Accurate Dielectric Measurements Research Articles

The complex permittivity of PEDOT:PSS.

High permittivity materials are required for efficient organic photovoltaic devices, and the addition of the conjugated polymer composite poly(3,4-ethylenedioxythiophen) polystyrene sulfonate (PEDOT:PSS) to dielectric polymers has been shown to significantly heighten their permittivity. The permittivity of PEDOT:PSS at the optical and microwave frequencies has been investigated, but PEDOT:PSS layers are mainly used for low-frequency device applications, where accurate dielectric property measurements are hindered by their high electrical conductivity and the problems arising from the metal-polymer interfaces. Here, we determine the complex relative permittivity (εr*=εr'-jεr″) of PEDOT:PSS layers perpendicular to the layer plane in the 10-2-106 Hz range by combining data from the reactive energy estimations and electrochemical impedance spectroscopy, and discover that: εr' at <1Hz is ultra-high (∼106) decreasing with frequency to ∼5 at 106 Hz; the experimental data fit the Cole-Cole dielectric relaxation model by considering multiple relaxation mechanisms; PEDOT:PSS polarizes nonlinearly and εr' increases with the intensity of the applied external field; low frequency εr' increases with both thickness and temperature of the layer, opposite trend of temperature-dependence prevails at >103Hz; the dielectric properties of PEDOT:PSS are highly anisotropic and the in-plane εr' at 1.0kHz is three orders of magnitude higher than the vertical εr'; and that the εr'' decreases proportional to the reciprocal of frequency (1/f). The latter finding provides an explanation for the ubiquitous pink noise accompanying signals transmitted through organic conductor links. The described methodology can be adopted for investigations on other conjugated polymers.

Application of the body of revolution finite-element method in a re-entrant cavity for fast and accurate dielectric parameter measurements

In dielectrometry, traditional analytical and numerical algorithms are difficultly employed in complex resonant cavities. For a special kind of structure (a rotating resonant cavity), the body of revolution finite-element method (BOR-FEM) is employed to calculate the resonant parameters and dielectric parameters. In this paper, several typical resonant structures are selected for analysis and verification. Compared with the resonance parameter values in the literature and the simulation results of commercial software, the error of the BOR-FEM calculation is less than 0.9% and a single solution time is less than 1 s. Reentrant coaxial resonant cavities loaded with dielectric materials are analyzed using this method and compared with simulation results, showing good agreement. Finally, in this paper, the established BOR-FEM method is successfully applied with a machined cavity for the accurate measurement of the complex dielectric constant of dielectric materials. The test specimens were machined from polytetrafluoroethylene, fused silica and Al2O3, and the test results showed good agreement with the literature reference values.

Investigation of Thermal Effects in Coaxial Probe Method and Dielectric Characterization of Glycerol up to 140°C

Dielectric properties of a material are of paramount importance in many areas of knowledge. All scientific fields that deal with the interaction of electromagnetic radiation with matter demand this information. Consequently, suitable methods for obtaining accurate dielectric data are required. In the microwave community, the dielectric metrology with the coaxial probe method is well known due to its simplicity and practicality. It is also pointed out as a broadband and wide temperature range technique. Despite this, a detailed study of the implications of thermal effects at calibration with this technique has not been plainly reported in the literature. In the present work, a concise exposition of the problem is made. Using air, short and water as standards, we have analyzed a set calibration procedures with different thermal conditions and compared them with the isothermal case. With the aid of statistical analysis, we have made recommendations for precise and accurate dielectric measurements at high temperatures. Using glycerol as material under test (MUT) we also demonstrated that measurements at temperatures up to 50°C above the calibration temperature can provide reliable data. Thus, measurements of glycerol up to 140°C were performed by calibrating the system with water at 90°C. This important issue was raised due to the lack of standard material data in the literature for temperatures above 100°C. From the measured data we also have derived the Debye model parameters of glycerol from 30 to 140°C.

