N McN Alford

Dielectric loss of oxide single crystals and polycrystalline analogues from 10 to 320 K

The key factors influencing microwave dielectric loss are examined. A comparison is made between single crystals and polycrystalline analogues. Measurements of the temperature dependence of microwave dielectric losses in various materials are reported, for temperatures between 20 and 300 K. Single crystal and polycrystalline TiO2, LaAlO3, MgO and Al2O3 are considered. The temperature dependence of dielectric losses of certain single crystals (MgO and Al2O3) are found to be in good agreement with the theory of intrinsic losses for temperatures above 100 K. At lower temperatures losses due to defects and grain boundaries dominate. The absolute value of the loss predicted by theory is considerably lower than measured values. # 2001 Published by Elsevier Science Ltd. All rights reserved.

On the temperature coefficient of resonant frequency in microwave dielectrics

Abstract Developing a material with a temperature coefficient of the resonant frequency (TCF) of zero is probably the most difficult aspect of research into microwave dielectric ceramics. There are a host of high-quality-factor (Q) high-relative-permittivity (ϵr) materials whose TCF is far too high to be usable, notably TiO2 (ϵr = 104; Q = 14000 at 3GHz: TCF = +427 ppm K−1). The challenge is to find ways of tuning TCF while maintaining values of ϵr and Q suitable for use in microwave components. This paper explains, in terms of octahedral tilting and ϵr dilution, the tunability of TCF or, more precisely, the temperature coefficient of ϵr in Ca and Sr titanate-based ceramics. Examples of the structural changes that occur during tuning are also shown.

Specific heat of La1−xCaxMnO3−δ

Abstract Detailed specific heat measurements from 4.5 to 300xa0K are reported on a series of polycrystalline La1−xCaxMnO3−δ samples, with x=0.10, 0.33 and 0.62. The samples were also investigated using electrical transport, magnetic measurements and low temperature NMR. The x=0.10 composition does not show a well defined transition in the specific heat at the ferromagnetic ordering temperature, but the low temperature specific heat shows a ferromagnetic T3/2 spin wave term, and no electronic linear term as expected for an insulator. The x=0.33 composition undergoes a transition to a ferromagnetic metal with a well defined peak in the specific heat, suggestive of an underlying first order transition at Tc; at low temperatures a linear electronic specific heat term is observed but no spin wave ferromagnetic term. High temperature specific heat data on the x=0.62 sample shows only one anomaly, suggesting simultaneous onset of charge ordering and antiferromagnetism. Low temperature data indicates that the magnetic phase is an A-type antiferromagnetic insulator with no electronic contribution. The NMR data at 4.2xa0K on this compound shows a motionally narrowed peak indicating short range ferromagnetic interactions or possibly magnetic polarons at 4.2xa0K.

Temperature compensated niobate microwave ceramics with the columbite structure, M2+Nb2O6

Abstract Of the niobate ceramics with the formula M 2+ Nb 2 O 6 , several compounds (M 2+ =Zn, Mg, Ca and Co) have Qf values (at f =1–10 GHz) between 40u2008000 and 90u2008000 GHz, offering potential in dielectric resonator applications. However, their temperature coefficient of resonant frequency ( τ f ) values are too high for commercial development, at between −50 and −90 ppm. This paper details the doping of these materials with dielectric ceramics having a large positive τ f , in an attempt to reduce the overall τ f to zero, whilst maintaining a high quality factor ( Q ). It was also found that doping increased the relative permitivity ( e r ) of the niobates. Several materials have been made with near-zero τ f , such as 90% CoNb 2 O 6 /10% CaTiO 3 ( τ f =+2.0 ppm, e r =25.2 and Qf =21u2008700 GHz), and 94% CoNb 2 O 6 /6% TiO 2 ( τ f =+4.4 ppm, e r =29.6 and Qf =20u2008300 GHz)

Influence of oxygen vacancies on magnetoresistance properties of bulk La0.67Ca0.33MnO3-δ

Abstract Bulk polycrystalline samples of La 0.67 Ca 0.33 MnO 3− δ were annealed in controlled oxygen atmospheres. Two types of conditions were chosen, firstly with the aim of taking oxygen from the grain boundaries and secondly with the aim of removing oxygen uniformly from the sample. Reducing conditions increase the overall resistivity and suppress the Curie–Weiss temperature ( T c ). A second peak at temperatures below T c appears in the resistivity and magnetoresistivity when the samples are deoxygenated. However, the presence of this peak does not strongly influence the low-temperature magnetoresistance.

Structure and microwave dielectric properties of La(Mg0.5Ti0.5)O3-CaTiO3 system

(1-x)La(Mg 0.5 Ti 0.5 )O 3 (LMT)-xCaTiO 3 (CT) [0<x<1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT-0.1CT composition was refined using P2 1 /n space group, which allows taking into account B-site ordering. The compounds having x≥0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content.

Processing, properties and devices in high-Tc superconductors

The exploitation of high-Tc superconductors will ultimately depend on successful manufacturing methods. This paper describes methods which have been used to produce a wide range of superconducting devices. These include wire, antennae, microwave resonators, flux transformers, solenoid coils and gradiometers. The properties of the superconducting ceramics are described and the design and manufacture of prototype devices are discussed.

Techniques for microwave measurements of ferroelectric thin films and their associated error and limitations

This paper examines the problem of evaluating the microwave properties of thin ferroelectric films patterned as planar capacitors. Two types of microwave measurements of ferroelectric thin films are considered: reflection- and resonance-type measurements. Algorithms are presented for evaluation of capacitance-permittivity and dielectric loss. Using sensitivity analysis, the error and limitations associated with these measurements are estimated. The end result is a series of formulae that use the network analysers measurement data to calculate the capacitance-permittivity, the dielectric loss and the associated error.

Influence of grain size on magnetoresistance properties of bulk La0.67Ca0.33MnO3−δ

Abstract The magnetic and magnetoresistive properties of six polycrystalline bulk samples of nominal chemical composition La 0.67 Ca 0.33 MnO 3− δ and varying grain size (3.3xa0μm–20.4xa0μm), were investigated. Although the Curie temperature, T c , is independent of grain size, the variation of the saturation magnetisation with grain size depends on the magnetic history. At 20xa0K and 8xa0T applied field, the magnetisation decreases as the grain size increases. An approximate linear variation of resistivity and magnetoresistivity with grain size allows the use of a simple 1D model to extract the intrinsic grain magnetoresistance for these materials at T c and to separate the grain and grain boundary resistivity contributions. The relationship between resistivity and magnetisation is discussed.

Anisotropic and cyclic mechanical properties of piezoelectrics—compression testing

Abstract The mechanical stressing of PZT produces irreversible deformation by the irreversible switching of 90° domains. This leads to highly anisotropic deformation behaviour for poled materials. The threshold stresses required to produce this switching are relatively small,