Temperature Coefficient Of Resonant Frequency Research Articles

Sintering behavior and microwave dielectric properties of CaTi1-x(Nb1/2Al1/2)xO3

CaTi1-x(Nb1/2Al1/2)xO3 with x=0.1-0.5 ceramics were processed through solid state sintering. X-rays diffraction (XRD) patterns of the compositions showed that the samples have orthorhombic crystal structure with symmetry (Pbnm). The symmetry was further confirmed using Raman spectroscopy. A total of 13 Raman modes were detected, which were in agreement with the XRD results. Microstructure analysis of the samples showed porosity in the samples, presumably due to the substitution of Al, having high melting point. As the concentration of Al and Nb increased, relative permittivity (er), quality factor (Q×fo) and temperature coefficient of resonance frequency decreased. Optimum microwave dielectric properties were achieved for the composition x=0.5 sintered at 1650 °C for 8 h i.e., er ~27.09, Q×fo ~17378 GHz and tf ~ -2.5 ppm/°C.

High sintering and dielectric performance: The improved (Mg, Zn)3B2O6 ceramics with the help of the DFT calculation

With the help of the first principle calculation, the solid-state reaction experiment was conducted to investigate the alteration in the sintering and the microwave dielectric properties of Mg3B2O6 ceramic with many Zn2+ substitutions. These properties were characterized using the scanning electron microscopy, network analyzer, X-ray diffraction, Raman spectroscopy, energy-dispersive spectroscopy, and thermomechanical and differential-thermal analyses. The coexistence of Mg3B2O6, Mg2B2O5 and ZnO ceramics could be observed with increasing Zn2+ addition, and the lattice distortion occurred in the Mg2B2O5 and Mg3B2O6 ceramics due to the substitution of Mg2+ with Zn2+. The electron density and the bond property of the MgO6 octahedron changed, and a quantitative method was used to discuss the variation in sintering, substitution and phase formation properties. The densification window was decreased to 1100 °C, and the dielectric properties improved with the formation of a three-phase borate solid solution (dielectric constant = 6.73, quality factor = 112,000 GHz at 16 GHz (Q = 7000), temperature coefficient of resonant frequency = −61.2 ppm °C−1, and relative density = 97.0%).

Ultra low loss (Mg1 − xCax)2SiO4dielectric ceramics (x = 0 to 0.15) for millimeter wave applications

Abstract(Mg1 − xCax)2SiO4dense ceramics (x ≥ 0.15) were prepared, and their microwave dielectric characteristics were investigated together with the structure evolution. The sintering temperature for Mg2SiO4ceramics was reduced significantly with Ca2+substitution. (Mg1 − xCax)2SiO4ceramics exhibited a small increase in dielectric constant (εr) correlated with increased crystallite size, and ultra‐high quality factorQfvalue was achieved throughout the compositional range. Temperature coefficient of resonant frequency (τf) was considerably tuned from −70 ppm/°C to −33 ppm/°C, and this improvement was deeply linked with the decreased bond valance. At x = 0.075, (Mg1 − xCax)2SiO4ceramics exhibited the best combination of microwave dielectric characteristics:εr = 7.2,Qf = 199,800 GHz at 26 GHz,τf = −33 ppm/°C. The present ceramics could be expected as promising candidate of dielectric materials for millimeter wave applications.

Microwave Dielectric Properties and Sintering Behavior of K20 Series Temperature‐Stable 0.91MgTiO3‐0.09(Ca0.8Na0.1Ce0.1)TiO3 Composite Ceramics

