Temperature Coefficient Of Resonant Frequency Research Articles

Structure dependence of dielectric characteristics in Li2Mg3Ti1-x(Al0.5Ta0.5)xO6 ceramics

To investigate the effects of (Al0.5Ta0.5)4+ substitution on the crystal structure and dielectric characteristics of Li2Mg3TiO6, a set of Li2Mg3Ti1-x (Al0.5Ta0.5)xO6 (x = 0.0–0.12) samples were fabricated via solid-state process. XRD and Rietveld refinement results confirmed the formation of single rock salt structure in the space group Fm3m (No.225) within 0.0 ≤ x ≤ 0.12. Subsequently, some intrinsic factors were introduced to evaluate the structure dependence of dielectric characteristics in Li2Mg3Ti1-x (Al0.5Ta0.5)xO6 ceramics based on the unit cell parameters. The variation trend of dielectric constant (εr) was explained by the relative density and average ionic polarizability. The quality factor (Q × f) showed the same trend as the packing fraction and lattice energy, indicating the intrinsic lattice anharmonicity might play vital roles in dielectric loss. Additionally, the temperature coefficient of resonant frequency (τf) was strongly associated with the bond energy of Etotal. Typically, the Li2Mg3Ti0.91(Al0.5Ta0.5)0.09O6 sample obtained at 1310 °C possessed remarkable dielectric performances: εr = 15.06, Q × f = 169,600 GHz and τf = −27.45 ppm/°C.

Preparation, phase assemblage and microwave dielectric properties of AlPO4-BPO4-SiO2 ternaries

Microwave dielectric ceramics with low dielectric permittivities (Ɛr<6), high quality values and temperature sable resonator frequencies are strongly desired with the development of millimeter wave communication. In this paper, a compositional design for low-k materials was proposed from the point view of crystal chemistry. AlPO4-BPO4-SiO2 glass-ceramics were prepared by traditional solid state and sol-gel processes, respectively. The sintering behaviors, phase assemblages, microstructures and microwave dielectric properties of AlPO4-BPO4-SiO2 ternaries have been studied. The solid solubility, glass forming ability and crystallization of the AlPO4–BPO4–SiO2 ternaries can be understood by considering the structural flexibility via the degree of ionicity i of the bonds in the ternaries. All compositions demonstrate low dielectric permittivity less than five and negative temperature coefficient of resonant frequency. Maximum Q × f value could be obtained for the 0.45AlPO4–0.45BPO4–0.10SiO2 composition prepared by sol-gel process after sintering at 1175 °C/2 h: Ɛr∼4.16, Q × f∼59,519.

Microwave Sintering and Microwave Dielectric Properties of (1-x)Ca0.61La0.26TiO3-xNd(Mg0.5Ti0.5)O3 Ceramics.

The (1–x)Ca0.61La0.26TiO3-xNd(Mg0.5Ti0.5)O3 [(1–x)CLT-xNMT, x = 0.35~0.60] ceramics were prepared via microwave sintering. The effects of sintering temperature and composition on the phase formation, microstructure, and microwave dielectric properties were investigated. The results show that the microwave sintering process requires a lower sintering temperature and shorter sintering time of (1–x)CLT-xNMT ceramics than conventional heating methods. All of the (1–x)CLT-xNMT ceramics possess a single perovskite structure. With the increase of x, the dielectric constant (ε) shows a downward trend; the quality factor (Qf) drops first and then rises significantly; the resonance frequency temperature coefficient (τf) keeps decreasing. With excellent microwave dielectric properties (ε = 51.3, Qf = 13,852 GHz, τf = −1.9 × 10−6/°C), the 0.65CLT-0.35NMT ceramic can be applied to the field of mobile communications.