Quantification of the Sensing Radius of a Coaxial Probe for Accurate Interpretation of Heterogeneous Tissue Dielectric Data

Accurate tissue dielectric measurements are crucial for the development of electromagnetic diagnostic and therapeutic devices that are designed based on estimates of the dielectric properties of diseased and healthy tissues. Although the dielectric measurement procedure is straightforward, several factors can introduce uncertainties into dielectric data. Generally, uncertainties are higher in the dielectric measurement of heterogeneous tissues, due to the fact that there is no standard procedure for acquiring and interpreting the dielectric data of heterogeneous tissues. Uncertainties related to tissue heterogeneity can be minimized by estimating the probe sensing volume, defined by the sensing depth and radius, and characterizing the tissue distribution within that volume. While several studies have investigated the sensing depth, this paper focuses on examining the sensing radius. Both dielectric measurements and numerical simulations with heterogeneous porcine tissues in the microwave range of 0.5–20 GHz have been conducted to quantify the sensing radius and the dielectric contribution of each tissue within the sensing volume. Experiments demonstrate that the sensing radius, which depends on the individual dielectric properties of the constituent tissue types, can be smaller than the probe radius. This paper further quantitatively demonstrates that the dielectric contribution of a particular tissue depends on both its location within the sensing volume and its dielectric properties. This study provides fundamental knowledge for accurately interpreting dielectric data of heterogeneous tissues, with the aim of supporting medical device development.

Note: Development of a new Bridgman-type high pressure cell for accurate dielectric measurements.

We developed a new high pressure cell which allows us to perform accurate dielectric measurements above 3 GPa. The developed cell is based on Bridgman type opposed anvil cells with electrodes immersed into holes in the anvils in order to make the electrode area constant under compression. As a feasibility experiment, dielectric measurements for H2O were conducted up to 6.6 GPa and obtained dielectric constants are well consistent with previous studies.

A 16 × 16 CMOS Capacitive Biosensor Array Towards Detection of Single Bacterial Cell.

We present a 16 × 16 CMOS biosensor array aiming at impedance detection of whole-cell bacteria. Each 14 μm × 16 μm pixel comprises high-sensitive passivated microelectrodes connected to an innovative readout interface based on charge sharing principle for capacitance-to-voltage conversion and subthreshold gain stage to boost the sensitivity. Fabricated in a 0.25 μm CMOS process, the capacitive array was experimentally shown to perform accurate dielectric measurements of the electrolyte up to electrical conductivities of 0.05 S/m, with maximal sensitivity of 55 mV/fF and signal-to-noise ratio (SNR) of 37 dB. As biosensing proof of concept, real-time detection of Staphylococcus epidermidis binding events was experimentally demonstrated and provides detection limit of ca. 7 bacteria per pixel and sensitivity of 2.18 mV per bacterial cell. Models and simulations show good matching with experimental results and provide a comprehensive analysis of the sensor and circuit system. Advantages, challenges and limits of the proposed capacitive biosensor array are finally described with regards to literature. With its small area and low power consumption, the present capacitive array is particularly suitable for portable point-of-care (PoC) diagnosis tools and lab-on-chip (LoC) systems.

Improvement in the Accuracy of Dielectric Measurement of Open-Ended Coaxial Resonators by an Enhanced De-Embedding of the Coupling Network

An improvement of the accuracy of dielectric measurements of the open-ended coaxial resonator method is described. The technique is based on an empirical technique for de-embedding the coupling network excited by electric probes. By this procedure, the influence of the coupling structure on the resonance can be precisely eliminated independently of the coupling conditions, which guarantees a high accuracy in the permittivity determination of materials by open-ended coaxial resonators. The technique is applicable to materials with a wide range of dielectric constants and losses. The results of dielectric measurements are compared with those obtained using other standard methods.

