Cr+6 Controlled Nucleation in SiO2-MgO-Al2O3-K2O-B2O3-F Glass Sealant (SOFC)

Abstract

This study highlights the strong effect of chromium (Cr 6+) as a nucleating agent for SiO 2-MgO-Al 2 O 3-B 2 O 3-K 2 OF glass sealant. In the process, Al 2 O 3 from the base glass was gradually substituted by K 2 Cr 2 O 7 , which considerably tuned the crystallization characteristics, microstructure, thermal, and mechanical properties. The distinctive feature of this study is the induction of nucleation and crystallization in glass matrix on addition of Cr 6+ content performing only annealing heat-treatment (600 • C). The melt-quenched SiO 2-MgO-Al 2 O 3-B 2 O 3-K 2 OF glass with zero Cr 6+ content is found amorphous, which in presence of Cr 6+ content became crystalline with MgCr 2 O 4 , K 3 CrF 6 , MgF 2 , and mullite (3Al 2 O 3 .2SiO 2) phases. Glassy features in DSC and dilatometric thermal properties (T g , T d , thermal expansion) were attained in glass without Cr-content, but glass-ceramic-like features obtained for Cr-containing glasses. Large thermal expansion (>11 × 10 −6 /K) was achieved for such glasses, and that is compatible with the CTE of solid oxide fuel cell (SOFC) components (electrode, interconnect, etc.) at the operating temperature (700-900 • C). FESEM study revealed the development of 200-500 nm sized crystallites in 2 mol% K 2 Cr 2 O 7-containing glass microstructure and that became more compact sorted with 10-50 µm sized crystals in higher Cr-doped glasses. Higher crystallinity was thus ascertained for higher Cr-containing glasses, which influenced the corresponding density and mechanical properties. From nano-indentation measurements, the hardness and Young's modulus were estimated to be 0.6 (±0.5) and 25 (±10) GPa, respectively, for base glass and in the range of 3.3 to 8.4 and 58 to 94 GPa, respectively, for Cr-containing glassceramics. Hardness measured from micro-indentation tests for the base glass was 3.63 (± 0.18) GPa, which increased to 3.94-6.08 GPa for Cr-containing glass-ceramics. Due to the typical microstructure and compatible thermal and mechanical properties, 2 mol% K 2 Cr 2 O 7-doped SiO 2-MgO-Al 2 O 3-B 2 O 3-K 2 OF glass can be useable as high temperature sealant (like SOFC).

Nanocrystalline microstructure in Sm3+ and Gd3+ doped K2O–MgO–Al2O3–SiO2–F glass-ceramic sealant (SOFC)

“…Such a compact morphology is ascribed for its high density value (3.02 AE 0.02 g cm À3 ). 6,20 During the heat-treatment at 950 1C, the nucleating agents such as Sm 3+ or Gd 3+ ions can either support the heterogeneous phase separation or these can cause the accumulation in a specific microphase or nanophase of the phase separated glass. 3,10 Those trivalent ions Sm 3+ or Gd 3+ having large ionic field strength (charge/radius) and smaller critical nuclei tend to form 'cluster', which makes a minority of oxide ions involved in the Sm-O-Sm or Gd-O-Gd linkages, and hence isolated from the silicate glass network (Si-O-Si).…”

“…10 Thus, the maximum density is estimated for G-3 as 3.06 AE 0.02 g cm À3 . 6,20 The dilatometry of the heat-treated samples (i.e., glassceramics) was carried out in order to investigate the thermal expansion-sealant (SOFC) behavior. 21,22 A plot of the CTE of the glass-ceramics between 400 1C and 800 1C is presented in Fig.…”

“…4 In addition to the flexibility to develop an improved fine-grained microstructure with desired thermal properties, these glassceramics exhibit consistent reproducibility of properties due to the homogeneity of the as-cast melt glass. [4][5][6] They typically contain a finite quantity of crystalline ceramic phase produced by the controlled nucleation of highly viscous glass forming melts. 5 Mica glass-ceramics with dense nanocrystalline-grained microstructure and improved thermal properties are interesting materials for high temperature sealing applications such as solid oxide fuel cells (SOFCs).…”

“…in SOFC cell. 6,7 Moreover, they possess layered crystalline structure that can avert the generation and growth of micro-crack during thermal recycling operation performed at high temperature. [6][7][8] However, the large thermal shock resistivity for those mica glassceramics arises due to the wide thermal expansion value.…”

“…6,7 Moreover, they possess layered crystalline structure that can avert the generation and growth of micro-crack during thermal recycling operation performed at high temperature. [6][7][8] However, the large thermal shock resistivity for those mica glassceramics arises due to the wide thermal expansion value. [9][10][11] Moreover, the wide thermal expansion in the K 2 O-MgO-B 2 O 3 -Al 2 O 3 -SiO 2 -F glasses is obtained due to the layered structure of the mica crystals, which permit structural relaxation along the planes of the flat structural network.…”