Everett E. Crisman

High temperature stability of indium tin oxide thin films

A robust high temperature strain gage based on indium-tin-oxide (ITO) has been used to measure static and dynamic strains at temperatures up to 1400 °C. These thin film, ceramic strain gages have several advantages over metal strain gages including a large gage factor and increased chemical and electrical stability at very high temperatures. Electron spectroscopy for chemical analysis (ESCA) of ITO films deposited onto high purity alumina substrates and subjected to temperatures up to 1400 °C indicated that the ITO films had undergone an interfacial reaction with the substrate. In addition, the interfacial reaction appears to have been responsible for high temperature stabilization through the formation of an ITO/Al2O3 solid solution. When similar ITO films were deposited onto alumina with a platinum diffusion barrier, there was no evidence of an interfacial reaction. Thermodynamic calculations indicated that bulk ITO may not be stable in air ambients at temperatures above 1300 °C but the alumina substrates stabilized the ITO to temperatures well beyond this value.

An apparent n to p transition in reactively sputtered indium–tin–oxide high temperature strain gages

Abstract A robust strain gage based on alloys of indium–tin–oxide (ITO) has been developed, which is capable of measuring strain at temperatures up to 1450 °C. These thin film sensors are ideally suited for in-situ strain measurement in harsh environments since they are non-intrusive, have minimal impact on vibration patterns due to their negligible mass and are robust enough to withstand the high ‘g’ loading associated with rotating components. Thus, this ITO strain gage is well suited to meet instrumentation requirements in advanced propulsion systems. Static strain tests performed at temperatures as high as 1400 °C have resulted in a relatively large and repeatable piezoresistive response. However, in the vicinity of 950 °C, a change in sign of the piezoresistive response from −G to +G was observed, suggesting that the active ITO strain element had been converted from a net ‘n-carrier’ to a net ‘p-carrier’ semiconductor. The ‘n’ to ‘p’ transition has been shown to be reversible over many temperature cycles from room temperature to 1400 °C. This repeatability implies that the carrier species is the predominate factor controlling the observed changes in the resistance and gage factor. Consistent with this change in sign of the gage factor was a change in the sign of the slope of emf vs. temperature (dV/dT) for hot probe measurements made on the same ITO films that were thermally cycled over the temperature range (600 °C to 1300 °C). This finding supports the premise that change in sign of the gage factor from −G to +G occurred within the same temperature range (∼950 °C) and that a change in the charge carrier type was responsible for observed transition in both cases.

Large pyroelectric response from reactively sputtered aluminum nitride thin films

We report the pyroelectric response of c-axis oriented, undoped, wurtzite, aluminum nitride reactively sputtered onto polished silicon wafers. The voltage between a metallic contact on the AlN surface and the n + -doped silicon substrate was monitored during pulsed infrared, radiant heating. From analysis of the data, a pyroelectric voltage coefficient, P v , in excess of 0.5 × 10 6 V/m/K was extracted for films in the 600 to 2500 A thickness range.

Thermoelectric properties and microstructure of Cu-In-O thin films.

Combinatorial chemistry techniques were used to study the thermoelectric properties of sputtered thin films in the system copper oxide (CuO) and indium oxide (In2O3). Seven hundred seventy thin film thermocouples or combinatorial library elements were simultaneously deposited, each with a unique spatially dependent chemistry, based on the relative position of the thermocouples to each sputtering target. The resulting thermoelectric properties of each element were determined along with electrical resistivity as a function of composition. Energy dispersive spectroscopy was used to identify the composition of each thermo-element, and electron and X-ray diffraction were used to determine the degree of crystallinity and phases present. Transmission electron microscopy was used to characterize the microstructure of selected thermo-elements. A change in sign of the thermoelectric voltage was observed in the thermo-element containing 40.0 atomic percent indium, which suggests a change in the dominant carrier type occurred, from p-type to n-type. Based on this finding, the fabrication of thermoelectric p-n junctions using the same base Cu-In-O semiconductor appears feasible.

Smart optical waveguide sensors for cumulative damage assessment

A strain gage is being developed, based on optical modulation that is capable of gage factors on the order of 500 for stains in excess of 2000 (mu) (epsilon) . The strain sensing element is a coated, hollow, glass waveguide of dimensions 0.5 mm ID X 1mm X OD X 100mm long. Since the geometry is compatible with standard telecommunication optical fiber such gages it can be readily incorporated into smart system arrays for damage assessment in structure such as buildings, roads and bridges. Optical fibers bring the excitation light signal to and the response signal for the sensing element. The small diameter glass tubes act as the substrate for a multiple thin film layers which can be optimized to provide the maximum dynamic range for a predetermined strain excursion. The sensor respond to bending strain by attenuation the optical intensity of the excitation signal. The gage elements exhibit little or no hysteresis and are insensitive to temperature. Also, they are environmentally stable and are not affected by factors such as corrosion or electromagnetic fields. The preliminary experimental result will be presented for this type of strain gage system operating to 2000 (mu) (epsilon) . Also, the model for the physical process will be discussed.

