Laser-assisted embedding of all-glass optical fiber sensors into bulk ceramics for high-temperature applications

https://doi.org/10.1016/j.optlastec.2020.106223Get rights and content

Highlights

  • A novel laser-assisted sensor embedding process was developed.

  • All-glass optical fiber IFPI sensors were embedded into Al2O3 ceramics.

  • The embedded sensor was demonstrated for high temperature sensing application.

  • The capability of sensor for ceramic structural status monitoring was evaluated.

Abstract

We develop a laser-assisted sensor embedding process to embed all-glass optical fiber sensors into bulk ceramics for high-temperature applications. A specially designed two-step microchannel was fabricated on an Al2O3 substrate for sensor embedment using a picosecond (ps) laser. An optical fiber Intrinsic Fabry-Perot Interferometer (IFPI) sensor was embedded at the bottom of the microchannel and covered by Al2O3 slurry which was subsequently sintered by a CO2 laser. The sensor spectrum was in-situ monitored during the laser sintering process to ensure the survival of the sensor and optimize the laser sintering parameters. By testing in furnace through high temperature, the embedded optical fiber shows improved stability after CO2 laser sealing, resulting in the linear temperature response of the embedded optical fiber IFPI sensor. To improve the embedded IFPI sensor for thermal strain measurement, a dummy fiber was co-embedded with the sensing fiber to improve the mechanical bonding between the sensing fiber and the ceramic substrate so that the thermal strain of the ceramic substrate can apply on the sensing fiber. The response sensitivity, measurement repeatability and high-temperature long-term stability of the embedded optical fiber IFPI sensor were evaluated in this work.

Introduction

Due to their brilliant mechanical and thermal properties, ceramic materials have been widely applied as the critical components of systems working in high temperature, such as energy production systems, high temperature heating equipment, and aerospace facilities [1], [2], [3], [4], [5]. Since these systems normally work under the extremely harsh conditions for a long period, the evaluation of their structural health is necessary for system maintenance and optimization. An embedded sensor is one of the effective ways to accomplish this objective. The real-time information of the part, such as temperature and strain, can be continuously collected through in-situ monitoring of the embedded sensors during system operation [6].

All-glass optical fiber sensors are among the promising candidates for structural status monitoring under harsh environment [7], [8]. In addition to the well-known advantages such as compact size, high spatial resolution, fast response and immunity to electromagnetic interference, the all-glass optical fiber sensors are robust to operate under high temperature. For example, the optical fiber Intrinsic Fabry-Perot interferometer (IFPI) has shown great long-term high-temperature stability up to 1100 °C for over 1200 h [9], [10]. Since the optical fiber IFPI sensor is highly sensitive to the tensile stress applied to the optical fiber, this sensor is capable of sensing the thermal strain of the components if the fiber is well attached to the parts [11].

In general, attaching the sensor to the part without damaging the optical fiber is crucial in the fabrication of the optical fiber sensor embedded components. One of the common methods is to mount the fiber sensors on the surface of the finished parts with robust protectors. This technique has been proposed for years to monitor the health status of concretes [7], [8], [12]. However, the surface-mounted method usually results in poor attachment between the sensors and the components, leading to offset between the sensing signals and the real variation of the part [13], [14]. In addition, for harsh environment application, the sensors are usually mounted far away from the operating points to avoid damage on the optical fiber. In this way, the sensor only detects the variation of the part indirectly with low spatial and temporal resolution [6].

Additive manufacturing (AM) has been developed to embed the optical fiber sensors into the bulk materials during the part fabrication. In this way, the optical fibers are buried inside the components, which significantly improves the attachment between the sensors and parts and protects the sensors under harsh environment [15], [16]. AM methods have been developed to embed the glass optical fiber sensors into the metallic components for high temperature applications [17], [18]. The main challenge for this internal sensor embedding process is the thermal expansion mismatch between the glasses and the metals. At rising temperature, large thermal strain applied on the optical fiber from the metallic parts will delaminate the fibers from the components and degrade the performance of the embedded sensors [17]. In addition, the AM methods are only suitable to embed the glass optical fiber sensors into the materials whose melting point is lower than the working temperature of the fused silica glasses. It is still quite difficult to embed the all-glass optical fiber sensors into the high-temperature ceramics, such as Al2O3 and yttria-stabilized zirconia (YSZ), using the AM methods, since the glass cannot survive the sintering temperature of most ceramics.

