Corrosive Degeneration of Autoclaves for the Ammonothermal Synthesis: Experimental Approach and First Results

Abstract

The ammonothermal growth of different bulk group III nitride materials enables cost-efficient production of high-quality single crystals. Despite its advantages this process is limited due to the corrosive ammonothermal environment. This environment consists of supercritical ammonia with acidic or basic additives at high temperatures and pressures in high-pressure cells made of a nickel-base alloy. As a consequence of corrosion, large amounts of metallic impurities are detectable in the grown crystals. The mechanisms of corrosion are not well understood. A method is presented to investigate corroded autoclaves, caused by the interaction with supercritical fluid, together with first results regarding the degeneration of autoclave material.

Ammonothermal Synthesis of Nitrides: Recent Developments and Future Perspectives

“…Most of these materials are applicable for syntheses in ammonobasic environment, whereas severe corrosion can occur with acidic mineralizers. [158,159] Liners or capsules are used to prevent corrosion of the autoclaves and to avoid incorporation of transition metalsi nto the products.R ecently,n umerous materials were screened in terms of their chemicals tability, using pure NH 3 ,N aNH 2 ,o rN H 4 Cl. [155] Another study investigated the behavioro fN H 4 X( X= F, Cl, I) solutions on diverse metallic samples.…”

Different corrosion behaviour of autoclaves made of nickel base alloy 718 in ammonobasic and ammonoacidic environments

“…To determine if there are preferred positions of corrosive attack regarding the total length, the samples were located at different positions at the autoclaves. To obtain appropriate surfaces for investigating the microstructure samples have been grinded and polished as described in [19].…”

“…Furthermore we are able to evaluate lifetime limiting factors like stress corrosion cracking. The results represented here are based on the method described in a preliminarily study [19].…”

“…Therefore the first indentation was made in a distance of 20 m to the surface of the autoclave because the indenter itself has a diagonal of approx. 18 m. Second, it is nessesary to find the minmal force so that the hardness value is not influenced by the applied force and the local microstructure of the alloy [19]. The measured sub-surface hardness profiles of autoclave 1, 2 and 3 is shown in Fig.…”

Stability of materials in supercritical ammonia solutions

“…While these alloys provide sufficient corrosion resistance for basic supercritical ammonia solutions, they are ill-suited for acidic growth environments due to substantial etching of the autoclave material. 9,10 Inconel 625 provides improved corrosion resistance but is not recommended for long-term use. A partial solution to this dilemma is provided by using a corrosion resistant liner such as platinum, 11 though this introduces design challenges and a significant increase in equipment cost.…”