Effect of zirconium deficiency on structure characteristics, morphology and microwave dielectric properties of Li2Mg3Zr1-xO6 ceramics

To suppress the impurity phases and porous microstructure caused by lithium volatilization, the Li2Mg3ZrO6 and Zr-deficiency Li2Mg3Zr1-xO6 (x = 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were successfully synthesized via the solid-state method assisted with atmosphere-controlled sintering. The influences of non-stoichiometric on structure characteristics, morphology and microwave dielectric properties of Li2Mg3Zr1-xO6 ceramics were investigated. The XRD and SEM results proved that the Zr-deficiency restrained the formation of impurity phases and remarkably improved the densification of Li2Mg3Zr1-xO6 samples. The variation trend of dielectric constant (εr) was explained by the relative density and theoretical dielectric polarizability. The quality factor (Q × f) was strongly associated with the impurity phases and relative density. Additionally, the temperature coefficient of resonant frequency (τf) showed the same trend as the total bond energy Etotal, indicating the bond energy might play vital roles in thermal stability of Li2Mg3Zr1-xO6 samples. Typically, the Li2Mg3Zr1-xO6 sample obtained at x = 0.06 possessed remarkable dielectric performances: εr = 13.13, Q × f = 116,400 GHz (10.14 GHz) and τf = −26.30 ppm/°C.

Design and analysis of magnetostrictive sensors for wireless temperature sensing.

Magnetostrictive transducers are commonly used as actuators and sonar transducers, and in remote non-destructive evaluation. Their use in wireless thermometry is relatively unexplored. Since magnetostriction-based sensors are passive, they could potentially enable long-term near-field thermometry. While the temperature sensitivity of resonance frequency in magnetostrictive transducers has been reported in previous studies, the origin of the temperature sensitivity has, however, not been elucidated. Here, we identify material properties that determine temperature sensitivity and identify ways to improve sensitivity as well as the detection technique. Using a combination of analytical and computational methods, we systematically identify the material properties that directly influence the temperature coefficient of resonance frequency (TCF). We first experimentally measure the shift in resonance frequency due to temperature changes in a Metglas strip to be 0.03% K-1. Using insights from theory, we then experimentally demonstrate a fivefold improvement to the TCF by using Terfenol in place of Metglas as the magnetostrictive sensor material. We further demonstrate an alternate temperature sensing technique that does not require measuring the resonance frequency, consequently reducing instrument complexity. This work provides a general framework to analyze magnetostrictive materials and the sensing scheme for near-field wireless thermometry.

Ultra-high quality factor of Mg6Ti5O16-based microwave dielectric ceramics with temperature stability

Mg6Ti5O16-based ceramics with ultra-high quality factor and temperature stability were prepared by a conventional solid-state reaction route. The effects of Ca2+ on phase composition, microstructure, and microwave dielectric properties of Mg6Ti5O16 ceramics were systematically investigated. X-ray diffraction (XRD) patterns and energy dispersive spectroscopy (EDS) analysis confirmed that the ceramics were composed of MgTiO3, MgTi2O4, and CaTiO3 with all Ca2+ contents. A kind of microwave dielectric ceramics based on Mg6Ti5O16 was developed, and the microwave properties of Mg6Ti5O16 ceramics can be effectively improved by doping of Ca2+. After sintering at 1360 °C for 4 h, Mg6Ti5O16 ceramics doped with x = 0.35 Ca2+ exhibited an ultra-high quality factor of 100432 GHz (at 9 GHz) and a dielectric constant of 19.87. The resonant frequency temperature coefficient was − 3.0 ppm/°C during the temperature range of 25–85 °C. It is a promising material in the field of microwave communication.

A Study of the Effect of Sintering Conditions of Mg0.95Ni0.05Ti3 on Its Physical and Dielectric Properties.

