Abstract: Pressure measurement with excellent stability and long time durability is highly desired, especially at high temperature and harsh environments. A polymer-derived silicoboron carbonitride (SiBCN) ceramic pressure sensor with excellent stability, accuracy, and repeatability is designed based on the giant piezoresistivity of SiBCN ceramics. The SiBCN ceramic sensor was packaged in a stainless steel case and tested using half Wheatstone bridge with the uniaxial pressure up to 10 MPa. The SiBCN ceramic showed a remarkable piezoresistive effect with the gauge factor (K) as high as 5500. The output voltage of packed SiBCN ceramic sensor changes monotonically and smoothly versus external pressure. The as received SiBCN pressure sensor possesses features of short response time, excellent repeatability, stability, sensitivity, and accuracy. Taking the excellent high temperature thermo-mechanical properties of polymer-derived SiBCN ceramics (e.g., high temperature stability, oxidation/corrosion resistance) into account, SiBCN ceramic sensor has significant potential for pressure measurement at high temperature and harsh environments.
Keywords: polymer-derived ceramics (PDCs); silicoboron carbonitride (SiBCN) ceramic pressure sensor; piezoresistivity; high temperature and harsh environment sensor
Pressure sensor is highly desired to provide online health monitoring and improve the safety of modern industry, especially for those of high temperature and harsh environments such as turbine engines and coal gasification [1,2]. Sensors based on piezoresistive behavior of materials are one of the most adopted ones for pressure sensing due to its facile sensor design and easy fabrication. However, due to the serious drift effect and degradation at high temperature, the polymer based piezoresistive pressure sensors are not suitable for pressure measurement at the circumstance of high temperature and/or room temperature for long time [3–5]. On the contrary, ceramic based piezoresistive pressure sensors possess excellent stability and long time durability [6,7], even at high temperatures [8–10]. Nevertheless, poor sensitivity is the obvious drawback
of these ceramic based pressure sensors due to their limited piezoresistive coefficient (gauge factor, K) [11,12].
Polymer-derived ceramics (PDCs) obtained by thermal decomposition of polymeric precursor own excellent thermal stability up to 2000 ℃ , and oxidation resistance/ corrosion resistance (better than that of SiC and Si3N4) [14–16]. Meanwhile, PDCs exhibit extreme high piezoresistivity with K up to 4000–7000 [17–19], which is much higher than that of the commonly used ceramics of Si (K = 37.5)  and SiC (K = 30) . Therefore, PDCs are a promising candidate for pressure sensing with remarkable sensitivity and
stability, especially at high temperature and harsh environments. These unique properties are ascribed to their unique microstructures with a nanodomain structure
consisting of Si-based amorphous matrix and highly disordered carbon clusters, named free-carbon phase [20,21]. Specially, silicoboron carbonitride (SiBCN) is one of the most promising PDCs for ultra-high-temperature applications, due to its excellent stability up to 2000 ℃ and prominent creep resistance . For its superior properties, SiBCN ceramics have been extensively investigated including synthesis [22–24], structural evolution [25–28], high temperature stability [13,29,30], etc. However, the piezoresistivity and
pressure sensing ability of SiBCN ceramic have not been reported yet.
In this study, a piezoresistive pressure sensor made of polymer-derived SiBCN ceramic is reported for the first time. The piezoresistivity of SiBCN ceramic sensor and relative sensing performance of stability, repeatability, and response time were discussed, indicating the promising potential application at high temperature and harsh environments of SiBCN ceramic pressure sensor.
2. 1 Fabrication of SiBCN ceramic sensor head
The SiBCN ceramic sensor head was synthesized by using a commercially available liquid precursor, polyborosilazane (PSNB, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China). Briefly, the obtained PSNB precursor was cross-linked by heattreatment at 200 ℃ for 4 h in vacuum oven without any other curing agents to acquire infusible solid, which was subsequently ground to fine powder of ~1 μm by high energy ball milling (8000M, SPEX SamplePrep, United States). Then the powder was pressed into disks with the size of φ12 mm × 3 mm using uniaxial pressing at 20 MPa for 2 min. The disc like green bodies were pyrolyzed at 1000 ℃ for 4 h in a tube furnace with the heating and cooling rate of 5 ℃/min to yield amorphous SiBCN ceramics. Finally, these ceramic samples were post-treated at temperature of 1300 ℃ for 4 h to tune the piezoresistivity of SiBCN ceramics. The entire pyrolysis and post-treatment process were performed in the flowing of ultra-high purity N2 to minimize possible oxygen contamination.
