Development of micro dual temperature–heat flux sensing probe using Pt Thin film for transient heat measurements up to 1400 °C.

Abstract

Thin-film heat flux sensors are critical measurement tools in hypersonic wind tunnel aerodynamic thermal environment tests. However, they suffer from limitations such as poor erosion resistance, low operating temperature (<1200 °C), and large size, which affect wind tunnel testing efficiency. In this paper, a Pt thin-film temperature-heat flux sensor (TFT-HFS) is proposed. A 2 mm alumina micropillar was used as the substrate to enable the arrayed fabrication of the TFT-HFS. Meanwhile, an electrohydrodynamic jet printing technique was employed to deposit the protective layer and regulate the stress within the sensitive layer, effectively improving the stability and high-temperature performance of the sensor. The test results show that the TFT-HFS can operate above 1400 °C with a repeatability error less than 1% FS. Under static laser calibration, the measurement error below 1.7% FS and a repeatability error below 0.6% FS. It exhibits a good linear relationship, with a correlation coefficient of R² = 0.993. During the rapid heating test, the heat flux exceeded 3.5 MW/m², the peak temperature reached 1588.2 °C, and the temperature error less than 0.75% FS. Furthermore, the fabricated TFT-HFS demonstrates an ultrafast response time of 0.2 ms. The proposed TFT-HFS provides a feasible and effective approach for high-temperature transient, high spatial-resolution, and high-precision heat flux measurements.

Introduction

During high-speed flight, the surfaces of vehicles are subjected to severe aerodynamic thermal shocks. Accurate measurement of aerodynamic thermal parameters—such as temperature and heat flux on components like turbine blades of aero-engines, inner walls of combustion chambers, and surfaces of gas turbines has become a research hotspot^1–6. Conducting aerodynamic thermal environment tests in hypersonic wind tunnels is a key technological approach for verifying the structural reliability design and thermal protection schemes of thermal-end components of aircraft^7–9. Since these components are typically exposed to high-temperature (>1000 °C) and strong airflow erosion environments, conventional measurement techniques and sensing materials are prone to oxidation or failure under such extreme conditions, thereby affecting the accuracy of heat flux measurements^10–12. Therefore, the development of thin-film heat flux sensors with high temperature resistance, fast response, excellent stability, and high spatial resolution is of great significance.

Typical heat flux sensors used for aerodynamic thermal environment measurements in wind tunnels include thin-film Gardon gauges and thin-film thermopile heat flux sensors^13–21. The Gardon gauge, composed of constantan foil, is relatively large, with a radius typically exceeding 5 mm, which can disturb the surface flow field²². When operating at temperatures above 250 °C, the nonlinearity of the Gardon gauge increases, and its dynamic response time is on the order of several hundred milliseconds^23–25. Wang et al. ²⁶ designed a sandwich-structured heat flux sensor based on a Pt/Pt-13Rh thermopile, which can measure heat fluxes up to 110 kW/m², operate at temperatures as high as 1000 °C, and has a response time of 15.24 ms. Fu et al. ²⁷ developed a novel heat flux sensor based on a W-5Re/W-26Re thermopile, which can operate in an atmospheric environment at 1000 °C with a pulsed heat flux of 1000 kW/m². Wang et al. ²⁸ fabricated a multilayer thin-film heat flux sensor based on a Pt/Pt-13Rh thermopile on a high-temperature alloy substrate, capable of operating at 1000 °C with a heat flux density of 167.1 kW/m² and a response time of 10.18 ms. Lu et al. ²⁹ proposed a composite-protection-layer heat flux sensor based on an ITO/In₂O₃ thermopile; by depositing an Al₂O₃-YSZ protective layer on top of the sensitive layer, it can withstand operating temperatures up to 1200 °C, with a heat flux density of 286.619 kW/m² and a response time below 1.2 s. Although significant progress has been made in measuring heat flux using thermopiles, most studies still rely on precious metal thermopiles, with operating temperature limits not exceeding 1200 °C and response times mostly at the millisecond level. Further research is still required to improve their high-temperature tolerance and response performance. In addition to the aforementioned types of heat flux sensors, coaxial thermocouples and Pt-resistance heat flux sensors are also employed for transient heat flux measurements in extreme environments due to their small size, high natural frequency, fast response, and low measurement error^30–34. Common E-, K-, and J-type coaxial thermocouples typically have a maximum measurement temperature below 900 °C, with a heat flux measurement range less than 2 MW/m^2,35. Chen et al. ³⁶ proposed a novel G-type coaxial dual-parameter sensor, which was successfully tested under a 1250 °C supersonic flame environment, achieving a heat flux response time of 1.15 ms. Goswami et al. ³⁷ fabricated Pt thin-film resistors on quartz substrates. Calculations indicate the sensor response time is 15.377 ms, with a heat flux density reaching 77.5 kW/m² at an operating temperature of 330 K. However, the sensitive material was prepared by manually coating a slurry, resulting in poor thickness uniformity and difficulty in mass production.

