Temperature Stable Microwave Dielectric Ceramics Research Articles

High-Q, temperature-stable, and medium-permittivity composite ceramics of tungstenbronze-type and perovskite titanates through layer-stacking process

High-Q and temperature-stable ceramics are indispensable in microwave engineering. However, medium-permittivity (εr ∼ 60, named as K60) microwave dielectric ceramics that meet the above demands are yet to be synthesized. High-Q Ba4Sm9.33Ti16.5Al2O54 (BSTA) and Ca0.6Sm0.27Ti0.8Al0.27O3 (CSTA) ceramics have large resonant frequency temperature coefficients (τf) with opposite signs. The composites of the two are expected to have good dielectric properties, but there will be obvious diffusion when they are compounded with each other by sintering after directly mixing the raw powder, which would lead to severe deterioration in dielectric properties. In this work, the undesired diffusion can be effectively avoided by stacking two or three ceramic layers together, which are achieved by two methods named adhesive bonding and spark-plasma-sintering (SPS) cofiring. The finite element method (FEM) analysis with COMSOL Multiphysics proposed in this work can precisely predict the microwave dielectric properties of layered ceramics, helping to determine the ratio of different layers to reach near-zero τf. The adhesive-bonded layered ceramics exhibit excellent dielectric properties: εr = 62.2, Q×f = 21,400 GHz, and τf = –0.7 ppm/°C. The SPS-cofired layered ceramics can be fabricated in one time, partially solving the problem of sintering mismatch between different layers, together with superior properties of εr = 59.8, Q×f = 19,780 GHz, and τf = –0.6 ppm/°C. These layered ceramics could fill the gap of high-Q and temperature-stable K60 microwave dielectric ceramics. The layer-stacking process and related FEM simulations can be used to design more high-Q and temperature-stable microwave dielectric ceramics and devices.

Design and Fabrication of Wideband Dielectric Resonator Antenna Using Low Loss Ultra‐Low Sintering Temperature Li6B4O9 Microwave Dielectric Ceramic for Wireless Communication Applications

AbstractLow dielectric constant (ɛr), high quality‐factor (Q × f), and temperature stable microwave dielectric ceramics with ultra‐low sintering temperature are the preferred choice of researchers for the development of low‐cost and high‐performance dielectric resonator antenna (DRA), which have wide application prospects for wireless communication technology. In this work, wideband DRAs fabricated using ultra‐low sintering temperature, low‐loss, and high‐quality Li6B4O9 microwave dielectric ceramics are reported. Li6B4O9 ceramic synthesized via solid‐state reaction at sintering temperature of 620 °C demonstrates excellent microwave properties with optimum ɛr = 5.95, high Q × f = 38,700@12.4 GHz, and thermal coefficient of the resonant frequency (TCF ≈ −68.6ppm/○C). For the first time, performance comparison of two Li6B4O9‐based cylindrical DRAs with different ceramic aspect (radius to height) ratios excited by slot coupled microstrip line is realized. In the first DRA with ceramic aspect ratio of 0.864, DRA HE11δ mode is excited and bandwidth of 20.01% , 95% radiation efficiency, and 6.3 dBi maximum gain is attained. In the second DRA with ceramic aspect ratio of 2.367, slot and dielectric resonator resonances are merged to attain wide bandwidth of 38.15% over which antenna broadside radiation performance is preserved. The proposed DRAs can be utilized for high‐capacity, high‐speed wireless communication applications.

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.

