A method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material and the structure of the low-temperature temperature-sensitive fiber grating.

Abstract

This invention relates to the field of fiber optic sensing technology, specifically to a method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating, and the low-temperature temperature-sensitive fiber grating structure thereof. The method includes the following steps: preparing and pre-treating an optical fiber core layer; coating the outer periphery of the optical fiber core layer with nano-sized zirconium hafnium to obtain a stress transfer layer; preparing a low-temperature negative thermal expansion coefficient sensitizing material powder and coating it on the outer periphery of the optical fiber stress transfer layer to obtain a negative thermal expansion material sensitizing coating layer; constructing a porous structure on the surface of the obtained negative thermal expansion material sensitizing coating layer to form a porous layer on the surface of the negative thermal expansion material; and encapsulating a composite shell structure on the outside of the porous layer on the surface of the negative thermal expansion material to obtain a metal sheath layer. The fiber grating structure provided by this invention, for adapting to the production of temperature sensors, can simultaneously solve the failure problems of traditional sensors in the 4K ultra-low temperature region, the interference problems of electrical sensors in strong electromagnetic field environments, and the sealing failure problems caused by material brittleness in cryogenic environments.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating and the structure of the low-temperature temperature-sensitive fiber grating. Background Technology

[0002] Fiber Bragg gratings (FBGs) are modulation structures formed within the core of optical fibers with periodic refractive indices. They possess a range of outstanding advantages, including small size, light weight, intrinsic safety, resistance to electromagnetic interference, corrosion resistance, long lifespan, and ease of wavelength division multiplexing (WDM). These advantages have made them one of the most crucial passive devices in modern fiber optic sensor networks. FBG sensors, by demodulating the linear response of their Bragg wavelength to external physical quantities such as temperature and strain, enable quasi-distributed measurement and long-term remote online monitoring of multiple parameters and points. They have demonstrated enormous application potential in aerospace, smart grids, petrochemicals, and civil engineering.

[0003] In the field of temperature sensing, traditional electrical sensors such as platinum resistance thermometers (Pt100 / Pt1000), thermocouples (K-type, J-type, etc.), and thermistors (NTC / PTC) have been widely used for decades. Their advantages mainly lie in their mature technology, relatively low cost, and standardized and easy-to-implement measurement circuits. However, their inherent characteristics limit their application in certain modern industrial scenarios: First, their signal transmission method based on metal wires makes them highly susceptible to interference from strong electromagnetic fields, leading to signal distortion or drift; second, the long-term stability of the sensors is affected by factors such as material oxidation and metal creep, with significant performance degradation in high-temperature, high-humidity, or corrosive environments; third, the signal transmission loss is high and the distance is short, making it difficult to construct large-scale, long-distance distributed sensor networks; finally, in hazardous environments such as flammable and explosive environments (e.g., oil depots, coal mines) or high-voltage environments (e.g., inside transformers), there is a risk of electrical sparks causing accidents, resulting in insufficient inherent safety.

[0004] Fiber Bragg grating temperature sensors ingeniously utilize optical signals as information carriers, fundamentally overcoming many bottlenecks of traditional electrical sensors. Their core advantages lie in: 1. Safe and explosion-proof, the sensor head does not need to be powered on, the light source is far away from the monitoring site, and there is no risk of electric sparks; 2. Extremely strong anti-electromagnetic interference capability. The light waves are not affected by electromagnetic environments such as microwaves, radio frequencies, and lightning, and are suitable for strong electromagnetic environments such as high-voltage substations and large motors. 3. It has good chemical stability and is made of corrosion-resistant quartz glass, making it suitable for harsh environments such as chemical plants and marine environments; it has a long signal transmission distance and low loss, enabling remote monitoring over distances of several kilometers or even longer. 4. Strong multiplexing capability: Through wavelength division and time division technologies, dozens to hundreds of gratings can be connected in series on a single optical fiber to achieve quasi-distributed temperature field measurement, which greatly reduces system cost and complexity. In addition, its fast response speed and small size make it easy to embed in composite materials or inside structures for in-situ monitoring. Because pure silica fiber gratings have a low temperature sensitivity coefficient (approximately 10 pm / ℃) and exhibit cross-sensitivity to temperature and strain, extensive and in-depth research has been conducted by scholars both domestically and internationally to address the challenges of high-precision temperature measurement and stress interference elimination. Besides employing special optical fibers and gratings, the main technical approaches can be categorized into two main types: encapsulation-based sensitization and structural / algorithm-based desensitization.

[0005] The most common method is encapsulation-based sensitivity enhancement. This method encapsulates a fiber optic grating in a material with a high coefficient of thermal expansion (CTE), utilizing the material's thermal expansion to apply additional strain to the grating, thereby amplifying the temperature-induced wavelength shift. Early studies often employed metal encapsulation; for example, Zhan et al. used aluminum groove encapsulation to increase sensitivity to 3.6 times that of a bare grating. However, the metal encapsulation itself easily becomes a stress transmission channel. On the other hand, polymer encapsulation has been found to have a higher CTE. However, polymers suffer from aging, nonlinear thermal expansion, and the potential introduction of chirp. To balance sensitivity and mechanical protection, composite structure encapsulation has been proposed.

