Enhancing the microwave dielectric performance of SrSm2Al2O7 ceramic by Sr2+ nonstoichiometry and sintering aid addition

Sr_1+xSm₂Al₂O_7+x (0 ≤ x ≤ 0.05) ceramics were prepared by a conventional solid-state reaction method. Slight Sr²⁺ nonstoichiometry dramatically enhanced the microwave dielectric performance of the ceramics. Compared with the stoichiometric material, Sr-deficient ceramics show greatly enhanced microwave dielectric properties. For x = 0.03, the ceramics exhibited good microwave dielectric properties of ε_r = 18.31, Q × f = 78,000 GHz and τ_f = 2.28 ppm/°C. ZnO and LiF sintering aids were added to the ceramic to reduce the presintering temperature and enhance the microwave dielectric properties of the ceramics. After 0.25 wt% ZnO and 0.25 wt% LiF were added, the ceramics exhibited microwave dielectric properties of ε_r = 19.40, Q × f = 81,400 GHz and τ_f = 3.27 ppm/°C.     

Microwave dielectric properties of (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics

The densification, microstructural evolution and microwave dielectric properties of (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics with x=0, 0.25, 0.5 and 0.75 are investigated in this study. The sintering temperature of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ is significantly reduced from 1575 °C to 1400 °C as the x value increases from 0 to 0.25 and 0.50; this result is accompanied by the formation of the (Ba_1−ySr_y)WO₄ phase and a small quantity of second phase surrounding the grains. The grain size of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics is increased by raising the Sr²⁺ content, which significantly lowers the sintering temperature. The microstructure of the (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic displays the smallest average grain size of approximately 0.8 μm, with a narrow grain size distribution. Without long annealing time, very high Q×f values are obtained for the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics sintered at 1400–1575 °C for a duration of only 2 h. The (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic sintered at 1400 °C results in the best microwave dielectric properties, including ε_r of 20.6, Q×f of 152,600 GHz and τ_f of +24.0 ppm/°C.     

Microwave dielectric properties and infrared reflectivity spectra analysis of two novel low-firing AgCa2B2V3O12 (B = Mg,Zn) ceramics with garnet structure

Two Ag-containing microwave dielectric ceramics AgCa₂B₂V₃O₁₂ (B?=?Mg, Zn) with garnet structure were prepared through solid-state reaction method. Dense ceramics were obtained at low sintering temperatures, 665?°C for AgCa₂Zn₂V₃O₁₂ and 730?°C for AgCa₂Mg₂V₃O₁₂. Their microwave dielectric properties were characterized for the first time and analyzed by means of packing fraction, bond valence, octahedral distortion, Raman spectra and infrared reflectivity spectra. Both compounds displayed high chemical compatibility with Ag electrodes. Additionally, thermally stable ceramics with near-zero temperature coefficients of resonance frequency (τ_f) were achieved by forming ceramic composites with CaTiO₃.     

Structural,infrared reflectivity spectra and microwave dielectric properties of the Li7Ti3O9F ceramic

A cubic rock salt structured ceramic, Li₇Ti₃O₉F, was fabricated via the conventional solid-state reaction route. The synthesis conditions, sintering characteristics, and microwave dielectric properties of Li₇Ti₃O₉F ceramics were investigated by X-ray diffraction (XRD), thermal dilatometer, Scanning Electron Microscopy (SEM) accompanied with EDS mapping, and microwave resonant measurements. Rietveld refinement, selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) confirmed that Li₇Ti₃O₉F adopts a cubic rock-salt structure. The ceramic sintered at 950?°C presented the optimal microwave properties of ε_r?=?22.5, Q×f?=?88,200?GHz, and τ_f?=??24.2?ppm/^oC. Moreover, good chemical compatibility with Ag was verified through cofiring at 950?°C for 2?h. These results confirm a large potential for Li₇Ti₃O₉F ceramic to be utilized as substrates in the low temperature cofired ceramic (LTCC) technology. This work provides the possibility to exploit low-temperature-firing ceramics through solid solution between oxides and fluorides.     

