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The microwave dielectric properties and microstructure of Mg3B2O6 ceramics with the addition of 0–5 wt% Mg2B2O5 were investigated. Scanning electron microscopy of the Mg3B2O6 ceramics sintered at 1310°C revealed the presence of anisotropic, exaggeratedly grown grains with a crystallographic fault that extends along the direction of the anisotropy. The most pronounced, exaggerated grain growth was observed for Mg3B2O6 with 5 wt% of Mg2B2O5. The anisotropic grains were examined by electron backscatter diffraction. The analysis of the Kikuchi pattern showed that the crystallographic fault is a (011) twinning plane. The duration of annealing was found to have no significant effect on the dielectric permittivity, while the Q×f values increased two times with a prolongation of the annealing from 20 to 160 h. The highest Q×f value of 240 000 GHz was measured for Mg3B2O6 ceramics with 5 wt% of Mg2B2O5. These ceramics consisted of 10 times larger grains than the Mg3B2O6 ceramics without the added Mg2B2O5, which also showed high Q×f values of 200 000 GHz. The decrease of the amorphous phase during the annealing could be one of the reasons for this improvement of the Q×f values.

The nucleation and crystallization of MgO–B2O3–SiO2 (MBS) glasses with various TiO2 additions (0–15 wt%) were studied by means of a nonisothermal, thermal analysis technique, X‐ray diffraction, and electron microscopy. The effect of the TiO2 on the nucleation rate was determined from the dependence of the inverse temperature at the differential scanning calorimetry (DSC) peak (1/Tp; Tp→crystallization peak temperature) on the nucleation temperature (Tn). A significant increase in the nucleation rate was observed for the MBS glasses with a TiO2 content greater than 3 wt%. The TiO2 was also found to influence the crystallite growth mechanism. In the pure MBS glass the crystallization of the Mg2B2O5 was 3‐dimensional (3D) bulk with a diffusion‐controlled crystal growth rate. The value of both the Avrami parameter (n) and the morphology index (m) was 1.5 and the crystallization activation energy (E) was in the range 410–440 kJ/mol. The change of the nucleation rate (a) from zero (a = 0) to a constant nucleation rate (a = 1) was observed at 3 wt% TiO2. The change from a diffusion‐ to an interface‐controlled growth rate occurred at 5 wt% TiO2, where n and m became 4 and 3, respectively. According to the analysis of the DSC data using the modified Kissinger method, a small TiO2 concentrations (1 wt%) lowered E, whereas E increased with a further increase in the TiO2 concentration, exceeding at 5 wt% TiO2 the E of pure MBS glass.

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