微波消解-火焰原子吸收光谱法测定氧化铝中的氧化钠

氧化铝中氧化钠含量的测定,一般采用高温熔结,用水浸出钠盐后,用原子吸收火焰光度法测定,该方法的测定周期比较长,而且高温熔结时对铂金皿的损耗比较大。文章提出了试样用磷酸和硫酸混合溶液,在高压微波消解器中进行处理,将样品在较短的时间内消解,并在试液中加入一定量的氯化铯,消除钠的电离干扰,采用原子吸收光谱法直接测定,测试结果与标准样品推荐值比较,结果令人满意。     

Aluminum Yttrium Oxide Metal Matrix Composite Synthesized by Microwave Hybrid Sintering: Processing,Microstructure and Mechanical Response

The current study is focused on the microstructure, phase transition, and mechanical properties of the aluminum yttrium oxide (Al–Y₂O₃) composite material. Microwave hybrid sintering using Y₂O₃ nanoparticles as reinforcement at various (i.e., 0.5, 2, 3.5 and 5) wt% was used. Simultaneous thermal analysis (STA) and X-ray photoelectric spectroscopy (XPS) were used to investigate the chemical interaction between Al and Y₂O₃. This research will aid in gaining a better knowledge of the changes in thermal characteristics and compositional changes that occur throughout the microwave hybrid sintering process. The insight into material properties reveals that intermetallic Al₃Y and Al₂O₃ are generated during the synthesis process, which was substantiated by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDX) analysis. The Al–Y₂O₃ composite material has a well-consolidated microstructure and improved mechanical characteristics. To further understand material behaviour, a robust and non-destructive depth sensing nano-indentation technology was being used. With the addition of 5 wt% Y₂O₃, the microhardness of composite material is enhanced by 1.62 times. Furthermore, with 5 wt% Y₂O₃, the produced composite's nano hardness and elastic modulus augmented by 2.43 and 1.8 times, respectively. It is caused by the presence of intermetallic in the composite material, as well as the prevalence of uniform reinforcement distribution.

     

Microwave Synthesis of Materials from Aluminum Oxide Powders

High-temperature synthesis with the use of microwave energy is used to fabricate corundum materials with a relative density of 95 – 99% of the theoretical value from aluminum oxide powders with different grain sizes and prehistory. The microstructure of the materials is composed of grains with a size close to the initial one and closed spherical pores.     

Stability of the Aluminum Silicates

From the ternary phase diagrams of Al₂O₃–SiO₂ with CaO, MgO, or FeO, it can be concluded that the free energies of formation of kyanite, andalusite, and sillimanite, according to the reaction Al₂O₃+ SiO₂= Al₂O₃.SiO₂, are of the order of magnitude of 0 to –10 kcal. rather than the previously accepted value of –40 to –45 kcal. However, this result may be expected from the general variation of free energies of formation with the ionic potential of the silicate-forming cation; this conclusion is supported by a plot for some silicates, carbonates, and sulfates.     

Aluminum Burn Rate Modifiers Based on Reactive Nanocomposite Powders

Aluminum powders have long been used as additives in propellants, pyrotechnics, and explosives. Aluminum has a high enthalpy of combustion but relatively low burn rate. Addition of reactive nanocomposite powders can increase the burn rate of aluminum and thus the overall reaction rate of the energetic formulation. Replacing only a small fraction of the fuel by a nanocomposite material can enhance the reaction rate with little change to the thermodynamic performance of the formulation. This research showed the feasibility of the above concept using nanocomposite powders prepared by arrested reactive milling (ARM), a scalable “top‐down” technique for manufacturing reactive nanomaterials. The nanocomposite materials used in this study were 2B+Ti, and Al‐rich thermites: 8Al+3CuO, and 8Al+MoO₃. The reactive nanocomposite powders were added to micrometer‐sized aluminum powder and the mixture was aerosolized and burned in a constant volume chamber. The combustion atmosphere was varied using oxygen, nitrogen, and methane. The resulting pressure traces were recorded and processed to compare different types and amounts of modifiers. Additives of nanocomposite powders of 8Al+MoO₃ and 2B+Ti to micrometer‐sized aluminum were found to be effective in increasing both the rate of pressure rise and maximum pressure in the respective constant volume explosion experiments. It was observed that 20 wt.‐% of additive resulted in the best combination of the achieved burn rate and pressure.     

