedly and the grain size of Y₂O₃ ceramics decreased markedly by cation diffusivity mechanism and the lattice parameter of Y₂O₃ ceramics slightly decreased. The highest transmittance (at wavelength 1100 nm) of the 5.0 mol% ZrO₂–Y₂O₃ ceramic (1.0 mm thick) sintered at 1860 °C for 8 h reached 81.7%, very close to the theoretical value of Y₂O₃.     

Ultrafast preparation of Al2O3–ZrO2 multiphase ceramics with eutectic morphology via flash sintering

Herein, Al₂O₃–ZrO₂ multiphase ceramics with a eutectic molar ratio were prepared using the flash sintering technique at 1200 °C in 2 min. The samples were divided into two regions according to the microscopic morphology: multiphase ceramics zone and eutectic ceramics zone. The multiphase ceramics zone uniformly distributed irregular Al₂O₃ and ZrO₂ phases. The eutectic zone presented typical eutectic morphology, where different morphologies of ZrO₂ was uniformly embedded in the Al₂O₃ matrix, mainly showing fine lamellar, rod-like and irregular network morphology. Overall, flash sintering is a promising route to prepare high-performance multiphase ceramics.     

Fabrication and characterization of highly transparent ZrO2-doped Tm2O3 ceramics

Highly transparent polycrystalline Tm₂O₃ ceramics were successfully fabricated by vacuum sintering at temperatures from 1650 to 1850 °C for 8 h using commercial Tm₂O₃ and ZrO₂ (1 at%) powders as starting materials. It is the first time that ZrO₂ was reported as a sintering additive to prepare Tm₂O₃ transparent ceramics. The effects of sintering temperature on the optical transmittance and microstructure of Tm₂O₃ transparent ceramics were studied. The desired Tm₂O₃ ceramics with relative density of 99.8% and an average grain size of approximately 9.7 μm were obtained at 1800 °C and the in-line transmittance reached 75% at 880 nm and fluctuated around 80% from 2100 to 2400 nm, respectively. This study demonstrated that Tm₂O₃ transparent ceramics with higher in-line transmittance and smaller grain size could be prepared by using ZrO₂ as sintering additive at a relatively lower vacuum sintering temperature compared to those already reported in open literatures.     

The origin of microstructural inhomogeneity in vacuum-sintered ZrO2-doped Lu2O3 transparent ceramics

Highly transparent Lu₂O₃ ceramics were prepared by using co-precipitation combined with vacuum sintering method with ZrO₂ as sintering aid. The effect of ZrO₂ on densification, transmittance, and microstructure evolution of the Lu₂O₃ ceramics was carefully studied. It was found that the addition of ZrO₂ was very effective in improving densification of Lu₂O₃. The highest transmittance (at 600 nm) of the 3 at.% ZrO₂ doped Lu₂O₃ ceramic (1.6-mm thickness) sintered at 1800°C reached 80.1%. Microstructural inhomogeneity was found after vacuum sintering with larger grain sizes at the central. The microstructural inhomogeneity mainly occurred in the final stage of sintering. Doping ZrO₂ mitigated microstructural inhomogeneity and decreased markedly the grain size of Lu₂O₃ ceramics. Raman measurements indicated that the disordered structure was formed due to oxygen vacancy, and the oxygen vacancy concentration at the central was higher than at the peripheral. The oxygen vacancy concentration gradient is the dominant factor governing nonuniform microstructure evolution during vacuum sintering. Furthermore, a uniform microstructure was obtained by the inhibition of oxygen vacancy formation by the Lu₂O₃/ZrO₂ double powder bed.     

Catalytic oxidation of CO on Sodium-treated NiO/Al2O3 solid

The influence of Na-pretreatment of γ-Al₂O₃ on the catalytic oxidation of CO over NiO/Al₂O₃ was investigated. The effect of such treatment on the concentration of the catalytically active phase (NiO) in the uppermost surface layers of the catalyst was also studied by the X-ray photoelectron spectroscopic investigation of various solids.The results revealed that the addition of Na₂O (0.45 wt%) decreased the catalytic activity (reaction rate constant per unit surface area) of the pretreated solid. However, the activity was found to increase monotonically by increasing the amount of sodium added in the range 1.35–6 wt%. The sodium-pretreatment of γ-Al₂O₃ resulted in a decrease in the concentration of Ni on the catalyst surface to an extent proportional to the amount of Na2O present. These findings have been attributed to blocking of some sites of Al2O3 surface by Na+ ions thus decreasing the amount of Ni²⁺ ions retained on its surface. The observed increase in catalytic activity of sodium-treated solid (containing more than 0.45 wt%) might be due to dissolution of some Na⁺ ions in the NiO lattice which lowered its Fermi-level, leading to an increase in the degree of p-type semiconducting character of the doped NiO catalyst.     

