Impedance spectroscopy and magneto-dielectric analysis of La3+ substituted Co2Z hexaferrite

Single phase Ba₃Co₂Fe₂₄O₄₁ (Co₂Z) and Ba_2.8La_0.2Co₂Fe₂₄O₄₁ (La-Z) were prepared by solid state ceramic method. The dielectric, impedance and electric modulus were systematically investigated as a function of temperature from 50 °C–250 °C and in the frequency range of 1 Hz–1 MHz. Frequency dependent imaginary impedance and electric modulus implies that La³⁺ substitution significantly affect hopping and relaxation mechanism of charge carriers. Cole-Cole plots (Z" vs Z') has been fitted with suitable equivalent circuit for both Co₂Z & La-Z. The obtained grain and grain boundary resistances confirmed that charge carriers experienced lower resistance in La-Z. Arrhenius plots of relaxation time (τ) vs 1/T showed parallel conduction mechanism in system. Complex electric modulus spectrum distinguished the capacitive contribution of grain, grain boundary and electric effect. A very high magnetodielectric effect at low frequencies has been observed for La-Z.     

Effect of sintering time on electronic properties of strontium hexaferrite

Polycrystalline M-type strontium hexaferrite was synthesized by a solid-state reaction, and the change in physical properties according to the change of sintering conditions was studied. Rietveld refinement analyses on the strontium hexaferrite sintered in different conditions indicate negligible change in the crystal structure, and the results from FE-EPMA indicate little change in stoichiometry. However, the results from FE-SEM measurements show grain growth in the ones sintered for longer time. In addition, this grain growth affects the decrease in the coercive field. In addition, results of impedance measurements show drastic difference in relaxation behavior. Double relaxation peaks appear in frequency domain in all samples. Double relaxation peaks appear due to grain and grain boundary effects, and two factors were confirmed through Z-view data fitting. As the grain size increases by longer sintering, the relaxation frequency shifts to the high frequency region, and overall normalized resistance decreases. From this result, only electrical properties were controlled without magnetic change through sintering condition control.     

Low-temperature sintering and microwave dielectric properties of 3Z2B glass–Al2O3 composites

A potential low temperature co-fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and Al₂O₃ filler was investigated with regard to phase development and microwave dielectric properties as functions of the glass content and sintering temperature. The densification mechanism for 3Z2B–Al₂O₃ composites was reported. The linear shrinkage of 3Z2B glass–Al₂O₃ composites exhibited a typical one-stage densification behavior. XRD patterns showed that a new crystalline phase, ZnAl₂O₄ spinel, formed during densification, indicating that certain chemical reaction took place between the 3Z2B glass matrix and the alumina filler. Meanwhile, several zinc borate phases, including 4ZnO·3B₂O₃, crystallized from the glass matrix. Both of the reaction product phase and crystallization phases played an important role in improving the microwave dielectric properties of composites. The optimal composition sintered at 850–950 °C showed excellent microwave dielectric properties: ?_r = ∼5.0, Q·f₀ = ∼8000 GHz, and τ_f = ∼−32 ppm/°C at ∼7.0 GHz.     

Low-temperature sintering and microwave dielectric properties of Ca5Co4(VO4)6 ceramics

A novel low temperature firing high Q microwave dielectric ceramic Ca₅Co₄(VO₄)₆ was prepared by the conventional solid-state reaction method. The phase purity, microstructure, and microwave dielectric properties were investigated. The Ca₅Co₄(VO₄)₆ ceramic sintered at 875 °C exhibited excellent microwave dielectric properties: Qxf = 95,200 GHz (at 10.6 GHz), τ_f = −63 ppm/°C, ɛ_r = 10.1, and its ɛ_r corrected for porosity was calculated as 11.1.     