Influence of Ti4+ substitution for Ta5+ on the crystal structure,Raman spectra,and microwave dielectric properties of Ba3Ta4-4xTi4+5xO21 ceramics

This paper systematically investigated the influence of Ti⁴⁺ substitution for Ta⁵⁺ on the phase composition and microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1, 0.2, and 0.3) ceramics with hexagonal tungsten bronze-like structures. X-ray diffraction and Rietveld refinement results indicated that single-phase Ba₃Ta₄Ti₄O₂₁ could be obtained only with the x values of 0.1 and 0.2, and a secondary phase was detected at an x value of 0.3. The valence state of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.2) ceramics was analyzed through X-ray photoelectron spectroscopy. Increasing Ti⁴⁺/Ta⁵⁺ ratios could reduce sintering temperature and improve the microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ solid solutions. However, the dielectric properties, particularly the quality factor, of Ba₃Ta_4-4xTi_4+5xO₂₁ ceramics deteriorated severely as a result of oxygen vacancy defects caused by the transition of the valence state from Ti⁴⁺ to Ti³⁺ when x = 0.2 and the coexistence of the secondary phase when x = 0.3. Infrared reflectivity spectroscopy was performed to explore the intrinsic dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics. The measured and extrapolated microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics sintered at 1240 °C for 6 h were ε_r ~ 46.5, Q × f = 13,900 GHz, τ_f ~ +49.4 ppm/°C, and ε_r ~ 44, Q × f = 34,850 GHz.     

Sintering characteristic,microstructure and microwave dielectric properties of the borax-added Sr3(VO4)2 ceramics

Na₂B₄O₇·10H₂O (borax) doped Sr₃(VO₄)₂ ceramics for ULTCC applications were synthesized by the solid-state reaction. The influence of borax addition on the sintering characteristic, microstructure, and microwave dielectric properties of Sr₃(VO₄)₂ ceramics was investigated in detail. The result indicated that borax was an effective sintering aid for the Sr₃(VO₄)₂ system and a suitable amount of borax dramatically reduced the sintering temperature of Sr₃(VO₄)₂ ceramics from 1000 to 675 °C. Meanwhile, borax addition prevented the Q × f value from getting degenerated and improved the τ_f value of Sr₃(VO₄)₂ ceramics in the case of ultra-lower sintering temperatures. Novel Sr₃(VO₄)₂ + 1 wt% borax ceramic sintered at 675 °C with optimum properties of Q × f = 19,200 GHz, ε_r = 15.9, and τ_f = 10.9 ppm/°C was achieved in this study.     

Microwave dielectric properties of Ca1-xBaxMgSi2O6 ceramics

Ca_1-xBa_xMgSi₂O₆(x = 0–0.4) ceramics were prepared through a traditional solid-state reaction sintering route with various sintering temperatures. The effects of substituting Ba²⁺ for Ca²⁺, the relative density, phase composition, crystal morphology, and microwave dielectric properties of Ca_1-xBa_xMgSi₂O₆ (x = 0–0.4) ceramics were thoroughly studied. X-ray diffraction patterns indicate a single phase was formed in the samples when x ≤ 0.2, and the second phase BaMg₂Si₂O₇ appeared at x = 0.4. As the amount of Ba²⁺ substitution increases, the Q×f value first increases and then decreases due to the combined effects of FWHM of peak v₁₁ and atomic packing density, and the ${\epsilon }_r$ value was increased continuously which was closely corrected with the relative density and molecular polarization. The ${\tau }_f$ value improved slightly with the substituting Ba²⁺ for Ca²⁺. Typically, the Ca_0.88Ba_0.12MgSi₂O₆ ceramic can be well sintered at 1275 °C for 4 h with a maximum relative density of 99.3%, and possesses optimal microwave dielectric properties: ${\epsilon }_r=7.49$, $Q\times f=64310$ GHz, ${\tau }_f=-44.02$ ppm/°C.     

Microwave dielectric properties of Li3A3Te2O12 (A = Y,Yb) garnets for low temperature cofired ceramic technologies

Te⁶⁺-containing microwave dielectric ceramics Li₃A₃Te₂O₁₂ (A = Y, Yb) with low firing temperatures were prepared using the solid-state reaction method. Li₃Y₃Te₂O₁₂ and Li₃Yb₃Te₂O₁₂ can be obtained as garnet single-phase ceramics with low sintering temperatures of 940 ℃ and 950 ℃, respectively. The cations, such as Li⁺, Te⁶⁺, and Y³⁺/Yb³⁺, fully occupied the tetrahedral, octahedral, and dodecahedral sites of garnet structure, respectively. Li₃A₃Te₂O₁₂ (A = Y, Yb) ceramics exhibit ε_r ～ 7.83 ± 0.2 and 5.94 ± 0.2, Q × f ～ 47,800 ± 500 GHz and 41,800 ± 500 GHz, and τ_f ～ –47 ± 3.0 ppm/°C and –76 ± 3.0 ppm/°C, respectively. The full width at half-maximum (FWHM) values of the A_1g Raman mode correlate negatively with the Q × f values. Moreover, Li₃A₃Te₂O₁₂(A = Y, Yb) ceramics possess good chemical compatibility with the Ag electrode, making them promising candidates for low-temperature cofired ceramics (LTCC) technologies.     

