Stable Dielectric Properties Research Articles

Influence of atmospheres on the dielectric properties of calcium copper titanate ceramics

AbstractCalcium Copper Titanate Ceramics (CaCu3Ti4O12; CCTO) are useful as capacitors because of the large dielectric constant. This study investigated the effect of testing atmospheres such as air and dry N2 on the stability and reproducibility of the electrical properties displayed by CCTO ceramics. Solid‐state reaction method is used to synthesize phase pure CCTO powders, which are used to fabricate dense samples by sintering. Samples with different microstructures are obtained by sintering at different temperatures of 1070 and 1100°C. AC impedance spectroscopy is used to study the effects of testing atmospheres, temperatures, microstructures, and frequency on the dielectric properties of the sintered samples. Highly irreproducible dielectric properties are observed upon testing in ambient conditions. Stable and reproducible dielectric properties are only obtained in dry N2. Re‐exposing the same CCTO samples back into ambient air after treating in dry N2 restored irreproducibility. The results from this study suggested that the role of moisture/air in the ambient on the dielectric properties should not be over‐looked and is expected to play a key role in controlling the stability of dielectric properties of CCTO. This new approach of characterizing the CCTO ceramics can be used as guidelines to eliminate hysteresis due to ambient atmosphere and to produce stable and reproducible dielectric properties useful as capacitors.

Three-Dimensional Printed Piezoelectric Array for Improving Acoustic Field and Spatial Resolution in Medical Ultrasonic Imaging.

Piezoelectric arrays are widely used in non-destructive detecting, medical imaging and therapy. However, limited by traditional manufacturing methods, the array’s element is usually designed in simple geometry such as a cube or rectangle, restricting potential applications of the array. This work demonstrates an annular piezoelectric array consisting of different concentric elements printed by Mask-Image-Projection-based Stereolithography (MIP-SL) technology. The printed array displays stable piezoelectric and dielectric properties. Compared to a traditional single element transducer, the ultrasonic transducer with printed array successfully modifies the acoustic beam and significantly improves spatial resolution.

Sulfonyl‐Containing Polyimide Dielectrics with Advanced Heat Resistance and Dielectric Properties for High‐Temperature Capacitor Applications

AbstractPolymer dielectrics, with advanced dielectric properties and heat resistance, are critical for high‐temperature capacitors in various applications. However, the high performance of heat resistance and dielectric properties are quite difficult to achieve all together due to their mutual implication. Here, by intensively investigating the correlation between molecular structure and properties, polyimide dielectrics with i) enhanced dielectric constant by introducing sulfonyl group, ii) low dissipation factor by introducing flexible linkage, and iii) highTg(glass transition temperature) by retaining an aromatic structure, are obtained. The sulfonyl‐containing polyimides with different flexible linkages exhibit simultaneously a high dielectric constant (4.50–5.98), low dissipation factor (0.00298–0.00426), and outstanding breakdown strength (most above 500 MV m−1), as well as superior heat resistance (Tg: 244–304 °C). Specifically, the polyimide (SPI‐1) with sulfonyl group in diamine moiety and para‐para linkage shows stable dielectric properties up to 150 °C, and the discharged energy density and charge–discharge efficiency can be as high as 7.04 J cm−3and 91.3% at 500 MV m−1, respectively.

Flexible acrylic-polyurethane/copper composites with a frequency and temperature-independent permittivity

Capacitors with a high dielectric constant and low dielectric loss are highly demanded for electronic applications, such as electric devices and energy storage. To enhance the performance of those electronic applications, it is of great significance to achieve capacitors with stable dielectric property both in a broad ranges of frequency and temperature. In this research, flexible Acrylic-polyurethane/copper (APU/Cu) composites with different filler fractions were successfully synthesized. Excellent flexibility of APU/Cu composites is maintained even when Cu particles reached to 19.11 vol%. The dielectric and thermal properties of APU/Cu composites were systematically investigated. Enhanced dielectric constant and lower dielectric loss were achieved. Meanwhile, it is found that the permittivity fluctuated slightly with the increase of frequency and temperature, suggesting that permittivity was independent on frequency and temperature over a broad range. This research shed lights on the application of APU/Cu composites in flexible thin-film capacitors and stretchable electronic devices.

