xO4 Ceramics Research Articles

Tailoring properties of Ni0.50Co0.50DyxFe2–xO4 ceramics using microwave non-thermal plasma for high-frequency devices

Tailoring properties of Ni0.50Co0.50DyxFe2–xO4 ceramics using microwave non-thermal plasma for high-frequency devices

The structural characteristics and microwave dielectric properties of (Mg0.5Ti0.5)3+ substituted Sr0.6Ca0.4LaAlO4 ceramics

Achieving both high Qf and near-zero τf values for microwave dielectric ceramics in the microwave frequency band is challenging. In this work, the effect of (Mg0.5Ti0.5)3+ substitution on the structural characteristics and microwave dielectric properties of Sr0.6Ca0.4LaAlO4 ceramics was systematically investigated. The Sr0.6Ca0.4LaAl1-x(Mg0.5Ti0.5)xO4 (x = 0–0.5) ceramics were successfully synthesized. The average bond length of Sr/Ca/La–O for Sr0.6Ca0.4LaAl1-x(Mg0.5Ti0.5)xO4 ceramics is related to the dielectric constant (εr). The relative density of the ceramics and FWHM of the Raman peak have strong effects on the Qf values. There is a dependence of the [AlO6] octahedral distortion on the τf values, which provides new possibilities for tuning the temperature coefficient of the K2NiF4-type ceramics. When x is 0.4, the excellent microwave dielectric properties are obtained as following: εr = 20.52, Qf = 103,600 GHz (at 7.79 GHz), and τf = −3.77 ppm/°C. The above work provides a deeper insight into the relationship between the crystal structure characteristics and the microwave dielectric properties of the K2NiF4-type ceramics.

Degree of inversion of A/B lattice sites and microwave/millimeter wave/terahertz dielectric properties of MgAl2-x(Zn0.5Mn0.5)xO4 ceramics

In this work, spinel-structured MgAl2-x(Zn0.5Mn0.5)xO4 (0 ≤ x ≤ 0.08) single-phase ceramics were prepared through a solid-state reaction route. The substitution of (Zn0.5Mn0.5)3+ for Al3+ at the octahedral site affected the degree of inversion of A/B lattice sites, bond length/strength/valence, and covalency of metal-oxygen bond in the tetrahedron and hence microwave dielectric properties of MgAl2O4. The variation in εr and tanδ of ceramics is investigated in the millimeter wave-terahertz frequency band by combining infrared reflection spectrum and terahertz time-domain spectroscopy. A high Q×f value of 111,010 GHz @ 12.01 GHz, low εr = 8.3, and slightly lower τf = −60 ppm/°C is obtained for MgAl1.98Zn0.01Mn0.01O4 ceramics, which is tuned by adding a small amount of SrTiO3. The composite ceramics exhibited a near-zero τf (2.8 ppm/°C), high Q×f (55,400 GHz @ 11.15 GHz), and low εr (= 8.5), showing a great potential application prospect for 5G/6G wireless communication.

Structure, microwave dielectric properties and bond characteristics of Mg2Ti1-x(Al1/2Ta1/2)xO4 ceramics

The microwave dielectric ceramics Mg2Ti1-x(Al1/2Ta1/2)xO4 (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) are synthesized via solid-state reaction. The formation of pure spinel structural phase is confirmed by XRD refinement patterns. The intrinsic dielectric characteristics of Mg2Ti1-x(Al1/2Ta1/2)xO4 ceramics are researched by complex chemical bond theory (P–V–L theory) and infrared reflectivity spectra. The interaction of extrinsic (relative density) and intrinsic (dielectric polarizability) factors cause a change in εr. The lattice energy determines the variation of the Q × f. Both the τf and the total bond valence show a same changing trend. The improved microwave dielectric properties were obtained in Mg2Ti0.8(Al1/2Ta1/2)0.2O4 (εr = 14.2, Q × f = 186,060 GHz, τf = −39.27 ppm/◦C) when sintered at 1390 °C for 4 h.

