Microwave Dielectric Loss Research Articles

Tetragonal Tungsten Bronzes in Ba(M2+ 1/3Nb2/3)O3 Microwave Ceramics

For the first time, the dielectric properties of the 9:1:7 TTB phases Ba9M2+Nb14O45 (M = Co, Mg, Zn) which form in Ba-deficient high-Q perovskite niobates Ba(M2+ 1/3Nb2/3)O3 have been measured in the radio-frequency and microwave range. The low-temperature dielectric relaxation found in Ba9M2+Nb14O45 was shown to give a strong contribution into their microwave dielectric loss. The measured data were used for a simple simulation of the Q-factor of a perovskite-TTB composite. Calculated parameters have been shown to correspond well to the measured data.

Microwave and Infrared Dielectric Response of Temperature Stable (1−x)BaMoO4–xTiO2 Composite Ceramics

The (1−x)BaMoO4–xTiO2 (x = 0.0, 0.2, 0.3, 0.338, 0.4, 0.5, 0.66) ceramics were synthesized by the conventional mixed‐oxide process. The sintering behaviors, phase composition, chemical compatibility with silver, and microwave dielectric properties of pure (1−x)BaMoO4–xTiO2 ceramics and 0.662BaMoO4–0.338TiO2 ceramic with H3BO3–CuO addition were studied. Infrared reflectivity spectra of (1−x)BaMoO4–xTiO2 (0.2 ≤ x ≤ 0.4) composites were measured in the range of 50–4500 cm−1 at room temperature. X‐ray diffraction analysis reveals that scheelite BaMoO4 and rutile TiO2 phase coexist with each other at 1275°C and both of them do not react with silver (Ag) at 850°C. When the mole fraction of TiO2 (x value) is 0.4, a temperature stable microwave dielectric material is obtained, with εr = 13.8, Q × f = 40 500 GHz (f = 8.02 G), and τf = −6.13 ppm/°C. Complex dielectric spectra gained from the infrared spectra were extrapolated down to microwave range, and they were in good agreement with the measured microwave permittivity and dielectric losses. With 5 wt% H3BO3 and 1 wt% CuO addition, the 0.662BaMoO4–0.338TiO2 ceramics can be sintered well below 900°C, and possess good microwave dielectric properties with εr = 14, Q × f = 48 360 GHz, and τf = +13.9 ppm/°C.

Influence of the various doping elements on the microwave dielectric loss of silica

This article presents an evaluation of the influence of various doping elements on the dielectric loss of silica glass. Based on the Anderson–Stuart model, the temperature dependence of the electrical conductivity activation energy was investigated; then, comparisons of microwave dielectric loss of silica doped by various elements such as Li 2O, Na 2O, K 2O, CaO, MgO and Al 2O 3 are presented. At 1373 K, the dielectric loss of silica doped with Li 2O was found to be 10 times more than that the undoped one. The order in the impact of the doping elements on the microwave dielectric loss was found to be Li 2O > Na 2O > K 2O > MgO > CaO > Al 2O 3. This work is of importance with regard to applications that employ silica glass at conditions that involve especially high temperature and also in the microwave frequency range.

Annealing Effects on Conductivity and Microwave Dielectric Loss of MgTiO3 Ceramics

MgTiO3 ceramics were annealed in various atmospheres of air, O2 and N2 at a low temperature of 800 °C and the influences of annealing on the conductivity and microwave dielectric loss were investigated. The conductivity variation with the annealing atmosphere is consistent with the defect equilibrium 2OO×↔2VO•• + O2↑+ 2e', indicating n-type conductance for MgTiO3. Annealing in air/O2 is favorable for eliminating oxygen vacancies and electron defects and thus decreases the conductivity. The N2-annealing increases the contents of oxygen vacancies and electron defects as well as the conductivity. Annealing in air/O2/N2 reduced the microwave dielectric loss irrespective of the contrary effects of air/O2-annealing and N2-annealing on the defects and conductivity, suggesting the frozen defects associated with the oxygen loss during the high-temperature sintering and the low-temperature N2-annealing have negligible effects on microwave dielectric loss. Other factors, such as the release of thermally induced strain, may be responsible for the reduction of microwave dielectric loss due to the low-temperature annealing in air/O2/N2.

