Research on the Withstand Voltage Properties of Cr/Mn-Doped Al₂O₃ Ceramics in Vacuum

Abstract

Al₂O₃ ceramics are widely used in vacuum electronic devices. However, surface flashover in a vacuum during the application of high voltage significantly influences their reliability and restricts the development of vacuum electronic devices. The secondary electron emission yield (SEY) and surface resistivity of ceramics are the main factors affecting the vacuum withstand voltage of ceramic materials. In this study, the bulk density, microstructure, and surface properties—including SEY and surface resistivity—of Al₂O₃ ceramics were tested. The relationship between these properties and the vacuum withstand voltage of the ceramics was investigated. The influence of the addition ratio of Cr₂O₃ to MnO₂ and the sintering temperature was investigated. The results show Cr/Mn-doped Al₂O₃ ceramics, with appropriate amounts of Cr₂O₃ and MnO₂ and sintered at suitable temperatures, exhibit low SEY, high withstand voltage, and excellent stability in vacuum.

1. Introduction

As vacuum electronic devices evolve towards higher power, higher frequency, and miniaturization, their working voltage increases significantly, putting higher demands on the voltage withstand capabilities of the ceramic materials used in these devices. Ceramic materials are widely used as electronic materials due to their excellent properties [1,2,3]. Due to their excellent mechanical properties, electrical properties, and chemical stability, Al₂O₃ ceramics have a wide range of applications in the field of electronic materials. In vacuum electronic devices, Al₂O₃ ceramics can be used as insulating components, structural support components, and power transmission components [4,5,6]. Studies indicate that during the operation of vacuum electronic devices, flashovers often occur at relatively low voltages, significantly lower than the dielectric strength of the vacuum and the insulator between the two electrodes [7,8]. When the operating voltage of a vacuum electronic device reaches a certain level, surface flashover will occur on the insulator within the device, causing the device to malfunction or even suffer damage [9,10,11,12,13,14]. Al₂O₃ has a relatively high secondary electron emission yield (SEY), which makes it prone to surface flashover during operation when used in vacuum electronic devices. This affects the withstand voltage of the devices and restricts the advancement of vacuum electronic devices [15,16,17,18,19]. Consequently, enhancing the withstand voltage properties of Al₂O₃ ceramics has become a pressing challenge in the field of vacuum electronics.

The mechanism of vacuum surface flashover is complex. The process of the widely accepted Secondary Electron Emission Avalanche (SEEA) theory can be described as follows [20,21,22]: surface flashover typically initiates with the electron emission at the cathode triple junction (CTJ), which is the boundary between the insulator, the cathode, and the vacuum. Some of these electrons strike the surface of the insulator, causing the emission of secondary electrons. A portion of secondary electrons strike the surface again, generating tertiary electrons. When the secondary electron yield (SEY) is larger than one, this process may occur continuously, leading to an SEEA ultimately. The final stage is the desorption and ionization of gases on the insulator surface, leading to surface flashover when the electric field is sufficiently strong. According to the SEEA theory, the SEY of ceramic is a critical factor influencing its surface flashover voltage.

Additionally, as an electronic material, the electrical properties of Al₂O₃ ceramics are crucial. In electric vacuum devices, the surface resistivity of ceramic is generally high, which will hinder the leakage of charges accumulated on the ceramic surface, enhance the intensity of the local electric field, and reduce the withstand voltage. Thus, reducing the resistivity of the ceramic to a certain level is necessary to enhance charge dissipation and improve the withstand voltage property in a vacuum.

Researchers have conducted extensive research on improving the withstand voltage properties of Al₂O₃ ceramics. Tangal S. Sudarshan et al. [23] studied the influence of Cr₂O₃ coating on the vacuum surface flashover performance of Al₂O₃ ceramics under direct current (DC) and pulsed voltage conditions. Zheng Jiagui et al. [24] prepared a Cr₂O₃ coating on an Al₂O₃ insulator, studied the SEY of the material, and analyzed the mechanism of its action. Hosono, T. et al. [25] studied the surface roughness and electric field distribution characteristics of Al₂O₃ and proposed a method for controlling the surface charge of the insulator. Zhang Hao et al. [26] investigated the effect of a small amount of Cr₂O₃ on the flashover voltage of glass-ceramics. They also utilized ANSYS to simulate the surface electric field distribution. The results indicated that the addition of Cr₂O₃ changed the surface electric field.