AbstractRecently, the progress of miniaturization and integration for dielectric communication devices is being motivated by the rapidly‐developed communication technology. Microwave dielectric ceramics, the foundation of dielectric communication devices, have been receiving great attention. In this study, 0.91MgTiO3‐0.09(Ca0.8Na0.1Ce0.1)TiO3 ceramic (0.91MT‐0.09CNCT) was synthesized by the traditional solid‐phase process to tune the temperature coefficient of resonant frequency (τf) to zero. Compared with 0.95MgTiO3–0.05CaTiO3 ceramic, a Q×f value increased by 15 % was achieved in 0.91MT‐0.09CNCT ceramic, meanwhile, the sintering temperature of which was significantly lowered by 225 °C. In general, the τf value is related to the (Ca0.8Na0.1Ce0.1)TiO3 content and sintering temperature. At the same time, the 0.91MT‐0.09CNCT ceramic sintered at 1175 °C has competitive dielectric properties: ϵr=20.4, Q×f=66,000 GHz, τf=0.2 ppm/°C, which suggests it a promising candidate for dielectric communication devices with low cost and green synthesis.

Structure and microwave dielectric characteristics of Hf1‐xTixO2 ceramics

AbstractHf1‐xTixO2 dense ceramics were prepared by a standard solid‐state reaction process, and the microwave dielectric properties in a wide frequency range were determined together with structure evolution. With increasing x, the structure gradually changed from HfO2‐based solid solution (monoclinic in space group P21/c, x ≤ 0.05) to HfTiO4 (orthorhombic in space group Pbcn, x = 0.5), and the two phase regions were determined for 0.1 ≤x < 0.5 and x = 0.55. The microwave dielectric properties of HfO2 ceramics at 10 GHz were determined as εr = 14, Qf = 24 500 GHz, τf = –50 ppm/℃. With Ti‐substitution, the microwave dielectric characteristics, especially the Qf value, could be significantly improved, and the best combination of microwave dielectric characteristics was achieved at x = 0.05: εr = 17, Qf = 84 020 GHz (at 7.8 GHz) and 151 260 GHz (at 27.9 GHz), and τf = –47 ppm/℃. The smallest temperature coefficient of resonance frequency (τf = –4.8 ppm/℃) was obtained for x = 0.55 together with a Qf value of 52 370 GHz at 5.0 GHz and 60 390 GHz at 17.9 GHz, where the dielectric constant was 40. Moreover, Hf1‐xTixO2 microwave dielectric ceramics might be very competitive in the future of mobile communication technology due to their excellent compatibility with Si.

Study on low temperature sintering behavior and dielectric properties of BaTi4O9/La2O3-B2O3-CaO microwave dielectric ceramic

In this paper, La2O3-B2O3-CaO (LBC) glass ceramics were sintered at low temperature to density BaTi4O9 (BT4) microwave dielectric ceramics. The effects of composition ratio and sintering temperature on the micromorphology, phase composition and microwave dielectric properties of LBC/BT4 multiphase ceramics were investigated. Experimental results showed that LBC microcrystalline glass and BT4 happened between the ceramic interface reaction obviously, and with the increase of microcrystalline glass content and sintering temperature increases, density, dielectric constant epsilon εr value and the quality factor Qf value were increased after the first decrease. And the resonant frequency temperature coefficient of τf value with the increase of LBC content decreases, and with the increase of sintering temperature, it goes up and then it goes down. When the content of LBC glass ceramics is 50 wt%, LBC/BT4 multiphase ceramics sintered at 875 °C for 2 h have good microwave dielectric properties εr = 26, Qf = 8000 GHz, τf; = +150 ppm/°C.

Sintering behaviour and microwave dielectric properties of MgO-2B2O3-xwt%BaCu(B2O5)-ywt%H3BO3 ceramics

This study investigates the bulk density, sintering behaviour, and microwave dielectric properties of the MgO-2B2O3 series ceramics synthesised by solid-state reaction. According to the X-ray diffraction and microstructural analyses, the as-prepared MgO-2B2O3 ceramics possess a single-phase structure with a rod-like morphology. The effects of different quantities of H3BO3 and BaCu(B2O5) (BCB) on the bulk density, sintering behaviour, and microwave dielectric properties of the MgO-2B2O3 ceramics were investigated. Accordingly, the optimal sintering temperature was obtained by adding 30 wt% H3BO3 and 8 wt% BCB. We also reduced the sintering temperature to 825 °C. Furthermore, the addition of 40 wt% H3BO3 and 4 wt% BCB increased the quality factor, permittivity, and temperature coefficient of resonance frequency of MgO-2B2O3 to 44,306 GHz (at 15 GHz), 5.1, and −32 ppm/°C, respectively. These properties make MgO-2B2O3 a viable low-temperature co-fired ceramic with broad applications in microwave dielectrics.

