State-of-the-Art and Practical Guide to Ultrasonic Transducers for Harsh Environments Including Temperatures above 2120 °F (1000 °C) and Neutron Flux above 1013 n/cm2

Abstract

In field applications currently used for health monitoring and nondestructive testing, ultrasonic transducers primarily employ PZT5-H as the piezoelectric element for ultrasound transmission and detection. This material has a Curie–Weiss temperature that limits its use to about 210 °C. Some industrial applications require much higher temperatures, i.e., 1000–1200 °C and possible nuclear radiation up to 1020 n/cm2 when performance is required in a reactor environment. The goal of this paper is the survey and review of piezoelectric elements for use in harsh environments for the ultimate purpose for structural health monitoring (SHM), non-destructive evaluation (NDE) and material characterization (NDMC). The survey comprises the following categories: 1. High-temperature applications with single crystals, thick-film ceramics, and composite ceramics, 2. Radiation-tolerant materials, and 3. Spray-on transducers for harsh-environment applications. In each category the known characteristics are listed, and examples are given of performance in harsh environments. Highlighting some examples, the performance of single-crystal lithium niobate wafers is demonstrated up to 1100 °C. The wafers with the C-direction normal to the wafer plane were mounted on steel cylinders with high-temperature Sauereisen and silver paste wire mountings and tested in air. In another example, the practical use in harsh radiation environments aluminum nitride (AlN) was found to be a good candidate operating well in two different nuclear reactors. The radiation hardness of AlN was evident from the unaltered piezoelectric coefficient after a fast and thermal neutron exposure in a nuclear reactor core (thermal flux = 2.12 × 1013 ncm−2; fast flux 2 (>1.0 MeV) = 4.05 × 1013 ncm−2; gamma dose rate: 1 × 109 r/h; temperature: 400–500 °C). Additionally, some of the high-temperature transducers are shown to be capable of mounting without requiring coupling material. Pulse-echo signal amplitudes (peak-to-peak) for the first two reflections as a function of the temperature for lithium niobate thick-film, spray-on transducers were observed to temperatures of about 900 °C. Guided-wave send-and-receive operation in the 2–4 MHz range was demonstrated on 2–3 mm thick Aluminum (6061) structures for possible field deployable applications where standard ultrasonic coupling media do not survive because of the harsh environment. This approach would benefit steam generators and steam pipes where temperatures are above 210 °C. In summary, there are several promising approaches to ultrasonic transducers for harsh environments and this paper presents a survey based on literature searches and in-house laboratory observations.

High Temperature Ultrasonic Transducers: A Review

“…Furthermore, AlN is hard to obtain due to the difficulties during production. A difficulty of obtaining a thin layer of AlN, growing high-quality crystals, and weak piezoelectric properties are the main disadvantages of AlN [ 25 , 68 , 69 ].…”

“…Although, AlN piezoelectric crystals could be used for non-destructive evaluation, because after 55 h at 550 °C, at standard atmospheric conditions, the echo amplitude remained almost the same. The ultrasonic transduction of these crystals was not affected by 24 h of exposure to 950 °C and 48 h exposure to 1000 °C [ 25 , 66 , 67 ].…”

“…The properties of the bismuth titanate elements may be improved manufacturing piezoelectric composites with supplements of other piezoelectric materials. For example, Tittmann et al concluded that PZT/Bi 4 Ti 3 O 12 and Bi 4 Ti 3 O 12 /LiNbO 3 composites could work at high temperatures of 675 °C and 1000 °C, respectively [ 25 ].…”

“…Application of ultrasonic techniques for such conditions requires ultrasonic transducers suitable for operation not only at elevated temperatures but also at high levels of gamma ray dose and neutron fluence, high pressure, and is resistant to corrosion [ 23 , 24 , 25 ].…”

Sensors for Ultrasonic Nondestructive Testing (NDT) in Harsh Environments

“…The sixth paper by Tittmann et al [6] originates from a long record of research at Penn State University into ultrasonic transducers for use in nuclear reactors. A very thorough review is provided of the piezoelectric materials that could be used under conditions of temperatures over 1000 • C, neutron fluence up to 10 20 cm −2 , and gamma ray dose rates of up to 10 9 Rem/h.…”