Microwave Dielectric Ceramics Research Articles

Crystal structure, bond characteristics and microwave dielectric properties of Ce2[Zr1-x(Sr1/3B2/3)x]3(MoO4)9 (B = Ta, Sb) solid solutions

A series of Ce2[Zr1-x(A)x]3(MoO4)9 (A = Sr1/3Ta2/3, Sr1/3Sb2/3; x = 0.02, 0.04, 0.06, 0.08 and 0.10) (abbreviated as CZ1-xTx and CZ1-xSx) microwave dielectric ceramics were studied systematically with respect to microstructures, sintering behaviors, crystal structures, microwave dielectric properties and infrared vibrational modes. Density measurements and scanning electron microscopy (SEM) reveal the dense and homogeneous microstructures. X-ray diffraction (XRD) results confirm the formation of solid solutions belonging to the hexagonal system with a R3‾c space group, and the lattice volume increases with raising the doping content. Furthermore, the complex chemical bond theory was used to calculate the intrinsic bond parameters, and the correlations between the microwave dielectric properties and intrinsic bond parameters were established. Finally, far infrared (FIR) spectra reveal that the main polarization contribution stems from the absorptions of the phonon oscillations in these two groups of ceramics. Typically, the optimum microwave dielectric properties are achieved for CZ0.9T0.1 and CZ0.94S0.06 ceramics with εr = 10.24, Q × f = 92,009 GHz (at 9.70 GHz), τf = −8.84 ppm °C−1 and εr = 10.40, Q × f = 82,696 GHz (at 9.69 GHz) and τf = −8.04 ppm °C−1, respectively. Interestingly, the CZ0.9T0.1 ceramic exhibits the maximum Q × f value in the ion doped Ce2Zr3(MoO4)9 ceramics while retaining a good τf = −8.84 ppm °C−1.

Sintering behavior, crystal structure, and microwave dielectric properties of a novel diopside SrMgGe2O6 ceramic and adjustment of its τf value

A solid-state process was used to synthesize novel microwave dielectric SrMgGe2O6 ceramics. X-ray diffractometry revealed that these ceramics have monoclinic structures that belong to the C2/c space group. A dense SrMgGe2O6 ceramic was obtained at the optimal sintering temperature of 1225 °C, the ceramic exhibited excellent microwave dielectric properties, with εr = 8.58, Q×f = 56800 GHz, and τf = −59.06 ppm/°C. The τf value is affected by the bond valence and bond energy of the Ge–O bond. The addition of CaTiO3 or TiO2 successfully adjusted the τf value to approximately zero, with the optimal CaTiO3- and TiO2-included composite ceramics exhibiting excellent dielectric properties, with εr = 11.74, Q×f = 22056 GHz, τf = +0.18 ppm/°C, and εr = 12.32, Q×f = 20930 GHz, τf = −2.27 ppm/°C, respectively.

Entropy regulation in LaNbO4-based fergusonite to implement high-temperature phase transition and promising dielectric properties

High-entropy effect is a novel design strategy to optimize properties and explore novel materials. In this work, (La_1/5Nd_1/5Sm_1/5Ho_1/5Y_1/5)NbO₄ (5RNO) high-entropy microwave dielectric ceramics were successfully prepared in the sintering temperature (S.T.) range of 1210–1290 ℃ via a solid-phase reaction route, and medium-entropy (La_1/3Nd_1/3Sm_1/3)NbO₄ and (La_1/4Nd_1/4Sm_1/4Ho_1/4)NbO₄ (3RNO and 4RNO) ceramics were compared. The effects of the entropy (<i>S</i>) on crystal structure, phase transition, and dielectric performance were evaluated. The entropy increase yields a significant increase in a phase transition temperature (from monoclinic fergusonite to tetragonal scheelite structure). Optimal microwave dielectric properties were achieved in the high-entropy ceramics (5RNO) at the sintering temperature of 1270 ℃ for 4 h with a relative density of 98.2% and microwave dielectric properties of dielectric permittirity (<i>ε</i>_r) = 19.48, quality factor (<i>Q</i>×<i>f</i>) = 47,770 GHz, and resonant frequency temperature coefficient (<i>τ</i>_f) = –13.50 ppm/℃. This work opens an avenue for the exploration of novel microwave dielectric material and property optimization via entropy engineering.