Kinetics of Metal-Ceramic Composite Formation by Reactive Penetration of Silicates with Molten Aluminum

Wetting and reactions between molten Al and silicate substrates (particularly mullite) are studied to determine both how substrate tensity and p (O₂) influence wetting behavior, reaction rates, composition, and reaction product microstructure and what key steps control penetration kinetics. Guidelines are provided for using reactive penetration or infiltration when fabricating metal/ceramic composites. For dense substrates, a reactive penetration process occurs. For a certain range, the chemical reaction between Al and the ceramic is a limiting kinetic step resulting in fast reaction rates. Maximum dense mullite substrate reaction rates are between 1000° and 1200°C independent of p (O₂), unlike fused silica, which has faster penetration rates at higher temperatures. For mullite, reaction layer microstructure evolution halts reaction at higher temperatures. For porous substrates, reactive infiltration alone occurs. Either a critical temperature or p (O₂) must be reached before infiltration starts.     

Low‐Temperature Synthesis of Aluminum Nitride from Transition Alumina by Microwave Processing

Aluminum nitride (AlN) was synthesized by carbothermal reduction and nitridation method from a mixture of various transition alumina powders and carbon black using 2.45 GHz microwave irradiation in N₂ atmosphere. We achieved the synthesis of AlN at 1300–1400°C using 2.45 GHz microwave irradiation for 60 min. Our results suggest that θ‐Al₂O₃ is more easily nitrided than γ‐, δ‐, and α‐Al₂O₃. On the other hand, nitridation ratio of samples synthesized in a conventional furnace under nitrogen atmosphere were zero or very low. These results show that 2.45 GHz microwave irradiation enhanced the reduction and nitridation reaction of alumina.     

Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.     

Evolution of Structure in Sintering Ceramics Based on Aluminum Oxide with a Eutectic Additive

The aspects of sintering of corundum-based ceramics with a eutectic additive of the MnO – Al₂O₃ – SiO₂ system are considered in the context of the evolution of its structure. In sintering in the presence of a small quantity of eutectic, the behavior of the system is largely different before and after the emergence of the liquid phase. Before the liquid phase formation, the process is determined by the properties of the material located at the site of contact between the particles, and afterwards the system becomes sensitive to the state of the surface of the contact between the solid phase and the eutectic liquid. The latter is due to the fact that the eutectic composition is virtually saturated with respect to aluminum oxide.     

Composite Wear-Resistant Material Based on Aluminum Oxide

A wear-resistant composite material based on Al₂O₃ and ZrO₂ obtained by hot pressing and microarc oxidation is described.     

New Ceramic Materials Based on Aluminum Oxide

A classification of the liquefying additives used for fabrication of sintered alumina ceramics is given. The basic principles guiding the chooice of additives for fabrication of high-performance aluminum-oxide materials with a low sintering temperature are formulated. Based on these principles, a range of eutectic systems is proposed which, used as additives to alumina, enables one to fabricate dense ceramic materials at calcination temperatures of 1300 – 1550°C. Specific sintering features of these ceramics are discussed.     

Porous Permeable Ceramics Based on Aluminum Oxide

The effect of additives (within limits of 15 – 30%) of disperse alumina with different content (up to 2.3%) of TiO₂ and MgO on properties of porous permeable materials based on electromelted corundum of different degrees of dispersion is investigated. Porous ceramics with open porosity up to 40% and bending strength up to 25 MPa is obtained, which has high filtration efficiency and chemical stability.     