[0006] In addition, during the practical application and engineering of fiber Bragg grating temperature sensors, some technologies focus on improving the packaging structure to achieve stable sensitivity enhancement. For example, Chinese patent application number 2012102164986 discloses a technique of fixing the grating in a relaxed arc shape within a module and then inserting a protective tube. Its core advantages are convenient assembly and stress isolation, but the sensitivity enhancement effect is not its primary goal. Chinese patent application number 2011103080131 discloses a technique of directly packaging with corrosion-resistant steel pipes, which has a simple structure but limited stress isolation effect. The above-mentioned existing technologies are all based on the encapsulation and sensitivity enhancement of materials with positive expansion coefficients (such as aluminum alloys and polymers). Although they effectively improve the temperature sensitivity coefficient, they still have significant limitations in practical applications, especially in low-temperature applications.

[0007] First, the temperature sensitization coefficient (CTE) of materials with a positive coefficient of thermal expansion decreases significantly at low temperatures, even approaching zero, leading to a sharp weakening of the sensitization effect. For example, the temperature sensitization coefficient of bare FBG at 93K is only 2.19 pm / K, far lower than 9.18 pm / K at room temperature (Optics & Precision Engineering, 2022, 30(1):56). Although polymer encapsulation (such as polyimide and epoxy resin) can achieve a CTE of 200 × 10⁻⁶ at room temperature... -6While the CTE (Coefficient of Thermal Expansion) is high, its thermal expansion behavior exhibits strong nonlinearity below the glass transition temperature (Tg), leading to unstable sensitivity in the low-temperature region. Secondly, there is the stress mismatch at the material-fiber interface. The difference in shrinkage rates between the positive CTE material and the silica fiber causes stress concentration at the interface at low temperatures, triggering fiber coating peeling or grating chirp, and in severe cases, even damaging the grating structure. Metal encapsulation (such as aluminum channels or bimetallic sheets) undergoes plastic deformation due to thermal cycling (e.g., the creep rate of aluminum > 0.8%), resulting in a decline in sensitivity enhancement performance after repeated thermal cycling. Finally, material performance degradation is unavoidable: metal materials undergo plastic deformation during thermal cycling (creep rate of aluminum > 0.8% after > 200 cycles), leading to sensitivity decay. Although liquid metal encapsulation (such as Ga-In alloys) improves stress distribution, its volatile components undergo phase separation above 150℃, forming oxide cavities after long-term use. Aluminum alloys show an 83% decrease in CTE at -196℃ and a fracture toughness of only 4 MPa·m. 1 / 2 Polymer materials face aging challenges (epoxy resin yellowing index > 2.5 per year), especially in low-temperature and liquid nitrogen / liquid helium cryogenic regions, where polymer materials become severely embrittled.

[0008] Current technologies for temperature sensors using positive thermal expansion materials for sensitivity enhancement suffer from technical bottlenecks such as a sharp drop in sensitivity, material embrittlement, and stress interference in the low-temperature region. To address these issues, a method for fabricating a fiber grating temperature sensor is proposed to obtain a fiber grating structure based on a negative thermal expansion material. Summary of the Invention

[0009] The purpose of this invention is to address the problems existing in the background technology by proposing a method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material and the structure of the low-temperature temperature-sensitive fiber grating.

[0010] The technical solution of this invention: a method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating, characterized by comprising the following steps: S1. Prepare the fiber core layer, and treat the outer peripheral surface of the fiber core layer by sequentially passing it through Piranha solution and argon plasma bombardment. S2. Nanoscale zirconium hafnium oxide is coated on the outer peripheral surface of the optical fiber core to obtain a stress transfer layer; S3. Prepare a low-temperature negative thermal expansion coefficient sensitizing material powder with a particle size range of 0.2-0.8μm and a specific surface area >35m2 / g. Coat the above-mentioned low-temperature negative thermal expansion coefficient sensitizing material powder on the outer peripheral surface of the optical fiber stress transfer layer by a layered coating method to obtain a negative thermal expansion material sensitizing coating layer. S4. Construct a porous structure on the surface of the negative thermal expansion material sensitizing coating to form a porous layer on the surface of the negative thermal expansion material. S5. A microcrystalline glass-Invar laminated composite shell structure is encapsulated on the outside of the porous layer on the surface of the negative thermal expansion material to obtain a metal cladding layer.

[0011] Preferably, the fiber core layer is made of any one of homogeneous doped quartz glass, pure quartz glass, or polymer.

[0012] Preferably, the coating thickness of the stress transfer layer is 20–100 nanometers.

[0013] Preferably, the stress transfer layer is prepared using the sol-gel method, and the specific steps are as follows: Equimolar ratios of zirconium alkoxide and hafnium alkoxide were dissolved in anhydrous alcohol solvent, deionized water was added and acid was used to catalyze hydrolysis and polycondensation, and the mixture was aged to obtain a transparent and uniform sol. The pretreated fiber core was then vertically immersed in the sol and pulled to form a liquid film on the surface of the fiber core. After gelation at room temperature, the fiber is placed in a tube furnace and gradually heated to 400-600℃ in an air atmosphere for heat treatment, resulting in a ZrHfO4 coating on the surface of the fiber core.