The SiBCN ceramic pressure sensor head was cut from the above mentioned sample with dimension of ~6 mm × 6 mm × 1.7 mm. For piezoresistivity characterization and sensor test purpose, both square surfaces (6 mm × 6 mm) of SiBCN ceramic sensor head were polished to 1 μm finish and coated with silver paste (5002-AB, SPI, Pennsylvania, USA) as electrodes. Then a universal testing system (ZQ-990B, Zhiqu, Dongguan, Guangdong, China) was used to apply compressive stress. The resistance was measured by a high temperature insulation resistance testing system (HRMS-900, Partulab, Wuhan, Hubei, China).
2. 2 Sensor design, assembling, and test setup
In order to convert the output resistance change into voltage signal, a half Wheatstone bridge circuit is designed, as shown in Fig. 1, which composed of the SiBCN ceramic sensor head (R) and standard resistance (R0) for matching purpose. A constant voltage of V0 = 9 V was supplied by the laboratory dedicated electronic regulated power source, and the relative output voltage (Vout) was recorded by digital multimeter, as described by
Vout = V0R0 / R0+R (1)
The range of output voltage (Vr) can be expressed as
Vr = Vmax - Vmin = V0R0 / ( R0+Rmin ) - V0R0 / ( R0+Rmax ) (2)
where Vmax and Vmin are the voltages of the matched resistor when the sensor head with the minimum and maximum resistance, Rmin and Rmax, respectively (corresponding to the maximum and minimum pressure applied on the sensor head).
Fig. 1 Circuit of the designed pressure sensor.
Then the resistance of matching resistor can be selected by optimizing the sensitivity of the sensor by Eq. (3):
Solving Eq. (3), the optimal resistance of the matching resistor R0 can be calculated by Eq. (4) to be 1 MΩ, with the resistance of Rmax and Rmin to be 2.92 and 2.76 MΩ, corresponding to the maximum (7 MPa) and the minimum (0 MPa) pressure applied on the sensor (size of the sensor head: ~6 mm × 6 mm × 1.7 mm).
The as obtained SiBCN ceramic sensor head and matching resistor were assembled by a stainless steel case with alumina plates on both sides of the sensor head for isolating purpose, as shown in Fig. 2. The inverse T-shaped stainless steel is used as an indenter to apply the extra pressure. A spring was used to insure the closed contact of electrodes and to fasten the sensor head.
Fig. 2 SiBCN ceramic pressure sensor: (a) structure design and (b) assembled sensor.
The assembled SiBCN ceramic pressure sensor was tested at room temperature with the pressure up to 10 MPa. The overall sensor test setup was illustrated schematically in Fig. 3. The pressure sensor testing system mainly consists of three parts, namely, voltage regulator power supply, loading & control system, and output signal recording system. The excitation voltage of the regulated power supply is 9 V, at which the signal variation of pressure sensor can be well detected. The output pressure is provided by an electric universal testing machine with a pressure application rate of 0.5 mm/min (ZQ-990B, Zhiqu, Dongguan, Guangdong, China, equipped with a load cell of 500 N in the maximum). The output voltage signal was recorded by a digital multi-meter (VICTOR-86C, Victor Hi-Tech Co., Ltd., Shenzhen, China).
Fig. 3 Schematic drawn of SiBCN ceramic pressure sensor test setup.
3 Results and discussion
3. 1 Giant piezoresistivity of polymer-derived SiBCN ceramic
The pressure–resistance relationship of polymer-derived SiBCN ceramic is presented in Fig. 4. It can be seen that the resistance of SiBCN ceramic sensor head decreases monotonically with increasing applied pressure indicating that SiBCN ceramic is adequate for pressure sensing.
Fig. 4 Pressure–resistance relationship of SiBCN ceramic sensor head.
K defined as Eq. (5) is a crucial parameter to evaluate the resistance variation ability with applied pressure, which also reflects the sensitivity of a pressure sensor. The larger the K is, the higher sensitivity of the sensor is.
where R is the resistance, σ is the applied stress, and Eis the Young’s modulus of the material, which can be estimated by the following equation :
where E0 is Young’s modulus of fully dense SiBCN material (170 GPa) , and p is the porosity of the specimen measured to be ~23.7 vol%. Calculated from Eq. (6), the Young’s modulus of SiBCN ceramic sensor head studied here is ~102 GPa. Then, the relative K of SiBCN ceramic sensor head can be calculated using Eq. (5) and is shown in Fig. 5. Interestingly, the SiBCN ceramic exhibits an extremely high K up to 5500, due to the unique structure of PDCs [32,33], which is much higher than that of Si  and SiC  material, indicating that SiBCN ceramic is a promising material for pressure sensing application with high sensitivity.
