IRG 3: Non-Carbon Based Sensors
Develop materials that enable new robust and stable, physical and chemical sensing devices that remain operational in harsh conditions in collaboration with NASA GRC.
Sub-Theme A. Sensors for Harsh Environments. Physical Sensors
Co-PIs: Ram Katiyar, Gerardo Morell, Luis F Fonseca
A. UV Sensors:ZnO-based UV sensors
§ Thin films of ZnBeMgO were deposited by PLD method on Al2O3 substrate and are being analyzed by different techniques.
§ We synthesized the targets of ZnO, Zn0.8Be0.1Mg0.1O and Zn0.7Be0.1Mg0.2O. We made the targets by mixing powders of Zinc oxide, beryllium oxide and magnesium oxide in the corresponding molar ratios and we used solvent as 2-methoxyethanol, mixing all together for 24 hrs in the low energy ball milling machine. The powders were calcined at 800 °C for 4 hrs and later put in the hydrostatic press under 16 tons of pressure for making the target. Once again we put the target in the furnace for sintering at 1200 °C x 8 hrs.
§ Thin films were grown on (0001) oriented Al2O3 substrate by pulsed laser deposition (PLD) technique using an excimer laser (KrF, 248 nm, 10 Hz repetition frequency) with laser energy 250 J/cm2. The substrate temperate was maintained at 650 °C. The deposition was done under oxygen pressure of 2-15 mTorr.
B. Thermal Sensors
We did temperature dependence measurements of the thermoelectrical properties of single CrSi2 nanowires using specialized measuring microdevices. The chipcarrier holding the microdevice was mounted in a cylindrical cryostat where the temperature can be adjusted from 80K to 340K. System was pumped down to 2x10-6 Torr for thermal isolation and covered with a copper radiation shield. We measured Seebeck coefficient (S), thermal conductivity (k) and electrical conductivity () between 100K and 320K. For completeness purposes we also calculated the dimensionless figure of merit, ZT, from the above mentioned parameters , where ZT is defined as:
Figures IB-1 show (a) S , (b) k, (c) , (d) ZT, versus temperature. Lines are a guide for the eye. The thermal conductivity here reported is obtained from the total heat resistance including the contribution from the contact thermal resistance. The total heat conductivity increases with temperature and reaches a plateau above 200K. Both the overall behavior and order of magnitude matches the values obtained in reference . For example, at T=311K, our value is k=3.68 W/m K while in ref  they report k~6 W/m K. We attribute the difference to the anisotropic behavior of the thermoelectrical properties of CrSi2 . Bulk measurements show that the heat conductivity of CrSi2 along the direction perpendicular to the c-axis (this is growth direction of our nanowires) is smaller than along the c-axis of the crystal (this is the growth direction for the nanowires studied in ref ) .
Sub-Theme B: SENSORS FOR SAFETY MONITORING: GAS SENSORS
Co-PIs: Wilfredo Otaño, Maria M Martinez-Iñesta, Gerardo Morell.
A. Pd Nanostructures for New Ultrasensitive Hydrogen Gas Sensors
A set of samples was prepared in our laboratory to study the Pd nanoshells sensors chemical gas response. The samples were prepared by electrospinning a polyethylene oxide mat of fibers on top of an oxidized silicon substrate followed by sputtering deposition of Pd 10nm thick and deposition of gold contacts. The mats of fibers were also deposited using a mask such that they were deposited in the middle of the wafer and the Pd on top of the fibers (type 2).
B. Synthesis of Pd nanowires using zeolites as templates
The nanowires appear in general to be connected and to have a 4.95 nm mean diameter. It is evident, however, that, the apparent aligned single crystal structure of the nanowires when inside the zeolite (shown in previous reports), is affected during the dissolution process. Care is being taken to find a dissolution process that does not affect the nanowire structure so significantly.
A drop of these recovered nanowires was placed in one of the interdigitated electrodes provided by Dr. Gary Hunter and the sensor was tested at the laboratory facilities of of Prof. Wilfredo Otaño at UPR-Cayey. Figure IIB-2 shows the current and the resistance of the sensor as a function of time upon exposure to 1% H2 by volume. The operating voltage was 1 V so that the circuit follows Ohm’s law. It is evident that the resistance of the sensor increases when it is exposed to H2 showing an average response time of 10.12 s. The fact that there is an increase in resistance is similar to the behavior of bulk platinum [1-2] and is in contrast to results obtained by Favier et al  using Pt mesowires. Our interpretation is that the mesowires had a disconnected structure that was connected with the swelling of the Pd when going to the b phase which caused the decrease in resistance. In our case the Pd nanowires is connected throughout so the phenomena that dominates is the change in increase in resistance when going from the a phase to the b phase. However, one of the apparent advantages of using nanowires is that the response times are significantly faster than the minute range responses of bulk Pd sensors.