Exploring a Thermal Contact-Based Strategy for Temperature Control in Low-Field TD-NMR Systems

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    \textbf{20\textsuperscript{th} NMR USERS MEETING – October, 06-10, 2025 – Angra dos Reis, RJ, Brazil}
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    \textbf{Exploring a Thermal Contact-Based Strategy for Temperature Control in Low-Field TD-NMR Systems}
    
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    \textit{Gustavo H. O. Machado, Bruno Trebbi,  Rodrigo H. S. Garcia, Edson L. G. Vidoto,     Araldo L. I. deMoraes, Ademir Morai, Claudio J. Magon, Luciano L. Felippe, Luis A. O. Nunes, João G. S. Filho,Eduardo R. deAzevedo}
    
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    Instituto de Física de São Carlos, Universidade de São Paulo, Avenida Trabalhador São-carlense, nº 400, Parque Arnold Schimidt,  CEP 13566-590, São Carlos, Brasil. \\
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\noindent\textbf{Keywords:} instrumentation for NMR, variable temperature NMR, temperature controller, high thermal inertia systems for NMR.

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\paragraph{} Because many NMR parameters are highly sensitive to temperature, controlling and stabilizing the sample temperature is essential in NMR experiments. This becomes even more critical in studies that rely on molecular dynamics, where temperature is the most important external parameter to be precisely controlled and stabilized in order to ensure accurate measurement of dynamical properties.

\paragraph{} The progress of time-domain NMR (TD-NMR) at low magnetic fields for applications requiring sample temperature control demands equipment capable of providing an extended variable temperature range with effective stabilization. Currently, the most common sample temperature control systems rely either on thermal baths or vented systems involving the injection of cold nitrogen gas combined with flow-based heating. While the former is limited in its achievable temperature range, the latter offers lower temperature resolution and may result in significant internal temperature gradients within the sample. 

\paragraph{} Contact-based thermal controllers are commonly used in thermal measurements as well as in optical and electron paramagnetic resonance (EPR) experiments \textsuperscript{(1)}. These systems typically rely on direct contact between the sample and a metal cold finger—usually made of copper—whose temperature is regulated by a combination of cold gas (or liquid) and heating elements. Depending on the design, they can offer highly stable temperature control over a wide range. However, this type of system is rarely used in conventional NMR, mainly due to the limitations associated with placing a metal cold finger inside or even near the NMR detection coil, a scenario that might change with the introduction of ceramic materials combining high thermal and low electrical conductivities.

\paragraph{}Following this approach, a strategy for constructing a thermal contact-based temperature controller is presented, along with a description of the first prototype. The core concept relies on the high thermal inertia of a copper cylinder placed in an evacuated and reflective environment. By using a copper cold finger, it becomes possible to vary the sample temperature very slowly, allowing experiments to be conducted while the temperature changes by only a few millikelvins. However, as previously mentioned, it is essential to avoid direct contact between the copper cold finger and the sample, due to its proximity to the RF coil region of the spectrometer. To address this, a dielectric ceramic material with good thermal conductivity was used at the tip of the cold finger to make contact with the sample holder. Additionally, to control the temperature of the copper block, a conventional automated system—based on cold gas (or liquid), heating elements, and a power unit with Proportional–Integral–Derivative (PID) control is used.

\paragraph{}Concerning the current stage of development, a fully functional homemade power unit with PID control has been built, including custom control software. Initial external tests regarding heat transfer through the cold finger have been carried out, and the first prototype is nearing completion. Therefore, in this poster, we intend to present the project design, the development of the power unit, the results of the heat transfer tests, and the upcoming first tests with the completed prototype.

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\noindent \textbf{REFERENCES}

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\noindent (1) P. Heller, \textit{Nuclear-Magnetic-Resonance Studies of Critical Phenomena in MnF\textsubscript{2}. I. Time-Average Properties}, \textit{Physical Review}, \textbf{1966}, \textit{146.2}, 403.

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\noindent \textbf{Acknowledgements:} PETROBRAS, CNPQ, FAPESP

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