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We investigate the potential of nanovacuum devices utilizing graphene edges as field emitters, with their work function modulated by a nearby gate on the graphene surface. Unlike metals, the semi-metallic nature of graphene enables modulation of the Fermi level and work function via the surface field. This modulation alters the potential barrier for field emission. Our simulation study reveals that device operation critically depends on two screening factors—horizontal and vertical. Horizontally, work function modulation occurs when the emitter edge is within the critical screening length from the gate edge. Vertically, the effectiveness of work function modulation diminishes beyond the second layer of multi-layer graphene due to surface field screening by the first layer. Our simulations demonstrate that maintaining the vacuum channel on tens of nanometer scale enables transistor-like operation of the device, with remarkably high cut-off frequencies and maximum oscillation frequencies ranging from 0.45 to 0.71 and 32.9 to 40.5 THz, respectively, under source–drain bias from 90 to 100 V. [ABSTRACT FROM AUTHOR]

For a high capacitance and high lifetime reliability of multilayer ceramic capacitors for automotive applications, the activation energy on thermal activation process can typically be calculated by using Arrhenius based Prokopowicz–Vaskas equation as a method for lifetime prediction. In this study, it is clearly observed that the activation energy shows to be constant in the range of ~ 1.5 eV for the prototype MLCCs, higher than the activation energy values of ~ 1.0 eV related to the motion or diffusion of oxygen vacancies reported in the previous literature. The activation energy value of ~ 1.5 eV for three prototype MLCCs is close to a half the energy band gap (Eg/2 ≈ 1.6 eV) of BaTiO3 obtained from specific environment, where oxygen vacancies are stabilized by external containment such as the effect of rare earth oxide additives. Due to an obvious difference in activation energy values, it difficult to explain the conduction mechanism for failure by only oxygen vacancy migration. Therefore, the concepts of electronic processes and oxygen vacancy should be considered together to understand conduction mechanism for failure of BaTiO3-based MLCCs in thermal activation processes. It can be useful as an indicator for future MLCC development with high lifetime reliability. [ABSTRACT FROM AUTHOR]

The effect of excess Ba concentration on the dielectric nonlinearity was investigated in Mn and V-doped BaTiO3 multi layer ceramic capacitors (MLCC) under the same grain size condition, which was described by the Preisach model utilizing the first order reversal curve (FORC) distribution. The high-field dielectric constant and its ac field dependence dramatically changed increasing to a maximum and then decreasing with the increase of Ba concentration. The saturation polarization which scales to the magnitude of spontaneous polarization also showed similar behavior. These results indicate that the dependence of the dielectric constant on the Ba concentration is associated with the variation of both domain wall contribution and the magnitude of the spontaneous polarization, which could be correlated with the same dependence on the Ba concentration of the reversible FORC distribution at zero bias and the irreversible FORC distribution near origin. In the corresponding bulk specimens of the dielectrics of MLCC, almost the same amount of the Ba2TiSi2O8 second phases were detected irrespective of Ba concentration, which shows that the excess Ba incorporate into BaTiO3. Thus, low and high Ba concentration corresponds to Ba-deficient and Ba-rich or Ti-deficient BaTiO3, respectively, which results in a small spontaneous polarization and low domain wall density. The intermediate Ba concentration for the maximum dielectric constant is supposed to be near stoichiometric condition in the ABO3 structure corresponding to large spontaneous polarization and high domain wall density. The excess Ba concentration and its resultant A/B stoichiometry is a crucial factor controlling dielectric properties. [ABSTRACT FROM AUTHOR]

