The trend towards high integration, density, and power in electronic circuits has led to more stringent requirements for the dielectric strength, mechanical strength, and thermal conductivity of dielectric materials. The research for dielectric materials with high and reliable dielectric strength and properties is of great significance for engineering applications in various fields such as dielectric capacitors^[1-2], high-power microwave device components^[3-4] and substrates^[5]. In the past decade, great efforts have been made to study the dielectric strength of different materials. Polymers usually have low dielectric constants, dense microstructure, and amorphous state, which are the best choices for high dielectric strength^[6-7]. While ceramic materials have become a research hotspot due to their excellent temperature stability, aging stability, and extended service life^[8-9]. However, characteristics like high dielectric constant, porous and coarse grain structure make ceramics easier to breakdown at low electric field, which are usually lower than one-tenth of that of polymers^[10]. Therefore, searching for ceramics with high dielectric strengths is of great importance. At present, chemical composition regulation, including ion doping to modify phase structure, construct defects, or regulate microstructure, has been extensively studied and proven to be simpler and more feasible^[11-12].

For MgO ceramics, lower dielectric constant (~10), higher bandgap (nearly 7.77 eV) and simple rock salt crystal structure contribute to high dielectric strength as 232 kV/mm^[13-14]. However, the actual value is usually much lower than the theoretical one, which requires the regulation of the non-intrinsic factors, such as the ceramic microstructure. Former studies have shown that the addition of 1%-1.5% (in molar) ZrO₂ and CaCO₃ is proved effective to inhibit abnormal grain growth of MgO and promote pore discharge, leading to high density and mechanical strength, which are beneficial to enhance the dielectric strength^[15-16]. Therefore, ZrO₂ reinforced MgO ceramics possess great potential in high-voltage microwave dielectric application. However, sintering temperature of the above-mentioned MgO-based ceramics is higher than 1500 ℃, which limits their application in microwave capacitors and insulators. Fortunately, previous researches have indicated that MnCO₃ is effective in lowering the sintering temperature of ceramics and enhancing the density^[17⇓-19]. Hence, MnCO₃ is utilized here to reduce the sintering temperature of MgO-based ceramics, and their relevant application potential substrate materials are worth exploring.

In this work, MgO-1%ZrO₂-1%CaCO₃-x%MnCO₃ (MZCM_x, x = 0, 0.25, 0.50, 1.00, 1.50, in molar) ceramics were prepared, and ZrO₂ and MnCO₃ are used as reinforcing phase and sintering additive, respectively. The microstructure evolution and dielectric properties were studied and discussed in detail. The results show that the doping of MnCO₃ significantly reduces the sintering temperature of MgO by ion substitution to activate the lattice and reduce the sintering activation energy, which affects and changes the sintering behaviour and dielectric properties of MgO.

Light magnesium oxide (MgO, 99.650%), ZrO₂ (99.980%), CaCO₃ (99.657%) and MnCO₃ (99.916%) were mixed and ball milled for 6 h with ethanol by a planetary ball mill, then the dried powders were pressed into ϕ13 mm × 2 mm pellets and ϕ13 mm × 10 mm cylinders at a pressure of 200 MPa. Finally, these pellets and cylinders were sintered at 1520 ℃ (for pure MgO ceramics), 1540 ℃ (for MZCM₀ ceramics), 1400 ℃ (for MZCM_0.25 and MZCM_0.50 ceramics), and 1350 ℃ (for MZCM_1.00 and MZCM_1.50 ceramics) for 2 h, respectively. The sintered pellets were machined into ϕ10.0 mm × 0.5 mm and coated with silver electrode (ϕ8.0 mm) for capacitance and dielectric strength measurement. The cylinders were machined into ϕ10.0 mm × 5.5 mm samples by plane and outer edge processing for microwave performance test.

