Aaron Jay Knobloch

CO2 detection using carbon nanotube networks and micromachined resonant transducers

A CO2 sensor using single-walled carbon nanotubes (CNT) and a microelectromechanical system (MEMS) resonator is demonstrated. The MEMS transducer comprises a membrane driven into transverse vibrations by means of Lorentz forces. A downshift in the resonant frequency of the device is measured by a laser vibrometer when changes in the stress state of the CNT film∕membrane structure are caused by adsorption of CO2 onto the nanotubes. The sensor has shown excellent sensitivity, linearity, and recovery over a broad range of concentrations (0–15vol%). In comparison to resistive, dielectric, and gravimetric CNT transducers, this sensor displayed remarkable intrinsic selectivity in the presence of interferences.

Remote driven and read MEMS sensors for harsh environments.

The utilization of high accuracy sensors in harsh environments has been limited by the temperature constraints of the control electronics that must be co-located with the sensor. Several methods of remote interrogation for resonant sensors are presented in this paper which would allow these sensors to be extended to harsh environments. This work in particular demonstrates for the first time the ability to acoustically drive a silicon comb drive resonator into resonance and electromagnetically couple to the resonator to read its frequency. The performance of this system was studied as a function of standoff distance demonstrating the ability to excite and read the device from 22 cm when limited to drive powers of 30 mW. A feedback architecture was implemented that allowed the resonator to be driven into resonance from broadband noise and a standoff distance of 15 cm was demonstrated. It is emphasized that no junction-based electronic device was required to be co-located with the resonator, opening the door for the use of silicon-based, high accuracy MEMS devices in high temperature wireless applications.

Observability analysis for surface sensor location in encased battery cells

Compact battery pack design and cooling requirements present significant challenges for the placement of temperature sensors. Typically, temperature sensors are located near the edge of the battery, and away from the hottest regions, which leads to slower response times and increased errors in the prediction of interior cell temperature. New sensor technology allows for sensor placement between cells to improve sensor performance. With the ability to place sensors anywhere on the exterior of the cell, an observability analysis is necessary to determine the optimal locations for these sensors. The analysis is performed using a spatial discretization of a validated electrothermal model. This model describes a 5 Ah Li-ion cell harvested from a Ford C-max 2013 pack. Given that the spatial discretization of the heat partial differential equation (PDE) governing the system results in singular values that are very small (numerically zero), two methods are presented in this paper to quantify observability and address the issue of optimal sensor placement. The optimal sensor placement between cells yields a 240% improvement (for 3 sensors on the surface) and 15% improvement (for one sensor on the surface) in observability over existing sensor placement near the top edge of the cell. The pack geometry and airflow conditions impact sensor placement. It is preferable to place the sensors towards the outlet side of the airflow as opposed to the inlet side, resulting in a 13% improvement in observability.

Eddy current sensor for in-situ monitoring of swelling of Li-ion prismatic cells

In-situ monitoring an on-board rechargeable battery in hybrid cars can be used to ensure a long operating life of the battery and safe operation of the vehicle. Intercalations of ions in the electrode material during charge and discharge of a Lithium Ion battery cause periodic stress and strain of the electrode materials that can ultimately lead to fatigue resulting in capacity loss and potential battery failure. Currently this process is not monitored directly on the cells. This work is focused on development technologies that would quantify battery swelling and provide in-situ monitoring for onboard vehicle applications. Several rounds of tests have been performed to spatially characterize cell expansion of a 5 Ah cell with a nickel/manganese/cobalt-oxide cathode (Sanyo, Japan) used by Ford in their Fusion HEV battery pack. A collaborative team of researchers from GE and the University of Michigan has characterized the free expansion of these cells to be in the range of 100×125 microns (1% of total cell...

MOEMS pressure sensors for geothermal well monitoring

The technology for enhanced geothermal systems (EGS), in which fractures connecting deep underground wells are deliberately formed through high pressure stimulation for energy generation, is projected to enormously expand the available reserves of geothermal energy in the U.S. EGS could provide up to 100,000 MWe within the U.S. by the next 50 years. Pressure measurements, in particular, are important for determining the state of the fluid, i.e., liquid or steam, the fluid flow, and the effectiveness of the well stimulation. However, it has been especially difficult to accurately measure pressure at temperatures above ~200°C at a distance of 10 km below ground. MEMS technology has been employed for many years for extremely accurate pressure measurements through electrical readout of a MEMS fabricated resonator. By combining optical readout and drive at the end of a fiber optical cable with a MEMS resonator, it is possible to employ these highly accurate sensors within the harsh environment of a geothermal well. Sensor prototypes based on two beam and four beam resonator designs have been designed, fabricated and characterized for pressure response and accuracy. Resonant frequencies of the sensors vary between ~15 kHz and 90 kHz depending on sensor design, and laboratory measurements yielded sensitivities of frequency variation with external pressure of 0.9-2.2 Hz/psi. An opto-electronic feedback loop was designed and implemented for the field test. The sensors were packaged and deployed as part of a cable that was deployed at a geothermal well over the course of 2½ weeks. Error of the sensor versus the reference gage was 1.2% over the duration of the test. There is a high likelihood that this error is a result of hydrogen darkening of the fiber that is reducing the temperature of the resonator and, if corrected, could reduce the error to less than 0.01%.

