Journal
JOURNAL OF MATERIOMICS
Volume 7, Issue 3, Pages 478-487Publisher
ELSEVIER
DOI: 10.1016/j.jmat.2020.11.005
Keywords
Dielectric breakdown mechanism; MgO-Based ceramics; Microwave substrate; Excessive liquid-phase sintering
Funding
- National Natural Science Foundation of China [11774366]
- International Partnership Program of Chinese Academy of Sciences [GJHZ1821]
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Glass-free MgO-based microwave dielectric ceramics were synthesized with LiF and Al2O3 as additives, showing altered ceramic morphologies and densities with excessive LiF addition, leading to adverse effects on physical properties. Additionally, LiF dramatically reduced dielectric breakdown strength and altered the breakdown behavior, with optimal properties achieved in ceramics modified with 1 wt% LiF sintered at 950 degrees C.
Glass-free MgO-based microwave dielectric ceramics (1-x) wt% (0.98 MgO-0.02 Al2O3)-x wt% LiF (x = 0.5, 1, 2, 3, 4) were synthesized where LiF and Al2O3 were utilized as sintering additive and reinforcement phase in MgO matrix respectively. It was found that ion substitution is apt to occur between LiF and MgO, leading to the formation of oxygen vacancies and MgF2. Nevertheless, different from ordinary liquid-phase sintering, morphologies of ceramics were distinctly altered with grains changing from polyhedron to sphere-like shape and densities underwent obvious decrease when excessive amount of LiF was introduced and we call it excessive liquid-phase sintering. Grain boundary weakening caused by this circumstance would exert an adverse effect on physical properties. Moreover, LiF addition dramatically reduced dielectric breakdown strength and altered the dielectric breakdown behavior of MgO-based ceramics, which is dominated by electrical breakdown mechanism. Combination of good properties were achieved in 1 wt% LiF modified MgO-based ceramics sintered at 950 degrees C which exhibited superior microwave properties (epsilon(r) = 9.56, tan delta = 9.2 x 10(-5), Qf = 124,600 GHz), high flexural strength (184.5 MPa), high thermal conductivity (21.3 W/(m . K)), high coefficient of thermal expansion (similar to 12 ppm/degrees C) and moderate electrical properties (E-b = 35.9 kV/mm, p = 4.9 x 10(12) Omega cm). (C) 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V.
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