Synthesis and characterization of Al2O3 - ZrO2-based eutectic ceramic powder material dispersion-hardened with ZrB2 and WB particles prepared by SHS

Method of self-propagating high-temperature synthesis (SHS) was used to obtain ceramic powder material based on Al₂O₃-ZrO₂-WB/ZrB₂ system with size of particles ranging from 10 to 200?µm. The routes of chemical reactions taking place in the process of synthesis depending on the source mixture composition have been determined. It was shown that the formation of zirconium boride strengthening phase depends on the content of source components В and W and combustion temperature of mixture. The analysis of microstructure has shown that powder particles feature complex composite structure consisting of oxide eutectics of Al₂O₃-ZrO₂, individual phases of aluminum oxide and zirconium oxide as well as particles of WB and/or ZrB2. The technological properties of produced powders have been investigated.     

Ultrafine Si/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites obtained by combining methods of mechanical activation and self-propagating high-temperature synthesis

Mechanical activation of a mixture of silicon oxide and aluminum is studied by methods of infrared spectroscopy, differential thermal analysis, and x-ray diffraction. If a SiO₂/Al mechanocomposite is used as a precursor, then a Si/Al₂O₃ composite with a small grain size and a uniform distribution of the components can be obtained by the method of self-propagating high-temperature synthesis.     

Synthesis of Al3BC in air from mechanically activated Al/B/C powder mixtures

A ternary aluminum borocarbide, Al₃BC was prepared for the first time by self-propagating high-temperature synthesis (SHS) induced by mechanical activation of Al/B/C powder mixtures in air. The effect of mixing molar ratio of Al/B/C and grinding time on the formation of Al₃BC was investigated. On the other hand, Al₃BC was also formed by mechanical activation and subsequent annealing of Al/B/C=3/1/1 powder mixture. The lattice constants of Al₃BC obtained in two methods were compared.     

High-temperature synthesis of composite materials based on (Cr,Mn, V)–Al–C MAX phases

Composite materials based on (Cr, Mn, V)–Al–C MAX phases were obtained by self-propagating high-temperature synthesis (SHS). Regularities of synthesis of composite materials from mixtures containing chromium (III) oxide, manganese (IV) oxide, vanadium (V) oxide, calcium (IV) oxide, aluminum, and carbon powders were studied. The synthesis of 30-g blend was carried out in an SHS reactor with a volume of 3 l under Ar pressure (5 MPa). Variation in the amount of the starting reagents markedly affected the process parameters, phase composition, and microstructure of combustion products. The combustion products were characterized by XRD, SEM, and EDS analysis. For Cr–Al–C system, MAX Cr₂AlC phase in addition to chromium aluminide Cr₅Al₈ and chromium carbides (Cr₇C₃, Cr₃C₂) was detected. SEM studies showed that Cr₂AlC has a laminated structure with layer thickness varying from 3 to 20 nm. XRD pattern of Mn–Cr–Al–C composite material were found to have signals belonging (Cr_xMn_1–x)₂AlC solid solution, Mn₃AlC, and Cr₂Al. It was shown that V–Al–C composite material contains nano-layered MAX V₂AlC phase and particles VC_х, VAl₃.     

Catalytic oxidation of methane on CuO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/FeAlO/FeAl cermet catalysts

The activity of plates of CuO/Al₂O₃/FeAlO/FeAl structured cermet catalysts is compared by varying their alumina content. The catalysts were prepared by impregnation of cermet supports obtained by mechanochemical activation of powder mixtures of an alumina precursor [20–50% (wt.)], iron, and aluminum, followed by hydrothermal treatment and calcination. It is shown that increasing the content of the alumina precursor (product of thermal activation of gibbsite) increases the specific surface area of the support and the mesopore and macropore volumes and reduces its mechanical strength. The content of the active component (CuO) also increases, resulting in an increase in the specific activity of catalyst despite a reduction in the effectiveness of using the active component. The activity of catalysts with a moderate concentration of alumina is sufficient to initiate methane oxidation.     

Rapid fabrication of Al4SiC4 using a self-propagating high-temperature synthesis method

As a promising high-temperature ceramic, aluminum silicon carbide (Al₄SiC₄) has attracted much attention. Al₄SiC₄ is usually synthesized at high temperatures with a long reaction time in an electric furnace. Self-propagating high-temperature synthesis (SHS) is a promising technique for rapid synthesis. In this study, Al₄SiC₄ was prepared by the SHS method from a mixture of silicon, aluminum and carbon black with the addition of poly(tetrafluoroethylene) (PTFE) as an exothermic promoter. The experimental results showed that the use of a high-pressure Ar atmosphere could retain the gaseous materials in the pellet mixture, and the PTFE additive promoted the formation of silicon carbide. In addition, the oxide layer present on the surface of silicon particles inhibited the reaction between silicon and carbon. As a result, high-purity Al₄SiC₄ could be synthesized from aluminum, silicon, and carbon black with 15 wt% PTFE under 1.0 MPa Ar atmosphere in several seconds by the SHS method.     