Crystal structure and magneto-dielectric properties of Co-Zr co-substituted Co2Z hexaferrites

A series of Ba_1.5Sr_1.5Co_2+xZr_xFe_24-2xO₄₁ hexaferrites (x = 0.00, 0.01, 0.03, 0.05, 0.07, and 0.09) were successfully prepared by the conventional solid-state reaction method. It was found that as the Co²⁺ and Zr⁴⁺ ions entered the hexaferrite structure, the lattice parameters increased, whereas the relative density increased when x = 0.00-0.03 and then decreased. A suitable amount of substitution increased the DC resistivity, reduced the magnetic and dielectric losses, and made the ${\mu }^{\prime }$ and ${\epsilon }^{\prime }$ closer to each other. At x = 0.03, the relative density and DC resistivity of the samples reached their maxim. Besides, both the magnetic and dielectric losses were lowest within the frequency range of 10 MHz-1 GHz. Meanwhile, the hexaferrite was impedance matched to free space, and the miniaturization factor was about 15. Therefore, this low-loss ferrite with almost equal permeability and permittivity could be meaningful for antenna miniaturization and high-frequency applications.     

Low-temperature sintering and microwave dielectric properties of Nd(Co1/2Ti1/2)O3 ceramics using glass addition of oxides

The microstructures and microwave dielectric characteristics of complex perovskite Nd(Co_1/2Ti_1/2)O₃ ceramics with 60P₂O₅–15ZnO–5La₂O₃–5Al₂O₃–5Na₂O–5MgO–5Yb₂O₃ (PZLANMY) additions (1–4 wt%) prepared through the conventional solid-state route were investigated. It was found that Nd(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1210 °C owing to the sintering aid of PZLANMY-glass addition. At 1300 °C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 1 wt% of PZLANMY-glass addition possess a dielectric constant (ε_r) of 27, a Q×f value of 64,000 GHz and a temperature coefficient of resonant frequency (τ_f) of ?29 ppm/°C. The PZLANMY-glass doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices that require low sintering temperature.     

Low-temperature sintering and electrical properties of BBSZ glass-doped ZnO-based multilayer varistors

The effect of Bi₂O₃-B₂O₃-SiO₂-ZnO glass (denoted as BBSZ) content on the densification, microstructure, and electrical properties of ZnO-based multilayer varistors (MLVs) has been investigated. In ZnO-based (MLVs), BBSZ acted as a promoter in ZnO grain growth, which increased the grain size of ZnO, resulting in a decrease in breakdown voltage. Results showed that the adding of BBSZ glass had an improvement in densification and overall properties of MLVs. It was found that a maximum value of 5.45 g/cm³ of the bulk density and the optimum properties (V_1mA = 27.28 V, α = 40.33, I_L = 0.75 μA, I_p = 35 A) of ZnO-based MLVs were achieved with 3 wt% BBSZ glass sintered at 925°C for 3 hours.     

Growth of self-textured barium hexaferrite ceramics by normal sintering process and their anisotropic magnetic properties

In this report a simple and low-cost technique for growing highly textured barium hexaferrite ceramics without the need for flux or seed crystals to achieve grain orientation is demonstrated. Plate-like shaped barium hexaferrite particles were synthesized using a solid-state reaction process and then aligned under a weak magnetic field, followed by uniaxial compaction. The aligned hexaferrite particles appear to serve as seeds, forming textured grains during sintering. The development of texture was verified by X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and vibration sample magnetometer (VSM) measurements. The prepared high-quality hexaferrite ceramics exhibited good anisotropic magnetic properties, comparable to those of single crystal counterparts. A mechanism for the formation of the self-textured grain growth of the barium hexaferrite ceramics, which involves grain boundary and lattice diffusion and interface reaction processes, is proposed.     

Enhanced magnetodielectric properties of Co2Z ferrite ceramics for ultrahigh frequency applications achieved via Cr3+ ion doping

