The thermal stability of p-type Ba(8)Ga(16)Ge(3)0 clathrates grown from gallium flux has been tested by heat treatment in low pressure Ar atmosphere at 400, 600, and 800 degrees C. Significant gallium loss was observed for all samples during heat treatment. The treatment at 400 degrees C does not significantly change the sample properties, and the samples remain p-type and comparable to the untreated, as-prepared, sample. At 600 degrees C the sample switches from extrinsic p-type to extrinsic n-type, presumably due to significant loss of Ga, and shows a high thermopower but a reduced electrical conductivity compared to as-made n-type samples. Surprisingly, after a thermal treatment at 800 degrees C, the crystal structure seemingly loses less Ga, only reducing the hole concentration to near intrinsic levels and thus has a negative impact on ZT. Regardless of the heat treatment temperature of the p-type samples the thermal conductivity remained exceptionally low, for some samples 0.9 W/m K. Heat treatment can thus greatly affect the thermoelectric properties of p-type Ba(8)Ga(16)Ge(3)0, but the crystal structure remains intact. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3236635
The effects on the thermoelectric properties of Mg 2 Si when adding TiO 2 nanoparticles have been evaluated experimentally. A batch of Mg 2 Si was prepared through direct solid state reaction and divided into portions which were mechanically mixed with different amounts of TiO 2 nanoparticles ranging from 0.5 to 3 vol% and subsequently sintered to disks. All materials showed n-type conduction and the absolute value of the Seebeck coefficient was reduced with increasing amount of TiO 2 added, while the electrical resistivity was greatly reduced. The thermal conductivity was surprisingly little affected by the addition of the nanoparticles. An optimum value of the thermoelectric figure-of-merit ZT ¼ TS 2 r/k was found for the addition of 1 vol% TiO 2 , showing almost three times higher ZT value than that of the pure Mg 2 Si. Larger TiO 2 additions resulted in lower ZT values and with 3 vol% added TiO 2 the ZT was comparable to the pure Mg 2 Si. The sintering process resulted in reduction or chemical reaction of all TiO 2 to TiSi 2 and possibly elemental titanium as well as reduced TiO x . The increased electrical conductivity and the decreased Seebeck coefficient were found due to an increased charge carrier concentration, likely caused by the included compounds or titanium-doping of the Mg 2 Si matrix. The low observed effect on the thermal conductivity of the composites may be explained by the relatively higher thermal conductivity of the included compounds, counter-balancing the expected increased grain boundary scattering. Alternatively, the introduction of compounds does not significantly increase the concentration of scattering grain boundaries.
7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (mTBD) has useful catalytic properties and can form an ionic liquid when mixed with an acid. Despite its potential usefulness, no data on its thermodynamic and transport properties are currently available in the literature. Here we present the first reliable public data on the liquid vapor pressure (temperature from 318.23 K to 451.2 K and pressure from 11.1 Pa to 10 000 Pa), liquid compressed density (293.15 K to 473.15 K and 0.092 MPa to 15.788 MPa), liquid isobaric heat capacity (312.48 K to 391.50 K), melting properties, liquid thermal conductivity (299.0 K to 372.9 K), liquid refractive index (293.15 K to 343.15 K), liquid viscosity (290.79 K to 363.00 K), liquid-vapor enthalpy of vaporization (318.23 K to 451.2 K), liquid thermal expansion coefficient (293.15 K to 473.15 K), and liquid isothermal compressibility of mTBD (293.15 K to 473.15). The properties of mTBD were compared with those of other relevant compounds, including 1,5-diazabicyclo(4.3.0)non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]undec-7ene (DBU), and 1,1,3,3-tetramethylguanidine (TMG). We used the PC-SAFT equation of state to model the thermodynamic properties of mTBD, DBN, DBU, and TMG. The PC-SAFT parameters were optimized using experimental data.
The effects on thermal and electrical properties of adding small amounts of TiO 2 nanoinclusions to bulk Ba 8 Ga 16 Ge 30 clathrate have been investigated. The thermal properties were analysed using the transient plane source technique and the analysis showed a significant decrease in thermal conductivity as the volume fraction of TiO 2 increased from 0 vol. % to 1.2 vol. %. The introduction of TiO 2 nanoparticles caused a shift in the peak value of the Seebeck coefficient towards lower temperatures. The maximum value of the Seebeck coefficient was, however, only little affected. The introduction of TiO 2 nanoparticles into the bulk Ba 8 Ga 16 Ge 30 resulted in an increased electrical resistivity of the sample, thus simultaneously reducing the charge carrier contribution to the thermal conductivity, partly explaining the decrease in total thermal conductivity. Due to the large increase in resistivity of the samples, ZT was only somewhat improved for the material with 0.4 vol. % TiO 2 while the ZT values of the other materials were lower than for the reference Ba 8 Ga 16 Ge 30 material without TiO 2 nanoparticles. The combined results are consistent with a scenario where the nanoparticle introduction causes a light doping of the semiconductor matrix and an increased concentration of phonon scattering centres.
Transient measurements of thermal conductivity are performed with hot disc sensors on samples having a thermal conductivity variation adjacent to the sample surface. A modified computational approach is introduced, which provides a method of connecting the time-variable to a corresponding depth-position. This allows highly approximate-yet reproducible-estimations of the thermal conductivity vs. depth. Tests are made on samples incorporating different degrees of sharp structural defects at a certain depth position inside a sample. The proposed methodology opens up new possibilities to perform non-destructive testing; for instance, verifying thermal conductivity homogeneity in a sample, or estimating the thickness of a deviating zone near the sample surface (such as a skin tumor), or testing for presence of other defects.
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