Microwave Cavity Perturbation Measurements Research Articles

Temperature Correction Using Degenerate Modes for Cylindrical Cavity Perturbation Measurements

Microwave cavity perturbation measurements are a useful way to analyze material properties. Temperature changes can be introduced during these measurements either intentionally or as a result of some other process. The microwave cavity itself also has a temperature-dependent response, which can affect the results. A common method to correct is to use another resonant mode separately to the measurement mode, which is not affected by the sample. Instead of using independent modes, this paper describes a method to use split degenerate TMm10 modes of cylindrical cavities. TMm10 consists of two modes with identical field patterns with a relative rotation between them and identical resonant frequencies. A strategically placed perturbation reduces the frequency of one of the TMm10 modes and affects the coupling of both modes by reconfiguring the fields. This can be used for temperature correction by placing a sample such that both modes are equally coupled. The lower frequency, the perturbed mode is used as a measurement mode. The higher mode is used as a reference for temperature correction as it is unaffected by the sample. This technique was verified by measuring the permittivity of pure water using an aluminum microwave cavity resonator at 3.96 GHz. The temperature was swept between 20 °C and 60 °C, and the results was verified against the literature.

Microwave Permittivity of Trace sp2 Carbon Impurities in Sub-Micron Diamond Powders.

Microwave dielectric loss tangent measurements are demonstrated as a method for quantifying trace sp2-hybridized carbon impurities in sub-micron diamond powders. Appropriate test samples are prepared by vacuum annealing at temperatures from 600 to 1200 °C to vary the sp2/sp3 carbon ratio through partial surface graphitization. Microwave permittivity measurements are compared with those obtained using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electron energy loss spectroscopy (EELS). The average particle size remains constant (verified by scanning electron microscopy) to decouple any geometric dielectric effects from the microwave measurements. After annealing, a small increase in sp2 carbon was identified from the XPS C 1s and Auger spectra, the EELS σ* peak in the C 1s spectra, and the D and G bands in Raman spectroscopy, although a quantifiable diamond to G-band peak ratio was unobtainable. Surface hydrogenation was also evidenced in the Raman and XPS O 1s data. Microwave cavity perturbation measurements show that the dielectric loss tangent increases with increasing sp2 bonding, with the most pertinent finding being that these values correlate with other measurements and that trace concentrations of sp2 carbon as small as 5% can be detected.

Temperature Correction for Cylindrical Cavity Perturbation Measurements

The need for accurate material property measurements using microwave cavities requires a form of compensation to correct for changes in temperature and other external influences. This paper details a method for temperature correcting microwave cavity perturbation measurements by monitoring two modes; one which is perturbed by the sample and one which is not (referred to as a nodal mode). The nodal modes used (TM310 and TE311 for an axial sample in a cylindrical cavity) are subject only to sample-independent influences. To demonstrate this technique, the bulk permittivity of a PTFE rod has been measured under varying temperature conditions. The results show that without correction, the measured temperature-dependent dielectric constant has large variations associated with the stepped and linear temperature ramping procedures. The corrected response mitigates systematic errors in the real part. However, the correction of the imaginary part requires careful consideration of the mode coupling strength. This paper demonstrates the importance of temperature correction in dynamic cavity perturbation experiments.

Dielectric properties of myoglobin at 10 GHz by microwave cavity perturbation measurements

We report on the temperature dependence, at microwave (mw) frequency, of the imaginary part of the dielectric constant (ε″) in myoglobin powder samples with different hydration levels (h). The measurements have been performed by the cavity perturbation technique, in the range of temperature 80–345 K. The sample is located inside a glass capillary along the axis of a cylindrical copper cavity, resonating in the TE011mode at 9.6 GHz, where the mw electric field has a node. By measuring the variation of the quality factor of the resonant cavity, one can extract the imaginary part of the dielectric constant. At temperatures higher than 230 K we observe an evident increase of the dielectric losses with increasing temperature; the effect scales almost linearly with hydration, indicating that it must be attributed to a relaxation of water in the hydration shell of the protein. Furthermore, ath≥0.18, we observe a clear peak in the ε″ vs.Tcurve, that shifts towards lower temperatures upon increasing hydration; this shows that the activation enthalpy of the hydration water relaxation decreases with hydration. More in general, our data show that the technique of microwave cavity perturbation allows one to study the dynamics of water molecules in the hydration shell of proteins and to extend information obtained with dielectric techniques to the mw frequencies.

