The charge of quartz glass and soluble sodium silicate fill a cylindrical crucible, which implements the geometry of the "unlimited" cylinder (Fig. 1). Three thermocouples (1,2,3) with bare junctions are placed in the charge and are located on the average level. The coordinates of the junctions are precisely fixed.
Fig.1 Scheme of the experiment
Fig. 2. Temperature profiles
The temperature changes in time in three coordinates are recorded during the heating. Mathematical processing of the results was performed using a specially designed program. The program reproduces using the iterative method the temperature field profile, which occurs in the given evolution of the temperature field, and also performs the calculation of the effective temperature conductivity . The reproduced temperature profiles are shown in Fig. 2. Temperature curves as time functions are shown in Fig. 3 (1 − center, 2 − 1/R, 3 − R). Deviation from the monotonicity of the curves flow indicates the appearance of thermal effects. More clearly this is shown in Fig. 4.
The effective temperature conductivity of a quartz glass powder up to t ~ 600°C is practically a constant value (fig. 5, curve 1). Such a curve for a quartz glass powder with an admixture of sodium silicate is far from monotonicity (Fig. 5, curve 2). At the initial heating stage, an undervoltage of effective temperature conductivity is stated. This is explained by the endothermic effect of adsorption moisture removal. Then, in the range of 200-450°C, there is another decrease in the effective temperature conductivity. This is caused by the endothermic effect of the removal of already chemically bound helium water. Starting from t ~ 650°C, another endoeffect is observed, due to the melting of the eutectic.
Fig. 3. Temperature change in time
Fig.4. Changing the heating speed in time
Fig. 5 (curve 3) also shows the dependence of the effective temperature conductivity of the final material.
Fig. 5 Effective temperature conductivity
Fig. 6. Effective thermal conductivity
Fig. 6 shows the dependence of the effective heat conductivity of the final material. It is calculated on the basis of the values of effective temperature conductivity through the known values of the specific heat of quartz glass and bulk mass of porous material. The value for relatively low temperatures remains at the level of 0.3-0.4 W/(m·K). This material is characterized by thermal insulation properties, fire resistance and heat resistance.