By modifying the surface of insulating materials, it is possible to specifically improve surface properties and enhance withstand voltage without changing the structure of the insulator [18,19]. However, this approach has certain limitations in practical use: firstly, surface modification of insulators with complex shapes is difficult; secondly, it is challenging to control the evaporation of the coating during use. For insulators with complex structures, bulk doping is easier to achieve, and the material’s properties can maintain relative stability. Among commonly used additives, the SEY of Cr₂O₃ [20] is very low, while the resistivity of MnO₂ is also low. The addition of Cr₂O₃ and MnO₂ to Al₂O₃ ceramics affects not only their withstand voltage property in vacuum but also their sintering behavior and microstructure.

As the working voltage of electrical vacuum devices increases significantly, ceramic materials with higher withstand voltage are required. Meanwhile, the process of surface flashover in a vacuum is extremely complicated, and there are also many factors that affect the withstand voltage of ceramic materials. Hence, further systematic and in-depth research is needed on the surface properties and microstructure of ceramic materials, as well as their relationship with withstand voltage properties. This will provide technical support for the preparation of ceramic materials with high withstand voltage in a vacuum.

This paper systematically investigates the effect of sintering temperature and the Cr₂O₃ to MnO₂ addition ratio on the microstructure, SEY, resistivity, and withstand voltage of Al₂O₃ ceramics. By adjusting the Cr₂O₃ to MnO₂ addition ratio and optimizing the sintering temperature, we have obtained Cr/Mn-doped Al₂O₃ ceramics with excellent performance.

2. Experimental Procedures

2.1. Samples Preparation

We chose Al₂O₃ ceramics as the base material. The addition of SiO₂ and CaO as sintering aids helped improve the sintering properties of the ceramics. Different proportions of Cr₂O₃ and MnO₂ were added as additional dopants, while the total amount of Cr₂O₃ and MnO₂ added remained constant. After the ceramic raw materials were mixed uniformly, a powder with good fluidity was prepared through spray granulation. Then, it was formed into a ceramic green body with a certain form by cold isostatic pressing. Then, the green body was placed in a high-temperature muffle furnace for sintering in an air atmosphere. The preparation process of the Al₂O₃ ceramics is displayed in Figure 1. The sintering parameters are presented in

Table 1. Sintering program of the Al₂O₃ ceramics.

Temperature, °C	25–520	520–520	520–1100	1100–1300	1300–*	*–*
Time, h	5	2	3	2	3	4

, where “*” denotes the sintering temperature. The samples sintered at different temperatures with varying proportions of Cr₂O₃ and MnO₂ are numbered and listed in

Table 2. Numbers for samples with varying addition ratios of Cr₂O₃ to MnO₂ and sintering temperatures.

Sample	Cr2O3: MnO2	Sintering Temperature, °C
A-0	-	1645
CMA30	3:1	1623
CMA20	2:1	1623
CMA11	1:1	1590
CMA12	1:1	1611
CMA13	1:1	1623
CMA14	1:1	1643
CMA05	1:2	1623
CMA03	1:3	1623

.

2.2. Samples Characterization

2.2.1. Characterization of Microstructure, Bulk Density and Resistivity

The microscopic morphology and phase composition of the ceramic samples were observed and analyzed using a scanning electron microscope (SU3800, HITACHI, Japan) and an X-ray diffractometer (D8 Advance, BRUKER, Germany). The bulk density of the ceramic samples was measured using a densitometer based on the Archimedes’ drainage method. The surface and volume resistivity of ceramic samples were tested using a high resistivity meter (SM7120, Hitachi, Japan). The dimension of the resistivity test sample was Φ40 mm × 2 mm.