Effect of phase, chemical bond and vibration characteristics on the microwave dielectric properties of temperature-stable Zn1−x(Li0.5Bi0.5)xMoxW1−xO4 ceramics

Zn1−x(Li0.5Bi0.5)xMoxW1−xO4 (x = 0–0.25) ceramics were prepared using the solid-state reaction process. The results show that the existence of the (Li0.5Bi0.5)MoO4 secondary phase significantly increases the τf value and improves the sintering behavior. When sintered at 755 °C, the sample with x = 0.05 exhibits the highest Q×f value (37,551 GHz), while the ceramics with x = 0.15 and 0.20 display excellent τf values, − 6.569 and − 3.097 ppm/°C, respectively. Additionally, X-ray diffraction refinement, Raman and infrared spectroscopy, and the complex bond valence theory were used to investigate the relationship between the phase, structure, and vibration characteristics and the microwave dielectric properties. Good microwave dielectric properties were obtained for the Zn0.85(Li0.5Bi0.5)0.15Mo0.15W0.85O4 ceramic sintered at 755 °C: εr = 18.937, Q×f = 20,022 GHz, and τf = −6.569 ppm/°C, which provides a promising candidate for low temperature co-fired ceramic technology.

Ultra-high quality factor and low dielectric constant of (Zn0.5Ti0.5)3+ co-substituted MgAl2O4 ceramic

In this study, MgAl2O4-based ceramics with high quality factor (Qf) and low dielectric constant (εr ≤ 10) were obtained by fabricating MgAl2-x(Zn0.5Ti0.5)xO4 (x = 0–0.5) ceramics via conventional solid-state reaction method. Excellent microwave dielectric properties were achieved for samples at x = 0.5 and sintered at 1550 °C, i.e., εr = 9.86, Qf = 263 900 GHz (five times better than that for x = 0 sample) and τf = −92 ppm/°C. The X-ray diffraction (XRD) patterns displayed characteristic peaks of MgAl2O4 with spinel structure. MgTi2O5 and MgTiO3 were considered as secondary phases. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and relative density analysis indicated that ultra-high Qf values were dominated by dense microstructure, secondary phase and cation vacancies; whereas εr values were mainly affected by secondary phase and ionic polarizability. MgAl2-x(Zn0.5Ti0.5)xO4 ceramics with excellent microwave dielectric properties have potential application in millimeter-wave communication, dielectric filters, dielectric antennas and resonators.

Preparation and characterization of Ni0.6CoxMn2.4−xO4 (0.2 ≤ x ≤ 1.4) NTC ceramics with low resistivity and high B value

The effects of Co content on the phase structure, micromorphology and electrical performance of Ni0.6CoxMn2.4−xO4 ceramics were studied using transmission electron microscope (TEM), X-ray powder diffractometer (XRD), scanning electron microscope (SEM), energy-dispersive spectrometer (EDS) and resistance/temperature measurements. TEM and high angle annular dark field (HAADF)-mapping results show that the calcined powders were nano-sized and the elements were uniformly distributed. For x < 0.98, the ceramics exhibited single cubic spinel structure. Our SEM–EDS results confirm that a rock-salt phase (Ni,Co)O is formed when 0.98 ≤ x ≤ 1.4. The lattice parameter of the spinel phase decreases with increasing Co content. The room-temperature resistivity of the ceramics decreases from 1100 to 219 Ω cm and then increased to 325 Ω cm as Co was added, while the B value decreased slightly from 3588 to 3030 K. The Ni0.6CoxMn2.4−xO4 ceramics are promising negative temperature coefficient materials for temperature monitoring in electric vehicles owing to their low resistivity and high B values.

