Showing papers on "Relative permittivity published in 2004"

Novel high capacitance materials:- BaTiO3:La and CaCu3Ti4O12

Abstract: A review is given of the origins of high permittivity in two groups of materials, La-doped BaTiO3 and a new barrier layer capacitor material, CaCu3Ti4O12. Factors that influence permittivity include: dopant, doping mechanism, processing conditions and grain size. La-doped BaTiO3 has high permittivity due to its ferroelectric nature at low temperatures and a novel doping mechanism: A-site substitution linked to the creation of B-site vacancies for charge compensation. Permittivities of 25,000 have been achieved, which can be increased further to ∼36,000 by additional doping with Zr. The value of impedance spectroscopy to characterize materials that have heterogeneous electrical microstructures is illustrated with the example of CaCu3Ti4O12; the high permittivity is not a bulk effect, as widely stated in the literature, but is a thin layer effect typical of a barrier layer capacitor. By attention to processing conditions to achieve large grain sizes, effective permittivities as high as 300,000 have been obtained.

High tunability barium strontium titanate thin films for rf circuit applications

Abstract: Large variations in the permittivity of rf magnetron sputtered thin-film barium strontium titanate have been obtained through optimization of growth conditions for maximum dielectric strength and zero-field permittivity in a parallel-plate capacitor structure. Using nominal target compositions of Ba0.5Sr0.5TiO3, and Pt electrodes on c-plane sapphire substrates, adjustment of the O2 partial pressure during deposition was used to vary the excess Ti incorporation into the films, which influenced the low-field permittivity, loss tangent, and dielectric strength. By balancing the benefits of a high permittivity with dielectric strength and loss, we have produced films capable of sustaining short-duration fields greater than 4MV∕cm with over 13:1 (>90%) change in dielectric constant, and greater than 5:1 tunability in bias fields under 1MV∕cm.

The morphotropic phase boundary and dielectric properties of the xPb(Zr1∕2Ti1∕2)O3-(1−x)Pb(Ni1∕3Nb2∕3)O3 perovskite solid solution

Abstract: The solid solution between the normal ferroelectric Pb(Zr1∕2Ti1∕2)O3 (PZT) and relaxor ferroelectric Pb(Ni1∕3Nb2∕3)O3 (PNN) was synthesized by the columbite method. The phase structure and dielectric properties of xPZT-(1−x)PNN where x=0.4–0.9 and the Zr∕Ti composition was fixed close to the morphotropic phase boundary (MPB) were investigated. With these data, the ferroelectric phase diagram between PZT and PNN has been established. The relaxor ferroelectric nature of PNN gradually transformed towards a normal ferroelectric state towards the composition 0.7PZT-0.3PNN, in which the permittivity was characterized by a sharp peak and the disappearance of dispersive behavior. X-ray diffraction analysis demonstrated the coexistence of both the rhombohedral and tetragonal phases at the composition 0.8PZT-0.2PNN, a new morphotropic phase boundary within this system. Examination of the dielectric spectra indicates that PZT-PNN exhibits an extremely high relative permittivity near the MPB composition. The permittivity shows a shoulder at the rhombohedral to tetragonal phase transition temperature TRT=195°C, and then a maximum permittivity (36 000 at 10kHz) at the transition temperature Tmax=277°C at the MPB composition. The maximum transition temperature of this system was 326°C at the composition x=0.9 with the relative permittivity of 32 000 at 10kHz.

High microwave permittivity of multiwalled carbon nanotube composites

Abstract: Complex permittivity spectra of the multiwalled carbon nanotube (MWNT)-epoxy composites with mass concentrations up to 25.9% were measured from 10 MHz to 20 GHz. The composites exhibit high real and imaginary relative permittivities over broad bandwidth, showing strong dependence on the MWNT loading. It is observed that distinct resonance takes place at ∼1.5 GHz, with the corresponding resonant frequency shifts downward as the MWNT concentration increases. The origin of the dielectric properties is explained in terms of the dielectric relaxation/resonance and electrical conduction behavior of the nanotube composites, substantiated by the agreement between simulation and experimental data.