The influence of tetragonality (c/a) of the dielectrics of BaTiO3-based multi-layer ceramic capacitors (MLCC) on the direct current (dc) bias characteristics was investigated. The tetragonality can be reduced by decreasing the grain size, and it can be further decreased for the same grain size condition by increasing the measurement temperature or the A/B ratio in the ABO3 structure. Fine grain samples showed higher dielectric constants than those of coarse grain samples under a high dc-bias field over the entire temperature range below Curie temperature (TC), and they became similar to each other above TC. Whereas the dielectric constants under a high dc-bias field increase slightly with the increase of temperature for the fine grain samples, those of coarse grain samples systematically increase, of which the tetragonality is also supposed to change more distinctly. In fine grain samples, the dielectric constants under a high dc-bias field can be further enhanced by increasing the A/B ratio in ABO3 structures through the excess Ba addition that results in a decrease of tetragonality. All these results demonstrate that the reduction of tetragonality increases the intrinsic dielectric constants along the c-axis direction, and thus is a crucial factor for the enhancement of dielectric constant under a high dc-bias field condition where the domain alignment with the c-axis parallel to the dc-bias field occurs. [ABSTRACT FROM AUTHOR]

Dielectric relaxation behavior of acceptor (Mg)-doped BaTiO3 ceramics was investigated with the increase of Mg concentration up to 0.6 mol. % in the temperature rang of 120 ∼ 540 °C. In the high temperature range above 320 °C, the activation energies of dielectric relaxation (Eτ) showed nearly similar values of ∼ 1.2 eV irrespective of Mg concentration. However, in the low temperature range below 320 °C, they continuously decreased from ∼ 1.2 eV and then saturated to ∼ 0.4 eV with the increase of Mg concentration. The activation energies of electrical conduction (Eσ) of the bulk grain evaluated by impedance analysis also showed almost the same behavior. Such coincidence demonstrates that the observed dielectric behaviors are caused by the space charge polarization at grain boundaries by conducting charge carriers. The disappearance of the dielectric relaxation in submicrometer fine-grain specimen also supports this mechanism. The variation of Eτ and Eσ with the increase of Mg concentration in the low temperature range was supposed to be caused by the dominant hopping conduction between ionized acceptor (MgTi″) and neutral or hole-trapped acceptor (MgTi×). [ABSTRACT FROM AUTHOR]

Bulk electrical conductivity of acceptor (Mg)-doped BaTiO3 ceramics were evaluated via an impedance spectroscopy analysis for materials with Mg concentrations up to 0.6 mol % and over a temperature range of 120–540 °C. At high temperatures beyond 320 °C, the activation energies of the bulk electrical conduction (Eσ) slightly decreased from 1.2 to ∼1.0 eV with an increase in the Mg concentration. However, in the low temperature range below 320 °C, there is a continuous decreased from 1.2 eV and then this saturated to ∼0.4 eV with the increase in Mg concentration. The calculated bulk conductivities based on a defect chemistry model explains conductivity for temperatures above 320 °C. The model demonstrates a predominant electronic conduction in the low concentration range below 0.1 mol % Mg and mixed conduction with ionic and electronic carriers in the intermediate and in high acceptor concentration range of 0.2–0.6 mol % Mg. However, the decrease in Eσ with the increase in Mg concentration in the low temperature range could not be explained by the defect chemical model. Such deviation however could be explained by a transition to hopping conduction between ionized acceptor (MgTi″) and neutral or hole-trapped acceptor (MgTi×) with the increase in Mg concentration in the low temperature range. [ABSTRACT FROM AUTHOR]

Bulk electrical conductivity of acceptor (Mg)-doped BaTiO3 ceramics were evaluated via an impedance spectroscopy analysis for materials with Mg concentrations up to 0.6 mol % and over a temperature range of 120–540 °C. At high temperatures beyond 320 °C, the activation energies of the bulk electrical conduction (Eσ) slightly decreased from 1.2 to ∼1.0 eV with an increase in the Mg concentration. However, in the low temperature range below 320 °C, there is a continuous decreased from 1.2 eV and then this saturated to ∼0.4 eV with the increase in Mg concentration. The calculated bulk conductivities based on a defect chemistry model explains conductivity for temperatures above 320 °C. The model demonstrates a predominant electronic conduction in the low concentration range below 0.1 mol % Mg and mixed conduction with ionic and electronic carriers in the intermediate and in high acceptor concentration range of 0.2–0.6 mol % Mg. However, the decrease in Eσ with the increase in Mg concentration in the low temperature range could not be explained by the defect chemical model. Such deviation however could be explained by a transition to hopping conduction between ionized acceptor (MgTi″) and neutral or hole-trapped acceptor (MgTi×) with the increase in Mg concentration in the low temperature range. [ABSTRACT FROM AUTHOR]

