Aluminum sintering aids

It is technically easier to manufacture a molded component from SiC-powder by slip casting or dry pressing, working it mechanically and then sintering pressureless at 1950 to 2000°C. Due to the low sintering activity of silicon carbides, such processes have only been recently successfully carried out with the advent of fine particulate SiC-powders (specific surface area > 5 m /g) with low oxygen-contents (< 0.2%). Boron or aluminum and free carbon or boron carbide are added as sintering aids. 

Aluminum nitride powder synthesized by high-temperature routes can be sintered to a density of more than 97% of the theoretical density by adding calcium or yttrium compounds as sintering aids [52-54]. Therefore, new processing routes to aluminum nitride predominantly aim at powders with lower quantities of cationic impurities. In addition, the formation of aluminum nitride coatings [55-58] or fibers [59] has become subject of extensive research. 

Aluminum is an effective sintering aid for B4C and SiC ceramics if combined with elemental boron and carbon. Phase relations in the B-C-Al-Si system may hence indicate suitable procedures to initiate transient liquid phase or enhanced solid-state sintering. Furthermore, liquid A1 may be used to infiltrate porous B4C bodies acting as a reinforcing phase. 

Villalobos GR, Sanghera JS, Aggarwal ID (2005) Degradation of magnesium aluminum spinel by lithium fluoride sintering aid. J Am Ctaxim Soc 88 1321-1322... 

Similarly, an experimental, quasi-stoichiometric, oxygen-free SiC fiber has been produced from a Si-AI-C-0 precursor. In this process, aluminum is introduced into the polymer as aluminum (III) acetylacetonate, and acts like boron as a SiC sintering aid during heat treatment at 1800°C [40]. 

The presence of a sintering aid (preferably boron or aluminum) to allow sintering at lower temperatures and thereby limit grain growth within the monolith.2.". 5... 

While REPEL or conventionally sintered (CS) materials are the common forms, hot pressed material (HP) of almost theoretical density is also available. The HP artifacts are usually obtained at 2100°C and 50 MPa pressure with either aluminum or boron carbide, B4C, added as a sintering aid. Hot pressing with up to 1.5% aluminum produces equiaxed microstructures with grains about 2 /im in diameter, but a different microstnicture, with elongated grains 20-40 fim long, is produced in the presence of B4C. 

Use of sintering aids that density the material by liquid-phase sintering as a liquid is formed between aluminum oxide in the AIN powder and the additives. 

Well known sintering aids in category (1) are alkali-earth oxides or rare-earth oxides such as Y2O3 (17,18) and CaO (19,20). These can be added not only as oxide but also as nonoxide compounds such as halide, nitride, carbide, nitrite, or carbonate. Some of the transition elements such as NiO and Ti02 can be classified as category (2) additives. Rare earth or alkali-earth oxide additive reacts with aluminum oxide of AIN powder (i.e. oxide layer of AIN powder) to form aluminate liquid at a high temperature Eq. (4) and promotes liquid-phase sintering of AIN powder. 

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