Annealing Effects on Conductivity and Microwave Dielectric Loss of MgTiO3Ceramics

MgTiO3 ceramics were annealed in various atmospheres of air, O2 and N2 at a low temperature of 800 °C and the influences of annealing on the conductivity and microwave dielectric loss were investigated. The conductivity variation with the annealing atmosphere is consistent with the defect equilibrium 2OO×↔2VO•• + O2↑+ 2e', indicating n-type conductance for MgTiO3. Annealing in air/O2 is favorable for eliminating oxygen vacancies and electron defects and thus decreases the conductivity. The N2-annealing increases the contents of oxygen vacancies and electron defects as well as the conductivity. Annealing in air/O2/N2 reduced the microwave dielectric loss irrespective of the contrary effects of air/O2-annealing and N2-annealing on the defects and conductivity, suggesting the frozen defects associated with the oxygen loss during the high-temperature sintering and the low-temperature N2-annealing have negligible effects on microwave dielectric loss. Other factors, such as the release of thermally induced strain, may be responsible for the reduction of microwave dielectric loss due to the low-temperature annealing in air/O2/N2.

Influence of thermal treatments on the low frequency conductivity and microwave dielectric loss of CaTiO3 ceramics

We prepared CaTiO3 by solid state reactions as a model for Ti-based microwave dielectric ceramics to investigate the influence of thermal treatment on conductivity and microwave dielectric loss. The intrinsic defect concentrations of CaTiO3 may be modified by air-quenching, air-annealing and thermal treatment in various atmospheres such as O2, Ar and air. CaTiO3 has p-type conductance implying that calcium vacancy VCa× may be the dominant defect. The variation in defect concentration influences the low frequency conductivity but not the microwave dielectric loss. The VCa× concentration in the air-quenched sample was found to be lower than that in the air-annealed one. However, the former showed ∼8% deterioration and the latter showed a 14% improvement in the Q × f value. The data, therefore, suggests that other factors such as thermally induced strain rather than intrinsic defects may influence the microwave dielectric loss.

Influence of Humidity and of the Electric and Magnetic Microwave Radiation Fields on the Remediation of TCE-contaminated Natural Sandy Soils

The influence of moisture content (15% w/w) on the remediation (vaporization) of trichloroethylene (TCE) present in natural sands, chosen as a TCE-polluted model system for soils, was investigated with regard to applied microwave power levels, the depth of the sand sample, and the dielectric factors. The heating process occurring in the sand samples arises through the microwave conduction loss heating and dielectric loss heating mechanisms. The characteristic relevance of the electric and magnetic microwave radiation fields to the heating mechanisms was also examined. Heating by the magnetic microwave radiation field was considerable when magnetite was added to the dry and wet sand samples as the microwave absorber. Optimal microwave conditions are reported for a single-mode microwave applicator. Near-quantitative elimination of the TCE contaminant was achieved for sandy soils within a very short time.

Measurements of Microwave Dielectric Properties of (1-x)TiO2-xCaTiO3 and (1-x)TiO2-xSrTiO3 thin Films By The Cavity Perturbation Method

This research studies the measurements of dielectric properties of thin films at microwave frequencies by the cavity perturbation method. The measured frequency is in X-band (8.2–12.4 GHz). The titanium dioxide based (1-x)TiO2-xCaTiO3 and (1-x)TiO2-xSrTiO3 thin films deposited by the RF reactive magnetron sputtering method are adopted for the measurements. It was found that the dielectric constants of the films are increased as the compositions of CaTiO3 or SrTiO3 are increased. The microwave dielectric constants and loss tangents of films are measured. The accuracy of measurements is discussed.

Intrinsic microwave dielectric loss of lanthanum aluminate

The intrinsic dielectric properties of LaAlO₃ were investigated to understand the microwave properties of several materials containing LaAlO₃. In this study, LaAlO₃ single crystals were prepared by the Czochralski method. The temperature dependence of the dielectric properties and neutron inelastic scattering of the single crystals were measured. From these data, the intrinsic dielectric properties were evaluated and it was found that the dielectric loss of the LaAlO₃ includes two types of dielectric loss. One is a phonon absorption-related loss and the other is a component of the loss arising from Debye- type orientation polarization. The latter affects the room temperature dielectric loss in materials containing LaAlO₃. The present study suggests that avoiding this polarization loss is an important goal in decreasing the total dielectric loss.

Influence of Nonstoichiometry on Extrinsic Electrical Conduction and Microwave Dielectric Loss of BaCo1/3Nb2/3O3Ceramics

The microwave (MW) dielectric loss (or quality factor,Q) of BaCo1/3Nb2/3O3(BCN) ceramics exhibits a strong dependence on sample‐processing conditions and is not always consistent with the degree of 1:2 B‐site cation ordering. Impedance spectroscopy results reveal that these BCN ceramics exhibit remarkably different electrical properties due to low levels of nonstoichiometry induced during ceramic processing. These compositional changes are difficult to detect by common chemical techniques but can have a significant influence on the electrical properties, and in particular on the extrinsic electrical conduction andQ. The influence of extrinsic conduction induced by ceramic processing should therefore be considered when optimizingQin MW dielectric ceramics.