In this paper, a TFT-HFS based on a Pt thin-film probe is proposed and successfully fabricated in an array form. Alumina ceramic is used as the substrate, and signal leads are routed through the interior of the pillars to enhance the reliability of signal transmission and high-temperature resistance of the sensor. An alumina protective layer is deposited on the surface of the Pt thin film using electrohydrodynamic jet printing technology, which improves the stability of the sensitive material under high-temperature conditions. The temperature performance of the sensor was tested, showing good stability in the range of 25–1400 °C. Static calibration of the TFT-HFS was carried out using a laser device, and the results demonstrated high measurement accuracy. In addition, dynamic calibration was performed using a rapid heating system, verifying the sensor’s fast response and large heat flux measurement capability. The proposed sensor meets the requirements for high-temperature, transient, and high-precision measurements of thermal components in hypersonic wind tunnels.

Design and simulation

The heat transfer of TFT-HFS follows the one-dimensional semi-infinite body heat conduction theory, in which voltage and heat flux are mutually converted, enabling the transformation of the voltage signal time history into the heat flux density time history^38–40. This relationship is illustrated schematically in Fig. 1a, and can be expressed by the Cook–Felderman equation:

$$q\left(t\right)=\frac{\sqrt{\rho C_{\mathit{Pk}}}}{2\sqrt{\pi }{\alpha }_R{E}_0}\left[\frac{2E\left(t\right)}{\sqrt{t}}+{\int }_0^t\frac{E\left(t\right)-E\left(\tau \right)}{{\left(t-\tau \right)}^{\frac{3}{2}}}d\tau \right]$$ 1

Fig. 1.

TFT-HFS Structure and Simulation. a Schematic diagram of the heat transfer principle of the TFT-HFS, (b) Schematic diagram of the TFT-HFS structure, (c) Overall temperature distribution, (d) Potential contour map, (e) Relationship between output voltage and thermal flux, (f) Thermal stress simulation model for TFT-HFS structures, (g) Thermal stress simulation results curve for TFT-HFS structure

In the equation, q(t) represents the heat flux at time t, E₀ is the initial voltage across the two ends of the TFT-HFS, and E(t) is the voltage at time t. $\sqrt{\rho C_{\mathit{Pk}}}$ represents the thermal physical parameter of the TFT-HFS. Based on the experimentally measured voltage–time data, the transient heat flux value at time t can be calculated.

The temperature measurement principle of the TFT-HFS is based on the linear relationship between the resistance of the metallic sensitive layer and temperature:

$$\Delta R={\alpha }_R\Delta T{R}_1$$ 2

In the equation, ΔR is the change in the resistance of the TFT-HFS, α_R is the temperature coefficient of resistance (TCR) of the TFT-HFS, ΔT is the temperature variation, and R₁ is the initial resistance of the TFT-HFS before the experiment.

When a constant current is applied across the two ends of the TFT-HFS, the resistance of the sensitive layer changes linearly with temperature, resulting in a corresponding change in the voltage across its two ends:

$$\Delta U=U_2-U_1=I\Delta R$$ 3

In the equation, ΔU represents the change in voltage of the TFT-HFS, U₁ is the voltage of the TFT-HFS before the temperature change, U₂ is the voltage of the TFT-HFS after the temperature change, and I is the applied constant current.