Cation distribution and bond energy of intermediate spinel Mg(Zn0.5Ti0.5)2xGa(2−2x)O4 microwave dielectric ceramics

Substitution of Ga3+ by [Zn0.5Ti0.5]3+ in MgGa2O4 improved the comprehensive properties of the resultant spinel. A continuous solid solution with a spinel structure was formed in Mg(Zn0.5Ti0.5)2xGa(2−2x)O4 (0 ≤ x ≤ 1); here, Zn2+ occupied the tetrahedral 8a site while Ti4+ occupied the octahedral 16d site, resulting in a decrease in the degree of inversion λ from 0.86 to 0. The optimal sintering temperature decreased from 1460 °C to 1220 °C, and the dielectric constant (εr) increased from 8.8 to 14.9; these changes could be ascribed to the greater polarizability of [Zn0.5Ti0.5]3+ relative to that of Ga3+. The temperature coefficient of resonant frequency (τf) of the material remarkably increased from −64.1 ppm/°C to −16.8 ppm/°C, which may be attributed to the increase in dielectric polarizability/molecular volume, bond energy, and bond valence in B-sites. The high-quality factor (Q × f) slightly decreased from 89,253 GHz to 73,432 GHz with decreasing λ and packing fraction. Optimal microwave dielectric properties were obtained from the sample with x = 1 and sample sintered at 1220 °C; in this case, εr = 14.9, Q × f = 73,432 GHz, and τf = −16.8 ppm/°C.

Microwave dielectric properties and low-temperature sintering mechanism in (Ca,Bi)(Mo,V)O4 ceramics

In this work, high-quality (Ca1-xBix)(Mo1-xVx)O4 (0.15 ≤ x ≤ 0.5) ceramics with a tetragonal scheelite structure were prepared at sintering temperatures of 800–925 ℃. Their microstructures, alternating current impedance, and microwave dielectric properties were studied comprehensively. Through scanning electron microscopy and transmission electron microscopy analyses, the low-temperature sintering mechanism was clarified to be the dissolution–precipitation process of small particles in the liquid phase of the grain boundary. The increase in point defects led to an increase in extrinsic loss, which might be the main extrinsic factor affecting the Q×f (Q = quality factor = 1/dielectric loss, f = resonant frequency) value. The (Ca0.65Bi0.35)(Mo0.65V0.35)O4 ceramic sintered at 875 ℃ for 4 h exhibited superior microwave dielectric properties, with permittivity (εr) ~ 17, Q×f value ~ 12080 GHz (at 9.86 GHz), and near-zero temperature coefficient of resonance frequency (τf) ~ + 3.6 ppm/℃; thus, the ceramic has great potential for the LTCC application.

Multilayer co-fired microwave dielectric ceramics in MgTiO3-Li2TiO3 system with linear temperature coefficient of resonant frequency

To address the nonlinear variation of resonant frequency in a MgTiO3-CaTiO3 system over a wide temperature range (-40–105°C), MgTiO3-Li2TiO3 system ceramics with multilayer architectures are designed. Trilayer MgTiO3-Li2TiO3-MgTiO3 ceramics (mass ratio=0.2:0.56:0.2) sintered at 1275°C possess a linear and near-zero temperature coefficient of resonant frequency (τf+=-3.0 ppm/°C (25–105°C), τf-=2.7 ppm/°C (-40–25°C)). Compared with that of random distribution-type MgTiO3-Li2TiO3 ceramics, the Q × f value of trilayer ceramics is significantly improved (Q × f=80,000 GHz (@7.6 GHz)). The electric field distribution is examined by the eigenmode solver of High Frequency Structure Simulator software to analyze the effect of the Li2TiO3 content and the stacking scheme on the dielectric properties. The cross-sectional morphologies show a dense intermediate layer formed by ion diffusion, which acts as a glue to bond each layer. This work can be conceived as a new strategy for developing 5G wireless communication components with linear temperature-related resonant frequency (-40–105°C) and low dielectric loss.