Ultra-low dielectric loss lithium-based, temperature stable microwave dielectric ceramics

The low loss and small density of lithium-based microwave dielectric ceramics (MWDCs) make them promising for communication devices such as dielectric filters, antennas, and substrates. In this study, (1- x )[Li 2 Ti 0.98 Mg 0.02 O 2.96 F 0.04 −1 wt%Nb 2 O 5 ]- x Li 3 Mg 2 NbO 6 (LTMN 0.02 -LMN, 0 ≤ x ≤ 0.1) MWDCs fabricated via solid solution reaction were successfully synthesized to obtain a near-zero temperature coefficient of the resonant frequency (τ f ). The X-ray diffraction pattern and Rietveld refinement results showed that a good solid solution was formed and that the phase composition of the LTMN 0.02 system gradually changed from monoclinic to cubic. Scanning electron microscopy images revealed that the microstructure of LTMN 0.02 improved as the LMN content increased. Ultimately, desirable dielectric properties were obtained by sintering at 1220 °C with x = 0.08: ε r ∼18.1, Q × f ∼105,661 GHz, and τ f ∼ + 1.3 ppm/°C. Furthermore, the highest Q × f value (∼131,157 GHz) was achieved in the x = 0.04 composition with relatively homogeneous grains. The high-performance LTMN 0.02 -LMN ceramics developed in this study provide a new alternative material for the dielectric components of communication systems.

Microwave dielectric property adjustment of CoZrNb2O8 ceramics by CaTiO3 addition

Temperature stable microwave dielectric ceramics CoZrNb2O8-xCaTiO3 (x = 1/60 ~ 1/10) were fabricated by a solid-state technique. The phase formation and sintering behavior were detected for the purpose of structure–property relationship exploration. Appropriate rate of CaTiO3 addition was benefit to the sintering temperature decrease of CoZrNb2O8. From x = 1/50 to x = 1/10, unexpected CaNb2O6 and Ti2Nb10O29 phases both formed, consequent on the chemical reaction between CoZrNb2O8 and CaTiO3. Co-based oxide might separate out in the form of liquid phase and decrease the sintering temperature. Increasing CaTiO3 ration degraded the quality factor Q × f, but at the same time enhanced the dielectric constant er and temperature coefficient of resonant frequency τf. According to the Lichtenecker mixing rule, the estimated microwave dielectric properties basically agreed well with the measured values at first. However, excessive CaTiO3 incorporation caused lattice defects and gave noticeable deviation. In current work, CoZrNb2O8-1/40CaTiO3 composite displayed a rather small τf with Q × f= 64,600 GHz at 1225 °C.

Sintering characteristics and microwave dielectric properties of a new temperature-stable ceramic of Ce0.5Sm0.5O1.75

The novel temperature-stable microwave dielectric ceramics of Ce0.5Sm0.5O1.75 were fabricated via the mixed oxide reaction method, with the detail information including crystal phase, microstructure, sintering behavior and microwave dielectric properties being systematically investigated. For crystal phase, all the diffraction patterns were indexed as cubic fluorite-type structure with Fm-3m (225) space group and exhibited a single phase with no obvious peak position shift. The grain growth was analyzed by density and average grain size. For microwave dielectric properties of Ce0.5Sm0.5O1.75 ceramics, the dielectric constant value (er) was dependent on density. The quality factors (Q × f values) were investigated considered with average grain size. And the temperature coefficient of resonant frequency (τf value) had no significant change during the sintering temperature range, which was found consistent with the trend of the dielectric constant value. The Ce0.5Sm0.5O1.75 ceramics sintered at 1475 °C for 4 h exhibited good microwave dielectric performances with er = 18.21, Q × f = 70,500 GHz, and τf = − 16.4 ppm/°C.

Improved Microwave Dielectric Properties of LiNb0.6Ti0.5O3 Ceramics with Zr Substitutions

The influences of Zr4+ substituted for Ti4+ on the microwave dielectric properties of LiNb0.6Ti(0.5−x)ZrxO3 (0.00 ≤ x ≤ 0.14) ceramics prepared via conventional solid-state method have been studied and are discussed. X-ray diffraction analysis indicated that the Li1.075Nb0.625Ti0.45O3 phase was formed, while a small trace of ZrO2 was observed when x reaches 0.08. With the increase of Zr4+ substitution, variation of the dielectric constant (er) was dominated by phase constitutions and densification. The quality factor (Q × f) showed a continuously decrease tendency, which was attributed to the increasing number of grain boundaries and the content of the secondary phase. The temperature coefficient of the resonant frequency (τf) value shifted from 30.60 ppm/°C to − 14.50 ppm/°C. Temperature-stable microwave dielectric ceramics was obtained with er = 63.14, Q × f = 4626 GHz and τf =1.38 ppm/°C, at x = 0.10.