Crystal structure and dielectric properties of germanate melilites Ba2MGe2O7 (M = Mg and Zn) with low permittivity

Two melilite-structured ceramics, Ba₂MgGe₂O₇ and Ba₂ZnGe₂O₇, were fabricated using the simple solid-state sintering route and their microwave dielectric properties were reported for the first time. Rietveld refinements revealed that both ceramics crystallized in a tetragonal system with a space group P-42₁m. In contrast to Ba₂MgGe₂O₇ that was single phase in the whole sintering range, Ba₂ZnGe₂O₇ exhibited a second phase BaZnGeO₄ which was confirmed by SEM and Raman analysis. The sintering temperature could optimize the relative density and dielectric properties. Dense Ba₂MgGe₂O₇ and Ba₂ZnGe₂O₇ ceramics could be obtained at 1280?°C and 1180?°C with low relative permittivity(ε_r) of 7.76 and 9.0, high quality factor (Q?×?f) of 20,700 and 13,950?GHz, and negative temperature coe?cient of resonant frequency (τ_f) of ?55 and ?75?ppm/°C, respectively. Their differences in microwave dielectric properties were analyzed based on ionic polarizability and packing fraction. The thermal stability of Ba₂MgGe₂O₇ was tuned through the solid solution formation with TiO₂ and the τ_f value was adjusted successfully.     

A novel low-fired,temperature-stable,and low-cost (1−x)BaCu(B2O5)−xTiO2 microwave dielectric ceramic

BaCu(B₂O₅) is a typical microwave dielectric ceramic (MDC) with a low sintering temperature, but it exhibits a large negative temperature coefficient of resonant-frequency (τ_f) which makes it difficult to use in wireless communications. We employ TiO₂ to improve its temperature-stability of resonant-frequency, and reveal the effects of TiO₂ on the densification and the microwave dielectric properties of BaCu(B₂O₅). Here we show that BaCu(B₂O₅) can be well-sintered at 825 °C with proper TiO₂ additions; we find that the TiO₂ grains homogeneously distribute in the boundaries of BaCu(B₂O₅) grains, resulting in the τ_f compensation of BaCu(B₂O₅). Enhanced temperature-stability of resonant-frequency can be achieved by increasing the content of TiO₂ properly. A novel temperature-stable (1-x)BaCu(B₂O₅)–xTiO₂ (x = 0.20) MDC (τ_f =?0.8 ± 3.0 ppm/°C, ε_r = 8.8 ± 0.36, Q×f = 28,612 ± 1170 GHz) is obtained using some low-cost raw materials. Our results provide the underlying insights needed to guide the design of temperature-stable MDCs for wireless communication applications.     

Structural dependence of microwave dielectric performance of wolframite structured Mg1-xCaxZrNb2O8 ceramics: Crystal structure,microstructure evolution,Raman analysis and chemical bond theory

Wolframite-structured Mg_1-xCa_xZrNb₂O₈ (0?0.1) ceramics were synthesized through a solid-state procedure. Crystal refinement indicates a pure Mg_1-xCa_xZrNb₂O₈ ceramic with a wolframite structure. Ca²⁺ substitution led to the redshift of the Ag mode at approximately 896 cm^?1. Additionally, Ca²⁺ substitution could promote grain growth and contribute to microstructure evolution from a polyhedral shape to a rod shape. According to chemical bond theory, an appropriate Ca²⁺ concentration can increase NbO bond iconicity and NbO bond lattice energy, which contributed to the great improvement in the dielectric constant and Q × f value. Additionally, the τ_f value was affected by the bond valence and thermal expansion coefficient of the MgO bond. The Mg_1-xCa_xZrNb₂O₈ (x = 0.04) ceramics exhibited great improvement in the Q × f value: ε_r = 25.21, Q × f = 116,000 GHz (@7.17 GHz) and τ_f = ?24.4 ppm/°C, which provides enormous potential for future millimeter-wave applications.     