Mg0.95Ni0.05TiO3 ceramics were prepared by traditional solid-state route using sintering temperatures between 1300 and 1425 °C and holding time of 2–8 h. The sintered samples were characterized for their phase composition, micro-crystalline structure, unit–cell constant, and dielectric properties. A two-phase combination region was identified over the entire compositional range. The effect of sintering conditions was analyzed for various properties. Both permittivity (εr) and Q factor (Qf) were sensitive to sintering temperatures and holding times, and the optimum performance was found at 1350 °C withholding time of 4 h. The temperature coefficient of resonant frequency (τf) in a range from −45.2 to −52 (ppm/°C) and unit–cell constant were not sensitive to both the sintering temperature and holding time. An optimized Q factor of 192,000 (GHz) related with a permittivity (εr) of 17.35 and a temperature coefficient (τf) of −47 (ppm/°C) was realized for the specimen sintered at 1350 °C withholding time of 4 h. For applications of 5G communication device (filter, antennas, etc.), Mg0.95Ni0.05TiO3 is considered to be a suitable candidate for substrate materials.

Phase evolution, crystal structure, and microwave dielectric properties of gillespite‐type ceramics

AbstractA gillespite‐structured MCuSi4O10 (M = Ba1‐xSrx, Sr1‐xCax) ceramics with tetrahedral structure (P4/ncc) were prepared by solid‐state reaction method. X‐ray diffraction and thermogravimetry with differential scanning calorimetry (TG‐DSC) were employed to study the phase synthesis process of BaCuSi4O10. Pure BaCuSi4O10 phase was obtained at 1075°C and decomposed into BaSiO3, BaCuSi2O6, and SiO2 when calcined at 1200°C. The relationships between the crystal structure and microwave dielectric properties of MCuSi4O10 ceramics were revealed based on the Rietveld refinement and P‐V‐L complex chemical bond theory. The dielectric constant (εr) decreased linearly with decreasing total bond susceptibility and ionic polarizability. Quality factor (Q × f) was closely dependent on bond strength and lattice energy. The temperature coefficient of resonant frequency (τf) was controlled by the stability of [CuO4]6− plane in MCuSi4O10. Optimum microwave dielectric properties were obtained for SrCuSi4O10 when sintered at 1100°C for 3 hours with a εr of 5.59, a Q × f value of 82 252 GHz, and a τf of −41.34 ppm/°C. Thus, SrCuSi4O10 is a good candidate for millimeter‐wave devices.

Effects of Ni2+ substitution on the crystal structure, bond valence, and microwave dielectric properties of BaAl2–2xNi2xSi2O8–x ceramics

BaAl2−2xNi2xSi2O8−x (x = 0, 0.005, 0.01, 0.02, 0.03) ceramics were prepared using traditional solid phase reaction method. The microwave dielectric properties, including permittivity (εr), quality factor (Q × f), and temperature coefficient of resonant frequency (τf), were discussed based on the bond valence theory. The first-principle calculation was adopted to determine the site (Ba, Al, and Si) where doping element (Ni2+) would be inclined to occupy. The substitution of Ni2+ for Al3+ contributed to the breaking of Al-O and Si-O bonds and then facilitated the BaAl2Si2O8 (BAS) hexacelsian-celsian transformation. Moreover, this substitution could change the bond strength between cation and oxygen anion due to the variation of the bond valence, which reasonably explained the variation of εr, Q × f, and τf values. Well-sintered and completely transformed celsian ceramics can be obtained after doping with Ni2+. When x = 0.01, compact BaAl1.98Ni0.02Si2O7.99 ceramic exhibited highly promising microwave dielectric properties: εr = 6.89, Q × f = 53, 287 GHz and τf = -25.31 × 10−6 /°C.

