Dependence Of Microwave Dielectric Properties Research Articles

Calibration-free method to study temperature dependence of microwave dielectric properties of nematic liquid crystals

ABSTRACTThe present article shows an original transmission-reflection-based method to characterise the microwave dielectric properties of liquid crystals. This new method offering broadband capabilities of transmission-reflection method without the need for vector network analyser calibration. It is based on two measurements in the frequency range: one measurement is performed with the empty cell, the other with the cell filled with the material. The analysis of the two recorded scattering matrices allows to eliminate all error terms and to extract real and imaginary parts of the complex dielectric constant. In this paper, we first present this method, its validation in comparison to other characterisation methods. Secondly, we present an appropriate measuring cell which allows measurements in the frequency range 26 GHz-40 GHz which is also well-suited for temperature-dependent measurements. This method is used to perform dielectric microwave measurements on 5CB liquid crystal. Finally, we discuss the temperature-dependent microwave dielectric properties (permittivity, losses and dielectric anisotropy).

Structure dependence of microwave dielectric properties in Zn2-xSiO4-x-xCuO ceramics

In this work, the Zn2-xSiO4-x-xCuO (x = 0, 0.04, 0.08, 0.12, 0.16 and 0.20) ceramics were synthesized through solid state reaction. The dependence of microwave dielectric properties on the structure was investigated through X-ray diffraction (XRD) with Rietveld refinements, Scanning electron microscope (SEM) and Raman spectra. The melting of CuO can reduce the densification temperature of Zn2-xSiO4-x ceramics. In comparison with x = 0, the x = 0.08 ceramics were densified at 1150℃ and the excellent microwave dielectric properties with low dielectric constant (εr = 6.01), high quality factor (Qf = 105 500 GHz) and τf = −28 ppm/°C, were obtained. The εr, Qf and τf value are dominated by covalency of Si-O bond and secondary phase, crystallinity and lattice energy, respectively. This provides a theoretical basis to further adjust the microwave dielectric property (especially τf value) from the structural point of view.

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

Mg(Ti1-xNbx)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(Ti1-xNbx)O3 (x = 0–0.09) ceramics were systematically studied. The structural variations and influence of Nb5+ doping in Mg(Ti1-xNbx)O3 were also systematically investigated by X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction and its Rietveld refinement results confirmed that Mg(Ti1-xNbx)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(Ti1-xNbx)O3 (x = 0–0.09). This paper also studied the different sintering temperatures for Mg(Ti1-xNbx)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 τf from −47 ppm/°C to −40.1 ppm/°C is due to the increment in cell polarisability. Another reason for the increased τf is the reduction in the distortion degree of the [TiO6] octahedral, which was also confirmed by Raman spectroscopy. Mg(Ti0.95Nb0.05)O3 ceramics sintered at 1350 °C for 2 h possessed excellent microwave dielectric properties of εr = 18.12, Q × f = 163618 GHz and τf = −40.1 ppm/°C.

Bond characteristics and microwave dielectric properties of (Li0.5Ga0.5)2+ doped Mg2Al4Si5O18 ceramics

Cordierite ceramic is one kind of important electronic materials for radio frequency circuit. In this work, Mg2-x (Li0.5Ga0.5)xAl4Si5O18 (0 ≤ x ≤ 0.1) ceramics with pure cordierite phase, were successfully synthesized via the traditional solid state reaction method. The dependence of microwave dielectric properties on the crystal structure was analyzed in detail by the complex chemical bond theory (P–V-L theory) and classical dielectric polarizability theory. The dielectric constant (εr) was mainly related to the density, bond ionicity and ionic polarizability of Mg2-x (Li0.5Ga0.5)xAl4Si5O18 ceramics. The quality factor (Q × f) was in relationship to the lattice energy, atomic packing fraction and centrosymmetry of [Si4Al2O18] hexagonal ring in cordierite crystal structure. The temperature coefficient of resonant frequency (τf) value was dominated by the bond energy and distortions of [MgO6] octahedron. The maximum values of Q × f = 50,560 GHz and εr = 4.72 were obtained for undoped Mg2Al4Si5O18 and Mg1.98Li0.01Ga0.01Al4Si5O18, respectively.