Thermal Response and TC f of GaN/AlN Heterostructure Multimode Micro String Resonators From −10 °C Up to 325 °C

We report on the first experimental characterization and analysis of the thermal response and temperature coefficient of resonance frequency (TC f) of gallium nitride/aluminum nitride (GaN/AlN) heterostructure micro string resonators, in a wide temperature range from -10 °C up to 325 °C. Thanks to its excellent electrical and mechanical properties and chemical inertness, GaN has recently stimulated growing interests in GaN microelectromechanical systems (MEMS) for emerging high-power, high-temperature, and harsh-environment applications. GaN films on Si wafers often require AlN buffer layers, thus the residual tensile stress profile in the GaN epilayers and GaN/AlN hetero-layers can play a key role in affecting the MEMS specifications and performance. Here we design and fabricate GaN/AlN heterostructure micro string resonators with length L = 100, 200 and 300 μm to probe the stress and thermal effects on resonance behavior. All out-of-plane flexural modes show clear string behavior, and the multimode resonance frequencies downshift almost linearly with increasing temperature up to 325 °C. The linear temperature dependence and TC f values of GaN/AlN heterostructure resonators can be directly employed for thermal sensing. Comparison among different devices indicates that higher tensile stress levels contribute to smaller TC f values, suggesting strain engineering may be exploited for intentionally regulating the TC f.

Effects of LiF on crystal structure, cation distributions and microwave dielectric properties of MgAl2O4

In this work, MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics were synthesized by solid-state reaction method. The phase information of a spinel structure was delineated by XRD. The crystal structure of MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics were discussed with the help of Rietveld refinements. As x increased from 0 to 6.4 mol. %, the tetrahedral sites in MgAl2O4 ceramics gradually were occupied by Al3+ cations, which is confirmed by 27Al NMR and Raman spectra. Such the occupation of Al3+ cations increased the covalency (fc/s) value of M-O (M=Mg, Al) bond in MO4 tetrahedral of MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics. LiF is mainly used as a sintering aid to improve the formation of the MgAl2O4 ceramics. In order to obtain high Q×f value spinel ceramics, LiF must be removed before the final stage of densification. In addition, the dielectric constant (εr), grain size and quality factor (Q×f) value are significantly affected by the LiF content. However, the temperature coefficient of resonance frequency (TCf) values were independent of LiF content. MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics with x = 6.4 has excellent microwave dielectric properties when sintered at 1575 °C for 8 h: εr= 8.36, Q×f= 99,900 GHz, TCf= − 61.57 ppm/°C.

EFFECTS OF La2O3 AND Bi2O3 ADDITIONS ON THE MICROWAVE DIELECTRIC AND ANTENNA PROPERTIES OF SrBi2Nb2O9 CERAMIC MATRIX

This work presents a comparison between the dielectric properties of the SrBi2Nb2O9 (SBN)ceramic matrix and the individual additions of Bi2O3and La2O3, suggesting the material that would be most suitable for microwave applications. The SBN sample was synthesized by solid-state reaction, and the X-ray diffraction method (XRD) was used for the structural characterization of the materials analyzed in this study. The thermal stability of the samples was measured experimentally by studying the temperature coefficient of resonant frequency (τf) the results confirm that the individual additions of 15wt% of Bi2O3or La2O3improved the thermostability of the composites. The Hakki-Coleman method was used to obtain the dielectric properties in the microwave range, and it is possible to observe that the dielectric permittivity of the composites increased, while their loss tangents decreased. The numerical simulation demonstrated the functioning of the materials as dielectric resonating antennas (DRA), with a reflection coefficient below -10 dB in all samples.