Phase formation, microstructure, magnetic properties, and dielectric properties of Ba_1.5Sr_1.5Co₂Fe₍₂₃−_x)Cr_xO₄₁ (0.0 ≤ x ≤ 1.0) ceramics, in which Fe³⁺ ions were substituted by Cr³⁺ ions, were systematically investigated. X-ray diffraction results reveal that Z-type hexagonal ferrite was formed by sintering at 1250 °C, and Cr³⁺ ions successfully enter lattice without destroying crystal structure. Analysis of the microstructure reveals that Cr³⁺ ion doping has significant effect on crystal micromorphology. Samples with x = 0.4 have the most homogeneous micromorphology and the highest sintering density of 5.12 g/cm³. In addition, under the influence of external magnetic field, all samples exhibit typical soft magnetic character and hysteresis characteristics, with saturation magnetization up to 63.86 emu/g (x = 0.6). Particularly, compared with undoped sample, Cr-doped samples have outstanding magnetic–dielectric properties. Firstly, with increasing Cr³⁺ amount, real part of the permeability (μ′) reaches the maximum value of 10.70 at x = 0.4, while cutoff frequency exceeds 2 GHz, and Snoek constant reaches ∼19.50 GHz. Furthermore, due to more homogeneous microstructure, samples with x = 0.4 have low magnetic loss and can maintain high quality factor (Q) over a broad frequency range. Moreover, real part of the permittivity (ε′) reaches the maximum value of 16.90 at x = 0.6, and dielectric loss remains lower than 0.013 for frequencies below 0.7 GHz. Consequently, magnetic–dielectric materials prepared in this work are expected to have extensive application prospects for ultrahigh-frequency devices.     

Low-temperature sintering of BF-PT-BZ ternary solid solutions with enhanced piezoelectric properties

The 0.34BiFeO₃-0.46PbTiO₃-0.2BaZrO₃ (BF-PT-0.2BZ) ternary solid solutions were prepared by the solid-state reaction methods. Different amounts of Li₂CO₃-Bi₂O₃ (LB) additives were introduced into the based materials after the calcination process. Upon using LB additives, the sintering temperature of BF-PT-0.2BZ ceramics was lowered to 950°C, while the relative density was enhanced to the highest of about 97% for LB of 1 wt%. XRD results indicate that BF-PT-0.2BZ ceramics exhibit the perovskite structure without detectable second phases. SEM images reveal that BF-PT-0.2BZ ceramics are well densified and the grain size is enhanced with the addition of LB. Moreover, the piezoelectric properties are enhanced significantly, achieving the highest d₃₃ and bipolar strain of 350 pC/N and 0.53%, respectively, for BF-PT-0.2BZ ceramics with LB of 1 wt%. Our results indicate that low-temperature sintered BF-PT-0.2BZ ceramics with excellent piezoelectric properties have promising applications in multilayer piezoelectric actuators.     

Low-temperature sintering of PZT ceramics

The required sintering temperature of Pb(Zr_0·52Ti_0·48)O₃ ceramics (abbreviated as PZT 52/48) can be lowered to about 1000°C by incorporating Li₂CO₃, Na₂CO₃ or Bi₂O₃. A dielectric constant of about 1000 and a planar coupling factor of between 45% and 65% are obtained in PZT 52/48 ceramics sintered at 1025°C, with added Li₂CO₃ and Bi₂O₃. The optimal amount of the additives, which can be deduced from the densification, the dielectric and piezoelectric properties of the sintered PZT 52/48 ceramics, is 0·375 wt% of Li₂CO₃ together with an equal mole fraction of Bi₂O₃. A planar coupling factor of 65% is obtained. This is explained, with the aid of X-ray diffraction (XRD) analysis, by a maximum c/a ratio and consequently by a large spontaneous polarization. The PZT 52/48 ceramics sintered with Li₂CO₃ and Bi₂O₃ under the optimal conditions can have ε₃₃^T of about 1000, k_p higher than 60%, Q_m around 100 and tan δ less than 2·0%.     

Low-temperature sintering of microwave ceramics with high Qf values through LiF addition