BaTiO3-Based ceramics for tunable microwave applications

Abstract: The dielectric properties of BaTi0.09Ga0.05Nb0.05O3 (BTGN) and Ba0.60Sr0.40TiO3 (BST) ceramics prepared by the conventional solid-state route have been investigated. Their relative potential for microwave (MW) tunable applications was assessed by the figure of merit (K) defined as K = epsilon(ro) - epsilon(rv)/epsilon(ro) (.) tan delta(o), (where er and srv are the relative permittivity at zero and 20 kV cm(-1) at 10 kHz and tan delta(o) is the loss at 10 kHz or similar to1 GHz without DC bias). Fine-grained (similar to2-3 mum) BTGN ceramics fired at 1500degreesC in air exhibit dielectric behavior characteristic of relaxor-type materials, with relative permittivity, epsilon(r), decreasing from similar to3082 to similar to2116 and dielectric loss, tan delta, increasing from 0.0035 to 0.0542 at 10 kHz and similar to1 GHz, respectively. In contrast, large-grained (20-100 mum) BST ceramics exhibit a frequency independent epsilon(r) of similar to5000 and show little variation of tan delta with frequency (0.0012 at 10 kHz and similar to0.0048 at 0.6 GHz). At 10 kHz, K-BTGN = 91 and K-BST = 367, whereas at MW frequencies K-BTGN = 6 and K-BST = 92. The large decrease in K-BTGN at MW frequencies is attributed to a substantial increase of tan delta. The applicability of another relaxor-type BaTiO3-based ceramic, Ba(Ti0.70Zr0.30)O-3, which was recently proposed as promising material for tunable MW applications, is also discussed. It is demonstrated that BaTiO3-based ferroelectric-relaxors may exhibit good tunable characteristics at 10 kHz; however, they are not competitive with BST for high K-factor MW applications.

Calibration of Capacitance Probe Sensors using Electric Circuit Theory

Abstract: Capacitance probe sensors are an attractive electromagnetic technique for estimating soil water content. There is concern, however, about the influence of soil salinity and soil temperature on the sensors. We present an electric circuit model that relates the sensor frequency to the permittivity of the medium and that is able to correct for dielectric losses due to ionic conductivity and relaxation. The circuit inductance L is optimized using sensor readings in a modified setup where ceramic capacitors replace the sensor's capacitance plates. The three other parameters in the model are optimized using sensor readings in a range of nonconductive media with different permittivities. The geometric factor for the plastic access tube gp is higher than the geometric factor for the medium gm, indicating that most of the electromagnetic field does not go beyond the access tube. The effect of ionic conductivity on the sensor readings is assessed by mixing salts in three of the media. The influence is profound. The sensor frequency decreases with increasing conductivity. The effect is most pronounced for the medium with the lowest permittivity. The circuit model is able to correct for the conductivity effect on the sensors. However, as the dielectric losses increase, the frequency becomes relatively insensitive to permittivity and small inaccuracies in the measured frequency or in the sensor constants result in large errors in the calculated permittivity. Calibration of the capacitance sensors can be simplified by fixing two of the constants and calculating the other two using sensor readings in air and water.

Crystalline Structure and Dielectric Properties of Li1+x−yNb1−x−3yTix+4yO3M-Phase Solid Solutions

Abstract: The structure and dielectric properties of the so-called M-phase solid solutions in the Li2O–Nb2O5–TiO2system have been investigated Detailed studies of the lattice parameters of these phases agree well with structure models based on intergrowths of LiNbO3slabs with a titanium-rich corundum-type layer The relative permittivity ranges from ∼80 to ∼55; microwave quality factors increase with the titanium content, reaching values of Q×f= 9000 at 6 GHz The temperature coefficient of the resonant frequency changes sign within the solid solution region, thus permitting tunability to a zero value All the compositions can be sintered to high density at temperatures ≤1100°C

Microwave permittivity and dielectric relaxation of a high surface area activated carbon

Abstract: Carbonaceous materials are amenable to microwave heating to varying degrees. The primary indicator of susceptibility is the complex permittivity (e*), of which, the real component correlates with polarization, and the imaginary term represents dielectric loss. For a given material, the complex permittivity is dependent upon both frequency and temperature. Here we report the complex permittivity of a high surface area coconut shell activated carbon which is commonly used in analytical chemistry and a wide variety of industrial separations. Associated polarization-relaxation phenomena are also characterized. Broadband measurements were made using a high temperature compatible open-ended coaxial dielectric probe at frequencies between 0.2 and 26 GHz, and across the temperature region between 24 °C and 191 °C.