Correlation between structure characteristics and dielectric properties of Li2Mg3-xCuxTiO6 ceramics based on complex chemical bond theory

A series of Li2Mg3-xCuxTiO6 (x = 0.00–0.20) ceramics were synthesized by a simple solid-state reaction. The influences of Cu2+ substitution on the microwave dielectric characteristics, structure characteristics and morphology of Li2Mg3TiO6 ceramics were investigated. The XRD results demonstrated that all samples displayed the pure Li2Mg3TiO6 phase with rock salt structure. The SEM results exhibited that a microstructure evolution from porous structure to nearly compact structure appeared with increasing Cu2+ content, indicating that the pores originated from lithium volatilization could be suppressed by Cu2+ substitution. Subsequently, the dependence of microwave dielectric properties on intrinsic factors was evaluated based on refinement results and P–V–L theory. The monotonic increase of dielectric constant (εr) was mainly correlated with the polarizability. The variation of quality factor (Q × f) was dependent on the total systematic lattice energy and packing fraction. In addition, the evolution tendency of the temperature coefficient of resonant frequency (τf) was explained by the linear thermal expansion coefficient α and EMg/Cu–O bond energy. All results suggested that the intrinsic factors play decisive roles on dielectric characteristics.

Influence of Ni2+ Substitution for Zn2+ on Microwave Dielectric Properties of Zn1−xNixZrNb2O8 Ceramics

Dependence of microwave dielectric properties on the crystal structure of Zn1−xNixZrNb2O8 was investigated as a function of NiO content (x = 0, 0.02, 0.04, 0.06, 0.08). Pure single phase with monoclinic wolframite structure was obtained through solid-state reacti. The unit-cell volume of the specimens decreased with NiO content because of the smaller ionic radius of Ni2+ than Zn2+. Dielectric constant (K) of the sintered specimen was affected by bond-ionicity. Quality factor (Q × f0) was analyzed depending on the lattice-energy. With the increase of Ni2+, temperature coefficient of resonant frequency (τf) of specimens increased due to the increase of B-site bond energy. The sample of Zn1−xNixZrNb2O8 (x = 0.08) sintered at 1250°C for 4 h showed excellent microwave dielectric properties, er of 25.72, Q × f0 value of 73224 GHz, and τf of − 39.14 ppm/°C.

Structural dependence of microwave dielectric properties of spinel structured Mg2(Ti1-Sn )O4 solid solutions: Crystal structure refinement, Raman spectra study and complex chemical bond theory

Abstract Mg2(Ti1-xSnx)O4 (x = 0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg2(Ti1-xSnx)O4 average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg2(Ti1-xSnx)O4 compounds were obtained ( e r = 12.18, Q × f = 170,130 GHz, τ f = −53.1 ppm/°C, x = 0.2).

Effect of Isovalent Substitution on Microwave Dielectric Properties of Mg4Nb2O9 Ceramics

The dependence of microwave dielectric properties on the isovalent substitution of (Sn 1/2 4+ W1/26+)5+ (electronegativity difference from O2−, X = 2.16) and (Ti 1/2 4+ W1/26+)5+ (X = 1.95), with higher electronegativity differences than Nb5+ (X = 1.6) at Nb5+-sites of Mg4Nb2O9 ceramics was investigated. As expected from the electronegativity values, the quality factor of the specimens reached the highest value (274000 GHz) at X = 0.05 because Mg4Nb1.95(Sn1/2W1/2)0.05 O9 had a higher average bond valence than the other specimens. The dielectric constants (K) of the specimens were affected by the theoretical dielectric polarizability. The K value of specimens decreased with increasing x because Nb5+ has a higher dielectric polarizability (3.97 A3) than the substitution ions such as (Sn1/2W1/2)5+ (3.015 A3) and (Ti1/2W1/2)5+ (3.065 A3). The effects of the structural characteristics with isovalent substitution on the microwave dielectric properties are also discussed.

