High Permittivity Materials Research Articles

Colossal dielectric permittivity of BaTiO3-based nanocrystalline ceramics sintered by spark plasma sintering

In pursuit of high permittivity materials for electronic application, there has been a considerable interest recently in the dielectric properties of various perovskite oxides like calcium copper titanate or lanthanum doped barium titanate. When processed in a particular way, this later material present at ambient temperature and at f = 1 kHz unusual interesting dielectric properties, a so called “colossal” permittivity value up to several 106 with relatively low dielectric losses. Moreover and contrary to what is classically expected and evidenced for this type of materials, no temperature dependence is observed. This behavior is observed in nanopowders based ceramics. An assumption to explain the observed properties is proposed. These results have important technological applications, since these nanoceramics open a new route to the fabrication of very thin dielectric films.

A Novel Wideband Dielectric Resonator Antenna

A novel wideband dielectric resonator antenna is proposed. This antenna is comprised of a dielectric hemispherical shell made of high permittivity material and a ground made of low permittivity material. A conformal patch is used as the excitation structure. A quasi-omnidirectional radiation pattern in the upper half space and an impedance bandwidth up to 40% can be achieved in the simulation results. And the parameters affected the resonant frequency and bandwidth are studied.

Optical properties of epitaxial SrHfO3 thin films grown on Si

The perovskite SrHfO3 can be a potential candidate among the high-permittivity materials for gate oxide replacement in future metal-oxide semiconductor field-effect transistor technology. Thin films of SrHfO3 were grown by molecular beam epitaxy and compared with SrTiO3 films. Their optical properties were investigated using spectroscopic ellipsometry and analyzed with respect to their structural properties characterized by x-ray diffractometry, atomic force microscopy, and transmission electron microscopy. A band gap of Eg=6.1±0.1eV is measured optically, which renders this material better suited for gate dielectric applications than SrTiO3 with Eg∼3.4eV. At similar equivalent oxide thickness, SrHfO3 also exhibits lower gate leakage current than SrTiO3 does.

Strain-Gradient-Induced Polarization inSrTiO3Single Crystals

Piezoelectricity is inherent only in noncentrosymmetric materials, but a piezoelectric response can also be obtained in centrosymmetric crystals if subjected to inhomogeneous deformation. This phenomenon, known as flexoelectricity, can significantly affect the functional properties of insulators, particularly thin films of high permittivity materials. We have measured strain-gradient-induced polarization in single crystals of paraelectric SrTiO3 as a function of temperature and orientation down to and below the 105 K phase transition. Estimates were obtained for all the components of the flexoelectric tensor, and calculations based on these indicate that local polarization around defects in SrTiO3 may exceed the largest ferroelectric polarizations. A sign reversal of the flexoelectric response detected below the phase transition suggests that the ferroelastic domain walls of SrTiO3 may be polar.

High permittivity and low dielectric loss in ceramics with the nominal compositions of CaCu3−xLa2x∕3Ti4O12

Ceramics with the nominal compositions of CaCu3−xLa2x∕3Ti4O12 were prepared by the conventional solid-state reaction technique. It was found that these ceramics have the properties of rather high dielectric permittivity and sufficiently low dielectric loss. In particular, CaCu2.9La0.2∕3Ti4O12 ceramics sintered at 1050°C for 20h showed the high dielectric permittivity of 7500 with weak frequency dependence below 1MHz, the low dielectric loss less than 0.05 in the wide frequency range of 120Hz–200kHz (with the minimum 0.012 at 20kHz), and the small temperature coefficient about ±15ppm∕°C between −80 and 125°C. From the analysis, it was suggested that CaTiO3 secondary phase due to Cu deficiency and La doping plays the important roles in the observed excellent dielectric properties in CaCu3−xLa2x∕3Ti4O12 ceramics. The results indicated that they are a very promising class of high-permittivity materials for practical applications.

Phosphonic Acid‐Modified Barium Titanate Polymer Nanocomposites with High Permittivity and Dielectric Strength

