The preparation of core–shell Al2O3/Ni composite powders by electroless plating

Core–shell nanostructured Ni-coated Al₂O₃ composite powders were synthesised by using the electroless plating method. The influence of the chemical components and powder concentration in the Ni coating was investigated by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction techniques. The results show that the concentration of the plating components plays an important role in the formation of core–shell Al₂O₃/Ni composite powders. The nickel content in the composite powders could be effectively controlled by adjusting the nickel chloride content and the concentration of NaH₂PO₂·H₂O in the plating solution. The nanostructure of the crystalline Ni coatings was observed to be very attractive for achieving good bonding between ceramic particles and matrices for composite production.     

The role of K2O on sintering and crystallization of glass powder compacts in the Li2O–K2O–Al2O3–SiO2 system

The effects of K₂O content on sintering and crystallization of glass powder compacts in the Li₂O–K₂O–Al₂O₃–SiO₂ system were investigated. Glasses featuring SiO₂/Li₂O molar ratios of 2.69–3.13, far beyond the lithium disilicate (LD-Li₂Si₂O₅) stoichiometry, were produced by conventional melt-quenching technique. The sintering and crystallization behaviour of glass powders was explored using hot stage microscopy (HSM), scanning electron microscopy (SEM), differential thermal (DTA) and X-ray diffraction (XRD) analyses. Increasing K₂O content at the expense of SiO₂ was shown to lower the temperature of maximum shrinkage, eventually resulting in early densification of the glass-powder compacts. Lithium metasilicate was the main crystalline phase formed upon heat treating the glass powders with higher amounts of K₂O. In contrast, lithium disilicate predominantly crystallized from the compositions with lower K₂O contents resulting in strong glass–ceramics with high chemical and electrical resistance. The total content of K₂O should be kept below 4.63 mol% for obtaining LD-based glass–ceramics.     

Effect of MgO/Al2O3 ratio on the crystallization behaviour of Li2O–MgO–Al2O3–SiO2 glass-ceramic and its wettability on Si3N4 ceramic

The composition governs the crystallization ability, the type and content of crystal phases of glass-ceramics. Glass-ceramic joining materials have generated more research interest in recent years. Here, we prepared a novel Li₂O–MgO–Al₂O₃–SiO₂ glass-ceramic for the application of joining Si₃N₄ ceramics. We investigated the influence of the MgO/Al₂O₃ composition ratio on microstructure and crystallization behaviour. The crystallization kinetics demonstrated that the glasses had excellent crystallization ability and high crystallinity. β-LiAlSi₂O₆ and Mg₂SiO₄ were precipitated from the glass-ceramics, and the increase of MgO concentration was conducive to the precipitation of Mg₂SiO₄. Among the glass-ceramic samples, the thermal expansion coefficient of LMAS2 glass-ceramic was 3.1 × 10^?6/°C, which was very close to that of Si₃N₄ ceramics. The wetting test showed that the final contact angle of the glass droplet on the Si₃N₄ ceramic surface was 32° and the interface was well bonded.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

Effect of ZnO on the crystallization behavior and properties of SiO2–CaO–Al2O3–Fe2O3 glass-ceramics prepared from simulated secondary slag after reduction of copper slag

The feasibility of preparing low-cost glass-ceramics from Zn-containing dust and secondary molten slag generated during the carbothermal reduction of copper slag was investigated. Analytical-grade agents, such as ZnO, Fe₂O₃, SiO₂, CaO, and Al₂O₃, were used to simulate the dust and secondary slag. The effect of ZnO content on the crystallization behavior, structure, and mechanical properties of the glass-ceramics was investigated through X-ray diffraction analysis, scanning electron microscopy-energy dispersive spectrometry, differential scanning calorimetry, Fourier transform infrared spectroscopy, and Raman spectroscopy. The results showed that with increased ZnO content from 0 to 6 wt%, the crystallization activation energy of base glass increased from 386.05 to 425.89 kJ/mol. Meanwhile, the average value of the crystal growth index increased from 1.91 to 4.10, and the highest crystallization rate of the glass-ceramics increased from about 1.44 to 23.11 mm³/min. The increased ZnO in glass-ceramics promoted the precipitation of gehlenite, but inhibit the crystallization of anorthite. When the ZnO content was 6 wt%, the comprehensive properties of the glass-ceramics were better; the flexural strength, microhardness, volume density, water absorption rate, and open porosity were 58.67 MPa, 738.35 HV, 2.92 g/cm³, 0.44% and 1.27%, respectively.     

