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Abstract

The past fifteen years has seen the rapid development of various wireless communication
devices, which operate at microwave or milimeterwave frequencies (mobile phones, wireless
local-area networks (LAN), Bluetooth, etc.). At the same time, this progress has stimulated
the development of new materials and process technologies, which enable the size reduction
and increased functionality of electronic components. Low-temperature co-fired ceramic
(LTCC) technology is one of the most promising approaches for fabrication of low-cost,
miniaturised and integrated electronic components. Due to simultaneous co-firing of the
substrates and electrodes the most commonly used dielectric substrates are based on glassceramics
materials. In spite of very advanced features of the LTCC technology, the
development of new LTCC modules is accompanied by several challenges and new materials
that could fulfil various demands. In the scope of this dissertation MgO-B2O3-SiO2 (MBS)
based glass and its transformation to glass-ceramics was systematically investigated in order
to evaluate this material for potential dielectric LTCC substrate applications. Among the
compounds that are expected to crystallize from the MgO-B2O3-SiO2 glass the borates
(Mg2B2O5, Mg3B2O6) have not been so intensively investigated in terms of dielectric
properties as silicates (MgSiO3, Mg2SiO4). However, the knowledge of the dielectric
properties of individual crystalline components is important for the interpretation of the
properties of the MgO–B2O3–SiO2 glass-ceramics. From this reason the first part of the
dissertation is dedicated to the synthesis, sintering and dielectric characterization of Mg2B2O5
and Mg3B2O6 ceramics. Mg2B2O5 (6.2-7.0) and Mg3B2O6 (6.8 - 7.4) did not differ
significantly in permittivity, while nearly 10-times difference was observed in Qxf-values.
With regard to the Qxf values Mg3B2O6 (240 000 GHz) surpasses the Mg2B2O5 ceramics,
which exhibited Qxf-values 10 000-30 000 GHz. The microstructural characteristics and the
synthesis conditions for the preparation of high-Q Mg3B2O6 ceramics was investigated in
details. 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 anisotropic grains were examined by
electron backscatter diffraction (EBSD). The analysis of the Kikuchi pattern showed that the
crystallographic fault is a (011) twinning plane.
The control over the microstructure and dielectric properties of the glass-ceramics is
possible when the nucleation and crystallization processes are well understood. The overall
crystallization processes in MBS glass were studied by means of non-isothermal analysis
technique, X-ray diffraction and scanning electron microscopy. For MBS glass nucleation
occurred in the temperature range from 600 to 750 °C with the maximum nucleation rate at
700 °C, whereas the nucleation and crystal growth processes overlapped at 700 ≤ T ≤ 750 °C.
The analyses of the non-isothermal data by the most common models (Ozawa, Kissinger,
Matusita, Ozawa-Chen, Šatawa) revealed that the crystallization of Mg2B2O5 was threedimensional
bulk with diffusion controlled crystal growth rate with n = m = 1.5 and activation
energy for crystallization (E) of 420-450 kJ/mol. The addition of TiO2 nucleating agent to the
MBS glass in an amount of 10 wt.% was found to facilitate the formation of nuclei and
changed the crystallization mechanism to bulk crystallization with an increasing number of
nuclei (m = 3, n = 4).
The crystallization product obtained during non-isothermal experiment differed from that
obtained after isothermal heat-treatment at the temperatures higher than 950 °C. In the timescale
of dynamic differential scanning calorimetry (DSC) measurement only Mg2B2O5
crystallize, while the silicon remained in the glassy matrix till the melting. In contrast, during
isothermal heat-treatment, the main crystalline phases of Mg2B2O5 and MgSiO3 appeared at
800 and 950 °C, respectively. Warwickite (Mg3TiB2O8) and boron doped rutile (TiB0.024O2)
phase additionally crystallized in TiO2-added MBS glass at 1000 °C.
The addition of TiO2 nucleating agent was found to increase the permittivity and improve
the Qxf-values. The permittivity increased from 6.1 in MBS glass to 6.9 in MBS Glass with
10 wt.% of TiO2. The improvement of the Qxf-values, which reached the maximum value of
16 5000 GHz in TiO2-added glass-ceramics was ascribed to the enhanced crystallization
caused by the TiO2 addition. The results of dielectric characterization showed that MgOB2O3-
SiO2 based glass-ceramics in some respect surpass the commercially used glassceramics
substrates.