Low temperature sintering and microwave dielectric properties of Zn<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramic

Crystal structure and dielectric properties of Zn₃Mo₂O₉ ceramics prepared through a conventional solid-state reaction method were characterized. XRD and Raman analysis revealed that the Zn₃Mo₂O₉ crystallized in a monoclinic crystal structure and reminded stable up to1020 °C. Dense ceramics with high relative density (~ 92.3%) were obtained when sintered at 1000 °C and possessed good microwave dielectric properties with a relative permittivity (ε _r ) of 8.7, a quality factor (Q?×?f) of 23,400 GHz, and a negative temperature coefficient of resonance frequency (τ _f ) of around ??79 ppm/°C. With 5 wt% B₂O₃ addition, the sintering temperature of Zn₃Mo₂O₉ ceramic was successfully lowered to 900 °C and microwave dielectric properties with ε _r ?=?11.8, Q?×?f?=?20,000 GHz, and τ _f = ??79.5 ppm/°C were achieved.     

Solid-state reactions in the system Li<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>

The formation mechanisms of Li _x Na_{1 ?x} Ta _y Nb_{1 ? y} O₃ perovskite solid solutions in the Li₂CO₃-Na₂CO₃-Nb₂O₅-Ta₂O₅ system have been studied by x-ray diffraction, differential thermal analysis, thermogravimetry, IR spectroscopy, and mass spectrometry at temperatures from 300 to 1100°C. The results indicate that the synthesis of Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions involves a complex sequence of consecutive and parallel solid-state reactions. An optimized synthesis procedure for Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions is proposed.     

Enhancing the microwave dielectric properties of Sr<Subscript>2</Subscript>Ca<Subscript>3</Subscript>Ta<Subscript>4</Subscript>TiO<Subscript>17</Subscript> ceramics by Zr Substitution

The compositions in Sr₂Ca₃Ta₄Ti_1?xZr_xO₁₇ (0?≤?x?≤?0.12) series were designed and fabricated by solid state sintering method. All the compositions formed single phases and crystallized in an orthorhombic crystal structure. Zr substitution led to the enhancing of the microwave dielectric properties by tuning the τ_f value through zero and increased the Q_uf_o value from 12,540 to 14,970 GHz with a slight decrease in ε_r. In the present study, a good combination of ε_r ~?51, Q_uf_o ~?145,43 GHz and τ_f ~ 3 ppm/°C were obtained for Sr₂Ca₃Ta₄Ti_0.90Zr_0.1O₁₇ ceramic sintered at 1575 °C for 4 h.     

Magnetization reversal and tunable exchange bias behavior in Mn-substituted NiCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single phase samples of Ni(Cr_1?xMn _x )₂O₄ (x = 0–0.50) were synthesized by using sol–gel route. Investigation of structural, magnetic, exchange bias and magnetization reversal properties was carried out in the bulk samples of Ni(Cr_1?xMn _x )₂O₄. Rietveld refinement of the X-ray diffraction patterns recorded at room temperature reveals the tetragonal structure for x = 0 sample with I4₁/amd space group and cubic structure for x ≥ 0.05 samples with \( {\text{Fd}\bar{3}\text{m}} \) space group. Magnetization measurements show that all samples exhibit ferrimagnetic behavior, and the transition temperature (T_C) is found to increase from 73 K for x = 0 to 138 K for x = 0.50. Mn substitution induces magnetization reversal behavior especially for 30 at% of Mn in NiCr₂O₄ system with a magnetic compensation temperature of 45 K. This magnetization reversal is explained in terms of different site occupation of Mn ions and the different temperature dependence of the magnetic moments of different sublattices. Study of exchange bias behavior in x = 0.10 and 0.30 samples reveals that they exhibit negative and tunable positive and negative exchange bias behavior, respectively. The magnitudes of maximum exchange bias field of these samples are found to be 640 and 5306 Oe, respectively. Exchange bias in x = 0.10 sample originates from the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic moment. The tunable exchange bias behavior in x = 0.30 sample is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.     

