Flexible screen-printed temperature sensor based on Mn-Co-Ni metal oxide powder lled PVB polymer

There has recently been renewed interest in wearable devices and electronic skin because of the demand in real-time monitoring of human body temperature. This work developed a exible paper-based temperature sensor by screen printing technology. The sensing layer is composed of Mn-Co-Ni metal oxide powders lled with Polyvinyl butyral (PVB). The exible temperature sensor shows extremely high sensitivity (3.14%° C − 1 ) at human body temperature (25 to 45° C). It also exhibits excellent durability (less than 0.25%) during the long-term aging tests, which indicates that the exible temperature sensor has great potential in wearable devices and electronic skin.


Introduction
The past decade has been the rapid development of Internet of Things (IoT) technology. It is expected that billions of devices will be integrated into the Internet [1] and the sensors are the vital element in Computer-Human Interaction (CHI) technology [2]. As an essential physiological parameter, the temperature is one of the critical indicators of health system evaluation. So real-time monitoring of human body temperature is of great signi cance in the eld of health care. Most of the traditional temperature sensors, such as the mercury thermometer and infrared thermometer, are di cult to perform effective long-term detection of the human body. Therefore, exible temperature sensors with ultra-thin thickness, low modulus, light weight, high exibility and stretchability have attracted widespread attention [3]. Jin Jeon reported a exible wireless temperature sensor based on the nickel-lled binary polymer PEO/PE composite, with a sensitivity of 0.3V/° C at the temperature of 35 to 42° C [4]. Qingxia Liu developed a high-performance exible temperature sensor consisting of polyethyleneimine/reduced graphene oxide bilayer and the sensitivity in the temperature range of 25 to 45° C is 1.30%° C − 1 [5]. Jin Pan reported a exible temperature sensor array with polyaniline/graphene-polyvinyl butyral lm, which shows a sensitivity about 1.20%° C − 1 at the temperature range of 25 to 80° C [6]. Guanyu Liu reported a exible temperature sensor based on graphene oxide and the sensitivity of the sensor is 0.6435%° C − 1 over the temperature range of 30 to 100° C [7]. Wen-Pin Shih reported a graphite-based exible temperature sensor array of polydimethylsiloxane composites. The sensitivity is 0.042 K − 1 and 0.286 K − 1 when the volume fraction of graphite is 25% and 15%, respectively [8]. Most of the exible temperature sensors use graphene or metal powder as sensor materials. These materials are expensive and the sensitivity coe cient is no more than 1.3%° C − 1 , which is challenging to satisfy the demand of human real-time temperature monitoring.
In this paper, a exible temperature sensor with high sensitivity and low-cost was developed on the paper substrate through a screen-printing method. Polyvinyl butyral (PVB), Mn-Co-Ni metal oxide powder, and silane coupling agent (KH550) [9] were used as the adhesive, temperature sensing material, and surfactant respectively. Mn-Co-Ni metal oxide is a kind of negative temperature coe cient thermistor with a spinel structure, which has the characteristics of high sensitivity, excellent reliability, and low cost. In addition, the temperature coe cient of resistance (TCR) is usually ten times higher than that of other temperature sensors [10]. The composition of the exible temperature sensor was characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The morphology was obtained by scanning electron microscope (SEM). The most attractive performance of the sensor is the high sensitivity of 3.14%° C − 1 between 25 to 45° C, which is much higher than that of the graphene composites temperature sensor (1.3%° C − 1 ). Further durability tests in long-terms temperature and mechanical aging prove that the exible temperature sensor has great potential in the health care eld.  Table 1. Then, mixing them in a defoaming mixer (THINKY ARE-310, JAPAN) at a rate of 2000 r/min for 10 minutes to obtain a black ceramic slurry. The exible lm was printed by the screen printer (AT-25PA Dongyuan, China). The ethanol organic reagent was volatilized by drying at 50° C for 2 hours. Ag slurry was plated on both ends of the exible lm to make the electrode.

Characterization of the exible temperature sensor
The structure of the inorganic phase was tested by the X-ray diffraction (D8 ADVANCE Bruker, Germany).
The functional groups of the organic phase were tested by the Fourier transform infrared spectroscopy (WGH-30/30ABruker, Germany). The surface structure and element distribution of the exible lm were observed by scanning electron microscope (SUPRA 55VP Zeiss, Germany). The conductivity of the lm at different temperatures was measured on the probe station (SM-4, SEMISHARE ELECTRONIC CO., LTD).  Figure 1 shows the structure of the proposed temperature sensor, which consists of a paper substrate, PVB-MCN sensing layer, and Ag electrode. The exible lm can be printed on the ber paper [11] substrate by the screen printing, and then the wires are printed on the lm to prepare a exible temperature sensor.
It can be seen that the exible temperature sensor is simple in structure and convenient to manufacture, which is suitable for mass production.  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 exible lms. Therefore, it is necessary to do chemical modi cation 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. γaminopropyltriethoxysilane (KH550) is often used in the surface modi cation of inorganic particles [13].

Structural analysis
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.

Microstructure analysis Figures 3a) b) c) d) shows the microstructure of exible lms. 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 signi cantly. Fig. 3e) shows the element distribution of the exible lms. 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 exible lm. 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 exible lm. This further approves that it can effectively improve the bonding strength between ceramic particles and PVB by adding the silane coupling agent. Figure 4a) b) c) d) show the relationship between resistivity and temperature of exible lms. It can be seen that as the temperature increases, the resistivity shows a downward trend, and all of the exible lms exhibit a negative temperature coe cient (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 e ciency in the semiconductor is much improved, thus the material resistance decreases rapidly. The sensitivity of the exible lm can be calculated by the formula (1).

Electrical performance and application
It can be seen from Table 2 that the exible lm has a high-temperature coe cient of resistance (greater than 3.1% C -1 ) in the temperature range of the human body. Furthermore, the exible lm still exceeds a temperature coe cient of resistance of 1.4%° C at the wide temperature range from 25 to 80 degrees Celsius. At the same time, there is no signi cant hysteresis during cooling and heating.
It can be seen from Fig. 4a) that in the exible lm 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 exible lms. It can be seen that there is a linear relationship between resistivity and temperature for all of the exible lms and the resistivity increases as the solid content increases. It was found that compared with the 20% solids exible lm, the 30% solids exible lm has lower resistivity and better temperature dependence, which is bene cial to convert temperature signals into electrical signals. At the same time, the mechanical properties of exible lms with a solid content of 30% are better than the exible lms with a solid content of 40% and 50%. Therefore, the exible lm with a solid content of 30% is considered to have the most practical value.
In order to further study the practicality of the exible lm with a solid content of 30%, ve 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 exible lm of 30% solids was aged for 1000 minutes at the temperature of 40 to 80° C. It was found that the exible lm 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 exible lm during the folding process is negligible (0.2366% at 30 °, 0.2054% at 60 °, 0.1814% at 90 °, 0.1434% at 120 °). shows that the exible temperature sensor has great potential in wearable devices and electronic skin.