By measuring the change in voltage, the change in resistance can be determined, and thus the temperature variation of the measured flow field can be obtained:

$$\Delta T=\Delta U/I{\alpha }_R{R}_1=\Delta U/{\alpha }_R{U}_1$$ 4

The one-dimensional semi-infinite body heat conduction model requires that the thickness of the sensitive thin film be much smaller than that of the substrate. Under this condition, the heat absorbed by the sensitive thin film can be neglected, and the thin-film temperature can be considered equal to the surface temperature of the sensor. During the testing period, the substrate is regarded as a semi-infinite body. In addition, the temperature gradient along the sensor surface is much smaller than the temperature gradient perpendicular to the surface, so the heat transfer can be treated as one-dimensional conduction. The substrate material of the TFT-HFS determines its tolerance to extreme operating conditions, including its intrinsic high-temperature resistance, insulation performance, thermal conductivity, and machinability. Considering these factors comprehensively, Al₂O₃ was selected as the substrate material. Al₂O₃ can withstand temperatures up to 1600 °C, meeting the requirements for testing in high-temperature environments. Moreover, its coefficient of thermal expansion (CTE) is 7.8 × 10⁻⁶/°C, which is close to that of platinum (Pt), thereby minimizing the adverse effects caused by mismatched thermal expansion coefficients. Based on the one-dimensional semi-infinite body assumption, the minimum length of the sensor substrate must satisfy the following equation⁴¹:

$$x=4{\left(\mathit{kt}/\rho c\right)}^{0.5}$$ 5

In the equation, $x$ is the characteristic length required to satisfy the one-dimensional semi-infinite body assumption; $\rho$ is the material density; $c$ is the specific heat capacity; $k$ is the thermal conductivity; and $t$ is the heat-conduction time. A heat-transfer duration of 10 ms commonly used in hypersonic wind tunnels is adopted. Substituting the ceramic-substrate parameters given in Table 1 into Eq. (5) yields a minimum substrate length of 0.39 mm.

Table 1.

Materials of the TFT-HFS

Component	Parameter	Value
Substrate	Density	3950 kg/m3
	Specific heat	770 J/(kg·K)
	Heat conductivity coefficient	30 W/(m⋅K)
	Length	15 mm
	Diameter	2 mm
Pt sensitive layer	Thickness	1 μm
	Line width	150 μm
	CTE	8.8 × 10−6/°C

The structural schematic of the TFT-HFS is shown in Fig. 1b. From top to bottom, it consists of an alumina protective layer, a Pt sensitive layer, an alumina substrate, and Pt wire leads. The alumina protective layer effectively suppresses the sublimation and agglomeration of the Pt sensitive layer at high temperatures, thereby improving the stability of the sensor³⁸. The Pt sensitive layer is designed in an S-shaped pattern with a total length of 8.6 mm. Compared with a straight-line structure, the S-shaped design effectively increases the length of the temperature-sensitive element, thereby enhancing its sensitivity. Inside the alumina substrate, lead holes with a diameter of 0.3 mm are arranged at a spacing of 0.8 mm. The signal leads are routed through the interior of the substrate, which, compared with external routing methods such as silver paste coating or film deposition, results in a more compact overall structure. This internal routing method eliminates the need for additional external insulation protection during installation and helps reduce electromagnetic interference, thus improving signal transmission reliability.

To further verify the rationality of the designed structure of the TFT-HFS, COMSOL finite element simulation was employed to analyze the sensor model. The solid heat transfer field and electric current field were coupled to realize the thermoelectric conversion process. The initial temperature was set to 293.15 K, and a vertical heat flux of 10 kW/m² was applied to the upper surface of the sensor. The simulation results are shown in Fig. 1c. In Fig. 1c, the overall temperature distribution is presented. When heat flux is applied to the upper surface of the sensor, heat is conducted downward through the alumina protective layer, resulting in a higher surface temperature at the protective layer compared to the underlying coverage region. Figure 1d shows the current and electric potential distribution of the sensitive thin film. A parametric sweep of the input heat flux, ranging from 3 kW/m² to 30 kW/m², was carried out, and the output results are shown in Fig. 1e. The results indicate that the output voltage increases with the increase in heat flux, and the relationship exhibits an almost linear trend. Based on these simulation results, it can be concluded that the TFT-HFS structure is reasonable, as a given heat flux input produces a corresponding voltage output, confirming the feasibility of the design. Furthermore, thermal stress simulations were conducted on the Pt sensing layer at temperatures ranging from ambient to 1400 °C, with results presented in Fig. 1f, g. It can be observed that, in the absence of the protective layer, the Pt film is subjected to compressive stress arising from the mismatch in the coefficients of thermal expansion between the film and the substrate. In addition, when only the Pt film and the protective layer are considered, the Pt film still experiences compressive stress. Therefore, when the effects of both the substrate and the protective layer are taken into account, the Pt film remains consistently under compressive stress. Such compressive stress is believed to inhibit the formation of surface voids and agglomerated structures in the Pt film, thereby suppressing film degradation at elevated temperatures⁴².