Temperature stable relative permittivity from −60 to 200 °C in 75BaTiO3–(25 − x)BiMg0.5Ti0.5O3–xNaNbO3 ceramics with X9R like characteristics

75BaTiO3–(25 − x)BiMg0.5Ti0.5O3–xNaNbO3 (x = 0–0.2) ceramics were obtained by conventional solid-state reaction method. X-ray diffraction patterns of the samples showed the formation of cubic structure. No evidence of secondary phase was found within the detection limit of XRD facility which was in agreement with the Raman spectra of all the samples. Temperature dependence of relative permittivity and dielectric loss for the sample with x = 0.10 showed a temperature stable relative permittivity of ∼840 ± 15% over the temperature range −62 to 192 °C and dielectric losses <0.02 in a wide temperature range (−28 to 374 °C). The wide temperature stable dielectric properties of x = 0.10 sample suggest that it could be a promising candidate material for X8R/X9R type capacitors.

High temperature stable dielectric properties and enhanced energy-storage performance of (1− x)(0.85Na0.5Bi0.5TiO3 − 0.15Ba0.8Ca0.2Ti0.8Zr0.2O3)− xK0.5Na0.5NbO3 lead-free ceramics

Novel high temperature ceramic capacitors (1− x)(Na0.5Bi0.5TiO3 − 0.15Ba0.8Ca0.2Ti0.8Zr0.2O3)− xK0.5Na0.5NbO3 were synthesized in the solid-state reaction route. The influence of K0.5Na0.5NbO3 modification on dielectric behavior, energy-storage properties, ac impedance and temperature stable dielectric performance were systematically investigated. The reduced grain size and enhanced relaxor properties are obtained with the addition of KNN. The content of x = 0.1 exhibits a stable permittivity (~ 1630) and dielectric loss (< 0.05) over a relatively broad temperature range (66–230 °C). A variation in permittivity within ± 15% can be observed over a pretty wide temperature range of 66–450 °C. Beyond that, this ceramic shows enhanced energy-storage properties with the density (Wrec) of 0.52 J/cm3 and efficiency (η) of 80.3% at 110 kV/cm. The possible contributions of the grain and the grain boundary to the ceramic capacitance are discussed by the ac impedance spectroscopy.

Temperature‐stable dielectric and energy storage properties of La(Ti0.5Mg0.5)O3‐doped (Bi0.5Na0.5)TiO3‐(Sr0.7Bi0.2)TiO3 lead‐free ceramics

AbstractA new type of (0.7−x)Bi0.5Na0.5TiO3‐0.3Sr0.7Bi0.2TiO3‐xLaTi0.5Mg0.5O3 (LTM1000x, x = 0.0, 0.005, 0.01, 0.03, 0.05 wt%) lead‐free energy storage ceramic material was prepared by a combining ternary perovskite compounds, and the phase transition, dielectric, and energy storage characteristics were analyzed. It was found that the ceramic materials can achieve a stable dielectric property with a large dielectric constant in a wide temperature range with proper doping. The dielectric constant was stable at 2170 ± 15% in the temperature range of 35‐363°C at LTM05. In addition, the storage energy density was greatly improved to 1.32 J/cm3 with a high‐energy storage efficiency of 75% at the composition. More importantly, the energy storage density exhibited good temperature stability in the measurement range, which was maintained within 5% in the temperature range of 30‐110°C. Particularly, LTM05 show excellent fatigue resistance within 106 fatigue cycles. The results show that the ceramic material is a promising material for temperature‐stable energy storage.

Effects of MnO addition on the stable dielectric properties of BaTiO3- (Bi0.5Na0.5)TiO3-Ta2O5 ceramics

The dielectric composition of 0.9BaTiO3-0.1(Bi0.5Na0.5)TiO3 with 4 mol% Ta2O5 sintered at 1230 °C revealed a dielectric constant (K) of 1179, tanδ values of 1.32% and 8.56%, and temperature coefficients of capacitance (TCCs) of 0.6% and − 14.05% at 25 °C and 200 °C, respectively, which meet the X9R specifications. In this study, various amounts of MnO additive were investigated for their ability to reduce the tanδ values of the compounds at 25 °C and 200 °C. Results indicated that the microstructure consists of fine equiaxed grains with a perovskite structure and second phases including plate-like Ba6Ti17O40, granular Ba3Ta3.2Ti5O21, and large granular Ba4Ti2O27 grains, depending on the MnO content. The MnO additive dramatically reduces tanδ values in the measured temperature range, even though it does not significantly change the microstructure for up to 4.5% addition. The 0.9BaTiO3-0.1(Bi0.5Na0.5)TiO3 with 4 mol% Ta2O5 and 0.01% MnO showed a K of 1107, tanδ values of 1.16% and 0.23% and TCCs of − 4.32% and − 13.81% at 25 °C and 200 °C, respectively. The dielectric properties of the formulations prepared in this study were highly repeatable, which confirmed their immense potential for use in X9R capacitors.