Strontium Zirconate and Strontium Titanate Ceramics for High-Voltage Applications: Synthesis, Processing, and Dielectric Properties

Abstract: The synthesis, processing, and electrical properties of SrZrO3 and SrTiO3 materials have been examined. Phase-pure powders of SrZrO3 and SrTiO3 materials were synthesized using the Pechini method. Powder processing routes that used water and 2-propanol as carrier fluids were developed to achieve high green densities, which resulted in sintered densities of >99% of the theoretical density. The relative permittivity and average breakdown strength for carefully processed SrZrO3 were 60 and 40 V/μm; the corresponding values for SrTiO3 were 400 and 35 V/μm. The higher breakdown strengths suggest that these materials can be used in high-voltage capacitor applications.

Arrangements of microstrip antennas having dielectric substrates including meta-materials

Abstract: A slot fed microstrip patch antenna (300) includes a conducting ground plane (308), the conducting ground plane (308) including at least one slot (306) A dielectric material is disposed between the ground plane (308) and at least one feed line (317), wherein at least a portion of the dielectric layer (313) includes magnetic particles (324) The dielectric layer between the feed line (317) and the ground plane (308) provides regions having high relative permittivity (313) and low relative permittivity (312) At least a portion of the stub (318) is disposed on the high relative permittivity region (313)

Effective permittivity of nanocomposite powder compacts

Abstract: Advances in nanotechnology have led to a variety of new materials with strong potential applications to microwave and millimeter-wave components, e.g. dispersions of nanoscale particles, nanoparticle-filled polymers, self-assembled nanolattices of magnetic particles. More specifically, the properties of nanocomposites can be tailored for operation as insulators, ferro- and ferrimagnetic materials, highly conductive materials as well, for specific applications. In this study, we have investigated the electromagnetic response at microwave frequencies, using frequency domain network analysis, of cold-pressed powder compacts made of Ni, /spl gamma/-Fe/sub 2/O/sub 3/, Co, and ZnO nanosized powders. Effective complex permittivities of composites over the frequency range (100 MHz-10 GHz) as a function of composition were studied. Within the frequency range of measurements the real and imaginary parts of the effective permittivity of nanocomposites exhibit spectra which can be analytically well represented by power laws. The associated power law exponents, which are similar for the real and imaginary parts of the permittivity, are in the range 0.05-0.20 in agreement with data in the published literature. The dependences of the effective permittivity vs. composition are compared to those obtained from the effective medium theory of Bruggeman, which is found not to be adequate for all nanocomposites studied.

Preparation, characterization and microwave dielectric properties of Ba(B1/2′Nb1/2)O3 [B′ = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb and In] ceramics

Abstract: Microwave dielectric resonators (DRs) based on Ba(B 1/2 ′Nb 1/2 )O 3 [B′=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In] complex perovskites have been prepared by conventional solid state ceramic route. The dielectric properties (relative permittivity, e r ; quality factor, Q ; and resonant frequency, τ f ) of the ceramics have been measured in the frequency range 4–6 GHz using resonance methods. The resonators have relatively high dielectric constant in the range 36–45, high quality factor and small temperature variation of resonant frequency. The dielectric properties are found to depend on the tolerance factor ( t ), ionic radius ( r ), and lattice parameter ( a p ).