Crystal structure, Raman spectra, and microwave dielectric properties of high-Q Li2ZnTi3O8 systems with Nb2O5 addition

Li2ZnTi3O8(LZT)-xwt.%Nb2O5 (x = 0, 1, 2, 3, 4) microwave dielectric ceramics were synthesized by the conventional solid-phase sintering method. The effect of Nb2O5 addition on crystal structure, Raman spectra, and dielectric properties were studied. X-ray diffraction results indicated that the maximum crystallinity of LZT ceramics were obtained at 1075 °C, and that Nb5+ diffused into the LZT matrix completely to form a stable solid solution. XRD refinement results demonstrated that the substitution of Nb5+ for Ti4+ caused the cell volume to increase. The microscopic morphology of LZT-xwt.%Nb2O5 (x = 0, 1, 2, 3, 4) ceramics were further investigated by scanning electron microscopy, and results indicated that Nb2O5 could promote the grain growth and sintering of LZT ceramics. Extrinsic and intrinsic dielectric losses were studied through Raman and impedance spectroscopy. The highest value of quality factor × resonant frequency (Q × f) was attributed to the increase in relative density and reduced number of oxygen vacancies (VO••), as well as the decrease in damping behavior. Moreover, the dielectric constant (εr) showed the same trend as cell polarization and the temperature coefficient of resonant frequency (τf) was inversely proportional to cell volume. The LZT ceramics with 3 wt% Nb2O5 addition sintered at 1075 °C for 4 h exhibited the optimal microwave properties, i.e., εr = 26.52, Q × f = 66 304 GHz, and τf = - 14.2 ppm/°C.

Structural investigation and improvement of microwave dielectric properties in Ca(HfxTi1-x)O3 ceramics

Calcium hafnium titanate, Ca(HfxTi1-x)O3 (CHT) was prepared using the conventional solid state reaction route. The X-ray diffraction patterns were used for analyzing the phase compositions. The XRD patterns revealed the arrangement of a single phase perovskite structure and therefore the structure is transformed from non-centrosymmetric orthorhombic (Pbnm) to centrosymmetric cubic (Pm-3m) at 0.4 ≤ x. Lattice parameter, crystallite size, densities and porosity were calculated. The SEM morphology of CHT at various hafnium (x), which is comprises of round-rod shaped grains with small pores microstructure. The substitution of Hf4+ ions over Ti4+, the microwave dielectric constant (ε r) decreases from 145 to 52, while the quality factor (Qxf) increases from 8105 to 24305 GHz and temperature coefficient of resonant frequency tuned towards zero τ f ∼ 4.5 ppm °C−1. Good combination of microwave dielectric material properties was accomplished for the composition x = 0.8 (at 3 GHz).

A low-firing melilite ceramic Ba2CuGe2O7 and compositional modulation on microwave dielectric properties through Mg substitution

A melilite Ba2CuGe2O7 ceramic was characterized by low sintering temperature and moderate microwave dielectric properties. Sintered at 960 °C, the Ba2CuGe2O7 ceramic had a high relative density 97%, a low relative permittivity (εr) 9.43, a quality factor (Q×f) of 20,000 GHz, and a temperature coefficient of resonance frequency (τf) −76 ppm/°C. To get a deep understanding of the relationship between composition, structure, and dielectric performances, magnesium substitution for copper in Ba2CuGe2O7 was conducted. Influences of magnesium doping on the sintering behavior, crystal structure, and microwave dielectric properties were studied. Mg doping in Ba2CuGe2O7 caused negligible changes in the macroscopic crystal structure, grain morphology, and size distribution, while induced visible variation in the local structure as revealed by Raman analysis. Microwave dielectric properties exhibit a remarkable dependence on composition. On increasing the magnesium content, the relative permittivity featured a continuous decrease, while both the quality factor and the temperature coefficient of resonance frequency increased monotonously. Such variations in dielectric performances were clarified in terms of the polarizability, packing fraction, and band valence theory.

