Quantifying Heating Power and Internal Temperatures in Induction-Heated Magnetic Nanoparticle Samples

When magnetic nanoparticles are exposed to high-frequency magnetic fields, they can generate heat through induction heating via magnetic hysteresis losses. This property enables remote heating by embedding nanoparticles in a target object and applying an external alternating magnetic field. Applications span from cancer treatments to electrification of thermo-catalytic reactions, CO₂ capture and storage, and endometriosis therapy. When nanoparticles are developed and application possibilities are explored, it is important to quantify the heating power of the particles and to understand the nanoscale temperature distribution, which are the two topics of this PhD project.

Measuring heating power
The nanoparticle heating power is an important quantity, as it allows comparison between nanoparticles and calculation of therapeutic doses. Specifically in cancer treatment, insufficient heating power is a key limitation, driving interest in designing nanoparticles with higher heating efficiency. Unfortunately, the most widespread measurement method, non-adiabatic AC calorimetry, used to quantify heating powers suffers from systematic errors of up to 40% as reported from an inter-laboratory round-robin test by the RADIOMAG consortium in 2021.

This thesis aims to improve the reliability of heating power measurements using non-adiabatic AC  calorimetry. First, the effect of using insulation in non-adiabatic AC calorimetry setups is investigated. It is found that the insulation can cause non-linear heat losses with respect to the temperature difference, if this is defined incorrectly as the difference between the sample and room temperature due to heating of the insulation material. This leads to incorrect estimation of heating powers by the corrected slope method when the temperature immediately outside the sample is not taken into account, which is typically not done. This is a significant finding, as the corrected slope method is regarded as the most reliable method for measuring the heating power, and insulation is frequently used.

Secondly, this thesis revisits the non-adiabatic AC calorimetry data from the RADIOMAG round-robin inter-laboratory study. This re-examination of data from 21 laboratories identified four common instrumentation challenges: i) Insufficient temperature resolution, ii) Field-sensitive thermometers, iii) Non-physical heat oscillations, and iv) Non-linear heat losses similar to those identified due to the use of sample insulation. Excluding data from laboratories with insufficient quality reduced the standard deviation of heating power by up to 38% compared to the original RADIOMAG results.

Nanothermometry
Heat is generated locally inside magnetic nanoparticles during induction heating, raising the question of whether local nanoscale temperature increases (nanoscale hotspots) can occur at and near the nanoparticles, in addition to bulk temperature rise. This remains debated with theoretical works predicting negligible hotspot temperature differences (<0.1 mK), while experimental reports generally claim local temperature differences even up to 200 K.

In this PhD project, two methods for measuring the internal nanoparticle temperature were investigated. The first method is X-ray diffraction thermometry, previously reported only once for induction-heated magnetic nanoparticles. This technique probes thermal expansion in both nanoparticles and support material, enabling simultaneous temperature measurement. Three studies were performed with X-ray diffraction thermometry and induction heating: i) Nanoparticles on powder support, ii) Nanoparticles in aqueous solutions, iii) An in operando study with induction-heated, catalytically active nanoparticles. Across all studies, it was found that temperatures could be determined with sub-Kelvin resolution, even for water. From the first study, it was found that no significant hotspots existed. Investigations of hotspots from the last two studies are pending due to challenges with sample changes. Despite these challenges, both experiments demonstrate proof of concept for the methods.

As an alternative nano-thermometry method, inelastic neutron scattering thermometry using the detailed balance factor was explored. The conducted study is a preliminary study to investigate the feasibility of the method. Currently, the method overestimates temperature by about 30 K and has a noise level of 30 K, but further analysis may reduce these errors.