Then the fracture toughness KIC was

2.4. Sintering and crystallization
The sintering and crystallization kinetics of the gel-glasses were evaluated by hot stage microscopy (HSM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Based on the HSM and DSC results, isothermal heat treatments at different temperatures were designed to sinter and crystallize the glass powder compacts.
2.4.1. Hot stage microscopy (HSM)
To identify the optimal temperature for sintering, the glasses were analyzed using Pentobarbital heating microscope (Misura HSM ODHT – Expert System Solutions). First, 30 mg of glass powder was manually pressed into a stainless steel mold at 25 MPa for 5 min. The sample (3 mm height×3.2 mm diameter) was placed on an alumina plaque and inserted inside the furnace. The measurement was performed at 10 °C/min up to 1400 °C. In the heating microscope, a light source was projected onto the sample, and a digital camera was used to capture the dimensional changes of its shadow.