Low‐temperature sintering and thermal stability of Li₂GeO₃‐based microwave dielectric ceramics with low permittivity

Abstract A low‐permittivity dielectric ceramic Li₂GeO₃ was prepared by the solid‐state reaction route. Single‐phase Li₂GeO₃ crystallized in an orthorhombic structure. Dense ceramics with high relative density and homogeneous microstructure were obtained as sintered at 1000‐1100°C. The optimum microwave dielectric properties were achieved in the sample sintered at 1080°C with a high relative density ~ 96%, a relative permittivity εr ~ 6.36, a quality factor Q × f ~ 29 000 GHz (at 14.5 GHz), and a temperature coefficient of resonance frequency τf ~ −72 ppm/°C. The sintering temperature of Li₂GeO₃ was successfully lowered via the appropriate addition of B₂O₃. Only 2 wt.% B₂O₃ addition contributed to a 21.2% decrease in sintering temperature to 850°C without deteriorating the dielectric properties. The temperature dependence of the resonance frequency was successfully suppressed by the addition of TiO₂ to form Li₂TiO₃ with a positive τf value. These results demonstrate potential applications of Li₂GeO₃ in low‐temperature cofiring ceramics technology

A reduced sintering temperature and improvement in the microwave dielectric properties of Li₂Mg₃TiO₆ through Ge substitution

In this paper, we demonstrate a strategy to reduce the sintering temperature of titanates through an appropriate amount of Ge substitution for Ti. A series of Li₂Mg₃Ti1-xGexO₆ (x = 0.04, 0.06, 0.08, 0.10, 0.12) ceramics, prepared by a solid-state reaction method, are reported. By controlling the content of the Ge substitution, the sintering temperature of Li₂Mg₃TiO₆ was significantly reduced to 1140 °C. When x = 0.10, the Li₂Mg₃Ti1-xGexO₆ ceramics sintered at 1140 °C for 6 h displayed excellent values of εr = 13.7, Q × f = 131,500 GHz and τf = −34.2 ppm/°C. In addition, the temperature stability was successfully adjusted to be close to zero by adding CaTiO₃ to form a composite ceramic. A temperature stable ceramic 0.96Li₂Mg₃Ti0.9Ge0.1O₆-0.04CaTiO₃ with τf = −3.5 ppm/°C, εr = 14.9 and Q × f = 68,900 GHz was obtained when sintered at 1180 °C. The good dielectric performances of the CaTiO₃-modified Li₂Mg₃Ti0.9Ge0.1O₆ ceramics makes them possible candidates for substrates in microwave integrated circuits

Structural, thermal and microwave dielectric properties of the novel microwave material Ba₂TiGe₂O₈

Abstract Rapid developments of microwave dielectric materials have emerged in recent years due to their wide-spread applications and the revolution in wireless communications. However, many commercial microwave materials are based on titanates, niobates and tantalates which have the disadvantages both of costly raw materials and high sintering temperatures. These result in a production process which is not energy efficient. In this paper we develop a BaO-TiO₂-GeO₂ ternary system derived from the BaO-TiO₂ binary system to obtain low temperature co-fired microwave dielectric ceramics for high frequency applications. The Ba₂TiGe₂O₈ ceramics were prepared via the conventional solid-state route. The X-ray diffraction results showed that Ba₂TiGe2O₈ belongs to the orthorhombic, Cmm2 space group. The ceramics exhibited a densification of 96.3% after being sintered at 1060 °C. They also exhibited a relative permittivity (εr) of 12.7, a quality factor (Q×f) of 9060 GHz (at 10 GHz), a temperature coefficient of resonant frequency (τf) of −30 ppm/°C and a coefficient of thermal expansion (CTE) of 11.0 ppm/°C. In addition, the Raman spectra and ionic polarizability of Ba₂TiGe₂O₈ unit cells were investigated

Ultralow loss CaMgGeO₄ microwave dielectric ceramic and its chemical compatibility with silver electrodes for low-temperature cofired ceramic applications

