A temperature sensor with practically no effect at all on the temperature of the object measured has been created in the laboratory by researchers at the University of São Paulo (USP) and the University of Campinas (UNICAMP) and is awaiting a patent for commercial production.

The sensor consists of a system made of titanium dioxide (TiO2) doped with thulium ions (Tm3+). Because it's capable of measuring a very broad spectrum of temperatures, it can be used in manufacturing, where temperatures sometimes reach very high levels. It can also be used for biological processes, which are highly sensitive to the slightest temperature variations.

When excited by a laser pulse, the material emits light at wavelengths that vary depending on the temperature of the environment. A very precise measurement of the wavelength is used to determine this temperature. Variation in the wavelength of the emitted light is linear between 80 and 750 Kelvin — the device remains stable throughout this temperature range.

The material is in the form of a thin film and can theoretically be used as a coating for any surface, be it flat or curved, smooth, or rough. The material can also be presented as microparticles or nanoparticles.

A simple application would entail coating a plastic substrate with the sensor and fixing it to a patient's skin. It's important to note that titanium dioxide is abundant, easy to obtain, and biocompatible and is already used in many medical prosthetic devices.

Other potential applications of the sensor range from the identification of hotspots in electronic equipment to the detection of viral or bacterial infection in specific regions of an organism.

As a thin film, the material can be as small as a few square centimeters or it can reach several square meters for use as a surface coating on components of land vehicles, aircraft, or power grid transformers. As micrometric or nanometric particles, it can be dispersed in a liquid medium while remaining solid.

It should also eventually be possible to encapsulate the laser emitter, temperature sensor, and wavelength detector with a radio communicator inside a small pill. Swallowed with a little water, the pill would send back temperature data while moving along the digestive tract until its ejection from the organism at the opposite extremity.

Because the device is optical, the temperature can be obtained without direct physical contact. A laser beam can be projected onto the sensor and the response observed. By measuring the wavelength of the light emitted by the sensor, the temperature of the object can be determined with great precision.

The wavelength varies approximately 2 picometers (2×10-12 m) per degree of temperature. Spectroscopy can be used to detect this tiny wavelength variation. However, the need for a dedicated detector is a limiting factor at the present stage, in terms of both the cost and portability of the device.

The researchers believe, however, that as technology advances, it will be possible to fabricate an integrated device with a semiconductor laser, temperature sensor, and detector.

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Sensor Technology Magazine

This article first appeared in the March, 2018 issue of Sensor Technology Magazine.

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