Wireless Sensing in Hostile Environments

Mauricio Pereira da Cunha Dept. of Electrical and Computer Engineering, Laboratory for Surface Science & Technology

University of Maine Orono, ME, 04469, U.S.A.

mdacunha@maine.edu

Abstract—Microwave acoustic devices have long been shown to provide sensitive platforms for physical and gas sensors. Piezoelectric substrates introduced over the past two decades, such as the langasite (LGS) family of crystals, gallium orthophosphate, and aluminum nitride have enabled the exploration of microwave acoustics sensing at temperatures above 500°C. However the ability of the substrate to withstand high temperatures is only one of the requirements for sensor operation in harsh environments. Other prerequisites are: the development of stable high-temperature device electrodes; appropriate packaging; and resilience of the entire system to thermal shock and thermal cycling. In this paper, the wireless operation of microwave acoustic wave sensors in harsh environments is reviewed, and recent achievements are highlighted. Particular focus is given to surface acoustic wave (SAW) sensor technology because it has the advantage of being wireless, battery-free, and allows interrogation of arrays of multiple devices. During the last decade, wireless hostile environment microwave acoustic sensors that can detect gases and monitor temperature and pressure have transitioned from laboratory proof-of-concept to commercial devices and systems. Recent accomplishments in thin film technology and packaging are extending the technology to even higher temperatures, beyond 1000°C. These technological advances are enabling wireless microwave acoustic sensor applications in hostile environments including power plants, turbine engines, oil/gas refineries, and high temperature industrial processing.

Keywords - high temperature sensors; wireless sensors; harsh environment SAW devices; thin film technology; characterization of harsh environment materials.

I. INTRODUCTION Several modern industrial applications in the energy,

aerospace, and material processing industry sectors are in need of a new class of wireless sensors that can operate for long time periods in harsh high temperature environments. For example, wireless sensors are needed for diagnostics and prognostics of components in turbine engines, power plants, high temperature industrial machinery, oil/gas exploration equipment, and space vehicles. Among the limitations of direct-wired sensors for use in harsh high temperature environments, one can list: (i) reliability problems due to degradation and breakage of physical connections over time; (ii) extra weight due to all the wires and connections; (iii) complicated and costly sensor installation and maintenance; (iv) difficult to be placed in rotating parts; and (v) limited overall number of sensors that can be monitored due to complexity of the wiring. Assuming that a wireless sensor connection can be consistently and

reliably established, several advantages become readily apparent, such as the freedom to place sensors in more versatile independent locations, the capability to request information from multiple sensor devices with the same interrogator, and a reduction in sensor system size and weight.

Implementation of the desired harsh environment wireless sensors faces several challenges. Depending on the application, sensor requirements can include: (i) accurate performance from room temperature up to 1400°C; (ii) reliable operation during exposure to pressures ranging from atmospheric to thousands of psi; (iii) endurance to abrupt and sudden pressure and temperature variations (pressure and thermal shock); (iv) operation under physically or chemically aggressive environments, including vibration, corrosive gases, and/or high energy particulate bombardment; and (v) survivability for several thousands of hours, if not for the entire life of the equipment, with adequate endurance, precision, and stability.

For successful wireless operation in such environments, the sensors need a source of energy. Battery operation is often inadequate: if they operate at temperatures of several hundred degrees Celsius, they still require maintenance and add weight to the system. Energy scavenging, although a viable approach, can quickly increase the system complexity, weight, and cost. In addition, energy scavenging and can pose great challenges to acquire and deliver enough energy for wireless communication under harsh environment conditions. A desirable wireless technology for hostile environments should operate with battery-free sensors, and be capable of accessing and interrogating multiple sensors by assigning respective identities (ID tags) to different sensors within a large array.

This paper reviews wireless microwave acoustic sensors for operation in hostile environments, and discusses recent progress and future challenges. Microwave acoustics, and in particular surface acoustic wave (SAW) sensor technology, has proven to respond to the demanding requirements listed above. Other wireless sensor technologies that utilize a variety of semiconductor, optical, and micro-electro-mechanical devices can also potentially address some of the wireless harsh environment operational needs, and are under development by many research groups. However, a discussion and comparison of all these sensor technologies is beyond the scope of this work and can be found elsewhere [1]. Section II reviews important accomplishments towards development of harsh environment wireless microwave acoustics sensors during the last two decades, highlighting some of the groups involved and their respective achievements. This section also addresses

This work has been supported by the U.S. Department of Energy Award #: DE-FE0007379TDD. A disclaimer statement is included in the Acknowledgments Section as required by the sponsor.

