ASR&D Chemical Sensor Development
Chemical sensor development at ASR&D has focused on the gas detection needs of our customers, including the detection of hydrogen, methane, hypergolic propellants, and humidity. The sensors demonstrated by ASR&D provide rapid, reversible responses to these vapors at room temperature, with quantitation possible for selected vapors and concentration ranges. Sensors can be optimized for leak detection or for real time vapor concentration monitoring.
Arrays of sensors can be combined to identify and quantify concentrations of individual components in mixtures of VOCs. Detail on specific sensors can be found in our NASA reports under resources, but the following illustrate the capability of SAW devices to produce highly responsive vapor sensors.
Humidity sensors are being developed in collaboration with the Borguet group at Temple University. Sensors have been demonstrated using three humidity sensitive coatings, polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), and LiCl doped titanium dioxide (TiO2) nanoparticle films. These materials are deposited in a thin film (under 1 micron) on the surface of the SAW sensor, and when exposed to an environment, rapidly reach an equilibrium partitioning of water vapor from the air into the film. PVA and PVP are non-conductive films, and operate based on changes in mass loading and viscoelastic properties when the film absorbs water vapor. The LiCl doped TiO2 films experience changes in conductivity when exposed to water vapor, in addition to the effects mentioned above. Figure 1 shows a typical response for a LiCl doped TiO2 coated SAW sensor when exposed to varying levels of relative humidity (RH). The red curve in Figure 1 is the response of an Omega RH-32 sensor, used as a reference. Notice that the SAW (blue curve) responds very rapidly to changes in RH, and the measurement is much less noisy than that of the reference sensor. The low noise level may be due to these measurements being performed on an Agilent E5070B network analyzer, and this may degrade somewhat with a different reader, but it is interesting to note that the sensor itself exhibits very low noise. Another interesting feature shown in Figure 1 is that the speed of the SAW sensor response allowed us to identify a vapor leak between our purge line and our device under test in the vapor generator system used for these tests. In Figure 1, the dashed vertical lines (labeled “vent”) correspond to the carrier gas and humid flow being diverted to a vent line, while the humidity level is mixing to a new RH. Under these conditions, no vapor should have been flowing onto the device under test, until the solid vertical lines (sample gas to sensor), when the new RH sample would be introduced to the sensor. Examination of Figure 1 however, shows that the SAW sensor is clearly detecting and responding to the new (higher or lower) RH level during the mixing period, when the vapor should have been venting. The rapid response of the SAW sensors enabled ASR&D personnel to identify and fix this leak. Notice that the reference sensor, with a rated response time of about 5 seconds (one of the fastest available at the time of purchase), exhibits almost no response during the mixing (vent) phase, and equilibrates to new RH levels comparatively very slowly. Additional development work on these sensors is ongoing.
Figure 1. Comparison of SAW humidity sensor response (blue) with reference humidity sensor from Omega (red). This is scaled raw sensor data, and illustrates the rapid, fairly linear response of the SAW device to changes in RH. The rapid response of the SAW sensor uncovered a previously unidentified leak in our vapor generator system.
Hydrogen sensors based on nanocluster palladium (Pd) are also being developed in collaboration with the Borguet group at Temple University. These patented sensors utilize a sensing bilayer consisting of a self assembled siloxane monolayer, and nanocluster Pd films. The device response is based on changes in electrical conductivity of the Pd films upon exposure to hydrogen gas. Due to the nanostructured nature of these films, hydrogen gas induced swelling and surface stretching of the clusters induces changes in the electron tunneling current between clusters, modifying the film conductivity. When optimized, these changes in film conductivity can produce changes in SAW device velocity and attenuation. SAW attenuation for a sample sensor upon repeated exposure to varying concentrations of hydrogen gas is shown in the figure below.
Figure 2: Vapor testing of one of ASR&D's SAW hydrogen sensors. Note the very rapid response to gas exposure (under 1 second), and the slower, but complete reversibility of the sensor. All testing was conducted at room temperature. The red line shows the gas concentration used, referenced to the right vertical axis, showing that hydrogen gas at 8%, 4%, and 2% all produce virtually identical responses. This indicates these sensors are saturating, even at the 2% level, suggesting they may be sensitive to substantially lower gas concentrations. The first two red pulses represent exposure of the sensor to nitrogen, demonstrating that the sensor is not responsive to changes in gas flow rate or system transients associated with gas switching.