A link betweenp-type electrical conduction and microwave dielectric loss in highly ordered Ba(Co1/3Nb2/3)O3ceramics

BaCo1/3Nb2/3O3ceramics, with a high density and a similar, high degree of 1:2 B-site cation ordering, exhibit very different quality factors, Q. The ceramics exhibitp-type behavior with higher conductivity and lower Q for samples processed in O2as compared with those processed in air. It is proposed that unavoidable Co loss during high-temperature ceramic processing leads top-type doping that must be compensated by oxygen vacancies to impede hole formation. The composition exhibiting only intrinsic conduction and optimized Q is not achieved with processing in atmospheric oxygen due to filling of oxygen vacancies and hole formation during cooling.

A Viable Route for Dense TiO2with a Low Microwave Dielectric Loss

Dense, high‐purity TiO2was prepared from commercially available powders. These powders were deagglomerated, colloidally stabilized in aqueous NH3, followed by screening to remove residual agglomerates. Homogeneous compacts were formed using colloidal‐filtration followed by sintering. TiO2with >99.5% density and 400 MPa flexural strength was obtained at a sintering temperature as low as 1000°C. The dielectric loss of this TiO2was 1.4 × 10−4at 6.4 GHz at room temperature. This implied that the product of the quality factor and frequency, (Q×f )=f/tan δ, was 46 000 GHz, ∼7.6 times higher than 6000 GHz (Q<2000 at 3 GHz) reported previously. This high‐quality factor is attributed to the low sintering temperature as well as a homogeneous dense microstructure with 2.2 μm grain size. The process described can be applied to realize complex TiO2structures in new microwave antenna concepts, that require components with a high dielectric constant and low dielectric loss.

A new low-loss dielectric using CaTiO 3-modified (Mg 0.95Mn 0.05)TiO 3 ceramics for microwave applications

The microwave dielectric properties of the (1 − x)(Mg 0.95Mn 0.05)TiO 3– xCaTiO 3 ceramic system prepared by mixed oxide route have been investigated. The microstructures of the ceramics were characterized by SEM. Ilmenite-structured (Mg 0.95Mn 0.05)TiO 3 and perovskite-structured CaTiO 3 were coexisted and the two-phase system was confirmed by the XRD and EDX analysis. The microwave dielectric properties are strongly related to the density and matrix of the specimen. Combination of (Mg 0.95Mn 0.05)TiO 3 and CaTiO 3 forms a two-phase system and leads to a near-zero τ f. With increasing x, the microwave Q × f decreased and ɛ r increased. A new microwave dielectric material, 0.93(Mg 0.95Mn 0.05)TiO 3–0.07CaTiO 3 possesses excellent microwave dielectric properties with a dielectric constant ( ɛ r) of ∼22.67, a Q × f of ∼90,700 GHz (where f = 9 GHz, is the resonant frequency) and a τ f value of ∼0.8 ppm/°C at 1270 °C for 4 h. It is proposed as a suitable material candidate for applications requiring low microwave dielectric loss.

Low-Loss Perovskite Niobates Ba(M1/3 2 + Nb2/3)O3: Composition, Structure, and Microwave Dielectric Properties

A slight deviation from the compositional stoichiometry in both A-site and B-site deficient perovskites Ba(M 1/3 2 + Nb 2/3 )O 3 (M – Co, Zn, Mg) was found to promote cation ordering processes in studied materials. The highest magnitudes of the Qxf product have been obtained in the non-stoichiometric BCN (Qxf = 90 000 GHz), and BMN (Qxf = 150 000–200 000 GHz). A significant “extrinsic” contribution to the microwave dielectric loss has been found in Co and Mg -containing perovskites. Possible structural factors responsible for the variation of the Q-factor in studied systems have been discussed.

Response of Ag thick film microstrip equilateral triangular patch antenna to overlay of moisture laden soybean

PurposeThe purpose of this paper is to report on the Ku band microwave characteristics of moisture laden soya seeds using overlay technique.Design/methodology/approachKu band (13‐18 GHz) moisture dependent microwave permittivity, conductivity, penetration depth of moisture laden soybean (Glycine Max) using overlay on Ag thick film equilateral triangular patch antenna are studied. The change in the frequency response of the patch antenna due to change in moisture content of the soybean overlay has been used to obtain the various microwave properties.FindingsThe permittivities obtained are in the range expected of moisture laden soybean. As moisture content increases microwave dielectric constant, dielectric loss, and conductivity of soybean increases. Only the amplitude data have been used here.Originality/valueKu band characterization of soybean has been done using overlay technique. The thick film patch antenna is sensitive even to ∼4 percent moisture content in the overlay material. This can be used for even moisture sensing at low moisture levels. This paper is believed to be an original research report.