The behaviors of the electrical resistance at room and high (200 °C) temperatures of acceptor (Mg)-doped BaTiO3 ceramics with the increase of acceptor concentration were investigated. A series of coarse-grained specimens with different acceptor concentrations that were sintered at various oxygen partial pressures was prepared. The critical acceptor concentrations, beyond which the room temperature resistance increases abruptly, were experimentally evaluated and they were found to increase with a decrease in oxygen partial pressure during sintering. Each defect and electron concentrations at sintering and room temperature as a function of acceptor concentration were theoretically calculated. The results calculated could explain the experimentally observed behavior of the resistance versus acceptor concentration. The high (200 °C) temperature resistances under high electric field showed resistance degradation critically depending on acceptor concentration in the very small concentration range below ∼0.1 mol %, which occurred easily with the increase of acceptor concentration. It was explained that such behavior was caused by the variation of potential barrier of grain boundaries for the migration of oxygen vacancies as a function of acceptor concentration. [ABSTRACT FROM AUTHOR]

The behavior of the bulk electrical resistivity of donor-doped BaTiO[sub 3] with the increase of donor concentration was investigated. A series of coarse-grained specimens with different donor concentrations were prepared by adjusting the oxygen partial pressure during sintering. Then they were heat treated in air for an extended period in order to reach the equilibrium defect distribution corresponding to each heating temperature, which was followed by fast cooling. The critical donor concentrations, beyond which the bulk resistivity increases abruptly, were experimentally evaluated and they were found to decrease with a decrease in the equilibrium temperature in air. Each defect and electron concentrations as a function of donor concentration were theoretically calculated based on the model that the resistivity anomaly is caused by the partially ionized cation vacancies (V[sup '', sub Ti]) at high temperature, which trap electrons, and become fully ionized cation vacancies (V[sup '', sub Ti]) as specimens are cooled to room temperature. The results calculated could explain the experimentally observed behavior of the bulk resistivity versus donor concentration. The behavior of the bulk electrical resistivity of donor-doped BaTiO[sub 3] with the increase of donor concentration was investigated. A series of coarse-grained specimens with different donor concentrations were prepared by adjusting the oxygen partial pressure during sintering. Then they were heat treated in air for an extended period in order to reach the equilibrium defect distribution corresponding to each heating temperature, which was followed by fast cooling. The critical donor concentrations, beyond which the bulk resistivity increases abruptly, were experimentally evaluated and they were found to decrease with a decrease in the equilibrium temperature in air. Each defect and electron concentrations as a function of donor concentration were theoretically calculated based on the model that the... [ABSTRACT FROM AUTHOR]

Abstract: The influence of temperature on the variation in dielectric nonlinearity and domain structures was investigated for the (Ba0.95Ca0.05)(Ti0.83Zr0.17)O3 (BCTZ)‐based multilayer ceramic capacitor that shows a diffuse phase transition. Whereas the dielectric constant (εr) vs temperature shows a broadened maximum peak at low ac driving field, such a peaked behavior disappears at high ac driving field due to an abrupt increase in dielectric constants at low temperatures. Such low temperature effect can be associated with an enhanced spontaneous polarization (PS) and a significant increase in irreversible domain wall contribution to polarization representing normal ferroelectric behavior based on the Preisach analysis. No ferroelectric domain contrasts were observed at room temperature through transmission electron microscopy. However, they appeared and became more and more distinct with the decrease in temperature, and the crystal structure also changed from cubic to rhombohedral with increased lattice constants. It demonstrates that the dramatic increase in the dielectric nonlinearity with decreasing temperatures originates from the corresponding changes in domain and crystal structure, where the polar‐micro‐regions of BCTZ at room temperature change to normal ferroelectric domains at low temperatures. [ABSTRACT FROM AUTHOR]