1337978-1-4673-5686-2/13/$31.00 ©2013 IEEE 2013 Joint UFFC, EFTF and PFM Symposium

10.1109/ULTSYM.2013.0342

 

(a) (b)

Fig.1. Pt/Zr electrodes on a LGS SAW devices (4 μm electrodes; mark to space 1:1): (a) as fabricated; and (b) de-wetting after heating to 1000°C in air 40 hours. Insets in (b): magnified picture and a scanning electron microscopy (SEM) picture of the electrode region; Pt-film agglomerates and becomes discontinuous. [23] [26] [27]

relevant technological aspects concerning the implementation of the wireless harsh environment acoustic wave (AW) sensor technology, namely: (i) the piezoelectric materials involved; (ii) the electrodes required for transduction; (iii) strategies and techniques for wireless interrogation; (iv) important aspects of packaging for static and rotating parts in such environments; and (v) different sensor types (gas, physical) and applications. Recent progress obtained on thin film electrodes, packaging, piezoelectric materials, wireless interrogation, and harsh environment tests are addressed in Section III. Section IV discusses the main challenges for the technology, and Section V concludes the paper.

II. HISTORIC PERSPECTIVE AND ASPECTS CONCERNING WIRELESS HARSH ENVIRONMENT AW SENSORS

The purpose of this section is to emphasize some of the critical materials, device designs, and wireless-related work that have led to the development of harsh environment microwave sensors. The subsections below address the current relevant aspects of the technology.

A. Piezoelectric Crystals & Films for Harsh Environments Early in the effort of fabricating harsh environment

microwave acoustic devices, in particular SAW sensors, the major difficulties identified were related to substrate integrity, electrode survivability, and packaging [2]. The more popular and well-established piezoelectric substrates used in SAW technology, such as lithium niobate (LiNbO3, LNO), lithium tantalate (LiTaO3, LTO), and quartz (SiO2, QTZ) have shortcomings in performance at elevated temperatures. Both LNO and LTO suffer from material dissociation above 300°C, which severely degrades piezoelectric coupling. In addition, LTO is pyro-electric and LNO is not very resilient to abrupt variations in temperatures. Quartz undergoes a α→β transition near 576°C, which renders the material no longer piezoelectric active [3]; this transition temperature can be even lower under harsh environment conditions, such as high pressure and vibration. Still quartz has been used in frequency control applications for downhole, oil and gas exploration.

Piezoelectric materials that have better potential for high temperature operation are: gallium orthophosphate (GaPO4, GPO); aluminum nitride and gallium nitride (AlN, GaN); and the langasite family of crystals (LGX), a nomenclature used to refer to langasite (La3Ga5SiO14, LGS), langanite (La3Ga5.5Nb0.5O14, LGN) and langatate (La3Ga5.5Ta0.5O14, LGT). Investigation of LNO, GPO, and LGS materials for wireless applications at 2.45 GHz [4], revealed that LNO has significantly lower losses than GPO and LGS, with the losses being 3.9dB/μs, 29dB/μs, and 17dB/μs, respectively. Although LNO offers lower propagation losses at higher frequencies, LNO is limited to 300°C due to material instability as discussed above. Work on AlN, GaN, and AlXGa1-xN piezoelectric thin films suggest that they are also appropriate and promising for harsh environment applications [5], [6], but there is potential limitation of use in air to below 700°C due to film oxidation at higher temperatures. New piezoelectric calcium gallium germinate, langasite isomorphs crystals, such as Ca3NbGa3Si2O14 (CNGS), Ca3TaGa3Si2O14 (CTGS),

Sr3NbGa3Si2O14 (SNGS), and Sr3TaGa3Si2O14 (STGS), have been successfully grown and also show promising performance for harsh environment applications [7], [8], [9], but large high quality single crystals of these materials are not readily available and also their full set of acoustic constants and temperature coefficients have yet to be fully characterized at high temperatures.

