Double cation (KMg)3+ co-substitution to regulate negative thermal expansion property of ln2W3O12

To improve the negative thermal expansion (NTE) performance of ln₂W₃O₁₂, a novel series of NTE (KMg)_xln_2-xW₃O₁₂ ceramics were fabricated via the solid-state method. The effects of (KMg)³⁺ substitution on the phase composition, microstructure and thermal expansion property of the ln₂W₃O₁₂ ceramics were characterized using X-ray diffraction (XRD), Raman spectrometer (Raman), X-ray photoelectron spectrometer (XPS), scanning electron microscopy (SEM), transmission electron microscope (TEM) and thermal mechanical analyzer (TMA). Results indicate that (KMg)³⁺ can partially replace In³⁺ in In₂W₃O₁₂ and form a new phase K_xMg_xln_2-xW₃O₁₂ with monoclinic symmetry. For x = 0.5, pure monoclinic (KMg)_0.5ln_1.5W₃O₁₂ ceramics is prepared and shows strong NTE. Its coefficient of thermal expansion is −7.89 × 10⁻⁶ °C⁻¹ in 30–700 °C, in addition, no phase transition was observed over the entire testing temperature range. These research results indicate that double cations co-substitution is an effective strategy to improve the NTE property of ln₂W₃O₁₂ through crystal structure modulation. This strategy could be extended to the performance modulation of other NTE materials.     

Sc- and W-substitution and tuning of phase transition in the Al2Mo3O12

Al₂Mo₃O₁₂ is a typical negative thermal expansion (NTE) material, whose thermal expansion behavior depends on its crystal phase. The thermal shock caused by temperature-induced phase transition limits its wide application. The two series of Al_{2. x}Sc_xMo₃O₁₂ (0 ≤ x ≤ 1) and Al₂Mo_3-xW_xO₁₂ (0 ≤ x ≤ 2.5) solid solutions with controllable phase transition temperature were synthesized via single cation substitution at the A or B position. The problem of thermal shock caused by the change of temperature is effectively solved in the synthesized Al_1.6Sc_0.4Mo₃O₁₂ and Al₂Mo_0.5W_2.5O₁₂, showing stable NTE performance above room temperature, and the coefficients of thermal expansion of which are ?2.19 × 10^?6 °C^?1 in 100–550 °C and ?4.25 × 10^?6 °C^?1 in 85–500 °C, respectively. A-site cation substitution is a more effective way to tune the thermal expansion properties of Al₂Mo₃O₁₂, which is attributed to the fact that the bond strength of A-O is weaker than that of B–O in the compound.     

Effect of strontium substitution on the structure and dielectric properties of unfilled tungsten bronze Ba4-xSrxSmFe0.5Nb9.5O30 ceramics

The effects of Sr²⁺ substitution for Ba²⁺ on phase structure, microstructure, dielectric and electric properties for Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2, 3 and 4) ceramics were systematically researched. X-ray diffraction patterns show that Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ (x = 0, 1, 2 and 3) ceramics are tetragonal tungsten bronze compound with a space group of P4bm, while the sample for x = 4 is an orthorhombic structure compound. The result can be corroborated by the analysis of Raman spectroscopy. As the Sr²⁺ contents increase from 0 to 3, the full width at half maximum of Raman lines of all samples increase gradually, indicating that the degree of lattice distortion increase. All tetragonal tungsten bronze ceramics exhibited a broad permittivity peaks, accompanied by frequency dispersion, indicating all samples are relaxor. The electrical properties of BSSFN ceramics were further studied by complex impedance spectroscopy. XPS spectrum shows that Fe²⁺ and Fe³⁺ coexist in Ba_4-xSr_xSmFe_0.5Nb_9.5O₃₀ ceramics, and their proportion varies with the concentration of Sr²⁺.     