Colored rubber vulcanizates with some magnetic properties

A new composite based on natural rubber vulcanizates loaded with the newly prepared iron oxide–aluminum oxide (Fe₂O₃·Al₂O₃) fillers were prepared and their physical and magnetic studies were investigated. The prepared fillers were evaluated as reinforcing fillers with some magnetic properties; these properties were dependent on the ratio of iron oxide to aluminum oxide in each prepared ratio of these fillers. Rheological properties of rubber mixes containing (1Fe₂O₃:3Al₂O₃) and (1Fe₂O₃:1Al₂O₃) fillers exhibited better properties than mixes containing (3Fe₂O₃:1Al₂O₃) and (α‐Fe₂O₃), which showed almost the same behavior. Physical properties such as tensile strength, stress at 100 and 200% strain, Young's modulus, and hardness were increased by increasing the volume fraction of the investigated fillers concentration in the mixed vulcanizates. Measured rheological and physical results were inversely related to the magnetic properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:494–505, 2006     

Effect of partial substitution of alumina–chromium slag for Al2O3 on microstructures and properties of Al2O3–SiC–C trough castables

Alumina–chromium slag (ACS), a cheap and abundant refractory raw material comprising aluminum–chromium oxides and β-Al₂O₃, is a byproduct of ferrochrome smelting. For this reason, we investigated the relationships between composition and mechanical properties, abrasion resistance, oxidation resistance, and resistance to iron slag erosion for Al₂O₃–SiC–C trough castables in which ACS was substituted for alumina. Due to the presence of β-Al₂O₃ in ACS, the aluminum-chromium slag reacted with SiO₂ to form a low-melting phase of albite and promoted the formation of mullite, which filled the pores at high temperatures and reduced the porosity, thereby promoting densification and strengthening of the sample. The cold mechanical properties of the sample and the normal temperature wear resistance were enhanced, but the high-temperature mechanical properties and the resistance to iron slag corrosion of the sample were impaired. According to the results of the anti-oxidation experiment, the presence of β-Al₂O₃ in the ACS reduced the porosity and made the sample more dense, which remarkably improved oxidation resistance of the sample. For industrial production requirements, ACS substitution should not exceed 48?wt% due to of thermomechanical properties and anti-slag corrosion performance in Al₂O₃–SiC–C trough castables.     

Study on the preparation of the ceramic composite-lined steel pipe with the SHS reaction system of Al-Fe2O3-Cr2O3

The Al₂O₃-Fe-Cr composite was prepared and lined on the inner surface of the seamless steel pipe with the gravitational separation SHS method by using Al, Fe₂O₃ and Cr₂O₃ powders as the raw materials for improving the properties of the Al₂O₃-Fe-Cr composite-lined steel pipe. The effect of the amount of Cr₂O₃ addition on the preparation process in the SHS reaction system of Al-Fe₂O₃-Cr₂O₃ was discussed, and the micro-structure and properties of the Al₂O₃-Fe-Cr composite were observed and investigated. The results showed that a transition structure was formed in the SHS, which could improve the bonding quality of the composite-lined steel pipe; with the addition of Cr₂O₃, the Fe-Cr alloy formed and could improve the anti-corrosion ability of the Al₂O₃-Fe-Cr composite; the SHS speed decreased and the thickness and hardness of the Al₂O₃-Fe-Cr composite increased with the increase of the Cr₂O₃ addition, and then the abrasion resistance was improved.     

Direct synthesis of NiAl intermetallic matrix composite with TiC and Al2O3 reinforcements by mechanical alloying of NiO–Al–Ti–C powder mixture

NiAl intermetallic matrix nanocomposite with TiC and Al₂O₃ was directly fabricated by mechanical alloying of NiO, Al, graphite, and Ti as the starting powder mixture. The phase and morphological evaluations during mechanical alloying were characterized by X-ray diffraction and scanning electron microscopy equipped by energy-dispersive X-ray spectroscopy, respectively. The thermal behavior of the 40 h milled powder was obtained via differential thermal analysis. Followed by 10 h milling, no new phases were formed; but after 20 h milling, Al reduced a fraction of NiO, NiAl and Al₂O₃ were formed, and the released energy promoted TiC formation. When milling time reached 40 h, the remained raw materials were completely consumed and NiAl/TiC–Al₂O₃ nanocomposite was formed. Therefore, the main stage to synthesize NiAl/TiC–Al₂O₃ nanocomposite was the reduction of NiO by Al. The microstructural evaluation revealed formation of a homogeneously distributed composite and thermal analysis showed that the synthesized product was stable.     