Effect of Ni<Superscript>2+</Superscript> substitution on crystal structure and microwave dielectric properties for MgZrNb<Subscript>2</Subscript>O<Subscript>8</Subscript> ceramics

Monoclinic structured Mg_1?xNi_xZrNb₂O₈ (0?≤?x?≤?0.12) ceramics were synthesized for the first time through traditional solid-state reaction process and pure phase were obtained in all range. Rietveld refinement was used to analyze the crystal structure. With the increase of Ni²⁺ substitution amount, ε _r decreased, Q?×?f rose first then fell, τ _f shifted for the positive direction. Bond ionicity, lattice energy and bond energy were separately calculated to investigate the correlations with microwave dielectric properties. Typically, ceramics samples with the composition of Mg_0.92Ni_0.08ZrNb₂O₈ sintered at 1280 °C for 4 h exhibited the optimum microwave dielectric properties: ε _r ?=?24.58, Q?×?f?=?74534.1 GHz, τ _f ?=???49.11 ppm/°C, which could be a promising material for application.     

Influence of Nd doping on microwave dielectric properties of SrTiO<Subscript>3</Subscript> ceramics

Sr_1?x Nd _x TiO₃ (x?=?0.08–0.14) ceramics were prepared by conventional solid-state methods. The analysis of crystal structure suggested Sr_1?x Nd _x TiO₃ ceramics appeared to form tetragonal perovskite structure. The relationship between charge compensation mechanism, microstructure feature and microwave dielectric properties were investigated. Trivalent Nd³⁺ substituting Sr²⁺ could effectively decrease oxygen vacancies. This reduction and relative density were critical to improve Q?×?f values of Sr_1?x Nd _x TiO₃ ceramics. For ε _r values, incorporation of Nd could restrain the rattling of Ti⁴⁺ cations and led to the reduction of dielectric constant. The τ _f values were strongly influenced by tilting of oxygen octahedral. The τ _f values decreased from 883 to 650 ppm/°C with x increasing from 0.08 to 0.14. A better microwave dielectric property was achieved for composition Sr_0.92Nd_0.08TiO₃ at 1460 °C: ε _r ?=?160, Q?×?f?=?6602 GHz, τ _f ?=?883 ppm/°C.     

Structure and electrical properties of MnO doped (Ba<Subscript>0.96</Subscript>Ca<Subscript>0.04</Subscript>)(Ti<Subscript>0.92</Subscript>Sn<Subscript>0.08</Subscript>)O<Subscript>3</Subscript> lead free ceramics

In this work, (Ba_0.96Ca_0.04)(Ti_0.92Sn_0.08)O₃–xmol MnO (BCTS–xMn) lead-free piezoelectric ceramics were fabricated by the conventional solid-state technique. The composition dependence (0 ≤ x ≤ 3.0 %) of the microstructure, phase structure, and electrical properties was systematically investigated. An O–T phase structure was obtained in all ceramics, and the sintering behavior of the BCTS ceramics was gradually improved by doping MnO content. In addition, the relationship between poling temperature and piezoelectric activity was discussed. The ceramics with x = 1.5 % sintering at temperature of 1330 °C demonstrated an optimum electrical behavior: d ₃₃ ~ 475 pC/N, k _p ~ 50 %, ε _r ~ 4060, tanδ ~ 0.4 %, P _r ~ 10.3 μC/cm², E _c ~ 1.35 kV/mm, T _C ~ 82 °C, strain ~0.114 % and \(d_{33}^{*}\) ~ 525 pm/V. As a result, we achieved a preferable electric performance in BaTiO₃-based ceramics with lower sintering temperature, suggesting that the BCTS–xMn material system is a promising candidate for lead-free piezoelectric ceramics.     

Low temperature sintering and microwave dielectric properties of Li<Subscript>6</Subscript>Mg<Subscript>7</Subscript>Ti<Subscript>3</Subscript>O<Subscript>16</Subscript> ceramics with LiF additive for LTCC applications

Li₆Mg₇Ti₃O₁₆ ceramics were prepared by the conventional solid-state method with 1–5 wt% LiF as the sintering aid. Effects of LiF additions on the phase compositions, sintering characteristics, micro-structures and microwave dielectric properties of Li₆Mg₇Ti₃O₁₆ ceramics were investigated. The LiF addition could effectively lower the sintering temperature of Li₆Mg₇Ti₃O₁₆ ceramics from 1550 to 900 °C. For different LiF-doped compositions, the optimum dielectric permittivity (ε _r ) and quality factor (Q·f) values first increased and then decreased with the increase of LiF contents, whereas the temperature coefficient of resonant frequency (τ _f ) fluctuated between ??14.39 and ??8.21 ppm/°C. Typically, Li₆Mg₇Ti₃O₁₆ ceramics with 4 wt% LiF sintered at 900 °C exhibited excellent microwave dielectric properties of ε _r ?=?16.17, Q·f?=?80,921 GHz and τ _f ?=???8.21 ppm/°C, which are promising materials for the low temperature co-fired ceramics applications.     

