## Materials and Methods
### The Characterization of Raw and Sintered Samples
#### 2.3.5 The Mechanical Properties of the Sintered Samples  
The characterization of the mechanical properties consisted of flexural strength, fracture toughness (crushing strength), and microhardness determination. Measurements of flexural and compressive strength were conducted on solid samples obtained by pouring the GC samples in the form of viscous paste into molds of desired dimensions and heating at 800 \({}^{\circ}\)C.  
The operating temperatures of the heating equipment in which the examined adhesives were to be applied did not exceed 400-500 \({}^{\circ}\)C; therefore, it was not necessary to set the sintering temperature to any higher than that. However, since these compositions were also examined for their application as sealants for IT-SOFCs that operate at temperatures between 500 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, the sintering temperature was set to be 800 \({}^{\circ}\)C. Moreover, thermal stability tests were previously conducted on temperatures up to a maximum of 800 \({}^{\circ}\)C, so practically, it is unknown whether these materials would be stable at higher temperatures. For the purpose of flexural strength (modulus of rupture) determination, cylindrical solid samples of all GC compositions with dimensions \(\Phi\)5.5 mm \(\times\) H30 mm were prepared and a three-point bending test method using a Universal Testing Machine (Model: Instron 1185, Instron, Boston, MA, USA) with a crosshead speed of 1 mm/min was performed in accordance with the Chinese national standard GB/T3001-2007 [29]. Cylindrical samples were initially air-dried at room temperature for 24 h and then heat-treated at 800 \({}^{\circ}\)C for 8 h prior to the measurements. The distance between the supports in the test was set to be 20 mm. For the calculation of flexural strength values (\(\sigma\)) of the samples, the following equation, Equation (1), was applied:  
\[\sigma_{\mathrm{f}}=(\mathrm{L^{*}F})/(\pi\,\mathrm{R^{3}}), \tag{1}\]  
where \(\sigma_{\mathrm{f}}\) is the flexural strength of the sample (MPa); F is the flexural force leading to the sample failure (N); L is the distance of the supporting pin (<mm); and R is the radius of the cross section of the sample (mm). Crushing-strength (fracture toughness) measurements were conducted on cylindrical solid samples of all GC compositions with dimensions of \(\Phi\)6 mm \(\times\) H6 mm. The test method using the Universal Testing Machine (Model: Instron 1185) with a speed of crosshead of 1 mm/min is presented in Figure 3. The samples were initially cured at ambient temperature (20 \({}^{\circ}\)C) for 24 h and then heated at 800 \({}^{\circ}\)C for 8 h before testing. For the calculation of crushing-strength values (\(\sigma_{\rm c}\)), the following formula was used (Equation (2)):  
\[\sigma_{\rm c}=\rm F/S, \tag{2}\]  
where \(\sigma_{\rm c}\) is the crushing strength of the sample (MPa); F is the maximum load at which the sample breaks apart (N); and S is the pressed area of the sample (mm\({}^{2}\)). For the measurements of microhardness both, the raw and sintered GC samples were pressed at 200 MPa and prepared for the tests in the form of pellets (solid samples). All of them were cylindrically shaped with dimensions of \(\Phi\)16 mm \(\times\) H2 mm. Sintered solid samples were prepared by heating at 800 \({}^{\circ}\)C for 8 h. In order to obtain plane-parallel surfaces for the test, the obtained solid samples were polished employing SiC paper with a P2000 grain size. The indentation test method (Vickers hardness test) was applied for an evaluation of the microhardness of the solid samples. The experiments were carried out using a Buehler Micromet Microindentation Hardness Tester (Model Micromet 5101, Buehler, Lake County, United States). For each raw and sintered GC solid sample, three measurements were performed and average values were calculated. Since the raw solid samples were estimated to be very soft, a small load of 0.25 N was applied in the tests, while for the sintered samples, the load was set to be higher, 0.5 N. The time of indentation was 15 s.