[0014] Preferably, the low-temperature negative thermal expansion coefficient sensitizing material powder is any one of ZrW2O8 powder, ScF3 powder, or Ag3[Co(CN)6] powder.

[0015] Preferably, the low-temperature negative thermal expansion coefficient sensitizing material powder is ZrW2O8 powder. The method for forming a negative thermal expansion coefficient sensitizing coating layer of ZrW2O8 powder on the outer peripheral surface of the stress transfer layer is as follows: Prepare the bottom layer slurry, transition layer slurry, and functional layer slurry separately; wherein: The composition ratio of the bottom slurry is ZrW2O8@SiO2 = 60%:40%; The transition layer slurry composition ratio is ZrW2O8@Al2O3 = 75%: 25%; The functional layer slurry is pure ZrW2O8; Auxiliary solvents were added to the preparation of the bottom layer slurry, transition layer slurry and functional layer slurry respectively, and they were stirred respectively to ensure that the viscosity and conductivity of the preparation of the bottom layer slurry, transition layer slurry and functional layer slurry met the requirements of electrostatic jet deposition. First, the bottom layer slurry is sprayed, then the transition layer slurry is sprayed, and finally the functional layer slurry is sprayed. Each layer is sprayed only after the target thickness is reached. After each layer is deposited, a short-term pre-curing is performed to reduce interlayer miscibility and form a controlled diffusion transition zone. After deposition, a variable-temperature heat treatment is performed.

[0016] Preferably, the variable-temperature heat treatment method involves rapidly cooling to 77K with liquid nitrogen, then heating to 620℃ at a rate of 10K / min and holding at that temperature for 45min.

[0017] Preferably, one method for constructing a porous structure on the surface of the negative thermal expansion material sensitizing coating layer is as follows: Electrochemical etching is performed in an electrolyte containing hydrofluoric acid to directly dissolve the material and form pores. Then, annealing is performed at 480°C in a nitrogen atmosphere to obtain a porous layer on the surface of the negative thermal expansion material with a porosity of not less than 21%.

[0018] Another preferred method for constructing a porous structure on the surface of the negative thermal expansion material sensitizing coating is as follows: A polyamide thin film is covered as an auxiliary layer on the surface of the functional layer of the negative thermal expansion material sensitizing coating. A CO2 laser beam is used to scan and ablate the coating along a preset path to create a conical hole with a pore size of 13-16 μm and smooth boundaries, resulting in a porous layer on the surface of the negative thermal expansion material with a porosity of 15%-22%.

[0019] Preferably, the method for preparing and encapsulating the microcrystalline glass-Invar laminated composite shell structure is as follows: Thin sheets of microcrystalline glass and Invar are alternately stacked and solid-state bonded at a set temperature by anodic bonding or low-melting-point glass solder to form a dense and sealed stacked composite shell, in which Invar is used as the structural layer and transition layer, and microcrystalline glass is used as the functional layer that comes into contact with the outside world. The integrated fiber core, stress transfer layer, negative thermal expansion material sensitizing coating layer, and negative thermal expansion material surface porous layer are then placed in the center of the laminated composite shell and flexibly fixed at the ends with low modulus silicone.

[0020] Preferably, solid bonding is performed by anodic bonding or by low-melting-point glass solder at a temperature of less than 450°C.

[0021] Preferably, a corrugated pipe is installed on the outside of the metal sheath layer.

[0022] Preferably, the bellows is made of Fe-Ni-Cr shape memory alloy, and the axial elastic modulus of the bellows is 3.8 MPa in the installed state.

[0023] The present invention also discloses a low-temperature temperature-sensitive fiber grating structure, which is prepared by the above-mentioned method for preparing a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating. The low-temperature temperature-sensitive fiber grating structure includes an optical fiber core layer. A stress transfer layer that coats the surface of the fiber core to buffer thermal stress and enhance interfacial bonding; A negative thermal expansion material sensitizing coating layer is applied to the surface of the stress transfer layer to generate thermal strain opposite to that of the optical fiber at low temperatures, thereby amplifying the optical temperature response of the optical fiber. A porous layer on the surface of a negative thermal expansion material, used to provide gas or liquid channels for rapid heat exchange and reduce transverse stress transmission; And a metal sheath layer covering the porous layer of the negative thermal expansion material.

[0024] Preferably, the low-temperature temperature-sensitive fiber Bragg grating structure further includes a bellows covering the outer layer of the metal sheath for absorbing and buffering external lateral stress.