Relationship between microwave and lattice vibration properties in Ba(Zn1/3Nb2/3)O3-based microwave dielectric ceramics

Abstract: The dielectric properties of (1 − x)Ba(Zn1/3Nb2/3)O3–xBa (Ga1/2Ta1/2)O3 (BZN–xBGT) microwave (MW) ceramics, with x between 0 and 0.2, and those of 0.9Ba(Zn0.6Co0.4)1/3Nb2/3O3–0.1Ba(Ga0.5Ta0.5)O3 (BZCN–BGT) were studied at MW, terahertz (THz) and infrared (IR) frequencies at temperatures from 10 to 300 K. At room temperature, the temperature coefficient of resonance frequency (τf) near 3 GHz decreases from 28 ppm K−1 in undoped BZN to 2 ppm K−1 in BZN–0.2BGT and reduces to zero in BZCN–BGT. The addition of BGT to BZN depresses the dielectric Q value, but incorporation of Co improves the Q values, yielding Q ~ 30 000 at 3 GHZ in BZCN–BGT. The relative permittivity (e') exhibits only limited variation with composition (e' values in the range 34.4–36.0). IR and THz spectra as well as the low-temperature MW dielectric measurements revealed a weak dielectric relaxation below phonon frequencies, possibly arising from charges caused by inhomogeneous distribution of the B-site ions with differing valences. The IR reflectivity spectrum of BZN–0.2BGT is significantly different (smeared) compared with other compositions, which may be caused by disorder on the B sites and by an amorphous phase at the grain boundaries.

Broadband complex permittivity measurements of dielectric substrates using a split-cylinder resonator

Abstract: We discuss a theoretical model describing the split-cylinder resonator for non-destructive measurement of a dielectric substrate's relative permittivity and loss tangent. This improved model properly accounts for the fringing electric and magnetic fields in the dielectric substrate. Previously, the split-cylinder resonator has been used for single-frequency permittivity and loss tangent measurements using only the fundamental TE/sub 011/ resonant mode. By including high-order TE/sub 0np/ modes, we demonstrate how to measure the relative permittivity and loss tangent of dielectric substrates over an extended frequency range. We validated the new model by measuring the permittivity and loss tangent of fused-silica substrates from 10 to 50 GHz and comparing with results obtained with circular-cylindrical cavity, a dielectric-post resonator, and several split-post resonators.

Complex permittivity, permeability and electromagnetic wave absorption of α-Fe/C(amorphous) and Fe2B/C(amorphous) nanocomposites

Abstract: The complex permittivity, permeability and electromagnetic (EM) wave absorption properties have been investigated for resin compacts containing 75 wt% composite powders of α-Fe/C(a) and Fe2B/C(a). The real and imaginary parts ( and er') of relative permittivity for the resin composites remained almost constant in the 0.05–20.05 GHz range. The imaginary part of the relative permeability (μr') exhibited a wide peak in the 1–9 GHz range for α-Fe/C(a) and in the 2–18 GHz range for Fe2B/C(a). Both resin composites exhibited good EM wave absorption properties (RL < −20 dB) in the 4.4–8.3 GHz range with an absorber thickness of 1.9–3.4 mm, and in the 7.5–16 GHz range for an absorber thickness of 1.2–2.2 mm, respectively.

Microwave Dielectric Properties of Low-Fired Ba5Nb4O15

Abstract: The effect of B2O3 on the sintering temperature and microwave dielectric properties of Ba5Nb4O15 has been investigated using X-ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba5Nb4O15 and B2O3 led to formation of second phases, BaNb2O6 and BaB2O4. The addition of B2O3 to Ba5Nb4O15 resulted in lowering the sintering temperature from 1400° to 925°C. Low-fired Ba5Nb4O15 could be interpreted by measuring changes in the quality factor (Q×f), the relative dielectric constant (ɛr), and the temperature coefficient of resonant frequency (τf) as a function of B2O3 additions. More importantly, the formation of BaNb2O6 provided temperature compensation. The microwave dielectric properties of low-fired Ba5Nb4O15 had good dielectric properties: Q×f= 18700 GHz, ɛr= 39, and τf= 0 ppm/°C.