GPO crystals, which have been available since the 1990s, are stable up to 930°C, where they undergo a phase change. The acoustic constants and temperature coefficients are available in the literature [10], [11] making GPO an attractive material for sensor applications up to ~900oC. The LGX crystals do not exhibit any phase changes up to their melting points of around 1470°C, and have been shown by X-ray diffraction studies to be stable up to at least 1300°C [12]. High temperature LGS coefficients can be inferred from references [13] and [14]. For LGT, a full set of high temperature material constant have been recently reported [1], [15], thus enabling the design and investigation of high temperature AW devices [16], [17], [18]. In reference [19], anelastic properties of LGS and LGT have been studied for high temperature operation, targeting frequency control and mass deposition devices. Langasite crystals and devices have been exposed to abrupt shocks in temperature and pressure, and to turbine engine vibration environments without compromising the material integrity [20] [21].

B. Thin Film Electrodes High-temperature microwave acoustic sensor devices

contain thin-film electrodes, typically 50nm to 200nm thick, which must operate under the harsh conditions [2]. The performance and thermal stability of these electrode materials depends strongly on temperature and on the environment (i.e., either oxidizing or reducing) [22]. Materials such as titanium (Ti), iridium (Ir), and palladium (Pd) can all be used for high-temperature applications, but these materials show significant loss of mass between 700°C and 1000°C in air.

Platinum electrodes do not show any significant mass loss between 700°C and 1000°C, but Pt electrode films dewet and agglomerate into crystallites on piezoelectric crystal substrates such as LGS above 700°C, thus leading to an electrical open-circuit [23], [24], [25], [26], [27] (Fig 1). To overcome this

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Fig. 2. Wireless SAW possible operation: (a) Frequency Stepped Continuous Wave (FSCW); (b.) Code Division Multiply Access (CDMA); (c) Frequency Division Multiple Access (FDMA). [36], [38]

agglomeration problem, different materials and electrode structures have been introduced. In particular, PtRh/ZrO2 co-deposited and multilayered nanocomposite film architectures have been successfully used to delay the agglomeration process, thus extending the temperature of electrode operation close to 1000°C [23], [24], [25], [26], [28]. Protective ceramic passivation layers, or capping layers, have also been used [23], [24], [29]. In order to push sensor operation to even higher temperatures, additional electrode materials and techniques are currently being explored [30], [31] as discussed in Section III.

C. Wireless Sensor Interrogation and Tagging Wireless microwave acoustic interrogation dates to the

beginning of the 1970s [32]. Wireless interrogation was reintroduced during the 1990s [33], [34] for temperature and pressure sensor applications, and used up to 350°C [4].

The versatility of SAW technology allows acoustic sensors to be interrogated in three possible dimensions: time, frequency, or code. The interrogation in the time domain uses signals reflected in the device structure from different locations, with the specific locations of the reflectors being responsible for creating a code or tag identity [35]. This interrogation method is commonly referred to as Frequency Stepped Continuous Wave (FSCW). The direct implementation of a coded transducer in the substrate, where each particular code identifies a tag, is normally referred to as Code Division Multiply Access (CDMA), and can be implemented by direct spreading [36] or orthogonal coding [37]. Finally, if the identification is done in frequency, for instance by a SAW resonator central frequency, the interrogation method is referred to as Frequency Division Multiple Access (FDMA) [38], [39]. Fig. 2 depicts the three possible wireless interrogation methods. Due to the inherent tagging, the FSCW,

CDMA, and FDMA techniques allow the interrogation of multiple devices, giving the wireless SAW technology the attractive feature of both functioning as a sensor and assigning a unique identification to the sensing location. The three systems have been investigated at high temperatures (750 to 925°C range) at the University of Maine (UMaine) using SAW LGS coded transducers and resonators [36] [38]. Reference [38] discusses LGS SAW resonators operating at six different frequencies that were designed, fabricated and tested up to 925°C as temperature sensors.

The flexibility of the SAW technology allows for wireless sensing on both static and moving parts, as shown in Fig. 3. Tests have been performed by UMaine and Environetix Technologies Corp. to validate the applicability of SAW sensors to operating within different types of turbine engines. Specific engines that were instrumented with SAW sensors include: Armfield CM-4, TMC85-1, GE CT-7, and JetCat P-70, P-80, and P-200. In a JetCat turbine engine, where temperatures can reach up to 750°C, wireless interrogation and real time temperature monitoring were achieved on rotating parts up to 53,000 gs [20] [21], as shown in Fig. 4. This wireless SAW sensor achievement relied strongly on packaging development as discussed in Section D below.