Effect of molybdenum on structural, optical and microwave dielectric properties of copper tungstate

This article covers the structural and the optical properties of CuMoxW1−xO4 ceramics for x = 0, 0.02, 0.04, 0.06 and 0.08 prepared by solid-state reaction route. Chemical composition and microstructure of the prepared CuMoxW1−xO4 ceramic samples were characterized by means of X-ray diffraction, scanning electron microscopy, UV–Vis spectrophotometry and FT-IR spectroscopy. X-ray diffraction analysis confirmed the formation of monophasic wolframite structure whose cell volume is increased with increased Mo content. The Reitveld refinement of X-ray powder revealed the triclinic structure with $$P\overline{1}$$ symmetry. A reduction in optical band gap from 2.4 to 2.0 eV is obtained with increased Mo concentration. This reduction in band gap suggested its possible utilization in the visible region of electromagnetic spectrum. The photoluminescence (PL) peaks are shifted towards lower energy. The microwave dielectric parameters were measured by the Hakki–Coleman method. The dielectric constant (e′) and dielectric loss (tan δ) decreased from 17.049 to 12.375 and 0.0156 to 0.0129, respectively, with the increase in Mo content.

Bond characteristics, vibrational spectrum and optimized microwave dielectric properties of chemically substituted NdNbO4

Abstract Coordination chemistry, bond state and vibrational spectrum of co-substituted microwave dielectric NdNb1-x(Zr0.5W0.5)xO4 ceramics (x = 0.01∼0.05) were investigated. Raman spectra and XRD refinement showed a solid solution was formed. The compressed and elongated chemical bonds are responsible for the variations of crystal parameters and cell volume. Calculated chemical bond parameters indicated bond covalency, lattice energy and Nb-site bond energy act on the fluctuations of the permittivity, quality factor and temperature coefficient, respectively. Meanwhile, the infrared vibrational spectrum is fitted to quantify the contributions of observed IR mode to the intrinsic loss. Compact ceramic possesses excellent properties: er ∼ 19.2, Q × f ∼ 55282 GHz and τf ∼ -11.36 ppm/°C with x = 0.04, at 1250°C.

Improved sinterability and microwave dielectric properties of [Zn0.5Ti0.5]3+‐doped ZnAl2O4 spinel solid solution

AbstractLow‐permittivity ZnAl2‐x(Zn0.5Ti0.5)xO4 ceramics were synthesized via conventional solid‐state reaction method. A pure ZnAl2O4 solid‐state solution with an Fd‐3m space group was achieved at x ≤ 0.1. Results showed that partial substitution of [Zn0.5Ti0.5]3+ for Al3+ effectively lowered the sintering temperature of the ZnAl2O4 ceramics and remarkably increased the quality factor (Q × f) values. Optimum microwave dielectric properties (εr = 9.1, Q × f = 115,800 GHz and τf = −78 ppm/°C) were obtained in the sample with x = 0.1 sintered at 1400°C in oxygen atmosphere for 10 h. The temperature used for the sample was approximately 250°C lower than the sintering temperature of conventional ZnAl2O4 ceramics.

Phase composition, Raman spectra, infrared spectra and dielectric properties of Li2MgTi1-x(Mg1/3Nb2/3)xO4 ceramics at microwave frequency

The Li2MgTi1-x(Mg1/3Nb2/3)xO4 (0 ≤x ≤ 0.5) ceramics were prepared by the conventional solid-state method. The relationship among phase composition, substitution amount and microwave dielectric properties of the ceramics was symmetrically investigated. All the samples possess the rock salt structure with the space group of Fm-3m. As the x value increases from 0 to 0.5, the dielectric constant linearly decreases from 16.75 to 15.56, which can be explained by the variation of Raman spectra and infrared spectra. The Q·f value shows an upward tendency in the range of 0 ≤x ≤ 0.3, but it then decreases when x > 0.3. In addition, the temperature coefficient of resonant frequency (τf) is shifted toward zero with the increasing (Mg1/3Nb2/3)4+ addition. By comparison, the Li2MgTi0.7(Mg1/3Nb2/3)0.3O4 ceramics sintered at 1400 °C can achieve an excellent combination of microwave dielectric properties: εr= 16.19, Q·f = 160,000 GHz and τf = −3.14 ppm/°C.

