## 2 Thermal Conductivity Methods
### General Statements
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multirow{2}{*}{**Temperature Range (K)**} & \multicolumn{2}{c}{**Thermal Conductivity Range (N m\({}^{-1}\) K\({}^{-1}\))**} & \multirow{2}{*}{**Measure Time (s)**} & \multirow{2}{*}{**Advantages**} & \multirow{2}{*}{**Disadvantages**} \\ \hline \multicolumn{7}{c}{STEADY-STATE METHODS} \\ \hline Guarded hot plate [b] & 113–973 & 0.01–6 & From hours to weeks & \begin{tabular}{c} High \\ accuracy, \\ adjustable to the \\ sample \\ thickness \\ \end{tabular} & \begin{tabular}{c} Time \\ -consuming, large \\ specimen, not suitable \\ for film-like \\ samples \\ \end{tabular} \\ \hline Heat flow & 173–473 & \textless{}0.3 & \begin{tabular}{c} Shorter than \\ guarded hot plate \\ \end{tabular} & \begin{tabular}{c} Simple \\ construction and \\ operation \\ \end{tabular} & \begin{tabular}{c} Measurement \\ uncertainty, \\ non-absolute method \\ \end{tabular} \\ \hline \multicolumn{7}{c}{TRANSIENT METHODS} \\ \hline Transient plane source [c] & 238–1273 & 0.001–1800 & Few seconds & \begin{tabular}{c} Rapid and \\ precise, wide \\ temperature and \\ thermal conductivity \\ range \\ \end{tabular} & \begin{tabular}{c} Require \\ entirely planar side of \\ sample, not \\ suitable for \\ powders or \\ granules \\ \end{tabular} \\ \hline Laser flash [c] & 153–3073 & 0.1–1000 & 1–2 & \begin{tabular}{c} Wide \\ temperature range, \\ small sample, fast, \\ accuracy at high \\ temperature \\ \end{tabular} & \begin{tabular}{c} More expensive, not \\ suitable for \\ insulation \\ materials \\ \end{tabular} \\ \hline Modulated DSC [c] & 190–670 & 0.1–1 & 10–100 & Sensitive and accurate & \begin{tabular}{c} Expensive than \\ conventional DSC \\ \end{tabular} \\ \hline \multicolumn{7}{c}{Wide temperature range,} \\ \multicolumn{7}{c}{30\(\omega\) method [c] 308–773} & 0.2–20 & Few seconds & \begin{tabular}{c} Wide \\ temperature range, \\ acceptable \\ accuracy, \\ and insensitive to \\ black-body \\ radiation \\ \end{tabular} & \begin{tabular}{c} Not suitable for \\ electrically conductive \\ samples \\ \end{tabular} \\ \hline Transient hot-wire [a] & 123–473 & 0.005–500 & \
\begin{tabular}{c} \(10^{-3}\)–1 \\ (fluid) \\ 10 \\ (solid) \\ \end{tabular} &
\begin{tabular}{c} Limited to materials \\ with low electrical \\ conductivity (bare \\ wire), delicate thin wire \\ \end{tabular} \\ \hline \hline \end{tabular}  
* Considered as a primary method. [b] Considered as a primary method for low thermal conductivity measurements. [c] Considered as a secondary method.  
\end{table} 
Table 1: Summary of some steady-state and transient methods for measuring thermal conductivity [4,10]. A non-steady-state or transient technique records a measurement during a heating process and takes advantage of temperature variation over time, leading to a reduction of the heat convection within fluids due to the use of short-time periods on which this effect can be easily identified. Compared to the steady-state techniques, these measurements can be performed relatively quickly [10]. In addition, the amount of sample needed for the measurements is small [4]. Numerous solutions have been derived for the transient heat conduction equation by using one-, two-, or three-dimensional geometries. Generally, transient methods employ probes or wires and work at high temperature and pressure [3; 4]. The most extended unsteady methods--transient plane source, laser flash, modulated DSC, 3\(\omega\)u method, or transient hot-wire (THW)--are also summed up in Table 1, along with their principal characteristics. Furthermore, methods can be classified as primary and secondary depending on their metrological quality [1; 3]. According to the Comite Consultatif pour la Quantite de Matiere, a primary method is a completely described and understood method in which uncertainty and standard results are accepted [1].