Dopant Distribution in Grain‐Boundary Films in Calcia‐Doped Silicon Nitride Ceramics

Abstract

Quantitative analyses of the local chemistry of amorphous films at the grain boundary (GB) were taken on hot isostatically pressed high-purity Si 3 N 4 doped with various amounts of calcium (up to 450 ppm). This work was mainly accomplished by using spatially resolved electron energyloss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. The amount of calcium segregation, quantified in terms of GB excess, saturated in the films at a bulk-doping level of ∼220 ppm. Extra additives did not stay at the triple-point glass pockets, where the calcium was almost expelled completely; instead, the additives stayed at intersections between the films and pockets. Otherwise, the calcium distribution was uniform along and across GB films. The latter was determined from simulations of EELS profiling. At grain/pocket interfaces, a much-lower segregation level occurred, ranging from one-half to one-tenth of the level at the GB. This observation indicates different segregation mechanisms in the two cases. Also, the calcium segregation in GB films changed the film composition dramatically, because more N 3− ions were introduced and replaced O 2− ions, to maintain the local stoichiometry. Reduction of the Van der Waals force has been proposed as being the origin of the film expansion with increasing calcium content.

Electron Energy-loss Spectroscopy Characterization of ∼1 nm-thick Amorphous Film at Grain Boundary in Si-based Ceramics

“…Employing the empirical difference of 0.25-0.3 nm between the structural and chemical widths as experienced in the previous section, the structural widths for these films should be about 3.2 nm, 1.1 nm and 0.6 nm, respectively, if HREM observations were carried on the same films. Nonetheless, the film 3 matches well to the previously observed of equilibrium thickness for the same material, 16) while the film 2 matches to the typical equilibrium thickness in the undoped Si 3 N 4 ceramics, 15) leaving the much wider film 1 matches to none.…”

“…10,11) This advanced spatially-resolved EELS method, the socalled ''spectrum separation'' approach, can give a chemical composition and a width of amorphous film in an indirect way, which yields quantitative information from area smaller than electron probe. [12][13][14] The analytical electron microscopy (AEM) results revealed not only that the inter-granular amorphous films in silicon nitride (Si 3 N 4 ) were made of a novel oxynitride phase instead of silica, 11,15) but also that such films may have different widths in a given ceramic material, in contrary to the previous high-resolution electron microscopy (HREM) observations. [15][16][17] These observations favor the later diffuse-interface and the multi-layer-absorbate pictures to improve or to replace the wetting picture.…”

“…[12][13][14] The analytical electron microscopy (AEM) results revealed not only that the inter-granular amorphous films in silicon nitride (Si 3 N 4 ) were made of a novel oxynitride phase instead of silica, 11,15) but also that such films may have different widths in a given ceramic material, in contrary to the previous high-resolution electron microscopy (HREM) observations. [15][16][17] These observations favor the later diffuse-interface and the multi-layer-absorbate pictures to improve or to replace the wetting picture. [18][19][20] The new AEM approach on grain boundary has also practical benefits for microstructure researches.…”

“…However, quantitative analysis of Ca segregation to grain boundary, measured as excess, 11,15) indicates a different scenario. As seen in the lower part of Fig.…”

“…It is well studied that amorphous films in this Si 3 N 4 has an equilibrium thickness of 1:1 AE 0:1 nm in this model material. [7][8][9]15) Conventional AEM analysis detected oxygen segregation to grain boundary, indicating that the inter-granular film is silicate-based but could not tell whether nitrogen is also present. The ''spectrum separation'' method probes the ELNES for Si-L 2;3 edge and distinguishes the ELNES patterns of grain and film.…”