[0025] Compared with the prior art, the above-mentioned technical solution of the present invention has the following beneficial technical effects: This technical solution utilizes the large and linear negative thermal expansion coefficient and excellent low-temperature structural stability of negative thermal expansion materials to achieve low-temperature sensitization of fiber optic gratings. The fabricated fiber grating structure consists of an optical fiber core, a stress transfer layer, a negative thermal expansion material sensitizing cladding layer, a porous layer on the surface of the negative thermal expansion material, a metal cladding layer, and a bellows. The metal cladding layer employs a microcrystalline glass-Invar composite structure combined with a shape memory alloy bellows to absorb transverse stress, achieving high-sensitivity temperature response and stress decoupling at low temperatures. Furthermore, the microcrystalline glass-Invar composite structure also exhibits high radial thermal conductivity. The innovative use of a negative thermal expansion material to fabricate the negative thermal expansion material sensitizing cladding layer and the porous layer on the surface of the negative thermal expansion material forms a temperature-sensitizing functional layer. The porous structure of the negative thermal expansion material surface can disperse external stress, and the negative thermal expansion material sensitizing cladding layer exhibits a high negative thermal expansion coefficient (up to -15.2 × 10⁻⁶) in the low-temperature region. -6 The fiber optic grating structure fabricated using this technology significantly improves the low-temperature sensitivity enhancement effect by improving the linearity of temperature (K / °C). The temperature sensor fabricated using this technology can achieve a sensitivity of up to 50 pm / °C in a wide temperature range of 4K–350K, resist electromagnetic interference, and has a vibration noise suppression ratio of not less than 40 dB. The magnetostrictive wavelength drift is less than 0.05 pm (compared to a magnetostrictive wavelength drift of more than 12 pm in traditional metal-encapsulated systems). The fabricated temperature sensor is suitable for extreme environments such as superconducting magnets and aerospace cryogenic systems, and is also suitable for applications in superconducting quantum computers and LNG transportation pipelines.

[0026] The fiber grating structure provided by this invention for adapting to the production of temperature sensors can simultaneously solve the problems of traditional sensor failure in the 4K ultra-low temperature region, electrical sensor interference in strong electromagnetic field environments, and sealing failure caused by material brittleness in cryogenic environments, and has extremely high application value. Attached Figure Description

[0027] Figure 1 This is a perspective view of one embodiment of the present invention; Figure reference numerals: 101, fiber core; 102, stress transfer layer; 103, sensitizing coating layer of negative thermal expansion material; 104, porous layer on the surface of negative thermal expansion material; 105, metal sheath layer; 106, corrugated tube. Detailed Implementation

[0028] Example 1, as Figure 1 As shown, the present invention proposes a low-temperature temperature-sensitive fiber optic grating structure, which includes an optical fiber core layer 101, a stress transfer layer 102, a negative thermal expansion material sensitizing cladding layer 103, a negative thermal expansion material surface porous layer 104, a metal cladding layer 105, and a corrugated tube 106. The fiber core 101 is made of any one of homogeneous doped quartz glass, pure quartz glass, or a polymer represented by polymethyl methacrylate (PMMA-POF). When using homogeneous doped quartz glass or pure quartz glass as raw material to produce the fiber core 101, a homogeneous quartz glass core rod is prepared using any one of the following processes, but not limited to MCVD / PCVD / OVD / VAD, and then drawn into fibers. When using the aforementioned polymers as raw material to produce the fiber core 101, a fiber core 101 is prepared using a preform polymerization-drawing or casting-drawing process. The fiber refractive index of the prepared fiber core 101 ranges from 1.4 to 1.7. The stress transfer layer 102 is attached to the outer peripheral surface of the optical fiber core layer 101 for efficient transfer of low-temperature interface stress. The material of the stress transfer layer 102 is nano-sized zirconium hafnium oxide (ZrHfO4), and the thickness of the stress transfer layer 102 obtained by coating is 20-100 nanometers. The process of depositing the stress transfer layer 102 on the outer peripheral surface of the optical fiber core layer 101 using nano-sized zirconium hafnium oxide (ZrHfO4) is a sol-gel process. A negative thermal expansion material sensitizing coating layer 103 is attached to the outer peripheral surface of the stress transfer layer 102. The material of the negative thermal expansion material sensitizing coating layer 103 is selected, but is not limited to, ZrW2O8, ScF3, or Ag3[Co(CN)6]. A high-performance porous structure is constructed on the surface of the negative thermal expansion material sensitizing coating 103 to form a porous layer 104 on the surface of the negative thermal expansion material. A metal sheath layer 105 is encapsulated on the outside of a porous layer 104 on the surface of a negative thermal expansion material. The metal sheath layer 105 is a microcrystalline glass-Invar laminated composite shell structure, using Invar as the structural and transition layer. The extremely low thermal expansion coefficient difference between Invar and optical fiber (quartz) ensures minimal interfacial stress over a wide temperature range, preventing debonding and grating chirping. The microcrystalline glass, as the functional layer in contact with the external environment, utilizes its high thermal conductivity (typically >2 W / (m·K)) to achieve rapid temperature response and uniform distribution. The microcrystalline glass-Invar laminated composite shell structure can be fabricated using precision machining combined with low-temperature sealing. Specifically, thin sheets of microcrystalline glass and Invar are alternately laminated, and solid-state bonding is performed using anodic bonding or low-melting-point glass solder at a set temperature (below 450°C to avoid damaging the grating) to form a dense and sealed laminated composite shell. The obtained metal sheath layer 105 is encapsulated on the outside of the porous layer 104 on the surface of the negative thermal expansion material, and the ends are flexibly fixed with low modulus silicone, which not only completes mechanical protection, but also ensures stress isolation and efficient heat conduction. The high thermal conductivity and high strength properties of the metal sheath layer 105 are used to protect the internal bonding. The bellows 106 is wrapped around the metal sheath layer 105. The bellows 106 is a Fe-Ni-Cr shape memory alloy bellows with an axial elastic modulus of 3.8 MPa. During installation, the bellows is axially pre-compressed by 15% from its free length. This pre-compression allows it to store a large restoring force, thereby tightly gripping the composite shell radially inward. In other words, by utilizing its superelasticity, a constant and controllable preload is applied during installation to overcome the thermal expansion mismatch of the layers and maintain close contact between the interfaces during temperature cycling.