Preparation and characterization

Preparation of TFT-HFS

The fabrication process of the TFT-HFS is illustrated in Fig. 2, and the detailed steps are as follows: (1) The alumina substrate is sequentially cleaned with acetone, anhydrous ethanol, and deionized water to remove organic contaminants and ensure a clean surface. (2) High-temperature Pt paste is evenly injected into the lead-through holes on the substrate, and Pt wires are inserted into the holes simultaneously. After filling, the substrate is placed in a muffle furnace for high-temperature sintering at 1000 °C for 1 hour to ensure complete sintering of the Pt paste. (3) The upper surface of the substrate after high-temperature sintering is ground and polished using a metallographic polishing machine (BRIGHT, UK) at a rotation speed of 1200 r/min, with alumina polishing solution as the polishing medium. The end-face roughness is kept uniform and controlled at 40 nm. (4) A metal mask is placed on the substrate surface to pattern the Pt sensitive layer, enabling array deposition and improving the consistency of the Pt sensitive layer. (5) The Pt sensitive layer is deposited using a magnetron sputtering process (JCP600, Techno, China) under a working pressure of 0.5 Pa and power of 200 W, utilizing DC sputtering. After sputtering, the sample is annealed in a muffle furnace at 600 °C for 30 minutes to enhance the stability of the Pt sensitive layer, completing its fabrication. (6) An Al₂O₃ protective layer is deposited on the Pt sensitive layer using electrohydrodynamic jet printing technology^43,44. The schematic diagram of the printing process for the Al₂O₃ protective layer is shown in Figure S1 of the Supporting Information. Al₂O₃ printing ink is filled, and the printing parameters are set for layer-by-layer deposition. Al₂O₃ printing ink consists of Al₂O₃ sol and Al₂O₃ mixture solution, as shown in Figure S2 of the supporting information. After each layer is printed, low-temperature sintering is performed to remove organic components in the film and prevent cracking. Once deposition is complete, the sensor is annealed in a muffle furnace at 1000 °C for 2 h to finalize the fabrication of the Al₂O₃ protective layer. The flowchart of the printing process for the Al₂O₃ protective layer is shown in Figure S3 of the Supporting Information. (7) Finally, the lead packaging of the TFT-HFS is completed using the four-wire connection method, finalizing the overall fabrication of the sensor.

Fig. 2. Process flow for fabrication of TFT-HFS.

It includes the lead packaging of the Al₂O₃ substrate, metal mask installation, sputtering of the Pt sensitive layer, high-temperature annealing, printing of the Al₂O₃ protective layer, high-temperature annealing, and packaging of the TFT-HFS

Characterization of TFT-HFS

The S-shaped Pt sensitive thin film was characterized and analyzed using several microscopic techniques. The S-shaped sensitive pattern was observed using a tool microscope (Olympus, STM7), as shown in Fig. 3a. The surface morphology of the thin film was examined using a laser confocal microscope (ZEISS, LSM980), as shown in Fig. 3b. The results indicate that the S-shaped sensitive pattern is intact with clear boundaries, and the thin film shows no warping or peeling. Figure 3c shows the cross-sectional profile curve of the line width of the S-shaped sensitive pattern. The cross-sectional thickness of the S-shaped pattern structure is about 1 μm, and the cross-sectional line width is 150 μm, which is consistent with the design dimensions. The surface morphology of both the Pt sensitive layer and the Al₂O₃ protective layer was further analyzed using a scanning electron microscope (SEM, SU8220, Japan), as shown in Fig. 3d, e. The observation results indicate that the surface of the Pt sensitive layer is dense, pore-free, and has uniform grain size. The surface of the Al₂O₃ protective layer exhibits a microcrack-like microstructure, which is due to the further crystallization of Al₂O₃ into α-Al₂O₃ after annealing at 1000 °C for 2 h. The α-Al₂O₃ structure enhances the sintering resistance and thermal stability of the protective layer⁴⁵.

Fig. 3.

