CVD-diamond-based thermocouple for high sensitive temperature measurements

Abstract

The possibility to realize a high sensitive thermocouple by means of boron doped chemical vapour deposition (CVD) diamond was investigated. The thermoelectric power of p-type diamond, grown by plasma enhanced CVD was studied for films of electrical resistivity in the 0.2-40 Ω cm range in order to asses the dependence of thermocouple sensitivity on the doping level. The p-type diamond films were prepared by CH₃OH + B₂O₃ vapour addition to a 1% CH₄-H₂ gas mixture during the growth. The conductive films were then tested tracing the I-V characteristic in order to study the conduction properties of the films. An appropriate experimental setup was built to evaluate the thermoelectric properties of the grown samples for different temperatures imposed between two ends of the samples. Firstly, the output voltage was measured maintaining a reference temperature of 273 K at one end and varying the second temperature between 275.5 and 360.5 K. A constant value of the temperature drop of 5 K was then used for an accurate evaluation of the thermoelectric properties of the diamond films for different value of the average temperature. The measurements provided values of thermoelectric power in the range 0.3-0.6 mV/K while conductivity increases. These values showed different decreasing behaviour with increasing temperature for different resistivity of the sample. In particular, more relevant changes in thermoelectric power were measured for high resistive sample

Thermoelectric transport properties of boron-doped nanocrystalline diamond foils

“…Boron forms a deep acceptor level at 0.37 eV above the valence band edge, which decreases to about 0.1 eV with increasing dopant concentration [14,15]. Astonishingly, only very few reports on Seebeck coefficients in boron-doped CVD-diamond can be found in the literature, and no reports show data below 250 K or above 350 K [7,[16][17][18]. A recent theoretical approach [19] calculates Seebeck coefficients from experimental Hall and resistivity data in a p-type CVD-single-crystal [20].…”

Configurational, electronic entropies and the thermoelectric properties of nanocarbon ensembles

“…Such a re-sult would be consistent with large increases in Seebeck coefficient as a function of boron doping observed in polycrystalline diamond. 15 On the other hand, a large increase in the fraction of sp 2 bonded carbon due to conversion of UNCD to nanographite is expected to lead to an order of magnitude increase in electrical conductivity. The two combined effects, if realized, would yield ZT values in the range of 1-5.…”

“…Boron doping of diamond 15 as well as of carbon nanotubes 16 is known to result in an increase in the Seebeck coefficient. Presumably boron states induce an acceptorlike feature near the top of the valence band.…”

Thermoelectric power factors of nanocarbon ensembles as a function of temperature

“…19,20 The difference in behavior of ␣ between NGEs and NDEs is probably due to a difference in the effect of boron on the thermoelectric properties of these two allotropes of carbon. Here we wish merely to point out that continued annealing up to 12 h of NDEs at 1700 K gives ␣ values that increase with heating time approaching those observed with NGEs ͓compare Figs.…”