2.2.2. Test of Secondary Electron Emission Property

When electrons bombard ceramics, secondary electrons are emitted from the ceramic surface. Additionally, ceramics are dielectric materials, and when they are bombarded by electrons, charges accumulate on their surface [27]. The accumulated charges will change the electric field on the ceramic surface, thereby affecting the testing of secondary electron emission properties. To eliminate this effect, the three-gun method is employed in this study to measure the secondary electron emission properties of ceramic materials. The diagrammatic sketch of the system to test the secondary electron emission property using the three-gun method is shown in Figure 2. The testing principle of this system is as follows: The measuring gun emits primary electrons perpendicularly to the sample. These primary electrons incident on the ceramic and excite secondary electrons, while a portion of these electrons accumulate on the ceramic surface. In this testing system, charge neutralizer gun 1 is grounded. When the SEY of the ceramic is greater than 1, positive charges accumulate on the ceramic surface, and neutralizer gun 1 is turned on to neutralize the positive charges on the ceramic surface. When the SEY of the ceramic is less than 1, neutralizer gun 2 is turned on. Charge neutralizer gun 2 operates at a potential of–V_d, a voltage that ensures the SEY of the ceramic exceeds 1. Once positive charges accumulate again on the ceramic surface, neutralization gun 1 is activated to ensure that the surface potential of the ceramic remains at zero. By working in coordination, the two charge neutralizer guns effectively eliminate the charge accumulation on the ceramic surface, ensuring that the surface potential of the ceramic remains stable at the ground potential, thereby guaranteeing the accuracy of secondary electron emission measurements [28]. The dimension of the test sample for secondary electron emission property is Φ26 mm × 1 mm. The vacuum level in the chamber is better than 1 × 10⁻⁴ Pa. The incident electron energy ranges from 0 to 3500 eV, with an incident current of 0.1 μA.

2.2.3. Test of Withstand Voltage in Vacuum

The testing system for withstand voltage of Al₂O₃ ceramics in vacuum is shown in Figure 3a. The test electrodes are circular plate electrodes made of stainless steel, with the dimensions of Φ66 mm × 10 mm. A photograph of the plate electrodes and sample is shown in Figure 3b. The dimension of the samples for withstand testing is Φ26 mm × 5 mm. The vacuum level in the test chamber is better than 1 × 10⁻⁴ Pa. The test voltage is a negative DC voltage. The testing procedure is as follows: Firstly, the samples undergo thorough conditioning: applying a voltage to the sample until surface flashover occurs, then continuing to increase the voltage to the maximum value. This voltage is then applied repeatedly three times to complete the conditioning of the samples. Subsequently, the voltage is re-applied to the sample with an increasing rate of 500 V/s until a flashover occurs on the ceramic surface. Then, the voltage gradually decreases until the flashover disappears. This process repeats three times. If no flashover occurs on the sample during these repetitions, the voltage at this point is considered the withstand voltage and is denoted as U_ho.

3. Results and Discussion

3.1. Thermal Analysis

The thermal properties of the basic and the Cr/Mn-doped (Cr₂O₃: MnO₂ = 1:1) Al₂O₃ ceramics are analyzed by TG-DSC (Thermogravimetric Analysis and Differential Scanning Calorimetry) curves. In Figure 4, the left vertical axis of the TG-DSC curve represents the mass change of the sample, while the right vertical axis corresponds to the DSC curve. The upward and downward peaks, respectively, indicate exothermic and endothermic phenomena occurring in the sample during the heating process. Both basic Al₂O₃ ceramic and doped Al₂O₃ ceramic had an endothermic peak before 800 °C, accompanied by a significant mass loss on the TG curve, which was caused by the decomposition of calcium carbonate. Furthermore, there is an obvious endothermic peak between 1300 and 1400 °C on the DSC curve, which corresponds to the initial sintering temperature of Al₂O₃ ceramic. From Figure 4, we can see that the initial sintering temperature of the Cr/Mn-doped Al₂O₃ seems to have dropped by 28 °C from 1359 °C to 1331 °C.

This is because Cr₂O₃ and MnO₂ are both hexagonal close-packed structures [29], which is the same as α-Al₂O₃, and solid solutions of Cr₂O₃ and MnO₂ may form with the Al₂O₃ lattice [30,31]. Moreover, the differences in electronic structure and electronegativity between Mn⁴⁺ and Al³⁺ may distort and activate the Al₂O₃ lattice, facilitate the diffusion of particles, and reduce the sintering activation energy, thus promoting sintering and crystalline grain growth. In addition, adding MnO₂ to Al₂O₃ ceramics reduces the low eutectic point of the liquid phase generated during the sintering process, which helps to reduce the sintering temperature of the ceramic.

