High temperature electromagnetic acoustic transducer

Abstract

A novel electromagnetic acoustic transducer (EMAT) is provided. The EMAT includes an electrically insulating backing plate defining at least one open surface channel; an electrical conductor positioned within the open surface channel to form at least a portion of an electromagnetic coil; wherein the open surface channel constrains movement of the electrical conductor relative to the backing plate; a retaining element configured to maintain the electrical conductor within the open surface channel during operation; and a magnetic field source configured to provide a bias magnetic field for ultrasonic generation.

Description

HIGH TEMPERATURE ELECTROMAGNETIC ACOUSTIC TRANSDUCERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application Serial No. 63 / 779,567, filed March 28, 2025, the contents of which is hereby incorporated by reference in its entirety.STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under DE-NE0009176 awarded by the U.S. Department of Energy. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present invention relates to ultrasonic transducers.BACKGROUND OF THE INVENTION

[0004] Conventional ultrasonic transducers typically require a liquid couplant or high mechanical pressure to transmit ultrasonic energy into a structure. These requirements are undesirable for permanently installed monitoring systems, especially at elevated temperatures or where coatings are present. Electromagnetic acoustic transducers (EMATs) generate and receive ultrasound through electromagnetic coupling without requiring a couplant, but practical long-duration high-temperature EMAT operation is limited by (i) coil oxidation and stability, (ii) maintaining small and repeatable lift-off, (iii) mechanical wear and damage at the interface, (iv) electromagnetic interference, and (v) permanent magnet demagnetization at elevated temperature. A need exists for a robust, manufacturable EMAT capable of long-term operation at high temperatures with high transduction efficiency at low drive power.SUMMARY OF THE INVENTION

[0005] Certain exemplary aspects of the invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention.

[0006] In a first embodiment of the invention, a novel electromagnetic acoustic transducer (EMAT) is provided. The EMAT includes an electrically insulating backing plate defining at least one open surface channel; an electrical conductor positioned within the open surface channel to form at least a portion of an electromagnetic coil; wherein the open surface channel constrains movement of the electrical conductor relative to the backing plate; a retaining element configured to maintain the electrical conductor within the open surface channel during operation; and a magnetic field source configured to provide a bias magnetic field for ultrasonic generation.

[0007] In one embodiment, the backing plate comprises a ceramic material for operation at elevated temperature. In another embodiment, the retaining element comprises a wear plate. In one embodiment, the retaining element comprises a fixative. In another embodiment, the retaining element comprises a combination of a fixative and a wear plate.

[0008] In one embodiment, the wear plate comprises a ceramic material. In another embodiment, the wear plate comprises a low electrical conductivity metallic alloy material. In one embodiment, the conductor comprises a material selected from the group consisting of copper, silver, gold, platinum, and combinations thereof. In another embodiment, the conductor comprises at least one wire. In one embodiment, the conductor comprises a sheet of material cut into a coil shape.

[0009] In another embodiment, the magnetic field source comprises a permanent magnet. In one embodiment, the electromagnetic coil forms a butterfly coil geometry wherein the magnetic field source has a footprint and portions of the electromagnetic coil beneath the footprint of the permanent magnet are routed in straight segments. In another embodiment, the electromagnetic coil forms a spiral coil geometry wherein the magnetic field source has a footprint and the electromagnetic coil is routed in a spiral pattern beneath the footprint of the permanent magnet. In one embodiment, the electromagnetic coil forms a racetrack coil geometry.

[0010] In another embodiment, the EMAT further includes an electrically conductive electromagnetic shield positioned between the electromagnetic coil and the permanent magnet. In one embodiment, the electromagnetic shield is separated from the electromagnetic coil by a standoff distance selected to minimize the reduction of eddy currents induced in a metallic test component due to the presence of the shield.

[0011] In another embodiment, the magnetic field source is a permanent magnet of prismatic shape having a length-to- width ratio equal or greater than 1 : 1 with the magnetization direction in the long axis of the prismatic shape. In one embodiment, the magnetic field source comprises apermanent magnet rated for operation at elevated temperature exceeding 250 °C. In another embodiment, the permanent magnet comprises a samarium-cobalt material. In one embodiment, the permanent magnet includes a protective metallic coating.