Fig. 5 K of SiBCN ceramic as a function of pressure.
3. 2 SiBCN ceramic sensor performance
In order to fully investigate the performance of SiBCN ceramic pressure sensor, the feasibility, accuracy, repeatability, and stability were addressed systematically by using the testing platform shown in Fig. 3. The variation of output voltage as a function of the applied pressure was recorded in Fig. 6. The output voltage increases monotonously with the pressure increasing due to the outstanding piezoresistivity of SiBCN ceramics. The voltage–pressure curve shows a perfect linearity, which explores the great feasibility and reliability of SiBCN ceramic pressure sensor.
Fig. 6 Output voltage and pressure relationship of SiBCN ceramic pressure sensor.
In order to verify the accuracy and response ability of SiBCN ceramic pressure sensor, the step loading-unloading experiment was considered, as shown in Fig. 7. The applied pressure increased steeply from 0 to 10 MPa with an interval of 1 MPa (black line in Fig. 7) and the output voltage shows a similar trend correspondingly, varies from 1.09 to 1.22 V. Meanwhile, the unloading cycle shows the same trend with the opposite direction. Specially, the output voltage increased synchronously even with the abrupt increasing of pressure at the top point of Fig. 7. The loading–unloading cycle reveals that the SiBCN ceramic sensor possesses excellent stability, accuracy, and fast response speed.
Fig. 7 Loading–unloading cycle of SiBCN ceramic pressure sensor.
To further evaluate the repeatability and stability of SiBCN ceramic sensor, the 50 cycles of loading– unloading test was carried out, as shown in Fig. 8. From the loading and unloading process (Fig. 8(a)), it can be concluded that the output voltage follows up pressure increase and/or decrease closely without any delay and obvious deviation, indicating that SiBCN ceramic sensor possesses excellent repeatability. The maximum and minimum output voltages of each cycle were plotted in Fig. 8(b). The maximum and minimum output voltages of each cycle remain constant, no obvious drift, meaning that the SiBCN ceramic sensor owns excellent stability.
Fig. 8 Repeatability evaluation of SiBCN ceramic pressure sensor: (a) 50 loading–unloading cycles and (b) maximum and minimum output voltage of each cycle.
In this study, a piezoresistive pressure sensor with polymer-derived SiBCN ceramic as sensing material and stainless steel as frame is designed and tested. The polymer-derived SiBCN ceramic head exhibits a giant piezoresistivity of K as high as 5500, which insures a high sensitivity of the as designed pressure sensor. In addition, the SiBCN ceramic sensor exhibits excellent accuracy, repeatability, and stability in the pressure range of 0–10 MPa. Combining with the excellent high temperature thermo-mechanical properties, the polymerderived SiBCN ceramic pressure sensor is very promising to be used at the high temperature and harsh environments.
 Smith G. The application of microtechnology to sensors for the automotive industry. Microelectron J 1997, 28: 371–379.
 Prosser SJ. Advances in sensors for aerospace applications.Sensor Actuat A: Phys 1993, 37–38: 128–134.
 Tung TT, Robert C, Castro M, et al. Enhancing the sensitivity of graphene/polyurethane nanocomposite flexible piezoresistive pressure sensors with magnetite nano-spacers. Carbon 2016, 108: 450–460.
 Park SJ, Kim J, Chu M, et al. Flexible piezoresistive pressure sensor using wrinkled carbon nanotube thin films for human physiological signals. Adv Mater Technol 2018, 3: 1700158.
 Liu H, Dong MY, Huang WJ, et al. Lightweight conductive graphene/thermoplastic polyurethane foams with ultrahigh compressibility for piezoresistive sensing. J Mater Chem C2017, 5: 73–83.
 Masheeb F, Stefanescu S, Ned AA, et al. Leadless sensor packaging for high temperature applications. In Technical Digest. MEMS IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems. Las Vegas, NV, USA: IEEE, 2002: 392–395.
 Li M, Tang HX, Roukes ML. Ultra-sensitive NEMS-based cantilevers for sensing, scanned probe and very highfrequency applications. Nat Nanotech 2007, 2: 114–120.
 Zaitsev AM, Burchard M, Meijer J, et al. Diamond pressure and temperature sensors for high-pressure high-temperature applications. Phys Stat Sol (a) 2001, 185: 59–64.
 Ned AA, Okojie RS, Kurtz AD. 6H-SiC pressure sensor operation at 600 ℃. In 1998 Fourth International High Temperature Electronics Conference. Albuquerque, NM, USA: IEEE, 1998: 257–260.
 Kurtz AD, Ned AA. Hermetically sealed ultra high temperature silicon carbide pressure transducers and method for fabricating same. U.S. Patent 6058782, May 2000.
