Let me provide a general answer which may help you select materials. The thermal conductivity is proportional to the heat capacity. Therefore, whenever you have T-dependent variation in the heat capacity you can expect variation in the thermal conductivity. The T-dependence of the lattice is typically not so exciting and follows a Debye model behavior. Selection of material with respect to the Debye Temperature can help you to to see an effect. Magnetic transitions contribute to heat capacity as well.

Also there is the celebrated Wiedemann Franz law for metals relating thermal conductivity with heat conductivity. Therefore, if you have a T-dependent metal to insulator transition (Mott insulator ) as you can have in some oxides (nickelates for instance) you should see large effects in the T-dependence of the thermal conductivity. 

thanks for your reponse but it does not address my question. i need a material with a large thermal conductivity variation (i.e., being a thermal insulator to a high thermal conductor material) in temperatures below 100oC.

How does this not address your question? I tried to address exactly that. 

I would recommend PbTe. Its total thermal conductivity changes about 2 W/K m in your desired temperature range.

I could develop such material for you. If you need, please, email [email protected] 

Efim Litovsky, www.itlinc.com 

phase change materials and their dispersions  with nanomaterials could provide large thermal conductivity contrasts.  E.g. hexadecane with CNT or graphene.

There are some recent  publications on such materials.

A agree with John Philip that  apparent thermal conductivity can to be changed strongly during  phase and chemical transformation. See E. Litovsky, V. Issoupov, S. Horodetsky, J. Kleiman, Express Methods for Determination of Apparent and True Thermophysical Properties of Materials within a Temperature Range of – 150 °C ... 1800 °C, Proceedings of the 31st International Thermal Conductivity Conference and the 19th International Thermal Expansion Symposium, May 2013, DEStech Pupbl. However these changes are caused by latent heat effects during transient processes. Conductive/true thermal conductivity can be almost the same. If I understand correctly the task of  Mr. R. Boukhanouf he needs change of true thermal conductivity.

Another way of tuning thermal conductivity is by using magnetic nanofluids, which we have demonstrated  some years ago.  Papers can be down loaded from my RG site. Here by  varying magnetic field, you tune the thermal conductivity.

J. Philip*, Shima.P.D. Baldev Raj, “Nanofluid with tunable thermal properties”

Applied Physics Letters 92, 043108 (2008) doi:10.1063/1.2838304

P.D.Shima, J. Philip* and B. Raj “Magnetically controllable nanofluid with tunable thermal conductivity and viscosity ” Applied Physics Letters 95, 133112 (2009) doi:10.1063/1.3238551

Here is one reference on phase change materials.

S. A. Angayarkanni & J. Philip*, Tunable Thermal Transport in Phase Change Materials Using Inverse Micellar Templating and Nanofillers, J. Phys. Chemistry C 118, (2014) 13972−13980. http://dx.doi.org/10.1021/jp503209y

thanks John for pointing out the excellent publication. my application for a variable thermal conductivity materials is rather for an application in a building envelope- to act as a dynamic insulation- with thermal conductivity changing according to ambient conditions. a layer of the material is to be incorporated within existing construction materials.

If you consider a thermoelectric material then you will have a system where the thermal conductivity is partly influenced by an imposed electrical current passing  through it. This is a result of Onsager coupling laws (and Wiedemann-Franz is a part of this). Nevertheless, if you find a material where two thermodynamics potential are coupled (like Temperature and chemical potential in thermoelectricity), you may modify the heat conductivity by forcing the conjugated flux of the second pontential.

But I do not believe that the resulting effect would be important.  

thank you Chris for your insight. thermoelectric is an active system which needs a constant supply of power. my interest is in passive materials that change their thermal conductivity according to ambient temperature to be used as a construction building materials - this to reduce energy consumption for air conditioning.

Since you are looking for a material that changes its thermal conductivity as a function of temperature, the only reasonable option would be to allow the material to go through a phase transition at the temperature of interest. Organic phase change materials may not be a good candidate for this as they only change their conductivity from ~0.15 W/mk to 0.3 W.mK. However, salt hydrates change their thermal conductivity significantly when they transition for solid to liquid or vice versa. Take calcium chloride hexahydrate (CaCL2.6H2O) as an example: the conductivity of its liquid phase (above 29 degC) is about 0.4-0.5 W/mK whereas the conductivity of its solid phase is more than 1 or 1.5 W/mk.

The same is true for water (above 0 degC; ~0.6 W/mk) whereas ice has around ~2 W/mK.

You could find many salt hydrates in the temperature range of 25-35 degC as suitable candidates for thermal comfort that has the same feature upon its phase transition.