Robert J. Lad

Interaction of organophosphorous compounds with TiO2 and WO3 surfaces probed by vibrational spectroscopy

The interaction of organophosphorous compounds with TiO2 and WO3 high surface area powders has been studied by thin film infrared spectroscopy. Room temperature adsorption of dimethyl methyl phosphonate (DMMP), dimethyl hydrogen phosphonate (DMHP), and trimethyl methyl phosphonate (TMP) is through hydrogen bonding of the PO functional group to the hydroxyl groups of the metal oxide surface. At higher reaction temperatures, these hydrogen bonded organophosphorous compounds dissociate and form covalently attached species. Above 200°C, the methoxy groups desorb from the surface while the methyl groups remain stable. Above 300°C, a stable phosphate surface complex is formed. The presence of the phosphate complex may be responsible for poisoning effects observed during DMMP gas exposure of chemiresistive sensors operating in this temperature range.

High temperature stability of langasite surface acoustic wave devices

High temperature acoustic wave (AW) devices capable of operating above 600degC and in hostile environments have opened potential applications for monitoring industrial processes, power plants, and aerospace systems. The authors have reported on the development of thin film electrodes and protective ceramic layers to allow surface acoustic wave (SAW) device operation up to 800degC on langasite (LGS) crystals. This success motivated further study of the electrode material and protective ceramic overlayer, as well as investigations of long term performance, temperature cycling and shock behavior, which are reported in this work. Among the results reported are: behavior of a co-deposited Pt/Rh/ZrO2 composite electrode structure up to 1000degC; investigation of oxygen rich and nitrogen rich SiAlON protective ceramic layers; long term (4080 hours, or about 5frac12 months) operation of a two-port SAW resonator at 800degC; cyclical thermal tests between room temperature and 850degC; and thermal shock tests of crystals between 700degC and room temperature.

Heteroepitaxial growth of tungsten oxide films on sapphire for chemical gas sensors

The performance of chemiresistive gas sensors made from semiconducting metal oxide films is influenced by film stoichiometry, crystallographic structure, surface morphology and defect structure. To obtain well-defined microstructures, heteroepitaxial WO3 films were grown on r-cut and c-cut single crystal sapphire substrates using rf magnetron Ar/O2 reactive sputtering of a W target. On r-cut sapphire, an epitaxial tetragonal WO3 phase is produced at a 450°C deposition temperature whereas 650°C growth stabilizes an epitaxial monoclinic WO3 phase. On c-cut sapphire, a metastable hexagonal WO3 phase is formed. RHEED and X-ray diffraction indicate that the films have a ‘polycrystalline epitaxial structure’ in which several grains are present, each having the same crystallographic orientation. STM analysis of the film surfaces reveals morphological features that appear to be derived from the substrate symmetries. The monoclinic phase has a step/terrace growth structure, has the smallest mosaic spread in XRD rocking curves and exhibits the highest degree of reproducibility suggesting that it is the best suited for sensor applications. Measurements of film conductivity versus temperature indicate that the charge transport mechanisms are also dependent on the crystallographic phase and microstructure of the WO3 films.

Detection and quantification of nitric oxide in human breath using a semiconducting oxide based chemiresistive microsensor

Abstract Nitric oxide (NO) is recognized as playing a critical role in an ever-increasing list of diseases. An important requirement for more extensive utilization of the potential diagnostic value of NO concentrations in human breath is the development of low-cost, reliable NO monitoring devices. This paper describes a promising approach to meet this requirement using a semiconducting metal oxide based chemiresistive sensor. We have shown that it is possible to monitor NO levels in human breath samples with a WO 3 based thin film chemiresistive sensor element. The sensor element is highly sensitive to nitrogen dioxide (NO 2 ). Monitoring of NO is achieved via oxidation of the NO component in breath samples by an oxidizing agent such as alumina supported potassium permanganate (KMnO 4 ). Human breath contains a large number of organic compounds that can interfere with the response of the sensor element as well as NO 2 . Molecular sieve filter materials such as silicalite are used to remove these interfering compounds from breath samples without affecting their NO concentrations. Verification of this monitoring scheme is demonstrated with data which correlates sensor response with NO concentrations in human breath samples, as determined by a chemiluminescence NO analyzer.

Defects and morphology of tungsten trioxide thin films

Abstract Tungsten trioxide is a wide band-gap n-type semiconductor that has been used as a sensing material in chemiresistive gas sensors. The microstructure and morphology are important characteristics that have a large influence on the sensitivity, selectivity, and stability of the sensor. We have produced tungsten trioxide thin films 15–600 nm thick by reactive r.f. magnetron sputtering onto r-cut sapphire substrates. The microstructure of the films was characterized by reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Films with local epitaxy or randomly oriented textures were produced by controlling the substrate deposition temperature and by post-deposition annealing treatments. All films were found to be dense with low porosity. Grain boundaries were identified in the films with randomly oriented texture and these films were composed of either the monoclinic or orthorhombic crystallographic phase. No grain boundaries were found for the locally epitaxial films. These films were discontinuous during early growth, exhibited evidence of crystallographic shear planes, and had a cubic crystallographic phase.