Surface morphology characterization of the TFT-HFS. a S-shaped Pt sensitive film morphology, (b) Three-dimensional morphology of the S-shaped Pt sensitive film, (c) Morphology curve of the Pt sensitive layer, (d) Microscopic morphology of the Pt sensitive layer, (e) Microscopic morphology of the Al₂O₃ protective layer, (f~i) The surface SEM images of the unprotected Pt films after annealing at 800 °C, 1000 °C, 1200 °C, and 1400 °C. j~m The surface SEM images of the Pt films with printed protective layer after annealing at 800 °C, 1000 °C, 1200 °C, and 1400 °C

Figure 3f~m show SEM images of the unprotected Pt sensing layer and the Pt sensing layer with a protective layer after annealing at 800 °C, 1000 °C, 1200 °C, and 1400 °C for 30 minutes, respectively. It can be observed that, after annealing at 1000 °C, the surface of the unprotected Pt sensing layer exhibits numerous voids of varying sizes. The agglomeration of the Pt film is driven by surface diffusion along the free surface of the film⁴⁶. The occurrence of such agglomeration leads to a reduction in the stability of the Pt sensing layer and may even result in its failure⁴³. In contrast, the Pt sensing layer with a protective layer does not show any agglomeration on the surface after annealing at 1000 °C. Agglomeration of the Pt film with the printed protective layer does not occur until 1400 °C. Therefore, the deposition of an Al₂O₃ protective layer effectively suppresses the agglomeration of the Pt film and enhances the high-temperature stability of the Pt sensing layer.

The cross-sectional structure of the TFT-HFS was also examined. As shown in Fig. 4a, the sensor consists of multiple layers, from top to bottom: Al₂O₃ protective layer, Pt sensitive layer, and Al₂O₃ ceramic substrate. The image demonstrates that the interfaces between the layers exhibit excellent adhesion and that the overall structure is dense and compact. Through EDS analysis of the cross-sectional interface (Fig. 4b), it was observed that, in addition to Pt, Al, and O, elements such as Si, Mg, and C were also present. This is due to the presence of Si, Mg, and C in the Al₂O₃ substrate and the alumina solution. Based on these observations, it is inferred that, after high-temperature annealing at 1000 °C, the Al₂O₃ protective layer contains not only α-Al₂O₃ crystals but also amorphous phases that include Si, Mg, and C elements, which are introduced from the sub-strate and precursor solution. The crystal phases of the Pt sensitive layer and the Al₂O₃ protective layer were analyzed using an X-ray diffractometer (XRD, SmartLab 9, Japan). As shown in Fig. 4c, the Al₂O₃ protective layer is amorphous before annealing, whereas it transforms into stable α-Al₂O₃ after annealing. The crystal orientations of the Al₂O₃ film are identified as (012), (104), (110), (113), (024), (116), (018), (214), and (300). The peak positions are consistent with the α-Al₂O₃ phase reported in the International Centre for Diffraction Data (ICDD) PDF card No. 83-2080⁴⁷. This demonstrates that after annealing at 1000 °C, the crystalline phase of the alumina film transforms into α-Al₂O₃, a crystal structure that is crucial for maintaining the stability of the film in high-temperature environments. As shown in Fig. 4d, the Pt crystals exhibit a preferred orientation along the (111) plane. As shown in Fig. 4e, after annealing at 1000 °C, the (111) peak of both the unprotected Pt sensing layer and the Pt sensing layer with a protective layer shifts to higher angles and becomes narrower, indicating that the Pt film is under compressive stress and that grain growth has occurred. The CTE of Pt (8.8 × 10⁻⁶/°C) is larger than that of Al₂O₃ (7.2 × 10⁻⁶/°C). At high temperatures, the Pt film tends to expand relative to the substrate and the protective layer, resulting in compressive stress within the Pt film. The presence of compressive stress can suppress surface mass transport and pore formation in the film, thereby enhancing the stability of the Pt film^42,48.

Fig. 4.