The chemical composition of ceramic powders was measured by an X-ray fluorescence spectrometer (Axios, PANalytical, Netherlands). The density of the sintered ceramic samples was tested by Archimedes method, and the crystal structure of the materials was analyzed by an X-ray diffractometer (XRD, PANalytical 9430070 99991 series). The morphology and chemical composition of the ceramic samples were characterized by a field emission scanning electron microscope (SEM, Magellan400, FEI, USA). The dielectric strength of the samples (at least 8 pieces for each composition) at room temperature was tested using high-voltage source (WISMAN, DL120P600, 0-120 kV). The temperature and frequency dependence of the dielectric constant and dielectric loss were evaluated via a broadband dielectric Novocontrol Alpha spectrometer (Novocontrol Technologies, Germany). Vector network analyzer (Agilent E8362B) and temperature chamber were used to measure the microwave dielectric properties of the samples. The quality factor (Q × f) of samples was estimated by the TE01δ mode dielectric resonator method. The resonant frequency temperature coefficient τ_f was calculated by the following equation:

where f₈₅ and f₂₅ represent the resonant frequencies at 85 and 25 ℃, respectively. ∆T represents the temperature difference, 60 ℃. The capacitance temperature coefficient τ_c could be calculated using a similar equation:

where C₂ and C₁ represent the capacitances at T₂ and 25 ℃, respectively.

Fig. 1 shows the dependence of densities of MZCM_x (x = 0, 0.25, 0.50, 1.00, 1.50) ceramics versus x and sintering temperature. According to the changing trend of density, the optimal sintering temperature for sample densification can be obtained, which is 1520 ℃ for pure MgO, 1540 ℃ for x = 0, 1400 ℃ for x = 0.25, 0.50, and 1350 ℃ for x = 1.00, 1.50, respectively. The optimal sintering temperature decreases with the increase of x.

The sintering theory of ceramics points out that the growth process of grains conforms to the kinetic equation:

where G₀ and G_t represent the initial grain size as well as the grain size after sintering for t time at T temperature, R is the ideal gas constant, n and Q represent the grain growth index and the grain growth activation energy, respectively. To solve these two key parameters, the above equation can be deformed:

where G is the average grain size of the ceramic, and k₀ is the constant. The grain growth index n can be solved using the graphical method by controlling the sintering temperature and sintering time of the ceramics to make the logarithmic fitting curves of the average grain size to the sintering time and temperature, respectively^[20].

The calculation results of pure MgO ceramics show that the grain growth index n = 3 of pure MgO ceramics corresponds to the volume diffusion process, and the grain growth activation energy Q is about 556.9 kJ/mol^[20]. Fig. 2 is the solution of grain growth index and grain growth activation energy of MZCM_1.00 ceramics. The grain growth index n of MZCM_1.00 ceramics sintered at 1300 ℃ is 1.97 (≈ 2). Among them, the size of n represents different diffusion mechanisms in the sintering process of ceramics. n = 2 corresponds to the grain boundary diffusion process, while the grain boundary diffusion mode can be regarded as a fast channel for material diffusion. Therefore, MZCM_1.00 ceramic grain growth gradually changes from the grain boundary diffusion mode to the intrinsic bulk diffusion mode of MgO with the increase of temperature, implying that the introduction of Mn significantly promotes the grain growth process during the initial stage of ceramic sintering. Meanwhile, due to the change of grain growth index during the sintering process of ceramics, n = 2.5 is taken to further solve the grain growth activation energy. The average grain growth activation energy is about 301.56 kJ/mol, which is much lower than that of pure MgO ceramics, further proving the promotion effect of Mn on the sintering process of MgO ceramics.

Fig. 3 shows the XRD patterns of MZCM_x ceramics. A composite structure composed of cubic-MgO (PDF#45-0946), cubic-ZrO₂ (PDF#49-1642) and glass phase can be clearly observed while the diffraction peaks of cubic-ZrO₂ weaken with the increase of MnCO₃ content, which means the decrease of cubic-ZrO₂ content. Firstly, Ca²⁺ solid dissolves into MgO lattice completely, and then the introduction of MnCO₃ results in the replacement of Ca²⁺ and the formation of glass phase.

Fig. 4 reveals the morphological evolution and the grain size distribution of different thermal etched MZCM_x samples. Initially, the average grain size of pure MgO ceramics is high up to 22.94 μm, and the addition of ZrO₂ and CaCO₃ forms the nano-sized second phase at the grain boundaries of MgO, leading to the grain size decreasing to 5.95 μm. After that, with the doping of MnCO₃, the content of the second phase decreases, resulting in an increase for grain size, but the overall size is still smaller than that of pure MgO. The ion vacancy concentration significantly affects the grain growth and sintering process. Similar condition happens to Cooper et al.^[21], who found that increasing the concentration of U⁴⁺ vacancies could significantly promote the growth of UO₂ grains. The sintering process of MgO is dominated by anion diffusion, and the doping of MnCO₃ further reduces the oxygen vacancy concentration and inhibits the mass transfer process during the later stages of sintering, resulting in an increase in grain size.

