Neil Mcneill Alford

Measurements of Permittivity, Dielectric Loss Tangent, and Resistivity of Float-Zone Silicon at Microwave Frequencies

The complex permittivity and resistivity of float-zone high-resistivity silicon were measured at microwave frequencies for temperatures from 10 up to 400 K employing dielectric-resonator and composite dielectric-resonator techniques. At temperatures below 25 K, where all free carriers are frozen out, loss-tangent values of the order of 2times10-4 were measured, suggesting the existence of hopping conductivity or surface charge carrier conductivity in this temperature range. Use of a composite dielectric-resonator technique enabled the measurement of materials having higher dielectric losses (or lower resistivities) with respect to the dielectric-resonator technique. The real part of permittivity of silicon proved to be frequency independent. Dielectric losses of high-resistivity silicon at microwave frequencies are mainly associated with conductivity and their behavior versus temperature can be satisfactory described by dc conductivity models, except at very low temperatures

Properties and applications of thick film high temperature superconductors

Melt processed YBa/sub 2/Cu/sub 3/O/sub x/ thick films display low surface resistance, moderate performance in fields and can be applied to three-dimensional (3-D) substrates with ease. The processing and properties of such films are described. Possible applications are examined and prototype devices are described. These include high Q, low frequency resonators for cellular communications filters, low phase noise oscillators, magnetic resonance imaging receiver coils, low noise magnetic shields, coils, flux transformers, and antennas.

Ceramic Dielectrics for Microwave Applications

Publisher Summary This chapter reviews material properties, processing, and physics of dielectric loss that are important for application of ceramic dielectrics for microwave. A wide range of materials can be considered to be ceramic dielectrics. Materials available for use in dielectric resonators have dielectric constants ranging from 10–100. In recent years good progress has been made in processing ceramic dielectric materials. As the mobile and satellite communications industry has grown, dielectric resonators have become an important technology. Although there has been considerable progress in processing, there is still no satisfactory model to accurately describe the dielectric loss of polycrystalline ceramics. To improve the properties of existing materials, a more in-depth understanding of the physics of the loss is required. The drive toward miniaturization requires the development of materials with a higher dielectric constant. Recent advances in cryogenic technology have opened new possibilities for ceramic dielectric resonators. This may even resurrect interest in materials that were previously discarded as impractical. The frequency range important for microwave communications is 500 MHz–30 GHz. Two main types of resonator are currently in use. The choice of resonator type depends on the operating frequency. For higher frequencies, above 2 GHz, dielectric resonators can be used. The aim of a significant portion of the research into dielectrics is to develop materials with higher dielectric constants that can be used to produce dielectric resonators for lower frequency applications.

Oxygen transport during formation and decomposition of AgNbO3 and AgTaO3

A thermogravimetric method was used to analyze intermediate processes involved in the formation and decomposition of AgNbO 3 and AgTaO 3 perovskites. Critical parameters that control the kinetics of the formation are associated with oxygen transport. The Nb 2 O 5 crystal structure has a capacity to trap molecular oxygen, which evolves during the decomposition of Ag 2 O that is present in a starting mixture. The formation of the perovskite phase involves a simultaneous reaction of three species: O 2 , Ag, and Nb 2 O 5 /Ta 2 O 5 . As the trapped molecular oxygen is in the immediate vicinity of the reaction site, the kinetics of the reaction is significantly accelerated. An absence of the molecular oxygen in the solid-state phase cannot be compensated for with an increase in a partial pressure of oxygen in the gas phase, that is, application of oxygen atmosphere.

Highs and lows in superconductivity

The other day I was discussing magnetic fields in superconductors with a friend of mine, a fellow Scot as it happens. He knows a thing or two about critical currents and magnetic fields in these materials and was pointing out the huge differences in what is perceived to be a high magnetic field. To people working on superconducting quantum interference devices (SQUIDs), a few millitesla is a high field. However, those working on magnets consider 20–30 T to be high. It is therefore important for all of us working in superconductivity to understand exactly what each section of the community means when talking about high or low currents or fields. This book attempts to explain the issues in straightforward terms.