New microwave dielectric ceramics based on the CaRNbWO8 (R = Y, La, Ce, Nd or Sm) system

New microwave dielectric ceramics based on the CaRNbWO8 (R = Y, La, Ce, Nd or Sm) system

Crystal structure, far-infrared spectroscopy, and microwave dielectric properties of two novel vanadates CaRECeV3O12 (RE = Nd, Sm)

Two novel low-εr CaRECeV3O12 (RE = Nd, Sm) microwave dielectric ceramics were prepared to utilize the solid-state method. According to XRD and Raman spectroscopy data, the CaRECeV3O12 (RE = Nd, Sm) ceramics had a tetragonal zircon-type with an I41/amd space group. Dense CaNdCeV3O12 and CaSmCeV3O12 ceramics sintered at 1225 °C and 1200 °C showed εr ∼11.85 ± 0.1 and 11.33 ± 0.1, Q × f ∼40,100 ± 500 GHz and 43,290 ± 500 GHz, τf ∼ −27.2 ± 0.5 ppm/°C and −22.4 ± 0.5 ppm/°C, along with αL of 9.4 ppm/°C and 8.5 ppm/°C, respectively. The measured εr and corrected εr(corr) values were larger than εr(C-M) (9.41 and 9.47) in CaNdCeV3O12 and CaSmCeV3O12 due to the cations being, on average, under-bonded and the overall structures being in a slightly expanded state. The slightly higher |<d>| value of CaNdCeV3O12 led to its higher εr(corr) value, even though it had a lower αD/Vm value. The sign and magnitude of τf values for CaNdCeV3O12 and CaSmCeV3O12 were mainly determined by their positive temperature coefficients of dielectric polarizability ταm of +36.2 ppm/°C and +32.1 ppm/°C, respectively. The inherent dielectric constant and dielectric loss were examined using far-infrared reflectivity spectra, Raman spectra, and the packing fraction.

NaYW2O8: A novel glass-free microwave dielectric ceramic for LTCC application

Microwave dielectric ceramics are important materials for wireless communication devices because of their excellent dielectric properties. Tungstenate compounds with multiple scheelite structures can be used in low temperature co-fired ceramic (LTCC) devices because of their low densification temperature and high-quality factor. In this study, NaYW2O8 ceramics (NYW) were synthesized by a solid-state reaction method at a low densification temperature below the Ag melting point. The microstructure, morphology and microwave dielectric properties of NYW ceramics were investigated, and the effects of packing fraction and lattice energy on microwave dielectric properties were studied. The results showed that the microwave dielectric properties of NYW ceramics were influenced by the grain size and porosity, and deteriorated with the increase of porosity. In addition, the stacking fraction and lattice energy shared a similar trend with the quality factor. The NYW ceramics sintered at 925 °C exhibited excellent microwave dielectric properties with εr= 10.8, Q × f = 49,800 GHz (at 10.5 GHz), and τf= −26.5 ppm/°C. In particular, the NYW ceramics were found to have an excellent chemical compatibility with Ag. The above results indicate the NYW ceramics are candidate in LTCC applications.

Effect of Nb5+ doping on microwave dielectric properties of MgTa2O6 ceramic

As the main carrier for microwave circuits, microwave dielectric ceramics are receiving a lot of attention as the main carrier for the current 5G and pre-developed 6G communication technologies. A series of Nb5+ doped MgTa2O6 ceramics were synthesized by the solid-state reaction method X-ray diffraction confirmed that a single tetragonal structure can be formed when the doping amount is lower than 0.03, a heterogeneous structure of tetragonal and orthorhombic crystal structure is formed when the doping amount is lower than 0.15, and the tetragonal structure is completely transformed into orthorhombic structure when the doping exceeds 0.15. SEM results also show the transformation process of the grains of tetragonal structure to orthorhombic structure. The optimum microwave dielectric properties were obtained for 9% Nb5+ doped samples: εr = 24, Q×f = 71,000 GHz (at 8.9 GHz), TCF = 15 ppm/oC.