3.2. Bulk Density and Micromorphology

We tested the bulk density and microstructure of Al₂O₃ ceramics to illustrate the effects of Cr₂O₃ and MnO₂ as additives on the sintering properties of Al₂O₃ ceramics. From Figure 5, we can see that as the amount of MnO₂ addition increases, the volume density of the doped Al₂O₃ ceramics increases at first and then decreases. On the one hand, due to the relatively high density of MnO₂, the addition of MnO₂ can contribute to the increase in the bulk density of Al₂O₃ ceramic. On the other hand, the addition of MnO₂ promotes the sintering of Al₂O₃ ceramic, leading to an increase in the bulk density. Later, with the additional amount of MnO₂ increasing continuously, the density of the doped Al₂O₃ ceramic actually decreases. At the same time, the volume density of the sample (Cr₂O₃: MnO₂ = 1:1) slightly increases at first and then decreases with the increase in sintering temperature. From the thermal analysis of the Al₂O₃ ceramics, it can be seen that the addition of Cr₂O₃ and MnO₂ promotes ceramic sintering and lowers the sintering temperature of the ceramic. When the sintering temperature is 1623 °C, the volume density of sample CMA12 reaches the maximum value of 3.781 g/cm³. Subsequently, as the sintering temperature continued to increase, the bulk density of the ceramic (Cr₂O₃: MnO₂ = 1:1) decreased because of the occurrence of overburning.

Figure 6 shows the SEM of the Cr/Mn-doped Al₂O₃ ceramic. When the additional amount of MnO₂ increases, the grain size of the ceramic increases. The MnO₂ added to Al₂O₃ ceramics forms a limited solid solution with Al₂O₃, which promotes the sintering of the ceramics. In addition, as an additive, MnO₂ can reduce the eutectic point of the liquid phase during the sintering process of ceramics, which also promotes ceramic sintering. However, when an excessive amount of MnO₂ is added, abnormal grain growth occurs in the ceramic. We can observe that in CMA05, especially in the CMA03 sample, there are large-sized grains locally, which will lead to uneven microstructure and reduced density of the material. Figure 6f shows that there are many pores on the grains of the CMA03 sample. This is due to the rapid sintering of the ceramic caused by the addition of a large amount of MnO₂, which results in the pores not having enough time to be expelled during the sintering process.

3.3. Phase Composition

To demonstrate the microstructure of the Al₂O₃ ceramic, the XRD analysis of the samples was performed. From Figure 7, it can be seen that there is only α-Al₂O₃ crystal phase in the basic ceramic A-0. The major crystal phase in Cr/Mn-doped Al₂O₃ ceramics is (Al, Cr)₂O₃. Cr₂O₃ and Al₂O₃ crystals had the same lattice type, and Cr³⁺ and Al³⁺ had the same ionic valence and similar ionic radius. Cr₂O₃ dissolved into the Al₂O₃ lattice, and Cr³⁺ replaced Al³⁺, forming a continuous Al₂O₃-Cr₂O₃ solid solution [30].

Figure 8 shows that there is also a MnAl₂O₄ phase in the Cr/Mn-doped Al₂O₃ ceramic except for the major crystal phase of (Al, Cr)₂O₃. With a certain amount of MnO₂ added to the Al₂O₃ ceramic, the MnAl₂O₄ phase will precipitate during the sintering process, which may affect the crystal structure of the ceramics. By normalizing and magnifying the peak of MnAl₂O₄ at 2 Theta = 37.8 degrees, as shown in Figure 8, it is found that the relative content of MnAl₂O₄ in the ceramic increases with the increase in the additive amount of MnO₂. Meanwhile, the sintering temperature has almost no effect on the type and amount of crystal phases in ceramics.

3.4. Secondary Electron Emission Property

According to the SEEA theory, the SEY of ceramics is the direct cause of the surface flashover in a vacuum, Therefore, to increase the vacuum voltage holdoff capacity of ceramics, it is necessary to reduce their SEY. We tested the SEY of the basic Al₂O₃ ceramic and the Cr/Mn-doped Al₂O₃ ceramic, and the secondary electron emission curves are exhibited in Figure 9. We can see that the SEY of Cr/Mn-doped Al₂O₃ ceramics is significantly lower than 6.388 of the basic Al₂O₃ ceramic, which is mainly attributed to the addition of Cr₂O₃ with extremely low SEY. The Cr/Mn-doped Al₂O₃ ceramics with low SEY are less prone to SEEA phenomena, which is beneficial for improving their withstand voltage in a vacuum. Additionally, as the additive amount of MnO₂ and sintering temperature increases, the SEY of the ceramics increases slightly.