Composite Powders with the Pseudoeutectic Structure in the Al2O3 – TiN System

The processes occurring in the synthesis of composite powders in Al₂O₃ – TiN system are considered for two variants: the first variant is the reaction of carbothermic reduction of TiO₂ mixed with Al₂O₃ with simultaneous nitration, and the second one is direct nitration of metallic titanium dispersed in aluminum oxide under high-frequency heating in a “cold” crucible. The synthesis produces submicron composite powders, which are promising for application in construction ceramics.     

Ethanol‐Water‐Derived Sucrose‐Coated‐Al2O3 for Submicrometer AlON Powder Synthesis

Fluffy and homogenous sucrose‐coated‐γ‐Al₂O₃ structured precursor was prepared by drying ethanol‐water sucrose/Al₂O₃ suspension, in which the ethanol content of 85 vol% was optimized. Using the C/Al₂O₃ mixture pyrolyzed from such precursor with 23.2 wt% sucrose, single‐phase AlON powder was synthesized by two‐step carbothermal reduction and nitridation method at 1550°C for 2 h and 1700°C for another 1.5 h. The particle size of the AlON powder was around 0.6–1.0 μm. Compared with those synthesized by the traditional approaches with mechanical C/Al₂O₃, Al/Al₂O₃, or AlN/Al₂O₃ mixtures, the synthesis temperature was reduced about 50°C, and the AlON powder was fine and exhibited good dispersity. Such superiority of this method was attributed to that the pyrolyzed carbon film on Al₂O₃ particle greatly restrained Al₂O₃ coalescence during the thermal treatment.     

Research on the preparation of Ni/Al2O3 powder by fluidized crystallization granulation–hydrogen reduction

This paper proposes a new process route for the preparation of Ni/Al₂O₃ composite powder through fluidized crystallization granulation–hydrogen reduction and investigates the effects of process feasibility, nickel concentration, nucleation method, and hydrogen reduction on Ni/Al₂O₃-coated powder. The decrease in nickel concentration, heterogeneous nucleation, and natural air-drying properties of precursor particles was beneficial to the formation of a complete and uniform cladding layer, and the increase in hydraulic retention time could improve the thickness of the cladding layer. The optimum process involves a nickel concentration of 50 mg/L; hydraulic residence time of 75 h to ensure heterogeneous nucleation; two-step hydrogen reduction of precursor particles to obtain a complete and uniform Ni/Al₂O₃ cladding layer; and a powder surface nickel content of 59.5%.     

Al2O3–W nanocomposites obtained by colloidal processing and in situ synthesis of nano-metal particles

The preparation process and the properties of Al₂O₃-W nanocomposites obtained by gelcasting are reported. The novelty of the synthesis path is the formation of nano-tungsten particles in an in situ reduction of water-soluble precursor during pressureless sintering. The use of water-soluble salt as a tungsten precursor ensured highly homogenous distribution of reinforcing particles and good adhesion between ceramic and metal phases. The maximum content of tungsten in the composites was approximately 0.5 wt%. The size of the reinforcing particles was less than 100 nm. Presence of metallic tungsten and tungsten carbide (W₂C) in the composites lead to the improvement of mechanical parameters: an increase of hardness by about 10 % and of fracture toughness by about 20 %, compared to the reference sample of pure aluminum oxide.     

Self-propagating high-temperature synthesis of spinel-type pigments

The process of producing spinel-type pigments from a mixture of Al₂O₃ and Co₂O₃ with aluminum powder ASD-4 and MgO and ZnO additives by the method of self-propagating high-temperature synthesis (SHS) is investigated. A blue and an ultramarine pigment are obtained. Their phase composition, IR spectra, and structural and color characteristics are studied. It is demonstrated that the SHS method is promising for producing heat-resistant spinel-type pigments of a blue-sky-blue color range. __________ Translated from Steklo i Keramika, No. 2, pp. 20–21, February, 2006.     