High configurational entropy for low phase transition temperature and thermal expansion of A2M3O12 oxide ceramics

Transverse vibrations of bridging atoms in framework structure oxides contribute to negative thermal expansion (NTE), increasing the configurational entropy. Herein, the configurational entropy of NTE (Al_1/3Fe_1/3Cr_1/3)₂Mo₃O₁₂ (AFCM) is tuned by introducing ZrMg and W to AlFeCr and Mo sites to lower NTE. The NTE of ((Zr_1/2Mg_1/2)_x(Al_1/3Fe_1/3Cr_1/3)_(1-x))₂Mo₃O₁₂ (ZMAFCM) reduce obviously with increasing the content of ZrMg and also the phase transition temperatures (PTTs) (x = 0∼0.5). For ((Zr_1/2Mg_1/2)_x(Al_1/3Fe_1/3Cr_1/3)_(1-x))₂(Mo_1/2W_1/2)₃O₁₂ (ZMAFCMW), the NTE and PTTs reduce at a faster rate than that of ZMAFM. The configurational entropy increases with the content of ZrMg firstly (x = 0∼0.4) and then decreases. The possible mechanism of thermal expansion change is related to the enhanced lattice configuration, high entropy. The inconsistent transverse vibrations of bridging oxygen atoms could reduce their contribution to NTE, especially for high entropy. The PTT of high configurational entropy oxides is reduced obviously due to the influenced on the effective electronegativity. The investigation paves a high entropy way to lower thermal expansion and PTT of A₂M₃O₁₂ oxide ceramics and explores the further mechanism of NTE.     

Improving room temperature negative thermal expansion performance of Fe2-xScx(MoO4)3

Negative thermal expansion (NTE) performance of Fe₂(MoO₄)₃ is only found in a high-temperature range due to its monoclinic-to-orthorhombic (M-O) phase transformation temperature (PTT) at 503.5°C. To stabilize the orthorhombic phase of Fe₂(MoO₄)₃ at room temperature, a series of Fe_2-xSc_x(MoO₄)₃ (0≤x≤1.5) (abbreviated as F_2-xS_xM) were fabricated via solid-state reaction. Results indicate that the M-O PTT of Fe₂(MoO₄)₃ is successfully reduced from 503.5°C to 34.5°C by A-site cation substitution of Sc³⁺. The regulation mechanism is considered to be the decrease in electronegativity of (Fe_2-xSc_x)⁶⁺ in F_2-xS_xM. Both variable temperature X-ray diffraction (XRD) and thermal mechanical analysis (TMA) analysis results indicate that F_0.5S_1.5 M exhibits anisotropic NTE in 100–700°C. The results indicate that it can effectively improve the densification of Sc-substituted F_0.5S_1.5 M ceramics by two-step calcination process. Furthermore, higher second-step calcination temperature is beneficial for the formation of single-phased orthorhombic F_0.5S_1.5 M. The NTE response temperature range of F_0.5S_1.5 M ceramics second-step sintered at 1000°C is broadened to 30–600°C, and the corresponding coefficient of thermal expansion is -5.74 × 10⁻⁶°C⁻¹. The ease in the proposed design and preparation method makes NTE F_0.5S_1.5 M potential for a wide range of applications in precision mechanical, electronic, optical, and communication instruments.     

Negative thermal expansion coefficient of Sc-doped indium tungstate ceramics synthesized by co-precipitation

Co-precipitation was successfully applied to synthesize the Sc³⁺ doped In_2-xSc_x (WO₄)₃ (x = 0, 0.3, 0.6, 0.9 and 1.2) compounds. The composition- and temperature-induced structural phase transition and thermal expansion behaviors of Sc³⁺ doped In₂(WO₄)₃ were investigated. Results indicate that In_2-xSc_x (WO₄)₃ crystalizes in a monoclinic structure at 300 °C for x ≤ 0.3 and changes into hexagonal structure for x ≥ 0.6. Hexagonal In_1.1Sc_0.9(WO₄)₃ displays negative thermal expansion (NTE) with an average linear coefficient of thermal expansion (CTE) of −1.85 × 10⁻⁶ °C ⁻¹. After sintering at 700 °C and above, a phase transition from hexagonal to orthorhombic phase was observed in In_2-xSc_x (WO₄)₃ (x ≥ 0.6). Sc³⁺ doping successfully reduce the temperature-induced phase transition temperature of In_2-xSc_x (WO₄)₃ ceramics from 250 °C (x = 0) to room temperature (x = 0.9). When x = 0.9 and 1.2, the average linear CTEs of In_2-xSc_x (WO₄)₃ ceramics are −5.45 × 10⁻⁶ °C⁻¹ and -4.43 × 10⁻⁶ °C⁻¹ in a wider temperature range of 25–700 °C, respectively.     

