Wet Milling of Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 Powder Mixtures

Wet milling of Al₂O₃-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al₂O₃ and Fe₂O₃/Al/Al₂O₃ powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe₂O₃/Al/Al₂O₃ powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al₂O₃ powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.     

New Strategies for Preparing NanoSized Silicon Nitride Ceramics

We report the preparation of nanosized silicon nitride (Si₃N₄) ceramics via high-energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultrafine β-Si₃N₄ and sintering additives of 5-mol% Y₂O₃ and 2-mol% Al₂O₃ was prepared by high-energy mechanical milling. After milling, the powder mixture was mostly transformed into a non-equilibrium amorphous phase containing a large quantity of well-dispersed nanocrystalline β-Si₃N₄ particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600°C for 5 min at a heating rate of 300°C/min. The fully densified sample consisted of homogeneous nano-Si₃N₄ grains with an average diameter of about 70 nm, which led to noticeable high-temperature ductility and elevated hardness.     

Formation of Al2O3–TiC Composite Nano-Particles Synthesized from Carbon-Coated Precursors

The formation of nano-sized alumina–titanium carbide (Al₂O₃–TiC) composite powders from a carbon-coated titanium dioxide–aluminum (TiO₂–Al) mixture was investigated. The carbon-coated TiO₂–Al mixture altered the mechanism of the reaction, compared with standard mixtures, to produce high-quality nano-sized Al₂O₃–TiC powders. Data synthesized from intermediate temperatures indicate that these products form via Ti₂O₃ and Al₃Ti. TEM images of the Al₂O₃–TiC powders showed fine size (50–100 nm), narrow size distribution, and lack of agglomeration. DSC data for the carbon-coated TiO₂–Al mixture showed a single endothermic and four successive weak exothermic reactions as the carbon coating moderated the heat release during the reaction.     

Al2O3─SiC Composites from Aluminosilicate Precursors

Al₂O₃ and SiC composite materials have been produced from mixtures of aluminosilicates (both natural minerals and synthetic) and carbon as precursor materials. These composites are produced by heating a mixture of kaolinite (or synthetic aluminosilicates) and carbon in stoichiometric proportion above 1550°C, so that only Al₂O₃ and SiC remain as the major phases. A similar process has also been used for synthesizing other composite powders having mixtures of Al₂O₃, SiC, TiC, and ZrO₂ in different proportions (all compounds together or selective mixtures of some of them), as desired. The microstructure of hot-pressed dense compacts, produced from these powders, revealed that the SiC phase is distributed very homogeneously, even occasionally within Al₂O₃ grains on a nanosize scale. The homogeneous distribution of SiC particles within the system produced high fracture toughness of the hot-pressed material (K_IC∼ 7.0 MPa · m^1/2) and having Vicker's hardness values greater than 2000 kgf/mm².     

Carbothermal Synthesis of Nanocrystalline Aluminum Nitride Powders

A new precursor technique for the carbothermal synthesis of nanocrystalline AlN powder has been developed. A precursor that contains an intimate mixture of nanocrystalline Al₂O₃ and carbon has been synthesized by using a chemical pyrophoric reaction. The formation of AlN starts at 1473 K, and complete conversion has been observed at temperatures >1673 K. The synthesized AlN particles are nanocrystalline (<100 nm) in size.     

Preparation of Al2O3–AlN–Ni Composites

A method is proposed to prepare Al₂O₃-AlN-Ni composites. The composites are prepared by sintering Al₂O₃/NiAl powder mixtures at 1600°C in a mixture of nitrogen and carbon monoxide. The presence of NiAl particles raises the green density of Al₂O₃/NiAl powder compacts. During sintering, NiAl reacts with nitrogen to form AlN and Ni inclusions. A volume expansion accompanies the reaction. Because of the high green density and the reaction, the volume shrinkage of the Al₂O₃-AlN-Ni composite decreases with the increase of added NiAl content.     

Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3

The formation process of barium hexaaluminate (BaO 6Al₂O₃) from BaCO₃/γ-Al₂O₃ powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two-step solid-state reaction from BaCO₃ and Al₂O₃ via formation of BaO·Al₂O₃. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide-derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al₂O₃ proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m²/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al₂O₃.     

TiB2/TiC nanocomposite powder fabricated via high energy ball milling

TiB₂/TiC nanocomposite powder was fabricated via high-energy ball milling processing, and microstructure development of the powder mixtures was monitored by X-ray diffraction, transmission electron microscopy. Ball milling B₄C and elemental Ti powder mixture at ambient temperature resulted in the formation of TiC prior to the formation of TiB₂. The bulk of TiC and TiB₂ formation accomplished after 5 h of milling. The final product consisted of nanosized TiC particles and microscale TiB₂ particles.     