Temperature stable microwave dielectric ceramics in Li2ZnTi3O8–based composite for LTCC applications

A temperature stable low temperature co-fired ceramic (LTCC) was fabricated by the powder mixture of Li2ZnTi3O8 ceramic, TiO2 τf-tailoring dopant and B2O3–La2O3–MgO–TiO2 (BLMT) glass sintering aid, and the sintering behavior, activation energy, phase composition, microstructure and microwave dielectric properties of the composite were investigated in the composition range (wt%) of 5 BLMT–(95-x) Li2ZnTi3O8−X TiO2 (x = 0, 1, 2, 3, 4 and 5). The sintering behavior results showed that all composites could be well sintered at 910 °C for 2 h through liquid-phase sintering. The activation energy of Li2ZnTi3O8 ceramic was calculated to be 520.9 ± 40.46 kJ/mol, while 5BLMT–93Li2ZnTi3O8–2TiO2 (in wt%) composite was reduced to 330.98 ± 47.34 kJ/mol. The XRD results showed that Li2ZnTi3O8 and TiO2 phase stably existed in all sample and a new phase LaBO3 was crystallized from BLMT glass during sintering process. As x increases, the rutile TiO2 phase increased in composite, which could adjust the temperature coefficient of resonant frequency (τf) to near-zero owing to the opposite τf value to other phases. And simultaneously dielectric constant (er) demonstrated gradually increase, whereas the quality factor (Q × f) decreased gradually. The composite with x = 2 had an optimal microwave dielectric properties with er = 25.3, Q × f = 32,800 GHz, and τf = − 0.54 ppm/°C. The corresponding fitting equations of er, Q × f and τf on the x value were obtained by the Origin software, indicating that the dielectric properties of the composite could be precisely controlled by varying the content of TiO2. In addition, the good chemical compatibility of this material with Ag electrode made it as a potential candidate for LTCC technology.

Novel Al2Mo3O12-based temperature-stable microwave dielectric ceramics for LTCC applications

Novel Al2Mo3O12-based temperature-stable microwave dielectric ceramics for LTCC applications were designed and prepared.

Low‐firing and temperature stable microwave dielectric ceramics: Ba2LnV3O11 (Ln = Nd, Sm)

AbstractLow‐firing and temperature stable microwave dielectric ceramics of Ba2LnV3O11 (Ln = Nd, Sm) were prepared by solid‐state reaction. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the phase purity, crystal structure, sintering behavior, and microstructure. The XRD patterns indicated that Ba2LnV3O11 (Ln = Nd, Sm) ceramics belong to monoclinic crystal system with P21/c space group in the whole sintering temperature range (800°C ‐900°C). Both ceramics could be well densified at 880°C for 4 hours with relative densities higher than 96%. The Ba2LnV3O11 (Ln = Nd, Sm) samples sintered at 880°C for 4 hours exhibited excellent microwave dielectric properties: εr = 12.05, Q × f = 23 010 GHz, τf = −7.7 ppm/°C, and εr = 12.19, Q × f = 27 120 GHz, τf = −16.2 ppm/°C, respectively. Besides, Ba2LnV3O11 (Ln = Nd, Sm) ceramics could be well co‐fired with the silver electrode at 880°C.

