G. Bernhardt

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.

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.

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.

Recent advances in harsh environment acoustic wave sensors for contemporary applications

There is a significant need for wireless sensor systems capable of operation up to 1100°C and beyond, in abrasive or corrosive harsh environments, in particular for the energy, steel, aerospace, oil and gas exploration industries. These environments and applications preclude the use of batteries and normally require wireless and multiple sensor interrogation. The University of Maine and Environetix Technologies have successfully responded to these needs by researching and developing surface acoustic wave (SAW) sensors based on the langasite family of crystals and co-deposited Pt/Rh/ZrO2 thin-film electrode technology. This paper reports on the recent achievements, which include: long term operation in furnace and technology validation in jet-engine static and rotating parts up to 53,000 gs; stable and repetitive wired and wireless responses of temperature sensors; multiple wireless sensor interrogation; and associated packaging (tests run in the 200°C to 1000°C range).

Stable electrodes and ultrathin passivation coatings for high temperature sensors in harsh environments

Sensor operation in harsh environments up to 1000degC requires robust packages including stable electrodes and protective coatings. We have developed nanostructured ultra-thin (< 100 nm) Pt-10%Rh / ZrO2 electrode structures grown by e-beam co-evaporation that operate at temperatures approaching 1000degC. X-ray diffraction (XRD), resistivity, and electron microscopy (EM) studies indicate incorporation of ZrO2 within the film delays recrystallization, maintaining a stable morphology. We have also developed ultra-thin (< 50 nm) SiAlON passivation coatings that mechanically protect the sensor surfaces, yet allow interaction with the environment. Different SiAlON stoichiometeries were produced by rf magnetron sputtering of Al and Si targets in O2/N2/Ar mixtures. The SiAlON films are amorphous and extremely smooth (< 1 nm rms) and remain so even after extended annealing at 1000degC. Our results are applicable to a wide range of high temperature sensor configurations.

Characterizing the mechanism of improved adhesion of modified wood plastic composite (WPC) surfaces

To have a better knowledge of the phenomena that affect the adhesion characteristics of wood plastic composites (WPCs) a series of surface treatments was performed. The treatments consisted of chemical, mechanical, energetic, physical, and a combination of energetic and physical WPC surface modifications. After each treatment, the composite boards were bonded using a commercial epoxy adhesive, and bond shear strength was determined according to ASTM D 905. All the surface treatments, except the mechanical one, were performed and presented in a previous paper (W. Gramlich et al., J. Adhesion Sci. Technol. 20, 1873–1887 (2006)). Mechanical treatment and surface characterization for all the treatments were performed in the present study. The surface characterization included application of thermodynamic and spectroscopic techniques. Most of the surface treatments improved the adhesive bondability of wood plastic composites and, particularly, the smoothest WPC surfaces increased the shear strength by 100% with respect to the control. Thermodynamic measurements indicate that the WPCs low surface energy of about 25 mJ/m2, is likely due principally to the surface migration of a lubricant component used in the extrusion formulation. The surface energy increased over 45% with respect to the control samples after the chemical treatments. X-ray photoelectron spectroscopy analysis indicated that high oxidation levels of the WPC surfaces resulted in high surface energy and high bond shear strength.

Forced Air Plasma Treatment (FAPT) of Hybrid Wood Plastic Composite (WPC)–Fiber Reinforced Plastic (FRP) Surfaces