Another interesting aspect of wireless SAW sensors is that they can be interrogated either by field coupling of by signal radiation. Field coupling is appropriate for harsh environment sensor interrogation applications where the sensor and the respective antenna need to be positioned in very close proximity to a metallic body. This metal ground plane prevents proper operation of radiating antennas, especially if the available size is very limited with respect to the wavelength. One disadvantage of field coupling is the limited distance of interrogation, which can be a few centimeters in practice.

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Fig. 4. First demonstration of wireles SAW sensor interrogation on the integrally bladed rotor (IBR) of an operating JecCat turbine engine. Tempertaure of the IBR in the exhaust section of the turbine measured wirelessly in real time during operation with a passive SAW sensor. [20]

When the antenna does not need to be placed in very close proximity and low profile with respect to a metallic part, the radiating signal solution can be used which extends the wireless interrogation range to several meters.

D. Sensor Packaging: Static and Rotating Applications In the previous sections, attention has been devoted

primarily to substrates, thin film electrodes, and different interrogation alternatives to implement the wireless microwave acoustic sensor technology. Sensor packaging, however, is another critical component for successful harsh environment sensing. The sensor assembly needs to involve ways to attach the device in the targeted location with the minimum required profile, and at the same time withstand significant temperature excursions up to 1000°C, and absorb shocks in temperature, pressure, and vibration that are present in typical environments in turbine engines, power plants, and industrial manufacturing equipment. In addition, depending on the application, the packaging must perform on static components or rotating parts operating up to several thousands of rpm’s, thus compounding

the challenge.

Up to the 300°C limit, where LNO sensors can be used, more conventional metallic and surface-mount device (SMD) ceramic packages can be also used [2], [40]. However for operation up to 1000°C, steel and the commercial SMD packaging are inadequate. In many situations, Inconel stock can be machined to fit particular locations and to respond to the specific need [20] [21]. Other mounting techniques can involve a combination of platinum wire (25 to 100 μm dia.), ceramic epoxies, and/or thermo-spray depending on the application and target location to secure the sensor. Successful attachments to static parts in furnaces, turbine engines, and power plan equipment, as well as attachment to rotating parts in JetCat turbine engines have been accomplished using these techniques. For rotating parts, the attachment survived up to 53,000 g-force tangential load, at 90,000 rpm for the particular engine used and up to 750°C. Additional tests performed at an independent contractor [21] confirmed the packaging survivability under different testing conditions, namely: (i) exposed to tangential loads ramping from 14,000g to 58,000g and at both 425°C and 650°C; (ii) under 58,000g and 650°C for an hour; and (iii) under temperature snaps between 425°C and 650°C at constant 24,300 rpm.

E. SAW Sensors for Gas Detection and Physical Monitoring Applications Gas sensors fabricated from SAW devices rely on changes

in selective films or coatings placed in the acoustic wave path. Using the sensor packaging methods discussed above, SAW gas sensors have been demonstrated in high-temperature hostile-environments. Langasite SAW sensors fabricated with platinum, platinum/trioxide (Pt/WO3), and palladium have been used to detect C2H4 and H2 in N2 carrier up to 450°C [41], [42] [43]. Langasite was also used for the detection of O2 up to 700°C [44]. For the detection of HF down to 1ppm in dry air, quartz resonators employing Pt electrodes were used [45].

Regarding physical sensors, SAW devices have been used in harsh environments for the detection of temperature and pressure [46], [47], [48], [49], [23] [24] [21] [25]. LNO SAW wireless temperature sensors, operated up to a 350°C maximum temperature, are discussed for power lines, steel industry equipment, and high-speed high-voltage motors [40] and [50]. SAW sensors are also used as contact-less torque and temperature sensors in car engines [51]. Pressures up to 225

 

 

 

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Fig 3. Attachment and packaging used up to 1000°C to meet diverse application requirements in (a.) static parts; (b.) rotating parts. [21].