Microwave ceramic with low-sintering temperature is one of the most important classes of material to realize the integration and miniaturization of microwave devices. In this work, in order to simultaneously realize low-temperature sintering and good microwave dielectric properties, CaMgSi₂O₆–xLiF was sintered at various sintering temperatures using LiF as a sintering aid. In comparison to CaMgSi₂O₆ (x = 0) sintered at 1250°C, desirable microwave dielectric properties of ε_r = 7.45, Qf = 64 800 GHz, and τ_f = −34 ppm/°C and good chemical compatibility with the Ag electrodes, were achieved sintered at 900°C when adding 2 wt% LiF into CaMgSi₂O₆. Furthermore, a secondary phase, Li₂MgSiO₄, occurred at x ≥ 1 wt%, and the densest microstructure was obtained at the x value of ~2 wt%. We propose that the high Qf value and the low-sintering temperature were obtained through moderate LiF addition, which promotes densification and provides Li as the acceptor dopant. By further verifying in Mg₂SiO₄ ceramic, our study demonstrates that the approach of adding LiF can realize low-temperature sintering without jeopardizing the excellent microwave dielectric properties, and can potentially be applied in a wide range of low-temperature sintering of electronic ceramics.     

Low-temperature sintering and microwave dielectric properties of CaWO4 ceramics for LTCC applications

Microwave dielectric properties of CaWO₄ ceramics were investigated as a function of H₃BO₃ and/or Bi₂O₃ content and sintering temperature. For a single addition of H₃BO₃ (1 ≤ x (wt.%) ≤ 5), the density of specimen increased up to 3 wt.% H₃BO₃, and then decreased. The dielectric constant (K) and the quality factor (Q × f) of the specimens sintered at 850 °C showed lower value than those of specimens sintered above 900 °C due to the poor sinterability. With the increase of H₃BO₃ content of 0.5 wt.% Bi₂O₃–yH₃BO₃ (5 ≤ y (wt.%) ≤ 20), the sintering temperature of CaWO₄ ceramics could be effectively reduced from 1100 to 850 °C without degradation of dielectric properties. For the specimens sintered at 850 °C for 30 min, K was not changed remarkably with Bi₂O₃–H₃BO₃ content; however, Q × f value increased up to 9 wt.% H₃BO₃ of 0.5 wt.% Bi₂O₃–yH₃BO₃, and then decreased. The temperature coefficient of resonant frequency (TCF) shifted to the positive value with increasing Bi₂O₃–H₃BO₃ content. Typically, K of 8.7, Q × f of 70,220 GHz and TCF of −15 ppm/°C were obtained for the specimens with 0.5 wt.% Bi₂O₃–9 wt.% H₃BO₃ sintered at 850 °C for 30 min.     

Low-temperature sintering and electrical properties of (K,Na)NbO3 based lead-free ceramics with high Curie temperature

Lead-free ceramics (1 ? x)(K_0.48Na_0.52)NbO₃–(x/5.15)K_2.9Li_1.95Nb_5.15O_15.3 (x = 0.3–0.6, KNN–KLN100x) were prepared by conventional sintering technique at a low temperature of 960 °C. The effects of KLN contents on microstructure, dielectric, and piezoelectric properties were investigated. After the addition of KLN, the sintering performance and Curie temperature of the ceramics were markedly improved. The ceramics with x = 0.3 exhibited very good piezoelectric properties: d₃₃ = 138 pC/N, k_p = 45.03%, T_c = 495 °C, the dielectric constant at room temperature ?_r (RT) = 478 and the maximum dielectric constant ?_r (max) = 5067. These results indicated that the KNN–KLN100x lead-free ceramics sintered at low temperatures are promising for high temperature piezoelectric applications.     

Enhanced microwave absorption features of Ba3Co2Fe24O41 hexaferrite by high lanthanium doping concentration

Controlling material structure and its electromagnetic properties, including complex permittivity and permeability, could enhance the microwave absorption performance of the material in terms of reflection loss and effective absorption bandwidth. In this study, La-substituted barium hexaferrite, Ba_3−xLa_xCo₂Fe₂₄O₄₁ (x = 0, 0.1, 0.3, and 0.5) compounds were successfully prepared using the solid-state reaction method, and their corresponding microstructures, static magnetic properties, and electromagnetic features in 2–18 GHz were investigated. The doping of La content increased saturation magnetization, coercivity, and remnant magnetization. The Ba_2.7La_0.3Co₂Fe₂₄O₄₁ epoxied sample with 3.5 mm thickness possessed an excellent microwave absorption of −47.3 dB at 3.52 GHz, and its corresponding effective absorption bandwidths were 3.75 GHz (2.25–6 GHz) and 0.57 GHz (17.43–18 GHz). It is shown that doping with various La concentrations on Ba₃Co₂Fe₂₄O₄₁ can be used as an effective technique to tune the performance of microwave absorbers based on barium hexaferrite.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Touch-free reactive flash sintering of dense strontium hexaferrite permanent magnet