Reduction of microwave dielectric losses in KTa 1 − xNb xO 3 thin films by MgO-doping

KTa 1 − x Nb x O 3 (KTN) thin films were grown by pulsed laser deposition on sapphire and MgO substrates. Their structural and high frequency dielectric characteristics evidenced the strong influence of the substrate and suggested possible KTN/MgO interdiffusion that could be responsible for the lower dielectric losses obtained on this substrate. Both undoped and 6% MgO-doped KTN thin films were then grown on sapphire. Dielectric measurements performed at 12.5 GHz by a resonant cavity perturbation method evidenced reduction of losses by MgO-doping. Loss tangent (tan δ) was reduced by a factor of 3 in comparison with undoped films grown on sapphire.

Do Grain Boundaries Affect Microwave Dielectric Loss in Oxides?

Reducing loss in microwave dielectrics is critical to improving performance in wireless communications systems. Grain boundaries in polycrystalline microwave dielectric ceramics have long been suspected of increasing dielectric loss. They are often cited as the main contributor to the observed difference in dielectric losses between single crystals and polycrystalline ceramics. The exact configuration of grain boundaries is problematic to quantify in practice and their influence on the dielectric loss difficult to distinguish from other defects such as porosity, oxygen vacancies, impurities, and dislocations. Here we measure the sensitivity of a single grain boundary in a magnesium oxide bi‐crystal to the polarization of an applied microwave field as a function of temperature.

Influence of sintering process on the microwave dielectric properties of Bi(V 0.008Nb 0.992)O 4 ceramics

Relationship between sintering process, relative density (shrinkage), grain morphological parameters and microwave dielectric properties of Bi(V 0.008Nb 0.992)O 4 ceramics was studied. Influence of incubation time and sintering temperature on the microwave dielectric properties was discussed. The experiential logarithmic relationship and the parabola relationship were used to simulate the relationship between microwave permittivity, different incubation time and different sintering temperatures, respectively. Modification of Bosman and Havinga's correction was used to eliminate the influence of pores on the permittivity of ceramics. The experimental, calculated and modified results corresponded well with each other. Morphological parameters of grains and pores were found to have dominant influence on microwave dielectric loss and the qualitative relationship was given. The best microwave dielectric properties were obtained in ceramics sintered at 830 °C for 2 h with heating ratio of 3 °C min −1 with permittivity about 40 and Qf reaching 18,500 GHz while relative density was only about 93.5%.

Low-Loss Microwave Ceramics Based on Non-Stoichiometric Perovskites Ba(Co1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3

The effect of both A-site and B-site non-stoichiometry on the microstructure and microwave dielectric properties of the perovskite niobates Ba(A 2 + 1/3 Nb 2/3 )O 3 (A 2 + = Co, Zn) has been examined. Both the chemical composition of the ceramics and their sintering regimes have been shown to significantly influence the type and the amount of impurity phases which consecutively demonstrate a prevailing effect on the microwave dielectric loss. The highest magnitudes of the Qxf product have been obtained in the Co-deficient BCN (Qxf = 100 000 GHz) and Ba-deficient BZN (Qxf = 90 000 GHz) which both demonstrate noticeable amount of secondary Ba-rich phases.

Dielectric loss caused by oxygen vacancies in titania ceramics

Undoped TiO 2 exhibited deterioration in microwave (MW) dielectric loss as it reached its maximum density due to the reduction of Ti 4+ to Ti 3+ causing oxygen vacancies at high sintering temperatures. By adding small amounts of acceptor dopants with ionic radii between 0.5 and 0.95 Å, reduction during sintering was suppressed. The upper limit of ionic radius was discrete with almost no observed effect using dopants >0.96 Å ionic radius. In addition, the microwave dielectric loss of undoped TiO 2 could be improved by annealing at 1500 °C for 10 h in air, presumably as a result of re-oxidation. High loss samples exhibited a dark ‘core’ to the naked eye which was absent in low loss ceramics. Transmission electron microscopy revealed that grains in the dark core contained planar defects attributed to the condensation of O vacancies onto specific crystallographic planes, in a manner similar to that observed in Magnelli phases.