[0012] In a second embodiment of the invention, a method of manufacturing an electromagnetic acoustic transducer (EMAT) is provided. The method involves: providing an electrically insulating substrate having a surface; forming at least one open surface channel in the surface of the substrate; positioning an electrically conductive wire within the open surface channel such that the wire defines at least a portion of an electromagnetic coil extending along the surface of the substrate; and retaining the electrically conductive wire within the open surface channel using a retaining element configured to maintain the position of the wire relative to the substrate during operation of the electromagnetic acoustic transducer.

[0013] In one embodiment, the retaining element comprises an electrically insulating wear plate disposed over the open surface channel. In another embodiment, the retaining element comprises an electrically insulating fixative securing the wire within the open surface channel. In one embodiment, forming the open surface channel comprises machining, laser forming, molding, or additive manufacturing.

[0014] In another embodiment, positioning the electrically conductive wire forms a substantially planar coil geometry adjacent to the substrate surface. In one embodiment, the substrate comprises a ceramic material suitable for elevated-temperature operation.

[0015] In a third embodiment of the invention, a method of performing ultrasonic monitoring of a component is provided. The method involves: providing an electromagnetic acoustic transducer as described above; positioning the electromagnetic acoustic transducer adjacent to a surface of a metallic component; energizing the electromagnetic coil to generate ultrasonic waves within the metallic component; and receiving ultrasonic signals using the electromagnetic acoustic transducer to evaluate a condition of the metallic component.

[0016] In one embodiment, the component is monitored at a temperature greater than 250 °C. In another embodiment, the ultrasonic monitoring comprises measuring wall thickness using time-of-flight measurements. In one embodiment, the electromagnetic acoustic transducer is permanently installed on the component for long-duration monitoring. In another embodiment, energizing the electromagnetic coil comprises applying a low-power excitation signal.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The objects and advantages of the disclosed invention will be further appreciated in light of the following detailed descriptions and drawings in which:

[0018] FIG. 1 is a schematic showing an exploded view of an EMAT assembly 100 including a wear plate 110, electrical conductor 120, electrically insulating backing plate 130, electromagnetic shield 140, permanent magnet 150, cable 160, and outer casing 170.

[0019] FIG. 2 is a schematic showing a sectioned view of the backing plate 130. Channels 210 are present to accommodate electrical conductors to form an electromagnetic coil. Holes 220 may also be present to allow pass-through of the conductors.

[0020] FIG. 3 is a schematic showing a bottom view of the backing plate depicting the routing of the open channels 310 for the ‘butterfly coil’ arrangement. Also shown is the footprint of a cylindrical permanent magnet 320, and the direction of ultrasonic excitation 330 marked by the double headed arrows. Holes for electrical connection to the electrical conductors that are to be embedded in the channels are shown in 340.

[0021] FIG. 4 is a schematic showing a bottom view of the backing plate depicting the routing of the open channels 410 for the ‘spiral coil’ arrangement. Also shown is the footprint of a cylindrical permanent magnet 420, and the direction of ultrasonic excitation 430 marked by the double headed arrows. Holes for electrical connection to the electrical conductors that are to be embedded in the channels are shown in 440.

[0022] FIG. 5 is a graph showing ultrasonic a-scan measurements taken on an A508 ferromagnetic steel at a range of temperatures up to 550 °C.

[0023] FIG. 6A is a graph showing ultrasonic a-scan measurements taken on a 316 austenitic stainless steel at a range of temperatures up to 550 °C.