Ultrawideband, Stable Normal and Cancer Skin Tissue Phantoms for Millimeter-Wave Skin Cancer Imaging.

This work introduces new, stable, and broadband skin-equivalent semisolid phantoms for mimicking interactions of millimeter waves with the human skin and skin tumors. Realistic skin phantoms serve as an invaluable tool for exploring the feasibility of new technologies and improving design concepts related to millimeter-wave skin cancer detection methods. Normal and malignant skin tissues are separately mimicked by using appropriate mixtures of deionized water, oil, gelatin powder, formaldehyde, TX-150 (a gelling agent, widely referred to as "super stuff"), and detergent. The dielectric properties of the phantoms are characterized over the frequency band of 0.5-50GHz using a slim-form open-ended coaxial probe in conjunction with a millimeter-wave vector network analyzer. The measured permittivity results show excellent match with ex vivo, fresh skin (both normal and malignant) permittivities determined in our prior work over the entire frequency range. This work results in the closest match among all phantoms reported in the literature to surrogate human skin tissues. The stability of dielectric properties over time is also investigated. The phantoms demonstrate long-term stability (up to 7 months was investigated). In addition, the penetration depth of millimeter waves into normal and malignant skin phantoms is calculated. It is determined that millimeter waves penetrate the human skin deep enough (0.6 mm on average at 50GHz) to affect the majority of the epidermis and dermis skin structures.

Design of h-BN-Filled Cyanate/Epoxy Thermal Conductive Composite with Stable Dielectric Properties

The thermal conductive cyanate ester/epoxy (CE/AG80) composites with hexagonal boron nitride (h-BN) reinforced were manufactured and investigated. The low and stable dielectric constant/loss of the composites with respect to both frequency (500 Hz-100 kHz) and temperature (50-350 °C) were discussed as functions of the content of h-BN. It was found that the low and stable dielectric constant/ loss of the composites can be maintained in the whole frequency range at room temperature, and the mutations of constant/loss at the frequency of 1 kHz occurred above 280 °C for all of the samples. Moreover, a remarkable enhancement of the thermal conductivity (0.54 W/mK a 1.8-fold enhancement) was achieved with the introduction of h-BN and significantly outperformed traditional thermal conductive fillers. The significant improvements of the thermal conductivity of the composites were analyzed by the Agari’s model and the parameters of C1 and C2 were also calculated.

Simplified Normalization of C-Band Synthetic Aperture Radar Data for Terrestrial Applications in High Latitude Environments

Synthetic aperture radar (SAR) applications often require normalization to a common incidence angle. Angular signatures of radar backscatter depend on surface roughness and vegetation cover, and thus differ, from location to location. Comprehensive reference datasets are therefore required in heterogeneous landscapes. Multiple acquisitions from overlapping orbits with sufficient incidence angle range are processed in order to obtain parameters of the location specific normalization function. We propose a simpler method for C-band data, using single scenes only. It requires stable dielectric properties (no variations of liquid water content). This method is therefore applicable for frozen conditions. Winter C-band data have been shown of high value for a number of applications in high latitudes before. In this paper we explore the relationship of incidence angle and Sentinel-1 backscatter across the tundra to boreal transition zone. A linear relationship (coefficient of determination R 2 = 0.64) can be found between backscatter and incidence angle dependence (slope of normalization function) as determined by multiple acquisitions on a pixel by pixel basis for typical land cover classes in these regions. This allows a simplified normalization and thus reduced processing effort for applications over larger areas.

Advances in lead‐free high‐temperature dielectric materials for ceramic capacitor application

Ceramic capacitors with upper operating temperatures far beyond 200°C are essential for high-temperature electronics used in deep oil drilling, aviation, automotive industry and so on. Recent advances in existing lead-free dielectrics for potential high-temperature capacitor applications are reviewed and grouped into three categories according to the parent component of the solid solution. Their desirable temperature stabilities were summarised comprehensively. However, there are still some limitations in the current research, such as achieving low loss in a wide temperature range and maintaining stable dielectric properties with different frequencies or at different voltages. Furthermore, the successful implementation of multilayer ceramic capacitors is one of the biggest challenges, which will have far-reaching impacts on the realisation of high-temperature capacitor application in the future.

