Very high thermal stability with excellent dielectric,and non-ohmics properties of Mg-doped CaCu<Subscript>3</Subscript>Ti<Subscript>4.2</Subscript>O<Subscript>12</Subscript> ceramics

In this work, the nominal CaCu_3?xMg_xTi_4.2O₁₂ (0.00, 0.05 and 0.10) ceramics were prepared by sintering pellets of their precursor powders obtained by a polymer pyrolysis solution method at 1100 °C for different sintering time of 8 and 12 h. Very low loss tangent (tanδ)?<?0.009–0.014 and giant dielectric constant (ε′) ～?1.1?×?10⁴–1.8?×?10⁴ with excellent temperature coefficient (Δε′) less than ±?15% in a temperature range of ??60 to 210 °C were achieved. These excellent performances suggested a potent application of the ceramics for high temperature X8R and X9R capacitors. It was found that tanδ values decreased with increasing Mg²⁺ dopants due to the increase of grain boundary resistance (R_gb) caused by the very high density of grain, resulting from the substitution of small ionic radius Mg²⁺ dopants in the structure. In addition, CaCu_3?xMg_xTi_4.2O₁₂ ceramics displayed non-linear characteristics with the significant enhancements of a non-linear coefficient (α) and a breakdown field (E_b) due to Mg²⁺doping. The high values of ε′ (14012), α (13.64) and E_b (5977.02 V/cm) with very low tanδ value (0.009) were obtained in a CaCu_2.90Mg_0.10Ti_4.2O₁₂ ceramic sintered at 1100 °C for 8 h.     

Giant dielectric, low dielectric loss, and non-ohmic properties of nanocrystalline CaCu3Ti4O12

A simple polymer pyrolysis method has been successfully used to prepare CaCu₃Ti₄O₁₂ (CCTO) nanoparticles by calcination the obtained precursor powder at a low temperature of 800 (CCTO-1) and 850 °C (CCTO-2) in air for 4 h. The XRD results show that both of the calcined powders (CCTO-1 and CCTO-2) are pure having perovskite structure with the crystallite sizes, as evaluated by the XRD line boardening technique, of 47.5 and 75 nm, respectively. The particle sizes as estimated from the bright field images of TEM were found to be in the range of 10–35 and 7–52 nm for CCTO-1 and CCTO-2, respectively. The further sintering of CCTO-1 and CCTO-2 at 1,050 °C in air for 6 h, CCTO-1A and CCTO-2A, are also pure with perovskite structure as indicated by the XRD results. The measurements of the dielectric constant ( $ \varepsilon^{\prime } $ ε ′ ) and the low loss tangent (tanδ) at 1 kHz and 20 °C of CCTO-2A were found to be ~11,472 and ~0.0438, respectively. In addition, the CCTO-2A sample shows a small temperature coefficients ( $ \left| {\Updelta \varepsilon^{\prime } } \right| < 15\,\% $ | Δ ε ′ | < 15 % ) in a wide temperature range from ?50 to 110 °C. The non-Ohmic properties non-linear coefficient (α) of CCTO-1A and CCTO-2A were observed and the non-linear coefficient (α) of them determined in the range of 1–10 mA cm^?2 were found to be 12.00 and 7.26, respectively. Moreover, the breakdown field (E _b ) of CCTO-1A and CCTO-2A ceramics obtained at J = 1 mA cm^?2 were calculated and found to be 811 and 1,342 V cm^?1, respectively.     

Improvement of Temperature Stability,Dielectric Properties and Nonlinear Current-Electric Field Characteristic of CaCu 3 Ti 4.2−x Sn x O 12 Ceramics

The dielectric properties and the nonlinear current density–electric field (J–E) relationship of CaCu3Ti4.2−xSnxO12 (x = 0.00, 0.05 and 0.10) ceramics at various sintering temperatures are presented. Excellent dielectric properties with a very low tanδ ∼ 0.008–0.020, a giant ε′ ∼ 6495–16,975, and stability of Δε′ of < ± 15% over the temperature range of −60 to 210°C are obtained in a CaCu3Ti4.15Sn0.05O12 ceramic sintered at 1080°C and CaCu3Ti4.10Sn0.10O12 ceramics sintered at both 1080°C and 1100°C. Additionally, all ceramics exhibited a nonlinear J–E relationship. A maximal nonlinear coefficient (α) of ∼ 1044.4 is obtained in the CaCu3Ti4.15Sn0.05O12 sintered at 1080°C. X-ray diffraction and field emission scanning electron microscopy techniques were used for structural and microstructural evaluation of all ceramics. Elemental mapping with energy dispersive X-ray spectroscopy confirmed the presence of Sn4+ dopant at the main CaCu3Ti4O12 site and in minor TiO2 phases of all Sn4+ doped CaCu3Ti4.2O12 ceramics. This mixed phase plays an important role to increase grain boundary resistance (Rgb) and significantly improves the thermal stability of dielectric properties, as well as the nonlinear J-E relationship.     