Characterization of the thin film thermal flow sensor. a Cross-sectional morphology of the TFT-HFS, (b) EDX curve, (c) XRD patterns of the Al₂O₃ protective layer before and after annealing, (d) XRD curve of the Pt sensitive layer, (e) XRD curve of the Pt sensitive layer after annealing at 1000 °C

Results and discussion

Test of high temperature performance of TFT-HFS

To investigate the temperature characteristics of the Pt thin-film heat flux sensor, the experimental test was conducted using the setup shown in Fig. 5a. The TFT-HFS was placed inside a muffle furnace, with the temperature set in the range from room temperature to 1000 °C, and a heating rate of 10 °C/min. The four-wire method was used for the connection. The constant current source (IT2805, ITECH, China) was connected to two leads of the sensor using Pt wires to provide a constant current of 1 mA. The digital multimeter (DMM6500, KEITHLEY, China) was connected to the other two leads of the sensor to measure the output voltage. Finally, the TCR of the Pt TFT-HFS was calculated to be 1964.1 ppm/°C according to Eq. (1). The resistance-temperature curve is shown in Fig. 5b. Linear fitting indicates that the correlation coefficient R² = 0.999, demonstrating a good linear correlation between resistance and temperature. The maximum temperature limit of the TFT-HFS was tested, and the results are shown in Fig. 5c. The maximum withstand temperature of the sensor without a protective layer reached only 960 °C, while the sensor with a printed alumina protective layer achieved a maximum withstand temperature of 1440 °C, representing a 50% improvement. This indicates that the protective layer significantly enhances the temperature resistance of the TFT-HFS. The repeatability of the sensor significantly affects its measurement accuracy and reliability. Therefore, repeatability tests were conducted on the temperature characteristics of the TFT-HFS. The muffle furnace was set to heat from room temperature to 1300 °C at a heating rate of 10 °C/min, with three repeated heating cycles under identical conditions. The test results are shown in Fig. 5d, e. The three cycles exhibited consistent trends, with a repeatability error of less than 1%, indicating that the Pt TFT-HFS has good repeatability. Figure 5f presents the thermophysical parameter curve of the thin-film heat flux sensor, and its output satisfies the following equation:

$${\left(\sqrt{\rho C_{p}k}\right)}_2={\left(\sqrt{\rho C_{p}k}\right)}_1\frac{T_{10}-T_c}{T_c-T_{20}}$$ 6

Fig. 5.

High-temperature performance test of the TFT-HFS. a Schematic diagram of the high-temperature testing device, (b) Resistance-temperature test curve from room temperature to 1000 °C, (c) Resistance-temperature test curves of TFT-HFS with and without protective layer, (d) Cycling test curve from room temperature to 1300 °C, (e) Cycling error curve, (f) Calibration curve of thermal physical parameters

In the formula, T₁₀ represents the initial temperature of water, T₂₀ is the temperature of the TFT-HFS at room temperature, and Tc is the temperature of the TFT-HFS after being immersed in water. ${\left(\sqrt{\rho C_{p}k}\right)}_1$ represents the thermal properties parameters of water, which is 1580 J/m²·k·s^1/2.

The calibration of thermophysical parameters is shown in Figure S4. During the test, water was heated to a specified temperature T₁₀(60 °C), and the sensor to be tested was positioned at a certain distance directly above the container. The TFT-HFS was then quickly immersed into the water for a short period, during which an instantaneous heat transfer occurred, causing a step change in resistance. Subsequently, the sensitive structure completed heat exchange with the water and reached a steady-state equilibrium, at which point the temperature change of the sensor was recorded. The calibration result of the thermophysical parameter ${\left(\sqrt{\rho C_{p}k}\right)}_2$ of the TFT-HFS is 8041.2 J/m²·k·s^1/2. Since the TFT-HFS is based on the one-dimensional semi-infinite body assumption, the response time is defined as 63% of the time required to reach steady state from room temperature²⁴. As shown in the Fig. 5f, the time to reach 63% is 0.2 ms. Therefore, this TFT-HFS can be used for transient heat flux monitoring in hypersonic wind tunnels.

Static and dynamic performance testing of the TFT-HFS

To verify the heat flux response characteristics of the TFT-HFS, static and dynamic testing platforms were constructed separately, as shown in Fig. 6a, b. A laser (VCL-980 nmM1-30 W, Blueprint, Beijing) was used for the static calibration of the sensor, with a power range of 0–30 W.During the test, the TFT-HFS was mounted on a fixture and connected to a source meter and a digital multimeter using the four-wire method. The laser served as the heat source, heating the surface of the sensor, and the power was increased in 3 W increments. The source meter provided a constant current of 1 mA, while the digital multimeter recorded the output voltage of the TFT-HFS. The collected output data were then substituted into Eq. (1) to calculate the heat flux value. To compare measurement accuracy, a commercial standard sensor (HT50-20, ITI) was simultaneously mounted at the same position on the fixture and subjected to identical heating power, ensuring that both sensors were exposed to the same heat flux environment. The output values from the commercial standard sensor were collected and compared with the measurements from the TFT-HFS to complete the accuracy error calibration.