Effects of (Cr0.5Ta0.5)4+ on structure and microwave dielectric properties of Ca0.61Nd0.26TiO3 ceramics

The Ca0.61Nd0.26Ti1-x(Cr0.5Ta0.5)xO3 (CNT-CTx) ceramics with orthorhombic perovskite structure were prepared using the conventional solid-state method. The X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectra (XPS) were employed to investigate the correlations between crystal structure and microwave dielectric properties of CNT-CTx ceramics. The XRD results showed that all CNT-CTx samples were crystallized into the orthorhombic perovskite structure. The SEM micrographs indicated that the average grain size of samples depended on the sintering temperature. As (Cr0.5Ta0.5)4+ concentration increased, there was a significant decrease in the average grain size of samples. The short range order (SRO) structure and structural distortion of oxygen octahedra proved to exist in CNT-CTx crystals according to the analysis of Raman spectra results. The microwave dielectric properties highly depended on the full width at half maximum (FWHM) of Raman spectra, oxygen octahedra distortion, reduction of Ti4+ to Ti3+ and bond valence. At last, the CNT-CT0.05 ceramic sintered at 1420 °C for 4 h exhibited the good and stable comprehensive microwave dielectric properties: relative permittivity of 96.5, quality factor of 14,360 GHz, and temperature coefficient of resonant frequency of +153.3 ppm/°C.

Stable dielectric properties of Na1+xBiTi6O14+0.5x (x = −0.02, − 0.01, 0.01, 0.02) ceramics with low loss and high operating temperature

New lead-free dielectric ceramics, Na1+xBiTi6O14+0.5x (x = −0.02, − 0.01, 0.01, 0.02), were prepared by the conventional solid-state method. Micro-structural and electrical properties of Na1+xBiTi6O14+0.5x were studied. XRD showed all the samples exhibited a single structured phase. Grains decreased at the beginning, then grew with the increasing x content in SEM. Impedance spectra (IS) analysis evidenced the phenomena that the dielectric permittivity increased firstly, then decreased, while the loss had the opposite trend. Z* plots showed that Na1+xBiTi6O14+0.5x ceramics were a kind of dielectrics. The activation energy (Ea) could be calculated in the range of 1.53–1.65 eV, which indicated they were dielectric ceramics. Na1+xBiTi6O14+0.5x ceramics (x = 0.01) sintered at 1040 °C showed prominent dielectric properties with a dielectric constant of 25.76, loss of 0.07%, Ea of 1.65 eV, density of 3.463 g/cm3, impedance of 0.532 MΩ cm, -Z′′max of 0.1958 MΩ cm, capacitance of 5.56 pF/cm (600 ℃), which were enhanced much compared with other samples. The existence of dielectric properties with high dielectric constant, low dielectric loss and wide operating temperature range makes it possible to develop the ceramics into high-temperature capacitors.

Temperature stable dielectric properties of lead-free BiFeO3–BaTiO3 modified with LiTaO3 ceramics

LiTaO3 (LT)-modified (1 − x)(0.675BiFeO3–0.325BaTiO3) + 0.6 mol% MnO2 + 0.4 mol% CuO (BFBTMC–xLT, where x = 0–0.030) ceramics were synthesized by solid state reaction method. The crystal symmetry, ferroelectric response, and temperature stable dielectric properties were investigated as a function of different LT-content. The X-ray diffraction study of BFBTMC–LT piezoceramics revealed no remarkable variation in the crystal symmetry. The value of maximum polarization (Pm ≈ 24 μC/cm2) and remnant polarization (Pr ≈ 18 μC/cm2) were obtained at BFBTMC–0.010LT composition. The maximum temperature of permittivity (Tmax) decreased by the addition of LT-content in BFBTMC materials. The un-doped sample was ferroelectric; a change to relaxor behavior occurred by the addition of LT-content, with broad frequency dependent curves of dielectric mid-value permittivity (εrmid) and tangent loss as a function of temperature. A significant enhancement occurred at 3 mol% LT-modified BFBTMC ceramics, with εrmid = 5000 ± 9.5% (250–450 °C at 1 kHz). These results are favorable for practical applications in high temperature dielectrics.

