Characterization of bismuth titanate thick films fabricated using a spray-on technique for high temperature ultrasonic non-destructive evaluation

Recent shifts towards green power generation have led to renewed interest in nuclear energy. However, the recent incident with the Fukushima Dai-ichi power plant has been a grim reminder that safety is of utmost importance. Similarly, monitoring of jet engine turbine blades has seen a renewed interest because of incidents such as US Airways Flight 1549, which suffered turbine damage upon take off and a water landing in the Hudson river needed to be performed. As a result, both of these two major industries have set goals for developing and implementing new in-situ systems for damage detection. Inspection via ultrasonic non-destructive evaluation (UNDE) is one of the leading methods being considered for such systems due to the light weight, relatively low cost, and low system footprint required for UNDE. However, UNDE in such real world systems is a non-trivial task due in large to the high temperature environments involved with such systems.;Utilization of a spray-on deposition technique of ferroelectric bismuth titanate (Bi4Ti3O12) is investigated. The spray-on technique has several advantages compared to more standard methods of transducer fabrication, particularly for high temperature systems. The primary advantage of this technique is that transducers can be deposited directly onto the structure requiring investigation. Eliminates the need to develop a means of maintaining reliable high temperature mechanical coupling between the transducer and structure under evaluation. Additionally, the ability to analyze structures with complicated geometries with greater ease can potentially be achieved. Moreover, this method allows for fabrication of thick film transducer and therefore transducers with center frequencies typical to industrial UNDE (1-20 MHz) can be realized. Bismuth titanate has been chosen as the material to be investigated due to its high Curie temperature (685°C) and because it has already been shown that fabrication of bismuth titanate based ultrasonic transducers via the spray-on deposition method is possible.;Since initial results with conventional sintering methods were unsuccessful, microwave sintering was pursued and is capable of producing films with superior quality. Industrial partners showed strong interest for on-site deposition transducers. However, with on-site deposition, microwave sintering may not be a viable option. As a result, a crude, but very successful, method of sintering with a blow torch was developed and transducers fabricated in this fashion showed equivalent electromechanical strength.;Moreover, the entire temperature range which bismuth titanate fabricated with the spray-on technique had yet to be explored. In this work it has been shown that spray-deposited bismuth titanate transducers are capable of generating ultrasonic signals with strengths equivalent to room temperature signals up to 650°C for short periods of time. Above this temperature, thermal depoling of the sample occurs rapidly as temperature increases and signal strength diminishes. Furthermore, little has been done to date to characterize material properties bismuth titanate films deposited with the spray-on technique. Initial results indicated that higher electromechanical coupling could be achieved with films deposited in this fashion when compared to their bulk counterparts due to the fact larger field strengths can be employed during poling without breakdown. Typically, the electromechanical properties of substrate supported films are determined via curve fitting methods since the standards outlined by the Institute of Electrical and Electronic Engineers are not valid for the boundary conditions inherent to substrate supported films. However, as it is demonstrated in this work with the Mason model, due to the loading on the transducer from the substrate and the porosity of the films, these techniques cannot be employed as no electromechanical resonance is observed in the impedance spectrum of the transducer. Values for the d33 of the films were approximated by using a commercial d33 meter and suggest d33 values within the range of 13-16 pC/N are being achieved. Extrapolation of field strengths applied to the films fabricated herein during poling to numbers reported for bulk bismuth titanate imply that these values for the d 33 are fairly accurate. Conductivity measurements were made of the bismuth titanate films and were found to be equivalent to bulk bismuth titanate, thus eliminating lower conductivity as a possible reason as to why larger field strengths are attainable. It is well known that if thermal breakdown is the mechanism responsible for breakdown within a material, larger field strengths can be achieved in samples as thickness decrease due to the fact that localized heating can be dissipated more quickly in thinner materials. Therefore, thermal breakdown is posited here as the breakdown mechanism in bismuth titanate and it has been calculated that breakdown strength of the material increases inversely proportional roughly to the cube root of the thickness of the samples.