Fig. 6.

TFT-HFS test setup. a Static calibration device, (b) Dynamic calibration device

To verify the dynamic heat flux response of the TFT-HFS, a rapid heating test platform was independently built. This setup operates based on the principle of resistive heating, where a constant current (0–500 A) is applied to the heat-conducting material to achieve dynamic cyclic heating under a high heating rate. During the test, the TFT-HFS was attached to the heat-conducting plate using ceramic adhesive. The input current was then adjusted to heat the plate. Based on the one-dimensional heat conduction principle, the surface of the TFT-HFS experiences an instantaneous temperature change. At this moment, the source meter provides a constant current of 1 mA, and the digital multimeter records the instantaneous output voltage. The collected output data are then substituted into Eq. (3) to calculate the heat flux value. Meanwhile, an Impac IGAR 12-LO infrared thermometer (Germany) was used to monitor the surface temperature of the sensor in real time.

The resistance variation of the TFT-HFS under different power loads is shown in Fig. 7a, with each loading duration exceeding 10 min. It can be observed that as the loading power increases, the output of the TFT-HFS shows a gradual upward trend and remains stable. The commercial standard heat flux sensor was calibrated, and its heat flux density outputs in the power range of 0–30 W are shown in Fig. 7b, c. The output of the commercial heat flux sensor HT50-20 increases linearly with the increase in loading power, exhibiting a strong linear correlation with R² = 0.997. When the power reaches 30 W, the maximum heat flux density is 70.18 kW/m². The relationship curve between heat flux density and output resistance is shown in Fig. 7d. It can be seen that the output resistance of the TFT-HFS varies linearly with the heat flux density, with a linear correlation of R² = 0.993. The heat flux densities of the TFT-HFS calculated under different power levels were compared with those of the commercial heat flux sensor, as shown in Fig. 7e. It can be observed that both exhibit consistent growth trends. When the power is 3 W, the heat flux densities are 3.92 kW/m² and 3.96 kW/m², respectively. When the power reaches 30 W, the maximum heat flux density of the TFT-HFS is 71.06 kW/m², with a maximum measurement error of less than 1.7% FS. In addition, the TFT-HFS was repeatedly loaded with a power of 30 W for three cycles, resulting in an average output heat flux density of 70.03 kW/m² and a repeatability error of less than 0.6% FS, as shown in Fig. 7f. Based on these results, it is demonstrated that the TFT-HFS exhibits excellent measurement accuracy and repeatability, meeting the requirements for precise heat flux monitoring of wind tunnel models.

Fig. 7.

Static calibration of the TFT-HFS. a Output resistance of the TFT-HFS under different power levels, (b) Output of the HT50-20 heat flux sensor, (c) Heat flux collected by the HT50-20 under different power levels, (d) Output resistance of the TFT-HFS under different heat flux densities, (e) Comparative calibration curves, (f) Repeatability test of the TFT-HFS

The TFT-HFS was subjected to rapid heating, with input currents set to 150 A and 200 A. The test video available in supporting materials (Rapid Heating Test. MP4). The output results are shown in Fig. 8a. As the input current increased, the output of the TFT-HFS also increased gradually. The output data in Fig. 8a were substituted into Eq. (2) to calculate the temperature acquired by the sensor, and the results are shown in Fig. 8b. The maximum recorded temperatures were 1370.4 °C and 1588.2 °C, respectively. Subsequently, the data from Fig. 8b were substituted into Eq. (1) to calculate the heat flux density, as shown in Fig. 8c. It can be observed that as the input current increases, the measured heat flux density of the TFT-HFS also increases. The maximum heat flux densities reached 3346.3 kW/m² and 3577.5 kW/m², respectively. As illustrated in Fig. 8d, when the input currents were 150 A and 200 A, the temperatures recorded by the infrared thermometer were compared with those measured by the TFT-HFS, showing a full-scale maximum error of less than 0.75% FS. In addition, repeatability tests were carried out on the TFT-HFS under a loading current of 150 A. The test results are shown in Fig. 8e and f. The maximum heat flux densities of the TFT-HFS are approximately 3346.3 kW/m², 3366.7 kW/m², and 3369.3 kW/m², and the maximum deviation of the heat flux density is less than 0.68%. The maximum testing temperatures of the TFT-HFS are 1370.4 °C, 1370.5 °C, and 1372.4 °C, with a maximum deviation of less than 0.15%. Therefore, it can be indicated that the TFT-HFS exhibits good repeatability and stability.

