Reaction Sintering of Kyanite and Alumina to Form Mullite Composites

The rate at which North Carolina kyanite of various grain sizes decomposes into mullite and glass in the range 1350° to 1600 °C. is given. Kyanite from other sources is also discussed.

Sintering of Silicon Carbide with nano-sized alumina as well as mullite as an additive has been studied. Nano-sized alumina produced by several routes were first analyzed and then investigated. This alumina along with yittria as well as mullite was then added to SiC powder in different proportions and sintered under ambient pressure of argon and then also hotpressed under 20 MPa uniaxial pressure using a graphite mold. The microstructure of the sintered pellets obtained from the two techniques shows interesting difference in the distribution and morphology of the different grains and phases. The shrinkage curves plotted for the hotprcssed SiC also predict the presence of a very small amount of liquid phase

The powerful method for alumina-containing stuff processing is offered in the article for materials both naturally occurring and technology-related in order to produce the alkali-free high-level alumina hydroxide in the capacity of the raw materials for high alumina stuff obtaining, which includes both conventional technical alumina, high-alumina chamotte, fused corundum or mullite, as well as some alternative kinds of alumina products. Ref. 46. Tab. 3.

In-situ alumina-zirconia composite bodies were fabricated by heat treatment of gibbsite-zircon-kaolinite mixture at 1450℃. The current research investigated crystallization behavior and mechanical properties of the mentioned mixture in the presence of 5 wt.% MgO as an additive. X-ray diffraction (XRD) results showed that alumina, zirconia, and magnesium aluminosilicate were crystallized during the heat treatment at 1250-1550℃. It was expected that mullite and zirconia were crystallized as the final phases; however, the addition of 5 wt.% of MgO changed the behavior of the mentioned mixture during the heat treatment at 1250-1550℃. Energy diffractive X-Ray spectroscopy (EDS) reported that after heat treatment at 1450℃, an Al3+-rich aluminosilicate phase was formed as the matrix of the composite. Crystallization of alumina and zirconia and the existence of the amorphous aluminosilicate phase formed a composite with appropriate hardness and mechanical strength. The diametral tensile strength and Vickers microhardness values of the final composite were 130±7 MPa and 7.49 ± 1.2 GPa, respectively.

Fly ash could be utilized as alumina and silica sources material. It could be treated by the following processes of demagnetisation, sinterization, grinding, mixing with crushed bricks as well as fired cement to create acid based on castable refractory. Sinterization changes alumina and silica signifi- cantly into mullite. The standard measurement of pyrometric cone equivalent (PCE) is used to under- stand the temperature resistance of the studied castable refractory based fly ash. The temperature resistance seems to increase after contacting with high temperature at longer time or repeatedly until reaching its Si-Al stabilization phase.

Alumina, Mullite and Spinel, Zirconia

On examine les conditions de croissance et quelques proprietes de la mullite aciculaire formee sur une plaque d'alumine en vue de la modification superficielle de l'alumine par de fins cristaux aciculaires de mullite

Dense β, β″-alumina ceramics were fabricated by the reaction sintering in the Na2O-MgO-Al2O3 system using α-alumina compacts infiltrated with Na2O and MgO. High-density β, β″-alumina ceramics with uniform texture were prepared at 1600°C by liquid phase sintering through controlling the pore volume of α-alumina compacts and the amount of infiltrated Na2O and MgO. High-porosity α-alumina compacts were superior to low-porosity ones in introducing a large amount of Na2O in pores, and enhanced the densication rate through the formation of liquid phase of Na2O-Al2O3 system. The effects of MgO content on the densification and phase composition of β- and β″-phases were also discussed.

Home-grown dissertations on ceramics obtained from aluminum oxide and methods for obtaining them in the period from 2000 are reviewed. Fine construction-grade ceramic is predominantly studied. The emphasis is on technological aspects. The first part of the review includes the sintering of Al2O3 with the addition of heteroelement oxides, reactive sintering with the addition of aluminum, the production of cermets of the type Al2O3, and the production of reinforced composites with an aluminum oxide matrix.

Effect of dual liquid phase sintering aids on the densification and microstructure of low temperature sintered alumina ceramics

A new process for the production of spinel-based materials by reaction sintering is presented. Oxidation of aluminum is accelerated by using magnesium as an alloying element as well as by the contact to magnesia. Thus, the composition used here leads to a maximum of the reaction kinetics. The effects occurring during the oxidation process have to be properly controlled, in particular the time of wetting of magnesia by molten aluminum. The reaction can be completed at low temperatures (900–1000°C). The fairly microstructure is characterised by a closed porosity, leading to a gastight material. There is only very little volume change during reaction sintering, qualifying the material e. g. for assembling with metallic or ceramic components.

Production of NiAl–matrix composites by reactive sintering

Mullitization temperatures and mechanical properties of reaction‐bonded mullite composites were investigated using silicon carbide (SiC) of two different particle sizes (180 nm and 2.5 μm) as one of the starting components. The smaller SiC particle size resulted in earlier mullitization, lower final densities, and lower strength of these composites. The sintering shrinkage of these composites was investigated. Low‐to‐zero shrinkage was rendered possible via volume expansions that were associated with the oxidation of SiC and aluminum in the green material. Green bodies that contained 55 vol% aluminum were compacted to 70% of the theoretical density. The materials showed a linear sintering shrinkage of <1% and had a four‐point bend strength of 430 MPa. Samples that were made from precursors with the coarse (2.5 μm) SiC were covered by a porous outer layer after firing in air. This layer led to anisotropy in shrinkage. The porosity of this outer layer was attributed to the oxidation of residual SiC during sintering and the trapping of gaseous oxidation products. Samples that were made from the fine (180 nm) SiC did not exhibit such a layer and showed isotropic shrinkage.

Reaction sintering in the Na2O–MgO–Al2O3system was studied with α–alumina green compacts containing Na2O and MgO in the pores of a controlled structure to produce β, β″–alumina polycrystalline at low temperatures. HCOONa and (CH3COO)2Mg were infiltrated into the alumina powder compacts (average particle size 0.2 μm) with relative densities of 51 to 66% at the compositions of Na2O/MgO/Al2O3= 1/0.10/4.0–7.6. The alumina powder compacts were formed by filtration of the aqueous colloidal suspensions through gypsum molds. Phase change from α to β, β″–alumina proceeded fast at low temperatures in the samples with high Na2O/Al2O3ratios. However, no significant densification occurred below 1500 °C owing to the volume increase of powder compacts associated with the phase change of α to β, β″–alumina. At 1600 °C, the green compacts were rapidly densified to relative densities above 99%. This densification was related to the liquid phase sintering based on the partial decomposition of β″–alumina. The dense β, β″–alumina polycrystalline consisted of needle-like grains of 2–5 μm in length and 1–2 μm in width.

A novel MgAl2O4/MgO ceramic core with water-collapsibility