Ultra-stable X9R type CaCu3-xZnxTi4.1O12 ceramics

CaCu_3-xZn_xTi_4.1O₁₂ (x?=?0.00, 0.05 and 0.10) precursor powders were prepared by the polymer pyrolysis (PP) solution method. Ultra-stable X9R type capacitor with very low loss tangent (tanδ) ~0.017 varied within a value of less than 0.05 in a wide temperature range of ?60 to 150?°C and high dielectric constants (ε^′) ~9200 with Δε′ ≤?±?15% in a wide temperature range of ?60 to 210?°C was achieved in CaCu_2.95Zn_0.05Ti_4.1O₁₂ (Zn05-1) ceramic obtained by sintering the precursor powder (x?=?0.05) at 1060?°C for 8?h. A major role for the validity of ε^′ and tanδ in these wider temperature ranges was suggested to originated from the very high grain boundary resistance (R_gb ~413,190?Ω?cm), resulting from the effect of Zn²⁺ doping and TiO₂-rich at grain boundary. With the excellent dielectric properties of (Zn05-1) ceramic, it was suggested to be applied for X8R and X9R capacitors. Interestingly, improvements of nonlinear properties with very high nonlinear coefficient (α ~ 25.94) and breakdown field (E_b~ 3146.25?V.cm^?1) values were achieved in (Zn05-1) ceramic, as well.     

Very low loss tangent and giant dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics prepared by the sol–gel process

The pure phase of CaCu₃Ti₄O₁₂ (CCTO) powder can be successfully synthesized by the sol–gel process. CCTO ceramic samples were synthesized at different sintering temperatures of 1015 and 1050?°C and sintering times of 8 and 10 h. X-ray diffraction results indicated a pure phase for all ceramic samples. Rietveld refinements were adopted for the calculation of lattice constants. Scanning electron microscopy micrographs revealed the effect of sintering conditions on the microstructural evolution of ceramic samples. X-ray absorption near edge spectroscopy was performed to determine the oxidation state of Cu and Ti ions in ceramic samples. The dielectric and non-linear current voltage properties of CCTO ceramic samples were systematically investigated. Interestingly, very low loss tangent (tanδ?<?0.017 at 30?°C and 1 kHz) and giant dielectric constant (ε′?～?10,942) with temperature coefficients less than ±15% in a wide temperature range of ?60 to 125?°C were obtained in the CCTO ceramic sample sintered at 1015?°C for 10 h (CCTO1-10). This suggests a potential use for CCTO1-10 sample in capacitor applications. All CCTO ceramic samples display non-linear characteristic with non-linear coefficient (α) and breakdown field (E _b ) values in the range of 5.69–11.02 and 1415–4294, respectively.     

Hydrodynamics and Heat Transfer Analysis of Airflow in a Sinusoidally Curved Channel

For heat transfer enhancement in heat exchangers, different types of channels are often tested. The performance of heat exchangers can be made better by considering geometry composed of sinusoidally curved walls. This research studies the modeling and simulation of airflow through a units long sinusoidally curved wavy channel. For the purpose, two-dimensional Navier Stokes equations along with heat equations are under consideration. To simulate the fluid flow problem, the finite element-based software COMSOL Multiphysics is used. The parametric study for Reynolds number from to and the period of vibration P from to are observed. The surface plots, streamline patterns, contours, and graphs are presented for the velocity field magnitude, temperature, and pressure against the Reynolds number as well as period of vibration. The results are compared with various literature. It is found that due to the creation of periodic contraction regions the velocity magnitude of the flow is continuously increasing with the increase of Reynolds number, on the contrary the pressure is decreasing from inlet to outlet of the channel. Also, a periodic variation in the pressure distribution along the vibrating boundaries has been found with an average increase of for the high Reynolds number. A novel work was done by expressing the rotation rate per second in terms of local Reynolds number for the recirculating regions found due to the periodic oscillation of the boundaries. The average temperature near the outlet where a fixed temperature is imposed initially is decreasing with an increase in Reynolds number. The convection process is weakened due to an increase of periodic vibration of boundaries.     