[0024] FIG. 6B is a graph showing an a-scan taken at 550 °C after 2000 hours of exposure at that temperature.DETAILED DESCRIPTION

[0025] One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerousimplementation-specific decisions must be made to achieve the developers’ specific goals, such as compliance with system -related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0026] Conventional ultrasonic testing requires the use of liquid couplants, or high mechanical pressure between the transducer and the component under test. This is unattractive for long term monitoring applications, particularly at high-temperatures, and is not feasible when coatings are present. An electromagnetic acoustic transducer utilizes electromagnetic coupling between the transducer and the component under test and therefore does not require a mechanical transmission path. It is therefore considered “non-contact.”

[0027] The present invention relates to ultrasonic transducers, and more particularly to electromagnetic acoustic transducers (EMATs) which may be used for elevated-temperature operation and long-term structural health monitoring. Examples of monitoring include measuring wall thickness of a component using time-of-flight measurements or monitoring defects such as cracks. In one embodiment, the present invention is an electromagnetic acoustic transducer configured for long-duration ultrasonic monitoring at elevated temperatures. The EMAT of the present invention includes a backing plate defining one or more open surface channels that receive an electrically conductive track. The conductor is held in position by the use of a fixative or a rigid cover plate disposed over the backing plate to mechanically retain the conductor. A ceramic wear plate may be used as the rigid cover plate to provide a thin, electrically insulating lift-off between the conductor and a test component. A magnetic field source is provided. A “magnetic field source” is a source producing magnetic field such as an electromagnet or a permanent magnet. In one embodiment, the magnetic field source is a permanent magnet. A permanent magnet is positioned to provide a bias magnetic field, and in some embodiments is configured to achieve a high permeance coefficient to reduce irreversible demagnetization at elevated temperatures. An electrically conductive electromagnetic shield may be positioned between the coil assembly and the magnet to reduce acoustic reverberations within the magnet. The stand-off spacing between the shield and the conductor should be chosen so as to shield the magnet without reducing the efficiency of the EMAT through the generation of eddy currents that are counterflowing to thecurrent in the conductive tracks. The EMAT provides efficient ultrasonic generation and detection at low excitation levels and at temperatures up to at least about 550 °C.The Device

[0028] An EMAT assembly of the present invention can be used for permanently installed or long-duration ultrasonic monitoring at elevated temperatures. The EMAT avoids liquid couplants and high clamping pressure requirements by generating ultrasound through electromagnetic coupling. The structure and materials are chosen to maintain performance and mechanical integrity while minimizing lift-off and maintaining coil geometry control.

[0029] In one embodiment, the EMAT of the present invention includes an electrically insulating backing plate, a conductor formed into a coil, an electrically insulating wear plate and / or a fixative, a permanent magnet, and optionally, a metallic electromagnetic shield between the coil and the magnet. The electrically insulating backing plate 130 (FIG. 1) defines open surface channels 210 (FIG. 2) configured to receive an electrical conductor. In embodiments anticipating operation at elevated temperature, the backing plate may be ceramic. The conductor is seated in the channels such that the conductor forms a coil that can be assembled by pressing the conductor into the open surface channels 120 (FIG. 1). In one embodiment, the conductor is composed of a metallic wire and the wire should be suitable for the operational temperature. In the case of operation up to 550 °C, the wire may be fine silver. The electrically insulating wear plate can be disposed over the backing plate to retain the conductor and provide a thin, electrically insulating interface toward the test component. The wear plate 110 may be ceramic, enabling operation up to 550 °C. In another embodiment, a fixative may be used to secure the conductor, either in addition to the wear plate or without it.

[0030] The permanent magnet provides a bias magnetic field. In different embodiments, the magnet 150 is configured with a high permeance coefficient geometry and / or magnetic circuit features to reduce demagnetizing field and high-temperature loss. The combination of magnetic material and permeance coefficient should be suitable for the operational temperature. In one embodiment, a 2:1 length to diameter ratio cylindrical magnet is used. This shape can provide better performance in higher temperatures, such as temperatures up to 550 °C. In one embodiment, the magnet material is SmCo T55O.

[0031] In one embodiment, a metallic electromagnetic shield is located between the coil and the magnet. The shield 140 has a stand-off spacing from the conductor. In one embodiment, the conducting wires pass through holes in the backing plate to enable connection to a cable. The cable may be mineral insulated, which is useful for operation up to 550 °C. In one embodiment, the EMAT is housed in an outer casing 170.