The process of the SEEA theory involves the electrons being emitted from the CTJ, accelerated by the electric field, and then colliding with the ceramic surface to excite secondary electrons. These secondary electrons escape from the ceramic surface, under the control of the electric field, and collide with the ceramic again. When the electric field is sufficiently strong, this process can become self-sustaining, leading to a rapid increase in the number of electrons and the occurrence of an electron avalanche. A large number of electrons collide with the gas adsorbed on the ceramic surface, causing the gas to desorb and be ionized by the high-energy electrons, generating a large amount of plasma. Ultimately, surface flashover occurs along the ceramic surface in a vacuum. The main factors influencing the electron avalanche process are the energy of the primary electrons and the ionization energy of the lattice molecules or atoms with which they collide. The ease of collisional ionization is typically quantified by the collisional ionization coefficient α. This coefficient represents the number of ionization events caused by an electron traveling a unit distance under the electric field. The expression of α is as follows:

α = Aexp(−(BV_i)/λE)

(1)

where V_i is the ionization energy of the collided lattice molecule or atom, λ is the mean free path of the electron, and E is the electric field strength. A and B are constants related to the material. From the above expression, it can be seen that the α is directly affected by the mean free path of electrons. The longer the mean free path of electrons, the longer the time they are subjected to the electric field and the more energy they gain between collisions, making collisional ionization more prone to occur. From the microstructure of the ceramic shown in Figure 6, it can be seen that the grain size increases with higher MnO₂ addition and sintering temperature. This leads to a longer time for the electron to be accelerated by the electric field during its escape, enhancing the likelihood of ionization and increasing the SEY consequently.

3.5. Resistivity

As electronic materials, the electrical properties of ceramics have a crucial impact on the performance of devices [32]. The resistivity of ceramic material, especially the surface resistivity, is another key factor influencing its vacuum dielectric strength. Al₂O₃ ceramics generally have high resistivity, and their high surface resistivity inhibits the leakage of charges accumulated on the ceramic surface. At the same time, it also leads to localized electric field enhancement, making vacuum surface flashover more likely to occur [27,33]. Therefore, in order to improve the withstand voltage of Al₂O₃ ceramics, it is necessary to reduce their surface resistivity to a certain extent.

As can be seen from

Table 3. Resistivity of the Al₂O₃ ceramics.

Sample	Rs (Surface Resistance), Ω	Rv (Volume Resistance), Ω·cm
A-0	7.41 × 1015	2.35 × 1016
CMA30	4.78 × 1014	1.23 × 1016
CMA20	1.56 × 1014	5.23 × 1015
CMA11	7.55 × 1014	6.46 × 1015
CMA12	7.21 × 1014	3.81 × 1015
CMA13	5.32 × 1014	4.81 × 1015
CMA14	5.62 × 1014	3.48 × 1015
CMA05	8.94 × 1013	8.37 × 1014
CMA03	3.44 × 1012	1.64 × 1012

, with the increase in the additional amount of MnO₂, the R_s (surface resistance) and R_v (volume resistance) of the Cr/Mn-doped Al₂O₃ ceramic show a downward trend. The R_s and R_v of Al₂O₃ ceramics decreased by one order of magnitude when a small amount of MnO₂ was added. With a further increase in the amount of MnO₂ added, the R_s and R_v of Al₂O₃ ceramics can decrease by three to four orders of magnitude, reaching 10¹² Ω. The influence of MnO₂ addition on the resistivity of Al₂O₃ ceramics can be attributed to the following reasons: MnO₂ added to Al₂O₃ ceramics can dissolve in the Al₂O₃ crystal lattice, forming a solid solution. However, the ionic valence of Mn⁴⁺ and Al³⁺ are different. When Mn⁴⁺ replaces the position of Al³⁺, it forms an inequivalent substitutional type solid solution. To maintain electrical neutrality, vacancies are generated at the original Al³⁺ sites. These vacancies form defect centers in the ceramic, serving as charge carriers that facilitate charge transport, thereby reducing the resistivity of the ceramic.

3.6. Withstand Voltage in Vacuum

The voltage holdoff capacity in vacuum of the ceramics was expressed by the withstand voltage. Five ceramic samples are tested for each group, and the average value of U_ho is then calculated as the final result of the withstand voltage. The standard deviations of the samples in each group were calculated to characterize the voltage withstand stability of the Al₂O₃ ceramics.

Table 4. Value of the withstand voltage and the stability of the Al₂O₃.