Stabilization of Indium Tin Oxide Films to Very High Temperatures

Thin film strain gages based on indium-tin-oxide (ITO) are being developed to measure to static and dynamic strain at temperatures approaching 1500°C. These ceramic strain gages exhibit excellent oxidation resistance and high temperature stability, surviving more than 25 hours of testing in air at 1470°C. Electron spectroscopy for chemical analysis (ESCA) studies indicated that interfacial reactions between ITO and alumina can increase the stability of ITO at elevated temperature. Solid state diffusion of aluminum into the ITO at these temperatures can produce a very stable ITO/Al 2 O 3 solid solution [1, 2]. To determine the nature of the interfacial reaction product, ITO films were deposited onto both Al 2 O 3 and AlN surfaces and thermally cycled to 1500°C. AlN films were used to reduce/eliminate oxygen transport to the interface, so that aluminum-indium interactions alone could be studied. ITO films were deposited onto Al 2 O 3 and AlN films, which were rf sputtered on platinum-coated alumina substrates. The resulting ESCA depth files showed that an interfacial reaction had occurred between the ITO and the Al 2 O 3 and AlN. The presence of two new indium-indium peaks at 448.85 and 456.40eV, corresponding to the indium 3d5 and 3d3 binding energies were observed in both cases; i.e. the AlN and the Al 2 O 3 . These binding energies are significantly higher than those associated with stoichiometric indium oxide. In addition, aluminum doped ITO films were formed by co-sputtering from multiple targets and electrical stability of these films was compared to undoped ITO films over the same temperature range (25–1500°C) [1–4].

Surface etching of YBCO films by xenon difluoride

We have demonstrated that xenon difluoride (XeF2) etches thin films of YBa2Cu3O7−x, the superconducting form of yttrium–barium–copper–oxide (YBCO), during dry etch processing. Both c-axis and mixed a∕c-axis YBCO films show evidence of such etching with a axis being more reactive. Profiles of YBCO films examined by ESCA show that the surfaces of both etched and unetched films are barium rich at the expense of yttrium. After XeF2 etching, fluorine was found to be present to a depth of at least 40nm. Despite the etching and the presence of fluorine in the YBCO films, the superconducting transition temperature, Tc, was unaffected by the XeF2 treatment.

An optical device for measuring bending strain to 5000 microstrain and compatible with optical fiber installations

An optical sensor is described which can be attached to a structure and used as a gage for measuring bending strain. This device can be adjusted to maximize the gage factor for predetermined strain ranges. The sensor consists of glass capillaries coated on the outer surfaces with an optical absorbing layer followed by a reflecting layer. A mechanical strengthening layer can be included to extend the range of strain response. A source laser beam from an optical fiber is injected into one end of the gage. The light remaining in the beam after traveling through the gage is collected via another optical fiber. The optically active layer is adjusted during manufacture to provide a predetermined gage factor. For a given thickness of the absorber layer, the detected light is proportional to the amount of bending. Thus, by rigidly affixing the sensor to a structural member, the strain experienced by the member can be monitored.

Temperature-insensitive smart optical strain sensor

An optical strain gage, employing a hollow polyimide-coated glass capillary tube, is currently under development. The capillary tube serves as a waveguide, in which an optical signal is attenuated in an amount proportional to applied bending strain. The capillary is incorporated into an optical fiber link which acts as both the source of signal and as the return path to a photodiode detector. The inherent compatibility of this optical strain sensor with fiber optic telecommunication systems makes it amenable for incorporation into intelligent systems for the continuous monitoring and damage assessment of bridges, highways, piers, airframes, and buildings. By applying various thin films to the interior and/or exterior surfaces of the waveguide, the strain gage can be optimized for specific strain ranges. This optical strain sensor exhibits advantages in comparison to commercially available metal foil (resistance) strain gages, including gage factors 100 times larger and temperature insensitivity for operating temperatures ranging from -25 degrees Celsius to +51 degrees Celsius.

The microwave behavior of an anisotropic negative index medium

Free space microwave measurements are reported for a split ring and post type metamaterial which exhibits negative refraction in a frequency band between 13.5 and 14.5 GHz. Varying azimuthal angles and magnitudes are achieved by changing the polarization of the transmitter and receiver relative to each other and to the anisotropic axes of the material. The amplitude of the cross- polarized transmission has been measured at 50% of the co- polarization level. The maximum amplitude was achieved at a polarization angle of 20 degrees relative to the initial polarization. This polarization conversion indicates there are other losses besides ohmic losses.