[0029] Example 2: A method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material, proposed in this invention, is used to fabricate the low-temperature temperature-sensitive fiber grating structure described in Example 1. The method specifically includes the following steps: S1. Fabrication of optical fiber core layer 101; The fiber core 101 is made of any one of homogeneous doped quartz glass, pure quartz glass, or a polymer represented by polymethyl methacrylate (PMMA-POF). If homogeneous doped quartz glass or pure quartz glass is used as raw material to produce optical fiber core layer 101, a homogeneous quartz glass core rod is prepared by any of the processes, but not limited to MCVD / PCVD / OVD / VAD, and then drawn into fiber. If the above-mentioned polymer is used as a raw material to produce the optical fiber core 101, the optical fiber core 101 is produced by a fiber forming process using a preform polymerization-firing or casting molding-firing route. The refractive index of the fiber core 101 is in the range of 1.4 to 1.7. Furthermore, the outer surface of the prepared optical fiber core layer 101 is cleaned. The cleaning method for the outer surface of the optical fiber core layer 101 is as follows: a Piranha solution with a volume ratio of H2SO4:H2O2=3:1 is used to clean the outer wall of the optical fiber core layer 101 to remove all organic contaminants and moderately hydroxylate the outer surface of the optical fiber core layer 101. Then, argon plasma is used to bombard the outer surface of the optical fiber core layer 101 to deeply clean the outer surface of the optical fiber core layer 101 while controlling the surface energy of the optical fiber core layer 101. This facilitates the tight bonding between the stress transfer layer 102 and the optical fiber core layer 101 in the next step and can suppress the propagation of microcracks at the interface between the stress transfer layer 102 and the optical fiber core layer 101 caused by thermal cycling.

[0030] S2. Nanoscale zirconium hafnium oxide (ZrHfO4) is coated on the outer peripheral surface of the optical fiber core layer 101 to obtain a stress transfer layer 102 with a thickness of 20-100 nanometers; the stress transfer layer 102 is used for efficient transfer of low-temperature interface stress. In an optional embodiment, nano-sized zirconium hafnium oxide (ZrHfO4) is deposited on the outer periphery of the optical fiber core 101 using a sol-gel process to obtain a stress transfer layer 102. The stress transfer layer 102, using nano-sized zirconium hafnium oxide (ZrHfO4) as a transition layer, is bonded to the optical fiber core 101, enabling it to form strong chemical bonds (such as Si-O-Zr / Hf bonds) and tight physical bonds with the quartz optical fiber (mainly composed of SiO2) at the atomic / molecular scale. This enhanced chemical and physical bonding effect ensures that even at extremely low temperatures of -269°C, the interfacial bonding strength remains above 20 MPa, providing a robust and reliable foundation for subsequent stress transfer. This is something that traditional organic polymer coatings (which become brittle, shrink, and peel off at low temperatures) cannot achieve. This transition layer achieves: 1) Thermal strain transfer efficiency ≥95% (traditional polymer coatings have a transfer rate of <40% at -100℃); 2) The stress transfer layer 102 locks the displacement synchronization between the fiber core layer 101 and the negative thermal expansion material sensitizing cladding layer 103, with a hysteresis effect of <0.05%.

[0031] In an optional embodiment, the transition layer 102 is prepared using a sol-gel method, and the preparation method is as follows: Equimolar ratios of zirconium alkoxide (such as zirconium isopropoxide) and hafnium alkoxide (such as hafnium isopropoxide) are dissolved in anhydrous alcohol solvent (such as ethanol or isopropanol), and a small amount of deionized water and acid (such as hydrochloric acid or nitric acid) are added to catalyze hydrolysis and condensation. After aging, a transparent and uniform sol is obtained. The pretreated fiber core 101 is then vertically immersed in the sol and pulled at a constant speed to form a liquid film on the surface of the fiber core 101. After room temperature gelation, the material is placed in a tube furnace and gradually heated to 400–600℃ in an air atmosphere for heat treatment, which decomposes the organic components and densifies the oxides. Finally, a ZrHfO4 coating with a thickness of 20–100 nanometers is obtained on the surface of the fiber core 101. This method has simple equipment, easy control of stoichiometry, and is suitable for uniform coating of bent optical fibers.