Fabrication and characteristics of Ce-doped Y3Fe5O12 ceramics by hot-press sintering with oxygen/nitrogen atmosphere

Infrared transparent Ce-doped Y₃Fe₅O₁₂ (Ce_: YIG, Ce_xY_3-xFe₅O_12, x = 0, 0.12, 0.24, 0.36) ceramics were successfully produced by the solid-state reaction using a hot-press sintering process from the Y₂O₃, Fe₂O₃, and CeO₂ powders. The phase structure, microstructure, infrared transmittance, and magnetic and magneto-optical properties of the Ce-doped Y₃Fe₅O₁₂ ceramics were measured and analyzed. The in-line transmittances of the Ce-doped Y₃Fe₅O₁₂ ceramics with the x = 0, 0.12, 0.24 (L = 0.5 mm) at 1550 nm were about 72%, 66.5%, and 57.6%, respectively. In the state of saturation magnetization, the Faraday rotation angle per centimeter (θ_F) of Ce_xY_3-xFe₅O₁₂ (x = 0, 0.12, 0.24) ceramics measured by the light extinction method was 182.5, −410.4, and −958.3 deg./cm, respectively. The change of the θ_F was about −142.5 deg./cm when per 1at.% Ce was substituted in the dodecahedral site of YIG materials. The (Ce_0.24Y_2.76)Fe₅O₁₂ ceramics were determined as the optimized composition for its excellent infrared optical and magneto-optical properties.     

Structural and dielectric properties,and nonlinear electrical response of the CaCu3-xZnxTi4O12 ceramics: Experimental and computational studies

CaCu_3-xZn_xTi₄O₁₂ ceramics (x = 0, 0.05, 0.10) were successfully prepared by a conventional solid-state reaction method. Their structural and dielectric properties, and nonlinear electrical response were systematically inspected. The X-ray diffraction results indicated that single-phase CaCu₃Ti₄O₁₂ (JCPDS no. 75–2188) was obtained in all sintered ceramics. Changes in the lattice parameter are well-matched with the computational result, indicating an occupation of Zn²⁺ doping ions at Cu²⁺ sites. The overall tendency shows that the average grain size decreases when x increases. Due to a decrease in overall grain size, the dielectric permittivity of CaCu_3-xZn_xTi₄O₁₂ decreases expressively. Despite a decrease in the dielectric permittivity, it remains at a high level in the doped ceramics (~3,406–11,441). Besides retention in high dielectric permittivity, the dielectric loss tangent of x = 0.05 and 0.10 (~0.074–0.076) is lower than that of x = 0 (~0.227). A reduction in the dielectric loss tangent in the CaCu_3-xZn_xTi₄O₁₂ ceramics is closely associated with the enhanced grain boundary response. Increases in grain boundary resistance, breakdown electric field, and conduction activation energy of grain boundary as a result of Zn²⁺ substitution are shown to play a crucial role in improved grain boundary response. Furthermore, the XPS analysis shows the existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺, indicating charge compensation due to the loss of oxygen lattice. Based on all results of this work, enhanced dielectric properties of the Zn-doped CCTO can be explained using the internal barrier layer capacitor model.     