Forced atmospheric (air) plasma treatment (FAPT) was applied to wood plastic composite (WPC) and continuous glass fiber reinforced plastic (FRP) surfaces to improve their adhesive bonding properties. The FRP was composed of oriented continuous E-glass fibers in a polypropylene matrix, while the WPC was fabricated using wood flour, polypropylene and additives. The FAPT was applied using two levels of discharge length projected from the discharge head (2.5″ and 1″) to ionize the air, oxidize the surfaces and improve wettability. The treatment was performed by passing the electrode over either surface, five or ten times. Surface characterization consisted of thermodynamic (surface energy determination), chemical (X-ray photoelectron spectroscopy), mechanical (shear strength) and microscopic (atomic force microscopy (AFM)) analysis. The results indicate that the acid–base component of the surface energy for both WPC and FRP after FAPT correlates with an increase in wettability. X-ray photoelectron spectroscopy was performed on wood regions and non-wood regions of the WPC surfaces; the oxygen concentration increased to a larger extent in the non-wood regions. Bonding shear strength measurements indicated increases of 50% after FAPT on WPC surfaces (2.5″ discharge length, 1 pass) and up to 200% for the hybrid WPC–FRP. Atomic force microscopy measurements using a silicon tip probe showed increases in adhesive force interactions up to 56% on WPC surfaces post-FAPT.

Electrode optimization for a lateral field excited acoustic wave sensor

Lateral field excited (LFE) acoustic wave sensors have a distinct advantage over the standard, AT-cut, quartz crystal microbalance (QCM) in which the thickness shear mode (TSM) is excited by electrodes on the sensing and reference surfaces. The TSM in LFE sensors is excited by placing electrodes only on the reference surface of the crystal, leaving the sensing surface free. Due to the lack of a shielding electrode on the sensing surface, the TSM electric field is able to penetrate into an analyte selective film or an adjacent liquid thus enabling the detection of mechanical and electrical property changes in the film or liquid. Several electrode designs on the reference surface are examined and evaluated by measuring each sensors admittance and resonant frequency when exposed to a variety of analytes and liquids. These results are compared to the results obtained using a standard QCM.

Thin films and techniques for SAW sensor operation above 1000°C

High temperature (300°C to 1400°C) wireless sensors have applications in energy exploration and generation, harsh environment industrial processing, and aerospace engineering. Existing technology developed at the University of Maine allows the fabrication of surface acoustic wave (SAW) langasite (LGS) sensors with Pt-Rh/ZrO2 electrodes that can deliver long-term stable operation up to 850°C. Since LGS remains piezoelectric up to its melting point of ~1400°C, it is desirable to extend the current SAW sensor temperature range of operation. In addition, it is desirable to diminish the SAW interdigital transducer (IDT) electrode dimensions to increase the wireless frequency of operation towards the GHz range. In this work, new thin film electrode materials have been investigated to allow the operation of SAW one-port resonators up to 1000°C and beyond. In particular, alternative Pt/Al2O3 and Pt-Rh/HfO2 thin film electrode compositions are presented, which yield operation of SAW resonator sensors up to 1100°C. In addition to a previously used capping layer, an interfacial layer has been added between the LGS and the electrodes to delay any interdiffusion between the materials and extend the temperature and/or time of sensor performance. Finally, it is also reported in this work that exposure of untreated SAW device electrodes with 120 nm thick and 2μm wide Pt-Rh/ZrO2 co-deposited IDT fingers to temperatures above 850°C can create long platinum-rich nano-whiskers. These structures short-circuit the SAW interdigital (IDT) fingers, rendering the device unusable. The short-circuit problem was solved by the use of multilayered electrode structures and the used of the capping layer.

Lateral field excitation of well structures in quartz

Recently, there has been interest in the fabrication of multiple Quartz Crystal Microbalances (QCMs) on a single substrate, to create a sensor array. The QCMs are ldquowell structuresrdquo etched on an AT-cut quartz substrate, to isolate the devices from one another. However, such devices are ultimately subject to the limitations of the QCM configuration, requiring electrodes and wires on the sensing surface of the crystal substrate. The Lateral Field Excited (LFE) sensor is a novel sensing device that only requires electrodes on the backside of the substrate. With a bare sensing surface, the LFE sensor is a better choice for implementing a sensor array. The purpose of this paper is to present preliminary results on LFE well structures in quartz, which would be the fundamental building block of an LFE sensor array. Several well structures of varying depth and diameter were etched into AT-cut quartz crystals. The transverse shear mode was successfully excited in the LFE well structure, and data on the frequency response and center frequency vs. well depth and diameter are presented.