Thermal Conductivity of β‐Si₃N₄: III, Effect of Rare‐Earth (RE = La, Nd, Gd, Y, Yb, and Sc) Oxide Additives

Abstract

␤-Si 3 N 4 ceramics sintered with a series of rare-earth (RE ‫؍‬ La, Nd, Gd, Y, Yb and Sc) oxide additives were fabricated by hot pressing and subsequent annealing. Their microstructures, lattice oxygen contents, and thermal conductivities were evaluated. Mean grain size increased, while lattice oxygen content decreased, and hence, thermal conductivity increased with decreasing ionic radius of the rare-earth element. In all cases, a marked change was observed in the order of ionic radius from La to Nd to Gd, and a little change was observed below them. Rare-earth oxide additives significantly influenced the thermal conductivity of ␤-Si 3 N 4 , unlike in the case of AlN.

Textured silicon nitride: processing and anisotropic properties

“…Figure 35(c) shows that the thermal anisotropy increases gradually with increased Y 2 O 3 : SiO 2 molar ratio, which seems to correlate with the increased grain size. Actually, this should be attributed to the increased aspect ratio with increased Y 2 O 3 : SiO 2 molar ratio, thereby resulting in a higher degree of orientation during HP and subsequent annealing [62,135]. However, it is found that thermal anisotropy is limited to below 30% in hot-pressed Si 3 N 4 regardless of the processing conditions.…”

“…They showed that the thermal conductivity along the direction parallel to the HP is 36% lower than that along the direction perpendicular to the HP at 20 • C. Moreover, this value was found to decrease to 15% at 1300 • C. Kitayama et al conducted a series of studies on the thermal conductivity of hot-pressed Si 3 N 4 , including the roles of the starting powders [53], lattice oxygen [58] and RE oxide additives [62], as summarized in figure 35. Figure 35(d) shows the two cases for measuring thermal conductivity by the laser flash method.…”

“…Textured β-Si 3 N 4 ceramics exhibit an interesting thermal anisotropy: the thermal conductivity is higher in the direction of the elongated grain alignment than in the direction perpendicular to the grain alignment [39,45,47,52,53,58,59,62,65,72]. Thus, texturing has become another important method for improving the thermal conductivity of β-Si 3 N 4 ceramics.…”

“…Using a more refractory additive composition (Lu 2 O 3 -SiO 2 ), Zeng et al [72] reported the formation of textured Si 3 N 4 exhibiting a bending strength of ≈740 MPa at both room temperature (RT) and 1500 • C along the direction perpendicular to the grain alignment, that is, no strength degradation occurs even at 1500 • C, indicating better HT strength retention than that of untextured materials. Moreover, textured Si 3 N 4 exhibits unique anisotropic thermal conductivity [39,45,47,52,53,58,59,62,65,72], tribological and wear properties [54,55,63,66] and erosion behavior [67,69]. For example, Watari et al [52] showed that thermal conductivities of 155 and 52 Wm −1 K −1 could be achieved in highly textured Si 3 N 4 ceramic along the directions parallel and perpendicular to the grain alignment, respectively.…”

Potential use of only Yb2O3in producing dense Si3N4ceramics with high thermal conductivity by gas pressure sintering

“…To enhance the thermal conductivity, sintering additives should play a dual role of promoting densification and removing lattice oxygen. Among the rare-earth oxides, Y 2 O 3 and Yb 2 O 3 are the most promising additives for enhancing the thermal conductivity of β-Si 3 N 4 ceramics [12]. Because of its economic advantage over hot isostatic pressing and the improved thermal and mechanical properties compared with Si 3 N 4 ceramics produced by pressureless sintering, gas pressure sintering has been widely used to produce β-Si 3 N 4 ceramics with high thermal conductivity.…”