Thermal enhancement of up-conversion luminescence in Lu2W2.5Mo0.5O12: Er3+, Yb3+ phosphors

Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors were synthesized through high temperature solid state method. Under 980 nm laser excitation, the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ compounds show thermal enhancement of up-conversion luminescence (UCL), which is attributed to the lattice contraction and distortion from negative thermal expansion (NTE) of Lu₂W_2.5Mo_0.5O₁₂ host enhancing the energy transfer of Yb³⁺ to Er³⁺, eliminating the energy transfer of Er³⁺ to Er³⁺ through Er³⁺ single-doped Lu₂W_2.5Mo_0.5O₁₂ phosphors without thermal enhancement of UCL. The green luminescence intensities at 693 K of the Lu_1.98-xW_2.5Mo_0.5O₁₂: 0.02Er³⁺, xYb³⁺ (x = 0.2, 0.3, 0.4) samples are 4.6, 4.3 and 7.0 times as that of 302 K, respectively. And through fluorescence intensity ratio (FIR) technique, the corresponding maximum absolute sensitivities are 0.00741, 0.00744 and 0.00723, respectively. The green monochromaticity of UCL spectra in Er³⁺/Yb³⁺ co-doped samples increase with the increasing of temperature, and the possible UCL mechanism with temperature was discussed. The results indicate that the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors can be applied at a high temperature as optical thermometer with a good green monochromaticity.     

Structural dependence of microwave dielectric properties in ilmenite-type Mg(Ti1-xNbx)O3 solid solutions by Rietveld refinement and Raman spectra

Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were prepared by the conventional solid-state reaction method. The phase composition, sintering characteristics, microstructure and dielectric properties of Ti⁴⁺ replacement by Nb⁵⁺ in the formed solid solution Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were systematically studied. The structural variations and influence of Nb⁵⁺ doping in Mg(Ti_1-xNb_x)O₃ were also systematically investigated by X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction and its Rietveld refinement results confirmed that Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics crystallised into an ilmenite-type with R-3 (148) space group. The replacement of the low valence Ti⁴⁺ by the high valence Nb⁵⁺ can improve the dielectric properties of Mg(Ti_1-xNb_x)O₃ (x = 0–0.09). This paper also studied the different sintering temperatures for Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics. The obtained results proved that 1350 °C is the best sintering temperature. The permittivity and Q × f initially increased and then decreased mainly due to the effects of porosity caused by the sintering temperature and the doping amount of Nb₂O₅, respectively. Furthermore, the increased Q × f is correlated to the increase in Ti–O bond strength as confirmed by Raman spectroscopy, and the electrons generated by the oxygen vacancies will be compensated by Nb⁵⁺ to a certain extent to suppress Ti⁴⁺ to Ti³⁺, which was confirmed by XPS. The increase in τ_f from ?47 ppm/°C to ?40.1 ppm/°C is due to the increment in cell polarisability. Another reason for the increased τ_f is the reduction in the distortion degree of the [TiO₆] octahedral, which was also confirmed by Raman spectroscopy. Mg(Ti_0.95Nb_0.05)O₃ ceramics sintered at 1350 °C for 2 h possessed excellent microwave dielectric properties of ε_r = 18.12, Q × f = 163618 GHz and τ_f = ?40.1 ppm/°C.     

Expanding negative thermal expansion range of ZrMnMo3O12 to cover room temperature by introducing V5+

Solid solutions of Zr_1+xMn_1-xMo_3-2xV_2xO₁₂ (0 ≤ x ≤ 0.5) are developed with reduced phase transition temperature (from 362 to 160 K) by introducing V⁵⁺ into ZrMnMo₃O₁₂. Zr_1+xMn_1-xMo_3-2xV_2xO₁₂ adopt monoclinic (P2₁/a) and orthorhombic (Pbcn) structure at room temperature (RT) for x ≤ 0.1 and x ≥ 0.2, respectively. The formation of bond V–O induces a larger average effective negative charge on oxygen to enhance the repulsive force between them and then strengthens the bond of Mo–O, which reduces the phase transition temperature due to the reduction in effective electronegativity and expands negative thermal expansion (NTE) range covering RT. NTE property in a wide temperature range (from 160 to 673 K) for Zr_1.5Mn_0.5Mo₂VO₁₂ is realized, implying great potential for applications. The NTE property of the materials is induced by low-frequency phonons.     