Near‐Zero Thermal Expansion Ba1−xSrxZn2Si2O7‐Based Microwave Dielectric Ceramics for 3D Printed Dielectric Resonator Antenna with Integrative Lens

AbstractHigh‐gain, low‐weight, wide bandwidth, and miniaturized lens antennas with stable properties against temperature are intensely required in extreme environments such as aerospace field. However, high‐performance microwave dielectric ceramics with near‐zero thermal expansion and frequency shift with temperature are very rare; moreover, the requirement of 3D printing processes also restricts their developments. Ba0.4Sr0.6Zn2Si2O7with negative coefficient of thermal expansion (CTE), which originates from the stretched [ZnO4] tetrahedral chain to the twisted one alongb‐axis with temperature, and CaTiO3with positive temperature coefficient of resonant frequency (τf) can effectively adjust both CTE and τfof Zn1.8SiO3.8to near‐zero value in 0.95(0.9Zn1.8SiO3.8–0.1Ba0.4Sr0.6Zn2Si2O7)–0.05CaTiO3ceramic. Then, a Ku‐band Luneburg lens integrative antenna fabricated by stereolithography 3D printing technology exhibits an average gain of 8.06 dBi at 10.45–11.39 GHz and 10.3 dBi at 12.27–13.45 GHz, which has potential applications in temperature‐stable satellite communication.

Boosting densification and microwave dielectric properties in cold sintered BaF2 ceramics for 5.8 GHz WLAN applications

BaF2 is a low-firing fluoride with excellent microwave dielectric properties, while the densification of BaF2 ceramics is challenging via conventional sintering. In this work, dense BaF2 ceramics with 95.3~98.5% relative densities have been prepared via the cold sintering (150–600 MPa, 150 °C, 1 h) and following post-annealing at 900 °C. The scanning electron microscope images demonstrate closely packed microstructures, and the chemical compatibility between BaF2 ceramics and Ag electrodes is identified. The optimum Qf (82,320 GHz) is 1.53 times higher than that obtained via conventional sintering (53,654 GHz), with a dielectric permittivity, εr = 7.3, and a temperature coefficient of resonant frequency, τf = –107.9 ppm/°C. Furthermore, a BaF2-based microstrip patch antenna is fabricated, which realizes an S11 = –16.5 dB, a high gain of 5.84 dB, and total efficiency of −0.38 dB at 5.78 GHz. The exceptional microwave dielectric properties and antenna performances indicate that BaF2 ceramics are promising candidates for 5.8 GHz wireless local area network applications.

Effects of (Mg1/3Sb2/3)4+ substitution on the structure and microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics

Ce2[Zr1−x(Mg1/3Sb2/3)x]3(MoO4)9 (0.02 ⩽ x ⩽ 0.10) ceramics were prepared by the traditional solid-state method. A single phase, belonging to the space group of Ru2a48erline 3 c, was detected by using X-ray diffraction at the sintering temperatures ranging from 700 to 850 °C. The microstructures of samples were examined by applying scanning electron microscopy (SEM). The crystal structure refinement of these samples was investigated in detail by performing the Rietveld refinement method. The intrinsic properties were calculated and explored via far-infrared reflectivity spectroscopy. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by Phillips-van Vechten-Levine (P-V-L) theory. Ce2[Zr0.94(Mg1/3Sb2/3)0.06]3(MoO4)9 ceramics with excellent dielectric properties were sintered at 725 °C for 6 h (εr = 10.37, Q×f = 71,748 GHz, and τf = −13.6 ppm/°C, εr is the dielectric constant, Q×f is the quality factor, and τf is the temperature coefficient of resonant frequency).

