Crystal-size dependence of the spark-plasma-sintering kinetics of ZrB2 ultra-high-temperature ceramics

The crystal-size dependence of the spark-plasma-sintering (SPS) kinetics of ZrB₂ ultra-high-temperature ceramics (UHTCs) was investigated. It was found that refining the starting powder enhances the SPS kinetics, reducing the onset temperatures of sintering and of the intermediate and final sintering regimes, as well as promoting a greater maximum shrinkage rate at lower temperatures. This enhancement was only relevant with reduction in crystal size to the nanoscale. Finally, the implications for low-temperature sintering of ZrB₂ UHTCs are discussed.     

Effect of high-energy ball-milling on the spark plasma sinterability of ZrB2 with transition metal disilicides

A wide suite of powder mixtures of ZrB₂ with five different additions of transition metal disilicides (MeSi₂; 5, 17.5, or 30 vol% MoSi₂; 17.5 vol% TaSi₂; or 17.5 vol% ZrSi₂) were prepared by high-energy ball-milling (HEBM) for different times, and their optimal densification temperatures were evaluated by dilatometric spark-plasma-sintering (SPS) tests to compare their sinterability. SPS densification tests at the so-determined optimal densification temperatures were also performed in selected cases. It was found that HEBM progressively enhanced the sinterability, especially once the ZrB₂ particle sizes were refined to the ultrafine range or below (<250 nm). It was also observed that the sinterability was enhanced (i) with the greater addition of MeSi₂, and (ii) with the lower refractoriness of the MeSi₂. Nonetheless, under long-time HEBM, the addition of harder, more refractory MeSi₂ or of softer, less refractory MeSi₂ is equally effective in terms of sinterability, both enabling low-temperature densification (SPS at ∼1200 °C).     

Electrical properties and microstructure of Y-doped BaTiO3 ceramics prepared by high-energy ball-milling

The effects of corn-starch content and high-energy ball-milling time on the microstructure and electrical properties of porous Y-doped (Ba, Sr)TiO₃ samples were investigated. All the (Ba, Sr)TiO₃ samples at room temperature crystallized in the tetragonal structure and the crystal structure was independent of the corn-starch content and ball-milling time. We found that the corn-starch additive and the ball-milling time affected the microstructure and the electrical properties. The (Ba, Sr)TiO₃ samples exhibited a large PTCR jump (>10⁵) due to the high porosity. The PTCR jump of the samples increased with increasing corn-starch content, while it decreased with increasing ball-milling time. In addition, the resistivity increased with increasing corn-starch content, while it decreased with increasing ball-milling time.     

ZrO2 removing reactions of Groups IV-VI transition metal carbides in ZrB2 based composites

In this paper, ZrO₂ removing reactions of Groups IV-VI transition metal carbides (MCs, M = Hf, Nb, Ta, W, Ti and V) in ZrB₂ based ultra-high temperature ceramics (UHTCs) are investigated. Distinct roles of various MCs were observed during this process. According to thermodynamic analysis and experiment verifications, the sequence for oxide removing ability of different MCs is WC > VC > NbC > TaC > HfC > TiC. The importance of this study concerns the establishment of a map of reactivity of the transition metal carbides against ZrO₂, in order to choose the proper additive for the densification of ZrB₂-composites.Utilizing right reactions and controlling proper sintering atmospheres, high density ZrB₂-SiC ceramic could be eventually obtained by pressureless sintering at temperatures (around 2000-2200 °C).     

Surface densification of porous ZrB2-39 mol.% SiC ceramic composites by a laser process

40% porous composites ZrB₂-39 mol.% SiC were irradiated under a mobile laser beam, with a low power of 90 W, in a protective atmosphere of flowing argon. Quasi-full densification of the surface zone was obtained, with thicknesses of more than 20 μm. Temperature reached ca. 2500 °C and a fusible phase rich in SiC appeared, which partially dissolved the ZrB₂, favouring its sintering. The liquid phase filled the porosity present between the ZrB₂ grains, and migrated towards the surface and the bulk of the samples. After cooling, the liquid phase demixed, according to the ZrB₂-SiC phase diagram, and the phases ZrB₂ and SiC interweaved in a eutectic-like solid phase, co-existing with granular zones mainly composed of ZrB₂. Only few cracks were observed. Traces of free carbon were found at the surface, while oxygen never penetrated inside the samples, despite the presence of traces of this element in the surrounding atmosphere. These two last observations were explained by a thermodynamic study. The pellets so obtained could find applications in the fields of aerospace and of low-temperature protonic ceramic fuel cells.     

Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

Fabrication and characterization of ZrB2–SiC ceramic electrodes coated with a proton conducting, SiO2-rich glass layer

3.5 μm thin layers of dense, crack-free, proton conducting, SiO₂-rich glass have been developed on ZrB₂–SiC ceramic composites, by thermal oxidation at 1400 °C for 30 min in air. A conductivity of 2 mS cm⁻¹ at 25 °C was found, as measured by AC impedance and steady-state voltammetry, and was estimated at ca. 2 × 10⁻² S cm⁻¹ at 80 °C. A striking behaviour of the oxidized ZrB₂–SiC composites is also pointed out: underneath the glass layer, there is a porous layer rich in electronic conductive ZrB₂, without well-defined interface between them, i.e., exhibiting a composition gradient in oxygen. In other words, protonic half-fuel cells could be fabricated under such conditions, for future use in hydrogen or direct alcohol fuel cells.     

Characterization of the structure of TiB2/TiC nanocomposite powders fabricated by high-energy ball milling

TiB₂/TiC nanocomposite powders were successfully prepared by high-energy ball milling of the powder mixtures of Ti and B₄C. X-ray diffraction analysis showed that the TiC phase was not produced until the milling time was up to 24 h and only a minimal amount of TiB₂ was generated, even after 48 h of milling. The critical grain size of Ti milled for the reaction between Ti and B₄C was 31.2 nm. Transmission electron microscopy clearly indicated that the resulting powder mixture obtained after milling for 48 h and annealing at 800 °C for 30 min was composed of nanosized TiC and TiB₂ particles.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Cf/SiC 复合材料的 ZrB2-SiC/SiC 超高温陶瓷涂层研究

采用两步包埋法在Cf/SiC复合材料表面制备了Zr B_2-SiC/SiC超高温陶瓷涂层。借助SEM、XRD对涂层的微观结构及物相组成进行了分析研究,并进行了高温静态氧化和热震测试。研究表明,1500°C氧化5 h后,涂层表面覆盖有平整的玻璃相氧化层,氧化失重率为6.4%;热震测试10次后涂层的氧化失重率为14%。Zr B_2-SiC/SiC涂层能有效提高Cf/SiC复合材料的高温抗氧化性能。     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Influence of TiO2 and CeO2 on the hydrogenation activity of bulk Ni2P

TiO₂- and CeO₂-promoted bulk Ni₂P catalysts were prepared by impregnation and in-situ H₂ temperature-programmed reduction method. The prepared catalysts were characterized by XRD and XPS. The hydrogenation activities of the catalysts were studied using 1.5 wt.% 1-heptene in toluene and 1.0 wt.% phenylacetylene in ethanol as the model feeds. The results indicate that bulk Ni₂P possesses low hydrogenation activity but is tunable by simply controlling the content of the additives (TiO₂ or CeO₂), suggesting that TiO₂ and CeO₂ are effective promoters to enhance the hydrogenation activity of Ni₂P.     

Enhancement of oxygen storage capacity by reductive treatment of Al2O3 and CeO2–ZrO2 solid solution nanocomposite

Oxygen storage capacity (OSC) of CeO₂–ZrO₂ solid solution, Ce_xZr_(1−x)O₄, is one of the most contributing factors to control the performance of an automotive catalyst. To improve the OSC, heat treatments were employed on a nanoscaled composite of Al₂O₃ and CeZrO₄ (ACZ). Reductive treatments from 700 to 1000 °C significantly improved the complete oxygen storage capacity (OSC-c) of ACZ. In particular, the OSC-c measured at 300 °C reached the theoretical maximum with a sufficient specific surface area (SSA) (35 m²/g) after reductive treatment at 1000 °C. The introduced Al₂O₃ facilitated the regular rearrangement of Ce and Zr ions in CeZrO₄ as well as helped in maintaining the sufficient SSA. Reductive treatments also enhanced the oxygen release rate (OSC-r); however, the OSC-r variation against the evaluation temperature and the reduction temperature differed from that of OSC-c. OSC-r measured below 200 °C reached its maximum against the reduction temperature at 800 °C, while those evaluated at 300 °C increased with the reduction temperature in the same manner as OSC-c.     