Synthesis,lattice energy and microwave dielectric properties of BaCu2-xCoxSi2O7 ceramics

BaCu_2-xCo_xSi₂O₇ solid solutions with orthorhombic structure (Pnma) were prepared by solid-state reaction method. The phase synthesis process, structural evolution and microwave dielectric properties of BaCu_2-xCo_xSi₂O₇ ceramics were investigated. Single BaCu₂Si₂O₇ phase was obtained when calcined at 950 °C for 3 h and was decomposed into BaCuSi₂O₆ phase when calcined at 1075 °C for 3 h. The sintering process was effectively promoted when Cu²⁺ was replaced by Co²⁺ and the maximum solubility of BaCu_2-xCo_xSi₂O₇ was located between 0.15 and 0.20. P-V-L complex chemical bond theory and Raman spectra were used to explain the structure-property correlations of BaCu_2-xCo_xSi₂O₇ ceramics. The corrected dielectric constant (ε_r-corr) of BaCu_2-xCo_xSi₂O₇ ceramics decreased monotonously with the susceptibility (Σχ^μ) and ionic polarizability of primitive unit cell. The quality factor (Q × f) increased with bond strength and lattice energy (U_cal), especially the lattice energy of the Si-O bond. The temperature coefficient of resonant frequency (τ_f) was determined by the susceptibility and lattice energy of the Cu/Co-O bond. The following optimum microwave dielectric properties were obtained at x = 0.15 when sintered at 1000 °C for 3 h: ε_r = 8.45, Q×f =58958 GHz and τ_f = -34.4 ppm/°C.     

Structure and microwave dielectric properties of BaAl2−2xLi2xSi2O8-2x ceramics

BaAl_2-2xLi_2xSi₂O_8-2x (x = 0, 0.005, 0.0075, 0.01, 0.02, 0.03) ceramics were synthesized by solid-state sintering method. Based on density functional theory, the first-principle calculations provided by the Cambridge Sequential Total Energy Package (CASTEP) software were introduced to the BaAl₂Si₂O₈ (BAS) system. In an effort to confirm the site occupied by Li⁺, we discussed the formation energy and final energy of different positions of Li⁺ doped BAS. The result demonstrated that Li⁺ should substitute Al³⁺ to promote the hexacelsian-to-celsian transformation with the aid of generated oxygen vacancies. The sintering behavior, crystal structure, surface appearance, and microwave dielectric properties of samples were investigated. Completely transformed celsian could be obtained when x = 0.005–0.03, which lowered the sintering temperature from 1400 °C (x = 0) to 1300 °C (x = 0.03), as well as strikingly improved the compactness, quality factor (Q × f) value and temperature coefficient of resonant frequency (τ_f) of BAS ceramics. When x = 0.1, unveiling the significant effects of Al-position ion substitution, BaAl_1.98Li_0.02Si₂O_7.98 ceramic sintered at 1350 °C for 5 h exhibited a supreme Q × f value of 48,620 GHz, and the ε_r and τ_f values were 6.99 and -23.29 × 10^?6 °C^?1, respectively.     

Contrasting microwave dielectric properties of zircon-structured AEuV2O8 (A = Bi,La) ceramics

Two zircon-structured ceramics AEuV₂O₈ (A = Bi, La) were prepared with optimal sintering temperatures of 900 °C and 1375 °C, respectively. They exhibited sharp contrast performances with medium ε_r ～ 28.7 ± 0.1, Q × f ～ 14,000 ± 300 GHz, and large positive τ_f ～ 75.7 ± 2.0 ppm/°C for BiEuV₂O₈, whereas low ε_r ～ 10.4 ± 0.1 Q × f ～ 25,100 ± 300 GHz, and negative τ_f ～ ? 40.7 ± 2.0 ppm/°C for LaEuV₂O₈. The rattling effect at the A-site was more dominant in determining microwave dielectric properties than that of compressed V⁵⁺ at the B-site. It resulted in the higher ε_r, lower Q × f and τ_ε, and larger τ_f of BiEuV₂O₈ than those of LaEuV₂O₈. Besides, their Q × f was related with the relative density, bond valence and FWHM of the B_1 g Raman mode.     

Synthesis of CaAl2xB2O4+3x: Novel microwave dielectric ceramics with low permittivity and low loss

Novel CaAl_2xB₂O_4+3x (x = 0.25, 0.5, 0.75, and 1) ceramics were prepared via solid-state reaction method. The investigation is concentrated on sintering behavior, phase composition and dielectric properties of CaAl_2xB₂O_4+3x ceramics with various ratios of Al₂O₃ content. The optimal sintering temperature and the content of CaAl₂B₂O₇ are highly related to the variation of x value. CaAl_2xB₂O_4+3x ceramics possess excellent microwave dielectric properties: ε_r = 5.8 ± 0.05, Q × f = 63,338 ± 2690 GHz (@13.57 GHz), and τ_f=-29 ± 2 ppm/℃ when sintered at 940 ℃ for x = 0.5. Comparing the properties of CaAl_2xB₂O_4+3x ceramics with reported borate ceramics, the ceramics prepared in this study with low dielectric loss and low sintering temperature have promising prospects in LTCC technology.     

