Casey Mungle

The 500 MHz to 5.50 GHz complex permittivity spectra of single-wall carbon nanotube-loaded polymer composites

Abstract The 500 MHz to 5.50 GHz complex permittivity spectra of a thick-film polymer loaded with 0–23 wt% single-wall carbon nanotubes is measured. At 500 MHz, as the weight percentage loading of the carbon nanotubes increases from 0 to 23% the real permittivity is found to increase by a factor of ∼35, and the imaginary permittivity by a factor of 1200. The spectral magnitudes decrease rapidly from the 500 MHz value over the measured frequency range. Experimental data are in qualitative agreement with values predicted using an effective medium theory for materials comprised of elongated cylindrical conductors [A.N. Lagarkov, A.K. Sarychev, Phy. Rev. B 53 (1996) 6318].

Effect of purification of the electrical conductivity and complex permittivity of multiwall carbon nanotubes

In this work we report on the complex permittivity spectra and electrical conductivity of both as-fabricated and graphitized multiwall carbon nanotubes (MWNTs). The high-temperature annealing removes the Fe3C catalyst particles present in the as-fabricated material, enabling the intrinsic MWNT properties to be measured. The permittivity spectra of 1 wt % MWNT-polystyrene composite films are measured from 75 to 1875 MHz. Comparison of measurements with an appropriate effective medium model shows that the residual catalyst inclusions in the core of the nanotube increase the average electrical conductivity by approximately a factor of 3.5.

Time domain characterization of oscillating sensors: Application of frequency counting to resonance frequency determination

A frequency counting technique is described for determining the resonance frequency of a transiently excited sensor; the technique is applicable to any sensor platform where the characteristic resonance frequency is the parameter of interest. The sensor is interrogated by a pulse-like excitation signal, and the resonance frequency of the sensor subsequently determined by counting the number of oscillations per time during sensor ring-down. A repetitive time domain interrogation technique is implemented to overcome the effects of sensor damping, such as that associated with mass loading, which reduces the duration of the sensor ring-down and hence the measurement resolution. The microcontroller based, transient frequency counting technique is detailed with application to the monitoring of magnetoelastic sensors [C. A. Grimes, D. Kouzoudis, and C. Mungle, Rev. Sci. Instrum. 71, 3822 (2000)], with a measurement resolution of 0.001% achieved in approximately 40 ms.

Magnetoacoustic remote query temperature and humidity sensors

In response to an externally applied time-varying magnetic field, freestanding sensors made of magnetoelastic thick or thin films mechanically oscillate. These oscillations are strongest at the characteristic resonant frequency of the sensor. Depending upon the physical geometry and the surface roughness of the magnetoelastic sensor, these mechanical deformations launch an acoustic wave that can be detected remotely from the test area by a microphone. By monitoring changes in the characteristic resonant frequency of a magnetoacoustic sensor, multiple environmental parameters can be measured. In this work we report on the application of magnetoacoustic sensors for the remote query measurement of temperature, the monitoring of phase transitions and, in combination with a humidity-responsive mass-changing Al2O3 ceramic thin film, the in situ measurement of humidity levels.

Simultaneous measurement of liquid density and viscosity using remote query magnetoelastic sensors

Earlier work [C. A. Grimes et al., Smart Mater. Struct. 8, 639, (1999)] has shown that upon immersion in liquid the resonant frequency of a magnetoelasticsensor shifts linearly in response to the square root of the liquid density and viscosity product. It is shown that comparison between a pair of magnetoelasticsensors with different degrees of surface roughness can be used to simultaneously determine the liquid density and viscosity.

Measurement of temperature and liquid viscosity using wireless magneto-acoustic/magneto-optical sensors

Remote query magneto-acoustic and magneto-optical sensors are used to measure liquid temperature, viscosity and density. Sensors comprising magnetoelastic Metglas(R) 2826MB thick-films, alloy composition Fe/sub 40/Ni/sub 38/Mo/sub 4/B/sub 18/, oscillate in response to an externally applied, time-varying magnetic field. The sensor oscillations are strongest at the characteristic mechanical resonant frequency of the sensor. Depending upon the physical geometry and surface roughness of the magnetoelastic films, the mechanical sensor-vibrations launch an acoustic wave that can be detected remotely using a hydrophone or microphone. Furthermore, the sensor oscillations act to modulate the intensity of a laser beam reflected from the sensor surface. The sensor vibrations were optically monitored using a photo detector placed in the path of a laser beam back-scattered off the sensor ribbon. Using a Fast Fourier Transform, the signal obtained in the time-domain from acoustical or optical detectors is converted into the frequency-domain from which the resonant frequency of the sensor is determined. The resonant frequency shifts linearly with temperature and, when immersed in a liquid, with the frictional damping forces associated with liquid viscosity and density, thus allowing a remote measurement of temperature and liquid viscosity.

Control of a magnetoelastic sensor temperature response by magnetic field tuning

A method is reported for controlling the frequency-temperature response of ribbon-shaped magnetoelastic sensors by adjusting the strength of an applied dc biasing field. By controlling the temperature response, a magnetoelastic sensor can be made temperature independent, facilitating their application as pressure, stress, and chemical sensors. Conversely, the temperature dependency of the magnetoelastic sensor can be maximized for use as temperature sensors. In this work we show the temperature response of the magnetoelastic sensor can be negative, positive, or zero by varying the dc biasing field. Experiments have been conducted to illustrate the shift in temperature response with dc biasing field.

Magnetism-based sensors

This paper begins with an overview of nanostructured magnetoelastic materials, namely the amorphous ferromagnetic alloys, detailing how the material structure gives rise to unique magnetic and physical properties suitable for sensor applications, such as a high magnetostriction coefficient, high magnetoelastic coupling, as well as low coercive force and anisotropy field. With the correlation between the material structure and the magnetic and physical properties established, we then show how these unique properties are utilized for measuring multiple physical parameters such as stress/strain, liquid density and viscosity, fluid flow velocity, coating elasticity, ambient temperature, and chemical analyte concentrations including glucose, pH, carbon dioxide, and ammonia.

Control of temperature response for a magnetoelastic sensor with magnetic field tuning

In this paper, temperature control response for a magnetoelastic sensor with magnetic field tuning were investigated. Frequency-temperature response of the magnetoelastic sensor, within the temperature range 20 /spl deg/- 60/spl deg/, can be controlled by varying the magnitude of the dc biasing field.

Wireless remote-query environmental monitoring using magnetoelastic sensors

Magnetoelastic sensors, made of amorphous metallic glass ribbons or wires, have been used to measure various environmental parameters such as temperature, humidity, viscosity, and chemical concentration including pH, carbon dioxide, and ammonia. The parameter of interest is determined by remote detection of the shift in the resonant frequency of the magnetoelastic sensors, which is dependent upon several factors including stress, pressure, temperature, and magnetic field. This paper describes the operating principles of the magnetoelastic sensors and presents several proven applications, as well as methods for optimizing the sensor performance.