Temperature stable microwave dielectric ceramics in Li1.33xZn2-2xTi1+0.67xO4 (0.75 < x < 1) cubic spinels and their chemical compatibility with silver

A series of microwave dielectric ceramics in Li1.33xZn2-2xTi1+0.67xO4 (x = 0.8, 0.9, 0.95) system prepared by a solid-state reaction method were reported. Raman spectroscopy was employed to investigate the structure and microwave dielectric properties. Dense ceramics were obtained at relatively low sintering temperatures (940–1040 °C). As x increased, the sintering temperature decreased from 1040 °C at x = 0.8, to 960 °C at x = 0.9, and then to 940 °C at x = 0.95. The microwave dielectric properties of Li1.33xZn2-2xTi1+0.67xO4 were found to be sensitive to the composition and densification. A ceramic with x = 0.95 exhibited reasonably good microwave dielectric properties with εr = 26.4, Q × f = 16,040 GHz and τf = −5.3 ppm/°C when sintered at 940 °C. The chemical compatibility with silver of the x = 0.95 sample was evaluated for LTCC applications.

Structure evolution and microwave dielectric characteristics of Ca[(AlxGa0.5-xNb0.5)0.5Ti0.5]O3 ceramics

The microwave dielectric characteristics of Ca[(AlxGa0.5-xNb0.5)0.5Ti0.5]O3 (x = 0, 0.1, 0.2, 0.4, 0.5) ceramics were investigated together with the structure evolution. Distorted perovskite structure in space group Pbnm with b-b-c+ tilting was determined for the present solid solutions, where the microstructures were remarkably affected by Al-content. Owing to the Al-substitution which was accompanied by the subdued octahedral tilting, the Qf value reached up to 40,000 GHz with increasing x, the dielectric constant (εr) remained at around 50, and the temperature coefficient of resonant frequency (τf) could be adjusted to near zero. Low-loss temperature-stable microwave dielectric ceramics with medium dielectric constant were obtained at x = 0.2–0.5, which exhibited Qf values of 35,900–40,000 GHz, εr of 49.2–53.4, and τf of 0.6–3.5 ppm/°C.

Novel temperature stable high-εrmicrowave dielectrics in the Bi2O3–TiO2–V2O5system

Temperature stable microwave dielectric ceramics were obtained for compositions with 0.45BiVO4–0.55TiO2sintered at 900 °C withεr∼ 86, aQf∼ 9500 GHz and TCF ∼ −8 ppm per °C. It is promising for both low temperature co-fired ceramic technology and dielectrically loaded micro-strip patch antennas substrates.

Microwave Dielectric Properties of Temperature-Stable BaLn2(MoO4)4–TiO2 (Ln = Ce, Nd, and Sm) Ceramics

A series of temperature-stable microwave dielectric ceramics (1 − x)Ba Ln2(MoO4)4–xTiO2 (Ln = Ce, Nd, and Sm; 0.4 ≤ x ≤0.55) were prepared by solid-state reaction. Sintering behavior, phase composition, microstructure, and microwave dielectric properties were investigated. X-ray powder diffraction and scanning electron microscopy revealed that the main phases of the BaLn2(MoO4)4–TiO2 ceramics were the monoclinic BaLn2(MoO4)4 phase and the rutile TiO2 phase. Study of the materials’ microwave dielectric properties revealed that the permittivity (e r) and the temperature coefficient of the resonant frequency (τ f) increased gradually with increasing x value whereas quality factors (Q × f) decreased. For BaLn2(MoO4)4–TiO2 (Ln = Ce and Nd), τ f values could be adjusted to near zero, relative permittivity (e r) was 13.2–14.3, and Q × f values were between 11,950 and 45,720 GHz (at 9.11–9.83 GHz) when x = 0.45 to 0.55. For BaSm2(MoO4)4–TiO2, excellent microwave dielectric properties were obtained when x = 0.4 to 0.5, τ f values could be adjusted to near zero, e r was 13.1–13.9, and Q × f values were between 25,520 and 63,130 GHz (at 9.31–10.11 GHz).

