What is the impact on the sensor at the critical point where the coating material softens or hardens?
1. Definition of Physical Concepts: Glass Transition Temperature (T_g)
The critical point at which a coating material softens or hardens is known as the Glass Transition Temperature, abbreviated as T_g in materials science.
- Below T_g: The polymer material is in a Glassy State, where molecular chain segments are “frozen.” The material exhibits high hardness, a high elastic modulus, and good rigidity.
- Above T_g: The polymer material enters the Rubbery State (also known as the elastic state), where molecular chain segments begin to move. The material rapidly softens, its elastic and shear moduli decrease sharply, and it exhibits high plasticity and flexibility macroscopically.
2. Impact of Glass Transition on Fiber Optic Sensors (Especially FBG Sensors)
This critical temperature point decisively impacts the measurement accuracy, stability, and physical lifespan of fiber optic sensors, primarily in the following four aspects:
(1) Cliff-like Drop in Strain Transfer Efficiency
For Fiber Bragg Grating (FBG) strain sensors, the coating is the crucial medium for transferring strain from the external structure to the quartz fiber core.
- Below T_g: The rigid coating provides extremely efficient and stable tangential force transfer, ensuring that the fiber and the measured structure deform together.
- Once the temperature crosses T_g upwards: The coating material rapidly softens, and its Young’s modulus and shear modulus decrease by several orders of magnitude. At this point, severe shear creep and viscoelastic slip occur within the coating, preventing external strain from being effectively and linearly transferred to the fiber core. The sensor’s strain measurement sensitivity significantly decreases, leading to substantial measurement errors.
(2) Nonlinear Temperature Response Caused by Abrupt Change in Coefficient of Thermal Expansion (CTE)
When polymer materials undergo glass transition, their Coefficient of Thermal Expansion (CTE) undergoes an abrupt change. Typically, the CTE in the rubbery state is several times higher than in the glassy state.
- When FBG sensors measure temperature, the thermal expansion of the coating itself applies axial stretching to the fiber through the thermoelastic effect, contributing to a portion of the temperature-induced wavelength shift.
- As the temperature sweeps across T_g, the discontinuous change in CTE causes a sudden “inflection point” or “step” in the sensitivity of the FBG reflection wavelength’s change with temperature (temperature sensitivity). This severely disrupts the linearity of the temperature calibration curve (usually a stable linear or quadratic polynomial relationship).
(3) Mechanical Protection Failure and Microbending Loss
- At extremely low temperatures (far below T_g): Some coating materials (like ordinary polyacrylates), due to intense cold contraction and hardening, exert uneven axial or radial compressive forces on the fiber, leading to microbending losses within the fiber and increased optical signal attenuation.
- At high temperatures (above T_g): The softened coating loses its ability to resist lateral mechanical pressure. Any slight external force, such as squeezing, bending, or shearing, can directly act on the fiber’s glass cladding, not only causing severe microbending losses but also making the fiber core highly susceptible to fatigue fracture under stress.
(4) Creep and Hysteresis
In the rubbery state above T_g, materials exhibit strong viscoelastic behavior. When the sensor is subjected to cyclic loads or long-term static strain, the material undergoes slow stress relaxation and creep. This results in significant hysteresis in the sensor’s output (i.e., the wavelength is inconsistent for the same physical quantity during loading and unloading), making it difficult for the sensor to perform high-precision dynamic monitoring.
3. Engineering Solutions and Related OFSCN® Technical Products
To overcome or mitigate the negative impacts of the glass transition temperature of polymer coatings, high-level optical engineering applications require the selection of appropriate coating media based on the operating temperature range:
Solution A: For Conventional Temperature Ranges
In the conventional temperature range (-40\ {}^\circ\text{C} to 100\ {}^\circ\text{C}), polyacrylate is typically used for coating:
- OFSCN® Polyacrylate Fiber Bragg Gratings / FBG Strings (Bare): The re-coating material is standard polyacrylate, which can maintain stable basic protection and sensing performance within this temperature range. However, once the operating temperature approaches or exceeds 100\ {}^\circ\text{C}, the coating will completely soften and begin thermal degradation.
Solution B: For Wide Temperature Ranges and High Stability (Ultra-High T_g)
If the measurement environment involves high temperatures or requires extremely high strain transfer accuracy across a wide temperature range, the coating needs to be replaced with Polyimide. Polyimide has a very high glass transition temperature (typically above 300\ {}^\circ\text{C}, or even carbonizes without a distinct classical T_g phase), thus maintaining high hardness and stable mechanical properties over an extremely wide temperature range:
- OFSCN® Polyimide Fiber Bragg Gratings / FBG Strings (Bare): The re-coating material is polyimide, with an operating temperature range of -200\ {}^\circ\text{C} to 300\ {}^\circ\text{C}, offering excellent creep resistance and extremely high strain transfer efficiency.
Solution C: Complete Elimination of Glass Transition Effects (Inorganic Metal Coatings)
In extreme environments such as ultra-high temperatures (e.g., 700\ {}^\circ\text{C}) or ultra-high vacuum, any organic polymer coating will thermally decompose. In such cases, inorganic metals must be used as coating layers to completely eliminate the glass transition issues associated with polymers:
- OFSCN® Gold-coated Optical Fiber: Utilizes metallic gold as the coating material, with an operating temperature range of -270\ {}^\circ\text{C} to 700\ {}^\circ\text{C}. Metallic coatings do not possess the T_g characteristics of polymers and exhibit continuous and extremely stable mechanical and physical properties across a very wide temperature range.