Processing and Properties of ZrO2-Containing Reaction-Bonded Aluminum Oxide with High Initial Aluminum Contents

The reaction-bonded aluminum oxide (RBAO) process relies upon the oxidation of Al/Al₂O₃ powder compacts, and many of its associated advantages stem from the presence of the aluminum in the green powder. Higher aluminum contents in the starting powders allow for higher green strengths, densities, and lower overall shrinkage, all while producing a fine-grained, high-strength sintered material. However, it is evident that the reaction and sintering of ZrO₂-containing RBAO with higher aluminum contents are more challenging. Therefore, in this study, the effects of aluminum content on the processing, structure, and properties of RBAO ceramics were comprehensively characterized. It was found that RBAO samples with high aluminum contents were more prone to cracking during reaction and even when successfully fired were not able to be sintered to full density. Despite these characteristics, RBAO samples with increasing aluminum contents showed no significant degradation in mechanical properties.     

Compositionally Graded Aluminum Oxide Coatings on Stainless Steel Using Laser Processing

A 1.5 mm thick fully dense alumina coating with a composition gradient from 100% Ni–20 wt% Cr at the substrate to 100% alumina on top has been developed on a 316 stainless steel sheet using Laser Engineered Net Shaping (LENS^™). The gradient coatings showed hardness in the range of 1800–2000 Hv, one of the highest reported so far due to high-density layers. During laser deposition, α-Al₂O₃ found to grow along the deposition direction with coarse columnar structure. The inherent advantage of this approach is to control simultaneously both location and composition leading to better interfacial properties of coatings.     

Processing of a Silicon-Carbide-Whisker-Reinforced Glass-Ceramic Composite by Microwave Heating

A calcium magnesium aluminosilicate-based glass that contained 10 wt% of silicon carbide whiskers (SiC _w ) as reinforcement was prepared by tape casting, followed by sintering either in a conventional furnace or in a microwave oven. The results were consistent with retardation of glass sintering through whisker bridging. The glass, by itself, was sintered to almost-full density at 750°C for 4 h by conventional furnace sintering; the best sintered composite, with an estimated density of ∼90%, was obtained at 800°C with a dwell time of 4 h. Sintering at a temperature of >800°C did not improve the densification but rather resulted in severe whisker oxidation. A reduced densification rate was observed for the samples that were sintered in nitrogen. By contrast, in the microwave oven, almost-full density for the glass and ∼95% of the theoretical density for the composite were obtainable at 850°C for 15 min, which represented a reduction of ∼10 h of the total processing time and a reduced SiC _w oxidation.     

Preparation of Fully Cubic Calcium-Stabilized Zirconia With 10 mol% Calcium Oxide Dopant Concentration by Microwave Processing

A novel, fast, and cheaper method has been developed for the synthesis of fully cubic calcium-stabilized zirconia (ZrO₂) of composition Ca_0.1Zr_0.9O_1.9 by dissolution of calcium oxide in monoclinic ZrO₂ for the first time using microwave energy. In this process, the precursors have been prepared by the mixed-oxide method taking the constituents in their stoichiometric ratio. The samples have been allowed to absorb microwave radiation in the presence of a polymeric susceptor. The susceptor absorbs the microwave radiation at room temperature and increases the temperature of the sample, where it starts interacting with microwave radiation. The susceptor burns off at a higher temperature without reacting with the sample. The cubic Ca_0.1Zr_0.9O_1.9 has been prepared at a temperature of 1100°C within 5 min.     

Alkaline Corrosion Resistance of Anodized Aluminum Coated with Zirconium Oxide by a Sol-Gel Process

To improve the alkaline corrosion resistance of aluminum, composite films were prepared that consisted of a porous layer of anodically grown aluminum oxide filled with zirconium oxide, with a zirconium oxide coating layer that was deposited thereon via the sol—gel process, using a dip-coating technique. The alkaline corrosion resistance of these composite films was extremely improved when this coating layer was placed on an anodic oxide film. Comparisons of the composite film and conventional anodic oxide film showed that the alkaline corrosion resistance of the composite film was increased by a factor of 24–50. Because these composite films, which have high corrosion resistance, indicated a vibration phenomenon of voltage in the duration time curve of the electromotive force measurement, the composite film had a self-repairing action for alkaline corrosion.