Corundum dissolution in concentrated sodium hydroxide solution

Abstract

Efforts to achieve high performance in turbine engines have been limited by the ceramic materials used in the investment casting of hollow, high pressure turbine engine airfoils. The process requirements for casting temperatures and soaking time are greater than 1,700 ºC and 16 hours, respectively, exceeding the permissible ceramic/molten metal contact time limitation. Generally, the ceramic used for casting cores must be able to serve as the material for the casting mold in the investment casting of nickel based superalloy turbine airfoils (e. g. turbine blades and vanes) with internal air cooling passages.Corundum (α-alumina) has been considered as a candidate for investment casting of hollow, high pressure turbine engine airfoils for its excellent properties, such as high chemical metallurgy stability and creep resistance, which can provide better dimensional tolerance in internal passages of airfoils [1][2] . However, it is very difficult to leach out the core made in α-alumina from the casting. Many methods and apparatus have been employed with leaching solution of potassium Abstract: The corundum (α-alumina) core has been considered as a suitable candidate for investment casting of hollow, high pressure turbine engine airfoils due to its excellent properties. However, the efficiency of removing alumina cores in concentrated caustic solution cannot meet the needs of industrial production. In this paper, the effects of temperature and initial solution concentration on dissolution of α-alumina were studied by the classical weight-loss method. The fractal kinetic model was developed in order to describe α-alumina dissolution, assuming that the nonporous particles shrank during reaction process. The results show that the dissolution rate increases with increasing reaction temperature and initial solution concentration. Especially, the initial solution concentration has a significant influence on α-alumina dissolution rate at a higher reaction temperature. The activation energies decrease with increasing initial solution concentration, and the chemical reaction is the rate-controlling step.

Introducing Porosity into Refractory Molybdenum Boride through Controlled Decomposition of a Metastable Mo–Al–B Precursor

“…10 The Mo 2 AlB 2 −AlO x nanolaminate was chosen to provide nanoscale mixing of Mo 2 AlB 2 , a metastable phase that could be transformed to the stable α-MoB phase, and alumina, which could be dissolved in aqueous sodium hydroxide. 32 To synthesize the Mo 2 AlB 2 −AlO x nanolaminate precursor, a literature procedure was used. 10 Briefly, partial room temperature Al deintercalation from the stable compound MoAlB produced MoAl 1−x B, a destabilized derivative that crystallized to form the metastable compound Mo 2 AlB 2 upon heating at 600 °C for ∼4.5 h. Continued heating at 600 °C for two additional hours facilitated the decomposition of Mo 2 AlB 2 into the Mo 2 AlB 2 −AlO x nanolaminate, which contains Mo 2 AlB 2 interleaved with amorphous alumina, both as sheets having thicknesses of 1−3 nm.…”

“…We approached the synthesis of mesoporous α-MoB powder using the thermal decomposition of a previously reported Mo 2 AlB 2 –AlO x nanolaminate, which was synthesized by chemical and thermal processing of MoAlB . The Mo 2 AlB 2 –AlO x nanolaminate was chosen to provide nanoscale mixing of Mo 2 AlB 2 , a metastable phase that could be transformed to the stable α-MoB phase, and alumina, which could be dissolved in aqueous sodium hydroxide . To synthesize the Mo 2 AlB 2 –AlO x nanolaminate precursor, a literature procedure was used .…”

“…10 Mo 2 AlB 2 − AlO x serves as a topotactic template to mesoporous α-MoB as thermal energy causes the collapse of the nanolaminate to produce AlO x inclusions in α-MoB, which can be dissolved in aqueous sodium hydroxide under mild pressure. 32 Porosity is validated by observing that mesoporous α-MoB, as compared to bulk MoB, had increased catalytic activity for the HER. The ability to design mesoporous refractory metal borides through a bulk-scale thermal decomposition pathway of metastable intermediates provides a powerful synthetic framework that overcomes the diffusion limitations inherent to high-temperature solid-state reactions, which generally prohibit the formation of mesoporosity in refractory compounds.…”

“…This mesoporous α-MoB material forms via a controlled decomposition pathway from a series of metastable intermediates that includes MoAl 1– x B, Mo 2 AlB 2 , and Mo 2 AlB 2 –AlO x , all of which are ultimately derived from the stable compound MoAlB. ,, The direct precursor to mesoporous α-MoB is Mo 2 AlB 2 –AlO x , which is a heterostructured nanolaminate containing alternating nanoscopic layers of Mo 2 AlB 2 and alumina . Mo 2 AlB 2 –AlO x serves as a topotactic template to mesoporous α-MoB as thermal energy causes the collapse of the nanolaminate to produce AlO x inclusions in α-MoB, which can be dissolved in aqueous sodium hydroxide under mild pressure . Porosity is validated by observing that mesoporous α-MoB, as compared to bulk MoB, had increased catalytic activity for the HER.…”

Chemical and Mechanical Properties of Metakaolin-Based Geopolymers with Waste Corundum Powder Resulting from Erosion Testing

“…Since one of the objectives of this study was to evidence the eventual effects of Al 2 O 3 presence in geopolymer 3D networks, prior to the introduction of recycled corundum powder to the geopolymers, the ability to dissolve corundum in a basic environment was studied. As seen in several papers [ 22 , 23 ], dissolution is directly proportional to the adsorption of the OH- ions. Moreover, for this reason, an 8 M NaOH solution was chosen.…”