Lubricants such as engine oil play an important role in preventing machine wear and damage. Monitoring the deterioration of lubricating oils is a significant technical issue in machine maintenance. In this study, a sensor for monitoring engine oil viscosity was developed using a metal-core piezoelectric fiber/aluminum composite. This composite is a piezoelectric ceramic that is reinforced by a metal matrix; it is expected to be utilized in harsh environments such as the inside of an engine. An active type measurement method was employed to monitor variations in the viscosity of glycerin solution as a model liquid. In this method, a self-generated vibration is correlated to the viscosity of a liquid by measuring the damped vibration amplitude and the variation in the resonance frequency. The results showed that the vibration had a high sensitivity to the liquid viscosity; further, it was observed that the shift in resonance frequency correlated to a wider range of measurable viscosity. Both measured parameters indicate that the metal-core piezoelectric fiber/aluminum composite is a viable sensor for engine oil monitoring.

This paper describes the characterization of a metal-core piezoelectric ceramics fiber/aluminum composite as a metal-based piezoelectric composite. Piezoelectric materials, especially piezoelectric ceramics are generally used as excellent transducer materials. However, there are serious disadvantages, as they are very brittle and need a complicated electrode system with an adhesion layer to generate piezoelectricity. Therefore, the application of piezoelectric ceramics is limited. In order to solve these problems, a metal-core piezoelectric ceramics fiber/aluminum composite was developed. The metal-core piezoelectric fiber is not as brittle as bulk ceramics, but it is still too brittle to be embedded in an aluminum matrix using a conventional process. Therefore, the interphase forming/bonding method was applied to embed it in an aluminum matrix without fracture. Using this successful approach, a simple electrode system was formed between the metal core of the embedded fiber and the matrix. As this material system is expected to be used as a robust sensor and energy harvester, its output voltage and power characteristics were evaluated with vibration test equipment and compression vibration equipment. According to the results, the output voltage generated from the specimen is proportional to its strain, and dependent on its direction. It was also found that the output power generated from the specimen increases with the square of its strain, and in proportion to its frequency, and the calculated maximum output power reaches approximately 3.4 mW when the specimen undergoes 0.2 % strain and 600 Hz frequency by vibration. As this output power is generated from the single embedded fiber, it is suggested that the energy for driving a wireless module can be secured by embedding multiple fibers. Consequently, a composite embedded with multiple fibers will be able to be used as a wireless strain sensor owing to its strain measurement and energy-harvesting capabilities.

It is well known that the curing residual stress induced during a fabrication process has a great influence on the performance of piezoelectric composite devices. The purpose of this work was to evaluate the residual thermal stress of lead zirconate titanate piezoelectric fiber aluminum (Al) matrix (piezoelectric fiber/Al) composites generated during fabrication numerically and experimentally and to understand the effect of the residual thermal stress on the electromechanical response. The three-dimensional finite element method was employed, and the residual stress generated during the solidification process of the Al matrix was calculated. The output voltage was also calculated in the analysis when putting stresses on the composite materials in the length direction of the piezoelectric fiber. It was shown that the cooling from higher temperatures increases the electromechanical conversion capability. Furthermore, we also performed the simulation, and we recorded the output voltage under concentrated load to investigate its application as a load position detection sensor, and we also discussed the influence of the position by changing the modeling with a different fiber position in the Al. The residual stress of hot press molded piezoelectric fiber/Al composite was then measured, and the comparison was made with the calculated values. The simulation results revealed that our model predictions reproduced and explained the experimental observations of curing residual stress. After this study, similar models of composite materials can be analyzed by this simulation, and the result can be used to design piezoelectric composite materials.

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