This work presents an extension of the touch-free flash sintering technique. In the proposed technique, chemical reaction and sintering occur in a single step, without the use of electrodes, in the presence of electric and magnetic fields. We show that a dense, single-phase strontium hexaferrite magnet can be produced from a mixture of commercial carbonate and oxide powders in a single step in a little more than a minute. This new technique implies significant reduction in energy and time consumption (primarily because of ultrafast processing) relative to conventional sintering.     

Low-temperature sintering and microwave dielectric properties of Ba5Nb4O15 with ZnB2O4 glass

The Influence of ZnB₂O₄ glass addition on the sintering temperature and microwave dielectric properties of Ba₅Nb₄O₁₅ has been investigated using dilatometry, X-ray diffraction, scanning electron microscopy and network analyzer. It was found that a small amount of glass addition to Ba₅Nb₄O₁₅ lowered the sintering temperature from 1400 to 900 °C. The reduced sintering temperature was attributed to the formation of ZnB₂O₄ liquid phase and B₂O₃-rich liquid phases such as Ba₃B₂O₆. The Ba₅Nb₄O₁₅ ceramics with ZnB₂O₄ glass, sintered at a low temperature, exhibited good microwave dielectric characteristics, i.e., a quality factor (Q × f) = 12,100 GHz, a relative dielectric constant (ɛ_r) = 40, a temperature coefficient of resonant frequency (τ_f) = 48 ppm/°C. The dielectric properties were discussed in terms of the densification of specimens and the influence of glassy phases such as Ba₃B₂O₆ and ZnB₂O₄.     

Low-temperature Oxidation of Carbon Monoxide on Co/ZrO2

A 10%Co/ZrO₂ catalyst prepared by impregnation was tested for its activity for the oxidation of CO to CO₂ in excess oxygen. Activity tests showed that conversion could be obtained at temperatures as low as 20 °C. Time-on-stream studies showed no loss of activity in these experiments, indicating that this catalyst is stable in the experimental oxidizing conditions. The activation energy for the CO to CO₂ oxidation reaction was calculated as E_a = 54 kJ/mol over this catalyst. Characterization of the material by thermogravimetric analysis, temperature-programmed techniques, X-ray photoelectron spectroscopy, and laser Raman spectroscopy indicate that Co₃O₄ is present on monoclinic ZrO₂ after the calcination of the catalyst.     

Low-temperature sintering and microwave dielectric properties of B2O3-added ZnO-deficient Zn2GeO4 ceramics for advanced substrate application

ZnO-deficient Zn_2-xGeO_4-x ceramics with 0.05?≤?x?≤?0.15 were synthesized because a ZnO secondary phase is formed in the stoichiometric Zn₂GeO₄ ceramics synthesized using micrometer-sized ZnO and GeO₂ powders. The Zn_1.9GeO_3.9 ceramic sintered at 1000?°C showed a homogeneous Zn₂GeO₄ phase with good microwave dielectric properties: ε_r of 6.8, Q?×?f of 49,000?GHz, and τ_f of ?16.7?ppm/°C. However, its sintering temperature was still too high for it to be used as an advanced substrate for low-temperature co-fired ceramic devices. Therefore, various amounts of B₂O₃ were added to the Zn_1.9GeO_3.9 ceramics to reduce their sintering temperature. Owing to the formation of a B₂O₃-GeO₂ liquid phase, these ceramics were well sintered at low temperatures between 925?°C and 950?°C. In particular, 15?mol% B₂O₃-added Zn_1.9GeO_3.9 ceramic sintered at 950?°C showed promising microwave dielectric properties for advanced substrates without the reaction with an Ag electrode: ε_r?=?6.9, Q?×?f?=?79,000?GHz, and τ_f?=??15?ppm/°C.