Resistivity and Dielectric Measurements of Pb(Mn&lt;sub&gt;0.5&lt;/sub&gt; W&lt;sub&gt;0&lt;/sub&gt;&lt;sub&gt;.5&lt;/sub&gt;)O&lt;sub&gt;3&lt;/sub&gt; Ceramics with Guard Ring Electrode

Polycrystalline Pb(Mn0.5W0.5)O3, PMW, a ferroelectric oxide was prepared by high temperature solid state reaction route. In order to reduce the measurement error caused by edge capacitance, laboratory-made three-terminal ring electrode (including a guard terminal) was constructed. A guard ring electrode eliminated parallel resistance paths that could cause errors in materials resistance measurements. Guard ring electrodes were useful devices which enable accurate dielectric measurements to be made on small sample of insulating material. The dielectric constant (ε′) and loss tangent (tan δ) of PMW ceramics were measured as a function of frequency ( 1 kHz- 100 kHz ). The volume resistivity and surface resistivity of PMW ceramics were also investigated. The smallest value of volume and surface resistivity were found to be 1.6 × 10-4 MΩ and 0.1281 MΩ, respectively.

Effect of Molecular Flexibility on the Nematic-to-Isotropic Phase Transition for Highly Biaxial Molecular Non-Symmetric Liquid Crystal Dimers.

In this work, a study of the nematic (N)–isotropic (I) phase transition has been made in a series of odd non-symmetric liquid crystal dimers, the α-(4-cyanobiphenyl-4’-yloxy)-ω-(1-pyrenimine-benzylidene-4’-oxy) alkanes, by means of accurate calorimetric and dielectric measurements. These materials are potential candidates to present the elusive biaxial nematic (NB) phase, as they exhibit both molecular biaxiality and flexibility. According to the theory, the uniaxial nematic (NU)–isotropic (I) phase transition is first-order in nature, whereas the NB–I phase transition is second-order. Thus, a fine analysis of the critical behavior of the N–I phase transition would allow us to determine the presence or not of the biaxial nematic phase and understand how the molecular biaxiality and flexibility of these compounds influences the critical behavior of the N–I phase transition.

High-frequency dielectric measurements

The demands on dielectric material measurements have increased over the years as electrical components have been miniaturized and device frequency bands have increased. Well-characterized dielectric measurements on thin materials are needed for circuit design, minimization of crosstalk, and characterization of signal-propagation speed. Bulk material applications have also increased. For accurate dielectric measurements, each measurement band and material geometry requires specific fixtures. Engineers and researchers must carefully match their material system and uncertainty requirements to the best available measurement system. Broadband measurements require transmission-line methods, and accurate measurements on low-loss materials are performed in resonators. The development of the most accurate methods for each application requires accurate fixture selection in terms of field geometry, accurate field models, and precise measurement apparatus.

Generation and use of optimised databases in microwave characterisation

A method for optimising the sampling points in a database used to characterise a coaxial probe for dielectric measurement is described. The optimised databases are related to the forward operator that maps the model parameters (the material dielectric properties) to the observation (measured data). The optimised sampling method is shown by simulation to improve the accuracy of dielectric measurements, in the presence of measurement noise, when a neural network is trained using the database compared with a regularly sampled database.

Quantitative measurements of dielectric spectra with microdielectric fringe-effect sensors.

The parallel-plate method is a gold standard for measuring dielectric properties of materials. However, it requires sampling of the material under testing (MUT), which makes it less suitable for real time, dynamic, and in situ measurements. The alternative to the parallel-plate method is to use the microdielectric fringe-effect (FE) sensors, which can be placed inside the process or laboratory equipment to provide rapid, on-line, and noninvasive characterization of the dielectric properties. An additional potential advantage of the FE measurements is the ability to obtain spatially localized and interfacial measurements, which may be important in some applications. Unfortunately, interpretation of the FE sensor measurements is difficult because of the spatial nonuniformity of the electrical excitation field created by the FE sensor and the extraneous contributions from the sensor substrate and unknown stray elements. The objective of this study is to summarize the theoretical basis of the dielectric measurements using planar interdigitated sensors and to use it in the development of a new method for obtaining quantitative measurements with FE sensors. As the first step, the basic correlation between the impedance measurements obtained with the FE sensor and the dielectric properties of the MUT is elucidated. The theoretical results are then used to analyze the contribution of the sensor substrate and unknown stray components to the overall measurements. A novel calibration method to eliminate extraneous contributions is then proposed. The application example demonstrates the application of the developed method to the measurement of the dielectric permittivities of a polydispersed cis-polyisoprene samples. The results are compared with those obtained using the parallel-plate measurements and show excellent agreement. Experimental comparison with the alternative calibration methods is also performed, indicating significant improvement in accuracy of dielectric measurements over a broad range of frequencies.

