Thermal Segregation of Cations in Iron Aluminate Spinels

Abstract

The observation of crystalline grains of both Si2N20 and MgSiO, situated in the noncrystalline phase suggests that they crystallized from that phase. The principal pieces of evidence are the ill-defined shape of many of the Si2N20 grains, the fact that the grains are considerably smaller than the average size (= 1 p m ) of the grains away from the noncrystalline pockets, the spherulitic morphology of the MgSiO, grains (characteristic of the early stages of crystallization in glass ceramics6), and the fact that Si2N20 and MgSiO, have formed, whereas the phases in the same material prior to the rapidly cooling silicon nitride alloy were found (Part I) to be Si3N4, Si2N20, and MgzSi04. The formation of MgSiO, in the intergranular phase cannot be readily explained from phase equilibria considerations as only the subsolidus phase relations are known. However, it is possible that it forms as an intermediate phase during a crystallization sequence of the intergranular phase.The fact that the same elemental partitioning was detectable in the light and dark regions of the intergranular noncrystalline phase (see Fig. 2 of Part I) in the same alloy after hot-pressing, but without additional rapid cooling, indicates that phase separation can occur under normal fabrication conditions. As glasses in the SiO,-MgO miscibility gap can be crystallized by conventional glass-ceramic processes using a nucleating agent, it may be possible to take advantage of the observed phase separation to crystallize the intergranular phase in silicon nitride ceramics. IV. Conclusions(1) Phase separation in the noncrystalline phase (as evidenced by elemental partitioning and morphology) of a hot-pressed and rap-idly cooled silicon nitride was detected by high spatial resolution microanalysis techniques in the transmission electron microscope.(2) Small crystallites of SizN20 and MgSiO, are found in the noncrystalline phase. The presence of Si2N20 indicates the possibility that the noncrystalline phase once incorporated nitrogen. EELS measurements also indicate that the nitrogen content of the noncrystalline phase is probably <5 at.%.(3) The compositional range of the phase separation is determined to be close to the miscibility gap of the system SiOz-MgO and the presence of nitrogen could extend the gap into the Si2N20-Si02-MgSi0, ternary phase field.(4) The fact that noncrystalline regions having compositions similar to the two separated phases have also been detected in both the hot-pressed silicon nitride and the hot-pressed ternary eutectic composition (Part I) indicate that the intergranular phase in conventionally hot-pressed silicon nitrides containing MgO may also undergo phase separation under suitable circumstances.Thermal segregation experiments were performed on open and encapsulated wafers of Fe(Fel.slAlo.49)04 spinels using CO, laser radiation as a heat source. The correlation of "apparent" marker movement and cation segregation led to the conclusion that the free exchange of gaseous oxygen at the external spinel surfaces is necessary ...

Kinetic demixing in yttria-doped zirconia part II - theoretical analysis

“…When the sample is placed under a temperature gradient, one can assume, in agreement with both literature data [6] and our results obtained with semiconducting oxides and doped-alumina [8][9], that the flux of charged species "i" is due mainly to the driving force of the electrochemical potential gradient (Vrl3. If one neglects the correlation effects, the flux of charged species "i" may then be written, with respect to the laboratory reference frame, as:…”

“…First, one can recall that in oxides cations and anions diffuse primarily by different path ways [3][4][5][6]. Their mobilities are due to the presence of point defects and they differ by several orders of magnitude.…”

Behavior of MgFe2O4 Films on MgO in an Electric Field

“…A GENERALIZED thermodynamic potential gradient can provide a strong driving force which can lead to the kinetic demixing of an oxide solid solution, or to the kinetic decomposition of a multicomponent oxide, at temperatures where the structural elements are mobile. 1 This driving force could be an oxygen potential gradient, 1-7 a temperature gradient, 8,9 a nonhydrostatic stress, 10 or, as in the case of the present study, an applied electric field. [11][12][13][14][15][16][17] Kinetic demixing and decomposition become issues of practical importance when oxides are utilized for their high-temperature properties, e.g., their chemical inertness and electrical insulating properties.…”

On the Soret effect in binary nonstoichiometric oxides—kinetic demixing of cuprite in a temperature gradient

“…Under working conditions, these materials are often exposed to external forces causing both reactions and transport within the materials. Prominent examples are electromigration or kinetic demixing in electric fields [1][2][3][4][5], kinetic demixing in chemical potential gradients [6][7][8][9], stress-driven segregation [10,11] or thermodiffusion ( [12][13][14][15][16][17][18][19][20], see [21,22] for reviews).…”