Characterization of Zn doped MgTiO<Subscript>3</Subscript> ceramics: an approach for RF capacitor applications

In the present study the effect of Zn substitution on densification, microstructure, microwave and broad band dielectric properties of MgTiO₃ ceramics were investigated. The (Mg_1?x Zn _x )TiO₃ (x?=?0.01–0.07) ceramics have been prepared by the conventional solid-state reaction method. The sintering conditions were optimized to obtain the best dielectric properties with maximum relative densities. The microwave dielectric properties are heavily influenced by the amount of x concentration. The optimum dielectric properties of ε _r ~ 17.34, Q?×?f _o ~ 274 THz, τ _f ~ -40.3 ppm/^oC is obtained for (Mg_0.95Zn_0.05)TiO₃ ceramics sintered at 1275?°C. The broad band dielectric properties of (Mg_0.95Zn_0.05)TiO₃ ceramics were measured in the frequency range of 1–100 MHz, and temperature range of 133–483 K. Interestingly, the broad band dielectric properties show relaxation behaviour with frequency. The higher temperature dielectric spectrum of (Mg_0.95Zn_0.05)TiO₃ (MZT) ceramics displayed a distinct dispersion, which is shifting towards a lower frequency side. The observed dielectric relaxation behavior is analyzed using Cole–Cole plot. Furthermore, voltage dependent capacitance behavior at different frequencies is studied for the MZT sample, and it’s interesting to note that the capacitance is stable with the variation in voltage. The electrical conductivity study is carried out as a function of frequency and temperature for MZT sample and the activation energy is calculated by using Arrhenius equation, which is found to be 0.07 eV at 10 MHz. The obtained dielectric response of MZT ceramics are suitable for dielectric resonator and type-1 RF capacitor applications.     

Effect of sintering temperature on microstructures and microwave dielectric properties of Ba<Subscript>2</Subscript>MgWO<Subscript>6</Subscript> ceramics

The microwave dielectric properties of Ba₂MgWO₆ ceramics were investigated with a view to the use of such ceramics in mobile communication. Ba₂MgWO₆ ceramics were prepared using the conventional solid-state method with various sintering temperatures. Dielectric constants (? _r ) of 16.8–18.2 and unloaded quality factor (Q _u  × f) of 7000–118,200 GHz were obtained at sintering temperatures in the range 1450–1650 °C for 2 h. A maximum apparent density of 6.76 g/cm³ was obtained for Ba₂MgWO₆ ceramic, sintered at 1650 °C for 2 h. A dielectric constant (? _r ) of 18.4, an unloaded quality factor (Q _u  × f) of 118,200 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?34 ppm/°C were obtained when Ba₂MgWO₆ ceramics were sintered at 1650 °C for 2 h.     

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.     

Dielectric relaxation phenomena in the compound: LiCo<Subscript>3/5</Subscript>Mn<Subscript>1/5</Subscript>Cu<Subscript>1/5</Subscript>VO<Subscript>4</Subscript>

Solution-based chemical method has been used to produce LiCo_3/5Mn_1/5Cu_1/5VO₄ ceramics. The formation of the compound is checked by X-ray diffraction analysis and it reveals an orthorhombic unit cell structure with lattice parameters of a = 9.8262 Å, b = 3.0706 Å, c = 14.0789 Å. Field emission scanning electron micrograph indicates a polycrystalline texture of the material with grains of unequal sizes (~0.2 to 3 μm). Complex impedance spectroscopy technique is used to study the dielectric properties. Temperature dependence of dielectric constant (ε _r) at various frequencies exhibits the dielectric anomalies in ε _r at T _c (transition temperature) = 245, 255, 260 and 265 °C with (ε_r)_max. ~458, 311, 214 and 139 for 50, 100, 200 and 500 kHz, respectively. Frequency dependence of tangent loss at various temperatures shows the presence of dielectric relaxation in the material.     