[0032] The EMAT is designed to provide sufficient sensitivity at low-power excitation while operating at temperatures up to about 550 °C for long-term monitoring such as thickness measurement by ultrasonic time-of-flight. In one embodiment, each excitation is performed with less than 100 Volts. In another embodiment, each excitation is performed with about 24 Vpp and about 500 mApp. The lower power requirements enabled by the present invention means you could use battery power (which is important in remote sensors). Further, it means that an intrinsic safety certification is viable.

[0033] In one embodiment, the present invention is used at temperatures from 250-550 °C. In another embodiment, the present invention is used at temperatures from 280-550 °C. In one embodiment, the present invention is used at temperatures from 300-550 °C.Backing plate with open channels for conductor placement

[0034] A backing plate defines one or more open surface channels sized to receive an electrical conductor. In one embodiment, the conductor is a wire and the channels are approximately 0.3 mm x 0.3 mm, with 0.25 mm separation between channels, though other dimensions may be used. The channels provide precise conductor spacing and geometry control. In various embodiments, the open channels are formed using machining, laser forming, molding and / or additive manufacturing.

[0035] In some embodiments, a wire is pressed into the surface channels to form the conductive tracks. This assembly approach improves manufacturability compared with designs requiring threading through complex passages, and can also place the coil in a single plane closer to the test component. The backing plate may also include holes 220 (FIG. 2) that allow portions of the conductor to pass through. Example materials for the backing plate include ceramics, including machinable glass-ceramics (e.g., Macor™) and zirconia-based ceramics. Other useful materials include silicon nitride, alumina, aluminum nitride, silicon carbide, and polymers such as PEEK, glass reinforced PEEK or carbon reinforced PEEK.Wear plate or fixative for minimal lift-off and mechanical protection

[0036] The conductor may be secured into position using an electrically insulating fixative, such as an adhesive, epoxy, enamel or cement. Alternatively, or in addition to a fixative, a wear plate is positioned against the backing plate such that the conductor is mechanically retained in the open channels. Useful materials for the wear plate include low electrical conductivity metallic alloy materials, ceramics, and zirconia.

[0037] The wear plate may be clamped, captured by a perimeter lip, bonded, cemented, or otherwise secured. The wear plate protects the conductor from mechanical damage and provides a thin, electrically insulating separation (Tift-off) between the conductor and a test component. In some embodiments, a zirconia wear plate enables very small lift-off (e.g., 1 mm or less).Conductor materials and geometries

[0038] The conductor may comprise a high-conductivity metal or alloy suited to the operating temperature, including inert or oxidation-resistant materials such as copper, silver, gold, platinum, or suitable alloys. For high-temperature use, materials such as high purity copper, silver, gold, or platinum may also be used. The conductor may be retained by the mechanical constraint of the channel, adhesives / cements / enamels compatible with temperature, and / or by the wear plate.

[0039] In one embodiment a ‘butterfly coil’ arrangement 310 (FIG. 3) is used whereby the conductors beneath the magnet footprint 320 are routed straight to create a straight eddy-current path in the test component. When used in combination with an out-of-plane magnetic bias, a high purity linearly polarized shear bulk is generated 330. The conductors return path is routed outside of the magnet footprint where the magnetic field is lower to reduce unwanted mode generation. This approach also has a large ultrasonic aperture to produce a columnated ultrasonic wavefield. When used in combination with a periodic permanent magnet array a shear guided wave is generated.

[0040] In another embodiment a ‘spiral coil’ arrangement is used (FIG. 4) where the conductor is routed in a spiral underneath the footprint of the magnet 410. When used in combination with an out-of-plane magnetic bias, a radially polarized shear bulk wave is generated 430.

[0041] In another embodiment a racetrack coil is used. A racetrack coil is an electrical coil having two generally straight parallel conductor segments joined by curved end portions, forming anelongated closed loop resembling a racetrack and producing a directional magnetic field primarily along the straight sections.