Recent progress in dielectric resonator antenna: Materials, designs, fabrications, and their performance

Dielectric resonator antenna (DRA) has secured an esteemed position in the field of antenna engineering technology due to its versatile features, including compactness, light-weight, low loss, high radiation efficiency, ease of excitation, diverse feeding techniques, multiple excitation modes, ease of integration with passive and active microwave integrated circuit components, a wide variety of available materials, simple fabrication techniques, and high degree of flexibility over wide frequency range. This article presents a comprehensive up-to-date review of research carried out in the field of DRA technology which includes an overview of DRA technology, its history, present status, and prospects. The basic parameters of microwave dielectric ceramic materials and the classification of materials based on utilization and material parameters are briefly discussed. The applications of DRA in different fields of science, engineering, and technology, overview of different reported DRA shapes, and the feeding techniques used to excite different modes of DRA are mentioned. Based on the control of DRA permittivity, shape/geometry, feeding techniques, and structural modifications, performance parameters engineering of DRA and its achievable performance in terms of low profile, compactness, high gain, wide beam, circular polarization, wide bandwidth, DRA decoupling methods for multiple input multiple output arrays, dielectric patch antenna, filtering DRA, and reconfigurable DRA are discussed. Different techniques used to attain and enhance the particular performance of DRA are highlighted. This review article also dilates upon the fabrication processes and characterization methods of DRA microwave dielectric ceramics, which play a very vital role in controlling the performance and properties of DRA.

Novel Method to Achieve Temperature-Stable Microwave Dielectric Ceramics: A Case in the Fergusonite-Structured NdNbO4 System.

Microwave dielectric ceramics with permittivity (εr) ∼ 20 play an important role in massive multiple-input multiple-output (MIMO) technology in 5G. Although fergusonite-structured materials with low dielectric loss are good candidates for 5G application, tuning the temperature coefficient of resonant frequency (TCF) remains a problem. In the present work, smaller V5+ ions (rV = 0.355 Å, with coordination number (CN) = 4) were substituted for Nb5+ (rNb = 0.48 Å with CN = 4) in the Nd(Nb1-xVx)O4 ceramics, which, according to in situ X-ray diffraction data, lowered the fergusonite-to-scheelite phase transition (TF-S) to 400 °C for x = 0.2. The thermal expansion coefficient (αL) of the high-temperature scheelite phase was +11 ppm/°C, whereas for the low-temperature fergusonite phase, it was + 14 < αL < + 15 ppm/°C. The abrupt change in αL, the associated negative temperature coefficient of permittivity (τε), and the minimum value of εr at TF-S resulted in a near-zero TCF ∼ (+7.8 ppm/°C) for Nd(Nb0.8V0.2)O4 (εr ∼ 18.6 and Qf ∼ 70,100 GHz). A method to design near-zero TCF compositions based on modulation of τε and αL at TF-S is thus demonstrated that may also be extended to other fergusonite systems.

Synergistic modification of properties in Ba[(Zn0.8Mg0.2)1/3Nb2/3]O3 ceramics through ordered domain engineering

AbstractThe ordered domain engineering was investigated for Ba[(Zn0.8Mg0.2)1/3Nb2/3]O3 microwave dielectric ceramics to synergistically modify the physical properties especially the temperature coefficient of resonant frequency τf and quality factor Q value together with the thermal conductivity. The ordered domain structure could be tailored and controlled by the post‐densification annealing, and the fine ordered domain structures with high ordering degree and low‐energy domain boundary were obtained in the present ceramics annealed around 1400°C for 24 h, where the Qf value was improved from 51 000 to 118 000 GHz, τf was suppressed from 30 to 25.5 ppm/°C. Moreover, the thermal conductivity at room temperature was increased from 3.79 to 4.30 W m−1 K−1, and the Young's modulus was improved from 98 to 214 GPa. The present work provided a promising approach for synergistic modification of physical properties in Ba‐based complex perovskite microwave dielectric ceramics.

Dielectric constant prediction of perovskite microwave dielectric ceramics via machine learning

With the development of communication technology, microwave dielectric ceramics are in increasingly urgent need. Perovskite ceramics, as a kind of microwave dielectric ceramics with large dielectric constant span, have broad application prospects. Predicting material properties before experiments can greatly accelerate the development of materials. Although the existing methods, including classical theory and density functional theory, are of practical use for dielectric constant prediction, unsatisfactory universality and predictability limit rational design of microwave dielectric ceramics. This work aims to develop an uncomplicated method to quickly predict the dielectric constant of perovskite ceramics. According to the element and content of the compound, the dielectric constant can be accurately predicted by our machine learning model. Moreover, the model provides prediction results that are consistent with the experiment, but are completely different from those calculated by C-M equation.