Layered composite thick films for dielectric applications

Electrophoretic deposition has been successfully used for the preparation of multilayer composite thick films of the temperature stable microwave dielectric ceramics BaLa4Ti4O15 (BLT) (ɛr = 44, Qf0 = 44,000 GHz) and Ba4Nd9.33Ti18O54 (BNT) (ɛr = 82, Qf0 = 10,000 GHz). The composites exhibit permittivity intermediate between the end members that may be tuned by modifying the ratio between and/or thickness of each layer, thereby tailoring the films for specific applications. In demonstrating an alternative way to tailor functional properties, these results have broad implications for a large number of thick film-based devices.

Two novel A4B3O12-type microwave ceramics with high-Q and near-zero τf

Two novel La3+ and Ti4+ cosubstituted Sr3LaNb3O12 ceramics, Sr2La2TiNb2O12 and SrLa3Ti2NbO12, were prepared and the effect of substitution on the microwave dielectric properties was investigated. Results showed that as the amounts of substitution increased, dielectric constant (εr) increased from 36 to 41, and temperature coefficient of resonant frequency (τf) improved from −9 to 3 ppm/°C, whereas the quality factor (Q×f) decreased from 45,000 to 33,600 GHz. A temperature-stable microwave dielectric ceramics SrLa3Ti2NbO12 with τf of 3 ppm/°C, εr of 41, Q×f of 33,600 GHz was obtained; it might be a promising candidate for practical application in base stations if the quality factor is enhanced beyond 40,000 GHz.

Effects of CaTiO&lt;SUB&gt;3&lt;/SUB&gt; on Microstructures and Properties of (1-x)ZnAl&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;4&lt;/SUB&gt;-xMg&lt;SUB&gt;2&lt;/SUB&gt;TiO&lt;SUB&gt;4&lt;/SUB&gt;(&lt;I&gt;x&lt;/I&gt;=0.21) Microwave Dielectric Ceramics

ZnAl2O4-Mg2TiO4-CaTiO3 ceramics were prepared by the solid-state reaction.The effects of CaTiO3 on phase compositions,microstructure and microwave dielectric properties were investigated.It shows that CaTiO3 can effectively improve the sinterability of the ceramics and lower the densification temperature by about 150℃.ZnAl2O4-based spinel,CaTiO3,MgTi2O5 and Zn2Ti3O8 phases are included in the ZnAl2O4-Mg2TiO4-CaTiO3 system,however,Zn2Ti3O8 phase disappears when the sintering temperature is higher than 1400℃.As CaTiO3 content increases,CaTiO3 phase in the sintered body raises while MgTi2O5 phase reduces.Furthermore,CaTiO3 can adjust obviously the temperature coefficient of resonant frequency of the(1-x)ZnAl2O4-xMg2TiO4(x=0.21) ceramics.When 6mol% CaTiO3 is added to the(1-x)ZnAl2O4-xMg2TiO4(x=0.21) system,a temperature-stable microwave dielectric ceramics sintered at 1400℃ with an er of 11.8,a Q·f of 88080GHz and a τf of-7.8×10-6/℃ can be obtained.

Synthesis of temperature-stable microwave dielectric ceramics NiNb 2O 6 with TiO 2 addition

NiNb 2O 6 ceramics with 8–10 mol% TiO 2 were investigated. A mixture of raw materials was sintered into ceramics by bypassing calcination and subsequent pulverization stages. The addition of TiO 2 resulted in the formation of a Ni 4Nb 2O 9 phase and phases with the same structure as that of a Ni 0.5Ti 0.5NbO 4 phase. An increase in the TiO 2 content resulted in an increase in the content of the Ni 0.5Ti 0.5NbO 4-like phase, especially at high sintering temperatures. The dielectric constant increased from 29.2 at 1170 °C to 31.1 at 1230 °C in NiNb 2O 6 ceramics with 9 mol% TiO 2. Qxf reached 36800 GHz in NiNb 2O 6 ceramics with 8 mol% TiO 2. It decreased as the TiO 2 content increased with the content of the Ni 0.5Ti 0.5NbO 4-like phase with a low Qxf. τ f between −2 and 2 ppm/°C could be obtained in NiNb 2O 6 ceramics with 9 and 10 mol% TiO 2.