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.     

Effects of ionic polarizability and crystal structure on microwave dielectric properties of RE2O3 (RE = La,Eu) ceramics with opposite τf and high Q

Hexagonal La₂O₃ and monoclinic Eu₂O₃ ceramics were prepared, and their microwave dielectric properties were investigated. La₂O₃ sintered at 1400 °C exhibited promising microwave dielectric properties of ε_r = 18.6, Q×f = 71,400 GHz, and a negative τ_f of − 35.1 ppm/°C, while Eu₂O₃ sintered at 1500 °C possessed relative lower ε_r and Q×f values of 17.9 and 35,000 GHz, respectively, with an abnormally positive τ_f of + 19.6 ppm/°C. The difference in their microwave dielectric properties is mainly due to lattice-induced strain, which can be characterized by bond valence. To investigate the degradation of RE₂O₃ (RE = La, Eu) ceramics in air, a series of La_2−xEu_xO₃ (x = 0.5, 1, and 1.5) ceramics were prepared. The results of the present study suggest that the introduction of Eu³⁺ effectively prevents the decomposition of La₂O₃.     

Preparation and microwave dielectric properties of BaLi1+xF3+x (x = 0–0.01) ceramics

BaLi_1+xF_3+x (x = 0–0.01) were successfully mechanosynthesized by a simple ball-milling process. The effects of excessive LiF and sintering method and/or annealing atmosphere on its sintering behavior, microstructure, and microwave dielectric properties have been investigated in this paper. The mechanosynthesized powder can be densified with relative densities of ∼95 % after sintering at 750–800 °C/2 h in N₂. The obtained ceramics exhibit excellent optimized microwave dielectric properties with ε_r of ∼11.46 ± 0.06, Q×f values of 83175 ± 1839 GHz and τ_f of ∼ − 70 ± 3 ppm/°C at the x = 0.006 composition. Its Q×f value could be improved to 94603 ± 2037 GHz) by post-annealing in N₂ after post annealing at 700 °C/2 h. The Q×f value could be further improved to (120,098 ± 2344 GHz) by hot-pressed sintering (HPS). Sintering in the ambient atmosphere or O₂ leads to lower Q×f values than those of the counterparts sintered in N₂ due to the introduction of F-vacancies by oxidation_, while little variation in ε_r andτ_f.     

Correlation between crystal structure and microwave dielectric properties of two garnet-type ceramics in rare-earth-free gallates

Two garnet-type rare-earth-free ceramics, Ca₃M₂SiGa₂O₁₂ (M = Sn and Zr), were prepared through a solid-state reaction method. The relationship between crystal structure and microwave dielectric properties was investigated. The larger deviation of ε_r from ε_theo in Ca₃Zr₂SiGa₂O₁₂ could be ascribed to the rattling Zr⁴⁺. The increase in packing fraction and the decrease in FWHM enhance the Q × f value by substituting Zr⁴⁺ with Sn⁴⁺. The smaller oxygen bond valence in Ca₃Zr₂SiGa₂O₁₂ indicates a smaller τ_f value. Good microwave dielectric properties are obtained with ε_r = 9.14 ± 0.02, Q × f = 106,800 ± 1700 GHz and τ_f = -45.8 ± 1.8 ppm/°C for Ca₃Sn₂SiGa₂O₁₂ and ε_r = 11.98 ± 0.03, Q × f = 84,200 ± 1500 GHz, and τ_f = -32.8 ± 1.4 ppm/°C for Ca₃Zr₂SiGa₂O₁₂. Furthermore, near-zero τ_f values of +5.7 ± 1.9 ppm/°C and +4.5 ± 1.6 ppm/°C appear in 0.95Ca₃Sn₂SiGa₂O₁₂-0.05CaTiO₃ and 0.96Ca₃Zr₂SiGa₂O₁₂-0.04CaTiO₃, respectively.     

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

Low-permittivity ZnAl_2-x(Zn_0.5Ti_0.5)_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 [Zn_0.5Ti_0.5]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 (ε_r = 9.1, Q × f = 115,800 GHz and τ_f = −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.