[0032] S3. Prepare particles with a size range of 0.2–0.8 μm and a specific surface area >35 m². 2 / g of low-temperature negative thermal expansion coefficient sensitive material powder is calcined at 450-650℃ to eliminate the internal stress of the low-temperature negative thermal expansion coefficient sensitive material powder. Then, the above-mentioned low-temperature negative thermal expansion coefficient sensitive material powder is coated on the outer peripheral surface of the optical fiber stress transmission layer 102 by a layer coating method to obtain negative thermal expansion material sensitive coating layer 103. In an optional embodiment, the low-temperature negative thermal expansion coefficient sensitizing material powder is any one of ZrW2O8 powder, ScF3 powder, or Ag3[Co(CN)6] powder.

[0033] Furthermore, the low-temperature negative thermal expansion coefficient sensitizing material powder is selected from ZrW2O8 powder. The method for forming the negative thermal expansion coefficient sensitizing coating layer 103 on the outer peripheral surface of the stress transfer layer 102 using ZrW2O8 powder is as follows: Three types of spray slurry / sol systems were prepared: bottom layer slurry (mixed according to the volume fraction of the bottom layer), transition layer slurry (mixed according to the volume fraction of the transition layer), and functional layer slurry (pure powder or high content system). The composition and proportions of the three types of sprayed slurries prepared are as follows: The volume ratio of the bottom layer is: ZrW2O8@SiO2 = 60%:40%, that is, the volume fraction ratio of ZrW2O8 to SiO2 is 60%:40%. The volume ratio of the transition layer is ZrW2O8@Al2O3 = 75%:25%, meaning the volume fraction ratio of ZrW2O8 to Al2O3 is 75%:25%. Functional layer: pure ZrW2O8; It should be noted that when preparing the three types of spray slurries—bottom layer volume ratio, transition layer volume ratio, and functional layer—it is necessary to add auxiliary solvents and perform ultrasonic stirring to ensure that the viscosity and conductivity of the three types of spray slurries meet the requirements for stable spraying. Large particle agglomerates are removed by filtration before spraying. During deposition, the layers are sprayed sequentially: first the bottom layer, then the transition layer, and then the functional layer; each layer is sprayed only after it reaches the target thickness, in order to form a gradient transition, thereby reducing cracking and delamination caused by sudden changes in interfacial thermal stress and elastic modulus. The spraying process employs electrostatic spraying deposition, where the fiber core layer 101 with a stress transfer layer 102 is fixed on a movable platform. The distance between the nozzle and the stress transfer layer 102 is adjusted, and the substrate / matrix temperature is set to promote solvent evaporation and film formation. Specifically, the nozzle diameter is 50–200 μm, preferably 100 μm; the substrate temperature is 80–180 °C, preferably 130 °C; the relative moving speed is 0.1–1.0 mm / s, preferably 0.3 mm / s; the voltage during electrostatic spraying is 6–12 kV, preferably 8 kV; the target layer thickness is approximately 120 ± 5 μm after spraying; and the interlayer element diffusion band is less than 3 μm. Under the action of high voltage power supply, a stable Taylor cone is formed at the nozzle end and atomized microdroplets are sprayed. The microdroplets are deposited on the surface of stress transfer layer 102 under the traction of electric field to form a coating. First, the bottom layer is deposited, then the transition layer is deposited, and finally the functional layer is deposited. After each layer is deposited, it can be dried / pre-cured for a short time to reduce interlayer miscibility and form a controlled diffusion transition zone. After the deposition is completed, variable temperature heat treatment is performed to remove organic matter and eliminate internal stress. The variable temperature heat treatment method is to apply the obtained fiber core layer 101 with negative thermal expansion material sensitizing coating layer 103.

[0034] S4. Construct a porous structure on the surface of the negative thermal expansion material sensitizing coating 103 to form a porous layer 104 on the surface of the negative thermal expansion material. In an optional embodiment, if the negative thermal expansion material sensitizing coating layer 103 is made of ZrW2O8 material and prepared using an electrostatic spraying deposition process, then an electrochemical etching and surface activation process is used to construct a porous structure on the surface of the negative thermal expansion material sensitizing coating layer 103. The specific method is as follows: Electrochemical etching is performed in an electrolyte containing hydrofluoric acid (current intensity of 6 mA / cm² per unit area, processing time of 25 min) to directly dissolve the material and form pores. To improve the mechanical stability of the porous layer, it is then annealed at 480℃ in a nitrogen atmosphere. This process achieves a high porosity (≥21%), but precise control is required to prevent pore wall cracking.

[0035] Furthermore, for applications requiring high precision and customized pore sizes, a laser-induced pore-forming process is employed. Specifically, a polyamide thin film is applied as an auxiliary layer to the surface of the functional layer of a negative thermal expansion material sensitizing coating layer 103 made from pure ZrW2O8. A CO2 laser beam (120W power) is then used to scan and ablate the surface along a preset path (at a rate of 0.4mm). 2 The method, which produces tapered holes with a diameter of 13–16 μm and smooth boundaries, and a porosity of 15%–22%, is a non-contact processing method that can effectively avoid thermal stress concentration and achieve controllable porosity with regular structure.