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Fig. 5. Differential frequency of devices versus applied pressure measured at different temperature ranges (A) 17°C - 26°C, (B) 234°C - 258°C (C) 488°C - 516°C. Inset shows the sensor used for the experiment. [48]

Fig. 6. Six distinct wireless SAW sensors (S1 +, S2 ○, S3 □, S4 ◊, S5 ×, S6 *) responses measured at 900°C furnace set-point temperature. [38]

psia and 500°C have been measured [48] (Fig. 5). Work at the University of Maine and Environetix Technologies has demonstrated several aspects of wireless and wired temperature LGS SAW sensor operation including (i) validation of long-term sensor operation up to 5½ months, (ii) accuracy during repetitive sensor cycling between room temperature and in excess of 900°C (Fig. 6), and (iii) wireless temperature monitoring within several laboratory furnaces, on static parts in turbine engine exhaust, and on rotating turbine blades (Fig. 7) [23] [24] [21] [20]. A recent review of companies offering microwave acoustic sensor technology with interest in wireless and hostile environment applications can be found in [52].

III. RECENT PROGRESSES

A. Electrodes Stability at 1000° C and Higher One of the current UMaine and Environetix efforts aims at

obtaining wireless SAW sensor operation at temperatures in the vicinity of 1000°C and beyond, where they can be routinely deployed in power plant and aerospace environments. As discussed in Section II, the LGX family of crystals has proven survivability and material consistency up to 1300°C, thus offering the possibility of wireless sensor operation to this temperature. In trying to achieve operation at higher temperatures, however, the important issue of thin-film electrode performance needs to be addressed.

The PtRh/ZrO2 nanocomposite films developed at UMaine that exhibit excellent performance up to 850oC [23] [24] [21] [25] have long term performance limitations in the vicinity of 1000°C, thus motivating the need to investigate additional electrode materials. A series of alternative co-deposited and multilayered structures has been examined [30], [31] including Pt-Co, Pt-Rh/CoO, Pt-Ni, Pt-Rh/NiO, Pt-Cr, Pt-Al, Pt-Al/Pt/ZrO2, Pt-Al/Pt/Nb, Pt-Al/Pt/Cr, Pt-Al/Pt/ZrO2, Pt/Al2O3, and Pt-Rh/HfO2. Out of this initial screening work, Pt/Al2O3 and Pt-Rh/HfO2 yielded the best electrode performance. Fig. 8a shows the ramping temperature profile and the center frequency response of a LGS SAW Pt-Rh/HfO2 resonator up to 1050°C, and Fig. 8b reveals stable device response at 1000°C.

B. Interfacial Layer at the Electrode / LGS Interface Approaching 1000°C, interdiffusion and chemical reactions

can occur across the electrode / LGS interface. It was previously verified [29] that heating LGS to 800°C in a reducing (vacuum) environment depletes the bare langasite surface of Ga, whereas in an oxidizing (air) environment the LGS surface composition remains close to the bulk

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Fig 7. Temperature measured by thermocouple (green) and by the SAW sensor (blue). (a.) WIRED SAW sensor; (b) WIRELESS SAW sensor. Note discrepance in the wired due to the heat sinking effect of the cable, which is not present in the wireless test under the same conditions. [20].

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composition when heated to the same temperature. At 1000°C, interdiffusion becomes much faster leading to potentially more rapid electrode degradation. A SiAlON capping layer has already been used [23] [24] [21] [25], and more recently an Al2O3 interfacial layer fabricated by atomic layer deposition (ALD) was introduced [30] as a means of diminishing the strong interaction between the LGS surface and the thin-film electrode. Fig. 9 [31] shows scanning electron microscopy (SEM) backscatter images of a focused ion beam (FIB) cross-sectioned SAW device with Pt-Rh/HfO2 electrodes after heating at 850°C for 4 hrs (Fig. 9a) and at 1000°C for 4 hrs (Fig. 9b). As can be seen from these images, layered electrode structure is still organized at 850°C, but significant distortion of the electrode layering is observed at 1000°C. The inclusion of the ALD Al2O3 interfacial layer extends the period of time that a device can operate at the highest temperatures before failure [30], thus also extending the stability and reliability of the wireless SAW sensors at lower temperatures.

C. Capacitive Coupling at High Temperatures While a protective or capping insulating layer over the

entire sensor surface [23], [24], [25], [26], [28] extends the life of the Pt-based electrodes by delaying agglomeration, it also make it more difficult to electrically access the bonding pads since the surface is covered with an insulating layer. Breaking through the insulating capping layer is not ideal, since it reduces the bonding quality and performance and creates defects that become reaction sites at temperatures of 1000°C and higher. Patterning a window in the protective layer is also not ideal, since agglomeration still takes place in the exposed area and compositional changes can occur near the bonding region [31].