Microwave dielectric properties and cation distributions of Zn1-3xAl2+2xO4 ceramics with defect structures

The spinel-structured Zn1-3xAl2+2xO4 (x=0–0.2) ceramics having defective structures were synthesized using the molten salt method, and their microwave dielectric properties and cation distributions were assessed. The 27Al solid-state nuclear magnetic resonance spectra of these ceramics demonstrate that they have an intermediate spinel structure in which the tetrahedral site occupancy increases from 0.03 to 0.64 as x increases. Moreover, crystal structure refinements suggest that cation vacancies are located at octahedral sites for x=0.1 and 0.2. Based on these data, the introduction of cation vacancies at octahedral sites appears to enhance the preferential occupation of tetrahedral sites by Al3+. The εr of these ceramics slightly decreased from 8.5 to 8.2 with increasing x, while the Q·f value increased significantly, from 127,532 to 202,468GHz, upon the introduction of cation vacancies. An intermediate spinel structure with preferential occupancy of tetrahedral sites by trivalent cations exhibits an enhanced Q·f value.

Structural and electrical properties of NixMn3 − xO4 system ceramics for infrared sensors

In this study, NixMn3−xO4 ceramics were prepared using a conventional mixed oxide method for application in infrared sensors, and the dried powder was calcined at 900°C for 2h. All specimens were sintered in air at 1200°C for 12h and annealed at a rate of 2°C/min to 800°C, subsequently quenching to room temperature. It has been shown that all specimens showed the formation of a cubic spinel phase without any second phase. From the surface micrographs and density results, all specimens showed a high density and low porosity. In terms of the temperature coefficient of resistance (TCR=1/R∗dR/dT), all specimens showed values −4%/°C above and the typical NTCR (negative temperature coefficient of resistance) properties, displaying decreased electrical resistance with an increase in temperature. The resistivity of the Ni0.79Mn2.21O4 specimen at room temperature was approximately 2602Ω·cm. The responsivity and detectivity of the Ni0.79Mn2.21O4 ceramics sintered at 1200°C were 4.56∗10−3V/W and 2010cmHz1/2/W, respectively.

Preparation and characterization of LixZn2−xVxSi1−xO4 ceramics

In this work, LixZn2−xVxSi1−xO4 (x=0.9, 0.8, 0.7, 0.5, 0.3, 0.2, and 0.1) ceramics were obtained by the conventional solid-state reaction method, using a different sintering temperatures for each x. We report on the compositional and morphological characterization of LixZn2−xVxSi1−xO4 samples, as well as on the influence of the composition on the photoluminescence and dielectric properties. The highest emission intensity was attained for x=0.9, while the best microwave dielectric properties (εr~7.8 and Qf~21,122GHz) were achieved for x=0.8.

Effect of La substitution on structural and magnetic properties of microwave treated Mg0.35Cu0.05Zn0.60LaxFe2−xO4 ceramics

A series of La substituted MgCuZn ferrite powders (Mg0.35Cu0.05Zn0.60LaxFe2−xO4 with, x=0.00,0.01,0.03 and 0.05) were synthesized by a novel microwave sintering method. The effect of La-substitution on the structural, physical and magnetic properties has been studied. X-ray diffraction confirmed the formation of single phase cubic spinel lattice for all the compositions studied. The lattice constant increased with increasing La content, might be attributed to the larger ionic radius of La3+ than the Fe3+. X-ray density of each sample is higher than the corresponding bulk density of sintered sample. The grain size was estimated using SEM micrographs. Room temperature saturation magnetization and magnetic initial permeability were found to increase with La substitution in Fe up to x=0.01 and then decreased. The role of lanthanum in modifying structural and magnetic properties of microwave treated MgCuZn ferrites system has been explained suitable.