Structure and microwave dielectric properties of epitaxial 0.7CaTiO3-0.3NdAlO3 dielectric thin films deposited by pulsed laser deposition

High quality epitaxial 0.7CaTiO3-0.3NdAlO3 (CTNA) dielectric thin films with complex perovskite structure are deposited on SrTiO3 substrates by pulsed laser deposition. In a combination with XRD and aberration-corrected TEM, the strain state and oxygen octahedral rotation are analyzed in detail. The substrate temperature dependence of microwave dielectric properties has been investigated by a split-post dielectric resonator method. The dielectric constant and dielectric loss tangent of the CTNA thin film deposited at 700 °C are 417 and 0.048 at 10 GHz, respectively. The dielectric response of CTNA films derived from far-infrared reflectance spectroscopy shows that the frequency of phonon mode TO1 centered at 174.5 cm−1 is shifted to 176.6 cm−1 with increasing substrate temperature. In contrast, the damping factor decreases with increasing substrate temperature. The substrate temperature dependence of dielectric response mainly results from the strain-induced oxygen octahedral rotation.

Dependence of microwave dielectric properties on the complex substitution for Ti-site of MgTiO 3 ceramics

Effects of the structural characteristics of MgTi0.95(Sn0.05-xZrx)O3 (0 ≤ x ≤ 0.04) and Mg(Ti0.99-xSnx)Zr0.01O3 (0 ≤ x ≤ 0.06) on microwave dielectric properties were investigated. Complete solid solutions consisting of single-phased ilmenite structures with overall compositions of Mg(Ti0.99-xSnx)Zr0.01O3 and MgTi0.95(Sn0.05-xZrx)O3 (x ≤ 0.03) were obtained. However, a secondary phase was detected in MgTi0.95(Sn0.05-xZrx)O3 specimens at x = 0.04. The temperature coefficient of resonant frequency (TCF) of the specimens was affected by distortion of the Ti-site octahedra. The dielectric constants (K) of the specimens were affected by the theoretical dielectric polarizabilities of constituent ions of the ilmenite structure. The quality factor (Qf) was dependent on the Ti-site bond valence, which was a function of the bond strength between the cation and oxygen ion in the oxygen octahedron. The quality factors of MgTi0.95(Sn0.02Zr0.03)O3 and Mg(Ti0.95Sn0.04)Zr0.01O3 improved owing to the well-ordered structure, which was confirmed by analysis of Raman spectra.

Crystal structure and microwave dielectric characteristics of Zr-substituted CoTiNb2O8 ceramics

Co(Ti1-xZrx)Nb2O8 microwave dielectric ceramics were synthesized via the conventional solid-state reaction route. The dependence of microwave dielectric properties on the crystal structure was discussed. The phase transitions were analyzed using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. A series of composition-induced phase transitions were confirmed via the sequence: tetragonal rutile structure→coexistence of rutile and wolframite phase→monoclinic wolframite structure. For the Co(Ti1-xZrx)Nb2O8 compounds, Zr substitutions from 0 to 1 led to a decrease in the εr from 62.4 to 23.8. In contrast to εr, Q×f increased considerably, which was explained in terms of packing fraction. The τf was correlated with oxygen octahedral distortion and B-site bond valence. A near zero τf of 4.4ppm/°C was obtained in the CoTi0.4Zr0.6Nb2O8 ceramics with an εr of 29.9 and a high Q×f of 72,833GHz.

Crystal structure, Raman spectroscopy and microwave dielectric properties of layered-perovskite BaA2Ti3O10 (A = La, Nd and Sm) compounds

Dependence of microwave dielectric properties on the structural characteristics of BaA2Ti3O10 (A = La, Nd and Sm) was investigated. All the compounds were prepared by solid-state reaction method and the structural parameters were obtained from Rietveld refinement of powder diffraction data of the compounds. The dielectric constant (ɛr) increased due to decrease in molar volume of the compound, which was observed as a result of smaller ionic radii. Quality factor (Q × ƒ) of BaSm2Ti3O10 (BST) and BaLa2Ti3O10 (BLT) were found to be lowest (10,490 GHz) and highest (20,860 GHz) respectively, which was explained in terms of average bond strength between B-site cations and oxygen anions. Raman spectroscopy was also used to analyze the B–O bond strength with the help of Ag mode. The temperature coefficient of resonant frequency (τƒ) was correlated with respective octahedral-distortion in the structure.