Steam reforming of bio-oil derived small organics over the Ni/Al2O3 catalyst prepared by an impregnation–reduction method

An impregnation–reduction method has been developed in this study to prepare the Ni/Al₂O₃ catalyst for steam reforming of bio-oil model compounds for hydrogen production. This method can alleviate the interactions between nickel species and an alumina carrier and promote metal dispersion and utilization of the nickel species, achieving a highly active, selective and stable reforming catalyst. Moreover, this method is very simple and fast. Only the impregnation and reduction steps are involved and the catalyst can be prepared in situ. In addition, coke formation in steam reforming is not uniform on a catalyst bed. Coking in the topmost catalyst layer is much more serious.     

Synthesis of menthols from citral on Ni/SiO2–Al2O3 catalysts

The one-pot synthesis of menthols from citral was studied on Ni/SiO₂–AlO₃ catalysts containing 3.6%, 8.8% and 11.4% Ni. The yield of menthols increased with the amount of Ni up to 94% on Ni(11.4%)/SiO₂–AlO₃, reflecting the diminution of byproducts formation via acid-catalyzed reactions. The sample deactivation was studied by performing two consecutive catalytic tests. Results showed that Ni(11.4%)/SiO₂–AlO₃ was a stable, active, and highly selective catalyst because it contained the appropriate density and strength of bifunctional acid/Ni⁰ active sites to efficiently promote the hydrogenation/isomerization pathway involved in the reaction network while avoiding coke formation.     

Effect of solid loading on properties of Y2O3–Al2O3–Nd2O3 powder mixtures obtained by planetary ball milling and ceramics based on them

The effect of the solid loading (41–50 wt%) of the slurry on granulometric composition and physico-chemical characteristics of Y₂O₃–Al₂O₃–Nd₂O₃ powder mixtures obtained by planetary ball milling has been studied for the first time. It was shown that the particle size distribution of powder, its Zeta potential, and specific surface area depend on the solid loading of the milled slurry and, consequently, on the interparticle distance during milling. The interparticle distance decreases from 200 nm to 142 nm with an increase of solid loading in the range of 41–50 wt%. It was shown that for the solid loading of 47 wt%, the convergence of particles to a distance comparable to their median diameter promotes subsequent clustering of particles. This facilitates the sintering of highly-homogenous ceramics. It was found that solid loadings in the 46–50 wt% range is useful for obtaining high-quality Nd:YAG transparent ceramics. The lowest optical losses optical losses of 1 × 10^?3 cm^?1 and the highest in-line transmittance of 84.1%@1064 nm were obtained for 1 at.% Nd:YAG transparent ceramics (22 × 3 × 4 mm³) prepared from slurries with 47 wt% solid loading (taking all other ball milling parameters fixed). If the interparticle distance in the powder is higher (solid loading of 41 wt%) than the median particle diameter, the ceramics are characterized by significant residual porosity due to the survival of large particles (insufficient milling).     

Promotional effect of H2O on the activity of In2O3‐doped Ga2O3–Al2O3 for the selective reduction of nitrogen monoxide

Effect of metal oxide additives on the catalytic performance of Ga₂O₃–Al₂O₃ prepared by the sol–gel method for the selective reduction of NO with propene in the presence of oxygen was studied. Of several metal oxide additives, the addition of In₂O₃ enhanced drastically the activity of Ga₂O₃–Al₂O₃ for NO reduction by propene in the presence of H₂O. In addition, the activity of In₂O₃‐doped Ga₂O₃–Al₂O₃ catalyst was extremely intensified by the presence of H₂O below 350°C. The promotional effect of H₂O was interpreted by the suppression of undesirable propene oxidation and the removal of carbonaceous materials deposited on the catalyst surface. We also found that close interaction of In₂O₃ and Ga₂O₃ is necessary for the enhancement of activity by H₂O. A lot of hydrocarbons except methane and oxygenated compounds served as good reducing agents, among which propene and 2‐propanol were the most efficient ones. In₂O₃‐doped Ga₂O₃–Al₂O₃ catalyst was capable of reducing NO into N₂ quite efficiently in the presence of H₂O at a very high space velocity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of mechanochemical treatment of Ni–Al(OH)3 on the preparation of carbon nanomaterials by thermal CVD

Carbon nanomaterials have been prepared by the thermal chemical vapor deposition (CVD) method in which C₂H₂ gas was deposited on the Ni–Al(OH)₃ mixture pretreated by mechanochemical treatment with a high-energy mill. As the duration time of grinding for the Ni–Al(OH)₃ mixture by the mixer mill is increased, amorphous Al(OH)₃ and smaller Ni particles agglomerated into spheres. With unground and 120-min ground mixture of Ni–Al(OH)₃, carbon nanomaterials were prepared at 500 and 600 °C. As a result, carbon nanomaterials prepared on a ground mixture have more uniform morphologies than those of the unground mixture. The characterization of Ni–Al(OH)₃ mixture and as-prepared carbon samples was done by X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectroscopy and current–voltage (I–V) measurement.     