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.     

Effects of Nb2O5 addition on the sinterability,microstructure and mechanical behaviour of ZTM-A12O3

ZTM–Al₂O₃ ceramics doped with Y₂O₃, MgO and Nb₂O₅ were studied. These ceramics could achieve high density at a sintering temperature as low as 1390°C. Results showed that there was a critical Nb₂O₅/ZrO₂ weight ratio. When Nb₂O₅/ZrO₂ was lower than the critical value, Nb₂O₅ addition had little influence on the sintering, otherwise the sinterability could be considerably improved. The microstructure of all the samples were homogenous; no obvious difference in microstructure had been brought about by Nb₂O₅. Despite this, X-ray diffraction measurement showed a continuous increase in m-ZrO₂ with Nb₂O₅, due to a deficiency of oxygen vacancy in ZrO₂. As a result, the mechanical properties of the ceramics, especially toughness, were greatly improved, because of the enhancement of microcrack toughening mechanism by m-ZrO₂.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

High-gravity activated SHS of large bulk Al2O3/ZrO2 (Y2O3) nanocrystalline composites

Large bulk Al₂O₃/ZrO₂ (Y₂O₃) composites were in-situ prepared by SHS under varied high-gravity from ZrO₂ + Y₂O₃powder blends with an added thermit mixture. Investigated was the effect of high gravity on the microstructure, crystal growth, and properties of synthesized materials. The XRD data suggest that high gravity did not bring about any change in the phase constitution of the composite ceramics and that the ceramic matrix was composed of α-Al₂O₃, t-ZrO₂, and m-ZrO₂. SEM and EDS data show that, with increasing level of high gravity, the morphology of the ceramic microstructures transformed from the cellular eutectics to the rodshaped colonies, and the volume fraction and aspect ratio of the rod-shaped colonies increased while the rodshaped colonies were refined. Above 200 g, the microstructures of composite ceramics developed as the randomlyorientated rod-shaped colonies with a symmetrical triangular dispersion of tetragonal ZrO₂ fibers of 300 nm in the average diameter. Relative density, hardness, flexural strength and fracture toughness simultaneously reached the highest values of 98.6%, 18.6 GPa, 1248 MPa, and 15.6 MPa m^1/2 as the maximum high-gravity level of 250 g was achieved. An increase in the relative density and hardness of the ceramics with increasing gravity level was attributed to the acceleration of gas escape from SHS melts and the elimination of shrinkage cavity in the ceramics under the action of high-gravity field. The increase in fracture toughness results from the enhancement of the coupled toughening mechanisms while the increase in flexural strength comes from the refinement of the microstructures, decrease in critical defect size, and achievement of high fracture toughness.     

Effect and mechanism of ZrO2 doping on the cracking behavior of melt-grown Al2O3 ceramics prepared by directed laser deposition

Directed laser deposition (DLD) is a new method for rapidly preparing melt-grown ceramics, but cracking problem greatly limited its application. In this study, cracking behavior of Al₂O₃ ceramics was suppressed by doping ZrO₂. Crack suppression mechanism of ZrO₂ doping in melt-grown ceramics was also analyzed. Process parameters which are prone to generating cracks were adopted in the experiments, and they contribute to showing the crack clearly. Results show that ZrO₂ doping has remarkable crack suppression effects. It is most obvious when ZrO₂ content is 37 mol%. Compared with those of pure Al₂O₃ ceramics, crack density reduces by 43.2%, and the number of longitudinal main cracks reduces by 63.2%. Doping of ZrO₂ forms dense composite microstructure with primary α-Al₂O₃ grains discretely distributing in eutectic continuous matrix. Therefore, initial crack sources are effectively reduced. Morphology of primary Al₂O₃ grains transforms from cellular to dendritic, which changes crack propagation mode from inter-granular to trans-granular. Mismatch of thermo-physical properties of different phase promotes the arrest, deflection, and bridging phenomena in crack propagation, contributing to crack suppression. On the basis of ZrO₂ doping, we have realized the preparation of crack-free eutectic ceramic (37 mol%ZrO₂) samples through further process optimization. The maximum size of the sample reaches 230 mm.     