Materials with high dielectric permittivity are important in electronic components such as capacitors, gate dielectrics, memories, and power-storage devices. Conventional highpermittivity materials such as barium titanate (BT) can be processed into thin films by using chemical solution deposition yielding a relative permittivity (er) of about 2500 and relatively low dielectric loss but require high-temperature sintering, which is not compatible with many substrate materials. Polymer-based dielectrics, such as biaxially oriented polypropylene (BOPP), have good processability with high dielectric strengths (∼ 640 V lm) suitable for high-energy-density capacitors, but the storage capacity (ca. 1–1.2 J cm) is limited by the low er (ca. 2.2) of these materials. [6] Various approaches to high-er materials based on nanocomposites containing metal particles or other conductive materials have been pursued. Such nanocomposites have afforded huge er values but the resulting materials are limited by the high-temperature processing required, high dielectric loss, or low dielectric strength. Polymer/ceramic nanocomposites in which high-er metal oxide nanoparticles such as BT and lead magnesium niobate–lead titanate (PMN–PT) are incorporated into a polymer host are of significant current interest. The combination of high-er nanoparticles with high-dielectric-strength polymer hosts offers the potential to obtain processable highperformance dielectric materials. Simple solution processing of BT particles in a polymer host generally results in poor film quality and inhomogeneities, which are mainly caused by agglomeration of the nanoparticles. Addition of surfactants, such as phosphate esters and oligomers thereof, can improve the dispersion of BT nanoparticles in host polymers and consequently the overall nanocomposite film quality. However, in such systems, residual free surfactant can lead to high leakage current and dielectric loss. Thus, approaches to bind surface modifiers to BT nanoparticles via robust chemical bonds are highly desirable. Ramesh et al. have reported on the use of trialkoxysilane surface modifiers for the dispersion of BT nanoparticles in epoxy polymer hosts resulting in nanocomposites with reasonably high er, up to 45. [12] With the objective of identifying ligands that can form stable bonds to a BT surface through coordination or condensation, we have investigated a series of different ligand functionalities. In this Communication, we report that phosphonic acid ligands effect robust surface modification of BT and related nanoparticles and that the use of particles modified with suitable phosphonic acid ligands leads to well-dispersed BT nanocomposite films with high er and high dielectric strength. We have investigated the binding of a variety of ligands to the surface of BT nanoparticles, as the stability of the binding on the surface is vital to effective surface modification. We examined the following set of ligands, each bearing an aliphatic octyl chain with a different terminal binding group: C8H17-X, where X = PO(OH)2 (OPA), SO2ONa (OSA), Si(OCH3)3 (OTMOS), and CO2H (OCA). Trialkoxysilanes are widely used surface modifiers for silicate, indium tin oxide, and other metal oxide surfaces. Phosphonic acids have been reported to modify TiO2, ZrO2, and indium tin oxide surfaces and are thought to couple to the surface of metal oxides either by heterocondensation with surface hydroxyl groups or coordination to metal ions on the surface. Carboxylic acid and sulfonic acid groups may also bind to the surface in a similar manner. A sample of each ligand was mixed with BT nanoparticles (30–50 nm, 0.5 mmol ligand/ g BT) in an ethanol/water solution and stirred at 80 °C, followed by extensive washing with ethanol or water and centrifugation to remove excess and/or physisorbed ligand. The treated BT nanoparticles were dried and characterized by using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). Figure 1a shows a comparison of FTIR spectra in the C–H stretching region for the BT nanoparticles treated with the ligands described above, followed by washing. These results C O M M U N IC A IO N

Measuring the effect of field strength on noise amplification factor

AbstractAs shown previously, parallel imaging techniques should benefit significantly from higher field strengths due to a reduction in noise amplification (g) factor. The intent of this study was to measure directly the role of field strength in this reduction, comparing the measured g factors for two commercial eight‐channel head array coils of similar mechanical design at 3T and 7T using low and high permittivity phantoms. The results indicated that the lower g factors result primarily from the RF standing wave pattern and the greater spatial complexity of the coil sensitivities in high permittivity materials at high frequency, although the physical design of the array elements plays an important role in controlling the growth of noise amplification. A simplified method of g factor measurement for an array coil is also presented. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 31B: 51–59, 2007

Dielectric permittivity and Cr 3+ impurity ion probe luminescence in SrTiO 3 sol–gel ceramics

In order to inspect bulk properties of the SrTiO 3 ceramics grains, dielectric permittivity and temperature shift of a zero-phonon R-emission line of the Cr 3+ impurity-ion probes were studied in sol–gel SrTiO 3:Cr (3 × 10 −2%) ceramics of the average grain size ∼24 μm. We found that non-dispersive permittivity (100 Hz to 1 MHz) increases on cooling less steeply than in single crystals reaching ∼5000 at 30 K. At lower temperatures the dispersion appears and the permittivity saturates at ∼8 K ( ɛ′ ≈ 7000, 400 Hz). However, the temperature shift of the R-line in ceramics remains practically identical to that for SrTiO 3 single crystals, in which R-line shifts with temperature is Δ ν ∼ 1/ ɛ′ ∼ ω TO 1 2 . This indicates that the bulk of the grains in ceramics exhibits properties inherent to SrTiO 3 quantum paraelectric, which agrees with the “generalized brick-wall” approach developed recently to describe polarization response of granular high permittivity materials.

