Microwave-frequency Dielectric Properties Research Articles

Microwave frequency material properties of PBS 9501 and PBX 9501 and small scale heating experiments

This work reports the microwave frequency dielectric properties of PBX 9501 and one of its representative mocks, PBS 9501, within 1-20 GHz. From these measurements it is shown that the binder system has a strong influence on microwave heating of such compositions resulting in significant temperature gradients within the individual HMX or sugar crystals at high microwave heating rates. Using the measured dielectric properties, COMSOL 4.3 Multiphysics was used to simulate and optimize a microwave applicator with a high electric field to input power ratio. The simulated applicator design indicated subsecond heating to decomposition for PBX 9501 and was validated with small scale experiments on both PBS 9501 and PBX 9501. At approximately 2.45 GHz and 100 W applied power, PBS 9501 decomposition was observed shortly (< 34 ms) after a measured surface temperature of 70 °C (binder system melts). Finally, rapid heating of PBX 9501 was also shown in the optimized cavity.

Microwave‐induced thermoacoustic scanning CT for high‐contrast and noninvasive breast cancer imaging

A fast thermoacoustic computed tomography system with a multielement linear transducer array was developed to image biological tissues with circular scanning. The spatial resolution of the imaging system and the spectra of the thermoacoustic signals were analyzed. A modified integration backprojection algorithm using velocity potential was employed to recover the direct energy deposition distribution, signal processing methods, and reconstruction algorithms were validated by imaging a phantom. The differences of the microwave-frequency dielectric properties between malignant and normal adipose-dominated tissues in the breast are considerable, and the absorption contrast can reach as large as 6:1 at 1.2 GHz. An experiment of human breast tissue with a tumor was performed with this system; the thermoacoustic images reconstructed by a limited-field-filtered backprojection algorithm and a modified integration backprojection algorithm were also compared with a mammogram. Our results show that the system can provide a rapid and noninvasive approach for high-contrast breast cancer imaging.

A large-scale study of the ultrawideband microwave dielectric properties of normal, benign and malignant breast tissues obtained from cancer surgeries

The development of microwave breast cancer detection and treatment techniques has been driven by reports of substantial contrast in the dielectric properties of malignant and normal breast tissues. However, definitive knowledge of the dielectric properties of normal and diseased breast tissues at microwave frequencies has been limited by gaps and discrepancies across previously published studies. To address these issues, we conducted a large-scale study to experimentally determine the ultrawideband microwave dielectric properties of a variety of normal, malignant and benign breast tissues, measured from 0.5 to 20 GHz using a precision open-ended coaxial probe. Previously, we reported the dielectric properties of normal breast tissue samples obtained from reduction surgeries. Here, we report the dielectric properties of normal (adipose, glandular and fibroconnective), malignant (invasive and non-invasive ductal and lobular carcinomas) and benign (fibroadenomas and cysts) breast tissue samples obtained from cancer surgeries. We fit a one-pole Cole–Cole model to the complex permittivity data set of each characterized sample. Our analyses show that the contrast in the microwave-frequency dielectric properties between malignant and normal adipose-dominated tissues in the breast is considerable, as large as 10:1, while the contrast in the microwave-frequency dielectric properties between malignant and normal glandular/fibroconnective tissues in the breast is no more than about 10%.For more information on this article, see medicalphysicsweb.org

A large-scale study of the ultrawideband microwave dielectric properties of normal breast tissue obtained from reduction surgeries

The efficacy of emerging microwave breast cancer detection and treatment techniques will depend, in part, on the dielectric properties of normal breast tissue. However, knowledge of these properties at microwave frequencies has been limited due to gaps and discrepancies in previously reported small-scale studies. To address these issues, we experimentally characterized the wideband microwave-frequency dielectric properties of a large number of normal breast tissue samples obtained from breast reduction surgeries at the University of Wisconsin and University of Calgary hospitals. The dielectric spectroscopy measurements were conducted from 0.5 to 20 GHz using a precision open-ended coaxial probe. The tissue composition within the probe's sensing region was quantified in terms of percentages of adipose, fibroconnective and glandular tissues. We fit a one-pole Cole–Cole model to the complex permittivity data set obtained for each sample and determined median Cole–Cole parameters for three groups of normal breast tissues, categorized by adipose tissue content (0–30%, 31–84% and 85–100%). Our analysis of the dielectric properties data for 354 tissue samples reveals that there is a large variation in the dielectric properties of normal breast tissue due to substantial tissue heterogeneity. We observed no statistically significant difference between the within-patient and between-patient variability in the dielectric properties.

Microwave frequency dielectric properties of hexagonal perovskites in the Ba 5Ta 4O 15–BaTiO 3 system

The dielectric properties of the hexagonal perovskites Ba 8Ta 4+0.8 x Ti 3− x O 24 ( x = 0, 0.4, and 0.8), Ba 10Ta 8−0.8 x Ti x O 30 ( x = 0.6, 0.9, and 1.2) and Ba 5Sr 2Ta 4ZrO 21 were studied at microwave frequencies. XRD analysis did not reveal the presence of impurity phases in the obtained samples and the lattice parameters of solid solutions were linearly dependent on composition. These systems show a relatively high permittivity (27 < ɛ < 44) with a low dielectric loss (10,000 < Q × f < 31,000 GHz). Solid solutions Ba 10Ta 8−0.8 x Ti x O 30 with disordered face-sharing octahedra show higher Q × f values compared to ordered Ba 8Ta 4+0.8 x Ti 3− x O 24 compounds. Filling of empty vacancy layers by multicharged cations in the Ba 5(Ta, Ti) 4+ δ O 15 system (0 < δ < 1) along with Ba 5Sr 2Ta 4ZrO 21 intergrowth leads to decrease a quality factor. The effect of increasing Ti content in the pseudobinary Ba 5Ta 4O 15–BaTiO 3 system on microwave dielectric properties is discussed.