Performance of an adaptive liquid microlens controlled by a microcoil actuator

We present an optofluidic system based on electromagnetic manipulation of a ferrofluid to tune a liquid lens. Both studies of the dynamics of fluid transport and of the optical properties of the liquid lens have been carried out. Thermal and magnetic field simulations of the microcoil actuators are presented. Proof-of-principle experiments demonstrating the adaption of the focal length of the lens have been carried out. It is shown that the lens adaption proceeds in a reversible and reproducible manner, given that the ferrofluid plug moves with a speed below a specific threshold value. Furthermore, the time delay between the actuation and the deflection of the lens surface is studied.     

La0.5Sr0.5TiO3 nanopowders prepared by the hydrothermal method

La_0.5Sr_0.5TiO₃ nanopowders were prepared by the hydrothermal method. The influence of processing parameters, including KOH concentration, reaction temperature and reaction time on the obtained products were studied. The structure and morphology of the obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD results show that pure phase La_0.5Sr_0.5TiO₃ nanopowders can be successfully synthesized with 2 M KOH concentration at a low temperature of 220 °C for 24 h. In addition, the product has a plate-like shape with particle sizes in the range of 25–100 nm as estimated by TEM.     

Improved non-ohmic and dielectric properties of Cr3+ doped CaCu3Ti4O12 ceramics prepared by a polymer pyrolysis solution route

CaCu_3-xCr_xTi₄O₁₂ (x?=?0.00–0.20) ceramics were prepared via a polymer pyrolysis solution route. Their dielectric properties were improved by Cr³⁺ doping resulting in an optimal dielectric constant value of 7156 and a low tanδ?value of 0.092 in a sample with x?=?0.08. This might have resulted from a decrease in oxygen vacancies at grain boundaries. XANES spectra confirmed the presence of Cu⁺ ions in all ceramic samples with a decreasing Cu⁺/Cu²⁺ ratio due to an increased content of Cr³⁺ ions. All CaCu_3-xCr_xTi₄O₁₂ ceramics showed nonlinear characteristic with improvement in both the breakdown field (E_b) and its nonlinear coefficient (α). Interestingly, the highest values of α, ～ 114.4, and that of E_b, ～8455.0?±?123.6?V?cm^?1, were obtained in a CaCu_3-xCr_xTi₄O₁₂ sample with x?=?0.08. The improvement of dielectric and nonlinear properties suggests that they originate from a reduction of oxygen vacancies at grain boundaries.     

Thermal stability improvement of dielectric properties and non-ohmic characteristic of CaCu3+xTi4O12 ceramics via a Cu-nonstoichiometric approach

In this work, the effects of Cu composition on the thermal stability of the dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics obtained via a polymer-pyrolysis chemical process were studied. The mean grain sizes of Cu-stoichiometric (x = 0), Cu-deficient (x < 0) and Cu-excess (x > 0) CaCu_3+xTi₄O₁₂ ceramics were found to be ~3.2, ~3.4 and ~3.7 μm, respectively. Interestingly, very good dielectric properties (0.020 ≤ tanδ ≤ 0.038 and 4000 ≤ ε′ ≤ 7065) were attained in CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.1, excluding x = 0.2) ceramics. Moreover, the variation of dielectric constant (ε′) within a limit of ±15% (Δε′_{± 15%}) over a wide temperature range (T_R) of ?70 – 220 °C with low tanδ < 0.05 (tanδ_<0.05) over a T_R of ?70 to 80 °C were achieved in a CaCu_2.8Ti₄O₁₂ ceramic. These results suggest that this ceramic could be applicable for X9R capacitors and energy storage devices that require high thermal stability. Additionally, the nonlinear properties of Cu-nonstoichiometric ceramics could be improved when compared with those of the Cu-stoichiometric material. The incremental changes of dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics revealed the significant role of Cu composition on grain boundary resistance (R_gb), which was confirmed by impedance spectroscopy analysis. In addition, XANES results revealed the proper ratios of Cu⁺:Cu²⁺ and Ti³⁺:Ti⁴⁺ found in these ceramics, indicating the semiconducting behavior of these grains.