In situ synthesis of NbB2–Al2O3 nanocomposite powder by mechanochemical processing

This research is based on the production of NbB₂–Al₂O₃ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of niobium, aluminium and boron oxide powders was subjected to high-energy ball milling. The structural evaluation of powder particles after different milling times was conducted by the X-ray diffractometry (XRD), scanning electron microscopy, and transmission electron microscopy. The results showed that during ball milling the Nb/Al/B₂O₃ reacted with a combustion mode producing NbB₂–Al₂O₃ nanocomposite. The XRD analyses exhibited that the NbB₂–Al₂O₃ nanocomposite was formed after 10?h milling time and increasing milling time up to 30?h had no significant effect other than refining the crystallite size. In the final stage of milling, the crystallite sizes of NbB₂ and Al₂O₃ were estimated to be less than 50?nm.     

Investigating the effect of WO3 on the crystallization behavior of SiO2–B2O3 – Al2O3–Na2O – CaO–ZnO high VIS-NIR reflecting glazes

White glass enamels with high solar reflectance and containing different WO₃ concentrations have been prepared and characterized with regard to their optical, mechanical and microstructural characteristics. Upon addition of WO₃ to a glass containing SiO₂, B₂O₃, Al₂O₃, Na₂O, CaO, and ZnO, the crystallization of scheelite follows a crystallization mechanism of bulk type where scheelite grows in one-dimension in a patterned morphology dominated by the heating rate and the concentration of WO₃. Octahedral bipyramids and arrow-like crystals appeared in enamels containing WO₃ concentration above 6%. The presence of scheelite crystals with different orientations also leads to slight variations in hardness and Young modulus thus obtaining Hv values between 8 and 8.8 GPa and E values between 72 and 83 GPa. Similarly, the optical properties such as whiteness, brightness and solar reflectance increase with the presence of scheelite, and the highest solar reflectance occurs for the enamel containing arrow-like and bipyramidal crystals.     

Characterization of MoO3–V2O5/Al2O3 catalysts for selective catalytic reduction of NO by NH3

MoO₃–V₂O₅/Al₂O₃ catalysts were characterized by B.E.T., XRD, LRS, XPS and TPR and the effect of MoO₃ addition to alumina supported vanadia catalysts on the catalytic activity for the selective catalytic reduction of NO by ammonia was investigated. Upon the addition of MoO₃, catalytic activity was enhanced and the particle size of V₂O₅ which is shown by the results of B.E.T., XRD and Raman spectroscopy decreased. This was one reason for increased catalytic activity. The results obtained by XPS and TPR showed that MoO₃ addition to alumina supported vanadia catalysts increased the reducibility of vanadia and this was the another reason for synergy effect between MoO₃ and V₂O₅ in MoO₃–V₂O₅/Al₂O₃ catalysts.     

Fabrication of WSi2–Al2O3 and W5Si3–Al2O3 composites by combustion synthesis involving thermite reduction

Formation of WSi₂–Al₂O₃ and W₅Si₃–Al₂O₃ composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO₃+2Al and 0.6WO₃+0.6SiO₂+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al₂O₃. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi₂/Al₂O₃ from 0.8 to 4.0 were achieved for the production of the WSi₂–Al₂O₃ composites. Due to the lower formation exothermicity, the W₅Si₃–Al₂O₃ composites were produced with a narrower range of W₅Si₃/Al₂O₃ from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi₂ and unreacted W in the as-synthesized W₅Si₃–Al₂O₃ composites.     

Preparation of Al2O3–SiC composite powder from kyanite tailings via carbothermal reduction process

ABSTRACT

Al₂O₃–SiC composite powders were prepared from kyanite tailings mixed with 20% excess carbon coke via carbothermal reduction (CR) reaction. The optimised synthesis condition for synthesising Al₂O₃–SiC composite powders was at 1600°C for 6?h. The equilibrium relationship curves of the condensed phases were presented and the temperature dependence of the phase composition was also studied. The results show that irregular Al₂O₃ and SiC grains first formed at 1500°C, and the elements C, O, Al, and Si randomly distributed in the each crystal particles. The amount of Al₂O₃–SiC composites increased with the increasing synthesis temperature and reaction time. Finally, Al₂O₃–SiC composite bulk materials were further prepared by pressureless sintering using the synthesised Al₂O₃–SiC composite powders as raw materials, and their mechanical properties were investigated in detail. All these results indicate that the CR method can offer a niche application for the development of kyanite tailings.     

