sintering behavior and microwave dielectric properties of MgO-2B 2 O 3 - xwt %H 3 BO 3 - ywt %BCB ceramics

The bulk density, sintering behavior and microwave dielectric properties of MgO-2B 2 O 3 series ceramics synthesized by solid-state reaction method were systematically studied in this paper. X-ray diffraction and microstructural analysis revealed that the as-prepared MgO-2B 2 O 3 ceramics possessed a single-phase structure with rod-like morphology. Through the investigation of the effects of different dosages of H 3 BO 3 and BCB on bulk density, sintering behavior and microwave dielectric properties of MgO-2B 2 O 3 ceramics, the optimum sintering temperature was obtained at an addition of 30 wt %H 3 BO 3 and 8 wt %BCB and the sintering temperature was reduced to 825 o C. The addition of 40 wt %H 3 BO 3 and 4 wt %BCB increased the quality factor Q×f , permittivity ε r and temperature coefficient of resonance frequency τ f of MgO-2B 2 O 3 to 44,306 GHz, 5.1 and -32 ppm/ o C, respectively, meeting the criteria of low-temperature co-fired ceramics.


Introduction
Due to the low manufacturing cost, short development cycle, and potential for miniaturization and incorporation of electronic devices, low-temperature co-fired ceramics (LTCC) is the trend for electronic components manufacture in the wireless communication and broadcasting industry [1][2][3][4]. However, most of the high Q×f dielectric materials are manufactured at high sintering temperatures. High sintering temperatures not only hinder their incorporation with low melting electrode and polymer based substrates but also lead to huge energy consumption and volatile components evaporation. For practical application, LTCC requires not only excellent microwave dielectric properties but also low sintering temperature and good co-fire matching between ceramics and electrodes [5][6][7][8][9][10]. Materials with low melting point are often added in order to lower the firing temperatures. However, the microwave dielectric properties were lowered by the addition of low melting point materials.
By contrast, the sintering temperature of magnesium oxide was effectively reduced by adding appropriate amount of sintering aid [34] such as B2O3, H3BO3 and BaCu(B2O5) (BCB).
However, the microwave dielectric properties of MgO-2B2O3-xwt%H3BO3-ywt%BCB ceramics have not been investigated systematically. This is the objective of this research.

Results and discussion
SEM images of the MgO-2B2O3-10wt%H3BO3-ywt%BCB (y = 2, 4, 6, and 8) ceramics sintered at optimal temperatures are shown in Figure 2(a-d). It can be seen that the MgO-2B2O3 crystals possessed a rod-like shape and refined with the increase of BCB content. As the BCB content increased to 8 wt %, the MgO-2B2O3 crystals exhibited a glassy phase. Moreover, the porosity increased with the increase of BCB content.
Bulk density, ɛr, Q×f, and τƒ of the MgO-2B2O3-10wt%H3BO3-ywt%BCB (y = 2, 4, 6 and 8) ceramics sintered at different temperatures are shown in Figure 3. It can be seen that the bulk density initially increased with the increase of sintering temperature except for the sample with 2 wt% BCB which showed a slight decrease as shown in When sintering temperature increased from 900 °C to 950 °C, the Q×f of the      The dielectric properties depend on relative density, crystal structure, and other phase content [39,40]. The relative εr of MgO-2B2O3-xwt%H3BO3-4wt%BCB ceramics was consistent with the change in bulk density. The maximum εr was found to be 4.81 at a H3BO3 content of 10 wt%. Also, an overall increase in relative εr was observed for H3BO3-containing ceramics when H3BO3 content increased to 20 wt%, 30 wt% and 40 wt%. Hence, the εr was influenced by the variation of H3BO3 content, sintering temperature and bulk density.

Conclusions
In ceramics, which is highly desirable for microwave equipment and devices.