Fabrication of black-colored CuO–Al2O3–ZrO2 ceramics via heterogeneous nucleation method

We have reported the fabrication of black-colored CuO–Al₂O₃–ZrO₂ ceramics via a heterogeneous nucleation method. The as-prepared ZrO₂ ceramics exhibit a deep and genuine black color with a uniform color distribution. PEG2000 has been used as the dispersants for the preparation of ZrO₂ nanopowder solutions. Then Cu and Al hydrates have been introduced into the solutions with tailored pH values for the fabricated coated ZrO₂ powders, to induce the heterogeneous nucleation of the colorants within ZrO₂ matrixes. CuO and Al₂O₃ transformed from Cu and Al hydrates act as the black colorant and colorant stabilizer, respectively, which make the fabrication of black ZrO₂ at a relatively low sintering temperature of 1200 °C. The fabricated black-colored ZrO₂ ceramics are characterized by scanning electron microscopy, X-ray diffraction and optical reflectance spectrum. It can be believed that current work could present a facile and cost-saving method for fabrication of black-colored ZrO₂ ceramics without using any toxic chromophore elements.     

Microstructure and properties of large bulk Al2O3/ZrO2 (Y2O3) composites prepared by SHS under high gravity

For Al₂O₃/ZrO₂ (4Y) composites prepared by SHS under high gravity, the correlation between the microstructure and properties of the materials was investigated by adjusting the ZrO₂ (4Y) content. The results indicated that, as the volume fraction of ZrO₂ (4Y) was below 37%, the composite ceramics were mainly composed of the rod-shaped and randomly-orientated colonies in which nano-micrometer tetragonal ZrO₂ fibers were embedded; as the volume fraction of ZrO₂ (4Y) was above 40%, the composite ceramics in the matrix of micrometer sphere-like tetragonal ZrO₂ grains were obtained. The mechanical properties showed that Al₂O₃/33% ZrO₂ (4Y) not only had the maximum values of the relative density and hardness because of the low solidification temperature and the highest volume fraction of the colonies but also the maximum flexural strength value due to the small size of defects and high fracture toughness supported by the crack-deflection and crack-bridging toughening mechanisms.     

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Effect of the Y2O3–ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane

Nickel catalysts with a load of 5 wt.% Ni, supported on pure ZrO₂ and ZrO₂ stabilized with 4 mol%, 8 mol% and 12 mol% of Y₂O₃, were prepared by the polymerization method. The samples were characterized by X-ray diffraction (XRD), temperature-programmed reduction with hydrogen (TPR-H₂), specific surface area (BET) and electronic paramagnetic resonance (EPR) and tested as catalysts for carbon dioxide reforming of methane. The XRD patterns showed the presence of the oxide precursor (NiO) and the tetragonal phase of a Y₂O₃–ZrO₂ solid solution. According to the TPR-H₂ analysis, the reduction of various NiO species was influenced by the composition of the support. Catalytic tests were conducted at 800 °C for 6 h, and the composition of the gaseous products and the catalytic conversion rate depended on the composition of the Y₂O₃–ZrO₂ solid solution and its influence on the supported NiO species. A direct relation was observed between the variation in the support, the nickel species supported on it and the performance in the catalytic tests.     

A comparative study of thermal conductivity and thermal emissivity of high temperature solar absorber of ZrO2 /Fe2O3 and Al2O3/CuO ceramics

Oxide ceramics are considered as promising high temperature solar absorber materials. The major aim of this work is the development of a new solar absorber material with promising characteristics, high efficiency and low-cost processing. Hence, this work provides a comparative and inclusive study of densification behavior, microstructure features, thermal emissivity and thermal conductivity values of the two new high temperature solar absorbers of ZrO₂/Fe₂O₃ and Al₂O₃/CuO ceramics. Ceramic composites of ZrO₂/(10–30 wt%) Fe₂O₃ and Al₂O₃/(10–30 wt%) CuO were prepared by pressureless sintering method at a temperature of 1700 °C/2hrs. Identification of the solar to thermal efficiency of the composites was evaluated in terms of their measured thermal emissivity. Thermal efficiency and heat transfer homogeneity were investigated in terms of thermal conductivity and diffusivity measurement. The results showed that both composites exhibited comparable densification behavior, homogenous and harmonious microstructure. However, Al₂O₃/10 wt% CuO composite showed higher thermal and solar to thermal efficiencies than ZrO₂/Fe₂O₃ composites. It gave the lowest and the best thermal emissivity of 0.561 and the highest thermal conductivity of 15.4 W/m. K. These values proved to be the best amongst all those of the most known solar absorber materials made from the expensive SiC and AlN ceramics. Thus, Al₂O₃/CuO composites have succeeded in obtaining outstanding properties at a much lower price than its other competitive materials. These results may strongly identify Al₂O₃/CuO composites as promising high-temperature solar absorber materials instead of ZrO₂ and the other carbide and nitride ceramics.