Fabrication and Electrical Characterization of Ba(1-x)SrxTiO3 Based Thin Films

ABSTRACTOver the past several years there has been a tremendous growth and development of thin film deposition technology in the electronics industry. Ferroelectric thin films have been recognized for their unique dielectric properties and appear to be desirable for tunable microwave device applications. Among the most promising candidates for such applications are Ba(1−x)SrxTiO3[BST] and BST-based thin films. In this work pure BST and acceptor doped BST-based thin films were fabricated on (100) MgO substrates via pulsed laser deposition [PLD]. X-ray diffraction (XRD) in conjunction with the atomic force microscope (AFM) were used to analyze the film crystalinity and surface morphology. The dielectric properties were characterized at both 100 kHz and 20 GHz. The MIM capacitor configuration was used to attain the dielectric properties at 100 kHz and the microwave measurements, S11 reflection parameters, were achieved via interdigitated capacitor design with Au/Ag top electrodes. The parallel resistor-capacitor models were used to determine the microwave capacitance and Q factors and the permittivity was calculated using a modified conformal-mapping partial-capacitance method using the dimension of the capacitors. Our results demonstrated that the low frequency and microwave frequency dielectric properties were strongly influenced by the film composition. Specifically, the Mg doping served to lower the dissipation factor, permittivity, and tunability of the BST based films at both frequencies. This work demonstrates that the BST based thin films possessed excellent microstructural, structural, and dielectric properties. The structure-process-property correlations of the pulsed laser deposited BST and acceptor doped BST-based thin films are discussed in detail.

Effects of environmental exposure on solvent- and water-based epoxy systems

The dielectric properties of latex water-cast and conventional solvent-cast epoxy neat resin systems were investigated under three formation conditions: solvent-based as-made, water-based as-made, and water-based following a boiling fluid extraction procedure to remove residual surfactant. Samples from each of these three groups were cast, cured, and exposed to controlled humidities in an environmental chamber at room temperature and elevated temperature, a steam bath at various temperatures, and soaked in boiling water. Composite laminates, press fabricated using solvent and latex-based epoxy impregnated glass cloth, were subjected to similar environmental exposure. The microwave frequency dielectric properties of these materials, after each environmental exposure, were measured at 9.4 GHz. These dielectric measurements revealed that under identical exposure conditions, water-borne epoxy systems absorbed more water than solvent-based systems and also allowed more free water into the samples. Extraction of water and soluble components in the resin had no effect on the dielectric properties of the water-borne samples. The presence of high quantities of free water at moisture fractions beyond 4.0 wt %, suggests that the differences between the dielectric properties of solvent and water-borne epoxy systems are due to void morphology, which was confirmed by optical and electron microscopy. ©1999Kluwer Academic Publishers

Microwave properties of YBa2Cu3O7-x/SrTiO3 planar capacitors

The microwave properties of SrTiO3 (STO)/CeO2 varactors deposited on (1102) oriented Al2O3 substrates, using either YBCO or Ag/YBCO electrodes, was investigated. Measurements were conducted using a microstrip resonator at a frequency of 3 GHz, over temperatures ranging from 78–300 K, and bias voltages up to 260 V across a 20 μm capacitance gap. The temperature and bias voltage dependencies of capacitance and loss of the varactors were analyzed. Below the superconducting transition temperature, the microwave frequency dielectric properties of the STO film (tunability and tanδ) were dominant in establishing the varactor properties for both the YBCO and Ag/YBCO electrode structures. At T = 78 K, the capacitance ratio (= C (0 V)/C (260 V)) was 1.6 for both electrode structures. Tanδ was also identical for both structures, with tanδ = 0.04 – 0.05 under zero bias, and reduction of tanδ by a factor of 1.3 under high dc bias conditions observed.

Microwave frequency dielectric properties of poly(vinylidene fluoride) films

AbstractThe frequency dependence of dielectric constant ϵ′, dielectric loss ϵ″, and dielectric anisotropy were determined for poly(vinylidene fluoride) (PVDF) in microwave frequencies from 4 to 13 GHz. The ϵ′ and ϵ″ for PVDF films decreased with increasing frequency. Both ϵ′ and ϵ″ were larger in the transverse direction than in the machine direction or draw direction, but the values at 12 GHz were smaller than those observed at 4.0 GHz. The angular dependence of ϵ″ at microwave frequency reflects the orientational distribution of molecules in the amorphous region. The orientation function was determined to be about 0.04 and 0.01 for uniaxially and biaxially stretched PVDF, respectively. © 1995 John Wiley &amp; Sons, Inc.