[0042] In another embodiment a meander coil is used. A meander coil is an electrical conductor arranged in a serpentine pattern of alternating parallel segments connected by transverse turns, producing a spatially periodic magnetic field along its length.Permanent magnet materials and configuration

[0043] The EMAT may include one or more permanent magnets configured to generate a static bias magnetic field in a region of a conductive test object. The magnet assembly may comprise any suitable permanent magnet material, including but not limited to rare-earth permanent magnets (for example, neodymium-iron-boron or samarium-cobalt magnets) and non-rare-earth metallic permanent magnets (for example, aluminum-nickel-cobalt magnets), or any other magnet material capable of providing a suitable magnetic field under the intended operating conditions, including elevated-temperature environments. In certain embodiments, materials having increased temperature stability or coercivity may be preferred; however, the invention is not limited to any particular magnet composition. The magnet may comprise a single element or multiple elements arranged in an array, stack, or segmented configuration. Field-shaping components such as pole pieces, flux concentrators, yokes, or magnetic circuits formed from ferromagnetic materials may be provided to control field strength, distribution, and orientation. The magnets may be arranged to produce a uniform field, a localized field, or a spatially varying field, including configurations such as opposing poles, side-by-side poles, circumferential arrangements, or Halbach-type arrays. The assembly may be configured to generate a magnetic field oriented normal to, parallel to, or at an angle relative to a surface of the test object. In some embodiments, the magnetic circuit may be configured to reduce internal demagnetizing fields within the magnet material, for example by providing flux return paths or other arrangements that minimize self-demagnetization, thereby reducing the risk of irreversible demagnetization, particularly at elevated temperature. Additionally or alternatively, the magnet itself may be shaped to reduce its demagnetizing factor, for example by elongation along the direction of magnetization, by providing a high aspect ratio, or by other geometries that reduce self-demagnetization. The magnet assembly may further include protective coatings, encapsulation, thermal barriers, mechanical supports, or cooling structures to enable operation in harsh or high-temperature environments. In some embodiments, the biasmagnetic field may alternatively or additionally be generated by an electromagnet or by a combination of permanent magnets and electromagnets. In one embodiment, the permanent magnet has a shape with a length-to- width ratio equal or greater than 1:1, with the increasing ratio being in the length direction (e.g. twice as long as it is wide).

[0044] A permanent magnet provides a static bias field. To reduce irreversible demagnetization at high temperature, the magnet may be configured to have a high permeance coefficient geometry (i.e., low self-demagnetizing field) by:1) selecting a magnet geometry with relatively high aspect ratio (e.g., for an axially magnetized cylinder, length-to-diameter > about 2:1)2) coupling the magnet to magnetically permeable materials to reduce external magnetic reluctance, and / or3) arranging magnets and flux return structures to increase the effective permeance coefficient.4) a periodic permanent magnet array may be used for guided wave generation.

[0045] In one embodiment, the magnet is any magnetic material rated for operation at elevated temperature exceeding 250 °C. In another embodiment, the magnet material is selected from the group of neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo), and aluminum-nickel-cobalt (Alnico). For NdFeB magnets, useful materials include those with temperature grade suffixes: M, H, SH, UH, EH, AH. These can be suitable up to about 200°C. SmCo magnets are inherently more thermally stable so any version is suitable. SmCos type magnets typically have a maximum operating temperature of around 250-300°C, SnizCoi? have a maximum operating temperature of around (300-550°C).