[0036] S5. Encapsulate a microcrystalline glass-Invar composite shell structure on the outside of the porous layer 104 on the surface of the negative thermal expansion material to obtain a metal cladding layer 105. In an optional embodiment, a high thermal conductivity and high strength encapsulation of the fiber Bragg grating is achieved by using Invar as the structural and transition layer. The extremely low difference in thermal expansion coefficient between Invar and optical fiber (quartz) ensures that the interfacial stress is minimized over a wide temperature range, preventing debonding and grating chirping. Microcrystalline glass serves as the functional layer in contact with the outside world, utilizing its high thermal conductivity (typically >2W / (m·K)) to achieve rapid temperature response and uniform distribution. The fabrication and installation of the microcrystalline glass-Invar laminated composite shell structure adopts a combination of precision machining and low-temperature sealing process. Specifically, thin-film microcrystalline glass and Invar are alternately laminated and solid-state bonded at a set temperature by anodic bonding or low-melting-point glass solder to form a dense and sealed laminated composite shell. Then, the integral fiber core layer 101, stress transfer layer 102, negative thermal expansion material sensitizing coating layer 103, and negative thermal expansion material surface porous layer 104 are placed in the center of the laminated composite shell and flexibly fixed at the ends with low-modulus silicone, which not only completes the mechanical protection but also ensures stress isolation and efficient heat conduction. The temperature is set to be less than 450℃ to avoid damaging the grating.

[0037] S6. To further eliminate stress interference, a bellows 106 is installed on the outside of the metal sheath layer 105. In an optional embodiment, the bellows 106 is made of Fe-Ni-Cr shape memory alloy with an axial elastic modulus of 3.8 MPa. During installation, the bellows 106 is axially pre-compressed by 15% from its free length. This pre-compression allows it to store a large restoring force internally, thereby tightly gripping the metal sheath layer 105 radially inward. That is, by utilizing its superelasticity, a constant and controllable preload is applied during installation to overcome thermal expansion mismatch between the layers and maintain close contact between the interfaces during temperature cycling.

[0038] In summary, this technical solution is based on the fabrication of fiber optic temperature sensors using materials with negative thermal expansion coefficients. Unlike materials with positive thermal expansion coefficients, materials with negative thermal expansion coefficients exhibit a reverse sensitivity enhancement mechanism at low temperatures. When the temperature rises, the negative thermal expansion material contracts, generating axial tensile force: F NTE =E NTE ·A·α NTE ·ΔT; where E is Young's modulus and A is the cross-sectional area. The wavelength shift Δλ of the grating Bragg is caused B / λ B =(α f +ξ)·ΔT+(1-pe )·ε NTE εNTE is positively correlated with ΔT, thus achieving a sensitivity jump; The design of a fiber optic temperature sensor based on materials with negative thermal expansion enables the fiber optic grating to exhibit ultra-large linear response in the low-temperature region: A cladding layer is formed using materials with a multiplier coefficient of negative thermal expansion at low temperatures (such as ZrW₂O₈, ScF₃, Ag₃[Co(CN)₆], etc.), which exhibits significantly enhanced negative expansion characteristics (α = -15.2 × 10⁻⁶) in the temperature range of 4 K to 77 K. 6 / K), which is 1.7 times larger than the value at room temperature. The material undergoes a dramatic axial expansion as the temperature decreases, applying a compressive strain Δε=-αnet·ΔT·L to the grating (L is the effective working length).

[0039] The document sensor fabricated using the fiber grating obtained by the above method can achieve a sensitivity of up to 50 pm / ℃ in a wide temperature range of 4K–350K, resist electromagnetic interference (magnetic drift <0.05 pm), and is suitable for extreme environments such as superconducting magnets and aerospace cryogenic systems.

[0040] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited thereto. Various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention.

Claims

1. A method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating, characterized in that, Specifically, the following steps are included: S1. Prepare the fiber core layer (101) and treat the outer peripheral surface of the fiber core layer (101) by sequentially passing it through Piranha solution and argon plasma bombardment. S2. Nanoscale zirconium hafnium oxide is coated on the outer periphery of the fiber core (101) to obtain a stress transfer layer (102). S3. Prepare particles with a size range of 0.2–0.8 μm and a specific surface area >35 m². 2 / g of low-temperature negative thermal expansion coefficient sensitive material powder is coated on the outer peripheral surface of the optical fiber stress transfer layer (102) by a layered coating method to obtain negative thermal expansion material sensitive coating layer (103). S4. A porous structure is constructed on the surface of the negative thermal expansion material sensitizing coating layer (103) to form a porous layer (104) on the surface of the negative thermal expansion material. S5. A microcrystalline glass-Invar laminated composite shell structure is encapsulated on the outside of the porous layer (104) on the surface of the negative thermal expansion material to obtain a metal cladding layer (105).