Preparation, structure and microwave dielectric properties of novel La2MgGeO6 ceramics with hexagonal structure and adjustment of its τf value

A novel La2MgGeO6 ceramic was synthesized through a solid-state reaction process within a sintering temperature range of 1450–1550 °C. By a combination of X-ray diffraction and Rietveld refinement analyses, the ceramics were found to have a pure hexagonal phase structure belonging to space group R3/146. The scanning electron microscopy images revealed that the ceramic grains were closely connected. The effects of internal (lattice energy, valence bond, and fraction packing) and external factors (density) on the microwave properties of ceramics were also studied. The ceramic exhibited excellent microwave dielectric performances, with a relative permittivity (Ɛr) of 21.2, a quality factor (Q × f) of 52 360 GHz, and a temperature coefficient of resonant frequency (τf) of −44.2 ppm/°C, when sintered at 1500 °C for 4 h. The τf value of the La2MgGeO6 ceramic doped with CaTiO3 could be adjusted to zero. Particularly, 0.2La2MgGeO6-0.8CaTiO3 ceramics have good microwave dielectric properties with τf = +2.1 ppm/°C, Q × f = 15 610 GHz, and Ɛr = 40.3.

Direct preparation of magnesium borate (Mg2B2O5) ceramic from boron-bearing slag: Super-gravity separation and microwave dielectric properties

In this study, a novel method was developed for direct preparation of magnesium borate (Mg2B2O5) ceramic from boron-bearing slag. The amorphous boron was efficiently enriched into Mg2B2O5 via crystalline phase transformation in the condition of B/S (B2O3/SiO2) = 1.20 and wt.% (MgO) = 36.00. High-purity Mg2B2O5 crystals were fully recovered from molten slag by super-gravity separation at G ≥ 500 and t ≥ 240 s. The Mg2B2O5 ceramic was directly prepared at various temperature by using the recovered Mg2B2O5, and the Mg2B2O5 ceramic displayed a favourable microwave dielectric properties including the dielectric constant (εr) of 6.05, the quality factor (Q × f) of 38,147 GHz and the temperature coefficient of resonant frequency (TCF) of -20.98 ppm/K. Compared with synthetic methods that use high-purity oxides, this study provides a green and economical method for direct utilisation of industrial slag to prepare microwave dielectric materials.

Two novel garnet Sr3B2Ge3O12 (B = Yb, Ho) microwave dielectric ceramics with low permittivity and high Q

Two cubic garnet-structured Sr3B2Ge3O12 (B = Yb, Ho; called SYG and SHG) ceramics synthesised by the high-temperature solid-phase reaction method were investigated in terms of phase formation, sintering behaviour and microwave dielectric performance. At their optimal sintering temperatures, SYG had low dielectric constant (εr) = 9.30, high quality factor (Q × f) = 129,360 GHz and negative temperature coefficient of resonant frequency (τf) =−42 ppm/℃, whereas SHG exhibited low εr = 9.23, Q × f = 104,600 GHz, and comparatively near-zero τf =−26 ppm/℃. The difference in microwave dielectric performance of both ceramics was discussed by the packing fraction, full width at half maximum (FWHM) of A1g Raman mode at 779 cm−1 (775 cm−1) and bond valence. The intrinsic dielectric properties of SYG and SHG ceramics were investigated by the infrared reflectivity spectra, and their negative τf values were tuned to near-zero.

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

BaAl2-2xLi2xSi2O8-2x (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 (τf) of BAS ceramics. When x = 0.1, unveiling the significant effects of Al-position ion substitution, BaAl1.98Li0.02Si2O7.98 ceramic sintered at 1350 °C for 5 h exhibited a supreme Q × f value of 48,620 GHz, and the εr and τf values were 6.99 and -23.29 × 10−6 °C−1, respectively.