Fig. 8.

Dynamic testing of the TFT-HFS. a–c Output resistance, output temperature and heat flux density of TFT-HFS under 150 A and 200 A current loads, (d) Temperature error measured for TFT-HFS under 150-amp and 200-amp current loads, (e, f) Output heat flux density and temperature of the TFT-HFS during repeated testing under a 150-ampere current load

Table 2 presents a detailed comparison of the performance parameters of our Pt TFT-HFS with those of similar sensors reported in the literature. Although extensive research has been conducted on heat flux sensors with significant breakthroughs achieved, most of these sensors operate at temperatures below 1200 °C. Thin-film Gordon gauges require external water-cooling devices when used in high-temperature environments. Thin-film thermopiles typically utilize noble metal materials, involve complex fabrication processes, and exhibit millisecond-level response times. Existing Pt thin-film heat flux sensors exhibit poor erosion resistance, limiting their applicability in extreme environments. Therefore, the Pt TFT-HFS developed in this study successfully overcomes these limitations, enabling stable and repeatable operation over a wide temperature range from 25 °C to 1400 °C, and achieving rapid and accurate heat flux measurements with performance superior to that of commercial gSKIN® heat flux sensors.

Table 2.

Performance comparison of developed sensors with those reported in the literature

Type	Size	Temperature	Heat flux	Response time	Ref
Thin-film Gordon gauge	>5 mm	/	/	>100 ms	23
Pt/Pt-13Rh thin film thermopile	10 mm	1000 °C	110 kW/ m2	15.24 ms	26
ITO/In2O3 thin film thermopile	48 mm	1200 °C	276.659 kW/m2	1.2 s	29
Pt thin film	6 mm	330 k	77.5 kW/ m2	15.377	37
gSKIN®	10 mm	150 °C	150 kW/ m2	0.7 s	49
Pt thin film	2 mm	>1400 °C	>3.5 MW/ m2	<0.2 ms	This study

Conclusion

In this study, a TFT-HFS based on a Pt thin-film was developed. Compared with the currently reported Pt thin-film heat flux probes, this work achieved array fabrication of the Pt thin-film probe. Alumina ceramic was used as the substrate, and the lead wires were routed through the inner columns, effectively improving the signal transmission reliability and high-temperature resistance of the sensor. An alumina protective layer was deposited on the surface of the Pt thin film using electrohydrodynamic jet printing technology. Characterization results showed that the protective layer was dense and stable with an α-Al₂O₃ structure, which effectively suppressed Pt thin-film agglomeration and enhanced the stability of the sensitive material under high-temperature conditions. High-temperature performance tests demonstrated that the Pt thin-film probe can achieve accurate temperature measurements up to 1440 °C, exceeding the highest temperature detection capability of Pt thin films reported in the literature and showing a 50% increase compared with unprotected Pt thin-film probes. In addition, the Pt thin-film probe exhibited excellent repeatability and stability, with three heating cycles tested between 25–1300 °C, resulting in a repeatability error of less than 1%. Thermophysical parameter calibration indicated that the response time of the Pt thin-film probe was less than 0.2 ms. Static and dynamic calibration platforms were constructed for comprehensive performance evaluation. Static testing results showed that when the applied power reached 30 W, the maximum heat flux density of the thin-film heat flux sensor was 71.06 kW/m², with a maximum measurement error of less than 1.7% FS compared with a commercial heat flux sensor. When repeatedly loaded at 30 W for three cycles, the repeatability error was less than 0.6% FS. Dynamic testing results revealed that under a 200 A current load, the maximum measured temperature reached 1588.2 °C, and the maximum heat flux density reached 3577.5 kW/m². When compared with the temperatures recorded by an infrared thermometer, the full-scale maximum error was less than 0.75% FS. These results demonstrate that the Pt thin-film probe developed in this study possesses excellent potential for application in monitoring surface temperature and heat flux in hypersonic wind tunnel models, offering a reliable solution for extreme thermal environments.

Conflict of interest

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41378-026-01274-5.