Preparation of ZrO2/Gd2O3 composite ceramic materials by coprecipitation method

High burnup is a goal for further development of advanced nuclear power in the future. However, along with the increase of burnup, it becomes more diffidult to control reactor reactivity, which affects the operation safety of the nuclear reactor. Al₂O₃/B₄C burnable poison materials widely used in pressurized water reactor currently will not meet the requirements of burnable poison materials in high burnup nuclear power. Because of the better performance of ZrO₂/Gd₂O₃ burnable poison materials than that of Al₂O₃/B₄C, this paper studies the preparation of ZrO₂/Gd₂O₃ composite ceramic materials by the coprecipitation method. The experimental results show that at the sintering temperature of 1500–1650 °C, ZrO₂/Gd₂O₃ composite ceramic grains are small, compact and uniform with the generation of homogeneous solid solution. At 1600 °C, ZrO₂–10%Gd₂O₃ has the highest density and mechanical strength.     

Electrocatalytic activity of IrO2-RuO2 supported on Sb-doped SnO2 nanoparticles

Electrocatalytic IrO₂-RuO₂ supported on Sb-doped SnO₂ (ATO) nanoparticles is very active towards the oxygen evolution reaction. The IrO₂-RuO₂ material is XRD amorphous and exists as clusters on the surface of the ATO. Systematic changes to the surface chemical composition of the ATO as a function of the IrO₂:RuO₂ ratio suggests an interaction between the IrO₂-RuO₂ and ATO. Cyclic voltammetry indicates that the electrochemically active surface area of IrO₂-RuO₂ clusters is maximised when the composition is 75 mol% IrO₂-25 mol% RuO₂. Decreasing the loading of IrO₂-RuO₂ on ATO reduces the electrochemically active surface area, although there is evidence to support a decrease in the clusters size with decreased loading. Tafel slope analysis shows that if the clusters are too small, the kinetics of the oxygen evolution reaction are reduced. Overall, clusters of IrO₂-RuO₂ on ATO have similar or better performance for the oxygen evolution reaction than many previously reported materials, despite the low quantity of noble metals used in the electrocatalysts. This suggests that these oxides may be of economic advantage if used as PEM water electrolysis anodes.     

Fabrication of SiCN-Sc2Si2O7 coatings on C/SiC composites at low temperatures

SiCN-Sc₂Si₂O₇ environmental barrier coatings were fabricated on the surface of C/SiC composites at low temperatures by adding Li₂CO₃ as sintering aids. With this addition, the fabrication temperature could be lowered about 100-200 °C. The shrinkage of the polysilazane-Sc₂Si₂O₇ bars with and without Li₂CO₃ was tested by dilatometer. The results indicate that the shrinkage speed of the polysilazane-Sc₂Si₂O₇ bar with Li₂CO₃ is faster than the one without Li₂CO₃, indicating that the Li₂CO₃ greatly promotes the sintering of polysilazane-Sc₂Si₂O₇. Water-vapor corrosion behavior of the SiCN-Sc₂Si₂O₇ coated C/SiC composites was carried out at 1250 °C. The results reveal that the SiCN-Sc₂Si₂O₇ coatings can effectively protect the C/SiC composites. The corrosion resistance of SiCN-Sc₂Si₂O₇ coatings is not degraded by adding Li₂CO₃.     

TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO₂, CeO₂ and CeO₂-doped TiO₂ have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N₂-physisorption, Transmission Electron Microscopy (TEM) and UV-Vis spectrophotometry. The TiO₂ and CeO₂-doped TiO₂ nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10-13 nm for Ce doping concentrations of 5-50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO₂. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce^3+/4+ in the TiO₂ matrix, which generated an n-type impurity band.