Microstructure and dielectric properties of novel MgTiO3-x wt% MgAl2O4 microwave dielectric composite ceramics

Microstructure and dielectric properties of novel MgTiO3-x wt% MgAl2O4 microwave dielectric composite ceramics

Optimization of Zn2GeO4 microwave dielectric ceramics via Cu substitution and B2O3 sintering aid for LTCC application

Optimization of Zn2GeO4 microwave dielectric ceramics via Cu substitution and B2O3 sintering aid for LTCC application

Preparation and characterization of the temperature-stable CaO-Li2O-Nd2O3-TiO2 microwave dielectric ceramics

Preparation and characterization of the temperature-stable CaO-Li2O-Nd2O3-TiO2 microwave dielectric ceramics

Effects of chemical ordering and homogeneity on microwave dielectric properties of LaGaO3-SrTiO3 compounds

(1-x)LaGaO3-xSrTiO3 (x = 0–0.6) microwave dielectric ceramics were synthesized by using conventional solid-state reaction method and sintered at 1450 °C with different sintering conditions. The variations of microstructure, relative density, and phase formation were observed by different composition and sintering conditions. The microwave properties, such as permittivity/dielectric constant (εr), quality factor (Qf), and temperature coefficient of resonant frequency (TCF, τf) at a high frequency were investigated in terms of lattice distortion, chemical ordering, and chemical homogeneity according to the composition and equilibrium sintering conditions. A maximum Qf ∼64,900 GHz was observed at x = 0, and the best τf ∼ 0 ppm/°C was obtained at x = 0.4. Consequently, it was found that the permittivity is related mainly to the lattice distortion, whereas the quality factor is primarily affected by chemical ordering and chemical homogeneity of the microstructure-induced composition.

Co‐optimization of microwave dielectric properties in Ba(Mg1/3Ta2/3)O3 complex perovskite ceramics through ordered domain engineering

AbstractIt is an important subject to improve the temperature coefficient of resonant frequency (τf) and thermal conductivity (κ) of microwave dielectric ceramics without reducing the Qf value. Ordered domain engineering was applied to realize the previous objectives in Ba(Mg1/3Ta2/3)O3 ceramics. With the increasing ordering degree from 0.835 to 0.897, the optimized Qf value was obtained. Meanwhile, near zero τf from 11.9 to 5.6 ppm °C−1 was achieved, together with increased κ from 5.5 to 7.6 W m−1 K−1, and enhanced dielectric strength from 801 to 921 kV cm−1. The noticeable ordered domain structure with large ordered domains (∼100 nm) and low‐energy domain boundaries was revealed in Ba(Mg1/3Ta2/3)O3. The consequent weakened phonon scattering rises the thermal conductivity. The increased bond covalency and oxygen distortion in ceramics with higher ordering degree were suggested as a cause of enlarged bandgap, which enhanced the dielectric strength. The reduced τf is dominated by the less “rattling” space of the cations in the ordered state by inducing more positive τε. The reduced τf, optimized thermal conductivity, and Qf value in the present work indicate that the ordered domain engineering could open up a new direction for the optimization of microwave dielectric ceramics.

Simultaneously achieving high permittivity and low loss in CuO/Nb2O5 substituted Li2O-TiO2-Nb2O5 low-temperature microwave dielectric ceramics

To cope with the growing challenges in device miniaturization increase, there is an urgent need to develop high K materials with excellent microwave properties. Here, novel (Cu1/3Nb2/3)4+ substituted LiNb0.6Ti0.5O3 (LNT) ceramics (LNT-xCN, 0≤x ≤ 0.20) were successfully fabricated via solid-state reaction method. The use of CuO as an effective burning agent enabled one to successfully enhance ceramic densification and reduce the sintering temperature to 950 °C. Outstanding microwave dielectric performance (εr∼65.21, Q × f∼9215 GHz, τf ∼ -24.06 ppm/°C) was achieved in LNT-0.16CN ceramic. In particular, the Q × f value was improved nearly 1.44 times relative to that of pure LNT ceramic (6385 GHz@1100 °C). More importantly, the variation of microwave dielectric parameters not only was related to bulk density, densification and polarization, but also to LN/[Ti2O3]2+layers and the squeezing pressure of oxygen octahedron [NbO6]. Encouragingly, the characteristics most compatible with those of silver were found in LNT-0.16CN ceramic, making a suitable substitute material for the next-generation 5G communication devices.