Structural and dielectric properties of PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions synthesized at high pressures and temperatures

(1 ? x)PbMg_1/3Nb_2/3O₃ · xPbZrO₃ (1 ? x)PMN · xPZ) solid solutions have been synthesized at a pressure of 5 GPa and temperatures from 1300 to 1700 K, and their structural and dielectric properties have been studied. The composition dependences of the average unit-cell parameter and dielectric permittivity for the solid solutions indicate that the PMN-PZ system has a morphotropic phase boundary near x = 0.65. The solid solutions have a cubic structure for x < 0.65, a rhombohedral structure in the range 0.65 < x < 0.9, and an orthorhombic structure (similar to that of PbZrO₃) for x > 0.9. The temperature and frequency dependences of dielectric permittivity suggest that the (1 ? x)PMN · xPZ samples with x < 0.65 consist of two ferroelectric phases: a relaxor with antipolar dipole order and a normal ferroelectric with a diffuse phase transition. The effect of annealing temperature on the ferroelectric state of the samples with x < 0.65 is examined. In the composition range 0.65 < x < 0.9, the samples have normal ferroelectric properties, independent of annealing temperature.     

Phases and morphotropic regions in the PbNb<Subscript>2/3</Subscript> Mg<Subscript>1/3</Subscript>O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> system

Ceramic samples of solid solutions (1 ? x) PbNb_2/3Mg_1/3O₃ · xPbTiO₃ (0 ≤ x ≤ 1.0, Δx = 0.0025–0.05) are prepared by the columbite method. A detailed x?T phase diagram of the system is constructed (isothermal join at 25°C), and dielectric, piezoelectric, and elastic properties are investigated. It is established that the region of the morphotropic phase transition is positioned in the range 0.28 < x ≤ 0.43 and consists of a series of narrower regions. Inside one phase (cubic, rhombohedral, tetragonal), regions are found in which a qualitative and quantitative difference in structural and electrical parameters is observed. An interpretation of the observed effects in the context of the defect structure of the objects is suggested.     

Enhanced magnetoelectric coupling in La-modified Bi<Subscript>5</Subscript>Co<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>15</Subscript> multiferroic ceramics

Multiferroic properties of La-modified four-layered perovskite Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ (0 ≤ x ≤ 1) ceramics were investigated, by analyzing the magnetodielectric effect, magneto-polarization response and magnetoelectric conversion. X-ray diffraction indicated the formation of pure Aurivillius ceramics, and Raman spectroscopy revealed the Bi ions displacement and the crystal structure variation. The enhancement of ferromagnetic and ferroelectric properties was observed in Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ after La modification. The evidence for enhanced ME coupling was determined by magnetic field-induced marked variations in the dielectric constant and polarization. A maximum ME coefficient of 1.15 mV/cm·Oe was achieved in Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ ceramic, which provides the possible promise for novel magnetoelectric device application.     

Crystal structure,microstructure and microwave dielectric properties of novel MgAl<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>10</Subscript> ceramic

In the present work, a novel MgAl₂Ti₃O₁₀ ceramic was obtained using a traditional solid-state reaction method. X-ray diffraction and energy dispersive spectrometer showed that the main MgAl₂Ti₃O₁₀ phase was formed after sintered at 1300–1450 °C. With rising the sintering temperature from 1300 to 1450 °C, the bulk density (ρ), relative permittivity (ε _r ) and Q?×?f value firstly increased, reached the maximum values (3.61 g/cm³, 14.9, and 26,450 GHz) and then decreased. The temperature coefficient of resonator frequency (τ _f ) showed a slight change at a negative range of ??94.6 to ??83.7 ppm/°C. When the sintering temperature was 1400 °C, MgAl₂Ti₃O₁₀ ceramics exhibited the best microwave dielectric properties with Q?×?f?=?26,450 GHz, ε _r ?=?14.9 and τ _f ?=???83.7 ppm/°C.     

Ultralow thermal conductivity of cerium-doped Nd<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> over a wide doping range

In this paper, we report an ultralow thermal conductivity and a high-temperature phase stability of the (Nd_1?x Ce _x )₂Zr₂O_7+x system over the temperature range from room temperature to 1600 °C and over a wide composition range (0.2 ≤ x ≤ 0.8), and the (Nd_1?x Ce _x )₂Zr₂O_7+x system is therefore considered a strong candidate material for the fabrication of next-generation high-temperature thermal barrier coatings. The observed thermal conductivities (0.65–1.0 W/mK) are about 60–40% lower than those of undoped Nd₂Zr₂O₇ over the same temperature range (100–700 °C) and indicate a glass-like behavior. For comparison, the variation in the thermal conductivity with the temperature of the (Gd_1?x Ce _x )₂Zr₂O_7+x system with similar point defects was also measured, and the observed behavior was almost the same as that of undoped Gd₂Zr₂O₇ and was mostly determined by phonon–phonon scattering (λ ∝ 1/T). The effect of point defect scattering and strong phonon scattering sources (rattlers) on the thermal conductivity is also discussed in this paper. The results of this study suggest that the ultralow thermal conductivity of (Nd_1?x Ce _x )₂Zr₂O_7+x can be attributed to the presence of rattlers because of the large difference between the ionic radii of the Nd³⁺ and Ce⁴⁺ ions.     