Gd3+ doping induced microstructural evolution and enhanced visible luminescence of Pr3+ activated calcium fluoride transparent ceramics

Transparent Pr³⁺ doped Ca_1-xGd_xF_2+x (x = 0, 0.01, 0.03, 0.06, 0.10, 0.15) polycrystalline ceramics with fine-grained microstructures were prepared by the hot-pressing method. The dependence of microstructure, optical transmittance, luminescence performances and mechanical properties on the Gd³⁺ concentrations for Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics were investigated. The Gd³⁺ ions show positive effects on the microhardness of Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics as a result of the decrease in the grain sizes. Excited by the Xenon lamp of 444 nm, typical visible emissions located at 484 nm, 598 nm and 642 nm were observed. Furthermore, the incorporation of Gd³⁺ ions can greatly enhance the photoluminescence performance owing to the improvement in the concentration quenching effect. The quenching concentration of Pr³⁺ ions in CaF₂ transparent ceramics increased to 1 at.% as a result of the positive effect of Gd³⁺ codoping. The energy transfer mechanism of Pr³⁺ in the Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics has been investigated and discussed.     

Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y_2.8Yb_0.2)(Al_4.8Sc_0.2)O₁₂ (YS-YAG) showed the lowest thermal conductivity (1.58 Wm^?1K^?1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10^?6 K^?1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.     

Effects of W6+ substitution on crystal structure and microwave dielectric properties of Li3Mg2NbO6 ceramics

Li₃Mg₂(Nb_1-xW_x)O_6+x/2 (0 ≤ x ≤ 0.08) ceramics were synthesized by the solid-state reaction route. The effects of W⁶⁺ substitution on the phase composition, microstructure and microwave dielectric properties of Li₃Mg₂NbO₆ ceramics were investigated systematically. The XRD results showed that all the samples formed a pure solid solution in the whole doping range. The SEM iamges and relative density revealed the dense structure of Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics. The relationship between the crystal structure and dielectric properties of Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics was researched through polarizability, average bond valence, and bond energy. The substitution of W⁶⁺ for Nb⁵⁺ in Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics significantly promoted the Q × f values. In addition, the increase of W⁶⁺ content improved the thermal stability of the Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics. The Li₃Mg₂(Nb_0.94W_0.06)O_6.03 ceramics sintered at 1175 °C for 6h possessed excellent properties: ε_r ~ 15.82, Q × f ~ 124,187 GHz, τ_f ~ −18.28 ppm/°C.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Crystal structure,vibration characteristics and microwave dielectric properties of low loss MgMo1-xWxO4 solid solution ceramics

Solid-phase method was used to synthesize MgMo_1-xW_xO₄ (x = 0–0.15) ceramics. The influences of substitution Mo⁶⁺ with W⁶⁺ on crystal structure, vibration characteristics and microwave dielectric properties of MgMo_1-xW_xO₄ ceramics were comprehensively studied. X-Ray diffraction illustrated all samples exhibit single-phase monoclinic wolframite structure when x = 0–0.15, in which W⁶⁺ replaces Mo⁶⁺ sites formed solid solution. W⁶⁺ effectively improves sintering properties of the MgMoO₄, the average grain size and relative density were increased. Raman characterization reveals that suitable W⁶⁺ substitution amount leads to reduction of v₁ A_g peaks FWHM and the enhancement of specific v₃ A_g peak for Mo/WO₄ tetrahedron, which improves the ordered distribution of the crystal structure. The above combined effect results in the increased Q × f value, but has little influence of W⁶⁺ substitution on ε_r and τ_f for MgMoO₄. When x = 0.09, MgMo_0.91W_0.09O₄ ceramic sintered at 1050 °C has optimal microwave dielectric performance: ε_r = 7.21, Q×f = 90,829 GHz, τ_f = ?67 ppm/°C.     