Abstract A new ultralow dielectric loss cofired CaMgGeO₄ dielectric material with olivine structure was fabricated by the solid-state route. The X-ray patterns, Rietveld refinement, and microstructure revealed the characteristics of the synthesized material. CaMgGeO₄ ceramic belongs to the orthorhombic system with a Pbmn space group. Sintered at 1300 °C for 6 h, the ceramic exhibited a densification of 96.5%, an ultrahigh quality factor (Q × f) of 124 900 GHz (tan δ = 1.24 × 10–4) at a frequency of 15.5 GHz, a permittivity (εr) of 6.71, and a temperature coefficient of resonant frequency (τf) of −73.7 ppm/°C, and the average coefficient of thermal expansion of CaMgGeO₄ was 12.4 ppm/°C. The sintering temperature of the CaMgGeO₄ ceramic was reduced from 1300 to 940 °C with the addition of 5 wt % B₂O₃. The CaMgGeO₄ + 5 wt % B₂O₃ ceramics exhibited favorable microwave dielectric performances: Q × f = 102 000 GHz (at 16.4 GHz), εr = 5.80, and τf = −64.7 ppm/°C, respectively. In addition, the CaMgGeO₄ ceramic did not react with Ag electrodes, which could be advantageous in low-temperature cofired ceramic multilayer microwave devices

Ultralow temperature cofired BiZn₂VO₆ dielectric ceramics doped with B₂O₃ and Li₂CO₃ for ULTCC applications

Abstract In this paper, BiZn₂VO₆ doped with sintering aids of B₂O₃ and Li₂CO₃ is investigated in order to broaden the options for ultralow temperature cofired ceramics (ULTCC). The sintering behavior, microstructure, and microwave dielectric properties are studied. In combination with 1 wt% B₂O₃ and 5 wt% Li₂CO₃ dopants, the sintering temperature of the BiZn₂VO₆ ceramics was reduced from 780°C to 600°C. The co‐doped BiZn₂VO₆ ceramics exhibited a low relative permittivity (εr) of 8.9 and a quality factor (Q × f) of 13 000 GHz at a microwave‐range frequency of 9 GHz. The temperature coefficient of resonant frequency (τf) was measured to be −97 ppm/°C. The average linear coefficient of thermal expansion (CTE) was 7.2 ppm/°C. With the low sintering temperature, the co‐doped BiZn₂VO₆ ceramics are compatible to be cofired with cost‐effective aluminum electrodes. This was proven in a reaction test between the BiZn₂VO₆‐B₂O₃‐Li₂CO₃ and aluminum powders, in which no chemical interaction could be detected. These promising properties make the B₂O₃‐Li₂CO₃ co‐doped BiZn₂VO₆ an ideal candidate for ULTCC applications

Structural, infrared reflectivity spectra and microwave dielectric properties of the Li₇Ti₃O₉F ceramic

Abstract A cubic rock salt structured ceramic, Li₇Ti₃O₉F, was fabricated via the conventional solid-state reaction route. The synthesis conditions, sintering characteristics, and microwave dielectric properties of Li₇Ti₃O₉F ceramics were investigated by X-ray diffraction (XRD), thermal dilatometer, Scanning Electron Microscopy (SEM) accompanied with EDS mapping, and microwave resonant measurements. Rietveld refinement, selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) confirmed that Li₇Ti₃O₉F adopts a cubic rock-salt structure. The ceramic sintered at 950 °C presented the optimal microwave properties of εᵣ = 22.5, Q×f = 88,200 GHz, and τf  = −24.2 ppm/°C. Moreover, good chemical compatibility with Ag was verified through cofiring at 950 °C for 2 h. These results confirm a large potential for Li₇Ti₃O₉F ceramic to be utilized as substrates in the low temperature cofired ceramic (LTCC) technology. This work provides the possibility to exploit low-temperature-firing ceramics through solid solution between oxides and fluorides