Structure–property relationships of perovskite-structured Ca0.61Nd0.26Ti1-x(Cr0.5Nb0.5)xO3 ceramics

A series of Ca0.61Nd0.26Ti1-x(Cr0.5Nb0.5)xO3 (CNTCNx) (0 ≤ x ≤ 0.1) ceramics were prepared via a solid state reaction method. All CNTCNx samples were crystallized into the orthorhombic perovskite structure. The SEM micrographs indicated that the average grain sizes of samples depended on (Cr0.5Nb0.5)4+ concentration. And as (Cr0.5Nb0.5)4+ concentration increased, the average grain size of samples decreased significantly. The short range order (SRO) structure and structural distortion of oxygen octahedra proved to exist in CNTCNx crystals from Raman spectra analysis results. The microwave dielectric properties highly depended on the B-site bond strength, oxygen octahedra distortion, reduction of Ti4+ to Ti3+ and internal strain η. At last, the CNTCN0.06 ceramic sintered at 1400°C for 4h exhibited good and stable comprehensive microwave dielectric properties of εr = 92.3, Q × f = 13,889GHz, τf = + 152.8ppm/°C.

Stable dielectric, elastic, and piezoelectric properties of Yb3+:CNGS crystals at high temperature

In this study, Yb3+:Ca3NbGa3Si2O14 (Yb3+:CNGS) single crystals with different doping concentrations (2 and 5 at%) were grown using the Czochralski method. The dielectric, elastic, and piezoelectric coefficients at room temperature and elevated temperature were measured and calculated according to the impedance method for the first time. Crystals show excellent piezoelectricity and exhibit favorable temperature stability from room temperature to 800 °C. The transmittance spectrum of the Yb3+:CNGS crystals is also recorded, and the results prove the high optical quality of these crystals.

The relationship between the molecular structure and properties of novel poly(arylene ether nitrile ketone)s with high-temperature and stable dielectric properties

Three new poly(arylene ether nitrile ketone)s (PENKs), special engineering plastics, with different molecular structure were prepared. The obtained PENKs were characterized by FTIR spectroscopy. The thermodynamic and heat resistance behavior of PENKs were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. It was noticed that the glass transition temperature (T g ) and initial decomposition temperature (T i ) of the PENKs were higher than 160 and 480°C, respectively. The main reason was due to their molecular structure, in which the stronger intermolecular forces improved the heat resistance. Moreover, the crystallinity of PENKs decreased with increasing the nitrile group (–CN) content, which indicated that the regularity of molecular chain decreased. The dielectric property indicated that the dielectric constant (K) of PENKs increased with–CN content because of its strong polarity. Meanwhile, the T g of PENKs was higher than traditional poly(ether–ketone) (PEK) and poly(ether–ether–ketone) (PEEK). It was important that the solubility of PENKs was well improved. Besides, the tensile strength of PENKs was higher than that of common poly(arylene ether nitrile) (PEN).

High-temperature dielectrics based on (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xNaNbO3 system

The microstructure, phase structure, ferroelectric and dielectric properties of (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xNaNbO3 (BNT-BT-xNN) ceramics were investigated as candidates for high-temperature capacitors with a working temperature far beyond 200 °C. The decreased polar rhombohedral phase shown in XRD and the weakened macroscopical polarization in P-E test indicate that NN addition can dilute the mutual coupling of polar nanoregions (PNRs), which is very conducive to flattening the curves of temperature dependent dielectric permittivity at the expense of a small reduction in dielectric permittivity values. Among all manufactured samples, those with composition of x = 0.10 showed a high permittivity of more than 2700 (1 kHz) and energy storage density of 0.61 J/cm3 (7 kV/mm) at 150 °C with a ΔC/C150°C varying no more than 15% in a wide temperature range of 54°C–318 °C. All of these exhibit wonderful temperature stable dielectric properties and indicated the promising future of BNT-BT-xNN system as high-temperature ceramic capacitors.

Improved temperature stability of dielectric properties for BaZr0.134Ti0.866O3 ceramics by hydro-phase method at atmospheric pressure

ABSTRACTMicrospheres BaZr0.134Ti0.866O3 particles (317.7 ± 8 nm) were obtained using a hydro-phase method at atmospheric pressure, which controls both uniformity and particle size. Fine-grained BaZr0.134Ti0.866O3 ceramics were successfully prepared by sintering. Dielectric temperature stability of the ceramics is effectively improved, which can meet the X8R specification, with ∆ϵ/∆ϵ25°C ≤ ± 15% in the temperature range of −55 to 150°C. The results are attractive for the applications in multilayer ceramic capacitors.