In recent years, the 3ω method based on the harmonic detection principle has made rapid development. The principle of 3ω method is that, the micro metal belt heater on the sample surface can generate a temperature fluctuation with an angular frequency of 2 ω when driving by sinusoidal alternating current with an angular frequency of ω. The amplitude and phase of the fluctuation are affected by the thermal properties of the metal belt and sample. Temperature fluctuations then cause resistance fluctuations with an angular frequency of 2ω, which interact with the 2ω sinusoidal alternating current and generate a 3ω harmonic voltage containing relevant information of the thermal physical parameters of the samples. However, the traditional 3ω methods still have many confinements. Before each measurement, preparation such as depositing micro metal heating belt as detector, smoothing surface, insulating conductive samples are needed. The costly samples and detector can not be reused after each measurement, resulting in tremendous waste of resource

After years of unremitting efforts, heat and mass transfer center, IET, finally developed a new 3 ω method with independent detector. The metal heating belt is firstly deposited on polyimide film, and then covered with another layer of the same polyimide film after connecting wire. Thus, an independent detector is made, which can not only be used to measure film, crystal, liquid, powder and other materials, but also the composite and porous material which the traditional 3 ω method are not applicable to. Besides, the detector can be reused, greatly reducing the cost of measuring

When measuring thermal conductivity of PMI foam with the new 3ω method, we only need to clamp and fix the detector in the middle of the two pieces of same foam material to form a "sandwich" structure and put the sample and detector in a high precision constant temperature box with required temperature. According to the measurement results, the thermal conductivity of PMI foam with higher density is larger in the range of room temperature to 100 ℃. For the foam with the same density, the thermal conductivity increases with temperature linearly. Because of the large sample pore diameter, heat transfer between gas and solid and radiation increases with temperature, which results the effective thermal conductivity increases linearly. In addition, due to large pore sizes, heat transfer of gas and radiation is independent of density, so the effective thermal conductivity is proportional to the solid content in the sample, and increases with density.