X-ray photoelectron spectroscopy (XPS) is used to characterize the changes in oxygen vacancy concentration, and MZCM_1.00 and MZCM_1.50 ceramic samples are selected to test due to the limited doping concentration of MnCO₃. The samples were subjected to argon ion etching to eliminate adsorbed oxygen on the sample surfaces prior to testing for the oxygen element, and the results of the fitted analyses are shown in Fig. 5. Oxygen in the samples mainly exists in the forms of lattice oxygen (LO) and oxygen vacancy (OV), and doping of Mn ions leads to a reduction of the ratio of the vacancy oxygen peak area to the lattice oxygen peak area, implying that the concentration of oxygen vacancies decreases in the systems. It is worth noting that the oxygen vacancies in the ceramic systems prepared in this work originate from intrinsic oxygen vacancies due to the susceptibility of MgO to Schottky defect reactions^[22].

The Weibull distribution is widely used to describe the dielectric strength^[12]. In this model, the probability of dielectric breakdown is defined as follows:

where E_i, E_b and β are specific breakdown strength of single sample, characteristic breakdown strength and shape parameter that indicates the width of distribution, respectively. Fig. 6(a) illustrates the Weibull distribution of the dielectric strength for MZCM_x ceramics, in which X_i and Y_i are two parameters which can be described as follows:

where i and n are sample sequence in specific order and total amount of samples for each composition. The fitting dielectric strengths of MZCM_x are 113.95, 99.4, 96.2, 92.3, and 85.2 kV/mm for x = 0, 0.25, 0.50, 1.00, and 1.50, respectively. The fine grain and dense structure of MZCM₀ ceramics allow for a significant increase in dielectric strength. And doping of MnCO₃ results in reduction in dielectric strength due to the weakened second phase and intergranular bonding strength^[23]. Fig. 6(b) shows the temperature dependence of the dielectric constant and dielectric loss of MgO-based dielectric ceramics at 1 MHz, respectively. The dielectric losses of all samples are less than 0.1% in the tested temperature range, presenting a low loss characteristic. In addition, the dielectric properties of all samples maintain good stability in the temperature range of 50-300 ℃, which is beneficial to their application as core materials in extreme environments.

The dielectric properties of MgO-based ceramics are characterized in this work, which are shown in Table 1. MZCM_1.00 ceramics exhibit low τ_c in the temperature range of -25-85 ℃, which further proves the good temperature stability. The microwave dielectric properties of the samples were measured at a resonant frequency range of 10.2-10.5 GHz. In particular, MZCM_1.00 ceramics have a Q × f of up to 216642 GHz, and τ_f is relatively close to zero (-12.54×10^-6 ℃^-1), which is conducive to the integration and miniaturization of electronic circuits^[24]. The addition of MnCO₃ reduces oxygen vacancies, resulting in the creation of free electrons. Therefore, the overabundant MnCO₃ causes excessive electrons, increasing dielectric loss and decreasing Q × f ^[25].

In summary, MnCO₃ doped MgO-ZrO₂-CaCO₃ high- voltage dielectric ceramics are designed and prepared. The ultrahigh dielectric strength of 92.3 kV/mm was achieved in MZCM_1.00 ceramics, and the sintering temperature is significantly reduced to 1350 ℃. The remarkable improvement of dielectric strength is mainly due to two main factors. Firstly, MgO has the potential to achieve high dielectric strength. Secondly, the introduction of ZrO₂ and CaCO₃ modifies the microstructure, eliminates external factors, and fully utilizes its dielectric properties. At the same time, MnCO₃ can reduce the sintering temperature of MgO ceramics by accelerating mass transfer caused by ionic vacancies. The series of ceramics with high dielectric strength and medium sintering temperature also show high Q × f, which makes the MgO-based ceramic a candidate material for microwave applications. This MgO-based ceramic also has the advantages of simple components and preparation process, displaying the great potential for the application in dielectric materials of high-power microwave transmission devices.