The microwave characterization of single crystal lithium and calcium fluoride at cryogenic temperatures.

The whispering-gallery mode method is used for very accurate permittivity and dielectric loss measurements of single crystal lithium fluoride (LiF) and calcium fluoride (CaF2) over the temperature range of 4.5 K to 300 K. The absolute uncertainty in the real part of permittivity was estimated to be less than 0.1%, and it was limited principally by uncertainty in dimensions of the samples. Dielectric losses were measured with uncertainties of about 10% limited by the accuracy of Q-factor measurements. The measured materials exhibited dielectric losses between 2-4 x 10(-7) near 5 K. The relative permittivity was evaluated as 6.502 (4.9 K) to 6.844 (296 K) at 17.5 GHz for CaF2 and 8.534 (4.6 K) to 9.063 (300 K) at 13.5 GHz for LiF.

Complex permittivity of some ultralow loss dielectric crystals at cryogenictemperatures

Whispering gallery modes were used for very accurate permittivity and dielectric loss measurements of ultralow loss isotropic and uniaxially anisotropic single crystals. Several materials including sapphire, YAG, quartz, and SrLaAlO4 were measured. The total absolute uncertainty in the real part of permittivity tensor components was estimated to be ±0.1%, limited principally by the uncertainty in sample dimensions. Imaginary parts of permittivities were measured with uncertainties of about 10%, limited by the accuracy of Q-factor measurements of whispering gallery modes. It has been observed that, for most crystals, dielectric losses can be approximated by a power function of absolute temperature only in limited temperature ranges. At temperatures between 4-50 K, losses are often affected by impurities, which are always present in real crystals.

Open-ended coaxial probe for high-temperature and broad-band dielectric measurements

A stainless steel open-ended coaxial probe was developed to measure the complex permittivity of solid dielectric materials at elevated temperatures and over a broad frequency range. The spring loading of the inner conductor insured that the probe maintained contact with the sample up to 1000/spl deg/C and eliminated errors due to differential thermal expansion of the probe. Comparison with an industry standard probe demonstrated that the spring-loaded probe accurately and reproducibly measured the complex permittivity of several samples over a broad frequency range of 0.3-6 GHz at room temperature. At temperatures up to 1000/spl deg/C, dielectric measurements of a glass ceramic and of a porous alumina composite performed with both a spring-loaded probe and a resonant cavity agreed to within 8% for the real part and 15% for the imaginary part of the complex permittivity. The probe's insensitivity in measuring low-loss materials constrained accurate dielectric measurements to materials with tan /spl delta//spl ges/0.05. Finally, optimization of an open-ended probe by varying the probe dimensions is presented.

Resonators for accurate dielectric measurements in conducting liquids

The compact, rugged, re-entrant radio-frequency resonator [A. R. H. Goodwin, J. B. Mehl, and M. R. Moldover, Rev Sci. Instrum. 67, 4294 (1996)] was modified for accurate measurements of the zero-frequency dielectric constant (relative electric permittivity) εr of moderately conducting liquids such as impure water. The modified resonator has two modes with frequencies near 216/εr MHz and 566/εr MHz. The results for εr at both frequencies were consistent within 0.0002εr, verifying that the low-frequency limit had been attained with water samples with conductivities in the range 100–2500 μS/m. The results for water and for the insulating liquid cyclohexane were within 0.0005εr of literature values. The present analysis is based on a simplified equivalent circuit that accounts for the loading of the resonator by the external instrumentation. This circuit can easily be generalized for a resonator with three or more modes. The present resonator has a thick gold plating on its interior surfaces. With the plating, the quality factors Q of the resonances varied in a predictable way with frequency and temperature. Predictable Qs were essential for obtaining accurate values of εr.

