Terbium and europium aromatic carboxylates in the polystyrene matrix: The first metal-organic-based material for high-temperature thermometry

Highlights

• The first metal-organic composite material for thermometry at elevated temperature obtained.

• It is based on a physical mixture of lanthanide complexes doped into a polymer.

• Photoluminescence quantum yield at room temperature is very high (75 %).

• The spectrum of the obtained material is temperature-dependent.

• Sensitivity of 2,2 %/°C, temperature resolution of 0,92 °C/% were reached.

Abstract

We proposed and successfully demonstrated for the first time the possibility of using a finely dispersed mixture of terbium and europium carboxylates, doped into a polystyrene polymer matrices, as a luminescent thermometer at the elevated temperature (25–200 °C). The sensitivity of 2.2 %/°C was reached.

Introduction

The high-precision luminescent thermometry is indispensable in both science and in the industry when it is required to measure the temperature of small, fast-moving, or hard-to-reach objects, or when physical contact is impossible [[1], [2], [3]]. The most ambitious application of luminescent thermometry in the high-temperature range is the measurement of the temperature of internal components of combustion engines, aircraft engines, and rocket engines during their design phase, such as pistons. Moreover, high-temperature luminescent thermometers are promising for temperature mapping of various surfaces [[4], [5], [6]]. The high precision of luminescent spectroscopy makes it promising for temperature measurements [7,8].

There are several ways to record the temperature change using luminescent data, among which the luminescence intensity ratio (LIR) is the most convenient temperature response [[9], [10], [11]]. Indeed, thermometers based on it are not subject to external conditions, i.e. layer thickness, excitation radiation intensity, etc., thus, they are self-calibrating [[12], [13], [14], [15], [16], [17], [18]]. The most significant parameter of the luminescent thermometer is its relative sensitivity, determined as S_r = 1/LIR·dLIR/dT, which characterizes the accuracy of temperature measurement at a given temperature [2,12,19,20].

Therefore, the ideal material for high-temperature luminescence thermometry should demonstrate the following properties: high sensitivity, high intensity of luminescence, narrow luminescence bands with constant position, and high thermal stability.

The most promising materials for luminescence thermometry are coordination compounds of lanthanides, in particular terbium and europium. Their advantages include narrow stationary emission bands and high luminescence intensity, which, despite the low absorptive capacity of the lanthanides themselves, is achieved due to the “antenna” effect [[21], [22], [23], [24], [25], [26], [27]]. Combining compounds of several lanthanides, it is possible to reach the most accurate temperature response LIR for temperature measurement. However, coordination compounds of lanthanides with rare exceptions demonstrate low-temperature stability. That is why every work devoted to lanthanide-based high-temperature thermometry utilizes only purely inorganic compounds with high thermal stability, but very low luminescence brightness [[28], [29], [30]].

Some lanthanide aromatic carboxylates, long studied in our group [31], possess both the required temperature stability (up to 400 °C) and bright luminescence, satisfying all the properties of an ideal luminescent thermometer. Therefore, the aim of this work is to test the possibility of using terbium and europium carboxylates for high-temperature luminescence thermometry.

As an object of study we have chosen a finely dispersed mixture of Eu(mfb)₃BPhen and Tb(czb)₃ (Hmfb is monofluorobenzoic acid, BPhen is bathophenanthroline, Hczb is 4-(9H-carbazol-9-yl)benzoic acid, Fig. 1), doped into a polymer to ensure film formation and uniform distribution of complexes. It was shown earlier in our research group that these coordination compounds demonstrate bright luminescence and high thermal stability [32,33]. As a polymer, both polystyrene (PS) and poly(methyl methacrylate) (PMMA) were chosen for their availability, thermal stability, and good mechanical properties. Ratio of terbium (at 545 nm) and europium luminescence (at 612 nm) was chosen as the temperature response: LIR = I(545 nm))/I(612 nm)) [[12], [13], [14]].