Dielectric properties of myoglobin at 10 GHz by microwave cavity perturbation measurements

We report on the temperature dependence, at microwave (mw) frequency, of the imaginary part of the dielectric constant (e �� ) in myoglobin powder samples with different hydration levels (h). The measurements have been performed by the cavity perturbation technique, in the range of temperature 80-345 K. The sample is located inside a glass capillary along the axis of a cylindrical copper cavity, resonating in the TE011 mode at 9.6 GHz, where the mw electric field has a node. By measuring the variation of the quality factor of the resonant cavity, one can extract the imaginary part of the dielectric constant. At temperatures higher than 230 K we observe an evident increase of the dielectric losses with increasing temperature; the effect scales almost linearly with hydration, indicating that it must be attributed to a relaxation of water in the hydration shell of the protein. Furthermore, at h 0.18, we observe a clear peak in the e �� vs. T curve, that shifts towards lower temperatures upon increasing hydration; this shows that the activation enthalpy of the hydration water relaxation decreases with hydration. More in general, our data show that the technique of microwave cavity perturbation allows one to study the dynamics of water molecules in the hydration shell of proteins and to extend information obtained with dielectric techniques to the mw frequencies.

Accuracy of microwave cavity perturbation measurements

Techniques based on the perturbation of cavity resonators are commonly used to measure the permittivity and permeability of samples of dielectric and ferrite materials at microwave frequencies. They are also used to measure the local electric- and magnetic-field strengths in microwave structures including the shunt impedances of cavity resonators and the coupling impedances of slow-wave structures. This paper reexamines the assumptions made in the theory of these techniques and provides estimates of the errors of measurement arising from them.

Microwave electrodynamics of the electron-doped cuprate superconductors Pr 2− xCe xCuO 4− y and Nd 2− xCe xCuO 4− y

The pairing-state symmetry of the electron-doped cuprate superconductors is thought to be s-wave in nature, in contrast with their hole-doped counterparts which exhibit a d-wave symmetry. We re-examine this issue based on recent improvements in our electron-doped materials and our measurement techniques. We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth of Pr 2− x Ce x CuO 4− y and Nd 2− x Ce x CuO 4− y crystals. Our data suggest that these materials do not have a large, or standard, s-wave gap in their excitation spectrum.

Microwave electrodynamics of electron-doped cuprate superconductors

We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth, lambda(T), and conductivity, sigma(T) of Pr(2-x)Ce(x)CuO(4-delta) (PCCO) crystals, as well as parallel-plate resonator measurements of lambda(T) in PCCO thin films. Penetration depth measurements are also presented for a Nd(2-x)Ce(x)CuO(4-delta) (NCCO) crystal. We find that Deltalambda(T) has a power-law behavior for T<T(c)/3, and conclude that the electron-doped cuprate superconductors have nodes in the superconducting gap. Furthermore, using the surface impedance, we have derived the real part of the conductivity, sigma(1)(T), below T(c) and found a behavior similar to that observed in hole-doped cuprates.

The synthesis, solid state conductivity and X-ray crystal structure of [Fe{ η5-C 5H 5)( E- η5-C 5H 4–CHCH-9-C 16H 9)][1,4-{(CN) 2C} 2C 6H 4

The new compounds [Fe( η 5-C 5H 5)( E- η 5-C 5H 4–CHCH-9-C 16H 9)], [Fe( η 5-C 5H 5)( E- η 5-C 5H 4–CHCH-9-C 16H 9)][(CN) 2CC(CN) 2] and [Fe( η 5-C 5H 5)( E- η 5C 5H 4–CHCH-9-C 16H 9)][1,4-{(CN) 2C} 2C 6H 4] have been synthesized and characterized by their mass, IR and 1H NMR spectra. The crystal structure of the latter compound contains 1,2-ferrocenyl-pyrenyl-ethene and 1,4-{(CN) 2C} 2C 6H 4 (TCNQ) molecules alternating in stacks characteristic of π– π* electron donor–acceptor complexes. The pyrene and TCNQ moieties are nearly parallel, angle of tilt 5.3°, with interplanar separations of 3.33 and 3.42 Å. Microwave cavity perturbation measurements have been used to show that this solid behaves as a normal semi-conductor below 176 K.

Towards the Synthesis of Atomic Scale Wires

ABSTRACTRecent work1 has highlighted the possibility that through the introduction of metals into the one-dimensional channels of zeolite L, it may be feasible to engineer charge transport along the channels to produce a unique compound comprising a precise, assembled array of ultrafine, atomic-scale conducting wires embedded within the aluminosilicate framework. Using electron spin resonance (ESR), and microwave cavity perturbation measurements, we examine the properties of these remarkable materials as a function of composition as they approach the insulator to metal transition.