Assessment of carbon contamination in MgAl₂O₄ spinel during spark-plasma-sintering (SPS) processing

Abstract

Carbon contamination caused during spark-plasma-sintering (SPS) processing was investigated in the MgAl 2 O 4 spinel by Raman spectroscopy. Although the carbon contamination became remarkable around the sample surfaces directly contacting the carbon paper, it sensitively changes with the SPS conditions, particularly for the heating rate. For the slow heating rate of 10°C/min, the carbon contamination can be detected around the surface regions rather than inside. For the high heating rate, however, a large amount of the carbon contamination was detected even inside in addition to the significant contamination around the surfaces even though the sintering temperature is the same and the processing time is shorter as compared to those of the slow heating rate. The present results suggest that the carbon contamination is not caused by diffusion processes, but caused by evaporation of the carbon phase from the carbon paper/dies, which were used in the SPS process. For the high heating rates, the carbon evaporation is enhanced due to the rapid heating and goes into the samples through open pore channels. The evaporated carbon is encapsulated into the closed pores during the heating process and remains along the grain junctions as glassy carbon.

On the first evidence of exchange-bias feature in magnetically contrasted consolidates made from CoFe2O4-CoO core-shell nanoparticles

“…The spectra are consistent with the signature of two main contributions, a sextet (major) and a doublet (minor) with an average isomer shift < δ > of 0.40 mm.s −1 and an average hyperfine field (<B hyp > of 49.0 T) (see Figure SI-4 in the supporting information section). The sextet is quite consistent with ferric cations involved in the ferrimagnetic spinel structure but some of them with a partial electron delocalization (explaining the inner wing of the left outer line), while the quadrupolar doublet must be assigned to paramagnetic ferrous species: they result probably from a small contamination by a paramagnetic phase non detected by XRD, such as siderite FeCO 3 (that would be compatible with the observation of traces of carbon as often reported on SPS sintered ceramics 44,45 ) and/or the replacement of the CoO phase by its solid solution Co 1-x Fe x O (x close to zero) (that would be compatible with the diffusion of iron cation from spinel to adjacent rock-salt grains because an interatomic diffusion in SPS sintered nanocomposites was also reported 34 ).…”

Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

“…[42][43][44] Morita et al found that during the annealing, the carbon phases generate CO/CO 2 gases by reacting with oxygen and form many pores due to the inner pressure of the generated gases. 45,46 Such pores dramatically degrade the optical and laser properties. 41 | 699 et al reported that preheating the starting powder in air can improve transmittance without increasing its grain size.…”

Predicting final stage sintering grain growth affected by porosity

“…However, those methods involve pressure and specific atmospheric conditions during sintering (argon pressure or vacuum with graphite contact, for respectively HIP and SPS). For SPS, graphite pollution has been widely reported to be a limiting transparency phenomenon [11,12]. Dopants were also studied for the sintering of spinel, notably LiF [13][14][15].…”