One solution to electrically access the SAW interdigital transducer (IDT) is by means of capacitive coupling [31]. Fig. 10 shows a schematic of the layers and also a fabricated sensor. The architecture consists of an ALD Al2O3 interfacial layer grown on top of the LGS surface, followed by an adhesion layer, electrode layer, capping layer, and finally a thick-film Au or Pt bond pad layer. The frequency response shown in Fig. 8 came from a similarly fabricated one-port LGS SAW resonator sensor.

D. Packaging In order to have wireless sensors perform in a reliable and

stable way, it is fundamental to have the devices properly assembled and installed in the hostile environment. Section II listed some of the previous packaging achieved by the UMaine / Environetix group on static and rotating parts of turbine engines, energy generating equipment, and other structures. In this section, some of the latest achievements are listed and discussed.

Fig. 11 shows a sensor test fixture and the combustor and test section of the National Energy Technology Laboratory (NETL, Dept. of Energy, WV) Aerothermal Facility (Morgantown, WV, U.S.A.) [53]. Parts of the packaging and the interrogating antenna structure mounted onto the test coupon were exposed to the maximum gas flow yielding a

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Pt thick  film  (~ 10  um)

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(b) Fig. 9 SEM backscatter image of a FIB cross-sectioned device (a.) heated at 850°C for 4 hrs.; (b.) heated at 1000°C for 4 hrs.[31]

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temperature of around 1100°C. The packaging included Inconel plates and shim stock, ceramic alumina, ceramic epoxies, Inconel cables, and thick film Pt screen printed antenna. Another type of sensor probe packaging was recently prepared for a test in a General Electric GE-CT7 helicopter turbine engine at GE’s facilities. The sensor probes are shown in Fig. 12 together with a typical Blackhawk helicopter, which employs this type of engine. Three different types of wireless sensor probes were prepared and qualified by Environetix to be

tested in the last stage of the compressor session of the engine. One of the probes was covered with a closed cap and the other two were open to allow gas flow through their structure.

E. Recent Sensor Tests in Hostile Environments The tests reported in this section used both Vector Network

Analyzers and a dedicated Environetix interrogator electronics/software system (EVHT-100) developed to address sampling capability requirements, portability, and multiple resonator based sensor interrogation (Fig. 13, top). Fig. 13 (bottom) also shows, as an example, the wireless interrogator system used to access the probes in the GE CT-7 turbine engine. For the particular tests reported in this section, LGS wired and wireless SAW temperature sensors operating from 190 to 350MHz have been used.

Fig. 14 shows results of tests conducted inside a GE CT-7 using the wireless covered sensor shown in Fig. 12. The plot compares the temperature sensor response with the engine speed variation, thus successfully enabling the correlation between variations in rotation with temperature inside the compressor, wirelessly monitored in real time.

Fig. 15 shows recent wireless and wired temperature sensor results of a 7h test performed within the NETL Aerothermal Facility. The maximum gas temperature in the chamber reached 1100°C, thus exposing the test coupon, sensor fixtures, and packaging to that temperature. The sensors themselves were attached to the metallic surface of the test coupon inside the chamber and thus were exposed to a maximum temperature of around 850°C.

As another recent implemented system, Fig. 16 shows a pipe test fixture, mounted on top of a propane burner, and instrumented with six wireless temperature LGS SAW sensor tags interrogated by the Environetix’s EVHT-100 system. The fixture served as a prototype for further implementation of temperature monitoring at multiple locations within an energy sector industrial environment. The pipe reached a maximum temperature of ≈700°C in the hotter spots and all the sensors reported consistently with respect to each witness thermocouple. In this application, the distance between the interrogating and sensor antennas reached up to about 14m (45 ft), which is more than adequate for the targeted industrial

Substrate

Interfacial layer (diffusion)Interfacial layer (adhesion)

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Fig. 10 Schematic and implementation fo of multilayer film capacitive coupling architechure.[31]

 

Fig. 11 Schematic and implementation fo of multilayer film capacitive coupling architechure.[31]

SAW device location

Fig. 12 Wireless sensor probes prepared for test in a GE-CT7 blackhawk helicopter engine. Courtesy of Environetix Technologies Corp.