Sample	Uho, kV					Uho, kV	Standard Deviation, kV
A-0	18	23	26	31	26	25	4.76
CMA30	23	27	28	32	30	28	3.39
CMA20	22	30	28	27	30	29	3.28
CMA11	23	25	23	29	25	25	2.45
CMA12	28	25	30	31	28	28	2.30
CMA13	30	32	28	29	35	31	2.77
CMA14	35	27	26	36	37	32	5.26
CMA05	24	28	34	23	18	25	5.98
CMA03	19	33	23	24	30	26	5.63

shows the withstand voltage in vacuum of the base Al₂O₃ ceramics and the Cr/Mn-doped Al₂O₃ ceramics.

From Table 4, we can see that the addition of Cr₂O₃ and MnO₂ to Al₂O₃ ceramics can significantly increase the value of the withstand voltage and the stability of Al₂O₃ ceramics. Al₂O₃ ceramics with an appropriate amount of Cr₂O₃ and MnO₂ and sintered at suitable temperatures exhibit high withstand voltage and stability. The U_ho of CMA13 reached 31 kV, which was 24% higher than that of the basic Al₂O₃ ceramic. In addition, its standard deviation reached the minimum value, indicating a high voltage withstand stability. The U_ho of the Al₂O₃ ceramics was improved by the addition of Cr₂O₃ and MnO₂ for the following reasons: Firstly, the SEY of the Cr/Mn-doped Al₂O₃ ceramics is significantly lower than that of the basic Al₂O₃ ceramics, making it less likely for SEEA to occur when a voltage is applied. As a result, the withstand voltage increases. Secondly, the resistivity of the CMA11 sample is reduced by one order of magnitude compared to the basic Al₂O₃ ceramic, facilitating charge dissipation from the ceramic surface. The decrease in resistivity can accelerate the leakage of charges accumulated on the ceramic surface, reduce the probability of surface flashover, and improve the surface withstand voltage. Finally, by adding an appropriate amount of Cr₂O₃ and MnO₂ and sintering at a suitable temperature, Al₂O₃ ceramics with uniform and compact microstructure can be prepared, which can enhance the voltage withstand stability of the ceramic.

The U_ho of the CMA14 sample reached the maximum value of 32 kV, but the stability decreased, which may lead to poor reliability of vacuum devices in use. This is because the sintering temperature was too high, resulting in the oversintering of the ceramic. The microstructure and density of the ceramic became worse. With the crystalline grains in the samples growing abnormally, many closed pores appeared in the grains and at the grain boundary, leading to the uneven distribution of grain size [34,35]. These defects in the material become traps for capturing charges and form equilibrium states in the process of dielectric polarization [36]. This may be a contributing factor to the poor voltage withstand stability of the doped Al₂O₃ ceramic sintered at 1643 °C.

The withstand voltage and stability of CMA05 and CMA03 samples are significantly reduced. Although the R_s of CMA05 and CMA03 samples decreased by two to three orders of magnitude, which is more conducive to the release of surface charges on the ceramic, the microstructure and density of CMA05 and CMA03 samples deteriorated, leading to a decline in their surface withstand voltage performance. In this study, the CMA03 and CMA05 samples did not exhibit the beneficial effect of reduced resistivity on the withstand voltage performance of the ceramics in a vacuum.

4. Conclusions

In this study, the influence of the addition ratio of Cr₂O₃ on MnO₂ and the sintering temperature was investigated, and the SEY and resistivity of Al₂O₃ ceramic were reduced by adding Cr₂O₃ and MnO₂. Cr/Mn-doped Al₂O₃ ceramics with excellent withstand voltage properties were prepared. The main conclusions are as follows:

(1)

Cr₂O₃ and MnO₂ have an influence on the microstructure and the sintering performance of Al₂O₃ ceramics. Al₂O₃ ceramics with an appropriate amount of Cr₂O₃ and MnO₂, sintered at 1623 °C, exhibit uniform and dense microstructure and high voltage withstand stability.

(2)

The addition of Cr₂O₃ and MnO₂ significantly reduced the SEY and R_s of Al₂O₃ ceramics. Compared to basic Al₂O₃ ceramics, the SEY of Cr/Mn-doped Al₂O₃ ceramics decreased by about 50%, while the R_s of Cr/Mn-doped Al₂O₃ ceramics decreased by up to three orders of magnitude.

(3)

The withstand voltage properties of the prepared Cr/Mn-doped Al₂O₃ ceramics are significantly improved. The U_ho of the CMA13 sample reached 31 kV, which was 24% higher than that of the basic Al₂O₃ ceramics. In addition, the voltage withstand stability of the CMA13 sample was relatively high.