Solid-State FET-Based Sensors Capable of Measuring Acidity of Lubricants
Authors: Hyodo Takeo , Yuto Masahiro , Tanigawa Hiroaki , Tsuruoka Mioko , Sakamoto Yuki , Ueda Taro , Kamada Kai , Shimizu Yasuhiro 
Published: May 2020

Introduction

The deterioration of various kinds of machine lubricants and engine oils has been estimated through total acid number (TAN), and the value of TAN has been commonly measured by color-indicator titration and potentiometric titration, according to ISO standards (e.g., No. 6619 and 7537). Presently, we cannot directly monitor the TAN value on site as well as in real time, which leads the difficulty in the efficient operation of various machines and their automatic maintenance. Some kinds of devices capable of measuring nuclear magnetic resonance (NMR) [1], infrared (IR) adsorption [2], impedance [3], or electrochemical potential [3–6] of lubricants and oils to estimate their deterioration have been developed, but compact, maintenance-free, and low-cost devices capable of directly monitoring their TAN values are quite convenient for controlling mechanical systems utilized in various fields. Our group have recently attempted to develop new TAN-monitoring sensors employing a general pH meter based on ion-sensitive field-effect transistor (IS-FET) technology [7–9]. In this presentation, we would like to introduce the new concept, fundamental properties, and the operating mechanism of the sensors.

Experimental

Figure 1 shows schematic cross-sectional drawing of a sensor tip, which consists of an IS-FET and a reference electrode (Horiba, Ltd., ISFET pH electrode: 0040-10D, sensor chip: 0141) [10]. 1-Propanol solution containing 5.0 wt% cation-conducting polymer (Tokuyama Corp., CS-7P) was used as an original coating agent, and it was appropriately diluted with 1-propanol to form three kinds of diluted CCP solution (2.5 wt%, 1.7 wt%, and 1.3 wt%). The CCP solution (20 mm3) was dropped on the surface of the sensor tip, and then the CCP film was uniformly coated on the while surface containing the IS-FET and the reference electrode, after moderately drying at room temperature. The obtained films were expressed as CCP(m), where m stands for the concentration of the CCP solution used (wt%). Subsequently, the surface of the sensor tip was treated with 1 M NaCl aqueous solution (100 mm3) several times in order to exchange H+ for Na+ in the CCP(m) films, until the pH value reached up to ca. 6. As soon as some kinds of lubricants (20 mm3) were dropped on the surface of the obtained CCP(m) sensors, variations in pH of the lubricants measured by the CCP(m) sensors with time were measured with a pH meter (Horiba, Ltd., D-52).

Results and Discussion

Figure 2 shows response transients of the CCP(2.5) sensor to typical fresh and deteriorated lubricants (Oil(n), n: oxidation period for deterioration (h)) at 30°C. TAN values of the lubricants oxidized for 0, 504, 800, and 1104 h were 0.19, 0.23, 0.30, and 0.35 (mgKOH/g), respectively, and the fresh lubricant corresponds to Oil(0). After the initial pH value was controlled at ca. 6, the pH value gradually increased immediately after dropping the Oil(0) on the sensor surface (i.e., starting the pH measurement of the lubricant), and it reached a constant value of ca. 7.5. On the other hand, the pH value decreased soon after dropping other oxidized Oil(n)s on the sensor surface, and the pH value decreased with an increase in the oxidation period. Consequently, the pH value of all the Oil(n)s was largely dependent on the oxidation period and the TAN value. In addition, the response speed of the CCP(2.5) sensor to all the Oil(n)s was much faster than the deterioration rate of general lubricants under common usage conditions. These results show that the sensor is really promising as a device monitoring the deterioration of various lubricants on site as well as in real time. We will report effects of thickness of the CCP film on the response behavior and response transients of CCP(m) sensors to other kinds of lubricants and discuss their operating mechanism in this presentation.

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Figure 1