 Kervran Y, de Sagazan O, Crand S, et al. Microcrystalline silicon: Strain gauge and sensor arrays on flexible substrate for the measurement of high deformations. Sensor Actuat A: Phys 2015, 236: 273–280.
 Phan HP, Dao DV, Tanner P, et al. Thickness dependence of the piezoresistive effect in p-type single crystalline 3C-SiC nanothin films. J Mater Chem C 2014, 2: 7176–7179.
 Riedel R, Kienzle A, Dressler W, et al. A silicoboron carbonitride ceramic stable to 2000 ℃. Nature 1996, 382: 796–798.
 Wang YG, An LN, Fan Y, et al. Oxidation of polymerderived SiAlCN ceramics. J Am Ceram Soc 2005, 88: 3075–3080.
 Wang YG, Fei WF, An LN. Oxidation/corrosion of polymer-derived SiAlCN ceramics in water vapor. J Am Ceram Soc 2006, 89: 1079–1082.
 Wang YG, Fei WF, Fan Y, et al. Silicoaluminum carbonitride ceramic resist to oxidation/corrosion in water vapor. J Mater Res 2006, 21: 1625–1628.
 Zhang LG, Wang YS, Wei Y, et al. A silicon carbonitride ceramic with anomalously high piezoresistivity. J Am Ceram Soc 2008, 91: 1346–1349.
 Li N, Cao YJ, Zhao R, et al. Polymer-derived SiAlOC ceramic pressure sensor with potential for high-temperature application. Sensor Actuat A: Phys 2017, 263: 174–178.
 Cao YJ, Yang XP, Zhao R, et al. Giant piezoresistivity in polymer-derived amorphous SiAlCO ceramics. J Mater Sci2016, 51: 5646–5650.
 Colombo P, Mera G, Riedel R, et al. Polymer-derived ceramics: 40 years of research and innovation in advanced ceramics. J Am Ceram Soc 2010, 93: 1805–1837.
 Fu SY, Zhu M, Zhu YF. Organosilicon polymer-derived ceramics: An overview. J Adv Ceram 2019, 8: 457–478.
 Zhao H, Chen LX, Luan XG, et al. Synthesis, pyrolysis of a novel liquid SiBCN ceramic precursor and its application in ceramic matrix composites. J Eur Ceram Soc 2017, 37: 1321–1329.
 Kong J, Wang MJ, Zou JH, et al. Soluble and meltable hyperbranched polyborosilazanes toward high-temperature stable SiBCN ceramics. ACS Appl Mater Interfaces 2015, 7: 6733–6744.
 Thiyagarajan GB, Devasia R. Simple and low-cost synthetic route for SiBCN ceramic powder from a boron-modified cyclotrisilazane. J Am Ceram Soc 2019, 102: 476–489.
 Sarkar S, Gan ZH, An LN, et al. Structural evolution of polymer-derived amorphous SiBCN ceramics at high temperature. J Phys Chem C 2011, 115: 24993–25000.
 Liao N, Jia DC, Yang ZH, et al. Enhanced mechanical properties and thermal shock resistance of Si2BC3N ceramics with SiC coated MWCNTs. J Adv Ceram 2019, 8: 121–132.
 Kousaalya AB, Kumar R, Packirisamy S. Characterization of free carbon in the as-thermolyzed Si-B-C-N ceramic from a polyorganoborosilazane precursor. J Adv Ceram 2013, 2: 325–332.
 Chen YH, Yang XP, Cao YJ, et al. Effect of pyrolysis temperature on the electric conductivity of polymerderived silicoboron carbonitride. J Eur Ceram Soc 2014, 34: 2163–2167.
 Ramakrishnan PA, Wang YT, Balzar D, et al. Silicoboron– carbonitride ceramics: A class of high-temperature, dopable electronic materials. Appl Phys Lett 2001, 78: 3076–3078.
 Ding Q, Ni DW, Wang Z, et al. 3D Cf/SiBCN composites prepared by an improved polymer infiltration and pyrolysis. J Adv Ceram 2018, 7: 266–275.
 Kingery WD, Bowen HK, Uhlmann DR. Introduction to Ceramics. New York, U.S.: John Wiley and Sons, 1976.
 Wang YS, Zhang LG, Fan Y, et al. Stress-dependent piezoresistivity of tunneling-percolation systems. J Mater Sci 2009, 44: 2814–2819.
 Wang YG, Ding J, Feng W, et al. Effect of pyrolysis temperature on the piezoresistivity of polymer-derived ceramics. J Am Ceram Soc 2011, 94: 359–362.