Measurements of the Dielectric Properties of Cheddar Cheese

An experimental study of the dielectric properties of cheddar cheese at microwave frequencies has been made at 20 °C. Measurements were made using rectangular waveguide transmission line cells in five frequency bands from 750 MHz to 12.4 GHz. The physical and chemical properties of the cheese were also measured and correlations with the dielectric properties were investigated. Cheddar cheese was chosen for this study because it is a homogeneous product which allows accurate dielectric measurements to be made. This makes it a useful product for testing models which predict the dielectric properties offoods based on knowledge of their chemical and physical properties.

Reentrant radio-frequency resonator for automated phase-equilibria and dielectric measurements in fluids

A reentrant rf cavity resonator has been developed for automated detection of phase separation of fluid mixtures contained within the cavity. Successful operation was demonstrated by redetermining the phase boundaries of a CO2+C2H6 mixture in the vicinity of its critical point. We developed an accurate electrical model for the resonator and used helium to determine the deformation of the resonator under pressure. With the model and pressure compensation, the resonator was capable of very accurate dielectric measurements. We confirmed this by remeasuring the molar dielectric polarizability Aε of argon and obtained the result Aε=(4.140±0.006) cm3/mol (standard uncertainty) in excellent agreement with published values. We exploited the capability for accurate dielectric measurements to determine the densities of the CO2+C2H6 mixture at the phase boundaries and to determine the dipole moment of 1,1,1,2,3,3-hexafluoropropane, a candidate replacement refrigerant. Near the operating frequency of 375 MHz the capacitor in the resonator has an impedance near 14 Ω. This low impedance is more tolerant of electrical conductivity within the test fluid and in parallel paths in the support structures than comparable capacitors operating at audio frequencies. This will be an advantage for operation at high temperatures where some conductivity must be expected in all fluids. Of further value for high-temperature applications, the present rf resonator has only two metal–insulator joints. These joints seal coaxial cables; neither joint is subjected to large mechanical stresses and neither joint is required to maintain precise dimensional tolerances. The resonator is rugged and may be operated with inexpensive electronics.

Open-ended metallized ceramic coaxial probe for high-temperature dielectric properties measurements

A metallized ceramic coaxial probe has been developed for high temperature complex permittivity measurements. The probe is made of alumina and metallized with a 3.0-mil-thick layer of moly-manganese, and a 0.5-mil-thick protective coating of nickel plating. It is shown that based on carrying out the network analysis calibration procedure up to 1000/spl deg/C, and on actual dielectric properties measurements, the probe provides accurate dielectric measurements over a broad frequency range (500 MHz to 3 GHz) and for temperatures up to 1000/spl deg/C. An uncertainty analysis based on two different calibration techniques was also given to help quantify possible measurement errors.

A generic approach for permittivity measurement of dielectric materials using a discontinuity in a rectangular waveguide or a microstrip line

A generic approach is proposed to accurately measure microwave and millimeter-wave properties of dielectric materials. This novel technique first determines the scattering parameters of a discontinuity containing a material having a wide range of complex dielectric permittivity /spl epsiv/ that is known a priori (the direct problem). The discontinuity is embedded in a rectangular waveguide or a microstrip line. A unified theory, which is essentially based on a combination of the boundary integral equation technique with a modal expansion approach, is presented to tackle the direct problem. A class of generic diagrams are obtained for interrelating the dielectric permittivity /spl epsiv/ to its S-parameters, and a simple analytical expression is deduced to solve the inverse problem. The proposed analytical formulation is easy to use and much more rapid than its iterative counterparts. It demonstrates a completely new and efficient strategy for accurate complex dielectric measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>