In this work, the frequency dependent dielectric properties of polycrystalline MgB₂ as an effect of sintering temperature, elevated oxidation and Si and C addition were investigated. The x-ray diffraction (XRD) patterns, lattice properties and surface morphology of the samples were analyzed and correlated to the dielectric properties as a function of frequency (100 Hz to 10 MHz) measured at room temperature.

The in situ samples were studied in the first part. They showed negative permittivity as a result of negative capacitance indicating the metallic nature of these samples. These samples had comparable oxide phase but improved crystallinity was found in samples sintered at higher temperature. The samples sintered at higher temperature showed negative real part of complex dielectric permittivity (Ɛ’) as low as -6 x 10⁵ because of reduced defect density and better grain connectivity, leading to a higher alternating current (AC) conductivity (σ_AC  2.5 S/m) and dielectric loss ( tan δ  -2000). Next, the laboratory-made MgB₂ was sintered at various temperatures (ex situ). These ex situ samples showed positive real part of complex dielectric permittivity behavior as a result of more oxide phase present and weak grain coupling. The ex situ samples sintered at higher temperature have improved crystallinity and more oxide. However, the improved grain connectivity in these samples had led to lower real part of complex dielectric permittivity value but higher dielectric loss and AC conuctivity. The ex situ samples were also compared to the in situ sample in terms of XRD patterns, lattice structure, microstructural properties and dielectric behavior.

The MgB₂ samples reacted at different oxidation level were also studied. Borates were detected as secondary phases in highly oxidized samples. The presence of more oxides and borides in these  samples had resulted in higher real part of complex dielectric permittivity value (Ɛ’  35) and reduced AC conductivity (σ_AC  10^-7 S/m). The low dielectric loss (tan δ  10^-2) shown in highly oxidized samples can be utilized for practical application. Finally, MgB₂ reacted with Si and C related compounds were investigated. MgO and Mg₂Si phases were detected in samples reacted with Si related compounds. The samples reacted with SiC had shown severely distorted lattice structure with high C substitution level as a result of active free C substitution released from the Mg-SiC reaction. This had also led to higher real part of complex dielectric permittivity value and reduced AC conductivity and dielectric loss in this sample.

The AC conductivity of all these samples can be well described by the Drude model or power law model. The Argand diagrams were plotted for these samples and equivalent circuits were modeled from these plots. The circuit models indicated the electrical behavior is primarily due to combined effect of bulk and grain boundary for all samples except the sole bulk response for the in situ samples. The dielectric properties may agree with the XRD data indicating the presence of impurity phases and distorted lattice structure in these samples.

Figure 1: Field emission scanning electron microscopy images of (a) 85CX, (b) SiC10, (c) Si+C10, (d) Si7 and (e) C3.

Figure 2: Frequency dependent dielectric permittivity for (a) 85CX, (b) SiC10, (c) Si+C10, (d) Si7 and (e) C3.

*Abstract of the thesis (PhD) by Tan Kwee Yong

For further information, please contact:

Chen Soo Kien, PhD
chensk@upm.edu.my

Date of Input: 30/09/2025 | Updated: 30/09/2025 | roslina_ar