[0046] An example of a useful samarium-cobalt rated for operation at elevated temperature exceeding 250 °C is neodymium. In one embodiment, the magnet material is Sm2Coi7 EEC 18-T550, which is suitable for operation at temperatures up to at least about 550 °C. The magnet may optionally have a protective coating (e.g., nickel). In another embodiment, the external reluctance of the magnetic circuit is further reduced by the inclusion of magnetically permeable materials forming a partially closed loop, for example through the inclusion of a core or yoke.Electromagnetic shield with stand-off spacing

[0047] An electrically conductive electromagnetic shield may be positioned to reduce undesirable ultrasonic generation within the magnet and / or to mitigate electromagnetic interference. In various embodiments, the shield may be copper or silver foil or sheet. Because eddy currents generated in the shield are in the opposite direction to the current in the coil conductor, the net eddy current induced in the test component is reduced by the presence of the shield. To mitigate this a standoff spacing is provided between the coil conductor and the shield to limit this loss. In one embodiment, the stand-off distance is from 0.1mm to 5mm. In another embodiment, the stand-off distance is from 1mm to 2mm. The shield may be cut or shaped to avoid interference with conductor feedthroughs.Cable and outer housing for high-temperature installation

[0048] In one embodiment, a cable connects the coil to instrumentation. The cable may be mineral insulated (MI) cable with two conductors or a coaxial configuration. The MI cable may be particularly useful for high temperature environments. The assembly may include an outer housing configured to contact the monitored component (pipe, vessel, etc.) and protect internal components. In one embodiment, the outer housing material is stainless steel, titanium, Inconel or combinations thereof.Monitored / Test component

[0049] Exemplary components suitable for monitoring with the disclosed high-temperature electromagnetic acoustic transducer (EMAT) system include, but are not limited to, metallic or electrically conductive components operating at elevated temperature, such as pipes, pipelines, pressure vessels, storage tanks, boilers, heat exchangers, reactor vessels, furnace tubes, ducts, structural members, plates, turbine components, and rotating or stationary machinery parts. The system may also be applied to components during manufacture, processing, forming, heat treatment, or transport, including workpieces that are stationary or moving, continuous or discrete, on a production line. Such workpieces may include, without limitation, slabs, billets, blooms, ingots, plates, sheets, strips, rods, bars, rails, wires, tubes, and extrusions. Monitoring may be performed in situ, during operation, or at any stage of production, including on hot, coated, oxidized, or otherwise untreated surfaces.EXAMPLESExample 1

[0050] An EMAT according to the present invention was used to measure wall thickness by transmitting and receiving ultrasonic reflections from an internal surface and measuring time-of-flight. Operation is achievable at low excitation levels, while the EMAT and test piece are heated to elevated temperatures up to at least about 550 °C. FIG. 5 shows ultrasonic a-scan measurements taken on an A508 ferromagnetic steel at a range of temperatures up to 550 °C. The EMAT and material were entirely enclosed in the furnace with the cable extending out to provide connection to the measurement electronics. A spiral coil configuration was used. 24 Vpp open circuit excitation voltage was used with 450 mApp current.

[0051] FIG. 6A shows ultrasonic a-scan measurements taken on a 316 austenitic stainless steel at a range of temperatures up to 550 °C. The EMAT and material were entirely enclosed in the furnace with the cable extending out to provide connection to the measurement electronics. A butterfly coil configuration was used. 400 Vpp, and 8 App. FIG. 6B shows an a-scan taken at 550 °C after 2000 hours of exposure at that temperature.

[0052] Although not described in detail herein, other steps which are readily interpreted from or incorporated along with the disclosed embodiments shall be included as part of the invention. The embodiments that have been described herein provide specific examples to portray inventive elements, but will not necessarily cover all possible embodiments commonly known to those skilled in the art.

Claims

What is claimed is:

1. An electromagnetic acoustic transducer (EMAT) comprising:a. an electrically insulating backing plate defining at least one open surface channel; b. an electrical conductor positioned within the open surface channel to form at least a portion of an electromagnetic coil; wherein the open surface channel constrains movement of the electrical conductor relative to the backing plate;c. a retaining element configured to maintain the electrical conductor within the open surface channel during operation; andd. a magnetic field source configured to provide a bias magnetic field for ultrasonic generation.