Properties of surface waveguides derived from inversion of fundamental and higher mode dispersive GPR data

Thin surface layers of high-permittivity material (e.g., water-saturated soil) can cause a pronounced dispersion of ground-penetrating radar waves. The dispersion characteristics depend on the permittivity and thickness of the effective surface waveguide and the permittivity of the material below it. As for the analogous seismic case, the values of these parameters can be estimated by applying a scheme that includes calculating the phase-velocity spectra, picking dispersion curves from the spectra, and inverting the dispersion curves using a combined local and global minimization procedure. Here, this scheme is extended by incorporating higher order modes. Explicit expressions for the cutoff frequencies of the fundamental and higher order transverse electric (TE) and transverse magnetic (TM) modes are derived. These expressions demonstrate that the cutoff frequency decreases as the thickness and/or permittivity of the waveguide increase and/or the permittivity of the lower half-space decreases. In addition, the TE cutoff frequencies are shown to be generally lower than the TM cutoff frequencies. Numerical modeling of realistic models demonstrates the presence of the higher order modes with the correct cutoff frequencies. Application of the modified inversion scheme to synthetic and field data demonstrates its efficacy in providing the required physical property information. In particular, better-constrained models are obtained by including higher order modes in the inversion

Middle- and high-permittivity dielectric compositions for low-temperature co-fired ceramics

Various middle- and high-permittivity dielectric material systems for LTCC were examined. By designing several borosilicate glass frit systems carefully, we tried to lower the temperature of densification to 875 °C in selected host dielectric materials still maintaining acceptable dielectric properties. The effects of glass addition on the densification, electrical properties, and phase changes in the host dielectric materials were examined.

Sci‐Fri PM Imaging‐08: Development of Micro‐Strip Coils and “Direct” Method for Bilateral Breast Imaging

In this work, we present a position‐independent decoupling method using micro‐strip transmission line coils. Micro‐strip transmission line coils consist of a grounded back plane together with the conductive strip bonded to either side of a low‐loss, high permittivity material. This two‐part structure exhibits the shielding decoupling properties and excellent decoupling over a range of geometries. Its asymmetrical signal response operates so that signals from each individual breast originate primarily from the coil facing the breast. In this work we also present a parallel imaging method similar to Partially Parallel Imaging with Localized Sensitivities (PILS) method for bilateral breast MRI by taking advantage of the isolation properties of micro‐strip coils to allow independent imaging of both breasts in the time usually required to scan a single breast without resorting to traditional SENSE techniques.

Properties of surface waveguides derived from separate and joint inversion of dispersive TE and TM GPR data

Pronounced dispersion of ground-penetrating radar (GPR) waves is observed at locations distinguished by thin surface layers of high-permittivity material (e.g., water-saturated soil). The dispersion characteristics depend on the permittivity and thickness of the effective surface waveguide and the permittivity of the material below it. We introduce a scheme for estimating the values of these parameters from dispersed transverse-electric (TE) and transverse-magnetic (TM) GPR data that is analogous to recently developed methods for analyzing dispersed Rayleigh waves recorded on multichannel seismic data. Our scheme involves calculating phase-velocity spectra, picking dispersion curves from the spectra, and then inverting the dispersion curves for the subsurface material properties by using a combined local- and global-minimization procedure. Application of this new scheme to synthetic and field data demonstrates its efficacy in providing the required physical property information. Where the surface layer is relatively distinct and uniform, inversions of the resulting high-quality dispersed TE data provide all required parameters. In more heterogeneous environments, joint inversions of the TE and TM data, which usually include information in different frequency ranges, may be required.

Novel evaluation method for complex high permittivity of BaTiO 3 families at microwave frequency

A simple but practically accurate evaluation technique is proposed for the complex dielectric constant of high permittivity materials in microwave region using the discontinuity structure in coaxial line. The test fixture is designed using the accurate theoretical analysis, i.e., the extended spectral domain approach and mode matching method (ESDMM). The complex permittivity is estimated by inversely fitting the measured S-parameters of test fixture loaded with specimen to the most probable permittivity values of specimen using ESDMM simulation. The new method provides an easy way to evaluate the high complex permittivity exceeding 10,000, which is difficult to be measured by currently used measurement techniques.