Pressureless Sintering of Alumina-Titanium Carbide Composites

The densification of Al₂O₃-TiC composites is detrimentally affected by chemical reactions between Al₂O₃ and TiC. These reactions must be suppressed in order to promote sintering. In this study, the specific reactions occurring in Al₂O₃-TiC composites were modeled, using thermodynamic calculations, and verified by experiments. The reaction between Al₂O₃ and TiC was suppressed by the use of specially prepared embedding powders allowing pressureless sintering to closed porosity. The Al₂O₃-TiC composites were subsequently hot isostatically pressed to > 99% of theoretical density without encapsulation. Typical flexural strength and fracture toughness of Al₂O₃-30 wt% TiC composites were 690 MPa and 4.3 MPa · m^1/2, respectively.     

Size Control of α-Al2O3 Platelets Synthesized in Molten Na2SO4 Flux

α-Al₂O₃ platelet powders were synthesized in molten Na₂SO₄ flux. The size of α-Al₂O₃ platelets was significantly reduced when partially decomposed rather than pure Al₂(SO₄)₃ was used as the source of Al₂O₃; a further reduction in the platelet size was realized through additional seeding with nanosized α-Al₂O₃ seeds. The addition of microsized α-Al₂O₃ platelet seeds significantly influenced the platelet morphology of the final powder, as well. The platelet size of the final powder was in direct proportion to the size of the platelet seeds, and was in reverse proportion to the cube root of the platelet seed content.     

Preparation of a Highly Conductive Al2O3/TiN Interlayer Nanocomposite through Selective Matrix Grain Growth

An electroconductive TiN/Al₂O₃ nanocomposite was prepared by a selective matrix grain growth method, using a powder mixture of submicrosized α-Al₂O₃, nanosized γ-Al₂O₃, and TiN nanoparticles synthesized through an in situ nitridation process. During sintering, a self-concentration of TiN nanoparticles at the matrix grain boundary occurred, as a result of the selective growth of large α-Al₂O₃ matrix grains. Under suitable sintering conditions, a typical interlayer nanostructure with a continuous nanosized TiN interlayer was formed along the Al₂O₃ matrix grain boundary, and the electroconducting behavior of the material was significantly improved. Twelve volume percent TiN/Al₂O₃ nanocomposite with such an interlayer nanostructure showed an unprecedentedly low resistivity of 8 × 10⁻³Ω·cm, which was more than two orders lower than the TiN/Al₂O₃ nanocomposite without such an interlayer nanostructure.     

Effect of Milling Conditions on the Strength of Alumina–Silicon Carbide Nanocomposites

Processing effects of wet ball-milling on the microstructure and fracture strength of Al₂O₃/5 vol% SiC nanocomposites were investigated. Homogeneous microstructure and a high fracture strength of 1200 MPa could be achieved under the milling condition of relatively low wear contents of Al₂O₃ grinding media, maintaining the homogeneity of the ball-milled powders. The fracture strength decreased with increasing wear content. Degradation of fracture strength was caused by abnormal grain growth related to wear particles from the Al₂O₃ balls.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Consolidation of Alumina—Zirconia Mixtures by a Colloidal Process

The relationship between the dispersion of colloidal powder particles in Al₂O₃–ZrO₂ suspensions and the microstructures of consolidated compacts was examined. Suspensions were prepared from Al₂O₃ powder and ZrO₂ sol with average particle sizes of 390 and 62 nm, respectively. The dispersion was controlled by pH and salt concentration adjustments. The compacts composed of completely separated Al₂O₃ and ZrO₂ layers were obtained from well-dispersed suspensions with pH values below about 4 and salt concentration of 0.0527 M. An increase in pH or salt concentration resulted in macroscopically uniform compacts. The compacts made from suspensions with pH values above about 7, however, were composed of a mixture of Al₂O₃ and ZrO₂ agglomerates, with one acting as a matrix and the other a dispersed phase. Suspensions with a pH value of 4.5 and optimum salt concentrations resulted in compacts with microscopically uniform microstructure. Above or below these salt concentrations, ZrO₂ agglomerates were distributed in an Al₂O₃ matrix. The optimum concentration was dependent on solid content. In addition, the dispersion of mixed suspensions was compared with those of single-component suspensions. The ZrO₂ particles formed three-dimensional networks during agglomeration, which reduced the component separation in suspensions and during consolidation.     

Precipitation of Magnesium Aluminum Spinel from Alumina-Matrix Solid Solution: I, Fundamental Concept and Precipitation Behavior

Al₂O₃ ceramics with magnesium aluminum spinel dispersion particulates were produced by a precipitation treatment of Al₂O₃-matrix solid solution, (Al_1—2x,Ti _x ,Mg _x )₂O₃. We successfully constructed the precipitation process for synthesizing the nanocomposite, using the solubility dependence of titanium and magnesium in Al₂O₃ on the valence of titanium. Changing of the valence of titanium from 4+ to 3+ was accomplished by controlling the heating atmosphere, and, thereby, magnesium precipitation was promoted. The precipitation behavior was characterized using X-ray diffractometry, and the microstructure was observed using transmission electron microscopy. We confirmed that magnesium aluminum spinel nanosized particulate were precipitated in the Al₂O₃ grain.     