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Fig. 15. Test results for a wireless and a wired LGS SAW sensors in the fixture of Fig. 5. Solid line: wireless SAW sensor; dashed line: wired 3.5µm SAW sensor; ∆ : gas thermocouple; ○ : thermocouple near wireless SAW sensor; □ : Thermocouple near wired 3.5µm SAW sensor.

Fig. 16. Prototype pipe test structure with an array of six sensor mounted on the steel pipe and wireless interrogated by the Environetix’s EVHT-100 system. Courtesy of Environetix Technologies Corp.

Fig. 14. Test performed in a GE CT-7 turbine engine, comparing variations in engine rotation with variations in engine temperature measured by wireless SAW sensors and the Environetix EVHT-100 system. Courtesy of Environetix Technologies Corp.

application.

IV. CURRENT CHALLENGES Significant progress has been achieved during the past two

decades in designing, packaging, and fabricating wireless microwave acoustic sensors and interrogating systems for hostile environments. As demanding applications in areas such as aerospace, energy, and industrial process have continued to increase, the requirement for high temperature operation has risen from several hundred degrees Celsius to above 1000°C. New approaches in substrate selection, thin film electrodes, and packaging must continue to be explored.

Since hostile environment wireless sensor technology is relatively new, most of the solutions encountered face the process of adaptation to the market, including the understanding from customers and application engineers of technology features and current limitations, wireless performance compromises, and packaging alternatives. Placing the sensors and interrogating systems in harsh environment equipment and condition is often challenging, not only due to the hostile environment conditions, but also due to the fact that most of the equipment has not been originally designed with this new harsh environment wireless sensor technology in mind. Retrofit, in some instances of expensive and delicate equipment, can involve significant barriers. However, for several applications, the hostile environment wireless sensor technology is ready and moving from research and development to field applications.

The identification of major driving markets, close interaction with potential interested customers, establishment of standards driven by technology offer and demand, and fine tuning of specifications and product development are paths that will significantly help establish and expand the field.

On the technical side, as wireless hostile environment applications expand, one can envision the need to extend research on piezoelectric substrates or thin films, alternative stable harsh environment electrodes for use beyond 1000°C, novel approaches for packaging, and further development in

Fig.13 Dedicated wireless interrogator used for field tests (left) and schematics for the equipment insertion in the GE-CT7 test. System specifications: sampling rates from 1Hz to 100KHz; operating frequency 100MHz to 1GHz. Courtesy of Environetix Technologies Corp.

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wireless interrogation and signal processing in harsh environments. Additionally, pseudo SAWs, plate modes, and interfacial waves, for example, which are acoustic modes other than SAW and BAW, should be explored as a way to provide alternative platforms for wireless hostile environment sensing applications.

V. CONCLUSIONS This paper has provided a review of wireless microwave

acoustic sensors for hostile environment applications, with focus on SAW sensor technology. Background work developed over the past two decades has been listed and briefly discussed. Important technological challenges regarding piezoelectric substrates, , thin film electrode materials, wireless interrogation methods, sensor attachment and packaging, and examples of sensor applications have been presented.

Recent achievements have been highlighted, including: progress with SAW sensor electrodes for operation at 1000°C and above; thin film techniques for hostile environment applications, including interfacial and capping layers; capacitive coupling electrical access; packaging; and recent tests in turbine engines and power plant environments. Challenges regarding technology insertion and expansion have been discussed, in an attempt to motivate and engage researchers and users. An increasing number of applications and opportunities are being revealed for wireless sensing in hostile environments, and the technology is continuously growing to respond to these requests and demands.

ACKNOWLEDGMENT The author would like to acknowledge R.J. Lad for helpful

discussions and reviewing of this manuscript. The work has benefited from the enthusiastic participation of many current researchers (S. Moulzolf, D. Frankel, G. Bernhardt, M. Call, A. Maskay) and past researchers (R. Behanan, T. Moonlight, P. Davulis, A. Canabal, J. Thiele) at the University of Maine, and C. Neslen from the Air Force Research Laboratories. The material supplied by Environetix Technologies Corporation, in particular by D. McCann, T. Pollard, and E. McCarthy, is greatly appreciated. The author would also like to thank NETL/DOE personnel, in particular S. Lawson, D. Straub, and S. Maley, for discussions and guidance on the experiments at the NETL facility, and J. Riley and M. Tucker for conducting the tests at the NETL Aerothermal Facility. This work was supported by U.S. Department of Energy Award #: DE-FE0007379TDD. Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and

opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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