Densification, flexural strength and dielectric properties of CaO-MgO-ZnO-SiO2/Al2O3 glass ceramics for LTCC applications

Novel glass ceramics for LTCC applications with high flexural strength can be achieved by CaO-MgO-ZnO-SiO2(CMSZ) glass cofiring with Al2O3. The sintering shrinkage behavior, crystalline phases, mechanical and dielectric properties, and thermal expansion of the CMZS/Al2O3 glass ceramic were determined. The X-ray diffraction results revealed that multiphases (CaMgSi2O6, Al2Ca(SiO4)2 and ZnAl2O4) formed in the sintering process of the CMZS/Al2O3 glass ceramic. The flexural strength of CMZS/Al2O3 glass ceramics first increases and then decreases with increasing Al2O3 content. The CMZS/Al2O3 glass ceramic with 50 wt % Al2O3 sintered at 890 °C for 2 h achieved the best performance, with a maximum flexural strength of 256 MPa, dielectric constant (εr) of 7.89, dielectric loss (tan δ) of 3.41 × 10−3 (12 GHz), temperature coefficient of resonance frequency (τf) of −29 ppm/°C, and the CTE value of 7.93 × 10−6/°C.

The effect of co-substitution on the microwave dielectric properties of Ba6-3xNd8+2xTi18O54(x=2/3) ceramics

In this work, the microwave dielectric properties of Ba4(Nd1-yBiy)28/3Ti18-x(Al1/2Ta1/2)xO54(0≤x≤2, 0.05≤y≤0.2) ceramics co-substituted by A/B-site were studied. Firstly, (Al1/2Ta1/2)4+ was used for substitution at B-site. At 0≤x≤1.5, the above mentioned ceramic was found to exist in single-phase tungsten bronze structure, but at x = 2.0, the secondary phase appeared. Although the dielectric constant decreased by doping the (Al1/2Ta1/2)4+, but the quality factor was observed to improve by 40% and the temperature coefficient of resonant frequency decreased by 75%. Based on the above results, Bi3+ was introduced to Ba4Nd28/3Ti17(Al1/2Ta1/2)O54. The introduction of Bi3+ reduced the sintering temperature, greatly improved the dielectric constant, and ultimately decreased the temperature coefficient of resonant frequency, but it led to deterioration of quality factor. At last, with appropriate site-substitution content control (x = 1.0,y = 0.15), excellent comprehensive properties (εr = 89.0, Q × f = 5844 GHz @ 5.89 GHz，TCF = +8.7 ppm/°C) were obtained for the samples sintered at 1325 °C for 4 h.

Microstructure and Microwave Dielectric Properties of Low-Temperature Sintering Ca5−yLi2yNi4(VO4)6 Ceramics

The influence of sintering temperature and the quantity of lithium content on microstructure and microwave dielectric properties of the Ca5−yLi2yNi4(VO4)6 were investigated. All samples were fabricated by the solid-state reaction method with a sintering temperature of 935 °C–995 °C for 4 h. The results showed no second phases formed from the XRD pattern in all samples and dense surface morphology with a sintering temperature of 965 °C. For specimens with y = 0.10 sintered at 965 °C, the permittivity, quality factor (Qf), and temperature coefficient of resonant frequency (τ f ) were 9.7, 25,000 GHz –52 ppm/°C, respectively. Thus, these modified dielectric materials possess a high potential for applications in low-temperature co-fired ceramic.

Temperature Stable, High-Quality Factor Li2TiO3-Li4NbO4F Microwave Dielectric Ceramics

In this work, (1-x)Li2TiO3-xLi4NbO4F ceramics were prepared by the conventional solid-state ceramic route. With the increase of Li4NbO4F content, the phase structure transformed from ordered monoclinic to disordered cubic. By increasing Li3NbO4F content, the temperature coefficient of resonant frequency (τf) was successfully adjusted closer to zero, while the dielectric constant (εr) and microwave quality factor (Qf) decreased to some degree. Outstanding microwave dielectric properties with a εr = 18.7, Qf = 61,388 GHz (6.264 GHz), and τf = 0.9 ppm/°C were obtained for 0.9Li2TiO3-0.1Li4NbO4F ceramics sintered at 1050 °C for 2 h, which indicated that these ceramics are suitable for practical applications in the field of microwave substrates and components.