Novel ultra-low loss and low-fired Li8MgxTi3O9+xF2 microwave dielectric ceramics for resonator antenna applications

Low temperature sintered Li8MgxTi3O9+xF2 microwave dielectric ceramics with x = 2−7 were developed based on a newly designed pseudo ternary phase diagram of the Li2TiO3–MgO–LiF system. Dense solid solution ceramics (of relative density >96 %) with cubic rock-salt structure, accompanied by a small amount of secondary phase MgO, were obtained in the temperature range of 800−925 °C. With increasing Mg2+ content, the value of εr decreased, whereas that of τf remained nearly constant, and the Q×f increased to a maximum at x = 5. The Li8Mg5Ti3O14F2 ceramic sintered at 875 °C exhibited superior microwave dielectric properties with εr= 16.8, Q× f= 119,700 GHz, and τf= −41.6 ppm/°C. The good compatibility with Ag electrodes highlights the promising prospects of this ceramic in low-temperature co-fired ceramic technology. Furthermore, a dielectric resonator antenna fabricated based on a Li8Mg5Ti3O14F2 ceramic exhibited an outstanding S11 of −34.7 dB and a broad bandwidth of 360 MHz at the desired resonant frequency of 5.98 GHz.

Research Progress of Ba(Zn1/3Nb2/3)O3 Microwave Dielectric Ceramics: A Review

Ba(Zn1/3Nb2/3)O3 (BZN) microwave dielectric ceramics have attracted great attention due to their high-quality factor (Q), near-zero temperature coefficient of resonant frequency (τf), and suitable dielectric constant (εr), making them promising materials for application in microwave devices. Due to their superior dielectric properties, composite perovskite ceramics are widely used in the field of microwave communication, base stations, navigation, radar, etc. This article summarized the latest research progress of BZN ceramics and discusses the main preparation methods and performance modifications. Furthermore, the problems faced by BZN ceramics and solutions to improve their performance, as well as their potential applications, are analyzed. This article provides a reference for the design and preparation of BZN ceramics.

Relationship between bond characteristics and microwave dielectric properties of REVO4 (RE = Yb, Ho) ceramics

Two novel low- ε r REVO 4 (RE = Yb, Ho) microwave dielectric ceramics with the symmetry of the zircon structure, space group I 4 1 / amd , were prepared using the solid-state method. Dense REVO 4 (RE = Yb, Ho) ceramics sintered at 1200 °C and 1160 °C performed ε r ∼ 12.3 ± 0.1 and 13.3 ± 0.1, Q × f ∼ 28,200 ± 300 GHz and 24,100 ± 300 GHz, τ f ∼ −18.8 ± 0.5 ppm/°C and −17.4 ± 0.5 ppm/°C, along with thermal expansion coefficient ( α L ) of 9.0 ppm/°C and 8.1 ppm/°C, respectively. Bond valence results indicated that the slightly rattling RE 3+ cations at the A-site and compressed V 5+ at the B-site occurred in both ceramics. The positive deviations ( Δε r ) of porosity corrected ε r (Corr) from those calculated by the Clausius-Mosotti equation ε r (C-M) , 8.1% for YbVO 4 and 17.7% for HoVO 4 , were observed, implying that the rattling effect of RE 3+ in dodecahedral A-site were greater than those of compressed V 5+ in tetrahedral B-site. Rattling effect also led REVO 4 (RE = Yb, Ho) to develop higher ε r , and smaller τ ε and τ αm , then closer to zero τ f values than other zircon-structured REVO 4 (RE = Ce, Nd, Sm, Eu) ceramics with large negative τ f . The differences in sintering temperature and microwave dielectric performance of both ceramics were discussed using the packing fraction, full width at half maximum (FWHM) of Raman modes and Phillips-Van Vechten-Levine (P–V-L) theory.