Impedance spectroscopy of Bi4Ti3O12 ceramic produced by self-propagating high-temperature synthesis technique

Abstract: Bi 4 Ti 3 O 12 (bismuth titanate—BIT) ferroelectric ceramic was synthesized by self-propagating high-temperature synthesis (SHS) technique. The microstructure, electric and dielectric properties were determined and the results were compared to a reference sample produced by solid state reaction. The powders from SHS had agglomerated particles with average size of 200 nm. Bi 4 Ti 3 O 12 100% crystalline phase was reached after SHS reaction followed by sintering at 1050°C/2 h. The sintered bodies presented high relative density (98%) and small grain size (around 5 microns). The electrical and dielectric properties of the samples were studied using the impedance spectroscopy technique, and the three observed semicircles in the complex impedance diagrams were associated to the bulk, plate boundaries and grain boundaries of the sintered ceramics. The samples produced through SHS presented the same bulk conductivity as the reference one, and lower conductivity of the grain and plate boundaries. The same dielectric constant e ′=200, at 300 °C and f =1 MHz, was found for SHS sample and for the reference. For higher temperatures, at this same frequency, SHS samples presented higher relative permittivity than the reference, and for temperatures above T c both samples presented the same Curie–Weiss parameters. The differences observed in the electrical conductivity and relative permittivity, were attributed to the differences in the grain size and defect concentration. The anomaly in the permittivity curves near 550 °C is also discussed.

Ferroelectric and dielectric anisotropy in high-quality SrBi2Ta2O9 single crystals

Abstract: High-quality SrBi2Ta2O9 single crystals were obtained by a self-flux solution method. The crystals were naturally oriented with [001] direction perpendicular to the major face and edges parallel to [110] axes. The dielectric and ferroelectric properties of the crystals were investigated along the c axis and in the ab plane of the orthorhombic unit cell. The ferro–paraelectric phase transition was observed at TC=355°C. The anisotropy of dielectric permittivity, i.e., the ratio between average permittivity in the ab plane and along c axis was about 10 at TC and decreased to ∼2 at room temperature. Saturated hysteresis loops were observed for switching in the ab plane. The spontaneous polarization along ferroelectric a axis was estimated to be ≈20μC∕cm2.

A spiral antenna sandwiched by dielectric layers

Abstract: An infinitesimally thin spiral antenna, sandwiched by bottom and top dielectric layers having the same relative permittivity, is analyzed under the condition that the dielectric layers are of finite extent and the antenna is backed by an infinite conducting plane. As the thickness of the top dielectric layer increases, the input impedance and axial ratio (AR) vary in an oscillatory fashion, with a period slightly larger than one-half of the guided wavelength of a wave propagating in an unbounded dielectric material. These oscillatory variations are reduced by adding a layer, called the "anti-reflection layer (ARL)," to the top dielectric layer. A representative spiral antenna with an ARL shows a frequency bandwidth of approximately 11% for a 3-dB AR criterion, having a gain of approximately 13 dBi (6 dBi higher than a printed spiral antenna without the top dielectric and ARL) and a voltage standing wave ratio of less than 1.2.

Seawater dielectric permittivity model from measurements at L band

Abstract: The knowledge of the seawater dielectric properties is very important in the areas of remote sensing. The models mainly used today seems not to be enough accurate for the actual applications. A new model has been derived from measurements at L Band.

Enhancement of the dielectric permittivity of Ta2O5 ceramics by CO2 laser irradiation

Abstract: The tantalum-pentoxide ceramics with significantly enhanced dielectric permittivities around 60–70 (up to 7162) at 1MHz and 20°C were prepared by CO2 laser irradiation The dielectric loss and the temperature coefficient of dielectric permittivity could be 00128 and 721ppm∕°C, respectively Dielectric properties were investigated over the temperature range from −60 to 100°C at 1MHz The frequency dependence of the dielectric properties was also investigated between 1kHz and 1MHz at 20°C The stabilization of high-temperature phase at RT and crystallographic orientation in the ceramics by laser irradiation can explain the strong enhancement of the dielectric permittivity

Electric-Field-Controlled Permittivity Ferroelectric Composition for Microwave LTCC Modules

Abstract: A low-temperature cofired ferroelectric ceramic composition for electrically tunable radio frequency devices is introduced. A sintering temperature of 950°C for BaSrTiO3–MgO was obtained following the combined addition of B2O3 and Li2CO3. The effects of these sintering aids on densification, microstructure, and ferroelectric properties were investigated. The permittivity and dissipation factor values were 234 and 0.0010, respectively, measured at 286.3 K, 1 kHz. The electrically tunable ferroelectric structure for microwave measurement was fabricated throughout the tape-casting process. Measured permittivity was 130 at 26 GHz and tunability >15% (4 V/μm).