2. The EMAT of claim 1, wherein the backing plate comprises a ceramic material for operation at elevated temperature.

3. The EMAT of claim 1, wherein the retaining element comprises a wear plate.

4. The EMAT of claim 1, wherein the retaining element comprises a fixative.

5. The EMAT of claim 1, wherein the retaining element comprises a combination of a fixative and a wear plate.

6. The EMAT of claim 3, wherein the wear plate comprises a ceramic material.

7. The EMAT of claim 3, wherein the wear plate comprises a low electrical conductivity metallic alloy material.

8. The EMAT of claim 1, wherein the conductor comprises a material selected from the group consisting of copper, silver, gold, platinum, and combinations thereof.

9. The EMAT of claim 1, wherein the conductor comprises at least one wire.

10. The EMAT of claim 1, wherein the conductor comprises a sheet of material cut into a coil shape.

11. The EMAT of claim 1, wherein the electromagnetic coil forms a butterfly coil geometry wherein the magnetic field source has a footprint and portions of the electromagnetic coil beneath the footprint of the magnetic field source are routed in straight segments.

12. The EMAT of claim 1, wherein the electromagnetic coil forms a spiral coil geometry wherein the magnetic field source has a footprint and the electromagnetic coil is routed in a spiral pattern beneath the footprint of the magnetic field source.

13. The EMAT of claim 1, wherein the electromagnetic coil forms a racetrack coil geometry.

14. The EMAT of claim 1, wherein the magnetic field source comprises a permanent magnet.

15. The EMAT of claim 14, further comprising an electrically conductive electromagnetic shield positioned between the electromagnetic coil and the permanent magnet.

16. The EMAT of claim 15, wherein the electromagnetic shield is separated from the electromagnetic coil by a stand-off distance selected to minimize the reduction of eddy currents induced in a metallic test component due to the presence of the shield.

17. The EMAT of claim 1, wherein the magnetic field source is a permanent magnet of prismatic shape having a length-to- width ratio equal or greater than 1 : 1 with the magnetization direction in the long axis of the prismatic shape.

18. The EMAT of claim 1, wherein the magnetic field source comprises a permanent magnet rated for operation at elevated temperature exceeding 250 °C.

19. The EMAT of claim 18, wherein the permanent magnet comprises a samarium-cobalt material.

20. The EMAT of claim 18, wherein the permanent magnet includes a protective metallic coating.

21. A method of manufacturing an electromagnetic acoustic transducer (EMAT), comprising:providing an electrically insulating substrate having a surface; forming at least one open surface channel in the surface of the substrate; positioning an electrically conductive wire within the open surface channel such that the wire defines at least a portion of an electromagnetic coil extending along the surface of the substrate; and retaining the electrically conductive wire within the open surface channel using a retaining element configured to maintain the position of the wire relative to the substrate during operation of the electromagnetic acoustic transducer.

22. The method of claim 21, wherein the retaining element comprises an electrically insulating wear plate disposed over the open surface channel.

23. The method of claim 21, wherein the retaining element comprises an electrically insulating fixative securing the wire within the open surface channel.

24. The method of claim 21, wherein forming the open surface channel comprises machining, laser forming, molding, or additive manufacturing.

25. The method of claim 21, wherein positioning the electrically conductive wire forms a substantially planar coil geometry adjacent to the substrate surface.

26. The method of claim 21, wherein the substrate comprises a ceramic material suitable for elevated-temperature operation.

27. A method of performing ultrasonic monitoring of a component, comprising:a. providing an electromagnetic acoustic transducer according to claim 1;b. positioning the electromagnetic acoustic transducer adjacent to a metallic component;c. energizing the electromagnetic coil to generate ultrasonic waves within the metallic component; andd. receiving ultrasonic signals using the electromagnetic acoustic transducer to evaluate a condition of the metallic component.

28. The method of claim 27, wherein the component is monitored at a temperature greater than 250 °C.

29. The method of claim 27, wherein the ultrasonic monitoring comprises measuring wall thickness using time-of-flight measurements.

30. The method of claim 27, wherein the electromagnetic acoustic transducer is permanently installed on the component for long-duration monitoring.

31. The method of claim 27, wherein energizing the electromagnetic coil comprises applying a low-power excitation signal.

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