Development of a Wireless Temperature Sensor Using Polymer-Derived Ceramics

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Development of a Wireless Temperature Sensor Using Polymer-Derived Ceramics
A temperature sensor has been developed using an embedded system and a sensor head made of polymer-derived SiAlCN ceramics (PDCs). PDC is a promising material for measuring high temperature and the embedded system features low-power consumption, compact size, and wireless temperature monitor. The developed temperature sensor has been experimentally tested to demonstrate the possibility of using such sensors for real world applications.

1. Introduction
Accurate temperature measurements are crucial for many applications, such as chemical processing, power generation, and engine monitoring. As a result, development of temperature sensors has always been a focus of microsensor field. A variety of materials have been studied for temperature sensor applications, for example, semiconducting silicon and silicon carbide. Silicon based sensors are typically used at temperatures lower than 350°C due to accelerated material degradation at higher temperature [1, 2]. Silicon carbide based sensors are better than silicon based sensors in high temperature measurement and can be applied in temperatures up to 500°C [3–5].

Polymer-derived SiAlCN ceramics (PDCs) are another widely studied material that demonstrate properties such as excellent high temperature stability [6] as well as good oxidation/corrosion resistance [7]. PDCs have been considered as a promising material for measuring high temperature [8]. Our early works have showed that PDC sensor head can accurately measure high temperature up to 830°C [9] using data acquisition system from National Instruments. The cost and size of the sensor system must be significantly reduced before it can be deployed for real world applications. In this paper, we develop a temperature sensor using PDC and an embedded system. Comparing to the National Instruments data acquisition equipment used in the previous paper, the newly developed embedded sensor is much smaller (9.7 dm3 versus 0.3 dm3), lighter (5.97 kg versus 0.19 kg), and cheaper (approximately $8000 versus $170). A WiFi module is also added so the temperature measurement can be transmitted wirelessly. The embedded board and WiFi module used in this paper are commercially available. The experiments in this paper demonstrate the possibility of deploying PDC based sensors for real world applications.

2. Fabrication of the PDC Sensor Head
In this study, the PDC sensor head is fabricated by following the procedure reported previously [9]. In brief, 8.8 g of commercially available liquid-phased polysilazane (HTT1800, Kion) and 1.0 g of aluminum-tri-sec-butoxide (ASB, Sigma-Aldrich) are first reacted together at 120°C for 24 hours under constant magnetic stirring to form the liquid precursor for SiAlCN. The precursor is then cooled down to room temperature, followed by adding 0.2 g of dicumyl peroxide (DP) into the liquid under sonication for 30 minutes. DP is the thermal initiator which can lower the solidification temperature and tailor the electrical properties [10]. The resultant liquid mixture is solidified by heat-treatment at 150°C for 24 hours. The disk-shaped green bodies are then prepared by ball-milling the solid into fine powder of ~1 μm and subsequently uniaxially pressing. A rectangular-shaped sample is cut from the discs and pyrolyzed at 1000°C for 4 hours. The entire fabrication is carried out in high-purity nitrogen to avoid any possible contamination.

Pt wires are attached to the sensor head by two ceramic fasteners on the two mounting holes on the diagonal of the sensor head. To improve the conductivity, both mounting holes are coated with Pt plasma; see Figure 1.

To measure temperature using the PDC sensor, the processor needs to perform the following tasks: () supply voltage to the circuit through DAC7724; () sample the circuit output using AD7656 and convert the output to temperature measurement; and () transmit data to readers from the RS232 port.

The input signal to the conversion circuit is a sinusoidal signal of ±10 V. The sinusoidal signal can bypass the parasitic capacitor in series to the PDC probe. The noise from the furnace coil can also be greatly subdued. The sensor output voltage is approximately sinusoidal as well and its magnitude can be computed using Fast Fourier Transformation (FFT) or curve fitting using recursive least square method (RLSM) [11]. Comparing to FFT, RLSM is more computationally efficient but may have numerical instability because TMS320F28335 only supports IEEE 754 floating-point arithmetic. Here we prefer FFT for fast prototyping purpose because Texas Instruments provides FPU library that performs floating FFT routines on C2000 series microcontroller. Next we explain how the sensor works.

A high-priority interrupt service request (ISR1) based on a CPU timer continues reading a look-up-table and drives the DAC7724 to generate the input signal . The frequency of is controlled by the frequency of ISR1. ISR1 also samples circuit output from AD7656 and adds the data to a 1024-point buffer if there is no FFT running. Once the buffer is filled up, ISR1 stops writing the buffer and the FFT routine starts. The FFT routine is implemented in another slower low-priority interrupt service (ISR2). Once the FFT routine is completed, ISR2 will give ISR1 the permission to clean and write the input buffer again. The magnitude from the FFT is used as the circuit output . The software flowchart is shown in Figure 4.
 
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