Microwave dielectric properties of cordierite-diopside glass-ceramics

The dependence of microwave dielectric properties on the crystallization behavior of (1-x) Mg2Al4Si5O18 – xCaMgSi2O6 glass-ceramics was investigated as a function of the CaMgSi2O6 content (x) and heat-treatment temperatures. The crystallization behaviors of the specimens were affected by the peak temperature of crystallization, the crystalline phases, and degree of crystallization. The amounts of crystalline phases of the specimens were quantitatively evaluated by X-ray diffraction analysis using a combination of the Rietveld and the reference intensity-ratio method. For the specimens with x = 0.1 heat-treated at 1160 °C for 3 h, a single phase of α-cordierite (Mg2Al4Si5O18) was detected and showed the maximum degree of crystallization. The overall dielectric constant (K) of the specimens was affected by the density and the K of diopside. The quality factor (Qf) of the specimens was dependent on the crystalline content of α-cordierite through the entire range of compositions. The measured dielectric constants of the specimens are also compared with those predicted by various dielectric mixing models.

Cation Distributions and Microwave Dielectric Properties of Spinel-Structured MgGa2O4 Ceramics

The Mg2+ and Ga3+ cation distributions in the MgGa2O4 lattice were characterized by the refinement of the crystal structure and the firing temperature dependence of microwave dielectric properties was described in this study. The crystal structure refinement of MgGa2O4 ceramics fired at different temperatures indicated that the degree of inversion x, which represents the Mg2+ and Ga3+ cation distributions in the 8(a) and 16(d) sites in (Mg1-xGax)[MgxGa2-x]O4, slightly decreases from 0.88 to 0.84 with increasing firing temperature from 1500 to 1600 °C. This implies that the Mg2+ cation preferentially occupies the 8(a) site, i.e., the tetrahedral site, with increasing firing temperature. The dielectric constant (εr) of the MgGa2O4 ceramics fired above 1520 °C was almost constant (εr = 9.2), whereas their Q·f significantly increased from 92,000 to 298,000 GHz, depending on the firing temperature. Such an increase in the Q·f may be related to the Mg2+ and Ga3+ cation distributions in the MgGa2O4 lattice.

Effects of Crystallization Behavior on Microwave Dielectric Properties of CaMgSi2O6Glass-Ceramics

Dependence of microwave dielectric properties on the crystallization behaviors of CaMgSi₂O 6 (diopside) glass-ceramics was investigated with different heat treatment methods (one and/or two-step). The crystallization behaviors of the specimens, crystallite size and degree of crystallization, were evaluated by differential thermal analysis (DTA), scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis by combined Rietveld and reference intensity ratio (RIR) methods. With an increase in heattreatment temperature, the dielectric constant (K) and the quality factor (Qf) increased due to the increase of the crystallite size and degree of crystallization. The specimens heat-treated by the two-step method had a higher degree of crystallization than the specimens heat-treated by the one-step method, which induced improvement in the quality factor (Qf) of the specimens.

Microwave dielectric properties of scheelite structured low temperature fired Bi(In1/3Mo2/3)O4 ceramic

Abstract A Bi(In1/3Mo2/3)O4 ceramic was prepared via the solid state reaction method. The pure monoclinic phase was formed at around 650 °C. Ceramic samples with relative densities above 97% were obtained when sintering temperature was around 840 °C. The best microwave dielectric properties were achieved in the Bi(In1/3Mo2/3)O4 ceramic sintered at 840 °C for 2 h with permittivity ∼25.2, Qf of 40,000 GHz and temperature coefficient of resonance frequency ∼−65 ppm/°C at 8.2 GHz. The temperature dependence of microwave dielectric properties was also studied in a wide temperature range from −250 °C to +120 °C. The Qf value increased with the decrease of temperature and reached a maximum of 150,000 GHz at −250 °C.

