Figure 1 shows the structure of the proposed temperature sensor, which consists of a paper substrate, PVB-MCN sensing layer, and Ag electrode. The flexible film can be printed on the fiber paper[11] substrate by the screen printing, and then the wires are printed on the film to prepare a flexible temperature sensor. It can be seen that the flexible temperature sensor is simple in structure and convenient to manufacture, which is suitable for mass production.
3.1 Structural analysis
Figure 2(a) shows the phase structure of the flexible films at different contents. It can be seen that the pure phase ceramic powder has a distinct spinel structure. With the decrease of solid content, the intensities of all diffraction peaks tend to weaken, which indicates the decrease of the inorganic phase.
Figure 2(b) shows the FTIR spectra of the flexible film at different contents. It can be seen that the absorption peak of the organic functional group appears at different positions, where characteristic peak at 3458 cm−1 is attributed to the functional group from the silane coupling agent and hydroxyl groups on the surface of the ceramic particles. The absorption peak at 3390 cm−1 corresponds to the NH groups from the silane coupling agent while the CH2 groups at positions 2960, 2930, and 2850 cm−1 are functional groups from the carbon chain in silane coupling agent and PVB. The peaks at 1557 and 1106 cm−1 are attributed to the bonds between the silane coupling agents. The absorption peak at 1641cm−1 corresponds to the carbon-oxygen bond formed during the dehydration condensation reaction between PVB and the ceramic particle. And the carbon-nitrogen double bond at 992 cm−1 corresponds to the chemical bond formed between silane coupling agent and PVB, which confirms that the silane coupling agent indeed played the role of adhesion. The absorption bands between 2000 and 2230 cm−1 correspond to the diamond substrate in the instrument. All the vibration peaks gradually decrease with the increase of the amount of the ceramic particles, which proves the decrease of the organic phase.
Figure 2(c) shows the reaction mechanism between different components. Ceramic particles have a strong tendency to aggregate because of the high surface energy [12] and the aggregation of ceramic particles will dramatically reduce the mechanical properties of flexible films. Therefore, it is necessary to do chemical modification of the ceramic for improving the dispersion performance. The silane coupling agents with polar groups at one end of the molecule can react with the hydroxyl groups of the ceramic particles while the groups at the other end can crosslink with organic polymers. γ-aminopropyltriethoxysilane (KH550) is often used in the surface modification of inorganic particles [13]. The silanol groups at one end of the silane coupling agent undergo a hydrolysis polycondensation reaction with the hydroxyl groups on the surface of the ceramic particles, forming a carbon-oxygen bond. While, the organic group at the other end of the silane coupling agent reacts with the organic functional group of the PVB, forming a carbon-nitrogen double bond. The bond bridge which formed between ceramic particles and PVB by adding silane coupling agent can help the coupling process between them.
3.2 Microstructure analysis
Figures 3a) b) c) d) shows the microstructure of flexible films. It can be seen that the size of the ceramic particles is about 4 to 5 μm with a narrow distribution. As the solid content increases, the density of the ceramic particles increases significantly. Fig. 3e) shows the element distribution of the flexible films. It can be seen that all the elements are distributed uniformly and the ceramic particles are homogenously mixed with the PVB without visible agglomeration, which proves the high uniformity of the flexible film. Although small portions of the silane coupling agents were used as the surfactant, the distribution of the silicon element is uniform, indicating that the silane coupling agent is distributed on the surface of the ceramic particles and the PVB molecule. Fig. 3f) is the microstructure model of the flexible film. This further approves that it can effectively improve the bonding strength between ceramic particles and PVB by adding the silane coupling agent.
3.3 Electrical performance and application
Figure 4a) b) c) d) show the relationship between resistivity and temperature of flexible films. It can be seen that as the temperature increases, the resistivity shows a downward trend, and all of the flexible films exhibit a negative temperature coefficient (NTC). MCN powder is a semiconductor material with a spinel structure, which is very sensitive to temperature. When the external temperature rises, the carrier transport efficiency in the semiconductor is much improved, thus the material resistance decreases rapidly. The sensitivity of the flexible film can be calculated by the formula (1).
TCR = (R − R 0 )/(R 0 ·∆T) (1)
It can be seen from Table 2 that the flexible film has a high-temperature coefficient of resistance (greater than 3.1% C-1) in the temperature range of the human body. Furthermore, the flexible film still exceeds a temperature coefficient of resistance of 1.4%° C at the wide temperature range from 25 to 80 degrees Celsius. At the same time, there is no significant hysteresis during cooling and heating.
It can be seen from Fig. 4a) that in the flexible film with a solid content of 20%, the resistivity at room temperature is as high as 2.8 GΩ. With the increase of the inorganic phase, the resistivity gradually decreases to 56 MΩ. Figure 4e) is an Arnius diagram of the flexible films. It can be seen that there is a linear relationship between resistivity and temperature for all of the flexible films and the resistivity increases as the solid content increases. It was found that compared with the 20% solids flexible film, the 30% solids flexible film has lower resistivity and better temperature dependence, which is beneficial to convert temperature signals into electrical signals. At the same time, the mechanical properties of flexible films with a solid content of 30% are better than the flexible films with a solid content of 40% and 50%. Therefore, the flexible film with a solid content of 30% is considered to have the most practical value.
In order to further study the practicality of the flexible film with a solid content of 30%, five temperature cycling tests were performed in the human temperature range (30–40° C), and the average change rate was found to be less than 0.15%. Figure 4g) shows that the flexible film of 30% solids was aged for 1000 minutes at the temperature of 40 to 80° C. It was found that the flexible film has excellent temperature stability (0.1266% at 40° C, 0.1364% at 50° C, 0.1422% at 60° C, 0.1646% at 40° C). This is because the spinel structure only shows lattice relaxation at 200° C or higher, which is much lower than the human body temperature range. Figure 4h) shows the resistance change after bending for 1000 times at different angles. It is found that the resistance change of the flexible film during the folding process is negligible (0.2366% at 30 °, 0.2054% at 60 °, 0.1814% at 90 °, 0.1434% at 120 °).
Table 2 The TCR,B value and activation energy of flexible temperature sensor.
content
|
TCR25/45(%)
|
TCR25/75(%)
|
B25/75(K)
|
Ea(eV)
|
20%
|
3.153
|
1.811
|
4906
|
0.4233
|
30%
|
3.142
|
1.557
|
3129
|
0.2700
|
40%
|
3.136
|
1.645
|
3591
|
0.3099
|
50%
|
3.115
|
1.713
|
4034
|
0.3481
|