Bond characteristics and microwave dielectric properties on Zn3-xCux(BO3)2 ceramics with ultralow dielectric loss

The crystal structure and microwave dielectric properties of Zn3-xCux(BO3)2 (x = 0–0.12) ceramics prepared via a traditional solid-state reaction method were investigated by means of X-ray diffraction (XRD) utilizing the Rietveld refinement, complex chemical bond theory, and Raman spectroscopy. XRD showed that all samples were single phase. The samples maintained a low permittivity, even at higher Cu2+ contents, which is conducive to the shortening of signal delay time, and intimately related to the average bond ionicity and Raman shift. Moreover, proper Cu2+ substitution greatly reduced the dielectric loss associated with the lattice energy. Cu2+ entering the lattice optimized the temperature coefficient of resonance frequency (τf) values and improved the temperature stability of samples by affecting the bond energy. Optimal microwave dielectric properties were: εr = 6.64, Q × f = 160,887 GHz, τf = −42.76 ppm/°C for Zn2.96Cu0.04(BO3)2 ceramics sintered at 850 °C for 3 h, which exhibited good chemical compatibility with silver and are therefore good candidate materials for Low temperature co-fired ceramic applications.

Enhancement of densification and microwave dielectric properties in LiF ceramics via a cold sintering and post-annealing process

LiF is a low-firing fluoride with excellent microwave dielectric properties, however densification of LiF ceramics is challenging owing to their low surface free energy. In this study, a cold sintering process (150 °C, 250 MPa, 60 min) was employed to pre-densify LiF ceramics to 78 % relative density. Post-annealing treatments between 650 °C and 800 °C led to significant grain growth which was accompanied by an increase in relative density to 92 %. The microwave quality factor (Qf) increased with increasing annealing temperature to a maximum of 110,800 GHz at 800 °C, 1.5 times higher than the value obtained via conventional sintering (78,800 GHz), with relative permittivity εr = 8.2 and temperature coefficient of resonant frequency, τf =–135 ppm/°C. Such high values of Qf and its compatibility with Ag electrode suggest that cold sintered LiF has great potential as a component or additive in low temperature co-fired ceramic formulations.

Microstructure and microwave dielectric properties of Ba([Mg1−xZnx]1/3Ta2/3)O3 solid solution ceramics

The phase structure, microstructure and microwave dielectric properties of Ba([Mg1−xZnx]1/3Ta2/3)O3 (BMZT, 0 ≤ x ≤ 0.25) ceramics were researched as a function of ZnO addition by the traditional solid-state ceramic route. X-ray diffraction (XRD) analysis showed that there were two phases: main crystalline phase Ba(Mg1/3Ta2/3)O3 (BMT) and secondary phase Ba0.5TaO3. SEM photograph indicated that ZnO working as a sintering aid promoted the grain growth and densification. As increasing x from 0 to 0.25, the dielectric constant (er) increased continuously from 23.7 to 25.9, and the Q × f value initially increased from 62,300 GHz to the peak value of 158,900 GHz (x = 0.2) and declined to 96,500 GHz thereafter. Meanwhile, the temperature coefficient of resonant frequency (τf) initially declined to 0.15 ppm/°C (x = 0.2) and then increased to 2.9 ppm/°C. Ceramics with superior microwave dielectric properties: er = 25.7, Q × f = 158,900 GHz (f = 8 GHz) and τf = 0.15 ppm/°C were obtained at x = 0.2.

Structures and microwave dielectric properties of Ti-doped CeO2 ceramics with a near-zero temperature coefficient of resonant frequency

A systematic study on structures and microwave dielectric properties of Ti-doped CeO2 ceramics with various Ti contents synthesized via the traditional solid-state reaction method was carried out in this work. A cubic fluorite structural solid solution was formed in the Ti content range of 0.05–0.1. X-ray photoelectron spectroscopy and Raman results indicated Ti3+ ions and oxygen vacancies appeared easily when the sintering temperature was beyond 1200 °C, which would significantly impact the quality factor (Q×f) property. The dielectric constant (εr) was positively correlated to the relative density and Ti content for all the samples. The temperature coefficient of resonant frequency (τf) increased with the Ti content increasing, and came up to a near-zero value of −2.49 ppm/°C at the 0.875CeO2-0.125TiO2 composition with the εr of 27.38 and Q×f of 12,950 GHz, which could meet the actual requirements of microwave devices.