Effects of Crystal Structure on the Microwave Dielectric Properties of ABO4 (A = Ni,Mg,Zn and B = Mo,W) Ceramics

The dependence of microwave dielectric properties on the crystal structure, bond character, and electronic characteristics of AMoO4 and AWO4 (A = Ni, Mg, Zn) ceramics was investigated. The dielectric constant (K) of specimens was principally affected by the dielectric polarizabilities, molar volume, and electronic oxide polarizabilities. MgMoO4 and AWO4 (A = Ni, Mg, Zn) display a single phase monoclinic wolframite structure, whereas ZnMoO4 is a single phase triclinic wolframite structure. The quality factor (Qf) of AWO4 was higher than that of AMoO4 (A = Mg, Zn); these results were attributed to the packing fraction due to effective ionic size. The temperature coefficient of the resonant frequency (TCF) of the specimens was dependent on the cations' bond valence between the cation and oxygen ions. This suggests the ability to tailor ABO4 microwave K, Qf, and TCF via ionic design rules.

Effect of Crystal Structure on Microwave Dielectric Properties of (Ni1/3B2/3)1-xTixO2 (B=Nb and Ta)

Dependence of microwave dielectric properties on the crystal structure of (Ni1/3B2/3)1-xTixO2 (B5+=Nb, Ta, 0.3≤x≤0.6) were investigated. Single phase of tetragonal rutile structure was detected through the entire range of compositions (0.3≤x≤0.6). Dielectric constant (K) and the temperature coefficient of resonant frequency (TCF) of (Ni1/3B2/3)1-xTixO2 (B5+=Nb, Ta) increased with an increase of TiO2 content due to the increase of bond length ratio of (dapical)/(dequatorial), and the octahedral distortion of rutile structure, respectively. The specimens with smaller Ti content and/or B5+=Ta showed larger Qf value than those with larger Ti content and/or B5+=Nb.

Relationships between crystal structure and microwave dielectric properties of (Zn 1/3B 2/35+) xTi 1 − xO 2 (B 5+ = Nb, Ta) ceramics

Dependence of microwave dielectric properties on the crystal structure of (Zn 1/3B 2/3 5+) x Ti 1 − x O 2 (B 5+ = Nb, Ta) ceramics was investigated as a function of Zn 1/3B 2/3 5+O 2 (B 5+ = Nb, Ta) content (0.4 ≤ x ≤ 0.7). Dielectric constant ( K) and the temperature coefficient of resonant frequency ( TCF) of sintered specimens were strongly dependent on the structural characteristics of oxygen octahedra in rutile structure. Cation rattling and the distortion of oxygen octahedra were dependent on the bond length ratio of apical ( d apical)/equatorial ( d equatorial) of oxygen octahedra. The quality factor ( Qf) was dependent on the reduction of Ti ion as well as the microstructure of the sintered specimens.

Effects of structural characteristics on microwave dielectric properties of (1 − x)Ca 0.85Nd 0.1TiO 3– xLnAlO 3 (Ln = Sm, Er and Dy) ceramics

Dependence of microwave dielectric properties on the structural characteristics of (1 − x)Ca 0.85Nd 0.1TiO 3– xLnAlO 3 (Ln = Sm, Dy and Er) ceramics were investigated as a function of LnAlO 3 content (0.05 ≤ x ≤ 0.25). For the specimens with SmAlO 3, a single phase with orthorhombic perovskite was obtained through the entire composition, however, Dy 2Ti 2O 7 and Er 2Ti 2O 7 were detected as a secondary phase along with the orthorhombic perovskite phase for the specimens with DyAlO 3 ( x = 0.25) and ErAlO 3 (0.10 ≤ x ≤ 0.25), respectively. With an increase of LnAlO 3 content, the dielectric constant decreased due to the smaller ionic polarizability of LnAlO 3 than Ca 0.85Nd 0.1TiO 3. The temperature coefficient of the resonant frequency (TCF) decreased with LnAlO 3 content resulted from an increase of oxygen octahedral distortion.