2. The method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 1, characterized in that, The coating thickness of the stress transfer layer (102) is 20–100 nanometers.

3. The method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 2, characterized in that, The stress transfer layer (102) was prepared using the sol-gel method, and the specific steps are as follows: Equimolar ratios of zirconium alkoxide and hafnium alkoxide were dissolved in anhydrous alcohol solvent, deionized water was added and acid was used to catalyze hydrolysis and polycondensation, and the mixture was aged to obtain a transparent and uniform sol. The pretreated fiber core (101) was then vertically immersed in the sol and pulled to form a liquid film on the surface of the fiber core (101); After room temperature gelation, the material is placed in a tube furnace and gradually heated to 400-600℃ in an air atmosphere for heat treatment, resulting in a ZrHfO4 coating on the surface of the fiber core (101).

4. A method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 1 or 3, characterized in that, The low-temperature negative thermal expansion coefficient sensitizing material powder is any one of ZrW2O8 powder, ScF3 powder, or Ag3[Co(CN)6] powder.

5. The method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 4, characterized in that, The low-temperature negative thermal expansion coefficient sensitizing material powder is selected from ZrW2O8 powder. The method for forming a negative thermal expansion material sensitizing coating layer (103) on the outer peripheral surface of the stress transfer layer (102) with ZrW2O8 powder is as follows: Prepare the bottom layer slurry, transition layer slurry, and functional layer slurry separately; wherein: The composition ratio of the bottom slurry is ZrW2O8@SiO2 = 60%:40%; The transition layer slurry composition ratio is ZrW2O8@Al2O3 = 75%: 25%; The functional layer slurry is pure ZrW2O8; Auxiliary solvents were added to the preparation of the bottom layer slurry, transition layer slurry and functional layer slurry respectively, and they were stirred respectively to ensure that the viscosity and conductivity of the preparation of the bottom layer slurry, transition layer slurry and functional layer slurry met the requirements of electrostatic jet deposition. First, the bottom layer slurry is sprayed, then the transition layer slurry is sprayed, and finally the functional layer slurry is sprayed. Each layer is sprayed only after the target thickness is reached. After each layer is deposited, a short-term pre-curing is performed to reduce interlayer miscibility and form a controlled diffusion transition zone. After deposition, a variable-temperature heat treatment is performed.

6. The method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 4, characterized in that, The method for constructing a porous structure on the surface of the negative thermal expansion material sensitizing coating (103) is as follows: Electrochemical etching is performed in an electrolyte containing hydrofluoric acid to directly dissolve the material and form pores. Then, annealing is performed in a nitrogen atmosphere at 480°C to obtain a porous layer (104) on the surface of the negative thermal expansion material with a porosity of not less than 21%.

7. The method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 4, characterized in that, The method for constructing a porous structure on the surface of the negative thermal expansion material sensitizing coating (103) is as follows: A polyamide film is covered as an auxiliary layer on the functional layer surface of the negative thermal expansion material sensitizing coating layer (103). A CO2 laser beam is used to scan and ablate the coating along a preset path to create a conical hole with a pore size of 13-16 μm and smooth boundaries, resulting in a porous layer (104) on the surface of the negative thermal expansion material with a porosity of 15%-22%.

8. The method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 1, characterized in that, The fabrication and encapsulation method of the microcrystalline glass-Invar laminated composite shell structure is as follows: Thin sheets of microcrystalline glass and Invar are alternately stacked and solid-state bonded at a set temperature by anodic bonding or low-melting-point glass solder to form a dense and sealed stacked composite shell, in which Invar is used as the structural layer and transition layer, and microcrystalline glass is used as the functional layer that comes into contact with the outside world. The integrated fiber core (101), stress transfer layer (102), negative thermal expansion material sensitizing coating layer (103), and negative thermal expansion material surface porous layer (104) are then placed in the center of the stacked composite shell and flexibly fixed at the ends with low modulus silicone.

9. The method for fabricating a low-temperature temperature-sensitive fiber grating based on a negative thermal expansion material coating according to claim 1, characterized in that, A bellows (106) is installed on the outside of the metal sheath layer (105).

10. A low-temperature temperature-sensitive fiber Bragg grating structure, fabricated using the method for preparing a low-temperature temperature-sensitive fiber Bragg grating based on a negative thermal expansion material as described in any one of claims 1-9, characterized in that, The low-temperature temperature-sensitive fiber grating structure includes an optical fiber core layer (101). A stress transfer layer 102 is coated on the surface of the fiber core (101) to buffer thermal stress and enhance interfacial bonding; A negative thermal expansion material sensitizing coating (103) is coated on the surface of the stress transfer layer (102) to generate thermal strain opposite to that of the optical fiber at low temperature to amplify the optical temperature response of the optical fiber. A porous layer (104) on the surface of a negative thermal expansion material sensitizing coating (103) is used to provide gas or liquid channels for rapid heat exchange and reduce transverse stress transmission. And a metal sheath layer (105) covering the surface of the porous layer (104) of the negative thermal expansion material.

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