Recently, sapphire optical fiber has been successfully embedded into the alumina ceramics using the AM method [19]. Since the melting point of single crystal sapphire is over 2000 °C, sapphire optical fibers can survive the post-sintering process of the 3D-printed alumina ceramics and have potential to work on temperature over 1500 °C. However, due to the lack of cladding layers, the sapphire optical fibers are normally multi-mode fibers with large modal volume, which complicates the interrogation of sapphire optical fiber sensors [20], [21]. In addition, the high optical loss of the sapphire fibers also limits it for high-performance sensing applications.

Compared to the AM methods, laser processing technologies are promising to overcome the challenge of embedding the all-glass optical fiber sensors into the high-temperature ceramics. Laser has shown its unique capability for high-resolution processing of ceramic materials [22], [23], [24]. Ultrafast laser has been developed for machining micro structures like microchannels on bulk ceramics with a resolution of up to several microns [25], [26]. Since the pulse duration is shorter than the typical thermalization time of materials, the ultrafast lasers can machine the materials without thermally degrading the mechanical strength of the parts [27]. In addition, fast, precise and flexible heat treatment on ceramic materials has been realized using the CO2 laser. The laser heating effective zone can be precisely controlled in three dimensions with ultrahigh heating and cooling rate. The material properties, such as density and cracking propagation, can be flexibly fine-tuned through adjusting the laser processing parameters [28], [29]. Both of these laser technologies are promising to accomplish embedding glass optical fiber sensors into finished ceramic products, resulting in the improvement of flexibility and efficiency in the fabrication of sensor-embedded smart ceramic components.

Here we propose a laser-assisted sensor embedding process to embed the all-glass optical fiber sensors into bulk ceramics. A specially designed two-step microchannel was machined on an Al2O3 substrate for sensor embedment using a picosecond (ps) laser. An IFPI sensor, which was fabricated on a glass single mode optical fiber by the femtosecond (fs) laser irradiation, was embedded to the bottom of the microchannel and covered by the Al2O3 slurry. The filled Al2O3 slurry was subsequently sintered by a CO2 laser to seal the sensor inside the part. The design of the two-step microchannel was based on the shape of the optical fiber and the heating depth of the CO2 laser. During the laser sealing process, the spectrum of the optical fiber IFPI sensor was in-situ monitored to ensure the survival of the sensor and optimize the laser sintering parameters. The microstructure of the sensor-embedded Al2O3 substrate was presented to evaluate the laser sealing quality. By high-temperature measurement in a furnace, the high-temperature response, repeatability and long-term stability of the embedded optical fiber IFPI sensor were investigated.

Section snippets

Optical fiber IFPI sensor fabrication

The schematic of the optical fiber IFPI sensor was shown in Fig. 1. The IFPI sensor was formed by two internal partial reflectors created by a femtosecond (fs) laser at the core of a single-mode glass optical fiber (SMF-28, Corning Inc.). Owing to the non-linear effect of the ultrashort laser pulses, the fs laser can locally modify the refractive index of the optical fiber at the focused laser spot. The spot size of the focused fs laser beam is ~1 μm, which is smaller than the diameter of the

Ps laser machined two-step microchannel

To firmly attach the optical fiber to the ceramic substrate, the microchannel needs to have a similar shape of the optical fiber to host the fiber at the bottom. In addition, to protect the optical fiber during the laser sealing process, the distance between the top of the optical fiber and the surface of the substrate should be slightly larger than the sintering depth of CO2 laser on the Al2O3 slurry, which is about 100 μm as reported previously [31]. Since the diameter of the standard optical

Conclusion

In summary, an all-glass optical fiber IFPI sensor was successfully embedded into a commercial Al2O3 ceramic substrate using the laser-assisted sensor embedding process. The two-step structure effectively reduces the general aspect ratio of the laser-machined microchannel to provide precise control on the channel shape, which is essential to fit the optical fiber inside the ceramic substrate. After sealing by the CO2 laser sintering, the high-temperature stability of the embedded optical fiber

CRediT authorship contribution statement

Jincheng Lei: Methodology, Writing. Qi Zhang: Software. Yang Song: Validation. Jianan Tang: Software. Jianhua Tong: Supervision. Fei Peng: Supervision. Hai Xiao: Conceptualization, Supervision, Project administration.

Declaration of Competing Interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the Department of Energy [Grant number DE-FE0031826].

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