Layer‐by‐layer growth of epitaxial SnO2 on sapphire by reactive sputter deposition

Epitaxial films of stoichiometric tin oxide were grown on sapphire (1102) substrates by reactive sputter deposition. X‐ray diffraction showed the films to have a single (101) orientation. Lateral registry of film growth with respect to the substrate lattice was demonstrated by low energy electron diffraction. Atomic force microscopy was used to examine surface morphology and roughness. The films are extremely flat, having a rms roughness of 3 A over a 4×4 μm2 area. Atomic steps, observed on the sapphire substrate and attributable to a 0.24° miscut, were also observed on the surface of a 400 A film. The results indicate that the film grew via a layer‐by‐layer growth mechanism which was controlled by diffusion of the adatoms to the step edges.

Performance of Zr and Ti adhesion layers for bonding of platinum metallization to sapphire substrates

Single crystal sapphire wafers with <1 nm root mean square (RMS) roughness are ideal substrates for chemiresistive sensors that utilize ultra-thin (<50 nm thick) semiconducting metal oxide (SMO) films. Platinum metallization on a highly polished sapphire platform to form electrodes, heater, and a resistive temperature device (RTD) requires the use of a very thin (<20 nm) buffer layer, such as Ti or Zr, to achieve good adhesion at the Pt/sapphire interface. Using AES, secondary ion mass spectroscopy (SIMS), XRD, and wire bond tests before and after annealing treatments, we have found that Zr has superior performance as an adhesion layer compared to Ti. At temperatures of 200–700°C, required for RTD and SMO film stabilization as well as prolonged sensor operation, there is significant migration of Ti through the Pt film, whereas the Zr layer is less mobile. The Pt/Zr/sapphire architecture also minimizes delamination failure of wire bonds to the sensor device.

Microstructural effects on the friction and wear of zirconia films in unlubricated sliding contact

Abstract Friction and wear of sapphire and steel counterfaces sliding on unlubricated thin films of zirconia were studied using conventional multiple pass pin-on-disk testing and single pass testing with a ‘friction microprobe’. The zirconia films were grown by electron-cyclotron-resonance (ECR) oxygen plasma assisted deposition on r-cut (0112) sapphire using different growth parameters to produce films with varying degrees of order. The microstructure of the zirconia films is correlated to the widely varying observed friction and wear characteristics. Anisotropic stress effects produced during scratch tests are also presented.

Controlled growth of WO3 films

We have used reactive rf magnetron sputtering of a tungsten target in Ar/O2 mixtures and direct electron-beam evaporation of WO3 pellets with and without the presence of an electron cyclotron resonance (ECR) oxygen plasma to grow WO3 films on α-Al2O3(1012) single-crystal substrates (r-cut sapphire). The WO3 films exhibit a range of microstructures depending on deposition conditions. Using any of the deposition methods, the films are amorphous when grown at room temperature. Postdeposition annealing in O2 induces the formation of a random polycrystalline microstructure and an increased surface roughness. Growth of crystalline WO3 films can be achieved at deposition temperatures above 200 °C. During electron-beam evaporation of WO3 at 600 °C, reflection high-energy electron diffraction observations indicate that a tetragonal phase of WO3 grows epitaxially on r-cut sapphire with the (100) tetragonal plane coincident with the rectangular mesh of the r-cut sapphire substrate. Deposition of WO3 using plasma sp...

P4L-1 Enabling Very High Temperature Acoustic Wave Devices for Sensor & Frequency Control Applications

The introduction of piezoelectric crystals capable of acoustic wave (AW) excitation at high temperatures (> 600degC) has opened new possibilities for harsh environment applications, such as combustion engines, industrial processes, and gas/oil extraction. Significant remaining challenges are the fabrication of electrode thin films as well as appropriate packaging capable of withstanding such harsh environments. Thin film electrodes utilizing platinum over zirconium (Pt/Zr) developed by the University of Maine research team for surface acoustic wave gas sensors proved to be inappropriate for long term operation above 700degC, due to the de-wetting phenomenon of thin film Pt. In this paper the fabrication and testing of thin film electrodes and AW devices for longer term operation (from a few hours to months) in high temperature environments (up to 1000degC) have been investigated. The techniques used to overcome the problem of AW device electrode failure at temperatures above 600degC include: multilayered film architectures, alloy compositions, high temperature processing, and protective ceramic overlay films. In particular Pt, zirconium (Zr), ZrO2, Pt/Rhodium, and Pt/Au films have been examined alone or in combinations as the electrode materials, and ultra-thin SiAlON coatings have been used to extend electrode lifetime and to provide device protection in harsh environments. It has been found that the combination of layered and alloy electrodes retarded or prevented de-wetting of the Pt film, and extended the long-term AW device operation from 600degC to at least 950degC. These results indicate the feasibility of very high temperature AW device operation, and open up new opportunities for AW device applications in harsh environments.