Phase Stability in Nanocrystalline Zirconia

Zirconia can be considered to be one of the most important ceramic materials because of its large range of industrial applications (catalysis, coatings, spacecraft shielding, paint additives, oxygen sensors, fuel cells, nuclear fuel matrices, an alternative high permittivity material to replace silicon oxide as a gate dielectric in MOS devices). Many of these applications require the use of zirconia in a nanocrystalline form. It is now well established that a monoclinc to tetragonal phase transition is trigged by the grain size of zirconia. The mechanism of this phase transition in zirconia is not yet clearly understood. Several experiments point out that the thermodynamic properties of nanocrystalline solids are particle-size dependent. Size-related effects like the reduction of the melting temperature and displacement of the phase boundaries can be predicted. Zirconia can be considered a textbook example for describing these effects. In this ceramic several polymorphic transformations occur with the change of external parameters (Temperature, pressure, …). In this paper, the behaviour of the tetragonal to monoclinic martensitic phase transition within Landau theory framework in particular will be discussed, pointing out the peculiar effects related the small grain size of the nanoparticles. Neutron diffraction experiments will illustrate the of these arguments and provide some insight to the understanding of the behaviour of nanocrystals in severe environments, such as in nuclear reactors or in space applications.

Experimentally demonstrated filters based on guided resonance of photonic-crystal films

We demonstrate a guided resonance filter based on photonic crystals (PhCs), which are fabricated in a high-permittivity material. The resulting spectra from a three-dimensional analysis of the structure and experimental measurement results show sharp dips and flattop transmissions. These provide promising properties in constructing sensitive and compact wavelength-selective devices, such as wavelength-division multiplexers.

Temperature dependence of the permittivity and loss tangent of high-permittivity materials at terahertz frequencies

An analysis including the temperature dependence of the permittivity and loss tangent of three low-cost and high-permittivity materials (zirconium-tin-titanate, alumina, and titanium-dioxide) in the terahertz frequency range is presented. Such dielectric materials find varied applications in microwave and terahertz systems and components. Their effective use under varying environmental conditions or in space applications requires a detailed knowledge about temperature dependencies. Here, measurements using broad-band terahertz time-domain spectroscopy are presented in the temperature range from 10 to 323 K. It is shown that zirconium-tin-titanate and alumina provide a good thermal stability of the permittivity, whereas the permittivity of titanium-dioxide exhibits a strong dependence on the temperature.

Wideband coplanar waveguide-to-rectangular waveguide transition using fin-line taper

This letter introduces a new wideband coplanar waveguide-to-rectangular waveguide transition. The transition uses a uniplanar circuit in line with the waveguide, which eases the design and fabrication. The design does not require airbridges. Simulations and measurements of X-band (8.2-12.4 GHz) transitions based on both a low- and high-permittivity material (/spl epsiv//sub r/= 2.33 and 10.8) show that the transition works fine over the full frequency band. For /spl epsiv//sub r/= 2.33 the measured return and insertion loss of a back-to-back transition are more than 16 dB and less than 0.4dB, respectively. The corresponding values for /spl epsiv//sub r/= 10.8 are more than 10 dB and less than 1.0 dB, respectively, over 90% of the frequency band. The measured insertion loss values indicate losses of less than 0.14 dB and 0.36 dB at the center frequency for a single transition on a substrate with /spl epsiv//sub r/= 2.33 and 10.8, respectively.

Comparative study on electrical properties of atomic layer deposited high-permittivity materials on silicon substrates

Deep level transient spectroscopy, capacitance–voltage and conductance transient measurement techniques have been applied in order to evaluate the electrical quality of thin high-permittivity oxide layers on silicon. The oxides studied included HfO2 film grown from two different oxygen-free metal precursors and Ta2O5 and Nb2O5 nanolaminates. The interface trap densities correlated to the oxide growth chemistry and semiconductor substrate treatment. No gap state densities induced by structural disorder were measured in the films grown on chemical SiO2. Trap densities were also clearly lower in HfO2 films compared to Ta2O5–Nb2O5.

Novel Evaluation Method of the Complex Dielectric Constant of High-Permittivity Material at Microwave Frequency

A novel accurate measurement method for high-permittivity materials in the microwave region, by adopting a discontinuity structure in the coaxial transmission line, is proposed. Electromagnetic field analysis is carried out using the extended spectral domain approach in combination with the mode-matching method (ESDMM). Numerical results obtained by ESDMM are compared with the values obtained by the finite element method (FEM) and excellent agreement is observed. However, ESDMM is more efficient in numerical computations and can be utilized for the iterative fitting of the measured values to identify the material. A simple but practically effective characterization method for high-permittivity materials at microwave frequency was developed.

Polar Ceramics in RF-MEMS and Microwave Reconfigurable Electronics: A Brief Review on Recent Issues

Properties and fabrication status of microdevices for microwaves based on polar ceramics are reviewed. We discuss bulk acoustic wave devices with AlN films, rf-MEMS capacitive switches with high permittivity materials, and tunable ferroelectrics. The relevant properties of ferroelectrics for microwave applications are summarized with emphasis on composites and thin films.