Phase and structure formation mechanisms of SHS synthesized composite coatings

High performance composite coatings were synthesized by self-propagation high-temperature synthesis followed by gravitational-separation process based on thermite reaction. The phase, structure and composition of generated composite coatings were investigated, and formation mechanism was studied by thermodynamic analysis. Results showed that phase composition of Al-Fe₂O₃ reaction system consisted of Al₂O₃, Fe and FeAl₂O₄. In Al-Fe₂O₃/Al-Cr₂O₃ composite reaction system, Fe-Cr alloy was formed and FeAl₂O₄ phase disappeared, which could improve the corrosion resistance of composite coatings. Furthermore, the addition of SiO₂ in SHS reaction favored the formation of low-melting point phase Al₂O₃·SiO₂, which filled into voids of Al₂O₃ dendrites and reduced the porosity of composite coatings, thus improving their strength and densification level. Moreover, the generated transition structure in different reaction systems could buffer the residual stress to promote the binding between the composite coating and steel pipe.     

Novel Ti2O3–Al2O3 raw materials with controllable Ti2O3 content from the aluminothermic reduction of ilmenite

Titanium-containing refractories have excellent performance, but the high cost limits their application. A series of Ti₂O₃–Al₂O₃ raw materials with different Ti₂O₃ contents were prepared by aluminothermic reduction of ilmenite, while the generated ferrotitanium alloy can be used as raw materials for special steels. Regulating the amount of aluminum added in the system regulates the degree of titanium reduction, and the formed ferrotitanium alloy can achieve separation from the oxides. With decreasing aluminum content of, the Ti₂O₃ content increased, and the continuous distribution of the corundum area decreased, resulting in a continuously distributed Ti₂O₃ area. Our results indicated that the molar ratio of aluminum to ilmenite should be higher than 1.4 to achieve slag iron separation. The reaction model for the aluminothermic reduction was established, and the formation mechanism of Al₂O₃ and Ti₂O₃ in the system was discussed.     

Conditions for Preparation of Oxide Components and Their Effect on Properties of Al2O3 – ZrO2 – SiC Composite

Results of a study of phase formation in the Al₂O₃ – ZrO₂ – SiC system sintered under vacuum and in a reducing medium at 1350 – 1550°C are reported. Conditions for preparation of Al₂O₃ and ZrO₂ powders from hydroxides are specified and the effect of specific surface, temperature, and holding time on the reaction between oxide and carbide components is considered. Results of microstructural, x-ray phase, thermogravimetric, and chemical analyses of precursor materials and end products are discussed. Optimum composition (20% SiC, 15% ZrO₂, 65% Al₂O₃ ) and dispersity of the mixture for obtaining a composite with a strength of about 200 MPa by a method other than high-temperature toughening are determined.     

The effect of Al content on the wettability between liquid iron and MgOAl2O3 binary substrate

In the present study, the wettability between liquid iron with two different Al contents and MgOAl₂O₃ binary substrates was studied in reducing atmosphere. The contact angles between liquid iron with 18?ppm Al and MgO, MgO·Al₂O₃, Al₂O₃ were 133.5°, 113.7°, 126.9° respectively. With the variation of the substrate composition, the contact angles for the intermediate binary phases of the three components (MgO, MgO·Al₂O₃, Al₂O₃) obeyed the Cassie theory. In the experiment using iron with 370?ppm Al, all the contact angles were higher than that using low Al-containing iron. The surface of the iron drop was covered with an oxide layer, which mainly consisted of many small particles. With the variation of the substrate gradually from MgO to Al₂O₃, the composition of the oxide layer changed from MgO·Al₂O₃ to CaOAl₂O₃. The formation of the oxide layer prevented the spreading of liquid iron, leading to the increase of the contact angle.     

TPR study of the mechanism of rhenium promotion of alumina-supported cobalt Fischer-Tropsch catalysts

The effect of small amounts of Re on the reduction properties of -alumina supported cobalt catalysts has been studied by temperature-programmed reduction (TPR). An intimate mixture of Co/Al₂O₃ and Re/Al₂O₃ catalysts showed a promoting effect of Re similar to that for coimpregnated Co-Re/Al₂O₃. A loose mixture of Co/Al₂O₃ + Re/Al₂O₃ did not show any effect of Re on the reduction of cobalt. However, when the loose mixture of Co/Al₂O₃ + Re/Al₂O₃ was pretreated with Ar saturated with water before the TPR, a promoting effect of Re on the reduction of Co was observed. It is suggested that Re promotes the reduction of cobalt oxide by hydrogen spillover, and that no direct contact between Re and Co seems to be necessary in order to obtain the promoting effect as observed by TPR. It is also shown that the presence of a high temperature TPR peak at 1200 K assigned to cobalt aluminate is mainly a result of Co-ion diffusion during the TPR and not during calcination.