Effects of Microstructure Evolution on High-Temperature Mechanical Deformation of 95Sn-5Sb

Lead-free solders have garnered much attention in recent years due to legislation banning the use of lead in electronics. As use of lead solders is phased out, there is a need for lead-free alternatives for niche applications such as high temperature environments where traditionally high lead solders are used. Electronics and sensors exposed to high-temperature environments such as those associated with deep well drilling require solder interconnects that can withstand high thermal-mechanical stresses. In an effort to characterize solder alloys for such applications, this study focuses on deformation behavior of the Sn95-Sb5 solder under high-temperature exposures (from 298°K to 473°K). As compared to conventional high-temperature Pb-based solder 90Pb–10Sn, Sn95–Sb5 exhibited very high tensile strength and modulus, as well as superior creep properties despite its lower melting temperature. Importantly, high-temperature deformation was shown to be influenced by the presence of the second phase (SnSb) distributed within the Sn-rich matrix. These second phase precipitates appeared to be dissolved into the Sn-rich phase above 453°K, which converted the solder into a single-phase alloy and resulted in a change in its deformation mechanism. Furthermore, as the service temperature is of such high homologous temperature (T > 0.5Tm), creep deformation will contribute significantly toward the life of the solder joint during thermal cycling. In order to characterize the creep behavior and to identify controlling mechanism(s), creep tests were carried out, from which the stress exponent and activation energy were determined. In this study, detailed microstructures under high-temperature are presented in conjunction with the corresponding mechanical behavior to further understand the controlling deformation mechanisms.Copyright

A Piezoelectrically-Actuated Valve for Modulation of Liquid at High Flow Rate Under High Pressure

A MEMS valve implementation is presented as part of a comprehensive liquid fuel modulation system that targets improved NOx emissions in gas turbines. The target metrics require fuel delivery at high flow rates (>10 ml/s) and bandwidths (>100 Hz) against high pressures (>75 psi). A prototype microvalve has been designed and fabricated with conventional micromachining techniques coupled with an off-the-shelf piezoelectric actuator. The device has dimensions of 40 mmtimes11 mmtimes2.5 mm. Test results demonstrate flow modulation of 15 ml/min at 20 psid and 50 Hz, when the applied sinusoidal voltage to the piezo actuator and therefore the orifice area modulated by the lever arm, was varied between 84 V to 120 V.

Experimental Study of a Novel Silicon Carbide MEMS Ignition Device

This paper presents a robust, low power MEMS igniter built using low pressure chemical vapor deposited (LPCVD) polycrystalline Silicon Carbide films. The MEMS igniter design is based on a 5 mum thick, low stress membrane composed of doped and undoped SiC layers making up the resistive heaters and passivation layer respectively. Experimental tests using an optical pyrometer to measure temperature indicate that this igniter can achieve temperatures beyond 1400degC, with less than 10 W power input, and a time response of less than 0.1 sec. Reliability tests were performed to characterize the igniter behavior as a function of time and determine the lifetime of the devices. Lifetime of the igniter at temperatures greater than 1300degC was limited due to the growth of unstable crystobalite oxide layers resulting in membrane fracture. Reliability significantly improved when operation of the igniter was limited to temperatures below 1 100degC.

Measurement of piezoelectric coefficient of gallium nitride using metal-insulator-semiconductor capacitors

Gallium nitride cantilevers with metal-insulator-semiconductor capacitors were fabricated and characterized as a function of strain to determine the effective piezoelectric constant. These cantilevers were tested at room temperature through a combination of cantilever bending experiments, strain gauge measurements, and finite element modeling. By measuring the shift in flatband voltage to applied strain the magnitude of the piezoelectric charge induced is determined. The effective piezoelectric constant e31′ was determined from these measurements to be −0.57±0.03C∕m2.

Remote excitation and readout of a high Q silicon resonator

This paper presents the first published report of a totally passively driven actuation and readout of an electromechanical resonator using inductive coupling. The goal of this work is to remotely excite and read a MEMS comb drive resonator with a focus on low power operation while simultaneously maximizing standoff distance without the use of active electronics at the sensor location. Initial measurements focused on the determining the relationship of the received signal level with the drive parameters. Reading the resonator through integrated piezoresistors, the drive response showed the expected dependence of both the RF power and AM modulation depth and the coil separation matched a simple model of a 6 cm coil radius and B4 dependence. Measurements of an entirely wireless (read and driven) resonator were made to explore the standoff capability with a practical limit of ∼15 dBm RF power to both the drive and receive systems. A standoff of 9 cm was demonstrated limited by power input to the device. The effect of read and drive coil position was also studied.