Effects of LiF addition on the sintering behavior and microwave dielectric properties of Li<Subscript>2</Subscript>Mg<Subscript>3</Subscript>SnO<Subscript>6</Subscript> ceramics

Li₂Mg₃SnO₆ (abbreviation for LMS) ceramics doped with 1–4 wt% lithium fluoride (LiF) were prepared by the conventional solid-state reaction method. The effects of LiF addition on the phase compositions, sintering behaviors and microwave dielectric properties of LMS ceramics were investigated. A small amount of LiF addition could effectively decrease the sintering temperatures due to the liquid phase in the sintering process and induced no apparent degradation of the microwave dielectric properties. The optimized quality factor values for each composition firstly increased and then decreased with the increase of the LiF content. Whereas, the optimized dielectric permittivity increased with increasing of the LiF content. Distinguished microwave dielectric properties with a dielectric constant (ε _r) of 11.13, a quality factor (Q·f) of 104,750 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?10.83 ppm/°C were obtained for LMS ceramics sintered at 950?°C doped with 3 wt% LiF, which showed that the materials were suitable for the low temperature co-fired ceramics applications (LTCC).     

Modulus spectroscopy of lead potassium titanium niobate (Pb<Subscript>0.95</Subscript>K<Subscript>0.1</Subscript>Ti<Subscript>0.25</Subscript>Nb<Subscript>1.8</Subscript>O<Subscript>6</Subscript>) ceramics

The modulus Spectroscopy of Lead Potassium Titanium Niobate (Pb_0.95K_0.1Ti_0.25Nb_1.8O₆, PKTN) Ceramics was investigated in the frequency range from 45 Hz to 5 MHz and the temperature, from 30 to 600 °C. XRD analysis in PKTN indicated a orthorhombic structure with lattice parameters a = 18.0809 Å, b = 18.1909 Å and c = 3.6002 Å. The dielectric anomaly with a peak was observed at 510 °C. Variation of ε^I and ε^II with frequency at different temperatures exhibit high values, which reflects the effect of space charge polarization and/or conduction ion motion. The electrical relaxation in ionically conducting PKTN ceramic analyzed in terms of Impedance and Modulus formalism. The Cole–Cole plots of impedance were drawn at different temperatures. The dielectric modulus, which describes the dielectric relaxation behaviour is fitted to the Kohlrausch exponential function. Near the phase transition temperature, a stretched exponential parameter β indicating the degree of distribution of the relaxation time has a small value. From the AC conductivity measurements the activation energy near phase transition temperature (T _C°C) has been found to different from that of the above and below T _C. The temperature dependence of electrical modulus has been studied and results are discussed.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on the microstructure and electrical properties of LaMnO<Subscript>3</Subscript>-based NTC thermistors

A study to develop a new system of negative temperature coefficient thermistors for wide temperature range, A series of Mn-based perovskite-structured ceramics of composition (LaMn_1?x Al _x O₃)_0.9(Al₂O₃)_0.1 has been synthesized by conventional solid state reaction at 1350?°C. The X-ray diffraction patterns showed that for all the samples, the substitution of manganese by aluminum up to x?=?0.1 preserved the rhombohedral perovskite LaMnO₃-like phase. For x?=?0.2, apart from the LaMnO₃-like structure, a second perovskite phase based on the cubic LaAlO₃ structure was formed. For x?=?0.3 and 0.4, the phase present was LaAlO₃ -type structure. The grain sizes of the sintered body detected by scanning electron microscope were decreased with increasing Al₂O₃ content. The resistivity increases with increasing the Al content. The obtained values of ρ _25?°C and B _25/50 and E _a are in the range of 10–13103 Ω cm, 1813–2794 K, 0.156–0.241 eV, respectively. The resistance variation (ΔR/R) was <0.241% and the minimum value (0.0483%) was obtained for aging at 125?°C at 500 h. The aim of this work was explored new composite ceramics materials, which could be used as potential candidates for wide temperature range from ?100 to 500?°C thermistors applications.