IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 58, . 9, SEPTEMBER 2011 1994 0885–3010/$25.00 © 2011 IEEE Thick-Film Acoustic Emission Sensors for Use in Structurally Integrated Condition- Monitoring Applications Andrew J. Pickwell, Robert A. Dorey, and David Mba Abstract—Monitoring the condition of complex engineering structures is an important aspect of modern engineering, elimi- nating unnecessary work and enabling planned maintenance, preventing failure. Acoustic emissions (AE) testing is one method of implementing continuous nondestructive structural health monitoring. A novel thick-film (17.6 μm) AE sensor is presented. Lead zirconate titanate thick films were fabricated using a powder/sol composite ink deposition technique and mechanically patterned to form a discrete thick-film piezoelec- tric AE sensor. The thick-film sensor was benchmarked against a commercial AE device and was found to exhibit comparable responses to simulated acoustic emissions. I. I A  emission (AE) sensors are an important tool in nondestructive testing (NDT). NDT is used in many fields, including structural and process monitor- ing and biomedical research, with industries such as aero- space, nuclear, and civil engineering making widespread use of NDT by AE monitoring [1]–[4]. AE monitoring is now being applied to both static and nonstatic systems [5]. The drive for smaller, more cost-effective devices is paramount and one solution to this challenge is the use of micro-electromechanical systems (MEMS). MEMS de- vices enable AE technology to be deployed in space- or weight-critical applications for which the commonly used commercial bulk-piezoelectric AE sensors [1], [2], [6] are inappropriate. MEMS manufacturing methods also enable devices to be low cost and disposable, enabling deploy- ment in situations where device recovery is difficult. In this work, an 8-mm-diameter, 17-μm-thick film acoustic emission sensor is deposited directly onto a Kovar (Carpenter Technology Corp., Reading, PA) plate acting as the test structure. The diameter of 8 mm was chosen because it is comparable to the existing commercial sen- sor. The response of the thick-film device to these simu- lated acoustic events was compared with responses of one of the smallest commercially available sensors for bench- marking purposes. The thick-film and commercial devices were then exposed to simulated acoustic emissions. The thickness of 17 μm is 1/140 the thickness of some of the smallest available commercial devices [7]. II. M Lead zirconate titanate (PZT) sol and composite slurry (PZT sol and powder) were produced with a Zr/Ti ratio of 52/48 [8], [9]. A Kovar substrate with dimensions 100 × 100 × 3.2 mm was cleaned in an acetone/isopropyl alcohol ultrasonic bath. A 17.6-μm PZT thick film was deposited onto the Kovar using a 4(2C+5S) composite spin-coating technique [10]. A spin speed of 2000 rpm was used. After each deposition, the film was dried at 310°C for 180 s and pyrolised at 525°C for 60 s. Once the composite film was complete, sintering was carried out at 720°C for 20 min, following a ramp rate of 5°C/min. The PZT was masked and then patterned by powder blasting. To produce the final sensing elements, a mask was applied and an 8-mm- diameter top electrode, of 11.2-nm-thick Cr and 150-nm- thick Au, was sputtered onto the PZT surface. The PZT was then heated to 135°C and poled by a corona poling process at 16 kV with a needle-to-sample gap of 30 mm for 15 min. The PZT was left to cool under the electric field to prevent stress depoling. A BNC connector was connected to the top electrode and to the Kovar base plate. The BNC was connected directly to the commer- cial pre-amplifier. This was done to minimize the signal loss and interference caused by electrical resistance in the system before amplification. The poled thick-film device had a piezoelectric constant (d 33 ) of 38 pC/N, a capaci- tance of 2.18 nF, and a dielectric loss of 0.102, measured at 100 kHz. The relative permittivity of the film was cal- culated to be 86.21. The thick-film device deposited on the Kovar plate con- sists of a circular single element; this provides high sensi- tivity to AE regardless of the sensor orientation relative to the incoming wave [11]. As such, a single-element PICO sensor from Physical Acoustics Corporation (PAC; Princ- eton Junction, NJ), with an operating frequency range of 200 to 750 kHz, was chosen for comparative testing. An- other reason for selecting the PICO sensor was the small size (4 × 5 mm) of the device, because it is often used in situations similar to those for which the thick-film sen- sor is designed. A commercial PICO sensor was mounted 32 mm from the center of the Kovar plate using grease as a coupling agent, see Fig. 1. Signals from both the PICO and the thick-film PZT sensors were monitored using MI- TRA, a commercial transient recorder-analyzer system from PAC (2001). The PICO and thick-film sensor signals were pre-amplified by 40 and 60 dB, respectively, using variable gain 2/4/6 pre-amplifiers from PAC. A lower- Manuscript received December 8, 2010; accepted June 11, 2011. The authors are with Cranfield University, Cranfield, Bedfordshire, UK (e-mail: r.a.dorey@cranfield.ac.uk and d.mba@cranfield.ac.uk). Digital Object Identifier 10.1109/TUFFC.2011.2043