Preparation,X-ray phase analysis and dielectric properties of Pb(Fe1-xCox)2/3W1/3O3 and Pb(Co1-yFey)1/2W1/2O3 (0 ≤ x,y ≤ 1) systems

In this work, the Pb(Fe_1-xCo_x)_2/3W_1/3O₃ (PFCW) and Pb(Co_1-yFe_y)_1/2W_1/2O₃ (PCFW) ceramics with 0 ≤ x, y ≤ 1 were successfully fabricated by a solid-state reaction process. X–ray diffraction phase analysis indicate the formation of two different series of solid solutions with a perovskite structure and with the substitution limits of Fe for Co (in PFCW) and Co for Fe (in PCFW) are x = 0.35 and y ≈ 0.05, respectively. Based on the results of dielectric study of the PFCW ceramics, it was shown that a crossover from relaxor ferroelectric to ferroelectric with a diffuse phase transition takes place at x = 0.10. In the case of PCFW ceramics, the observed dielectric maxima correspond to the phase transitions at 320 K and 256 K. The peculiarities of the temperature dependencies of the thermally stimulated depolarization currents of PFCW and PCFW solid solutions were studied and discussed.     

Effect of substituting Al3+ for Ti4+on the microwave dielectric performance of Mg2Ti1-xAl4/3xO4 (0.01 ≤ x ≤ 0.09) ceramics

In this paper, Mg₂Ti_1-xAl_4/3xO₄ ceramics (0.01 ≤ x ≤ 0.09) were synthesized through conventional solid-state ceramic route. The cubic spinel structure, microstructure and microwave properties of Mg₂Ti_1-xAl_4/3xO₄ (x = 0.01, 0.03, 0.05, 0.07, 0.09) ceramics were investigated by X-ray diffraction, Raman spectra, infrared spectra. Rietveld refinements confirm that a spinel structure phase with space group Fd-3m is formed. The variation of the permittivity was concerned with the ionic polarizability, and the value of τ_f was influenced by the bond valence. Both Q × f values and relative density showed an identical trend. Intrinsic properties of Mg₂Ti_1-xAl_4/3xO₄ ceramics were analyzed by infrared spectra and Raman spectra. In addition, the Mg₂Ti_1-xAl_4/3xO₄ ceramic sintered at 1420 °C for 4 h possessed optimal dielectric properties (ε_r = 14.65, Q × f = 182347 GHz, τ_f = −57.7 ppm/°C) when x = 0.09.     

Low-temperature firing and microwave dielectric properties of MgNb2-xVx/2O6-1.25x ceramics

MgNb_2-xV_x/2O_6-1.25x (0.1≤x≤0.6) ceramics with orthorhombic columbite structures were prepared at low-temperature by a solid-phase process. The phase component, microscopic morphology, low-temperature sintering mechanism and microwave dielectric performance of MgNb_2-xV_x/2O_6-1.25x ceramics were comprehensively investigated. Low-temperature sintering densification of dielectric ceramics was achieved via the nonstoichiometric substitution of vanadium (V) at the Nb-site. In contrast to pure MgNb₂O₆ ceramics, the sintering temperature of MgNb_2-xV_x/2O_6-1.25x (x = 0.2) ceramics was reduced by nearly 300 °C owing to the liquid-phase assisted sintering mechanism. The liquid phase arises from the autogenous low-melting-point phase. Meanwhile, MgNb_2-xV_x/2O_6-1.25x (x = 0.2) samples with nonstoichiometric substitution could achieve a more than 900% improvement in the Q × f value, compared with stoichiometrically MgNb_2-xV_xO₆ (x = 0.1, 0.2) ceramics. Finally, MgNb_2-xV_x/2O_6-1.25x dielectric ceramics possess outstanding microwave dielectric properties: ε_r = 20.5, Q × f = 91000, and τ_f = -65 ppm/°C when sintered at 1030 °C for x = 0.2, which provides an alternative material for LTCC technology and an effective approach for low-temperature sintering of Nb-based microwave dielectric ceramics.