Multi-layer ceramic capacitor (MLCC) is a highly promising dielectric capacitor due to its high capacitance, favorable dielectric temperature stability, and high breakdown strength. The conventional MLCC uses expensive silver and palladium metals as inner electrode materials. In recent years, base metals such as nickel or copper are employed as inner electrodes to reduce the related costs. However, these metals must be co-fired with the dielectric layer in a reducing atmosphere. BaTiO₃ ceramics are the principal dielectric materials used in the manufacture of MLCC. The sintering of BaTiO₃ ceramics in a reducing atmosphere generates a significant number of oxygen vacancies and free electrons, which significantly impair the insulation performance and reduce the insulating properties. Also, the pronounced dielectric peak of BaTiO₃ ceramics occurs at the elevated temperature end (i.e., 125 ℃), leading to an inadequate temperature stability.

Recent research focus on the relaxation properties and dielectric properties of BaTiO₃-Bi(Me)O₃(Me = Fe³⁺, Co³⁺, Y³⁺, etc.) solid solutions. The co-substitution of the A and B sites via introducing cations with different ion radius and valence states is shown to disrupt the long-range ferroelectric ordering, induce diffusive phase transitions, and broaden the dielectric temperature profile. Furthermore, Zn²⁺, Mg²⁺, Mn⁴⁺, Co³⁺, and other rare-earth element ions are introduced into BaTiO₃-based ceramics as host-accepted dopants with the objective of forming defective dipoles with oxygen vacancies, which can bind the free charge movement. Among these, multivalent ions can bind electrons, thereby modifying the valence and regulating the electron concentration within the ceramics. This reduces the likelihood of conversion of Ti⁴⁺ to Ti³⁺.

In this paper, 0.95BaTiO₃-0.05BiCoO₃ dielectric ceramics were used as a matrix. A small amount of MnO₂ was selected for doping and modification, and subsequently sintered in a reducing atmosphere. The structure, micro-morphology, dielectric, and insulating properties were characterized, and the mechanism of anti-reduction was investigated.

In the preparation of the ceramics with a composition of 0.95BaTiO₃–0.05BiCoO₃–x% (in mass) MnO₂ (x=0.1, 0.2, 0.3, 0.4, and 0.5), the materials were sintered at 1275 ℃ for 2 h under a reducing atmosphere (99.5%N₂+0.5%H₂, in volume fraction), forming dense ceramic samples. Subsequently, the ceramic samples were thinned and polished, and silver paste electrodes were applied to the both sides to measure their electrical properties.

The physical structure of the samples was analyzed using a model X'Pert Pro X-ray diffractometer (XRD, PANalytical Co., the Netherlands). The microscopic morphology of the sample section was determined by a model 450 FEG field emission scanning electron microscope (FE-SEM, FEI Quanta Co., USA). The dielectric properties of the samples were measured by a model PolyK PK-CPT1705 low temperature dielectric tester. The resistivity of the ceramic samples was tested by a model Concept 80 low frequency module (Novocontrol Co., Germany). The ferroelectric properties of the samples were analyzed by a model PK-CPE1801 ferroelectric polarisation return and dielectric breakdown test system (PolyK Co., USA). The elemental valence analysis was conducted by a model 250Xi X-ray photoelectron spectrometer (XPS, EscaLab Co., USA). The thermally stimulated depolarization currents (TSDC) of the samples were carried out by a model Concept 80 TSDC (Novocontrol Co., Germany).

All the ceramic samples exhibit a pseudo-cubic perovskite structure. The crystal surface spacing of the ceramics increases as the amount of MnO₂ doping increases. This indicates that Mn⁴⁺ is reduced to Mn³⁺ or Mn²⁺ during sintering in a reducing atmosphere. The radius of Mn³⁺ or Mn²⁺ is larger than that of Ti⁴⁺ (i.e., 0.0605 nm, CN = 6) ionic radius, leading to the crystal lattice distortion and increased cell volume. The cross section of the ceramics reveals fine grains (i.e., 0.24–0.32μm), clear grain boundaries, and a close arrangement between grains. The change in valence of Mn effectively inhibits the concentration of free electrons. When x=0.5, the insulation resistivity increases to 1.84×10¹³ Ω·cm, the breakdown voltage reaches 210 k V/cm, thus improving the anti-reduction performance of ceramics. The introduction of Mn reduces the dielectric loss of the ceramics and improves their dielectric temperature stability. The optimum dielectric properties are achieved when x=0.5 (i.e., a dielectric constant of 2354, a dielectric loss of 0.0075, and ΔC/C(25 ℃)≤±15% at–55 to 154 ℃). The XPS spectra reveal the presence of both divalent and trivalent Co ions in the ceramics. The result of TSDC test shows a TSDC peak associated with trapped charge, which is related to the change in valence states of the variable valency acceptor ions Co and Mn. Two TSDC peaks are also related to defect dipoles, which are formed by Co and Mn with oxygen vacancies in the ceramic sample.

The effect of MnO₂ doping on the phase structure, microstructure, and electrical properties of 0.95BT–0.05BC ceramics was investigated. The results demonstrated that Mn⁴⁺ ions underwent a conversion to Mn³⁺ or Mn²⁺ ions during sintering process in a reducing atmosphere, having the larger ionic radius. This transformation led to an expansion of the cell volume. Mn doping significantly impacted the broadening of the Curie peak and enhanced the dielectric temperature stability of 0.95BT–0.05BC ceramics. Specifically, as x = 0.5, the dielectric constant at 25 ℃ was 2354, and the dielectric loss was 0.0075. Furthermore, at–55 to 154 ℃, ΔC/C(25 ℃)≤±15%. The resistivity at room temperature increased to 1.84×10¹³ Ω·cm, and the breakdown voltage increased to 210 k V/cm. The XPS spectra revealed the presence of both divalent and trivalent Co ions in the ceramic, indicating that Co ions could capture free electrons and reduce their valence state. The result of TSDC test further demonstrated the existence of two types of defective dipoles, which were associated with the acceptor substitution of Co and Mn. The combination of these two factors reduced the carrier concentration in the ceramics, thereby hindering the migration of oxygen vacancies and free electrons and improving the anti-reduction performance.