Intragranular Particle Residual Stress Strengthening of Al2O3–SiC Nanocomposites

Al₂O₃/SiC ceramic nanocomposites were fabricated from nanocrystalline Al₂O₃ (10 nm in diameter) and SiC (15 nm in diameter) powders, and a theoretical model of intragranular particle residual stress strengthening was investigated. The SiC nanoparticles in the Al₂O₃ grains create a normal compressive stress at the grain boundaries and a tangential tensile stress in the Al₂O₃ grains, resulting in the "strengthening" of the grain boundaries and "weakening" of the grains. The model gives a good explanation of the experimental results of the authors and others which are difficult to be explained by the existing strengthening models, i.e. the maximum strength is normally achieved at about 5 vol% of SiC particles in the Al₂O₃–SiC ceramic nanocomposites. According to the model, there exists an optimum amount of SiC for strengthening, below which the grain boundaries are not fully "strengthened" and the fracture is mainly intergranular, above which the grains are "weakened" too much and the fracture is mainly transgranular, and at which the fracture is a mixture of intergranular and transgranular.     

Synthesis of Novel Niobium Aluminide-Based Composites

A reactive sintering process has been used to produce almost fully dense composites with interpenetrating networks of NbAl₃ and Al₂O₃. The process involves the reaction synthesis of niobium aluminides and Al₂O₃ from compacts of intensively milled aluminum and Nb₂O₅ powder mixtures. During carefully controlled heating under an inert atmosphere, the oxide reduction by aluminum to form niobium aluminides and Al₂O₃ proceeds at temperatures below the melting point of aluminum. At temperatures of >1000°C, the reaction-formed niobium aluminides and Al₂O₃ sinter. The present paper discusses processing parameters, such as attrition milling, the heating cycle, and the metal:ceramic ratio in the starting mixture, that control microstructure development and mechanical properties.     

Synthesis of AlN Powder by Carbothermal Reduction-Nitridation of Various Al2O3 Powders with CaF2

We investigated the effect of characteristics of raw Al₂O₃ powder on the synthesis of AlN by the carbothermal reduction-nitridation method, in which CaF₂ was added as a catalytic material. Four types of Al₂O₃ powders were selected. An Al₂O₃/C molar ratio of 0.29 was fixed, and the amount of CaF₂ was varied from 3 to 30 wt%. The carbothermal reduction-nitridation was conducted from 1350° to 1450°C in N₂ flow. The nitridation rate tended to increase with decreasing particle size of raw Al₂O₃ and was found to depend on the Al₂O₃ synthesizing method. The particle sizes of the synthesized AlN increased somewhat with increasing reaction temperatures. However, even though different particle sizes of Al₂O₃ powders were used, AlN powders synthesized under the same conditions exhibited almost the same particle size, round shape, and narrow size distribution. From XRD analysis, CaO·6Al₂O₃ and CaO·2Al₂O₃ were identified as intermediate compounds during these reactions. The above phenomena suggest that the synthesis mechanism of AlN powder by carbothermal reduction-nitridation of Al₂O₃ with CaF₂ addition was the nitridation of the intermediate compounds through the liquid phase of the system CaF₂-CaO·6Al₂O₃-CaO·2Al₂O₃.     

Novel Synthesis of Mullite Powder with High Surface Area

Boehmite (AlO(OH)) solid-solution gel, which yields stoichiometric mullite (3Al₂O₃2SiO₂) at high temperatures, has been prepared by the hydrazine method. The formation process leading to 3Al₂O₃2SiO₂ is discussed. The as-prepared powder and powders heated below 1200°C consist of very fine particles showing needlelike morphology, whereas the particles of mullite powder show thin prismatic morphology. The mullite powder after heating at 1300°C has a high surface area (87 m²/g).     

Carbothermal Synthesis of Aluminum Nitride Using Sucrose

Several aluminum oxides (α-Al₂O₃, θ-Al₂O₃, and AIOOH) were examined to study the differences in reaction behavior and powder characteristics during carbothermal nitrida-tion to AIN using sucrose and carbon black. The reaction conditions investigated were carbon-to-alumina ratio, reaction temperature, and time. Carburized sucrose resulted in Full conversion to AIN and produced a uniform powder morphology using a near-istoichiometric ratio of C:Al₂O₃ while carbon black required higher C:Al₂O₃ ratios (i.e., >4:1) for full conversion and led to agglomeration of the AIN powder. The most favorable reaction temperature was 1600°C, with the reaction time to full conversion being dependent on the type of Al₂O₃. The particle and agglomerate size of the AIN powders did not change significantly with reaction time. However, the particle size and morphology were strongly dependent on that of the initial AI₂O₃ with sucrose, whereas agglomeration of the AIN occurs when using carbon black. A solid–solid reaction mechanism is proposed.