Effect of the sintering temperature on microstructure and properties of Al2O3–Cu–Ni hybrid composites obtained by PPS

In the present research, the influence of sintering temperature on the microstructure and properties of Al₂O₃–Cu–Ni hybrid composites prepared by the Pulse Plasma Sintering (PPS) technique were described. In this research, three temperatures have been selected: 1250°C, 1300°C, and 1350°C. SEM observations were carried out to determine the distribution of the metallic phase in the composite depending on the sintering temperature. The conducted experiments and microscopic observations enabled a better understanding of the phenomena occurring between the ceramic matrix and metallic phase in the obtained materials. The mechanical properties like a hardness and fracture toughness were measured. The technology applied allowed us to obtain ceramic-metal composites with a homogeneous microstructure. It was found that the sintering temperature influences the selected physical and mechanical properties of the composites produced. It was found that samples produced at 1300°C are characterized by the highest relative density and the mechanical properties.     

NO presence effects on the reduction of N2O by CO over Al–Pd–Co oxide catalyst

Nitrous oxide (N₂O) is known as one of the greenhouse gases of which the emission levels are to be controlled and also an ozone-depleting material in the stratosphere. For more than a last couple of decades, various catalysts have been investigated for the reduction of N₂O emission. However, most of those catalysts require reaction temperatures as high as 450 °C even with the use of effective reducing agents. Furthermore, NO, which is usually present with N₂O, significantly interferes with the removal efficiency of N₂O. Al–Pd–Co oxide catalyst which is a type of mixed metal oxides (MMO) has shown 100% conversion of N₂O at temperatures as low as 200 °C using CO. This paper examines the effect of NO on the reduction of N₂O with CO over Al–Pd–Co oxide catalyst. The experiments were carried out in the temperature range of 100–500 °C with space velocity of 10,000–50,000 h^?1. Though the efficiency of N₂O reduction is decreased significantly when NO is present, the efficiency increases when sufficient CO is supplied. In this study, the reduction mechanisms of N₂O and NO by CO have been confirmed, and it was shown that MMO catalyst can simultaneously remove N₂O and NO using CO with high efficiency.     

Preparation of ZrB2–SiC–Al2O3 composites by SHS method with aluminothermic reduction

Ternary composites composed of ZrB₂, SiC, and Al₂O₃ with a broad composition range were produced by PTFE-activated combustion synthesis involving aluminothermic reduction of ZrSiO₄, SiO₂, and ZrO₂. Two solid-state combustion systems were investigated: one comprised ZrSiO₄, SiO₂, Al, B₄C, and C and the other consisted of ZrSiO₄, ZrO₂, Al, B₄C, and B powders. It was required for the powder compacts to adopt 3 wt% PTFE as the reaction promoter to initiate and sustain their combustion reaction. The influence of the gas-phase diffusion transport induced by PTFE on the SHS process was confirmed and the activation mechanisms were proposed. With the assistance of PTFE, self-sustaining combustion was established with a distinct combustion wave characterized by a reaction temperature of 1210–1380 °C and a propagation velocity of 2.27–3.16 mm/s. Based on the XRD analysis, the products were ZrB₂–SiC–Al₂O₃ composites with traces of Si, ZrO₂, or ZrC. The synthesized products exhibited a granular morphology. The particle size of the composite powders was in the range of 1–2 μm.     

Lithium concentration effect on crystallization kinetics and spectral properties of Cr-doped Li2O–K2O–Al2O3–B2O3 glass-ceramics

Alkali-aluminaborate glass-ceramics doped with Cr ions are synthesized by volume crystallization. According to non-isothermal DSC method three parallel processes occur in material: 2D Avrami-Yerofeev nucleation, 2D and 3D crystallization. During the heat treatment, the LiAl₇B₄O₁₇ crystalline phase is formed. With Li₂O content rising crystallinity of the material increases from 27 to 69% and the crystalline field strength Dq/B of Cr³⁺ increases from 2.25 to 3.55. The photoluminescence spectra possess intense bands at 685, 700, and 715 nm for glass with 6.8 mol.% Li₂O and higher and its decay kinetics is described by the sum of two exponentials. The maximum luminescence QY obtained is 50% at 16.1 mol.% Li₂O. The highest conversion efficiency of the 532 nm LED luminescence obtained by glass-ceramics with chromium is 10%. Thus, Cr-doped alkali-alumina-borate glass-ceramics are a promising material for use in the design of radiation sources for the red and NIR spectral regions.